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Processing of wood-thermoplastic composites

Article  in  International Journal of Material Forming · August 2009

DOI: 10.1007/s12289-009-0654-y

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 PROCESSING OF WOOD-THERMOPLASTIC COMPOSITES

Mladen Šercer1*, Pero Raos2, Maja Rujnić-Sokele1

1
Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb
2
Mechanical Faculty Slavonski Brod, University of Osijek

ABSTRACT: Natural and wood fibre plastic composites (WPC) are among the most rapidly growing markets within
the plastics industry. They are manufactured by combining either wood or other natural fibers such as flax, hemp, jute
or kenaf with polymers. Thermoplastic resins, such as polypropylene, polyethylene or polystyrene, are mixed with other
materials (such as wood), and form a composite product. The resulting composite can be easily processed into various
shapes and can be recycled. The experiments were made to study the behaviour of the wood flour-polypropylene
composite during injection moulding, and to what extent the mechanical properties deteriorate after several processing
cycles, i.e. mechanical recycling. Regarding efficiency of energy recovery, the heating value of wood-thermoplastic
composite with different contents of wood and thermoplastics was determined as well.

KEYWORDS: wood-thermoplastic composite, processing,

1 INTRODUCTION are the most widely used short fibres, but can also be
obtained from agro-bases from different parts of the
In recent years, interest in natural fibre composites for plant such as bast, seed or leaf fibres. [4]
industrial applications in western countries has increased
significantly. This is partly due to the environmental
concern about man made composites (like recyclability),
but mostly due to their potential in cost reduction. In
addition to being biodegradable, natural fibres are in
general lighter than man-made fibres, and when used as
reinforcements, they have mechanical properties
comparable to those of glass fibre reinforced plastics.
They are available as agricultural resources in many
countries in the developing world. Also they are Figure 1: Classification of natural/biofibres [5]
renewable, nonabrasive to processing equipment and can
be incinerated. [1,2] Cellulose is the basic structural component of all plant
fibres. It is the most important organic compound
2 NATURAL FIBRE COMPOSITES produced by plants and the most abundant in the
Natural fibres can be classified as: 1) seed fibres (such as biosphere. The cellulose molecules consist of glucose
cotton, kapok), 2) bast fibres (like flax, hemp, jute, units linked together in long chains, which in turn are
kenaf, ramie), 3) leaf fibres (agaves like sisal, pineapple, linked together in bundles called microfibrils. The tensile
Manila hemp), 4) fruit fibres (like coconut) 5) wood strength of the cellulose microfibrils is enormous, being
fibres and 6) grasses and reeds (wheat, oat or corn) the strongest known material with a theoretically
(Figure 1). [3] estimated tensile strength of 7.5 GPa. [6]
In general natural fibres can be classified into two Hemicelluloses are also found in all plant fibres.
categories, particulate and fibrous. Particulates have an Hemicelluloses are polysaccharides bonded together in
aspect ratio (ratio of the length to diameter) of relatively short, branching chains. They are intimately
approximately unity. In general no significant associated with the cellulose microfibrils, embedding the
strengthening is expected, although the elastic modulus cellulose in a matrix. Hemicelluloses are very
and some other properties may be improved. Wood hydrophilic (i.e., containing many sites to which water
flour, ground rice hulls, ground corncob, etc. can be can readily bond). [6]
classified as particulate. Fibres can be considered to be Lignin is the compound which gives rigidity to the plant.
short when the aspect ratios vary between that of the Lignin is a three-dimensional polymer with an
particulates and that of continuous fibres. Wood fibres amorphous structure and a high molecular weight. Of the

