International Journal of Applied Glass Science 312)122-136(2012) 1112041 N TERNATIONAL JOURNAL OF Applied GI ass sC丨ENCE Glass Fiber-Reinforced Composites: From Formulation to application Joy M. Stickel and Mala Nagarajan* Owens Corning Science d Technology, 2790 Columbus Rd, Granville, Ohio 43023 Glass fiber-reinforced composite materials are attractive because their properties can be tailored to meet the specific needs a variety of applications. The mechanical and thermal properties of a composite generally follow the rule of mixtures glass hiber is the major component at 70-75% by weight (50-60% by volume), selection of the correct glass product is criti- al. Glass fiber reinforcement is available in many forms, including continuous rovings, chopped fibers, fabrics, and nonwoven mats. In addition to form, selection of a reinforcement product involves choosing a glass type, chemistry on the glass(sizing) filament diameter, and tex. Glass formulation or type governs mechanical, thermal, and corrosion properties, whereas sizing protects the glass during handling and gives compatibility with the resin system. Filament diameter and strand tex are choser to balance physic erties and manufacturing efficiency. A signifcant amount of tensile strength, up to 50%, may be lost from a pristine single filament to a multi-filament roving. To minimize this degradation, the utmost care and consistency must be exercised in the fber forming process. This, coupled with selection of a high-performance glass formulation, enables use of composites in highly demanding applications, such as pressure vessels and ballistic armor Introduction fiberized, the glass be treated as gently and carefully as possible. An organic coating(sizing) is applied to the The properties of a composite material are gov- glass surface during the forming process that lubricates erned by the properties of the fiber used to reinforce it. and protects the glass by minimizing abrasion when When tensile strength and toughness must be maxi- individual filaments contact one another. Extreme care mized,glass fiber is the reinforcement of choice. Inno- is taken throughout the manufacturing process vations in glass fiber formulation allow for strengths on whether the glass fiber product is wet or dry,continu lly important, for high- ous or chopped, from forming through pack-out-to volume manufacturing by a direct-melt process at tensile strength is pre To ensure that the properties of the glass formul: By exercising this care in manufacturing, glass fiber tion are realized in a composite part requires that once tensile strength can be translated to composite laminate tensile strength. The term"composite" is used through out this article to refer to polymer or resin-based Ceramic Sociery and wiley Periodicals, Inc composite materials. Two strength-driven applications
Glass Fiber-Reinforced Composites: From Formulation to Application Joy M. Stickel and Mala Nagarajan* Owens Corning Science & Technology, 2790 Columbus Rd, Granville, Ohio 43023 Glass fiber-reinforced composite materials are attractive because their properties can be tailored to meet the specific needs of a variety of applications. The mechanical and thermal properties of a composite generally follow the rule of mixtures. As glass fiber is the major component at 70–75% by weight (50–60% by volume), selection of the correct glass product is critical. Glass fiber reinforcement is available in many forms, including continuous rovings, chopped fibers, fabrics, and nonwoven mats. In addition to form, selection of a reinforcement product involves choosing a glass type, chemistry on the glass (sizing) filament diameter, and tex. Glass formulation or type governs mechanical, thermal, and corrosion properties, whereas sizing protects the glass during handling and gives compatibility with the resin system. Filament diameter and strand tex are chosen to balance physical properties and manufacturing efficiency. A significant amount of tensile strength, up to 50%, may be lost from a pristine single filament to a multi-filament roving. To minimize this degradation, the utmost care and consistency must be exercised in the fiber forming process. This, coupled with selection of a high-performance glass formulation, enables use of composites in highly demanding applications, such as pressure vessels and ballistic armor. Introduction The properties of a composite material are governed by the properties of the fiber used to reinforce it. When tensile strength and toughness must be maximized, glass fiber is the reinforcement of choice. Innovations in glass fiber formulation allow for strengths on par with carbon fiber, and equally important, for highvolume manufacturing by a direct-melt process. To ensure that the properties of the glass formulation are realized in a composite part requires that once fiberized, the glass be treated as gently and carefully as possible. An organic coating (sizing) is applied to the glass surface during the forming process that lubricates and protects the glass by minimizing abrasion when individual filaments contact one another. Extreme care is taken throughout the manufacturing process — whether the glass fiber product is wet or dry, continuous or chopped, from forming through pack-out — to ensure that tensile strength is preserved. By exercising this care in manufacturing, glass fiber tensile strength can be translated to composite laminate tensile strength. The term “composite” is used throughout this article to refer to polymer or resin-based composite materials. Two strength-driven applications *Mala.Nagarajan@owenscorning.com © 2012 The American Ceramic Society and Wiley Periodicals, Inc International Journal of Applied Glass Science 3 [2] 122–136 (2012) DOI:10.1111/j.2041-1294.2012.00090.x
amics.org/lAGS Glass Fiber-Reinforced Composit 123 for composites, pressure vessels an Raw discussed. Although very different Materials demand high strength, excellent ngth of the load-bear- Furnace g composite prevents a pressure vessel from exploding and prevents a projectile from penetration of an armor panel. In both cases, lives are at stake. The strength of the composite is critical for safety, Is imperative that the glass fiber reinforcement the source of Batch House Forehearth strength- be consistently high 自 Coating Fabrication Winding Glass Fiber Forming Process Fig. I. Fibergl As discussed in the previous paper, Strength of Formulation also governs whether a glass can be Higb Performance Glass Reinforcement Fiber, assuming fiberized. In some formulations, a small amount of fibers are treated equally and care important contributor to glass fiber tensile strength is strength or modulus may have to be sacrificed to widen formulation. There are a variety of glass fiber types as the forming temperature range there by allowing large defined in American Society for Testing and Materials scale industrial production by a direct melt process. (ASTM)D578: Standard Specification for Glass Fiber Direct melt furnaces of this type typically produce Strands and International Organization for Standardiza- 20,000 tons or more annually, and are preferred to tion(ISO)2078: Textile glass- Yarns- Designation maximize manufacturing efficiency and product consis- p standards describe key characteristics of each tency. A high-level schematic of direct-melt fiberglass manufacturing is given in Fig. I Raw materials that arrive by rail or truck ranges. Although there is a multitude of glass fiber batched on site and conveyed to the furnace for melt- types available, the most important from a tensile trength perspective are listed in Table I ing. Molten glass slowly cools as it travels through a channel and out to multiple forehearths, where it is further conditioned before fiberizing. Each forehearth Table L. Common Glass Fiber Types Used in feeds multiple forming positions ing or fibe ng fb Description through tiny orifices in a precious metal bushing. Labo- ratory bushings can have as few as one hole, whereas Alumino-borosilicate family of glasses high-throughput production bushings typically have inally developed for electrical 1000 or greater. Figure 2 shows a glowing hot glass applications; has found widespread fiber bushing and a close-up image of fibers exiting a use in nearly all glass fiber-reinforced bushing. The first direct-melt furnace for continuous fiber production, developed by Owens Corning in E-CR Corrosion-resistant E-glass; equal or 1961, had bushings with throughputs on the order of better mechanical properties and little 10 Ib/h. Bushing technology development at Owens or no cost disadvantage versus standard E Corning has focused on reducing the amount of High-strength glass with performance high-cost alloy required while increasing bushing ther intermediate to e and s mal-mechanical stability, and thus operational life and A family of glasses composed primarily of efficiency. Today, Advantex" glass fiber bushings are the oxides of magnesium, aluminum, capable of throughputs exceeding 300 Ib/h with life- and silicon; S-glass was developed for times greater than a year. high strength and modulus plus superior thermal and corrosion performance registered trademark of Owens Corning
