Biodegradable bio-based polymers, obtained from renewable resources, represent an important alternative to petrol-derived non-degradable polymers. Thus bio-based polymers have become an important issue both for academia and industry. Typical examples are poly(lactic acid) (PLA), and polyhydroxylalkanoates (PHA), in particular polyhydroxylbutyrate (PHB) and its copolymers poly(hydroxylbutyrate-co-valerate) (PHBV).
Polyhydroxyalkanoates are a wide family of polyesters, obtained by different bacteria cultivated under stressful conditions, with properties quite similar to conventional plastics [1
]. The commercially marketed PHBV copolymers have good mechanical properties [4
] and good resistance to solubility in water [5
]. PHB and PHBV are also highly biodegradable and biocompatible [6
]. Unfortunately, the relatively high cost [3
], compared to other biodegradable polymers such as PLA, has somehow hindered research activity on the use of PHB and PHBV in commodity applications such as packaging and service items, and restricted their use to high-value applications, such as those in medical and pharmaceutical sectors.
In order to achieve products with particular properties for different applications, biocomposites can be utilized. Biocomposites are a special class of composite materials, obtained by mixing natural fibers to bio-based polymers. Biocomposites represent an environmentally friendly and low-cost alternative to conventional petroleum-derived materials. Their properties have been reviewed in several books and articles [9
]. The additional benefit offered by natural or bio-based fibers is that, besides being biodegradable, they also exhibit a lower density, which makes biocomposites economical and lightweight [18
The mechanical performances of a fibers reinforced biocomposite result from both the matrix and the fiber properties [19
], and are strongly dependent on the fiber/matrix interphase [21
]. The tensile strength is more sensitive to the matrix/fiber adhesion, whereas the modulus depends in general on both the matrix and the fiber properties. The percentage of fibers, their aspect ratio (length to width ratio) and orientation, and the fiber–matrix adhesion are crucial elements responsible for the final properties of natural fiber reinforced composites. The transmission of the applied stress to the fibers occurs at the interface, which explains the necessity of a good matrix/fiber adhesion. Often biocomposites made of hydrophobic polymers and hydrophilic natural fibers are characterized by poor adhesion, which results in limited mechanical properties, due to the tendency of the fibers to aggregate during processing. In addition, the hydrophilic nature of the lignocellulosic fibers usually causes moisture absorption, which worsens processability and induces formation of porous products. The fiber aspect ratio strongly influences the tensile modulus and the fracture properties. Fibers with low aspect ratio and irregular shape in general behave as fillers, and not as reinforcement.
The poor compatibility between fiber and biopolymeric matrix can be improved by modifying the fiber surface properties [17
]. Physical and chemical methods can be utilized. Some physical treatments, as for example stretching, can enhance the interface polymer/natural fibers, without changing the chemical composition of the fibers. Plasma or corona treatment can induce compatibilization between hydrophilic fibers and hydrophobic matrix, through formation of free radicals and surface cross-linking. On the other hand, chemical methods utilize coupling agents to modify the surface composition of the fibers, or chemical treatments to increase the surface roughness or to reduce the fiber hydrophilic nature.
Several types of natural fibers have been used to produce PHBV based biocomposites, for which thermal and mechanical properties were investigated [8
]. Organic wastes have also been sometimes utilized as reinforcement or additives for different polymers [28
]. Significant amounts of organic wastes from industry and agriculture remain unutilized, so that the use of organic residue materials in biocomposites can represent a sustainable method to produce materials for different applications, characterized also by reduced cost, meeting even a circular economy approach.
