From waste to functional additives: thermal stabilization and toughening of PVA with lignin

Abstract

Blend films of poly(vinyl alcohol) (PVA) and a graft copolymer (GL) of acrylic acid (AA) with eucalypt lignosulfonate calcium (HLS) were prepared by using a solution casting method. The structure of GL/PVA blend films was confirmed by X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM), which indicate that GL/PVA is a homogeneous system due to the strong interactions between PVA and GL. Differential scanning calorimetry (DSC) results indicate that only one glass transition temperature (T_g) can be seen over the entire blending ratio. Meanwhile, the T_g was enhanced and the melting point (T_m) was depressed when GL was added to PVA. With the addition of 3 wt% GL, the onset decomposition temperature (T_o) was increased by 102 °C compared to that of PVA. Compared with pure PVA, the GL/PVA exhibited a remarkable improvement in mechanical properties: the tensile strength and Young’s modulus of GL/PVA with 5 wt% GL were 39% and 285% higher than that of pure PVA, respectively. These results show that a new melt-processing method of PVA may be developed by the addition of GL in PVA.

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1 Introduction

Poly(vinyl alcohol) (PVA) is widely used in many applications such as medical treatments, construction materials, and household industries because of its low cost and many excellent properties including mechanical performance, solvent resistance, biocompatibility and biodegradability.¹ Until now, two technologies, thermal processing and solution processing, have been adopted for PVA. Despite its excellent water solubility, the casting procedure of PVA is still time-consuming and uneconomical. Thus, there is particular interest in obtaining PVA products through melt extrusion or injection. Because of the strong inter- or intramolecular hydrogen bonding in PVA, its thermal processing window is very narrow, resulting in simultaneous decomposition during the extrusion process.² Lowering PVA melting point by the incorporation of plasticizers could resolve this problem to some extent.^3,4 Nevertheless, this approach is unsatisfactory for entirely overcoming the problem of the thermal decomposition of PVA without sacrificing it’s physical properties. In contrast to plasticization, the improvement of PVA thermal stability with fillers might offer a promising way for achieving its melt extrusion. For example, magnesium chloride, calcium oxide and silica show a thermal retardation effect on PVA.^2,5,6 Similar results were also observed with the addition of nano-fillers.^7–11 However, their effects on improving PVA thermal stability are rather limited. Usually, the maximum (T_max) or onset decomposition (T_onset) temperatures are improved by no more than 40 °C, meanwhile remarkable deterioration of mechanical properties is commonly observed. Interestingly, recently enhancement of the thermal properties and toughness of PVA was synchronously achieved with a low loading of melamine via hydrogen bond self-assembly.¹² It was demonstrated that T_max and T_onset of PVA were increased by 13 °C and 16 °C, respectively, with only 0.5 wt% melamine addition; due to the formation of a physically crosslinked network via H-bonding, the tensile strength and toughness of PVA were improved by 22% and 200%, respectively.

Lignin, the second most abundant biomacromolecule that exists in the plant kingdom, is relatively inexpensive and widely available.¹³ Nevertheless, most lignin is burned as a low cost fuel, leading to the waste of resources and growing environmental problems.¹⁴ Accordingly, great endeavors have been made to research its broader applications. Yeo et al. used modified lignin as a filler in polypropylene composites.¹⁵ Binary blends of alkaline lignin with three poly(vinyl alcohol) samples were investigated.¹⁶ Eucalyptus lignosulfonate calcium (HLS)/PVA blends were researched by Ye, but the compatibility between them was limited.¹⁷ In this work, a graft copolymer of acrylic acid (AA) and eucalypt lignosulfonate calcium with abundant aromatic structures and hydroxyl groups was prepared¹⁴ and used to improve the thermal stability of PVA via H-bonding interactions.

2 Experimental

2.1 Materials

Poly(vinyl alcohol) (PVA : DP = 1700, degree of hydrolysis 99%) was provided by Sichuan Vinylon Factory, SINOPEC (China). Eucalyptus lignosulfonate calcium (HLS, ∼96%) was purchased from Aladdin (China). Acrylic acid (AA), potassium persulfate (K₂S₂O₈), absolute ethyl alcohol and hydroquinone were provided by Kelong Chemicals Co. Ltd. (China).

The graft copolymer (GL) of AA with HLS was prepared using the same methods as our previous paper¹⁴ and the experimental conditions are shown in Table 1.

