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Background Information: Equipment – Mixing, Compounding, and Dosing
CHAPTER 21 Background Information: Equipment - Mixings Compounding^ and Dosing
21.1 Incorporation of Additives
Additives are introduced into polymer compounds is by means of physical mixing, using a paddle-blade mixer or roll calender, for powdered materials, or a compounding extruder, for thermoplastic melts. The key criteria are the type and nature of additive and the point at which it should be introduced, the degree of precision in measurement of polymer/additive ratios and the work that has to be carried out to ensure good homogeneous mixing, with even dispersion of the additive. Modern polymer compounds, with many different types of additive, present strong challenges to the designers of the system. Other equipment, particularly dosing systems, can be mounted on the extruder, and is increasingly used for precise direct metering of very small quantities of additives such as liquid colour systems. The last few years have seen major developments in more efficient design of mixers and extruders. There has also been important development on the materials side, to make additives easier and more suitable for compounding, opening up to smaller compounders the possibility of making special compounds. This will be mainly to offer a range of colours, while only needing to hold stocks of natural material, or to add or boost specific properties, such as UV stability, anti-static properties or flame retardancy. To facilitate in-plant compounding, most suppliers have developed systems that efficiently and reproducibly deliver a controlled package of additives to a compound, using either a specialized concentrate or a masterbatch formulation. Compounding as a separate industry was originally devoted mainly to the addition of colouring, especially for special colour matches. This gradually developed to skill in short-run production, with high efficiency in distribution, to meet the demand from plastics processors for small volumes of special grades. Many independent compounders now operate under franchise from polymer producers, while also marketing their own formulations. From this base, compounders have extended their activities to production of technical compounds, incorporating special additives and chopped glass-fibre
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reinforcement. Modern compounding equipment is based mainly on extrusion mixing and incorporates a number of points at which other materials can be introduced, at various stages in the process, possibly including satellite extruders to produce blends. In the most sophisticated operations, there are facilities for reactive compounding, in which additives and other polymers are chemically reacted with the main polymer system. Modern compounding may involve the introduction of several different ingredients and, for all equipment, the crucial aspect is to produce homogeneous mixing, often with very precise measurement of the various components. Most suppliers of compounds include a wide choice of such packages in their standard range and can readily formulate customized systems, to meet a specific requirement. Some of the polymer manufacturers have also made available advanced additive delivery systems that they have often developed originally for their own use.
21.2 Mixing Thermosets
Thermosets are mainly viscous liquid resins, presenting their own compounding requirements. Liquid mixing systems of varying degrees, from the most basic to highly sophisticated are used, depending on the type of resin, additives, and volume output required. The amount of additive by weight which can be incorporated will depend on the particle size, density and oil absorption properties. For example, addition of porous high oil absorption fillers (such as diatomaceous silicas) and chopped glass can greatly increase the viscosity of a resin system at low filler loadings of only 1-50 phr. Medium weight granular fillers, such as powdered aluminium and alumina may be used at loadings of up to 200 phr. The non-porous lower oil absorption fillers, such as aluminium oxide, silica and calcium carbonates can be incorporated at levels of 700-800 phr without making the formulation unworkable. Loadings can be increased by adding a diluent, but this may not always be desirable, as the diluent may detract from other desired properties. Organotitanates can be added to improve filler wetting, enabling higher loadings at the same viscosity. Fine particles are easier to incorporate and there is a lesser tendency for them to settle. Coarse and heavy fillers will settle and *cake' on standing, but this may be countered by adding lightweight fillers such as the colloidal silica compounds. It goes without saying that the crucial aspect is to obtain a completely homogeneous mixture of polymer and additive. This is a difficult target, bearing in mind the viscosity of the polymer and the often particulate nature of the additive. To make things easier, additive producers have and still improve their products to facilitate mixing. Pre-heating fillers before mixing with polymer or resin can improve the mixing rate, reduce energy consumption in processing, reduce wear on equipment and improve product quality, it is reported. Addition of fillers cold has a number of
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disadvantages, not the least of which is the difficulty of maintaining consistent mixing cycles or production rates during seasonal changes in temperature or humidity, especially if the filler is stored out of doors. During winter, unheated fillers may have greater levels of condensed and absorbed surface moisture than in the summer and can require as much as 1 5 20% longer mixing time. Moreover, since mixing relies heavily on frictional shear heating to achieve plasticizing and dispersion, there can be excessive wear on compounding equipment blades, screws, barrels, and walls. High friction can also break down the aspect ratio of the filler, degrading properties of the finished compound.
21.3 Mixing Thermoplastics
Additives can be introduced at various stages in production and moulding of a thermoplastics compound, depending on the type and form of the additive. Comonomers may be also regarded as additives and introduced during polymerization, depending on the polymer matrix, but most additives conferring special properties, such as reinforcement, colouring or modification, are added to the polymer matrix in a secondary compounding stage. Depending on volumes, this may be carried out by polymer producers themselves, in their own in-plant facilities, or under contract, or by independent compounding organizations. Large special-purpose extruders are used, designed for very efficient mixing with close control over temperature and shear rate, and high output. An important qualification is that the compounds are heat sensitive, calling for a closed system with accurate control over temperature. Regarding the latter, it should also be remembered that the shear action of compounding may produce its own heat, which must also be considered. 27.3.7 Dry mixers
The basic form of mixer operates on a batch process, using dry materials such as powders, or pastes. For the simplest work, a tumble mixer design has been used for many years, for addition of colour or mixing with masterbatch concentrate. More technical demands, however, have called for mixers normally using the Z-blade design, giving precise control over ingredients and times. This system is widely used in production of PVC compounds, which can be mixed in dry powder or paste form. In this sector, there has is always been an argument about whether it is better for the processor to mix compounds in-house, or buy in compounds ready-made. The answer lies very much in the type and volume of work, and particularly in the cost of handling hazardous substances with the safety and consistency that is demanded in processing today. In PVC processing, volume is a clear criterion, and it is unusual to be a large processor without having in-house mixing facilities. Centrally made PVC compounds are one of the strongest growth sectors of the business, and all resin producers have invested strongly in this sector.
