WO2019159383A1

WO2019159383A1 - Pressure-sensitive adhesive sheet, surface protective sheet, and method for producing pressure-sensitive adhesive sheet

Info

Publication number: WO2019159383A1
Application number: PCT/JP2018/014528
Authority: WO
Inventors: Kousuke Yonezaki; Hakaru Horiguchi
Original assignee: Nitto Denko Corporation
Priority date: 2018-02-13
Filing date: 2018-04-05
Publication date: 2019-08-22

Abstract

[Problem] Provided is a PSA sheet suited for adherends having large surface areas. [Solving Means] A PSA sheet is provided. The PSA sheet is formed by fusing at least two initial PSA sheets at their edge faces. The PSA sheet comprises a substrate layer and a PSA layer provided to at least one face of the substrate layer.

Description

PRESSURE-SENSITIVE ADHESIVE SHEET, SURFACE PROTECTIVE SHEET, AND METHOD FOR PRODUCING PRESSURE-SENSITIVE ADHESIVE SHEET

The present invention relates to a pressure-sensitive adhesive sheet, a surface protective sheet, and a method for producing a pressure-sensitive adhesive sheet.
The present application claims priority to U. S. Provisional Patent Application No. 62/629,834 filed on February 13, 2018, the entire contents of which are incorporated herein by reference.

In processing and transporting various articles, in known techniques to prevent damage (scratches, contamination, corrosion, etc.) of their surfaces, protection sheets are bonded to the surfaces for protection. The objects to be protected vary widely. For instance, protection sheets are used on glass plates bearing Low-E (Low-Emissivity) layers as well. Low-E-layer-bearing glass plates are preferably used as building materials such as window glass because of the effects of the Low-E layers to improve the efficiency to cool down and heat up indoor spaces. In producing such a glass plate, usually, until a Low-E-layer-bearing glass plate and another glass plate are assembled into pair-glass (dual-pane glass) with the Low-E layer surface on the inside, a protection sheet is applied via its adhesive layer to the bear Low-E layer surface which will be otherwise left exposed. This protects the Low-E layer from damage, wearing, degradation, corrosion, etc. Documents related to this type of conventional art include EP2150669B1 (Patent Document 1) and WO2016/139318A1 (Patent Document 2).

In these applications, as a removable bonding means, pressure-sensitive adhesive (or PSA; the same applies hereinafter) can be preferably used. In general, pressure-sensitive adhesive has characteristics of being in a soft solid (viscoelastic) state in a room temperature range and easily adhering to adherend under some pressure. A surface protection sheet using PSA typically has a PSA layer on one face of a substrate sheet formed of a material such as resin and is constituted so as to achieve a protection purpose when applied via the PSA layer to an adherend (an object to be protected). Conventional art documents disclosing PSA sheets usable as surface protection sheets include JP2017-186517A (Patent Document 3), JP5719194B2 (Patent Document 4), JP2012-131976A (Patent Document 5) and JP3571460B2 (Patent Document 6). JP2010-194947A (Patent Document 7) and JP2012-86582A (Patent Document 8) are conventional art documents related to methods for bonding sheet members by laser irradiation.

As the adherend (e.g. a building material such as window glass), in view of efficiency of production, transportation, etc., objects having large surface areas are used, among which some have surface widths as large as about 2.6 m or larger, or even about 3 m or larger. For an adherend having such a large surface area (or a large-surface adherend), a method where several PSA sheets are layered with partial overlaps to entirely cover the adherend surface is used. In this method, however, the several PSA sheets need to be seamlessly placed when they are applied, making the placement work complicated. It may be possible to employ a method in which several PSA sheets are connected with PSA tape, etc. In any case, however, a connection between PSA sheets forms an uneven segment due to their partial overlap. This is likely to give rise to wrinkling and lifting which cause permeation of water such as moisture, making such a method unsatisfactory in view of protection.

As a solution, it is desirable to produce a PSA sheet that covers the entire surface of the large-surface adherend. With respect to the length direction, the PSA sheet is not usually limited in size when it is supplied from a roll or the like; however, with respect to the width direction, it is limited by the PSA sheet manufacturing equipment, etc. Thus, production of a PSA sheet sufficient both in length and in width requires large-scale equipment that allows production of a wide PSA sheet. In addition, adherend objects (e.g. glass plates used for building materials) are not necessarily produced in the same size in a large quantity and their sizes can be different each time. Such a case may require an on-demand supply of a certain quantity of PSA sheets that meet the size requirement and may not be suited for handling by large-scale manufacturing equipment.

The present invention has been made in view of such circumstances with an objective to provide a PSA sheet and a surface protective sheet suited for adherends having large surface areas. Another objective of this invention is to provide a method for producing a PSA sheet, the method allowing production of large-surface PSA sheets in a required quantity.

The present description provides a PSA sheet (final PSA sheet) obtainable by fusing at least two initial PSA sheets (or PSA sheet-workpieces, or workpiece-PSA sheets) via their edge faces. The PSA sheet typically comprises a substrate layer and a PSA layer provided to at least one face of the substrate layer.

The PSA sheet in this embodiment has a larger surface area than the respective initial PSA sheets and is able to cover a larger adherend surface. Such a PSA sheet is suited for an application in which it is applied to a large-surface adherend. According to this embodiment, as compared to a conventional method using several PSA sheets, no gaps are formed in the final PSA sheet (between initial PSA sheets), whereby placement adjustment is not required. It does not require a component (a third component) to fill gaps when such gaps are formed, either. Moreover, unlike indirect joining via a joining member such as PSA tape or the like, joining by fusion (typically by direct fusion) may provide a PSA sheet that has sufficient joint strength and is of high quality with its adhesive face, back face, etc., as smooth as those of the initial PSA sheets before joined. Such a PSA sheet has good adhesive properties; for instance, when used as a protective sheet, it may provide good protection.

In a preferable embodiment of the art disclosed herein (including the PSA sheet, the surface protective sheet and the method for producing the PSA sheet; the same applies hereinafter), the at least two initial PSA sheets have long lengths and are fused at edge faces of their width directions (fused at their lengthwise edge faces). Long initial PSA sheets fused at edge faces of their width directions may form a PSA sheet that extends sufficiently both in length and in width. Such a PSA sheet is particularly suitable for an application where it is applied to a large-surface adherend.

In an embodiment, the PSA sheet disclosed herein and the initial PSA sheet are identified by having a long side and a short side with respect to its plane (sheet face). By definition, the long side is longer than the short side and the short side is shorter than the long side. For instance, the short side may be approximately perpendicular to the long side. The length direction of the PSA sheet is in the direction along the long side and the width direction is in the direction perpendicular to the length direction. Thus, as used herein, the “width” is defined as the length in the direction perpendicular to the length direction. Typical examples of the PSA sheet disclosed herein include a PSA sheet that is described as having a long length, a band shape, and a rectangular shape. The long side is a line segment that runs almost linearly. The short side is not limited to a straight line and can be a curve, zig-zag, etc. In another embodiment, the PSA sheet and the initial PSA sheet may be have square planar shapes or other shapes.

In a preferable embodiment of the art disclosed herein, the PSA sheet has first and second faces forming its outer surface. Here, the second face is the opposite face of the first face (on the reverse side of the first face). At least either the first face or the second face is an adhesive face. The PSA sheet also has a fused segment between the at least two initial PSA sheets, which is free of an uneven segment either on the first face or on the second face. The PSA sheet is free of an uneven segment such as a projection on either face. Thus, for instance, the adhesive face of the PSA sheet is likely to tightly adhere to an adherend without wrinkling, lifting, etc., caused by an uneven segment. When the PSA sheet is stored in a roll form, defects arising from an uneven segment will not occur.

In a preferable embodiment of the art disclosed herein, the adhesive face is essentially free of a recess in the fused segment between the at least two initial PSA sheets. In this description, “being essentially free of a recess” indicates absence of a recess; or if any, a recessed depth of 1 μm or less. In this embodiment, the PSA sheet can tightly adhere to an adherend without a gap between itself and the adherend. The use of the PSA sheet as a surface protective sheet can prevent entry of a foreign substance into the protected surface via a recess as a channel for entry. For instance, it can prevent permeation of water such as moisture, providing excellent waterproofness. This is particularly preferable for an application involving protection of a surface (e.g. a Low-E layer surface of a Low-E layer-bearing glass plate) that may undergo the sort of degradation and alteration upon permeation of water.

In a preferable embodiment of the art disclosed herein, the PSA sheet has a long length. The fused segment between the at least two initial PSA sheets runs in the length direction of the PSA sheet. The PSA sheet in such an embodiment may be wider than the respective initial PSA sheets, extending sufficiently not only in length, but also in width.

In a preferable embodiment of the art disclosed herein, the fusion is thermal fusion by laser irradiation (the at least two initial PSA sheets are thermally fused by laser irradiation). The thermal fusion by laser irradiation enables continuous high-quality joining, allowing efficient production of a PSA sheet with sufficient joint strength. In a typical embodiment, two or more initial PSA sheets are fused solely by thermal fusion by laser irradiation without using a joining member that may form an uneven segment. Thermal fusion by laser irradiation is also advantageous in that it preferably enables continuous seamless joining without forming a channel that allows passage of water and the like in the thickness direction.

In a preferable embodiment of the art disclosed herein, the substrate layer is a resin layer that comprises a thermoplastic resin as its primary component. With the use of the thermoplastic resin as the primary component of the substrate layer, the initial PSA sheets are preferably joined by fusion. The substrate layer is more preferably formed from at least one species of resin selected from the group consisting of polyolefinic resin, poly(vinyl chloride)-based resin and polyurethane-based resin.

In a preferable embodiment of the art disclosed herein, the substrate layer has a thickness of 25 μm to 150 μm. When the substrate layer’s thickness is at or above the prescribed value, the edge faces to be fused will have larger surface areas and sufficient joint strength can be readily obtained. When the substrate layer’s thickness is limited to or below the prescribed value, fusion by a laser beam and the like can be efficiently carried out.

In a preferable embodiment of the art disclosed herein, the PSA layer satisfies at least one of the following: it is an acrylic PSA layer comprising an acrylic polymer as the base polymer or a rubber-based PSA layer comprising a rubber-based polymer as the base polymer; and the PSA layer has a thickness of 1 μm to 20 μm. In an embodiment that satisfies one or each of the above, the effects of the art disclosed herein can be preferably obtained.

The PSA sheet according to a preferable embodiment shows an initial peel strength of 0.01 N/20mm to 5 N/20mm to a glass plate. When the to-glass initial peel strength is 0.01 N/20mm or greater, the PSA sheet can tightly adhere to an adherend. Because the peel strength is limited to or below 5 N/20mm, the PSA sheet is easily removed after applied, allowing easy re-application and efficient removal. The PSA sheet having such properties is preferable as a surface protective sheet. An increase in bonding area attributed to an increased surface area may lead to heavy peel; and therefore, in view of the efficiency of removal, it is desirable that the initial peel strength is limited to or below a prescribed value.

In a preferable embodiment of the art disclosed herein, the PSA sheet has a long length. The PSA sheet has a width of about 2.6 m or greater. In such an embodiment, the PSA sheet can cover the entire surface of an adherend having a width of 2.6 m or greater.

