Polymers (EVA, SIS, SBS, SEBS)
- EVA Stabilization
- SIS Stabilization
- SBS/ SEBS/ SEPS 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.
Figure 1 : Gel Content of EVA After Compounding in a
Sigma Blade Kneader at 170°C
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
*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