US2670663A - Process for making crush resistant boxboard - Google Patents

Description

Patented Mar. 2, 1954 PROCESS FOR MAKING CRUSHRE'SISTANT BOXBOARD Elwood P. 'Wenzelberger, Dayton, and Kurt A. Wohl,.Cincinnati, Ohio, assignors to The Weston xPaper and ,Manufacturing 00., Dayton, Qhi a: orper tione Ohio Application May 3, 1951, Serial N 0.224326 @ur inventionrelates tothe process of manufacturing paper and particularly boxboard; and relates to the-process,-product and apparatus for practicing the process and producing the product to secure decreased weight or increased strength, or both decreased weight and increased str n th- Ourinvention relates primarily to fiber treatmentan'd conversion into a substantially standardized product for producing drypaper of uniformstrength characteristics.

-' This invention deals with dry strengthpaper.

111 171118 art "the problem competitively is to produce ,boxboard of decreased weight or increased strength, referring to strength 'as resistance to crush! or both. Thevariationsdue to the v riety of, raw mat als used nm u a tu e ssari y present an unusua ly diflieu t p em in the. con rol ofthe weight and strength and partic lar y in the efiective redu ion of wei h and increase/of crush-strength c nomically on xisting nap rnmaking and corruga ed, boxboard mak ngamachines and in-conne tion with, th me hanismsusediorpreparin th stoekpri t delivery o the machines, Due tothenatureo manuia tureof boxboar in which th a er sh ets ere first mamuactured on one machine andithen corrugated inpart and thecorrugated sheet is assembled between ,twoflatsheets on stillothermachinesand,heat is applied on both machines, to I the paper stock, 1 an additional set of variables is. intrqduceizl,intov the problem.

tan obiectof-this invent ont e-a l to mee-t thesei-varying-conditions and at the same me to s-somodify the rush strength and Wei ht of he pap r as to produce an impr v d produc and; one that is moreeconomical as the sales pr ce n st vary in pro or ion toth w gh and tre g h.

itisr-afurth r Qbject to r v de pr uct th t can be so. manufactured, while at the, same, time retaining-most. of-the desired characteristics of ink; absorption for the printing on the boxboard and other characteristics,requiredby the trade.

Itis an:-object of this invention to provide an additive compound and methodof making; and to provide 1 a method. of introducing this compound into the. pa er-stock, to treat, the i r control the stock ralue, 1 control its absorptioniand edsomtienr-Qn t est ek seaso r a late thegr esultin reng h an -we h ed th 123 and l f -i fth il '0. Vi Q QQ1ym-E i fititan f the compound on hedb rsg eien y. by cstages. sensthe inane machine and the later 1 treetmentfien: the eer -ugating-z mach ne, so

2 that the ultimate polymerization will be accomplished for producing the ultimate in increase of strengthland saving in the weight with the approximate conclusion of the manufacture 0 the completed boxboard.

It is a further object to provide for a mechanism for adequately introducin and incorporating the compound so as to bring about the maxi-1 mum saving in weight .and increase in crush strength of the dry paper,

It is particularly an object of this invention to provide for a new combination which is precooked and is introduced as a pre-cooked combie nation constituting a new composition of matter introducedinto the fiber stock under controlled pH mmciition.v Webring about the combinat on of th scc mposi ionof. matter withhe p per stock itself and chemical means to control ,allgalinityand acidity.

e unex ected result is that the new compos on effects the desired resu s f increased stren th resi tance to crush and reduced weigh althoughthelccmponents ,from which the new composition is manufactured ,in themselves .do not produce the same result.

The particular object of this invention is'to provide for the combination of water soluble urea. formaldehyde resins and starch pre-cooked sufficientlyto pro-polymerize this composition of matterintoa new form, preferably in thepresence oilpapermakers alum, aluminum sulphate (A12 (SO4)3), or its equivalent, foracidifyingthe resin and starch polymerization composition, to facilitate vthe.polymerization prior to introductionintothe fiber stockby controlling the pH.

