TPR TPU

TPR or thermoplastic rubber is typically a copolymer that merges the properties of Polystyrene (PS) and butadiene (synthetic) rubber. This material was developed to provide much of the property benefit of synthetic rubber while allowing processing (and reprocessing) in the more precise and controllable injection molding process. TPR delivers outstanding fatigue resilience, chemical stability, impact strength, and moderate recyclability, with the right processing.

TPU or thermoplastic polyurethane is a wide classification of polyurethane polymers that share properties such as: elasticity, transparency, wear resilience, and very high resistance to oils. The materials are thermoplastic, with a range of degrees of elastomeric behavior. Their high strength and resilience are derived from a polymer chain structure consisting of alternating hard and soft segments.

This article will further compare TPR vs. TPU, their applications, uses, physical properties, and alternative materials.

What is TPR?
TPR (thermoplastic rubber) typically consists of a blending of 23% PS and 77% butadiene. It is prepared by emulsion polymerization of constituent polymers. The process makes an inseparable material that contains an elastomeric, thermoset phase, and a distributed, rigid matrix of thermoplastic material. The result derives properties from both components but is able to be processed with extreme precision in injection molding equipment. For more information, see our guide on What is Thermoplastic Rubber.

Typical rubbers are synthesized or refined in a partially polymerized form, and then heat is applied to finish the cross-linking process. In the case of TPR, the butadiene component is manufactured fully cross-linked and produced as a fine powder. The PS component then substitutes for the cross-linking by bonding the rubber into a strong polymer matrix. Flexibility results from the elasticity of the butadiene component, where the intra-particle rubber bonding is essentially inelastic.

TPR does not match the performance of vulcanized rubbers, so it is not suitable for tire manufacture, having too low a tear modulus. However, the significantly better ozone-, weathering-, and UV-resistant properties of TPR mean that it finds wide application across many product sectors.

What is TPU?
TPU polymers are made of block structures with longer, [soft" regions of low polarization, and short, [hard" sections of much higher polarization. Covalent linkages between the two segment types make well-integrated chains that derive properties from both chain elements. By altering the molecular weights and ratios of the constituent parts, a wide range of properties can be derived in chemically (almost) identical materials. The glass transition temperatures of the two components can be similar or disparate, and they can be chemically altered to be higher or lower. These processing issues allow the production of a range of thermal properties of the resulting material family.

The higher polarization of the harder elements induces strong attraction, which induces pseudo-crystalline regions, located in a highly elastic matrix. The pseudo-crystalline regions behave as cross-linking elements, which account for the high elastic modulus of the family, whereas the longer, softer chains moderate this effect, allowing a range of hardness/elasticity to be manufactured.

This cross-linking effect diminishes to zero as the glass transition temperature of the harder component is exceeded. The family behaves as a fully thermoplastic material group that can be processed in ordinary injection molding equipment. TPUs can be recycled by melting and reforming, although the degradation in chain length/integrity is significant on remelting.

TPR vs. TPU: Applications and Uses
Listed below are the typical industries and the common TPR applications:

Auto Manufacture: Door and window seals, transmission/suspension parts, fender inserts, exterior and interior trim, instrument panels, AC and engine air ducts, grommets, drive belts, fluid pipes, floor mats, O-rings.
Construction: Door and window seals, hydraulic seals, plumbing seals.
Industrial: Vibration dampers, pipes, manifolds, seals, suspension bushes, shock absorbers, roof membranes.
Consumer: Refrigerator seals, handgrip overmolds, mobile phone covers, switch panels, vibration dampers.
Medical: Air tubes, syringe seals, breathing masks and plenums, seals, valves, and catheters.
Electronics: Encapsulation, power leads, high-quality cables, mobile phone shock protection, and seals.
Footwear & Sports Equipment: Diving flippers, snorkels, masks, ski-pole grips, ski-boot components, and shoe soles.
Listed below are the typical industries and the common TPU applications:

Automotive: Automotive interior parts (good surface finish, durability, wear resistance; and low cost).
Agriculture: ID tags for animals (great flexibility, tear and weather resistance, and temperature tolerance. Great for encapsulating RFID).
Piping and Plumbing: Seal profiles and O-rings, tubes, belts, and hoses. Specialist polymers with optimal melt-flow properties, extrusion-adapted high resistance to hydrolysis by hydraulic and other oils and organics, resistance to compression at high temperatures, high toughness, flexibility, and resistance to tearing.
Textile: Used for conveyor belts, inflatables, and military equipment. Wide range of processing options, and good mechanical, chemical, and thermal properties.
Sports Equipment: Extreme flexibility, high-impact and temperature resistance, transparency, and environmental tolerance.
There are common market sectors and ostensibly common product areas that can use either TPR or TPU materials. In general, the commonalities do not imply interchangeability, as each application tends to exploit a narrow property of one or another material. For example, the automotive sector tends to use TPRs for extreme flexibility and weathering properties, and TPUs for resilience, wear resistance, and ergonomic applications

TPR vs. TPU: Recyclability and Sustainability
Both materials are derived from petrochemical sources, and their sustainability and recycling credentials are quite similar. TPU is highly recyclable; its waste can be processed back into high-quality raw material. Although, it tends to be second-grade due to thermal decomposition. TPU is also biodegradable and will generally break down in landfill/compost conditions over 3–5 years. It leaves no toxic residues when it breaks down. TPUs are also slowly becoming available and can be manufactured from bio-sourced monomers. 

TPR is also highly recyclable, but like most polymers, the recycled material is second-grade. They are stable materials that degrade extremely slowly in the natural environment. Algal-derived monomer source materials for TPRs are also becoming available.

TPR vs. TPU: Cost
TPRs are generally very low-cost materials, in the range of $1.60 to $2.00 per kg. TPUs are a little more expensive, at $2.00 to $4.00 per kg.

Alternative Materials to TPR and TPU
For injection molding use, a wide variety of thermoplastic elastomeric materials are available, with an equally wide spectrum of properties and costs. While these are not interchangeable across the whole range, there are many commonalities of properties that result in options at the specification stage. Listed below are some alternative materials:

Thermoplastic Vulcanisates (TPE-V or TPV).
Thermoplastic Polyolefins (TPE-O or TPO).
Thermoplastic Copolyesters (TPE-E, COPE, or TEEE).
Thermoplastic Polyether Block Amides (TPE-A).
Styrenic Block Copolymers (TPE-S).
Melt-Processable Rubber (MPR).
Fluoropolymer Elastomers (FPE).
When a switch to thermosetting polymers is an option, there are more material choices, as listed below:

Vulcanized natural rubber (NR) (latex, vulcanized to form Buna rubber).
Polyisoprene (IR).
Polychloroprene (CR).
Butadiene rubber (BR).
Nitrile (butadiene) rubber (NBR).
Thermoset rubbers bring new properties and new restrictions to the options list. They can, however, achieve remarkable results whenever specific properties are sought and the processing issues do not prevent use.