AIR1-CT92-0106 Improvement of a Local Preservation Process for Wood Products with Reduced Environmental Impact

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AIR Cluster VIII - Wood Products : Solid Wood Products : Wood (Lignocellulose)

	Proposal No:	AIR1-CT92-0106
	Date Prepared:	September 1999
	Source:	Final summary and synthesis reports December 1995

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Final summary and synthesis reports December 1995

Summary

Introduction This three-year project focused on a wide range of aspects of local preservation systems. The background of this research approach is the situation in which in some countries different kinds of solid preservatives are already used in joinery and glulam industry, often without sufficient scientific knowledge on diffusion capacity, efficacy or environmental impact.

Objectives The main objective was the extension of knowledge on behaviour of active ingrediants in pill or rod treated window frames, glulam and poles. It involved the following activities.

• Development of models for simulation of diffusion, in order to be able to quickly predict the efficacy from pills or rods, instead of execution of long term test methods.
• Assessment of diffusion and efficacy in Scots pine, Norway Spruce, Sitka Spruce and Douglas fir of boron, bifluoride based preservatives and TBTO pills, with tests carried out in various conditions of moisture.
• The improvement of pills/rods performance for window frames, glulams, poles treatment, derived from experiments.
• Assessment of the hazard for leaching of active ingredients of pills or rods from the wood through open joints.
• Assessment of the influence of active ingredients on the adhesion of paints, on a possible etching of glass or destruction of glues or rubbers and on possible corrosion of metal fixings. The methods used in all cases was simulation of "worst case" scenarios.

The optimisation of treatments (preventive and remedial) of commodities as windows, glulams and poles was derived from the following tests:

• assessment of the penetration of active ingredients through different types of glue lines
• determination of sensible areas of different European L-joint window designs, assessment of the efficacy of the treatments by accelerated tests
• determination of the best implantation of rods for remedial treatment of glulams without lost of mechanical properties of the beam
• determination of the best position of pills or rods in secondary treatment of poles in soil

The impact on environment is expected to be low by reducing load in preservative in wood. For this, different tasks were carried out:

• evaluation of toxicity and ecotoxicity data of the different active ingredients with handling and storage advertisements in particular for workers
• as toxicity is reduced when active ingredients stay into wood cells, leaching and emission into air from the different kinds of commodities were studied

Activities

Computer modelling The modelling work was focused on diffusion of boric acid into Scots pine. For the development of the diffusion model three actions were taken:

• review of literature and existing knowledge on quantitative diffusion rates of boric acid
• assessment of the key factors to be included in the model
• experimental runs using the prototype model(s) and assessment of accuracy levels of predicted diffusion areas with existing results

Test runs were performed using the experimental BioComposites model and an adapted DIANA model (finite element analysis) by TNO.

Diffusion and efficacy tests These criteria were assessed with the four preservatives chosen during the first meeting in the three solid wood species cited before. Different methods were used by the partners. Long term exposures (six months and more) to high moisture conditions simulated open joints’ exposure, in order to assess the influence of dynamic moisture conditions and the influence on efficacy and permanence of the active ingredients released from the pill into the wood (unsteady state conditions). In another set of experiments, large parallelepipedic samples were put in wet vermiculite to accelerate the diffusion they simulated - horizontal exposure at steady state flow. Finally, different shaped specimen from large parallelepipedic blocks to L-joint samples were exposed either outside or in various climatic rooms under accelerated weathering conditions.

To assess experimentally the diffusion area, three methods were used with the aim to find correlation between them. A simple staining reaction with a specific colouring reactive was used for the boron, bifluoride and tin preservatives. On the same test specimens it has been possible to carry out efficacy tests by exposure to fungal strain (EN 113, or a screening mycological test) and also a chemical analysis of the compounds in the wood. In all tests carried out, the suitability of the staining reaction was proved with a correspondence to the extent of the active ingredients under its threshold value. However a gradient probably exists at the border zone. As around one hundred samples were used for tests in this study, it was of benefit to use staining test instead of long mycological tests or chemical analysis.

Improvement in performance of pills or rods In order to assess the leaching potential, a worst case leaching experiment was carried out on commercially produced, open L-joints. The pills were inserted at positions that were currently used in the Netherlands. In total, 700 litres of demineralised water was sprayed during ten days. The wood was analysed for residual active ingredients. Evaluation of the influence of the active ingredients on paint performance was executed by cross cut tests (cf ISO 2409) on the test blocks of the long term "open joint" exposure as described before. The effects of boric acid and bifluorides on polysulfite rubbers and butylene rubbers used in double-glassed systems confirmed the literature that indicates that no problem will occur.

