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CHAPT E R 4 MAT E RIALS 4.1 I ntrod uction Selection of a polymeri c material possessing the opti mum fi re safety characteris· tics fo r a particular appl icati on is a complex and difficult task because so many factors m ust be considered. General fi re-consciousness i n system des i gn, the use of structu ral materi als with improved fi re safety characteristics, fi re and explosio n pote ntial detection equ i pment, and fi refighti ng procedu res a n d equ i pment a re equal l y i m portant aspects of cop i ng wi th fire hazards and must be viewed togethe r and analyzed i n a systems approach to t h e problem. I n additi on, i n evaluating pol y meric materia l s with improved fire safety characteristics, competing fi re safe ty requ i rements must be assessed and trade·offs made (e .g., a decrease in ease of ignition or fl ame s pread that a lso leads to an i ncrease in the production of smoke or toxic com busti on products might not be tolerable i n situations where egress is l i mited ) . 4.1 .1 F ire Safety Characteristics of Polymeric Materials No organic pol ymeric material can withstand intense and prol onged heat with· out degradation, even in the absence of oxygen. Given sufficient oxygen and energy i nput, all organic materials wi l l bu rn. ( Meta ls also w i l l exh i bi t some undesi rable cha racteristics under these cond itions. ) There are, however, several methods for reducing the fi re hazard of pol ymeric materials incl u d i n g : 1 . Developing a n d u s i n g po l ymers with fi re safety ch aracte ristics that are inh e r· ently better than those of the wel l-known materials. (Some mate rials of th i s type a re ava i l able com mercially now, but most are very expensive. ) 2. I m prov i ng th e fi re safety characteristics of ava i l able, low-cost materials with fire retardants ( i.e., by coating the surface of the materials or by i ncorporat· ing a fire reta rdant i nto the bu l k material at some appropriate processi n g stage ). 3. Com bi ning two or more materials i n a way that uti l i zes the best properties of each (e.g., pl aci ng stee l pl ates on each side of a pl ywood slab). Pol ymers may be fire retarded by i ntroducing fi l l ers such as a l u m i na trihydrate or active compounds into the bu l k material. The former lowers the fi re load eith e r b y absorbing heat o r by d i l uting the fu el. The l atter, usual l y halogen , phosphoru s , nitroge n, antimony o r boron compounds, may be used i n synergistic combination . One m ay also d isti ngu ish between reactive and non-reactive retardants accordi n g to whether or not they form cova lent bonds with the pol ymer. 62
MAT E R I A LS The bu rn i ng of a pol ymeric so l i d is essenti a l l y a th ree-stage process consisting of a heati ng phase, a therma l pyrolytic phase, and an i gnition phase. The behavior of a pol ymer duri ng th e i n itial heating phase depends considerably on its com position. ° ° Thermoplastic com positions gene ra l l y wi l l mel t between 1 00 C and 250 C. The l oss of rigidity that occu rs at the softe n i n g poi nt of such materials and th e su bse q uent decrease in me lt viscos i ty as the temperature increases in many cases al low these l iquids to recede from the ignition sou rce at a sufficiently rapid rate to prevent thei r su bsequent pyrolysis and ignition. This phenomenon apparently h as l ed to some erroneous conclusions based o n smal l-sca le tests concern ing the flam · mabi l ity of such compos iti ons in a rea l fi re situation. Therm osets and most natu ral polymers, such as wood and ce l l u l ose, remain essenti a l l y u nch anged di mensiona l l y d u r i n g th is early heating phase . At some l ate r stage i n the heati ng phase, thermal decomposi tion occu rs with the evol ution of gaseous products whose flammabi l ity wi l l depend u pon th e chem ical com position of the origi nal material. The temperatu re and rate at wh ich th is stage occurs depends u pon the thermal stabil ity of the material and the chem ica l de composition reactions occurring u nder the existing fi re conditions. The fl am· mabil ity of a sol id is determi ned largely by its behavior at this stage in the bu rning p rocess. The establ ishment of a sel f-susta i n i n g flame is predom inantly dependent upon the generation of sufficient fuel gases from thermal py rolysis to produce a flammable oxygen-fuel m i xture close enough to the sol i d fuel so th at sufficient heat can be transferred from the fl ame to the sol i d su rface by rad iation or convection to sustai n pyrolysis at an acceptable rate. Th is means th at the flame zone usua l l y is spati al l y removed by some smal l distance from the fuel su rface. A separation of flame and solid fu el is necessary in order to al low d i l ution of the pyrolytic fuel gases with sufficient oxygen to make the m i xture fl ammable. ° Pyrolysis genera l l y proceeds i n th ree cl ose ly rel ated stages. Between 1 00 C and ° 250 C, sufficient thermal energy is ava i l able on l y for such low energy reactions as the el i m i n ation of fu nctional grou ps, usua l l y from the end of the ch ain, and of ° s ma l l molecu les l i ke water and hydrogen hal i de . Between 250° C and 500 C, suffi· c ient e nergy becomes ava i l a ble to break the h ighest energy chem ical bond usual ly contained i n the structure of most pol ymers. These reactions often can lead to the "u nzipp i ng" of polymer chains and the d issemi nation of flammable monomer or sma l l chemical fragments. Both products can sustain gas-phase flame reactions; however, in some cases, these fragments w i l l recombine, which leads t o the formation o f aromatic condensed r i n g syste ms th at a re sta ble under the pyrol ytic cond itions. When this happens, a th i rd stage of the pyrolysis begi ns and the aromatic con densed structu res are conden sed fu rther at ° te m peratu res near 500 C with the eventual e l i minat i on of most elements other th an carbon. The res ult is a carbon ch ar that is highly insulating and difficu lt to i gn ite at n ormal oxygen concentrati ons. If the char can be mai nta i ned in a viscoel astic state during the i n termedi ate py rolysis stage, the gases evolved wi l l be trapped in the 63
M I N ES AN D B U N K E R S viscous liquid and the char wi l l expand into a carbon foam. The formation of th is special type of pyrolytic char is cal led intu mescence. Such char-form ing reactions are desi rable because they conve rt a flammable polymer to a less flammable ch ar wh ile s i m u ltaneously reducing the quantity of flammable gases. I f such a conversion can proceed because of the nature of the polymer structu re i n the absence of phosphorus, halogen or heavy metal addi tives, h ighly toxic by-p roduct gases are el i m i nated and the off-gases are no more tox ic than carbon diox i de or carbon monox i de. With increasi ngly higher temperatu res, the rate of production of the gaseous degradation products i ncreases unti l a m ixture with the oxygen of the ai r is reached that exceeds the fl amma bi l i ty l i m i t and i gnition occu rs. Conti nued burn ing at this stage is dependent u pon the transfer of sufficient heat from the flame to the condensed phase to mai nta i n an adequ ate supply of fl ammable gaseous decom posi tion products and, of cou rse, u pon the presence of a su pply of oxygen i n the surroundi ng atmosphere sufficient to su pport com bustion. The chem ica l reacti ons, genera l l y occurri ng i n the gas phase at flame te mperatu res, are free-radical in natu re. 4. 1 .2 Fire- R etardant Mechanisms There are fou r m ajor mechanisms for altering the flammabi l ity of common com mercial pol ymers : 1 . Alteration or reducti on of the heat of com busti on of the total pol ymer composition. 2. I n h i bi tion of the gas-phase com bustion reactions. 3. Alteration of the condensed-phase pyrolytic reactions to enhance the forma tion of ch ar. 4. Appl ication of an i ntu mescent coating (ex posu re of the coating to the ther mal fl ux of a fi re expands the coati n g i nto therm al l y sta ble intu mescent ch ar, which then protects a su bstrate from the ignition source). 4. 1 . 3 Economic Factors Cost is an i m portant aspect of fire hazard reducti on. Therm a l l y stable polymers with superior fire safety ch aracteristics may be too ex pensive for routine u se. The appl ication of fi re-retardant coati ngs is sometimes a cost-effective approach ; how ever, it is l im i ted to a rel atively smal l number of uses. The cost of fi re retardati on by the i ncorporation of a fi re retardant in the pol y mer varies greatl y, depending on the particu l a r com pound or treatment used and the performance level desi red. F i re retardation norm a l l y increases the cost of the material, except when the des i red measu re of protection can be obtained with i nexpensive inert fi l lers. 4.2 Specific Polymeric Materials 4.2. 1 Wood 64
