In-House Coloring of Terluran Standard (World Grades)

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1 In-House Coloring of Terluran Standard (World Grades)

2 In-House Coloring of Terluran Standard (World Grades) Introduction Choosing the coloring solution that is right for your business is critical to success in today s competitive environment. Today, many plastics processors elect to color natural resin for their manufacturing operations in-house rather than purchase pre-colored resin. In-house coloring is often a cost-effective solution in operations involving small production runs or frequent color changes. Large processors can often save money by buying uncolored resin in bulk quantities, which can be fed from a central location, such as a silo, to different molding machines. The colorants are purchased as masterbatches (concentrates), dry or liquid color, in small lots, thus making storage and handling easier and cheaper. Although attractive from a cost viewpoint, in-house coloring of most ABS resins is often difficult because of wide variation in the resin s inherent color. By contrast, BASF s new Terluran World Grade ABS resins, produced in Korea since March 1999, and in Mexico from the second half of 1999, have a consistent, very light natural color a prerequisite for successful in-house coloring. Another reason processors may be reluctant to self-color ABS resin involves its frequent use in the manufacture of high-quality parts, typically demanding good mechanical properties and optical properties such as gloss and uniformity of color. Methods of coloring resin in-house In-house coloring is accomplished by adding a suitable colorant in the form of a pigment powder, liquid dye or resin-based color masterbatch directly to the molding compound, at the molding machine, immediately prior to plasticizing. While all three forms of colorant are used in extrusion, most injection molders use a masterbatch. The use of liquid dyes is relatively uncommon in either process. Powder pigments Powder pigments are without doubt the least expensive method of coloring plastics. One advantage is that, aside from the colorant itself, no other substance is added to the resin. On the other hand, much skill and know-how is required to achieve an accurate color match. Pigment powders, which are usually metered gravimetrically, are also more difficult to handle than other types of colorant. There is also the risk of agglomeration of the primary pigment particles, which causes colored specks in the molded part. In addition, the relatively short length of injection screws limits the homogenization capacity of injection-molding machines, which in turn limits the amount of pigment that can be incorporated in the resin. Liquid dyes The advantage of liquid dyes is that the colorant is already pre-dispersed; the task is simply to distribute the colorant throughout the resin melt by a process of mixing. The dye is metered by peristaltic pump directly onto the pellets as they enter the barrel of the plastication unit. The disadvantage is that addition rates of more than 1% cause feed problems on the injection molding machine. Liquid dyes also must be handled carefully to prevent contamination of the workplace one critical reason why the use of dyes is relatively uncommon. no of lots below a certain Yellowness Index GP-22 Mean GP-22 Mean-Sdev Mean+Sdev Conv. ABS Mean Conv. ABS Mean-Sdev Mean+Sdev Mean = 19.2 Sdev = 0.9 Mean = 38.7 Sdev = 4.1 Figure Yellowness Index

3 Masterbatches Masterbatches are color concentrates in which the colorant (pigment or dye) is well-dispersed in a carrier resin. Typically, masterbatches are added at a rate of 2% and 4%, but sometimes much more if a higher pigment content is required. The disadvantage of masterbatches is the contamination of the natural resin material by the carrier resin, which may adversely affect the mechanical, thermal, rheological and optical properties. For this reason, the carrier resin and the natural resin material must be compatible preferably belonging to the same class of polymer. The big advantage of masterbatches is ease of handling. Masterbatches come in pellet form and are mainly metered volumetrically. However, gravimetric metering units allow more accurate dosing (see figure 2). Unlike powders and liquids, masterbatches are very clean and easy to handle, making color changes very easy. In-house coloring of Terluran World Grade ABS with masterbatch As already mentioned, compared with pre-colored molding compounds, inhouse coloring offers potentially large cost savings through: Bulk buying of uncolored resin Silo storage Central drying Centralized supply to molding machines Quick color changes Lower storage costs (masterbatches) Less capital tie-up The cost savings achieved through the above outweigh the extra cost of employing masterbatches, the biggest advantage of which is greater flexibility when it comes to color changes. Fig. 2: Dosing and blending unit Fig. 3: Vacuum cleaner part

4 The performance of mixing elements in in-house coloring Injection screws found in practice can have widely differing geometries. The shorter the screw, the worse their homogenizing capacity. Even those with an L/D ratio of are often unable to distribute the masterbatch sufficiently to provide streak-free molded parts. Other factors are the type and amount of masterbatch employed. Fig. 4: Static mixer Fig. 5: Faceted mixing torpedo More problems can be expected with a masterbatch addition rate of 2% than at, for example, 6%. Trials carried out by BASF indicate that the use of a mixing device produces a much better surface finish with respect to streaks in the molded part. By employing a static mixer it was possible to greatly reduce the extreme streaks produced by a 17 D screw; the result was a finish comparable to that produced by a (mixerless) 22 D screw. The homogenizing performance of three mixing elements faceted, static and Twente mixers was assessed by comparing the surface finish of flat molded parts (here, the lid of a refrigerator foodstorage box) injection molded from Terluran GP-22 containing only 0.2% (4% is normal) of a blue masterbatch. The results were compared with the surface finish produced by a 22 D mixerless universal screw. A static mixer is a nozzle device fitted to the end of the barrel. It comprises a number of individual cylindrical segments housed within the nozzle body (see photo). Between adjoining segments there is a small cavity. Polymer melt entering the nozzle is mixed as it is forced from cavity to cavity via four small channels drilled in different angles in each of the segments. The faceted mixing torpedo (see photo) is fitted to the end of the injection screw. The torpedo features diamond-shaped studs that mix the ABS melt and masterbatch through the screw s rotation and axial movement during the plastication phase of the molding cycle. The Twente mixer (a device developed at Twente University in Holland) is a combined non-return valve and mixing torpedo that is fitted to the end of the screw (see photo). The valve consists of a collar 2 to 3 times longer than a conventional check ring which is able to slide and rotate on the torpedo. The collar is perforated with rows of evenly spaced holes. The surface of the torpedo contains cavities. Due to this design very homogenous mixing of resin and masterbatch is achieved through the rotation and horizontal movement of the screw. The use of either a faceted or Twente mixer requires that the screw be shortened by about 1 D. Exact changes should be reviewed with the specific machine/screw manufacturer. Fig. 6: Twente Mixer

5 As figure 7 shows, the static mixer produced no visible improvement with regards to streaks. By contrast, the faceted mixer produced much better results but is still far from achieving the homogenous melt required for streakfree molded parts. Only with the Twente mixer was it possible to produce a streak-free surface finish on the molded part. As trials in practice have shown, the use of a suitable distributive mixing device is a must for producing good quality parts from in-house colored ABS. Thanks to the Twente mixer s very good distributive mixing capacity, a 5 15% reduction in the amount of masterbatch is possible. Standard screw static mixer faceted mixing torpedo Twente Mixer Fig. 7: Terluran GP % blue masterbatch

6 Do you have any technical questions about BASF s styrene-copolymers? We will be happy to give you answers at our Copo-Infopoint: The reference: Internet: Terluran World Grades KTTP 9905 e BASF Aktiengesellschaft Ludwigshafen Germany