Rubber to Metal Bonding Using Metallic Coagents

Rubber to Metal Bonding Using Metallic Coagents 

Metallic coagents are reactive substances, which improve the effectiveness of peroxide crosslinking. Most of them belong to the group of methacrylates or derivatives containing allyls, but polymeric materials with a high content of vinyl groups are also known to 

The crosslinking efficiency of many peroxide-initiated free radicals is low. These labile radicals can be converted to more stable radicals by contact in situ in the rubber mix with polyfunctional monomers to form a three-dimensional network. Crosslinking 

In addition these materials act as plasticisers during processing and in some cases also act as hardening agents. The following chapter discusses the use of such agents in bonding applications for a variety of rubbers. 

Coupling agents are molecular bridges at the interface between two dissimilar substrates, usually but not limited to, an organic filler and an organic polymer matrix. 

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When metallic coagents are used in rubber compounds to increase bonding to metals and reinforcing materials, they also change the physical properties of the rubber compound. Therefore, their use as coagents and their effect on the properties of the rubber compound will be discussed in Sections 8.2.2 and 8.2.3. 

Three techniques to bond rubber to metal and synthetic fibres using metallic coagents have been developed [25]. They are ... 

Several papers have been published that describe the use of zinc diacrylate (ZDA) and ZDMA as coagents to improve the physical properties of rubbers [10, 11, 12, 13 and 14]. In this context, they are referred to as metallic coagents. They differ from conventional rubber coagents, like triallyl cyanurate (TAC) and TRIM, in that they have ionic bonds that become part of the crosslink network. The ionic bonds allow for a flexible and more forgiving network, particularly at high crosslink levels. This can lead to rubber products with good dynamic properties and a good combination of physical properties. 

The reactive dispersions (CD-627, CD-628, PRO 1825) are experimental materials developed by Sartomer Company for evaluation in this study. They are proprietary products that consist of a metallic coagent dispersed at various concentrations in different acrylic monomers. A viscous solution or paste is formed which can be used in place of an adhesive to form adhesive bonds during the curing step of the rubber compoimd (peroxide cure only). They contain no solvents and are 100% reactive. The metallic coagent is a small particle-size powder that is not soluble in monomers, but does form a stable dispersion. 

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. 

A further 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 compoimd. The uncured compound is then formed into sheets by calendaring and then cut into 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... 

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. 

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