20 июля, 2015 
Malyar A. Stabilization of Polymers (Elastomers)
Typically, the polymers used in adhesive formulations have only a minimal level of stabilization to endure isolation/coagulation, drying/finishing, and warehouse storage. As an adhesive producer, it is most desirable to understand the level of performance of a stabilized polymer and determine whether additional stabilization is needed to provide the necessary level of performance.
1. Stablilization of Ethylene-Vinyl Acetate Copolymers
Ethylene-vinyl acetate (EVA) copolymers are used in HMAs. The EVA acts as the binder, contributing cohesive strength to the adhesive formulation. Typically, an EVA used in a HMA is approximately 18-28 mol% vinyl acetate (VA). In an EVA copolymer, the crystalline polyethylene (PE) region provides strength, compatibility with the wax, and the desired high-temperature properties. The amorphous region containing both VA and PE provides compatibility with the tackifier.
Figure 3 shows the performance during static oven aging at 170°C (338°F) of a stabilized EVA polymer. The base stabilization of the EVA polymer by the producer using AO-1 provided an unsatisfactory level of stability. The presence of skinning and a more pronounced level of discoloration in the base AO-1-stabilized EVA requires additional antioxidant to meet the performance needs of a HMA. Upon the addition of AO-2 to the base polymer, it is clear that the stability of the EVA is improved significantly, skin formation is not observed, and color development is reduced substantially. The formation of insoluble gel as a result of cross-linking is also reduced dramatically with the addition of AO-2, as shown in Fig. 4.
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Figure 3 Discoloration of EVA polymer: Gardner color, days at 170°C (338°F). |
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Figure 4 Gel formation in EVA polymer: gel content, percent insoluble in toluene at 25°C (77°F). |
2. Stabilization of SIS Thermoplastic Elastomers (Styrene-Isoprene-Styrene Block Copolymer)
Thermoplastic elastomers (TPEs) with blocks of polydiene rubber are subject to degradation at the carbon-carbon double-bond sites and require proper stabilization. In SIS block copolymers, chain scission is the predominant degradation mechanism. In an SIS block copolymer, the addition of a more effective stabilizer, AO-3, alone or blended with a secondary antioxidant, PS-1, can provide a significantly superior performance over AO — 1 alone or with PS-1. Resistance to discoloration after static oven aging at 80°C (176°F) is improved dramatically (Fig. 5). Viscosity stabilization (melt flow index stability) (Fig. 6) is also improved drastically using AO-3/PS-1.
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