,ltrisa further specific object to likewise con; trol the. acidity upon the introduction of this prey polymerized composition into the stock.

It is an additional object to utilize the heaton the papermaohine for further polymerization of the composition of matter in the presence of a um.-in-the stock to bring about, not only fur!- ther polymerizationof the. composition, but to dorso on thestock-so as to control and secure adsorption in orabsorption on or into the fibers.

It is an object to control absorption and adsorption. ofthe composition with the fiber and to -provide a controlled balance between absorm t en and adsorpt :The pr -po er aa i nei h comp ti on ri r to t in o ction in e; pa er fiber-is to he; u pose ofpro id nsa meieeular struc ure tha per t 'en r e th mole u e ia et efibe ee ella c atthe fi ers. he eh eii etii ceentrol edrbalan e iides r d between-a er tienanda orp ion It is still a further object to provide for the completion of this polymerization in the completed sheet by the application of the heat in connection with the corrugation of the sheet and final assembly of the corrugated sheet with the liners as a part of the boxboard manufacture.

It is an object of this invention to control the pH value at each stage to regulate the polymerization rate and to regulate the incorporation of this composition of matter into and upon the fiber stock to form the resulting paper and boxboard.

It is a particular object to accomplish this result for dry strength, particularly crush strength, of the finished paper.

It is an object to provide a method involving time, temperature, and viscosity control, plus control of the chemical reactions indicated above, and the control of the pH values for regulating and accelerating the reaction and the polymerization and unity of the composition with the paper fibers into a new composition.

Due to the fact that heat is applied to the paper in both the making of it originally and later during the corrugation and assembly into boxboard, this new composition is particularly adaptable to progressive polymerization until the final result is achieved in the completed boxboard, but it will be understood that the invention is equally applicable to conditions in which polymerization is completed in one stage when used with a product where a single heating takes place. In both instances it is an object of this invention to be able to utilize the existing heat employed in paper making and boxboard making to conclude and complete the polymerization which has been initially started in the step of combining starch and resin in the initial prepolymerization operation to produce the composition of matter which is introduced later into the stock to treat the fibers. At this stage, the composition is readily combined with the stock fibers. Later, the stage polymerization completes the incorporation into the fibers while completing the boxboard.

It is a further object to provide a method of introduction of this composition into the paper making line for its maximum incorporation and to insure the ease of control of the pH values.

It is a further object of this invention to provide for proportions of this new composition of matter which have a low ratio compared to the resulting dry paper stock.

It is a further object to provide a highly cationic resin of water soluble urea formaldehyde composition in combination with starch and alum (the latter optionally) under an elevated temperature sufiicient to bring about pro-polymerization and the formation of a new composition of matter; and to combine such partially polymerized composition with paper fibers; and then to complete the polymerization of the composition on or in the fibers as the boxboard is made.

The drawing shows a flow sheet indicating the various steps in connection with the practice of these inventions.

