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Text | Afang
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Polypropylene (PP) is a thermoplastic synthetic resin with excellent performance, which is widely used in electrical insulation materials, furniture, chemical resistant materials, automobiles, building materials, textiles and interior decoration materials due to its high cost performance and excellent comprehensive properties.
However, the PP oxygen index is only 17%, which is easily ignited in the air and the droplet phenomenon is serious during the combustion process, which further spreads the fire. Therefore, the research of flame retardant PP with high flame retardant efficiency, excellent mechanical properties and environmental protection and economy has become a development trend.
Next, let's explore the influence of PAPP-MPP-PER-PER composite flame retardant on the flame retardant effect and mechanical properties of PP, and discuss its mechanism of action in depth.
One. Experimental part
Pentaerythritol (PER, 99%), Pyrophosphate (PAPP), Yuntianhua Co., Ltd.; Melamine phosphate (MPP), Zhenjiang Xingxing Flame Retardant Co., Ltd.; Polypropylene PP), Sinopec Guangzhou Branch; Antioxidant (Irganox1010), BASF, Germany; Lubricants (EBS), KLK Malaysia; Anti-drip agent (SN3307PF), Guangzhou Entropy Energy Innovative Materials Co., Ltd.; Plastic injection molding machine (TTZ90SeIIS type), Donghua Machinery Co., Ltd.
High speed mixer (SHR-100A type), Zhangjiagang Xinyuan Machinery Manufacturing Co., Ltd.; Thermogravimetric analyzer (TGDTG, type F209, NETZSCH, Germany; Limiting oxygen index test (LOI, 5801A type), combustion testing machine (5402 type, smoke density tester (5920 type) Suzhou Yang Eat Walch Testing Technology Co., Ltd.: Universal ability testing machine (UTM-1422 type), impact testing machine (HIT-2492 type, Chengde Jinjian Testing Instrument Co., Ltd.; Cone Tester, FTT UK.
Polypropylene granules were dried in a blast drying box at 100C for 6h, and flame retardants and additives were dried at 75C for 4h. Subsequently, the PP particles, flame retardants and auxiliaries were mixed evenly with a high-speed mixer according to the formula shown in Table 1, melt-plasticized and extruded by a twin screw extruder, and then air-cooled into the pelletizer to pelletize and prepare flame retardant modified masterbatches. The prepared PP masterbatch was dried at 100C for 6h, and the plastic injection molding machine was used to make a standard spline for testing.
The universal capacity testing machine is used to test the tensile properties according to GBT2567 and the bending properties according to GBT2567; The impact testing machine was used to test the notch impact performance according to GBT1043.1, the notch type of the specimen was A type, and the pendulum impact energy was 5.5J; The limiting oxygen index of the sample was measured by automatic oxygen index meter: the combustion testing machine was used to test the UL-94 combustion grade of the spline, and the thickness of the spline was 1.6mm; The thermal stability of flame retardant modified PP was determined by thermogravimetric analyzer, N2 atmosphere, flow rate 30cm/min, heating rate of 10K/min, temperature range 30~800C; The smoke density of the spline is measured by smoke density meter, the radiation intensity is 25kW/m, and there is no ignition flame: the combustion and heat release data of the sample are tested by a cone calorimeter, and the radiation power is 25kW/m.
Two. Study results
In the flame retardant research of polymer materials, the addition of flame retardants can give the material excellent flame retardant properties, but often deteriorate the mechanical properties of the material. Therefore, how to optimize the combustion performance and mechanical properties has always been the focus of flame retardant material research. The effects of PAPP, MPP, PAPP-MPP, PAPP-MPP-PER-PER compound flame retardants on the mechanical properties of flame retardant PP are shown in Figure 1 and Table 2.
As shown in Figure 1 and Table 2, the notched impact strength and tensile strength of flame retardant PP gradually decrease as the content of composite flame retardant increases. With the increase of flame retardant addition, during processing, the flame retardant powder destroys the continuity of the molecular chain of the PP matrix, reduces the interaction between the molecular chains, and makes it easier to slide between the molecular chains under the tensile and impact of external forces, resulting in a decrease in notched impact strength and tensile strength.
However, it can be seen that after the increase of composite flame retardant, its bending strength has been improved to a certain extent, and it has decreased with the increase of flame retardant dose. The analysis believes that flame retardants such as MPP and PAPP are oligomers, which play a certain role in plasticizing PP processing, so that the bending strength of flame retardant PP is improved compared with pure PP. However, with the increase of the amount of PER in the content of flame retardant, the continuity between the molecular weight of PP is destroyed, and the bending strength is reduced to a certain extent.
