Lactide Synthesis Essay

7.3.1.3

Poly(Ethylene Terephthalate-co-Lactic Acid)s and Poly(Ethylene Terephthalate-co-Glycolic Acid)s

Lactic acid and glycolic acid are two of the most relevant renewable-based hydroxy acids due to their use in the production of two of the most spotlighted commodity plastics, namely poly(lactic acid) (PLA) and poly(glycolic acid) (PGA), respectively. Typically, PLA is obtained in high molecular weights by ring opening polymerization [115–117]; displaying glass transition temperatures ranging from 45°C to 65°C, whereas its melting temperature varies between 150°C and 200°C [118] closely depending on crystallinity and molecular weight. PGA is a rigid thermoplastic material with high crystallinity (46–50%) and glass transition and melting temperatures around 35–45 and 220–233°C [118], respectively.

Both polyesters, and a range of their copolymers, have historically comprised the bulk of published material on biodegradable polyesters and have a long history of use as synthetic biodegradable materials in a number of applications [118–120]. In particular PLA is nowadays one of the most successful biopolyesters, which can be processed through a large number of techniques in line with the different fields of application that span from packaging and textile, through the biomedical field [115,116,120]. PGA finds several applications especially within the biomedical field. Indeed, it was the first biodegradable polyester used as a medical suture in the 1960s [120]. Most probably attracted by these relevant characteristics several authors investigated the use of PLA [30,47,49,65–68] and/or PGA [48] as the aliphatic counterpart of several PET-aliphatic copolyesters.

Poly(ethylene terepthalate-co-lactic acid), designated hereafter by PET-co-PLA, was routinely prepared by means of slightly different approaches to general aromatic/aliphatic copolyesters syntheses. One approach involved the direct use of PLA and BHET (or oligomeric PET), the former obtained from glycolyzed postconsumed PET, in a polytransesterification reaction (Scheme 7.2a) [30,49,68]. Additionally, PET-co-PLA was also routinely obtained in a similar approach to PET-co-PS copolyesters, that is, from TPA (or its DMT derivative), LA, and EG (Scheme 7.2b) using a titanium-derived catalyst and high temperature in a typical two-step approach [30,47,49,65–68]. Another approach [30] used postconsumed PET and oligomeric PLA directly (Scheme 7.2c).

In some investigations, the role played by different catalysts in the synthesis of PET-co-PLA was assessed [65], namely p-toluene sulfonic acid (p-TSA), triphenyl phosphate (TPP), Ti(OBu)4, antimony trioxide (Sb2O3), tin(II) 2-ethylhexanoate (SnOct2), and tin(II) chloride (SnCl2), among which Sb2O3 and SnOct2 demonstrated to be the most effective. Some studies also focused on the influence of an additional diol [47], including PD and BD, on the properties of the ensuing PET-co-PLAs. In this vein, Namkajorn [47] described that BD provided the copolymers with the highest molecular weights (ca. 35 kDa for LA/DMT/BD ∼1/2/4 in mol) compared to shorter-chain diols (ca. 11.7 kDa for LA/DMT/PD ∼1/2/4 in mol). This behavior is associated with a higher chain flexibility of BD when compared with the other diols, which caused a higher reactivity of BD hydroxyl groups toward carboxylic acid functional groups. Additionally, in this investigation [47] the role played by the aromatic/aliphatic content was assessed for DMT in 20–29 mol%. These copolyesters were essentially semicrystalline in nature, displaying the highest Tg and Tm when the DMT feed was the highest (31 and 210°C vs. 26 and 191°C for 29 and 25 mol% of DMT, respectively), and also a reduction of their solubility was observed.

The synthesis of PET-co-PLA by direct transesterification just by mixing waste PET bottle flakes with PLA [30] (Scheme 7.2c) was carried out in an attempt to solve the problem of postconsumer soft drink bottle degradation. The reaction was carried out at 140 or 170°C, in the presence of dibutyltin(IV) oxide and o-nitrophenol, as catalyst and solvent, respectively. This approach revealed to be an effective strategy to enhance hydrolytic degradation of the ensuing copolyester compared to PET. The weight loss percentage during degradation essays increased with the increasing amount of PLA used, for example, at 7 days it increased from 0% in PET to 21.7% in PET-co-PLA with 90% weight PLA feed [30].

In the same line of research, Olewnik et al. [68] used a similar synthetic approach but using BHET, obtained from postconsumer PET, and oligomeric poly(L-lactic acid) (PLLA) to produce PET-co-PLLA copolyesters (Scheme 7.2a). These copolyesters possessed amorphous character when PET and PLLA were incorporated in equimolar amounts, exhibiting accordingly a Tg close to 60°C below PET. In 2007 the same authors [49] also investigated the hydrolytic degradation of PET-co-PLLAs at pH 7.40 and 7.35 and at 45 and 60°C using different buffer solutions. Results have showed that the mass of PET-co-PLLAs decreased almost linearly with time and also that copolyesters with equimolar amounts of PET/PLLA exhibited the fastest weight loss, reducing over 60% of its initial weight, after 180 days in a phosphate solution, at 60°C. One investigation from 2010 [67] introduced a slight change in PET-co-PLAs-derived copolyesters using additionally diethylene glycol. However, a depression on thermal properties was observed compared to PET or even to PET-co-PLA copolyesters.

The incorporation of PGA into the PET backbone (Scheme 7.3) to produce poly(ethylene terephthalate-co-glycolic acid), hereafter abbreviated to PET-co-PGA, was investigated by Olewnik and Czerwin´ski [48]. This study confirmed the degradation susceptibility of PET copolyesters incorporating these linear aliphatic moieties and their lower Tm when compared to the homopolyester counterparts. These authors claimed that the disruption of the PET chain by incorporating a sequence length of ∼1 GA unit per ∼4 TPA to lower Tm to 166°C, compared with 265°C of PET [4], or in this matter also of the PGA (∼220°C). In terms of degradation the autocatalytic action of carboxylic acid end groups of GA significantly accelerated the hydrolysis rate.

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