Preparation and Characterisation of PBAT-Based Biocomposite Materials Reinforced by Protein Complex Microparticles ^†

Abstract

In this work, new biodegradable composite materials based on poly (butylene adipate terephthalate) (PBAT) reinforced with zein–TiO₂ complex microparticles were prepared and characterised by electron microscopy and tensile and dynamic-mechanical tests. The composite pellets were prepared by solvent casting with different filler contents, namely 0, 5.3, 11.1 and 25 part per hundred resin (phr), to modify and modulate the properties of the final materials. Scanning electron microscopy (SEM) images showed homogeneous dispersion of the filler, without microparticles aggregation or phase separation between filler and matrix, suggesting a good interphase adhesion. According to tensile tests, Young’s modulus showed an improvement in the rigidity and the yield stress presented an increasing trend, with opposite behaviour compared to other composites. Dynamic-mechanical analysis (DMA) results exhibited increasing storage modulus values, confirming a greater rigidity with a higher filler percentage. The glass transition temperature showed a slightly increasing trend, meaning the presence of an interaction between the two phases of the composite materials. Overall, the produced PBAT composites showed similar properties to low-density polyethylene (LDPE), proving to be promising and more sustainable alternatives to traditional polymers commonly adopted in agri-food fields.

1. Introduction

Among materials for packaging, plastics are the most widely used, thanks to their lightness, good mechanical behaviour, barrier properties and low cost, among others [1]. Amongst traditional plastics, the most employed are polypropylene (PP), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyethylene terephthalate (PET) and polystyrene (PS), which however are not eco-sustainable due to the problems related to their end-of-life disposal [2].

In the last few decades, increasing attention has been devoted to the study and employment of bioplastics in order to reduce the environmental impact and increase sustainability. Since bioplastics generally present poorer properties when compared to traditional plastics, the realisation of composite materials represents a valid way to improve and modulate their characteristics. The major downside of biopolymers being their high cost [3], the use of natural biodegradable fillers is a possible solution to reduce the production costs and at the same time to preserve their degradability [4].

Poly (butylene adipate terephthalate) (PBAT) is a 100% biodegradable polymer produced from fossil resources [5], although recently it has been reported that its monomers can be obtained from renewable sources [6,7,8]. PBAT presents similar properties and processability to polyethylene (PE), especially high flexibility [9,10].

Zein is a prolammine protein which can be extracted in pure form from corn. Its use in polymers has been studied since the 20th century [11] as it is considered a safe biocompatible and biodegradable material [12]. Zein can be formed into films and displays good barrier properties, thanks to its hydrophobic nature [13]. However, protein films are usually tough and brittle and cannot be used as is, but protein in the form of particles can be used as the reinforcing phase in the realisation of composites which are based on a flexible polymer matrix [14], such as PBAT in this case.

Of late, zein has been functionalised in protein–TiO₂ complexes for packaging, environmental and medical applications [15].

The aim of this work was to design and fabricate biocomposites based on PBAT loaded with microparticles of a zein–TiO₂ complex. The so-obtained composites have been characterised in terms of their mechanical and dynamic-mechanical properties.

2. Materials and Methods

PBAT (MAgMa Spa) pellets were dissolved into pure chloroform. The zein–TiO₂ complex had been previously prepared with a composition of 50–50 wt%, by first dissolving the zein (Sigma-Aldrich, St. Louis, MO, USA) in ethanol at 50 °C and then adding TiO₂ (Carlo Erba, Emmendingen, Germany) under constant stirring until a homogeneous phase was obtained. After casting and ethanol evaporation, the recovered material was milled and sieved at 25 μm, and the so-obtained powder was homogeneously dispersed into the polymer solution at the concentrations of 0, 5.3, 11.1 and 25 phr. After solvent evaporation, the obtained films were used for the production of different loaded composite samples, named PBAT, PBAT + 5.3P, PBAT + 11.1P and PBAT + 25P, respectively.

Dumbbell specimens, model 1BA according to the UNI EN ISO 527 standard, of each composite were produced by injection moulding and their mechanical properties were characterised.

Uniaxial tensile test (UTT) results allowed the evaluation of the characteristic parameters, such as Young’s modulus E, yield stress σ_Y, elongation at break ε_B, stress at break σ_B and toughness T.

DMA measurements were carried out according to ASTM D7028 standard with a single cantilever clamp for the determination of the storage modulus (E′), the loss modulus (E″) and the loss factor (tanδ).

SEM images were acquired to investigate the internal microstructure of the composites, by means of a field emission gun SEM (FESEM, Nova Nano SEM 450, FEI company, Hillsboro, OR, USA). SEM was performed on the central cross-section of the specimens obtained through a cryo-fracture.

