UV-curable Acrylate Coatings Research Articles

Self-Healing UV-Curable Acrylate Coatings for Wood Finishing System, Part 2: Impact of Monomer Structure and Self-Healing Parameters on Self-Healing Efficiency

Wood is increasingly used in construction for the benefits it brings to occupants and for its ecological aspect. Indoor wood products are frequently subject to mechanical aggressions, their abrasion and scratch resistance thus need to be improved. The coating system ensures the wood surface protection, which is, for wood flooring, a multilayer acrylate UV-curable 100% solid system. To increase the service life of wood flooring, a new property is studied: self-healing. The objective of this study is to observe the impact of monomer structure on self-healing efficiency and the effect of self-healing parameters. A previous formulation was developed using hydrogen bond technology to generate the self-healing property. In this paper, the assessment of the formulation and the self-healing parameters’ impact on self-healing efficiency as well as the physicochemical properties are presented. The composition of the monomer part in the formulations was varied, and the effect on the conversion yield (measured by FT-IR), on the Tg and crosslinking density (measured by DMA) and on mechanical resistance (evaluated via hardness pendulum, indentation, and reverse impact) was analyzed. The self-healing efficiency of the coatings was determined by gloss and scratch depth measurements (under constant and progressive load). It was proven that monomers with three acrylate functions bring too much crosslinking, which inhibits the chain mobility necessary to observe self-healing. The presence of the AHPMA monomer in the formulation permits considerably increasing the crosslinking density (CLD) while keeping good self-healing efficiency. It was also observed that the self-healing behavior of the coatings is different according to the damage caused. Indeed, the self-healing results after abrasion and after scratch (under constant or progressive load) are different. In conclusion, it is possible to increase CLD while keeping self-healing behavior until a certain limit and with a linear monomer structure to avoid steric hindrance. Moreover, the selection of the best coatings (the one with the highest self-healing) depends on the damage.

Self-Healing UV Curable Acrylate Coatings for Wood Finishing System, Part 1: Impact of the Formulation on Self-Healing Efficiency

In the wood flooring sector, good surface mechanical properties, such as abrasion and scratch resistance, are prerequisite. Surface wood protection is provided by finishing systems. Despite coating improvement, scratches formation on wood flooring is unavoidable. A new approach to increase service life is to confer the self-healing property to the finishing system. The most common coatings used for prefinished wood flooring are acrylate UV curable 100% solids coatings. They usually have good mechanical properties and high cross-linking density. The objective of this study was to develop and evaluate an intrinsic self-healing formulation, which is applicable to wood flooring. For this purpose, acrylate formulations were developed with monomers and oligomers carrying hydroxyl groups. To meet the requirements of wood application, hardness, and polymerization conversion of coatings were evaluated. König pendulum damping tests provide information on coating hardness and flexibility. Results around 80 oscillations is acceptable for UV curable wood sealer. The chemical composition was studied by FT-IR spectroscopy while dynamical mechanical analysis (DMA) was performed to determine glass transition temperature and cross-linking density. The self-healing behavior was evaluated by gloss and scratch depth measurements. The formulation’s composition impacted the hydrogen binding quantity, the conversion, the Tg and the cross-linking density. The (hydroxyethyl)methacrylate (HEMA) monomer provided self-healing and acrylated allophanate oligomer allowed self-healing and cross-linking. This study demonstrated that it is possible to combine high cross-linking density and self-healing property, using components with low steric hindrance.

UV-curable acrylic coatings containing biomacromolecules: A new fire retardant strategy for ethylene-vinyl acetate copolymers

This work aims at demonstrating the suitability of selected biomacromolecules, namely caseins and deoxyribonucleic acids, as low environmental impact flame retardant additives for UV-curable coatings. To this aim, Bisphenol A hydroxyl ethyl diacrylate was utilized as UV-curable system, in the presence of a suitable photoinitiator (i.e. 2-hydroxy-2-methyl-1-phenylpropan-1-one), for coating ethylene-vinyl acetate copolymer thick plates. The flame retardant features of the coatings were evaluated in the presence of caseins or deoxyribonucleic acids – DNA – (at 10 and 15 wt.% loading) or a mixture of the two biomacromolecules (15 wt.% total loading) embedded in the UV-curable coating system, by means of forced combustion (i.e. cone calorimetry) tests. The coatings containing the biomacromolecules (namely 10 wt.% of DNA or 15 wt.% of casein) showed a decrease of peak of heat release rate and an increase of the time to peak as compared to the unfilled UV-cured counterparts. The proposed strategy may represent a possible starting point for the development of green and durable alternatives to the use of standard and more environmental impacting flame retarded coatings.

