US3849184A

US3849184A - Treatment of paperboard

Info

Publication number: US3849184A
Authority: US
Inventors: A Roberts; M Roberts
Original assignee: Lever Brothers Co
Current assignee: Lever Brothers Co
Priority date: 1969-04-18
Filing date: 1971-12-16
Publication date: 1974-11-19
Anticipated expiration: 1991-11-19
Also published as: BE749147A; DE2018088A1; DE2018088B2; AT298228B; CH532685A; GB1301100A; NL7005635A; NL158865B; FR2045439A5; DE2018088C3

Abstract

A coated paperboard of improved physical strength properties is provided where at least one surface of a paperboard, preferably containing a major proportion of low strength cellulosic fibres, is coated with an aqueous dispersion, the solids content of which comprises a water-soluble lignosulphonate, a non-reactive hydrophobic waxy material and, optionally, a high molecular weight polysaccharide. The improved physical strength characteristics are retained to a considerable extent even under tropical conditions of humidity and temperature.

Description

United States Patent Roberts, deceased TREATMENT OF PAPERBOARD Allan Roberts, deceased, late of Upminster, England by Margaret Agnes Roberts, executrix Inventor:

Assignee: Lever Brothers Company, New

York, N.Y.

Filed: Dec. 16, 1971 Appl. No.: 208,896

Related US. Application Data Continuation-impart of Ser. No. 27,121, April 9, 1970, abandoned.

Foreign Application Priority Data Apr. 18, 1969 Great Britain 19836/69 US. Cl. 117/155 UA, 106/117, 106/123, 106/201, 117/154, 117/156, 117/158,

Int. Cl D2lh l/40 Field of Search 106/117, 123, 201; 117/156, 158, 155 UA, 154; 162/172, 180

References Cited UNITED STATES PATENTS 8/1919 Grosvener 106/212 Warth, A. l-l., The Chemistry & Technology of Waxes, 1956, pp. 636-640, TP 670 W25 1956 A4, Copy 145.

Primary Examiner-William D. Martin Assistant ExaminerM. R. Lusignan Attorney, Agent, or Firm-Lever Brothers Company [57] ABSTRACT A coated paperboard of improved physical strength properties is provided where at least one surface of a paperboard, preferably containing a major proportion of low strength cellulosic fibres, is coated with an aqueous dispersion, the solids content of which comprises a water-soluble lignosulphonate, a non-reactive hydrophobic waxy material and, optionally, a high molecular weight polysaccharide. The improved physical strength characteristics are retained to a considerable extent even under tropical conditions of humidity and temperature.

4 Claims, No Drawings TREATMENT OF PAPERBOARD This application is a continuation-in-part of our copending application Ser. No. 27,121, now abandoned, filed Apr. 9, 1970.

This invention relates to an improved process for the coating (surface application) of paperboard and is particularly concerned with an economic process of such a nature which provides a product of improved physical strength properties which are retained to a considerable extent even under tropical conditions of humidity and temperature.

FIELD OF THE INVENTION In this specification the term paperboard" shall be understood as referring to a non-laminated product of basis weight within the range 130 850 g/m comprising a ply or a plurality of intimately bonded plies consisting essentially of cellulosic fibrous material, that is made on a board machine. The process of this invention is applicable to a paperboard possessing a caliper (i.e. thickness) in the range 0.004 0.050 inch (0.1 1.3 mm) and particularly applicable to a paperboard of caliper 0.008 0.050 inch (0.2 1.3 mm).

The process of this invention is advantageously applied to a paperboard that is prepared from a cellulosic pulp comprising a major proportion of low strength fibrous material. These paperboards such as unlined chipboard, chemical pulp lined chipboard and corrugating medium are used for the production of carton board, rigid board, pasted board and corrugated board.

The term physical strength properties as applied to paperboard can be measured in relation to, for example, stiffness, rigidity, Concora strength or Mullen value. The stiffness of a paperboard is defined as the degree of resistance offered by the paperboard when it is bent by application of a force under specified conditions. Its value is conveniently determined by the method set out in B.S. 3748/1964, using an instrument such as a Taber tester. The Taber tester measures stiffness as a bending moment which is expressed in terms of Taber units. The value of this term is dependent upon a number of factors including the thickness of the paperboard, the method by which the paperboard is produced, the elastic modulus of the cellulosic fibrous material used to form the paperboard and the direction in which the measurements have been made.

