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FAIR-CT95-0089
Natural Resins as a Potential Wood Processing Agent |
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Contract No: | FAIR-CT95-0089 |
| Date Prepared: | January 1999 | |
| Source: | Final Report Abstract & Executive Summary |
Wood as a renewable material has a high economic importance throughout Europe due to its outstanding characteristics. However wood is hygroscopic and is liable to cracking due to swelling and shrinking. When exposed to the elements, high moisture content in combination with wood destroying organisms causes severe destruction of cell wall carbohydrates in non-durable European wood species. Up to now toxic chemicals, toxic not only to wood destroying organisms but also to human beings, have been used to preserve wood. Problems are created not only due to the environmental impact of chemically treated wood during its service life but also during the disposal of treated timber after its useful life. These problems need to be solved since bans and restrictions for the use of toxic wood preservatives have been announced in many European countries.
Over the last few years attempts have been made to improve the durability and dimensional stability of wood by chemical modification and impregnation with resin monomers. This project made an effort to introduce a range of environmentally safe resins, which can protect wood, are safe to handle and which are easily disposable. The overall aim of the project was to find potential natural resins and to develop integrated resin systems to protect wood and to act as alternatives to conventional toxic biocides.
Objectives
The specific objectives were:
Activities
The project covered the production chain from pre-selection of resin systems to thoroughly tested end-use model products, bringing together producers of resins and wood scientists. The work was divided into several research tasks with expected outcomes. In the final report results and the major findings have been highlighted in eight Chapters. Chapter I describes, how potential resins for further investigations were formulated after substantial screening for their efficacy against wood destroying fungi and for their treatability. Several tall oil compounds, melamine resin, DMDHEU (dimethyloldihydroxyethyleurea) and the mixture of linseed oil/ tall oil/maleic anhydride passed this selection phase. Acceptable application methods are essential for any of the wood preservatives. Deep penetration, even distribution through wood micro structure, penetration into cell wall and resistance against leaching and evaporation are desired properties for wood preservatives.
Basic methods were developed (Chapter 2) and then scaled up to fit in industrial processes (Chapter5). Material properties of treated timber are very important and may reflect to the usability of developed resinated wood. Properties, such as dimensional stability, paintability and glueability, fate of resins during service and mechanical properties were studied and are reported in Chapter 3. Chapter 4 deals with resistance of resin systems against wood destroying organisms and wood boring insects. Standard tests and modified methods together with enzymatic research were used to assess the performance of resinated wood and to investigate fundamental aspects. These basic studies need to be verified in service with large dimensional wood. Chapter 6 deals with these aspects. However, due to the very short duration of the current project, only preliminary results could be produced. Chapter 7 deals with waste management of resin treated wood.
Three potential methods of possible disposal of treated wood waste were studied: Combustion experiments concerned topics related to optimum burning parameters in order to minimise harmful emissions to air. The disposability of resin treated wood waste was also evaluated in a composting study. Experiments were also carried out to examine the usability of resin treated chips as raw-material for the particle board industry.
The increasing concern about the use of nonrenewable resources and environmental pollution means that in future decisions environmental aspects will become a regular part of choosing construction materials. Life-cycle analysis (LCA), is a potentially powerful method of quantifying environmental impacts of materials. Some model products treated with resins were evaluated in terms of their environmental effects from "gradle to grave" using LCA method, as reported in Chapter 8.
Results
The project studied the whole range of natural based resins and their use as wood protecting agents, with the aim to find suitable resins and to develop integrated resin systems to use for wood protection as alternatives to conventional biocides. Information was produced on:
From tall oil compounds many of the studied resins showed potential for wood protection. Due to the limited duration of the project and huge choice of different potential resin combinations the selection was made taking account the environmental profile of resin candidate, not only their maximised biological performance. Only four different systems were examined for their properties and only one was scaled up. Melamine together with two drying oils were removed from further assessments.
