First, let's talk about general suspension method PVC resin, such as the SG5 type. Since all processing revolves around the resin, it is recommended for interested readers to refer to the book PVC Processing Technology published by the Chlor-Alkali Industry Press, with a blue cover-if I recall correctly, that's the name.
The polymerization degree of type 5 resin is around 1000, while the 800-grade resin has better fluidity.
In general, suspension method resins are typically porous particles internally with a membrane on the surface to achieve better processability.
For polyolefin materials during thermal processing, long-chain molecules break into shorter ones, effectively generating oligomers, which provide self-lubrication. As a result, the fluidity of olefin processing improves with appropriate increases in temperature and shear.
For PVC, theoretically, the viscosity decreases first and then increases with rising temperature and shear. From a practical processing standpoint, PVC resin's viscosity is positively correlated with heat, meaning that higher temperatures improve fluidity.
As the resin transitions from solid to viscoelastic to molten states, the molecular kinetic energy increases, intermolecular forces weaken, and viscosity consistently decreases.
Under the heat and shear of processing, PVC molecules undergo degradation rather than depolymerization, which is a key difference from olefin materials. Specifically, PVC macromolecular chains release hydrogen chloride, forming double bonds, which increases melt viscosity and reduces processability. This characteristic results in initial difficulty in melting, followed by poor fluidity and increased degradation, leading to delayed plasticization initially and accelerated plasticization later on.
Based on the above description, PVC, under normal circumstances, generally exhibits the characteristic of decreased viscosity under thermal shear, although not as significantly as olefin materials. However, PVC extrusion processing involves a continuous process from solid powder to molten state, repeatedly cycling, and another key factor to consider is the screw extrusion process.
This can explain why early plasticization increases the overall current, as the resin viscosity decreases, while the melt viscosity is higher. Early plasticization extends the melting zone into the feeding zone, causing an overall increase in extrusion current.
Simply increasing lubrication theoretically delays overall plasticization, lowering current, but more rigid parisons entering the high compression zone result in increased current later, along with deteriorated physical properties. In practice, the current may decrease slightly, but not significantly.
The challenge is how to reduce processing current and maintain adequate physical properties in a high-filling system.
First, it's crucial to select a large-diameter, high-speed extruder that is less sensitive to shear, though this increases investment costs. Alternatively, modifying the screw structure to enhance root shear and thrust while reducing end mixing shear is another option. Reducing mold pressure is also a solution, as is minimizing the use of filters.
Next, prioritize raw materials, using inert or highly fluid powders.
Only then should the formula be adjusted.
The principles for reducing viscosity in a formula include:
In terms of lubricant selection, the traditional ratio recommended by experienced operators is 2:3 for stearic acid to OPE, but I suggest a 2:1 ratio-using more internal lubricant and less external lubricant.
For internal lubricants, choose more polar varieties, such as GMS40, which offers better fluidity than stearic acid 1840, or EBS, which performs better than PE wax. Ester-based lubricants are highly efficient at lower amounts. In previous formulations requiring both fluidity and physical performance, a certain brand of lignite wax was used, resulting in a dosage one-fourth of the original GMS40, yet with significantly improved surface gloss, reduced exudation, and enhanced low-temperature impact strength-an impression that remains vivid.
If the current increases significantly when switching powders, formula adjustment becomes very difficult, requiring drastic measures. In my experience, simply increasing the amount of lubricant can worsen early-stage plasticization, causing issues like material overflow. It is more effective to improve the flow properties of the powder directly.
The impact of powder on processing is comprehensive. In simple terms, it delays early-stage plasticization and advances late-stage plasticization. The impact on melt fluidity is a trade-off process, directly related to surface structure and fluidity.
This characteristic results in poor processing performance in high-filling systems, which is why I emphasize the need to promote early-stage plasticization to avoid issues like material overflow and low feeding ratios, while delaying late-stage plasticization by promoting fluidity with a large amount of internal lubricant and minimizing external lubricant for demolding.