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Monday, 23 September 2019
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Raw materials & technologies, Raw materials, Coatings binders

Biobased monomers set new trends in wood coatings and fire retardants

Tuesday, 3 July 2012

In order to look at the possibility of the use of 1,3-propanediol for exterior wood coatings, a comparison has been made of polyurethane dispersions (PUD) in which the polyester composition is the investigated parameter. Encapsulation is the trend for fire retardants.

Enhancing weathering properties of exterior wood coatings by renewable resources for PUDs

Source: Fraunhofer

Enhancing weathering properties of exterior wood coatings by renewable resources for PUDs

Source: Fraunhofer

Andreea Daniliuc and her colleagues from Fraunhofer-Institute for Wood Research (WKI) improved UV-curing wood exterior coatings made from diols. Synthetic routes are available to use renewable products. Improved fire retardants can be made from encapsulated ammonium polyphosphate. These outcomes will be published in the next European Coatings Journal on July 5, 2012.

The polyester composition of PUD’s investigated

In order to look at the possibility of the use of 1,3-propanediol for exterior coatings, a comparison has been made of polyurethane dispersions (PUD) in which the polyester composition is the investigated parameter. The work involved replacing 1,6-hexanediol (1,6-HDO) by 1,3-PDO.

After polyesterification by melt condensation, the PUDs were produced according to the acetone process, followed by a chain extension with a diamine. After removing the acetone, the polyurethane dispersion was adjusted to a solids content of 40 %.

Further renewable resources for 1,3 – propandiol

Biotechnologically produced 1,3-propanediol (1,3-PDO) was used as a starting component in the polyester synthesis. 1,3-PDO is already produced from corn sugar by means of biotechnology. Work has been carried out on the conversion of glycerol to 1,3-PDO, which should lead to a more cost-effective process.

Bio-sourced pentaerythritol triacrylate is using commercial pentaerythritol and bio-sourced acrylic acid. If these building blocks are available in the future, waterborne UV-curing polyurethane dispersions with a natural content of 55 to 80 % will be possible.

Excellent properties of renewable UV-wood coating

UV-dispersions produced in the laboratory were formulated with a photo-initiator (a-hydroxyketone), defoamer and a wetting agent.

The coating has a gloss of 95 % measured at 60 °. And its wood-grain-enhancement properties are excellent.

Hydrolysis of 1,3 – PDO and 1,6 – HDO examined

Hydrolysis resistance is an important criterion for good weathering properties of a paint film. The polyester component has the dominant influence on the paint’s performance. Therefore, to assess the influence of the diol component of the hydrolysis, two polyester based on 1,3-PDO and 1,6-HDO respectively were examined for their hydrolysis resistance. In each variant, the acid component was also varied.

The PDO and HDO - based PUD improved weathering resistance

The hydrolysis of polyester polyols was measured by the reduction of its molecular weight over time, as the hydrolysis process breaks the ester bonds of polyester polyols.

To improve the durability of exterior wood coatings, for example, the hail resistance, the use of a blend of pure acrylics with a flexible polyurethane dispersion is highly recommended. Both the 1,3-PDO and the 1,6-HDO-based PUD significantly extended the durability of the acrylic dispersions tested.

Novel fire retardants by encapsulation

Moving on to fire-retardant coatings for wooden facades, intumescent fire-retardant coatings (IFRC) provide fire protection for flammable substrates by the formation of a voluminous, insulating protective layer by the simultaneous carbonization and  foaming of the ingredients.

These so-called intumescent coatings consist of a polymeric binder, an inorganic acid source (e.g. ammonium polyphosphate), a carbon-rich source (e.g. polyalcohols such as starch, pentaerythritol) and a blowing agent (e.g. melamine).

To ensure the moisture resistance and temperature-controlled release of ammonium polyphosphate (APP), various APP-composites with carnauba wax and synthetic waxes with different melting ranges were prepared by hybridization process. This process applies fine powder onto the surface of a core powder. The system operates by mechanical forces such as impact and shear. It is therefore not just restricted to chemical laws.

Encapsulated APP enhances the moisture resistance without any influence on intumesence

The encapsulation of APP with wax provides improvement in moisture resistance of coated wood panels as shown by artificial weathering results according to EN 927. Certain samples remained unchanged. Their fire retardant performance being identical to unexposed samples after 240 hrs. The protective wax layer did not negatively influence the intumescent process.

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