WO1996027047A1

WO1996027047A1 - Kraft-substitute bristol paper products

Info

Publication number: WO1996027047A1
Application number: PCT/CA1996/000119
Authority: WO
Inventors: Richard W. Detrick; Martin Fairbank
Original assignee: Abitibi-Price Inc.
Priority date: 1995-03-01
Filing date: 1996-02-29
Publication date: 1996-09-06
Also published as: CA2212865A1; AU4711196A

Abstract

The present invention provides a kraft-substitute bristol paper product, comprising from 70 to 95 per cent mechanical pulp fibres and from 5 to 30 per cent chemical pulp fibres. The paper product has a caliper of from 0.006 to 0.020 inches and weighs from 20 to 30 per cent less than the corresponding conventional kraft paper product of equivalent caliper. In an embodiment a novel file folder is provided. The invention further provides a process for preparing a kraft-substitute bristol paper sheet having a density in the range of 0.5 to 0.75 g/cm3 and a tear strength of from 700 to 2300 mN.

Description

Titles KRAFT-SUBSTITUTE BRISTOL PAPER PRODUCTS

FIELD OF THE INVENTION

The invention relates generally to kraft- substitute bristol paper products comprising from 70 to 90 per cent mechanical pulp fibres and from 10 to 30 per cent chemical pulp fibres and having a caliper of from 0.006 to 0.020 inches. The bristol paper products of the invention have the necessary physical characteristics to substitute for kraft products, including products such as file folders, return postcards and index dividers, and yet weigh from 20 to 30 per cent less than the corresponding conventional kraft paper product of equivalent caliper. The invention also relates to a process for preparing kraft-substitute bristol paper sheets by providing a furnish of 70 to 95 per cent mechanical pulp and 5 to 30 per cent chemical pulp, depositing the furnish on the wire of a paper machine to form a web and running the web through the paper machine to form a paper sheet. BACKGROUND OF THE INVENTION Bristol paper products are paper and paperboard products which are generally 0.006 inches or greater in thickness and which are strong, stiff, smooth, tear resistant and foldable. The physical properties of bristol paper make it suitable for paper products requiring strength and tear resistance, which will maintain their shape and which can be folded without tearing or splintering. Examples of products which are manufactured from bristol paper includes for example, file folders, return postcards, index dividers, sales tags, garment tags and cards.

Bristol paper products are traditionally made primarily from chemical pulp, in particular kraft pulp. Accordingly, commercial bristol paper products are also frequently referred to as kraft products. Chemical pulp is prepared by chemically treating wood chips in an aqueous solution at elevated temperature - 2 -

and pressure to dissolve sufficient lignin in the cell walls to allow the cellulose and he icellulose fibres to be separated with relatively little or no mechanical action. The two major chemical pulping processes are the kraft and sulfite processes.

In the sulfite process, wood chips are digested with a mixture of sulfurous acid and bisulfite ions to solubilize the lignin. The sulfites combine with the lignin to form salts which dissolve in the cooking liquor. The sulfite process produces somewhat brighter pulp, but weaker paper sheets than the kraft process. The sulfite process has been largely superseded by the kraft process, which has become the dominant pulping process worldwide.

Kraft pulp for bristol paper products is prepared with an alkaline pulping liquor containing, for example, sodium hydroxide and sodium sulfide. When cooked in this liquor the alkali results in fragmentation of the lignin into smaller segments. The cellulosic fibres in kraft pulp are long, flattened and deficient in lignin compared to the fibres produced by the mechanical process and yield a dense pulp.

The use of pulp containing predominantly kraft pulp fibres has been considered essential for bristol paper products, where bending stiffness, folding endurance, brightness, smoothness, tear strength and low surface compressibility are required features of the paper product. Although the use of predominantly kraft pulp imparts strength and smoothness, kraft pulp is dense and provides relatively heavy paper products which are expensive to handle and ship.

The use of kraft pulp in the manufacture of bristol paper products raises a number of environmental concerns. Kraft pulping has a relatively low yield from the wood or raw material of from between 40 and 50%. Accordingly from 50 to 60 % of the original wood is discarded as waste during chemical processing. Thus, chemical pulping is an inefficient use of natural forest resources. Kraft pulp is dense and dark and usually requires extensive bleaching and refining before it can be used in the manufacture of kraft paper products.

