The Potential of Bio-Based Polylactic Acid (PLA) as an Alternative in Reusable Food Containers: A Review

Abstract

The biodegradable biopolymer polylactic acid (PLA) has been used in the recent past in single-use packaging as a suitable replacement for non-biodegradable fossil fuel-based plastics, such as polyethylene terephthalate (PET). Under FDA and EU regulations, lactic acid (LA), the building block of PLA, is considered safe to use as a food contact material. The mechanical, thermal, and barrier properties of PLA are, however, major challenges for this material. PLA is a brittle material with a Young’s modulus of 2996–3750 MPa and an elongation at break of 1.3–7%. PLA has a glass transition temperature (Tg) of 60 °C, exhibiting structural distortion at this temperature. The water permeability of PLA can lead to hydrolytic degradation of the material. These properties can be improved with biopolymer blending and composites. Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), for instance, increases the thermal stability of PLA while decreasing the water permeability by up to 59%. Polypropylene (PP) is one of the most common plastics in reusable food containers. This study will compare PLA-based blends and composites to the currently used PP as a sustainable alternative to fossil fuel-based plastics. The end-of-life options for PLA-based food containers are considered, as is the commercial cost of replacing PP with PLA.

1. Introduction

The harmful effects of the production and disposal of plastics on the environment and public health have drawn attention from industry and regulatory bodies in recent years. Over the past four decades, the global demand for plastics has quadrupled [1]. This is predicted to grow in the future, with a calculated 5.6% compound growth rate per annum [2]. In the past, there has been a focus on the harmful impact of plastic pollution and incineration. There is now a growing awareness of the substantial environmental and health impacts of fossil fuel-based plastic production, such as the release of greenhouse gases and particulate matter emissions [3]. Fossil fuel combustion during plastic production alone accounted for up to 88% of the total carbon footprint of plastics in 2015, while end-of-life (EoL) stages, including incineration, landfill, and recycling, accounted for 6% of plastic’s total carbon footprint [4]. Packaging accounts for approximately 36% of all plastic use, with food packaging accounting for approximately 50% of total plastic packaging sales [5].

The most common plastics used in general food packaging are polyolefins, i.e., polyethylene (PE) and polypropylene (PP), followed by polyethylene terephthalate (PET) and polycarbonate (PC) [6] (Figure 1). There are many advantages to using these plastics in food packaging and containers. They offer design flexibility and chemical resistance, and are lightweight and inexpensive [5]. PP offers an advantage in microwavable, dishwasher-friendly plasticware compared to other fossil fuel-based plastics, such as PE. It exhibits high thermal resistance with a melting point (Tm) of 160–166 °C [7] compared to PE (Tm = 110–130 °C). High-density polyethylene (HDPE) is commonly used as a food container for microwavable applications, with high thermal resistance and good barrier properties. HDPE has poor aging properties compared to PP and will show stress cracking over time [8]. Although PET has a high melting point (Tm = 245–265 °C), it exhibits brittleness at frozen food temperatures and has a higher moisture vapor transmission rate and water uptake (%) compared to PP [9]. PC is not recommended as a food container for elevated temperature applications, as the migration level of bisphenols could exceed safe levels [10]. This study will focus, therefore, on the development of a biodegradable biopolymer to replace PP, the most functional plastic currently on the market, in relation to microwavable, reusable food containers (

Table 1. The advantages and disadvantages of commonly used food contact plastics [7,8,9,10,11,12].

Type of Food Contact Plastic	Advantages	Disadvantages
High-Density Polyethylene (HDPE)	Heat and cold resistant, solvent resistant, high mechanical strength, UV resistant, good moisture barrier, chemical resistant	Poor weathering, stress cracking occurs over time
Low-Density Polyethylene (LDPE)	Better extensibility and processability compared to HDPE, chemical resistance, low-temperature resistance	Poor mechanical strength, low moisture barrier, poor weathering, not heat resistant
Polyethylene terephthalate (PET)	High mechanical strength, high gas barrier, chemically resistant	Lower moisture barrier compared to HDPE, oxidation over time leading to shorter shelf life
Polycarbonate (PC)	Excellent mechanical strength, good insulator, good weathering	Bisphenol A migration into food upon heating
Polypropylene	Heat and cold resistant, solvent resistant, good moisture and gas barriers, high mechanical strength, chemical resistant, good weathering	Impact strength decreases at low temperatures, degraded by UV
Polylactic Acid (PLA)	Made from renewable resources, non-toxic, better UV resistance compared to LDPE, better thermal properties compared to other biobased plastics, biodegradable	Brittle, low moisture and gas barriers, not heat resistant

).

Reduction in food waste is an important benefit of food packaging, prolonging the shelf life of food products through chemical (e.g., oxygen and moisture barriers) and biological (e.g., bacteria barrier) protection. There is, however, a significant environmental impact associated with the production and disposal of PP. Gross CO₂ emissions for the manufacture of 1 kg of PP is calculated to be 3.1 kg [13,14]. A further 2.1 CO₂e/kg PP is emitted during the incineration [15]. There is current research on the use of specific bacterial strains to biodegrade fossil fuel-based plastics such as PP. The most promising of these studies found that a mixture of four strains of bacteria, obtained from waste management landfills and sewage treatment plants, could degrade PP strips up to 56.3% after 140 days [16,17]. As of now, there are no known isolated enzymes capable of degrading PP. It could be possible to improve the biodegradation of PP by blending it during the manufacturing process with degradable additives, such as PLA [18].

To minimise the carbon footprint of food packaging manufacture and waste disposal, bio-based, biodegradable plastics have risen in popularity in recent years. Bio-based plastics are produced from renewable resources, e.g., sugar in plants. The monomers are extracted from the biomass and polymerized to produce a replacement for an existing plastic (e.g., bio-based polyethylene terephthalate, PET) or a novel plastic (e.g., polyhydroxyalkanoates, PHAs, or polylactic acid, PLA) [19]. Certain bio-based plastics are also biodegradable, meaning the plastic can be degraded by living organisms. In this way, bioplastics can have a lower carbon footprint than traditional plastics by using renewable resources during production and undergoing biodegradation as an EoL option.

When replacing traditional, petrochemical-based plastics with bioplastics, a range of material properties must be considered, including mechanical and thermal properties, chemical resistance, barrier properties, processing ease and cost, and EoL options. Bioplastic production currently accounts for 1% of global plastic production. Developments in this sector had stagnated due to the COVID-19 pandemic [20]. Since 2021, global production capacities of both biodegradable, biobased polymers and fossil fuel-based polymers have continued to grow. PLA is a biobased, industrially compostable polymer that can be synthesized by the polymerization of lactic acid (LA) produced by bacterial fermentation of feedstock [21]. A wide range of feedstock can be used to produce LA, including first-generation feedstock (e.g., corn) or second-generation feedstock (e.g., food waste). Short-chain polysaccharides are extracted from the feedstock. An example of sustainable sugar production from second-generation feedstock uses wheat straw and biogas digestate [22]. The sugar units are then fermented by microorganisms to yield lactic acid. This lactic acid is polymerized to PLA. Energy consumption occurs throughout this process, but biopolymers such as PLA have the environmental benefit of carbon uptake during feedstock growth, as shown by various life cycle assessments (LCAs) of PLA [23,24,25]. This positive carbon credit does not occur with fossil fuel-based plastics.

