Keywords

1 Introduction

Plastics are among the most common materials in Construction and Demolition sector (CaD), one of the most challenging in reaching the ambitious objectives proposed by the European Commission’s Waste Framework Directive 2008/98/EC in terms of recycling and eco-compatibility (EC 2015, 2018). Among the plastics used in CaD sector, a significant amount is composed by geosynthetics. Geosynthetics are polymeric products, such as geotextiles, geogrids, geonets etc., used in contact with soil and/or rock and mainly applied in civil and environmental engineering (Wiewel and Lamoree 2016). The global market of geosynthetics is approximately of 6,125 million square meters per year, for 5.76 USD Billion value and still growing (Müller and Saathoff 2015). Most of geosynthetics are fabricated from petroleum-based polymers such as the polyolefin and polyester family. Polypropylene covers almost the whole total production (~90%), whereas polyethylene accounts for the remaining part (Wiewel and Lamoree 2016). Surely, petroleum-based polymers are high performing in terms of mechanical resistance and durability, but the exposure to common environmental conditions over time could lead to micro-plastics particles (below 5 mm) releasing and leaching of eco-toxic compounds (Browne et al. 2007). The increase of environmental consciousness has led to an increasing interest in using natural and biodegradable products, such as the biopolymers, to replace the synthetic materials. The recent research highlighted how biopolymers can provide a good and cost-effective alternative to the conventional materials in several engineering applications, such as the construction of temporary roads, the basal reinforcement of embankments or the control of surface erosion (Maneecharoen et al. 2013). Until now, a wide variety of biodegradable composites reinforced with natural fiber composites (bamboo, banana, hemp, etc.) has been proposed and tested with encouraging results (Pickering et al. 2016); blending with polymeric matrix was also shown to be a viable option (Farrington et al. 2005). Among the available biodegradable biopolymers, poly(lactic acid) or polylactide (PLA) is one of the most common. PLA is a thermoplastic, high-strength, high-modulus biopolymer that can be easily processed to fabricate a wide range of products. PLA has already been investigated/used for several applications such as food technology, medical engineering, pharmaceutical, packaging and agriculture (Garlotta 2001), whereas very few are the studies focusing on geocomposites production (Bhatraju and Kumar 2018). Producing geosynthetics with PLA and/or PLA-based composites can potentially ensure several advantages in terms of sustainability and eco-compatibility. In fact, PLA is (i) prepared from renewable agriculture-based feedstock, which are fermented into lactic acid; (ii) recyclable and compostable (Drumright et al. 2000); and (iii) slowly degradable (Rasal et al. 2010) into non-toxic substances, as water, carbon dioxide and humus (Tuominen et al. 2002). The aim of this work is to investigate the chance to produce geosynthetics for geotechnical applications with PLA, in view of its high sustainability and favorable mechanical properties. Moreover, PLA is easily available on the market and suitable for prototypes construction. Specific objectives are to investigate the chemical and mechanical characteristics of PLA and to test the tensile behavior of small-scaled model geogrids produced through a 3D printing process.

2 Materials and Methods

2.1 Selection of Biopolymer

An accurate analysis of the wide spectrum of biodegradable and non-toxic materials already available on the global market and, in particular, of those commercialized as consumables for 3D printers was conducted. The research focused on those biopolymers characterized by elevate mechanical properties, comparable with petroleum derivatives. Neat PLA was selected for the present study because it is surely the most diffuse biopolymer in the field of 3D printing and shows also an ultimate tensile strength (TS) variable between 25 and 70 MPa and an average Young’s modulus of 3500 MPa (Muller et al. 2017). PLA is readily available on the market as cheap filament with standard diameter of 1.75 mm or 2.85 mm, suitable for 3D printers.

2.2 Physical, Mechanical and Chemical Characterization of Biopolymer

Neat PLA filament (Orbi-Tech©, Germany) was selected and characterized in detail, besides the physical properties provided in the producer’s technical sheet, such as a glass transition temperature of 50 °C, a melting temperature of 210 °C and a Young’s modulus of 3310 MPa.

