Metallic coagents

Properties Liq. insol. in water m.w. 226.28 dens. 1.010 b.p. 290 C flash pt. (COC) 124 C ref. index 1.4520 Toxicology TSCA listed Storage Light-sensitive Uses Peroxide crosslinker/coactivator for elastomers, PVC plastisols, syn. resins, cast acrylic sheet/rods coagent for rubber compding. impregnant for metal and wood composites, adhesives, and glass-reinforced... 

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... 

Metallic salts of acrylic acid and methacrylic acid also fimction as coagents in the same way as the methacrylate esters do. They are also highly reactive in the presence of free radicals and readily form a polymeric crosslink network with rubbers. Several examples are reported where they have been used as coagents in peroxide-cure formulations for both saturated and unsaturated rubbers. 

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. 

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. 

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. 

Figure 8 Ionic crosslinks formed with metallic coagents...

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. 

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. 

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

Although the metallic coagents are not adhesives by themselves, there are applications where these techniques may be applied to improve the adhesion of rubber to metals and synthetic fibres. 

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. 

Table 8.16 Tensile properties obtained with metallic coagents in EPDM ...

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... 

The EPDM formulation described in Section 8.3.1.1, with and without 10 phr of metallic using coagent, was cured in contact with Nylon/polyester warp fabric for 20 minutes at 160 °C. RFL treated fabric was used in the control sample. Peel adhesion (180° angle)... 

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