IL31561A - Strengthened impregnated fibrous material and process of producing same - Google Patents

Description

inpsjn τ'ϊπη'» ρτιηη AISO Ί»Ο ηοιη Strengthened impregnated fibrous material and process of producing same 1) IMMA OEL LICHTENS "EIN 2) RUTH YRA WEI3B3RG 3) FREDRIC A ELIZABETH STILIMN 4) DAVID RICHARD GOLDSTEIN and 5) LOUISE M. GOLDSTBII doing business as STRUCTURAL PAPER ΟΟϋΡΑΙΥ C. 29861 LICHTENSTEIN ET AL This invention relates to the impregnation of fibrous material including felted cellulosic materials such as paperboard and corrugated container board for the purpose of discouraging or preventing rodents from gnawing through the material. The invention is particularly useful in the manufacture of shipping or storage containers for food and for the erection of static storage facilities such as bins and the like for food intended for human consumption. To impart rodent resistance to such fibrous material the present invention provides the impregnation of the material with potassium alum or ammonium alum within relative narrow ranges of conditions to accomplish the required degree and uniformity of penetration.

In British Patent No. 779,619 there is disclosed the impregnation of fibrous products with potassium alum or ammonium alum to secure certain advantages as set forth in that patent. Some of the advantages thus set forth are increased strength, fire resistance and, as appropriate, water resistance. In said British patent there are set forth certain ranges of degree of impregnation and certain ranges of conditions to be maintained in the process for achieving best increases in strength, fire resistance and in some cases water resistance of the impregnated material. It has now been found that by operating within ranges which in some instances differ in various respects from those best calculated for the purposes of the patent aforesaid, a very-high degree of rodent resistance may be achieved at a low cost. Also it may be achieved without significant deterioration of appearance of the product, the latter being of sub-stantial importance in the food industry.

Rodents eat a significant amount of the food supply of the world during the shipment or storage thereof particularly in the very areas where starvation of the human population is a constant threat. The need for rodent resietant packaging and storage materials is very great. A good method of testing rodent resistance of a packaging or constructional material for the shipping and storage of food consists of the following steps. The rodent is placed in one end of a cage and food is placed in the other. The ends are separated by a vertical barrier made of the material to be tested. The barrier is raised above the floor of the cage a sufficient amount so that the rodent is aware of the food. Furthermore, the raising of the barrier gives the rodent an edge on which to secure his teeth in an at-tempt to gnaw through the barrier to reach the food. The rodent is not fed for a considerable period of time prior to being put into the test cage. During the test, while the rodent is in the cage, it is given unlimited water, i.e. ad-libitum. The time required for the rodent to go through the barrier is an inverse measure of the rodent proofness of the material. In the extreme the rodent never gnaws through the barrier and dies. In tests of the material of the present invention in the vast majority of cases the rodent completely failed to gnaw the material and in and did not persist in gnawing to such extent as to accomplish passage through the barrier.

While the reasons for failure of the rodent to gnaw through or to substantially damage the alum impregnated barrier of the present invention are not fully understood it is considered possible that the taste and the astringency of alums which human beings find to be objectionable are also objectionable to rodents. Also it is possible that some characteristic of the alum impregnated barrier arouses an instinctive sense of danger to the rodent. The latter seems possibly to be indicated by the many instances in which a rodent deposited urine on an area where it had abandoned attempts to gnaw. This procedure is already known to be followed by rats for labeling a particular area or substance as dangerous.

As disclosed in said British Patent No. 779,619 fibrous products are impregnated by immersing the same in a bath made up of molten ammonium or potassium alum diluted with water and maintained at an elevated temperature. The relative amounts of molten alum and water and the temperature of the bath are maintained within rather narrow ranges to achieve the type of impregnation best calculated to achieve the increase in strength and the other characteristics provided in said patent. In a general sense the ranges set forth in said patent impart a great deal of stiffness to the impregnated product but when considered from the standpoint of very small increments of area or thickness of the product do not necessarily result in uniformity of distribution of the impregnant within the strength of the product, the alum may be quite highly concentrated in the regions near the opposite major surfaces of the product and quite low in concentration or absent altogether in the central regions of the thickness of the thick-ness of the product. Also, the relatively high specific gravities and relatively high temperatures constituting the preferred ranges set forth in said patent result in somewhat uneven distribution patterns in the zones of highest concentration of the impregnant. Such unevenness exists be-tween surface increments or thickness increments which are sufficiently small when compared with the size and thickness of the product as to be of no consequence from the standpoint of strength or stiffness of the final product.

However, from the standpoint of rodent resistance it has been discovered that the development of maximum strength or stiffness is not required and that the minimizing of unevenness of distribution is essential. In the tests aforesaid it was observed that unevenly or shallowly penetrated fibrous products, some exhibiting great strength in accordance with the teachings of said patent were not effective rodent barriers inasmuch as they were damaged by gnawing in some areas or were delaminated or peeled after which the rodent sometimes was able to work his way through the barrier.

