WO2016147150A1

WO2016147150A1 - Rigid polyvinylchloride compositions for shrink films for use in lpg cylinder tamper evident seals

Info

Publication number: WO2016147150A1
Application number: PCT/IB2016/051524
Authority: WO
Inventors: A. Krishna; C. N. Rajendrakumar; Meenam Mohit VERMA; N. Subramaniyam; A. Sreerama MURTHY; P. Krishnan KUTTY; N Nandakumar
Original assignee: Lpg Equipment Research Centre
Priority date: 2015-03-19
Filing date: 2016-03-18
Publication date: 2016-09-22

Abstract

The present invention provides a rigid PVC formulation comprising, (a) 50 to 100 % by weight of medium molecular weight Suspension-PVC (S-PVC) resin; (b) 1.4 to 2.8% by weight of at least one alkyl tin mercaptide stabilizer; (c) 1 to 2 % by weight of at least one solid alkaline earth metal lubricant; and (d) 0.4 to 0.8 % by weight of at least one solid organic lubricant; wherein the rigid PVC formulation can be used to make rigid PVC shrink films for non-packaging applications such as tamper evident seals used as closures in LPG cylinders to prevent misuse/tampering of LPG gas. The present invention further provides processes for making the rigid PVC shrink films by the double bubble extrusion process using an extruder with a rotating die.

Description

RIGID POLYVINYLCHLORIDE COMPOSITIONS FOR SHRINK FILMS FOR USE IN LPG CYLINDER TAMPER EVIDENT SEALS

FIELD OF THE INVENTION

[0001] The present invention relates to uniquely formulated un-plasticized polyvinyl chloride (PVC) formulation(s) which can be used to make rigid PVC (RPVC) shrink films for non- packaging applications such as tamper evident seals (TES) used as closures in LPG cylinders to prevent misuse/tampering of LPG gas. The un-plasticized PVC compositions find applications in rigid PVC pipes, profiles, and blister films besides shrink film seals.

BACKGROUND OF THE INVENTION

[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.

[0003] Among all the commodity thermoplastics, polyvinylchloride (PVC) stands out as the most versatile polymer for manufacturing a wide spectrum of very soft to vary hard plastic products like flexible hoses, rigid pipes, wire & cable insulation and sheathing, shoe soles, soft and hard profiles and stretch and shrink films. PVC when blended with the right choice of additives, it could be converted to useful plastic products either by the extrusion or injection process. Another advantage of using PVC thermoplastic is that it is less susceptible to flammability in relation to other thermoplastics like, Polyolefins, Styrenix and Acrylic polymers.

[0004] The process of sealing Liquefied Petroleum Gas (LPG) cylinders, after gas filling, involves hot air to shrink the PVC seal. PVC shrink films which are currently used in preformed PVC seals in LPG cylinders contain primary plasticizers in PVC composition (also referred to as PVC-P composition) which do not make them tamper evident as they fall into the category of semi-rigid films which soften when heated by pouring hot water above 70°C. In other words, the already shrunk film partially expands due to the effect of heat on the liquid plasticizer whose viscosity decreases and the PVC-P film's rheology changes to that of a flexible film. This was established when the installed seal at atmospheric temperature became loose on treating with hot water. The loosened seal could be effortlessly removed from the cylinder valve for pilfering the gas. The said removed seal, to a great extent, contracted back to its original state as temperature dropped close to atmospheric ambient conditions. That is, on cooling the viscosity of the plasticizer in the PVC matrix reversed to its former state regenerating sufficient shrinkage of the expanded seal. This was evidenced when the said dislodged film seal shrunk once again after placing it over the cylinder valve & treating it with hot water. After removing and recapping the seal on to the LPG cylinders, the seals apparently looked un-tampered to the consumers. The pilferage of liquefied petroleum gas due to wrongly formulated PVC-P compositions and poorly designed shrink seals has caused immense monetary losses to the public by black marketers. As the market for shrink seals is highly competitive, the present day manufacturers shared with inventors of this patent that the currently used PVC compositions include a primary plasticizer and low cost filler(s) to reduce price of the raw materials (PVC Compounds) used in making shrink films. These films do not meet the requirements of permanent and irreversible shrink properties which are prerequisites for tamper proof shrink seals for LPG cylinders. Further, the present day manufacturers use an outdated single bubble blown film extrusion process, drawbacks of which include variation in film thickness contributing to inconsistent mechanical properties, dislodging of expanded seal by pouring hot water facilitating softening of PVC-P shrink film, and shrinkage in both horizontal and vertical directions.

[0005] Thus, there is a need in the art to develop new rigid PVC (RPVC) formulations which can be used in rigid PVC shrink films. Such shrink films must be used to develop and design tamper evident shrink RPVC seals for LPG cylinders. Further, shrinkage of such rigid PVC shrink films must be irreversible and permanent when they are subjected to heat at about 80°C and above either using hot water or hot air. Moreover, such new rigid PVC shrink films must have unidirectional shrink property.

[0006] The present invention satisfies the unmet existing needs, as well as others, and generally overcomes the deficiencies found in the prior art.

OBJECTS OF THE INVENTION

[0007] Accordingly it is an object of the present invention to provide a rigid PVC (RPVC) shrink film that can overcome one or more aforementioned limitations of currently used PVC-P semirigid shrink film among others.

[0008] It is an object of the present invention to provide a RPVC formulation that is substantially devoid of any plasticizer, liquid lubricant and filler that hamper the permanent shrink property of PVC seals. [0009] It is an object of the present invention to provide a RPVC film whose shrink property is not only permanent but also irreversible at a wide shrink temperature range between 80 °C and 130 °C with good mechanical and optical properties.

[0010] It is an object of the present invention to provide a RPVC film whose shrink property is substantially unidirectional i.e., horizontal direction, unlike in currently used PVC-P formulations, where it is both in horizontal and vertical directions.

