Metallic coagents for rubber-to-metal adhesion.

The manufacture of conventional metal-reinforced rubber products requires both an adhesive to bond the metal to the rubber and a separate curing system to increase the mechanical properties of the rubber. This entails an intensive, time-consuming series of procedures: first the metal surface has to be prepared, then the adhesive has to be applied and dried, next the elastomer compound has to be prepared. The product is then molded and cured, and finally post-cure treated.

In contrast, elastomers that are peroxide-cured with metallic coagents yield stronger rubber-to-metal bonds without the use of external adhesives or a separate curing step, and demonstrate improved performance properties. The rubber compound is simply prepared with the coagent and cured directly against the metal surface. During curing, metallic coagents develop adhesive bonds at the metal-rubber interface, while simultaneously producing strong crosslinks in the rubber.

Metallic coagents are effective crosslinkers for both saturated and unsaturated elastomers and can be used over a wide concentration range to tailor mechanical properties for a variety of applications (ref. 1). In addition to improving mechanical properties, they also significantly increase the adhesion of vulcanized rubber to metal substrates. The following excerpts from a study conducted by Sartomer demonstrate the superior adhesion and performance properties achieved by metallic coagents when used with a variety of rubbers.

Experimental

Materials and formulations

In all the experiments pertaining to nitrile rubbers, a masterbatch containing 100 phr Hycar 1042 (a general purpose nitrile rubber with good processing characteristics and high physical properties), 65 phr N365 carbon black, 15 phr dioctyl phthalate, 5 phr zinc oxide and 1 phr stearic acid was used.

A master batch containing 100 phr Nordel 1040, 100 phr N762 carbon black, 50 phr Sunpar 2280, 5 phr zinc oxide and 1 phr stearic acid was utilized in all experiments related to EPDM rubbers.

The rubbers were compounded according to the formulations given in table 1 using a laboratory six-inch two roll mill. The masterbatches described above were masticated on a two roll mill until a flux was created at the nip of the rollers. At this point, the Agerite Resin D, dicumyl peroxide and coagent were slowly added to the flux roll. The band was then sheeted and folded and then rebanded for mixing. The coagent concentration was varied as noted. The compounded rubbers were then cured in a chelsea mold for 20 minutes at 160[degrees]C - 166[degrees]C.

[TABULAR DATA 1 OMITTED]

The remaining formulations were compounded in a similar manner to the masterbatches with the dicumyl peroxide being added last.

Measurement

Physical tests were conducted for all molded compounds after molding and again after heat aging at 100[degrees]C for 70 hours. Tensile strength, modulus and elongation were evaluated according to ASTM method D-412 using a Thwing Albert model 1451-42 tensile tester at a crosshead speed of 20 in./min.

Shore A hardness tests were determined for samples after molding using a hand-held Shore A durometer.

Compression set was ascertained by compressing a one inch diameter specimen built up with four plies to 50% original thickness for 22 hours at 100[degrees]C. The specimen was then removed and the permanent set measured as a percentage of original thickness (ASTM D395). Rubber-to-metal adhesion was determined by tensile testing a rubber specimen cured between two metal coupons. The tensile test was run at 1.0 IPM crosshead speed and the force in pounds to break the lap joint in shear was measured. The rubber specimen, approximately 0.030 inch in thickness, was cured at 160[degrees]C - 166[degrees]C for 30 minutes between two 1 x 3 x .030 inch metal coupons overlapped 1.0 inch in a plaque mold under 30,000 psi pressure. The metal coupons were methanol washed and dried before curing.

Results and discussion

Metallic coagent physical properties

The zinc diacrylate and zinc dimethacrylate metallic coagents used for this study are S-633 and S-634 respectively. They are free flowing, 100% reactive solids that can be readily compounded with a variety of elastomers (figure 1). They exhibit excel excellent high temperature stability as well as outstanding fluid resistance. They derive many of their unique physical and mechanical properties from the ionic bonds that are formed between the zinc cations and the carboxylate anions (figure 2). Metal cations, particularly zinc, are known to increase the mechanical properties of metal-neutralized ionomers by forming ionic crosslinks (ref. 2). The same ionic crosslink mechanism is believed to occur with elastomers that are cured with metallic coagents. Ionic bonds exhibit both good heat aged stability and the ability to slip along the hydrocarbon chain and reform.

