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Polymethyl methacrylate (PMMA), also known as acrylic or acrylic glass, is a thermoplastic synthesized by polymerization of methyl methacrylate. Following the discovery of methacrylic acid in 1865, the initial interest in the polymerized products was ignited by Otto Röhm’s 1901 doctoral thesis. Eventually, by the 1930s, concurrent development of polymethyl methacrylate took place in England by Rowland Hill and John Crawford (Imperial Chemical Industries), in Germany by Otto Röhm (Rohm and Haas AG), and in the United States by the DuPont Company (Bauer, 2000). Their products are available under the trademark names Perspex, Plexiglas, and Lucite, respectively. By the end of World War II, PMMA was commercialized for civilian markets.
Acrylic is often used as a cost-effective alternative to polycarbonate or used in conjunction to reap the benefits of both the materials. In comparison to glass, both the materials provide superior properties while being relatively cheaper. The use of acrylic or polycarbonate boils down to the application and the cost. In general, most benefits associated with polycarbonate can also be found in acrylic. Acrylic tends to have better UV resistance and does not turn yellow over time as opposed to polycarbonate. Further, it does not contain the potentially harmful compound bisphenol-A which is often found in the latter.
Physical & Chemical Properties
|Melting Point||:||160 °C (320 °F)|
|Tensile Strength||:||9400 PSI|
|Magnetic Susceptibility||:||−9.06×10−6 (SI, 22 °C)|
|Ignition Temperature||:||460 °C (860 °F)|
|Heat Deflection Temperature||:||95 °C (203 °F) at 0.46 MPa (66 PSI)|
|Water Absorption Ratio||:||Maximum of 0.3 to 0.4% by weight|
|Refractive Index||:||1.4905 at 589.3 nm|
|Young’s Modulus||:||3200 N/mm2|
|Hardness||:||M80 to M100|
|Chemical Resistance||:||Good resistance to aqueous solutions of most laboratory chemicals, detergents, cleaners, dilute inorganic acids, alkalis, and aliphatic hydrocarbons|
|Optical Properties||:||Clear, almost glass-like appearance|
|Other||:||Polishable, UV tolerant, brittle under load, prone to scratching, inert, rigid|
In comparison to the production of other plastics, such as polypropylene, acrylic production requires many intermediate stages to obtain methyl methacrylate for polymerization. The most common method for production of methyl methacrylate is by reacting acetone with sodium cyanide. The product of the reaction, acetone cyanohydrin, is then reacted with methyl alcohol to obtain the main monomer for acrylic production, methyl methacrylate. Acrylic manufacturing can also make use of the copolymerization process using monomers such as methyl acrylate, and acrylonitrile to produce acrylic plastic with different properties.
The manufacturing process for acrylic can either use emulsion polymerization, suspension polymerization, solution polymerization, or bulk polymerization. The underlying process involves radical chain polymerization of methyl methacrylic usually along with a comonomer. The bulk polymerization methods (batch cell and continuous) are used for producing sheets and rods or tubes of acrylic, while a suspension or solution polymerization yields acrylic powders and beads. Production of emulsion acrylic is done via emulsion polymerization. Anionic polymerization approach can also be used for the production of acrylic.
The processing conditions and the catalysts used to enable the production of polypropylene with desirable properties suitable for specific applications. Property variations can be achieved by introducing additives or varying the crystallinity in the polymer.
In general, two categories of polypropylene, homopolymers, and copolymers, are available commercially. Polypropylene homopolymer consists of only the monomer propylene in the polymer chain while the copolymer version typically uses the comonomer ethylene. The copolymer category is further divided based on the arrangement of the comonomer. The block copolymer polypropylene, typically, contains 5 to 15 % ethylene comonomer units which are arranged in regular patterns. On the other hand, the random copolymer grade has an irregular, random arrangement of the ethylene units (copolymer level is generally 1 to 7%). In comparison to the general-purpose grade homopolymer polypropylene, the copolymer category polypropylene offers many advantages which can include impact resistance, more flexibility, and enhanced clarity. Another grade of polypropylene, the impact copolymer, are usually formed by the addition of ethylene-propylene rubber to homopolymers or random copolymers (copolymer level range from 5 to 25 %) to produce a product with increased low-temperature impact strength. Impact copolymers can also be formed using ethylene-propylene-diene, polyethylene, or plastomers.