____________________
* Corresponding author: Ivana Lučića 5, HR-10000 Zagreb, Croatia, phone +38516168338, fax +38516150081, msercer@fsb.hr
three main constituents in fibres, it is expected that lignin for North America and 19 % CAGR for Europe from
would be the one with least affinity for water. Another 2002 to 2010. [8]
important feature of lignin is that it is thermoplastic (i.e., Nearly 50 % of the combined North American/European
at temperatures around 90 °C it starts to soften and at market and 65 % of the North American market is
temperatures around 170 °C it starts to flow). [6] extruded decking. The building products segment, which
The combined effect of the three main constituents includes decking as well as railing, shingles, and window
results in properties which are unique for plant fibres. and door profiles, has 80 % of market demand in North
The most important are: [6] America. Generally, the trend is that higher quality and
• Very good strength properties, especially tensile higher cost products like railings, windows and car parts,
strength. In relation to its weight the best bast fibres are growing their volume. Although injection moulding
attain strength similar to that of Kevlar. of WPC is currently small compared to extrusion, the
• Very good heat, sound and electrical insulating potential in injection moulding is huge. Growing
properties. applications include automotive, packaging, and building
• Combustibility. From a waste point of view, products such as shingles and siding. [8,9]
combustibility is an advantage. Products can be
disposed of through burning at the end of their 3.1 WOOD
useful service lives and energy can simultaneously The origin of wood fibres can be: sawmill chips,
be generated. sawdust, wood flour or powder, cutter shavings, pulp or
• Biodegradability. As a result of their tendency to wood residues. [9] Wood is usually pine, maple or oak,
absorb water, fibres will biodegrade under certain and can be wood flour with fibre length-to-diameter ratio
circumstances through the actions of fungi and/or from 1/1 to 5/1 or wood fibres with ratio greater than
bacteria. 10/1. Wood fibres act more reinforcing in the composite
• Dimensional stability. As a consequence of the than wood flour that acts more as a filler. [10]
hygroscopicity of the fibres, products and materials
based on plant fibres are not dimensionally stable 3.2 THERMOPLASTICS
under changing moisture conditions. This is the The primary drawback of the use of natural fibres is the
greatest disadvantage in relation to industrial use of lower processing temperature permissible due to the
plant fibres. However, if necessary, this may be possibility of lignocellulosic degradation and/or the
controlled at an extra cost by a number of known possibility of volatile emissions that could effect
treatments (e.g., heat treatments or chemical composite properties. The processing temperatures are
modification procedures such as acetylation). thus limited to about 200 ºC although it is possible to use
• Reactivity. The hydroxyl groups present in the cell higher temperatures for short periods. This limits the
wall constituents not only provide sites for water type of thermoplastics that can be used with agro-fibres
absorption but are also available for chemical to commodity thermoplastics such as polyethylene (PE),
modification (e.g., to introduce dimensional polypropylene (PP), polyvinyl chloride (PVC) and
stability, durability, or improved oil/heavy metal polystyrene (PS). [4]
absorption properties).
3.3 ADDITIVES
3 WOOD-THERMOPLASTIC Additives are used to improve the performance and
COMPOSITES production of the composites. Coupling agents are an
Traditionally, wood composites have been produced to essential ingredient when designing a natural fibre
substitute for solid wood in a variety of different reinforced composite because of the incompatibility
applications. These materials were produced of wood between the hydrophilic natural fibres and the
elements in different forms (i.e. fibres, particles, strands hydrophobic thermoplastic matrix. Especially in the case
or lumber) that are adhered with thermosetting resin. of e.g. PP and cellulose, where one component is polar
More recently, a new class of these materials has and the other non-polar, compatibilisation is required in
emerged that combines wood with thermoplastic order to improve the adhesion of the wood fibre and the
polymers. The development of these materials has been matrix polymer. Coupling agents are used to improve the
driven primarily by the need for durable wood materials bonding between wood and plastics by creating chemical
that can function in exterior environment without bonds across the interface the two. The most well known
preservative treatment. In recent years, composites coupling agent is MAPP (maleated polypropylene). [11]
produced from wood and thermoplastic resins have seen Besides improving mechanical properties such as tensile
rapid increases in commercial production. [7] strength, elastic module and impact strength it also
About 87 % of WPCs use wood fibre or flour; the reduces absorption of water and improves resistance to
remaining 13 % use natural fibres such as kenaf, hemp, high temperatures. [12]
jute or flax. The European market uses more natural
fibres, while North America, with more readily available
wood sources, uses more wood fibre. The WPC market
continues to grow rapidly, with 14 % CAGR estimated
3.4 PROCESSING OF WOOD-THERMOPLASTIC medium density fibreboard (MDF), one of the most
COMPOSITES commonly used materials in this application.
The manufacture of thermoplastic composites is often a Table 1: Partners in EUREKA E!2819 FACTORY
two-step process. The raw materials are first mixed ECOPLAST project
together in a compounding process, and the compounded
material is then formed into a product. Manufacturing
options for wood-thermoplastic composites include sheet Project coordinator
or profile extrusion, injection moulding, calendering, ISOKON / ISOSPORT GMBH - Slovenia
thermoforming and compression moulding. [13] Project partners
Extrusion is by far most common processing method for TECOS, Slovenian Tool and Die Development
wood-thermoplastic composites, so processors use a Centre, Slovenia
variety of extruder types and processing strategies. Faculty of Mechanical Engineering, University
Injection or compression moulding has advantages when of Ljubljana, Slovenia
processing of a continuous piece is not desired or a more PIEP, Department for Innovation in Polymer
complicated shape is needed, but the total weight is Engineering, University of Guimarães, Portugal
much less than what is produced with extrusion [13] Faculty of Mechanical Engineering and Naval
When fillers are added to either an extrusion or injection Architecture, University of Zagreb, Croatia
moulding process, changes occur which alter the TVK, Tiszai Vegyi kombinat Rt, Hungary
processing parameters, fibre orientation, and mechanical Budapest University of Technology and
properties of the product. Similar to extrusion processes, Economics, Hungary
filled polymers have higher melt viscosities and Rettenmaier & Soehne Gmbh + co, Germany
temperatures, increased fibre damage during mixing and INTELASTICO Industrias Tecnical de
moulding, and a decrease in melt strength. Other issues Plasticos, SA, Portugal
associated with moulding such as skincore behaviours
and fibre orientation are important issues for IM filled
thermoplastics. [7]
Injection moulding of WPC is not very different from
conventional injection moulding, but attention must be
given to some details. Before the processing, the material
must be thoroughly dried in order to avoid the swelling
of the product after processing.