Referring to the accompanying flow sheet, the following examples indicate variations of the basic features of our invention. It will be understood that our objective is to improve the dry strength and primarily the crush strength properties of boxboard by increasing crush strength and reducing weight; and, in doing so, to adapt this process and its composition of matter to the specific operating conditions of producing Statement of introduction steps The process is as follows: I designates a source of papermakers alum which is introduced into the cooker 2 containing a suitable amount of water 5 and into which the water soluble formaldehyde condensation resin is introduced as at 3 together with starch as at 4. The mixture is heated at elevated temperatures supplied by the steam 6. A typical cook comprises introducing gallons of water to the mixer tank 2; adding 125 pounds of 50% resin and mixing thoroughly. Then there is added 62.5 pounds of starch slowly which again is mixed thoroughly. Alum is then added to the mixture as required to the desired pH. The exact amount however varies and the amount to be added should be sumcient to give the desired pH of 5.5 to 6.5. The mixture while agitated is heated to gelation (166 to 180 F.). When the gelation starts a temperature of approximately F. is maintained for approximately 25 minutes. It is understood that a longer or shorter time may be chosen depending on character of paper to be treated and the degree of molecular size desired of the resin starch mix; we may use from 5 minutes to 50 minutes. On the completion of the heating, the steam is shut off and the mixture is diluted to 300 gallons with cold Water. For convenience we cook at approximately 10% total solids and then dilute to 5% solids for use. We emphasize this is a typical cook: the procedure and proportions can be varied considerably. The solution diluted with the water I is stored in the tank 8. A certain amount of overflow 9 may be added from the weir box Ill to which the composition is added through the lin I0 and then sent to the dividing head II. This pre-cooked, pre-polymerized starch-resin composition prepared in the presence of alum is added on a one to one solids ratio to provide a 2% total solids based on dry paper Weight. In practice, this composition is metered into the stock as it flows from the weir box or head box into the dividing head I l.

The second alum solution is prepared and added as follows: Turning to the right hand side of the flow sheet there is added alum I2 and water l3 to the alum dissolving tank 14. The mixture then passes to the alum storage tank I5 which is supplied with overflow water through the line 16 from the alum weir box H. The alum solution from the storage tank 15 is introduced by the line l8 into the alum weir box (I to produce a pH of 5.5 to 7.0, preferably not above 6.9. This alum solution is introduced by th lines I9, 20 and 21 into the dividing head 22 which is provided with dilution water 23. The dividing head 22 receives the starch-resin composition which has been pre-cooked and pre-polymerized in the presence of alum through the lines 24, 25 and 26.

Turning to the left of the flow sheet, we now describe the preparation and supply of the paper fibers. Water 21 is delivered to the digester 28 which is supplied with steam 29. In this digester is introduced straw 36 as an example, it being understood that other materials may be used; and the digesting chemicals 3! of conventional type. The pulp passes through the line 32 to the multiple beater 35 and. thence through the line 34 into the beater chest 35 where it is stored.

5 Itiis withdrawn-cthmng'h thei lineiifiiintwzonemr more Jordans 31 from whence the pulpspaaes; through the line storage machine chest.39where. it is.slo wly.. agitated. Jt thence passes through .into .the "dividing? headliZL Where it is mixed with. water. andthe. Ezra-polymerized. preecooked. composition. and. withjthe additional alumfor the. maintenanceiof'the. pH;value in1jthe cylinder vats of; f'rom 6i7 $66.95 ,(preferably 6.9). 'Thence theimpregnatejd' and. coated "fiber passes: to the. screens and to the cylinder 'vats 43 with their cylinders L44 diagrammatically shown under the general title .ofrcylin'deri machine 45. It williberealizedthat this 'diagram js merely our outlinetmindicatethe several steps. In practice the cyl'inder is mounted in the vat. Fromthis point the. treated .stockffrom' each cylinder is laminated andthe 3 QLYisheet passes; out as wetsheet at; andithence to theidryi'ng rolls 4}, 'slitter reroll '48" and finish roll 49'. TThe'temperature to which the combined" stock and composition are. subjected on. the machine varies according to the'machinebutgenerally thegtem- Derature is as follows nQrmaItJDrocess water temperatures on the wetendofthe'machin tof 60 to 70F. .Thedrier temperature "rangesbetween: 23 57 1F. to 2.85"-'F.'from" the :wetend I to" the oalendersr The time the paperjis "exposed 'to these temperatures isdependenhon thespeed' of running but usually the total time is'pabout -l minute. The higher temperature could'be; used satisfactorily. In the -'making-- of 'boxboard this temperature and time may not? bersuffi'cient to complete the polymerization of the composition absorbed and adsorbed in the fi-ber. "This polymerization can be completed on; the -b0xboard, corrugating and fabricating --maeh ine indicated at -50 in whichthe temperatureof the heater 'is carried at approximately 340 F. at 300 ft. perminute or faster. Thisternpera tu-re:and-speedwill vary with the corrugating machine. *The sheet corrugated -:inthe usual manner is then treated with adhesiveand a liner sheet applied to" one -01: both sides of the corrugation depending upon the. type of boxboar'd'tdbe produced. This final-heat;treatment-completes"the polymerizationfar-enough to providethe reducedweight of the completed board and the" high crush strength which constitute the "desired-"results i from this process. We wish" to emphasize that thedetails of this process will varygreatly from'machineto machine "but with the foregoing teaching to one skilled in'the art the adaptation of this invention to varying conditionspanbe effected' with the minimum difficulty.

