GB2070622A - Silica and fibrous polyolefin thickening agent - Google Patents

Abstract

A thickening agent for organic liquids such as styrene-polyester resin liquids used in iaminating or gel coating applications comprises a mixture of a finely divided silica and a finely divided fibrous polyolefin.

Description

SPECIFICATION Silica and fibrous polyolefin thickening agent This invention relates to thickening agents and organic liquids thickened with such agents. More particularly, the thickening agents of this invention comprise a mixture of a finely divided silica and a finely divided fibrous polyolefin.

A wide variety of finely divided inorganic and organic materials have been used to increase the viscosity of organic liquids for use in various applications such as paints, coatings, lubricating oils, and molding compositions. Synthetic amorphous silicas such as silica aerogels and pyrogenic silicas have commonly been used, for example to thicken liquids such as paraffin oils and polyester, alkyd, and epoxy resins in the production of greases from oils, resinous gel coats, and other similar applications.

Relatively large amounts of many silica and other conventional thickening agents have been required to provide the increase in viscosity of organic liquids required for the formation of thixotropic gels and certain other applications. The use of such amounts may adversely affect the properties of the thickened organic liquid that are desirable for the application and make the cost of the thickening agent economically prohibitive. Thus, there is a great need for materials with improved thickening efficiency to provide a greater viscosity increase when used in the same or smaller proportions than known thickeners.

In accordance with the present invention, it has been discovered that a composition which comprises a mixture of finely divided silica and finely divided polyolefin fibers has unexpectedly superior efficiency in thixotropic thickening of a wide variety of organic liquids compared to the thickening efficiency of the silica component when used alone and avoids the agglomeration that can occur when the polyolefin component is used alone. The fibers have a fibrillar structure and an exceptionally high surface area.

The finely divided silica used in the present invention is generally a substantially dehydrated synthetic amorphous silica. The water content is generally from about 1 to about 5 weight percent as measured by loss in weight after heating for 1 hour at 1 7500F (9550C). These synthetic amorphous silicas generally have surface areas of greater than about 50 square meters per gram and commonly of greater than about 1 50 square meters per gram. The surface areas are determined by the nitrogen adsorption method described in Brunauer, Emmett, and Teller, 60 J. Am. Chem. Soc. 309 (1933). The method is run to a P/PO of 0.967 so that pores of from 14 to 600 angstroms in diameter are measured.

The synthetic amorphous silicas generally have an aggregate weight median particle diameter of less than about 50 microns and preferably of less than about 10 microns. This aggregate silica particle diameter is the size to which the ultimate silica particles having an average size of from about 10 to about 50 millimicrons coalesce by a combination of chemical reaction, physical attraction and mechanical interaction.

Silica aerogels, pyrogenic silicas, and mixtures thereof are highly preferred synthetic amorphous silicas for use in the thickening agents of this invention because their mixtures with a finely divided fibrous polyolefin have significantly superior thickening efficiency. These amorphous silicas comprise chemically similar polymerized silica molecules and have some differing and some similar physical properties. Because of this basic chemical similarity, the silica art has adopted the method of synthesis as the principal means of differentiating between the various types of synthetic amorphous silicas.

Silica aerogels are the most preferred synthetic amorphous silica for use in the thickening agent of this invention. A silica aerogel is typically prepared by mixing sodium silicate and sulfuric acid to form an acidic silica hydrosol, allowing the hydrosol to set to a hydrogel, treating the hydrogel with ammonium hydroxide, washing the hydrogel substantially free of sodium and ammonium compounds, and drying the washed hydrogel in a manner so that there is no substantial shrinkage to the silica structure. A useful drying technique employs a fluid energy mill which concurrently dries and sizes the silica aerogel to the desired particle size range.Silica aerogels may also be prepared without ammonium hydroxide treatment by use of a drying step in which the hydrosol or the washed hydrogel is heated in the presence of an organic solvent, such as ethyl acetate, to at least the critical temperature of the solvent and thereafter the solvent is slowly released from the system. The silica aerogel products have relatively low surface areas and large pore volumes and average pore diameters.

