ULTRATHIN CHEMICALLY RESISTANT MARKER AND METHOD OF PRODUCING THEREFOR

Abstract: An ultrathin chemically resistant marker for use on printed circuit boards, electrical wire and cable, and the like comprises at least one layer of a chemically resistant, self-­ amalgamating adhesive film (4) and printed matter (10) which together cooperate to form a monolithic, chemically resistant, environmentally sealed marker. The adhesive film is made of a material selected from the group of polymers which adhere to a substrate upon the application of heat, and preferably also pressure, and which are resistant to caustic solutions and solvents, such as methylene chloride. The ad­ hesive film has a chemically resistant front and rear surface (6, 8) wherein the rear surface (8) is for adhering to a sub­ strate. One of the front or rear surfaces (6, 8) contains the printed matter. There is also disclosed a method for produc­ ing an ultrathin chemically resistant marker.

Description: This invention relates to the field of markers and particularly to those markers which are very thin and also have the combined properties of adhesion and resistance to a variety of chemicals. In conjunction with various electrical and electronic components, identifying markers are necessary for providing identification or other relevant information about the particular component. For example, individual wires in a bundle or harness may be identified by wraparound markers. Here, wraparound markers are used for identification. Other components, such as printed circuit board laminates, need markers that serve the purpose of identification but that are also resistant to a variety of chemicals. Printed circuit board laminates, for example, go through a series of chemical baths that all but destroy presently available markers. Since printed circuit board laminates must maintain their identification for process or lot traceability, the fact that presently available markers are unsatisfactory because of delamination of the markers due to adhesive failure represents a severe problem to the electronics industry. Additionally, since the laminates may be stacked closely together to form a multilayer printed circuit board, a very thin marker is practically a necessity. A very thin marker would be extremely desirable in other areas of application as well, for example, to avoid excessive buildup when marking densely bundled electrical wires in a harness or when marking the outside of the bundle or harness itself. In the latter situation, the smallest outer diameter possible is desirable. Presently available marker materials are generally papers or plastics (eg. polyethylene) which are used in conjunction with an adhesive, usually a contact adhesive. An adhesive is never used alone as a marker material but always in conjunction with one of the previously mentioned substrate materials. These materials are unsatisfactory since firstly they are not sufficiently chemically resistant and secondly they dry out and fall off at elevated temperatures. Another method of marking articles is to use heat transfer labels. For example, US Patent No. 4426422 describes a laminate for heat transfer labels. In these labels a carrier (or support) such as an elongated web of paper or plastic film or air laid plastic fibre sheet carries a multilayer 'label' to be transferred onto a substrate. The multilayer label may consist of adhesive and protective layers of a mixture of at least two interspersed polymers with an imprint layer between them. The protective and adhesive layers are applied as a lacquer of the polymers dissolved in a solvent onto the support or carrier. The support is coated with a release layer (such as wax) which permits the 'label' to be transferred to the substrate. The present invention provides a marker that is particularly suitable for use on printed circuit boards, electrical wire and cable and the like. The invention provides an ultrathin chemically resistant marker comprising at least one adhesive thermoplastic polymeric film and printed matter on at least one principal surface thereof, said adhesive film having a thickness of 0.0254 mm to 0.254 mm and being made of a material selected from the group consisting of polymers having the properties of self-amalgamation, adhesion to a substrate when applied at a temperature of less than about 315°C (600°F), flow characteristics when heated such that printed matter legibility is maintained, optical clarity, and chemical resistance to methyl carbitol and methylene chloride at temperatures up to about 88°C (190°F); said adhesive film and said printed matter cooperating to form a monolithic, environmentally sealed marker. The term "self-amalgamating" should be understood to mean that the adhesive film fuses together within itself without significant flowing. Self-amalgamating should further be understood to means that any bond line between layers of adhesive film dissolves and then disappears. Thus, a multilayer marker will become a monolithic marker after the self-amalgamation. Further, when printed matter is applied between the layers of adhesive film, the printed matter becomes sealed from the environment while still being legible after the self-amalgamation. The adhesive thermoplastic polymeric material, according to the invention, should satisfy the following requirements. It is necessary that the material adhere to a substrate when applied at a temperature less than about 315°C (600°F) because at temperatures higher than about 600°F, they may be delamination of the substrate material which, for example, may be a printed circuit board laminate. For best results, it is preferred that the material be applied under pressure. So as to maintain print legibility, there should be a minimum flow of the adhesive material at the adhering temperture. Also, for print legibility, the material should have optical clarity so that the printed matter can be read through the material. It is also necessary that the material be chemically resistant to chemical baths of caustic solutions and solvents, particularly methylene chloride, up to temperatures of about 190°F. Finally, there should preferably be minimum transference of the chemicals making up the chemical bath when the material is tranferred between the various chemical baths. This transference, which is unusually high in the prior art materials, is believed due to first, the degradation of the marker material and, second to the ionic attraction of the chemicals to the marker material. The materials of the instant invention are generally relatively inert and thus avoid this transference. There are a number of materials that will satisfy these requirements and that will also achieve the objects of the invention. Generally, the preferred materials are modified ethylene vinyl acetate and modified polyolefins such as polyethylene, polypropylene, polybutene, polyisoprene, and their polymers. The ethylene vinyl acetate and polyolefins are modified by attaching (e.g., by grafting) strong polar groups such as anhydrides, acid groups, and cyano groups. The more preferred materials are polyethylene and polypropylene modified by grafting acrylic acid groups. The most preferred materials are ethylene vinyl acetate copolymers modified by grafting anhydride groups. As will become more apparent hereafter, these materials are the most chemically resistant and most adhesive of all the materials evaluated. Thus, these materials when incorporated into a marker will have the most advantageous properties over the prior art markers. Further, copolymers may be polymerized in which one of the monomers contains the strong polar groups. It is believed that these copolymers will exhibit the same advantageous properties as the above-described modified polymers. It is preferred that the adhesive film be very thin so that one layer by itself or several layers of material combined would not cause a noticeable buildup due to the markers. Preferably, the thickness should be not more than about 0.254mm (.010 inch) and, more preferably, should be less than about 0.127mm (0.005 inch). For some applications the preferred thickness may be less than about 0.0254mm (0.001 inch). While the marker as just described will perform satisfactorily in practice it is preferred that the adhesive film be surface treated, such as by cross-linking. Surface cross-linking should be understood to mean that the adhesive film is cross-linked to a depth less than the thickness of the adhesive film. When the adhesive film is surface cross-linked this limits the flowability of the surface of the film so as to avoid obscuring the printed matter. It should be understood, however, that the printed matter is still readable even when the film is not surface cross-linked. Also, when the adhesive film is only surface cross-linked the adhesive quality of the film is not affected. Surface crosslinking may be accomplished by methods known to those skilled in the art such as exposing the adhesive film to a low-energy beam of electrons or to ultra-violet radiation. Of course, when surface crosslinking, the adhesive film may require certain additives known to those skilled in the art which aid and promote the crosslinking. Several embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a perspective view of a first embodiment of the ultrathin chemically resistant marker. Figure 2 is a perspective view of a second embodiment of the ultrathin chemically resistant marker. Figure 3 is a plan view of the ultrathin chemically resistant marker attached to a planar substrate. Figure 4 is a perspective view of the ultrathin chemically resistant marker wrapped around a substrate. Figure 5 is a perspective view of a second embodiment of the ultrathin chemically resistant marker wrapped around a substrate. Referring to the drawings in more detail, and particularly referring to Figure 1, there is shown the ultrathin chemically resistant marker 2. There is the adhesive film 4 having a front surface 6 and a rear surface 8. The rear surface 8 contains the printed matter 10. However, while not shown, it is within the scope of the invention to have front surface 6 contain the printed matter. The printed matter 10 may be added by any known printing means such as a typewriter or nonimpact printer. When the material is cross-linked the depth of the cross-linking will be less than the thickness 12 of the adhesive film. Figure 3 shows the marker attached to a substrate 14. The marker will be planar. It should also be understood that planar can mean substantially planar or nearly planar. Figure 4 shows the marker 2 wrapped around a substrate 16. In this case a first edge of the marker 18 is adjacent to a second and opposite edge 20 of the marker. The printed matter has been printed on the back side of the marker. Alternatively, the front of the marker could have contained the printed matter, particularly