Wood Protection with Dimethyloldihydroxy ... - ACS Publications

Chapter 17

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Wood Protection with DimethyloldihydroxyEthyleneurea and Its Derivatives Yanjun Xie,1,2 Andreas Krause,3 and Holger Militz*,1 1Department

of Wood Biology and Wood Products, Georg August University Göttingen, Buesgenweg 4, D-37077 Goettingen, Germany 2Key Laboratory of Bio-based Material Science and Technology, Northeast Forestry University, 26 Hexing Road, Harbin 150040, People’s Republic of China 3Mechanical Wood Technology, University Hamburg, Leuschnerstr. 91, D-21031 Hamburg, Germany *E-mail: [email protected].

The low-molecular-weight N-methylol compounds, dimethyloldihydroxyethyleneurea (DMDHEU) and its derivatives, have been successfully used to modify wood. The N-methylol compounds can penetrate and react in wood cell walls. The reaction modes may be crosslinking of cell wall polymers by DMDHEU and/or self-condensation of DMDHEU within the cell wall. As a result, the modified wood exhibits a permanent cell wall bulking; the swelling and shrinkage is reduced, depending on the modification levels. This causes an improved anti-swelling efficiency of up to 70%. Modification does not substantially influence the equilibrium moisture content of wood but improves the durability against white, brown, and soft rot fungi. The treatments also enhance the wood’s surface hardness and compression strength, but do not change its flexural properties. The adhesion of coatings on the modified wood is greater than on the untreated wood and the weathering properties of both uncoated and coated wood are improved. The simple processing, enhanced material properties, and acceptable production cost make this modification technique applicable to industry.

© 2014 American Chemical Society In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Introduction Chemical modification can improve the properties of wood and impart a protection efficacy comparable to that given by preservatives (1). The chemicals used do not contain heavy metal elements and are able to react with wood cell wall polymers or condense in wood micro-structures. As a result, there is little risk of chemical leaching and, therefore, protection to wood can last for a long-term service period. Consequently, chemical modification has been recognized as an important alternative to the use of tropical hardwood species or preservatives in the wood protection industry. Chemical modification can generally be classified as either cell wall modification, or filling of large cell cavities, or a combination of both (2). Cell wall modification refers to the process whereby wood cell wall constituents are altered through reactions with reactive low molecular weight monomers or oligomers, or by heating under high temperature conditions. Filling of cell cavities is the process by which chemicals are deposited in the large cell cavities such as lumens to block the physical passages thereby reducing water/moisture access to wood cell walls. The protection mechanisms of wood modification are mainly proposed effected through bulking of cell walls, reduction of moisture content, and/or changes of molecular structure of the cell wall polymers (3). Various chemical modification techniques have been investigated for many years, particularly acetylation (4) and treatments with melamine (5). Among the most promising chemicals used for wood modification are N-methylol compounds, which are widely used in the textile industry to improve cotton or other cellulose-based fabrics. They enhance wash- and wear-properties and help fix color or other agents to fibers (6). Dimethyloldihydroxyethyleneurea (DMDHEU) was the most widely used N-methylol compound in the textile industry, but due to formaldehyde emissions in the process and from the textiles, low-formaldehyde containing agents were developed (7). Modification of wood with DMDHEU or its derivatives could be applicable to both solid lumber and wood based composites (8, 9). The mode of action is based on DMDHEU cross-linking with wood compounds and self poly-condensation within the cell wall. Technically, the modified material is a wood polymer composite with the appearance and texture of solid wood (10).

Chemical Agents and Reactions Chemical Agents Various N-methylol compounds have been developed by the textile industry over the past 40 years (11), but only DMDHEU and its derivatives were widely accepted. The reactive functional groups in the molecule are the two N-methylol groups (Figure 1). The molecule is also partially methylolated to mDMDHEU to reduce the formaldehyde emissions from DMDHEU, but this reduces the reactivity. 288 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 1. The chemical structure of dimethyloldihydroxy-ethyleneurea (DMDHEU as R=H) or its methylolated derivative (mDMDHEU as R=CH3). DMDHEU can be modified by an Ν,Ο-acetalization with an alcoholic compound to prevent hydrolytic release of formaldehyde (12). Formaldehyde emissions can be further reduced by adding formaldehyde scavengers, such as citric acid, chitosan or glyoxal (13). Formaldehyde free finishing agents, such as dihydroxydimethylimidazolidinone (DHDMI), are also used for finishing of textiles. However, this compound has a low reactivity and is thus not suitable for wood modification (10). Various catalysts are used to enhance the reactivity of cross-linking agents (10, 14). One of the best catalysts is magnesium chloride MgCl2 which is used in the reported results below. Mechanism of Reaction and Treatment of Wood The chemical reaction mechanism has been extensively investigated by textile researchers (6). The N-methylol group reacts with hydroxyl groups to form acetal bonds. The following reactions can occur: • • • •

