Wood Coating

20 European Coatings JOURNAL 07|08 l 2012 www.european-coatings.com

Technical PaperWood coatings

Andreea DaniliucBarbora DeppeOlaf DeppeStefan FriebelDirk KruseClaudia Philipp

Improved, UV-curing wood exterior coatings can be made from diols. Synthetic routes are available to use renewable products. Improved fire retard-ants can be made from encapsulated ammonium polyphosphate.

In order to look at the possibility of the use of 1,3-pro-panediol 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 polyurethane dispersions (PUD) were produced according

New trends in wood coatings and fire retardantsBiobased monomers and high performance coatings

Contact:M.Sc. Andreea DaniliucFraunhofer-Institut für Holzforschung WKI Braunschweig/Germany T +49 531-2155-308 [email protected]

to the acetone process, followed by a chain extension with a diamine. After removing the acetone, the polyure-thane dispersion was adjusted to a solids content of 40%.Biotechnologically produced 1,3-propanediol (1,3-PDO) was used as a starting component in the polyester syn-thesis. 1,3-PDO is already produced from corn sugar by means of biotechnology [1]. Work has been carried out on the conversion of glycerol to 1,3-PDO, which should lead to a more cost-effective process [2].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. In series 1, the same mole fractions of adipic acid and phthalic anhydride were used. In series 2, the molar ratio of adipic acid to phthalic an-hydride was 1:2.


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Hydrolysis 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. The acid groups created reduce the pH value. Since the hydrolysis is autocatalysed by the pH value, the mixture was buffered to a constant value of pH 4. The change in molecular weight of the polyester was deter-mined at 80 °C. The reduction of the molecular weight indicated that the molecular weight distribution index of the polymer only changed little. Thus, the hydrolysis of the ester groups was shown to be independent of the molecular weight and occured from the chain ends.Taking the molecular weight/initial molecular weight ratio (Mn/Mn0) as a quantitative measure, the molar mass of 1,3-PDO-based polyester polyols decreased by hydrolysis to 47% (series 1) and 61 % (series 2) of the original value compared to 49 % (series 1) and 66 % (series 2) for 1,6-HDO based polyesterpolyols. The hydrolysis rate was much lower than originally expected, because of the smaller molecular structure of 1,3-PDO. The higher proportion of phthalic anhydride had a much stronger influence than the choice of the diol in series 2 (see Figure 1), because the phenyl group sterically shielded the ester bondings. Additional studies have shown that the hydrolysis stabil-ity can be further enhanced by other sterically hindered monomers such as isophthalic acid and neopentyl glycol, but this, however, reduces the elasticity of the film.The 1,3-PDO and 1,6-HDO-based PUD were formulated as exterior wood varnishs. They were mixed with a com-mercially available acrylic resin in a ratio of 1:3. Pine sap-wood was then coated with these products. The total film thickness was approx. 100 µm. The coated samples were weathered using fluorescent UV lamps and water in a QUV machine according to DIN EN 927-6.The weathered samples are shown in Figure 2. After the QUV weathering, the pure acrylic dispersion displays were visually slightly more damaged than the two PU blends. The durability of the two PU blends cannot be visually distinguished using this weathering method.In a further test, pine sapwood was coated with pure 1,3-PDO-PUD and with a 3:1 blend with a pure acrylic dis-persion. The coated samples were subjected to natural weathering according to DIN EN 927-3. The results after 15

Results at a glance In order to look at the possibility of the use of

1,3-propanediol for exterior wood coatings, a com-parison has been made of polyurethane dispersions (PUD) in which the polyester composition is the investigated parameter.

Ways of making the constituents from renewable sources are being developed

Improved fire retardants can be made from encap-sulated ammonium polyphosphate (APP)

Figure 1: Variation over time of Mn/Mn0 of polyester polyols in a buffered solution

Figure 2: Results after 2016 h QUV-weathering (DIN EN 927-6) left: acrylate; centre: blend with 1,3-PDO-PUD; right: blend with 1,6-HDO-PUD. The right-hand sample of each series is the non-weathered reference sample.

months weathering are shown in Figure 3. The plane are-as of the samples are in both cases free from any damage.Since the initial elasticity of the PU-blends is higher than that of the pure acrylics, the visual damage occurs at a later stage. 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 dis-persion is highly recommended. Both the 1,3-PDO and the 1,6-HDO-based PUD significantly extended the dura-bility of the acrylic dispersions tested.