for composites, pressure vessels and ballistic armor, are discussed. Although very different, both applications demand high strength, excellent durability, and most importantly, reliability. The strength of the load-bearing composite prevents a pressure vessel from exploding and prevents a projectile from penetration of an armor panel. In both cases, lives are at stake. The strength of the composite is critical for safety, so it is imperative that the glass fiber reinforcement — the source of strength — be consistently high. Glass Fiber Forming Process As discussed in the previous paper, Strength of High Performance Glass Reinforcement Fiber, assuming fibers are treated equally and carefully, the next most important contributor to glass fiber tensile strength is formulation. There are a variety of glass fiber types as defined in American Society for Testing and Materials (ASTM) D578: Standard Specification for Glass Fiber Strands and International Organization for Standardization (ISO) 2078: Textile glass — Yarns — Designation. These standards describe key characteristics of each glass type and prescribe acceptable compositional ranges. Although there is a multitude of glass fiber types available, the most important from a tensile strength perspective are listed in Table I. Formulation also governs whether a glass can be fiberized. In some formulations, a small amount of strength or modulus may have to be sacrificed to widen the forming temperature range there by allowing large scale industrial production by a direct melt process. Direct melt furnaces of this type typically produce 20,000 tons or more annually, and are preferred to maximize manufacturing efficiency and product consistency. A high-level schematic of direct-melt fiberglass manufacturing is given in Fig. 1. Raw materials that arrive by rail or truck are batched on site and conveyed to the furnace for melting. Molten glass slowly cools as it travels through a channel and out to multiple forehearths, where it is further conditioned before fiberizing. Each forehearth feeds multiple forming positions. Forming or fiberizing is achieved by pulling fibers through tiny orifices in a precious metal bushing. Laboratory bushings can have as few as one hole, whereas high-throughput production bushings typically have 1000 or greater. Figure 2 shows a glowing hot glass fiber bushing and a close-up image of fibers exiting a bushing. The first direct-melt furnace for continuous fiber production, developed by Owens Corning in 1961, had bushings with throughputs on the order of 10 lb/h. Bushing technology development at Owens Corning has focused on reducing the amount of high-cost alloy required while increasing bushing thermal-mechanical stability, and thus operational life and efficiency. Today, Advantex® glass fiber bushings are capable of throughputs exceeding 300 lb/h with lifetimes greater than a year.† Table I. Common Glass Fiber Types Used in Composite Applications1 Glass type Description E Alumino-borosilicate family of glasses originally developed for electrical applications; has found widespread use in nearly all glass fiber-reinforced composites E-CR Corrosion-resistant E-glass; equal or better mechanical properties and little or no cost disadvantage versus standard E R High-strength glass with performance intermediate to E and S S A family of glasses composed primarily of the oxides of magnesium, aluminum, and silicon; S-glass was developed for high strength and modulus plus superior thermal and corrosion performance Fig. 1. Fiberglass manufacturing schematic. † Advantex® is a registered trademark of Owens Corning. www.ceramics.org/IJAGS Glass Fiber-Reinforced Composites 123
International Journal of Applied Glass Science--Stickel and Nagarajan Vol.3,No.2,201 Fig. 2. Glass fiber bushing(L), close-up image of bushing tips(R) Fig 3. Type 30 or direct roving() and multi-end or assembled roving(R After exiting the bushing, glass fibers are rapidly Chopped fiber applications will not be discussed in cooled, gathered, coated with sizing, and in the case of depth in this article, but fibers can also exit the bushing continuous, high-strength fibers, fed to a winder. and be fed directly to a chopper instead of a winder Winders are configured to produce direct, ready-to-use Chopped fibers are available wet or dry in a variety of rovings or an intermediary form called a forming cake. lengths. These are used as inputs for nonwoven mats After drying, direct or Type 30 rovings (pictured in and both thermoset and thermoplastic compounds. A Fig. 3)are ready for use by the composite fabricator or schematic illustrating how continuous and chopped glass fabric weaver. The glass fiber strand is typicall glass fiber reinforcements are produced is given in pulled from the inside of the Type 30 package, but it Fig. 4 can also be pulled from the outside if the fabricator has ropriate u Assembled or multi-end rovings are produced Key Product Parameters for Continuous Fibers ng several Type 30 or forming cake inputs, and can be produced with or without an interior bobbin or A combination of bi allow multiple strands or yarns to be combined into a linear density, of the bundle of filaments. These Pilana tube. Assembled rovings, shown on the right in Fig. 3, chopper pull rate controls filament diameter and tex, ngle spool, which results in higher glass application eters, filament diameter and tex, are critical to the efficiencies for the fabricator. Note the multiple strands performance of the glass fiber and thus to the compos and the smaller inside diameter of the assembled roving ite as a whole. For this reason, filament diameter and spool versus the Type 30 spool tex must be carefully monitored and controlled during
After exiting the bushing, glass fibers are rapidly cooled, gathered, coated with sizing, and in the case of continuous, high-strength fibers, fed to a winder. Winders are configured to produce direct, ready-to-use rovings or an intermediary form called a forming cake. After drying, direct or Type 30 rovings (pictured in Fig. 3) are ready for use by the composite fabricator or glass fabric weaver. The glass fiber strand is typically pulled from the inside of the Type 30 package, but it can also be pulled from the outside if the fabricator has the appropriate unwinding equipment. Assembled or multi-end rovings are produced using several Type 30 or forming cake inputs, and can be produced with or without an interior bobbin or tube. Assembled rovings, shown on the right in Fig. 3, allow multiple strands or yarns to be combined into a single spool, which results in higher glass application efficiencies for the fabricator. Note the multiple strands and the smaller inside diameter of the assembled roving spool versus the Type 30 spool. Chopped fiber applications will not be discussed in depth in this article, but fibers can also exit the bushing and be fed directly to a chopper instead of a winder. Chopped fibers are available wet or dry in a variety of lengths. These are used as inputs for nonwoven mats and both thermoset and thermoplastic compounds. A schematic illustrating how continuous and chopped glass fiber reinforcements are produced is given in Fig. 4. Key Product Parameters for Continuous Fibers A combination of bushing design and winder or chopper pull rate controls filament diameter and tex, or linear density, of the bundle of filaments. These parameters, filament diameter and tex, are critical to the performance of the glass fiber and thus to the composite as a whole. For this reason, filament diameter and tex must be carefully monitored and controlled during Fig. 2. Glass fiber bushing (L), close-up image of bushing tips (R). Fig. 3. Type 30 or direct roving (L) and multi-end or assembled roving (R). 124 International Journal of Applied Glass Science—Stickel and Nagarajan Vol. 3, No. 2, 2012
