The automotive industry utilizes a wide variety of thermoplastic elastomers (TPEs), with the most prominent ones being thermoplastic olefin (TPO), thermoplastic vulcanizate (TPV), thermoplastic polyester elastomer (TPEE), thermoplastic polyurethane (TPU), and thermoplastic styrenic elastomer (TPS). TPE materials are processed similarly to thermoplastics, offering significant production benefits, while their applications resemble those of rubber in practical use. TPE materials not only reduce system costs, decrease component weight, enhance part performance, and improve recyclability, but they also enhance the vehicle's appearance. In the 1990s, the use of TPE in foreign automobiles grew rapidly, with some rubber parts previously made from ethylene propylene diene monomer (EPDM) being replaced with TPE.
The primary application of TPE in automobiles is for sealing strips, as they offer similar performance to EPDM, better tear resistance, and are lighter in weight, which aligns with the current automotive industry's goal of weight reduction. Additionally, TPE is widely used in automotive hoses and other parts. It is reported that Mitsubishi Corporation in Japan used SANTOPRENE for the rear windshield seal of car bumpers, which not only reduced costs by 5% to 10% compared to rubber but also improved material recyclability and durability. With the rapid development of the global automotive industry, the production and consumption of automobiles have surged, leading to an astonishing number of vehicles being scrapped each year. As a result, countries like those in Europe and America have begun to enact regulations mandating that automotive manufacturers use recyclable materials, thus prompting a trend towards TPE-ization of automotive rubber components.
Currently, the commonly used thermoplastic elastomers in automobiles are mainly divided into four categories: polyolefin-based TPEs (TPO and TPV), polyester-based TPEs (TPEE), polyurethane-based TPEs (TPU), and styrenic-based thermoplastic elastomers (TPS). TPO is mainly used in car bumpers, interior trim surface materials, door and window seals, mud flaps, steering gear box covers, fuel lines, etc.; TPV is primarily found in high-end passenger cars for buckle fasteners, corner anti-impact profiles, windshields, air dams, and various air and water hoses, door and window waterproof seals, oil seals, gaskets, etc.; TPEE is used in constant velocity joint boots, handbrake rollers, seat belt components, door latches, control system protective covers, gear and rack covers, support device covers, bushings for spherical joints, and dust protection covers. TPU, known for its excellent wear resistance and bending fatigue resistance, is used for connecting shaft sleeves, sealing rings, and gaskets. TPS is mainly used for automotive hoses, sealing pads, and adhesives.
1. The Application of TPO Thermoplastic Elastomers in Automobiles
TPO is a physically blended polyolefin-based thermoplastic elastomer, consisting of polyolefins (commonly polypropylene) and unvulcanized or slightly vulcanized polyolefin elastomers (ethylene-propylene-diene monomer rubber) through mechanical blending or dynamic partial (complete) vulcanization blending. It exhibits excellent resistance to polar fluids, but poor resistance to non-polar hydrocarbons and halogenated carbon fluids, and its performance retention at high temperatures is not ideal.
TPO is currently the most actively used TPE in automobile manufacturing, especially since the advent of dynamically vulcanized TPO in the 1980s, followed by the development of fully cross-linked dynamically vulcanized TPO. Due to its outstanding physical properties, a certain degree of oil resistance, and excellent aging resistance, its applications are increasingly valued. At present, the largest use of TPO in automobiles is in the manufacture of bumpers and as interior surface materials. Monsanto Company in the United States was the first to develop and industrialize the production of TPO, which is now widely used in manufacturing automotive bumpers and dashboards. In China, TPO developed by institutions such as the Beijing Research Institute of Chemical Industry has been industrialized and applied in the automotive industry.
2. Application of TPV Thermoplastic Elastomers in Automobiles
TPV is a dynamically vulcanized thermoplastic olefin elastomer, produced by cross-linking two or more polymers that exhibit a synergistic effect through dynamic full vulcanization or melt blending, facilitated by functional groups. It combines the excellent properties of each component polymer, such as resistance to fluids, resistance to compression set, and fatigue resistance to flexing. Generally, it maintains good performance at high temperatures. Santoprene, developed by Monsanto Company in the United States using the dynamic vulcanization method, has been widely applied. This rubber has a tensile strength of up to 26.9 MPa, an elongation at break of 600%, and a tear strength of 1.12 KN/m, though it has the drawback of high permanent deformation upon stretching. Santoprene products can operate within a temperature range of -40 to +150°C. This material is used in the suspension systems of modern automobiles and is expected to be used in the manufacture of reinforced tubing, folding bellows, and glass sealing products, among others.
To reduce costs and maintain aesthetics, manufacturers both domestically and internationally have begun using TPV materials to produce automotive door and window seals. TPV is convenient for coloring and recycling. High molecular weight TPV has good abrasion resistance and a low friction coefficient, making it suitable as an extrusion material for automotive glass run channel seals, as well as a burr-free corner material for automotive rubber seals.