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An alternative design employs one or two kneading rotors, working inside a fixed stator housing. This can be of various sizes, and offers considerable scope regarding the geometry of the rotor flights and stator walls, to optimize the mixing process. Continuous PVC compounding is claimed by Colortronic for its Novablend system with automated gravimetric blender and liquid doser, with a specially designed horizontal mixing screw and a high-speed Waeschle mixing turbine. Precision blends and flexible formulations can be produced with greater accuracy and at lower cost. Unlike conventional systems, the system creates sufficient nominal friction to raise the temperature of the PVC powder high enough to absorb liquid stabilizers for rigid PVC. A mixer particularly suited to free-flowing powders, has been introduced by Hosokawa Mikron BV. Operating by the use of shear forces, the Cyclomix is conical and is filled to about 50% of its capacity. A central high-speed rotating shaft is driven from the mixer cover (so removing seals and bearings from the area of the product). Paddle-shaped mixing elements rotate close to the inner wall and their high speed plus the conical shape of the mixing vessel transport the product from the bottom of the chamber to the top, intermixing the particles by friction against the wall. On reaching the top, the product flows down again into the centre. Special features of the Cyclomix are: fast intensive mixing, widespread applications, self-emptying, no seals and bearings in the product zone and good control over the product temperature. A feeder unit with flexible polyurethane walls has been developed by Brabender Technologic, which offers high reliability particularly for poorly flowing ingredients. The Flex-Wall uses massage paddles instead of agitators and can be employed as a 'stand alone' volumetric feeder or as a loss-in-weight feeder in conjunction with a Brabender scale. Removal of the screw tube and dismounting of the screw is rapid and, if necessary, the flexible hopper can easily be removed for further dry or wet cleaning. The feeder is particularly suited to systems where processes have to be interchanged frequently, where the entire hopper can be exchanged together with its contents, without the need for emptying. 27.3.2 Calendering Calendering is a method of mixing in which polymer and additives are introduced into a stack of rollers, which apply work and heat and progressively reduce the thickness of the mix to that of sheet or film, with the facility also of adding an embossed surface. The equipment, which has been 'borrowed' from the rubber industry, is usually large and costly, but laboratory-scale systems are also available, for small-scale testing of compounds and formulae. 27.3.3 Extrusion compounding Extrusion compounding is the main method of compounding thermoplastics. The base polymer is fed continuously by means of a hopper into a plasticizing
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chamber or barrel, in which it is worked by means of a rotating Archimedes-type screw, while heat is applied by elements in the barrel and screw, as required. At appropriate locations there are ports through which additives can be fed as required. Glass fibre, for example, requires specific handUng. Pigments may be added on the shop floor (enabling raw materials stocks to be rationalized to natural material only), but there may be potential health hazards. When the required homogeneous mix has been achieved, the output of the machine is extruded as strands of polymer melt, which are cooled in a water bath and are chopped into free-flowing granules, for packaging and shipment. As the geometry of the granule has an important influence on processing properties, the design and technology of the granulating stage is critical. The system offers great flexibility in the speed and geometry of the screw and the number and location of heating elements. Single or co-rotating twin-screw systems are used. Much has been learnt during the past few years and several new machine designs have been introduced to the market. There is also a trend now towards modular construction, giving compounders a large degree of flexibility in rapidly modifying a machine for a special grade. 213.4 Compounding
mineral fillers
With the increasing use of mineral fillers to improve mechanical properties of a compound, the demands on the compounding extruder have risen. Fillers can be added in concentrations ranging from 20 to 80% by weight, meaning that it is not possible to have a single standard feed system. They can be fed at different positions through one of a number of side feeders. All fillers contain air and moisture, which must be removed during compounding, along with other volatiles. Most machines therefore have at least one degassing zone in the melt section, downstream from the filler addition, usually operating under vacuum. 27.3.5 Fine talc masterbatch
Among recent compounding developments by Buss Compounding Systems is production of masterbatch concentrates using fine grades of talc, especially for automobile components, where the industry is looking for a system capable of dosing a talc masterbatch directly into a base polymer, at the injection moulding machine. This calls for fine talc masterbatches at 70-80% loadings, but fine talc is difficult to compound due to the low bulk density of the powder. A Buss MKS 70 mm 20L/D machine was adapted, with the kneading chamber configured to permit vertical feeding of polymer and filler in the first barrel module, and side-feeding of filler and vacuum degassing downstream. A split feed technique was used, introducing one-third of the talc together with the polymer at the first inlet and the rest downstream, directly into the melt. The interaction between the fixed pins and reciprocating screw flights in the Buss kneader, giving multiple mixing as the material moves down the chamber, provides a high degree of surface renewal and optimum wetting and degassing. Samples at 70% loading have been successfully produced.