MATER I A LS By vol ume, more wood is used in mines than any other polymeric m ateria l . Serving principa l l y to su pport walls and ove rheads, ti m be r is more readi l y ign ited than coa l, bu t i t has been accepted because of its cost-effectiveness, slow ch arri ng rate, and good strength retention under fi re . F i re-retardant treatments and coati ngs avai la ble for use with l u m ber a n d wood base products are descri bed in Vol u me 1 M ateri als-State of the Art, Chapter 2, Sections 3 and 4.3, and Ch apter 7. The chemical treatments reduce the h azard of i nitial i gn ition; decrease the rate of su rface fl ame spread, the initial rate of heat release, and the total smoke development ; and render the m ateri al less su bject to after-fl aming and afterglow. These treatments i ncrease the cost of the wood prod u cts by 50 to 1 00 percent and have some effect on moistu re absorption, appear ance, strength, mach i n i ng, glu i ng, and fi nishing characteristics. Most of these treat ments also are l i m ited to a pplications in low-moistu re envi ronments, and a m ine atmosphere typica l l y has a relative humi dity of 90 percent. There is also some question about the ro le of flame retardants in wood i n producing toxic gaseous products during combustion. The use of fi re-retardant coati ngs for wood genera l l y has bee n m ore l i m ited th an the chemica l treatments because thick fi l ms appl ied i n several coats are req u i red and spec i a l care m ust be taken i n h igh-h u m i dity envi ronments. In work recently completed at DeBel l and Rich ardson ( Bau m 1 977), fire-retardant po l y i m i de/fungi- cide formu l ations for coating mi ne ti m be rs were screened. The curable pol y i m i de appeared to be fl ame retardant and evolved a m i n i m u m of fu mes when ex posed to a flame. The question of whether rot increases the fl ammabil ity of ti m be r sti l l re mai ns ope n. At the Pittsburgh M i n ing and Research Center, prech arri ng of wood to a depth of 2 m m is be i ng evalu ated as a means for making it less fl ammable. Evaluati on using l aser ignition reveals that a com bi n ation of zinc chloride i m pregnation plus precharri ng is the best treatment. 4.2.2 Thermoplastics 4.2.2. 1 Polyv inyl Chloride The flammabi l i ty characteristics of and fi re-retardation meth ods for polyvinyl chloride ( PVC) and its form u l ations are described i n Vol u m e 1, Section 5.3.3. PV C is one of the lowest priced of the commercial therm opl astic polymers and has been sold co mmerci a l l y for more than 35 years. It can be formul ated into a wide variety of com positions with properties varying from soft elastomers to tough rigid poly mers. (The appl ications of PVC i n m i nes are discussed below i n Section 4.3). It is the least flammable of the low-cost l arge-vol ume therm oplastics because of its h i gh (g-eate r than 50 percent by we i ght) ch lorine content. The flammabil ity of the many ava i l a ble commercial formul ations can vary widely from rel atively l ow in the absence of a n outs i de fl ux to rapid burni ng, depending on the natu re of the m ate ri als added (e.g., plastici zers, fi l l ers, i m pact modifiers, and reinforcements). 65
M I N ES A N D B U N K E R S When exposed t o a flame or t o excessive heat, P V C c a n emit hydrogen ch l oride at relatively l ow temperatu res in a h igh ly endothermic process. This, together with i ts high chl ori ne content, accounts for the low flam mabi l i ty of the u ncom pounded pol y mer si nce the hydrogen chloride keeps oxygen away from the su rface of the plastic. Decompos i tion products vary according to the amout or type of the com pou ndi ng ingredients used du ring fabrication but m ay incl u de benzene, hydro carbons, char, and other fragments. Chlori nated or phosphorus-based plasticizers particularly ph osphates and ch lori nated paraffins, also are used i n large quantities to reduce the fl ammabil ity of p lasticized com positi ons. Phosphates, particularly tricresy l phosph ate, cresy l di phenyl phosphate and 2-ethy lhexyl d i phenyl phosph ate, trad itiona l l y have been added to PVC as plastici zers. They a lso en hance fi re retardance ; fl ame-out times are exce l lent. By far the largest usage for PVC in m ine appl icati ons is in p i pe ; pi pe fitti ngs ; condu i t, wi re, and cable coati ngs; and belt conveyors. 4.2. 2.2 Styrene Polymers The general types of commercial styrene pol ymers and the i r fl am mabi l i ty ch ar acteristics are descri bed in Volu me 1 , Section 5.3.2. Of the various po lymers, copolymers, graft pol ymers and blends in th is grou p, the high-im pact polysty renes ( H I PS) and the bl end/graft co polymer of acry lonitri le/bu tadiene/sty rene (ABS) are the most wi dely used. F i re-retarded versions of both of these materials, genera l l y obtai ned by the add ition o f hal ogenated additives (ch lorinated a l i phatics a n d a l i cyclics and decabromo di phenyl ether) with or without anti mony oxide, also are used. Part or a l l of the halogen component may be incorporated in the form of halogenated pol ymers, such as PVC, in blends. The cost of the bette r fi re-reta rded materials is about twice that of the materials that are n ot f i re reta rded and th eir appl icati on is l i m i ted by cost cons i derati ons. The major styrene po lymer in pi pe, fitti ngs, and conduits is ABS. Low wei gh t, easy fitting, low corros ion, and competitive cost are its major advantages over metal. The fire safety as pects of ABS p i pe have been discussed wide l y. O bviously, buried waste p i pe presents no fi re hazard, but the use of ABS pi pe i n ex posed l ocations cou ld lead to propagation of fi res u nder some conditions and shou l d be avoided in m ine appl ications. 4.2. 2. 3 Polyolefins The polyolefins, pri ncipal l y low-density pol yethylene ( P E ) , h i gh -density po l y ethylene and polypropy lene (PP), comprise a U. S. market in excess of 1 0 bi l l i o n pou nds. The amount used i n m i nes is re l atively s m a l l but growing. The po lyolefins ca n be ta i lored and form u l ated to offer a gre at variety of proper ties including vari ati ons in processabi l i ty, heat res istance, rigidity, l ight stabi l i ty , printa bi l i ty, fri cti on, static properties, foam sta bi l i ty , strength , and tough ness as 66
M ATER I A LS wel l as fl ammabi l ity (see Vol u me 1 , Section 5.3. 1 ). Chemica l l y, po l yolefins are very similar to paraffin wax, and they bu rn in m uch the same way - i.e., they i gn i te eas i ly, bu rn with a sm oky flame, and melt as they burn. Pol yolefins produce less smoke than pol ysty rene, and the degree of melting and dripping can be enhanced or decreased by choice of molecu lar weigh t, cross l inki ng, fi llers, additives, etc. The mechanism of bu rn i ng, products of com bu stion, and flame- retarding form ul ations are descri bed i n Volu me 1, Section 5. 3. 1 . F lame- retardant form u lations general l y are produced by compounding with highly hal ogenated organic compou nds i n com bi nation with antimony oxide. These formu l ations ex hi bit i mproved resistance to i gnition in low thermal en ergy envi ron ments. F l ame spread rates also can be reduced, but a l l known fire-retardant pol y olefin com positions bu rn read i l y in a fu l l y developed fire. Electri c cable coati ngs are m ade from the various density grades of pol yethy lene, som eti mes cross-l i n ked and someti mes parti a l l y foamed for modified dielectric properties. Col d water pipe, cisterns, tan ks, and waste p ipe are com mon appl ica tions for medi u m- and high-density P E and PP. Pol ypropylene often is used i n drai nage fitti ngs. Polyethylene can be chlorinated in the presence of l i !tl t or a free radical cata lyst. The ch l orine content, and therefore the prope rties of the product, can vary con siderably depen di ng on the extent of chlori nation and the reaction conditions. F lammabi l ity decreases directl y with increase i n the ch lorine conte nt. The fl a mm a bi l i ty characteristics of these materi als resem blE' th ose of poly (vinyl chlori de) and pol y (vinyl i dene chloride) . Hydrogen ch loride is a m ajor com bustion product. Pol yethyl ene can be ch l orosu l fonated by methods similar to th ose u sed in the chl ori nation al ready descri bed. The reaction is represented by : As in the chlorination , the properties of the com position can be varied widely by contro l l ing the extent of ch l orosu l fonation and the reaction conditio ns. As ex pected, the flammabil ity of the polyethylene is reduced as the ch lorosu l fonyl ch lo ride content is increased. Fl ammabi l ity has not been th orough ly stu died alth ough hydrogen chl oride and sulfu r d iox i de are major products of com bustion. The volume of chl ori nated an d chlorosu l fon ated polyeth ylene used i n mines is rel atively l ow, but the l atte r is used extensive l y as fi re-resistant electrical wire and cable insu l ation i n ra i l cars and other pl aces where flam mabi l i ty control is essenti al. 4.2.2.4 Acryl ics 67
