US2349698A - Fiberboard - Google Patents

Description

vFaterratedA.May 23, 1944 FIBEItBOARDv Ernest R. Boiler, Hudson, and Raymond F.

Remler, Lakewood, Ohio, Font de Nemours & Company,

assgnors to E. I. du Wilmington,

Del., a corporation of Delaware Application July 1, 194'1, seria1No.400,'z00

z claims. (cl. 1an-saus) This invention relates to flberboard and processes for producing it, and is more particularly directed to corrugated berboard comprising a paperboard corrugated member impregnated on at least one side with sodium silicate, the ratio between the percentage of sodium silicate on the outside .002 inch of the paper board and in the next lower .002 inch being from about 1.2 to 3.0,

and one or more paper linerboards which optionally may be treated with sodium silicate in the same manner as the corrugating medium, and is further directed to processes for producing such fiberboard.

Recently an increasing amount of attention has been devoted to the strength characteristics of corrugated berboard shipping containers. There is a growing realization that damages in transit and in storage to goods packaged in such containers can be greatly reduced by observing certain minimum strength standards for the containers, and that these standards can be directly related to the conditions of use. These standards require that the containers be able to stand up under compression tests and other such tests made on the finished containers, on the corrugated berboard from which the container is made, and on the materials from which the fiberboard is made.

Fiberboard container manufacturers have endeavored to meet the strength requirements for their products in two ways, first by improving and controlling their operations for .producing corrugated berboard and for fabricating it into containers, and second by raising the quality and strength properties of the paperboard used in manufacturing the fiberboard containers. Both of these lines of attack must be followed to produce the highest quality of product. Unfortunately, the use of high quality, high strength paperboard has heretofore entailed higher cost, since to obtain such paperboard it has been neces'sary to use either more 'bers or higher quality fibers. This higher cost is probably the chief factor militating against greater and more general improvement in the strength properties of berboard containers.

Other methods cf adding strength and stiliness to paperboard have been suggested, such as treatment with sodium silicate, and such methods have found a limited usefulness as for nsfance in coating butter boxes and similar containers, but brittleness caused by such treatment makes\the treated paper inapplicable for use as a corrugating medium in corrugated iiberboard. The sharp bending to which a corrugating mcdium is subjected by the corrugating rolls in the manufacture of berboard makes it imperative that brittleness be absent and processes heretofore known for coating paper with sodium silicate have not been free from this troublesome property.

It is an object of this invention to provide improved corrugated fberboard suitable for use in making containers. Another object is to provide a high quality of corrugated berboard assembled from paper of a quality heretoforeconsidered inferior. Another object is to provide fiberboard which for a given strength employs a minimum amount of ber stock. Another object is to provide processes for producing such improved berboard lwithout interference with accepted assembling techniques and machinery. Other objects will appear hereinafter.

The foregoing and other objects of this invention are accomplished by vtreating a paperboard on at least one side with a sodium silicate solution of such a concentration and SiO2:NazO weight ratio that with the paperboard used an impregnation of decreasing concentration from outside to inside of the paperboard is obtained, the ratio between the percentage of sodium silicate in the outside .002- inch and in the next lower .002 inch of the side treated being from about 1.2 to 3.0, and assembling the impregnated paperboard into a laminated board. In a preferred embodiment of this invention the impregnation is accomplishedin the course of manufacture-of the paper by applying a sodium silicate solution at a point between the end of the driers and the end of the calender rolls, the concentration and SiO2:NazO weight ratio of the silicate solution used being so related to the porosity, moisture content, and ber character of the paperboard at the point of application that penetration of the silicate into the paper occurs to the extent necessary to give the desired concentration gradient.

In the drawing there is illustrated one particular embodiment of this invention in which a laminated liberboard is made up of two paperboard plies joined with an adhesive lm. In this embodiment the lower paperboard ply has been treated on its lower side with a graduated silicate impregnant, indicated by the dotted portion, the greater density of dots near the lower edge of the bottom paperboard indicating the greater concentration of sodium silicate in that portion ci the paperboard and the variation of density of dots being illustrative of -the fact that the percentage of sodium silicate in the bottom .002 inch of the paperboard and in the next .002 inch is from about 1.2 to 3.0. It will be understood that the drawing illustrates but a single embodiment of the invention, and that any desired number of plies might be used, that one or more of the plies might be corrugated, and that some or all of the4 plies might be impregnated on both sides or on a single side with the graduated silicate irnpregnant.

