Study on wood powder-high filler modified recycled polypropylene material

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Research on the processing technologies and equipment for wood-plastic composites. Wood-plastic composites possess the properties of both wood and plastics. It has the appearance of natural wood and can be sawed, nailed, bonded, and painted. At the same time, it is insect-resistant and corrosion-resistant, has low water absorption, does not deform easily, does not crack, and possesses good mechanical properties; it is hard, strong, durable, wear-resistant, and has stable dimensions. Generally, the hardness of wood-plastic composites is 2 to 8 times higher than that of untreated wood, their wear resistance is 4 to 5 times higher, and it is even higher than that of marble. The use of various additives endows it with many special properties; for example, adding a foaming agent to the raw materials of wood-plastic composites during extrusion processes can reduce the density of the product while improving its toughness and impact resistance. Furthermore, it is an environmentally friendly material that can be recycled and reused; its raw materials are inexpensive and abundant. It offers significant benefits in terms of reducing environmental pollution, protecting forest resources, and promoting economic development, which is why it has attracted the attention of many researchers. Since the 1990s, extensive research on wood-plastic composites has been conducted in North America and Europe, and in recent years, research on this material has also increased steadily in China. At the same time, its production and application have also seen rapid growth; in North America and Europe, the consumption of wood-plastic composites reached 680,000 tons in 2002, and it is expected to grow at rates of 14% and 19% by 2010, far exceeding the overall growth rate of the plastics industry during the same period. 1 Development trends and molding methods of wood-plastic composites? Based on the further progress in research and application of wood-plastic composites, it can be predicted that their processing technologies will exhibit the following development trends: (1) Diversification of raw materials ； (2) Extremely high wood powder filling level ； (3) Specialization of equipment processes ； (4) Upgrading of products. The main molding methods used in its industrial production at present include: extrusion molding, injection molding, and thermo-pressing molding. Due to its short processing cycle, high efficiency, and simple molding process, extrusion is more widely used in industrial production compared to other processing methods. The extrusion molding of wood-plastic composites can be divided into one-step and multi-step methods. The one-step process involves the mixing, devolatilization of wood-plastic composites, and extrusion of the final product to be carried out continuously within one or a set of devices. The multi-step method involves carrying out the mixing, devolatilization, and extrusion of wood-plastic composites in different equipment; first, the raw materials are mixed to form intermediate wood-plastic pellets, which are then extruded into finished products. When the one-step method is used, poor devolatilization and dehydration will result in a significant reduction in the mechanical properties of the product due to the presence of bubbles ； At the same time, it requires that the structure of the product not be too complex. Therefore, in the actual processing, the one-step method is often limited by the high moisture content of the wood powder and the complex structure of the products. Currently, the multi-step method is primarily used in industrial production both domestically and internationally. The Plastic Machinery Research Institute at Beijing University of Chemical Technology has been conducting research in this area for many years, and several production lines have been put into industrial use. 2 Key Issues to Be Solved: The key technology in processing wood-plastic composites lies in ensuring that, even with a high content of wood powder, the material retains high fluidity and permeability. This allows the plastic melt to bond effectively with the wood powder, thereby achieving the desired mechanical properties and other functional characteristics of the composite. Ultimately, this enables the production of products with high performance at lower manufacturing costs. Therefore, three issues need to be addressed during the extrusion molding process: (1) Raw materials—how to improve the compatibility at the interface between plastic and wood powder ； (2) Equipment and processing techniques for product molding—how to maintain stable feeding, achieve effective devolatilization, improve the dispersion of wood powder within the system, set an appropriate extrusion temperature, and establish sufficient molding pressure to ensure the product’s performance ； (3) Design of molding dies and cooling setting technology. 3 Molding Process 3.1 Selection of Material Formulation 3.1.1 Selection of Polymers? The plastics used in the processing of wood-plastic composites can be either thermosetting plastics or thermoplastic plastics. Thermosetting plastics such as epoxy resins. Thermoplastic plastics such as polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC). However, due to the poor thermal stability of wood fibers, only thermoplastics with processing temperatures below 200°C are widely used, especially polyethylene. The selection of plastic polymers is primarily based on the inherent properties of the polymer, product requirements, availability of raw materials, cost, and the degree of familiarity with it. For example, polypropylene is mainly used in automotive products and daily household items, while polyvinyl chloride is primarily used in building doors and windows, as well as flooring materials. Furthermore, the melt flow rate (MFI) of plastics also has an impact on the properties of composite materials. Under the same processing conditions, a higher MFI of the resin results in better overall impregnation of the wood powder, as well as a more uniform distribution of the wood powder. The impregnation and distribution of the wood powder affect the mechanical properties of the composite materials, especially their impact strength. According to statistics, PE wood-plastic composites still dominate the market, accounting for about 65%. PVC wood-plastic composites account for about 16%, while PP wood-plastic composites account for about 14%. 