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213.6 Single- and twin-screw
extruders
Two basic designs of extruder are in use today, equipped with one or two screws. The single-screw design is generally considered a more simple but rugged machine, with good endurance, and is best suited for long continuous runs of the same formulation. The twin-screw design, however, is more complex but gives greater flexibility in mixing special compounds. In polymer mixing, a distinction can be made between distributive and dispersive mixing. The former method aims to improve the spatial distribution of the components, without cohesive resistance playing a role. It is also called simple or extensive mixing. Dispersive mixing has to overcome cohesive resistances to achieve finer levels of dispersion. It is also called intensive mixing. Dispersive mixing is the more difficult and, in single-screw extruders, is thought to be inefficient because the mixing is achieved primarily in shear and the compound is exposed to the mixing action of the high stress region only once. Twin-screw extruders provide a uniform and reliable feed of the polymer and mineral fillers, with effective degassing in the melt zone. There must be complete wetting of the filler by the polymer melt and good dispersive mixing of the fillers into the melt, while the heat and shear history of the matrix is reduced to a minimum, to avoid degradation. Specialized screw kneaders have been developed, based on co-rotating twin-screw machines in single- or two-stage versions. The screw speed does not influence the physical properties, provided that there is constant torque utilization. Additionally, the number of filler additions has only a negligible effect, giving the compounder good flexibility and permitting adjustment of rate to suit market demands. While most of the development work has been carried out on filled polypropylene, Werner and Pfleiderer has also been working with polyethylene butylate, nylon 6 and 66, and ABS. The company's MEGAcompounder is a twinscrew design that is being produced with capacity to supply extra torque, making it possible to raise the screw speed from 400 to 500 rpm, so allowing an increase of about 50% in throughput. Higher speeds have been run with nylon without adversely affecting the properties. Krupp Werner & Pfleiderer has a number of new developments. The ZSK 50 Mc filling a gap in the range, with 2 5 0 - 1 0 0 0 kg h~^ output, has a new design concept with tie-rod tensioning of the block barrels and three-point support for base-frame and gearbox. The biggest yet co-rotating twin-screw (ZSK 380 Mc), with a screw diameter of 380 mm and output up to 75 tonnes per hour, it uses a special drive concept and new measuring devices can examine the entire bore length within half a day, to identify barrel wear without dismantling. The machine also features improvement in internal mixers: an HESC (high-efficiency supercooling) four-wing rotor, interchangeable with existing rotors, and a PES 5 contour-milled rotors for better quality and higher output with hydraulic feed ram replacing pneumatic ram (for energy savings and reproducibility) with simpler cooling via the piston rod. A new disposable mixer that is claimed to reduce costs of production of reactive resins has been developed by Sulzer Chemtech, Switzerland. The design
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is based on a unique arrangement of mixing baffles, giving a mixer with a low pressure drop that can achieve the required degree of mixing with only half the volume needed by conventional spiral mixers. The degree of mixing can be predicted mathematically, leading to consistently high quality. The advantages add up (it is claimed) to lower waste, improved use of material, and lower cost. Sulzer also claims a reduction of as much as 40% in the use of masterbatch by fitting a new design of static mixer to the nozzle of an injection moulding machine. The mixer improves the homogeneity of the melt, allowing reductions to be made in use of pigment without the usual penalty of streaks appearing in the moulded part. Pay-back times of 4 - 6 weeks are not unusual, claims the manufacturer, arguing that companies often use more masterbatch than is necessary, to compensate for poor mixing. In addition to improving the use of masterbatch, the mixer can be used to reduce cycle times, to increase the amount of regrind that can be added to the mix and to reduce overall scrap rates. It is designed to produce a very low pressure drop and, because of this, can be used with shear-sensitive materials such as PET. 213.7 Adjustable screw
geometry
In twin-screw extruders, the elongational flow, which is the mixing action created, achieves more efficient dispersive mixing, but the main mixing action occurs not in the intermeshing region but in the region between the pushing flight flank and the extruder barrel. Twin-screw machines make good dispersive mixers because this space is wedge-shaped, creating elongational flow as the mix is forced through the flight clearance. Developments by Rauwendaal Extrusion Engineering, force the material through the high-stress regions several times, creating elongational flow in two ways. The company's CRD mixer uses a slanted pushing flight flank, to stretch the material as it is forced through the flight clearance, and the flanks have tapered slots, which serve to increase distributive as well as dispersive mixing. The same company has also introduced an adjustable grooved feed extruder, which overcomes some of the disadvantages of the existing non-adjustable design. Grooved feed machines have been in use since the 1960s, giving higher throughput, better stability and the ability to process very high molecular weight polymers, but with disadvantages in higher load on the motor, greater likelihood of wear and high pressure in the grooved region. APV has developed extruders with a segmented agitator system to process all types of plastics compounds. The machines have a 'clamshell' design of barrel that opens hydraulically, to give easy access for efficient cleaning or maintenance of screws. A new, segmented liner system allows rapid replacement of liners and reduced maintenance costs. Liner materials are selected to match the specified processing duty in each extruder zone. Melt temperature, specific energy input, and residence time can all be precisely controlled. High-torque or high-free-volume versions allow the machine specification to be matched to the requirement and an option of 1200 rpm drive maximizes output potentials and adds extra versatility to the capabilities of the Advanced Compounder.
