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COLORING PROCESSES Introduction Coloration of plastic materials is accomplished by depositing a colorant on the surface of the plastic part or incorporating the colorant into the plastic itself. Standard coating, dyeing, and printing techniques are used for surface coloration (see COATING METHODS, POWDER TECHNOLOGY; COATING METHODS, SURVEY). This article deals with colorants that are incorporated into the plastic. The three basic forms of colorants used in plastics are raw pigments or dyes, color concentrates, and color compounds. Dyes and pigments are typically dry powders. Some pigment suppliers also offer single pigment dispersions containing only pigment and polymer. These products offer increased color strength, dispersion, and ease of use. Dyes and pigments are sold into many different levels of the supply chain. Pigments and Dyes. The largest volume of raw pigment and dye is supplied to color houses that provide value-added products and services to molders and extruders. In turn the extruders and molders manufacture colored parts, bottles, and fibers. Some of the raw pigments and dyes are incorporated directly into finished parts via molding and extrusion; however, this is not common. Color Concentrate. Color concentrate or masterbatch consists of a carrier resin that is highly loaded with colorants and additives. It is designed to match a reference colored sample when let down or reduced at a specified ratio, ie 25:1, 50:1 (resin/concentrate). When let down, the concentrate colors the resin as required. Color Compound. A color compound is a system of colorants, additives, and resin that requires no letdown or addition of materials. It is ready to be processed into finished parts via molding or extrusion. A compound can be colored with dry pigment and dye or color concentrate. Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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Color Houses Color houses offer expertise in coloring of plastics. Whether a compounder or a concentrate house, they provide a high level of service to their customers. They supply a colorant and resin system that will match both the shade desired and the physical properties needed for the life of a product. The physical form of the compound or concentrate and equipment used in manufacturing colored plastic objects can vary. Color houses can provide liquid, powdered, or pelletized systems that can be converted using all types of equipment (media mills, extruders, continuous mixers). The functions and equipment typical of a color house are detailed in the following. Color Labs. Color matching is a complicated art and/or science. Color matchers must balance the designers’ aesthetic desires with the engineers’ required physical properties. This is not always an easy task. It is often difficult to achieve the color and physical properties in the desired resin. Interaction between designers and engineers provides a forum for selecting colorants and end part colors based on economics and/or performance. This improves the ability to achieve a good and reproducible color match. Color labs are outfitted with laboratory size equipment that simulates the larger machines used for production internally and by their customers. Typical processing equipment found in the lab are small extruders, two-roll mills, banburry mills, and media mills. Small rotational, injection and blow molding machines are used to duplicate the customers’ process. Instruments and computers are required for testing physical properties and color. Most labs have a computercontrolled color measuring system and a light booth to evaluate color. The spectrophotometer with computer is initially used to assist in colorant formulation and later as a quality control (QC) tool to provide certification of the quality of match to standard. The light booth provides a standardized set of conditions to visually observe color and appearance. Product literature, provided by the pigment supplier, is kept in the color lab for reference. General information regarding cost, compatibility, FDA approval, heat stability, lightfastness, and migration resistance is used for initial colorant selection. Laboratory quantities of a large variety of colorants are kept on hand. The samples are required for physical testing, QC adjustment evaluation, and color matching. The color lab is usually divided into two parts: color development and quality control. The color development group works closely with the sales and marketing departments. Anything, from a competitive color concentrate to bottles, films, or injection-molded parts, can be submitted to the lab for color-matching. Pantone® books or chips and other color standards are also used to specify or select targets. Specifications regarding resin, letdown ratios, weatherfastness, and price are typically submitted with the target. In the next step, a starting formulation of colorants, additives, and resin is developed. The computer generates formulas on the basis of a database of known spectral curves of colorants. The formulas are reviewed to ensure that colorants meet heat stability, weatherfastness, and migration and chemical resistance requirements, at a cost that is competitive. Lab samples are produced, evaluated, and submitted to the customer for approval. The customer can be internal or

