The comprehensive performance of wood plastic composite materials
is very good, and the variety of wood-plastics used in outdoor buildings and landscapes is increasing and increasing. As manufacturing techniques and processes continue to advance, the appearance and quality of the products increase significantly. However, there are many wood-plastic enterprises, and even many large-scale enterprises. Due to the lack of targeted research, there are inherent defects in material design.
Thermoplastic polymer materials are viscoelastic materials, which undergo stress creep under long-term stress, which is determined by the characteristics of thermoplastic polymer materials. The main influencing factors are long-term stress and ambient temperature. The composite material made of polymer material and plant fiber is wood-plastic material. Because of the plant fiber content, the creep resistance of polymer composite system is greatly improved. The combination of the advantages of both polymer materials and plant fibers makes the wood-plastic materials have a comprehensive advantage over other materials in outdoor applications. This is also the root cause of the rapid growth and development of wood-plastic materials in landscapes and outdoor buildings.
Deformation of wood plastic material
After the use of outdoor wood-plastic buildings and landscapes for a period of time, some wood-plastics with long scales will gradually deform and sag, which is a common defect in outdoor wood-plastic buildings and landscapes. The wood-plastic material is a beautiful straight line when it is built. The deformation factor of the bending deformation after several cold and heat tests is as follows:
Temperature stress and creep
In summer, the temperature rises and the wood-plastic material thermally expands. The longer wood plastics thermal expansion is larger, because both ends are fixed, the temperature stress is also large (stress and expansion change in direct proportion), when the expansion reaches a certain value, wood plastics occurs instability and bending deformation and with the thermal expansion increases the bending degree (bending deformation). After the wood plastic composite is bent, the temperature stress is released; The neutral surface stress is zero, due to bending, the concave surface generates compressive stress, and the convex surface generates tensile stress. Due to the long-term effects of the tensile and compressive stresses, the convex and concave surfaces respectively generate tensile and compressive creep, so that the curvature becomes large, and the full shrinkage recovery cannot be achieved when the temperature is lowered.
Water absorption expansion creep
During the rainy season, the wood plastic composite water swells and the wood plastic composite material produces a slight creep elongation, which is unrecoverable; And with the alternating wet and dry cycle, the creep elongation is periodically generated and superimposed. As the wood inside the wood plastic reaches the possible limit of water absorption expansion and the binding force of the polymer material, the increment of the water-swelling creep elongation gradually decreases to zero, reaching the creep limit. Creep of wood plastic composite decking is a comprehensive result of temperature stress creep superposition and water absorption expansion creep. When both ends of the wood plastic composite decking
are fixed and restrained, the beautiful linear material gradually shows obvious bending when built with the superposition of various creep of wood plastic composite decking
; And after a number of alternate cold and heat reach the size of the basic stability, bending no longer increase.
Solution of water absorption expansion creep
The hygroscopic properties of wood – plastic materials are small, and the hygroscopic properties of plant fibers are large. It is important to treat plant fibers effectively to greatly reduce their moisture absorption. In the formulation design and process design of wood plastic composite materials, the water absorption of wood plastic composite materials must be reduced as much as possible, and finally, the strict water absorption rate is used as the quality index.
Solution of temperature stress and creep
The temperature stress and creep are determined by the material’s own characteristics. Improvement must be found from the polymer material itself. Controlling and reducing linear expansion coefficient of the wood plastic composite decking can effectively control and reduce temperature stress. The outdoor building temperature is usually below 70 ° C. When the Tg (glass transition temperature) of the polymer material is lower, the stress creep is more likely to occur. The appearance, color and durability of wood-plastic materials and their design have improved the outdoor wood-plastic buildings and landscapes.
After a period of use, the surface of wood-plastics often has chalking and fading, which affects the ornamental. Wood-plastic materials used outdoors are exposed to the sun and rain, and the environment is demanding; Generally, inorganic color materials with excellent natural aging resistance are used to ensure the durability of coloring. The powdery fading phenomenon often occurs, first is the wood plastic surface layer polymer aging, the second is often a small pH value of rain (commonly known as acid rain) erosion of inorganic color. In the design of wood plasticity, the anti-ageing auxiliaries are usually evenly distributed. Aging is actually gradually developed from the outside to the inside, so improving the aging resistance of wood plastic composite materials
surface layer is an important way to solve the problem of flouring; At the same time in the color formula to enhance the ability of acid rain resistance.