Patent Publication Number: US-7595115-B2

Title: Repulpable corrugated boxboard

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Ser. No. 10/351,324, filed Jan. 27, 2003, now U.S. Pat. No. 7,070,865, issued Jul. 4. 2006, which in turn is a continuation of U.S. Ser. No. 09/662,167, filed Sept. 14, 2000, now U.S. Pat. No. 6,541,556, issued Apr. 1. 2003, which in turn is a continuation of U.S. Ser. No. 09/032,851, filed Mar. 2, 1998, now U.S. Pat. No. 6,143,113, issued Nov. 7, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     i) Field of the Invention 
     This invention relates to a composition for forming a repulpable, water resistant, grease resistant, water vapor permeable coating on a porous paper sheet material; a repulpable coated paperboard liner for use in corrugated paperboard packaging manufacture, a method of producing a repulpable coated paperboard liner; a repulpable corrugated paperboard and a method of producing a repulpable corrugated paperboard. 
     ii) Description of Prior Art 
     Perishable food products such as frozen meat, fish and vegetables are shipped, stored and displayed for sale in packages formed of corrugated paperboard coated with a water resistant or repellant material, the most widely used coating being a wax coating. The wax coating renders the packages resistant or repellant to water and impermeable to water vapor. 
     Such packages, however, have a serious disadvantage in that they are essentially non-repulpable. Repulping refers to the art of recycling waste paper products including paperboard to provide a reusable wood pulp. Recyclability of waste paper products is of growing importance in efficiently employing available wood fibre resources and in curtailing the amount of waste which is either combusted or disposed of in landfill sites. 
     The problem in recycling wax-coated paperboard is well established and is described in, for example, U.S. Pat. Nos. 5,626,945 and 5,562,980 and PCT International Publication WO 96/22329. 
     An additional problem or manufacturing limitation with existing water repellent coatings, such as wax coatings, for corrugated paperboard packaging, is that the wax coating can only be applied to the finished or assembled corrugated paperboard. 
     Corrugated paperboard comprises an assembly of a pair of spaced apart linerboards with a corrugated medium sandwiched therebetween and adhered thereto. In particular, the tips of the flutes of the corrugated medium are adhered by an adhesive to the inwardly facing surfaces of the linerboards. The most widely used adhesives are aqueous starch-based adhesives which meet Government standards, e.g., FDA standards, for food packages. After assembly the adhesive is dried liberating water vapor which escapes through the porous uncoated linerboards. 
     If the wax coating were to be applied to the linerboards prior to assembly of the corrugated paperboard, the vapor impermeable wax coating would prevent the escape of the water vapor from the adhesive and the water vapor would be trapped and condense in the interior of the corrugated paperboard. 
     Consequently employing conventional water vapor impermeable coatings such as the wax coatings, necessitates complete assembly and formation of the corrugated paperboard before application of the vapor impermeable coating. This places significant restrictions on the manufacturing process. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a composition for forming a water-resistant, grease resistant, water vapor permeable coating on a porous paper sheet material. 
     It is a further object of this invention to provide such a composition having the quality that the thus coated porous paper sheet material is repulpable. 
     It is a further object of this invention to provide such a composition which forms such a coating which is also heat resistant. 
     It is still another object of this invention to provide a repulpable coated paper substrate for packaging, especially a coated paperboard liner for use in corrugated paperboard packaging manufacture. 
     It is yet another object of this invention to provide a method of producing the afore-mentioned repulpable coated paper substrate for packaging, especially a coated paperboard liner. 
     It is a further object of this invention to provide a repulpable corrugated paperboard. 
     It is a still further object of this invention to provide a method of producing the afore-mentioned repulpable corrugated paperboard. 
     In accordance with one aspect of the invention there is provided a composition for forming a water resistant, grease resistant, water vapor permeable coating on a porous paper sheet material, comprising: i) a styrene-acrylate copolymer; ii) a C 14 -C 18  fatty acid complex of a metal ion having an oxidation state of at least 3, and iii) an aqueous vehicle. 
     In accordance with another aspect of the invention there is provided a repulpable coated paperboard liner for use in corrugated paperboard packaging manufacture comprising: a) a paperboard sheet having opposed first and second surfaces, b) a water resistant, grease resistant, water vapor permeable coating on at least one of said surfaces, said coating comprising a styrene-acrylate copolymer and a C 14 -C 18  fatty acid complex of a metal ion having an oxidation state of at least 3. 
     In accordance with still another aspect of the invention there is provided a method of producing a repulpable coated paperboard liner for use in corrugated paperboard packaging manufacture comprising: a) providing a paperboard sheet having opposed first and second surfaces, b) coating at least one of said surfaces with a composition comprising: i) styrene-acrylate copolymer, ii) a C 14 -C 18  fatty acid complex of a metal ion having an oxidation state of at least 3, and iii) an aqueous vehicle, and c) drying said composition to form a repulpable, water resistant, grease resistant, water vapor permeable coated linerboard. 
     In accordance with yet another aspect of the invention there is provided a repulpable corrugated paperboard comprising: a) first and second coated paperboard liners and a corrugated medium therebetween, each of said liners having opposed inner and outer surfaces, and said corrugated medium being adhesively secured to said inner surfaces, b) at least one of said outer surfaces having a water resistant, grease resistant, water vapor permeable coating thereon, and c) said corrugated medium being water vapor impermeable. 
     In accordance with a further aspect of the invention there is provided a method of producing a repulpable corrugated paperboard comprising: a) providing paperboard sheet having opposed first and second surfaces, b) coating at least one of said surfaces with a coating composition which dries to form a water resistant, grease resistant, water vapor permeable coating, c) drying said composition to form a repulpable, water resistant, grease resistant, water vapor permeable coated paperboard liner, d) adhesively securing a water vapor impermeable corrugating medium between a pair of paperboard liners, at least one of said liners being a said coated paperboard liner, with a water-based adhesive, and e) curing said adhesive, allowing water vapor of said water-based adhesive to escape through said at least one liner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     i) Descriptions 
