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Reprinted from Textile Research Journal, Vol. 52, No. 7, July 1982 Pruned in U. &. A~ Properties and Applications of Celanese PBI--Polybenzimidazole Fiber #1 D. R. Coffin and G. A. Serad \ *>{ yv Jr Celanese Fibers Company, Charlotte, North Carolina 28232, U.S.A. H. L. Hicks and R. T. Montgomery Celanese Fibers Marketing Company, Charlotte, North Carolina 28232, U.S.A. ABSTRACT f % Celanese PBI is a derivatized polybenzimidazole fiber possessing a unique balance * of properties that permit it to meet critical cost/performance criteria in demanding \r\ \ \ environments. The fiber, a modification of previously documented earlier versions, T \ / has excellent textile, tactile, thermal, and chemical resistance properties and sig- yl nificantly reduced Same shrinkage, all of which are fully described. These properties ' /Yr' Pcrinit the fiber to be used as an alternative to asbestos, in high temperature filtration fabrics, in thermal protection clothing, and in numerous other high fern* perature and chemical resistance applications. Several specific applications are described along with performance results. A new production facility will provide commercial .quantities beginning in early 1983. Introduction A facility to produce one million pounds per year The introduction of polybenzimidazole (PBI) fi ber, scheduled for commercial production in early 1983, marks a new step forward in high performance lgjjm. It has emerged as a specialty fiber with unique *f|pK)naU chemical, and textile properties that are useful in a variety of critical performance applica tions. ; ' ' '. - . . PBI fiber has ah interesting history which Jackson of Celanese PBI fiber is currently under construction at Rock Hill, South Carolina, and commercial quan tities of the fiber will be available in early 1983. This paper reviews the preparation of PBI fiber and describes its thermal, chemical, and textile prop erties. Several end-use applications are also dis cussed, relating relevant fiber properties to specific end uses. reviewed previously [4]. It was first used in the space program for fabrication of brake parachutes, tether Fiber Preparation lines, and non-flammable protective suits for astro PBI polymer (poly(2,2'-m-phenyiene-5,5'-bibenz- nauts. Investigations of PBI's performance demon imidazole)) is prepared by the reaction of tetra- strated that it had potential for critical industrial as aminobiphenyl and diphenyiisophthalate (Figure j) well as aerospace applications, such as asbestos re in a two-stage, high temperature reaction carried out placement, high temperature filtration, and protec in an inert atmosphere [1]. Equimolar amounts of tive apparel Each application has specific end-use the two monomers are charged to the reactor and requirements that are satisfied by one or more of heated. In the first reaction stage, water and phenol PBI's properties. are evolved as byproducts, causing the polymer mass Development and field trials are underway to dem to foam. The foam is heated an additional one to two onstrate the effectiveness of PBI as a replacement hours at 273s to 300"C and then crushed. In the for asbestos in high temperature applications such second reaction stage this low molecular weight pre as gloves, conveyor belts, and plastic composites. PBI polymer is heated an additional two to three hows is also expected to be an effective fabric bag material at 375* to 400*C. "Hie resulting polymer has an in for filtration of stack gases prior to atmospheric ex herent viscosity of 6.7 to 0.8 dl/g as measured in haust. It offers enhanced protection in clothing for concentrated sulfuric acid. foundary workers, firefighters, pilots, and others who The fiber process following polymerization is risk occupational exposure to fire. Other applications shown in Figure 2. The polymer is put into solution are also being investigated in which PBI may be an under pressure using dimethylacetamide as the sol active alternative to other fibers currently in use. vent. The solution is filtered and converted into fiber (-5173/82/0700-0466*01.00/0 1982 Textile Research Institute July 1982 using a high temperature dry-spinning process. The fiber is subsequently drawn at elevated temperatures to produce the desired mechanical properties. It is then sulfonated in a sequence of steps and made into staple using conventional crimping and cutting tech niques. ,, HU-W {U-toCPfllfNC) -*-T JMCmImOCOU Figure 1. Synthesis of FBI polymer. 