Abstract
This study investigates the influence of three fatty acids (lauric acid, palmitic acid, and stearic acid) on biodegradable polymer blends based on poly(lactic acid) (PLA) and poly(butylene succinate) (PBS), containing different weight ratios (100:0, 100:2, and 100:4) of fatty acids on the transparency, mechanical properties, morphology, contact angle, and water vapour permeability. All of the blends were pressed into thin films and tested. The experimental results showed that the properties of the samples varied with chain length and amounts of the fatty acids. Thus, it could be concluded that use of fatty acids opens up new ways for the plasticisation of PLA/PBS blends for use as new bioplastics.
GRAPHICAL ABSTRACT
Keywords
Introduction
In recent years, fatty acids have been extensively studied as additives in biodegradable films. Owing to their lepidic nature, they are expected to help reduce the water vapour permeability (WVP) of a hydrophilic film. Moreover, they have proved to have some impact on other film properties. The most common materials for the formulation of biodegradable films are polysaccharides, proteins, and lipids, and the combination of these allows for producing blends of improved characteristics. Aiming at the reduction of the use of chemical additives in the food industry, there has been growing interest recently on the use of natural food additives.
Nowadays, consumer trends are changing continuously and terms such as sustainability or environmentally friendly materials are becoming more widely appreciated. Traditional packaging materials are considered as waste when their main function has concluded. These large amounts of synthetic polymers need to be processed and recycled, which needs a significant investment on the part of every country. Thus, natural polymers constitute a ‘green’ alternative to petroleum-based plastics due to their biodegradability and compatibility with food products. In this sense, Poly(lactic acid) (PLA) (Figure 1(a)) and Poly(butylene succinate) (PBS) (Figure 1(b)) have been used to obtain edible films and coatings which show adequate functional properties, which lead to a lengthening of the foodstuffs’ shelf-life. PLA is one of the polymers that are most widely used to obtain edible films or packaging. As another polymer, PBS produces films with adequate gas barrier properties. However, due to its hydrophilic nature, it acts as a poor barrier against water vapour transfer. To overcome this disadvantage, lipid materials are added in order to increase film hydrophobicity.
The chemical structure of (a) PLA, (b) PBS, (c) LA (C12), (d) PA (C16), and (e) SA (C18).
Fatty acids are usually carboxylic acids with long hydrocarbon chains. They can be classified according to whether or not they contain double bonds; saturated fatty acids have no double bonds whereas unsaturated fatty acids have one or more double bonds. However, most fatty acids are saturated and conform to the general molecular formula of CH3(CH2) x COOH. Fatty acids can reduce WVP of films satisfactorily. However, these lipids can negatively modify other relevant properties of the films due to their phase separation during film drying. Many authors have studied the effects of fatty acid addition on biodegradable polymer films. Bertan et al. [1] studied the effects of adding increasing amounts of lauric acid (LA) on the properties of homogenised films made from gelatin, triacetin, and blends of palmitic and stearic acids. They concluded that addition of LA led to decreases in WVP and tensile strength. Fakhouri et al. [2] also found that addition of fatty acids (palmitic acid (PA), LA, myristic acid, capric acid, caproic acid, and caprylic acid) to biopolymer films composed of lipophilic starch and gelatin gave rise to a decrease in WVP. Additionally, it was found that incorporation of these fatty acids also gave rise to increases in some properties such as elongation and opacity.
The main objective of this work has been to study the incorporation of different types and amounts of fatty acids (LA (Figure 1(c)), PA (Figure 1(d)), and stearic acid (SA) (Figure 1(e))) into blends of PLA and PBS and to examine and evaluate the characteristics of the blended films. Evaluation of the film properties was carried out in terms of WVP, mechanical properties, structural properties, opacity, morphology, water contact angle, and thermal characteristics. The results obtained demonstrate how inexpensive fatty acids can be used effectively to modify the properties of biodegradable polymers for use in food packaging applications.
Experimental
Materials
PLA in pellet form was supplied by NatureWorks LLC, U.S.A. (IngeoTM Biopolymer 2003D). PBS in pellet form was supplied by Mitsubishi Chemicals, Japan (FZ91PD). The fatty acids, LA, PA, and SA were supplied by Loba Chemie Pvt. Ltd., India.