ramics.org/lAGS Glass Fiber-Reinforced Composit Forehearth Water spray WIlIIApplicat Traverse Direct-draw oppe Direct chopped strand forming. These parameters will be discussed in depth in S-glass fibers are available across the full range, but the following section, along with another important fac- finer filament diameters are favored for applications tor mentioned briefly earlier: sizing. Sizing, also known requiring the highest tensile strength as a binder or finish, is the chemistry applied to the Finer filaments- made in production into multi glass to give compatibility with the resin matrix in filament strands show higher strengths than their is critical to the performance of the finished com- comes a lower likelihood of failure-inducing law ri c it will be used. Like filament diameter and tex, coarser counterparts because with their lower volum ensure consistent sizing composition Ppl posite, so it is equally critical that glass fiber producers fibers, especially of high-performance, high-melt viscos ity formulations, are significantly more difficult to form for this same reason Solid inclusions, whether from the refractory that lines the furnace, from devitrified glass Filament diameter arising from a cool spot in the forehearth, from con- amination, or from an inhomogeneity in the batch, Filament diameter is set and monitored by winder can cause filament breakage that cascades across th ed or pull rate. It is also verified periodically by bushing. Larger filaments can tolerate lar microscopy. Filament diameter of continuous glass inclusions, thus making the forming process is some fibers typically ranges from 9 to 24 um, although it what more forgiving and easier can be as low as 3 um. For conventional E- or E-CR glass, as described in Table I, diameters are usually at 'This is not true for pristine fibers; as discussed in the previous artide, filament diameter the higher end of this range. High-performance R oes not affect pristine fber tensile strength
forming. These parameters will be discussed in depth in the following section, along with another important factor mentioned briefly earlier: sizing. Sizing, also known as a binder or finish, is the chemistry applied to the glass to give compatibility with the resin matrix in which it will be used. Like filament diameter and tex, sizing is critical to the performance of the finished composite, so it is equally critical that glass fiber producers ensure consistent sizing composition and application. Filament Diameter Filament diameter is set and monitored by winder speed or pull rate. It is also verified periodically by microscopy. Filament diameter of continuous glass fibers typically ranges from 9 to 24 µm, although it can be as low as 3 µm. For conventional E- or E-CRglass, as described in Table I, diameters are usually at the higher end of this range. High-performance R- or S-glass fibers are available across the full range, but finer filament diameters are favored for applications requiring the highest tensile strength. Finer filaments — made in production into multi- filament strands — show higher strengths than their coarser counterparts because with their lower volume comes a lower likelihood of failure-inducing flaw.§ Fine fibers, especially of high-performance, high-melt viscosity formulations, are significantly more difficult to form for this same reason. Solid inclusions, whether from the refractory that lines the furnace, from devitrified glass arising from a cool spot in the forehearth, from contamination, or from an inhomogeneity in the batch, can cause filament breakage that cascades across the entire bushing.3 Larger filaments can tolerate larger inclusions, thus making the forming process is somewhat more forgiving and easier. Fig. 4. Fiberglass forming process.2 § This is not true for pristine fibers; as discussed in the previous article, filament diameter does not affect pristine fiber tensile strength. www.ceramics.org/IJAGS Glass Fiber-Reinforced Composites 125
International Journal of Applied Glass Science-Stickel and Nagarajan Vol.3,No.2,201 impossible d locais un it re that every filament is the same length and iformly and in the absence of a resin matrix to load transfer between the filaments this method yields low values with poor repeatability. For these reasons, ASTM D2343 impregnated strand testing is preferred. The tensile strength data in Fig. 5 was generated on 4000-filament rovings utilizing the same multi-com patible sizing, MCX21. Rovings were produced and tested during April 2011. Sample size was 30 or for each filament diameter. As shown, there is FiLament Dameter (n) tion in tensile strength on the order of 15% wl ter is increased from 9 to I' Fig. 5. Impregnated strand tensile strength verus flament In addition to higher strengths another reason fine high-performance fibers are preferred is for downstream The tex of a roving is its linear density in g/km processability in composite applications. These fibers Tex typically ranges from 300 to 4800 and is governed are significantly stiffer, up to 25%, relative to standard by the bushing design(the number of holes)and th reinforcing fibers. In composite processes like filament desired filament diameter. For example, the rovings later, rovings must used to generate the data in the previous section were pass through guide eyes and around tight radii en route produced on the same 4000-hole bushing. For 9 um to the composite part. Finer filaments will more readily filaments, a 675 tex roving strand is produced. When bend around these contact points whereas coarser fila- 17 um filaments are required, the winder speed is ments can break and fuzz reduced and the resultant bundle tex is 2400 Figure 5 shows impregnated strand tensile strength Clearly, a 2400 tex strand will provide significantly data, generated by ASTM D2343, for epoxy-impreg- more glass-on-the-part coverage for a given distance nated Owens STrand"S-glass when making (Owens Corning, Toledo, OH, USA)at different fila- rovings are often preferred for low to medium strength ment diameters. XStrand", developed for inc ndustrial applications where part manufacturing efficiency may applications, is part of a family of high-perfc rmance be the more important driver to the composite fabrica- reinforcements(HPR) with improved strength, stiffness, tor. In addition, fabricators may have physical limita and temperature stability versus conventional reinforce- tions in terms of shop floor or creel rack space, leading ments.Other products in the HPR line include Flite- them to prefer input rovings of higher tex. For this Strand, ShieldStrand", and WindStrand. Target reason, high-performance reinforcements like XStrand application areas are aerospace, defense/security, and S, are frequently offered as multi-end rovings: tensile ind energy, respectively strength can be preserved as a result of the fine filament The ASTM D2343: Standard Test Method for Ten- diameter and manufacturing efficiency maximized by sile Properties of Glass Fiber Strands, Yarns, and Rovings combining multiple ends Used in Reinforced Plastics method is used frequently as Tex consistency is important to ensure consistent an indicator of glass tensile performance. Single, pristine glass fiber content in a composite part. This is of particu filament tensile testing does not accurately represent lar significance in high-performance applications, such as how a glass will perform when bundled into a roving of aerospace components, which demand the highest several thousand-filaments. Unfortunately, rovings can- strength at the lowest weight. In defense and security not accurately be tested when un-impregnated with applic /ing correct, consistent tex in a rovin resin. A dry strand tensile testing procedure (ASTM ensures that weight and tensile strength are balanced D2256: Test Method for Tensile Properties of Yarns by the both the reinforcement fabric and the armor made Single-Strand Method) does exist. However, because Tex measurement is a standard quality control