3.1.2 Selection of additives: Due to the high water absorption and strong polarity of wood powder, while most thermoplastic plastics are non-polar and hydrophobic, the compatibility between the two is poor and the adhesion at the interface is weak. Therefore, appropriate additives are often used to modify the surfaces of the polymer and wood powder in order to enhance the interfacial affinity between wood powder and resin. Furthermore, wood flour with a high filler content disperses poorly in molten thermoplastics and often exists in an aggregated state, resulting in poor melt flow and difficulties in extrusion molding. Surface treatment agents must be added to improve flowability for easier extrusion molding. At the same time, various additives also need to be added to the plastic matrix in order to improve its processability and the performance of the final product, as well as to enhance the bonding between wood powder and the polymer and the mechanical properties of the composite material. 3.1.2.1 Coupling agents: Coupling agents enable strong interfacial bonding between plastics and the surface of wood powder ； At the same time, it can reduce the water absorption of wood powder and improve the compatibility and dispersion between wood powder and plastic, thereby significantly enhancing the mechanical properties of the composite material. Common coupling agents include: isocyanates, cumene peroxide, aluminates, phthalate esters, silane coupling agents, maleic anhydride-modified polypropylene (MAN-g-PP), and ethylene-acrylate (EAA). Generally, the amount of coupling agent added is 1wt% to 8wt% of the amount of wood powder added. For example, silane coupling agents can enhance the adhesion between plastic and wood powder, improve the dispersion of the wood powder, and reduce its water absorption, while alkaline treatment of the wood powder only improves its dispersion but does not affect its water absorption or its adhesion to plastic. It should be noted that the malate coupling agent reacts repulsively with the stearate lubricant; using them together results in a decrease in product quality and yield. 3.1.2.2 Plasticizers For some resins with high glass transition temperatures and high melt flow viscosities, such as rigid PVC, it is difficult to compound them with wood powder; therefore, plasticizers are often added to improve their processability. The molecular structure of plasticizers contains both polar and non-polar components; under high-temperature shear forces, they can penetrate into the polymer molecular chains. The polar components attract each other, resulting in a uniform and stable system. Meanwhile, the insertion of the longer non-polar molecules reduces the attraction between polymer molecules, thereby facilitating processing. Plasticizers that are often added to wood-plastic composites include dibutyl phthalate (DOS), among others. In PVC wood powder composites, the addition of the plasticizer DOP can reduce the processing temperature, minimize wood powder decomposition and smoking. As the content of DOP increases, the tensile strength of the composite decreases while its elongation at break increases. 3.1.2.3 Lubricants? Wood-plastic composites often require the addition of lubricants to improve the flowability of the melt and the surface quality of the extruded products. The lubricants used can be divided into internal lubricants and external lubricants. The selection of the internal lubricant is related to the base resin used; it must have good compatibility with the resin at high temperatures, and it should also exert a plasticizing effect by reducing the intermolecular cohesion energy and decreasing the friction between molecules, thereby lowering the melting viscosity of the resin and improving its melt flow properties. In plastic molding, external lubricants actually serve to lubricate the interface between the resin and wood flour; their main function is to facilitate the sliding of resin particles. Typically, a lubricant possesses both internal and external lubricating properties. Lubricants have a certain impact on the service life of molds, barrels, and screws, the production capacity of extruders, the energy consumption during the production process, the surface finish of the products, and the low-temperature impact resistance of the profiles. Commonly used lubricants include: zinc stearate, ethylene bisstearamide, polyester wax, stearic acid, lead stearate, polyethylene wax, paraffin wax, and oxidized polyethylene wax. The American company Struktol has introduced several new lubricants: TPW-012 for PVC wood powder composites, and TPW-101 for polyolefin wood powder composites. These lubricants can increase productivity by 20–25% compared to traditional zinc stearate/ethylene bisstearamide lubricants. The TPW-113 lubricant contains coupling agents that significantly improve processing properties ； Honeywell’s Optipak300 possesses both lubricant and coupling agent properties, enabling it to improve product performance and increase production volume for PE and PP wood-plastic composites. 