Materials

P(LA-co-CL)-2000 molecular weight polyol was purchased from Shandong Huakai Resin, isophorone diisocyanate (IPDI) and 1,4-butanediol (BDO) were purchased from Macleans Biochemical Technology, Shanghai, dibutyltin dilaurate (DBTDL) from Chemical Reagent Factory, Guangzhou, di-n-butylamine from Lingfeng Chemical Reagent Factory, Shanghai, and isopropanol from Yongda Chemical Reagent Limited Company, Tianjin.

Preparation of PLA Acid-Based Polyurethane

The polyurethane sample that used in the analysis was synthesized by a two-step method with the co-intercalated polyol of PLA and PCL as the soft segment, the IPDI and chain extender BDO as the hard segment. The preparation of PLA-based polyurethane is shown in Fig. 1.

Methods of Thermal Degradation Behavior

It is assumed that the thermal decomposition reaction conversion rate and time variation of PLA-based polyurethane follows by Equation (1):

Fig. 1.

Preparation of PLA-based polyurethane.

In the formula, α is the conversion rate; k is the reaction rate constant; and n is the reaction order. When the time variation is small enough, the non-isothermal thermal decomposition reaction can be regarded as isothermal. According to the Arrhenius principle [10], the Equation (2) can be obtained as follows:

T is the absolute temperature of pyrolysis; A is the pre-factor; β is the heating rate; Eα is the activation energy of the thermal decomposition reaction; R is the gas molar constant 8.314 J/(K·mol); f(α) is a reaction mechanism function that depends on the control reaction mechanism. And α can also be named the standardized mass of volatile substances, which can be expressed by the following formula:

Where m₀ is the initial mass of the sample, m_t is the mass of the sample at any time t, and m_f is the final mass of the sample after the pyrolysis reaction. Several research models of thermal degradation kinetic behavior are shown as follows.

Characterization

Thermal Degradation Behavior and Kinetics

The thermogravimetric (TG) and derivative thermogravimetry (DTG) curves of PLA-based polyurethane at different heating rates are illustrated in Fig. 2. The thermal decomposition processes are divided into three stages by means of split-peak fitting according to Gaussian curve. And their characteristic data are determined from the TG, DTG, first time derivatives and second time derivatives curves of DTG at different heating rates. Here using DTG curve at the heating rate of 5ºC/min as an example (Fig. 3), the extrapolated onset decomposition temperature was obtained by extrapolating the slope of the DTG curve in correspondence with the first local maximum in first derivative curve and down to the baseline of the DTG curve [18].

Fig. 2.

TG and DTG curve of PLA-based polyurethane at different heating rates.

Fig. 3.

Determination of decomposition characteristic data of PLA-based polyurethane.

The peak temperature, T_p, is determined by DTG peak, where the maximum decomposition rate is obtained and corresponded to the minimum of the second derivative curve of the DTG curve. The weight loss percentage corresponding to T_p is represented with WL_p. The end temperature, T_e, is defined by extrapolating the slope of the DTG curve corresponding to the local minimum of the first derivative curve in this region and down to the baseline of the DTG curve. The weight loss percentage corresponding to T_e is marked with WL_e. According to above, the decomposition characteristics data of PLA-based polyurethane are presented in Table 2 at different heating rates.

In Figs 2 and 3 and Table 2, when the PLA-based polyurethane is degraded at a heating rate of 20ºC/min., its degradation starts from 294.15ºC and ends at 415.74ºC with a degradation rate of (mass loss rate) 93.57%. The degradation process is divided into three stages: the first stage is from 294.15 to 318.16ºC, with a degradation rate of 2.41%; the second stage is from 318.16 to 358.67ºC, with a degradation rate of 26.38%, and the third stage is from 358.67 to 415.74ºC, with a degradation rate of 66.16%. With the increase of degradation warming rate, the initial and end temperatures of degradation of the specimens increase, the degradation temperature range become larger and the degradation rate decreases; the degradation process still shows three stages, and the peak temperature of the degradation heat absorption peak of each stage move to the high temperature area, and the degradation rate of different stages do not show regular changes, which indicates that the change of warming rate has a complex effect on the degradation behavior of different stages of the degradation process. The reason for this result may be due to the difference in sensitivity to temperature of sample components, heat transfer barrier effect between broken chain segments, and further degradation mechanism of degradation product fragments at different degradation stages with the change of warming rate [19, 20].

In order to better understand the thermal decomposition process of PLA-based polyurethane, the non-isothermal kinetic integration FWO method, KAS method and differential FD method are used to analyze the kinetics of polyurethane and obtain the activation energy of kinetic parameters. The related fitting and calculation results are shown in Table 3 and Figs 4 and 5.

