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Polymethyl methacrylate, polymers obtained by polymerization of acrylic acid and its esters are collectively referred to as acrylic resins, and corresponding plastics are collectively referred to as polyacrylic plastics. Among them, polymethyl methacrylate is the most widely used. The abbreviation code of polymethyl methacrylate is PMMA, commonly known as plexiglass. It is the most excellent and reasonably priced variety among synthetic transparent materials so far.
Polymethylmethacrylate (PMMA for short, Acrylic in English), also known as acrylic or plexiglass, is often called agaric in Hong Kong. The number average molecular weight of its cast polymer is generally 2.2× 104, the relative density is 1.19 to 1.20, the refractive index is 1.482 to 1.521, the moisture absorption is below 0.5%, and the glass transition temperature is 105°C. It has the advantages of high transparency, low price, easy machining, etc. It is a commonly used glass substitute material. Novel design, advanced technology, elegant color, beautiful appearance. PMMA is lighter in weight and has a lower density than glass: PMMA has a density of about 1150-1190 kg/m3, which is half of glass (2400-2800 kg/m3). The weight of a material of the same size is only half that of ordinary glass, and 43% of metallic aluminum (belonging to light metal). The mechanical strength of PMMA is relatively high: the relative molecular weight of plexiglass is about 2 million, it is a long-chain polymer compound, and the chain forming the molecule is very soft. Therefore, the strength of plexiglass is relatively high, tensile and impact resistant The capacity is 7-18 times higher than ordinary glass. The tensile strength is 6-7 kgf/mm2, and the compressive strength is 12-14 kgf/mm2. The impact resistance is better than that of polystyrene; it also has the characteristic of not being easily broken. There is a kind of plexiglass that has been heated and stretched, in which the molecular chains are arranged in a very orderly manner, which significantly improves the toughness of the material. Use nails to penetrate the plexiglass, even if the nail penetrates, the plexiglass No cracks are produced, and the plexiglass will not break into fragments after being broken down by bullets. Therefore, the stretched plexiglass can be used as bulletproof glass and also as a cockpit cover on military aircraft.
Visible light: high visible light transmittance: PMMA is currently the best polymer transparent material, with a visible light transmittance of 92%, which is higher than that of glass . Ultraviolet light: Quartz can completely transmit ultraviolet light, but it is expensive. Ordinary glass can only transmit 0.6% of ultraviolet light, but PMMA can transmit 73%. PMMA cannot filter out ultraviolet (UV). Ultraviolet light can penetrate PMMA, and some manufacturers coat the surface of PMMA to increase the effect and properties of UV light filtering. On the other hand, PMMA has better stability when irradiated with ultraviolet light than polycarbonate. Infrared: PMMA allows infrared rays with a wavelength of less than 2800nm to pass. Longer wavelength IR, less than 25,000nm, can basically be blocked. There is a special colored PMMA, which can allow specific wavelengths of IR to pass through, while blocking visible light (used in remote control or thermal induction, etc.).
The monomer of polymethyl methacrylate is methyl methacrylate, which is a colorless liquid with fragrance. It has a boiling point of 101°C and a density of 0.940 g/cm3 (25°C). Industrially, methacrylic acid is prepared by acetone cyanohydrin method or isobutylene catalytic oxidation method, and then obtained by esterification. It is easy to polymerize and needs to be stored below 5°C, or add about 0.01% hydroquinone polymerization inhibitor for storage. Distill it before use to separate out the polymerization inhibitor. Polymethyl methacrylate can be dissolved in organic solvents such as its own monomer, chloroform, acetic acid, ethyl acetate, acetone, etc. Because it can be dissolved in its own monomer, its bulk polymer is very transparent (see bulk polymerization).
It adopts new type polyester material, which is formed by hot bending or flat edge, and the light source is built in the metal support, which has great visual impact. Weather resistance: The panel is coated with high-concentration ultraviolet absorber, and the metal base is sprayed with imported automobile paint. Acrylic has high transparency. Acrylic has high transparency, can maintain long-term weather resistance, never fade, and has a service life of 5-8 years. Durability: The product has good protection for the built-in light source and prolongs the service life of the light source product. Reasonable: Reasonable design, rainproof and moisture-proof; open structure, easy maintenance. Impact resistance: 200 times that of glass products, with almost no risk of breaking. Light transmittance: up to 93%, excellent light transmittance, soft light, and dazzling. Flammability: non-flammable, can continue to burn after leaving the fire. Aesthetics: exquisite craftsmanship, all fonts have a mirror effect, the base has no wrinkles, no seams, and all rivets are not exposed. Energy saving: good light transmission performance, correspondingly reducing light source products, saving electricity and reducing use costs.
