Processing of Fluoroelastomers

6 Processing of Fluoroelastomers

6.1

Introduction

Processing methods used for other synthetic elastomers can be applied to fluoroelastomers, sometimes with considerable adjustment to take account of special characteristics of the polymers and their compounds. The slow relaxation rates of fluoroelastomers cause difficulties in mixing, extrusion, and injection processes normally run at high shear rates. Many curatives and additives are insoluble in fluoroelastomers, so special procedures may be necessary to get adequate dispersion in compounds for reproducible curing. Many fluoroelastomer compounds give problems in molding operations, including the opposite situations of undesired sticking to mold surfaces, and of inadequate adhesion to metal inserts. The relatively low volume of fluoroelastomer parts production requires that equipment used for other high-volume elastomers be adapted to fluoroelastomer processing.

6.2

Mixing

Fluoroelastomer compounding is usually carried out in relatively small batch mixing equipment, since materials costs are high and production volumes are low. However, most mixing has shifted from open rubber mills to internal mixers as volume has increased and quality control has become more stringent. A major consideration for a production facility handling several other elastomers is to avoid contamination of fluoroelastomer compounds. Strict cleanup procedures are necessary to assure that hydrocarbon elastomers, oil, grease, and other incompatible contaminants are removed from equipment before processing fluoroelastomers.

6.2.1

Compounding Ingredients

Ingredients should be kept in sealed containers stored in cool, dry areas. Particular attention should be paid to metal oxides and hydroxides that may interact with moisture and carbon dioxide in ambient air. Excessive moisture pickup by polymer, filler, and other additives may cause erratic curing and flaws such as porosity in fabricated parts. Special forms

of some ingredients must be used to get adequate dispersion and curing performance. Necessary uniform dispersion of curatives is particularly difficult in compounds cured with the bisphenol system. Bisphenol AF crosslinker and quaternary phosphonium salt accelerators are high-melting solids that must be micropulverized to fine particles for dispersion in compounds. Because many fabricators would have problems in attaining the uniform dispersion necessary for reproducible curing, polymer producers offer these curatives already mixed with fluoroelastomer in the form of concentrates or precompounds. For example, DuPont Dow sells the VDF/HFP dipolymer Viton® E-60 as a gum polymer to be mixed with curative concentrates and also as a precompound, Viton® E-60C, with Bisphenol AF (BpAF) and benzyl triphenyl phosphonium chloride (BTPPC) in the proper amounts for curing. The curative concentrates VC-30, 50% BpAF in dipolymer, and VC-20, 33% BTPPC, are readily incorporated by fabricators in the amounts chosen for desired cure characteristics. Similar curative concentrates are offered by other fluoroelastomer suppliers. DuPont Dow and Dyneon also offer precompounds containing these curatives in the form of a mixture of BTPP+BpAF- salt with additional BpAF (weight ratio BpAF/BTPP+ about four). The isolated mixture is a low-melting glass that is readily dispersed (offered by DuPont Dow as VC-50). Fluoroelastomer suppliers offer a number of bisphenolcurable precompounds, often including processing aids, for various applications. These offerings give fabricators assurance of reproducible curing characteristics and considerable flexibility in compounding for particular processing characteristics and vulcanizate properties.

6.2.2

Mill Mixing

Two-roll mills have been used for rubber compounding since the middle of the nineteenth century. Originally they were also used for mastication of natural rubber, to break down high molecular weight fractions. However, such breakdown is generally not desirable for synthetic elastomers, including fluoroelastomers, which are designed to have molecular weight distributions optimized for various process-

104 ing methods and end uses. Mills are suited to lowvolume production of specialty fluoroelastomer compounds, but have been largely replaced with internal mixers. In many production operations, mills are used for sheeting off stock from internal mixers or for warm-up of compounds for sheet feed to extruders or calenders. A typical rubber mill is shown in Fig. 6.1.[1] The mill consists of two closely spaced parallel, horizontal rolls made from hard castings supported by strong bearings in a mill frame. The counter-rotating rolls are driven at different speeds to maintain a friction ratio of 1.05 to 1.25, transporting the rubber over the top of the roll to the nip area, then through the nip with small adjustable clearance (usually 2–6 mm) to subject the stock to high shear stresses. To get good mixing, the amount of stock and mill clearances used should result in formation of a smooth band on one roll, with a rolling bank of stock in the nip. The surface speed of the slow roll is about 50 cm/s, allowing the mill operator to cut the band diagonally and fold the cut portion over the remaining band for blending. The mill rolls are hollow to allow flow of coolant for control of roll and stock temperatures. A number of safety features are incorporated into mill design, including shutoff switches and brakes to stop the rolls quickly, means to move the rolls apart, and guards to keep hands and tools away from the nip area. Stringent operator training and adherence to safe procedures are necessary to avoid the inherent hazards involved in mill operation. A typical mill mixing procedure is given in a 1975 DuPont product bulletin.[2] The fluoroelastomer described is a VDF/HFP dipolymer precompound, Viton® E-60C, containing about 2 phr Bisphenol AF and 0.55 phr BTPPC accelerator. The medium-viscosity polymer was designed with a considerable high

Figure 6.1 Rubber mixing and sheeting mill.[1]

FLUOROELASTOMERS HANDBOOK molecular weight fraction to impart enough cohesive strength for good mill mixing. A batch size of about 40 kg is recommended for a production-scale mill (about 50 cm diameter and 150 cm length). The compound recipe contains 100 phr E-60C precompound, 30 phr MT black, 6 phr calcium hydroxide, and 3 phr magnesium oxide. The clean mill is cooled to about 25ºC and the nip is adjusted to about 3 mm. The polymer is added to the mill for banding. Ordinarily, the fluoroelastomer bands on the fast roll, but may be forced to the slow roll by increasing the temperature slightly on the slow roll. The nip is adjusted to about 5 mm to get a rolling bank in the nip. The banded polymer is cut about three times from each side to get a uniform sheet on the roll. The powdered ingredients are preblended and added at a rapid uniform rate across the width of the nip. Loose filler that falls through the nip is swept from the pan and added to the batch before cutting the sheet. Further mixing is carried out by cutting and blending the sheet about four times from each side. The mixed sheet is cut off the mill and cooled. About fifteen minutes of milling time is usually adequate for the total operation described. Cooling of the slab is accomplished by dipping in a water tank, or by water spray or forced air. If water cooling is used, it is important to dry the stock with forced air before storing it. Mill mixing is difficult, especially on a production scale, for a number of gum fluoroelastomers. Polymers with narrow molecular weight distribution and low ionic end group levels may not have adequate cohesive strength to form a smooth, holefree band on a single roll. When addition of powdered ingredients is attempted, the stock and loose fillers may drop off the rolls into the pan. Subsequent consolidation of such a batch is time consum-

6 PROCESSING OF FLUOROELASTOMERS ing and messy at best. Very high molecular weight fluoroelastomers undergo significant breakdown during initial passes through a tight nip of a cold mill, with resultant deterioration of vulcanizate physical properties. On the other hand, bimodal blends (formed by latex mixing before isolation) have excellent milling characteristics, with negligible breakdown of high molecular weight fractions. High viscosity elastomers with considerable long-chain branching and gel fractions may also break down during milling, possibly improving subsequent processing characteristics (e.g., extrusion).

