8 Minute Read
By Tayla Pritchard
Thermoplastics are a class of materials that have revolutionised countless industries and become an integral part of our daily lives. These versatile polymers possess the unique ability to soften when heated and harden when cooled, allowing them to be moulded, shaped and reshaped multiple times without losing their inherent properties. This characteristic not only makes thermoplastics highly recyclable but also opens up a world of possibilities for manufacturers and designers.
From the packaging that keeps our food fresh to the components in our vehicles and electronic devices, thermoplastics are present everywhere in modern society. Their strength-to-weight ratio, durability, and resistance to various environmental factors have made them indispensable in industries such as aerospace, automotive, construction, and healthcare.
The first thermoplastic, called Parkesine, was invented in 1856 by Alexander Parkes. This material was made from cellulose and could be shaped when heated and hold its form when cooled. Although it didn't succeed commercially due to high costs and some safety issues, Parkesine marked the beginning of the plastics industry. It laid the groundwork for future thermoplastic developments, leading to the wide variety of plastic materials we use today.
Polyethylene, a widely used plastic, has an interesting history that started in the 1930s. It was discovered by accident in 1933 by chemists Eric Fawcett and Reginald Gibson at Imperial Chemical Industries (ICI) in England. They created a white, waxy substance while experimenting with ethylene gas. After five years of research, ICI began commercial production of polyethylene in 1939. During World War II, it was used to insulate radar cables. After the war, polyethylene became popular for many products, including packaging and toys. In the 1950s, new types like high-density polyethylene (HDPE) were developed, further increasing its use in everyday life.
Figure 1: A visualisation of the chemical structure of polyethylene showing the repeating monomer units.
Thermoplastics are polymeric materials characterised by their ability to undergo reversible phase transitions between solid and liquid states upon heating and cooling. These materials consist of long molecular chains held together by weak intermolecular forces, typically Van der Waals interactions.
When heated above their glass transition temperature (Tg) or melting point (Tm), the molecular chains gain sufficient mobility to slide past one another, allowing the material to flow and be reshaped. Upon cooling, the chains become less mobile, and the material solidifies, retaining its new form. This cycle can be repeated multiple times without significant degradation of the polymer's chemical structure or mechanical properties.
Common thermoplastics include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and polyethylene terephthalate (PET), each exhibiting specific thermal, mechanical, and chemical properties that make them suitable for a wide range of industrial and consumer applications.
Figure 2: After polyethylene, the chemistry of polymers and plastics improved to allow different types of polymers with different characteristics and properties.
Polymers are processed through various methods, with master batching, compounding, and dry blending being three common techniques. Here's an overview of each method and their differences:
Master batching involves creating a highly concentrated mixture of pigments or additives encapsulated in a carrier resin. This masterbatch is then metered into the base polymer at predetermined ratios (typically 1-5%). This method is cost-effective and flexible, often used for colouring plastics or adding specific properties. For example, creating colour concentrates for plastic packaging.
Compounding is a more thorough process of melt blending the base polymer with colours and/or additives, resulting in a homogeneous final product. This technique alters the physical, thermal, electrical, or aesthetic properties of the polymer. Compounding produces excellent colour distribution and consistency but can be more costly. An example of compounding is its use in producing automotive components that require specific performance characteristics.
Dry blending is the simplest technique, involving the mixing of the polymer base with raw colour pigments and/or additives without further processing steps. While this method is cost-effective and quick to prepare, it can result in lower quality products with poor dispersion and colour consistency. Dry blending is often employed in less demanding applications, such as the production of simple coloured plastic utensils.
After initial processing, thermoplastics can be used in a variety of moulding techniques, such as injection and extrusion moulding.
Injection moulding is a popular manufacturing process used to create parts by injecting melted materials, mainly thermoplastic plastics, into a mould. The process involves several steps: plastic pellets are heated until they melt, then the molten plastic is injected into a mould under high pressure. Once it cools and solidifies, the finished part is ejected from the mould. This method uses specialised machines that include an injection unit, a mould, and a clamping unit. Common items produced through injection moulding include car parts like bumpers, medical devices such as testing kits, electronic device cases, and food containers.
Figure 3: An injection moulding machine used to create moulds from thermoplastic materials.
