Heat stabilizers for PVC processing

   In most cases heating to processing temperature leads to partial thermal destruction of PVC.  Characteristic sign of PVC destruction when heated is its darkening at temperatures above 100°С, especially during processing in the temperature range of 160-190°С. Color change is accompanied by the polymer cross-linking. Destruction proceeds faster in presence of oxygen than in inert gas. PVC destruction can be evaluated by HCL emission rate, however in practice it is normally judged by the material color change. During processing of unplasticized PVC compositions  by extrusion and injection molding material heat destruction leads to  product color change and to bubbles.  In  case of polymer mass “scorching” during processing partial cross-linking occurs which results in higher melt viscosity.  

   Stabilization purpose is to retain initial properties of polymer materials. In principle polymer stabilization can be achieved by two methods: by introduction of stabilizers and by PVC modification by physical and chemical methods. Besides, stabilizers have an effect on the processing rates and the performance characteristics of the finished goods. Introduction of stabilizers delays start of PVC destruction and during this period of delay which is called “induction time” there is no noticeable HCL emission.  It is important that molten state time does not exceed induction time at processing temperature. So, it is necessary to control PVC plasticization time.

   PVC stabilizers designed to prevent these adverse events are supplied as individual compounds (mainly stearates and other heavy metal – and alkaline-earth metal salts), and as complex stabilizers which include substances strengthening their action and providing long-term stability (costabilizers) and other additives.

   An ideal PVC stabilizer should be able to: bind emitted HCL, inhibit oxidation reaction, cross-linking, protect double-bonds in PVC chains, absorb ultraviolet radiation. All these goals can be achieved by using mixed stabilizers (complex stabilizers). Use of several types of correctly selected stabilizers together with lubricants not just results in a cumulative effect but it has a much greater effect than each component separately. 

   A distinctive feature of PVC processing is that heavy metal compounds are the only really efficient stabilizers. All these materials are toxic to greater or lesser extent. In Germany and CIS countries it is allowed to use lead stabilizers in limited quantities (1% max.) for potable water system pipes. In Italy, the USA and Japan they use stabilizers which do not contain heavy metals, though such systems are more expensive and technically more sensitive.

    Normal content of heat stabilizers is 1-3%.

Heat stabilizers for PVC processing

• Chimstab-BK, Chimstab-BKTs, Chimstab-BTs, Chimstab-BK-p
  
• Chimstab-KTs