Biodeterioration-tests on wood/plastic composites - CHIMICA OGGI ...

Biocides
20
INA STEPHAN
RUDY PLARRE
<strong>Biodeterioration</strong>-<strong>tests</strong> on
wood/plastic composites
ABSTRACT
The purpose of this article is to raise awareness about the
possibilities for biodeterioration of wood/plastic composites
(WPCs). It will introduce the European standards (EN) and
technical specifications (TS) with which the durability of
WPCs against microorganisms and termites can be tested.
INTRODUCTION TO WOOD/PLASTIC
COMPOSITES (WPC)
WPCs are blends of wood particles (or in some cases other
cellulosic particles) into a thermoplastic matrix. Typically 50
to 70 percent wood and 30 to 50 percent polymer are the
ratio limits in which the materials are mixed. They can be
used in outdoor applications (e.g. decking, cladding, window
/ door profiles, fences, playground equipment, footbridges,
highway sound barriers, shutters, roofing shingles) as well
as in indoor applications (e.g. floors, panels, door frames,
furniture, skirting boards, shelving,
storage equipment).
The predictions for the growth of
the market are consistently high.
The actual numbers for selected
countries are quoted in table 1 (1).
Plastic materials used in WPCs
nowadays are mainly polyethylene
(low, medium and high density PE),
polyvinylchloride (PVC) and
polypropylene (PP). In general
plastics used in WPC have to be
suited to being processed at below 200° C in order not to
damage the wood particles in the mixture (1).
The slenderness of the wood particle defines the characteristics
the wood has in a WPC. While wood flour (about 0.3 to
0.4 mm diameter) acts as a filler in the WPC, wood fibres
or shavings can add structural stiffness to the material (1).
Besides wood and plastic, additives such as UV-stabilisers
or absorbers and lubricants can be used as coupling agents.
Furthermore, preservatives, e.g. fungicides, insecticides
and/or algicides can be incorporated depending on the
intended use of the product.
ACCESSIBILITY OF WPC FOR ORGANISMS
Microorganisms
Wood is a natural, renewable resource. Therefore it is
biodegradable by many organisms in nature. As a general
rule it can be said that wood with a high natural durability
is harvested from slow growing tree species, while wood
with low natural durability is harvested from fast growing
tree species. Consequently, wood with a low natural durability
is more accessible on the market and cheaper. To delay the
biodegradation process of this wood by fungi several options
Table 1. Annual use in tonnes of WPC materials in selected
countries.
Ina
Stephan
Rudy
Plarre
are possible: a) use of biocides, b) use of modified wood
or wood with a high natural durability c) keeping the wood
moisture content below 25 percent. In this context, the plastic
component of a WPC has the potential to envelope the dry
wood particles and thus keep the moisture away from it.
Nevertheless, <strong>tests</strong> in the field as well as in the laboratory
demonstrated that while moisture uptake is delayed in WPCs,
it cannot always be prevented (2).
To induce wood decay and mass loss by microorganisms,
the moisture content of the wood fibre is critical. Below a
level of 25 percent decay is usually not possible. Therefore,
several authors have focused on the water uptake of different
WPC materials (3, 4). While the water uptake over the whole
of the product dimension might be below 25 percent it has
to be considered that the water is not homogenously distributed
in the WPC material. It can be assumed that a) the plastic
component does not take up any significant amount of water
and b) that there is a moisture gradient with higher levels
of moisture in the outer layer, therefore allowing and favouring
fungal attack in certain parts of the material. This would
lead to the effect that the outer
appearance and surface feel of a
material can change under moist
conditions whereas the stability of
a product is reduced by fungal
decay at a slower rate, depending
on its overall dimensions (5).
Surface growth
For many customers the surface
appearance is often a reason for
deciding for or against a product.
This is especially true where either large areas of the material
are seen or the product is a conspicuous location, e.g. with
deckings for terraces. Algal growth and moulds living on
debris and moisture collecting on surfaces are a common
cause for complaints. Even though the WPC-material may
not be metabolised by these organisms, they are known to
build biofilms that could damage plastics and additives by
releasing e.g. organic acids. Research on such interactions
with WPCs is still in its infancy. However, it is already evident
that biofilms make surfaces slippery and decelerate drying
of underlying material, thus enhancing fungal decay.