3. Results and Discussion

In Figure 1, SEM images at different magnifications of PBAT and PBAT + 25P are reported as representative samples. The morphology of the filler particles (as visible in detail in Figure 1d) emphasises the protein–TiO₂ complex nature, showing bright-white areas corresponding to the TiO₂ portion and a greyer part representing the zein protein.

The images display homogeneous dispersion of the protein complex within the polymer matrix, even at a high concentration of the filler, with no aggregation of the particles and no phase separation (Figure 1b). Indeed, a region with an intermediate shade of grey is visible at the grain boundaries. This suggests the formation of an actual interfacial layer bonding the PBAT matrix to the protein due to the presence in the protein structure of both polar and non-polar functional groups [16], able to interact with the polymer macromolecules. Therefore, good adhesion and interaction between the phases can be supposed.

UTT results on the prepared biocomposites indicated a pronounced increase, up to 47%, in the E modulus with increasing filler content. An important result is the increasing trend shown by σ_Y, significantly different from what is traditionally displayed by other composite materials [17]. The obtained results can be interpreted as additional evidence of the good interaction between the phases involved in the biocomposite [18]. The characteristic parameters ε_B, σ_B and T showed a decrease with increasing filler content, related to the increased stiffening of the composites. The representative values are reported in

Table 1. Young’s modulus (E), yield stress (σ_Y), stress at break (σ_B), elongation at break (ε_B) and toughness (T) values of poly (butylene adipate terephthalate) (PBAT) and protein complex composites.

Sample	E (MPa)	σY (MPa)	σB (MPa)	εB (MPa)	T (MJ/m3)
PBAT	126 ± 12	8.1 ± 0.2	13 ± 1	4.0 ± 0.8	45 ± 6
PBAT + 5.3P	131 ± 10	8.4 ± 0.2	12 ± 1	3.7 ± 0.3	35 ± 4
PBAT + 11.1P	149 ± 4	8.8 ± 0.2	11 ± 1	3.5 ± 0.2	32 ± 3
PBAT + 25P	186 ± 11	8.9 ± 0.1	9 ± 1	2.7 ± 0.2	22 ± 2

.

The E’ modulus obtained from DMA analysis as a function of the temperature exhibited a linear increase with increasing filler content in the composite, thus confirming the stiffening effect obtained by the addition of high amounts of filler (Figure 1c).

Other authors have investigated similar biocomposite systems, based on biopolymers reinforced with natural filler particles at different concentrations [19], finding a similar increasing behaviour of E’ compared to the system studied in the present work. The enhanced modulus in composite materials can be attributed to the restricted mobility of the polymer chains due to the physical presence of the filler particles and to the chemical interaction at the interface between the polymer and the particles [20].

The glass transition temperature (T_g) was calculated as the temperature corresponding to the peak of the tanδ curves, defined by the ratio between E″ and E’ moduli. The values of T_g for the different composites show a slight increase as the filler content increases, confirming the interaction between matrix and filler, as observed in other composite systems [21].

Table 2. DMA representative results of PBAT and protein complex composites.

Sample	E’ @ 0 °C (MPa)	E’ @ 20 °C (MPa)	E’ @ 40 °C (MPa)	Tg (°C)
PBAT	273 ± 60	202 ± 50	155 ± 50	−20.4 ± 0.9
PBAT + 5.3P	297 ± 60	224 ± 70	182 ± 60	−20.3 ± 0.8
PBAT + 11P	319 ± 70	239 ± 60	192 ± 70	−18.3 ± 0.5
PBAT + 25P	395 ± 50	294 ± 50	235 ± 60	−17.9 ± 0.5

displays the discussed results of DMA tests.

4. Conclusions

Among the different biopolymers, PBAT is one of the most studied and promising biodegradable plastic materials. In this work, it was employed in the fabrication of biocomposites reinforced with a zein–TiO₂ complex at different concentration. The addition of different amounts of filler enabled modulation of the material properties.

The filler particles were homogeneously dispersed, as emerged from SEM images of the analysed samples, and with the presence of an interface connecting layer between the protein complex and the polymer matrix. The protein complex appeared to have a stiffening effect on the polymer matrix, with an increase of the E and σ_Y, suggesting, therefore, an effective good interfacial interaction between the phases.

The stiffening effect was confirmed by the increasing trend observed in the E’ modulus calculated from DMA analysis. Moreover, T_g values increased with increasing filler content, validating the hypothesis of an interface layer bonding the matrix and the reinforcing particles.

According to the obtained results, the biocomposites can be considered as a valid and more sustainable alternative to the non-biodegradable, fossil-based plastics generally used in the packaging field, such as LDPE.