Continuous mixing of polymeric compounds.
As has been the case in many of the past expansions in the industry, automotive has been the spark which kindles the flame of progress. Disregarding the tire sector, it is those automotive applications which relate to the seals, gaskets and numerous other elastomeric parts found in and on automobiles which have created a small revolution in the rubber industry. Quality requirements have become more stringent and rubber parts which are visible on automobiles must have the same smooth surface quality as the metal and plastic parts which can be seen. The rubber parts producer must not only design the part, but he must also assure its durability, and produce it at a low cost.
The raw material manufacturers have taken these requirements in stride. The advent of thermoplastic elastomers was the first to invade the hallowed traditions of the rubber industry. Today we see friable bales, pelletized forms of EPDM, new free flowing forms of EPDM and products resulting from metallocene catalysis. All of these are niche products, having the one common capability of being mixed in continuous type machinery. In some instances the free flowing forms or friable bales will be used for the continuous process, while in other cases they will be used to reduce mixing cycles in batch mixers.
Whether one chooses the batch or continuous approach to one's mixing requirements, if the choice is available, the suitability of the process must satisfy the purpose for which the compound has been developed. The machinery chosen must also fulfill the user's economic requirement for profitability. This will include basic machinery cost, the cost of installation, staffing requirements and the projected utility costs.
The machinery choice must also consider the level of quality required. It is true that quality is always important, but the specific level of quality must be related to the product's application requirements. The one factor which cannot be compromised is uniformity. Regardless of the level of quality required, uniformity and reproducibility cannot be overlooked. If the mixed products lack these qualities, manufacturing processes will be unpredictable, scrap rates will increase and resultant labor costs will suffer.
Capability to deliver optimum uniformity is one of the most important requirements of any mixing system, whether it is batch or continuous. The mixer must be able to provide the necessary level of dispersive and distributive mixing required to yield target results.
In order to determine whether the batch or continuous system is the best selection for a given application, both the advantages and disadvantages of each must be thoroughly investigated. Although either system may perform satisfactorily, there are many cases where one may not be suitable at all. This must be known in advance to prevent costly expenditures for systems which may not be the best choice. Fortunately, most of the manufacturers of these types of machinery have process laboratories where the client can determine in advance whether his compound can be suitably mixed on a particular type of mixer. Table 1 charts out the advantages and disadvantages of each type of system. Following the same logic, one must also fully understand the advantages and disadvantages of a continuous processing line when selecting machinery (table 2).
Table 1 - advantages, disadvantages of batch mixing Advantages * Raw materials may be handled in irregular sizes and shapes. * Materials can be fed manually or automatically into the mixer hopper with little or no preparation. * May be operated manually or with a simple PLC. * Can mix compounds intensively, requiring high shear, or less intensively by means of speed control or special rotor designs. * Can be operated economically for either short runs or long extensive ones. * Very rugged machinery which lasts many years before requiring refurbishing. Disadvantages * Frequent loading and unloading results in inefficient use of electrical supply. * Variation in heat history as large batches are directed through mills or other batch-off processes. * Potential loss of chemicals as mixer displaces large quantities of air during the mixing process. * Labor intensive, usually requiring an operator in attendance at all times. * High installation costs relating to multi level and heavy weight requirements. * Batch to batch variations resulting from subtle differences in weighments. * Requires second machine to form or handle the batch after mixing. Table 2 - advantages, disadvantages of continuous mixing Advantages * Power consumption is steady, minimizing energy costs on a pound to pound basis. Energy consumption is steady without great peaks. * Mixing reaches a steady state, allowing fine machine adjustments to zero in on certain required parameters, (temperature, power.etc.) * Little operator attendance required during lengthy production runs. * Uniform heat history resulting from continues smaller discharge than experienced from batch mixers. * Uniform out put following premixed ingredients and steady state runs. * Reduced installation costs. Disadvantages * Unable to process materials not available in free flowing forms. * Requires sophisticated weighing and metering equipment. * Difficult to operate manually, requiring automatic controls. * Not cost effective for short production runs. * Output normally characterized by higher discharge temperatures than experienced from batch mixers. * Requires special machine configurations for changes in compounds.
