Wood-plastic composite
Wood-plastic composites ( WPC , English for wood-plastic composite materials , also English wood (fiber) polymer composites , wood (fiber) polymer composites') are thermoplastically processable composite materials, which are made of different proportions of wood - typically Wood flour, plastics and additives are produced. They are mostly processed using modern plastics technology such as extrusion , injection molding , rotational molding or pressing techniques , but also using thermoforming processes .
In addition to WPC, the names are wood-plastic composite (HCD) , thermoplastic fiber (TPF) and wood plastic or high-tech timber common.
Definition and demarcation
The term WPC is used differently internationally. The above description is an attempt to establish a generally accepted definition for the German-speaking area.
Wood can be replaced in the materials by other plant fibers (e.g. kenaf , jute or flax ) without significantly changing the material properties. Such products are sometimes still referred to as WPC. In this case, however, the term natural fiber-reinforced plastic should be used for better delimitation .
WPC belong to the subgroup of biocomposites within biomaterials . In order to be counted among the biomaterials, their wood content should be over 20%, but this is consistently achieved in Germany and Central Europe (see below).
construction
Typically, WPC in Germany and Central Europe consist of a wood fiber or wood fiber content of 50% to 90% and a plastic matrix made of polypropylene (PP) or, less often, of polyethylene (PE). WPC with a 50 percent polyethylene content dominate the North American market. Mixtures with other plastics and bio-based plastics are technically possible, but not very common. Due to the thermal sensitivity of the wood, processing temperatures are only possible below 200 ° C. Thermal transformations and decomposition of the wood occur at higher temperatures.
The addition of additives optimizes special material properties: bond between wood and plastic, flowability , fire protection, color design and - especially for outdoor applications - weather and UV protection as well as biocides for the purpose of pest resistance.
A new development in this sector is a mixture of polyvinyl chloride (PVC) and wood fibers of 50% each. WPC based on thermosetting plastics such as modified melamine resin are in development. As Bamboo Plastic Composites (BPC) composites are referred to, in which the wood fibers by bamboo fibers are replaced.
properties
Previous long-term studies and cases of damage from practice have shown that WPC can be damaged by UV radiation, the effects of moisture and temperature as well as fungal attack.
advantages
The advantages of the material over traditional wood-based materials such as chipboard or plywood are the free, 3-dimensional malleability of the material and the greater resistance to moisture . Compared to solid plastics, WPC offer greater rigidity and a significantly lower coefficient of thermal expansion .
disadvantage
The breaking strength is reduced compared to sawn timber. However, fittings can be provided with reinforcing inserts to compensate for this.
The water absorption of molded parts without a final coating is higher than that of solid plastic parts or molded parts with foil or flow coating.
standardization
A new standard for WPC is currently being developed in Europe. The standard with the designation: "EN 15534 composite materials made of cellulose-containing materials and thermoplastics (usually called wood-polymer materials (WPC) or natural fiber composite materials (NFC))" contains several parts. Some have already been published:
DIN EN 15534-1: 2014-04 Composite materials made from materials containing cellulose and thermoplastics (usually called wood-polymer materials (WPC) or natural fiber composites (NFC)) - Part 1: Test methods for the description of molding compounds and products
DIN EN 15534-4: 2014-04 Composite materials made from cellulose-containing materials and thermoplastics (usually called wood-polymer materials (WPC) or natural fiber composites (NFC)) - Part 4: Requirements for profiles and panels for floor coverings
DIN EN 15534-5: 2014-04 Composite materials made from materials containing cellulose and thermoplastics (usually called wood-polymer materials (WPC) or natural fiber composites (NFC)) - Part 5: Requirements for profiles and panels for wall coverings
Other parts are still under construction.
Decking
Since the end of 2007 there have been specified quality criteria for this product under the quality seal for decking boards made of wood-polymer materials. The system developed by the quality association for wood-based materials e. The quality mark awarded by V. takes into account certain material properties for durability and stability ( flexural strength , weather resistance, dimensional stability and slip resistance) as well as the origin of the product. The product is intended for outdoor use. For certified products, only wood fibers are used that come 100% from verifiably FSC or PEFC- certified forestry . The incorporated polymer or polymer mixture must consist of 100 percent plastic that has been processed for the first time .
Manufacturing
WPC is processed using methods that are well established in plastics production. The vast majority of WPC production takes place with profile extrusion . However, injection molding , plate pressing and rotational molding are also used. Nonwoven forming processes are common in the automotive industry.
With profile extrusion, it can be a one-step or two-step process. All other procedures take place in two stages. The first stage of the two-stage process is used to mix the WPC base material (often in the form of granules), in the second stage the desired shape is created.
