IntroductionWorldwide mass production of consumer plastics continues to be dominated by petroleum-based polymers. Recent programs such as Blue Planet 2, have increased awareness regarding the detrimental impact upon the environment of traditional carbon-based polymers; highlighting the importance of finding alternative resources into the public domain. The environmental, economic and safety challenges have provoked scientists and producers to partially substitute petrochemical-based polymers with biodegradable ones ; mitigating climate change by minimising the environmental impact of a product throughout their life cycle. This has led to the further development of the ‘bioplastic industry’, a notable step towards a holistic goal to ultimately replace ‘fossil carbon’ with renewable carbon through biobased polymers.
3 Biobased polymers are sustainable polymers made from bio-derived sources such as biomass as an alternative to conventional fossil fuel resources. Due to this they are considered to be renewable and are classified by their respective carbon offset; atmospheric CO2 does not increase after incineration. Polysaccharides are being intensively studied for use as sustainable biobased polymers. Polysaccharides currently studied include starch, chitin, chitosans, xylans and mannans , but for the purpose of the article, the focus will be upon the benefits and limitations of cellulose (Figure 1a ) and the starch derivative polylactic acid (Figure 1b ) for use as sustainable biobased polymer, focusing upon manufacturing methods and the resulting applicational uses.Poly(Lactic Acid)Poly(lactic acid), or PLA, is an aliphatic polyester produced by the chemical synthesis of bio-derived monomers (commonly ?-hydroxy acids). It was the second biodegradable polymer ever synthesised , discovered by Wallace Carothers at DuPont in 1932.
A low molecular weight PLA product was produced through the heating of lactic acid under vacuum whilst removing the condensed water by-product. 1,2 At the time, the main limitation was an inability to produce a product with a larger molecular weight; this led to the discontinuation of further studies. Later on, a high-molecular weight PLA was synthesised by ring-opening polymerisation of the lactide. The first ever use of PLA was in combination with polyglycolic acid (PGA) as a suture material known as Vicryl in 1974.2 ProductionThe biodegradable thermoplastic is produced by the condensation polymerisation of lactic acid.3 Lactic acid (2-hydroxy propionic acid), the single monomer in PLA, is derived from the fermentation of sugars from renewable carbohydrate sources such as corn, sugarcane or tapioca.3, The characteristics of this monomer allow PLA to be manufactured with a wide range of properties.1 Lactic acid is a chiral compound with two asymmetric centres allowing for two optically active configurations: L(+) and D(-) stereoisomers (Figure 2)1, produced by homo and heterofermentation respectively.
2 The homofermentative method is preferred due to its greater yields, up to 99.5% of the L isomer, and fewer by-products hence pure L-lactic acid is used in PLA production.1,2 As previously mentioned, low-molecular weight PLA is made by direct condensation polymerisation of the lactic acid whereas high-molecular weight PLA is synthesised through ring-opening polymerisation of a lactide. This is because the direct condensation route is an equilibrium reaction.14 There are therefore difficulties when removing trace amounts of water at the later polymerisation stages which in turn limits the molecular weight achieved. For ring-opening polymerisation, the two active forms of lactic acid results in three potential forms of the intermediate cyclic lactide dimer (as shown in Figure 2).1 The meso-form is optically inactive in contrast to the other two which are optically active.
The ratio of these three forms is readily adjusted in the production process to control the stereochemical structure of the product.1, A family of polymers are made as a result of the ring opening polymerisation of these lactides which contains different isomer ratios and molecular weights.1 Figure 3 gives an overview flow diagram of a typical PLA manufacturing process. PLA is one of the most environmentally friendly bioplastics available today; made from 100% bio-based resources with multiple end-of-life options, it is both 100% recyclable as well as biodegradable.15 In addition, the manufacturing process is extremely efficient with only a meagre 1.6kg of sugar required to produce 1kg of PLA.
15 An additional benefit is that the total CO2 consumption from the cradle to factory is more than its emission to the environment. Properties, Uses and ApplicationsDue to its degradation mechanism, PLA is often used in environmental applications where recovery of the product is not practical, such as in agriculture in mulch films and bags.14 Ironically, however, the main growth of PLA applications has not been due to the biodegradability of the material.
14 The versatility of the compound and the ability to modify its stereochemical properties to suit multiple functions was the primary reason for its further research. The thermal, mechanical and biodegradation attributes of the lactic acid polymer depend on the choice and distribution of the L and D stereoisomers within polymer chains.1 Ultimately, this results in several distinct forms of polylactide, including Poly-L-lactide (PLLA) and Poly-D-lactide (PDLA), all of which possess different properties and uses.15 PLA resins can be tailor-made for different fabrication processes, including blow moulding, thermoforming, film forming or fibre spinning.14 This is achieved, as previously mentioned, by controlling the molecular parameters during the process such as branching, D-isomer content and molecular weight distribution.14 Polymers with high levels of the L-stereoisomer produce crystalline products while high D-levels (>15%) result in an amorphous product while both can have a molecular weight ranging from a few thousands to over a million.1 Furthermore, modifications can also be used to tailor properties further for specific uses through the addition of modifiers, blending, copolymerising and physical treatments of the polymer.
