Techniques for bonding rubber to metal using metallic coagents.

We have previously reported on the use of metallic coagents as crosslinkers for peroxide cured elastomers (ref. 1). They are effective crosslinkers for both saturated and unsaturated elastomers and may be used over a wide concentration range to tailor mechanical properties for a variety of applications. We have now found that, in addition to improving the mechanical properties of rubber, they also increase the adhesion of rubber to metal substrates and synthetic fibers during vulcanization. The metallic coagents discussed in this article are zinc diacrylate (ZDA) and zinc dimethacrylate (ZDMA).

Conventional metal reinforced rubber products require 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. After this step, the rubber compound has to be prepared, molded and then cured. In most cases a post-cure step also is required. The metallic coagents offer several alternative ways of bonding rubber to metal that are far less intensive and time consuming. This is due in part to the fact that they form strong rubber-to-metal bonds without the use of external adhesives or a separate curing step. Any procedure that places the metallic coagents and peroxide between the metal surface and rubber with applied pressure may be used. Three techniques for applying this technology for rubber-to-metal adhesion are:

* In the uncured rubber compound as an internal adhesion promoter;

* in a thin adhesive strip that functions as a tie layer upon curing; and

* in a reactive dispersion that can be applied as a viscous liquid or paste to either the metal or rubber prior to curing. Although the metallic coagents are not adhesives per se, there are many rubber applications where these techniques may be applied to improve adhesion.

Experimental

Materials

A masterbatch containing 100 phr Hycar 1042, 65 phr carbon black N365, 15 phr dioctyl phthalate, 5 phr zinc oxide and 1 phr stearic acid was used in all experiments pertaining to nitrile rubbers. Hycar 1042 is a general purpose NBR (acrylonitrile content is 33%) with good processing characteristics and high physical properties. A masterbatch containing 100 phr Nordel 1040 EPDM (4% 1,4-hexadiene, 55% ethylene), 100 phr carbon black N762, 50 phr Sunpar oil 2280, 5 phr zinc oxide and 1 phr stearic acid was used in all experiments related to EPDM rubbers.

The Hypalon compound contained 100 phr Hypalon 40, 44 phr T(HRL)D-90, 5 phr carbon black N550, 40 phr Whitex clay, 20 phr Kenflex A, 15 phr Sundex oil 790, 6 phr paraffin wax, 5 phr of DiCup 40KE and 5 phr ZDA (control was without the ZDA). ZDA is zinc diacrylate containing a non-nitroso scorch retarder to provide scorch safety during processing. The EVA compound contained 100 phr Elvax 240 (28% vinyl acetate, melt index of 43), 25 phr Si[O.sub.2] Hi Sil 230, 3 phr zinc oxide, 3 phr stearic acid, 3 phr TOTM, 2 phr Perkadox 1440 and 3 phr ZDA.

The silicone formulation contained 100 phr GE Silicone 6140, 1 phr GE 6916 HA, 2 phr Triganox 10145 and 10 phr ZDA (control was without ZDA).

The natural rubber formulation contained 100 phr natural rubber SMR CV60, 22 phr carbon black, 5 phr Si[O.sub.2] Hi Sil 230, 4 phr zinc oxide, 3 phr TOTM, 2 phr Resin D, 1 phr stearic acid, 5 phr DiCup 40KE and 10 phr ZDA (control was without ZDA).

The liquid rubbers that were used in the sealant applications were Trilene 56 EPDM (13% DCPD, 49/51 ethylene/ propylene, molecular weight 5,200), DPR 40 natural rubber (360-550 Poise viscosity at 38 [degrees] C, molecular weight 40,000), R45HT polybutadiene (hydroxyl terminated polybutadiene, 2,800 molecular weight) and NBR 1312LV (33% acrylonitrile, low molecular weight liquid rubber).

The reactive dispersions (PRO 1785, PRO 1786, PRO 1771, PRO 1825) are experimental materials developed by Sartomer for evaluation in this study. They are proprietary products that consist of a metallic coagent dispersed at various concentrations in different acrylic monomers. The metallic coagent is a small particle-size powder that is insoluble in monomers, but forms a stable dispersion in the mon-omers selected for this study.

