Polyurethane modified epoxy acrylate resins containing ε-caprolactone unit
Introduction
UV-curable technology has been of great interest in recent years on account of its environmental-friendly trait. It has advantages of fast curing, no volatile organic compounds (VOC) and low energy consumption during the curing process [[1], [2], [3], [4], [5], [6], [7], [8]]. UV-curable coating formulations often consist of photoinitiators, reactive diluents and photo-sensitive resins, which determine the main mechanical properties of the coatings [9].
Epoxy acrylate resin is a widely used UV-curable resin, which possesses fast curing speed, low cost and exhibits a high gloss, good chemical resistance after curing [10]. However, the high viscosity of epoxy acrylate resin, which is due to non-covalent interactions between the molecules, restricts its applications in certain specialized fields such as electronic and aerospace [11,12]. Numerous efforts have been made to modify the epoxy acrylate and eliminate these issues [[13], [14], [15]].
Modifications of epoxy acrylate consist of physical and chemical modification. Physical modification of epoxy acrylate resin requires the addition of pigments [16], fillers [[17], [18], [19]] and particles [20,21] into the formulations. These approaches have increased the mechanical properties of the cured coatings. However, the compatibility of additives with the resin is not good. Chemical modification involves the introduction of certain flexible segments into the main or side chain, which include silicon-based, polyurethane-based and phosphorous-based segments. Giilary Bayramoglu used silane, with a terminal NCO group, reacts with the epoxy acrylate. Compared with unmodified epoxy acrylate resins, the silane modified resins showed enhanced thermo-stability, higher elongation at break and tensile strength [22]. Oprea Stefan et al. prepared cured coatings with excellent toughness and hardness through the reaction of diisocyanate with the hydroxyl group of epoxy acrylate [23]. A series of epoxy acrylate oligomers, containing organophosporous, were synthesized by Yongxia Ren et al., which improved flame retardant and lower surface drying time of the cured coating [24]. Among them, polyurethane-based modified epoxy acrylate displays excellent mechanical properties, as well as chemical and water resistance. However, the hydrogen bond between the macromolecular chain contributes to the high viscosity of polyurethane-based modified epoxy acrylate. Furthermore, due to diminished molecular chain mobility, polyurethane modified epoxy acrylate displays a considerable volume shrinkage resulting in poor adhesion to the substrate [25,26].
ε-Caprolactone (ε-CL) is a biocompatible raw material and has been widely used as a monomer in high performance polymer materials. Polycaprolactone(PCL) synthesized via ring opening polymerization(ROP) of ε-CL has been used as environment friendly materials in simple processes. The copolymer of lactide and ε-CL is often employed as a surgical suture material [[27], [28], [29], [30]]. Ekin et al. synthesized a novel hydroxyalkyl carbamate oligomer, which further reacted with ε-CL to produce a novel H-type block copolymer. They discovered that the block copolymers were excellent candidates as surface modifying additives, such as drug encapsulation [31]. Calvo et al. presented that biodegradable polyester resins, which were prepared via ROP of ε-CL, displayed enhanced degradation rate (showing a mass loss of 50% within 77 days) than linear poly(ε-CL) [32].
In this paper, a series of polyurethane modified epoxy acrylate resins containing various content of ε-CL were synthesized. The introduction of a flexible carbon chain into the molecule via ROP of ε-CL increases film toughness and reduces resin viscosity. To investigate the effect of ε-CL unit on the mechanical properties of the films, we analyzed the dynamic thermo-mechanical properties and tensile strength of UV cured films. Also, the basic properties of the prepared coatings (thickness, gloss, impact, adhesion force, pencil hardness and pendulum hardness) were characterized.
Section snippets
Materials
The chemical’s purity and purchase source are as follows: isophorone diisocyanate (IPDI, 97%, Shanghai Aladdin Biochemistry Technology Co., Ltd), hydroxyethyl acrylate (HEA, 98%, Shanghai Aladdin Biochemistry Technology Co., Ltd), ε-caprolactone (ε-CL, 98%, Shanghai vita Reagent Co., Ltd), dibutyltin dilaurate, Tin (II) 2-ethyl-hexanoate ((DBTDL, Sn(Oct)2, 99%, Shanghai Aladdin Biochemistry Technology Co., Ltd), 2-hydroxy-2-methyl-phenyl propanone, diphenylmethanone (I-1173, BP, 98%, Changzhou
Synthesis and characterization of EA-PUA and EA*-PUA
The structures of EA*-PUA and EA-PUA were identified by FTIR and 1H NMR. The FTIR spectra (Fig. 1a) shows the disappearance of the stretching vibration band at 2270 cm−1 (NCO) and appearance of the bending vibration band at 710 cm−1 NH) due to the reaction of OH and NCO. The double bond (CC) absorption band at 1625 cm−1 is present indicating that the epoxy acrylate structure is not decomposed. The NMR spectra (Fig. 1b) presents the proton shifts of the amide bond at 6.8–7.3 ppm and double bond
Conclusion
A series of polyurethane modified epoxy acrylate resins containing various ε-CL unit content were successfully synthesized. The introduction of the long carbon chain decreases density of hydrogen bond interaction. Compared with polyurethane modified epoxy acrylate without ε-CL unit, the viscosity of modified resins decreases up to 80%. Meanwhile, the toughness of the cured films increased by 30% because of the flexible long chain which decreases the crosslinking density of films. Furthermore,
Statement of author
All authors contribute equally.
Conflict of interest
The authors declared that they have no conflicts of interest to this work.
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgments
The authors acknowledge the financial support by the Fundamental Research Funds for the Central Universities (JUSRP 51719A), MOE & SAFEA for the 111 Project (B13025).
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