Plant fiber-polymer composites and methods of making

Abstract

The present application belongs to the technical field of biomass composite material processing, and discloses a plant fiber-polymer composite material and a preparation method. The plant fiber-polymer composite material is prepared by plasticizing and extruding molding of raw materials containing the following components: 50-88 parts of plant fiber, 0.5-8 parts of biomass crosslinking agent, 0.5-5 parts of low-temperature water-soluble polymer, 10-50 parts of water, and 0.05-3 parts of metal ions; wherein the metal ions and the biomass crosslinking agent form a reversible crosslinking network in the system through coordination; the existence of the reversible crosslinking network enables the composite material to be plasticized and continuously extruded at a temperature of 20-70 DEG C through shearing. The metal ion-biomass crosslinking agent coordination network in the present application provides high structural strength under static conditions and reversibly dissociates under a shearing field, thereby realizing the shearing-driven plasticization of a high-viscosity, high-fiber-content system in a low-temperature interval of 20-70 DEG C.

Description

Technical Field

[0001] This invention relates to the field of biomass composite material processing technology, specifically to plant fiber-polymer composite materials and their preparation methods. Background Technology

[0002] Current processing of plant fiber-polymer composites primarily employs heat-driven plasticization, requiring the system to be heated to the polymer's melting temperature or above its glass transition temperature to achieve fluidity. Even with the introduction of low-temperature soluble PVA or gelatin plasticizers, high processing temperatures are still necessary to ensure continuous extrusion and molding stability, resulting in high energy consumption. Furthermore, this process easily triggers thermal degradation and browning of cellulose, hemicellulose, and proteins, thereby weakening the material's mechanical strength and dimensional stability. On the other hand, plant fibers themselves are rigid, have strong surface polarity, and poor interfacial compatibility. At low temperatures, the system exhibits high viscosity and poor fluidity, making continuous extrusion difficult. Existing technologies typically improve rheological behavior by increasing temperature, adding plasticizers, or optimizing screw parameters, but these methods still primarily rely on polymer self-plasticization, offering limited benefits for interfacial stress transfer and the construction of dense structures, making it difficult to establish a stable processing window at low temperatures.

[0003] While existing studies have reported coordination / complexation of metal ions with biomass macromolecules such as gelatin, chitosan, and polysaccharides, most of these studies have focused on gels, pharmaceutical hydrogels, or tissue engineering materials, with an emphasis on biocompatibility and gel strength. However, they have not used this type of coordination to regulate the processing rheological behavior and extrusion stability of composite materials.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] This invention aims to overcome the shortcomings of the prior art and provide a novel plant fiber-polymer composite material and its preparation method. Based on the synergistic mechanism of "metal ion-induced coordination crosslinking" and "shear-driven reversible plasticization," this method achieves active and precise control of the rheological behavior of the composite material within a range significantly lower than the traditional melting temperature. This simultaneously achieves multiple objectives: energy saving during low-temperature processing, protection of fiber integrity, high-filling continuous extrusion, and improved interfacial bonding and mechanical properties.

[0006] To achieve the above objectives, the first technical solution adopted by the present invention is as follows:

[0007] Plant fiber-polymer composite material is obtained by plasticizing and extruding raw materials containing the following components, wherein the components are in parts by mass: 50-88 parts plant fiber, 0.5-8 parts biomass crosslinking agent, 0.5-5 parts low-temperature water-soluble polymer, 10-50 parts water, and 0.05-3 parts metal ions.

[0008] In this system, metal ions and biomass crosslinking agents form a reversible crosslinking network within the system through coordination. The presence of this reversible crosslinking network enables the composite material to be plasticized and continuously extruded at a temperature of 20~70°C through shearing.

[0009] Preferably, the biomass crosslinking agent is a biomass raw material containing functional groups that can undergo coordination / complexation reactions with metal ions;

[0010] The biomass cross-linking agent comprises at least one of collagen, gelatin, keratin, silk fibroin, soy protein, casein, zein, starch, alginate, and chitosan.

