Introducing Versatile Vinyl
The term vinyl originated from the Latin word “vinum,” which means wine. The first to coin this term was Hermann Kolbea, a chemist-scientist from Germany.
Vinyl is mainly produced from ethylene (based on crude oil) and chlorine (based on sodium chloride salt). It is desired for its versatility, controllable hardness and flexibility. It was developed as a durable and inexpensive material to manufacture various items and coatings.
Monomers of the vinyl group include vinyl acetate, vinyl fluoride and vinyl chloride, which on polymerization produces polyvinyl acetate (PVAc), polyvinyl fluoride (PVF) and polyvinyl chloride (PVC), respectively. PVC is popularly known as vinyl.
Based on innovative formulations, vinyl can have widely different properties. For example, it can be colored or clear, and flexible or rigid. Besides its use as a coating, vinyl is used for making numerous consumer items. Copolymers of vinyl chloride and vinyl acetate are used in architectural coatings.
Advantages of Vinyl Coatings
PVC (vinyl) coatings are widely preferred for the following properties:
- Excellent electrical insulator
- Weather, moisture and mildew resistant
- Fire and flame retardant
- Ultraviolet (UV) resistant
- Chemically non-reactive
- Suitable for cold and hot climates
- Available in visually delightful designer colors
- A combination of desired hardness with desired flexibility
- Several custom surface textures (matte/glossy/foams)
- Low temperature customization
- Abrasion/scratch resistance
- Chemical corrosion resistance
Vinyl Ester Coatings and Linings
Vinyl ester coatings and linings are useful because of their excellent resistance to organic chemicals and acids wherever metal surfaces are exposed to severe corrosive conditions such as in chemical plants. However, vinyl ester can shrink nearly 8% during curing, and any potential thermal expansion of metal substrates must be kept in mind during the design stage when choosing an appropriate coating.
Vinyl esters have very good adhesion to steel, but thorough surface preparation is a must. (Surface preparation is discussed in the article Substrate Surface Preparation for Corrosion Prevention.) A coarse profile, built by abrasive blasting, is useful.
Based on the application requirements, vinyl ester proprietary coatings can be custom designed to tolerate:
- A wide ambient temperature range
- Abrasion and impact load conditions
- Exposure to moisture and chemicals
Uses for Vinyl Coatings
Vinyl coatings are used for:
- Handles on screwdrivers and spanners, because of the grip provided and insulation properties
- Steel chains, so that links don’t create friction by sticking to each other
- Plating racks
- Crab traps used in seawater
- Stainless steel cables
- Wire rope used as aircraft cables
- Medical component parts and instruments
- Auto and machinery parts
- Outdoor furniture
- Levers and clamps
- Electrical conduits
- Plumbing components
- Electromechanical equipment
Functions of Vinyl Coatings
Metallic parts and tools are universally coated with vinyl, especially if visual appeal is important or a subsequent fabrication activity is likely to damage the surface.
Vinyl coatings provide protection against corrosion, abrasion, scratching and moisture, and insulate against electricity and heat.
Other functions include providing a non-slip surface, cushioning, color coding, impact resistance and weather resistance.
Forms of Vinyl Coatings
Vinyl coatings can be applied as a water-based product, a powder coating or as a liquid dip coating system. (Learn more about power coatings in The 5 Advantages of Powder Coatings.) They can be applied to metal and nonmetals. Different formulations can be designed and developed depending upon the application. One coat of PVC is adequate because it is flexible, has good adhesion (removal is very difficult) and it is incompatible with other coatings.
Popular forms of vinyl coatings are plastisol, dip and powder.
Plastisol is the PVC in a fluid condition dip coated and subjected to a thermal curing process to produce a resilient yet flexible coating on metallic surfaces. The coating material consists of dispersions of PVC resin in chemical plasticizers, with the fillers and modifiers needed to attain the properties specified by the end user. Being very versatile, plastisol is used in a variety of applications.
Hot and cold dip processes are used for corrosion prevention as well as aesthetics. For certain complex geometrical shapes (e.g., metallic blower wheels of fans and HVAC equipment), complete coverage cannot be attained by spray coating or brushing. In applications such as these a uniform coating can be attained only by a hot dip process. Dip coating of PVC has been popular for many decades. It has been used to produce chain-link fences and metal storage racks. A one-step dip is very cost-effective.