     Certain terms are employed herein to identify characteristics of the coating composition, the coated paperboard liner and the corrugated paperboard. These terms are believed to be clear in the context of the invention, to a person skilled in the art but are further elaborated here. 
     The terms “water repellent” and “water resistant” refer to the tendency of the coating to repel, block or, in any event, not transmit or absorb any significant quantity of liquid water in normal use. In other words, these terms identify a liquid water-blocking property of the coating sufficient for packing intended for perishable frozen foods. 
     The term “grease resistant” refers to the character of the coating in repelling, blocking or, in any event, not transmitting or absorbing any significant quantity of grease or oil. 
     The term “water vapor permeable” refers to the character of the coating in permitting passage therethrough of water in vapor form. 
     The term “water vapor impermeable” refers to the character of the corrugating medium in not permitting passage therethrough of water in vapor form. 
     The term “repulpable” refers to the character of the coated paperboard liner and the corrugated paperboard, whereby the paper fibre component of the paperboard liner and the corrugated paperboard can be readily recovered as a pulp suitable for use in paper product manufacture, the coating composition of the invention not presenting any significant obstacle to such pulp recovery. The term is to be viewed in the context that the conventional wax coated paperboard liners employed in corrugated paperboard packages for perishable frozen foods, are considered essentially non-repulpable based on a number of factors including the difficulty in separating the wax coated paper fibres and the contamination of any fibre pulp produced with wax particles which form stickies rendering the pulp unsuitable for paper manufacture. 
     As such the term “repulpable” contemplates absence or substantial absence of wax or comparable materials which would render the paperboard liner and corrugated paperboard non-repulpable. 
     The term “heat resistant” refers to the character of the coating in not becoming soft or tacky at the manufacturing temperatures encountered in the manufacture of corrugated paperboard, more especially at temperatures below about 200° C. 
     In the present invention the corrugated paperboard has the necessary characteristics for packaging of perishable frozen foods, including water-repellency, grease resistance and water vapor impermeability. The water repellency and grease resistance, but not the water vapor impermeability, are provided by the coated paperboard liners; the water vapor impermeability is provided by the corrugating medium. 
     The employment of the coating composition of the invention which provides a coating for the porous paperboard liners, which is water vapor permeable, permits considerable variation in the manufacture of the corrugated paperboard. 
     Thus the corrugated paperboard may be assembled employing pre-coated paperboard liners which are water vapor permeable so that post-coating of the manufactured corrugated paperboard is not required, thereby considerably simplifying the manufacturing process of the corrugated paperboard. 
     ii) Coating Composition 
     The coating composition of the invention forms a water resistant, grease resistant, water vapor permeable coating on a porous paper sheet material, such as paper linerboard employed in the manufacture of corrugated paperboard. 
     The coating composition has particular application for the coating of the paper linerboard of corrugated paperboard employed in packaging in which the paperboard is required to be water resistant or repellent and grease resistant, such as in packaging for perishable frozen foods. 
     a) Copolymer 
     The coating composition comprises a styrene-acrylate copolymer and a C 14 -C 18  fatty acid complex of a metal ion having an oxidation state of at least 3, in an aqueous vehicle. 
     The copolymer is, in particular, a copolymer of styrene and an alkyl acrylate in which the alkyl moiety has 1 to 6 carbon atoms. Butyl acrylate is especially preferred as the comonomer of styrene. 
     One particular commercially available source of the copolymer is the styrene/butyl acrylate copolymer dispersion available under the Trade-mark ACRONAL 296 D from BASF and which is described as a dispersion in water having a content of the copolymer of about 50% , (50%±1), a pH of 7.5 to 9, a viscosity at 23° C. (ISO 3219) of 300 to 650 mPas, and an apparent Brookfield viscosity at 23° C. of about 10,000 mPas. 
     The dispersion is further described as being anionic, having a density of about 1.04 g/cm 3 , and having an average particle size of particles of the copolymer of about 0.1 μm. 
     The dispersions of the styrene-acrylic copolymer may typically contain 40 to 60%, preferably about 50%, by weight of the copolymer. 
     b) Complex 
     The complex of the metal ion is in particular a complex of a metal having an oxidation state of at least 3, for example, an ion of iron, titanium, chromium or vanadium. The C 14 -C 18  fatty acid of the complex may be, for example, tetradecanoic acid also known as myristic acid; or octadecanoic acid also known as stearic acid. The complex may comprise a mixture of C 14 -C 18  fatty acids. 
     A commercially available class of complexes suitable in the invention is that available under the Trade-mark QUILON of DuPont Company which class comprises solutions of a chemically reactive complex in which a C 14 -C 18  fatty acid is coordinated with trivalent chromium. The vehicle of the solutions is primarily an alkanol, usually isopropanol. 
     Particular complexes include chromium pentahydroxy(tetradecanoato)di- available in solution under the Trade-marks QUILON C and C-9 and which are catalogued by CAS Registry No. 65229-24-5; tetradecanoato chromic chloride hydroxide available in solution under the Trade-mark QUILON M and catalogued by CAS Registry No. 15659-56-0; and octadecanoato chromic chloride hydroxide available under the Trade-mark QUILON S and catalogued by CAS Registry No. 15242-96-3. 
     These complexes are considered to have a structure of the following form 
     in which R is a fatty acid radical of 13 to 17 carbon atoms and R′ is the alkyl group of the alkanol vehicle, for example, isopropanol. 
     When the alcoholic solution of the complex is diluted with water, aquo groups replace the coordinated alcohol groups and some of the chlorine atoms. The chlorine atoms enter solution as chloride ions and the complex acquires a positive charge; the complexes may polymerize through hydroxyl bridges, as a result of hydrolysis or neutralization. 
     The different grades of QUILON are described by DuPont Company as having the following typical properties set forth in Table I 
     
