Reinforcing Fibers for Plastics (Glass, Carbon, Natural): 5 Fiber Types, Loading and Selection

Reinforcing fibers are high-aspect-ratio additives, glass, carbon, aramid, natural and basalt fibers, that carry mechanical load inside a polymer and make up 15 to 30 wt% of a typical reinforced plastic. That 15 to 30 wt% band is the composition range published for reinforcements by Chea and co-workers in 2025, adapted from the plastic-additive inventory of John Hahladakis and colleagues in 2018. A fiber only carries that load if the melt transfers it across the interface, so which fiber, which length and which coupling chemistry does a compound actually need?

Fiber and filler are separated by geometry rather than by chemistry: a blocky calcium carbonate particle with an aspect ratio of 1 to 3 fills, while a wollastonite needle at 5 to 30 and a drawn filament reinforce. Reinforcing fibers are one of the 2 families in the fillers and reinforcements group of the 43 families of plastic additives, and they differ from fillers by aspect ratio, not by chemistry. Reinforced thermoplastics are therefore a formulation question, not a material class.

This reference covers what a reinforcing fiber is and how the FRP, GRP and GFRP terminology maps onto it, how load transfers from matrix to fiber and why fiber length decides the result, the sizing and coupling chemistry every fiber needs, the 5 fiber types with their sourced property values, the short, long and continuous fiber forms, how much fiber each polymer carries, how to select a fiber in 6 steps, what reinforcement does to the stabilizer and flame-retardant package, how fiber content is measured, how fibers and their sizings are regulated in the European Union and the United States, who supplies them, and the 8 pages of this family.

Table 1. The 5 reinforcing fibers for plastics at a glance. The Fiber column carries the single link to each type page on this site.

Property values are single published figures for one grade of each fiber, not specification limits. Where our source library holds no verified value, the cell says so rather than carrying a handbook estimate.

What Is a Reinforcing Fiber in Plastics?#

A reinforcing fiber is a high-aspect-ratio additive that takes over part of the mechanical load in a polymer, because stress transfers from the matrix to the fiber through shear at the interface, and it therefore raises tensile strength and modulus rather than only cost or stiffness. Reinforcements sit at 15 to 30 wt% of the finished plastic product in the composition ranges of Chea and co-workers, 2025, which is a narrower and higher band than the 0 to 50 wt% that fillers as a group occupy. Both standard references treat the class as its own subject: the Hanser Plastics Additives Handbook carries a chapter titled "Fillers and Reinforcements", and Murphy's Additives for Plastics Handbook gives "Fillers" and "Reinforcements" two separate chapters.

Where does that leave calcium carbonate, talc and the other mineral powders? Reinforcement is a function, not a chemistry, so 3 minerals sold as reinforcements are covered on the filler side of this site: wollastonite with an aspect ratio of 5 to 30, mica at 20 to 100 and talc at 5 to 40 all raise stiffness and, in the case of the acicular grades, strength. This page covers the drawn and grown fibres, glass, carbon, aramid, natural and basalt, and the sizing and coupling chemistry that makes them work.

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

A filler displaces polymer volume and changes stiffness, density and cost, while a reinforcement carries load: the dividing property is aspect ratio, which runs from 1 to 3 for blocky calcium carbonate up to 20 to 100 for mica platelets and far higher for a drawn fiber. Loading levels follow the same split. Fillers occupy 0 to 50 wt% of the plastic product and calcium carbonate in polypropylene is typically dosed at 20 to 40 %, while reinforcements concentrate in the 15 to 30 wt% band, because a fiber that is present at 5 % changes little and one at 60 % cannot be moulded.

Particle shape sorts the candidates into 3 classes, listed below with one aspect ratio from our source library for each.

• Near-spherical and blocky particles. Calcium carbonate at an aspect ratio of 1 to 3 is the archetype, and the powders that displace polymer volume are compared on fillers for plastics.
• Platy particles. Talc at 5 to 40, kaolin at 10 to 30 and mica at 20 to 100 raise modulus and lower warpage, and they align in the flow direction during moulding.
• Needles and fibers. Wollastonite reaches an aspect ratio of 5 to 30 and is sold as a mineral reinforcement; the acicular mineral with an aspect ratio of 5 to 30 is covered on wollastonite in plastics, while a drawn glass or carbon filament is longer again.

