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Review
S-PVC grain morphology: A review Reza Darvishi, Mohsen Nasr Esfahany, and Rouhollah Bagheri Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02478 • Publication Date (Web): 20 Oct 2015 Downloaded from http://pubs.acs.org on October 23, 2015
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Industrial & Engineering Chemistry Research
S-PVC grain morphology: A review Reza Darvishi, Mohsen Nasr Esfahany*, Rouhollah Bagheri Department of Chemical Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran.
Abstract Among the properties required for producing PVC resin of good quality, the particle morphology (size, primary particles, agglomerates, skins, pores, etc.) plays an important role in processing and performance of products. Three main physical transitions are involved in self-structuring the grains of suspension PVC: nucleation, aggregation and growth. All of these processes are affected by many physical transitions and chemical phenomena which are inter-related and many are complex functions of polymerization conditions (i.e. agitation, temperature, stabilizers, etc.). Because of this multi-scale complexity, the relationships between suspension polymerization conditions and morphology have yet to be wellunderstood. In spite of all these difficulties, a literature survey of the available research nevertheless provides a comprehensive and descriptive insight into mechanisms governing PVC particle formation accounting for the effect of the process variables. Keywords: PVC, Suspension polymerization, morphology. Introduction Poly (vinyl chloride), PVC, is the second most commonly used plastic material, and its production is the third largest, next to polyethylene and polypropylene1. The great versatility and cost effectiveness of PVC explains its large use in many rigid, semi-flexible and flexible (plasticized) applications such as pipes, fittings, non-food packaging, medical equipment, bottles and a variety of extruded profiles. Currently 80 % of the global PVC production is by suspension polymerization in a batch-wise stainless steel reactor2. Vinyl chloride monomer is dispersed into small droplets in a continuously stirred aqueous phase, consisting of de*
Corresponding author: M. Nasr Esfahany,
[email protected] Tel: +98 (31) 33915631 Fax: +98 (31) 33912677 ACS Paragon Plus Environment
Industrial & Engineering Chemistry Research
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mineralized water, suspending agents (protective colloids) and a buffer salt3,4. The polymerization is started in monomer phase by the addition of an oil-soluble initiator. Thermally decomposing initiator molecules create PVC chains which precipitate inside VCM phase at relatively short chain lengths. As a result, polymerization medium is divided into the monomer- and the polymer-rich phases inducing several physical transitions during the polymerization5. Figure 1 shows the morphological development of PVC grain in relation to reactor pressure with conversion evolution. For the conversions less than 0.1 %, aggregation of the precipitated macro-radicals, initially forms nano-domains which are slightly stabilized by negative charge resulting from HCl cleavage6,7. In the next stage, these nano-sized particles rapidly coagulate to generate the nuclei of the primary particles, also called domains at conversions between1-5 %. During a rather wide rangeofconversions5%
