POLYMER NANOCOMPOSITE FORMATION BY EMULSION SYNTHESIS

Abstract: The formation of a nanocomposite by emulsion polymerization is described. The invention includes the nanocomposite latex, a solid nanocomposite of a layered silicate mineral intercalated with an emulsion polymer and blends of the solid nanocomposite with other polymers.

Description: POLYMER NANOCOMPOSITE FORMATION BY EMULSION SYNTHESIS Field of the Invention This invention relates to composite materials having reduced permeability to small molecules, such as air, and which has enhanced mechanical properties. More particularly this invention relates to layered silicates intercalated with an emulsion polymer. Background of the Invention Layered clay minerals such as montmorillonite are composed of silicate layers with a thickness of about 1 nanometer. Dispersions of such layered materials in polymers are frequently referred to as nanocomposites. Recently, there has been considerable interest in forming nanocomposites as a means to improve the mechanical properties of polymers. Incorporating clay minerals in a polymer matrix, however, does not always result in markedly improved mechanical properties of the polymer. This may be due to the lack of affinity between the layered silicate materials and the organic polymers. Thus it has been proposed to use ionic interactions as a means of incorporating clay minerals in a polymer. In this regard, see for example U.S. Patent 4,889,885 and U.S. Patent 4,810,734. This type of approach, unfortu¬ nately, has limited usefulness. Indeed, a more direct, simple, and economic approach to preparing nanocomposites is highly desirable. One object of the present invention is to provide a latex comprising a layered silicate intercalated with an emulsion polymer. Another object of the present invention is to provide a composite material formed from a dispersion latex of a layered silicate and an emulsion polymer which material has reduced permeability to small molecules such as air, and improved mechanical properties. These and other objects, features and advantages of the present invention will become more apparent from the description which fol1ows. Summary of the Invention In one embodiment of the present invention, a latex is provided comprising water and a layered mineral intercalated with a polymer emulsion. Another embodiment of the present invention provides a nanocomposite comprising a layered mineral intercalated with an emulsion polymer. Another aspect of the present invention comprises a blend of a first polymer with a nanocomposite composed of a layered mineral intercalated with an emulsion polymer. The process for producing the latex of the present invention comprises forming a dispersion of a layered mineral in water including a swelling agent such as an onium salt, adding a polymerizable monomer or monomers, such as an olefin or diene, with a polymerization initia¬ tor to the dispersion, and thereafter polymerizing the monomer or monomers to form a latex comprising water and a polymer nanocomposite. The preparation of this latex comprises yet another embodiment of the present invention. A composite material formed from the latex of the present invention has improved mechanical properties and reduced air perme¬ ability to small molecules such as air making it particularly useful in a range of applications, particularly as a tire liner and as inner tubes, barriers, films, coatings and the like. Detailed Description Any natural or synthetic layered mineral capable of being intercalated may be employed in the present invention; however, layered silicate minerals are preferred. The layered silicate minerals that may be employed in the present invention include natural and artificial minerals capable of forming intercalation compounds. Nonlimiting examples of such minerals include smectite clay, mont- morillonite, saponite, beidellite, ontronite, hectorite, stevensite, vermiculite, and hallosite. Of these mont orillonite is preferred. The swelling agent used in the practice of the present invention is any compound capable of intercalating the layered mineral and thereby increasing the distance between the layers. Particularly preferred swelling agents are hydrocarbyl oniu salts represented by the formulae A _ M+R1R2R3R4 anc | A'Py+R 4 where A" denotes an anion such as halide, OH", NO3-, SO4- and the like; M denotes N, S, P; Rl, R2, R3 and R4 independently denote hydrogen alkyl, aryl or allyl groups, which may be the same or different, provided at least one of which is other than hydrogen; and Py denotes the pyridinium or alkyl substi¬ tuted pyridinium group. It will be readily appreciated that some of the above mentioned swelling agents are also emulsifying agents. However, in those instances when the swelling agent is not an emulsifying agent preferably an emulsifying agent will be employed in carrying out the polymerization. Optionally, of course, another emulsifying agent may be used even when the swelling agent has emulsifying properties. In either event, the emulsifying agent will be one typically used in emulsion polymerization processes. Cationic emulsifying agents and non-ionic emulsifying agents are preferred. The polymers and copolymers referred to herein as emulsion polymers are those formed by emulsion polymerization techniques. Included are polymers based on one or more