Custom mixing of silicone rubber.
The silicone polimer is based on what has become one of the most versatile molecules available to man since its commercial development around 1940, solving materials challenges across many industries in a wide variety of physical forms. The family of organo-silicon compounds is based on a molecular backbone of alternating silicon and oxygen atoms. Depending on the length of the polymer chain and the organic groups attached to the silicon atoms, silicone materials can range from extremely low-viscosity liquids to heavy greases, solid resins and curable rubbers.
The first silicone rubbers to be produced on a commercial scale offered resistance to extreme temperatures and other forms of degradation, but could not provide the physical strength and elongation properties common to many organic elastomers. The ability to manipulate the polymer chain and develop literally thousands of new and application-specific materials has since shown silicones to be a most versatile elastomer.
This material's development has initiated a trend well illustrated by the rising use of silicones in automotive applications (figure 1). While the auto industry averaged less than 1/4 pound of silicone rubber per car in the early 1970s, longer warranty requirements and rising underhood temperatures resulting from tighter emission controls and more aerodynamic body styling have boosted that average to nearly three pounds in the 1990 model year. Other industries are seeing more gradual increases in the use of silicone rubber for gaskets, sealing materials and insulation, typically for reasons of temperature resistance, longevity, fuel/solvent resistance and for the inherent inertness and low toxicity of silicone products (figure 2).
Some organic elastomers continue to provide greater physical strength than silicones, and certainly offer an initial cost advantage. But the concept of life cycle costing has begun to replace earlier evaluations of a material's true price, prompting a rising number of engineers to specify silicones in original equipment applications. Increasingly harsh operating environments, coupled with tougher standards and regulations affecting end products, are adding to the percentage of molded and extruded parts made from silicone rubber.
A versatile polymer
Unlike its natural rubber counterparts, silicone rubber is the result of relatively few ingredients. Mined quartz, or silica, (SiO2) is reduced to silicon metal in an electric arc furnace, then converted to chlorosilanes through a direct process reaction with methyl chloride. The resulting polymer chain can be manipulated in a number of ways to affect physical properties and processing characteristics.
The silicon-oxygen linkage in the silicone polymer chain is the same strong Si-O-Si bond that occurs in sand, quartz and glass. That bond is the basis for the excellent temperature resistance of silicones, as well as their ability to withstand exposure to ultraviolet light, ozone and other forms of weathering (figure 3).
Organic polymer chains, such as those occuring in natural rubbers, often have double carbon bonds that are quickly degraded by UV light, ozone, heat and other environmental conditions (figure 4).
The versatility of the silicone polymer permits the tailoring of materials to meet very specific application criteria. The silicones industry can be said to have an extreme `outside-in' orientation, meaning that application requirements are responsible for driving the development of specific materials which have been formulated to provide the necessary physical properties and processing characteristics.
For this reason, although the percentage of a rubber fabricator's base represented by silicones may be growing, his level of expertise and comfort in the development and processing of new materials may not be keeping pace. That expertise is slow to advance when each application may require a new material, with distinct processing characteristics.
The growing use of silicone rubber has led many fabricators who were once dedicated to molding or extrusion of organic elastomers to include silicones among their resources. Silicone elastomers typically mold easily and quickly, with no post-cure operations required. They do not require high pressures and offer very precise molding, with little scrap. Although the material cost exceeds that of most organic elastomers, the processing advantages offset this difference to some degree, often making the cost per part of silicone rubber competitive with the cost of making the same part from an organic material.
As the silicone rubber industry began to mature, applications for the new elastomers grew quickly in number and diversity. In the automotive, aerospace and appliance industries, as well as a wide range of other applications, silicones provided physical properties unmatched by organic elastomers. The ability to tailor material properties to specific application requirements greatly increased the utility of these elastomers, resulting in the development of thousands of different material formulations.
The use of silicones quickly became application-driven, as material suppliers manipulated physical properties to meet the needs of the many new and developing applications for silicone rubber. This fact highlighted several growing concerns. For one, the existence of so many different products made it difficult for fabricators to amass significant experience with any one material.
In addition, material consistency became a key issue. Its significance is twofold: obviously, lot-to-lot variation of materials causes variation in the physical properties of the end product, which may affect the acceptability of the final part. Of equal concern to fabricators is the fact that processing characteristics are also affected by slight variation in material formulation; two batches of the same material mixed at different times may not process identically. Therefore, the mixing operations has become a critical function of fabricating with silicone rubber.
