摘要
采用磷酸/尿素体系对双螺杆挤压制备的玉米纳米淀粉进行改性,制备了淀粉基含磷氮阻燃剂(P⁃CNS);然后通过浸渍阻燃剂法制备阻燃纸,采用垂直燃烧法、极限氧指数法和热重分析等研究了阻燃纸的性能。结果表明,P⁃CNS对纸张具有良好的阻燃效果,当P⁃CNS浸渍浓度为10%时,阻燃纸的垂直燃烧余长占纸样总长度的33.6%,极限氧指数值达到28.2%,氮气气氛下650℃时的残炭率从原纸的9.7%提高到23.2%,且抗张指数和耐破指数相比原纸仅分别降低9.7%和12.9%。
纸张主要由植物纤维抄造而成,具有可持续性和生态友好性,在包装、建筑、装饰、电子设备等领域的应用广
淀粉是自然界中取之不尽、用之不竭,且可生物降解的可再生资源,通常由25%左右的直链淀粉和75%左右的支链淀粉组
HYU⁃JF⁃3氧指数测定仪,东莞鸿进检测仪器有限公司;Vertex 70v傅里叶变换红外光谱仪,德国Bruker公司;X' Pert Pro X射线衍射仪,荷兰PANalytical公司;STA⁃8000综合热分析仪,美国PerkinElmer公司;Nova NanoSEM 230扫描电子显微镜,美国FEI公司;WZL⁃300B卧式电脑拉力仪、BSM⁃1600耐破度测定仪,杭州轻通博科自动化技术有限公司;TD5⁃A纤维解离机、TD7⁃PFI打浆机,咸阳通达轻工设备有限公司;ZCX⁃A型纸页成型器,长春市小型试验机厂。
根据文献[
将CNS(32.4 g,0.2 mol)、磷酸(4.62 g,0.04 mol)、尿素(9.61 g,0.16 mol)和100 mL蒸馏水加入到三颈圆底烧瓶中,用集热式磁力搅拌器在135℃下搅拌、油浴加热3 h,得到棕黄色液体,真空抽滤除去不溶物,溶液在烘箱干燥后磨碎得到P⁃CNS。P⁃CNS的合成路线如

图1 P⁃CNS的合成路线
Fig. 1 Synthetic route of P⁃CNS
取30 g绝干质量的硫酸盐针叶木浆,在0.5 L水中浸泡4 h后以1.0%浓度在纤维解离器中疏解30000转,再以10%浓度在PFI打浆机中进行打浆处理至打浆度约为35°SR,然后将浆料稀释至总体积为2000 mL,移入纤维解离器中疏解10000转,使纸浆纤维悬浮液充分解离。按照TAPPI标准,利用纸页成型器制备湿纸样,然后在105℃下经平板干燥制得定量为90 g/
采用浸渍法制备阻燃纸,将阻燃剂P⁃CNS溶解于蒸馏水中,配置不同质量分数(5%、10%、15%、20%、25%和30%)的溶液,25℃条件下,原纸在P⁃CNS溶液中浸渍60 s,至溶液完全渗透至纸样内部后,将浸渍纸样放入50℃烘箱干燥2 h,将取出后纸样放置在恒温恒湿室平衡48 h备用。处理纸样分别记为WP⁃5、WP⁃10、WP⁃15、WP⁃20、WP⁃25、WP⁃30。
根据UL94⁃2018标准方法对阻燃纸进行垂直燃烧测试,纸样大小为125 mm × 13 mm。采用本生灯,点火(10±0.5)s后移开火源,同时记录余焰时间t1,余焰停止时应立即点火(10±0.5)s并移开火源,记录余焰时间t2和余灼时间t3。结束后记录试样剩余长度。
根据GB/T 2406—1993进行LOI值测试,纸样大小为120 mm × 10 mm。采用极限氧指数仪,调整点火器火焰长度为(16±4)mm,在纸样顶端着火后立即移去点火器,开始计时或观察纸样被烧去的长度。若30 s内不能点燃,继续增加氧浓度,换样再次点燃。
采用KBr压片法,将试样与KBr混合,混合物在研钵中充分研磨,取适量置于压片器中压制成薄片,然后放入仪器进行测试分析,测试条件为:分辨率2 c
测试条件为:Cu⁃Kα发射电压为45 kV,扫描范围2θ=0~50°,扫描速率为1°/min,分辨率0.02°,样品的结晶度(CI,%)按下式计算。
式中,I002为002晶峰的强度,Iam为非结晶区的晶峰强度。
原纸(WP)与阻燃纸(WP⁃5、WP⁃10、WP⁃15、WP⁃20、WP⁃25和WP⁃30)的阻燃剂P⁃CNS上载率和垂直燃烧结果见
LOI值是指在规定的条件下,材料在氧氮混合气流中进行有焰燃烧所需的最低氧浓度,可以判断材料与火焰接触时燃烧的难易程度。LOI值高表示材料不易燃烧,LOI值低表示材料容易燃烧,一般认为LOI值<22%属于易燃材料,LOI值在22%~27%属于可燃材料,LOI值>27%属于难燃材料。

