Introduction To PVC Mixing Technology

In the PVC (polyvinyl chloride) processing technology, mixing technology plays a vital role as the first process in the production of PVC products. However, many companies often do not pay enough attention to the mixing process, which leads to misunderstandings among workers about mixing technology, thinking that it is enough to just mix the materials evenly. This article will introduce the role, principle and process of PVC mixing in detail to help readers fully understand PVC mixing technology.

1. The basic principle of PVC mixing

PVC products are made of a polydisperse system consisting of PVC resin, stabilizer, modifier, filler, colorant and other additives, which are mixed and then used for molding. The main purpose of mixing is to mix these raw materials evenly to form a dry blend with high apparent density, good fluidity, dryness and looseness.

The mixing process includes two meanings: mixing and dispersion:

1. Mixing: Change the distribution of two or more components in space. Through mixing, materials of different components can be evenly distributed, ensuring the consistency and stability of subsequent processing.

2. Dispersion: refers to the change in the physical properties of one or more components in the mixture, such as particle size reduction or dissolution in other components. Dispersion helps the components to fully contact and react, and improves the performance of the product.

Mixing and dispersion are generally carried out and completed at the same time. During the mixing process, through mechanical actions such as crushing and grinding, the particle size of the mixed material particles is continuously reduced to achieve the purpose of uniform dispersion.

2. PVC mixing process

The PVC mixing process mainly includes two processes: hot mixing and cold mixing.

1. Hot Mixing Technology

1.1 Definition of hot mixing

The role of hot mixing of PVC is crucial. Hot mixing is a process in which the material is rapidly dispersed along the inner wall of the mixer and falls from the center under the push of the high-speed rotating stirring blade, forming a vortex-like motion. Under the shearing effect caused by the collision and friction between the material and the blade and the inner wall of the mixer, the material transforms from a solid, single-phase, inhomogeneous state to a multi-phase, homogeneous, partially gelled state, and the material temperature continues to rise.

1.2 Main functions of hot mixing

• The material produces a certain degree of gelation (pre-plasticization): many substances with low melting points (such as lubricants, etc.) melt and penetrate or adhere to the PVC surface.

• The material obtains a good initial gelation and dispersion effect.

• The material obtains a larger apparent density and enhances its flowability: it is conducive to its transportation and improves extrusion efficiency.

• Remove moisture and low-volatile components from the raw materials: eliminate the impact of these components on product quality.

The hot mixing end temperature is a key factor affecting the quality of the mixture. After long-term experiments and exploration, the most suitable hot mixing end temperature is currently 110-120℃. If the hot mixing temperature is too low, the PVC material is unevenly plasticized, and the volatile matter cannot be completely removed, affecting the performance; if the hot mixing temperature is too high, the PVC material consumes more stabilizers in the mixing tank, which is prone to degradation and even paste, affecting production.

1.3 Hot Mixing Process

The hot mixing process includes three processes: diffusion, convection, and shearing. Diffusion makes the distribution between material components more uniform; convection and shearing use mechanical force to make the material components evenly dispersed. As the temperature continues to rise, PVC particles continue to absorb stabilizers, lubricants, etc., and are gradually coated by various components. When the material temperature rises to about 100°C, it is easy to remove moisture and other volatile substances in PVC resin and additives; when the material rises to 110-120°C, a uniform and stable dry mixed powder can be formed.

Hot mixing is also the gelation process of PVC mixture. Unmixed PVC resin has different particle sizes at room temperature, with more small particles, which can easily cause uneven plasticization during extrusion molding. During the hot mixing process, PVC resin gradually melts, plasticizes, and recrystallizes to form a network morphology structure. When the material temperature reaches 115°C, the small particles of PVC resin become large and uniform, the small particles gradually disappear, and the edges of the particles become transparent and translucent due to partial gelation.

1.4 Hot Mix Discharge Temperature

The hot mix discharge temperature determines the uniform plasticization process of the PVC material. Generally speaking, most PVC product manufacturers use 120℃ as the rated temperature index for hot mixing. When the amount of calcium carbonate is large, the hot mix discharge temperature can be appropriately increased to facilitate the balanced adsorption of calcium carbonate by the PVC material and the low melting point components, thereby increasing the apparent density.

1.5 Hot Mixing Time

Hot mixing time is also an important parameter that needs to be controlled. When mixing continuously, if the time for hot mixing to reach the discharge is short (such as less than 7 minutes), the interval between each batch of materials should be extended; when the mixer is used for a long time, if the temperature rises too slowly (such as more than 12 minutes still can not reach the discharge temperature), it can be unloaded in time and the thermocouple and temperature gauge can be checked for failure. The wear of the mixer blades also needs to be checked regularly, and they should be replaced in time if they are severely worn.

1.6 Hot Mixing Addition Order

The hot mixing addition order has a certain influence on the mixing quality and mixing tank wear. The usual addition order is: add PVC resin and heat stabilizer at low speed; add impact modifier, processing aid, internal lubricant at high speed and 60℃; add filler, titanium dioxide, ultramarine, external lubricant, UV powder, masterbatch, antioxidant, etc. at high speed and 80℃.

1.7 Mixing volume

The mixing volume is also a factor that cannot be ignored. Generally, each batch of materials should be controlled at about 75% of the volume of the hot mixing tank to avoid mixing time that is too long (or too short), resulting in excessive plasticization (or uneven plasticization) of the material.

2. Cold mixing technology

The working principles of cold mixing and hot mixing are different. The barrel of the cold mixer has an interlayer, in which cooling water flows; and the volume of the cold mixer is generally about 3 times that of the hot mixer. The hot mixed material entering the cold mixer is in a relatively spacious environment and the rotating blades are constantly stirring and tumbling. Its heat is absorbed by the cooling water in the interlayer of the inner wall of the barrel, and it gradually cools down.

PVC materials should be cold mixed immediately after hot mixing to prevent the hot mixed material from being stored at high temperature for a long time and degrading. At the same time, if the hot mixed material is not cold mixed, it is easy to reabsorb moisture when cooled in a natural state, which will have an adverse effect on extrusion. Cold mixing can not only prevent the phenomenon of water absorption and moisture return when PVC materials are cooled at high temperatures, but also is the process of further expelling moisture between molecules of PVC materials during the heat release process.

The rated temperature of cold mixing discharge is generally 40℃, and the cooling water temperature is usually controlled at 13-15℃.

Summary: PVC mixing technology is an important link in the PVC processing process. By deeply understanding the role, principle and process of PVC mixing, the mixing process can be optimized, and product quality and production efficiency can be improved. I hope this article can be helpful to readers and contribute to the development of the PVC processing industry.

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