water immiscible, free radical polymerizable, monomers such as olefinic monomers and especially styrene or paramethyl styrene, butadiene, isoprene, chloroprene, and acrylonitrile. Particularly preferred are styrene rubber copolymers, i.e., copolymers of styrene and butadiene, isoprene chloroprene and acrylonitrile. Especially preferred, in the practice of the present invention are homopolymers and copolymers having a glass transition temperature less than about 25 β C, a number average molecular weight above 5,000g/mole and especially above 15,000g/mole. Also, the preferred polymer will contain some unsaturation or other reactive sites for vulcanization. The latex of an intercalatable mineral having an emulsion polymer intercalated in the mineral is prepared by forming a disper¬ sion of the layered mineral in water and including the swelling agent. Typically, the mineral is first dispersed in water by adding from about 0.01 to about 80 grams of mineral to 100 grams of water and preferably, about 0.1 to about 10.0 g of mineral to 100 g of water, and then vigorously mixing or shearing the mineral and water for a time sufficient to disperse the mineral in the water. Then the hydrocarbyl onium salt is added to the dispersion, preferably as a water solution, and with stirring. The amount of the onium salt used in the process of the present invention depends on the type of layered material and monomers used as well as process conditions. In general, however, the amount of onium salt used will be in the range of the cation co-exchange capacity of the layered mineral to about 10% to about 2,000% of the cationic exchange capacity of the layered mineral. Next, the polymer latex is formed by adding to the mineral dispersion an emulsifying agent, if desired or necessary, the appropriate monomer or monomers, and free a radical initiator under emulsion polymerization conditions. For example, styrene and isoprene are polymerized in the mineral dispersion using a free radical poly¬ merization initiator while stirring the reactants. The copolymeri- zation typically is conducted at a temperature in the range of about 25 β C to about 100 β C and for a time sufficient to form the polymer latex, followed by termination of the reaction. The latex described above can be used to form coatings or films following standard.techniques employed for forming such materi¬ als. Additionally, the nanocomposite of the layered silicate mineral and the polymer may be recovered by coagulating the latex, and drying the solid composite. The solid composite can then be formed into tire inner-liners or inner tubes using conventional processing techniques such as calendaring or extrusion followed by building the tire and molding. In one embodiment of the present invention the nancomposite is dispersed with another polymer, such as a styrene-rubber copolymer by blending on a rubber mill or in an internal mixer. Preferably the nanocomposite will be blended with a polymer formed from the same monomer or monomers used in forming the nanocomposite. The amount of the nanocomposite in the polymer typically will be in the range of about 0.1 to about 70 wt.%. In producing tire inner liners the polymer blended with the nanocomposite of this invention preferably will have a molecular weight of greater than about 10,000 and some unsaturation or other reactive sites so that it can be vulcanized or cross-linked in the bulk state. The invention will be more clearly understood by reference to the following examples. Example 1 A layered silicate, montmorillonite clay (18g), was slurried with water (450g) which had been degased by sparging with nitrogen. The slurry was stirred overnight at 23 β C. The clay was dispersed in the water in a Waring blender for three minutes and then degased further. Dodecyl trimethyl ammonium bromide (25.7g) was dissolved in degassed water (250g) and added to the clay slurry. Isoprene (35g), styrene (15g), and azobisisobutyronitrile (AIBN) (0.25g) as initiator were blended and then added to the clay slurry. The mixture was mechanically stirred for 20 hours at 23 β C and for 26 hours at 65 β C at which time polymerization was terminated with a 5g aliquot of a mixture of (0.24g) 2,6-di-tert-butyl-4-methylphenol , (1.6g) hydro- quinone, (0.8g) tetrakis [methylene(3,5-di-tert-butyl-4-hydroxy- hydrocinnamate)] methane and 200 ml ethanol . The net result was the formation of an emulsion containing a layered silicate having a styrene-isoprene copolymer latex intercalated in the layered mineral. Example 2 A solid nanocomposite was formed from the latex of Example 1 by adding an excess of methanol to the latex, separating the solid from the liquid aqueous phase and washing the solid six times with methanol, followed by drying for about 18 hours at 60 β C under vacuum and for 48 hours at 23°C in vacuum. Example 3 A portion of the solid nanocomposite (20 grams) of Example 2 was then melt blended at 130 β C in a Brabender mixer for 5 minutes with a styrene-isoprene copolymer (20 grams) that was synthesized identi¬ cally but had no clay. The blend of nanocomposite and the clay-free styrene-isoprene copolymer was cross-linked by roll milling the blend with stearic acid (1 phr), zinc oxide (3.9 phr), and tetra ethyl thiuram disulfide (accelerator) (1 phr) at 55 β C for ten minutes. Then the blend was hot pressed into 20 mil films