Colloidal silica - latex polymer nanocomposites for high performance coatings.INTRODUCTION Nanotechnology will have a significant impact on the paint and coatings market through two channels: the improvement of existing surface coatings and the innovation of new products that deliver the right blend of performance and aesthetics. Nanotechnology is the study of material properties with particle size below 100 nm. Hybrid organic-inorganic composites have attracted widespread attention in the domain of material science. (1) The controlled combination of organic and inorganic composite colloids is of increasing interest as it enables chemists to create new nano-structured materials with unusual morphology, composition, and properties originating from existing building blocks. [ILLUSTRATION OMITTED] Long lasting performance and good aesthetics are two key performance components for exterior paint. Over time, exterior exposure leads to the accumulation of dirt, air pollutants, dust particles, and debris on the paint film. This results in a dirty appearance which no longer meets the expectations of the consumer. It is well known in the industry that a high level of dirt resistance can be achieved through the use of hard polymers. But, these polymers can also jeopardize film formation, which can then also require high levels of coalescing coalescing (kō  les´ing),n a joining or fusing of parts. solvents. Chemically, there is a wide range of nano material particles that are of interest for use in coatings. To date, the main focus in architectural coatings has been on incorporating nano Si[O.sub.2], [Al.sub.2][O.sub.3], and ZnO to produce functional properties such as scratch resistance, mildew resistance, and increased durability in the paint film. This article will document hybrid polymer technology which combines the advantage of one polymer species with another to synergistically syn·er·gis·tic adj. 1. Of or relating to synergy: a synergistic effect. 2. Producing or capable of producing synergy: synergistic drugs. 3. create a higher performing class of polymers. Further, this article will discuss silica-latex hybrid technology and its applications, particularly in the design of dirt pick-up resistant coatings. BACKGROUND In recent years, considerable effort has been devoted to the design and controlled preparation of hybrid organic-inorganic composites using colloidal particles as building blocks. Among colloidal colloidal of the nature of a colloid. colloidal bath a bath containing gelatin, bran, starch or similar substances, to relieve skin irritation and pruritus. materials, silica has been studied extensively because of its well known uses in a variety of applications including high performance coatings, photonic devices, magnetic fluids, and paper coating. (2) Furthermore, the incorporation of inorganic materials such as colloidal silica at the nanoscale level can enhance the fire retardancy and mechanical strength of organic polymers and their thermal properties. Including an organic polymer phase in the organic-inorganic matrix reduces the brittleness found with pure inorganic materials, resulting in reduced stress cracking during the drying process of coatings. Unfortunately, the simple mixing of organic and inorganic components at the macroscopic macroscopic /mac·ro·scop·ic/ (mak?ro-skop´ik) gross (2). mac·ro·scop·ic or mac·ro·scop·i·cal adj. 1. Large enough to be perceived or examined by the unaided eye. 2. level is tedious, very energy intensive, and usually not very successful. There are various methods to make silica/organic nano composites, but in order to achieve the desired improvement by adding nanoparticles, the targeted organic polymer must display specific interaction tendencies with the inorganic particles. Heterogeneous polymerization polymerization Any process in which monomers combine chemically to produce a polymer. The monomer molecules—which in the polymer usually number from at least 100 to many thousands—may or may not all be the same. , such as emulsion polymerization, has been used to prepare organic-silica composites by conducting polymerization in the presence of silica particles. (2-9) Thermodynamic ther·mo·dy·nam·ic adj. 1. Characteristic of or resulting from the conversion of heat into other forms of energy. 