No aspect ratio is published here for glass or carbon fiber, because our source library holds filament diameters for those fibers but no measured length-to-diameter ratio in a compound. Particle size, aspect ratio, surface area and oil absorption are set out as measured properties on filler properties: particle size and aspect ratio.

Are FRP, GRP, GFRP and fiberglass the same thing?#

FRP, GRP, GFRP and fiberglass all name the finished composite, a polymer matrix with fibers in it, while this page covers the fiber itself as an additive in a compound. FRP abbreviates fibre-reinforced plastic, GRP glass-reinforced plastic and GFRP glass fiber reinforced polymer, and "fiberglass" is the North American trade word for the same family of products. The four terms therefore answer the question "what is fibre reinforced plastic" at the level of the article, not at the level of the formulation, and a compounder buying chopped strand is one step upstream of all of them.

The border of this reference follows from that distinction. Laminated FRP wall panels and sheet, GFRP rebar, GRP pipe and fiberglass insulation are building products and are not covered here, whatever the search term they share with this page. Reinforced thermoplastic and thermoset compounds, the fibers inside them and the additives that go with them are covered in full.

How Do Reinforcing Fibers Strengthen a Polymer?#

Reinforcing fibers work by shear transfer: the polymer deforms, shear stress builds along the fiber surface, and the fiber, which is far stiffer than the matrix, takes the load the matrix cannot carry. E-glass illustrates the stiffness gap with a modulus of 76.0 GPa against the 1 to 3 GPa range of an unfilled commodity thermoplastic, and carbon composite grades reach 150 to 250 GPa. Nothing in that mechanism is chemical: the fiber does not scavenge radicals, absorb light or release water, and it changes the compound only through geometry and the interface.

Load transfer runs in 3 steps, listed below in the order in which they happen under stress.

1. The matrix deforms first. The polymer around the fiber strains under the applied load, because it is the compliant phase.
2. Shear stress builds along the interface. The difference in strain between matrix and fiber is carried as shear at the fiber surface, which is why the sizing and the coupling agent decide how much load ever reaches the fiber.
3. The fiber carries the stress. Tensile stress accumulates along the fiber from both ends toward its middle, and the fiber fails in tension rather than pulling out only when it is long enough for that stress to reach its own breaking strength.

The stiffness of a reinforced compound is normally modelled with the Halpin-Tsai equations of John Halpin and John Kardos, published in Polymer Engineering and Science 16 (1976) 344 to 352, in which the shape factor rises with aspect ratio. The equations describe how modulus scales with fiber content and shape; they do not replace a supplier data sheet, and no worked Halpin-Tsai example for a fiber compound is published in the sources this page draws on.

Why fiber length decides how much strength a fiber delivers#

A fiber delivers its full tensile strength only above its critical length, the length at which interfacial shear can build up the fiber's own breaking stress, which is why the same glass grade reinforces a long-fiber compound more effectively than a short-fiber one. Below that length the fiber pulls out of the matrix and the compound fails at a lower stress, while stiffness, which depends on strain rather than on failure, suffers far less. The critical length scales with fiber diameter, and the diameters involved are small: 5 to 10 µm for carbon filament and 10 to 20 µm for basalt.

Fiber length is not a constant of the compound, because every melt step shortens it. A chopped strand entering a twin-screw extruder leaves it shorter, and the moulding machine shortens it again, which is the reason the long-fiber thermoplastic route exists at all: its pellets are 10 to 12 mm long with the fibers running the full pellet length, and the fiber length surviving in the finished part is 6.35 mm (0.25 in) or more. Pellet geometry and part properties are covered on long fiber thermoplastics (LFT). No numeric critical length for a named glass and polymer pair is published in the sources used here, so the relationship is stated without a value.

