Polymeric coupling agent

A coupling agent for chemically linking metal substrates to thermoset polymers is composed of a hydrophobic polymer backbone having functional groups attached thereto at spaced intervals, the functional groups having the ability to form chemical bonds to both the metal substrate and the thermosetting polymeric resin. In a preferred embodiment, the hydrophobic polymeric coupling agent comprises mercaptoester functional groups placed along a strong, hydrophobic polymeric backbone. This polymeric backbone preferably comprises polyethylene resulting in an ethylene mercaptoester (EME) copolymer.

BACKGROUND OF THE INVENTION 
This invention relates to a method of improving the adhesion between metal 
substrates and thermoset resins using a polymeric coupling agent. More 
particularly, this invention relates to a method of improving the adhesion 
between metal substrates (e.g. steel and copper) and thermoset resins 
(e.g. epoxy) utilizing a polymer composed of a strong, hydrophobic polymer 
backbone having functional groups attached thereto at spaced intervals 
that will chemically bond to both the metal and thermoset resin. This 
invention also relates to a preferred method of making the novel polymeric 
coupling agent. 
It is well known in the adhesive bonding art that water is extremely 
detrimental to metal/thermoset adhesion systems. It is believed that 
diffusion of water into the interfacial region between the polymer and 
metal will reduce the strength of the metal/polymer bond for several 
reasons. For example, since water molecules are very strong hydrogen 
bonding agents, they can readily break non-covalent bonds between the 
metal and polymer and form new hydrogen bonds with the oxide surface of 
the metal. A weak water layer results, reducing the strength of the entire 
adhesion system. Water can also weaken the interfacial region by 
initiating corrosion and/or hydration reactions with the base metal, 
oxides or the polymer itself. 
Epoxy resins are well known examples of thermosetting polymeric material 
whose bonds with metals are adversely affected by water. Thus, while an 
epoxy resin will normally exhibit a high strength and strong bond to metal 
surfaces such as steel, the integrity of the epoxy/metal bond is 
significantly reduced when exposed to high humidity or water. 
Attempts have been made to improve the bonding between polymers and metals 
(particularly the adverse effect of water on the bond) by the use of 
intermediate compounds which are designed to form chemical bonds with both 
the metal substrate and the polymeric resin. In essence, these 
intermediate compounds chemically link or bridge the polymer to the metal 
substrates. These linking compositions have consisted of various low 
molecular weight coupling agents which have the ability to form chemical 
bonds across the metal/polymer interface. U.S. Pat. Nos. 4,448,847 and 
4,428,987 to Bell et al, the contents of which are fully incorporated 
herein by reference, both disclose the use of such low molecular weight 
coupling agents. U.S. Pat. No. 4,448,847 describes a method of improving 
the adhesion of epoxy resins to steel substrates using either a 
beta-diketone or mercaptoester coupling agent. U.S. Pat. No. 4,428,987 
describes a method of improving the adhesion of epoxy resins to copper 
substrates using benzotriazole, benzothiazole and related compounds as the 
coupling agent. 
While the low molecular weight coupling agent of the type described 
hereinabove have improved the durability of polymer/metal bonds, these 
compositions nevertheless suffer from certain deficiencies and 
disadvantages. For example, in order to achieve the greatest effect, the 
prior art coupling agents should be coated onto the metal substrate in a 
single layer. However, single layer thicknesses are extremely difficult to 
produce. As a consequence, bond failure can result between layers of the 
coupling agent itself thereby dramatically reducing the bond strength of 
the overall polymer/metal bond. In addition, low molecular weight coupling 
agents are normally quite water permeable. Therefore, their presence can 
actually promote the infiltration of destructive water molecules into the 
interfacial region. 
Low molecular weight coupling agents also provide little help in 
alleviating internal stresses in metal/thermoset adhesion systems. It will 
be appreciated that significant stresses can develop during post cure 
cooling of a thermoset such as an epoxy, primarily due to the thermal 
expansion coefficient mismatch between the metal and the polymer. This 
interfacial stress is believed to be the cause of failure in a number of 
adhesion systems. 
