Alumina and zirconia acrylate nanocomposites coatings for wood flooring: Photocalorimetric characterization

doi:10.1016/j.porgcoat.2007.09.013

Progress in Organic Coatings

Volume 61, Issue 1, January 2008, Pages 76-82

https://doi.org/10.1016/j.porgcoat.2007.09.013 Get rights and content

Abstract

Radiation curable coatings are presently the standard in wood flooring industries, although important improvements can still be brought to these coatings. In this work, nanocomposites coatings for wood flooring were prepared from various acrylate reactives. Nanoparticles were added in the neat acrylate formulation prepared from two acrylate monomers, two acrylate oligomers, a defoaming agent and a photoinitiator. Particle size characterization was performed by a dynamic light scattering technique. Reinforcing agents and coupling agents addition effects on acrylate resin conversion were studied by photo-calorimetry (photo-DSC). For each nanocomposite sample, heat of reaction and induction time were determined from exotherms and these datas were used to study the effects of reinforcing agents and coupling agents on curing kinetics of radiation curable nanocomposite coatings. Photo-DSC studies show that nanoparticles and coupling agent clearly affect coatings polymerization. In fact, zirconia nanoparticles tend to decrease polymerization. Alumina nanoparticles do not affect negatively curing coatings. Silane coupling agent affects positively the curing of acrylate coatings, although zirconate coupling agent tends to decrease it.

Introduction

Many nanocomposites studies were published in the last decade. Thermoplastic polymers like polyolefins (PP and PE), polyurethanes, polyimides and nylons are among the most studied matrices [1]. Their uses in the automotive and aerospace industries explain the widespread interest for new and improved products. In many cases, fillers and coupling agents are added in polymers to enhance properties. The most used fillers are aluminum oxide (Al₂O₃), clay, calcium carbonate (CaCO₃), silica (SiO₂) and titanium dioxide (TiO₂). Studies show that their addition in polymers is often related to a great improvement of mechanical properties. Improved scratch and impact resistance, Young's modulus, modulus of rupture are among the most researched properties for thermoplastic nanocomposites. Flame, fire and moisture resistance seem to be improved by clay introduction and exfoliation in thermoplastic polymers [2]. Thermoset nanocomposites studies are fewer than those performed for thermoplastic matrices. Processes with the first ones are more difficult to develop and applications, even if they cover a wide range of industries, are less diversified than for thermoplastics. Adhesives and coatings are two very widespread industries which are both under continuous development and use large quantities of thermoset polymers, especially epoxies and acrylates. The uses of reinforcing agents in these industries are quite important and this is one of the reasons why nanocomposite studies are popular presently. Epoxy nanocomposites studies in the last few years shown a strong increase. Montmorillonite exfoliation in epoxy polymer was one of the major interest [3], [4]. Exfoliated nanocomposites were shown to increase the Young's modulus [5] and fracture toughness [6]. Nanocomposites were also prepared with carbon fiber and silica [7]. Those prepared with carbon nanofiber presented higher temperature performance capability, better mechanical performance, an extreme environment corrosion resistance and an improved dimensional control compared to neat epoxy. Epoxy–silica nanocomposites showed elastic modulus improvement and epoxy glass transition increased.

Acrylate thermoset nanocomposites studies started to be published in 2000 [8]. These studies are largely due to the growth of UV radiation curable coatings in many industries. These coatings, developed in the last 1970s, now have a wide market share in all the important coatings areas (metal, plastic and wood) and are still under development. UV curing is an important technology for printing inks, overprint varnishes, adhesives, food packaging and in the electronic devices industries. UV coatings present an important advance for the finishing technology. In fact, their curing rate and mechanical performance make the UV coating technology very popular for many wood industries [9]. While wood flooring industries have switched to UV technology 10 years ago, they are still looking for mechanical resistance improvement, especially with regard to scratch and wear resistance. To obtain a strong and resistant coatings, curing has to be well done. The photo-calorimetry technique is certainly among the best ways to evaluate and to quantify UV curing.

In the last 5 years this technique appeared to be a state of the art technique to study radiation curable coatings. A small number of studies have been published on photo-DSC technique, and the technique already has shown a great analytical potential. Popular topics studied with photo-DSC are oxygen inhibition [10], [11], [12] of photocured systems, acrylate polymerization kinetics [13], influence of monomers and oligomers structure on photocuring rate and new photoinitiators efficiency [14], [15]. Recently, a few studies on acrylate nanocomposite were reported. Cho et al. [16] have studied the effects of different loadings of silica nanoparticles in acrylate formulations. They studied the heat of reaction and curing rate as a function of silica loading. Similar studies were performed for clay nanocomposites coatings [17]. In this study, nanoclay particles were added in the formulation as well as alumina and zirconia nanoparticles in a typical wood UV curing formulation. Coupling agents were also considered in those formulations. Photo-DSC was used to study the effect on polymerization rate and amount. The effects of nanoparticles and coupling agents introduction on curing of radiation curable coatings were investigate by the aim of Photo-DSC.

Section snippets

Materials

Basic formulation was prepared from two acrylate monomers and oligomers. The acrylate monomers which were used are 1,6 hexanediol diacrylate (HDODA, SR 238, 9 cps) and tripropylene glycol diacrylate (TRPGDA, SR 306, 15 cps), two bifunctional monomers. HDODA is a low viscosity and fast curing monomer. TRPGDA has a low volatility and viscosity. The oligomers chosen are an aliphatic polyester-based urethane hexaacrylate oligomer (CN 968, 18,000 cps) and a difunctional bisphenol A-based epoxy acrylate

Data reduction

Photo-DSC allows the obtention of an exotherm related to the radiation curing of the acrylate coatings. Thermosetting resins have two general kinetics models: nth-order and autocatalytic models. The first one can be expressed by the following equation: $\frac{d α}{d t} = k {(1 - α)}^{n}$ where dα/dt is the reaction rate given in s⁻¹, α the extent of reaction or the fraction conversion after time t, k the specific rate constant and n is the reaction order. The radiation curable coatings studied here follow an

Zetasiser experiments

Fig. 4 presents an example of the results obtained with Zetasizer Nano ZS for a dispersion of Nanobyk in HDODA, one of the monomer used in this study. In this case, particle size distribution is relatively sharp so only one peak is present on the resulting graph and the absence of peaks at higher sizes show absence of aggregation. Table 3 presents the results obtained for the different nanofillers in HDODA monomer at different concentrations. Mean particle size (nm) are presented in this table.

Conclusion

Coatings formulations were prepared with 4% (w/w) alumina and zirconia nanoparticles. Two coupling agents were added in these formulations, silane and zirconate. They were added by in situ and ex situ technique. Zetasizer experiments show that commercial dispersion of alumina nanoparticles present a very narrow particle size distribution compared to dispersions performed in our laboratory. From the exotherms obtained for each formulation, heat of reaction and induction times were determined.

Acknowledgements

Authors wish to thank Economic Development Canada, Forintek Canada Corp., the National Sciences and Engineering Research Council of Canada trough its industry-university grant program and Chemcraft International.

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Alumina and zirconia acrylate nanocomposites coatings for wood flooring: Photocalorimetric characterization

Abstract

Introduction

Section snippets

Materials

Data reduction

Zetasiser experiments

Conclusion

Acknowledgements

Polymer

Polymer

Polymer

Polymer

Polymer

Polymer Nanocomposites and their Applications

Clay-containing Polymer Nanocomposites

Nanotechnology

Polym. Int.