Polymethyl methacrylate

General-purpose plastics General-purpose plastics refer to a class of plastic materials that are not included in the scope of general plastics and special plastics, and their application range is between general plastics and special plastics, the main varieties include polymethyl methacrylate (PMMA), polyurethane (PU), fluoroplastics and chlorinated polyethers. General-purpose plastics often have their own obvious characteristics, such as PMMA has good transparency, while fluoroplastics and chlorinated polyethers are ranked first and second respectively in corrosion resistance.
1. Polymethyl methacrylate
(1) Introduction to polymethyl methacrylate
Polymethyl methacrylate is commonly known as "plexiglass", and the English abbreviation is PMMA.
PMMA was first industrialized in 1932 by ICI in the United Kingdom, and large-scale production by Rohm & Huss in the United States in 1936, and is still the world's largest producer.
The biggest feature of PMMA is good transparency, with a light transmittance of 90%~92%, which is comparable to inorganic glass. In addition, it has good weather resistance, high surface hardness and excellent comprehensive properties, and is mainly used for optically transparent products.
Half of PMMA is used for casting and extruding sheets, and the rest is used for injection molded parts.
(2) Performance of polymethyl methacrylate
1. Optical properties PMMA is a highly transparent amorphous thermoplastic, which has excellent optical properties, with light transmittance of 90~92%, refractive index of 1.49, and can transmit most of the ultraviolet and infrared rays.
2. Mechanical properties PMMA is a light and tough material, which has excellent tensile strength, bending strength and compressive strength at room temperature, but the impact strength is average, and the notch is sensitive, the surface hardness is average, it is easy to scratch, the wear resistance is low, and the anti-silver grain ability is poor.
3. Thermal performance The oxygen index of PMMA is 17.3, which is a flammable material, and has the smell of flowers and fruits when burned, and the heat-resistant temperature is not high, and the long-term use temperature is only 80 °C.
4. Electrical properties Due to the large polarity in the molecule, its electrical properties are not as good as PE, and the dielectric constant is larger, so it is mainly used as a high-frequency insulating material.
5. Environmental performance PMMA has good weather resistance, and the performance degradation is very small when used outdoors for a long time.
The existence of ester groups in PMMA makes it generally resistant to solvents, only alkalis, dilute acids and water-soluble inorganic salts, long-chain alkanes, oils, alcohols and gasoline, etc., and is not resistant to aromatic hydrocarbons and chlorinated hydrocarbons, specifically carbon tetrachloride, benzene, xylene, dichloroethane and chloroform.
(3) Molding and processing of polymethyl methacrylate
1. Processing characteristics
PMMA is a non-Newtonian fluid, and the change in viscosity is mainly affected by the speed of the screw. The viscosity of the melt is higher than that of PE and PS, and the sensitivity to temperature is also higher than that of other non-Newtonian fluid plastics. The molding temperature is between 180~230 °C, and the processing temperature range is relatively narrow, and it will decompose if it exceeds 260 °C.
Before processing, it needs to be dried to make its water content less than 0.02%.
The melt viscosity of PMMA is large, and internal stress is easy to occur during molding. In order to obtain a product with high dimensional accuracy, annealing must be performed. The treatment conditions are: temperature 85°C, slow cooling is sufficient.
2. Processing method
PMMA can be processed by two methods: polymerization molding and plasticizing molding.
(1) Polymerization molding is mainly for casting molding, the liquid MMA monomer and catalyst are injected into the mold together, and the products of different thicknesses are kept warm for a certain time at an appropriate temperature (generally 40~60 °C), and slowly cooled.
Casting molding can be used to produce products such as flat plates, round bars and round tubes.
(2) Plasticizing molding Plasticizing molding includes injection molding, extrusion, thermoforming, etc.
(4) Application range of polymethyl methacrylate
1. Lighting and lighting are often used for lampshades and glass. Window panes and windshields on various vehicles such as airplanes, ships, and automobiles, as well as others such as instrument windows, display windows, advertising windows, lighting ceilings, etc.
2. Optical instruments All kinds of optical lenses such as glasses, magnifying glasses and lenses, etc., information transmission materials such as optical discs, optical fibers, etc.
3. Medical materials are used for dental materials such as dental trays, dentures and prosthetic materials.
4. Daily necessities All kinds of product models, specimens and arts and crafts, etc., all kinds of buttons, hairpins, children's toys, pen holders, drawing instruments, etc.