Blend preparation and compression moulding
Both the PLA and PBS pellets were initially dried in a vacuum oven at 45°C to constant weight in order to remove any residual moisture before blending. The weight ratio of PLA to PBS in the blends was predetermined and fixed at 90:10. The preweighed PLA and PBS were melt-blended at 180°C using a high torque mechanical stirrer for 20 min under an inert nitrogen gas atmosphere. Then, different ratios of each fatty acid (100:0, 100:2, and 100:4) were added to the melt blend and stirring continued for a further 15 min. Before compression moulding, the blends were dried at 45°C in a vacuum oven for 12 h. Samples were then hot-pressed into thin films by compression moulding at 180°C for 3 min under a pressure of 1000 psi. The moulded specimens were first cooled down to 50°C and then further cooled down to room temperature before removing from the mould.
Characterisation
Thermal properties
Thermal analysis was carried out on a differential scanning calorimeter (Perkin-Elmer DSC7). An empty aluminium pan was used as a reference. Samples of 3–5 mg in weight were cut from the films, placed in the sample pan, and a temperature scan performed under a nitrogen atmosphere from 0 to 200°C at a heating rate of 10°C/min. From the thermograms obtained, the glass transition temperature (Tg), crystallisation temperature (Tc), and melting temperature (Tm) of each of the films with different ratios of each fatty acid were determined.
Thermal stability of the films was evaluated by thermogravimetric analysis (Perkin-Elmer TGA7). Approximately 10 mg samples were cut from the films and TGA analysis performed at 10°C/min from 50 to 500°C in a nitrogen atmosphere. The weight ratio change of the samples as a function of temperature was recorded.
Mechanical properties
Tensile properties were measured using a Lloyds LRX + Universal Testing Machine according to the ASTM Standard Method D638-14 (2014). Each film sample was cut into a rectangular shape (10 mm × 100 mm) and tensile testing carried out at a crosshead speed of 10 mm/min and initial grip separation of 50 mm. Ten samples for each composition were conditioned at 50% relative humidity and 25°C before testing. From the stress–strain curves obtained, the tensile strength, elongation at break, and initial modulus were determined.
Hydrophobicity
Contact angle measurements of the surface hydrophobicity of the films were estimated by the sessile drop method. A droplet of distilled water (100 µl) was placed on the surfaces of the samples and images taken immediately using a digital camera (Digital USB Microscope 50×–1000×). The contact angle between the drop and the surface of the film was measured using ImageJ software. Measurements were taken in at least three different positions for each specimen.
Water vapour permeability
WVP of each film was determined gravimetrically according to the ASTM Standard Method E 96-16 (2016). This method is also known as the desiccant method in which water vapour penetrates through the film and is absorbed by the desiccant. The aluminium test cup was filled with 10.0 g silica gel, covered with the film, and then placed in an incubator (50 ± 2% relative humidity at 30 ± 1°C). The weight of the cup was recorded using an analytical balance (±0.0001 g) at 1-h intervals for a period of 24 h and then every 12 h until the weight became constant.
The WVP of the film was calculated using the following formula:
Opacity
Opacity of the films was determined using a UV-visible spectrophotometer (Perkin-Elmer Lambda 25 UV/Vis Spectrometer). Each sample was cut into the shape of a rectangle (10 mm × 30 mm) and placed on the inner side of a transparent cuvette. Opacity was measured in terms of percentage light transmittance at a wavelength of 600 nm.
Morphology
The morphology of the fracture surfaces of each sample was studied by scanning electron microscopy (JOEL Model JSM 5910-LV). The sample was affixed to the stub with double-sided carbon tape and then coated with a layer of gold before imaging.
Results and discussion
Thermal properties
The thermal responses of the films with increasing temperature are shown in Figure 2 and the derived data summarised in Table 1. At below the glass transition temperature (Tg), polymer molecules are in their glassy state. When the samples were heated to above Tg, the polymer changed from the glassy state to the rubbery state, resulting in a change from being hard and rigid to soft and flexible. As seen in Figure 2 and Table 1, it is clear that the Tg of the films decreased as the fatty acid content increased. In addition, it was also found that the shorter the fatty acid chain length, the greater the reduction in Tg. This is due to the greater ability of shorter chains to insert themselves between the polymer molecules resulting in an expansion of free volume [3]. The Tg values of the samples were in the following order: PLA/PBS/LA(4) < PLA/PBS/LA(2) < PLA/PBS/PA(4) < PLA/PBS/PA(2) < PLA/PBS/SA(4) < PLA/PBS/SA(2). When the polymers were heated up to their crystallisation temperatures (Tc), the solid-phase polymer chains rearranged themselves to form crystalline regions. From the results in Table 1, it can be seen that the added fatty acids decreased the Tc of the films by helping the polymer chains to move more easily for crystallisation to occur.
