不同碳黑含量PMMA的热降解行为和动力学分析
Thermal Degradation Behavior and Kinetics Analysis of PMMA with Different Carbon Black Contents
通讯作者: 王传洋,教授,jyniigata@163.com,研究方向为高分子材料加工方法及关键技术等
责任编辑: 黄青
收稿日期: 2021-08-13 修回日期: 2022-04-01
基金资助: |
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Corresponding authors: WANG Chuanyang, Tel:
Received: 2021-08-13 Revised: 2022-04-01
Fund supported: |
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作者简介 About authors
龙庆,女,1997年生,硕士生
用熔融共混法制备不同碳黑(CB)含量的聚甲基丙烯酸甲酯(PMMA)材料,根据固态反应动力学研究了这种材料的热降解行为。在不同升温速率的非等温热重分析(TGA)实验的基础上用Friedman、FWO、KAS和Freeman-Carroll四种方法构建了不同碳黑含量PMMA的热降解动力学模型,并将TGA实验数据与求解模型对比验证了模型的可靠性。结果表明:添加碳黑使PMMA的热分解温度和活化能提高,碳黑含量(质量分数)为0.1%时达到峰值;随着碳黑含量的提高PMMA的活化能先增大后减小,其最大提高量为17.76 kJ·mol-1,表明加入碳黑能在一定程度上提高PMMA的热稳定性。
关键词:
Polymethyl methacrylate (PMMA) with different carbon black (CB) contents were prepared by melt blending. Based on the solid-state reaction kinetics, the thermal degradation behavior of PMMA with different CB contents was explored. Through non-isothermal thermogravimetric analysis (TGA) experiments under the conditions of different heating rates, the thermal degradation kinetic models of PMMA with different CB contents were built by four methods, including Friedman, FWO, KAS and Freeman-Carroll. The accuracy of models was verified by comparing with TGA experiments. The results show that PMMA with CB has higher thermal decomposition temperature and activation energy compared with pure PMMA. The PMMA with 0.1% CB presents the highest thermal stability. The activation energy of PMMA first rises and then falls down with increasing CB content, and the maximum increment is 17.76 kJ·mol-1, which proves that adding CB improves the thermal stability of PMMA to a certain extent.
Keywords:
本文引用格式
龙庆, 王传洋.
LONG Qing, WANG Chuanyang.
国内外学者对碳纳米填料对聚合物热学性能的影响,进行了大量研究。2006年Juan Li等[6]通过热重分析研究了多壁碳纳米管(MWNTs)/聚酰胺6(PA6)复合材料的热降解行为,并用Kissinger法计算出纯PA6、p-MWNTs/PA6和f-MWNTs/ PA6复合材料在空气和氮气中的降解活化能。结果表明,MWNTs的存在明显提高了PA6在空气中的稳定性,但是对PA6在氮气中的热降解行为影响不大;2009年Mi Wang等[7] 研究了聚硅氧烷接枝多壁碳纳米管/聚碳酸酯复合材料的性能,发现大量相互连接的碳纳米管形成了能阻止热量传递的碳层屏障,从而提高了底层聚合物材料的热稳定性;2017年Yamamoto T[8]通过静电相互作用将PMMA颗粒吸附在碳纤维表面,促进了碳纤维在PMMA中的分散、扩散和界面粘合,从而提高了PMMA材料的机械性能。但是目前使用的材料大多是价格昂贵的碳纳米管和碳纤维填料,且制作工艺复杂,难以在批量生产中应用。
碳黑填充剂具有良好的经济性、在聚合物基体中具有良好的分散特性以及对聚合物材料优异的增强作用。鉴于此,本文研究不同含量的碳黑对PMMA的热稳定性的影响,并在固态反应动力学的基础上求解和验证不同碳黑含量的PMMA的热降解动力学模型。
1 实验方法
实验用材料为PMMA(ALTUGLAS® V040)。不同碳黑含量的PMMA由纯PMMA母粒与不同质量分数的碳黑粉末(0%~0.25%,质量分数)共混注塑而成。共混成型的PMMA主要物理性能参数,列于表1。
表1 PMMA的物理性能
Table1
Material | Density/kg∙(m3)-1 | Molar specific heat capacity/J∙(kg∙K)-1 | Thermal conductivity /W∙(m∙K)-1 | Viscous flow temperature/℃ |
