Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
1997-10-30
2001-11-13
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S168100, C162S168300, C162S181200, C106S465000, C106S499000
Reexamination Certificate
active
06315867
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention concerns the provision of dispersing agents and their use. In particular it concerns dispersing agents for use in processing particulate solid materials for use as fillers and pigments to be incorporated into sheet materials which are to be brought into contact with foodstuffs.
Particulate solid materials for use as fillers and pigments in sheet materials are commonly prepared for use in suspension in water or an aqueous medium. Operations which are required to render a particulate solid material suitable for these purposes include grinding to increase the particle fineness of the product, and particle size classification to select a product with the range of particle sizes which is most suitable for its desired application. These operations are often most conveniently and efficiently carried out when the particulate material is in suspension in an aqueous medium. However, if these operations are to be successfully performed, it is necessary to ensure that individual particles of the particulate material are not attracted together by electrostatic charges, to form aggregates or agglomerates of particles. For this purpose a dispersing agent is added to the aqueous medium in which the operations are performed, in order to confer a substantially uniform electric charge of the same polarity on the whole of the exposed surface of the particles. In this way the individual particles repel one another, and the formation of aggregates is substantially avoided.
A class of dispersing agents which has been found to be especially suitable for use in processing particulate solid materials in suspension in an aqueous medium is that of the anionic polyelectrolytes, and, in particular, water soluble salts of poly(acrylic acid), such as sodium or ammonium polyacrylate. These polyacrylates generally have a relatively low molecular weight; for example their weight average molecular weight is generally less than 20,000, and preferably less than 10,000.
The United States Food and Drug Administration has imposed an upper limit of 0.25% by weight, based on the weight of the dry particulate material, for the amount of a sodium polyacrylate dispersing agent to be used in the processing of a particulate material destined for use as a filler or pigment in a sheet material which will be brought into contact with certain categories of foodstuffs.
Unfortunately, such a low upper limit restricts the usefulness of polyacrylates for dispersion of materials for incorporation in food grade packaging.
Various species of water soluble chemicals are known in the prior art for use as dispersing agents. The purpose of the present invention is to provide a composition, not disclosed or suggested in the prior art, effective as a dispersing agent in the dispersion of fine particulate material for incorporation in food grade packaging.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method of dispersing a solid particulate material in an aqueous medium by addition to the aqueous medium of a dispersing agent wherein the solid particulate material is to be used as a filler or pigment in a sheet material which is required to come into contact with foodstuffs, wherein the dispersing agent is a composition comprising:
(1) up to 0.25% by weight, based on the dry weight of the dry particulate solid material, of a water-soluble salt of a poly(acrylic acid) having a weight average molecular weight not greater than 20,000, and
(2) from 0.02% to 1.5% by weight, based on the dry weight of the particulate solid material, of either (a) an anionic polyelectrolyte which comprises a water-soluble salt of a copolymer of acrylic acid with acrylamide; or (b) a water-soluble condensed phosphate salt.
DESCRIPTION OF THE INVENTION
The dispersing agent may be added to the aqueous medium before, during or after addition thereto of the particulate material.
The water-soluble salt of poly(acrylic acid) preferably is present in the dispersing agent composition when in use in the said method in an amount of from 0.2% to 0.25% by weight based on the dry weight of the particulate material present. The water soluble salt of poly(acrylic acid) preferably has a weight average molecular weight in the range of from 1,000 to 10,000. The cation in the water-soluble salt of poly(acrylic acid) is preferably an alkali metal, especially sodium, or ammonium.
The weight ratio of (1) to (2) in the dispersing agent composition is desirably in the range of from 1:0.1 to 1:5.
The weight ratio of (1) to (2) may be in the range 1:1 to 10:1 where (2) is a condensed phosphate salt and in the range 1:1 to 1:5 where (2) comprises a copolymer of acrylic acid with acrylamide.
The water-soluble salt of a copolymer of acrylic acid with acrylamide preferably has a weight average molecular weight not greater than 20,000, and most preferably in the range of from 1,000 to 10,000. The copolymer preferably comprises acrylic acid and acrylamide in a molar ratio of from 99.8:0.2 to 75:25, more preferably from 99.5:0.5 to 90:10.
The water-soluble condensed phosphate salt may be, for example, a hexametaphosphate, a pyrophosphate or a tripolyphosphate.
The particulate solid material will generally be an inorganic material, and may be any particulate inorganic material which is used as a filler in paper making, or as a paper coating pigment. Examples of such inorganic materials include, kaolin or china clay, whether in its natural hydroxylated state or calcined, natural or precipitated calcium carbonate, calcium sulphate, titanium dioxide, talc, mica, silica, aluminium silicate or the like.
The dispersing agent composition used in the method in accordance with the first aspect of the invention makes it possible to prepare suspensions of particulate solid materials which are at least as fluid, for a given solids concentration, as equivalent suspensions which are prepared by using the optimum amount of a water-soluble polyacrylate alone as a dispersing agent, but unexpectedly have the advantage that, if the particulate solid materials are destined for use as a filler or coating pigment for paper which comes into contact with a foodstuff, the materials meet approved regulatory standards.
The compounds specified above for use in the composition comprising the dispersing agent used in the method according to the first aspect are known per se. However, their use together in the dispersion of particulate materials for food grade products is unknown and surprisingly and beneficially allows such products to be processed and produced efficiently for that application as exemplified hereinafter.
The aqueous suspension in which the particulate solid material is dispersed in the method according to the present invention may beneficially comprise a suspension of high solids concentration, eg. wherein the solids present constitute at least 70 per cent by weight of the suspension, eg. at least 75 per cent by weight of the suspension. The suspension may comprise a composition for coating a paper or like cellulosic sheet or sheet containing material or a dispersed pigment suspension to be employed in such a composition.
Coating compositions for use in coating cellulosic sheet materials vary depending upon the materials to be coated which vary throughout the world depending upon the geography of the region in which the material is produced. As noted above, such compositions may vary from layer-to-layer in a single layer or multi-layer coated product.
The composition of each layer may include as adhesive or binder, depending on the type of composition concerned, any one or more of the hydrophilic adhesives known or used in the art, eg. selected from starches and other polysaccharides, proteinaceous adhesives, and latices.
The amount of adhesive or binder present in the composition of a given coating layer depends upon whether the composition is to be applied as a relatively dilute or concentrated pigment-containing suspension to the material to be coated. For example, a dilute pigment-containing composition (binder-rich composition) co
Chen Shih-Ruey Thomas
Garforth William Leonard
Matz Gary F
Skuse David Robert
Chin Peter
Finnegan, Henderson Farabow, Garrett and Dunner L.L.P.
Imerys Minerals Limited
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