Nanoparticle-filled polymers

[0002] The invention relates to polymer resins incorporating filler materials of particle size 1-100 nm and having a narrow particle size distribution.

[0003] It is well known that the presence of fillers in a polymer resin can improve mechanical and chemical properties of the resin. For example, Moloney et al. (Journal of Materials Science, 1987, 22, 381) reported that, in epoxy systems, by increasing the volume fraction of fillers and by using higher modulus fillers, such as micron-size silica, alumina, and silicon carbide, the modulus of the composites was also increased. Filled or reinforced polymers have been used for many years in applications as varied as synthetic rubbers, epoxy-fiberglass composites, and paints.

[0004] However, despite the advantages of using filled polymers in such application, in many cases, the properties fall short of the ideal. For example, articles fabricated from or coated with polymers, filled or unfilled, are easily scratched. Plastics used in optical applications such as eyeglass lenses are typically coated to improve scratch resistance, yet resistance to scratching remains less than desired. A second example is magnetic tape, which is also susceptible to scratching, leading to a loss in quality. In addition, conventional fillers can embrittle polymers.

[0005] Therefore, there is a continuing need for polymers with improved properties, including scratch resistance. There is also a need for polymers having increased modulus while maintaining good ductility. There is a further need for polymers having improved dimensional stability.

[0006] It has been unexpectedly discovered that incorporation of a filler of very small particle size with a narrow particle size distribution results in a polymer having improved mechanical properties, including increased toughness and scratch resistance and improved dimensional stability.

[0007] In one aspect, the present invention relates to a polymer nanocomposite comprising about 50-99 weight % polymer resin and about 1-50 weight % nanoparticles having particle size from about 1 nm to less than about 100 nm, and a narrow particle size distribution. The polymer resin is preferably an epoxy, polycarbonate, silicone, polyester, polyether, polyolefin, synthetic rubber, polyurethane, nylon, polystyrene, polyphenylene oxide, and polyketone or a copolymer or blend thereof. More preferably, the polymer resin is an epoxy. Even more preferably, the epoxy is a diglycidyl ether of bisphenol A. The nanoparticles are preferably metal nanoparticles, metal oxide nanoparticles, metal nitride nanoparticles, metal carbide nanoparticles, metal sulfide nanoparticles, metal fluoride nanoparticles, and metal chloride nanoparticles, more preferably, metal oxide nanoparticles, and even more preferably, titanium dioxide nanoparticles. Preferably, the particle size of the nanoparticles ranges from about 1 nm to about less than 50 nm. Preferably, the polymer nanocomposite comprises about 70-5 weight % polymer resin and about 5-30 weight % nanoparticles having particle size from about 1 nm to less than about 100 nm, and a narrow particle size distribution.

[0008] In another aspect, the present invention relates to an epoxy nanocomposite having improved mechanical properties comprising 50-99 weight % epoxy resin and 1-50 weight % quasi-spherical particles having particle size from about 1 nm to less than about 100 nm and a narrow particle size distribution.

[0009] In yet another aspect, the invention relates to a scratch-resistant coating comprising about 50-99 weight % polymer resin and about 1-50 weight % nanoparticles having particle size from about 1 nm to less than about 100 nm, and a narrow particle size distribution. In a preferred embodiment, the scratch-resistant coating comprises about 85-95 weight % polymer resin and about 5-15 weight % nanoparticles.

[0010] In yet another aspect, the invention relates to a polymer nanocomposite stamp comprising about 50-99 weight % polymer resin and about 1-50 weight % nanoparticles having particle size from about 1 nm to less than about 100 nm, and a narrow particle size distribution. In a preferred embodiment, the polymer nanocomposite stamp comprises about 85-95 weight % polymer resin and about 5-15 weight % nanoparticles.

[0011] FIG. 1 shows particle size distribution for TiO₂nanoparticles useful in the present invention, as determined by transmission electron microscopy.

[0012] FIG. 2 is a scanning electron micrograph showing the scratch morphology of an unfilled epoxy.

[0013] FIG. 3 is a scanning electron micrograph showing the scratch morphology of a microparticle-filled epoxy.

[0014] FIG. 4 is a scanning electron micrograph showing the scratch morphology of a nanoparticle-filled epoxy.

[0015] FIG. 5 is a scanning electron micrograph showing a close-up of the scratch morphology of an unfilled epoxy.

[0016] FIG. 6 is a scanning electron micrograph showing a close-up of the scratch morphology of a microparticle-filled epoxy.

