DE69514542T2 - COATING MATERIALS WITH WAXY SUBSTANCES - Google Patents

Abstract

A process of coating a heat-conductive strip article, e.g. aluminum or aluminum alloy strip (14) used in the automotive industry, with a solid wax-like coating material, preferably a wax-type lubricant. The process involves heating a solid wax-like coating material that melts in a temperature range of about 30 to 100 DEG C to form a melt having a viscosity in a range of about 50 centipoise or less and flowing the melt onto a moving surface of the strip article through an elongated slot (15) in a movable coating head having an extended surface adjacent to the slot to form a coating layer on the article. The coating head is pushed towards the surface of the strip article as the melt is flowed as a coating onto the surface from the slot (15) to reduce a coating thickness by pressing the extended surface of the coating head onto the coating as the coating is formed. The process allows a coating of a wax-like material to be formed to a desired thickness and yet allows the wax-like material to solidify shortly after application to the strip article (14) so that the strip can be coiled without providing a cooling run of undue length or special cooling devices.

Description

Technical area

This invention relates to the coating of strip-shaped objects with waxy materials.

In particular, the invention relates to a coating process particularly suitable for coating aluminum and aluminum alloy strips, such as those used for automotive sheet metal in stock, with waxy lubricants.

State of the art

The use of aluminium and aluminium alloys in the manufacture of car bodies and other automotive parts is of increasing interest today because of manufacturers' and governments' efforts to reduce fuel consumption and air pollution. This has led to the use of adhesives for joining aluminium parts, since welding aluminium is problematic and inefficient. However, conventional lubricants used for coating sheet surfaces during sheet metal manufacture tend to be incompatible with the adhesives suitable for the joining processes, and new lubricants have been developed to eliminate this problem. These new lubricants usually have wax properties, i.e. they are solid at room temperature, but melt to low viscosity liquids (i.e. liquids with a viscosity below 50 centipoise and often below 30 centipoise) at slightly elevated temperatures (i.e. 30 to 100ºC, or more commonly 30-50ºC, depending on the type and formulation of the lubricant).

Currently, a heated roll coater is typically used to apply such lubricants, but this is partially inefficient because the distribution of the coating material over the strip surface becomes variable due to the tendency of the low viscosity lubricant to flow in irregular patterns over the metal surface following its initial application.

In addition, sufficient heat is transferred to the metal sheet by the coater during the coating process so that solidification of the lubricant is retarded. Unless special precautions are taken, such as installing a separate cooling zone downstream of the coater apparatus, the lubricant may still be in a molten state when the sheet reaches the reeling machine used to reel the strip article after coating. If the lubricant is still molten when the strip is reeled, the reeling pressure acting on the strip surface will cause the lubricant film to become unacceptably thin, with excess material being squeezed out through the sides of the reel. In addition, the very low strip surface friction can cause the reel to become unstable and "telescoping" (shifting into one another) during the reeling operation.

Consequently, there is a need for an improved method of coating metal surfaces with waxy materials of this type.

Description of the invention

It is an aim of the invention to be able to efficiently coat sheet metal objects with waxy coating materials, in particular lubricants.

Another aim of the invention is to be able to apply waxy coating materials, in particular lubricants, in relatively thick layers to sheet metal objects.

Another aim of the invention is to make it possible to avoid the inevitable cooling of a strip-shaped article after coating with waxy coating materials, in particular lubricants, before rolling up, without a great risk of tearing the coating layer and destabilising the shape of the roll as it is being formed during rolling up.

According to the invention there is provided a method of forming a coating layer of a solid waxy coating material on a surface of a thermally conductive strip-shaped article, in which the solid waxy coating material is heated to melt and a coating layer is formed on the advancing surface of the strip-shaped article by applying the melt to the surface and cooling the melt to form a solid coating layer, characterized in that the melt flows onto the advancing surface from an elongated, laterally open slot of a movable coating head having an extended surface adjacent to the slot oriented at an angle to the strip-shaped article so as to define a coating gap extending in the direction of advance of the strip-shaped article and the coating head is pushed against the surface of the strip-shaped article against opposing hydrodynamic forces exerted by the coating material on the extended surface in the gap as the melt flows from the slot onto the surface to control the thickness of the coating layer.

The present invention is based on the unexpected discovery that waxy coating materials, and in particular waxy lubricants used in the automotive and related industries, can be applied by means of a coating apparatus equipped with a "floating" coating head with an elongated, side-open slot. The success of this process is astonishing because coating apparatus of this type, although known for other applications, have been considered unsuitable for coating low viscosity liquids because excessive leakage of such liquids occurs and uniform coatings are not formed. In addition, low viscosity liquids usually do not generate sufficient hydrodynamic force to prevent metal-to-metal contact of the coating head with the moving strip surface. However, in the process of the invention, the partial solidification of the waxy coating materials in the coating gap appears to take place in a sufficiently reliable manner so that effective coating can be achieved despite the initial low viscosity of the coating material.