Cross-linking with hydroxyl groups of wood Hydrolysis of N-methylol groups to formaldehyde and NH-groups Condensation with NH groups to form methylene bonds Condensation with hydroxyl groups of alcohols to form ether bonds

Reactions of N-alkoxymethyl compounds are subjected to a general acid catalysis (6). The main goal in modifying wood with N-methylol compounds is to achieve both a high extent of cross-linking with wood components coupled with self-condensation in the wood cell wall. The treatment procedures for textiles and solid wood are different. Wood tends to form cracks after treatment due to drying stresses. Also, since wood will undergo structural changes at temperatures above 130°C, relatively mild reaction processes are necessary. The typical treatment consists of following steps: • • • •

Impregnation of wood with an aqueous solution containing agent and catalyst Drying the wood to below fiber saturation point (optional) Curing at temperatures above 90°C and below 130°C Conditioning the modified wood to a final equilibrium moisture content (optional) 289 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Dry wood is normally impregnated. During impregnation, the agent is incorporated into the wood cell wall. The curing at high temperatures leads to the formation of cross-links between wood hydroxyl groups and N-methylol groups, and to poly-condensation between N-methylol groups (Figure 2).

Figure 2. Schematic reaction of DMDHEU with hydroxyl groups of wood cell walls and condensation of DMDHEU. Uniform distribution of DMDHEU within the wood is required when treating large size wood since uneven distribution will lead to heavy cracking of the treated wood when it is dried after treatment. Therefore, a novel curing process which employs superheated steam was developed (15). Wood with large pores and low extractive content are suitable for treatment with this modification technique.

Properties of Treated Wood Moisture Content and Dimensional Stability Incorporation of N-methylol compounds in the cell walls influences the moisture sorption behavior of wood. At 20 °C and 65% relative humidity, beech wood (Fagus sylvatica) treated with 22.5% DMDHEU and 1.5% MgCl2*6H2 O, had an equilibrium moisture content (EMC) of 9.3%, lower than the EMC of untreated wood (13%), as calculated based on the dry mass of modified wood. The reduced EMC can be attributed to incorporation of resin molecules into the wood cell walls (bulking). Increasing amounts of N-methylol resin reduced the pore size and numbers in the cell walls of beechwood (16), which caused a reduced free space for water accessibility. In addition, the reactive N-methylol groups of DMDHEU may also react with the OH groups of wood cell walls, thereby blocking the water absorption sites of cell walls (17). The calculation of EMC for modified wood has been the subject of some debate. Some recommend that EMC is calculated from the mass of wood before treatment to exclude the effect of chemicals deposited in the wood (18, 19). DMDHEU-treated wood exhibits comparable, sometimes even higher equilibrium moisture contents than the untreated wood using this calculation method (20). This is mainly because each incorporated DMDHEU molecule also contains two non-reactive OH groups that are accessible to water (Figure 1). The trace, but highly hygroscopic magnesium chloride (as catalyst) in wood can also adsorb water, thereby offseting the reduction in the moisture sorption of cell wall substances (10). The moisture sorption behavior of DMDHEU-modified wood is also supported by the production of more energy during the sorption process compared to the untreated controls, as determined by isosteric method and solution calorimetry (20). 290 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Swelling behavior of treated wood differs considerably from untreated wood compared to the minor effect on the EMC. Monomeric N-methylol compounds are able to penetrate the cell wall and bulk the wood cell wall in a permanently swollen state. This bulking effect can increase the volume of treated beech wood up to 10% compared to the volume of untreated wood. Consequently, the swelling and shrinking of wood is considerably reduced. The complementary effects of bulking and cross-linking result in an anti-swelling efficiency (ASE) of up to 70%. The correlation between EMC and the swelling/shrinking of treated beech wood is not linear, however, unlike untreated wood (21). EMC increases more than swelling with increasing relative humidity (22, 23).