Chemical structure of PLA samples was obtained through optical purity analysis and 1H-NMR spectroscopy. Such procedure verified the chemical purity of the lactic acid monomer, the presence of carboxylic acid impurities that could retard the polymerisation of lactic acid and could affect the other properties. Specific optical rotation ([α]D) was registered with a JASCO P-2000 polarimeter, at 25 °C with a wavelength of 589 nm. PLA samples were solubilized in chloroform (CHCl3) at a concentration of 1.00 g/dL. Equation 1 defines the percentage of optical purity (OP):

$$OP\left( \% \right) = \frac{{\left[ \alpha \right]_{589}^{25} }}{ - 156} \cdot 100$$
(1)

where [α] is the specific rotation observed and −156 is the specific rotation for L-lactic acid enantiomerically pure PLA, at a concentration of 1 g/dL at 25 °C (Marques et al. 2012). 1H NMR spectra were recorded at 25 °C with a Bruker AV 600 spectrometer at 600 MHz. Samples were prepared solubilizing PLA in CDCl3 in a range of concentration of 1–10 mg/mL.

Mechanical tests were conducted through a Universal Testing Machine (MTS tensile test machine) mounting TH240 g universal clamps (with a distance between the jaws of 10 cm) that avoided sample damage at the clamping points (ASTM D638). Tensile force was exerted by a system of gears at a rate of 10 mm/min and it was measured as a function of the strain by a load cell (F.S. = 5000 N). The tensile force (N) at the point of rupture was taken as the peak load, and the related stress (MPa) was calculated by dividing the breaking force by the cross-sectional area of the filament (mm2). For model geogrids, the stress (kN/m) was calculated by dividing the breaking force by the sample’s width.

2.3 Prototype Samples Building and Testing

A common approach adopted in geotechnical research is based on model test results. However, due to the cost and difficulties associated with model preparation, full-scale model tests/studies are rare compared to small-scale tests. 3D printing has become a revolutionary technology that has also been adopted in geotechnical research. For instance, Stathas et al. (2017) proposed and successfully fabricated standard prototypes of model geogrids at a scale of 1:10 through a 3D printing process for the centrifuge model testing under 1-g and 10-g condition.

According to the technical specifications of the geotechnical literature (e.g., Springman et al. 1992; Viswanadham and König 2004) the tensile strength of model geogrids used in 1:N reinforced-soil models must be 1/N2 of that of full-scale geogrids when the test is carried out under 1-g condition, or n/N2 of that of prototype (geogrids) under n-g condition in centrifuge tests. Note that N represents the scale of a reduced model (e.g., 1:10), and n is the level of gravity (e.g., 10-g) to which the model is subjected (usually n is set equal to N).

For this study two different model geogrids, uniaxial and biaxial, were designed and tested on a scale of 1:5 so that they could be used to investigate the behavior of geosynthetics-reinforced soil structures in 5-g centrifuge models. The design adopted for the geogrids was obtained modifying the scale of the design adopted by Stathas et al. (2017) from 1:10 to 1:5. The prototypes were produced with a Sharebot NG 2 printer using the Fused Filament Fabrication, a common 3D printing process for thermoplastic materials that uses a continuous filament (as shown in Fig. 1). The dimensions of the prototype samples were 50 mm width ×150 mm length, and 1 mm thick. Tensile tests were carried out on the raw filaments characterized by 1.75 and 2.85 mm diameter and on the 3D printed geogrids samples.

Fig. 1
figure 1

3D printing process of model geogrids from filament to sample geogrid

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3 Results

3.1 Chemical and Mechanical Characterization of Biopolymers

In terms of chemical properties, PLA samples showed an average [α]D value of −128.70 ± 1.40° and an OP of 81.96 ± 0.52%. The data obtained, compared with the literature (Marques et al. 2012), showed how the PLA sample was mainly composed by the L-lactic acid enantiomer with a not substantial amount of impurities.

PLA structure was confirmed by the NMR analysis. In the protonic spectrum a quartet and a doublet were observed at 5.15 and 1.57 ppm. Those signals are typical of the CH and CH3 groups of the polymeric chain. In the spectrum, were also visible a weak quartet at 4.35 ppm (compatible with the terminal CH) and weak high-field peaks.

From the mechanical point of view, the observations during the tensile tests exhibited the common characteristics of the stress-strain curve of plastic materials as shown in Fig. 2. These features can be identified as three principal stages:

Fig. 2
figure 2

a Stress-strain curve of the seven tests (1, 2, …7) conducted on 1.75 mm PLA filaments; b stress-strain curve of the five tests (1, 2, …5) conducted on 2.85 mm PLA filaments

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  • The linear elastic region, where the stress is proportional to the strain and the material undergoes only elastic deformation. The end of the stage is the initiation point of plastic deformation. The stress component of this point is defined yield strength.

  • The strain hardening region, which starts as the strain goes beyond yielding point, and ends at the ultimate strength point that is the maximal tensile stress shown in the stress-strain curve (that corresponds to the maximum tensile strength, TS).