Especially preferred silica aerogels for use in the thickening agent of this invention have a weight median particle diameter of from about 2 to about 10 microns, a surface area of from about 300 to about 400 square meters per gram, a pore volume of at least about 1.2 cubic centimeters per gram, and an average pore diameter of from about 1 50 to about 250 angstroms. The pore volume is determined by the same B.E.T. nitrogen adsorption method used to determine surface area. The average pore diameter in angstroms is calculated from the pore volume in cubic centimeters per gram and surface area in square meters per gram in accordance with the equation 4 x pore volume x 104 average pore diameter = diameter= surface area The preferred pyrogenic silicas are sometimes also referred to as fumed silicas.Pyrogenic silicas are prepared by volatilizing and recondensing silica in high temperature arc or plasma jet processes or by charging vapors of a silicon compound, such as silicon tetrachloride, silicon tetrafluoride, or silicon sulfide into a high temperature hydrolyzing flame.

The fibrous polyolefin used in the thickening agent of this invention may be a polymer of a variety of olefins and is generally a crystalline or partially crystalline high density polyalkene. Fibrous polymers of lower aliphatic alkenes containing from about 2 to about 6 carbon atoms are generally employed.

Preferably, the polyolefin is selected from the group consisting of polyethylene, polypropylene, and mixtures thereof. Fibrous polyethylene is especially preferred. Other olefins which may be employed include diolefins such as butadiene and isoprene and alpha-olefins such as 1 -butene, 1 -pentene, 1-dodecene, and 4-methyl-i -pentene. In addition to fibrous homopolymers of these olefins, fibrous copolymers and block copolymers may be formed by polymerization of olefin mixtures. Preferably, the fibrous polyolefin has a viscosity average molecular weight of greater than about 400,000 and more preferably of greater than about one-half million.These molecular weights correspond to a preferred intrinsic viscosity of greater than about 4.0 dl/gram and a more preferred intrinsic viscosity of greater than about 5.0 dl/gram and a melt index of zero as measured by ASTMD-1 238-62T. The preferred fibrous polyethylene softens at a temperature of from about 120 to about 1300C (248-2660F) and melts at a temperatures of from about 130 to about 13500 (266-2750F).

The fibers are made up at least in part of fibrils and thus have a fibrillar structure. Some of the fibers are made up of bundles of macrofibrils which are generally larger than about 1 micron in diameter and some of the macrofibrils have portions that are made up of microfibrils having a diameter of less than about 1 micron. Preferably, the polyolefin fibers are highly fibrillated (i.e. branched) and have an exceptionally high surface area of greater than about 1 square meter per gram and preferably of greater than about 5 square meters per gram. The surface area of the fibers typically ranges from about 5 to about 1 5 square meters per gram. The surface area is measured by gas adsorption techniques, such as the nitrogen B.E.T. method, of samples rinsed in isopropanol, dried in a 450C (1 130F) oven, and vacuum dried.

Suitable high surface area fibrous polyolefins may be prepared, for example, by direct conversion of an olefin monomer gas. In these processes, a monomeric olefin is polymerized at a relatively rapid reaction rate in a reaction medium in which the polyolefin to be formed is swellable or soluble to a significantly measurable extent in the presence of a coordination catalyst under conditions of high shear stress. Representative polymerization processes of this type are disclosed in U.S. Patents 3,891 ,6 10 and 3,849,387. The fibrous polyolefin may also be prepared by the process of U.S. Patent 3,743,272, in which a polyolefin is dispersed in a precipitant under conditions of shear stress to form polyolefin fibers having a microfibrillar structure, a high surface area, and a size and morphology similar to natural cellulosic fibers.

The fibers produced by the polymerization tend to be interconnected or bundled together. The fibers can be refined or beaten to separate discrete fibers from the bundles by conventional defibering or shredding techniques in an apparatus such as a disc refiner, Claflin refiner, Hollander beater or Dynapulper.