467 /mbs Figure 2. FBI fiber process* A significant improvement in this PBI fiber com pared with that produced previously is its low shrink age when subjected to heat or Same; this is achieved through the sulfohation. The sulfonation reaction, mechanism of sulfonation, and structure of the fiber product were examined using Fourier Transform In frared Spectroscopy (FTIR) [2 J. la the non-dcrivatized PBI fiber, NH bond deformation is observed at 1530 cm-1 (Figure 3a). Following sulfuric acid treatment, this absorption is shifted to 1565 cm"* (Figure 3b), consistent with the formation of an amidine cation (Figure 4a). The reaction is reversible at this point, and the amidine structure can be re moved by washing the fiber with water. The amidine-containing fiber is then heated for a short period (less than 30 seconds) at 475-500*C. One possible reaction is sulfonation of the imidazole nitrogen (Figure 4b). Another is sulfonation of the benzene ring fused to the imidazole ring (Figure 4c), Treatment of the fiber with NH4OH causes no re action. In addition, the infrared spectrum shows strong absorptions at 1200 cm-1 and 1040 cm"1 (Figure 3c), which are consistent with the predicted formation of an aryl sulfonic acid. There are also Figure 3. Infrared spectre of a) drawn PBI fiber, b) acid treated PBI fiber, and e) acid treated heat set PBI fiber. H H ; a, H Mot & r -COOt'O - <OCt>o * MO-f " . { " **oJ * H -- " FlOURS 4. Proposed tulfonstion mecbsniimfor PBI fiber. changes in the aromatic substitution region of the spectrum, indicating an additional substituent on this ring. Hence, the treated fiber appears to con tain a pendant aryl sulfonic acid group as shown in Figure 4c. N1BCO001306 468 Textile Research Journal Fiber Properties fiber has a unique combination of thermal, ^.Iv ' -jemical, and textile properties. All fiber properties ' :and applications subsequently described refer to the sulfonated PBI fiber. It has excellent thermal sta bility, is resistant to the effects of many chemicals (including strong acids, bases, and organic chemi cals), processes well on conventional textile equip ment, and when made into a fabric is comfortable to wear. Textile Properties The textile properties of PBI fiber are summarized below. These properties are comparable with many commercial man-made fibers and allow PBI staple to be processed on conventional textile equipment into woven, knitted, and needle-punched fabrics. mined the limiting oxygen index of PBI to be 41% [6]. This compares favorably with their values of 28% for Nomex, 23% for nylon 66. 20% for cotton, and 21% for polyester. ' Thermogravimctric analysis (TGA) measures the weight loss of a material exposed to elevated tem peratures. The test can be conducted either isothcr- mally or using a programmed temperature rise. Fig. ure 5 shows the weight loss of PBI fiber in air and nitrogen as a function of temperature using a pro grammed heating rate of 20"C/min. The weight loss that occurs from 25-I25C is due to loss of mois ture. Rapid degradation starts at about 450C in air and accelerates above 52SC, The amount of deg radation above 525C is considerably less in nitro gen, with over half of the sample weight remaining at 975C. " Textile properties of Celanese PBI fiber Denier per filament Tenacity '. Breaking elongation Initial modulus Crimps per unit length Percent crimp Density oisture regain (at 68 F, 65% RH) <^ling-water shrinkage )Ior 1.5 denier 3.1 g/d 30% 45 g/d 12/in. 30% 1.4 g/cm1 15% 0.5% gold Figure J. Programmed temperature TGA of PBI fiber io air and nitrogen at 20*C/rain. The moisture regain of PBI is. unusually high for an organic fiber, and garments made from PBI are very comfortable to wear. Comfort was demonstrated in a study conducted by the Gillette Research In stitute where PBI and cotton blouses were compared 3]. The study concluded that the PBI and cotton blouses were essentially equivalent in comfort. Figures 6 and 7 show the isothermal weight loss for PBI fiber in air and nitrogen as a function of time at temperatures from 200C to 350C. The rates of weight loss are low, indicating fairly long life for PBI at the exposure temperatures anticipated in long term, high temperature applications* Thermal Stability An outstanding characteristic of PBI is its excel lent stability in flame and high temperature environmeats. This stability can be assessed in several ways, including limiting oxygen index, thermogravlmetric analysis, and exposure to controlled high tempera ture and flaming environments. PBI does not bum in air and does not exhibit any afterglow in the manner of many other high-temperaturc fibers. After flame exposure, PBI fabrics remain