The Influence of PVC Additives on the Plasticization Process

I. Concept of PVC Plasticization

Polyvinyl Chloride (PVC) resin is a thermoplastic amorphous polymer material that undergoes a transition from the glassy state → highly elastic state → viscous flow state under the combined action of temperature and shear force. From a rheological perspective, the plasticization of PVC precisely refers to this state transition process.

At room temperature, PVC resin exists in a glassy state; in the later stage of extrusion processing, however, it melts and enters the viscous flow state. Therefore, the plasticization process can also be simply understood as the process of resin softening, melting, and flowing.

II. Analysis of PVC Plasticization from Different Perspectives

From the perspective of PVC flow structural units (continuous phase): The plasticization process is manifested as the gradual decomposition of PVC resin aggregates from larger particles into secondary particles, primary particles, and ultimately depolymerization into molecular chains.

From the perspective of energy transfer: Plasticization is the process by which extrusion equipment converts electrical energy into mechanical shear energy and thermal energy, using these energies to break down and melt resin particles.

From the perspective of interfacial interaction: Mechanical shear force or thermal energy is transmitted to the resin surface through interfacial forces between powder particles, leading to particle breakdown and melting.

From a mathematical perspective: Plasticization is a process variable used to describe the melting behavior of PVC resin.

III. Key Evaluation Indicators of Plasticization

In the PVC plasticization process, two key indicators are typically emphasized:

Plasticization rate and plasticization uniformity.

Combined with rheological curves, the PVC plasticization process and its influencing factors can be analyzed. As a process variable, plasticization exhibits different rates. A fast plasticization rate means the resin completes melting in a short time and at a high speed. Rheologically, this is usually reflected by a short time interval between the minimum torque and peak torque in the rheological curve.

From the interfacial perspective, rapid plasticization indicates strong interfacial interactions between particles in the dry blend, enabling efficient transmission of mechanical shear force and thus rapid breakdown and melting of the resin under high shear conditions.

IV. Typical Characteristics and Control Strategies of PVC Plasticization

PVC plasticization exhibits the following typical characteristics:

- Slow initial progress

- Significant acceleration in the later stage

- Poor plasticization uniformity

- Low melt fluidity

These characteristics are closely related to the chain reaction nature of PVC thermal degradation and its viscosity-increasing property with rising temperature.

Therefore, the conventional control strategy for PVC plasticization is:

Promote plasticization in the early stage and inhibit it in the later stage,

thereby improving melt uniformity, thermal stability, fluidity, and overall flow stability.

V. Factors Affecting Plasticization Rate

The plasticization rate (typically characterized by the slope of the rheological curve from the minimum torque to the peak torque) essentially depends on the interfacial interactions between powder particles. Therefore, any raw materials or process conditions that can enhance interfacial interactions between particles will promote PVC plasticization.

It is generally believed that materials such as acrylic processing aids (ACR), calcium stearate (CaST), and oxidized polyethylene wax (OPE Wax) can promote plasticization by enhancing interfacial interactions.

During extrusion, such formulations often result in melt bodies that are yellowish-green and relatively soft. In certain shear-sensitive equipment, a similar plasticization-promoting effect can also be achieved by increasing the feed ratio. Additionally, titanium dioxide, calcium carbonate, and chlorinated polyethylene (CPE) also exhibit a certain plasticization-promoting effect.

VI. The Influence of Lubricants on Plasticization

There is no controversy in the industry that external lubricants have a plasticization-retarding effect.

But do internal lubricants promote plasticization?

Internal lubricants, typically represented by stearic acid, have the following characteristics:

- Low melting point

- Contain carboxyl groups with Lewis acid-base properties

- Certain compatibility with PVC

During processing, internal lubricants melt and adsorb on the surface of PVC resin particles or melt molecular chains, thereby reducing interfacial interactions and exerting a plasticization-retarding effect—this is also verified by rheological analysis.

However, the main function of internal lubricants is not to promote plasticization, but to regulate plasticization uniformity and melt fluidity. In actual processing, the controllability of the processing process and the uniformity of products are often more important than the mere plasticization rate.

VII. Functions and Limitations of External Lubricants

External lubricants (such as olefin-based lubricants) have no Lewis acid-base properties, poor compatibility with PVC, and high melting points. At appropriate temperature conditions, their efficient plasticization-retarding ability has attracted attention.

However, due to their incompatibility with PVC melt, external lubricants mainly exist freely between the melt and the metal surface, providing excellent mold release performance but having limited effect on improving internal melt fluidity.

This may lead to the following problems:

- Excessively high melt pressure

- Increased internal melt temperature

- Increased stabilizer consumption

- Aggravated thermal degradation of resin

Therefore, external lubricants usually need to be used in combination with internal lubricants.

VIII. The "Positive Effects" of Internal Lubricants

Although internal lubricants act on polymer chains in PVC melt and seemingly retard plasticization (which may appear unfavorable for processing), this effect can actually:

- Enable more uniform transmission of mechanical shear

- Reduce internal interfacial interactions of the melt

- Minimize thermal degradation of resin

- Improve melt fluidity and thermal stability

- Enhance melt uniformity

Ultimately, it improves the consistency and quality of products, which is beneficial for both the processing process and the end products.

IX. The True Positioning of ACR Processing Aids

Although acrylic processing aids (ACR) can promote plasticization, their large molecular weight mainly improves melt flow stability rather than significantly enhancing melt fluidity or overall thermal stability.

In addition, ACR processing aids mainly function during the processing process and have limited direct improvement on the mechanical properties of products.

It should not be expected that ACR will significantly improve the mechanical properties of products; its core value lies in enhancing the uniformity of mechanical properties.

X. Conclusion

It is hoped that the above content will be helpful to professionals engaged in the PVC extrusion processing industry.