GB2234990A

GB2234990A - Fibrous sheet material

Info

Publication number: GB2234990A
Application number: GB9015915A
Authority: GB
Inventors: Robert Morrow; Kevin Drury
Original assignee: Tioxide Group Ltd
Current assignee: Tioxide Group Ltd
Priority date: 1989-08-18
Filing date: 1990-07-19
Publication date: 1991-02-20
Anticipated expiration: 2010-07-19
Also published as: GB9015915D0; GB2234990B; GB8918861D0

Abstract

An opacifying agent for paper comprises mixed anatase titanium dioxide and extender in which the mixed particulate material is coated with one or more hydrous oxides of, inter alia, Ti, Al, Si, Ce, Zr, Zn, Sb or Sn. Preferably the agent is coated with the combination of hydrous titania, alumina and hydrous silica and the extender is clay. The paper products have improved opacity, less tendency to yellow and seem to be brighter.

Description

FORMAL SPECIFICATION FIBROUS SHEET MATERIAL This invention relates to fibrous sheet material and particularly to cellulosic sheet material e.g. paper.

Titanium dioxide and extenders such as clays have been used as additives in paper. Titanium dioxide, usually in the anatase form, has been used to pigment paper and extenders or other fillers used to give body to the paper. During the paper making process significant losses of these additives occurs and this gives rise to the need for extensive recovery processes and to potentially serious effluent problems.

According to the present invention cellulosic fibrous sheet material contains as an opacifying agent a mixed particulate material of pigmentary anatase titanium dioxide and one or more extenders which mixed particulate material is coated with one or more hydrous oxides.

The opacifying agent used in the products of the invention usually has an weight average particle size when measured by light scattering of from 2.75 microns to 3.44 microns with 90% of said agent having a size greater than 1.03 microns at an average size of 2.75 microns and 90% of said agent 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 90two of the agent having a size greater than 1.15 microns at an average size of 3.04 microns and 90% of the agent 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 an anatase 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 anatase titanium dioxide pigment which is used to form the composite products of the present invention can be made either by the so-called "sulphate" process or the "chloride" process. Usually pigment prepared by the sulphate process will be used. 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 but usually the extender is a clay such as china clay. The extender can be milled prior to mixing with the titanium dioxide pigment, if desired.

The mixed particulate mixture which is to be coated can contain a wide range of proportions of the titanium dioxide pigment to extender and can be within the weight ratio of 2:1 to 1:15 for TiO2:extender preferably 2:1 to 1:10. 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 mixed particulate material 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 the total amount can be up to 35% by weight as oxides on weight of particulate material.

More usually total amounts of hydrous oxide in the range 5% to 20% by weight as oxide preferably 7% to 15Q/o 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 a separate coating. A coating including a hydrous oxide of silicon, a hydrous oxide of aluminium and a hydrous oxide of titanium has been found to be particularly useful.

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 co-addition 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 hydrous 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 a hydrous oxide of titanium and of aluminium then a mixed solution of titanyl sulphate and aluminium sulphate can be used. Aluminium sulphate or an alkali metal aluminate can be used as the source of a hydrous oxide of aluminium.

The coated particulate material can be milled by fluid energy milling to produce a product having the desired particle size. The product has a particle size such that the weight average 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 microns. Fluid energy milling can be effected using steam or air as the milling fluid and the milling conditions employed with steam is a temperature of the steam of from 1500C to 350"C and a steam to particulate weight ratio of from 0.5:1 to 1.7:1. Preferably the steam temperature is from 1700C 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.

The products of the invention can have a wide variety of forms, such as sheet papers and boards having a range of qualities.

All the common paper making ingredients can be added to the pulp prior to sheeting if desired. Typical ingredients are size, starch and retention aids such as polyacrylamides. The cellulosic material can be that from a wide variety of sources such as wood pulp, linen and other vegetable sources.

Conveniently the cellulosic sheet material contains from 0.5% to 40%, usually 2% to 15% by weight of the opacifying agent on weight of cellulosic fibrous material.

Products of the present invention can be produced by any of the usual paper making processes. Usually this involves making an aqueous dispersion of the cellulosic fibres, opacifying agent and other ingredients and bringing this so-formed "stock" into contact with a grid through which the liquid drains to leave the paper sheet which can be subjected to the usual finishing processes, e.g. drying.

The products of the present invention have been found to have an increased opacity when compared to a product containing a non-coated particulate mixture of said extender and anatase titanium dioxide. The products have a reduced tendency to be yellow in colour and the products seem to be brighter than known products. As stated the use of the opacifying agent has reduced the amount of pigment and/or extender which is not retained within the paper and the retention in the paper can be improved by up to three times more usually up to twice.

The invention is illustrated in the following Example.

Example Uncoated calcined anatase 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 on TiO2 as a dispersing agent.

The dispersion was milled for one hour in a laboratory sand mill and after milling the product had an average particle size of 0.4 micron.

An aqueous dispersion (slurry) of a china clay 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 15 mins producing a product having an average particle size of 1.1 microns.

The two milled aqueous dispersions obtained were mixed (equal weight of pigment and clay) and diluted with water to a concentration of 220 gpl of mixed. titanium dioxide pigment and clay. The mixture was heated to 50"C and held at this temperature throughout the following 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 Al203 on 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, washed with water in an amount of 10 litres water per kilo of particulate material, repulped and finally filtered.

The product contained soluble salts in the filtrate in the range 0.25 to 0.35 gpl.

The coated, filtered material was then dried in an oven. The dried material was then milled in a laboratory fluid energy mill at an air to pigment ratio of from 5:1 to 10:1 to produce the opacifying agent.