Formulations

The rubbers were compounded according to the formulations described above using a laboratory six-inch two roll mill. All masterbatches 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, DiCup and coagent were slowly added to the flux roll. The band was then sheeted and folded and then rebanded for mixing. This process was repeated many times to ensure thorough mixing. The coagent concentration was varied as noted. The compounded rubbers were then cured in a compression mold for twenty minutes at 160 [degrees] C-166 [degrees] C. The remaining formulations were compounded in a similar manner to the masterbatches with the DiCup being added last.

Compounding and curing

The ZDA and ZDMA coagents can be mixed with most rubbers using a two roll mill or a standard internal mixer. Curing the resulting rubber compound against the metal substrate creates the adhesive bond as well as the mechanical properties of the rubber. The rubber formulations used in most adhesive tests were mixed on a two roll mill and sheeted out to a thickness of 0.08 mm. Strips of the rubber sheeting were then placed between the appropriate metal panels and cured in a compression mold for 20 minutes at 165 [degrees] C. The shear adhesion values and the mechanical properties were determined according to ASTM test methods D816-55 and D412-80, respectively.

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 determined according to ASTM method D-412 using a tensile tester at a crosshead speed of 51 cm/min. Shore A hardness tests were determined for samples after molding using a hand-held Shore A durometer.

Cure characteristics, which include scorch time, cure rate and torque values, were measured over a 20 minute period at 160 [degrees] C using an oscillating-disk rheometer according to ASTM method D-1084.

Compression set was determined by compressing a 2.5 cm 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 as illustrated in figure 1. The tensile test was run at 2.5 cm/min. crosshead speed and the force in kg to break the lap joint in shear was measured. The rubber specimen, approximately 0.08 cm thick, was cured at 160-166 [degrees] C for 30 minutes between two 2.5 x 7.6 x 0.08 cm metal coupons overlapped 2.5 cm in a plaque mold under 207 MPa pressure (figure 1). The metal coupons were methanol washed and dried before curing.

[Figure 1 ILLUSTRATION OMITTED]

Adhesion peel tests were run by curing a 0.64 cm thick rubber specimen against a metal coupon and then tensile testing at 180 [degrees] angle of pull with 2.5 cm/min. crosshead speed. The rubber specimen was hand pressed against the metal coupon.

Results and discussion

Metallic coagents

Metallic coagents are defined as the metal salts of acrylic and methacrylic acids. The two products discussed in this article, ZDA and ZDMA, are the difunctional zinc salts of acrylic acid and methacrylic acid as shown in figure 2. They are free flowing, 100% reactive solids, and can be readily compounded with a variety of elastomers. Additional characteristics include low odor, scorch protection with a non-nitroso scorch retarder and improved physical properties. They also exhibit excellent high temperature stability, as well as outstanding fluid resistance.

[Figure 2 ILLUSTRATION OMITTED]

They derive many of their unique physical and mechanical properties from the ionic bonds that are formed between the zinc cation and the carboxylate anions. 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 ZDA and ZDMA. 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 tear 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 with sulfur curing 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. The crosslinks formed with metallic coagent in peroxide cured elastomers are "ionic" due to the zinc carboxylate bonds (figure 3). These ionic bonds exhibit both good heat aged stability and the ability to slip along characteristics of both the peroxide and sulfur crosslink systems, giving high tensile and tear strength and excellent heat aged properties.

[Figure 3 ILLUSTRATION OMITTED]

Adhesion

When using ZDA and ZDMA as internal adhesion promoters, it is important to remember that they do crosslink with the elastomer during curing and, therefore, change the physical properties of the elastomer. An example of this is shown in table 1 for the EPDM formulation containing 2 phr of the metallic coagents. In this example, both the modulus and tensile strength increased significantly, even at a concentration of 2 phr. Shear adhesion also increased significantly at the 2 phr concentration, especially with ZDA.

Table 1

Coagent   Modulus      Tensile       Adhesion to
(2phr)    (MPa)     strength (MPa)   steel (MPa)

None      0.55           3.83           0.50
ZDA       1.10           9.14           3.44
ZDMA      1.03           8.66           2.14

ZDMA, while not as effective as ZDA, also increases rubber-to-metal adhesion in peroxide cured elastomers as illustrated in table 1. In applications where high tear strength, abrasion resistance, exceptional scorch safety and slow cure are needed, ZDMA can be used. Metallic monomers based on other metal salts, such as calcium and magnesium, also increase rubber-to-metal adhesion, but not as effectively as ZDA and ZDMA.