[0017] FIG. 7 is a scanning electron micrograph showing a close-up of the scratch morphology of a nanoparticle-filled epoxy.

[0018] FIG. 8 is a graph of weight percent change over time for unfilled epoxy, microparticle-filled epoxy, and nanoparticle-filled epoxy samples in boiling water.

[0019] FIG. 9 is a graph of volume percent change over time for unfilled epoxy, microparticle-filled epoxy, and nanoparticle-filled epoxy samples in boiling water.

[0020] The polymer resins used in the present invention include epoxy, polycarbonate, silicone, polyester, polyether, polyolefin, synthetic rubber, polyurethane, nylon, and polystyrene, and polyvinylaromatic, acrylic, polyamide, polyimide, phenolic, polyvinylhalide, polyphenylene oxide, and polyketone and copolymers and blends thereof. Copolymers include both random and block copolymers. Polyolefin resins include polybutylene, polypropylene and polyethylene, such as low density polyethylene, medium density polyethylene, high density polyethylene, and ethylene copolymers; polyvinylhalide resins include polyvinyl chloride polymers and copolymers and polyvinylidene chloride polymers and copolymers, fluoropolymers; polyvinylaromatic resins include polystyrene polymers and copolymers and poly α-methylstyrene polymers and copolymers; acrylate resins include polymers and copolymers of acrylate and methacrylate esters, polyamide resins include nylon 6, nylon 11, and nylon 12, as well as polyamide copolymers and blends thereof, polyester resins include polyalkylene terephthalates, such as polyethylene terephthalate and polybutylene terephthalate, as well as polyester copolymers; synthetic rubbers include styrene-butadiene and acrylonitrile-butadiene-styrene copolymers and; polyketones include polyetherketones and polyetheretherketones.

[0021] A variety of diluents and additives which are well known to those skilled in the art are typically admixed with the polymer resins including water, oils, coupling agents, diluents and dispersants.

[0022] In a preferred embodiment, the polymer resin is an epoxy resin. Any conventionally used epoxy resin may be used in the present invention. Examples of suitable epoxy resins include glycidyl ethers such as bisphenol-A-diglycidyl ether (DGEBA), glycidyl ethers of bisphenol S and bisphenol F, butanediol diglycidyl ether, bisphenol-A-extended glycidyl ethers, phenol-formaldehyde glycidyl ethers (epoxy novolacs), and cresol-formaldehyde glycidyl ethers (epoxy cresol novolacs); expoxidized alkenes such as 1,2-epoxyoctane, 1,2-epoxydecane, 1,2,13,14-tetradecane diepoxide, 1,2,7,8-octane diepoxide; epichlorohydrin; cycloaliphatic epoxides such as limonene dioxide, α-pinene oxide, dicyclopentadiene dioxide, 4-vinylcyclohexene dioxide, 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexene carboxylate, cyclohexene oxide, cyclopentene oxide, cycloheptene oxide, cyclooctene oxide, 1,2,5,6-cyclooctadiene dioxide; glycidyl ethers such as diglycidyl phthalate; epoxidized vegetable oils such as epoxidized linseed, soybean, safflower, sunflower, rapeseed, canola oil; silicone epoxy resins; epoxidized polymers such as epoxidized polybutadiene, epoxidized polyisoprene and epoxidized polystyrene-co-butadiene; glycidyl ester epoxy resins, glycidyl amino epoxy resins, linear aliphatic epoxy resins, heterocyclic epoxy resins halogentated epoxy resins and liquid crystalline epoxies. These epoxy resins may be used singly or as a mixture of two or more. A preferred epoxy-functional monomer is DGEBA.

[0023] The nanoparticle-filled epoxy resins for use in the present invention may be cured by heating, using conventional curing agents. Illustrative of suitable curing agents are aromatic amines, acid anhydrides, dicyandiamide, and imidazole compounds.

[0024] The resins may also be cured using radiation, including UV or visible light, e-beam or x-ray irradiation. Exemplary initiators or curing agents which may be used to initiate photopolymerization are diaryliodonium salts, triarylsulfonium salts, diaryliodosonium salts, dialkylphenylsulfonium salts, dialkyl(hydroxydialkylphenyl)sulfonium salts and ferrocenium salts. Typically, the photoinitiator is employed in concentrations ranging from 0.1 to 10% by weight based on the monomer or monomers.

[0025] The polymer resin may be in the form of a neat resin, a dispersion in water or organic solvent, or a solution in an organic solvent.