The PSA sheet according to a preferable embodiment has a first section formed of the first initial PSA sheet and a second section formed of the second initial PSA sheet. Here, the first and second initial PSA sheets are included in the at least two initial PSA sheets. Each of the first and second sections comprises a substrate layer and a PSA layer provided to at least one face of the substrate layer. One edge of the substrate layer in the first section is joined to one edge of the substrate layer in the second section. One edge of the PSA layer in the first section is continuous with one edge of the PSA layer in the second section. This embodiment preferably brings about the effects of the art disclosed herein.

In a more preferable embodiment, both the first section formed of the first initial PSA sheet and the second section formed of the second initial PSA sheet have long lengths. The first and second sections are continuous, with one edge face of the width direction of the first initial PSA sheet fused to one edge face of the width direction of the second initial PSA sheet. The resulting PSA sheet formed in one piece may be wider than the respective initial PSA sheets and extend sufficiently not only in length, but also in width.

In the PSA sheet according to an embodiment, the first section and the second section have an equal width. In another embodiment, the first section and the second section have different widths. By suitably selecting the widths of the first and second initial PSA sheets corresponding to the first and second sections, a PSA sheet having a desirably large width can be efficiently obtained without relying on large-scale PSA sheet manufacturing equipment.

In a preferable embodiment, the PSA species of the first and second sections are of equal type. The resin species forming the substrate layers in the first and second sections are of equal type as well. In such an embodiment, the PSA sheet has uniform properties over the entire sheet face. When the first and second initial PSA sheets corresponding to the first and second sections are of equal type in terms of the PSA species, the two are likely to undergo thermal fusion and good joint strength is likely to be obtained.

In this description, that “the PSA species are of equal type” does not require them to have an equal composition, but rather indicates that they belong to the same category when they are classified by their types of base polymer such as being acrylic or rubber-based, etc. They do not need to have an equal monomer composition or PSA composition. For instance, when the base polymers in the PSAs in two different areas are both acrylic polymers, the PSA species of these two sections are of equal type. Other than this, that “PSAs are the same” means literally that they are the same PSA, basically having an equal composition. The PSA in the first section more preferably comprises the same base polymer as that of the PSA in the second section and particularly preferably has essentially the same composition as that of the PSA in the second section.

Similarly, in this description, that “the resin species forming substrate layers are of equal type” does not require them to have an equal composition, but rather indicates that they belong to the same category when they are classified by their types of base polymer such as polyolefinic resin, poly(vinyl chloride)-based resin, polyurethane-based resin, etc. They do not need to have an equal monomer composition or resin composition. For instance, when the base polymers in substrate layers in two different areas are both polyolefinic polymers, the resin species forming the substrate layers are of equal type. Other than this, that “resins are the same” means literally that they are the same resin, basically having an equal composition. The substrate layer in the first section more preferably comprises the same base polymer as that of the substrate layer in the second section and particularly preferably has essentially the same composition as that of the substrate layer in the second section.

The present description provides a surface protective sheet formed from a PSA sheet disclosed herein. In the surface protective sheet, the PSA sheet is an adhesively single-faced PSA sheet wherein the PSA layer is provided solely to one face of the substrate layer. The PSA sheet disclosed herein is preferably used as a surface protective sheet.

This description provides a method for producing the PSA sheet. The production method comprises a step of obtaining at least two initial PSA sheets; a step of joining the two initial PSA sheets by abutting the two initial PSA sheets at their edge faces and fusing the abutted edge faces (abutted segment). In a preferable embodiment, the joining step is a step of joining the two initial PSA sheets by abutting the two initial PSA sheets at their edge faces and subjecting the abutted segment to laser irradiation so as to fuse the edge faces.

By joining initial PSA sheets by fusion as described above, the resulting PSA sheet can have a larger surface area while maintaining comparable quality to the original initial PSA sheets before joined. By joining the edge faces, the resulting PSA sheet surface may be free of an uneven segment. As the means of fusion, thermal fusion by laser irradiation is typically employed to achieve a high-quality joint. The PSA sheet obtained by this method has a larger sheet area with good adhesive properties. For instance, when used as a protective sheet, it may provide good protection. In particular, the fusion (preferably thermal fusion by laser irradiation) can preferably achieve a continuous seamless joint and prevent formation of a channel that allows passage of water and the like in the thickness direction. This may be an important property in an application that requires surface protective properties. Because it is free of such seams (gaps), no third component to fill the seams is necessary. In terms of surface protection and efficiency of the protection work, it is superior to the conventional art where several PSA sheets are applied with overlaps. By employing the method, a large surface area can be obtained from initial PSA sheets smaller than the actual size required and initial PSA sheets that are already available can be effectively used. In addition, when no large-scale equipment is available for producing a PSA sheet having a large surface area, conventional equipment can be used to fabricate initial PSA sheets and the method described above can be then employed to produce a PSA sheet having a desirable size.

In a preferable embodiment of the PSA sheet production method disclosed herein, the two initial PSA sheets are both long (have long lengths). In the joining step, lengthwise edge faces of the two initial PSA sheets are abutted and fused. This method allows effective use of a PSA sheet that has been prepared (obtained) in advance with a smaller width than required. Even when a large-scale system is not available for manufacturing a wide PSA sheet, after a PSA sheet is fabricated with an available system, a PSA sheet having a desirably wide width can be produced by the aforementioned method.

Fig. 1 shows a perspective diagram illustrating a general constitution of the PSA sheet according to an embodiment. Fig. 2 shows schematic cross-sectional diagrams to illustrate the PSA sheet production method according to an embodiment; (a) and (b) illustrate how two initial PSA sheets are abutted, (c) illustrates thermal fusion by laser irradiation, and (d) shows a PSA sheet obtained by laser thermal fusion. Fig. 3 shows a schematic cross-sectional diagram to illustrate a modification example of the PSA sheet production method, corresponding to Fig. 2(c). Fig. 4 shows an enlarged cross-sectional SEM image of the thermally fused segment in the PSA sheet according to Experimental Example. Fig. 5 shows an enlarged cross-sectional SEM image showing part of Fig. 4.

Preferred embodiments of the present invention are described below. Matters necessary to practice this invention other than those specifically referred to in this description can be understood by a person skilled in the art based on the disclosure about implementing the invention in this description and common technical knowledge at the time of application. The present invention can be practiced based on the contents disclosed in this description and common technical knowledge in the subject field.

<Constitution of PSA sheet>
As used herein, the term "PSA" refers to, as described earlier, a material that exists as a soft solid (a viscoelastic material) in a room temperature range and has a property to adhere easily to an adherend with some pressure applied. As defined in C. A. Dahlquist, "Adhesion : Fundamental and Practice" (McLaren & Sons (1966), P. 143), the PSA referred to herein is a material that has a property satisfying complex tensile modulus E* (1Hz) < 10⁷ dyne/cm² (typically, a material that exhibits the described characteristics at 25 °C). The concept of PSA sheet herein may encompass so-called PSA tape, PSA labels, PSA film, etc. The PSA sheet disclosed herein can be in a roll or in a flat sheet. Alternatively, the PSA sheet may be processed into various shapes.

The PSA sheet disclosed herein has a PSA layer on a substrate layer (support substrate). Fig. 1 shows a cross-sectional structure of the PSA sheet according to an embodiment. PSA sheet 1 is in an embodiment where a PSA layer 20 is provided on one face (first face) 10A of a substrate-layer sheet 10; for use, the surface (adhesive face) 20A of PSA layer 20 is applied to an adherend. PSA layer 20 is placed to cover the entire surface of the first face 10A of substrate layer 10. The adhesive face 20A of PSA layer 20 forms the first face 1A of PSA sheet 1. When PSA sheet 1 is used as a surface protective sheet, the surface 20A of PSA layer 20 is applied to an object to be protected. The back face 10B (the second face, i.e. the opposite face of the first face 10A) of substrate layer 10 is also the back face (second face) 1B of PSA sheet 1, forming the outer surface of PSA sheet 1. PSA sheet 1 prior to use (i.e. before applied to an adherend) may be in an embodiment where the surface 20A of PSA layer 20 is protected with a release liner having a release face at least on the PSA layer side. Alternatively, in the PSA sheet 1, the second face 10B of substrate layer 10 is in contact with the surface (adhesive face) 20A of PSA layer 20 to protect the surface. In this embodiment, the second face (back face) 10B is a release face. The PSA sheet can be a substrate-bearing double-faced PSA sheet having a PSA layer on each face of the substrate layer. In this case, each face of the substrate layer is entirely covered with a PSA layer.

PSA sheet 1 is formed by fusing two initial PSA sheets. In particular, PSA sheet (final PSA sheet) 1 has a first section 2 formed of the first initial PSA sheet and a second section 4 formed of the second initial PSA sheet. One edge face of the first initial PSA sheet and one edge of the second initial PSA sheet are directly fused so that the first section 2 and the second section 4 are continuous. In this embodiment, as the means of fusion, thermal fusion by laser irradiation is employed. Thus, the PSA sheet disclosed herein can be called a fused PSA sheet (fused PSA sheet) as well.

Both the two initial PSA sheets as materials of PSA sheet 1 have long lengths (tape forms) and they are fused edge-face to edge-face both along their lengthwise direction. By the edge-face-to-edge-face joining, PSA sheet 1 is less likely to form an uneven segment on the adhesive face or on the back face. In this embodiment, the two initial PSA sheets used are of equal type. Thus, the two initial PSA sheets are equal in terms of their layer structures, materials of which they are formed, their shapes and sizes (lengths, widths and thicknesses). Because of this, of the PSA sheet 1, the first section 2 (first initial PSA sheet) and the second section 4 (second initial PSA sheet) have an equal thickness, with the substrate layers and the PSA layers in the respective areas having equal thicknesses. Such an embodiment is preferable as it does not form an uneven segment. Between the two sections, differences in thicknesses of PSA sheet, substrate layer and PSA layer are preferably within about ±10 % ranges (e.g. ±5 % ranges).

In top view of PSA sheet 1, fused segment 6 (the border between the first section 2 and the second section 4) of the two initial PSA sheets runs linearly in one direction; in stereo view, it is a portion formed by the linear segment extending in the thickness direction. More specifically, fused segment 6 continuously extends in the plane of PSA sheet 1, joined seamlessly. In this embodiment, PSA sheet 1 has a long length and fused segment 6 extends in the length direction of PSA sheet 1. The PSA sheet may not be long, having instead a square shape, etc.

The first and

second faces

1A and 1B of PSA sheet 1 form the outer surface of PSA sheet 1 as its main surface (meaning that it is not an edge face). When direct fusion is employed, fused segment 6 of PSA sheet 1 does not have an uneven segment such as a projection (e.g. a segment projecting to a height of about 10 μm or more, preferably about 5 μm or more, typically 1 μm or more) either on the first face 1A or on the second face 1B. Adhesive face 20A in fused segment 6 is free of a recess that may form a gap after applied to an adherend; or if any, the recess has a depth of about 1 μm or less (e.g. 0.5 μm or less). This can prevent defects (e.g. with respect to the tightness of adhesion to adherend, etc.) caused by an uneven segment and further prevent permeation of water into the sealed face due to a recess, etc.