Biobased UV-coatingsThe method of synthesis of UV-curable polyester polyu-rethane dispersions for furniture coatings was as follows. The first step was the synthesis of a polyesterpolyol. The diol component was predominantly 1,3-propanedi-ol. Other biosourced diols such as glycerol, pentaeryth-ritol or castor oil may also be used. The main building blocks were Diacids such as phthalic acid, isophthalic acid and adipic acid. In principle biosourced diacids such as furandicarboxylic acid, succinic acid and difatty acids can also be used, i.e. the polyesterpolyol can be made of 100% natural resources. The second step was the addi-tion of an isocyanate such as isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI) or others to give an isocyanate-terminated polyesterpolyol. For dis-persions that require a high light fastness, aliphatic iso-cyanates are preferred to aromatic ones. The advantage of IPDI is that it gives particularly good film hardness. A



self-emulsified dispersion was achieved by the addition of dimethylol propionic acid (DMPA). In the next step, the NCO-groups of the polyurethane prepolymer were react-ed with hydroxyacrylates such as 2-hydroxyethyl acrylate (HEA), 2-hydroxyethyl methacrylate (HEMA) and pentae-rythritol triacrylate (PETA) or a mixture thereof. Typically, the level of double bonds was in the range of 1.8 to 3.0 mmol/g polymer. After neutralising with, for example, triethylamine, the PU-resin was dispersed in water.

Renewable sourcesTo date, hydroxyacrylates have been made from petro-leum. Figure 4 illustrates the future scenario for “bio” method for the production of 2-hydroxy acrylate. A dem-onstration plant for biobased acrylic acid production is due for completion this year with a production plant fol-lowing in 2013 [3]. Bioethanol can be used to make eth-ylene oxide as part of the process. Bio-sourced pentaerythritol triacrylate is using commer-cial pentaerythritol and bio-sourced acrylic acid as shown in Figure 5. 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.UV-dispersions produced in the laboratory were formu-lated with a photo-initiator (a-hydroxyketone), defoamer and a wetting agent. The total solvent requirement (butyl glycol) is less than 25 g/l to achieve a minimum film-forming temperature of < 5 °C.The coating has a gloss of 95 % measured at 60 °. And its wood-grain-enhancement properties are excellent. The lacquered wooden plate was subjected to a standard test DIN 68861-1 (for furniture surfaces) and rated as category 1B. The pendulum hardness according to König (DIN EN ISO 1522) lies between 70 and 133 s.

Sugarbased methacrylate monomers for acrylic dispersions

Novel saccharide monomers exist for acrylic dispersions. For example, radically polymerisable sugar methacrylates were synthesized in two-step reactions using low-molecular sac-charides, which were copolymerised with commercial acrylic monomers based on renewable materials. These vinylsac-charides, also from renewable materials, are an alternative to petrochemical raw materials such as MMA. 3-O-Methacryloyl-1,2:5,6-di-O-isopropylidene-α-d-glucofuranose (MDG), a monomer suitable for the radical emulsion polymerisation, can be synthesized from glucose in a two-step reaction by esterification [3]. The acetalisation of glucose to diacetonglucose (DAG) is state of the art. Protect-ed sugars, which still have one free-hydroxyl group, can be converted to polymerisable monomers by esterification for example with methacrylic anhydride. During the work, this reaction was adapted to give lab-scale batches of 8 kg with 90 % yield and 97 % purity without distillation. Generally, the homopolymers of these sugar methacrylates exhibit very high glass-transition temperatures (Tg >160 °C) compared to commercial monomers such as MMA (Tg = 105 °C) [4].The aim is the synthesis of waterborne binders for wood and wood-based materials based largely on renewable resources which are present at more than 40 % based on the solid content of the binder. The properties of acrylic

Figure 3: Results after 15 months natural weathering (DIN EN 927-3) left: pure 1,3-PDO-PUD; right: 3:1 blend with acrylic / 1,3-PDO-PUD

