FR2722806A1 - Highly filled fibre-reinforced sheet prodn. useful for economy - Google Patents

Abstract

In the prodn. of a highly filled, fibrous-reinforced sheet by continuously feeding an aq. dispersion of solids, including fibres, binder, cationic flocculant (I) and at least 50 wt.% filler, w.r.t. solids, over a moving porous support surface: (a) the filler is calcium carbonate, and (b) (I) has a charge density of at least 2 equiv. cationic N/kg polymer. Also claimed are: (i) a fibrous-reinforced sheet; (ii) a method of opacifying white paper on a white, highly filled, fibrous-reinforced sheet; and (iii) the opacified paper or sheet per se.

Description

: 1 Field of the Invention The present invention relates to a sheet

  reinforced with fibers, highly charged and with a process for preparing the sheet. More particularly, the present invention relates to a fiber reinforced sheet which is highly charged with calcium carbonate and to a

  process for preparing the sheet.

  Sheets which include a relatively small amount of fiber and a relatively high amount of fillers are well known. An example of such a sheet comprises cellulose fibers in an amount of from about 5 to about 25% by weight, from about 50 to about 85% by weight of mineral filler such as talc, and from about 2 to about 30% 15 by weight of a binder. Such sheets can be used in a variety of applications, for example, as a backing for vinyl flooring, sealing paper, laminate bases for articles such as, for example, book covers and covers, and thermoplastic composites. compression molded and / or laminated, such as, for example, a paper-reinforced thermoplastic composite article described in US Patent No. 5,188,888. The sheets manufactured for use in such applications generally have a thickness of about 254 at about 12700pm (10 to about 500 mils) and wide from 1.8 to

3.0 m (6 to 10 feet) or more.

  The properties which must be present in such sheets vary, depending on the particular implementation in which the sheet is used. However, in general, the sheet should have the following properties: sufficient tear strength and sufficient tensile strength to withstand being snapped up during processing and handling; adequate cohesive strength to resist delamination during treatment; sufficient flexibility to allow the sheet to be shaped into a roll and, in the case of use of vinyl flooring, to be covered; resistance for indentation; and dimensional stability to changes in humidity and temperature that are encountered in the environment in

which one uses the sheet.

  The type of fibers encountered in the sheet contributes mainly to properties such as tensile strength, tear resistance and dimensional stability in the sheet. The binder contributes

  mainly with cohesion properties, a binder which acts to make the fibers and the filler of the sheet adhere together. The function of the load is mainly resistance to indentation. The fibers, fillers and binders contribute to the flexibility properties.

  The filler is a particularly important constituent of the sheet simply because it contributes in a large proportion to the constitution of the sheet. To perform its function satisfactorily, it must be relatively inexpensive, communicate the properties

  desired to the sheet and not to shift to any significant degree the properties imparted to the sheet by its other constituents.

  The most commonly used process for preparing such sheets is the basic process used for making paper, comprising the use of conventional paper making equipment such as, for example, a Fourdrinier machine. This process includes forming a highly dilute aqueous suspension (e.g. 1% by weight solids) of fibers, filler and other constituents and continuously feeding a stream of such suspension onto a conveyor belt porous moving horizontally. The method includes the use of constituents which are effective in forcing the retention of the particulate solids constituting the sheet on the porous belt as water drips through the porous belt in significant amounts; means for compacting the wet sheet and removing additional water therefrom; and means for

  evaporate the residual water from the leaf in order to dry it.

  One of the problems encountered in the manufacture of a fiber reinforced sheet, loaded using the aforementioned type of process (papermaking) and the pressure drop across the openings of the conveyor belt porous. Typically, the size of the filler particles is smaller than the openings of the porous belt. Although a certain amount of charge is retained in the wet mass of solids deposited on the belt by virtue of the fact that the fibers block the

  openings in the porous belt, a significant amount of charge can be transported with the water flowing through the porous belt. Of course, this is undesirable for several reasons, including the pressure drop from the product.

  The present invention is directed to improved means for increasing the charge retention in the sheet during its manufacturing process. 35 Developments reported US Patent No. 4,225,383 describes a fiber reinforced sheet which is made using a "papermaking" type process which contains about 50 to about 80% filler. This patent describes that such sheets can be made by including in the aqueous suspension from which the sheet is formed about 2 to about 30% by weight of a binder which comprises an organic polymer insoluble in water, film-forming under the form of a latex stabilized by ions having no more than 0.7 milliequivalents of bound filler per gram of polymer in the latex. Such polymers include anionic polymers, i.e., polymers which carry a negative charge due to their contained ionizable groups such as carboxylate groups. This patent also describes the use in the suspension of a flocculating agent which has a charge opposite to that of latex. Consequently, a cationic flocculant, i.e. a flocculant carrying a positive charge, is suggested for use in combination with an anionic polymer. The aforementioned patent 4,225,383 describes the use of a variety of fillers in the preparation of the highly charged sheets described in the invention, including, for example, magnesium hydroxide, magnesium silicate, carbonate magnesium, talc, zinc oxide, calcium sulfate, calcium carbonate and the like. Development work has shown that it is