Basic laboratory treatment parameters were developed for two water based resin systems, melamine resin and DMDHEU. Tall oil resins were studied when dissolved in pinene turpentine or in "white spirit" and drying oils without solvent. When improving the fixation of drying oils a new resin combination was discovered. The mixture of drying oil together with tall oil resin and with small portion of maleic anhydride (TOMLO) greatly decreased exudation of oil when heated. This new innovation was also selected for further assessments.
Resin treatments did not considerably change strength properties of wood. Only DMDHEU showed lower MOR and melamine somewhat higher values. All the examined resin treatments decreased water uptake significantly, but only DMDHEU showed superior ASE values. Some what diverse results were obtained when assessing glueability and paintability. However, working systems could be found for every resins. Various amounts of resins leaches out in water, but only linseed oil to a greater extent. DMDHEU and melamine were found to emit formaldehyde exceeding the E I limit value of 0. 125 Mg per cubic metre.
Bio-assays showed that the systems had a positive effect against fungi. However, none of the resins were suitable for use in European Hazard Class 4 for timber buried below ground. DMDHEU and one of tall oil compounds seemed to be efficient against wood boring insects. In addition TOMLO together with DMDHEU and two tall oil resins showed good performance against termites.
Scaling up the processes is a critical phase for industrial adoption. Work includes results for DMDHEU, melamine, one tall oil resin and TOMLO. Processes were optimised successfully for all the mentioned resins. Concerning work with tall oil resin hot oil treatment was "re-discovered".
Service testing with large dimensional wood samples is continuing. Preliminary results indicate that drying oils including tall oil resins greatly decrease water uptake. However, more time is required in order to produce the final results.
Combustion results indicate that resin treated wood with melamine, DMDHEU and tall oil resin can be bumt. Compared to untreated wood some extra care is needed in adjusting furnace parameters. Chips from resin treated wood can be easily used at up to 10% for UF or PMDI bonded particle boards. Melamine resin treated wood could be composted as well as untreated wood. The use of DMDHEU plus a tall oil resin appeared sufficient for many for common uses of wood. However, in case of this specific tall oil resin phytotoxic problems may occur.
The life cycle assessment study showed that although no comprehensive winner has emerged, it was clear that the final tall oil resin made a particularly strong case overall. It achieved the lowest impact scores in five of the eight categories in the Hazard Class 2 application and six in Hazard Class 3 and 4. It was the only system to clearly score better than CCA in a wide range of impact categories.
The project evaluated the use of resins derived from natural raw materials as wood protecting agents. Characters, such as treatability, material properties, biological and weather resistance, waste management and finally LCA from cradle to grave of final resins were studied in details.
Resin selection
From a huge number of potential candidate resins four different resins were chosen:. These were were DMDHEU (dimethyloldihydroxyeythyleneurea), etherified melamine urea, decarboxylated tall oil rosin (PS o46) and TOMLO (mixture of linseed oil, TOR ester and maleic anhydride). The latter being developed during the project. The primary selection was based on biological screening experiments and treatability trials. The secondary selection for final LCA was made in the final year of the project after multi-faceted testing.
In case of tall oil based compounds the most bio-resistant metal salts had to be rejected because of their unfavourable environmental characteristics. From several potential candidates the selected material was TOR resin and a non-volatile ester mixture which enabled application by "hot oil" treatments or "Boultonising" methods. This enables drying of green timber under vacuum followed by penetrative vacuum/pressure treatment in a single process. This method seems suitable for drying round timbe, reessulting in a high quality surface appearance.
Chemical analysis
Advance chemical and microscopic technology were used to assess retention, distribution and bonding of resin systems in the wood matrix. Confocal laser fluorescence scanning microscopy (CSLM) was used to detect resins in wood and showed that resins were not only locate in the lumen of cells lumen but also in rays and cell wall. Both FI-IR spectroscopy and SEM-EDX techniques were used to determine resin distribution and retention in wood. DMDHEU showed decreasing concentration from surface to centre, but it did penetrate into the bulk of the wood. It was also noticed that it distributes evenly throughout the cell wall. SEM-EDX analysis gave clear evidence that melamine resin penetrates into the cell wall including the middle-lamella. It was observed that the effect of TOR and maleic anhydride on the curing of linseed oil in the case of small blocks of wood was successful. However, separation of TOMLO components occurred when large wood samples were treated. Maleic anhydride penetrated throughout the wood, but the other components showed less penetration.