Figure 4: Possible synthesis route for bio-based 2-hydroxy acrylate

Figure 5: Possible synthesis route for bio-based pentaerythritol triacrylate

Figure 6: Synthesis of a sugar methacrylate starting from glucose


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dispersions can be achieved using an appropriate recipe and production process. Thus, binders were produced both by con-ventional emulsion polymerisation and mini-emulsion poly-merisation. The lab-scale product thus generated is mainly suitable for furniture lacquers [5]. The acrylic dispersions were applied on glass. Film formation took place at ambient condi-tions. The films were exposed to a light resistance tester for 72 h. No yellowing occurred. The heat stability was tested by various thermal-analysis methods, such as DSC and TGA-MS. As shown in Figure 8, the loss of mass is low up to 100° C (< 2 wt- %) and is dominated by water (blue lines). Acetone of the protecting groups (green lines) is released in the same amount as water at 130° C (~2 wt- %) and dominates up to 200° C. Subsequently, condensation reactions take place and water is released again. In general, MDG-containing copoly-mers are heat stable and for this reason they were tested in fire retardant coatings (see below). The coatings of these new binders exhibit a chemical re-sistance of 1B according DIN 68861-1 without mustard. A two-layer system of primer and topcoat both consisting of MDG-copolymers show excellent wood warmth (Figure 9). Some other properties such as hardness or gloss need to be improved. This project was funded by „German Federal Ministry of Food, Agriculture and Consumer Protection” (BMELV) via “Agency for Renewable Resources” (FNR) within the frame-work “Technische Kunststoffe und Spezialpolymere aus nachwachsenden Rohstoffen”.

Novel fire retardants by encapsulationMoving on to fire-retardant coatings for wooden facades, intumescent fire-retardant coatings (IFRC) provide fire pro-

tection for flammable substrates by the formation of a vo-luminous, 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),

Figure 7: Degrada-tion profile of a MDG-containing copolymer. The percentage (%) scale of the ordinate is related only to the black TGA-curve

Figure 8: Steam-roller model coated with two-layer system of primer and topcoat both consisting of MDG-copolymers



a carbon-rich source (e.g. polyalcohols such as starch, pen-taerythritol) and a blowing agent (e.g. melamine). In spite of the many advantages, fire retardant coatings lack sufficient weathering stability. Therefore, their ap-plication is limited to indoor use (r.H. < 70 %). Due to the requirements of national building codes, there is a strong market demand for fire-protected wooden facades [6].Substances essential for the intumescent reaction are sensitive to hydrolysis. For example, ammonium polyphosphate (APP), as part of intumescent coating, is hydrolyzed to water-soluble monoammonium phos-phate when exposed to relative high humidity. The re-sulting leakage of phosphate ions leads to a decrease in fire performance. APP in crystal form II has better water resistance and higher thermal stability (decomposed at temperature higher than 275° C) than in crystal form I [7].

Figure 9: SEM of used substances and their composites (1000x magnification)

Figure 10: Laboratory fire tests with topcoats

APP II surface treated with different resins and agents has very low water solubility and improved thermal sta-bility and is commercially available. However, to provide protection against fire, phosphor-carbonaceous foam has to form at an early stage of the fire. To ensure the moisture resistance and temperature-controlled release of APP, various APP-composites with carnauba wax and synthetic waxes with different melting ranges were pre-pared 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[8]. 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. Cer-tain samples remained unchanged. Their fire retardant performance being identical to unexposed samples after 240 hrs. The protective wax layer did not negatively influ-ence the intumescent process.Furthermore, to improve the fire retardant performance, pentaerythritol (PER) was brought by encapsulation of APP to its immediate proximity. After ammonia was released from APP, the polyphosphoric acid dehydrated the PER. Additionally, the APP-PER-composite was encapsulated with carnauba and synthetic waxes of different melting range to protect it from moisture. Laboratory-scale fire tests showed better fire retardant performance of intu-mescent coatings (IC) with APP-PER-synthetic wax with melting range from 67 to 118° C than the reference con-taining pure APP and PER. This result was also confirmed by TGA measurements. The fire-retardant performance of APP-PER-carnauba wax composites, with melting range from 70 to 89° C, was lower than the reference. Figure 9 shows scanning electron microscope (SEM) images of the pure substances and the prepared composites [9]. » 10.a) pure APP » 10.b) pure PER » 10.c) APP coated with 5 % PER » 10.d) APP coated with 5 % PER and 2 % carnauba wax