The rigidity of the paperboard is defined as the degree of resistance offered by the paperboard when it is deformed by application of an edgewise force under specified conditions. Such a determination of rigidity is set out in TAPPl Standard T472 (Su 68). Measurement of this term is conveniently made by using a ring-crush holder on a Gaydon instrument.

The term Concora strength" is a measurement of the resistance to crush that is offered by a paperboard (corrugating medium) to a force applied at right angles to the corrugating plane. The measurement is conveniently made by following the method set out in TAPPl Specification T809 (Su 66) and using a Gaydon instrument.

It is an object of this invention to produce at economical cost, a paperboard which shows improved physical strength properties in respect of at least one of the above defined parameters, particularly under conditions of high humidity. Paperboards having these improved properties can be made from high strength pulp but are expensive particularly when compared with paperboards prepared from waste paper.

PRIOR ART Much effort has been directed to the development of processes for the coating of paper or paperboard. For example, US. Pat. No. 1,31 1,964 (Grosvenor) teaches the preparation of a paper suitable for lithographic or photogravure processing whereby a paper is coated with a combination of starch and wax. Canadian Patent Specification 731,509 (Blackmore) teaches a method for the creation of rigid tubes or cores where an aqueous solution comprising a lignosulphonate and an animal glue is applied to paperboard as an adhesive. Neither of these specifications suggestions however, that a paperboard of improved physical strength properties can be obtained by the processes they describe.

The addition of certain water-soluble or waterswellable high molecular weight polysaccharides has long been known as a means for improving the resistance to deformation of a paperboard. This process leads also to an increase in the stiffness of paperboard. Any beneficial improvement is however substantially lost under conditions of high humidity.

The application of an aqueous solution containing a lignosulphonate salt to a paperboard has also been proposed (TAPPl Vol. 45, p.192a (1962)). This material is, however, highly hydroscopic and any paperboard thus treated will lose its improved stiffness when exposed to conditions of high humidity.

The addition of wax to a paper laminate as an adhesive to improve the waterproofing properties is described in Warths textbook The Chemistry and Technology of Waxes. This addition however does not improve the physical strength properties of the paper laminate to any significant extent.

SUMMARY OF THE INVENTION:

In one aspect of the present invention there is provided a coated paperboard comprising:

1. A paperboard consisting of a ply or of a plurality of intimately bonded plies consisting essentially of cellulosic fibrous material; and

2. a coating on at least one surface thereof comprisa. a water-soluble lignosulphonate salt and b. a non-reactive, hydrophobic waxy material having a melting point higher than the temperature of application of said coating to said paperboard,

said paperboard having a thickness of 0.004 0.050 inch and a basis weight of 850 g/m, said watersoluble lignosulphonate salt and said non-reactive hydrophobic waxy material being present in an amount effective to improve the physical strength characteristics of said paperboard.

In a further aspect of this invention there is provided a process for coating at least one surface of a paperboard with an aqueous 5 60% w/w dispersion, the solids content of said dispersion comprising a watersoluble lignosulphonate salt and an effective amount of a non-reactive hydrophobic waxy material, the melting point of said waxy material being such that the said dispersion remains stable at the temperature of application to said paperboard.

DESCRIPTION OF THE INVENTION For paperboard containing no lining of high strength cellulosic fibrous material (unlined chipboard) it is preferable to coat both surfaces with the aqueous dispersion, particularly if the paperboard has a caliper approaching 0.050 inch (1.3 mm), in order to obtain an improvement in physical strength properties. For paperboard which is lined on one side with a high strength cellulosic fibrous material (for example bleached and unbleached soft wood sulphite or sulphate pulp), the unlined surface is advantageously coated with one or more applications of the aqueous dispersion.

For improving the stiffness of the paperboard it is preferred that the solids content of the aqueous dispersion should comprise 95 99 percent by weight of the lignosulphonate salt and 5 1 percent by weight of the non-reactive hydrophobic waxy material. The stiffness and resistance to deformation and burst of a paperboard can be improved by coating at least one surface with an aqueous dispersion, the solids content of which comprises 47.5 95 percent by weight water-soluble lignosulphonate salt, 5 1 percent by weight of the non-reactive hydrophobic waxy material and 47.5 4 percent by weight of a water-swellable, high molecular weight polysaccharide. Representative examples of such a material include corn starch, potato starch, pregelatinised starch, oxidised starch, starch ethers such as hydroxy ethyl starch, esterified bifractional starch or a mannogalactan gum such as guar gum. The resistance to burst of the paperboard is indicated by determination of its Mullen value as set out in B5. 3137/1959.

