The Adhesion of Electrodeposits to Plastics
The 1966 Carl E. Huessner Gold Medal Award was given to Dr. Edward Saubestre and co-workers for Best Paper appearing in Plating in 1965, and their paper is republished here in a series on the AES/AESF/NASF Best Paper Awards. This paper is a comprehensive treatise on the Jacquet peel test, a primary test method for determining adhesion on plated plastics.
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Edward B. Saubestre, Lawrence J. Durney, Juan Hajdu & Edwin Bastenbeck
West Haven, Connecticut, USA
The 1966 Carl E. Huessner Gold Medal Award for Best Paper appearing in Plating in 1965
Presented at the AES 52nd Annual Convention in New York, New York, July 13, 1965
Originally published as E.B. Saubestre, et al., Plating, 52 (10), 983-1000 (1965)
Editor's Note: This paper is part of a series on the AES/AESF/NASF Best Paper Awards. In 1965, Edward B. Saubestre and co-workers received the Carl E. Huessner Gold Medal Award for Best Paper appearing in Plating. A printable PDF version is available by clicking HERE.
The use of electrodeposited plastics is growing rapidly, especially in automotive and appliance fields. With this growth, there arises a need for valid testing techniques, to measure such properties as corrosion resistance, adhesion of coatings, etc. Adhesion testing of plated plastics is generally done by means of pull testing, using various adaptations of the Jacquet test. Although this is a valid test, it is subject to widespread misapplication due to lack of understanding of the fundamentals involved in this test. Some of the pertinent equations relating to pull testing are developed in the paper, wherein it is shown that a specific parameter (which includes pulling force, width of the test strip, thickness of coating, and modulus of elasticity) measures a combination of adhesion and ductility. Practical implications for commercial plated plastics are given.
Nature of the metal-to-plastic bond
A. General considerations
Any discussion of the nature of the bond between a plastic and a metal must begin with the seemingly obvious question, Where does the bond break? The seemingly equally obvious answer, Between the plastic and the metal, turns out to be rarely, if ever, correct. Bikerman1 has shown why failure of adhesion can rarely occur exactly at an interface, based on considerations of crack propagation. In the specific case of the Jacquet Test for peeling of metals from plastics, it will be shown below that dimensional analysis also shows that the break cannot occur at the plastic-metal interface.
The break may actually occur in any one of three places, of which two are of practical importance. The bond may break within the metal, within the plastic, or along a relatively weak boundary layer between the two which forms at one stage or another within the plastic body. Primarily because of tensile strength and Young's Modulus considerations, a break within the metal is most unlikely in most practical situations. Therefore, the bond breaks either in the body of the plastic itself, or along a weak boundary layer within the plastic. Bikerman2 refers to bonds that break within the plastic as "proper bonds" and those that fail along weak boundary layers as "improper bonds."
In practical terms (referring to metal-plated plastics), the former refers to "good" adhesion of metal-to-plastic, the latter refers to "poor" adhesion. Obviously, then, to eliminate "poor" adhesion of metal-to-plastic, it is necessary to investigate what causes the formation of weak boundary layers, and then to eliminate those causes. There are two schools of thought on why failure of the metal-to-plastic bond can occur in a weak boundary layer in the plastic, and it is probable that both approaches are of practical significance for plated plastics.
The first which might be called the "molecular theory" school,3 claims that bond failure is caused primarily by a weak chemical-electrical interaction between metal and plastic. According to this school, strong attraction between the atoms of metal and the polymer molecules of plastic is required for a strong bond. A valence bond between the different substances found across the interface would be ideal, but interaction between dipoles in the two phases is more likely. This hypothesis has been objected to by Bikerman4 on the ground that substances (such as polyethylene) which contain no dipoles adhere well to metals, with which no chemical reaction is possible; furthermore, he points out that when a brittle polar, a brittle non-polar, and a ductile non-polar material are compared, the two brittle materials behave similarly, while there is no similarity in the behavior of the two non-polar materials. On the other hand, the "molecular theory" helps to account for a number of readily observed phenomena. It explains, for example, how oxidation of polyethylene produces a stronger joint with metals,5 because of dipole interactions between the metal and polar groups produced by oxidation in the polyethylene. It could also explain why use of chromic acid-containing oxidizing acids improves adhesion of metals to ABS plastics.6 (See below, however, for an alternative explanation). It also explains why polyethylene, which normally bonds poorly to copper, can be made to produce a good bond to it by chemically oxidizing the copper surface so as to produce a thin layer of black cupric oxide.7 A further objection has been made to the "molecular theory"4 that bond strengths are not nearly as high as they should be were dipole interactions effective in establishing overall bond strength. This objection, however, has been met by pointing out that bond strengths are not as high as might be expected in terms of intermolecular forces because, in part, of the large size of the long-chain polymer molecules.8 The configurations which these adopt to give maximum interaction with the metal sterically hinder other molecules trying to attach themselves to the surface,9 thus limiting the amount of surface available for bonding. In summary, according to the "molecular theory," the way to avoid poor bonding of metals to plastics is to create dipole interactions at the interface between metal and the plastic boundary layer. In practical terms, this would generally mean the creation of oxidative linkages such as the cited methods for oxidation of polyethylene and ABS before metal bonding.
