The effect of polyhedral oligomeric silsesquioxane dispersant and low surface energy additives on spectrally selective paint coatings with self-cleaning properties
Introduction
Optical [1], [2], [3], [4] and thermal [5] properties of Thickness-Insensitive Spectrally Selective (TISS) paint coatings that can be applied onto metal and non-metal surfaces are well documented. The oldest version of TISS paint coatings is a black coating consisting of a black spinel pigment dispersed in silicone or polyurethane resin binders, providing black TISS coatings with solar absorptance (as) of about ~0.91 [6]. Recently, colored TISS paint coatings have also been developed from pigments of various shades, imparting TISS coatings with appropriate colors for use as coatings for façade solar absorbers, where aesthetic appearance is important [3], [4]. Low thermal emittance of black and colored versions of TISS paint coatings is assured by the addition of metallic flake pigment. The lowest eT values achieved for both types of coatings have been eT=0.35–0.41 [5].
Even though spectral selectivity has the largest impact on the solar-to-thermal conversion efficiency of solar thermal collectors, merely the improvement of their spectral selectivity is not enough for unglazed solar façade absorbers, and other properties, such as abrasion resistance of the absorber coatings, their UV stability and self-cleaning properties should also be considered in order to retain the persistency of spectral selectivity, high solar-to-thermal conversion efficiency and long (>25 years) exploitation time of the solar thermal system. This is not an easy task and can only be achieved using solar absorber coatings that preferentially exhibit multifunctional properties. Accordingly, we synthesized various nanocomposites based on POSS, simultaneously possessing chemical reactivity and low surface energy, and applied them as dispersants for making pigment dispersions and for achieving self-cleaning properties of TISS black coatings expressed by their hydrophobicity and oleophobicity.
The first part of this study deals with the properties of resin binders. In this study, fluoropolymer resin binder (Fig. 1) was selected. Fluoropolymers have not been used extensively as binders for solar absorber paint coatings merely because their temperature stability is inferior to that of silicones. In glazed solar collectors, stagnation temperatures can be up to 160–170 °C, even when the spectral selectivity is moderate (i.e., as ~0.90, eT~0.40) [5]. Moreover, the high cost of fluoropolymers also does not favour their commercial use particularly because, in general, spectrally selective paint coatings are expected to be cheaper than CVD coatings with high spectral selectivity (Sunselect, Tinox, etc.) used today.
On the other hand, the extremely high resistance of fluoropolymers to UV radiation (survival time >30 years) and their expected inherent water-repellent properties [7], make these binders interesting candidates for TISS paint coatings, particularly for unglazed façade solar absorbers. For such systems, temperature stability is not a critical issue because stagnation temperatures for unglazed solar thermal systems, even for the highest spectrally selective coatings, never surpass 120 °C.
Another aspect that favours the use of fluoropolymers as binders for TISS paint coatings for façade solar absorbers is their inherent low thermal emittance. Fluoropolymers are known to have low absorption in the thermal infrared spectral region, which originates from the presence of weakly absorbing –CF groups dominating their molecular structure [8]. Accordingly, in this study, we firstly determined the infrared absorption properties of the selected fluoropolymeric binder and compared them to the corresponding absorptions of polyurethanes and silicones used so far for TISS paint fabrication.
The second part of this study deals with preparation of the pigment dispersions, which represent the key material for making paints and the corresponding TISS coatings. In general, the pigment dispersion, i.e. the distribution of pigment particles in the selected polymer resin binder (including fluoropolymers), and their particle size has by far the highest impact on the achieved spectral selectivity of any paint coating. In TISS paints, the pigment loadings (black and Al flakes) (expressed by the Pigment Volume Concentration ratio—PVC) are much higher (up to 40%, i.e., at least two times higher) than ordinary paints, in which the pigment particles are fully surrounded by the resin binder. However, excessive use of binder inevitably enhances the thermal emittance of the paint coatings. Because the use of a binder cannot be avoided, it is essential to reduce its amount as far as possible, without hampering the mechanical properties of the coatings. This can be achieved only for paints made of pigment dispersions consisting of pigment particles wrapped around with properly selected dispersant molecules, which are covalently adsorbed onto their surface and also ensure the compatibility of the binder with functionalized pigment particles with the binder molecules. Due to the compatibility of the binder with the functionalized pigment particles, they can form closely packed particle layers with the highest possible opacity for solar radiation (high solar absorptance as) and since the dispersant molecules allow the linking together of particles with the smallest amount of resin binder, the ensuing coating exhibits low thermal emittance (eT). In order to make highly spectrally selective TISS paint coatings, IB7 T7(OH)3 POSS molecules (Fig. 1) were applied as dispersant for the black spinel pigment in Lumiflon resin binder and used for making paints and TISS paint coatings.