It willbe notedthat the unexpected 'an'd'unpH ineither the. pro-treatment. or. therstock' system of the machine or both. There. are various materials which are. suitable substitutes fq 'ralum such as acids. and-acid, salts. ispecificsexamples are. acetic acid, hydrochloric.-acidpsulphurickacid,

ammonium-chloride. or. emhate ammcniumtnk tratesor phosphate;diammonium phosphate; so-

dimrrrbisulphate orrbisnlphite;

iWefindutha-t, awater-solubleurea formaldehyde orzsmelamine formaldehyde condensation "resin wilkrbens'eful. -Anyfilm" forming cooking-starch isiuseful.

:Ihe pHvvalues, given are those we find partic-- ui'arlly adaptable to-.conditi:ons'= where wheat and oattstraw predominate. "On the "machines 1 the lower pH could-possiblybe used advantageously on 1 other types ofcpulpand may i vary depending on thel-typeof pulp and chemicals. used'for digestingthe paper. :We: realize that pro-cooking and pre polymerizing the "starch and resin in the presence or alum-not only-hastens: the formation of -a '-balanced colloidal solution between the starch-resin and water :but: it formsv a means: or adjusting the molecular composition of the productas polymerization takes place so as tofacilitate the absorption and adsorption of the compositionpnto or into. the. paper fiber to produce thejdesired result.

WewishltQ emphasize thatv the problem solved bythisinvention. has nothing .toido with wet strength paper wherethe. problem is to improve tensile. strengthwhen wet. as compared, with ,the same ,kindof paper when. wet. which has had no treatment. The problem here is to'. provide. in.- creased crush strength and reduced weightof dry paper. It is well known that treatments for improved, wet. strength paper by increasing: tensile strength, frequency and usually do resultininfluencing thedry strength, but in. any eventthe strength therein questionis tensileand'not crush.

.To secure the resultof this inventionit is preferred to have affiber characteristic of wheat and cat straw which should at least predominate, butthisinvention is adaptable'to avarietyrof papermakingrfibers. ;.It need only be adjusted to such. conditions to secure the desired 1 result.

For-instance, if soya bean strawis utilized as'a source offiber'for the pulp, foaming-may. occur which interferes with machine operationan'dif a defoaming agent suchas kerosene is used, it is injurious to the product of this invention because is. used.

Basically our objective is to produce a molecular resin starch icombination with a carefully controlled pH, precooked and pare-polymerized =to=a controlled degree and then combined with the fiber stock so as" to' have a controllable balance of absorption and adsorption in or on the'fiber due to therange of molecule size controllable 'by the pre polymerization: --and after incorporation of this'composition has been effected, the fiber and this compositicn are so treated by heat ofthe machines in the formation of the paper and boxboard that thecrush strengthis' progressively increased until .the. paper is. finished and polymerization is simultaneously completed. One reason i for-this progressive polymerization in boxboard manufacture according to this invention is. that premature setting of the resin to the ultimate completely polymerized condition before the laminate reached the corrugation machine. wouldibe' undesirable as it would. defeat satisfactory corrugation and would result inbreaking of the fibers and its completion by corrugation into boxboard. It should be kept in mind that the greater the molecular size, the greater the dry crush strength and probable reduction in weight, but this rule is modified by the fact that if the molecular size progresses to such an extent that only the fiber is coated on the outside by adsorption and there are not suilicient smaller molecular sizes there will be no desirable absorption to carry the molecules of the composition into the fiber to add that internal strength as well as the coating producing external strength so desirable in a balanced fiber treatment. It will be understood that the foregoing is our theory and we do not wish to be bound to such a theory but state it for the purpose of making the best and most complete disclosure within the realm of our knowledge.