Durability against Biological Decay Protection of wood against biological decay is one of the main objectives of N-methylol modification. While DMDHEU-modified wood has enhanced fungal resistance, the protective effect by DMDHEU is not based on a biocidal effect but on wood modification (24, 25). The mechanism of protection against biological decay is generally assumed to be based on: (1) the EMC in modified wood is below the value required by fungi; (2) deposition of DMDHEU in the cell wall causes a reduction in pore diameter of cell wall smaller than the diameter of decay enzyme, which cannot access to interior of cell wall; and (3) grafting of DMDHEU onto cell wall polymers makes the modified wood non-recognizable by fungal enzymes.

Brown and White Rot Decay Resistance Durability tests against brown and white rot fungi was done according to EN113 using beech and pine sapwood impregnated with mDMDHEU and diethyleneglycol (DEG) with magnesium chloride as the catalyst. A negative relationship between the chemical loading with DMDHEU/DEG and wood mass loss was observed; weight percent gains of more than 15% to 20% assure complete protection against decay by four fungi species (Figure 3). Consequently, this study confirmed that DMDHEU modification improves wood durability against basidiomycetes.

Soft-Rot Decay Resistance The durability of treated pine sapwood against soft rot decay was investigated in laboratory (ENv807) and field tests (EN252). As expected, the laboratory results showed that the resistance of modified beech wood in soil contact depended on chemical loading (WPG). The difference in decay resistance between wood treated with DMDHEU vs. mDMDHEU was minor. The laboratory tests indicated that beech wood treated with DMDHEU or mDMDHEU was classified as highly durable. 291 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 3. Mass loss of mDMDHEU/DEG-treated wood due to various fungi after 16 weeks incubation in an EN113 decay test.

Figure 4. Rating of treated pine after 3 years of ground contact in field exposure according to EN252. DMD and DMD/DEG content is expressed as WPG. DMD = DMDHEU. Column = mean value, line = maximal and minimal rating. 292 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Reliable conclusions about the durability of modified wood can be only attained after ground-contact outdoor tests. Those tests, however, require considerable exposure time to obtain meaningful results. The results presented below have only been exposed for the relatively short period of three years and, thus, the results are only preliminary (Figure 4). The failure-rate of untreated pine sapwood samples in the test indicated a normal infestation by fungi in the test field. In contrast, only a few of the DMDHEU/DEG treated samples exhibited minor evidence of decay, and DMDHEU-treated samples at a 24% WPG had no decay. Based on these results, modified pine sapwood at WPG above 15% can possibly be classified as highly durable, independent of the specific DMDHEU-agent employed.

Mechanical Properties Modification with DMDHEU increased hardness up to four times at a high-level, compared to untreated wood (Figure 5). The combined treatments of thin veneer strips of Scots pine with DMDHEU and magnesium chloride caused a strength loss of up to 50% in zero-span (mainly measuring the intra-fibre strength) and up to 70% in finite-span test mode (mainly measuring the inter-fibre shear strength). Acid-catalyzed hydrolysis of polysaccharides and crosslinking were the main reasons for the losses (26). DMDHEU-treated beech wood blocks exhibited an increase in compression strength of up to 65% (Figure 6a), but a decrease in tensile strength of up to 40% by increasing the DMDHEU concentration using magnesium chloride as the reaction catalyst (27). Only a slight decrease of MOR in bending was observed. This minor change in MOR can be explained by the increased compression strength on the top layer of wood sample, which compensated the loss of tensile strength on the bottom layer during the static bending test (28). The most adverse effects of DMDHEU treatments may be a reduction in the dynamic mechanical properties. Impact strength decreased with increasing DMHDEU concentration; at the highest concentration of 2.3 mol l-1, treated beech wood showed up to 80% loss in impact strength (Figure 6b). Scanning electron microscope (SEM) observation revealed that some microfibril bundles were pulled out on the fractured surface of the untreated control (Figure 7a), while samples treated to 28% weight percent gain (WPG) exhibited a regular fractural surface with some fragments (Figure 7b). The results show that DMDHEU-treated wood is brittle.

Surface Properties Wood used outdoors can be susceptible to biotic and abiotic damage. One target of chemical modification is to create durable wood. For example, the acetylation of wood enhances the weathering resistance of wood compared to untreated controls (29). Acetylation is compatible with coatings and improves coating properties, such as adhesion or drying rate (30). 293 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 5. Brinell hardness perpendicular to grain of pine sapwood treated to increasing levels of DMDHEU.

