Abstract:PVC injection molding products such as electronic equipment, electronic components and other fields are developing very rapidly. Usually PVC injection molded parts appearance and physical properties are closely related to its processing, PVC processing aids can improve the processing properties of PVC a class of additives, mainly core-shell acrylate compounds (ACR). , preventing molten PVC from adhering to the equipment. This paper focuses on the advantages and functions of ACR-type processing aids in the field of hard PVC injection molding, and states the key points of processing aid selection.
Preface
Rigid PVC can be made into special shapes through injection molding, which can be used for electrical housings, fittings, valve components, etc., widening the width and breadth of PVC applications. For injection molding, the biggest challenge is the special cavity structure of the abrasive tool, the narrow flow channel resulting in processing difficulties with the cost of raw materials, appearance, performance and other contradictions, PVC is no exception.
The processing aids discussed in this paper are limited to core-shell acrylate processing aids, which usually have a molecular weight of 100. The processing aids discussed in this paper are limited to core-shell acrylate processing aids, which usually have a molecular weight in the range of 100,000-60,00,000, and are added in low amounts, with only 0.5 to 2 acrylate processing aids per 100 parts of PVC resin, but they can significantly improve the processing performance of hard PVC.
Enhancing the processing properties of PVC can be done in various ways, such as enhancing the plasticizing and melting process of PVC resin; improving the way the melt enters the mold; improving the adhesion of the PVC melt to metal surfaces, etc. ACR processing aids can have one or more of these functions, depending on the ACR processing aid molecular composition, molecular weight and structure. However, the diversity of PVC processing aids brings the complexity of processing aid selection, and it is not simple to choose the right processing aids according to the product requirements. This paper systematically introduces the characteristics of the application of processing aids, providing guidance and basis for the selection of processing aids.
Experimental part
Two sets of formulations were used to examine the different effects of processing aids. One, the formulation of PVC (K=60) was chosen to describe the effect of processing aids on PVC melt plasticization, jet flow, gate whitening and metal adhesion properties. Second, for areas with high mold filling requirements, PVC resin with K=51 was chosen to examine the effect of processing aids on its melt viscosity. The specific formulations are shown in the following table:
Table I PVC formulation table
Formulation 1 | Formulation 2 | |
PVC（K=60） | 100 | |
PVC（K=51） | 100 | |
Organotin stabilizer | 2 | 2 |
Glyceryl stearate | 0.6 | 0.6 |
Montan wax | 0.2 | 0.2 |
Acrylate processing aids as variables include different compositions and different molecular weights. The molecular weight of low molecular weight acrylate processing aids is less than 100,000; the molecular weight of medium molecular weight processing aids is 100,000 - 500,000; the molecular weight of high molecular weight processing aids is 1 million to 3 million; and the molecular weight of very high molecular weight processing aids is 3 million - 6 million.
Some of the experimental results were tested by means of a Harker torque rheometer in which the PVC was present in the form of a dry blend in these formulations. For tests such as gate whitening and jet flow were first tested by twin-screw granulation and subsequently on an injection molding machine under the experimental conditions specified in the discussion.
The role of acrylate processing aids
The role of acrylate processing aids in PVC can be summarized into three points: 1) promoting the melting of PVC resin; 2) changing the rheology of PVC melt; 3) improving the lubricity of PVC. Some acrylate processing aids have only one of the above functions, while some have two or even all of the above functions. The most important thing is to understand the need to do PVC processing, which function of acrylate processing aids need to be selected, and thus what type of processing aids to choose. This article focuses on the functions of processing aids and their advantages in injection molding, which leads to how to choose processing aids.
Promotes the melting of PVC resin
One of the most important characteristics of PVC resins is their granularity, both suspension-polymerized and propriety-polymerized PVC has a granular state. The granules have an external diameter of about 100 microns and are composed of numerous submicron particles with a diameter of about 1 micron inside, the structure is shown in the first diagram in Figure 1.
Figure 1. The plasticization process of PVC resin
Under the combined action of shear and heat, PVC can be turned into a melt with processability and formability. This process destroys the PVC particle structure, releases the primary particles, and requires the molecules in the primary particles to interact and diffuse to form a molten material with some cohesion, a process usually regarded as the change of PVC from a granular material to a smooth fluid, as shown in Figure 1. However, granular PVC is not a good conductor of heat and shear, with the result that the entire PVC plasticization process is not uniform, and in some places it remains granular unplasticized PVC resin, while in some places it has become an overplasticized melt and the resin has degraded.
PVC processing aids are designed to facilitate the transfer of heat and shear from PVC and promote homogeneous melt plasticization of PVC. Typically, acrylic processing aids are present in the form of a powder that is uniformly dispersed in the PVC resin. With its excellent compatibility and high molecular weight, it increases friction throughout the system, thus promoting melting and improving the homogeneity of plasticization.
PVC is a plasticized melt process that can be tested with a Harker torque rheometer. Figure 2 shows the torque rheology diagrams for two PVC compounds with K=60, one curve for a PVC compound without processing aids and the other for a PVC compound with 2 additional parts of medium molecular weight processing aids. Figure 3 shows the digital photographs of the PVC blends at different time points. The top row of Figure 3 has no processing aids added and the bottom row has 2 processing aids added.
Figure 2. Torsional rheology diagram of PVC mixes
Figure 3. Photographs of PVC mixture in the torque rheometer at different time points
By comparing Figure 2 and Figure 3, it is clear that PVC processing aids improve the PVC melt-plasticization process, producing a viscous and smooth melt in about 90 s, while formulations without added processing aids still produce a fragile melt after 90 s, and the melt is not complete. In extrusion or injection molding processes, the residence time of PVC in the equipment is generally very short. PVC resin melt rate enhancement is very important to improve surface roughness and smoothness at the nozzle or die. In addition, processing aids can provide glossiness to the melt. (The melt glossiness is shown at 150s in the presence of processing aids, while the melt glossiness is shown at 240s in the absence of processing aids.)
Processing aids increase the uniformity of melt plasticization because it reduces molding defects in injection molding, improves the yield and increases the consistency of the product.
In short, the benefits of processing aids promote PVC fusion, shorter plasticization time, finer and glossy product surface, and more uniform product appearance and quality due to increased plasticization uniformity.