Wood Protection with Dimethyloldihydroxy ... - ACS Publications

Chapter 21

Wood Protection with DimethyloldihydroxyEthyleneurea and Its Derivatives Downloaded by UNIV OF ARIZONA on August 4, 2012 | http://pubs.acs.org Publication Date: April 2, 2008 | doi: 10.1021/bk-2008-0982.ch021

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Andreas Krause , F. Wepner, Y. Xie , and Holger Militz 1

Institute of Wood Biology and Wood Technology, Georg August University, Buesgenweg 4, D-37077 Goettingen, Germany Hamberger Industriewerke GmbH, Postfach 10 03 53, D-83003 Rosenheim, Germany

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The cross-linking reagent D M D H E U (dimethyloldihydroxyethyleneurea) and its derivatives were used to chemically modify wood. The mode of action is based on D M D H E U cross-linking with the wood compounds and selfpolymerization within the cell wall. The modified material is a polymer composite with the appearance and texture of solid wood. The impregnation causes a permanent bulking of the cell wall and reduces the swelling and shrinkage properties, with the dimensional stability thus considerably increased. This is reflected by an improved anti shrink efficiency (ASE) of up to 70 %. In addition, high durability against white, brown and soft rot fungi are obtained. The treatments also enhance the wood's surface hardness and weathering properties.

© 2008 American Chemical Society In Development of Commercial Wood Preservatives; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Downloaded by UNIV OF ARIZONA on August 4, 2012 | http://pubs.acs.org Publication Date: April 2, 2008 | doi: 10.1021/bk-2008-0982.ch021

Introduction The objective of wood modification is to improve some of the properties of wood, such as low dimensional stability, UV/sunlight degradation and biological durability. (7). The two basic methods to modify wood are thermal or chemical processes. Various chemical modification techniques have been investigated for many years, particularly acetylation (2) and treatments with melamine (3). One of the most promising chemicals was N-methylol based agents. N-methylol compounds 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 (4). D M D H E U 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 (5). Chemical wood modification with D M D H E U or its derivatives are applicable to both solid lumber and wood based composites. The mode of action is based on D M D H E U cross-linking with wood compounds and its self polycondensation within the cell wall. Technically, the modified material is a wood polymer composite with the appearance and texture of solid wood (6). One of the main advantages of DMDHEU-based modification is an increase of dimensional stability. Impregnation causes a permanent bulking of the cell wall and, thus, reduces subsequent dimensional changes of the wood. Tests with D M E U (dimethylolethyleneurea) and D M D H E U resulted in anti-shrink efficiencies (ASE) of up to 70% (6-13). In addition, high durability against white, brown and soft rot fungi are obtained (9,14-17), but the mode of action against basidiomycetes is not fully understood (18). Also, only a slight increase in termite resistance was demonstrated (19). A final advantage is that D M D H E U is a non-toxic and environmental benign chemical (20). However, despite numerous advantages, the treatment of wood with D M D H E U poses some problems. The basic difficulty is the tendency for larger size wood to crack after treatment (5,75). Furthermore, some mechanical properties, such as bending strength or impact strength, are reduced (27).

Chemical Agents and Reactions Chemical Agents Various N-methylol compounds have been developed by the textile industry in the past 40 years (22), but only D M D H E U and it's derivatives were

In Development of Commercial Wood Preservatives; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


358 widely accepted. The reactive functional groups in the molecule are the two N methylol groups (Figure 1 A). To reduce the formaldehyde emissions from D M D H E U , the molecule is also partially methylolated to m D M D H E U (Figure IB). The reactivity of methylolated D M D H E U is lower than that of D M D H E U , however. By an Ν,Ο-acetalization with an alcoholic compound, D M D H E U can be modified. This reaction prevents the hydrolytic release of formaldehyde (23). Formaldehyde emissions can be further reduced by adding formaldehyde scavengers, such as citric acid, chitosan or glyoxal (24). Formaldehyde free finishing agents, such as dihydroxydimethylimidazolidinone (DHDMI), are also used for finishing of textiles. However, it has a low reactivity and is thus not suitable for wood modification (6). Various catalysts are used to enhance the reactivity of cross-linking agents (6,13). One of the best catalysts was magnesium chloride (MgCl ) which is used in the reported results below. 2