On the other hand, since unacceptably long setting times are required for conventional coating machines for the production of waxy coatings, similar problems were to be expected when slot-type coaters were used to be used for this purpose. However, it has been surprisingly found that apparatus of the "floating head" type permit the formation of coatings which solidify rapidly, perhaps because the hot coating head does not directly contact the strip-shaped object and so does not cause excessive heating of the strip.

A preferred example of the type of apparatus used in the process of the present invention suitable for single-side coating of a strip-shaped article is disclosed in U.S. Patent No. 4,675,230, issued June 23, 1987, to the same assignee as the present application. In addition, a related apparatus for double-side coating of sheet material is disclosed in the same assignee as the present application's pending PCT Patent Application No. PCT/CA94/0029, filed May 26, 1994, and published December 8, 1994 as WO 94/27739.

The process of the invention is effective for coating any strip-shaped article made of a material with sufficiently good thermal conductivity so that the heat from the waxy coating can quickly dissipate and the liquid coating thereby quickly solidifies. While the minimum thermal conductivity that achieves this result varies depending on a number of factors (e.g. type and temperature of the coating material, speed of advance of the strip-shaped article, etc.), metals, and in particular aluminum and aluminum alloys, are generally suitable for coating according to the process of the invention.

Numerous "waxy coating materials" are suitable for use in the present invention, but wax-type lubricants are particularly preferred. These materials are solid at room or ambient temperature and form liquids with low viscosities, generally below 50 centipoise and more often below 30 centipoise, when heated to temperatures in the range 30-100°C, in most cases 30-50°C. Wax materials containing olefin, e.g. alcohol esters of fatty acids or fatty diamides, are particularly suitable for use in the present invention and specific examples of suitable wax-type lubricants include the following: glycerol monolaurate, pentaerythritol monostearate, ethylene glycol monolaurate, glycerol monopalmitate, ethylene glycol monostearate, glycerol dipalmitate and propylene glycol distearate.

Since, according to the invention, rapid cooling and solidification of the coating material is necessary during and immediately after coating, the waxy coating material is preferably heated before coating only to a temperature that is the lowest possible in order not to risk premature solidification of the material in the coating apparatus. Temperatures of 30-100°C and more preferably 30-50°C are generally suitable.

As stated above, the coating must be solid before the strip article can be rolled up; so the strip should preferably be fed at a speed which allows solidification before the coating reaches the reeling machine, and for a given strip article and coating material this will vary with the distance between the coating apparatus and the reeling machine. Of course, the highest productivity is achieved when the strip is fed at or near the maximum speed which allows complete solidification before reaching the reeling machine, but lower feed rates can be used if desired, provided that a flat smooth coating can still be produced. Minimum speed at which suitable coating is still possible depends on such factors as the size of the coating gap, the nature of the strip-shaped object and the type of coating material. Suitable speeds can be determined by simple trials and experiments.

The strip article itself should preferably be quite cool when the coating is applied to ensure rapid solidification of the coating. However, pre-cooling of the strip article is generally not necessary, as a strip at ambient temperature is usually suitable.

By applying the waxy coating material in the manner of the invention, rapid and effective coating with waxy coating materials, and in particular wax-type lubricants, can be achieved without the disadvantages of using conventional equipment.

Short description of the drawings

Figure 1 is a side view, partially in cross-section, of a coating apparatus suitable for practicing a preferred embodiment of the invention.

Fig. 2 is a highly simplified schematic representation of an apparatus for carrying out a two-sided coating according to another preferred embodiment of the invention.

Fig. 3 is a graph showing the test results obtained as explained in Example 1.

Fig. 4 is a graph showing the test results obtained as explained in Example 2.

Best way to carry out the invention

Fig. 1 shows in simplified form an apparatus suitable for carrying out a preferred embodiment of the invention. The apparatus 10 consists of a coating head 12 of the type described in U.S. Patent 4,675,230, but modified in the manner described below, which applies a layer of wax-type lubricant 13 to an aluminum strip 14 which is fed around an unheated back-up roller 16 in the direction of arrow A. The coating head 12 generally has an open-sided, extended elongated coating slot 15 which extends across the entire width of the strip at a position in the direction of strip advance where the strip is pressed firmly against the surface of the back-up roller 16. The end of the coating head 12, opposite the strip 14, has an extended coating surface 18 adjacent the outer opening of the slot 15 at least on the downstream side of the slot, provided with a downstream edge 20. The extended surface 18 is disposed at an angle (normally in the range 0.1 to 5º, more preferably 0.5 to 1º) to the surface of the strip 14, thus forming a coating gap 19 which converges in the direction of strip advance.