Potato wastes are biomasses rich in starch and lignocellulosic constituents. After extraction of starch, the potato pulp accumulates in high amounts—approximately 0.75 tons of pulp arises per ton of purified starch. Within the European Union, about 140,000 tons of dried potato pulps are produced annually in the starch industry [29
]. The original potato pulp contains water up to about 90%, but de-watering processes generally results in an increase in the dry matter up to about 90 wt %. Dried potato pulp, which consists mainly of lignocellulosic fibers, starch and, at a lesser extent, of proteins, can be used as filler for reinforcement of biopolymers. As the starch content that remains after potato processing can be quite high, similar to that of the lignocellulosic fibers, the potato pulp powder can be defined as a mixture of lignocellulosic fibers and starch, both with hydrophilic structure. The cost of the potato pulp powder is low, which makes it even more interesting for industrial utilization [30
]. Potato pulp powder has never been utilized to produce biocomposites with PHBV. In the present work, biocomposites made of PHBV and 20 wt % of potato pulp powder have been produced by extrusion, followed by injection molding, and characterized in terms of thermal, mechanical and morphological properties. Preliminary tests indicated that potato pulp powder could be added and easily processed with PHBV up to a percentage of about 20 wt %. Due to the high cost of PHBV, in order to investigate the cheapest formulation, in the present study the properties of the PHBV based biocomposite with 20 wt % of potato pulp powder were analyzed. To make the preparation of the biocomposite simpler, a plasticizer, acetyl-tri-n-butyl citrate (ATBC) was used. ATBC is in general an efficient plasticizer of PHAs [31
]. It is derived from naturally occurring citric acid, is non-toxic and accepted for contact with food [32
]. In addition, in an attempt to improve the adhesion between PHBV and potato pulp and reduce the tensions at the interface, in the present study fiber coating with bio-based and petroleum-based waxes was employed and investigated.
2. Materials and Methods
Commercial grade polyhydroxyalkanoate (PHI002™) was supplied in pellets by Naturplast® (Caen, France). The material is a copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) with 5 wt % valerate content, characterized by a density of 1.25 g/cm3 and a melt flow index (190 °C, 2.16 kg) of 10–20 g/10 min.
The plasticizer acetyl-tri-n-butyl citrate (ATBC) was purchased from SigmaAldrich S.R.L. (Milan, Italy).
The calcium carbonate (CaCO3) OMYACARB® was an inert filler supplied by the company OMYA (Oftringen, Switzerland). The powder, which has fine granulometry with particle size distribution centered at 12 μm, is used to facilitate the removal of the injection molded specimen from the mold.
The dried potato pulp (PP) powder was provided by the company (SüdStärke, Schrobenhausen, Germany). The moisture content was about 3 wt %, and the composition of the dry matter: cellulose 16 wt %, hemicellulose 7 wt %, lignin 20 wt %, starch 25 wt %, pectin 17 wt %, proteins 7 wt %, ash 5 wt %.
Wax-based additives Aquacer 561, Aquacer 581, Aquacer 593 and Hordamer PE 02 were provided by BYK Additives & Instruments (Wesel, Germany). They are (i) non-ionic aqueous emulsions of bee wax (Aquacer 591), (ii) non-ionic aqueous emulsions of carnauba wax (Aquacer 581), (iii) non-ionic aqueous emulsion of a modified polypropylene wax (Aquacer 593), and (iv) anionic aqueous emulsion of polyethylene (Hordamer PE 02).
2.2. Composite Preparation
Biocomposites of PHBV with potato pulp powder were prepared by adding 20 wt % of PP powder to a polymeric matrix, constituted by the biopolymer PHBV, with concentration 85 wt %, the plasticizer ATBC, with concentration 10 wt %, and CaCO3
, with concentration 5 wt %. For comparison, pure PHBV and the PHBV mixed only with the plasticizer ATBC were also processed in the same way. For the preparation of the PHBV based biocomposites with potato pulp powder coated with natural waxes, 20 mL of wax aqueous emulsion with concentration 5% w
were added to 19 g of PP powder. The mixture was carefully hand blended for sufficient long time. The PHBV based samples investigated in the present study, with the relative composition, are listed in Table 1
Before processing, PHBV and the PP powder, non-coated and coated, were dried at a temperature of 60 °C for at least 24 h. The PHBV based matrix and biocomposites were prepared by using a MiniLab II HAAKE Rheomex CTW 5, a co-rotating conical twin-screw extruder, which allows the mixing of the different components. The molten materials were transferred from the mini extruder through a preheated cylinder to a mini injection molder (Thermo Scientific HAAKE MiniJet II), which allows us to prepare dog-bone tensile bars specimens to be used for thermal and mechanical characterization. The dimensions of the dog-bone tensile bars were: width in the larger section: 10 mm, width in the narrow section: 4.8 mm, thickness 1.35 mm, length 90 mm. The extruder operating conditions adopted for all the formulations are reported in Table 2
. After preparation, all the samples were stored in a desiccator and analyzed the day after in order to avoid physical ageing effects on the physical properties investigated.