2. Of or relating to thermodynamics. and kinetic factors relating to the type of polymerization process play a key role in determining the ultimate structure. Polymer/silica nanocomposite colloids with various morphologies have been created through emulsion polymerization using a polymerizable silane silane or silicon hydride Any of a series of inorganic compounds of silicon and hydrogen with covalent bonds and the general chemical formula SinH(2n + 2). such as methacryloxypropyl trimethoxy silane (TMS TMS Transcranial Magnetic Stimulation (alternative medicine for depression) TMS Test Match Special (sports - cricket) TMS Texas Motor Speedway TMS Transportation Management System TMS Toyota Motor Sales ) and a methacrylate-terminated macromer as a coupling agent. (5) Depending on the synthesis strategy and the nature of the mineral particle, a wide range of polymer morphologies have been obtained. (5) The anchoring TMS monomer monomer (mŏn`əmər): see polymer. monomer Molecule of any of a class of mostly organic compounds that can react with other molecules of the same or other compounds to form very large molecules (polymers). can be simply reacted on the silica particle surface via silanol functionality. Additionally, organosilane-modified silica particles containing an active double bond have been reacted with styrene sty·rene n. A colorless oily liquid from which polystyrenes, plastics, and synthetic rubber are produced. Also called vinylbenzene. via emulsion polymerization to produce a nanosilica-latex hybrid. The resulting organic-inorganic composite latex was stable in a wide range of ionic strength environments. (5), (6) Zhang (10) and Bourgeat-lami (5) have successfully incorporated silanol functional groups on latex particles using TMS as a functional comonomer co·mon·o·mer n. One of the compounds that constitute a copolymer. via semi-continuous emulsion and mini-emulsion polymerization. Further, Zhang (10) discovered a controlled method of synthesizing silanol functional latex particles via mini-emulsion polymerization. These silanol functionalized latexes were reacted with colloidal silica to prepare novel hybrid particles and hollow-sphere silica polymer composites. (11) The alkyl alkyl /al·kyl/ (al´k'l) the monovalent radical formed when an aliphatic hydrocarbon loses one hydrogen atom. al·kyl n. modified-nanosilica/polyacrylate composite latex prepared by emulsion polymerization exhibited a defect-free surface as measured by atomic force microscopy (AFM (Atomic Force Microscope) A device used to image materials at the atomic level. AFMs are used to solve processing and materials problems in electronics, telecom, biology and other high-tech industries. ) and better thermal properties than a polyacrylate composition as measured by thermogravimetric analysis (TGA See TARGA. TGA - Targa Graphics Adaptor ). (7) The organic polymer matrix encapsulated with silica particles has also been prepared by phase-inversion emulsification. (1), (2) Atom transfer free radical polymerization (ATRP ATRP Atom Transfer Radical Polymerization ATRP Advanced Technology and Repair Program (USAF) ) has also been used to make polymer-coated inorganic silica particles. (1) Styrene acrylic-silica polymers were produced by modifying silica particles with oleic acid oleic acid /ole·ic ac·id/ (o-le´ik) a monounsaturated 18-carbon fatty acid found in most animal fats and vegetable oils; used in pharmacy as an emulsifier and to assist absorption of some drugs by the skin. functionality, resulting in a stable silica-organic composite. (2) Octaphenylsilisequioxane (ONPS) has been used to react with hydroxyl hydroxyl /hy·drox·yl/ (hi-drok´sil) the univalent radical OH. hy·drox·yl n. The univalent radical or group OH, a characteristic component of bases, certain acids, phenols, alcohols, carboxylic acrylate Noun 1. acrylate - a salt or ester of propenoic acid propenoate salt - a compound formed by replacing hydrogen in an acid by a metal (or a radical that acts like a metal) emulsion. (9) These thermoset A polymer-based liquid or powder that becomes solid when heated, placed under pressure, treated with a chemical or via radiation. The curing process creates a chemical bond that, unlike a thermoplastic, prevents the material from being remelted. See thermoplastic. ONPS-modified nanocomposites exhibited excellent thermal and mechanical properties. Core-polystyrene/shell-silica nanocomposite latex particles were also synthesized using cationic cationic having qualities dependent on having free cations available. cationic detergents are wetting agents that disrupt or damage cell membranes, denature proteins and inactivate enzymes. silica sol. (6) The silica particles (25 nm average diameter) form a monolayer mon·o·lay·er n. 1. A film or layer one molecule thick formed at the interface between water and either oil or air by a substance such as a partially esterified fatty acid that contains both hydrophobic and hydrophilic groups in the same shell of 30 nm thickness on polystyrene particles. It was found that the incorporation of more than 10 wt% silica content enhances thermal stability due to the strong interaction between silica and polystyrene molecules. (6) In another article, (11) a new class of nano Si[O.sub.2]/acrylic resin composite latex was synthesized by introducing nano Si[O.sub.2] particles