For maximum rodent resistance the distribution of the alum penetrant in the fibrous structure of the product must be quite even in all three dimensions of each element of the product. By this it is meant that the distribution should be substantially uniform in concentration in the least devoid of sharply changing layers of concentration through the thickness of the fibrous product. Theoretically the concentration of penetrant throughout the thickness of the product could be made uniform or substantially so but from a practical standpoint absolute uniformity would be difficult if not impossible to achieve. The present invention provides ranges of operating procedures and other techniques to achieve the type of penetrant distribution required for maximum rodent resistance.

It has been found that for both ammonium alum and potassium alum the specific gravity and the temperature of the alum-water bath should be lower than or at least at the lower ends of, the ranges set forth in said patent in order to achieve the evenness and depth of penetration best cal-culated for rodent resistance. Also, it has been found desirable to utilize penetration-promoting substances and techniques as will be explained below.

The present invention is applicable to any fibrous product which is customarily used in the container and paperboard fields such as fiber board, corrugated board or corrugating medium made of any of the usual fibrous materials such as virgin pulps from wood, sisal, straw etc, or secondary pulps such as used in chipboard, newsboard and the like or mixtures of any of these pulps. Also it is applicable to containers, container elements, tubes and the like made from such materials. A particular product with which the present invention is very advantageous is corrugated board made up of one or more liner boards laminated with one or more plies of corrugating medium. When such same in the treating bath tinder conditions such that it will be certain that the bath liquid will flow throughout the lengths of all of the flutes, thus to expose all surfaces of all elements of the corrugated board to penetration by the bath liquid.

Liner boards used in the manufacture of corrugated board sometimes have one surface which is smoother than the other. Particularly when the smoother surface has been calendered it is a customary practice to arrange the less smooth surface inside the structure to take advantage of the better "pasting" characteristics of such surface, which is more easily penetrated by the adhesive. For the purposes of the present invention it is preferable to arrange the less smooth surface, when there is a difference, to lie outside the corrugated structure thus to take advantage of the greater penetrability of such surface to promote uniformity and depth of penetration of the outer surfaces by the alum-water bath. If so desired the paperboard may be deliberately manufactured in such a way as to have both surfaces readily penetrable by the alum-water bath. For example, calendering of either surface may be omitted.

The rapidity, evenness and depth of penetration may be enhanced when so desired by steaming the fibrous product just prior to immersion into the alum-water bath. It also has been observed that there is a tendency for the upper surface regions of the alum-water bath to have a higher concentration of alum than the remaining regions. Thus when a fibrous product is withdrawn from the bath there is some tendency toward the deposit, on the surface r alum which sometimes crystallizes and degrades the appearance of the final product. The steaming of the product before immersion can be carried out by arranging nozzles supplied with low-pressure steam, say 50 psig or less, directed upon the product as it enters the bath. Steam nozzles also may be directed onto the upper surface of the bath to cause local dilution of the surface portions of the bath to minimize the superficial deposit just mentioned. If the steam nozzles are also directed on the product as it emerges from the bath the steam will be effective to even out the surface distribution of the alum, thus promoting the rodent resistance thereof while also improving the surface appearance by minimizing surface bloom.

Other or further penetration-promoting procedures may be employed if so desired. For example although sodium alum, Na^SO^.Alg (S0^)^. iH20 is not suitable as an impregnant for the present invention, with either ammonium alum, A12(S0^)^. (NH^)2S0^.2!|H20, or potassium alum, Al2(S0^)^.K2S0^.2iH20, penetration is enhanced by adding to the alum-water bath a sodium ion source. The mass percent of sodium added should be from about f0 to about 6 of the total mass of the alum in the bath. Lesser quantities of such additive have little effect while greater quantities cause the final product to be undesirably weak or limp. The sodium may be added in the form of sodium silicate or sodium sulfate, for example.

Another helpful procedure which may be used if so desired is to add a suitable quantity of wetting agent to the alum-water bath, whether or not sodium has been added as readily at the temperature and pH of the particular bath may be used, for example, sodium dihexyl sulfo succinate may be added in the proportion of about 0.2$ to 1,0% (based on total weight of the bath)„ The addition of sodium and/or a wetting agent to the alum-water bath may be made with the specific gravity and the temperature of the bath maintained at any point within the best or acceptable ranges which will be discussed below. Since different fibrous products may have somewhat different characteristics with regard to imbibing or accepting penetration by the molten alum the use of one or both of such penetration-promoting additives will be most useful when the particular fibrous product at hand is relatively difficult to penetrate. Also such addition may make it possible to operate at somewhat higher specific gravities in the treatment of fibrous products of different characteristics. In a very general sense the achievement of deep and evenly distributed penetration appears to be accomplished when the specific gravity and temperature of the bath are both maintained toward the lower regions of the ranges set forth below and the use of additives frequently is very helpful in operations in the upper regions of said ranges.

For uniformity of impregnation the ammonium alum-water bath temperature is best maintained from about 9^0C. to about 99°C. when the specific gravity of the bath lies within the ranges given below. The comparable temperatures for the use of potassium alum are from about 93°C. to about 99°C. These temperature ranges give best results. Accept of 99°Go to 10i-°G. For potassium alum the analogous temperatures are 97°C. to 102°C.