Solid state CP-MAS 13C NMR spectra were recorded for different resin systems. From NMR data it is apparent that there is no strong covalent bond between wood and DMDHEU and melamine resin. There is a possibility of the formation of bond of the type Wood-O-DMDHEU. No cell wall penetration nor chemical bonds were found when drying oils, including tall oil resins, were used.
Moisture uptake
Most resin systems decreased liquid water uptake. However, equilibrium moisture contents in constant climate room did not differ from the values for untreated wood samples. Only DMDHEU significantly improved dimensional stability due to its considerable cell-wall bulking. Slight cell-wall bulking was also observed for melamine resin and Na-salt of tall oil rosin (PS 032).
Weather resistance
In order to assess the long term weather resistance of different systems several resins were also tested for out of ground exposure without surface coating using the Lap-joint test (ENV 12037). The results suggested that the main colour changes occur within the first seven months. It is interesting to notice that the assessment of the anti-swell-efficiency (ASE) in the laboratory revealed low dimensional stability of wood treated with oils, whereas in the field oil treated lap-joints changed their dimension only slightly and few cracks have been detected so far. This results could reflect any of a number of reasons:
Other findings suggested that wood that contains a high level of water or organic solvents requires careful drying in order to avoid cracks. The use of solvents should therefore be reduced or avoided. Discoloration was not prevented by any of the resins tested.
Leaching of protection chemicals
The long term performance of the resin systems depends on the leachability of the system. None of the tall oil resins showed superior performance with leachability between 10 and 20 % at the lowest. Part of the leaching figures may be due to loss of remaining organic solvents that pressumanly leach out. The loss of TOMLO was about 10%. but hemp and linseed oil were not very resistant against water leaching. DMDHEU did not leach out after curing. However, curing loss of the chemical is significant. Melamine resin showed leaching loss of 10 kg/m3 of the original retention.
Emissions
In addition to the fate of resins when soaked in water, emissions to air were also measured for one tall oil compound (PS 308), melamine resin and DMDHEU. Considerable amounts of organic components from the solvent were detcted from PS 308. The result was expected, because of the relatively high boiling point of pinene turpentine and mineral oil based solvents. On the contrary only small quantities of VOCs were detected from DMDHEU and melamine resins. However, formaldehyde emissions from DMDHEU and melamine resin samples exceeded the El limit value of 0.125 mg/m3 (expressed as chamber air concentration of formaldehyde with sample loading of 1 m2/m3 and air exchange rate of lh-1). The emissions from melamine were especially high.
Gluing trials
Two types of adhesives were used to assess the glueability of resin treated wood. The adhesives were PVAC (polyvinylacetate), which represents type D3 of glue and PUR (polyurethane), which is a D4-type of glue. For most of the resins PVAC was the most critical. Only DMDHEU showed 'acceptable results with both of the glue types. Removal of the outer zone improved adhesion. Only hemp seed oil, one of the two tested tall oil resins and TOMLO were not at all compatible with PVAC but only hemp seed oil failed when PLJR was used as an adhesives
Weathering trials
Weathering trials of resin treated and painted samples were carried out in laboratory scale and in field using large samples. Resin treated blocks were painted with water- and solvent-born stains and paints. Artificial ageing of resin treated wood is not extremely different from control material however some resin treated wood shows more cracks. The application of standard coatings (white opaque paint and brown translucent stain) was not found to cause problems in combination with resin treated wood. The different coating showed differences between waterbome and solventbome coatings. When using more demanding coatings, clear finishing systems or light coloured coatings, almost all combinations of treatment and coating fail..
Exterior trials with painted and resin treated wood samples are ongoing. Inspection after 9 months exposure was not able to confirm the effectiveness of resin treatments due to the short exposure time. However, there were some indications that cracks in resin treated samples are smaller than in untreated samples. Particularly TOMLO treatment seems to prevent graying.
