Thermoplastic Polyurethanes - Versatile and Durable for Today's and Tomorrow's Products
By Roger Huarng and Stephane Morin, Alliance Polymers and Services
Thermoplastic polyurethanes (TPUs) can be found just about anywhere, filling the void between flexible rubber and rigid plastic or simply serving as a replacement for other materials. Applications are limitless because TPUs are known for their extremely high wear resistance, good compression set, excellent tear strength, high elasticity, high transparency, great low temperature performance, durable environmental resistance and excellent processability. Combine this with their resistance to water and high temperatures, oils and grease while maintaining superior mechanical property performance, TPUs should be on everyone’s list who involved with product design, production and marketing. TPUs are a material that can add value and appeal.
Thermoplastic polyurethanes are not only the most versatile of any group of plastics, but they also offer the widest variety of technology platforms with their ability to be formulated into an infinite number of variations in chemistry and structure (stiffness, hardness and densities) to meet the most challenging needs of a particular application. And they are phthalate free. Of special note is that most polymerization of TPUs is done by the customer, not the chemical manufacturer, thus giving the processor broad control over the finished product.
By way of background, thermoplastic polyurethanes, part of a larger group of elastomers called thermoplastic elastomers or TPEs, were developed in the early part of the 20th century. They didn’t go fully commercial until the 1950’s, but are noted to be the first major elastomers to be processed by thermoplastic processing methods. Today, they are broken down into two types: 1) polyester-based TPUs, derived from adipic acid esters for applications requiring high abrasion resistance, excellent mechanical properties and 2) polyether-based TPUs for applications calling for good mechanical properties, excellent low temperature flexibility and great hydrolysis resistance.
Applications Abound Because TPUs Offer Great Features and Benefits
Generally applications include all types of consumer and industrial products (where a soft, grippy surface and rubbery, colorful or clear finish is desired) to seals and gaskets, appliances, transportation (automotive, truck, recreational vehicles), medical products, furnishings, inflatables, marine and agricultural equipment, military equipment, electronics and building and construction products, plus coatings and adhesives, among others.
There’s good reason for the popularity of thermoplastic polyurethanes. Depending upon the grade selected, they offer designers, processors and marketers a slew of benefits and features because they are:
- Tough and durable, unparalleled with their abrasion resistance
- Well suited for both soft and rigid applications
- Offer good color stability with UV additives
- Formulated for applications calling for good-to-excellent transparency
- Resistant to oil, grease, fuel, solvents and chemicals
- Cut and tear resistant
- Offer low temperature flexibility and fatigue resistance
- Resistant to creep in load bearing applications
- Available in halogen and non-halogen (fire retardant) grades
- Depending on the grade, capable of being injection molded, extruded, blow molded, calendared, slush molded or thermoformed
- Readily available in a variety of hardnesses, ranging from 50 Shore A to 85 Shore D
- Fungus and microbial resistant, especially polyether TPUs
- Good for applications calling for ultrasound transparency
- Suited for both soft and rigid applications
- Light weight (especially as a metals replacement)
- Provided with good elasticity qualities
- Hydrolytically stable and naturally antimicrobial (polyether TPUs)
- Assured to provide good color stability with UV additives
- Capable of offering excellent adhesion strength to many different substrates in overmolding soft-touch applications
Polyurethanes move quickly to the top of any preferred materials list of a designer, processor or manufacturer when the goals are, among other things, toughness/tensile strength, good aesthetics, appealing hand feel (hapstic), abrasion resistance and low-temp flexibility. Add to this other beneficial physical properties which include good impact resistance and tear strength, plus a wide selection of hardnesses.
In recent years, new thermoplastic polyurethane offerings coming onto the market have pushed the performance envelope – these are non-plasticized soft TPUs, low density TPU foams for products like shoe soles and cable, high heat TPUs for wire and cable applications, TPUs with a low coefficient of friction and new formulations to compete with flexible PVC, PP (polypropylene) and polyester in elastic nonwovens.
Newer, non-plasticized, soft TPUs with special polyester chemistry, crystallize quickly (for short molding cycles and for facilitating thin-wall extrusion), yet have the same molding characteristics as plasticized TPUs with the same hardness (60 to 75 Shore A). The difference is there is no gradual increase in hardness with time, particularly useful in overmolding applications. Current products applications include bearing dust covers and seals and mounting-plate rings, where requirements call for pressure deformation resistance, ball joint seals and numerous other applications.
The inherent tacky nature of soft TPUs has seen the development of special grades that don’t require typical additives in order to reduce or eliminate tackiness. They feature a lower coefficient of friction while maintaining transparency. These new offerings have a coefficient of friction about 90% lower than previous grades, and offer a wider procession window in blow molding and sheet extrusion processing. Since tackiness is eliminated without lubricants, waxes or inorganic fillers, the potential for blooming or adverse affects on performance, transparency, surface roughness or adhesion on the end product is eliminated.
Recent developments in light TPU foams, with density as much as 60% less than previous TPUs, open new doors for improving existing applications in cable jacketing, tubing, profiles and sheet.
New grades of both polyester- and polyether-based formulations that boost performance have come onto the scene, offering high elasticity as well as high strength along with good sealing capabilities. The primary advantages of these new TPUs over non-elastic polypropylene or PET materials are their ability to make bi-directionally elastic non-wovens in a single step and end-products that provide better sealing, recovery and filtering performance.
Still not convinced by the maverick performance of polyurethanes? According to industry data, the popularity of polyurethanes accounts for more than 15.4 US tons or 14 million metric tons sold and used annually, with a high growth rate over the past 25 years, and yes, including the recession starting in 2008. TPUs continue to grow at ambitious rates (8% or more). As a point of interest, the top applications for TPUs are building and construction, transportation, furniture and bedding and appliances, closely followed by packaging, textiles, fibers and apparel and machinery and foundry. Look into today’s TPUs to see what you – and your products – may be missing. The evidence is most convincing for a bright future for TPUs, taking the lead in applications calling for dependable performance, recyclability, ruggedness and design freedom.
TPUs (thermoplastic polyurethanes) have achieved widespread usage and popularity in thousands of products, owing to their durability, softness and colorability among other benefits. In addition, they are easy to process.