How to apply and cure silicone coatings - tutorial

Dow Corning

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Tutorial – How to apply and cure silicone coatingsTypical Application MethodsTypical methods for the application of silicone coatings to fabric include:• Knife coating (knife-over-roll, knife-over-gap/air or knife-over-blanket)(Figure 1)-W ith knife-coating methods, the outcome is influenced by bladetype and angle as well as the substrate. (Figure 2)Figure 4 – Rotogravure coating.•D ip/immersion coating or kiss coating (lick roll) (Figure 3)•Rotog ravure coating (Figure 4)-T he gravure process ensures that the topcoat is delivered at aconstant application rate and is not affected by substrate variability.This method facilitates higher line speeds.• Extrusion (Figure 5)• Spraying (Figure 6) Figure 1 – Knife coating.The use of opposing knives orknife-over-air methods facilitatessubstrate penetration. Figure 5 – Extrusion.Squeeze-roll, reverse-roll andknife-over-roll methods facilitatefilm forming. In squeeze- andreverse-roll methods, roll pressuregoverns the deposition of the film.Knife-over-roll, on the other hand,relies on blade type and angle.Figure 2 – Knife shapes.Figure 6 – Spraying.Figure 3 – Dip/immersion coating or kiss coating.Liquid silicone rubbers (LSRs)LSRs are usually sold as two-component systems (kits). Mix ratios varyfrom 1:1 to 10:1.For laboratory trials, LSR can be mixed with a laboratory mixer or by hand.Pot life will vary depending on the amount of inhibitor in the formulation. Toavoid air bubbles, the mixture must be ...

Informations

• Publié par: Uprou
• Nombre de lectures: 30
• Langue: English

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Tutorial – How to apply and cure silicone coatings

Typical Application Methods Typical methods for the application ofsilicone coatings to fabric include: • Knifecoating(knife-over-roll, knife-over-gap/air or knife-over-blanket) (Figure 1) - Withknife-coating methods, the outcome is influenced by blade type and angle as well as the substrate. (Figure 2) • Dip/immersioncoating or kiss coating(lick roll) (Figure 3) • Rotogravurecoating(Figure 4) - Thegravure process ensures that the topcoat is delivered at a constant application rate and is not affected by substrate variability. This method facilitates higher line speeds. • Extrusion(Figure 5) • Spraying(Figure 6) Figure 1 – Knife coating. The use ofopposing knives or knife-over-air methods facilitates substrate penetration. Squeeze-roll, reverse-roll and knife-over-roll methods facilitate film forming. In squeeze- and reverse-roll methods, roll pressure governs the deposition ofthe film. Knife-over-roll, on the other hand, relies on bladetype and angle.

Figure 2 – Knife shapes.

Figure 3 – Dip/immersion coating or kiss coating.

Figure 4 – Rotogravure coating.

Figure 5 – Extrusion.

Figure 6 – Spraying.

Liquid silicone rubbers (LSRs) LSRs are usually sold as two-component systems (kits). Mix ratios vary from 1:1 to 10:1. For laboratory trials, LSR can be mixed with a laboratory mixer or by hand. Pot life will vary depending on the amount ofinhibitor in the formulation. To avoid air bubbles, the mixture must be put under vacuum before use. In production settings, static mixers are preferred over dynamic mixers. Dynamic mixers can add excessive heat or cause the filler to separate. LSR Application Methods • Conventionalcoating machines can be used. • Forthick layers use knife-over-roll. 2 •Forfnielayers(upto40g/m)useknife-over-airorgravure. • Additionof solventmakes thinner coatings possible. • Screen-printingis also possible for some materials. Curing LSRs • WithLSRs, deep-section cure is possible. • Potlife varies according to inhibitor/catalyst level; most LSRs have a 24-hour pot life. • MostLSR coatings cure in 1 to 2 minutes at 160-180°C (many organic systems are solvent-based and require a temperature gradi- ent to gently remove the solvent prior to cure). • Thereare no reaction byproducts and only a small amount (1-2 percent) ofvolatile silicones (cyclics); consequently, recycling air is not a problem. • TheLSR’s platinum catalyst can be poisoned by contaminants: Lewis bases (such as amines, ammonia, neoprene, sulfur, mercaptides, vinyl groups, Buna rubber). • Ifthe LSR Part B becomes contaminated with water, strong acids or bases, there is a potential for the release ofhydrogen gas.

Room-Temperature-Vulcanizing (RTV) Elastomers RTV Application Methods When the RTV is used at 100 percent solids as a sealant or adhesive, it is applied via extrusion using a pump system with a follower plate and an extrusion head at the point ofuse. When the RTV is used as a coating, or ifit is being applied to an uneven substrate, it is diluted with solvent and sprayed. Airless spraying is pre- ferred. Air contact should be avoided before application as the product will skin over in 5-10 minutes. Cleaning • Uncuredsilicone can be removed using white spirits. • Curedsilicone is very difficult to remove; strong acid or base may be required. • Curedand uncured silicone may be removed by Digesil NC (available from American Digesil Co. – telephone [phone]).

Figure 7 – Typical LSR coating line.

Take-up Roll

Curing RTVs • RTVelastomers do not cure under a nitrogen blanket. • Becausethe RTV reacts with moisture in the air, the surface cures first. Consequently, for proper cure, thick sections (>5 mm) should be avoided. • Curetime for a 1-mm layer is approximately 1 hour at room temperature. • Thecoating cures faster at higher temperatures and humidity. • Smallamounts ofacetic acid byproduct are emitted during cure; consequently, ventilation is necessary. • Post-cureat high temperature improves coating properties.

HighConsistency Silicone Rubbers (HCRs)

HCRs are applied via calendering (which produces sheets ofrubber of uniform thickness) or via solvent dispersion. Toluene is the solvent most typically used.

These coatings are typically applied to fiberglass, but may be applied to nylon and polyester as well.

HCRs employ a heat-activated cure system in which peroxide crosslinks organic groups via free-radical addition. Cure rate can be varied by temperature, peroxide type and concentration.

Physical properties vary with gum type, filler level and peroxide type.

Technical Support

Contact the Industrial Textiles Technical Support team at [e-mail].

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