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What are thermoplastics and thermosetting plastics?

We have heard this question so many times. So really What are Thermoplastics and Thermosetting Plastics?

Thermosets are a group of polymers when melted undergo an irreversible reaction chemically and cannot be remelted for reuse. Once these materials are cured in the moulding process, they form a very high cross-linked 3-dimensional structure, hence they do not melt when reheated.
Thermosets are usually not used in their original form (resins in powder or liquid forms), they need to be mixed with other chemicals to be processed to form the solid part which upon curing does not melt even when heated. Depending on the specific requirements of the end-user, a resin is mixed with hardeners, plasticizers, reinforcement fillers, and curing agents to meet end requirements. So really what are thermoplastics & thermosetting plastics? Let us read on and find out.

How many types of Plastics are there?

Plastics can be classified into 2 basic forms-
1. Thermosetting plastic ( once formed in the required shape- thermoset plastic cannot be remelted, if reheated it will char and burn), Thermoset plastics usually cater to niche applications, mainly in heat resistance.
2. Thermoplastics can be remelted and reused.
Therefore, thermoplastics are much more versatile and useful than thermosets.

What are the different processes to mould thermoset plastics and thermoplastics?

There is a whole range of ways to mould plastics:

  • Compression Moulding is a commonly used process for thermoset plastics
  • Reaction Injection Moulding (RIM) is a process consisting of two-part reactive products, Polyurethanes (PU) this reactive process creates the internal pressure that fills the mould with the polyurethane plastic
  • Blow Moulding is a process where molten plastic is extruded in the form of a tube and trapped into a Mould and upon inflating this tube it fills into the shape of the mould. Typical applications range from simple water bottles, and water tanks to complex automotive fuel tanks etc.
  • Injection Moulding is the most common process in the plastic industry. This process is used for making articles for everyday used products like toothbrushes, plastic buckets, mugs etc to critical medical devices and automotive parts etc… Basically nearly everything is plastic. The injection moulding process forces molten plastic material under high pressure into a closed mould to form the part in the required shape, design and size.
  • Injection Stretch Blow Moulding– this process is a combination of injection moulding and blow moulding. In this process, the thermoplastic (commonly PET) is injection moulded to form a preform. This preform is then transferred to a blow moulding station which consists of multi-cavities of the required shape of the bottle. Here the heated preform is inflated, cooled and removed. The best example of this process is the commonly used carbonated drinks bottles and mineral water bottles.
  • Rotational Moulding is another process, where thermoplastic powders are loaded into the interior ( cavity) of a preheated mould that revolves on the end of an articulated arm and the powder sticks to the walls of the hot mould. Rotational moulding is mostly used for manufacturing large tubs, troughs and large containers, especially custom-made items. Rotational Moulding is a fairly primitive process that is great for small runs and small markets.
  • Expanded Polystyrene Moulding In this process, beads of polystyrene are treated with a blowing agent and using steam to expand many times( about 40 times) their original size and fuse together in a closed mould. Typical applications are the protective white blocks that are light in weight and are used for packaging electronic, medical and high-value articles.
  • Slush Moulding is where a liquid thermoplastic resin (usually Polyurethane) is poured into a hot hollow mould, here it forms a skin in contact with the walls of the cavity. The excess liquid is drained off and the cured part is removed from the mould. This is the preferred process by manufacturers as it gives greater freedom in the component design. Slush moulding is used to produce a variety of automotive interior components such as automotive door trim consoles, instrument panels, passenger airbag covers etc.
  • Dip Moulding is a process where a metal part is dipped into molten PVC which leaves a thin residue on the metal surface which is enough to make a cushioned handle grip. Typical examples are the soft feel handles on metal tools and other equipment.
  • Finally, other methods of processing plastics are, forming, which is different from moulding

What is thermoset plastic?

Thermosets are a group of polymers when melted undergo an irreversible reaction chemically and cannot be remelted for reuse. Once these materials are cured in the moulding process, they form a very high cross-linked 3-dimensional structure, hence they do not melt when reheated.

The thermosets are usually not used in their original form (resins in powder or liquid forms), they need to be mixed with other chemicals to be processed to form the solid part which upon curing does not melt even when heated. Depending on the specific requirements of the end-user, a resin is mixed with hardeners, plasticizers, reinforcement fillers, and curing agents to meet the end requirements.

What are the properties of thermosets?

Components made using thermosets, which have a highly crosslinked structure, due to which when heated to a high temperature, they do melt or lose their shape which is best suited for large and solid shaped components. They are light in weight, have good insulating properties, and dimensional stability, are highly rigid and do not deform under heavy loads. Components made using thermoset resin cannot be welded by gas, ultrasonic or laser. The best method is using adhesives or a mechanical fastener for areas where low strength of bonding is required.