Place of introduction of the pres-polymerized composition Keeping in mind that thoroughly mixing of this composition of matter is important to incorporate it with the stock, we have employed its introduction into the open trough leading to the Jordan after the flow regulating dam; into the stock stream from the Jordan to the machine chest; into the machine head box after the gate regulating overflow back to the machine chest; and into the dilution box at the end of the machine head box.

It is found that the preferable point for addition of chemicals is at the dividing head of the machine head box where dilution with water takes place. This head box is located between the machine chest and cylinder vats. The dilution water creates sufiicient agitation to mix the chemicals into the pulp.

While the pre-cooked partially polymerized starch and resin combination can be introduced into any one of the vats it has been found preferable to introduce the composition into all of the vats equally.

pH values The question of pH values is an important one both in the initial reaction of the starch and the water soluble urea formaldehyde resin, and in the stock to which the starch-resin product is added. In the pre-cooked mixture it is desired to have a pH range of 5.3 to 6.5. While this varies accord- Temperature control is important in connection with the pre-cooking. A satisfactory cook of starch, alum and resin is eiiected at 175 for five minutes after gelation and then held for 20 minutes at that temperature. This period may vary and th temperature may vary with the type of resin employed. With such a combination the proportions of resin and starch are 1% resin and 1% starch making a 2% mixture of solids based on the dry paper weight.

The word cook used herein indicates heating and holding of the components to temperature as required and does not indicate a boiling tem- 8, perature. This holds true throughout the process.

EXAMPLE I The following is a typical composition and method of preparation of a pre-cooked starch and water soluble urea formaldehyde resin:

Cook th starch and water and a water soluble urea formaldehyde resin and bring the composition to boiling point and thereafter cook at F. for five minutes and then hold for twenty minutes at that temperature. The composition of matter so formed is then cooled and used as an additive as indicated as follows:

The pH value of the composition is maintained at 5.7 by the addition of alum or its equivalent such as any acid salt or any material which gives excess of hydrogen ions such as sodium, bisulphate, hydrochloric acid, acetic acid, phosphoric acid, nitric acid, oxalic acid or any mineral acid or any of the strongly organic acids or any acid salt.

The alum added or its equivalent depends upon the pH value to b maintained.

This composition is then added as indicated to the stock where the pH value ranges from 6.5 to 6.95, although an even lower pH may be used under some circumstances. This pH value is controlled by the second addition of alum or its equivalent.

The stock is then passed through the paper machine and is manufactured into finished sheets. The heat of such manufacture brings about further polymerization of the starch resin composition. Thereafter the corrugation of the sheets and the assembly with the front and back liner sheets involve the application of heat which completes the polymerization and the reaction between this composition of matter and the paper fibers to form a boxboard of high crush strength and reduced weight.

Principles of cooking the paper fiber to bringabout absorption. In,

this way one can control the formation of average molecular size thus governing the degree of absorption and adsorption in or on the fiber.

For instance if a short cook composition is introduced into the machine chest with slow agitation and mixing is conducted for an hour we find that greater absorption is secured, but if it is added at the head box which holds a much small-- or amount and there is movement continuously in and out of the head box, creating shorter time of contact we find that greater adsorption or surface coating and lesser penetration result. The resulting strength and characteristics of the paper can be accordingly modified. If there is a short cook, the paper will be more fiexible because of greater absorption, while, with the longer cook, the composition of matter stays more on the surface of the stock and provides a stifiening effect on the fiber.