4 EUREKA E! 2819 FACTORY

ECOPLAST
EUREKA project E! 2819 FACTORY ECOPLAST
(2002-2005) is combining natural fibres with
thermoplastics to create new recyclable compounds for
consumer products and audio components. As world
markets expand, especially in developing countries, the
need for new materials to satisfy production Figure 2: Loudspeaker box (45 % PP + 50 % wood +
requirements continues to grow. 5 % MAPPt)
Partners in the EUREKA FACTORY ECOPLAST
project (Table 1) decided to join efforts to develop a 4.1 INJECTION MOULDING
palletised compound suitable for extrusion and injection
The selection of correct parameters is crucial for quality
moulding, combining two or more material components
of injection moulded products. Correct temperature is of
in such a way that the resulting compound is better than
the most importance, since at temperatures above 200 °C
any of the individual components alone.
wood fibres start to degrade. When composite with high
The project results included defining precise
percentage of wood is injected (more than 50 %), the
technological parameters for extrusion and an optimal
injection speed has to be increased, otherwise the melt
palletising process for making compounds for injection
does not fill the cavity completely. Mixtures were
moulding and extrusion. The new materials are suitable
comprised of different combinations of two different
for use in the manufacture of a wide variety of products,
kinds of polypropylene (manufacturer TVK, Hungary),
including vacuum cleaner and lawn mower parts, storage
two different kinds of wood, soft and hard (manufacturer
boxes or golf tees. Acoustic properties also became a
Rettenmaier & Söhne, Germany) with mass share from
focus of investigation. Results showed that the wood
40 to 60 % and two different coupling agents. [12]
fibre-filled composites developed under the project are
The tests showed that addition of wood fibres improves
well suited to use in loudspeaker boxes (Figure 2). Both
some mechanical properties, but choosing the right
damping of sound radiation and sound wave resistance
coupling agent is of the most importance, since it can
for the material are comparable to those displayed by
improve performance up to 30 - 40 % in comparison to
 poorly chosen one. By adding wood to thermoplastics, Direct Manufacturing of Polymeric Products that are
one important quality is gained - the touch and warmth included in program Rapid Production – From Vision to
of wood, which is especially important for floor decking, Reality supported by the Ministry of Science, Education
laminates, garden furniture etc. Even though viscosity is and Sports of the Republic of Croatia. The authors would
rising with the addition of wood flour, the tests have like to thank the Ministry for the financing of this
shown that there are no major obstacles for injection project.
moulding. [12]
REFERENCES
4.2 RECYCLING AND RECOVERY OPTIONS [1] Kandachar, P: Agricultural Fibres for Product
The recycling of thermoplastics usually decreases the Design & Development. In Proceedings of the
majority of mechanical properties with the increase in Workshop on Collaborative Open Source Design of
the number of treatments, so the tests were conducted to Appropriate Technologies, 2001.
determine to what extent the mechanical properties [2] Status and Market Report – TRA Polymer
deteriorate after several processing cycles, i.e. Materials, Cluster3: Polymer Composites, 1999.
mechanical recycling of wood-polypropylene composite. [3] Schuh, T.G.: Renewable Materials for Automotive
The material was processed 10 times in total with same Applications, Daimler-Chrysler AG, Stuttgart, 2000.
processing parameters. The tested material has not [4] Sanadi, A., Caulfield, D. F, Rowell, R. M.:
shown behaviour typical of the thermoplastics regarding Lignocellulosic/plastic composites, The Fibril