The preparation of pigment dispersion is a complex task, which can be achieved by either of the functionalisations of pigment particles: modification by physical interaction (i) or modification by chemical interaction (ii), both can be performed either in solutions or in melts [9], [10].
Surface modification based on physical interaction (i) is usually done with surfactants or macromolecules, which are attached to the surface of the pigment. The principle of surfactant treatment is the preferential adsorption of a polar group of a surfactant or macromolecules, due to electrostatic interactions. They lead to a reduction of interactions between the pigment particles, preventing their agglomeration and enabling their incorporation into the polymer binder matrix.
For the production of commercial TISS paints [(Solarect-Z24®, is manufactured by Color d.d. (SI)], organic dispersant have been used exclusively as described in patents [11], [12], [13]. The same procedure is also described for the preparation of Thickness-Sensitive Spectrally Selective (TSSS) coatings on a water-based binder system [14]. The latter differs from the previous ones only in the use of a different dispersant, which is suitable for an aqueous medium. The optimal PVC ratios for all paints mentioned above are between 16% and 20%, while the thickness of the applied coatings is expressed by the weight (in g) of dry coating per unit of area (m2), e.g., 2 g/m2 (up to 2 μm).
On the other hand, reaction of the pigment surface by chemical reactions (ii) relies on chemical interactions between the modifiers and the pigment surface. Modifiers are usually alkylakoxysilanes [10] or grafting polymers. Specifically, silane modifiers have hydrolysable groups while grafting is achieved by polymers having organofunctional ends. Silanes with the general formula R′-SiOX3 (R′=alkyl chain and X=Cl or alkoxy) in the presence of water readily form reactive silanol groups required to form the Si–O–Metal bonding (M=Si, Ti, Mn…) on the pigment (usually oxide of transition metals, or just TiO2 or silica) [15]. A variety of R′ exists, enabling the functionalization of pigment particles in order to match their compatibility with the polymer binder matrix [9]. The functionalization of nanosilicas is a typical example of using silanes for providing stable and non-agglomerated dispersion of nanoparticles in various resin binders. However, they have not so far been used for the preparation of TSSS or TISS paint coatings [16]. In this study IBTMS was also used as an example of a simple silane dispersant for making pigment dispersions and its effect was compared to the corresponding TISS paint coatings.
The use of trisilanol isobutyl (IB7 T7(OH)3 POSS), bearing also Si–OH functionality such as IBTMS, represents a step forward in making pigment dispersions. IB7 T7(OH)3 POSS have recently been tested and used in the modification of nanosized TiO2 particles pigments, which were then incorporated in polypropylene (PP) polymer [17]. The results revealed that the silanol groups of the IB7 T7(OH)3 POSS cage are bound to the surface of the TiO2 particles [17], creating a layer of isobutyl POSS that improved the compatibility with the PP matrix. Importantly, it was also demonstrated that functionalization with the IB7 T7(OH)3 POSS leads to a reduction in the average size of the TiO2 (from 70 nm for neat TiO2 to 50 nm for chemically treated particles), thereby increasing the covering efficiency the TiO2/PP blends. Chemically similar trisilanol phenyl POSS has also been incorporated in PMMA with good success [18] and blended into polycarbonate (PC) but, conversely, trisilanol isooctyl POSS makes bulk PC opaque, indicating poor dispersion of the POSS [19].
IB7 T7(OH)3 POSS belongs to the family of more common octasilsesquioxanes (T8), discovered in 1946, when the molecule was the first produced from the cohydrolysis of methyltrichlorosilane with dimethylchlorosilane [20], [21]. In brief, octameric T8 consists of cluster-like oligomers of the type (R–SiO3/2)n (n=6, 8, 10, 12, …). They have a cage-like structure, in contrast to the open cage structure of trisilanol IB7 T7(OH)3 POSS, with a stable inorganic Si–O3/2 core surrounded by organic corona (represented by substituent R), resembling in this respect organically functionalized nanosized particles of SiO2 [22], [23]. The diameter of these monodispersal particles ranges from 1 to 3 nm, depending on the composition of the cage. The substituents can be varied widely to provide a range of different properties or to increase or reduce compatibility with a polymer matrix, or they can be made reactive to allow copolymerization or graft polymerization with a wide range of monomers. For example, POSS have been already used for the modification of thermoplastics [24], [25], [26], [27], polyolefines [28], polyurethanes [29], [30], olefins [31], [32], acrylates [33], [34] and thermosets, the latter mainly in relation to epoxy resins [35], [36]. In general, these copolymers exhibit organic/inorganic hybrid properties, demonstrating to various degrees an enhanced modulus, stiffness, flame retardancy and thermal stability in comparison with the base materials.