formed in-situ hydrolysis hydrolysis (hīdrŏl`ĭsĭs), chemical reaction of a compound with water, usually resulting in the formation of one or more new compounds. of tetraethylorthosilicate in emulsion polymerization. (5b) The properties of the latex were studied by scanning electron microscopy and differential thermal analysis Differential thermal analysis (or DTA) is a thermoanalytic technique, similar to differential scanning calorimetry. In this technique, the heat flow to the sample and reference remains constant, as opposed to temperature. . The results revealed that the resulting latex composite possessed an interpenetrating network structure of inorganic and organic phases. (11) In other work, Mayer et al. reacted chlorobenzyl modified latex particles with amine amine (əmēn`, ăm`ēn): see under amino group. amine Any of a class of nitrogen-containing organic compounds derived, either in principle or in practice, from ammonia (NH3). and thiol thiol: see mercaptan. function nano inorganic particles. (12) In summary, the nano particles are so small that the bulk of the polymer matrix is in the "interphase interphase /in·ter·phase/ (in´ter-faz) the interval between two successive cell divisions, during which the chromosomes are not individually distinguishable. in·ter·phase n. ," resulting in significantly improved coatings properties. The interphase exhibits different chemical and physical properties than either the bulk or the polymer particle. Interphase properties can be drastically different than the polymer matrix itself and therefore could become drastically different than bulk properties. DISCUSSION Buildings with premium architectural design or those that are historically important are in need of high quality coatings for maintenance and preservation. Many have published investigations on the development of exterior coatings with excellent resistance to dirt pick-up, mainly influenced by the hydrophilicity and hardness of the coating surface. In this regard much attention has been paid to organic-inorganic nanocomposites because in these hybrid systems the flexibility of the organic component can be combined with the hardness of the inorganic component. The more widely used inorganic component has typically been silica gel due to its availability. These systems have been investigated in a wide variety of application fields including medicine, electronic adhesives, separation membranes, and surface coatings. In general, three different ways to prepare latex polymer Si[O.sub.2] nanocomposites have been described in the literature. In the first case, emulsion polymerization is carried out in the presence of colloidal silica gel compositions. In the second case, the surface of the colloidal silica gel particles is previously functionalized and then used during emulsion polymerization. In the case of the present work, a functional emulsion polymer is prepared in such a way that an effective interaction of the polymer particles with the silica gel particles is tailor-made. It is on this route that we concentrate and describe our results. PREPARING THE EMULSIONS To investigate the effect of the level of silica nano particles on the dispersion and resulting paint properties, a series of binders was prepared with silica levels ranging from 0% to 40% (total solids content). The base polymer consisted of butyl butyl /bu·tyl/ (bu´t'l) a hydrocarbon radical, C4H9. bu·tyl n. A hydrocarbon radical, C4H9. butyl a hydrocarbon radical, C4H9. acrylate, methyl methacrylate methyl methacrylate (meth´il methak´rilāt), n an acrylic resin, CH2 = C(CH3)COOCH3, derived from methyl acrylic acid. Monomer is the single molecule and polymer is the polymerization product. , and methacrylic acid methacrylic acid /meth·a·cryl·ic ac·id/ (meth?ah-kril´ik) an organic acid that polymerizes easily to form a ceramic-like mass. Its esters, methyl and polymethyl methacrylate, are used in the manufacture of acrylic resins and plastics. . A second series was made with a functionalized base polymer. Dispersion properties are shown in Table 1. Results show that the glass transition temperature The glass transition temperature is the temperature below which the physical properties of amorphous materials vary in a manner similar to those of a solid phase (glassy state), and above which amorphous materials behave like liquids (rubbery state). ([T.sub.g]) increases with increasing silica level. This is primarily due to restricted mobility of the polymer chains near the polymer silica interface. At a 40% silica level, an increase in minimum film formation temperature (MFFT MFFT Minimum Film Forming Temperature (polymer temperature transition testing instrumentation) ) of about 5[degrees]C was observed. Particle sizes measured by laser aerosol spectroscopy were in the range of 120-130 nm.