Mechanical properties
Resin treatments intended to protect wood should not cause changes in mechanical properties.Therefore MOE (modulus of elasticity) and MOR (modulus of rupture) were evaluated on default free and treated test specimens. The main findings can be summarised as follows:
Biological protection
On introducing a new type of wood protection system such as the natural resins, one of the important properties to establish is their biological efficacy. Only when their spectrum of activity against a wide range of biological challenges has been defined can their suitability for particular end uses be recommended through the European Hazard Class System. The approach taken in evaluating the biological activity of the resin systems was to use a combination of conventional European Standard testing (EN) and more investigative assessment techniques. European Standard tests were used to predict performance against, brown rot, white rot, soft rot (soil microfungi) and insects (Hylotrupes bajalus and termites). Other techniques included biochemical assays, non-standard laboratory assessments and non- standard field trials. The biochemical assays included measurement of the effects of resin treated wood on enzyme production in brown rots and the activity of enzyme preparations.
The non standard laboratory tests were assessments of performance against brown rot, white rot, soft rot and unsterile soil using miniaturised samples. The non-standard in-ground and above-ground field trials also involved associated isolation and identification of the colonising micro organisms in order to identify the ability of members of the natural microflora, not included in conventional testing, to develop in resin treated wood.
The combined results of the standard tests for Hylotrupes bajalus indicate that melamine, LBA (linseed based anhydride), DMDHEU, and PS310 showed the best potential as wood protecting systems. The results of the termite standard tests indicate good performance from TOMLO, DMDHEU, PS3 10 and PS308. Taken together the promising formulations were: LBA, TOMLO, DMDHEU and PS3 10.
Combined together the results of the standard tests for brown rot, white rot and soft rot (soil microfungi) suggest that none of the resin systems compared with standard wood preservatives if interpretation of the results is applied in the conventional manner. However, when criteria normally used for interpretation of 'Natural Durability' assessment are employed, several of the formulations (PS308, PS325 and to a lesser degree melamine and DMDHEU) showed potential as wood protecting systems equivalent to 'moderately durable' wood for use in European Hazard Class 3. Even using this method of interpretation none of the resins is suitable for use in European Hazard Class 4. In-ground test results (EN 252) with larger dimensions are ongoing but no reliable results could be obtained after six months exposure.
The results of the investigations into the effects on fungal enzymes suggest that all of the resin systems tested, melamine, PS308, DMDHEU and linseed oil, protected the wood to a degree from degradation through the action of enzymes. The proposed mode of action was that the resin treatments either inhibited hydrolytic enzyme production or modified the wood substrate which inhibited the catalytic action of the enzymes.
The results of the assessments of performance against white rot, brown rot, soft rot and non-sterile soil using miniaturised samples showed a variation in performance of the resins. This varied from that of PS046 which did not protect the wood from attack by any of the fungi to that of the highest concentration of TOMLO which was effective in all tests except beech in non-sterile soil. DMDHEU and melamine were effective in pine but were not so good in beech.
The observations made on the frequency of organisms isolated from the in-ground and above-ground field trials indicated that TOMLO treated pine was significantly less well colonised than the other samples. In addition the TOMLO treated beech was no better than the other samples. These results very firmly support those obtained in the miniaturised laboratory tests. However, TOMLO was not used in the enzymology experiments nor was it used in the Hylotrupes tests. TOMLO treated wood performed well against termites but was not one of the better resin treatments in the standard fungal testing. The explanation for this may be simply the variation in sample sizes in different tests and the penetration problems with TOMLO, which were discovered when scaling up the process.
The observations made in the study of microbial colonisation of resin treated samples in the field suggested that the colonisation was inhibited effectively by the two tall oil resins, PS038 and PS046, and CCA. Further observations would clarify this part of the work on the biological performance of resin treated wood.
