Durability of alternatives to CCA-treated wood Results from field tests after 11 years exposure

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1 Proceedings IRG Annual Meeting (ISSN ) 213 The International Research Group on Wood Protection IRG/WP THE INTERNATIONAL RESEARCH GROUP ON WOOD PROTECTION Section 3 Wood Protecting Chemicals Durability of alternatives to CCA-treated wood Results from field tests after 11 years exposure Pia Larsson Brelid 1) and Marie-Louise Edlund 2) 1) SP Technical Research Institute of Sweden Box 569, SE Stockholm, Sweden 2) Ödeen & Co AB Svedalavägen 16 SE Johanneshov, Sweden Paper prepared for the 44 th IRG Annual Meeting Stockholm, Sweden 16-2 June 213 Disclaimer The opinions expressed in this document are those of the author(s) and are not necessarily the opinions or policy of the IRG Organization. IRG SECRETARIAT Box 569 SE Stockholm Sweden

2 Durability of some alternatives to CCA-treated wood Results from field tests after 11 years exposure Pia Larsson Brelid 1) and Marie-Louise Edlund 2) 1) SP Technical Research Institute of Sweden Box 569, SE Stockholm, Sweden 2) Ödeen & Co AB Svedalavägen 16 SE Johanneshov, Sweden ABSTRACT The present study was initiated as a consequence of restrictions against the use of CCA-type wood preservatives in Sweden in the 199s. New copper-based formulations were introduced on the market and to some extent, also alternatives to preservative-treated wood, such as thermally and chemically modified and linseed oil treated wood as well as heartwood of non-tropical naturally durable wood species like oak, pine and larch. For most of the alternatives to CCA, no or very limited documentation on durability properties was available at that point. Field trials in and above ground were therefore started at test sites in Sweden and Hawaii, USA. Results after 11 years testing in Sweden and 9 years in Hawaii are presented, and the main conclusions are: All the natural durable species tested were severely attacked by decay after 11 years exposure in Sweden, both in and above ground, and after 9 years above ground exposure in Hawaii For the alternative treatments acetylation performed best, both in and above ground and is the only treatment, preservative treated wood included, that obtained a durability comparable with CCA-A. Thermally modified wood had initially no visible sign of decay, but lost a good deal of its strength during treatment. After prolonged exposure, however, both in ground and close to ground the fungal degradation increased and after 11 years it is severely attacked. A low level of linseed-oil treatment gave almost no protection. Linseed oil-treated wood with a high retention of linseed oil performed well, but because of the poor appearance the use in practice seems limited. Wood treated to Use Class 3 according to EN is not recommended for use in ground and consequently, most of them performed well above ground, less good in ground but better than the naturally durable wood. Of the chromium and arsenic free preservatives Impralit KDS was the least successful, much likely due to its comparatively lower copper content. The different test methods gave the same order of ranking of the three groups of materials tested, although the rate of degradation differed. Keywords: durability, natural durability, copper-organics, acetylation, thermal treatment, linseed oil, decay, field testing 2