5

Mechanism of Reaction and Treatment of Wood The chemical reaction mechanism has been extensively investigated by textile researchers (4). 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 N H groups to form methylene bonds Condensation with hydroxyl groups of alcohols to form ether bonds

These reactions of N-alkoxymethyl compounds are subjected to a general acid catalysis (4). The main goal in modifying wood with N-methylol compounds is to achieve both a high extent of cross-linking with wood components and for it to selfpolymerize in the wood cell wall. The treatment procedures of textiles and of 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, relative mild reaction processes are necessary. The typical treatment consist 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


359

CH OR 2

HO

A

Figure

OH 1. A = DMDHEU

B

RO


R = H or CH

3

(dimethyloldihydroxy-ethyleneurea).

Β = modified (methylolated)

•

OR

DMDHEU

Conditioning the modified wood to a final equilibrium moisture content (optional)

Dry wood (Figure 2A) is normally impregnated. During impregnation, the agent is incorporated into the wood cell wall (Figure 2B). The curing at high temperatures leads to the formation of cross-links (cl) between wood hydroxyl groups and N-methylol groups, and to polycondensation (pc) between N methylol groups and N H groups (Figure 2C). When treating large size wood, it is necessary to obtain an uniform distribution of D M D H E U within the wood. A n uneven distribution will lead to heavy cracking of the treated wood when it is dried after treatment. Therefore, a novel curing process, which uses superheated steam, was developed (25). Wood in any dimensions can be treated with this modification.

Properties of Treated Wood Moisture Content and Dimensional Stability The sorption behavior of wood treated with D M D H E U or m D M D H E U is significantly changed compared to untreated wood. The equilibrium moisture content (EMC) at a specific relative humidity is influenced by the amount of the N-methylol compound in the wood as well as the type and amount of catalyst employed. The hygroscopic nature of wood is mainly due to hydroxyl groups (26) of the cellulose and hemicelluloses. The N-methylol compounds further contain two to four hydroxyls and, consequently, are also hydroscopic as monomers (Figure 1). Consequently, investigations on Scots pine sapwood (Pinus sylvestris) and European beech (Fagus sylvatica) impregnated with D M D H E U but without curing showed an increased E M C compared to untreated wood at the same relative humidity. However, upon further reaction of the N methylol compounds so that polymerization and cross-linking has occurred, the



C

Figure 2. Scheme of modification of wood with DMDHEU. A = dry wood before treatment; B= impregnated wood; C = wood after reaction; cl = cross-linking; pc = polycondensation.

B


ON

361 number of free hydroxyl groups was reduced with a concurrent decrease in hygroscopicity. Beech wood was impregnated with an aqueous solution containing 22.5% D M D H E U and 1.5% M g C l * 6 H 0 and cured at 120°C for 24h. At 20°C and 65% relative humidity (rh), the treated beech wood showed an E M C of 9.3%, significantly lower than the E M C of untreated wood (13%). In addition to the effect of hydroxyl groups of wood, the pore structure in wood affects the hygroscopic behavior of wood. At humid climate conditions of 80% to 100% rh, water vapor condenses in the capillaries of the cell wall and is partly responsible for the E M C (27). The reduced water uptake for the modified wood is partially explained by reduced capillary condensation, which is induced by the D M D H E U deposited within the cell wall and the cross-linking effect (6). The sorption behavior of treated wood is also dependent on the type and the amount of catalyst employed. Hygroscopic substances such as M g C l lead to an increased moisture content compared to untreated wood (6,28). Along with the changed E M C , the swelling behavior of treated wood differs from untreated wood. Monomeric N-methylol compounds are able to penetrate into the cell wall and fix the wood cell wall in a permanently swollen state. This bulking effect can increase the volume of treated beech up to 10% compared to the volume of untreated wood. Consequently, the swelling and shrinking of wood is significantly reduced. The complementary effects of bulking and cross-linking result in an anti-shrink efficiency (ASE) of up to 70%. The correlation between E M C and the swelling/shrinking of treated beech wood is not linear, however, unlike untreated wood (29). With increasing relative humidity, E M C increases more than swelling (30,31).