  possible, in accordance with the description of patent 4 225 383, to prepare highly loaded sheets using a filler such as raw talc (approximately 50% by weight of

  magnesite and about 50% by weight of talc). (Raw talc is a filler widely used in the manufacture of sheets which are used, for example, as a support for vinyl floor coverings). However, development work has also shown that it is not possible to efficiently manufacture a highly loaded sheet (for example around 80% by weight of filler) when the filler is calcium carbonate. Experience has shown that critical quantities of calcium carbonate are not retained with the other ingredients which accumulate in the sheet and are entrained with the constituent water of the suspension when it flows through the porous belt on which the suspension is deposited. This is a significant drawback in that calcium carbonate cannot be used as a filler in highly charged sheets since it costs about a third of the price of raw talc. The present invention relates to the provision of a highly charged, fiber reinforced sheet which comprises a filler consisting essentially of carbonate

calcium.

  Summary of the Invention In accordance with the present invention, there is provided, in a method for forming a highly charged, fiber reinforced sheet, wherein the sheet is formed from an aqueous dispersion of solids which comprises fibers, a filler and a binder, and a cationic flocculating agent by continuously feeding a stream of such an aqueous suspension onto a moving porous support surface, wherein the filler comprises at least about 50% by weight of the solid portion of the dispersion, improvement including use in dispersion

  aqueous of a filler consisting essentially of calcium carbonate and of a cationic polymeric flocculant having a charge density of at least about 2 equivalents of cationic nitrogen per kilogram of polymer.

  In a preferred form, the cationic flocculant is an acrylamide polymer and a cationic acrylate ester of

  essentially linear polymer structure with a weight average molecular weight of at least about 5 million.

  Another aspect of the present invention includes providing a fiber reinforced sheet comprising: (A) about 5 to about 25% by weight of fibers; (B) at least about 50% by weight of filler based on calcium carbonate; (C) about 2 to about 30% by weight of resin-based binder; and (D) about 0.01 to about 0.5% by weight of a cationic polymeric flocculant having a charge density of at least about 2, preferably at least about 3.5, equivalents of cationic nitrogen per kilogram of polymer. In a preferred form, the sheet comprises at least about 70% by weight of calcium carbonate, very particularly preferably about 75 to about 85% by weight of calcium carbonate. While in a preferred form the

  the above sheet has a tensile strength of at least about 3.6 kg per centimeter in width (20 pounds per inch of width), more preferably at least 7.1 kg per cm in width (40 pounds per inch of width).

  A further aspect of this invention relates to a method for imparting opacity to white, highly charged, white fiber-reinforced paper or sheet which comprises a filler, such as calcium carbonate, and which is prepared from a dilute aqueous dispersion of fibers and filler comprising in the dispersion carbon black in an amount sufficient to make the paper or sheet opaque and to maintain the white appearance of the sheet or paper. In addition, the present invention includes within its scope the supply of white paper or a fiber-reinforced, highly charged, white sheet which tends to lack opacity where a sufficient amount of carbon black is included in the sheet for communicate opacity to the sheet or white paper while keeping

  the white appearance of the sheet or paper.

  There are many advantages which are provided by the present invention. It allows a manufacturer to use, in relatively large quantities, calcium carbonate which is one of the inexpensive fillers available in the industry. The invention allows this to be done in a manner which uses available equipment which can be operated under processing conditions which have become industry standards. So there is no need to invest in equipment or make

  processing changes by adopting the use of the present invention. In addition, the highly charged sheet containing calcium carbonate of the present invention has properties which meet accepted standards.

  The invention also provides cost-effective and efficient means for imparting opacity to paper or fiber-reinforced sheet, highly loaded in such a way that the white color thereof is

essentially preserved.

  Detailed description of the invention

  The classes of the constituents constituting the sheet of the present invention are known. They comprise fibers, a filler, a binder and a cationic polymer flocculant having a charge density of at least about 2 equivalents of cationic nitrogen per kilogram of polymer (also designated in the invention under the name of "flocculant

at high charge density ").

