Bio-based Unsaturated Polyesters

CHAPTER

BIO-BASED UNSATURATED POLYESTERS

20

Qiong Li1,2, Songqi Ma1, Xiwei Xu1,3 and Jin Zhu1 1

Key Laboratory of Bio-Based Polymeric Materials Technology and Application of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, P.R. China 2University of Chinese Academy of Sciences, Beijing, P.R. China 3School of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, P.R. China

20.1 INTRODUCTION Over the past few years, shortages of oil resources and environmental pollution have become increasingly serious. Bio-based polymer materials made from a wide range of renewable, lowprice, and natural raw materials are highly valued by industry and academia [1 5]. Unsaturated polyester resins (UPRs), as the most used type of resin for reinforcing fiber matrices, possess outstanding characteristics such as good mechanical processing, heat and chemical resistance, and excellent electrical conductivity, therefore, they are widely used in transportation, construction, coatings, electronics, and other areas and are considered as indispensable polymer materials [6,7]. With the deepening of the concept of sustainable development and environmental protection, a large number of attempts have been made to use natural fibers as alternatives to glass fibers to reinforce UPRs and obtain bio-based composites [8 15]. There are plenty of crude fibers existing in nature including plant fibers such as hemp, cotton, jute, flax, ramie, sisal, among others, as well as silk and animal hair. Therefore the synthesis of UPRs from bio-based raw materials has potential applications in natural fiber reinforced composites. However, current commercially available UPRs are mainly synthesized from petrochemical products [16,17], containing two main components, namely UPs and reactive diluents. UP is a linear polymer compound possessing ester bonds and carbon carbon double bonds, and which is synthesized by the polycondensation of dibasic organic acids (including at least one unsaturated dibasic organic acid) with diols (Fig. 20.1). These diacids and diols are mostly extracted and produced from fossil resources. The most conventionally used reactive diluent for UPs is styrene, which is a low viscosity and highly reactive solvent that can be miscible with UPs to reduce the viscosity of resin and provide excellent fiber impregnation; on the other hand, the carbon carbon double bonds of styrene participate in the free radical polymerization of UPs (Fig. 20.2), thus increasing the cross-link density of the system and improving the material properties [18]. Besides its unsustainable

Unsaturated Polyester Resins. DOI: https://doi.org/10.1016/B978-0-12-816129-6.00020-X © 2019 Elsevier Inc. All rights reserved.

515

516

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

FIGURE 20.1 Synthetic route of UPs.

O O

O

O

O O

O

O

O

O

O

O

UPE Styrene

Styrene

O UPE

O

O O

O

O

O

Styrene

O

O

O

O

R

O

R

FIGURE 20.2 Cross-linking of UPs with styrene [18].

sources, styrene is a suspected carcinogen [19,20], thus, the environmental and health issues triggered by its high volatility cannot be ignored. Efforts should be invested in the development of bio-based reactive diluents that can replace styrene. Bio-based UPRs can be developed from the aspects of bio-based UPs and bio-based reactive diluents. Besides their sustainable resources, quite a few bio-based UPs are biodegradable and biocompatible, and as a result, many achievements has been made in bio-based UPs [21 24]. Over the past few decades, the main raw materials used for bio-based UPs have been itaconic acid (IA), isosorbide, furan compounds, and vegetable oils. Thus in this chapter, the research progress of bio-based UPs from these chemicals are summarized and discussed separately. Other bio-based monomers for UPs are also mentioned during the introduction of UPs from these main monomers. The development of bio-based diluents is discussed as well.

20.2 ITACONIC ACID-BASED UNSATURATED POLYESTERS

517

20.2 ITACONIC ACID-BASED UNSATURATED POLYESTERS IA, also known as methylene succinic acid, contains carbon carbon double bonds and two carboxyl groups in its molecular structure, which makes it extremely similar to maleic acid as shown in Fig. 20.3. Therefore it can replace maleic acid or maleic anhydride in the synthesis of bio-based UPs via polycondensation with diols (Fig. 20.4). At present, IA is prepared mainly from agricultural and sideline products such as straw, sugar cane, sugar beet, starch, distiller’s grains, among others, and suitable fermentation species (Candida, Aspergillus itaconicum, Aspergillus terreus, etc.) are selected for large-scale fermentation production [25 27]. The world market for IA is expected to grow beyond US$216 million by 2020 on account of the growing demand for biomass resources [28]. Nowadays, IA is generally used to synthesize bio-based UPs. Information on the reported IA-based UPs are listed in Table 20.1.

20.2.1 POLYMERIZATION OF ITACONIC ACID-BASED UNSATURATED POLYESTERS 20.2.1.1 Catalyst for itaconic acid-based unsaturated polyesters In 1991, IA-based UPs were reported by Singh et al. [41] IA and PEG-600 were polymerized with p-toluenesulfonic acid (pTSA) as a catalyst and hydroquinone as an inhibitor. The obtained UPs were used as precursors for biosoluble vaccine-loaded hydrogel microspheres. Cross-linking in this case was triggered by an aqueous solution of ammonium persulfate. O OH

HO

HO

OH O O

O Itaconic acid

Maleic acid

FIGURE 20.3 Chemical structure of itaconic acid and maleic acid. O OH

HO

+

O

HO m

O H

O

O O

FIGURE 20.4 UPs from the polycondensation of itaconic acid and diols.

m OH n

OH

Table 20.1 The Raw Materials, Catalysts, and Molecular Weight (Mn) of the Reported Itaconic Acid UPs and the Thermal and Mechanical Properties of Cured UPs Comonomers

Catalysts

Mn (g/mol)

Tg ( C)

Tensile Strength (MPa)

Itaconic acid

No catalysts No catalysts pTSA, DBTL pTSA, DBTL Ti(OBu)4 Ti(OBu)4 TBT Ti(OBu)4

1140 to 2200 940 748 to 1247

225.6 to 18.4 4 68 to 100

0.22 to 18.20 2.05 50 to 120

229.3 to 225.5

MSA

4800 to 6000 243 to 215 B1500 49,188 to 52,529 3100 to 5600

3 to 12.2

DBTO, TBPB

2182 to 2791

58.8 to 76.2

CaLB

2030 to 6650 11,900 854 to 13,288 390 to 3690

237.7 to 8.1 262.4 249.6 to 235.9

10,900 to 16,800

260 to 3

Dimethyl itaconate Itaconic anhydride

Adipic acid, trimethylolpropane [29] Succinic acid, sorbitol [29] Ethylene glycol, 1,4-butanediol, 1,6-hexanediol [30] 1,4-Butanediol, glycerol [31] 1,4-Butanediol, succinic acid [32] Succinic acid, 1,4-butanediol [33] Sebacic acid, 1,3-propanediol, diethylene glycol [34] Sebacic acid, succinic acid, 1,4-dibutanediol, 1,3-propanediol [35] Neopentyl glycol, isophthalic acid trimethylolpropane, adipic acid [36] Diethylene glycol, neopentyl glycol, trimethylolpropane, isophthalic acid, adipic acid; styrene, methyl methacrylate, butyl acrylate [37] 1,4-Cyclohexanedimethanol PEG [29] Adipic acid, 3-methyl-1,5-pentanediol [29] Dimethyl succinate, 1,4-butanediol [38] Succinic anhydride or glutaric anhydride, 1,4-butanediol, 1,6-hexanediol, 1,8-octanediol, 1,10-decanediol [39] 3-Methyl-1,5-pentanediol, maleic anhydride [40]

CaLB

Nf2NH

249.4 to 239.7 257 to 71.5

4.5 to 8.2 0.6 to 0.87

2.70 to 16.59 2.04

pTSA, p-toluenesulfonic acid; DBTL, dibutyltin dilaurate; Ti(OBu)4, titanium(IV) tetra-n-butoxide; TBT, tetra-n-butyl titanate; MSA, methanesulfonic acid; DBTO, dibutyltin oxide; TBPB, tert-butyl peroxy benzoate; CaLB, lipase B from Candida antarctica; Nf2NH, bis(nonafluorobutanesulfonyl)imide.