[0011] It is a further object of the present invention to produce a monolayer RPVC shrink film by double bubble extrusion process using an extruder with a rotating die.

[0012] These and other objects, which are achieved by the present invention, by systematic study of presently used PVC-P formulation and developing novel PVC-U formulations for RPVC shrink films for tamper evident LPG cylinder seals, are disclosed herein.

SUMMARY OF THE INVENTION

[0013] The prequalification for the development of RPVC shrink film is based on the failure of plasticized shrink PVC film in terms of permanent and irreversible shrinkage, which is inevitable for tamper evident seal.

[0014] The present invention provides a rigid PVC formulation including:

(a) 50 to 100 % by weight of medium molecular weight Suspension-PVC (S-PVC) resin;

(b) 1.4 to 2.8 % by weight of at least one alkyl tin mercaptide stabilizer;

(c) 1 to 2 % by weight of at least one solid alkaline earth metal lubricant; and

(d) 0.4 to 0.8% by weight of at least one solid organic lubricant;

wherein the rigid PVC formulation can be used in making rigid PVC shrink films for tamper evident seals in LPG cylinders.

[0015] In one embodiment of the present invention, the rigid PVC formulation further includes a processing aid comprising 1.5 to 3% by weight of at least two acrylic polymers.

[0016] The rigid PVC formulation of the present invention can further include 2 to 4% by weight of at least one liquid epoxy stabilizer.

[0017] In some formulations, in accordance with embodiments of the present disclosure, internal and external lubricants can be used in addition to solid alkaline earth metal lubricant(s) and solid organic lubricant(s). In other formulations, in accordance with embodiments of the present disclosure, solid alkaline earth metal lubricant(s) and solid organic lubricant(s) can act as internal and external lubricants respectively. [0018] In one embodiment of the present invention, the rigid PVC formulation can further include at least one impact modifier including 3 to 6% by weight of a terpolymer in which butadiene is a rubbery component.

[0019] In an embodiment of the present invention, the rigid PVC formulation can further include 0.29 to 0.58 % by weight of a combination of three polyolefin based lubricants.

[0020] The present invention also provides a process for making a rigid PVC shrink film including the steps of:

(a) mixing 50 to 100% by weight of medium molecular weight S-PVC resin, 1.4 to 2.8% by weight of at least one alkyl tin stabilizer in a high speed heating mixer to form a hot homogenized mixture;

(b) cooling the hot mixture in a cooler mixer to form a cold mixture;

(c) sieving the cold mixture in a classifier to form a free flowing dry blend; and

(d) converting the blend into shrink film by the Double Bubble Film Extrusion Process to form the rigid PVC shrink film.

[0021] In one embodiment, the mixing step of the process can further include at least one processing aid, one or more lubricants and at least one impact modifier.

[0022] In another embodiment of the present invention, the cold mixture can be sieved in a classifier with 40 mesh to remove agglomerates above 350 microns.

[0023] In one embodiment of the present invention, the blend obtained in step (c) can be extruded in a rotating die extruder.

[0024] The rigid PVC shrink films of the present invention can be used in tamper evident seals used as closures in LPG cylinders to prevent misuse/tampering of LPG gas. The rigid PVC compositions of the present invention can also find applications in rigid PVC pipes, profiles, and blister films besides shrink film seals.

[0025] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments.

BRIEF DESCRIPTION OF DRAWINGS

[0026] The diagrams are for illustration only, which thus is not a limitation of the present invention, and wherein:

[0027] FIG. 1 illustrates an exemplary die which has the contours of a LPG cylinder seal in accordance with embodiments of the present disclosure. [0028] FIG. 2A and 2B illustrate exemplary perspective views of a preformed PVC-P seal and a preformed RPVC (PVC-U) seal respectively with representative transverse direction, machine direction, direction of cutting 1 and direction of cutting 2 in accordance with embodiments of the present disclosure.

[0029] FIG. 3 illustrates an exemplary view of a sample seal obtained after cutting the plain preformed PVC-P seal and/or plain preformed RPVC (PVC-U) seal illustrated in figures 2A and 2B in accordance with embodiments of the present disclosure.

[0030] FIG. 4A illustrates an exemplary plot of % shrinkage in MD Vs. Time Interval in minutes exhibiting shrinkage of 1 ^st sample of PVC-P film (80 microns thick) in the Machine Direction (MD) at 95 °C for different time intervals in accordance with embodiments of the present disclosure.

[0031] FIG. 4B illustrates an exemplary plot of % shrinkage in TD Vs. Time Interval in minutes exhibiting shrinkage of 1 ^st sample of PVC-P film (80 microns thick) in the Transverse Direction (TD) at 95 °C for different time intervals in accordance with embodiments of the present disclosure.

[0032] FIG. 4C illustrates an exemplary plot of % shrinkage in MD Vs. Time Interval in minutes exhibiting shrinkage of 2^nd sample of PVC-P film (80 microns thick) in the Machine Direction (MD) at 95 °C for different time intervals in accordance with embodiments of the present disclosure.

[0033] FIG. 4D illustrates an exemplary plot of % shrinkage in TD Vs. Time Interval in minutes exhibiting shrinkage of 2^nd sample of PVC-P film (80 microns thick) in the Transverse Direction (TD) at 95 °C for different time intervals in accordance with embodiments of the present disclosure.

[0034] FIG. 5A illustrates an exemplary plot of % shrinkage in MD Vs. Time Interval in minutes exhibiting shrinkage of 1 ^st sample of RPVC film (60 microns thick) in the Machine Direction (MD) at 95 °C for different time intervals in accordance with embodiments of the present disclosure.