Why every reinforcing fiber needs a sizing and a coupling agent#

A drawn glass filament is chemically inert and abrasive, so it leaves the bushing with a sizing of 0.5 to 2.0 wt% on the fiber that combines a film former, a silane and a lubricant, and it is that silane which later bonds the fiber to the polymer. The size is applied from an aqueous bath, and Gelest gives the working conditions for the silane component as 0.5 to 2.0 % silane at pH 5.5, cured at 110 to 120 °C for 20 to 30 minutes. 3-Aminopropyltriethoxysilane (CAS 919-30-2, EC 213-048-4) is the amino-silane used for glass fiber and mineral fillers in polyamide, PBT, polycarbonate, phenolics and epoxies: its ethoxy groups hydrolyse to silanols, which condense with the surface M-OH groups of the glass to form Si-O-M bonds, while the primary amine reacts with or hydrogen-bonds to the matrix. The chemistry of the silane coupling agents in that size is set out separately, with dosing for the integral-blend route at 0.2 to 1.0 wt% of the total mix.

Polyolefins are the exception that proves the rule, because polypropylene offers the silane nothing to react with. In a glass-fiber polypropylene compound the coupling agent therefore sits in the matrix rather than on the fiber: maleic anhydride grafted polypropylene is added at 0.5 to 2 wt% in the published short-glass studies, and 5 wt% in one hybrid-fiber study. Its succinic anhydride groups react or hydrogen-bond with the Si-OH of glass, the OH of cellulose and the amine end groups of polyamides, while the polypropylene backbone entangles with the matrix and transfers the stress across. The grafting chemistry behind them is on maleic anhydride grafted polymers, and size formulations by matrix are compared on coupling agents for glass fiber.

Table 2. Sizing and coupling chemistry by matrix.

5 Types of Reinforcing Fibers for Plastics#

The 5 types of reinforcing fibers used in plastics are glass fiber, carbon fiber, aramid fiber, natural fiber and basalt fiber, and glass carries the great majority of commercial thermoplastic compounds. No tonnage or market share is attached to that statement anywhere on this page, because no sourced figure for the ranking of fibers by volume exists in our source library. Mineral reinforcements such as wollastonite and mica, and hollow glass microspheres, sit on the fillers hub instead.

1. Glass fiber#

Glass fiber is a drawn silicate filament and the default reinforcement of thermoplastics: E-glass reaches 3,445 MPa tensile strength and 76.0 GPa modulus at a density of 2.58 g/cm3. Two further compositions carry measured values here, and the 3 are listed below with the property set that separates them.

• E-glass. 3,445 MPa tensile strength, 76.0 GPa modulus, density 2.58 g/cm3, and the composition used in most reinforced plastics.
• S-2 glass. 4,890 MPa tensile strength, 85.5 GPa modulus, density 2.46 g/cm3, the high-strength composition of the three.
• C-glass. 3,300 MPa tensile strength, 69.0 GPa modulus, density 2.49 g/cm3, the chemically resistant composition.

Glass leaves the bushing already coated, since the sizing at 0.5 to 2.0 wt% on the fiber is applied in line, and the compounds that follow it are the most heavily regulated of the five. Glass fibers hold their own entry on the Union list of Regulation (EU) No 10/2011 as FCM 38, Ref 55520; 21 CFR 177.2410 lists glass fiber, with asbestos fiber, diatomaceous earth, mica and carbon black, as an adjuvant in phenolic moulded articles; and the International Agency for Research on Cancer placed continuous filament glass in Group 3 in Monograph volume 81 of 2002. Loading runs high in engineering compounds: flame-retarded polyamides carry 10 to 50 % glass in the Clariant Exolit OP data. Alkali-resistant AR-glass reinforces concrete rather than plastics and stays outside the scope of this site.

2. Carbon fiber#

Carbon fiber is a 5 to 10 µm filament made by carbonising a polyacrylonitrile, rayon or pitch precursor, and it gives the highest stiffness of the 5 reinforcements, with 150 to 250 GPa modulus in civil-engineering composite grades. The polyacrylonitrile route passes through an oxidative stabilisation step at about 300 °C in air before carbonisation, which is what converts a textile precursor into a carbon skeleton. The strength figure that goes with those moduli, about 3 GPa, is a composite value for civil-engineering grades and not a single-filament value, which this page does not convert into a filament value.

