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Polymethylmetacrylate (PMMA) - ACRYLIC GLASS

History

Acrylates are being called as of organic or acrylic glasses because of its parameters coming close to parameters of glass. This is where the business name came from - for hard and limpid panels and blocks produced from 1933 out of methylesters of the methylacrylic acid - produced by the Röhm & Haas company in Darmstadt under the name PLEXIGLAS that has spread all over the world. Today, the Czech name "plexisklo" is commonly used by the plastics public to describe acrylic materials, mostly polymethylmethacrylate (PMMA).

Advantages

Polymethylmethacrylate advantages are as follows:
- High hardness, rigidity and strength
- Homopolymers are fragile, copolymers are tough
- High quality of surface: high shine, scratch resistance, possibility of glazing
- Excellent optical characteristics, especially high clarity; copolymers are slightly faded to yellow
- High heat resistance
- Good electric and dielectric characteristics
- Resistant to weak acids and alkalis, non-polar solvents, fats, oils and water
- High weather-aging resistance
- Easy processing and machining

Disadvantages

Among disadvantages belong:
- Tendency to impact breaking
- Flammability

Copolymers

Polymethylmethacrylate is a fragile material. Use mixing or grafting polymerization with polybutadiene or other elastomers to suppress this characteristics and produce tough materials. However, this causes loosing of optical quality, and such PMMA becomes milkily clouded.

 

You can use copolymerization to build the acrylonitrile into the PMMA, and this way increase resistance to solvents. If the amount of acrylonitrile exceeds 50 %, the toughness increases at the most. Limpid products are faded to yellow because of creation of cyclic structures.

Production

Polymethylmethacrylate is produced using block or suspension polymerization of methylmethacrylate. Both ways lead to limpid materials. When using suspension polymerization, beads are created. Block polymerization is there to produce blocks and panels or other-way shaped semi-shapes. With rotation forms, we can manufacture pipes as well. Block polymerization leads to high values of polymerization grade. This improves mechanical characteristics of panels, and especially quality of their surface.

Design

Polymethylmethacrylate as a raw material for further thermoplastic processing is supplied as a granulate. It is limpid in its natural form, copolymers and modified types are clouded. It can be colored to beautiful transparent or opaque colors. Raw materials are supplied mostly as a panels or blocks, but also as pipes or different types of profiles.

Modulus of elasticity

PMMA excels mostly in high rigidity. Its modulus of elasticity lies between 2300 and 3300 MPa. With increasing temperature PMMA looses its rigidity like other thermoplastics. It means the modulus of elasticity decreases with increasing temperature; the higher the molar weight is, the slower the decrease of modulus of elasticity is. It is apparent, that the decrease of rigidity with increasing temperature is the slower, the higher the molar weight is.

 

Other mechanical characteristics

As with other thermoplastics, PMMA strength decreases with increasing temperature. The impact strength and impact value is not particularly high compared to other thermoplastics. As with polystyrene, it is nearly independent on temperature, i.e. PMMA keeps its fragility even at higher temperature. This arises from its low ductibility of 2 - 4 %. PMMA characteristics at dynamic fatigue is apparent from the Wöhler's curve of bending strength dependence on the count of cycles.

Thermal characteristics

PMMA glass transition temperature is 106 °C and up to 115 °C for cast intermediates. The Vicat B softening point depends on the value of mean molar weight and lies between 84 and 111 °C. This implies that PMMA belongs to thermoplastics with good thermal resistance. In a long term, the products can be permanently used up to temperatures of 70 - 100 °C.

Optical characteristics

PMMA excels in optical characteristics. It is colorless, limpid and absorbs nearly none of visible light spectrum. The light permeability is 92 % and the refractive index is 1.491. It is the most light-permeable plastic. The permeability can be preserved even when PMMA is additionally colored with transparent dyes. The surface glaze is excellent, too. The PMMA panels absorb UV light. Panel of only 2 mm of thickness can absorb all of the UV light.

Electrical characteristics

PMMA is good insulator for low frequencies. At high frequencies, its insulation properties are worse than for polyethylene and polystyrene. Dissipation factor and surface resistance doesn't change during common usage. That's why PMMA is used with advantage for electric-industry parts. From the other side, there is a surface charge being created on surfaces of PMMA parts, so often the anti-static additives must be used.

Chemical resistance

PMMA is resistant to weak acids and alkalis, salty solutions, aliphatic hydrocarbons, non-polar solvents fats, oils, water and detergents. It is attacked by strong acids and alkalis, benzene, polar solvents, ketones, esters, ethers, aromatic hydrocarbons and chlorohydrocarbons. It is prone to corrosion under tension. This decreases its usability at higher temperatures or when in long-term contact with solvents.

Weather resistance

Resists to the impact of both weather and UV light. The weather impact doesn't leave fades to yellow or loosing the surface glaze at any climatic condition. However, for long-term outer usage it is recommended to perform UV stabilization. It nearly doesn't absorb water.

Flammability

PMMA is very easily combustible. It burns with lucent flame even after the flame source has been removed. Combustion-produced gases have typically sweetish smell. Based on UL 94 standard it is classified as HB class. Self-ignition point is between 400-465 °C.

Food usage suitability

PMMA is suitable from the physiological point of view. It can be used for products becoming into contact with food or drinking water.

Thermal moulding

The PMMA panels can be easily thermal-moulded. Moulding temperatures are between 150 - 180 °C based on molar weight, for high molar-weighted types can rarely reach up to 200 °C.

Machining

PMMA can be easily cutting-operated. You can cut it, as well as mill, grind, drill and glaze. Cutting speed must be selected so as the material wouldn't smelt. You can cut it for example with a laser beam with advantage.

 

Surface finish

Polishing and printing can be performed even without prior surface finish. The printing durability is so high that it can be followed with thermal moulding. The products can be plated either with polishing or using vacuum evaporation.

Bonding

PMMA intermediates can be bonded with advantage, because it is polar, easily soluble plastic. Solvent, reactive, fusion and second adhesives can be used for sticking. The most suitable is to use solvent adhesives with low amount of dissolved PMMA, approximately 10-15 %. Solvent adhesives adjusted in such a way are called adhesive lacquers. Their advantage is that they don?t leave a vestige of sticking. The adhesive layer is highly transparent and the bonded place is nearly invisible. Excellent are adhesives based on acetone and chloroform. However, be aware these are highly flammable liquids, so the safety measures must be consistently kept.

Bonding with other plastics

You can bond PMMA with other plastics, too. Without any problems, you can bond PMMA with both crystalline and tough polystyrene, polycarbonates, cellulose derivatives, ABS terpolymers or SAN copolymers. Suitable solvents and adhesive lacquers are used to do it. Other plastics are bonded to PMMA using reactive and fusion adhesives. Today, there is a lot of suitable adhesives on the market. To make unfailing joint, bonded surfaces must be perfectly clean with any dust and dirt removed. As a cleaning agent, use either water with detergents or alcohols. Even the methylmethacrylate monomer is used to bond PMMA, that creates solid bonded joint using polymerization activated with chemical initiators or UV light.

Use

PMMA is suitable for all the applications taking advantage of its benefits, mostly optical, i.e. high transparency. It is used to produce transparent covers, light and display covers. It is very popular in automotive for production of tail light covers, tachometers or triangular reflectors. In optics it is used as a raw material for production of apparatus lens or eye-glasses. In consumer industry it is used to manufacture vases, bowls, costume jewellery, promotional pieces, advertising stools and stationery. PMMA is intensively used for production of acrylic sanitary products. Thermal moulding of panels is used to produce modern tubs, basins, shower baths, etc.

 

 

 

 




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