GB2226017A

GB2226017A - Opacifying agent

Info

Publication number: GB2226017A
Application number: GB8925934A
Authority: GB
Inventors: Elma Napier
Original assignee: Tioxide Group Ltd
Current assignee: Tioxide Group Ltd
Priority date: 1988-12-13
Filing date: 1989-11-16
Publication date: 1990-06-20
Anticipated expiration: 2009-11-16
Also published as: GB8925934D0; GB8829076D0; AU611065B2; AU4543689A; GB2226017B

Abstract

A composite opacifying agent is formed of pigmentary titanium dioxide and one or more extenders. The composite is coated with one or more hydrous metal oxides and has a specified particle size range. The agent is produced by fluid energy milling the coated particulate mixture with steam and specified steam/particulate material weight ratios. The agent is useful in matt finish aqueous paints. Extenders may be clay, BaSO4, SiO2 or CaSO4. Metal oxides are e.g. those of Ti, Al, Si, Ce, Zr.

Description

FORMAL SPECIFICATION OPACIFYING AGENT AND ITS PREPARATION This invention relates to an opacifying agent and its method of manufacture and particularly to a coated mixture including titanium dioxide.

According to the present invention an opacifying agent comprises a mixed particulate material of pigmentary titanium dioxide and one or more extenders which particulate material is coated with one or more hydrous metal oxides and said material has an average weight particle size when measured by light scattering of from 2.75 microns to 3.44 microns and with 90% of said material having a size greater than 1.03 microns at an average size of 2.75 microns and 90% of said material having a size greater than 1.49 microns at an average size of 3.44 microns.

According to the invention also a method for the manufacture of an opacifying agent comprises forming a particulate mixture of pigmentary titanium dioxide and one or more extenders and said particulate mixture having a coating on the particles thereof of one or more hydrous oxides and subjecting the particulate mixture to fluid energy milling with steam as a grinding medium at a temperature of from 1500C to 3500C and at a steam to particulate material weight ratio (S:P) of from 0.5:1 to 1.7:1 to produce said particulate material having a particle size falling within the range of average weight particle size of 2.75 microns and with 90% by weight of the particles greater than 1.03 microns to an average weight particle size of 3.44 microns and with 90% of said material having a size greater than 1.49 microns when measured by light scattering.

Surprisingly it has been found that an advantageous opacifying agent formed of a c & oated particulate mixture of an extender or extenders and pigmentary titanium dioxide of the specified size has an opacifying effect in matt finish aqueous paints superior to similar co-coated particulate material having a particle size such as is produced by conventional fluid energy milling as practised in the titanium dioxide industry. Milling to a fine size actually has been found to produce an opacifying agent of reduced opacifying effectiveness. This requirement of less fluid energy milling has economic advantages as well as producing the surprisingly technical advantageous product. However some products prepared at the lowest steam:pigment ratio do exhibit grittiness which renders their use in the higher quality paints unadvisable.

The opacifying agent of the invention has as described previously an average weight particle size when measured by light scattering of from 2.75 microns to 3.44 microns and with 90% of said material having a size greater than 1.03 microns at an average size of 2.75 microns and 90% of said material having a size greater than 1.49 microns at an average size of 3.44 microns. Preferably however the average particle size is from 3.04 microns to 3.36 microns with 90% of the material having a size greater than 1.15 at an average size of 3.04 microns and 90% of the material having a size greater than 1.47 microns at an average size of 3.36 microns.

Opacifying agents of the present invention comprise a mixture of a titanium dioxide pigment which itself is not independently coated prior to mixing and an extender or extenders which is compatible with the titanium dioxide and which also is not independently coated prior to mixing.

