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AIR2-CT93-1059
A study of the factors governing the performance of preservatives used for the prevention of sapstain on seasoning wood with regard to the establishment of European Standards |
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Contract No | AIR2-CT93-1059 |
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Summary
The project consisted of four linked core tasks and related sub-tasks. The objective of the first three tasks was to define the limits of any possible standard test designed to assess the preventative action of anti-sapstain treatments in Europe and to establish the scientific basis for preparing such standards. Three core tasks set out to establish the limits of any proposed and to demonstrate the relationships that exist between a laboratory test, field test and actual service. These core tasks were designed to take into account the variables that exist within the community with regard to climate, wood species, saw mill practice and causal organisms. A forth core task was concerned with studying novel non-toxic methods of control. Task 1 consisted of studying the relationship between wood species and fungal groups supplied by the participants from the various regions of Europe. Although some species specificity was noticed, it was possible to evolve a laboratory procedure which utilised a set of moulds and a set of stains selected from those supplied which gave reproducible results when used to test fungicides on the most susceptible timbers, namely Scots, or Corsican pine. The proposed protocol was shown to give results applicable to all wood species and regions of Europe. A rapid screening technique was also proposed. In task 2 a series of field trials carried out on different pine species across Europe allowed the development of a field protocol. Essential elements of the method include the use of an internal "positive control" and specific validation criteria. Field and laboratory results were shown to give acceptable comparisons and did the service trials where chemicals were used in practice in task 3. Additional studies showed that the variations in uptake in the trials did not greatly influence the results, but the staking of treated packs should be limited to a single layer. A parallel study in task 4 on novel treatments indicated that the inhibitory effects of Streptomyces griseoviridis and some lichens merit further study. As a result of this work recommendations were made to CEN TC38 to develop laboratory and field standards based on this project.
Introduction
The major proportion of sawn timber produced in the CEC is subject to sapstain and mould growth. For the production of high quality timber this loss of aesthetic qualities is an obvious problem. However, within Europe the problem of mould and stain on lower priced package and pallet board timber currently presents an even greater problem. In countries such as Sweden producing high quality building and furniture, timber kiln drying has expanded considerably with around 80% of current production being so dried. Approximately 1 million cubic metres per annum of Swedish timber is still air dried and requires chemical treatment for protection during seasoning. Although kiln drying can prevent fungal growth, re-wetting during transport and storage and subsequent fungal development remains a problem. Wrapping of packs to prevent wetting from rain can also cause condensation making chemical protection a necessity. In Finland most timber is kiln dried but chemical treatments are still the norm to prevent stain due to this accidental wetting. In addition to these problems, chemical treatment is also practised in Sweden to prevent the development of thermotolerant and thermophyllic moulds during kilning which presents a major health problem in some mills.
In Southern Europe, air drying is still very common and with much of the production for package and pallet boards no drying is carried out. Portugal produces 350,000 cubic metres of sawn Maritime Pine per annum, the majority of which is treated against sapstain. Effective chemical treatment is essential if, for instance, Portuguese timber is to be able to compete with other materials for pallet production and box timber. Kiln drying would not only prove prohibitively expensive but would lead to rejection of much of the timber in a market where size and distortion tolerances are critical due to automatic production of many of the final products. In South West France the production of Maritime Pine depends on the widespread use of chemical treatment due to climatic conditions. Production in 1990 was 1.66 million cubic metres of sawn timber and 4.3 million cubic metres of logs.
In the recent past the industry worldwide has relied almost exclusively on the use of Sodium Penta chlorophenoxide (NaPCP) for sapstain control. This chemical is highly effective and very forgiving of bad application and poor quality control at the saw mills. Sweden banned the use of Na PCP at sawn-tills for sapstain control in 1978 and since that date an ever increasing number of specifiers are not prepared to accept Na PCP treated timber. The Netherlands banned its use in 1989 but still allows its import on treated wood. In France its use is still permitted but the need for change is considered urgent to allow free circulation of treated wood in Europe. Irrespective of legislation, (eg. 9th Amendment directive 76/769/CEE) customer pressure is such that alternative, more environmentally acceptable treatments, are being demanded throughout Europe. As a result there has been worldwide interest in the development of' Na PCP replacements. Within Europe this has been pioneered in Sweden and Finland, but recently major effort has been concentrated in Portugal where UK Dutch and German interests are demanding pallet and packaging timber free of Na PCP.