SUMMARY OF THE INVENTION 
The above-discussed and other problems and deficiencies of the prior art 
are overcome or alleviated by the method of the present invention wherein 
the adhesion between metals and thermoset polymers is improved using a 
polymeric coupling agent. In accordance with the present invention, a 
coupling agent for chemically linking metal substrates to thermoset 
polymers is composed of a hydrophobic polymer backbone having functional 
groups attached thereto at spaced intervals, the functional groups having 
the ability to form chemical bonds to both the metal substrate and the 
thermoset polymeric resin. The polymer backbone preferably comprises an 
aliphatic or aromatic hydrocarbon backbone polymer since these types of 
polymers exhibit the necessary and requisite hydrophobicity. 
In a preferred embodiment, the polymeric coupling agent of the present 
invention comprises mercaptoester functional groups placed along a strong, 
hydrophobic polymeric backbone. This polymeric backbone preferably 
comprises polyethylene resulting in an ethylene mercaptoester (EME) 
copolymer. 
The use of a hydrophobic polymeric coupling agent provides many features 
and advantages over prior art low molecular weight coupling agents. For 
example, the presence of the polymer backbone gives the interfacial region 
greater physical strength, even though more than a monolayer of coupling 
agent is present. The polymeric coupling agent also permits relief of 
interfacial stresses caused by mismatch of properties (such as thermal 
expansion coefficient) at the interface, because of the improved 
viscoelastic properties of high molecular weight polymer coupling agents 
relative to the prior art low molecular weight coupling agents. Also, the 
hydrophobicity of the polymeric backbone provides greatly improved 
resistance of the adhesive bond to the deleterious effects of water. 
The above-discussed and other features and advantages of the present 
invention will be appreciated and understood by those skilled in the art 
from the following detailed description and drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention comprises a coupling agent for forming chemical bonds 
across a metal/thermoset interface. This coupling agent comprises a 
hydrophobic polymeric backbone having functional groups at spaced 
locations along the polymer backbone which have the ability to chemically 
bond to the metal substrate and the thermoset polymer. While not limited 
thereto in its utility, the polymeric coupling agent of the present 
invention is particularly well suited for improving the adhesion of 
thermoset resins, particularly epoxy resins, to steel and/or copper metal 
substrates. 
In accordance with the present invention, the polymeric backbone of the 
coupling agent may comprise any suitably strong, hydrophobic polymers. 
Because of their strength and hydrophobicity, suitable polymeric backbones 
include any substantially aliphatic or aromatic hydrocarbon backbone 
polymer. Examples include, but are not limited to polyethylene and simple 
derivatives thereof and polypropylene. The functional groups attached at 
spaced locations to the polymer backbone must have the ability to 
chemically bond to a metal substrate and to the intended polymer. Examples 
of suitable functional groups which may be grafted onto the hydrophobic 
polymer backbone include, but are not limited to beta-diketones, 
mercaptoesters, benzotriazole, benzothiazole, primary amines and 
carboxylic acids and any combination of these functional groups. 
The polymeric nature of the novel coupling agents in accordance with the 
present invention provide significant advantages over the more common low 
molecular weight coupling agents such as those disclosed in U.S. Pat. Nos. 
4,448,847 and 4,428,987. Unlike low molecular weight compounds, polymeric 
coupling agents are capable of bearing a load, which make the 
effectiveness of the coupling agents much less sensitive to the thickness 
of the coupling agent layer. In addition, low molecular weight coupling 
agents are normally quite water permeable. Therefore, their presence can 
actually promote the infiltration of destructive water molecules into the 
interfacial region. Since the polymeric coupling agent of the present 
invention can be tailored to consist of reactive hydrophilic groups on a 
hydrophobic hydrocarbon backbone, their use will reduce the water 
permeability of the interphase. 
Moreover, unlike the low molecular weight compounds, the viscoelastic 
properties of the polymeric coupling agents of the present invention 
enable them to help relieve internal stresses in metal/thermoset adhesion 
systems. This is advantageous since significant stresses can develop 
during the post-cure cooling of the thermoset, primarily due to the 
thermal expansion coefficient mismatch between the metal and the polymer. 
The crosslinked structure and the elevated glass transition temperatures 
of most thermosets limit their stress relaxation ability. For example, 
epoxy resins have thermal expansion coefficients which exceed that of 
steel by an order of magnitude. Hence, a large degree of stress can 
develop. Interfacial stress has been cited as the cause of failure in a 
number of adhesion systems. 