Table 1 Experimental conditions for grafting reactions of HLS and AA

Sample	HLS (g)	AA (g)	H2O (ml)	K2S2O8 (g)	Hydroquinone (g)	T (°C)	Time (min)
GL	1.6	8	18.67	0.24	0.2	50	20

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2.2 Film preparation

Blend films were prepared by a solution casting method. A calculated amount of PVA and GL were dissolved in distilled water at 90 °C for 3 h. The dissolved solution was casted onto PTFE Petri dishes and dried at 70 °C in a vacuum oven for 15 h to completely eliminate water. The dried films were named as GL/PVA and labelled with the prefixes 5, 15, 25, 35, 50, which represent the weight ratio of lignin graft copolymer to the total sample dry weight. For example, 5GL/PVA represents 5 wt% GL in the film (weight is based on the total quantity of dry PVA and lignin graft copolymer).

2.3 Characterization

Before characterization, except for mechanical tests, all the samples were dried under vacuum (P < 0.3 MPa) at 105 °C for 5 h to eliminate water.

2.3.1 Fourier-transform infrared (FT-IR) spectroscopy. FTIR spectra of samples were measured using a FT-IR spectrophotometer (Nicolet 560) using attenuated total reflection. The measurements were carried out at 4 cm⁻¹ resolution with 32 scans in the frequency of range of 4000–650 cm⁻¹.

2.3.2 Differential scanning calorimetry (DSC). Glass transition temperature (T_g) and melting temperature (T_m) of the blends were determined by using a Mettler Toledo DSC1 (DSC, Mettler Corp. Switzerland) with a scan rate of 10 K min⁻¹ over a temperature range from −40 °C to 300 °C. The flow rate of the nitrogen atmosphere was kept at 50 ml min⁻¹ during the process.

2.3.3 X-ray diffraction measurements (XRD). X-ray diffraction patterns were recorded in an angular range of 3–50° (2θ) by using an X’Pert Pro XRD diffractometer equipped with Cu-Kα radiation operated at 50 kV and 35 mA. The measurements were performed at a scanning speed of 2θ = 0.06° s⁻¹.

2.3.4 Scanning electron microscope (SEM). The morphologies of fracture surfaces were analyzed by SEM (JSM-5900LV) in order to identify the compatibility between GL and PVA in the blends. Prior to characterization, blends were immersed in liquid nitrogen for 3 h to obtain freeze-fractured surfaces.

2.3.5 Thermogravimetric analysis (TGA). The onset (T_o) and maximum decomposition (T_max) temperatures of PVA and blends were determined using thermal gravimetric analysis (TGA, model SDT Q600). A sample (3–4 mg) was equilibrated at 30 °C and then was heated from 30 to 600 °C at a heating rate of 10 °C min⁻¹. The flow rate of the nitrogen atmosphere was maintained at 100 ml min⁻¹ during the whole process.

2.3.6 Mechanical measurements. The tensile strength and elongation at break of PVA and blend films were tested at room temperature using a tensile tester (Instron 5567). Crosshead speed was set at 20 mm min⁻¹. The initial gauge length of the specimen was 20 mm. The width of each tensile sample is 4 mm. Thicknesses of the films were measured with a micrometer in triplicate. The films were stored at 25 °C with RH of 54% for 1 week before testing. The data for each sample was calculated from the average value of five specimens. Tensile toughness (U_t) can be calculated by integrating the area under the tensile curves, as expressed by following equation.¹²
where σ and ε respectively refer to the tensile stress and strain at failure.

3 Results and discussion

3.1 FTIR analysis

FT-IR spectroscopy is a sensitive tool for monitoring changes in the interactions of blends.¹⁸ The FTIR spectra of PVA and blends with the addition of lignin graft polymers are illustrated in Fig. 1. For neat PVA, the wide absorption peak centered at 3313 cm⁻¹ is related to the –OH stretching vibration due to the formation of intermolecular/intramolecular hydrogen bonding. After blending with the graft polymer from lignin and acrylic acid, a red shift of –OH peak absorption was observed, suggesting strong interactions between PVA and the lignin graft polymer. In 5GL/PVA, this wide –OH absorption band is located at 3305 cm⁻¹, declining by 8 cm⁻¹ in comparison with pure PVA. Upon loading 15 wt% GL, it finally decreased to 3297 cm⁻¹, which can be attributed to the interactions between the –OH group and carbonyl group from lignin graft polymers. This hypothesis could be proved by the changes in the absorption peak of PVA emerging at 1736 cm⁻¹ which was assigned to the typical peak of ester carbonyl (–C=O) structures. With increasing loading of GL, this peak absorption showed similar behavior that the –OH band did, shifting gradually from 1736 cm⁻¹ to 1728 cm⁻¹. This indicates that –C=O groups in PVA also could form interactions with –OH or –COOH groups of modified lignin.