Advantages of Thermosets

  • Thermosets are resistant to high temperatures.
  • Thermosets are highly flexible to design parts and have high levels of dimensional stability
  • Thermosets have the capability to be moulded in thick to thin wall components
  • Thermosets have an excellent aesthetic appearance
  • Thermosets are very cost-effective

What is an example of a thermoset plastic?

Thermosets are available as phenolic resins, amino resins, vinyl ester, polyester resin, polyurethane and silicone resins.

What are thermosets used for?

Thermoset plastics are used to manufacture construction equipment panels, electrical & electronic housings and components, electrical insulators, heat shields, circuit breakers and parts, motor components, and disc brake components.

What is thermoplastic?

Polymers contain linear or branched structures to hold the molecules together with a relatively weak intermolecular force, due to which the material softens when heated and returns to its original state when it is cooled. This process( reprocessing) can be repeated several times using post-processing techniques, but the original properties will be lost or altered to a great extent on repeated reuse. 

A polymer resin that becomes soft when it is heated and becomes hard when it is cooled is called Thermoplastic material. These thermoplastic materials can be recycled without any changes in their chemical properties. Any thermoplastic material when heated in a controlled manner, its physical properties change and it becomes a homogenized liquid which can be shaped into the required article using different moulds and thermoplastic processing machinery.

Where does Thermoplastic come from?

Man-made plastics are known as synthetic plastics. The main source of synthetic plastics is crude oil. Coal and natural gas are also used to produce plastics. These gases are broken down into monomers, which are chemical substances consisting of a single molecule and when thousands of these monomers are linked together by a process called Polymerisation, the compound is called a polymer (thermoplastic).

How does thermoplastic work?

A thermoplastic polymer is made up of many repeated units called monomers, that are weakly linked together into chains or branches via electrical bonds called Van Der Waals forces. These weak neutral molecules of a thermoplastic polymer are arranged in repeated strands that look like a mixed strand of pearls.

Whenever a thermoplastic polymer is heated in a controlled manner, it tends to become soft and pliable, which allows manufacturers to use various moulds and processing machinery to form it in the desired shape and size. Thermoplastic polymers can be repeatedly heated and cooled several times, which means they are highly recyclable materials.

Thermoplastics examples – Most commonly used thermoplastics

Low-Density Polyethylene (LDPE)

It is one of the most common thermoplastic materials used by many injection moulders, rotational moulders, and the extrusion industry.
The advantages of using LDPE is because of its low cost, very good chemical resistance, and good impact strength at low temperatures, It is FDA compliant and can be extruded as a very thin film for packaging purposes. It also offers very good processability.
LDPE also has the following limitations -they possess very low strength, stiffness, very poor UV resistance, susceptible to environmental stress cracking.
LDPE finds itself vastly used in the packaging sector for the food industry, squeeze bottles. Consumer goods and household articles, toys, agricultural liners & trash bags, hose pipes etc.

High-Density Polyethylene (HDPE)

Another commonly used thermoplastic material by many injection moulders and the extrusion industry.
The advantages of using HDPE are its low cost, excellent chemical resistance, low-temperature resistance, good electrical and insulating properties, very low water absorption and it is FDA compliant.
HDPE has few limitations, they possess very low strength, stiffness, very poor UV resistance, low heat resistance, and susceptibility to environmental stress cracking.
HDPE finds itself vastly used in the Packaging sector for crates, barrels, caps & closures, Bottles, Drums, etc. In the automotive sector, they are vastly used for blow moulded fuel tanks.
In the extrusion sector, they are used for manufacturing pipes & tubes and related fittings. Tapes and filaments for the woven industry and the agricultural industry.

Polypropylene (PP) Homopolymer and Copolymer

One of the most commonly used plastic materials by the industry for consumers and has few engineering applications. The advantages of polypropylene are its low cost and good chemical resistance to most inorganic acids, alkalis and salts. They possess good environmental stress cracking resistance when they come in contact with alcohols, esters, or polar hydrocarbons.

They have good mechanical properties at higher temperatures, and good electrical properties, They are FDA compliant and can be steam stabilised.
The limitations of PP are that they become brittle when exposed to temperatures below -20 deg C, and they have a low upper service temperature between 90*C – 120*C. They have very poor resistance to UV unless stabilised, they have limited impact resistance, low scratch resistance and poor paint adhesion.

Acrylonitrile Butadiene Styrene (ABS)

Acrylonitrile Butadiene Styrene (ABS) is used in a wide range of industries and applications, from automotive to piping. The advantages of ABS are its toughness and rigidity, good strength, impact resistance, and temperature resistance, good insulating properties. ABS can be easily moulded and provide a colourfast, glossy effect with a high-quality surface finish. ABS can be electroplated.
The limitations that ABS possesses are poor weathering resistance, and very poor resistance to solvents ( aromatic, ketones & esters). The common grades of ABS burn very easily and are not flame-retardant.

