In recent years, UV-curing technology has been widely used in painting, printing, adhesives, and 3D printing due to its advantages of fast curing, energy efficiency, and environmental-friendliness. Traditional UV-curable coatings are organic systems composed of oligomers and monomers. These coatings usually exhibit the benefits of good film-forming characteristics, favourable flexibility and adhesion properties, and low cost. However, they suffer from low hardness, poor heat and wear resistance and high volume shrinkage during curing. Introducing nanofillers into UV-curable coatings can effectively improve the performance of the coating and impart new functionalities to the film. For example, the addition of nanomaterials such as silica [1–4], zirconia [5–7], montmorillonite [8, 9], graphene [10–12], and POSS [13, 14] can effectively improve the thermal and mechanical properties of UV-curable nanocomposites. Meanwhile, the addition of silver nanoparticles [15], Fe3O4 [16], carbon nanotube cellulose [17], and polyaniline/multiwalled carbon nanotubes [18] can expand the application of UV-curable nanocomposites in the fields of antibacterial, microwave absorption, adsorbents, and sensors, respectively. Therefore, the utilization of UV-curable nanocomposite coatings has received extensive attention.
Epoxy acrylate (EA) is widely applied as an oligomer in several products, including UV-curable formulations. Cured EA films exhibit high hardness, good thermal stability, and substrate adhesion; however, such films have shortcomings, such as poor toughness and high brittleness. Nanofillers such as organonanoclay [19–23], polyhedral oligomeric silsesquioxane (POSS) [14, 24–27], silica [28–31], cellulose nanocrystals (CNC) [32], alpha-zirconium phosphate [33, 34], graphene oxide [35], halloysite nanotubes [36], polyphosphazene nanotubes [37], multiwalled carbon nanotubes [38], and layered double hydroxide [39] have been used to enhance the mechanical, thermal, and flame-retardant properties of EA-based UV-curable coatings. Zhang et al. [32] prepared UV-cured EA/CNC nanocomposites and found that when 8 wt % CNC was added, the storage modulus increased by 54% compared with pure EA. Li et al. [37] reported that at a POSS-functionalized polyphosphazene nanotube loading of 3 wt %, the glass transition temperature and storage modulus of the nanocomposites increased by 16°C and 88%, respectively, compared with those of pure EA. Xing et al. [33] found that alpha-zirconium phosphate nanometer sheets could effectively improve the flame-retardant properties of UV-curable EA-based coatings. However, these nanofillers did not significantly improve the flexibility of the EA-based nanocomposites. In addition, the transparency of films usually decreases with the addition of nanofillers.
Aramid nanofiber (ANF) is a new type of polymer nanofiber that has been developed in recent years. ANF retains rigid structure, excellent mechanical properties, and thermal stability of aramid fiber and acquires new characteristics because of its nanofiber structure, making it a potential 'reinforcing unit' for building high-performance composite materials. Thus, ANF has attracted considerable attention in composite reinforcement, flexible electronics, and battery separators. Research has revealed that ANF can effectively improve the mechanical performance of polymers, such as epoxy resin [40–43], rubber [44–47], polyurethane [48–50], poly(vinyl alcohol) (PVA) [51], polysulphone [52], polyvinyl chloride [53], and polyamide [54]. Guan et al. [53] found that compared with neat PVA, the toughness and tensile strength of PVA nanocomposites with 5 wt % loading of ANFs increased by 149% and 79%, respectively. Nasser et al. [54] determined that the tensile strength and elastic stiffness of ANF-reinforced polyamide nanocomposites increased by more than 62% and 27%, respectively, compared to those of neat polyamide. ANFs established connections with the polymer matrix through hydrogen bonding in these studies. To further enhance the mechanical properties of nanocomposites, modified ANFs have been prepared to establish interfacial chemical interactions with the polymer matrix. Jung et al. [42] prepared epoxy functionalized ANFs (EANFs) using 3-glycidoxypropyltrimethoxysilane to reinforce epoxy resin; the fracture toughness of nanocomposites increased by 440% compared with that of neat epoxy resin. Zhang et al. [45] synthesized epichlorohydrin-modified ANFs, which can interact with carboxylated acrylonitrile butadiene rubber/styrene-butadiene rubber via covalent bonding and π–π conjugation and found the incorporation of modified ANFs increased the tensile and tear strengths of nanocomposites by 87.2% and 194.7%, respectively.
In a previous study [55], we firstly prepared UV-curable EA/ANF nanocomposite films. The inclusion of ANF substantially enhanced the strain at break and tensile strength of the nanocomposites. Compared to neat EA, a 36% improvement in tensile strength and a 194% improvement in strain at break are observed for 0.05 wt % and 0.1 wt % ANF-reinforced nanocomposite, respectively. However, due to the surface effect of nanoscale fiber and the poor interaction between ANFs and EA matrix, ANFs could form aggregation easily, adversely affecting the composites' mechanical properties. In this work, to further enhance the interaction between ANFs and EA matrix, we prepared methacrylate-functionalized ANFs (mANFs) by adding hydroxypropyl methacrylate (HPMA) as a modifier during the polycondensation of terephthaloyl chloride (TPC) and p-phenylenediamine (PPD). The reaction mechanism of HPMA with PPD and TPC is depicted in Scheme 1. The functionalized ANFs attach to the EA matrix through π–π conjugation and hydrogen bonding, participate in photo-initiated radical polymerization, and interact with the EA matrix through covalent bonds. The mechanical properties of mANF-reinforced nanocomposites were dramatically improved compared with those of ANF-reinforced nanocomposites. Fourier-transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM) were used to examine the chemical structure and morphology of the synthesized mANFs. The inner morphology, microstructures, and crystallinity of as-synthesized EA/mANF films were revealed by XRD and TEM. In order to determine the effect of mANFs on the curing kinetics, Real-time FTIR was employed. To examine the transmittance of the EA/mANF films in the visible and UV areas, UV-vis transmission spectra were measured. Tensile tests were performed to assess the effect of mANFs on films' mechanical characteristics. SEM studied the morphologies of a fracture surface.