US 3189502 A
Description (OCR text may contain errors)
. June 1-5, 1965 Y J. R. LITTLE 3,139,502
METHOD OF MAKING IMPREGNATED CORRUGATED PAPERBOARD V SHEETS.ON A CORRUGATQR MACHINE Filed March 3, 1961 DOUBLE BACKER CORRUGATOR INVENTOR. JOHN R. LITTLE ATTORNEY United States Patent 3,189,502 METHODOF MAKING IMPREGNATED GORRU- GATED PAPERBOARD SHEETS ON A CORRU-' This invention relates to the continuous in-line manufacture on a corrugator machine of double-face corrugated paperboard sheets or box blanks which are given increased moisture resistance by impregnation of a wax or modified wax penetrant. In some important aspects the invention is an improvement in the subject matter of my copending US. patent application, Serial No. 785,408, filed January 7, 1959, now Patent 2,982,333.
In my aforesaid copending application a process is disclosed for in-line manufacture of a moisture-resistant double-face corrugated paperboard web on a corrugator machine by applying a wax penetrant to the corrugated me dium of the single-face board over a wax resist adhesive which had been previously applied to thecrests of the flutes of the corrugated medium. Excellent adhesion of the second or double-face liner to the flutes of the corrugated medium is achieved by following my teaching as compared to the unacceptable results that occur when the wax penetrant is applied before application of the adhesive. The resultant corrugated paperboard web of my invention has a highly desirable concentration of wax in the corrugated medium inner ply in contrast with prior moisture-resistant corrugated paperboards made by dipping in wax or otherwise coating with wax from the outside in less efficient and more costly batch operations.
I have now discovered improvements to the process of my previous invention that permit trimmed sheets and box blanks, evenly impregnated throughout their surface areas, their thicknesses and widths to be produced in an in-line operation on the corrugator machine.
I have also discovered a method of applying on the corrugating line a less expensive and improved rosinmodified wax penetrant to the liners of a corrugated paperboard web, particularly the double-face liner where the rosin constituent would seriously impair the bonding by waterproof starch adhesive of the double-face liner to the single-face board if conventional practices of combining these elements were followed.
In the practice of the invention according to my aforesaid copending application the production of commercially acceptable impregnated sheets and box blanks'on the corrugator machine was not practicable because "of the failure of the wax penetrant to migrate evenly throughout the surface areas of the corrugated paperboard web andparticularly to the edges thereof. Attempts to apply larger quantities of the penetrant to the corrugated medium resulted in a spilling out of the penetrant from the side of the corrugated board and caused unacceptable deposits of the penetrant to be built up on the conveyor section of the conventional corrugator machine. Accordingly, for the production of impregnated sheets and box blanks a post-corrugator operation was used in which'the before the combining step on the double backer.
7 Ice -out-to,the edges wasv inhibited and trimming of the edges in excess of the normal trim loss in corrugated boa'rd op: erations was required.
I have overcome the problem of uneven and incomplete migration that necessitated the post-corrugator maturing step by applying wax penetrant to the liner webs, in addition to the corrugated medium as in my previous teaching; In this connection, it was surprising to find that the double-face liner web will adhere in an entirely satisfactory manner to the flutes of the wax impregnated corrugated medium of the single-face board when the wax penetrant is applied to the outer face of the double-face liner web and strikes through to the inner face thereof before that face contacts the adhesive on the flutes or even when the wax penetrant is applied directly to the inner face of the double-face liner web. As previously mentioned, if a wax penetrant is applied to the corrugated medium before the adhesive, the wax somehow prevents the adhesive from achieving an acceptable bond to the double-face liner web, but I have now discovered that the adhesive can achieve a fully acceptable bond with a different web, i.e., a liner web, even though wax is previously applied thereto.
lnan important and presently preferred aspect, my 7 present invention contemplates utilization of rosin-modified waxes so as not only to save penetrant costs, but also to impart certain desirable characteristics to the end product while at the same time avoiding mechanical interference with adhesion between the corrugated medium and the liners of the board. plated a differentiation in the impregant mixtures employed for application to the liners (or at least the doubleface liner) on the one hand and for application to the corrugated medium on the other hand. In the case of the rosin modified waxes, I have discovered that it is of great importance in order to achieve a successful board combination using conventional waterproof starch adhesives that the modified wax penetrant be applied to the exterior or outer face of the double-face liner web at such time during the combining-operation that contact between the adhesive covered crests of the single-face board be made with the double-face liner before the modified wax penetrant strikes through into the inner face of the double-face liner and reaches the interface between the crests of the corrugated medium and the double-face liner.