The paperboard used in this invention can be prepared from mechanical pulp, a hybrid of chemical and mechanical pulp, a pulp derived from secondary fibres, pulp screenings or any mixtures of these materials. A pulp derived from secondary fibres, for example, mixed waste paper, container waste or news waste is a preferred material since the strength properties of such a material are not noticeably improved.

The upper limit of the solids content of the aqueous dispersion is primarily determined by the viscosity and the stability of the dispersion at its application temperature. The lower limit of solids content is determined by the consequential pick-up of solids by the paperboard during application of the dispersion. A preferred range of solids content varies from 20 40% w/w.

The preferred range of application temperature is between and 40C and should be below the melting point ofthe hydrophobic waxy material. The higher the application temperature the greater will be the likelihood of the coating dispersion breaking down with consequential deposition of gross amounts of waxy mateial onto the paperboard. The parameters of the process of this invention are desirably adjusted so that the amount of lignosulphonate and waxy material taken up by the paperboard varies between 0.5 and 15 percent by weight of the paperboard.

The lignosulphonate salt should be soluble in water and desirably contain a minimum of hydroscopic impurities such as monoor disaccharides. Calcium, sodium, magnesium, ammonium and aluminium lignosulphonate salts are commonly available. Sodium lignosulphonate or calcium lignosulphonate are preferred materials for the process of this invention.

The hydrophobic waxy material can be of an animal, vegetable, petroleum or synthetic origin and should be such as to form a stable dispersion with the aqueous lignosulphonate solution and have a melting point such that the dispersion remains stable at the temperature of application to the paperboard. Further, the waxy material should remain chemically stable in the presence of paperboard, water and lignosulphonate salt. Representative examples of a waxy material of animal origin include beeswax and lanolin. Carnauba wax is an example ofa wax of vegetable origin while of the many types of petroleum waxes there may be included ozokerite, montan wax, microcrystalline petroleum waxes and various grades of paraffin hydrocarbon waxes. Low molecular weight polyethylene or polypropylene, hydrogenated castor oil, cetyl alcohol or oxidised montan wax are examples of waxes of synthetic origin.

The waxy material can be dispersed directly into the aqueous solution of the lignosulphonate salt. It is also convenient to form an initial aqueous dispersion of the waxy material with the aid of a surface active agent which is inert to the lignosulphonate salt and subsequently add this dispersion to the lignosulphonate solution. The solid lignosulphonate salt can if desired also be added to a suitable aqueous dispersion of the waxy material. The amount of waxy material preferably used in the aqueous dispersion lies between 1 and 2 percent by weight of the solids content of the dispersion.

The aqueous dispersion of lignosulphonate and waxy material is conveniently applied to the surface of a paperboard by an on machine process or an off machine process. The paperboard can be either partially or completely dried prior to the application. The aqueous dispersion is preferably applied to the paperboard by means of a size press, a calender water-box or a spray. The pick-up of the dispersion by the paperboard depends upon the temperature of application, the solids content of the applied dispersion, the degree of sizing of the base paperboard and the machine speed.

The paperboard prepared according to the process of the present invention exhibits an improved stiffness and rigidity (even after subjection to tropical humidity conditions) which is higher than in the case of similar paperboards which have not been thus treated.

The invention is further illustrated with reference to the following examples.

COMPARATIVE EXAMPLE 40 parts by weight calcium lignosulphonate were dissolved in parts by weight water at 50C. 4 5 mls of the solution were metered onto the surface of a sample of an air dried paperboard (derived entirely from a waste paper pulp) measuring 33 X 22.9 cm and processing a basis weight of 558 g/m.

A sample of this coated paperboard (20.3 X 17.8 cm) was cut out, dried at 105C and weighted. Small test pieces were cut from this sample and equilibrated at Rl-l/20C (temperature conditions) and RHI34C (tropical conditions). The cross directional (CD) and machine directional (MD) stiffness of these equilibrated test pieces were determined in Taber units by means of the method described in 8.8. 3748/1964.

