e-dokumen.com

Viton FluoroelastomersPrecautions Related to Compounds and Vulcanizates Ensure that no scorched or cold material remains ininjection systems that could increase back pressure.Exothermic Decompositions Ensure that machine location and guards minimize therisk of damage or injury, should an incident occur.Three potential causes of sudden exothermicdecomposition of the FKM class of elastomers, whichincludes Viton , have been identified. Occurrence is rare,but conditions giving rise to the possibility should beunderstood. Avoid contamination of processing equipment withother materials or cleansing compounds, especially anythat might leave residual particles of aluminum or othermetals (see subsection following).Diesel EffectFinely Divided Metals and Metal OxidesThis is manifested by a sudden and vigorousdecomposition during processing, causing a rapid andnoisy emission of charred material from the equipment.It is not related to any specific grade of Viton or formof compound and has been known to occur with othertypes of elastomers. Individual production incidents havebeen investigated and the phenomenon studied underlaboratory conditions, as in Appendix 1.Finely divided metals should not be used as compoundingingredients for Viton fluoroelastomers or allowedto contaminate compounds based on them. Stockscontaining such powders, especially aluminum andmagnesium, have been known to undergo vigorousexothermic decomposition at moderately elevatedtemperatures. Compounds containing bronze powderhave been exposed to temperatures as high as 275 C(527 F) without incident, but their incorporation is notrecommended.All evidence suggests that this type of thermaldecomposition arises from heat generated by adiabaticcompression of air pockets in the compound during rapidcompression. Thus, the phenomenon is usually observedwith injection molding; although, examples from transferor compression molding and extrusion have occurred.Predictably, high compound viscosity is a contributoryfactor.Some metal oxides dispersed at high concentrations influoroelastomers, and other elastomers, such as naturalrubber, SBR, polychloroprene, and ethylenepropylenecopolymers, may undergo exothermic decompositionin the region of 200 C (392 F). For example, amasterbatch of lead oxide (litharge)/Viton 80/20decomposed exothermically when heated above 200 C(392 F), yielding metallic lead as one of the by-products.This type of decomposition has also been observed incompounds of Viton containing excessively high levels( 15 phr) of metal oxide, such as magnesium oxide.Study of Appendix 1 shows exothermic reaction ofuncured compounds of Viton beginning at about316 C (601 F), as determined by differentialthermal analysis (DTA). Thermogravimetric analysis(TGA) indicates that 50% weight loss occurs at about475–500 C (887–932 F), and that Viton is completelyvolatilized at 550 C (1022 F). Any operation thatsubjects a compound of Viton (or any other FKM-typefluoroelastomer) to temperatures above 316 C (601 F)can initiate rapid decomposition. Calculations show thata pocket of air can easily be heated to 500 C (932 F) ifcompression is rapid enough.Accelerated rate calorimetry (ARC) tests were runon various combinations of magnesium oxide andfluoroelastomer that contained a bisphenol cure system.Four mixtures ranging from 9 to 40 phr magnesium oxidewere tested in a calorimetric unit pressurized to 2.1 MPawith pure oxygen.To minimize the possibility of exothermic decompositionof compounds of Viton from the diesel effect, we suggest:The ARC plots of “Self Heating Rate” versus Temperatureshow that, under conditions of very high oxygenconcentration, once a reaction begins, it becomes a“runaway” reaction with a relatively large release of energy.At these very high oxygen concentrations (designedto ensure complete combustion in the calorimeter),the results can be used to establish the total oxidationpotential of the system. The kinetics of the reaction thatwould take place in conventional rubber equipment arenot well understood. Use the lowest viscosity compound consistent withoverall processing and end-use requirements. Incorporate an effective process aid, such asVPA Nos. 1, 2, or 3 or carnauba wax, in the compound. Avoid unnecessarily fast injection, transfer, or moldclosing speeds. Ensure absence of unnecessarily tight constrictions inthe flow path, e.g., dies, sprues, runners, and gates.ARC measurements indicate that as the level ofmagnesium oxide increases in a fluoroelastomer4