<Method for producing PSA sheet>
Described next is a method for producing the PSA sheet disclosed herein. The production method comprises a step of obtaining at least two initial PSA sheets; and a step of joining the two initial PSA sheets by abutting the two initial PSA sheets at their edge faces and fusing the abutted edge faces (abutted segment). While referencing to Fig. 2 and showing a method for producing PSA sheet 1 according to this embodiment as an example, the production method disclosed herein is described below. However, the art disclosed herein is not limited to this production method.

As shown in Fig. 2(a), first and second

initial PSA sheets

50 and 60 are obtained for use in producing PSA sheet 1. The first and second

initial PSA sheets

50 and 60 are formed as initial PSA sheets having release liners in which the surfaces of the respective PSA layers 52 and 62 are protected with

release liners

53 and 63. As

release liners

53 and 63, polyethylene terephthalate (PET) film whose PSA layer-side surface has been treated with a release agent is used. Other features about the first and second

initial PSA sheets

50 and 60 are as described earlier. Thus, further details are omitted here. The initial PSA sheet (workpiece) is also called a PSA sheet material.

As shown in Fig. 2(b), the first and second

initial PSA sheets

50 and 60 obtained are abutted so that their edge faces 50C and 60C face each other. In particular, edge face 50C of the first initial PSA sheet 50 and edge face 60C of the second initial PSA sheet 60 are placed face to face at a distance of about 100 μm or less (e.g. about 50 μm or less). Edge faces 50C and 60C may be partially in contact or may be in contact without a break. In this embodiment, the first and second

initial PSA sheets

50 and 60 are both long and each of edge faces 50C and 60C is an edge face of the width direction (a lengthwise edge face). For high-quality fusion, the edge faces of the initial PSA sheets may be subjected in advance to a process such as cutting to form straight edge lines and smooth edge faces. Such cutting is not limited to a straight line mode and can be carried out in a curved line mode, a wavy line mode, and so on. It can be slanted as well in the thickness direction.

Subsequently, as shown in Fig. 2(c), the border between the first and second

initial PSA sheets

50 and 60 which is the segment formed by abutting edge faces 50C and 60C (i.e. the segment to be joined) is covered on top and bottom with

cover films

70 and 80. As the

cover films

70 and 80, a thermosetting resin film, a thermoplastic resin film having a higher melting point than the corresponding PSA sheet, a glass plate and the like are used. In this embodiment, as

cover films

70 and 80, a PET film having a thickness of about 20 μm to 500 μm (preferably about 25 μm to 150 μm) is used. For high-quality fusion, the widths of

cover films

70 and 80 are preferably about 5 mm or greater (e.g. 10 mm or greater). With the use of

cover films

70 and 80 having smooth surfaces, the first and second

initial PSA sheets

50 and 60 can be joined to a flush surface free of an uneven segment.

With the abutted segment enclosed in between

cover films

70 and 80, the first and second

initial PSA sheets

50 and 60 are loaded on a stage (not shown in the drawings). In this embodiment, both the first and second

initial PSA sheets

50 and 60 are loaded so that substrate layers 51 and 61 are on top with PSA layers 52/62 and release liners 53/63 are placed in this order toward the bottom. The abutted segment of the first and second

initial PSA sheets

50 and 60 is fastened (fixed in place) with a fastening member (not shown in the drawings) which is pressed over cover film 70 placed atop. As the fastening member, a transparent glass plate and the like are used. The fastening pressure exerted by the fastening member is not particularly limited. It can be selected from a range of, for instance, about 0.5 kgf/cm² to 100 kgf/cm² (preferably 1 kgf/cm² to 20 kgf/cm²). Assist gas can be supplied to the segment to be fused (e.g. the segment subject to laser irradiation) to make the cover films to tightly adhere to the initial PSA sheets.

The abutted segment of edge faces 50C and 60C is fused to join the first and second

initial PSA sheets

50 and 60. In particular, the abutted segment of edge faces 50C and 60C is irradiated by a laser beam R to thermally fuse the first and second

initial PSA sheets

50 and 60. According to thermal fusion by laser irradiation, high-quality fusion can be achieved. More specifically, by laser irradiation, at edge faces 50C and 60C, the substrate layers 51 and 61 of the first and second

initial PSA sheets

50 and 60 are thermally fused together and their PSA layers 52 and 62 are also thermally fused together. PSA layers 52 and 62 may be viscoelastic bodies that deform (liquefy and spread) later to undergo autohesion even without thermal fusion. From the standpoint of the smoothness of the adhesive face, it is preferable that PSA layers 52 and 62 are to be thermally fused.

Cover films

70 and 80 are removed when appropriate after the thermal fusion is completed.

The method of laser irradiation is not particularly limited. It can be carried out by employing a known or conventional method or making a suitable modification thereto if necessary, by scanning the segment to be fused by a laser beam or by sending two initial PSA sheets through a laser beam. By this, the first and second

initial PSA sheets

50 and 60 are continuously fused; and in the fused segment 6, no channel (gap) is formed to allow passage of water and the like in the thickness direction. As the laser beam, a semiconductor laser is used in this embodiment, but it is not limited to this. Other than this, various types of laser beam can be used, such as an Nd-YAG laser, a fiber laser, a carbon dioxide laser, etc. As the oscillating method, a pulse laser can be employed, such as a CW laser (continuous wave laser) and a femtosecond laser. From the standpoint of the permeability relative to the resin material, the depth reached by the laser beam, the ease of fusion, etc., the laser beam preferably has a wavelength in the near-infrared range (specifically, in a range of 800 nm to 2000 nm). To increase the heat efficiency by the laser beam, the cover films and the edge faces to be fused may be provided (coated, etc.) with a laser absorber.

As described above, as shown in Fig. 2(d), PSA sheet 1 wherein two initial PSA sheets are fused and joined is produced. The resulting first face 1A (adhesive face 20A) and second face (back face) 1B of the PSA sheet 1 are smooth, including the fused segment 6. In particular, the first section 2 formed of the first initial PSA sheet 50 and the second section formed of the second initial PSA sheet 60 are free of an uneven segment and are essentially free of a recess as well. The first section 2 and the second section 4 are joined at the fused segment 6, having sufficient strength. The fused segment 6 is joined seamlessly and continuously. Thus, when PSA sheet 1 is used as a surface protective sheet, it does not require the sort of a third component for gap filling (possibly a known or conventional powder or liquid coating such as those used in conventional art where several PSA sheets are applied with overlaps). In the resulting PSA sheet 1, the adhesive face 20A remains protected with

release liners

53 and 63. PSA sheet 1 thus obtained is used after arbitrarily cut to a suitable size or stored in a roll form, etc.

In this embodiment, the PSA sheet is fabricated by fusing two initial PSA sheets. However, the number of initial PSA sheets can be just two or more; for instance, three or more, or even four or more initial PSA sheets can be joined by fusion.

In this embodiment, the initial PSA sheets are joined by thermal fusion by laser irradiation. However, in the art disclosed herein, various techniques capable of fusing initial PSA sheets can be employed. A typical fusion technique is thermal fusion performed with respect to thermoplastic resins in PSA sheet materials. Thermally melted PSA sheet materials mix together and solidify again to be fused and joined. Examples of such thermal fusion techniques include hot pressing in addition to laser irradiation. Hot pressing can be carried out by pushing a heat plate heated to a certain temperature against a segment to be fused, optionally over a cover film, etc. In the art disclosed herein, in the fusion process, various subsidiary materials such as meltable fastening materials can be used; however, because initial PSA sheets are fused at edge faces, the use of such a material is not necessary. Because no fastening materials or various subsidiary materials are used; and therefore, there is no uneven segment and sufficient joint strength can be obtained without using such a material.

In this embodiment, the joining of initial PSA sheets is carried out as seamless, continuous fusion. However, the art disclosed herein is not limited to this. For instance, in an application requiring no adherend surface protection, it is possible to select an embodiment where edges to be joined are fused intermittently at certain intervals or an embodiment where only certain areas of opposing edge faces are fused together.

In this embodiment, the two initial PSA sheets have long lengths, but the art disclosed herein is not limited to this. In accordance with the shape of the PSA sheet to be produced, they can be in arbitrary shapes such as square shapes. In this embodiment, the fused faces (the abutted edge faces) of the respective long initial PSA sheets are edge faces of their width directions (lengthwise edge faces), but they can be instead edge faces of their length directions (widthwise edge faces).

In this embodiment, the first section (first initial PSA sheet) and the second section (second initial PSA sheet) have an equal layer structure (a single-faced PSA sheet having a substrate layer) and are of equal type. However, the materials of the first section (first initial PSA sheet) and the second section (second initial PSA sheet) are not particularly limited as long as they can be fused together. From the standpoint of the ease of fusion, the joint strength, etc., the PSA species of the first section (first initial PSA sheet) and the second section (second initial PSA sheet) are preferably of equal type. It is more preferable that the same PSA is used. Similarly, the resin species forming the substrate layers of the first section (first initial PSA sheet) and the second section (second initial PSA sheet) are preferably of equal type. It is particularly preferable that the substrate layers of the first section (first initial PSA sheet) and the second section (second initial PSA sheet) have an essentially equal composition.

In this embodiment, the long first section (first initial PSA sheet) and second section (second initial PSA sheet) have an equal width, but they can have different widths as well. For instance, two kinds of initial PSA sheets differing in width can be used to produce three kinds of final PSA sheets differing in width.

This embodiment uses release liners, but they are not necessary. In such a case, by directly covering the adhesive face of the PSA sheet with a transparent glass plate or cover film, the initial PSA sheets can be fixed in place without a problem. For instance, as in the modified example shown in Fig. 3, cover film 70 is placed over the substrate layers 51 and 61 and cover film 80 is placed in contact with the surface 20A of PSA layer 20 via no release liner. By this, while the abutted segment of edge faces 50C and 60C are enclosed in between

cover films

70 and 80, the abutted segment can be irradiated by a laser beam R to thermally fuse the first and second

initial PSA sheets

50 and 60. Cover film 80 placed on the adhesive face side preferably has a releasable surface on one face (the face opposing the adhesive face) and cover film 70 placed on the backside also preferably has a releasable surface on one face in view of easy release from the back face. Accordingly, cover

films

70 and 80 can be release liners. When the fastening member serves as a cover film as well, a cover film is not separately required, allowing a fusion operation such as laser irradiation to be performed without a cover film or with a cover film placed only on top of the PSA sheet.

<Dimensions of PSA sheet, etc.>
When the PSA sheet has a long length, the width of the PSA sheet is not particularly limited. It is suitably about 1 m or greater. A preferable PSA sheet has a width of, for instance, about 2 m or greater, or even about 2.6 m or greater. The width of the PSA sheet may be greater than 2.6 m (e.g. 3 m or greater). The maximum width of the PSA sheet is not particularly limited. From the standpoint of the manufacturing, the handling properties, etc., it is usually suitably about 5 m or less, for instance, about 4 m or less. The long PSA sheet has a length (the distance in the length direction) equal to or greater than the width.

The thickness of the PSA sheet disclosed herein is not particularly limited. From the standpoint of the handling properties, the lightness of weight, etc., it is usually suitably about 1000 μm or less (typically about 300 μm or less, e.g. about 150 μm or less). In an embodiment, the thickness of the PSA sheet is preferably about 120 μm or less, more preferably about 100 μm or less, yet more preferably about 75 μm or less, or possibly, for instance, less than 60 μm. The thickness of the PSA sheet can be typically greater than 20 μm, preferably greater than 30 μm, or more preferably greater than 40 μm, for instance, greater than 45 μm.
As used herein, the thickness of the PSA sheet includes the thicknesses of the PSA layer and the substrate layer, but excludes the thickness of the release liner.