In addition to higher strengths another reason fine high-performance fibers are preferred is for downstream processability in composite applications. These fibers are significantly stiffer, up to 25%, relative to standard reinforcing fibers. In composite processes like filament winding, which will be discussed later, rovings must pass through guide eyes and around tight radii en route to the composite part. Finer filaments will more readily bend around these contact points whereas coarser filaments can break and fuzz. Figure 5 shows impregnated strand tensile strength data, generated by ASTM D2343, for epoxy-impregnated Owens Corning XStrand® S-glass rovings (Owens Corning, Toledo, OH, USA) at different filament diameters. XStrand®, developed for industrial applications, is part of a family of high-performance reinforcements (HPR) with improved strength, stiffness, and temperature stability versus conventional reinforcements. Other products in the HPR line include FliteStrand®, ShieldStrand®, and WindStrand®. Target application areas are aerospace, defense/security, and wind energy, respectively. The ASTM D2343: Standard Test Method for Tensile Properties of Glass Fiber Strands, Yarns, and Rovings Used in Reinforced Plastics method is used frequently as an indicator of glass tensile performance. Single, pristine filament tensile testing does not accurately represent how a glass will perform when bundled into a roving of several thousand-filaments. Unfortunately, rovings cannot accurately be tested when un-impregnated with resin. A dry strand tensile testing procedure (ASTM D2256: Test Method for Tensile Properties of Yarns by the Single-Strand Method) does exist. However, because it is impossible to ensure that every filament is the same length and loads uniformly and in the absence of a resin matrix to facilitate load transfer between the filaments, this method yields low values with poor repeatability. For these reasons, ASTM D2343 impregnated strand testing is preferred. The tensile strength data in Fig. 5 was generated on 4000-filament rovings utilizing the same multi-compatible sizing, MCX21. Rovings were produced and tested during April 2011. Sample size was 30 or greater for each filament diameter. As shown, there is a reduction in tensile strength on the order of 15% when filament diameter is increased from 9 to 17 µm. Tex The tex of a roving is its linear density in g/km. Tex typically ranges from 300 to 4800 and is governed by the bushing design (the number of holes) and the desired filament diameter. For example, the rovings used to generate the data in the previous section were produced on the same 4000-hole bushing. For 9 µm filaments, a 675 tex roving strand is produced. When 17 µm filaments are required, the winder speed is reduced and the resultant bundle tex is 2400. Clearly, a 2400 tex strand will provide significantly more glass-on-the-part coverage for a given distance when making a composite laminate, so higher tex rovings are often preferred for low to medium strength applications where part manufacturing efficiency may be the more important driver to the composite fabricator. In addition, fabricators may have physical limitations in terms of shop floor or creel rack space, leading them to prefer input rovings of higher tex. For this reason, high-performance reinforcements like XStrand® S, are frequently offered as multi-end rovings: tensile strength can be preserved as a result of the fine filament diameter and manufacturing efficiency maximized by combining multiple ends. Tex consistency is important to ensure consistent glass fiber content in a composite part. This is of particular significance in high-performance applications, such as aerospace components, which demand the highest strength at the lowest weight. In defense and security applications, having correct, consistent tex in a roving ensures that weight and tensile strength are balanced in both the reinforcement fabric and the armor made from it. Tex measurement is a standard quality control Fig. 5. Impregnated strand tensile strength versus filament diameter. 126 International Journal of Applied Glass Science—Stickel and Nagarajan Vol. 3, No. 2, 2012
amics.org/lAGS Glass Fiber-Reinforced Composites viscosity, affinity for the glass, and applicator physical Tex on Position 2 parameters, such as roll speed and material. This amount varies by product form. Chopped strands for making wet-formed mats generally have the least sizing, measured as loss on ignition (LOD), whereas rovings for spray-up or sheet molding compound have the most. 6 Typical LOI or strand solids values for high-perfor mance single- or multi-end rovings in the 0.5-1.0% range. LOI measurement is a standard quality control evaluation that is included on all Owens Corning certificates of analysis. Moisture is removed first to a 50a0snsa01300 ensure an accurate measure of solids. Moisture content is also reported on quality certificates and must not Fig. 6. Tex variation in Owens Corning Shields exceed 0.15% s rovings. The specific components used in a sizing are selected to give the glass fiber reinforcement compati- evaluation and is reported on all Owens Corning certifi- bility with a certain family of resins. Common resins cates of analysis. Figure 6 is a tex control chart from sev- for use in composites include polyester, vinyl ester, eral months of production of ShieldStrand S roving epoxy, phenolic, and polyurethane, plus a vast array of (Owens Corning). Target tex is 362, and lower and thermoplastic fiber with the corred upper control limits are 333 and 391 tex, respectively. is critical to composite mechanical performance and These values were determined in accordance with the durability. A fiber with an inappropriate sizing will not military detail specification for glass fiber-reinforced form an optimal bond with the resin and load will not composite armor, MIL-DTL 64154B, which requires tex be transferred efficiently amongst the load bearing variation to be within +8% fibers. Furthermore, these poorly bonded regions will weee tex is the most common measure of glass strand be the weak link in the system and failure may initiate ght used worldwide. In the United States, yield is within them once load is applied. Even if part failure also used as an indicator of strand weight. Yield is does not occur, the weak interphase will be susceptible reported in yards per pound (yd/Ib). Yield is inversely to microcracking, which allows ingress of environmen- proportional to tex, and can be calculated by dividing tal media and can accelerate corrosion 496055 by tex. Sizing and Resin Compatibility Iry aafter st h boll a glass fiber sizing is a water-based coating applied resin matrix in which it will be used. Sizings protect 2 the glass and improves processing both in fiber forming 50.6 and in composite manufacturing. Sizings are made up of four primary components: film formers, lubricants, coupling agents, and water. Other additives sometimes used in sizings include anti-statics, wetting agents, chopping aids, antioxidants, and pH modifiers. Sizings generally contain 0.05-10% solids, and the remaining balance is water. Sizings may also be nonaqueous, but these are not as common as water-based formulations he amount of active solids that ends up on the Fig. 7. ASTM D2344 short-beam shear testing of Strand S glass surface is related to the sizings solids content