3.1.2.4 Colorants: During the use of wood-plastic composites, soluble substances in wood powder tend to migrate to the surface of the product, causing it to lose its color and eventually turn gray. Under certain usage conditions, different products may also develop black spots or rust spots. Therefore, colorants are also widely used in the production of wood-plastic composites. It enables the product to have a uniform and stable color, with slow fading. The American company Americhem has industrialized the production of various colorants and continues to improve them. 3.1.2.5 UV Stabilizers The use of UV stabilizers has also seen rapid development as demands for the quality and durability of wood-plastic composites increase. It prevents the polymer in composite materials from degrading or having its mechanical properties decline. Commonly used ones include hindered amine light stabilizers and UV absorbers. 3.1.2.6 Antimicrobials: To ensure that composite materials maintain a good appearance and optimal performance, it is often necessary to add antimicrobials. The selection of a bacteriostatic agent takes into account various factors such as the type of wood powder, the amount added, the fungi present in the environment in which the composite material is used, and the moisture content of the product. For example, zinc borate can prevent corrosion but not algae growth. 3.1.2.7 Foaming agents: Wood-plastic composites possess many advantages, but the combination of resin and wood flour reduces their ductility and impact resistance; the material is brittle, and its density is nearly twice that of traditional wooden products, which limits its widespread use. The foamed wood-plastic composite, thanks to its favorable pore structure, can blunt crack tips and effectively prevent crack propagation, thereby significantly improving the material’s impact resistance and ductility, while also **reducing the density of the finished product**. There are many types of foaming agents, and the two commonly used chemical foaming agents are: endothermic foaming agents (such as sodium bicarbonate NaHCO3). ) and exothermic blowing agents (azodicarbonamide AC) have different thermal decomposition behaviors, which exert distinct effects on the viscoelasticity of the polymer melt and the foam morphology. An appropriate blowing agent should be selected based on the usage requirements of the product. For example, AC can be chosen as a blowing agent for PVC/wood flour foaming, with a usage ratio of 0.5wt%-1wt%. Reedy International has newly developed a blowing agent specifically designed for wood-plastic composites of the SAFETC TFPE-504 grade; it features a high gas generation rate and low usage amount, thereby reducing production costs. 3.2 Setting of process parameters 3.2.1 Screw speed From the theoretical formulas for solid transport and viscous fluid transport, it can be seen that the speed is proportional to the production capacity. Therefore, increasing the rotational speed can effectively boost production capacity, but there are many limitations on raising the screw speed during the extrusion processing of wood-plastic composites. For example, in PVC wood flour composites, both PVC and wood flour are thermosensitive; too high a screw speed can lead to the degradation and gelation of the material. At the same time, the screw speed also affects the residence time of the material and the extrusion pressure. Only by meeting the requirements regarding the extrusion temperature, shear strength, mixing quality, and extruder power of the material can the rotation speed be increased to the maximum extent in order to boost productivity. 3.2.2 Extruder temperature and pressure During the extrusion processing of wood-plastic composites, the control of the extruder’s temperature and pressure is also very important. If the extrusion temperature is too high, the material tends to degrade; moreover, such high temperatures result in lower viscosity of the melt, leading to insufficient extrusion pressure. This in turn causes the surface of the product to be rough and its strength to be low, affecting the quality of the extruded product. Too low a temperature results in poor plasticization of the plastic, preventing it from fully enveloping the wood powder, which in turn affects the strength of the product. At the same time, melt fracture is sensitive to die temperature; both excessively high and low die temperatures can cause melt fracture. Appropriately reducing the temperature of the extruder, increasing the die pressure, and lowering the screw speed can effectively improve the processing properties of wood-plastic composite systems. The temperature settings for each stage during the actual processing are as follows: Stage I: 150–170℃ ； Phase II: 160～190℃ ； Stage III: 170～195℃ ； Section IV: 180～195℃ ； Nozzle die section: 180～205℃. The temperature in each section should be as stable as possible, and the total residence time should be less than 15 minutes. 4 Molding Equipment 4.1 Issues to Be Addressed in Molding Equipment 4.1.1 Feeding Problems Since most of the wood fibers added are in powder form, and wood powder has a porous structure that makes it difficult to feed it into the extruder screw, moreover, an ideal mixture cannot be formed between the plastic matrix and the filler materials, which prevents them from being uniformly added to the extruder. As a result, phenomena such as \"bridging\" and \"clumping\" often occur during the feeding process. This phenomenon is even more apparent, especially when the wood powder contains a high amount of moisture. The instability of the feed not only directly leads to low extrusion output but also causes fluctuations in extrusion, resulting in reduced extrusion quality. At the same time, due to the interruption in feeding, the residence time of the material inside the barrel is prolonged, which causes the material to burn and change color, affecting the internal quality and appearance of the products. Therefore, strict control over the feeding method and amount is necessary; forced feeding devices and starvation feeding are generally used to ensure stable extrusion. 