2

2

2

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 mechanism of protection against biological decay has not been completely clarified. It is generally assumed that the protective property of D M D H E U is not based on a biocidal effect but on wood modification (18,32).

Brown and White Rot Decay Resistance Durability tests against brown and white rot fungi, done according to EN113, studied treated beech and pine sapwood. The impregnation solution was based on m D M D H E U and diethyleneglycol (DEG) with magnesium chloride as the catalyst. Based on EN84, the wood was leached with water before incubation. Pine sapwood was exposed to brown rot fungi and beech wood to white rot fungi. Figure 3 shows a negative relationship between the chemical loading with D M D H E U / D E G and wood mass loss.



Figure 3. Mass loss of mDMDHEU/DEG-treated wood due to various fungi after 16 weeks incubation time, according to EN113.

WPG

25%

Trametes versicolor

Coniophora puteana

Gloeophyllum trabeum

Poria placenta


363 The results clearly revealed that weight percent gains (WPG) of more than 15% to 20% assure complete protection against decay by the four fimgi species. Consequently, this study confirmed that D M D H E U 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 depends on chemical loading (WPG). The difference in decay resistance between wood treated with D M D H E U vs. m D M D H E U was minor. The laboratory tests indicated that beech wood treated with D M D H E U or m D M D H E U complies with durability class 1-2. 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 fimgi in the test field. In contrast, only few of the D M D H E U / D E G treated samples exhibited minor evidence of decay, and DMDHEU-treated samples at a 24% W P G did not show any indications of decay. Based on these results, modified pine sapwood at WPG's above 15% can possibly be classified at a high durability class, independent of the specific DMDHEU-agent employed.

Mechanical Properties Wood is an excellent material for numerous engineering applications due to its good strength-to-weight properties. However, some wood modification processes, particularly heat treatment, can decrease the mechanical properties to a critical extent (33). Chemical treatment can also decrease the mechanical properties, although the negative impact is less than in the heat treatment process. For example, tensile strength tests (zero span method) with thin strips of DMDHEU-treated Scots pine sapwood yielded losses of approximately 40% (28). The treated wood showed an increased brittleness which is reflected by a reduced impact strength. Fortunately, D M D H E U enhances some of the mechanical properties of wood, such as hardness and modulus of elasticity. The hardness of wood mainly depends on the species and on the density. In some applications, such as parquet flooring, hardness is a decisive property. Use of pine sapwood is limited in flooring applications, as it is naturally a soft In Development of Commercial Wood Preservatives; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


sapwood

heartwood

5% DMDHEU

24% DMDHEU

5% DMD/DEG

18% DMD/DEG

Figure 4. Rating of treated pine after 3 years ground contact in field according to EN252. DMDHEU and DMD/DEG content is expressed as WPG. DMD = DMDHEU. Column = mean value, line = maximal and minimal rating

Ο

1

3

4


365 2

material (approx. 15N/mm Brinell hardness). Modification with D M D H E U showed that the hardness of untreated wood can be increased up to 4 times by a high-level DMDHEU-treatment (Figure 5), but a disadvantage is a loss in flexibility.


Surface Properties One goal of the chemical modification is to stabilize the surface of uncoated and coated wood and thus increase its weathering properties. For example, the acetylation of wood enhances the weathering resistance of wood compared to untreated controls (34). The acetylated wood is compatible with finishes and improves the coating properties, such as adhesion or drying rates of finishes (35). Improvement of weathering resistance or surface behavior of wood treated with D M D H E U has not yet been clearly shown (77).

