Acrylic plastic polymers are formed by fusing monomers (methyl metharcrylate) with a catalyst. Catalyst normally can be found in forms like organic peroxide. The catalyst starts the reaction and enters into monomers to keep the process on going, but does not become a part of it in the end product.
Acrylic products can be found in three common forms: flat sheets, elongated tubes, and powders. Powders are commonly used in industry for molding purposes. They are manufactured by the process called suspension polymerization, in which the reaction takes place between small tiny droplets of monomer suspended in solution (mixture of catalyst and water). The result is grains of polymer with tightly controlled molecular weight, very suitable for molding and extrusion.
Acrylic plastic sheets are formed by the process known as bulk polymerization. In this process, the monomer and catalyst are poured into a mold where the whole reaction will be taking place in. 2 methods of bulk polymerization may be used, let it be batch cell or continuous. The most cost effective method is batch cell, adapted to manufacture acrylic sheets in thickness from 0.06 to 6 inches. Batch cells can also be used to make rods and tubes. The continuous method is relatively more expensive to implement, utilizing higher end equipments and less labour. They are used to make thinner sheets of acrylics. Refer to the diagram below for a better understanding.
Batch Cell Bulk Polymerization
- The mold for producing acrylic sheets is assembled from 2 plates of polished glass. They are then separated by a flexible window frame spacer. This spacer is sitted along the outer perimeter of the surface, to form a sealed cavity in between. The spacer is actually flexible to allow the mold cavity some tolerance to shrink during the polymerization process. This is essential to compensate for the volume contraction of the materials to go from molecules to linked polymers. In certain manufacturing house, they make use of metal plates instead of glass plates.
- The open corner of the mold cavity is filled with a pre measured solution of methyl methacrylate monomer and catalyst. The prepolymer solution is partially formed polymer chain to help the polymerization process to react faster. The solution flows through the cavity to fill it. The the mold is sealed and heat is applied to help the polymerization process. The process goes into automation as it will generate heat my itself. Then, to cool down the acrylic, the heat is fanned off in air ovens and water bath. Then it goes through acuring process that involves vaporization in several hours.
- Excess materials around the edges will be trimmed off. Masking tapes will then be applied to the surface of all finished acrylic sheets for protection purposes to deal with accidental damages during shipping and handling. The paper or film is often marked with the material’s brand name, size, and handling instructions. Conformance with applicable safety or building code standards is also noted
Acrylic Sheets Quality Control
Acrylic Sheets need to be stored and handled with care to prevent contamination of the material, or unsafe chemical reactions. Temperature control is especially crucial in the polymerization process because they might be chemically unstable under different temperatures. During the entire process, the temperature of the reacting material is monitored and controlled to ensure the heating and cooling cycles are the proper temperature and duration.