Halogen Free Flame-Retardant Glass Fiber Reinforced PA 66 / PPO Alloys

Halogen-free ame retardants are ideal plastic additives that meet carbon neutral requirements. In this work, halogen free ame retardant with glass ber reinforced PA66/PPO composites were prepared by using coated red phosphorus (FRM-150B) and phosphorus-silicon ame retardant (WR6002). The mechanical properties, heat resistance, friction and wear properties and ame retardancy were carried out to evaluate the performances of composites using as structural parts that require heat resistance, dimensional stability and accuracy. It was found that the friction coecient and wear volume of the composites were improved with the contents of glass ber increased, as a result, PA66/PPO composites was obtained with excellent comprehensive performance when the content of compatilizer is 7%, the glass ber was 30%, the content of FRM is 8% and the phosphorous-silicon ame retardant is 16%. The ame retardant effect of FRM-150B and WR6002 in PA66/PPO was presented in the condensed phase, the results showed that the composite material with 16% WR6002 forms a carbon layer with excellent thermal stability. On the other hand, the mechanical properties of composites were hardly affected, has important prospects in automotive components and household appliances


Introduction
Polyphenylene Oxide (PPO) also known as poly (2, 6-dimethyl-1, 4-phenylene oxide),is one of the ve major engineering plastics with good thermal stability and mechanical strength. Polyphenylene ether has ame-retardant properties due to its molecular structure, and the oxygen index is 28 [1][2] . The glass transition temperature (Tg) of polyphenylene ether is 210°C, which can maintain its good mechanical properties, ame retardant properties and electrical properties in a large temperature range. The dielectric properties of PPO are the best among the ve engineering plastics [3][4][5] . In addition, the dimensional stability of polyphenylene oxide is good at plastic cage, but the disadvantage of polyphenylene oxide is that it has a large melt viscosity and it is di cult to process. And the products are prone to stress cracking due to residual stress.
Nylon 66 is a thermoplastic, one of the ve major engineering plastics, too, and it mostly used in electronic devices and automotive parts due to its excellent mechanical properties, outstanding molding characteristics [6] , good chemical resistance and electrical insulating features [7][8][9][10][11][12] . However, the oxygen index of nylon 66 is only 24, which is belongs to the ammable class. The ame retardancy of nylon 66 is very poor, and it will burn with the release of smoke and severe melting and dripping during the combustion process, which is the reason that greatly limits its application range [13][14] . However, nylon 66 has excellent processing uidity due to it contains a large number of amide bonds in its main chain [15] . Moreover, with low melt viscosity. But the high-water absorption is the major disadvantage of nylon 66.
PPO and PA66 has a similar melting point. The high melt viscosity of PPO could be deduced by incorporating PA66. However, the mechanical properties of the simple blend need to be improved due to the poor interface compatibility between PA66 and PPO. Therefore, some compatibilizers, such as high impact polystyrene and styrene-butene copolymer, were introduced to enhance the compatibility of the PPO/PA66 [2][3]5] .
Although pure polyphenylene oxide will extinguish by itself within a period of time after ignition. However, the ame-retardant performance of the materials would be greatly reduced. after blended with PA66. It is necessary to perform ame retardant modi cation treatment on composites [7] . In order to meet the market's requirements for ame retardant performance in electronic devices and other aspects. Though halogen-based ame retardants still occupy the main market, its threat on environment and human health has attracted more and more people's attention.
As the country pays more and more attention to environmental protection, halogen-free ame retardants such as phosphorous ame retardants, nitrogen ame retardants etc. rise in response to the proper time and conditions [10][11][12][13][14]16] .
In this study, the coated red phosphorus and phosphorus-silicon ame retardants were blended into PA66/PPO, respectively. The glass bers were added during the extrusion of the composites to strengthened the mechanical properties. Then, a halogen-free ame-retardant glass ber reinforced PA66/PPO with excellent comprehensive performance was prepared [18][19][20][21][22][23][24][25][26][27][28][29] . The oxygen index test, vertical combustion test, friction and wear performance, mechanical properties and thermal properties were carried out to appraise effect of blending.

Synthesis of HIPS-g-MAH
The HIPS resin was dried in a vacuum drying oven at 95°C for 12 hours, and then the HIPS resin and maleic anhydride were melt-blended in an internal mixer at 180°C for 8 minutes at a speed of 20 r/min. Finally, the melt-blended polymer was irradiated with a dose of 10 kGy in the gamma rays emitted by the co source. PA66 and PPO were dried under a vacuum oven at 110°C for 4 hours prior to extrusion. The required additives and ame retardant materials were uniformly mixed in different proportions. And the additives, PA66 and PPO were extruded through a Giant SHJ-36 twin-screw extruder and pelletized. The pellets were dried in a vacuum oven at 120°C for 4 hours and then injection molded into samples with an injection molding machine (PL860/260, Wuxi Haitian Machinery Co., Ltd.). The detailed processing conditions for the extrusion and injection molding are listed in Table 1. mm×4 mm, the notch shape is a V-shaped mouth of 45 °, There are ve patterns in each group, and the average value is calculated. The morphology of the fracture surface obtained after broken by izod impact tester were observed by scanning electron microscopy (SEM, S-4800 Japan) at an accelerating voltage of 15 kV. The surface of samples was sputter-coated with gold before examination. The friction coe cient and wear rate were conducted on a High-speed reciprocating friction and wear testing machine according to GB/T3960-2016 using the (MGW02, Lanzhou Zhongke Kaihua Technology Development Co., Ltd.) with the spline size was 80 mm×10 mm×4 mm. And the load of the sample is 10 N, the running speed was 500 rpm, the reciprocating friction time was 1h, the wear radius was 3 mm. The thermogravimetric analysis (TGA) was carried out on the STA-409 PC synchronous thermal analyzer (NETZSCA, Germany) using a heating rate of 10°C/min in nitrogen atmosphere.  Table 2 and Table 3, respectively. Obviously, the PA66/PPO composites was highly ammable 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 ame 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 ammable 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 ame retardancy of the composites.