The titanium dioxide pigment which is used to form the composite products of the present invention can be made either by the so-called "snIphate" process or the "chloride" process and can be anatase titanium dioxide or rutile titanium dioxide. Usually pigment prepared by the sulphate process will be used when this is easily available. The titanium dioxide pigment can be milled e.g. by sand milling prior to mixing with the extender pigment if desired.Any so-called extender can be used providing it is compatible with the titanium dioxide pigment and typical extenders are the various clays such as china clay, barium sulphate, silica and calcium sulphate. The extender can be milled prior to mixing with the titanium dioxide pigment, if desired.

The particulate mixture to be coated can contain a wide range of proportions of the titanium dioxide pigment to the total amount of extender and can be within the weight ratio of 2:1 to 1:2 for TiO2:extender.

Preferably, however, approximately equal weights are present in the mixture prior to coating. The mixture can be prepared by any suitable method such as dry mixing but conveniently an aqueous dispersion of the titanium dioxide pigment can be mixed with the extender also in the form of an aqueous dispersion.

The particulate material of the invention is coated with one or more hydrous oxides and typically the coating may be of one or more hydrous oxide of titanium, aluminium, silicon, cerium, zirconium, zinc, antimony or tin. The coating can contain amounts of hydrous oxides within a wide range of amounts and can be up to 25%, or more e.g. up to 35% by weight of the coating as oxides on weight of particulate material. More usually amounts of hydrous oxide in the range 1% to 10% by weight as oxide preferably 2.5% to 10% are present based on the weight of the particulate material. Mixed coatings of two or more different hydrous oxides can be present either as a homogeneous mixture or as separate coatings.A coating including both a hydros oxide of titanium and a hydrous oxide of aluminium has been found to be particularly useful and, if desired, a small amount of a hydrous oxide of silicon can be present also.

The coating of the particulate mixture is effected usually in the wet state by mixing an aqueous dispersion of the particulate mixture with a hydrolysable compound of the desired metal or of silicon and hydrolysis effected to deposit the chosen hydrous oxide or hydrous oxides. The hydrolysable compound can be one which is acidic or alkaline in character, usually acidic, which is hydrolysed on neutralisation or rendering alkaline or acid as is appropriate.

Coating of pigments can be carried out by a number of different techniques depending on the acidity or alkalinity of the initial aqueous dispersion to be coated and of the particular hydrolysable compounds used.

Usually when acidic hydrolysable compounds are added to an aqueous dispersion of particulate material the hydrolysis and precipitation of the coating is effected by subsequently adding an alkali or base to change the pH of the dispersion. When the hydrolysable compound is alkaline in nature then hydrolysis and precipitation can be effected by adding an acid or by adding an acidic hydrolysable compound. Many different techniques can be used as appropriate. Alternative methods of coating such as coaddition of acidic and alkaline reacting hydrolysable compounds, simultaneous addition of one or more hydrolysable compounds and an acid or an alkali can be used also.

Typically the water soluble hydrolysable compound is, in the case of a metal compound, a salt such as a halide or the salt of a mineral acid.

When a bydrous oxide of silicon is present in the coating this may be deposited from an alkali metal silicate such as sodium silicate. When the coating is to include hydrous oxides of titanium and of aluminium then a mixed solution of titanyl sulphate and aluminium sulphate can be used.

Alkali metal aluminates can be used as the source of a hydrous oxide of aluminium, if desired.

The coated particulate material is milled by fluid energy milling to produce the product having the desired particle size. The product has a particle size such that the average weight particle size is in the range 2.75 micron and with 90% of said material having a size greater than 1.03 micron to 3.44 micron with 90% of said material having a size greater than 1.49 micron. Fluid energy milling is usually effected using steam as the milling fluid and the milling conditions employed with steam is a temperature of the steam of from 1500C to 3500C and a steam to particulate weight ratio of from 0.5:1 to 1.7:1. Preferably the steam temperature is from 170"C to 300"C. The pressure of the steam is such as to produce the desired degree of superheat in the steam.Preferably the steam to particulate material weight ratio is from 0.75:1 to 1.3:1 and most preferably 0.90:1 to 1.1:1.

Any type of fluid energy mill can be employed and, if desired, this can incorporate an impact mill surface or surfaces or opposed impinging streams of milling fluid carrying the particulate material to be milled.

