Temperature control for polymer processing.
The first Banbury mixer was installed in a tire company in 1916. Since then, many design changes have taken place. Not only have rotor speed levels and connected power been increased, but there have been significant improvements in the design of the heating and cooling passages within the machine castings. Cooling passages are now designed using finite element methods to achieve optimum machine cooling without compromising mechanical integrity. The traditional "first batch effect" in batch mixing is a phenomenon which has been significantly reduced in today's mixers, mainly through the understanding and application of temperature control. In addition, through the understanding and utilization of temperature control, facilities have been able to standardize mixing procedures whether their plants are in the far north or in warmer southern climates. This has not been accomplished by throttling water valves, but by understanding and removing those obstacles which contribute to temperature variations. Process temperature control may be divided into three categories:
* raw materials temperature;
* machine temperature; and
* process temperature.
Control and understanding of each of the above temperatures is tantamount to achieving optimum quality at the lowest cost, consistently. Recognition of the importance of temperature control in each of the above phases of operation can assure process repeatability, and result in continuing product quality and uniformity. The latter is perhaps the most sought after characteristic in today's products. Product performance requirements have reached levels previously considered impossible. A tire which will outlast the life of an automobile is now becoming a reality. The demanding requirements of polymeric products associated with space exploration also are examples of product performance which require improvements in process control. In order to achieve these levels of quality and performance, one must achieve precise control of all the key elements of processing among which temperature is a major factor.
Raw materials temperature
The polymers, fillers, oils and additives are individually major sources of concern regarding possible process variations. For example, polymers (especially elastomers) are excellent insulators, and retain heat or cold for long periods of time. Heated storage areas called "hot rooms" have been used in facilities, such as tire plants, to maintain rubber at controlled temperatures for processing.
In recent years, some of the new mixing facilities have installed microwave pre-heating ovens for conditioning polymers and compounds prior to initial and secondary stages of mixing. In some cases, not only are polymer bales being preheated, but so are slabs of first pass being reprocessed into productives (final mixes). Smaller facilities are not able to justify these luxuries of polymer conditioning, and must live with problems associated with varying polymer temperatures. Unfortunately, in many of today's operations due to cost limitations, emphasis is placed upon reduced inventories. The policy of "just in time supplies" has tended to prevent storage and temperature stabilization of raw materials resulting in the introduction of a variable which is difficult to overcome. A facility in a warm climate, receiving raw materials transported to them from a cold region, will suffer the same anxiety as the individual in the cold climate who pays little attention to storage areas and materials temperatures.
Let us consider some of the potential problems created by inconsistent raw materials temperatures.
Inconsistent polymer feed temperatures will negate the benefits of power mixing. Polymer bales charged to the mixer at inconsistent temperatures will demand erratic levels of energy for initial mastication. These unpredictable energy levels will void the target energy levels which have been prescribed in mixing by energy consumption. If a mixing cycle is started with a bale of rubber, which has been stored in a cold storage area, or is partially frozen, much of the energy which has been targeted for various mixing stages will now be consumed in masticating the cold bale of rubber. Although not recommended as a planned means of mixing, when a large quantity of rubber must be used at an unusually low temperature, the mixing cycle may be adjusted by ignoring power consumption until a temperature threshold of 45 to 55[degrees]C is reached. At this time, one can then begin to integrate power with satisfactory end results. Although the cold polymer creates energy inefficiency, this method will at least produce good compound in an emergency. This method of operation can be rather easily programmed with a modern PLC controller. Inconsistent rubber temperature is not always associated with raw polymer exclusively. For instance when production requirements take sudden increases, cooling of first pass mixes may be shortened. Rubber is then directed for final mix at a higher than typical temperature which may significantly affect final viscosity, and standard processing characteristics causing an increase in product scrap rates. Results of controlled tested programs in which rubber was mixed from bales conditioned for 48 hours at a range of temperatures from -10 to 35[degrees]C, clearly indicated the wasted energy consumed and the erratic results at the lower temperature ranges. Inconsistent and low polymer temperatures will cause:
* extended mixing cycles;
* wasted energy;
* variation in physical and rheological properties;
* smaller batch sizes.
Each of these effects of cold polymers can be translated into lost profits.
