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Are polyurethane materials resistant to high temperatures? In general, polyurethane is not resistant to high temperatures, even with a regular PPDI system, its maximum temperature limit can only be around 150°. Ordinary polyester or polyether types may not be able to withstand temperatures above 120°. However, polyurethane is a highly polar polymer, and compared to general plastics, it is more resistant to heat. Therefore, defining the temperature range for high-temperature resistance or differentiating different uses is very critical.
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So how can the thermal stability of polyurethane materials be improved? The basic answer is to increase the crystallinity of the material, such as the highly regular PPDI isocyanate mentioned earlier. Why does increasing the polymer's crystallinity improve its thermal stability? The answer is basically known to everyone, that is, structure determines properties. Today, we would like to try to explain why the improvement of molecular structure regularity brings about an improvement in thermal stability, the basic idea is from the definition or formula of Gibbs free energy, i.e. △G=H-ST. The left side of G represents free energy, and the right side of the equation H is enthalpy, S is entropy, and T is temperature.
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The Gibbs free energy is an energy concept in thermodynamics, and its size is often a relative value, i.e. the difference between the starting and ending values, so the symbol △ is used in front of it, as the absolute value cannot be directly obtained or represented. When △G decreases, i.e. when it is negative, it means that the chemical reaction can spontaneously occur or be favorable for a certain expected reaction. This can also be used to determine whether the reaction exists or is reversible in thermodynamics. The degree or rate of reduction can be understood as the kinetics of the reaction itself. H is basically enthalpy, which can be approximately understood as the internal energy of a molecule. It can be roughly guessed from the surface meaning of the Chinese characters, as fire is not

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S represents the entropy of the system, which is generally known and the literal meaning is quite clear. It is related to or expressed in terms of temperature T, and its basic meaning is the degree of disorder or freedom of the microscopic small system. At this point, the observant little friend may have noticed that the temperature T related to the thermal resistance we are discussing today finally appeared. Let me just ramble on a bit about the entropy concept. Entropy can be stupidly understood as the opposite of crystallinity. The higher the entropy value, the more disordered and chaotic the molecular structure is. The higher the molecular structure's regularity, the better the molecule's crystallinity is. Now, let's cut a small square off the polyurethane rubber roll and regard the small square as a complete system. Its mass is fixed, assuming that the square is made up of 100 polyurethane molecules (in reality, there are N many), as its mass and volume are basically unchanged, we can approximate △G as a very small numerical value or infinitely close to zero, then the Gibbs free energy formula can be transformed into ST=H, where T is the temperature, and S is the entropy. That is, the thermal resistance of the polyurethane small square is proportional to the enthalpy H and inversely proportional to the entropy S. Of course, this is an approximate method, and it is best to add △ before it (obtained through comparison).
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It is not difficult to find that the improvement of crystallinity can not only reduce the entropy value but also increase the enthalpy value, that is, increasing the molecule while reducing the denominator (T = H/S), which is obvious for the increase of temperature T, and it is one of the most effective and common methods, regardless of whether T is the glass transition temperature or the melting temperature. What needs to be transitioned is that the regularity and crystallinity of the monomer molecular structure and the overall regularity and crystallinity of the high molecular solidification after aggregation are basically linear, which can be approximately equivalent or understood in a linear way. The enthalpy H is mainly contributed by the internal energy of the molecule, and the internal energy of the molecule is the result of different molecular structures of different molecular potential energy, and the molecular potential energy is the chemical potential, the molecular structure is regular and ordered, which means that the molecular potential energy is higher, and it is easier to produce crystallization phenomena, like water condensing into ice. Besides, we just assumed 100 polyurethane molecules, the interaction forces between these 100 molecules will also affect the thermal resistance of this small roller, such as physical hydrogen bonds, although they are not as strong as chemical bonds, but the number N is large, the obvious behavior of the relatively more molecular hydrogen bond can reduce the degree of disorder or restrict the movement range of each polyurethane molecule, so hydrogen bond is beneficial to improving thermal resistance.