Mutsuga et al. [79] reported on the leaching of lactic acid, lactide, and oligomers from PLA at different temperatures. PLA sheets procured from different manufacturers were placed into glass tubes with 100 mL water, and water with 4% acetic acid or 20% ethanol, and the presence of migrated compounds was measured with liquid chromatography/mass spectrometry after several fixed periods. They note that migration into the mixture was present but low for 20 or 40 °C, but that at 60 °C or higher there are significant migrant levels due to the decomposition of the PLA. Lactide migration levels were raised to 0.24 mg.cm² at 60 °C, 0.64 mg.cm² at 80 °C, and 4.12 mg.cm² at 95 °C and only at that higher temperature oligomers were identified at 1.98 mg.cm². After the migration test at 95 °C, the sample turned cloudy. It was observed that LA was calculated to be formed at 95 °C from lactide produced by PLA degradation. The authors note that migrant levels were worse for samples with a higher ratio of D-lactide. For the lower temperatures, the amount of lactic acid is lower than those present in some common food ingredients, making PLA suitable for food packaging applications [49,79]. However, the leaching of material may interfere with sensitive lab applications, particularly those involving higher temperature chemical processes.

Plasticisers may be used to increase the ductility of brittle polymers. For PLA, the glassy brittle polymer can be plasticised using its own monomer (lactic acid/lactide) to increase its flexibility. Sinclair reports the tensile properties for different percentages of lactide plasticiser [80]. The data for different percentage of plasticiser illustrate the broad range of properties that can be obtained, and some similarities to conventional thermoplastics can be noted. This allows PLA to compete as a sustainable packaging material; however, the authors note that degradation is increased with increasing plasticiser [80].

In addition to using plasticisers to improve ductility, other additives may be used with PLA to improve or tailor other properties. Polymer blends are another approach for achieving desired properties. PLA can be blended with several other polymers. For example, PLA can be blended with rubbery polycaprolactone (PCL) to increase elongation at break with reduced tensile strength and stiffness [81,82]. Impact modifiers can be added to improve the impact strength, and nanoscale clay particles can be added to improve stiffness [83]. Additives may be mixed to achieved the desired balance of properties, as illustrated in Figure 3 [83].

Additives can also be used to improve the heat resistance. Heat resistance is evaluated by several methods characterizing how the material’s resistance to deformation changes with temperature. The Heat Deflection Temperature (HDT) for PLA, defined as the temperature at which a defined specimen deflects 250 µm under a specified load and heating rate, is 55 °C, and the Vicat Softening Temperature (VST) for PLA, defined as the temperature at which a specimen is penetrated to 1 mm by a flat ended 1 mm² area pin under a specified load and heating rate, is 65 °C [84]. Heat resistance can be improved by increasing the crystallinity (using nucleating agents or processing strategy), by polymer blending, or by other reinforcement additives. Using nucleation additive dibenzoylhydrazide with PLLA, Kawamoto et al. [85] were able to achieve a HDT of 124 °C. The stiffness and Izod impact strength were also improved, 4.1 GPa and 7.9 kJ m⁻², respectively. In terms of processing, Tábi et al. [86] produced PLA samples using injection molding with different mold materials. Additively manufactured epoxy-based molds were able to deliver higher crystallinity due to their slowing cooling rates compared to steel molds. Using PLA containing nucleation agents (talc and polyethylene glycol (PEG)) with both mold types at room temperature, the VST was improved from 60–65 °C for the steel molds, to 118–124 °C for the epoxy-based molds. The strength of the interfacial bond between adjacent layers dictates the mechanical characteristics of 3D components manufactured using the fused filament fabrication (FFF) process. When PEGs are added to FFF-printed PLA components, the interlayer bond strength is increased, lowering the mechanical anisotropy from 32% for pristine PLA parts to 16% for PLA/PEG parts. Additionally, PEGs with a molecular weight greater than 8000 g/mol have a significant impact on the mechanical characteristics of PLA components [87]. Hriţuc et al. [88] looked into the 3D printing processes that use polylactide (PLA) wire to make parts in a wide range of shapes and sizes. The fused deposition modelling process had some differences between the desired dimensions and the real dimensions that were made. Taguchi models used in the study show that in the case of tubular parts made of PLA, printing speed and plate temperature have the biggest impact on the height and diameter of 3D printing.

PLA can also be used as a matrix in composite materials. Composite materials are made up of two or more different component materials with advantageous properties. For polymers, reinforcement with stiff fibres is a popular approach. Fibre-Reinforced Plastics (FRPs), using fibres such as glass or carbon in the polymer matrix, can provide high specific stiffness, specific strength, impact strength, and damping [89]. Their ability to deliver high strength-to-weight ratios has made FRPs popular in automotive, aerospace, and wind turbine applications [89,90,91,92,93,94,95]. Glass fibres have been used with PLA, providing increases in tensile strength, flexural strength, impact strength, and heat deflection temperature of 183%, 134%, 331%, 313%, respectively, using 30% glass fibre [96]. Carbon fibres have also been used with PLA, giving increases in tensile strength, tensile modulus, flexural strength, and flexural modulus of 73%, 438%, 53%, and 400%, respectively, with 30% carbon fibre [97]. It was reported by Agüero et al. [98] that they made and characterised green composites that used PLA-based fillers and additives that came from the linen processing industry. They showed that the waste from flax (Linum usitatissimum L.) or byproducts can be used to get renewable raw materials that can be used to make green composites for market applications like rigid food packaging and food-contact disposable items in the circular economy and bioeconomy.