3 1. INTRODUCTION In the 199s, increased concern about the safety and environmental impact of preservatives and preservative treated wood led to restriction of the use of chromated copper arsenate (CCA) treated wood in Sweden. New copper based preservatives emerged on the market and the industry quickly changed from producing CCA treated wood to wood impregnated with the new copper based preservatives. Also, the use of naturally durable wood species and chemically modified wood gained increased interest on the Swedish market. Wood preservatives that are intended for use according to the Nordic wood preservation classification system, see NWPC Document No 1:213, have to be tested in accordance with a set of standards specified in NWPC Document No 2:21. Alternatives to preservative treated wood like wood treated with linseed oil, chemically modified wood and naturally durable wood are marketed as environmentally friendly materials. Convincing documentation of their environmental friendliness as well as performance from durability point of view was very limited when this study was initiated in 21. Very few of the alternatives had been tested according to the same set of standards. Therefore it was not so easy to compare their durability with the durability of preservative treated wood. This study was conducted in order to give some comparative figures of their durability and it included both a laboratory test and in- and above ground field tests. The laboratory test was reported by Edlund in 24 and results from field trials after 5 years exposure have also been presented (Edlund and Jermer 27). The in ground trials are still running as well as the Swedish above ground trial, whereas the above ground trial in Hawaii was terminated after 9 years exposure. 2. MATERIALS The wood material in this study comprises eight different untreated wood species, including the Scots pine (Pinus sylvestris) sapwood reference, with durability classes according to EN 35-2 (CEN 1994), Table 2.1, and four different alternative treatments listed in Table 2.2 and described below. In addition Scots pine sapwood treated with four different chromium and arsenic free preservatives to retentions approved for above-ground use and CCA in two retentions intended for in and above ground use respectively are included. The preservative-treated wood is specified with formulations and retention levels applicable at the time of treatment (Table 2.3). Acetylated wood Through chemical reaction of wood hydroxyl groups with acetic anhydride, covalently bonded acetyl groups are formed on the wood polymers, resulting in increased dimensional stability and fungal resistance. The acetylation was performed in SP s pilot plant reactor and the level of acetylation expressed as wood acetyl content was 2.5 to 21.2 % (Larsson Brelid 1998). Thermally treated wood The treatments were conducted by Stora Enso Timber at their plant in Honkalahti, Finland. A maximum temperature 22 C was held during five hours and the total process time was four days. This level of treatment was classified by the producer as suitable for timber to be used for outdoor exposure above ground, i.e. Use Class 3 according to EN (CEN 1992). It should be noted that compared with the Thermowood process used today where the temperature not exceeds 212 C and the total process time is 36 hours, the comparatively higher temperature and longer process time led to a reduction in bending strength of about 5% (Bengtsson et al 22). 3

4 Table 2.1: Untreated specimens included, and their durability class according to EN 35-2 (CEN 1994) Untreated wood species Scientific name Durability class according to EN 35-2 Scots pine sapwood (Ref.) (Pinus sylvestris) 5 Scots pine heartwood Scots pine heartwood (from Gotland)* (Pinus sylvestris) (Pinus sylvestris) Spruce (Picea abies) 3-4 Siberian larch heartwood Beech Oak heartwood Aspen (Larix sibirica) (Fagus sylvatica) (Quercus Robur) (Populus tremula) *Sweden s biggest island in the Baltic sea Table 2.2: Specimens treated with alternative methods Specimen Scientific name Treatment details Acetylated pine (Pinus sylvestris) Acetyl content: % Thermally modified pine (Pinus sylvestris) 22 C/5hours Thermally modified spruce (Picea abies) 22 C/5hours Linseed oil treated pine (high retention) Linseed oil treated pine (low retention) Wood polymer composite (PowerWood) # Weight Percent Gain (Pinus sylvestris) WPG # : 19-5 (Pinus sylvestris) WPG: (Pinus sylvestris) WPG: 1 Wood polymer composite ( Powerwood ) The treatment was carried by Wood Polymer Technologies ASA (now Kebony ASA) in Norway. The wood is treated with a vinyl polymer that is polymerised in the wood cavities and there is no bonding to the wood. The monomer is hydrophobic and does not swell the wood. The weight gain was about 1 %. Linseed oil treated wood Scots pine sapwood treated with linseed oil by Järlåsa Färgindustrier AB in Sweden according to a patent pending method. Two different levels of retention were achieved. The low retention (linseed oil low) varied between 19 and 5 % weight gain and the high retention (linseed oil high) between 68 and 88 % weight gain. 4

