注塑級聚乳酸熱分解動力學分析

羅越峰，溫樺浩，廖正福

注塑級聚乳酸熱分解動力學分析

羅越峰，溫樺浩，廖正福*

（廣東工業(yè)大學 材料與能源學院，廣州 510006）

研究注塑級聚乳酸材料的熱分解動力學，準確理解注塑級聚乳酸的耐熱穩(wěn)定性，為開發(fā)耐高溫阻燃注塑級聚乳酸（PLA）材料提供理論依據(jù)。通過非等溫熱重分析法，采用5、10、15、20、25 ℃/min的升溫速率，研究注塑級PLA在氮氣氣氛中的熱分解行為，利用1種微分法和3種積分法進行詳細的動力學計算。比較相關系數(shù)及標準偏差，選取KAS等溫積分法，以相對偏差A=|1–c/e|為目標函數(shù)，利用16種熱分解動力學機理模擬計算注塑級PLA熱分解最合適的反應機理。得到了注塑級PLA熱分解所需的活化能和指前因子，其中Kissinger、Madhusudanan-Krishnan-Ninan（MKN）、Kissinger-Akahira-Sunose（KAS）和Flynn-Wall-Ozawa（FWO）法計算所得注塑級PLA活化能分別為177.01、174.43、173.01和173.28 kJ/mol。指前因子分別為25.84、26.69～33.75、25.83～32.89和26.38～32.94。確定了隨機成核和隨后生長反應機理（A1/4），ln(/2)=ln[4.75×109/–ln(1–)]–2.08×104/是描述注塑級PLA熱分解最合適的反應機理。

注塑級聚乳酸；熱分解動力學；非等溫分析；反應機理

注塑級聚乳酸（PLA）及其復合材料因其優(yōu)異的透明度、生物相容性、生物降解性和可加工性已成為當今研究最為廣泛的綠色塑料種類之一[1-4]，但是未改性注塑級PLA的熱變形溫度（HDT）只有58 ℃左右，遠低于通用塑料PS和PP等[5]，使注塑級PLA的應用受到了很大限制。另外，改善PLA材料的熱穩(wěn)定性還可以改善其降解速率、成炭速率和出炭率等[6-7]，提高其阻燃性能。因此，研究注塑級PLA的熱降解動力學，對準確研究其耐熱機理，進而開發(fā)高耐熱阻燃注塑級PLA材料，拓寬其應用具有重要意義。

目前為止，關于聚乳酸復合材料熱解動力學的研究已有廣泛報道[8-12]，但有關注塑級PLA熱分解動力學的研究并不多，且不同PLA結構、分析方法和熱失重氣氛等研究結果也存在較大差異。氮氣氣氛中，金玉順等[13]利用Freeman-Carroll、Achar和KAS法計算得到星形聚L-乳酸的熱分解表觀活化能值范圍為128.61～134.79 kJ/mol，ln為23.46～25.28；付春華等[14]利用FWO和Friedman法計算得到D, L-聚乳酸熱降解活化能值分別為97.78、90.64 kJ/mol；Alhulaybi等[15]利用Friedman、FWO、KAS和Starink法計算得到PLA熱分解活化能值分別為97、109、104和104 kJ/mol。空氣氣氛中，韓宇辰等[16]利用FWO和Kissinger法求得反應活化能值分別為104.9、109.2 kJ/mol。通過Ozawa等失重百分率法求得熱解反應的平均活化能值為91.4 kJ/mol和ln為14.9～18.2。本文擬通過非等溫熱重分析法，研究注塑級PLA在氮氣氣氛中的熱分解行為，并通過Kissinger、FWO、KAS和MKN法進行熱分解動力學分析，以期為注塑級PLA耐熱穩(wěn)定性研究提供一些參考依據(jù)。

1 實驗

1.1 材料和儀器

主要材料和儀器：注塑級聚乳酸（PLA），3052D，美國Nature Works LLC；AL-204電子天平，METTLER TOLEDO；DZF-6020真空干燥箱，上海博迅實業(yè)有限公司醫(yī)療設備廠；SDT-2960熱重分析儀，美國TA公司。

1.2 表征方法

熱失重曲線在美國TA公司SDT-2960熱重分析儀上記錄，掃描溫度為室溫～800 ℃，N2氣氛，流速為20 mL/min，升溫速率分別為5、10、15、20、25 ℃/min。