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Figure 21.1. For better compounding efficiency, recent barrier screw designs by Davis-Standard include (top) DSB-V, with variable-pitch barrier flight, and DSB-V1, with a dual-barrier design and variable lead barrier flight. (Photograph: Davis-Standard)
21.4 Colour Dosing
Colour changes in seconds are claimed by Colortronic GmbH, for a low-rate gravimetric additive feeder, Graviblend S. It provides feed rates of 8 0 - 1 0 0 g h ~ \ according to bulk density, and is designed with quick-action clamps for the dosing hopper and screw on the micro ingredient feeding system, to allow the user to change dosing systems and make colour changes quickly and efficiently. Any cleaning required for the dosing hopper and screw can be carried out offline. The unit has been designed for continuous feeding of bulk material for applications where precise low feeding rates for granules are needed, such as fibre, cable, filament and tubing, and laboratory lines. Its construction and advanced technology brings optimization to low-rate-feeding applications, with compact construction for easy and quick installation. Low weight causes good mass resolution of the metered material and a special damping system offers reliable protection against mechanical disturbances such as vibrations. A significant improvement in accurate blending of ingredients in liquid colour dosing is claimed by Maguire Products, USA. The company has been working on liquid colour systems since its formation and, in 1996, it developed a system in which the colour was fed directly to the weigh chamber of a gravimetric blender. This differs from the conventional systems, which rely on either dosing the liquid colour directly into the feed throat of the extruder or moulding machine, or employ a process to pre-coat the plastic pellets with colour before they are dosed. The company has also developed new software for accurately controlling very small additions of highly concentrated ingredients, which are typically added at a rate of 1% or lower. It can also control the addition of poorly flowing waxy additives, such as blowing agents, and long glass fibre concentrates, where the geometry of the pellet may inhibit consistent flow. A liquid colorant metering system from Hanna Group offers a simple, clean and less expensive alternative to cleaning machinery and shop floors. It incorporates a three-way valve that temporarily diverts colorant from the moulding equipment to a special calibration section for weighing, keeping this
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operation separate. Normally liquid colorant tube sets are disposable, as part of the tubing wears out, sometimes after several weeks, as a result of being flattened by the peristaltic pump. The Hanna system, however, makes it possible for the processor to replace only the worn part. Offering a new level of user input and sophisticated programming techniques to increase productivity and profits, the latest version of the ProPalette colour formulation system by GretagMacbeth is designed specifically for the plastics, paint and coatings industries. For 'single-hit' colour matching at all levels of opacity, the latest version (which gives a seamless transition for existing users of the system) has database improvements which offer users considerable flexibility with data, as well as improvements to the user interface giving better efficiency. Innovative colour utilities are also provided, which streamline the analysis of the data and improve the productivity of the system.
21.5 Recent Developments
Machinery manufacturer Farrel has developed a two-stage compounder that it claims will have the highest dispersive mixing capability of any unit yet developed. It combines a two-stage continuous mixer with a single-screw extruder, and is aimed at a wide variety of new and demanding applications,
Figure 21.2. Powerful shearing and homogenizing of sensitive materials, retaining vital rheological properties, is provided by Farrel Corporation's Advex. (Photograph: Farrel Corporation)
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such as dispersion of pigments in fibre grade matrices, direct injection of plasticizers, and liquid stabilizers into PVC, polymer alloying, and processing of temperature-sensitive materials and rubber-based products. Reflecting current and expected market demands, it is also designed for the addition of wood flour and high levels of inorganic fillers, and for mixing of nano-composites. The company is also developing dedicated two-stage mixing rotors for specific applications. Key features are extra mixing length and residence time, with a greatly expanded processing window. There is a variable operating LD ratio from 5:1 to 10:1, and the counter-rotating mixer has rotor speeds of up to 1000 rpm. As well as using rotor speed, fill factor and temperature as control variables, the machine also introduces the possibility of variation in clearances of the chamber wall
Figure 21.3. Looking towards a new market demand, the Davis-Standard Woodtruder combines in a single system the latest plastics extrusion technology with technology for processing wood fibre. (Photograph: Davis-Standard)
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between the two stages. This allows selection of the intensity of first-stage mixing. In the second stage, a variable orifice can be adjusted to provide increased dispersive mixing, vacuum venting or downstream addition of fibres, additives or supplementary polymers. Vacuum venting is used to remove volatiles and moisture from the two-stage extruder. With its high speed and variabiUty of fill factor in two stages, the machine can disperse higher concentrations of low bulk density fillers, while the high frequency of product surface renewal gives high levels of moisture removal. Farrel claims that a 10% moisture content in wood flour can be reduced to less than 1% over a 5D process length, using atmospheric venting. Two-stage mixing, vacuum venting in the mixing stage, and use of a vented extruder will reduce the level much further. Among latest developments from Werner and Pfleiderer has been the adaptation of a slurry process (originally used for rubber/SAN compounding to produce ABS) for the production of nano-composites, in which the problem of handling such tiny particles is solved by delivering them in a water slurry. A special screw design makes it possible to incorporate long glass-fibre strands into a polypropylene compound, and further development delivers the compound not as extruded strands but as a compact hot extrudate, which can be fed directly to a hydraulic press for moulding a complete part such as an automobile front end. The next step will be direct extrusion of the long glass fibre compound through a sheet die onto a stack of cooling rolls. Whereas extrusion compounding with glass fibre is a continuous process, yet another development envisages integration with the injection moulding process, using a screw configuration that, in a twin-screw layout, can provide enough mixing/melting to allow the incorporation of glass rovings into the polymer melt even at low speeds, as during the first rotation of the screws at the beginning of the plasticizing cycle.