supple and maintain integrity. Furthermore, PBI generates little or no smoke when exposed to Ijjune or high temperature. Steutz et at have deter Figure 6. Isothermal TGA of PBI fiber in air. N1BCO001307 JULY 1982 1 469 mm ^0' FIGURE 7. Isothermal TGA of PBI fiber in nitrogen. Figure 9. Effect of exposure time on fabric strip strength at 23 2*C in air, While weight loss in high temperature environ ments is indicative of fiber stability, the property of most interest for many applications is strength re tention after exposure to elevated temperatures. Ta ble I gives representative values for the strength re tention of FBI fiber after exposure to 300C and 4Q0C for 30 and 60 minutes. PBI also retains its strength well when tested during exposure at elevated temperatures, as shown in' Figure 8. Only above 300"C does the tensile strength of the fiber begin to decrease. Figure 9 shows the strip tensile strength of a 322 g/m2 PBI fabric after thermal aging at 232C for periods up to 8 weeks. The fabric retains 95% of its strength after 2 weeks of exposure in air and 66% after 8 weeks. Field trials indicate that PBI also holds up well for short term contact exposures at temperatures as high as 815C. The low thermal shrinkage occurring when PBI fiber is exposed to high temperature or flame is a definite asset in maintaining the physical integrity of protective clothing. The fiber shrinkage, as mea sured by thermomechanical analysis, is approxi mately 3% when the temperature is raised from room temperature to 500C at 20"C/min. As shown in Figure 10, PBI fabric shows no linear shrinkage after 24 hours at! 50C, and only about 3% after the same period at. 316C. When subjected to a flame (--600"C) PBI fiber shrinks 5-10%; however, the level of thermal shrinkage does not increase with longer exposure. This corresponds to approximately 3% fabric shrinkage. Table I. PBI strength retention after exposure to hot air.. Exposure conditions . % Tensile strength retained % Elongation retained 300*C 30 minutes 60 minutes 400*C . 30 minutes 60 minutes 100 too 95 80 too too 50 30 Figure 10. Effect of temperature on linear shrinkage . of PBI fabric after 24 hours of exposure.' Figure 8. Break strength of PBI fiber in air st elevated temperatures. These results all verify the excellent thermal st btlity and low thermal and flame shrinkage of Pi These properties emphasize PBFs utility in rmu high temperature applications. Chemical Resistance In addition to its thermal properties, PBI lijv exhibits outstanding resistance to many chemica. These include strong acids and bases, organic chen.icals. and high pressure saturated steam. i l ' t. f i If ` f NIBCO001308 Textile Research Journal (I gives the tensile strength retention values .iber after exposure to strong acids. At these /P nt conditions the tensile strength is. largely ^ ;cted. Table III shows similar data for PBI in - ions of strong inorganic bases. Here too, strength .ntion is good. These results illustrate PBI's re* tance to extreme acid and base exposures--sig.ficant attributes for both gaseous and liquid filtra* ion applications. Applications PBI fiber's combination of textile, thermal, and chemical properties offers many commercial oppor tunities in the high performance fiber market. Us textile properties (tensile, crimp, fiber diameter, and length) allow it to be processed on cotton, woolen, or core-spun systems, a versatility that simplifies PBf's use. The fiber can be processed in 100% form and into engineered yams and fabrics. Blending with Table II. Tensile strength after immersion in inorganic acids. other fibers and yarns can achieve an optimum cost/ performance ratio. PBI's thermal properties (high Concen tration, Acid . % Sulfuric add Sulfuric acid Hydrochloric add Hydrochloric acid Nitric odd Nitric acid . 50 so 35 10 70 10 Temper Tensile ature, Time, strength c houir retained. % 30 144 70 24 30 144 70 24 30 144 70 48 90 90 95 90 100 90 flame and contact*heat resistance, low shrinkage) make it a candidate for asbestos replacement in spe cific end uses. Alternatively, PBI's thermal properties and resistance to acid make it attractive as a high temperature (150"C-200"C) filtration- fabric for coal-fired boilers. Development and end-use trials are being conducted for asbestos replacement, high tem perature filtration, and apparel/industrial fabrics, PBI's unique properties enable cost effective use Table HI. Tensile strength after immersion in inorganic bases. in many high-performance fiber applications. Be cause it performs longer and better compared with Concen Temper Tensile tration, ature, Time, itrength Base % *C hours retained, % Sodium hydroxide to 30 144 Sodium hydroxide 10 . 