Suspension powder polymerization

Apparent BET surface areas for hypercrosslinked polystyrenic materials can be as high as 2,090 m g in some cases [25] and these materials can be produced as monoliths, powders, suspension polymerized beads, or by surfactant-free emulsion polymerization as spherical particles with diameters of around 500 nm [35]. Some care must be exercised when interpreting gas sorption isotherms for HCPs using sorbates such as nitrogen and argon as they exist in a non-classical [38] physical state and can exhibit unusual swelling characteristics (Fig. 3). 

In general, the suspension polymerization can be distinguished into two types, namely, the bead and powder suspension polymerization [4]. In the former process, the polymer is soluble in its monomer and smooth spherical particles are produced. In the later process, the polymer is insoluble in its monomer and, thus, precipitates out leading to the formation of irregular grains or particles. The most important thermoplastic produced by the bead suspension polymerization process is PS. In the presence ofvolatile hydrocarbons (C4—C6), foamable beads, the so-called EPS, are produced. On the other hand, PVC, which is the second largest thermoplastic manufactured in the world, is an example of the powder type suspension polymerization. 

The powder suspension polymerization is the most important polymerization process for manufacturing PVC. The main advantage of this process is that large (e.g., 300-500 pm). 

One of the most important issues in modeling the suspension polymerization process is the evaluation of the physical and transport properties of the reacting system as well as the calculation of composition and partitioning of the different species (e.g., monomer(s), polymer, initiator(s), etc.) in the various phases present in the system. In a suspension polymerization process, one can identify, at least, three phases the dispersed phase (e.g., polymerizing monomer droplets), the continuous aqueous phase and the gas phase. The dispersed phase can be either homogeneous (if the polymer is soluble in its monomer) or heterogeneous (if the polymer is insoluble in its monomer). In the powder suspension polymerization, the dispersed phase consists of two different phases the polymer-rich and the monomer-rich phase. The continuous aqueous phase contains only small amoimts of monomer and, finally, the gas phase contains monomer and water vapors. 

The term in the brackets corresponds to the monomer fugadty in the monomer-rich phase in the case of the powder suspension polymerization (i.e., VCM polymerization). For the EPS process, the fugacities of the -pentane in the three phases should also be equal ... 

Suspension Polymerization. Suspension polymerisation yields polymer in the form of tiny beads, which ate primarily used as mol ding powders and ion-exchange resins. Most suspension polymers prepared as mol ding powders are poly(methyl methacrylate) copolymers containing up to 20% acrylate for reduced btittieness and improved processibiUty are also common. 

The discovery of PTFE (1) in 1938 opened the commercial field of perfluoropolymers. Initial production of PTFE was directed toward the World War II effort, and commercial production was delayed by Du Pont until 1947. Commercial PTFE is manufactured by two different polymerization techniques that result in two different types of chemically identical polymer. Suspension polymerization produces a granular resin, and emulsion polymerization produces the coagulated dispersion that is often referred to as a fine powder or PTFE dispersion. 

The suspension polymerization of 65% acrylamide aqueous solution dispersed in n-hexane (aqueous phase -hexane = 1 5) in the presence of a stabilizer (sorbitan monostearate, 1.4% with respect to -hexane) and an initiator (2,2 -azo-bis-A/, A/ -dimethyleneisobutylamide chloride) carried out at 65°C for 3 h, with subsequent holding at 110°C, yields a powdered product with the granule size of 0.5 mm, while the addition of Na2S04... 

Suspension polymerization also is used When acrylic monomers or their mixtures with other monomers are polymerized while suspended (usually in aqueous system), the polymeric product is obtained m the form of small beads, sometimes called pearls or granules. Bead polymers are the basis of the production of molding powders and denture materials. Polymers derived from acrylic or methacrylic acid furnish exchange resins of the carboxylic acid type. Solutions in organic solvents furnish lacquers, coatings and cements, while water-soluble hydrolysates are used as thickeners, adhesives, and sizes. 

Aqueous suspension polymerization requires the usual additives, such as free radical initiators, colloidal dispersants (not always), and chain transfer agents to control molecular weight. After the process is completed, the suspension contains spherical particles approximately 100 pm in diameter. Suspension polymers are available as free-flowing powder or in pellet form for extrusion or injection molding.58... 

The powdered polymers from emulsion or suspension polymerizations intended to be used for solvent-based coatings are often milled into finer particle size with higher surface area for easier dissolution when used as coatings for metal and other substrates.58... 

The nature of the polymerization reactor also depends upon the desired form of the product (pellet, powder, bead, etc.). For example, extruder reactors (Stuber and Tirrell, 1985) are best suited to producing pellets, sheets, and coatings. The beads that may be directly useful in processing are best produced by the suspension polymerization process. The round beads, however, may not have suitable bulk-flow properties and are dangerous if spilled. Alternate shapes and the appropriate methods of production are, therefore, often employed. 

Note that suspension polymerization is only superficially related to emulsion polymerization, which was outlined in Chapter 8. In suspension processes the coagulation of the dispersion is controlled by agitation plus the action of a water-soluble polymer and/or a fine particle size inorganic powder. The role of water is to act primarily as a heat transfer medium. In vinyl chloride suspension polymerization the specific heat of the monomer and polymer are about equal and are one-quarter that of water, on an equal weight basis. Thus, at the typical 1.5/1 water/vinyl chloride mass ratio the heat capacity of the aqueous phase is about six times that of the organic phase. Another use of water is, of course, to keep the viscosity of the reaction medium at a useful level. Water/monomer ratios of 1.5/1 to 1.75/1 provide a good compromise between suspension concentration and viscosity. 

Hydrocarbon oil is added to the dispersion polymerization reactor to stabilize the polytetrafluo-roethylene emulsion. Temperature and agitation control are easier in this mode than suspension polymerization. Polytetrafluoroethylene fine powder and dispersion are produced by this technique. 

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