The development of safer anti-stain chemicals and their assessment has been frustrated by the lack of established standard methods of test, making it impossible to compare published information. As a result it was necessary to carry out excessive testing in any potential new market. This lack of standard procedures has also lead to new products entering the market on the strength of manufacturers claims, only to give problems in use. National requirements are also being set up to police the chemical treatment carried out on treated timber in countries importing the timber. TC 38 of CEN requested that the subject of sapstain testing be placed on their program. It was therefore very urgent to complete this work so that CEN could approach their work from a sound scientific basis after this prenormative work.
Objectives
The main aims was to define the limits of any possible standard test designed to assess the preventative action of anti-sapstain chemical treatments in Europe and to establish the scientific basis for preparing such standards as requested by CEN TC 38 Resolution 27 Antwerp Oct. 1992. Also to investigate novel non-toxic treatments in a separate related study. This will make it possible to establish laboratory and field test methods relevant to the different parts of the community and the different wood species grown therein. The first part of the project consists of three linked core tasks, essential sub-tasks and associated sub-tasks directly related to the core project but of significant interest in specific regions of the community. These three core tasks set out to establish the limits of any proposed standard test approach, and to demonstrate the relationships that exist between any proposed laboratory assessment, field test and actual practice. The first three core tasks take into account the variables that exist within the community with regard to climatic factors, wood species, saw mill practice and casual organisms.
Task 1 and its associated sub-tasks consist of laboratory studies with materials supplied from the different geographical regions of the community. The work will establish the basis for acceptable and meaningful laboratory procedures on, which a future standard could be based. Task 2 and its associated sub-tasks extend the study to field trials in different parts of the community with different, economically important softwoods. Task 3 put into practice the findings that can be drawn from Tasks 1 and 2. This would essentially validate or not the procedures and establish the limits of any future standards. The ultimate objective of the work is to make recommendations to CEN TC 38 regarding the suitability and nature of European Norms designed to assess the effectiveness of treatments for prevention of sapstain and mould in green, sawn timber.
Activities
Laboratory Tests The principal laboratory tasks were designed to develop a meaningful, reproducible laboratory procedure. The trials were based on established procedures, using miniboards of green test timbers treated with a range of selected, representative chemicals. The boards were treated with mixed spores of a range of test fungi supplied by the various participants. A full combination of variables with a range of fungicides, wood species and fungi from each geographical region of the EU were examined. The object was to establish the relationships between these variables in order to make a universally acceptable recommendation for the laboratory test method. Several other factors of importance arose during the work and were studied in order to make strong recommendations. The main task of developing a laboratory procedure was studied in a series of experiments, loosely described in the main laboratory task as "Wood fungal interactions", leading to defining a specific laboratory protocol.
Trails were conducted to investigate softwood species variation and the response of mould and stain fungi when 4 woods were treated with 3 anti-sapstain formulations at 4 concentrations. The most sensitive wood species were consistently Scots pine with Corsican pine in a few cases. The least sensitive to colonisation was Spruce. Radiata pine was the least susceptible pine species and on occasion showed less colonisation than the Spruce. The full range of comparisons between wood species and source of fungal inoculum was performed in the lab with Copper 8 quinolinolate. From these results it was shown that timber species was the overriding factor in test design as previously shown in the initial comparative trail. The timbers were placed into susceptible and less susceptible groupings, based on the degree of colonisation, at a given concentration, of either the mould or stain inoculum. These groupings were: Group 1 (susceptible): Scots & Corsican pine. Group 2 (less susceptible): Radiata & Maritime pine.
The fungi could also be placed into similar groups (vigorous and less vigorous) based on the timber they were isolated from. The mould (and to a lesser extent stain) groups showed a lack of species specificity. The less vigorous Staining sets showed a degree of specificity to their host timbers (notably Maritime pine) but this observation had little relevance in selecting a staining group for the testing of new formulations.
