The introduction of Acrylic plate

Acrylic

Acrylic, also known as PMMA or Acrylic, is derived from the English acrylic (acrylic plastic). The chemical name is polymethyl methacrylate. It is an important plastic polymer material that has been developed earlier. It has good transparency, chemical stability and weather resistance, easy to dye, easy to process, and beautiful in appearance. It is used in the construction industry. Has a wide range of applications. Plexiglass products can usually be divided into cast plates, extruded plates and molding compounds.

use

Acrylic products include acrylic sheets, acrylic plastic pellets, acrylic light boxes, signboards, acrylic bathtubs, acrylic artificial marble, acrylic resin, acrylic (latex) paint, acrylic adhesives, etc., with a wide variety of products. Commonly seen acrylic products are acrylic products assembled from raw materials such as acrylic pellets, plates, or resins through various processing methods, combined with various parts of different materials and functions. As for the commonly heard acrylic fiber, acrylic cotton, acrylic yarn, acrylic nylon, etc., it refers to man-made fibers made by the polymerization of acrylic acid, and has nothing to do with acrylic products. Among them, the acrylic sheet that people often say is the polymethyl methacrylate Polymeric Methyl Methacrylate (PMMA) sheet, which is polymerized by "Methyl Methacrylate (MMA)". Or it is extruded from acrylic pellets through an extruder. In the past, the board was commonly called plexiglass. Acrylic is derived from the English acrylic, which means PMMA board made of organic compound MMA. Its transparency and light transmittance are like glass. Because all sheets made of transparent plastics such as PS, PC, etc. or inferior recycled MMA are collectively referred to as plexiglass. For the sake of distinction, PMMA panels made of high-quality pure MMA are called acrylic panels to distinguish them from ordinary plexiglass panels.

Types of plexiglass (acrylic) sheets

There are many types of acrylic sheets. Common boards include: transparent board, dyed transparent board, milk white board, color board; special board: bathroom board, cloud board, mirror board, cloth board, hollow board, impact board, flame retardant board, super wear-resistant board, Surface pattern board, frosted board, pearl board, metal effect board, etc. Different performance, different colors and visual effects to meet the ever-changing requirements.

1. Acrylic panels are divided into cast panels and extruded panels according to the production process. According to the transmittance, they can be divided into transparent panels, semi-transparent panels (including dyed transparent panels), and color panels (including black and white and color panels); according to performance Impact board, anti-ultraviolet board, ordinary board and special board such as high impact board, flame retardant board, frosted board, metal effect board, high wear-resistant board, light guide board, etc.

A: Casting plate: high molecular weight, excellent rigidity, strength and excellent chemical resistance. Therefore, it is more suitable for processing large-size identification plaques, and the time in the softening process is slightly longer. This kind of board is characterized by small batch processing, incomparable flexibility in color system and surface texture effect, and complete product specifications, which can be used for various special purposes.

B: Extruded plate: Compared with the cast plate, the extruded plate has a lower molecular weight, slightly weaker mechanical properties, and higher flexibility. However, this feature is conducive to bending and hot forming processing, and the softening time is shorter. When processing large-sized plates, it is beneficial to various rapid vacuum forming. At the same time, the thickness tolerance of the extruded plate is smaller than that of the cast plate. Because the extruded plate is mass-produced and automated, the colors and specifications are inconvenient to adjust, so the variety of product specifications is subject to certain restrictions.

2. There is another kind of acrylic recycled board called recycled acrylic scrap, which is thermally degraded to obtain recycled MMA (methyl methacrylate) monomer, which is then obtained after chemical polymerization. After a rigorous process, the pure MMA monomer can be re-obtained, and there is no difference in quality from the newly synthesized monomer. However, the purity of the produced degradable monomers is not high, and the quality and performance of the sheet are very poor after the sheet is formed.

Summary: The extruded plate uses granular raw materials, which are extruded after being dissolved at high temperature, while the cast plate is directly cast with MMA monomer (liquid). Although the extruded plate is relatively smooth and smooth in appearance, it is because it is formed when the granular raw material is formed. To complete the polymerization. When processed into plates, its structure and performance are weak, and it is not suitable as a material for outdoor marking products. It is only suitable for indoor products such as crystal letters or product brackets. In addition, since most of the extruded panels do not have the function of UV protection, their outdoor use life is not the same as that of the cast panels. The color will gradually fade and it is easy to become brittle until it breaks. The casting plate is to complete the structural polymerization during the processing of the plate, during which the ultraviolet absorber is added, which has extremely high strength and UV function. The outdoor service life is more than 5 years or even 10 years, and the color is always bright as new during use.