In the making of the paper sheet the temperature of the drying rolls ranges from 235 to 300 F. as the stock moves through the machine. Depending on the length of the driers and their canacmvmr drying". mm. 500 't'eetofi sheefl' per minute theitemperature of thesheetiranges frem 70- 280 F;: an the corrugating'machinethe temperature runs approximately 300-340 F; in

thedrying: ovens with thepaper travelling: at 300 to 560 feet per minute again depending onthe particular machine;

The steps ofa' typical example of our'invention are as. follows:

Thisis a %:solution' ofea'ch component Water)"; gms'-' 1365? Sol'iition A; gms; 6-

Stir: and add Alum (25% sol.)- to pI-I of 5.7-. The -final mixture. contains 1 starch and 1 resin on: finishedpaper weight.

III

The steps of a typicall example of. our invention are as follows:

starch msi 10.0

Water soluble urea formaldehyde resin' 45 solids; gms' 22-;2

Water-4.. grns;s 1 70.0"

water) gms 1365} Solution A gms;s 6

Stir and stand: 3 minutes, .add: alum 1 (25% sols) to pH 5.6.

EXAMPLE IV Thestepsof a-typicalexample-of our'-inven-- tion -are asfollowsz Reduce-to 'pH 5.0 with papermakers alumaiid heat ten minutesat-175 Fi Stiaw. pulp gms. solids in 1'3501gms. water) 7 m 1365 Solution A--- gms 6 starch and 2% starch resin on" finished paper:

Eight.

C'o'rn' pearls'tarch; lbs" f Water "gal".

Mix well and add Thisis -a-'2.5% solution-"of each (resin andstarcli) in solution (total 5 'Ihissolution is=fed into the-pulp at the divid r ingshead of: the papermachineatthe head box at a rate to add 1% of each component "based; on=the air dr y'paper being madeon the ma'chirfe. Alum .is added: continuouslyahere (25 soluiv tiori simultaneously .to h'oldi the 1 pH? at? about fiil to-6;95-..

On a paper machine' producing-'55 tons per day the additions as above would amount 30556 lbs; per minute of the-fresin-star-ch solutiorii- Abo-ve resirisstarch sohitions have also been usecll by: addingito straight bogus stock (scrappaper) with: equally improved results: eves he'- resultsr obtained oma straight untreated ayee C uringpof theresiw 'I-h'e cooking- Deiiod of tlie starch 1 and resin i s the first st''p' i11 cori'tiolli'rig' th extent to Whilf there will be obtairied' absorption on adsorption iH' or 'on the fiber Asa rule slowagitation fof aconsiderable period? approximately an hour, of the starch resiii ompositionwith the um as inthe machine chest will bring about thepiedomination of absorption} whereasby adding" at the head box where shorter time" of contact re sults" wi'll givegreater adsorption: As" a rule" the. shorter the-cook; .tliesmal'l'e'r the mol-e'cularstmc ture and the'greater' thea'bflity' to enetrate the fibf'by" the Starch resin pre c'ooked combine; tion to bringabout greater absorption,.wherea's" thelo'r'igei: thec'o'ok; the'largerth'e m'oleoiile size. which Will -brin about'greater adsorption wehave foiind that a-loiiger' o'rishorter diara tio'fi of'fiimofcm'i-tacf Of'fih'e resin starchsolli tioiiwiththe'fib'ei's W'fllT also govern the degree of relative absorption .oi'i adsorption evenafter theaverage'molec'iilar sizes have been controlled and established in the 'pre-cooli of the resin and. station;

The heat used in..manufaoti1re-- ofpaper is? largely consumed in the dryingof the: paper-so that'the heatv is effective to alesser deg-1-ee, for-= further polymerization; and -euring. -,ofthe/resin as compared with thecorrugating= operation ..where the paper runsovercorrugating rollsamlover steam heated driersto dry the-gIue-appIied to. the corrugations a typical 'machine'these 3 driers normally-runabout- 34-0- F. The papere travels about- 450 pen minute and thedr-iers are about 40 -'1ong.- Thetemperature-pfi the *paperv 11. is such that the paper is too hot to touch, but being in rapid motion it is not possible to determine the actual temperature of the paper.

