Thermal Degradation And Stabilization Of PVC
Polyvinyl chloride is used as a third-most widely produced plastic formed by the polymerization of vinyl chloride and has tremendous applications all around. It is thermally degraded by dehydro- chlorination, in the absence of oxygen and evolves HCl. Thermogravimetry (TG/DTG) coupled with evolved gas analysis (MS detection) of volatiles was used to characterize the thermal behavior of commercial PVC cable insulation material during heating in the range 20-800°C in air and nitrogen, respectively. In addition, simultaneous TG/FTIR was used to elucidate chemical processes that caused the thermal degradation of the sample. There are two general approaches to the stabilization of polymers by modification of molecular structure and the use of additives. This degradation must be controlled by the addition of stabilizers. The mainly used stabilizers are Schiff bases, Alkyltin Stabilizers, Mixed Metal Stabilizers, Alkyl Phosphites Stabilizers, ? -Diketones Stabilizers, Epoxidized Fatty Acid Esters Stabilizers, Hydrotalcites Stabilizers, Tin Stabilizers, Organotin Mercaptides and Organotin Sulfide Organotin Carboxylates. The heat stabilizer must prevent the dehydrochlorination reaction. Among these Schiff bases were find best regarding environmental effects.
1.1 Background to the Study
Poly (vinyl chloride) is commonly abbreviated as PVC, It is the third-most widely pro- duced plastic,after polyethylene and polypropylene. PVC is used in construction because it is more effective than traditional materials such as copper, iron or wood in pipe and profile ap- plications. It can be made softer and more flexible by the addition of plasticizers, the most widely used being phthalates. In this form, it is also used in clothing and upholstery, electrical cable insulation, inflatable products and many ap- plications in which it replaces rubber. 
Pure polyvinyl chloride is a white, brittle solid. It is insoluble in alcohol, but slightly soluble in tetrahydrofuran.  PVC is useful because it resists two things that hate each other: fire and water. Because of its water resistance it’s used to make rain coats and shower curtains, and of course, water pipes. It has flame resistance, too, because it contains chlorine. When you try to burn PVC, chlorine atoms are released, and chlo- rine atoms inhibit combustion.
Structurally, PVC is a vinyl polymer. It’s similar to polyethylene, but on every other car- bon in the backbone chain, one of the hydrogen atoms is replaced with a chlorine atom. It’s pro- duced by the free radical polymerization of vinyl chloride. PVC can also be prepared by treating acetylene with hydrochloric acid (HCl).  (Figure 1)
Figure 1: Synthesis of poly (vinyl chloride)
Polyvinylchloride (PVC) is a plastic that is used in a variety of applications. For electrical wire coatings, the application of concern for this thesis, the PVC formulations typically include numerous additives. At a minimum the formulations include aplasticizer and thermal stabilizers. Plasticizers are utilized to increase the flexibility of the polymer. Thermal stabilizers are utilized to prevent degradation of the polymer backbone by either substitution of the stabilizer for labile chlorides or scavenging the HCl released from the degradation process.4 HCl released as part of the degradation process can catalyse further degradation by coordinating to labile chlorides and promoting their elimination from the backbone.
Phthalates such as diisodecyl phthalate (DIDP), bis(2-ethylhexyl) phthalate (DEHP), and diisononyl phthalate (DINP) are the most commonly utilized plasticizers for PVC formulations. In fact, they represent 85% of the global plasticizer consumption (6 Mton/year).5 However, phthalate plasticizers are derived entirely from crude oil, which is a nonrenewable fossil fuel whose costs are constantly increasing (Figure 1).6 Additionally, the leaching of low weight phthalate plasticizers such as DEHP from the polymer is facing increased scrutiny due to health concerns.7
1.2 Statement of Problem
Bio-based plasticizers derived from the oils of plants or from starch – such as castor, soybean, palm, grapeseed, sunflower and linseed – are being investigated as replacements for phthalate plasticizers. Plasticizers derived from plant oils contain double bonds which are often epoxidized to increase solubility in the PVC polymer.8-10 The use of bio-based stabilizers is hoped to introduce long-term sustainability to the PVC market due to the renewability of the feed-stock used for plasticizer production.
In order to maintain polymer performance after processing, thermal stabilizers such as metal soaps of stearates (ex. zinc stearate, calcium stearate) are also added to the PVC formulation.4-5 While the thermal degradation and stabilization of PVC in the presence of phthalate plasticizers has been studied, the research is largely segmented and focused on model studies in reaction solvents. This study details the integration of model studies and bulk PVC studies on the thermal degradation and stabilization of the polymer in the presence of stearate stabilizers and phthalate plasticizers. A comprehensive mechanism of the stabilization and degradation via zinc and calcium stearates is described.
1.3 Aims and Objectives
The aim of this study is to investigate thermal degradation and stabilization of PVC. The objectives are as follows;
- To identify the stages and processes of thermal degradation of PVC
- To characterize the thermal behavior of PVC
- To review the stabilization process and materials for PVC
5.0 Summary and Conclusion
?-Diketones, Epoxidized Fatty Acid Esters, Alkyl Phosphites, Hydrotalcites, Tin Organotin Mercaptides, Organotin Sulfides and Organotin Carboxylates are mainly used as thermal stabilizers; these have adverse effect on environment. The Schiff bases used for thermal stabilization for PVC are comparatively better in this respect. The most recently studied inorganic compounds are layered double hydroxides (LHDs) and inorganic salts rare earth. The stabilizing effect of these compounds is attributed to their HCl scavenging ability, re- placement of labile chlorine atoms and the electrostatic interaction between the chlorine atoms of the polymer chain and the positive layer of the hydrotalcite, thereby weakening the ability of the chlorine atoms to initiate dehydrochlorination. Synergism of stabilizer action between these classes of thermal stabilizers and Ca-Zn thermal stabilizers will eventually lead to evolution of ideal PVC thermal stabilizers.
From the foregoing, future efforts would continue on the search for compatible, non-toxic and environmentally friendly organic and inorganic compounds that could function as PVC thermal stabilizers. In our own work we will focus on preparation of different PVC stabilizers and the effect of PH on their stabilizing effect.
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