3.1 Flame retardancy of PA66/PPO/FRM blends
Although numerous tests have been developed to examine the flame retardant properties of blends, the vertical burning test (UL-94) and LOI test are the most commonly used methods for testing the flame retardant properties of polymer materials. The limit oxygen index (LOI) and vertical burning testing (UL-94) were carried out to investigate the flame retardancy of the PA66/PPO/FRM blends. The corresponding results were listed in Table 2 and Table 3, respectively. Obviously, the PA66/PPO composites was highly flammable with a low LOI value (24 vol.%) and no rating at the UL-94 testing accompanied by severe melt dripping. It can be seen from Table 2 that the oxygen index of the material reached 39.5% when 10 parts of coated red phosphorus added, and the flame retardant passed V-0 ratting. Because the appropriate red phosphorus was added to the composites, a membrane will be formed in the carbon layer to prevent external oxygen and some combustibles when the sample was burned, thereby an effective preventing combustion can be achieved. However, the limit oxygen index of the composites was decreased with more FRM added. This phenomenon maybe caused by the fact that red phosphorus itself was a kind of flammable material. it would release a lot of heat during combustion when too much red phosphorus added, which will make the material easy to burn, and decrease the flame retardancy of the composites.
WR6002 is a commercialized flame retardant that contains both phosphorus and silicon. Phosphorus and silicon can form a denser carbon layer during combustion, thereby achieving better effects in flame retardant. Phosphorus-based flame retardants and silicon-based flame retardants are non-toxic green flame retardants with good smoke suppression effects. They can form an expanded carbon layer during combustion to achieve heat insulation and oxygen isolation to prevent the burning of materials. Table 2, 3 and the Fig. 1 were the results of the flame retardant properties of PA66/PPO/30% GF with different content of phosphor-silicon flame retardants. The analysis of the table showed that as the number of phosphor-silicon flame retardants increases, the limiting oxygen index of the composite material gradually increase. When the glass fiber content was 30% and the flame retardant content is 20%, the limiting oxygen index of the material reaches the maximum 35.7%, and the flame retardant grade was V-0, which meets the flame retardant standard. However, in conjunction with the analysis of the mechanical properties of the material in Fig. 5, the overall performance of the materials is optimal when 30% glass fiber is added and the content of the phosphor-silicon flame retardant is 16%.
Table 2
LOI and UL-94 test results of the PA66/PPO/FRM-150B blends
Sample
|
LOI (%)
|
UL-94
|
PA66/PPO/30%GF
|
25
|
No
|
PA66/PPO/30%GF/8%FRM
|
40.29
|
V-1
|
PA66/PPO/30%GF/10%FRM
|
39.488
|
V-0
|
PA66/PPO/30%GF/12%FRM
|
38.272
|
V-0
|
PA66/PPO/30%GF/14%FRM
|
36.65
|
V-0
|
Table 3
LOI and UL-94 test results of the PA66/PPO/WR6002 blends
Sample
|
LOI (%)
|
UL-94
|
PA66/PPO/30%GF
|
25
|
No
|
PA66/PPO/30%GF/4%P-Si
|
31
|
V-2
|
PA66/PPO/30%GF/8% P-Si
|
33
|
V-1
|
PA66/PPO/30%GF/12% P-Si
|
33.6
|
V-1
|
PA66/PPO/30%GF/16% P-Si
|
34.6
|
V-0
|
PA66/PPO/30%GF/20% P-Si
|
35.7
|
V-0
|
3.2 The influence of compatibilizer on the properties of PA66/PPO composite
Compared with the query data, it was found that MAH has the characteristic peak of C = O of cyclic anhydride at about 1850 cm− 1 and 1780 cm− 1, and the characteristic peak of carboxycarbonyl (C = O) after hydrolysis of cyclic anhydride at about 1700 cm− 1. The graft compatibilizer HIPS-g-MAH has corresponding characteristic peaks of maleic anhydride in corresponding places, indicated that the graft compatibilizer has the characteristic peak of maleic anhydride, indicating that HIPS and MAH have successfully undergone the grafting reaction. Compared with pure HIPS, the purified reaction blend has three more characteristic peaks, and these peaks were just the characteristic peaks of maleic anhydride, indicated that the high impact polystyrene was grafted with maleic anhydride. The above infrared spectra prove that the grafting reaction between high impact polystyrene and maleic anhydride has formed the graft HIPS-g-MAH.