Fig. 1 The FT-IR spectra of PVA, 5GL/PVA, and 15GL/PVA.

3.2 Thermal properties and glass transition temperature

Fig. 2 shows the DSC heating thermograms of PVA films according to the concentration of GL. It can be seen that there were two chain segment motion platforms located at around 80 °C and 130 °C for pure PVA. Obviously, the former platform represents the movement of PVA chains in the amorphous region and the latter platform is mainly caused by the crystalline relaxations of PVA.^19,20 The pure PVA displayed a T_g of 74.9 °C and amorphous GL displayed a T_g of 108.59 °C (Fig. 2A). Moreover, it can be clearly seen that only one single compositional dependent glass transition temperature (T_g) can be seen over the entire blend ratio for GL/PVA blends. It can be interpreted that the compatibility between GL and PVA is very good and the system is homogeneous and single phase which is in accordance with the results of SEM.^21,22 After GL was incorporated, T_g was gradually enhanced. For the composite film with 35 wt% GL, the T_g was increased to 94.1 °C with an increment of 19.2 °C, compared with pure PVA. This can be ascribed to the strong hydrogen bonding interactions between GL and PVA, which can restrict the free movement and arrangement of PVA chains.¹² Besides, the melting point (T_m) gradually decreased as GL content was increased (from 229 °C for pure PVA to 214 °C for 35GL/PVA), which was in accordance with the previous literature (Fig. 2B).¹⁷ The depression of T_m indicates that there may be strong interactions in the blend system.^23,24 Thus, it can be considered to be GL in the blend system that contributes to the formation of strong hydrogen bonding with PVA molecules.

Fig. 2 DSC heating thermograms of PVA films as a function of GL content (heating rate = 10 K min⁻¹).

3.3 XRD analysis

Within the angular range of 5–40 °C, there are two main crystalline maxima which may be indexed as 101 at 2θ = 19.88°, and 200 at 2θ = 22.76° for PVA (Fig. 3). The former crystalline peak, which is stronger, is often used for qualitative judgment of the variation of crystallinity for PVA.²⁵ With rising GL content, this diffraction peak decreased in intensity indicating that the ordered arrangement of PVA main chains in the crystalline region could be disturbed by the incorporation of GL, the crystallization process of PVA can be suppressed and the crystallinity can decrease.²⁶ It can be inferred that GL/PVA is a compatible system, and there are strong interactions between GL and PVA. This result is in accordance with our previous results.

Fig. 3 X-ray diffraction patterns of PVA films with different concentrations of GL.

3.4 Scanning electron microscopy

In order to confirm the compatibility of blends, the fracture surfaces of pure PVA as well as GL/PVA with different compositions (15GL/PVA and 50GL/PVA) were characterized by SEM (Fig. 4). Pure PVA shows a very smooth fracture surface. Interestingly, it is noticed that blend films also show smooth and homogeneous surfaces even with the incorporation of 50 wt% GL. GL is homogeneously dispersed in the PVA matrix. As reported for PVA blends with alkali lignin, starch and cellulose and so on, the aggregation of particles and deteriorated compatibility are commonly observed due to a lack of strong interactions.^27–29 Our previous studies demonstrated that PVA/lignosulfonate blends with strong interactions (the q value in the Kwei equation is −62.4 ± 10.0) still become partly immiscible with a 50 wt% addition of lignosulfonate.¹⁷ Nevertheless, the absence of particle aggregation is observed in 50GL/PVA blend, indicating that the modification of lignosulfonate by grafting is an effective method for improving blend compatibility. It could be explained by the fact that the inter-hydrogen bonding between PVA and lignin is enhanced by adopting this simple grafting method. This good compatibility is obviously beneficial for achieving higher thermal stability and mechanical properties of blends, as proved by later discussions. SEM micrographs of GLPVA blend membranes.

Fig. 4 SEM micrographs of GLPVA blend membranes.