Table 3 shows the effects of flame retardants of different components on the flame retardant properties of PP. It can be seen from Table 3 that the pure PP limiting oxygen index (LOI) of unflame retardant modification is 18.8%, and the UL-94 test result is not flame retardant (NR), and there is serious melting phenomenon. When PAPP and MPP are used alone, the LOI of PP has been improved, and 24% of MPP can reach V-0 level when used alone, and there is no melting drop.
When PAPP-MPP-PER was added to form a composite flame retardant, due to its synergistic effect, compared with pure PP, the LOI increased from 18.8% to 29.0%, and the combustion grade was increased to V-0, and there was no melting drop. When the addition amount of PAPP-MPP-PER compound flame retardant was 24%, UL-94 increased from V2 (1.6mm) to V-0 (1.mm) LI from 25.5% to 29.0%, an increase of 13.7% compared with the PAPP flame retardant system. It can be seen that the flame retardant efficiency has been significantly improved after the formation of pyrophosphate flavor-based expansion flame retardant system with melamine polyphosphate (MPP) and pentaerythritol (PER) as gas source and carbon source supplementation.
Figure 2 shows the effect of the addition of flame retardant in different PAPP-MPP-PER-PER compounds on the flame retardant PP retardant performance. It can be seen from the figure that with the increase of flame retardant addition, the flame retardant performance is improved (the combustion grade has reached V-0 level, and LO shows a gradual increase trend). The analysis believes that with the addition of flame retardant, the flame retardant forms an effective expanded carbon layer during combustion through decomposition and heat absorption, charcoal covering, foaming and expansion, making it difficult for the combustible gas generated by thermal decomposition of PP to overflow, and insulating and isolating oxygen to the material, thereby inhibiting the cracking of PP and achieving the flame retardant effect. When the amount of flame retardant added reaches 30%, the limiting oxygen index reaches 30.8%, and the UL-94 combustion grade reaches V-0 level (1.6m).
In order to further explore the effects of PAPP, MPP and PER flame retardants on the combustion performance of PP, TGDTG was used to analyze the thermal degradability of flame retardant modified PP, and the results are shown in Figure 3, and the corresponding thermal degradation data are shown in Table 4.
It can be seen from the figure that pure PP begins to decompose at 338.24C, the maximum thermal decomposition temperature is 464.81C, the maximum thermal weight loss rate (MMLR) is 21.52%/min, and the residual carbon rate is only 0.01% at 800C, which is almost completely decomposed. When PAPP and MPP are used separately, the initial decomposition temperature of modified PP is reduced due to the decomposition of PAPP and MPP when heated, and MMLR is also reduced, and when PAPP-MPP is compounded, the initial temperature and MMLR are further reduced.
When PAPP-MPP-PER was added to compound flame retardant, MMLR decreased to 9.13% and increased the residual carbon content to 10.37% due to its synergistic effect. Compared with the addition alone, the flame retardant efficiency of MPP and PER, as gas and carbon sources supplement to form pyrophosphate paizyl expansion flame retardant system, its flame retardant efficiency has been significantly improved.
With the addition of PAPP-MPP-PER-PER compound flame retardant from 20% to 30%, the flame retardant performance of modified PP was improved (the residual carbon rate showed a trend of increasing early, and MMLR showed a trend of gradual decrease).
It shows that with the increase of the content of compound flame retardant, the flame retardant effectively forms an expanded carbon layer covering the surface of the matrix during the combustion process through decomposition and heat absorption and charcoal covering, so that the combustible gas generated by thermal decomposition of PP is difficult to overflow, and the material plays the role of heat insulation and oxygen isolation, thereby inhibiting PP cracking and achieving the effect of flame retardant. When the amount of PAPP-MPP-PER compound flame retardant was 24%, the residual carbon rate reached 10.37%, which was 131.99% higher than that of the PAPP flame retardant system. When the amount of flame retardant added reaches 30%, the residual carbon rate reaches 13.73%, and the MMLR is reduced to 9.06%.
Table 5 shows the smoke density test data of different flame retardants and different amounts of modified PP, where DS is the maximum specific optical density during combustion, which is used to characterize the total amount of smoke released during combustion. It can be seen from Table 5 that the maximum specific optical density of pure PP without flame retardant modification reaches 593.9.
Using PAPP and MPP separately, the Dm was reduced to 580.5 and 588.9 respectively due to the flame retardant effect of PAPP and MPP. When the amount of PAPP-MPP-PER compound flame retardant is 24%, due to its synergistic effect, DS is further reduced to 489.4, which is reduced by 15.7% compared with the PAPP flame retardant system D580.5, and the content of PAPP-MPP-PER-PER compound flame retardant increases from 20% to 30%, and the D of modified PP is gradually decreasing, and when the amount of flame retardant is increased to 30%, DS is reduced to 423.9, which is 26.9% lower than pure PP.
