Acrylic or PMMA (Poly Methyl Methacrylate) is widely used in many applications due to its unique properties. It has properties like light transmission up to 92%, toughness, lightweight, good impact strength, good UV tolerance, scratch resistance and good dimensional stability. It is used in outdoor applications as it is weather resistant. It can maintain its original colour and finish for many years.
Acrylic is available in sheet and resin form. The two main types of acrylic sheet are cast and extruded. These are manufactured in plants using different processes. During the production process, various modifiers can be added to the acrylic to get the desired properties.
The sheet can be modified to get properties like flame retardant, light diffusion, UV light filtering, improved impact strength and more. Dyes can also be added to get the desired colour. The different types of sheets available are fluorescent sheets, coloured sheets, tinted sheets, mirror sheets, prismatic sheets and more.
Types of PMMA sheets
Cast sheets
The cast sheets are produced using the process called bulk polymerisation or mass polymerisation. In this pure liquid MMA (Methyl Methacrylate) is used along with an initiator. The liquid monomer is poured into a flat mold made up of two sheets of toughened glass which are sealed with a rubber gasket. It is then heated using hot baths or ovens for the monomer to polymerise. The PMMA sheet is removed from the mould.
This process produces sheets with good surface quality. These sheets can be easily fabricated and machined.
Extruded sheets
These sheets are made from PMMA resin and MMA. First, the process of polymerisation is carried out using the MMA. Then PMMA resins are fed into an extruder which melts the resins. The molten PMMA resins are pushed into a dye to take the form of a thin sheet. It is then cooled using rollers to get the PMMA sheet.
The sheet produced using this method cannot achieve the optical quality of the cast sheets. During the fabrication process of extruded sheets care has to be taken to use the right tools with accuracy. If required, cooling lubricant can be used.
Though these sheets provide an economical solution for thermoforming. They can be easily thermoformed to make acrylic parts.
Producing acrylic resins
The two main processes used to produce PMMA resins are the mass process and the suspension process. Resins are produced by the polymerisation of MMA.
In the mass process, an initiator is added to the MMA and heated. As it is heated it becomes more viscous and produces molecular masses. The polymer is then converted into granules.
In the suspension process, MMA is dispersed into water under controlled agitation. It is dried after centrifugation to get polymer beads.
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