(San Jose, CA), to develop commercial processes for the production of bio‐PET. The source of petrochemical terephthalic acid is primarily from the oxidation of p‐xylene, obtained from the catalytic reforming of naphtha. More than 98% of the p‐xylene produced globally is converted to terephthalic acid and the global demand for purified terephthalic acid is expected to exceed 60 million tons by 2020, being most of it used for PET production. As a result of this cost‐competitive demand, bio‐based terephthalic acid made at equal purity and cost as the petroleum derived terephthalic acid would have a clear market advantage as well as a lower price volatility due to the non‐dependence on petrochemical p‐xylene [77]. However, bio‐PET is currently produced from plant‐based monoethylene glycol (bio‐MEG) whereas terephthalic acid is still derived from petroleum. Toyota Tsusho (Nagoya, Japan) and China Manmade Fibers Corporation (Taipei, China) jointly founded a company in November 2010 that manufactures bio‐MEG made from plant derived bioethanol [78]. Other companies such as SCG Chemicals (Bangkok, Thailand) also produce bio‐MEG from residues of agricultural activities including molasses, hay, and bagasse [79]. The first partially bio‐PET product was commercialized by The Coca‐Cola Company (Atlanta, USA) under the trade name of PlantBottle™, where a 30 wt% of bio‐MEG was used for the production of bio‐PET (Company). At the moment, some researchers are working on the synthesis of bio‐based terephthalic acid to obtain fully bio‐PET. This process is based on an integrated method to convert forest residues to isobutanol [80], which can be processed into p‐xylene [81], the precursor of terephthalic acid. Although the 100% bio‐based bottle was released in Milan in June 2015, the so‐called PlantBottle™, shown in Figure 1.5, has not reached yet the price parity to equal the price of producing current Coca‐Cola PET bottles [82]. PepsiCo (New York, USA) also announced the use of a PET bottle made entirely with renewable resources coming from waste carbohydrate biomass obtained from the food industry such as orange peels, oat hulls, corn husks, and potato scraps [83].
Similar to bio‐PE, bio‐PET is not biodegradable but it has the same properties as conventional PET made from natural gas or oil feedstocks. Current partially bio‐PET is used to make a number of products including drinking water and soda bottles, making them environmentally friendly and a new packaging alternative. Products made from bio‐PET have the same qualities as regular PET in its distinctive functions, weight, appearance, and it can also be recycled and reused [79]. This material can be recycled, incinerated, or landfilled, but it can also be intended for disposal by composting, where it undergoes soil degradation to CO2 and water [9]. Enzymatic hydrolysis of aromatic/aliphatic polyesters was first demonstrated in the 1990s for PET with several esterases, lipases, and especially cutinases [84, 85]. Nevertheless, PET hydrolysis by enzymes is a relatively slow process, since the biocatalysts are specialized to attack natural polyesters such as cutin and were not designed by nature for degrading manmade synthetic polyesters in the first instance [85].
Figure 1.5 Image of the PlantBottle™ made up to 30% from biomass and 100% recyclable.
Source: Courtesy of the Coca-Cola Company (Atlanta, USA).
1.3.7 Poly(ethylene furanoate)
A potential green substitute for terephthalic acid is 2,5‐furandicarboxylic acid (FDCA), which is a bio‐based building block that can be polymerized with bio‐MEG to form a new 100% bio‐based polyester called poly(ethylene furanoate) (PEF) [85]. PEF can be synthesized by polycondensation, ROP, and solid‐state polymerization. Polycondensation is the most commercially relevant method but it results in long exposure times to high processing temperatures, around 200 °C, which increases the production cost and, even more importantly, leads to thermal degradation and discoloration of the biopolymer. Solid‐state polymerization (SSP) is a milder process, though a bottle‐grade PEF has not been achieved yet [86]. SSP involves heating of the starting partially crystalline polyester at a temperature between its Tg and Tm, which is used mainly for PET manufacturing to get over its relatively low MW [87]. The company Avantium is developing PEF using bio‐MEG and FDCA coming from the dehydration of carbohydrates [77]. In 2016, it was announced a new technology involving a highly efficient separation technology and catalyst that would result in economically feasible production of FDCA starting from 2016. The planned industrial production capacity is estimated to exceed 300 000 tons per year while the company has established collaborations with major endusers from the food and beverage industry [84].
PEF shows excellent thermal properties and lower Tm values than PET because of which it can be processed at lower temperatures. It has the ability to withstand high temperature due to its higher Tg as well as thermal stability up to 320 °C. PEF outperforms the barrier properties of PET. Specifically, PEF's O2 barrier is more than ten times higher than that of PET, the CO2 barrier is four times higher, and the water vapor barrier is two times higher [88]. Other attractive properties of PEF include excellent mechanical strength, reduced carbon footprint, and ability to formulate in films, fibers, and almost replace PET in water bottles [87]. PEF can be recycled in very similar ways to PET [88]. Recently, the enzymatic synthesis of FDCA‐based polyesters was studied by the groups of Loos and Boeriu with Candida antarctica lipase B [89, 90]. However, apart from numerous reports around the emerging topic of PEF production and application, there is no information on enzymatic hydrolysis of PEF. Nevertheless, especially for bio‐based PEF, biotechnological processing for functionalization and chemical recycling of monomers could have a large potential to replace harsh chemicals [84].
1.3.8 Poly(ɛ‐caprolactone)
Poly(ɛ‐caprolactone) (PCL) is a biodegradable aliphatic polyester derived from the chemical synthesis of crude petroleum [91]. It is obtained from ROP of ɛ‐caprolactone in presence of metal alkoxides and also through the polycondensation of 6‐hydroxyhexanoic acid. PCL is commercially available under the trade names CAPA (Solvay, Belgium), Tone (Union Carbide, USA), Celgreen (Daicel, Japan), and many others [92].
Some features of PCL include biodegradability, good solubility, flexibility, low Tm (approximately 60 °C) and a Tg of around −60 °C, and easy processing. PCL is used in flexible packaging materials in the form of films or coatings for extending the shelf life of food products. However, due to its high price and long biodegradability cycles, PCL is commonly blended with other biopolymers, such as chitosan and starch [93]. Among them, starch has been proposed as a reinforcing agent to improve the mechanical strength of PCL [94]. There are also several studies on the hydrolysis and biodegradability of PCL. The degradation process of PCL takes place through hydrolysis, thereby leading to molecular fragmentation or chain scission [94]. Moreover, enzymes and fungi easily biodegrade PCL. However, to improve the degradation rate, several copolymers with lactide or glycoside have been developed [94].
1.3.9 Thermoplastic Starch
Starch is a natural polysaccharide that can be obtained from a great variety of crops such as cassava and corn. It is considered as one of the most promising biopolymers for food packaging due to its availability, biodegradability, and low price [95]. Starch is composed of a mixture of two polymers, namely amylose and amylopectin. Amylose is a linear polysaccharide composed entirely of D‐glucose units joined by α‐1,4‐glycosidic linkages. Amylopectin is a branched‐chain polysaccharide composed of glucose units linked primarily by α‐1,4‐glycosidic bonds but with occasional α‐1,6‐glycosidic bonds, which are responsible for the branching. Relative percentages of amylose and amylopectin in starch are in the range 10–30% amylose and 70–90% amylopectin. The starch having a crystallinity between 20% and 40% is termed as semicrystalline in which the amorphous region of starch contains amylose and the branching points of amylopectin [96]. Native dry starch has a limited range of applications