What is ABS plastic - Acrylonitrile, Butadiene, Styrene Plastic

What is ABS Plastic?

ABS is a synthetic thermoplastic resin that is made of three-dimensional monomers which are acrylonitrile, butadiene, and styrene. These monomers have different properties due to their chemical structure.

ABS is a two-phase polymer blend. A continuous phase of styrene-acrylonitrile copolymer provides the materials -

• Rigidity
• Hardness
• Heat resistance.

The toughness of ABS is the result of sub-microscopically fine poly-butadiene rubber-grafted polymer particles uniformly distributed in the SAN matrix.

Acrylonitrile has an acrylic group (CN). This group can form strong Hydrogen bonds. So, it gives strength to the ABS polymer. The bulky group in the styrene monomer gives rigidity. Butadiene monomer has a C=C bond; so, it gives flexibility. And also, it can absorb external impacts.

The properties of the final polymer can vary, by varying the proportions of the three components. Generally, 15% to 35% of acrylonitrile, 5% to 30% of butadiene, and 40% to 60% of styrene are mixed together.

Manufacturing processes of ABS plastic

Mainly there are three manufacturing processes for ABS plastic. These are blending, co-coagulation and copolymerization.

01. Blending

Styrene-butadiene rubber (SBR) and Styrene acrylonitrile (SAN) are blended together by specific blending machines. Generally, the twin-screw extruder is used to blend SBR and SAN. But sometimes SAN and SBR can be mixed in an internal mixture first and then mixed by using a single screw extruder.

02. Co-coagulation

SAN and polybutadiene are mixed together in a latex state. Then the mixture is let to be coagulated. This process is called co-coagulation. After drying the coagulum, ABS granules are made.

3. Co-polymerization

Co-polymerization for the production of ABS resins can be largely divided into two processes. They are listed below.

• the mass polymerization process
• the emulsion polymerization process

Mass polymerization or bulk polymerization is the process that is carried out by mixing undiluted acrylonitrile peroxide monomer with styrene rubber. The free radical initiator which is dissolved in the monomer phase is used as the initiator. The polymerization process is occurring in a special chamber. After the devolatilization process, ABS polymer will be pelletized.

Emulsion polymerization is the process in which polymerization occurs in small droplets which are called emulsions. Generally, water is used as the continuous phase. Polybutadiene latex, styrene monomer, and acrylonitrile monomer are dispersed in water. An initiator that is dissolved in the monomer is used. monomers are dispersed in the continuous phase by mechanical agitation. To stabilize the monomer droplets surfactant is added.

Properties of ABS

ABS polymer is a graft polymer. Polyacrylonitrile (PAN), Polystyrene (PS), and Styrene acrylonitrile (SAN) chains are grafted to the polybutadiene (PB) backbone.

Chemical properties

ABS show excellent resistance to diluted acid, diluted alkalis, oils, and greases. There is moderate resistance to aliphatic hydrocarbons. ABS shows poor resistance to aromatic hydrocarbons, halogenated hydrocarbons, and alcohols.

Physical properties

The reasons for the widespread acceptance of ABS are listed below.

• high impact resistance
• good stiffness
• excellent surface quality
• high dimensional stability at elevated temperatures

The main disadvantages of ABS are listed below.

• lack of transparency,
• poor weathering resistance
• poor flame resistance

Processing of ABS

ABS is processed into end products by most thermoplastic processing methods such as injection molding, extrusion, blow molding, calendaring, and vacuum forming.

ABS may absorb up to 0.3% moisture and therefore it must be stored under dry conditions.

There is a greater tendency to degradation than Polystyrene (PS) during processing. So, it is very important not to overheat. And avoid too high screw speed and back pressure during molding.

Being an amorphous material, ABS has a low molding shrinkage.

01. Injection molding

In both protective and decorative applications productions, molded ABS products are used.

Examples:-

• safety helmets
• communications equipment
• camper tops
• automotive instrument panels and other interior components
• pipe fittings
• business machines
• home-security devices and housings for small appliances

Typical Injection molding conditions for ABS are,

• Recommended melt temperature range for ABS - 220-280 °C
• The typical mold temperature for ABS is 40–90 °C
• Injection pressures range for ABS - 69-138 MPa
• L/D (Length to diameter ratio) ratio - 20:1
• Compression ratio - 2:1 to 3:1

02. Extrusion

Some of the Extruded shapes are listed below.

• weather seals
• glass beading
• refrigerator breaker strips
• conduit
• pipe for drain-waste-vent (DWV) systems

Some of the main, largest application areas Of ABS plastics are Pipe and fittings manufacturing.

Typical extrusion conditions for ABS are,

• Melt temperatures 205-250°C.
• Temperature profile 192-250°C.
• Recommended screw L/D ratio 20:1 to 36:1.
• Recommended compression ratio 2.5:1 to 3:1.

03. Thermoforming / Vacuum forming

Over a temperature range of 130(403.15K) to 190°C (463.15K), ABS can be thermoformed. The optimum conditions depend on the below-mentioned factors.

• material grade
• part of design
• draw ratio
• sheet thickness
• forming technique

Typical vacuum-formed products from extruded ABS sheets are listed below.

• refrigerator liners
• luggage shells
• tote trays
• mower shrouds
• boat hulls
• large components for recreational vehicles