One-step synthesis of novel renewable multi-functional linseed oil-based acrylate prepolymers and its application in UV-curable coatings

https://doi.org/10.1016/j.porgcoat.2020.105820Get rights and content

Highlights

  • Novel bio-based prepolymers were prepared by one-step reaction from linseed oil.

  • Low viscosity and high reactive UV-curable bio-based prepolymers were synthesized.

  • The composite UV-curable films had good mechanical properties and thermal stability.

  • The linseed oil-based prepolymers have promising applications in UV-curable coatings.

Abstract

The use of renewable resources and their derivatives provides a "green" approach to synthesizing UV-curable raw materials, and it possesses many advantages, such as environmental protection and energy conservation. Linseed oil has become a research hotspot because of its high content of double bonds and biodegradability. In this study, boron trifluoride diethyl ether was used as a catalyst to successfully introduce acrylic acid into the double bond of linseed oil long chain. Therefore, the UV-curable acrylated linseed oil prepolymers (ALO) were prepared in one step. The characterizations of 1H NMR and FT-IR were employed to verify the successful preparation of ALO. The viscosity of ALO was measured via rheological test, and it was turned out to be 803 mPa·s, indicating the prepolymers had a good processing property. The synthesis mechanism of ALO catalyzed by boron trifluoride diethyl ether was also proposed. Furthermore, different kinds of UV-curable films were prepared by mixing ALO with polyurethane acrylate resin (PUA-2665), trimethylolpropane triacrylate (TMPTA), and photoinitiator (PI-1173). As for the cured films, their thermal stability was investigated by thermogravimetric analysis (TGA), the dynamical mechanical properties were probed via dynamic mechanical analysis (DMA), and the mechanical properties were measured by tensile test. It was found that increasing TMPTA content promoted crosslinking density, leading to the improvement in thermal stability, storage modulus, and tensile strength of the cured films. Besides, they also exhibited excellent hardness, strong adhesion, and outstanding water (solvents) resistance on wood surface. Therefore, this study provides a novel one-step approach for the synthesis of UV-curable materials from vegetable oils, and the as-prepared high-performance films have potential for broad applications.

Introduction

As we know, UV-curable coatings are composed of different components, generally including prepolymers, reactive diluents, and photoinitiators. Since appeared in the 1960s, UV-curable coatings are developed rapidly due to the advantages of “5E” (Efficient, Environmentally friendly, Energy saving, Enabling, and Economical) [[1], [2], [3]]. Above advantages can be explained specifically as follows. First, curing process can be accomplished within a few seconds, indicative of a high curing efficiency. Second, reactive diluents that can polymerize with prepolymers to form cured films are generally used to replace common volatile solvents, which decreases the use of organic solvents and exhibits environmentally-friendly feature. Third, UV-curable coatings can be cured at room temperature, avoiding the process of curing at high temperature, which helps to save energy. Also, UV-curable coatings can be applied onto heat-sensitive substrates, and thus they can be used as plastic coatings, paper coatings, etc. At last, compared with thermal curing, the equipment for UV curing has a more compact volume, and the curing process performs a higher efficiency and needs less energy consumption, which makes UV curing become economical [[4], [5], [6], [7]]. Nowadays, UV-curable coatings have penetrated into our daily life. However, with the improvement of living standards, the requirements for UV-curable coatings become stricter [8,9]. In the past, most of UV-curable raw materials were prepared from petrochemical products. One major drawback of them lies in the excessive dependence on the upstream supply chains, which leads to unstable supply of raw materials and high prices. Besides, petrochemical products are non-renewable, which causes more pressure on environmental supervision [[10], [11], [12]].