21.1 Incorporation of Additives
Additives are introduced into polymer compounds is by means of physical mixing, using a paddle-blade mixer or roll calender, for powdered materials, or a compounding extruder, for thermoplastic melts. The key criteria are the type and nature of additive and the point at which it should be introduced, the degree of precision in measurement of polymer/additive ratios and the work that has to be carried out to ensure good homogeneous mixing, with even dispersion of the additive. Modern polymer compounds, with many different types of additive, present strong challenges to the designers of the system. Other equipment, particularly dosing systems, can be mounted on the extruder, and is increasingly used for precise direct metering of very small quantities of additives such as liquid colour systems. The last few years have seen major developments in more efficient design of mixers and extruders. There has also been important development on the materials side, to make additives easier and more suitable for compounding, opening up to smaller compounders the possibility of making special compounds. This will be mainly to offer a range of colours, while only needing to hold stocks of natural material, or to add or boost specific properties, such as UV stability, anti-static properties or flame retardancy. To facilitate in-plant compounding, most suppliers have developed systems that efficiently and reproducibly deliver a controlled package of additives to a compound, using either a specialized concentrate or a masterbatch formulation. Compounding as a separate industry was originally devoted mainly to the addition of colouring, especially for special colour matches. This gradually developed to skill in short-run production, with high efficiency in distribution, to meet the demand from plastics processors for small volumes of special grades. Many independent compounders now operate under franchise from polymer producers, while also marketing their own formulations. From this base, compounders have extended their activities to production of technical compounds, incorporating special additives and chopped glass-fibre
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reinforcement. Modern compounding equipment is based mainly on extrusion mixing and incorporates a number of points at which other materials can be introduced, at various stages in the process, possibly including satellite extruders to produce blends. In the most sophisticated operations, there are facilities for reactive compounding, in which additives and other polymers are chemically reacted with the main polymer system. Modern compounding may involve the introduction of several different ingredients and, for all equipment, the crucial aspect is to produce homogeneous mixing, often with very precise measurement of the various components. Most suppliers of compounds include a wide choice of such packages in their standard range and can readily formulate customized systems, to meet a specific requirement. Some of the polymer manufacturers have also made available advanced additive delivery systems that they have often developed originally for their own use.
21.2 Mixing Thermosets
Thermosets are mainly viscous liquid resins, presenting their own compounding requirements. Liquid mixing systems of varying degrees, from the most basic to highly sophisticated are used, depending on the type of resin, additives, and volume output required. The amount of additive by weight which can be incorporated will depend on the particle size, density and oil absorption properties. For example, addition of porous high oil absorption fillers (such as diatomaceous silicas) and chopped glass can greatly increase the viscosity of a resin system at low filler loadings of only 1-50 phr. Medium weight granular fillers, such as powdered aluminium and alumina may be used at loadings of up to 200 phr. The non-porous lower oil absorption fillers, such as aluminium oxide, silica and calcium carbonates can be incorporated at levels of 700-800 phr without making the formulation unworkable. Loadings can be increased by adding a diluent, but this may not always be desirable, as the diluent may detract from other desired properties. Organotitanates can be added to improve filler wetting, enabling higher loadings at the same viscosity. Fine particles are easier to incorporate and there is a lesser tendency for them to settle. Coarse and heavy fillers will settle and *cake' on standing, but this may be countered by adding lightweight fillers such as the colloidal silica compounds. It goes without saying that the crucial aspect is to obtain a completely homogeneous mixture of polymer and additive. This is a difficult target, bearing in mind the viscosity of the polymer and the often particulate nature of the additive. To make things easier, additive producers have and still improve their products to facilitate mixing. Pre-heating fillers before mixing with polymer or resin can improve the mixing rate, reduce energy consumption in processing, reduce wear on equipment and improve product quality, it is reported. Addition of fillers cold has a number of
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disadvantages, not the least of which is the difficulty of maintaining consistent mixing cycles or production rates during seasonal changes in temperature or humidity, especially if the filler is stored out of doors. During winter, unheated fillers may have greater levels of condensed and absorbed surface moisture than in the summer and can require as much as 1 5 20% longer mixing time. Moreover, since mixing relies heavily on frictional shear heating to achieve plasticizing and dispersion, there can be excessive wear on compounding equipment blades, screws, barrels, and walls. High friction can also break down the aspect ratio of the filler, degrading properties of the finished compound.
21.3 Mixing Thermoplastics
Additives can be introduced at various stages in production and moulding of a thermoplastics compound, depending on the type and form of the additive. Comonomers may be also regarded as additives and introduced during polymerization, depending on the polymer matrix, but most additives conferring special properties, such as reinforcement, colouring or modification, are added to the polymer matrix in a secondary compounding stage. Depending on volumes, this may be carried out by polymer producers themselves, in their own in-plant facilities, or under contract, or by independent compounding organizations. Large special-purpose extruders are used, designed for very efficient mixing with close control over temperature and shear rate, and high output. An important qualification is that the compounds are heat sensitive, calling for a closed system with accurate control over temperature. Regarding the latter, it should also be remembered that the shear action of compounding may produce its own heat, which must also be considered. 27.3.7 Dry mixers
The basic form of mixer operates on a batch process, using dry materials such as powders, or pastes. For the simplest work, a tumble mixer design has been used for many years, for addition of colour or mixing with masterbatch concentrate. More technical demands, however, have called for mixers normally using the Z-blade design, giving precise control over ingredients and times. This system is widely used in production of PVC compounds, which can be mixed in dry powder or paste form. In this sector, there has is always been an argument about whether it is better for the processor to mix compounds in-house, or buy in compounds ready-made. The answer lies very much in the type and volume of work, and particularly in the cost of handling hazardous substances with the safety and consistency that is demanded in processing today. In PVC processing, volume is a clear criterion, and it is unusual to be a large processor without having in-house mixing facilities. Centrally made PVC compounds are one of the strongest growth sectors of the business, and all resin producers have invested strongly in this sector.
