Acrylic resins include thermosetting or thermoplastic plastic substances obtained from methacrylic acid, acrylic acid or other related compounds. These resins are extensively used in mobile telephones, television screens, furniture, in domestic paints and these are also mainly used in aquariums. It is an economical, effective and also a transparent material. At room temperature, these plastics are ductile and are derived from products such as acrylic acid obtained from a refining oil byproduct polypropylene.
An emulsified form of acrylic resin is polymethyl acrylate (PMA), which is used for textile finishes, lacquer, mixed with clay and adhesives to gloss paper. Polymethyl methacrylate (PMMA) is used to make hard plastics with a range of light transmitting properties.
Molecular formula - (C5O2H8)n
Density - 1.18 g/cm3
Melting Point – 160°C (320°F)
IUPAC ID - Poly(methyl 2-methylpropenoate)
Molecular Formula - (C4H6O2)n
Density - 1.22 g/cm3
Glass transition temperature – 6°C (43°F)
Chemical and Physical Properties of Acrylic Resins
Acrylics are clear and excellent light transmitters and are also colored easily. It is possible to find them in all shades including fluorescents. The surface luster of these materials is excellent. Acrylics are hard and weather resistant in spite of having a low softening point. Acrylics feature high scratch resistance and low moisture absorption characteristics.
Cleaning fluids and gasoline may damage acrylic however household alkalies and non-oxidizing acids do not impact them.
One of the most commonly used acrylic resin is PMMA and its properties are detailed below:
- PMMA features unique optical properties. It has a high transparency of around 92% light transmission in the visible range from 380 to 780 nm. On an inner surface, the angle of total reflection is 41 to 42° hence enabling its use in fiber optic filament, light conductors, etc.
- The vitreous transition temperature of PMMA is from 110 to 135 °C, it is an uncrystallized polymer and is rigid, hard and brittle at ambient temperature and has a low shock resistance.
- PMMA is hygroscopic and under extreme conditions, the absorbed water acts as a plasticizer that will change the material properties. It has a fairly limited creep.
- At a high critical stress PMMA, crazing may occur. PMMA is considerably scratch-resistant and can be polished easily
- PMMA is odorless, tasteless and can be used with food
- The electrical properties can be impacted sharply by humidity absorption. Arcing resistance is high.
- PMMA is combustible and burns without the production of a high amount of smoke. When molded, its retraction is less. Since its co-efficient of linear expansion differs from that of metals and it has a very low elasticity, metal inserts should not be used in an injected PMMA component.
- The maximum operating temperature is less than 80° C
- PMMA can be used for silk screening and vacuum applied metallization.
Another key acrylic resin is PMA and its properties are detailed below:
- PMA has a refractive index of 1.479
- PMA features good flexibility and adhesion
- It has an elongation of up to 750%
- Its tensile strength is 6.9MPa
- PMA is usually soluble in acetone, ethyl acetate, dichloroethane, xylene, etc.
- PMA cannot be dissolved in saturated hydrocarbons, ethanol, carbon tetrachloride, toluene, etc.
Manufacturing Process of Acrylic Resins
A monomer, for example, methyl methacrylate is made to react with a catalyst during the production of these acrylic resins. A typical catalyst is organic peroxide.
Three forms of acrylic plastics are available. These include elongated shapes such as tubes and rods, flat sheets as well as molding powder. A technique called suspension polymerization is used to make molding powders wherein the reaction occurs between tiny monomer droplets suspended in a solution of catalyst and water. Grains of polymer are formed that have a strictly controlled molecular weight suitable for extrusion or molding.
Bulk polymerization is used to form acrylic plastic sheets. Here, the catalyst and the monomer are poured into a mold in which the reaction is taking place. There are two types of bulk polymerization, which are continuous or batch cell.
Batch cell is very common as it is simple and can be used to make sheets of thickness 0.06 to 6 inches and widths from 3 ft to several hundred feet. Rods and tubes can be produced using the batch cell method. The continuous method is rapid and involves a less amount of labor. It is possible to make thin sheets with small widths using the batch cell method.
Applications of Virgin Material
Acrylic resin is used in the following applications:
- Used as the sole resin or an additive in solvent-based adhesives and water-borne coating systems
- Used as watch crystals, optical lenses, aircraft windshields, outdoor signs and sky lights
- PMMA can be cast to resemble marble, it is also used to make sinks, countertops, and other fixtures.
- Useful in clothing, blankets, carpets made from acrylic fibers, adhesives, contact lenses, dentures, floor waxes, food preparation equipment, disposable diapers, sanitary napkins, paints
- Acrylics can be used as a lens material. Also, they find applications in cameras, binoculars and eye glasses. They are also used in car headlights and stop lights. Acrylic plastics are used in dishes, lighting fixtures, carpets, floor waxes, piano keys, skylights and beverage dispensers.
Environmental Impacts of Acrylic Resin
The key cause for environmental problems due to acrylic plastics is that consumers do not dispose these properly. Plastic usage has helped develop a large number of disposable products that has increased the amount of disposed trash. Nearly 25% of waste is landfills comprise plastics. Acrylics or most plastics degrade very slowly hence enter the ocean and coastal ecosystems posing a threat to birds and marine creatures. Acrylic is obtained from fossil fuel and petroleum products that are considered to be non-eco friendly. It is possible to recycle certain forms of acrylic but not all. In typical usage conditions, off-gassing of acrylic does not occur and environmentally it functions just like glass.
Acrylic plastic is not recycled easily. Among recycled plastics, it is considered as a Group 7 plastic and mostly not collected for recycling. It is possible to form large pieces into useful objects in case they have not suffered crazing, stress or cracking. These are not readily biodegradable. Some of them are highly flammable and must be protected from combustion sources.
PMMA, the key type of acrylic, can be recycled in several ways. This normally involves subjecting the resin to pyrolysis. It is possible to recover the monomer from PMMA scrap by depolymerization. PMMA has been successfully depolymerized by contacting with molten lead resulting in MMA with a purity more than 98%.
From an environmental perspective, the use of lead is undesirable. Since it is very difficult to remove and then cannot be used afterwards. Hence new methods of recycling are being devised.
Applications of Recycled Acrylic
Recycled acrylic PMMA sheets are used in the following applications:
- Construction – soundproof windows and doors, windows, mining mask and telephone booths
- Medical: baby incubators, a specific variety of surgical medical devices
- Public goods: handicrafts, sanitary facilities, cosmetics, frame,
- Surface of instrument panel and cover
- Lighting: fluorescent lamps, chandeliers, street lamp
- Advertisement sector – signs, light boxes, signage, exhibition
- Transport: cars, trains, and other vehicles, doors and windows