6.2.3

Internal Mixers

Even for modest production scales, internal mixers have largely replaced mills for fluoroelastomer compounding. Well-designed laboratory mixers have become available in recent years, allowing reliable development of compounds with small amounts of elastomers. Mixing is accomplished inside a closed chamber with rotating kneading rotors. The major type is the Banbury mixer, developed in the early twentieth century and shown in Fig. 6.2.[3] This design has tangential rotors that do not intermesh. Since the paths of the rotor tips do not touch, the rotors can be driven at different speeds. Dispersive mixing is accomplished in high-shear tapering nip regions between rotor tips and the mixer wall. Distributive mixing occurs by transfer of material from one rotor to the other and around the mixing chamber. Most Banbury mixers have two-

105 wing rotors, but four-wing designs have been developed for faster mixing. In the 1930s, mixers with intermeshing rotors were developed, such as the Shaw Intermix shown in Fig. 6.3.[3] Tangential and intermeshing rotor geometries are shown in Fig. 6.4. Intermeshing rotors provide dispersive mixing in the nip between the rotors and facilitate transfer of material from rotor to rotor. Modern internal mixers are available in a wide range of sizes and have variable speed rotors with special helical profiles and cooling for control of batch temperature and energy input.[4] Ram position and pressure can be controlled to promote optimum mixing. With the sensors and controllers provided, computer-controlled mixing lines have been developed, as shown in Fig. 6.5.[5] Such systems include controls of ingredient feeds and mixed compound takeoff equipment.

Figure 6.3 Shaw Intermix.[3]

(a) Figure 6.2 Banbury mixer.

[3]

(b) �

Figure 6.4[3] Tangential���� and intermeshing���� rotor designs.

106

FLUOROELASTOMERS HANDBOOK

Figure 6.5 Mixing line with computer control.[5]

Conditions for Banbury mixing of a VDF/HFP dipolymer compound are described in Ref. 2. The recipe is the same as that in the mill mixing example of Sec. 6.2.2, a Viton® E-60C precompound mixed with 30 phr MT black, 6 phr calcium hydroxide, and 3 phr magnesium oxide to get a medium hardness stock. The mixer used is a 3D Banbury with a 600-hp DC drive, mixing chamber capacity of about 80 liters, and two-wing rotor design. Total compound weight of 104 kg with specific gravity of about 1.8 results in a fill factor of about 0.75. The mixer and associated auxiliary equipment are cleaned to avoid potential contamination of the fluoroelastomer mix. Full cooling water is applied to rotors and shell, rotor speed is set at 30 rpm, and ram pressure is set at 0.4 MPa. The fluoroelastomer precompound in sheet form (75 kg) is added and the ram put down. Then the ram is raised and the blended powder ingredients are added. The ram is put down and the batch is mixed for about two minutes; measured mixer temperature increases from 30ºC to about 75ºC. The ram is then raised to allow unconsolidated material to be swept down into the mix. The ram is put down and mixing is continued for another minute, as temperature increases to 100ºC. The ram is raised to allow a final sweep, and then is put down for continued mixing for 15–30 seconds. The batch is dumped to a mill for cooling and sheeting. Total mixing time is 3–4 minutes, and final stock temperature is no more than about 120ºC.

Some modifications of Banbury mixing procedure may be necessary for other fluoroelastomer product forms, or curatives. For polymer in the form of pellets, an “upside down” charging procedure is recommended, with fillers and other powdered ingredients added first, followed by the elastomer pellets. If the curing system has short scorch times (e.g., diamine or some peroxide systems), the curatives may have to be added in a second mixing pass after incorporation of other ingredients and cooling of the stock. More detailed procedures and a trouble-shooting guide are given in a recently updated Processing Guide[6] for fluoroelastomers.

6.3

Extrusion

Extruders of varying design are used for fluoroelastomers and their compounds. As described in Sec. 4.8, dewatering and drying extruders are used in production of fluoroelastomer gums. In the DuPont continuous polymerization process, precompounds have been made by continuous feed of curatives along with isolated polymer to an extruder with high-shear mixing elements. Such compounding requires close control of all feeds on an instantaneous basis, since material goes through the extruder essentially in plug flow, with minimal back mixing. The compounding extruder, thus, provides mainly dispersive mixing, with little distributive mixing. With the growth of the precompound market

6 PROCESSING OF FLUOROELASTOMERS and resulting proliferation of products, along with requirements for close control of precompound composition, precompound production was switched to more versatile batch internal mixer systems. Short extruders may also be used to take warm isolated polymer from a dryer, or mixed stock from an internal mixer, and form it into sheet. However, the main use for extruders in fluoroelastomer processing is to convert mixed stock into forms suitable for curing. Extruded solid cord or tubing may be cut into preforms for press molding of seals. Extruded heavywalled tubing may be cured in an autoclave for hose applications. Extrusion through cross-head dies is used for coating of wire and cable, and for hose veneer layers on mandrel supports. Fluoroelastomer suppliers offer specially designed polymers and compounding for fast, smooth extrusion of profiles with good dimensional control. In Vol. 2 of this� ���� ��������� ������� Ebnesajjad[7] describes many design and operating aspects of extruders used for melt-processible fluoropolymers. Parts of a typical single-screw extruder are shown in Fig.6.6.[7] Features of a typical

107 extrusion screw are indicated in Fig. 6.7.[7] However, the operating conditions used for fluoroplastics, with temperatures of 200ºC to 400ºC, are not applicable to extrusion of fluoroelastomers, except for a few specialty thermoplastic fluoroelastomer products. Since elastomers are essentially amorphous, viscous liquids, melting is not required. Extrusion of compounds must be carried out at temperatures below about 120ºC to avoid premature curing. Early screw extruders for rubber had short barrels, with length/diameter ratios (L/D) 6:1 or less, and required hot feed, using mills to break down and preheat the rubber to reduce its viscosity before extrusion. After World War II, extruder manufacturers started to develop machines with longer barrels (L/D = 12:1 or more) to handle cold feed of rubber strip.[8] Modern cold feed extruders are suitable for most synthetic rubbers, including fluoroelastomers. The following description of these extruders is based largely on a 1985 review by Kemper and Haney.[8] From Fig. 6.6, elastomer fed to the extruder is moved through the barrel by a screw to a die to get