The fibrous polyolefin can be fluffed by passing the fibers several times through a high-speed material fan. The fluffing operation by itself does not dry the fibers to any great extent but the fluffed fibers can be dried by various hot air systems to a moisture content of less than about 2 weight percent.

The fluffed fibrous polyolefin having a moisture content of from about 45 to about 55 weight percent is preferred for preparing the thickening agent of this invention because of its convenience in handling.

Generally, the finely divided polyolefin used in this invention has an average fiber length of less than about 900 microns and a diameter of less than about 10 microns. Average fiber length is the average by weight measured in a Bauer-McNett classifier in accordance with TAPPI Standard Test No.

T-233 SU-64. The length to diameter ratio of the fibers is greater than about 1 to 1 and generally is greater than about 5 to 1.

Preferably, the fibrous polyolefin is reduced in size for use in the thickening agent of this invention so that it has a major dimension of not greater than about 50 microns and preferably of less than about 10 microns. The minor dimension of the preferred fibers ranges from less than about 5 to less than about 1 micron. The length to diameter ratio of the fiber aggregate particles is preferably greater than about 10 to 1 and more preferably greater than about 50 to 1. The fibrous polyolefin generally contains a major amount, e.g., greater than 90 percent by weight, of fibers having lengths of from about 5 to about 10 microns and diameters of less than about 1 micron. Minor amounts, e.g., less than about 10 percent, of larger fibers or agglomerates of the smaller fibers having major dimensions of up to about 50 microns and minor dimensions of from about 5 to about 10 microns can be detected in a microscopic examination. The finely divided silica used in the thickening agent of this invention generally has a weight median particle diameter of less than about 50 microns and preferably less than about 10 microns.

Any apparatus suitable for the reduction of the silica and fibrous polyolefin to the desired size may be used. The feed to the apparatus may be a silica hydrogel or a silica aerogel produced by drying the hydrogel. The gel and polyolefin components are preferably broken up, as by cutting or shredding, into pieces of about 1/8 inch (3.175 mm) or less in size to aid feeding into the apparatus. The polyolefin fibers and silica may be mixed in a conventional manner to form the thickening agents of this invention.

Preferably, the components are mixed prior to incorporation in the organic liquid and mixing methods, such as blending or tumble mixing, are sufficient. Although lower thickening results, the silica and polyolefin components may be mixed in the liquid to be thickened if desired.

In a preferred embodiment, the mixture of the thickening agent of this invention is prepared by simultaneous size reduction of the silica and polyolefin in the presence of the other by a crushing, milling, or grinding operation in such a manner as to cause fracture of the particles and formation of freshly exposed surfaces and provide an intimate mixture of the components. Suitable apparatus for simultaneous size reduction in which freshly formed surfaces are exposed includes, for example, ball mills, vibration mills, pot mills, hammer mills, gyratory crushers, pulverizers, speedline mills, sand grinders, colloid mills, and micron mills.

The preferred method for preparing the mixture comprises simultaneous fluid energy milling of the components. In this method, the polyolefin and silica are suspended in a moving gaseous medium and additional gas is continuously introduced in a plurality of high velocity streams directed inwardly into the mill in such a way as to cause extreme turbulence and attrition and fracturing of the suspended silica and polyolefin. The comminuted fibrous polyolefin-silica mixture is continuously removed from the mill along with the gaseous medium and separated from the suspending gas. Air and steam are the preferred suspending gases and are also preferably used as the supplemental turbulence-creating gas because of inexpensiveness and ready availability.

In the operation of the fluid energy mill, using air as the gaseous grinding medium, suitable pressures of the suspension air range from about 100 to about 500 pounds per square inch (35 kg/cm2) gauge and preferably from about 110 to about 300 pounds per square inch (7.7 to 21 kg/cm2) gauge.

The auxiliary turbulence-creating air can be injected into the whirling body of polyolefin and silica at pressures which may range from about 100 to about 500 pounds per square inch (7 to 35 kg/cm2) gauge and preferably are between about 110 to about 250 pounds per square inch (7.7 to 1 7.6 kg/cm2) gauge. The suspension and auxiliary air should be at a temperature low enough to avoid softening or melting of the polyolefin. Preferably, the air temperature is from about 50 to about 1200F (10--490C).