Sustainable composites can be achieved by combining PLA with eco-friendly additives [99,100]. Cali et al. [101] report on the use of PLA matrices with agricultural waste fillers to create composite filaments which can be used in additive manufacturing (AM). Combining an eco-friendly, biodegradable polymer with biodegradable waste materials that might otherwise go unused to improve the mechanical properties of produced parts is a boon for the sustainability of composite parts, and of additive manufacturing as a technique. The authors used the filaments to successfully produce two biomedical prototypes [101]. Similarly, Matsuzaki et al. [102] describe a method where jute fibre is fed into a heated nozzle with pure PLA filaments during the AM process, achieving modulus and strength increases of 157% and 134%, respectively, compared to pure PLA. By combining PLA with poly (ε-caprolactone) (PCL), a biodegradable, water insoluble polyester, López-Rodríguez et al. [103] found that an increase of PCL led to a decrease in the Young’s modulus and the tensile strength of the composite (from 56.8 MPa at 0% PCL to 12.5 MPa at 80% PCL). At a composition ratio of 80% PLA and 20% PCL this blend was found to have similar mechanical properties to PS. Composite blends of PLA and silk fibroin (SF) containing 2–10 wt.% of PLA dispersed in a SF matrix displayed an increase in Young’s modulus (from 2876 MPa at 0% SF to 3480 MPa at 90% SF), tensile strength (from 23.2 MPa at 0% SF to 28.5 MPa at 90% SF), and hydrophobicity in comparison to neat PLA [104]. The mechanical properties of blends containing poly(butylene succinate) (PBS) and PLA were investigated by Chen et al. [105] and Wang et al. [106]. PBS is typically added into a blend with PLA in order to increase the toughness of the material without compromising the biodegradability of the plastic material [107,108,109]. More recently still, Rojas-Martínez et al. [110] have published results showing that PLA blended with keratin and chitosan can be 3D printed into scaffolds. This year, Brounstein et al. [111] reported the blending of PLA with TiO₂, ZnO, and ceramics (up to 30 wt.%) to produce antimicrobial composites. Banerjee et al. [112] evaluated the many kinds of nanoparticles employed in the manufacture of PLA nanocomposites, including nanoclay, nanocelluloses, carbon nanotubes, and graphene, covering all key processing, characterisation, and application elements. Dedukh et al. discussed new developments in the usage of composite materials manufactured from PLA in bone surgery and the use of 3D printing to create implants [113,114,115]. The usage of PLLA and PLA blends will rise as we learn more about how to adjust the mechanical response of this essential class of materials. These bioresorbable polymers have the capacity to breakdown under biomedically relevant situations. This degradation is regulated by the moleculer weight and orientations, crystallinity, and the chemical and load environment [116]. The impact of acrylated epoxidized soybean oil (AESO) addition on the mechanical, thermal, and thermomechanical characteristics of PLA components formed by injection moulding was studied by Quiles-Carrillo et al. [117] PLA components with 2.5–7.5 wt.% AESO exhibited a significant increase in elongation at break and impact-absorbed energy, but their tensile and flexural strength, as well as thermomechanical characteristics, were preserved or slightly enhanced. It also had a stronger thermal stability and a reduced crystallinity. Thus, using AESO to generate toughened PLA materials of great interest in rigid packaging, automotive, or building and construction applications might be deemed an ecologically acceptable alternative. PLA is gaining a reputation for being a flexible material, from which composites and blend materials can be produced, in order to fine tune the required physical and chemical properties. The key factor for consideration for these developments is not only the improved properties of the PLA, but also the degradation pathways of the materials and end destination of the additives.

The industrial production of PLA today is mainly based on the ring opening polymerization (ROP) of lactide because polycondensation of lactic acid requires rather harsh conditions, i.e., high temperatures (180–200 °C), vacuum (at least 5 mbar) and long reaction times to obtain PLA of high molecular weights [118,119]. In contrast, ROP works at rather quite mild conditions (max 130 °C, reaction times of several hours) and yields PLA with narrow PDI and high molecular weights of up to 100 kDa, which are important for reasonable mechanical properties. Metal alkoxides as for example tin(II) octoate are preferred as industrial catalysts as they give high molecular weight and no loss of optical purity [118,119]. The chiral integrity is crucial for the properties of PLA (as discussed in Section 3.2). Gupta et al. [120] reviewed the uses of PLA and its potential value in a variety of emerging technologies, including orthopaedics, drug administration, sutures, and scaffolds, and have piqued researchers’ curiosity in this innovative field. Additionally, they addressed developing PLA using a range of catalysts to meet a variety of performance needs.