  • The necking region, where the local cross-sectional area becomes significantly smaller than the average. During this stage irreversible plastic deformation of the material occurs.

Mechanical characterization of PLA shows how the 1.75 mm filament has a peak tensile load of 124.99 ± 5.72 N, corresponding to TS of 51.96 ± 2.38 MPa. Tests conducted on 2.85 mm filament highlighted a peak tensile load of 303.99 ± 31.38 N, TS of 47.65 ± 4.92 MPa and a shorter plastic deformation phase, compared with 1.75 mm filament (Fig. 2b).

3.2 Prototype Samples Building and Testing

Mechanical tests conducted on the uniaxial geogrid prototypes gave a maximum tensile load of 681.54 ± 163.67 N and a TS of 12.66 ± 3.20 kN/m. Figure 3 shows all the tests and highlights the different shapes of the stress-strain curves. The variability in the trends of the curves could be due to the 3D printing process that produced prototypes with microscopic imperfections within a tolerance margin depending on the printer and caused by a possible variation of the extrusion temperature.

Fig. 3
figure 3

a stress-strain curve of the seven tests (1, 2, …7) conducted on uniaxial prototype geogrid b stress-strain curve of the five tests (1, 2, …5) conducted on biaxial prototype geogrid

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Biaxial prototype geogrids showed an average peak load of 192.92 ± 72.76 N. Consequently, a lower ultimate tensile strength was obtained (3.79 ± 178 kN/m).

A certain variability was still recorded between the stress-strain curves. In particular, tests [1] and [2] showed a long plastic deformation phase, while its nearly absent in the other cases (Fig. 3b). As expected, if tested in machine direction, uniaxial prototype geogrids exhibited a higher tensile strength than the biaxial samples that, conversely, provided a more uniform tensile in two orthogonal directions instead of a single one.

The obtained results were compared with those measured in the work of Stathas et al. (2017), once converted to 1:5 scale. The comparison highlighted that the uniaxial prototype geogrids of neat PLA pointed out a greater tensile resistance (12.66 kN/m) respect than the same ones produced with Verowhiteplus, a photopolymerising resin composed of acrylic compounds, as shown in Fig. 4a, despite the greater tensile strength (58 MPa) of the raw material compared with PLA. PLA-based biaxial geogrid resulted slightly more resistant than Verowhiteplus-based (Fig. 4b), with a tensile strength of 3.79 kN/m versus 2.4 kN/m. However, despite the lower tensile strength, acrylic resin resulted to be more elastic than PLA in both uni- and bi-axial prototypes.

Fig. 4
figure 4

Comparison between uniaxial (a) and biaxial b model geogrid with literature (Stathas et al. 2017) at 1:5 scale

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4 Discussion and Conclusions

In the present study, the chemical and mechanical properties of PLA, a biodegradable biopolymer, were investigated in view of its use for geosynthetics production, as an alternative to petroleum-based polymers. PLA emerged as the best candidate for this application among the variety of biodegradable materials available on the market and already largely used in other fields, such as packaging, that we analysed prior to the study. The chemical and mechanical characterization showed that sample PLA is composed of approximately 82% L-lactic acid and has a tensile strength of 51.96 ± 2.38 MPa in form of 1.75 mm filament and 47.65 ± 4.92 MPa in form of 2.85 mm filament.

The production of small-scaled standard geogrid through a 3D printer allowed a robust comparison with other produced with different materials. In particular, PLA-made uniaxial geogrids resulted to be approximately 50% more resistant than Verowitheplus ones at the same scale. The PLA biaxial geogrid performance, even if higher than the models realized by Stathas et al. (2017), showed a high variability, probably due to the difficulty to reproduce this specific geometry during the 3D printing process. Despite the higher tensile strength, PLA prototypes resulted to be less elastic than those produced with the acrylic resin that present an elongation at break up to 25%.

Finally, the biodegradable PLA or PLA-based polymers could be reliable and robust materials for the use in the field of civil, geotechnical and bioengineering. Further investigations on their mechanical degradation under different field conditions (water, soil, rooted-soil, etc.) and estimation of durability, will allow to establish if these materials may actually represent an alternative to the petroleum-based products that cause serious impacts on the environment. Clearly, there is still a gap in terms of tensile strength between biopolymer-made and petroleum-based geosynthetics, but their use in combination with live plants—where the root system development initially supports and then gradually substitutes the biodegradable products in soil reinforcement—represents a promising research field.