Comparison of the CD stiffness of a coated and uncoated paperboard showed that under temperature conditions there was an improvement of 31% [(254 194) X /194]. After equilibration under tropical conditions a comparison of coated and uncoated test pieces showed that there was an improvement in CD stiffness of 19% [(187 157) X l00/l57]. The basis weight of the coated paperboard prepared by this experiment was 570 g/m Samples of paperboard similar to that used in the above experiment were coated with an aqueous dispersion containing an amount of paraffin wax (melting point about 40C) equivalent to 3 percent by weight of the calcium lignosulphonate used. The coated samples were dried, weighed and their CD and MD stiffness determined under temperature and tropical conditions. Comparison of the CD stiffness of the coated test piece (190 units) and uncoated test piece (194 units) indicated that no significant improvement in stiffness had been brought about by this coating treatment. Similarly, comparison of the CD stiffness of the coated test piece under tropical conditions (165 units) and the uncoated test piece (157 units) showed that, again, there was no significant increase. The basis weight of the coated paperboard was 559.5 g/m Samples of paperboard similar to that used in the above experiments was treated with an aqueous dispersion containing an amount of calcium lignosulphonate and an amount of paraffin wax equal to that used in the above two experiments. These samples were dried at 105C, weighed and equilibrated under temperate and tropical conditions.

Comparison of the CD stiffness of a coated and uncoated test piece which had been equilibrated under temperate conditions showed that there had been an increase of 32% [(257 194) X 100/194]. Comparison of the CD stiffness of the coated test piece and uncoated test piece which had been equilibrated under tropical conditions showed that there had been an increase of 34% [(211 157) X 100/157]. The basis weight of the coated sample was 570 g/m EXAMPLE 1 35 parts by weight calcium lignosulphonate (containing 'percent sugar impurity) were dissolved in 65 parts by weight water at a temperature of 50C with suitable agitation. 0.88 part by weight of a 40% w/w aqueous dispersion of paraffin wax (melting point 40C) was subsequently stirred into this solution. The wax had been converted into an aqueous dispersion with the aid of 2 percent by weight calcium lignosulphonate. The two surfaces of an unlined paperboard of basis weight 670 g/m and a caliper of 0.040 inch (1.0 mm) which was made solely from a recovered waste paper pulp were coated with the above prepared aqueous dispersion at 38C. The dispersion was applied by means of calender water-boxes, one application per face of the paperboard. A sample of the coated paperboard was dried at 105C, weighted and subsequently equilibrated under temperate conditions (65% RH/C) and its CD stiffness determined in Taber units. The coated sample had a basis weight of 680.5 g/m and a stiffness of 406 Taber units.

Compared with untreated paperboard of the same basis weight that had also been made from similar recovered waste paper pulp on the same machine and at similar production rate and equilibrated under similar conditions of humidity and temperature, the increase in CD stiffness was 27% (from 320 406 Taber units).

This experiment was repeated using an aqueous dispersion containing an equal amount of calcium lignosulphonate and equal amount of cetyl alcohol in place of the paraffin wax. A similar increase in CD stiffness was maintained under conditions of tropical humidity.

By applying the above prepared dispersion to the surfaces of the paperboard and using two water-boxes per face a paperboard having a basis weight of 694.5'g/m and a CD stiffness of 464 Taber units was obtained. The increase in CD stiffness was approximately 45 percent (from 320 464 Taber units).

EXAMPLE 2 A dispersion having a composition as described in Example 1 was prepared. This dispersion was used to coat the unlined surface of a white lined paperboard of caliper 0.021 inches (0.52 mm) and having a basis weight of 370 g/m A sample of the treated paperboard was dried at C, weighed and equilibrated under tropical conditions of humidity and temperature (90% RH/40C) and its CD stiffness was determined. The coated paperboard had a basis weight of 377.5 g/m The CD stiffness of the treated board had improved by 22 percent when compared with the CD stiffness of untreated paperboard equilibrated under similar tropical conditions.

EXAMPLE 3 20 parts by weight calcium ligno-sulphonate containing 10 percent sugar impurity were dissolved in 80 parts by weight water. 0.5 part by weight of a 40% w/w aqueous dispersion of paraffin wax, melting point 40C, (containing 2 percent calcium lignosulphonate by weight of the wax as a dispersing agent) was added to this solution with agitation. A corrugating grade paperboard of 0.009 inch (0.21 mm) caliper and having a basis weight of 136 g/m was coated on one side with this prepared dispersion at 25C. The coated paperboard had a basis weight of 140 g/m The coated paperboard was equilibrated at 65% RH/20C, subsequently corrugated and then subjected to a crush test using a Gaydon crush tester.