The previous work verified the influence of different contents of compatibilizer on the tensile strength and impact strength of the matrix material. The experiment shows that with the increase of the compatibilizer content, the tensile strength of the PA66/PPO composite material first increases and then decreases. When the content of the compatibilizer is 7%, the tensile strength of the material reaches 71.95 MPa. This is because HIPS-g-MAH is used as a compatibilizer. Maleic anhydride is a functional monomer. It will react with the terminal amine groups of PA66 to form HIPS-g-PA66 at the two-phase interface, which makes the interface layer thicker, reduces the interfacial tension and improves the adhesion. However, because the added amount of the compatibilizer is too high, unreacted compatibilizer may accumulate in the interface layer, which will affect the tensile properties of the material. The notched impact strength of the composite material increases with the increase of the compatibilizer content.
The measurement of thermal deformation temperature (HDT) and the measurement of tensile strength and impact strength were carried out to study the thermal and mechanical properties of PA66 and FRPA66. It can be seen from Fig. 2 that the tensile strength and impact strength of composites have been greatly improved with the increase of glass fiber content. When the glass fiber content is 30%, the impact strength of the composites reaches the maximum and the performance is optimal. It was obvious that the blending of glass fiber in the PA66/PPO matrix not only improved the bonding force of the interface, but the physical properties of the composites were improved. The matrix material is firmly bonded to the matrix after being blended with the glass fiber, and the material will preferentially transfer the load to the glass fiber when it was loaded. The interface plays the role of transferring stress and gradually spreading the load to the entire composites, greatly improved the load bearing capacity, and the size has become relatively stable.
It can be found from Fig. 3 that the thermal deformation temperature of the composites was 70°C when the glass fiber content was 0. With the addition of glass fiber, the heat distortion temperature of composites has been greatly improved, and with the increase of glass fiber content, the heat distortion temperature of composites has been continuously improved. The reason was that the glass fiber itself has high heat resistance, and after blending with the matrix material, it increases the cohesive force of the components of the composites. Moreover, the glass fiber plays a role in the heterogeneous nucleation of the composites, which allows the molecular chains of the matrix to be arranged in an orderly manner within a short distance, and the movement was hindered, so more energy was required to make the polymer chains slip and movement. Therefore, the overall heat resistance of the material was improved, and the heat distortion temperature is also greatly increased.
3.3 Friction and Wear Performance
Polymers are more prone to adhesive wear when subjected to friction. Figure 4 shows the friction coefficient graph and the analysis graph of the wear rate of composites with different glass fiber content. It can be seen from Fig. 3 that with the glass fiber content increased from 0 to 10%, the friction coefficient of the composites decreased from 0.47 to 0.34, and the friction coefficient has been improved with the glass fiber content increased. The friction coefficient is 0.3, and the wear rate also drops from 14.4–1.1% when the glass fiber content is 30%. This is because glass fiber is a kind of wear-resistant material.