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external. Internal, if they are manufacturing an end use product, or external, if the concentrate or compound is sold to a molder or extruder. The QC group is focused on the evaluation of the color concentrate or compound made in the plant. Grab samples are taken during the production run. The samples (molded chips, film, fiber) are prepared and evaluated versus standards. If required, adjustments are made to fine-tune color and properties. Incorporation of Colorants. There are a variety of points in the manufacturing process for the introduction of colorants. All involve similar steps: premixing, dispersion, and letdown. Colorants, and more specifically pigments, require more attention than other additives that can be incorporated simultaneously, eg plasticizers, antioxidants, flame retardants, fillers, and impact modifiers. This is true as the manufacturing process of pigments results in different particle sizes and particle size distributions. Primary particles (true single crystals of pigment) are uniform in size, shape, and distribution but they can combine to form aggregates and agglomerates. Aggregates and agglomerates are usually created during the drying of the pigment. The water is evaporated and the crystals come in contact with one another. Van der Waal, electrostatic, magnetic forces, and, at times, atomic bonding are responsible. Aggregates are primary pigment crystals randomly joined at their surfaces. Their interior surfaces are not available to polymer or plasticizers, and aggregates as a result are difficult to separate. Agglomerates are primary crystals joined at corners or edges with interior surfaces available. They are more readily dispersed than aggregates. The finer the dispersion and the better the incorporation, the lower is the impact on a polymer’s performance. Properties like impact and tensile strength are lowered when agglomerates or aggregates are present. Surface problems such as specks in film and injection-molded parts are also a result of poor dispersion. Poor incorporation can lead to other typical color problems of low strength and inconsistency. Processing issues such as screen pack plugging and low throughput can also be avoided if a robust process is employed. A robust process includes several steps: wetting the pigment surfaces, breaking down aggregates, and agglomerates, and distribution of the particles in the resin. The methods to accomplish this are numerous. In general, a premix is made and milled or extruded and then is let down and extruded, calendered, or molded into a final part. Many of the processes and equipments are used in more than one phase of the coloring process.

Mixing Mixing is usually the first step in the manufacturing of a color concentrate or compound. The goal of mixing is to achieve a homogeneous blend of polymer and colorants. In a dry or liquid mix, pigments are not fully dispersed. Remaining agglomerates preclude direct use in thin cross-section parts like film and fiber. The agglomerates are less of a problem in thick cross-section parts like injectionmolded containers. In any case the undeveloped pigment provides economic motivation to process further. High Shear Mixing Dry Powders and Resin. High shear mixing is required for achieving fiber and film quality dispersion. The shear impacts the pigment particle onto the surface of the resin and waxes. The air on the surface is