Between the paper machine and the corrugating machine the paper is rolled while warm and this residual temperature continues the slow curing of the resin.

After the delivery of the boxboard from the boxboard making machine there is suflicient residual heat in the boxboard so that the cure of the resin in some instances will continue (even at room temperature) to bring about the final cure.

Generally speaking temperatures above 250 F. will bring about the final and accelerated cure of the resin. The speed of cure is a function of and is governed by the pH values as a high pH (alkaline) gives more stability and less tendency for rapid cure than a low pH (acid condition). These conditions are controlled by using the proper amount of papermakers alum or other similar chemicals.

In the instant case when straw is the basis of the stock it is essential that the acidity be controlled as indicated, in order to prevent any destruction of the strength of the stock itself which would negative the strengthening effect of'the additive composition.

However, in the event that the composition is employed with stock other than straw we have found it possible to increase the amount of alum and thereby accordingly modify the pH values because the effect of alum in some cases is not deleterious per se on the strength of the stock.

Thus in the event that bogus (any scrap paper and mixed boxboard) or other paper making fibers were utilized as the basis of the stock on the run then the pH values would be accordingly adjusted as indicated because the destructive effect of incorporating alum would not be as marked as in the case of using virgin straw.

It is understood that the use of alum under these circumstances for the primary purpose of effecting the polymerization of the resin and setting it up in combination with the starch and its combination with the paper fibers is .(aside from) its well known function of acting as a size precipitant for rosin size and other sizing materials.

In various places throughout the text the phrase to increase strength and lower weight is used. It must be understood that this is proportional, that is if we lower the weight, strength changes proportionally to the reduction in weight. In other words we lower the weight to a point where strength remains the same as it was without any treatment. By the same token if we maintain constant weight then strength (crush resistance) increases to its maximum. This resolves itself into an economic feature. By maintaining uniform crush resistance (strength), we can reduce weight and save costs. If we maintain uniform weight compared with untreated sheets then at the same cost or very slightly additional due to chemical costs, we attain increased quality of crush resistance.

In increasing strength we do not refer to tensile strength, but mean resistance to crush force. The term crush resistance can be employed instead of strength as we have no reference to tensile, burst or tear strength when we speak of this property called crush resistance, although increased crush resistance will influence these properties of tensile and burst to some extent. This differentiates this result from wet strength which is essentially a tensile strength when wet.

There are, also, variations as to preliminary preparation of the cooking, the pulp treatment, etc. before reaching the paper sheeting stage. Some plants use spherical steam heated digesters, others use chemical pulpers and various other types of digesters.

In the case of bogus paper it may be pulped in the beater or it may be treated in a hydraulic pulper or by other mechanisms.

Also, in a cylinder machine, the number of cylinders may vary from one cylinder up to four or even five cylinders or more.

In our flow sheet and description thereof, there are shown two weir boxes for controlling the rate of addition of the chemicals to the paper pulp. This again is a typical set up. Instead of weir. boxes we may use proportioning pumps or flow regulating meters or any other devices suitable to control and regulate a constant flow.

This also is true of the dividing head of the boxes. Any other method may be used to furnish three or more (or less) uniformly controlled feeds such as three separate pumps, three meters or any other suitable method.

The chemical addition agents, besides being suitable for straw stock and bogus can also be applied to any other type of stock including jutes, kraft, rag, wood, chestnut or any other grade of pulp.