The results of this project illustrate the problems of applying current European Standard testing and indeed non-standard methods of assessment to new protection systems. The correct interpretation of test results is vitally important if we are not to lose potential protection systems before they have even been developed. It is also of equal importance not to retain products that will fail in service. The standard test data has to provide an accurate estimate of the expected service life, giving due consideration to the balance between biological efficacy and the benefits in other areas, suggesting that some of these resin systems deserve further investigation.
Disposal
Waste management of melamine resin, DMDHEU and tall oil compound (PS 308; sodium salt of disprop. TOR) was studied in work that included both re-cycling and disposal experiments.
Combustion
Combustion trials were carried out in a pilot furnace with pre-oven, boiler and a fluidised degassing reactor. The maximum capacity of the furnace was 0.035 MW. As the result of the experiment it can be stated that all the resin treatments had a slight negative impact on combustion and the accepted threshold values for emissions cannot always be met. The effect of the resin treatments could be seen as higher emissions of CO, NOx and total carbon. On the other hand, emission of cyanides, PCDD/Fs and PAHs were not a problem.
Composting
The investigation of composting was focused on the question of whether resin treated wood can be degraded in an establish waste management process. Common household wastes from the compost plant "Biitzberg" of the MVA Stapelfeld were used as raw material. The waste was sieved at the compost plant (wire mesh 80 mm) and used on the same day for starting the experiments. About 43-52 kg wastes were mixed with 10% (w/w) untreated, melamine, DMDHEUor tall-oil-treated Pine chips. In general the addition of untreated and of treated wood chips did not negatively influence the composing process and the differences in quality between the combinations tested were low. The addition of melamine DMDHEU treated wood slightly enhanced the fermentation temperature which was judged as a positive effect.
In the case of tall-oil chips after 7 weeks of composting the aqueous extract still showed phytotoxic effects which was judged as a negative. The untreated chips, however, as well as the treated ones were hardly degraded after 7 weeks of composting in common kitchen and garden wastes, which was not expected. Therefore the material has to be ground to fine chips to guarantee a degradation in a common composting cycle. The composting of tall-oil treated wood may cause phytotoxic problems in a common process and this point should be considered in further investigations.
It can be concluded that the characteristics of compost with the addition of a melamine resin is not very different from the characteristics of a compost with addition of untreated wood. That applies to the root growth as well as to the composting process itself. Microscopical examination after a 6-week decay phase revealed differences in the wood chips. Untreated Scots pine was slightly more degraded by soft rot and bacteria than wood treated with DMDHEU, melamine resin or tall-oil.
Panel production
The production of panel products is one of the most promising possibilities for the recycling of resin treated wood. Hence, it was thought to be of prime importance to investigate the influence of resin treated chips on the quality of chipboards. Two general types of single layer panels were prepared: a non-water resistant panel type bonded with an urea formaldehyde binder containing 10% chips of resin treated wood and 90% of untreated chips and a water resistant type bonded with a phenol-formaldehyde binder or an isocyanate binder, exclusively produced with chips of resin treated timber. The first variant is more realistic because enough material might not be available for producing panels from pure treated wood. Investigations aimed to show whether the addition of treated wood might influence the mechanical properties of common LJF bonded particle panels. In the second case the production of water resistant panels free of biocides was examined. It was concluded that resin treated wood can be used for UF and PMDI bonded chipboards up to the level of 10% without any problems. The melamine and tall-oil treated wood can be used without mixing it with untreated wood chips for the production of PF bonded panels. The PF bonded DMDHEU panels showed a reduced but still sufficient internal bond compared to chipboards made from from pure pinewood. For the production of PMDI bonded panels from pure tall-oil treated wood further investigations are necessary as well as for PF bonded panels made from DMDHEU treated wood without addition of untreated chips.
Life cycle assessment
The Life Cycle Assessment (LCA) was carried out for potential resin systems, which passed all stages of development during the project. The resins were DMDHEU, melamine resin, TOMLO (mixture of linseed oil, tall oil resin and maleic anhydride) and tall oil resin (decarboxylated TOR). Alongside these materials, a traditional preservative system; CCA was assessed on an equivalent basis as a reference.
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