       
         
           
               
             
               
                 TABLE I 
               
             
            
               
                   
               
               
                 QUILON chromium complexes 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 C 
                 C-9 
                 H 
                 L 
                 L-11 
                 M 
                 S 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Appearance 
                 blue- 
                 blue- 
                 dark- 
                 dark- 
                 blue- 
                 dark- 
                 dark- 
               
               
                   
                 green 
                 green 
                 green 
                 green 
                 green 
                 green 
                 green 
               
               
                   
                 liquid 
                 liquid 
                 liquid 
                 liquid 
                 liquid 
                 liquid 
                 liquid 
               
               
                 Odor 
                 alcoholic 
                 alcoholic 
                 alcoholic 
                 alcoholic 
                 alcoholic 
                 alcoholic 
                 alcoholic 
               
               
                 Chromium 
                 5.7 
                 9.2 
                 9.2 
                 9.2 
                 11.0 
                 5.7 
                 5.7 
               
               
                 as Cr. wt. % 
               
               
                 Chloride as 
                 7.8 
                 12.7 
                 12.6 
                 12.7 
                 15.2 
                 7.8 
                 7.8 
               
               
                 Cl, wt. % 
               
               
                 Fatty Acid 
                 11.8 
                 21.2 
                 19.0 
                 21.2 
                 25.2 
                 11.7 
                 14.8 
               
               
                 (C 14 -C 18 ) 
               
               
                 wt. % 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Boiling point, approx. 
                 ° C. 
                 82 
                 — 
                 82 
                 82 
                 — 
                 82 
                 82 
               
               
                   
                 ° F. 
                 180 
                 — 
                 180 
                 180 
                 — 
                 180 
                 180 
               
               
                 Freezing Point, 
                 ° C. 
                 −47 
                 — 
                 2 
                 4 
                 — 
                 −50 
                 −35 
               
               
                   
                 ° F. 
                 −53 
                 — 
                 36 
                 39 
                 — 
                 −58 
                 −31 
               
               
                 Flash Point (TOC) 
                 ° C. 
                 4 
                 — 
                 −3 
                 −2 
                 — 
                 1 
                 2 
               