It will be understood that we desire to comprehend within our invention those modifications and substitutions of materials that are found necessary in adapting these inventions to various conditions of use and particularly where the claims specify a particular resin or other material it will be understood that equivalent substitutes of material may be employed some of which are indicated herein as typical substitutes. The specification and the claims therefore of a specific material is meant in the generic sense of any materials of that class having those similar characteristics.

We claim:

1. The method of making a water-dilutable potentially thermosettable starch-resin reaction product which comprises adding from about 62.5 to about 73.0 pounds of raw starch to an aqueous solution of about to 146 pounds of a 50% solids water-soluble urea-formaldehyde resin precondensate in about to gallons of water at room temperature, adding sufficient alum to adjust the pH of the mass to about 5.3

to 6.5, heating the mass at said pH and with agitation to a temperature in the range of 166 F. to F. and maintaining the mass at said elevated temperature for from about 20 to 25 minutes.

2. A water-dilutable potentially thermoset table raw starch-urea formaldehyde resin reaction product made by the method of claim 1.

3. The method of making boxboard having controlled crush resistance and weight which comprises (1) incorporating an aqueous dilution of a water-dilutable potentially thermosetting raw starch-urea formaldehyde reaction product in the paper stock in an amount to provide the stock with a total solids content of 2% of the raw starch modified-resin based on the dry weight of the paper, said 2% being made up of 1% of raw starch and 1% or" the urea-formaldehyde resin, (2) adjusting the pH of the mixture of paper stock and diluted starch-modified resin to 6.7 to 6.95, (3) sheeting the mixture, (4) laminating a plurality of sheets to obtain a multilayer product, (5) passing the laminate through a drying zone, (6) passing the dried laminate through a corrugating zone, (7) and then passing the corrugated laminate through a finishing zone in which an adhesive is applied to at least one surface of the corrugated laminate and a liner sheet is applied to the adhesive-coated surface, the temperature to which the mixed fibers and heat-curable starch-modified resin are subjected from the wet end of the sheeting machine to the exit end of the finishing zone being increased progressively and being correlated with the rate at which the mixture, in the sheeted form and then in the laminated form, is passed to the exit end of the finishing zone so that conversion of the resin to thermoset condition is efiected progressively but is not completed before the boxboard is ready for removal from the finishing zone, said aqueous dilution of the starch-resin reaction product being obtained by adding from about 62.5 to about 73 pounds of raw starch to an aqueous solution of about 125 to 146 pounds of a 50% solids water-soluble urea-formaldehyde resin precondensate in about 135 to 140 gallons of water at room temperature, adding sufficient 14 alum to adjust the pH of the mass to about 5.3 to 6.5, heating the mass at said pH and with agitation to a temperature in the range of 166 F. to F., maintaining the mass at the elevated temperature for from about 20 to 25 minutes, and diluting the mass with 300' to 350 gallons of water.

ELWOOD P. WENZELBERGER. KURT A. WOHL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 109,621 Hutchins Nov. 29, 1870 1,582,841 Lorenz Apr. 27, 1926 2,121,076 Ellis June 21, 1938 2,221,200 Perry Nov. 12, 1940 2,275,314 Pierson Mar. 3, 1942 2,302,309 Glarum Nov. 17, 1942 2,302,310 Glarum Nov. 17, 1942 2,338,602 Schur Jan. 4, 1944 2,400,820 Glarum May 21, 1946 2,407,071 Gill Sept. 3, 1946 2,450,377 Kesler et a1 Sept. 28, 1948 2,544,714 Moore Mar. 13, 1951 2,555,057 Porter May 29, 1951 2,590,013 Huntzicker Mar. 18, 1952 7 2,601,671 Wilson et a1 June 24, 1952 2,629,699 Moore Feb. 24, 1953 OTHER REFERENCES Collins, Paper Ind. & Paper World, June 1943, pp. 263-269.

Maxwell, Paper Trade Journal, May 13, 1943, page 41.

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