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An alternative design employs one or two kneading rotors, working inside a fixed stator housing. This can be of various sizes, and offers considerable scope regarding the geometry of the rotor flights and stator walls, to optimize the mixing process. Continuous PVC compounding is claimed by Colortronic for its Novablend system with automated gravimetric blender and liquid doser, with a specially designed horizontal mixing screw and a high-speed Waeschle mixing turbine. Precision blends and flexible formulations can be produced with greater accuracy and at lower cost. Unlike conventional systems, the system creates sufficient nominal friction to raise the temperature of the PVC powder high enough to absorb liquid stabilizers for rigid PVC. A mixer particularly suited to free-flowing powders, has been introduced by Hosokawa Mikron BV. Operating by the use of shear forces, the Cyclomix is conical and is filled to about 50% of its capacity. A central high-speed rotating shaft is driven from the mixer cover (so removing seals and bearings from the area of the product). Paddle-shaped mixing elements rotate close to the inner wall and their high speed plus the conical shape of the mixing vessel transport the product from the bottom of the chamber to the top, intermixing the particles by friction against the wall. On reaching the top, the product flows down again into the centre. Special features of the Cyclomix are: fast intensive mixing, widespread applications, self-emptying, no seals and bearings in the product zone and good control over the product temperature. A feeder unit with flexible polyurethane walls has been developed by Brabender Technologic, which offers high reliability particularly for poorly flowing ingredients. The Flex-Wall uses massage paddles instead of agitators and can be employed as a 'stand alone' volumetric feeder or as a loss-in-weight feeder in conjunction with a Brabender scale. Removal of the screw tube and dismounting of the screw is rapid and, if necessary, the flexible hopper can easily be removed for further dry or wet cleaning. The feeder is particularly suited to systems where processes have to be interchanged frequently, where the entire hopper can be exchanged together with its contents, without the need for emptying. 27.3.2 Calendering Calendering is a method of mixing in which polymer and additives are introduced into a stack of rollers, which apply work and heat and progressively reduce the thickness of the mix to that of sheet or film, with the facility also of adding an embossed surface. The equipment, which has been 'borrowed' from the rubber industry, is usually large and costly, but laboratory-scale systems are also available, for small-scale testing of compounds and formulae. 27.3.3 Extrusion compounding Extrusion compounding is the main method of compounding thermoplastics. The base polymer is fed continuously by means of a hopper into a plasticizing
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chamber or barrel, in which it is worked by means of a rotating Archimedes-type screw, while heat is applied by elements in the barrel and screw, as required. At appropriate locations there are ports through which additives can be fed as required. Glass fibre, for example, requires specific handUng. Pigments may be added on the shop floor (enabling raw materials stocks to be rationalized to natural material only), but there may be potential health hazards. When the required homogeneous mix has been achieved, the output of the machine is extruded as strands of polymer melt, which are cooled in a water bath and are chopped into free-flowing granules, for packaging and shipment. As the geometry of the granule has an important influence on processing properties, the design and technology of the granulating stage is critical. The system offers great flexibility in the speed and geometry of the screw and the number and location of heating elements. Single or co-rotating twin-screw systems are used. Much has been learnt during the past few years and several new machine designs have been introduced to the market. There is also a trend now towards modular construction, giving compounders a large degree of flexibility in rapidly modifying a machine for a special grade. 213.4 Compounding
mineral fillers
With the increasing use of mineral fillers to improve mechanical properties of a compound, the demands on the compounding extruder have risen. Fillers can be added in concentrations ranging from 20 to 80% by weight, meaning that it is not possible to have a single standard feed system. They can be fed at different positions through one of a number of side feeders. All fillers contain air and moisture, which must be removed during compounding, along with other volatiles. Most machines therefore have at least one degassing zone in the melt section, downstream from the filler addition, usually operating under vacuum. 27.3.5 Fine talc masterbatch
Among recent compounding developments by Buss Compounding Systems is production of masterbatch concentrates using fine grades of talc, especially for automobile components, where the industry is looking for a system capable of dosing a talc masterbatch directly into a base polymer, at the injection moulding machine. This calls for fine talc masterbatches at 70-80% loadings, but fine talc is difficult to compound due to the low bulk density of the powder. A Buss MKS 70 mm 20L/D machine was adapted, with the kneading chamber configured to permit vertical feeding of polymer and filler in the first barrel module, and side-feeding of filler and vacuum degassing downstream. A split feed technique was used, introducing one-third of the talc together with the polymer at the first inlet and the rest downstream, directly into the melt. The interaction between the fixed pins and reciprocating screw flights in the Buss kneader, giving multiple mixing as the material moves down the chamber, provides a high degree of surface renewal and optimum wetting and degassing. Samples at 70% loading have been successfully produced.