Paperboard impregnated with sodium silicate according to a process of this invention has considerably improved properties for use in laminated board, particularly corrugated board and spiral or convolute-wound berboard tubes. The treated paperboard has improved stiffness without being brittle. Hence it may be used as a cor- EXAMPLE Strawboard .010 inch in thickness and with a basic weight of 35 lbs. per 1000 sq. ft. was treated with` sodium silicate solution having a speciiic gravity of 20 B. and a SiO2:Na.2O weight ratio of 3.25 'Ihe silicate was applied from water boxes on the calender stack of a paper making machine. 'I'he equivalent of 4.5 lbs. of 40 B. solution was applied per 1000 sq. ft. of strawboard. Application of the silicate in this manner did not necessitate any reduction of speed or other changes in the normal operation on the paper making machine.

The silicate-treated strawboard had a ratio of silicate in the outside .002 inch to that in the next lower .002 inch of 1.48 and a ratio of silicate in the second .002 inch to that in the the third .002 inch of 2.04. The treated strawboard had a crushing strength Iof 36.0 lbs. in the machine direction' and 22.3 lbs. in the cross direction, whereas the untreated strawboard had a crushing strength of 25.8 lbs. in the machine direction and 16.4 lbs. in the cross direction. The strength increase by reason of the silicate treatment, therefore, was 39 per cent in the machine direction and 36 per cent in the cross direction.

'Ihe silicate-treated strawboard was used as the corrugating material in fabricating corrugated board, A flute. on a, conventional pressure-type corrugating machinev with 16 pointA jute linerboard on both the single and double face sides. Sodium silicate was used as the corrugating adhesive. No deviation from the c'ustomary corrugating practice was necessary in fabricating this board from the silicate-treated strawboard. Similarly corrugated board was fabricated from the untreated control strawboard under the same' conditions. Flatwise crush tests were conducted on specimens of' these two boards. The strength of the board constructed With the ,untreated straw-board was 23.3 lbs/sq. in. and that of the board fabricated from the silicate-treated strawboard was 28.3 lbs./sq. in., the silicate treatment of the corrugating medium accounting for a `21 per cent increase in crushing strength.

The improved character of the berboard made with silicate-treated corrugating medium oi the example was further demonstrated by be attained.

manufacturing boxes from the corrugated board, boxes also being made from thetlberboardhaving untreated corrugating medium for comparison. These boxes were No. 2 can case size. boards in this fabricating operation were noted. The boxes were tested in compression, both end to end and top to bottom, ten specimens being tested in each direction. The boxeswere set up,v

all aps glued, vand thenconditioned to '7.2 per cent moisture content before testing. The compression strength of the boxes fabricated from the untreated strawboard was found to be 773 lbs. end to end and '793 lbs. top to bottom. The compression strength of the boxes constructed from the silicate-treated strawboard was found to be 997 lbs. end to end and 970 lbs. top to bottom, representing an advantage of 28 per cent and 23 per cent respectively for the latter boxes, While in the foregoing example certain materials and conditions were employed, it will be understood that the advantages of the invention may be secured with other materials and under other conditions.

To obtain a gradient of sodium silicate concentration in paperboard treated according to a process of this invention, the concentration and SiOzzNazO weight ratio should be correlated with the properties of the paperboard at the point of application. The factors requiring correlation are enumerated below.

1. concentration of silicate solution The concentration of silicate solution applied may be considerably varied, the particular conf centration used in a specic instance depending upon the condition of the paperboard at the point of application. The concentration may be such that the silicate solution has a specific gravity in thev range of from about 10 to 30 B., a gravity of about 20 B. being in most instances preferable.