For the effect on glass a test was set up in which glass slides were submersed in difluoride solutions of different concentrations. In addition the glass slides were exposed to the vapours of the same solutions. The results show that difluorides in lignids are capable of etching of glass. Only very strong concentrations can cause etching through vapour. In practice normally no problems will occur with preservatives etching glass. Boric acid does not have any negative influence on glass.

For the effect on glue a weathering test with laminated beams was set up. Both the (ASTM) weathering test and an exposition test showed that boric acid and difluorides have no negative effect on PVAc-glues. On the other hand, the action of a boron solution on the kinetic of curing of several glues (resorcine, PUR, vinylic) tested by differential scanning calorimetry showed a small delay, measured by the exothermic temperature, only for high boron concentrations.

The possible impact on glue lines was assessed by shear tests carried out on glulam samples in which diffusion took place with accelerated wetting. Only a few weakening, comparable to those observed after an ageing cycle test.

The extent of corrosion was investigated by contact of different kind of fixings with solutions of preservative implants and in situ in wood treated with these implants.

Optimisation of treatments To appraise if the glue line is a barrier for the active ingredients diffusion, the implant was installed in a piece of glulam in various ways: at a tilt angle of 45°, perpendicularly, in a medium lamella or in two lamellae. Samples were JUSTIFY to diffuse in vermiculite in accelerated moistening and diffusion was appraised with staining reactives at the end of the test. Results showed that some glues permitted passage of boron and bifluorides compounds very easily; resorcine in particular. The diffusion of active ingredients through PVAc - glue layers in glulam was also investigated. From the test results, it is clear that the diffusion of difluoride is not hindered at all by the glue. Boric acid, however, is partly hindered by PVAc glue. This may mean that in laminated beams of larger dimensions, the use of more boric acid pills in different lamellae is necessary.

Ways to optimise the treatments for three typical types of window frames were also investigated. In a L-joint test of typical Dutch design pills were placed vertically in the joints. The joints, painted with acrylic or alkyd paints, were made from spruce or pine. Part of the joints was glued, another part was maintained open. The joints were exposed to the Dutch climate for a three-year period. One of the main results from this test is that the diffusion in the vertical members is very poor. Since we know that in practice wood rot in joinery often starts in the bottom part of the vertical members, this is not acceptable. In the other tests for this subtask different positions of the pills were investigated.

Other tests were set up with L-joints of typical Irish design and with L-joints of typical French design. In both cases a "curative" and a "preventive" pill position was tested. Due to logistical problems in the project these tests started late and no result were available.

Accelerated diffusion tests carried out on window L-joints in Scots pine and Fraké showed that with implantation of one pill or rod in the sill and one in the post, protection of the joint was effective, but not for all preservatives tested. Similar tests were installed in entire buildings.

When used in remedial treatments of laminated beams, the effect of boreholes drilled in the vertical sides of the beam with a density as high as three times that recommended in order to get the lethal amount, was to reduce slightly the stiffness of the beam. This effect can be regarded, in most of the cases, as having a negligible increase in the beam deflexion. The influence of the orientation of the drill-holes as well as that of the drill diameter was also investigated. The presence of drill-holes in the upper and lower lamellae affects the stiffness more, as might be expected since these zones are heavily stressed in bending. The influence of the drill-holes on the strength of the beam was also investigated. It was found that the ultimate bending strength of a lamella can be significantly reduced by the presence of a borehole in the finger-joint, this being more important for narrow lamellae than for larger one.

Concerning poles, Sitka spruce telephone poles (near Dublin) were previously treated with creosote and erected in 1985. After having measured the depth of creosote, the moisture content, three implants were installed per pole at 45° angle relative to the pole placed into the soil. They were examined 6 months later, at the same time the soil around the pole was also analysed.

Impact on environment Data concerning toxicity and ecotoxicity of boron compounds, difluorides and tin compounds were obtained from the literature as were safety precautions for handling and storage. Leaching from various products was assessed. Leaching was investigate in the soil around poles or in vermiculite used in experiments with wood specimens. Leaching was observed from L-joints exposed in laboratory tests and under Dutch weather. No emission into air was observed for boron compounds. Very low emission of bifluorides was observed by at an evaporation rate of 0.5 % of the total pill content after 18 months.

Exploitation The two boron compounds and the bifluoride pills have been considered as diffusable, and efficient under certain conditions. No real impact on materials used in manufacturing and corrosion was observed and no real impact on environment and human health was recognised. Leaching can be avoided with some precautions. Hence, these kind of preservatives can be used locally when a risk is recognised in a given product. As preventive and remedial treatments, they can be used in glulam treatments and the windows industry for which non-durable and unimpregnated wood from species such as Spruce and Douglas fir can be used.

As this type of treatment is largely used at this time (in Europe and in the Netherlands in particular), results obtained in this project will permit an improvement in the use of such treatments and creation of new ones, with the aim of keeping in mind and deriving benefits from the negative points obtained in some results.