5 Table 2.3: Preservative formulations and retentions. Preservative Chemical formulation % m/m Retention according to NWPC* kg/m 3 Retention according to AWPA** kg/m 3 Stakes Gr prox Stakes Impralit KDS 4 Cu(OH) 2 CuCO. 3 2H 2 O # 1.6 Didecylpolyethoxyammoniumborate (polymeric betain) 5. H 3 BO 3 4. Gr prox Kemwood ACQ 19 Copper tetra-amminedihydrogencarbonate Benzalkoniumchloride (BAC) 4.8 Tanalith E-7 Copper hydroxycarbonate Tebuconazole.24 Propiconazole.26 H 3 BO Wolmanit CX-8 Cu(OH) 2 CuCO Cu(HDO) H 3 BO 3 4. EN Ref. CCA CuSO. 4 5H 2 O AB class retention K 2 Cr 2 O As 2 O 5.2H 2 O 2. EN Ref. CCA Ditto A class retention # * Retentions are based on total formulation of the preservative as specified by the producer ** Retentions are based on AWPA Standard U1-6. Impralit KDS 4 was not approved by AWPA when the test started; the basis for retention is CuO+Polymeric betain+h 3 BO 4 3. TEST METHODS 3.1 General Field testing was carried out according to EN 252 (CEN 1989) and a ground proximity multiple layer method proposed by Bror Häger, the inventor of the K33 CCA formulation (Häger 1979, Nilsson 1993). The test procedures are briefly described below. 3.2 EN 252 stake test Ten stakes (25 x 5 x 5 mm) of each material were installed in two test fields, the Simlångsdalen old test field (in collaboration with the Swedish University of Agricultural Sciences) and SP s test field in Borås. Both test fields are located in South-Western Sweden. Brown rot, white rot and soft rot all occur in the Borås field. The Simlångsdalen old test field has been used since 1943 and was formerly agricultural land. This field is dominated by brown rot, but white rot and soft rot appear as well. 5

6 The stakes were inspected annually and the extent of decay is graded according to the rating in Table 3.1. By adding the rating decay for the stakes of each group and dividing the sum by the number of stakes, the average index of decay for each group of stakes is calculated by multiplying with 25. When all stakes in a group have failed (average index of decay = 1), the average life time is determined. Guidance of the evaluation procedure is found in NWPC Information No 23/9. The test method is intended for wood material to be used in ground contact but is used in this study to accelerate the degradation. Table 3.1: Grading system according to EN252 Definition of Rating Decay index condition Sound - no decay Slight decay 1 25 Moderate decay 2 5 Severe decay 3 75 Very severe decay (stake rejected 4 1 Figure 3.1: Stake test at SP s test field in Borås 3.3 Ground proximity multiple layer field test This method is intended to simulate exposure near and above the ground. Two parallel tests were set up, one at SP s test field in Borås and one at a test site in Hilo, Hawaii. The latter was chosen to get faster information on the performance of the samples tested. Each test unit consists of ten samples, (22 x 95 x 25 mm), that are stacked two by two in five crossed layers, with the bottom layer on the ground, Figure 3.2. To avoid weed growth around the stacks the ground has been covered with a geotextile, permeable for micro-organisms. One stack of each material was exposed at each test site, as demonstrated in Figure 3.2. The evaluation with respect to decay reported here is focussed on the top and bottom layers only. The same grading system as for the EN 252 stakes was used, see Table 3.1. Although the middle layers have also been inspected, reporting has been restricted to the bottom and top layers. It did not make sense to try to get any additional information from the progress of decay of the middle layers as explained in Edlund and Jermer (27). 6

7 a) b) Figure 3.2: Ground proximity multiple layer sample test set up at site in a) Hilo, b) Borås. 4. RESULTS AND DISCUSSION 4.1 EN 252 The results after 6 and 11 years exposure in the Borås test field are presented in Fig Index of Decay (%) years 11 years 2 1 Figure 4.1: Index of Decay for the stakes exposed according to EN 252 in Borås. The result shows that even the most naturally durable species tested had lost more than 6% of their strength already after 6 years and after 11 years in ground contact all the naturally durable species were more or less destroyed. 7

8 Among the alternative treatments, only acetylated wood obtained durability comparable with CCA-A. The stakes with other treatments were extensively degraded after 11 years exposure. Thermally modified stakes were difficult to evaluate according to usual method (picking with a knife) as the thermal treatment itself reduces the strength of the material. Even though the stakes had lost strength during the first years of exposure, no biological attack could be detected. After prolonged exposure, however, both in ground and close to ground the fungal degradation increased and after 11 years it is severely attacked. The biological resistance and strength loss are correlated to treatment temperature. In this study, material treated to a very high temperature was used and thus obtained high strength loss already before the test started. In later industrial production the treatment has been optimized with respect to different end uses. Most preservative treated stakes performed better than the natural durable and most alternative treated stakes. However, all the stakes but the CCA-A were treated to a retention intended for above ground use. Looking at the test results from the stake tests in Borås and Simlångsdalen the correlation is in general very good. The difference in performance between the two test fields is small, as indicated from a comparison of the results after 11 years exposure in Fig These differences can to a great extent be explained by the fact that the fungal flora is different in the two test fields. In order to get relevant and reliable test data from stake tests, more than one test site with different fungal flora should preferably be used. The most remarkable result is that the CCA reference preservative behaves so differently in the two test fields Index of Decay (%) Simlångsdalen Borås 1 Figure 4.2: Index of decay after 11 years exposure of according to EN 252 in Borås and Simlångsdalen. Detailed information of the index of decay over time, for all groups of stakes in both test fields, is presented in the Appendix, Figures A1 to A6. 8