It is explained that with the addition of PAPP-MPP-PER-PER-compound flame retardant, the flame retardant effectively forms an expanded carbon layer covering the surface of the matrix during the combustion process through decomposition and heat absorption and charcoal covering, making it difficult for the combustible gas generated by PP due to thermal decomposition to overflow and isolating the external heat and oxygen, thereby inhibiting PP cracking and achieving the effect of smoke suppression.
In order to further study the effects of PAPP, MPP and compound flame retardants on the combustion performance of PP, the conical calorimetric test of pure PP and the above flame retardant modified PP was carried out, Figure 4 and Table 6 are the cone test curve and test data of PP material can be seen from Figure 4 and Table 6, pure PP burns rapidly after ignition, and the heat release rate quickly reaches the maximum, and the peak heat release rate (pkHRR) is 599.79kW/m2.
The papp and MPP were used separately, due to the flame retardant effect of PAPP and MPP, the pkHRR was reduced to 226.47kWm and 305.71kW/m, respectively, and when the PAPP-MPP-PER was added to form a compound flame retardant, pkHRR was further reduced to 157.81kW/m due to its synergistic effect, which was 30.3% lower than the 226.47W/m of the PAPP flame retardant system. It was shown that the addition of compound flame retardants showed a significant effect on reducing heat release in polypropylene systems.
Polypropylene materials have a large combustion smoke, which has always been a short board for the application of this type of materials, and the lower total smoke emission (TSR) value and smoke formation rate (SPR) value mean that the smoke toxicity hazard is low. As shown in Table 5 and Figure 6, after adding 24% PAPP, MPP, PAPP-MPP, PAPP-MPP-PER-PER, flame retardant modified PP TSR decreased by 16.37%, 22.37%, 55.91% and 79.22% compared with 1006.12m2/m of pure PP, respectively, and the SPR of flame retardant modified PP decreased by 54%, 58%, 72% and 86% compared with 0.050m%/s of pure PP.
When the amount of PAPP-MPPPER compound flame retardant was 24%, the TSR was reduced to 209.09m2/m3, which was 75.1% lower than that of the PAPP flame retardant system, and the SPR was reduced to 0.007m/s, which was reduced by 695% compared with the PAPP flame retardant system.
It can be seen from Figure 5 that after the pure PP test, the material is basically all burned, and there is almost no carbon layer left on the tin foil, and when PAPP and MPP are added separately, a certain thickness of carbon layer is formed, and there are multiple fine carbon pores on the surface of the carbon layer. When PAPP and PER are combined and used, a complete and dense carbon layer is produced, because MPP is released by the thermal decomposition process of gas source, and the carbon layer expands greatly, but due to the rapid generation of gas on the surface of the carbon layer, there are more cracks on the surface of the carbon layer, and the carbon layer cracks.
After adding PER as a carbon source, the height of the carbon layer increased to a certain extent, and the biggest change was that the surface of the carbon layer was more uniform and there was no large crack. Mainly because the substances produced by the thermal decomposition of PAPP cover the surface of the polymer during the combustion process, and react with MPP, dehydrate into a thick layer, and crosslink to form a dense carbon layer. The carbon produced by PER decomposition is covered in time in the cracks of the carbon layer caused by MPP decomposition, which makes the carbon layer denser and harder, and the flame retardant effect is significantly improved, which is consistent with the cone quantity test results.
Three. conclusion
1. The addition of PAPP-MPP-PER flame retardant has a great influence on the mechanical properties of PP, which reduces the notched impact strength and tensile strength of modified PP, but the bending strength is improved to a certain extent.
2. Compared with PAPP alone, MPP and PER are added, so that the flame retardant smoke suppression effect of the PAPP-MPPPER compound system is significantly improved, when the flame retardant addition amount is 24%, the PAPP-MPP-PER compound system compared with the PAPP flame retardant system, UL-94 grade (1.6mm) reaches V-0 grade, LOI increases by 13.7%, the residual carbon rate at 800C increases by 131.99%, and the smoke density DSmax is reduced by 15.7%. pkHHR decreased by 30.3%, TSR by 75.1%, and SPR by 69.5%.
3. The compound flame retardant composed of PAPP-MPP-PER can significantly improve the flame retardant smoke suppression performance of PP composites, when 30% PAPP-MPP-PER composite flame retardant is added, the LOI reaches 30.8%, UL-94 reaches V-0 grade (1.6m), the residual carbon rate is 13.73% at 800 °C, and the smoke density DSax is reduced by 26.9% compared with pure PP.