20.2 ITACONIC ACID-BASED UNSATURATED POLYESTERS

519

Yousaf et al. [29] reached a milestone in the synthesis and application of IA-based UPs. They combined IA with trimethylolpropane and a second dicarboxylic acid to form UPs through direct polycondensation without any catalysts and inhibitors (Fig. 20.5). In spite of the lack of a catalyst, the molecular weights were legitimately up to 2200 g/mol, and the polydispersity indices (Ð) were fairly narrow. It was quite remarkable, because the unsaturated double bond of IA commonly leads to undesirable side-reactions. If no inhibitors are used to quench the radicals arising at elevated temperatures, the radicals will eventually bring about gel formation during the polycondensation process. Using an enzyme catalyst, lipase B from Candida antarctica (CaLB), a series of UPs with higher molecular weights (2030 11,900 g/mol) were further synthesized. Nevertheless, it would take a long time (48 hours), which may lead to the degradation of the enzymes. The carbon carbon double bond of IA can undergo a photocuring reaction on the molecular chain structure of UPs. The UV-curing cross-linked systems exhibited Young’s modulus, tensile strength, and elongation at break of 0.17 398.14 MPa, 0.11 18.20 MPa, and 5% 198%, respectively. As expected, the material performance was dependent on the IA/adipic acid ratio. With increasing adipic acid content, the Young’s modulus and tensile strength of the materials decreased due to its long flexible chain. The monomers used in the materials have previously been segmented in biocompatible polymers and the cytotoxicity is expected to be small, making them suitable for drug delivery, tissue engineering, and other biomedical applications. IA-based UPRs can replace the type of resins established in the field of radically curable resins. However, one of the biggest challenges of IA-based UPs is the more complex synthesis process, especially under industrial-scale polycondensation conditions. The α,β-unsaturated double bond of IA is inclined to side reactions, which lead to the gelation of polyester resin under standard conditions. It was reported in earlier research that a large amount of the aliphatic diols in UP result in low resin conversion and gelation [42]. Consequently, the synthesis of itaconate-based UPs could exhibit limitations in molecular weight. In previous works, multifarious catalytic studies were performed to clarify the reasons for this behavior. The results demonstrated that the choice of catalyst has a decisive influence on the side reactions that occur

O

O OH

HO

OH

+ HO

O

+ HO

O

OH OH

No catalyst 120–150°C

OR

O H

O

O O

O

O n

O O

FIGURE 20.5 Polycondensation of branched UPs based on itaconic acid [29].

OH m OR

520

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

during the polycondensation reaction [42,43]. Robert et al. [44] reported that the polycondensation reaction with IA would not lead to UP under industry-related conditions. It was demonstrated that Bronsted acids such as methanesulfonic acid (MSA) were inappropriate catalysts for polycondensation with diols due to contradictory side effects, and water-resistant Lewis acid (Zn(OAc)2) is the catalyst of choice for the polycondensation reaction of IA, particularly with 1,3-propanediol and 1,4-butanediol. Although the comparative reaction kinetics were slightly slower than in an MSA system, the target products were obtained with .99% conversion after 9 hours, and there was no evidence of gelation. Besides the classic chemical catalysts, IA-based UPs can also be prepared through enzymatic catalysis. Jiang et al. [38] utilized CaLB as a catalyst to synthesize UPs with high bio-based content from succinic acid, itaconate, and 1,4-butanediol. The resulting UPs possessed high molecular weights of up to 13,000 g/mol. The amount of itaconate did not significantly affect the Tg and thermal stability of the obtained UPs (Fig. 20.6). In a slightly diverse enzymatic approach, Yamaguchi et al. [39] used a lipase-catalyzed ring-opening addition condensation polymerization (ROACP) of itaconic anhydride and ethylene glycol (EG) at 25 C for 2 hours to obtain UPs with molecular weights ranging from 560 to 3690 g/mol (Fig. 20.7). Gardossi et al. [45] developed a novel covalent immobilized enzyme catalyst that combined CaLB with an epoxy-functional methacrylic resin. They demonstrated that this catalyst can be used for the polycondensation of dimethyl itaconate (DMI) with butanediol. In addition, the catalyst was indeed recovered with little loss of catalytic reactivity. However, only a low molecular weight was obtained even after 96 hours of polymerization. In a sequent study, they investigated the influential factors of enzymatic catalysis [46]. The polycondensation reaction was more detailed to improve the reactivity.

O

O O

O

+

O

O

+

O

OH

HO

O

Diethyl succinate

Itaconate

1,4-Butanediol

Stage-1: Oligomerization 80°C, 2–14 h, atm, N2 Stage-2: Polycondensation 80°C, 94 h, highvacuum O

O O

O

O

O xO

FIGURE 20.6 Polycondensation of UPs based on itaconic acid by lipase catalysis [38].

O y

20.2 ITACONIC ACID-BASED UNSATURATED POLYESTERS

521

O HO O

O

O

Lipase O

–H2O

O

O

O

OH

O

FIGURE 20.7 Synthesis of different UPs by lipase-catalyzed ring-opening addition condensation polymerization [39].

The key factor for high reactivity was to optimize the mass transfer and to evenly disperse the enzyme. The authors proved that the reactivity can also be affected by the structure and concentration of the diol used. Theoretical and experimental studies both indicated that the rigid cyclic diol, 1,4-cyclohexanedimethanol, would be the more promising monomer for higher molecular weight UPs as compared to 1,4-butanediol. Brannstrom et al. [47] conducted a more comprehensive study of the performance of different esterification catalysts for the synthesis of renewable UPs from DMI, dimethyl succinate (DMS), and 1,4-butanediol (BD). Six esterification catalysts were chosen to assess the synthesis of itaconate-based UPs. Considering electivity and efficiency, the three most suitable catalysts were CaLB, pTSA, and Ti(OBu)4, which were employed to study how catalytic mechanisms influenced the synthetic order of UPs. The selectivity of CaLB and Ti(OBu)4 to the ester groups in DMI showed a significant difference during the transacylation of DMI. CaLB reduced the reaction rate by the conjugated ester group of DMI, and DMS was the more suitable monomer for CaLB in contrast to DMI. On the contrary, there was no such effect observed when Ti(OBu)4 was used as a catalyst. It was significant to understand the performance of different esterification catalysts for the synthesis of itaconatebased UPs in an effective way. The choice of CaLB as a catalyst allowed the UP to possess more itaconate at the end. The compound Ti(OBu)4 produced a UP with more random itaconate ester bonding in the polyester chain. While the properties of the two UPs showed no obvious differences after cross-linking. CaLB was the most suitable catalyst as the polycondensation proceeded at the low temperature of 60 C at which there was no need for radical inhibitors and low energy demand. In addition, the obtained polyester showed no color change, which usually happens in UPs with organometallic catalysts, and the use of immobilized enzymes facilitated the removal and recycling of the catalyst. Therefore CaLB was a satisfactory choice for the sustainable synthesis of itaconic-based UPs.

20.2.2 APPLICATION OF ITACONIC-BASED UNSATURATED POLYESTERS 20.2.2.1 Coatings Organic coatings have played a crucial role in the modern chemical industry as coverings to protect, decorate, and functionalize substrate materials. Due to increasing concerns regarding the limited fossil resources and environmental issues caused by the use of fossil resources, bio-based coatings using renewable bioresources have captured great attention. IA-based UPs can be crosslinked under radiation or heating due to their carbon carbon double bonds, which make them suitable to be used as the binders for coatings [3,30,31,48]. In a previous work, several carboxyl-terminated bio-based UPs were synthesized from IA and different diols [EG, 1,4-butanediol (BDO), and 1,6-hexandiol (HDO)] to produce bio-based

522

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

O OH

HO

+

HO

R

OH

O

HO pTSA 160ºC

HO

HO

DBTL inhibitor O H

HO

O O

O O

R

O

R

OH OH

OH

OH

OH O

FIGURE 20.8 Synthesis of COOH-terminated bio-based UPs [30].

waterborne UV-curable coatings (Fig. 20.8) [30]. The prepolymer was first formed in the presence of 0.5 wt.% pTSA. After 2 hours, 1 wt.% of dibutyltin dilaurate (DBTL) was added as a second catalyst, followed by the removal of water under vacuum. p-Hydroxyanisole (0.5 wt.%) acted as a free radical inhibitor to prevent the unsaturated double bond cross-linking of IA. The molecular weight of the obtained UPs were between 750 and 1250 g/mol. They were neutralized with sodium hydrogen carbonate and then a water-based emulsion was formed (40 wt.% solids content). After adding waterborne photoinitiators and a curing promoter, they were cured under UV radiation. The waterborne UV-cured coatings based on these UPs exhibited high hardness, outstanding water resistance, and solvent resistance. However, they were inferior in the performance of adhesion and flexibility. To overcome this fault, different contents of glycerol were incorporated into the molecular chain of the UPs through melt polycondensation with IA and 1,4-butanediol to enhance the adhesion (Fig. 20.9). Furthermore, flexible and reactive acrylated epoxidized soybean oil (AESO) was introduced during the coating preparation process to improve the flexibility of the UV-cured waterborne coatings based on IA-based UPs [31]. Besides the outstanding adhesion and superior flexibility, the obtained UV-cured waterborne coatings also exhibited high hardness and excellent solvent resistance. In these works, water was used to disperse the IA-based UPs; however, during the curing process the water had to be removed to achieve favorable properties. The water evaporation often required time and energy. Thus it was also attempted to prepare coatings without water. As shown in Fig. 20.10, AESO, a reactive multifunctional monomer with low viscosity, was mixed together with IA-based UPs; the mixtures could be used to prepare coatings and transferred to molds. After thermal curing, they presented excellent coating properties such as solvent resistance, adhesion, flexibility, water absorption, and hardness [48]. Besides carboxyl-terminal bio-based UPs from IA for coatings, researchers also reported hydroxyl-terminated bio-based UPs for wood coating applications [36,49,50]. Philipp et al. [36]

