Issue 20, 2012 - Research Progress on PP-based WPC Composites

The resource crisis and ecological problems have triggered an upsurge
in research on sustainable development and ecological protection around the world.
Wood-plastic composite material (WPC) is a new type of functional material obtained by adding organic cellulose filler such as wood, bran, bamboo chips and nut hard shell in the form of powder, fiber and shavings as reinforcement or filler, adding to thermoplastic or thermosetting plastics for compounding, combining the advantages of wood materials and plastics, with the advantages of renewable raw materials, good dimensional stability, corrosion resistance, aging resistance, recyclability, no pollution, etc.
, and is considered to be the new material
with the most development potential.

Polypropylene (PP) is one of the important general plastic materials, with excellent mechanical properties, good chemical stability, low price, non-toxic and easy processing and molding characteristics, is an excellent wood plastic material matrix, so the research of PP based wood plastic composites has received extensive attention
.

1.
Wood flour-filled PP matrix composite

Yang Lingling of South-Central University for Nationalities et al.
studied the interface morphology of PP-based wood-plastic composites, and used two different coupling agents to surface the
wood powder.
The results show that the performance of the composite is improved.
With the increase of mixing time, the impact strength and tensile strength of the material show a trend
of first increasing and then decreasing.
Liu Wenpeng, Key Laboratory of Rubber and Plastic Materials and Engineering, Ministry of Education, Qingdao University of Science and Technology, et al.
studied the effects of coupling agent, compatibilizer, wood flour dosage and wood filler type on the mechanical properties of
wood-plastic composites with PP-based resin.
The results show that the mechanical properties of the composites can be improved by treating wood flour with silane coupling agent or directly adding compatibilizers.
The increase of wood flour reduces the impact strength of the composite material, and the bending strength, bending modulus and tensile strength are greatly improved
.
Gong Jingyu, School of Materials Science and Engineering, Chongqing University of Technology, et al.
used PP as the matrix to prepare wood-plastic composites
by thermoforming method.
The results show that with the increase of wood flour content, the tensile strength of the composite is improved, while the impact performance is significantly reduced
.
Join 0.
When the aluminate coupling agent is 5 points, the processability of the material is also improved
.

Nie Hengkai, Department of Materials, Xuzhou Industrial Vocational and Technical College, et al.
studied the effects of
wood powder, compatibilizer and rubber powder dosage on the mechanical properties and processing properties of wood-plastic composites prepared with PP-based resin.
The results show that with the increase of wood flour dosage, the tensile strength of the wood-plastic system first increases and then decreases, and the overall trend of the impact strength of the system is gradually increasing.
When maleic anhydride grafted PP (PP-g-MAH) is used for 10 minutes, the comprehensive mechanical properties of the system are the best
.
H.
P.
S.
AKHALIL ET AL.
studied the effects
of different sizes of fillers on the water absorption and mechanical properties of wood flour/PP composites.
The results show that the mechanical strength of the composite material after adding wood flour is lower than that of pure PP, and the tensile performance of wood flour in the composite material is better
than that of 212μm and 300μm when the addition size of wood flour is 100μm.
Yu Wei of Shanghai Jiao Tong University et al.
used the in situ reaction composite method to study and prepare long-branched PP/wood flour composites
.
The results show that the long branching reaction improves the impact strength and tensile strength of the wood plastic materials, and the maximum amplitude reaches 29.
44% and 27.
75%,
respectively.
V.
N.
Hristov et al.
studied wood-plastic composites
prepared with PP-g-MAH as compatibilizer and wood flour as filler.
The results show that the tensile strength, elongation at break and impact strength of the materials increase to different degrees with the increase of modifier content
.

2.
Other materials are filled with PP matrix composites

Zhang Donghui, College of Engineering, Nanjing Agricultural University, et al.
used the molding process to prepare rice straw powder/PP wood-plastic composites
.
The results show that when the mass fraction of rice straw flour is 50% and the particle size is 60 mesh, the comprehensive mechanical properties of the composite material are better
.
Jin Cuixia, College of Chemistry and Chemical Engineering, Guizhou University, et al.
prepared wood fiber/PP composite microcellular foam materials
by chemical foaming injection molding.
The results show that the addition of wood fiber significantly improves the mechanical properties of the material, and the impact strength and flexural strength of the material are higher than those of the unfoamed material
.
Dang Wenjie of Northeast Forestry University and others used PP-G-MAH and SEBS-g-MAH to strengthen and toughen wood
fiber/PP.
The results show that the addition of PP-G-MAH and SEBS-g-MAH enhances the adhesion between the wood fiber and the PP matrix, so that the two are more tightly bonded, and then improves the mechanical properties
of the wood-plastic composite.
Adrian et al.
prepared PP/wood fiber composites, compared the effects of wood fiber treatment with MAH and direct addition of MAH grafted PP (MAPP) on material properties, and the results showed that both methods could improve the dispersion of wood fiber in the material, while the addition of MAPP also improved the creep resistance of the material
.

Gao Hua et al.
of Northeast Forestry University used MAH to graft and modify the PP/PE mixture, and then used the grafted blend as the matrix to composite with wood fibers to prepare wood-plastic composites
.
The results showed that the flexural strength and notchless impact strength of the composites increased greatly after grafting modification, and when the MAH dosage was 1%, the flexural strength increased by 50.
4%, the notchless impact strength increased by 90.
8%, and the flexural strength and notchless impact strength of the composites prepared from waste plastics increased by 40.
2% and 53.
4%,
respectively 。 Bledzki et al.
found that PP-based resins with high melt index help to improve the morphology and distribution of cells, and found that the length, geometry and content of wood fibers have a certain influence
on the size, shape and distribution of cells in composites.
The exothermic blowing agent helps PP/wood fiber composites to obtain good micro-cell structure
.
They also found that the hardwood fiber composite samples using MAPP had better performance than the softwood fiber composite samples, with tensile strength and impact strength improved by 50% and 20%
at the maximum.

3.
Conclusion

PP-based wood-plastic composite materials are green and environmentally friendly, both PP and plant fiber advantages, not only have excellent physical properties, good machinability, anti-corrosion and easy coloring and other advantages, but also wood perception, and can be like wood for cutting, cutting, drilling, nailing, planing and other secondary processing, but also recycling and reuse, biodegradation, conducive to environmental protection, good social and economic benefits, can be widely used in construction, automobiles, warehousing, transportation, agriculture, military and amusement facilities and other fields, In the continuous pursuit and yearning for new environmental protection materials, resource conservation, environmental protection today, showing strong vitality and development prospects
.
However, China's research started late, there are not many industrialized products, and the key issues that need to be studied in the future are: (1) Improve the interface properties of
wood-plastic composites.
Because plant fibers have strong polarity, and common resin matrices are usually non-polar and non-hydrophilic, the compatibility between plant fibers and resin matrices is very poor, and the interfacial bonding strength is low, which affects the mechanical properties
of PP wood-plastic composites.
(2) Improve the dispersion of plant fibers
in the matrix resin.
Since hydrogen bonds can be formed between hydroxyl groups, there is a strong interaction between plant fibers, which makes their dispersion in the resin matrix extremely poor, and it is difficult
to achieve uniform dispersion.
To achieve the above objectives, we should strengthen the research of interface, and at the same time strengthen the adjustment and optimization of cost performance, so that PP wood-plastic composites can develop
in the direction of lightweight, high performance, high added value, high fiber content and functionalization.