Temporary coatings
Temporary protective coatings for semi-finished products and alloys sensitive to wear and humidity.
Temporary coating are products used to cover an artifact to protect it from atmospheric or mechanical agents. Unlike real coating, temporary coatings are designed to be removed from the need, for example before painting. These protectives can be used between a manufacturing process and another, in order to not damage themselves during transport or in order to prolong storage without having oxidation problems. The application of these coatings is simple and can take place by diving, vaporization and spraying.
For functionality we can say that they have the same purpose as traditional oily applications (able to preserve the pieces for a long time), with the advantage of being able to be removed more easily.
Regarding the environmental impact, the first category to mention is the temporary water-based coating, removable by simple jet of slightly alkaline hot water or alcoholic solutions. This is possible thanks to particular acrylic resins (which can also be combined with others to guarantee different performances) soluble in water which, once dried, can solve again in water solution. These products are used by immersion or applied manually by brush (for small artifacts), after which they are made dripping until complete drying. The aid of ventilated or hot air can serve to accelerate the process.
Typical formulations contain, in addition to the resins specified above, also thixotropic additives to increase the viscosity and de-airing to reduce the formation of foam that could otherwise create application problems. Many companies also add dyes to differentiate the artifacts or to distinguish them better from the raw ones.
Other types of temporary coatings for metals are:
Polyurethane-based coatings. Resistant to water, abrasion and corrosion, they are made up of:
- Liquid polyurethane (can be derived from synthetic or natural sources).
- Organic solvents, to aid in application and improve its viscosity;
- Additives to improve abrasion resistance, such as silicon or zirconium metal oxides;
- Additives to improve corrosion resistance, antioxidants and UV stabilizers;
- Additives to improve elasticity, such as plasticizers that make the coating less susceptible to chipping or cracking;
- Catalysts, to accelerate the polymerization of the polyurethane.
In general, polyurethane-based temporary coatings should be designed to be easily removed without causing damage to the underlying metal surface. Solvents can be used for removal (based on the type of polymer cross-linking) or by mechanical action or peel-off.
Silicone-based coatings. Resistant to high temperatures, water and abrasion, they are made up of:
- Liquid silicone or powder;
- Organic solvents (for example, xylene, toluene) or water emulsions;
- Additives to improve resistance to corrosion and wear (for example, zinc or cobalt salts);
- Additives to improve the adhesion and hardness of the coating (for example, aluminum or sodium silicate);
- Additives to improve resistance to chemical action (for example, polysiloxanes or acrylic polymers);
- Additives to improve resistance to high temperatures.
Removing a silicone coating
can be complex, which is why it is not used much as a temporary coating.
The peelable coatings
They can be made with polymers such as polyethylene, polypropylene or PVC, which are applied to the metal surface and subsequently removed as needed by mechanical detachment (peel-off).
A basic formulation of a polyethylene coating is as follows:
- Polyethylene granules (dissolved in an organic solvent): 80%
- Organic solvent (for example, xylene): 15%
- Antistatic additive: 2%
- Anti-aging additive (for example, antioxidant): 2%
- Additive to improve adhesion (e.g. calcium silicate): 1%
A basic formulation of a polypropylene coating is as follows:
- Thermoplastic polypropylene granules: 70%
- Organic solvent: 15%
- Additive to improve abrasion resistance: 5%
- Additive to improve corrosion resistance: 2%
- Additive to improve elasticity: 3%
- Catalyst: 5%
A basic formulation of a PVC coating is as follows:
- PVC granules: 70%
- Organic solvent (for example, dichloromethane): 20%
- Plasticizer additive (for example, di-butyl phthalate): 5%
- Anti-aging additive (for example, antioxidant): 2%
- Additive to improve adhesion (e.g. calcium silicate): 3%