The utility of PLA as a replacement material for single-use food packaging has been reported [26,27,28] and it is currently the leading bioplastic in this sector, with investments in PLA production expected to increase in the US and Europe in the coming years [24]. However, the reusability of PLA for food storage is still largely unknown. As plastic production accounts for the majority of the carbon footprint of plastics [4], a transition from single-use plastics to reusable plastics must be considered. A further reduction in the carbon footprint of plastic production and EoL options can be achieved by switching from fossil fuel-based plastic, such as PP, to bioplastic, such as PLA. An estimated 3.1–5.2 kgCO₂e/kg plastic is emitted during PP production [14,15] and incineration/landfill EoL options, while 1.8–3.7 kgCO₂e/kg plastic is emitted during PLA production and incineration/landfill EoL options [3,29] (Figure 2). This review will investigate the feasibility of PLA and its blends/composites to make reusable food containers, particularly in comparison to the common reusable plastic, PP.

2. Food Storage Container International Regulations

2.1. Introduction

Regulations surrounding food contact materials (FCM) exist due to the possibility of a transfer of chemicals or substances between the materials and their contents. It is important to note that the following regulations described in Section 2.2 and Section 2.3, the United States Food and Drug Administration (FDA) and European Commission (EC) regulations, are in relation to the monomer LA and not the polymer polylactide. The monomer LA is produced by microbial fermentation of biomass [30]. The polymer PLA is formed from LA through various methods such as ring-opening polymerization or direct polycondensation [30,31] (Scheme 1). PLA can be hydrolysed into its substituent components, such as lactide, other oligomers of PLA, and LA, during the intended use of the material [32]. Lactide and PLA oligomers will then be hydrolysed to LA [31]. Therefore, LA is the focus in the evaluation of PLA as a suitable FCM under the relevant regulations and requirements from various governing bodies and scientific communities.

2.2. Food and Drug Administration (FDA): Code of Federal Regulations

The Select Committee on GRAS (Generally Recognized as Safe) Substances (SCOGS), established by the FDA, affirmed LA as a GRAS substance in 1978 in SCOGS Report Number 116 [33]. L(+)-LA was classified as a Category 1 substance by the SCOGS (21 CFR Regulation 184.1061), while D(-)-LA was classified as a Category 1 substance for non-infants (those over 12 months old) and a Category 4 substance for infants. Category 1 states that “There is no evidence in the available information on [the substance] that demonstrates, or suggests reasonable grounds to suspect, a hazard to the public when they are used at levels that are now current or might reasonably be expected in the future”. Category 4 states that “The evidence on [the substance] is insufficient to determine that the adverse effects reported are not deleterious to public health should it be used at former levels and in the manner formerly practiced” [34].

Under the Code of Federal Regulations (CFR), Title 21, Chapter 1, Subchapter B, Part 184.1061, LA is recognized as a GRAS substance as a “direct human food ingredient” and can be “used in food with no limitation other than current good manufacturing practice” [35,36]. These regulations concern LA as a direct food substance. Under the CFR, Title 21, Chapter 1, Subchapter B, Part 184.1(a), “Ingredients affirmed as GRAS in this part [Part 184] are also GRAS as indirect human food ingredients” [37].

The FDA Code of Federal Regulations states that an FCM that is expected to migrate into food may be exempt from regulation as a food additive if the total migration levels fall below the threshold of regulation (dietary concentration of 0.5 parts per billion/day or 1.5 µg/person/day) and the migrant is non-carcinogenic and will have no adverse environmental effects [38].

The FDA’s food contact notification (FCN) program is the primary system in which a food contact substance (FCS) is evaluated, and companies are required to submit an FCN to the FDA prior to marketing the product [39]. The company Total Corbion PLA, now TotalEnergies Corbion (Zuid-Holland, The Netherlands), is listed as the notifier for FCN No. 1926 [40]. The FDA issued a finding of no significant impact for FCN No. 1926, “for the use of … polylactide (PLA) polymers, optionally containing up to 16 weight percent D-lactic acid polymer units (CAS Reg. No. 9051-89-2) for use as components of food-contact articles”, with the added note that “the FCS is not for use in contact with infant formula and human milk”. NatureWorks LLC also holds two FCNs (475 and 178), in which the polylactide is intended to come into contact with all food types [41,42].

2.3. EU Regulations

Under EC Regulation No. 10/2011 on plastic materials and articles intended to come into contact with food, LA is affirmed for use as an additive or polymer production aid in the manufacturing of FCM. It is also approved to be used as a “monomer or other starting substance or macromolecule obtained from microbial fermentation” [43]. General measures for FCMs under EC legislation are set out under Regulation (EC) No. 1935/2004 and Regulation (EC) No. 2023/2006. Regulation (EC) No. 1935/2004, which applies to all FCMs, emphasizes the requirements of the acceptable inertness of an FCM, to prevent any substances transferring to food at an unsafe level for human health. Regulation (EC) No. 2023/2006 sets out the principles of good manufacturing practices that apply to every material intended for use as an FCM [44].

Under EU regulations, LA is an authorized food additive (E270) according to Annex II and Annex III of Regulation (EC) No. 1333/2008, belonging to group 1 additives. Its use is permitted in several food categories, mainly at quantum satis (no maximum level specified). Categories of interest that require restrictions are detailed in

Table 2. Categories of food with LA restrictions according to Regulation (EC) No. 1333/2008, Annex II and III [45].

Category	Maximum Level (mg/L or mg/kg as Appropriate)	Restrictions/Exceptions
Infant Formula	Quantum satis	L(+) form only
Follow-on Formula	Quantum satis	L(+) form only
Processed Cereal Based Food and Baby Foods for Infants and Young Children	5000	Only for pH adjustment, L(+) form only
Other Foods for Young Children	Quantum satis	L(+) form only
Lactic Acid Esters of Mono- and Diglycerides of Fatty Acids	5000	Only biscuits and rusks, cereal-based foods, baby foods

[45].

The Joint FAO/WHO Expert Committee on Food Additives (JECFA) has not placed a limitation on the use of LA as a flavouring agent for foods [46]. The European Commission’s Scientific Committee on Food (SCF) agrees with JEFCA’s ADI (Acceptable Daily Intake) status of “not limited” in relation to LA. They also noted that “there is some evidence that babies in their first three months of life have difficulties utilizing small amounts of DL- and D(-)LA. Adults metabolize D(-)-lactate without difficulty.”

3. PLA as a Feasible Replacement for PP

3.1. Operational Parameters for Food Storage Containers

For a material to work as a suitable replacement for currently used food container plastics, it will need to withstand a variety of temperature changes, heating methods, and safety requirements. These requirements have been posed as the following questions (Figure 3):

• What effect will heating in a microwave have on the material?
• What effect will dishwashing have on the material?
• Will mechanical properties be maintained after cooling in fridge (4 °C) or freezer (−20 °C)?
• Will the material leach into food at unsafe levels over a range of temperatures?
• Does the material have sufficient barrier properties to exclude oxygen?
• Is the material optically transparent?

3.1.1. Microwaving

Microwave ovens use microwaves (wavelengths ranging from 1 mm to 30 cm) generated by a magnetron to heat food. This is achieved through the interaction of the microwaves with water molecules within the food. The microwaves cause water molecules in food to vibrate, which produces heat, thus cooking the food [47]. Due to this, the maximum temperature that food placed in a microwave should reach will be the boiling point of water (100 °C). A material used as a container for heating food would need to withstand these elevated temperatures while retaining mechanical, chemical, and thermal stability.