               
                   
                 ° F. 
                 39 
                 — 
                 27 
                 29 
                 — 
                 34 
                 36 
               
               
                 Density at 
                 g/ml 
                 0.953 
                 1.050 
                 1.015 
                 1.025 
                 1.125 
                 0.922 
                 0.924 
               
               
                 20° C.(68° F.) 
                 (Mg/- 
               
               
                   
                 m 3 ) 
               
               
                   
                 lb/gal 
                 8.1 
                 8.7 
                 8.7 
                 8.6 
                 9.3 
                 7.8 
                 7.9 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Solubility in Water 
                 Complete 
                 Complete 
                 Complete 
                 Complete 
                 Complete 
                 Complete 
                 Complete 
               
               
                 Comodity 
                 indefinite 
                 negligible 
                 indefinite 
                 indefinite 
                 indefinite 
                 indefinite 
                 may 
               
               
                 stability at 
                   
                 sludging 
                   
                   
                   
                   
                 sludge 
               
               
                 storage 
                   
                   
                   
                   
                   
                   
                 after 5 
               
               
                 temps. 
                   
                   
                   
                   
                   
                   
                 months 
               
               
                 below 32° C.(90° F.) 
               
               
                 and above freezing 
               
               
                 point 
               
               
                   
               
            
           
         
       
     
     Especially good results were achieved with QUILON C which also provides coating compositions displaying good storage stability. 
     The complex of the invention is more especially a so-called Werner complex. 
     The alcoholic solution of the complex may suitably contain about 3 to about 15%, preferably about 4 to about 12%, more preferably about 5 to about 10%, by weight of the metal ion, especially chromium and about 8 to about 28%, preferably about 10 to about 25%, by weight of fatty acid; and may typically contain chloride in an amount of about 5 to about 20%, preferably about 7 to about 15%, by weight. 
     c) Other Components 
     The aqueous vehicle of the coating composition comprises the water of the aqueous dispersion of the copolymer and the alkanol of the solution of the complex. 
     The coating composition may additionally comprise an inert particulate filler, for example, clay and may contain water in addition to that derived from the aqueous dispersion of the copolymer, which in this Specification is referred to as “additional water”. The additional water is employed to lower the viscosity, if desired. 
     In general, the coating composition may contain in weight % to a total of 100%: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Aqueous dispersion of styrene- 
                 35 to 99.5% 
               
               
                   
                 acrylate copolymer 
               
               
                   
                 Alcoholic solution of complex 
                 0.5 to 5%    
               
               
                   
                 Particulate filler 
                 0 to 35%  
               
               
                   
                 Additional water 
                 0 to 25%. 
               
               
                   
                   
               
            
           
         
       
