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What is wood-plastic composite

Wood-plastic composites (WPCs) are composite materials made of wood fiber/wood flour and thermoplastic(s) (includes PE, PP, PVC etc.).


PP (polypropylene)

PE (polythene) is elastic, but its thermal oxidation stability is bad. Without oxygen, PE’s decomposition temperature is about 300°C. However, with oxygen, it can be oxidized when the temperature goes beyond 50°C. After it is oxidized, PE turns yellow and becomes less stable. Therefore, when it is installed outdoors, it will gradually suffer damage from sunshine.

PP is known for able to stand heat and high temperature, and the most widespread use of it should be lids or tops on all kinds of beverage bottles. PP can function properly under 100°C. Therefore, PP is a much better substitute for PE when installed outdoors.  Since it is better to use PP than PE outdoors? Then why don’t  most companies produce WPC outdoor decking with PP? Is it because PP is more expensive than PE? No. On the contrary, PP is actually a little cheaper than PE. The real reason for it is that PP is hard to process. Japanese companies spent about 20 years combining PP and wood powder to produce WPC. However, WPC has only a history of ten years in China. Plus, most Chinese companies do not pay much attention to scientific research. They mainly focus on how to lower their cost so that they can sell their product cheaply. One more thing, producing WPC with PP is much slower than with PE. In general, the production speed of many Chinese-made WPC outdoor decking is about 3-5 meters/minute. However, the production speed of WPC outdoor decking containing PP is limited to 0.3-0.4 meter/minute.



Chemical additives seem practically "invisible" (except mineral fillers and pigments, if added) in the composite structure. They provide for integration of polymer and wood flour (powder) while facilitating optimal processing conditions.[1]


In addition to wood fiber and plastic, WPCs can also contain other ligno-cellulosic and/or inorganic filler materials. WPCs are a subset of a larger category of materials called natural fiber plastic composites (NFPCs), which may contain no cellulose-based fiber fillers such as pulp fibers, peanut hulls, bamboo, straw, digestate, etc.


In recent years, people in the flooring industry starts referring to WPC as a type of floor that has a basic structure of top vinyl veneer plus a rigid extruded core (the core can be made without any wood fiber). WPC is now an established product category within LVT.[2] This type of WPC is different than the WPC decking and is not intended for outdoor usage.

 

Contents  

1 Uses

2 Production

3 Advantages

4 WPC sandwich boards

5 Fire hazards

6 See also

7 References


Uses

Wood-plastic composites are still new materials relative to the long history of natural lumber as a building material. The most widespread use of WPCs in North America is in outdoor deck floors, but it is also used for railings, fences, landscaping timbers, cladding and siding, park benches, molding and trim, window and door frames, and indoor furniture.[3] Wood-plastic composites were first introduced into the decking market in the early 1990s. Manufacturers claim that wood-plastic composite is more environmentally friendly and requires less maintenance than the alternatives of solid wood treated with preservatives or solid wood of rot-resistant species. These materials can be molded with or without simulated wood grain details.[4]


Production

Wood Plastic Composites (WPCs) are produced by thoroughly mixing ground wood particles and heated thermoplastic resin. The most common method of production is to extrude the material into the desired shape, though injection molding is also used. WPCs may be produced from either virgin or recycled thermoplastics including HDPE, LDPE, PVC, PP, ABS, PS, and PLA. Polyethylene based WPCs are by far the most common. Additives such as colorants, coupling agents, UV stabilizers, blowing agents, foaming agents, and lubricants help tailor the end product to the target area of application. Extruded WPCs are formed into both solid and hollow profiles. A large variety of injection molded parts are also produced, from automotive door panels to cell phone covers.


In some manufacturing facilities, the constituents are combined and processed in a pelletizing extruder, which produces pellets of the new material. The pellets are then re-melted and formed into the final shape. Other manufacturers complete the finished part in a single step of mixing and extrusion.[5]


Due to the addition of organic material, WPCs are usually processed at far lower temperatures than traditional plastics during extrusion and injection molding. WPCs tend to process at temperatures about 50 °F (28 °C) lower than the same, unfilled material, for instance. Most will begin to burn at temperatures around 400 °F (204 °C).[6] Processing WPCs at too high of temperatures increases the risk of shearing, or burning and discoloration resulting from pushing a material that’s too hot through too small of a gate, during injection molding. The ratio of wood to plastic in the composite will ultimately determine the melt flow index (MFI) of the WPC, with larger amounts of wood generally leading to a lower MFI.