20.2 ITACONIC ACID-BASED UNSATURATED POLYESTERS

HO

O

OH

+

OH HO

523

HO

OH

OH O

pTSA 120°C

Prepolymerization –H2O

Oligomers

DBTL 120°C

O O

O

O

O HO

Polycondensation –H2O

O nO

O

O OH

OH m

O

Bio-based unsaturated polyesters

FIGURE 20.9 Synthesis of bio-based UPs [48].

synthesized hydroxyl-terminated bio-based UPs via polycondensation from IA and other bio-based monomers such as adipic acid, cyclohexane-1,2-dicarboxylic anhydride, 1,6-hexanediol, 1,3-propanediol, and neo-pentane glycol using p-hydroxyanisole and butylated hydroxytoluene as inhibitors and MSA as a catalyst. The reactions were proceeded in a Dean Stark apparatus at 120 C 180 C within 8 hours (Fig. 20.11A). The molecular weights of the synthetic UPs were between 3100 and 5600 g/mol. The obtained IA-based UPs were reacted with various diisocyanates such as methylenediphenyl diisocyanate (MDI) or hexamethylene diisocyanate (HDI) to prepare water-based polyurethane for wood coating applications, in which dimethylolpropionic acid worked as an emulsifier followed by a reaction with trimethylamine (Fig. 20.11B). Other special coatings based on IA-based UPs were also developed. Acrylic-modified UPs were designed with IA in combination with unsaturated resins and further graft copolymerized with 20% 40% acrylic monomers (Fig. 20.12) [37]. The acrylic-modified polyester resin coatings had superior mechanical properties such as hardness, adhesion, flexibility, and impact resistance, and improved weather ability when the grafting of acrylates on the UPs increased. In a previous work, monomethyl itaconate was used as a green alternative to acrylic acid, where it was reacted with epoxidized soybean oil through a ring-opening reaction to get an IA-based polyester (IESO) [3]. Compared with the AESO from acrylic acid and epoxidized soybean oil, IESO had no volatile organic compounds (VOC) problem as does AESO, and the IESO-based UV-cured coatings possessed comparable, even higher properties than those of the AESO-based ones.

524

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

O HO

O O

R

O

OH O

O

O

O

O

OH O

R = –CH2–CH2–, –CH2–CH2–CH2–CH2–, –CH2–CH2–CH2–CH2–CH2–CH2–

+

O

O O O

O

O O

OH

O O

OH

AESO

O

O O

TBPT Δ

Cross-linked polymers

FIGURE 20.10 Copolymerization of AESO PIE/PIB/PIH/PIBG [48].

20.2.2.2 Elastomers and composites Shibata et al. [32,33] demonstrated that IA-based UPs can be used to produce organic inorganic hybrid composites. 1,4-Butanediol, succinic acid, IA, or maleic anhydride were condensed to synthesize polybutylene succinate diacetate oligomers containing IA or maleic acid units (Fig. 20.13). The oligomer was polymerized with benzoyl peroxide as an initiator to obtain a UPR that is insoluble in chloroform but retains a certain degree of biodegradability [32]. When a methacrylate-substituted polysilsesquioxane (Me-PSQ) (Fig. 20.13) was copolymerized with the oligomer, the thermal stability, storage modulus, and Tg of the resin can be significantly increased, for example, the Td5% (5 wt.% weight loss temperature) of the hybrid composites was up to 405 C much higher than that of the pure polyester (333 C) [33]. IA also played an important role in producing polyester elastomers. Wei et al. [35] synthesized a fully bio-derived elastomer, namely a bio-based engineering elastomer (BEE) from succinic acid, sebacic acid, 1,3-propanediol, and 1,4-butanediol with 5 15 mol.% of IA in proportion to the total acid

20.2 ITACONIC ACID-BASED UNSATURATED POLYESTERS

O

(A)

OH

HO

O

+

O

H 2O

O

R

OH

R'

+ H 2O

525

OH

MSA inhibitor O H

O

R'

O

O

R'

O

O O

HO

(B)

R'

R

OH

HO

OH

R'

OH n

O

O OH

HO

O

O OH

HO

OH :

O

O

O

O :

HO

R

+

O

HO

OCN

OH

NCO

+

HO

OH

HO

OH

HO

HO HN OCN

O

O O

O

NH N H

O

O

O

O

O

O

NH

O NH

O

O

NH NCO

O

NET3 H 2O

HNET3 O HN OCN

O

O O

O

NH N H

O

O O

O

O

NH

O NH

O

O

NH O

NCO

Prepolymer dispersed in water

H 2N

NH2

UV-curing polyurethane dispersion

HO

OH : Polyester polyol derived from itaconic acid

OCN

NCO : HDI, MDI

FIGURE 20.11 Synthesis of (A) hydroxyl-terminated bio-based UPs from itaconic acid and (B) water-based polyurethane dispersions derived from polyester polyols [36].

526

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

HO

COOH

+

COOH

HOOC

COOH

OH

OH

+

+ HO

HO

OH

O

OH

Catalyst 225°C O O HO

O

O

O O

O O

O

OH

HO

Polyester O OH

HO O

O O HO

O O

O

O

O

O

O

O O Itaconic acid based unsaturated polyesters

O

140°C

O

OH

O

Acrylic monomers +initiator O

O O O HO

O

O O O

O

O

O O

O

O

O

O

OH

O

O O O Acrylic-modified polyester

FIGURE 20.12 Acrylic-modified polyester [37].

content. The bio-based elastomer exhibited thermomechanical properties comparable to those of conventional petrochemical-derived elastomers. Hydroquinone monomethyl ether and phosphoric acid were used as inhibitors. The monomers were refluxed at 180 C for 2 hours before titanium butoxide was added as a catalyst and the reaction was continued at 220 C at reduced pressure. This bio-based UP showed a number-average molecular weight (Mn) of 33,000 g/mol, and could

20.2 ITACONIC ACID-BASED UNSATURATED POLYESTERS

O OH

HO

+

527

O OH

HO

+

OH

HO O

O Ti(OC 4H 9 )4 O O

O O

O

O O m

On OCH 3 H3 CO Si OCH3 OCH 3

(CH3 )4

N+OH –

H2O/EtOH

R Si O O Si O n R ME-PSQ

TESPM O R=

O

FIGURE 20.13 Synthesis of itaconic acid-based UPs and Me-PSQ [32,33].

be cross-linked with various amounts of dicumyl peroxide as the thermal initiator. Silica nanoparticles were subsequently imported into the BEE matrix to achieve meaningful strengthening and enhanced environmental stability. The obtained BEE nanocomposite was a material like transparent rubber with a low Tg of around 254 C and a high tensile strength of 20 MPa far beyond that of reported biodegradable elastomers and related composites and meeting the requirements for the majority of rubber products.

20.2.2.3 Medical application As previously mentioned, Yousaf et al. [29] reported biocompatible IA-based UPs and the cytotoxicity was expected to be small, making them suitable for drug delivery, tissue engineering, and other biomedical applications. Reports subsequently recommended IA-based UPs as excellent perspective biomaterials. Guo et al. [34] utilized 1,3-propanediol, sebacic acid, and IA as the main raw material to synthesize bio-based poly(propylene sebacate), and diethylene glycol to regulate the flexibility of the polyester (Fig. 20.14). The resulting UPs were found to be promising shape memory polymers with excellent shape recovery and fixity (near 100% and independent of thermomechanical cycles). In vitro cell viability assay and the degradation observed indicated that the materials were potentially biocompatible and biodegradable, making them potential biomaterials. Tang et al. [40] studied the polycondensation of maleic anhydride and itaconic anhydride with 3-methyl-1,5-propanediol at low temperatures using bis(nonafluorobutanesulfonyl)imide (Nf2NH) as a catalyst (Fig. 20.15). The resulting UPs possessed high molecular weights above 16,000 g/mol. The hydrogel obtained from the UV cross-linking of UP and methyl methacrylate can be used in drug-release systems and other biomaterials.

528

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

O OH +

HO

HO

O

Ο 8 OH

O

+ HO

OH

OH + HO O

0.5 wt.% Ti(IV) butoxide 0.5 wt.% 4-hydroxyphenol O HO

O 8

O O

O

O

O n

O

O

O

O 8 n O

O

OH

FIGURE 20.14 Ti(IV)-butoxide-catalyzed polycondensation [34].

O O

O

+

O

O

+

HO

OH

O Nf2 NH 60–80ºC

O HO

O

O

O

O

O

O n

O

OH m

FIGURE 20.15 Low temperature polycondensation using Nf2NH as a catalyst [40].