The thickness of the substrate layer constituting the PSA sheet disclosed herein is not particularly limited. The thickness of the substrate layer can be, for instance, about 800 μm or less (typically about 250 μm or less). In an embodiment, the thickness of the substrate layer (typically, non-foamed resin film) is preferably about 150 μm or less, more preferably about 100 μm or less, or yet more preferably less than 65 μm, for instance, less than 55 μm. When the substrate layer’s thickness is limited to or below a certain value, thermal fusion (preferably by laser irradiation, etc.) can be efficiently carried out. With decreasing thickness of the substrate layer, the PSA sheet tends to exhibit greater conformability to the adherend shape while its lifting and peeling tend to be inhibited. From the standpoint of adherend protection and handling properties, etc., the thickness of the substrate layer can be typically about 10 μm or greater, preferably about 25 μm or greater, more preferably greater than about 30 μm or greater, or yet more preferably greater than 40 μm. When the substrate layer’s thickness is at or above a certain value, the surface area of the edge face to be thermally fused increases and sufficient bonding strength is likely to be obtained.

No particular limitations are imposed on the thickness of the PSA layer constituting the PSA sheet disclosed herein. From the standpoint of preventing adhesive transfer to the adherend, the thickness of the PSA layer is usually about 50 μm or less, suitably about 30 μm or less, preferably about 15 μm or less, or more preferably about 8 μm or less (e.g. less than 6 μm). In another embodiment, from the standpoint of the ease of removal, etc., the thickness of the PSA layer is suitably about 5 μm or less, about 4 μm or less, or possibly, for instance, 3 μm or less. From the standpoint of the adhesion, the thickness of the PSA layer is usually suitably about 0.5 μm or greater, preferably about 1 μm or greater, or more preferably greater than 2 μm. The thickness of the PSA layer is greater than 3 μm, for instance, greater than 4 μm.

The PSA sheet may be in a PSA sheet roll form. The PSA sheet roll may be in an embodiment where the PSA sheet is wound in its length direction, having a substrate layer with the first and second faces and a PSA layer provided on the first face. With respect to a PSA sheet roll in which the PSA sheet disclosed herein is wound in the length direction, the diameter is not particularly limited. From the standpoint of the ease of winding, it is advantageous that the diameter of the PSA sheet roll is not excessively large. From such a standpoint, the diameter of the PSA sheet roll is usually suitably about 1 m or smaller, or preferably about 50 cm or smaller. In view of the use, storage, efficient transportation, etc., the art disclosed herein can be favorably implemented in an embodiment of the PSA sheet roll having a diameter of about 5 cm or larger (e.g. about 15 cm or larger).

<Properties of PSA sheet>
The PSA sheet disclosed herein suitably exhibits an initial peel strength of about 0.01 N/20mm or greater to a glass plate, determined at a tensile speed of 0.3 m/min at 180° peel angle. The PSA sheet showing such initial peel strength adheres well to an adherend in relatively short time and is less likely to lift off the adherend. When the PSA sheet disclosed herein is used as a surface protective sheet, it may provide good protection. In an embodiment, the initial peel strength can be about 0.05 N/20mm or greater (e.g. about 0.1 N/20mm or greater). In another embodiment, the initial peel strength can be about 0.5 N/20mm or greater (e.g. about 1 N/20mm or greater). The maximum initial peel strength is not particularly limited. From the standpoint of light peel, it is usually suitably about 5 N/20mm or less, or preferably about 2.5 N/20mm or less (e.g. about 2 N/20mm or less). In an embodiment, the initial peel strength can be about 1 N/20mm or less (e.g. 0.4 N/20mm or less). The to-glass-plate initial peel strength is determined by the method described below.

〔Initial peel strength to glass plate〕
The PSA sheet to be measured is cut to a 20 mm wide by 100 mm long strip to prepare a test piece. In a standard environment at 23 °C, 50 % RH, with a 2 kg rubber roller moved back and forth twice, the test piece is press-bonded to a glass plate as the adherend. The sample is stored in the standard environment for 30 minutes. In the same standard environment, using a universal tensile tester, the initial peel strength (N/20mm) is determined at a tensile speed of 0.3 m/min, at 180° peel angle. As the glass plate, a cut blue plate available from Matsunami Glass Ind. (1.35 mm thick, 100 mm by 100 mm) can be used. The initial peel strength can also be determined, using a similar product or other commercial glass plate as the adherend.

After applied to a glass plate and stored at 50 °C for seven days, the PSA sheet disclosed herein preferably exhibits an aged peel strength less than about 11 N/20mm, determined at a tensile speed of 0.3 m/min at 180° peel angle. With the PSA sheet satisfying this property, even when it is applied to the adherend for a relatively long time, the aged adhesive strength is sufficiently suppressed and its light peel from adherend can be maintained. Thus, it shows excellent efficiency of removal from adherends. This is particularly meaningful when the PSA sheet disclosed herein is used as a surface protective sheet. According to the PSA sheet showing an aged peel strength of about 5 N/20mm or less (more preferably about 2 N/20mm or less), greater efficiency of removal can be achieved. From the standpoint of inhibiting lifting and peeling while the adherend is protected (e.g. during processing of the adherend with the PSA sheet applied thereon), the aged peel strength is usually suitably about 0.05 N/20mm or greater, preferably about 0.1 N/20mm or greater, or more preferably about 0.3 N/20mm or greater. The aged peel strength is determined by the method described below.

〔Aged peel strength to glass plate〕
The PSA sheet to be measured is cut to a 20 mm wide by 100 mm long strip to prepare a test piece. In a standard environment at 23 °C, 50 % RH, with a 2 kg rubber roller moved back and forth twice, the test piece is press-bonded to a glass plate as the adherend. The sample is stored in an environment at 50 °C for seven days and then in a standard environment at 23 °C, 50 % RH for one hour. Subsequently, in the same standard environment, using a universal tensile tester, the aged peel strength (N/20mm) is determined at a tensile speed of 0.3 m/min, at 180° peel angle. The glass plate used as the adherend is the same as the one used in the initial peel strength measurement.

The PSA sheet disclosed herein has an initial peel strength P₁ (N/20mm) and an aged peel strength P₂ (N/20mm), preferably showing an increase in aged adhesive strength (P₂ - P₁, the difference between the aged peel strength P₂ and the initial peel strength P₁) of 8.5 N/20mm or less. A limited increase in aged adhesive strength may suggest, in addition to suppression of the increase in aged adhesive strength, the absolute value of the aged adhesive strength is limited to a level that does not compromise the efficiency of removal when the initial adhesive strength is limited. The PSA sheet satisfying this property is likely to provide excellent efficiency of removal. The increase (P₂ - P₁) in aged adhesive strength is more preferably 5 N/20mm or less, yet more preferably 3.5 N/20mm or less, or particularly preferably 1 N/20mm or less (typically 0.5 N/20mm or less). P₂ - P₁ (the increase in aged adhesive strength) is usually 0 N/20mm or greater, but the PSA sheet disclosed herein is not limited to an embodiment that shows an increase in aged adhesive strength.

While no particular limitations are imposed, the PSA sheet disclosed herein may have a ratio of aged peel strength P₂ (N/20mm) to initial peel strength P₁ (N/20mm) (i.e. a P₂/P₁ ratio value) of 5 or lower. A small P₂/P₁ ratio value indicates a small increase in peel strength with aging. By this, initial adhesion and light peel during removal are favorably combined. From such a standpoint, the P₂/P₁ ratio is preferably 4 or lower, more preferably 3 or lower, or yet more preferably 2 or lower, for instance, 1.8 or lower, 1.5 or lower, or even 1.3 or lower. The P₂/P₁ ratio is typically 0.8 or higher; it can be, for instance, 1 or higher.

The PSA sheet disclosed herein preferably exhibits an aged peel strength in humidity (aged in-humidity peel strength) less than about 11 N/20mm, determined at a tensile speed of 0.3 m/min at 180° peel angle, after applied to a glass plate and stored at 40 °C and 92 % RH for seven days. With the PSA sheet satisfying this property, even when it is used in an embodiment where it is exposed to a highly-humid environment, the aged adhesive strength is sufficiently suppressed and its light peel from adherend can be maintained. With the PSA sheet having an aged in-humidity peel strength of about 5 N/20mm or less (more preferably about 2 N/20mm or less, e.g. about 1.5 N/20mm or less), greater efficiency of removal can be achieved. From the standpoint of inhibiting lifting and peeling to prevent permeation of water such as moisture while the adherend is protected (e.g. during processing of the adherend with the PSA sheet applied thereon) with a chance of exposure to a highly-humid environment, the aged in-humidity peel strength is usually suitably about 0.05 N/20mm or greater, preferably about 0.1 N/20mm or greater, or more preferably about 0.3 N/20mm or greater. The aged peel strength is determined by the method described below.

〔Aged in-humidity peel strength to glass plate〕
The PSA sheet to be measured is cut to a 20 mm wide by 100 mm long strip to prepare a test piece. In a standard environment at 23 °C and 50 % RH, the test piece is press-bonded to a glass plate as the adherend, with a 2 kg rubber roller moved back and forth twice. The sample is stored in an environment at 40 °C and 92 % RH for seven days and then in a standard environment at 23 °C and 50 % RH for one hour. Subsequently, in the same standard environment, using a universal tensile tester, the aged in-humidity peel strength (N/20mm) is determined at a tensile speed of 0.3 m/min, at 180° peel angle. The glass plate used as the adherend is the same as the one used in the initial peel strength measurement.

<PSA layer>
The type of PSA forming the PSA layer (including the PSA layers in the first and second sections, thus possibly the PSA layer of an initial PSA sheet; the same applies hereinafter unless otherwise noted) disclosed herein is not particularly limited. The PSA layer may be formed from a PSA composition including, as the base polymer (the primary component among the polymers, i.e. a component accounting for 50 % by weight or more), one, two or more species selected among various polymers (adhesive polymers), such as acrylic, polyester-based, urethane-based, polyether-based, rubber-based, silicone-based, polyamide-based, and fluorinated polymers. The art disclosed herein can be preferably made, for instance, as a PSA sheet having an acrylic PSA layer or a rubber-based PSA layer.

The “acrylic PSA layer” here refers to a PSA layer including an acrylic polymer as the base polymer. Similarly, the “rubber-based PSA layer” refers to a PSA layer including a rubber-based polymer as the base polymer. The “acrylic polymer” refers to a polymer whose primary monomer (the primary component among the monomers, i.e. a component that accounts for 50 % by weight or more of the total amount of the monomers forming the acrylic polymer) is a monomer having at least one (meth)acryloyl group per molecule. Such a monomer may be referred to as an “acrylic monomer” hereinafter. As used herein, the “(meth)acryloyl group” comprehensively refers to acryloyl group and methacryloyl group. Similarly, the “(meth)acrylate” comprehensively refers to acrylate and methacrylate. Acrylic and rubber-based PSA layers are described below as favorable examples, but the PSA layer used in the art disclosed herein is not limited to these.