2. Sz'Oz:Na2O ratioof silicate 'I'he SiOztNazO weight ratio of the silicate uscd should be from about 3.00 to 3.50 and preferably 3. Porosity of paperboard The porosity of paperboard treated is an important factor governing the concentration of silicate solution to be used. Ordinarily the porosity of the paperboard dependson the fibers from which the board is made and the extent to which the board has been compacted, as by heavy calcndering.' Such paperboards as kraft, straw, jute, an d chipboard vary, considerably in the rates at'which they are penetrated by silicate solutions. Chipboard, for instance, being relatively dense and non-porous, requires the use of a relatively dilute silicate solution. say 15 to 20 B. or less, in order that the desired concentration gradient from outside to inside may On the other hand. strawboard. being relatively porous, takes up the silicate solution rapidly and a more concentrated solution, say 20 to 25 B. or even higher, proves more satisfactory under ordinary conditions.

No differences in behavior of thetwo- 4. Moisture content yof paper The moisture content of the paperboard at the point of application of the silicate solution has a tent may vary considerably, preferably being about 6 to 8 per edit where the paper leaves the driers and where it leaves the calender rolls, and being higher than this at various intermediate pointsbecause of water applied from water boxes on the calender rolls. At the lower moisture contents relatively dilute silicate solutions may be applied whereas with higher moisture contents in the paper a more concentrated silicate solution should be used.

5. Size in .paper Other conditions being equal, the amount of size in paperboard treated with a silicate solution lniluences the distribution of the silicate thru the paperboard. 1 For paperboards containing considerable amounts of size -such as kraft it is ordinarily necessary to use a relatively dilute silicate solution, say 10 to 20 B., although more lconcentrated solutions can be 4used when a wetting agent is employed in the solution to facilitate penetration. With unsized paperboard, on the other hand, a more concentrated solution, say 20 to 30 B., should be employed, `else the distribution of the silicate in the paper is not graduated but rather, is uniform.

' 6. Amount of silicate solution applied 7. Thickness 0f Paper The thickness of a paperboard treated with silicate according to this invention should be in ex- -cess of .004 inch for paper coated only on one side and .in excess of .006 inch for paper coated on both sides. Ordinarily the paperboard used as corrugating medium will be considerably thicker than this, but it is noted that for corrugated board of equal strength one may start with a treatment is to be applied. Thus a strawboard corrugating medium weighing 32 lbs. per 1000 square feet, when treated with sodium silicate, has a strength equivalent to a strawboard weighing 34 lbs. per 1000 square feet. This represents a substantial saving to the paperboard manufacturer since the saving of iiber thus made possi- -ble more than offsets the cost of the treatment.

While in the foregoing description of this invention there have been shown numerous processes and products, it will be -understood that without departing from the spirit of this invention those skilled in the art may employ various processes for treating paper with sodium silicate and produce numerous treated products.

We claim:

'1. In a process for producing laminated berboard, the steps comprising treating at least one Y side of a paperboard, which is at least .004 inch lighter weight corrugating medium ifv a silicate l thick, with a sodium silicate solution having a specific gravity of from about 10 to 30 Baum and an SiOzzNazO weight ratio of from about 3.00 to 3.50, controlling the distribution of the silicate inthe paperboard so that the ratio between the percentage of sodium silicate in the outside .002 inch and the next lower .002 inch is from about 1.2 to 3.0, such control being accomplished by varying the silicate solution concentration inversely with the amount of size and directly with the moisture content and porosity of the paperboard, and the total amount of silicate solution applied being equivalent to from about 1 to 5 pounds of 40 Baum solution per 1000 sq. ft. of paperboard, and assembling the impregnated board into a laminated board.

2. In a process for producing corrugated fiberboard, the steps comprising treating at least one side of a paperboard, which it at least .004 inch thick, with a sodium silicate solution having a specific gravity of from about 10 to 30 B. and

an SiOzzNaO weight ratio of from about 3.00 to 3.50, controlling the distribution of the silicate in the paperboard so that the ratio between the percentage of sodium silicate in the outside .002 inch and the next lower .002 inch is from about '1.2 to 3.0, such control being accomplishedby varying the silicate solution concentration inversely with the amount of size and directly with. the moisture content and porosity of the paperboard, the total amount of silicate solution applied being equivalent to from about 1 to 5 pounds of 40 B. solution per thousand square feet of paperboard, corrugating the impregnated Paperboard, and assembling it into a corrugated board.

ERNEST R. BOLIER. RAYMOND F. REMLER.

Images