The coating head 12 has an internal channel 22 which communicates with the coating slot 15. This channel is fed with molten waxy coating material through a heated pressure hose 24 which communicates with a pressure vessel 25. The vessel contains coating material 13 which is kept molten by the supply of heat from a surrounding hot water jacket 26 through which hot water is continuously passed through the inlet 26a and outlet 26b. The vessel is pressurized by compressed air which is supplied through the inlet 27 at the top of the container. The inner channel 22 in the coating head contains an elongated rod-shaped electric heater 28 for the purpose of preventing solidification of the coating material in the coating head. The pressure exerted on the coating material in the container 25 is sufficient to force the molten coating material out of the slot 15 onto the surface of the strip 14 where the extended surface 18 of the coating head adjacent to the slot spreads the coating material and measures its thickness.

The coating head 12 forms part of or is attached to a metal block 32 which is supported by a floor 34 having a flat upper surface on which the metal block 32 rests, the block being supported for sliding movement relative to the floor in a generally horizontal direction as indicated by the double arrow B. A number of vertical opening slots 36 (Fig. 1 shows only one of these), extending horizontally in the direction of the arrow B, are formed in the body of the block 32 at the rear of the channel 22 at spaced locations along the length of the block. A number of bolts 38 (only one of these is shown) each extend through these slots and are bolted into the floor at one end while having enlarged bolt heads 38a at the other end for securing the block 32 to the floor 34. Interaction between the bolt shafts 38b and the side walls of the slot 36 prevents lateral movement of the block 32 relative to the ground, but the extension of the slot allows the block 32 to move in the direction of arrow B over the full range of head position in operation.

The base 34 is mounted on a support frame 40 for rotation about a horizontal axis 42 in order to remove the block 32 together with the base (e.g. with a suitable pneumatic device, not shown) from the position shown in Fig. 1. shown position to a position remote from the path of strip advance. An arm 44 secured to the frame 40 and located beneath the base 34 carries a screw 46 directed upwardly from the arm and pressing against the lower surface of the base 34, thus permitting adjustment of the angular orientation of the head 12 in its operative position.

The frame 40 is fixed in position relative to the axis of the roller 16, the frame and roller both being mounted on a common support assembly (not shown). Thus, the axis 42 is fixed in position relative to the axis of the roller 16 and when the floor 34 is in the operating position shown in Fig. 2, adjusted with the screw 46 to achieve a desired angular orientation, the roller 16 holds the advancing strip 14 opposite the slot 14 at a fixed distance from the floor 34.

The apparatus further includes means acting between the base 34 and the head 12 to apply a constant load to the head and so urge the head against the opposite surface of the strip 14. This load applying means comprises a number of air cylinders 17 (they may generally be of conventional construction and only one of these is shown) fixedly secured to the base 34 at the rear of block 32. As shown, the cylinders 17 are secured to the rearward facing ledge members 50 of the base. Activation of the cylinders (which may generally be of conventional construction) causes the block 32 and coating head 12 to be pressed against the surface of the strip 14. This load is counteracted by the fluid pressure (hydrodynamic force) of the molten (or partially solidified) coating material 13 acting on the extended surface 18 so that the head 12 "floats" on the layer of material 13. This provides a Metering orifice is defined between the downstream edge 20 of surface 18 and the strip surface, the size of the metering orifice (for a given coating material) being determined by the magnitude of the load exerted by the cylinders, which is generally 0.18-27 kg/cm (1-150 lbs./linear inch) of strip width, and more preferably 0.9-9 kg/cm (5-50 lbs./linear inch). No direct mechanical contact occurs between the coating head 12 and the strip 14, thus preventing heat transfer to the surface of the strip and defacement thereof.