Table 1-Basic Properties of Prepared Emulsions
Emulsion Functionalized Si Solid MFFT
Sample [O.sub.2] Content ([degrees]C)
(%) (%)
1 Yes 0 48.4 3.0
2 Yes 10 47.6 3.0
3 Yes 20 46.9 3.5
4 Yes 30 46.1 4.0
5 Yes 40 45.5 7.0
6 No 0 48.2 2.0
7 No 10 47.4 2.0
8 No 20 46.7 3.0
9 No 30 46.0 3.5
10 No 40 45.4 7.0
Emulsion Functionalized [T.sub.g] Aerosol-dw PSCS-dw
Sample ([degrees]C) (nm) (nm)
1 Yes 4.8 127 136
2 Yes 14.3 127 132
3 Yes 14.9 125 137
4 Yes 13.5 125 133
5 Yes 12.5 124 135
6 No 8.2 129 134
7 No 11.7 127 130
8 No 13.5 130 127
9 No 12.7 125 135
10 No 13.5 126 120
The influence of increasing amounts of colloidal Si[O.sub.2] on the mechanical properties of emulsion films is shown in Figure 1. As expected, the elongation at break is reduced with increasing amounts of Si[O.sub.2] and the tensile strength of the films is increased. This influence on the elasticity behavior is stronger when the latex polymer is functionalized. By correct selection of the latex polymer's [T.sub.g] and the Si[O.sub.2] amount, flexible but tough films can be obtained. [FIGURE 1 OMITTED] It was also important to investigate the influence of the functionalization of the polymer on the distribution of the Si[O.sub.2] particles, both in the wet emulsion samples and in dry films prepared from them. For this purpose, transmission electron microscopy “TEM” redirects here. For other uses, see TEM (disambiguation). Transmission electron microscopy (TEM) is an imaging technique whereby a beam of electrons is transmitted through a specimen, then an image is formed, magnified and directed to appear either (TEM TEM 1. transmission electron microscope. 2. triethylenemelamine. 3. transmissible encephalopathy of mink. ) was used to characterize the materials. This investigation showed that when the latex polymer was not functionalized (as in Emulsion Sample 10 of Table 1), the colloidal silica gel particles agglomerate agglomerate Large, coarse, angular rock fragments associated with lava flow that are ejected during explosive volcanic eruptions. Although they may appear to resemble sedimentary conglomerates, agglomerates are igneous rocks that consist almost wholly of angular or rounded and are not homogeneously distributed. This also leads to differences in the Si[O.sub.2] distribution in the films, which is, of course, the most decisive factor when the materials are used as a binder for coatings. Indeed, as it can be seen in Figure 2b, the TEM images of the cross section of the films from the emulsion sample (Table 1, Emulsion Sample 5) showed that the colloidal Si[O.sub.2] units are homogeneously distributed in the films only when the latex polymer is functionalized. [FIGURE 2 OMITTED] EFFECT ON PAINT PROPERTIES Due to the inorganic phase, nano hybrid emulsions would be the ideal binders for exterior paints as they reduce the thermoplastic A polymer material that turns to liquid when heated and becomes solid when cooled. There are more than 40 types of thermoplastics, including acrylic, polypropylene, polycarbonate and polyethylene. behavior of standard exterior paints, leading to less dirt pick-up and a cleaner surface. Therefore, the nano hybrid emulsion was tested in the standard exterior wall paint recipe detailed in Table 2. Table 2--Standard Exterior Wall Paint Recipe Ingredient Parts (by weight) Water 141 Tylose MH 30000 YP 2 2 Agitan 281 4 AMP 90 4 Additol VXM 6208 3 Titandioxide Kronos 2160 200 Omyacarb 2 GU 50 Omyacarb 5 GU 100 Glimmer MUN 101 100 Texanol 15 Nano hybrid emulsion (45% solids) 400 Tafigel PUR 40 (1:1 in water) 6 Total 1025 Pigment Volume Concentration (PVC) 65.85% Investigations with SEM showed that by using nano hybrid emulsions it is possible to create a real