Viscosity Stability
An important requirement for adhesives and their raw materials is viscosity stability.
It is essential through all stages of the product's life cycle:
- Processing
- Storage
- Shipment
- Use
- Aging
In formulated adhesives, the resins, plasticizers, and other polymers present must
be protected. Depending on the predominant degradation mechanism, the product may
either decrease or increase in viscosity if not stabilized. For polymers where cross-linking
predominates, an increase in viscosity is often accompanied by gel formation or
skin formation.
Chain scission,
leading to a molecular weight decrease on aging manifests itself by a decrease of
viscosity.
The most appropriate stabilizer combination will depend on the selection of other
formulation ingredients, as well as on the aging conditions. Suitable phenolic antioxidants
in combination with thiosynergists or multifunctional antioxidants exhibit the best
performance.
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SIS-HMA Stabilization
Melt Viscosity measured at 170°C
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Melt Viscosity of SBS-HMA after oven aging at 170°C
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Skin Formation
For certain hot melt adhesives, high temperature aging may lead to skin formation.
This occurs with the cross-linking reaction. Skinning is based on irreversible oxidative
damage to the polymer and cannot be corrected by subsequent mixing or addition of
stabilizers. During processing, skin formation in the adhesive can lead to blocking
of pipes and irregular coating. The addition of the proper additive package during
the manufacturing of the hot melt significantly retards skin formation.
Note: Originally atmospheric conditions become anaerobic under a cross linked skin,
due to reduced diffusion of oxygen.