In the first stage (0<α≤0.15), the linear fits based on the FWO and KAS methods, respectively, are almost parallel, which indicates the activation energy of degradation calculated for lactic acid-based polyurethanes at different conversion rates is approximated as a constant, and consequently implies the possibility of single reaction mechanism (or the unification of multiple reaction mechanisms). In the second and third stages, the parallelism of the straight lines fitted according to the above two methods deteriorated, and the slope of the straight lines changed more and more obviously as the conversion rate became larger, and the activation energy gradually decreased accordingly (the results are shown in Fig. 5a), which predicted that the thermal degradation reaction mechanism changed at α>0.15, and there might be autocatalytic effects in the degradation mechanism, leading to more complicated modeling and simulation of the degradation process. The trends of the lines fitted by the FWO and KAS methods show that the thermal degradation of PLA based thermoplastic polyurethane belongs to a possible single reaction mechanism at low conversions (α≤0.15) and to a variable mechanism when higher conversions are reached (α>0.15).

As the goodness of fit (GOF) considered, The R² values range of fitting linear of FWO method is from 0.9443 to 0.9948 while that of KAS method is between 0.9103 and 0.9907 at different conversion rate (The results are presented in Fig. 5b). The R² values, reflecting reliability of linear fit, are closer to 1 at moderate conversions (0.15≤α≤0.65) than at low conversions (α<0.15) or high conversions (α>0.65). However, the lines fitted with the differential FD method show non-parallelism at different stages of the whole thermal decomposition process, revealing the non-constant characteristics of activation energy calculated at different conversion rate. Furthermore, there is a large dispersion of the R² values of the fitted lines, which are far from 1. Therefore, it is not appropriate to use the Friedman method as one of the kinetic models for the thermal decomposition of PLA-based polyurethane.

Fig. 4.

The fitting curves of PLA based-polyurethane by FWO a), KAS (b) and FD c) methods at different conversion rates.

Considering the above results together, the conversion rates range from 0.15 to 0.65 (R²>0.98) is emphasized, rather than the entire decomposition process, which may offer a simplified and more meaningful way to model the thermal decomposition behavior of PLA-based polyurethane. As a result, the average apparent activation energies calculated by FWO and KAS are 24.28 kJ/mol and 19.24 kJ/mol at different conversion rate.

In order to determine the decomposition reaction mechanism of PLA-based polyurethane, the thermal decomposition process was explored according to the Criado reaction mechanism model and the Coast-Redfern method (C-R method). Based on the different reaction mechanisms g(x) in Table 1 and the experimental thermal decomposition curves of PLA-based polyurethanes at a heating rate β of 10ºC/min, the Z(x)/Z(0.5) master curve can be plotted using Equation (7). Fig. 6 shows the Z(x)/Z(0.5) master and the experimental curves of the PLA-based polyurethane, respectively. It can be seen that the trend of experimental sample curve, is close to the master curves of R2, R3, A2, A3, and A4, and nearly overlaps the master curve (Z(R2)/Z(0.5)) of A2. This result indicates that the thermal decomposition behavior of PLA-based polyurethane complies with A2 reaction mechanism model, and belongs to the nuclear growth mechanism [21]. For further verification, the corresponding reaction mechanism functions were substituted into the C-R method, and the relevant kinetic parameters of the experimental samples were calculated. The results are presented in Fig. 7 and Table 4.

Fig. 5.

Activation energy calculated and fitting reliability vs α according to FWO and KAS methods.

It can be observed that the activation energy (23.11 kJ/mol) calculated by the reaction mechanism function A2 is closest to that obtained by FWO method (24.28kJ/mol). While the activation energies calculated by other mechanism functions are very different from those calculated by the FWO method, indicating that the thermal decomposition mechanism function of PLA-based polyurethane is g(α) = [–ln(1–α)]½, which is consistent with the nuclear growth mechanism.

Fig. 6.

Master plots of different kinetic models and experimental data at 10 K/min. calculated by Eq. (7) for thermal decomposition of polylactic acid-based polyurethane.

Fig. 7.

Linear fitting curves of thermal decomposition of polylactic acid-based polyurethane by Coast-Redfern methods.

Fig. 8.

IR spectra of decomposition products of PLA-based polyurethane under different temperatures at a heating rate β of 10ºC/min.: Full view a) and projection b).

TG-FTIR Analysis

TG-IR result of PLA-based polyurethane is shown in Fig. 8.

