Glass Fiber

What is Glass Fiber

Glass fiber, also called fibreglass or textile glass, is a material made of extremely fine filaments of glass, drawn from a molten silicate melt and cooled fast enough to remain amorphous rather than crystalline. Individual filaments used for reinforcement are typically 5 to 24 microns in diameter, finer than a human hair, and are gathered immediately after forming into a coherent bundle called a strand. The same bulk glass that is brittle and weak as a window pane becomes strong and flexible as a thin fiber, because drawing it down reduces the number and size of surface flaws that would otherwise initiate fracture.

The reason glass works so well as a fiber is the SiO2 network. Silicon and oxygen atoms form SiO4 tetrahedra that link into a three-dimensional glassy network; pure silica, the basis of high-purity quartz material, softens near 1,200 degrees Celsius and only becomes freely fluid near 1,713 degrees Celsius. Industrial glass fiber adds oxides of aluminium, calcium, boron, and magnesium to lower the melting and forming temperature into a workable range and to tune chemical durability and electrical behaviour. The chemistry of these additions defines the lettered glass families discussed in Chapter 2.

The modern industry has clear historical milestones. Edward Drummond Libbey exhibited a dress woven with glass fiber in 1893. The breakthrough came in 1932 and 1933 when Games Slayter at Owens-Illinois developed a practical process for fine glass wool, with first commercial production in 1936. In 1938 Owens-Illinois and Corning Glass Works merged to form Owens-Corning Fiberglas Corporation, which began producing E-glass that year, S-2 glass in 1968, ECR-glass around 1980, and boron-free Advantex E-glass in 1997. From a niche insulation product, glass fiber grew into a multimillion-tonne reinforcement industry.

It is important to separate glass fiber from its two relatives. Glass wool is a loosely tangled, low-density mat of short fibers used for thermal and acoustic insulation, where trapped air, not fiber strength, does the work, much like the stone-based rock wool it competes with in building insulation. Continuous glass filament, the subject of this guide, is drawn as long, aligned, strong fiber for structural reinforcement. The third relative, optical fiber, is an ultra-pure silica fiber engineered for light transmission rather than mechanical load, closer in purpose to optical glass than to structural reinforcement, and is covered separately. This page focuses on reinforcement-grade continuous and chopped glass fiber.

Four properties make glass fiber the default composite reinforcement. First, high specific strength: E-glass reaches roughly 3,445 MPa tensile strength at a density of only 2.58 g/cm3, far stronger per unit weight than structural steel. Second, electrical insulation, the original reason for the E (electrical) designation, which underpins its use in circuit boards and high-voltage parts. Third, dimensional stability and low creep, holding shape under load and temperature. Fourth, low cost: bulk E-glass roving prices near 2 USD/kg, roughly one tenth the cost of standard carbon fiber, which is why glass dominates volume composite production.

Glass Composition Types

Glass fiber families are identified by a letter that signals the property the composition was designed to deliver. ASTM D578 codifies the major composition families, and choosing the wrong one is the most expensive selection mistake: a circuit board needs low alkali for electrical stability, a chemical tank needs acid resistance, and concrete cladding needs alkali resistance, and no single glass satisfies all three. The table below compares the key engineering properties of the main families.

E-glass (electrical) is an alumino-borosilicate with less than 1 percent alkali oxides by weight. It supplies roughly half of all glass fiber produced. The low alkali content gives stable electrical insulation, good water resistance, and adequate chemical durability at low cost, making it the default for printed circuit board substrate, general FRP composite laminates, and most structural composites. Its boron oxide content lowers the forming temperature, but boron-bearing E-glass has weaker acid resistance, which drove the development of ECR.

S-2 glass (strength) is a magnesium-alumino-silicate that drops boron and lime and raises silica and alumina (roughly 25 percent alumina versus 8 percent in E-glass) plus magnesia. The result is about 40 percent higher tensile strength (4,890 MPa), higher modulus, lower density, and a softening point near 1,056 degrees Celsius versus 846 for E-glass. S-2 is the choice for ballistic armor, helicopter rotor components, and high-pressure vessels, but it costs roughly an order of magnitude more than E-glass, near 20 USD/kg, so it is used only where strength-to-weight pays for itself.

