Glass Fiber Reinforced Plastics: 5 Glass Types, Loading, Coupling and Selection

Glass fiber reinforced plastic is a compound in which chopped or continuous glass fibers, typically 10 to 50 wt% of the compound, carry the mechanical load that the polymer matrix transfers to them across a silane-treated interface. E-glass reaches about 3,445 MPa tensile strength against roughly 30 to 80 MPa for the unfilled engineering thermoplastics it goes into, so the question is never whether glass is stronger, but whether the interface between glass and polymer transfers the load at all. That interface is a coating called sizing, applied to the fiber at 0.5 to 2.0 wt%, and it is the single variable that decides whether a glass-filled compound performs like a reinforced material or like an expensive, brittle filler.

Glass fibre is the reinforcing member of a compound, which puts it in the same group of plastic additives as fillers, pigments and stabilizers, even though it is neither dissolved in the melt nor dispersed as a fine particle. Its job is structural rather than cosmetic or protective: it raises stiffness, strength and heat deflection temperature in a way no particulate additive can match.

Glass is one of six fibre classes covered under reinforcing fibers for plastics, and this page works through them in the order a compounder actually decides them: which of the 5 glass types to specify, which physical form to buy it in, how much of it the compound needs, why the sizing and coupling agent decide whether any of that works, how the choice changes by polymer, what it does to the rest of the additive package, how the result is measured, and what the food-contact, REACH and IARC status of glass fiber is.

Key figures

• E-glass: 3,445 MPa tensile strength, 76.0 GPa modulus, 2.58 g/cm3 density
• Glass content in flame-retardant polyamide: 10 to 50 wt% of the compound (Clariant Exolit brochure)
• Sizing on the fiber: 0.5 to 2.0 wt%, applied at the bushing
• EU food contact: glass fibres are FCM No 38 under Regulation (EU) No 10/2011, with no specific migration limit

What Is Glass Fiber Reinforced Plastic?#

Glass fiber reinforced plastic, also written GFRP or GRP, is any plastic whose mechanical properties come from glass fibers embedded in and bonded to the polymer matrix rather than from the polymer alone. This page covers glass fibre as a compounding additive for plastics. Structural glass-fibre composites such as GFRP rebar, GRP pipe and FRP wall panels are a construction topic and are only summarised at the end. Reinforcements as a class typically make up 15 to 30 wt% of the finished plastic products surveyed in the literature, a lower share than the 10 to 50 wt% range quoted for glass specifically, because that survey average blends glass, carbon, mineral and natural-fibre reinforced parts together.

Particulate additives that do not carry load, such as calcium carbonate, talc or titanium dioxide, are covered under fillers for plastics, and the distinction between the two categories is not cosmetic. It determines which additive raises which property, and it is the first decision a formulator makes before specifying a glass grade at all.

What is the difference between a filler and a reinforcement?#

A filler is dispersed in the polymer to change cost, density or stiffness, while a reinforcement has an aspect ratio high enough for the matrix to transfer load into it, which is what raises tensile strength rather than only modulus. Fillers are particulate, typically 5 to 70 wt% of the compound, and near-spherical fillers improve isotropy and flow while reinforcing very little. Platelet fillers such as talc (aspect ratio 5 to 40) and mica (20 to 100) raise stiffness and cut warpage. Fibres, with an aspect ratio from 20 to over 1,000, raise strength and modulus but bring anisotropy with them, because a fiber only carries load along its own axis.

GFRP, GRP, FRP and fiberglass: are they the same thing?#

GFRP, GRP and fiberglass-reinforced plastic all name the same class of material, while FRP is the wider term that also covers carbon, aramid, basalt and natural fiber composites. "Fiberglass" is the common name for the same glass-reinforced plastic in consumer and trade use. The spelling splits by region: "glass fiber reinforced plastic" is the more common US form and "glass fibre reinforced plastic" the more common UK and international form, and both refer to the identical material.

