1,3-Propanediol and its Application in Bio-Based Polyesters for Resin Applications
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Polyester resins are important raw materials for a myriad of applications especially in the field of coatings and radical curing polymers, such as wood and powder coatings, molding compounds, and UV-curing applications. In addition, polyols derived from polyester resins are precursors for the synthesis of polyurethanes and polycarbonates. Besides dicarboxylic acid, diols are used as monomers in these polyesters. To date most of the diols utilized are derived from petrochemical feedstock. To increase the bio-based content of polyester resins, novel diols derived from renewable resources are of special interest. In this respect, 1,3-propanediol has drawn considerable attention over the last years. It is accessible via microbial fermentation of glucose from starch at a competitive price in sufficient amounts. Therefore, 1,3-propanediol could be a valuable alternative to petrochemical diols, such as 1,6-hexanediol and neopentylglycol, which are currently used as diols in most resin applications. This article gives a brief overview over the utilization of 1,3-propanediol in high molecular weight polyesters for plastic application followed by a more detailed discussion of the most relevant work in the field of polyester resins derived from 1,3-propanediol.
Keywords1,3-Propanediol Polyester resins Coatings Unsaturated polyesters
The last decade has seen a growing interest in bio-based materials. Especially in the polymer field, the demand for alternatives from renewable resources is continuously increasing. This trend is driven by the finite nature of fossils fuels, but also a rising consumer awareness and lately the problem of rising accumulation of (micro) plastic in the environment, in particular in the oceans. The latter results in an increasing attention towards the transition to a circular economy and the development of bio-degradable plastics . Against this background, bio-based polyesters are of special significance . They are derived from renewable resources and exhibit in many cases a higher bio-degradability in comparison to other bio-based plastics, such as bio-polyethylene. The most prominent examples in the field of bio-based polyesters are polylactic acid (PLA) [3, 4, 5] and polyhydroxyalkanoates (PHA) [4, 6, 7, 8, 9], which are both commercially available. Especially PLA is employed in a variety of applications, such as packaging, composite materials and biomedical applications [5, 10, 11, 12, 13, 14].
In addition, other bio-based polyesters have been in the focus of both academic as well as industrial research and are in different stages of commercial availability. A range of bio-sourced monomers, such as furandicarboxylic acid (FDCA), succinic acid, isosorbide, etc. have been used and there are a range of extensive book chapters and review articles that cover this topic of polymer and material science [15, 16, 17, 18, 19, 20, 21, 22]. Among these building blocks, 1,3-propanediol (PDO) has been successfully applied in a several bio-based polyesters.
Even though this diol is accessible by synthetic pathways, for example from acrolein  or ethylene oxide  these methods suffer from high production costs and undesired and irritant impurities. This led to low interest in this monomer from an industrial viewpoint and therefore very limited applications. However, the situation changed when PDO was made accessible through microbial fermentation of glucose [25, 26]. This biotechnological pathway led to an increased availability of PDO at a competitive price and in turn to more applications of this bio-based building block in polymer materials.
In addition, a lot of work was dedicated towards aliphatic polyesters derived from PDO (Fig. 1b) [36, 37, 38, 39]. These polyesters are especially interesting due to their high susceptibility towards enzymatic degradation [37, 40]. This makes these polymers promising candidates for degradable plastics, as well as pharmaceutical applications.
Both the aromatic, as well as aliphatic polyesters derived from PDO have been studied extensively and are the subject of several comprehensive review articles [22, 41, 42, 43, 44, 45]. PDO has also been used as monomer for polyester resins in the coatings field. To the best of our knowledge, no review for this type of polyesters derived from PDO for binder resins has been made. Herein, we try to give an overview of the most important work undertaken in this area in the last years.
2 Polyester Resins from 1,3-Propandiol
2.1 Polyester Polyols for Polyurethanes and Polycarbonates
In another study the influence on the properties of PU-dispersions were examined when PDO was used to replace 1,6-hexanediol (HDO) in the polyester polyol (Fig. 2) . In a first step, the polyesters were synthetized by polycondensation of the two respective diols with adipic acid and phthalic anhydride. The hydrolysis resistance of the two different polyols was examined at a buffered pH value of 4 and 80 °C. In both cases, the molecular weight of the polyesters decreased slowly, with no significant difference when PDO or HDO are used as building block. Both polyesters were then used as polyol compounds for PU-dispersions synthesized by the acetone process. The solid content of the final dispersions was adjusted to 40%. The PU-dispersions were then blended with a commercially available acrylic resin and the blend was coated on wood panels. Weathering test revealed a significant difference in the performance between the blends and the pure acrylic resin used as a reference, with the blend outperforming the acrylic resin.
In 2012 Schirp et al.  reported the use of PDO as diol component in PU-dispersions derived from fatty acid-derived polyester polyols. The latter were synthesized by reacting either fatty acids or fatty acid methyl esters with trimethylolpropane (TMP), isophthalic acid and PDO. The resulting dispersions exhibited a good particle size distribution and stability. In addition, the authors were able to show that the surface structure of the coated and dried dispersion strongly correlate to the amount of 2,2-bis(hydroxymethyl)propionic acid (bis-MPA).
