Purpose of Fiber Sizing
Fiber sizing is a multifunctional surface treatment applied to glass fiber, carbon fiber, and other reinforcement filaments immediately after forming. It serves both processing and structural roles:
- Sizing protects filaments during high-speed weaving/winding, reduces breakage, binds filaments into cohesive bundle
- For composites: sizing controls fiber-matrix adhesion — wrong sizing can halve composite strength
- Two-stage: temporary processing aids (lubricants, antistats) + permanent coupling agents (silanes)
Without correct sizing, filament damage during textile processing causes strand breakage and surface flaws that act as stress concentrators in the cured composite laminate. For structural composite applications — aerospace, wind blade, automotive — sizing chemistry is a critical engineering parameter, not a commodity.
Glass Fiber Sizing Formulation
A glass fiber sizing bath is an aqueous dispersion of several functional components applied at 0.5–1.5% LOI (loss on ignition) on fiber weight:
| Component | Function | Typical Level |
|---|---|---|
| Silane coupling agent (KH-550/560/570) | Matrix adhesion | 0.2–0.5% on fiber |
| Film-former (PU or epoxy emulsion) | Bundle integrity | 1–3% |
| Lubricant (PEG ester, mineral oil emulsion) | Process lubricity | 0.1–0.5% |
| Antistatic agent | Static dissipation | 0.05–0.2% |
The film-former is the major component and must be compatible with the end-use matrix resin. Epoxy-compatible film-formers are used for aerospace and electrical laminates; PU film-formers for woven fabrics and filament-wound pipes; polyester-compatible formers for pultrusion profiles. The silane coupling agent is the minority component but disproportionately controls composite mechanical properties.
Silane Selection by Matrix Resin
Coupling efficiency requires chemical compatibility between the silane functional group and the matrix resin's cure chemistry:
| Matrix | Silane | Functional Group |
|---|---|---|
| Epoxy | KH-560 (γ-glycidoxy) | Epoxy |
| Polyester / vinylester | KH-570 (γ-methacryloxy) | Methacryloxy |
| Polyurethane | KH-550 (γ-amino) | Amino |
| Polyolefin (PP, PE) | A-171 (vinyl) | Vinyl |
| Phenolic | KH-550 or KH-792 | Amino / diamine |
Mismatched silane–resin pairs produce weak interphase bonds that fail preferentially under hygrothermal aging. Short-beam shear strength and interlaminar shear strength (ILSS) tests are standard benchmarks for sizing qualification. Properly sized glass fiber composites retain 80–90% dry strength after 1000 h water immersion; unsized or incorrectly sized fiber may retain only 40–50%.
Carbon Fiber Surface Treatment
Carbon fiber requires different surface chemistry from glass fiber because the graphitic surface has inherently low surface energy and few reactive sites:
- Carbon fiber is treated by anodic oxidation (electrochemical oxidation) to introduce –COOH and –OH groups, then sized
- Sizing for CF typically epoxy-compatible (KH-560 or epoxy emulsion film-former)
- Silane loading on CF: 0.05–0.15% (much lower than GF due to smaller fiber diameter and higher surface area)
Anodic oxidation is performed in-line immediately after carbonization and graphitization, before the sizing bath. The oxidation depth is shallow (1–5 nm) but sufficient to convert the inert graphite basal planes at the fiber surface into a reactive, wettable interphase. Post-oxidation sizing then bonds covalently to the newly introduced surface oxygen groups and to the matrix resin, creating a continuous load-transfer path from fiber to resin.