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Polyurethane coatings

Polyurethane is a polymer made from organic units joined by urethane links. It is formed through an exothermic reaction between compounds with multiple hydroxyl groups (such as diols or polyols) and isocyanates with multiple isocyanate groups. The properties of polyurethanes, such as strength, flexibility, and toughness, can be tailored for different applications through adjustments to the molecular structure or crosslinking ratio. Common uses include coatings, adhesives, elastomers, and foams.

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POLYURETHANE COATING
WHAT IS POLYURETHANE? • Polyurethane (PUR and PU) is a polymer composed of organic units joined by urethane links. • While most polyurethanes are thermosetting polymers that do not melt when heated, thermoplastic polyurethanes are also available. • Polyurethanes are made by the exothermic reactions between alcohols with two or more reactive hydroxyl(-OH) groups per molecule (diols, triols, polyols) and isocyanates that have more than one reactive isocyanate group (-NCO) per molecule (diisocyanates, polyisocyanates). Urethane Linkage
CHEMISTRY PU are carbonic acid derivatives. PU are formed by addition polymerization reaction of diisocyanates with di or polyfunctional hydroxy compounds, or other compounds having a plurality of active hydrogen atom For example a diisocyanate reacts with a diol: Generalized urethane reaction
CURING • The curing of two-component polyurethane coating systems can be carried out at room temperature. • The drying times can vary considerably depending on the types of polyols and polyisocyanates used. • Elevated temperatures (such as after giving a flash-off time of 10 minutes, PU paint can be baked at 80°C) are often used for forced drying in production line coatings to speed up curing time.
PROPERTIES The properties of PUs greatly depend on the structure of the backbone. They can be tailored to have • High strength • Flexibility • Toughness • Low thermal conductivity Most urethanes also have good resistance to oil, (aromatic) hydrocarbons, oxygen, and ozone. Two major drawbacks of PURs are their • Susceptibility to microbial attack • Tendency of aromatic urethanes to discolor (yellow) when exposed to UV light.
MODIFICATIONS • By “undercrosslinking” which means using an NCO to OH ratio less than 1.0 / 1.0, the polyurethane film generally becomes softer and perhaps more flexible, less weather resistant, and less resistant to solvent and chemicals • By “overcrosslinking” using an NCO to OH ratio greater than 1.0 / 1.0, the resultant films tend to be harder and more chemical resistant. • Another possibility for changing the properties of a coating is to use a blend of polyols or polyisocyanates.
APPLICATION PROCEDURE IN AUTOMOTIVE INDUSTRY • Wear a complete paint suit to keep paint and other toxic products out of your personal system and clothing. • Wear a respirator with two air filters, and some kind of head covering and goggles. • Keep your area clean, you’ll create less work-by keeping dust and dirt out of the painted surfaces.
APPLICATION PROCEDURE IN AUTOMOTIVE INDUSTRY • Each panel should be primered and block-sanded with required grit(sand) paper prior to painting. • The component should be properly placed so that it stays stable. • Make sure the body is dirt and dust free. • After pouring your paint into the spray can, make sure you stir it thoroughly and often. • Filter the paint as it enters the paint gun reservoir. • Note that this is a HVLP gun. Paint filters are fairly standard, but don’t forget to grab them when you pick up your paint. • Wear the protective gloves, which can be changed often.
APPLICATION PROCEDURE IN AUTOMOTIVE INDUSTRY • Spray gun nozzles are adjustable (the inner fittings are changed) depending on the paint being used. • Check the parameters (such as pressure, gun span and gun volume) of the HVLP spray gun to make sure they are as per required standard • The gun should never be too close to the body and can be tested beforehand to make sure the pattern is appropriate. • If the gun spits at any time, stop and clean out the nozzle with the proper solvent and start again. • Avoid contact between the air hose or spray gun and the wet painted panel or vehicle-or you will have to repair it by letting it dry and sanding out the flaw.
QUALITY CONTROL Some of characteristics that are tested include the • pH value • Specific gravity • Viscosity • Thickness • Appearance • Color • Odor • Dry time • Film thickness • Hardness • Gloss Polyurethane fibers are tested for things such as elasticity, resilience, and absorbency. Polyurethane foams are checked to ensure they have the proper density, resistance, and flexibility.
USES • Building & construction • Transportation • Furniture & bedding • Appliances • Packaging • Textiles, fibers & apparel • Machinery & foundry • Electronics • Footwear
FINISHED GOOD PROPERTIES