Characteristics of raw materials
It is colorless and transparent, with a light transmittance of 90%---92%, strong toughness, more than 10 times larger than silica glass.
Good optics, insulation, processability and weather resistance.
Dissolved in organic solvents such as carbon tetrachloride, benzene, toluene dichloroethane, chloroform and acetone.
It has high transparency and brightness, good heat resistance, toughness, rigidity, and rigidity. The heat distortion temperature is 80℃ and the bending strength is 110Mpa.
The density is 1.14-1.20g/cc, the deformation temperature is 76-116℃, and the molding shrinkage rate is 0.2-0.8%.
Coefficient of linear expansion 0.00005—0.00009/℃, heat distortion temperature 68--69℃ (74--107℃).
Polymethyl methacrylate can be cast, injection, extrusion, thermoforming and other processes.
Casting is used to form plexiglass plates, bars and other profiles, that is, the profiles are formed by bulk polymerization. After casting, the products need to be post-processed, and the post-processing conditions are heat preservation at 60℃ for 2h and at 120℃ for 2h
Injection molding uses pellets made by suspension polymerization and molding on a common plunger or screw injection molding machine. Table 1 shows the typical process conditions of polymethyl methacrylate injection molding. Process parameters Screw injection molding machine plunger injection molding machine barrel ℃ temperature rear 180-200 180-200 middle 190-230 front 180-210 210-240 nozzle temperature ℃ 180-210 210-240 mold temperature ℃ 40-80 40-80 plexiglass plexiglass injection pressure MPa 80-120 80-130 holding pressure MPa 40-60 40-60 screw speed rp.m-1 20-30 injection molded products also need post-processing to eliminate internal stress, processing at 70- It is carried out in a hot air circulation drying box at 80°C. The processing time depends on the thickness of the product, and generally takes about 4 hours.
Polymethyl methacrylate can also be extruded, and plexiglass plates, rods, tubes, sheets, etc. can be prepared from particles produced by suspension polymerization. However, the profiles prepared in this way, especially plates, have a small molecular weight due to the low polymer molecular weight. The mechanical properties, heat resistance, and solvent resistance are not as good as cast-molded profiles, and its advantage is high production efficiency, especially for pipes and other molds when the casting method is used. Difficult to manufacture profiles. Single-stage or double-stage exhaust extruder can be used for extrusion molding, and the ratio of screw length to diameter is generally 20-25. Table 2 shows the typical process conditions of extrusion molding. Process parameters Sheet and rod screw compression ratio 2 2 Barrel ℃ temperature rear part 150-180 150-180 middle part 170-200 170-200 front part 170-230 170-200 extrusion pressure MPa 2. 8-12. 4 0 ．7-3.4 Temperature of the feed port ℃ 50-80 50-80 Die temperature ℃ 180-200 170-190
Thermoforming is the process of making plexiglass plates or sheets into products of various sizes and shapes. The blank cut to the required size is clamped on the mold frame, heated to soften it, and then pressurized to make it close to the mold surface , Get the same shape as the molding surface, and trim the edge after cooling and shaping to obtain the product. Pressurization can be done by vacuum drawing or directly pressurizing a convex mold with a profile. The thermoforming temperature can refer to the recommended temperature range in Table 3. When using rapid vacuum low-draft molding products, the temperature should be close to the lower limit, and the deep drawing products with complex shapes should be close to the upper limit temperature. Normal temperature is generally used.
As a transparent material with excellent properties, polymethyl methacrylate is widely used in the following aspects: 1. Lamps, lighting equipment, such as various household lamps, fluorescent lampshades, car tail lights, signal lights, road signs.
2. Optical glass, such as manufacturing various lenses, mirrors, prisms, TV screens, Fresnel lenses, and camera translucent lenses.
3. Prepare dials, covers and dials of various instruments and meters.
4. Prepare optical fiber.
5. Commercial advertising window, billboard.
6. Aircraft cabin glass, bulletproof glass for aircraft and automobiles (with interlayer material required). 7. Various medical, military and architectural glass.