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