     
     Amounts of the alcoholic solution above 5% result in loss of stability and shorten the shelf life of the coating composition. 
     The particulate filler which is suitably a clay filler such as that available under the Trade-mark Omnifil provides heat resistance in the coating. 
     Such heat resistance is advantageous to the extent that heat is employed in the manufacture of the corrugated paperboard, for example, in the adhesion of the coated liner board to the corrugated medium, where the coating on the coated linerboard may be in direct contact with a heated pressure roll or platens which pressure roll or platens may typically be at a temperature of about 177° C. 
     Preferably the filler is present in an amount of 10 to 15%, by weight. Amounts of the filler above 35%, by weight, result in deterioration in the water and grease resistance. 
     The coating composition applied to porous paper sheet material, such as linerboard for use in corrugated paperboard manufacture, forms a water resistant, grease resistant coating which is permeable to water vapor, and the coated paper sheet material is pulpable, the coating separating readily from the paper sheet material during repulping. 
     The coating composition is applied in a dry coating weight of 5 g/m 2  to 25 g/m 2 . Typically the lower coating weights will be employed for smooth surface paper substrates and higher coating weights for more porous surface paper substrates. 
     It will be understood that the coating composition of the invention is a wax-free composition, although small amounts of wax might be tolerated to the extent that the coated linerboard still retains its repulpability and the coating is water vapor permeable. 
     iii) Water Impermeable Coating 
     The coating composition of the invention is particularly employed for coating linerboard for use in corrugated paperboard manufacture. 
     In such case the corrugated medium of the invention is coated to produce a water vapor impermeable barrier to provide the required water vapor impermeability in the corrugated paperboard. 
     The coating employed should be one which renders the corrugated medium repulpable so that the repulpable coated corrugated medium in conjunction with the repulpable coated linerboard is repulpable. 
     The coating providing water-vapor impermeability may be, for example, the coating described in WO 96/22329, published Jul. 25, 1996, S. Berube, the teachings of which are incorporated herein by reference. In general the afore-mentioned publication describes a moisture vapor barrier coating for a paper substrate, which produces a repulpable coated paper substrate, the moisture vapor barrier coating is more especially based on polyvinylidene chloride, although other polymer materials are also described. The polymer is employed in emulsion from and may additionally contain an additive such as hydrated aluminum silicate, calcium carbonate or vinyl acetate homopolymer. 
     Another preferred moisture vapor barrier containing composition for forming the water-vapor impermeable coating comprises a combination of polyvinylidene chloride, styrene-butadiene copolymer and an acrylic polymer. This composition is employed as a dispersion of the three polymers in an aqueous vehicle and may be applied between a pair of paper substrates which are then laminated together, the composition forming a water-vapor impermeable coating or layer therebetween. This laminate is then corrugated to form the corrugated medium. 
     Alternatively the composition may be applied to one or both sides of a paper substrate to form a coated substrate which is then corrugated to form the corrugated medium. It is also possible to apply the composition as a coating to a preformed corrugated medium. 
     The water impermeable coating is wax-free, although small amounts of wax might be tolerated to the extent that the resulting water impermeable corrugated medium retains its repulpability. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic representation in cross-section of corrugated paperboard of the invention; and 
         FIG. 2  illustrates schematically the production of linerboard in accordance with the invention and assembly of corrugated paperboard. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS WITH REFERENCE TO THE DRAWINGS 
     With further reference to  FIG. 1 , a corrugated paperboard  10  of the invention comprises coated linerboard  12  and coated linerboard  14  with laminated corrugated medium  16  therebetween. 
     Coated linerboard  12  comprises linerboard  18  having a coating  20  thereon. Coating  20  is water-resistant, grease resistant and water vapor permeable. 
     Coated linerboard  14  comprises linerboard  22  and coating  24  of the same character as coating  20 . 
     Laminated corrugated medium  16  comprises paper layers  26  and  28  with a vapor barrier  30  therebetween. Vapor barrier  30  is water vapor impermeable. 
     The flute tips  32  of laminated corrugated medium  16 , are adhered to the inner faces  34  and  36  of linerboards  18  and  22  respectively by adhesive  38  which, in particular, is derived from an aqueous starch-based adhesive composition. 
     The vapor barrier  30  renders the corrugated paperboard  10  water vapor impermeable and the coatings  20  and  24  render the corrugated paperboard water resistant and grease resistant. Coatings  20  and  24  separate readily from linerboards  18  and  22  respectively during repulping to enter solution or form fine fragments that are readily separated from the pulp fibers of the linerboard. Similarly, vapor barrier  30  readily separates from the paper layers  26  and  28  of corrugated medium  16  during repulping to form fragments which are readily separated from the pulp fiber of the paper layers  26  and  28 . In this way the corrugated paperboard  10  is repulpable. 
     With further reference to  FIG. 2 , there is shown schematically corrugated paperboard apparatus  40  including corrugating coater assembly  42 , corrugator  44 , liner coating assembly  46 , liner coating assembly  48  and board fabricators  50  and  54 . 
     Paper sheet  26  is fed from supply roll  52  to corrugating coater assembly  42 . The corrugating coater assembly  42  includes rolls  56  and  72 , a coating roll  58  mounted for rotation in a coating bath  60  containing coating composition  62  to form a vapor barrier  30 . 
     In  FIG. 2  there is further shown a second supply roll  76  of paper sheet  28  and roll  80 . There could, in addition, be included a second full corrugating coating assembly  42  to provide a coating of vapor barrier  30  to paper sheet  28 . 
     Corrugator  44  includes corrugating rolls  84  and  86  both of which have corrugating teeth  85 , and feed roll  82 . 
     Liner coating assembly  46  includes a coating bath  94  and a coating roll  96 . Coating bath  94  contains coating composition  98  and coating roll  96  is mounted for rotation so that a lower portion thereof rotates through coating composition  98 . 
     Liner coating assembly  46  further includes rolls  100 ,  110  and  112 , a pair of guide rolls  108  and drying unit  106 . 
     A supply roll  90  feeds linerboard  18  to the nip between roll  100  and coating roll  96 . 
     Liner coating assembly  48  is of the same form as liner coating assembly  46  and the same parts are shown with the same integers raised by 100 so that no further description of liner coating assembly  48  is required. However, assembly  48  additionally includes guide rolls  202  in the feed from supply roll  190  to coating bath  194 . 