A paper was prepared incorporating the above prepared opacifying agent by the following method.

A mixture of equal weights of pulps prepared from softwood and hardwood was mixed with water and beaten in a Hollander beater to a Canadian Standard freeness of 250 ml. The stock was then diluted to a 1% fibre consistency.

The diluted stock (two litres) was placed in a British Standard disintegrator and stirred for 500 revolutions. To the stirred stock there was added a sizing agent in an amount of 0.3% by weight of fibre and stirred for 500 revolutions. The sizing agent was a ketene dimer emulsion containing HCl/formaldehyde resin and was available under the name "Aquapel". Then an amount of the opacifying agent was added (as described later) and stirred for 1000 revolutions.

The furnish so obtained was diluted to 0.4to fibre consistency (5 litres total volume) and sheets of paper prepared using a British Standard handsheet machine. The papers were dried (bone dry) on a rapid glazing machine (Kodak model 24) and then stored at a constant temperature (22"C) and under 50% relative humidity. The dried sheets had a substance of 60 grams per square metre.

Paper sheets were then tested to determine the following properties.

Opacity. The papers were tested by measuring the contrast ratio using an Erichsen colourmeter 526.

Wax opacity. Sheets of paper were dipped in hot wax and heated in an oven for 75 minutes at 75"C The contrast ratio was then measured.

Brightness. The brightness was measured at a wavelength of 457 nm on a Hunterlab Colorquest Spectrocolourmeter. Retention. Sheets of paper were heated in a muffle furnace at 700"C for one hour and the amount of ash determined. The Go retention was determined using the following equations: % ash = wtofash x100 initial wt of paper % retention = % ash x 20 x100 (100 - % ash) x (opacifier loading) Colour Index. This is measured as a ratio of the yellowness/blueness using a Hunterlab Colorquest Spectrocolourmeter and the lower values tend to indicate a bluer or less yellow paper.

Using the above described method two opaciting agents were prepared. Opacifying agent A was prepared from a mixture of anatase titanium dioxide and an uncalcined clay obtainable under the name Speswhite. Opacifying agent B was prepared from anatase titanium dioxide and a calcined china clay obtainable under the name Ansilex. Papers were prepared at two different loadings on fibre employing the above described method from opacifying agents A and B and also by using a dry mixture of the uncoated anatase titanium dioxide and the respective china clay for comparison purposes. The papers were tested as described and the results are shown in the following tables.

Table 1 Opacifying Agent A at 3% loading on fibre Agent A Dry Mixture Retention 47.8 21.2 Opacity 88.8 86.09 Wax Opacity 61.11 49.56 Colour Index 5.22 5.70 Brightness 83.22 49.56 Table 2 ODacifving Agent A at 6% loading Agent B Dry mixture Retention 51.2 20.4 Opacity 93.72 89.78 Wax Opacity 71.25 60.56 Colour Index 4.32 4.81 Brightness 85.87 83.32 Table 3 Opacifving Agent B at 3% loading Agent B Dry mixture Retention 60.4 27.1 Opacity 91.66 89.61 Wax Opacity 64.96 55.62 Colour Index 4.91 5.03 Brightness 84.31 82.87 Table 4 Opacifying Agent B at 6% loading Agent B Dry mixture Retention 52.1 30.2 Opacity 94.92 90.48 Wax opacity 72.04 63.79 Colour Index 4.02 4.2 Brightness 86.60 84.64 These results show quite clearly that the use of the co-coated pacifying agent is advantageous over the uncoated mixture.

Claims

1. Cellulosic fibrous sheet material containing as an pacifying agent a mixed particulate material of pigmentary anatase titanium dioxide and one or more extenders which mixed particulate material is coated with one or more hydrous oxides.

2. Sheet material according to claim 1 in which the weight ratio of titanium dioxide to extender is from 2:1 to 1:15.

3. Sheet material according to claim 2 in which the ratio is from 2:1 to 1:10.

4. Sheet material according to claim 1, 2 or 3 in which the mixed particulate material is coated with one or more hydrous oxides of titanium, aluminium, silicon, cerium, zirconium, zinc, antimony or tin.

5. Sheet material according to any one of claims 1 to 4 in which the total amount of said hydrous oxides can be up to 35% by weight as oxide on weight of particulate material.

6. Sheet material according to claim 5 in which the total amount of said hydrous oxides is from 5% to 20% by weight as oxide.

7. Sheet material according to claim 6 in which the total amount of said oxide is from 7% to 15% by weight as oxide.

8. Sheet material according to any one of claims 1 to 6 in which the particulate material is coated with hydrous titania, hydrous silica and hydrous alumina.

9. Sheet material according to any one of the preceding claims in which the extender is a clay.

10. Sheet material according to any one of the preceding claims in which the pacifying agent has a weight average particle size when measured by light scattering of from 2.75 microns to 3.44 microns and with 90% of said agent having a size greater than 1.03 microns at an average size of 2.75 microns and 90% of said agent having a size greater than 1.49 microns at an average size of 3.44 microns.

11. Sheet material according to claim 10 in which the agent has an average particle size of from 3.04 microns to 3.36 microns with 90% of the agent having a size greater than 1.15 microns at an average size of 3.04 microns and 90% of the agent having a size greater than 1.47 microns at an average size of 3.36 microns.

12. Sheet material according to any one of the preceding claims containing cellulosic fibrous material and said opacifying agent in an amount of from 0.5% to 40% by weight of the weight of said cellulosic fibrous material.

13. Sheet material according to claim 12 in which the opacifying agent is present in an amount of from 2% to 15% by weight of the opacifying agent.

14. Sheet material according to claim 1 when obtained substantially as described in the foregoing Example.