Synergistic flame retardant effect of lignin containing silicon-nitrogen with ammonium polyphosphate on polylactic acid
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摘要: 聚乳酸(PLA)作为生物基可降解塑料逐渐成为研究热点,但由于其极易燃烧,在包装、电器等领域的应用受到限制,为了解决此类问题,对碱木质素(Lig)改性合成了含硅-氮元素的木质素(Si-NLig),通过TGA分析发现,Si-NLig在空气中的初始降解温度(T5%)提高了20℃,且高温残留量由2.3%提高至25.5%。将 Si-NLig作为成炭剂,与聚磷酸铵(APP)复配,通过熔融共混法制成阻燃聚乳酸材料(Si-NLig-APP/PLA),对其阻燃性能、力学性能、燃烧行为等进行了研究。研究表明:Si-NLig与APP质量比为1∶4,10wt%的添加量可使Si-NLig-8%APP/PLA的极限氧指数(LOI)值达到27%,UL-94垂直燃烧达到V-0级别,而同等条件下Lig-8%APP/PLA的LOI值为26%,UL-94仅为V-2级别。同时,与Lig-8%APP/PLA相比,Si-NLig-8%APP/PLA的热释放速率峰值(PHRR)降低了27%;残炭的拉曼图谱分析发现Si-NLig-8%APP/PLA的石墨化程度比Lig-8%APP/PLA提高了36.7%,为其良好的阻燃性能提供了理论依据。Si-NLig的引入使阻燃PLA力学性能得到了改善,拉伸强度提升了21%。可见Si-NLig在无卤阻燃PLA领域中具有潜在的应用前景。Abstract: Polylactic acid (PLA) as a biobased degradable plastic has increasingly become a research hotspot. However, PLA's application in packaging and electrical appliances is limited due to its high combustibility. In order to solve the above problem, lignin containing silicon-nitrogen synergistic (Si-NLig) was synthesized by modification of alkali lignin (Lig), and its TGA results showed that the temperature corresponding to mass loss 5wt% of material (T5%) increased by 20℃ and the high temperature residue increased from 2.3% to 25.5% in air. And Si-NLig was used as a charring agent, and compounded with ammonium polyphosphate (APP) to prepared flame retardant polylactic acid (Si-NLig-APP/PLA) by melt blending and hot-press molding, and the PLAs' flame retardant properties, mechanical properties, and combustion behavior, were investigated. The results showed that addition of 10wt% Si-NLig-APP with the mass ratio 1∶4 made Si-NLig-8%APP/PLA reach the limiting oxygen index (LOI) value of 27% and the vertical burning UL-94 V-0 level, while the LOI value of Lig-8%APP/PLA with Lig as charring agent at the same condition was 26% and the vertical burning UL-94 only passed V-2 rating. Meanwhile, the peak heat release rate (PHRR) reduced by 27% compared to Lig-8%APP/PLA. Raman spectroscopy was used to characterize the structure of the residual char after cone calorimeter testing, it was found that the degree of graphitization of Si-NLig-8%APP/PLA increased by 36.7% compared to that of Lig-8%APP/PLA, which provides a theoretical basis for its good flame retardancy. Introduction of Si-NLig led to the enhancement of mechanical properties of the flame retardant PLA with tensile strength increasing by 21%. It can be seen that Si-NLig has potential application prospect in the field of halogen-free flame retardant PLA.
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图 6 PLA ((a), (b))、Lig-8%APP/PLA ((c), (d))、10%APP/PLA ((e), (f))、Si-NLig-8%APP/PLA ((g), (h))、 Si-NLig-12%APP/PLA ((i), (j))、Si-NLig-16%APP/PLA ((k), (l))的冲击横截面SEM图像
Figure 6. SEM images of fracture surface of PLA ((a), (b)), Lig-8%APP/PLA ((c), (d)), 10%APP/PLA ((e), (f)), Si-NLig-8%APP/PLA ((g), (h)), Si-NLig-12%APP/PLA ((i), (j)) and Si-NLig-16%APP/PLA ((k), (l))
表 1 聚乳酸(PLA)和阻燃PLA配方
Table 1. Formulations of polylactic acid (PLA) and flame retardant PLA
Sample PLA/
wt%APP/
wt%Li/
wt%Si-NLig/