(Acrylic polymer)
A preferable example of the acrylic polymer is a polymer of a starting monomer mixture that includes an alkyl (meth)acrylate (or a monomer A hereinafter) and may further include another monomer (or a monomer B hereinafter) that is copolymerizable with the alkyl (meth)acrylate. The acrylic polymer typically has a monomer unit composition corresponding to the monomer composition of the starting monomer mixture.

A preferable monomer A is an alkyl (meth)acrylate represented by the next general formula (1):
CH₂=C(R¹)COOR² (1)
Here, R¹ in the formula (1) is a hydrogen atom or a methyl group. R² is an alkyl group having 1 to 20 carbon atoms. Hereinafter, such a range of the number of carbon atoms may be indicated as “C_1-20.” From the standpoint of the polymerization reactivity, polymerization stability, etc., an alkyl (meth)acrylate wherein R² is a C_1-16 alkyl group is preferable, and an alkyl (meth)acrylate wherein R² is a C_1-12 (typically C_1-10, e.g. C_1-8) alkyl group is more preferable.

Examples of an alkyl (meth)acrylate with R² being a C_1-20 alkyl group include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, n-nonyl (meth)acrylate, isononyl (meth)acrylate, n-decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate, eicosyl (meth)acrylate, etc. These alkyl (meth)acrylates can be used solely as one species or in a combination of two or more species.

Examples of compounds that can be used as the monomer B may include functional group-containing monomers such as carboxy group-containing monomers (e.g. acrylic acid (AA)), acid anhydride group-containing monomers, hydroxy group-containing monomers (e.g. 2-hydroxyethyl (meth)acrylate), amide group-containing monomers, imide group-containing monomers, amino group-containing monomers, epoxy group-containing monomers, cyano group-containing monomers, keto group-containing monomers, monomers having nitrogen-containing rings, and alkoxysilyl group-containing monomers. These functional group-containing monomers may be useful for introducing crosslinking points into the acrylic polymer or for increasing the cohesiveness of the acrylic polymer. Functional group-containing monomers can be used solely as one species or in a combination of two or more species.

Other examples of compounds that can be used as the monomer B include vinyl ester-based monomers such as vinyl acetate; aromatic vinyl compounds; non-aromatic ring-containing (meth)acrylates; aromatic ring-containing (meth)acrylates; olefinic monomers; chlorine-containing monomers; isocyanate group-containing monomers; alkoxy group-containing monomers; and vinyl ether-based monomers. These can be used singly as one species or in a combination of two or more species. As the monomer B, one, two or more species can be used among polyfunctional monomers such as 1,6-hexanediol di(meth)acrylate. When using such a polyfunctional monomer, its amount used is not particularly limited. It is usually suitably about 2 % by weight or less (more preferably about 1 % by weight or less) of the total monomer content.

The monomer A content in the total monomer content can be, but is not particularly limited to, for instance, about 50 % by weight or greater; it is suitably about 60 % by weight or greater, preferably about 70 % by weight or greater, more preferably about 80 % by weight or greater, or yet more preferably about 85 % by weight or greater. With the inclusion of the monomer A in a prescribed amount, a PSA sheet having good adhesiveness can be favorably obtained. The art disclosed herein can be preferably implemented, for instance, in an embodiment where the monomer A content in the total monomer content is about 90 % by weight or greater. In an embodiment, the monomer A content can be about 95 % by weight or greater, or even about 97 % by weight or greater. In an embodiment using a monomer A and a monomer B together, from the standpoint of suitably obtaining the effects of the monomer B, the monomer A content in the total monomer content can be, for instance, 99.9 % by weight or less; it is usually preferably 99.5 % by weight or less, more preferably 99 % by weight or less, or about 97 % by weight or less (e.g. 95 % by weight or less).

When an aforementioned functional group-containing monomer is copolymerized in the acrylic polymer, the ratio of the functional group-containing monomer to all the monomers forming the acrylic polymer is usually preferably about 0.1 % by weight or higher (typically about 0.5 % by weight or higher, e.g. about 1 % by weight or higher), and preferably about 40 % by weight or lower (typically about 30 % by weight or lower, e.g. about 20 % by weight or lower).

(Rubber-based polymer)
In another preferable embodiment, the PSA layer can be a rubber-based PSA layer. Examples of the base polymer include natural rubber; styrene-butadiene rubber (SBR); polyisoprene; butene-based polymer synthesized with a butene (1-butene or cis- or trans-2-butene) and/or 2-methylpropene (isobutylene) as the primary monomer(s); A-B-A block copolymer rubber and a hydrogenation product thereof, e.g. styrene-butadiene-styrene block copolymer rubber (SBS), styrene-isoprene-styrene block copolymer (SIS), styrene-isobutylene-styrene block copolymer rubber (SIBS), styrene-vinyl-isoprene-styrene block copolymers (SVIS), hydrogenated SBS (styrene-ethylene/butylene-styrene block copolymer (SEBS)), and hydrogenated SIS ( styrene-ethylene-propylene-styrene block copolymers (SEPS)). These rubber-based polymers can be used singly as one species or in a combination of two or more species.

(Tg of base polymer)
The PSA layer’s base polymer (an acrylic polymer in an acrylic PSA layer) is not particularly limited in Tg. The Tg of the base polymer can be, for instance, about -70 °C or higher. In the PSA sheet according to a preferable embodiment, the base polymer of the PSA layer has a Tg of about -65 °C or higher. According to a base polymer having such a Tg, a PSA layer having good adhesive properties can be favorably formed. In an embodiment where the base polymer has a Tg of about -50 °C or higher (more preferably about -35 °C or higher), greater effects can be obtained. The Tg of the base polymer is usually suitably 0 °C or lower, preferably about -5 °C or lower, more preferably about -15 °C or lower, or yet more preferably about -20 °C or lower (e.g. about -25 °C or lower). In the PSA sheet according to another preferable embodiment, from the standpoint of the adhesion, autohesion, etc., the base polymer of the PSA layer has a Tg of about -35 °C or lower, more preferably about -40 °C or lower, or yet more preferably about -45 °C or lower (e.g. about -55 °C or lower). The base polymer’s Tg can be adjusted by suitably changing the monomer composition (i.e. the monomer species used in the synthesis of the polymer and their ratio).

In the present description, the Tg of a polymer refers to the value determined by the Fox equation based on the Tg values of homopolymers of the respective monomers forming the polymer and the weight fractions (copolymerization ratio by weight) of the monomers. As shown below, the Fox equation is a relational expression between the Tg of a copolymer and glass transition temperatures Tgi of homopolymers of the respective monomers constituting the copolymer.
1/Tg = Σ(Wi/Tgi)
In the Fox equation, Tg represents the glass transition temperature (unit: K) of the copolymer, Wi the weight fraction (copolymerization ratio by weight) of a monomer i in the copolymer, and Tgi the glass transition temperature (unit: K) of homopolymer of the monomer i.

For the glass transition temperatures of homopolymers used for determining the Tg, the values found in known documents are used. For instance, with respect to the monomers listed below, for the glass transition temperatures of their homopolymers, the following values are used:
2-ethylhexyl acrylate: -70 °C
n-butyl acrylate: -55 °C
ethyl acrylate: -20 °C
methyl acrylate: 8 °C
n-butyl methacrylate: 20 °C
methyl methacrylate: 105 °C
2-hydroxyethyl acrylate: -15 °C
4-hydroxybutyl acrylate: -40 °C
vinyl acetate: 32 °C
styrene: 100 °C
acrylic acid: 106 °C
methacrylic acid: 228 °C
acrylonitrile: 104 °C

With respect to the Tg values of homopolymers other than the examples listed above, the values given in Polymer Handbook (3rd edition, John Wiley & Sons, Inc., Year 1989) are used. With respect to a monomer for which two or more values are listed in the Polymer Handbook, the highest value is used. When no values are given in the Polymer Handbook, values obtained by the measurement method described in Japanese Patent Application Publication No. 2007-51271 are used.

(Synthesis of base polymer)
The method for obtaining the base polymer (e.g. an acrylic polymer) is not particularly limited. Known polymerization methods can be suitably employed, such as solution polymerization, emulsion polymerization, bulk polymerization, and suspension polymerization. Alternatively, it is also possible to employ photopolymerization involving irradiation of light such as UV (typically carried out in the presence of a photopolymerization initiator) and active energy ray irradiation polymerization such as radiation polymerization involving irradiation of radioactive rays such as β rays and γ rays. As the monomer supply method in solution polymerization and emulsion polymerization, a suitable method can be employed among the all-at-once method where all the starting monomer mixture is supplied in one portion, gradual supply method, portion-wise supply method, etc. The polymerization temperature can be suitably selected in accordance with the monomer species, the solvent species, and the polymerization initiator species used, etc. The polymerization temperature is usually suitably about 20 °C or higher, preferably about 40 °C or higher, more preferably about 50 °C or higher; it can also be about 60 °C or higher, about 65 °C or higher, or even about 70 °C or higher. The polymerization temperature is usually suitably about 170 °C or lower (typically about 140 °C or lower), or preferably about 95 °C or lower (e.g. about 85 °C or lower).

The solvent (polymerization solvent) used in solution polymerization can be suitably selected among heretofore known organic solvents. For instance, it is preferable to use aromatic compounds (typically aromatic hydrocarbons) such as toluene, acetic acid esters such as ethyl acetate, aliphatic or alicyclic hydrocarbons such as hexane and cyclohexane, and the like.

The initiator used in the polymerization can be suitably selected among known or commonly-used polymerization initiators in accordance with the monomer species and the type of polymerization method. For instance, azo-based polymerization initiators such as 2,2’-azobisisobutyronitrile can be preferably used. Other examples of the polymerization initiator include persulfates such as potassium persulfate; peroxide-based initiators such as benzoyl peroxide; substituted ethane-based initiators; and aromatic carbonyl compounds. Yet other examples of the polymerization initiator include redox initiators by the combination of a peroxide and a reducing agent. These polymerization initiators can be used singly as one species or in a combination of two or more species. The polymerization initiator can be used in a usual amount. For instance, it can be selected from a range of about 0.005 part to 1 part by weight (typically about 0.01 part to 1 part by weight) to 100 parts by weight of the total monomer content.

The surfactant (emulsifier) used in emulsion polymerization is not particularly limited. Commonly-known anionic surfactants, nonionic surfactants and the like can be used. A surfactant having a radically polymerizable functional group can also be used. For the surfactant, solely one species or a combination of two or more species can be used. The amount of surfactant is usually preferably about 0.1 part by weight or greater (e.g. about 0.5 part by weight or greater) to 100 parts by weight of the total monomer content; and it is preferably about 10 parts by weight or less (e.g. about 5 parts by weight or less) to 100 parts by weight of the total monomer content.

In the emulsion polymerization, as necessary, various heretofore known chain transfer agents (which can be considered also as a molecular weight-adjusting agent or polymerization degree-adjusting agent) can be used. For the chain transfer agent, solely one species or a combination of two or more species can be used. As the chain transfer agent, mercaptans can be preferably used, such as n-dodecyl mercaptan, t-dodecyl mercaptan, and thioglycolic acid. When using a chain transfer agent, its amount can be, for instance, about 0.01 part to 1 part by weight to 100 parts by weight of the total monomer content. The art disclosed herein can also be preferably practiced in an embodiment that uses no chain transfer agent.