evaluation and is reported on all Owens Corning certifi- cates of analysis. Figure 6 is a tex control chart from several months of production of ShieldStrand® S roving (Owens Corning). Target tex is 362, and lower and upper control limits are 333 and 391 tex, respectively. These values were determined in accordance with the military detail specification for glass fiber-reinforced composite armor, MIL-DTL 64154B, which requires tex variation to be within ±8%. Tex is the most common measure of glass strand weight used worldwide. In the United States, yield is also used as an indicator of strand weight. Yield is reported in yards per pound (yd/lb). Yield is inversely proportional to tex, and can be calculated by dividing 496055 by tex. Sizing and Resin Compatibility A glass fiber sizing is a water-based coating applied during forming to give the glass compatibility with the resin matrix in which it will be used. Sizings protect the glass and improves processing both in fiber forming and in composite manufacturing. Sizings are made up of four primary components: film formers, lubricants, coupling agents, and water. Other additives sometimes used in sizings include anti-statics, wetting agents, chopping aids, antioxidants, and pH modifiers. Sizings generally contain 0.05–10% solids, and the remaining balance is water.5 Sizings may also be nonaqueous, but these are not as common as water-based formulations. The amount of active solids that ends up on the glass surface is related to the sizing’s solids content, viscosity, affinity for the glass, and applicator physical parameters, such as roll speed and material. This amount varies by product form. Chopped strands for making wet-formed mats generally have the least sizing, measured as loss on ignition (LOI), whereas rovings for spray-up or sheet molding compound have the most.6 Typical LOI or strand solids values for high-performance single- or multi-end rovings in the 0.5–1.0% range. LOI measurement is a standard quality control evaluation that is included on all Owens Corning certificates of analysis. Moisture is removed first to ensure an accurate measure of solids. Moisture content is also reported on quality certificates and must not exceed 0.15%. The specific components used in a sizing are selected to give the glass fiber reinforcement compatibility with a certain family of resins. Common resins for use in composites include polyester, vinyl ester, epoxy, phenolic, and polyurethane, plus a vast array of thermoplastics. Selecting a fiber with the correct sizing is critical to composite mechanical performance and durability. A fiber with an inappropriate sizing will not form an optimal bond with the resin and load will not be transferred efficiently amongst the load bearing fibers. Furthermore, these poorly bonded regions will be the weak link in the system and failure may initiate within them once load is applied. Even if part failure does not occur, the weak interphase will be susceptible to microcracking, which allows ingress of environmental media and can accelerate corrosion.7 Fig. 6. Tex variation in Owens Corning ShieldStrand® S rovings.4 Fig. 7. ASTM D2344 short-beam shear testing of XStrand® S rovings in vinyl ester resin. www.ceramics.org/IJAGS Glass Fiber-Reinforced Composites 127
128 International Journal of Applied Glass Science--Stickel and Nagarajan Vol.3,No.2,201 Figure 7 illustrates the effect of the sizing selection pre Short-beam shear strength testing was performed per th d air spraying. For ASTM D2344: Standard Test Method for Short-Beam fiberglass flament process is used and Strength of polymer Matrix Composite Materials ar rovings are continuous thre Their Laminate using curved specimens made from chopped. FFU requirements for filament winding rov Owens Corning XStrand S rovings Short-beam shear ngs include good payout, low fuzz, and fast wet out testing does have limitations: it measures only apparent and opening of strands upon introduction of resin interlaminar shear strength, it obtains neither a pure Two strength-dominated composite applications nor uniform shear stress state, and it does not measure will be discussed in the next section. The product shear modulus. For these reasons, other test methods forms, sizings, and processes used to maximize and are preferred when shear properties design data are maintain tensile strength in each will be described. needed. Advantages of ASTM D2344 are that specimen preparation and testing are simple, rapid, and inexpen- Composite Processing and Selected Applications sive. Interlaminar shear strength values obtained are a good indicator of the fiber-matrix interfacial bond, and thus of the quality of the composit Pressure Vessels The two rovings shown in Fig. 7 are identical in Pressure vessels are used to contain pressurized Alu glass formulation and tex, and curved beam test speci- or gases. Most fluid-containing vessels are main- mens,14 per sample, were produced at th e same fib tained at fairly low internal pressure, which means that content(75% by weight) in a vinyl ester resin. Samples tank walls can be relatively thin. Steel is the material of differed only in sizing chemistry. MCX21 is a multi- choice for most low-pressure applications unless the compatible sizing, meaning that it has functionality in vessel will be in contact with corrosive a variety of resin types, including vinyl ester, whereas contained within it or in its external service environ- EPX15 is considered epoxy-compatible only. Data has ment. For such corrosion applications, composite mate- been normalized to the dry or as received shear strength rials are preferred of the MCX2I product. For high-pressure storage of compressed gase The multi-compatible MCX2I roving clearly per- composite materials offer the most competitive forms superior to the epoxy-compatible EPX15 in a strength-to-weight ratios. Applications include com vinyl ester resin system. Furthermore, the MCX21 pressed natural gas(CNG), hydrogen fuel, paintball more than 80% of its original shear bond and breathing air for personal use and emergency res- h after 96 h of hot-wet aging in a boil tank. cue. Within CNG, which this discussion will focus on on the other hand, suffers poor initial bonding is a wide range of sub-segments and pressure vessel strength and rapid deterioration upon aging. As shown, sizes. At the low end, passenger vehicles will have tanks these specimens retained only about 50% of their shear in the 50 L range, whereas bulk hauling modules will ength through the 96 h boil have pressure vessels the size of shipping containers. alos n addition to compatibility with resin, sizing is Although metal is dominant with more than 90% selected for compatibility with the composite man- share, with the current number of CNG pressure ves- facturing process. The same family of resins, and sels on the market close to 30 million and an annual sometimes the same specific resin system, can be used growth rate of greater than 25% in select regions, the in multiple composite processes, such as compression 10% share belonging to composites represents an molding and pultrusion. Although some sizings can also be used in multiple processes, most are tailored to meet the specific fitness-for-use(FFU) requirements of one process. Vessel Types For example, polyester resins are used frequently There are four general pressure vessel types, for boat hulls and for pipes. a glass fiber sizing with described in Table II, that are recognized by domestic polyester resin compatibility would be required for both and international design and qualification standards applications, but the overall sizing package would be Type I vessels, which are all steel (typical)or quite different. Boat manufacture utilizes the spray up Im, are generally the least expensive to manuf
