A processing guide to silicone rubber extrusions.
It is impossible to cover each and every processing variable in the space allotted for an article of this nature. The general scope of this article will deal with the processing of silicone robber profile extrusion, and wire and cable industries.
Compounding and freshening
The wide selection of reinforced gums and finished compounds available today provides fabricators with the broad flexibility they need. Reinforced gums are pure silicone robber polymers, plus silica filler and processing aids. Silicone rubber is fundamentally very easy to formulate. With the addition of one or two filler types, plus heat-age additives, pigment and a suitable vulcanizing agent, fabricators can design a compound to meet their total requirements.
While the equipment used to process silicone rubber is much the same as that used for organic rubbers, it is not advisable to use the same mill for both organic and silicone. A clean work area, preferably in its own separate location, is ideal since contaminated silicone rubber can have decreased physical and electrical properties. If it is not feasible to equip a separate area for silicones, precautions should be taken to thoroughly segregate contaminating materials from contact with the silicone rubber and its compounding ingredients.
The silicone industry offers non-milling compounds which can be fed directly into an extruder. However, various applications demand high strength stocks which do not fall into this non-milling product category. These latter compounds require a milling or freshening operation before extrusion. The gamut of stocks that do require milling will also display a variation in plasticity after milling. The firm stocks, or those exhibiting high green strength, very often can be removed from the mill roll with the aid of a doctor blade or mill knife; however, the softer, or low green strength materials, tend to adhere to the mill roll, and most often the use of a full length scraper blade is a necessity. The scraper blade should be constructed of nylon, since nylon possesses the durability required and will not be honed from the roll, as will steel, thus eliminating a safety hazard. The use of a nylon blade will also eliminate streaks in the compound sometimes caused by a metal blade. Since silicone rubber inherently will cling to the fast roll, the scraper blade should be installed on this roll. Should a compound tend to follow the slow roll, check the nip of the rolls. Usually, tightening the rolls and reducing the nip will force the compound to the fast roll. The mill rolls should be water cooled to prevent scorching the compound, particularly those containing bis (2,4 dichlorobenzoyl) peroxide, or benzoyl peroxide.
Compounding silicone rubber requires few ingredients to develop required properties. Very often when one or two general purpose reinforced gums are blended with a variety of fillers, they will provide the flexibility required.
Many have theorized that since the fabricator would not strain his compounds as a separate operation before using them, the resulting compounds would not offer the ultimate in quality. It is true that organic contaminants will affect various properties of silicone and even prevent cure; however, a compounding area ideally set up for silicone, proper house-cleaning techniques and adequate mixing know-how have proven that both electrical and physical properties of compounds mixed by the fabricator can, and do, equal the typical properties of those offered by the suppliers. In some cases, compounded reinforced gums are superior to compounds or pure gums. Assuming that the compound has been properly formulated, a quality extruded product can be maintained by removing the small amount of undispersed particles that might be included from the compound with the use of a screen pack in the extruder.
The following basic procedures for compounding silicone rubber will provide a quality product with minimum effort.
* Carefully weigh quantities of each ingredient to be used.
* Place pure or reinforced silicone gum (R-gum) on mill rolls, adjust rolls to obtain desired nip and allow gum or R-gum to bond on fast roll and to thoroughly freshen. Pure silicone gum usually requires little or no freshening before filter addition; however, since reinforced gums contain some reinforcing silica filler, they should be allowed to thoroughly freshen. An indication of freshness is usually the point of transfer to the fast roll. Thorough freshening of the R-gum provides ultimate efficiency in filler addition.
* Reinforcing fillers (fine particle silicas), if required, are then added to the mix. Some filler will drop through the rolls into the pan during mixing, and should be picked up and re-added to the mix before subsequent filler additions. A rubber squeegee is a useful tool for scraping the filler from the drop pan. Brushes are not advisable since some bristles may fall from the brush and get into the compound. Since reinforcing fillers have a larger surface area than extending fillers, they must be thoroughly blended into the gum, or R-gum, to assure maximum gum wetting of the filler. As a rule of thumb, the entire quantity of silica filler should not be added at one time, but rather in two or three separate additions, each followed by adequate cross-blending of the batch. This assures proper dispersion and prevents formation of hard filler lumps. If the reinforcing silica filler is added to the gum too fast, hard particles of high filler and low gum content will roll on top of the compound bank and result in poor dispersion. When adding reinforcing silica fillers, a reasonably tight nip insures both good mixing at optimum mixing speed.
