How Fusion Influences PVC Product Quality and the Role of Stabilizers

Fusion, also called gelation, is a key focus in the production of PVC products. What does it entail, and which factors can influence it? To understand this, we need to delve into the fundamental characteristics of PVC, which stands out from common plastics due to its particulate nature.

• In suspension polymerization, PVC manufacturers employ a suspending agent to reduce interfacial tension of PVC droplets in the suspension. This promotes better dispersion and creates smaller droplet sizes. Additionally, it allows for control over both porosity and bulk density of the PVC resin.
• These suspending agents become a permanent part of the final PVC particles. They need to be broken in the extruder. The following figure shows a representation of PVC particles:

Image source: Fillot, L.-A., Gauthier, C., & Masenelli-Varlot, K. (2006). U-PVC Gelation Level Assessment, Part 1: Comparison of Different Techniques. Journal of Vinyl & Additive Technology

Each S-PVC grain has a hard, porous outer layer that houses clusters of primary particles. Inside each primary particle there are numerous PVC chains and crystalline structures. Let’s visualize this structure with an analogy:

• An S-PVC grain: a full mesh bag of mandarins
• A primary particle: a single mandarin
• PVC chains: mandarin carpels

During the fusion process, this mesh bag of mandarins transforms into a plate comprising these individual mandarin sections.

Heat and shear are powerful tools for melting and breaking down the S-PVC grains, revealing the PVC chains and crystallites inside. The best process temperature to achieve enough fusion is 180-190°C for UPVC applications, while it is 140-160°C for plasticized PVC applications.

The next phase of fusion involves the formation of a three-dimensional network composed of PVC chains. This network plays a crucial role in the fusion process. The number of entanglements directly correlates with the degree of fusion achieved. Methylene chloride can attack and dissolve PVC chains if PVC gelation is not enough. Hence, it is used in a common basic test in manufacturing plants to assess the gelation of UPVC products Here’s a visual representation of entanglements and tridimensional network:

Image source: Fillot, L.-A., Gauthier, C., & Masenelli-Varlot, K. (2006). U-PVC Gelation Level Assessment, Part 1: Comparison of Different Techniques. Journal of Vinyl & Additive Technology

A very useful method to simulate the rheology of compounds in an extruder, which includes fusion, is by using torque rheometers also called laboratory kneader, plastograph or plasticorder. With a stable formula and consistent mixing conditions, the graph should display minimal fluctuations. The fusion time and the measured maximum torque represents the fusion process. It indicates the amount of time and energy needed for the process to break down the PVC grains and establish a three-dimensional network.

• It’s important to note that this data may not precisely mirror the actual extrusion process due to the influence of shear and mechanical forces. Furthermore, these parameters can exhibit variations among machines and even before and after maintenance procedures.

M. Schiller; “PVC Additives”, 2nd editition, C. Hanser Publisher, Munich, Germany (2022), p. 84

The optimum fusion level for rigid PVC application is in the range of 65-70%. Within this range, the product has the best mechanical properties. To assess the percentage of fusion, one can use the DSC method. Increasing the degree of fusion typically leads to enhanced tensile strength and better results in methylene chloride test. However, once you surpass 70 percent of fusion, the impact resistance starts to decrease. This decline is due to an excessive level of entanglements, which hinders the ability to absorb impact energy effectively.

Apart from PVC, other components within the formula can impact the fusion, primarily by changing the rheology within the extruder. It’s crucial to recognize that any parameter capable of modifying the shear and heat applied to the compound has the potential to affect the fusion process. The following table is a straightforward guide for a better understanding of the impact of each component role.

As previously mentioned, a one-pack stabilizer consists of a complex blend of components. Each component, as indicated in the table, can either delay or promote fusion. The quality of stabilizer is determined by the precise dosage of each component. One of the most frequent fusion-related issues can be attributed to the stabilizer formula, such as an overabundance of external lubricant or its origin. Consequently, the quality of a one-pack stabilizer has a substantial impact on the final product.

Baumann has set up a state of the art laboratory with torque rheometers (Brabender) and DSC for testing the fusion process of each one-pack stabilizer batch. These tests are customized to match the final product’s requirements, ensuring consistent fusion time and degree.

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