Preparation of PA66/PPO composite
WR6002 is a commercialized ame retardant that contains both phosphorus and silicon. Phosphorus and silicon can form a denser carbon layer during combustion, thereby achieving better effects in ame retardant. Phosphorus-based ame retardants and silicon-based ame retardants are non-toxic green ame 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 ame retardant properties of PA66/PPO/30% GF with different content of phosphor-silicon ame retardants. The analysis of the table showed that as the number of phosphor-silicon ame retardants increases, the limiting oxygen index of the composite material gradually increase. When the glass ber content was 30% and the ame retardant content is 20%, the limiting oxygen index of the material reaches the maximum 35.7%, and the ame retardant grade was V-0, which meets the ame 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 ber is added and the content of the phosphor-silicon ame retardant is 16%.  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 puri ed 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 veri ed the in uence 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 rst 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 ber content. When the glass ber 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 ber 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 rmly bonded to the matrix after being blended with the glass ber, and the material will preferentially transfer the load to the glass ber 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 ber content was 0. With the addition of glass ber, the heat distortion temperature of composites has been greatly improved, and with the increase of glass ber content, the heat distortion temperature of composites has been continuously improved. The reason was that the glass ber 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 ber 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.

Friction and Wear Performance
Polymers are more prone to adhesive wear when subjected to friction. Figure 4 shows the friction coe cient graph and the analysis graph of the wear rate of composites with different glass ber content. It can be seen from Fig. 3 that with the glass ber content increased from 0 to 10%, the friction coe cient of the composites decreased from 0.47 to 0.34, and the friction coe cient has been improved with the glass ber content increased. The friction coe cient is 0.3, and the wear rate also drops from 14.4-1.1% when the glass ber content is 30%. This is because glass ber is a kind of wear-resistant material.

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 signi cantly 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 ber added, the mechanical properties of the ame-retardant composites have been signi cantly improved. When the content of coated red phosphorus is 8%, the impact strength of the composites without glass ber was 5.8 kJ/m 2 , and the impact strength of glass ber reinforced red phosphorus ame retardant material reached 7.08 kJ/m 2 , increased 22%, a composite with excellent comprehensive mechanical properties was obtained.

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 gure that although the mechanical properties of the material enhanced after the glass ber added. However, due to the "candle heart effect" of glass ber, 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 ame retardant. The T max of the PA66/PPO/GF was 472°C, but after WR6002 added, the T max was improved to 504°C. With the addition of ame 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 ame retardant addition. This is because with the addition of the coated red phosphorus ame retardant and phosphorus silicon ame retardant, the material can form polyphosphate during combustion, which plays a role in blocking oxygen and heat transfer. WR6002 is a phosphorous silicon ame 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 ame 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 re ned 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 ber. From the D and E, it can be seen that a clear cavity structure after the glass ber was broken. After the glass ber was pulled out, it can be clearly seen that the adhesive materials were taken out, indicating that the glass ber has improved the phase of the composites. Capacitance improves the mechanical properties of composites. After the ame retardant added, the morphology of the fracture site has changed signi cantly. It can be seen from Fig. 8C that the fracture surface of the composites after adding the ame retardant was bark-like, which will damage the mechanical properties of the blends.
Based on the above results, a ame-retardant mode of action was proposed (Fig. 9). The Si-O-Si will promote the degree of carbonization of ame-retardant materials in providing good ame 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 ame 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 ame retardant effect both in gas phase and condensed phase.

Conclusions
In summary, by adding irradiated HIPS-g-MAH and glass ber to PA66/PPO, the composite material was compatibilized and modi ed by glass ber, and PA66/PPO was reinforced and modi ed. The composite material was modi ed by adding ame retardant (IFR) FRM-15B and WR6002 respectively. The ame retardant e ciency and mechanical properties of PA66/PPO have been signi cantly improved. Different contents of glass ber and compatibilizer have a signi cant impact on the mechanical properties of PA66/PPO composites. According to the experimental data, when the compatibilizer content is 7% and the glass ber content is 30%, the composite material can obtain the greatest interfacial adhesion and the best mechanical properties. As the content of red phosphorus coated in the composite material increases, the oxygen index of the composite material rst increases and then decreases. When FRM-15B is added to the composite material, a glass-like isolation layer of metaphosphoric acid is formed on the surface of the material during combustion to block the combustion. When the content of FRM-15B is 10%, the ame retardant performance reaches the V0 level, the ame retardant performance is the best, and the impact on the mechanical properties of the composite material is minimal. After research, it is found that the oxygen index of the spline after adding glass ber is higher than that of the spline without glass ber, which proves that glass ber can increase the ame retardant performance of the material while enhancing the mechanical properties of the material. Experiments show that the ame retardant ability of phosphorous silicon ame retardant is weaker than that of coated red phosphorus. However, when the phosphorus-silicon ame retardant content is 16%, the composite material can also be extinguished in a short time. The ame retardant performance of the composite material after adding 16% phosphorous silicon ame retardant reaches the V0 level, and the carbonization after combustion and the isolation layer on the surface of the material are better than the blended material with red phosphorus. In addition, the overall performance of composite materials is better than that of adding coated red phosphorous materials. Figure 1 Photographs of different IFR samples after calcination. The effect of glass ber content on the friction and wear properties of PA66/PPO