Profile extrusion
One-step process
In the one-step process, also known as direct extrusion, wood particles and plastic granulate are spatially separated or fed to the extruder at the same place at the same time. The plastic is melted in the extruder primarily by frictional heat. The mixture of wood and plastic is intensively mixed and extruded into a profile by the tool. The wood particles must not exceed an upper moisture limit, which is usually not more than 8%. During the extrusion process, the moisture is extracted so that the WPC compound has a moisture content of less than one percent when it leaves the mold. Planetary roller extruders , conical, counter-rotating twin-screw extruders , which simultaneously fulfill the task of intensive mixing and the pressure build-up required for extrusion, are used for single-stage processes . The most important use after the crowd is decking.
Two-step process
In the two-stage extrusion process, the mixing, possibly also the drying, as well as the compacting is carried out in one unit and the extrusion by a tool in another unit. Both can be directly coupled or spatially and temporally separated.
For mixing and compacting, co-rotating, parallel twin-screw extruders (compounders), heating-cooling mixers or pelleting presses as well as grinding processes are used. The WPC intermediate products produced in this way can be used as raw materials for profile extrusion, injection molding or sheet molding. Only free-flowing, plastic-coated wood particles are suitable for rotational molding.
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Compounders work continuously and produce a very homogeneous mixture, but usually cannot build up enough pressure for profile extrusion. The pressure already compresses the wood matrix. The residence time in the compounder is a few minutes. The mixture must reach the melting temperature of the thermoplastic. If this temperature is above 160 ° C, thermal degradation of the wood can begin. Compounders work continuously and WPC pellets that are geometrically largely uniform can be produced as an intermediate product.
It is also customary to feed the WPC compound into an extrusion unit, such as a single-screw extruder or a melt pump, in the melt phase. The pellets produced allow simple and even dosing with the usual dosing devices. Extruded pellets have a moisture content below 1% and facilitate further processing.
- Heating-cooling mixers work discontinuously. Wood and plastic particles are mixed in the heating mixer and heated up to the melting point of the thermoplastic. The dwell times in the heating mixer are usually longer than in the compounder. At the melting point, the thermoplastic is absorbed by the wood fibers and the WPC mixture is transferred to a cooling mixer. It is cooled and coarse agglomerates are broken into smaller pieces. The WPC intermediate product is a dry, non-compacted granulate with an irregular grain shape.
- In pelleting presses , wood particles and plastic particles are compacted together under pressure. When pressing through a perforated die, frictional heat is generated in order to melt the plastic part. A homogeneous mixing as in the compounder or heating-cooling mixer does not take place, so that the pellets produced in this way are not suitable for all further processing processes. The wood portion is compacted by the high pressure during pressing.
- In the grinding process , wood and plastic parts are ground at the same time. A connection with the wood particles takes place at a grinding temperature that is matched to the plastic content. Depending on the grinding method and process management, clumped or individual, free-flowing wood particles can be produced.
Injection molding
In injection molding , the material must be homogeneous and easy to dose so that all parts of the melt have good flowability. Mainly compounded WPC pellets with a low wood content compared to extrusion goods are used. They are melted in a plasticizer and injected “shot by shot” into a metal mold in a discontinuous process under high pressures of up to 2400 bar. The WPC mass solidifies when it cools. Injection molding can also take place in one work step in the “Injection Molding Compounder”.
Due to the wood particle geometry and the low impact strength, the wall thicknesses in injection molding are designed to be thicker than with pure plastic granules. The significantly higher heat resistance, which gives the mass rigidity at higher temperatures, is advantageous. WPC molded parts can therefore be demolded at higher temperatures.
Plate presses
The first continuously operating presses are being tested for the plate press process. The WPC pre-product is sprinkled on a belt to form a cake. Similar to established wood-based materials processes, this is fed into a continuous press, where it is compressed and heated. The WPC particles are melted and combine to form a plate. In the following part of the press the plate is cooled down.
Rotational molding
In rotational molding hollow bodies are produced. A free-flowing powder is tumbled three-dimensionally into a hollow shape while it is heated from the outside. The powder sinters on the hot surface and forms an even layer that corresponds to the inner contour of the tool. In 2005 an armchair was presented as a WPC rotary product for the first time.
History and market development
A forerunner of modern WPC was Bois Durci , which consisted of wood flour and various animal proteins . Bois Durci was replaced by the first modern plastics , specifically Bakelite , a composite of phenol-formaldehyde resin and a filler material for which wood flour , rock flour or textile fibers were used. The first commercial product made from Bakelite with wood flour was a joystick for Rolls-Royce from 1916.
As early as the early 1960s, technologists in Germany and France began to develop and manufacture WPC on the basis of PVC and cellulose , waste paper and ground wood using the VINYPAL process. At that time, however, the processes and products did not have a market in Europe and were classified as "replacement products" and not as new materials.
