Polymers (EVA, SIS, SBS, SEBS)

1. EVA Stabilization
2. SIS Stabilization
3. SBS/ SEBS/ SEPS stabilization

EVA Stabilization

After exposure to temperatures up to 170-200°C, typical adhesive grade ethylene vinyl acetate (EVA) polymers (vinyl acetate content: 18-40%) undergo discoloration together with gel formation and a viscosity increase.

During static aging at 170°C, the viscosity increase can be reduced by the use of relatively low levels of antioxidants. The same antioxidants also reduce gel formation after dynamic aging (Figure 1) and hinder color development.

SIS Stabilization

Unsaturated elastomers such as styrene-isoprene-styrene (SIS) and styrene-butadiene-styrene (SBS) block copolymers contain double bonds in their structure. Therefore, they are more sensitive to oxidation than saturated polymers. SIS and SBS, however, behave quite differently during degradation as illustrated in Figure 2: Torque Development in a Brabender Plasticorder. SBS is predominantly cross-linked by oxidation, leading to gel formation (torque increase), whereas SIS displays mainly chain scission, leading to molecular weight decrease (torque decrease).

Figure 2 : Torque Development of SIS & SBS at 180°C in a Brabender Plasticorder

The difference in the degradation mechanisms must be considered when preparing hot melt adhesives.

Stabilization of SIS

Controlling the degradation of neat SIS during mixing at high temperatures and under high shear forces is more difficult than SBS. As a result, it is necessary to use an effective processing stabilizer, stabilizer system, or a multifunctional antioxidant to efficiently stabilize neat SIS under these conditions.

Figure 3 : Melt Flow Index of SIS After Oven Aging at 70°C

*Sample containing BHT is completely destroyed after seven days aging: MFI cannot be measured (sample is almost liquid).

SBS/ SEBS/ SEPS stabilization

SBS copolymers, as with EVA, have the tendency to crosslink under thermo-oxidative conditions. Simulating compounding conditions in a Brabender Plasticorder, cross-linking is observed as an increase in torque (viscosity). Induction time, which is the time to an increase in torque of 1Nm after the minimum, is a useful parameter for evaluating the processability of SBS.

Induction Time After Brabender Plasticorder Aging of SBS at 160°C