The strength of the adhesive bond increases as the concentration of the metallic coagent is increased. The effect of ZDA concentration on the adhesion of EPDM to steel is shown in table 2. Shear adhesion increased from 0.5 MPa to 11.34 MPa as the ZDA concentration was increased from 2 phr to 20 phr. In all cases, cohesive failure is the primary mode of failure for the adhesive bond. It should also be noted that as the ZDA concentration was increased from 2 phr to 20 phr, the modulus and hardness of the rubber also increased.

Table 2

ZDA concentration       2 phr      20 phr

Adhesion, MPa            0.50       11.34
Modulus, MPa              1.1        2.13
Shore A hardness           55          80

As shown in figure 4, strong rubber-to-metal bonding can be obtained with ZDA as an internal adhesion promoter for a variety of rubbers such as silicone rubber, EVA, EPDM, NR and CE. All of the compounds contained five phr of ZDA and were cured with peroxide. In each case, shear adhesion was significantly increased with the ZDA cure system over the peroxide control.

[Figure 4 ILLUSTRATION OMITTED]

Cohesive failure is the predominant mode of failure in each rubber compound. Therefore, as the ZDA concentration is increased, the rubber compounds become stronger due to additional crosslinking, resulting in an increase in shear adhesion. This is shown for EPDM in figure 5. As the ZDA concentration was increased from 0 to 20 phr, the shear adhesion increased from approximately 0.55 MPa to over 0.31 MPa at 20 phr ZDA.

[Figure 5 ILLUSTRATION OMITTED]

The ZDA cure system also promotes good adhesion to other untreated metals including aluminum, zinc, brass and stainless steel (figure 6).

[Figure 6 ILLUSTRATION OMITTED]

In addition to promoting adhesion to metals as described above, ZDA also increases bonding to other polar materials, including various fibers and fabrics. An example of this is shown in figure 7 for EPDM bonded to nylon/polyester fabric. This is a common fabric used in various belt applications and must adhere to the rubber for good performance. Normally, the fabric undergoes an RFL treatment to enhance adhesion before being placed in contact with the uncured rubber stock. The RFL treatment is a tedious process and the treated fabric has a limited shelf-life. Figure 7 shows that EPDM containing 10 phr ZDA adheres strongly to the untreated fabric. Peel adhesion for EPDM with ZDA is double that obtained for the conventional RFL treated fabric without ZDA. Both adhesive systems are better than the untreated fabric control. A practical use of ZDA as an internal adhesion promoter is shown in table 3 for a pumpable sealant based on liquid EPDM. This formulation not only gives excellent adhesion to steel, but bonds strongly to oily steel as well. In this example, lap shear adhesion was run using steel coupons, overlapped 2.5 cm, with a spacer to form a uniform sealant thickness of 0.18 cm. Light pressure was applied so that the upper coupon would bear on the spacer. The sealant was then cured at 160 [degrees] C for 25 minutes. To test for bonding to an oily surface the steel coupons were coated with ASTM 3 oil, the excess oil was removed, and then the sealant was applied and cured as described above. This type of performance is particularly attractive for automotive applications where oily surfaces and oil contamination are prevalent. Normally, extensive cleaning is required to prepare the metal surface for adhesion with conventional adhesives and sealants.

[Figure 7 ILLUSTRATION OMITTED]

Table 3 - EPDM sealant

Formulation                 ZDA       Control

Trilene 56 EPDM              100          100
Ca[CO.sub.3]                  50           50
Zinc oxide                   7.5          7.5
Stearic acid                 1.0          1.0
Maglite D                    2.0          2.0
Agerite resin D              0.5          0.5
t-Butyl perbenzoate          7.7          7.7
ZDA                           20            0
Lap shear adhesive
To steel, MPa          4.88 C(1)       0.14 C
To oily steel, PSI        4.26 C   0.21 50% C

