The synthetic adhesives used in manufacturing fibreboard cause both emissions and waste problems. Alternatively, enzyme systems naturally responsible for biosynthesis and coupling of wood cells can be used. A new project describes how these enzymes are used in practice and how the technology can be transferred from the lab to the industry. The technology is the first step towards creating a general method that can reduce the environmental impact of adhesives in fibreboard.
Background and objective
Environment-friendly bonding of wood
Today, fibreboard is a material commonly used to produce both furniture and indoor installations. The board is made by triturating wood into single fibres and then adding a synthetic adhesive and pressing the fibres together at approximately 200 °C. The synthetic adhesives used in this production are typically based on the substances formaldehyde, urea, phenol and melamine. As the use of formaldehyde is particularly problematic, only a very low amount of this substance may be released subsequent to production. The use of synthetic adhesives also means that possible fibreboard waste may not be burnt, but must be disposed of as special waste.
Consequently, new methods are needed for coupling and bonding wood fibres. For a number of years, experiments have been conducted with new adhesives based on waste materials from the paper industry, etc. But developments in biotechnology have given the industry the possibility of using the same enzyme systems as nature uses to help create and couple cells and cell walls in wood (see box 1 on enzymes and wood).
The objective of the project is to exploit and develop the possibilities of using enzymes to couple and bond fibres in fibreboard. Thus, the objective is to create a product that is more environment-friendly than fibreboard made with synthetic adhesives. One advantage of using enzymes rather than synthetic adhesives to bond the boards is that they pose no emission or waste problems, etc.
The study
From lab tests to industrial plants
The study was conducted in cooperation with the Laboratory for Plant Fibres at the Royal Veterinary and Agricultural University, Novo Nordisk A/S and Junckers Industrier A/S in Køge, Denmark.
The starting point of the study was MDF – Medium Density Fibreboard – made from beech wood at Junckers Industrier A/S. On the basis of lab tests, MDF has been produced by using the enzyme laccase to activate the fibres in the beech wood. Against this background, the reaction mechanisms causing the bonding effect, as well as the effect of laccase on wood fibres’ chemical and physical structure, have been studied. As a follow up to the lab work, the process has been up-scaled and tested under conditions corresponding to those at a small industrial plant.
A large part of the study encompasses areas that have yet to be studied scientifically. Consequently, the study not only serves a practical purpose but can also be seen as basic research.
Enzymes and wood
Enzymes are catalytic proteins, meaning they can speed up chemical processes without being used themselves. Therefore, only a very tiny amount of enzyme is needed to catalyse the conversion of large amounts of material. Biologically, enzymes are involved in practically all chemical processes, able to catalyse both the synthesis and degradation of organic materials. In other instances, the advantages of using enzymes are that they can catalyse highly specific reactions and that, because they occur naturally, they entail only very few or no environmental or health risks.
Wood consists of three main components: cellulose, hemicellulose and lignin. In a tree, much of the lignin content is found in the outer layer of the cell walls, where it couples the individual cells and stiffens the cell wall. The aim of this project was to use the enzyme that catalyses the coupling of lignin between cell walls to couple single fibres to a board. This enzyme is called laccase. The enzyme acts to aerate or oxidise lignin. Aerating removes electrons, creating single electrons called radicals. Since electrons preferably occur in pairs, the radicals will try to pair up, thus creating chemical bonds (gluing) between the individual fibres.
Main conclusions
Environment-friendly fibreboards in the works
The project developed a production process for enzyme-treated MDF. However, the production equipment and the moisture properties of MDF require further development before the enzyme-treated boards can be produced industrially in a quality comparable to that of synthetically bonded boards. To this end, a number of tests need to be conducted at the pilot plants, after which the production process has to be implemented at the factories. Consequently, it will probably take some years and a good deal of money before industrial production of the environment-friendly boards can commence. But then, fibreboard, in contrast to existing boards, which are bonded with synthetic adhesive, will be so environment-friendly that it can be burnt immediately. At the same time, the emission of formaldehyde will be minimal both during and after production.
The greatest obstacle to producing enzyme-treated fibreboard is making the enzyme act without water, for enzymes usually require a certain amount of water in order to act. The water affects both the enzyme’s three-dimensional shape and the transport of the enzyme to and from its substrate (lignin). Today, however, only 50 per cent water is used to make fibreboard, all of which is bound in the fibres. Thus, there is no free water. The study introduced a number of minor changes in the existing production process to solve this problem:
/ An extra process step where the enzyme is added in a so-called atmospheric refiner (see figure 1).
/ A 15-30 minute pause where the enzyme can react with the fibres.