and cured for 20 minutes at 130 β C. The films were tested on a Mocon 2/20 for oxygen transmission at 30 β C. The results are given in Table I below. Also shown in Table 1 were the results obtained with a film formed from a styrene-isoprene copolymer that had been synthesized identically but had no clay. (Comparative Example 1) Uniaxial tensile properties were also measured on mini- tensile film specimens using an Instron tester. The stress-strain measurements were performed at room temperature and at an extension rate of 0.51 mm/min and the results are shown in Table 2 below. Also shown in Table 2 and labeled as Comparative Example 1 are the tensile properties obtained for a polystrene-isoprene copolymer that was synthesized identically to that in Example 1 but had no clay. TABLE 1 fc Mocon 2/20 <? 30° C TABLE 2

Claims: CLAIMS : 1. A latex comprising: water and a layered mineral intercalated with a an emulsion polymer. 2. The latex of claim 1 wherein the layered mineral is a natural or synthetic mineral selected from the group consisting of smectite clay, montmorillonite, saponite, beidellite, montronite, hectorite, stevensite, vermiculite, and hallosite. 3. The latex of claim 1 wherein the polymer is formed from a free radical poly erizable olefinic monomer or monomers 4. The latex of claim 1 wherein the polymer is a styrene- containing copolymer. 5. The latex of claim 4, wherein the copolymer contains a comonomer selected from the group consisting of butadiene, isoprene, chloroprene and acrylonitrile. 6. The latex of claim 5 wherein the layered material is montmorillonite. 7. A latex comprising: water and a natural or synthetic layered mineral inter¬ calated with a polymer or copolymer, wherein the layered mineral is selected from the group consisting of smectite clay, montmorillonite, saptolnite, beidellite, montronite, hectorite, stevensite, vermi¬ culite, and hallosite and wherein the polymer or copolymer is formed from a free radical polymerizable olefinic monomer or monomers. 8. The latex of claim 7, wherein the olefinic monomer or monomers are selected from the group consisting of styrene, para- methylstyrene, butadiene, isoprene, chloroprene and acrylonitrile. 9. A nanocomposite comprising a layered mineral inter¬ calated with an emulsion polymer. 10. The nanocomposite of claim 9 wherein the layered mineral is selected from the group consisting of smectite clay, montmorillonite, saponite, beidellite, montronite, hectorite, stevensite, vermiculite, and hallosite. 11. The nanocomposite of claim 10, wherein the polymer is formed from a free radical polymerizable olefinic monomer or monomers. 12. The nanocomposite of claim 11, wherein the polymer is a styrene-containing copolymer. 13. The nanocomposite of claim 12, wherein the styrene- containing copolymer is a copolymer of styrene or paramethyl styrene with a monomer selected from the group consisting of butadiene, isoprene, chloroprene, and acrylonitrile. 14. The nanocomposite of claim 13 wherein the layered mineral is montmorillonite. 15. A polymer blend which comprises: a first polymer and a nanocomposite of a layered mineral intercalated with an emulsion polymer. 16. The blend of claim 15 wherein the first and emulsion polymers are formed from the same monomer or monomers. 17. The blend of claim 16 wherein the first and emulsion polymers are copolymers. 18. The blend of claim 17 wherein the amount of nano¬ composite in the blend is in the range from about 0.1 to about 70 wt.%. 19. The blend of claim 18 wherein the copolymer is a copolymer of styrene or paramethyl styrene with a monomer selected from butadiene, isoprene, chloroprene and acrylonitrile. 20. A process for producing a latex including a nano¬ composite material which comprises: dispersing a layered mineral in water to form a dispersion; adding a swelling agent to the dispersion; and thereafter polymerizing a free radical polymerizable olefinic monomer or monomers in the presence of the dispersion under emulsion polymerization conditions to form a latex including the nanocomposite material. 21. The process of claim 20 wherein two monomers are co- polymerized, one being a styrene or paramethylstyrene monomer and the other being butadiene, isoprene, chloroprene, or acrylonitrile. 22. The process of claim 21 wherein the swelling agent is a hydrocarbyl onium salt. 23. The process of claim 22 wherein the hydrocarbyl onium salt has the formula A-M+R1R2R3R4, or A"Py+R4 wherein or A" is an anion; M is N, S, or P; Rl, R 2 , R3, and R 4 independently denotes the same or different hydrogen, alkyl, aryl or allyl groups, and Py denotes a pridinium or an alkyl substituted pyridium group. 24. The process of claim 23 wherein the polymerization is conducted in the presence of an emulsifying agent at a temperature in the range of about 5 β C to about 100°C for a time sufficient to form the latex. 25. The process of claim 24 including adding a coagulating agent to the latex to coagulate solid nanocomposite and thereafter separating the solid nanocomposite.

Inventors: CHLUDZINSKI JAMES J, ELSPASS CHESTER W, HSEIH DONG-TSAI, KRESGE EDWARD N, PEIFFER DENNIS G, CHLUDZINSKI, JAMES, J., ELSPASS, CHESTER, W., HSEIH, DONG-TSAI, KRESGE, EDWARD, N., PEIFFER, DENNIS, G.
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1996
Main classification: C08L7
IPC codes: C08F2, C08K3, C08L9, C08L101