Siloxane Hardcoats
Exxene siloxane coatings are thermal-cure materials that produce a hard, rigid surface with a glassy appearance. The polymeric content consists of modified polysilsesquioxane sol-gel in an alcohol or glycol ether solution. Solutions are low-viscosity, fast-drying liquids supplied at 15-30% solids and do not require refrigerated storage. Coatings adhere to a variety of substrates, including polycarbonate.
Performance Characteristics
Silicone (polysiloxane) Hardcoat ings provide superior abrasion resistance and inertness to hostile chemical and environmental conditions. These coatings are applicable to many substrates, but the majority of applications are on nonmetallic surfaces, especially plastics.
Scratch-Resistant Organometallic Coating
The fully cured organometallic polymer provides a semi-crystalline barrier several microns in thickness and eliminates scratching and abrasion normally encountered with uncoated plastics.
Optical Clarity and Chemical Resistance
It exhibits excellent light transmission and actually improves the optical properties of most substrate materials by eliminating substrate surface roughness, which contributes to reduced clarity in the uncoated state. Chemical resistance is dramatically enhanced through encapsulation of the substrate surface with the cross-linked siloxane matrix.
Silicone Hardcoats for Plastic and Lenses
Silicone Hardcoats are routinely used on cast or extruded plastic sheets and injection-molded or cast lenses, windows, and structural housings. These coatings are especially suited for application upon extruded polycarbonate sheets and injection-molded polycarbonate lenses.
Application
Most applications benefit from the minimum film thickness required to obtain the desired abrasion resistance, with an average of three to six microns. Rigid polysiloxane layers are prone to crack or craze and may delaminate if applied in excessively thick layers on thin polycarbonate sheet.
Controlling Thickness in Siloxane Coatings
All siloxane Hardcoats can be applied by dip, flow, spin, and spray methods. Flow coating of large surface areas often requires dilution and large flow volume to limit the coating thickness and prevent cracks that originate from the areas of thicker coating.
Substrates
The plastics that are most commonly coated with polysiloxanes are polycarbonates, polyarylates, polysulfones, and polyamides/nylons. Coupling agent or primer treatments facilitate use on more challenging polymeric substrates such as vinyl, urethane, polyester, cellulosic, and polyolefin.
Temperature Effects on Siloxane Curing
Siloxanes can be cured at temperatures as low as 50 degrees Celsius; however, they require several hours at such low temperatures. Higher temperatures, appropriate to the substrate, will accelerate the rate of cure to one hour or less. Exxene Hardcoats are most often cured at 82 to 130 degrees Celsius for 20 to 45 minutes.
Siloxane Hardcoat Products
S-24NFPA
A primerless Hardcoat was specially developed to meet the abrasion and flame exposure requirements of the National Fire Protection Association on polycarbonate and polycarbonate copolymers, including PC-TMC. It can be supplied with 20% or 25% solids in isobutanol and is primer compatible on substrates not requiring NFPA certification.
S-25 Universal
S-25 Hardcoat formulated to adhere to a variety of rigid and semi-rigid substrates without the use of a primer. The S-25 provides improved adhesion and resistance to crazing on thinner, more flexible substrates. It is suggested for use on polycarbonate, polyarylate, polysulfone, ABS, ADC resin, and polyamide/nylon. It is supplied at 25% solids in isobutanol.
S-28-1
S-28-1 is a general-purpose Hardcoat for rigid and semi-rigid polycarbonate. A primer is suggested for maximum chemical and weathering resistance. It is supplied at 25% solids in isobutanol.
S-30AP Chemical Biological Barrier
S-30AP is a siloxane-modified nanoparticle sol designed to provide maximum chemical resistance against chemical warfare compounds and other corrosive agents. It is supplied at 25% solids in isobutanol and provides a highly chemical resistant surface on rigid polycarbonate; it requires Exxene SP-1 Primer.
S-500 Weatherable Hardcoat
The S-500 siloxane Hardcoat is a weatherable and UV-absorbing coating specialized for use on polycarbonate components in exterior architectural, aerospace, and automotive applications. It is supplied at 25% solids in isobutanol and produces a semi-rigid coating; it requires Exxene SP-1 Primer.