Surprisingly, it has been found that the waxy coating material can be successfully coated onto the metal strip even though it usually has a low viscosity in the range of less than 50 centipoise, e.g. 20 to 30 centipoise, using the type of apparatus described in which the coating thickness is controlled by a dynamic load control mechanism. While this type of coating apparatus can in principle be used to apply any liquid to a strip article, in practice this apparatus is not effective for low viscosity liquids. The coating gap 19 between the extruder head and the strip article is selected for a given coating material according to the viscosity, speed and desired coating thickness to ensure that the forces developed are within a practically controllable range. For very low viscosity liquids, low speeds and coating thicknesses, the required width of the gap may become too wide to be practical. However, we have surprisingly found that the device is suitable for the application of molten waxy materials with low viscosity. Because no part of the heated extruder head 12 directly touches the strip-shaped article 14, the Theory, the strip article does not heat excessively above ambient temperature before receiving the layer of molten waxy material, and because of the high thermal conductivity of the sheet metal article (particularly when made of aluminum or aluminum alloy) and the relatively small thickness of the layer of coating material, the coating material begins to solidify immediately when it contacts the strip article within the coating gap between the surface of the article and the coating head. This increases the effective viscosity of the coating sufficiently to make the coating apparatus effective and to permit relatively thick layers of low viscosity lubricant (e.g., thicknesses in the range 1-100 µm, and more preferably 2-25 µm) to be applied at relatively low coating speeds (e.g., about 15 m/min. (50 ft/min.)). In practice, the lubricant is found to be completely solidified within a few decimetres of the coating head.

By heating the container 25, the pressure hose 24 and the coating head 12 (by the heater 28), the coating material 13 can be maintained in a molten state within the above-mentioned viscosity range before it is applied as a coating to the strip 14.

Above the roll 16, the strip can quickly cool the molten waxy material below its solidification point, since it has not been excessively heated by the coating procedure, and so the strip can be rolled up in a conventional manner without the need for a separate cooling step.

Although the illustrated apparatus is designed for one-sided coating, the invention can also be used for two-sided coating by using apparatus of the type described in the above mentioned application, modified so that they can be fed with molten waxy coating material as in the apparatus described for single-side coating.

Fig. 2 shows an example of an apparatus suitable for two-sided coating. The metal strip 14 to be coated is continuously advanced longitudinally parallel to its length by a roller 200 along a path indicated by arrows A which extends successively around spaced guide rollers 201, 202, 203 which are rotatably supported in axially fixed positions (by a structure not shown). The rollers 201 and 202 cooperatively define a rectilinear portion 205 of the path in which the major surfaces of the advancing strip are substantially flat. At one point on this path portion 205, coating material is applied to the two main surfaces 206 and 207 of the strip 14 by two coating heads 12, 12' (aligned with one another and facing the two main surfaces of the strip-shaped object) to achieve a continuous layer or coating of the waxy material on each of the strip surfaces. After passing the roller 203, the coated strip is rolled up again, e.g. on a driven take-up roller 208, which represents the device for advancing the strip through the coating path.

The coating devices 12 and 12' may each be the same as the coating head 12 described for the previous embodiment and may be similarly fed with coating material and pushed against the strip surface.

In this embodiment, molten coating material is applied simultaneously to both sides of the strip at approximately the same location, but Nevertheless, solidification of the applied coating occurs within the coating gaps and thick layers of lubricants can be applied to both sides of the strip 14.

Finally, it should be emphasized that the process of the invention is not limited to the coating of wax-type lubricants on automotive strips in stock, but can also be used for similar applications in which a waxy material is applied as a layer to a strip-shaped article, e.g., the coating of aluminum can ends in stock with protective layers.

The following examples, which are not intended to limit the scope of the invention, illustrate the invention in more detail.

example 1

The tests were carried out on an automotive sheet (aluminum alloy) of 0.09 cm (0.036 inch) thickness using a 0.762 m (30 inch) two-side coating apparatus of the type shown herein (equipped with 5 cylinders 17 for applying load) and AL070 wax type lubricant (ethylene glycol monolaurate having a melting point of approximately 35 to 37°C) applied at a temperature of 44°C. The coating material was fed to the coating heads at a pressure of 34.5 kPa (5 pounds per square inch) and the coating heads were pressed against the strip surface with an average force of 6.25 kg/cm (35 pounds per linear inch) of strip width. The test results are shown in the graph of Fig. 3, which shows the distribution of the coating thickness on the upper and lower surfaces of the sheet (when the sheet is fed horizontally by the apparatus) across the width of the sheet. The vertical Arrows in the graph indicate the positions of the five load air cylinders which were set at pressures of 0, 138, 138, 138 and 0 kPa (0, 20, 20, 20 and 0 pounds per square inch, respectively) of air pressure. The speed of strip travel was 15 m/min. (50 ft/min.), the width of the measuring field 0.8 cm (0.3 inch) and the angle of the coating head 0.5º. Under these conditions the weight of the film theoretically calculated for a coating liquid having a viscosity of 30 centipoise is approximately 0.1 g/m². The fact that the actual thickness of the coating ranged from 1 to 3.5 g/m² shows that the invention is capable of producing films of greater than theoretical thickness. It is believed that the coatings thus produced have an acceptable thickness and uniformity for use in automotive applications.