Following the preceding lists of advantages and disadvantages, one can begin to grasp the complexity of making the best machinery selection for a given material or process. This must be a very meticulous process based upon testing, costs and potential reliability of the supplier.
It is generally agreed that optimum dispersion occurs during the most intensive level of shear. In most cases this is achieved by directing the matrix through the most tortuous path at its lowest temperature and highest viscosity. This is a complex and sometimes difficult achievement. Two levels of mixing are necessary to yield a completely useful rubber compound. These are described as intensive and extensive. The first provides the necessary requirements for physical and rheological properties, while the latter is responsible for providing the uniformity necessary for manufacturing procedures, and product reliability. Following this logic, longer residence time at reduced temperatures may produce the best product. This lower temperature, longer mixing cycle is characteristic of the batch mixer. To some degree, this also explains the typically lower energy requirements of a continuous system, where much less heat transfer is experienced through machine cooling. Generally, if a mixing process takes longer than 20 to 30 seconds of residence time, it may not be a candidate for a continuous mixing system. L/D increases of continuous mixers will improve residence time to some extent, but this approach has its limitations.
Concern over residence time is a valid issue following increased activity in reactive mixing of rubber and thermoplastic rubber compounds, as well as recent innovations regarding reactions with filler linking agents.
With the advent of tailored polymers, the parts producer will be supplied with polymers which have been designed for molding, calendering or extruding, as well as for specific properties such as good compression set, improved tensile strength and good tear resistance. The compounder will still have to do his job in adding those chemicals which will provide the required physical properties. This will, to a great extent, simplify formulating and direct more emphasis toward costs associated with the manufacturing process. Taking some of the ingenuity of formulating away from the compounder may or may not be practical, but it will result in more reliable costs for raw materials and move the total cost emphasis toward mixing efficiency.
Although the technology for free flowing forms of elastomers has been available for some time, it has only been recently that they have become more readily available. Today, many classes of EPDM are available in pellet form and friable bales. The projected availability of more free flowing elastomers following recent developments in polymerization technology are making continuous mixing of rubber compounds more viable than ever before. Compounds from these elastomers are usually relatively simple formulae, containing comparatively easy to disperse ingredients. Some will be suitable to be prepared into pre-blends to further simplify the weigh/feed metering process. Formulae of these types will address large volume markets, such as automotive, where long runs will be the norm, and most adaptable to continuous mixing systems.
The tire industry, on the other hand, still depends on natural rubber as its primary elastomer. Compounds are characterized by high loadings of difficult to disperse carbon blacks. These compounds require excellent dispersion and good viscosity reduction which can only be obtained through the use of chemical additives and re-milling steps. Recently, new tire compounds have been designed which contain sophisticated filler and linking agents. These compounds require longer residence times than would be practical with continuous mixing systems.
Several types of continuous mixers are available, which have been popular in the plastics industry for many years. With certain modifications, these mixers have been used to mix a very selective range of rubber compounds. There are many types of continuous mixers to choose from. Most follow the designs of co-rotating twin screw devices, capable of being customized with a large variety of screw elements and barrel segments. Some continuous mixers are distinctly different. Following is a brief description of three types of continuous mixing machines.
FCM continuous mixer
This mixer is a twin rotor counter rotating device. It is a relatively short mixing machine with a length to diameter ratio of approximately 5:1. The machine consists of two rotors within a smooth bore mixing chamber similar to the arrangement of a Banbury mixer. The mixer has a feed hopper at one end through which raw materials are fed into the mixer. There is a discharge gate at the opposite end, through which the mixed product is discharged.
The unique design of the rotors was patterned to some extent from the design of the Banbury rotors. The rotors consist of a feed section which resembles extruder screws. The sole purpose of this section is to propel raw materials into the mixer. Next are forward and reverse helices which are designed to provide both the intensive and extensive mixing. Beyond these are the elliptical paddles which are responsible for pumping the material through the discharge gate opening.