Corrugated paperboard made according to the presently preferred aspects of my invention shows substantial improvements in strength and service characteristics as compared with the best of prior continuous impregnating proposals. More particularly, compression strength is improved under standard, high humidity, and 24-hourwet conditions. Mullen tests are also substantially improved as are wet puncture properties. Improvement in the latter respect is particularly striking.
Thus in its broad aspects my invention makes it feasible for the first time to manufacture in a continuous process moisture-resistant, fully trimmed sheets, or scored and trimmed box blanks, of corrugated paperboard in which a penetrant is distributed throughout the thickness and width of the board as it comes off the corrugating line, and preferably to utilize in the process arosin modified w ax' penetrant together with: a conventional waterproof starch adhesive to.pr 0duce less expensive and more improved ,mois ture-resistant corrugated paperboard sheets and box blanks than herefore attainable.
Patented June 15, 1 965 In particular, there is contem- 1 Reference is. made ,to the accompanying drawing "in" 3 gating rolls 11 and 12; an adhesive mixture 13 is applied to the 'crests formed on the'web 10, and the first or singleface liner 14 is combined with the corrugating medium or web in the conventional manner. In the particular apparatus illustrated, the adhesive 13 is picked up by the adhesive roll 15 which applies the adhesive to the corrugated sheet. A doctor roll 16 is adjustable in relation to the adhesive roll 15 to suitably adjust the amount of adhesive being applied.
The resulting single-face combination is carried acrossa suitable span 17 which may be of considerable length According to the present invention several of the four I faces of the various paper components fed into the double backer (at which is accomplished the combining operation which results in the double-face paperboard) may be subjected to the application of adhesive and/or impregnant in a manner to be described specifically below. In this connection it will be understood that the single-face corrugated paperboard web 17 comprises two faces including the face 18, identified with the single-face liner 14, and the face 19, identified with the exposed side of the corrugated medium. The double-face liner 28 also, of course, comprises two faces including the face 31, which is to become the outer face of the double-face liner when it is combined with the single-face web, and the opposed face 30 which is to become the inner face of the double-face liner, which inner face is positioned against the crests or the fiutes of the corrugating medium. According to the present invention, penetrant may be applied at the faces 18, 19 of the single-face combination and preferably to the outer face 31 of the double-face liner, and when the penetrant is paratfin, it may be applied to face 30 of liner 28. This operation comprises a roller coating opertion. Suitable doctor bars or scraper 34 may be employed -to control the amount of wax penetrant applied to web 28. Adhesive is applied to theexposed crests of the corrugated medium as described below.
As shown in the drawing, the penetrant may be applied to'the fiat face 18 of the single-face corrugated paperboard web by a. roller 21, this operation comprising a roller coating operation. Penetrant is applied to the face 31 of the double-face liner by the roller coater 33. Suitable doctor bars or scrapers 22 and 34 may be employed to control the amount of penetrant applied to the respective web faces.
' adhesive applying operation, but no substantial pressure is applied at this point in order toavoid crushing the corrugations. The adhesive applying step may be in itself substantially identical to, the corresponding adhesive apply- .-ing step performed in conventional double backer machines.
The application of penetrant to the corrugated medium is preferably accomplished by spraying or projecting it as indicated at the spray head 27 shown in the drawing, although the invention also contemplates the use of dipping or other applying operations.
The application to thefaces 18 and 31is preferably 4 accomplished by roller coating as with the rolls 21 and 33 although the invention in certain broadaspects contemplates spraying or other applying operations at these points. However, the operation should be such that a high degree of uniformity of penetrant application occurs f across the width of the web.
The'penetrant which is employed may be a paperpenetrant moisture-resistance-enhancing wax. Paraflin or paraffin type waxes or petroleum waxes maybe employed, among others. The wax may be amorphous, crystalline, or micro-crystalline or combinations thereof, with or without additional modifiers or additives.
In preferred aspects of the invention, it is contemplated that the penetrant applied by the rolls 21 and 33 will be a rosin-modified wax, that is, a wax including a substantial proportion of rosins, such as'tall oil rosins combined with paraffin or the like. The rosin modifier may comprise a majority of the mixture. A preferred rosin-modified wax may comprise a mixture of three parts rosin to one part paratlin where the rosin comprises a modified tall oil rosin produced from whole tall oil in the separation of fatty acids from the rosin by fractional distillation and is composed of about 40% rosin acids, about 22% unsaponifiables (primarily decorboxylated rosin products), about 36% rosin acid products having reacted or otherwise tied up carboxyl groups (probably comprising rosin acid, esters, lactones and ,anhydrides), and about 2% free fatty acids.