Figure 7 illustrates the effect of the sizing selection. Short-beam shear strength testing was performed per ASTM D2344: Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminate using curved specimens made from Owens Corning XStrand® S rovings. Short-beam shear testing does have limitations: it measures only apparent interlaminar shear strength, it obtains neither a pure nor uniform shear stress state, and it does not measure shear modulus. For these reasons, other test methods are preferred when shear properties design data are needed. Advantages of ASTM D2344 are that specimen preparation and testing are simple, rapid, and inexpensive. Interlaminar shear strength values obtained are a good indicator of the fiber-matrix interfacial bond, and thus of the quality of the composite.8 The two rovings shown in Fig. 7 are identical in glass formulation and tex, and curved beam test specimens, 14 per sample, were produced at the same fiber content (75% by weight) in a vinyl ester resin. Samples differed only in sizing chemistry. MCX21 is a multicompatible sizing, meaning that it has functionality in a variety of resin types, including vinyl ester, whereas EPX15 is considered epoxy-compatible only. Data has been normalized to the dry or as received shear strength of the MCX21 product. The multi-compatible MCX21 roving clearly performs superior to the epoxy-compatible EPX15 in a vinyl ester resin system. Furthermore, the MCX21 retains more than 80% of its original shear bond strength after 96 h of hot-wet aging in a boil tank. EPX15, on the other hand, suffers poor initial bonding strength and rapid deterioration upon aging. As shown, these specimens retained only about 50% of their shear strength through the 96 h boil. In addition to compatibility with resin, sizing is also selected for compatibility with the composite manufacturing process. The same family of resins, and sometimes the same specific resin system, can be used in multiple composite processes, such as compression molding and pultrusion. Although some sizings can also be used in multiple processes, most are tailored to meet the specific fitness-for-use (FFU) requirements of one process. For example, polyester resins are used frequently for boat hulls and for pipes. A glass fiber sizing with polyester resin compatibility would be required for both applications, but the overall sizing package would be quite different. Boat manufacture utilizes the spray up process, so rovings must maintain adequate integrity through chopping and compressed air spraying. For fiberglass pipe, the filament winding process is used and rovings are continuous throughout the part, not chopped. FFU requirements for filament winding rovings include good payout, low fuzz, and fast wet out and opening of strands upon introduction of resin. Two strength-dominated composite applications will be discussed in the next section. The product forms, sizings, and processes used to maximize and maintain tensile strength in each will be described. Composite Processing and Selected Applications Pressure Vessels Pressure vessels are used to contain pressurized fluids or gases. Most fluid-containing vessels are maintained at fairly low internal pressure, which means that tank walls can be relatively thin. Steel is the material of choice for most low-pressure applications unless the vessel will be in contact with corrosive media, either contained within it or in its external service environment. For such corrosion applications, composite materials are preferred. For high-pressure storage of compressed gases, composite materials offer the most competitive strength-to-weight ratios. Applications include compressed natural gas (CNG), hydrogen fuel, paintball, and breathing air for personal use and emergency rescue. Within CNG, which this discussion will focus on, is a wide range of sub-segments and pressure vessel sizes. At the low end, passenger vehicles will have tanks in the 50 L range, whereas bulk hauling modules will have pressure vessels the size of shipping containers. Although metal is dominant with more than 90% share, with the current number of CNG pressure vessels on the market close to 30 million and an annual growth rate of greater than 25% in select regions, the 10% share belonging to composites represents an attractive opportunity. Vessel Types There are four general pressure vessel types, described in Table II, that are recognized by domestic and international design and qualification standards. Type 1 vessels, which are all steel (typical) or aluminum, are generally the least expensive to manufac- 128 International Journal of Applied Glass Science—Stickel and Nagarajan Vol. 3, No. 2, 2012
amics.org/lAGS Glass Fiber-Reinforced Composites Table Il. Pressure Vessel Types, Construction, and Cost-Performance Type 1 Type 3 f Market share(%) 93 Metal Metal liner reinforced Metal liner reinforced with Resin impregnated with resin impregnated resin impregnated continuous continuous filament continuous filament filament(full wrap) with a nonmetallic (hoop wrap Most commonly CrMo steel CrMo steel with glass Aluminum with HP glass HDPE liner with fiber or carbon carbon Indicative $3 to S5 $5 to S7 $9 to $14 S11 to $18 ost -USS/L Indicative 0.91 0.81.0 04-0.5 0.3-0.4 ture, but are the heaviest. Types 24 are composite vessels. Type 2 vessels have a heavy liner and utilize Rovin Tensioner composite rel nforcemer nt only in the hoop or circum ferential Types 3 and 4 have a full compos supply Resin bath ite overwrap, providing both circumferential and axial inforcement. The difference between Type 3 and 4 is in the liner. Type 3 cylinders have a metal(typically Tool Shuttle Axle aluminum) liner that may share a small fraction of the internal pressure load. Type 4 cylinders have a plastic (typically high-density polyethylene [HDPE) liner that serves only as a substrate for the load-bearing compos ite. In general, in order from Type I to Type 4, as Type number increases, price increases, and weight Semi-finished decreases. The indicative cost data in the table ii will component Track ange with raw material prices, but the relative cost ratios between the four vessel types should remain fairly Fig. 8. Filament winding proces schematic c Filament Winding Process by Owens Corning in the 1960s. A schematic of the filament winding process is given in Fig. 8. Rovings Before discussing why composites work in pressure are fed through tensioning devices to a resin impregna- vessel applications, it is of value to discuss how compos- tion bath. Impregnation is achieved either by feeding ite pressure vessels are made. Steel liners are made from rovings through the bath or by feeding them over a blanks using a deep draw process. Aluminum liners can metering roll that rotates through the bath to pick up a be made from blanks, but are more typically made from defined amount of resin. The impregnated rovings are seamless extruded tube stock. Ends are heat spun to then gathered together and placed onto the rotating shape into hemispherical domes, and in most cases, part in a high-precision, computer-controlled process necks are formed in the same processing step so that The number and tex of rovings used depends on the tire end cap is one piece. For Type 4 tanks, HDPE size of the part to be fabricated and the width of the liners are made by blow molding or rotational molding. band desired. mposite outer layer is created using filament A single-axis(or axle) winder would be adequate ding, a continuous fiber process that was developed for making Type 2 vessels, but a multi-axis winder is