<PSA composition>
The PSA layer of the PSA sheet disclosed herein can be formed from various forms of PSA compositions. Examples of the forms of PSA compositions include a solvent-based PSA composition containing the PSA (adhesive component(s)) in an organic solvent, a water-dispersed PSA composition containing at least part of the PSA dispersed in an aqueous solvent, an active energy ray-curable PSA composition formulated so as to cure with active energy rays such as UV rays and radioactive rays to form PSA, and a hot-melt PSA composition which is applied in the molten state by heating and forms PSA when it cools to near room temperature.

(Crosslinking agent)
In the art disclosed herein, the PSA composition used to form the PSA layer preferably includes a crosslinking agent. With the use of crosslinking agent, the cohesive strength can be suitably adjusted. The type of crosslinking agent used is not particularly limited. Examples include oxazoline-based crosslinking agents, aziridine-based crosslinking agents, isocyanate-based crosslinking agents, epoxy-based crosslinking agents, melamine-based crosslinking agents, peroxide-based crosslinking agents, urea-based crosslinking agents, metal alkoxide-based crosslinking agents, metal chelate-based crosslinking agents, metal salt-based crosslinking agents, carbodiimide-based crosslinking agents, hydrazine-based crosslinking agents, amine-based crosslinking agents, and silane coupling agents. These can be used solely as one species or in a combination of two or more species. For instance, it is preferable to use one, two or more species selected from a group consisting of oxazoline-based crosslinking agents, aziridine-based crosslinking agents, isocyanate-based crosslinking agents and epoxy-based crosslinking agents.

The crosslinking agent content (the total amount of crosslinking agent) in the PSA composition disclosed herein is not particularly limited and can be suitably selected in view of the composition and the molecular weight of the base polymer so as to obtain favorable properties after crosslinked. While no particular limitations are imposed, the amount of the crosslinking agent used to 100 parts by weight of the base polymer (typically an acrylic polymer) is usually about 0.01 part by weight or greater, suitably about 0.1 part by weight or greater, or preferably about 1 part by weight or greater (e.g. about 2 parts by weight or greater). From the standpoint of the adhesion, etc., the amount of the crosslinking agent is usually suitably about 15 parts by weight or less (preferably about 10 parts by weight or less, e.g. about 5 parts by weight or less) to 100 parts by weight of the base polymer.

The PSA composition may include, as necessary, various optional additives generally known in the field of PSA compositions, such as tackifier such as rosin-based tackifier, peel-adjusting agent such as a phosphate, viscosity-adjusting agent (viscosifier, etc.), crosslinking accelerator, plasticizer, softener, filler, anti-static agent, anti-aging agent, UV-absorber, antioxidant and photo-stabilizing agent. With respect to these various optional additives, heretofore known species can be used by typical methods. Because these additives do not characterize the present invention in particular, details are omitted.

(Formation of PSA layer)
As for the method for providing the PSA layer to a support substrate which forms the substrate layer, it is possible to employ a direct method where the PSA composition as described above is directly provided (typically applied) to the support substrate and subjected to a curing treatment; a transfer method where the PSA composition is applied to a suitable release face (e.g. a releasable surface of a transfer sheet) and subjected to a curing treatment to form a PSA layer on the surface followed by applying and transferring the PSA layer to the support substrate; and so on. The curing treatment may include one, two or more processes selected among drying (heating), cooling, crosslinking, supplemental copolymerization reaction, aging, etc. The curing treatment referred to herein also encompasses, for instance, a process (heating process, etc.) simply to allow a PSA composition containing a solvent to dry, a process simply to cool down (solidify) a heat-melted PSA composition. When the curing treatment includes two or more processes (e.g. drying and crosslinking), these processes may be performed at once or stepwise.

The PSA composition can be applied, for instance, using a commonly used coater such as a gravure roll coater, reverse roll coater, kiss roll coater, dip roll coater, bar coater, knife coater and spray coater. From the standpoint of accelerating the crosslinking reaction, increasing the productivity, etc., the PSA composition is preferably dried with heat. The drying temperature may vary depending on the object (a support substrate, etc.) to which the PSA composition is applied, but it can be, for instance, about 40 °C to 150 °C.

(Gel fraction)
The weight fraction (gel fraction) of the ethyl acetate-insoluble portion of the PSA layer disclosed herein is not particularly limited. It can be, for instance, about 40 % or higher (typically about 50 % or higher). In an embodiment, from the standpoint of obtaining at least certain cohesive strength, the gel fraction of the PSA layer is suitably about 60 % or higher, preferably about 80 % or higher, or more preferably about 90 % or higher. The gel fraction of the PSA layer can be, for instance, about 95 % or higher (e.g. about 98 % or higher). With increasing gel fraction, the cohesion of the PSA tends to increase while the aged adhesive strength tends to be suppressed. The maximum gel fraction is theoretically 100 %. In some embodiments, the gel fraction can be, for instance, about 98 % or lower, or even about 95 % or lower (e.g. about 90 % or lower). The gel fraction can be adjusted by the selection of, for instance, the base polymer composition, the polymerization method and conditions for the base polymer, the molecular weight of the base polymer, the presence of a crosslinking agent as well as its type and amount used if any, and so on. The gel fraction is determined by the method described below.

(Degree of swelling)
The degree of swelling of the PSA layer disclosed herein is not particularly limited and can be usually about 30-fold or less. From the standpoint of obtaining at least certain cohesive strength, the degree of swelling is suitably about 20-fold or less, preferably about 15-fold or less, or more preferably about 12-fold or less, for instance, about 10-fold or less, or even about 8-fold or less. The minimum degree of swelling is theoretically 1-fold; it can be usually about 3-fold or greater, for instance, about 5-fold or greater. The degree of swelling can be adjusted, for instance, through the molecular weight of the base polymer, the type pf crosslinking agent (distances among functional groups) and its amount used, etc. The degree of swelling is determined by the method described below.

〔Determination of gel fraction and degree of swelling〕
A PSA layer sample (weight: W₁) weighing approximately 0.1 g is wrapped into a pouch with a porous polytetrafluoroethylene membrane (weight: W₂) having an average pore diameter of 0.2 μm, and the opening is tied with twine (weight: W₃). As the porous polytetrafluoroethylene membrane, trade name NITOFLON (registered trademark) NTF 1122 (product of Nitto Denko Corp.; 0.2 μm average pore diameter, 75 % porosity, 85 μm thickness) or an equivalent product can be used. The resulting package is immersed in 50 mL of ethyl acetate and stored at room temperature (typically 23 °C) for 7 days. Subsequently, the package is taken out, and any residual ethyl acetate is wiped off the outer surface. The package weight (W₄) is measured. The package is then dried at 130 °C for 2 hours and the package weight (W₅) is measured. The gel fraction and the degree of swelling of the PSA layer can be determined by substituting the respective values into the following equation:
Gel fraction (%) = [(W₅-W₂-W₃)/W₁] × 100
Degree of swelling (fold) = (W₄-W₂-W₃)/(W₅-W₂-W₃)

<Substrate layer>
As the substrate layer (including the substrate layers in the first and second sections, thus possibly the substrate layer of an initial PSA sheet; the same applies hereinafter unless otherwise noted) of the PSA sheet disclosed herein, resin film, a rubber sheet, a foam sheet, a composite of these, etc., can be used. Examples of the rubber sheet include natural rubber sheets, butyl rubber sheets, polybutadiene rubber sheets. Examples of the foam sheet include polyurethane foam sheets, and polychloroprene rubber foam sheets. In the art disclosed herein, the substrate layer is preferably a resin layer that comprises a thermoplastic resin as the primary component (component accounting for the highest content among the resins in the substrate layer, preferably a resin component accounting for 50 % by weight or more). The use of thermoplastic resin as the primary component of the substrate layer preferably enables thermal fusion/joining of initial PSA sheets.

The art disclosed herein can be preferably applied to a PSA sheet wherein the substrate layer is a resin film. The concept of “resin film” here refers to film typically obtained by molding a thin layer from a resin composition primarily including resin components as described below; it should be distinguished from so-called non-woven and woven fabrics. In other words, the concept of resin film excludes non-woven and woven fabrics. A resin film (non-foamed resin film) which is essentially not foamed can be preferably used. Here, the non-foamed resin film refers to resin film that has not been deliberately subjected to a foaming process. In particular, the resin film may have an expansion ratio lower than about 1.1 (e.g. lower than 1.05, typically lower than 1.01).

Examples of the resin components forming the resin film include polyolefinic resins (polyethylene, polypropylene, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, etc.), poly(vinyl chloride)-based resins (typically soft poly(vinyl chloride)-based resin); poly(vinyl acetate)-based resin, poly(vinyl alcohol)-based resin, polyurethane-based resins (ether-based polyurethane, ester-based polyurethane, carbonate-based polyurethane, etc.), urethane (meth)acrylate-based resin, thermoplastic elastomers (olefinic elastomer, styrene-based elastomer, acrylic elastomer, etc.), polyester-based resins (polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, etc.), polycarbonate-based resin, polyamide-based resin, and polyimide-based resin (thermoplastic polyimide, polyether imide, polyamide-imide, etc.), cellulose-based resin (triacetylcellulose, etc.), poly(methyl methacrylate)-based resin, norbornene-based resin, polyoxymethylene-based resin, polyether ether ketone-based resin, polystyrene-based resin, and polymethylpentene. Among these resins, solely one species or a combination of two or more species can be used.

While no particular limitations are imposed, in the PSA sheet according to an embodiment, it is preferable to use a substrate layer that includes, as its primary component(s), one, two or more species of resin selected from the group consisting of polyolefinic resin, poly(vinyl chloride)-based resin, polyurethane-based resin, thermoplastic elastomer and polyester-based resin (typically a substrate layer including such resin in an amount exceeding 50 % by weight). In another embodiment, in view of the performance, ease of handling, costs, etc., a substrate layer including a polyolefinic resin layer, polyester-based resin layer or polyvinyl chloride-based resin layer can be preferably used. Among the resin materials, in view of the heat stability, the lightness of weight, etc., polyolefinic resins, polyurethane-based resins and olefinic elastomers are preferable; in view of the handling properties, etc., polyolefinic resins and olefinic elastomers are particularly preferable.

The PSA sheet disclosed herein can be preferably made in an embodiment having a substrate layer that includes a polyolefinic resin as the primary component, that is, an embodiment wherein the substrate layer is polyolefinic resin film. The substrate layer in such an embodiment may bring about excellent thermal fusion. For instance, it is preferable to use polyolefinic resin film in which 50 % by weight or more of the entire substrate layer is polyethylene (PE) resin or polypropylene (PP) resin. In other words, in the polyolefinic resin film, the combined amount of PE resin and PP resin may account for 50 % by weight or more of the entire substrate layer.

The PP resin may include, as the primary component, various polymer species (propylene-based polymers) that include propylene as a monomer unit. The PP resin may be formed essentially of one, two or more species of propylene-based polymer. The concept of propylene-based polymer here includes homopolypropylene as well as a random copolymer of propylene and other monomer(s) (random polypropylene) and a block copolymer (block polypropylene). The concept of propylene-based polymer here includes, for instance, the following species:
Propylene homopolymer (homopolypropylene), for instance, isotactic polypropylene;
Random copolymer (random polypropylene) of propylene and other α-olefin(s) (typically, one, two or more species selected from ethylene and α-olefins having 4 to 10 carbon atoms), preferably random polypropylene synthesized with propylene as the primary monomer (i.e. the monomer accounting for 50 % by weight or more of the total monomer content);
Block copolymer (block polypropylene) of propylene and other α-olefin(s) (typically, one, two or more species selected from ethylene and α-olefins having 4 to 10 carbon atoms), preferably block polypropylene synthesized with propylene as the primary monomer (i.e. the monomer accounting for 50 % by weight or more of the total monomer content).