Chemical pulping also generates large amounts of environmental pollutants which may be troublesome and expensive to dispose of. Volatile malodorous sulfur compounds such as hydrogen sulfide, methyl mercaptan, dimethyl sulfide and dimethyl disulfide are produced as undesirable polluting by-products of the process for preparing chemical pulps.

Particularly toxic pollutants are generated by bleaching chemical pulp. The whiteness or brightness of pulp varies over a wide range, depending on the type of wood pulped and the pulping process itself. As noted above, Kraft pulp tends to be quite dark, having a brightness in the range of from 15 to 30, which is unacceptable for most bristol paper products. Accordingly, kraft pulps used in the manufacture of bristol paper products are usually bleached by one or more of a number of chemical treatments, such as chlorination, alkaline extraction, chlorine dioxide, oxygen, hypochlorite, peroxide or ozone.

Chlorination and alkaline extraction have traditionally been employed in the initial stages of kraft pulp bleaching to delignify the pulp and remove the chromophores. The chlorination of pulp forms a host of chlorinated organic compounds, such as polychlorinated dioxins and dibenzofurans which are extremely toxic. Environmental and health concerns mandate that the generation and release of these chemicals by pulp and paper mills be reduced and that more environmentally sound substitute kraft paper products be developed.

Mechanical pulp has not been used as a major component of bristol paper products as the fibres produced by the mechanical grinding process are short, and weak. The fibres of mechanical pulp retain the original cell wall lignin and, as a result, yield paper products having high bulk, opacity, compressibility and surface roughness but relatively low mechanical strength and density. Paper prepared from mechanical pulp is considered to be weak and mechanical pulp has been used primarily to manufacture lower quality paper grades, such as newsprint, where strength is not required. Typically, paper machines for manufacturing these low grade papers from mechanical pulp are not equipped with size presses to apply surface treatments to the paper, as the low quality rough grades, such as newsprint and cardboard are not coated.

There is a need for bristol paper products having the necessary minimum physical characteristics, such as stiffness, foldability and strength, to substitute for traditional kraft products, but which may be manufactured from pulp which is less wasteful of wood resources. Accordingly, there is also a need for a more economical and less polluting process for preparing these products.

{■flTMM^V QF THE INVENTION It is an object of the present invention to provide a bristol paper product, having acceptable minimum strength characteristics to substitute for a corresponding kraft product of equivalent caliper, but which is made primarily from mechanical pulp. It is a further object of the invention to provide such a bristol paper product which is less dense and thus weighs less than the kraft product of equivalent caliper for which it may be substituted. It is yet a further object to provide a bristol paper product which utilises wood resources more efficiently and which may be manufactured by a process which results in fewer toxic by-products than the chemical pulping processes, such as the kraft process.

It is also an object of the present invention to provide a bristol paper product having a caliper of from 0.006 to 0.020 inches and having the requisite minimum stiffness, tear strength, density, foldability for a bristol product, while being less dense and thus lighter than the corresponding conventional kraft paper product of equivalent caliper.

The present inventors have surprisingly determined that a kraft-substitute bristol paper product, having the necessary minimum physical properties to substitute for a conventional kraft paper product of equivalent caliper can be prepared from a furnish containing from 70 to 95 per cent mechanical pulp and 5 to 30 per cent chemical pulp. An aspect of the invention provides a kraft- substitute bristol paper product comprising from 70 to 95 per cent mechanical pulp fibres and from 5 to 30 per cent chemical pulp fibres, wherein the paper product has a caliper of from 0.006 to 0.020 inches and weighs from 20 to 30 per cent less than the corresponding conventional kraft paper product of equivalent caliper. In an embodiment, the paper product comprises from 70 to 80 per cent mechanical pulp fibres and from 20 to 30 per cent high yield sulphite pulp fibres. In a preferred embodiment, the paper product comprises from 80 to 95 per cent mechanical pulp fibres and from 5 to 20 per cent kraft pulp fibres. It is contemplated that, in an embodiment, the mechanical pulp fibres are unbleached and the bristol paper product has a brightness of from 45 to 75. In a further embodiment, the paper product has a density of from 0.5 to 0.75 g/cm³ and a tear strength of from 700 to 2300 milliNewtons (mN) .

