Effects of Additives on Fusion Parameters of Rigid PVC Formulations M. Moghri, H. Garmabi, M. Akbarian Polymer Engineering Department, Amirkabir University of Technology, Hafez Ave., Tehran, Iran The effects of varying amounts of tribasic lead sulfate (TBLS), dibasic lead stearate (DBLS), neutral lead stea- rate (NLS), calcium stearate (Ca.St.), stearic acid (St.Ac.), and paraffin wax (PW) on the fusion properties of a rigid PVC formulation were investigated. The materials showed varying effects on fusion. Thus, fusion time was lowered and fusion factor increased by TBLS, DBLS, and Ca.St., while the reverse was true with NLS, St.Ac., and PW. The behavior of the materials is discussed in terms of their effects on the transfor- mation of PVC grains. J. VINYL ADDIT. TECHNOL., 14:73– 78, 2008. ª 2008 Society of Plastics Engineers INTRODUCTION Although poly(vinyl chloride) (PVC) is one of the most versatile thermoplastics, it suffers from poor thermal stability and undergoes thermal degradation during proc- essing and use. Aiming at the improvement of the thermal stability, many classes of thermal stabilizers, such as lead compounds, soaps, organotins, and fully organic stabil- izers, are known. In fact, the main roles of heat stabilizers are to bond the emitted hydrogen chloride and to replace the labile chlorine atoms in the PVC chains [1, 2]. These processes obviate the dehydrochlorination of PVC to some extent. Another group of additives used in PVC compounding is lubricants. Lubricants, although used at relatively low concentration, are, like the heat stabilizers, indispensable in formulating rigid PVC compounds. In fact, many would argue convincingly that proper lubrica- tion balance is even more important than stabilizer selec- tion for ensuring successful processing [3]. The pioneering study by Berens and Folt [4] in 1967 reported the discovery of particulate structure in PVC. Later, Collins and Krier [5] used the concept of particu- late structure in order to explain more about the unusual rheological behavior of PVC. Since these initial studies, a hierarchy of structural order has been revealed. In fact, PVC shows a variety of levels of morphology [1]. Powder particles known as grains are irregular in shape and are about 150 lm in diameter. Each grain consists of many primary particles of about 1 lm in diameter and also some agglomerates [1, 6–8]. These primary particles are loosely packed together, thereby giving the grain its po- rous nature. Primary particles are, in turn, composed of still smaller structures or domains which are 0.1 lm in di- ameter and microdomains with a 0.01-lm diameter. These aforementioned structures contain about 5–10% of crystal- linity. It should also be added that some authors have shown that the virgin powder can possess an even higher crystallinity [7]. In processing, grain boundaries must be eliminated, and the primary particles must be altered and compacted together. After significant interdiffusion, the boundaries of submicroparticles disappear, and a three-dimensional net- work of polymer chain entanglements and primary and secondary crystallinity is formed which guides the proces- sor to achieve a product with good mechanical properties. The aforementioned process is referred to as fusion or gelation of PVC [7], which is accomplished during proc- essing through the combined action of heat, pressure, and shear. Fusion is taken to imply the extent to which individual powder grains have fused together to form a continuous matrix. The fusion process is essentially a destruction of the original coarse powder structure and the buildup of a new fine structure based on micro and submicro particles. The fusion itself is dependent to a high degree upon the additives and formulation changes. Additionally, control- ling the fusion time is very important, so that the melting does not take place too late or too early [9]. The rheological and fusion behavior of PVC com- pounds plays an important role in the processing opera- tions and in the development of physical properties in the processed material. These processes are governed by the complex morphological structure of PVC resin, compound composition, and processing conditions. It has been shown by several investigators that a complete destruction of particulate structure is not a prerequisite for optimum properties [10]. Correspondence to: Hamid Garmabi; e-mail: garmabi@aut.ac.ir DOI 10.1002/vnl.20148 Published online in Wiley InterScience (www.interscience.wiley.com). Ó 2008 Society of Plastics Engineers JOURNAL OF VINYL & ADDITIVE TECHNOLOGY——2008