20.3 ISOSORBIDE-BASED UNSATURATED POLYESTERS Isosorbide, a solid diol, contains a unique bicyclic rigid structure. Isosorbide is commercially produced via a number of different methods including the enzymatic hydrolysis of starch and the catalytic dehydration of sorbitol. Sorbitol can be obtained by the hydrogenation of starch, sucrose, or glucose [51,52]. Fig. 20.16 is the synthetic route of obtaining isosorbide from glucose [53]. In terms of the structure of isosorbide, the two tetrahydrofuran rings are connected with 120 degrees V-type, and form a chiral diol structure with two hydroxyl functional groups. Therefore isosorbideprepared polymers possess excellent thermomechanical properties [54 57]. In contrast to isosorbide, it would result in a low Tg if aliphatic bio-based monomers were to be incorporated such as succinic acid (SA) [58 61] and 1,3-propylene glycol [58,62,63] into UP structures. UPs with comparatively high Tg have been prepared by means of introducing aromatic diacids, chemical

20.3 ISOSORBIDE-BASED UNSATURATED POLYESTERS

OH HO HO

OH

OH OH

H2

O

HO OH

OH

Glucose

O

–2H 2O

OH

OH HO

OH

529

Sorbitol

O Isosorbide

FIGURE 20.16 Synthetic route of obtaining isosorbide from glucose [53].

OH

H O

+

O

O

O

O

+

OH

HO

O HO

O

H Ti(n-Btu) – H2 O O H

HO

O

O O

H

O

O

O H

m H O O

O

H O O

O

O H n

H O

FIGURE 20.17 Synthesis of poly(isosorbide maleate-co-succinate)s [57].

modification of monomers, or forming a rigid skeletal structure. However, rising oil prices and pollutant emissions during chemical processing are leading to increased interest in polymers with high Tg equipped from renewable resources. Therefore scientists place great emphasis on isosorbide as a promising monomer to be used in a polycondensation process to obtain high Tg UPs. Jasinska et al. [57] reported an example of the use of isosorbide in synthetic bio-based UP for coating applications. As shown in Fig. 20.17, a series of bio-based UPs were synthesized from isosorbide, succinic acid, and maleic anhydride. The obtained UPs exhibited limited molecular weights (830 1770 g/mol), and there were a majority of hydroxyl end-groups and a small amount of COOH end-groups in the structures. After being cross-linked with 2-hydroxyethy methacrylate, N-vinyl-2-pyrrolidinone, methacrylamide, and acrylic acid in the presence of a peroxide initiator (2-butanone peroxide) and an accelerator (cobalt (α) 2-ethylhexanoate), they showed high Tg of up to 103 C. Although the obtained materials were inferior in thermal stability, they are still promising materials for coating applications. The synthesis of isosorbide-containing UPs could also be catalyzed by enzymes. Naves et al. [64] utilized isosorbide to copolymerize with diethyl adipate and fractions of different unsaturated diesters (diethyl itaconate, diethyl fumarate, diethyl glutaconate, and diethyl hydromuconate) to synthesize UPs in one step using CaLB as a catalyst (Fig. 20.18).

530

IS

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

O

HO

∗

O

O O

OH

O

O

+ AD

O

O O

O

O

O

∗ O

O

4

CH2

O n

m O O

C 2 H5 O

4 OC 2 H5

∗

or IT

O

CH2

O

OC2 H5 or

C2 H 5O

∗

O O

∗

O

O

O

O

O O

4

O

O n

m

OC2 H 5

C2 H5 O

n

O O

or O

O

O

4

O

O GL

O

∗ O

m

Cyclohexane:toluene (6:1) azeotropic distillation OC2 H5

O

O

CaLB

O

O

O

O O

C2 H 5O

FU

O

O

or

∗

HM O

O

O

O

OC 2H 5

C 2H 5 O O

O O

O

O

O 4

∗ O

O m

O n

FIGURE 20.18 Schematic representation of copolycondensation reactions with 1:4,3:6-D-diahydrohexitols (IS), diethyl adipate (AD) and fractions of itaconate (IT), fumarate (FU), trans-glutaconate (GL), and trans-β-hydromuconate (HM) diesters [64].

As shown in Table 20.2, high molecular weights (Mw of 9350 15,900 g/mol) were achieved for the UPs in which fumarate or glutaconate diesters were added in 5 or 10 mol.% ratios against diethyl adipate. While the other UPs obtained from isosorbide, diethyl adipate and diethyl itaconate or diethyl hydromuconate showed low molecular weights (Mw of 630 680 and 2000 2200 g/mol, respectively). To achieve fully bio-based UPs, Goerz et al. [65] used isosorbide to condensate with IA and SA (Fig. 20.19). They combined an equivalent mixture of dicarboxylic acids with glycol in toluene, adding 0.5 mol% sulfuric acid as a catalyst and 0.25 mol.% phenothiazine as an inhibitor. Nevertheless, a normal polycondensation route which was carried out in a Dean stark apparatus at 140 C for 48 hours, leading to UPs with extremely low yields below 10% and with low molecular

20.3 ISOSORBIDE-BASED UNSATURATED POLYESTERS

531

Table 20.2 Characterization of Unsaturated Homopolyesters and Copolyesters Derived From Isosorbide [64] Molar Ratio Code

Monomers

Feed

Product (AD:UDa)

Mw (g/mol)

PDI

Yield (%)b

DPc

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

IS:AD IS:IT IS:AD:IT IS:AD:IT IS:FU IS:AD:FU IS:AD:FU IS:AD:FUe IS:GL IS:AD:GL IS:AD:GL IS:AD:GLe IS:HM IS:AD:HM IS:AD:HM

1:1 1:1 1:0.9:0.1 1:0.95:0.05 1:1 1:0.9:0.1 1:0.95:0.05 1:0.95:0.05 1:1 1:0.9:0.1 1:0.95:0.05 1:0.95:0.05 1:1 1:0.9:0.1 1:0.95:0.05

n.d. n.d. 0.97:0.03 0.97:0.03 n.d. n.d. 0.96:0.04 0.92:0.08 n.d. 0.90:0.10 0.93:0.07 0.97:0.03 n.d. 0.9:0.1 0.97:0.03

27,900 600d 630d 680d 350d 11,400 15,900 10,500 1100 9350 14,800 10,350 420d 2200 2200

1.5 1.5 2.4 2.4 1.1 2.3 1.5 1.9 1.1 2.5 1.5 1.9 1.1 1.7 1.68

76 n.d. 37 34 n.d. 70 70 75 58 56 62 70 n.d. 51 64

108 2.3 2.3 2.3 1.4 44 62 41 4.3 36 58 40 1.7 8.6 7.8

a

Molar ratio of adipate:unsaturated diester (AD:UD) in the product as determined by 1H NMR. Determined before precipitation step. c Degree of polymerization estimated by considering the molecular weight of the isosorbide adipate repetitive unit. d Peak molecular weight. e Reactions in the presence of hydroquinone 1% with N2 bubbled for 2 hours before heating. n.d. not determined. b

weights (500 800 g/mol). Using microwave irradiation for 4 hours rather than heating, the yield and molecular weight were somewhat increased to 17% and 1200 g/mol, respectively. But neither were further increased with increasing catalyst content and prolonging reaction time. In addition to the low reactivity of isosorbide, the most reasonable explanation might be the poor solubility of these UPs in toluene, as a result, the oligomers were able to precipitate in the reaction process. Consequently, the UPs were cross-linked with DMI through an inductive radical reaction triggered by the diazo-based initiator VA-65. The Tg of the cured thermosetting resin increased significantly to 74 C, and an elastic memory shape function was obtained. In contrast to the SMPs synthetized by Guo et al. [34], the materials was proven to be highly brittle due to the rigid isosorbide. Subsequently, Goerz et al. [66] utilized bio-derived dinitrones based on isosorbide to cross-link the UPs with a 1,3-dipolar cycloaddition. Styrene is a diluent generally used in UPs. Thus Sadler et al. [54,67] synthesized a series of isosorbide-containing UPs from isosorbide, EG, maleic anhydride, saturated dibasic acid (hexane diacid, suberic acid, and sebacic acid) as well as phthalic acid (Fig. 20.20), and systematically investigated the properties of their blends with styrene. The investigation indicated that the resulting UPs were miscible with styrene when the isosorbide monomer content was low, forming low

HO

O

O

O

HO

+ O

HO

+

OH O

OH

OH O

Toluene H2 SO4

140ºC MW

O O

O

O n

O

O

O

O

O

O

m

O

O

V-65

O O

O O

Cross-linked polymers

FIGURE 20.19 Synthesis route of obtaining isosorbide-based UPs [65]. (A)

O

OH

O

+

O

+

O

+

O

HO

OH

O

O

HO

O

O

O

O

O

O O

O

O

O

O

O

O

n

Linker

(B)

End group

O O

O

O

HO

O O

O

O

O

CH 2 x

O O

O

O

O

O

O

x H2 C

O

O

O

O

x

O

H 2C

n

Repeat unit FIGURE 20.20 Typical structure of unsaturated polyester prepared from rigid isosorbide [68,69].