Absorption peaks located at 1154 cm^-1, 1234 cm^-1, 1350 cm^-1, 1454 cm^-1, 1569 cm^-1, 1755 cm^-1, 2275cm^-1, 2340 and 668 cm ^-1, 2937 and 2968 cm^-1, 3500 to 3700 cm^-1 are found in the IR spectra of the thermal decomposition products collected at different temperatures, respectively represent the characteristic functional groups of ‒C‒O‒ (alcohol group), ‒C‒O‒ (carboxyl group), ‒C‒N, ‒CH₃, ‒NH‒, ‒C(=O)‒, ‒NCO, CO₂, ‒CH₃ and ‒CH₂‒, ‒OH or ‒NH‒. In the range of 150‒250ºC, the absorption peaks representing ‒NH‒ at 1569 cm^-1, ‒NCO at 2275 cm^-1, CO₂ at 2340 cm^-1 and ‒OH at 3500‒3700 cm^-1 appear weaker. The result indicates that the thermoplastic PLA-based polyurethane contains a small amount hydrones and free isocyanates, which react with each other to produce CO₂. With the increase of decomposition temperature, the ‒C‒O‒ group at 1154 cm^-1, ‒C‒NH‒group at 1350 and 1569 cm^-1, ‒C(=O)‒OH‒ group at 1755, 1234 and 3500-3700 cm^-1, and ‒CH₃ and ‒CH₂ groups at 1454, 2968 and 2937 cm^-1 show an increase and then decrease in the intensity of the absorption peaks between 350-450ºC; the intensity of CO₂ peaks located at 2340 and 668 cm^-1 show two increases and then decreases between 300-380ºC and 380-450ºC, respectively, and the peak temperatures are located at 350 and 410ºC, respectively, the reason for this result is that the ester groups in the soft segments of urethane and PLA-based polyurethane break at different temperature stages to form CO₂, respectivThis result indicates that the temperature range of 350-450ºC is the main stage for the breakage of PLA-based polyurethane chain segments, where the breakage of carbamate and ester groups occurred successively, forming a large amount of CO₂, carboxyl carbon chains, amino carbon chains and hydrocarbon carbon chains. While further breakage of these chain segments leads to weakening of the intensity of the corresponding absorption peaks. When the decomposition temperature is between 500 and 800ºC, only the absorption peaks representing ‒ NH‒, ‒CH₃ and ‒CH₂‒, ‒OH groups and CO₂ appear, but the peak intensity become significantly weaker; the absorption peak of representing ‒C(=O)‒OH groups is basically absent, which indicates that when the decomposition temperature is greater than 500ºC, the main thermal decomposition behavior occurred is the further decomposition of the broken fragments into small molecules of hydrocarbons, alcohols and amines, while the carboxylic acid-based chain segments are converted to CO₂.

Fig. 9.

The chromatogram of polylactic acid-based polyurethane at 500ºC.

PY-GC/MS

The cleavage at high temperature can be used to investigate the scission laws of the chemical bonds on main chain of the PLA-based polyurethane [22, 23]. According to the results of PY-GC/MS and the chemical structure of polyurethane, the pyrolysis products of PLA-based polyurethane at 500ºC are inferred and listed in Fig. 9 and Table 5. Demonstratively the thermal decomposition of polylactic-based polyurethane mainly generates carbon dioxide, isocyanate, amine, alcohol, olefin and a small amount low molecular ester through the fracture of chemical bonds at different positions. Correspondingly it consists of three pathways for the chemical bond-fracturing of polylactic-based polyurethane according to the results of Fig. 9 and Table 5. Firstly, the urethane segments are cracked into isocyanates and polyester polyols through the fracture of carbon-oxygen bonds on the right side of the ether bond in the ester groups. Furthermore, the isocyanates are broken into primary amines, carbon dioxide and so on. The polyester polyols are disassociated to carboxylic acids and fatty alcohols, which generates olefins or cyclic ether by dehydrating carbon dioxide or hydrone at high temperature. Secondly the carbamates are ruptured into carbamic acids and unsaturated polyester by the breaking of carbon-oxygen bonds on the left side of the ether bond in the ester groups. The carbamic acids are broken into primary amines and carbon dioxide while the unsaturated polyester is splitted into aldehydes or olefins [24, 25]. Thirdly, the carbamate chains are directly converted into amines and carboxylate esters containing aliphatic rings by carbon dioxide removing from carbamate structures.

Decomposition Mechanism

The thermal degradation mechanism of PLA-based polyurethane materials can be divided into three stages according to the results of TG-IR and PY-GC-MS. The first stage is mainly represented of volatilizing of hydrones and unreacted isocyanates, which react with each other to generate carbon dioxide in the temperature range of 150-250ºC. The second stage is located at 300-450ºC, which is the main temperature region for carbamate chain segment bond breakage. Here the chemical bond in the NH‒C(=O)‒O‒ structures break at different positions, forming carbon dioxides, hydroxyl segments, carboxyl segments, isocyanates and amino segments. Furthermore, the ester group structures in the co-polyester with PLA and PCL crack into carbon dioxides, hydrocarbon molecules, acids, alcohols, etc. The third stage is above 450ºC, taking on further cracking of the fracture products and forming to small molecular alcohols, hydrocarbons, carbon dioxides and so on. The decomposition mechanism of polylactic acid-based polyurethane is deduced into Fig. 10.

Fig. 10.

The breaking mechanism of the main bond in polylactic acid-based polyurethane.