Process characteristics

1. Polymethyl methacrylate contains polar pendant methyl groups and has obvious hygroscopicity. The water absorption rate is generally 0.3%-0.4%. It must be dried before molding. The drying condition is 80℃-85℃ for 4-5h .

2. Polymethyl methacrylate has obvious non-Newtonian fluid characteristics in the temperature range of molding processing. The melt viscosity will decrease significantly with the increase of shear rate, and the melt viscosity is also very sensitive to temperature changes. Therefore, for the molding process of polymethyl methacrylate, increasing the molding pressure and temperature can significantly reduce the melt viscosity and achieve better fluidity.

3. The temperature at which polymethyl methacrylate starts to flow is about 160°C, and the temperature at which it starts to decompose is higher than 270°C, which has a wide processing temperature range.

4. Polymethyl methacrylate has a higher melt viscosity and a faster cooling rate, and the product is prone to internal stress. Therefore, the process conditions are strictly controlled during molding, and post-processing is also required after the product is molded.

5. Polymethyl methacrylate is an amorphous polymer, and the shrinkage rate and its variation range are small, generally about 0.5%-0.8%, which is conducive to forming plastic parts with high dimensional accuracy.

6. The cutting performance of polymethyl methacrylate is very good, and its profile can be easily machined into various required sizes.

Processing technology

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 product needs to be post-treated. The post-treatment conditions are heat preservation at 60°C for 2h and heat preservation at 120°C 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.

Injection molding products also need post-processing to eliminate internal stress. The processing is performed in a 70-80°C hot air circulating drying oven. 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, pipes, 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 molecular weight of the polymer. 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 vented extruder can be used for extrusion molding, and the ratio of screw length to diameter is generally 20-25. Table 2 is the typical process conditions of extrusion molding.

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 , To obtain 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 drafting or by directly pressurizing a convex mold with a profile. The thermoforming temperature can refer to the recommended temperature range in Table 3. When adopting 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. Normally, the normal temperature should be used.

Mainly for hollowing out and engraving formed acrylic or colored acrylic materials. Ordinary laser engraving and cutting machines can meet the engraving and hollowing needs of most acrylic products.

Acrylic (ACRYLIC), the common name of special treatment plexiglass. The research and development of acrylic has a history of more than one hundred years. The polymerizability of acrylic acid was discovered in 1872; the polymerizability of methacrylic acid was known in 1880; the synthesis method of propylene polypropionate was completed in 1901; the aforementioned synthetic method was used to try industrial production in 1927; the methyl ester industry was in 1937 The manufacturing development is successful, thus entering large-scale manufacturing. During the Second World War, because of its excellent toughness and light transmittance, acrylic was firstly used in the windshield of aircraft and the field of vision mirror in the cab of the tank driver. The birth of the world's first acrylic bathtub in 1948 marked a new milestone in the application of acrylic. .

1. It has crystal-like transparency, light transmittance above 92%, soft light, clear vision, and acrylic colored with dyes has a good color development effect.

2. Acrylic sheet has excellent weather resistance, high surface hardness and surface gloss, and good high temperature performance.

3. Acrylic sheet has good processing performance, which can be processed by thermoforming or mechanical processing.

4. Transparent acrylic sheet has a light transmittance comparable to glass, but the density is only half that of glass. In addition, it is not as fragile as glass, and even if it is broken, it will not form sharp fragments like glass.

5. The abrasion resistance of acrylic sheet is close to that of aluminum, it has good stability and is resistant to corrosion by various chemicals.

6. Acrylic sheet has good printability and sprayability. The use of appropriate printing and spraying techniques can give acrylic products an ideal surface decoration effect.

7. Flammability: It is not spontaneously combustible but is flammable, and does not have self-extinguishing property.

1. Hardness

Hardness is one of the parameters that best reflects the production process and technology of cast acrylic sheet, and it is an important part of quality control. The hardness can reflect the purity of the raw material PMMA, the weather resistance of the sheet and the high temperature resistance. The hardness directly affects whether the sheet will shrink and bend, and whether the surface will crack during processing. Hardness is one of the hard indexes for judging the quality of acrylic sheet, and the average Dallow's hardness value is about 8 or 9 degrees.

2. Thickness (Acrylic Tolerance)

Acrylic sheet thickness has acrylic tolerance, so the control of acrylic tolerance is an important manifestation of quality management and production technology. Acrylic production has an international standard ISO7823

Tolerance requirements for cast plates: Tolerance = ± (0.4 + 0.1 x thickness)

Tolerance requirements for extruded plates: Tolerance=< 3 mm Thickness: ± 10 %> 3 mm Thickness: ± 5 %3, strict transparency/whiteness raw material selection, advanced formula follow-up and modern production technology to ensure The sheet has excellent transparency and pure whiteness. Crystal clear after flame polishing.