where a corrosive environment is not expected. Figure 5 shows a modification of the wraparound marker of Figure 4. In this case, the first edge 18 of the marker is no longer adjacent to second and opposite edge 20. With this embodiment, second edge 20 extends substantially beyond first edge 18 so as to provide an overlapped area. The overlapping is provided so that, for example, printed matter may be applied to the front surface of the marker near first edge 18 and then overlapped with that part of the marker near second edge 20. In this way, the printed matter is protected from the environment. In another embodiment of the invention the marker may comprise at least a second layer of the adhesive film which contacts the one layer of adhesive film so that the printed matter is between the layers of adhesive film. In this embodiment the layers of adhesive film and printed matter cooperate to form a multilayer marker. When the layers of adhesive film are subjected to heat, and preferably also to pressure, the layers of adhesive film become a single layer with the printed matter in the middle thereof, due to the self-amalgamation discussed above. Thus, the multilayer marker actually results in a monolithic, environmentally sealed marker. It should be noted that when there are two or more layers, each layer may be the same material as each of the other layers or may be made of different material, just so long as all the layers are made from the preferred materials. This embodiment of the marker is illustrated in Figure 2. In Figure 2 there is shown an ultrathin chemically resistant marker 22 having one layer of adhesive film 24 having a front surface 26 and a rear surface 28. There is also a second layer 30 of the adhesive film which will contact the rear surface of the first layer 24 of adhesive film. The second layer has a front surface 32 and a rear surface 34. Located between the two layers of adhesive film is the printed matter 36. It is not important to the invention whether the printed matter is printed on the front surface 32 of adhesive film 30 or the rear surface 28 of adhesive film 24. It is desirable, however, that the printed matter be located between the layers of adhesive film. The two layers of adhesive film may be connected such that the multilayer marker may be formed by simply folding the two layers together along fold line 38. However, it is also contemplated within the scope of the invention that the two layers of adhesive film may be entirely separate and that the markers are formed by simply placing one layer of adhesive film over another layer of adhesive film. This particular embodiment is not shown. As with the first embodiment of the ultrathin chemically resistant marker, the marker 22 may be attached to a substrate 14 or may be wrapped around a substrate 16 as previously described with respect to the single-layer embodiment of the marker. It may be found desirable to crosslink the material. When there are two layers of material, the top layer of material may be crosslinked through its entire thickness. Under certain circumstances, it may be found desirable to only surface-crosslink both layers. In either of the embodiments the adhesive film is attached to the substrate 14 by heating the adhesive film which causes the adhesive to adhere to the substrate. When the marker is wrapped around a substrate such as substrate 16, the respective ends 18,20 of the marker may be joined by heating only at the ends or heat may be applied around the entire circumference. It bears repeating here that while the markers of this invention may begin with two or more distinct layers of adhesive film, after application of heat, and preferably also pressure, the layers of adhesive film self-amalgamate so that only a monolithic or single-layer marker results. This is true also in the case of the wraparound marker where the layers may partially or entirely overlap. There is also disclosed according to the invention a method of manufacturing an ultrathin chemically resistant marker. The method comprises the steps of first forming at least one adhesive film. The film may be formed by any of the known methods such as, for example, blowing. The adhesive film may be any of the adhesive thermoplastic polymers described earlier. The adhesive film has a chemically resistant front and rear surface wherein the rear surface is for adhering to a substrate. The next step in the method is adding printed matter to one of the front and rear surface of the adhesive film. Printed matter may be added by any of the well-known methods such as a typewriter, or nonimpact printer. The adhesive film and the printed matter cooperate together to form a marker. When it is desired to adhere the marker to a substrate, then the method may further comprise heating the marker to a temperature less than about 315°C to cause the adhesion of the marker to a substrate. It is preferred that pressure also be applied to promote the adhesion. Preferred materials are polyethylene and polypropylene modified by grafting acrylic acid groups. The most preferred material is ethylene vinyl acetate modified by grafting anhydride groups. An optional step in the method is crosslinking the film to a depth less than its thickness. The reason for this surface crosslinking is as discussed above. The method may further comprise the steps of forming at least a second adhesive film and then contacting the first adhesive film with the second