Copper Adhesive Composition

This application is not related to any other application owned by the same asignee but it is an improvement over commonly assigned U.S. Pat. No. 3,862,265, entitled "POLYMERS WITH IMPROVED PROPERTIES AND PROCESS THEREFOR" filed Apr. 3, 1972, which is incorporated herein in its entirety, by reference, and it is relevant to a copending, commonly assigned application with the same filing date as Ser. No. 340,061, entitled "FASTENER ARTICLE" Ser. No. 340,446 filed Mar. 12, 1973, now Ser. No. 659,317, filed Feb. 19, 1976 by the same inventor.

Copper is a notoriously difficcult substrate to adhere materials to since it has a highly oxidized surface and bonds can readily rupture through the failure of an oxide layer when oxygen diffuses through the adhesive. Certain epoxy adhesives conventionally known are used for bonding materials to copper but they usually give rise to weak intermediate layers which ultimately result in bond failure. Whatever success is obtained by the art in adhering materials to a copper substrate, usually results from the use of bright, clean copper which has been mechanically abraded or which has been vapor degreased and acid etched.

Conventionally a feric chloride/nitric acid acqueous solution is used to obtain the bright copper. The copper surfaces can also be primed with a silane such as an amino silane to improve copper bonding.

A recent U.S. Pat. No. 3,342,647, teaches that bonding of copper sheets can be obtained by using urea, formamide, or hexamethylene tetramine which is decomposed thermally and conventional adhesive then applied to such an activated surface.

Conventional type adhesives which are used to bond copper include epoxides hardened with polyamine, or an ionomer resin a class of polymers in which ionized carboxyl groups create ionic crosslinks, such as Surlyn obtained from DuPont. If hot-melts are to be used, then nylon-type polyamides or polyhydroxy ether polymers can be used.

Specifically formulated acrylic acid or a glycidyl acrylate grafts containing a flexible, e.g. elastomeric component, can be used as copper adhesive without elaborate copper surface preparation.

In the Steinkamp and Grail Ser. No. 240,494 application, and the copending application entitled "FASTENER ARTICLE", a detailed description is given to a new class of grafted polymers which are prepared preferably by a novel technique and which demonstrate unusual adhesion capabilities for numerous substrates. Surprisingly it has been found that most of the compositions prepared according to the Ser. No. 240,494 application exhibited excellent adhesion properties over a wide range of substrates but do not have minimum adhesion capabilities towards copper.

It has been discovered and forms the basis of this invention that certain highly specific compounds specially formulated and falling within the generic class described in the above mentioned applications, will exhibit outstanding adhesion to copper. Indeed in a direct comparison with the DuPont Surlyn the specific products of the invention are at least equivalent in one instance and highly superior in other instances.

The speicific novel formulations set forth herein will vary in their adhesive values towards copper over a range slightly beter than Surlyn to a range more than twice the adhesive value of Surlyn.

Generally speaking the special copper adhesion thermo-plastic compositions of the invention can be classified as either polyethylene based or polypropylene based.

When they are polyethylene based, the significant factor in their formulation and composition is that direct grafting with either a glycidyl acrylate component or an acrylic acid component to a polyethylene base, without additional modification of the base formulation, results in compositions which do not adhere well to copper.

In order to obtain maximum copper adhesion values with a polyethylene based system, there must also be included in the formulation a minor amount of an elastomeric component. Such an elastomeric component can be any elastomer which is compatible with polyethylene, generally low unsaturated varieties being preferable. Suitable examples include ethylene p lene copolymers, butyl rubbers, polyisobutylene, ethylene vinyl acetate, and the like.

Very generally, it will be found that the polyethylene based copper adhesive composition of the invention will comprise 60 to 97, preferably 80 to 95, and most preferably 85 to 92 weight percent of the low density polyethylene blended with a balance of elastomer such as butyl rubber, polyisobutylene, or ethylene propylene copolymer.

This blend is then grafted with from about 0.02 to 20, preferably 0.1 to 10, and most preferably 0.2 to 8 weight percent of acrylic acid or glycidyl acrylate. The grafting is preferably carried out in accordance with the procedures of Steinkamp and Grail, Ser. No. 240,494, which has been incorporated herein in its entirety by reference.

When referring to polypropylene based copper thermoplastic adhesive formulation, it is also important to note that direct grafting or incorporation of acrylic acid or glycidyl acrylate into the polypropylene per se results in little or minimal copper adhesive values, unless molt temperatures of at least 500.degree. F. are used.