Viton Fluoroelastomerscompound containing curing agents, the temperatureat which exothermic reaction takes place decreases. Atlevels of magnesium oxide above 25 phr, the decreasein activation temperature is even more pronounced. Thetemperature predicted by the lower concentrations ofmagnesium oxide for the onset reaction at 40 phr ofmagnesium oxide is 140 C (284 F), but the actualmeasured onset was only 106 C (223 F).Studies under laboratory conditions indicated that: Commercial compounds containing MT carbon blackor relatively high levels of iron oxide (a potentialdehydrofluorinating agent) were thermally stable for atleast 24 hr at 320 C (608 F) by differential scanningcalorimetry (DSC) and even to 350 C (662 F) whenheated in an adequate air stream. When tested by accelerating rate calorimetry (ARC) in asealed system without air flow, initiation temperaturescould be as low as 200 C (392 F) for compoundscontaining iron oxide and MT black and in the region of110–140 C (230–284 F) for compounds containingiron oxide, MT, and ground anthracite fillers. Regardlessof filler system, times in excess of 24 hr were requiredfor the maximum rate of heat generation to be reached.Even in the absence of curing agents, exothermicdecomposition reactions have been observed in instancesin which very high levels of magnesium oxide were mixedinto fluoroelastomer gum polymer. One known examplewas a case in which an exothermic reaction occurred in aproduction-scale internal mixer, in which a manufacturerattempted to incorporate 80 phr magnesium oxide intoViton A-100. Surface contamination with silica-containing materialsmight exert a catalytic effect.Reaction Onset Temperature Based on this evidence and broad experience, webelieve that oven fires during post-cure can be avoidedto occur if recommended practice is followed, i.e.:45MgO (pph)40140 C35301510502.00– Ensure a rate of air flow to provide at least 10complete changes per hour positively vented toatmosphere, preferably through a gas scrubber unit.170 C2520106 C190 CNote: See Appendices 2 and 3 for a discussion ofvapor emissions from curing and post-curing.206 C2.102.202.302.402.502.602.70– Keep loading to a maximum 10% of the chambervolume.2.801000/T ( K)– Load so that air can freely circulate around the partsbeing post-cured.Additional information is given in the paper, “TheExothermic Decomposition of Litharge-ElastomerDispersions,” by M.B.M. Simpson, presented at the RubberDivision of the American Chemical Society, Atlanta,Georgia, March 26–29, 1979.– Ensure that post-cure ovens are fitted with hightemperature cut-outs, checked between each curecycle and set to a maximum 25 C (77 F) above thedesired temperature.More detailed information may be found in the paper, “AnInvestigation of Fluoroelastomer Post-Cure Oven Fires,”by R.E. Tarney and E.W. Thomas, ACS Rubber DivisionMeeting, Fall 1991, Detroit.Oven Fires During Post-CurePress cured vulcanizates based upon Viton fluoroelastomers often require a subsequent air ovenpost-cure, typically for up to 24 hr at 200–250 C(392–482 F), to develop optimum properties. Thisoperation is generally uneventful. Occasionally, fireshave occurred during post-cure that appear to involveexothermic decomposition of the exposed vulcanizates.This would not be expected from the thresholdtemperature of approximately 316 C (601 F) forexothermic reactions in uncured compounds based uponViton , as shown in Appendix 1.Vapors From High Temperature Curing and Post-CuringEvolution and Area VentilationAs with all elastomers, vapors are evolved fromcompounds based upon Viton during cure or post-cureat suggested temperatures. Area and local ventilationin work areas should be controlled. Ventilation shouldbe adequate to prevent exposure of personnel to suchvapors at levels greater than are permitted by national5

Viton Fluoroelastomersand local regulations. Anyone accidentally inhalingsignificant vulcanization vapors should be immediatelymoved to fresh air, given artificial respiration if indicated,and examined promptly by qualified medical personnel.Please review local legislation for the occupationalexposure levels in your country. Data in Appendix 2indicates that the level of emission is likely low, with themajority released during post-cure.It is very difficult to provide direct correlations of workplace concentrations of any vulcanization by-productgas to the possible evolution figures given in this bulletin,as many factors are involved (size and surface areaof parts, processing temperatures, cycle times, sizeof room, efficiency of ventilation, etc). The processormust ensure that, under his/her unique conditions,atmospheric concentrations at a minimum comply withthe requirements of local regulatory agencies.Peroxide-curable fluoroelastomers typically containeither bromo- or iodo-substituted cure site monomersand/or iodine modification of polymer chain ends. Thisbeing the case, methyl halides may be liberated duringcure. These include methyl bromide and/or methyl iodide.The SDS for the specific product to be used should beconsulted. Both methyl bromide and methyl iodide areregulated as potentially hazardous air contaminants in theEuropean Union, the U.S., and most industrialized nations.Information on methyl halide evolution from various typesof peroxide-curable Viton may be found in Appendices 3and 4.Evolution of Hydrogen FluorideRegardless of the Viton type or the nature of thevulcanization system, cure or post-cure near or above200 C (392 F) may liberate small quantities