2 Industrial processing methods use PLA to produce packaging in order to reduce environmental impact (i.e. films, laminates, containers (bottles and cups), wrappings).2 It is classified as generally recognised as safe (GRAS) by the United State Food and Drug Administration (FDA) and is safe for all food packaging applications. PLA also has a number of practical medical applications due to its degradable properties and low toxicity, including as dissolvable sutures and bone fracture internal fixation devices during surgery.
It is superior to nondegradable materials by eliminating the need to remove implants and any issues with long term biocompatibility. Various other medical uses also include: drug releasing microparticles, nanoparticles and porous scaffolds for cellular applications (i.e. for the growth of neo-tissue).21 Three dimensional porous scaffolds of PLA have been created to culture various cell types to be used in: cell-based gene therapy for cardiovascular diseases, muscle tissues, bone and cartilage regeneration.21 PLA does have a number of disadvantages. Due to its poor thermal properties, the applicability of PLA is limited at high temperatures.
20 In addition, despite its faster degradation time, specific micro-organisms present in composting plants and a slightly elevated temperature are required for degradation to occur, so degradation time can in fact increase.20 This has started the debate of whether or not PLA, despite being biobased, can be classified as biodegradable. CelluloseCellulose is the most abundant organic polymer on earth, making it an exciting possibility in sustainable biobased polymer development. It has received attention due to beneficial properties such as high abundance, low costs and flexibility during the manufacturing process. Furthermore, it has a potential use as a biobased, sustainable alternative to more commonly used but environmentally harmful industrial synthetic polymers.
However, in order to be favored for industrial purposes, both the manufacturing method and disposing of the material must be environmentally friendly and economically viable when compared to traditional synthetic polymers, whilst the material itself must be able to perform as well as, if not better than, the petroleum polymer it is replacing. A 2013 paper proposed cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) as potential biobased polymers. ProductionCNCs are often sourced from biological sources such as bleached wood pulp, cotton, and bacteria cellulose.24 Several CNC manufacturing methods have been proposed. One suggested method required strong acid hydrolysis to remove both non-cellulose and the majority of amorphous cellulose components from the source leaving a highly crystalline structure. However, a recent paper highlighted that acid hydrolysis may have negative environmental effects negating from the benefits of a biobased polymer when compared to traditional, petroleum sourced polymers.
Instead, an ‘eco-friendlier’ method using bacterial sourced cellulose treated with cellulase to form bacterial cellulose nanocrystals (BCNCs) was cited. Though limited by enzymatic activity, benefits included increased thermal stability and improved mechanical properties when compared to nanocrystals formed using the acid hydrolysis method. 7,25Multiple methods have been proposed to manufacture CNFs including TEMPO – mediated oxidation, multi-pass high pressure homogenization, enzymatic hydrolysis, or direct mechanical fibrillation.24 Often, the method chosen was dependent upon the morphology and dimensions of the CNFs required. Traditional CNF manufacturing methods used to have significant energy requirements, consequently limiting the availability of CNF as a desirable alternative biobased polymer. Over the past 5 years, two methods have been researched to decrease the energy consumption thus making nanofibrillation easier – using new mechanical processes or using chemical modification of cellulose fibers.
6 Like CNC manufacturing, CNF manufacturing can have adverse effects upon the environment. The esterification and etherification process7 was stated to require a large volume of solvents containing toxic reagents meaning the waste water discarded required treatment. Therefore, a TEMPO-mediated oxidation method (TEMPO/NaBr/NaClO) (Figure 4) was proposed which required lower temperatures and allowed the pH of the medium to be kept at 10.
The method was limited however due to not allowing length and length distribution of the CNFs to be determined and products lacking thermal stability when combined with other biobased polymers. Properties, Uses and ApplicationsFor biobased polymers to be used however, they need to be capable of performing the roles of petroleum polymers meaning their properties must be equal to or exceed those of the resource that they are replacing. Thus, CNFs are being developed as ‘drop-in’ polymers – biobased polymers which exhibit similar properties to, and can therefore be used instead of, petroleum-based polymers. CNCs and CNFs applications contrast due to the variance in polymer structures: needle-like crystalline CNCs and the longer, flexible fibre networks of CNFs.24A commonly referred to application of biobased polymers is in sustainable, environmentally friendly and preferably biodegradable food packaging. For a packaging material to be considered, it must have suitable heat and UV resistance whilst also maintaining sufficient gas and moisture resistance.15, Feasibility of use of cellulose both as coatings for various biopolymer28, and synthetic polymers , , and as stand-alone biofilms , has been analysed across a range of papers.
