The choice of PEX carrier is more difficult than it might be assumed a priori, although some principles of choice of PEX carrier seems quite obvious. For example, as a carrier can not use a PEX that is incompatible with the PEX contained in concentrate. For example, low density polyethylene (LDPE) is incompatible with the PEX of acrylonitrile, butadiene and styrene (ABS). When the joint processing of LDPE and ABS in the finished product will be observed stratification, and the product will have low mechanical properties: for example, the shock strength. However, small amounts of LDPE are compatible with ABS: LDPE simply dissolved in the matrix of ABS, without adverse effects. Should such a case, recommend the use of LDPE as a vehicle for its coloring concentrate on ABS? Definitely – no. However, pigments, weighed in plastic bags on the stage of preparation of concentrates of coloring, and sometimes they break through and fall into the uterine mix based on ABS. This “pollution” of the PEX is permissible as long as long as the number of “pollutant” is insignificant and the “polluter” is well distributed in the matrix.
Paraffins – another excellent example of selecting the base PEX. They may be good carriers for coloring concentrates, but in case of misuse can cause serious problems in the finished product. For example, ethylene-bis-stearamidny wax (Eb) is widely used as a carrier. However, if the content of Ebbs in the finished product is great, there may be problems with the sliding screw extruder, migration of pigment on the surface, changing shape of the product, interior discoloration, poor adhesion with the stencil dyeing, as well as other problems associated with the external decoration of the PEX product. Ebbs also reacts with polycarbonate (PC), causing a rupture of macromolecular chains of the latter, which reduces the melt viscosity of the PEX and reduces the mechanical characteristics of the finished product. Paraffins and paraffin additives such as UV stabilizers are good speakers, because they have low melting temperatures and good processed by casting and extrusion. They also allow a very high concentration of pigments. However, these carriers can not be used to produce dry mixtures with PEXs, which must be loaded into the tube processing equipment in the dried form. The carrier will melt during drying and can turn into one big ball carrier with the PEX.
To achieve good tsvetootdachi concentrate should melt near or below the temperature of the final compound, as well as to have viscosity that is close or lower relative to the PEX, which is the custom. Usually pre-painted PEX concentration of pigment is low enough, so the impact on the viscosity of the PEX is negligible. When the concentration of pigment and its dispersion in the carrier system viscosity increases to as long as shear stress and viscosity of the system will not be so large that the heating due to friction can lead to PEX degradation during compounding. This phenomenon may occur because the pigment content in the PEX is so large that the PEX chains between the particles of the pigment is not enough to ensure the sliding material. An indirect measure of the surface area of the pigment is maslopogloschenie, it can characterize the degree of filling of the PEX pigment without any problems with the implementation of its color. For example, particles of carbon black has a surface area of 20 to 1000 m2 / g, whereas the majority of organic pigments, the figure ranges from 10 to 100 m2 / g, and for inorganic pigments, from 5 to 50 m2 / g. For comparison: a pound phthalocyanine pigment has a surface area of 5 acres. This means that the PEX carrier at 50% concentration of the phthalocyanine blue pigment to be distributed over an area of 5 acres to cover the surface of pigment particles. Suppose what a pound of PEX contains about 1 pint of pellets. Imagine how thin should be a layer to cover 5 acres and then understand why 30-35%-ing the content of the phthalocyanine pigment is its largest real concentration.
To prevent a too high viscosity of some of the carriers during the filling of the pigment, the drafters of the coloring compositions are three possibilities:
1) use low molecular weight grade of PEX with high melt flow rate compared with paintable PEX;
2) use plastikatory and other additives to improve the sliding of the melt of the PEX by reducing its viscosity;
3) to hold the pigment content in the concentrate at the lowest possible level.
In practice, all three possibilities. If low molecular weight grade of PEX is not available, the PEX is treated with additives to break the macromolecular chains, leading to greater melt flow rate. In the case of the PC as a carrier of the concentrate can use a secondary PEX material obtained by processing the CD-ROM drive. It has a low viscosity, and processing further reduces its molecular mass, thus providing an even higher rate of flow of the melt. In addition, the secondary PEX has low cost and therefore is widely used in the production of concentrates of coloring. Application plastikators also considered to be highly effective and widely used in the production of concentrates. All these techniques lead to a positive outcome. At the same time, they are causing all sorts of problems. Thus, reduction of molecular weight of the carrier may reduce the impact resistance of the finished product below the permissible level. Plastikator can migrate to the surface during the casting products in the form and cause the appearance of blemishes. They may cause the dissolution of organic pigments and their interaction with dyes, leading to a change in tone color. If plastikator migrate to the surface of the concentrate (as they tend to do so), they may affect the achievement of color pigments, by what is called the “syndrome of watermelon seeds – if you Squeeze watermelon seeds with your fingers along the edges, seed slip out of your hands. In equipment for processing of PEXs is something similar. Plastikator provides sliding of particles concentrate on the surface of PEX pellets instead of divide them into smaller particles. As a result, concentrate poorly distributed in the volume of the PEX, and realized a weak staining. But perhaps the greatest challenge to the use plastikators is their evaporation and the emergence of odor during processing. Plastikatory may evaporate during the recycling process, causing a strong fumes in industrial facilities that is harmful to the operator. It is known that plastikatory quickly dissolve in the PEX at elevated temperatures. If their content in the concentrate is large, with cooling of the finished product they can migrate to the surface. This is especially noticeable in the dark staining colorant or the use of colorants, migrating at the same time plastikatorom.
Plastic and paraffin additives at the beginning of the recycling process usually provide a good distribution of pigment particles in the PEX melt, but at the stage of casting in a form they can cause a change in shape of products due to overheating of the inner regions of the PEX. Most of the shape defects observed when using crystalline or amorphous PEXs, which recrystallized after melting. Particularly noticeable in the recrystallization of thick-walled products, which are cooled more slowly than thin-walled products.
The main advantage of using concentrates is that they are less polluted than the original pigments and dyes.