  The fibers impart mechanical strength and other properties to the sheet. They are insoluble in water and are capable of being dispersed in the aqueous medium which functions as a liquid vehicle for the various constituents from which the sheet is formed. The fibers can be natural or synthetic materials. Two or more different types of fibers can be used to prepare the sheet. Examples of such fibers are cellulosic fibers, glass fibers including, for example, chopped glass fibers, blown glass fibers and centrifuged glass fibers, and rock wool fibers, ceramic fibers and fibers. synthetic softwoods, for example polyester, polyether, polypropylene and nylon fibers. The sheet of the present invention can be efficiently produced without the use of asbestos fibers which are associated with

health hazards.

  It is preferred to use a mixture of cellulosic fibers and glass, the latter acting to impart good dimensional stability to the sheet. The cellulosic fibers can be, for example, cellulose pulp fibers, either virgin or secondary (recycled), and either bleached or unbleached. Such cellulosic fibers can be produced, for example by any method of

  manufacture of suitable chemical or chemical paste-

  mechanical. The glass fibers are preferably chopped glass fibers which are bonded for hydro-

  dispersibility, for example, with a polyester coating. Any suitable form of calcium carbonate can be used as a filler in order to impart the desired properties to the sheet and to reduce its cost. Various forms of calcium carbonate are known, for example as described in US Pat. No. 4,596,661. It is assumed that the most widely used source of calcium carbonate will be limestone which is crushed and ground to an appropriate particle size, by example using a roller mill. However, the source of calcium carbonate may

  be any naturally occurring calcium carbonate or precipitated calcium carbonate.

  The calcium carbonate can be of any suitable particle size. In general, the particle size of the calcium carbonate will be in the range of about 1 to about 30 µm in equivalent spherical diameter. A limestone ball mill can be used to produce calcium carbonate which has a relatively high content of fine particles, i.e. less than 3 microns, which is desirable to impart opacity to the sheet. With a good advantage, dry ground limestone was used having the following particle size distribution: 0% by weight> 100 μm; % by weight to 35% by weight of between 15 pm and 100 pmn; 50% by weight of between 7 and 15 pm; and 15 to 35% by weight

<3 pm.

  Excellent results have been achieved using

  calcium carbonate as the sole filler in the sheet.

  However, one or more other fillers can be used in combination with calcium carbonate. When using one or more other fillers, it is recommended that they constitute about 1 to about 10% by weight of the solids content of the aqueous dispersion. Typically, the filler is a particulate material which is essentially insoluble in water. Examples of other fillers that can be used are clay, magnesium hydroxide,

  magnesium carbonate, titanium dioxide, zinc oxide, barium sulfate, calcium sulfate, amorphous silica, aluminum silicate, magnesium silicate, diatomaceous earth, alumina trihydrate , talc and vermiculite.

  The binder works to adhere the various constituents of the sheet together and to communicate cohesion and other desired properties. The binder typically includes solid, water-insoluble resin particles which are dispersed in the suspension and which are film-forming in nature, for example, the types of solid resins used in water-based paints. The most suitable source of the binder is a latex, namely, an aqueous dispersion of resin solids having a size which

is colloidal in nature.

  For use in the present invention, the resin must be anionic, i.e. carry a negative charge. Any available means can be used to formulate a negatively charged resin or to impart a negative charge to the resin. In a preferred form, the resin is prepared from a monomer or a mixture of monomers, at least one of which comprises an ionizable group, for example a negatively charged carboxylate, sulfate or sulfonate group. As an example of the nature of the filler carried by the resin, it is from approximately 0.03 to approximately 0.7 milliequivalent of filler per gram of resin in a latex.

of it.

  It is assumed that the anionic resins which will be most widely used as a binder in the present invention will comprise a polymer (a homopolymer or a higher polymer) prepared from one or more monomers which comprise ethylenic unsaturation. Examples of suitable polymers include styrene-butadiene resins, acrylic resins, ethylene-vinyl acetate resins, acrylonitrile resins, acrylonitrile-butadiene-styrene resins, poly (vinyl chloride) resins, and poly resins (vinylidene chloride). The preferred binder is a styrene resin

  carboxylated butadiene supplied in the form of a latex.

  As desired, a coagulant which functions to impart positive charges to the fibers and fillers which anchor these materials to the negatively charged sites of the anionic resin binder should be included in the suspension. Examples of coagulants which can be used are water-soluble mineral salts such as aluminum sulphate (alum), sodium chloride, and magnesium chloride, and water-soluble cationic resins, for example quaternized amines of low molecular weight, low molecular weight polyacrylamides and low molecular weight poly (ethylene imine). The low molecular weight polyacrylamide resin can also function to impart wet strength properties to the sheet, such as, for example, resins of the type which impart permanent strength to a sheet. It is preferred to use a mixture of coagulants comprising a metallic or trivalent mineral salt, for example an alum and a polymeric coagulant, for example a wet strength resin based on

  classical cationic polyacrylamide.