Although polymer temperature is perhaps the most critical of all of the raw materials temperatures to control, one cannot discount the importance of knowing the effect of temperature of two other major raw materials in the rubber formulation. These are the fillers, such as carbon black, calcium carbonate, silicas, and the oils or plasticizers. Fillers in bulk quantities are usually stored in large unheated silos or containers. Carbon black and some of the non-black fillers usually are 40% or more of the rubber formulation. As in the case of the polymers, these raw materials temperatures can have a significant affect upon productivity, quality and uniformity of the mixed compounds. Although suspected for many years as being another major cause of product inconsistency, only lately have rubber companies been taking action to provide fillers at consistent and moderate temperatures. Temperature control of this class of materials also reduces the possibility of fillers absorbing moisture as they experience rapid temperature changes.
Oils and plasticizers
Oils and liquid plasticizers must also be stored, and used at controlled temperatures. In most cases, lower temperatures will increase the viscosity of many fluids. This inconsistency will not only affect the accuracy of metering and weighing these liquids, but will have distinctly different affects as they enter the batch at some prescribed point of introduction. Cold fluids will usually take longer to become incorporated into the batch. This fact not only affects productivity but also distribution of the oil within the matrix of the mix.
Control of raw material temperatures can be evasive. Many of the problems associated with mixing usually occur when seasons change. This is most noticeably as cold weather approaches.
In the earliest designs of internal mixers, heat transfer of the various machine components, although adequate for the times, would not have satisfied the demands of today. Compounders have considerable freedom with curing systems, and the use of plasticizers and softeners to compliment and simplify manufacturing systems. Usually a mixer was supplied with the coldest water available. Water supplies varied from city water, to well water, to tower water, and in some cases, chiller water. Variations in water supply temperatures could be significant from day to day, plant to plant, and even from the beginning to the end of a day. As a result of these variables, users would suffer from:
* seasonal mixing variations;
* first batch effect;
* unpredictable power consumption;
* erratic quality.
Wise mixer operators were aware of these variables, and restored to "cracking" water valves to reduce water flow, and permit the mixers to reach equilibrium more rapidly. Unfortunately, not all operators were wise and much variation was experienced in mixing operations, these were passed off as a necessary evil. Cold water and cooling were considered essential to any mixing operation. Cooling was utilized as low as could be achieved. Mixing cycles were usually designed by reacting to temperature plateaus as they occurred through the mixing cycle.
In the late 1960s and early 1970s, major advancements were achieved in heat transfer capabilities of internal mixers. Drilled sides were designed and successfully applied to mixer design (figure 1). The new design for cooling passages within the mixer chamber walls achieved a level of cooling not previously experienced. As opposed to the earlier designs for cooling by cored and spray sides, this design provided a dimension which required compounders to re-think their mixing procedures. Cooling passages were designed by finite element analysis to achieve maximum heat transfer without compromising mechanical integrity (figure 2). Prior to this development, the machinery supplier and chemist preached the advantage of chilled water because of poor heat transfer. With this improved heat transfer, cold water and chillers are no longer necessary except for a very narrow range of materials.
Compounders agree, most mixing occurs when a polymer and compound is at its highest viscosity. This is true. However, to accomplish mixing, the compound must drag along the walls of the mixer. The surface must be able to transfer heat as the compound generates temperature by shear. Unfortunately, not all polymers react the same to a single wall temperature. As improved heat transfer began to exhibit unexpected mixing results, it became necessary to provide an explanation. Experiments demonstrated not only the benefits of mixing with elevated water temperature, but that various polymers reacted differently to various metal temperatures. These experiments began by investigating the coefficient of friction of various elastomers over a range of temperatures similar to those which cooling water could deliver to the metallic elements of the mixer (figure 3).
When mixing with a modern drilled design mixer, experimental studies proved that elevated water temperatures did not cause a product to reach its optimum temperature more rapidly, due to the grossly improved heat transfer capabilities. Later, research programs strengthened by empirical evaluations, exhibited shorter cycles due to the fact that the correct machine temperature will take a compound more rapidly to target temperatures and permit maintaining and controlling at that temperature due to the improved heat transfer. The shearing action between the rotor and wall of the mixer is far more effective when the polymer viscosity can be controlled along with the heat transfer. One factor, which has been difficult to quantify, is the application of a temperature differential between the mixer rotors and mixer sides. Using a differential temperature has demonstrated improvements in many mixing trials. These improvements have manifested themselves both in quality and productivity performance.