wt%PLA 100 0 0 0 10%APP/PLA 90 10 0 0 Lig-8%APP/PLA 90 8 2 0 Si-NLig-6.4%APP/PLA 92 6.40 0 1.60 Si-NLig-3.33%APP/PLA 90 3.33 0 6.67 Si-NLig-6%APP/PLA 90 6 0 4 Si-NLig-6.67%APP/PLA 90 6.67 0 3.33 Si-NLig-8%APP/PLA 90 8 0 2 Si-NLig-12%APP/PLA 85 12 0 3 Si-NLig-16%APP/PLA 80 16 0 4 Note: APP—Ammonium polyphosphate. 表 2 由XPS图谱得到的Lig和Si-NLig的元素组成
Table 2. Element contents of Lig and Si-NLig obtained by XPS spectra
Sample C/at% O/at% N/at% Si/at% Lig 81.16 18.41 0.43 — Si-NLig 80.16 15.81 1.28 1.45 表 3 Lig和Si-NLig空气氛围下的热降解参数
Table 3. Thermal degradation parameters of Lig and Si-NLig in air
Sample T5%/℃ Tmax/℃ Vmax/(%·min−1) Residue at 550℃/% Tmax1 Tmax2 Tmax3 Vmax1 Vmax2 Vmax3 Lig 212 270 371 500 2.8 4.0 15.9 2.3 Si-NLig 232 317 474 556 7.5 5.4 5.3 25.5 Notes: T5%—Temperature corresponding to mass loss 5wt% of material; Tmax—Temperature corresponding to the maximum thermal degradation rate; Vmax—Degradation rate at the maximum degradation temperature. 表 4 Lig和Si-NLig氮气氛围下的热失重参数
Table 4. Thermal degradation parameters of Lig and Si-NLig in N2
Sample T5%/℃ Tmax/℃ Vmax/(%·min−1) Residue at 700℃/% Lig 222 350 5.5 41.3 Si-NLig 228 377 3.7 48.6 表 5 PLA和阻燃PLA阻燃性能测试结果
Table 5. Results of flame retardant performances for PLA and flame retardant PLA
Sample LOI/% UL-94 vertical burning t1/s t2/s Rating Dripping Ignition cotton PLA 19.5 >60 — NR Yes Yes 10%APP/PLA 28 2 1 V-0 Yes No Lig-8%APP/PLA 26 5 2 V-2 Yes Yes Si-NLig-6.4%APP/PLA 27 3 1 V-2 Yes Yes Si-NLig-3.33%APP/PLA 25 6 1 V-2 Yes Yes Si-NLig-6%APP/PLA 27 9 7 V-2 Yes Yes Si-NLig-6.67%APP/PLA 27 4 0 V-2 Yes Yes Si-NLig-8%APP/PLA 27 2 0 V-0 Yes No Si-NLig-12%APP/PLA 28 1 0 V-0 Yes No Si-NLig-16%APP/PLA 30 0 0 V-0 Yes No Notes: t1, t2—Duration of sample burning; LOI—Limiting oxygen index; NR—No rating. 表 6 PLA和阻燃PLA力学性能测试结果
Table 6. Test results of mechanical properties of PLA and flame retardant PLA
Sample Tensile strength/
MPaElongation at break/
%Impact strength/
(kJ·m−2)PLA 70.0±4.6 14.2±0.9 5.6±0.3 10%APP/PLA 39.8±1.1 10.7±1.4 4.5±0.1 Lig-8%APP/PLA 51.9±2.9 9.9±0.7 4.7±0.4 Si-NLig-8%APP/PLA 62.8±3.1 8.3±1.1 5.5±0.4 Si-NLig-12%APP/PLA 65.8±4.3 8.1±0.6 4.5±0.1 Si-NLig-16%APP/PLA 49.1±3.9 8.0±0.6 3.8±0.5 表 7 PLA和阻燃PLA空气氛围下的热降解参数
Table 7. Thermal degradation parameters of PLA and flame retardant PLA in air
Sample T5%/℃ Tmax/℃ Vmax/(%·min−1) Residue at 800℃/% PLA 337 371 65.7 0 Lig-8%APP/PLA 335 372 59.4 8.4 Si-NLig-8%APP/PLA 337 368 61.3 8.4 表 8 PLA和阻燃PLA的锥形量热仪测试分析结果
Table 8. Cone calorimeter testing analysis results of PLA and flame retardant PLA
Sample TTI/s tPHRR/s PHRR/
(kW·m−2)MHRR/
(kW·m−2)THR/
(MJ·m−2)FPI/
(m2·s·kW−1)Mean COY/
(kg·kg−1)Mean CO2Y/
(kg·kg−1)PLA 74 200 411 137.4 85.1 0.180 0.024 4.803 Lig-8%APP/PLA 61 150 356 97.6 45.7 0.171 0.080 4.162 Si-NLig-8%APP/PLA 58 110 269 99.7 48.5 0.216 0.054 4.231 Notes: TTI—Time to ignition; PHRR—Peak heat release rate; THR—Total heat release; MHRR—Mean heat release rate; tPHRR—Time to peak HRR; FPI—Fire performance index; COY—Carbon monoxide yield; CO2Y—Carbon dioxide yield. 表 9 Lig-8%APP/PLA和Si-NLig-8%APP/PLA炭层的拉曼图谱分析结果
Table 9. Raman spectroscopic analysis results of residual carbon in Lig-8%APP/PLA and Si-NLig-8%APP/PLA
Sample AD AG AD/AG Lig-8%APP/PLA 1024 327 3.13 Si-NLig-8%APP/PLA 1290 651 1.98 Notes: AD—Integral area value of D peak ; AG—Integral area value of G peak. -
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