M I N ES A N D B U N K E R S The acry l i cs are polymers formed from acry l ic esters accordi ng to the form u l a : (R H) = or meth acry l i c (R CH) = {CH2 - COOR' �� X R' ( R' CH3 C. where re presents an alkyl radi ca l . The major plastic in the group is th e homo pol ymer of methy l methacrylate = ), a crystal clear material that softe ns at about 1 00 ° Pol y ( m ethyl methacrylate) ( P M M A) ign ites readi l y and softens as it burns. B u rn i ng rate, fuel load, and smo ke production are less than for pol ysty rene. I n burning, PMMA undergoes "unzi pping" pyrolysis ( reverti ng to monomer) from the heat of the ignition source, the heat of com bustion , or other envi ron mental energy . The volati l e products of pyrolysis then bu rn i n the gas phase. Al though halogen and antimony compou n ds have been used to reduce the burn i ng rate and ease of ign ition of PMMA, less effort has been devoted to its fi re reta rdation than to that of other po lymers. Th is is due partly to the fact th at for most appl ications i t is difficu l t to affect the "u nzi pping" depolymeri zation mech a n ism so cha racteristic of this polymer. F i re-retardi ng additives also usu a l l y i m pa i r the excel l ent transparency a nd agi ng characteristics o f the polym er. When acryl ics a re used as glazing, particularly in rel ati vely l a rge areas, th e pote n tial fi re hazard shou l d be ana lyzed carefu l l y . There is probably l ittl e justi ficatio n for the use o f acryl ics i n mi nes. 4.2.2.5 Nylon Nylon is the generic name for synthetic pol y mers with amide groups occu rri ng repetitively i n the main chain (see Vol u me 1, Section 5.3. 5) . The properties of 20 variants of a l i phatic nylon are ta bul ated in the Modern Plastics Encyclopedia ( 1 975). Nylon pl asti cs play a ro le i n m i nes. They appear i n smal l molded parts su ch as wi ndow and door hardware, spigots, va lves, and appl i ance com ponen ts. Nylon fi l m has been proposed for use as air ducti ng. Many tests i ndicate that nylon plastics have low flammabil ity . They genera l l y self-extinguish afte r exposu re t o a n i gn i tion sou rce becau se they dri p when ign ited, which removes the flame front and hot polymer from the burn ing part. If dri pping is prevented, the nylon burns with a smoky flame. F i re-retarded formul ations genera l l y are prepared using phosphorus or halogen con taining addi tives with or without addition of anti mony or iron oxides. U nder some circu mstances, halogens greatl y increase the flammabil ity of po lyamides, pre sumably by greatly accel erati ng the degradati on to vol ati l e fuel fragments . The addition of dri p promoters, such as thiourea, has been proposed, and hydrated 68
MATER I A LS alumina also has been used. None of these systems, however, prevents a nylon from burning in a fu l l y developed fi re. Many molding an d extrusion compositions contai n glass fi bers or particul ate min era l fi l lers (as much as 60 percent by weight) to enhance ce rtain engineeri ng properties. Such fi l led materi als may burn more read i l y than thei r unfi l led cou nter parts because the fi l l e rs tend to red uce dri pping. The highly fi l led m aterials, how ever, h ave a l ower fue l va lue. Nyl ons cu rrentl y are use i n mi nes in re lative l y smal l items that have not posed serious fi re safety problems. However, larger items (e.g., tan ks, large casti ngs, and other structu ra l appl ications) are being conside red, and more attenti on needs to be given to analysis of the fi re hazards that might be introduced. 4.2.2.6 Polycarbonates Pol ycarbonates are a cl ass of polymers genera l l y consi dered to be extremely tough and to com bi ne clarity with h igh im pact resistance. (These mate rials are d iscussed in Volume 1 , Secti on 5. 3.9. ) The polycarbon ates are significantl y less f l ammabl e than u n modified styrene, olefi n, or acry l i c polymers. Thei r fi re resist ance h as been improved by the addition of hal ogenated materi als as additives or copol ymers. It is the tough ness, clarity, and im pact resistance of the pol ycarbonates th at make them attractive for use as hel mets, face shiel ds, an d glazing material in m ine appl icatio ns. Many of the housi ngs, handles, etc., of smal l app l i ances and tools also are of polycarbonate. Before applying polycarbon ate glazing in large areas, how eve r, an ana lysis shou l d be made of the effect of its use on the fire safety of the system. 4.2.2. 7 Acetals The acetals are l inear homopolymers or copo lymers of formaldeh yde (i.e., pol y methylene oxide) : [-�-o-] n ( Copolymers employ eth ylene oxi de or other sim i l a r comonomers ) . Acetal parts f i n d sign i ficant appl icati ons in pl u m bing fixtu res ( bal l cocks, faucets, showerheads, etc. ) , wi ndow and door hardware, and handles and mech an ical com ponents in appl iances. They are used in re lative l y smal l amounts in m i n i ng. (Acetal chemistry and flammabi l i ty properties are rev iewed in Vol u m e 1 , Section 5.3. 7 ) . The aceta l p l astics are strong, stiff, a n d tough . They are consi dered t o be engi neering resins because of the i r predictable design , processi ng, an d end use ch aracter- 69
M I N ES A N D B U N K ERS istics. The acetals are easi l y com busti ble. They bu rn with very l i ttle smoke and a non l u m i n ous flame and l ittl e oxygen is requ i red, Molecu lar wei gh t and percen t of f i l l e r dictate the amount of dri pping that wi l l occu r. The p roducts of com bustion are carbon d i ox i de, wate r and some carbon monoxi de. Phosphorous- based systems have been suggested as flame retardan ts for aceta ls, but no commercia l success has been ach ieved in th is directi on. On the oth er h and, acetals are used i n rel ative ly smal l parts where they do not present m ajor fire h azards. 4. 2. 2.8 Polyesters The pol yesters cons i de red here are the l i near therm oplastic pol y (ethylene terephthalate) ( P ET) , pol y ( tetramethylene te rephth alate ) ( PTMT), and thei r m odi fications. (The cross- l i n ked styrenated pol yesters are d iscussed with thermosetti ng m aterials in Section 4. 2.3.3. ) These polymers bu rn with a smoky fl ame accom pa n ied by melting and dri ppi ng and l ittl e char formation. F i re-retarded grades gen e ra l l y are prepared by incorporati ng halogen-conta i n i ng materials as part of the polymer molecu les or as addi tives. Metal ox i de synergists frequently are incl u ded. These fire-retarded syste ms are resistant to small igniti on sou rces i n low h eat flux e nvi ron ments but sti l l bu rn readi l y i n fu l l y developed fi res. These po lyesters are being incre asingly used in com ponents of m i ne su rface veh icles. A recent exa mple is the re placement of zi nc die cast head l a m p moldings by pol yester moldi ngs. 4.2.3 Thermosetting Resins Thermoset polymers are disti ngu ished from the thermoplastics discussed above in that they become chemica l l y cross- l i n ked duri n g the fi nal moldi ng. The fi nal product is set i nto shape by pri m ary chemical bonds and, for most practical pur poses, no longer can be melted, reshaped, or dissolved. Because of the i r brittle nature, thermosets are used almost excl usively in con junction with various inorganic or organic fi brous reinforcements and various types of powdered fi l lers. These rei nforcements and fi l lers ofte n com prise m ore th an h a l f of t h e fi nal com position a n d can alter t h e fl ammabi l i ty o r fi re safety o f t h e total com posite signi ficantl y; therefore, it is i mportant to consider the tota l com position before deciding u pon the flammabi l ity characteristics or fire safety of th ese mate ri a ls. Because of thei r cross- l i n ked natu re, thermosets general ly do not soften or dri p when exposed to a flame. Such resi ns are i nherently fire retardant and wi l l pass many com mon l aboratory tests without th e need of a fi re-retard ant modification or addi tive. Thei r fi re retardance, however, is a function of the mech anical stabil ity of the insulati n g cha r and is l i m i ted by the resistance of elemen tal carbon to oxida tion. 4.2.3. 1 Phenol ic Resins and Molding Compou nds 70
