During the processing of hard PVC, its particle structure will undergo important changes: At lower processing temperatures, the particles disintegrate into primary particles due to the action of heat and shearing knives; with increasing temperature, primary particles can be All smash, the crystal melts, and the boundary disappears, forming a three-dimensional network. This process is called melting (fusion) or gelation (gelation) and is generally called 'plasticization'.
For the explanation of the melting and melting process of PVC, the first is that the particles break up to release the primary particles, which are further crushed by heat, shearing, or deformed and compacted, and then the connection point provided by molecular entanglement or by means of melting. Crystallization during recrystallization connects primary particles together to form a three-dimensional network
The change of PVC particle structure will inevitably affect the performance of the product. As the processing temperature increases, the strength and stiffness of the hard PVC gradually increase to reach the maximum value, but the notched impact strength passes through the maximum value and then decreases. The overall performance of the PVC pipe is the best value. It is obtained when the melting degree is 60%~70%.
Melting can be measured by flux, microscopy, thermal analysis, rheology, etc., but rheology is the most widely used. Experiments have shown that the flowability of plasticized PVC drops sharply with increasing processing temperatures. This phenomenon can be indicated by a capillary rheometer with an increased extrusion pressure at a constant flow rate or by a decrease in the melt index value at a constant load with a melt indexer. On the melt indexer, the die The length-to-diameter ratio is 1~1.5, the load is 20~450N, and the melt flow rate of the soft and hard PVC is measured at 140 °C~150 °C. The melt flow rate of the sample is g/10min, and the melt flow rate curve is flat. The relative distance between the straight parts is pulped, ie the melting degree is calculated.