A preferred polymeric coupling agent particularly well suited for improving 
the bond adhesion between epoxy resins and steel or copper substrates is a 
copolymer of ethylene and mercaptoester having the general formula: 
##STR1## 
where n=1, 2 or 3. Reference is made to U.S. Pat. No. 4,448,847 for a more 
detailed description of the mercaptoester group suitable for use in this 
invention. Also, reference should be made to U.S. Pat. No. 4,448,847 for a 
thorough discussion of preferred epoxy resin and coupling agent 
formulations. It will be appreciated that the mercaptoester group has been 
found to chemically bond with both steel and copper substrates (in 
addition to its ability to bond to epoxy resin). 
Ethylene mercaptoester (EME) copolymers are preferably prepared starting 
from ethylene vinyl acetate (EVA) copolymers (supplied by DuPont (Elvax) 
MW: 70,000-90,000) in a two step reaction with ethylene vinyl alcohol 
(EVAlc) serving as the intermediate as shown below. In the first step, EVA 
(14-50% vinylacetate) is saponified using sodium methoxide in a refluxing 
methanol/xylenes solution to produce the EVAlc intermediate. Once washed 
and dried, the EVAlc copolymer is reacted at 100.degree. C. for 1 hr. with 
mercaptoacetic acid in the presence of an acid catalyst (toluene sulfonic 
acid monohydrate) yielding the EME product. The second step of the 
reaction is driven to the desired product by removing the water byproduct 
using a Dean-Stark trap. 
##STR2## 
As will be discussed in the following non-limiting examples, it has been 
found that the EME copolymers chemically react with epoxide rings of epoxy 
resins and with free iron ions to form chemical links between a steel 
substrate and an epoxy resin. Similarly, chemical linking will also occur 
between the EME copolymer and a copper substrate. 
EXAMPLES 
A. Peel Strength 
EME copolymers were synthesized from ethylene-vinyl acetate copolymers as 
discussed hereinabove, first by hydrolysis of acetate groups to the 
alcohol, followed by esterification with mercaptoacetic acid to give the 
mercaptoester copolymer. Peel adherends (1".times.4") were cut from 
4".times.12" 1010 SAE 20 mil thick carbon steel plates (Q Panel) using a 
squaring sheet metal shear blade. The plates were wiped with a damp cloth 
and acetone degreased before undergoing the specific pretreatments. A 
diglycidyl ether of bisphenol A epoxy resin (Epon 1001, Shell Development 
Co.) was dissolved (40 wt % solids) in an equal weight solvent mixture of 
xylenes, cellosolve and MIBK prior to mixing with Versamid 115 (Miller 
Stephenson Chemical Co.) (80 phr) polyamide curing agent. Pressure 
sensitive polyethylene tape was obtained from Minnesota Mining and 
Manufacturing Co. Three-mil thick, 6".times.5/16" bending beam adherends 
were cut from 1010 CRS steel foil (Precision Brand) using a sheet metal 
shear blade. Trimethylolpropane trithioglycolate (TTTG), 95+% low 
molecular weight mercaptoester coupling agent (Evans Chemetics) was used 
as received for the control group experiments. To provide ease of handling 
and insure identical treatments to every sample, the steel plates were 
placed in glass racks (capacity: 30 samples) prior to the pretreatment 
procedures. The 1".times.4" steel plates were prepared for bonding by 
first degreasing for 15 min. in an acetone bath followed by 15 min. 
exposure to a 70.degree. C., 3 wt % aqueous citric acid bath with pH 
adjusted to 4.0 using ammonium hydroxide. A water rinse followed by 
immersion for one (1) minute in a xylenes bath completed the pre-coupling 
agent pretreatments. All pretreatments were carried out in a 
nitrogen-purged glove box. The pretreatments are designed to provide a 
fresh oxide layer for bonding. The EME coupling agents were applied to the 
steel plates from solution (0.25 wt % in xylenes/methanol) at 60.degree. 
