Development and Function of Plastic Processing Aid
The global plastics industry has developed very rapidly with an average annual growth rate of 4% to 6%, surpassing the growth rate of global GDP. The main reason for this growth is that plastic materials continue to replace traditional materials such as metals, wood, and minerals. In fact, the various additives added to the resin are also very helpful for the successful application of plastic materials. Among the various types of additives used, polymer impact modifiers and processing aids provide the polymer with the most unique and valuable properties, while also improving the processability of the product. Toughening treatment, rheological properties control, appearance aesthetics, processing performance and economic factors are important properties of nature. A variety of these additives have been used for many years, after a long period of development derived a wide range of varieties. One of the main reasons for this is the wide variety of emulsion polymerization processes that allow scientists to continually design suitable polymer components, polymer structures, polymer morphology, and polymer molecular weight / molecular weight distribution. Due to the low cost of production, and the resulting emulsion product is easy to separate, emulsion polymerization is still very attractive in commercial production.
There are many plastic materials that have very limited applications because they either do not possess the required physical properties or their processing properties are very poor. Processing aids are used to enhance the melt processability of plastics, increase yields, and reduce parking Repair time and provide better quality products. In the 1950s Rohm and Haas pioneered the first commercially available processing aid product to be used in rigid PVC production. Since then, this unprecedented technology has quickly become familiar to the industry and has given rise to the production craze in the PVC industry. Since the 1980s, such research and development efforts have begun to be targeted at other thermoplastics and polymer blends. Most processing aids are usually added to PVC, whereas processing aids are rarely used in other thermoplastics (0.5% to 5%), but nevertheless, these processing aids can significantly Improve processing performance, while not having a significant impact on other application performance. Depending on their functionality, processing aids can be classified as fluxes, melt rheology modifiers, lubricants and dispersion promoters. In fact, each type of processing aids have more than one effect. The functionality and effectiveness of any processing aid depends on its chemical composition, polymer construction, polymer molecular weight, and the type of polymer matrix.
PVC processing aids
It is well known that for thermoplastic resins, the mechanical properties of the final product are closely related to the homogeneity of the polymer melt during the conversion. Unlike most other thermoplastic resins, rigid PVCs can not be processed directly due to their inherent particle structure. It requires a long processing time at high temperatures, which in turn often leads to thermal degradation. Processing aids bring many benefits to PVC resins, primarily associated with the melting process and melt rheology during processing. Processing aids help improve the bond and uniformity of the melt, increasing melt strength, melt ductility and melt elasticity. The composition of the processing aid and its polymer structure will affect the compatibility of the additive with the PVC and will change some properties such as fluxing and lubricating properties. On the other hand, the molecular weight and molecular weight distribution of processing aids play a key role in controlling the rheological properties of the melt. The most common processing aids are methacrylate polymers. Polymethylmethacrylate (PMMA) -based polymers have a high glass transition temperature (Tg) and have excellent compatibility with PVC materials, which are favorable for generating and transferring localized shear heat to In the melting phase to promote the melting of PVC.
In addition to the rheological properties of the melt, improved dispersibility, improved process efficiency and overall balance of properties (especially melt strength corresponding to viscosity) are the major directions and goals for the development of new processing aids. This trend of development requires, on the one hand, that processing aids can achieve the same effect with a small amount of usage, and in applications requiring uniform color and transparent materials, the materials are also required to be more easily dispersed and more uniform and transparent.
1, to speed up the melt and melt uniformity
The most common way to monitor the PVC melt process is to use a Brabender Plasticorder or a Haake rheometer. Figure 2.1 shows the plot of the plasticizing torque versus the time during PVC plasticization. The melt temperature is recorded for each stage. The "A" point shows the compression peak, which reflects the compression and thickening of the powder. The "B" point indicates the start of plasticization, followed by the plasticized peak. Point "C" appears in the plasticization of PVC into a melt moment. The time difference from point "A" to point "C" is called the "plasticizing time." The torque measured at point "C" is called the "plasticizing torque." PVC does not melt completely at this stage, most of the melt is in the primary particle state. Plasticization continued, the torque began to decline in the "D" point after the torque appears almost constant, the torque is called balance torque. The equilibrium torque can be roughly characterized as an estimate of melt viscosity. As heating and shearing continue, after the "E" point, the PVC chains undergo dehydrochlorination and cross-linking, causing the torque to rise again. The time difference between point "A" and point "E" is called "degradation time." Factors such as PVC recipe type, process temperature, shear rate and load level strongly influence the melting curve.
If the melting time is shortened, the result shows that the PVC particle structure is not completely broken and has little to do with the homogeneity of the melt. However, the surface finish on the flat baffles of a roll mill can provide a rough estimate. Only 2% acrylic processing aid was added to tin-stabilized PVC (K = 61) and the PVC material remaining on the press roll was very clear, smooth and uniform at 180 ° C processing temperature with the same baffle surface smooth. Conversely, without the addition of processing aids, the melt on the roll is very non-uniform and there are many cracks on the surface of the baffle. The processed sheet in both cases is shown in Figure 2.2. The sheets using the processing aids are strong, have no pinhole defects, and are free from air striations and melt fracture. However, the PVC film, which has not been modified by processing aids, tends to tear, crumble and lose its integrity. The homogeneity of the PVC melt can be detected by transmission electron microscopy. In addition, the degree of melting of the PVC can be determined by differential scanning calorimetry (DSC). This technique actually reflects the degree of gelation and is related to the degree of plasticization of the PVC sample.
