Characteristic and Application of Common Plastics - Part 3
PMMA (Polymethyl Methacrylate)
Polymethyl methacrylate can be formed in various ways, including polymerization of acrylic acid with or without its esters and is commonly known as acrylic resins; among the other varieties, PMMA is most commonly used. PMMA is a popular synthetic transparent material and is also called acrylic glass; it has a harmonizing blend of quality and cost. PMMA can dissolve in organic solvents, including phenol and benzyl ether, and can obtain excellent quality thin films by spin coating. It has good dielectric properties and can be a dielectric layer in organic field-effect transistors.
PMMA is a polymeric material composed of methyl methacrylate, a high-molecular-weight polymer compound. It was developed early in the history of significant kinds of thermoplastic. Acrylic glasses include colorless clear or colored clear, pearl, tortured varieties of acrylic. It is known as various names: acrylic, perspex, and Plexiglas with the special features of a high degree of transparency, excellent chemical stability, good mechanical properties, and outstanding capacity in the capacity of weathering resistance. Easy processability and attractive appearance make PMMA an advanced, dyeable, easy-to-process plastic; it is also sometimes known as gel glass and its style as acrylic glass.
PMMA resin is non-toxic and environment-friendly, and it can produce tableware and sanitary ware; it has excellent chemical stability and weatherability. When the resin material is broken, the ground will not produce sharp fragments, which cause the US, Japan, and other countries to produce the law that the PMMA resin should be used in the building of schools and kindergartens. Through lots of street signs, advertising lightboxes, and telephone booths, urban construction has been notably speeded up all over the country. Green and eco-friendly, PMMA resin found wide uses in the outdoor colorful building materials of the Beijing Olympic projects.
Electrical Properties
Due to the polar ester groups along its backbone, the electrical performance of PMMA is not as good as the plastics which bear non-polar groups, such as polyolefins and polystyrenes. However, PMMA still has good dielectric and electrical insulation properties. It is worthwhile to note that PMMA and other acrylic plastics possess great arc resistance, hence they do not allow conductive lanes of arcing marks to be left on the surface. The secondary transition temperature is 20°C, corresponding to the temperature at which ester groups begin to reorient. Below 20°C the ester groups are frozen in place, so the material will now have better electrical performance than above 20°C.
Physical Properties
The mechanical properties of PMMA are really quite good and it is considered as one of the general plastics, besides the polyolefins, polystyrene, and polyvinyl chloride plastics. The tensile, bending, and compressive strengths are higher than polyolefins, polystyrene, and polyvinyl chloride plastics, although the impact toughness is relatively low and a bit better than that of polystyrene. Cast PMMA sheets, like those used in aviation-grade acrylic sheets, have better mechanical properties and approach those of engineering plastics like polyamide and polycarbonate.
Typically PMMA's tensile strength is of the order of 50-77 MPa, while its bending strength is of the order of 90-130 MPa, and the maximum values are as high as or higher than that of a number of engineering plastics. Its elongation at break is only of the order of 2%-3 per cent, showing that it is hard and brittle; and has notch sensitivity and cracks at stresses well below the tensile strengths. The fractures in PMMA, unlike in polystyrene and ordinary glass, are not sharp and highly irregular. Whereby the secondary transition temperature is about 40°C, where above that temperature toughness and ductility will improve. PMMA has low surface hardness and easily gets scratched.
The strength of PMMA decreases under constant stress, whereby PMMA with tensile orientation, or oriented acrylic glass, has the mechanical property in which it has improved very much; notch sensitivity also becomes low. Heat resistance in PMMA is not too high, with a glass temperature at 104°C, and the continuous use temperature ranges between 65 and 95°C, depending on working conditions. Heat distortion temperature is around 96°C at 1.18 MPa, the Vicat softening point is around 113°C. Its heat resistance could be improved by copolymerization with methyl methacrylate or diester methacrylic acid. Also, the cold resistance of PMMA is not very well. The brittle temperature is on the order of 9.2°C. The polymer possesses quite moderate thermal stability—better than that of polyvinyl chloride and polyoxymethylene but less good than that of polyolefins and polystyrene. The thermal decomposition temperature is slightly above 270°C, with a melting temperature of around 160°C, which allows a relatively wide range of melt processing temperatures.
The thermal conductivity and specific heat capacity of PMMA are moderate among plastics, 0.19 W/m·K and 1464 J/kg·K respectively.
