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Continuous compounding of color masterbatches.

Color is as necessary to plastics as it is to life. Consider the plain white expanded polystyrene coffee cup.

While fast-food containers are littering highways and parking lots everywhere, the public has deemed the coffee cup an environmental nuisance and seeks to ban it. A moderate brown or green color, such as the paper industry has used for years in their coffee cups, would have resolved the issue.

This article discusses the processing requirements of organic and inorganic pigments, liquid colorants, and polymers, along with the different types of processing equipment. The co-rotating intermeshing twin-screw extruder, with its versatility in screw and barrel setup, is shown to be a viable option for the continuous production of color master-batches. Pigment Processing Requirements

Every color pigment has its specific physical properties-size, surface area, adhesion to plastic or itself-that should be available from the supplier. This information determines the specific processing requirements for each pigment group that must be understood before the compounding equipment for color masterbatching is selected. It is worth mentioning that there is no "ultimate" compounding equipment available for the color masterbatching industry. Recent publications list a number of types: * Banbury-type batch blenders; * Two-roll mill batch blenders; * Single-screw extruders with special shear sections; * Co-rotating intermeshing twin-screw extruders; and * Counter-rotating twin-screw extruders.

Organic pigments: This group presents the most challenging task for the masterbatch compounder. Chromophore groups absorb certain frequencies in the visible light spectrum, to give their characteristic colors. The particle sizes of primary pigment crystals are from 0.01 to 0.1 micrometers. Because this is the range of the wavelengths of visible light, light is not reflected, and organic pigments are always transparent.

One gram of organic pigment can have a surface area of 20 to 100 M[sup. 2]. For example, one gram of phthalocyanine blue has a 60- to 70-m[sup. 2] surface area, and one gram of organic yellow has a 20-M[sup. 2] surface area. This surface must be wetted out by polymer, which explains why masterbatch concentrations are kept lower (20% to 30%) when these types of pigments are used. Small particles also have higher surface forces (molecular adhesion) that cause agglomerates.

Inorganic pigments: These are produced in a burning process at 1000[deg.]C, which gives them good heat stability-about 500[deg.]C. Because the particle size is 1 to 5 micrometers, light is reflected, and products appear opaque. Their surface area per gram is 5 to 10 M[sup. 2]. In general, they are easy to disperse (low surface forces). However, agglomerates can form during the melting process.

Liquid colorants and dyes: These produce transparent effects with a maximum strength and brilliance. Most dyes are miscible with the polymer and require only a thorough distributive mixing for good homogeneity. However, some liquid colorants are immiscible, and masterbatch preparation becomes more challenging (as described later). Insufficient mixing will cause immediate bleed-out at the die.

Pigment properties: * Fastness to light. Especially when Ti0[sup. 2] is used in the formulation, a photo reaction can take place, which causes fading of the original color. * Heat stability tests are conducted for 5 min at the highest possible temperatures: 180[deg.]C to 240[deg.]C for organic pigments, and 240[deg.]C to 260[deg.]C for inorganic pigments, including lead chromate. * Color strength is a linear function of the pigment surface area. A high wet-out surface gives good color strength. * Dispersion hardness. Pigments are available from 0 to 50 hardness. Below 30 hardness, a twin-screw extruder can generate enough shear for dispersion. Above 30 hardness, a combination of two-roll mill and twin-screw extruder is used. The Two-Roll Mill

The two-roll mill is the ultimate equipment for dispersing a color pigment. The nip clearance can be reduced at every pass, and it has ample surface area to cool the batch before the next pass through the high shear area. However, its disadvantages are a dusty environment, a labor intensive operation, a hazardous working area, and batch-type accuracy. Single-Screw Extruders - Single-screw extruders are the most commonly used machines for continuous extrusion processes. However, in color masterbatch compounding, they are limited to easily dispersible pigments with 10% to 20% pigment ratios. Several processing disadvantages have been reported when a pigment-resin preblend is added to a single-screw extruder [G.M. Gale, SPE ANTEC Tech. Papers, 37, 95 (1991)]. * In the early stages of compaction, before melting starts, a fragile, solid mass is formed, which resembles a sintered material with large patches of segregated pigments Fig. 1). * The usual melting behavior of a rolling melt pool in the compression zone-starting at the trailing side of the screw flight and steadily growing in width until the channel is full-does not appear to occur. Instead, the separation of polymer granules caused by a coating of color pigments results in a certain deformation under heat and stress without achieving any fluxing. This may contribute to pigment agglomeration between solid polymer particles. * At some point, far downstream of the normal melting location, the sintered mass receives sufficient conducted heat to become fully molten, and normal rotating channel flow occurs. However, the agglomerates are entrained in the melt, and even a special high shear mixing section (Maillefer type) cannot properly disperse the pigments. Intermeshing Twin-Screw Extruders

The disadvantages of the two-roll mill and the single-screw extruder can be eliminated with the intermeshing twin-screw extruder. The melting does not depend on heat conducted from the barrel, but is accomplished in the nip areas of kneading blocks or other restricting elements. Because of the grinding action between solid pellets and pigments applied by the intermeshing of the screws, melt is generated by frictional heat buildup, and at no time is a sintered mass formed.

Both counter- and co-rotating twinscrew extruders are quite common in this market. This article focuses on the co-rotating version because it offers features that allow it to process the widest range of color masterbatches.