Other rosin derivatives may be employed where substantially all fatty acids have not been separated from the rosin and which comprise -90% rosin acids, with the balance being free fatty acids and minor amounts of unsaponifiables. say about 2%. The preferred range of rosin acids in these tall oil rosin derivatives is from about to I As used herein the term rosin-modified wax is intend ed to include micro-crystalline or parafiin waxes or the like combined with the rosin or rosin derivativesas mentioned above and with other appropriate rosins or rosin derivatives such as for example those mentionedin US. Patent No. 2,443,221 to Bergstein. However it is to be noted that the Bergstein procedures are to be distinguished from the present invention inasmuch as Berg stein contemplates the application of a penetrant to paper which is so cold as to be merely at room temperature whereby striking into the paperboard is minimized (Bergstein, col. 4, line 69). Also Bergstein contemplates the use of at the most minor amounts of rosins (Bergstein,
col. 5, line 40) as distinguished from the contemplation of the present invention where a majority of the rosinmodified penetrant may be comprised of the rosinous material.
The penetrant such as parafiin applied by the spray head 27 should melt within a sufiiceintlylow range or at a sufliciently low point to avoid premature actuation of the adhesive mixture. That. is, the melting point of the penetrant should not be greatly in excess of the temperature at which the adhesive mixture sets or gels. A typical gelling temperature for a waterproof starch ad hesive is 145 F. A penetrant melting temperature of about 140 F. is preferred in this instance, although temperatures as high as 200 F. or more have been successfully used withwaterproof starch adhesives when the amount of penetrant applied per given weight of paper is relatively low, say about 13 pounds per 1000 square feet of corrugated ply 10.
The melting point ormelting range of the several pcnetrants should exceed the environmental temperatures to which the final product will be subjected. Temperatures of F. are very minimal. Minimum temperatures F. or F. are more desirable lower limits.
The invention may be employed on corrugating lines running at normal production rates of about to 300 or 400 feet per minute where the liner web 17 is subjected to adhesive-setting heat at the double backer (heated to about 325 F.) alonga length of 30 or 40 feet or more, as along the hot plate 36. All customarilj used weights of' webs may be employed including, for example, ,co'r'rugating medium of 26 pounds per 1000 square feet'(bef ,ore corrugation)- and liners of '26, 33, 42, 4 7, 69 or 90- pounds per 1000 square feet.
for a corrugated medium or ply weighing about; 39 poundsper 1000 square feet (26 pounds per 1000 square feet prior to corrugation) there may be applied a total of about 10 to 50 pounds of penetrant per 1000 square feet. A preferred range presently appears to be about to pounds of penetrant per 1000 square feet. For" special applications the range may vary from 5 to 80 pounds of penetrant per 1000 square feet. Of the total amounts applied, about from 3 to 12 pounds of penetrant per 1000 square feet may be applied to the face 18 and from 3 to 12 pounds of penetrant per 1000 square feet may be applied to the face 31.
It will be understood that penetrant may be applied according to the invention only at selected areas of the paperboard being manufactured. In this connection the sprayer or coater at the penetrant applying station associated with the corrugated medium may be activated and deactivated as desired without interfering with the remainder of the manufacturing process merely by turning the spray on and otf, or by shielding a portion of the width of the corrugated medium from the spray. The application of penetrant to the faces 18 and 31 may be similarly activated and deactivated as desired without interfering with the remainder of the manufacturing process, as by placing the rolls 21 and 33 into and out of operative contact with the faces 18 and 31 or by moving them laterally to contact only a portion of the width of the faces. Thus, changeovers back and forth between the manufacture of paperboard including the moisture resistant penetrant and untreated paperboard may be effected without shutting down the corrugating line.
The adhesive mixture applied by the roll 26 at the double backer may or may not be the same as the adhesive mixture applied by the roll 15 immediately following the corrugating step, but in any event will consist of a heat-activatable, free-water-containing adhesive mixture having sufficient body to stand up momentarily on the tips of the corrugation until the liner or facing is pressed on into position. This necessary property of body is well recognized in the art. See, for example. Technology of Adhesives by John Delmonte, Reinhold Publishing Corp., 1947, p. 454. Reference is also made to the starch, including waterproof starch, silicate and silir cate clay adhesive mixtures for corrugated paperboard discussed in Technology of Adhesives," supra. All of these adhesive mixtures comprise a bodied unset heatactivatablc free-water-containing adhesive mixture for application to the corrugated web of the paperboard and, as such, all of them constitute resists to penetrants employed according to the present disclosure.