Carbon fiber enters thermoplastic compounds in the same forms as glass, as milled fiber, chopped strand and continuous tape, and it competes with glass on stiffness per unit weight rather than on cost. Its regulatory record here is thin by comparison: no entry for carbon fiber appears on the Union list of Regulation (EU) No 10/2011 in the sources checked, which is a statement about the sources rather than a prohibition, since a substance can sit outside the Union list and still be used outside food contact. Grades, precursors and the comparison with glass are covered on carbon fiber reinforced plastics.

3. Aramid fiber#

Aramid is an aromatic polyamide fiber, CAS 24938-64-5, defined since the United States Federal Trade Commission ruling of 1974 and the ISO definition of 1977 as a fiber in which at least 85 % of the amide linkages are attached to two aromatic rings. That definition, rather than a property value, is what identifies the class: our sources hold no tensile strength, modulus or density for any aramid grade, so none is stated here.

Capacity figures exist only for the para-aramid subclass and only as a dated point: world capacity stood at 41 thousand tonnes a year in 2002, growing 5 to 10 % a year, and nothing more recent is recorded here. Aramid has no entry on the Union list of Regulation (EU) No 10/2011 in the sources checked and no United States food-contact citation in our source library. The grade families, the applications and the property data that this page cannot supply are the subject of aramid fibres in plastics.

4. Natural fiber#

Natural fibers are plant and wood-derived reinforcements, including wood flour, sawdust and cellulose, used to cut density and cost in polypropylene and polyethylene compounds, and their processing window is limited because cellulose degrades. A wood-plastic composite runs at least 28 °C below the unfilled resin and at a ceiling of about 200 °C for that reason, and its burning point sits at about 204 °C. One published recycled formulation shows the proportions: 34 % recycled HDPE with 60 % sawdust, plus a coupling wax, stearic acid and pigment. Maleated polypropylene at 1 to 3 wt% is what bonds the polar cellulose surface to the non-polar polyolefin, and the full WPC package is on additives for wood-plastic composites.

Food-contact status is the point on which natural fiber differs most sharply from glass. Cellulose is listed as FCM 553, Ref 14500, while Regulation (EU) 2023/1442 deleted untreated wood flour and fibres, FCM 96, from the Union list: articles first placed on the market before 1 February 2025 may be sold until stocks are exhausted, and wood-flour products may continue where an application was filed before 1 August 2024. The canonical academic reference for the class remains the review by Omar Faruk, Andrzej Bledzki, Hans-Peter Fink and Mohini Sain in Progress in Polymer Science 37 (2012) 1552 to 1596. The wider plant-fiber set is treated on natural fiber composites.

5. Basalt fiber#

Basalt fiber is melt-spun from volcanic rock containing more than 46 % silica and a low iron content, and it reaches 2.8 to 3.1 GPa tensile strength with an 85 to 87 GPa modulus, above E-glass, at a density of 2.67 g/cm3. Its elongation at break is 3.15 % and its filament diameter 10 to 20 µm, so it sits geometrically between carbon and the coarser glass products. The fiber is drawn without additives, which distinguishes it from a glass composition adjusted by batch chemistry.

Basalt has no entry on the Union list of Regulation (EU) No 10/2011 in the sources checked and no cost, availability or supply figure in our source library, so none is given. The comparison with E-glass at equal density, and the sizing chemistry it shares with glass, are covered on basalt fiber reinforcement.

Short, Long and Continuous Fiber: How Fiber Form Changes the Compound#

Reinforcing fibers reach the polymer in 3 forms: short chopped fiber compounded on a twin-screw extruder, long-fiber pellets of 10 to 12 mm in which the fibers run the full pellet length, and continuous fiber in tapes and laminates. The form decides the surviving fiber length in the part, and the surviving length decides how much of the fiber's tensile strength the compound can use. A short-fiber compound is the cheapest to make and the easiest to mould; a long-fiber pellet keeps 6.35 mm (0.25 in) or more of fiber in the finished part; a continuous laminate keeps all of it and gives up injection moulding in exchange.

Table 3. The 3 fiber forms and what each one is for.

Compounding is where the form is either preserved or destroyed. Fiber is fed downstream rather than into the main hopper during plastic compounding, so that it spends the shortest possible time under shear, and the same logic governs screw design and the position of the side feeder. Screw and feeder choices are on twin-screw compounding and side feeding. This page gives no screw-design, side-feeder or fiber-breakage numbers, because none is recorded in our source library. Two forms sit at the edges of the family: rotomoulded composites carry about 10 % glass with nano-fillers at 3 % or below, and milled fiber behaves more like a mineral filler than like a reinforcement.