The unfilled high-viscosity silicone polymer, also known as a gum, is first processed through the use of heat and process aids in the mixing stage, where reinforcing fillers are added. The resulting base can be used to develop many different products, depending on the additives, pigments, fillers and catalyst added. Each distinct blend of these ingredients is a unique compound, from which parts can be molded, extruded or calendered. Materials developed for one specific application, customer or process are categorized as custom compounds.
The case for custom mixing
As silicone chemistry advanced into the 1970s and scientists learned to manipulate the molecular structure of the polymer further, materials engineers found it somewhat easier to meet physical property specifications. There are typically a number of material formulations which will cure to an elastomeric product meeting the physical property requirements of a given application. But optimizing the formulation to provide maximum processing speeds, fastest cure times and a minimum of waste began to surface as the newest challenge. Consistency of the compounds became a key issue, especially in sensitive extrusion operations, and many fabricators began to recognize the limitations of in-house mixing.
The two primary methods for compounding silicone rubber are the two-roll mill and the internal blade mixer. There are several variations on the latter, including top-discharge and bottom-discharge units. One of the obstacles faced in the mixing of a silicone rubber compound is the capacity of the mixing equipment. A common 60" mill is capable of handling 100-pound lots of silicone, but most production runs require substantially more material. This introduces one of the biggest problems in fabrication: lot-to-lot variation.
Though each material batch may fall well within the tolerances established for the application, slight changes from one batch to the next can disrupt production and interfere with consistent, efficient processing. Slight changes in release values or shrinkage can cause parts to stick in a mold, for example. Even minor inconsistencies in extrusion grade materials can drastically affect plasticity and, therefore, extrusion rate.
The use of statistical process control (SPC) is one method of reducing variation in the mixing process, but SPC requirements from end-use customers can further complicate in-house mixing. Certifying that a process and product are in control under statistical guidelines means documenting data on raw materials, compounds and final product. In order to collect that data, a fabricator mixing his own material would require the services of a laboratory with appropriate testing and data collection capabilities.
Another concern in compounding silicones in-house is contamination, especially for multi-elastomer fabricators. Trace amounts of organic materials or other chemicals trapped in mixing or processing equipment can render an entire batch of silicone compound unusable. In-house mixing also requires substantial inventories of the raw materials, additives and catalysts necessary to produce the various silicone compounds. Some of these are flammable, which can cause storage problems.
In the early years of development, silicones were not well understood, and the equipment to mix custom compounds was not common among rubber fabricators. Organic elastomers usually require high shear for adequate mixing, but silicones are much more pliable, and equipment for mixing organic elastomers is generally not well suited to the mixing of silicone rubber.
As a new material, silicone rubber was limited to use in specialized, low-volume applications. As suppliers improved on physical properties, many industries began to take advantage of the unique properties of silicone, and the markets for silicone rubber entered a rapid growth phase. A number of these new applications involved high volumes, and led some fabricators to establish mixing operations in-house. At that time, the required equipment for compounding silicone elastomers included an appropriate mill or mixer, an extruder and basic testing capabilities.
But over the last several years, the trend away from commodity-type products toward specially-formulated materials has contributed to the growth of firms specializing in the custom compounding of silicone elastomers. What was once a fairly straightforward process of compounding the necessary ingredients to produce desired physical properties has become a complex operation, further complicated by the sophisticated testing and documentation requirements currently in place in most industries. These requirements, coupled with more demanding operating environments for silicones, have encouraged the growth of the custom compounder.
The primary and immediate advantage to the fabricator taking advantage of custom mixed materials is reduced variation. This is accomplished in a number of ways. First, large batch mixing technology has made it possible to compound up to 4,000 pounds of silicone rubber at one time. The production run, which might require an in-house mixing operation of twenty batches to complete, can be supplied by one consistent lot of material from the custom mixer. In addition, because the custom compounder has dedicated its resources to silicones, the opportunity for contamination from other elastomers is eliminated.
Because the custom mixer's strategy is to meet individual customer requirements rather than supply a comprehensive line of standard offerings from which customers must choose, several thousand quality control tests are typically performed each month. While several fabricators may order the same product, they often require different certification. Some may need to meet stringent UL standards or ZZR specifications for military applications. By utilizing the systems in place at the custom mixer for documenting this certification, the fabricator avoids a complex array of tolerances, test results and material inventory. He is free to concentrate resources on design and fabrication.