2.3. Composite Characterization
The thermal stability of the potato pulp powder and selected samples was investigated by thermogravimetric analysis (TGA), carried out on about 10 mg of sample by using a Perkin Elmer TGA 7 (Waltham, MA, USA), under nitrogen flow (35 mL/min), at a heating speed of 10 K/min from 50 °C to 600 °C.
The morphology of the potato pulp powder and PHBV based matrix and biocomposites was investigated by scanning electron microscopy (SEM) with an FEG-Quanta 450 ESEM instrument (Waltham, MA, USA). The micrographs of samples fractured with liquid nitrogen and etched with gold were collected. Backscattered electrons generated the images whose resolution was provided by beam deceleration with a landing energy of 2 kV.
Differential scanning calorimetry (DSC) measurements were performed with a Perkin Elmer Calorimeter DSC 8500 (Waltham, MA, USA) equipped with an IntraCooler III as a refrigerating system. The instrument was calibrated in temperature with high purity standards (indium, naphthalene, cyclohexane) according to the procedure for standard DSC [34
]. Energy calibration was performed with indium. Dry nitrogen was used as purge gas at a rate of 30 mL/min. The samples were analyzed from −85 °C to 200 °C at the heating rate of 10 K/min.
Tensile tests on the samples prepared with the injection molder were performed at room temperature, at a crosshead speed of 10 mm/min, by means of an INSTRON 5500 R universal testing machine (Canton, MA, USA), equipped with a 10kN load cell and interfaced with a computer running the Testworks 4.0 software (MTS Systems Corporation, Eden Prairie, MN, USA). At least five specimens were tested for each sample in according to the ASTM D 638, and the average values reported.
The PP power utilized to produce biocomposites acts as filler, and not as reinforcement, for the PHBV matrix. The loss in mechanical properties is however not very high: the elastic modulus decreases by about 20% when the PP powder concentration is 20 wt %, with the result that this biocomposite still presents properties valuable for practical applications. The adhesion between the PP powder and the polymeric matrices is poor, as attested by the tensile strength, which has been found to be smaller for the biocomposite with respect to the PHBV matrix. The decrease is of about 40% when the PP powder concentration is 20 wt %. Also the ductility of the biocomposites is slightly smaller with respect to that of the PHBV polymeric matrix, because the incorporation of the PP powder promotes microcracks formation at the polymer/filler interface.
An interesting result obtained by the present study is that a simple surface coating of the PP power with a bio-based wax, carnauba wax, can markedly improve the mechanical properties of the PHBV based biocomposite. Adhesion between the polymeric matrix and the filler improves also when another natural wax, bee wax, is used as surface coating of the PP powder. The use of bio-based waxes does not hinder biodegradability: on the contrary it allows to maintain a high content of bio-based components in the biocomposites.
In conclusion PP powder, which is an organic waste from the production and extraction of starch and no-food competition biomass, can be suitable for processing in the melt with the bio-based and biodegradable PHBV, presenting no main problems in processing. Due to the low aspect ratio of the PP powder, there is not a reinforcing effect on the polymeric matrix, but rather a moderate loss in mechanical properties that anyway still meet the technical requirements indicated for rigid packaging production. All the components of the biocomposites, including the natural waxes, are in the list of food contact approved substances, thus even food packaging application can be considered. The addition to PHBV of potato pulp powder up to about 20 wt % offers the possibility to reduce the cost of the final products, considering the relatively high cost of this polymer, and to make available the utilization and valorization of an abundant agro-food biomass such as potato pulp, according to the principles of the circular economy.