A novel star-shaped, cardanol-based bio-prepolymer: Synthesis, UV curing characteristics and properties of cured films

Highlights

• Cardanol-based, star-shaped bio-prepolymer.

• UV-curable prepolymer with phosphazene core.

• Green and inherent fire-retardancy.

• Excellent thermal stability of UV-cured films.

Abstract

Starting from renewable Cardanol, a novel inherently fire-retardant UV curable bio-based prepolymer (AEHCPP) with phosphazene core and six Cardanol arms are prepared in this report. The chemical structure of all newly prepared intermediates compounds and AEHCPP are well-characterized using nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). UV-curable mixture consisting of AEHCPP and different diluents were formulated and their photopolymerization dynamics were investigated on Photo-calorimetry (photo-DSC). The thermal properties of crosslinked coatings were estimated using thermal analysis technology. Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) results show that glass transition temperatures of all cured films are above room temperature (>30 °C), the 5% weight loss temperature in nitrogen is higher than 266 °C. The fire-retardant properties are estimated by TGA and all calculated Limiting Oxygen Index (LOI) values are over 24, implying the excellent fire-resistance of cured films.

Introduction

Coatings are functional film-like materials which are applied to the substrates’ surface to protect them from external chemical erosion and physical damages. Among all the coating-formation technologies, UV-curing process is regarded as a green route for its “5E″ characteristics. (”5E” means Environmental-friendly,Efficient,Energy-saving, Economy and Enabling). Normally, the environmental-friendly character of UV-curable coatings always refers to its extremely low VOC emission, little attention was paid to whether the components of coating are renewable or not. However, from the viewpoint of sustainability, the components of UV coating should also be eco-friendly. But the majority prepolymers currently utilized in UV coating is fossil-based reactive prepolymers and diluents. Considering the non-renewable properties of fossil resources, there is an urgent need to replace the fossil-based reactive prepolymers with renewable ones [[1], [2], [3]]. (see Scheme 1)

Vegetable oils, including both edible and non-edible oil, are regarded as suitable feedstocks for bio-based UV coating due to their structural feature of unsaturated alkyl chains [[4], [5], [6]]. The frequently employed vegetable oils are soybean oil, sunflower oil, castor oil, and Cardanol. Vegetable oils do not exhibit high UV-curing reactivity. However, most of them can be easily converted to UV curable prepolymers by introducing photosensitive acrylate groups into their structures via various chemical routes. The first method is the direct reaction of plant oil with acrylic acid catalyzed by strong acid [7,8]. The second one, namely the most common-used method, is epoxy ring-opening acrylation reaction between epoxidized plant oil and acrylic acid catalyzed by organic bases [[9], [10], [11], [12], [13]]. Moreover, acryloyl chloride was also used to prepare acrylated vegetable oil derivatives in presence of an acid absorbent from hydroxyl group-containing castor oil [14,15].

For practical application, improving the fire-retardant property of UV coatings is an urgent task to enlarge its utilization scope. Coatings which are lack of fire resistance can't provide effective protections to certain substrates (such as wood, polyolefin, and et al.) under fire condition. In fact, some efforts have been done to prepare green fire-retardant UV coatings. The most widely adopted method is introducing green fire-retardant elements, such as phosphorus, silicon, nitrogen and boron, into UV coating by chemical bonding [[16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]]. since these elements generates less toxic and corrosive decomposition products than halogen-based ones under fire condition. Among them phosphorus atom is the most popular choice to enhance the fire-retardant of UV coatings [[33], [34], [35], [36], [37], [38], [39], [40], [41]]. Though nitrogen atom [23,42] alone can also act as a fire-retardant element, the well-established method is to prepare phosphorus-nitrogen type UV coatings for its synergistic effect [38,[43], [44], [45], [46]]. Recently, silicon atom [[47], [48], [49], [50], [51], [52], [53], [54], [55]] also attracted many attentions in preparing green fire-retardant UV-coating, this type of coatings showed high thermal stability and expected flexibility. Besides, boron atom [19,25] is also regarded as a green fire-retardant component suitable for raising the fire-resistant level of UV-coatings.

Above-mentioned techniques have also been used to prepare bio-based fire-retardant UV coatings. Güngör's researches [33] shows that vinyl phosphonic acid can reacted directly with epoxidized soybean oil (AESO) to form phosphorus-containing soybean-oil derivative. Based on this phosphorus-modified natural oil, a new kind of organic–inorganic hybrid UV coating was prepared with attractive thermal stability. As expected, the cured coating shows excellent fire-retardancy and the LOI value of coating raised to 23.5 from 19 with improving phosphorus content. Obara and colleagues [56] prepared a castor oil–based UV coating and studied the effect of the methacrylate-substituted polysilsesquioxane (POSS) on cured film. They founded that both mechanical properties and thermal stability of cured film were reinforced by increasing the amount of modified POSS particles, but higher content of POSS showed negative effect on flexibility of the coating. Fire-resistant UV coating was also use as surface coating of renewable biopolymer, such as cotton textile [57], to increase the fire properties of biopolymers. For example, Hu's group reported that the LOI value of cotton textile could be increased to 24.5 after coated by tri(acryloyloxyethyl) phosphate and triglycidyl isocyanurate acrylate mixture.

As subsidiary products during cashew production, cardanol is not suitable for food purpose. Nevertheless, it is widely used as raw material in chemical industry. Cardanol consists of phenol unit and unsaturated long alkyl chain, which makes it full of tailorability [58]. In this paper, a novel approach to prepare inherently fire-retardant bio-based prepolymer is reported. By combining phosphazene and natural Cardanol together by chemical bonds, a star-shaped UV curable prepolymer with phosphazene core and six Cardanol arms which were modified by acrylic acid was prepared. The UV coating was formulated by prepolymer and diluents, and its photo-curing characteristics were probed on photo-DSC. The UV-cured coatings displayed excellent thermal stability. The highest char yield at 850 °C in nitrogen was 28.9%, and the corresponding LOI value based on empirical equation was 29.1.