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partially displaced and thus the pigment is easier to “wet out” (break down) in the dispersion phase of processing. Powdered resin and waxes are included as carriers for the colorants in this step. High shear mixing is accomplished in a Henschel® or Hobart® type mixer. These equipment manufacturers are known throughout the food and plastic industries for jacketed kettle-type mixers with high speed impellers. The rotors have a mixing speed of up to 3600 rpm revolutions per minute. Most blends are run on low speed to slowly mix the ingredients and then at high speed to homogenize. A good vortex is required. A charge of no more than 80% of the mixer’s capacity is good practice. Mixers of this type can also be used to completely flux or melt the resin and incapsulate the colorants. Low Shear Mixing of Powders and Resin. Low shear mixing is suitable for easier dispersing inorganic pigments. These pigments have a large particle size and are in turn easy to de-agglomerate. Low shear mixing is preferred for pearlescents and metallics as these products can be destroyed in high shear environments. Dyes are suitable for low shear mixing, as they only need to be distributed evenly throughout the polymer. Low shear mixing can be accomplished with anything from a drum tumbler to a variety of planetary mixers. Planetary mixers have low speed screws or paddles that mix the ingredients in the bin or hopper. A charge of no more than 80% of the mixer’s capacity is good practice. Mixing Liquids. Carriers like mineral oil and plastisol are typical. Mineral oil is used in a variety of liquid color application. Plastisols find use in calendered and slush-molded PVC applications. The process provides good mixing but a low level of dispersion. Some of these blends are suitable for end-use applications. Most are milled to provide a higher quality of dispersion. A spindle or cowles mixer is used to mix colorants into liquids. The spindle is equipped with a sawtooth blade. The blade turns at 1000–5000 ft/s. Average mixing time is an hour. Flushing. The Sigma blade mixer is used almost exclusively by pigment manufacturers. It is often referred to as a “flusher.” It is named after the flush process, used by pigment manufacturers to incorporate pigment into a polymer. The goal of the process is to take the pigment while in its aqueous phase and transfer it into a plasticizer or polymer. Under temperature and shear, the pigment has a higher affinity for the polymer than water. The pigment migrates, into the polymer and the water is “flushed” (displaced) to the surface and poured off. The lid on the mixer is closed to pull vacuum and the dispersion is dried at an elevated temperature. It is allowed to cool and is cryogenically ground. This process avoids drying of the pigment during its manufacture and thus there is no opportunity to form the “hard to disperse” agglomerates. The outcome is a highly loaded, up to 60%, pigment dispersion. It is nondusting, has excellent dispersion, and offers high throughput rates. Concentrate houses utilize these products as single pigment concentrates or mix them with other flushes and traditional pigments to make high quality color matches for film and fiber. Flushing is also used to increase the pigment concentration in a typical dry color concentrate. With its high loading and low molecular weight carrier, flushing aids wetting of the dry color while at the same time increasing the pigment loading.

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Dispersion Methods for Liquids Milling is used to disperse pigment that is incorporated into a premix. The term “milling” is generally reserved for liquid or paste systems. There are several types of mills used to fully disperse pigment into polymer. Three-roll mills, media mills, and ball mills are the most common. They are used in making color dispersions for cast acrylics, epoxies, and plastisols. Viscosity is the key to a good grind and/or dispersion in liquid systems. The colorant’s loading and surface area are important factors. In general, higher loadings of low surface area pigment and lower loadings of a high surface area pigment are desired for a good grind. High surface area pigments can cause vehicle demand problems. Low viscosity mixes may run with little shear. Three-Roll Mill. The three-roll mill was designed for the ink industry in order to incorporate pigment into liquid carriers. Its main use in plastics is for the manufacture of high quality plastisol dispersions and liquid colors. The typical pigment levels for these concentrates range from 25 to 50%. The pigment level is largely dependent on its surface area and the customer’s requirements. The mill is configured as a series of three rollers horizontally positioned. They are separated only by a small nip, which can be opened or closed to control the level of shear. The premix is milled and agglomerates are reduced as it is passed through the nip from roller to roller. The finished paste is scraped off the front roll. Several passes may be required to obtain a desired level of dispersion. The viscous paste is packed out in buckets. Media Mill. The media mill, formerly designed for the ink and paint industries, is used in plastics, for the manufacture of liquid color or paste dispersions. Suitable ink or paint grade colorants along with dispersion aides are useful in solving viscosity and flocculation problems. Buehler, Netzch, and Schold are a few of the manufacturers of media mills. A premix is pumped through a cavity containing steel, ceramic, or glass shot. A variety of shot sizes are used. An impeller agitates the shot and the impact reduces agglomerate size.

Dispersion Methods and Equipment for Solids Two-Roll Mills. Two-roll mills are key to making high quality PVC and rubber concentrates. These mills are easily cleaned. The batch process is ideal for producing small volumes; thus, a variety of colors and materials can be run without much equipment down time. The two-roll mill rollers are parallel and horizontally mounted. The speeds and directions of rotation are different. The gap or nip between rolls can be controlled. Heat is applied or can be developed by friction. The dry premix is forced down through the gap and allowed to form a band or sheet on one of the rollers. There is little mixing, and so the banded material is usually cut and reintroduced manually to promote mixing (1). Leaving the material on the mill for a longer period of time can maximize dispersion. On occasion, the majority of the resin is added separately and allowed to band. The colorant blend is then slowly added. This can help contain the pigments and additives. The powders quickly stick to the banded resin and are incorporated as the material passes through the nip.