Surprisingly, no reports of the modification of other pigment particles used for the manufacture of commodity paints can be found in the literature, even though POSS silanols possess several distinct advantages over traditional silane surface modifiers. They can be applied in a single step onto the pigment surface, since they posses stable silanol moieties. They do not react readily with one another by forming siloxane bonds (Si–O–Si), but preferentially lead to the formation of only a monolayer of POSS on the pigment surface. By avoiding the formation of oligomers (and other condensation products), the agglomeration of the pigment particles is thus minimized. It should be noted that a single POSS molecule has three reactive silanol groups per molecule, which enables the establishment of a robust pigment/POSS interfacial bonding, exceeding that which is usually met for simple silane coupling agents. Strong interactions are also expected due to the enormous surface area to volume ratio of POSS. The polymer–POSS interaction increases dramatically compared to other coupling agents, such as silanes. Accordingly, in this study, black pigment was functionalized with IB7 T7(OH)3 POSS and corresponding dispersions were used for making TISS paint coatings having the same composition as those which have been already reported [3], [4]. In order to establish the effect of dispersant SEM, micrographs of the novel TISS paint coatings were measured and their solar absorptance and thermal emittance values established from the hemispherical reflectance spectra measurements. The IB7 T7(OH)3 POSS that was chemically bonded onto the pigment surface was established from the thermogravimetric (TGA) measurements combined with infrared spectra measurements of the un-functionalized and treated, washed and unwashed pigment.
The last part of this study was devoted to assessment of the wetting properties of TISS paint coatings made by the addition of synthesized heteroleptic POSS compounds to paint made of pigment dispersion prepared with the aid of IB7 T7(OH)3 POSS dispersant and Lumiflon binder, aiming to achieve the self-cleaning properties of corresponding solar absorber coatings. It is important to note that the hydrophobic properties of TISS paint coatings can already be provided using IB7 T7(OH)3 POSS dispersant, since the functionalized pigment particles can exhibit inherent water-repellent properties. However, hydrophobicity itself is not sufficient for obtaining self-cleaning properties and coatings must also be oleophobic. This led us to add various POSS-bearing perfluoro groups to the resin binders and to compare the effect of their addition to that obtained by the commercial agent Zonyl [DuPont]. Although among the various POSS functionalized with perfluoro groups, octameric perfluorodecyl T8 POSS is the material with the lowest surface energy [37], the corresponding POSS lacks the reactive groups needed for anchoring the POSS into the paint system. Accordingly, we synthesized heteroleptic POSS consisting of amino (AP), isooctyl (IO) and perfluoro (PF) groups (nominal chemical composition AP2IO4PF2 T8 POSS) attached to the same –(SiO3/2)8 core (Fig. 1). The functionalization of POSS with amino groups stemmed from the fact that they readily react with 1,6-disiocyanatohexane (DICH), which served to cross-link the resin binder. Homogeneous distribution of the POSS in the fluoropolymer binder matrix was thus achieved, hindering or even completely preventing the POSS from being expelled onto the paint surface [38]. It should be noted that AP2IO4PF2 T8 POSS has already been used as a textile finish, imparting superhydrophobicity to cotton fabrics (water contact angle θwater=150°) and notable oleophobicity (θn−hexadecane=125°) properties [39].
Section snippets
Instrumental
IR absorption spectra of the polymer binders and POSS were deposited on silicon wafers and recorded on a Bruker IFS 66/S with a resolution of 4 cm−1 and 32 scans for each sample. IR spectra of functionalized pigment F 3060 was recorded in KBr pellets prepared by mixing the pigment with the appropriate amount of KBr.
The optical properties of the paint coatings were determined from the measured IR absorption and reflectance spectra. Reflectance spectra in the visible (VIS) and near-infrared (NIR)
Average absorption coefficient of commercially available binders
In order to make absorber coating with the smallest possible thermal emittance, it is necessary to reduce the amount of thermally emitted radiation to the maximum extent. This can be achieved by optimizing (i.e., decreasing) the amount of resin binder in the coating as reported previously [3], [4] and using a binder, which has an inherently small thermal emittance.
The Lumiflon resin binders belong to a family of fluoropolymers, which in many aspects structurally resemble the well-known
Conclusions
The TISS paint coatings studied in this work can be ranked among solar materials that have multifunctional properties; they were spectrally selective and they repelled water and oils. It was demonstrated that these properties were obtained due to the use of POSS nanocomposites, which exhibited inherent multifunctionality due to their specific chemical composition and structure; they consisted of various groups that simultaneously imparted hydrophobicity and oleophobicity to TISS paint coatings,
Acknowledgements
This work was financed by Slovenian research agency in the frame of MATERA-MULTIFUNCOAT Project (Contract no. 3211-09-000029) and Programme P1-0030. The authors wish to thank Miha Stainbuecher for technical support.
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