4.1.2 Exhaust issues: Since wood powder contains a large amount of small-molecule volatile substances and moisture, which can easily cause defects in the final products, pre-treatment cannot eliminate them completely. Therefore, more attention must be paid to the exhaust system design of wood-plastic composite extruders compared to those of conventional plastic extruders. To a large extent, the better the exhaust performance, the better the quality of the extruded products; multi-stage exhaust systems can be employed if necessary. 4.1.3 Screw structure: During the extrusion of wood-plastic composites, the screw structure has a significant impact on product quality. A proper screw design can reduce friction between the screw and wood fibers, achieving adequate shear and dispersion effects while preventing poor fiber dispersion or fiber damage. For example, reducing the diameter of the screw in the metering section helps improve melt flow, and shortening the residence time in this section minimizes material degradation. Compared to conventional screws, screw designs with pins help prevent the accumulation of wood powder and enhance dispersion and mixing. 4.2 Main extrusion equipment Currently, the main equipment available for the extrusion molding of wood-plastic composites includes single-screw extruders, tapered twin-screw extruders, and parallel twin-screw extruders. 4.2.1 Single-screw extruder A single-screw extruder can carry out the tasks of transporting and plasticizing materials. However, the conveying action of single-screw extruders relies primarily on friction. Due to the fluffy structure of wood powder, it is difficult to feed it into the extruder screw; as a result, the material stays in the barrel for a longer time. Moreover, the presence of wood powder increases the viscosity of the polymer melt, thereby increasing the difficulty of extrusion ； At the same time, its exhaust performance is poor ； Its mixing and plasticizing capacity is also weak. Therefore, single-screw extruders face significant limitations in the extrusion of wood-plastic composites; the single-screw extruders used must have screws with a special design, offering strong capabilities for material transport and mixing/softening. The materials are often mixed and granulated prior to extrusion. 4.2.2 Twin-screw extruders: Twin-screw extruders transport materials based on the principle of positive displacement; there is no pressure backflow, making feeding easier ； It provides good exhaust performance, enabling the complete removal of volatile components from wood powder ； The screws mesh with each other, and the intense shear force results in better mixing and plasticization of the material ； The residence time of the material is short, so wood powder will not burn. Therefore, the main processing equipment for wood-plastic composites at present is the twin-screw extruder, which can be divided into parallel twin-screw extruders and conical twin-screw extruders. 4.2.2.1 Parallel twin-screw extruder The parallel twin-screw extruder can directly process wood powder or plant fibers. Therefore, it is possible to complete the drying of the wood powder before mixing it separately with the molten resin. Another type of parallel twin-screw extruder can also be used; its front section serves as a dehydration and devolatilization unit. Wood powder is fed into the main feed port of the extruder where it undergoes dehydration and devolatilization, after which plastic resin and additives are added through a side feeder for plasticization and extrusion. Therefore, the extruder is relatively long, with a screw length-to-diameter ratio (L/D) of 44–48, of which 2/3 is used for water removal and devolatilization. This melting processing step results in a lower melting temperature, avoiding the risk of sintering of the molten material. It also helps to ensure the uniformity of the wood chip mixture in the next step and to control the ratio of wood chips to plastic. However, there are certain requirements regarding the moisture content of wood powder. The heterophasic twin-screw extruder produced by Dawia-Standard has an L/D ratio of 28:1, offering a high pressure building capacity at the die head, and it combines the capabilities for processing wood chips as well as plastic extrusion. There are currently three screw diameter options for Woodtruder available on the market, namely 94mm, 114mm, and 140mm. The maximum WPC production rate of the Woodtruder with a screw diameter of 140 mm is 900 Kg/h. NFM Company manufactures co-rotating twin-screw extrusion systems that offer high torque and screw speeds, along with a wide processing range. These systems can process plastics such as PVC, PE, and PP, as well as composites made from various wood powders, without the need for prior drying or mixing of the materials. This reduces equipment costs and the space required for processing. They can be used for direct extrusion or granulation, and the product diameters range from 26mm to 240mm. 4.2.2.2 Tapered twin-screw extruders: Compared with “mixing” type equipment, counter-rotating tapered twin-screw extruders are considered low-speed, low-energy consumption “profile” type equipment. The feeding section of its tapered screw has a large diameter, allowing for continuous compression of the material. It can reduce the residence time of the material inside the barrel, and the small diameter of the metering section results in less shear on the molten material, which is a significant advantage when processing thermosensitive wood-plastic composites. Compared to ordinary conical twin-screw extruders, to meet the requirements of processing thermosensitive resins, there are many new features and demands: the screw must be able to operate over a wide range of processing conditions, cause minimal cutting of