[0035] FIG. 5B illustrates an exemplary plot of % shrinkage in TD Vs. Time Interval in minutes exhibiting shrinkage of 1 ^st sample of RPVC film (60 microns thick) in the Transverse Direction (TD) at 95 °C for different time intervals in accordance with embodiments of the present disclosure. [0036] FIG. 5C illustrates an exemplary plot of % shrinkage in MD Vs. Time Interval in minutes exhibiting shrinkage of 2^nd sample of RPVC film (60 microns thick) in the Machine Direction (MD) at 95 °C for different time intervals in accordance with embodiments of the present disclosure.

[0037] FIG. 5D illustrates an exemplary plot of % shrinkage in TD Vs. Time Interval in minutes exhibiting shrinkage of 2^nd sample of RPVC film (60 microns thick) in the Transverse Direction (TD) at 95 °C for different time intervals in accordance with embodiments of the present disclosure.

[0038] FIG. 6A illustrates an exemplary TGA analysis of PVC-P film in accordance with embodiments of the present disclosure.

[0039] FIG. 6B illustrates an exemplary TGA analysis of RPVC film in accordance with embodiments of the present disclosure.

[0040] FIG. 7A illustrates an exemplary plot of % shrinkage in MD Vs. Time Interval in minutes exhibiting shrinkage of plain preform PVC-P seal (80 microns thick) in the Machine Direction (MD) at 95°C for different time intervals in accordance with embodiments of the present disclosure.

[0041] FIG. 7B illustrates an exemplary plot of % shrinkage in TD Vs. Time Interval in minutes exhibiting shrinkage of plain preform PVC-P seal (80 microns thick) in the Transverse Direction (TD) at 95 °C for different time intervals in accordance with embodiments of the present disclosure.

[0042] FIG. 8A illustrates an exemplary plot of % shrinkage in MD Vs. Time Interval in minutes exhibiting shrinkage of plain preform RPVC seal (60 microns thick) in the Machine Direction (MD) at 95°C for different time intervals in accordance with embodiments of the present disclosure.

[0043] FIG. 8B illustrates an exemplary plot of % shrinkage in TD Vs. Time Interval in minutes exhibiting shrinkage of plain preform RPVC seal (60 microns thick) in the Transverse Direction (TD) at 95 °C for different time intervals in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION [0044] Unless the context requires otherwise, throughout the specification which follow, the word "comprise" and variations thereof, such as, "comprises" and "comprising" are to be construed in an open, inclusive sense that is as "including, but not limited to."

[0045] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0046] As used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the content clearly dictates otherwise. It should also be noted that the term "or" is generally employed in its sense including "and/or" unless the content clearly dictates otherwise.

[0047] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term "about." Accordingly, in some embodiments, the numerical parameters set forth in the written description are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable.

[0048] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. "such as") provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.

[0049] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.

[0050] Reference will now be made in detail to the exemplary embodiments of the present invention.

[0051] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

[0052] In one aspect, the present invention provides a rigid PVC formulation including:

(b) 1.4 to 2.8%by weight of at least one alkyl tin mercaptide stabilizer;

(c) 1 to2 % by weight of at least one solid alkaline earth metal lubricant; and

(d) 0.4 to 0.8% by weight of at least one solid organic lubricant;

[0053] In one exemplary embodiment, the PVC used in the present formulation is a medium molecular weight S-PVC resin having an average degree of polymerization of 1,000 +/- 50 and an apparent bulk density between 0.51 - 0.59. It is suitable for making RPVC films by a blown film process involving either a single bubble or double bubble process.

[0054] The term "alkyl tin stabilizer" as used herein, means mono or dialkyl tin mercaptans, represented by the following formulas A or B respectively:

Formula A Formula B

wherein,

Ri can be methyl, ethyl, butyl, or octyl; R₂ can either be R3-0-C(=0)-R₄ or R3-C(=0)-0-R4, wherein R3 can be a C1-C20 hydrocarbyl group; R4 can be a linear or branched hydrocarbyl group; and R4 can be a linear or branched hydrocarbyl group or hydrogen.

[0055] The term "alkyl tin mercaptans" as used herein also encompasses mixed mono-/di-alkyl tin mercaptans.

[0056] As used herein by "hydrocarbyl" is meant a hydrocarbon group containing carbon and hydrogen.

[0057] In one exemplary embodiment of the present invention, the alkyl tin mercaptide can be selected from but not limited to methyl tin mercaptans, dimethyl tin mercaptans, octyl tin mercaptans, dioctyl tin mercaptans. The methyl tin stabilizers in accordance with the aspects of the present invention can be selected from but not limited to methyl tin dodecylmercaptides, dimethyl tin dodecylmercaptides, methyl tin mercaptoesters such as methyl tin 2-ethylhexyl thioglycolates. The octyl tin stabilizers in accordance with the aspects of the present invention can be selected from but not limited to octyl tin dodecylmercaptides, dioctyl tin dodecylmercaptides, octyl tin mercaptoesters such as octyltin 2-ethylhexyl thioglycolates.

[0058] In one exemplary embodiment of the present invention, the alkyl tin stabilizer is methyl tin mercaptitide represented by the following formula:

[0059] The alkyl tin stabilizers in accordance with the aspects of the present invention can act as thermal stabilizers for heat sensitive PVC polymers which deteriorate at their processing temperatures. These stabilizers impart good transparency to the RPVC films of the present invention.

[0060] The rigid PVC formulation can further include a processing aid comprising 1.5 to 3% by weight of at least two acrylic polymers. The RPVC formulation of the present invention can further include 2 to 4% by weight of at least one liquid epoxy stabilizer such as epoxidized soya oil that protects the PVC polymer by reacting with hydrogen chloride, which is formed due to thermal degradation of the PVC during melt processing and thus improves thermal stability of the PVC during processing and use.

[0061] In the formulations of some embodiments of the present disclosure, internal and external lubricants can be used in addition to solid alkaline earth metal lubricant(s) and solid organic lubricant(s). In other formulations of embodiments of present disclosure, solid alkaline earth metal lubricant(s) and solid organic lubricant(s) can be internal and external lubricants respectively.