ABS does not have good chemical resistance, however, and should not be used in applications requiring electrical insulation or UV resistance.
ABS materials can be processed using any of the standard thermoplastic processing methods. It is mostly used in appliances, telephone handsets, computers and other office equipment housings. ABS can be used in musical instruments, medical device housings/ casings, motorcycle helmets, toys, enclosures for electronics and electronic assemblies, luggage, and interior and exterior automobile trims.

Polycarbonate (PC)

Thermoplastic polymers containing carbonate groups in their chemical structures are called Polycarbonates. Polycarbonate is an amorphous polymer and they exhibit outstanding strength, stiffness, impact resistance, and very good heat resistance and can be combined with flame retardant materials. Polycarbonate is thermally resistant up to 135°C. Polycarbonate being a good electrical insulator and having heat-resistant and flame-retardant properties, they are used in various products for the electrical and telecommunications hardware industry. Polycarbonate’s optical clarity makes it ideal for applications such as signboards, architectural glazing, face shields, skylights and various components for the automotive industry. Polycarbonate can be easily worked, moulded and thermoformed.

Polyamide (Nylon)

Nylon is a synthetic polymer, which means they have no basis in organic material like most thermoplastics. Nylons are semi-crystalline and are very tough materials, they also exhibit good heat resistance, chemical resistance and self-lubricating properties. Nylons, because of these properties, have replaced metal parts in many applications. For example, in the automotive industry, the air intake manifold usually manufactured by Aluminium casting has been successfully replaced by Nylon, as it is tough, lightweight, corrosion-resistant and has a smooth finish on the internal surface, unlike castings which have a very rough finish.

Nylon’s self-lubricating properties are useful for making bushings and different types of gears. In the power tools and electrical industry, because Nylon exhibits good electrical insulation, corrosion resistance and toughness properties, Nylon is a choice of manufacturers for parts requiring high loads. Electrical Insulators, Electrical Switch housings and power tool housing are a few examples of the use of Nylons.
In the consumer industry – the toothbrush bristles are a very good example of nylon being used in our day-to-day life. Kitchenware- like bowls, is another area where nylons are used.
Nylons can be processed by injection moulding, Rotational moulding, casting and extrusion.

What is a thermoplastic used for?

Thermoplastic materials are useful for a variety of applications, including consumer goods, machine and mechanical parts, medical equipment, packaging and storage materials etc.

Is Thermoplastic Strong?

Thermoplastics have very wide-ranging properties. Depending upon their chemistry they can be as soft as rubber, or as strong as aluminium.

Is Thermoplastic Safe?

Any thermoplastic material can be potentially or inherently toxic or safe depending on several factors. Burning of any thermoplastic produces toxic fumes including many harmful substances.

What is a good example of a common thermoplastic?

Low-density polyethene, High-density polyethene, Polypropylene, Nylons, ABS, and Polystyrene are examples of thermoplastic materials which are used in our day-to-day life.

Tooth Brush, Buckets & Mugs, Food containers, Mineral Water & aerated soft drink bottles, Crates, Plastic bags, Safety Helmets, mobile phone casings etc, are a few examples of Thermoplastics.

Thermoplastics – Commodity vs Engineering plastics

Commodity Plastics
Households and consumer items which are produced in large quantities for low-end applications, which do not demand any critical properties come under this category. The commodity plastic raw materials are low cost and used for various applications like – pails, buckets, furniture, household items, ropes, barrels, bags and covers etc. Few of the common commodity plastic raw materials used are – Low-Density Polyethylene(LDPE), High-Density Polyethylene(HDPE), Polypropylene(PP), Acrylonitrile Butadiene Styrene (ABS).

Engineering Plastics
This group of materials – Engineered thermoplastics to suit the needs of the industry. When compared to commodity plastics, they cater to demanding applications where mechanical and thermal properties are required to be met by the injection moulded components. Properties like heat resistance, Flame retardance, high mechanical strength, self-lubrication, etc are under this range of polymers. A few of the common Engineering plastic raw materials are Nylons ( Polyamides), Acetal -Polyoxymethylene (POM), Polycarbonate (PC) etc..and alloys of these materials.

What is the basic difference between thermoplastics and thermosets?

Thermoplastic vs thermoset

Once a thermoset plastic is moulded, it will always remain in a permanent solid state, it cannot be melted or reprocessed, whereas a thermoplastic can be remelted back into a liquid and formed into the same shape or a different shape.

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