MATER I A LS Two genera l types of fi rst-stage phenol ic resin are prod uced depending on the catalyst, the phenol /forma l dehyde ratio, an d the reaction conditio ns. These are called resoles and novol acs, respective l y. The physical properties of the pheno l ic res i ns vary widely depending on the ty pe, kind, and amount of fi l l er used; the kind of rei nforcement used, the phenol /formal dehyde ratio, the type of curing cata lyst; a nd other formu l ati on variables. Phe no l /formal dehyde polymers are practical l y a l ways used in conju nction with f i l lers or fi brous reintorcements. The modified polymers are char-form i ng an d do n ot readi l y support combustion i n the absence of external energy inpu t (see Vol u me 1 , Section 5.4. 1 ). The major use of pheno l i c resi ns is i n resi n- bonded wood for plywood sheeti ng, and such composites genera l ly are less flamma ble than corresponding systems based o n wood al one. The second largest appl ication of phenolic resins is as binders in f i be rgl ass insul ati on. About 1 0 percent resi n ( based on the gl ass ) is used to bond th e gl ass fibers to give dimensional sta bi l ity to the i nsu l ating material. I n th is configu ra· t i on ( i .e., l a rge su rface area and inert su bstrate ), the resin can burn more read i l y t h a n i n a dense sol i d form such a s a lami nate. The system d oe s not readily propa gate a flame but ca n propagate fire by "pu n ki ng" (glowing com bustio n ) . "Pun king" can be overcome by use of various nitrogen-conta in ing reactants ( melam ine, di· cyandi am i de, etc. ) in the phenolic resin. These compos itio ns are u sed for special a ppl ications. Molded phenol ics have been used extensively i n vari ous electrica l appl ications due to their com bination of desi ra ble electrical, mech an ical, and fi re-resista nce p roperties. I n some applications, these resins are bei ng replaced by special thermo pl astics that have l ower fabrication cost. 4.2.3.2 Urea/Formaldehyde and Melamine/formaldehyde Resins The basic chemistry, properties, and appl ications of u rea/form aldehyde and melami ne/formaldehyde resins are summari zed in Volume 1 , Section 5.4.7. The only signi ficant use of these amino resins i n mines is fo r resin-bonded wood applica t i ons. 4.2.3.3 Unsaturated Polyester Resins The unsaturated polyester resins are prepared by condensing a satu rated d i basic alcohol and both a saturated and an u nsatu rated d ica rboxyl ic aci d into a prepoly mer (or fi rst-stage ) resin. The l atter then is dissolved in a vinyl monom er, usual l y sty rene. The cu red res i n is produced by free radical copolymeri zation o f the sty rene monomers and the unsaturated aci d resi dues. Phthal i c anhydride is used most wide l y as the satu rated acid component. The res i ns usu a l l y are compounded with a rei nforcing fabric (genera l l y glass cloth or m at) with or without fi l l e r before cu ring. The burn i ng characteristics of unsaturated polyesters ca n be modified by the 71
M I N ES A N D B U N K E R S add ition of i no rganic fil lers ; the addition of organic fire retardants ; the chem ical modification of the acid, alcohol, or unsatu rated monomer com ponent; and the chemical combi nation of organometa l l i c compounds with the resin. A wide varia· tion in fl ammabi l i ty characteristics can be achieved by using one or more of these modifi cations. Flame spread ratings of 25 or less, as measured by ASTM Test E-84, have been attai ned by using ch lorendic acid with antimony oxide as synergist. Such low fl ame spread rati ngs now can be o btai ned i n the a bsence of opacifying anti· mony by using the more efficient bromi ne-su bstituted monomers. Both fire-retarded and unretarded polyester resin form u l ations yiel d copious amounts of smoke when exposed to fi re because sty rene is the major product of pyrol ytic decomposition and styrene bu rns with a very smoky fl ame. The h igh s mo ke values have been reduced onl y marginally by the use of rel atively large amounts of i n organic fi l lers such as alumina hydrate . The rel ative tox icity of halogenated polyester resi ns has been a su bject of con siderable discussion ever since thei r i ntroduction in 1 953. The chlorine contained in these com positions genera l l y has been shown to be converted largely, if not quanti· tativel y, i nto hydrogen chl oride, Reinforced pol yester formu lations fi nd appl ications i n panels, pipes, ducts, and tan ks. The flammabi l i ty hazards associ ated with these materials i n min ing appl ica tions is incompletely defined at th is ti me, General ly, a h igh degree of fi re retard· ance can be incorporated into u nsaturated pol yester compositions using avai lable technology, 4.2.3.4 E poxy Resins Epoxy resins genera l ly are prepared by reacting a first-stage polyfu nctional epoxy compou nd or resin with a basic or acidic cross- l i n ker (or "hardener") to yiel d a thermoset product cross- l i n ked by ether or ester l i n kages. The basic epoxy resin can be prepared in a variety of ways alth ough the most com mon is the reaction of a po lyphenol i c compound with epich l orohydrin. The fl ammabi l ity characteristics of these resins can be significantly im proved by addi ng halogen and antimony com pounds, the former as either an additive or copolymer. The halogen compou nds genera l l y increase the tendency for generatio n o f smo ke on ignition . E poxy resins frequently are loaded heav i l y with inorga n ic f i l l e rs, and hydrated alumina as a fi l ler can decrease flammabi l i ty. Epox y resins are used principa l l y to seal pi pe joi nts, as plasti c-based pai nts, as morta r for bonding either new or o l d concrete, and as epoxy com posites fo r electrical app l ications. Little or no fire hazard is introduced by the use of epox ies in construction d u e t o the nature o f the com positions and their appl ications, locations, and quan tities. 4.2.3.5 F u ra n Resins Although fu ran resins cu rrently a re l ittle u sed in mining applications, they m ight be used more extens ively in the futu re. Furan res ins are prepared by reacti ng 72
MATER I A LS furfury l alcohol and an aldehyde - most frequently formaldehyde. Urea is often used as a modifying agent. The resins are hardened in situ with an acidic su bstance added just before appl ication. A typical curing agent wou l d be p-toluenesu l fon ic acid. Although no specific literature reference has been found th at descri bes fi re retardant methods for furan resins, fi re-retardant form u l ations are ava i l able com mercially. 4.2.3.6 Amine Resins Am ine resins are thermoset resins prepared by the reaction of an amino com  pou nd with an aldehyde. The reacti ve amino groups (-N H 2 or - N H -) are charac teristi ca l l y present as am ides. The two m ost i m portant commercial m aterials are based on urea and mel am ine used with formal dehyde. Little work h as been done to develop fire retardance i n amino resins because of their rel atively h igh heat res istance and low flammabi l i ty and their predominant u se i n app l i cations where flamma bi l i ty is rel atively u n i m portant. A variety of phos phorus and boron compounds have been used to reduce fl ammabil ity when re qui red. The various amino resins find l i mi ted appl ication as coati ngs on a variety of metal and reinforced plastic panel su bstrates. Hardness, durabi l ity, abrasion re· sistance, and easy colorabi l ity are the prime reasons for their use. 4.2.4 E lastomers The fi re safety aspects of elastomers are largely dete rm ined by their chemical structu re. From this poi nt of view they may conven iently be assigned to the several disti nct groups descri bed below. A more com plete descri ption of these compounds is foun d i n Vol u me 1, Chapter 5. 4.2.4. 1 Hydrocarbo� Based Elastomers The h ydrocarbon- based el astomers pri ncipa l l y comprise natu ral ru bber, syn thetic cis-polyisoprene, pol y bu tadiene, styrene-butadiene ru bber ( SB R ) , butyl ru b ber, and ethylene-propy lene ru bber. These ru bbers are l ow-cost m ateri als with good mechanical properties and, thus, are used i n large volume appl icati ons such as automobi le and truck tires. SBR ru bbers are wi dely used in belting. They do, however, bu rn readi ly and produce much smoke . F i re-reta rdant additives reduce flame spread and ease of ign ition from low energy ign ition sou rces but do not prevent burn i ng in an i ntense fi re situation. Alu mina trihydrate is receiving i n tensive study as a fi l ler to reduce fl ammabi l i ty and smoke productio n in these elastomers. A good future seems to exi st for fluorocarbon ru bbers as a coating and for pri mary construction in hose and cable. 4.2.4.2 Chlorine-Containing Elastomers The chlori ne-contain ing elastomers include polych loroprene, rubber hydro- 73
M I N ES A N D B U N K E RS chl oride, ch l orinated ethylene polymers and copol ymers (ch lorinated polyolefi ns ) , a n d epich l orohydrin ru bbers. These materials are sign i ficantly more fi re reta rdant than the straight hydrocarbon ru bbers, but they generate extensive black smoke and hydrogen ch loride gas whe n exposed to a fu l l y devel oped fire. Ch l ori nated elastomers, particul a rl y polychloroprene (more com mon ly des i g· nated as Neoprene ), are wi de l y used where fire retardance is i m portant. Some l a rge-scale appl i cati ons are in electrical insulati on , foamed seat cush ions, and con veyor bel ts. 4. 2.4. 3 Nitri le Rubbers Nitrile ru bbers are copolymers of butadiene and acrylonitri le. The rati o of butadiene to acrylonitrile is s i m i l a r to the ratio of butadiene to styrene in SB R . The cyanamide grou p i mparts to these el astomers some of the pro perties of the halo gen-containing ru bbers but also constitutes a potential toxicol ogical haza rd. 4.2.4.4 Polyurethane Elastomers Pol yureth anes are polymers containing the grou p - N H-C0-0-. They are formed typica l l y through the reacti on of a d i isocyanate and a glycol. Because a variety of gl ycols or esters can be cou pled with different d i isocyanates, a large vari ety of l i near pol ymers can be obtai ned in this way. These elastomers are cross l i nked by including a control led amount of a polyfunctional monomer (e.g., a tri isocyan ate or tri hydric alcohol ) in the reaction. F i re-retardant grades, genera l l y based on bromi ne-and/or phosphorus-contai n i ng add i tives, are avai lable, bu t they sti l l bu rn in intense fi res. Smo ke generation is genera l ly less than with hydrocarbon el astome rs, but some hydrogen cyanide gas may be generated. The major use of polyu rethane el astom ers is in foams and in bel ts when extreme tou ghness is needed. A future use i n m i nes is for strata rein forcement. (The uses of pol yurethane foams are described below i n Section 4.3.4 ) . Reaction i njection molded ( R I M ) pol yurethanes are fi nding increasing use on automobi le exteriors in such appl ications as front and rea r fender exten sions, fro nt fender s k i rts and panels, and front or rear fen der fi l lers ; appl icati ons in uti l i ty veh i cles can be expected to fol low. 4.2.4.5 Polysulfide Rubbers The polysul fide ru bbers, also known as th i okols, are pol ymers composed of al i phatic hyd rocarbon chains con nected by di·, tri·, and tetra-sulfi de l i n ks. Becau se of the i r outstandi ng resistan ce to hydrocarbon solvents, they are used extensive ly as sealants i n a i rcraft fuel tanks and pressuri zed ca bi ns but have only l i m ited app l i c a bil i ty elsewhere. 4. 2.4. 6 Sil icone R u bbers Si l i cone el astomers ge nerate re lative l y l i ttle smo ke, are reasonabl y fi re retardan t 74
