UV-radiation curing is one of the most effective processes to rapidly produce composite materials at ambient temperature. Mineral charges, such as silica particles or glass fibers, can be introduced into photocurable resins to get hard and abrasion-resistant materials that can be used as protective coatings, dental fillers, sealants, composite membranes and fiber-reinforced plastics.1-4 The abrasion and scratching resistance of UV-cured coatings was found to be greatly improved by incorporation into the polymer matrix of acrylate-functionalized colloid silica.5-7 In all of these applications, the spatial and temporal control afforded by photopolymerization, together with the short setting time, make this environment-friendly technology very attractive for rapid and inexpensive processing of polymeric composites.

In conventional composite materials, the filler and the polymer are combined on a micronic scale, which often leads to insufficient adhesion between the organic matrix and the reinforcing filler. Composite materials that exhibit a change in structure and composition over a nanometer length scale have been shown to afford remarkable property enhancements with respect to stiffness and strength, heat resistance and gas barrier properties.8 Most of the nanocomposite materials are based on linear polymers and show, therefore, insufficient chemical and heat resistance. When the mineral filler, usually layered silicate such as clay, is incorporated into the melted polymer, thermodegradation of the organoclay may occur, thus affecting the nanocomposite performance.

The basic idea behind this work was to produce nanocomposite materials by photoinitiated in situ polymerization of multifunctional monomers and oligomers containing a nano-scale filler, in order to make use of the unique advantages of UV-curing technology:9,10

• a solvent free-formulation, with essentially no emission of volatile organic compounds;
  
• fine control of the swelling time to ensure a perfect interpenetration of the resin into the lattice layers of the clay mineral;
  
• operations at ambient temperature in the presence of air to avoid thermal degradation;
  
• ultrafast curing by using the highly reactive acrylate-based resins and adequate photoinitiators;
  
• fine control of the polymerization rate in a large domain, by adjustment of the light intensity;
  
• the production of cured polymers very resistant to heat and chemicals because of their high crosslink density; and
  
• a wide range of mechanical properties, from soft and flexible composite materials to hard organic glasses, by a proper choice of the telechelic oligomer.

In a recent work,11-14 we successfully used photopolymerization to produce polyacrylate/clay nanocomposites at ambient temperature. We report here how these nanocomposite coatings materials have been synthesized by UV-radiation curing, focusing on both the defoliation of the mineral filler and the photopolymerization of the resin, as well as on some of the properties of the UV-cured nanocomposite.

Experimental

A typical UV-curable resin was made of a telechelic acrylate oligomer (Ebecryl 284 or 8402 from UCB), a reactive diluent (25 wt% of hexanediol diacrylate) and a photoinitiator (a combination of Darocur 1173 and Irgacure 819 from Ciba SC). The mineral filler selected was a natural clay (Bentonite, Montmorillonite K-10), which was made organophilic by treatment with an alkylammonium salt11, or Nanomer I-30 E from Nanocor. The filler was dispersed into the UV-curable resin and exposed to ultrasound for seven hours in the dark to ensure an effective defoliation. Another type of nanocomposite material was made by using as a filler acrylated silica nanoparticles (10 to 50 nm size), Highlink® NaNOG from Clariant.15

Samples were exposed to the UV radiation of a medium-pressure mercury lamp, either on a Minicure line from IST (power output of 80 W/cm and light intensity of 500 mW cm-2), or on a Novacure irradiation device from EFOS equipped with an optical guide where the light intensity could be varied between 15 and 400 mW cm-2. All the irradiation experiments were performed at ambient temperature in the presence of air.

The degree of polymerization of the UV-irradiated sample was evaluated by using Infrared spectroscopy (IR) to measure the decrease of the acrylate double bond at 810 cm-1 for thin films (£ 50 µm) or at 6,160 cm-1 for 2-mm-thick plates. The presence of the clay filler did not affect the monitoring of these bands. In some experiments, the sample was exposed simultaneously to the UV beam and to the IR beam, thus allowing conversion versus time curves to be directly recorded by this technique (real-time IR spectroscopy). Low-angle X-ray diffraction was used to evaluate the efficiency of the organophilic treatment of the mineral filler and quantify the widening of the clay galleries, as well as to assess the extent of defoliation after incorporation of the organoclay into the UV-curable resin.

Results and Discussion
Exfoliation of the Filled Resin

To obtain a true nanocomposite material, it is essential that the 1-nm-thick silicate platelets be uniformly dispersed within the polymer matrix. This can only be achieved by rendering the mineral filler organophilic, so as to allow the UV-curable resin to penetrate into the clay galleries and make them fall apart. Exchanging the alkali cations (Na+, K+, Ca++) by cationic surfactants, such as alkylammonium salts, proved to be an effective way to make the clay compatible with the acrylate resin. This treatment leads to a widening of the clay galleries from 12 Å to 18 Å, as shown by the shift toward small angles of the X ray diffraction pattern (Figure 1).