Figure 6.6 Typical single-screw extruder with a vented barrel.[7]

Figure 6.7 Conventional screw design.[7]

108 the desired extrudate cross section. The screw is driven at controlled speed with a motor and gear reducer. The drive must be capable of supplying adequate torque over a wide speed range (up to 200 rpm) with precise speed control. In most modern extruders, a variable speed DC drive is used. Torque of DC drives decreases with increasing speed; this matches the lower torque required as polymer viscosity also decreases with increasing shear rate. The shank of the screw connects to the drive mechanism supported by a thrust bearing, which must withstand the force on the screw from the back pressure of the rubber being forced through the die at the other end of the barrel. For steady operation of a cold feed extruder, the design of the hopper and feed section must be adequate to assure uniform, uninterrupted feed. Machine features may include non-restrictive guards, roller feed assists, deep screw flights in the feed section, uniform temperature control, and alarms to warn of loss of feed. However, rubber strip with constant width and thickness must be properly introduced into the extruder. Ordinarily the strip is fed through power assist rollers to one side of the screw. The output of an extruder varies with the inside diameter (D) of the barrel. Common sizes are 60, 90, 115, and 150 mm (2.5, 3.5, 4, and 6 inches), with output approximately doubling with each size increment. Length (L) of the barrel is measured from the end of the feed throat section to the die. Cold feed extruders have L/D of at least 12:1. Extruder barrels are made of high-strength steel with thickness capable of resisting pressures of some 70 MPa (10,000 psi). Usually a high-strength steel alloy liner is provided for wear and corrosion resistance. A heating and cooling system is provided to control temperatures of the internal surface of the barrel and the external surface of the screw. The objective of temperature control is to adjust the coefficient of friction so that the rubber slips along the screw while adhering slightly to the barrel surface. Usually, a modern extruder has at least five temperature-controlled zones: three for the barrel, one for the screw, and one for the head. In most rubber extruders, an ethylene glycol/water mixture is circulated through jackets around the barrel and through the core of the screw. Electric immersion heaters and a heat exchanger for cooling are used to get a maximum temperature near 120ºC.

FLUOROELASTOMERS HANDBOOK The screw controls rubber output rate and stability, temperature rise, backpressure, uniformity of mixing, and compression of the compound into a solid mass. The ideal screw design that would accomplish all these tasks efficiently for a wide range of compounds doesn’t exist, so design compromises are necessary in practice. The simplest screw designs, such as that shown in Fig. 6.7, have three discrete sections. The feed section has a relatively deep channel with a constant pitch (helix angle). The compound is compressed in a transition zone with a reduction in channel depth and/or helix angle. Material is pumped to the die by a metering section with constant channel depth and pitch. Screw flights are only partially filled in the feed zone, but are completely filled in the metering zone. More complex screw designs have been devised to optimize output with minimal temperature rise and improved mixing of compound. These may include mixing sections with special elements or extended length, and barrier sections that promote mixing or set up regions of low pressure for venting of volatiles without loss of compound. Ordinarily, a vented barrel design such as that shown in Fig. 6.6 results in significant reduction in output. The original clearance between barrel and screw for rubber extruders is about 0.08 mm per 25 mm (3 mils per inch) of barrel diameter. To minimize wear, screw flights may be hardened or made with wear-resistant materials. As shown in Fig. 6.6, a breaker plate and screen pack are positioned at the entrance to the head to generate back pressure on the screw and to remove foreign particles from the compound. The extruder head directs the rubber through a shaping pin and die, and has a streamline design with accurate temperature control to provide uniform delivery to the die. A straight head, shown in Fig. 6.8,[8] is used for extrusion of profiles such as cord or tubing. A tubing die is shown in more detail in Fig. 6.9.[7] A crosshead die, shown in Fig. 6.10,[8] is used for coating wire or extruding veneer on a mandrel as the inner layer of fuel hose. Careful design is necessary to obtain uniform rubber flow and concentric coating of the wire or mandrel. Figure 6.11[7] is a schematic of an extrusion line for wire coating, or hose veneer, showing auxiliary equipment for feeding the wire, or mandrel, and for taking up the coated material. Dimensions of the extrudate must be monitored and controlled to get the desired shape before curing of the rubber.

6 PROCESSING OF FLUOROELASTOMERS

109

Figure 6.8 Straight extruder head.[8]

Figure 6.9 Tube die.[7]

Figure 6.11 Extrusion line.[7]

Figure 6.10 Cross-head.[8]

110 Conditions for extruding preforms of a medium viscosity bisphenol curable VDF/HFP dipolymer compound are suggested in Ref. 2. A relatively cool barrel and screw are used to keep stock viscosity high enough to minimize entrapment of air. Care is taken to ensure that the stock is dry, especially to remove any surface condensate that may have formed on material taken from cold storage. Approximate temperatures suggested are 30ºC for the screw, 55ºC for the barrel, 65ºC for the head, and 95ºC for the die. Low screw speeds are suggested to assure extrusion smoothness. As with fluoroplastics,[7] a fluoroelastomer compound shows melt fracture when shear rate through the die exceeds a critical value related to the characteristic relaxation rates of the polymer chains. Extrusion conditions must be adjusted to get the desired cross section for accurate size of preforms for compression molding. Ordinarily, the stock has sufficient scorch resistance so that startup material and stock left in the extruder head at shutdown can be recovered for reuse. Extruded cord or tubing may also be cured in an autoclave under steam pressure (0.55 to 0.70 MPa to get curing temperatures of 155ºC to 165ºC) for an hour or more. Ram extruders such as the Barwell Precision Preformer[9] are widely used to make blanks of rubber compound with suitable shape and weight for use as preforms in compression molding. Typical barrel capacities are 40, 60, or 80 liters of compound. Various die designs (usually for rod, strip, or tubing extrusion) are available for extrudate diameters up to 190 mm. Ram pressures are usually up to 35 MPa. Depending on design, rubber compound can be loaded either at the front or rear of the machine. Rear loading allows the die assembly to remain in place for more efficient resumption of extrusion. A variable speed rotary cutter at the die face allows for cutting of preforms to accurate size. With manual controls, preform accuracy of ±1.5% by volume can be attained. Machines with weigh-scale loop feedback controls can achieve ±1% accuracy. For extrusion of a medium viscosity fluoroelastomer compound, the stock is usually warmed on a mill before charging, barrel temperature is set at about 90ºC and the die at about 70ºC. A screen is used to remove large particles of contaminants and to increase pressure at the die so that air bubbles are not extruded. Barwell extruders are particularly useful for process-

FLUOROELASTOMERS HANDBOOK ing of high-cost specialty fluoroelastomers used for limited volumes of precision molded parts.