MATER I A LS i n a i r, an d, when bu rned, have low fuel val ue. They burn sl owly and produce n o f l am i ng drip. They a r e rel atively expensive (less s o than fl uorocarbons, b u t more so than hydrocarbon ru bbers) and thei r mechanical properties are m arginal for many appl ications. Si l icones, however, do offer a most prom ising combi nation of fire safety aspects, physical pro perties, and cost, and they are used in electrical insu l a tion a n d seat cush ions. 4.2.4.7 Phosphonitrilic E lastomers Phosphon itri l ic el astomers rep resent another example of "i norganic el astom ers ". The phos phoru s· nitrogen back bone : � ]--( c;> = p N }- X or [ b I R s uppl ies the flexi bi l ity requ i red for el astomeric properties and contri butes l ittle ' fuel va l ue. The various side grou ps ( R, R ) affect many of the characteristics of the e l astomers incl udi ng fl ammabi li ty (e. g., long hydrocarbon side cha i ns wou l d i n c rease flammabi l ity whereas fl uoroca rbon side chains wou l d not contri bute to flam mabi l ity but cou l d contribute to undesi rable pyrolysis products ). These ph osphon itri lic materials are in the earl y stages of devel opment, and much n eeds to be done to defi ne thei r uti l ity and appl ica bi l ity for various uses. Th is i ncl udes the defi nition of the com busti on and pyrolysis products contri buted by the phos phorus and nitrogen. They represent, however, one of the main hopes for a l ow-smoke, l ow-fl ammabi l ity elastomer. 4.2. 5 Foams Pol ymeric foams genera l l y are complex multicomponent systems that m ay con tain fi bers and various fil lers. Pol y meric foams can be divided into rigi d and flexi ble foams. Sy ntactic foa ms are another type that are essentially polymers su rrou nding t i ny hollow spheres of a nother polymer or gl ass. F lex i bl e foams genera l l y have an open cel l structu re whereas rigi d foa ms usual l y have a cl osed-cel l structu re. Since more of a foam's su rface is exposed to atmospheric oxygen, its rate of pyrolysis and burn ing is greater than that of the base pol ymer. The low therm al conductivity of foams tends to concentrate the heat on the su rface of the structu re rather than to dissi pate it to underlying material or su bstrate. The result is rapid h eating and py rolysis of the surface material when ex posed to a flame. Th is often l eads to an extremely rapid fl ame spread rate ; however, other factors may m oderate t h is effect considera bly (e.g., the sma l l amou nt of potenti a l l y flammable m aterial per unit vol ume i n low-density foams res u l ts i n a ve ry sma l l amount of tota l heat 75
M I N ES A N D B U N K E R S being avai lable per unit a rea for fl ame propagation ) . I f the foa m material is a thermoplastic such as polystyrene, the heat o f a flame rapidly melts the foam adjacent to i t, and the material may recede so fast from the flame front that there is n o real i gn ition. A highly cross l i nked therm oset foam, on the other hand, behaves i n an enti re l y different manner. Si nce l ittle or no melting occurs, the surface does not recede from the flame front, and the foam is rapidly i gn i ted. The fl ame then spreads if the foam is fl ammable. Under the same condi tions, a fi re- retarded foam pyrolyzes rapidly i n the vicin ity of the fl ame, leaving a carbonaceous char on the surface of the materi al. Th is high ly i nsu lating char pro tects the remainder of the materi a l from the effects of the flame. Since carbon itsel f is com busti ble, t h e continued impi ngement o f a radiant heat flux can generate con ti nued com bustion, but the l ow density of the su rface char generally does not produce sufficient heat to sustain burn ing i n the a bsence of su rface heat radiation. 4.2.5.1 Polyurethane Foams Polyu reth anes are the reaction products of a dihydroxylic or polyhydroxy l ic com pound or resi n and a diisocyanate or polyi socyanate. Polyurethane foams are prepared by modifying the fun da menta l reaction of an isocyanate and an alcoh ol to produce a permanent cel l u lar structure in the basic polyurethane du ri ng i ts poly merization by the control led introduction of a gas phase. ( The appl ications of polyurethane foams i n m ines are discussed below in Section 4.3.4). An i m portant reacti on is the trimeri zation of isocyanate to produce an iso cyan urate ring: / co......._ [ ] 3 R NCO Cata lyst R - N N - R 'co "ccf ' ' N I R The isocyanu rate ri n g i ntroduces a trifunctional cross- l i n k. It is therm a l l y m ore stable than the ureth ane from which it is derived and can be used to reduce th e flamma bi l ity of polyurethanes. lsocyanu rate structures can be produced by the use of excess isocyanate i n the presence of an isocyan u rate catalyst such as a tertia ry ami ne. The ce l l u lar nature and low thermal stability of polyureth ane foam s genera l l y i nfluence their fl ammabil ity. Because of the l o w thermal conductivity , the h igh surface heat flu x generated from an i gn iti on sou rce, can cause a l most i nstantaneous conversion of a polyurethane to fl amm able gases. Th is often results in very rapi d surface flame spread a n d h i gh fla m i n g tem peratu res once the surface i s ign ited. I n general, fi re retardance i s i mparted to pol yu rethane foams by the chemical inco r porati on of halogen and/or phosphorus compounds. 76
M ATER I A LS Although polyurethanes themse l ves are nontoxic, the pyrolytic com bustion gases have been shown to contain consi derable quantities of tox ic gases. Significant amounts of hydrogen cyanide have been detected in pol yu reth ane com busti on products, but its relative toxicity in gaseous mixtures contai n i ng l arge amounts of carbon monoxide has not been defi n i tely esta blished. 4.2.5.2 Polystyrene Foams Low-density po lystyrene foam is used as thermal insu l ation, and h igh density foam is used for structu ral appl ications. The major fi re hazards from ce l l u l ar poly styrene are the potential for h igh bu rn ing rate, high smoke production, and rapid flame spread. These are, of cou rse, high l y dependent on location, geometry , orien tation, and rel ati onsh ips to other materials. I gnition and bu rn ing rates also are affected by com position (e.g., use of flame retardants) , ign iti on sou rce, and thermal env i ronments. There is l ittl e justification for the use of polysty rene foam in mining appl icati ons. 4.2.5.3 Poly(vinylchloride) Foams F lexi ble pol y(vinylch l oride ) ( PVC) foams find greatest appl ication in coated fabrics, cloth ing, and seati ng where they pro bably have l ittle effect on fire safety . H igh-density foams of va rying flex i bi l ity are used for flooring in nonresidential bui l di ngs, and the flammabi l ity hazard of PVC i n this application can vary depend i ng on the ty pe and amount of pl asticizer used in the com position. R i gid vinyl foamed extru ded sha pes are being used i ncreasingly as exte rior and i nte rior tri m, window casi ng, sandwich core material, an d siding. The flammabi l ity of such materials is low, however, because of the high density and thermal con ductivi ty of most of the products, the absence of signi ficant am ou nts of flammable plasticizers, and the h igh concentration of i nert fi l lers general ly used in the form u l a tions. 4.2.5.4 Rubber Foams Practica l l y any el astomer can be made into a flex i ble foam. When a chem ical blowing agent is used i n a dry-compounding reci pe, the foam ru bber genera l l y is referred to as sponge ru bber. Sponge ru bber is made mostly from natu ra l and styrene-butadiene ru bber although silicone and fl uorocarbon sponge ru bbers also are ava i lable. Latex foam ru bber is made by beati ng air i nto com pounded ru bber latex . F l u or ocarbons with or without a i r are used as foaming agents in som e processes. Natu ral or styrene-butadiene rubber or blends of the two are widel y used. The approaches to fi re retardation i n these materials are general ly the same as those employed with the same bu l k (i.e., non-foamed) el astomer, except th at post· t reatments s i m i l ar, in principle, to those appl ied to wood and wood prod ucts are possi ble because of the cel l u lar nature of foams. 77