1.3 PLA熱分解動力學簡析

研究物質熱解機理的熱分析方法分為等溫動力學方法和非等溫動力學方法。其中，非等溫動力學又分為無模型動力學方法和模型擬合動力學方法[17]。無模型方法是材料熱解過程動力學研究中最常用的方法[18-23]，也稱為等轉換法。無模型方法分為微分法和積分法2類，微分法由于采用瞬時速率值、差分等變換方法，對實驗噪聲敏感，往往使得計算數(shù)值不穩(wěn)定，而積分法可以有效避免這種現(xiàn)象[24]。熱重分析（TGA）方法簡單、準確，是獲得熱解特性和動力學參數(shù)的最佳方法[25-29]。

固體的熱分解過程是一種非均相的熱分解反應體系，一般可描述為：

固體（solid）→最終分解殘留物（solid）+揮發(fā)物（gas）

定義熱解失重轉化率為：

(2)

式中：為指前因子，s?1；為表觀活化能，kJ/mol；為氣體常數(shù)，=8.314 J/(mol·K)；為絕對溫度，K。非均相固體熱分解反應的分解速率定義如下：

對樣品線性升溫時，升溫速率dd，則式（3）變?yōu)椋?/p>

式（3）和式（4）是基于物質熱失重數(shù)據(jù)進行熱分解動力學分析的2個基本微分方程。熱分解動力學研究的任務就是設法獲得式（3）和式（4）中表征某個熱解反應過程的動力學三因子：()，并以此來對熱分解曲線進行擬合和預測，進而展開熱分解過程的模擬設計與參數(shù)控制。

1.4 微分動力學參數(shù)計算

1.4.1 Friedman等轉化率微分法

1.4.2 Kissinger等轉化率微分法

1.5 積分動力學參數(shù)計算

1.5.1 MKN等溫積分法

1.5.2 KAS等溫積分法

1.5.3 FWO等溫積分法

Flynn-Wall-Ozawa（FWO）方法基于阿倫尼烏斯定律和Doyle的近似理論[38]，是等轉換熱分析方法中常用的積分方法之一[39-40]（式（9））。通過lg對1/作圖，由斜率可計算值，由截距和機理函數(shù)()可求得ln值。

2 結果與分析

2.1 PLA的熱失重分析

圖1為PLA在不同升溫速率下的TG-DTG曲線。可以發(fā)現(xiàn)，PLA僅有一段熱失重（320～390 ℃）。溫度低于320 ℃時，TG曲線平直，表明PLA沒有發(fā)生熱分解。在320～390 ℃，TG曲線斜率驟變，PLA發(fā)生熱分解，樣品熱失重迅速。隨著溫度的繼續(xù)增加，當溫度達到390 ℃以上，PLA熱分解基本結束，樣品不再質量損失，最大質量損失率達到97%以上。隨著升溫速率從5 ℃/min增加到25 ℃/min，PLA熱解溫度（）也隨之增加（見表1），表明PLA熱解溫度與升溫速率有關。原因在于熱解速率滯后于升溫速率，需要較高溫度加以補償。

圖1 TG-DTG曲線

表1 不同升溫速率下熱解參數(shù)

Tab.1 Thermal deposition parameters at different heating rates

2.2 PLA的熱分解活化能計算

為了更好地理解PLA的熱分解過程，分別利用Kissinger、MKN、KAS和FWO法，對表1熱解參數(shù)進行線性擬合得到等轉化率Arrhenius圖（圖2a～d）。根據(jù)直線斜率，可分別求得PLA的熱分解活化能（見表2）。可以發(fā)現(xiàn)，用MKN、KAS、FWO方法計算得到的活化能并非常數(shù)，隨轉化率變化。在整個熱失重區(qū)間，MKN方法求得介于148.81～179.03 kJ/mol，平均值為173.28 kJ/mol；KAS方法求得介于148.51～178.76 kJ/mol，平均值為173.01 kJ/mol；FWO方法求得介于150.69～180.02 kJ/mol，平均值為174.43 kJ/mol；Kissinger方法求得的平均值為177.01 kJ/mol。比較4種方法，MKN、KAS、FWO方法計算得到的活化能平均值相近，而Kissinger法由于只采樣一個溫度點而得到的活化能偏大。同時，MKN方法的R介于0.955 0～ 0.993 0，平均值為0.981 7；KAS方法的2介于0.954 7～0.992 9，平均值為0.981 6；FWO方法的2介于0.959 9～0.993 7，平均值為0.983 5；Kissinger方法的2為0.988 8。4種方法模擬所得線性相關系數(shù)R均接近于1，表明數(shù)據(jù)間線性關系優(yōu)良，4種方法計算均具有良好的準確性，所得活化能結果是可靠的。

