The increasing demand for environmentally sustainable products has stimulated a growing research effort in the last years towards the production of new bio-sourced materials, characterized by a lower environmental impact with respect to the traditional fossil-fuel counterparts [1
]. In this context, poly(lactic acid) (PLA) is considered one of the most promising candidates for replacing petrochemical-derived polymers in several fields, including packaging, biomedical, and electronic industries [4
]. The leading position of PLA is related to its relevant physico-chemical properties, coupled with its bio-sourced origin, compostability, and even biocompatibility [6
]. However, in order to obtain high added value PLA-based materials, potentially suitable for high-tech applications, some drawbacks of this biopolymer need to be overcome. In particular, as clearly reported in the literature, its hydrophobicity and lack of modifiable side chain groups, together with its brittleness (elongation at break is below 10%) and low gas barrier properties, limit its complete exploitation for industrial sectors such as automotive, biomedical, electronics, and packaging [7
]. To accomplish this objective, different strategies have been pursued, including the formulation of blends [9
], composites [11
], and nanocomposites [13
], using a huge variety of either polymeric or non-polymeric dispersed phases. In particular, with the aim to obtain highly sustainable materials, natural and renewable fillers are commonly being selected for the production of PLA-based composites [15
]. As an example, composites based on PLA and nanocellulose fibrils with high lignin content have been obtained through film casting and hot pressing. The resulting biocomposites showed a significant improvement in mechanical, thermal, and water vapor barrier properties, owing to the effective coupling between PLA and the fillers embedded at nanoscale level [17
]. Tesfaye et al. explored the possibility to recycle PLA biocomposites containing different amounts of silk nanocrystals, by subjecting the materials to a repetitive extrusion process [18
]; interestingly, a remarkable stabilizing action of the embedded nanocrystals against PLA degradation occurring during multiple processing cycles was observed.
Recently, biochar (BC) has attracted increasing interest as far as its utilization as filler for polymer-based composites, either thermoplastics or thermosets, is concerned [19
]. BC is usually obtained from the pyrolysis of agricultural and forestry wastes [21
], and its structure can be tuned depending on the pyrolysis conditions; in particular, by adjusting the temperature and the oxygen flow, the degree of internal porosity and the presence of different functional groups anchored to the surface can be tailored [22
]. The interest towards BC lies in its intriguing properties, such as high chemical and thermal stability, great electric properties, and very large surface area [23
], combined with its cost effectiveness and low environmental impact [24
]. For instance, electrically conductive composites based on ultra high molecular weight polyethylene and BC derived from bamboo charcoal were formulated through high-speed mechanical mixing and the hot-pressing method [25
]. These composites, characterized by a peculiar segregated morphology, exhibited a low electrical percolation threshold and superior thermal stability with respect to the neat matrix. Das et al. formulated polypropylene-based systems containing different amounts of BC produced from landfill pine wood waste [26
]. A beneficial effect of BC on the matrix flame behavior was documented; more specifically, the peak of heat release rate and smoke production were remarkable reduced as a result of the BC addition. Besides, the char layer formed after combustion was proven to be effective in enhancing the insulation properties of the formulated composites. Interestingly, the BC derived from the carbonization of starch-based packaging material was melt mixed with post-consumer poly(ethylene terephthalate), to produce a composite filament suitable for 3D printing applications [27
]. The introduction of BC particles improved the processability of the matrix, giving rise to composite systems with improved mechanical and thermal properties with respect to the neat polymer matrix.
Generally speaking, biochar is currently exploited as a cheap, functional material, already employed for several applications (i.e., as an adsorber in functional clothing, as storage for volatile nutrients, as energy storage in batteries, and as an insulating material in the building industry, among others) and undoubtedly it is a way for recovering wastes at the end of life, offering them a new added value [28
Few reports dealing with the formulation of BC-containing composites based on biopolymers are present in the literature [29
]. For instance, biodegradable poly(butylene adipate-co
-terephtalate) was melt compounded with BC derived from agriculture commodities to produce composites potentially suitable for food packaging applications [31
]. BC addition was found to enhance the thermo-mechanical properties of the polymer matrix, notwithstanding its hydrophobicity. Additionally, the presence of BC particles from bamboo charcoal was demonstrated to be effective in stabilizing the PLA matrix against UV irradiation.