Water-blown sponge-like TPV has been successfully developed abroad and has been applied to automotive rubber seals. This represents a new trend in materials that will replace sponge rubber for automotive seal production in the future. Additionally, due to the simplicity and convenience of TPV coloring processes, it is often used as an exterior decorative layer material for automotive rubber seals. It can be co-extruded and coated at the die head and die of the extruder, or after the rubber seal is vulcanized, a TPV film can be extruded and coated onto the seal at high temperatures. When using the former method, due to the significant difference in extrusion temperatures between TPV and rubber, the extrusion line's die head requires a complex structure with multiple temperature control points that do not interfere with each other. At this time, to counteract the vulcanization temperature of rubber seals in microwave and hot air vulcanization tanks, and to maintain the performance of TPE materials, special high-temperature resistant TPE materials must be used. TPV materials have distinctly superior performance, but compared to EPDM rubber, their high cost impacts their widespread application and promotion.
3. Application of TPEE Thermoplastic Elastomers in Automobiles
TPEE (Thermoplastic Polyester Elastomer) is a block copolymer containing polyester hard segments and polyether soft segments. The polyether soft segments, along with the amorphous polyester, form an amorphous phase, while the partially crystalline polyester hard segments form crystalline microdomains that act as physical crosslinking points. TPEE possesses the elasticity of rubber and the strength of engineering plastics; the soft segments provide elasticity, making it rubber-like, and the hard segments provide processability, making it plastic-like. Compared to rubber, it offers better processing performance and a longer service life; compared to engineering materials, it also has high strength, but with better flexibility and dynamic mechanical properties.
In the automotive industry, TPEE is mainly used for constant velocity (CV) joint boots. Previously, these boots were made from CR (Chloroprene Rubber), which had poor flex fatigue resistance and often cracked when used in high-temperature environments, making replacement laborious and wasteful. With increasing demands for heat resistance, TPEE has begun to be used in some European, American, and Japanese vehicles. TPEE materials meet the requirements for weather resistance, oil resistance, and heat resistance, while also achieving the goal of lightweighting. In other aspects, steering rack boots, suspension boots, and gearbox control dust covers can all be manufactured using TPEE with blow molding methods; various automotive connecting sleeves, handbrake pulleys, and dust sealing rings can be made from TPE materials using injection molding, and wire harness and cable sheaths can be produced using TPEE with extrusion molding methods.
4. Applications of TPU Thermoplastic Elastomers in the Automotive Industry
TPU (Thermoplastic Polyurethane) is known for its high strength, high wear resistance, high elasticity, aging resistance, and oil resistance. It is widely used in automotive components, such as in shift rod handles, various coupling joint sleeves and washers, various wiring harness connectors, spiral retractable wires, cable sheaths, gear belts, protective sleeves and gaskets for steering tie rods, suspension joint hinges, hydraulic and pneumatic components, suspension diaphragms, shock absorption diaphragms, shock absorbers, spring limiters, etc. Leveraging the high strength and high load-bearing capacity of polyurethane elastomers, the Goodyear Tire & Rubber Company in the United States has developed the second generation of TPU, commercially known as Estaloc. This product retains the characteristics of the first generation TPU Estaloc and incorporates hollow glass microspheres as a filler, which increases the glossiness by more than 15%, making it suitable for manufacturing automotive side panels and shock-absorbing pads. Many hydraulic rubber parts used in vehicles are made of TPU material. For protective sheaths, door and window seals, fuel line hoses, and lining materials, the Japanese company Nippon Zeon recommends using the polyester thermoplastic elastomer Elasta for manufacturing.
5. Application of TPS Thermoplastic Elastomers in Automobiles
Styrene-based thermoplastic elastomers (TPS) are tri-block or multi-block copolymers with polystyrene segments as the hard segments and polydiene as the soft segments. TPS exhibits excellent tensile strength and elasticity, maintaining a degree of flexibility even at very low temperatures. It has outstanding electrical properties and breathability, but poor heat resistance and oil resistance.
Styrenic block copolymer (SBC) type TPEs have several characteristics that set them apart from other types: the hardness of these materials can range from Shore A30 to 70, allowing for the production of products that closely resemble rubber in properties; they have excellent low-temperature resistance and are compatible with resins and asphalt, making them highly suitable for modification of resins and asphalt; they have good water and chemical resistance, and with the development of hydrogenated SEBS and SEPS, their heat and aging resistance have significantly improved; they also have excellent adhesive properties with high initial tack. As a result, they are widely used as adhesives and sealants (such as adhesive tapes and hot melt adhesive strips) and plastic modifiers (such as impact-resistant polystyrene HIPS, modified polyphenylene oxide MPPO, etc.), especially in the automotive manufacturing industry. TPE based on SEBS is primarily used in the automotive industry for car sealing systems.
Although thermoplastic elastomers cannot yet fully replace traditional vulcanized rubber materials, their application has seen significant progress in recent years, particularly with the development of dynamic vulcanization technology, which has opened up broad prospects for their use in the automotive industry.