In particular embodiments, the paper product may be a file folder, return postcard or index divider. In preferred embodiments, a file folder is provided having a caliper of from 0.010 to 0.011 inches and weighing from 90 to 115 pounds per ream.

In further embodiments, the invention provides a kraft-substitute bristol paper product comprising a postcard having a machine direction/cross-machine direction stiffness ratio of from 1.5 to 2.5 and a machine direction/cross-machine direction tear strength ratio of from 0.3 to 0.7.

In yet a further embodiment, the invention provides a file folder comprising 80 to 90 per cent, preferably 84 to 86 per cent, unbleached groundwood pulp fibres and from 10 to 20 per cent, preferably 14 to 16 per cent, kraft pulp fibres, having a caliper of from 0.0100 to 0.0110, preferably 0.0104 to 0.0106 inches and a weight of from 90 to 115, preferably 100 to 106 pounds, per ream and a tear strength of from 1100 to 1500 mN. The present invention also relates to a process for preparing a kraft-substitute bristol paper sheet having a density in the range of from 0.5 to 0.75 g/cm³ and a tear strength of from 700 to 2300 mN, comprising? mixing unbleached and unrefined mechanical pulp and chemical pulp to obtain a furnish comprising from 70 to 90 per cent mechanical pulp and from 10 to 30 per cent chemical pulp; depositing a jet of the furnish on the wire of the forming section of a paper machine to form a web and running the web through the paper machine to produce the paper sheet.

In an embodiment of the process the furnish has a brightness of from 45 to 70. In a further embodiment of the process the furnish is deposited on the screen at a jet/wire ratio of approximately 1.0. DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a range of kraft-substitute bristol paper products and to the process for preparing these products including specialty products, such as file folders, cards, postcards, return postcards, and tags. In a preferred embodiment of the invention, the kraft-substitute bristol paper product is a file folder.

As hereinbefore mentioned, the present invention relates to a kraft-substitute bristol paper product comprising from 70 to 95 per cent mechanical pulp fibres and from 5 to 30 per cent chemical pulp fibres. The kraft-substitute bristol paper product has a caliper of from 0.006 to 0.020 inches and weighs from 20 to 30 per - 7 -

cent less than the corresponding conventional kraft product of equivalent caliper.

The term bristol paper product as used herein refers to paper or paperboard which is 0.006 inches or greater in thickness and which is strong, tear resistant and foldable. Conventional kraft bristol paper products are traditionally made from close to 100% per cent kraft pulp fibres. The physical properties of bristol make it suitable for paper products requiring strength and tear resistance and which will maintain their shape. Examples of products which may be made from the kraft-substitute bristol include file folders, return postcards, index dividers, drawing pads, coating base papers, ticket base papers, sales tags, garment tags and cards. A kraft-substitute bristol paper product refers to a bristol paper product which has the necessary minimum requisite physical characteristics, such as stiffness and foldability to be used as a substitute for a corresponding conventional kraft paper product, but which has no more than 30, preferably 20, most preferably 15 per cent kraft pulp fibres, as described in more detail below.

Corresponding conventional kraft paper products refers to bristol paper products which are made up predominantly of kraft pulp fibres. Conventional kraft paper products generally have from 80 to 100 per cent kraft pulp fibres, depending upon the specific application. Such conventional products are well known and are commercially available.

Pulp fibres are cellulosic fibres from which paper products are manufactured. These may be obtained from a variety of raw materials, primarily wood or from other natural sources, such as cereal straws, bamboo and bagasse. In the paper-making process, the raw material is broken up and prepared into a pulp, which consists of an aqueous suspension of cellulosic pulp fibres.

