Page 27 - Handbook of Surface Improvement and Modification

22 Scratch and Mar Resistance

2.2.9 SILICA

GENERAL INFORMATION
Name: silica CAS #: 7631-86-9 EC #: 231-545-4
Solids content, wt%: 16-34
Product composition: surface treated silica nanoparticles; carrier: hexanediol diacrylate,
1-methoxy-2-propanol acetate, or 1-methoxy-2-propanol
PHYSICAL PROPERTIES
State: liquid/dispersion Odor: acrylic-like, solvent-like Color: colorless, translucent,
clear to light yellow
o
o
Melting point, C: 0-<4 Boiling point, C: 100-140 pH: 9.8
2
3
Surface area, m /g: 220 Particle size, nm: 50 Density, kg/m : 1090-1390
Vapor pressure, kPa: 0.00337-0.025 Surface tension, mN/m: 43.5
Viscosity, mPas: 10-50
HEALTH & SAFETY
o
o
Autoignition, C: >200 Flash point, C: 45-79 Rat oral LD50, mg/kg: >5000
Rabbit dermal LD50, mg/kg: >2000 to >5000 Carcinogenic: no
3
OSHA, PEL, mg/m : 80 UN/NA class: 1993
USE & PERFORMANCE
Outstanding properties: Improves the scratch and abrasion resistance of coating surfaces without
negatively affecting properties such as gloss and transparency. It also increases chemical resis-
tance. Provides the so-called “immediate reflow effect” due to the silica nanoparticles which are
evenly distributed in the coating and which act like shock absorbers. They absorb the impact
energy and only release it very slowly. This results in an elastic coating surface with a consistent
hardness. Mechanical impact leaves hardly any trace and the coating surface is virtually undam-
aged. It, therefore, provides perfect long-term protection.
Recommended for polymers: PU, acrylates and acrylate copolymers
Recommended for products: automotive refinish coatings and OEM coatings, wood and furniture
coatings, industrial coatings, architectural coatings
Concentrations used, wt%: 0.5-6
Food approval FDA: 175.105/adhesives, 176.180/components of paper and paperboard in contact
with dry food

Sol-gel coating is a low-cost and low-temperature process, applied to almost any type of
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substrate of any shape. The properties of sol-gel coatings can be easily modified through
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chemical formulation and hybridization with organic or inorganic compounds. The
improvement of scratch behavior by relatively hard and stiff coatings produced by sol-gel
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processes can be achieved on relatively soft and compliant substrates. The sol-gel pro-
cessing temperature can be as low as 100°C, therefore it can be especially useful on poly-
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meric substrates which have very low glass transition temperatures. Major applications
include plastic lenses, automobile topcoat, safety windows, display panels, etc. Figure
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2.18 shows typical failure modes of sol-gel coatings. The stresses are induced by the
joint action of contact-induced bending, the friction traction from sliding, and the pileup
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of the substrate due to plastic deformation. On increased addition of colloidal silica, the