[0062] In another embodiment of the present invention, the rigid PVC formulation can further include 0.29 to 0.58 % by weight of a combination of three polyolefin based lubricants.

[0063] The internal lubricants as used in the RPVC formulation of the present invention can be selected from the group comprising of dicarboxylic acid esters of alkaline earth metals such as calcium stearate, oleo chemical fatty acid derivatives, and combinations thereof. In one exemplary embodiment, the internal lubricant can be a commercially available internal lubricant called FINALUX - G 120 which is an oleo chemical fatty acid derivative. Internal lubricants facilitate the movement of one polymer chain against another during melt processing and prevent friction created by polymer chains in motion during extrusion. They not only improve the fusion/gelling of the PVC compound but also reduce the melt viscosity for easy flow of the molten PVC in the extruder.

[0064] The external lubricants as used in the RPVC formulation of the present invention can be selected from the group comprising of low density homo-polymer polyethylene waxes, Sasolwax paraffin waxes, Sasolwax microcrystalline waxes and Fischer- Tropsch Waxes. In one exemplary embodiment of the present invention, the external lubricant is a commercially available external lubricant called A-C® 6A which is a low density homo-polymer polyethylene wax. It has an amorphous structure and low melt viscosity and acts as a strong external lubricant. It is specially required when processing rigid PVC compositions in which the base PVC resin has a K value of 65-67 which gives high melt viscosity when subjected to the extrusion process. Consequently, this high melt viscosity of the PVC extrudate leads to stickiness (adhesion) of the molten PVC to metal surfaces of the blown film extruder. A-C® 6A, during processing of the RPVC composition, migrates to the PVC melt-metal interface where a film layer is formed resulting in reduced shear and improved surface quality. [0065] The RPVC formulation of the present invention, during extrusion, may stick to the metal surfaces and may not fuse completely. For this reason, the RPVC formulation of the present invention can further comprise AC® polyethylene waxes as fusion promoters. In one exemplary embodiment, the AC® polyethylene wax can be AC®316 A which has a crystalline structure, high melt viscosity and functional groups which facilitate strong fusion of the PVC polymer and its additives. It is an oxidized high-density polyethylene wax homo-polymer which improves metal release and the increased melt homogeneity results in enhanced dimensional stability of the extruded blown film.

[0066] In some embodiments, the RPVC formulation of the present invention can further comprise a commercially available blend of oxidized hydrocarbon waxes known as FINAWAX PE 3. FIN A WAX PE 3 functions not only as a viscosity reducer to facilitate the motion of the hot PVC melt in the extruder but also facilitates fusion and thereby contributes to eliminate the problems of flow lines in rigid PVC blown films made either by the single or double bubble process.

[0067] In some embodiments, the RPVC formulation of the present invention can further comprise a processing aid selected from a class of commercially available acrylic polymers.

[0068] In one exemplary embodiment, the commercially available acrylic polymer used in the RPVC formulation of the present invention can be PARALOID™ K-120N which is in lower- dusting free-flowing powder form that improves the melt processing characteristics of rigid vinyl compounds. It reduces melt fracture and gels and fish eyes in the blown film. It prevents surface of the PVC melt from tearing when it exits the extruder die.

[0069] In one exemplary embodiment, the commercially available acrylic polymer used in the RPVC formulation of the present invention can be PARALOID™ K-175 which is an acrylic polymer that functions as both an external lubricant and a processing aid in vinyl compounds. Its acrylic composition enables PARALOID™ K-175 processing aid to provide external lubrication without imparting the incompatibility often found in vinyl compounds containing conventional lubricants. PARALOID™ K-175 processing aid is a valuable ingredient in clear vinyl formulations for applications such as rigid PVC blown films in the extrusion process. The lubricity of a hot PVC melt has a strong effect on its thermal stability. PARALOID™ K-175 facilitates excellent release of the viscous PVC melt from metal surfaces of the screw and barrel of the extruder. It greatly reduces the parting (shear) forces generated between the PVC melt and the metal surfaces which results in improved thermal stability of the molten PVC in the extruder. [0070] The rigid PVC formulation of the present invention can further comprise 3 to 6 % by weight of an impact modifier comprising a terpolymer in which butadiene is a rubbery component.

[0071] More particularly, the RPVC formulation of the present invention can comprise MBS (methylmethacrylate-butadiene-styrene) impact modifiers selected from a commercially available PA ALOID™ BTA series. In one exemplary embodiment, the impact modifier is PA ALOID™ BTA-730 which increases toughness of the blown film and prevents its failure during mechanical stresses due to defects such as notch, crack and un-molten particles. Specifically, in the blown film extrusion, where two bubbles are blown, there are chances of film tearing during the process. This can be prevented using PARALOID™ BTA-730 which imparts high impact strength to film/sheet applications. It increases toughness and prevents failure during mechanical stresses due to defects, such as, notch, crack and un-molten particles. In the double bubble blown film extrusion process, it not only imparts flexibility to the blown film but also contributes to consistently maintain the strength and dimensional stability of the blown film by energy absorption during stress created by the air pressure stretching the inflated film bubble in the cross direction.

[0072] The present invention also provides processes for making the rigid PVC shrink films which can be used in rigid PVC seals for LPG cylinders.

[0073] In one aspect, the present invention provides a process for making a rigid PVC shrink film including the steps of:

(a) mixing 50 to 100 % by weight of medium molecular weight suspension-polyvinylchloride (S- PVC) resin, 1.4 to 2.8 % by weight of at least one alkyl tin mercaptide stabilizer, 1 to 2 % by weight of at least one solid alkaline earth metal lubricant, 0.4 to 0.8 % by weight of at least one solid organic lubricant, in a high speed heating mixer to form a hot mixture;

(b) cooling the hot mixture in a cooler mixer to form a cold mixture;

(c) sieving the cold mixture in a classifier to form a blend; and

(d) converting the blend into shrink film by a conventional Double Bubble Film Extrusion Process to form the rigid PVC shrink film.