2, melt strength, ductility and flexibility
Melt strength is a parameter that reflects the elastic and elongation viscosities. Ductility describes the ability of a PVC melt to undergo elongation or tensile deformation without rupture. Elasticity refers to the ability to recover to the original The trend of the state. The three rheological properties are highly correlated and difficult to describe separately. Taking into account the three properties of tensile strength, elongation and elasticity, we define the "toughness" of the melt. PVC can not withstand higher stresses and stretches without the use of polymer processing aids. Acrylic copolymers, commonly used as processing aids, are generally well compatible with PVC and, by virtue of their long molecular chains and interactions, produce harder, more flexible melts. The improvement of rupture stress and ductility makes PVC material resistant to defects induced by rupture.
It is difficult for the processor to measure the melt strength quantitatively. The Gottfert Rheotens melt strength meter uses a geared similar gear-driven traction unit that draws the fully melted melt from a right-angle (vertical drop) extruder. As the extruder's extrusion rate remains stable, the driven discharge speed begins to accelerate until the melt (extruded profile) breaks. Because of this, all the rheological properties of PVC melt can be quantitatively measured and recorded. Extrusion swelling is another measure of melt elasticity. When the polymer is deformed, there is a tendency for the polymer to return to its original state after the external force is removed. Often we can observe this behavior: Swelling occurs when the extruded material leaves the die orifice. The extent of swelling is closely related to the recoverable deformation (x) or elasticity of the polymer, which is generally defined as the swollen ratio (the diameter of the extruded material / the diameter after leaving the die), or also as a fixed length Extruded material weight ratio. As shown in Figure 2.3, the weight of the extruded material depends on the processing conditions associated with the concentration of the processing aid. As predicted, swollen extrusion also has a very close relationship with the molecular weight of the polymer. As the melt enters and exits the die during extrusion, the melt's elasticity is a very important factor in determining the melt's stability. With the addition of processing aids, the higher the pressure at the die inlet, the higher the melt elasticity exhibited.
For the current processing aids, developing ultra-high molecular weight materials specifically for PVC foaming applications is one of the more advanced directions. If appropriate processing aids are used, the cellular structure of the extruded foam will be more uniform and the fracture will also be reduced. Prior to chipping, a low density foam with good cell structure and good surface quality (appearance quality) can be obtained since the PVC melt can withstand very large stretches and stretches. As shown in Figure 2.4, the foaming material foaming density, cell uniformity and surface quality, the molecular weight of 8 × 106 ultra-high molecular weight processing aid and the molecular weight of 6 × 106 the same type of processing aid phase Than the effect is almost 30% better. The effect of the molecular weight of the processing aid on the melt strength of the PVC is shown in Figure 2.4. If improper processing aids are used, the foam will have very large cells, a poor exterior structure, and poor air tightness (burst).
3, melt viscosity
Many thermoplastic resins have excellent physical properties and high service temperatures, however, their melt viscosities are usually also high. High melt viscosity is not conducive to the processing of materials, often reducing production efficiency, affecting product quality. Especially in injection molding, high melt viscosity is an absolute challenge, as any material must be filled with thin-walled structures, through narrow runners or complex structural shapes. The vast majority of ultra-high molecular weight processing aids will increase melt viscosity. However, some data indicate that low concentrations of standard acrylic processing aids do not have a significant effect on melt viscosity. On the other hand, the combined use of various functional aids balances the rheological properties of the melt and the homogeneity of the melt. Rigid PVC compounds have successfully solved this problem. Many application parts, management equipment, electronics housings are made from PVC resins with added processing aids and impact modifiers. As mentioned earlier, the equilibrium torque measured by a Haake rheometer can be viewed as a rough approximation of the melt viscosity with proper control, as can many modern analytical rheometers such as capillary rheometers The melt viscosity was measured.
Lubricants are often used during processing to prevent the plastic melt from adhering to the metal surface. There are many disadvantages to using nonpolymeric lubricants, including mold fouling, transparency, migration, and delayed melting. Lubricating processing aids facilitate metal removal, reduce mold fouling, improve melt homogeneity and minimize melt-down delay. Lubricating processing aids both have the function of a lubricant and a processing aid. Compared with the traditional processing aids, these processing aids and polymer matrix compatibility is less. As a result of being immiscible with the resin, a clear cloudy haze appears. However, this opacity can be corrected by proper adjustment of the refractive index. Commercially available PVC lubricity processing aids, such as Paraloid K-175, reduce melt fracture, reduce shear stress and improve surface quality without affecting the clarity of the polymer matrix.