C. under nitrogen. A one-min. wash in a xylenes/methanol bath was the 
final pretreatment step. Ellipsometry and X-ray photoelectron spectroscopy 
results indicated that the coupling agent thickness was approximately 50 
Angstroms. Epon 1001/Versamid 115 (five-mil dry thickness) films were 
applied to the pretreated samples one hour after mixing at room 
temperature using a thin film applicator (Gardner Labs). Pretreated steel 
adherends were kept under an inert atmosphere until just prior to 
application of the epoxy film. The films were cured for 7 days in air at 
room temperature. Post curing for 9 hours at 80.degree. C. was found to be 
necessary to remove the residual solvent and complete the crosslinking 
reactions. The back and sides of the samples were masked with polyethylene 
tape prior to 57.degree. C. distilled water bath exposures. 
Following specified water exposures, samples were scribed to a width of 0.7 
in. with a razor blade and immediately tested for adhesion strength using 
a 90.degree. peel test apparatus. The 90.degree. peel test fixture was 
attached to the cross head of an Instron tensile test machine. The sample 
stage slides freely on low friction bearings, enabling the peel angle to 
remain at 90.degree. as the crosshead moves downward. A TM-S Instron 
tensile tester equipped with either a 500 g, 2000 g or 50 lb load cell and 
a chart recorder was used to perform the measurements. The peel rate for 
all tests was 0.4 in./min. The average peel force was taken from the chart 
recorder output of peel force vs. debonded length. All of the peel test 
variables (i.e. crosshead speed, peel angle, epoxy thickness, epoxy 
composition) were chosen so as to eliminate as many extraneous 
contributions to the peel force as possible. 
Table 1 shows that the initial adhesion strength was enhanced by almost an 
order of magnitude over the control (i.e., no polymeric coupling agent) by 
employing the EME 90 coupling agent. On the other hand, the more 
hydrophobic EME 23 coupling agent was the most successful at protecting 
the steel adherend from corrosion reactions. Corrosion protection was 
extended over a period three times as long as that achieved in the control 
systems. 
The peel strength results in Table 1 indicate that maximum initial (dry) 
adhesion is obtained by incorporating a large concentration of polar 
reactive moieties along the polymer coupling agent backbone. However, such 
practice is not without its consequences. The increased hydrophilicity 
associated with an increase in polar group concentration reduces the 
ability of the coupling agent/resin system to protect the steel adherend 
from corrosion. The presence of an electrolyte at the steel surface is 
required to initiate the corrosion reactions. Apparently, water molecules 
can reach the steel surface in sufficient concentration to provide the 
necessary electrolyte, more readily when a coupling agent that is less 
resistant to water permeation is present in the interphase. 
Table 1 also shows that adhesion tests were completed on specimens treated 
with a low molecular weight multifunctional mercaptoester compound (TTTG). 
The TTTG coupling agent contains a significantly higher concentration of 
mercaptoester groups by weight than EME 90. As shown in Table 1, peel 
specimens constructed with TTTG exhibited a greater rate of strength loss 
and a lesser degree of corrosion protection in the presence of hot water 
than those prepared using EME 90. However, this is not surprising since 
the TTTG interlayer is expected to be more water permeable than that of 
EME 90. On the other hand, the initial adhesion strength of the EME 90 
samples exceeded that of the TTTG samples. This contradicts previous 
observations indicating that a greater concentration of reactive groups 
leads to greater initial adhesion strength. The ability of EME 90 to 
relieve, at least in part, interfacial stresses due to its visoelastic 
nature has been hypothesized as one reason for this contradiction. Other 
results based on bending beam data indicate that the EME polymeric 
coupling agents can reduce the interfacial stresses present in steel/epoxy 
adhesion systems. It is believed that the extent of stress relief achieved 
can significantly enhance the resulting adhesion strength. 
B. Effect of Coupling Agent Thickness 
Referring now to Table 2 and FIG. 1, 140 A thick coupling agent layers, 
steel/EME/epoxy peel specimens were prepared as described hereinabove with 
EME copolymers containing 23-90 wt % mercaptoester units. As observed with 
the 50 A thick samples of TABLE 1, the dry adhesion strengths increased 
and the corrosion protection decreased with an increase in coupling agent 
functionality. The dry strengths of the EME 47 and EME 90 samples were 
enhanced from 284 to 1165 g/in and 729 to 2465 g/in respectively by 
increasing the thickness of the coupling agent layer from 50 to 140 A. On 
the other hand, the degree of corrosion protection obtained did not appear 
to be strongly dependent on the thickness of the coupling agent used. 
