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Matéria (Rio de Janeiro)

versão On-line ISSN 1517-7076

Matéria (Rio J.) vol.24 no.3 Rio de Janeiro  2019  Epub 16-Set-2019

http://dx.doi.org/10.1590/s1517-707620190003.0718 

Articles

Stabilization effect of kraft lignin into PBAT:Thermal analyses approach

Lara Basilio Tavares1 
http://orcid.org/0000-0001-7316-0388

Derval dos Santos Rosa1 
http://orcid.org/0000-0001-9470-0638

1Engineering, Modeling and Applied Social Sciences Center, ABC Federal University, Santo André, São Paulo, Brazil.


ABSTRACT

Sterically hindered phenols are an important class of synthetic antioxidants for polymers, commonly incorporated to inhibit polyolefins or polyesters oxidation. However, synthetic antioxidants use is under restricted regulation due to their potential health risk and environmental impacts. Thus, great interest has been attracted to the phenol-based natural antioxidant such as lignin. This work aimed to evaluate the thermostability of kraft lignin (KL) as a natural antioxidant when incorporated into poly(butylene adipate-coterephthalate) (PBAT) matrix. PBAT-KL films (0%, 1%, 3%, 5% and 10%, by mass) were prepared by twin-screw extrusion followed by thermo-compression. These materials were examined in detail for their effect on the thermal stability using the oxidative induction time (OIT), oxidative induction temperature (OITemp) and thermogravimetric analysis (TG). According to the results, lignin-containing compositions have improved thermal resistance in oxidative environment, compared to neat PBAT. However, 10 wt.% lignin composition presented the lowest thermostability among lignin-containing compositions. The results showed that kraft lignin promotes the thermal stabilization of PBAT films at the optimal concentration of 1% by mass.

Keywords Poly(butylene adipate-co-terephthalate); Kraft lignin; Biodegradable polymer; Thermal analysis; Oxidative degradation

1. INTRODUCTION

Lignin is an abundant natural polymer with complex phenolic structure. After cellulose, lignin is the second in abundance among polymers in nature. It can be found in plants, in a proportion of 15% to 40% by mass [1]. Lignin is obtained from lignocellulose separation as a by-product, and processes such as paper manufacturing and biofuel production are the largest producers [2]. It is an amorphous, cross-linked and three-dimensional phenolic polymer consisting of methoxylated phenylpropane structures [3]. Kraft lignin (KL), a derivative of native lignin, is obtained as a by-product from a pulping process at a large scale [4]. Although the annual lignin extraction in pulp operation around the world is estimated at 60 million [4], the compound is treated as a waste and burned for steam and power generation [6], and less than 2% is recovered for utilization as a chemical product [2,7]. Lignin is an effective free radical scavenger that stabilizes the reactions induced by oxygen and its radical species. It is not harmful or cytotoxic, biodegradable, environment-friendly, biocompatible [8], which make it a promising material for food packaging and cosmetic applications as an antioxidant [9]. Several studies reported lignin thermal stability effect when incorporated in polyolefins such as polypropylene [10, 11], and biodegradable polymers, including poly(acid lactic) [12, 13] and poly(butylene adipate-co-terephthalate) (PBAT) [14, 15]. Despite few works concerning thermal stability study of lignin into PBAT [14, 16], none of them has a deep investigation focused on the thermal effects of lignin incorporation into PBAT matrix in oxidative atmosphere.

Biodegradable polymers are a potential solution to environmental problems. PBAT is an alternative for packaging .

The objective of this study was to evaluate the thermostability of kraft lignin as a natural antioxidant when incorporated into PBAT matrix, using different thermal analysis methods including oxidation induction time (OIT), oxidation induction temperature (OITemp) and thermogravimetric (TG) analysis.

2. MATERIALS AND METHODS

2.1 Materials

Kraft lignin (KL) was supplied by Suzano Papel & Celulose, Brazil; obtained as a by-product from pulp and paper manufacturing (brown color; pH 3.8; 95% solid content; 2% ash content). Poly(butylene adipate-coterephthalate) (PBAT) was supplied by BASF Brazil under the trade name Ecoflex® F Blend C1200 (mass density 1.26 g.cm-3 at 23 ºC; melt rate flow 3.3 g.10 min-1 at 190 ºC, 2.16 kg; melting point 115 ºC).

2.2 PBAT and PBAT-KL films preparation

Kraft lignin and PBAT were dried at 80 °C and 60 °C, respectively, for 24 h before use. KL (0%, 1%, 3%, 5% and 10% by mass) was incorporated into PBAT by a twin-screw extruder (THERMO HAAKE Minilab Rheomex, model CTW5) under temperature profile of 132, 135, 135, 138, 138 and 140 ºC from the feeder to the die and 60 rpm. After extrusion, samples were thermo-compressed under a heating press (140 ºC, 1.5 ton, 90 s) between two poly(tetrafluoroethylene) (PTFE) sheets and a steel mold of 1.5 mm. Table 1 summarizes the compositions.

Table 1 PBAT and kraft lignin (KL) compositions under study. 

CODE PBAT(%) KL (%)
PBAT_L0 100 0
PBAT_L1 99 1
PBAT_L3 97 3
PBAT_L5 95 5
PBAT_L10 90 10

2.3 PBAT and PBAT-KL films characterization

2.3.1 Oxygen induction time (OIT)

OIT is an accelerated test to assess the oxidation stability of polymers, where samples are exposed to an oxidant atmosphere at an elevated and constant temperature (above melting temperature), in order to verify the time needed for their oxidation occur by varying the heat flow. Measurements were carried out based on ASTM D3895-14 [20], using DSC Q-200 equipment (TA Instruments). Samples of 5.5 ± 0.5 mg were heated in a nitrogen atmosphere until 220 ºC. After 5 minutes of isotherm at 220 °C, the nitrogen atmosphere was switched to oxygen (both at 50 mL.min-1) and then the temperature was held for 120 minutes or until the peak of DSC curves appeared. Aluminium pans were used without any cover. All samples were measured in duplicate.