How Much Fiber Does a Reinforced Compound Contain? Loading by Polymer#

A reinforced plastic contains 15 to 30 wt% fiber in the finished product, and flame-retarded polyamide compounds run from 10 to 50 % glass. The 15 to 30 wt% range comes from the review by John Hahladakis and colleagues at the University of Leeds, published in the Journal of Hazardous Materials in 2018, as tabulated by Chea and colleagues in 2025. The 10 to 50 % figure for flame-retarded polyamide is supplier data from the Clariant Exolit OP brochure, and it is the widest sourced loading range in our source library for any single polymer family. Every loading on this page is weight per cent of the compound, not volume per cent, and the two differ substantially because glass is roughly twice as dense as polyamide.

Table 4. Fiber loading by polymer in reinforced compounds.

Fiber is bought with 4 penalties, listed below, and each one is a specification decision rather than a defect.

• Anisotropy and warpage. Short fibers align with the flow, so shrinkage differs along and across it, which is why low-warpage mineral grades exist alongside glass.
• Surface finish. Fiber close to the skin shows through as a matt or streaked surface, and a paintable part usually needs a process answer rather than a formulation one.
• Wear on the screw, the barrel and the tool. Abrasion scales with the hardness and the loading of the reinforcement, and the machine specification changes with it.
• Density above 1 g/cm3. Glass at 2.58 g/cm3 pulls a polyolefin compound upward, which matters for part weight and for float-sink sorting at end of life.

What does GF30 mean?#

GF30 means the compound contains 30 wt% glass fiber, so a PA66 GF30 pellet is 70 wt% polyamide 66 and 30 wt% glass. The same convention produces GF15, GF50 and, for carbon, CF30, and it is always a weight fraction of the compound rather than a volume fraction. PA66 GF30 is the grade behind the heat-ageing data quoted on this site, which makes it the reference point for comparisons here rather than a claim about what the market buys most.

Which Reinforcing Fiber Suits Each Polymer and Part?#

The reinforcing fiber follows the property target: glass for strength and stiffness at the lowest cost, carbon where stiffness per unit weight decides, basalt where a higher modulus than E-glass is wanted, natural fiber where density and renewable content matter, and aramid where the part is specified against a fiber identity rather than a published modulus. The matrix polymer constrains that choice immediately afterwards, because the coupling chemistry that works in polyamide is not the one that works in polypropylene, and a food-contact part narrows the sizing options to a single authorised silane.

Table 5. Property target and the fiber that answers it.

Application decides how much of that table applies. Structural car parts combine a fiber decision with a stabilizer, flame-retardant and surface decision taken at the same time, and the complete package for them is on additives for automotive plastics. Printed parts are the newest case of the same question, since glass- and carbon-filled filaments are covered on additives for 3D printing filaments, where the fiber has to survive an extrusion step in the printer as well as in the compounding line.

How to select a reinforcing fiber in 6 steps#

Select a reinforcing fiber in 6 steps: define the mechanical target, fix the matrix and its processing temperature, choose the fiber type, choose the fiber form and length, match the sizing and coupling agent, then set the loading and confirm it by test. The order matters, because a decision taken at step 2 removes options at step 5 and a decision taken at step 5 constrains the loading at step 6.

1. Define the mechanical target and the direction it acts in: tensile strength, modulus, heat deflection temperature or wear, measured along the flow or across it.
2. Fix the matrix polymer and its processing temperature before choosing a fiber, since a wood-plastic composite runs at a ceiling of about 200 °C while a glass-filled polyamide runs far above it.
3. Choose the fiber type against that target, using the property values in Table 5: carbon for stiffness per unit weight, glass for strength at the lowest cost, basalt for modulus, natural fiber for density and renewable content.
4. Choose the fiber form and length, between short chopped fiber, a long-fiber pellet of 10 to 12 mm that leaves 6.35 mm or more in the part, and continuous fiber.
5. Match the sizing and the coupling agent to the matrix: an amino-silane size for polyamide and PBT, an epoxy-silane size for the food-contact PET, PC and PBT route, and 0.5 to 2 wt% of maleated polypropylene in the matrix for polypropylene. You should match the interface using coupling agents for filled and reinforced plastics rather than raising the loading to compensate for a weak one.
6. Set the loading, then confirm fiber content and properties by test: ash content to ASTM D5630 or ISO 3451, tensile to ISO 527, impact to ISO 179 or ASTM D256 and heat deflection temperature to ISO 75.