The effectiveness of the cation exchange has been confirmed by thermogravimetric analysis. For the untreated bentonite, a 10% weight loss was observed above 100 °C due to the release of the water absorbed, without much weight change as temperature was raised up to 600 °C, as shown in Figure 2. For the organoclay, a similar but less important initial behavior was observed (5 wt% loss), but it was followed by a continuous decrease of the sample weight up to 25% at 600 °C, which was attributed to the thermodegradation of the organic moiety.

When the organoclay is introduced into the UV-curable acrylic resin, at a typical concentration of 5 wt %, exfoliation occurs within a few hours, as demonstrated by the following observations:

• a total disappearance of the X-ray diffraction band of the organoclay and thus of the crystalline structure, which is not the case for the untreated clay (microcomposite), as clearly shown in Figure 3;
  
• a much slower sedimentation of the mineral particles in the nanocomposite resin than in the microcomposite resin13 ;
  
• a greater transparency because light scattering by the nanoparticles is much reduced, compared to the microparticles in the non-exfoliated sample; and
  
• transmission electron microscopy pictures of the nanocomposite show both isolated particles (exfoliation) and stacks of silicate platelets (intercalation).14

The liquid resin containing the randomly distributed nanosize silicate platelets was then hardened by a short UV exposure to obtain a nanocomposite material.

UV-Curing of the Filled Resin

The UV-curable polyurethane-acrylate (PUA) resin used in this study was previously shown to polymerize rapidly upon UV irradiation, with formation of a tight tridimensional polymer network.10 The addition of the organoclay was found to have no significant effect on the polymerization kinetics, similar conversion versus time curves being recorded by RTIR spectroscopy upon UV exposure for the two samples, as shown in Figure 4. It can be seen that, after a fast start, the crosslinking reaction is slowing down and the acrylate conversion is levelling off at a value of 80%, most probably because of severe mobility restrictions in the glassy polymer formed. An even faster and more complete polymerization was achieved by rising the light intensity from 100 to 500 mW cm-2 and performing the photopolymerization on a Minicure UV-line (Figure 4). This effect was attributed to an increase of the sample temperature caused by the ultrafast exothermal polymerization (85% conversion in 0.1 s) and the resulting enhanced molecular mobility.16

It was even more pronounced for thick samples, as shown by the nearly complete polymerization achieved in a 1-mm-thick plate UV-irradiated at a light intensity of 50 mW cm-2 in the presence of air, where the temperature was found to rise up to 120 °C within seconds, as shown in Figure 5. Here again, the addition of the organoclay was found to have no slowing down effect on the curing reaction, mainly because of the remarkable transparency of the nanocomposite material.

Relatively thick nanocomposite samples (up to 1 cm) have been obtained by this UV-curing process, because the acylphosphine oxide photoinitiator used (Irgacure 819) undergoes a fast photobleaching reaction, thus allowing the incident UV radiation to penetrate progressively deeper into the sample upon irradiation and promote a frontal polymerization.17 This photoinitiator has also the distinct advantage of absorbing in the near-UV region (350-400 nm), thus being well suited for producing such nanocomposite materials by simple exposure to sunlight. Up to 95% of the acrylate double bonds did polymerize within 20 seconds, due to a substantial rise of the sample temperature upon such solar curing.

A similar study has been carried out by using as UV-curable resin a cycloaliphatic diepoxide (Araldite CY-179 from Ciba SC). In the presence of a diaryliodonium salt (Irgacure 250 from Ciba SC), the photoinitiated cationic ring-opening polymerization proceeds effectively upon UV exposure on the Minicure line: 70% conversion within 0.4 s, or a UV dose of 200 mJ cm-2 (Figure 6). Here again, the addition of the organoclay (Nanomer I-30 E) was found to have no significant effect on the polymerization kinetics. Such UV-cured epoxy nanocomposites proved to be very hard and scratch resistant, which makes them well suited for coating applications on rigid substrates. Epoxy coatings were found to become more flexible and more resistant to impact by addition of small amounts (3 wt%) of silicate nanoparticles. Their resiliency increased from 19 kJ m-2 for the UV-cured epoxide to 46 kJ m-2 for the nanocomposite material.13

Another type of nanocomposite material was synthesized by photopolymerization, in which the mineral filler consisted of acrylated silica nanoparticles (Highlink® NaNOG). As these acrylate double bonds undergo copolymerization with the acrylic resin, they will be chemically bonded to the tridimensional polymer networks, as represented schematically in Figure 7. Here again, the mineral filler was found to have no detrimental effect on the UV-curing process, even at high loads (30 wt%).5

Properties of UV-Cured Nanocomposites

According to recent morphology and rheology investigations, 18,19 in situ-generated anisotropic nanoparticles assemble to form a skeleton-like superstructure, which is considered to account for the enhanced properties. The true nanocomposites synthesized by UV radiation curing will therefore exhibit the distinct characteristics that are typically found in this kind of materials.8 Compared to the conventional microcomposites made of the same components, nanocomposite photopolymers show:

• a lower permeability to gas due to a labyrinth effect;
  
• an excellent chemical resistance owing to their high crosslink density, which makes them totally insoluble;
  
• a greater thermal resistance;
  
• a higher flame retardancy; and
  
• improved stiffness and strength.