• GFRP (glass fiber reinforced polymer / plastic): glass-specific, used interchangeably with GRP
• GRP (glass reinforced plastic): the UK and construction-industry term for the same material as GFRP
• FRP (fiber reinforced plastic / polymer): the broader category that includes glass, carbon, aramid, basalt and natural fibers
• Fiberglass: the everyday name for glass fiber reinforced plastic, used for both the fabric and the finished composite

Is fiberglass a plastic?#

No: fiberglass is a composite, not a plastic, but it is roughly half plastic, because the glass fibers only work when a polymer matrix holds them in place and transfers load into them. The glass supplies the strength and stiffness; the polymer matrix supplies the shape, the environmental protection and the load path that reaches the fibers in the first place. Neither component performs the composite's job alone.

How Does Glass Fiber Reinforce a Polymer?#

Glass fiber reinforces a polymer by taking over the load: the matrix deforms, shear stress builds along the fiber surface, and the fiber carries the stress its own strength can bear, which is why the quality of the interface decides everything. H.L. Cox described this shear-lag load transfer in British Journal of Applied Physics 3 (1952) 72. Halpin and Kardos, Polymer Engineering and Science 16 (1976) 344-352, model how a compound's modulus depends on fiber modulus, aspect ratio and orientation, using a shape factor of about twice the aspect ratio for aligned fibers. Fu, Feng, Lauke and Mai, Composites Part B 39 (2008) 933-961, remains the canonical review of how particle size, interfacial adhesion and loading combine to set composite strength.

Load transfer and critical fiber length#

A short glass fiber only reaches its own tensile strength when it is longer than the critical fiber length lc = σf·d / (2τ), the length Kelly and Tyson defined in 1965, where σf is the fiber strength, d its diameter and τ the interfacial shear strength. Kelly and Tyson published the relationship in the Journal of the Mechanics and Physics of Solids 13 (1965) 329. Below lc, a fiber pulls out of the matrix before it breaks, so it never contributes its full strength.

This one equation explains both the sizing and the coupling agent: raising τ shortens lc, so shorter surviving fibers still carry full load, which is the entire justification for treating the glass surface with a silane rather than leaving it bare. No numeric critical-length value for a specific commercial compound is verified in our source base, so this page states the relationship rather than a length in millimetres for any named product.

Why fiber length falls between the roving and the molded part#

The fiber in a molded part is always shorter than the fiber that entered the extruder, because screw elements, the die and the gate break the strands, and that attrition is the reason long fiber thermoplastics exist as a separate product class. Where the fibre enters the screw is a plastic compounding decision on its own: compounders side-feed glass fiber downstream of the melting zone rather than through the main feed throat, so the fiber meets a melt that is already flowing instead of the full shear history of the screw's melting section.

Side feeders and screw design are covered in full under twin-screw compounding of additives, which sets out how feed position, screw element choice and residence time each affect how much of the incoming fiber length survives to the finished part.

What glass fiber costs you: anisotropy, warpage and weld lines#

Glass fiber buys strength, stiffness and heat deflection temperature, and it charges for them in anisotropy, warpage, weld-line strength, machine wear and density. This is the most repeated question buyers ask about glass fiber reinforced plastic, and the answer is a list of trade-offs rather than a single number.

• Anisotropic shrinkage and warpage, because fibers align with the flow direction during molding and shrink less along their length than across it
• Weld lines, where two flow fronts meet and the fibers on either side do not cross, leaving a plane with little reinforcement
• Lower notched impact strength in some systems, even as tensile modulus rises, because the fiber-matrix interface becomes the weak point under sudden load
• Fiber attrition during compounding and molding, which shortens the reinforcement below its as-supplied length
• Higher wear on screws, barrels, dies and gates, because glass is harder than the polymer it reinforces
• Higher compound density, since E-glass at 2.58 g/cm3 is denser than any unfilled thermoplastic, which matters for float-sink sorting in recycling

Glass raises the heat deflection temperature of semi-crystalline polymers more strongly than almost any other additive, because the rigid fiber network resists creep near the matrix's softening point even under a 1.8 MPa test load. Notched impact strength does not always follow the modulus upward, which is why a glass-filled grade that looks stiffer on a data sheet can still need an impact modifier if the part sees sudden loads.

What Are the 5 Types of Glass Used to Reinforce Plastics?#

The 5 glass compositions used to reinforce plastics are E-glass, E-CR glass, S-2 and R-glass, C-glass and D-glass, and E-glass accounts for the great majority of reinforced thermoplastic and thermoset compounds. A sixth composition, AR-glass, is alkali-resistant and is formulated for cement and concrete reinforcement, which places it outside the scope of a plastics-additive reference.