Insect damage
Several insect species can damage wood or wood containing
materials. By far the most important insect group worldwide
causing destruction of wood are termites, especially soil
inhabiting and dry wood termites (6). They are known to
cause damage throughout the tropics, sub-tropics and temperate
regions (7). Water is essential for termite survival, however,
only few termite species demand a minimum moisture content
of the wood they attack, since they either utilise independent
water sources in the soil or physiologically compensate low
moisture contents by metabolic water production.
Termites have a complex social organisation with definite
chimica oggi • Chemistry Today • vol 26 n 3 / May-June 2008

patterns when feeding in groups using all resources to the
maximum (8). Although some preferences for certain wood
species may occur (9-11), all cellulose containing material can
be used as food and energy sources. Termites, therefore, feed
on any wooden material.
As termites, especially subterranean termites, search for
food, they may penetrate and damage many noncellulose
materials as well, including plastics, even if these do not
serve as food source and can not be digested (12). The
sharp mouth parts of worker termites can abrade or break
apart virtually any material and the high
reproductive rate in termites instantly replaces
workers with blunt mandibles.
It is therefore not surprising that most plastics
failed in <strong>tests</strong> for resistance to termites (13-
16). Nevertheless, differences exist between
different plastics and their susceptibility to
termite attack. Condensation-plastics like
polyesters appear more resistant than additionplastics
like polyethylene, polystyrene and
polyurethane (13). However, it is not only the
mechanical hardness of plastics that characterises
durability against termite attack, the surface
structure is just as important. Smooth, even
levelled surfaces, are more likely to prevent
termite attack, but the smallest cracks and
crevices or ruffled surfaces and edges provide
immediate access for termite mandibles. The
same holds true for WPCs. Once the surface
is broken, destruction is only a matter of time
and termite population density, unless the
material is protected by either an effective
termiticide or a repellent that prevents continuous
feeding. Although it was shown that drywood
termites are capable of attacking plastics (13),
it is considered unlikely that this group of
Wood/plastic
composites (WPC) are a
fairly new category of
materials on the market.
The authors point out
biological procedures to
characterise and specify
different WPC-materials.
While moisture is
essential for growths
of algae and fungi,
termites can also attack
dry materials.
Whether biocides
are needed for
protection of a materials
depends
on the environment
it will be exposed to
during its use.
termites would infest plastics to establish colonies as they do
in pure wooden structures. For WPCs this still needs to be
demonstrated.
USE CLASSES FOR WOODEN MATERIALS
Before deciding on measures to protect the WPC it is important
to decide where the material has to perform and to what
environment it will be exposed. In the field of wood protection,
the definition of use classes (former hazard
classes) was found to be useful and could now
also be applied to identify the intended use for
WPC-materials (17, 18). These use classes
cover the following general service situations:
Use class 1 for interior and covered, use class
2 for interior and not covered, use class 3 for
exterior above ground, use class 4 for exterior
in the ground or fresh water and use class 5
for use in salt water. Some minor subdivisions
in the use classes may occur.
TESTING OF WPCS FOR BIOLOGICAL
DURABILITY
In Europe, CEN with its Technical Committee
249, Working Group 13, has been working on
laboratory methods to characterise WPCs since
2003. Chapter 8.6 of the technical specification
TS 15534-1released in 2006, “Resistance against
biological agents” (19), deals with the different
forms of microbiological decay (Annex C: termites,
Annex D: wood destroying basidiomycetes,
Annex E: soft rotting micro-fungi, Annex F:
discolouring micro-fungi and algae).
Biocides

Biocides
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The test methods described in TC 15534-1 are intended to
be used for the characterization and specification of WPCs.