3.1.2. Dishwashing

Dishwashers have become a staple in most domestic settings due to their ease of use and ability to clean a variety of utensils and dishes with the press of a button. The working principle of most common dishwasher models is that rotating arms spray a mixture of hot water and detergent, cleaning dirt and debris from the items in the dishwasher. Dishes are then rinsed with water and finally dried using residual heat in the machine. The temperatures and exposure times to this environment vary widely between settings, models, and brands. Most modern models are equipped with an eco-setting that uses temperatures as low as 30 °C for longer periods of time compared to the average temperature and runtime of a dishwasher cycle of 55 °C for 30 min.

3.1.3. Sterilizing

Sterilizers are typically commonplace in domestic settings with new-born children, where it is recommended that bottles and other items be cleaned to protect the child from infections while their immune system is still weak. Sterilizers work on the simple principle of steam cleaning, where water is heated to form steam, which kills bacteria. Sterilizers typically operate at around 100 °C, with exposure times varying widely across models and brands. Microwave sterilization operates on the same principle as steam sterilization with the water converted to steam using microwave radiation.

3.2. Mechanical Properties of PLA and PP

PP exhibits better ductility and impact strength than pure PLA, whereas PLA has been described as a brittle material [48]. Comparisons of the general mechanical properties of PLA and PP are given in

Table 3. The literature values of Young’s modulus (MPa), ultimate tensile strength (MPa), elongation at break (%), and impact strength (J/m²) of PLA and PP, respectively. Refs. [7,30,48,49,50].

Polymer	Young’s Modulus (MPa)	Ultimate Tensile Strength (MPa)	Elongation at Break (%)	Impact Strength (J/m2)
PLA	2996–3750	40–59	1.3–7	1300
PP	1300	31–45	50–145	3000–6500

.

Molecular weight and the degree of crystallinity influence the mechanical properties of plastics. There are two stereoisomers of PLA: L(+)LA and D(-)LA. An increased concentration of D(-)LA in the plastic will lead to decreased impact strength and ductility and increased brittleness [49]. Natural aging can also affect the strength and brittleness of polymers. Ageing can occur through hydrolysis, photodegradation, and natural physical processes [51,52,53]. These physical processes include the movement of the polymer chains over time into more stable, enthalpic arrangements and occur at temperatures below the glass transition temperature (T_g) of the polymer [54,55]. This process occurs faster at a temperature close to the T_g of the polymer. For PLA, the T_g is 55–63 °C [56], meaning natural aging will occur fastest at these temperatures.

Pastor et al. reported that this aging process can be stopped by freezing PLA at −24 °C or lower, possibly ad infinitum [53]. Freezing did not lead to negative effects on mechanical properties, and when PLA samples were defrosted, no measurable changes were observed in the properties of PLA.

3.3. Thermal Properties of PLA and PP

The semi-crystalline structure of these polymers produces a complex combination of thermal transitions occurring in the crystalline phase and the amorphous phase. The presence of the crystalline phase can impose restrictions on the mobility of the amorphous phase, impacting the mechanical performance of such semi-crystalline polymers at elevated or reduced temperatures [57,58,59]. PLA exhibits a relatively low glass transition temperature (T_g) and melting temperature (T_m) of approximately 55–65 °C and 145–180 °C, respectively [60,61,62]. In comparison, PP can be found in amorphous (atactic) and semi-crystalline forms (isostatic) with a T_g of −18–0 °C and a T_m of 156–170 °C [57,63]. The thermal properties of PLA and PP are detailed in

Table 4. The literature values of the glass transition temperature (T_g), crystallization temperature (T_c), melting temperature (T_m), degree of crystallization (X_c), heat deflection temperature (T_HDT) and thermal conductivity (λ) of PLA and PP, respectively [46,50,51,52,53,54].

Polymer	Tg (°C)	Tm (°C)	Xc (%)	THDT (°C)	λ (W/mK)
PLA	55–63	148–168	10–22	55	0.2–0.26
PP	−18–0	156–170	30–45	57–90	0.18–0.2

.

The T_g of a polymer is a phenomenon of amorphous and semi-crystalline polymers where the material transitions from a glassy state (at temperatures below T_g) to a rubbery state (at temperatures above T_g). This is due to the disordered chains of the amorphous state gaining enough energy to exhibit mobility at this temperature. At working temperatures ranging from 0–150 °C, PP food containers will maintain their relative ductile consistency and will not deform upon heating within this range. Neat PLA, with a T_g of 60 °C, will undergo a transition around this temperature, becoming deformed when heated (Figure 4). Although T_g of PP is below that of PLA, both materials exhibit a close heat deflection temperature (HDT) of 57–90 °C and 55–60 °C, respectively [62,64,65,66].

At temperatures above its T_g (~60 °C), PLA has been found to have a decrease in Young’s modulus. Dynamic mechanical analysis (DMA) by Suryanegara et al. on amorphous PLA described a decrease in the Young’s modulus from 3500 MPa below its T_g to 4 MPa at 80 °C with a recovery to 200 MPa at 100 °C [67]. Analysis was also carried out on crystalline PLA, with a decrease from 3896 MPa below T_g to 505 MPa at 80 °C and a further decrease to 293 MPa at 120 °C [67]. Research by Ruz-Cruz et al. described the same decrease in Young’s modulus after reaching T_g and recovery of the Young’s modulus at 90 °C [68]. A slight increase was noted in the Young’s modulus as T_g was approached, due to crystallization of the polymer with recovery occurring at higher temperatures [68]. Lopez-Rodriguez et al. described the same phenomenon as for crystalline PLA (PLLA), with a large decrease in the Young’s modulus occurring at T_g from around 2500 MPa to 250 MPa at 100 °C, with a further decrease to <250 MPa at 120 °C [47,67].

Similarly, DMA performed on PP showed that the Young’s modulus decreased at temperatures greater than T_g. The Young’s modulus of isotactic PP decreases by ~90% when the temperature is increased from 25 °C to 125 °C [69]. Brylka et al. reported a decrease from 3500 MPa at T_g to ~500 MPa at 100 °C, similarly to PLA [70]. Comparisons of DMA conducted on both polymers show similar behaviours at higher temperatures with large decreases in Young’s modulus for both polymers, although the change in Young’s modulus for PLA is greater than that for PP. PLA and PP appear to have similar heat deflection temperature (HDT) ranges, though this does not account for differences in crystallinity, processing parameters, or other factors that can affect this temperature [71,72].

PLA and PP have similar thermal conductivities of 0.07–0.24 W/m·K and 0.17–0.27 W/m·K, respectively [7,65,73,74,75]. PLA can act as a thermal insulator with a thermal conductivity value of less than 0.1 W/m·K [73]. Short annealing times (~1 h) at lower temperatures (~280 K) during PLA processing showed low thermal conductivities (0.074–0.086 W/m·K) comparable to commercial thermal insulators [73].

3.4. Migration and Sorption Properties

Migration is the term used to describe the transfer of food packaging material into food. This material is referred to as non-intentional added substances (NIAS) [76,77,78]. The identity, toxicology, and quantity of migrated material into food items under conditions of intended use are considered [79]. Nano- and microplastics (NMPs) and the negative health effects of NMP ingestion due to plastic food packaging or containers have gained the attention of consumers in recent years. NMPs have been found in drinking water, dairy products, and sugar, to name a few [80]. Higher temperatures during food processing and prolonged periods of food storage increase the likelihood of monomer or oligomer migration [81].

