How Modified Carbon Fibers Enable Polyamide (PA) To Achieve Dual Leap In Mechanical And Thermal Stability?

PA6/CF composite materialThe main components are CF, PA6, and the interface layer. The interface layer is responsible for connecting CF and PA6 and transmitting external forces. Good compatibility between the CF and PA6 phases is a prerequisite for ensuring the excellent performance of the interface layer. It can increase the contact area and physical and chemical interlocking between CF and PA6, reduce defect generation, and directly affect the mechanical properties of PA6/CF. However, the surface of CF is smooth and chemically inert, resulting in weak interfacial bonding ability with PA6.It is easy to become a weak bonding area, making it difficult to fully utilize its mechanical performance advantages.。Therefore, improving the surface activity of CF and enhancing the interfacial performance between CF and PA6 is of great significance for enhancing the mechanical properties of PA6/CF.

Methods for improving CF surfactants includeOxidation method, chemical grafting method, and sizing method.Among them:

Polyurethane (PU)With a chemical structure similar to PA6 and good thermal stability, it is an ideal modifier for the compatibility of CF and PA6.Sizing agent。

KARSLI and others studied the effects of different sizing agents on the modification of CF on the mechanical properties of CF/PA66, and the results showed that:PU as a sizing agent has the greatest effect on improving the tensile modulus of composite materials.Zhao et al. modified CF using a waterborne PU sizing agent, significantly increasing the polar functional groups on the CF surface. The flexural strength of CF/PA6 was improved by 28.6% compared to when CF was not added.

2.1.1 Micromorphology of CF

Figure 1 shows the SEM photographs of CF under different treatments. It can be seen from Figure 1 that the untreated NCF surface is relatively smooth, although some small particles can be observed, their quantity is relatively small. These small particles are the original sizing agents of CF. The DCF treated with acetone exposes the original surface of CF, which has a few shallow grooves on its surface, but overall the surface is still relatively smooth. The SCF treated with acetone and then sized with PU has a large number of small particles of sizing material, and there are also many grooves and wrinkles.

2.1.2 Contact Angle Between CF and PA6

Table 2 shows the contact angles between CF and PA6 under different treatment methods. From Table 2, it can be seen that:

The contact angle between DCF and PA6 after removing the original sizing agent with acetone is 108.6°.

The contact angle between untreated NCF and PA6 is 68.9°.

The contact angle between SCF treated with acetone and then with a water-based PU sizing agent and PA6 is 46.3°.

The contact angle between CF and PA6 without any surface coating of other modifiers is the largest. The contact angle between CF and PA6 modified by PU sizing agent is the smallest, which is reduced by 62.3° compared to DCF, indicating that CF modified by PU sizing agent has the best compatibility with PA6.

This is because the surface of CF, which has not been modified or coated with any modifiers, is inert and has low polarity, resulting in poor compatibility with PA6. The surface polarity of CF coated with sizing agents is increased, enhancing compatibility with PA6, and the contact angle becomes smaller. The original sizing agent on CF has poor compatibility with PA6, or the coating amount of the original sizing agent is limited, leading to better compatibility with PA6 after the PU sizing agent coats the CF.The contact angle between SCF and PA6 is the smallest, followed by the contact angle between NCF and PA6.。

Figure 2 shows the single fiber tensile strength of CF under different treatment methods. From Figure 2, it can be seen that the single fiber tensile strength of DCF is 4.9 GPa, that of NCF is 5.1 GPa, and that of SCF is 5.3 GPa, indicating that SCF has the highest single fiber tensile strength, while DCF has the lowest.

This is because the sizing agent forms a tight coating layer on the CF surface, which can fill the original grooves and defects of the CF, thus improving the integrity of the CF and enhancing its tensile strength. Moreover, the aqueous PU sizing provides a more complete and tighter coating on the CF surface (as shown in Figure 1), resulting in the highest tensile strength of the SCF. Therefore, the aqueous PU sizing agent has a better modification effect on CF.

Figure 3 shows the interlaminar shear strength of PA6/CF. It can be seen from Figure 3 that with the increase of SCF content, the interlaminar shear strength of PA6/CF first increases significantly and then slightly decreases, reaching a maximum when the mass fraction of SCF is 15%.

