The utilization of PVC wood-plastic panels heavily relies on their performance. Should the performance of PVC wood-plastic panels be subpar, people will naturally refrain from using them, thus diminishing their value. Hence, strict measures are implemented during the production of PVC wood-plastic boards to avert any issues that may compromise their performance. The melt flow rate and molecular weight distribution of the raw materials utilized in PVC wood-plastic board production are critical factors influencing their performance. Consequently, great attention is devoted to these aspects throughout the manufacturing process.
The relative molecular weight of a material can be indicated by its melt flow rate, which is inversely proportional to the former. A small melt flow rate implies a larger relative molecular weight, which, in turn, translates into better melt strength of the parison. By improving weight sag and enhancing tensile strength, impact strength, and hot deformation temperature, a higher relative molecular weight augments the quality of the final product. Conversely, a higher relative molecular weight means higher viscosity, which impairs fluidity and complicates processing. Additionally, such materials display high shrinkage, which can cause large contraction in the parison prior to molding.
It is crucial to appropriately control the molecular weight distribution in order to achieve desired properties in plastic processing. Wide molecular weight distribution offers better fluidity compared to narrow distributions, making the processing easier to control. This results in reduced die pressure, decreased fracture tendency of the parison melt, improved processing performance, and increased extrusion speed.
However, wide molecular weight distribution also implies the presence of relatively low and high molecular weight fractions. When the proportion of low molecular weight fractions is too high, it leads to a decline in the mechanical properties and thermal stability of the products. Moreover, the grading effect during the flow process causes the low molecular weight fractions to concentrate more on the surface of the extruded parison. This gives the appearance of small white particles on the surface, giving it a sprayed texture. Furthermore, the rough inner wall of the blow-molded products and the presence of these white particles can block the air valve, potentially leading to the failure of the air passage system.
Conversely, when the proportion of high molecular weight fractions is too high, it becomes difficult to plasticize the material. This results in incomplete plasticization of the billet surface, leading to a decrease in appearance quality. Currently, the trend is shifting towards using resins with bimodal distributions instead of single peak distributions. Resins with bimodal distributions offer improved processability and melt strength under similar conditions. Additionally, they exhibit significantly enhanced environmental stress cracking resistance.
To summarize, controlling molecular weight distribution is vital in plastic processing. Wide distributions offer better fluidity and processing control, while bimodal distributions provide improved processability and enhanced properties. However, an imbalance in the proportions of low and high molecular weight fractions can lead to decreased product quality and potential operational issues.