  A suspension which comprises the coagulant of the above-mentioned type is not capable of being formed into a sheet

  highly charged which is the subject of the present invention.

  The retention of coagulated particles on the porous support on which the suspension is deposited is insufficient. In addition, the nature of the coagulated particles which are formed is such that it interferes with the drainage of water through the porous support. When this appears, it is necessary to reduce the speed of the moving support in order to allow the drainage of additional water. This is disadvantageous because the production speed is slowed down. The cationic flocculant with a high charge density used in the implementation of the present invention functions to form the coagulated particles of the suspension in soft flakes. In some embodiments within the scope of the present invention, it has been observed that the flakes are similar in structure and size to snowflakes. The flakes are in fact discrete clusters which are larger than the coagulated particles and large enough to resist passage through the openings of the porous support surface on which they are retained. Likewise, the nature of the flakes is such that the water in the suspension drips freely and rapidly through the openings of the support surface and at a speed such as the normal operating speeds of the porous support surface in

movement can be maintained.

  As mentioned above, the cationic flocculant for use in the practice of the present invention has a charge density of at least about 2 equivalents of cationic nitrogen per kg of polymer. It is assumed that the most widely used flocculants will have a charge density in the range of about 2 to about 5.2 or 5.2 equivalents mentioned above. In a preferred form, the charge density is at least about 3.5 equivalents

  of cationic nitrogen per kilogram of polymer. Resins or cationic polymers are known which have such charge densities, for example as described in US Pat. Nos. 5,098,520 and 5,178,730.

  Such a polymer can be prepared, for example by polymerizing an ethylenically unsaturated cationic monomer or monomers, with or without other monomer (s) (typically non-ionic) using quantities of monomers such that the resulting polymer has the desired charge density. Examples of cationic monomers which can be used to prepare cationic polymers with high charge density are: dialkylaminoalkyl acrylates or methacrylates, or acrylamides or methacrylamides in the form of the acid salt, or preferably in the form of 'a quaternary ammonium salt. An example of a monomer which can be polymerized with the cationic monomer is an acrylamide. Examples of cationic polymers include quaternized and non-quaternized copolymers of dimethylaminoethyl acrylate or methacrylate and acrylamide, polyethyleneimines,

  polyamine epichlorohydrin polymers and homo- and copolymers (with acrylamide) of diallyldimethylammonium chloride.

  Other patents which refer to cationic polymers having a high charge density include: US Patents 3,658,474, 3,962,332; 4,174,279; 4,396,513; and 4,711,727. Various of these patents describe different bases for the charge densities of the cationic polymers described herein. For example, the aforementioned patent No. 4,396,513 describes a charge density based on the molar ratio of the cationic monomer used to prepare the polymer. For example, a polymer made from 80 to 90 mol% of a cationic monomer is considered to have a charge density of 80 to 90%, which is referred to as having a very high charge density. The cationic polymers described in this patent include those

  prepared by polymerizing a 2- quaternary ammonium salt

  (meth) acryloyloxyethyl-N, N-dimethylamine with an acrylamide.

  Examples of commercially available cationic flocculants with high charge density which can be used in the practice of the present invention are described below. WT-24761, which is sold by Calgon20 Corporation, is a high molecular weight cationic polyelectrolyte in the form of a cationic polymer latex which has a charge density of about 2 to about 5.2 equivalents of cationic nitrogen per kilogram of polymer. Latex is a white opaque liquid having a density of 1.03 (8.6 pounds per gallon) and a viscosity of about 1500 to 3500 cps. This cationic polymer is described by its manufacturer to be effective for use in various liquid / solid separation processes such as sludge thickening, clarification, flotation, vacuum filtration and centrifugation. (It is assumed that this polymer is the equivalent of Calraid's Hydraid CMP-452 which is described as being effective for use in liquid / solid separations involving a paper machine, retention, clarification, and draining of a paper machine, 35 and implementation of dough collector). Optimer 7194 and Optimer 7195, which are sold by Nalco Chemical Company, are described as high charge, high molecular weight cationic flocculants which are in the form of latex. These cationic flocculants are described as being effective for use in various types of draining processes. They have a charge density of about 2 to about 5 equivalents of cationic nitrogen per kilogram of polymer. Although a variety of cationic flocculants with high charge density are commercially available in the form of latex (colloidal dispersions of resin solids), the flocculants can be treated according to known methods for solubilizing them, as recommended by their suppliers. So we can add flocculants to

  the suspension in the form of a solution.