ture, but are the heaviest. Types 2–4 are composite vessels. Type 2 vessels have a heavy liner and utilize composite reinforcement only in the hoop or circumferential orientation. Types 3 and 4 have a full composite overwrap, providing both circumferential and axial reinforcement. The difference between Type 3 and 4 is in the liner. Type 3 cylinders have a metal (typically aluminum) liner that may share a small fraction of the internal pressure load. Type 4 cylinders have a plastic (typically high-density polyethylene [HDPE]) liner that serves only as a substrate for the load-bearing composite. In general, in order from Type 1 to Type 4, as Type number increases, price increases, and weight decreases. The indicative cost data in the Table II will change with raw material prices, but the relative cost ratios between the four vessel types should remain fairly constant. Filament Winding Process Before discussing why composites work in pressure vessel applications, it is of value to discuss how composite pressure vessels are made. Steel liners are made from blanks using a deep draw process. Aluminum liners can be made from blanks, but are more typically made from seamless extruded tube stock. Ends are heat spun to shape into hemispherical domes, and in most cases, necks are formed in the same processing step so that entire end cap is one piece. For Type 4 tanks, HDPE liners are made by blow molding or rotational molding. The composite outer layer is created using filament winding, a continuous fiber process that was developed by Owens Corning in the 1960s. A schematic of the filament winding process is given in Fig. 8.10 Rovings are fed through tensioning devices to a resin impregnation bath. Impregnation is achieved either by feeding rovings through the bath or by feeding them over a metering roll that rotates through the bath to pick up a defined amount of resin. The impregnated rovings are then gathered together and placed onto the rotating part in a high-precision, computer-controlled process. The number and tex of rovings used depends on the size of the part to be fabricated and the width of the band desired.11 A single-axis (or axle) winder would be adequate for making Type 2 vessels, but a multi-axis winder is Fig. 8. Filament winding process schematic.10 Table II. Pressure Vessel Types, Construction, and Cost-Performance9 Type 1 Type 2 Type 3 Type 4 Market share (%) 93 4 <2 <2 Structure Metal Metal liner reinforced with resin impregnated continuous filament (hoop wrap) Metal liner reinforced with resin impregnated continuous filament (full wrap) Resin impregnated continuous filament with a nonmetallic liner Most commonly used CrMo steel CrMo steel with glass fiber Aluminum with HP glass or carbon HDPE liner with carbon Indicative cost -US$/L $3 to $5 $5 to $7 $9 to $14 $11 to $18 Indicative weight -Kg/L 0.9–1.3 0.8–1.0 0.4–0.5 0.3–0.4 www.ceramics.org/IJAGS Glass Fiber-Reinforced Composites 129
International Journal of Applied Glass Science--Stickel and Nagarajan Vol.3,No.2,201 required for Type 3 or 4 vessels to place windings paral lel or near-parallel to the primary axis along the length 4500 y=09x38 of the tank. These tanks require complex winding pro- grams with layers at a variety of angles, ranging from pure hoop(88-89, as measured relative to the primary y axis)to low-angle helicals of approximately 10. As the angled windings must anchor around the pressure vessel end bosses to prevent slipping, the composite thickness 51500 is greater here than in the cylindrical section. Ideally fill- ing valves, pressure relief devices, and other accessories are placed before filament winding so that the cured Fig. 10. Impregnated strand tensile to hoop strength translation Why Tensile Strength Matters to impregnated strand or roving form. Figure 9 shows In a cylindrical pressure vessel, hoop or circumfer- a correlation between pristine, single filament and pressure by the cylinder radius and dividing by the wall are shown in the chart, and relative per .ass types ntial stress is calculated by multiplying the internal impregnated strand tensile strength. Four glass types thickness. Hoop stress governs pressure vessel design as clearly illustrated. Efficient strength translation like this it is twice the stress in the axial or longitudinal orienta- suggests that the glass fiber sizing has done its job: the tion. One of the key qualification tests for pressure glass has been well protected and shows good compati- vessels is burst performance, which involves internally bility with the epoxy resin. Figure 10 is an extension of Fig. 9 showing how In a Type 2 cylinder, there is equal load share between impregnated strand tensile strength translates to perfor the steel liner and the composite hoop wrap, however mance in a finished pressure vessel as measured by as the composite reinforcement does not cover the hoop strength. Additional reinforcing fibers are domed end caps, the steel liner is fully responsible for included in this plot, and generally lie along the same all axial stress. In Type 3 and 4 cylinders, the liner trendline, again suggesting that the sizing is functioning ries very little, if any, of the internal pressure, so the as intended to provide good protection, wet out, and tolerable hoop stress or burst pressure is almost exclu- fiber-matrix bonding. The single data point that is off sively a function of the composite' s strength of the curve, K49 aramid, likely has an issue with its To provide hoop strength to a pressure vessel, a sizing that has caused processing difficulty or inade lass fiber must first translate its pristine tensile strength quate resin compatibility. Failure must occur at a burst pressure that is at least the service pressure (3000 or 3600 psi are typical or approximately 200 or 250 bar) multiplied by 02 afety fa afety factors prescribed by Table Ill. Design Stress Ratios( Safety Factors for Composite CNG Cylinders from ANSI NGV2 and ISO 11439 Type 2 Type 3 Type 4 ANSI ISO ANSI ISO ANSI ISO Glass Pristine Filament Tensile Strength (MPa) Glass2.652.753.503.653.503.65 Aramid2.252.353.003.103.003.10 Fig. 9. Pristine flame pregnated strand tensile strength Carbon2.252352.252.352.252.35
required for Type 3 or 4 vessels to place windings parallel or near-parallel to the primary axis along the length of the tank. These tanks require complex winding programs with layers at a variety of angles, ranging from pure hoop (88–89°, as measured relative to the primary axis) to low-angle helicals of approximately 10°. As the angled windings must anchor around the pressure vessel end bosses to prevent slipping, the composite thickness is greater here than in the cylindrical section. Ideally filling valves, pressure relief devices, and other accessories are placed before filament winding so that the cured composite laminate can anchor them in place. Why Tensile Strength Matters In a cylindrical pressure vessel, hoop or circumferential stress is calculated by multiplying the internal pressure by the cylinder radius and dividing by the wall thickness. Hoop stress governs pressure vessel design as it is twice the stress in the axial or longitudinal orientation. One of the key qualification tests for pressure vessels is burst performance, which involves internally pressurizing a cylinder at a constant rate until it fails. In a Type 2 cylinder, there is equal load share between the steel liner and the composite hoop wrap, however as the composite reinforcement does not cover the domed end caps, the steel liner is fully responsible for all axial stress. In Type 3 and 4 cylinders, the liner carries very little, if any, of the internal pressure, so the tolerable hoop stress or burst pressure is almost exclusively a function of the composite’s strength. To provide hoop strength to a pressure vessel, a glass fiber must first translate its pristine tensile strength to impregnated strand or roving form. Figure 9 shows a correlation between pristine, single filament and impregnated strand tensile strength. Four glass types are shown in the chart, and relative performance is clearly illustrated. Efficient strength translation like this suggests that the glass fiber sizing has done its job: the glass has been well protected and shows good compatibility with the epoxy resin. Figure 10 is an extension of Fig. 9 showing how impregnated strand tensile strength translates to performance in a finished pressure vessel as measured by hoop strength. Additional reinforcing fibers are included in this plot, and generally lie along the same trendline, again suggesting that the sizing is functioning as intended to provide good protection, wet out, and fiber-matrix bonding. The single data point that is off of the curve, K49 aramid, likely has an issue with its sizing that has caused processing difficulty or inadequate resin compatibility. Failure must occur at a burst pressure that is at least the service pressure (3000 or 3600 psi are typical, or approximately 200 or 250 bar) multiplied by a design safety factor. Safety factors prescribed by Fig. 9. Pristine filament to impregnated strand tensile strength translation. Fig. 10. Impregnated strand tensile to hoop strength translation. Table III. Design Stress Ratios (Safety Factors) for Composite CNG Cylinders from ANSI NGV2 and ISO 11439 Type 2 Type 3 Type 4 ANSI ISO ANSI ISO ANSI ISO Glass 2.65 2.75 3.50 3.65 3.50 3.65 Aramid 2.25 2.35 3.00 3.10 3.00 3.10 Carbon 2.25 2.35 2.25 2.35 2.25 2.35 130 International Journal of Applied Glass Science—Stickel and Nagarajan Vol. 3, No. 2, 2012