[0011] Preferably, the metal ions include Ca²⁺, Mg²⁺, Fe³⁺, Al³⁺, Zn²⁺, and Ti. 4 At least one of the following: ⁺

[0012] Preferably, the low-temperature water-soluble polymer is polyvinyl alcohol, with a degree of polymerization of 500-1200 and a degree of alcoholysis of 80%-88%.

[0013] The second technical solution adopted in this invention is:

[0014] Methods for preparing plant fiber-polymer composite materials include:

[0015] Plant fibers, biomass crosslinking agents, low-temperature water-soluble polymers, water, and a metal ion source that provides metal ions are mixed to form a mixture containing a reversible crosslinking network precursor.

[0016] At 20~70℃, a shearing force is applied to the mixture, and the reversible cross-linked network is reversibly dissociated by the shear force, thereby giving the mixture fluidity and plasticizing it.

[0017] The plasticized material is extruded into shape.

[0018] Preferably, prior to the mixing, the plant fibers are further subjected to a pretreatment, which includes at least one of drying, particle size control, and surface modification treatment, so as to control the moisture content of the plant fibers to below 12 wt% and enhance their surface reactivity.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] This invention abandons traditional plasticizing methods that rely on high temperatures to melt polymers or strong chemical reagents to degrade fibers. Instead, it creatively utilizes the unique rheological properties of a metal ion-biomass crosslinking agent coordination network. This network provides high structural strength under static conditions and reversibly dissociates under a shear field, thereby achieving shear-driven plasticizing of high-viscosity, high-fiber-content systems within a low-temperature range of 20–70°C. This fundamentally avoids thermal degradation damage to plant fibers caused by high temperatures and eliminates the damage to components and complex post-processing problems caused by strong chemical environments. Thanks to the supporting and regulating effect of the reversible network, the method of this invention can achieve stable and continuous extrusion of high-fiber-content composite materials at temperatures far below those of traditional melt processing. Processing energy consumption is significantly reduced, process stability is good, and it is easy to scale up production on conventional twin-screw extruders. Detailed Implementation

[0021] The technical solutions of the present invention will be clearly and completely described below with reference to embodiments and comparative examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] The first embodiment of the present invention provides a plant fiber-polymer composite material, which is obtained by plasticizing and extruding a raw material containing the following components, wherein the amount of the components is in parts by mass: 50-88 parts of plant fiber, 0.5-8 parts of biomass crosslinking agent, 0.5-5 parts of low-temperature water-soluble polymer, 10-50 parts of water, and 0.05-3 parts of metal ions.

[0023] In this system, metal ions and biomass crosslinking agents form a reversible crosslinking network within the system through coordination. The presence of this reversible crosslinking network enables the composite material to be plasticized and continuously extruded at a temperature of 20~70°C through shearing.

[0024] The plant fibers form the skeleton and reinforcing phase of the product, and their high content is fundamental to achieving high rigidity, low cost, and high bio-based content. Suitable fibers include wood fibers (such as wood flour and bamboo flour), agricultural waste fibers (such as wheat straw, rice husks, and distiller's grains), and bast fibers (such as hemp and flax). To optimize the final product performance, the fibers can undergo conventional pretreatment, such as drying (controlling moisture content <12%), pulverizing (target particle size 50~500 μm), and alkali treatment to improve surface activity.

[0025] The biomass cross-linking agent is one of the key reactive components in constructing a reversible cross-linked network. It must contain functional groups capable of coordinating / complexing with metal ions, such as hydroxyl, carboxyl, amide, amino, and phenolic hydroxyl groups. Examples include proteins / peptides (such as collagen, gelatin, keratin, silk fibroin, soy protein, casein, zein, etc.) and polysaccharides (starch, alginate, and chitosan, etc.). Gelatin is a preferred embodiment due to its rich content of such groups, wide availability, and controllable cost. Chitosan and sodium alginate are also effective alternatives or blending options.

[0026] In this invention, the core function of the "low-temperature water-soluble polymer" is that it can dissolve or fully swell in the aqueous phase within a processing temperature range of 20~70°C, providing the necessary initial viscosity, adhesion, and film-forming properties to the composite system, thereby assisting in encapsulating plant fibers, stabilizing the mixture, and participating in the formation of a continuous phase. Preferably, low degree of polymerization (500~1200), partially alcoholyzed (80%~88%) polyvinyl alcohol (PVA) is used, such as PVA0588 or PVA1788. This type of PVA provides suitable viscosity and film-forming properties at the processing temperature, helping to encapsulate fibers and stabilize the mixture, while its lower dosage reflects the reduced reliance on traditional polymers in this product.