Monitoring of thickness and conversion of thick pigmented UV-cured coatings by NIR hyperspectral imaging

Hyperspectral imaging using a near-infrared (NIR) camera was used for in-line monitoring of thickness and conversion of white pigmented UV-cured acrylate coatings. In particular, it was applied for the quantitative determination of both parameters as well as for the evaluation of the homogeneity of their spatial distribution. Quantitative data were obtained by means of chemometric approaches. Calibration models were built up on the basis of the partial least squares (PLS) regression algorithm using reference data of coatings with thicknesses in the range from about 20–200 μm and with a conversion between about 15 and 80%. Reference values for the conversion were determined by NIR reflection spectroscopy utilizing the acrylate band at 1620 nm. The resulting accuracies (in terms of the root mean square error of prediction, RMSEP) were found to be 8.7 μm for the prediction of the layer thickness and 2.5% for the acrylate conversion. The developed methods were also used for the visualization of the spatial distribution of both parameters across the surface of the UV-cured coating and the detection of inhomogeneities within the layers. The performance of this approach was demonstrated with coatings that formed a pronounced heterogeneous surface structure after UV curing resulting from shrinkage. All NIR measurements were carried under conditions that were similar to those in technical coating processes. Consequently, the results of this study prove that hyperspectral imaging has considerable potential for in-line process and quality control.

Fouling release of UV-cured acrylic coatings: Set-up of an in vitro test with Rhodotorula mucilaginosa

Acrylic coatings were prepared by UV-induced crosslinking of two different diacrylates monomers having the same molecular weight but containing either a polyethyleneoxide chain or an alkylic chain. The fouling behavior of the UV-cured coatings was assessed by a simple set-up, monitoring the growth of Rhodotorula mucilaginosa and the adhesion of the film formed by the pigmented yeast. The effect of the monomers, the photoinitiator and the polymers on the microorganism growth was studied. The monomers partially inhibited the yeast growth, the coatings showed fouling release activity: yeast could grow on both coatings but with different morphology, they detached easily by washing depending on the type of coating. By examining the physico-chemical and mechanical properties of the polymers, the fouling release behavior was found mostly dependent on the surface tension and the water uptake of the coatings.

Multifunctional antistatic and scratch resistant UV-cured acrylic coatings

Organic–inorganic antistatic hybrid acrylic coatings were prepared. Trialkoxy-silyl ammonium salt was added to the photocurable formulations in order to introduce an antistatic additive which could be covalently linked to the hybrid network through a co-condensation reaction involving the alkoxy groups. The influence of the antistatic additive on the radical photopolymerization reaction of the acrylic resin was evaluated by real time FTIR, keeping the inorganic precursor content constant at 20phr and increasing the trialkoxy-silyl ammonium salt in the range between 5 and 15phr. When the samples were cured under nitrogen atmosphere, a complete conversion of acrylic double bonds was achieved after 90s of irradiation both for the acrylic resin and the formulations containing the ammonium salt. The scratch behavior of coatings was investigated by carrying out of progressive load scratch test. The penetration depth (Pd) and the residual depth patterns were investigated for all the hybrid films. A consistent improvement of penetration resistance by increasing alkoxy-ammonium salt content was noted in terms of Pd. Inorganic component and antistatic additives increase ε′ and σAC values of the hybrid coatings and decrease the resistivity ones, showing their efficiency for increasing antistatic properties of coatings, improved with respect to the pure acrylic resin.

Study of the effect of organically functionalized silica nanoparticles on the properties of UV curable acrylic coatings

In this work the properties of UV curable coatings were improved modifying the polymeric matrix adding properly functionalized silica inorganic nanoparticles. Mild steel sheets were used as substrate. Pre-formed silica nanoparticles, prepared by sol–gel chemistry from methyltriethoxysilane (MTES) and methacryloxypropyltrimethoxysilane (MPTMS) precursors, were dispersed into UV curable acrylic oligomers: by irradiation a polymeric matrix embedding the nanoparticles was obtained. Unfilled UV cured coatings were also produced for comparison. The DSC measurements revealed different thermal stabilities among the samples, depending on the presence of the functionalized nanoparticles. The barrier properties (investigated by means of electrochemical impedance spectroscopy, EIS) were demonstrated to be strongly affected by the presence of the nanoparticles. In particular, a proper functionalization of the silica nanoparticles leads to noticeable improvement of the barrier properties of the coatings against ions permeation. The effect of the functionalized nanoparticles on the coating properties was also investigated performing water vapor diffusion measurements through the different coatings. The experimental measurements highlighted that the appropriate functionalization of the silica particles leads to noticeable improvements of the barrier properties against water and ions permeation of the composite polymeric coating with respect to the unfilled UV curable resin.