Values are reference data for the glass itself, not for a finished compound. Cells marked "not in our sources" are gaps in our source base; we do not publish estimated values.

1. E-glass: the standard reinforcement#

E-glass is an alumino-borosilicate glass with under 1% alkali oxides, and it is the composition behind almost every glass-filled thermoplastic compound on the market. Its reference tensile strength of 3,445 MPa and modulus of 76.0 GPa are the values the rest of this page uses as the baseline for glass fiber, at a density of 2.58 g/cm3.

2. E-CR glass: acid resistance#

E-CR glass is an acid-resistant variant of E-glass, used where the compound meets acids in service. It shares E-glass's general role as a reinforcement but is specified when the application involves acidic process fluids or acidic soil and water exposure that would attack standard E-glass more quickly.

3. S-2 glass and R-glass: high strength#

S-2 glass and R-glass are the high-strength compositions: S-2 reaches about 4,890 MPa and 85.5 GPa, roughly 40% more tensile strength and 12% more modulus than E-glass, at a lower density of 2.46 g/cm3. That comparison is arithmetic on the glass fiber's own reference values, not on a finished compound, and it explains why S-2 and R-glass appear in aerospace and other weight-critical composites rather than in general-purpose injection molding compounds, where cost rules over the last increment of strength.

4. C-glass: chemical resistance#

C-glass is chosen for chemical resistance and electrical insulation and reaches about 3,300 MPa tensile strength at 69.0 GPa modulus. Its density of 2.49 g/cm3 sits between E-glass and S-2 glass, and its composition is what gives it corrosion resistance in surfacing applications rather than raw structural performance.

5. D-glass: low dielectric constant#

D-glass has a low dielectric constant, which is why it appears in printed circuit boards and 5G laminates rather than in structural compounds. Its role is electrical, not mechanical: D-glass is specified when signal transmission properties matter more than tensile strength or modulus.

Which Glass Fiber Forms Go into Plastics Compounds?#

Glass fiber reaches a compound in four forms, chopped strand, milled fiber, continuous roving and mat or fabric, and the form sets the fiber length in the finished part more than any process setting does. Hollow glass microspheres in plastics are a related glass product but belong to the filler category rather than the reinforcement category, because their near-spherical shape gives isotropy and flow rather than load transfer.

1. Milled fiber, ground to the shortest lengths of the four forms
2. Chopped strand, cut to a few millimetres for injection-molding compounds
3. Long fiber thermoplastic (LFT) pellets and continuous roving, retaining the longest fiber length reaching the finished part
4. Mats and fabrics, continuous reinforcement for thermoset laminates rather than pelletised compounds

Chopped strand for injection molding compounds#

Chopped strand is the standard form for injection-molding compounds: bundles of sized filaments cut to a few millimetres, side-fed into the extruder and broken down further during compounding and molding. It is the form behind the 10 to 50 wt% glass loadings standard in reinforced polyamide, PBT, PET and PPS, and it survives compounding with enough residual length to remain a true reinforcement rather than a filler, provided fiber attrition through the screw and gate is controlled.

Milled fiber#

Milled fiber is ground glass fiber with an aspect ratio too low to reinforce much: it adds stiffness and dimensional stability, and it behaves more like a mineral filler than like a reinforcement. No fiber length or loading value for milled glass fiber is established in our source base, so this page does not state one; the distinguishing fact is functional, not dimensional: milled fiber falls below the aspect ratio a compound needs for the fiber-carries-the-load mechanism to dominate.

Long fiber thermoplastics and continuous roving#

Long fiber thermoplastic pellets are 10 to 12 mm long with the fibers running the full length of the pellet, which leaves 6.35 mm (0.25 in) or more of fiber in the molded part. That residual length is far above what chopped-strand compounds typically retain after compounding and molding, and it is the reason LFT grades reach higher impact strength and better load-bearing performance at equal glass content. Pellet formats and processes are on long fiber thermoplastics (LFT), which covers pultrusion-wire coating and the direct LFT process separately.

