Sizing is a fraction of a percent of a carbon fiber's weight and a large fraction of how it behaves. The film former in that size decides whether tow chops cleanly or fuzzes, whether a bundle feeds or clogs, and whether the fiber-matrix interface transfers load or delaminates. This is the selection logic we walk through with fiber producers and processors, in the order that avoids dead ends.
What the size is doing
Carbon filaments run a few microns in diameter, and a tow carries thousands of them. Sizing is applied on the fiber line after surface treatment, usually at a fraction of a percent up to a few percent by weight, higher for chopped products. The film former forms a continuous film over the filaments and does three jobs at once: protects them from abrasion in winding, weaving, and conveying; consolidates the tow into a bundle that can be handled as one thing; and puts a resin-compatible surface between the carbon and the matrix so the cured composite transfers load into the fiber.
Because those three jobs pull in different directions, there is no single carbon fiber sizing. The selection is a sequence.
First: the matrix
Compatibility with the matrix resin is the decision everything else follows. For epoxy systems, prepreg, filament winding, pultrusion, and infusion, epoxy film formers are the default, and the epoxide equivalent weight is the lever: low-EEW grades built on liquid epoxy resin give a soft film and fast solubility in the matrix, while high molecular weight grades give a tougher, more protective film.
For thermoplastic matrices, the question is what survives the melt. Higher molecular weight epoxy film formers are matched to PET, PBT, and polycarbonate. Polyamide 6 takes a different route: polyurethane dispersions built on polycaprolactone polyols, which stay compatible with the polyamide melt. Above those temperatures, at PPS and PEEK processing conditions, standard epoxy sizings degrade; we would rather say that plainly than sample you something that chars. That application is development territory, and we can take it up with the manufacturer's technical team.
Second: Tg, which sets the hand
The film former's glass transition temperature is the practical control over how the sized tow feels and behaves, what the trade calls hand. The carbon fiber range spans roughly minus 20 to plus 40 degrees C in Tg. At the soft end, tow stays flexible: it spools without springback, survives textile handling, and chops cleanly. At the stiff end, the bundle is compact and stable, holds its form in conveying and dosing, and resists fuzzing under shear. Converters chopping fiber, weavers, and compounders each land at a different point on that ladder, which is why the range exists.
Working a sizing selection now?
Tell us the fiber, the matrix, the process, and the hand you need. We will recommend a starting grade from the FILCO carbon fiber range and ship a sample from US stock.
Request a SampleThird: the bath details
Two properties decide whether a film former runs well in a carbon fiber size bath. It has to be non-ionic: surface treatment leaves the filament oxidized and charged, and non-ionic waterborne emulsions stay stable against that surface and against the other bath components. And the particle size has to suit filaments a few microns across; carbon fiber grades run fine, starting around 200 nanometers, so the emulsion penetrates the tow and coats filaments evenly rather than sitting between them.
Fourth: heat downstream
Some sized fiber sees real temperature after the size bath: drying ovens, compounding, and downstream cures. Epoxy novolac film formers carry higher functionality than standard epoxies and are the grades to reach for when the size has to hold up under higher-temperature demands. A reactive polyurethane option is also available where the film former should crosslink during downstream processing rather than stay thermoplastic.
Sourcing, and where these grades come from
The FILCO film formers are made by COIM in Italy, a line built over three decades of supplying sizing chemistry to the glass fiber industry and extended to carbon fiber. We stock FILCO grades in the eastern United States, ship samples for line trials, and can bring the manufacturer's technical team into grade selection and, where the catalog does not fit, development work. The full line, including the glass and basalt side and the mat binders, is on the fiber sizing film formers page. If your fiber is recycled or you are re-coating converted fiber, the selection changes in ways covered in re-sizing recycled and converted carbon fiber.
Frequently asked
What does sizing do on carbon fiber?
It protects filaments a few microns across from abrasion, holds the tow together so it can be wound, chopped, or woven, and couples the fiber surface to the matrix resin so the composite transfers load into the reinforcement. Typical add-on is a fraction of a percent to a few percent by weight.
Why do carbon fiber sizings need to be non-ionic?
Surface treatment leaves an oxidized, charged filament surface, and an ionic emulsion can destabilize against it. Non-ionic waterborne emulsions stay stable through the size bath and coat evenly.
How does glass transition temperature affect a sized tow?
Tg sets the hand. Low-Tg grades give a soft, flexible tow that chops cleanly and spools well. High-Tg grades give a stiffer, compact bundle that holds its form. The carbon fiber range spans roughly minus 20 to plus 40 degrees C so the hand can be matched to the process.
Can epoxy-sized carbon fiber go into thermoplastics?
Into some. High molecular weight epoxy grades suit PET, PBT, and polycarbonate, and polycaprolactone-based polyurethane dispersions are matched to polyamide 6. At PPS and PEEK melt temperatures, standard epoxy sizings degrade; that calls for development work, not a catalog grade.
Selecting a size for your line?
Send the fiber, the matrix, and the process. We will match a film former, ship a sample from US stock, and back the trial with technical support.
View the fiber sizing page