磷氮阻燃聚乳酸热分解动力学的研究

姚露 ,  刘媛 ,  罗越峰 ,  廖正福

塑料科技 ›› 2025, Vol. 53 ›› Issue (09) : 66 -71.

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塑料科技 ›› 2025, Vol. 53 ›› Issue (09) : 66 -71. DOI: 10.15925/j.cnki.issn1005-3360.2025.09.012
理论与研究

磷氮阻燃聚乳酸热分解动力学的研究

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Study on Thermal Decomposition Kinetics of Phosphorus-nitrogen Flame Retardant Polylactic Acid

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摘要

利用非等温加热方式的热分析技术研究磷氮阻燃聚乳酸(PLA)在N2气氛中的热分解行为,分别采用Kissinger方程、Madhusudanan-Krishnan-Ninan(MKN)方程、Kissinger-Akahira-Sunose(KAS)方程和Flynn-Wall-Ozawa(FWO)方程模拟计算阻燃PLA热分解动力学参数,活化能(Ea)计算结果依次为187.62、180.06、179.80、180.95 kJ/mol;指前因子自然对数(ln A)计算结果依次为41.56、24.39~34.60、24.27~34.51和24.92~34.67。最后,选取相对简单的KAS等温积分法,以相对偏差(DR)为目标函数,并利用16种热分解动力学机理数值进行模拟。结果表明:A1/4(随机成核和随后生长)反应机理,即ln(βT2)=ln[-2.54×1010ln(1-α)]-2.16×104T,是描述磷氮阻燃PLA热分解最适反应机理。研究结果对于准确理解PLA的阻燃机理,进而开发高效PLA阻燃体系具有重要意义。

Abstract

The thermal decomposition behavior of injection-grade polylactic acid (PLA) containing phosphorus-nitrogen flame retardant was studied by non-isothermal thermogravimetric analysis under nitrogen atmosphere. The kinetic parameters of the thermal decomposition of the PLA/IPOI system were obtained by using a differential method and three integral methods, which include Kissinger method, Madhusudanan-Krishnan-Ninan (MKN) method, Kissinger-Akahira-Sunose (KAS) method, and Flynn-Wall-Ozawa (FWO) method. The activation energy (Ea) calculated results by these methods was 187.62, 180.06, 179.80, 180.95 kJ/mol, respectively, and the pre-exponential factors (ln A) calculated results by these methods was 41.56, 24.39~34.60, 24.27~34.51 and 24.92~34.67, respectively. Finally, based on relative deviation (DR) as the objective function, KAS isothermal integral method was selected for simulation calculation, and 16 values of thermal decomposition kinetic mechanism were used to simulate. The numerical simulation results showed that the random nucleation and subsequent growth reaction mechanism (A1/4), ln(βT2)=ln [-2.54×1010ln(1-α)]-2.16×104T, was the best fit reaction mechanism to describe the thermal decomposition of the flame-retardant PLA/IPOI system. The research results are of great significance for accurately understanding the flame retardancy mechanism of PLA and developing efficient flame retardancy system of PLA.

Graphical abstract

关键词

磷氮阻燃聚乳酸 / 热分解动力学 / 非等温热重分析 / 反应机理

Key words

Phosphorus-nitrogen flame retardant polylactic acid / Thermal decomposition kinetics / Non-isothermal thermogravimetric analysis / Reaction mechanism

引用本文

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姚露,刘媛,罗越峰,廖正福. 磷氮阻燃聚乳酸热分解动力学的研究[J]. 塑料科技, 2025, 53(09): 66-71 DOI:10.15925/j.cnki.issn1005-3360.2025.09.012

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聚乳酸(PLA)及其复合材料作为一类透明度高、生物相容性好、可降解且易加工的绿色塑料,近年来受到广泛关注[1-3]。然而,PLA的易燃特性极大地限制其应用范围,因此如何提高其阻燃性能已成为亟待解决的关键问题[4-5]。在众多阻燃改性方法中,磷-氮膨胀阻燃体系(IFR)成为目前常用且效果显著的手段[6-8]。其中,9,10-二氢-9-氧杂-10-磷杂菲-10-氧化物(DOPO)及其衍生物因分子结构中含有联苯环、菲环、O=P—O键而在构建磷氮膨胀阻燃体系阻燃PLA中具有优异的阻燃性能[9-12]。更为关键的是,采用DOPO类反应型阻燃剂对PLA进行阻燃改性后,PLA主链的化学结构并未发生显著的改变,其可降解性得以保留[13],仍具有绿色环保的优点。这对于控制热分解、达到生产所需产品和废物回收利用具有重要价值。此外,材料的阻燃性能并非仅由其热稳定性单一因素决定,热分解速率、热分解机理、成炭速率以及残炭率等动力学因素同样发挥至关重要的作用[14-15]。尽管此前对PLA及其复合材料热解机理及其动力学的研究较多[16-22],但磷氮阻燃PLA体系的热解机理及其相关动力学参数的研究较少[23-25]。本文采用非等温热重分析技术,结合Kissinger方程、Flynn-Wall-Ozawa (FWO)方程、Kissinger-Akahira-Sunose (KAS)方程和Madhusudanan-Krishnan-Ninan (MKN)方程对磷氮阻燃PLA体系在氮气气氛中的热解行为进行深入分析,以期为研发高效PLA阻燃体系提供参考。