It is only when an ethylene/propylene copolymer component either elastomeric or in the form of a reactor copolymer or a similar elastomeric component such as described above, e.g. butyl rubber, polyisobutylene and the like, is introduced in minor quantities into the polypropylene based formulation, followed by grafting with either acrylic acid or glycidyl acrylate that outstanding copper adhesion results can be obtained.

The other important criteria when operating with polypropylene based formulations, is that the grafting monomer (5) of choice are the glycidyl acrylates, since in those situations exceptionally high adhesion values to copper can be obtained, especially when compared to a standard of the industry such as Surlyn.

Very generally, it is found that blends of 60 to 97, preferably 80 to 95, and most preferably 85 to 92 weight percent of a polypropylene blended with a balance of elastomer or reactor copolymer and then grafted with from about 0.02 to 20, preferably 0.1 to 10, and most preferably 0.2 to 8 weight percent of acrylic acid or, most preferably, glycidyl acrylate, will adhere to copper or copper alloy substrates. Glycidyl acrylate grafts are particularly oustanding for achieving maximum copper adhesion.

The components of the copper adhesion compositions can be blended together and grafted, or they can be grafted separately and then blended and formulated together. The person skilled in the art will choose whichver is most convenient.

Additional details need not be given here since the Ser. No. 240,494 application has complete descriptions.

Although this application and invention is expressly directed to specific, narrowly-defined thermoplastic and adhesive compositions, which display outstanding adhesion for copper, it is to be noted that an important feature of this invention is that the preferred copper adhesion formulation contains either a reactor copolymer of ethylene and propylene or an elastomeric component in relatively minor proportions.

This apparently increases the mobility of the adhesive composition and promotes additional adhesion. However, the phenomenon is more broadly applicable than for just formulations prepared for copper adhesion.

It is applicable to all the grafted formulations disclosed not only in this applicaton, but in related copending applications, which are copending with Ser. No. 240,494.

Moreover, it has also been noted and discovered and is an important facet of the technology involved with these grafted formulations, that inclusion of a relatively minor, e.g. 2 to 30, preferably 5 to 20, most preferably 5 to 15 weight percent of an elastomeric component such as described above, results in a formulation, particularly as respects a polypropylene based formulation in which drastic quenching, such as water soaking to promote adhesion is rendered unnecessary.

In other words, very good and perhaps maximum adhesion values can be obtained with polypropylene and high density polyethylene based formulations in which a small elastomeric component is included without the necessity for a quench. Obviously this is a process feature of significant economic value.

The adhesive thermoplastic formulations of the invention can also be more precisely defined by additional criteria, i.e. their melt-flow rate (MFR) or their melt index (MI). Generally, as this number increases, the ability of the adhesive formulation to wet the copper substrate increases. Therefore, the MFR or MI of the particular formulation used will vary over a range of from aout 1 to 300, preferably 2 to 50, and most preferably 2 to 35.

Although glycidyl acrylate and acrylic acid monomers are especially preferred and demonstrated grating monomers for this invention, other closely related monomers would be expected to be equivalents.

The term reactor copolymer as used herein is intended to refer to isotactic crystalline polypropylene which has been modified as made in the reactor with ethylene sequences, either alternating or in blocks to improve its impact performance at low temperatures. This is a common commercial expedient.

It should also be emphasized that the elastomeric component can be added to the base polymer when in the form of an impact blend, sometimes incorrectly referred to as impact copolymer. This blend will usually have varying proportions of polypropylene, polyethylene and an elastomer such as ethylene propylene copolymers.

The invention will be further illustrated by the following examples:

In this example a series of formulations containing various quantities of polypropylene or polyethylene were blended together, either with or without any additional modifying elastomeric or reactor copolymer components.

Either films or pads were prepared from the formulation and they were melted between two copper strips, allowed to cook, and a standard ASTM peel value determination was made on these strips.

The results are summarized below in Table I.

As can be seen from the data in the above Table, excellent results are obtained with respect to only a few specially formulated compositions, since just directly grafting the polypropylene or polyethylene with either acrylic acid glycidyl acrylate is not adequate to provide outstanding adhesion properties to copper.

Under normal conditions except the glycidyl grafted polypropylene will adhere to copper if the melt temperature is at least 500.degree. F.

It is also an important part of the invention that the copper adhesive formulations will also adhere very well to many other common ferrous and nonferrous metals, as well as thermoplastics, thermosets and other surfaces. Examples of specific laminates include

In addition any of the metals listed can be bonded to each other with the adhesives of this application as well as the Steinkamp-Grail application.