wood fibers, and ensure that wood fibers remain evenly dispersed as well as complete melting of the material even when there is a low amount of resin present. Due to the low specific gravity and high volume of wood powder and plant fibers, the feeding area is larger in volume and longer than that of conventional models. When a large amount of wood powder or plant fibers is used, the molten resin becomes highly rigid; therefore, a gearbox capable of withstanding high back pressure is required, as well as a screw with strong pushing force. Screws that enable rapid compression and melting and have a short metering section are used to ensure that the wood fibers remain there for only a short time, thereby preventing them from breaking or having their properties degraded. However, because its screw is of a monolithic design, it is difficult to meet the requirements of different processing formulations, and its mixing efficiency is inferior to that of parallel twin-screw extruders. The main companies abroad that produce such equipment are C Incinatti Milacron and DURA. Among them, Extrusion Tek Milacron in the United States has actually deployed over 100 WPC extrusion machines, while C Incinatti Extruders has publicly displayed its series of tapered twin-screw extruders \"Titan\" for WPC applications. In addition to the aforementioned types of twin-screw extruders, the American company Battenfeld produces planetary roller twin-screw extruders that utilize metered feeding, offer precise temperature control, and provide excellent mixing capabilities, making them highly suitable for processing thermosensitive composite materials. 4.3 Extruder head and cooling setting system The extruder head is a crucial component that affects the quality of the extruded products. Due to the special properties of wood-plastic composites and the high content of wood powder, the extruded material has poor flowability and is difficult to cool; conventional molds and setting equipment are no longer sufficient to meet the requirements of such products. As a result, the design of the extruder head must not only ensure smooth transitions in the flow channels and proper distribution of flow rates, but also take into account the head’s pressure generation capacity and temperature control accuracy. Due to the poor flowability of wood-plastic composites, molds should avoid using structures such as flow-blocking elements; instead, the flow rate at various cross-sections of the flow channel should be adjusted by changing the dimensions of those channels ； When the mold strength is sufficient, minimize the number and size of the support ribs ； A higher compression ratio is also used to ensure a greater extrusion pressure, which facilitates molding. Given the thermal sensitivity of wood-plastic composites, the mold should have larger structural dimensions to increase heat capacity, thereby enhancing the temperature stability of the entire die head ； The dimension in the extrusion direction takes the smaller value, in order to reduce the residence time of the material inside the die head ； The heating and cooling devices of the machine head are also arranged appropriately, ensuring fast heating and cooling speeds as well as high precision. The UK has developed easily flowable die designs that eliminate the need for porous plates; they also allow for cooling of the inner surface of hollow profiles, reducing melt pressure and increasing production volume. At the same time, the degree of fiber orientation in wood-plastic composites has a significant impact on the properties of the resulting products; it is therefore necessary to design the flow channel structure appropriately in order to achieve the desired fiber orientation that meets the performance requirements of the products. Furthermore, under the same strength requirements, wood-plastic composite materials require a greater thickness than pure plastics, and they are often in the form of profiles with complex structures, which makes cooling them more difficult. Water cooling is generally used; for products with larger cross-sections or more complex structures, special cooling devices and methods are required. Located in Pittsburgh, Pennsylvania, Conair has developed high-strength spray cooling tanks designed specifically for wood fiber composites. By combining evaporation and cooling, these tanks achieve a higher cooling efficiency, enabling effective cooling during the production of large and heavy materials. 5 Conclusion: Wood-plastic composites have excellent properties and are developing rapidly. In its industrial production, multi-step extrusion processing is widely used. However, the addition of a high volume of wood flour makes extrusion processing difficult: on one hand, the poor interfacial compatibility between the plastic matrix and the wood flour, along with the low fluidity of the molten material, requires the use of coupling agents and lubricants during the processing of wood-plastic composites. Additionally, the increasing demands for certain properties of the products in practical applications necessitate the use of other appropriate additives during the processing of these composites ； On the other hand, wood powder has a porous structure and often contains a high amount of moisture, which makes it difficult to feed it in and to disperse and mix it with other materials. Additionally, the material tends to degrade easily, which imposes various special requirements on the setting of process parameters such as extrusion temperature and screw speed, as well as on aspects like the feeding system, exhaust system, screw design, and die structure of the extruder. Therefore, in the processing of wood-plastic composites, to obtain high-quality products it is necessary to select plastic resins and various additives appropriately, set the process parameters correctly, and use suitable processing equipment.