Weathering Resistance Thin veneers of pine sapwood were treated with D M D H E U to 48% weight gain and were artificially weathered for 72 h. After weathering, the losses in tensile strength of DMDHEU-treated veneers were lower than that of untreated veneers, likely due to reduced cellulose degradation. Scanning electron microscopy (SEM) also revealed that D M D H E U treatment is highly effective at reducing the degradation of wood cell wall during weathering. Specifically, the tracheids in untreated veneers become distorted within 48 hours of artificial weathering, whereas the tracheids of modified veneers retain their shape even after 144 hours of weathering (Figure 6). The stabilization effect increases with increasing the DMDHEU-content within the veneers (28). Flat-sawn panels of pine sapwood were modified with D M D H E U to 22% weight gain and naturally weathered for 18 months. The treated wood was found to have reduced discoloration and cracking on the wood surface compared to untreated wood (Figure 7). The surface erosion caused by weathering, especially in the less dense earlywood, is 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. Investigation of D M D H E U treated wood showed that the wettability of surfaces with several waterborne acrylic and solvent-borne alkyd finishes is similar to untreated wood (36). The drying rates of various finishes were not affected by the treatment, and the wet adhesion was significantly improved. Finally, modified pine sapwood coated with waterborne stains or oils showed



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0%

20%

40%

60%

80%

100%

WPG Figure 5. Brinell

hardness perpendicular

to grain of pine sapwood treated with

DMDHEU.

significantly less cracking of coating and wood after weathering for 2 years than untreated coated wood (57).

Treatment Of Wood Based Panels The treatment of solid wood with aqueous solutions at reaction temperatures of more than 100°C will likely be a complex process on commercial-size lumber, because of internal stresses within the wood. Also, many wood species, such as spruce (Picea abies) or oak (Quercus spp.), are difficult to impregnate. In contrast, small or thin wood parts, such as veneers or chips, can be easily impregnated for most wood species. Alternatively, these species could be modified with N-methylol compounds using veneers or as fibers (38) as the furnish, rather than solid wood. The curing and drying is relatively easy with veneers or fibers. Water evaporation is also very fast with



Figure 6.The cross sections of veneers after artificialy weathering of 144 h UV-light: A, untreated; B, treated with DMDHEU to 48% weight gain.



368

Figure 1. Pine sapwood after 18 month natural weathering. A: untreated; B: treated with DMDHEU (22% WPG). (See page 7 of color inserts.)

small wood pieces, so that the tendency to form cracks is reduced and high temperatures can thus be employed. A number of possible products have been examined. These include parquet flooring, veneers to obtain plywood with enhanced stability and weathering properties (31), and particleboards or fiberboards with enhanced properties (39).

Conclusion And Outlook New processes and chemicals enable wood to be treated with N-methylol compounds such as D M D H E U and its derivatives. Solid wood must be impregnated with aqueous solutions. Wood of small size, such as veneer or flakes, can also be treated.



369 Among the improved properties are dimensional stability, which depends on the concentration of the agent and the specific processing variables. The swelling and shrinking of wood can be reduced up to 70%. Mean A S E of 50% in an industrial process may be achievable. A durability which corresponds to the natural durability class 1 against fungal decay can be achieved. The treatment does not prevent the growth of molds and stains at surface, but it will reduce the growth of non-wood destroying molds to a high extent in exterior exposure because of the changed moisture behavior. The hardness of wood can be increased by several fold through the treatment, and may be useful as flooring material. The weathering behavior of treated wood is also improved. Less cracking and erosion of the wood surface, as well as greater dimensional stability in outdoor exposure, are also observed. The possibility of usage N-methylol compounds for modification of various wood-based products, such as veneers, fiber boards or plywood, has good commercial potential.

References 1. 2.

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