[0026] Nanoparticles for use in the present invention are typically manufactured by a gas condensation process, such as that described in U.S. Pat. Nos. 5,128,081 and 5,320,800, the contents of which are incorporated herein by reference. A gas condensation process for the preparation of nanoparticles typically involves evaporation of a metal precursor material from which the nanoparticles will be synthesized at gas pressures of less than one or equal to one atmosphere. The evaporated metal condenses into small particles in the gas atmosphere and the resulting nanoparticles are collected on a surface within the reactor. Any metal or metal compound capable of being volatilized may be used to form the nanoparticles for use in the present invention. Exemplary metals are titanium, copper, silver, gold, platinum, and palladium. The metal nanoparticles may be further subjected to a reactive gas atmosphere to form oxides, nitrides, carbides, sulfides, fluorides, and chlorides. Exemplary metal oxide nanoparticles are those composed of aluminum oxide, antimony tin oxide, cerium oxide, copper oxide, indium oxide, indium tin oxide, iron oxide, silicon dioxide, tin oxide, titanium dioxide, yttrium oxide, zinc oxide, barium oxide, calcium oxide, chromium oxide, magnesium oxide, manganese oxide, molybdenum oxide, neodymium oxide, and strontium oxide. Metal titanate and metal silicate nanoparticles including, for example, strontium titanate, barium titanate, barium strontium titanate, and zirconium silicate may also be used. Titanium dioxide nanoparticles are preferred for use as polymer fillers. Titanium dioxide nanoparticles of varying particle size, synthesized by a gas condensation process, are commercially available from Nanophase Technologies Corporation. Nanophase Technologies also manufactures the metal, metal oxide, metal titanate and metal silicate nanoparticles listed above. Carbon particles may also be used in the practice of the invention.

[0027] Nanoparticles utilized in the present invention have a particle size ranging from about 1 nm to about 100 nm, preferably from about 10 nm to about 50 nm. The size distribution of the nanoparticles is narrow. A narrow particle size distribution is defined as one in which greater than 90% of the particles have a particle size in the range of 0.2-2 times the mean particle size. Preferably, greater than 95% of the particles have a particle size in this range, and more preferably greater than 99%.

[0028] In the nanoparticle industry, it is customary to define a particle size distribution in terms of the mean particle size and the width of the distribution. The width of the distribution curve at one half of the maximum value is termed full width half max (FWHM). The relationship between the FWHM and mean particle size is used as a measure of broadness or narrowness of the distribution. For example, a distribution having a FWHM value that is greater than the mean particle size is considered relatively broad. As discussed above, the nanoparticles used in the present invention have a narrow particle size distribution. An alternative definition of narrow particle size distribution for the nanoparticles used in the present invention is a distribution in which the FWHM of the distribution curve is equal to the difference between the mean particle size plus 40% of the mean and the mean minus 40% of the mean. (This may be simplified to two times 40% of the mean, or 80% of the mean. Using this simplified formula, the FWHM is less than or equal to 80% of the mean.) Preferably, the FWHM is less than or equal to the difference between the mean plus 30% and the mean minus 30% (60% of the mean.) More preferably, the FWHM is less than or equal to the difference between the mean plus 20% and the mean minus 20% (40% of the mean.)

[0029] FIG. 1 is a graph showing the narrow particle size distribution of an exemplary sample of TiO₂nanoparticles. On the graph, the number of particles having a particular size is plotted against that particle size. The mean particle size of the sample is about 12 nm; greater than 99% of the particles have a particle size in the range of 2.4 nm-24 nm. The maximum number of particles shown on the graph is about 200, at 12 nm; half max, then, is about 100. The lower limit of the curve at half max is about 10 nm, the upper limit at half max is about 17.5, and the difference between the two is equal to a value of about 7.5 for FWHM. For this exemplary distribution, then, the mean plus 30% minus the mean minus 30% (15.6−8.4=7.2) is about equal to the FWHM of about 7.5 nm.

[0030] Nanoparticles useful in the present invention are typically equiaxed, such that their shape is quasi-spherical. The long axis of a particle is defined as the longest axis through a particle, and the short axis means the shortest axis through a particle. The long axis of the nanoparticles for use in the present invention is approximately equal to the short axis, resulting in a particle shape which is quasi-spherical. For at least 90% of the nanoparticles, the ratio of the length of the short axis to that of the long axis is at least 0.1, preferably 0.4, and more preferably 0.8.