nano structure on the paint surface (Figure 3). [FIGURE 3 OMITTED] Weathering tests were conducted to prove that the nano structure is still present and stable after nine months of outdoor exposure (45 degrees, south-facing) and after 400 cycles of artificial weathering including rain, freeze thaw, UV treatment, and temperature changes (Figure 4) from +60[degrees]C to -25[degrees]C in 240 min per cycle (Global UV tester). [FIGURE 4 OMITTED] The dirt pick-up behavior was also tested in comparison to a standard acrylic paint by using an artificial dirt pick-up test, which shows a significant lower dirt pick-up for a paint based on the nano hybrid emulsion (Figure 5). [FIGURE 5 OMITTED] These tests were repeated in a real outdoor environment. Outdoor exposure results from Istanbul after six months (90 degrees, south-facing, city near a broad highway) matched the laboratory data. [DELTA][L.sub.(Paint with Nano Hybrid Emulsion)] = -2.70 [DELTA][L.sub.(Paint with Standard Acrylic Emulsion)] = -3.81 No chalking was present for either paint. When calculating pigment volume concentration (PVC PVC: see polyvinyl chloride. PVC in full polyvinyl chloride Synthetic resin, an organic polymer made by treating vinyl chloride monomers with a peroxide. ), the inorganic portion of the nano hybrid emulsions must be taken into account. Due to the inorganic portion, it is possible that the nano hybrid emulsions could be more brittle. Therefore, cracking behavior must be assessed when the binders are used in new formulations and on different substrates. Table 3 shows the results of paints based on nano hybrid emulsions with respect to water vapor diffusion (lower Sd value) and water take-up in comparison to standard acrylic emulsion-based paints. The higher water permeability of the nano hybrid emulsion is desirable.
Table 3--Water Response of Two Emulsion Systems
Binder Water Water Contact
Vapor Permeability Angle
Diffusion in 24 Hours
sd (m) (Kg/[m.sup.2]
[h.sub.0.5])
Standard acrylic emulsion 0.88 0.01 82
Nano hybrid emulsion 0.05 0.16 70
CONCLUSIONS In this work, a nanostructure hybrid of acrylic latex and inorganic nano silica with controlled morphology was prepared by emulsion polymerization. It was also found that the glass transition temperature increases with an increased silica level due to restricted mobility of the polymer chains near the polymer silica interface. TEM was used to characterize the nano organic-inorganic hybrid. TEM images of the cross sections of the films synthesized showed that the colloidal Si[O.sub.2] units are homogeneously distributed in the films only if they are made from a functionalized latex polymer. Paints made with the hybrid material as a binder showed a nano structured surface. Both laboratory and real world exposure testing of these paints displayed excellent resistance to dirt pick-up, and a cleaner appearance. These functional and aesthetic properties when combined with the high water vapor permeability clearly show that the organic-inorganic hybrid systems (nano hybrid emulsions described in this article) make an excellent binder choice for high performance coatings. References (1) Castelvetro, V. and de Vita, C., "Nanostructured Hybrid Materials from Aqueous Dispersions," Advances Colloid colloid (kŏl`oid) [Gr.,=gluelike], a mixture in which one substance is divided into minute particles (called colloidal particles) and dispersed throughout a second substance. Interface Sci., 108-109, 167-185 (2004). (2) Mahdavian, R.A., Ashjari, M., and Makoo, B.A., "Preparation of Poly (styrene-methyl methacrylate methacrylate /meth·ac·ry·late/ (meth-ak´ri-lat) an ester of methacrylic acid, or the resin derived from polymerization of the