The second school of thought, which might be referred to as the "rheological theory,"10 explains bond failure in weak boundary layers of the plastic as being the result of a type of adhesive (or stress-strain) failure. Such adhesive failure in a weak boundary layer may occur owing to the formation of oils or low-molecular-weight polymers exuded from the bulk of the plastic into the surface layer, loosely bound oxides, moisture, gases or other substances in which shear or tensile failure may occur more readily than in the main body of the materials being joined. An outstanding example of this phenomenon is the deleterious effect of moisture on the bonding of metals to ABS plastics. A bake-out of moisture before conditioning and electroless plating of ABS, or immediately after the electroless plating step, or even after final plating with (for instance), copper, nickel, chromium, can considerably improve adhesion. In fact, it can be stated almost categorically that the adhesion of metals to ABS plastics improves with aging, owing to elimination of moisture in the weak boundary layer.6 Bikerman11 has given further weight to this argument by noting that "syneresis" can be a source of trouble, and that eliminating the result of syneresis will improve bond strength considerably.* For example, in the specific case of polyethylene investigated by Bikerman, it was found that low-molecular-weight hydrocarbons and oxygenated impurities only loosely bound to the surface constituted the principal phase of the weak boundary layer formed by this syneresis. These were removed11 and found to comprise less than one per cent of the plastic, and not to affect mechanical properties (Young's Modulus; breaking strength), or interfacial properties (contact angles for water were the same for treated or untreated polyethylene), but to eliminate IR spectroscopic peaks for carbonyl groups. The result was that poor bonding was converted to good bonding. This approach also offers another explanation of the effect of the chromic acid treatment of plastics.12 For example, it was shown that chromic acid treatment of polyethylene (which admittedly improved bonding to metals) did not result in the formation of an oxidized layer by IR spectroscopy.13 Bikerman11 concludes that the effect of the chromic acid treatment is to destroy some of the low-molecular-weight ingredients present in the weak boundary layer, thus eliminating the effect of syneresis in the surface layer. It has been suggested that another support for this approach is the fact that passing a stream of hot nitrogen gas over polyethylene will have the same effect as the treatments discussed above in improving bonding properties;14 i.e., evaporating impurities is the key phenomenon, since no oxidation can occur under these conditions. Some related observations dealing with fiber-elastomer adhesion have also appeared recently.15 On the other hand, the same author discussed the resistance of adhesive bonds to water, (water reduced adhesion to zero if the specimen were peeled apart when wet; if allowed to dry before adhesion testing, the bonds retained their original strength), and concluded that the reversible behavior noted could most readily be explained by postulating that adhesion was caused by hydrogen bonds. In the presence of liquids of high energy density, hydrogen bonds were correspondingly weakened.
B. Goal of the current investigation
From this review of the literature on the bonding of metals to plastics, the following conclusions are drawn:
- An adequate test method for evaluating the bonding of metals to plastics must involve rheological failure, since simple tensile failure at the metal-to-plastic interface can occur only in the case of so-called "improper bonds," which are of no commercial importance except in the very limited field of application for the production of what might be termed "plated-plastic-junk."
- Experiments should be conducted to determine, on a scientifically acceptable basis, the nature of the bond cleavage between plastic and metal.
- Experiments should be conducted to determine, on a practical (commercial) plane, which operational factors contribute to the development of good bonding between plastic and metal, and more particularly, in view of major industrial (especially automotive) interest, which factors lead to good bonding of copper to ABS plastics.
- Comparisons should be established experimentally between rheologically acceptable tests and other industrially acceptable tests for determining the bonding of metals to plastics, particularly ABS plastics.
- Simple chemical procedures should be evaluated for experimentally verifying the existence of weak boundary layers as previously discussed.
- A correlation should be made between methods of testing plated plastics and those for testing plated metals.
The following sections will detail our attempts to meet these six objectives.
C. The Jacquet test
The Jacquet, or "pull," test is the most commonly used industrial test for adhesion of metals to plastics. Figure 1 indicates the common geometry of the Jacquet test. A relatively thin metallic strip (generally, the electrodeposited coating under test) is pulled at a 90° angle from the underlying plastic substrate. The force required, either to initiate or to sustain at a steady rate, peeling of metal from plastic is recorded as the numerical value for the test (Appendix II contains further details). It is evident that either the plastic substrate or the metallic film must yield plastically in order to permit the radius of curvature which results from pull testing. Experimental data presented later in this paper will prove that the plastic yielding occurs entirely in the plastic film.