1 实验部分

1.1 主要原料

PLA,注塑级(半透明状颗粒),美国NatureWorks LLC公司;阻燃剂(IPOI),白色粉末,根据专利[26]自制。

1.2 仪器与设备

高速搅拌机,JYK-800Y,永康市鸿德五金制品厂;双辊开炼机,HY160D,上海恒驭仪器有限公司;平板硫化机,BD-8820-A,东莞市宝鼎精密仪器有限公司;热重分析仪,TGA2,瑞士Mettler Toledo公司。

1.3 样品制备及热分析方法

取100 phr的PLA以及1、3、5、8、10 phr的IPOI,用高速搅拌机搅拌混匀后,通过双辊开炼机混炼、平板硫化机压制得到测试样品。

1.4 热分析方法

使用热重分析仪采集热失重数据,测试条件如下:氮气流速为20 mL/min,温度范围为25~800 ℃,升温速率分别为5、10、15、20、25 K/min。

1.5 动力学研究方法

1.5.1 动力学方法原理

一般地,固体的热解过程可描述为非均相化学反应,即可表示为:固体(s)→残留物(s)+挥发气体(g)。显然,将固体热分解速率代替反应速率、热解失重率代替反应转化率,则可以通过化学反应动力学方程计算固体热解过程的动力学参数,包括活化能Ea、指前因子A(或指前因子自然对数ln A)等。因此,基于不同升温速率下的热失重分析结果,可以用阿伦尼乌斯公式准确地对固体材料的热分解过程进行动力学分析[27-28]

定义热解失重率(α)为:

α=W0-WtW0-W

则结合Arrhenius方程k(T)=Aexp (-EaRT),得到热分解速率(dαdt)为:

dαdt=k(T)f(α)=Aexp(-EaRT)f(α)

实验中,若升温速率为β=dTdt,则式(2)可表示为:

dαdt=(Aβ)exp(-EaRT)f(α)

式(1)~式(3)中:W0WtW分别为样品起始、t时刻、热解终时的质量,mg;f(α)为机理函数;Ea为表观活化能,kJ/mol;A为指前因子,s-1R为气体常数,8.314 J/(mol·K);T为绝对温度,K。

根据上式可以得到固体热分解动力学方程中的A(或ln A)、Eaf(α)。

1.5.2 微分动力学参数计算

根据升温方式不同,固体热解动力学分析方法可以分为等温和非等温两类方法。其中,非等温方法又分为无模型拟合动力学方法和模型拟合动力学方法[25]。在固体热解过程动力学研究中,无模型方法(又称为等转换法)应用最为广泛[29-34],该方法进一步分为微分法和积分法。无模型微分法采用瞬时速率值及差分等变换方法,对实验噪声较为敏感,容易导致计算结果不稳定,而无模型积分法则对实验噪声不敏感[35]

(1) Friedman等转化率微分法。

Friedman等转化率微分法(或称FD方法)是常用的等转化率微分方法[36],可表示为:

ln(dαdt)=ln[Af(α)]-EaRT

构建方程过程中未进行任何近似和假定,因而适用于各种热分解过程的动力学分析。利用式(4)ln(dαdt)-1T图,从斜率和截距可分别得到固体热分解的EaA(或ln A),但基线漂移对FD方法影响显著,会导致计算数据不够准确[37-38]

(2) Kissinger等转化率微分法。

Kissinger法也称最大转化率法,是通过式(2)在最大转化率d2αd2t=0的条件下作简单变换再取对数后整理而得的一种微分方法[39],可表示为:

ln(βTp2)=ln(AREa)-EaTp

式(5)中:Tp为最大失重速率温度,K。

利用式(5)ln(βTp2)-1Tp图,由斜率和截距可分别得到EaA(或ln A)。

1.5.3 积分动力学参数计算

通常认为,固体刚开始发生热解时的转化率可以忽略不计,积分区间T0~T可用0~T替代,使T0Texp(-EaRT)dT变换为0Texp(-EaRT)dT。定义x=EaRT,则有:

G(α)=(Aβ)T0Texp(-EaRT)dT=(AEaβR)xe-xx2dx

定义p(x)=xe-xx2dx,引入h(x)=p(x)x2ex,则有:

G(α)=AEae-xβRx2h(x)

计算表明,h(x)值由0缓慢增加,直到渐近值1[40]。显然,用h(x)替代p(x)更易于分析各种温度积分。对式(7)两边取对数,则有:

ln[G(α)]=ln(AEaβR)+ln[p(x)]=ln(AEaβR)+ln[h(x)]-2ln x-x

式(8)几乎涵盖所有温度积分方法的形式,采用不同方法处理ln[p(x)]项和ln[h(x)]项就形成各种温度积分方法。将ln[h(x)]项合并到-2ln x-x项中进行处理,即形成MKN类指数型温度积分方程[41-42];将ln[h(x)]项和ln(AEaβR)项合并进行处理,则形成KAS有理数型温度积分方程[43]。MKN方程和KAS方程分别为:

ln(βT1.884 318)=ln[A(Ea/R)-0.884 318G(α)]-1.001 928EaRT-0.389 677
ln(βT2)=ln[AREaG(α)]-EaRT

基于式(9)ln(βT1.884 318)-1T图,基于式(10)ln(βT2)-1T图,由斜率可得Ea,由机理函数G(α)和截距可得ln A。此外,Starink在总结MKN方程的基础上提出1个含有指数形式的普遍近似公式[44]

ln[p(x)]=aln x+bx+c

式(11)中:abc为常数,在不同积分方程中取值不同。取a=0时,可以得到Doyle近似方程[45]

ln[G(α)]=ln(AEaβR)-1.051 6EaRT-5.330 8

将阿伦尼乌斯定律应用于Doyle近似理论,即得到FWO方程[46-47],公式如下:

lg β=lg[AEaRGα]-0.456 7EaRT-2.315

lg β-1T图,由斜率可得Ea,由机理函数G(α)和截距可得ln A

2 结果与讨论

2.1 体系的热分解活化能计算

表1为FWO、KAS、MKN和Kissinger法的拟合结果。

表1可以看出,4个方程模拟所得线性相关系数R2均非常接近于1,表明实验数据线性关系优良,拟合结果可信。随着热分解转化率α的提高,热分解的Ea逐渐增加。当α10%时,因为P—C键的键能较低,PLA/IPOI阻燃体系热分解的Ea较小[48];当α15%时,阻燃剂IPOI存在脱水炭化效应,提高了PLA/IPOI体系的热分解Ea,从而使PLA的热稳定性增强[49]

此外,MKN、KAS和FWO这3个方程拟合计算所得的Ea比较接近,而且Ea随热解转化率α升高的变化趋势也相似,这与文献[23,50]的研究结果一致,进一步说明上述计算的Ea具有较高的可信度。而Kissinger方程拟合得到的Ea较大,这可能与只采一个温度样点具有偶然性有关。为此,本文选用相对简单的KAS方程来研究PLA/IPOI体系热分解最适反应机理。

2.2 热分解反应机理的确定

定义相对偏差DR=1-αcα(其中,αcα分别为理论计算和实验测定的热解转化率),当αcα之间越接近,DR越小,表明该反应机理更适合用于描述这种固体热解过程。

表2为16种机理函数的微分和积分形式。将表2中的16种反应机理方程逐个代入式(10)计算热分解转化率αc,分别得到各个机理在5种不同升温速率下不同α对应的DR图1为PLA/IPOI热分解机理模拟结果。

图1可以看出,5种不同升温速率中,以A1/4机理拟合计算所得的DR最小,也就是说,A1/4反应机理是描述注塑级PLA/IPOI磷氮阻燃体系热解最适合的反应机理,其机理方程为ln(βT2)=ln[-2.54×1010ln1-α]-2.16×104T,而且磷氮阻燃剂IPOI的加入仅提升PLA体系的热稳定性,并未改变体系的热解机理。

3 结论

利用非等温热失重分析技术探究PLA/IPOI磷氮阻燃体系在N2气氛中的热分解行为,并采用MKN、KAS、FWO和Kissinger共4个方程进行动力学模拟计算。结果发现,MKN、KAS、FWO和Kissinger模拟得出的R2均非常接近于1,表明实验数据间线性关系良好,计算结果具有可靠性;同时发现,MKN、KAS、FWO拟合计算得到的平均Ea相近,而Kissinger方程可能由于单一温度点采数据导致拟合得到的Ea偏大。最后,选取相对简单的KAS等温积分方程对16种热分解动力学机理的PLA/IPOI磷氮阻燃体系的热分解过程进行模拟,基于DR=1-αcα为目标函数获得最合适的反应机理,即随机成核和随后生长反应机理(A1/4),具体方程为ln(βT2)=ln[-2.54×1010ln1-α]-2.16×104T

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