To avoid the drawbacks from petroleum-based products, the market calls for the development of UV-curable raw materials with the features of renewability, low cost, and wide-ranging sources. Biomass materials, such as proteins and vegetable oils, have attracted worldwide attention [[13], [14], [15]]. Among them, vegetable oils have been extensively studied due to their advantages of environmental friendliness, low price, biodegradability, and sufficient source [16,17]. However, because the reactivity of the double bonds in the long chains of most vegetable oils is not high, it is necessary to chemically modify these vegetable oils [[18], [19], [20]]. Crivello et al. epoxidized a wide variety of vegetable oils to prepare the UV-curable monomers that could be efficiently polymerized. Different cationic photoinitiators were used to photopolymerize the monomers, and the resulting polymer films possessed good adhesion and mechanical properties [21]. Hubert et al. studied the photoacceleration of autoxidation process by using alkyd resins. They found that light could accelerate the oxidative drying of alkyd resins in the presence of colored compounds (such as methylene blue or rose bengal), which was because these dyes could serve as photo-sensitizers converting oxygen into its energetic singlet state [22]. Phalak et al. synthesized ricinamide triacrylate by sequentially reacting ricinoleic acid with diethanolamine and glycidyl methacrylate, and it showed good elongation property [23]. Liang et al. synthesized a hexa-functional acrylate prepolymers by chemically modifying tung oil, and all of the films cured by UV irradiation demonstrated high performances [24]. Because different vegetable oils contain different vegetable oleic acids, in order to synthesize a product with a specific structure, it is necessary to select a suitable vegetable oil as a raw material [15].

Linseed oil is a typical biomass material with environmentally-friendly and renewable features. It possesses the most double bond content among all the vegetable oils, and the unsaturated Cdouble bondC double bonds inherent in its molecular chains provide it with chemical reactivity [25,26]. It is noteworthy that, various modification reactions could be carried out on the reactive groups, such as double bonds, ester groups and propylene carbons, on the fatty acid carbon chain of linseed oil, which would help to introduce functional groups with stronger polymerization ability to improve its functionality and the degree of conjugation [27,28]. Generally, modification methods used include epoxidation, epoxy esterification, epoxy hydroxylation, double bond isomerization, triglyceride alcoholysis, hydrogenation, direct polymerization, and the like [[29], [30], [31]]. Also, linseed oil could be used for the fabrication of various resins, biodegradable materials, and organic chemicals. Chen et al. synthesized norbornyl epoxidized linseed oil via Diels-Alder reaction of cyclopentadiene with linseed oil at high pressure and high temperature, followed by the epoxidation with hydrogen peroxide. Three different divinyl ether monomers were used to dilute the as-prepared norbornyl epoxidized linseed oil, and the results showed that the addition of divinyl ether significantly improved the curing rate [32]. Zong et al. prepared epoxynorbornane linseed oils (ENLOs) with different norbornene contents. It was found that diluents could reduce the viscosity of the ENLO systems, significantly accelerate the polymerization rate of ENLOs, and increase their final conversion rate. The cationic photopolymerization of ENLOs was proposed to be controlled by diffusion [33]. Sahoo et al. also prepared epoxidized linseed oil acrylate and epoxidized linoleic acid acrylate using a two-step process. The results showed that the viscosity of the epoxidized linseed oil acrylate reached 26,500 cp, while the viscosity of the epoxidized linoleic acid acrylate was only 680 cp [26]. Díez-Pascual et al. prepared an epoxidized linseed oil acrylate by a two-step process, and then blended with titanium dioxide to obtain a nanocomposite coating, which demonstrated a certain bactericidal effect [34]. Wuzella et al. tested the properties of epoxidized linseed oil acrylate coatings, and they believed that the coatings would perform solvent resistance, scratch prevention, and good adhesion [35]. Although desired coatings were achieved in their work, the reaction steps they employed had reached two or three steps, which imposed constraints on their practical applications [36,37]. Therefore, more and more researchers are beginning to study efficient reactions that can be carried out in one step. If the active double bond can be introduced into the long chain of vegetable oils in one step, the utilization value of vegetable oils will be greatly improved. Zhang et al. carried out the reaction of acrylic acid and soybean oil by one step under different catalysts, explored the influence of different catalysts on the reaction process, and thermally polymerized the product with styrene to obtain a thermosetting film. The results showed that higher acrylation degree brought about harder thermosetting films [38,39]. To obtain sunflower oil acrylamido derivatization, the Ritter reaction was adopted by Eren et al., but the reaction was implemented under harsh conditions by using concentrated sulfuric acid as cosolvent at −20 °C [40]. Walther et al. mixed the alkyd resin with acrylic acid and N-bromo succinimide, and the resultant mixture was allowed to react for 7 days at ambient temperature to obtain the desirable acrylated alkyd resin bearing a bromine substituent [41].