One of the most significant features of this mixer is its starve fed characteristic. The output of the mixer is based on feed rate. Maintaining constant feed, one may increase rotor rpm to induce more shear, or close the discharge opening slightly and increase the discharge temperature. The mixer operates in a non-pressurized manner, thereby creating an environment of mixing alone, and not compromised by secondary functions such as forming. Following this characteristic, the FCM device can mix materials requiting high shear and high operating temperatures, but also those which require less shear and are more temperature sensitive.
There are more than 600-700 of these mixers operating in the world today. Most applications are in the plastics industry, although this mixer was originally designed as a continuous Banbury mixer. Machine sizes cover outputs from 100 to greater than 100,000 pounds/hour.
MVX (mixing venting extruder)
The MVX consists of a mixer containing two delta shaped counter rotating rotors, close coupled to a pumping extruder. The objective was to develop a machine which could mix a rubber compound and convert it to a useful form simultaneously. Although this mixer was originally designed for the plastics industry, it found its niche in the rubber industry. The feeding device is a simple pneumatic ram which forces free flowing materials into the rotors and mixing chamber. The ram is reciprocating and contributes to maintaining the projected production rate. As the materials are directed through the mixing chamber, they are subjected to six rolling banks which are interrupted by dams to prevent short circuiting of unmixed raw materials.
The extruder speed is the master of the unit's production rate. Venting is accomplished through the back end of the feed screw. Moisture and air may be removed in this manner. The head end of the extruder can be equipped with a strip or pelletizing head. Mixers have been designed for production rates from 1,000 to 10,000 pounds/hour.
Twin screw extruder
The FTX is a co-rotating twin screw extruder, having various configurations to address many different kinds of mixing requirements. These configurations are accomplished with the assembly of numerous specifically designed screw and chamber elements. This continuous mixing device became very popular for mixing engineering resins, which characteristically mix at very high temperatures. This is one of the positive features of this type of mixer which in many instances is a disadvantage. Unfortunately, because of the modular screw design, screw cooling is virtually impossible. This may present a serious drawback for mixing temperature sensitive materials. The chamber barrel sections are designed so that materials such as fibers may be fed in regions where best wetting can be achieved without damaging the fiber integrity. These barrel sections also permit vacuum venting as well as designs for introducing other additives downstream such as plasticizers. Following this modular design, there is almost no limit to the configurations and length to diameter ratios which may be obtained.
The most common screw elements used are the kneading and screw type, which represent the most and least intensive designs. New configurations have been recently introduced which suggest greater opportunities for mixing temperature sensitive materials such as rubber compounds. These elements are hexagonal polygons and FCM (CME) designs. Both permit mixing with lower heat generation, yet introduce flow patterns which assure good dispersion and uniform distribution. The FCM segments create both forward and reverse mixing patterns, while the octagonal segments split the flow streams to create a process of cross blending. These designs have suggested opportunities for the twin screw concept to succeed in mixing rubber compounds where previous attempts have failed.
In conclusion, it is apparent that the rubber industry is ready for continuous mixing. This does not suggest all batch mixers will become extinct immediately, following the tremendous investment involved. The increased availability of free flowing forms of raw materials and the introduction of newer designs of continuous processing machinery make this development inevitable. This will open up a new dimension of mixing technology. With long continuous runs, one will be able to witness mixing phenomena in a steady state, rather than in the current manner where the batch is experiencing continuous changes as it passes from the physical forms of raw materials to the finished mixed compound. Through this vehicle of continuous mixing, many of the secrets of mixing may be exposed and more fully understood, with the final result being higher quality rubber products.
"Continuous mixing of polymeric compounds" is based on a paper given at the Connecticut Rubber Group meeting April, 1998.
"Developments in the extrusion of tire components" is based on a paper given at the April, 2000 meeting of the Rubber Division and is being simultaneously published in Kautschuk Gummi Kunststoffe.
"Zero pressure extrusion" is based on a paper given at the September, 1999 meeting of the Rubber Division.
"Dispelling organic peroxides' myths and legends" is based on a paper given at the October, 2000 meeting of the Rubber Division.
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|Author:||Borzenski, Frank J.|
|Date:||May 1, 2001|
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