Stefan Sepeur

UV curable hard coatings on plastics

A photochemically curable transparent hard coating material has been developed to improve the wear resistance of organic polymers. Methacrylate functionalized silanes and nanoscaled boehmite particles were used for the preparation of the UV curable hard coatings by the sol–gel technique. The inorganic network is formed as a result of the controlled hydrolysis and condensation of the methacryloxysilanes in the presence of nanoparticles (particle size: 15 nm). The hydrolysis and condensation of the methacryloxy silane, for two hours in presence of boehmite nanoparticles (43 wt.%), at 100°C leads to a degree of condensation of 80% of the inorganic network, as determined by 29Si-NMR spectroscopy. The organic network is formed by photoinitiated polymerisation of the methacrylic-functions in only a few minutes, using UV light from a Hg high pressure lamp. Transparent coatings on plastic substrates (PC, PMMA) were produced by spray coating with good optical quality and thicknesses of several micrometers. The transparent coatings show excellent adhesion on PMMA and PC (GT/TT=0/0, DIN 53151), even without using primers, and also good abrasion resistance after the Taber abraser test with haze values of 10% after 1000 cycles (CS 10F rolls, 5,4 N, DIN 52347). The water exposition test (65°C, deionized water) and the Suntest, without using filters, did not result in any crack formation, loss of adhesion or yellowing within the test period of two weeks.

Aqueous wet coatings for transparent plastic glazing

Abstract Electrostatically stabilized particulate sols, e.g. aqueous silica or alumina sols, have been surface-modified to reduce the particle/particle interaction and to obtain electrosterically stabilized sols. Epoxy functionalized alkoxysilanes were used as surface modifiers for alumina, silica or zirconia particles to achieve water dispersible nanoparticulate coating materials. After evaporating the organic solvent (produced by hydrolysis and condensation) aqueous sol-gel coatings have been prepared with organic solvent contents below 5%. The storage stability of these materials has been investigated by 29 Si-NMR. An amino functionalized alkoxysilane was used as condensation catalyst and the nanocomposite material was thermally cured at 130°C after flow or spray coating on pretreated polycarbonate substrates. The coatings show excellent adhesion which was shown by cross cut and tape test. After 1000 cycles Taber abrasion test (CS-10F, 500 g, DIN 52347) losses of transmittance due to scattering of 2–6% were achieved with a coating thickness of about 5 μm.

Aqueous sol-gel derived nanocomposite coating materials

Sol-gel reactions, in general, have to take place in organic solvents in order to control the hydrolysis and condensation reaction. This leads to drawbacks in applications, since the organic solvent vapor concnetration has to be controlled for example in ocoating applications for environmental, health or security reasons. For this reason, a reaction route has been developed to produce coating precursors stable against water as solvent. To achieve this, conventional electrostatically stabilized sols obtained either by hydrolysis and condensation or commercially availabe sols (e.g. SiO2 sols) have been surface modified to reduce the particle-to-particle interaction and electrosterically stabilized precursors compatible to water have been obtained. By use of alkoxy silanes as surface modifiers, water-dispersable nanoparticulate liquid coating systems have been prepared, for example, with boehmite, SiO2 or TiO2 as nanoparticles to be crosslinked after coating thermally by inorganic condensation and organic condensation or polymerization reactions. Thus, aqueous sol-gel coating systems have been prepared with EtOH contents below 5%. In the paper, the basic reaction as well as material properties will be discussed.

Nanoscaled Filled Sol-Gel Hybrid Materials for Powder Coatings

The sol-gel process has been applied for the synthesis of inorganic-organic powder materials. For this purpose, low reactivity and, for reasons of comparison, nanoscaled sol-gel precursors and nanoscaled silica filler were used to synthesize a low molecular weight inorganic-organic composite which was meltable between 85°C and 100°C and curable at temperatures of 150°C - 200°C. The powders were used for electrostatic coating of aluminum sheets. The resulting coatings of the unfilled system exhibited excellent mechanical properties, e. g. a high abrasion resistance (weight losses between 6 mg and 10 mg per 1000 cycles by taber abrader test (ASTM D1044)) and high values for hardness (universal hardness (DIN 55676): 150 N/mm 2 ). The hardness of the materials was increased up to 210 N/mm 2 by the addition of silica, while the abrasion resistance remained unaffected.