ofhydrogen fluoride, which is toxic and can cause prolongedirritation to the respiratory tract. Hydrogen fluoride isregulated as an air contaminant in the United Statesunder OSHA CFR Title 29 1910.1000, which sets the8-hr time weighted average (TWA) exposure limit in any8-hr work shift of a 40-hr work week at 3 ppm.Some peroxide-cured fluoroelastomer compoundscan generate other potentially hazardous substances,such as formaldehyde, acetone, and carbon monoxide.Workplace exposure limits for these three peroxidecure reaction by-products have been established bythe American Conference of Governmental IndustrialHygienists (ACGIH) and government agencies, such asthe Occupational Safety and Health Agency (OSHA).Exposure to these peroxide-cure reaction by-productscan be managed with local ventilation, personal protectiveequipment, and administrative control. See the Chemourstechnical bulletin, “Proper Use of Local ExhaustVentilation During Processing of Plastics,” available onthe Chemours website.Values for the evolution of hydrogen fluoride duringpress and post-curing under laboratory conditionsare given in Table 3 of Appendix 2 for bisphenol andamine-cured compounds and in Table 3 of Appendix 3for peroxide-cured compositions. Note that the highestvalues obtained for any of the cure systems was 7 mg/kgcompound for simulated press cure and 700 mg/kgcompound for simulated post-cure.VulcanizatesDust From GrindingSpecific Cure System DerivativesAs indicated in Table 3, fine dust from the grinding orabrading of raw Viton polymers or products made fromthem can generate toxic decomposition products ifburned.Reference to Table 5, Appendix 2, shows that significantamounts of bisphenol AF, the active ingredient in Viton Curative No. 30, and triphenylphosphine oxide may beevolved from conventional bisphenol cure systems.As noted in Table 1, bisphenol AF is a skin irritant andrated as Xi (by European Directive 67/548/EEC asamended). It may sublime and crystallize in ventilationducting. Triphenylphosphine oxide with an ALD value(oral rat) of 1,500 mg/kg is rated T (toxic). Effectiveventilation is necessary to remove it. Compoundscontaining Viton Curative No. 20 will also generate smallamounts of benzene during vulcanization. The EuropeanUnion classifies benzene as a Category 1 carcinogen,i.e., “a substance known to be carcinogenic to man.”Occupational exposure is regulated via Member Statelegislation.Smoking in areas in which these conditions occur shouldbe prohibited, and operators should be cautioned not tocontaminate smoking materials with the dust. They shouldbe advised to wash their hands thoroughly before smokinganywhere.To avoid inhalation of dust particles, a respirator should beworn and adequate ventilation provided. Safety gogglesshould be worn to prevent dust from coming in contactwith the eyes.6

Viton FluoroelastomersTable 3. Combustion Products of Vulcanization of Viton Compound FormulationViton E-60CaViton B-910b100100Magnesium Oxide33Calcium Hydroxide633030PolymerMT Carbon Black (N990)Cure: 10 min at 177 C (351 F), press 24 hr at 232 C (450 F), ovenExcess O2Deficiency O2Excess O2Deficiency O2Carbon Monoxide (CO)—121—96Carbon Dioxide (CO2)1,8361,5141,9272,000Hydrogen Fluoride (HF)25116125088Carbonyl Fluoride (COF2)—63—23Fluoroform (CHF3 )—15Vinylidene Fluoride (H2CCF2)Hexafluoropropylene (C3F6)Combustion Products, mg/g ce—TraceMiscellaneous Low Molecular WeightFluoropolymer FragmentsA-type polymer containing O-ring levels of bisphenol-based curatives.aB-type polymer containing O-ring levels of bisphenol-based curatives.bcTrace 5 mg/g sampleAccidental BurningAll residues should be neutralized by treatment withcopious amounts of lime (calcium hydroxide solution).Solid treated residues should be disposed of in thesame manner as Viton polymers. Used gloves must bediscarded.As noted in the previous section, intentional burningof Viton or its vulcanizates is not advised, except inapproved incinerators, owing to possible formation oftoxic and corrosive combustion products.In the event of an accidental fire, fire fighters and cleanuppersonnel should be aware of the likely formation ofhazardous by-products. Data in Table 3 show that thesemay include hydrogen fluoride, carbonyl fluoride, carbonmonoxide, and low molecular weight fluorocarbonfragments. Personnel fighting such a fire must wearfacemasks and self-contained breathing apparatus. Allunprotected personnel must leave the area immediately.Anyone exposed to fumes from the fire should be movedto fresh air at once and given qualified medical attention.Table 3 shows quantitative laboratory measurementsof the combustion products of bisphenol-cured Viton E-60C and Viton B-910 under well-ventilated conditions(excess of oxygen) and poorly ventilated conditions(deficiency of oxygen). Because these data are from aone-time test under controlled conditions, they are notnecessarily representative of an actual fire situation. Theyprovide information on likely by-products of combustion oftypical