[0074] In one embodiment, the mixing step of the process can further include 2 to 4 % by weight of a liquid epoxy stabilizer, 1.5 to 3 % by weight of at least two acrylic polymers, 3 to 6 % by weight of a terpolymer in which butadiene is a rubbery component and 0.29 to 0.58 % by weight of a combination of three polyolefin based lubricants. [0075] In one embodiment, the ingredients in step (a) are heated up to 120 °C and cooled to 45 °C in step (b).

[0076] In another embodiment of the present invention, the cold mixture is sieved in a classifier with 40 mesh.

[0077] In one embodiment of the present invention, the blend obtained in step (c) is extruded in a rotating die extruder.

[0078] The existing methods of preparation of PVC-P shrink films can include extruding molten PVC through a circular rotating die, forming an inflated tubular bubble, which can then be expanded in transverse direction, cooled, collapsed and formed into rolls. The shrink property of the film is a function of the inflated air pressure of the bubble. This process gives film thickness variation of about ± 12 microns which adversely affects the tensile strength at break and elongation at break including uneven shrinkage of the seals when subjected to heat. The seals made from this PVC-P shrink film could be easily manipulated by pouring hot water to facilitate the tampering.

[0079] On the other hand, transverse direction (TD) shrinkage in the RPVC film made from the RPVC (PVC-U) formulation of the present invention is about 10% higher than that achieved in the existing PVC-P shrink film. The machine direction (MD) shrinkage of the PVC-P film is almost 7 times higher than that of RPVC (PVC-U) film, which is undesirable for the tamper evident seals (refer table 4 & table 5 and figures 4 (A, B, C, and D) & 5 (A, B, C, and D)). Also, the Vicat Softening Point (VSP) of the RPVC formulation of the present invention is about 20% higher than that of PVC-P formulation. This characteristic of the RPVC formulation of the present invention is a pre-qualification of resisting softening of the RPVC shrink seal. Therefore, it is insurmountable to tamper RPVC seals made from the RPVC formulation of the present invention as the shrinkage is, both permanent and irreversible when subjected to heat either by hot water or hot air at 80°C and above (refer table 6). Moreover, tensile properties of the RPVC shrink films of the present invention is higher than those of existing PVC-P shrink films which prevent breakage of the film during processing and service. The shrinkage as a function of time at a constant temperature of 95°C was tested on a plain preform test samples of the PVC-P and PVC-U (RPVC) film made from a die which has the contours of a LPG cylinder seal. The said die tool used for this exemplary experiment is shown in Figure 1. The plain preformed PVC-P seal, an exemplary perspective view of which is as illustrated in Figure 2A, wherein, 220 indicates Transverse Direction (TD) and 240 indicates Machine Direction (MD), and plain preformed RPVC (PVC-U) seal, an exemplary perspective view of which is as illustrated in Figure 2B, wherein, 260 indicates direction of cutting 1 and 280 indicates cutting direction 2, were cut to the required size, an exemplary view of which is as illustrated in Figure 3, wherein 320 indicates Transverse Direction (TD) and 340 indicates Machine Direction (MD), using scissors and cutter for conducting the shrinkage tests at different intervals of 1 min, 2 min & 3 min at 95°C in a water bath. The shrinkage test results of the preformed samples, as shown in Table 7 & Table 8 and figures 7 (A and B) & 8 (A and B), clearly show that the shrinkage of the RPVC seal in the MD is 2.5% for the different time intervals. However, this is not so in the case of PVC-P seals where the shrinkage is as high as 22.5% for the above time intervals. The results also substantiate the shrinkage result of the respective films. The shrinkage test was also conducted on the printed preform RPVC (PVC-U) seal in the same way as explained above. The shrinkage test results of the printed preformed samples, as shown in Table 9, clearly shows that the shrinkage of the RPVC seal in the MD is 2.5% for the different time intervals. These results also indicate that the shrinkage is not affected by preforming & printing.

EXAMPLES

[0080] The present invention is further explained in the form of following examples. However it is to be understood that the foregoing examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the invention.

[0081] The inventors of the present invention have developed a novel RPVC formulation by systematically analyzing the root causes for the failure of the existing PVC-P formulation used in existing defective seals of LPG cylinders. The composition of the typical PVC-P formulation used in existing defective PVC seals is as shown in Table 1.

[0082] An exemplary formulation in accordance with embodiments of the present disclosure was prepared by following method:

[0083] The S-PVC resin and the additives were weighed and mixed in a 500 L High Speed Heating Mixer. The blend was mixed up to 120°C and subsequently, cooled in a 1,500 liter capacity cooler mixer to obtain a free flowing powder blend. First of all, weighed quantity of PVC resin was added to the high speed mixer (Make: M/s NE Neoplast, Ahmedabad) and mixed at 1480 rpm till 65°C temperature was attained. Then weighed quantities of Methyl Tin stabilizer & ESBO were added to the mixer and mixed till the temperature of the mix attained 75°C. Now, weighed quantities of Calcium stearate, Partially esterified glycerol stearate were added to the mix and mixed till the temperature of the dry mix blend reached 85°C. After attaining the same, weighed quantities of Paraloid K120 N, Paraloid K175, Paraloid BTA-730, AC-316A, Finawax PE-3 & AC-6A were added to the dry mix blend and mixed till 120°C temperature was reached. It was then cooled in a cooler mixer between 28°C and 30°C. Subsequently, the cooled PVC dry blend powder was sieved in a classifier with 40 mesh and then taken up for the extrusion process by the Double Bubble Blown Shrink Film Process.

