Metallic coagents Saret

Metallic coagents are defined as the metal salts of acrylic and methacrylic acids. The two products discussed, Saret 633 and Saret 634, are the difunctional zinc salts of acrylic acid and methacrylic acid, as shown in Figure 8.1. 

Thus, with the Saret metallic coagents it is possible to gain the benefit of increased crosslink density and faster cure rates, while maintaining good scorch safety. 

The effect of metallic coagents on the tear strength of EPDM is displayed graphically in Figure 8.5. With both Saret 633 and Saret 634, the tear strength increased as the coagent... 

Saret 633 and Saret 634 (Sartomer Company) are the scorch-retarded versions of zinc diacrylate and zinc dimethacrylate, respectively. The Saret metallic coagents are used in rubber compounding to avoid premature curing during mixing. 

When using Saret 633 and Saret 634 as internal adhesion promoters, it is important to remember that they do crosslink with the rubber during curing and, therefore, change the physical properties of the rubber. An example of this is shown in Table 8.16 for the Nordel EPDM formulation containing 2 phr of the metallic coagents. In this example,... 

Saret 634, while not as effective as Saret 633, also increased adhesion in the example described in Table 8.16. In applications where high tear strength, abrasion resistance, exceptional scorch safety and slow cure are needed, Saret 634 may be the best choice. Metallic coagents based on other metal salts, such as calcium and magnesium, also increase rubber-to-metal adhesion, but not as effectively as Saret 633 and Saret 634. 

The strength of the adhesive bond increases as the concentration of the metallic coagent is increased. The effect of Saret 633 concentration on the adhesion of EPDM to steel is shown in Table 8.17. Shear adhesion increased from 0.5 MPa to 11.34 MPa as the Saret 633 concentration was increased from 2 phr to 20 phr. In all cases, cohesive failure, i.e., failure within the rubber, is the primary mode of failure for the adhesive bond. It should also be noted that as the Saret 633 concentration is increased from 2 phr to 20 phr, the modulus and hardness of the rubber also increases. As a result, Saret 633 cannot be simply added to a currently used rubber compound to replace an adhesive without modifying the formulation. It would be necessary to adjust the Saret 633 and/or filler concentrations to get the best balance of adhesion and physical properties. 

Saret 633 and Saret 634 can be used to create strong adhesive bonds between a variety of rubber compounds and untreated metals, synthetic fibres and fabrics. They also function as coagents by increasing the crosslink density of the rubber compound. Thus, they function as adhesion promoters as well as crosslinkers to enhance both the adhesive and mechanical properties of the cured rubber. One requirement is that they are used in peroxide cure compounds for optimum results. Metallic coagents have been used with sulphur to improve adhesion but they do adversely affect the cure state of a sulphur compound. 

Sartomer s Saret zinc diacrylate and zinc dimethacrylate coagents were evaluated in the peroxide vulcanisation of EPDM automotive belt and hose compounds. Cure characteristics and effects on mechanical properties, heat and water resistance and adhesion of EPDM to metal and synthetic fibre reinforcements and to other mbbers were investigated. The results of this study and other experiments with Saret coagents in rabber formulations are discussed. 6 refs. 

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