It ,is presently much preferred, particularly from the standpoint of performance under wet conditions, to employ a waterproof adhesive at the juncture of the corrugated medium and the double-face liner. Of the waterproof adhesives commonly used, the conventional waterproof starch adhesives are preferred from the standpoint of performance under wet conditions and from the standpoint of economics. An appropriate adhesive of this kind has a formulation as follows:
Solids, percent Water is added to the above mixture to provide suitable fluidity for application of the adhesive by roll 26 according to practice conventional in the corrugating art.
While a waterproof starchad'hesive, such-as-the above, is. advantageous from a cost standpoint, it is adversely affected by rosin modified wax penetrants in the webs when such penetrants are comprised of as little as 5% of the preferred rosins. .I have discovered, however, that such adhesives may be successfully employed to accomplish bonding when penetrants comprising the preferred rosins are applied to the outer face 31 of the double-face liner in the corrugating line, even when thewax penetrant includes as much as 75% or more of the preferred rosins, provided that such penetrant does not strike through into the inner face of the double-face liner before such liner is brought into contact with the adhesive coated crests of the single-face board.
The accomplishment of initial .contact between the waterproof starch adhesive and the face 30 of. the doubleface liner 28 prior to contact of the rosin-containing pene trant with the face 30 is critical to insure establishing an adequate adhesive bond.
Following all above described applying and combining steps, which effect the formation of the impregnated double-face corrugated paperboard combination, heat is applied in the conventional manner at the hot plate section 36 of the double backer. The heat applied at the hot plate section 36 is directed against face 31 of liner web 28 and penetrates through liner web 28 to activate the adhesive applied by roll 26 on the crests of the corrugations of the single-face web 17. This heat application step, which continues along the full length of hot plate section 36 sets the adhesive and causes the wax penetrants previously applied to each of the liner webs and the corrugated medium to spread evenly over the surface areas of the combined corrugated paperboard web and drivessuch wax penetrants into the corrugated web.
The achievement of satisfactory bonding upon the setting of the adhesive requires that the paper webs absorb or pass a small amount of moisture given off as steam by the heated adhesive. Since passing of such steam; through the double-face liner tends to be impeded by the penetrant which has only moments ago been applied in molten condition to the face 31, the moisture contentof the paper should be closely controlled so that it exceeds within the narrowest maintainable tolerances the mini-,' mums necessary for satisfactory corrugating and double backing operations. In general, a moisture contentof the paper before application of penetrant of less than 7% (the amount by which wet weight) is desirable. Moisture control may be accomplished by any suitable conventional means.
In the illustrated corrugating line. suitable moisture control means such as a preheat roll or drum and a steam bath (not shown) may be employed at the corrugator in the conventional manner. Additional steam baths (not shown) may be employed immediately before the preheat drum 29 which may be steam heated to from 300 F. to 350 F. or 375 F. and preferably about 335 to 350 F, Preheating of the double-face liner 31 as by, the drum 29 is important to the successful application of penetrant. However, excessive preheating at the drum 29 may result in premature strike through of rosin modified wax penetrant applied by the roll 33, although it is desirable in' any event to provide a heat control element such as the drum 29 relatively close to the roll 33 to achieve the necessary moisture control, the web must be heated to temperatures which may cause strike through, the application of steam and heat for moisture control purposes may be performed by-additional conventional means (not shown) located sufiiciently remotely ahead of the drum 29 to allow the double-face liner web to cool on its way tothe drum 29 so that the drum 29 need only maintain the web at the temperature necessary for successful penetrant application. I
A preheat roll or drum 20 may be provided as shown 7 at the double-backer for heating the single-face corrugated board. This roll and the coating rolls 21 and 33 may all be steam heated to a temperature of 300 F. to 350 F. or 375 F. or higher and preferably from about 335 F. to 350 F. If penetrant catch pans such as the pans 42 and 32 are employed beneath the rolls 21 and 33,
they may also be heated to like temperatures. The drums or rolls 23 and 35 may likewise be heated.
desired tear tape" 38 may be combined with the board at a combining wheel 39.