[0084] The RPVC shrink film was processed by the "double bubble" film extrusion process to achieve the properties ideal for making TES which should demonstrate pilfer proof and tamper evidence by irreversible and permanent shrinkage of the LPG cylinder seals which can be only unsealed by physically damaging the seal which becomes a consequence of tampering for pilferage of cooking or fuel gas stored in cylinders. Furthermore, in the Shrink Film Manufacturing Industry, the film is also processed by the "single bubble" blown film process which has drawbacks like high shrinkage in the machine direction (MD) which adversely affects the TES which should shrink maximum in the transverse direction (TD). A trial batch of RPVC shrink film processed by the single bubble blown film process has shown variations in the tensile strength. Sample 1 & sample 2 cut from 85 micron RPVC film processed by single bubble blown film process has tensile strength of 56.16 MPa & 33.7 MPa respectively. This clearly shows that the thickness in the RPVC film made by single bubble blown film process is inconsistent. However, the tensile test samples taken from RPVC film made by the double bubble blown film process gives consistent tensile strength values, as observed for sample 1 & 2 are 40.5 MPa & 39.8 MPa respectively. This is due to the consistency in the thickness of the RPVC film made by double bubble blown film process (± 5 microns). Hence single bubble blown film process is, therefore, not employed for making RPVC shrink film & tamper evident seals. Films formulated by the RPVC formulation in accordance with embodiments of the present disclosure were compared with the films formulated by the conventional plasticized PVC formulations with respect to three critical parameters namely Vicat Softening Point (VSP), Tensile Strength and Shrinkage as discussed in detail below - Critical Parameter 1 - Vicat Softening Point (VSP)

[0085] Higher the VSP, higher is the resistance to softening point which is essential to prevent easy removal of the shrink seal by applying heat. Comparison of the commercial PVC-P shrink film formulation (Table 1) currently in use and that of the novel RPVC shrink film formulation (Table 2) shows that the VSP increases by replacing the liquid plasticizer, DOP, with solid impact modifier which is a ter-polymer comprising methyl methacrylate-butadiene-styrene. The solid high molecular weight polymer used in the present invention does not soften the shrink film like the former, which is a liquid softener, when subjected to heat, although both contribute to the impact strength of the inflated bubble film preventing it from tearing during the manufacturing process. The VSPs of PVC-P and RPVC compounds were tested as per ASTM D 1525. The VSP of the former was found to be 81°C, while that of the latter was found to be 97.8°C. The higher VSP in the RPVC formulation for shrink film as compared to that of the PVC-P clearly shows that in the absence of plasticizer, the VSP increases substantially. In other words, the plasticizer in the shrink PVC composition reduces the glass transition temperature (Tg) and thereby softens the PVC material which is detrimental in TES and facilitates tampering. Accordingly, the objective of increasing the resistance to softening by application of heat either by hot air or water is achieved as envisaged in the critical parameter 1, that is, VSP.

Table 1: Typical PVC-P formulation used in existing defective PVC seals

[0086] The inventors have identified the root causes for failure of PVC material used in existing defective seals as specified below:

a. The Phthalate primary plasticizer reduces the softening point of the PVC material, becomes slightly elastic & facilitates easy removal of the seal after application of a source of heat such as hot water or hot air. It is not recommended for permanent shrink film material. b. The liquid organic stearate is a lubricant which solvates the PVC polymer and lowers the softening point of the PVC material. It must be eliminated from the formulation as it facilitates softening of the PVC material.

c. Extended low cost fillers hamper the shrink property of the PVC material.

Table 2: An exemplary RPVC formulation of the present invention

Critical Parameter 2 - Tensile Strength

[0087] During shrink film manufacturing by the bubble process, it is subjected to mechanical stresses, particularly stretching when the extruded tube of lower diameter is blown by air into a bubble of higher diameter to form a thin film. The film should have requisite tensile strength to resist the fracture of the film. The novel RPVC composition is designed to give higher tensile strength than that of the present commercial grade plasticized PVC formulation, as the former does not contain the liquid plasticizer, Di-octyl phthalate (DOP) and the extender filler Calcium Carbonate, both of which lower the tensile strength of the end product. The Tensile Strength of PVC-P film was found to be 32.2 MPa, while the Tensile Strength of RPVC film was found to be 40.9 MPa for 85 micron film and 44.37 MPa for 100 micron film. This clearly demonstrates that Rigid PVC film has better Tensile Strength than its Plasticized counterpart. This proves that RPVC film has better resistance to mechanical stresses than the PVC-P film during processing and conversion of the film into tamper evident seals. The lowering of the Tensile Strength in PVC-P film is attributed to the presence of plasticizer(s) which acts as a softener and thereby reduces the critical parameter of Tensile Strength essential for resistance to deformation leading to breakage of the film due to mechanical stresses.

Critical Parameter 3 - Shrinkage

[0088] Shrinkage plays the most critical role for conferring pilfer proof and tamper evident properties to shrink films used in LPG cylinder seals. The shrinkage of films is a function of both, temperature and time. The deciding factor for anti-theft, pilfer proof and tamper evidence of shrink film end products depends on inherent properties of the said film namely:

1. Shrinkage should be very high in the TD and very less in the MD to make seals for LPG cylinders.

2. Shrinkage should be irreversible when heat is applied to soften it for de-sealing the film.

3. Once shrunk, the shrinkage of the film should be a permanent feature which has to be subjected to mechanical damage to de-seal the film.