In this work, PLA-based biocomposites containing BC particles derived from spent ground coffee were formulated through two different processing methods, namely melt mixing and solvent casting. BC was introduced within the polymer matrix at different contents, ranging from 1 wt.% to 7.5 wt.%, aiming at evaluating the effect of the processing route and of the BC loading on the ultimate properties of the resulting biocomposites. The dispersion of the embedded BC particles and the extent of the established polymer–filler interactions were assessed through morphological and rheological analyses; besides, the thermal and mechanical behavior of the biocomposites was investigated, assessing the possible influence of the processing on the PLA intrinsic characteristics. Finally, the thermal stability of biocomposites was evaluated and compared to that of neat PLA.
2. Materials and Methods
Two different PLA matrices were used, PLA 3251D and PLA 4042D, both supplied by IngeoTM Natural Natureworks (Minnesota, MN, USA). PLA 3251D is an injection molding grade and has a melt flow index of 13 g/10 min (210 °C, 2.16 kg); conversely, PLA 4042D is suitable for film applications and shows a melt flow index of 6 g/10 min (210 °C, 2.16 kg).
A waste brewed coffee powder (Caffè Vergnano, Italy) was used as starting material for the production of biochar (BC). In order to remove the leftover chemicals in waste coffee powder after brewing, the sampled waste brewed coffee powder was washed many times with demineralized water. The sample was further centrifuged and filtered and the obtained waste brewed coffee powder was then dried in an oven at 90 °C for 10 h. The pyrolysis of the material was subsequently performed at 700 °C for 1 h in a nitrogen atmosphere (gas flow: 120 mL/min). The heating ramp rate of the tubular furnace was set at 5 °C/min; after the pyrolysis step, the material was manually ground in a mortar, achieving an average size of 10 μm. Further characteristics of the obtained BC particles are reported in [32
PLA and PLA-based composites containing 1, 2.5, 5, and 7.5 wt.% of BC were processed through two different methods, namely melt mixing and solvent casting. PLA 3251D was used to formulate solvent cast composites, owing to its lower average molar mass and enhanced solubility in the selected solvent as compared with PLA 4042D.
In melt mixing (MM), the BC powder was compounded with the PLA4042D matrix using a Brabender Plastograph mixer and operating at 170 °C, 100 rpm for 5 min. Neat PLA was subjected to the same processing. Specimens for the various characterization were obtained by means of a compression molding step, using a laboratory press (Collin Teach Line 200T, Maitenbeth, Germany), working at 170 °C, under a pressure of 100 bar for 2 min. Prior to the processing, PLA and BC were vacuum dried overnight at 70 °C. The melt mixed materials were designed as PLA+XBC_MM, where X indicates the BC loading (wt.%).
In the solvent casting (SC) process, PLA 3251D pellets were dissolved in chloroform with vigorous stirring at room temperature. BC was added after the complete dissolution of PLA and the obtained solutions were stirred for 5 h. The solutions were then cast onto a glass Petri dish and allowed to dry for about 24 h at room temperature. The resulting solvent cast films were placed in a vacuum oven at 40 °C for three days in order to remove all remaining solvent. The solvent cast systems were designed as PLA+XBC_SC (X accounts for wt.% BC loading)
2.3. Characterization Techniques
Rheological measurements were performed using an ARES (TA Instrument, New Castle, DE, USA) strain-controlled rheometer in parallel plate geometry (plate diameter: 25 mm). Strain sweep tests were carried out at 170 °C and ω = 1 rad/s. The complex viscosity and storage and loss moduli were measured performing frequency scans from 10−1 to 102 rad/s at 170 °C, under a nitrogen atmosphere. The strain amplitude was selected for each sample in order to fall in the linear viscoelastic region. Time sweep tests were carried out at in air at 170 °C, ω = 1 rad/s, γ = 10%, and for 3600 s.
