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AIR3-CT94-2000
High Performance Biocomposites with Improved Durability and Dimensional Stability from Non-Toxic Chemically Modified Lignocellulosic Fibres |
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Proposal No: | AIR3-CT94-2000 |
| Date Prepared: | September 1999, April 1998 | |
| Source: | Final technical report 1998 Second Consolidated Report 1996 |
Summary
Lignocellulosic fibres derived from wood and annual plants are known to be strongly-upgraded biologically (resistance to fungi and insects) and technically (dimensional stability and resistance to weathering) when the cell wall components are chemically modified by introduction of bulky, less-hydrophilic groups such as the naturally-occurring acetyl groups. By this upgrading, renewable fibres with different origins are transformed into a high-quality non-toxic fibre resource. The strongly-improved properties gained by chemical modification such as acetylation enable the manufacturing of moulded fibre composites, which can be used in applications where traditional lignocellulosic fibre-based products have shown insufficient technical properties and may have been replaced by plastics.
The general objectives of this project were to produce chemically-modified fibres - preferably achieved by acetylation - and manufacture high-performance, flat and complex-formed, moulded biocomposites from the fibres; and to evaluate the biological and technical properties of the industrially-produced composites as well as their market potential.
Commercial softwood fibre (mainly spruce) and beechwood fibre pulps used for the manufacture of medium-density fibre boards were included in the study, as were pilot plant-manufactured pulps of aspen wood, wheat straw and waste wood from intermediate forest trimming. Recycled paper was also used. Different chemical modifications were applied including acetylation, maleoylation, succinylation, phtaloylation, silylation, acrylation, oxidation with sodium periodate and treatment with an isocyanate (TMI). Heat treatment of wood wool was also carried out. Only acetylation was conducted in a large-scale operation of 100 kg batches: the rest of the treatments were performed to laboratory scale. The treated fibres were used to form flat laboratory boards which were tested according to EN standards.
Taking into account the results obtained from different testings, flat-boards and complex-shaped products produced from acetylated fibre showed the best results for biological and mechanical properties. Acetylation also showed the lowest level of cellulose degradation. Although many of the other modifications are still under development, only one of these treatments, namely modification with TMI, seems to give a level of improvement comparable to that obtained by acetylation. However, the treatment is technically more complicated and the TMI chemical is very expensive.
Acetylated boards showed a thickness swelling of 1.8% (EN 317, 24-hour water immersion) and a remaining thickness swelling after EN 321 of -0.1%, the residual internal bond strength (IBS) corresponding to 80-90% of the dry strength. The decay resistance in laboratory tests was excellent, with no weight losses for the acetylated boards according to prENV 12038. This good resistance to degrading fungi was further confirmed in field tests at seven international sites, provided that the level of acetylation was about 20% calculated as acetyl content.
Acetylated fibres of different origin were used to produce flexible fibre mats also containing thermoplastic fibres (5-8%) and powdered phenol-formaldehyde resin (10%). The fibre mats were used to industrially-manufacture products such as moulded flat-boards for painted door-skins, flat-boards with a decorative layer for flooring, exterior cladding and wet-room panelling. Complex-shaped products were produced in the form of automotive rear shelves, roof tiles and toilet lids. All products showed greatly-improved properties compared with standard products.
Introduction
Lignocellulosic fibres derived from wood and annual plants are known to be strongly-upgraded biologically (resistance to fungi and insects) and technically (dimensional stability and resistance to weathering) when the cell wall components are chemically-modified by introduction of bulky, less-hydrophilic groups such as the naturally-occurring acetyl groups. Chemical modification has been shown to be applicable not only to high-quality wood fibres, but also to fibres from fast-growing hardwood species and annual plants, and to waste-wood fibres and fibres recovered from waste paper products. By this upgrading, renewable fibres with different origins are transformed into a high-quality non-toxic fibre resource. The strongly-improved properties gained by chemical modification such as acetylation enable the manufacturing of moulded fibre composites which can be used in applications where traditional lignocellulosic fibre-based products have shown insufficient technical properties and may have been replaced by plastics.