Simultaneous In-Line Monitoring of the Conversion and the Coating Thickness in UV-Cured Acrylate Coatings by Near-Infrared Reflection Spectroscopy

Near-infrared (NIR) reflection spectroscopy was used for in-line analysis of the conversion and the coating thickness (5-20 μm) of UV-cured clear and pigmented acrylate coatings. The quantitative evaluation of the recorded spectra was carried out by partial least-squares (PLS) regression, in particular with the PLS2 algorithm, which allows simultaneous prediction of both parameters. The efficiency of this method was investigated in roll coating experiments at line speeds up to 100 m min(-1). It was shown that the method is able to compensate for the effect of accidental variations of the coating thickness, which inevitably occur upon changes of the line speed, on the prediction of the conversion. Accordingly, the conversion could be determined with a precision of ±2...3%, whereas the error in the measurement of the thickness was found to be about 0.5-1 μm.

Characterization of Nano-structured UV Cured Acrylic Coatings

This paper deals with the production and characterization of nano-structured UV curable acrylic coatings applied on mild steel substrates for corrosion protection purposes. UV curable acrylic coatings were modified in two ways: by adding pre-formed silica nanoparticles modified with MEMO molecules and by promoting an in situ formation of silica domains functionalized with MEMO molecules. The coatings were characterized by means of both chemical (FT-IR, DSC, UV-Vis absorbance, Gloss) and electrochemical techniques (EIS, Potential vs Time). The coatings showed different thermal stability as revealed by the DSC measurements. The coatings showed also different barrier properties depending on the presence of the nanoparticles or of the silica domains. It was proved that a convenient functionalization of the silica domains (or particles) lead to noticeable improvements of the barrier properties of the coating respect to the pure UV curable polymer.

Thermal and Mechanical Analysis of Cross-Linked Optical Fiber Coatings

A convenient route to enhancing the thermal degradation on-set temperature of existing commercial optical fiber coatings is presented. UV curable acrylate coatings were modified through the addition of a multi-functional cross-linking agent and are shown to increase their degradation temperature by 65degC without any degradation in the mechanical or optical properties of the resultant fiber. Such enhanced thermal robustness in coatings is important for optical fiber applications in high energy laser (HEL) systems and selected higher temperature sensing environments.

Effect of changes of the coating thickness on the in-line monitoring of the conversion of photopolymerized acrylate coatings by near-infrared reflection spectroscopy

Near-infrared (NIR) reflection spectroscopy was used for in-line monitoring of the conversion and the thickness of thin UV-cured acrylate coatings applied to polymer foils. Quantitative analysis of the spectroscopic data was performed either with the aid of PLS-based chemometric models or by band integration according to the Beer–Lambert law. Unintended changes of the thickness of the coating, e.g. caused by variation of the web speed, were found to preclude the correct analysis of the conversion by chemometric methods. In order to correct the conversion data for such changes, NIR spectra were recorded before and after UV irradiation. The conversion was determined from the ratio of the band integrals of the overtone of the acrylic double bond at 1620nm. It was shown that quantitative conversion data with high precision were achieved in this way. The method was used for in-line monitoring of the conversion in clear and pigmented coatings, which were applied to OPP foil by roll coating at line speeds up to 120mmin−1.

UV curing and matting of acrylate nanocomposite coatings by 172 nm excimer irradiation

For UV-curable acrylate coatings reinforced by silica nanoparticles, the effect of 172 nm excimer irradiation on the surface roughness has been studied. A dual UV lamp set-up consisting of a 172 nm excimer lamp and a mercury arc lamp allowed obtaining gloss levels down to 0.5 units (at 60°) depending on the acrylate formulation and curing conditions. Moreover, UV matt-finished sample showed enhanced surface hardness and increased chemical resistance. It is assumed that 172 nm excimer irradiation resulted in a higher network density via additional cross-linking reactions. To study the depth profile of acrylate conversion for coatings cured by the combination of a 172 nm excimer lamp (accountable for surface curing) and a mercury arc lamp (responsible for through curing), FTIR microscopy as well as (Ge)ATR-FTIR having an IR penetration depth of less than 0.5 μm have been applied. Providing the presence of a photoinitiator as well as the absence of oxygen inhibition, similar degrees of double bond conversion of about 90% were observed on the entire area of the cross-section of the coating, i.e. the wavelength of UV irradiation was found to have no significant impact on acrylate conversion.