This is because the sizing agent has a large number of active groups on the surface of SCF, which allows SCF to be evenly dispersed in PA6. The molecules of the PU sizing agent on the surface of SCF intertwine, mechanically bond, or form intermolecular hydrogen bonds with PA6, enabling effective bonding between SCF and PA6. As a result, SCF uniformly bears and transmits shear loads.With a certain amount of doping, the interlaminar shear strength of PA6/CF increases with the increase in SCF doping. When the SCF content is too high.SCF may come into direct contact or agglomerate, leading to stress concentration and the formation of internal defects during processing, preventing SCF from fully exhibiting its high mechanical performance advantages.The interlayer shear strength of PA6/CF has slightly decreased.。

Figure 4 shows the tensile strength and flexural strength of pure PA6 and PA6/CF. From Figure 4, it can be seen that with the increase of SCF content, the tensile and flexural strengths of PA6/CF initially increase significantly and then increase slowly. When the mass fraction of SCF is 15%, the tensile strength and flexural strength reach 218.2 MPa and 300.3 MPa, respectively, which are 316.4% and 198.8% higher compared to pure PA6. Increasing the SCF mass fraction to 20% results in tensile and flexural strengths of 219.0 MPa and 301.6 MPa, respectively, only 0.37% and 0.43% higher than PA6/15% CF.

This is mainly because the tensile strength and flexural strength of CF are much higher than those of PA6. Adding SCF to PA6 can transmit and bear destructive loads, especially after the CF surface is modified with a sizing agent, which improves the interfacial bonding between SCF and PA6.SCF significantly improves the tensile strength and flexural strength of PA6/CF.However,When the dosage of SCF is excessiveAt this point, the contribution of excessive SCF to performance has basically reached its limit. Excessive SCF can easily lead to a deterioration in dispersion uniformity, causing agglomeration, resulting in processing difficulties, internal defects, and stress concentration.The effect of SCF on improving the tensile strength and bending strength of PA6/DCF is limited, so the increase is minimal.。

Figure 5 shows the SEM images of the fracture surface of the tensile strength samples of PA6/CF. It can be seen from Figure 5 that...

When the mass fraction of SCF is 5%~15%SCF is uniformly dispersed in the PA6 matrix, with PA6 resin wrapped around the fractured SCF, and the amount of SCF increases.

When the mass fraction of SCF increases to20% Some SCF were pulled out, leaving holes, and there was very little PA6 resin wrapped around some of the SCF.

Because excessive SCF leads to agglomeration, some SCF cannot be completely encapsulated by the PA6 matrix and tends to cause defects in the composite material system, making it easy for SCF to be pulled out.This further verifies the reason why the tensile strength and flexural strength first increase significantly and then grow slowly.

Figure 6 shows the TG curves of pure PA6 and PA6/CF. From Figure 6, it can be seen that without adding SCF, the initial decomposition temperature (t₀) of pure PA6 is 346°C, and the temperature corresponding to the maximum decomposition rate (t) is..._maxThe temperature is 451℃, and the final mass retention rate (w)._losThe thermal decomposition of PA6 primarily involves the initial decomposition into aliphatic hydrocarbons and oligomers, ultimately leading to the formation of H₂O and CO₂. As the SCF content continues to increase, the thermal decomposition curve of PA6/CF shifts upward, with t₀, t._max: and : Translate the above content into English, directly output the translation result without any explanation._losThe increasing trend indicates that the thermal stability of PA6/CF has improved.

When the mass fraction of SCF is 5%, the t₀ of PA6/CF is 357℃._maxFor 465℃, w_los6.9%

When the mass fraction of SCF is 10%, the t₀ of PA6/CF is 369℃._maxAt 479℃, w_los 12.0%.

When the mass fraction of SCF is 15%, the t₀ of PA6/CF is 382°C, t_maxAt 493℃, w_losThe figure is 16.8%.

When the mass fraction of SCF is 20%, the t₀ of PA6/CF is 383℃, t_maxFor 495℃, w_los21.7%.

It can be seen that when the mass fraction of SCF is 5%~15%, the thermal stability of PA6/CF improves rapidly. Compared with pure PA6, t₀ increases by 36°C and t_max increases by 42°C. When the mass fraction of SCF is 20%, the rate of improvement in thermal stability of PA6/CF slows down, with t₀ and t_max increasing by only 1°C and 2°C respectively compared to when the mass fraction of SCF is 15%. Additionally, the increase in w_los of PA6/CF is basically close to the increase in SCF content.

The reason for the above results is that the heat resistance of CF contained in SCF is much higher than that of PA6. The addition of SCF improves the thermal stability of PA6/CF, thus increasing t₀ and t_max. When the mass fraction of SCF reaches 15%, the effect of SCF on improving the thermal stability of PA6/CF basically reaches its maximum. Therefore, further increasing the mass fraction of SCF to 20% does not significantly change the thermal stability of PA6/CF. Although the addition of SCF improves the thermal stability of PA6/CF, the PA6 molecules continuously decompose at high temperatures. SCF mainly inhibits the decomposition rate of PA6 and has little effect on the decomposition residue of PA6. In addition, the PU sizing agent on the surface of SCF also decomposes at high temperatures, resulting in an increase in the w_los value of PA6/CF, which is close to the variation of SCF content.