M I N ES A N D B U N K E R S 4.2. 5.5 Urea/Formaldehyde Foams Urea/formal dehyde foams are made by mechan ica l l y froth ing two aqueous re· action streams in a special appl icator gu n. Foam comes from the gun in fu l l y expanded form, much l i ke shav ing cream. I t sets i n 1 0 to 6 0 seconds, cures i n 2 to 4 hours, and dries in 1 to 2 days. The ou tstanding properti es of these foams are the i r rel atively good fi re safety ( l ow smoke production, low flame spread rate, and l o w fuel va lue), good insu l ation efficiency ( K factor = 0. 1 8 t o 0. 20) , good sound insu l a· tion, pest repel lence, injectabi l ity i nto i naccessi bl e cavities, and lack of pressu re bui l d- u p. Urea/formal dehyde foams have no flex ural strength an d poor di mension al stabi lity and shou l d not be left ex posed since they are eas i l y mechan ica l l y dam aged. Cu rrently avai l a ble formulations also suffer hydrol ytic instabi l i ty at h igh humidity, which almost certainly ru les ou t their use i n certa in mining applications where a h igh degree of moisture is present. 4.2.5. 6 Phenol/Formaldehyde Foams Phenol/formaldehyde foams have been known for a long time, but their com· mercial uti l i ty has been very l i m ited due to the fri abi l ity and corrosivity resu lti ng from the residual strong acid used in their preparation. Such foams do exh i bit the norma l good fi re- resistance characteristics of phen olic polymers but have a ten d· ency to undergo "pu n king" (glowing com bustion ) . Recently, progress has been made towards overcoming these shortcom ings, and slab stock, spray appl ications, l am i nates, and i njection mo lded foams are in various stages of commercial develop ment. These foams vary from 1 00 percent open-ce l l to 80 percent closed-ce l l m a teri als; they have K values ranging from 0. 1 9 to 0.23 depending on density and tem peratu re. Phenol ic foam roof insulation is reported to be the fi rst plastic foam to o btain a Class 1 rating for an insu l ated steel roof deck construction. 4.2.6 F i bers 4.2.6.1 Natural Fibers 4.2.6. 1 . 1 Cotton Cotton is essenti a l l y cel lu l ose, wh i ch is rich in m oderately reactive hydroxy l groups a n d wi l l burn under a wi de variety o f conditions. Cotton i n the form of fiber, yarn, or fa bric can be treated with fire retardants in order to reduce its flammabi l ity. Metal ox i des and organ ophosphorus com pou n ds currently are the successfu l potenti a l l y accepta ble, du rable fi re retardants for cotton and rayon fab rics. 4.2. 6. 1 .2 Wool F ibers Wool texti l es genera l l y are less flammable than cel lu losics and are used exten· sive ly i n carpeting and seat covers. H igh concentrations of hydrogen cyan ide h ave been found in the py rolytic off gases from wool products. F i re-retardant meth ods 78
MATE R I A LS for wool have not been studied as extens ively as those for cotton, but, genera l l y, the flammabi l ity of wool is decreased by treatment with organophosph orus com· pounds or specific salts of polyvalent metals. 4.2.6.2 Commodity Synthetic Fibers Synthetic fi bers represent the "commodity" items of the textile industry and are p roduced in larger tota l volu mes than cotton an d wool com bi ned. Such fi bers i ncl ude rayon, acetate, nylon, polyester, olefi n, and acry lic. No fi bers in th is group c an be consi dered to offer protection against di rect exposure to flame. B lended fabrics made of yarns contai n i ng two or more fi bers of different chem ical composition and properties have attai ned great commercial i m portance in tex tile markets. F i ber blends pose sy nergistic fi re hazards that often lead to u n expected flammabi l ity characteristics (e.g., the incl usion of even small amounts of c otton in a pol yester fiber garment can lead to a severe fl amm abi l i ty h azard because the nonfusi ble cotton prevents dri pping of the fusi ble polyester, th us increasi ng the fuel avai l a ble for burn ing). 4.2.6.2. 1 Specialty Synthetic Fiben F i bers formed from h igher- me l ti ng-poi nt or nonmelti ng polymers with a sign i fi· cant aromatic or heterocycl ic ring content resist i gnition, do not spread flame, an d, in garments, offer a wearer protection for a brief period against injury by direct flame i mpingement. The avai l a ble f i be rs with these properties are Nomex ( poly- [ m-pheny lene iso· p h t h a l amide] ) and Kevlar (poly [ p-phenylene tereph thal am ide] ) . A pol y benzim i dazole ( PB I ) fiber, that offers the weare r protection against flame impinge ment is avai lable in devel opmenta l quantiti es. 4.2. 6. 3 1 norganic F ibers I norganic fi bers are important for some end uses. G lass fi bers, for example, melt at about 5 1 5° C and do not bu rn . However, they ge neral ly are treated with organ ic fi nishes to enhance their resistance to abrasi on and to i mprove other fu nctional properties. The flammabil ity hazard of glass fibers i n actu a l use, therefore, is sign ifi· cantly modi fied by the presence of these organic materials. 4.2.7 F i re- Retardant Coatings The use of fi re-retardant coati ngs is one of the ol dest methods for protecti ng f l ammable and nonfl ammable su bstrates from reach ing ign ition or softening tem· peratu res. F i re- retardant coati ngs are either intu mescent or non-intumescent. Fi re retardant coati ngs are particu larly usefu l in reducing the fl ame spread ch aracter istics of a l most any type of organ ic su bstrate. The fi re-retardant coatings shou l d be formul ate d so as not to sustain combustion . However, su bstrates th at thermolyze to y ield flammabl e vapors cann ot be protected by coatings in the event of a fu l ly developed fi re. 79
M I N ES A N D B U N K E R S 4.2.7. 1 Al kyd Coatings The no�i ntu mescent, fi re-retardant coati n gs with the largest sales are based o n chl ori nated al kyds predom inantly prepared fro m ch lorendic anhydride o r tetra chl orophthal ic anhydride. By using the proper ch lori n ated aci ds, coati ngs that have properties comparable to those of conventi on al coatings and that are also fire reta rdant can be made. F i re-retardant additives also are comm on ly added to alkyd resi ns. Halogenated additives such as chlorinated paraffi ns are most commonly used in these coatings because of thei r low cost. Antimony ox i de is the most commonly used synergist i n these app l i cations. 4.2.7. 2 Intumescent Coatings I ntu mescence is defi ned as "an enlargi ng, swe l l i ng, or bu bbling up (as u nder the action of heat) . " I ntumescent coati ngs are used to protect flammable su bstrates such as wood and pl astics from reach ing i gn ition tem peratu res. They also protect nonflammable su bstrates, such as meta ls, by preventin g them from reach ing soften  i ng o r melting temperatu res. Conventi onal intu mescent coatings contain several ingredien ts that are necessary to bri ng a bout the i ntumescent action : ( 1 ) a cata lyst th at tri ggers the fi rst of several chemical reactions in the coati ng fi l m, (2) a carbon ific com pound th at reacts with the catalyst to form a carbon residue, (3) a spu m ific compou nd that decom poses producing large quantities of gas wh ich cause the carbon aceous char to foam into a protective layer, and (4) a resi n bi nder that forms a sk i n over the foam and keeps the trapped gases from escaping. Apart from these key ingredients, intumesce nt coati ngs also m ay include many other i ngredients used in conventi ona l coati n gs (e.g., pigments, dri ers, level ing agents, a n d th inners) . Nonconventiona l i ntu mescent coatings are those in wh ich the elements of in tumescence are bu i lt i nto the resin itself. ( See Vol ume 1, Section 8.4 ) . For ex ample, a clear intumescent epox y coating h as been prepared by the reaction of tri phenyl phosphite with an epox y resi n prepared from epic hlorohydrin and bisphenol A. The coating is prepared by adding the amine catal yst to th e prem ixed epoxy- (tri phenyl phosph ite ) resin ju st before it is app l ied. The coating consists of 1 00 pe rcent sol ids. 4.3 Specific Usage of Polymeric Materials i n Mi nes As mentioned earlier, wood used as structu ral ti m ber is the largest volume of polymeric material used in underground m i nes. Its appl ication and fi re safety ch ar acte ristics have been discussed in Section 4 . 2. 1 and wi l l not be repeated here . Coal also i s consi dered to be a natural polymeric m ateri a l and, o f course, i t consti tutes the su bstance and the product of a l l coa l m i nes. It is fl ammable a n d subject t o very rapid combusti on . Coal dust, free ly generated in m in i ng, i s not on ly easy to ignite but can also form an ex plosive m ixtu re with air. However, since the 80