DSC thermograms of the PLA/PBS film samples with and without fatty acids (second heating scans). Thermal properties of PLA/PBS films with different fatty acid contents.
Moreover, previous work has suggested that fatty acids may act as nucleating agents by initiating crystallisation at lower temperatures due to enhancement of the chain mobility [4]. While the results in Table 1 cannot confirm this nucleating effect, they do at least indicate that the shorter the length of the fatty acid chain (LA < PA < SA) the lower the Tc and the higher the ΔHc (∝ % crystallinity). These findings are consistent with shorter chain fatty acids having more molecular mobility which in turn helps them to facilitate crystallisation of the polymer chains.
As for the melting temperatures (Tm) of the films, as denoted by Tm1 for PBS, and Tm2 and Tm3 for PLA in Table 1, the effects of the type and content of the fatty acids are not significantly different. However, what slight differences there are could be due to the fatty acids somehow affecting the thickness of the lamellae formed during crystallisation. It is well known that Tm is directly related to the lamellae thickness and hence the amount of energy required to overcome the intermolecular forces in order for the lamellae to melt. The various heats of melting (ΔHm1, ΔHm2 and ΔHm3) given in Table 1 were calculated from the areas under the corresponding Tm peaks.
In general, additive distribution in polymers depends on a variety factors such as polymer-additive interaction, polymer–polymer interaction, polymer morphology, migration, and so on. If a fatty acid is to exert its action in a polymer blend, it is often advantageous that it is molecularly and evenly distributed. For all fatty acids, their melting temperatures are lower than the mixing temperature of the polymer blend, resulting in the possibility of the fatty acid inserts between polymer chains in the amorphous phase. For PLA/PBS/LA(2), PLA/PBS/PA(2), and PLA/PBS/SA(2), the melting peak of fatty acid cannot be observed. It indicates that these concentrations did not exceed the saturation. In contrast to PLA/PBS/LA(4), PLA/PBS/PA(4), and PLA/PBS/SA(4), the small melting peaks of fatty acids can be seen in DSC thermograms. This indicates that there is a chance of a phase separation, migration, or aggregation of fatty acids. It is also known as a heterogeneous plasticiser distribution. In addition, the results can imply that the systems of 100:4 of all fatty acids are not simple homogeneous blends but instead are complex multiphased blends.
Thermogravimetric analysis was also performed to evaluate the effect of fatty acid addition on the thermal stability of the PLA/PBS films. The weight loss curves for all of the formulations are compared in Figure 3 and the data summarised in Table 1. The results show that the degradation temperatures of the neat PLA/PBS blends are 350–411°C. The degradation range of all blends decreased because the fatty acid can evaporate at these temperatures. The boiling points of LA, PA, and SA are approximately 297, 351, and 361°C, respectively. However, the TGA analysis indicates that the addition of fatty acids had a little effect on the thermal stability of the PLA/PBS films. These results imply that PLA/PBS films plasticised with fatty acids can still be processed safely at the melting temperatures of PLA and PBS by the same conventional thermoplastic processing techniques that are used for the unplasticised blend.
TGA thermograms of the PLA/PBS film samples with and without fatty acids.
Opacity measurements
As shown in Figure 4, opacity measurements show that the presence of fatty acids in the films has only a relatively small effect on film transparency except for SA at the 4% level. This effect is due to the fatty acid dispersion in the films which affects the transparency by scattering and/or preventing light from being transmitted through the film [5]. The much lower percentage transmittance of the film containing 4% SA is probably due to the poorer dispersion of the SA due to its greater chain length.
Effect of fatty acid type and concentration on film transparency as measured at 600 nm. (The error bars are the standard errors of the mean of five measurements).