Weathering performance of thermoset and photoset acrylate coatings

Thermosetting and photosetting acrylate polymers have been tested for their resistance to accelerated weathering. The chemical changes occurring both upon curing and upon photoaging have been followed by infrared spectroscopy. In the photodegraded polymers some functional groups are lost (ether, amide, CH groups) and oxidation products are formed (carbonyl and hydroxyl groups). The UV-cured polyurethane-acrylate coatings were found to be more resistant to accelerated weathering than the melamine/acrylate thermosets currently used as automotive finishes. The light stability of both thermoset and photoset acrylate clearcoats was substantially increased by the addition of a hydroxy-phenyltriazine UV-absorber and a HALS radical scavenger. The weathering resistance of painted metallic panels has been markedly enhanced by means of properly stabilized UV-cured acrylic coatings.

Photo-discoloration of UV-curable acrylic coatings and the underlying wood

Wood specimens coated with four different UV-curable acrylic clear coatings were exposed to UV light. All samples turned yellow after the exposure. Among these specimens, the one coated with aromatic urethane modified acrylic coating showed the deepest yellowing. Uncoated wood specimens covered with free films of the coatings under study were used to simulate coated wood. After accelerated lightfastness tests, it was observed that the color and chemical changes of silicone and aliphatic urethane modified acrylic free films were very slight. However, the discoloration of the wood underneath these two films was quite distinct. This observation makes it obvious that the color change of wood coated with so-called durable coatings was primarily caused by the discoloration of wood underneath, not the coatings. The degree of discoloration of aromatic urethane and epoxy modified acrylic films increases with irradiation time. An increase in discoloration is also evident on the wood specimens underneath these two films. These facts show that in addition to inhibiting the discoloration of coatings, it is also necessary to reduce the intensity of ultraviolet light reaching the wood underneath the coatings.

In-situ generation of light-stabilizers upon photoaging of thermoset- or UV cured-acrylate coatings

A 2-hydroxy phenylbenzotriazole (HBTA) light-stabilizer was generated by photolysis of the 2-acetate derivative (ABTA) in an acrylate coating. The ABTA disappearance upon QUV aging was monitored by UV spectroscopy and shown to follow a single exponential law, with an initial quantum yield of 5.7 × 10 −3 mol einstein −1. A good correlation was found between the first order rate constants of ABTA photolysis and HBTA formation. The distribution profile of the photoproduct within the irradiated film exhibits a sharp decrease from the UV-exposed face toward the deep lying layers, due to a strong internal filter effect. In-situ photogeneration of UV absorber proved to be a technique particularly well suited to stabilization of UV-cured coatings, since the ABTA precursor has only a minor effect on the cure speed. The HBTA photoproduct formed upon QUV aging acts as an efficient UV absorber and to a large extent prevents the photodegradation of the polymeric support.

Fluoropolymer barriers to stress corrosion in optical fibers

Some recent studies on fluoropolymers that exhibit substantially increased barrier properties against the migration of moisture when compared to conventional fiber coating materials, and as a result may offer unique advantages for use in optical fiber applications, are reported. Moisture vapor permeability studies of cis-trans fluoropolyol polyacrylate indicate that it can offer up to an order of magnitude reduction in H2O permeability relative to commercially available UV curable acrylate coatings. Experimentally determined permeability coefficients were of the order of 10−12 g cm−2 s−1 cm/mm Hg and diffusion coefficients were of the order of 10−10 cm2 s−1. In addition to improved barrier properties, incorporation of fluorine into the polymer structure lowers the index of refraction of the polymer enabling a material to be tailored for use as a cladding in conjunction with silica glass. The index of refraction of cis-trans fluoropolyol polyacrylate was determined, using index matching oils, to be 1.437 (lower than the silica core) permitting its use as a cladding. Other material parameters relevant to fiber coatings have also been measured. The elastic modulus E of the material was determined to be 1010 dyn/cm2 with a slow drop to 108 dyn/cm2 at the glass transition temperature ∼45°C. In addition, an improved formulation of cis-trans fluoropolyol polyacrylate is presented that allowed in-line coating of the optical fiber in an oxygen environment.

Testing of gloves for permeability to UV-curable acrylate coatings.

The handling of UV-curable acrylate formulations used in the coating of optical fiber requires protective measures to prevent contact dermatitis and/or allergic dermatitis. To characterize the permeability of various glove materials to a UV-curable acrylate coating, a study was undertaken using a modification of a standard ASTM permeability test, which demonstrated that nitrile rubber gloves provided the best protection of those glove materials tested.

Testing of Gloves for Permeability to UV-Curable Acrylate Coatings

The handling of UV-curable acrylate formulations used in the coating of optical fiber requires protective measures to prevent contact dermatitis and/or allergic dermatitis. To characterize the permeability of various glove materials to a UV-curable acrylate coating, a study was undertaken using a modification of a standard ASTM permeability test, which demonstrated that nitrile rubber gloves provided the best protection of those glove materials tested.