  The technique recognizes the use in aqueous suspensions of the type involved in the invention of other ingredients which are typically used in small amounts and which can be considered as additives or possible ingredients. Examples of such materials include antioxidants, anionic or cationic mildews, and defoamers. The aqueous suspension used in the manufacture of the sheet of the present invention can be formulated from ingredients which are used in amounts which are known in the art. The aqueous suspension contains a very high proportion of water and a relatively small amount of matter which is dissolved therein or which is dispersed therein as solid particles (both dissolved matter and solid particles are included in the use in the invention of the term "solids content"). Typically, the solids content of the suspension will range from about 0.5 to about 15% by weight of the suspension based on the total mass of water and the solids content of the suspension. In order to make a sheet for use as a carrier for a vinyl floor covering, it is recommended that it be formed from a slurry which has a solids content of about 3 to about 6% in weight. The proportion of the constituents constituting the solids content of the suspension will generally comprise: (A) from about to about 25% by weight of fibers; (B) at least about 50% by weight of calcium carbonate; (C) about 2 to about 30% by weight of binder; (D) 0 to about 2% by weight of a coagulant; and (E) from about 0.01 to about 0.5% by weight of a

  cationic flocculant with high charge density.

  When present, an additive will generally constitute from about 0.001 to about 1% by weight of the suspension. In the manufacture of a sheet for use as a support for a vinyl floor covering, it is preferred that the solids content of the suspension comprises: (A) about 5 to about 15% by weight of fibers; (B) at least about 70% by weight, preferably about 75 to about 85% by weight of calcium carbonate; (C) about 5 to about 15% by weight of binder; (D) about 0.1 to about 2% by weight of a coagulant; and (E) about 0.01 to about 0.1% by weight of a high charge density cationic flocculant. In a preferred form, the suspension

  will also contain from about 0.05 to about 0.5% by weight of an antioxidant.

  The pH of the suspension must be such that the calcium carbonate is not degraded. For this purpose, the pH must be

  from at least about 7.0 and is typically in the range of from about 7.5 to about 8.5.

  By virtue of the excellent retention of the solid contents of the suspension on the porous support surface which is achieved by the use of the present invention, the proportion of ingredients constituting the highly charged sheet will closely correspond to the proportion of the solid contents of the suspension. Thus, the sheet will generally include: (A) about 5 to about 25% by weight of fibers; (B) at least about 50% by weight of calcium carbonate; (C) about 2 to about 30% by weight of binder; (D) 0 to about 2% by weight of a coagulant; and (E) about 0.01 to about 0.5 by weight of a cationic flocculant with a density of

high load.

  For a sheet which is used as a support for a vinyl floor covering it is preferred that it comprise: (A) about 5 to about 15% by weight of fibers; (B) at least about 70% by weight, preferably about 75 to about 85% by weight, of calcium carbonate; (C) about to about 15% by weight of binder; (D) about 0.1 to about 1% by weight of a coagulant; and (E) about 0.01 to about 0.1 by weight of a high charge density cationic flocculant. Sheet parameters such as density and thickness will tend to vary within relatively wide ranges depending on the particular implementation in which the sheet is used. For example, the density and thickness of the sheet may be respectively from about 0.48 to about 2.41 (30 to about 150 pounds / cu ft) and from about 254pm to about 1270 pm (10 to about 50 mils). For use as a backing for vinyl flooring, it is preferred that the density be from about 0.80 to about 1.61 (50 to about 90 lbs / cu ft) and that the thickness is about 381pm to about 1016 pm (15 to about 40 mils). One of the features of the present invention is the

  supply of a sheet highly loaded with calcium carbonate which has a combination of excellent properties.

  For example, the present invention can be used to form sheets which have a tensile strength of at least about 3.57 kg (1 cm wide strip) (20 pounds, 35 1 inch wide strip), evaluation at 23 C (74 F), and at least about 2.27 kg (1 cm wide strip) (5 pounds, 1 inch wide strip), measured at 177 C (350 F). Sheets with such mechanical strength characteristics are particularly useful as a support for vinyl flooring. As described above, the sheet of the present invention can be made using any suitable method which involves forming the sheet from a highly dilute aqueous suspension of constituents constituting the sheet. Examples of suitable methods are referenced

in U.S. Patent No. 4,225,383.

  As mentioned above, one of the advantages of the present invention is that the standardized processing steps do not have to be adversely modified to adapt the use of the invention. In this regard, it should be noted that the line speeds of the type generally used in the manufacturing process can be maintained or even increased. For example, in the manufacture of a vinyl flooring support, the porous support surface can operate at a speed of at least 30.5 m / min (100 feet per minute), and preferably operate at a

  speed of at least about 61 m / min (200 feet per minute).

