WO1989000545A1

WO1989000545A1 - Chemically delaminated mica containing mineral

Info

Publication number: WO1989000545A1
Application number: PCT/GB1988/000554
Authority: WO
Inventors: Stephen Ronald Francis; Shaun Michael Barrett; Michael Joseph Ludden
Original assignee: Raychem Limited
Priority date: 1987-07-10
Filing date: 1988-07-08
Publication date: 1989-01-26
Also published as: EP0375693A1; JPH02504138A

Abstract

A mineral comprises a chemically delaminated weathered mica having a principal X-ray reflection at approximately 10 Angstroms characteristic of the mica basal spacing. The mineral may be produced by washing a suspension of weathered mica having a principal x-ray reflection at 10 Angstroms with a solution of intercalating ions e.g. organoammonium ions and with water in order to swell the weathered mica, mechanically shearing the swollen weathered mica, and separating components having a thickness greater than 500 Angstroms from the suspension so formed. The dispersion so formed may be used in the production of paints and coatings, or for electrophoretic deposition onto a metallic surface e.g. a wire conductor, in order to provide a high temperature insulation.

Description

Chemically dela inated mica containing mineral

This invention relates to layered silicate minerals and to products that incorporate such minerals .

It is known that several 2:1 layer phyllosilicate minerals such as vermiculites form interlayer complexes with a wide range of charged and uncharged species of both organic and inorganic origin e.g. alkylammonium ions, amino acids and amino acid cations. The inclu¬ sion of intercalating species between the layers of the macrocrystal usually results in changes to the basal spacing which can be measured by X-ray diffrac¬ tion techniques. Under certain circumstances an addi¬ tional swelling can take place whereby further intercalation, by a wide range of polar and non-polar solvents, occurs. In special cases the degree of expansion can be so extensive as to produce 'gel-like' samples. The application of mild mechanical action to these extensively swollen systems can lead to the pro¬ duction of colloidal dispersions of the mineral on a dispersing solvent, this process being known as "chemical delamination" . This effect can be particularly apparent in a range of mica-type complexes containing n-alkylammonium ions, with water as a dispersing solvent. Whether additional interlayer expansion occurs depends on the layer change density separating successive layers on the mineral and the length of the alkyl chain of the associated intercallants.

Minerals with a surface charge density in the range of 0.5 to 0.9, saturated with certain short chain n-alkylammonium ions e.g. n-propyl, n-butyl and isoamyl, behave exceptionally well in that they show extensive interlayer swelling in water. Crystals which show this type of behaviour can increase in volume by up to 30 times their original volume and remain coherent and 'gel-like¹. This action can be used to produce a colloidal dispersion of thin high aspect ratio platelets. In the case in which the starting mineral is of a homogenous nature the composition of the colloid will be consistent. However, if mixed layer minerals are used then there can be a wide variation of platelet composition and characteristics throughout the colloidal dispersion. Fractionation techniques, including sedimentation, can be used to isolate components of the dispersion which exhibit dif¬ ferent chemical and physical characteristics from each other and from the parent mineral. If the colloid is subjected to mild mechanical shearing, delamination will occur in the swollen crystals along the macrosco- pically swollen cleavage plane where the interlayer forces are minimised. ^' Such a process is described for example in British patent No. 1,593,382 to ICI. According to the present invention novel mineral systems are formed by applying the chemical delamina- tion technique described above to weathered mica rather than to the minerals mentioned above.

The term "weathered mica" is used herein to describe the weathering products of natural mica and includes minerals of a mixed layer type that contain mica layers interspersed with other layers that are formed by weathering. The weathered layers may, in the broadest aspect, comprise any hydratable, layer lat¬ ticed, expandable silicate structure, e.g. hydrobiotite and hydrophlogopite layers, and preferably hydroph- logopite II layers although other layers may instead be present. The hydratable layers may comprise a major part of the original mineral although it is preferred for the major part (by weight) to be formed from unweathered mica layers .

In contrast with weathered mica having a higher degree of cationic replacement, the weathered mica mineral used in accordance with the invention exhibits an x-ray reflection at. approximately 10 Angstroms (in fact 10.03 A) characteristic of mica, and, also in contrast with weathered mica having a higher degree of cationic replacement, does not exhibit any reflection at 14.25 Angstroms characteristic of vermiculite. The mineral will normally also exhibit a reflection at approximately 11.7 Angstroms which is characteristic of the hydrated biotite or hydrated phlogopite basal spacing.