Euromould and Eurostain Comparative trials Based on the iteration trails, selected mixed fungal inocula were compared to each of the 6 timbers' host fungal sets for each of the timbers, with the aim of establishing selected Euro sets equal to or stronger than the host colonising fungi. The results confirmed that the standard Euro-mould or Euro-stain sets gave results which were equal to or more severe than any other combination, when tested on the most sensitive wood species i.e. Scots or Corsican pine.
Related studies During the work a suitable incubation system was devised which prevented the effects of volatile, which had been shown to adversely effect the results. There were two significant reservations concerning the laboratory test method. It was shown that living wood tended to be more susceptible to stain whereas irradiated timber, used in these tests tended to favour mould growth. This effect was less noticeable in the presence of fungicides and considered less important due to using the two groups of fungi separately in the recommended protocol. In addition mode of failure studies revealed a potential problem in interpreting laboratory results using a standard inoculum source.
Mould isolates from Sinesto B treated boards showed an increase in MIC value when tested in liquid culture against moulds originating from untreated timber. This observation was also recorded in miniboard trails. Mould isolates from Mitrol PQ8 treated boards do not show any increase in MIC value however when tested on miniboards, Penicillium sp. from treated timber did show an increase in MIC. When tested in liquid culture, there was no recordable difference in any of the MIC values of the Ophiostoma piceae isolates from treated timber when compared to the MIC value for the O. piceae isolate from untreated timber. At preservative levels below the MC values for stain fungi, isolates originating from boards treated with Sinesto B or Mitrol PQ 8 appeared to grow more vigorously in liquid culture than the isolate from untreated boards. The results indicated a degree of tolerance in isolates that have been previously exposed to the test preservatives. This observation was especially evident in the case of Trichoderma sp. and Penicillium sp. tested against Sinesto B in liquid culture. Mould fungi appear to be more adaptable to the presence of anti-sapstain preservatives than stain fungi.
However, in view of the fact that laboratory trials would appear to be less severe than field trials it was considered valid to propose a laboratory protocol, accepting that field trails should also be conducted after establishing effectiveness in the lab. The laboratory work made it possible to propose a laboratory protocol which was shown to be comparable to most field trails carried out in Task 2. Almost all laboratory test "fail-safe" in comparison to field trails, making the procedure a very valuable initial test method prior to field trails.
Protocol for laboratory procedure
Timber Freshly sawn, flawless sapwood of susceptible Pine species to be used (either Scots Pine or Corsican Pine). 40 x 40 x 8mm size with open faces being of radial longitudinal section. Prepared wood blocks should be irradiated. Blocks may be cold stored temporarily or refrigerated for up to 1 year.
Fungi Formulations for testing are to be challenged with 2 separate, mixed spore suspensions of selected European mould and sapstaining fungi. These fungi are: Trichoderma sp. isolated from English Corsican Pine, Cladosporium cladosporoides isolated from English Corsican Pine.
Penicillium sp. isolated from French Scots Pine, Aspergillus niger isolated from Swedish Scots Pine. Trichotecium roseum isolated from Spanish Radiata Pine. Ophiostoma piceae isolated from English Corsican Pine. Leptographium procurum isolated from English Corsican Pine. Ceratocystis coerulescens isolated from Swedish Scots Pine, LEC 4 isolated from Portuguese Maritime Pine. 35A isolated from Spanish Radiata Pine. PM1O isolated from French Maritime Pine.
Test Chemicals Internal standard - Copper-8-quinolinolate Concentrations:- 1.5% & 0.75%.
Test chemical:- 4 concentrations, 2x, x, 1/2x & 1/4x. (where x equals the manufacturers suggested effective concentration. A control set of wood blocks dipped in distilled water should be included.
Treatment Blocks should be weighed, treated by dipping in solution for 10 seconds, drained and weighed again. 5 replicate blocks should be allowed per concentration. Treated blocks are then placed on racks. Separate incubation chambers are used for each chemical. Blocks are conditioned for 24 hours after treatment.