A conventional slitter-scorer 40 may be positioned toward the output end of the corrugating line-for performing machine direction trimming of the corrugated paperboard. A conventional cut'olf 41 is located beyond the slitter-scorer 40 for severing the now slit and scored double-face corrugated paperboard into a succession pf box blanks. 1
The present invention makes it possible to approach the same minimal trim losses are are involved in dry corrugating. Even on corrugating lines where no penetrant in applied, slight misalignments of the various webs normally necessitate a trimming operation for correction, which trimming operation involves a trim loss in the order of inch or more. Thus, it will be understood that the additional trim loss necessary to produce at the output end of the corrugating line a uniformly treated impregnated paper represents well under /2 inch. The prior methods of producing impregnated corrugated paperboard, such as by maturing, cut and stacked sheets were unable to achieve trim loss of less than about 2 inches, due to the allowance necessary for any incompleteness in migration of the penetrant to the edges of the stacked sheets.
The fact that the rosin modified wax penetrant strikes through liner web 28 to the waterproof starch adhesive while it is being activated and set along hot plate section 36 has no adverse effect upon the setting of the adhesive I nor upon the bonding power of the adhesive as long as the rosin material was not in contact with the waterproof starch adhesive at the time of the initial contact of the adhesive with the liner web 28. In fact, it appears that the penetration of the rosin modified wax into liner web 28 and through the web face 30 results in a stronger and more waterproof adhesive bond apparently because of the protective coating afforded around the lines of the 1 adhesive by the rosin modified penetrant.
The application of a rosin-free wax impregnant to the liner web 28 has no adverse effect upon adhesion. The establishment of material contact between the waterproof starch adhesive and the face 30 of double-faced liner 28 prior to contact of the wax impregnant is not critical and, in fact, conversely this type of application results in a board of improved moisture resistance and also in good adhesive bonding between the liner and the web in contradistinction to the behavior and critical nature of the application step when the impregnant is the rosin-modified wax referred to. Y
The introduction of the rosin-wax penetrant modified with the described rosin and application to the web 17 by applicator head 27 resulted in objectionable interference with the bonding action of the preferred waterproof starch adhesive when the liner 28 is applied to web 17 at the beginning of hot plate section 36. It was discovered that when the adhesive used was the conventional polyvinyl acetate, the objectionable'adhesive interference did not occur. However, the additional expense incurred when using the described P.V.A. to overcome the described adhesive problem results in an uneconomical process.
The paperboard produced when the rosin modified impregnant is applied to the web at applicator head 27 undergoes an exothermicreaction, which, whenuncontrolled, produced enough heat to char the board, and when the reaction was allowed to continue, the board burst into open flame. The control and surveillance of this problem results in additional expense. This hazard is extremely objectionable in manufacturing plants.
The paperboard produced when the rosin modified impregnant is applied to the web 17 has, in addition to the cited disadvantages, the undesirable characteristics of stiffness and brittleness.
When these preferred practices as taught by my present I invention are followed and the rosin-modified wax penetrants are employed for the liner webs, particularly striking improvements in re-wet characteristics are achieved as compared to the best practices of the prior art including the practices taught in my aforesaid copending application Serial No. 785,408, filed January 7, 1959, now US. Patent 2,982,333. Such striking improvements are shown in the test results when the impregnated board is subjected to standard wet puncture tests, 24-hour wet conditions compression test, a wet bursting test, and a 24-hour wet-dryrewet test.
In one set of tests, different combinations of commercial grades and weights of liner paper were wound into rainbow rolls, that is, rolls comprising lengths of each of the several different liner papers spliced together end to end. These rainbow rolls were then run through a corrugating line to be combined with a corrugated medium to make double-face corrugated board. As each different liner paper passed through the corrugating line, continuous impregnation procedures were followed, first according to the best prior practices and then according to the present invention of applying rosin modified wax penetrants to the liner webs.
Thus there was produced a first set of corrugated board samples each differing from all the others as to the grade or weight of the liner paper but all similar in that they had been impregnated according to the best;
prior practices. of corrugated board samples corresponding as to liner papers with the first set of samples on a one-for-one basis but all impregnated according to the improved methods of the present invention.
There was also produced a second set Puncture resistance properties are measured in units i according to defined standards. Based on the wet puncture units achieved by the set of samples impregnated according to the best prior practice, the set of samples impregnated according to my present invention using rosin modified wax achieved an increase in unit ratings averaging better than 25%, when tested according to the procedures of the American Society for Testing Materials (A.S.T.M.), Test Method D-781-44T (1955).
