Basics of PVC Thermal Stability

Thermal Stability is a very important characteristic of PVC that defines its resistance to thermal degradation, which manifests as the appearance of color in processed PVC that at low levels has only aesthetic effects but at higher levels may seriously affect the mechanical properties of final products.

Thermal degradation then limits the recyclability and circularity of PVC products. For this reason, PVC Thermal Stability has been the subject of multiple R&D studies both to determine the factors that cause it and to find ways to avoid it by adding additives.

PVC starts degrading when exposed to high temperatures and shear during processing by an "unzipping" dehydrochlorination reaction that releases HCl which autocatalyzes the same reaction. It is accepted that dehydrochlorination initiates in labile groups present in PVC chains formed during radical polymerization under VCM-starved conditions, being identified tertiary chlorines and allylic chlorines as those that affect Thermal Stability the most.

Once labile groups are formed in PVC chains, the only way to prevent thermal degradation from starting is by adding Thermal Stabilizers, which are nucleophilic compounds with specific reactivity that abstract labile chlorines and replace them with more stable functional groups that resist elimination. In order to perform their function, Thermal Stabilizers must have suitable compatibility with PVC and must be added under conditions that minimize diffusion time but without starting thermal degradation (above Tg but below dehydrochlorination activation temperature).

What is not well understood is that there are significant variations in molecular weight and formation of labile groups depending on polymerization conditions, so primary particles are not homogeneous but rather have a gradual change from their center (formed at the beginning of polymerization) to their surface (formed at the end of polymerization). Polymerization conditions then affect both the final number and accessibility of labile groups capable of initiating degradation, as well as the time needed for Thermal Stabilizers to reach them and perform their function, thereby affecting final Thermal Stability of final product.

With a suitable combination and control of polymerization conditions, it is possible to minimize both the formation of labile groups in PVC chains and the time required for Thermal Stabilizers to homogeneously penetrate PVC mass and protect their labile groups, hence improving Thermal Stability and recyclability of PVC products.

Let me help you understand and tackle Thermal Stability problems of your PVC resins.

www.PVCExpertise.com

#PVC #ThrmalStability #Degradation #dehydrochlorination