Extrusion moulding is a process used to make long products with a fixed cross section. It starts with plastic pellets that are heated until they melt. The melted plastic is then pushed through a shaped opening called a die, creating a continuous piece. After it comes out, the plastic is cooled to harden it. This method is often used to produce items like pipes, hoses, tubes, and straws.
Polymers are rarely used in their pure form. They are often enhanced with various colours, additives, and fillers to form thermoplastic compounds which improve their properties, making them suitable for specific applications.
Pigments and dyes play a vital role in the polymer industry, mainly to add colour and to give a distinct look to plastic parts. For an even more alluring visual appeal, pearlescent pigments and glitter can be added to plastics to create shiny, sparkling effects. Geotech create a variety of special effect pigments in different colours, sizes and effects for use in a variety of industries. For thermoplastics, their Geopearl® range of pearlescent pigments is the most popular. Pearlescent pigments are made from mica flakes which are coated with metal oxides like titanium dioxide and iron oxide to give the desired colour. They work best in clear plastics, offering effects from soft shimmer to bright sparkle, typically using 0.5-1.0% of the pigment. For a glittery look, larger particles based on coloured Polyester, Geoglit® SP, or Aluminium, Metalloy®, can be used in very small amounts to enhance sparkle. Our blog on the different special effect options for use in thermoplastics explains in more detail.
Figure 4: The Geotech special effect pigments in thermoplastics give different effects from glitter and pearlescent to metallic or chrome.
Additives are used to enhance the performance and durability of polymers. Common additives include stabilisers, which protect against degradation from UV light and heat; plasticisers, which make polymers more flexible; and flame retardants, which reduce the risk of fire. Each additive is selected to convey specific properties, ensuring that the polymer meets the requirements of its intended use.
Fillers are materials added to polymers to improve their mechanical properties or reduce costs. Common fillers include calcium carbonate, glass fibres, and clay. These fillers can enhance strength, stiffness, and impact resistance, or simply make the polymer more economical by reducing the amount of expensive polymer required. Another example is Silatherm, from Quarzwereke High Performance Minerals, used as a filler in various applications to improve thermal conductivity.
Addressing the environmental issues associated with polymers requires a multifaceted approach. Here are some strategies:
Improving recycling rates and developing new recycling technologies are critical. Encouraging the reuse of polymer products can also reduce waste. Recycling of thermoplastics comes with its own challenges such as processability, degraded mechanical properties, and odour. Fortunately, there are technologies available to address this. Check out our blog on the Additives for Recycled Thermoplastics for more details.
Recycled fillers and pigments can also be used instead of virgin additives to increase the total recycled content of thermoplastics. An example is Emerald rCB, a recovered carbon black from UK manufacturer, Waverly Carbon. It can be used as a pigment to produce deep black or grey shades in polymers, providing a sustainable alternative to conventional carbon black. As a pigment, Emerald rCB not only provides colour but also contributes to the polymer’s mechanical properties, such as enhanced durability and UV resistance. To find out more about the benefits of recovered carbon black, check out our blog on how it is a more sustainable feedstock.
Figure 5: Recycled Carbon Black from Waverly Carbon offers a sustainable alternative with a deep black colour that can offer numerous advantages over traditional carbon black.
Ongoing research focuses on developing polymers that decompose more readily in the environment, potentially replacing traditional plastics in various applications. The incorporation of sustainable additives and reinforcing fillers can significantly enhance their mechanical properties. This approach aims to bridge the performance gap between biodegradable and traditional polymers. Lawrence Industries encourages manufacturers to use additives and fillers that are either sustainable or use materials that help improve the process of recycling.
Governments can drive significant change by incorporating measures like the UK Plastic Packaging Tax, which puts a charge on packaging with less than 30% recycled content. These types of policies can help to promote the responsible use of plastics, such as not using them for single usage and creating incentives for sustainable materials. This shift can lead industries to adopt more environmentally friendly practices, whilst still recognising the valuable role that plastics play in modern life.
Thermoplastics have undeniably changed our world, providing convenience and advancements across numerous sectors. Their versatility and durability have led to developments in medicine, technology, and everyday life. A wide range of fillers and additives is available to tailor the properties of thermoplastics to these different applications, including recycled plastics.
Tayla completed her undergraduate studies at Liverpool John Moores University and then went on to get an MSc in Medicinal Chemistry from Cardiff University. She joined Lawrence Industries in 2024 having gained some industrial experience as a chemical analyst. She now works alongside Koen in the polymer department offering technical expertise and product advice.
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