Conventional mills incorporating a cylindrical milling chamber with additional optional tangential injectors for milling fluid can be used to produce the opacifying agents of the present invention.

The milling conditions useful in the method of manufacture in accordance with the present invention can be contrasted with those comrentionally understood as being required to mill titanium dioxide pigment in the absence of an extender which have required a steam to pigment weight ratio of from 2:1 to 2.5:1. The use of milling conditions such as these with the coated particulate material of the present invention produces a product which is much smaller than that required by the present invention and of considerable less opacifying properties.

Opacifying agents of the present invention are of use in aqueous emulsion paints designed to produce on drying a matt surface finish. The paints may be thixotropic or not and if desired can have the form of "solid" paints i.e. having sufficient initial viscosity to appear solid prior to application to a surface.

In this specification all particle sizes have been determined employing a light scattering technique using apparatus known as Malvern Mastersizer manufactured and sold by Malvern Instruments Ltd. UK. The general description of the method of measurement employed is to be found in an article titled "Measurement of drop sizes" by Azzopardi B.J. in "International Journal of Heat Mass Transfer" 1979, Vol 22. No 9.

September. In particular the procedure identified as low angle laser diffraction. (Section 6, Optical Methods) is particularly referred to.

The invention is illustrated in the following Example.

Example Uncoated calcined rutile titanium dioxide pigment prepared by the sulphate process was dispersed in water at a concentration of 700 grams per litre (gpl) employing sodium silicate in an amount of 1.2% by weight SiO2 onTiO2 as a dispersing agent. The dispersion was milled for one hour in a laboratory sand mill and after milling the product has an average particle size of 0.4 micron.

An aqueous dispersion (slurry) of a calcined china clay (Polestar 200P sold by English China Clay Company) was prepared at a concentration of 600 gpl clay and using sodium silicate in an amount of 1.2% by weight SiO2 on clay weight as dispersing agent. The dispersion was milled in a laboratory sand mill for 1 hour producing a product having an average particle size of 1.1 microns.

The two milled aqueous dispersions obtained were mixed in equal weights and diluted with water to a concentration of 220 gpl of mixed titanium dioxide pigment and clay. The mixture was heated to 500C and held at this temperature throughout the coating process whilst stirring.

To the diluted mixture there was added a mixture of titanyl sulphate and aluminium sulphate containing 1.34% by weight TiO2 and 2.2% by weight A1203 own weight of mixed particulate material. The mixture obtained was stirred for 15 minutes and then sodium hydroxide solution was added to increase the pH to approximately 8.5 prior to stirring for a further 45 minutes. Sulphuric acid 10% w/w was then added in an amount to reduce the pH to 7.5 and finally the mixture was stirred for a period of 25 minutes.

The dispersion was filtered and washed with water in an amount of 10 litres water per kilo of particulate material. The product after washing contained soluble salts in the filtrate in the range 0.25 to 0.35 gpl.

The coated, filtered material was then spray dried. The dried material was then milled in a laboratory fluid energy mill at various steam to particulate material weight ratios employing steam at a temperature of 2800C.