Mats and fabrics for thermoset laminates#

Mats, woven fabrics and chopped rovings over 25 mm belong to thermoset processing, where sheet molding compound, bulk molding compound and hand lay-up build the laminate rather than a pelletised compound. Sheet molding compound typically uses chopped fibers over an inch long in a polyester or vinyl ester resin, and 21 CFR 177.2410 separately lists glass fiber as an adjuvant substance for phenolic molded articles, alongside diatomaceous earth and mica.

How Much Glass Fiber Does a Compound Need?#

Reinforced thermoplastic compounds normally carry 10 to 50 wt% glass fiber, and 30 wt% is the grade the market most often treats as standard for polyamide and PBT. That statement should be read carefully: the sourced range comes from Clariant's Exolit brochure for flame-retardant polyamide, and the extension of the same 10 to 50 wt% range to general-purpose PA, PBT, PET and PPS compounds is a handbook range rather than a value confirmed against a specific compounder's data sheet.

These are the only glass loading ranges our source library holds with a source. Grade-specific levels come from the compound supplier's data sheet.

Request quotes for glass fiber and glass-reinforced compounds: specify polymer, glass type, loading, form, sizing chemistry, volume and country through the plastic additive supplier finder. Readers who need silane and coupling-agent guidance by polymer alongside a loading reference can also download the Glass Fiber Compound Selection Chart, a free guide covering silane choice, coupling agent and loading range by polymer, available against an email, role and company.

What does "GF30" or "30% glass filled" mean?#

GF30 means 30% glass by weight of the whole compound, not of the polymer, which is why a GF30 grade contains roughly half that share by volume. Loading can be stated as wt%, vol% or phr, and vol% is what governs the mechanics of load transfer even though wt% is what governs cost and compound density.

Take a GF30 polyamide compound as a worked example: 30 wt% E-glass at 2.58 g/cm3 in a polymer with a density near 1.14 g/cm3 works out to about 15.8 vol% glass, computed as (30/2.58) divided by [(30/2.58) + (70/1.14)]. That figure is an arithmetic example, not a product specification, and the exact volume share shifts with the base polymer's own density. Weight, volume and phr conversions in general are set out on PHR (parts per hundred resin).

Glass loading by polymer#

Glass loading is set by the stiffness and heat deflection target, then checked against surface finish, flow length and cost, because every added percent of glass shortens the flow path and roughens the surface. Table T3 above lists every polymer-specific loading our source library holds with a source; formulators reading it should treat the general PA/PBT/PET/PPS row as a starting point to confirm against a named compound's data sheet, not as a specification.

• Stiffness and heat deflection target, the primary driver that sets the minimum glass content
• Surface finish, since higher glass content roughens the visible surface and shows fiber patterns
• Flow length and wall thickness, since glass shortens the distance the melt reaches before freezing
• Cost, since glass fiber and the coupling chemistry both add to compound cost per kilogram

Flow improvers for highly filled compounds sit under polymer processing aids: hyperbranched polyester flow modifiers raise the spiral flow of highly filled and glass-fiber PA, PBT and PC compounds, which lets designers hold thinner walls at a given glass loading than the base resin would otherwise allow.

Why Glass Fiber Needs Sizing and a Coupling Agent#

Glass fiber needs a sizing because bare glass and a polymer melt do not bond: without the silane layer applied at the bushing, the fiber slides in the matrix instead of carrying load, and the compound gains density without gaining strength. Raising the interfacial shear strength at the fiber surface is exactly what the critical-length equation predicts will happen when a sizing is present, because a higher shear strength shortens the fiber length a broken strand needs to reach before it carries its full tensile load.

The whole coupling-agent class, spanning silanes, titanates and maleated polyolefins, is compared under coupling agents for filled and reinforced plastics, which sets out why silanes work on glass and mineral fillers with surface hydroxyl groups but fail on carbon black, graphite and some carbonate fillers.

What is glass fiber sizing?#

Sizing is the coating applied to glass filaments at the bushing, 0.5 to 2.0 wt% of the fiber, and it contains four things: a silane coupling agent, a film former, lubricants and an antistat.

