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What Are Thermoplastics: Material Types and Processes

By sensXPERT

What Are Thermoplastics: Material Types and Processes

Thermoplastics - What are thermoplastics?

Thermoplastics are integral in various industries due to their versatility, recyclability, and adaptability. From the intricate components within automobiles to life-saving medical devices, thermoplastics are multifaceted polymers. Thermoplastics possess distinctive characteristics, notably the ability to melt and be molded when exposed to heat and revert to a solid state upon cooling. 

This article outlines what thermoplastics are, their chemical composition, the numerous types of thermoplastics, properties, and applications, and the most popular thermoplastics processing techniques.  

What Are Thermoplastics? 

Thermoplastics represent a class of polymers characterized by their distinctive response to temperature changes. Unlike thermosetting plastics, which undergo irreversible chemical changes upon heating and cannot be re-molded, thermoplastics soften when exposed to heat and solidify when cooled.  

This behavior allows thermoplastics to withstand multiple cycles of heating and cooling without sustaining major changes in their fundamental properties, which allows them to be easily recycled. 

Thermoplastics vs thermosets

Understanding the Chemical Composition of Thermoplastics 

Thermoplastics are formed through the process of polymerization, a chemical reaction during which small molecular units, known as monomers, join together to create long-chain polymers. One polymer chain comprises thousands of monomers.  

Additionally, the arrangement of a thermoplastic’s molecular structure influences whether it is an amorphous or semi-crystalline polymer.  

Amorphous Thermoplastics 

These polymers have a disordered or random molecular structure without a well-defined, repeating pattern. In amorphous thermoplastics, the polymer chains lack specific long-range geometric order, which often results in materials that are transparent or translucent. These types of thermoplastics also generally exhibit higher impact resistance due to their lack of a semi-crystalline structure.  

Amorphous thermoplastics find use in various applications, including transparent packaging materials, optical lenses, and consumer goods where impact resistance and transparency are crucial. 

Semi-Crystalline Thermoplastics 

In contrast to amorphous thermoplastics, semi-crystalline thermoplastics have a more ordered and structured molecular arrangement, resulting in the formation of crystalline regions within the material. The molecular arrangement of these polymers leads to the following properties: 

  • Strength and Rigidity: Semi-crystalline thermoplastics, such as polyethylene and polypropylene, tend to be stronger and more rigid due to the ordered arrangement of their molecules. 
  • Higher Meling Points: They often have higher melting points compared to amorphous thermoplastics 

Semi-crystalline thermoplastics are commonly used in applications where strength, rigidity, and higher melting points are essential, which includes structural components in automotive parts, pipes, and other engineering applications.  

Examples of Thermoplastics 

While the two main thermoplastic polymer structures are amorphous and semi-crystalline, there are several popular types of thermoplastics, each with distinct properties and applications.  

Types of Thermoplastics

Type

Acronym

Properties

Applications

Polyethylene

PE

Semi-crystalline
Good chemical resistance
Flexible and lightweight

Packaging materials
Water pipes

Polypropylene

PP

Semi-crystalline
High strength and rigidity
Resistance to chemicals and fatigue

Automotive components
Packaging materials

Polyvinyl chloride

PVC

Amorphous
Flexible or rigid, depending on form
Chemical resistance

Construction materials
Pipes and fittings
Medical devices – tubes and bags

Poly(ethylene terephthalate)

PET

Can be amorphous or semi-crystalline
Transparency (amorphous)
Mechanical strength (semi-crystalline)

Beverage bottles
Textile fibers
Films

Polyamide

PA – Nylon

Semi-crystalline
High tensile strength
Good chemical resistance

Textile industry
Automotive components
Dowels

Polystyrene

PS

Amorphous
Transparency
Lightweight

Packaging materials (including expanded PS)
Disposable utensils

Acrylonitrile-butadiene-styrene

ABS

Amorphous
Impact resistance
Good dimensional stability

Consumer electronics
Automotive parts
Housings/casings/surface parts

Polycarbonate

PC

Amorphous
Transparency
High impact resistance

Optical lenses
Eyewear, specifically protective eyewear/visors

Poly(methyl methacrylate)

