Thermoplastic elastomers are made by copolymerising two or more monomers through either block or graft polymerisation methods. Block techniques create long-chain molecules with various sequences, or blocks, of hard and soft segments. Graft polymerisation methods involve grafting one polymer chain to another as branches.
These techniques cause one of the monomers to develop a hard or crystalline segment which acts as a thermally stable component. This component softens and flows under shear unlike the chemical cross-links between the polymeric chains in conventional, thermoset rubber. Meanwhile, the other monomer develops a soft or amorphous segment that contributes to the rubbery characteristics of TPE.
Varying the ratio of the monomers used, as well as the lengths of hard and soft segments, allows the properties of the finished TPE to be controlled. However, graft methods offer more possibilities to vary the copolymer because both the backbone monomer and the grafted branches can be hard and glassy, rubbery, or somewhere in between. In both block and graft production methods the environmental and fluid resistance are entirely predictable.
TPEs are usually produced in pellet form and added to the same type of injection moulding machine as rigid thermoplastics. Colourants can be added to the compound during production or blended in automatically or manually at the injection moulding machine.
Thermoplastic elastomers have inherently low levels of toxicity and are compliant with medical and food regulations, making them safe for a range of applications. Their excellent mechanical properties, strong haptics and elasticity mean they have a soft and appealing feel that makes them perfect for a range of different consumer goods.
TPEs are safe to use, particularly at room temperature, and can be found in food contact applications like spoons for babies, as well as healthcare applications such as dental guards.
TPEs have a wide range of advantages, relating to their production, material properties, recyclability and applications. Some of these advantages, particularly over thermoset rubbers, are as follows:
- TPEs do not require large amounts of energy to produce, need little or no compounding and do not require reinforcing agents, stabilisers or cure systems. This means that there is no variation in match weighting and metering components, creating greater consistency in raw materials and fabricated items
- TPE compounds are easily coloured by most dyes, leading to a wide range of uses
- TPEs are resistant to low and high temperatures (-30d°C to +150°C), providing good thermal properties and material stability at a range of temperatures
- TPE materials can be recycled and reused like plastics, but also have the elastic properties of rubbers, which are not recyclable. TPEs can also be ground down and turned into 3D printing filament
- The material properties of TPEs ensure excellent flexural fatigue resistance, good electrical properties, strong tear and abrasion resistance, high impact strength and elongation, a low specific gravity, excellent resistance to weathering and chemicals, and low compression set.
- TPEs can also be co-injected and co-extruded with certain engineering plastics
Despite the many advantages, there are some disadvantages to TPEs when compared to some other materials, including conventional rubber. These include:
- Melting. Despite the good temperature resistance, TPEs will melt at elevated temperatures. This limits their use in high temperature applications. Conventional will also melt at high temperatures, but offers protection if exposure is brief. However, recent developments have given some TPEs the capability to be used at temperatures of 150 °C or higher.
- Hardness. There are a limited number of low hardness TPEs available, with most having a hardness of around 80 Durometer A or more. However, there are an increasing number of materials that are softer than 50 Durometer A, with some even being gel-like
- Drying. Most thermolastic materials require drying before processing, a step that is almost never undertaken on conventional rubber materials
- Cost. TPEs have a higher cost than many other plastics
- Processing Temperatures. If heated to a relatively high temperature, TPEs will tend to lose their rubbery properties.
- Creep. TPE materials can move and deform under the influence of sustained stress, such as that caused by pressure or temperature
TPEs are being used for a wide range of applications due to their versatility and properties. They can be processed on plastics machinery, with the moulding and extrusion processes having fast cycle times of as little as 20 seconds. They can also be reprocessed at the end of life.
TPEs are commonly found being used in the automotive industry and for domestic appliances, as well as for roofing materials, medical applications, and even shoe soles. TPE have also seen growth in the heating, ventilation and air conditioning industry due as well as being used for a range of different cable jackets.
Some typical applications include:
- Airbag Covers
- Cables and Wires
- Grips and Handles
- Plugs and Seals
- Power and Hand Tools
Thermoplastic Elastomers always have three essential characteristics:
- Ability to be moderately stretched and then, once the stress is removed, to return close to the original shape
- Can be processed as a melt at elevated temperatures
- No significant creep
Commercial TPEs are classed under six generic classes:
1. Styrenic block copolymers, TPS (TPE-s)
Styrenic block copolymers (TPE-S) consist of two-phase block copolymers with hard and soft segments. The styrene end blocks provide the thermoplastic properties while the Butadiene mid-blocks deliver the elastomeric properties. These materials are commonly used in footwear, adhesives, bitumen modification and seals and grips where resistance to chemicals and aging is a lower priority
2. Thermoplastic polyolefin elastomers, TPO (TPE-o)
Thermoplastic Polyolefins (TPE-O or TPO) are blends of polypropylene (PP) and un-crosslinked EPDM rubber, although a low degree of cross-linking is sometimes present to boost heat resistance and compression set properties. They are used in applications that require increased toughness over the conventional PP copolymers, such as in automotive bumpers and dashboards
3. Thermoplastic Vulcanizates, TPV (TPE-v or TPV)
Thermoplastic vulcanisates (TPE-V or TPV) offer improved performance over TPE-O. They are also compounds of PP and EPDM rubber, but have been dynamically vulcanised during the compounding step. They are increasingly being used for automotive seals, pipe seals, and other applications where a heat resistance of up to 120°C is required
4. Thermoplastic polyurethanes, TPU (TPU)
Thermoplastic polyurethanes (TPE-U or TPU) are based on polyester or polyether urethane types and are used in applications where excellent tear strength, abrasion resistance, and flex fatigue resistance are required. Example uses include shoe soles, industrial belting, wires and cabling
5. Thermoplastic copolyester, TPC (TPE-E)
Thermoplastic copolyesters (TPE-E or COPE or TEEE) offer increased chemical resistance and heat resistance up to 140°C. With good fatigue resistance and tear strength, they are used in automotive applications, wires and cables, and industrial hose applications
6. Thermoplastic polyamides, TPA (TPE-A)
Thermoplastic polyether block amides (TPE-A) offer good heat resistance, chemical resistance and bonding to polyamide engineering plastics. They are used for applications including cable jacketing and aerospace components
Thermoplastic elastomers are increasingly being used by medical device manufacturers for applications that require flexibility and elasticity. Replacing materials like PVC or thermoset rubber, TPE compounds are finding use for tubing, ventilator bags, pouches, masks, cushions, drip chambers, syringes, stoppers, seals, gaskets and dropper bulbs, among others.
The versatility of TPEs is one of the reasons they have seen such take-up for medical applications as they can meet end user requirements for a range of purposes. Test methods demonstrate that the optical properties of TPEs range from clear to opaque and hardness can range from gel-like to semi rigid. Different grades of TPE allow for different strength requirements, low temperature toughness, heat stability and resistance to chemicals or ultraviolet light. The ease of processing into film, sheets or tubing also makes TPEs an attractive material for medical use, while their chemical inertness is another plus for medical grade applications.
Thermoplastic elastomers have a great many desirable properties that lend themselves to a range of applications across industry. Easily processed and coloured, as well as being safe and flexible, TPEs are being used for an increasing number of medical applications as well as being widely used for consumer goods and in the automotive and aerospace industries.
With different types of TPE offering lightly different properties at a range of prices, commercial TPEs are replacing other materials such as conventional rubber. While there are some drawbacks with TPE, the advantages outweigh these for many uses. In addition, because they can be recycled and reprocessed, thermoplastic elastomers offer a more environmentally-friendly alternative to other plastics.