Based on tacticity, polypropylene can exist as isotactic, syndiotactic and atactic. Isotactic polypropylene is what is generally available commercially. Isotactic polypropylene has a high crystallinity that results in desirable physical, mechanical and thermal properties in its solid state. On the other hand, atactic polypropylene has low crystallinity that results in an amorphous polypropylene generally used as sealants, caulks and for modifying rubber, asphalt, polyethylene, and bitumen. Lastly, the syndiotactic polypropylene, in comparison to its counterparts exhibits excellent electrical, thermal and mechanical properties due to its low crystallinity, smaller spherulites, different crystal lattice and less residual catalysts (Yoshino et al., n.d.).
Commercially available acrylic, produced using the radical polymerization method, is atactic and completely amorphous. Acrylic can be modified to meet the requirements of different applications. The modification of the properties can be done by varying the amount of the comonomer, or by introducing additive and fillers.
Comonomers often used in shaping the properties of the final product include acrylate (stabilize the polymer), butyl acrylate (improve impact strength) and methacrylic (increase glass transition temperature). Additives such as plasticizers are added to improve the processing properties of the acrylic and its impact properties. Plasticizers may also be used to lower the glass transition temperature of acrylic. Dyes are also added to the clear product to produce acrylic of different colors and to improve its UV resistance further.
The durable, hard and rigid nature of acrylic, among other properties, makes it useful for construction of equipment and apparatuses such as incubators and most of the preclinical research mazes. Acrylic is also biocompatible and easy to manipulate, thus having many applications in the medical scenes.
Acrylic has seen extensive usage in the dentistry domain, often used for denture bases and dental implants. Its low modulus of elasticity, thermal and electrical passiveness, and porosity are similar to human dentine. Additionally, it has good teeth adhesion, is insoluble in body fluids, and has good aesthetic appearance and color stability thus making it apt for dental applications (Leigh, 1975; Frazer, Byron, Osborne, & West, 2005). However, concerns regarding toxicity of the material do exist (Chaves, Machado, Vergani, de Souza, & Giampaolo, 2012).
The transparent nature of acrylic makes it useful in making eyeglass lenses and hard contact lenses. The first intraocular lens made of polymethyl methacrylate was implanted in the year 1949 by surgeon Harold Ridley at the St Thomas’s Hospital, London (Apple, 2006). Despite the initial skepticism towards these lenses, development in this area continued resulting in the modern-day lenses.
Bone cement is yet another application of the acrylic. However, unlike what the name suggests, bone cement functions as a grout, essentially filling up spaces between the implant and the bone in an orthopedic surgery rather than functioning as glue. The first use of the material is credited to surgeon John Charnley, who used it in performing total hip arthroplasty in 1958 (Vaishya, Chauhan, & Vaish, 2013). The application also extends to cosmetic surgery (Mandlik et al., 2015; Antenello & Maas, 2015).
Acrylic is also used in the field of biomedical research and bioprocess chromatography.
Strengths & Limitations
- Cost-effective and light-weight alternative to glass
- Doesn’t contain BPA as opposed to polycarbonate
- Excellent optical properties
- Good resistance to UV
- Excellent resistance to weathering aging
- Good resistance to abrasions and scratching
- Completely recyclable
- Limited heat resistance
- Limited chemical resistance
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Mandlik, D., Gupta, K., Patel, D., Patel, P., Toprani, R., & Patel. K. (2015). Use of Polymethyl Methacrylate-Based Cement for Cosmetic Correction of Donor-Site Defect following Transposition of Temporalis Myofascial Flap and Evaluation of Results after Adjuvant Radiotherapy. Journal of reconstructive microsurgery, 31(9):668-73. doi: 10.1055/s-0035-1558988.
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