ECR-glass (electrical/chemical resistance) is an alumino-lime-silicate with less than 1 percent alkali oxides and, critically, no boron. Removing boron improves long-term acid and base resistance and eliminates boron emissions during melting, an environmental advantage. Owens Corning markets boron-free E-glass as Advantex, which combines E-glass electrical behaviour with ECR-grade corrosion resistance and slightly higher strength. ECR is now standard for chemical storage tanks, pipe, and corrosion-critical pultrusions.

AR-glass (alkali-resistant) is formulated with roughly 16 percent or more zirconia (ZrO2), which resists the high-pH attack of Portland cement pore solution. It is the correct reinforcement for glass-fiber-reinforced concrete (GFRC), decorative cladding panels, and thin cement boards, an application where it competes with other concrete fiber reinforcements, including steel fiber. C-glass (chemical) is an alkali-lime glass with high boron content for surface acid resistance, used in surfacing veils and corrosion barriers. D-glass (dielectric) has a low dielectric constant for radomes and high-frequency PCB, while A-glass (soda-lime) is a lower-cost, lower-durability glass used where electrical and corrosion demands are mild.

Manufacturing and Filament Forming

Continuous glass fiber is made by melting a batch of mineral oxides, then drawing the molten glass through a precision bushing into thousands of parallel filaments that are coated, gathered, and wound. Two melting routes exist: the direct melt process, now dominant, and the older marble melt process. The table below contrasts them, after which each forming step is explained.

Batching and melting. Raw minerals (silica sand, limestone, kaolin clay, colemanite or boric acid, fluorspar, and others) are weighed to the target composition and fed to a refractory-lined furnace held near 1,400 to 1,500 degrees Celsius. In the direct melt process the molten glass passes through a forehearth and is delivered straight to the forming bushings, eliminating the cost and energy of casting and remelting marbles. The marble melt process casts intermediate glass marbles of about 15 to 16 mm that are remelted at the fiber-forming plant, and survives mainly for specialty and high-purity glasses.

Fiberizing. The molten glass flows out of the forehearth through a bushing, a tray of platinum-rhodium alloy pierced with very fine orifices, anywhere from 200 to 4,000 tips in multiples of 200, and up to 8,000 in large bushings. Platinum-rhodium is used because it resists the corrosive molten glass and holds its dimensions at temperature, keeping every filament identical. The glass extrudes from each tip, then is mechanically attenuated, that is, drawn down, into a fine filament. Winder speed sets the diameter: continuous filament is pulled at roughly 1 km per minute, stretching the soft glass stream into filaments 4 to 34 microns across.

Sizing application. As the filaments cool below the glass transition they pass a sizing applicator, a roller or pad that wets each filament with an aqueous chemical coating at 0.5 to 2.0 percent by weight. Without sizing the filaments would abrade and shatter against each other and against guides. The sizing also gathers the filaments into a strand and carries the silane coupling agent that will later bond the glass to the synthetic resin matrix. Sizing chemistry is the single most application-specific variable in the whole process and is matched to the end resin.

Gathering, winding, and drying. The coated filaments are gathered over a gathering shoe into one or more strands, then wound onto a forming package (a doff) at high speed, or chopped wet. The wound package is dried in an oven to drive off the water and cure the sizing film. From this point the glass diverges into product forms: single-end (direct) roving for filament winding and pultrusion, assembled (multi-end) roving, chopped strand, chopped strand mat, woven roving, and fine yarns for weaving, all covered in Chapter 4.

Two quality consequences follow from the forming physics. First, filament diameter uniformity depends entirely on bushing temperature control and tip geometry, so a reputable supplier guarantees a narrow diameter band, which directly governs strength and resin wet-out. Second, surface flaws set fiber strength, so freshly formed fiber is strongest and any abrasion in handling lowers it; this is why sizing and careful unwinding matter as much as the glass chemistry itself.

Sizing, Forms, and Standards

Once the glass chemistry is set, two practical decisions remain: which sizing chemistry to use, and in which physical form to buy the fiber. Both are governed by the manufacturing process you will run and the resin you will use. ASTM D578 and the ISO 2078 nomenclature system give a common language for naming the product, while ASTM D2584 governs how the sizing content is measured.