6.4

Molding

Fluoroelastomer parts may be fabricated by compression, transfer, or injection molding. All these processes are used commercially, with a number of factors determining the choice for a particular compound or application. Such choices are not always optimum, since fabricators may be forced to use available equipment because of lack of capital funds for upgrading to more modern molding processes. Some general considerations, discussed in the following section, apply to all fluoroelastomer molding operations.

6.4.1

General Considerations

Cure characteristics of rubber compounds must include a delay in the onset of crosslinking to allow sufficient time for the stock to flow at elevated temperatures to fill mold cavities. Then the cure should proceed rapidly to minimize the required time in the mold. Special measurements of scorch time at high temperature may be necessary to assure that a compound is usable for injection molding, since the stock may be subjected to high temperatures for a considerable time before injection into the mold. Compounds should be designed for good mold release, and should not leave residues on mold surfaces, which could lead to subsequent sticking of parts and unacceptable surface quality. The choice of cure system plays a large part in this. The original diamine cure systems generally give mold dirtying and poor quality surfaces on parts after a few heats, thus these systems are little used. Bisphenol cures can be formulated for good release, and are widely used for molded parts. Peroxide systems give variable results. The relatively slow cures of fluoroelastomers with bromine cure sites often give demolding problems, while the fast cures with iodine cure sites can give clean demolding. Mold release agents may be incorporated into compounds. These agents are incompatible with the fluoroelastomer at molding temperatures, so that they migrate quickly to the interfaces between stock and mold surface to facilitate release. When such internal mold release agents are

6 PROCESSING OF FLUOROELASTOMERS effective for a given compound, they are preferable to external mold release agents, which must be sprayed on mold surfaces periodically. Volatiles may be released from the cured stock when the mold is opened, so adequate local ventilation should be provided to protect operators. A concern with peroxide cures is the release of methyl bromide and/or iodide. The amounts of these materials can be minimized by keeping the ratio of radical trap (usually TAIC or TMAIC crosslinker) to peroxide high enough so that methyl radicals are intercepted by the trap, rather than by halide groups on polymer chains. Peroxide decomposition also results in significant amounts of low molecular weight organic compounds, such as acetone and isobutene, which will be evolved on demolding of the hot cured parts. In bisphenol cures, inorganic base levels should be set high enough to avoid significant hydrogen fluoride evolution. For good control of dimensions and surface characteristics of parts, molds should close tightly and cleanly at the flash line. Surfaces should be free of nicks and pits. Hard chrome plating of mold surfaces is recommended to minimize mold fouling.[6] However, chrome plating at sharp edges may show excessive wear. Molds made of nickel chrome alloy have hard wearing surfaces with good release characteristics.[10] Mold platens which hold mating mold plates should be free of distortion. The platens should be provided with heaters that allow good control of mold temperature. Compared to other elastomers, fluoroelastomers have higher thermal expansion coefficients and are cured at higher temperatures, so higher shrinkage is usually observed in cured fluoroelastomer parts. Shrinkage increases with higher molding temperatures, and decreases with higher levels of filler and metal oxides in compounds. A bisphenol-cured VDF/HFP dipolymer compound with 30 phr MT black shows 2.5%–3.2% shrinkage after molding at 177ºC–204ºC. An additional 0.5%–0.8% shrinkage occurs after post curing in an oven at 204ºC– 260ºC, as water and other volatiles are removed.[2] Shrinkage may be higher for fluoroelastomers with higher fluorine content. For close control over dimensions, shrinkage should be measured for a given compound and molding conditions, to allow proper design of mold cavities. Some fabricators may use molds designed for nitrile rubber to make fluoroelastomer parts. This may necessitate restrictions on

111 fluoroelastomer composition, filler level, and cure temperature to get finished parts within size tolerances.

6.4.2

Compression Molding

Compression molding, depicted in Fig. 6.12,[11] is the oldest and simplest way of making rubber parts, and is widely used for fluoroelastomers. In this process, a piece of uncured rubber is placed in the mold cavity. This is usually a preform with weight slightly greater than that of the finished part. The mold is then closed and held under hydraulic pressure at the desired temperature until the part is cured. Finally, the mold is opened for removal of the part and attached flash (excess rubber that is subsequently trimmed from the final part). Compression molding has several advantages for fabrication of fluoroelastomer parts. Loss of expensive material may be minimized by careful control of preform size to keep the amount of flash low. The process is advantageous for relatively small production volumes of parts of any size. Equipment costs of molds, presses, and auxiliaries are low. Compression molding works best with stocks of medium to high viscosity. Thus, fluoroelastomers with high molecular weight may be processed readily to give parts with excellent mechanical properties and environmental resistance.

Figure 6.12 Compression molding process.[11]

112 Among the disadvantages for compression molding is high labor cost, since considerable operator attention is needed for preparing and loading preforms, closing the mold, and removing cured parts. Quality of parts may be variable, largely because of variations in mold cycle time associated with manual operations. Temperature control may be compromised by variations in the length of time the mold is open, so rate and state of cure may vary considerably, affecting part dimensions and physical properties. Other molding processes may be better for high volume production of standard parts and for production of intricate parts with long flow channels in the mold. For small-scale molding, as in laboratory preparation of parts for evaluation and measurement of properties, compression molds are in the form of two plates that are removed from the press for loading and unloading. For most production operations, the mating mold plates are attached to recesses in the mold platens. In either case, mold temperature is set and controlled by the press heating system. Actual mold temperature may be significantly lower than the set press temperature, so periodic monitoring of mold temperature is desirable to avoid undercured parts. For production of high-quality compression molded parts,[6] preforms should be carefully prepared. Weight should be 6%–10% higher than that of the finished part, and preforms should be dense and free of trapped air. Proper size is necessary to assure complete filling of the mold cavity with minimal flash. Trapped air could lead to blisters in the final parts. Stock viscosity should be high enough at molding temperature to force air from the mold cavities, but not so high that backrinding occurs on demolding. Proper mold filling is facilitated by delayed bumping of the press to higher pressure after the stock has been heated to get good flow. Backrinding, rough edges on parts, is caused by expansion upon demolding, usually at the parting line of the mold cavity. Poor mold flow and backrinding may also occur if the stock is too high in viscosity or is too scorchy (curing prematurely before the mold cavity is filled). Blisters of various kinds may appear in molded parts for a number of different reasons:[6] undispersed particles, contamination by a different compound, trapped air, inadequately dispersed processing aid, entrained water (e.g., from condensate on cold stored

FLUOROELASTOMERS HANDBOOK stock), poor dispersion of curatives, or undercure. Many of these problems can be avoided by proper mixing and storage procedures for the compound, and assuring that equipment cleanup is adequate to avoid presence of small amounts of nonfluorinated rubber compounds. Parts undercured in the mold may exhibit sponging, splits, or fissures after oven post curing. Possible corrective measures include increasing accelerator level in the compound, increasing mold temperature, and/or molding time. Parts with thickness greater than 5 mm are more likely to form fissures on post curing. In addition to the corrective measures mentioned, it may be necessary to ramp up the post cure oven temperature gradually to allow escape of volatiles without blowing the parts. Multiple cavity molds should be designed to assure uniform pressure and temperature are maintained for all cavities. Loading fixtures[12] are useful when a large number of cavities must be loaded individually by a gloved operator while the mold is hot. Such fixtures must be light in weight and easy to operate. For parts such as shaft seals, metal inserts, as well as rubber preforms, may be loaded more readily with a properly designed fixture. Less complicated fixtures may be used for unloading parts from a mold. Ebnesajjad[13] describes compression mold designs in more detail.