图2 P⁃CNS浸渍浓度对阻燃纸LOI值的影响
Fig. 2 Effect of P⁃CNS impregnation concentration on LOI value of flame retardant paper
P⁃CNS浸渍浓度对纸张抗张指数和耐破指数的影响如

图3 P⁃CNS浸渍浓度对阻燃纸抗张指数(a)和耐破指数(b)的影响
Fig. 3 Effect of P⁃CNS impregnation concentration on tensile index (a) and burst index (b) of flame retardant paper
结合垂直燃烧分析、LOI测试和纸张强度性能测试结果可知,当P⁃CNS浸渍浓度为10%时,阻燃纸燃烧时无余焰,阻燃等级达到UL⁃94 V0。P⁃CNS上载率随着P⁃CNS浸渍浓度的增大而增大,纸样中P⁃CNS含量逐渐增加,但纸样的强度指标逐渐降低,且P⁃CNS含量高,生产成本也高。综合考虑,P⁃CNS浸渍浓度为10%,P⁃CNS上载率在25%左右较合适,既能保证阻燃纸的阻燃性能达到UL⁃94 V0等级,也能最大限度地保留纸张的强度性能。
CNS改性前后的FT⁃IR谱图如

图4 CNS与P⁃CNS的FT⁃IR谱图
Fig. 4 FT⁃IR spectra of CNS and P⁃CNS
为了进一步揭示阻燃剂P⁃CNS在纸张凝聚相中的阻燃机制,对原纸残炭(WPC)和阻燃纸残炭(WPC⁃10)进行了FT⁃IR光谱分析,结果如

图5 WPC和WPC⁃10的FT⁃IR谱图
Fig. 5 FT⁃IR spectra of WPC and WPC⁃10
采用热重分析仪对WP和WP⁃10、WP⁃20和WP⁃30在氮气气氛中的热稳定性进行了分析,结果如

图6 WP、WP⁃10、WP⁃20和WP⁃30的TG (a) 和DTG (b) 曲线
Fig. 6 TG (a) and DTG (b) curves of WP, WP⁃10, WP⁃20, and WP⁃30
采用XRD分析了WP和WP⁃10的晶体结构,结果如

图7 WP和WP⁃10的XRD谱图
Fig. 7 XRD patterns of WP and WP⁃10
采用SEM分析了WP⁃10及其残炭WPC⁃10的表面微观形貌,结果如

图8 WP⁃10 (a和b)及其残炭WPC⁃10 (c和d)的SEM图
Fig. 8 SEM images of WP⁃10 (a and b) and its char WPC⁃10 (c and d))
3.1 采用磷酸/尿素体系对玉米纳米淀粉(CNS)进行改性,成功制备出淀粉基含磷氮阻燃剂(P⁃CNS),该阻燃剂可有效提高纸张的阻燃性能和热稳定性,具有凝聚相阻燃和气相阻燃机制,阻燃纸燃烧时表面会形成致密炭层,离开火焰能自熄。
3.2 采用10% P⁃CNS浸渍制备的阻燃纸,垂直燃烧余长为4.2 cm,占纸样总长度的33.6%,极限氧指数值为28.2%,达到难燃级别;抗张指数和耐破指数相比原纸分别降低了9.7%和12.9%;氮气气氛中650℃时的残炭率从原纸的9.7%增至23.2%;阻燃剂P⁃CNS对阻燃纸的纤维素晶型结构没有影响,其结晶度略有降低。
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