LA, lactide, and small oligomers are the most common migrants from PLA. It has been shown that thermal processing and exposure to moisture can lead to the formation of PLA oligomers [76]. Oligomers are not listed in the positive list of the EC 10/2011 Directive, and therefore, the total oligomer concentration must not exceed 60 mg/kg of food [43,82]. LA is listed as an authorized monomer without restriction. The leaching of substances from PLA has been tested at various temperatures [83]. The concentration of migrating substances from PLA in various aqueous solutions was determined via liquid chromatography and mass spectrometry at fixed times. It was found that PLA was stable at 20 °C and 40 °C for up to 180 days, with migration levels in the range of 0.2–15.00 µg/cm². At temperatures of 60 °C or higher, the migrant concentration increases due to the decomposition of PLA. Lactide migration levels were measured at 0.24 mg/cm² at 60 °C, 0.64 mg/cm² at 80 °C, and 4.12 mg/cm² at 95 °C. Small oligomers were also measured at 95 °C at a concentration of 1.98 mg/cm². It was noted in this study that PLA material with a higher ratio of D-lactide led to worse migration levels.

Migration tests of oligomers from a biodegradable PLA polyester were also conducted with different food simulants at 60 °C for 10 days [76]. Different solvent systems were used to simulate aqueous and fatty food products. Certain oligomers migrated into the food simulants at a concentration of 0.0006 mg/g for the aqueous simulant and 0.00035 mg/g for the fat simulant. The effect of the manufacturing process on oligomer migration was also investigated, and no notable change was observed [76]. However, the composition of the PLA material did affect the migration tests. According to EC 10/2011, if any physical changes in the packaging occur during the testing, the material cannot be used under the test conditions [43]. It was noted in this study that the test temperature was close to the T_g of PLA (50–60 °C) and that the PLA material might have better applicability for lower temperatures, such as storage at room temperature or in refrigerated conditions.

As previously indicated, LA is the final migrant from PLA into food when oligomers or lactides are the initial migrants, due to hydrolysis. LA naturally occurs in food products or is intentionally added and is generally recognized as safe (GRAS) by the FDA [84]. The sorption of small molecules into PLA and their resulting migration out of the PLA material is an issue to consider when developing PLA food contact material. The high water diffusivity of PLA [85] allows small molecules to permeate into the matrix. A recent study investigated the use of PLA-chitosan composites as metal absorbents for wastewater treatment [86]. The relatively low barrier properties of PLA pose a major challenge when using PLA as a food contact material. These properties will be discussed further in Section 3.5.

PP is considered safe for food storage. However, polyolefin oligomeric hydrocarbons (POH) are NIAS associated with PP. A study was conducted to determine the level of POH in vegetable soup when placed in the microwave after refrigeration [87]. After storage at 4 °C for four weeks, the POH concentration in soup samples was determined to be less than 0.2 mg/kg. The soups were microwaved for the specified time, and the difference in POH content was determined. The temperature of the soup after heating was between 60–80 °C, depending on the water content of the soup. The increased POH concentration after heating ranged from 0.1 to 6.2 mg/kg. Increased fat content of the soup led to greater migration of POH during heating. These oligomeric levels are considered safe.

3.5. Barrier Properties of PLA and PP

Moisture and heat can influence polymer mechanical properties, such as Young’s modulus and strength [88]. Rate of moisture diffusion is affected by the polymer’s polarity and degree of crosslinking [89]. The diffusion coefficient (calculated using Fick’s Law) of water for PP is 1.48 × 10⁻¹⁴ m²/s, while for PLA it is 5.60 × 10⁻¹² m²/s [90] (Figure 5). PLA has been shown to degrade under hydrolytic conditions when the diffused water molecules (e.g., from steam or heated water molecules already present in the PLA) break the ester bonds of the polymer backbone. This leads to a reduction in mechanistic properties [91].

A study found that the weight % (wt.%) moisture absorption of PLA did not exceed 1% at 21 °C or 70 °C over the course of 10 days. However, the decrease in moisture wt.% at 70 °C over 10 days indicated material desorption [88]. In this same study, the % crystallinity of the PLA immersed in water at 21 °C decreased from 41% to 35% after seven days. After 14 days of immersion, the crystallinity had increased to 53%, indicating the degradation of the amorphous region of the semi-crystalline PLA [88]. A study was carried out using 3D-printed PLA immersed and washed in Cidex OPA and chlorine at ambient temperature. After seven days of immersion in both cleaning agents, the weight of PLA had increased by approximately 1%. There were no significant changes in the tensile strength and stiffness of the PLA after immersion in the cleaning agents or with 25 cycles of cleaning and drying [92].

High barriers to oxygen and moisture are essential for food container materials to ensure the safety of the food product from external impacts. Food spoilage can occur through chemical means (i.e., light, moisture, oxygen) or biological means (i.e., microorganisms) [93]. The gaseous permeabilities of PLA have also been compared to those of PP [9]. Oxygen permeation at 30 °C was recorded at 1.5 × 10⁻¹⁰ (cm³(STP)·cm)/(cm²·S·cm·Hg) for PP, while PLA had a lower oxygen barrier at 3.3 × 10⁻¹⁰ (cm³(STP)·cm)/(cm²·S·cm·Hg). PLA also displayed a higher water vapor transmission rate, at 18–22 g-mil/10 in² over 24 h compared to 0.5 g-mil/10 in² for PP over the same time range. It is essential to improve these barrier properties if PLA is to be used as a food container material to prolong the shelf life of food products.

PLA is a non-toxic material, but it does not display antibacterial properties [94,95]. To ensure food safety, natural antibacterial additives are required to improve the antibacterial properties of PLA blends and composites. The antimicrobial agent can be incorporated into the bioplastic matrix and will slowly be released over time. This leads to a longer shelf life for food products in this type of plastic container [95].

3.6. Optical Transparency of PLA and PP

Amorphous plastics are the most common transparent plastics. Semi-crystalline plastics are composed of both crystalline and amorphous regions, leading to light scattering between the boundaries of these regions. This light scattering decreases the optical transparency of most semi-crystalline plastics. The transparency of polymers can be increased during the crystallization process by altering the regular arrangement of the side groups, thereby preventing the formation of large crystals [96]. PP’s optical transparency has also been increased by copolymerization, e.g., with ethylene [97]. The high optical transparency of neat PLA in the visible region (400–800 nm) has been previously noted [98,99]. PLA also exhibits transparency in the UV region (200–400 nm), which is not desirable for food packaging. UV shielding has been increased in PLA films using nanocomposites (e.g., cellulose) or polymer blends (e.g., poly(pentamethylene 2,5-furanoate)) [99,100].

Optical clarity can be improved based on the manufacturing process. Factors such as injection speed, melt temperature, mould temperature, and injection pressure can affect the optical transparency of the end plastic product produced via injection moulding [101].

4. Modifications of PLA

4.1. Methods to Improve PLA Ductility and Thermal Resistance

Due to PLA’s inherent brittleness and low thermal resistance, modifications are required to improve its applicability in the food storage/container market. The brittleness of PLA has been improved using plasticizers, copolymers, and biocomposites (Table 5). Through improving the ductility of PLA (increasing impact strength and elongation at break), the overall strength (Young’s modulus and ultimate tensile strength) may be reduced, and vice versa. This can be a drawback, depending on the end use of the polymer. PLA-PCL blends with 80/20 wt.% will provide a toughened blend of PLA. Though PLA/PCL is an immiscible blend, the compatibility between PLA and PCL can be improved by additives such as glycidyl methacrylate (GMA) and polyhydroxybutyrate (PHB) [102,103,104]. It is for this reason that chemical modification of PLA is least preferred for such applications. Polymer ductility has also been improved through the manufacturing process by altering the polymer chain orientation [105]. A brittle-to-ductile transition of PLA films through biaxial stretching showed the impact of chain orientation in the amorphous region on the mechanical properties of PLA [106].