• Silane coupling agent, the reactive layer that bonds to both the glass surface and the polymer matrix
• Film former, a polyurethane, epoxy or polypropylene dispersion that holds the filament bundle together
• Lubricants, which reduce filament-to-filament abrasion during winding and chopping
• Antistat, which prevents static buildup that would otherwise cause fuzzing and processing problems

An aqueous glass-fiber size bath typically runs 0.5 to 2.0% silane at around pH 5.5, cured on the fiber at 110 to 120 degrees Celsius for 20 to 30 minutes. Size-bath formulation is covered in detail under coupling agents for glass fiber, which goes into film-former chemistry and bushing-line process parameters this page does not repeat.

Which silane for which polymer#

The silane is chosen for the matrix, not for the glass: aminosilanes such as APTES (CAS 919-30-2) suit polyamide, PBT and phenolics, while the epoxy silane GLYMO (CAS 2530-83-8) is the sizing silane for PET, PBT, polycarbonate, thermoset polyester and epoxy vinylester. The mechanism is the same for every organofunctional silane: ethoxy or methoxy groups hydrolyse in the size bath to silanols, which condense with hydroxyl groups on the glass surface, while the organic functional group on the other end of the molecule reacts into the polymer. For APTES, that functional group is a primary amine that reacts with polyamide, epoxy, phenolic or PBT chain-end groups. For GLYMO, it is an epoxy ring that opens and reacts with amines, acids and hydroxyls in the curing or reacting matrix.

All organofunctional types are compared on silane coupling agents, including the titanate alternative used where silanes are ineffective, on fillers such as calcium carbonate, gypsum, barite, graphite and carbon black.

3-Aminopropyltriethoxysilane (APTES) carries a harmonised CLP classification, Acute Tox. 4 H302 and Skin Corr. 1B H314, that matters for handling the size bath even though the cured sizing on the finished fiber does not carry the same hazard statements.

GLYMO (KH-560) is the only silane the EU authorises specifically for glass-fibre sizing under EU 10/2011: FCM 1068 permits it only as a sizing component for glass fiber embedded in low-diffusivity plastics, with residues not detectable at 0.01 mg/kg for the substance and 0.06 mg/kg for each reaction product.

Maleated polypropylene for glass-filled PP#

Polypropylene has no functional group for the silane to react with, so glass-filled PP needs a second coupling step: maleic-anhydride-grafted polypropylene at about 0.5 to 2 wt%, whose anhydride groups bond to the silanol groups on the fiber while its PP backbone entangles with the matrix. The maleic anhydride groups on PP-g-MAH react with the Si-OH groups on the sized glass surface by esterification or hydrogen bonding, while the polypropylene backbone co-crystallises and physically entangles with the surrounding matrix. Grafting chemistry and grades are on maleic anhydride grafted polymers, which covers PP-g-MAH alongside PE-g-MAH, POE-g-MAH and SEBS-g-MAH.

Polybond 3200 carries 0.8 to 1.2% maleic anhydride and is grafted at 180 to 190 degrees Celsius, a process temperature at which beta-scission also raises the polymer's melt flow rate. In the EU, food-contact status runs through the grafting monomer itself: maleic anhydride is FCM 234, with an SML(T) of 30 mg/kg expressed as maleic acid (group 3).

Which Polymers Are Reinforced with Glass Fiber?#

Glass fiber reinforces polyamide, polypropylene, PBT, PET, polycarbonate and PPS among thermoplastics, and unsaturated polyester, epoxy and phenolic resins among thermosets. The additive consequences differ by polymer, because each matrix brings its own functional groups, processing temperature and failure mode to the fiber-matrix interface.

Glass-filled nylon (PA6 and PA66)#

Glass-filled nylon is the largest single use of glass fiber in thermoplastics: PA6 and PA66 with aminosilane-sized E-glass, stabilized with copper halide systems when the part runs hot. Glass fiber also nucleates the polymer as it crystallises, which makes talc less necessary as a separate nucleating agent in reinforced grades, and black compounds typically use nigrosine (Solvent Black 7, CAS 8005-02-5) rather than carbon black alone, because nigrosine gives the high gloss that covers the fibers at the surface. Grade names, properties and moisture effects specific to the polyamide family are on glass-filled nylon.