6.4.3

Transfer Molding

The transfer molding process, shown in Fig. 6.13,[11] involves using a piston and cylinder device to force rubber through small holes into the mold cavity. A piece of uncured compound is put into a part of the mold called the����, and a plunger then pushes the stock into the closed mold through a sprue. The mold is kept closed while the rubber cures. The plunger is then raised, and the�������������� material is removed and discarded. The mold is opened for removal of the part; then the flash and sprue material is trimmed off and discarded. Compared to compression molding, transfer molding provides better product consistency, shorter cycle times, and better bonding of rubber to metal inserts.[11] However, considerable material is lost as scrap in the transfer pads, sprues, and flash. The stock must have relatively low viscosity and adequate scorch safety for adequate flow into the mold.[6] The rapid transfer of compound from the pot through

6 PROCESSING OF FLUOROELASTOMERS

113

small sprues to the mold cavities imposes high shear and considerable heat generation, so the stock is heated quickly to curing temperature. Sprue size should be kept as small as is practical, to minimize damage to parts on demolding and tearing from the molded parts. However, sprues must be large enough to allow adequate flow of the compound. Somewhat lower mold temperatures may be usable for transfer molding, to get cure times comparable to those for compression molding. The basic three-plate multiple cavity transfer mold is more complex and expensive than a compression mold, but is better suited to molding intricate parts or securing inserts.[14] Only a single piece of rubber is used to fill all mold cavities in a heat, so preparation of preforms is much simplified. Since the mold is closed during filling, flash is minimized through gates and vents. Several transfer molding process variants and mold designs are described by Ebnesajjad.[13]

6.4.4

Injection Molding

Ram or piston injection units are also used in the rubber industry.[6] These are somewhat similar to the transfer molding process. The rubber compound is fed to a heated cylinder, warmed to a predetermined temperature, and is then forced by a hydraulic ram through a nozzle, mold runners, and restrictive gates into the heated mold cavity. Ram injection units are lower in cost than reciprocating screw units, but are less efficient, especially for high-viscosity stocks. An alternative to the horizontal machine shown in Fig 6.14 is a vertical ram or screw type machine with a horizontal mold parting line. This may be more desirable for complex mold designs[6] requiring runner systems or metal inserts. Vertical machines also take up less floor space. Of all the molding processes, injection molding[11] provides the maximum product consistency, most control of flash, and shortest cycle times. However, injection molding is not suited for all compounds and molding applications, has the highest investment cost in molds and auxiliary equipment, and typically has considerable scrap in runners and sprues. The process is most suited to production of high volumes of standard parts. Injection molding machines have been highly developed for molding of thermoplastics, and

Injection molding is the most advanced method of molding rubber products.[11] In this process, all aspects of how the rubber gets into the mold and is cured are automated. The main steps in a typical rubber injection molding process are shown in Fig. 6.14[11] for a reciprocating screw machine. The compound is usually fed to the screw as a continuous strip, but sometimes is fed as pellets from a hopper as in plastics processing. The strip is worked and warmed by the screw in a temperature-controlled barrel. As the stock accumulates at the front of the screw, the screw is forced backward a specified amount in preparation for a shot. Screw rotation is stopped, and the screw is pushed forward to inject a controlled amount into the closed mold. While the rubber cures in the heated mold, the screw is initially held in the injection position to maintain a predetermined pressure to consolidate the stock. Then after a preset time, the screw rotates again to refill the barrel. The mold is opened for part removal, then is closed Figure 6.13 Transfer molding for the next shot. process.[11]

Figure 6.14 Injection molding process.[11]

114 are finding increasing use in molding of thermosetting elastomer compounds. Quite different temperature profiles are required for the two types of materials. For thermoplastics operation, pellets are fed to a screw that plasticizes and melts the material at high temperature. The low-viscosity melt is injected into a cold mold to crystallize and solidify the plastic part. For rubber processing, the stock is fed to the screw and warmed to a temperature high enough to reduce the stock viscosity without curing. The stock is injected into a hot mold to effect rapid curing of the parts. Careful design of relatively low-viscosity elastomer compounds for a balance of scorch safety and rapid cure is necessary, along with proper setting and control of stock temperatures in different parts of the equipment. Typical operating conditions are listed in Table 6.1 for injection molding of fluoroelastomers parts with thickness less than 5 mm.[6] These conditions are applicable to molding of low-to-medium viscosity compounds with fast-curing bisphenol systems or with peroxide curing of fluoroelastomers with iodine cure sites. Open time could be longer if parts must be removed manually or if metal inserts must be inserted prior to the next shot (e.g., for molding of shaft seals). Cure times would be longer for parts with thicker sections or for slower cure systems. Higher mold temperatures may be possible with some compounds to get faster cures. Injection molding machinery is described in considerable detail by Ebnesajjad in Volume 2 of this handbook series.[15] A typical injection molding machine, shown in Fig 6.15,[15] consists of these major components: plasticization/injection section, clamping unit, mold including the runner system, and control systems for temperatures and mechanical actions. The functions of the clamp unit are to open and close the mold halves and to hold the mold tightly closed during injection of the fluoroelastomer compound. Injection pressures are high (depending on stock viscosity) to obtain rapid filling of the mold in a few seconds. Thus the force needed to hold the mold closed is very great, with the melt pressure inside the mold exerted over the entire projected area of cavities and feed systems at the mold parting line. Required clamping pressure is a complicated function of injection pressure, projected area, and part thickness. A conservative rule of thumb for fluoro-