The milled samples were tested to determine the average particle size in microns and other characteristic. The results are given in Table 1 below: TABLE 1 A B C D E F Steam:Pigment ratio 0.5:1 0.75:1 1:1 1.3:1 1.5:1 1.7:1 Average size 3.44 336 3.24 3.04 3.03 2.75 90% greater than 1.49 1.47 1.27 1.15 1.13 1.03 (microns) The milled coated samples were each tested in paints and compared with a standard containing titanium dioxide pigment coated with the same procedure as described herein and also uncoated calcined clay (Polestar 200P). The paints were prepared according to the following formula: % by weight TEST PAINT STANDARD PAINT Water 38.80 39.75 Cellulose thickener (Cellobond QP4400) 0.50 0.51 NH3 0.10 0.10 Sodium hexametaphosphate 0.40 0.41 (Calgon 25% solution) 25% Orotan Dispersant 0.40 0.41 Triton CF10 alkyl aryl polyether 0.10 0.10 Butyl carbitol acetate 2.00 2.05 Foamaster NXZ (blend of hydrocarbons, 0.10 0.10 waxes, fats, & non-ionic emulsifiers) Biocide (Nuodex) 0.10 0.10 Snowcal 60ML calcium carbonate 13.20 13.19 Milled Sample 28.20 Clay (Polestar 200P) - 13.09 Titanium dioxide - 14.09 Emultex VV536 vinyl acetate emulsion 16.10 16.09 100.00 100.00 The prepared emulsion paints were tested and compared with the standard. The opacity differences were measured using a colorometer (Pacific Scientific, Colorgard System/05) and contrast ratios were calculated from readings.

The results are as follows Paint Containing Contrast Ratio Change SamDle: A -1.08 B -0.13 C +0.4 D -0.38 E -1.39 F -1.82 Paints containing Samples A and B had a gritty appearance.

Claims

1. An opacifying agent comprising a mixed particulate material of pigmentary titanium dioxide and one or more extenders which particulate material is coated with one or more hydrous metal oxides and said material having an average weight particle size when measured by light scattering of from 2.75 microns to 3.44 microns and with 90% of said material having a size greater than 1.03 microns at an average size of 2.75 microns and 90% of said material having a size greater than 1.49 microns at an average size of 3.44 microns.

2. An opacifying agent according to claim 1 in which the average particle size is from 3.04 microns to 3.36 microns with 90tic of the material having a size greater than 1.15 microns at an average size of 3.04 microns and 90% of the material having a size greater than 1.47 microns at an average size of 3.30 microns.

3. An opacifying agent according to claim 1 or 2 in which the weight ratio of titanium dioxide to the total amount of extender is from 2:1 to 1:2 for TiO2:total extender.

4. An opacifying agent according to claim 1, 2 or 3 in which the extender is a clay, barium sulphate, silica or calcium sulphate.

5. An opacifying agent according to any of the preceding claims in which the hydrous metal oxide is a hydrous oxide of titanium, aluminium, silicon, cerium, zirconium, zinc, antimony or tin.

6. An opacifying agent according to any one of the preceding claims in which the total amount of hydrous metal oxide as coating is up to 35% by weight expressed as oxide on weight of particulate material.

7. An opacifying agent according to claim 6 in which the total amount of said coating is from 1% to 10% by weight expressed as oxide.

8. An opacifying agent according to any one of the preceding claims in which the particulate material is coated with a hydrous oxide of titanium and a hydrous oxide of aluminium.

9. An opacifying agent according to claim 8 in which a hydrous oxide of silicon is also present in the coating on the particulate material.

10. A method for the manufacture of an opacifying agent comprising forming a particulate mixture of pigmentary titanium dioxide and one or more extenders and said particulate mixture having a coating on the particles thereof of one or hydrous oxides and subjecting the particulate mixture to fluid energy milling with steam as a grinding medium at a temperature of from 1500C to 3500C and at a steam to particulate material weight ratio (S:P) of from 0.5:1 to 1.7:1 to produce said particulate material having a particle size falling within the range of average weight particle size of 2.75 microns and with 90% by weight of the particles greater than 1.03 microns to an average weight particle size of 3.44 microns and with 90% of said material having a size greater than 1.49 microns when measured by light scattering.

11. A method according to claim 10 in which the coated particulate material is milled with steam at a temperature of from 1700C to 3000C.

12. A method according to claim 10 or 11 in which the steam to particulate weight ratio is from 0.75:1 to 1.3:1.

13. A method according to claim 12 in which the steam to particulate weight ratio is 0.90:1 to 1

14. A method for the manufacture of an opacifying agent according to claim 10 substantailly as described in the foregoing Example.

15. An opacifying agent when prepared by the method of any one of claims 10 to 14.