However, one-step synthesis of UV-curable acrylated linseed oil prepolymers (ALO) has not been reported for the time being. Herein, a one-step method was proposed to prepare ALO from linseed oil (LO). The novel linseed oil-based 2.5-functional acrylated prepolymers were synthesized by using linseed oil, acrylic acid as raw materials, and boron trifluoride diethyl ether as a catalyst. FT-IR and 1H NMR characterizations were utilized to verify the successful preparation of ALO. A series of UV-curable coatings were prepared by homogeneously mixing ALO, trimethylolpropane triacrylate (TMPTA), polyurethane acrylate resin (PUA-2665), and photoinitiator (PI-1173). The effects of different TMPTA contents on the dynamic mechanical properties (via DMA), thermal stability (via TGA), mechanical properties (via tensile test) and the general properties of the cured films were investigated. It was found that, the glass transition temperature (Tg) and crosslinking density (νe) of the cured films were improved along with the rise of TMPTA content, which was due to TMPTA possessing a higher double bond density than ALO. Furthermore, the enhancement in crosslinking density contributed to the improvement in thermal stability and mechanical properties of the cured films. Notably, the cured films also exhibited strong adhesion, outstanding hardness, and excellent water (solvents) resistance. Therefore, this work provides a novel one-step approach for the synthesis of UV-curable materials from vegetable oils, and the high performance of the cured films makes them a good candidate for practical applications.

Section snippets

Materials

Linseed oil (LO) was procured from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Acrylic acid (AA), n-hexane, sodium bicarbonate and anhydrous magnesium sulfate were kindly provided by Tianjin Fu Yu Chemical Reagent Co., Ltd. (Tianjin, China). Boron trifluoride diethyl ether solution (BF3, 46.5 %) was purchased from Lingfeng Chemical Reagent Co., Ltd. (Shanghai, China). Polyurethane acrylate (PUA-2665, with a molecular weight of 3000 g mol−1 and a viscosity of 1000 mPa·s)

Characterization of LO and ALO

The FT-IR spectra of LO and ALO are shown in Fig. 1. As for the spectrum of LO, the peak at 3010 cm−1 was attributed to the characteristic stretching vibration of Csingle bondH from C = Csingle bondH [43]. The peak at 1747 cm−1 corresponded to the carbonyl groups, and the peak at 1655 cm−1 revealed the presence of Cdouble bondC groups in LO. In the FT-IR spectrum of ALO, it was found that, after the acrylated reaction, the peaks at 3010 cm−1 disappeared. In the meanwhile, the peak at 1724 cm−1 corresponding to the Cdouble bondO

Conclusion

In this work, a novel one-step method was employed to prepare linseed oil-based 2.5-functional acrylated prepolymers (ALO) by using linseed oil, acrylic acid as raw materials, and boron trifluoride diethyl ether as a catalyst. FT-IR and 1H NMR characterizations were used to verify the successful preparation of ALO, and rheometer was employed to measure the viscosity of ALO. The synthesis mechanism of ALO catalyzed by boron trifluoride diethyl ether was also proposed. Furthermore, the

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

CRediT authorship contribution statement

Yupei Su: Methodology, Validation, Formal analysis, Investigation, Data curation, Writing - original draft. Shuting Zhang: Methodology, Validation, Formal analysis, Investigation, Writing - original draft. Yanwu Chen: Investigation, Data curation. Teng Yuan: Methodology, Conceptualization, Resources, Writing - review & editing, Funding acquisition, Supervision, Project administration. Zhuohong Yang: Conceptualization, Resources, Writing - review & editing, Supervision, Project administration,

Declaration of Competing Interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51673075, 21808070), the Natural Science Foundation of Guangdong Province (2018A030310349), the Science & Technology Program of Guangdong Province (2016A010103027), the Science & Technology Program of Guangzhou City (201803030003, 201704030085), the Special Innovation Project form the Department of Education of Guangdong Province (2017GKTSCX100), the Science & Technology Program of Foshan City (2016AG101695), the

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