In the early 1970s, the Italian company GOR Applicazioni Speciali SpA developed interior door panels, hat racks and roof panels from a composite with 50% wood and 50% resin matrix.
Wood plastic composites have been developing rapidly since the early 1990s. The USA was the starting point for this material. today they are among the most successful new biomaterials in Germany too .
use
Wood plastic composites are mainly used in the construction, automotive and furniture industries: in outdoor areas for floor coverings (terraces, swimming pools, ...), facades and furniture, especially as a substitute for tropical woods. There are already several chair and shelf systems made of WPC on the market. Further applications are writing implements, urns and household appliances. In the technical area they are used as profiles for electrical insulation. In the European automotive industry, WPC are used in door panels and rear shelves. Thermoplastic injection-molded products made from the biomaterial WPC can only be found in Germany as "niche products".
Economical meaning
More than 2.4 million t of WPC were produced worldwide in 2012, mainly in North America (1.1 million t), China (900,000 t) and Europe (260,000 t). In Europe, Germany is the leading producer with around 100,000 t and also the leading mechanical engineering company. Other leading WPC producing countries in Europe are Austria, Benelux and Scandinavia. The most important producers come from the wood and plastics industry. The new WPC market study by Asta Eder Composites Consulting shows that the worldwide spread of the WPC material is continuing. The WPC market in the Southeast Asian market will show further growth in the coming years.
literature
- Asta Eder, Andreas Haider: Market opportunities for wood polymer composites in German-speaking countries. In: wood technology. 52 (2011), pp. 44–49 (PDF; 1.1 MB)
- Asta Eder Composites Consulting: WPC Publications (Further reading on the WPC market)
- Dominik Vogt, Michael Carus, Sven Ortmann, Christin Schmid, Christian Gahle (nova-Institut GmbH): Study "Wood-Plastic-Composites" wood-plastic composite materials. Series of publications "Renewable Raw Materials" Vol. 28. Landwirtschaftsverlag, Münster 2006, ISBN 3-7843-3397-4 .
- Kristiina Oksman Niska, Mohini Sain (Ed.): Wood-polymer composites. Woodhead Publishing, Cambridge 2008, ISBN 978-1-84569-272-8 .
- Hans Korte: From a single source. Technologies for the production of wood-plastic composite materials. In: HK wood and plastic processing. 4/2006, pp. 24-27.
- nova Institute: Third German WPC Congress : Congress Journal. Hürth 2009 (PDF; 1.8 MB)
Web links
- WPC platform, organization Austria
- Wood composites at VHI
- Quality association wood-based materials
- Fraunhofer Institute WKI
- Department of Wood Biology and Wood Products at the Georg-August University of Göttingen
- Center for Timber Industry at the University of Hamburg
- WPC portal with further information
Individual evidence
- ↑ Fachagentur Nachwachsende Rohstoffe eV (Ed.): Study “Wood-Plastic-Composites” wood-plastic composite materials. Volume 28, Landwirtschaftsverlag Münster, 2006, ISBN 3-7843-3397-4 .
- ↑ Information sheet on decking boards from TBHolz (KOHLWEY trade & services) in cooperation with the German Timber Trade Association, Wiesbaden, www.holzhandel.de, accessed in January 2016
- ↑ a b Geoff Pritchard: Two technologies merge: wood plastic composites. In: Plastics, Additives & Compounding. 6 (4), July-August 2004, ISSN 1464-391X / 04, pp. 18-21
- ↑ Alireza Ashori: Wood-plastic composites as promising green-composites for automotive industries! Bioresource Technology 99 (11), 2008, ISSN 0960-8524 , pp. 4661-4667.
- ↑ Anatole A. Klyosov: Wood-plastic composites. John Wiley & Sons, 2007, ISBN 978-0-470-14891-4 , p. 75.
- ↑ Nicole Stark, Laurent Matuana: Ultraviolet Weathering of Photostabilized Wood-Flour-Filled High Density Polyethylene Composites. Michigan 2003.
- ↑ Test regulations ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 547 kB)
- ↑ Craig M. Clemons, Daniel F. Caulfield: Wood Flour. In: Marino Xanthos (Ed.): Functional fillers for plastics. Wiley-VCH, Weinheim 2005, ISBN 3-527-31054-1 , p. 249.
- ↑ Hans Korte: From a single source. Technologies for the production of wood-plastic composite materials. In: HK wood and plastic processing. 4/2006, pp. 24-27.
- ↑ nova-Institut: Third German WPC Congress: WPC growth market. ( Page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. In: www.nachwachsende-rohstoffe.info from December 4, 2009.