Oligomeric lactic acid (OLA) was used as a plasticizer with PLA between 15 wt.% and 25 wt.% [107]. This increased the elongation at break from 4% for neat PLA to 315% for PLA:OLA 75:25 wt.%. Cinnamate esters (20 wt.%) increase the ductility of PLA, with an increase in elongation at break of 3.9% (neat PLA) to 339.4% [108]. The PLA-cinnamate ester composites were processed by injection moulding. The T_g of the resulting composites was, however, reduced from 61.7 °C to 36.1 °C. Biocomposites of corn starch maleate and epoxidized soybean oil (2:5 wt.%) were used to increase the elongation at break of PLA film (93 wt.%) from 3.63% to 36.75% [109]. The resulting composite was fully biodegradable within four weeks. Biocomposites of PLA/acetyl tributyl citrate and nanofibrillated cellulose (NCF) (4 wt.%) increased the impact strength, elongation at break, and tensile strength [110]. These examples all have the advantage of an end material that is 100% biodegradable.

Table 5. Examples of methods utilized to increase PLA impact strength and/or decrease brittleness.

Methods	Benefits	Drawbacks	Types Used	Results	Ref.
Plasticizers	Improved ductility, improved crystallization (Tg reduction)	Strength and stiffness reduction	Oligomeric LA (OLA) (25 wt.%)	Increased elongation at break from 4% to 315%	[107]
			Cinnamate esters (20 wt.%)	Increased elongation at break from 3.9% to 339.4%	[108]
Copolymerization	Increased impact strength	Ductility not improved/can decrease depending on composition	80/20 wt.% PLA/PCL blend (3 wt.% GMA)	Impact strength increased by 160%	[102,103,104]
			90/10 wt.% PLA/PBS blend	Elongation at break increased from 9% to 64%	[111]
Biocomposites	Increased ductility and impact strength, fully bio-based	Can increase moisture absorption	Corn starch maleate and epoxidized soybean oil	Increased the elongation at break of PLA film (93 wt.%) from 3.63% to 36.75%	[109]
			Nanofibrillated cellulose (4 wt.%)	Increased the impact strength, elongation at break, and tensile strength	[110]

Table 5. Examples of methods utilized to increase PLA impact strength and/or decrease brittleness.

Methods	Benefits	Drawbacks	Types Used	Results	Ref.
Plasticizers	Improved ductility, improved crystallization (Tg reduction)	Strength and stiffness reduction	Oligomeric LA (OLA) (25 wt.%)	Increased elongation at break from 4% to 315%	[107]
			Cinnamate esters (20 wt.%)	Increased elongation at break from 3.9% to 339.4%	[108]
Copolymerization	Increased impact strength	Ductility not improved/can decrease depending on composition	80/20 wt.% PLA/PCL blend (3 wt.% GMA)	Impact strength increased by 160%	[102,103,104]
			90/10 wt.% PLA/PBS blend	Elongation at break increased from 9% to 64%	[111]
Biocomposites	Increased ductility and impact strength, fully bio-based	Can increase moisture absorption	Corn starch maleate and epoxidized soybean oil	Increased the elongation at break of PLA film (93 wt.%) from 3.63% to 36.75%	[109]
			Nanofibrillated cellulose (4 wt.%)	Increased the impact strength, elongation at break, and tensile strength	[110]

The heat deflection temperature (HDT) of PLA is 55 °C, only ~10 °C lower than that of PP (65 °C) [64]. PLA, however, has high mobility around its T_g, therefore exhibiting low heat resistance [112]. The poor thermal stability, decreased strength around the HDT, and the high melt flow index (low melt strength) of neat PLA make it unsuitable for food packaging applications. Table 6 lists methods and materials used to increase PLA’s heat resistance. The multiamide nucleator (trademarked: TMC-328) has been shown to improve the crystallinity and rate of crystallization of PLA during the injection moulding manufacturing process. This led to an increase in the HDT of PLA up to 150 °C [113,114]. Ethylenebishydroxystearamide (EBHS) (1 wt.%), a biobased and biodegradable fatty amide, was used as a nucleating agent with PLA and increased the HDT to 93 °C [115]. Modified kenaf fibres blended with PLA at 40 wt.% increased the HDT up to 122 °C, with an overall increase in impact strength and tensile strength [116]. The thermal stability of PLA has been increased with biocomposites. Byun et al. created composite films of PLA and PLA-β-cyclodextrin inclusion complex (IC), and 7 wt.% IC led to modest increases in T_g (from 55.6 °C to 60.7 °C) and T_c (from 82.6 °C to 88.4 °C) [117]. However, the addition of IC decreased the tensile strength and the elongation at break of the PLA films.

Blending of polymers is also used to alter the properties of PLA. Blending is a common method used to improve the heat resistance, brittleness, and barrier properties of polymers. Blending usually utilizes a high T_g compound or a heat-resistant compound. Shu et al. produced a PLA/lignin-modified polyvinyl acetate (L-PVAc) blend of up to 20 wt.%, increasing the degree of crystallinity by 78.8% compared to pure PLA. The mechanical properties of the resulting composites were also improved, with an increase in impact strength and elongation at break of 298.2% and 167.5%, respectively [118]. There is a loss of optical transparency when using a lignin blend. Thermomechanical properties of PLA/PMMA blends have been investigated [119]. Increasing the blending of PMMA with PLA leads to an increase in T_g and a decrease in percentage crystallinity (X_c). A 50/50 PLA/PMMA blend has a T_g of 74 °C and a X_c of 0.1% compared to 10.8% of pure PLA. PMMA blends will decrease the biodegradability of the PLA material, however. Focusing on biodegradable blends, polycaprolactone (PCL) increases the thermal stability of PLA for an 80/20 PLA/PCL blend [120]. The thermal stability was further improved when the compatibilizer glycidyl methacrylate-grafted PLA was included in the manufacturing process up to 6 wt.%. This increased the X_c from 2% for neat PLA to 11%.

Table 6. Examples of methods utilized to improve PLA thermal stability.

Method	Mode	Types Used	Results	Ref.
Nucleating Agent Addition	Increased crystallinity, increased Tm, increased HDT, resistance to heat-induced distortions	Multiamide nucleator (TMC)	Improved crystallinity leading to improved HDT up to 150 °C	[113,114]
		EBHS (1 wt.%)	Improved HDT up to 93 °C	[115]
Fiber Reinforcement	Chain motion of PLA when heated is restricted	Kenaf fiber (40 wt.%)	Increased the HDT up to 122 °C, increase in impact strength and tensile strength	[116]
Composites	Increased thermal expansion in the polymer film	Cyclodextrin (7 wt.%)	Modest increases in Tg (55.6 °C to 60.7 °C) and Tc (82.6 °C to 88.4 °C)	[117]
Blending	Polymers with polar and flexible groups blended with PLA to improve interaction between the polymer chains	PLA/L-PVAc (80/20 wt.%)	Increase in Xc by 78.8%	[118]
		PLA/PCL (80/20 wt.%)	Increase in Xc from 2% for neat PLA to 11%.	[120,121]

Table 6. Examples of methods utilized to improve PLA thermal stability.