9 4.2 Ground proximity multiple layer field test The results from the ground proximity multiple layer test in Borås are presented in Fig As expected the bottom layers are much more heavily degraded, as compared to the top layers. For the naturally durable woods and most wood treated with alternative methods, the extent of decay in the bottom layer samples are close to the results obtained in the in ground test (EN 252). Linseed oil-treated wood with the high retention of oil performed well both in the stake test and the ground proximity test. However, the high retention of linseed oil has resulted in an unpleasant appearance and sticky surface of the wood making it difficult to use in practice. Surfaces exposed to the weather have been completely blackened. Overlapping surfaces are still sticky owing to bleeding caused by the high retention. The preservative treated wood performed better than in the EN 252. The top layer performed better, and in some cases the treated wood showed no sign of attack at all after 11 years exposure. 4 3,5 3 2,5 Rating 2 1,5 1 Bottom layer Top layer,5 Figure 4.3: Comparison of the extent of decay between the top and bottom layer for samples exposed for 11 years according to Ground proximity multiple layer field test in Borås. The photos in Fig. 4.4 show the appearance of the beech samples and the Scots pine sapwood samples treated with CCA-AB after 11 years in test. It can be seen that beech is heavily degraded while the CCA treated pine performed well, with just a slight attack in the bottom layer. 9

10 Figure 4.4: Ground proximity multiple layer stacks (left), and top and bottom layer (right) for a) beech (Fagus sylvatica) and b) CCA-AB treated Scots pine (Pinus silvestris), after 11 years exposure in Borås. Not surprisingly, the degradation is much more extensive in Hilo test field (Fig. 4.5.) If looking at the bottom layer, all of the treatments were severely decayed after 9 years exposure, except for the preservative treated wood. None of the preservative treated samples (class AB) matched the performance of CCA-A where no decay occurred.. 1

11 4 3,5 3 2,5 Rating 2 1,5 1 Bottom layer Top layer,5 Figure 4.5: Comparison of the extent of decay between the top and bottom layer for samples exposed for 9 years according to Ground proximity multiple layer test in Hilo. A comparison between the two test fields is presented in Fig The figure reveals that the degradation after 5 years in Hilo is more or less comparable with the degradation after 11 years exposure in the test field in Borås. 11

12 4 3,5 3 2,5 Rating 2 1, years Borås 5 years Hilo,5 Figure 4.6: Comparison of the extent of decay between test samples exposed according to Ground proximity multiple layer test between the field in Borås and the field in Hilo. Aspen was not included in the test set up in Hilo. 5. CONCLUSIONS In all tests, the same degradation pattern could be seen and findings from this study can be concluded as follows. The progress of decay is slower above ground compared to close to ground (bottom layer in the above ground test) and in ground. However, the ranking with respect to the performance is still roughly the same. The different test methods gave the same order of ranking of the three groups of materials tested, although the rate of degradation differed. All the naturally durable species tested are severely attacked by decay after 11 years exposure, both in and above ground in Sweden, and after 9 years above ground exposure in Hawaii. For the alternative treatments acetylation performed best, both in and above ground and is the only treatment, preservative treated wood included, that obtained a durability comparable with CCA-A. Thermally modified wood had initially no sign of visible decay, but lost a good deal of its strength during treatment. After prolonged exposure, however, both in ground and close to ground the fungal degradation increased and after 11 years it was severely attacked. 12