recycling, moreover, flexural strength was even better in Angle, Spring 1998 Newsletter, 8-12.
higher-generation materials, for the reasons not quite [5] Kozlowski, R. et al.: Application of fibrous plants
explicable. [14,15] Possible explanation for such with a special reference to composite and paper,
behaviour can be better homogenisation and bonding Biofiber Industry Advancement Workhop, 2005.
between wood fibres and polypropylene after each cycle, [6] Olesen, P.O., Plackett, D.V.: Perspectives on the
but without the study of composite morphology, the performance of natural plant fibres,
conclusions cannot be definitive. www.ienica.net/fibresseminar/olesen.pdf
Also, the experiments were conducted to determine the [7] Wolcott, M. P.: Production Methods and Platforms
heating value of WPC. [16] Two types of wood- for Wood Plastic Composites. In Non-Wood
thermoplastic composites have been used for testing, Substitutes for Solid Wood Products Conference,
with different contents of polypropylene and wood 2003.
fibres. Naturally, WPC with lower wood content has [8] Markarian J.: Material and processing developments
higher heating value, because the heating value of wood drive wood-plastic composites forward, Plastic
is significantly lower then the heating value of Additives and Compounding, 24-28, 2003.
polypropylene. Nevertheless, the energy value of wood- [9] Mali, J. et al.: Woodfibre-plastic composites, VTT
thermoplastic composites is comparable to energy value Technical Research Centre of Finland, 2003.
of polymers which amounts to about 37 MJ/kg, so their [10] Rangaprasad, R.: Wood Plastic Composites - an
presence in the waste stream will contribute to the overview, IPI Seminar on Synthetic Wood, 2003.
efficiency of energy recovery. [11] Rowell, R. et al.: Utilization of Natural Fibers in
Plastic Composites: Problems and Opportunities. In
5 CONCLUSIONS Leao, A. L., Carvalho, F. X. i Frollini E. (ed.):
Lignocellulosic-Plastics Composites, 23-51, 1997.
The aim of the project was to search for rheological [12] Kuzman, K. et al.: Development and
properties, compatibility between natural fibres and Characterisation of Eco-Friendly Thermoplastics E!
thermoplastics, injection moulding parameters and 2819 FACTORY ECOPLAST. In 3rd International
possible applications of the compound. Results from the Conference on Manufacturing Engineering, 207-
project showed that developed wood-thermoplastic 214, 2008.
composites are well suited to use in loudspeaker boxes. [13] Clemons, C.: Interfacing wood-plastic composites
The future of wood-thermoplastic composites will industries in the U.S., www.jobwerks.com/
ultimately depend on many factors, including new news/Archives/iwpc.html
product identification, product quality, consumer [14] Rujnić-Sokele, M., Šercer, M., Barić, G.:
reaction and perceptions, and success of research and Mechanical recycling of injection-molded wood-
development efforts. Nevertheless, new materials that thermoplastic composites, in ANTEC 2006, 278-
are cheaper and better need to be developed. Such 282, 2006.
materials should also be recyclable more easily, reducing [15] Rujnić-Sokele, M.; Šercer, M.; Bujanić, B.: Utjecaj
environmental impacts. One of the choices will recikliranja na mehanička svojstva drvno-
definitely be wood-thermoplastic composites. plastomernoga kompozita. Polimeri 25(1-2), 12-19,
2004.
ACKNOWLEDGEMENT [16] Šercer, M., Rujnić-Sokele, M., Bujanić, B.:
This work is part of the research included in projects Recycling and recovery of wood-thermoplastic
Increasing Efficiency in Polymeric Products and composites. In 6th Global Wood and Natural Fibre
Processing Development and Advanced Technologies of Composites Symposium, P19-1 – P19-6, 2006.

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