Method	Mode	Types Used	Results	Ref.
Nucleating Agent Addition	Increased crystallinity, increased Tm, increased HDT, resistance to heat-induced distortions	Multiamide nucleator (TMC)	Improved crystallinity leading to improved HDT up to 150 °C	[113,114]
		EBHS (1 wt.%)	Improved HDT up to 93 °C	[115]
Fiber Reinforcement	Chain motion of PLA when heated is restricted	Kenaf fiber (40 wt.%)	Increased the HDT up to 122 °C, increase in impact strength and tensile strength	[116]
Composites	Increased thermal expansion in the polymer film	Cyclodextrin (7 wt.%)	Modest increases in Tg (55.6 °C to 60.7 °C) and Tc (82.6 °C to 88.4 °C)	[117]
Blending	Polymers with polar and flexible groups blended with PLA to improve interaction between the polymer chains	PLA/L-PVAc (80/20 wt.%)	Increase in Xc by 78.8%	[118]
		PLA/PCL (80/20 wt.%)	Increase in Xc from 2% for neat PLA to 11%.	[120,121]

4.2. Methods to Improve the Barrier Properties of PLA

PLA has previously been investigated as a viable alternative to fossil fuel-based plastics in the food packaging industry. However, PLA’s weak barrier properties inhibit its use in this sector [122]. Techniques and materials to improve the barrier properties of PLA are given in Table 7. Due to the increased water absorption that occurs in PLA compared to PP, methods must be found to decrease the absorptivity and diffusion coefficient and/or increase the moisture barrier of PLA. The X_c can have an effect on the barrier properties of PLA. Specifically, the water vapor permeation coefficient (P_water kg·m/m²/s/Pa) decreased from 2.18 to 0.99 × 10¹⁴ when X_c was increased from 0 to 32% in PLLA films [123]. Nucleating agents have also been used to increase the gas barrier properties of PLA [124]. Nanoparticles of Fe₃NO₄.SiO₂ grafted to poly(3-hydroxybutyrate)-diol (PHB) were melt-mixed with PLA at concentrations of 5–20 wt.%. Both oxygen and water vapor permeability decreased, while the thermal and mechanical properties of the PLA composites increased slightly. The final composites are not fully biodegradable in this instance. The composites are, however, recyclable without any loss of nucleation efficiency.

Thermoplastic starch (TPS) blends have also been utilized to improve the permeability of PLA plastics. A PLA/TPS 50/49.25 blend with 0.75 wt.% of citric acid as a compatibilizer showed a 70% reduction in the water vapor permeability compared to the PLA/TPS 50/50 blend without compatibilizer [125]. However, the thermal properties of this blend were affected, particularly the crystallization temperature (T_c), which decreased to 88 °C from 117 °C for pure PLA. The biodegradable bioplastic PHBV has shown promising results in lowering the permeability of PLA. A PLA/PHBV 25/75 blend showed a 75% reduction in water permeability and an 81.5% reduction in oxygen permeability compared to pure PLA. Even at the much lower 25 wt.% of PHBV, the water and oxygen permeability coefficients for PLA were decreased by 22.7% and 35.3%, respectively. PHBV is used in favour of other polyhydroxyalkanoates (PHAs) due to its improved flexibility and ease of processability [126,127,128,129].

Table 7. Materials blended with PLA to improve barrier properties.

Material Used	Benefits	Drawbacks	Ref.
PLA/TPS (50/49.25), citric acid stabilizer (0.75)	Decrease in water vapor permeability (g/m/Pa/day), biodegradable	Decrease in thermal properties (Tc = 88 °C), brittleness at lower temperatures	[125]
PLA/PHBV (75/25)	Decrease in oxygen and water vapor permeabilities, thermal properties maintained, compostable properties maintained	Decreased mechanical properties (impact strength and strain at break), processability (cost)	[126,127,130]

Table 7. Materials blended with PLA to improve barrier properties.

Material Used	Benefits	Drawbacks	Ref.
PLA/TPS (50/49.25), citric acid stabilizer (0.75)	Decrease in water vapor permeability (g/m/Pa/day), biodegradable	Decrease in thermal properties (Tc = 88 °C), brittleness at lower temperatures	[125]
PLA/PHBV (75/25)	Decrease in oxygen and water vapor permeabilities, thermal properties maintained, compostable properties maintained	Decreased mechanical properties (impact strength and strain at break), processability (cost)	[126,127,130]

4.3. Composites to Improve the Barrier Properties of PLA

Polymer composites are typically made from at least two parts: the polymer matrix and the reinforcement material [131,132]. This reinforcement material can be nanoparticles, fibres, metals, ceramics, or other materials and polymers. These materials are added to improve the mechanical, thermal, and electrical properties of polymers. Nanoparticles are small materials with dimensions ranging from 100 nm to 1 nm. In polymer-based nanocomposites, nanometre-sized organic or inorganic particles are homogeneously dispersed in the polymer matrix. The addition of nanoparticles to a polymer material can increase the path length of diffusion through the polymer, a process known as tortuosity, thus increasing the barrier properties of the polymer [133].

4.3.1. Metals and Metal Oxide-Based Nanocomposites

Inorganic nanoparticles such as metal oxides and minerals have been investigated as nanocomposites (NC) with PLA due to their superior antimicrobial properties compared to organic nanoparticles and their ability to improve the mechanical and barrier properties of neat PLA. The scope of the NCs discussed in this section will be restricted to those with regulations and guidance regarding their use as an FCM under FDA and EFSA (European Food Safety Authority) regulations.

PLA/Ag nanocomposites have been investigated as a food packaging material due to the antimicrobial effects of silver (

Table 8. PLA/Ag Nanocomposites.

Material	Benefits	Drawbacks	Ref.
PLA/Ag	Reduction in OTR and WVP	Tensile strength and elongation at break decreased	[134,135]
PLA/CNC/Ag	Reduction in OTR and WVP Migration levels below FDA and EFSA levels for Ag	Tensile strength and elongation at break decreased	[134,135,136]
PLA/CNF/Ag	Reduction in WVP and water uptake, migration levels below EU food contact requirements	Elongation at break decreased, optical transparency is reduced with increased composite concentration	[137]

). Fortunati et al. investigated the combined use of silver nanoparticles and cellulose nanocrystals in PLA materials formed by solvent casting. A 60% reduction in oxygen transmission rate (OTR) and a 59% reduction in water vapor permeability (WVP) for a PLA composite with 5% surfactant-modified cellulose nanocrystals (s-CNC) and 1% silver nanoparticles were observed [134,135,136]. Overall and specific migration testing as described by EN 1186-1:2002 and EN 13130-1:2004 was performed on PLA/Ag and PLA/CNC/Ag nanocomposites, with all samples displaying migration under the limits described [134]. Other work performed by Yu et al. showed a similar reduction in WVP for nanocomposites containing CNC’s and Ag [137]. A WVP reduction of 60.1% and a water uptake reduction of 71.8% were observed for these composites. The authors suggest that incorporation of CNFs and Ag nanoparticles can depress the migration of two food simulants into the nanocomposite film because of the improved interfacial interaction between Ag nanoparticles and PLA matrix.

Metal oxides as nanoparticles have also been investigated for use in PLA nanocomposites for food packaging, with a heavy focus on tin oxide (TiO₂) and zinc oxide (ZnO) (

Table 9. PLA/metal oxide nanocomposites.

Material	Benefits	Drawbacks	Ref.
PLA/ZnO + Plasticizer	Reduction in OTR and WVP	Low opacity compared to neat PLA, elongation at break decreased	[138]
PLA/ZnO:Cu/Ag	Reduction in WVP and OTR, Young’s modulus and tensile strength reduced (improved ductility)	Tg reduced, crystallinity increased	[139]
PLA/TiO2	WVP reduced Opacity of film to UVC and UVB reduced	Elongation at break decreased	[140]

).