[0027] In addition to polyvinyl alcohol, theoretically, other water-soluble polymers that meet the following conditions may also be considered: (1) The introduction of the polymer shall not substantially destroy or weaken the dominant reversible crosslinking network composed of metal ions and biomass crosslinking agents and its shear reversibility. That is, the interaction strength between the polymer and metal ions should be weaker than or compatible with the coordination of biomass crosslinking agents and metal ions to avoid strong competitive chelation of metal ions, which would lead to irreversibility of the network or imbalance of recombination kinetics. (2) It must have good solubility or rapid swelling ability in the aqueous environment under the aforementioned low-temperature processing conditions (20~70℃) to ensure the uniformity and operability of the system in the early stage of processing. (3) Its main role is as a processing aid and auxiliary film-forming / adhesive, rather than a major contributor to the strength of the system.

[0028] Based on the above principles, the following types of polymers may be used as alternatives or blended with PVA: polyvinylpyrrolidone (PVP), polyacrylic acid (PAA) or copolymers thereof, polyacrylamide (PAM) derivatives, and water-soluble cellulose derivatives.

[0029] The water is an essential processing medium that dissolves the biomass crosslinking agent and PVA, ensures uniform dispersion of the reactants and ions, and provides the necessary plasticizing effect for the entire system at low temperatures. The specific amount of water used should be adjusted to ensure the system has the rheological window required for continuous shear plasticization at low temperatures, and can be adapted by those skilled in the art to achieve the desired dispersibility and plasticizing effect. Preferably, it is 10-50 parts, more preferably 20-40 parts, and even more preferably 30 parts.

[0030] The metal ions are another key reactive component in constructing the reversible cross-linked network. They include Ca²⁺, Mg²⁺, Fe³⁺, Al³⁺, Zn²⁺, and Ti. 4 At least one of the following: ⁺. They are usually added in the form of water-soluble salts (such as AlCl3, CaCl2). A synergistic threshold effect exists between the concentration ratio of metal ions and biomass crosslinking agents, a key discovery for achieving product characteristics. When the concentrations of both reach a certain range (e.g., gelatin ≥ 2 parts, Al³⁺ ≥ 0.5 parts), the mixture exhibits a significant viscosity surge under static conditions, increasing from below 2000 mPa·s to above 3000 mPa·s, indicating the formation of an effective network.

[0031] In this embodiment of the invention, the preferred mass of the metal ions is 0.05 to 3 parts, which covers the effective dosage range of various metal ions in forming a reversible coordination network. Different metal ions have different optimal dosages due to differences in their charge number, ionic radius, coordination ability, and interaction strength with specific biomass crosslinking agents. For example, when gelatin is used as the biomass crosslinking agent and Al³⁺ as the metal ion, experiments show that the suitable dosage is 2.0 to 2.5 parts, at which point the system exhibits significant viscosity transitions, a continuous network structure, and optimal material mechanical properties. For other metal ions (such as Ca²⁺, Mg²⁺, etc.), their effective dosage may fall within different ranges of the above-mentioned range. Those skilled in the art can determine the specific suitable dosage within this general range based on the type of metal ion and biomass crosslinking agent selected, through conventional rheological testing and performance evaluation.