To ensure good adhesion, surface preparation and a primer coating is sometimes necessary.
Steps in the Hot Dip Coating Process
1. Metal part cleaning
Tools and metal parts to be vinyl coated must be thoroughly cleaned, chromated or phosphated, or vapor degreased to improve adhesion to the surface. Cleaning ensures that primers and plastisols don't become contaminated.
2. Metal surface priming
Vinyl in the liquid (plastisol) condition does not adhere well to steel and other metals. Hence there is a need to prime the metal parts. A primer can be sprayed or dip coated on a cleaned and processed surface. However the primer dip tank should be monitored closely, because the viscosity may increase due to evaporation of the solvent. The surface of metal parts is coated with a primer, dried and preheated (baked) at a temperature of 177°C to 204°C (350°F to 400°F). Priming and soaking (baking) prepares the metal parts for hot dipping.
3. Metal part dipping
The primed and baked metal part is then dipped into the bath of liquid vinyl (plastisol). Upon withdrawal, the surfaces of the metal part will develop a semi-fused uniform layer of semiliquid vinyl (plastisol). The coating's thickness will vary according to the duration of the immersion, the metal's surface temperature, the formulation and the part's surface condition.
A hotter metal part that is immersed longer will develop a thicker coating. Ideally the part should be immersed slowly and vertically, and withdrawn in the same way to minimize bubbles in the bath. Air pockets are avoiding by choosing the best immersion angle and position of the metal part.
4. Fusion of the vinyl plastisol film
The metal part coated with a semi-liquid vinyl plastisol is cured into solid form at temperatures of 149°C to 177°C (300°F to 350°F). This may be done in a separate curing oven.
5. Cooling process
The cured hot metal part is susceptible to surface marring as it is removed from the baking oven. It should be cooled below 49°C (120°F) before handling. Cold air circulation can cool the parts.
The dip facility's design should ensure that the immersion tank is located near the baking (preheating) oven so that the temperature of the heated metal parts is maintained while being transferred to the tank.
Cold Coating and Powder Coating Processes
Certain solvent-based vinyl products don’t need heat to develop a layer on metal surfaces. Some of them are useful as patch compounds or repair vinyl coatings. Powder coating processes are also used to form thin coatings on metals. Plastisol putties on the other hand require heat to cure the coating.
Disadvantages of Vinyl Coatings
Vinyl coatings do have some limitations that make them unsuitable for some applications.
Can’t be removed easily
Before using a different coating system the complete removal of any vinyl coating is essential. However, vinyl coatings are not easily removed because when it cures the polymer bonds securely with the metal's surface and the film covers crevices and the corners of the substrate. Hence, completely removing the film is a challenge as it can be labor intensive and expensive.
Requires specialized procedures and sophisticated equipment
Coating a metallic part with vinyl requires specialized procedures and facilities, such as sophisticated equipment and tools. Either the metal part is immersed in a molten plastisol or it is coated by using a spray coating system. A durable film curing requires a baking oven. The temperature during powder coating must be carefully monitored. These aren't practical in a small home-based workshop.
Insufficient coating thickness with powder coating
As the powder coating method results in a thin coating, the durability of the vinyl film can’t be guaranteed. Pinholes may develop on the film due to abrasion by other object. These pinholes can result in pitting corrosion. (Learn more in All About Pitting Corrosion.)
Better advanced techniques exist for cutting tools
As far as cutting tools are concerned, advanced ceramic coatings can prove superior due to their abrasion resistance. Chemical vapor deposition (CVD) and physical vapor deposition (PVD) coating systems are available for different types of tools made of wear-resistant alloy steel.
Ceramic coatings may have higher durability as compared to vinyl coatings for marine environments. Chrome nickel plated tools also provide superior performance. Phosphate coated fasteners are used for indoor construction. Titanium alloys are useful for corrosion prevention as well as abrasion resistance, but may prove to be too expensive.
Vinyl coatings are cost-effective and versatile for tools and metal part applications, especially those that require heat or electrical insulation, corrosion and abrasion resistance, and flexibility.