[0089] The above three properties are achieved in the novel RPVC Shrink Film composition in accordance with embodiments of the present disclosure. To achieve these properties, the formulation should not contain the extender filler, Calcium Carbonate and the plasticizer (DOP) which are highly detrimental to our objective of creating the right PVC composition for shrink film seals. The Calcium Carbonate in the PVC composition reduces the polymer fraction whose chains contribute to the stretching and shrinkage of the film. The evidence for the presence of filler in PVC-P film is exemplified by the fact that the TGA of PVC-P film (Figure-6 A) conducted upto 800 °C in Nitrogen atmosphere showed the total weight loss as 92.89% (high residue as 7.11% ). However, in the case of RPVC film (Figure 6-B), the total weight loss is 96.53% (residue as 3.47%). In other words, in the absence of the filler(s), the polymer stretches more during the bubble formation and retains its memory of the stretching ratio when cooled. Subsequently, this film when heated by air or water between 80°C and 100°C will shrink to the extent of its stretching stored in its memory. The plasticizer in the PVC composition facilitates the softening of the shrunk film due to which the seal expands making way for removal of the seal for tampering. This means that a true RPVC composition is one in which there is zero phr of plasticizer. The tampered seal, after tampering, can be re-shrunk by the application of heat as it still maintains some residual shrink property. Furthermore, the film will be truly permanent only in the absence of the plasticizer. Unlike the PVC-P film, the shrinkage of the RPVC film keeps on increasing with increasing temperature & time and is both permanent & irreversible. Table-3: Shrinkage as function of temperature for PVC-P film (80 microns thick) and

RPVC film (60 microns thick)

s. Shrinkage Test results

Test

o PVC-P Film RPVC film

In Hot air Oven -Machine

A % Shrinkage % Shrinkage

direction

at 80 deg C 17.5 2 at 90 deg C 17.5 2 at 100 deg C 17.5 2 at 110 deg C ~ 2 at 120 deg C ~ 2 at 130 deg C ~ 2

In Hot air oven -Transverse

B

direction

at 80 deg C 34 48 at 90 deg C 38 49 at 100 deg C 40 52 at 110 deg C ~ 52 at 120 deg C ~ 52 at 130 deg C ~ 52

In hot water- Machine

C

direction

80 deg C, 10 sec immersion 15.4 1.5

80 deg C, 30 sec immersion 16.9 1.5

80 deg C, 60 sec immersion 20 3.1

90 deg C, 10 sec immersion 20 3.1

90 deg C, 30 sec immersion 21.5 3.1

90 deg C, 60 sec immersion 23.1 3.1

95 deg C, 10 sec immersion 21.5 3.1

95 deg C, 30 sec immersion 23.1 3.1

95 deg C, 60 sec immersion 23.1 3.1 D In hot water- Transverse

direction

80 deg C, 10 sec immersion 38.5 47.7

80 deg C, 30 sec immersion 43 49.2

80 deg C, 60 sec immersion 43 49.2

90 deg C, 10 sec immersion 46.2 49.2

90 deg C, 30 sec immersion 46.2 49.2

90 deg C, 60 sec immersion 46.2 49.2

95 deg C, 10 sec immersion 47.7 50.8

95 deg C, 30 sec immersion 47.7 50.8

95 deg C, 60 sec immersion 47.7 50.8

[0090] It is evident from table 3 that a rigid RPVC film formulated according to the embodiments of the present invention exhibits shrink property that is not only permanent but also irreversible at a wide shrink temperature range between 80°C and 130°C with good mechanical and optical properties.

[0091] RPVC film provided by the embodiments of the present disclosure exhibits substantially unidirectional shrink property i.e., in horizontal direction, unlike in currently used PVC-P formulations, where it is both in horizontal direction & vertical direction. The experimental data obtained after comparison of the shrinkage property of PVC-P and PVC-U is shown in table 4 and table 5 below (figure 4 (A, B, C, and D) & 5 (A, B, C and D)):

Table 4: Shrinkage of PVC-P film (80 microns thick) at 95°C for different time intervals s. Sample Time interval, Shrinkage in MD, Shrinkage in TD,

No No Minutes % %

1 1 1 22 47.69

2 2 1 21 47.69

3 1 2 23 47.69

4 2 2 22 47.69

5 1 3 22 47.69

6 2 3 22 47.69

7 1 4 18 46.16 8 2 4 18 46.16

9 1 5 15 44.61

10 2 5 15 43.07

Table 5: Shrinkage of RPVC (PVC-U) film (60 microns thick) at 95°C in water bath for different time intervals

092] It is clearly evident from table 4 and corresponding figure 4 (A, B, C & D) that, PVC-P films are of reversible and temporary nature.

Table 6: Shrinkage of RPVC (PVC-U) film (60 microns thick) in hot water at different temperatures

s. Sample Shrinkage in MD, Shrinkage in TD,

Temperature

No No % %

1 80 °C 1 2 49.3

2 80 °C 2 2 49.3

3 80 °C 3 2 49.3

4 80 °C 4 2 49.3

5 80 °C 5 2 49.3

6 90 °C 1 2 50.76

7 90 °C 2 2 50.76 8 90 °C 3 2 50.76

9 90 °C 4 2 50.76

10 90 °C 5 2 50.76

11 95 °C 1 3 52.3

12 95 °C 2 3 52.3

13 95 °C 3 3 52.3

14 95 °C 4 3 52.3

15 95 °C 5 3 52.3 3] From table 6, it is evident that RPVC (PVC-U) films are of irreversible and permanent nature.

Table 7: Shrinkage of plain preform PVC-P (without print) seal (80 microns thick) at

95 °C in water bath for different time intervals

Table 8: Shrinkage of plain preform RPVC (without print) seal (60 microns thick) at

95°C in water bath for different time intervals

Time Intervals, Minutes 1 2 3

% Shrinkage in MD 2.5 2.5 2.5

% Shrinkage in TD 48 50 48 Table 9: Shrinkage of preform RPVC (printed) seal (60 microns thick) at 95°C in water bath for different time intervals

[0094] It is evident from the Tables 5, 6, 8 and 9 and figures 5 (A, B, C, and D) & 8 (A and B) that RPVC film and preformed seals meet the requirement of TES as the shrinkage in the TD grips the LPG cylinder valve firmly with little shrinkage in the MD retaining its grip throughout the length of the valve. Based on above evidence, we conclude that RPVC shrink film, in accordance with embodiments of the present disclosure, is ideal for LPG cylinder TES.