The water permeability of such UV-cured acrylic coatings was significantly reduced by the addition of a few percent organoclay. The effect was particularly pronounced for silicone-acrylate coatings, placed in a 100% humid atmosphere, where the water uptake rate was cut by a factor of 3 for the clay-nanocomposite.14

The mechanical properties of the nanocomposite will depend mainly on the type of UV-curable resin selected, and they can therefore be modulated in a large range, depending on the considered application. One can thus produce either flexible and impact-resistant nanocomposites, or hard and scratch-resistant materials. Figure 8 shows some typical stress-strain curves obtained with a UV-cured polyurethane-acrylate (Ebecryl 8402) plate containing 3 wt% of a clay filler (microcomposite) or of an organoclay filler (nanocomposite). It can be seen that both the tensile strength and the elongation at break are superior in the nanocomposite material, by contrast to the results reported recently by Redmer et al. on similar UV-cured acrylate nanocomposites.20 The Persoz hardness of this glassy material was measured to be on the order of 350 s. Softer nanocomposites can be obtained by using a monoacrylate as reactive diluent. The viscoelastic properties (Tg and elastic modulus) were not much affected by the introduction of clay nanoparticles in the photopolymer, in agreement with data recently reported by Uhl et al.21

One of the key properties of organic coatings used to protect different types of materials (metals, plastics, wood, paper) is their resistance to scratching. In this respect, UV-cured acrylic coatings were found to outperform most of the thermoset acrylate coatings, like those used as automotive finishes. The addition of silica nanoparticles causes a significant improvement of the scratch resistance measured by either the Taber test (load applied on a diamond tip) or the Scotch-brite test (loss of transparency after 200 double rubs), as shown in Figure 9, in full agreement with previous observations.22

Concerning the optical properties, the presence of nanoparticles is reducing the transparency of nanocomposites, but much less than in a conventional composite material. Its importance will depend on the particle size (perfectly clear UV-cured nanocomposites were obtained by using a colloid silica-acrylate filler5,6). Clay-based composite materials often show some discoloration, but we succeeded in producing colorless UV-cured nanocomposites by using a synthetic beidellite as mineral filler.

Another effect of silicate nanoparticles is to reduce the gloss of the UV-cured coating, as shown in Figure 10. An organoclay content of 5 wt% proved to be already sufficient to make the gloss value drop from 90% to 20%. While this effect may be a drawback for some applications where glossy materials are desired (automotive topcoats), it can be an advantage in some others, like for wood coatings or floor finishes. Low-gloss UV-cured coatings are usually difficult to obtain without increasing too much the formulation viscosity. In this respect, organoclay appears as an effective and cheap matting agent; unlike the conventional silica-based matting agents, low amounts of this filler are sufficient to reduce sharply the gloss, because of the randomly distributed nanoparticles (Figure 10).

For organic coatings used in outdoor applications, the resistance to weathering is a critical issue. UV-cured clay nanocomposite coatings based on aliphatic polyurethane-acrylates proved to be very resistant to accelerated weathering, when properly stabilized with a UV absorber and a HALS-radical scavenger.23 They remain clear and glossy after as much as 5,000 h QUV-A exposure, without delamination from their metallic support14, thus ensuring an effective and long-lasting protection.

Conclusion

An original method has been developed to synthesize rapidly nanocomposite materials showing superior properties. Layered silicate-based nanocomposites have been produced at ambient temperature by UV-irradiation of a multifunctional acrylate resin filled with in situ-formed nanosilicate platelets. The clay mineral had to first be made organophilic by exchange of the intergallery alkali cations for alkylammonium cations, in order to improve compatibility with the resin and achieve effective exfoliation, which was demonstrated by TEM and X-ray diffraction. A short exposure to intense UV radiation proved to be sufficient to obtain a hard and insoluble polymer. The 1-nm-thick silicate particles dispersed in the polymer matrix are responsible for a significant improvement in some of the properties of the clay/acrylate nanocomposite, which is achieved already at a low filler content (5 wt%). Such performance, together with cost-effective and environment-friendly processing, are key factors for the success of these newly developed photopolymer nanocomposites, mainly in the coating industry.

This paper was presented at the Nano and Hybrid Coatings Conference sponsored by The Paint Research Association, January 2005, Manchester, UK. Conference proceedings are obtainable by contacting Janet Saraty, Conference Administrator, at j.saraty@pra.org.uk.

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