www.ceramics.org/ljags Glass Fiber-Reinforced Composites T3 T4 carbon fiber vessels T3 glass fiber vessels 0.70 T2 glass fiber vessels 占0.60 Fig. 12. Number of incidents related to CNG Pressure vessels by category,19842010.2 1.00 Steel would not be practical for several reasons Fig. 11. Cost-performance curve for pressure vessels. Cost data First, a steel tank of this size would almost certainly has been normalized to most expensive vessels designs. have to be produced in multiple pieces and welded Welded tanks are permitted by some standards, but ar less reliable and are subject to higher safety factors America ional Standards Institute(ANSI)/Cana- because of weakness at the welded joints. Second, large- dian Standards Association(CSA)NGV2: American size steel tanks could be too heavy for transport on national standard for compressed natural gas vehicle fuel unimproved rural roads, especially given the added wall containers and International Organization for Standardi- thickness and weight associated with welding. Carbon zation(ISO)11439: Gas cylinders- High-pressure cyl- fiber tanks, at the other end of the spectrum, would automotive wehicles standards are show i gas as fuel for likely be prohibitively expensive and unnecessary since inders for the onboard storage of natur ISO safety factors are slightly higher than ANSI, but n in Table Ill. only a fraction of the tank's life would be Glass fiber would represent the most attractive balance both follow the same trend of cost and weight in this application. afety factors were developed to ensure safe, reli- Should design safety factors be reduced, high- able operati on of co omposite cylinders under sustained strength glass fibers become an even more appealing and cyclic loading essentially, they are based on choice in CNG pressure vessels. There are solid argu- static and cyclic fatigue performance of fibers. Glass is ments that support separation of E- and S-glass in quali- heavily penalized compared with carbon fiber because fication standards such that different safety factors could the glass designation includes E-glass, which is weaker be applied to each. The compositional range for S-glass and much more susceptible to corrosion than high- formulations is narrow, whereas the range for E-glass is performance glass formulations, like XStrand" S very broad. Thus the tensile strength of E-glass shows Even at current safety factors, glass fiber solutions significantly more variability than S-glass, and depend offer value compared both to steel Type I tanks and to ing upon the supplier, can be less than half the strength ultra-light carbon fiber-reinforced Type 3 and 4 tanks. of XStrand or similar S-glass formulations Glass fiber-reinforced tanks offer a balance of cost- Stress-corrosion cracking (SCC) is one of the performance. Figure 11 illustrates the niche for glass primary issues observed in E-glass CNG tank fiber pressure vessel solutions (Fig. 12). The terms stress-corrosion, stress-rupture Glass-reinforced vessels make sense where weight is creep-rupture, and static fatigue all refer to the same important, but not the most critical customer require- phenomena: degradation of a material system under ment. An example is a large pressure vessel for bulk sustained, static tensile loading. Stress-corrosion occurs hauling of CNG. These vessels are typically filled at in air, but is accelerated in the presence of mor one location, then transported to a remote area lacking corrosive media. For CNG tanks. those media include where the gas is used to provide power and e tationary battery acid, windshield wiper Auid, anti-freeze, gas pipeline infrastructure. Here, they remain moisture from rain or snow, and deicing salts
American National Standards Institute (ANSI)/Canadian Standards Association (CSA) NGV2: American national standard for compressed natural gas vehicle fuel containers and International Organization for Standardization (ISO) 11439: Gas cylinders — High-pressure cylinders for the onboard storage of natural gas as fuel for automotive vehicles standards are shown in Table III. ISO safety factors are slightly higher than ANSI, but both follow the same trend. Safety factors were developed to ensure safe, reliable operation of composite cylinders under sustained and cyclic loading — essentially, they are based on static and cyclic fatigue performance of fibers. Glass is heavily penalized compared with carbon fiber because the glass designation includes E-glass, which is weaker and much more susceptible to corrosion than highperformance glass formulations, like XStrand® S. Even at current safety factors, glass fiber solutions offer value compared both to steel Type 1 tanks and to ultra-light carbon fiber-reinforced Type 3 and 4 tanks. Glass fiber-reinforced tanks offer a balance of costperformance. Figure 11 illustrates the niche for glass fiber pressure vessel solutions. Glass-reinforced vessels make sense where weight is important, but not the most critical customer requirement. An example is a large pressure vessel for bulk hauling of CNG. These vessels are typically filled at one location, then transported to a remote area lacking gas pipeline infrastructure. Here, they remain stationary where the gas is used to provide power and heat. Steel would not be practical for several reasons. First, a steel tank of this size would almost certainly have to be produced in multiple pieces and welded. Welded tanks are permitted by some standards, but are less reliable and are subject to higher safety factors because of weakness at the welded joints. Second, largesize steel tanks could be too heavy for transport on unimproved rural roads, especially given the added wall thickness and weight associated with welding. Carbon fiber tanks, at the other end of the spectrum, would likely be prohibitively expensive and unnecessary since only a fraction of the tank’s life would be in transit. Glass fiber would represent the most attractive balance of cost and weight in this application. Should design safety factors be reduced, highstrength glass fibers become an even more appealing choice in CNG pressure vessels. There are solid arguments that support separation of E- and S-glass in quali- fication standards such that different safety factors could be applied to each. The compositional range for S-glass formulations is narrow, whereas the range for E-glass is very broad. Thus the tensile strength of E-glass shows significantly more variability than S-glass, and depending upon the supplier, can be less than half the strength of XStrand® or similar S-glass formulations. Stress-corrosion cracking (SCC) is one of the primary issues observed in E-glass CNG tanks (Fig. 12). The terms stress-corrosion, stress-rupture, creep-rupture, and static fatigue all refer to the same phenomena: degradation of a material system under sustained, static tensile loading. Stress-corrosion occurs in air, but is accelerated in the presence of more corrosive media. For CNG tanks, those media include battery acid, windshield wiper fluid, anti-freeze, moisture from rain or snow, and deicing salts. Fig. 11. Cost-performance curve for pressure vessels. Cost data has been normalized to most expensive vessels designs. Fig. 12. Number of incidents related to CNG pressure vessels by category, 1984–2010.12 www.ceramics.org/IJAGS Glass Fiber-Reinforced Composites 131