[0032] When the system contains a certain concentration of biomass crosslinking agent and metal ions, the metal ions can act as crosslinking points, coordinating with multiple coordinating functional groups on different biomass macromolecular chains. This bonding is not a permanent covalent bond, but a dynamic and reversible physical crosslinking based on ion-dipole interactions or coordination bonds. As the density of crosslinking points increases, the originally independent biomass molecular chains are bridged by metal ions, forming a three-dimensional network structure that runs through the entire composite system. The reversibility of this network is reflected in the specific binding energy and kinetic characteristics of its crosslinking bonds (i.e., metal ion-ligand coordination bonds). Under the action of external energy (especially mechanical shear energy), these coordination bonds can dissociate; when the external force is removed or weakened, they can reform within a certain period of time. This dynamic equilibrium characteristic means that the network structure is not rigid and fixed, but can undergo a reversible sol-gel transition in response to external shear stimuli. Under static or low-shear conditions, this three-dimensional network imparts extremely high structural viscosity and yield stress to the mixture, enabling it to encapsulate and support high-content plant fibers, preventing fiber sedimentation and phase separation, and providing the necessary shape retention for subsequent extrusion. This is fundamental to achieving high filler uniformity and extruded preform integrity.

[0033] The plant fiber-polymer composite material provided by the embodiments of the present invention can be plasticized by shearing at 20~70℃, which is directly derived from the unique rheological behavior of the above-mentioned reversible cross-linked network. The realization process can be divided into three stages: (1) Shear-triggered network dissociation: When a sufficiently strong shearing force is applied to the mixture containing the network (such as in a twin-screw extruder), the shear force, as an energy input, preferentially acts on the relatively weak dynamic coordination bonds in the network. These bonds undergo directional and reversible breakage under the action of shear force. With the temporary failure of a large number of cross-linking points, the three-dimensional network structure that runs through the entire system is destroyed and dissociates into smaller clusters or independent molecular chains. Macroscopically, this microscopic change is manifested as a sharp decrease in the apparent viscosity of the mixture (i.e., significant "shear thinning" behavior), and the material rapidly transforms from a high-viscosity, almost non-flowable state into a viscoelastic fluid with good flowability. This process can occur effectively at low temperatures of 20~70℃ because it is driven by mechanical shear energy rather than thermal energy. (2) Necessity of low temperature environment: Maintaining low-temperature processing conditions of 20~70℃ is crucial to the present invention. First, this temperature range is far below the starting temperature of thermal degradation of plant fibers, perfectly protecting the integrity of the fibers. Second, this temperature range is precisely the key to maintaining the appropriate dissociation-reorganization equilibrium of the dynamic coordination bonds. If the temperature is too low, the molecular chain segment mobility is too weak, and the network dissociation and reconstruction are too slow, which is not conducive to efficient plasticization; if the temperature is too high, it may cause water to evaporate too quickly, change the properties of coordination bonds (such as forming more stable complexes) or trigger other side reactions, destroying the dynamic reversibility of the network, which is not conducive to shear plasticization control. (3) Dynamic equilibrium and continuous extrusion in shear field: In a continuous shear field, the dissociation and reorganization of the network are in a dynamic equilibrium. In the region with the highest shear intensity, dissociation is dominant and the material flows; when the material flows to the region with slightly weaker shear, the reorganization process begins to appear. This dynamic equilibrium enables the material to achieve stable conveying, homogenization and flow in the extruder, thereby ensuring the feasibility of continuous extrusion.

[0034] The second embodiment of the present invention provides a method for preparing plant fiber-polymer composite materials, comprising:

[0035] Plant fibers, biomass crosslinking agents, low-temperature water-soluble polymers, water, and a metal ion source that provides metal ions are mixed to form a mixture containing a reversible crosslinking network precursor.

[0036] At 20~70℃, a shearing force is applied to the mixture, and the reversible cross-linked network is reversibly dissociated by the shear force, thereby giving the mixture fluidity and plasticizing it.

[0037] The plasticized material is extruded into shape.

[0038] The embodiments of the present invention can achieve plasticization at temperatures far below conventional melting temperatures. The principle is based on the unique rheological behavior of the formed reversible cross-linked network under shear force: shearing causes reversible dissociation of coordination bonds, the network temporarily melts, thereby giving the system fluidity; when shearing stops or weakens, the network tends to rebuild.