3.5 Thermogravimetric analysis

TG and DTG curves of pure PVA, GL and GL/PVA blends were recorded in order to study the effect of lignin graft polymer on the thermal degradation behavior of PVA (Fig. 5). Interestingly, the onset decomposition temperature (T_o) and the maximum decomposition (T_max) of PVA were remarkably improved with the addition of GL. T_o of pure PVA and GL are 239 °C and 217 °C, respectively. By only adding 3 wt% or 5 wt% GL, T_o and T_max were significantly improved by about 102 °C and 109 °C, respectively, compared with pure PVA. With a further increase of GL, T_o and T_max were found to slightly decrease. As reported in the literature, 5 wt% of nano-montmorillonite improved the T_max of PVA by 40 °C and introduction of 3 wt% graphene oxide also gave the same level of improvement in the T_max of PVA(36 °C), shown in Table 2.^8,9 To the best of our knowledge, GL/PVA blends show a much better thermal stability compared with any other results from PVA blends with inorganic or organic fillers. After GL is incorporated, it is obvious that the onset decomposition temperature moves to a higher temperature, while the melting peak moves to a lower temperature, from the TGA results together with DSC profiles shown in Fig. 2. The temperature gap between T_o and T_m was increased, indicating a new way of thermal processing of PVA. As we all know, hydroxyl groups take part in the thermal degradation of PVA, and the stability of hydroxyl groups is crucial for the stabilization of PVA.³⁰ A possible explanation for the high thermal stability of GL/PVA blends is the introduction of aromatic structures of lignin into PVA chains via strong hydrogen bonding. Moreover, PVA blends even with high compositions of GL still showed good compatibility, as proved by SEM and DSC results. Notably, these functional fillers are derived from bio-based resources which are rather inexpensive and environmentally friendly.

Fig. 5 Thermal analysis of PVA, GL and GLPVA blending films: (A) TGA curves and (B) DTG curves.

Table 2 Comparison of nano-fillers and GL effect on PVA maximum decomposition temperature (T_max)

Sample	Enhanced Tmax (°C, compared with pure PVA)
PVA/graphite oxide 5 wt%^31	105
PVA/nano montmorillonite 5 wt%^8	40
PVA/graphite oxide 3 wt%^9	36
PVA/graphite 3.5 wt%^10	20–30
3 wt% GL/PVA	109

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3.6 Mechanical properties

Fig. 6(A)–(C) show the mechanical properties of the blend films. Pure PVA displays a tensile strength of 49.90 MPa, Young’s modulus of 0.71 GPa, elongation at break of 225.85% and a tensile toughness of 89.86 MJ m⁻³. However, low addition of GL had significant effect on the mechanical properties of PVA. With the incorporation of 5 wt% GL, almost all mechanical parameters achieved maximum values. The tensile strength and toughness were increased to 69.15 MPa and 130.19 MJ m⁻³, respectively, 39% and 45% higher than those of pure PVA. Meanwhile the elongation at break was slightly deceased to 218.86% and the Young’s modulus was sharply increased to 2.73 GPa. When the GL content reached 15 wt%, the Young’s modulus achieved the maximum value (3.28 GPa), 362% higher than that of pure PVA. With a further increase of GL, the mechanical parameters gradually decreased. At the 35 wt% GL level, the tensile strength and elongation at break decreased to 44.91 MPa and 125.42%, respectively. But the Young’s modulus was 196% higher than that of pure PVA. Su et al. reported that when the mass ratio of alkali and PVA was 2 : 10, the tensile strength and elongation at break were increased by 20% and 11%, respectively, compared with PVA.²⁷ Ye et al. recently observed that lignosulfonate (HLS)/PVA blend films also had better mechanical properties than pure PVA film due to the formation of strong hydrogen bonding and the rigidity of lignin structures.¹⁷ Moreover, adding a small amount of MA can significantly improve the strength, modulus, and toughness of PVA due to the formation of a physically crosslinked network via H-bond self-assembly.¹² Hence, the observed enhancements can be primarily attributed to the strong hydrogen bonding between GL and PVA and the rigidness of lignin particles.

Fig. 6 (A) Tensile stress–strain curves, (B) tensile strength and Young’s modulus and (C) elongation at break and tensile toughness at break for PVA and its blends as a function of GL content.

4 Conclusion

In this paper, modified lignin (GL) was used as a reinforcer for PVA and it was amazing that the properties of PVA were considerably changed. GL was homogeneously dispersed in the PVA matrix and resulted in the enhancement of many properties. The results of DSC show that only one T_g can be seen over the entire blend ratio indicating that the blend systems are homogeneous and miscible. The obtained TGA and DTG results suggest a very small amount (3% wt) of GL significantly alters the thermal properties of PVA and the thermal decomposition temperature can be higher than that of pure PVA by about 102 °C, which was attributed to strong hydrogen bonding. The mechanical properties of the blend films were also obviously improved. With addition of 5 wt% GL, the tensile strength and Young’s modulus were 39% and 285% higher than those of pure PVA. Moreover, the tensile toughness was also improved. To summarize, these results can be attributed to the strong hydrogen bonding between GL and PVA. This work may open a door to realize the melt processing of PVA.

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