MATER I ALS presence of coa l is unavoi dable in coal m i nes, it does not constitute a su bject for material selection. For these reasons, coal is not discussed in Volume 1 or in this chapter; however, its fire safety aspects are dealt with elsewhere i n th is vo l u me. 4.3. 1 Ventilation Cloth Brattice cloth is used to bl ock se lected passageways in order to direct in creased a i r into the working face or to serve as seals for emergency ve nti l ation procedu res (e.g., in the event of fi re or other m i ne disaster). It may be used in the fo rm of a curtain, an ai r bag, or a pa rach ute anchored to the mine roadway, cei l ing, or wal ls. Typica l ly, the brattice cloth has been made of jute, cotton, nylon, pol yester rein forced PVC, or neoprene. F i re- retardant variants are used. M u rphy ( 1 972) has com piled data i l lustrati ng that perform ance in a large-sca le trial in an experimental mine correl ates wel l with the fl ame spread i ndex determ ined using ASTM Method E- 1 62 and th at a fl ame spread index of 25 or less is reasonably safe for venti lation cloth. Fi re-reta rded versions of jute, cotton, and polyvinyl c h loride film and woven gl ass cloth used in the m i ne ex peri ment had flame spread i ndex values of less than 25 and flame propagation of not more than 5 fpm at 400 fpm air velocities. If for some reason superior performance i s requ i red, N om ex ®, Kevlar ®, or PB I fabrics shou ld be consi dered. Nylon or polyester fa brics wou ld provide excel lent wea r properties, but it is not certain whether ava i l able flame-retardant fo rm ulations wi l l meet the stri ngent coal mine safety requi rements. 4.3. 2 Conveyor Belts F rictional heati ng of conveyor belts is the cause of about 8 perce nt of reported fi res, and when a belt ignites, it may conduct the fire over great distances. Co n t ro l l ing this hazard by proper materials selection and by im proved operating pro cedures has been reasonably successfu l. A conveyor be l t consists of a convey ing cover, a carcass, and a bottom or p u l l ey cover. It is constructed of elastomers ( natural of synthetic ru bber, polyvinyl ch lo r i de, or neoprene), fabric (cotton, gl ass, as bestos, nylon, polyester, rayon), fi ber cord (cotton, nylon, rayon , polyester), and wi re cord ( brass or zi nc-coated stee l ) . The el astomer i s used for the top a n d bottom covers o f the belting a n d for bonding together the fabric and cords. The R ubber Manufactu rer's Association tabu l ation of e l astomers most com monly used in be lti ng is reproduced in Table 1 ; other less common materials are l isted in Table 2. Exami nation indicates th at neoprene wil l best meet fi re safety an d genera l performance requirements in m i ne applicati ons; p o l yvinyl ch l oride is a second choice. Selection wil l be dictated by econom ics. Once an appropriate el astomer is chosen, the fi ber to be used in the cord or fabric can be selected consi dering requ i red stren gth , dura bi l ity, humidity, and tem p e ratu re exposure as well as cost factors. Evi dence indicates th at the fi ber pl ays a secondary role in fire safety pe rformance; however, one must remem ber th at a 81
M I NES AND BUNKERS Tâ¢bl⢠1 . Rubbers Mon OJmmonly Ul fld in Stilting. ASTM Des ignat ion Common Dl4 18- 7 lA Name Compo s i t ion Gene r a l Prope r t ies CR Neo p r ene Cbl o ropr ena Good o aona a nd lun-checkin& r e l i l t&nc e , aood r ea t a tanca t o pe t ro l eua ba aed o il & and to abra s io n . A l s o aood f l am e r e s is t a nc e . U s ed e x re n s 1v a l y underaround 1n a ine conveyor bel t s d u e to i t l f l ame  r c s i a t onc e c b o r a , t e r i a t ic . NR Na t u r a l l a o p r ane , Ex c e l l en t r e s i s t a n c e to c u t tin& , &Ou & i ri& , Natural a nd abr a 1 io n . Good e l a s t ic i t y and r e 1 i l 1enc y . No t o il r e s i l t a n t . IR Po l y i sopr ene I a o pr en e , Saae proper t ie s â¢â¢ na tura l . s yn t h e t ic l lR Bu t y l h o bu tyl ene- Ex c el l ent r e a i l tance tO hea t . Vary &ood I so p r ene r e a ia t e nc a to o zo n e and ⢠& i na . Good r a 1 i 1 t a n c e to abra s io n . No t o il r a s i l tant . NIR Buna N Nitr ile Ex c e l l en t r ee i a t a nc e to va&etab l a , animal Bu tad i ene a nd p e t r o l eua o i l l . SBR S IR S t y r ene- Ex c e l l en t abralion r a l i a tanca a nd aood Bu tad iene r e s i l tanc e t o cu t t in& , aou& in& and teer 1n& . Good bea t r e e i s t a nc e . No t o il re⢠i a tan t . BR Po l ybu tad i ene Pol ybu tad iene A synthet ic rubber in tha &aneral purpoaa f ie l d . Ca n b e u a ed a lone or in b l and a w i t h Na tural or Styrene-Bu t ad iene Rubber . Has ex c e l l en t abr a a 1o n r aa 1a t a nc e and h i&h r e s i l ienc y , a nd excellent low t aa p e r a tu r e r a l 1 1 tanc a . NOTE : Adapted from Rubber Man u fac t urers ' Ass oc i a t ion , 1973 82
MATE R I ALS Table 2. Other Synthetic Rubberlike Materials Used in S.ltlng. ASTM De s i gnat ion common 0 14 1 8 - 7 1A Name compos ition Gene r a l Prope r t ies FMQ S i l icone Mod if i e d - Ex c e l l e n t h i g h and low temperature r a a ia - Po l y s i l ox a n e o tanc e . Ca n b e m a d e to g iv e f a ir o i l r e s i s ta nc e . Poor phys tc i a l proper t i a a a t room t em p e ra tu r e s . c� !!)·pa l o n Ch l o r o  Ex c e l l en t o zo n e , wea ther ing and a c 1d r e S u l ! on y l  s i s t 3 nc e . C o o d a h r � s i o n a nd hea t r e o 1 s  Po l y e t h y l ene t a nc e . Fa i r oil r n � s u nc e . AIR Ac r y l ic Ac r y l a t e  Ex c e l l en t f o r high t em p e r a t u r e �il and bu tad i e n e a ir . Poor wa t e r r e s i o t a nc e . Poo r c o l d f l ow . CllR Chl or ina t ed Chl o r ina t ed o r Same s ener a l proper t i a a aa Bu t y l ex c e p t a nd Brom ina t ed i t c a n b e a d h e r e J to o r u s ed in comb in BIIR Br om i n a t ad I s o b u t y l ene a t i o n w i th o t her po l ym e r & . Bu t y l I s o p r ene A. � Teflon Tetraf luoro Ex c el l en t h ig h t em p er a tu r e p ro p er t i e s , Kal-F e t h y l en a c h em i c a l r e s i s tance and phyo i c a l r e a in proper t ies . PVC Vinyl P o l yv inyl A thermo p l a s t ic mater ial which has very Ch l o r id e good a br a s i o n . Ex c e l l en t f l am e r e s i s  r u i na t a nc e . Al s o baa g o o d r a a i a tance to a n i  m a l and v eg e ta b l e o il s . L im i ted t em p e r  a tu r e r a ng e . AU U r e tha ne P o l y e s t er Ex c e p t ional abra s i o n , cu t and t ear EU Po l y e t he r r e s i s t a nc e . EPM Ethyl ene E t h y l ene A seneral pur po s e synthe t ic which haa P r o p y l ene Propylene sood a g ing , abr a s ion a nd heat res i s  Ru b b e r t a nc e . EPDH Ethyl ene Ethyl ene Same a s EPH . Propy l en e propyl ene Rubber d ie n e  Te r p o l ).,. e r co Hyd r i n Polychloro Ex c e l l e n t o il and o zona r a s i a t a nc e . methyl Cood f lame r e s i s t anc e a nd low perme Ox i r a ne a b il i t y to c a s u . Fa ir low t â¢p a r a  t u r e proper t i e s . ECO Hyd r in Ethyl ene Ox id e Ex c e l l e n t o i l a nd o aona r e s i s ta nc e . and F a 1 r f l ame r e s i s t a n c e and l o w pe�a Chl o r om e t h y l ab i l i t y t o g a s e s . Cood 1�" t empera t u r e Ox i r a n tl proper t i es . NOTE : Adapted from Rubber Man u fac turers ⢠As s oc i a t i on , 1973 83
M I N ES AN D B U N K E R S thermopl astic fi ber in a thermopl asti c elastomer belt m ight melt and separate i n the event of sli ppage and excessive pu l ley fricti on and, thus, avoid a fricti on-induced fire incident. Thermopl astic fi bers for th is purpose are either nylon or polyester; the thermopl astic e l astomer for this app l ication is polyvinyl chl oride. When using PVC, a plastici zer genera l l y is requ i red. G iven the ch oice of phthal ate ester or phosphate ester plasticizers, the former increases th e fl am mabi l i ty h azard whereas the phosphate esters reduce the fire hazard. The U. S. Bureau of Mi nes has devel oped a set of requirements, designated Part 1 8, under wh ich bel ts m ust pass two tests : one to measu re flame propagation after d i rect flame ign iti on and the other to measu re resistance to dru m friction ign ition. In current work at Factory Mutual Research Corporati on ( 1 976), attem pts are bei ng made to con duct be l t fire tests in a fu l l-scale simulated m ine ga l lery where cei l i ng radiation effects enter the situation. 4.3.3 E lectrical Conductor I nsulation The probl em of fi re safety of electrical con ductor insu lation in m ines is essen tial l y the same one faced by electric uti l ities and i n naval vessel s, industrial instal l a ti ons, and a l l ty pes of bu ildings. Polyvin y l ch lori de has been the pri nci pa l resin used for almost a l l appl ications since about 1 938. Above 600 volt rati ngs, PVC genera l l y is not used ; ru bber, neoprene or cross-l i n ked pol yolefi ns are employed. Cross- l inked pol yolefi ns are employed in a bout 5 percent of i nsta l l ations becau se the fl ammabi l ity codes or the design en gi neers specify cross- l i n ked products to i mprove protection agai nst overl oads and hot am bients or surges. These materi als must pass th e hori zontal flame test specified by Un derwriters Laboratories, I nc., i n U L 44, Paragraph 3.0. Large i ndustria l complexes and uti lities use cross- l i n ked polyolefins as insu lation in armored cables that are used excl usively for power distri buti on circuits. The a rmored cable prov ides improved crush resistance. When high rel i a bi l ity is required i n the presence of high temperatures or h igh cu rrent density, higher cost m aterials can be justified. These materi als must pass the horizonta l test specified i n UL 44, Paragraph 3.0, but the vertical test, F R- 1 , specified in UL 44, Paragraph 74, is optional. Nonarmored ( hypalon-jacketed or neoprene-jacketed) con structions are used for control circu its. PVC is used in about 75 percent of these applications ( U L 83, Pa ragraph 74) . Howeve r, once again, cross- l i n ked polymers ( U L 44, Paragraph 74) are insta l led for higher rel i abi l ity. Silicon e insulation is used in such pl aces as stee l m i l ls where conductor tem peratures above 90° C are fou nd. Si l icone ru bber also is used to justify a h igher tem peratu re of operation in the rewi ring of old bui l dings where space is l i m ited. This i nsu lation is requ i red to pass the hori zontal flame test specified in UL 44, Paragraph 3.0, and, in some insta nces, the ve rtical flame test F R- 1 specified in U L 44, Paragraph 74. The insul ation used in electric generating stations must pass the vertical flame 84