2.2 Alkyd Resins
PDO has also been used as diol component in bio-based waterborne alkyd resins. Acar and co-workers prepared acryl modified water reducible alkyd resins, where besides TMP, PDO was used as sole alcohol component . The solid content of the acrylic modified alkyd resins was 60 weight-%. The others blended the resin with different ratios of an acrylic co-polymer, which resulted in an improved performance of the final coatings. The best properties were found, when 40% (in respect to the alkyd resin) of the acrylic co-polymer was used. In a consecutive study, the same authors investigated the dilution effect of this kind of acrylic modified alkyd resin on the coating properties, such as film formation and thermal behavior .
2.3 Coil Coating
Polyester derived from FDCA, succinic acid, isosorbide and either PDO or 1,5-pentanediol (PeDO) have been examined in coil coatings applications by Lomelí-Rodríguez . Increasing the ratio of FDCA led to an increase in viscosity, molecular weight, but also in coloring. Polyesters synthesized from PDO instead of PeDO were less disperse (1.3–1.6 against 1.5–2.4 with PeDO). Increasing the amount of isosorbide led to a decrease in MW, therefore the amount of isosorbide must stay under 50 mol % of the diol component. The incorporation of FDCA as well as isosorbide had a positive effect on the Tg, as well as Tm, with isosorbide having a bigger influence. The use of PDO also improved the Tg and Tm, due to the lower chain flexibility of the shorter PDO chain. In addition, the polyester derived from IS and PDO exhibited a higher decomposition temperature compared to the similar polyester synthesized from PeDO.
2.4 Powder Coatings
PDO was also examined as diol component in carboxyl functional polyesters in the field of powder coatings . In this case, the powder coatings were formulated with triglycidyl isocyanurate (TGIC) as hardener. It was shown that the coatings based on polyester with PDO exhibited a superior impact resistance and flexibility in comparison with those without PDO. However, in this example PDO is the only bio-derived monomer used.
2.5 Unsaturated Polyesters
1,3-Propanediol was also used as the only diol building block for unsaturated polyester resins . These resins were then used as precursor for thermally curing composites with nanoprecipitated calcium carbonate (NPCC). As other components fumaric acid, succinic acid, and lactic acid were used, resulting in a polyester with a high amount of bio-based monomers. The resins were characterized my means of IR and NMR. In addition to these resins, the authors synthesized a novel reactive diluent as partial replacement for styrene. This reactive diluent was obtained by a reaction of lactic acid with 1,2-propandiol, followed by an end-capping reaction with itaconic anhydride. The resins were subsequently cured with different amounts of reactive diluents (styrene and the bio-based component) and NPCC. It could be shown that the bio-based reactive diluent had a major influence on the properties of the composites.
The structure of the polyesters as well as side reactions occurring during the polycondensation reaction were thoroughly studied by means of NMR spectroscopy. For example, the degree of mesomerization of the itaconic to the corresponding mesaconic acid species was analyzed. Furthermore NMR was used to elucidate the multiplet complexity of the protons in a-position to the C–O-bond of the ester linkage. In addition, the crosslinking mechanism of polyester itaconates was examined. According to their studies, the crosslinking proceeds through an oxo-Michael-addition, the so called Ordelt reaction [63, 64, 65]. However, the abilities of these polyesters to undergo radical crosslinking or post-polymerization modification was not further examined in the course of this study.
In a recent example, PDO was also used as building block for unsaturated polyesters with itaconic acid as dicarboxylic acid . Polyesters of this kind have been used as binder resins for wood coatings [51, 67]. However, the use of PDO under acid-catalyzed azeotropic polycondensation conditions led to undesired cross-linking and gelation. An effect that is not observed, when other diols, such as HDO or neopentylglycol were used. By screening at set of different Brønsted- and Lewis acids as condensation catalysts, it could be shown, that the use of Brønsted acids, such as methanesulfonic acid leads to an enhanced rate of a competing etherification reaction. This eventually leads in combination with some crosslinking through radical or polar pathways to the gelation of the polyester. This etherification is observed to a much smaller degree, when Lewis acids, such as Zn(OAc)2 or Ti(OBu)4 are used. By employing these catalysts, the gelation can be prevented and polyesters with 100% PDO can be synthesized, setting the stage for the replacement of petrochemical diols with the bio-based PDO. Polyesters of this type have been employed as binder resins in UV-curing offset inks, resulting in binder resins with a renewable content from 47 to 100%, replacing commonly used diols in the preparation of polyester polyols .
The aim of this mini review was to highlight the potential of PDO in the field of coating resins. Even though a lot of work has been dedicated to plastics derived from this interesting diol, such as PTT or PFT, examples in the coatings field are still somewhat scarce, especially in the scientific literature. The examples presented herein show that PDO can be used as building blocks in a broad field of application, such as polyurethane, polycarbonate, alkyd, and unsaturated polyester resins. In addition, some applications in powder and coil coatings were given. It is worth mentioning that most of the studies focus on other aspects than the mere replacement of traditional diols with PDO. However, the wide range of applications show that PDO can be a bio-based alternative to diols usually used as monomers in the field of polyester resins, such as HDO or neopentylglycol. Depending on the diol being substituted and the composition of the polyester, it can alter the Tg and flexibility of the polymers, allowing for new properties of the resulting materials. PDO can therefore be considered as an additional and valuable tool in the toolbox of polymeric chemistry, for plastics as well as coating materials.
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Conflict of interest
The authors declare no conflict of interest.
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