Mechanical properties

Polymethyl methacrylate has good comprehensive mechanical properties and ranks in the forefront of general-purpose plastics. The tensile, bending, and compression strengths are all higher than polyolefins, and higher than polystyrene, polyvinyl chloride, etc., and its impact toughness is poor. But it is also slightly better than polystyrene. The cast bulk polymerized polymethyl methacrylate sheet (such as aerospace plexiglass sheet) has higher tensile, bending, and compression mechanical properties, and can reach the level of engineering plastics such as polyamide and polycarbonate.

Generally speaking, the tensile strength of polymethyl methacrylate can reach the level of 50-77MPa, and the bending strength can reach the level of 90-130MPa. The upper limit of these performance data has reached or even exceeded some engineering plastics. Its elongation at break is only

2%-3%, so the mechanical properties are basically hard and brittle plastics, and have notch sensitivity, and are easy to crack under stress, but the fracture is not as sharp and uneven as polystyrene and ordinary inorganic glass. 40℃ is a secondary transition temperature, which is equivalent to the temperature at which the pendant methyl group starts to move. Above 40℃, the toughness and ductility of the material will be improved. Polymethyl methacrylate has low surface hardness and is easy to scratch. The strength of polymethyl methacrylate is related to the stress time, and the strength decreases as the time increases. After stretching and orientation, the mechanical properties of polymethyl methacrylate (oriented plexiglass) have been significantly improved, and the notch sensitivity has also been improved.

The heat resistance of polymethyl methacrylate is not high. Although its glass transition temperature reaches 104°C, the maximum continuous use temperature varies between 65°C and 95°C depending on the working conditions, and the heat distortion temperature is about 96 ℃ (1.18MPa), Vicat softening point is about 113℃. The heat resistance can be improved by copolymerization of monomers with propylene methacrylate or ethylene glycol diester acrylate. The cold resistance of polymethyl methacrylate is also poor, with an embrittlement temperature of about 9.2°C. The thermal stability of polymethyl methacrylate is moderate, better than polyvinyl chloride and polyformaldehyde, but not as good as polyolefin and polystyrene. The thermal decomposition temperature is slightly higher than 270℃, and its flow temperature is about 160℃. There is a wide range of melt processing temperature.

The thermal conductivity and specific heat capacity of polymethyl methacrylate are in the middle level of plastics, which are 0.19W/CM.K and 1464J/Kg.K, respectively.

Electrical properties

Since polymethyl methacrylate contains polar methyl ester groups in the side of the main chain, its electrical properties are not as good as those of non-polar plastics such as polyolefin and polystyrene. The polarity of the methyl ester group is not too great, and polymethyl methacrylate still has good dielectric and electrical insulation properties. It is worth pointing out that polymethyl methacrylate and even the entire acrylic plastics have excellent arc resistance. Under the action of an arc, the surface will not produce carbonized conductive paths and arc tracks. 20°C is a secondary transition temperature, which corresponds to the temperature at which the pendant methyl ester groups start to move. Below 20°C, the pendant methyl ester groups are in a frozen state, and the electrical properties of the material will be improved compared to those above 20°C.

Solvent resistance

Polymethyl methacrylate can withstand relatively dilute inorganic acids, but concentrated inorganic acids can make it corroded, and it can be resistant to alkalis, but warm sodium hydroxide and potassium hydroxide can make it corrode and can be salt-resistant And grease, resistant to aliphatic hydrocarbons, insoluble in water, methanol, glycerin, etc., but can absorb alcohol and swell and cause stress cracking. It is not resistant to ketones, chlorinated hydrocarbons and aromatic hydrocarbons. Its solubility parameter is about 18.8 (J/CM3) 1/2, and it can be dissolved in many chlorinated hydrocarbons and aromatic hydrocarbons, such as dichloroethane, trichloroethylene, chloroform, toluene, etc., vinyl acetate and acetone can also make it Dissolve.

Polymethyl methacrylate has good resistance to gases such as ozone and sulfur dioxide.

Weather resistance

Polymethyl methacrylate has excellent atmospheric aging resistance. After 4 years of natural aging test, the sample has a slight decrease in tensile strength and light transmittance, a slight yellowing in color, and a decrease in crazing resistance. Obviously, the impact strength has been slightly improved, and other physical properties have hardly changed.

Combustibility

Polymethyl methacrylate is easy to burn, and its limiting oxygen index is only 17.3.

The bonding of acrylic products is a very critical process in acrylic processing. How to show the clear and transparent characteristics of plexiglass, reflect the value of acrylic tobacco and alcohol packaging crafts, and maximize the grade and taste of acrylic crafts, bonding technology Played a pivotal role.