Tang et al. demonstrated that the addition of ZnO nanoparticles and plasticiser (ATBC) to a PLA matrix displayed an overall reduction of 39% and 33.5% in the WVP and OTR of the nanocomposite, respectively, when 9 wt.% of ZnO nanoparticles was used in combination with 10 wt.% of plasticiser [138]. However, reductions in the optical transparency and thermal properties (T_g, T_m) of the composite were observed with increased loading. The mechanical properties of the nanocomposite changed, with a reduction in elongation at break and an increase in the Young’s modulus when compared to neat PLA. Vasile et al. observed the effect of combining Cu-doped ZnO nanoparticles functionalized with Ag into a PLA matrix via melt blending with a plasticiser (ATBC/MB) [139]. It was found that for a blend containing 0.5 wt.% ZnO/Cu and 19.99 wt.% plasticiser, WVP and OTR were reduced by 29% and 92.25%, respectively. Migration levels of the nanoparticles (Zn, Ag, and Cu) into food simulants were below the maximum dosages recommended by European Commission Regulation No. 10/2011.

TiO₂ nanoparticles have also been investigated for their use in nanocomposites with PLA due to their antimicrobial and UV opacity properties. Feng et al. investigated the addition of TiO₂ nanoparticles to a PLA matrix with materials prepared by solution casting [140]. The WVP of the material was reduced by 29% in comparison to neat PLA when 1 wt.% of TiO₂ was added. Investigation of the migration of these nanoparticles found that the levels of migration were also below the migration limits recommended by the European Commission (Council European Directive 85/572/EEC) [141].

4.3.2. Clay-Based Nanocomposites

Clay-based nanocomposites, also known as mono-layer silicates (MLS), have been investigated as a nanocomposite material to improve the thermal and barrier properties of neat PLA. Its high relative abundance, strength, barrier properties, and low cost make clay nanocomposites an attractive additive [142]. Thellen et al. investigated the effect of Cloisite 25A, a natural montmorillonite modified with a quaternary ammonium salt, on the WPR and OTR of neat PLA [143]. The addition of 5 wt.% MLS with 10 wt.% plasticiser (acetyltriethyl citrate) reduced the WPR by up to 50% and the OTR by 48%. Mechanical testing of the PLA/MLS nanocomposite showed an increase in Young’s modulus of 30–40% and an increase in elongation at break of 16–40%. Typically, when nanomaterials are added to plasticiser, elongation at break is reduced. This was not observed to be the case in this research, however. The increased elongation at break was explained by the MLS-induced structural and morphological changes that increase chain mobility [143,144].

Mohsen et al. observed the effect of introducing nanoclay to a PLA matrix with no additional plasticiser at 1–6 wt.% [145]. Oxygen permeability was decreased by 38% for PLA/nanoclay (6 wt.%), while the WVP was decreased by 32%. The tensile strength and Young’s modulus of the composites were also improved, but the elongation at break had decreased. Optical transparency decreased linearly with the increased loading of nanoclay.

4.3.3. Cellulose-Based Nanocomposites

Cellulose-based nanoparticles, namely cellulose nanocrystals (CNC), nanofibers (CNF), and nanospheres (CNS), have been investigated as nanocomposite materials to improve the barrier properties of PLA. Natural fibres are quickly becoming another sustainable alternative to fossil fuel-based materials. Natural fibres are renewable, biodegradable, sequester CO₂, exhibit good mechanical properties, and are cost effective [146]. Natural fibres can be derived from sustainable resources such as industrial waste and agricultural waste [147]. Natural fibres such as banana peel have be shown to be a sustainable, functional alternative to synthetic materials for use in brake pads [146]. When flax fibres were mixed with TPS up to 50 wt.%, mechanical properties of the composite increased. An increase in stability during thermal degradation, water uptake, and biodegradation was also noted [148].

Fortunati et al. as mentioned previously, have investigated the use of cellulose nanocrystals with Ag, successfully reducing the WVP of the material by up to 59% compared to neat PLA [134,135,136]. The impact of PLA-CNC composites alone, using unmodified CNCs (10–50 wt.%), has also been investigated. PLA-CNC composites were prepared via solvent casting followed by compression moulding of films. At a CNC concentration of 50 wt.%, the oxygen diffusion coefficient decreased by 44.5%. Surface grafting CNC with poly (glycidyl methacrylate) (CNC-g-PGMA) prior to solvent casting with PLA improves the barrier properties compared to unmodified CNC [149].

Yu et al. noted an 84% decrease in water uptake and a 79% decrease in WVP for PLA/CNC composites containing 10 wt.% of CNC nanoparticles [150]. An increase in tensile strength and reduction in elongation at break in comparison to neat PLA were also observed. Yu et al. compared cellulose nanocrystals, nanofibers, and nanospheres (

Table 10. Cellulose-based nanocomposites of PLA [150].

Material	Results
PLA/CNC	Increase in tensile strength (210%) and Young’s modulus (250%), decrease in WVP (79.1%) and OTR (26%)
PLA/CNF	Increase in tensile strength (260%) and Young’s modulus (350%), decrease in WVP (66.5%)
PLA/CNS	Increase in tensile strength (130%) and Young’s modulus (140%), decrease in WVP (76.3%)

), finding that PLA/CNC nanocomposites have the greatest reductions in water uptake and WVP compared to PLA/CNS and PLA/CNF [150]. All nanocomposites displayed higher tensile strength and Young’s modulus and lower elongation at break values compared to neat PLA, with PLA/CNF nanocomposites having the largest increases in tensile strength (230%) and Young’s modulus (350%). This change in mechanical properties can be attributed to increased hydrogen bonding between the nanocellulose and PLA and a higher degree of crystallinity.

5. End of Life

Currently, 19% of all plastic is incinerated, with 50% ending up in landfills [151]. There is a calculated carbon footprint reduction associated with the use of PLA instead of PP from incineration EoL options alone. An amount of 5.25 kg CO₂e/kg of PP is emitted during the production and incineration of PP [14,15] compared to 3.65 kg CO₂e/kg PLA [3]. More sustainable EoL options are available with regard to bioplastics such as PLA, composting in particular. The EoL of PLA has been assessed numerous times under the guidance of Environmental LCA ISO 14040:2006, which describes the principles and framework for the LCA [152], along with ISO 14044:2006, which specifies requirements and provides guidelines for the LCA [153].

5.1. Recyclability of PLA

Studies conducted on the recycling of PLA have only been performed on a ‘laboratory scale’ and have not been introduced to mainstream recycling. PLA cannot be recycled alongside non-bioplastics such as PET. For example, trace elements of PLA in PET can render it unsuitable for mechanical recycling as it causes noticeable hazing and degradation of recycled PET [154]. To date, bioplastics only make up ~1% of total plastics, and so it is not feasible to develop a separate recycling stream [155].

However, these laboratory-scale assessments have yielded promising results for the future. LCA studies show mechanical recycling of PLA waste supports the reduction in environmental impacts such as climate change, human toxicity, and fossil fuel depletion [156]. Depending on the quality of PLA waste and regional conditions, all investigated recycling alternatives, namely mechanical recycling, solvent-based recycling, and chemical recycling, are suitable EoL options [157]. Mechanical recycling was found to use four times less energy than recycling through a chemical process, although no LCA studies have accounted for the degradation of material properties from mechanical processing and no standard for the resulting quality has been developed; reprocessing may result in various changes in mechanical properties [155]. These changes may include, but are not limited to, a decrease in tensile strength, an increase in brittleness, and altered thermal properties [56]. The environmental impacts of solvent-based recycling have also been considered for post-consumer waste PLA [157]. Solvent-based recycling is considered the EoL option with the greatest reduction in environmental impact, as mechanical recycling facilities are not available.