The complete polyamide additive package, including moisture management and the full stabilizer range, is on additives for nylon (polyamide). In heat-aging tests, PA66 GF30 compounded with 3% dipentaerythritol and 0.3% copper stabilizer retained 99.1% of its tensile strength after 500 hours at 210°C and 94.8% after 1,000 hours, against 78.2% for a copper-only control at 500 hours; BRUGGOLEN TP-H1804 separately gives over 5,000 hours of retention at 190°C in PA66 GF30. Copper halide systems of this kind are covered under heat stabilizers for nylon.

Glass-filled polypropylene#

Glass-filled polypropylene only performs when a maleated polypropylene couples the sized fiber to the non-polar matrix, which is the single biggest difference between glass-filled PP and glass-filled nylon. PP has no amine or hydroxyl end group for a silane to react into directly, so the coupling step described above for PP-g-MAH is not optional the way it can be for polyamide, where the amine end groups already give the silane something to react with. Coupling levels and grade choices specific to reinforced PP are on glass-filled polypropylene, and the full PP additive package is on additives for polypropylene.

Glass-reinforced PBT, PET and polycarbonate#

In PBT the interface is also the weak point in service: hydrolysis above 60 degrees Celsius attacks the fiber-matrix bond before it attacks the bulk polymer, so hydrolysis stabilization and glass reinforcement are one decision, not two. PBT melts at 223 degrees Celsius and is sensitive to hot water above that 60-degree threshold, which is why GLYMO sizing, the food-contact-cleared route for PET, PC and PBT, is specified as much for hydrolytic stability as for initial bond strength. Hydrolysis stabilization specific to this polymer is covered on additives for PBT.

Glass fiber content also changes the flame-retardant answer in these matrices: Exolit OP 1240 in glass-filled PBT reaches UL 94 V-0 at wall thicknesses from 0.4 to 3.2 mm with a CTI of 600 V. Glass in polycarbonate changes the flame-retardant choice the same way, as set out on additives for polycarbonate and PC/ABS.

Thermosets: unsaturated polyester, epoxy and phenolics#

Thermoset composites use the same E-glass with a different silane: methacryl or epoxy functional silanes that react into the curing resin rather than with polymer end groups. Unsaturated polyester, sheet molding compound and bulk molding compound formulations combine calcium carbonate or aluminium trihydrate filler with chopped glass, typically over an inch long in SMC. GLYMO serves the same role in thermoset polyester and epoxy bisphenol vinylester that it serves in PET and PBT: the epoxy-functional silane bonded to the glass surface reacting into the curing resin system.

How Does Glass Fiber Change the Rest of the Additive Package?#

Glass fiber is never a single-additive decision: it changes the flame retardant, the heat stabilizer, the colorant, the nucleating agent and the flow of the compound at the same time. Each of these downstream changes traces back to a property glass fiber brings with it, whether that is the wick effect in a flame-retardant system, the abrasive melt it produces, or the crystallisation nucleus it provides for free.

The wick effect and the phosphinate answer are covered under flame retardants for nylon, which explains why non-reinforced polyamide gives a non-dripping V-0 rating that reinforced polyamide does not, because the glass fiber itself provides a wick that draws molten polymer to the flame front. Glass fiber's own nucleating effect is why nucleating agents are often skipped in reinforced grades even where they would be standard in the unfilled resin.

How Are Glass Content and Fiber Length Measured?#

Glass content is measured by burning the polymer away and weighing what is left, under ISO 1172 for glass-reinforced plastics or the general ash methods ISO 3451 and ASTM D5630. Methods and specimen preparation in full are on fibre length and glass content measurement, which this section summarises for the reinforcement-specific case.

Glass content by calcination and ashing#

ISO 1172 burns off the matrix in a crucible and reports the residue as glass content, while ASTM D5630-22 gives the general ash procedure with a muffle method on 5 to 50 g and a rapid-ash method on 2 to 10 g. ASTM D5630-22 and ISO 3451 exclude halogenated polymers from the standard procedure, since combustion residues from those systems distort the ash weight. Crucible masses and the halogen exclusion are set out in full on ash content and filler content testing.