O

O

O

O

O O

O

O

O

OH O

20.4 FURAN-BASED UNSATURATED POLYESTERS

533

viscosity liquid resins suitable for composite processing. Increasing isosorbide content would reduce the solubility in styrene and increase the viscosity of the blends, though the Tg and storage modulus could be increased. This meant that increasing the isosorbide content can be an invaluable tool for developing novel high-performance UPs using styrene as a reactive diluent. To overcome the solubility issues of isosorbide-containing UPs and reactive diluents, Xu et al. [68] synthesized isosorbide-based dimethacrylate to be used as a reactive diluent together with isosorbide-containing UPs from isosorbide, 1,4-butanediol, succinic anhydride, and maleic anhydride. Isosorbide-based dimethacrylate presented excellent miscibility with isosorbide-containing UPs; the viscosity of mixtures was largely dependent on the isosorbide content of the UPs. After thermal cross-linking, the UPs showed Tg around 57 C 84 C, thermal stabilities of up to 250 C, and storage moduli varying between 0.5 and 3 GPa.

20.4 FURAN-BASED UNSATURATED POLYESTERS Furan chemicals can be produced from renewable bioresources. 5-Hydroxymethylfurfural (5-HMF), converted from carbohydrates such as glucose, fructose, sucrose, inulin, and cellulose is an important biomass-based platform compound. As shown in Fig. 20.21, 5-HMF was converted to 2,5-furandicarboxylic acid (FDCA) with a yield of up to 95% in the presence of the oxidoreductase of FDCA [69]. In recent years, furan-based polymers have attracted a great deal of attention due to the similar rigidity of the furan ring to that of the benzene ring and the better properties obtained from the furan ring than those obtained from the benzene ring. As an example, poly(ethylene 2,5-furandicarboxylate) (PEF) exhibited a higher Tg and better barrier properties to oxygen, water, and carbon dioxide permeability [70 72]. To achieve desirable properties, furan-based UPs were also reported [73 75]. Sousa et al. [73] synthesized furan-based UPs from FDCA, SA, fumaric acid (FA), and 1,3-propanediol by melt polycondensation with no catalyst. The obtained UPs with no purification were cross-linked using 2-hydroxyethylmethacrylate (HEMA) as a reactive diluent and benzoyl peroxide (BPO) as a thermal initiator (Fig. 20.22). The cross-linked UPs were thermally stable up to 230 C, and exhibited Tg of between 87 C and 104 C, and storage moduli at 25 C ranging from 390 to 614 MPa. In a previous work [74], FDCA was copolymerized with other bioresources, namely IA, SA, and 1,3-propanediol (PD) to produce fully bio-based UPs (Fig. 20.23). The UPs were cured together with a bio-based, nonvolatile reactive diluent, that is, guaiacol-methyl acrylate. O OH

HO HO

O

FIGURE 20.21 Synthetic route of FDCA [69].

O

O

HO

O

OH

HO

O

O HMF

O

HMFO

HFCA

FDCA

534

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

O

O

O O

OH

HO

+

O OH

HO

+

OH

HO

O

HO

OH

O

FA

FDCA

+

PD

SA N 2 Flux

O O

O

O

O O

O

O

O O

O

O

O

O

O O

BPO

HEMA HO

Cross-linked polymers FIGURE 20.22 Synthesis of unsaturated polyester from FDCA and its network structure of cross-linking products with HEMA [73]. Itaconic acid

FDCA O

O

O

Bio-based UPs

O O

O

O

n

O m

Greener

O

O

O O

O Cross-link point

O

n

O

Petroleum-based UPs O

O m

Rigid structure

FIGURE 20.23 Chemical structures of fully bio-based UPs from 2,5-furandicarboxylic acid and petroleum-based UPs from m-benzoic acid [74].

20.5 VEGETABLE OIL-BASED UNSATURATED POLYESTERS

O

O O

O

O

NHC

O

O O

O

HTP

O Diels-Alder reaction

FDMA

O

O O

535

O

O

O O

O O

O

H2 O

O O

O O

FIGURE 20.24 Proposed mechanism for self-crosslinking of PFMDA through the Diels Alder reaction and the inertness of the saturated polyesters toward such cross-linking [75].

The cross-linked UPs exhibited Td5% of 330 C, Tg of between 73.5 C and 141.7 C, flexural strengths of B122.8 MPa, and flexural moduli of 3521 MPa. The research indicates that these furan-based UPs have the potential to replace petroleum-based UPs. The double bond of furan can undergo a Diels Alder reaction to form a polyester with selfcuring properties. Chen et al., reported a novel furan-based UP synthesized via the proton-transfer polymerization of furfural dimethacrylate, which demonstrated a unique self-curing ability (without any additional curing agents) to form a cross-linked material (Fig. 20.24) [75]. These were able to undergo Diels Alder reaction based self-curing via three routes to produce more robust polyester materials including thermally induced, microwave-induced, and Lewis acid-catalyzed cross-linking. The AlCl3-catalyzed Diels Alder reaction produced the material with the highest degree of crosslinking and char yield at 650 C (31.4 wt.%) during thermal degradation by TGA.

20.5 VEGETABLE OIL-BASED UNSATURATED POLYESTERS Vegetable oil is one of the most important renewable raw materials in the chemical industry due to its abundant resources, low cost as well as environmental benefits. Due to their special structures, vegetable oils are inherently degradable. As an alternative to traditional petrochemical raw materials, vegetable oils have been used by a multitude of researchers to prepare resins and polymers. Since the beginning of the 20th century, researchers have successfully developed multifarious plant-based thermoplastics and thermosetting resins, various elastomers. Products with high vegetable oil contents show physical and chemical properties comparable to those of petroleum products. Vegetable oil-based polymers gradually replace petroleum-based polymers in light-curing coatings, packaging materials, medical equipment, daily necessities, building materials, molds, automobiles, agricultural equipment, insulation, marine equipment, industrial liners, and many other fields. Vegetable oils are a large class of natural organic compounds and are defined as a mixture of triglycerides of fatty acids [76]. The main components (around 95%) of vegetable oils are triglycerides of fatty acids (Fig. 20.25), while minor and exceedingly complex components are nonglycerin including phospholipids, linoleic acid, fatty alcohol, fatty acids, aliphatic hydrocarbons, pigment fat-soluble vitamins, and others.

536

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

O R2 O O R1

O O

R3 O

FIGURE 20.25 Chemical structure of triglycerides of fatty acids.

Table 20.3 Formulas and Structure of the Most Important Fatty Acids [77] Fatty Acid

Formula

Caprylic

C8H16O2

Carpic

C10H20O2

Lauric

C12H24O2

Myristic

C14H28O2

Palmitic

C16H32O2

Palmitoleic

C16H30O2

Srearic

C18H36O2

Oleic

C18H34O2

Linoleic

C18H32O2

Linolenic

C18H30O2

α-Eleostearic

C18H30O2

Ricinoleic

C18H34O3

Vernolic

C18H32O3

Structure

Natural vegetable oil contains more than 800 kinds of fatty acids, and there over 500 kinds of fatty acids have been identified. Table 20.3 shows the structures of the most universal fatty acids found in vegetable oils [77]. Vegetable oils are distinguished from the constitution of fatty acids. Information regarding the most common vegetable oils is provided in Table 20.4. Unsaturation, or the content of carbon carbon double bonding is usually defined by the iodine value (IV) [100]. IV is the amount of iodine absorbed per gram of sample and it is typically expressed in units of centigrams of iodine per gram of sample. A high IV indicates a large amount of carbon carbon double bond content or unsaturation. Vegetable oils are classified into three types according to their IV, that is, nondrying oils (,90), semidrying oils (90 130), and drying oils ( . 130) [78,79]. The (C x:y) nomenclature in Table 20.4 illustrates the fatty acid composition, where x corresponds the number of carbon atoms in the fatty acid and y corresponds the number of carbon carbon double bonds [80].

20.5 VEGETABLE OIL-BASED UNSATURATED POLYESTERS

537

Table 20.4 Degree of Unsaturation and Composition of Common Vegetable Oils [81] Fatty Acid Composition (%) Vegetable Oil

Double Bonds

Iodine Value mg/100 g

C 16:0

C 18:0

C 18:1

C 18:2

C 18:3

Soybean Palm Castor Corn Linseed Olive Sunflower Canola Cottonseed Peanut Rapeseed

4.6 1.7 3.0 4.5 6.6 2.8 4.7 3.9 3.9 3.4 3.8

123 139 50 55 81 91 118 128 . 177 76 88 125 140 100 115 98 118 84 100 100 115

10.6 44.4 2.0 10.9 6.0 9.0 7.0 4.0 21.6 11.1 3.8

4.0 4.1 1.0 2.0 4.0 2.7 4.5 1.8 2.6 2.4 1.2

23.3 39.3 7.0 25.4 22.0 80.3 18.7 60.9 18.6 46.7 18.5

53.7 10.0 3.0 59.6 16.0 6.3 67.5 21.0 54.4 32.0 14.5

7.6 0.4 0.5 1.2 52.0 0.7 0.8 8.8 0.7 11.0

Vegetable oils contain carbon carbon double bonds, aliphatic chains, and ester bonds, which makes them very similar to synthesized UPs. With their carbon carbon double bonds, they could be polymerized into cross-linked UPs with no modification. To get a higher reactivity, modifications should be performed to introduce other kinds of carbon carbon double bonds.

