The Effects of Initiators Mixture on Suspension Polymerization of Vinyl Chloride and its Comparison with other Productivity-Enhancing Procedures

Abstract

Molecular and morphological properties of poly(vinyl chloride) grains produced by suspension polymerization of VCM in the presence of a mixture of three kind of initiators (i.e. fast, mild and slow) (named as Cok process) was experimentally investigated in a pilot-scale reactor. The results obtained here were initially compared with a isothermal regular process (named as control process) and then with those obtained already for other productivity-enhancing polymerization techniques (i.e. nonisothermal and fast initiator dosage process). The results showed that, in contrast to nonisothermal and fast initiator dosage process, the addition of initators mixture at the beginning of the reaction has the smallest influence on molecular weight and polydispersity index compared to control process. It is obvioused that Cok-PVC grains have the lowest cold plasticizer absorption and porosity among these mentioned processes. Scanning electron microscopy (SEM) showed that the particles produced by Cok process are more regularly shaped, with a smoother surface compared with the control product.According to the literatures, all three productivity-enhancing techniques lead to an apparent quality enhancement, higher flowability and greater bulk density of 2 | P a g e final grains. While both processes of continous initiator dosage and nonisothermal polymerization broaden the particle size distribution of final PVC grains, applying initiator mixture produces particles with the same particle size distribution as control process. The SEM images processing showed that Cok process deccelerates the formation of a three-dimensional skeleton of primary particles relative to the control polymerization. In comparison with nonisothermal trajectory and continous fast initiator dosage system, the Cok polymerization process leads to the most delay of motionless conversions and fusion time as well.

In situ suspension polymerization of vinyl chloride/3-(trimethoxysilyl) propyl methacrylate (MPTMS)-intercalated Mg-Al-hydroxide layered double hydroxide: I. Grain properties

“…The porous grain skin of PVC grains can be interpreted by the early aggregation of particles preventing the newly formed PVC nano-domains from precipitating at the skin. 36,37 As the aggregation of LDH particles inside VCM droplet occurs earlier than MPTMS-LDHs. Early aggregation of LDHs at the early stage of polymerization increases the interstitial pores type of the primary particles where cannot be totally filled by polymer chains and thereby resulting in pores and defects in grain skin.…”

“…The Xm is strongly affected by both the interstitial pores and discrete zones between irregular shaped three-dimensional networks of primary particles. 37,38 The presence of particles as the nucleating site for primary particles decreases the interstitial pores types, and increases the discrete zones by early aggregation. As a consequence, the MPTMS-LDH-PVCNC5 samples will have the highest motionless conversion.…”

“…As will be discussed in more detail in the next section, primary particles were exposed to an earlier aggregation in polymerization with particles, resulting in the formation of an inner porous, strong network at the initial stages of polymerization, preventing the droplet from collapsing with increasing monomer conversion and from coalescence in sticky stage. 5,[36][37][38][39][40]42,43 As can be seen from the images, the morphologies of the irregular-shaped particles have started to change into a spherical shape, and the mean size of the particles reduced. The corresponding particle skins are visualized in the micrographs shown in Figure 7(d) to (f).…”

“…The SEM images were analyzed by using image processing software (ImageJ) to measure the critical conversion (after which Sauter mean diameter (D 32 , equation 1) of primary particles remain unchanged with conversion. [37][38][39][40] ) At least 120 primary particles for three different broken particles at each conversion were analyzed to calculate D 32 . The K-value was calculated from the dilute solution viscosity measurements of the PVC samples in cyclohexanone by an Ubbelohde viscometer according to ASTM D 1243-95.…”