Formulating for Tannin Stain Inhibition

April 23, 2013

Nathan Kofira

• Tannin staining is the discoloration of a painted surface due to the migration of water soluble Tannins (tannic acids) through the film

• May appear immediately or after prolonged exposure to humid conditions

• Tannins may be more concentrated around high pressure areas (i.e. – nails, strapping marks, knot holes)

Phenolic compounds found in many tree species used commercially. Two main categories, Hydrolyzable and Condensed

Tannin Staining An Introduction:

Hydrolyzable (HT) • i.e. Chestnut, Oak • Glucose or Gallic acid core • Hydrolyzed by mild acid or mild bases to yield carbohydrate & phenolic acids

Condensed (CT)

• i.e. Pine, Cedar, Merbau • Flavonoid or catechol core • Not readily hydrolyzable to simpler molecules

Tannin Staining Extractables from Wood Products

Softwoods Hardwoods

Species % Species %

Ponderosa Pine 5.9 American Elm 2.1

White Pine 7.0 Northern Red Oak 4.4

Redwood 10.8 White Oak 5.4

Western Red Cedar 15.0 Southern Red Oak 9.6

Extractables include tannins, oils, and rosin component Wood color is generally related to tannin content All staining is not caused by tannins

Wood/Bark Extractives; 3434Wood; Virgina Tech Department of Chemistry (2000)

WOOD CONDITIONING Natural ageing and washing with oxidative solution (i.e. Oxalic acid

bleaching)

BARRIER PROPERTIES Resin structure

New developments in modified acrylics mimic the natural binding properties of tannin-lignin and tannin-protein complexes

Ionization and solubility of tannins reduced when using cationics which can also mimic tannin-protein binding complexes

Tannin Staining Methods of Reducing Tannins:

Tannin Stain Inhibition Origin and Binding of Tannins:

Tannins (tannic acids) are high molecular weight polyphenolic molecules found in wood – Lignins, Cellulose, and Hemicellulose

CHELATION Multivalent ions combine multiple water soluble tannins of lower

Mw to create large complexes that are no longer water soluble

Tannin Staining Methods of Reducing Tannins:

Powder Liquid

Cations Aluminum

Barium Antimony

Aluminum Zirconium Aluminum Zirconium

Zinc

pH 7.3 9.5 13.0 9.0

Oil Absorption 38.8 35.4 N/A N/A

% Water Solubility 0.17 0.11 100 100 Density g/ml 2.77 3.10 1.23 1.30

Mean particle size µm 5.1 3.1 N/A N/A

The chelation process is essentially the reaction of slightly water soluble metal cations from inhibitive pigments with hydroxyl

groups on the phenolic rings present in all Tannins.

OH

OH

OH

HO

HO

HO

Zr

Zn

Al

Zn+²

Al+³

Zr+4

SOLUBLE

HO

HO

HO

OH

OH

OH

HO

HO

HO

HO

HO

HO

HO

HO

HO

Tannin Stain Inhibition Simplified Model of Chelation:

IN

Formulation & Reformulation

• Coalescing Agents • Ester Alcohols (i.e. Texanol) and Propylene Glycol are

preferred. • Increased H bonding results in increased effective Mw and

decreased mobility

• Glycol Ethers (i.e. Propylene Glycol Monobutyl ether) reduce tannin stain inhibitor efficiency.

Why?

Lewis Base Complexed metal ion rendered unavailable

Formulation & Reformulation

• Tropical wood contains more Condensed Tannins • Bleaching agents has minimal impact • Require higher loading of Tannin Inhibitive Pigments

• Temperate wood contains more Hydrolyzable Tannins • More likely to bleed after initial coating • Mw of the tannins can change over time increasing molibility • Blends of multivalent ions shown to be more effective

Formulation & Reformulation

• pH control of the coating can significantly impact the ionization of hydrolyzable tannins • Low pH powder Tannin Stain Inhibitors can be

used in mildly basic coating formulations • High pH liquid Tannin Stain Inhibitors can

replace neutralizing agents and buffers in formulations to effectively limit tannin mobility

• Zinc containing Tannins Stain Inhibitors can reduce

the dependency of Zinc oxide in wood coatings

Dr. Tony Gichuhi – Director, R&D and Technical Service

Jessica Olmos – Project Chemist

Acknowledgements

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