Sizing and the coupling agent. A sizing is a water-based blend of a film former, lubricant, antistatic agent, and, most importantly, a silane coupling agent. The silane has a silicon end that bonds to the glass surface and an organic end that bonds to the resin, forming a covalent bridge across the fiber-matrix interface. The right silane for an epoxy is different from the right silane for an unsaturated polyester or a polyolefin, and using a mismatched sizing can cut interlaminar shear strength dramatically. Loss on ignition (LOI), per ASTM D2584, burns off the organic coating and reports its weight fraction, typically 0.4 to 1.2 percent, confirming both the amount and that the correct product was supplied.

The table below summarises the main commercial forms of reinforcement-grade glass fiber and the processes each one serves.

Roving. Direct (single-end) roving is the simplest, strongest form: filaments collected into one strand in a single step, ideal for high glass content, aligned processes such as filament winding of pressure vessels and pultrusion of profiles. Assembled (multi-end) roving collects two or more strands into a larger, loosely held bundle better suited to spray-up and chopping. Roving linear density is quoted in TEX (grams per kilometer); 300 to 9,600 TEX covers most reinforcement uses, with 1,200 and 2,400 TEX common for filament winding.

Mats and fabrics. Chopped strand mat (CSM) scatters short strands, generally 25 to 50 mm long, randomly and bonds them with a powder or emulsion binder, producing isotropic but lower-strength laminate that wets out readily for hand lay-up of tanks and panels. Woven roving weaves coarse rovings into a heavy, grid-like plain or twill fabric for strong bidirectional reinforcement in large hand-laid parts. Fine yarns, woven into electrical-grade cloth such as the styles defined in IPC-4412, form the reinforcement of printed circuit board laminate.

Nomenclature and standards. ASTM D578 specifies continuous and staple glass fiber strands and the letter system for naming them: a typical yarn code such as ECG 75 reads as E composition (E), continuous filament (C), filament diameter code G, and a yardage number. Filament diameter is coded by letter under the ISO 1888 and industry convention: B is 3.5 microns, C 4.5, D 5, DE 6, E 7, G 9, H 10, and K 13 microns. ISO 2078 gives the international designation for textile glass yarns, rovings, and chopped strand, and these codes let buyers compare products across suppliers without ambiguity.

Key Specification Parameters

A glass fiber datasheet can list a dozen or more parameters, but only a handful drive a selection decision: glass type, filament diameter, linear density (TEX), tensile strength and modulus, density, sizing chemistry and LOI, moisture content, and softening or service temperature. Each is decoded below so a spec sheet can be read at a glance.

Filament diameter. Quoted in microns and often as a letter code, diameter governs both strength and processability. Finer filaments (5 to 9 microns, codes D to G) are stronger and weave into fine electrical cloth but cost more to produce; coarser filaments (13 to 24 microns, codes K and up) wet out faster and suit heavy reinforcement. Diameter uniformity, not just nominal size, is the quality signal: a tight band means consistent strength and predictable resin uptake.

Linear density (TEX). TEX is grams per 1,000 meters of strand or roving and is the unambiguous size measure. A 1,200 TEX roving is twice as heavy per length as a 600 TEX roving. The older English system quotes yards per pound and a yield number, but for cross-supplier comparison always convert to TEX. Roving for structural composites typically ranges from 300 to 9,600 TEX.

Tensile strength and modulus. Reported on the bare filament or strand, these are the headline mechanical numbers: E-glass near 3,445 MPa strength and 76 GPa modulus, S-2 glass near 4,890 MPa and 85 to 88 GPa. Note that strength is for pristine fiber; real laminate strength depends on glass content, fiber alignment, void fraction, and the fiber-matrix bond, so a strong fiber poorly wetted out yields a weak part. Glass modulus is much lower than carbon fiber (230 GPa or more), which is why carbon is chosen for stiffness-critical structures.

Density and specific properties. E-glass density is 2.58 g/cm3, S-2 glass 2.46 g/cm3, AR-glass 2.68 g/cm3. Because composites compete on weight, the strength-to-weight and stiffness-to-weight ratios, not the absolute numbers, decide whether glass or a more expensive fiber wins. Glass density is roughly 1.4 times that of carbon fiber, a reason carbon dominates aerospace primary structure.

