Beater Addition Gasketing
The Beater Addition process of manufacturing gasket materials is a wet process utilizing a cylinder or fourdrinier paper machine. The components are dispersed in water at low concentrations.
A wide range of natural or synthetic fibers can be employed. The fibers are beaten or refined, which fibrillates the main fiber into numerous fibrils. This significantly increases the fiber’s surface area and the additional fibrils impart high strength to the product. The main synthetic fiber used in this process is aramid, an example of a natural fiber is cellulose fiber. Aramid is the most commonly used fiber.
Binders are added to the fiber and/or filler slurry and deposited (precipitated) evenly throughout the mix, onto the fibers and fillers. Typical binders used in the BA method are SBR, Nitrile, Chloroprene, and Poly-Acrylates. Other resins may be added which chemically combine with the rubbers to increase strength or improve heat and/or chemical resistance.
The material is formed on a cylinder(s) or a moving wire screen. In this above process, the product is dried on dryer cans or in conveyor ovens to remove the water. The dried product can be passed through calenders to obtain the desired thickness and density. Post operations such as coating, curing, laminating and cutting to size are performed depending on the end user requirements.
The BA process can yield gasket materials ranging in thickness from .005″ to .250″ and densities ranging from 25 to more than 100 pounds/cubic foot. By controlling the composition, density and state of cure, properties such as compressibility, strength and fluid resistance can be varied to serve many industry requirements.
Dip Saturated Gasketing
The Dip Saturation method is most often applied to Beater Addition materials, and involves dipping a material into a latex, or resin saturant and impregnating the sheet by squeezing between rolls. Drying, calendering and other post operations may also be performed.
Laminated Beater Addition Products
These products were commonly used in automotive gaskets in the past (less so today). Beater sheet materials can be laminated to support materials such as stainless steel sheet (using either adhesives or mechanical methods such as using tanged steel). In some cases the beater sheet is saturated, in some cases it is not, depending on the application or the formulation of the beater sheet. In some cases a metal eyelet can be added around critical openings, such as cylinder bores, to improve sealing, reduce erosion or improve chemical resistance. Compressed elastomer based fiber sheet gasketing products were first introduced in the 1890’s. Since that time compressed gasketing has been the single most widely used non-metallic gasket in the world for sealing flanges because of its ability to seal effectively over an extremely wide range of service conditions.
In the manufacture of compressed gasketing, fibers (most commonly aramid fibers) are mixed with a variety of elastomers and fillers. A viscous dough is formed in mixers with the introduction of a suitable solvent. The dough is then formed into sheets using a specially designed two-roll calender called a “Sheeter”.
A “Sheeter” has one large steam heated roll and one smaller water cooled roll. Each roll revolves toward the other forcing the dough located in the nip onto the heated roll, where it continuously builds up until the desired thickness is reached. A controlled pressure is constantly applied by the cold roll. As the thickness increases the rolls slowly back off from one another until the finished thickness is achieved.
The percentage and type of each constituent used, process times and temperatures, mixing and roll speeds, and carefully controlled, constantly applied loads on the dough during the combined compression/curing cycle on the “Sheeter” are the critical factors that impact the sealing characteristics of the products made by this unique process.
Sheets are stripped from the large roll, after which they may be given a surface treatment with a “release agent” (sometimes called “anti-stick”), and may be branded and cut into smaller size sheet sizes. The most common sheet sizes are 150″ x 150″, 120″ x 120″ and 180″ x 60″. Likewise, the most common cut-down sheet sizes are 50″ x 50″ and 60″ x 60”, although any number of combinations can be furnished. The most used thicknesses are 1/64”, 1/32”, 1/16” and 1/8″; however, thicknesses up through 1/4″ are available. Wire inserted sheet is available in sheet sizes as large as 180″ x 60″ and in thicknesses of 1/32″ through 1/8″.
Cork gaskets are produced from sheet composed of bark from the cork oak (Quercus Suber) tree. Sheets are made by grinding the cork bark into granules, and bonding with protein or synthetic resin binders, forming into blocks or mats, and slicing into sheets. The result is a material with high compressibility, unusual crush resistance, negligible extrusion, good recovery, and a high degree of impermeability under relatively low bolt loads.