[0039] The mixture is sheared in an apparatus with shear mixing capabilities, most commonly and preferably a twin-screw extruder. The barrel temperature of the extruder's plasticizing section (or mixing section) is set and maintained within the range of 20°C to 70°C, preferably 30-50°C. This low temperature is a key feature distinguishing this invention from existing high-temperature processing (e.g., >140°C), aiming to maintain a suitable kinetic temperature for the coordination reaction while avoiding thermal degradation of plant fibers. The material is subjected to strong shearing under the conveying and kneading action of the screw. This shearing action (typically corresponding to the shear rate generated by the screw rotation speed) must be strong enough to overcome the energy barriers of the ionic crosslinking points, causing reversible dissociation of the network. Macroscopically, the material gradually transforms from an initial high-viscosity agglomerate or granular state into a uniform, continuous, and well-flowing viscoelastic fluid. This process is the essence of plasticizing in this invention—it does not rely on the thermal melting of polymer chains, but rather on the shear-induced rheology of the ionic network.

[0040] The plasticized viscoelastic fluid is continuously extruded into profiles (such as sheets, rods, and other profiles). After extrusion, the shear force is significantly reduced or disappears. At this point, because the temperature is still relatively low and the components in the system have not undergone chemical changes, the metal ion-biomass coordination bonds that were previously broken by shear force have the opportunity to recombine, i.e., the reverse crosslinking network is reconstructed. This reconstruction process is usually accelerated and completed in subsequent cooling and shaping (such as through cooling rollers, air cooling, or water cooling tanks, at temperatures of 10~30℃). The reconstructed network forms a three-dimensional support structure inside the composite material and tightly binds the plant fibers and polymer matrix, resulting in a composite material with high density, high strength, and excellent dimensional stability.

[0041] During the extrusion process, the extrusion parameters can be adjusted according to the composition, such as an extruder barrel speed of 60 rpm and a barrel temperature of 30~50°C.

[0042] Depending on the final application requirements, the extruded profiles can be further processed, such as heat treatment at 80~120℃ to eliminate internal stress and improve dimensional stability, or surface coating, embossing, and other decorative and functional treatments. These are all well-known post-processing technologies in this field.

[0043] The following examples illustrate the plant fiber-polymer composite material, its preparation method, and its properties in detail.

[0044] Example 1: Preparation of plant fiber-polymer composite materials

[0045] Plant fibers were dried to a moisture content of 5–12 wt% and a particle size of 50–500 μm. To improve hydrophilicity and interfacial bonding, the fibers were treated with NaOH (0.5–6 wt%) solution or bleached with H₂O₂ (0.5–5 wt%) at a solid-liquid ratio of 1:5–1:20 and reacted at 20–80°C for 0.5–6 h. After washing to neutrality, the fibers were dried to a moisture content of 3–10 wt%. After pretreatment, the hydroxyl groups on the fiber surface were exposed and the roughness increased, which facilitated the penetration and adsorption of water-soluble polymers and crosslinking agents.

[0046] Mix gelatin aqueous solution with PVA aqueous solution, with water content of 10-50 parts, stir and mix at 25-50°C for 1-30 minutes, add AlCl3 aqueous solution to induce preliminary complexation between gelatin molecules, improve the viscosity and structural stability of the system, and control the system to maintain plastic dispersion rather than gel solidification;

[0047] Pretreated plant fibers are added to the above solution system and sheared and mixed using a twin-screw mixer at 20-70°C for 3-10 min at a shear rate of 100-1000 s⁻¹. Under the action of the shear field, the metal ion micro-network undergoes reversible dissociation-reconstruction, and the system changes from a solid particle agglomeration structure to a plastic viscoelastic dynamic, thereby realizing continuous extrusion and molding;

[0048] The extruded profiles are cooled and shaped by cooling rollers, air cooling or water cooling to form plant fiber-polymer composite products with dense structure and high interfacial bonding strength.

[0049] Example 2: Investigating the concentration threshold and viscosity abrupt change behavior of gelatin and Al³⁺

[0050] Referring to Example 1, this example uses distiller's grains fiber as raw material. The low-temperature water-soluble polymer used is 5 parts polyvinyl alcohol (degree of polymerization 800, degree of hydrolysis 85%), 5 parts gelatin, and 30 parts water, with other conditions kept constant. Six sets of experiments were designed according to Table 1, varying the amount of AlCl3 (calculated as Al³⁺). The apparent viscosity (η) of each mixture at 25°C and shear for 10 min was measured using a rotational rheometer. The results are shown in Table 1.