20.5.1 UNMODIFIED VEGETABLE OILS AS UNSATURATED POLYESTERS For drying oils such as linseed oil, tung oil, poppy seed oil, perilla oil, walnut oil, and others, with IVs of .130, they can be easily cross-linked to a tough and solid film by the oxygen in air. Drying oils have been widely used in oil paint and some varnishes. Here emphasis has been placed on a semidrying oil, that is, soybean oil. Soybean oil is one of the most abundant and low-price vegetable oils with a global production of around 57.58 million metric tons in 2018 [82]. From the structure of the soybean oil illustrated in Fig. 20.26, there are mainly two fatty acids in it, namely oleic acid and linoleic acid. The average number of carbon carbon double bonds in soybean oil is 4.6 per triglyceride, making it an ideal prepolymer for polymerization. However, due to the relatively high molecular weight of the soybean oil multichain structure and the nonconjugated double bonds in the fatty acid chain, the reactivity is fairly low for free radical polymerization [83]. However, the carbon carbon double bonds of soybean oil allow for cationic polymerization to be carried out more easily. Commonly used cationic initiators are generally divided into two categories, namely protonic acid and Lewis acid, among which the most commonly used initiator for initiating the cationic polymerization of vegetable oils is boron trifluoride diethyl ether (BFE) (Fig. 20.27) [84]. In most cases, the product obtained by homopolymerization of natural vegetable oils generally has poor performance and no practical application value. Therefore it is indispensable to introduce olefin comonomers such as styrene, cyclopentadiene, dicyclopentadiene, divinylbenzene, and norbornadiene to copolymerize with soybean oil to achieve enhanced thermal and mechanical properties of a material.

538

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

O O O O O O

FIGURE 20.26 Chemical structure of soybean oil [83].

Initiation: BF3• OEt2 + H2 O

BF3• OH 2 + Et2 O

BF3• OH 2 + RHC CHR

R RH2 C C BF3OH H

Propagation: R RH2 C C BF3OH H

+ n RHC CHR

R R R R C C C C BF3 OH n H H H H

FIGURE 20.27 Initiation and propagation of cationic polymerization of BFE [83].

Research in this area has mainly been carried out by Larock’s research group [85 91]. Initially, Larcok et al. [84,86] applied soybean oil and divinylbenzene as raw materials, and BFE as an initiator to initiate cationic copolymerization. It was found that as the introduced amount of divinylbenzene increases, the resulting thermosetting materials transformed from a soft rubber to a hard plastic. However, due to the poor compatibility between the initiator and the soybean oil, the copolymerization system led to a heterogeneous reaction at the initial stage of the reaction, and the homopolymer of the divinylbenzene showed a phase separation phenomenon. To address this issue, they improved the initiator (BFE) through the addition of Norway fish oil ethyl ester, Soygold-1100, Soygold-2000, or THF, so that the soybean oil and divinylbenzene could be freely homogenized. The obtained bulk polymers exhibited higher conversion of oils to cross-linked polymers than those utilizing BFE alone [85]. In order to further reduce the effect of poor compatibility on the properties of the soybean oil divinylbenzene system, they used styrene as the main component of the copolymer and a small amount of divinylbenzene as a cross-linking agent to regulate the cross-link density [86,87,89]; the reaction mechanism is shown in Fig. 20.28 [92]. The results indicated that the performance of the obtained thermoset materials differed with different ratios of styrene and divinylbenzene. The resulting thermoset materials possessed Tg from 0 C to

20.5 VEGETABLE OIL-BASED UNSATURATED POLYESTERS

539

COO COO

+

+

COO Triglyceride

Styrene

Divinybeneze

BF3 OEt2

O O

O

O

O

O

FIGURE 20.28 Cationic polymerization of soybean oils with styrene and divinylbenzene [92].

105 C and Young’s moduli from 6 to 200 MPa. These properties are closely related to the crosslink density of the materials and the rigid structure of the polymers. In addition, they also used cationic copolymerization with soybean oil, divinylbenzene and styrene, and dicyclopentadiene or norbornadiene, and obtained a series of polymer materials with shape memory function. The materials were subjected to various deformation treatments above their Tg, and then they were placed in an environment below their Tg to fix the shapes. When the temperature rose above the Tg of the materials, the shapes of the materials were able to return to their original states [90].

20.5.2 MODIFIED VEGETABLE OILS AS UNSATURATED POLYESTERS 20.5.2.1 Methacrylated vegetable oils The reactivity of the double bond on the long chain of vegetable oil is low, and direct curing does not achieve sufficient cross-linking. The introduction of more active carbon carbon double bonds is an effective way to improve the performance of vegetable oil based materials. The most important application is the preparation of vegetable oil based acrylate resin. The main synthetic methods are (1) ring-opening reaction of epoxidized vegetable oil with methacrylic acid or hydroxyethyl methacrylate (Fig. 20.29A) [93]; (2) condensation reaction of acryloyl chloride with hydroxyl-containing vegetable oil [94] (Fig. 20.29B); (3) one-step condensation reaction of unsaturated vegetable oil with acrylic acid catalyzed by boron trifluoride ether [95] (Fig. 20.29C).

540

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

O

(A)

O

O OH

O

O

O

OH

OH

HO HO

O O

HO

O

O

O

O

O O

O

O

O

O

O

O

R1

R2

R1

O

R2

R1

R2

Ring-opening reaction of epoxidized vegetable oil O

(B)

HO

OH

O OH O Cl O O

O

O

O

O O

O Acrylation of castor oil monoglyceride

(C) X

M X

X

O

+

O

HO

OH

H C C

O

X

+ X

M

OH H

O

O X

H C C

Acrylation of vegetable oil [93].

H C C O O

Catalyzed by lewis acid

FIGURE 20.29

O

H C C O

20.5 VEGETABLE OIL-BASED UNSATURATED POLYESTERS

541

The ring-opening reaction of epoxidized vegetable oil with methacrylic acid is the most commonly applied method. These methods can give vegetable oils with more active double bonds, but the lower functionality of vegetable oils as well as the natural flexibility of their long aliphatic chains are not improved. Therefore it is virtually impossible to obtain ideal materials from acrylate vegetable oils by self-polymerization. Their performance needs to be improved by copolymerization with other monomers or by physical blend modification. At present, materials from the copolymerization of acrylated vegetable oils (such as linseed oil, castor oil, rapeseed oil, soybean oil, etc.) with reactive monomers have been widely reported [96]. The main objective is to enhance the thermal and mechanical properties by increasing the crosslink density and rigidity of vegetable oil based cross-linked networks with reactive monomers. Among the reported acrylated vegetable oils, AESO is the most commercially available and exploited one. AESO is obtained by a ring-opening reaction of acrylic acid with epoxidized soybean oil (Fig. 20.30). ASEO has been widely used in surface coatings, and in recent years many

O

Glyceol center O O

O O O

(A)

Formic acid H 2O 2

Epoxy group O

O

O

O

O

O

O

O

O O

Acrylic Acid

(B) O

O Acrylate O

O

O

O

O OH

O

O

O O

OH O

O OH Hydroxyl group O

FIGURE 20.30 Synthetic route of obtaining AESO [96].