/ An increase in the time of pressure of about 20 per cent.
/ An increase in the water content of the fibres before drying, from approx. 40 to 55 per cent. This requires extra energy in the drying process.
With these changes, enzyme-treated fibreboard came close to attaining the same quality as synthetically bonded fibreboard. The quality of fibreboard depends on both its strength and moisture properties, and while the modulus of rupture is practically identical for the two board types, the moisture properties are slightly poorer in enzyme-treated boards, i.e. they take up more water and expand more in connection with water take-up (see table 1).
Project results
Transition to industrial production
The results of the research-oriented part of the project concern both the bonding mechanisms of enzyme-catalysed bonding and the effect of laccase on wood fibres’ chemical and physical structure.
The following mechanisms are assumed to cause the bonding effect: The individual electrons created by the enzyme cross-links the fibre surfaces, which become more water repellent, and the individual fibre surfaces attain a greater mutual coupling ability, i.e. the bond better.
Evidence of the effect of laccase on wood fibres appeared from the following: Stable radicals formed in the cell wall, and low molecular lignin compounds were probably deposited on the fibre surfaces. The effect of laccase mainly depends on these low molecular lignin compounds. Thus, it has been proven that the reaction takes place in an interaction between the enzyme, fibres and low molecular lignin compounds. This observation may prove a key to understanding the biosynthesis and degradation of wood.
Production of fibreboard
When fibreboard is made at lab level, the beech fibres have to be treated with enzymes first. This is done by suspending the beech fibres in water and adding the enzyme. This is left for about an hour. The water/fibre ratio is approx. 20:1. The large amount of free water allows the enzyme to work optimally. 0.2-2 ppm (parts per million) active enzyme is added, depending on the amount of wood fibre. After the enzyme treatment, the fibres need to dry for about 18 hours at 40 °C. The dry fibres are used to form a mat, which will then be pressed to the required thickness at 200 °C for about five minutes.
The lab process differs greatly from industrial production of fibreboard. In industrial production the water/fibre ratio rarely exceeds 1:1, and it only takes about ten minutes from the time the wood is triturated into fibres until the board is completed. In contrast, it takes about 20 hours to make fibreboard at lab level. Therefore, one of the greatest challenges in the project was to get the process to function under industrial conditions. This work was carried out at a pilot plant. Figure 1 shows how the production process can look under industrial conditions.
After the wood has been chipped and triturated into single fibres in the defibrator, the enzyme is added in a so-called atmospheric refiner. This enables enzyme and fibres to mix well, even though only 50 per cent water is present. Next, the fibres are transferred to a rotating cylinder for 15-30 minutes – this is where the actual enzyme process takes place. The cylinder plays the same role as the free water, transporting the enzyme between the substrate molecules. Subsequently, the enzyme-treated fibres are pressed and dried in the same way as in a normal industrial process.
The temperature and the moisture are two factors requiring attention. If the temperature rises well above 70 °C, the enzyme will be ruined. And if the water content falls just a fraction below 50%, then the enzyme cannot act.
Treatment / Density (kg/m3) / MOE (Gia) / MOR (MPa) / IB (MPa) / T.S (%) / W.A (%)
Modulus of elasticity / Modulus of rupture / Internal bond strength / Thickness swell / Water absorption
Untreated / 820 / delaminated / delaminated / 0.33 / 146 / 224
UF-adhesive + wax / 835 / 4.18 / 42.8 / 0.99 / 34 / 52
Enzyme / 868 / 3.95 / 46.0 / 0.93 / 46 / 92
Enzyme + wax / 811 / 3.44 / 32.3 / 0.28 / 53 / 108
Table 1. Strength and moisture property of fibreboard.
The boards are untreated, bonded with conventional adhesive or enzyme-treated. As the untreated boards broke during measuring, all data could not be measured.
Properties of the boards
Subsequent to the production of the boards, their strength properties are tested by means of a mechanical test measuring their strength (aka modulus of rupture – MOR), stiffness (modulus of elasticity – MOE) and internal bond strength (IB). The last property expresses how well the fibres are bonded. The moisture properties of the boards are tested by dipping them into water for more than 24 hors and measuring the water absorption (WA) and thickness swell (TS), i.e. how much they expand in connection with water absorption.
The results of various board types appear from table 1. These clearly shows that enzyme treatment produces a bonding effect comparable to when conventional urea formaldehyde adhesive is used. But it is also evident that the moisture properties of the enzyme-treated boards are unsatisfactory. In part, this is because the wax normally used to improve the moisture properties cannot be united with the enzyme treatment. Consequently, either new wax types or alternative ways of improving the moisture properties need to be developed.