Fiber length, HDT and impact checks#

Four properties track whether the reinforcement is working: residual fiber length, tensile strength and modulus to ISO 527 or ASTM D638-22, Charpy impact to ISO 179 and heat deflection temperature to ISO 75 or ASTM D648.

• Residual fiber length, measured after ashing to check how much attrition the compounding and molding process caused
• Tensile strength and modulus, to ISO 527-1/-2 or ASTM D638-22
• Charpy impact strength, to ISO 179
• Heat deflection temperature, to ISO 75-1/-2 (flatwise, 80 x 10 x 4 mm specimens at 1.80, 0.45 or 8.00 MPa) or ASTM D648-18 (edgewise, specimens 3 mm and thicker, at 0.455 or 1.82 MPa, heated at 2°C/min to 0.25 mm deflection)

ASTM D648 states explicitly that its HDT data are not intended for design or for predicting endurance at elevated temperature; the standard measures a comparative deflection point, not a service-temperature rating. No numeric pass or fail target for any glass-filled compound is established in our source base, so the endpoint on any of these four tests is set by the individual compound specification, not by a universal glass-fiber value.

What Is the Regulatory and Health Status of Glass Fiber in Plastics?#

Glass fiber itself is authorised for food-contact plastics in the EU as FCM No 38 with no specific migration limit, and the regulatory questions that actually arise concern the sizing on the fiber, not the glass. In the US, 21 CFR 177.2410 clears glass fiber as an adjuvant substance in phenolic molded articles. The nuance that matters in practice is that the fiber, the sizing silane and any coupling agent in the compound each carry their own regulatory status.

Food contact: the fiber, the sizing and the silane#

The food-contact question splits in three: the glass (FCM 38, no SML), the silane in the sizing (GLYMO under FCM 1068 only for PET, PC, PBT, thermoset polyesters and epoxy bisphenol vinylester) and the coupling agent in the compound (maleic anhydride, FCM 234, SML(T) 30 mg/kg as maleic acid).

Cells marked "not held in our source base" are gaps we have not verified against a primary instrument. We do not publish a regulatory status we have not read.

SML, OML and the Union list mechanics behind every entry in this table are explained on EU 10/2011, which covers the generic 60 mg/kg SML and the 10 mg/dm2 OML that apply across the plastics-additive site.

REACH registration and the Annex V question#

Glass fiber sits differently from a chemical additive under REACH: ECHA CHEM lists 11 active registration dossiers for amorphous glass fiber (EC 610-130-5), while glass and ceramic frits are covered by an exemption in Annex V. A separate registration, "Glass, oxide, chemicals" (EC 266-046-0), carries one active dossier. Neither APTES, GLYMO, MEMO nor the titanate KR-TTS appears on the SVHC Candidate List as of 22 September 2026, while tris(2-methoxyethoxy)vinylsilane (CAS 1067-53-4), a different silane not used in glass-fiber sizing, has been listed as an SVHC under Article 57(c) since 17 January 2022. Registration, exemptions and the Candidate List mechanics are explained in full on REACH and plastic additives.

Is glass fiber carcinogenic?#

IARC places continuous filament glass, the form used to reinforce plastics, in Group 3, which means the evidence does not allow a judgement on carcinogenicity to humans (Monographs vol. 81, 2002). That Group 3 classification applies specifically to continuous filament glass and must not be read across to glass wool, which IARC evaluates as a different agent. The workplace limits for respirable crystalline silica do not cover glass fiber, which is a different agent with its own classification; dust from machining or grinding reinforced parts is handled under general dust and mechanical-irritation rules rather than the silica limits. No Prop 65 listing for glass fiber or glass wool is confirmed in our source base, so this page states none.

Substances with a confirmed hazard classification, rather than an unclassifiable one, are listed under toxic plastic additives, which is the page to check before assuming a glass-fiber compound carries a hazard the glass itself does not.

Who Supplies Glass Fiber for Plastics?#

Glass fiber for plastics is made by a small group of producers, with Owens Corning, Jushi, CPIC, Taishan, NEG and Johns Manville the names that recur in the trade literature, while the reinforced compound itself usually comes from a compounder rather than from the fiber maker. Buyers should specify the glass type, the form, the loading and the sizing chemistry, because the sizing decides whether the fiber suits their matrix, and a fiber that is a poor match for the resin performs no better than an unsized reinforcement regardless of the producer's name.