Objectives
The general objective of this project was to produce chemically-modified fibres - preferably achieved by acetylation - and manufacture high-performance bio-composites, flat as well as complex-shaped, from the fibres; and to evaluate the biological and technical properties of the industrially-produced composites as well as their market potential. A comparison is made with the properties achieved for composites manufactured with present technology using non- modified fibres, thereby visualising the improvements gained by the new techniques in various applications.
Activities
Commercial softwood fibre (mainly spruce) and beechwood fibre pulps used for the manufacture of medium-density fibre boards were included in the study, as were pilot plant-manufactured pulps of aspen wood, wheat straw and waste wood from intermediate forest trimming. Recycled paper was also used. Different chemical modifications were applied, including acetylation, maleoylation, succinylation, phtaloylation, silylation, acrylation, oxidation with sodium periodate and treatment with an isocyanate (TMI). Heat treatment of wood wool was also carried out. Only acetylation was conducted in a large-scale operation of 100 kg batches, the rest of the treatments being performed to laboratory scale. The treated fibres were used to form flat laboratory boards which were tested according to EN standards.
The chemical modification of fibres involves a treatment with reactive chemicals and in some cases in the presence of a co-solvent or swelling agent. In order to achieve a reaction with the wood components - and thereby obtain substitution of hydroxyl groups in the wood with bulky, less hydrophilic groups - the treatment is carried out at elevated temperature. It is then important that the treatment itself does not have too much impact on the cellulose structure of the cell wall which otherwise could result in inferior fibre strength.
Results
Determination of the extent of cellulose degradation caused by acetylation showed a slight drop in the average cellulose chain length as evaluated by viscosity measurements. However, the extent of cellulose hydrolysis is considered small, with no effect on board properties. All fibre samples modified by other methods showed a more substantial drop in viscosity as compared with unmodified samples, but it is difficult to judge at which level the decrease in cellulose chain-length becomes severe with respect to mechanical fibre-handling, board-forming and strength of final composite products.
Modified fibres and powdered phenol-formaldehyde resin were used to dry-form flat laboratory boards tested to different EN standards. Acetylation provided tremendous protection against all types of decay fungi, regardless of the lignocellulosic fibre source. The decay resistance in laboratory tests was excellent, with no weight losses for the acetylated boards according to prENV 12038. This good resistance to degrading fungi was further confirmed in field tests at seven international sites, provided that the level of acetylation was about 20% (calculated as acetyl content).
Treatment with a mono-isocyanate TMI treatment also seems to provide an excellent protection. Phtalyolation and oxidation with sodium periodate provide a good protection, while the other modification treatments seem to vary between moderate and zero in the protection they afford against fungal attack.
The performance of chemically-modified fibre boards was tested according to EN 317 and the first cycle of EN 321. Taking into account the results obtained in different testings, flat-boards produced from acetylated fibre showed the best results with respect to biological and mechanical properties. Although many of the other modifications are still under development, only one of these treatments - namely modification with TMI - seems to give a level of improvement comparable to that obtained by acetylation. However, the treatment is technically more complicated and the TMI treatment is very expensive.
Acetylated fibres of different origin were used to produce flexible fibre mats also containing thermoplastic fibres (5-8%) and powdered phenol-formaldehyde resin (10%). The fibre mats to press laboratory boards and to industrially-manufacture products such as moulded flat-boards for painted door-skins, flat-boards with a decorative layer for flooring, exterior cladding and wet-room panelling. A comparison between the laboratory boards and the industrial boards reveals that the press conditions with respect to temperature, press time and pressure were not ideal for the industrial boards and most likely could be improved.
The remaining swelling after cyclic testing of the two types of boards was determined and the acetylated laboratory boards were shown to have a significantly lower remaining swelling, accompanied by a high residual internal bond strength. These two properties are crucial for boards exposed to exterior or moist conditions. The properties of the boards manufactured on an industrial scale were, however, considered very good by the companies involved. An example of the application of flat industrially-manufactured boards is their use in exterior doors. The product has acetylated fibre-board as door-skin on both sides. It was found that the warping of the door after long-term exposure between indoor and snow-melt climates was almost zero. This performance was better than that obtained for standard doors produced with a paper laminate of high resin content as door-skin and much better than that obtained when using a commercial exterior-grade fibreboard.