13 A low level of linseed-oil treatment gave almost no protection. Linseed oil-treated wood with a high retention of linseed oil performed well, but because of the poor appearance the use in practice seems limited The progress of decay for copper-organics vary but is, with one exception, roughly the same as for CCA-AB (= Use Class 3 according to EN 335-1). The EN 252 test confirmed that class AB-treated wood is not suitable for use in ground contact. Impralit KDS was the least successful, much likely due to its comparatively lower copper content. 6. ACKNOWLEDMENTS The project was originally supported financially by the Swedish Wood Association (Svenskt Trä). The latest inspections of the field trials and compilation of results were part of WoodBuild, a research programme within the Sectoral R&D Programme for the Swedish forestbased industry. This programme was jointly funded by the government, industry and other stakeholders with interests related to the Swedish forest-based industry. The Swedish University of Agricultural Sciences, Dept of Forest Products, is gratefully acknowledged for hosting the field trial in Simlångsdalen and Viance LLC for hosting the ground proximity trial in Hilo. Part of the analyses carried out in 213 were performed within the framework of the European Project PerformWood (Performance standards for wood in construction delivering customer service life needs project number ) which is hereby gratefully acknowledged for the financial support. 6. REFERENCES Bengtsson, C, Jermer, J och Brem, F (22): Bending strength of heat-treated spruce and pine timber. Proceedings IRG Annual Meeting, IRG/WP CEN. (1989): EN 252. Field test method for determining the relative protective effectiveness of a wood preservative in ground contact. European Committee for Standardisation CEN. (1992): EN 335. Hazard classes of wood and wood-based products against biological attack. European Committee for Standardisation CEN (1994): EN 35-2 Durability of wood and wood based products. Natural durability of solid wood. Part 2 Guide to natural durability and treatability of selected wood species of importance in Europe. European Committee for Standardisation. Edlund, M-L, (24): Durability of some alternatives to preservative-treated wood. Proceedings IRG Annual Meeting, IRG/WP , 12pp. 13

14 Edlund, M-L, and Jermer, J, (27): Durability of some alternatives to preservative-treated wood. Proceedings IRG Annual Meeting, IRG/WP pp. Häger, B (1979): New field test for wood preservatives. Personal communication to the Nordic Wood Preservation Council. Larsson Brelid, P (1998): Acetylation of solid wood. PhD-thesis. Chalmers University of Technology. Nilsson, K (1993): Wood Protection Treatments. Comparative tests of a selection of traditional and modern treatments. Swedish Wood Preservation Institute Report No 168. (In Swedish, summary in English). NWPC (21): Conditions for approval of wood preservatives for industrial wood preservation in the Nordic countries. NWPC Document No 2:21. NWPC (212): Nordic Wood Preservation Classes and product requirements for preservativetreated wood. Part 1. Pine and other permeable softwoods. NWPC Document No 1:

15 APPENDIX Indeex of Decay (%) Pine Sapwood Spruce Oak Beech Aspen Pine Heartwood Pine Heartwood, Gotland Larch heartwood, Siberian Years Figure A.1: Index of decay vs. time for untreated wood samples exposed according to EN 252 in Simlångsdalen. Indeex of Decay (%) Pine Sapwood Acetylated Pine Powerwood Linseed oil "low" Linseed oil "high" Thermally Treated Spruce Thermally Treated Pine Years Figure A.2: Index of decay vs. time for modified wood samples exposed according to EN 252 in Simlångsdalen. 15

16 Indeex of Decay (%) Pine Sapwood Kemwood ACQ 19 Tanalith E7 Wolmanit CX-8 Impralit KDS 4 CCA-A CCA-AB Years Figure A.3: Index of decay vs. time for preservative treated wood samples exposed according to EN 252 in Simlångsdalen. Indeex of Decay (%) Pine Sapwood Spruce Oak Beech Aspen Pine Heartwood Pine Heartwood, Gotland Larch heartwood, Siberian Years Figure A.4: Index of decay vs. time for untreated wood samples exposed according to EN 252 in Borås. 16

17 Indeex of Decay (%) Pine Sapwood Acetylated Pine Powerwood Linseed oil "low" Linseed oil "high" Thermally Treated Spruce Thermally Treated Pine Years Figure A.5: Index of decay vs. time for modified wood samples exposed according to EN 252 in Borås. Indeex of Decay (%) Pine Sapwood Kemwood ACQ 19 Tanalith E7 Wolmanit CX-8 Impralit KDS 4 CCA-A CCA-AB Years Figure A.6: Index of decay vs. time for preservative treated wood samples exposed according to EN 252 in Borås. 17