Mineral and filler producers with an established directory presence are listed under mineral filler suppliers for plastics; a dedicated reinforcing-fiber directory is planned for this site.

Request quotes for glass fiber and glass-reinforced compounds: specify polymer, glass type, loading, form, sizing chemistry, volume and country through the plastic additive supplier finder.

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Where Glass Fiber Reinforcement Goes Beyond Compounding#

Most of what is published under the term GFRP describes structural composites rather than plastics compounding: rebar, pipe, tanks and wall panels made by pultrusion, filament winding or hand lay-up. These share the same E-glass this page describes, but the engineering discipline around them, including the process chemicals used to release laminates from a mold, is mold release agents for composites, a different question from compounding a pelletised thermoplastic.

GFRP rebar, GRP pipe and FRP panels (outside the scope of this site)#

GFRP rebar, GRP pipe and FRP wall panels are construction products made from the same E-glass, and they are governed by civil-engineering standards that fall outside the scope of a plastic additives reference. GFRP rebar reinforces concrete as a non-corroding alternative to steel rebar. GRP pipe carries fluids in chemical-process and water-infrastructure applications. FRP wall panels and cladding serve as corrosion-resistant building envelopes. This site does not cover their installation standards, design codes or service life.

Carbon, basalt, aramid and natural fibers compared#

Against the alternatives, glass is the mid-tier reinforcement: basalt fiber reaches a similar modulus at 85 to 87 GPa, carbon fiber roughly doubles to triples it at 150 to 250 GPa in civil-engineering grades, and aramid trades stiffness for toughness. No price value for glass fiber is established in our source base, so this comparison is made on mechanical properties only, never on cost.

Modulus, cost and conductivity are compared in full on carbon fiber reinforced plastics, which covers the electrically conductive behaviour that sets carbon fiber apart from every glass composition on this page.

Basalt fiber reinforcement reaches a similar modulus to S-2 glass at a slightly higher density, and its 10 to 20 micrometre filament diameter sits above the carbon fiber range.

Flax, hemp and jute options are on natural fiber composites, which trade the strength of glass and carbon for lower density and a renewable feedstock.

Aramid fibres in plastics trade stiffness for toughness: an aramid fiber is defined by having at least 85% of its amide linkages attached directly to two aromatic rings (FTC 1974, ISO 1977), and world para-aramid capacity reached 41 kt per year in 2002.

What are the disadvantages of glass fiber reinforced plastic?#

The disadvantages of glass fiber reinforced plastic are anisotropy and warpage, weak weld lines, abrasive wear on processing equipment, a higher compound density and fiber attrition that shortens the fibers during compounding and molding. Every one of these trade-offs traces back to the same source: a rigid, hard, directional fiber embedded in a matrix that is none of those three things on its own.

Is glass fiber reinforced plastic conductive?#

No: E-glass is an insulator and glass-filled compounds stay electrically insulating, which is the opposite of carbon fiber compounds, where the fiber itself conducts. C-glass is specified for electrical insulation and D-glass for its low dielectric constant, both applications that depend on glass fiber's non-conductive nature. Compounds that do need conductivity use carbon fiber, conductive carbon black, carbon nanotubes or steel fiber instead. No volume-resistivity value for a glass-filled compound is established in our source base, so this page states the qualitative answer only.

Can glass-filled plastics be recycled?#

Glass-filled thermoplastics can be reground and remolded, but every pass shortens the fibers further, so recycled glass-filled compounds lose the strength the reinforcement was added for. That fiber-shortening effect is the same attrition mechanism described earlier under fiber length in the finished part, repeated once for every additional processing pass. Density and sorting consequences for recyclers are covered on design for recycling, including the float-sink effect that a glass-filled compound's higher density has on sorting streams.

Who invented glass fiber?#

Games Slayter filed the glass-wool patent at Owens-Illinois in 1933, commercial glass fiber production started in 1936, and Owens-Corning Fiberglas was formed in 1938. That corporate history is why Owens Corning remains one of the producer names most often cited in the glass-fiber trade literature today, nearly a century after the original patent.

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