Industrial processes for the production of methacrylate-based (acrylic) polymers
Acrylic polymers are routinely produced by emulsion polymerization, bulk polymerization, and solution polymerization. Generally, industry uses free-radical initiation (including living polymerization methods) although PMMA (polymethyl methacrylate) can also be produced by anionic polymerization. Typically emulsion polymers have the lowest residual monomer level, because the polymerization process takes place in an aqueous (water-based) environment. Bulk polymers, cast polymers and continuously cast polymers also have very low residual monomer levels. Solution polymerization (solvent-based polymerization environment) products generally have the highest residual monomer levels, which are still relatively low.
Emulsion polymerization is a type of radical polymerization that usually starts with an emulsion incorporating water, monomer, and dispersant (surfactant). The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of acrylic monomers are emulsified (with surfactants) in a continuous phase of water. The name "emulsion polymerization" is a misnomer as polymerization rather than occurring in emulsion droplets actually takes place in the latex particles that form spontaneously early in the process. These latex particles are typically 100 nm in size, and are made of many individual polymer chains. Emulsion polymerization is used to manufacture several commercially important polymers. Many of these polymers are used as solid materials and must be isolated from the aqueous dispersion (latex or emulsion) after polymerization. In other cases the dispersion itself is the end product. These find applications in adhesives, paints, paper coating and textile coatings. They are finding increasing acceptance and are preferred over solvent-based products in these applications as a result of their eco-friendly characteristics due to the absence of VOCs (Volatile Organic Compounds) in them.
Bulk polymerization or mass polymerization is the most common polymerization process applied to acrylic monomers. It is carried out by dissolving an initiator in acrylic monomer and initiating the polymerization reaction by heating. The reaction is exothermic and a wide range of molecular masses are produced. The resultant polymers have high optical clarity making them suitable for lighting, displays, signage, optical and other applications where transparency is desired. These polymers also have the highest long-term design stress and superior weather-ability due to their high average molecular mass.
Bulk polymerized polymers are typically extruded into pellets for subsequent processing by industrial processors. All common forming processes may be used with these pellets, including injection molding, compression molding, and extrusion.
Extruded acrylic sheet is made through a process in which the molten acrylic polymer is forced between rollers which press it into sheets as it cools. It is a good choice for use in making signs, displays, and other uses where higher molecular weight cast materials are not essential. Cast PMMA sheets are produced by cell casting in which single sheets of acrylic are made by pouring acrylic syrup between two pieces of a mold, often made of glass or highly polished metal, which is then taken through a gradual heating and subsequent cooling process. The strength of the sheet produced by casting is higher than that by molding or extrusion owing to its extremely high molecular mass. Casting can also produce thicker sheet than is possible with extrusion processes. Cast products are often used for aquariums, awards, aircraft canopies or windows, and other products that require high performance or thick walls. This process also lends itself to lower volumes, and is often used for producing small quantities of colored sheet products. Less common is continuous cast sheet production, in which the acrylic syrup is poured between two parallel steel belts before polymerization and cutting to size. As the continuous polymerization process is quicker, the molecular weight of the resulting polymers is significantly lower than in cell casting, but still much higher than typical extrusion or molding grade acrylic polymers.
Solution polymerization is a method of industrial polymerization. In this procedure, the acrylic monomer is dissolved in a non-reactive solvent that contains an initiator. The reaction results in an acrylic polymer which is also soluble in the chosen solvent. Once the degree of polymerization is reached, the process is stopped and resultant polymer is either stripped of the solvent and supplied as a polymer pellet, or supplied in the solvent. Polymers supplied in the solvent are mainly used for applications where the presence of a solvent is desired, as is the case for varnish, adhesives or coatings.
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