TPU is a type of thermoplastic elastomer. One of its advantages is its wide hardness range, from Shore A 60 to Shore D 85, and its high elasticity across this entire hardness range. It is also recyclable and has good processability, making it widely used in footwear, automotive, electronics, industrial, and medical applications.
In e-cigarettes, TPU is gradually expanding its application, making it suitable for soft mouthpieces and seals, replacing silicone, and can also be used to create oil cups through injection molding.
Application of TPU in cigarette holders，OXBAR
Application of TPU in sealing ring，FEELMMax
TPU oil cup，美瑞新材
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I. What is TPU?
TPU stands for Thermoplastic Polyurethane, which is a type of polyurethane that can be plasticized when heated and dissolved in solvents. Compared to mixed-reaction and cast polyurethanes, thermoplastic polyurethanes have either no or very little chemical crosslinking in their chemical structure. Their molecules are essentially linear, yet there is a certain amount of physical exchange present.
The concept of physical exchange was first proposed by Schollenberge C.S. in 1958. It refers to the reversible 'connection points' between linear polyurethane molecular chains that become reactive upon heating or exposure to solvents. These connection points are not actual chemical crosslinks but act as if they were. Due to this physical crosslinking effect, polyurethanes form a multiphase morphological structure theory. The hydrogen bonds within polyurethanes reinforce their morphology and enable them to withstand higher humidity levels.
II. What are the Classifications of TPU?
Now that we know what TPU is, what are its classifications? Depending on the different criteria for classification, there can be many different types of TPU.
Lubrizol TPU material
For example, based on the soft segment structure, they can be divided into polyester type, polyether type, and butadiene type, which contain ester groups, ether groups, or butenyl groups, respectively. Based on the hard segment structure, they can be divided into urethane type and urethanamide type, obtained by glycol chain extender or diamine chain extender, respectively. The most common division is between polyester type and polyether type.
Based on whether there is cross-linking, they can be classified as purely thermoplastic or semi-thermoplastic. The former has a purely linear structure without cross-linking bonds; the latter contains a small amount of urethane esters and other cross-linking bonds. Based on the application of the finished product, they can be divided into profiles (various mechanical parts), pipes (jackets, rod profiles), films (thin sheets, thin plates), as well as adhesives, coatings, and fibers, etc.
III. How is TPU synthesized?
Although there are many classifications of TPU, they all belong to polyurethane in terms of molecular structure. So, how is it polymerized? According to different synthesis processes, it is mainly divided into bulk polymerization and solution polymerization.
Reaction for the generation of urethane groups
In bulk polymerization, it can be divided into prepolymerization and one-step methods based on the presence or absence of a pre-reaction: The prepolymerization method involves reacting diisocyanates with macromolecular diols for a certain period before adding chain extender to produce TPU; the one-step method involves mixing macromolecular diols, diisocyanates, and chain extenders together at the same time to react into TPU.
Solution polymerization is where diisocyanates are first dissolved in a solvent, then macromolecular diols are added to react for a certain period, followed by the addition of a chain extender to form TPU.
The type of soft segment, molecular weight, content of hard segments or soft segments, as well as the aggregation state of TPU, can affect the density of TPU, which is generally between 1.10 and 1.25, with no significant difference compared to other rubbers and plastics. At the same hardness level, polyether-based TPU has a lower density than polyester-based TPU.
IV. How is TPU processed?
The synthesized TPU particles need to undergo various processing steps to form the final product. The main methods for TPU processing are the melt method and the solution method.
Application of TPU rubber coating in electric tool housings
Fused processing uses common processes in the plastics industry, such as mixing, calendering, extrusion, blow molding, and molding (including injection, compression, transfer, and centrifugal molding); solution processing involves dissolving granules in a solvent or polymerizing them directly in a solvent to form a solution, which is then used for coating, spinning, etc.
TPU can generally be made into final products without the need for vulcanization cross-linking reactions, which can shorten the production cycle and allow waste materials to be recycled and reused.
V. How is the performance of TPU?
A material is widely used because it has excellent properties, and TPU is no exception.
Like cast polyurethane (liquid) and mixed polyurethane (solid), TPU has high modulus, high strength, high elongation and high elasticity, as well as superior wear resistance, oil resistance, low-temperature resistance, and aging resistance. Among them, high tensile strength, large elongation rate, and low long-term compression set are all significant advantages of TPU.
Lianjing Foamable TPU
The following will mainly discuss the mechanical properties of TPU from aspects such as tensile strength and elongation, resilience, hardness, etc.