PMMA

Amorphous
Optical clarity
UV resistance

Transparent plastics in optics and signage
Acrylic glass

Polyoxymethylene

POM

Semi-crystalline
Low friction
High stiffness and strength

Precision parts in machinery
Gears and bearings

Poly(lactic acid)

PLA

Amorphous
Derived from renewable resources
Biocompatible

Biodegradable packaging materials
Disposable cutlery and plates
Threads for medical sutures and implants

Poly(phenylene oxide)

(PPO) with High Impact Polystyrene (PPO+HIPS)

Amorphous
Good electrical insulator
Impact resistance (HIPS component)

Electronic components
Automotive parts

Polytetrafluoroethylene

PTFE – Teflon

Non-reactive
Low friction
High chemical resistance

Non-stick cookware
Seals and gaskets

Poly(vinylidene fluoride)

PVDF

Semi-crystalline
Excellent chemical resistance
High thermal stability

Chemical processing equipment
Electrical insulation

Polyetheretherketone

PEEK

Semi-crystalline
High temperature resistance
Excellent chemical resistance

Aerospace components
Medical implants

Poly(phenylene sulphide)

PPS

Semi-crystalline
Chemical resistance
High temperature stability

Electrical components
Automotive parts

Polyetherimide

PEI

Amorphous
High temperature resistance
Flame resistance

Aerospace components
Medical devices

Polyethersulfone

PESU, PES

Amorphous
Excellent chemical resistance
High temperature stability

Aerospace components
Medical devices

Polybenzimidazole

PBI

Amorphous
Excellent thermal stability
Flame resistance

High-temperature applications (flame-resistant)
Aerospace and automotive components

Processing Thermoplastics 

The processing of thermoplastics involves transforming raw polymer materials into a myriad of shapes and forms to meet the diverse needs of various industries. This process is marked by precision and adherence to specific manufacturing techniques to ensure that resulting products exhibit the desired properties and functionalities. 

Injection molding and extrusion are the most popular thermoplastic processing techniques, making up about 80% of the thermoplastic manufacturing market.  

Thermoplastic Injection Molding 

This process entails injecting thermoplastic material into a mold cavity, where it solidifies to form the final product. Injection molding is favored for its ability to manufacture high volumes of complex components with exceptional repeatability. 

Thermoplastic injection molding

The injection molding process begins with the feeding of thermoplastic pellets into a heated barrel. The material is melted and injected into a mold under high pressure. Once inside the mold, the material cools and solidifies, taking the shape of the mold cavity. Ejection mechanisms then release the final product, and the cycle repeats for subsequent production. 

Injection molding is prevalent in the manufacturing landscape and contributes to the production of a wide range of products, including automotive components, consumer electronics, medical devices, and household items.  

Thermoplastic Extrusion 

Extrusion is another popular method of thermoplastic processing. This process involves the continuous shaping of thermoplastic materials by forcing them through a die to create a consistent profile or shape. Extrusion is generally suitable for producing long, uniform products with a constant cross-section, which ranges from simple pipes and tubes to complex profiles used in construction and manufacturing. 

Thermoplastic extrusion

The process begins with the feeding of raw thermoplastic material into the extruder, a machine equipped with a rotating screw that transports and melts the polymer. The molten material is then forced through a die, determining the final shape. Post-extrusion, the shaped material undergoes cooling, sizing, and cutting to attain the desired dimensions and properties. 

Extrusion is used in various industries, most notably in the production of pipes, tubes, sheeting, and profiles. 

In previous articles, we discussed thermosets, elastomers, and composites.  

If you are interested in hearing our experts discuss material deviations and production anomalies, as well as the ways to combat them, watch our free on-demand webinar on “Enabling Automated Quality Control in Composites for Less Scrap, Higher Yield”.  

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