Under proper conditions, annealing can improve the performance of TPU to a higher level than at room temperature. #Best Practices

Shown here is a comparison of the elastic modulus of TPU with a nominal Shore hardness of 70D before and after annealing. The annealing process has increased the modulus of the material above room temperature.

Here, the annealing process reduces the room temperature stiffness of the TPU, giving it a better "feel" and maintaining a softer tactile response under lower than ambient conditions.

This figure shows the effect of annealing on the glass transition temperature of molded TPU

Polyurethane exhibits an incredible range of chemistry and structure, which may be unparalleled in the field of polymers. Although all polyurethanes are basically the product of a reaction between an isocyanate and an alcohol (polyol), the exact chemical properties of these components can vary greatly to produce a wide range of properties. In addition, changing the ratio of hard and soft blocks in the structure will produce a different balance of properties, which is usually expressed by surface hardness; the lowest Shore A grade is 60, and the highest Shore D grade is 75.

Polyurethane can be chemically cross-linked to form a thermoset plastic, or it can be thermoplastic. Although these materials are generally considered elastomers, it is reported that a class of rigid polyurethanes, commercially known as Isoplast, depolymerize when heated to their melt processing temperature and re-polymerize when they solidify in the mold... It is a very clever technique that most polymers cannot achieve.

When parts molded from TPU are annealed, polyurethane chemistry can improve the properties of these materials. The characteristics of molded TPU parts have not been fully developed. If the parts are stored at room temperature, the maturation process may take several weeks. During this maturation process, mechanical and thermal properties are improved due to the formation of physical crosslinks.

They do not have the durability of chemical cross-linking, they can be destroyed by high temperature, so the material will still melt. However, physical crosslinking can improve performance in a significant temperature range. Annealing increases the formation rate of physical crosslinks in the molded part.

Although the literature on annealing generally indicates that the process only reduces the time required to achieve the desired structure, the reality is that annealing under the right conditions can actually increase the performance to a level higher than what can be achieved at room temperature. The level, no matter how long it takes you to wait. Since even the softest TPU will not melt until the temperature reaches about 150 degrees Celsius (302 degrees Fahrenheit), the general guidelines for annealing these materials state that annealing at a temperature of 110 degrees Celsius (230 degrees Fahrenheit) 16-24 Hour. But for harder grades, it may not start to melt before 200 degrees Celsius (392 degrees Fahrenheit), so higher temperatures can be used.

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For different reasons, annealing different types of materials is usually strictly regarded as a technique to reduce the stress in the mold. In traditional thermosetting plastics (such as phenolic resins and polyimides) that are cross-linked by condensation reactions, the purpose is to increase the degree of cross-linking to exceed the degree of cross-linking that is easily obtained during the molding cycle. In this case, it is often referred to as post-baking or post-curing.

Thermoset parts are usually very thick, and the degree of crosslinking of the molded part may differ between the surface of the part and its core. Post-baking tends to balance this difference, which can reduce internal stress levels.

In semi-crystalline thermoplastics, annealing is performed above the glass transition temperature (Tg) of the polymer to produce higher crystallinity. In amorphous thermoplastics, annealing is performed at a temperature slightly below Tg, which is a stress reduction technique. In most cases, the purpose of practice is to improve performance at high temperatures.

However, in TPU materials, annealing can improve the performance at both ends of the temperature spectrum. Annealing has been shown to improve performance at high temperatures, but it also enhances low temperature performance by lowering the Tg of the polymer. The usable temperature range of elastomers is defined by these two benchmarks. Below Tg, the elastomer becomes hard and brittle, and no longer has the flexibility and damping properties required by this type of material. But when they are close to the melting point, the crystal structure that constitutes the hard segment of the structure begins to crack and the mechanical properties decrease. Annealing has been shown to improve high-temperature performance while also reducing Tg. 

Figure 1 shows a comparison of the elastic modulus of TPU with a nominal Shore hardness of 70D before and after annealing. The annealing process increases the modulus of the material above room temperature. This improvement solves the problem of deformation of molded parts during assembly at high temperatures. At the same time, the annealing process reduces the room temperature stiffness of the material, so that it has a better "feel" under these conditions and maintains a softer tactile response under lower than ambient conditions. This is shown in Figure 2. A property called viscous modulus or loss modulus is used to highlight the glass transition region and identify Tg. By convention, Tg is regarded as the peak temperature of the loss modulus. Figure 3 shows a comparison of this characteristic of formed and annealed materials.

The chemistry and physics behind this transition are interesting and a bit beyond the scope of this article. But the actual importance of this behavior cannot be overstated. In this case, the annealing process uses materials that are not suitable for the desired application at the high and low temperature ends of the spectrum, and transforms them into products that meet all application requirements. Therefore, it is beneficial to keep the annealing strategy in mind when evaluating the performance of TPU applications.

Mike Sepe is an independent global materials and processing consultant whose company Michael P. Sepe, LLC is located in Sedona, Arizona. He has 40 years of experience in the plastics industry and assists customers in material selection, design for manufacturability, process optimization, troubleshooting and failure analysis. Contact: (928) 203-0408 • mike@thematerialanalyst.com.

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