1. High tensile strength and large elongation rate
TPU has both excellent tensile strength and elongation rate. As can be seen from the data in the figure below, the tensile strength and elongation rate of polyether-type TPU far exceed those of polyvinyl chloride plastics and rubber.
Comparison of Tensile Strength between TPU and Other Materials
In addition, TPU does not require or uses very little additives during the processing process, which can meet the requirements of the food industry. This is something that other materials such as PVC and rubber struggle to achieve.
2. High temperature sensitivity to resilience
The resilience of TPU refers to the degree to which it rapidly returns to its original state after the deformation stress is relieved, expressed by recovery energy, i.e., the ratio of the work done in recovering from deformation to the work required to produce the deformation. It is a function of the dynamic modulus and internal friction of elastomers and is highly sensitive to temperature.
The relationship between TPU's resilience and temperatures below room temperature
The TPU soft segments consist of PTMG, PCL, and PBA, with relative molecular masses of 1000 and 2000, respectively; the hard segments consist of MDI-BDO, with hard segment contents of 48.2% (42.7% for PCL-1250) and 31.7%, respectively. Synthesized using a prepolymerization method with r0 = 1.05.
As expected, rebound decreases with decreasing temperature until a certain temperature, at which point elasticity rapidly increases. This temperature is the crystallization temperature of the soft segment, which depends on the structure of the macromolecular diol and is lower for polyether TPU than for polyester TPU. Below the crystallization temperature, the elastomer becomes very hard and loses its elasticity, resulting in rebound similar to bouncing off a hard metal surface.
3. Hardness ranges from Shore A60 to Shore D80.
Hardness is an indicator of a material's resistance to deformation, nicks, and scratches.
TPU hardness is typically measured using Shore A and Shore D durometers. Shore A is used for softer TPU, while Shore D is used for harder TPU.
Relationship between Shore A and Shore D
Because TPU's hardness can be adjusted by adjusting the ratio of soft and hard segments, it offers a relatively wide hardness range, from Shore A 60 to Shore D 80, spanning the hardness of rubber and plastic, while also exhibiting high elasticity throughout this range.
Some TPU properties change with changes in hardness. For example, increasing TPU's hardness results in increased tensile modulus and tear strength, increased stiffness and compressive stress (load capacity), decreased elongation, increased density and dynamic heat generation, and improved environmental resistance.
VI. Where can TPU be used?
In 1958, the American Goodrich Chemical Company (now Lubrizol) first registered the TPU trademark Estane. Over the following 40 years, over 20 trademarks have emerged worldwide, each with several product lines. Currently, major TPU raw material manufacturers include BASF, Covestro, Lubrizol, Huntsman, Wanhua Chemical, Shanghai Hengan, Ruihua, and Asahikawa Chemical.
As a high-performance elastomer, TPU has a wide range of downstream applications, including consumer goods, sporting goods, toys, and decorative materials. Here are a few examples.
1. Shoe Materials
TPU is used in shoe materials primarily due to its excellent elasticity and wear resistance. Footwear containing TPU is significantly more comfortable than conventional footwear. Therefore, it is widely used in high-end footwear, particularly athletic and casual shoes.
TPU midsole
TPU shoe material
2. Hoses
Due to their flexibility, excellent tensile strength, impact strength, and high and low temperature resistance, TPU hoses are widely used in China as gas and oil hoses for aircraft, tanks, automobiles, motorcycles, machine tools, and other mechanical equipment.
TPU conveyor hose
TPU pipe
3. Cables
TPU offers tear, abrasion, and bending resistance, and its high and low temperature resistance is crucial to cable performance. Therefore, in the Chinese market, TPU is increasingly used in high-end cables such as control cables and power cables to protect the sheathing of complex cable designs.
TPU cable material
TPU cable
4. Medical Devices
TPU is a safe, stable, and high-quality alternative to PVC. It does not contain harmful chemicals such as phthalates, which could migrate into the blood or other fluids contained in medical catheters or bags and cause adverse effects. Furthermore, our specially developed extrusion and injection molding grades can be easily integrated into existing PVC equipment with minimal modification.
Lubrizol TPU Medical Braces
TPU powder SLS printing heart model
5. Automotive Paint Protection Film and Color-Changing Film
Automotive paint protection film is a protective product based on aliphatic TPU film. It offers excellent properties such as high transparency, flexibility, stretchability, and yellowing resistance, making it a popular high-end product in automotive paint protection technology.
TPU car cover, photo courtesy of Hehe New Materials
6. E-cigarettes
In e-cigarettes, TPU is gradually expanding its application. It can be used to make soft mouthpieces and seals, replacing silicone, and can also be used to create oil cups through injection molding.
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