The global plastics industry is developing very rapidly, with an average annual growth rate of 4% to 6%, surpassing the growth level of global GDP. The main reason for this growth is that plastic materials continue to replace traditional materials such as metal, wood, and minerals. In fact, 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 polymers with the most unique and valuable outstanding performance, while also improving the processing performance of the product. Toughening treatment, rheological property control, aesthetic appearance, processing performance and economic factors are all important properties. All kinds of these additives have been used for many years, and a wide range of varieties have been derived after long-term development. One of the main reasons for this is the variety of emulsion polymerization processes, which makes scientists continue to design suitable polymer components, polymer structure, polymer morphology, and polymer molecular weight/molecular weight distribution. Due to the low production cost and the easy separation of the obtained emulsion products, emulsion polymerization is still very attractive in commercial production.
There are many plastic materials whose application range is very limited. This is because they either do not have the required physical properties or have very poor processing properties. Processing aids are used to enhance the melt processability of plastics, increase production, and reduce downtime. Overhaul time and provide better quality of products. In the 1950s, Rohm and Haas took the lead in developing the first commercially produced processing aid product, which was used in the production of rigid PVC. After that, this unprecedented technology was quickly known by the industry, and thus triggered a production boom in the PVC industry. Since the 1980s, such research and development work began to be carried out for other thermoplastic materials and polymer blend products. Most of the processing aids are usually added in PVC, while the processing aids used in other thermoplastics are very few (only 0.5% to 5%), but nonetheless, these processing aids can significantly Improve processing performance, while not having too much impact on other application performance. According to different functions, processing aids can be divided into fluxes, melt rheology modifiers, lubricants and dispersion accelerators. In fact, each type of processing aid has more than one effect. The functionality and application effect of any processing aid depends on its chemical composition, polymer structure, polymer molecular weight, and polymer matrix type.
PVC processing aids
As we all know, for thermoplastic resins, the mechanical properties of the final product are closely related to the homogeneity of the polymer melt during the conversion process. Unlike most other thermoplastic resins, rigid PVC cannot be processed directly due to its 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 resin, mainly related to the melting process and melt rheology during processing. Processing aids help to improve the bonding force and uniformity of the melt, and enhance the melt strength, melt ductility and melt elasticity. The components of processing aids and their polymer structure will affect the compatibility of the additives with PVC, and will change some properties, such as fluxing properties 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. Polymethyl methacrylate (PMMA) polymers have a high glass transition temperature (Tg), and it has excellent compatibility with PVC materials. These properties are conducive to the generation and transmission of local shear heat, thereby Promote the melting of PVC in the melting stage.
In addition to the melt rheological properties, improving dispersion performance, improving processing efficiency, and enhancing the overall balance of various properties (especially the melt strength corresponding to viscosity) are the main directions and goals for the development of new processing aids. On the one hand, this development trend requires processing aids to achieve the same effect with a small amount. In addition, in applications that require uniform color and transparent materials, they also require materials to be more easily dispersed and more uniform and transparent.
1, accelerated melting and melt homogeneity
The most common way to monitor the PVC melting process is to use a Brabender Plasticorder or Haake Rheometer. Figure 2.1 shows the curve of the plasticizing torque versus time during the PVC plasticizing process. The temperature of the melt at each stage is recorded. Point "A" shows the compression peak, which reflects the compression and thickening of the powder. Point "B" indicates the beginning of plasticization, followed by a plasticization peak. The "C" point appears at the moment when PVC is plasticized into a melt. The time difference from point "A" to point "C" is called "plasticizing time". The torque measured at point "C" is called "plasticizing torque". At this stage, PVC will not completely melt, and most of the melt is in the state of primary particles. Plasticization continues, and the torque begins to decrease. After the "D" point, the torque is almost constant. This torque is called the balance torque. The equilibrium torque can be roughly characterized as an estimate of melt viscosity. When the heating and shearing are continued, the PVC chain will undergo dehydrochlorination and crosslinking after reaching the "E" point, and the torque will rise again. The time difference between point "A" and point "E" is called "degradation time". Factors such as PVC formula type, processing temperature, shear rate and load level will strongly affect the melting curve.
If the melting time is shortened, the result shows that the PVC particle structure will not be completely broken, and it has nothing to do with the uniformity of the melt. However, the surface finish on the flat baffle of the roller mixer can provide a rough estimate. In the tin-stabilized PVC (K=61), only 2% acrylic processing aid is added. At a processing temperature of 180°C, the PVC material remaining on the pressure roller is very bright, smooth and uniform, and the surface of the baffle is the same smooth. On the contrary, if processing aids are not added, the melt on the roller is very uneven, and there will be many cracks on the baffle surface. The sheets processed in these two cases are shown in Figure 2.2. Sheets using processing aids have good strength, no pinhole defects, and no air streaks and melt fractures. However, the PVC film that has not been modified by processing aids is easy to tear, break, and lose its integrity. The homogeneity of the PVC melt can be detected by a transmission electron microscope (transmission electron microscope). In addition, the degree of melting of PVC can be determined by means of 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 elasticity
Melt strength is a parameter that reflects elastic deformation and elongation viscosity. Ductility describes the performance of PVC melt to undergo elongation or tensile deformation without breaking. Elasticity refers to the return to the original after the stress is removed. The trend of the state. These three rheological properties are very closely related, and it is difficult to describe them separately. Considering the three properties of tensile strength, elongation and elasticity, we define the "toughness" of the melt. If polymer processing aids are not used, PVC will not be able to withstand higher stress and stretch. Acrylic copolymers, which are usually used as processing aids, generally have good compatibility with PVC. With their long molecular chains and interactions, a harder and more elastic melt is formed. The increase in rupture stress and ductility enables PVC materials to resist the defects induced by rupture.
