Bio-based monomers for UV-curable coatings: allylation of ferulic acid and investigation of photocured thiol-ene network

Highlights

• Synthesis of Bio-Based photocured coatings.

• Allylation of ferulic acid.

• Thiol-ene photopolymerization.

Abstract

Ferulic acid (FA) is an unsaturated hydroxycinnamic acid that can be isolated from lignin. In this study, the biorenewable FA was allylated to result in a library of mono- or diallylated monomers, either having the inherent cinnamoyl double bond intact or saturated through hydrogenolysis. All monomers were photo-chemically cured with trimethylolpropane tris(3-mercaptopropionate) (TRIS) into crosslinked films in the presence of a photoinitiator (Irgacue 819). The reactivity of the FA-derived monomers toward TRIS was investigated in detail by photorheology and FT-IR spectroscopy to reveal details on the relative reaction rates for the different alkene groups. The thermo-mechanical properties of the crosslinked films were fully characterized by means of dynamic mechanical analysis (DMTA) and thermal calorimetry (DSC). It was demonstrated that the glass transition temperature of the final crosslinked network could be controlled by the addition of a monoallylated monomer. By increasing the content of the monoallylated compound, it was possible to observe a linear decrease of the Tg-values of the crosslinked films.

Introduction

Polymer scientists and industry are facing a challenging shift from fossil-based to bio-based materials; by the end of the next decade, many current petroleum-derived products should be replaced by less expensive and better-performing products based on renewable materials [1]. In literature, different renewable monomers are explored as precursors to polymers such as sugars, polysaccharides, vegetable oils, lignin, pine resin derivatives, and proteins [[2], [3], [4]]. Among them ferulic acid, Fig. 1, shows significant potential owing to its chemical structure and abundance. Ferulic acid, or (E)-3-(4-hydroxy-3-methoxyphenyl) prop-2-enoicacid, is a naturally occurring trans-cinnamic acid derivative (HCAs) [5] discovered in 1866 as a component of Ferula foetida Reg. (Umbelliferae). Its name, ferulic acid (FA), is derived from the plant from which it is extracted [6]. FA belongs to the p-hydroxycinnamic family together with caffeic acid and sinapic acid, Fig. 1. FA is a multifunctional compound, having three functional groups; a phenol, a cinnamoyl double bond, and a carboxylic acid-group.

FA is a component in lignin, the second most abundant biopolymer (after cellulose) on Earth, representing 30 % of the organic carbon in the biosphere [7]. Lignin can be obtained as a by-product from the pulp and paper industry, or from the production of food from crops and cereals, and is therefore readily abundant and hence, a scalable source of FA [8]. FA is usually concentrated in the bran of grains, peel of fruits, roots and peel of vegetables. In maize for example, the FA content is around 3–4 % of the dry weight [9]. FA is produced according to the phenylpropanoid pathway, it couples oxidatively to other FAs and FA-derivatives, lignin, and structural proteins within the cell wall. The links to lignin and proteins by ether bonds and to polysaccharides by ester bonds, act as universal connectors between cell wall polymers. FA has a key role in cessation of cell growth, anchoring lignin to cell wall polysaccharides, restricting the accessibility of plant pathogens. It influences properties such as adherence, extensibility, accessibility and biodegradability. FA plays an important role in the production of cellulosic ethanol, that is becoming an important supply for the fuel industry [10].

FA has been explored for a range of properties; as antioxidant, for anti-allergenic, anti-inflammatory, and antimicrobial purposes. It has been proposed to be used as an anticancer agent and, since it can absorb UV-light, in cosmetics to protect the skin [11,12].

FA has attracted interest by polymer chemists and has been evaluated as a promising bio-renewable monomer to substitute some of the fossil-based monomers already used, for instance in poly(ethylene terephthalate) (PET) [13]. In this potential application, FA is converted into a polyester but it can also be an interesting starting building block for polyurethanes, epoxides and phenolic resins as well [14]. FA provides versatile handles for further chemistries as the phenolic and carboxylic groups can be exploited for further functionalization. FA is particularly interesting as it is one of few biorenewable aromatic compounds that contain a double bond and may therefor find use in chain-wise polymerization, to replace meth(acrylates). The double bond can potentially be employed in photo-initiated (UV) polymerizations (i.e. radiation curing).

Radiation curing is attracting increasing interest especially in the field of coatings, as witnessed by the large number of books and review articles published in the last 20 years [[15], [16], [17], [18], [19], [20], [21], [22], [23]]. UV-curing shows important advantages such as high-speed cure, reduced energy consumption and absence of VOC emission as many systems are low viscous, without the need of any solvents. Among the different UV-curing processes, thiol-ene photopolymerization is attracting attention due to its “click” characteristics implying that very high conversions are reached [15,17,18]. The rationale for using free-radical thiol-ene polymerization is that it allows for ene-monomers that normally do not homopolymerize to be employed as comonomers in conjunction with the thiols. This is especially important when searching for bio-based unsaturated monomers which generally are di- or trisubstituted and hence rather challenging to homopolymerize due to steric reasons. Free-radical thiol-ene reactions have, for example, been demonstrated for unsaturated vegetable oils (1,2-disubstituted alkenes) [24,25], terpenes (1,1,2-trisubstituted and 1,1-disubstituted alkenes) [[26], [27], [28]], and unsaturated lignin-derivatives [29].

The reaction rate for differently substituted alkenes in a thiol-ene reaction strongly depends on the substitution pattern of the alkene, as for example presented by Hoyle [30]. This has to be taken into account when several alkenes are present in a curing system and can also be employed as a tool to design thermoset structures, as described by Bowman et al. for ternary monomer systems [31,32]. It should, however, in general be noted that enes that are less prone to homopolymerize provide more of a “click”-character to thiol-ene reaction compared to readily homopolymerizable monomers e.g. acrylates. The allyl ether-functionality utilized in the present study is very slow in free radical homopolymerization compared to acrylates and is thus very well suited for thiol-ene reactions [18]. Allylation of caffeic acid and other bio-based monomers has already been proposed in literature as a pathway to a new precursor platform for photo-curable biorenewable thermosets [33,34].

In this paper, we report the allylation of FA to achieve mono- or bifunctional allyl monomers, and their use in UV-initiated thiol-ene polymerization. The reactivity of the bio-renewable monomers towards trimethylolpropane tris(3-mercaptopropionate) was investigated in detail by photorheology and FT-IR analysis. The thermo-mechanical properties of crosslinked films were fully characterized by means of DMTA and DSC analysis. Finally, the influence of the addition of monoallyl-functional FA-derived monomers on kinetics and the properties of the crosslinked films were investigated.