Two of these steps should be taken together rather than in sequence. Steps 5 and 6 interact, because a compound with a properly coupled interface reaches a target at a lower loading than one without, and loading is what drives abrasion, warpage and density. The general method behind these steps is on how to select plastic additives.

How Reinforcement Changes the Rest of the Additive Package#

Adding fiber changes 3 other parts of the formulation: the stabilizer package, because reinforced parts run hotter and longer, the flame-retardant package, because a glass-filled compound needs its own system, and the coupling chemistry, which an unfilled compound does not need at all. A reinforced grade is therefore not an unfilled grade with fiber added to it, and the three changes are listed below with the value that demonstrates each.

• Stabilization. Long-term heat ageing is the property reinforced polyamide is bought for, and it depends on the stabilizer rather than on the glass: PA66 GF30 with 3 % dipentaerythritol retains 99.1 % of its tensile strength after 500 hours at 210 °C and 94.8 % after 1,000 hours, against 78.2 % for the copper-halide control, while the copper-halide system itself runs at 0.001 to 0.03 wt% copper with 0.1 to 5 wt% halide. The stabilizer choices are on antioxidants for plastics.
• Flame retardancy. Flame-retarded glass-filled polyamide needs its own phosphinate system rather than the one used in the unfilled grade: Exolit OP 1312 at 15 to 20 wt% takes glass-filled PA6 and PA66 to UL 94 V-0 at 0.4 to 3.2 mm, with a glow-wire flammability index of 960 °C, a glow-wire ignition temperature of 775 °C and a comparative tracking index up to 600 V, and PA66 GF reaches R22 HL3 under EN 45545-2 for rail. The phosphinate systems used in glass-filled polyamide are on halogen-free flame retardants.
• Coupling. A glass-filled polypropylene compound needs maleated polypropylene at 0.5 to 2 wt% that an unfilled polypropylene does not, and that addition changes melt flow as well as adhesion, since peroxide grafting lowers molecular weight by beta-scission.

Loading levels shift with all three, because the flame retardant and the fiber compete for the same volume in a compound that still has to fill a 0.4 mm wall, which is why the Clariant data quote their fire performance across a 10 to 50 % glass range. The additive package is therefore re-optimised whenever the glass content moves, rather than carried across unchanged from the unfilled grade.

How Are Fiber Content and Fiber Length Measured?#

Fiber content is measured by burning the polymer away and weighing what is left, under ASTM D5630-22 or ISO 3451, and the properties the fiber buys are measured by tensile testing to ISO 527, impact testing to ISO 179 or ASTM D256 and heat deflection temperature to ISO 75. ASTM D5630-22 covers ash contents of 0.01 % and above in materials that are stable to 900 °C, with Procedure A using a 5 to 50 g muffle sample and Procedure B a 2 to 10 g rapid-ash sample; halogenated and fluorinated polymers are excluded, and the method reports how much residue there is rather than what the residue is. All methods named here are indexed under testing plastic additives.

Table 6. What is measured, how and to which standard.

Ash content is the measurement that settles disputes, because it is the only one that returns the fiber level of an unknown compound directly. Two limits apply to it on a reinforced grade: the residue includes every inorganic component, so a compound carrying both glass and a mineral flame retardant returns a single combined figure, and ASTM D648 data are explicitly not for design or for predicting high-temperature endurance.

How Are Reinforcing Fibers Regulated in Plastics?#

Glass fiber is the only one of the 5 reinforcements with its own entry on the EU Union list, and in food-contact plastics the rules bite hardest on the sizing that coats it rather than on the glass itself. The reason is diffusion: a glass filament is an inert solid that does not migrate, while the silane, the film former and the lubricant on its surface are low-molecular-weight substances that can. Every instrument named on this page is summarised in plastic additive regulations, and the two layers that matter for a reinforced compound are food contact and chemical registration.