The PE resin can be various types of polymer (ethylene-based polymer) synthesized with ethylene as a monomer. The PE resin may be essentially formed of one, two or more species of ethylene-based polymer. The ethylene-based polymer can be an ethylene homopolymer or a copolymer (random copolymer, block copolymer, etc.) of ethylene as the primary monomer and other α-olefin(s) as secondary monomer(s). Favorable examples of the α-olefins include α-olefins having 3 to 10 carbon atoms such as propylene, 1-butene (which can be a branched 1-butene), 1-hexene, 4-methyl-1-pentene and 1-octene. For instance, it is preferable to use PE resin that includes, as the primary component, an ethylene-based polymer in which an α-olefin as the secondary monomer is copolymerized up to about 10 % by weight (typically up to about 5 % by weight).

The PE resin may include a copolymer of ethylene and a monomer (functional monomer) containing other functional group(s) in addition to a polymerizable functional group, copolymer of an ethylene-based polymer copolymerized with such a functional monomer, or the like. Examples of a copolymer of ethylene and a functional monomer include ethylene-vinyl acetate copolymers (EVA), ethylene-acrylic acid copolymers (EAA), ethylene-methacrylic acid copolymers (EMAA), ethylene-methyl acrylate copolymers (EMA), ethylene-ethyl acrylate copolymers (EEA), ethylene-methyl methacrylate copolymers (EMMA), and copolymers of ethylene and (meth)acrylic acid (i.e. acrylic acid and/or methacrylic acid) crosslinked by metal ions.

The PE resin is not particularly limited in density. The concept of PE resin here includes all of the following: high density polyethylene (HDPE), medium density polyethylene (MDPE), low density polyethylene (LDPE) and linear low density polyethylene (LLPDE). In an embodiment, the density of the PE resin can be, for instance, about 0.90 g/cm³ to 0.94 g/cm³. Preferable PE resins include LDPE and LLDPE. The PE resin may include one, two or more species of LDPE and one, two or more species of LLDPE. There are no particular limitations to the respective blend ratios of LDPE and LLDPE, or to the LDPE to LLDPE blend ratio. They can be suitably selected to form a PE resin having desirable properties. As the substrate layer of the PSA sheet disclosed herein, it is preferable to use polyethylenic resin film such as LLDPE film whose LLDPE content is higher than 50 % by weight (preferably about 75 % by weight or higher, e.g. about 90 % by weight or higher) and LDPE film whose LDPE content is higher than 50 % by weight (preferably about 75 % by weight or higher, e.g. about 90 % by weight or higher). Laminate resin film including such polyethylenic resin film as a component can be used as well.

The resin film (e.g. polyolefinic resin film) used as the substrate layer of the PSA sheet disclosed herein may include, as necessary, suitable components allowable in the substrate layer. Examples of additives that can be suitably added include filler, colorant (pigment such as inorganic pigment, dye), antioxidant, photostabilizer (including radical scavenger and UV absorber), antistatic agent, plasticizer, slip agent, and anti-blocking agent. Each additive can be added, for instance, in an amount similar to a typical amount in the field of resin film used as substrate layers and the like of PSA sheets.

The substrate layer may have a mono-layer structure or a multi-layer structure formed of two, three or more layers. In a multi-layer structure, it is preferable that at least one layer (preferably each layer) is formed of aforementioned resin film. For instance, in a preferable substrate layer, 75 % or more (more preferably 90 % or more) of the thickness is attributed to mono-layer or multi-layer (typically mono-layer) polyolefinic resin film. The substrate layer may be entirely formed of mono-layer or multi-layer polyolefinic resin film. From the standpoint of the cost-effectiveness, it is preferable to use a substrate layer formed of mono-layer resin film (e.g. LLDPE film, LDPE film, etc.).

The method for producing the substrate layer can be suitably selected among heretofore known methods and is not particularly limited. For instance, when resin film is used as the substrate layer, it is possible to use resin film fabricated by suitably employing a heretofore known general film-forming method such as inflation molding, extrusion, T-die cast molding, and calendar roll molding.

In an embodiment where at least one face (the PSA layer-side face) of the substrate layer is a resin film surface, the resin film surface can be subjected to a heretofore known surface treatment such as corona discharge treatment, plasma treatment, ozone exposure, flame exposure, UV irradiation, acid treatment, alkali treatment, and primer coating. These surface treatments may enhance the tightness of adhesion between the substrate layer and the PSA layer, or the anchoring of the PSA layer onto the substrate layer. In an embodiment using polyolefinic resin film as the substrate layer, it is particularly meaningful to provide these surface treatments.

<Release liner>
As the release liner used to protect the adhesive face, commonly-used release paper and the like can be used without particular limitations. For instance, a release liner having a release layer on a surface of a liner substrate such as plastic film and paper, a release liner formed from a low-adhesive material such as fluorinated polymer (polytetrafluoroethylene, etc.) and polyolefinic resin, and the like can be used. The release layer can be formed by subjecting the liner substrate to surface treatment with various release agents including silicone-based, long-chain alkyl-based, and fluorinated kinds as well as molybdenum sulfide.

<Applications>
The PSA sheet disclosed herein is preferably used as a surface protective sheet that is to be applied to surfaces of a metal plate, a coated steel plate, a synthetic resin plate, a glass plate and the like so as to prevent damage (scratches, contamination, etc.) to these surfaces while they are being processed or transported and to be eventually removed from the adherend at the end of the protection period. Such a surface protective sheet is typically formed of an adhesively single-faced PSA sheet having a PSA layer provided to one face of a substrate layer. The PSA sheet disclosed herein has at least a certain width; and therefore, it is preferably used in an embodiment where it covers the entire surface of an adherend having a width of, for instance, about 1 m or greater, or even about 2.6 m or greater. The PSA sheet according to a preferable embodiment may adhere well to an adherend surface and provide excellent waterproofness; and therefore, it is preferably used as a surface protective sheet on an article surface that needs to or is desired to avoid a contact with water such as moisture. Upon contact with water, the article surface may undergo a change in at least one aspect among appearance, quality, surface condition, etc.

The PSA sheet disclosed herein is favorable as a surface protective sheet for a glass plate used as a building material such as window glass, etc. The glass plate subject to application (protection) may have, for instance, a glass substrate and a coating layer layered on the glass substrate. The coating layer may include a metal layer, a metal oxide layer, a metal nitride layer, etc. In particular, the glass plate may have a Low-E layer on one face. In producing the glass plate, the Low-E layer surface may be left exposed until two glass plates including the Low-E-layer-bearing glass plate are assembled into pair-glass (dual-pane glass) with the Low-E-layer-side surface on the inside. The PSA sheet disclosed herein is preferably used to prevent the Low-E layer surface from suffering the sort of damage, degradation, and wearing. In particular, the Low-E layer usually includes a layer of metal such as silver. To protect such a Low-E layer from water which causes metal corrosion, the PSA sheet disclosed herein is preferably used. By this, the Low-E layer can be protected not only from damage, degradation and wearing, but also from corrosion. In other words, the PSA sheet can be used as a protective sheet for a Low-E layer surface. Low-E-layer-bearing glass plates have higher levels of heat blocking or thermal insulation as compared to conventional glass plates and can improve the efficiency to cool down or heat up indoor spaces; and therefore, they are widely used as building materials such as window glass. The art disclosed herein may indirectly contribute to energy saving and reduction of greenhouse gas emissions.

From the standpoint of the efficiency of removal, the PSA sheet disclosed herein is preferably used on an adherend having a large surface area on which the peel strength tends to be limited. The PSA sheet disclosed herein is preferably used in an embodiment where it covers the surface of an adherend having a width of about 1 m or greater, for instance, about 2 m or greater (or even about 3 m or greater). The length of the adherend surface is equal to or greater than the width. In a preferable embodiment, it is preferably used in an embodiment where it entirely covers the surface of one face of a large flat plate (favorably, a flat plate with a smooth surface). In particular, glass plates used for building materials such as window glass are becoming progressively larger in view of efficient production, transportation, etc. It is preferably used in an embodiment where it covers the entire surface of a glass plate (typically the entire Low-E layer surface of the Low-E-layer-bearing glass plate) having a large surface area (e.g. with a surface width above 2.6 m or even at or above about 3 m).

Matters disclosed by this description include the following:
(1) A PSA sheet obtainable by fusing at least two initial PSA sheets at their edge faces.
(2) The PSA sheet according to (1) above, wherein the at least two initial PSA sheets have long lengths and are fused at edge faces of their width directions (at their lengthwise edge faces).
(3) The PSA sheet according to (1) or (2) above, comprising a substrate layer and a PSA layer provided to at least one face of the substrate layer.
(4) The PSA sheet according to any of (1) to (3) above,
having first and second faces forming its outer surface, with the second face being the opposite face of the first face, and
having a fused segment between the at least two initial PSA sheets, with the fused segment being free of an uneven segment either on the first face or on the second face.
(5) The PSA sheet according to any of (1) to (4) above, whose adhesive face is essentially free of a recess in the fused segment between the at least two initial PSA sheets.
(6) The PSA sheet according to any of (1) to (5) above, wherein the PSA sheet has a long length and the fused segment between the at least two initial PSA sheets runs in the length direction of the PSA sheet.
(7) The PSA sheet according to any of (1) to (6) above, wherein the fusion is thermal fusion by laser irradiation (the at least two initial PSA sheets are thermally fused by laser irradiation).
(8) The PSA sheet according to any of (1) to (7) above, wherein the substrate layer is a resin layer comprising a thermoplastic resin as its primary component.
(9) The PSA sheet according to (8) above, wherein the substrate layer is formed from at least one species of resin selected from the group consisting of polyolefinic resin, polyvinyl chloride-based resin and polyurethane-based resin.
(10) The PSA sheet according to any of (1) to (9) above, wherein the substrate layer has a thickness of 25 μm to 150 μm.
(11) The PSA sheet according to any of (1) to (10) above, wherein the PSA layer is an acrylic PSA layer comprising an acrylic polymer as its base polymer or a rubber-based PSA layer comprising a rubber-based polymer as its base polymer.
(12) The PSA sheet according to any of (1) to (11) above, wherein the PSA layer has a thickness of 1 μm to 20 μm.
(13) The PSA sheet according to any of (1) to (12) above, showing an initial peel strength of 0.01 N/20mm to 5 N/20mm to a glass plate.
(14) The PSA sheet according to any of (1) to (13) above, with the PSA sheet having a long length and a width of about 2.6 m or greater.
(15) The PSA sheet according to any of (1) to (14) above, comprising a first section formed of the first initial PSA sheet and a second section formed of the second initial PSA sheet, wherein
the first section and the second section individually comprise a substrate layer and a PSA layer provided to at least one face of the substrate layer,
the substrate layer of the first section is joined at one edge thereof to one edge of the substrate layer of the second section (the substrate layers of the first and second sections are joined with each other at two edges, one from each substrate layer), and
the PSA layer of the first section is continuous at one edge thereof with one edge of the PSA layer of the second section (the PSA layers of the first and second sections are continuous with each other at two edges, one from each PSA layer).
(16) The PSA sheet according to (15) above, wherein both the first section formed of the first initial PSA sheet and the second section formed of the second initial PSA sheet have long lengths, and
the first section and the second section are continuous with one edge face of the width direction of the first initial PSA sheet fused to one edge face of the width direction of the second initial PSA sheet.
(17) The PSA sheet according to (15) or (16) above, wherein the first section and the second section have an equal width.
(18) The PSA sheet according to (15) or (16) above, wherein the first section and the second section have different widths.
(19) The PSA sheet according to any of (15) to (18) above, wherein the PSA species of the first section and the PSA species of the second section are of equal type; and the resin species forming the substrate layer of the first section and the resin species forming the substrate layer of the second section are of equal type.
(20) A surface protective sheet comprising the PSA sheet according to any of (1) to (19) above, wherein the PSA sheet is an adhesively single-faced PSA sheet having the PSA layer provided only on one face of the substrate layer.

