Surface Treatment

Surface Treatment with Silanes and Silicones

Surface treatment encompasses any chemical modification of a material's surface that changes its wetting, adhesion, electrical, or mechanical properties without significantly altering the bulk substrate. Silanes and silicones are the dominant chemistry for surface treatment because of their bifunctional nature: one end bonds to the substrate (via -OCH₃ or -OCH₂CH₃ alkoxy groups condensing with surface hydroxyls), the other end provides the functional property (hydrophobicity, oleophobicity, adhesion-promotion, antimicrobial, etc.).

The four largest surface-treatment markets globally are:

Glass-fiber sizing for composite reinforcement (>1 million tonnes glass fiber/year, 0.1–0.3 wt% silane)
Mineral filler treatment for polymer compounding (calcium carbonate, talc, kaolin, magnesium hydroxide, silica)
Pigment encapsulation for coatings, plastics, and printing inks
Stone, concrete, and masonry water-repellent treatment (covered separately under water repellency)

Glass-Fiber Sizing

Glass-fiber sizing is a multi-functional surface treatment applied at the glass-drawing stage (immediately after fiber formation, before wind-up). The sizing provides:

Fiber protection during processing (prevents fiber-fiber abrasion damage that destroys mechanical properties)
Adhesion to matrix resin in the final composite (the silane component bonds glass to resin)
Lubrication for processing operations (filament winding, weaving, pultrusion)

A typical glass-fiber sizing has 4–8 components:

Silane coupling agent (KH-550 / KH-560 / KH-570 / VTMOS, 0.05–0.30 wt% on glass) — the structural component
Film-former (epoxy or PU emulsion, 1–5 wt%) — protects the fiber surface
Lubricant (polyethylene glycol, fatty acid, 0.1–0.5 wt%) — handles processing
Anti-static agent (quaternary ammonium, 0.05–0.20 wt%) — for textile-style processes
Buffer (acetic acid + sodium hydroxide, pH 4.5–5.5) — silane hydrolysis catalyst

The choice of silane is dictated by the matrix resin: aminosilane KH-550 for epoxy and unsaturated polyester; epoxysilane KH-560 for epoxy; methacryloxysilane KH-570 for unsaturated polyester and acrylic; vinylsilane A-171 for peroxide-cured polyolefin matrices.

Mineral Filler Treatment

Calcium carbonate, talc, kaolin, mica, and other mineral fillers are surface-treated to improve their dispersion in polymer matrices and to provide adhesion-promotion. Treatment can be done two ways:

Pre-treatment (filler producer): silane is sprayed onto fluidized filler and dried at 80–120 °C. The filler ships with silane already grafted; the polymer compounder simply doses it into the formulation.

In-situ treatment (polymer compounder): silane and filler are added separately to a high-speed mixer; silane migrates to the filler surface and reacts during compounding. Less effective than pre-treatment but eliminates a process step.

Typical silane loadings:

Calcium carbonate (BET ~5 m²/g): 0.3–0.8 wt% silane
Talc (BET ~10 m²/g): 0.5–1.0 wt% silane
Kaolin (BET ~15 m²/g): 0.7–1.5 wt% silane
Magnesium hydroxide (BET ~20 m²/g): 1.0–2.0 wt% silane

The silane choice depends on the polymer matrix; for most engineering thermoplastics, KH-550 (aminosilane) or A-171 (vinylsilane) is the starting point. For specialty applications, methacryloxy or mercapto silanes are chosen based on cure chemistry.

Pigment Encapsulation

Inorganic pigments (TiO₂, iron oxide, ultramarine) are surface-treated to prevent agglomeration and improve dispersion in coatings, plastics, and inks. The treatment can be:

Single-component silane treatment (octyltriethoxysilane for hydrophobic dispersion in plastic compounds)
Multi-step treatment (silica encapsulation followed by silane functionalization, used in premium TiO₂ for high-durability coatings)
Silicone-resin encapsulation (creates a polymer shell around each pigment particle)

The largest single application is TiO₂ surface treatment for exterior architectural coatings: silica-alumina-silicone coating provides photostability against UV-induced photocatalysis, extending coating service life from 5–8 years (untreated TiO₂) to 15–25 years (premium-treated grades).

Application Methods

Industrial surface-treatment processes:

Spray-fluidized bed (mineral fillers): filler is fluidized in a heated chamber; silane mist is sprayed onto the fluidized particles; reaction proceeds for 10–30 minutes at 80–120 °C. The most uniform treatment method.

Ribbon blender / paddle mixer (mass-production fillers): silane and filler are blended at low speed; reaction is slower than fluidized-bed but equipment is cheaper.

Spray application (large surfaces): silane solution is sprayed directly onto substrate (concrete, stone, fabric, paper) and air-dried.

Sol-gel impregnation (porous substrates): silane solution penetrates the substrate by capillary action; hydrolysis and condensation produce a continuous gel phase within the substrate matrix.

Quality Control and Testing

Surface treatment quality is measured by:

Carbon content (LECO combustion analysis, indicates silane loading)
Methylene blue index (for silanol-rich fillers; declining MBI indicates silanol consumption by silane)
Water contact angle (hydrophobic treatments target above 130°; hydrophilic targets below 30°)
Resin compatibility (composite mechanical properties as a function of treated-filler loading)
FT-IR spectroscopy (confirms silane grafting via Si-O-Si signals at 1100 cm⁻¹)

Sourcing

Mineral fillers are produced by hundreds of regional suppliers worldwide. Surface-treated grades carry 10–30% premium over untreated. Silane chemistry has been commoditized; major suppliers (Dow, Evonik, Wacker) and Chinese producers offer equivalent quality for commodity treatments. For high-performance applications (medical-grade composites, military, aerospace), tradenamed silanes from major suppliers carry documentation packages required for regulatory approvals.