Description:
The introduction of metallic monomers as coagents in peroxide-cured systems can result in greater than 40% of the crosslinks containing ionic bonds. By using Saret SR633 and SR634, the benefits of adding ionic character to cured rubber compounds can be realized. These include increased modulus, improved tear strength, increased dynamic flex fatigue properties, and improved adhesion to metals and polar fabrics. The ionic bond is capable of breaking and reforming under strain and can reduce the stresses that would otherwise result in failure of non-reversible covalent bonds. New data quantifies the relative percentage of ionic crosslinks formed in these networks. The experimental results are consistent with the trends in physical property improvements that can be attributed to the ionic bond. The drawing below illustrates the ionic character introduced by Saret SR633 and SR634.

Product features and typical properties of Saret SR633 and SR634 are shown below:

Analytical Method:
An analytical method has been developed that allows for the quantification of the relative amount of ionic crosslinks formed in peroxide/metallic monomer-cured networks. Network swelling techniques are first used to calculate total crosslink density. The same system is then subjected to swelling in a weak acid solution that hydrolyses the Zn-O bond, leaving a network supported only by covalent bonds. After calculating the covalent crosslink density from a subsequent swelling step, the number of ionic crosslinks can be determined by subtraction. The figure below describes the procedure.

Ionic Crosslink Determinations:
Based on the above technique, the relative amount of ionic crosslinks obtained as a function of metallic monomer coagent loading was determined for a dicumyl peroxide-cured EPDM formulation. Flory-Rhener methodology was used to calculate crosslink densities; toluene was used as the swelling solvent. The results for Saret SR633 and SR634 are shown below. At low loading, approximately 15% of the crosslinks are derived through incorporation of the metallic monomers, while at elevated loadings, greater than 40% of the crosslinks contain ionic bonds.

Application Performance:
Metallic monomer coagents have been shown to provide several advantages in vulcanizate properties when compared to standard liquid monomers. The benefits provided by Saret SR633 and SR634 can be attributed to the ionic crosslink structure. The following EPDM rubber compound formulations were used for adhesion and tear strength testing.

The chart below demonstrates an increase in adhesion between an EPDM compound and steel as a function of SR634 loading. Adhesion correlates well to the percentage of ionic crosslinks in the system.

The following figure correlates tear strength and SR634 loading. Again the increase is attributed to a relative increase in ionic crosslink density.

Benefits Summary:
Metallic monomer coagents have been documented to produce performance improvements in rubber applications, including hoses, tires, rubber rolls, sporting goods, and other engineered products. It is now possible to quantify the relative amounts of crosslink density and to correlate these findings with the improvements in properties that can be attributed to the central ionic bond.

The listed properties are illustrative only, and not product specifications.  Sartomer Company disclaims any liability in connection with the use of the information, and does not warrant against infringement by reason of the use of its products in connection with other materials or in any process.