Figure 6. Effect of modification of beech wood with DMDHEU/MgCl2.6H2O (5wt% based on DMDHEU) on increase of compression strength (a) and retention of impact strength (b), respectively. 294 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 7. Micrographs of Scots pine latewood after impact fracture. The untreated wood exhibits a tough fracture surface (a) but a brittle fracture surface for the wood treated with DMDHEU to WPG of 20% (b).

Figure 8. Wet adhesion of coatings on wood determined by the pull-off method (PrENV 927-8). WF780, WF 950, and WF380 are coatings containing core-shell type acrylic binders and Novatech is an alkyd-based solvent-borne stain. The wood substrates are untreated controls and treated with 50% DMDHEU, respectively. 295 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

Coating Performance


DMDHEU-treated wood exhibit similar surface wettability with several waterborne acrylic and solvent-borne alkyd coatings to untreated wood (31). Both the drying rates of various coatings on the wood and the blocking effect between coated wood were unaffected by the DMDHEU treatment, but by the types of coatings (32). The wet adhesion was considerably improved due to treatment, which may extend the service life of coatings on the treated wood surface (Figure 8).

Figure 9. The appearance of untreated (a) and treated (b) pine sapwood after 18 months of natural weathering; the appearance of untreated (c) and treated (d) pine sapwood which were coated with water-borne acrylic stain and weathered outdoors for seven years in Goettingen, Germany. The wood treated with DMDHEU had a WPG of 22%.

Weathering Resistance Thin veneers of pine sapwood treated with DMDHEU to 48% weight gain and artificially weathered for 72hs experienced tensile strength losses that were lower than that of untreated veneers, likely due to reduced cellulose degradation (33). FT-IR spectroscopy suggested that DMDHEU slow the lignin photodegradation, but did not inhibit color change. SEM examination also revealed that DMDHEU 296 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


treatment is highly effective at reducing the degradation of wood cell wall during weathering. Specifically, the tracheids in untreated veneers became distorted within 48hs of artificial weathering, whereas tracheids in modified veneers retained their shape even after 144 hours of weathering. The stabilization effect increased with increasing DMDHEU-content within the veneers (33). Flat-sawn panels of pine sapwood modified with DMDHEU to 22% weight gain and naturally weathered for 18 months experienced reduction of discoloration and cracking on the wood surface compared to untreated wood (Figure 9a, b). The surface erosion caused by weathering, especially in the less dense earlywood, was lower in the treated wood. The modified panels also had less colonization by blue stain or molds. This latter observation may be due to a reduced hydroscopicity of modified wood, rather than a biocidal effect. Coatings on modified pine sapwood with waterborne stains or oils experienced significantly less cracking after weathering for 18 months than those on untreated wood (34). The appearance of modified and coated wood did not obviously change even after a seven-year outdoor weathering; however, the surface of untreated and coated wood had a large amount of cracking and staining (Figure 9c, d).

Conclusion and Outlook Treatments of permeable wood with low-molecular-weight N-methylol compounds, such as DMDHEU and its derivatives, can improve wood properties; the degree of improvement highly depends on the modifying levels. The swelling and shrinking of wood can be reduced up to 70% due to treatments. Mean ASE of 50% in an industrial process may be achievable when wood is treated at the proper chemical concentration. The treated wood exhibited resistance to decay fungi and can be graded as naturally durable material. The treatment did not prevent surface growth of molds and stains, but it reduced the growth of non-wood destroying molds to a high extent in exterior exposure. Surface hardness of wood was increased by several fold through treatment, and this characteristic makes it suitable for flooring material. Modification with DMDHEU did not influence the wettability or drying rate of coatings on the wood substrate, but did improve their wet adhesion. The modified wood exhibited less deformation, cracking and erosion on the surface during exposure outdoor. Synergistically protecting wood with surface coating, DMDHEU modification can extend the service life of wood during outdoor use. Modification with N-methylol resin has good commercial potential. The chemicals used are easily available industrial products and their cost is acceptable compared to the added value to wood. The treatment uses aqueous solution, thereby avoiding the pollution problem caused by solvent evaporation. Although there is an issue of formaldehyde release during the curing and use, this problem can be properly controlled by using modified chemicals and adjusting the curing parameters. Modification with N-methylol resins could be used to improve the quality of fast-growing wood species that have been extensively used in the wood industry in the countries such as China. 297 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

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299 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

Wood Protection with Dimethyloldihydroxy ... - ACS Publications

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