Figure 1: Production of enzyme-treated fibreboard
Wood chips are softened by pre-boiling them for 4 min. at 8 bar. Next, they are triturated into fibres at 8 bar and 170 °C in a defibrator. The enzyme is added in a refiner at atmospheric pressure and about 50 °C. The enzyme-treated fibres are transferred to a rotating cylinder for 15-30 min, after which they are dried. The dried fibres are formed into mats that are pressed into boards.
Bonding mechanisms
The basis of the project has been to study the effect of treating wood fibres with the enzyme laccase. One of the most interesting observations is that using laccase for oxidation causes stable radicals to form in the lignin contained in the fibres’ cell walls. This occurs even though radicals are normally extremely unstable, because radicals are unpaired electrons and electrons usually occurs only in pairs. However, when laccase is used, the lignin apparently locks the radicals in the cell walls, thus stabilising them. When the fibres are exposed to both high temperatures and pressure during pressing, the radicals in the cell walls will most likely form pairs with radicals on other fibres. This produces a chemical bonding between the fibres.
Scientific results
In addition to the results described in this article, the project has also produced some extremely interesting scientific results. The study has shown that electrons are capable of wandering through the cell wall. This means that the enzyme can oxidise, i.e. aerate large parts of the cell walls by simply oxidising the fibre surfaces. The mechanism resembles a pump that removes electrons form the cell wall. It has been, and still is, difficult to explain how a large enzyme that cannot penetrate the cell wall is capable of reacting with the components of the cell wall. The discovery that enzymes are capable of influencing the entire cell wall by simply being active on the fibres’ surface may give us insight into how enzymes work.
Other sources
Felby, C., L.S. Pedersen and B.R. Nielsen (1997): „Enhanced Auto Adhesion of Wood Fibers Using Phenol Oxidases”. Holzforschung, 51: 281-286.
Felby, C., B.R. Nielsen, P.O. Olesen and L.H. Skibsted (1997): „Identification and quantification of radical reaction intermediates by ESR-spectrometry of laccase catalyzed oxidation of wood fibers from beech” (Fagus sylvatica). Applied Microbiology and Biotechnology, 48: 459-464.
Facts:
The Danish EPA News Summary – no. 3 July 1999
Project title:
Bonding of lignocellulosic materials using enzyme-catalysed oxidative coupling
Performing organisation(s):
Novo Nordisk A/S with contributions from the Royal Veterinary and Agricultural University, Institute for Agricultural Science, Laboratory for Plant Fibres.
Author of the article:
Claus Felby, Novo Nordisk A/S
Printed publication:
„Enzymatic Bonding of Lignocellulosic Materials". Environmental project no. 442.
Danish EPA 1999. ISBN: 87-7909-198-9. Price DKK 75
Electronic publication:
No electronic publication available.
Financing:
Council for Recycling and Cleaner Technology, Novo Nordisk A/S, Junckers Industrier A/S
Further information:
Agriculture and Biotechnology Division, National Forest and Nature Agency, Phone: +45 39 47 20 00
The evaluations in this project article are the responsibility of the performing organisation. They do not necessarily reflect the opinion of the Danish EPA.
Printed publications are available from Miljøbutikken, Læderstræde 1-3, DK-1201 Copenhagen K, tel. +45 33 95 40 00, fax +45 33 92 76 90, e-mail
Fakta:
Projekttitel:
Limning af lignocellulosematerialer ved enzymatisk katalyseret
oxidativ kobling
Udarbejdet af:
Novo Nordisk A/S, med bidrag fra Den Kgl. Veterinær- og
Landbohøjskole, Institut for Jordbrugsvidenskab, Laboratoriet
for Plantefibre.
Artiklens forfatter:
Claus Felby, Novo Nordisk A/S
Trykt publikation:
„Enzymatic Bonding of Lignocellulosic Materials”.
Miljøprojekt nr. 442. Miljøstyrelsen 1999. ISBN: 87-7909-198-9.
Pris: 75 kr.
Elektronisk publikation:
Der findes ingen elektronisk publikation.
Finansiering:
Rådet vedr. genanvendelse og mindre forurenende teknologi,
Novo Nordisk A/S, Junckers Industrier A/S.
Yderligere oplysninger:
Landbrugs- og bioteknologikontoret, Skov- og Naturstyrelsen,
tlf. 39 47 20 00
Vurderingerne i projektartiklen står den udførende institution for. De er ikke nødvendigvis identiske med Miljøstyrelsens.
Trykte publikationer kan købes i Miljøbutikken, Læderstræde 1-3, 1201 København K,
tlf. 33 95 40 00, fax 33 92 76 90, e-mail .
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