All About Polyurethane Wood Coatings

21st February 2024
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Wood coatings can be divided into two categories: decorative coatings and furniture coatings, including nitrocellulose NC coatings, PU coatings, and PE coatings.

Wood coatings can be categorized according to the construction layers as follows:
Stains: used to color the surface of wood evenly.
Sealing primers: fill pores in wood, expel air from pores, seal the surface, and prevent wood from shrinking and swelling.
First coat primers: increase the adhesion of coatings and improve coating fullness.
Sanding primers: used for sanding needs and also fill pores.
Topcoats: main surface coatings for decoration and protection, including clear varnishes, transparent color coatings, and pigmented coatings, with both glossy and matte finishes available.
Gloss clear varnishes: enhance the decorative and protective properties of the entire coating film.

wood coating

Pros and cons of polyurethane wood coatings

There are five types of polyurethane coatings, including single-component moisture-curing, two-component hydroxyl-curing, two-component catalytic-curing, waterborne emulsion and water-dispersion polyurethane wood coatings. According to the construction procedure, they are divided into sealing primers, sanding primers,and topcoats.

Advantages:
(1) The coating film is glossy, full, and has excellent decorative properties. Coatings formulated with aliphatic isocyanates have good light and color retention (aromatic isocyanates tend to yellow, often used for indoor wood).
(2) High hardness and strong wear resistance of the coating film.
(3) Strong adhesion of the coating film.
(4) The coating film is resistant to water, boiling water, cosmetics, food, and chemical media, with excellent anti-staining properties.
(5) The coating film is heat-resistant, cold-resistant, and resistant to temperature changes.

Disadvantages:
(1) Inconvenient to use two-component systems, short usage cycle.
(2) Some varieties contain free isocyanate monomers, which can affect human health and environmental pollution.
(3) Relatively long drying time of the coating film.
(4) Coating film of aromatic isocyanate coatings tends to yellow.
(5) Strict construction requirements, sensitive to environmental conditions, moisture, and humidity.

Polyurethane Sealing Primer

The Polyurethane Sealing Primer consists of two components:

  • Component A: Polyester resin + Talc powder + Penetrating agent + Aldehyde resin + Solvent
  • Component B: TDI aromatic curing agent + Solvent

The two-component hydroxyl-curing sealing primer is the primary sealing primer for wood coatings and can be used in conjunction with nitrocellulose, unsaturated polyester, and other coatings.

Polyurethane Sanding Primer

The Polyurethane Sanding Primer consists of two components:

  • Component A: Polyester resin or short oil alkyd resin or acrylic resin + Nitrocellulose + Zinc hardener + Talc powder + Organic bentonite + Defoamer + Mixed solvents
  • Component B: HDI or TDI + Mixed solvents

The sanding primer is applied over the sealing primer and under the topcoat, serving as an intermediate layer. The two-component hydroxyl-curing sanding primer is only used in polyurethane systems.

Polyurethane Wood Topcoat

(1) Hydroxyl-Curing Polyurethane Wood Coatings:
A. Wood Clear Coat: This is the most widely used type of wood coating. Component A (B component) consists of saturated polyester resin, short oil alkyd resin, hydroxyl acrylate resin, synthetic fatty acid-modified alkyd resin, nitrocellulose, matting agent, wax powder, leveling agent, defoamer, mixed solvents. Component B (A component) consists of HDI, TDI, mixed solvents.

High-gloss coatings often use saturated polyester alkyd resin with high cross-linking density as the main component, with a high degree of cross-linking with the curing agent. Matte coatings should use hydroxyl components that are easy to mattify, with a moderate cross-linking density, and suitable matting agents should be selected. To maintain a good appearance of the coating film, resins with good compatibility and solvents with appropriate volatility should be selected, and corresponding additives should be used.

B. Two-Component Pigmented Coatings

(2) Catalytic-Curing Polyurethane Wood Coatings:
Consist of NCO-containing castor oil/glycerin or polyether prepolymers as component A, and amines, metal salts, or cycloaliphatic acid salts as curing catalysts as component B, mixed in certain proportions. Catalytic-curing polyurethane coatings cure quickly, unaffected by environmental factors or film thickness, have hard and glossy film, and are resistant to water and abrasion. They are commonly used for floors, sewing machine tables, etc.

(3) Moisture-Curing Polyurethane Wood Coatings:
These are single-component coatings formulated mainly with prepolymers containing NCO end groups. They are easier to apply compared to two-component coatings and have better wear resistance. The coating cures by reacting with moisture in the air to form urea bonds. The drying rate varies with humidity, and when the film is too thick or dries too quickly, carbon dioxide is produced during film formation, which is not easy to escape, leading to the formation of bubbles or pinholes.

(4) Oil-Modified Polyurethane Wood Coatings:
Also known as urethane oil or urethane alkyd wood coatings, these are single-component coatings with diisocyanate-modified drying oils (or alkyd resins) as film-forming materials. They are oxidative drying type coatings, with drying agents added to the composition. They are easy to apply and have better film properties than alkyd resin coatings and are generally used for indoor ordinary wood.

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