Advantages


WPCs do not corrode and are highly resistant to rot, decay, and Marine Borer attack, though they do absorb water into the wood fibers embedded within the material.[7] They have good workability and can be shaped using conventional woodworking tools. WPCs are often considered a sustainable material because they can be made using recycled plastics and the waste products of the wood industry. [8] They can be recycled easily in a new wood-plastic composite, much like concrete. One advantage over wood is the ability of the material to be molded to meet almost any desired shape. A WPC member can be bent and fixed to form strong arching curves. Another major selling point of these materials is their lack of need for paint. They are manufactured in a variety of colors, but are widely available in grays and earth tones.


WPC sandwich boards

WPC boards show good set of performance but monolithic composite sheets are relatively heavy (most often heavier than pure plastics) which limits their use to applications where low weight is not essential. WPC in a sandwich-structured composite form allow for combination of the benefits of traditional wood polymer composites with the lightness of a sandwich panel technology. WPC sandwich boards consist of wood polymer composite skins and usually low density polymer core which leads to a very effective increase of panel's rigidity. WPC sandwich boards are used mainly in automotive, transportation and building applications, but furniture applications are also being developed.[15] New efficient and often in-line integrated production processes allow to produce stronger and stiffer WPC sandwich boards at lower costs compared to traditional plastic sheets or monolithic WPC panels.[16]


Fire hazards

The types of plastics normally used in WPC formulations have higher fire hazard properties than wood alone, as plastic has a higher chemical heat content and can melt. The inclusion of plastic as a portion of the composite results in the potential for higher fire hazards in WPCs as compared with wood. Some code officials are becoming increasingly concerned with the fire performance of WPCs.[17][18]


References


1."WPC machines & equipment. WPC pellets and additives. Composite decking lumber.". wpc-composite-decking.blogspot.com.

2.Cruise, Mallory (2016-02-18). "WPC gains ground at Domotex Hannover 2016" (FCW Global February 2016). Floor Covering Weekly. Floor Covering Weekly. Retrieved 30 April 2016.

3.Clemons, C. (2002) "Wood-plastic Composites in the United States: The interfacing of two Industries" Forest Products Journal 52(6)

4.http://www.wpcextruder.com/news_show-13.html Introduction of WPC Wood Plastic Composite Products

5."Wood-Plastic Composites production requires high formulation quality the feeding system". ktron.com.

6."Wood Plastic Composites – Green Dot".

7.Stark, N. (2001) “Influence of Moisture Absorption on Mechanical Properties of Wood Flour-Polypropylene Composites.” Journal of Thermoplastic Composite Materials 14

8.Gibson, Scott (2008). “Synthetic Decking” [1]. Remodeling Magazine.

9."What are Ultra Polymers?". Solvay. 2014. Retrieved 2014-04-17.

10.Carraher, Charles (2014). Carraher's polymer chemistry. Boca Raton: Taylor & Francis. p. 232. ISBN 1-4665-5203-4.

11.Hamel, S. (2011) Modeling the Time-dependent Flexural Response of Wood-plastic Composite Materials Dissertation, University of Wisconsin–Madison

12.Morrell, J et al.(2006) “Durability of wood-plastic composites.” Wood Design Focus 16(3)

13.http://www.decksandfencesbyryan.com/articles/21-composite-vs-wood.html

15."WPC honeycomb panels". Renolit.com. Retrieved 2014-10-07.

16."Sandwich Panel Technology". EconCore.com. Retrieved 2014-10-07.

17.http://wpcinfo.org/techinfo/Fire_Studies.html Washington State University Wood Plastic Composites Information Center, "Fire Issues in Engineered Wood Composites for Naval Waterfront Facilities", 46th International SAMPE Symposium and Exhibition, Long Beach, California, May 2001

18.http://www.enn.com/business/article/24261 Environmental News Network, "California Fire Codes Put Focus on Plastic Decking Concerns" 5 Nov 2007

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