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Banburry Mixer. The Banburry mixer is most often used to produce highly loaded concentrates for olefins and PVC. The process can be used with or without a premix. A premix is recommended when making highly loaded concentrates and/or using high surface area pigments. The Banburry mixer is an internal, high shear compounder. Two cylindrical cavities intersect at an open feed throat that can be closed after the Banburry mixer is charged. Two intermeshing helical rotors turn at high rpm inside the cavities. A pneumatic ram seals the cavity for processing. Most are jacketed so they can be heated or cooled. Most are run cool, using shear to generate the heat to flux or melt the ingredients to form the concentrate. The fluxed material is unloaded via trapdoors in the bottom of the mixing chamber. The mix or molten mass can be fed either into an extruder and pelletized or onto a two-roll mill and sheeted out. Colors changes are rapid as the equipment is nearly self-cleaning. Continuous Mixer. A continuous mixer is similar to a Banburry mixer but it offers the advantage of a continuous process. Residence time can be controlled by the size of the discharge unit and/or the feed rates. The continuous mixer is known for its ability to produce high volume but tends to lack in the quality of dispersion achieved in a Banburry mixer. Extrusion Plastics processing operations often include extrusion (see EXTRUSION). Single Screw Extruders. The single screw extruder is the workhorse of the plastics industry. It is used to incorporate color into concentrates and compounds. It is also used to extrude rods, pipe, sheeting, film, siding, and other profiles. An extruder consists of a hopper, a heated barrel, a screw, and a die. The hopper is used to flood-feed the colorant and plastic premix into the feed section of the screw. An alternative to flood-feeding is to starve-feed the screw by metering in a controlled flow of material. Metering is not required for a single screw and has been found to have no impact on pigment dispersion. The feed section of the barrel can be cooled. The barrel can be heated at different temperature in different zones. The feed section of the screw is designed to convey the resin into the heated barrel, where melting takes place. The next zone is commonly referred to as the transition or compression section. The screw flights are smaller and decrease in size. External heating and heat from friction cause the melting to take place. The polymer melt is then conveyed, under pressure from the screw rotation, to the mixing and metering zones. Most extruders have a screen pack in front of the die to filter undispersed materials and trap contaminants. Typical downstream equipment consists of die face or strand pelletizing. Die face pelletizers are selected for high output extruders and “hard to run” materials. Twin Screw Extrusion. Twin screw extruders provide more mixing and dispersion energy. In general they are more versatile as they are manufactured using segmented barrels. The different sections can be vented or used as a port to “side stuff” product into the melt stream. The length can also be changed with respect to the segments. Twin screw extruders have two screws mounted side by side. The flights of one screw are intermeshing with the grooves in the other screw. Screw designs are similar to those in a single screw extruder in that different areas

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of the screws provide different functions. They differ in that they are usually modular and can be changed for a specific process or material. Screw elements are placed on a spline, as required. The elements can be for conveying, dispersing, mixing, devolatilizing, and kneading. A volumetric or gravimetric feeding system is required for a twin screw, as this type of extruder needs to be starve-fed.