The average particle size of the product can be varied by controlling the air velocity, temperature, and feed rate. The product can be separated from the suspending air in any suitable manner, preferably by the use of bag collectors, though cyclone and other kinds of separator can also be used.

The preferred fluid energy mill is the micronizer, in which relatively large particles are suspended in a gaseous medium and whirled around an enclosed base with additional gases introduced into the whirling body in a manner causing turbulence within the body and comminution and fracturing of the particles.

Co-milling of the fibers and the silica concurrently decreases the particle size of the silica, defibrillates (opens or unwinds) the fibers, and produces an intimate mixture. In the intimate mixture, the fibers and the silica particles are held together and cannot be separated by normal mechanical means so that something more than mere electrostatic attraction or mechanical impingement is present. It is believed that co-milling exposes active surfaces which bond the polyolefin and silica.

Various methods may be used to produce such an intimate mixture but fluid energy co-milling is preferred.

The polyolefin should be present in the mixture in an amount sufficient to provide a substantial increase in the thickening efficiency of the silica. The mixture generally contains from about 95 to about 5 weight percent polyolefin and from about 5 to about 95 percent silica. Preferably, the mixture comprises from about 25 to about 35 weight percent polyolefin and from about 75 to about 65 weight percent silica.

A preferred use for the mixtures of this invention is as an agent to thicken, i.e., form thixotropic gels, and/or increase the viscosity of organic liquids. The organic liquids which may be employed in the compositions of this invention are, for example, organic solvents, liquid organic film-formers, liquid organic resins, oleaginous liquids, and mixtures thereof. Such organic solvents may be solvents used in paint, varnish, or lacquer removers for example and include aliphatic and aromatic alcohols, ketones, and esters, such as ethanol, acetone, methyl ethyl ketone, ethyl acetate, or amyl acetate. The liquid organic film-formers generally comprise solutions of high molecular weight film-formers dissolved in organic solvents and are generally employed as adhesives, films, foils, paints, lacquers, and dopes.Such high molecular weight organic film-formers are exemplified by nitrocellulose, cellulose acetate, chlorinated rubber polyvinyl acetate, polyvinyl chloride, polyacrylic esters, cellulose butyrate, and cellulose propionate. When these liquid compositions are sprayed or spread on objects, the thickening agent of this invention will cause the formation of a thixotropic gel almost immediately on contact with the object and the gel will not run or drain.

The liquid resin compositions which may be employed with the silica and fibrous polyolefin thickening agent include plastisol compositions comprising halogenated vinyl or vinylidene resins. The thickening agent of this invention is especially useful for thickening thixotropic, polymerizable organic liquid resin compositions which are used in coating, filling, adhesive, and laminating operations. Such compositions include liquid alkyd or epoxy resins or solutions of solid alkyd, epoxy or polyester resins dispersed in a solvent (for example, styrene) which is usually copolymerizable with the polyester resin.

The mixtures of this invention are readily wetted and dispersed and give very great increases in viscosity at generally lower concentrations in curable liquid resins such as polyesters and polyepoxides and resin lattices such as paints.

The oleaginous liquids in which the present thickening agent may be used include oils of animal and vegetable origin such as, for example, cod liver oils, olive oil, corn oil, and lubricating oils such as hydrocarbon motor oils and mixtures thereof. The lubricating oils may be thickened with the thickening agent of this invention to provide gel-like bodies having a grease consistency.

The thickening agent of this invention may be incorporated into the organic liquid by any conventional dispersion method. Relatively low shear mixing methods such as hand stirring are often satisfactory but high shear dispersion mixers, such as roll mills, high speed blenders, or ultrasonic mixers may be preferred for certain organic liquids.

The amount of the thickening composition utilized in the liquid to be treated can vary greatly depending on the nature of the organic liquid, the dispersion method, and the degree of thickening desired and is a minor amount sufficient to increase the viscosity of the organic liquid. The amount of the thickening agent can generally vary from about 0.05 to about 10 percent but usually is from 1 to about 5 percent by weight of the liquid to be thickened.

