Thermoplastic polyurethane elastomer referred to as TPU, also known as PU thermoplastics, if according to the soft segment structure can be divided into polyester type, polyether type and butadiene type, they contain ester group, ether group and butenyl group, then polyester TPU and polyether TPU processing and what are the differences? Let me give you an analysis.
Polyurethane is a polar polymer that slowly absorbs moisture when exposed to air. The product surface is not smooth, bubbles are generated inside, and physical properties are reduced. Therefore, in order to ensure the performance of the product and prevent bubbles caused by moisture gasification during melt processing, it is generally necessary to dry the particles before TPU processing.Due to the invasion of polyester vulnerable to water molecules break, hydrolysis can generate acid and catalytic further hydrolysis of polyester, as is often the case, under the same conditions, polyester TPU is much higher than class water content of TPU, so in the drying process for polyester TPU is especially attention, attention should be paid to the thorough drying, strictly to control the drying conditions.
The holding period
Polymer melt in injection molding, whether in the pre-molding stage or injection stage, the melt must undergo the combined action of internal static pressure and external dynamic pressure.The holding stage, high pressure effect on the polymer melt, under the pressure, the free volume is compressed between the molecular chain segments, because the free volume decreases, and between the molecular chain of macromolecular chain near the viscosity increase molecular inter-atomic forces to strengthen the performance, in addition, since polyether type TPU ether bond cohesive energy is low, the rotation of the key barrier is small, which can lead to enhance the effect of molecular chain tight chain segment between is small, so when compressed, molecular chain relative displacement is larger, so the viscosity is able to change in a larger scope. In addition, as the carbon TPU molecular chain is much smoother than that of polyester TPU, it is difficult to form the permanent deformation of polyether TPU. Therefore, when holding pressure during the processing of polyether TPU, compared with polyester TPU, polyether TPU needs to control the holding pressure for a long time.
The processing time
Because in general, the molecular weight increase the molecular chain segments, longer molecular chain center of gravity move slower, segment the relative displacement between the offset the opportunity, the more the flexibility of the molecular long chain increased, around nodes increases, the chain of relief and slip is difficult, make flow resistance increases, the need of time and energy is also increasing, the sensitivity of viscosity to shear. Generally speaking, the molecular weight of clustering TPU is larger than that of clustering TPU, so the processing and molding time of clustering TPU is longer.
The processing temperature
In general, polyester TPU has a wider molecular weight distribution than polyether TPU, so the required temperature is higher during processing. Because the nitrogen-oxygen bond of polyether TPU is easy to break, it needs relatively low temperature to realize its processing.
Due to the high cohesive energy of polyester TPU molecules, the nitrogen-oxygen bonds in their molecular structure are difficult to break, so the processing of them, even breaking their molecular bonds, requires higher temperature and pressure.
Due to the large internal friction and molecular cohesion energy of polyester TPU, it is difficult to make it cool even if it returns to normal state, so a long cooling time is required.
As the cohesive energy of polyether TPU ether bond is low and the rotational barrier of the bond is small, with the increase of polyether's relative molecular weight, the chain is more pliable, and its molecular chain has a high degree of pliability, so it shows a good fluidity, while polyester TPU is slightly less.