vulcanizates.CleanupHigh Temperature Service (Above 275 C [527 F])Personnel handling residues of Viton polymers,compounds, or parts made from them that have beeninvolved in a fire must wear impervious acid-resistantgloves to protect the skin from these possibly highlycorrosive residues. Gloves may be of neoprene or polyvinylchloride (PVC) if temperatures are below the melting pointof the glove. Any liquid residues should be assumed to behighly acidic.In most applications, parts made from Viton can beexposed to temperatures up to 275 C (527 F) withoutsignificant decomposition or health hazard. As shown inTable 3 of Appendix 2, traces of evolved hydrogen fluoridecan be detected from compounds during vulcanization attemperatures close to 200 C (392 F), and vulcanizatesmust be expected to behave similarly. Even so, evolutionCombustion Products of Vulcanization7

Viton Fluoroelastomersof significant levels of hydrogen fluoride from vulcanizatesis unlikely, unless the vulcanizates are severely degraded.Based upon results from laboratory studies of factorsconducive to exothermic decomposition, the followingpractices should be observed:Laboratory tests supporting this were conducted in airusing very small quantities of test specimens preparedfrom a standard vulcanizate of Viton E-60C. The resultsmay not be valid if other compounding ingredients areincluded in the vulcanizate formulation or during exposureto other environments, as the effect of additionalingredients on decomposition cannot be predicted.1. Amines or compounds that could serve as potentialsources of amines should be avoided in formulations ofViton unless checked carefully. Viton curing agentsbased on amine carbamates (VC-1, VC-3, and VC-4)should be used only at the recommended levels.Concentrations of VC-1 exceeding 2 phr and VC-3above 4 phr may cause substantial heat buildup duringcompression molding. Such high levels are rarely, if ever,necessary.Use or testing of vulcanizates of Viton at temperaturesabove 316 C (601 F) in air or in other media should beavoided. This temperature represents the approximatelikely onset of exothermic decomposition for specificcompositions, as shown in Appendix 1, Figure 1B.2. Formulations of Viton containing N,N’dinitrosopentamethylene-tetramine-type blowingagents should not be used for the manufacture ofsponge exceeding 12.7 mm thickness. Temperaturesduring molding of such sponge compounds shouldnot exceed 153 C (307 F). Chemours experienceindicates azodicarbonamide can be used safely asa blowing agent for closed cell sponge of Viton atleast up to 25.4 mm thickness and temperatures to204 C (399 F). Additional compounding data onsponge of Viton may be obtained from your Chemoursrepresentative.If, for any reason, controlled exposure to temperaturesabove 316 C (601 F) is required, or is possiblethrough accident, the user should ensure that effectiveprecautions are taken to protect personnel from exposureto fumes or condensates. Appropriate medical treatmentshould be available for hydrogen fluoride burns or otherexposure from inhalation or contact with a decomposedpart or condensate. As previously noted, all equipment,etc., suspected of contamination with decompositionby-products should be liberally treated with lime water.Neoprene or PVC gloves should be worn at all timeswhen dismantling equipment or handling parts and thendiscarded.3. In extrusion, transfer, and injection molding, very stiffstocks of Viton should be avoided. Pre-warming willhelp prevent excessive frictional heat. For high viscositycompounds of Viton A-HV, minimum barrel and screwtemperatures during extrusion are likely to be above100 C (212 F); while for lower viscosity types, atemperature of 80 C (176 F) should suffice.It is the responsibility of the part manufacturer andend-user to determine that a specific application is safe,particularly at temperatures above 316 C (601 F).Note: These comments only apply to parts made fromFKM-type fluoroelastomers. They do not apply to Kalrez perfluoroelastomer or Teflon fluoropolymer resin parts,which behave differently at high temperatures.Note: Owing to the diversity of extruder, screw, and diedesigns, these temperatures are only a guide.4. Metal oxides, such as magnesium oxide (MgO)and calcium hydroxide, should not be used atconcentrations greater than 15 phr in fluoroelastomerpolymers and especially not in fluoroelastomercompounds containing curatives/cross-linking agents.Appendix 1Occasional Exothermic Decompositions DuringProcessing of Viton FluoroelastomerAbstractExperimental StudiesOccasional exothermic decompositions have beenobserved in the course of processing fluoroelastomercompounds. These exothermic events

Viton during injection, compression, or transfer molding or extrusion (see Appendix 1). Suggested maximum use limits are 2 phr for VC-1 and 4 phr for VC-3. Note: Additional information on Viton curing agents is contained in Chemours bulletin, "Viton Curing Agents for Viton Fluoroelastomers." Viton Curative No. 4 (VC-4) is