Thus, the mineral according to the invention may be regarded as formed from platelets that have a mica- ceous, or predominantly micaceous interior, and a sur¬ face that is formed from a hydrated silicate layer. The platelets preferably have an average thickness of not more than 500 Angstroms, more preferably not more than 300 Angstroms, especially not more than 200 Angstroms and most especially not more than 100 Angstroms, but preferably at least 20 Angstroms, more preferably at least 40 Angstroms and especially at least 60 Angstroms .

The mineral may be formed as a dispersion by a process which comprises:

(a) forming an aqueous dispersion of a weathered mica having a dominant x-ray reflection at approxima¬ tely 10 Angstroms characteristic of mica;-

(b) contacting the weathered mica with an aqueous solution of intercalating ions washing the disper¬ sion with water in order to swell the weathered mica;

(c) mechanically shearing the swollen weathered mica; and

(d) separating components having a thickness greater: than 500 Angstroms from the suspension so formed.

In this process the weathered mica ore is usually treated consecutively with an aqueous solution of an. alkali metal salt e.g. a sodium salt, and especially sodium- chloride, and an aqueous solution of a further salt, e.g. an organo substituted ammonium salt such as an n-butyl ammonium salt, in order to swell the ore for example as described in British Patent No. 1,065,385. The ore then swells up to 3.5 times its original size, usually to about 1.5 to 3 times its original size in water, after which it is delaminated for example by means of a mill, a mixer, an ultrasonic agitator or other suitable device to form the majority of the expanded mineral into a colloidal dispersion. The colloidal dispersion so formed can be fractionated by sedimentation into several cuts in order to separate components of the mineral having different thicknesses. With a mineral having a high degree of weathering, as one moves from the 'fines' to the more coarse fractions composition of the mineral changes significantly, the degree of hydration decreasing through successive layers, the K2O content increasing and the x-ray diffraction pattern moving closer to that of mica. When partially weathered micas are used, a distinctive increasingly micaceous compos.ition is identified as one moves to the coarse, unprocessable fraction of the mineral. Its x-ray diffraction pattern, TGA trace and elemental composition distinctly identify it as pure mica. In this case it is possible to form a dispersion of predominantly micaceous lamellae by selecting the appropriate fractions of the colloid i.e. by discarding the coarse mica fraction and optionally also the highly hydrated fines. It is therefore possible to generate a dispersion of mica-like platelets as iden¬ tified by x-ray diffraction utilising the chemical exchangability of weathered interlayers in partially weathered interstratified layered minerals.

In a typical process, the dispersion is permitted to stand for between 1 and 60 minutes, preferably 5 to 20 minutes, and the top fraction decanted to supply the working colloid. In many instances where partially weathered mica is employed, it will not be possible for all the mineral to be brought into suspension since the weathering process does not occur uniformly throughout the mineral, and the greater the degree of weathering, the greater- the proportion of mineral that can be dispersed. The particle size range of the decanted fraction, i.e. the particle dimensions parallel to the silicate layers, typically is between 1 and 250 urn, preferably between 1 and 100 urn.

Normally approximately 50% of the mineral will remain in dispersion and approximately 50% will form a sediment within a few minutes, any mineral that preci¬ pitates after a few minutes usually having substan¬ tially the same x-ray pattern as the mineral remaining in dispersion. The product that precipitates has a potassium content of approximately 9%, which corresponds to that of mica, while the material that remains in dispersion has a potassium content of approximately 6%.

If desired the water may be removed from the solids, e.g. by evaporation, or the mineral may be used in the form of a colloid, the choice depending on the use to which the mineral is put. In most cases, however, the mineral will be used in the form of the colloid after necessary adjustments have been made and additional components added. The colloid may for example be used to form coatings of various forms both for decorative and functional purposes. In one appli- cation as described in our copending international and European patent applications filed on even date herewith, claiming priority from British applications Nos. 8716303, 8716308, 8716309 and 8716310, the colloid is used to form a coating on the conductor of an electrical wire in order to enable the wire to function during a fire. In this case a binder, preferably in the form of a latex, is incorporated into the colloid, and the mineral is electrophoretically deposited on the wire. The colloid in this case preferably has^' a mineral concentration of at least 0.5 and especially at least 1% by weight although lower concentrations may be used provided that the concentration is not so low that flocculation occurs. The maximum concentration is pre¬ ferably 8% and especially 4% by weight, beyond which the relatively high viscosity of the suspension may lead to unreproducable coatings. The conditions that are employed to form the suspension will depend among other things on the particular type of mineral that is employed.