Molding Coloring may take place during a molding process. Injection Molding. Injection molding is the process of forcing molten plastic into a mold where it is to be formed. Similar to extrusion, the process begins with delivery of the plastic to the feed hopper. The plastic is usually pelletized and is normally added directly or fed into a hopper. The hopper flood-feeds the resin directly into the feed throat. The barrel is heated and temperature profiles are established, similar to extrusion. The screw is unique in that it can move both forward and backward. As the screw turns, melted polymer is forced to the front of the barrel. It is held in place by a check valve. The pressure of the melt forces the screw backward in the barrel. When the desired shot size is reached the screw stops rotating. The valve is opened and the screw is driven forward. The plastic is forced through a nozzle into the closed mold. The melt is held under pressure by the cushion of polymer in the barrel. This allows the melt to completely fill or “pack” the mold. The mold itself is held under great pressure. After the part is allowed to cool, it is ejected. The screw starts to turn and feed fresh polymer into the barrel and the process begins again. The screw and molding machine itself are designed for distributive mixing, and not dispersive mixing. Color concentrate is typically used for this reason. There is not enough shear to break down the agglomerates and aggregates of dry pigments. Dispersion problems and low value in use preclude dry pigment from most applications. Also, not many molders want to manage the color-matching process. By using precolored compounds or color concentrates, the color aspects are simplified and focus can be placed on molding. Precompounded color contains all the needed ingredients required for the molded parts. It is simply added to the hopper and molded. Compounds are used when an excellent shot-to-shot consistency is required. Concentrates require a letdown before molding. On the dry side, a typical letdown ratio is 25:1, or 25 parts of polymer mixed with 1 part of color concentrate. Viscosities or melt index of the polymers should be as alike as possible for the best mixing and color incorporation. The letdown ratio can be adjusted to achieve better economics or dispersion. Higher the loadings lower are the conversion charges for the concentrate, and the better the economics. Lower the letdown ratio less colored pellets required, resulting in a lower probability of good mixing and color consistency. On the liquid side, the color is let down in the feed section just above the screw or is introduced directly into the barrel. Here it quickly mixes with the polymer as it starts to soften. The liquid color is metered at a specified feed rate. This rate can be adjusted if required. Applications for injection molding are numerous, and all but a few are colored. A few end uses are plastic containers and lids for the packaging industry, parts and components for toys, automobiles, electronics, appliances, etc.

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Blow Molding. Blow molding is a process that utilizes internal air pressure to force the polymer outwards into a clamshell mold. This technique is a very practical way of molding hollow parts. Bottles for packaging are the main application; however, parts as large as plastic drums and garbage cans are made. The two types of blow molding are extrusion and injection blow molding. Color is incorporated using the same processes as in injection molding. Pelletized and liquid color concentrate are let down into virgin resin. For injection blow molding the color is incorporated during the molding of the preform. In extrusion blow molding the color is extruded into the polymer, forming the bubble of polymer or parison. Some colorants are known to plate out and stain the mold and/or blow pin. Formulations using proper pigments and additives can prevent this occurrence. The down time required to clean the mold and blow pin is usually found unacceptable. Extrusion Blow Molding. This is simple and economical. It can produce a variety of shapes, with fairly good control over wall thickness. The process is multistep. In the first step an extrusion of a tube of partially molten polymer is extruded down into a mold area. Then, the two halves of the mold close in on the tube of polymer. This cuts it from the extruder and captures the bubble of polymer inside. A blow pin (a hollow tube) is inserted and air is blown into the mold. The air forces the parison to fill the cavity (2). The flash or excess is then cut from the container ground and recycled when possible. Injection Blow Molding. This is a two-step process. The first step is injection-molding a preform to be used in place of the parison in extrusion blow molding. Next the preform is placed into the blow mold. The preform can be soft, coming straight from injection molding, or it may require preheating. This is dependent on the type of process involved. The second step is the introduction of air through a core rod that is inserted into the throat of the bottle or part. Air pressures are higher than those used in extrusion blow molding. Rotational Molding. The process of rotational molding is ideal for creating hollow parts. Polyethylene is the most common polymer used in rotational molding. Powdered plastic is introduced into a clamshell mold. Color and additives, usually a micro-pulverized blend, are added and the mold is closed. The mold is then rotated. The powder is distributed to the mold’s surface by the centrifugal force. The mold is heated and a melt skin forms on the mold’s surface. Spinning continues as the mold is allowed to cool. The mold is then opened, the part removed, and the process starts again. Lack of mixing or shear makes the type of concentrate supplied to the rotational molder unique. Typical color concentrates, in the pelletized form, is ineffective for rotational molding. The pellets lack adequate surface to melt in and color a part. Commonly all colorants and additives are blended and micro-pulverized by a concentrate house. Pigment strength achieved by micro-pulverizing is not equal to that of an extruded concentrate. The molder simply adds the blend at the time the mold is filled. This ensures that the required color is achieved. Fiber Most fibers are created by forcing molten polymer through a spinneret to form continuous strands of polymer. This is typically accomplished using an extruder