FLUOROELASTOMERS HANDBOOK plastics[15] is 0.79 tons per square centimeter of projected area; lower clamp pressures may be usable for fluoroelastomer compounds.[6] Clamp units must be robust to exert the required pressures, but also must open and close rapidly to minimize production time. Common types[15] are the direct hydraulic clamp (Fig. 6.16) and the toggle clamp (Fig.6.17). In either variation, the clamp unit features a fixed platen and a moving platen on which the two halves of the mold are attached. The fixed platen, with the injection half of the mold attached, is mounted rigidly on the machine base and is positioned adjacent to the nozzle of the injection unit. The moving platen carries the ejection half of the mold. The clamp also includes a tailstock platen that the pressure means reacts against to clamp the mold halves together. For this purpose, the fixed and tailstock platens are united by tiebars that also serve as guides for the moving platen. Some modern machines have been developed with other clamping arrangements with no tiebars. The injection unit consolidates the stock to form a fluoroelastomer melt with uniformly dispersed ingredients, and injects it into the mold under controlled conditions. Temperature in the feed system must be controlled well, high enough to get reasonable viscosity for rapid injection, but limited to avoid premature curing before filling the mold. Both screw and ram units are used, but the dominant form is the reciprocating screw injection unit shown in Fig. 6.18,[15] in which the screw is capable of both rotational and axial movement. For elastomers, stock is usually fed to the screw in strip form rather than as pellets from a hopper as shown. The screw should be designed for elastomer extrusion, as described in Sec. 6.3, with relatively high L/D. As indicated in Table 6.1, stock temperature in the extrusion section should be kept below about 120ºC to avoid scorch. The injection sequence was described at the beginning of this section (Sec.6.4.4) as involving four phases. In the melt preparation phase, the screw rotates and conveys the stock to the downstream end of the screw with the barrel nozzle closed by a valve or the presence of a previous molding. The accumulating stock forces the rotating screw back until sufficient melt is available for the next molding. Screw rotation then stops. In the mold filling phase, the barrel nozzle and the screw is pushed forward without rotating, to perform as a ram to inject the stock into the mold. The

6 PROCESSING OF FLUOROELASTOMERS �

115

Table 6.1 Fluoroelastomer Injection Molding Conditions[6] �

Machine Type

Ram

Screw

Feed zone

80–90

25–40

Middle zone

80–90

70–80

Front zone

80–90

80–100

90–100

100–110

165–170

165–170

205–220

205–220

165–170

165–170

14–115

14–115

Hold pressure

---

½ injection pressure

Back pressure

---

0.3–1

Maximum

Maximum

Total cycle

58–75

43–60

Clamp

48–65

33–50

Injection

3–5

3–5

Hold

---

10–15

Cure (includes hold)

45–60

30 – 45

Open – ejection of parts

10

10

Temperature, ºC Barrel

Nozzle Nozzle extrudate Mold Stock in mold Pressure, MPa Injection

Clamping pressure Time, seconds (for thin parts)

Figure 6.15 Typical injection molding machine.[15]

116

Figure 6.16 Typical direct hydraulic clamp unit.[15] A: Acuating plunger. B: Removable spacer. C: Mold. D: Injection nozzle. E: Fixed platen. F: Movable platen. G: Tiebar. H: Cylinder base plate. I: Clamping cylinder.

FLUOROELASTOMERS HANDBOOK

Figure 6.17 Typical toggle clamp unit.[15] A: Movable platen. B: Fixed platen. C: Mold. D: Front link. E: Rear link. F: Actuating cylinder. G:Tiebar, H: Crosshead link.

Figure 6.18 Typical reciprocating screw injection unit.[15]

high shear rates in the nozzle, sprue, runners, and gates heat the stock so that it reaches curing temperatures during the mold filling operation. In the holding phase, pressure is maintained on the filled mold. At the conclusion of the holding phase, while curing continues in the mold, the screw is again rotated to prepare melt for the next molding. In most elastomer injection molding operations, temperatures in the sprue and runners are high enough so that the stock cures and becomes scrap to be removed from the molded parts. For many of the small parts fabricated from fluoroelastomers, the fraction of such scrap is high and represents a sizeable cost. Cold runner systems have been devised to avoid such scrap losses.[8] In these systems, the sprue and runners are kept at temperatures high enough for plasticization and reasonable viscosity for

injection, but well below temperatures maintained in the mold for rapid curing. Compounds must be carefully designed for scorch times long enough to avoid significant curing during the hold times in the sprue and runners. Gates[15] are the entry points to the mold cavity from the runners. Size and position of gates control flow into the mold. Careful design is necessary to insure complete, symmetrical filling of mold cavities. The gate is usually small relative to the molding and upstream feed system for two reasons. One is that the gate serves as a thermal shutoff valve that cures quickly during the pressure hold phase and solidifies to prevent further flow. The second reason is that the small gate can be easily removed from the molded part without leaving much trace of its presence.

6 PROCESSING OF FLUOROELASTOMERS Most mold designs for injection molding are unique, depending on application, fluoroelastomer compound, and feed system (hot or cold runners). Some standard types can be distinguished, (e.g., twoplate, three-plate, or stack molds).[15] Mold designers must take into account a number of features, including venting, methods of ejecting parts from the mold, cleaning and sweeping of the mold surface between heats, heating methods, and shrinkage.[12] Power systems for injection molding machines must handle a wide range of mechanical movements with differing characteristics.[15] Mold opening is a low-force, high-speed movement, and mold closing is a high-force, low-speed movement. Extrusion involves high torque and low rotational speed, while injection requires high force and medium speed. The modern injection molding machine is a self-contained unit incorporating its own power source. Oil hydraulics have become established as the drive system for the majority of injection molding machines. In these systems, a reservoir of hydraulic oil is pumped by an electrically driven pump at high pressure, typically up to 14 MPa, to actuate cylinders and motors. High and low pressure linear movements are performed by hydraulic cylinders, and rotary movements are achieved by hydraulic motors. However, hybrid machines with the screw driven by electric motors and linear movements by hydraulic power are not uncommon. In recent years, all-electric machines using brushless servo motor technology to power the various movements have come into use. Capital cost is higher, but the electric machines have lower energy consumption, are inherently cleaner, and may have better precision and repeatability than hydraulic systems. Control systems for modern injection molding machines must be capable of handling the complex sequence of operations and necessary options.[15] The range of parameters and adjustments needed to control the process accurately and automatically is broad. Control is ultimately exercised by valves, regulators, and switches, but these are rarely under individual manual control. The norm is now electronic control with varying degrees of sophistication, ranging from partial control by programmable logic controllers up to fully centralized computer control. Injection molding machines are usually offered with choices of control options to suit a variety of end uses and budgets.

117 Troubleshooting injection molding problems[6] may be difficult, since a combination of factors may be involved. Each problem should be analyzed on an individual basis, considering the compound being used, preparation of the stock, the part being made, the injection molding machine and its operation, and the mold. Besides the general considerations noted in Sec. 6.4.1 on molding to avoid potential problems, the following problems are more specific to injection molding: • Air entrapment in the mold will prevent the mold from filling properly. Make sure the feed stock is free of air, provide sufficient back pressure at the nozzle to compress the stock in the barrel, increase injection time, lower injection pressure, and/or make sure the mold is sufficiently vented. • Distortion or rough surfaces of molded articles may result from scorched stock, too long an injection time, too hot a mold, or undersized runners and gates. • Excessive mold flash may be associated with too low stock viscosity, too high injection pressure, too long injection time, too large shot size, or a poor fitting mold. • Excessive nozzle flash may be caused by worn nozzle or nozzle bushing surfaces, too large a nozzle, too high injection pressure, or too low compound viscosity. • Long cure cycles may result from too low barrel or mold temperatures, or an inadequately formulated compound. • Poor knitting may be due to excessive mold release agent, too high mold temperature, too fast a cure rate, or inadequate stock flow.