Screw clearances in the nip areas of co-rotating types are typically kept three times tighter than in counter-rotating types. This provides excellent self wiping, necessary for frequent color changes. Because of the dynamically balanced rotation of both screws, co-rotating machines can be safely operated at twice the screw speed of counter-rotating machines. This can be essential for reaching the critical shear stresses needed for breaking up pigment agglomerates. Stages of relaxation can be incorporated within the same screw profile, allowing for back mixing in the axial direction and compensating for minor inhomogeneities in the preblend. Compounding Concepts - The most commonly used extruder setup is shown in Fig. 2. The resin-pigment pre-blend is fed to the main throat of the extruder with a gravimetric feeding device. Proper preparation of the preblend, which can contain several pigments and resins, and the advantages of polymer powder vs. polymer pellets have been reported previously, and the findings apply here [(R.L. Abrams, SPE ANTEC Tech. Papers, 37,9 1991)].

A 10-L/D barrel section is typically required for the melting and wet-out step. Additional heat in this area-300[deg.]C is not uncommon-an prevent secondary agglomerates. Figure 3 compares micrographs of 5-micrometer microtome cuts of 20% phthalocyanine blue dispersed in polypropylene in a 60-mm twin-screw extruder at 100-rpm screw speed, 90 lbs/hr, and at 220[deg.]C and 300[deg.]C barrel temperatures in the melting zone. The size of the agglomerates is shown to have decreased from 80 micrometers to less than 20 micrometers at the higher temperature.

The remaining two thirds of the barrel length is normally operated at cooler temperatures, just above the melting point of the polymer. In special cases the zones of dispersion, with several kneading blocks in a row, are even operated at room temperature to increase the viscosity and the resultant shear stress.

A vacuum vent is standard for removal of volatiles, moisture, or air that may have been incorporated during the mixing process. The discharge section is designed for the adequate pressurization and temperature homogenization of the melt.

When only one pigment is used in the masterbatch, e.g., Ti0[sup. 2] or carbon black, the side-feeding extruder setup (Fig. 4) can be of advantage. The resin is fed into the main throat, and after it is fully melted, the pigment is fed into the extruder with a side-fed unit. The entrained air can escape through a separate atmospheric vent upstream of the pigment addition point. Wet-out is achieved in distributive mixing sections, and the proper dispersion is carried out in conventional kneading blocks. This setup is very practical for high loadings (50% to 80%) of inorganic pigments. Because of the distributive mixing elements, it appears to avoid secondary agglomerates, compared with the conventional setup.

The extruder arrangement shown in Fig. 5 demonstrates the versatility of the system. Pigments are fed exclusively into the main feed throat; the polymer melt, coming from a reactor, enters in the second barrel section, followed by a series of distributive mixing elements (gear mixers) for the wet-out process at low stress levels. The traditional kneading blocks are then used for dispersive mixing (particle reduction). This arrangement eliminates the side-feed unit. Screw Elements

Both distributive and dispersive mixing are involved in masterbatch compounding. Distributive mixing is the regeneration of interfacial layers in a blend and does not require much energy to be accomplished. In this case it applies to the wet-out process. Dispersive mixing involves particle reduction and is achieved in high shear areas. Frictional heat buildup is the unpleasant consequence. It is typically required for the breakup of pigment agglomerates.

Gear-type distributive elements Fig. 6) have proven their benefits, particularly for compounding liquid colorants where the colors are immiscible with the polymer matrix and their viscosities are drastically lower than the viscosity of the base resin [M.H. Mack, SPE ANTEC Tech. Papers, 35,120 1989)]. The flow dynamics in this mixing element ensure better incorporation of colorant into the matrix than those of the kneading blocks, and lubrication" in the extruder is avoided.

Kneading blocks are used for dispersive mixing. They can be arranged in various degrees of restriction. The maximum restriction does not necessarily correlate with the optimum degree of dispersion because resulting viscosity changes can defeat the purpose. High shear kneading blocks are typically followed by moderate conveying sections, where the barrel cooling effect can compensate for the heat buildup.

Figure 7 shows an evaluation of both types of mixers in dispersing 5% orange liquid color in a fractional-melt-index HDPE. A homogeneous dispersion of color droplets was achieved with the gear mixers. The droplet size is too large in the kneading block product, which caused "bleed-out" from the polymer matrix. Quality Checks

The old engineering target applies for the color masterbatch as well as for the equipment layout: "As good as necessary, but not as good as possible! " If film grade quality is needed, do not go for the higher-quality-category fiber grade.

The quality control tests are rated accordingly. The filter test can easily separate both grades. A specified amount of masterbatch is run through a single-screw extruder equipped with a screen. The pressure increase due to buildup of pigment agglomerates gives a good indication of the particle size of undispersed agglomerates. Figure 8 is a typical diagram of pressure increases obtained in this case for nylon 6-organic blue samples. The target specification for a fiber grade product is 3 bars or lower, which can be accomplished at a specific energy input of 0.8 kWhr/kg or higher.

Films blown from a 4% masterbatch letdown are visually inspected for agglomerate size. Typically, 20 micrometers is the maximum tolerable agglomerate size in a film 100 micrometers thick. Film inspection cannot quantify color tone, strength, and development. These can be measured on a computerized colorimeter. Color Changes

Reducing downtime by improving cleaning procedures between color changes is important for economic operation. For single-screw extruders, the only way to change from a dark to a light color is to pull the screw and clean it with the barrel section. For co-rotating intermeshing twin screws, a purge resin can be used in many cases as long as there is not a plateout of pigments on the screws and barrel.

The critical areas to clean are those where dry pigments are present: the feed hopper, the feed barrel, and screws up to the melting zone. Soapy water sprayed into the feed hopper together with the purge resin can clean these areas without the necessity of pulling the screws. The soapy water wets out the dry pigments that are coating the screw flights in the solid conveying section. The foaming and the generated steam help remove the loose pigments. After melting, the hydraulic wiping action in the extruder will ensure the final clean-out. The cleaning process is finished when the foam bubbles in the feed throat are colorless.
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Author:Mack, Martin H.
Publication:Plastics Engineering
Date:Dec 1, 1991
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