93 2 ^o.tassium. hydroxide 10 25 24 95 65 SS existing fibers, a net savings may be realized. Asbestos Replacement Many fibers are competing as textile asbestos re placements. Asbestos' versatility is demonstrated by its wide variety of end uses. No one fiber is expected PBI fiber is very stable in steam. No loss in tensile properties occurs after 3 hours in 50 psig (150C) steam. In 140 psig steam (182#C) little or no strength loss occurs even after 16 hours of exposure. Resistance to organic chemicals is another valu* able attribute of PBI. The chemicals listed in Table IV have no effect on the tensile properties of the fiber to capture a dominant share of the asbestos market; rather certain fibers or blends of fibers will fill sectors where cost effectiveness can be realized. Some mar ket segments in which PBI is expected to be cost effective are high temperature gloves, glass manu facturing applications, and high temperature com posites. PBI trials in high temperature gloves have been after one week of exposure, even though some are in end uses where previously only asbestos would actually solvents for the precursor polymer. perform adequately, including gloves for foundries, aluminum extrusion, and metal treatment The Table IV. Tensile strength after immersion in organic, chemicals. gloves are typically subjected to severe abrasion and Short duration (5-20 seconds) temperatures up to 815C. In numerous field trials, a safety garment Compound - Acetic acid Methanol Perchloroethylene Dmethylacetttmide Dimethylformamide Dlmethylsulfoxide Kerosene Acetone jgasoline Temperiture. . *c ` 30 30 30 30 30 30 30 30 30 Time. hours 168 168 168 168 168 168 168 168 168 Tensile strength retained, % 100 100 100 too 100 100 too 100 100 manufacturer reported that gloves containing PBI outlasted asbestos two to nine times, and were cost effective. . .. PBI offers a suitable substitute for asbestos in sev eral areas of glass manufacturing. Potential end uses include conveyor belts, bumper pads, suction pads, and tong jaw covers. For a fiber to perform in these areas, it must withstand temperatures up to 425C and provide a resilient surface so as not to mar the glass. In one application, an. industrial textile man- NIBCO001309 July 1982 471 ufacturer concluded that a PBI conveyor belt was cost effective by outlasting asbestos two to one. No surface damage was detected in the glass. Similar results are expected in other areas of glass manu facturing. retained 70% of its original strength (see Figure 11). Strength retention stabilized after three to four months and did not decrease for the remaining test penod, suggesting excellent longevity for this appli cation. Flue-Gas Filtration PBI's chemical, thermal, and physical properties suggest that it will perform well as a flue gas filter fabric for coal fired boilers. Increased demand for an acceptable filter medium is expected as more in dustrial and utility plants convert to coal to avoid high petroleum costs. Burning coal presents three problems: fly ash col 0t4 * ttOtfTHt DTOIO lection, formation of acidic flue gases, and high Hue temperatures (150-200C). Low sulfur coal is used Figure 11. Strength retention (it, w X 0 of PBI fabric after exposure to coal-fired boiler five gas. where possible to minimize flue gas acidity and sub sequent acid rain. Fly ash has been traditionally col lected by electrostatic precipitators, but their re moval efficiency decreases when low sulfur coal Is used. Low sulfur coal ash has a reduced ionization potential requiring larger,. more expensive elec trostatic precipitators. Consequently, filter units equipped with fabric bags are being used to collect the ash at higher efficiency and lower cost. Field evaluations are currently in progress. These involve up to several hundred PBI bags located in various coal fired boiler baghouses. Some bags have operated for 17 months with no failures. Although development activity to date has concentrated on flue gas filtration of coal fired boilers, future tests will be conducted in other filtration end uses, for example, incinerators and high temperature dust collection. Few fabrics can withstand the acidic and high tem perature conditions encountered in coal fired boiler Apparel/Industriajl Fabrics flue gas. The filter bags must also withstand the PBI fiber is an excellent candidate for safety ap abrasion caused by periodic cleaning cycles to re parel, particularly flight suits and firefighter's uni move accumulated