Chemical pulp fibres refer to those pulp fibres which are obtained from wood or other raw materials principally using chemicals to dissolve away the lignin to release the fibres. Methods for preparing chemical pulp are well known in the art. The invention is by no means restricted to a particular type of chemical pulp, and it is contemplated that kraft, sulfite and high yield sulfite pulp fibres, most preferably kraft, may be useful in the paper products of the invention.

Sulfite pulp fibres may be prepared by the sulfite process, whereby wood chips are cooked in a digester with a mixture of sulfurous acid and bisulfite ions to solubilize the lignin. The sulfites combine with the lignin to form salts which dissolve in the cooking liquor. High yield sulfite pulp may be prepared following known procedures employing less severe cooking conditions combined with some mechanical fiberizing to separate the pulp fibres. High yield sulfite pulp fibres retain a higher proportion of lignin and hemicellulose than sulfite pulp fibres.

Kraft pulp may be prepared by standard procedures with an alkaline pulping liquor containing, for example, sodium hydroxide and sodium sulfide. In the kraft process wood chips are cooked in white liquor containing the active chemicals and the residual black liquor containing the reaction products of the lignin solubilization is removed for further processing. Under these conditions the wood fibres are de-lignified and the lignin is chemically fragmented and dissolved in the cooking liquor. Kraft pulp fibres typically contain very small amounts of lignin generally less than 5 per cent of the total weight. Cellulose and hemicellulose are also lost during this process.

The yield of pulp from chemical pulping, such as the kraft process is generally between from 40 to 60 per cent of the weight of the original wood and an improved yield of up to from 70 to 75 per cent may be achieved using the high yield sulfite process. Mechanical pulping is much more efficient and has about a 95 per cent yield from the wood.

Suitable equipment for digesting the wood is well known and includes stationary horizontal and spherical digesters and rotating vertical digesters. Continuous batch digesters, such as Messing and Durkee,

Kamyr or IMPCO digesters may be used.

Chemical pulps, especially kraft pulps, are generally used wherever strength is an important feature of the paper product or is required for applying a finish to the paper during processing. More particularly, kraft pulps are used where bending stiffness, folding endurance, brightness, smoothness and low surface compressibility are required features of the paper product.

Mechanical pulp fibres may be prepared by mechanical pulping processes, whereby the wood is pulped primarily using mechanical energy, with resulting high yields of pulp. Mechanical pulp typically contains from

5 to 35 per cent lignin based on the weight of the pulp.

Methods for preparing mechanical pulp are well known in the art. Stone groundwood pulp is prepared by pressing a block of wood lengthwise against a wet grindstone revolving at high speeds. Other suitable methods for preparing mechanical pulp are well known in the art and include pressurized groundwood PGW, refiner mechanical pulp RMP, thermo-refiner mechanical pulp TRMP, pressure refined mechanical pulp PRMP, chemi-mechanical CMP, theπno-mechanical pulp, pressure/pressure thermo- mechanical pulp PPTMP, chemi-refiner mechanical pulp CRMP, chemi-thermo-mechanical pulp CTMP, thermo-chemi-mechanical pulp TCMP and thermo-mechanical-chemi pulp TMCP.

The mechanical and chemical pulps, once prepared, may be further treated to screen out large particles and the pulps may be stored prior to further processing. The kraft-substitute bristol paper products of the invention may comprise from 70 to 95 per cent mechanical fibres and from 5 to 30 per cent chemical pulp fibres. Preferably, the paper products of the invention have from 80 to 95, more preferably 84 to 86, per cent mechanical pulp, preferably stone groundwood pulp fibres, most preferably unbleached and unrefined stone groundwood pulp fibres; and from 5 to 20, more preferably 14 to 16 per cent respectively kraft pulp fibres.

Where high yield sulfite pulp fibres are used as the chemical pulp fibres in the paper products of the invention, the appropriate percentage of chemical pulp selected should be higher, preferably from 20 to 30 per cent. As a rough guide, about twice as much high yield sulfite fibres are required to duplicate the results achieved with kraft pulp fibres.