  An exemplary sequence of process steps for forming the aqueous suspension is described below. With stirring, calcium carbonate is added to the water, which is preferably heated to a temperature of about 32 to about 43 C (90 to about 110 F). After which one can add, sequentially, the coagulant, the fibers, and possibly a polyacrylamide resin or an equivalent material which functions to improve the mechanical resistance in the wet state of the sheet and as a coagulant, and an antioxidant, if used . Additional water is added, with stirring, to the suspension to reduce its solids content, for example to about 3 to about 7% by weight. After which, the binder is added, preferably in the form of a latex, to the diluted suspension. Dilution facilitates careful mixing of the binder with the fiber / filler components. Then the cationic flocculant with high charge density is added to the suspension, with stirring.

Examples

  The examples below are given by way of illustration of the implementation of the present invention.

  Likewise, comparative examples are presented.

  Table 1 below includes a description of four aqueous suspensions, one of which can be used for

  forming a highly charged sheet which is an exemplary embodiment of the present invention (Example 1) and the others of which are comparative in that they include talc as filler instead of carbonate of

  calcium (examples C-2 and C-3) or that they comprise a cationic flocculant of the prior art (examples C-1, C-2 and C-3).

TABLE 1

  Quantity of ingredients, percentage by weight, Ingredient except water, as indicated Ex C-1 d ExC-2] ExC-3 Ex 1 Water (ml) 5000 5000 5000 5000

Talc (charge) - 79.7 80.04 -

  CaCO3 (filler) 79.73 - - 80.04 Alum (coagulant) 0.5 0.5 0.1 0.1 Cellulose fibers 7.4 7.4 7.4 7.4 Glass fibers 1.00 1.00 1.00 1.00 Polvacrvlamide (coagulant) 0.19 0.19 0.19 0.19 Antioxidant 0.15 0.15 0.15 0.15 Styrene-butadiene resin 10.99 11.00 11.00 11.00 (binder)

  Previous technical flocculant 0.05 0.13 0.13 -

  Cationic flocculant with high density - - - 0.08 of charge The talc used in the suspensions of examples C-1 and C- 3 was sold by Cypress Industrial Minerals under the name Vertal 8. The calcium carbonate used in the suspensions of examples C-1 and 1 were limestone crushed with rollers. The cellulose fiber consisted of Nordic bleached softwood kraft. The polyacrylamide coagulant is sold under the name Kymene 557H by Hercules Incorporated. The glass fibers were 7.5 µm in diameter and 3.2mm (1/8 ") long. The antioxidant is a 50% solids aqueous emulsion of 50% bisphenol

  crowded and 50% of a high molecular weight thioester.

  The styrene-butadiene resin is carboxylated and was added in the form of a latex sold by Dow Chemical Company. The prior art flocculant consisted of a medium molecular weight cationic polyacrylamide emulsion flocculant sold by Calgon Corporation under the name Hydraid TRP948. The high charge density cationic flocculant was Hydraid CMP-452 sold by Calgon Corporation. Each of the aqueous suspensions described in Table 1 above was formed into a sheet by adding the necessary aliquot of suspension to a standard sheet mold WILLIANS (Williams Standard Sheet Mol) d, by pressing the sheet formed between standardized blotting papers using a Williams Oil, 10 hydraulic press and drying against a heated drum dryer. Afterwards, the various properties of the leaves were evaluated and

  process conditions associated with the manufacture of the sheets. Table 2 below includes the results of the assessments.

TABLE 2

  Properties, conditions of Property values, treatment of sheets formed from treatment of suspensions of Ex Conditions C-1 | Ex C-2 Ex C-3 | Ex C-4 | Sample gauge 0.610mm 0.610mm 0.597mm 0.597mm l______________ _ (0.024 ") (0.024") (0.035 ") (0.0235") | ream weight (base 65.17 64.69 66.24 66.24 44.6m2 (480 sq ft)) Density (# cf) 69.25 68.87 70.42 70.42 Tensile strength, 2.54mm wide strip (1 "), at 27 24 27 29

23 C (74 F) #

  Elongation at 23 C (74 F) 4.8 4.8 5 4 (% c) Tensile strength, 2.54 cm wide strip (1 ") at 10 10 12 11

177 C (350 F) (#)

Extension to 177 C (350 F) 4 3

(%) _ ___ C) _ 4 3 3

  Plasticized resistance (#) 11 8 9 12 Tensile strength in the wet state 2.54cm strip 10 8 11 13 wide (1 "), 30 seconds (4) Mullen (psi) 48 43 49 59 Tear elmendorf 118 150 160 160 (gram-force) Rigidity 48, 52 60 52 Rigidity plasticized 16 o20 16 16 Williams draining, mesh sieve (loss in% 2.4 1.06 1.89 1.45 weight) Williams draining it 9 10 (seconds) We attract Pay attention to Table 2 above as regards the weight loss percentage values recorded for the nonaqueous constituents of the suspension passing through the 80 mesh screen. The value of 2.4% by weight recorded for the suspension of the Example C-1 is considered to be unsatisfactory for cost-effective sheet manufacturing on an industrial scale, however the loss value of 1.45% by weight for the suspension of Example 1 is considered to be satisfactory, and as it can be seen from the other values in Table 2, the suspension of Example 1 e st capable of being formed into a sheet which has comparable or better properties to the sheets formed

  from comparative suspensions.