It was found that the treated paperboard gave Concora values 40 percent higher than the uncoated corrugated paperboard (from 20.2 28.6 Kg).

EXAMPLE 4 20 parts by weight sodium lignosulphonate was dissolved in 80 part by weight water. 0.5 part by weight of a 40% w/w aqueous dispersion of paraffin wax (melting point 40C) was added to this solution with vigorous agitation.

A paperboard similar to that used in Example 3 was coated with the above prepared dispersion to give a paperboard having a basis weight of 140 g/m A sample of this treated paperboard was corrugated and its Concora value determined. lt was found that the treated paperboard gave Concora values approximately 40 percent higher than those obtained using a sample of untreated paperboard.

By replacing the paraffin wax used in this experiment with beeswax a similarly improved Concora value is obtained.

EXAMPLE 5 20 parts by weight calcium lignosulphonate and 6 parts by weight farina starch were dissolved in 74 parts by weight water. 0.5 part by weight of a 40% w/w aqueous dispersion of paraffin wax, melting point 40C,

(containing 2 percent calcium lignosulphonate by weight of the wax as a dispersing agent) was added to this solution with agitation. A corrugating grade paperboard of 0.009 inch (0.21 mm) caliper and having a basis weight of 136 g/m was treated on both sides at 25C with the above prepared dispersion which was applied at the size press of a board machine to give a paperboard having a basis weight of 140 g/m".

After equilibration at 65% RH/20C it was shown that the above treated paperboard had a rigidity value (28 Kg) which was about 30 percent higher than the rigidity value (21.4 Kg) of a similar untreated paperboard.

Similar results are obtained if the farina starch is replaced by an hydroxy-ethyl starch or a pre-gelatinised starch.

EXAMPLE 6 30 parts by weight of calcium lignosulphonate were dissolved in 70 parts by weight water at a temperature of 50C. 0.75 part by weight ofa 40% w/w aqueous carnauba wax dispersion were subsequently stirred into this solution.

A white lined paperboard of basis weight 378 g/m and a caliper 0.020 inch (0.50 mm) was coated on its unlined surface with the above dispersion by means of two calender water box applications. After equilibration at 65% RHl20C it was found that the CD stiffness had increased by 40 percent (from 62 to 87 Taber units) as compared with the CD strength of a similar but uncoated white lined paperboard which had been equilibrated under similar conditions. The coated paperboard had a basis weight of 385.5 g/m EXAMPLE 7 Three aqueous dispersions were prepared containing respectively a 5, 20 and 40 percent content of the cal-- cium lignosulphonate used in Example 1 and 1 percent per 0.027 inch (0.69 mm) was treated with a double:

application ofthe percent dispersion. The percentage pick-up of solids and the improvement in MD and CD stiffness (measured at 65% RH/20C) was determined and is tabulated below. This experiment was repeated with further samples of paperboard which were coated with the 20 and 40 percent aqueous dispersion respectively.

paraffin wax (melting point 40C). The solids content of this aqueous dispersion was about 17 percent. The basis weight of the coated paperboard was 241.6 glm The rigidity of a sample of the coated and uncoated paperboard was subsequently determined. The rigidity of the coated paperboard was 29.4 Kg while that of the uncoated paperboard was 26.8 Kg (both equilibrated at RH/20C).

EXAMPLE 9 White lined chipboard of basis weight 295 g/m and caliper 0.0175 inch was coated with an aqueous dispersion containing 40 percent by weight calcium lignosulphonate and 1 percent by weight paraffin wax (based on the calcium lignosulphonate). A single application of this dispersion was given to the unlined surface of this chipboard. The percentage pick-up of solids was 2.7 percent (8.1 g/m). The coated paperboard had a basis weight of 303.1 g/m and showed a 35 percent increase in CD stiffness.

A sample of the above white lined chipboard was also coated with an aqueous dispersion containing 40 percent by weight calcium lignosulphonate and 20 percent by weight paraffin wax (based on the weight of calcium lignosulphonate). The coated paperboard showed a lower increase (23 percent) in CD stiffness as compared with the board treated in the first part of this example. There was a 25 percent increase in the water resistance of this board.