As hereinbefore mentioned, the kraft-substitute bristol paper products of the invention have a caliper of from 0.006 to 0.020 inches, preferably from 0.008 to 0.016, more preferably form 0.010 to 0.011, most preferably from 0.0104 to 0.0106 inches. The appropriate caliper may be selected based on the desired specifications of a particular paper product.

The kraft-substitute bristol paper products of the invention have the requisite strength and characteristics to substitute for conventional kraft paper products and yet weigh from 20 to 30, per cent less than the corresponding conventional kraft products of equivalent caliper. Accordingly, the kraft-substitute bristol paper products are more economical to manufacture and to transport. Thus, not only are the paper products of the invention made from pulp which has a higher yield from the wood, but the paper products themselves require less weight of pulp to achieve the same caliper as the corresponding conventional kraft product. Accordingly, the paper products of the invention are less expensive to manufacture. The paper products of the invention may have a density of from 0.5 to 0.75 g/cm³.

In the competitive environment of the commercial paper industry, the weight of paper products may be a crucial factor for the distributor. For example, paper products are generally purchased by the distributor based on a per weight basis, such as tons of paper and the distributor pays a certain amount per ton in shipping costs. However, frequently, the distributor sells the paper product at a set price based on the number of units, such as reams of paper or number of file folders. Accordingly, if the distributor buys one ton of paper product that is 30% lighter than average, the distributor can sell the paper product at a competitive reduced price, for example 20% less, and still have increased profits of 10%.

Where tear strength is a necessary feature of the paper product, both the chemical and mechanical pulps may preferably be unrefined. Where the paper product is to be folded, the present inventors have determined that the chemical pulp fibres should preferably be kraft pulp fibres in order to prevent splintering at the fold.

In a preferred embodiment, the invention relates to a file folder made up of 80 to 90, preferably 84 to 86, most preferably 85 per cent unbleached groundwood pulp fibres and from 10 to 20, preferably 14 to 16, most preferably about 15 per cent kraft pulp fibres. The file folders of the invention have a caliper of from 0.0100 to 0.0110, preferably from 0.0104 to 0.0106, most preferably about 0.0105 inches and a weight of from 90 to 115, preferably 100 to 106 pounds per ream and a brightness of from 45 to 75 preferably from 60 to 65.

The file folders of the invention have an equivalent caliper to commercial file folders, such as Grand & Toy Permax file folders, but weigh approximately 25 per cent less. The present inventors have determined that the use of the small amounts of kraft pulp fibres detailed above, permit the file folders to hold a clean fold, without splintering. The file folders may be efficiently and economically prepared by the process described in more detail below, with a high yield from the wood, without the need for extensive bleaching or refining and with the generation of minimal toxic by products. Accordingly, the file folders may be produced and shipped more cheaply than conventional kraft file folders and are less harmful to the environment.

The invention still further relates to cards, such as tag cards, postcards and return postcards. The return postcards of the invention are made from stock having a basis weight of from 90 to 110g/m² and a caliper of at least 0.007 inches, preferably from 0.007 to 0.008 inches. This thickness helps to provide sufficient stiffness to enable the return postcards to be processed by post office sorting equipment without being damaged. Index dividers are contemplated as paper products of the invention. The index dividers may preferably have from 70 per cent groundwood pulp fibres and 30 per cent high yield sulfite pulp fibres and have a thickness of about 165 urn at a basis weight of about 103 g/m².

As hereinbefore mentioned, the present invention also relates to a process for preparing a kraft-substitute bristol paper sheet having a density in the range of 0.5 to 0.75 g/cm³ and a tear strength of from 700 to 2300, preferably 1100 to 1500 mN, comprising: mixing unbleached and unrefined mechanical pulp and chemical pulp to obtain a furnish comprising from 70 to 95 per cent mechanical pulp and from 5 to 30 per cent chemical pulp; depositing a jet of the furnish on the wire of the forming section of a paper machine to form a web and running the web through the paper machine to produce the paper sheet. Preferably, the furnish has a brightness of from 45 to 70.