  The effectiveness of the present invention has also been demonstrated in other ways. For example, the sample stock, which would normally be used in the Williams Sheet Mold, is exposed to a very high degree of tearing in a container with an agitator at 1300 rpm for 60 seconds. This reproduces the high shear that actually occurs when the suspension is pumped onto the moving porous support surface. With limestone and the flocculant Hydraid TRP- 948 (prior art), the drainage (drainage) increases from approximately 10 seconds to approximately 35 seconds (unacceptably low drainage) and the loss in weight percentage increases by approximately 1.5% about 15% (totally unacceptable). Using the flocculant of this invention, the drainage (drainage) increases from 10 seconds (unsheared) 25 to about 15 to 25 seconds (acceptable) and the weight percentage loss increases from about 1.5% to 5 - 9 % (acceptable). As a result, the improved flocculants are much more resistant to shear (from the suspension) than the flocculants of the prior art. As mentioned above, another aspect of the present invention relates to the use of carbon black for communicating opacity either to paper or to a fiber-reinforced, highly charged sheet, which has a white color, but tends to lack opacity, i.e., translucent for example. Opacity in paper or sheets of the above type is a desired characteristic in many applications where the paper or sheet is used. For example, with paper, opacity is a desired property when processing involves printing the paper on both sides. In the use of fiber reinforced sheet as a support for a vinyl floor covering it is not aesthetically desirable to have a support of transparent appearance; we

wish it to be opaque.

  Development work has revealed that the use of relatively small amounts of carbon black provides a significant increase in opacity with minor degradation of the white appearance of white paper or fiber-reinforced white sheets. Both paper and sheets of the aforementioned type have in common that they include fibers and a filler. Typically, the paper loading component constitutes about 5 to about 35% of the paper. On the other hand, the filler component of the highly charged fiber reinforced sheets constitutes a proportion

  top of the sheet, for example at least 50% by weight.

  The use of carbon black is effective for the purpose described in the invention in conjunction with both paper and sheet type products, and also with products which include a filler other than carbonate of

calcium.

  The amount of carbon black used for its "opacity" application will tend to vary depending on various parameters including, for example, the thickness of the paper or sheet, the proportion of ingredients making up the paper or sheet, and the whiteness of the paper or sheet. It is recommended to use about 0.0001 to about 0.05% by weight of carbon black in the paper or sheet, based on the total weight of the ingredients making up the paper or sheet, and adjustments can be made in quantity, if necessary, for the specific application. In the manufacture of a sheet of the type used as a support for a vinyl floor covering, it is recommended that the carbon black constitutes from about 0.0001 to about 0.05% by weight, more particularly preferably about 0.001 to about 0.01% by weight of the sheet. The retention characteristics of carbon black are excellent in that it is well retained with the other ingredients constituting the sheet or paper on the porous support surface in motion during the manufacture of the product. Consequently, the proportion of carbon black constituting the solids content of the aqueous dispersion for making the paper or sheet can correspond to the proportion of carbon black constituting the finished paper or sheet. In Table 3 below, the identity of the aqueous dispersions which comprise carbon black is set out.

  the interior (examples 2 and 3) and a comparative dispersion which does not include carbon black (example C-4). The dispersions described in Table 3 were formed into white sheets according to the base material described above in conjunction with the previous examples.

TABLE 3

  Quantity of ingredients, weight percentage, Ingredient except water, as indicated l _ __ __ __ __ _ _ Ex C-4 Ex2 Ex3 Water (ml) 5000 5000 5000

Talc (charge) 80.04 - -

  S | CaCO, (filler) - 80 80 Alum (coagulant) 0.1 0.15 0.15 Cellulose fibers 7.4 7.4 7.4 Glass fibers 1 1 1 Polvacrvlamide (coagulant) 0.19 0.19 0.19 Carbon black - 0.005 0.0025 Antioxidant 0.15 0.15 0.15 Stvrene-butadiene resin (binder) 11 11 11i

  Flocculant prior technique 0.14 - -

  Cationic flocculant with a density of - 0.14 0.14 high charge The opacities of the sample sheets formed from the aqueous dispersions described in Table 3 were evaluated by placing a hand between the back of the sheet and a ceiling light box and by examining the front of the sheet with an eye. The sheet made from the dispersion of Example C-4 was translucent in that the shadow of the hand and his fingers could be clearly seen through the sheet. On the contrary, nothing could be seen through the sheet made from the dispersion of Example 2, and the shadow of the hand could only be seen with difficulty through the sheet made from the

dispersion of example 3.