They are derived from existing test methods in the fields of
wood protection, plastics and coatings. The methods presented,
although used for many years for biocide-treated or untreated
material, were not primarily designed for WPCs. In the future
it is likely that the test methods will need to be adapted to
WPC materials and products.
Laboratory <strong>tests</strong>
Accelerated testing in the laboratory can be helpful to rank
resistance against fungal or algal growth of WPC-materials.
TS 15534-1 lists a variety of <strong>tests</strong> (19). Problems associated
with surface moulds on plastics (20) and algae on coatings
(21) are addressed in test standards which employ visual
estimations to rate or rank the results. Both standards are
regarded as being useful to estimate the susceptibility of
WPC surfaces to support surface growth on WPC-materials.
The following three standards and specifications (see below)
were all originally developed for testing durability of wood
and wood-based products. While the termite-standard (22)
requires the visual estimation of results, the two methods for
fungal decay determine the mass loss of wood in percent
based on the original mass of wood in a WPC (23, 24). The
appropriate way of preconditioning (e.g. UV-irradiation,
artificial weathering, leaching) of WPCs before testing is
still a much debated topic. For the simulation of water uptake
in practise (which might take some years in certain environments
before the material can be decayed) several standardised
options are mentioned in TS 15534 (25-27).
Field <strong>tests</strong> and the fungus cellar
No field <strong>tests</strong> are mentioned in TS 15534. Nevertheless, they
should be considered or recommended as a second tier after
laboratory <strong>tests</strong>. This is especially the case for the evaluation
of resistance to termites, since, under field situations, WPCs
may be either neglected as a source of food or attacked only
to a minimum when there is a choice between several food
sources including all sorts of natural, cellulose containing
materials. For testing WPC-materials or products against
microbial decay the test methods for preservative treated
wood can be adopted with success (28). However, since field
<strong>tests</strong> can take several years, methods for accelerated testing
in soil (e.g. fungus cellar) should be considered (29, 30).
ENHANCING THE DURABILITY OF WPCS
AGAINST ORGANISMS
As mentioned above, several steps can be considered to
improve the resistance of WPC to biological attack. These
considerations will certainly also be driven by cost and
intended lifespan. Wood waste from saw mills etc. is a cheap
raw material to substitute plastics with. Wood of fast growing
species is easier to source and therefore cheaper than wood
from slow growing species. Also, wood modification processes
exist (e.g. acetylation) that improve the resistance against
decay compared to non-modified wood (2). However, these
processes have to be performed before the wood comes in
contact with the plastic and therefore causes additional
production costs. Today, zinc-borate is the major biocide
used in WPCs against a wide range of organisms. It exhibits
fungicidal properties against surface moulds (31) and wood
decaying fungi and it is termiticidal. It is also thermally stable
and thus compatible with production process of WPC and
acts a flame retardant for plastics (3). Fungicides acting
against surface moulds only, e.g. Thiabendazole or DCOITbased
formulations, are also employed. Anti-algal properties
on the surface of WPCs can be introduced into a WPC
material e.g. by algicides such as Terbutryne.
CONCLUSION
Depending on the expected life time and type of exposure (see
use classes above), WPCs can become overgrown by and/or
attacked by microorganisms. Furthermore, termites are a hazard
to WPCs in certain regions of the world. Even if termites only
attack WPCs under natural field conditions to a minimum, they
can provide ports of entry for increased moisture and thus
favour microbial decay. Depending on the intended use of a
WPC material, its durability and performance should be tested.
TS 15534 lists some tools in this respect. Field <strong>tests</strong> and fungus
cellar <strong>tests</strong> should be considered as further option to characterise
the long-term stability of a WPC material or products. Further,
tailored test protocols would be advisable to optimise product
developments outside of a given standard.
REFERENCES AND NOTES
1. D. Vogt, M. Karus et al., nova-publications, nova-Institut GmbH (2006).
2. P. Larsson-Brelid, paper prepared for the 37th meeting of Int. Research
Group on Wood Preservation; Tromsoe, Norway, accessible under
www.IRG-wp.org, IRG/WP 06-40338 (2006).