EU: Union list entries and the sizing silane under Regulation (EU) No 10/2011#

Glass fibers appear on the Union list of Regulation (EU) No 10/2011 as FCM 38 without a specific migration limit, so the generic limit of 60 mg/kg and the overall migration limit of 10 mg/dm2 apply to the finished article. The entries relevant to a reinforced compound are set out below, taken from the Union list as consolidated on 16 March 2025.

Table 7. Union list entries for reinforcing fibers and their sizing silanes.

Three consequences follow for a formulator. A food-contact reinforced part is limited to the sizing chemistry that the Union list authorises for it, which for GLYMO means the low-diffusivity plastics named in its entry and nothing else; a wood-filled food-contact article lost its route in 2023 and depends on the transitional wording rather than on a live entry; and carbon, aramid and basalt fiber have no Union list entry in the sources checked, which is an absence of evidence rather than a prohibition. How the Union list works, and what an absence from it means, is explained on EU 10/2011. The substance-level record for the epoxy silane sits on 3-glycidoxypropyltrimethoxysilane (GLYMO).

REACH, CLP and US food-contact status#

Reinforcing fibers split in two under REACH: the silanes that size them are fully registered substances with their own CLP entries, while naturally occurring minerals that are not chemically modified are exempt under Annex V of Regulation (EC) No 1907/2006. The 3 routes that a reinforced compound has to clear are listed below.

• REACH registration. 3-Aminopropyltriethoxysilane carries 26 active registration dossiers and 3-glycidoxypropyltrimethoxysilane 28, both checked in ECHA CHEM on 22 September 2026, and neither is on the Candidate List at that date. Natural minerals that are not chemically modified are exempt under Annex V, which is why unmodified natural minerals carry no dossier under their natural identity, and the REACH position of glass fiber itself is not established in the sources checked. Registration and the Annex V exemptions are explained on REACH and plastic additives.
• CLP classification. 3-Aminopropyltriethoxysilane holds a harmonised classification under Annex VI of Regulation (EC) No 1272/2008, index 612-108-00-0, as Acute Tox. 4 H302 and Skin Corr. 1B H314, so the size bath is a hazard in the plant even where the sized fiber is not. GLYMO has no harmonised entry in ECHA CHEM as of 22 September 2026.
• United States food contact. 21 CFR 177.2410 lists glass fiber, with asbestos fiber, diatomaceous earth, mica and carbon black, as an adjuvant in phenolic moulded articles, which is a listing with conditions and not an approval of a compound. Every 21 CFR section that touches an additive is mapped on FDA food contact rules.

Who Supplies Reinforcing Fibers and Reinforced Compounds?#

A reinforced compound passes through 3 supplier layers: the fiber producer, the compounder that combines fiber, polymer and additive package, and the coupling-agent supplier whose silane or maleated polyolefin makes the interface work. Buyers usually meet the middle layer first, since a moulder specifies PA66 GF30 rather than a fiber grade, and the fiber decision has then already been taken inside the compound. Company profiles are in the directory of plastic additive manufacturers and suppliers.

Only the third layer is documented here by name. Momentive, Evonik, Dow, Shin-Etsu and Wacker are recorded as manufacturers of the amino-silane used in glass-fiber sizing, the same group of companies supplies the epoxy silane, and SI Group supplies the Polybond range of maleated polyolefins used to couple glass, talc, mica and wood in polypropylene. The mineral side, which overlaps the reinforcement question wherever wollastonite or mica replaces part of a glass loading, is listed under mineral filler suppliers.

No fiber-producer table is published on this page, and the reason is the method rather than the market. Our sources hold no verified record for any glass, carbon, aramid, natural or basalt fiber producer, and they hold no market size, tonnage or price figure for reinforcing fibers, so none is stated rather than repeating an unsourced list. Segment data that do exist for other additive families are on plastic additives market. That gap is an open item and will be filled with sourced company data before any supplier ranking appears here.