(21) A method for producing a PSA sheet, the method comprising
a step of obtaining at least two initial PSA sheets, and
a step of joining the two initial PSA sheets by abutting the two initial PSA sheets at their edge faces and fusing the abutted edge faces.
(22) The PSA sheet production method according to (21) above, wherein
both the two initial PSA sheets have long lengths, and
in the joining step, lengthwise edge faces of the two initial PSA sheets are abutted and fused together.

Several tested Examples related to the present invention are described below. In the description below, "part(s)" and "%" are by weight unless otherwise specified.

<Experimental Example>
〔Fabrication of initial PSA sheet〕
Into a reaction vessel equipped with a stirrer, a thermometer, a nitrogen inlet, a condenser and an addition funnel, was placed a monomer mixture formed of 45 parts of 2-ethylhexyl acrylate, 51 parts of n-butyl methacrylate and 4 parts of 2-hydroxyethyl acrylate as monomers; along with ethyl acetate as the polymerization solvent. The resulting mixture was allowed to stir under a nitrogen flow for two hours. Oxygen was thus eliminated from the polymerization system. Subsequently, to 100 parts of the monomer mixture, was added 2,2'-azobisisobutyronitrile at a ratio of 0.2 part and solution polymerization was carried out at 60 °C for 6 hours to obtain an acrylic polymer solution according to this Example.
To the acrylic polymer solution, for 100 parts of the acrylic polymer in the solution, was added 3 parts of isocyanate-based crosslinking agent (product of Covestro, trade name DESMODUR RFE, tris(p-isocyanatophenyl)thiophosphate, 27 % ethyl acetate solution) and mixed with stirring to prepare a PSA composition according to this Example.
Two sheets of 45 μm thick resin film formed of LDPE having one corona-treated face were obtained. The acrylic PSA composition was applied to the corona-treated face of the first sheet of resin film and allowed to dry at 90 °C for 1 minute to form a 3 μm thick PSA layer and obtain a 500 mm wide initial PSA sheet having an overall thickness of 48 μm. The resin film was oriented so that its TD is in the width direction of the PSA sheet. The non-corona-treated face (second face) of the second sheet of resin film was applied to the PSA layer and used as release liner.

〔Production of PSA sheet〕
Two such initial PSA sheets were obtained. As shown in Fig. 2, two

initial PSA sheets

50 and 60 were placed with their back faces on top and adhesive faces on bottom. Edge faces 50C and 60C of their width directions were abutted and

cover films

70 and 80 were placed on top and bottom of the abutted segment (segment to be fused). As the cover film 70 on top (on the backside of initial PSA sheets 50 and 60), was used a PET film (product name LUMIRROR S-10 available from Toray Industries, Inc.; 38 μm thick, 5 mm wide). As the cover film 80 on bottom (on the adhesive face sides of initial PSA sheets 50 and 60), was used a PET film (product name DIAFOIL MRF-38 available from Mitsubishi Chemical Corporation; 38 μm thick, 5 mm wide).
Of cover film 70 placed on top (i.e. on the substrate layer 51/61 side), one face was coated with a laser absorber (product name CLEARWELD LD120C available from Gentex Corporation). Cover film 70 was placed so that its laser absorber-coated face was in contact with

substrate layers

51 and 61. Atop this, a glass plate was placed as a fastening member. While applying a pressure of 10 kgf/cm², the abutted segment of the initial PSA sheets was scanned and irradiated by a laser beam (semiconductor laser, 940 nm in wavelength, 30 W output, spot diameter φ 2 mm) at a scanning speed of 100 mm/s to thermally fuse the two

initial PSA sheets

50 and 60 at their width-direction’s edge faces 50C and 60C.
A PSA sheet was thus produced, with the two initial PSA sheets fused at edges faces. The fused segment in the resulting PSA sheet was analyzed in SEM cross-sectional images (Figs. 4 and 5) and the fused segment was found to have a 0.9 μm deep recess, but no recess deeper than 1 μm.

〔Waterproofness test〕
The resulting PSA sheet including the thermally-fused areas was cut to a 20 mm wide by 100 mm long strip to prepare a test piece. In a standard environment at 23 °C, 50 % RH, with a 2 kg rubber roller moved back and forth twice, the test piece was press-bonded to a glass plate. As the glass plate, was used a cut blue plate (1.35 mm thick, 100 mm by 100 mm) available from Matsunami Glass Ind., Ltd. The sample was suspended in water at about room temperature (23 °C) and stored for seven days. With respect to the area protected with the PSA sheet, the surface condition of the glass plate was visually inspected. The protected face (the face to which the PSA sheet was applied) was found free of water permeation.

Although specific embodiments of the present invention have been described in detail above, these are merely for illustrations and do not limit the scope of the claims. The art according to the claims includes various modifications and changes made to the specific embodiments illustrated above.

1: PSA sheet
1A: first face
1B: second face
2: first section
4: second section
6: fused part
10: substrate layer
10A: first face of substrate layer
10B: second face of substrate layer
20: PSA layer
20A: adhesive face
50: first initial PSA sheet
50C: edge face
51: substrate layer
52: PSA layer
53: release liner
60: second initial PSA sheet
60C: edge face
61: substrate layer
62: PSA layer
63: release liner
70, 80: cover film
R: laser beam

Claims

A pressure-sensitive adhesive sheet obtainable by fusing at least two initial pressure-sensitive adhesive sheets at their edge faces, with the resulting pressure-sensitive adhesive sheet comprising a substrate layer and a pressure-sensitive adhesive layer provided to at least one face of the substrate layer.
The pressure-sensitive adhesive sheet according to Claim 1, wherein the at least two initial pressure-sensitive adhesive sheets have long lengths and are fused at edge faces of their width directions.
The pressure-sensitive adhesive sheet according to Claim 1 or 2, wherein the pressure-sensitive adhesive sheet has first and second faces forming its outer surface, the second face being the opposite face of the first face, and
the pressure-sensitive adhesive sheet has a fused segment between the at least two initial pressure-sensitive adhesive sheets, the fused segment being free of an uneven segment either on the first face or on the second face.
The pressure-sensitive adhesive sheet according to any one of Claims 1 to 3, wherein its adhesive face is essentially free of a recess in the fused segment between the at least two initial pressure-sensitive adhesive sheets.
The pressure-sensitive adhesive sheet according to any one of Claims 1 to 4, wherein the pressure-sensitive adhesive sheet has a long length and the fused segment between the at least two initial pressure-sensitive adhesive sheets runs in the length direction of the pressure-sensitive adhesive sheet.
The pressure-sensitive adhesive sheet according to any one of Claims 1 to 5, wherein the at least two initial pressure-sensitive adhesive sheets are thermally fused by laser irradiation.
The pressure-sensitive adhesive sheet according to any one of Claims 1 to 6, wherein the substrate layer is a resin layer comprising a thermoplastic resin as its primary component.
The pressure-sensitive adhesive sheet according to Claim 7, wherein the substrate layer is formed from at least one species of resin selected from the group consisting of polyolefinic resin, polyvinyl chloride-based resin and polyurethane-based resin.
The pressure-sensitive adhesive sheet according to any one of Claims 1 to 8, wherein the substrate layer has a thickness of 25 μm to 150 μm.
The pressure-sensitive adhesive sheet according to any one of Claims 1 to 9, wherein the pressure-sensitive adhesive layer is an acrylic pressure-sensitive adhesive layer comprising an acrylic polymer as its base polymer or a rubber-based pressure-sensitive adhesive layer comprising a rubber-based polymer as its base polymer.
The pressure-sensitive adhesive sheet according to any one of Claims 1 to 10, wherein the pressure-sensitive adhesive layer has a thickness of 1 μm to 20 μm.
The pressure-sensitive adhesive sheet according to any one of Claims 1 to 11, exhibiting an initial peel strength of 0.01 N/20mm to 5 N/20mm to a glass plate.
The pressure-sensitive adhesive sheet according to any one of Claims 1 to 12, having a long length and a width of about 2.6 m or greater.
The pressure-sensitive adhesive sheet according to any one of Claims 1 to 13, comprising a first section formed of the first initial pressure-sensitive adhesive sheet and a second section formed of the second initial pressure-sensitive adhesive sheet, wherein
the first section and the second section individually comprise a substrate layer and a pressure-sensitive adhesive layer provided to at least one face of the substrate layer,
the substrate layer of the first section is joined at one edge thereof to one edge of the substrate layer of the second section, and
the pressure-sensitive adhesive layer of the first section is continuous at one edge thereof with one edge of the pressure-sensitive adhesive layer of the second section.
The pressure-sensitive adhesive sheet according to Claim 14, wherein both the first section formed of the first initial pressure-sensitive adhesive sheet and the second section formed of the second initial pressure-sensitive adhesive sheet have long lengths, and
the first section and the second section are continuous with one edge face of the width direction of the first initial pressure-sensitive adhesive sheet fused to one edge face of the width direction of the second initial pressure-sensitive adhesive sheet.
The pressure-sensitive adhesive sheet according to Claim 14 or 15, wherein the first section and the second section have an equal width.
The pressure-sensitive adhesive sheet according to any one of Claims 14 to 16, wherein the first section and the second section are of equal type of pressure-sensitive adhesive species; and the substrate layer of the first section and the substrate layer of the second section are formed of equal type of resin species.
A surface protective sheet comprising the pressure-sensitive adhesive sheet according to any one of Claims 1 to 17, wherein the pressure-sensitive adhesive sheet is an adhesively single-faced pressure-sensitive adhesive sheet having the pressure-sensitive adhesive layer provided solely to one face of the substrate layer.
A method for producing a pressure-sensitive adhesive sheet, the method comprising
a step of obtaining at least two initial pressure-sensitive adhesive sheets, and
a step of joining the two initial pressure-sensitive adhesive sheets by abutting the two initial pressure-sensitive adhesive sheets at their edge faces and fusing the abutted edge faces.
The method according to Claim 19, wherein
both the two initial pressure-sensitive adhesive sheets have long lengths, and
in the joining step, lengthwise edge faces of the two initial pressure-sensitive adhesive sheets are abutted and fused together.