3. W. Wang, J.J. Morrell, Forest Products Journal 54, pp. 209-212 (2004).
4. C.M. Clemons, R.E. Ibach, Forest Products Journal 54, pp. 50-57 (2004).
5. J. Simonsen, C.M. Freitag et al., Holzforschung 54, pp. 2005-2008
(2004).
6. N.E. Hickin, The Insect Factor in Wood Decay, Hutchinson, London (1968).
7. M.J. Pearce., Termites, CAB International (19997).
8. T. Abe, D.E. Bignell et al., Termites: Evolution, Sociality, Symbioses, Ecology,
Kluwer Academic Publishers (2000).
9. S.S. Bampton, D. Butterworth et al., Material und Organismen 1,
pp. 185-199 (1965/66).
10. D. Butterworth, D. Kay et al., Material und Organismen 1, pp. 257-269
(1965/66).
11. R. Mannesmann, Material und Organismus 8, pp. 107-120 (1973).
12. R.H. Beal, J.K. Mauldin et al., United States Department of Agriculture
Home and Garden Bulletin 64 (1986).
13. G. Becker, Materialprüfung 5, pp. 218-232 (1963).
14. D. Butterworth, D. Kay et al., Material und Organismen 1, pp. 241-255
(1965/66).
15. U. Unger, A. Unger, Plaste und Kautschuk 31, pp. 241-247 (1984).
16. W. Unger, Holztechnologie 26, pp. 240-241 (1985).
17. DIN EN 599-1:1997 Durability of wood and wood-based products -
Performance of preventive wood preservatives as determined by biological
<strong>tests</strong> - Part 1: Specification according to hazard class (1997).
18. American Wood-Preservers’ Association Standard, ASTM U1-07 2007 Use
Catergory System: User Specification for treated wood (2007).
19. CEN/TS 15534-1:2006 Wood Plastic Composites (WPC) - Part 1: Test
methods for characterisation of WPC materials and products (2006).
20. ISO/FDIS 16869 “Plastics – Assessment of the effectiveness of fungistatic
compounds in plastic formulations” (2008).
21. EN 15458 “Paints and Varnishes - Laboratory method for testing the
efficacy of film preservatives in a coating against algae” (2007).
22. EN 117: 2005 “Wood preservatives - Determination of toxic values against
European Reticulitermes species - (Laboratory method)“ (2005).
23. ENV 12038:2002 “Durability of wood and wood-based products – wood
based panels - Methods for determining the resistance against wooddestroying
basidiomycetes” is suitable (2002).
24. CEN/TS 15083-2:2005 “Durability of wood and wood-based products -
Determination of the natural durability of solid wood against wooddestroying
fungi, test methods - Part 2: Soft rotting micro-fungi” (2005).
25. DIN EN 927-6:2006 Paints and varnishes - Coating materials and coating
systems for exterior wood - Part 6: Exposure of wood coatings to artificial
weathering using fluorescent UV lamps and water (2006).
26. DIN EN 321:2002 Wood-based panels - Determination of moisture
resistance under cyclic test conditions (2002).
27. DIN EN 152-1:1989 Test methods for wood preservatives; laboratory
method for determining the protective effectiveness of a preservative
treatment against blue stain in service (1989).
28. DIN EN 252:1990 Field test method for determining the relative protective
effectiveness of a wood preservative in ground contact (1990).
29. I. Stephan, M. Grinda et al., International Research Group on Wood
Preservation DocNo. IRG/WP 98 20149 (1998).
30. J. van Acker, paper prepared for the 37th meeting of Int. Research Group
on Wood Preservation; Tromsoe, Norway 2006, accessible under
www.IRG-wp.org, IRG/WP 06-20347 (2006).
31. B.Dawson-Andoh, L. Matuna, Holz als Roh - und Werkstoff 65, pp. 331-
334 (2007).
INA STEPHAN, RUDY PLARRE
BAM Federal Institute for Materials Research and Testing
Unter den Eichen 87
Berlin, 12205, Germany
chimica oggi • Chemistry Today • vol 26 n 3 / May-June 2008