Sep. 03, 2021
Wood plastic products have been widely used in various outdoor and indoor applications. Such as: wood plastic doors, door covers, blinds, cabinets, wardrobes, exterior wall hanging panels, ceilings, decorative wall panels, skirting lines, floors, floor joist, guardrail, pavilions, gallery frames, fences, fence post, leisure chairs , Flower racks, flower boxes, tree ponds and tree enclosures, air conditioning racks, air conditioning covers, road signs, transportation pallets, etc. In particular, plastic-wood materials are excellent in waterproof, moisture-proof, acid and alkali resistance, and corrosion resistance. They are particularly advantageous when used in seaside areas with high salinity. Wood-plastic profile is easy to use and flexible in application. It can not only replace the use of wood, but also is a new environmentally friendly material with excellent performance. The market prospect is very broad.
Wood plastic materials have excellent comprehensive performance, and there are more and more varieties and quantities of wood plastic products used in outdoor buildings and landscapes. With the continuous progress of manufacturing technology and process, the appearance quality of products has greatly improved. However, many wood-plastic companies, and even many large brand companies, have inherent defects in material design due to the lack of targeted research. These shortcomings often only gradually exposed the problems after two or three years of completion. At this time, it is too late to solve the problems. When tracking down issues, material design flaws are often difficult to find. After a period of use in outdoor wood-plastic buildings and landscapes, some longer-scale plastic wood will gradually bend and sag. This is a common defect at present. The wood-plastic material is beautifully straight when it is built, and gradually bends and deforms after several cold and heat.
Temperature stress and creep-The temperature rises in summer, and the wood-plastic material produces thermal expansion. Longer wood plastic composite decking board has a greater thermal expansion. Because the two ends are fixed, the temperature stress is also large (the stress and expansion are proportional to the deformation). When the expansion reaches a certain value, the wood plastic products becomes unstable and deforms and bends with the increase of thermal expansion. Degree (bending deformation) increases. If the installation is in a high temperature season, the temperature difference from the maximum temperature is small, and there is basically no bending and deformation due to thermal expansion. However, in the low-temperature season, the toughness and strength of wood-plastics decrease with temperature; because the two ends are fixed, the temperature stress caused by the cooling and shrinkage of wood-plastic reaches the maximum value, which often leads to the failure of the fixing.
Water-absorbing swelling and creep-In rainy season, wood-plastic swelling by absorbing water, wood-plastic material produces a small amount of creep elongation, which can not be restored; and with the alternating cycle of dry and wet, it periodically generates creep elongation and superimposes. As the wood inside the wood plastic reaches the possible water swelling limit and the binding force of the polymer material, the increment of the water swelling creep elongation gradually decreases to zero, reaching the creep limit. The creep of wood-plastic material is the comprehensive result of temperature stress creep superimposition and water-absorbing expansion creep. When the two ends of the longer wood-plastic board are fixed and restrained, with the superimposition of various creeps of the wood-plastic material, a beautiful straight line formed when it is built, gradually appearing obvious bending; and alternating between several cold and heat.
The design solution for the deformation of wood-plastic materials-the moisture absorption of the polymer base in the wood plastic material is very small, and the moisture absorption of the wood fiber is large; it is very important to effectively treat the wood fiber to greatly reduce the moisture absorption. In formula design and process design of wood-plastic materials, it is necessary to reduce the water absorption of wood-plastic materials as much as possible, and finally take strict water absorption as the quality index assessment; temperature stress and creep are determined by the characteristics of the material itself. Controlling and reducing the linear expansion coefficient of wood-plastic materials can effectively control and reduce temperature stress.
In order to make the composite decking boards drain away water and clean easily, suitable gap must be maintained during installation.The size of the gap between the head to head of the wood plastic (WPC) profile depends on the length of the profile, the temperature during installation, the thermal expansion coefficient of the profile (ζ≈0.9×10-4), and the highest possible local temperature.
Calculation method of head-to-head clearance:
For a L-meter-long profile, the highest possible local temperature in one year is Tmax, and the temperature during installation is T, then the gap between the head of this profile is installed Δ L=ζ×(Tmax-T)×L .
For example: a profile with a length of 2 meters, the temperature on the day of installation is 10°C, and the temperature on the hottest day of the year is 40°C, then the gap that should be left on the head of the profile when installing:
Δ L=ζ×(Tmax-T)×L=2000×(40-10)×0.9×10-4=5.4mm
In addition, to improve outdoor wood-plastic buildings and landscapes, inorganic pigments with excellent resistance to natural aging are usually used to ensure the durability of the coloring. The phenomenon of pulverization and fading often occurs because one is the aging of the polymer surface layer of wood plastic, and the other is the erosion of inorganic pigments by rain with low pH value (commonly known as acid rain). In the design of wood plastic performance, anti-aging additives are often evenly distributed. Aging actually develops gradually from the outside to the inside, so improving the aging resistance of the wood-plastic surface layer is an important way to solve the surface chalking and fading; at the same time, the ability to resist acid rain should be enhanced in the colorant formulation.
In the design concept of high-end wood-plastic products for outdoor buildings and landscapes, wood-plastic profiles have been upgraded from a uniform composite material to a double-layer functional composite material. Through technology, process, equipment improvement and appropriate cost increase, a double-layer distinguishing solution is proposed. It is beneficial to overcome technical difficulties and obtain high-grade wood-plastic profiles with high quality, high aesthetics, high performance and long life. The PVC/PVC double-layer wood-plastic profile is upgraded from a uniform composite material to a PVC-based surface layer/PVC-based core layer double-layer composite material, which is produced by dual-machine co-extrusion. Among them, the PVC-based surface layer focuses on solving the beauty of the surface and improving the life span; the PVC substrate core layer focuses on solving the dimensional stability, physical and mechanical properties, and optimizing costs. In short, according to research, PVC has better creep resistance and is more suitable as a resin base for outdoor wood plastic materials; the internal control standards of outdoor wood plastic manufacturers should be compared with indicators such as water absorption, water swelling, and dimensional change. Only by increasing the national and industrial standards by more than 25% can we better meet the requirements of outdoor buildings and landscapes; in order to effectively control the creep of outdoor buildings and landscape wood-plastic products.The manufacturer should detect and control the linear expansion coefficient. Minimize the use of large-scale wood plastic structures with strong restraints at both ends in outdoor buildings and landscapes.
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