Mechanical properties
Mechanical properties of the films were measured in terms of tensile strength, elongation at break, and modulus of elasticity, as shown in Figure 5. The results show a decrease in tensile strength upon the addition of the fatty acids, which can be attributed to their plasticising effect, thereby weakening the intermolecular interactions between the polymer chains. The elongation at break of the films was also observed to decrease slightly for the same reason. It has been reported that the fact that the fatty acids used here are solids at room temperature (and therefore at the temperature of the mechanical test) has an important effect on the film structure and, as a consequence, on the brittleness and strength [6]. The slight decreases in the modulus of elasticity of the films reveal that fatty acids induce a slight loss in the matrix strength. This can be attributed to complex formation of the fatty acids with the polymer matrix which reduces the cohesive forces of the polymer network. This is consistent with earlier work which reported that the incorporation of fatty acids brought about a reduction in the rigidity of the matrix due to the discontinuities introduced in the polymer network [7].
Effects of fatty acids on the tensile strength, elongation at break, and modulus of elasticity of the PLA/PBS.
Water vapour permeability
WVP of the PLA/PBS films with different fatty acid contents.
Contact angle
Contact angle is an indicator of the surface hydrophobicity, or wettability, of polymers. It is a well-known fact that the water contact angle increases with increasing surface hydrophobicity [11]. As demonstrated in Figure 6, it was found that the contact angle increased with both fatty acid concentration and chain length indicating an increase in hydrophobicity. It should also be mentioned that, since the contact angle is sensitive to only the upper most monomolecular layer of the sample which is in direct contact with the water, this shows that the fatty acid molecules are able to migrate to the film surface. These findings are consistent with those reported in a previous work [5] in which it was shown that an increase in the concentration of olive oil in chitosan-based films caused an increase in the contact angle values from approximately 60° with no olive oil to 65° with 15% olive oil. However, Hambleton et al. [12] also reported that films containing fat showed a decrease in the surface hydrophobicity because of the presence of polar ester-containing plasticisers in the recipe.
Contact angles of the PLA/PBS film samples with and without fatty acids.
Morphology
SEM can provide useful information regarding the microstructure of the films and the macroscopic interactions between the film components. Figure 7(a–d) shows the SEM micrographs of cross-sectional fracture surfaces of films both without fatty acid and with fatty acid in the weight ratio of 100:4. In the film without fatty acid (Figure 7(a)), the surface appears smooth which is characteristic of a brittle fracture. However, in the presence of fatty acid, the fracture surfaces appear rougher, as shown in Figure 7(b–d), which is characteristic of a more ductile fracture and is consistent with plasticisation and a lowering of the Tg.
SEM micrographs of cross-sections of the polymer films: (a) PLA/PBS, (b) PLA/PBS/LA (4), (c) PLA/PBS/PA (4), (d) PLA/PBS/SA (4). (Magnification × 500).
Despite their plasticising effect, fatty acid addition was unable to increase the elongation at break (Figure 5). This was probably due to the accompanying decreases in modulus and tensile strength counteracting the effect on % elongation. Apart from this, the roughened surfaces in Figure 7(b–d) appear to show homogeneous interfacial boundaries which suggest good polymer–fatty acid miscibility. The integration of fatty acid molecules in a polymer matrix has been previously reported [11, 12].
Conclusions
This study has aimed at developing a new understanding of the influence of a series of fatty acids (LA, PA, and SA) of varying chain lengths on the properties of films made from melt blends of PLA and PBS. The addition of the fatty acids was found to enhance the plastic behaviour of the films by decreasing the modulus of elasticity and tensile strength. This was a result of the films with fatty acids having increased free volume and enhanced polymer chain mobility. As far as the WVP and water contact angle measurements were concerned, the fatty acids added to the PLA/PBS blends did not change the WVP significantly but did increase the contact angle. In conclusion, this work has shown that the use of fatty acids is a convenient and inexpensive way of plasticising PLA/PBS blends. In doing so, it provides means of fine-tuning the properties so that they can be tailored to meet the specific requirements of various film packaging applications.
Footnotes
Acknowledgements
The research reported in this paper was supported by the Graduate School, Chiang Mai University, Chiang Mai, Thailand and Chiang Mai University. The authors also wish to thank the National Research University (NRU) Project under Thailand's Office of the Higher Education Commission for financial support.
Disclosure statement
No potential conflict of interest was reported by the authors.
Notes on contributors