What is claimed is: 1. A coated paperboard comprising: i. a paperboard consisting of at least one ply consisting essentially of cellulosic fibrous materials, and ii. a coating on at least one surface thereof comprismg: a. to 99 percent by weight of a water-soluble lignosulphonate salt and b. from 1 to 5 percent by weight of a non-reactive hydrophobic waxy material having a melting point higher than the temperature of application of said coating to said paperboard said paperboard having a thickness of 0.004 to 0.050 inch and a basis weight of to 850 g/m said coating being present in an amount effective to improve the physical strength characteristics of said paperboard.

2. A coated paperboard in accordance with claim 1, wherein said non-reactive hydrophobic waxy material wax, low molecular weight polyethylene, hydrogenated castor oil and cetyl alcohol.

Av improvement Av improvement Treatment with Pick-up in MD Stiffness in CD Stiffness 5% dispersion 0.5 7.1 8.6 20% dispersion 2.0 34.8 62.5 40% dispersion 4.7 53.6 106.0

EXAMPLE 8 3. A coated paperboard comprising A paperboard having middles and backs prepared? from waste paper pulp, a single lining of unbleached Kraft fibre and a basis weight of 226 g/m was coated; 65

on the unlined surface with an aqueous dispersion containing 540 parts by weight calcium lignosulphonate,; 312 parts by weight starch and 15.4 parts by weight i. a paperboard consisting of at least one ply consisting essentially of cellulosic fibrous materials, and ii. a coating on at least one surface thereof consisting essentially of a. 47.5 to 95 percent by weight of a watersoluble,

lignosulphonate salt and b. from 1 to 5 percent by weight of a nonnonreactive hydrophobic waxy material having a melting point higher than the temperature of application of said coating to said paperboard, and

c. 4 to 47.5 percent by weight of a polysaccharide selected from the group consisting of corn starch, potato starch, pre-gelatinised starch, oxidised starch and esterified bifractional starch, starch ether and a mannogalactan gum,

said paperboard having a thickness of 0.004 to 0.050

castor oil and cetyl alcohol.

Claims

1. A COATED PAPERBOARD COMPRISING: I. A PAPERBOARD CONSISTING OF AT LEAST ONE PLY CONSISTING ESSENTIALLY OF CELLULOSIC FIBROUS MATERIALS, AND II. A COATING ON AT LEAST ONE SURFACE OF A WATER-SOLUBLE LIGNOUS A. 95 TO 99 PERCENT BY WEIGHT OF A WATER-SOLUBLE LIGNOSULPHONATE SALT AND B. FROM 1 TO 5 PERCENT BY WEIGHT OF A NON-REACTIVE HYDROPHOBIC WAXY MATERIAL HAVING A MELTING POINT HIGHER THAN THE TEMPERATURE OF APPLICATION OF SAID COATING TO SAID PAPERBOARD SAID PAPERBOARD HAVING A THICKNESS OF 0.004 TO 0.050 INCH AND A BASIS WEIGHT OF 130 TO 850 G/M2, SAID COATING BEING PRESENT IN AN AMOUNT EFFECTIVE TO IMPROVE THE PHYSICAL STRENGTH CHARACTERISTICS OF SAID PAPERBOARD.

2. A coated paperboard in accordance with claim 1, wherein said non-reactive hydrophobic waxy material is selected from the group consisting of beeswax, lanolin, carnauba wax, montan wax, paraffin hydrocarbon wax, low molecular weight polyethylene, hydrogenated castor oil and cetyl alcohol.

3. A coated paperboard comprising i. a paperboard consisting of at least one ply consisting essentially of cellulosic fibrous materials, and iI. a coating on at least one surface thereof consisting essentially of a. 47.5 to 95 percent by weight of a watersoluble, lignosulphonate salt and b. from 1 to 5 percent by weight of a nonnon-reactive hydrophobic waxy material having a melting point higher than the temperature of application of said coating to said paperboard, and c. 4 to 47.5 percent by weight of a polysaccharide selected from the group consisting of corn starch, potato starch, pre-gelatinised starch, oxidised starch and esterified bifractional starch, starch ether and a mannogalactan gum, said paperboard having a thickness of 0.004 to 0.050 inch and a basis weight of 130 to 850 g/m2, said coating being present in an amount effective to improve the physical strength characteristics of said paperboard.

4. A coated paperboard in accordance with claim 3, wherein said non-reactive hydrophobic waxy material is selected from the group consisting of beeswax, lanolin, carnauba wax, montan wax, paraffin hydrocarbon wax, low molecular weight polyethylene, hydrogenated castor oil and cetyl alcohol.