adhesive film so that the printed matter is between the films. The forming of the second film may be by the same process as the first adhesive film. The manner in which the two films are contacted is not important to this invention. That is, two separate films may be contacted or a single large adhesive film may be folded together as previously described, to form two films. When the ultrathin chemically resistant marker is produced as above described the layer of adhesive film and the printed matter may cooperate to form a marker. Or in the case where there are two or more layers of adhesive film then the layers of adhesive film and the printed matter may cooperate to form a multilayer marker. Of course, in either case, the various layers of adhesive film self-amalgamate to form a monolithic marker. In one embodiment of the invention, the marker may be attached to a planar or substantially planar substrate. In another embodiment, the marker may be wrapped around a substrate to form a wraparound marker. The ultrathin chemically resistant markers of the instant invention have several advantages over the presently available markers. Of paramount importance are the adhesive and self-amalgamating qualities and chemical resistance of the markers this invention. It is, of course, known that the preferred polymers of the invention, as previously described may be used as adhesives or for laminating or coating a substrate. However, the adhesive and self-amalgamating qualities and chemical resistance of these polymers has never been recognized or appreciated in connection with their potential use as markers. The advantages of the marker of this invention will become apparent after reference to the test results which follow. Testing was conducted to evaluate materials for application as a chemical resistant marker for printed circuit boards and printed circuit board laminates. The marker is in the form of a printed thin adhesive film that would provide a permanent label when adhered to a printed circuit board or laminate surface. The label must be resistant to various chemicals and not transfer contamination when exposed to a series of baths, retain optical clarity, and possess minimum flow characteristics to eliminate print distortion. The most important criteria for testing was adhesion to a printed circuit board and chemical resistance. For board adhesion tests materials were first pressed into thin films, usually .0254 to .0381mm (2 to 3 mil) thick and print applied with a standard office typewriter. These markers were then heat-pressed into the surfaces of both fiberglass and copper-clad printed circuit boards. Melt temperatures of the markers had to be reached to insure good adhesion but without delaminating the board. The upper temperature limit is about 315 o C (600°F) Several materials failed because of their high melt temperatures. To pass the markers must not be able to be peeled or rubbed off of either surface (copper-clad board being the more limiting) and the print must remain legible. Chemical resistance was evaluated by visual inspection of the material after immersion in solvent for 30 minutes. Three solvents were tested: 1) Methylene chloride -- Solvent at room temperature, material tested in pellet form only. 2) Methyl cellosolve (ethylene glycol monomethylether), ethylene glycol, KOH, and water solution -- solvent heated to 88°C, material tested in pellet form, and as in place board marker. 3) Methyl carbitol (diethylene glycol monomethylether) ethylene glycol, KOH, and water solution -- solvent heated to 88°C, material tested in pellet form and as in place board marker. The methyl cellosolve and methyl carbitol solutions were rigorous solvents and partially destroyed the integrity of the circuit boards during the testing. Several materials discolored but remained intact in these solvents. To pass, markers had to remain adhered, intact, and legible. Two procedures were used to test soldering survivability of in-place markers. The first procedure is vapor phase reflow in which samples were immersed in FC70 (low molecular weight fluorinated polymer) atmosphere at 220 o C. for two to three minutes. The second procedure is wave/dip soldering in which samples were passed over a 300 o C. solder wave. In both procedures, the markers had to remain adhered and legible to pass. Table I lists the materials tested and their performance on a relative pass/fail basis. Of all the materials tested, only the Plexars passed all the given requirements. The Plexars are examples of ethylene vinyl acetate modified by grafting anhydride groups. They adhered well to both board surfaces though some flow of print was experienced. In methyl cellosolve solution the markers' edges separated from the copper-clad boards. This is probably a result of the damage to the surrounding copper surface by the solvent. Reichold 1016 and Reichold 1001 also performed well but failed adhesion to the copper-clad surface in methyl cellosolve solvent. The Reichold 1001 and 1016 materials are examples of polyolefin co-polymers modified by grafting acrylic acid groups. Under slightly less rigorous testing these latter two materials would have passed the test. In practice, it is believed that Reichold 1016, Reichold 1001, and similar materials would perform admirably. Overall, it can be seen that the modified polyolefins and the modified ethylene vinyl acetate performed superiorly to their unmodified counterparts.