Residual group O

542

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

Table 20.5 The Properties of the Reported AESO Systems Enhanced With Rigid Comonomers or Cross-Linkers

Code 1 2 3 4 5 6 7 8

Comonomers or Cross-Linkers Styrene [100] MAESO, styrene [101] Acrylated sucrose, hyperbranched acrylate [102] GASA, triallyl isocyanurate [103] Rosin-based monomer [104] 2,5-Furan diacrylate [105] TAHAs [106] Itaconic acid-based UPs [48]

Tg ( C)

Tensile Strength (MPa)

Elongation at Break (%)

1.6 43.6 115 130 20 58

9 23 41 44 2 27

7 27

21 71

4 26

5.8 16.5

40 1500

35 86

1.1 10.4 0.64 14 1.8 8.4 2.9 16.6

4.8 4.9 6.6 4.8

169 1205

39.2 78.1

Storage Modulus (MPa) 1922 2200

13.9 11.2 7.8 12.4

99 589

MAESO, maleinated acrylated epoxidized soybean oil; GASA, gallic acid based cross-linking agent; TAHAs, tannic acid based hyperbranched methacrylates.

researchers have used AESO to prepare high modulus polymers and composites [97 99]. Table 20.5 shows the properties of the reported AESO systems enhanced with rigid comonomers or cross-linkers. Fu et al. [100] synthesized AESO from soybean oil through the ESO-acrylic acid method and investigated the effect of the IV of ESO on the thermal and mechanical properties of AESO styrene thermosetting resins. They found that the impact strength and tensile strength declined with increases in the IV of ESOs. The lower IV of ESO led to a higher content of acrylic double bonds in AESO, corresponding to the more perfect network of the final thermoset resulting in increased mechanical properties. When the IV of the ESO was lower than 30 g iodine per 100 g oil, the AESO styrene thermosetting resins showed impact strengths .15 kJ/m2, tensile strengths of .23 MPa, and good thermal stability with an initial degradation temperature above 350 C. The hydroxyl groups formed during the preparation of AESO and the residual epoxy groups after the preparation of AESO can react with acids, amines, acid anhydrides, isocyanates, and others In order to further improve the performance of AESO styrene thermosetting resin, Wool et al. [97,101] made use of the reaction of maleic anhydride with the hydroxyl or epoxy groups in AESO to facilitate more carbon carbon double bonds, and finally a new type of AESO maleic acid half ester (MAESO) was obtained. The further modification of AESO with functional groups such as maleic anhydride rendered acid groups and added more unsaturation to the molecules (Fig. 20.31, route A); the carboxylic acid groups on the half-esters could react with residual epoxy groups or hydroxyl groups on the triglycerides to oligomerize triglycerides (Fig. 20.31, route B). Then it was copolymerized with styrene to obtain AESO/MAESO styrene thermosetting resin, resulting in a higher cross-link density than that of the AESO styrene thermosetting resin due to the presence of more double bonds. Therefore the storage modulus at room temperature, Tg, and tensile strength of the material were further improved. Dynamic mechanical analysis showed

20.5 VEGETABLE OIL-BASED UNSATURATED POLYESTERS

543

OH O O O

O

O

O O O

OH O

O

O

O O O

O O

(A)

O OH

OH

O O

O

O

OH O O

O

O O

OH O

OH O

O

(B)

O O

OH O

O

O

O O O

OH O

O OH

O OH OH

O O O

O

O

O

OH

O

O

FIGURE 20.31 Synthesis of maleinated acrylated epoxidized soybean oil. (A) Maleinization reaction and (B) oligomerization reaction [102].

storage moduli for these polymers ranging from 1.9 to 2.2 GPa at room temperature, and Tg in the range of 100 C 115 C. For the sake of developing AESO-based thermosets with higher bio-based contents as well as enhanced performance, a series of bio-based cross-linkers or reactive monomers have been reported in recent years. Chen et al. [102] synthesized acrylated sucrose (AS) to improve the properties of UV-curable coatings based on a commercial AESO (Ebecryl 860) together with commercial hyperbranched acrylates (HBAs). The HBAs-modified AESO-based coatings showed enhanced coating mechanical properties, Tg, and tensile moduli while decreased cross-link density and elongation at break. Incorporating AS together with HBAs, the toughness including impact strength and elongation at break of the coatings increased while the thermal stability and water resistivity decreased. The improved toughness was from the soft and hard microphase separation of AS in the coatings. These indicate that AESO-based coatings modified with AS and HBAs have balanced coating performances with relatively high bio-based contents of B50 wt.%. In a previous work [103], a tetrafunctional monomer was synthesized from bio-derived gallic acid and copolymerized

544

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

with AESO under UV radiation. The samples possessed high bio-based contents of 88 94 wt.% as well as high tensile moduli of up to 744 MPa, tensile strengths of B27 MPa, and Td5% of B293 C. By introducing gallic acid based monomer, the Tg was increased from 21 C for neat AESO to B71 C for the gallic acid based monomer-modified AESO, and the coating properties including pencil hardness, friction resistance, and adhesion were all significantly improved. Similarly, comonomers from rosin [104], 2,5-furan dimethanol [105], and tannic acid [106] were also explored to enhance the properties of AESO-based materials. Besides these small reactive molecules, bio-based prepolymers from IA were also utilized to improve the properties of AESO-based coatings [30]. The bio-based contents of the coatings were between 84.2 and 88.2 wt.% higher than neat AESO. The thermally cured coatings showed high thermal and mechanical properties with tensile strengths of B16.6 MPa, tensile moduli of B510.4 MPa, Tg of B78.1 C, as well as excellent coating performances including pencil hardness (B2 hours), water absorption (, 0.9%), adhesion (5B, the best), solvent resistance (MEK double rub .250), flexibility (0T, the best). In addition, in order to address the shortcomings of the low rigidity of vegetable oil coatings, there are also several reports on introducing inorganic nanomaterials into acrylate vegetable oil such as TiO2. Dı´ez-Pascual et al. [107] synthesized AESO from epoxidized linseed oil, crosslinked it with an acrylic monomer, reinforced it with anatase TiO2 nanoparticles, and then subjected it to UV irradiation to produce environment-friendly antimicrobial nanocomposite coatings. The nanoparticles did indeed enhance the resin barrier performance by reducing its water uptake, and oxygen and water permeability, while they increased the decomposition onset temperature, resulting in highly thermally stable coatings. In addition, the incorporation of TiO2 was beneficial for improving the tribological properties of the bioresin. Excellent mechanical properties were also attained, including storage and Young’s moduli, indentation and pencil hardness, Tg, and impact resistance. The Tg ranged from 33 C to 41 C, the water absorption was approximately 33% at a 7.5 wt.% loading, the elastic modulus was up to B1 GPa, and the hardness was around 75 MPa.

20.5.2.2 Vegetable oil maleates In addition to the acrylation of vegetable oil, maleic anhydride was also utilized to introduce relatively high reactive carbon carbon double bonds to vegetable oil. Wool et al. [108] introduced maleic anhydride type double bonds into soybean oil by the transesterification of soybean oil with glycerol to achieve soybean oil monoglyceride (SOMG), followed by the esterification of SOMG with maleic anhydride to obtain SOMG maleate half ester. A SOMG maleate half ester styrene thermosetting resin was successfully obtained by free radical copolymerization with styrene via bulk polymerization and emulsion polymerization. The reaction scheme is shown in Fig. 20.32. With 35 wt.% styrene, the obtained SOMG maleate half ester styrene thermosetting resin exhibited a Tg of 133 C, storage modulus of 0.94 GPa at 35 C, tensile strength of 29.36 Mpa, and a tensile modulus of 0.84 Gpa [109]. In order to obtain better performance in the SOMG maleate half ester styrene thermosetting resin, they used neopentyl glycol (NPG) and bisphenol-A (BPA) mixed with SOMG to react with maleic anhydride to get SOMG/NPG maleate half esters and SOMG/BPA maleate half esters, respectively. With the same content of styrene (35 wt.%), the SOMG/NPG maleate half ester styrene thermosetting resins showed Tg of 145 C, storage moduli of 2 Gpa at 35 C, tensile strengths of 15.65 Mpa, and tensile moduli of 1.5 Gpa, and the

20.5 VEGETABLE OIL-BASED UNSATURATED POLYESTERS

COO

HO

545

COO 230ºC–240ºC

COO

+

HO

HO Catalyst

COO

HO

HO

Soybean oil

Soybean oil monoglyceride (SOMG)

+ HOOC

C H

HOOC

C H

COO H C O

T<100°C

H C O

O

O

O

Maleic anhydride

SOMG meleate half esters

O O C O

O C O CH 2 CH

CH

C O CH COOH

CH

CH CH2 CH COOH

FIGURE 20.32 Synthesis and polymerization of soybean oil monoglyceride maleates [108].

SOMG/BPA maleate half ester styrene thermosetting resins showed Tg of 131 C and storage moduli of 1.34 Gpa at 35 C. They also investigated castor oil maleate half ester systems [110]. Due to the intrinsic hydroxyl groups in castor oil’s structure, more maleic anhydride could be introduced into the castor oil systems, and as a result, the castor oil based maleate half ester systems showed better properties than those of soybean oil-based systems. Mosiewicki et al. [110] prepared linseed oil monoglyceride (LOMG) and LOMG maleate half ester with the same method used for SOMG and SOMG maleate. They investigated the effect of styrene content (20, 40, and 60 wt.%) on the thermal and mechanical properties of LOMG maleate half ester styrene thermosets. Thermosets with 40 wt.% of styrene showed the better mechanical and fracture behavior, while the properties (Tg, tensile strength, modulus) were lower than those mentioned for soybean oil systems.

546

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

O O C

–(CH2O) n– EtAlCl 2 0ºC,2h

O C O O C O

O O C O C O O C O

CH2 OH

O O

O O C

O

, R

O C O O C O

Network polymer

CH 2 O CO CH CH COOH

FIGURE 20.33 Synthesis and polymerization of maleate half ester based plant oil triglycerides [111].