The material chosen for the binder should be inert, i.e. it should not corrode the conductor metal or react with the mineral coating and preferably it improves the bonding of the mineral layer to the con¬ ductor metal. It should also be electrophoretically mobile and non-flocculating. The binder may be disper- sible in the medium that is used to form the mineral suspension (water), for example it may comprise a water-dispersed latex, e.g. a styrene/butadiene/car- boxylic acid latex, a vinyl pyridine/styrene/butadiene latex, a polyvinyl acetate emulsion, an acrylic copo- lymer emulsion or an aqueous silicone emulsion. It is preferred to use binders in the form of emulsions because they may be dried quickly with only a few seconds residence time in the drying tower, whereas with aqueous solutions much longer drying times are necessary, and, if drying is forced, bubbles may be formed in the mineral layer that will cause imperfec¬ tions in the resulting dried layer. In addition at least some b'inders that are hydrophobic have the advan¬ tage that they can prevent or reduce the uptake of moisture by the mineral layer after it has been dried. This is particularly useful where the weathered mica has a relatively high degree of cationic replacement, i.e. where it contains a relatively high degree of weathered interlayers, so that undesired exfoliation of the mineral layer when subjected to a fire can be eli¬ minated. The binder is preferably non-curable since- curable binders do not significantly improve the per¬ formance of the wire and will normally reduce the speed at which the wire can be manufactured.

The binder is preferably used in quantities in the range of from 5 to 30%, and especially from 10 to 25% by weight based on the weight of the weathered mica. The use of smaller quantities may not sufficiently improve the processability of the conductor and/or may not improve the adhesion of the mineral layer to the metal conductor adequately while the use of larger quantities of binder may lead to the generation of too much char for the silicone layer to mask. Also, it is preferable not to use binders such as neoprene that generate large quantities of char. Preferably the binder has a carbonaceous char residue of not more than 15%, more preferably not more than 10% and especially not more than 5%. The process according to the invention provides a useful procedure for cleaving micaceous materials as compared with thermochemical or mechanical methods.

The use of a weathered mica mineral according to the invention in electrical applications has the advan¬ tage, as compared with more highly weathered products such as ideal hydrophlogopite or vermiculite, that the dielectric properties of the mineral e.g. dielectric loss , are improved due to the reduced water content and greater micaceous content, and that the mechanical strength and high temperature performance are improved, with little or no exfoliation of the mineral on heating.

The invention will now be described by way of example with reference to the accompanying drawings which are x-ray diffraction patterns of various mica¬ ceous minerals.

Referring to the accompanying drawings, figure 1 shows an x-ray diffraction pattern of mica. Only one reflection 1 can be seen located at approximately 10 Angstroms which is the characteristic basal spacing of mica.

Figure 2 is an x-ray diffraction pattern of the raw weathered mica that is used in the present inven¬ tion. In addition to the mica basal spacing reflection 1, a number of additional reflections are observed, including a reflection 2 at about 11.8 Angstroms which is characteristic of hydrophlogopite II or hydro- biotite. Figure 3 is an x-ray diffraction pattern of a ver- miculite raw material.

The mica of figure 1 will have a water content about 0.2% by weight as measured by thermogravimetric anaylsis at 400°C, and a potassium content of about 9.1% by weight. The mineral used according to the invention, (figure 2) has a water content of about 1% by weight and a potassium content of about 8.4%.

As the degree of weathering increases, so that the potassium content becomes about 3 to 5% and the water content increases to 6% by weight, the 10 Angstrom mica, and 11.8 hydrated biotite, reflections disappear, and new reflections at about 12.6 Angstrom, 3, and at 14.4 Angstroms, 4, appear, the latter reflection corresponding to the vermiculite basal spacing. The elemental composition data given herein were obtained by the method described in "Quantitative Scanning Electron Microscopy", D.B. Holt, M.D. Muir, P.R. Grant, I.M. Boswarva, 1974 Academic Press.

Figure 4 shows the diffraction pattern of the mineral formed by evaporation of the working colloid. By comparing figure 4 with figure 2, it can be seen that the intensity of the 10 Angstrom reflection 1 is decreased relative to that of the hydrobiotite reflec¬ tion 2, reflecting the removal of the unprocessed mica, during sedimentation of the colloid.