Claims: An ultrathin chemically resistant marker (2) comprising at least one adhesive thermoplastic polymeric film (4), and printed matter (10) on at least one principal surface thereof, said adhesive film having a thickness of 0.0254 mm to 0.254 mm and being made of a material selected from the group consisting of polymers having the properties of self-amalgamation, adhesion to a substrate when applied at a temperature of less than about 315°C, flow characteristics when heated such that printed matter legibility is maintained, optical clarity, and chemical resistance to methyl carbitol and methylene chloride at temperatures up to about 88°C; said adhesive film and said printed matter cooperating to form a monolithic, environmentally sealed marker. A marker (2) as claimed in claim 1, wherein said adhesive film (4) is optically clear. A marker (2) as claimed in claim 1 or claim 2, wherein said adhesive film (4) is cross-linked to a depth less than its thickness (12). A marker (2) as claimed in any one of claims 1 to 3, wherein said marker is planar when attached to a substrate (14). A marker (2) as claimed in any one of claims 1 to 3, wherein said marker is wrapped around a substrate (16) to form a wraparound marker. A marker (2) as claimed in any one of claims 1 to 5 further comprising a second adhesive film (30) having a thickness of 0.0254 mm to 0.254 mm and being made of material selected from the group consisting of polymers having the properties of self-amalgamation, adhesion to a substrate when applied at a temperature of less than about 315°C, flow characteristics when heated such that printed matter (10) legibility is maintained, optical clarity, and chemical resistance to methyl carbitol and methylene chloride at temperatures up to about 88°C; said second adhesive film being in contact with said first adhesive film (24) so that said printed matter is between said films, and said films and said printed matter cooperate to form a multilayer marker, wherein after application of said films to a substrate (14 or 16) at a temperature less than about 315°C, said films self-amalgamate to form a monolithic, environmentally sealed marker. A marker (2) as claimed in any one of claims 1 to 6, wherein said adhesive polymeric film (4, 24 or 30) comprises a polyolefin or an olefin copolymer modified by attaching polar groups. A marker (2) as claimed in claim 7, wherein said polar groups comprise anhydride groups, acid groups, or cyano groups. A marker (2) as claimed in claim 7 or claim 8, wherein said adhesive film (4) comprises polyethylene, polypropylene or an ethylene vinyl acetate copolymer. A marker (2) as claimed in claim 1, wherein the adhesive film (4) exhibits minimum transference of baths of methyl carbitol and methylene chloride when transferred between such baths. A method of producing an ultrathin chemically resistant marker (2) comprising the step of forming at least one adhesive film (4) having a thickness (12) of 0.0254 mm to 0.254 mm from a material selected from the group consisting of polymers having the properties comprising self-amalgamation, adhesion to a substrate when applied at a temperature of less than about 315°C, and chemical resistance to methyl carbitol and methylene chloride at temperatures up to about 88°C; and adding printed matter (10) to a principal surface of the marker. A method as claimed in claim 11, further comprising the steps of forming at least a second adhesive film (30) having a thickness of 0.0254 to 0.254 mm from a material selected from the group consisting of polymers having the properties comprising self-amalgamation, adhesion to a substrate when applied at a temperature less than about 315°C, and chemical resistance to methyl carbitol and methylene chloride at temperatures up to about 88°C; and contacting the first adhesive film (24) with said second adhesive film so that the printed matter is between the films. A method as claimed in claim 11, which produces a marker (2) as claimed in any one of claims 1 to 10. A marker (2) produced by a method as claimed in claim 11, wherein the adhesive films and the printed matter cooperate to form a multilayer marker. A method as claimed in any one of claims 11 to 13, further comprising the step of applying the marker (2) to a substrate (14 or 16) at a temperature less than about 315°C wherein said adhesive film self-amalgamates to form a monolithic, environmentally sealed marker.

Inventors: BIDDICK, KAREN M., GEN, TAMAR G., HANSEN, ERLING
Applicants: RAYCHEM CORPORATION (A DELAWARE CORPORATION)
Application year: 1985
Main classification: G09F3
IPC codes: G09F3, H05K1, H01B7