Crosslinking with peroxide results in the formation of a covalent bond. This carbon-carbon bond is quite rigid and stable and accounts for the lower tensile and strength of peroxide cured stocks compared with sulfur vulcanizates. The good heat stability of this covalent bond also explains the superior heat aged characteristics of peroxide cured systems. In contrast, polysulfide crosslinks formed in sulfur cure are thermally weak but are mobile under stress and can slip along the hydrocarbon chain. This mobility has been used to explain the superior tensile and tear strength in sulfur cured stocks.

Thus, metallic coagent-peroxide systems embody the best characteristics of both the peroxide and sulfur crosslink systems, yielding high tensile and tear strength and excellent heat aged properties.

Adhesion

The data in figure 3 show the lap shear adhesion of S-633 versus solvent based adhesives, commercial adhesion promoters and conventional sulfur and peroxide cure systems for bonding EPDM to steel. In an effort to compare each adhesive system on an equal basis, the EPDM in each case was formulated and cured to a 56 Shore A hardness.

It is normally very difficult to bond EPDM to most metals without extensive metal cleaning and the use of solvent based adhesives. This is illustrated in figure 3 where low adhesion was recorded for the samples cured with either peroxide alone or sulfur. As expected, an improvement in adhesion was obtained when either a polar coagent, a commercial adhesion promoter or a solvent based adhesive was added to the peroxide-alone formulation. S-633, without any special metal treatment, clearly outperformed all other materials tested, yielding a shear adhesion value of 9 MPa.

The S-633 cure system was also observed to promote good adhesion to other untreated metals including aluminum, zinc and stainless steel (figure 4). The effect of S-633 concentration on EPDM adhesion to these metals is shown in figure 5. Adhesion improved rapidly as the S-633 concentration increased from 0 to 20 phr. This means that strong adhesive bonds can be created economically at low coagent levels.

Figure 6 further demonstrates the strong rubber-to-metal bonds that can be obtained with silicone rubber, EVA, EPDM, natural rubber and Hypalon when they are formulated with S-633. In each case, the S-633 metal systems achieved significantly higher shear adhesion levels than the peroxide controls. S-634, while not as effective as S-633, also increases rubber-to-metal adhesion in peroxide cured elastomers as illustrated in figure 7. S-634 is ideal for applications where high tear strength, abrasion resistance, exceptional scorch safety and slow cure are needed.

Aged properties

The S-633 rubber-to-metal adhesive bond remains strong even when exposed to heat and moisture. Table 2 shows, no deterioration in lap shear strength under heat aging at 300[degrees]F and submersion under water at room temperature for 165 hours.

Mechanical properties

The mechanical properties of rubber cured with S-633 and 634 are superior to those obtained with conventional coagent systems. The S-634 is noted for its elongation and tear strength while S-633 gives excellent tensile strength and modulus as shown in table 3.

Conclusions

Based on the data reported in this article, it is evident that metallic coagents such as S-633 and S-634 can be used to create extremely strong adhesive bonds between a variety of rubbers and untreated metal substrates. Metallic coagents can be readily compounded into the rubber stock where they crosslink into the rubber upon curing with peroxides. Thus, they function as adhesion promoters as well as crosslinkers to enhance both the adhesive and mechanical properties of the cured rubber. In this study, S-633 demonstrated outstanding adhesion, but S-634 is a good alternative when abrasion resistance and tear strength are needed in addition to adhesion.

References

[1.] Richard Costin, Walter Nagel and Raymond Ekwall, Rubber Division Meeting, American Chemical Society, Washington, D.C., October9-12, 1990. [2.] H. Xie and Y. Feng, Polymer, 29, 1216, July (1988).