Studies On Particle Formation Processes In Suspension (Co)Polymerizaiton And Synthesis Of Thermally Expandable Polymer Microspheres

Aqueous suspension polymerization is one of the most important methods to produce polymer. Porous polymer powders would be formed for suspension polymerization system in which the polymer is not dissolved in its monomer, while dense polymer beads would be formed when the polymer can be dissolved in its monomer. Suspension polymerization in the presence of blowing agent is always adopted to prepare thermal expandable polymer microspheres. However, studies on the particle formation process and morphological control of core-shell structure of polymer microspheres in the suspension polymerization, and the relationship between the structure and thermal expansion properties of microspheres are lack till now. In this thesis, a new on-line laser particle size analysis system was established to monitor the mean size and size distribution of drops/particles in suspension polymerization, and effects of adding of blowing agent into methyl methacrylate (MMA), vinylidene chloride-methyl methacrylate (VDC-MMA), and vinylidene chloride-methyl methacrylate-acrylonitrile (VDC-MMA-AN) suspension polymerization on the particle formation processes were investigated. Moreover, the particle formation mechanism of suspension polymerization in presence of blowing agent was proposed based on the evolution of particle morphology and encapsulation efficiency of blowing agent in the polymerization, and effects of composition and character of shell polymer, and type and content of added blowing agent on the thermal expansion behavior of polymer microspheres were investigated.Firstly, an online laser particle size analysis system based on ORM technology was established and employed in monitoring the Sauter average size (d32) and size distribution of drop/particle in MMA liquid-liquid dispersion and suspension polymerization processes conducted at different agitation rates, poly(vinyl alcohol) (PVA) dispersant concentrations and initial oil compositions (viscosities). It showed that ORM technology could successfully applied in monitoring the variations of droplet/particle size and particle size distribution in liquid-liquid dispersion and polymerization process, and MMA suspension polymerization exhibited four characteristic intervals, i.e., transition, quasi-steady-state, growth, and identification stages. Increasing of agitation rate, PVA concentration, or reducing the viscosity of dispersed phase would lead the decreases of d32 and the width of size distribution of drops in MMA suspension polymerization. The dimensionless size ratio (d32/D) was related with the Weber number (We) as d32/D=0.06(1+5φ)Wee-0.6. When PVA concentration is too low or viscosity of initial dispersed phase is too greater, the duration of the quasi steady state and growth stages in particle formation process would be shorten, and d32 of polymer particle and size distribution width were obviously increased. VDC-MMA, VDC-MMA-AN suspension copolymerizations also exhibited four characteristic intervals and followed the bead suspension polymerization mechanism, similar to MMA suspension polymerization, and dense polymer beads with no porosity would be obtained.Secondly, effects of oil composition and adding of water additives on the final morphology and encapsulation efficiency of blowing agent in MMA, VDC-MMA, VDC-MMA-AN suspension (co)polymerization in presence of blowing agent were investigated. It was found that core-shell structure of polymer microspheres with encapsulated blowing agent could be prepared for MMA, VDC-MMA (feeding monomer composition 70/30) and VDC-MMA-AN suspension (co)polymerizations in the presence of blowing agent. Polymer microspheres with perfect core-shell morphology and greater encapsulated efficiency would be formed when a suitable concentration of crosslinking agent was added. Adding of suitable amounts of NaCl, citric acid and potassium dichromate would reduce the aqueous polymerization and the number of fine particles with low encapsulated blowing agent. Adding of blowing agent with boiling temperature closed to or greater than polymerization temperature, would favor to formation of core-shell structure microspheres, while the surface of microsphere would become irregular with many dents.Thirdly, variations of conversion, shell polymer composition, morphology, average size and size distribution of drops/particles, and encapsulation efficiency of blowing agent with time (conversion) in MMA, VDC-MMA and VDC-MMA-AN suspension (co)polymerizations were investigated. The results showed that the liquid-liquid dispersion behavior of MMA/pentane was similar to that of MMA, and d32 of drops was decreased as the mass fraction of pentane increased. Adding of blowing agent would reduce the gel effect of polymerization due to the decrease of monomer concentrations in the oil and polymer phases. Adding of blowing agent would also weakened the agglomerate behavior of droplet and affected the duration time of the four characteristics stage and the evolution of particle size in suspension polymerization process. From the microscopic aspect, the solubility of polymer in the oil phase consisted of residual monomer and blowing agent and swelling degree of oil phase in polymer would decreased as blowing agent added, resulting the early phase separation in polymerization. The primary particles and their aggregates would be formed through the coagulation of precipitated polymer chains, and be located at interface between oil and water phase to form non-continuous shell due to hydrophilic/hydrophobic balance of polymer and centrifugal force caused by agitation. More and more polymer primary particles would "fused" into the shell due to low glass transition temperature of swollen polymer. Finally, the continuous and thick shell and core-shell structure of polymer microspheres with encapsulated blowing agent would be formed.Finally, the diffusion behavior and thermal expansion performance of polymer microspheres with different composition have been studied. At the temperature of 50 ℃, the diffusion of blowing agent in polymer microspheres with feeding VDC/MMA/AN ratios of 50/40/10,60/20/20 and 50/30/20 followed the Fick diffusion mechanism, while the diffusion of blowing agent in polymer microspheres with feeding VDC/MMA/AN ratios of 70/20/10 and 70/10/20 followed the non-Fick diffusion mechanism and the microspheres were easy to lose blowing agent. The characteristics of shell polymer and blowing agent are the key factors influencing the expansion of the microspheres. PMMA and VDC-MMA-AN copolymer microspheres with encapsulated pentane exhibited good expansion properties, while VDC-MMA copolymer microspheres were difficult to expand. VDC/MMA/AN (50/30/20) copolymer microspheres with encapsulated isopentane, pentane, hexane or heptane all exhibited expansion ratio of about 30 times, while the temperature range for expansion was decreased as the boiling temperature of blowing agent increased. The expansion ratio was also increased with the increased content of encapsulated pentane. By adjusting the composition and Tg of shell polymer, types and contents of blowing agent, PMMA microspheres with the mediate expansion temperature and volume expansion ratio of 56 and VDC-MMA-AN copolymer microspheres with low expansion temperature and volume expansion ratio of 41 were prepared. The apparent density of microspheres after expanded are lower more than 30 times.