While various synthetic rubber compounds are employed as binders in the processing of gasket material, homogeneous rubber sheet also is used in many industrial applications. Rubber processed from a naturally occurring latex was the basis for development of synthetic rubbers but availability and property limitations restricted its broad application. The majority of synthetic rubbers used as gasket material are made by the polymerization of petroleum-based precursor monomers. Monomers also are mixed in various portions and copolymerized to produce material with a wide range of physical and chemical properties. One of the most prevalent is styrene-butadiene (SBR) rubber derived from the copolymerization of styrene and 1, 4 butadiene. SBR rubber was developed as an improved alternative to natural rubber and gaskets made of this material are commonly called Red Rubber gaskets. Some of the other more commonly used synthetic rubbers include butyl (HR), chloroprene (CR), ethylene propylenediene monomer (EPDM), and nitrile (NBR). Silicone and fluoroelastomer (FKM) elastomers are applied primarily in high temperature aggressive environments. All of these materials are compounded and processed into sheet from which gaskets can be cut. Formulations with various degrees of resistance to the effects of temperature, fluids, aging and abrasion can be compounded while retaining the resilience and other physical properties needed for gaskets.
Skived PTFE sheets are made by peeling off a continuous layer of material from a solid PTFE rod or thick-walled PTFE tube (commonly called billets) by virtue of horizontally pressing the billet into a fixed knife/blade. The process itself originated from the wood veneer industry and many machines used today are modified machines from the 1950’s and 1960’s. Some modern machinery for this industry has been developed but tends to limit the billet length (thus sheet width) and maximum skived thickness. The skiving process can produce a continuous sheet in thicknesses ranging from 0.4mm to 8mm (1/64” to 5/16”) with very good thickness consistency.
Filled PTFE sheets are made by two common processes: compression molding and skiving; and HS-10 calendering. The HS-10 method requires PTFE powder to be deagglomerated in a solvent with a large mixer. A filler is then blended with the PTFE resin and solvent. The resulting slurry is typically filtered to remove a portion of the solvent from the PTFE resin and fillers to form a cake. The cake is then squeezed through calender rolls to produce a sheet. The sheet is then dried to remove any remaining solvent followed by a sintering process to form a full-density PTFE sheet with homogeneously dispersed filler.
The compression molding process requires PTFE powder to be dry-mixed with filler at a very high intensity in a large volume mixer. This mixing process is referred to as compounding and is intended to homogeneously disperse the filler through the matrix. The compounded PTFE with filler is poured into a large mold and hydraulically compressed at high pressure to a size ratio of approximately 3:1. This compressed PTFE is called a billet and is then sintered to fully cure and densify. The cured billet is then skived (as described above) for sheet material or cut into gaskets or shapes on a turning lathe.
The phyllosilicates are a group of naturally occurring minerals that includes micas, biotite, muscovite, phlogopite, steatite, kaolin and vermiculite. All of the members of the group exhibit an extended sheet like structure and are found as flakes or plates that consist of a stack of numerous individual crystal sheets. All of the phyllosilicates are noted for their thermal and chemical resistance. Traditionally, mica has been used to produce a sheet gasketing material using a process like that used to produce beater addition gasketing and incorporating an elastomer binder. A process of chemically exfoliating vermiculite has been developed that produces a sheet gasketing material through a calendering process. Both common sealing types can be used as sheet gaskets, as the filler of spiral wound gaskets and the facing material for grooved metal gaskets.
Intercalation is a process that employs a high quality particulate graphite flake which is chemically treated, usually with mixtures of mineral acids to form an intercalation salt where acid is attracted into the space between the layers of the graphite material. This intercalation salt is then rapidly heated such that the acid turns rapidly to a gas, forcing the graphite crystal layers apart, resulting in an over eighty fold expansion in size compared with the raw flake material. The expansion produces a worm-like or vermiform structure with highly active, dendritic-like rough surfaces which makes them readily formable into sheet via either a molding or calendering operation. Since the forming of flexible graphite involves only mechanical interlocking of worm-like flakes, the resulting sheet product is essentially pure graphite which is typically well over 95% elemental carbon by weight, with a highly aligned structure.
Density and related material properties
Increasing the density of flexible graphite decreases compressibility and permeability; however it does increase the recovery, tensile strength, and abrasion resistance properties. Typical density of flexible graphite used for gasketing is 1.0g/cm3 (62.4 lb/ft3) in Europe and 1.12 g/cc (70 lb/ft3) in North America. Where a relatively large degree of conformability is needed and sealing loads are either relatively low, or the high compression caused by high load does not cause mechanical problems, then a lower density i.e. 0.8 g/cm3 (50 lb/ft3) can be used. Where high recovery, low compressibility and minimum permeability is needed, such as in a confined space with high internal gas pressure, a higher density i.e., 1.4 g/ cm3 (90 lb/ft3) is preferred. The ultimate density of flexible graphite is 2.2 g/cm3 (140 lb/ft3), which would occur at 50% compression of the 1.1 g/cm3 (70 lb/ft3) material. The edge surfaces of the flexible graphite sheet are more permeable than the flat surfaces. Since this edge permeability is inversely related to sheet density, gasket designs sometimes employ densification of edges near bores to the ultimate density to minimize any fluid penetration.