ADVANTAGES OF THE PRESENT INVENTION

[0095] The present invention provides a RPVC shrink film that will overcome many of PVC-P shrink film limitations.

[0096] The present invention provides a rigid RPVC formulation that is without any plasticizer, liquid lubricant and filler which hamper the permanent shrink property of the seal.

[0097] The present invention provides a rigid RPVC film whose shrink property is not only permanent but also irreversible at a wide shrink temperature range between 80 °C and 130 °C with good mechanical and optical properties.

[0098] The present invention provides a rigid RPVC film whose shrink property is mainly unidirectional i.e., horizontal direction, unlike in presently used PVC-P formulations, where it is both in horizontal direction & vertical direction.

[0099] The present invention provides a process to produce a monolayer RPVC shrink film by a double bubble extrusion process using an extruder with a rotating die.

Claims

We Claim:

1. A rigid polyvinylchloride (RPVC) formulation comprising:

(a) 50 to 100 % by weight of medium molecular weight suspension-polyvinyl chloride resin;

(b) 1.4 to 2.8 % by weight of at least one alkyl tin mercaptide stabilizer;

(d) 0.4 to 0.8% by weight of at least one solid organic lubricant; wherein the rigid PVC formulation is used in making rigid PVC shrink films for tamper evident seals in LPG cylinders.

2. The rigid polyvinylchloride formulation as claimed in claim 1, wherein the suspension polyvinylchloride resin has an average degree of polymerization of 1,000 ± 50 and an apparent bulk density ranging between 0.51 - 0.59.

3. The rigid polyvinylchloride formulation as claimed in claim 1, wherein the alkyl tin mercaptide stabilizer comprises mixture of mono-/di-alkyl tin mercaptans.

4. The rigid polyvinylchloride formulation as claimed in claim 1 or 3, wherein the alkyl tin mercaptide can be selected from the group comprising of methyl tin mercaptans, dimethyl tin mercaptans, octyl tin mercaptans, and dioctyl tin mercaptans.

5. The rigid polyvinylchloride formulation as claimed in claim 1, 3 or 4, wherein the alkyl tin mercaptide stabilizer can be represented by following structure:

6. The rigid polyvinylchloride formulation as claimed in any of the preceding claims, further comprises a processing aid comprising 1.5 to 3 % by weight of at least two acrylic polymers.

7. The rigid polyvinylchloride formulation as claimed in claim 6, wherein the processing aid can be selected from the group comprising of PA ALOID™ K-120N and PA ALOID™ K-175.

8. The rigid polyvinylchloride formulation as claimed in any of the preceding claims, further comprises 2 to 4 % by weight of at least one liquid epoxy stabilizer.

9. The rigid polyvinylchloride formulation as claimed in claim 8, wherein the at least one liquid epoxy stabilizer can be epoxidized soya oil.

10. The rigid polyvinylchloride formulation as claimed in any of the preceding claims, further comprising at least one internal and at least one external lubricant.

11. The rigid polyvinylchloride formulation as claimed in claim 10, wherein the internal lubricant can be selected from the group comprising of dicarboxylic acid esters of alkaline earth metals, oleo chemical fatty acid derivatives and combinations thereof and wherein the external lubricant can be selected from the group comprising of low density homo-polymer polyethylene waxes, sasolwax paraffin waxes, sasolwax microcrystalline waxes and fischer-tropsch waxes.

12. The rigid polyvinylchloride formulation as claimed in claim 10 or 11, wherein the internal lubricant can be FINALUX - G 120 and wherein the external lubricant can be AC® 6A.

13. The rigid polyvinylchloride formulation as claimed in any of the previous claims, further comprising FINAWAX PE 3.

14. The formulation as claimed in any of the preceding claims, further comprising at least one impact modifier comprising 3 to 6 % by weight of a terpolymer having butadiene as a rubbery component.

15. The formulation as claimed in any of the preceding claims, wherein the at least one impact modifier can be PARALOID™ BTA-730.

16. The formulation as claimed in any of the preceding claims, further comprising 0.29 to 0.58 % by weight of a combination of three polyolefin based lubricants.

17. A process for making a rigid polyvinylchloride shrink film comprising the steps of: (a) mixing 50 to 100 % by weight of medium molecular weight suspension-polyvinyl chloride resin, 1.4 to 2.8 % by weight of at least one alkyl tin mercaptide stabilizer, 1 to 2 % by weight of at least one solid alkaline earth metal lubricant, 0.4 to 0.8 % by weight of at least one solid organic lubricant in a high speed heating mixer to form a hot mixture;

(b) cooling the hot mixture in a cooler mixer to form a cold mixture;

(d) converting the blend into shrink film by the double bubble film extrusion process to form the rigid polyvinylchloride shrink film.

18. The process for making a rigid polyvinylchloride shrink film as claimed in claim 17, wherein the mixing step in (a), further comprises addition of 2 to 4 % by weight of a liquid epoxy stabilizer or 1.5 to 3 % by weight of at least two acrylic polymers or 3 to 6 % by weight of a terpolymer having butadiene as a rubbery component or 0.29 to 0.58 % by weight of a combination of three polyolefin based lubricants or combination of any of them.

19. The process for making a rigid polyvinylchloride shrink film as claimed in claim 17 or 18, wherein the step (a) comprises heating up to 120 °C.

20. The process for making a rigid polyvinylchloride shrink film as claimed in claim 17, 18 or 19, wherein the step (b) comprises cooling to 45 °C.

21. The process for making the rigid polyvinylchloride shrink film as claimed in claim 17 or 18 or 19 or 20, wherein sieving of the cold mixture obtained in step (a) is carried out in a classifier with 40 mesh.

22. The process for making the rigid polyvinylchloride shrink film as claimed in claim 17 or 18 or 19 or 20 or 21, wherein extruding of the blend obtained in step (c) is carried out in a rotating die extruder.