The custom mixing industry has adopted a global perspective on material quality, concentrating its efforts on total quality programs for unprecedented product consistency. At the heart of these programs are computerized information management systems which track raw material specifications, product tolerances, manufacturing data and test results.
A fabricator compounding in-house is likely to have unique specifications for each of its production runs, even when two customers use the same material. The process of assigning the right series of tests and tolerances to each material can be time-consuming and prone to error.
The custom mixer, however, automates this task. When a material sample and customer identifier are entered into the system, the software assigns the correct set of specs and required tests. Results from completed tests are automatically collected from lab equipment and fed into the computer. The implementation of SPC drastically reduces the sources of variation, and certification of the properties as tested is readily available. The ability to electronically transfer data also makes it possible for the fabricator to use the information as part of his own quality control program.
Some custom mixers also use the system to establish "alarm" specs, designed to flag variation before results stray outside of customer tolerances. This focus on process quality as a determiner of product quality represents a new approach for the custom mixing industry, one which has also extended to the raw material suppliers who provide the gum stock.
If the silicones industry can be said to have been application-specific in its initial orientation to the marketplace, the trend today is toward an industry that is increasingly process-specific. It is in the mixing of the rubber that adjustments can be made for optimum processing under the fabricator's particular set of circumstances, achieving the fastest cycle times and highest part quality with the least amount of scrap.
New product development
Perhaps the single greatest contribution of the custom mixer is in the development of new materials. The depth of experience in silicones affords the custom compounder many advantages over the multi-elastomer fabricator who mixes silicones in-house. There are relatively few sources for the raw materials to compound silicone rubber, and the larger suppliers have been involved in the industry for half a century. The relationship of the custom mixer to those technical resources, in some cases directly with the chemists who developed a new material, is an invaluable asset in the search for elastomers to satisfy new applications.
A fabricator bidding on a new product often approaches the custom compounder with a set of specifications and the knowledge that a silicone will be necessary to satisfy the application, and little else. The responsibility for product recommendations and samples generally falls to the custom mixer, who has the experience to know which compounds are most likely to satisfy the requirements and how to customize those compounds to run at maximum efficiency on the processing equipment available.
Because the compounder's technical staff must thoroughly understand the fabrication processes in order to make product modifications, he can also provide assistance on finishing techniques, adhesion promotion and assembly. Technical service includes new equipment recommendations and assistance in modifying existing equipment, as in the case of injection molders wishing to run liquid silicone rubbers (LSRs).
The larger compounders offer detailed fabrication guides, which include information on molding, extrusion, calendering and fabric coating with silicones. Compounders of the '70s and '80s were focused on supplying their fabricator customers with consistent elastomers that met specifications; the involvement of the custom compounder today often begins years before that, in the initial engineering stages.
As a complement to the technical support, the role of the custom compounder has also evolved as a result of the capabilities he can offer. In addition to the applications laboratories and extensive facilities of the raw materials suppliers, the equipment investment alone permits the custom mixer to provide services beyond the capacity of an in-house operation. This might include color matching services, for example, using a colorimeter for precise measurement and repeatability.
Certification of a material's suitability for specific applications can also simplify the fabrication of a new part. UL listed materials are available, which take advantage of flame retardant properties and low smoke technology. High-strength materials to consistently meet ASTM standards or ZZR specifications are an asset in some situations. Molding grade materials that satisfy FDA standards for appliance applications, without post-cure operations, can cut processing costs and simplify a fabricator's documentation.
Product quality has become such an issue that some compounders have begun to strain every lot of material as a matter of routine. Even large batches are extruded through a fine mesh screen to reduce contamination, which eases processing and further improves consistency.
For some fabricators, custom preforms direct from the compounder can streamline processing, especially in extrusion and transfer molding operations. The successful molding of any part depends largely on the shape of the uncured stock. The material should flow as the mold closes, reaching the flash points at the edges of the cavity at the same time. Custom preforms extruded to a specific shape and size can assure complete filling of the mold and minimize waste.
While fabricators and OEMs who process extremely large volumes of silicone may find the return on equipment capital attractive enough to warrant the investment, for the large majority of those supplying silicone shapes, custom compounds offer a viable option to the complex process of in-house mixing. As the use of these elastomers continues to rise in automotive and other industries, many fabricators are turning to the custom mixer for services and product quality unavailable from any other source. [Figure 1 to 4 Omitted]
Ross Shingledecker, Dow Corning STI and Robert Marshall, Furon IER Division