There also is accomplished an improved compression strength test, when tested according to procedures of Technical Association of Pulp and Paper Industry (TAPPI) Standard Method T804 M-45 for the 24-hour wet conditions compression test. Compression strength, measured in pounds per square inch, showed an increase averaging 10% above that obtained from board imin which a wax resist waterproof starch adhesive is applied to the exposed crests of the corrugated medium of a single-face board, molten paraflin ,waxis applied to the corrugated medium of the said single-face board and is prevented by the resist character of said waterproof starch adhesive from directly contacting the surfaces of said crests, a second liner web is contacted against the exposed crests of the corrugated web and heat is then applied to and through said liner web to activate and set said waterproof starch adhesive; the improvement which permits the continuous manufacture of fully. impregnated sheets of double-face corrugated board paperboard comprising:
(a) applying molten paraffin wax to the liner web of the single-face board before said'single-face board is combined into double-face board,
(b) applying molten parafiin wax to the outer face of the second liner web and allowing said wax to strike through to the inner face of the said second liner web before the said second liner web is combined into double-face board without thereby impairing the bonding of the said liner to the crests of the flutes of the single-face board,
(c) trimming the edges of the said double-face corrugated paperboard web after-the aforesaid heating and adhesive-setting step has effected impregnation of the wax throughout the thickness and width of the double-face corrugated paperboard web, and
(d) cutting said double-face corrugated paperboard web transversely into a succession of sheets.
2. The process of claim 1 wherein less than one-half inch is trimmed from each edge of the double-face corrugated paperboard web.
3. In the continuous in-line process of manufacturing on a cornigator machine moisture resistant double-face corrugated paperboard box blanks impregnated with wax in which a first liner web is combined with a corrugated medium to form a single-face board, a wax resist watera proof starch adhesive is applied to the exposed crests of the corrugated medium of the single-face board, molten parafiin wax is then appliedto the corrugated medium of the single-face board and is prevented by the resist character of said waterproof starch adhesive from directly contacting the surface of said crests, a second liner web is then contacted against the exposed crests of the corrugated web and heat is then applied to and through "said liner web to activate and set said waterproof starch adhesive; the improvement comprising: r
(a) applying a wax penetrant to the liner web of the single-face board before said single-face board is combined into double-face board, (b) applying molten parafiin wax to the outer face of the second liner web and allowing said wax to strike through to the inner face of said second liner web before the said web is combined into double-face board without thereby impairing the bonding of the said liner to the crests of the flutes of the singleface board, (c) trimming the edges of the said double-face corrugated paperboard web after the aforesaid heating and adhesive-setting step has effected impregnation 3 10 of the wax throughout the thickness and width of the double-face corrugated paperboard web, and (d) slitting and scoring the trimmed double-face corrugated paperboard web in the machine direction and severing it transversely to form a succession of box blanks.
4. In an in-line continuous process of manufacturing on a corrugator machine sheets of double-face corrugated paperboard impregnated throughout their thickness and width with moisture resistant impregnants in which a first linerv web and a corrugated medium are combined to form a single-face board, a wax resist waterproof starch adhesive is applied to the exposed crests of the corrugated medium of the single-face board, molten paraflin wax is then applied to the exposed crests of the single-face board and is prevented by the resist character of said waterproof starch adhesive from directly contacting the surface of said crests, a second liner is then contacted against the exposed crests of the corrugated web and heat is then applied to and through said liner web to activate and set said waterproof starch adhesive; the improvement com prising:
(a) spraying rosin-free molten paraflin wax to the exposed crests of the corrugated medium in an amount between about 5 and pounds per 1,000 square feet of the corrugated medium,
(b) applying between about 3 to 12 pounds per 1,000 square feet of a rosin-modified wax penetrant to the outer face of the second liner web before the second liner web is combined with the single-faceboard, said rosin-modified wax comprising up to at least three parts rosin to one part wax,
(c) contacting the inner face of said second liner web against the exposed crests of the corrugated medium: of the single-face board prior to penetration of said rosin-modified wax penetrant from said outer face to said inner face,
(d) trimming the edges of said double-face corrugated paperboard web after the aforesaid heating and adhesive-setting step has effected impregnation of the wax throughout the thickness and width of the double-face corrugated paperboard web, and
(e) cutting the double-face corrugated paperboard Web transversely into a succession of sheets.
References Cited by the Examiner UNITED STATES PATENTS EARL M. BERGERT, Primary Examiner. CARL F. KRAFFI, Examiner.