Characterization of mechanical behavior of UV cured urethane acrylate nanocomposite films loaded with silane treated nanosilica by the aid of nanoindentation and nanoscratch experiments

Highlights

• Dispersion of pristine and silane treated nanosilica in UV curable resins.

• Film formation of nanocomposites under UV radiation onto polycarbonate.

• Investigating the abrasion, scratch and indentation behavior of nanocomposite films.

• Discussing mechanical attributes from macro and nanoscale tests to the mechanism of deformation under loading.

Abstract

This work describes the mechanical behavior of different nanocomposite films containing non-treated and silane treated nanosilica loaded into a UV cured urethane acrylate resin. Nanoindentation, nanoscratch, micro-hardness and dynamic mechanical thermal analysis were utilized to assess the coatings. It was demonstrated that the surface treated nanoparticles had stronger interfacial interaction with the polymeric matrix and had improved storage modulus. Nanoindentation and microindentation results suggested that a homogenous reinforced structure was formed in the bulk and surface of hybrid coatings filled with treated nanosilica. This resulted in films with high abrasion resistance.

Introduction

Nanocomposite hybrid coatings have exceptionally good properties such as possibilities for protective applications including enhanced scratch resistance of optical filters on low-E window glazing and displays, protective topcoats and stacks of recording films in optical storage disks, and many more [1], [2]. They are of significant interest as scratch and abrasion resistant coatings on plastic substrates [3]. Scratch resistance of polymeric coatings can be increased by embedding nanofillers into them. Nanoparticles such as SiO₂, ZrO₂ and TiO₂ embedded in curable resins, have shown to represent high improved scratch and abrasion resistance of the resultant coatings [4], [5]. Besides, these particles do not reduce the transparency of coating, due to their small particle size relative to that of light wavelength. Nanosilica has been widely used in coating industries for the last few decades. The main properties investigated included mechanical reinforcement of polymeric films, improvement in scratch and abrasion resistance, enhancing the thermal and insulation properties, as well as increasing the resistance against UV radiation. In all of these applications a proper interfacial interaction is needed to ensure a good dispersibility [6]. Therefore, the interface of particle and the media in which they are incorporated plays a major role [7]. The pristine silica is inherently a hydrophilic particle. This imposes difficulties to properly wet the untreated silica into organic film formers. In other words, the applications of this filler are generally restricted to hydrophilic media.

Silane coupling agents (SCA) are often used to modify the silica surface due to their bi-functional structure. These materials are able to chemically link to silica surface and provide reactive functional groups through which the matrix binds tightly to the surface of silica. These functional groups can possibly guarantee maximum compatibility with resin system. Such particles carry organofunctional groups at their surface, including amines, vinyl, epoxy, mercapto, acrylic, etc. The attachment of SCA molecules to the nanosilica particles could decrease the aggregation of the particles due to the steric repulsion of grafted organic groups, and could improve the dispersibility of silica particles through increased interaction between silica and the matrix, resulting in improvement of scratch and abrasion resistance of coatings [8], [9], [10], [11]. The preparation and characterization of resin/silica nanoparticles for mechanical properties have been recently deeply investigated. Bauer et al. [12] studied UV-curable formulations with a high content of nanosilica particles to prepare a top coating with improved abrasion and scratch resistance. They reported that grafted tri-alkoxysilane molecules on the surface of nanoparticles comforted the particle dispersion in the matrix. This modification improves embedding and enhances the compatibility of nanoparticles into UV-curable acrylate resin. After UV-curing, acrylate composites revealed higher scratch and abrasion resistance as well as improved abrasion properties compared with the pure acrylate films.

Hedayati and co-workers [13] studied the effect of silica nanoparticles on tribological behavior of amorphous and semi-crystalline polyether–ether–ketone (PEEK) coatings. They found that the wear rates of both the semi-crystalline and amorphous nanocomposite coatings were lower than the pure ones but their coefficient of friction were slightly higher. Xiaohong and co-workers [14] also investigated tribological behavior of nanosilica to be used as additive in lubricant on wear testers. The results showed that they can evidently increase anti-wear ability and reduce the friction coefficient of lubricant. Effect of addition of hydrophobic nanosilica on viscoelastic properties and scratch resistance of an acrylic/melamine automotive clearcoat was investigated by Tahmassebi and co-workers [15]. Wang and Zhang [16] used a nanosilica to increase the tribological behavior of polycarbonate at micro and nanoscale. The scratch tests indicated that the nano-SiO₂/polycarbonate coating exhibited smaller scratch depth and lower frictional coefficient. Combined with the examination of infrared spectrum, the mechanisms of the improvements in mechanical and tribological properties of the coatings were analyzed.

The mechanical properties of the silica–acrylate nanocomposite hybrid coatings with respect to a degree of surface modification and a fraction of the silica nanoparticles have been reported by Soloukhin et al. [17]. Their analysis showed that filler content and chemical composition of the matrix influence the mechanical properties of the silica–(meth)acrylate nanocomposite hybrid coatings in a complex way. In the case of a tri-acrylate matrix, an increase of silica content from 10 up to 40 vol% does not increase the elastic modulus, although the hardness increases approximately by a factor of 1.3. After lowering of cross-linking density in the system by means of a di-acrylate, an increase in elastic modulus by a factor of 3.4 and in hardness by 4.2 takes place, when the silica content is increased from 10 up to 40 vol%

Wouters and coworkers [18] used urethane acrylate (U-Ac) as UV curable organic phase with sol–gel precursors to prepare hybrid scratch resistant films. Results showed that hardness and elastic modulus increased up to 0.2 and 3 GPa. Vu et al. [19] showed that incorporation of 23 vol% silica in UV-curable acrylate coatings could give a 2.5-fold increase of the modulus and higher abrasion resistance.

Various mechanical attributes have been used to indicate the scratch resistance of a coating including hardness, elastic modulus, residual stress and interfacial fracture toughness, etc. [20], [21]. The most significant method of measuring the mechanical properties of the coating is to deform it on a very small scale. A proper way to perform this is through an indentation experiment on a nanometer scale, commonly referred to as nano-indentation. This can also be combined with the scratch testing method [21], [22], [23]. The physical phenomena that take place during indentation are as follows. During loading and scratch step both elastic and/or plastic deformation occurs in a specimen resulting in formation of a hardness impression with the shape of the indenter. This impression has a projected contact area, which is associated with some contact displacement (or contact depth). During unloading, only the elastic portion of the displacement is recovered. Any attempt to resist these steps may improve scratch resistance. Elastic modulus and surface hardness play an important role in the scratch process, especially during the indentation stage. It was recognized that if elastic response dominates in a particular stage of the indentation process, not only the hardness of a material but also its elastic modulus could be evaluated from a single indentation test [24].

Hardness itself is a not well-defined characteristic [25], [26], [27]. Fink-Jensen [26], [27] defined hardness of a substance as “its ability to resist the temporary or permanent creation of a surface wholly or partly within its original boundaries, when the surface is locally subjected to compressive stresses that vary strongly along the boundary surface”. This is a complex statement to say that hardness is a measure of the resistance to permanent deformation or damage.

We have recently prepared a surface treated nanosilica having acrylic functionalities [6] using methacryloxy propyl trimethoxysilane (MPTMS). The impact of influencing parameters involved in treatment procedure such as the ratio of silane coupling agent to silica, hydrolysis ratio as well as treating bath pH were investigated. The overall aim of this surface treatment is to use the treated nanosilicas in UV curable urethane acrylate films to be utilized in scratch resistant nanocomposite coatings on transparent plastic substrates. This study aims at studying the tribological behavior of the nanocomposite films containing these treated particles.