Hollow latex particles functionalized with chitosan for the removal of formaldehyde from indoor air
Graphical abstract
Introduction
Formaldehyde, a colorless, flammable, and bad smelling gas, is a common precursor in production of more complex materials, e.g., phenol-formaldehyde and urea-formaldehyde resins, which are widely used as wood-binding products and insulating foam (Cogliano et al., 2005). Formaldehyde is also applied in plastics, coatings and textile finishing in spite of the fact that it is carcinogenic in humans (Cogliano et al., 2005, Formaldehyde TEACH Chemical Summary, 2013, http://www.epa.gov/teach/chem_summ/Formaldehyde_summary.pdf, 2013). When absorbed in the upper respiratory tract, it can be oxidized to formate and carbon dioxide or incorporated into biological macromolecules (Heack et al., 1985). At >6 ppm of formaldehyde, cellular proliferation increases and then amplifies the genotoxic effect (Cogliano et al., 2005). In addition, there is strong but not sufficient evidence that formaldehyde causes leukemia. Therefore, emitted formaldehyde gas in indoor air is considered hazardous and should be removed.
Among several methods used in the removal of formaldehyde in an indoor environment, e.g., decomposition using photocatalyst and physical adsorption by porous materials, one of the effective methods is chemical adsorption, where a re-emission is excluded due to strong chemical bonding (Agudo et al., 2004, Obee and Brown, 1995, Wada et al., 2005). Formaldehyde gas can be chemically adsorbed by polymeric amines, e.g., chitosan and polyethyleneimine (PEI) via a reaction of primary (1°) amines to form azomethine or Schiff base, a compound containing an aryl or alkyl carbon-nitrogen double bond (Da Silva et al., 2011, Gesser and Fu, 1990, Wada et al., 2005, Yang et al., 2011). Enhancement of formaldehyde adsorption was achieved by increasing the content of amines in the adsorbent. Nonporous TiO2 fiber functionalized with high PEI content showed high adsorbent efficiency, and was used as quartz crystal microbalance (QCM) sensors (Wang et al., 2012). Chitosan-hybridized acrylic resin was used as a binder in interior finishing coating for formaldehyde adsorption by employing the reaction between amino group of chitosan and carbonyl of formaldehyde (Wada et al., 2005). Most importantly, when the Schiff base is formed, adsorbed formaldehyde was not released from chitosan-hybridized acrylic resin film, even with heat treating the film at 60 °C for 2 h.
For preparation of chitosan-coated particles, colloidal particles having high surface area were used as a substrate to increase the amount of attached chitosan. As a cationic polymer, chitosan is conveniently coated onto oppositely charge colloidal particles, e.g., poly(styrene-co-acrylic acid) (P(St/AA)), via the Layer by Layer (LbL) technique. The thickness of the adsorbed chitosan layer increased when P(St/AA) particles containing a higher amount of carboxylic acid groups on the surface were employed (Du, Liu, Mu, & Wang, 2010). Chemical structure also has a strong effect on the coating efficiency. Compared to the high charge density linear poly(allylamine hydrochloride) (PAH), the lower charge density branched PEI formed a thicker layer on silica nanoparticles (Nypelo, Osterberg, Zu, & Laine, 2011).
In this study, carboxylated hollow latex (HL) polymer particles or void containing particles have hiding power due to multiple scattering between its surrounding medium and void, were used as a substrate for chitosan or PEI coating (Silva & Galembeck, 2010). The composite material was designed to combine both optical property and formaldehyde adsorption ability. Besides higher surface to volume ratio, the advantages of HL particles over conventional white pigments or opacifying agents are; lower density, better UV resistance, lower coefficient to thermal expansion, less agglomeration, and lower cost per gallon (Bourgeat-Lami, 2003, McDonald and Devon, 2002). Therefore, HL particles possessing high surface charge density and strong double shell are prepared via the template based strategy seeded emulsion polymerization, using P(St/AA) as a seed particle and methyl methacrylate (MMA)/divinyl benzene (DVB)/AA as monomers (Nuasaen & Tangboriboonrat, 2013). The spherical and non-collapsed HL particles having voids of ca. 280 nm in size are then coated with chitosan or PEI to produce HL-chitosan or HL-PEI particles via the LbL technique. The materials are used as formaldehyde adsorbent. The chemical composition, size, charge and morphology of HL-chitosan and HL-PEI particles are characterized by Fourier Transform Infrared (FTIR) spectroscopy, thermogravimetric analysis, dynamic light scattering, and transmission electron microscopy. Effect of type of polymeric amines on formaldehyde adsorption behaviors and the adsorption mechanism are also analyzed by FTIR spectroscopy.
Section snippets
Materials
Styrene (St) (Sigma–Aldrich, Purum) and methyl methacrylate (MMA) (Fluka, Purum) were purified by passing through a column packed with neutral and basic aluminum oxide (Fluka, Purum), while acrylic acid (AA) (Aldrich, Purum) was distilled before use. Divinyl benzene (DVB) (Merck, Synthesis), potassium persulfate (KPS) (Fluka, Puriss), and 40% formaldehyde (Carlo Erba, Analysis) were used as received. Polyethyleneimine (PEI), MW of 750,000 g/mol (Aldrich, 50 wt% aqueous solution) was diluted to 2
Analysis of amine functionalized HL particles
FTIR spectra of PEI, chitosan, HL, HL-PEI, and HL-chitosan are shown in Fig. 1.
The characteristic peaks at 3440 and 1558 cm−1 in Spectrum 1A correspond to NH stretching and NH vibration of PEI (Jenjob, Tharawut, & Sunintaboon, 2012). Spectrum 1B illustrates the characteristic bands at 3440 cm−1, relating to OH and NH stretching, whereas two bands at 1650 and 1558 cm−1 are attributed to CO of amide I and NH of amide II (He et al., 2012). The existence of PMMA, PAA, PS, and PDVB in HL particles are
Conclusions
HL-chitosan and HL-PEI particles, prepared by the seeded emulsion polymerization and the LbL technique, can chemically adsorb formaldehyde via the nucleophilic addition of amines to carbonyls of formaldehyde, followed by the elimination of water molecules. FTIR spectra reveal that the two species, i.e., carbinolamine intermediate and Schiff base, are generated when HL-chitosan is used in formaldehyde adsorption tests, whereas, the Schiff base structure is not clearly observed in HL-PEI. In both
Acknowledgements
Research Grant (RTA5480007) from The Thailand Research Fund (TRF)/Commission on Higher Education to P.T. and a scholarship from TRF through the Royal Golden Jubilee Ph.D. Program (Grant No. PHD/0190/2553) to S.N. are gratefully acknowledged.
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