Inoculation Blocks are sprayed with a spore suspension using an airbrush. Suspensions are made by scraping the surface of established cultures in 10mls of sterile Aerosol-OT solution. Spore counts should be between 1 x 106 & 1 x 108 per cm3.
Incubation Racks are stored at 100% relative humidity and at 25°C for 2 weeks.
Assessment The extent of fungal colonisation is evaluated according to the following scheme.
Visual assessment grading categories: 0. No growth, board is clean. 1. 0-25%-coverage. Traces of visible fungi apparent. 2. 25-50%-coverage. Fungi established. 3. 50-75%-coverage. Sporulation becoming heavy. 4. 75-100%-coverage. Full growth, underlying wood obscured.
Field trials
The objective of this part of the study was to define the limits of any possible standard field test designed to assess the preventive action of anti-sapstain chemical treatments in Europe. In order to do so several drafts of the test methodology were elaborated which led to a final document and recommendations to CEN TC 38. The existence of several non-standard test methods in the world and particularly in Europe, and the appearance in the last decade of many new anti-sapstain products made it necessary to establish the scientific basis for a standardised field test method.
Any such tests needs to assess the effectiveness of treatments for the common prevention of sapstain and mould in freshly sawn timber. It has to be relevant to the different parts of the Community and the different wood species grown therein. The experimental work consisted of carrying out spring and summer trials on local pine timber in different regions of the EU. After the completion of all the field tests and after a first evaluation of the results some points were clarified and a fifth draft was established. From this final draft, general recommendations regarding the implementation of a future Standard by CEN TC 38 were agreed upon. Agreement was reached concerning methods and materials as well as in the general design of the test procedure. The test set up and evaluation as well as the conditions for acceptance of the test results were initially established in accordance with the experience of the partners and the most common treatment and stacking, procedures followed in the European countries. The development of validation criteria was one of the most important aspects of the work. Validation criteria were adopted for both untreated and the reference preservative. The performance of the reference preservative was included as a positive control to ensure that a high level of risk has been present throughout the test, since the untreated controls will tend to fail even when the conditions are relatively mild and the treated material is not be stressed. At the start of the work any test was considered to be valid when the following conditions were met:
The research results however showed that this procedure could be improved. Firstly it was found that the relative performance of different preservative treatments could vary depending on whether close-stacked or open-stacked material was tested; therefore results from close-staked material could not be extrapolated and both situations should always be tested. Secondly, the use of NaPCP (the original reference preservative) has been restricted by recent legislation on chemical products/environment in a number of countries.
From the other products included in the trial tests, PQ8 was considered the most suitable alternative reference preservative since there is considerable experience with its use in the industry, its formulation is well known and it gives consistent test results. Moreover, in the 1.5% concentration its performance is equivalent to NaPCP at the same concentration, as confirmed by the mean rating of 1 evaluated in the treated boards. A single concentration of 1.5% was considered suitable, but lower and higher concentrations can be included for comparative purposes. Consequently, the initial conditions for validation were revised and a proposal made is that the test is valid if the following conditions are met:
After the first set of field trails were completed, it was observed by participants 1 and 5 working in co-operation, that the height of the stacks have an effect in colonisation, therefore the test protocol was amended to clearly state that the trial, should be set up as a monolayer. From the results obtained in the field trails no relation was found between the average recorded uptake of preservative and the average results obtained. Specific uptake studies with PQ8 on two wood species (Scots and Maritime pine) and with Basiment 545 on Maritime pine showed that for a certain dipping time, there is no relationship between retention and either the wood species or surface roughness of the boards when prepared with two different blades. Furthermore, dipping time from 30 seconds to 3 minutes appears to have no influence on retention.
The results for PQ8 and Maritime pine were further confirmed by work on differential absorption. No significant difference was found in the treatment solutions before and after treatment of several cubic meters of timber. This was shown by chemical analysis and biological testing of samples taken from the normal treatment tank in a sawmill. The field trails conducted allowed for significant evolution of the test method, particularly with regard factors affecting performance and validation criteria. The consortium felt confident in making recommendations for CEN TC 28 for a standard procedure suitable for susceptible pine species.
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