Currently, even the recycling of contaminated food contact non-biopolymeric packaging is not usually an option as typical recycling facilities are not equipped to dispose of the contaminated materials, and so most plastic ends up in landfill. Recycling is further complicated by the addition of other polymers or composites that have been added during the manufacturing process to improve the properties of PLA [158,159]. The composability of a PLA blend or composite must then be considered as the most suitable EoL option.

5.2. Compostability of PLA

PLA is less susceptible to environmental degradation than other aliphatic polyesters and will only degrade under industrial conditions [155]. For bioplastics such as PLA, these conditions require elevated temperatures of 55–60 °C with a high water content (60 wt.%) and the presence of oxygen. Under these conditions, several criteria must be met (Figure 6):

• At least 90% disintegration due to bacterial fermentation must take place within 12 weeks.
• A level of 90% mineralisation of the composted material must be achieved within less than 6 months.
• Material must not have a negative impact on the compost quality.

Once these conditions are met, the process continues at a milder temperature range [156]. The control of both moisture and temperature is a key factor in the degradation of PLA [160]. The rate of degradation also relies heavily on other factors such as polymer characteristics (composition, mobility, crystallinity, molecular weight, and presence of additives) [161]. Aspects of PLA such as these make it unsuitable for home composting.

Industrial composting of PLA usually occurs in two stages: fragmentation of polymer chains due to hydrolysis, followed by the uptake of the low-molecular weight chains by microorganisms. Composting has been suggested to be the optimal EoL option for contaminated PLA, such as food packaging and agricultural mulch films, which are difficult to recover through recycling [162]. Overall, PLA must be separated from other plastic sources and processed in a ‘closed composting environment’.

Composting studies have been undertaken with PLA blends. PLA/PHB blends at various wt.% were determined to be fully biodegradable under industrial composting conditions [156]. It was found that the disintegration and mineralisation processes could occur in parallel, particularly for thicker pieces of PLA/PHB material, and it was concluded that this blend does not leave microplastics in the compost at the end of the process. Biodegradation is associated with GHG emissions, with the full life cycle of PLA estimated to produce 3.3 kg CO₂e/kg plastic, where composting is the EoL option. This is compared to 3.7 kg CO₂e/kg plastic produced when landfill is the EoL option, with 60% biodegradation assumed [163].

5.3. Composting Worldwide

Currently, there is no international standard in relation to industrial composting. There are, however, a series of standards regarding the labelling of compostable products. They specify tests and requirements that must be carried out and met before a product can be certified as compostable, whether it be in terms of home composting or industrial composting.

For example, in the EU, for a packaging product to be certified as industrially compostable, it must meet the strict criteria of EU standard EN 13432, “Packaging: requirements for packaging recoverable through composting and biodegradation”, which states that after 12 weeks, at least 90% of the product should be converted to CO₂ and H₂O, and the remaining material should be able to pass through a 2 × 2 mm mesh [164]. The Australian government has a similar standard for industrially compostable bioplastics, AS 4736 (2006), with the addition of a worm test requirement [165]. However, a lack of uniformity between waste management systems at a local and national level is hindering the development of general policies and/or legislation that could promote the use of certified products [166].

The Seedling Logo (Figure 7) is assigned to a product that has been deemed biodegradable according to standards such as EN 13432 and AS 4736, stating the product is biodegradable in an industrial composting plant under controlled conditions, leaving only water, biomass, and carbon dioxide. The product must be formally certified by the national regulatory body [167].

6. PLA Material Processing Costs

An estimated per-unit cost of PLA-based material item production based on minimum order quantities (MOQ) of 50,000 and 100,000 in Europe was obtained. The costs ranged from EUR 0.202 to EUR 0.53 for 18 g containers, based on a MOQ of 50,000 from various European commercial suppliers. A price comparison was also carried out between the per-unit production costs of PLA and PP (

Table 11. Estimated price comparison of PLA versus PP as base material for MOQ of 50,000 and 100,000.

Item	Unit Price PLA MOQ 50,000	Unit Price PP MOQ 50,000	Unit Price PLA MOQ 100,000	Unit Price PP MOQ 100,000
11.7 g container	EUR 0.163	EUR 0.138	EUR 0.158	EUR 0.132
17.9 g container	EUR 0.202	EUR 0.161	EUR 0.196	EUR 0.155

). The per-unit price increase for PLA as compared to PP ranges from 13–25% for a 50,000 MOQ. The per-unit pricing considered here is for base material PLA. The final pricing for a bespoke bioplastic material would be expected to be higher.

7. Discussion

PLA has recently become a leading biodegradable biopolymer, particularly in the food packaging sector [24]. Its preferred use in single-use food packaging is due to its superior mechanical properties and ease of processing compared to other biopolymers, such as PHB [168]. Neat PLA is, however, not suitable as a replacement material for microwavable, dishwasher-friendly fossil fuel-based plastics, as outlined in this review. The properties of PLA, such as brittleness, low thermal resistance, and low barrier properties, make it unsafe for prolonged food storage or use at temperatures above 60 °C. Through blending PLA with suitable biopolymers and introducing biocomposites into the polymer matrix, these properties can be improved.

The biodegradable bioplastic PHBV, when blended with PLA (PLA:PHBV, 75:25), showed enhanced oxygen and water vapor barrier properties without a decrease in thermal properties [126,127,130]. A reduction in the mechanical properties of PLA (impact strength, strain at break) when blended with PHBV was noted, however. The low melt strength, low melt viscosity, and low crystallization rate (i.e., difficult processability) for these bioplastics must also be considered.

These drawbacks can be mitigated with the incorporation of cellulose nanocomposites [169,170]. Cellulose-based nanofibers, when incorporated into PLA film, showed an increase in barrier properties and material strength [150]. Cellulose fibres also improve the thermal properties of PLA [171]. Nanocomposites can have the added benefit of acting as an antibacterial agent, ensuring the safety of food [95,172]. Nucleating agents such as EBHS can also be utilized in the manufacturing process to improve the thermal resistance of PLA.

All components of the resulting polymer would be biodegradable and non-toxic. Given that the polymer is designed to be biodegradable, any product would not be as long-lasting as the PP containers currently used. However, the environmental impact of PP production and waste disposal must be considered. An LCA study of the resulting PLA-based bioplastic material is therefore needed to provide an evaluation of its environmental impact from cradle to grave and to compare it to the PP polymer. Lactic acid, the building block of PLA, has been produced through the bacterial fermentation of first-generation feedstock, but this is not the most sustainable method to produce PLA. By using land otherwise used for food production, some have questioned the overall positive impact of PLA production on climate change and sustainability [173]. There is now a shift to PLA production from second-generation feedstock, such as food waste [174,175], improving the overall waste reduction and sustainability of PLA production and usage.

8. Conclusions

The drawbacks of PLA as a reusable food container material, required to withstand elevated temperatures and protect food against moisture, oxygen, and microorganism contamination, have been outlined. The low barrier properties of PLA compared to fossil fuel-based plastics make the sorption of small molecules into the matrix and subsequent migration out of the matrix an issue for food contamination. The thermal distortion and degradation of neat PLA at elevated temperatures above Tg (60 °C) make it unsuitable for this purpose. Various processing methods, blends, and composites have been discussed that can improve these properties and make a biodegradable, bio-based polymer that can be reused as a food container and can withstand a wide temperature range. Blending PLA with PHBV, adding nanocomposites such as cellulose, and using nucleating agents such as EBHS can improve the properties of PLA to make it a functional reusable food container material. Pivoting from fossil fuel-based plastics to biobased plastics such as the PLA blend suggested here is a crucial step towards reaching the Sustainable Development Goals by 2030, as outlined by the United Nations.