In the preparation of thermoplastic carbon fiber, the industry commonly uses these four sizing agents.
In the preparation of thermoplastic carbon fiber composites, it is necessary to address the issue of interfacial bonding strength between the carbon fiber and the thermoplastic resin to ensure a better infiltration and bonding effect in subsequent processes. Sizing agents play an important role in this regard. However, different thermoplastic matrix materials possess varying physical and chemical properties, so experiments are needed to identify different types of sizing agents that meet the specific usage requirements.
In order to improve the compatibility of sizing agents with thermoplastic resin-based composites, the industry has conducted extensive research on various novel sizing agents for different thermoplastic resins, aiming to achieve a close structural resemblance and strong interactions between the sizing agents and the thermoplastic resins. After numerous experiments and comparative data assessments, it has been found that the following four sizing agents are particularly suitable: polyamide (PA), polyurethane (PU), polyarylether, and polyimide (PI).
1.Polyamide (PA) Sizing Agent
Polyamide (PA), also known as nylon, possesses excellent chemical stability, wear resistance, and mechanical properties. It is commonly used in special fibers, engineering plastics, and thermoplastic resin-based composite matrix resins. As PA has been widely utilized as a matrix resin for thermoplastic resin-based composites, selecting PA as a component of the sizing agent can enhance the interfacial compatibility of thermoplastic resin-based composites.
A solvent-based sizing agent was prepared by dissolving modified PA in polyols and applying it to de-sized T300 carbon fiber. This led to the fabrication of CF/PA66 composites. The good compatibility between the sizing agent and the nylon 66 matrix resin resulted in a synergistic effect of chemical bonding and physical adsorption, successfully improving the tensile strength and impact strength of the composites by 40.87% and 43.59%, respectively.
However, this method requires a significant amount of organic solvents, posing serious threats to environmental and production safety, and the energy consumption for drying solvents is considerable. Therefore, the focus of PA sizing agent research is gradually shifting toward more environmentally friendly water-based sizing agent systems. Currently, obtaining stable dispersed PA emulsions using surfactants and preparing PA aqueous sizing agents through hydrophilic modification are more mature approaches.
2.Polyurethane (PU) Sizing Agent
Polyurethane (PU) exhibits good compatibility and bonding strength with various thermoplastic resins due to its unique chemical structure, making it widely applicable as a sizing agent. By leveraging the similarities and compatibilities between the urethane and carbonate structures, PU can be used as a sizing agent for sizing the fibers in carbon fiber (CF)/thermoplastic polycarbonate (PC) composites through a solvent method.
The thermal stability of the polyurethane (PU) sizing agent is excellent; it begins to lose weight only at temperatures up to 270°C. This allows for chemical bonding with the carbonate structures in the polycarbonate (PC) matrix, resulting in an increase in interlaminar shear strength of the composites from 38.1 MPa to 62.9 MPa, representing a 65% improvement.
However, with the increasing emphasis on environmental issues, solvent-based PU sizing agents are gradually being replaced by water-based sizing agent systems. Emulsion dispersion is one of the commonly used methods for preparing water-based PU sizing agents. Water-based emulsion PU sizing agents can be stored for up to six months under normal temperature drying conditions, with heat resistance reaching 280–300°C, which can elevate the interlaminar shear strength of CF/PA66 composites to over 78 MPa, demonstrating a more significant enhancement.
Polyarylether Sizing Agent
Polyarylethers are polymers that contain aromatic rings and ether linkages. Well-known examples include polyether ether ketone (PEEK), polyphenylene sulfide (PPS), and polyethersulfone (PES). The rigid benzene rings and flexible ether linkages endow these materials with excellent mechanical and thermal properties, while also allowing some systems to be crystalline, enabling continuous use under high temperature and humid conditions. They are widely used as high-performance engineering plastics and thermoplastic resins in aerospace, electronics, energy, and medical fields.
However, the rigid and stable structure of polyarylethers, while providing many advantages, also makes it challenging for them to react with other active groups, leading to weak interfacial bonding with carbon fibers (CF). Therefore, modifying polyarylether systems and preparing sizing agents to enhance their bonding strength with CF and thermoplastic matrices has become a priority problem to address. Strong acid treatment is an effective method for introducing active groups into polyarylether molecules.
By employing a sulfonation treatment, sodium sulfonate structures (−SO3Na) were introduced into the PEEK system to prepare a sizing agent. The sulfonic groups can form hydrogen bonds with the groups on the fiber surface, and the sizing agent is compatible with the PEEK matrix, facilitating the wetting and infiltration of the matrix resin into the CF. The interlaminar shear strength of the composite material reached 78.2 MPa.
Additionally, a solvent-based hybrid sizing agent was prepared by modifying graphene oxide (GO) with a diamine structure similar to that of polyethersulfone (PES), which not only introduced active amino groups but also improved the thermal stability of the system. Various interactions such as chemical bonding, hydrogen bonding, polar attraction, van der Waals forces, and mechanical interlocking can achieve a strong bond between the sizing agent, GO, CF, and the PES matrix, resulting in a 74.1% improvement in the interfacial properties of the CF/PES composites.
4.Polyimide (PI) Sizing Agent
Polyimides (PI) are high-performance polymers that contain imide rings in their molecular backbone. They possess a highly rigid chain structure and excellent mechanical properties, making them one of the highest temperature-rated polymer materials. PIs have found widespread applications in aerospace, military equipment, electronic communications, and other fields. Among these, polyether imide (PEI) sizing agents, which contain flexible ether linkages, have gained considerable attention in recent years as high-temperature sizing agents due to their exceptional thermal stability, improved flexibility, better solubility, and compatibility with thermoplastic resins.
PI sizing agents can withstand high temperatures, meeting the molding and usage conditions for high-performance thermoplastic resin-based composites (such as CF/PES and CF/PEEK composites). However, similar to polyarylether sizing agents, the rigid and stable molecular structure of PI sizing agents results in low bonding capacity with carbon fibers (CF) and poor processability, necessitating chemical modification.
Modification of the PI sizing agent was performed using nanoparticles by dispersing multi-walled carbon nanotubes (MWCNT) into a dichloromethane solution of PEI. Using a solvent method, the surface of T300 grade CF fabric was treated. Research found that MWCNT in the mixed sizing agent effectively introduced a large number of active groups and could uniformly cover the fiber surface. After sizing, the imide rings in PEI could form polar interactions and hydrogen bonds with hydroxyl and carboxyl groups on the MWCNT surface, while π-π stacking interactions occurred between the aromatic rings of the MWCNT and the PEEK matrix resin. This modification significantly inhibited crack propagation, ultimately resulting in an interlaminar shear strength of 90.7 MPa for the composite material.
Strictly speaking, polyamide (PA), polyurethane (PU), polyarylether, and polyimide (PI) represent four categories of sizing agents, each tailored for different types of thermoplastic resins. These sizing agent systems typically undergo various modifications during use to effectively enhance the performance characteristics of thermoplastic carbon fiber composites. Additionally, it is essential to consider whether the experimental processes may cause significant negative impacts on the environment. To find optimal solutions, numerous experts and scholars both domestically and internationally are striving to identify the most suitable approaches.