3.4 Mechanical properties of PA66/PPO/GF/FRM
The mechanical properties of microcapsule-coated red phosphorus applied to PA66/PPO composites were shown in Fig. 5. It can be seen that after only adding coated red phosphorus, the tensile strength of the composites did not change significantly with the increase of the coated red phosphorus content. However, the impact strength of the material began to decrease as the content of coated red phosphorus increased. Because after coated red phosphorus added to the composites, these red phosphorus particles were likely to become stress concentration points, which weaken the resistance of the material when subjected to external forces. With the content of coated red phosphorus increased, the stress concentration point also increases, resulting in a gradual decrease in impact performance. After 30% glass fiber added, the mechanical properties of the flame-retardant composites have been significantly improved. When the content of coated red phosphorus is 8%, the impact strength of the composites without glass fiber was 5.8 kJ/m2, and the impact strength of glass fiber reinforced red phosphorus flame retardant material reached 7.08 kJ/m2, increased 22%, a composite with excellent comprehensive mechanical properties was obtained.
Figure 6. Tensile properties (left picture) and impact properties (right picture) of composites with different phosphorus-silicon (WR6002) flame retardant content.
3.5 TGA and DTG of blends
In an inert atmosphere (nitrogen), the thermal decomposition processes on the TGA curves displayed for PA66/PPO, PA66/PPO/GF, PA66/PPO/GF/P-Si, and PA66/PPO/GF/FR composites in Fig. 7. It can be seen from the figure that although the mechanical properties of the material enhanced after the glass fiber added. However, due to the "candle heart effect" of glass fiber, the weight loss of the material was increased compared to PA66/PPO. The introduction of WR6002 and FRM into the PA66/PPO/GF was found to reduce the temperature of the onset decomposition temperature of PA66/PPO/GF. The weight loss of the material increased from 28.8–52% after adding phosphorous and silicon flame retardant. The Tmax of the PA66/PPO/GF was 472°C, but after WR6002 added, the Tmax was improved to 504°C. With the addition of flame retardants, the more stable and higher quality carbon layer was formed. It can be seen from the curve that the carbon residue rate of the material also improved with the increase of the flame retardant addition. This is because with the addition of the coated red phosphorus flame retardant and phosphorus silicon flame retardant, the material can form polyphosphate during combustion, which plays a role in blocking oxygen and heat transfer. WR6002 is a phosphorous silicon flame retardant. Due to the joint action of Si-O-Si and phosphorous elements, the isolation layer formed was more stable than red phosphorous, which achieved a better flame retardant effect.
3.6 Fracture appearance of blends
SEM images of notch impact surface of different samples after test were presented in Fig. 8. It can be seen from A and B that the particle size distribution between the two dispersed phases before adding the compatibilizer was small, and the distribution was relatively clear, the parabolic dimple phenomenon was more obvious, and the two phases penetrate the uniform sea-island structure. With the addition of the compatibilizer, we can clearly see that the phase morphology of the blends is improved. The phase interface of the blends becomes blurred, which reduced the interfacial tension and refined the dispersed phase, resulting in enhanced material compatibility. The broken surface presents a co-continuous phase structure, and the thicker the two-phase interface layer, the more obvious the compatibilization effect, which is consistent with the trend of increasing mechanical properties. The C is the composites with glass fiber. From the D and E, it can be seen that a clear cavity structure after the glass fiber was broken. After the glass fiber was pulled out, it can be clearly seen that the adhesive materials were taken out, indicating that the glass fiber has improved the phase of the composites. Capacitance improves the mechanical properties of composites. After the flame retardant added, the morphology of the fracture site has changed significantly. It can be seen from Fig. 8C that the fracture surface of the composites after adding the flame retardant was bark-like, which will damage the mechanical properties of the blends.
Based on the above results, a flame-retardant mode of action was proposed (Fig. 9). The Si-O-Si will promote the degree of carbonization of flame-retardant materials in providing good flame retardancy of PA66/PPO blends. The cooperative of Si and P enhanced the stability of char layer, contributed to the phosphorus and silicon-rich cross-linked char, which play the flame retardant effect in condensed phase. In addition, released incombustible gases during the combustion, including ammonia and steam, could dilute the oxygen concentration and took the heat away, contributing to the gas-phase mode of action. Therefore, FRM exert the flame retardant effect both in gas phase and condensed phase.