MATER I A LS test F R- 1 spec ified in U L 44, Paragraph 74, and the vertical tray test specified i n I nstitute o f Electrica l and Electron ic Engi neers ( I E E E ) 383. Ethylene-propylene rubber ( E P A ) or cross-l i n ked polyolefi ns with hypalon or neoprene jac kets are used in foss i l fuel power stati ons, and E P A or cross- l i n ked polyo lefins contain ing halo genated addi tives with hypalon or neoprene jackets are used in nuclear in sta l l ations. Thermoplastic fluorpoly mers are begi n n i n g to fi nd increased u se in nuclear power plants. Cables frequ ently are grou ped (e.g., in trays or con du its ) in l arge fac i l ities such as industrial pl ants, hospitals, commercia l esta blish ments, and electric generati ng stati ons and often are al most i n accessi ble. A fire under these ci rcumstances can lead to l oss of power, dense smoke, destructi on of consi dera ble length of cable, etc. Many of the standard flammabi l ity tests are performed on single i nsulated con· ductors. The practice of grou ping cables does present a serious fi re hazard . (The rel atively recent fi re at a New York City telephone exch ange has served to em ph a size the hazard of vertical and grou ped cableways and the flammabi l i ty of PVC insu l ation i n an actua l fi re . ) I n the field, the conditions for fl ame propagation are very diffe rent when ca bles with flame-retardant insu l ation or covering are i nstalled i n l arge numbers. Some safeguards incl u de insta l l ation of automatic spri n k l ers, smoke detectors, use of i m proved flameproof coverin gs, an d an automatic carbon diox ide flooding system. Newman ( 1 976) has shown that PVC conduit or nonmeta l l i c sheathed cable exposed to fi re evo lved enough H CI to affect hu man escape potenti al from a bu i l d i ng. Under the same conditi ons, wi ring systems using steel raceways wou ld not adversely affect escape potential. 4.3.4 Sealants M i n e tunnel wa lls and cei l i n gs genera l l y are co mposed of eas i l y degradable shale, and seal ants are requ i red to protect them from the effects of air, moisture, and mechanical abrasion and to control methane diffu sion and gene ration. A sea lant that also serves as a thermal insulati on is desi ra ble . Ease of appl ication over i rregu lar and wet surfaces, rapid cureti me, and mechan ical tou gh ness are additional requ i re· ments. U rethane foa m appea red to meet all these requ i rements and its acceptance was rapid. However, fi re and tox icity hazards soon became apparent, and the use of u rethane foam as a sealant in mines now is essen tial ly proh i bited. Repl acements for urethane foa m are the su bject of ongoi n g investigations. Sprayed fi brous rei nforced cement is one possi ble sol ution, and sprayed water-base e poxy sea lant a lso may be effective ( F ran klin et al. 1 977). Sodi u m s i l icate is effec t i ve on sandstone wal l s. Coati ngs to protect ex isti ng polyureth ane sea l ant insta l l ations also are being s ought. Warner ( 1 97 5) reports that, in a fu l l-scale gal l ery test, concrete with steel fi bers was an effective fi re-i nhi biting coating over an ex isti ng u reth ane insta l l ati on . 85
M I N ES AN D B U N K E R S 4.3.5 Hydrocarbon Fuels Gasol ine is not used in U.S. underground mi nes, but diesel fuel traditi on ally h as been used i n metal and non meta l mines to power locomotives, shuttle cars , bu l l dozers, loadi ng mach ines, a n d other equ i pment. The use of diesel o i l in coal m i n es in similar appl ications is fairly recent ; therefore, fire and acc i dent statistics are meager. It is expected, however, that the fi re safety perfo rman ce of d iesel oil i n u ndergrou nd m i nes shou l d somewhat para l lel th at o f hydra u l i c fl u i ds. G as wel ding equ i pment (e.g. , oxygen-acetylene welders ) is common l y used i n mines, but special precautions are taken both before and during wel ding operations. 4.4 Conclusions and Recommendations Conclusion: Materials fl ammabi l ity hazards are of great concern in coal and m i n era l mines because of the l i mited access and egress routes, the need for forced venti lation, and the ever present energetic ign iti on sou rces. I f adequate attention is paid to access rou tes, compartmenta l i zation, and smoke venti l ation systems duri n g desi gn o f bu nkers, materials selecti on problems are the sam e a s in other bu i l d i n gs with comparable occupancies, Conclusion: In sel ecti ng polymeric materials with im proved fi re safety ch aracter istics, competin g functional, economic, and safety req u i rements must be reconci led. Conclusion: Wood represents the largest vol ume of poly meric m aterial used in mines. It ign ites more readi l y than coal and it rots readi l y i n a m ine envi ron ment. I t may b e that rotted wood is a greater fi re haza rd. Recommendation: R eserach to find a practical method for contro l l ing the ease of ign ition and fl ame spread of m i n e ti m bers shou l d be conti n ued. The method of precharri ng wood to a depth of 2 mm to reduce flammabi l i ty shou l d be eva l u ated in a fu l l -scale tri al. Conclusion: The venti l ation cloth used to direct a i r currents during normal working and emergency conditi ons can contri bute to the fi re safety hazard by igniting easi l y and spreading flame rapidly. Recommendation: The tentative gu ide l i ne specifyi n g that venti lation cloth ex hi bit a flame spread rati ng of less than 25 in ASTM Method E- 1 62 shou ld be conti nued unti l evi dence ind icates that it is not sufficient. F i bers (e.g., Nomex ® or PB I ) that can out-perform any now used for venti lation cloth are ava i l able and under devel opment. Conclusion: Conveyor bel ts are a frequent sou rce of fi res because of fricti on agai nst pulleys and accu m u lated spi l l age of conveyed m ateri als. The use of neo prene or plasticized PVC in belts has i mproved the i r fi re safety . Recommendation: Phosphate ester pl astici zers shou ld be used instead of phthal ate esters to fi re retard PVC, provided that the phosphate plasticizer does not increase tox ic smoke genera tion. Conclusion: The flammabi l ity an d smoke tox icity pro blems caused by electrical conductor insu l ation in mines are the same as those confronted in uti l ity applica tions, industrial insta l l ations, and aboard shi ps in that PVC cables i n grou ped cable ways wi l l bu m, spread fl ame, and emit tox ic hydrogen ch loride vapor. Recom- 86
MATE R I A LS menda tion: The present requi rements for cable flammabi l i ty perform ance sh ould be i ncreased. Conclusion: A practical replacement is needed for polyurethane foam as a seal ant for mine tunnel wal ls and ceil i ngs. The presentl y preferred sprayed steel fiber rei nforced concrete is effective but its appl i cation is cu m bersom e. Recom mendation: Mo re convenient meth ods fo r applying the sprayed fi ber rei nforced concrete should .be sought, and aqueous emulsions of cross l i n kable therm oset resins that can be used to seal mine tunnel wal ls and cei l ing shou ld be developed. 4.5 R eferences B. Baum, Study of Coati ngs of I m pro ved Fire and Decay Resistance of Mi ne Ti mbers, NASA· CR- 1 52009, Washi ngton, D.C., 1 977. J, C. Fran klin, et al., Pol ymeric Sealant Used to Stop Shale Degradation in Coal Mines, BuMin⢠- TPR- 103 Washi ngton, D.C., 1 977. Modern Plsstics Encyc/oP«Jis, McGraw-Hi/l ine., Naw York, N. Y., 1975 E. M. M urphy, Flame Spread Eval uation of Venti lation Cloth , Report of /nlltl$tigations 7625, U.S. Bu Mines, Washi ngton, DC, Apri l , 1 972. R, M. Newman, Summary Report, Factory Mutual Research Corp ., F NR C No. 22941 , Nor wood, Mass., 1 976, B. L. Warner, Eval uation of Materials for Protecting Existing Urethane Foam in Mines, BuMintl$ OFR-75-76 (PB 254682) , Washi ngto n , D.C., 1 975. 87