The majority of findings agreed cellulose coatings and films showed potential as an oxygen barrier when compared to purely synthetic polymers and also those polymers protected with an alternative, less environmentally friendly method (Figure 5). However, most papers focused upon dry conditions without investigating humidity’s impact, and often commented upon the negative impact of humidity leading to an increase in oxygen permeability. A 2018 paper went some way to tackle this limitation, citing previous work whereby a thermal treatment method decreased oxygen and water permeability of a CNF membrane twenty-five-fold and two-fold respectively at 175oC but where the food packaging application potential was limited due to restricted transparency of the membrane. In turn, further research was performed which led to improved water resistance and oxygen barrier properties in humid conditions, finding that treatment at 145oC led to oxygen permeability values one hundred times lower than most synthetic plastic films at fifty and eighty percent relative humidity.
The study still however indicated an increase in oxygen permeability with increased humidity. Some of the research above had focused upon the combined oxygen permeability between cellulose and synthetic polymers, however to better aid environmental endeavors, future research could focus upon the use of the cellulose coatings applied to biopolymer versions of polymers such as PET31 and PEO32 or on materials such as PLA. Following on from the importance of minimising the impacts of oxygen permeability, moisture permeability of food packaging is incredibly important when determining food shelf life. Papers have identified that when cellulose fibres were used to reinforce starch-based biofilms, a reduction of water affinity occurred compared to solely starch biofilms. However, when compared to industrially important synthetic polymers, the performance of cellulose based films and films, including some sort of cellulose reinforcement, was shown to be poorer (Figure 6).36 There has been research aimed at improving barrier properties of solely polysaccharide based multilayer films. Through the combination of the biopolymer guar gum (GG), a biodegradable polymer with low water permeability but which also lacks mechanical and tensile strength, with TEMPO oxidized cellulose nanofibrils (TOCNs), an improvement in both water barrier properties and tensile strength compared to a solely CGG film was seen.
The greatest improvement occurred using an eight layered multifilm. Another suggested barrier improvement method used an electrospinning technique. Although water repellent properties of the nanostructured cellulose-based films were improved through the addition of biopolymer coatings, the mechanical properties were compromised.
For example, a decrease in tensile stress-strain capability, and rigidity of the nanopapers was observed. However, the paper did suggest the loss in rigidity could increase the benefit of the material as food packaging. Future research suggested included the improvement of the processing to reduce coating thickness and identifying the how increasing production of the material to an industrial scale may impact upon the performance of the material.
Conclusions and OutlookHistorical research has shown that biobased polymers have been in commercial use for over a century. However, the convenience and cost-effective nature of crude oil and its technologies developed in the 1950s meant that biobased products were not prioritised. Increasing environmental pressures, the finite nature of crude oil and increased waste through landfill has led to a resurgence of interest in biobased polymers as a solution. In fact, biobased polymers have and continue to be at the forefront of environmental research today. Polymers such as PLA and CNCs/CNFs have shown potential for a wide variety of applications, including within the medical and food packaging industries, and currently look to be appealing replacements to traditional carbon-based polymers in large scale production.
The renewability of these resources however is offset by the logistical impacts of land usage both in the synthesis of these products and the growth of their respective sources.Competition for land with food crops or other resources become an issue when biobased polymers are produced on an industrial scale.20 In order to ensure these polymers are economically viable: biomass feedstocks must be developed along with new manufacturing routes to improve yields and further research into efficient downstream processing methods for recovery must occur. Research has suggested the use of genetically modified plants to increase yield of the source stock however this raises multiple issues surrounding the subject such as ethical or religious objections and a reduction in the gene pool. As suggested throughout the cellulose section, a major bottlenecks with respect to use of nanocellulose particles is the hydrophilic property influencing the susceptibility of nanocellulose to humidity. The change in hydrogen bonding within the structure as a result of changing humidity often leads to the degradation of cellulose nanofibril films in humid conditions with mechanical properties such as the tensile strength and young’s modulus showing a decline as humidity increases. Work has been performed to increase the resistance of cellulose based nanopapers to humidity37. Indeed, a 2017 paper modified cellulose nanopapers with lactic acid which led to an improved mechanical performance under increased humidity.
However, decreases in the modulus still occurred and indeed a comparable loss of tensile strength occurred suggesting that although the modified nanopaper coped better under humidity stress, work is required to further limit the negative impacts of humidity upon cellulose-based materials and thus increase the application potential of the material.Further research pathways looking to surpassing the current mechanical limitations of these polymers to widen their already broad range of applications. In both cases, industry and academia must successfully collaborate to further the field of biobased polymers until it becomes fully integrated in our modern society.