Eren et al. [111] developed another method to introduce maleic anhydride. They introduced 1.3 and 1.7 hydroxymethyl groups into sunflower oil and soybean oil by reacting with paraformaldehyde using a Lewis acid catalyst, respectively. Hydroxymethylated sunflower oil (HMSU) and hydroxymethylated soybean oil (HMSO) were further utilized to react with maleic anhydride (Fig. 20.33). The Tg of the free radical polymerized HMSU styrene and HMSO styrene systems were 30 C and 40 C, respectively.

20.6 BIO-BASED REACTIVE DILUENTS FOR UNSATURATED POLYESTERS Styrene is the most generally used diluent due to its low cost, high reactivity, low viscosity, and the excellent thermomechanical properties of the cross-linked resins. Nevertheless, its volatility and strong unpleasant odor cannot be ignored from an ecological and human health perspective [112,113]. Styrene was classified as a potential carcinogen by the National Toxicology Program (NTP) in 2011. It has been demonstrated that residual styrene can continue to emit during the service time of materials. Volatile styrene emissions jeopardize the environment as well as the people who manufacture and handle styrene-containing materials. The styrene emission upper limit as regulated by government is gradually reduced, and compliance with regulations becomes increasingly difficult. Thus people need to invest much energy in finding reactive diluents from economic, environment-friendly, and renewable resources. Plenty of bio-based diluents were synthesized to replace styrene [24,114]; the information of the reported bio-based diluents is illustrated in Table 20.6.

20.6 BIO-BASED REACTIVE DILUENTS FOR UNSATURATED POLYESTERS

547

Table 20.6 Information on the Reported Bio-based Reactive Diluents Bio-based Precursors

Reactive Diluents

Structure

Viscosity (cP) at 25 C

Styrene [115]

B0.8

Furfuryl alcohol

Furfuryl methacrylate [115]

460

Lauric acid

Methacrylate lauric acid [115]

72 6 2

Furoic acid

234 6 25

Guaiacol

Furoic acid glycidyl methacrylate (FA GM) [115] Methacrylated guaiacol (MG) [116]

Eugenol

Methacrylated eugenol (ME) [116]

27.9 6 1.3

Isoborneol

Isobornyl methacrylate (IBOMA) [118]

Lauryl alcohol

Lauryl methacrylate (LMA) [118]

Butanediol Phenol

Butanediol dimethacrylate (BDDMA) [118] Phenyl methacrylate [117]

2.4 6 0.04

Guaiacol

2-Methoxyphenyl methacrylate [117]

14.5 6 0.1

4-Propylguaiacol

4-Propyl-2-methoxyphenyl methacrylate [117] Dimethyl itaconate (DMI) [120]

29.6 6 0.2

Methanol/itaconic acid Butanol/itaconic acid Ethanol/itaconic acid Isopropanol/itaconic acid Soybean oil

17.1 6 1.2

4.32 6 0.08

Di-n-butyl itaconate (DBI) [120] Diethyl itaconate (DEI) [120] Diisopropyl itaconate (DiPI) [120] Acrylated epoxidized soybean oil [119]

Levulinic acid

Vinyl levulinate [121]

2.02 6 0.01

Fatty acid

Methacrylate fatty acid [122]

56 64

9,10-Dibromo stearic acid Ethyl cinnamate

9,10-Dibromo stearyl glycidyl methacrylate [123] Ethyl cinnamate [124]

834

Isosorbide

Isosorbide-methacrylate [68]

110

548

CHAPTER 20 BIO-BASED UNSATURATED POLYESTERS

Sadler et al. [115] synthesized furoic acid glycidyl methacrylate (FA GM) from furoic acid and glycidyl methacrylate as a reactive diluent to obtain a thermosetting resin with excellent mechanical properties. However, FA GM presented a high viscosity at 25 C, leading to resin blends with high viscosities. Stanzione et al. [116] employed three model lignin compounds, namely guaiacol, vanillin, and eugenol to esterify with methacrylic anhydride to obtain methacrylated guaiacol (MG), methacrylated vanillin (MV), and methacrylated eugenol (ME). Among these, MG and ME presented low viscosities at 25 C of 17 and 28 cP, respectively. Although their diluent ability is not as good as styrene, resins containing 50 wt.% of MG and ME possessed viscosities that satisfy molding applications. Isothermal weight loss measurement by thermogravimetric analysis (TGA) at 30 C demonstrated that MG and ME exhibited low volatilities of 1.05 wt.% loss and 0.96 wt.% loss in 18 hours, respectively, much lower than that of styrene (93.7 wt.% loss in 3 hours). The cured resin blends of standard vinyl ester resin VE828 with 50 wt.% MG and ME showed Tg of 127 C and 153 C, respectively, comparable to styrene systems. These indicate that MG and ME are sustainable and environment-friendly reactive diluents and have the potential to be used as alternatives to styrene. They also demonstrated that the purity of lignin-based diluents could significantly affect the viscosity of resin blends and the thermodynamic properties of cured resins [117]. Cousinet et al. [118] evaluated three bio-based methacrylates, namely butanediol dimethacrylate (BDDMA), isobornyl methacrylate (IBOMA), and lauryl methacrylate (LMA) as potential substitutes for styrene. IBOMA manifested a relatively high volatility of 12 wt.% loss after 15 hours at 30 C by isothermal weight loss measurement by TGA, and BDDMA and LMA presented low volatility with virtually no weight loss during the measurement. While, only BDDMA could dissolve the commercial UP from maleic anhydride and a mixture of diols including propylene glycol (PG). However, cured UP/BDDMA possessed a lower impact strength, flexural strength, and strain at break than those of cured UP/styrene and UP/MMA. Wu et al. [119] used a nonvolatile AESO as a substitute for styrene to dilute a commercial UP (PG MA) from propylene glycol and maleic anhydride. The PG MA/AESO systems containing 60 and 50 wt.% of AESO could reach viscosities of 10 Pa s, which is suitable for hand lay-up applications when heating to 45 C and 58 C, respectively. Glass fiber composites based on PG MA/AESO resin blends exhibited comparable or even superior tensile and flexural properties. Their Tg could reach 104 C, which was high enough to be used as indoor and outdoor fiber reinforced UP composites. For novel bio-based UPs, there are special segments from bio-based building blocks. The miscibility of the reactive diluents became especially important. Researchers have found that the reactive diluents from the same building blocks as the UPs could resolve the miscibility problem. As previously mentioned, Xu et al. [68] utilized isosorbide-based dimethacrylate as a reactive diluent to address the solubility of isosorbide-containing UPs in common reactive diluents. Panic et al. [120] used dialkyl itaconates [DMI, diethyl itaconate (DEI), dibutyl itaconate (DBI), and diisopropyl itaconate (DiPI)] as reactive diluents for IA-based UPs. The IA-based diluents and UPs manifested good miscibilities and could copolymerize well. While satisfactory performance and diluent ability could not be achieved at the same time for these systems. Resin blends of UP with DMI and DEI presented good thermal and mechanical properties while their viscosities were high. Resin blends of UP with DBI and DiPI exhibited low viscosities while their thermal and mechanical properties were poor.

REFERENCES

549

20.7 CONCLUSION Current research progress in bio-based UPs and reactive diluents has been presented. Bio-based UPs from IA, isosorbide, furan compounds, and vegetable oils were discussed separately. Although IA has no rigid ring in its structure, its UPs show excellent thermal, mechanical, and coating properties. Isosorbide and furan compounds, especially 2,5-furandicarboxylic acid, were used to produce bio-based UPs with high performance due to their rigid structures. By combining with other bio-based monomers, fully bio-based UPs could also be synthesized. Vegetable oils as the natural UPs could be cross-linked by cationic polymerization, and modified vegetable oils with more-reactive carbon carbon double bonds such as acrylated vegetable oils and vegetable oil maleates were also synthesized and could be cross-linked via free radical polymerization. Due to their flexible aliphatic chains, rigid monomers or fillers were often utilized together with vegetable oil based UPs, and bio-based cross-linkers were exploited to increase the bio-based contents as well as the performance of thermosets. Despite the fact that considerable works were proceeded to develop bio-based reactive diluents, a suitable bio-based substitute for styrene has not yet been identified. In future, comparisons between bio-based UPRs and conventional petroleumbased UPRs, and their application in composites where UPRs are mainly utilized should be systematically investigated. New bio-based diluents possessing low viscosity, high reactivity, low volatility, and good miscibility with UPs should simultaneously be invented. Bio-based functional UPRs with unique properties such as recyclability, flame retardancy, shape memory, and selfhealing should also be developed.

ACKNOWLEDGMENTS The authors are grateful for the financial support from National Natural Science Foundation of China (Nos. 51773216, 51473180), Youth Innovation Promotion Association, CAS (No. 2018335), and Chinese MIIT Special Research Plan on Civil Aircraft (No. MJ-2015-H-G-103).

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