Polyurethane System for Wood

PU coatings are considered as high end wood coating finishes. It provides rich colour, good appearance, smooth silky finish and most important protection. PU Wood Coating can either be clear or pigmented alkyd base or acrylic base system.

Performance Comparison for all solvent based wood coating systems is,

Polyurethane>Melamine>Nitrocellulose>Alkyd Enamel.

2K PU Alkyd systems are applied where yellowing of a coating after few months of application is not a big concern like interior wood finishes. It provides balance of properties like chemical, water and scratch resistance along with favourable cost dynamics and faster productivity.

2K PU Alkyd Clear or Pigmented Sealer/Primer

2 K PU Alkyd System

To formulate a two component PU alkyd clear or pigmented primer/sealer, we need to select resins and other raw material based on the physical and chemical properties requirement. Properties like good sanding ability, less absorption and high coverage. Adhesion of coating on wood substrates is not difficult as the wood is porous in nature so it absorbs the coating applied on it and also it has low surface energy thus wetting won’t be an issue.

So to get the excellent sanding property, we need to select fillers or special additives like stearate so that it should affect the transparency of the coatings if it’s a clear system and all other properties will depend upon the alkyd polyol selection. Roughly the RMC for PU Alkyd clear sealer would be around Rs. 95-100/ltr and for pigmented primers Rs. 110-120/kg.

Now for topcoats the most important properties that coating should have are stain, water and chemical resistance and high anti-scratch performance. For wetting of PU Alkyd topcoats on sealer/primer, it requires special wetting additives. Here the choice of alkyd polyol would be high molecular weight and high hydroxyl content resin to have better film performance. Also the hardener chemistry will decide the hardness development and drying characteristics. Generally aromatic hardeners are used as they are economical and yellowing is not a concern in case of alkyd system as it goes only in interior furniture’s. RMC for 2K PU Alkyd Clear Topcoat would be Rs. 110-120/ltr and for Pigmented Rs. 130-145/kg.

We have compared our products against market standard SIRCA and ICA 2K PU Alkyd products and the results were comparable in terms of technicality but commercially we were economical. Major Market of this system is with veneer manufacturers, furniture OEM and retail sector Pan India.

Similarly 2K PU Acrylic Wood Coating is based on 100% pure acrylic polyol cross-linked with anti yellowing hardeners. These systems are preferable were veneers or solid wood having open pore finish requirement. It gives water like clarity deep matt finishes thus preferable for light veneered wooden substrate. Being completely aliphatic chemistry, it is also preferable for outdoor wooden articles.

Only concern with the PU Acrylic wood coating its drying performance and manufacturing troubles. While formulating all 2K PU Acrylic products drying performance needs to be improved to a certain stage using accelerators which helps in faster curing. Acrylic polyol needs to be operated with certain temperature limit. Once the manufacturing temperature shoots above its glass transition temperature, resin loses its physical morphology and the coating gets gelled up.

I have rectified this problem by installing cooling reactor under high speed dispersers so that the temperature remains controlled.

Waterborne Wood Coatings

Waterborne coatings are advance category of coatings which are eco-friendly in nature due to negligible VOC content because of some amount of coalescing solvents which is very less toxic.

W aterborne Coating 2K Clear/Pigmented Interior Finish

For the interior formulation, choice of emulsion will dictate the coating performance properties like stain, water and detergent resistance and anti-scratch property. For 1K system self cross linking acrylic emulsion is preferred. Nowadays core shell technology self cross linking emulsions are being used in interior wood coating formulations for maintaining balance of all required properties. To have highly resistant interior wood coating, we can choose acrylic emulsion which can be cross linked with water base hardeners. This will be a 2K water base coating for wooden substrates and is costly thus goes into premium projects.

Now from last 4 years or so, all the export market based in Rajasthan, Delhi and Moradabad is using water base wood coating due to norms and regulations. I have customized many clear sealers and topcoats based on their requirement. One component waterborne wood coating is used in very bulk quantities as this is commercially viable and comparable to solvent base coatings. Roughly the RMC for water base clear sealer would be around Rs. 125-150/kg and for topcoats Rs. 150-165/kg.

For wood coating waterborne coating is the most preferable system for exterior application. Wood has its own natural tendency of expansion and contraction with change in surrounding climatic conditions, most prominently in exterior wooden furniture’s. Thus the coating should be flexible enough to sustain such behaviour at the same time should be hard enough to have various performance due to which waterborne chemistry is the best for this balance.

In case of waterborne exterior wood coatings, formulation based on pure acrylic emulsion and polyurethane dispersions ensure 100% protection of wood against UV resistance. Customers want more than 5 years warranty when it comes to exterior performance. To achieve that we need to have complete solution starting from isolator/impregnator application for blocking performance, then application of sealer or primer to fill up all pores of wood and lastly application of highly specialized waterborne PUD-acrylic topcoat which gives exterior durability. Normally the RMC for water base clear sealer would be around Rs. 190-210/kg and for topcoats Rs. 280-305/kg.