The following Example illustrates the invention:

A colloid was formed as follows: 800 gramms- of a weathered mica having a powder x-ray diffraction pat- tern as shown in figure 2, was washed with boiling water for about 30 minutes and the resulting liquid was decanted to remove the clay fraction. The mineral was then refluxed for 4 to 24 hours in saturated sodium chloride solution to replace the exchangable cations with sodium ions. This was then washed with distilled or deionised water to remove excess sodium chloride until no further chloride ions could be observed by testing with silver nitrate. The material was then refluxed for 4 to 24 hours with molar n-butyl ammonium chloride solution followed by further washing with distilled or deionised water until no chloride ions could be detected with silver chloride.

The swollen material was then worked in a Greaves mixer for 120 minutes to shear the mineral and was allowed to stand for 20 minutes to sediment the unpro¬ cessed mineral. The top fraction was used as the working colloid.

A colloid having 4% by weight weathered mica and 15% by weight carboxylated styrene-butadiene-styrene rubber based on the weight of the weathered mica, was used as a plating bath in the production of a coated wire. A 20 AWG wire was passed through a 40 cm long bath of the colloid at a speed of 5 metres minute"¹ while the weathered mica was electrophoretically depo¬ sited on the conductor at a 4.2V plating voltage and a 165 mA current. The coated wire was then passed through a drying tower to form a mineral layer of 30 micrometre dry thickness . The wire was then passed through a cross-head die by which a silicone elastomer was extruded and cured again as shown in the drawing to form a 80 micrometre thick silicone layer. Thereafter a 100 micrometre thick single wall insulation formed from low density polyethylene containing 8% by weight decabromodiphenyl ether and 4% antimony trioxide flame retardant was extruded onto the wire.

The wire was tested for circuit integrity by twisting three wires together and connecting each wire to one phase of a three phase power supply, and then heating the wire to 900°C for a test period of three hours in accordance IEC 331. The wire was able to sup¬ port 440V phase-to-phase for the entire test at 900°C without failing (i.e. without blowing a 3A fuse).

Claims

Claims :

1. A mineral comprising a chemically delaminated weathered mica having a principal x-ray reflection at approximately 10 Angstroms characteristic of the mica basal spacing.

2. A mineral as claimed in claim 1, which has a water content in the range of 0.7 to 5.5% by weight.

3. A mineral as claimed in claim 2, which has a water content in the range of from 1 to 4% by weight.

4. A mineral as claimed in any one of claims 1 to

3, which exhibits an x-ray reflection at approximately 11.7 Angstroms characteristic of the hydrated biotite basal spacing.

5. A mineral as claimed in any one of claims 1 to

4, which has a surface charge density in the range of from 0.5 to 0.9.

6. A mineral as claimed in any one of claims 1 to

5, which is in the form of particles of particle size (parallel to the mica layers) less than 300 micro¬ metres, preferably less than 200 micrometres and espe¬ cially less than 100 micrometres.

7. A mineral as claimed in any one of claims 1 to

6, which is in the form of particles of particle size (parallel to the mica layers) of at least 3 micro¬ metres, preferably at least 5 micrometres and espe¬ cially at least 10 micrometres.

8. A mineral as claimed in any one of claims 1 to

7, which is in the form of particles having a thickness (perpendicular to the mica layers) of not more than 500 Angstroms, preferably not more than 200 Angstroms and especially not more than 100 Angstroms.

9. A mineral as claimed in any one of claims 1 to

8, which is in the form of particles having a mean thickness (perpendicular to the mica layers) of at least 30 Angstroms, preferably at least 50 Angstroms and especially at least 60 Angstroms .

10. A mineral as claimed in any one of claims 1 to

9 , which has a potassium content in the range of from 5.5 to 9%, preferably from 7 to 8% by weight.

11. A mineral as claimed in any one of claims 1 to

10, which is in the form of an aqueous dispersion.

12. A mineral as claimed in any one of claims 1 to

11, which includes a binder.

13. A process for the production of a mineral which comprises:

(a) forming an aqueous dispersion of a weathered mica having a dominant x-ray reflection at approxima¬ tely 10 Angstroms characteristic of mica;

(b) contacting the weathered mica with an aqueous solution of intercalating ions and washing the suspension with water in order to swell the weathered mica; (c) mechanically shearing the swollen weathered mica; and

(d) separating components having a thickness greater than 500 Angstroms from the suspension so formed.

14. A mineral as claimed in claim 13, wherein the weathered mica is swollen during step (b) to not more than 3.5 times its original size.

15. A mineral as claimed in claim 13 or claim 14, wherein the components are separated in step (d) by sedimentation.

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