Suitable pulps for use in the process of the invention are described elsewhere herein. The mechanical pulp and preferably also the chemical pulp may be unrefined. Refining refers to the mechanical treatment of pulp, usually carried out in conical or disc type refiners, where the pulp fibres are modified by flowing parallel to bar crossings. The mechanical and chemical pulps are mixed or blended together, resulting in a furnish having from 70 to 95, per cent mechanical pulp and 5 to 30, cent chemical pulp. Blending of the pulp furnish may be carried out in a furnish beater or large chest with a pump having sufficient agitation to ensure a high turnover rate and adequate mixing. The resulting furnish has 15 to 30 per cent ligneous material.

The furnish may be prepared as described above and fed into a headbox on a paper making machine, such as a fourdrinier, double wire or cylinder machine.

In a fourdrinier machine, for example the furnish is fed from the head box onto the continuously moving fourdrinier wire in the forming section of the machine. As the slurry advances on the wire down the forming section of the machine an initial amount of the water in the pulp furnish drains through the wire into drainage units to form a web of pulp product supported on the wire. Close to the downstream end of the wire additional amounts of water are forcibly removed from the web by means of suction boxes in contact with the lower surface of the wire.

Once the web of pulp product reaches the end of the wire it is passed over a suction couch roll which extracts further water from the web of pulp product which emerges from the couch roll as a self supporting web of pulp product which can be peeled off the wire and subjected to further processing steps, such as pressing, drying and calendering to remove water and improve the surface and finish of the product.

The invention will be more fully understood by reference to the following examples. However, these examples are merely intended to illustrate embodiments of the invention and are not to be construed to limit the scope of the invention. E&Λ-KP ES BSΛMPIiB 1

File folders were prepared from groundwood pulp as follows. A groundwood pulp furnish, containing 70 to 85 per cent stone groundwood pulp and 15 to 30% per cent northern softwood kraft pulp or high yield sulphite, was used. The northern softwood kraft pulp was used without refining, so as to maximize the tear strength. The stone groundwood was used at a Canadian standard freeness of about lOOmL. No chemicals were added to the pulp.

A fourdrinier paper machine was used, running at a speed of about 550 feet per minute. Trial runs were performed using a range of basis weights from about 75 to 110 lbs/3000ft² and various amounts of calendering. When the file folder stock was calendered by a stack of six calender rolls to a constant caliper of 0.0105 inches and the basis weight was about 103 pounds per 3000 ft², the smoothness achieved was found to match that of competitive conventional kraft file folders.

The paper was converted into file folders using scoring dies. As the paper was less dense than conventional kraft file folders, increased pressures were used to obtain a good quality score line in the paper and thus a good quality folder. Paper stock containing 85% stone groundwood and 15 % kraft was generally found to give the best results. Paper stock containing 70% stone groundwood and 30 % high yield sulphite pulp gave poorer quality folds with stiff high yield sulphite fibres sticking out from the fold.

The file folders prepared from paper stock containing 85% stone groundwood and 15 % kraft as described above were 0.0105 inches in caliper but weighed only 103 pounds per 3000 ft² ream. This is in contrast to conventional kraft file folders, such as commercially available Grand & Toy Permax file folders, which are 0.0105 inches in caliper and weigh 137 pounds per ream. - 15 -

Accordingly, the novel file folders of the invention weigh approximately 25% less than conventional kraft file folders. EXAMPLE 2 Return Postcards

Return postcards were prepared from groundwood pulp as follows. A groundwood pulp stock, containing 70 per cent groundwood pulp and 30 per cent high yield sulphite pulp, was used. Return postcard stock was made with a basis weight of 106.5 g/m² and a thickness of 0.0076 inches. Kraft return post card stock is generally made at a minimum of 110 g/m² with a thickness of 0.0070 inches.