---|---|---|---|---|
PMMA | 1190 | 1470 | 0.21 | 220 |
用于热重分析实验的样品为5~10 mg不同碳黑含量的PMMA板。进行热重分析实验前,先将试样清洗20 min,然后在100~120℃干燥箱中干燥2 h;用Q500型热失重分析仪测试样品在程序升温过程中的热失重,测试温度范围为室温-600℃,升温速率5~20℃/min,氮气流速40 mL/min。所有TGA实验均进行两次,以确保实验结果的准确性。
2 热降解动力学求解
固态物质的热降解反应过程动力学,可表示为[19]
其中α为转化率;k为速率常数;f(α)为反应模型,是反应动力学机理函数关于转化率α的代数式,该反应模型通常是描述固态动力学反应的物理模型。
转化率α可表示为
其中W0为试样初始质量;Wt为试样在t时刻的质量;Wf为试样最终质量。本文用转化率预测PMMA的热降解。
可用Arrhenius(阿伦尼乌斯)方程表示速率常数
其中A为指前因子,min-1;E为活化能,kJ/mol;R为摩尔气体常数,由于实验采用氮气气氛,故这里取8.314 J/(mol·K);T为热力学温度,K。
将
TGA实验采用多个升温速率升温,升温速率可表示为
将
Li等[20]的研究结果表明:对于可用一步反应表征的固体热降解过程,可用n阶模型描述整个反应过程。因此,本文使用n阶模型表征PMMA的热降解过程,其热降解动力学机理函数为
其中n为反应级数。
Friedman法是一种微分等转化率法,其动力学方程为
在TGA曲线上截取不同升温速率β下相同转化率α时dα/dt-T的值,绘制
FWO法是一种积分等转化率法。对
式中G(α)为f(α)的积分式。在TGA曲线截取不同升温速率β下相同转化率α时T的值,绘制lgβ和1/T数据点并进行线性拟合,得到的直线其斜率即为-0.4567E/R,进而可计算出活化能E。
KAS法是一种积分等转化率法,其动力学方程为
在TGA曲线上截取不同升温速率β下相同转化率α时T的值,绘制ln(β/T2)和1/T数据点并进行线性拟合,得到的直线其斜率即为-E/R,进而可计算出活化能E。
Freeman-Carroll法是一种微分等转化率法,其动力学方程为
在TGA曲线上截取不同升温速率β下相同转化率α时dα/dt-T的值,绘制
使用得到的活化能,可计算Arrhenius方程中的指前因子A,
其中Tm为DTG峰值温度。
3 实验结果和分析
3.1 热重分析
图1给出了不同升温速率下PMMA/0% CB的TG/DTG曲线。热降解的起始温度范围为280~325℃,热降解的最高速率出现在350~390℃,高于405~440℃热降解基本完成。随着升温速率的提高,不同碳黑含量PMMA的起始分解温度、主降解阶段分解温度及终止分解温度均向高温侧移动。其原因是,试样相同的温度,升温速率越大经历的反应时间越短,反应程度越低。同时,升温速率还影响测点与试样、外层试样与内部试样间的传热温差和温度梯度,使热滞后现象加重和曲线向高温侧移动。
图1
图1
在不同升温速率下PMMA/0% CB的热稳定性曲线
Fig.1
Thermal stability of PMMA/0% CB with different heating rates (a) TG curves, (b) DTG curves
图2a~c给出了不同碳黑含量的PMMA在升温速率为5℃/min下的TG曲线。可以看出,不同碳黑含量PMMA的热降解过程都只有一个失重阶段,碳黑含量的变化不影响热失重曲线的形状,都为倒“S”型;随着碳黑含量的提高PMMA的热分解温度Tm 先提高后降低,并在碳黑含量为0.1%时达到峰值。碳黑含量小于0.1%时,提高碳黑含量可提高PMMA的热稳定性[7,21~26]。其原因是,碳黑是一种支链形式的无定形碳,是大量碳粒子通过碳晶体层相互点缀的结果[23]。因此,碳黑易于形成难以打破的三维网状碳层。碳黑纳米填料在PMMA外部形成了一层稳定的网状碳层,有效保护了底层聚合物材料,减轻了热降解程度,从而提高聚合物的热稳定性能[27,28];当碳黑含量高于0.1%时PMMA的热稳定性开始降低,因为碳黑颗粒极易在其一级和二级结构中团聚[29,30]。当碳黑含量过高时碳黑填料在聚合物基质中的分散和分布较差,使施加热载荷时填料和聚合物基体之间的热量耗散差[7,30],从而降低了材料的热稳定性,其对应的分子解释模型如图2d所示。
图2
图2
升温速率为5℃/min掺碳黑PMMA的热稳定性
Fig.2
TG curves of PMMA with different CB contents at a heating rate of 5℃/min (a) from 25 to 600℃, (b) from 350 to 390℃, (c) from 450 to 500℃, and (d) the molecular model
对降解产物质量的测量结果表明,残余物的质量变化与碳黑含量变化基本一致。这表明,掺有碳黑的PMMA并没有完全降解,不同碳黑含量的PMMA残余量随着碳黑含量的提高而增加,进而判断这些残余物是碳黑;纯PMMA的降解率接近100%,没有降解残余物。
3.2 热降解动力学
图3
图3
PMMA/0% CB四种等转化率曲线
Fig.3
Four iso-conversional rate curves of PMMA/0% CB (a) Friedman, (b) FWO, (c) KAS, (d) Freeman-Carroll
表2 Friedman、FWO、KAS方法下不同碳黑含量PMMA的活化能E
Table 2
CCB/% | Friedman | FWO | KAS | |||
---|---|---|---|---|---|---|
E/kJ·mol-1 | R2 | E/kJ·mol-1 | R2 | E/kJ·mol-1 | R2 | |
0 | 159.608 | 0.9981 | 166.755 | 0.9834 | 164.620 | 0.9813 |