Moreover, encapsulation overcomes chemical incompat-ibility such as sensitivity to acids of hydrogen carbon-ates. Hydrogen carbonate anions react with acidic binder or phosphate during the production of intumescent coat-ing releasing carbon dioxide. NaHCO3-wax composite creates the necessary protection layer and NaHCO3 acts as a blowing agent under heat impact and decomposes to carbon dioxide. Above 70 °C, non-protected NaHCO3 gradually releases CO2. However, the conversion is fast at 200 °C. Disposal of NaHCO3 and thus availability of CO2 is regulated by the melting range of used wax. No gas formation was observed during production of the IC with a wax-protection layer with content of 3 % or higher. The loss of mass determined by TGA was delayed, but the fire-retardant tests exhibit a decreasing in performance. The encapsulation level of wax in the composite was determined by DSC (Differential Scanning Calorimeter) from the calibration curve as function of normalized en-thalpy. For instance, for the synthetic wax with a melting range from 67 to 118° C, the equation y=-2,4x+0,2 was used and for carnauba wax with melting range from 70 to 89° C one of the of y= -1,1 -0,7. Two other methods were used to measure the encapsulation level: conduc-

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tivity measurements and gravimetric determination of water solubility. The core substances of the composites, such as NaHCO3 and APP, are by trend water-conductive salts.Further investigations were focused on particle size distribution suitable for a high-performance fire retardant coating, rheology changes associated with the introduced composites, long-term stability of composites, influence of shear force on the com-posites during dispersing and a long-term study of natural weathering coupled with fire retardant performance.

Topcoats reduce migrationThe migration of inorganic salts from the intumescent coating can be avoided by applying a topcoat. The higher its thickness, the better the humidity protec-tion, but at the cost of lower fire performance. The higher topcoat thickness also reduces the transparency of a wood intumescent coating.Several commercial topcoats were investigated. Their influence on fire-protection efficiency was examined in the laboratory. The results showed that, compared to the reference sample (a pigmented coating was used), all topcoats reduce the fire-protection performance (see Figure 10). However, significant differences can be seen between the chosen topcoats,. The systems TC 1, TC 2 and TC 7 have the smallest influence on the degree of fire protection observed in laboratory fire tests. Therefore, topcoats TC 1 and TC 2 were chosen for further testing for water absorption.In outdoor weathering trials, the results obtained are promising, but still do not meet the requirements. Nevertheless, despite better fire-retardant properties and humidity resistance that is superior to current commercially available sys-tems, there is still room of improvement.

Novel monomer MDG for fire retardants Modern flame retardants are adapted to the trend towards non-toxicity, sus-tainability and environmentally friendliness. Therefore, tailor-made saccha-ride-based binders were used to in the work on intumescent coatings. Moreo-ver, the polyhydroxylic compound in the intumescent mixture can be partially replaced by the saccharide-based binder. Hence, the filling grade of intumes-cent additives can be increased to improve the fire retardant performance. Waterborne saccharide-containing binders should be salt stable and critical pigment-volume concentration should be high. Several parameters such as melting viscosity, polymerization technique, surfactant system and the affili-ated particle size and distribution influence the intumescence reaction.Several 3-O-Methacryloyl-1,2 : 5,6-di-O-isopropylidene-α-d-glucofuranose (MDG)-containing binders were tested for suitability in intumescent coatings with 40 % intumescent mixture consisting of APP, pentaerythritol, melamine and titanium dioxide. Laboratory fire tests showed better fire retardant per-formance for MDG-containing acrylic dispersions with a particle size of 100 nm, 45 % of MDG in the copolymer and an increased non-ionic surfactant content in the surfactant mixture. Also MDG-containing binders with novel ita-conate comonomers prepared by mini-emulsion polymerization facilitate the intumescence reaction. Further investigation on to what extent saccharide-containing binders could function also as carbon source, are still in progress. í

REFERENCES[1] White G. M., Bulthius B., Trimbur D. E., et al, Patent WO 9910,356 1999.[2] Nakamura C.E., Whited G.M., Current Opinion in Biotechnology 2003, 14, 454[3] Klein J., Herzog D., Hajibegli A., Makromol. Chem., Rapid Commun. 1985, 6, 675-678.[4] Koch U., Yaacoub E.-J., Macromol. Chem. Phys. 2003, 204, 803–812.[5] Vymetalikova B., Deppe O., Zucker aufs Holz, Farbe und Lack, 2007, 10, 24-26[6] Östmann B., Foss A., Hughes A., et al., Dire Mater. 25, (2001) 95-104[7] Futterer T., Ammonium polyphosphate - a multi-use flame retardant, Speciality Chemicals

Magazine, 26 (2006), 34-36[8] www.nara-e.de/documents/render.php?aid=16[9] Derr L., Funktionelle Partikelmodifizierung und Charakterisierung, Diplomarbeit (2009).

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Wood Coating