Complete List of Reinforcing Fiber Pages on This Site (8 Pages)#

The reinforcing fiber family has 8 pages on this site, 5 for the fiber types, 1 for long-fiber thermoplastics and 2 for the polymers that use the most glass. Each is linked once from the tables above, so the list below repeats the page names with their publication status.

Table 8. The 8 pages of the reinforcing fiber family.

None of the 5 fibers has a substance page on this site, because a fiber is identified by grade and composition rather than by a single CAS number, so the family pages are the canonical entity pages for them. The silanes and coupling agents that go with them do have substance records, and they are in the plastic additives database.

Are Reinforcing Fibers Safe to Handle and Can Reinforced Plastics Be Recycled?#

Two questions follow a reinforced compound out of the plant: whether the fiber is a health hazard to the people handling it, and what happens to the fiber when the part is recycled. Both are questions about the loose fiber rather than about the moulded part, since a fiber bound in a matrix is neither inhalable nor separable. The handling exposure is to dry fiber in the compounding plant and to dust from cutting and machining, and the recycling question is about what a second melt pass does to fiber length. The measurement that links the two is the ash test, because it is what tells a recycler how much inorganic material an unknown regrind contains.

Is glass fiber carcinogenic?#

Continuous filament glass, the form used to reinforce plastics, sits in IARC Group 3, which means the evidence does not allow a classification of its carcinogenicity to humans (Monograph volume 81, 2002). That classification applies to continuous filament glass and to no other agent: glass wool, mineral wool and refractory ceramic fiber were assessed separately and their classifications are not transferable to reinforcing filament. The exposure that a compounding plant manages is respirable dust from cutting, machining and handling dry chopped strand, which is an occupational hygiene question rather than a product-safety one, and no Proposition 65 listing is stated here for any glass fiber form because none is confirmed in the sources checked.

What happens to fibers when a reinforced plastic is recycled?#

Fiber length falls at every melt step, so a regrind of a glass-filled compound is mechanically weaker than the virgin pellet even when the ash test shows the same glass content. Ash content and fiber content are the same measurement, but neither of them measures the property that actually changed, which is the surviving length distribution. A recycler buying on a glass percentage therefore buys a necessary but insufficient number, and the practical answer is to specify the mechanical property instead. The rules that follow from that for part design are set out on design for recycling.

Density is the second consequence and it acts on sorting rather than on properties. Glass at 2.58 g/cm3 raises the density of the compound it sits in, which moves a filled polyolefin toward and past the point where it sinks instead of floating, so the density thresholds published for polyethylene film sorting do not transfer to a reinforced compound and are not applied here. Restabilization of regrind, where the fiber survives the second pass better than the antioxidant reserve does, is on additives for recycled plastics.

Reinforcing fiber FAQs#

The 3 questions below are the ones asked most often about reinforcing fibers as additives rather than as building products, and each is answered from the values already given on this page.

Is fiberglass a plastic?#

No: fiberglass is a composite, made of glass fibers in a plastic matrix, so the glass is the reinforcement and the plastic is what holds it. Glass fiber itself is an inorganic silicate drawn into filament, with E-glass at 3,445 MPa tensile strength and a density of 2.58 g/cm3, and it is a plastic additive rather than a plastic. The word "fiberglass" is used in North American trade for the finished composite and sometimes for the fiber alone, which is the source of the confusion.

What is the difference between GFRP and CFRP?#

GFRP is reinforced with glass fiber and CFRP with carbon fiber, and the practical difference is stiffness: E-glass has a modulus of 76.0 GPa, while carbon-fiber composite grades reach 150 to 250 GPa. Density runs the other way, since E-glass sits at 2.58 g/cm3, so the stiffness advantage of carbon is larger again per unit weight. Both abbreviations name the composite rather than the fiber, and both compounds are built with a sizing and a coupling chemistry chosen for the matrix.

Is carbon fiber conductive?#

The electrical and thermal conductivity of carbon fiber is not established in our source library, so no value and no conductive-grade claim is stated here. What is established for carbon fiber is its geometry and its mechanics: a filament diameter of 5 to 10 µm, precursors of polyacrylonitrile, rayon or pitch, and composite grades reaching 150 to 250 GPa modulus. The conductivity question is answered with sourced values on the carbon fiber page rather than estimated on this one.

---