     Board fabricators  50  and  54  include adhesive applicators  51  and  53 , respectively. 
     Applicator  51  includes a bath  55  of an aqueous starch adhesive  57  and applicator rolls  59 ,  61 . Applicator  53  includes a bath  63  of the aqueous starch adhesive  65  and applicator rolls  67 ,  69 . 
     Fabricator  50  further includes pressure roll  71  which engages corrugating roll  84  of corrugattor  44 ; and fabricator  54  includes contact roll  75  and rider roll  77 . 
     Apparatus  40  further includes heater platens  124 , and feed roll  213 . 
     The apparatus  40  in  FIG. 2  may be employed to produce the corrugated paperboard  10  in  FIG. 1 . 
     In operation of the apparatus  40 , paper sheet  26  is fed from supply roll  52  over rolls  56  and  72  and between a nip formed between coating roll  58  and roll  64 . Coating roll  58  rotates through coating composition  62  in bath  60  and picks up coating composition  62  which is applied to paper sheet  26  as it passes between coating roll  58  and roll  64  to provide a wet coating of vapor barrier  30 . The thus wet coated sheet  29  is fed to corrugator  44 . 
     A separate paper sheet  28  is fed from supply roll  76  over roll  80  and feed roll  82  to corrugator  44 . In corrugator  44  wet coated sheet  29  and sheet  28  are laminated together with the vapor barrier  30  formed from coating composition  62  therebetween. 
     In corrugator  44  the coated sheet  29  and sheet  28  pass between the rotating corrugating rolls  84  and  86  where they are engaged by the teeth  85 , under pressure and at a temperature of 175° C. to 195° C. to form the corrugated medium  16  (see  FIG. 1 ). 
     At adhesive applicator assembly  50  an aqueous starch-based adhesive  57  is applied to the tips  32  (see  FIG. 1 ) of the flutes on one side of corrugating medium  16 . 
     This adhesive  57  is applied from roll  61  which picks up adhesive  57  from roll  59  a lower portion of which rotates through the bath  55  of adhesive  57 . The adhesive coated tips  32  are pressed into engagement with the uncoated face of coated linerboard  12 , the production of which is described below, between the pressure roll  71  and corrugating roll  84  to form a partial board  15 . The pressure roll  71  and the corrugating roll  84  are suitably both at a temperature of 150 to 190° C., preferably about 177° C., pressure roll  71  being in direct contact with coated linerboard  12 . 
     The partial board  15  is fed to fabricator  54  where adhesive  65  is applied to the tips  32  (see  FIG. 1 ) of flutes on the remaining, exposed side of corrugating medium  16 , in the same way as at fabricator  50 , and the adhesive coated tips  32  are pressed into engagement with the uncoated face of coated linerboard  14 , the production of which is described below, between the contact roll  75  and rider roll  77  to form precursor  17  of corrugated paperboard  10 . 
     At liner coating assembly  46 , linerboard  18  is fed from a supply roll  90  to the nip between coating roll  96  and roll  100 . Coating roll  96  rotates through the coating composition  98  in coating bath  94  and picks up coating composition which is applied to linerboard  18  at the afore-mentioned nip between rolls  96  and  100 . The thus coated linerboard  18  is fed through drying unit  106  where the coating composition is dried to form coated linerboard  12 . In the same manner, coated linerboard  14  is formed from linerboard  22  in liner coating assembly  48 . 
     Coated linerboard  12  is guided from drying unit  106  by guide rolls  108  and feed rolls  110  and  112  to board fabricator  50 . Coated linerboard  14  is fed in similar fashion to board fabricator  54 . 
     In precursor  17  the corrugated medium  16  is sandwiched between coated linerboards  12  and  14  and is adhered to the inner non-coated faces thereof by the aqueous starch-based adhesive. Precursor  17  is fed over roll  213  to heated platens  124  where water in the aqueous starch-based adhesive is vaporized and escapes from the percursor  17  through the coated linerboards  12  and  14  which comprise the porous linerboards  18  and  22  respectively and the vapor permeable coatings  20  and  24  thereon derived from coating compositions  98  and  198  respectively. 
     While  FIG. 2  shows apparatus  40 , composed of the different elements aligned and arranged for continuous manufacture of the corrugated paperboard, it will be understood that the operations illustrated could, in part, be carried out independently and at different sites. Thus the coated linerboards  12  and  14  could be produced at one site and the corrugated medium  16  could be produced at the same or a different site, whereafter the coated linerboards  12  and  14  and corrugated medium  16  could be shipped to yet another site for application of the adhesive to the corrugated medium  16  and fabrication of the corrugated paperboard  10  from the coated linerboards  12  and  14  and the corrugated medium  16  with the applied adhesive. 
     Thus in contrast to prior procedures the coatings on the linerboard which provide water resistance and grease resistance are applied to the linerboard prior to assembly of the linerboard with the corrugating medium to form the corrugated paperboard, and this facilitates fabrication of the different components of the corrugated board at different sites by different specialists. The corrugated paperboard is completed at the site of the board fabricator and the board fabricator does not need to apply coatings to the corrugated paperboard to provide the water and grease resistance. 
     It will be understood that arrangements other than that illustrated in  FIG. 2  are within the scope of the invention. Thus, as already indicated above, paper sheet  28  may likewise be coated with coating composition  62 . Similarly, while as illustrated only the faces of linerboards  18  and  22  which form the outer faces of the final corrugated paperboard  10 , are coated with coating compositions  18  and  22 , it is within the scope of the invention to coat both faces of linerboards  92  and  192  with the coating compositions to further enhance the water resistance and grease resistance. 
     The heated platens  124  conveniently operate at a temperature of about 150 to 180° C. to drive off the water of the aqueous starch-based adhesive. 
     The drying units  106  and  206  conveniently operate at temperatures of about 100 to 200° C. 
     EXAMPLES 
     Example 1 
     A coating composition for linerboard was produced by adding 57.3 kg of ACRONAL 296 D, an aqueous dispersion available from BASF, of a styrene-butyl acrylate copolymer to a tank. A premix was formed of 1.15 kg of QUILON C, described previously, and 34.38 kg of water (additional water), and the premix was added slowly to the aqueous dispersion of the copolymer in the tank, with agitation. A clay filler available under the Trade-mark OMNIFIL was added to the tank under agitation. The resulting liquid coating composition had a pH of 5 to 7 and a viscosity (Brookfield RVT Spindle #2 at 20 rpm) of 50 to 200 cps. 
     The coating composition was coated on paper linerboard by a conventional coater at a dry coating weight of 10 g/m 2 . 
     Example 2 
     The procedure of Example 1 was followed but employing the following components only: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 ACRONAL 296 D 
                  95 kg 
               
