Compounding Problems |
|
There are four problems that a compounder must solve when mixing plastics with additives. The first problem occurs when organic plastics are mixed with inorganic additives, such as glass fibers. The plastic automatically repels inorganic glass fillers and also forms clumps of glass and globs of plastic during mixing. The natural repulsion between the organic and inorganic matter is overcome by controlling the mixing process and the use of appropriate chemical coupling agents. Coupling agents have two chemical components that adhere to each other. One component easily coats, and wets out the surface of the plastic while the other part coats the glass fiber. The coupling agent is formulated for a specific plastic material and coated on the glass surface prior to mixing. The second problem involves the plastic molecule's tendency to break down from the heat. Many plastic materials are shear sensitive. The intensive mixing action coupled with heat and high pressure accelerates the breakup of shear sensitive plastics molecules. This problem is partially overcome by designing the mixer geometry so that it is less intensive. Both nylon and polycarbonate are shear sensitive materials and require special mixing techniques. The third problem is the complex flow behavior of plastic materials. Unlike water, which can be mixed at any speed without a change in its viscosity, plastics act differently. Viscosity is a measure of a fluids resistance to flow. Thick fluids have high viscosity while thin fluids a low in viscosity. Water does not change its viscosity. The water molecule is of uniform consistency, the plastics molecules is far more randomly sized with an entangled molecular structure. In turn, the plastic molecule exhibits higher resistance to flow. The heat transfer in molten plastics is very difficult. In order to melt plastic the mixing action must pulls apart the large plastic molecules and at the same time mix in the additives. If the shearing action is too high or too fast the molecules will break and decompose. Furthermore, when mixing action increases, the viscosity of the plastics decreases. With the increased mixing, the plastics molecule becomes untangled, grows farther apart, which creates more room to move. The increased distance between the molecules reduces the chain Van der Waal's force which allows the molecules to slip and melt. Heat generated from the mixing action helps to accelerate this process. As the plastic melts the heat required to maintain the melt decreases. However, if the mixing action is applied too fast, the long plastic molecules that do not have the time to untangle, will break and decompose. Additionally, shearing action will tend to physically break the lower viscosity molecules. The shearing and mixing action must be controlled and changed in relation to the change in viscosity of the plastic. The fourth problem occurs because of the physical nature of some additives and the difficulty to disperse them evenly throughout the mixture. Additives of various sizes with sharp edges or rough surfaces will increase the viscosity of the plastic melt by interfering with the slip between molecules. If the additives are not uniformly mixed, batch to batch uniformity difficult. These problems are addressed by:
Browse more: |