Sizing, LOI, and moisture. The datasheet should name the compatible resin system, whether a thermoset or a reinforced engineering plastic such as glass-filled nylon (epoxy, vinyl ester, unsaturated polyester, polyamide, polypropylene) and quote LOI (typically 0.4 to 1.2 percent) and moisture content (commonly below 0.1 percent). High moisture in roving causes processing defects and reduced bond. These three lines, easy to overlook, decide whether the fiber will actually bond to your matrix.

Thermal limits. Softening point sets the ceiling: E-glass softens near 846 degrees Celsius, S-2 glass near 1,056 degrees Celsius, but useful strength falls well before that, with E-glass retaining about half its strength near 370 degrees Celsius. The practical limit of a glass composite is almost always the resin glass-transition temperature (commonly 80 to 200 degrees Celsius), not the glass. For genuine high-temperature service, high-silica or silica fiber (over 96 percent SiO2) extends continuous use toward 1,000 degrees Celsius. The list below summarises the parameters that belong in every RFQ.

• Glass type: E, S-2, ECR, AR, C, or D, matched to electrical, strength, corrosion, or alkali demand.
• Filament diameter: in microns and letter code, balancing strength against wet-out.
• Linear density (TEX): always converted to metric for comparison.
• Tensile strength and modulus: bare-fiber values, read alongside expected glass content.
• Sizing / resin compatibility: the single most application-specific line on the sheet.
• LOI and moisture: per ASTM D2584, confirming sizing amount and dryness.
• Form and packaging: roving, mat, woven roving, chopped strand, doff or pallet.

Selection Decision Factors

To turn the preceding chapters into a purchase order, follow the decision sequence below. Most selection errors come not from a single wrong number but from deciding the wrong thing first, for example fixing a brand before defining the resin and process. These steps work as a fixed RFQ template.

1. Define the service environment first: electrical insulation points to E-glass or D-glass; acid or chemical exposure to ECR or C-glass; Portland-cement alkalinity to AR-glass; maximum strength-to-weight to S-2 glass. The environment, not the price, narrows the glass family.
2. Fix the manufacturing process: filament winding and pultrusion call for single-end direct roving; hand lay-up for chopped strand mat or woven roving; spray-up for assembled roving or chopped strand; injection or compression molding for chopped strand compounds. The process dictates the form.
3. Match the sizing to the resin: confirm the product is sized for your epoxy, vinyl ester, unsaturated polyester, or thermoplastic. This is the most common silent failure; a mismatched silane can halve interlaminar shear strength. Request the resin-compatibility line on the datasheet.
4. Set filament diameter and TEX: finer filaments for fine fabric and higher strength, coarser for fast wet-out and heavy reinforcement; pick a TEX that suits the machine and target glass content.
5. Specify the standard and grade: reference ASTM D578 for strand nomenclature, ISO 2078 for international designation, and IPC-4412 for electrical-grade woven cloth. Naming the standard removes ambiguity between suppliers.
6. Confirm quality data: require tensile strength, LOI per ASTM D2584, moisture content, and a corrosion or alkali strength-retention chart for corrosion-critical or GFRC use. Initial strength alone is not enough; ask for retained strength over time.
7. Check packaging and logistics: doff weight, pallet configuration, moisture-barrier packaging, and shelf life. Sizing ages, so confirm the storage life and conditions, especially for humidity-sensitive products.
8. Evaluate total cost of ownership: price per kilogram is only the start. Wet-out speed, scrap rate, consistency of diameter, and reject rate from voids or poor bond dominate the true cost of a composite part. A cheaper roving that wets out poorly raises labor and scrap beyond its saving.

One dimension buyers often overlook is supplier serviceability and continuity: lot-to-lot consistency, technical support for sizing selection, regional warehousing, and the ability to second-source an equivalent grade. Owens Corning, China Jushi, Nippon Electric Glass, Johns Manville, Taishan Fiberglass, and AGY operate global supply chains and publish full datasheets, which makes them dependable anchors for large or long-running programs. Whatever the brand, lock the qualified grade and sizing into the specification so a future lot cannot quietly substitute an incompatible product.