Return postcards are enclosed for example with magazines and are designed to be completed and returned by mail. Accordingly, the post cards must have sufficient stiffness as well as tear resistance in order that they can be processed in post office sorting equipment. The increased thickness of the return postcards prepared from groundwood pulp provides added stiffness for this purpose. The cards are always processed in "landscape mode", but depending on how they are cut, they can be "grain long" or grain short", i.e. the machine direction of the paper machine is either the longer or the shorter dimension of the rectangular postcard. Thus, the stiffness and tear strength need to be sufficient in both the machine direction and the cross-machine direction of the paper machine. This was achieved by making a sheet with a low machine-direction/cross-machine direction (MD/CD) fibre orientation ratio. The low MD/CD fibre orientation ratio was achieved by obtaining the same speed at the jet of stock exiting the headbox slice as the fourdrinier section wire. This is in contrast to conventional operation of the paper machine where the wire is run at a higher speed than the jet. This tends to orient a majority of the fibres in the machine direction and gives a typical MD/CD stiffness ratio of 2.5 to 5.0 and a MD/CD tear strength ratio of 0.25 to 0.4. Having the jet/wire speed ratio at close to 1.0 allowed the MD/CD stiffness ratio to be reduced to 1.5 to 2.5 and the MD/CD the tear strength ratio to 0.3 to 0.7. EXAMPLE 3

Index dividers

Index dividers were prepared from groundwood pulp as follows. A groundwood pulp stock, containing 70 per cent groundwood pulp and 30 per cent high yield sulfite pulp, was used. A stack of six calender rolls was used to give a smooth surface and to control the thickness to 165 μm.

The index dividers were prepared with a thickness of 165 urn at a basis weight of 103 g/m². In contrast, conventional index dividers made from 100 per cent kraft pulp have a basis weight of 120 g/m² at the same thickness.

Claims

WE CLAIM;

1. A kraft-substitute bristol paper product, comprising from 70 to 95 per cent mechanical pulp fibres and from 5 to 30 per cent chemical pulp fibres, wherein the paper product has a caliper of from 0.006 to 0.020 inches and weighs from 20 to 30 per cent less than the corresponding conventional kraft paper product of equivalent caliper.

2. A paper product as claimed in claim 1 wherein the paper product comprises from 80 to 95 per cent mechanical pulp fibres and from 5 to 20 per cent kraft pulp fibres.

3. A paper product as claimed in claim 1 wherein the paper product comprises from 70 to 80 per cent mechanical pulp fibres and from 20 to 30 per cent high yield sulphite pulp fibres.

4. A paper product as claimed in claim 2 wherein the mechanical pulp fibres are unbleached and wherein the paper product has a brightness of from 45 to 75.

5. A paper product as claimed in claim 2 having a density of from 0.5 to 0.75 g/cm³ and a tear strength of from 700 to 2300 mN.

6. A paper product as claimed in claim 2 wherein the paper product is a file folder, return postcard or index divider.

7. A paper product as claimed in claim 2 comprising a file folder having a caliper of from 0.010 to 0.011 inches and weighing from 90 to 115 pounds per ream.

8. A paper product as claimed in claim 2 comprising a postcard having a machine direction/cross-machine direction stiffness ratio of from 1.5 to 2.5 and a machine direction/cross-machine direction tear strength ratio of from 0.3 to 0.7.

9. A file folder comprising 80 to 90 per cent unbleached groundwood pulp fibres and from 10 to 20 per cent kraft pulp fibres, having a caliper of from 0.0100 to 0.0110 inches, a weight of from 90 to 115 pounds per ream and a brightness of from 45 to 75.

10. A file folder as claimed in claim 8 comprising 84 to 86 per cent stone groundwood pulp fibres and 14 to

16 per cent kraft pulp fibres, having a caliper of 0.0104 to 0.0106 inches, a weight of 100 to 106 pounds per ream and a tear strength of from 1100 to 1500 mN.

11. A process for preparing a kraft-substitute bristol paper sheet having a density in the range of 0.5 to 0.75 g/cm3 and a tear strength of from 1100 to 1500 mN, comprising: mixing unbleached and unrefined mechanical pulp and chemical pulp to obtain a furnish comprising from 70 to 90 per cent mechanical pulp and from 10 to 30 per cent chemical pulp; depositing a jet of the furnish on the wire of the forming section of a paper machine to form a web and running the web through the paper machine to produce the paper sheet.

12. A process as claimed in claim 11 wherein the furnish has a brightness of from 45 to 70.

13. A process as claimed in claim 11 wherein the stock is deposited on the screen at a jet/wire ratio of approximately 1.0.