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213.6 Single- and twin-screw
extruders
Two basic designs of extruder are in use today, equipped with one or two screws. The single-screw design is generally considered a more simple but rugged machine, with good endurance, and is best suited for long continuous runs of the same formulation. The twin-screw design, however, is more complex but gives greater flexibility in mixing special compounds. In polymer mixing, a distinction can be made between distributive and dispersive mixing. The former method aims to improve the spatial distribution of the components, without cohesive resistance playing a role. It is also called simple or extensive mixing. Dispersive mixing has to overcome cohesive resistances to achieve finer levels of dispersion. It is also called intensive mixing. Dispersive mixing is the more difficult and, in single-screw extruders, is thought to be inefficient because the mixing is achieved primarily in shear and the compound is exposed to the mixing action of the high stress region only once. Twin-screw extruders provide a uniform and reliable feed of the polymer and mineral fillers, with effective degassing in the melt zone. There must be complete wetting of the filler by the polymer melt and good dispersive mixing of the fillers into the melt, while the heat and shear history of the matrix is reduced to a minimum, to avoid degradation. Specialized screw kneaders have been developed, based on co-rotating twin-screw machines in single- or two-stage versions. The screw speed does not influence the physical properties, provided that there is constant torque utilization. Additionally, the number of filler additions has only a negligible effect, giving the compounder good flexibility and permitting adjustment of rate to suit market demands. While most of the development work has been carried out on filled polypropylene, Werner and Pfleiderer has also been working with polyethylene butylate, nylon 6 and 66, and ABS. The company's MEGAcompounder is a twinscrew design that is being produced with capacity to supply extra torque, making it possible to raise the screw speed from 400 to 500 rpm, so allowing an increase of about 50% in throughput. Higher speeds have been run with nylon without adversely affecting the properties. Krupp Werner & Pfleiderer has a number of new developments. The ZSK 50 Mc filling a gap in the range, with 2 5 0 - 1 0 0 0 kg h~^ output, has a new design concept with tie-rod tensioning of the block barrels and three-point support for base-frame and gearbox. The biggest yet co-rotating twin-screw (ZSK 380 Mc), with a screw diameter of 380 mm and output up to 75 tonnes per hour, it uses a special drive concept and new measuring devices can examine the entire bore length within half a day, to identify barrel wear without dismantling. The machine also features improvement in internal mixers: an HESC (high-efficiency supercooling) four-wing rotor, interchangeable with existing rotors, and a PES 5 contour-milled rotors for better quality and higher output with hydraulic feed ram replacing pneumatic ram (for energy savings and reproducibility) with simpler cooling via the piston rod. A new disposable mixer that is claimed to reduce costs of production of reactive resins has been developed by Sulzer Chemtech, Switzerland. The design
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is based on a unique arrangement of mixing baffles, giving a mixer with a low pressure drop that can achieve the required degree of mixing with only half the volume needed by conventional spiral mixers. The degree of mixing can be predicted mathematically, leading to consistently high quality. The advantages add up (it is claimed) to lower waste, improved use of material, and lower cost. Sulzer also claims a reduction of as much as 40% in the use of masterbatch by fitting a new design of static mixer to the nozzle of an injection moulding machine. The mixer improves the homogeneity of the melt, allowing reductions to be made in use of pigment without the usual penalty of streaks appearing in the moulded part. Pay-back times of 4 - 6 weeks are not unusual, claims the manufacturer, arguing that companies often use more masterbatch than is necessary, to compensate for poor mixing. In addition to improving the use of masterbatch, the mixer can be used to reduce cycle times, to increase the amount of regrind that can be added to the mix and to reduce overall scrap rates. It is designed to produce a very low pressure drop and, because of this, can be used with shear-sensitive materials such as PET. 213.7 Adjustable screw
geometry
In twin-screw extruders, the elongational flow, which is the mixing action created, achieves more efficient dispersive mixing, but the main mixing action occurs not in the intermeshing region but in the region between the pushing flight flank and the extruder barrel. Twin-screw machines make good dispersive mixers because this space is wedge-shaped, creating elongational flow as the mix is forced through the flight clearance. Developments by Rauwendaal Extrusion Engineering, force the material through the high-stress regions several times, creating elongational flow in two ways. The company's CRD mixer uses a slanted pushing flight flank, to stretch the material as it is forced through the flight clearance, and the flanks have tapered slots, which serve to increase distributive as well as dispersive mixing. The same company has also introduced an adjustable grooved feed extruder, which overcomes some of the disadvantages of the existing non-adjustable design. Grooved feed machines have been in use since the 1960s, giving higher throughput, better stability and the ability to process very high molecular weight polymers, but with disadvantages in higher load on the motor, greater likelihood of wear and high pressure in the grooved region. APV has developed extruders with a segmented agitator system to process all types of plastics compounds. The machines have a 'clamshell' design of barrel that opens hydraulically, to give easy access for efficient cleaning or maintenance of screws. A new, segmented liner system allows rapid replacement of liners and reduced maintenance costs. Liner materials are selected to match the specified processing duty in each extruder zone. Melt temperature, specific energy input, and residence time can all be precisely controlled. High-torque or high-free-volume versions allow the machine specification to be matched to the requirement and an option of 1200 rpm drive maximizes output potentials and adds extra versatility to the capabilities of the Advanced Compounder.