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or gear pump. A spinneret is a die that has one to several hundred holes. The tiny holes are sensitive to corrosion and clogging. Prefiltering of the molten polymer and frequent preventive maintenance to the die and filler are required. The polymer emerges in a semisolid state and solidifies in a process called spinning. This process is not the same as the “spinning” in the textile industry, where staple fibers are twisted into yarn. Stretching and orientation is the last step. The fibers can be stretched or “drawn” while solidifying or after they have hardened. Drawing orients the molecular chains along the major axis of the fiber. This creates a much stronger yarn. Most olefins are colored by adding color concentrates to the polymer melt during extrusion (solution-dyeing, mass-coloration, pigmentation). Other synthetic fibers are spun without color and dyed later. However, pigmentation is increasing because of the lower cost and better fastness properties. Pigmentation of nylon and polyester is experiencing rapid growth. No additional process water is required and no effluents are produced, as is the case in dye systems. Inventory flexibility is reduced with pigmented fibers. Coloration can also be added in the condensation process before polymerization. In the case of polyester, this can mean the colorants are exposed to temperatures reaching 290◦ C for 5–6 h. Only a few high performance pigments can withstand these conditions. The methods of spinning are (a) Melt, (b) Wet, and (c) Dry (3). Melt Spinning. This is used with thermoplastics like polypropylene, polyamide, and polyesters. The polymer is blended with dry, pelletized concentrate or flush containing the additives required to color and stabilize the fiber. The mix is extruded and forced through a screen pack to remove agglomerates. The polymer is then pressed through spinnerets. The small strands of polymer descend vertically through a cooling chamber and are stretched and wound. The fibers can be spun in different shapes (round, pentagonal, octagonal, etc) to achieve a variety of appearances and properties. Excellent dispersion is needed where pigments are used. After processing, remaining pigment agglomerates must not exceed 5 µm. Larger particles (pigments or additives) can lower the tensile strength and often cause failure as the fiber is stretched. With this in mind, dry pigment should be avoided. Pigment preparations, flushes or concentrates, are required. Wet Spinning. This is used to spin a filtered viscous polymer that has been dissolved in a solvent. The spinnerets are submerged and the fibers are forced into a chemical coagulation bath. The filaments precipitate from solution and solidify. This is termed wet spinning as the fibers are formed in a chemical bath. Acrylic, rayon, and spandex are produced with this type of spinning. In wet spinning a pigment’s heat stability is of less concern than in melt spinning, but solvent resistance is required. Dry Spinning. In dry spinning, the polymer is dissolved in a solvent, filtered, and forced through spinnerets. The fibers are then, by use of vacuum, pulled through an oxygen-free heated chamber. The polymer solidifies as the solvent evaporates. The filaments do not come in contact with a liquid and thus there is no need for drying. Solvent recovery is easy. Acetate, acrylic, and spandex fibers can be produced using dry spinning.

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BIBLIOGRAPHY 1. T. G. Webber, Coloring of Plastics, Wiley-Interscience, New York, 1979W. 2. R. J. Hernandez, S. E. M. Selke, and J. D. Culter, Plastics Packaging, Hanser, Munich, 2000. 3. H. K. Hunger, Industrial Organic Pigments, VCH, Weinheim, 1993.

GENERAL REFERENCES Society of Plastics Engineers, Color and Appearance Division, RETEC Papers 1994.

SCOTT HEITZMAN Sun Chemical