dust. Bags arc typically cleaned forms. PBI was used in the Apollo and Sky Lab by mechanical shaking, reverse air, oc compressed programs in Sight suits and underwear and is cur air, all of which create yam-to-yam and fabric-to- rently worn as escape suits by the Columbia space metal abrasion. Specially treated glass fabric is the shuttle astronauts during launching and landing. principal material currently used for filter bags. Flame and thermal resistance are key properties While performance is generally satisfactory, pro of Right suits and other protective apparel. PBI was longed use leads to strength reduction and eventual evaluated in JP-4 fuel fine pit tests conducted by the failure because of high temperature acid attack and U,S. Army at their Clothing, Equipment and Ma abrasion. Results from lab studies show that PBI - terials Laboratory in Natick, Massachusetts [5]. A fabric has good abrasion characteristics. Abrasion 153 gm/m5 PBI fabric was compared to 146 gm/m1 performance coupled with acid resistance and high Nomex and 339 gm/m1 fire retardant (FR) cotton temperature properties indicate PBI may be a can fabrics. Flight suits constructed from the fabrics didate for this application. were exposed to the JP-4 fuel fire for three seconds - PBI development is currently concentrated on a while being worn by mannequins equipped with tem particular technology, f.e., needle-punched felt bags perature sensors to predict body damage. The PBI in pulse jet cleaned baghouses. Compared with con clothed mannequin exhibited 2.05% body damage ventional woven glass fabrics, such a system offers (Class C .and D: burns) at a heat flux of 7.14 cal/ the possibility of higher air-to-cloth ratios, thus re cm1 s. Body damage rates for Nomex 452, Nomex quiring less fabric and supporting hardware. The 456 (E-U ), and FR cotton were 24.0% at 7.7 cal/ lower capital and operating costs are expected to cm2-s, 19.84% at 7-29 cal/cm1-*, and 34.2% at 6.5 make PBI cost effective in this application. cal/cm1 s, respectively. According to the Army, 20% Trials to date are encouraging. PBI fabric exposed body damage by Class C and D burns requires pro to coal fired boiler flue gas at I80*C for 13 months longed specialized medical treatment. * .* NIBCO001310 Textile Research Journal An effort is currently underway to upgrade fire men's protective apparel through the United States % Administration's "Project FIRES." PBI is one -the candidatesTteing evaluated .re:- the shell of the turnout jacket and pants. The vano'as test garments will be worn and compared by a ire departments across the United States by tSj ar.d of 1982. The applications discussed her? suggest that PBI will be a cost effective fiber W e believe these ap plications are just scratching the surface of ultimate end uses. Other applications to be evaluated are dam age control cloth to provide pcotaecton from molten metal, e.g,, during welding; atrera:: interior fabrics, such as seat foam encapsulated, and papermaker dryer fabric, which must restx; higE temperature and pH conditions. Summary Celancse initially developed PBI as a high tem perature fiber for the space program. Commercial opportunities for the fiber accelerated rapidly in the last three to four years due to the development of a dcrivatized PBI with a unique Meed of textile, ther mal, and chemical properties. PBI erd uses have been expanded from space and military to include asbestos replacements, high temperature filtration, and var- ious apparel/industriat applications. Given the prop erties of PBI and the needs of our technical society, many more applications are expected to emerge Commercial quantities of PBI will be available in early 1983 to satisfy these needs. Literature Cited l. Chcnevey, E. C., and Conciatori, A. B.. U.S. Pater. 3,549,603 to Celanese Corp., Process Tor the IVi- merization of Aromatic Polybenzimidazoles, Dec, II 1970. Davitt, R., and Kemmerer, R., unpublished research. Celanese Research Co., Summit, NJ. 3. Howard, M. E., Morin, C. J.. and Hollies, N. R, S_ A Comfort Comparison of Polyester Variations in Knc Blouses on Women, Gillette Research Institute. Jan. 15, 1980. ` 4. Jackson, R. R, PBI Fiber and Fabric--Propertic* arc Performance, Textile Res. J. 48, 314-319 (197S). 5. Sousa, J, A,, Caldarclla. G. J.. Rouch, J, F,, Levasscu:. L. A., and Remy, D. E., Assessment of Clothing ter.. Thermal Protection, Technical Report, Natick TR-8C 013, 1980, pp. 26, 30. 6. Steutz, D. E., DiEdwards, A. E.. Zitomer, F,, Barnes, B. P,, Polymer FlammabiHty--II, J. Poh Sci., Poiynt Ckem. Ed. 18, 987-1009 (1980). N1BCO001311