[0051] Table 1. Rheological and mechanical properties under different formulation ratios

[0052] .

[0053] Table 1 shows that when the Al³⁺ content is less than 2.0 parts, the system viscosity increases slowly with the increase of crosslinking agent content; when the Al³⁺ content reaches 2.0~2.5 parts, the system viscosity shows a significant jump, indicating that the metal ion-gelatin system forms a continuous complex network structure; when the Al³⁺ content further increases, the system viscosity continues to rise but the processing fluidity decreases, and local embrittlement occurs on the material cross-section. Among them, the Al³⁺ content of 2.5 parts shows the best overall performance, and the resulting material has a dense and continuous cross-sectional structure, with the bending strength and bending modulus reaching their maximum values.

[0054] Examples 3-5: Examples of different low-temperature water-soluble polymer systems

[0055] Referring to the preparation method of Example 1, the raw materials and dosages, and preparation process parameters in Examples 3-5 are shown in Table 2. The apparent viscosity, flexural strength, and flexural modulus of the obtained materials were measured during the shearing process in Examples 3-5, and the cross-sectional conditions of each sample were observed. The results are shown in Table 3.

[0056] Comparative Examples 1-3

[0057] Referring to the preparation method of Example 1, some raw materials or processes were modified to create a comparison with the example. The raw materials and amounts, and preparation process parameters of each comparative example are shown in Table 2. The apparent viscosity, flexural strength, and flexural modulus of the obtained materials during the shearing process of comparative examples 1 to 3 were measured, and the cross-sectional condition of each sample was observed. The results are shown in Table 3.

[0058] Table 2

[0059] .

[0060] Table 3

[0061] .

[0062] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A plant fiber-polymer composite material, characterized in that, It is prepared by plasticizing and extruding raw materials containing the following components, wherein the components are in parts by weight: 50-88 parts of plant fiber, 0.5-8 parts of biomass crosslinking agent, 0.5-5 parts of low-temperature water-soluble polymer, 10-50 parts of water, and 0.05-3 parts of metal ions. In this system, metal ions and biomass crosslinking agents form a reversible crosslinking network within the system through coordination. The presence of this reversible crosslinking network enables the composite material to be plasticized and continuously extruded at a temperature of 20~70°C through shearing. The biomass cross-linking agent comprises at least one of collagen, gelatin, keratin, silk fibroin, soy protein, casein, zein, starch, alginate, and chitosan. The metal ions include Ca. 2+ Mg 2+ Fe 3+ Al 3+ Zn 2+ Ti 4+ At least one of them; The low-temperature water-soluble polymer comprises at least one of polyvinyl alcohol, polyvinylpyrrolidone, polyacrylate, and polyacrylamide.

2. A method for preparing plant fiber-polymer composite materials, characterized in that, include: Plant fibers, biomass crosslinking agents, low-temperature water-soluble polymers, water, and a metal ion source that provides metal ions are mixed to form a mixture containing a reversible crosslinking network precursor. At 20~70℃, a shearing force is applied to the mixture, and the reversible cross-linked network is reversibly dissociated by the shear force, thereby giving the mixture fluidity and plasticizing it. The plasticized material is extruded into shape; The components, by mass, are: 50-88 parts plant fiber, 0.5-8 parts biomass crosslinking agent, 0.5-5 parts low-temperature water-soluble polymer, 10-50 parts water, and 0.05-3 parts metal ions. The biomass cross-linking agent comprises at least one of collagen, gelatin, keratin, silk fibroin, soy protein, casein, zein, starch, alginate, and chitosan. The metal ions include Ca. 2+ Mg 2+ Fe 3+ Al 3+ Zn 2+ Ti 4+ At least one of them; The low-temperature water-soluble polymer comprises at least one of polyvinyl alcohol, polyvinylpyrrolidone, polyacrylate, and polyacrylamide.

3. The method for preparing the plant fiber-polymer composite material as described in claim 2, characterized in that, Prior to the mixing, the plant fibers are pretreated, including at least one of drying, particle size control and surface modification, to control the moisture content of the plant fibers to below 12 wt% and enhance their surface reactivity.