EXAMPLE 1 250 grams of a silica aerogel and 200 grams of shredded polyethylene fibers having the properties shown in Table I were placed in a blender and mixed at high speed for 1 minute. This blending operation was repeated many times to accumulate 1 5 pounds (6.8 kg) of the mixture.

TABLE I Silica Aerogel Polyolefin Fibers Loss on Ignition 8.5 Specific Gravity 0.9 (% at 17500F9550C) Average Length (microns) 900 pH (5% Slurry in H2O) 7.0 Diameter (microns) SiO2 (% ignited basis) 99.6 Surface Area (m2/g) 8-12 Median Particle Diameter 2-3 (microns) Melt Index (g/1 0 min) 308 Surface Area (m2 g) 310.0 Moisture Content (%) 50 Oil Absorption (lb/1 00 Ib) 280.0 Softening Point (OC) 120-130 Bulk Density 8.0 Melting Point (OC) 130-135 (Ib/ft3, 6.01602 g/cm3) Buiking Value 16.66 (Ib/gal, 0.12 kg/l) Soluble Salts (%) 0.1 Avg.Pore Diameter (angstroms) 200.0 Pore Volume (cc/g) 1.4 6 pounds (2.7 kg) of the mixture was fed through a vibrating screw feeder and injected with compressed air at 127 pounds per square inch (8.9 kg/cm2) gauge and 85-900F (30--320C) into an 8 inch (20 cm) micronizer at a rate of 7.5 pounds (3.4 kg) per hour. Air at 95 to 1000F (35-380C) and under a pressure of 11 5 to 11 6 pounds per square inch (8.08 to 8.1 5 kg/cm2) gauge was injected into the whirling body of polyethylene fibers and silica aerogel to create a turbulent mass in the mill. The outlet air temperature was 95 to 1 050F (35-41 OC) and 4 pounds (1.8 kg) of the milled product were separated from the air stream in a bag collector.

The product was a white, odorless, flocculated powder and consisted of finely divided polyethylene fibers intimately mixed with finely divided silica. Microscopic examination of the powder showed that the fibers had a major dimension of less than 1 micron and a smaller dimension in a low millimicron range.

The blended mixture, the blended and milled mixture, and a sample of the same silica aerogel per se were used to thicken a liquid styrene-unsaturated polyester resin. The resin contained about 45 parts by weight of styrene and about 55 parts by weight of the unsaturated polyester resin. The liquid resin composition had a viscosity of 110 centipoises at 770F (250C) as measured in a Brookfield viscometer using a No. 4 spindle at 20 revolutions per minute.

Various concentrations by weight of each of the thickening agents were mixed with separate portions of the liquid resin for 5 minutes at 4000 revolutions per minute in an Eppenbach homogenizer.

The viscosity was then immediately measured in the same viscometer under the same conditions as the unthickened resin. The viscosities of the various thickened samples were also determined at 20 revolutions per minute in the same apparatus at 770F (250C) using a No. 4 spindle and the thixotropic index was calculated as the viscosity at 2 revolutions per minute divided by the viscosity at 20 revolutions per minute. The results are shown in Table II.

TABLE II VISCOSITY VS. CONCENTRATION IN STYRENE POLYESTER RESIN Concentration (weight parts thickening agent per one hundred Thixotropic Thixotrope weight parts of resin) 2 rpm 20 rpm Index Silica aerogel 1 200 1 50 1.3 2 600 230 2.6 3 2,050 470 4.4 5 6,600 1,080 6.1 Milled mixture 1 2,250 590 3.8 2 6,650 1,570 4.2 3 14,350 3,340 4.3 Blended mixture 1 1,020 370 2.8 2 3,900 940 4.1 3 7,500 1,760 4.3 The results clearly demonstrate that the use of the thickening agent of the present invention provides an unexpectedly higher increase in viscosity and in thixotropy when incorporated in organic liquids compared to the use of a silica aerogel per se in such organic liquids. The inclusion of the polyethylene fibers significantly reduced the amount of silica material required to thicken the liquid. A substantially higher viscosity and thixotropic index were achieved with the co-milled polyolefin and silica thickening agent compared to the blended mixture. It was also found that the polyethylene fibers agglomerated and floated to the surface of the resin when used alone.