Turning now to FIGS. 2 and 3, EME 90 solutions of various concentrations in 
xylenes were dip coated to 60C onto smooth steel substrates and allowed to 
air dry horizontally. The thickness of the resulting films were determined 
by ellipsometry. FIG. 2 shows that for concentrations ranging from 0 to 
0.4 wt % essentially a linear relationship exists between concentration 
and film thickness. 
Using FIG. 2 as a calibration curve, steel/EME 90/epoxy peel specimens with 
five different coupling agent thicknesses were prepared and tested under 
dry conditions. The epoxy thickness was held constant at 5 mils. The 
average peel strengths are plotted in FIG. 3. These data indicate that the 
peel strengths are quite strongly dependent on coupling agent thickness 
with the maximum strength of 4.9 lb/in occuring at an EME 90 thickness of 
approximately 140 A (average error p/m 13%). 
FIG. 3 shows that EME 90 copolymer coupling agent thickness has a strong 
influence on the dry adhesion strength. The strength increases as the 
coupling agent thickness increases up to approximately 120 to 160 A, but 
then decreases steadily for greater thicknesses. No clear cut explanations 
have been developed for this behavior. However, as the coupling agent 
layer becomes quite thick this region can become a weak link in the 
adhesion system since its strength related properties will be inferior to 
those of the epoxy resin. The development of a weak layer is believed to 
be the cause of the observed drop in peel strength at large coupling agent 
thicknesses. 
TABLE 1 
______________________________________ 
COR- 
PEEL STRENGTH (G/IN) ROSION 
IMMERSION TIME PRO- 
TREAT- (HRS 57 C WATER) TECTION 
MENT 0 1 3 5 11 24 (HRS) 
______________________________________ 
CON- 78 69 17 9 4 2 14 
TROL (57) (30) (24) (16) (5) 
EME 23 47 26 16 11 10 9 46 
(40) (31) (26) (23) (19) 
EME 47 284 139 47 36 16 13 32 
(196) (83) (71) (57) (44) 
EME 90 729 319 36 24 16 11 20 
(543) (311) (187) (66) (30) 
TTTG 684 106 28 11 8 3 16 
(650) (180) (17) (19) (9) 
______________________________________ 
*SAMPLES DRIED 1 HR UNDER VACUUM AT 50 C 
TABLE 2 
______________________________________ 
COR- 
PEEL STRENGTH (G/IN) ROSION 
IMMERSION TIME PRO- 
TREAT- (HRS 57 C WATER) TECTION 
MENT 0 1 3 5 11 24 (HRS) 
______________________________________ 
EME 23 44 51 30 27 23 18 45 
(31) (31) (32) 
(23) 
(20) 
EME 47 1165 1400 255 36 26 13 21 
(1565) (680) (390) 
(150) 
(41) 
EME 90 2465 2880 715 102 28 11 22 
(2960) (1620) 
(187) 
(180) 
(48) 
CITRIC 1675 344 67 28 13 3 11 
ACID (1022) (650) (590) 
(20) 
(5) 
______________________________________ 
AVERAGE ERROR .+-. 12% 
C. Chemical Bonding of Coupling Agent 
A spectroscopic analysis of the chemical interaction between EME and iron 
ions was conducted to determine the nature of the bonding. UV-visible 
spectroscopy experiments showed that both EME copolymer and model TTTG 
mercaptoester containing compounds have the ability to quantitatively 
scavenge iron ions from solution. These results suggest the presence of a 
specific chemical interaction between iron ions and the mercaptoester 
group. IR spectra of mercaptoester-iron model systems exhibited evidence 
that only the thiol group of the mercaptoester was involved in the 
bonding. A frequency shift of the carbonyl stretching band, which would 
indicate coordinate bonding through the carbonyl oxygen was absent from 
all spectra. However, a small decrease (approx. 5-10 cm.sup.1) in 
frequency of the carbonyl band of the TTTG-Fe.sub.2 O.sub.3 spectra 
suggest that the carbonyl oxygens of the mercaptoester groups help to 
stabilize the thiol-iron coordinate bond formation. 
While preferred embodiments have been shown and described, various 
modifications and substitutions may be made thereto without departing from 
the spirit and scope of the invention. Accordingly, it is to be understood 
that the present invention has been described by way of illustrations and 
not limitation.