               
                   
                 QUILON C 
                 1.5 kg 
               
               
                   
                 Water (additional water) 
                  3.5 kg. 
               
               
                   
                   
               
            
           
         
       
     
     The resulting coating composition had a pH of 5 to 7 and a viscosity (Spindle #2 at 20 rpm) to 200 to 500 cps. 
     The composition was coated on paperboard liner by a conventional coater at a dry coating weight of 15 g/m 2 . 
     Example 3 
     A repulpable corrugated medium was produced employing a vapor barrier composition containing the following in wt. %: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Dispersion of PVDC 
                 83.5% 
               
               
                   
                 SBR 65 
                 14.8% 
               
               
                   
                 STEROCOLL FD 
                 0.5% 
               
               
                   
                 Ammonia 
                 0.2%. 
               
               
                   
                   
               
            
           
         
       
     
     The PVDC (polyvinylidene chloride) dispersion was a commercially available dispersion, marketed under the Trade-mark SERFENE 2022; SBR is a styrene-butadiene copolymer available from BASF and STEROCOLL FD (Trade-mark of BASF for an acrylic acid polymer). 
     The composition was applied to a paper substrate precursor of the corrugated medium by a conventional coater or laminator at a dry coating weight of 15 g/m 2 , and the resulting coated substrate was laminated to an uncoated paper substrate between corrugating rolls at about 175° C. 
     The flute tips of the resulting corrugated medium were coated with an aqueous starch-based adhesive in conventional manner and the coated linerboard of Example 1 was adhered, under pressure, to opposed sides of the corrugated medium at the flute tips. The resulting corrugated paperboard assembly was dried on heated platens at about 150° C. and water vapor could be observed escaping from the corrugated paperboard. 
     Example 4 
     Following the procedure of Example 1 two coating compositions A and B for linerboard were produced and Domtar #26 paper linerboard was coated with different coating rods to produce different coating thicknesses. 
     The characteristics of the resulting coated linerboards was measured. The cobb test T441 is an evaluation of water resistance and the turpentine test T454 is an evaluation of grease or oil resistance. 
     The characteristics and formulae are set out below in Tables II, III, IV and V. 
     
       
         
           
               
             
               
                 TABLE II 
               
             
            
               
                   
               
               
                 Coating Composition A Characteristics 
               
            
           
           
               
               
               
               
               
            
               
                 film 
                   
                   
                   
                   
               
               
                 properties 
                 rod 10 
                 rod 15 
                 rod 20 
                 rod 25 
               
               
                   
               
               
                 coating 
                 8.5 
                 13.0 
                 17.0 
                 21.0 
               
               
                 weight g/m 2   
               
               
                 cobb test 
                 10 g/m 2   
                 10 g/m 2   
                  0 g/m 2   
                  0 g/m 2   
               
               
                 T441 30 min 
               
               
                 cobb test 
                 — 
                 10 g/m 2   
                 10 g/m 2   
                 10 g/m 2   
               