Cargill Inc. was the first to industrialize the ROP process from L-lactide in the early 1990s. The required lactide is produced starting from lactic acid in a continuous process; the LA is first condensed to produce a low molecular weight prepolymer PLA from which by controlled depolymerization produces the lactide. The typical operating conditions for the reactor were residence time about 1 h, vacuum pressure 4 mbar, temperature 210 °C, and catalyst amount 0.05 wt.% tin(II) octoate in the feed. The crude lactide is separated and purified by distillation as the specifications for lactide are stringent, especially in terms of free acidity, water content, and stereochemical purity [119]. The industrial PLA synthesis is often referred to as a two-step process because lactide synthesis and ROP are combined [119,121]. As an example, as a typical ROP process, Figure 4 shows that of NatureWorks based on the original Cargill-Dow patented process. NatureWorks produces the USA with a plant with a total capacity of 150,000 t/yr. The second largest plant is the 75,000 t PLA plant in Thailand under the joint venture of Total and Corbion companies [121]. For this later process Sulzer developed together Corbion a continuous process based on the use of static mixers, called SMR™ (Sulzer Mixing Reactor, Sulzer, Winterthur, Switzerland) [122]. This reactor is characterized by a precise control of heat transfer and mixing effects, which allow a high turnover and a consistently high polymer quality. Subsequent devolatization via a static degassing technology allows the elimination of volatiles in the PLA and thus the recycling of unreacted lactide [122].

PLA is used in a wide range of applications ranging domestic, engineering, agricultural, and biomedical sectors [49,123]. PLA fibres can be used in packaging, clothing, furnishings, bedding (pillows, mattresses etc.), and other applications such as hygiene products [60,124]. Lunt and Shafer, [124] noted the advantages of PLA fibres for garments: better wicking and moisture regain; more comfortable; good resilience; unaffected by UV; low flammability; lower stiffness leads to better drape/hang; good crease resistance; dyeability; and sustainability. Superior self-extinguishing behaviour and lower smoke generation compared to other polymer fibres like PET, along with greater resilience and better sustainability, make PLA attractive for furnishings such as drapes and upholstery [124]. The renewable and biodegradable nature of PLA makes it well suited for single-use applications, such as packaging and containers [59]. For example, Swiftpak produce PLA insulation bags [125] and PLA bottles EU produce PLA bottles for milk and water [126]. Common packaging polymers like PET, PVC, polyethylene, polystyrene, and polyamide are petrochemical based with poor degradability. PLA’s properties are adaptable as described above which give scope to tailor the material to a range of packaging products. The Food and Drug Administration (FDA) in America has approved PLA for use in all food packaging applications [73]. PLA has been a popular material for additive manufacturing or 3D printing [100,127,128]. The relatively low glass and melt temperature make PLA easy to process with thermal methods like Fused Deposition Modelling (FDM). For rapid modelling or prototyping, many parts may be produced and discarded, so good degradability and good recyclability as a thermoplastic are other advantages to PLA. As it is non-toxic to the human body, and a bioabsorbable polymer, PLA is attractive for medical applications [73]. It may be used for sutures, dental implants, and drug-delivery devices [129].

PLA’s biodegradable attributes makes it attractive for disposable single-use labware items; however, uptake of PLA in labware has been limited. Properties like temperature, UV, and chemical resistance may be limiting factors for this application. SP Scienceware subsidiary Bel-Art produces a range of “Earth-Friendly” spoons and sampling sticks [130,131]. Baden et al. [47] describe the use of additive manufacturing to produce labware in-house using PLA among other materials; however, the focus here was on longer term jigs and fixtures, rather than lab consumables. Gordeev et al. [132] investigated the use of AM to produce chemistry equipment from several engineering polymers including PLA. 3D printed test tubes of PP, PLA, Acrylonitrile Butadiene Styrene (ABS), and PETG were produced and characterised, and the suitability of the materials was assessed in the study. Polypropylene is the most suitable material for chemical experiments due to its high resistance to chemical reagents; PLA labware, on the other hand, has superior properties: it has almost no pores and is very tight, the material does not shrink significantly, and the material is convenient for additional mechanical post-processing. While PETG products are partially transparent, which is an undeniable benefit, the layered structure created during printing precludes the use of PETG printed labware owing to its high porosity. ABS’s limited chemical resistance severely limits its use in chemistry. As a consequence, PP and PLA are much more appropriate for printing labware than ABS or PETG. The following is a general order of the functioning of plastic materials for chemical applications: PP > PLA > ABS > PETG. The mild solvents were Et₂O, EtOH, hexane, and H₂O, and the aggressive solvents were acetone, MeCN, CH₂Cl₂, THF, toluene, and DMSO.