In the latest rotor design, cooling passages have now been introduced into the rotor tips as well as their cores (figure 4). This design ST Rotors (synchronous technology) not only has demonstrated improved heat transfer capabilities, but achieves a level of enforced order which prevents stagnation in any location of the mixing chamber. The benefits of this new design along with temperature control are manifested in:
* rapid, homogeneous discharge from the mixer;
* reduced mixing temperatures;
* lower Mooney scorch;
* improved building tack;
* improved carbon black dispersion.
One can safely say that the benefits which can be achieved by temperature control far surpass the cost of implementation. Although this discussion emphasizes temperature control benefits with respect to internal mixing, the rubber technologist can apply the concept to all types of polymer processing machinery. One most also realize three important facts with respect to the use of cooling water:
* The quality of water must have sufficient GPM to accomplish turbulent flow (table 1).
Table 1 - flow rates - GPM
1 3D 9/9D F80 F270/ F370 F620/ 11D 27D Sides 30 60 60 120 120 140 180 Rotors 20 35 40 40 60 100 150 Doortop 10 10 10 10 15 30 30
* Water must be at a controlled temperature.
* Water quality must be monitored to prevent hard water from forming scale deposits.
The optimum water temperature for mixing any compound may be easily determined after one has the opportunity to operate a mixer with a temperature control system. General settings will fall in the following ranges for optimum mixing of specific polymers: SBR, SBR/NR, PBD/NR and EPDM have temperature settings of 45[degrees] to 65[degrees]C; NR a setting of 35[degrees]C and NBR, CR settings of 20[degrees] to 35[degrees]C.
A three zone temperature control system will provide independent cooling to each of the three major zones of the mixer. The closed loop system normally recommended will have heat exchangers, and be either electrically or steam heated. With these three separate zones, the operator can compliment the slip/stick nature of the compound, minimize sticking, and affect heat transfer of the mixer (figure 5).
Process temperature is defined as the temperature or temperatures which a compound is subjected to during its path from raw materials to a product. These must be carefully controlled to protect the compound from premature reactions as in the case of scorch. In addition to prevention of scorch, other chemicals such as tackifiers, oil and processing aids can be negatively affected by excessive temperatures. Without question, process can seriously affect product quality, and lead to many processing problems which inevitably will have a negative impact on cost.
Much attention has been directed to achieving the most accurate indication of the internal temperatures within a mixer. Unfortunately, regardless of the accuracy of the instrument, actual internal temperature of material within the mixer, during the mixing cycle, has been an approximation. Because of this dilemma, users of these machines have often relied upon time, power consumption and torque to control various steps in the mixing process.
Due to the sliding design style of the discharge opening in the original Banbury mixer, thermocouples were installed in the mixer end frames. Since the nature of the mixer's rotor design was to direct material toward the center of the mixer, the end frame thermocouple could at times be in a void. Users with experience would always manually probe a batch after it discharged from the mixer to determine actual temperature. Using this as a guide, they would then be able to estimate the actual temperature of successive batches.
As the discharge region of the mixer was redesigned to be of the drop-door style, thermocouples were improved in response and durability. This design improved the accuracy of temperature determination. Two thermocouples were not installed in the doortop, for improved accuracy and back-up. Still, many characteristics of mixing could negatively influence temperature.
Among these are:
* incorrect batch sizes;
* very short mixing cycles;
* very cold water on the drop door;
* stiff, very high viscosity compounds.
As a result of the current design and the drop door installation, thermocouples will usually indicate, within a few degrees, the actual compound temperature (-3 to 5[degrees]F). It is still strongly recommended that thermocouple response and calibration should be a daily function in any mill room.
The quest for precision in temperature indication has continued. The desire for accuracy is prompted by the more frequent use of the mixer as a reactor, or where specific ingredients must reach a precise temperature during mixing. Success has been achieved by means of an infrared temperature measuring device (figure 6). This has by no means been an easy development, due to the usual function and environment which are common to mixers. The instrument must be rugged, and relatively insensitive to vibration, abrasion and corrosive chemicals. The infrared temperature measuring device has performed for nearly two years on a F80 mixer. It has exhibited accuracy and ruggedness mixing a wide variety of materials (figure 6).
Temperature, the most basic of process controls, is one of the most ignored. The benefits, which can be achieved by paying close attention to those elements of process control, can represent the greatest return on investment of any improvements which might be addressed in a mill room or mixing facility.
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|Author:||Melotto, Michael A.|
|Date:||Jul 1, 1995|
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