dry
As we know, polyurethane is a polar polymer that slowly absorbs moisture when exposed to air. It is melt-formed with hygroscopic TPU pellets, and the water vaporizes at the processing temperature, making the surface of the product smooth and internally produced. Bubbles, physical properties are reduced, so in order to ensure the performance of the product, to prevent bubbles caused by moisture vaporization during melt processing, before the TPU processing, it is generally necessary to dry the pellets.
Since the polyester is easily broken by the attack of water molecules, the acid formed by the hydrolysis can catalyze the further hydrolysis of the polyester. Generally, the polyester TPU has higher water content than the polyether TPU under the same conditions. A lot, so in the drying process should pay special attention to polyester TPU, pay attention to thoroughly dry it, strictly control the drying conditions.
Pressure holding stage
When the polymer melt is injected, whether it is in the pre-plastic stage or the injection stage, the melt is subjected to the combined action of internal static pressure and external dynamic pressure.
During the packing phase, the polymer melt will be subjected to high pressure. Under this pressure, the free volume between the molecular segments is compressed. Due to the decrease in the free volume between the molecular chains, the proximity of the macromolecular segments enhances the intermolecular forces. The apparent viscosity increases. In addition, since the polyether TPU has a lower cohesive energy of the ether bond and a smaller rotational barrier of the bond, the effect of the tight chain between the enhanced molecular chains is smaller, so when compressed, the molecular chain The relative displacement is large, and the viscosity appears to be able to vary over a wide range. In addition, since the polyether TPU molecular chain is much more flexible than the polyester TPU, the permanent deformation of the polyether TPU is difficult to form. Therefore, when the polyether TPU process is held, compared with the polyester TPU, the polyether TPU should control the longer holding time.
Processing time
In general, the molecular weight increases and the molecular chain lengthens, the molecular chain center of gravity moves slower, the relative displacement between the segments cancels more opportunities, the flexibility of the long chain of molecules increases, the entanglement points increase, the chain is released and slipped. Difficulty in moving, increasing the resistance of the flow process, increasing the time and energy required, showing the sensitivity of viscosity to shear. In general, polyester TPU has higher molecular mass than polyether TPU, so its processing The time required for molding will be longer.
Processing temperature
Since the polyester TPU usually has a wider molecular mass distribution than the polyether TPU, the temperature required in the process is higher. Since the nitrogen-oxygen bond of the polyether TPU is relatively easy to break, it needs to be relatively low. The temperature can be processed.
pressure
Since the polyester TPU has a large cohesive energy, the nitrogen-oxygen bond in the molecular structure is also difficult to break, so it requires higher temperature and pressure to process its molecular bond.
cool down
Due to the large friction in the polyester TPU, the molecular cohesive energy is large, so it is difficult to cool it even if it returns to normal state, so it takes a long cooling time.
fluidity
Since the cohesive energy of the polyether TPU ether bond is low, the rotation of the bond is small. As the relative molecular mass of the polyether increases, the chain is more compliant, and the molecular chain has a high degree of flexibility, so it shows a good Liquidity, while polyester TPU is a bit less.