               
                 T441 60 min 
               
               
                 turpentine 
                 30 minutes 
                 &gt;3 days 
                 &gt;3 days 
                 &gt;3 days 
               
               
                 test T454 
               
               
                 cobb test 
                 — 
                 — 
                  0 g/m 2   
                  0 g/m 2   
               
               
                 T441 30 min 
               
               
                 creased 
               
               
                 turpentine 
                 — 
                 — 
                  1 day 
                 — 
               
               
                 test T454 
               
               
                 creased 
               
               
                 heat 
                 — 
                 on metal: 
                 on metal: 
                 — 
               
               
                 resistance 
                   
                 200° C. 
                 200° C. 
               
               
                 (maximum 
                   
                 on liner: 
                 on liner: 
               
               
                 temperature) 
                   
                 190° C. 
                 190° C. 
               
               
                 10 psi, 120 sec. 
               
               
                 repulpability 
                 — 
                 &lt;= 1/16″ 
                 &lt;= 1/16″ 
                 &lt;= 1/162 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE III 
               
             
            
               
                   
               
               
                 Coating Composition A 
               
            
           
           
               
               
               
            
               
                   
                 Ingredient 
                 % by weight 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Acronal 296D (BASF) 
                 53.48 
               
               
                   
                 Quilon C (Dupont de Neymour) 
                 1.07 
               
               
                   
                 Omnifil (L. V. Lomas) 
                 13.37 
               
               
                   
                 Water 
                 32.08 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE IV 
               
             
            
               
                   
               
               
                 Coating Composition B 
               
            
           
           
               
               
               
            
               
                   
                 Ingredient 
                 % by weight 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Acronal 296D (BASF) 
                 57.31 
               
               
                   
                 Quilon C (Dupont de Neymour) 
                 1.15 
               
               
                   
                 Omnifil (L. V. Lomas) 
                 7.16 
               
               
                   
                 Water 
                 34.38 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE V 
               
             
            
               
                   
               
               
                 Comparison of Coating Composition 
               
            
           
           
               
               
               
            
               
                   
                 A 
                 B 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Solids Content % wt 
                 36.1% 
                 33.7% 
               
               
                   
                 pH 
                 6.2 
                 6.2 
               
               
                   
                 Viscosity cps 
                 100 
                 100 
               
               
                   
                 Density 
                 1.16 g/cm 3   
                 1.14 g/cm 3   
               
               
                   
                   
               
            
           
         
       
     
     The coated linerboard displayed good transmission of water vapor employing the a PERMATRAN-WIA water vapor transmission rate testing instrument from Mocon of Minneapolis. 
     Example 5 
     The procedure of Example 4 for composition B was repeated with fresh materials but employing only a #25 rod for a coating weight of 17 g/m 2  dry, the coated linerboard was dried at 200° C. for 1 minute. In the Cobb test and the turpentine test the coated surface of the coated linerboard of one sample was first heated at 190° C. for 60 seconds at 10 psi. The results showed no difference in the Cobb test and the turpentine test between the heated and unheated sample. The results are shown in Table VI below: 
     
       
         
           
               
               
               
             
               
                   
                 TABLE VI 
               
               
                   
                   
               
               
                   
                 film properties 
                 results 
               
               
                   
                   
               
             
            
               
                   
                 Cobb test T441, 60 minutes 
                 0 g/m 2   
               
               
                   
                 unheated sample 
               
               
                   
                 Cobb test T441, 60 minutes 
                 0 g/m 2   
               
               
                   
                 coated side heated at 190° C., 
               
               
                   
                 10 psi for 60 sec. 
               
               
                   
                 turpentine test T454 
                 &gt;3 days 
               
               
                   
                 unheated sample 
               
               
                   
                 turpentine test T454 
                 &gt;3 days 
               
               
                   
                 coating side heated at 190° C. 
               
               
                   
                 10 psi for 60 sec. 
               
               
                   
                 heat resistance maximum dwell time 
                 on liner: 120 seconds 
               
               
                   
                 190° C., 10 psi, one hot plate 
                 on metal &gt;300 seconds 
               
               
                   
                 heat resistance maximum dwell time 
                 on liner: 60 seconds 
               
               
                   
                 190° C., 10 psi, two hot plates 
                 on metal &gt;300 seconds 
               
               
                   
                 repulpability 
                 excellent 
               
               
                   
                   
               
            
           
         
       
     
     The test results showed that the fresh composition B performed as well as previously, in Example 4. Moreover, the coated surface properties are not affected by heat. The coating composition produces a coating which is highly flexible and heat resistant and the coated linerboard is repulpable.