Crosslinking efficiency

The crosslinking of ethylene-propylene copolymer rubber (EPR) in the presence of organic peroxides has been investigated by Natta and/or his coworkers (1-3) and others (4,5). Co-agents such as sulfur (3,4) and unsaturated monomers (6), including maleic anhydride (MAH)(3,7) have been utilized in an effort to increase the crosslinking efficiency in the EPR-peroxide system. 

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 with polyfunctional monomers to form a three-dimensional network. Crosslinking efficiency is thus increased by some 20%. In addition, these materials act as plasticisers during processing and in some cases also act as hardening agents. 

Figure 2. Crosslinking efficiency of unsaturated olefins in the presence of MFAs and MFMAs initiated by ionising radiation.

It is possible to uniquely determine both the scission efficiency Gis) and the crosslinking efficiency G(x) via Equations 9 and 10 providing both the number average molecular weight (M ) and the weight average molecular weight (Af ) can be measured. 

The companion paper (6) discusses the practical consequences of this reaction and other unique features of the chemistry of 1 and 2 on copolymer crosslinking (efficiency, rate and pot life) and substrate adhesion. 

The crosslinking efficiency of the peroxide process can be increased for some systems by incorporating small amounts of a comonomer containing vinyl groups into the polymer. This approach is used for polysiloxanes by copolymerization with small amounts of vinyltrimethylsilanol... 

It should be noted that AM is only a measure of an apparent crosslink density of compounds. It is beyond the scope of the present work to investigate in detail the effective crosslinking (physical and chemical). However, for a qualitative assessment it can be concluded that the apparent crosslink density decreases or is influenced by the E-beam irradiation of PTFE powder. PTFE500kGy-EPDM composites show much lower AM and hence lower apparent crosslink densities. It can be inferred that the state of cure and crosslinking efficiency are strongly dependent on irradiation dose. Table 3 shows the optimum curing time (f90, time required to reach 90% of the AM) as a function of PTFE loading and irradiation dose for different PTFE-filled EPDM composites. 

Gel type Approximate gelation time (min) Shear modulus (gcm 2) Equilibrium degree of swelling Effective crosslinking density (mol cm-3) Crosslinking efficiency (%)... 

L 6. Loan, L. D. Crosslinking efficiencies of dicumyl peroxide in unsaturated synthetic rubbers. J. Appl. Polymer Sci. 7, 2259 (1963). 

With an unaccelerated sulfur-natural-rubber system, the poor crosslinking efficiency results m sulfur being incorporated into the rubber network as long polysulfide crosslinks, cyclic monosulfides, and vicinal crosslinks, which are very close together and act physically as a single cross-link (see Fig. 2). 

In view of the abundance of unsaturation in polydiene elastomers it may be expected that this type of elastomers can be cured very efficiently with peroxides. This section will show that a very high crosslinking efficiency (= moles of crosslinks formed per mole of peroxide decomposed) can indeed be obtained. However, the substitution pattern of the unsaturation plays a major role in the actual efficiency found. 

Co-agents are multi-unsaturated compounds, which are used in the peroxide-curing of elastomers. When classical co-agents, such as triallylcyanurate (TAC), trimethylolpropanetrimethacrylate (TRIM) or diallylterephthalate (DATP), are added, the crosslinking efficiency is enhanced [98-102]. Various mechanisms for the increase of the crosslinking efficiency have been proposed. In all cases a fast reaction between the... 

Apart from the effect on the crosslinking efficiency, the use of co-agents in peroxidecuring also imparts the molecular structure of crosslinks. It has been reported that coagents with two or more unsaturated moieties can be incorporated as individual molecules between two elastomer strands to form crosslinks [103-109]. In this way the crosslink structure of peroxide-cured elastomers can be altered. Thus, apart from the expected benefits, such as improved crosslinking efficiency, decreased compound viscosity and faster cure, the use of co-agents may also provide a tool for manipulating mechanical properties. 

If the effect of co-agents on crosslinking efficiency is just the suppression of macroradical side reactions, such as chain scission and disproportionation, one should expect monofunctional co-agents to be as effective as their multi-functional analogues (if compared at the same molar level of unsaturation). This is definitely not the case, as will be demonstrated. 

The results clearly demonstrate that a diallyl functional co-agent such as DAO, provides a marked improvement in the crosslinking efficiency, whereas its monoallyl functional... 

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