6.5

Calendering

Uniform thin sheet of fluoroelastomer compounds (for end uses such as die cut gaskets, fabric lamination, and sheet stock) may be produced by calendering. In this operation, a stack of three or four rolls turn at the same surface speed to squeeze the elastomer stock through two or three nips to produce sheet of about 1 mm thickness per pass. A setup for making plied sheet on a cloth liner is shown

118

FLUOROELASTOMERS HANDBOOK

in Fig 6.19[2] The quality of calendered sheet depends largely on the viscosity of the fluoroelastomer compound at the calender.[6] The compound to be calendered should be uniform in dispersion, viscosity, temperature, and flow rate. The fluoroelastomer used should be high enough in molecular weight to give compounded stock with adequate green strength to form uniform bands on the rolls with no holes or tears. However, stock with too high viscosity may give difficulty in attaining consistent thickness across the width of the rolls. Within limits, stock temperature may be chosen to get viscosity in a reasonable range for good calendering characteristics. Use of internal process aids should be minimized, since high levels may lead to slipping or bagging of the stock on the rolls. Suggested roll temperatures are listed in Table 6.2 for fluoroelastomer compounds with different cure systems.[6] Mixed compound must be warmed on a mill with minimum shear to a temperature close to that of the top roll for strip feeding to the calender. The compound should be fed continuously and evenly across the width of the rolls, maintaining only a small bank in the nip between the first two rolls. Maximum roll speed should be 7–10 meters per minute; sheet thickness should be no more than 1.3 mm per pass.[2] Thicker sheet can be made by plying additional material to previously calendered sheet in successive passes, as shown in Fig. 6.19. The first pass is run to get about 1 mm thickness on a high-count cotton liner. In successive passes at lower roll speed, additional 1-mm plies are put on, with the sheet on a liner fed to the lower nip. The roll temperatures noted in Fig. 6.19 are somewhat higher than those suggested in Table 6.2; the higher temperatures are suited to a bisphenol-cured bimodal VDF/HFP dipolymer (Viton® E-60C) with higher green strength than most fluoroelastomers currently offered.

Figure 6.19 Calender operation for plied sheet.[2]

After calendering, the wrapped sheet stock should be allowed to stress relax in the liner for about 24 hours. It may then be rewrapped in the liner required to impart the desired surface texture to the cured sheet.[2] Curing is usually carried out in an autoclave with hot air or steam at temperatures near 170ºC; cure time should be long enough to assure that all the stock reaches curing temperature for an adequate time.[6] When steam is used, pressure should be raised and lowered slowly to prevent blistering. The stock should be wrapped with an outer impermeable layer (e.g., with a film such as PTFE or FEP fluoroplastic) to prevent direct contact with the steam. The liner should be stripped from the stock as soon as possible after curing. Post curing of the sheet is best done by festooning in a forced air oven. For sheets thicker than 6 mm, post cure oven temperature should be increased in steps to the final temperature to prevent blistering.

Table 6.2 Suggested Three-Roll Calender Temperatures[6]

Cure System

Top Roll (°C)

Middle Roll (°C)

Bottom Roll

Diamine (Diak #3)

45–50

45–50

Cool, ambient

Bisphenol

60–75

50–65

Cool, ambient

Peroxide

60–75

55–70

Cool, ambient

6 PROCESSING OF FLUOROELASTOMERS

6.6

Other Processing Methods

Relatively small volumes of fluoroelastomers are processed by other methods for specialty applications. Of these, latex and thermoplastic elastomers are discussed below.

6.6.1

Latex

Fluoroelastomer latex can be used for rubbercoated fabrics, protective gloves, and chemical or heat-resistant coatings. Most fluoroelastomer producers offer latex in limited quantities to processors skilled in latex applications. Typical latex products are based on VDF/HFP/TFE terpolymers (about 68% fluorine) which are readily polymerized to relatively stable dispersions containing 20%–30% solids. These dispersions are further stabilized by pH adjustment and addition of anionic or nonionic hydrocarbon soaps. A water-soluble gum (e.g., sodium alginate) is then added to increase particle size, allowing creaming (actually settling) to concentrated latex (about 70% solids); supernatant serum is discarded. The combination of added soap and gum prevents further particle agglomeration and stabilizes the latex to allow a storage life of several months. Biocides are usually added to prevent unwanted growth of microorganisms. Latex must be protected from freezing or excessively high temperatures during storage and shipping. Formulations used by processors for particular applications are proprietary. Compounding ingredients must be chosen carefully to avoid destabilizing the latex prematurely. Usually, a diamine (Diak #3) or polyamine curative is used with limited amounts of metal oxide and inert filler. Vulcanizate properties obtained from test compounds of Tecnoflon® TN Latex are shown in Table 6.3.[16] Tecnoflon TN is a VDF/HFP/TFE terpolymer (68% F); the latex is about 70% solids. In the compounding examples, a polyamine curative, triethylenetetraamine (TETA), is used with zinc oxide and, optionally, an inert mineral filler, Nyad 400 calcium metasilicate. Curing conditions are mild, chosen because curing is often necessarily carried out at a low temperature to protect substrates on which the compound may be deposited.

119 6.6.2

Thermoplastic Elastomers

Thermoplastic fluoroelastomers are offered commercially by Daikin. These products may be processed by conventional thermoplastics methods without curing. This allows flash from molding and other scrap to be recovered and reused. The materials are A-B-A block copolymers made by the Daikin “living radical” semibatch emulsion process using fluorocarbon diiodide transfer[17] as described in Ch. 4, Sec. 4.6.3. The center elastomeric B block soft segments are made in a first polymerization step. After removal of monomers and recharging a different monomer composition, the plastic A block hard segments are polymerized on the ends of the B blocks. The main commercial product is Dai-el® Thermoplastic T-530. This is described[18] as containing 85% soft segment of composition VDF/HFP/ TFE = 50/30/20 mole % or 33/46/21 wt % (70.5% fluorine) and 15% hard segments of composition TFE/E/HFP = 49/43/8 mole % or 67/17/16 wt %. The basic patent requires that hard segments have molecular weight of at least 10,000 Daltons, corresponding to a degree of polymerization (DP) of at least 140 units, sufficient for crystallization with melting point about 220ºC. Central soft blocks would then have molecular weight at least 110,000 Daltons, with DP = 110 units or more. The high fluorine content of the soft blocks gives the product excellent fluid resistance and a glass transition temperature of about -8ºC. The thermoplastic can be extruded and formed at temperatures above the melting range; after cooling, crystallization of the hard segments gives parts with good dimensional stability at temperatures up to about 120ºC. Typical applications include tubing, sheet, o-rings, and molded parts. Characteristics of T-530 are listed in Table 6.4.[19] To obtain better properties at high temperatures, the thermoplastic fluoroelastomer can be compounded with bisphenol or peroxide systems, molded, and cured at high temperature. Dai-el T-530 can also be compounded at about 90ºC, extruded or molded under high shear at temperatures below the crystalline melting point (110ºC–140ºC), then cured at higher a temperature (about 180ºC).[18] Part distortion would be difficult to avoid in such a process, however.