  In summary, it can be said that the present invention provides a practical means for improving

  significant the manufacture of products of the type described above

above.

Claims (20)

Claims   1. In a process for forming a highly charged fiber reinforced sheet, wherein the sheet is formed from an aqueous dispersion of solids, comprising fibers, a filler and a binder, and a cationic flocculating agent by supplying the same continuous a stream of such an aqueous dispersion on a moving porous support surface, in which the filler constitutes at least about 50% by weight of the solid content of the dispersion, the improvement comprises the use in the dispersion   aqueous of a filler consisting essentially of calcium carbonate and a cationic polymeric flocculant having a charge density of at least about 2 equivalents of cationic nitrogen per kilogram of polymer.   2. The method of claim 1, wherein the solids content of the aqueous dispersion constitutes from about 0.5 to about 15% by weight of the aqueous dispersion. 3. The method of claim 2, wherein the solids content of the aqueous dispersion constitutes approximately   3 to about 6% by weight of the aqueous dispersion.   4. The method of claim 2, wherein the solids content of the aqueous dispersion comprises (A) about 5 to about 25% by weight of fiber; (B) at least about 50% by weight of calcium carbonate; (C) about 2 to about 30% by weight of binder; (D) 0 to about 2% by weight of a coagulant; and (E) about 0.01 to about 0.5% by weight a flocculant.   5. The method of claim 2, wherein the solids content of the aqueous dispersion comprises (A) about 5 to about 15% by weight of fiber; (B) at least about 70% by weight of calcium carbonate; (C) about 5 to about 15% by weight of binder; (D) about 0.1 to about 2% by weight of a coagulant; and (E) about 0.01 to about 0.1% by weight of a flocculant.   The method of claim 3, wherein the solid content of the aqueous dispersion comprises (A) about 5 to about 15% by weight of fiber; (B) about 75 to about 85% by weight of calcium carbonate; (C) about 5 to about 15% by weight of binder; (D) about 0.1 to about 2% by weight of a coagulant; and (E) about 0.01 to about 0.1% by weight of a flocculant. 7. The method of claim 6, wherein the aqueous dispersion also comprises about 0.05 to about 0.5% of an antioxidant.   8. The method of claim 1, wherein said charge density is from about 2 to about 5.2 equivalent.   9. The method of claim 1, wherein said charge density is from about 3.5 to about 5 equivalents. 10. A fiber reinforced sheet comprising: (A) about 5 to about 25% by weight of fibers; (B) at least about 50% by weight of calcium carbonate; (C) about 2 to about 30% by weight of resin-based binder; and (D) about 0.01 to about 0.5% by weight of a cationic polymeric flocculant having a charge density of from about 2 to about 5.2 equivalents of cationic nitrogen per kilogram of polymer.   The sheet of claim 10 effective for use as a carrier for a vinyl floor covering, comprising: (A) about 5 to about 25% by weight of fibers; (B) at least about 70% by weight of calcium carbonate; (C) about 5 to about 15% by weight of binder; (D) about 0.1 to about 2% by weight of a coagulant; and (E) from about 0.01 to about 0.1% by weight of a cationic flocculant. 12. Sheet according to claim 11, comprising   about 75 to about 85% by weight of calcium carbonate.   13. Sheet according to claim 12, containing about 80% by weight of calcium carbonate.   14. The sheet of claim 10 wherein the charge density is at least about 3.5 equivalents.   15. Sheet according to claim 10 wherein the flocculant is a cationic polymer prepared by polymerizing   a 2- (meth) acryloyloxyethyl- quaternary ammonium salt   N, N-dimethylamine with an acrylamide.   16. Sheet according to claim 11, comprising   cellulosic fibers and glass fibers.   17. In a method of imparting opacity to white paper or a highly charged, white, fiber-reinforced sheet which comprises a filler and which is prepared from a dilute aqueous dispersion of fibers and filler, the improvement includes incorporating carbon black into the dispersion to make the paper or sheet opaque and to maintain the white appearance of the sheet or paper. 18. In white paper or a highly charged, white fiber-reinforced sheet, which tends to lack opacity, improvement includes the presence in the paper   or the sheet with a sufficient amount of carbon black to impart opacity to the paper or sheet and to maintain the white appearance of the sheet or paper.   19. Paper or sheet according to claim 18 comprising approximately 0.0001 to approximately 0.05% by weight of black of 0 carbon.   20. Paper or sheet according to claim 18 comprising about 0.001 to about 0.01% by weight of carbon black.