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Figure 21.1. For better compounding efficiency, recent barrier screw designs by Davis-Standard include (top) DSB-V, with variable-pitch barrier flight, and DSB-V1, with a dual-barrier design and variable lead barrier flight. (Photograph: Davis-Standard)
21.4 Colour Dosing
Colour changes in seconds are claimed by Colortronic GmbH, for a low-rate gravimetric additive feeder, Graviblend S. It provides feed rates of 8 0 - 1 0 0 g h ~ \ according to bulk density, and is designed with quick-action clamps for the dosing hopper and screw on the micro ingredient feeding system, to allow the user to change dosing systems and make colour changes quickly and efficiently. Any cleaning required for the dosing hopper and screw can be carried out offline. The unit has been designed for continuous feeding of bulk material for applications where precise low feeding rates for granules are needed, such as fibre, cable, filament and tubing, and laboratory lines. Its construction and advanced technology brings optimization to low-rate-feeding applications, with compact construction for easy and quick installation. Low weight causes good mass resolution of the metered material and a special damping system offers reliable protection against mechanical disturbances such as vibrations. A significant improvement in accurate blending of ingredients in liquid colour dosing is claimed by Maguire Products, USA. The company has been working on liquid colour systems since its formation and, in 1996, it developed a system in which the colour was fed directly to the weigh chamber of a gravimetric blender. This differs from the conventional systems, which rely on either dosing the liquid colour directly into the feed throat of the extruder or moulding machine, or employ a process to pre-coat the plastic pellets with colour before they are dosed. The company has also developed new software for accurately controlling very small additions of highly concentrated ingredients, which are typically added at a rate of 1% or lower. It can also control the addition of poorly flowing waxy additives, such as blowing agents, and long glass fibre concentrates, where the geometry of the pellet may inhibit consistent flow. A liquid colorant metering system from Hanna Group offers a simple, clean and less expensive alternative to cleaning machinery and shop floors. It incorporates a three-way valve that temporarily diverts colorant from the moulding equipment to a special calibration section for weighing, keeping this
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operation separate. Normally liquid colorant tube sets are disposable, as part of the tubing wears out, sometimes after several weeks, as a result of being flattened by the peristaltic pump. The Hanna system, however, makes it possible for the processor to replace only the worn part. Offering a new level of user input and sophisticated programming techniques to increase productivity and profits, the latest version of the ProPalette colour formulation system by GretagMacbeth is designed specifically for the plastics, paint and coatings industries. For 'single-hit' colour matching at all levels of opacity, the latest version (which gives a seamless transition for existing users of the system) has database improvements which offer users considerable flexibility with data, as well as improvements to the user interface giving better efficiency. Innovative colour utilities are also provided, which streamline the analysis of the data and improve the productivity of the system.
21.5 Recent Developments
Machinery manufacturer Farrel has developed a two-stage compounder that it claims will have the highest dispersive mixing capability of any unit yet developed. It combines a two-stage continuous mixer with a single-screw extruder, and is aimed at a wide variety of new and demanding applications,
Figure 21.2. Powerful shearing and homogenizing of sensitive materials, retaining vital rheological properties, is provided by Farrel Corporation's Advex. (Photograph: Farrel Corporation)
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such as dispersion of pigments in fibre grade matrices, direct injection of plasticizers, and liquid stabilizers into PVC, polymer alloying, and processing of temperature-sensitive materials and rubber-based products. Reflecting current and expected market demands, it is also designed for the addition of wood flour and high levels of inorganic fillers, and for mixing of nano-composites. The company is also developing dedicated two-stage mixing rotors for specific applications. Key features are extra mixing length and residence time, with a greatly expanded processing window. There is a variable operating LD ratio from 5:1 to 10:1, and the counter-rotating mixer has rotor speeds of up to 1000 rpm. As well as using rotor speed, fill factor and temperature as control variables, the machine also introduces the possibility of variation in clearances of the chamber wall
Figure 21.3. Looking towards a new market demand, the Davis-Standard Woodtruder combines in a single system the latest plastics extrusion technology with technology for processing wood fibre. (Photograph: Davis-Standard)
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between the two stages. This allows selection of the intensity of first-stage mixing. In the second stage, a variable orifice can be adjusted to provide increased dispersive mixing, vacuum venting or downstream addition of fibres, additives or supplementary polymers. Vacuum venting is used to remove volatiles and moisture from the two-stage extruder. With its high speed and variabiUty of fill factor in two stages, the machine can disperse higher concentrations of low bulk density fillers, while the high frequency of product surface renewal gives high levels of moisture removal. Farrel claims that a 10% moisture content in wood flour can be reduced to less than 1% over a 5D process length, using atmospheric venting. Two-stage mixing, vacuum venting in the mixing stage, and use of a vented extruder will reduce the level much further. Among latest developments from Werner and Pfleiderer has been the adaptation of a slurry process (originally used for rubber/SAN compounding to produce ABS) for the production of nano-composites, in which the problem of handling such tiny particles is solved by delivering them in a water slurry. A special screw design makes it possible to incorporate long glass-fibre strands into a polypropylene compound, and further development delivers the compound not as extruded strands but as a compact hot extrudate, which can be fed directly to a hydraulic press for moulding a complete part such as an automobile front end. The next step will be direct extrusion of the long glass fibre compound through a sheet die onto a stack of cooling rolls. Whereas extrusion compounding with glass fibre is a continuous process, yet another development envisages integration with the injection moulding process, using a screw configuration that, in a twin-screw layout, can provide enough mixing/melting to allow the incorporation of glass rovings into the polymer melt even at low speeds, as during the first rotation of the screws at the beginning of the plasticizing cycle.