指前因子ln是物質熱分解過程的另一個重要動力學參數(shù)。從圖2a～d的直線截距可以計算PLA熱分解過程的ln（如表2所示）。其中，MKN方法得到的PLA的ln介于26.69～33.75，平均值為32.44；KAS方法得到的PLA的ln介于25.83～32.89，平均值為31.58；FWO方法得到的PLA的ln介于26.38～33.18，平均值為31.92；Kissinger方法求得的ln為32.75。4種方法計算所得PLA的指前因子均相近，同時MKN、KAS和FWO法所得的活化能和指前因子值隨著變化的趨勢是一致的，這與文獻[16, 41]的報道是一致的，進一步證實上述活化能計算結果的可靠性。

上述結果表明，F(xiàn)riedman法受基線漂移的干擾影響非常顯著，往往導致計算數(shù)據(jù)不夠準確；Kissinger法由于只采樣一個溫度點而得到的活化能偏大，不夠準確；FWO和MKN法在=50%前受揮發(fā)性物質分解的影響較大，2平均值均小于KAS法，不夠準確。為此，選用相對準確、簡單的KAS法進行PLA的熱分解反應機理的確定。

圖2 等轉化率Arrhenius圖

表2 FWO、KAS、MKN和Kissinger法擬合計算的結果

Tab.2 Results of the fitting calculations by the FWO, KAS, MKN and Kissinger methods

2.3 PLA的熱分解反應機理的確定

表3 16種機理函數(shù)的微分和積分形式

Tab.3 Differential and integral forms of 16 mechanism functions

圖3 PLA反應機理模擬

3 結語

1）MKN、KAS、FWO和Kissinger 4種方法模擬所得線性相關系數(shù)2均接近于1，表明數(shù)據(jù)間線性關系優(yōu)良，4種方法計算均具有良好的準確性，所得活化能結果是可靠的。

因本文重點在于研究注塑級PLA材料的熱分解動力學，用于準確理解注塑級PLA的耐熱穩(wěn)定性，為開發(fā)耐高溫阻燃注塑級PLA材料提供理論依據(jù)。為此關于機理的合理性，正在進一步實驗驗證。

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Thermal Decomposition Kinetics Analysis of Injection Grade Polylactic Acid

LUO Yuefeng, WEN Huahao, LIAO Zhengfu*

(School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China)

The work aims to study the thermal decomposition kinetics of injection grade polylactic acid (PLA) and accurately understand the heat stability of injection grade polylactic acid (PLA), so as to provide theoretical basis for developing high temperature and flame retardant injection grade polylactic acid (PLA) materials. By non isothermal weight analysis, the heating rate of 5, 10, 15, 20 and 25 ℃/min was adopted to analyze the thermal decomposition behavior of injection grade PLA in nitrogen atmosphere. One differential method and three integral methods were used to carry out the detailed kinetics calculation. The correlation coefficient and standard deviation were compared. KAS isothermal integration method was selected, and with relative deviationA=|1–c/e|as the objective function, 16 kinds of thermal decomposition kinetics mechanisms were used to simulate and calculate the most appropriate reaction mechanism for the thermal decomposition of injection grade PLA. The activation energy and pre-exponential factors required for thermal decomposition of injection grade PLA were obtained. The activation energies of injection grade PLA calculated by Kissinger, Madhusudanan-Krishnan-Ninan (MKN), Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) were 177.01, 174.43, 173.01 and 173.28 kJ/mol, respectively. The pre-exponential factors were 25.84, 26.69-33.75, 25.83-32.89 and 26.38-32.94, respectively. The random nucleation and subsequent growth reaction mechanism (A1/4) are determined and ln(/2)=ln[4.75×109/–ln(1–)]–2.08×104/is the most appropriate description of injection grade PLA thermal decomposition reaction mechanism.

injection grade polylactic acid; thermal decomposition kinetics; non-isothermal analysis; reaction mechanism

TB324

A

1001-3563(2024)07-0045-08

10.19554/j.cnki.1001-3563.2024.07.007

2023-12-13

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