Chemical Properties
Chemical Reagents and Solvent Resistance
PMMA has good resistance to dilute inorganic acids but is erodable by concentrated inorganic acids. It is alkali-resistant but hot sodium hydroxide and potassium hydroxide cause erosion. PMMA is insensitive to salts and fats; it is non-dissolvable in water, methanol, or glycerol, though it becomes swollen and cracked by alcohol. It shows good power of acceptance to ketones, chlorinated hydrocarbons, and aromatics. The solubility parameter is about 18.8 (J/cm³)¹/², and it dissolves in a large amount of chlorine hydrocarbons and aromatics, for instance, dichloroethane, trihloroethylene, chloroform, toluene, in acetic acid, and acetone.
PMMA is ozone– and sulfur dioxide–resistant.
Atmospheric Aging
PMMA exhibits great atmospheric aging. According to the data of naturally aged samples over a period of 4 years, there is a small decrease in weight, tensile strength, light transmission, and light yellow color and slight reduction in resistance to silver streaks and slight increase in impact strength, while other physical properties remain almost unchanged.
PMMA exhibits a very high flammability, with a limiting oxygen index of 17.3.
Optical Properties
- Visible Light: PMMA has a high visible light transmittance of up to 92%, making it the best currently available high molecular transparent material, with better light transmittance than glass.
- Ultraviolet Light: Quartz has 100% transmission of ultraviolet rays but is quite costly. Normal glass transmits only 0.6% of ultraviolet rays, while PMMA can transmit 73%. PMMA does not filter UV rays; thus, manufacturers sometimes coat the surface of PMMA to enhance UV filtering effects. However, under UV exposure, PMMA is more stable than polycarbonate.
- Infrared Light: PMMA allows the transmission of infrared light with wavelengths less than 2800 nm. In many cases, it will block wavelengths of infrared rays that are longer than 25,000 nm. Special colored PMMAs can transmit specific infrared wavelengths pitched while blocking visible light for using in applications like remote controls or thermal sensors.
PMMA—also known as acrylic glass—is highly transparent, low-priced, smoothly processed mechanically, and has therefore found vast applications in numerous areas. This is one of the most ancient plastic materials developed; it is also a rather important kind of thermoplastic material, with comprehensive applications in many fields, because of its fine transparency, stability, and resistance to aging and weather; its dyeing and processing are convenient, and it is beautiful in appearance.
Advantages
- Beautiful Appearance: The PMMA produces a mirror-like finish and exquisite craftsmanship without wrinkles or seams.
- Visual Effect: Available in numerous colors so that they are of powerful visual effect.
- Light Transmission: Holds capability to transmit light with a maximum of 96%, therefore emanating first-class light clarity and a soft, bright distinguished appearance.
- Impact Resistance: They are almost not fragile and are almost 200 times stronger than regular glasses.
- Durability: It effectively protects the internal light sources and extends their lifespan.
- Weather Resistance: They retain original color qualities with high quality which last over 6 to 13 years.
- Flame Resistance: Can hardly ignite and is self-extinguishing.
- Energy Efficiency: Perfect light transmission eliminates the need for additional sources of light, which are draining in terms of energy costs and the energy itself.
- Design Rationality: Open structure design of the lamp board ensures water resistance and moisture-proofing. It is easy and simple to clean and maintain.
Disadvantages
PMMA is brittle and can easily have crack defects along with low surface hardness; thus, it is very prone to scratching. Any small scratch or defect can be eliminated through the process of polishing.
Raw Material Characteristics
It appears colorless and is transparent, with a 90%–92% light transmittance. It is very tough and ten times tougher than silica glass. It has excellent optical, insulating, and processing as well as weather-resistant properties. PMMA is dissolvable in organic solvents, including but not limited to carbon tetrachloride, benzene, toluene, dichloroethane, trichlorometh. Transparent: It exhibits high clarity, insight, and light transmittance. Heat Resistance: This polymer has high heat resistance. The hardness and rigidity of the polymer are also high. Heat distortion temperature = 80°C, Bending strength = 110MPa, Density = 1.14-1.20g/cm³ Molding shrinkage rate = 0.2%-0.8%, Linear expansion coefficient = 0.00005-0.00009/°C. The heat deformation temperature is equal to 68-69°C (74-107°C).
Application
- Lighting and Illumination Equipment: E.g., house lamps, bulb protections, car backlights and indicators, and traffic signs.
- Lenses, Mirrors, Prisms: TV screens, Fresnel lenses, and light condensing in cameras.
- Instrument and Meter Housings: Panels, covers, dials, and escutcheons.
- Pre-Fabricated Optical Fibers.
- Display Windows and Advertisement Boards.
- Cockpit Windows: In airplanes and bulletproof panes for aircraft and wagons (treated thermally, especially, to increase impact strength).
- Medical, Surgical, Dental, or Veterinary Instruments and Devices.