When adding fillers, a wider nip is more effective.
* Extending fillers, when used, are always added after the reinforcing filler. Add this filler in increments rather than the entire quantity at one time, blending between additions.
* The final addition to the compound is the catalyst. Be careful not to add catalyst if the compound has become too warm (100 [degrees] F maximum); a partial cure may develop, resulting in unusable rubber or catalyst depletion. If adequate cooling water is applied to the mill rolls, overheating is avoided. Several end passes of the entire batch will adequately disperse the catalyst.
In most cases, the catalyst is immediately added to the completed formulation, especially for stocks that do not require freshening before use. If the compound is stored beyond its shelf life, or if a high strength compound having a very limited freshened life (i.e., 1 to 2 days or less) is used, then add the catalyst during the subsequent freshening operation. Shelf life is the period of time between the freshening or milling operation when the compound becomes too firm to efficiently extrude. Compounds that have exceeded their shelf life limitation must be freshened or milled before use. Shelf life, however, does not determine the useful life of the compound. Since catalyzed material builds structure faster than uncatalyzed, the procedure of adding catalyst during freshening proves to be a time saver. Uncatalyzed compound with some structure will band to the mill rolls much faster than catalyzed material with medium to severe structure. The severely structured material may fall through the rolls several times before banding, resulting in excessive loss in mill time.
The type and quantity of catalyst will affect the freshened life characteristic of the compound. Bis (2,4 dichlorobenzoyl) peroxide allows a shorter shelf life and builds more structure faster than benzoyl peroxide. ("Structure" is the increase in compound viscosity caused by filler to polymer bonds.) Structure is also possible in uncatalyzed compounds, but to a lesser degree.
Those fabricators who employ the CV (continuous steam vulcanization) system and who find it feasible to use benzoyl peroxide, DiCup; Varox, rather than bis (2,4 dichlorobenzoyl) peroxide will realize the following advantages:
* Reduction in material cost;
* longer freshened life (i.e., a two-to-one or more with some compounds);
* higher processing speeds.
While compounding has many advantages for the fabricator, it also adds to his responsibility for quality. The pure gums and reinforced gums as supplied to the fabricator have been carefully checked for optimum quality; however, subsequent additions of fillers, catalyst and color pigments add variables such as cleanliness and accuracy of weighing and blending. A reasonable control for the fabricator is actual testing of his batches after the fillers have been allowed to properly wet out. Proper wet out time for reinforcing silica filled compounds is a minimum of three days. One to two days is sufficient if only extending fillers are used. "Wetting" is referred to as the absorption of the silicone polymer by the silica filler.
When the time is optimum for testing, it is advisable that ASTM slabs be used as the test medium. For bis (2,4 dichlorobenzoyl) peroxide and benzoyl peroxide, press conditions of 10 minutes at 280 [degrees] F are satisfactory. Oven curing and heat aging of the slabs should simulate the processing conditions to which the compound will be subjected, and the heat aging conditions should be according to the reference specification to which the extrudate will be manufactured. If properly organized and systematically maintained, quality testing will become routine and prove itself very efficient.
A Monsanto rheometer is an excellent piece of test equipment for characterizing compounds for CV and HAV cure systems. Quality testing should include measurements on tensile strength, percent elongation, Shore A durometer, tear strength (die B unless otherwise specified), specific gravity and electrical properties. The first four are self-explanatory and will immediately indicate if the compound meets specifications. The control test on specific gravity will indicate errors in compounding such as too little or too much filler.
The importance of the specific gravity check is underscored by its control on both your economy and ,specification requirements. It is possible to have suitable physical properties but experience loss of economy by not having added enough filler. On the other hand, a high specific gravity will indicate too much filler, which in turn can affect the quality of product properties of the finished construction, or may place the finished goods in a weight category that exceeds specification requirements.
Another useful tool in establishing time limits on the optimum age of a compound and its effect on efficient processing is the William's Plastometer. When you have established the two end points (i.e., the degree of softness at which a compound will extrude efficiently, and the maximum firmness you can tolerate), the William's plasticity measurement, when correlated with the optimum extrusion conditions, will serve as a useful time and money saving guide.
Ted Taylor is Senior Vice President and Managing Partner for Specialty Silicone Products. He has 36 years experience in silicones, which includes several positions at General Electric' Co.'s Silicone Products Division.
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|Title Annotation:||part 1|
|Author:||Taylor, Theodore C.|
|Date:||Aug 1, 1998|
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