(1) C = cohesive failure

The second technique is to form an adhesive strip containing the metallic coagent. The adhesive strip may then be used to bond another rubber to a metal surface. When the adhesive strip approach is used, the metallic coagents are milled into the rubber compound as described above. The uncured compound is then sheeted-out to form stable, thin strips that contain the metallic coagent, peroxide and other rubber additives. The rubber strip can then be applied between the uncured rubber compound and the metal surface and cured to form an adhesive bond. In this way, the rubber strip is functioning as a tie layer between the metal and rubber. Figure 8 illustrates the test method that was used to evaluate this approach for various rubber compounds. Although other rubbers could have been used, the adhesive strips used in this study were prepared using the EPDM masterbatch described previously and 10 phr ZDA. The EPDM compound was calendered to form thin strips approximately 0.8 mm thick. The strips were placed between the uncured rubber and the metal coupons as shown in figure 8. After curing, the test specimen was cooled and tested for lap shear adhesion.

[Figure 8 ILLUSTRATION OMITTED]

This technique works for bonding both peroxide and sulfur cured stock to various metals. An example of this technique is illustrated in table 4 with a strip containing ZDA in EPDM that is bonding sulfur cured EPDM to steel.

Table 4 - bonding sulfur cured cured EPDM to steel

                         Shear adhesion, MPa

No adhesive                      0.58
Commercial adhesive              4.68
EPDM ZDA strip                   5.78

The EPDM ZDA strip also may be used to bond other elastomers to various metals. Several examples of peroxide cured elastomers are shown in table 5. In each case, 0.8 mm of the EPDM ZDA strip was placed between the elastomer compound before curing. After curing at 160 [degrees] C for 20 minutes, the test specimen was cooled and tested for shear adhesion.

Table 5 - bonding peroxide cured elastomers to steel with the EPDM ZDA adhesive strip

Rubber                  Shear adhesion, MPa(a)

EPDM                            8.53 C
NBR                             5.98 C
CPE                             7.22 C
NR                              8.53 C

(a) C = cohesive failure

The third approach involves applying a dispersion of the metallic coagent as a reactive adhesive between the metal and the rubber stock. The reactive dispersions, as described previously, are made by mixing the metallic coagent in a liquid coagent to form a viscous liquid or paste. They may then be applied in place of an adhesive to the surface of the metal or the uncured rubber stock. The only requirement is that the rubber being bonded must contain peroxide as the curative. When the rubber is cured, peroxide in the rubber stock activates the reactive dispersion forming adhesive bonds between the rubber and the metal surface. Examples of this technique are shown in table 6 for several reactive dispersions versus a conventional solvent based adhesive for bonding peroxide-cured EPDM to steel.

Table 6 - bonding peroxide-cured EPDM to steel with reactive dispersions

                         Shear adhesion, MPa
No adhesion                      0.48
Solvent adhesion                 5.50
Reactive dispersion           3.44-5.50

Conclusions

ZDA and ZDMA can be used to create extremely strong adhesive bonds between a variety of rubbers and untreated metal substrates. As coagents, they are readily compounded into the rubber stock where they crosslink into the rubber when cured with peroxides. Thus, they function as adhesion promoters as well as crosslinkers to enhance both the adhesive and mechanical properties of the cured rubber. They also can be used in an adhesive strip to bond both sulfur and peroxide stocks to metal during curing. Another technique is to apply a dispersion of the metallic coagent as a reactive adhesive between the rubber stock and the metal prior to curing. ZDA is the best coagent for adhesion, but ZDMA is a good alternative when abrasion resistance and tear strength are needed in addition to adhesion.

References

(1.) R. Costin, W. Nagel and R. Ekwall, Rubber Division Meeting, ACS, Washington, D.C., October 9-12, 1990.

(2.) H. Xie and Y. Feng, Polymer 29 (1988) 1216.

Acknowledgements

"Techniques for bonding rubber to metal using metallic coagents" is based on a paper given at the May, 1998 Rubber Division meeting.

"Adhesion of EPDMs and fluorocarbons to metals using water-soluble polymers" is based on a paper given at the May, 1998 Rubber Division meeting.

Application of Raman microimaging in characterization of polymers" is based on a paper given at the May, 1998 Rubber Division meeting.

"Tread compounds with highly-dispersible silica" is based on a paper given at the May, 1997 Rubber Division meeting.

Richard Costin and Walter Nagel, Sartomer Co.