Polydopamine film coating on polyurethane foams as efficient “sunscreen”. Application to photocatalysis under UV irradiation
Graphical abstract
Introduction
Polyurethanes (PUs) are used in a large variety of commercial and technical applications, due to their high tensile strength, chemical resistance, good process ability and good mechanical properties (Sanders and Frish, 1962, Oertel, 1985, Hepburn, 1992). In particular, open cell polyurethane foams (OCPUF) are known as remarkable elastomers and are essentially used as filters in cooker hoods, aquariums, mattresses or vacuum cleaners. Based on a polydopamine coating strategy (Lee et al., 2007), we have shown the great interest of using OCPUF as a soft structured catalytic support (CS) for various catalytic reactions (Pardieu et al., 2016, Edouard et al., 2016, Lefebvre et al., 2017, Lefebvre et al., 2019, Ait-Khouya et al., 2019, Birba et al., 2020).
One of the main disadvantages of PU-based materials, however, is their high sensitivity to light, in particular to UV light. This is a major limitation, which relates to their use for outdoor applications or under UV radiation. PUs indeed undergo significant structural changes upon exposure to UV radiation, which cause deterioration of their physical and mechanical properties (Kachan et al., 2004, Davies and Evrard, 2007, Yang et al., 2003, Irusta and Fernandez-Berridi, 1999, Wilhelm and Gardette, 1998). For instance, a polyurethane made with an aromatic isocyanate will yellow upon exposure to UV light due to physical and chemical modifications of its surface. Although various analytical techniques or spectroscopies such as Fourier-Transform Infra-Red spectroscopy (FT-IR), scanning electron microscopy (SEM), or X-ray photoelectron spectroscopy (XPS) have been used to study the accelerated weathering degradation of PU systems, the exact mechanism of photolytic degradation of PU remains, to our knowledge, unsure (Kachan et al., 2004, Davies and Evrard, 2007, Yang et al., 2003, Irusta and Fernandez-Berridi, 1999, Wilhelm and Gardette, 1998, Rabek, 1995, Wilhelm et al., 1998, Gavorcin Bojsic et al., 1996, Yang et al., 2001).
In this work, we show that a polydopamine (PDA) guard film prepared by a simple eco-friendly dip-coating methodology in water efficiently protects OCPUF from UV radiation. For that purpose, pristine OCPUF and PDA-coated OCPUF (OCPUF@PDA) samples were compared before and after UV irradiation by FT-IR spectroscopy and on-line measure of volatile organic compounds emanation by gaseous chromatography and ATD-MS. Based on the interesting “sunscreen” properties of PDA, we next used OCPUF@PDA loaded with TiO2 nanoparticles (NPs) for photocatalytic applications.
For photocatalytic applications and water treatment, the catalyst is usually dispersed in an aqueous solution and used in slurry reactors. This type of reactors however generates major drawbacks: (i) the catalyst particles must be separated from the liquid phase, which involves a time and energy consuming process and (ii) the pressure drop is not negligible. To avoid these costly filtration/separation and catalyst recovery steps, the active phase can be anchored on a suitable support and used in fixed or fluidized bed reactors. In this context, Jain and Pradeep (Jain and Pradeep, 2005) used the bactericidal properties of silver nanoparticles anchored on a polyurethane foam for obtaining a logarithmic abatement of 5 over Escherichia Coli-contaminated water at a water flow rate of 0.5 L/min. However, the silver nanoparticles were directly coated on the foam by possible interactions with the nitrogen atoms of the polyurethane. As another example, Yamamoto and Imai (2004) prepared foam-based photocatalysts with the synthesis of anatase titania nanoparticles and open cellular structure by calcination of polyacrylamide gels containing titanyl sulphate and urea. The titania foams showed, under UV illumination, disappearance rates that were 2.5 and 4 times greater than those shown by fluffy commercial TiO2 P25 for methylene blue decomposition in water and gas phase acetaldehyde removal, respectively. The performances were attributed to the high accessibility and the high transparency of UV light due to an open cellular structure with wide channels and thin walls. Using the photocatalytic oxidation of 1,8-diazabicyclo[5.4.0]undec-7-ene in water as a model reaction, Ochuma et al. (2007) evidenced the interesting performances of a TiO2-coated alumina reticulated foam monolith inserted inside an annular photo-reactor compared to a slurry reactor. From these works, it clearly appears that there is an interest in developing a photo-reactor based on OCPUF@PDA materials. In this context, we present a new photoreactor based on OCPUF@PDA@TiO2 foams, which we have tested for water and air treatment.
Section snippets
Materials and methods
Commercial compounds were used as received. Dopamine hydrochloride, Tris base, were purchased from Alfa Aesar. Titanium (IV) oxide, anatase (99.9% - AB208992) was purchased from Sigma. TiO2 NPs present a specific surface and size of 100–220 m2/g and 30–50 nm, respectively. Polyurethane open cell foams were purchased from FoamPartner. Their characteristics are: (i) strut size: ; (ii) window size: ; (iii) cell size: ; (iv) density: 0.027–0.033 and (v) hydrodynamic porosity:
UV irradiation of OCPUF, OCPUF@PDA and OCPUF@PDA@TiO2
The behaviour of OCPUF, OCPUF@PDA and OCPUF@PDA@TiO2 foams under UV and UV irradiation was studied by FT-IR spectroscopy and GC measurements. The PDA content ranges typically from 3 to 5 wt% for OCPUF@PDA and ICP-AES analyses revealed an average Ti loading of 5112 ± 52 mg/kg for OCPUF@PDA@TiO2.
The vibrational changes in the FT-IR spectra of both OCPUF and OCPUF@PDA after UV irradiation can be seen in Fig. 1; important changes are observed in the FT-IR spectrum of OCPUF while no evident IR
Conclusions
Thanks to its lightweight, flexibility, good transport properties, low-pressure drop, and large pores that easily let the light go through it, polyurethane-based soft structured catalytic supports appear as ideal candidates to design original photo-reactors. However, due to the disadvantages of PU-based materials (i.e.: high sensitivity to light, in particular to UV light), polyurethane foams cannot be used directly. In this work, we show that a polydopamine film coating on the foam plays a
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors thank the SATT Conectus-Strasbourg (Projet ‘CleanFoam’), France and the Agence Nationale de la Recherche, France (ANR-16-CE07-0030-01) for financial support of this work. Dr L. Lefebvre is thanked for his help on sample preparation.
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