Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With chemical or physical modification of liberated fiber
Reexamination Certificate
1997-10-24
2003-06-17
Fortuna, Jose A. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With chemical or physical modification of liberated fiber
C162S010000, C162S181200, C162S181700, C162S181400, C106S461000, C106S463000
Reexamination Certificate
active
06579410
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to pigment materials and their preparation and use. In particular, it relates to new pigment material comprising a composite containing a fine particulate material such as titanium dioxide and to the preparation of such material and its use in operations to make or coat paper and the like sheet materials.
Titanium dioxide, herein “TiO
2
”, is an example of a fine particulate material which, unlike materials which are employed as bulk fillers, is employed for a specific function in paper making and paper coating operations. It offers excellent pigment opacity and brightness (which together result in excellent light scattering) and is therefore used to extend these properties in a paper making or coating composition. TiO
2
is a very expensive material and is consequently used only sparingly in paper making or coating compositions, eg. usually forming less than 5% by weight of the solids present in the composition. Because of its cost, TiO
2
needs to be employed as efficiently as possible.
The optimum particle size for TiO
2
pigment particles to give the best light scattering properties has been determined in the prior art to be about 0.2 &mgr;m to 0.3 &mgr;m. TiO
2
pigment material is normally supplied commercially in a form wherein the particles have this optimum size. The TiO
2
product may be supplied dry, in which case it requires dispersion in liquid media, eg. aqueous media, to wet and to deagglomerate or disperse the particles. Alternatively, TiO
2
may be supplied commercially in a pre-dispersed slurry form which may incorporate a relatively large amount of anionic stabilizer.
Since the TiO
2
particles employed in paper making are very fine and are usually dispersed with relatively high levels of dispersant to make slurries containing the material pumpable and to maximize the spacing between particles to give optimal light scatter, it is difficult to retain such particles when they are employed in a furnish or like composition in a paper or like sheet forming operation.
In order to improve TiO
2
retention in such operations, in order to minimize TiO
2
losses, various chemical retention aids are employed in the prior art. In general, such aids are expensive chemicals, eg. water soluble polymers, and the extent of use of such chemicals employed for the conventional retention of TiO
2
is considered to be very costly. Where the TiO
2
is supplied with anionic stabilizer large amounts of cationic chemicals (which may also serve as or be delivered together with retention aids) may be required to reduce the anionic loading.
TiO
2
particles (when used in a pigment-containing composition) have a tendency to agglomerate especially at higher loading levels, this effect being known as ‘crowding’. Use of retention aid chemicals can increase crowding which has been demonstrated in the prior art to have an adverse effect on the light scattering efficiency of the particles. This can result in degradation of the expected optical properties of commercially available TiO
2
material when used as a particulate pigment material in paper.
Furthermore, such added chemicals when used in substantial quantities to improve TiO
2
retention can have an adverse effect on the formation of the paper or other sheet being produced and can result in sheets of less than ideal quality in which the constituents of the sheet are not uniformly distributed.
Attempts have been reported in the prior art to counteract the effects of crowding of fine particulate material, especially TiO
2
, by forming composites of the particles of the material with various organic or inorganic materials to space the particles from one another. These attempts have been aimed primarily at improving the optical properties of the compositions to which the composite material is added.
The problem of maintaining a reasonable retention of TiO
2
particles in a paper making operation, whilst at the same time avoiding crowding and an adverse effect on sheet formation and quality caused by the addition of chemicals to facilitate retention, has not satisfactorily been solved in the prior art. One purpose of the present invention is to provide a novel solution to this problem.
Similar problems arise with the retention of other fine particulate materials, and it is another purpose of the present invention to provide a novel solution to the retention of such materials.
It is a further purpose of this invention to produce a novel pigment material which is useful in paper coating.
SUMMARY OF THE INVENTION
The present invention is concerned with the preparation and use of novel composites, not disclosed or suggested in the prior art, which can, amongst other things, improve the retention of fine particulate material such as TiO
2
in paper making compositions without seriously affecting optical or other properties, in some cases beneficially improving such properties also. The novel composites can also be used beneficially in paper coating.
According to the present invention in a first aspect there is provided a method of preparing and using a composite pigment material which incorporates a fine particulate material, which method comprises (a) treating an aqueous medium containing dispersed particles of a fine particulate material and also fibers by chemically precipitating by a precipitation reaction in the aqueous medium crystals of a white insoluble pigment compound thereby forming a composite pigment material comprising a composite matrix of fiber and precipitated crystals of the white pigment compound and particles of the fine particulate material dispersed and bonded within the matrix; and (b) adding the composite material to a composition for forming or coating a fibrous sheet material. The method may thereafter include (c) forming or coating a fibrous sheet material using the composition incorporating the composite material. The said aqueous medium employed in step (a) may comprise an aqueous suspension or slurry.
The fine particulate material and the fibers may be obtained from separate stocks and may be added together prior to step (a) to produce the composite pigment material. Either or both of these materials may be in dry or wet (eg. slurry) form when they are added together.
At least part of the fine particulate material may comprise fresh particles. By ‘fresh’ particles of fine particulate material is meant particles which have not previously been used in a sheet forming or coating or other operation.
The fibrous sheet material formed or coated in step (c) may comprise paper, paper board, card, cardboard, laminated paper and the like herein collectively called ‘paper’, wherein the fibrous sheet material comprises organic, eg. cellulosic fibers, and in many cases also inorganic filler comprising a particulate pigment material.
By “fine particulate material” is meant a particulate material wherein the particles have a size distribution such that at least 90% by weight have an esd (equivalent spherical diameter as measured in a known way by sedimentation) of less than 1 &mgr;m. At least 50 per cent by weight may have an esd less than 0.5 &mgr;m. The fine particulate material will generally be one which is more expensive than bulk filler materials (eg. kaolin and/or calcium carbonate) and one which is used in paper for a specific function (other than bulk filling) and generally has a poor natural retention in paper making.
The present invention is especially beneficial where the fine particulate material comprises TiO
2
, although the particulate material may alternatively, or in addition, be selected from calcined kaolin-containing material, eg. metakaolin, fine silica, eg. so called fumed silica, sodium silicate, aluminum silicate, sodium aluminum silicate, talc and so called plastic pigment materials, eg. produced from melamine formaldehyde. It should be noted that calcined material may contain aggregates of fine particles fused or sintered together but the fine particles which make up such aggregates may have the required particle size distribution properties if me
Bleakley Ian Stuart
Phipps Jonathan Stuart
Rey Paul A
Rodriguez Jose M
Toivonen Hannu Olavi Ensio
Fortuna Jos'e A.
Imerys Minerals Limited
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