[0031] Further, the surface of a nanoparticle utilized in the present invention is typically chemically clean, being composed essentially of pure metal, or metal oxide, sulfide, nitride, carbide, fluoride, or chloride. Nanoparticles made by wet chemical methods are often contaminated by residues from chemicals used in the process. Likewise, typical processes for the production of titanium dioxide particles involve the oxidation of TiCl₄to TiO₂. The surface of particles produced by this process contains residual chloride ions from the TiCl₄. Nanoparticles produced by a gas condensation process are not contaminated by process residues, because no solvents, reagents or intermediates are used. Therefore, nanoparticles for use in the present invention are preferably prepared by a gas condensation process, as described above.

[0032] Nanoparticles used in the present invention are crystalline materials, and are referred to as nanocrystalline. Each of these particles is composed of a single grain, that is, a single crystal consisting of atoms arranged in an orderly pattern. Nanocrystalline materials have grains containing thousands to tens-of thousands of atoms as compared to millions or trillions of atoms in the grains of conventional particles, and have a significantly higher percentage of atoms present on the surface of the particle.

[0033] The present invention is particularly useful as a scratch-resistant coating or part, and in electronic applications as a polymer nanocomposite stamp.

[0034] Mechanical and thermal properties of cured epoxy resins filled with a TiO₂nanoparticle filler at the 10% level (weight on total weight) were determined and compared to those for pure epoxy resins and for epoxies filled with TiO₂of micron dimensions.

[0035] Titanium dioxide particles of two different particle sizes, 10% by weight, were added at 60° C. to a commercial grade diglycidyl ether of bisphenol A from Shell Oil Company. Nanometer-size TiO₂particles (32 nm) of composition 90% anastase and 10% rutile were obtained from Nanophase Technologies Corporation. Micron-size TiO₂(0.24 μm, Tioxide Inc.) was chosen to match the surface chemistry and crystallographic composition of the nanometer-size TiO₂as closely as possible. The TiO₂fillers were dispersed in the epoxy by placing the mixture in an ultrasonic bath at 60° C. for about one hour. Hardener 1,3-phenyenediamine was melted and added to the epoxy/filler mix at ratio of 14.5 g per 100 g epoxy. The mixture was poured into a preheated silicone mold and cured at 75° C. for 2.5 hours and then at 120° C. for 2.5 hours.

[0036] Resistance to elastic and plastic deformation during abrasion was determined by scratch testing. Specimens of dimensions 2 cm×1 cm×0.5 cm were polished using a 400 grit paper prior to testing. Scratch testing was performed with a BYK Gardener SG-8101 balance beam scrape adhesion tester with a 4 kg load. Fracture and scratch morphology were observed using a JOEL-JSM 840 scanning electron microscope.

[0037] Results appear in Table 1. The nanoparticle-filled epoxy has the lowest scratch depth, followed by the unfilled epoxy and then the microparticle-filled epoxy.

[0038] Micrographs of exemplary scratch specimens are shown in FIGS. 2-7. The figures illustrate that the scratch profile for the epoxy containing micron-size particles showed greater deformation with more damage than the unfilled epoxy, while the nanoparticle-filled epoxy showed a 35% improvement in scratch resistance with the least plastic deformation and damage.

[0039] In FIG. 2, unfilled epoxy, the scratch shows some chip-off from the sides, and crack lines originating from the jagged edges. In FIG. 3, microparticle-filled epoxy, there is some particle pull-out from the groove, as well as extensive damage on either side of the track and a higher elevation on both sides of the track compared to the unfilled epoxy. In FIG. 4, nanoparticle-filled epoxy, the track is smooth, ridges beside the groove are less elevated, and there are no visible crack lines along the edges of the scratch.

[0040] Tensile testing was performed on an Instron Model 4204 at a stroke rate of 2.54 mm/min at room temperature. The gauge length of the tensile specimens was about 5 cm with a cross sectional area of about 11 mm². All samples were polished using a 400 grit paper prior to testing. Strain-to-failure, rupture stress and the 1% secant modulus were calculated.

[0041] Results above appear in Table 1. All of the samples showed approximately the same elastic modulus and rupture stress. However, the maximum strain-to-failure for the nanoparticle-filled epoxy was about 20% higher than that for the pure epoxy. The combination of strength, modulus and ductility gives greater toughness compared to the unfilled epoxy and the microparticle-filled epoxy.

[0042] Swelling behavior was determined in boiling water. Cubes 5 mm³were held in boiling water for five days. The samples were weighed and dimensions of the cube measured periodically, and weight percent and volume percent change was calculated.

[0043] FIGS. 8 and 9 show swelling results. Weight percent change for the filled and unfilled epoxies was about equal. However, there were significant differences in the volume percent change for the samples. The dimensional stability of the nanoparticle-filled epoxy was highest.