Complex-shaped products in the form of automotive rear shelves, roof tiles and toilet lids were also produced by the moulding of flexible fibre mats to good result.
Conclusions
A number of products have been industrially manufactured using multi-layer moulding techniques and acetylated fibre mats. The results obtained upon testing these products and laboratory-produced fibre-boards show that superior products can be produced from fibres chemically-modified by acetylation, and that this manufacturing can be done in a standard production line. High dimensional stability and resistance to biological degradation were characteristic features of the modified products.
Work and achievements
In general the programme outlined in the summary has been followed The work
has included two pilot plant mat-forming trials in order to evaluate different
parameters effecting the properties of the final composite and the intermediate
flexible fibre mat. Full scale production of fibre mats made from various fibre
species has been accomplished as well as final moulding of various products.
This activity is summarised on a task-by-task basis below.
Task 1. Production of moulded, high-performance fibre
composites.
Screening tests on flat boards have been performed and
evaluated. Most of the boards for final testing have been produced and a large
quantity of full sized industrial products have been manufactured from the
mouldable fibre mats produced
Task 2. Chemical modification of fibres from different sources.
Many acetylations have been successfully carried out. Various raw
materials used include waste wood (mixed wood from thinning of forest, wood
working mill residue and wood wool) as well as recycled paper fibre. Most of
the other chemical modifications, other than acetylation, have also been
performed at large laboratory scale.
Task 3. Analyses of modified fibre
Analyses of acetyl
content of acetylated fibres have been carried out. FTIRanalyses of fibre
modified by methods other than acetylation are partly finished. Determination
of the extent of cellulose degradation caused by the acetylation process has
been carried out for all fibre species and for softwood fibre modified by other
methods. Complementary analyses are being performed.
Task 4. Production of flexible, low-density fibre mats by
various techniques
Two pilot plant trials have been performed in order
to investigate important parameters before the full scale trial. The full scale
production of more than one tonne of mouldable mats has also been successfully
carried out and a first comparison of different matforming techniques made.
Task 5. Analyses of market requirements
A preliminary
analysis of market requirements has been made.
Conclusions
Acetylation of the fibre, irrespective of the source of lignocellulosic
material, has a very positive impact on the performance of fibre composite
products. For instance the thickness swelling in water is reduced by
approximately 90%, whereas the mechanical properties are maintained or slightly
improved. Screening results from cyclic climate testing according to EN 321
(three times 72 hours water-soaking to 24h freezing at below minus 18 degrees C
followed by 72h drying at 70 degrees C) show that more than 85 percent of the
transverse tensile strength remains after three full cycles. This value is to be
compared with the corresponding value of 20 to 40 percent obtained for boards
made from unmodified fibres. Succinylation and maleylation of the fibre are
other methods that result in boards with almost as good properties as obtained
for boards made from acetylated fibre. Succinylated fibre interact well with
polypropylene which is an interesting feature.
Determination of the extent of cellulose degradation caused by the acetylation process shows a slight drop in the DP (degree of polymerisation). However, the extent of cellulose hydrolysis is considered to be small and with no effect on board properties. All analysed fibre samples modified by other methods, show a much more substantial drop in viscosity, as compared to unmodified samples, but it is difficult to judge at which level the decrease in DP becomes severe.
Many of the modification methods other than acetylation are still at a stage of applying the method to fibre modification and up-scaling of the process, and it is too early to judge which of these treatments that may give the required level of improvement.
A number of products have been manufactured using multi-layer moulding technique and acetylated fibre mats. Such products are flooring boards, toilet lids, automobile panels, wet room wall panels, exterior doors, balcony fence boards and kitchen drawer fronts. The dimensional stability and mechanical properties of the products already tested were excellent. However, acetylation alone does not prevent water absorption and the compatibility with commercial paints and lacquers has changed. In a new project, paint formulas should therefore be developed in order to better suit the new materials produced.
© Copyright 2006 Policy Statements
Updated
by CPL Press:
03/07/2007
- biomatnet@biomatnet.org
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