0.05 | 161.461 | 0.9927 | 164.789 | 0.9898 | 162.575 | 0.9885 |
0.10 | 177.371 | 0.9979 | 175.778 | 0.9994 | 174.124 | 0.9994 |
0.15 | 169.740 | 0.9976 | 167.471 | 0.9870 | 165.371 | 0.9853 |
0.20 | 160.212 | 0.9894 | 158.353 | 0.9881 | 155.796 | 0.9864 |
0.25 | 157.274 | 0.9945 | 159.349 | 0.9970 | 156.827 | 0.9965 |
表3 Freeman-carroll法下不同碳黑含量下PMMA的反应级数n
Table 3
β/℃·min-1 | Freeman-Carroll | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
CCB/% | ||||||||||||
0 | 0.05 | 0.1 | 0.15 | 0.2 | 0.25 | |||||||
n | R2 | n | R2 | n | R2 | n | R2 | n | R2 | n | R2 | |
5 | 0.91 | 0.9998 | 0.62 | 0.9988 | 1.40 | 1.0000 | 1.38 | 1.0000 | 1.11 | 1.0000 | 0.38 | 0.9978 |
10 | 0.92 | 0.9998 | 1.07 | 1.0000 | 1.44 | 1.0000 | 1.37 | 1.0000 | 0.58 | 0.9990 | 0.95 | 0.9996 |
15 | 1.13 | 1.0000 | 1.52 | 0.9998 | 1.28 | 1.0000 | 1.44 | 1.0000 | 1.91 | 0.9986 | 1.20 | 1.0000 |
20 | 1.53 | 0.9996 | 0.95 | 0.9998 | 1.52 | 0.9996 | 1.60 | 0.9998 | 1.50 | 0.9996 | 1.45 | 0.9998 |
Average value | 1.12 | 0.9998 | 1.04 | 0.9996 | 1.41 | 0.9999 | 1.45 | 1.0000 | 1.27 | 0.9993 | 1.00 | 0.9993 |
用Friedman、FWO、KAS三种方法求解出的活化能值十分接近,但是其变化趋势不完全相同。Friedman法求解的PMMA的活化能随着碳黑含量的提高先增大后减小,用FWO和KAS法求解的活化能大小波动,其共同点是都在碳黑含量为0.1%时达到活化能峰值。用Friedman法求解出的不同碳黑含量的PMMA活化能变化趋势与前文中热分解温度的变化规律相同,因此取Friedman法求解值为最终活化能值。活化能,是分子从常态转变为容易发生化学反应的活跃状态所需要的能量,活化能值越高反应越难发生。这表明,碳黑含量的提高改变了PMMA发生化学反应需要的活化能,间接证实了碳黑能在一定程度上提高PMMA基材的热稳定性。
将活化能E值与热分解温度Tm 代入
表4 不同碳黑含量PMMA的指前因子A
Table 4
β/℃·min-1 | A | |||||
---|---|---|---|---|---|---|
CCB/% | ||||||
0 | 0.05 | 0.1 | 0.15 | 0.2 | 0.25 | |
5 | 2.8856 | 4.0788 | 87.2426 | 1.9197 | 20.8827 | 3.2088 |
10 | 2.8823 | 4.2802 | 88.1820 | 1.9237 | 21.2166 | 3.1756 |
15 | 2.7539 | 4.2433 | 94.9790 | 1.9435 | 20.1800 | 2.8766 |
20 | 2.7710 | 4.1112 | 82.3381 | 1.7478 | 19.1612 | 3.3272 |
Average value | 2.8232 | 4.1784 | 88.1854 | 1.8837 | 20.3601 | 3.1471 |
3.3 热降解动力学模型的验证
使用MATLAB 软件,将上述求出的不同碳黑含量的PMMA热降解动力学参数带入
图4给出了不同碳黑含量的PMMA在升温速率为5℃/min下转化率的模拟值与实验值的对比,其中矩形黑线是由TG实验结果变形而来的转化率曲线,圆形红线是由热降解动力学模型模拟出的转化率曲线。可以看出,实验值与模拟值二者大体上吻合。因此使用n阶模型和Friedman、Freeman-Carroll、直接求解A值法得到的不同碳黑含量的PMMA的动力学参数,能用于表征其热降解过程。
图4
图4
不同碳黑含量的PMMA热降解模型验证,升温速率为5℃/min
Fig.4
Validation of PMMA thermal degradation model with different CB contents at a heating rate of 5℃/min. (a) PMMA/0% CB, (b) PMMA/0.05% CB, (c) PMMA/0.10% CB, (d) PMMA/0.15% CB, (e) PMMA/0.20% CB, (f) PMMA/0.25% CB
4 结论
(1) 利用Friedman、Freeman-Carroll、直接求解A值法求解的热降解反应动力学模型,能再现不同碳黑含量的PMMA在氮气中的热重实验结果。
(2) 随着碳黑含量的提高PMMA的活化能先增大后减小,并在0.1%CB达到峰值。掺有碳黑PMMA的活化能,比纯PMMA提高了17.76 kJ·mol-1。碳黑能在一定程度上提高PMMA的热稳定性。
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