2.3.2 Oxygen induction temperature (OITemp)

Samples were characterized with respect to the energy released during the oxidation reactions using the same equipment described in 2.3.1. The samples were subjected to heating from 35 to 300 °C in an oxygen atmosphere (50 mL.min-1) at a heating rate of 10 °C.min-1. The samples have an average mass of 5.5 ± 0.5 mg and measurements were carried out in duplicate. Aluminium pans were used without any cover.

2.3.3 Thermogravimetric analysis (TG)

The thermogravimetric characterization of the samples was performed on a Netzsch equipment model STA 449F3, the average mass of 20.0 ± 0.5 mg and subjected to heating up to 500 °C at a heating rate of 10 °C.min-1 under an oxygen atmosphere (50 mL.min-1). All measurements were made in duplicate.

3. RESULTS AND DISCUSSION

Table 2 presents the OIT results for neat PBAT and PBAT-lignin compositions. According to the results, oxidative peak time was recorded at 6 min for neat PBAT, at 220 ºC. The lignin addition has improved the thermo-oxidative resistance. Compositions of 1 wt%, 3 wt% and 5 wt% lignin-containing did not show an exothermic peak, indicating these compositions are more effective to withstand heat and oxygen. However, PBAT_L10 presented an oxidative peak at 31 min, suggesting that 10 wt% of lignin addition reduce the thermostability in comparison to other lignin-containing samples.

Table 2 Oxidative induction time results for neat PBAT and PBAT-KL compositions at 220 °C. 

SAMPLE TIME (MIN)
PBAT_L0 6
PBAT_L1 >120
PBAT_L3 >120
PBAT_L5 >120
PBAT_L10 31

For a better understanding of kraft lignin efficiency as an antioxidant, oxidative induction temperature (OITemp) or dynamic OIT [ 21] was conducted. In this method, no gases change at a defined time or isotherm are employed. The samples are heated continuously under oxygen atmosphere up to an exothermic peak. Fig. 1 shows DSC curves obtained from OITemp analysis for neat PBAT and PBAT-lignin compositions and the average results for the extrapolated onset temperatures are described in Table 3. A similar trend was found when compared to OIT analysis. The beginning of oxidative reactions for lignin-containing compositions occurs at higher temperatures than for neat PBAT (214 ºC), indicating that lignin addition exhibit higher thermostability. Once again, PBAT_L10 presented the lowest oxidative temperature among lignin-containing compositions, 222 ºC, and PBAT_L1 presented the highest oxidative temperature, 299 ºC.

Figure 1 Oxidative induction temperature curves for neat PBAT and PBAT-KL compositions. 

Table 3 Onset oxidative induction temperature results for neat PBAT and PBAT-KL compositions. 

SAMPLE ONSET TEMPERATURE (°C)
PBAT_L0 214
PBAT_L1 299
PBAT_L3 245
PBAT_L5 259
PBAT_L10 222

The antioxidant activity of lignin was confirmed by thermogravimetric analysis in an oxidative atmosphere. Table 4 summarizes the initial degradation temperature considered at 10% mass loss (T10%), and the temperature at the maximum degradation rate (TMAX), determined by the maximum value of the first derivative (DTG), for neat PBAT and PBAT-lignin compositions. TG and DTG curves are shown in Fig. 2. All TG curves exhibit one single degradation step (Fig. 2a), as similarly reported by Xing et al. [14]. The lignin addition increased the T10% of neat PBAT, suggesting that KL incorporation inhibit oxidation. PBAT_L0 presented T10% at 339 ºC, followed by PBAT_L10 at 360 ºC. For compositions of 1, 3 and 5 wt% lignin, they did not exhibit significant variation, in which the composition PBAT_L1 was the most thermostable. However, as degradation takes place at higher temperatures, there is no significant difference between samples, as observed from DTG curves (Fig. 2b).

Figure 2 (a) TG and (b) DTG curves for neat PBAT and PBAT-KL compositions. 

Table 4 Onset oxidative induction temperature results for neat PBAT and PBAT-KL compositions. 

SAMPLE T10% (°C) Tmax (°C)
PBAT_L0 339 388
PBAT_L1 371 392
PBAT_L3 370 393
PBAT_L5 368 391
PBAT_L10 360 388

Our previous work [4] reported that 10 wt% lignin addition into PBAT matrix caused particles agglomeration and consequently phase separation, indicated by atomic force modulation (AFM) analysis. According to the results, KL incorporation has increased storage modulus (E’) in comparison to neat PBAT. However, 10 wt% KL addition has the lowest E’ result among lignin-containing compositions, due to lignin agglomeration. This effect might justify the lower thermostability presented for PBAT_L10 herein. Other work also reported lignin agglomeration in PBAT matrix for concentration up to 10 wt% [14]. The authors noticed microvoids in the sample, due to phase separation.

4. CONCLUSIONS

Kraft lignin was used as biobased antioxidant for biodegradable polymer PBAT. The lignin addition increased the oxidation induction time, oxidative degradation and thermal degradation temperatures of the matrix. Kraft lignin promotes the thermal stabilization of PBAT films at the optimal concentration of 1% by mass. However, the thermo stabilization effect for the composition of 10 wt% KL was the lowest among lignin-containing compositions, probably because of lignin phase separation.

AKNOWLEGMENTS

The authors acknowledge UFABC and CAPES for financial support

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Received: July 30, 2018; Accepted: February 26, 2019

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