Microstructures were observed using a LEO-1450VP scanning electron microscope (SEM, Zeiss, Ramsey, NJ, USA) (beam voltage: 20 kV) on the cross-sections of the investigated samples fractured in liquid nitrogen.
Differential scanning calorimetry (DSC) analyses were carried out using a QA1000 TA Instrument apparatus (Waters Lc, Milford, MA, USA). All the experiments were performed under dry N2 gas (20 mL/min) using samples of around 8 mg in sealed aluminum pans. All the materials were subjected to the following cycle:
First heating up from 30 to 200 °C at 10 °C/min;
Cooling down from 200 to 0 °C at 10 °C/min;
Second heating up from 0 to 180 °C at 10 °C/min.
All the thermal parameters were evaluated on the second heating scan, erasing the previous thermal history and evaluating Tg
(glass transition temperature), Tcc
(cold crystallization temperature), Tm
(melting temperature), and Xc
(crystallinity degree) in controlled conditions. Xc
was calculated as the ratio between the heat of fusion of the sample (ΔH = ΔHm
being the specific melting and cold crystallization enthalpies, respectively), considering only the polymer fraction, and the heat of fusion of a 100% crystalline PLA (93 J/g [33
Thermogravimetric analyses (TGA) were performed using a Pyris1TGA apparatus (Perkin Elmer, Shelton, CT, USA) (experimental error: ±0.5 wt%, ±1 °C). Samples (about 10 mg) were placed in alumina pans and runs were carried out in the range of 50–600 °C, with a heating rate of 10 °C/min, under N2 flow (35 mL/min). T5%, T10% (i.e., the temperatures at which 5% or 10% weight loss, respectively, occurs), and Tmax values (i.e., the temperatures corresponding to the peaks appearing in derivative - dTG - curves) were calculated; besides, the final residue at 600 °C was measured.
Tensile tests were performed using an Instron® 5966 dynamometer (Norwood, MA, USA). The measurements were carried out at two different crosshead speeds: 1 mm/min until 0.2% of deformation is reached, and then 100 mm/min up to sample breakage. At least five specimens for each system were tested and the were results averaged.
Two different processing methods, that is, melt mixing and solvent casting, were exploited for preparing PLA-based biocomposites containing BC particles obtained from pyrolysis of spent coffee grounds. The rheological results suggested that melt mixing method is more effective in dispersing BC particles owing to the high shear force that systems experienced during processing, though the high temperature, at which the processing is performed, caused the occurrence of degradation phenomena in PLA, leading to a severe reduction of the polymer molar mass. Conversely, the solvent casting method, carried out at room temperature, preserved PLA from degradation, but the weak dispersing stresses involved in this processing did not promote the establishment of strong polymer–filler interactions, owing to the bigger size of BC and its sponge-like structure. Furthermore, the selected processing route significantly affected the thermal behavior of formulated materials; in fact, melt mixed systems showed an amorphous structure, while solvent casting composites exhibited crystalline phases, owing to the nucleating effect of the embedded BC particles. Finally, irrespective of the processing strategy, an improvement of the tensile modulus was documented for all biocomposites, as compared with neat PLA.
From an overall point of view, the obtained results clearly showed that the ultimate properties of the PLA/BC biocomposites can be profitably modulated by selecting the content of BC and the suitable processing method, which are able to govern the morphology of the materials, as well as their rheological and mechanical behavior.
All these findings may suggest the suitability of BC for replacing some common fillers (such as carbon black, talc, and CaCO3) in the preparation of polymer biocomposites with tailored properties, useful for packaging, automotive, and electronic applications. Besides, the low production cost of this carbon filler, together with the possibility of deriving it from different waste sources, clearly represents an added value toward the circular economy approach. The combination of a waste-derived filler together with a biopolymer is thus a very effective tool for the improvement of materials sustainability.