120

FLUOROELASTOMERS HANDBOOK

A base-resistant thermoplastic fluoroelastomer has been developed by DuPont[20] using similar polymerization techniques. In this material, soft segments are of composition E/TFE/PMVE about 19/45/36 mole % with glass transition temperature –15°C, and soft segments have composition E/TFE about 50/50 mole % with DSC melting endotherm maximum about 250ºC. The thermoplastic fluoroelastomer is readily molded at 270°C to give good physical properties and excellent resistance to

fluids including polar solvents, strong inorganic base, and amines. This composition can be readily crosslinked with ionizing radiation after molding to obtain better properties, with no compounding required. Physical properties of the base-resistant thermoplastic fluoroelastomer are listed in Table 6.5; enhanced fluid resistance is shown in comparison with Dai-el T-530. However, this material has not been offered commercially.

Table 6.3 Typical Properties of Latex Compound[16]

Compound, phr Latex (100 phr rubber) Zinc oxide TETA Nyad 400

Filled

Gum

145

145

10

10

2.5

1.5

20

--

Sodium lauryl sulfate

1

1

Cr2O3

5

5

M100, MPa

2.0

0.8

TB, MPa

4.5

2.9

EB, %

300

800

M100, MPa

2.3

1.0

TB, MPa

5.1

5.2

EB, %

250

650

M100, MPa

5.3

2.3

TB, MPa

6.1

6.2

EB, %

180

450

Physical Properties Press cure (1 h, 90ºC)

Press cure (2 h, 90ºC)

Post cure (1 h, 50ºC)

6 PROCESSING OF FLUOROELASTOMERS �

121

Table 6.4 Characteristics of Dai-el® T-530 Thermoplastic[19] �

Property

Value

3

Density, g/cm

1.89

Hardness, JIS A

67

Melting point (approximate), ºC

220

Pyrolysis initiation temperature, ºC

380

Thermal conductivity, cal/cm·sec·ºC

3.6 × 10-4

Specific heat, cal/g·ºC

0.3

Low-temperature torsion test, Gehman T50, ºC

-9

Tensile strength, MPa

11

Elongation at break, %

650

Tear strength, kN/m

27

Rebound resilience, %

10

Compression set, 24 h at 50ºC, %

11

Electrical properties 5 × 1013

Volume resistivity, ohm-cm Dielectric breakdown strength, kV/mm

14

Dielectric constant, 23ºC, 1 kHz

6.6

Table 6.5 Properties of Base-Resistant Thermoplastic Fluoroelastomer[20]

Base-Resistant TPE

Dai-el® T-530

M100, MPa

3.4

--

TB, MPa

14.5

--

EB, %

510

--

M100, MPa

5.3

--

TB, MPa

16.9

--

EB, %

270

--

Compression set, % (pellets, 70 h/150ºC)

37

--

Polymer Compression molded

Irradiated, 15 MRad

Chemical Resistance, % wt gain after 3 days/25ºC Acetone

3.6

87.1

Methanol

0.0

0.8

Dimethyl formamide

0.5

48.2

Toluene

1.1

2.0

100.0

48.4

Trichlorotrifluoroethane Butylamine

1.9

Decomposed

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FLUOROELASTOMERS HANDBOOK

REFERENCES 1. �Farrel Mills and Calenders, www.farrel.com (2003) 2. �R. H. Burd, Processing Viton® E-60C Type Fluoroelastomers, DuPont Data Sheet V-J-3-401 (1975) 3. �Tyre School, www.tut.fi/plastics/tyreschool/moduulit (2003) 4. �Farrel F-Series Banbury Mixer, www.farrel.com (2003) 5. �R. Bond, The Component Manufacturer – The Roles of the Raw Material Supplier and the Machinery Manufacturer, paper given at ACS Rubber Division meeting, Detroit, Michigan, October 17-20 (1989) 6. �Processing Guide, Viton® Fluoroelastomer Technical Information bulletin VTE-H90171-00-A0703, DuPont Dow Elastomers (2003) 7. �S. Ebnesajjad, ��������������� Vol. 2: ����� ������������ ���������������� ���� ��������� ������� Chapter 8: Extrusion, William Andrew Inc., Norwich, NY, (2003) 8. �D. Kemper and J. Haney, An Overview of Modern Extrusion Technology, paper given at ACS Rubber Division meeting, Cleveland, OH, October 1-4, (1985) 9. �Barwell Preformers, www.barwell.com (2003) 10. �Prevention of Mold Staining and Sticking, DuPont Viton® Fluoroelastomer Data Sheet V-J-1-403, 1975. 11. �Molding Solutions, www.molders.com (2003) 12. �D. N. Raies, Important Factors in the Design of Molds for Compression, Transfer, and Injection Molding of Rubber, paper given at ACS Rubber Division meeting, Cleveland, OH, October 1-4 (1985) 13. �S. Ebnesajjad, ��������������� Vol. 2: ����� ������������ ���������������� ���� ��������� ������� Chapter 10: Other Molding Techniques, William Andrew Inc., Norwich, NY (2003) 14. �Rubber Molding, www.hawthornerubber.com (2003) 15. �S. Ebnesajjad, ��������������� Vol. 2: ����� ������������ ���������������� ���� ��������� ������� Chapter 7: Injection Molding, William Andrew Inc., Norwich, NY (2003) 16. �Tecnoflon TN Latex, Solvay Solexis Product Data Sheet, www.solvaysolexis.com (2003) 17. �M. Tatemoto, T. Suzuki, M. Tomoda, Y. Furukawa, and Y. Ueta, U.S. Patent 4,243,770, assigned to Daikin Kogyo Co. (January 6, 1981) 18. �M. Tatemoto, U.S. Patent 5,198,502, assigned to Daikin Kogyo Co., (March 30, 1993) 19. �Daikin Technical Information, Dai-el® T-530, www.daikin-america.com (2003) 20. �D. P. Carlson, U.S. Patent 5,284,920, assigned to DuPont Co. (February 8, 1994)