Example 2

Strip coating was carried out on stock aluminum cans of 0.03 cm (0.011 inch) thickness using a two-side coater with a 30 cm (12 inch) wide coating head (coating width 29 cm (11.5 inch)). Again, AL070 wax lubricant was used, but this time at an application temperature of 51ºC. Two load cylinders, each at one end with an inner cylinder diameter of 3.8 cm (1.5 inch), were used per coating head and each was pressurized with 310 kPa (45 pounds per square inch) of air. The average load on each coating head resulting from the cylinders was 2.5 kg/cm (13.9 pounds linear inch) of strip width. Each coating head had a field width of 0.63 cm (0.25 inch) and was set at an angle of 0.5º to the strip surface. The coating material was fed to each coating head at 24 kPa (3.5 pounds per square inch) pressure and the strip was fed at 15 m per minute (50 feet per minute). The graph in Fig. 4 shows the results. believes that the coatings thus produced have an acceptable thickness and uniformity for use in automotive applications.

Claims (14)

1. A method for forming a coating layer of a coating material (13) on a surface of a thermally conductive strip-shaped article (14) in which the coating material flows onto an advancing surface of the article from an elongated, laterally open slot (15) of a movable coating head (12) having an extended surface (18) adjacent to the slot oriented at an angle to the strip-shaped article to define a coating gap (19) converging in the direction of advance of the strip-shaped article (14), and the coating head (12) is pushed against the surface of the strip-shaped article (14) against opposing hydrodynamic forces exerted by the coating material (13) on the extended surface in the gap (19) as the coating material flows from the slot (15) onto the surface, to control the thickness of the coating layer, characterized in that a solid waxy coating material (13) is heated until a melt having a viscosity of 50 centipoise or less is formed and this melt is fed into the laterally open slot (15) as the coating material. 2. Method according to claim 1, characterized in that aluminium or aluminium alloy strips are used as thermally conductive strip-shaped object (14). 3. A method according to claim 1, characterized in that the solid waxy coating material (13) is a material which melts in a temperature range of 30-50°C to form a melt having a viscosity of 30 centipoise or less, and that the solid waxy coating material (13) is heated to a temperature in this range. 4. Method according to claim 1, wherein the strip-shaped object (14) has a second surface (207) and a layer of coating material (13) is simultaneously also formed on the second surface of this strip-shaped object (14) by flowing a coating material (13) onto this second surface (207) from an elongated, laterally open slot located in a second movable coating head (12') having an extended surface adjacent to the slot oriented at an angle to the strip-shaped object (14) to define a coating gap converging in the direction of advance of the strip-shaped object (14), and the second coating head (12') is pushed against the second surface (207) of the strip-shaped object (14) against hydrodynamic forces exerted by the coating material (13) on the extended surface of the second Coating head in the gap as the coating material flows from the slot onto the second surface (207) to control the thickness of the second coating layer, characterized in that a solid waxy coating material (13) is heated to form a melt with a viscosity of 50 centipoise or less and this melt is fed into the laterally open slot of the second coating head as coating material. 5. Method according to claim 1, characterized in that the coating head (12) is pressed against the surface with a force which produces a coating thickness in the range of 1-100 µm. 6. Method according to claim 1, characterized in that the coating head (12) is pressed against the surface with a force which produces a coating thickness in the range of 2-25 µm. 7. A method according to claim 1, characterized in that the coating head (12) is pushed against the surface with a force in the range of 0.18-27 kg/cm (1-150 lbs./linear inch) of strip width. 8. Method according to claim 1, characterized in that the coating head (12) is pushed against the surface with a force in the range of 0.9-9 kg/cm (5-50 lbs./linear inch) of strip width. 9. Process according to claim 1, characterized in that the melt is kept at a temperature below 100°C before it is applied to the strip-shaped object. 10. Process according to claim 1, characterized in that the melt is kept at a temperature below 50°C before it is applied to the strip-shaped object. 11. Method according to claim 1, characterized in that the strip-shaped object (14) is kept at ambient temperature before the application of the melt. 12. A method according to claim 1, characterized in that a lubricant is selected as the waxy coating material. 13. The method according to claim 12, characterized in that a lubricant selected from glycerol monolaurate, pentaerythritol monostearate, ethylene glycol monolaurate, glycerol monopalmitate, ethylene glycol monostearate, glycerol dipalmitate and propylene glycol distearate is used. 14. Method according to claim 1, characterized in that the coating head (12) is heated in order to prevent solidification of the melt before its application to the surface.