ester. See also acrylic resins, under resin. )/Si[O.sub.2] Composite Nanoparticles via Emulsion Polymerization. An Investigation into Compatibilization," Eur. Polym. J., 43, 336-334 (2007). (3) Park, J.P., Hoon hoon Austral & NZ slang Noun a loutish youth who drives irresponsibly Verb to drive irresponsibly , H., and Lim, K.T., "Synthesis and Characterization of Al[(OH).sub.3]/Polystyrene Nanocomposite Latex Particles by Emulsion Polymerization," Macromol. Symp., 249-250, 247-250 (2007). (4) Tutorskii, I.A., Malyavskii, N.I, Tkachenko, T.E., and Khameshina, G.V., "Structure and Strength Properties of Hybrid Zinc-Containing Latex Silica Composites," Inter. Polym. Sci. Technol., 28, 8 (2001). (5) a) Bourgeat-Iami, E., Herrera, N.A., Putaux, J., Perro, A., and Reculusa, S., "Designing Organic/Inorganic Colloids by Heterogeneous Polymerization," Macromolecule macromolecule, term that may refer either to a crystal such as a diamond, in which the atoms are identical and held by covalent bonds (see chemical bond) of equal strength, or to one of the units that compose a polymer. Symposium, 248, 213-226 (2007). b) Bourgeat-Iami, E., Lefebvre, F., and Novat, N., "Hybrid Latex Particle Coated with Silica," Macromolecules Macromolecules A large molecule composed of thousands of atoms. Mentioned in: Gene Therapy macromolecules , 34, 5737-39 (2001). (6) Lee, J., Hong, C.K., Choe, S., and Shim A small piece of software that is added to an existing system program or protocol in order to provide some enhancement. (jargon, memory management) shim - A small piece of data inserted in order to achieve a desired memory alignment or other addressing property. , S.E., "Synthesis of Polystyrene/Silica Composite Particles by Soap-Free Emulsion Polymerization Using Positively Charged Colloids," J. Colloid Interface Science, 310, 112-120 (2007). (7) Wang, Y., Zhang, Li, Y., Zhang R., Huang, Li, and He, W., "Synthesis and Characterization of Nano Silica/Polyacrylate Composite Latex," Polymer Composites, 202-208 (2006). (8) Wagner, G., Mustafa, M., Faatz, M., and Gorna, K., "Polymers and Inorganics: A Happy Marraige?" Macromolecular mac·ro·mol·e·cule n. A very large molecule, such as a polymer or protein, consisting of many smaller structural units linked together. Also called supermolecule. Research, Vol. 15, No. 2, 95-99 (2007). (9) Hu, L., Li, Q., and Geng, H., "Properties of Polyacrylic Ester Emulsion Modified with Octa(nitrophenyl)silsesquioxane," Proc. Asian International Conference on Advanced Materials, 319-22 (2006). (10) Zhang, S.W., Zhou, S.X., and Weng, Y.M., "Synthesis of Silanol-Functionalized Latex Nanoparticles through Miniemulsion Polymerization of Styrene and Methacryloxypropyltrimethoxysilane," Langmuir, 22, 4674-4679 (2006). (11) Yu-ping, L., Zhi-qiao, Z.R., and Jiang, X.H., "The Properties of Nano-Si[O.sub.2]/Acrylic Resin Composite Latexes with Interpenetrating Network Structure," J. Hunan University (Natural Sciences), Vol. 10, No. 34 (2006). (12) Cannizzo, C. and Mayer, C.R., "Covalent co·va·lent adj. Of or relating to a chemical bond characterized by one or more pairs of shared electrons. Hybrid Materials Based on Nanolatex Particles and Dawson Polyoxometalates," Advanced Materials, 17, 2888-2892 (2005). (13) Bagda, E., "Soiling of Exterior Wall Coatings," FATIPEC Congress, 25, 299 (2000). Presented at the 2008 American Coatings Conference, June 3-5, 2008, Charlotte, NC. *Industriepark Hochst, 65926 Frankfurt, Germany. [dagger]900 Gemini St.,Houston, TX 77058. by StephanKrieger, IvanCabrera, MarcRatering, and Thomas Fichtner Celanese Chemicals Europe GmbH* and Rajeev Farwaha Celanese Emulsions [dagger] |
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