Claims (28)

1. A thickening agent comprising a mixture of from 5 to 95 weight percent of finely divided silica and from 95 to 5 weight percent of finely divided polyolefin fibers having a fibrillar structure.

2. A thickening agent according to claim 1 in which the fibers have a surface area greater than 1 square meter per gram.

3. A thickening agent according to claim 1 or 2 in which the fibers have a surface area greater than 5 square meter per gram.

4. A thickening agent according to any one of claims 1 to 3 in which the silica has a weight median particle of less than 50 microns.

5. A thickening agent according to claim 4 in which the silica has a weight median particle diameter of less than 10 microns.

6. A thickening agent according to any one of the preceding claims in which the silica is a silica aerogel, a pyrogenic silica, or a mixture thereof.

7. A thickening agent according to claim 1 in which the silica is a silica aerogel.

8. A thickening agent according to claim 7 in which the silica aerogel has a weight median particle diameter of 2 to 10 microns, a surface area of 300 to 400 square meters per gram, a pore volume of at least 1.2 cubic centimeters per gram, and an average pore diameter of 1 50 to 250 angstroms.

9. A thickening agent according to any one of the preceding claims in which the fibers have a major dimension of less than 50 microns and a minor dimension of less than 5 microns.

10. A thickening agent according to claim 9 in which the fibers have a major dimension of less than 10 microns and a minor dimension of less than 1 micron.

11. A thickening agent according to any one of the preceding claims in which the polyolefin has a viscosity average molecular weight of greater than 400.000.

12. A thickening agent according to claim 11 in which the polyolefin has a viscosity average molecular weight of greater than one-half million.

13. A thickening agent according to any one of the preceding claims in which the fibers have a length to diameter ratio of greater than 5 to 1.

14. A thickening agent according to any one of the preceding claims in which the polyolefin is a crystalline or partially crystalline high density polyalkene.

1 5. A thickening agent according to claim 14 in which the polyolefin is a polymer of an aliphatic alkene containing from 2 to 6 carbon atoms.

1 6. A thickening agent according to any one of the preceding claims in which the polyolefin is polyethylene, polypropylene, or a mixture thereof.

1 7. A thickening agent according to claim 1 6 in which the polyolefin is polyethylene.

18. A thickening agent according to claim 17 comprising an intimate mixture of (a) from 75 to 55 weight percent of a silica aerogel having a weight median particle diameter of from 2 to 10 microns and (b) from 25 to 45 weight percent of polyethylene fibers having a fibrillar structure.

19. A thickening agent according to claim 1 8 in which the fibers have a surface area greater than 1 square meter per gram, a major dimension of less than 10 microns, and a minor dimension of less than 1 micron.

20. A thickening agent according to any one of the preceding claims which comprises from 25 to 35 weight percent polyolefin fibers.

21. A thickening agent according to claim 1 substantially as described in Example 1.

22. A composition of matter comprising a mixture of (a) an organic liquid and (b) a thickening agent as claimed in any one of the preceding claims.

23. A composition according to claim 22 in which the mixture has been prepared by co-milling the polyolefin and the silica of said thickening agent.

24. A composition according to claim 22 or 23 in which the thickening agent is present in an amount from 0.05 to 10 percent based on the weight of the organic liquid.

25. A composition according to claim 24 in which the thickening agent is present in amount of 1 to 5 percent based on the weight of the organic liquid.

26. A composition according to any one of claims 22 to 25 in which the organic liquid comprises a polyester resin or a polyepoxide resin.

27. A composition according to claim 26 in which the organic liquid is a styrene-polyester resin.

28. A composition according to claim 22 substantially as hereinbefore described.