It is very difficult for the processor to quantitatively measure the melt strength. The Gottfert Rheotens melt strength measuring instrument uses a strain gauge-controlled traction device similar to a gear drive to pull the completely melted melt out of a right-angle (vertical drop) extruder. When the extrusion rate of the extruder remains stable, the output speed of the transmission will start to accelerate until the melt (extruded profile) breaks. Because of this, various rheological properties of PVC melt can be quantitatively measured and recorded. Extrusion swell is another way to measure the elasticity of the melt. When the polymer deforms, the polymer tends to return to its original state after the external force is removed. Usually we can observe this behavior: when the extruded material leaves the die mouth, it will swell. The degree of swelling is closely related to the recoverable deformation (x) or elasticity of the polymer. This degree is generally defined as the swelling ratio (the diameter of the extruded material / the diameter after leaving the mold), or it may also be expressed as a fixed length The weight ratio of the extruded material. As shown in Figure 2.3, the weight of the extruded material depends on the processing conditions associated with the concentration of processing aids. As predicted, the extrusion swell is also very closely related to the molecular weight of the polymer. Since the melt must enter and exit the mold during extrusion processing, the elasticity of the melt is a very important factor in determining the stability of the melt. With the addition of processing aids, the higher the pressure at the entrance of the mold, the higher the melt elasticity exhibited.
As far as current processing aids are concerned, the development of ultra-high molecular weight materials specifically for PVC foaming applications is a more leading direction. If appropriate processing aids are used, the cell structure of the extruded foam will be more uniform and the breakage will be reduced. Before fragmentation, because the PVC melt can withstand very large extension and elongation, a low-density foam material with good cell structure and good surface quality (appearance quality) can be obtained. As shown in Figure 2.4, regarding the foaming density, cell uniformity and surface quality of the foamed material, ultra-high molecular weight processing aids with a molecular weight of 8×106 are comparable to processing aids of the same type with a molecular weight of 6×106. Compared with, the effect is almost 30% better. The effect of the molecular weight of processing aids on PVC melt strength is shown in Figure 2.4. If improper processing aids are used, the foam material will have very large cells, poor appearance structure, and poor air tightness (will burst).
3, melt viscosity
Many thermoplastic resins have excellent physical properties and high use temperature, but their melt viscosity is usually very high. High melt viscosity is not conducive to material processing, often reducing production efficiency and affecting product quality. Especially in injection molding, high melt viscosity is definitely an important challenge, because no matter what material must be filled with thin-walled structures, flow through long and narrow runners, or full of complex structural shapes. Most ultra-high molecular weight processing aids will increase melt viscosity. However, there are data showing that low concentrations of standard acrylic processing aids will not have a significant effect on melt viscosity. On the other hand, the combined use of multiple functional additives can also balance the rheological properties of the melt and the uniformity of the melt. Rigid PVC compounds have successfully solved this problem. Many application parts, operating equipment, and electronic and electrical enclosures are made of PVC resin added with processing aids and impact modifiers. As mentioned earlier, as long as proper control is taken, the equilibrium torque measured by the Haake rheometer can be regarded as a rough indication of melt viscosity, and it can also be measured by many modern analytical rheometers such as capillary rheometers. Determine the melt viscosity.
Lubricants are often used to prevent plastic melts from sticking to metal surfaces during processing. The use of non-polymeric lubricants has many disadvantages, including mold fouling, impact on transparency, migration, and delayed melting. Lubricating processing aids are beneficial for metal demolding, reducing mold fouling, improving melt uniformity, and having the least impact on melting delay. Lubricating processing aids have both the function of a lubricant and the function of a processing aid. Compared with traditional processing aids, these processing aids are less compatible with the polymer matrix. Because it is immiscible with resin, there is obvious haze. However, this turbidity can be corrected by appropriate adjustment of the refractive index. Industrial PVC lubricating processing aids, such as Paraloid K-175, can reduce melt fracture, reduce shear stress, and improve surface quality without affecting the transparency of the polymer matrix.
In recent years, more and more processing aids have been used to improve the processing properties of resins other than PVC. Some polymeric processing aids are prepared by emulsion polymerization methods, while others use other methods. It has been proven that acrylic processing aids can improve the melt strength and melt uniformity of thermoplastics such as polyolefins, polyesters, polycarbonates, and ABS/SAN blends. According to reports, methacrylate polymers can enhance the rolling processability of polyethylene. Alkyl methacrylate processing aids with higher alkyl carbon numbers can improve the melt strength of polypropylene and facilitate thermoforming operations when manufacturing containers and electrical enclosures. Lower molecular weight methacrylate processing aids can act as rheology modifiers in ABS resins, reducing melt viscosity and making the melt easier to process. Adding ultra-low concentration fluorocarbon processing aids when extruding linear low-density polyethylene (LLDPE) films can not only reduce melt viscosity but also eliminate melt fracture. Due to its high strength and good barrier properties, polyvinylidene chloride (PVDC) resins are often used in packaging applications, especially multilayer films and sheets. However, the stability of PVDC is obviously inferior to that of PVC, and PVDC usually degrades rapidly at the necessary processing temperature. Acrylic additives can reduce its thermal degradation while retaining most of its important properties. Polyvinyl alcohol (PVOH) is another thermoplastic with good barrier properties, but the material has very poor processing properties at high temperatures and under shear stress conditions. For this reason, the use of high molecular weight polymers as processing aids in PVOH can make melt processing very smooth without reducing the rigidity and barrier properties of PVOH. After adding low-concentration processing aids to aromatic polyesters such as polyethylene terephthalate (PET), its melt strength and melt viscosity can be significantly improved.