Catalyst-free self-healing fully bio-based vitrimers derived from tung oil: Strong mechanical properties, shape memory, and recyclability

Highlights

• TOTGE with terminal epoxy groups was prepared using tung oil.

• Catalyst-free self-healing vitrimers were prepared using TOTGE and citric acid.

• The vitrimers exhibited high properties due to the introduction of terminal epoxy groups.

• Vitrimers can complete network rearrangement with autocatalysis of −OH groups.

• The vitrimers exhibited self-healing, shape memory and reprocessing.

Abstract

Vegetable oil-based vitrimers are innovative renewable polymers that show promising applications. However, their applications are limited by their poor mechanical properties and the need for the use of a catalyst in their preparation. In this study, the renewable tung oil underwent the methyl esterification reaction, Diels-Alder reaction, and epoxidation reaction to obtain TOTGE with terminal epoxy groups. TOTGE was cured with citric acid (CA) to obtain catalyst-free, self-healing, high mechanical strength fully bio-based TOTGE–CA vitrimers. Due to the high reactivity of their terminal epoxy groups, the vitrimer networks have a high density of crosslinks, and thus show good thermal stability and mechanical properties. The curing agent CA introduces a large number of hydroxyl (−OH) groups into the vitrimers and these form abundant hydrogen (H) bonds. The −OH groups in the vitrimers act as a catalyst, allowing topological network rearrangement of the materials in the absence of an external catalyst. Due to their dynamic ester and H bonds, the vitrimers exhibit good self-healing, recyclability, and shape memory properties, making them promising candidates for use as repairable and recyclable adhesives.

Introduction

Vitrimers are a class of resins that can change their topology via dynamic bond exchange, resulting in their self-healing and reprocessable molding (Capelot et al., 2012b; Liu et al., 2017a; Pei et al., 2014). At high temperatures, vitrimers, like thermoplastic resins, are processable while at low temperatures they have a stable structure like traditional thermoset resins (Demongeot et al., 2017; Dhers et al., 2019; Snyder et al., 2018). It is because of the combination of their thermosetting and thermoplastic properties that vitrimers are deemed to be useful materials (Chen et al., 2019; Wang et al., 2018; Wu et al., 2017), and for this reason they have received increasing research attention.

To comply with sustainable development strategies, increasingly more bio-based vitrimers are being developed and widely used in coatings (Hao et al., 2019; Wang et al., 2020), adhesives (Wu et al., 2020a; Zhao and Abu-Omar, 2019), sensors (Niu et al., 2021; Xiong et al., 2020), and as materials for three-dimensional (3D) printing (Shi et al., 2017). Vegetable oil is a class of biomass raw material with a wide source and good biocompatibility. In the last decade, it has been widely used for the preparation of bio-based vitrimers (Auvergne et al., 2014; Gandini et al., 2016). Altuna et al. (Altuna et al., 2013) detailed the curing of citric acid (CA) and epoxidized soybean oil (ESO) to obtain fully bio-based vitrimers that exhibit self-healing and thermally-activated stress relaxation properties without the use of a catalyst. However, the glass transition temperatures (T_g) of the prepared vitrimers were only around 25 °C and their tensile strength values were less than 1 MPa. Wu and colleagues (Wu et al., 2020a) described the curing of ESO with glycyrrhetinic acid to prepare fully bio-based vitrimers that exhibit self-healing, shape memory, and degradable properties, however these materials have tensile strength values of only 3.10 MPa. Liu et al. (Liu et al., 2020b) reported the curing of ESO with 4,4′-dithiodiphenylamine to prepare dynamic disulfide bond-based self-healing vitrimers. However, since the epoxy groups in ESO are intramolecular epoxy groups, which have greater steric hindrance relative to the terminal epoxy groups, resulting in low crosslinking density, the tensile strength values of these materials were only 3.50 MPa. Even under high temperature, high pressure, and catalyst conditions, the low activity of the intramolecular epoxy groups leads to uneven curing, so that the cross-linked network cannot be completely cured (Altuna et al., 2011). This process results in the poor mechanical properties and low T_g of this type of vitrimer, thus limiting their application.

The introduction of terminal epoxy groups into polymers effectively reduces their steric hindrance and increases reactivity in these materials, resulting in a higher density of crosslinks, better mechanical properties, and higher T_g values of the resulting materials (Huang et al., 2013; Zhang et al., 2018). Huang et al. (Huang et al., 2013) demonstrated the introduction of terminal epoxy groups in dimer and acrylpimaric acids, which were then cured with methylnadic anhydride to prepare an epoxy resin with a T_g of 132 °C. Furthermore, Zhang et al. (Zhang et al., 2018) introduced terminal epoxy groups in castor oil derived sebacic acid and solidified it with ozone-treated kraft lignin to synthesize fully bio-based vitrimers. Due to the introduction of terminal epoxy groups, vitrimers with a high density of crosslinks were obtained after the starting materials were cured for only 3 h. The resulting bio-based vitrimers exhibited tensile strength values of up to 12.91 MPa and a T_g of 133 °C. These results confirm the possibility of preparing vegetable oil-based vitrimers that have good mechanical properties from vegetable oils that have terminal epoxy groups as raw materials. On the other hand, the toxicity, poor compatibility, and corrosiveness of the dynamic transesterification reaction (DTER) catalysts in vitrimers has presented a challenge in past research studies (Hao et al., 2020; He et al., 2019; Liu et al., 2019). The introduction of a large number of free −OH groups into such polymers is an effective way of solving this issue (Chakma and Konkolewicz, 2019; Han et al., 2018; Liu et al., 2020a). Natural product-derived CA has three carboxyl groups and one hydroxyl group, and the resulting polymers derived from the use of CA as a curing agent can undergo autocatalytic DTER due to the large number of −OH groups introduced into the materials by CA (Wang et al., 2020).

Tung oil (TO) is a renewable resource extracted from the seeds of the tung tree. It is widely used in coatings (Cao et al., 2015), epoxy resins (Huang et al., 2013) and toughening agents (Xiao et al., 2019) due to its easy modification and good flexibility. Similar to most vegetable oils, the fatty acids in tung oil are in the form of triglycerides and therefore tung oil is readily hydrolyzed. Tung oil contains about 80 % eleostearic acid (9, 11 13–octadecatrienoic acid), which has three conjugated double bonds and can easily undergo Diels–Alder reaction with dienophiles (Hilditch and Mendelowitz, 1951). Furthermore, Diels-Alder reaction adducts of tung oil with acrylic acid, maleic anhydride or fumaric acid are readily epoxidized to obtain epoxy resins with terminal epoxy groups.

In this work, tung maleic triacid (TMTA) derived from tung oil was epoxidized, and TO–based triglycidyl ester TOTGE with terminal epoxy groups was obtained. Catalyst-free self-healing fully bio-based TOTGE–CA vitrimers were synthesized by curing TOTGE with CA that contains −OH groups. Fourier-transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), tensile testing, and dynamic mechanical analysis (DMA) were used to study the effects that different ratios of carboxyl and epoxy groups and H–bonds have on TOTGE–CA vitrimers. Furthermore, stress relaxation experiments were used to show that the TOTGE–CA vitrimers can achieve effective DTER without the use of any additional catalyst. Thus, the vitrimers show self-healing, shape memory, and reprocessability properties. Moreover, they can be used as repairable and recyclable adhesives.