Ammonium alum melts at 93«5°C» and in its undiluted molten state has a specific gravity of about 1.550. Potassium alum melts at 92.0°Co and in its undiluted molten state has a specific gravity of about 1.70. For the present invention the specific gravity of the bath is made quite low by water addition. Typically in the case of ammonium alum it is made from about 1.32 to 1.375 for best results but acceptable results can be had within the range of about 1.325 to about I.I.52. For potassium alum best results are secured in the specific gravity range of from about 1.315 to about I.38O and acceptable results can be achieved from about 1.315 to about I.I.I.O.

In the best range for impregnation as regards temperature and specific gravity a typical fibrous product namely, corrugated board may be caused to pick up alum in well distributed form in an amount preferably in the range of from about 100 up to about 20$ of the weight of the unimpregnated product. This is regarded as the best range and will be referred to hereinbelow. Smaller pick up percentages down to about 50$ of the weight of the unimpregnated product are useful but as will be pointed out below are somewhat less rodent resistant. Pick up percentages substantially exceeding about 220$ of the weight of the unimpregnated product usually result in blooming or crystallization of the alum on the surfaces of the product and indeed may be harmful to a corrugated board, in particular, in that the crystals appear to weaken the original medium. Also, as will be pointed out below,high pick up ratios suc as just mentioned produce a product which is less rodent resistant than are the products made within the best ranges of all conditions. As already mentioned the use of wetting agents or the addition of sodium frequently will make it possible with particular types of fibrous products to operate toward the' upper portions of the pick up and specific gravity ranges with better than acceptable results.

In the best ranges of pick up ratio and temperature and specific gravity rodent attack occurs on 5 or less of the total area with no boring thrOugh. When operating within the best ranges of temperature and specific gravity but permitting the pick up ratio to rise substantially above 220$, rodent attack occurs on $0% or less of the surface area and some boring through occurs but usually not to such an extent as to permit passage of the rodent through any openings thus made. When the pick up ratio is permitted to drop down to about $0% but with the specific gravity and temperature lying within the best ranges, rodent attack occurs on $0% of the surface area, marginal boring through occurs on about half of the sample barriers and full boring through occurs only on about 10% of such barriers. Results of the latter type also occur when the specific gravity or temperature of the bath lies within the upper limits of the acceptable ranges. It will be apparent that reduction of full boring through to about 10% is nevertheless a substantial accomplishment when it is considered that standard corrugated board can not be regarded as having ditions of pick up percentage, temperature and specific gravity in about the same manner as is the rodent resistance. That is when the product is unevenly or superficially impregnated more than $ of the surface of the final product will be covered by crystalline impregnant. This also is particularly noted in operations at the higher temperatures and specific gravities especially in the absence of wetting agents or sodium. It is also noted when the percentage pick up is permitted to rise substan-tially above about 220$. Thus from a visual standpoint operations outside the preferred ranges usually will produce mottling or blooming observable by the unaided human eye and affecting more than 5% of the surface area.

If one wants, in the use of this invention, to a ply a waterproof coating to the alum impregnated product this may be done by passing the material either cold or hot through a flowing curtain of molten waterproof material. A typical waterproof material is vinyl acetate -ethylene copolymer lj? petroleum hydrocarbon resins 10 ; paraffin 1 . Any material that gives continuous film characteristics to the coating, such as paraffin-montan wax mixture is effective. The surface of the coated products will show to the unaided human eye a mottled appearance in which more than of the surface will have a different color or shade, if the alum impregnation is not carried out within the preferred or acceptable ranges set forth above. Such waterproof coatings may be desirable for containers or storage structures which are to be exposed to the weather or to other adverse moisture themselves flammable the addition of such coatings to the alum treated products of the present invention does not substantially degrade the fire resistant characteristics of the alum treated products. This is in considerable contrast with those fibrous products which are impregnated with flammable materials instead of being thinly coated as is the case in the waterproofing of the product of this invention. The waterproof coating appears to have little, if any, effect upon the rodent resistance of the alum impregnated material.

Impregnation of fibrous products in accordance with the present invention by immersing the same in a bath, as described above, is the preferable procedure. However acceptable results can be obtained by carefully controlled hot spray application. The hot spray application usually does not permit the production of products of best appearance but, if carefully controlled, sufficiently deep and sufficiently even penetration can be achieved to give the product at least acceptable rodent resistance characteristics.

Claims (7)

31561-2 φ Lichtenstein e al

1. A process for producing a strengthened impregnated rodent -resistant fibrous material such as paperboard which comprises: (I) impregnating said fibrous material by treating the same in an impregnating bath containing ammonium alum or potassium alum in molten form, (II) maintaining said impregnating bath, by the addition of water to said molten alum, at a specific gravity (a) within the range of 1.325 to 1.1+52 when ammonium alum is used, and (b) within the range of 1.315 to I.IJ4.O when potassium alum is used, (III) maintaining said impregnating bath at a temperature (i) within the range of ¾.°C. to 101+°C. when ammonium alum is used, and (ii) within the range of 93°C. to 102°G. when potassium alum is used, and (IV) terminating said treating of said fibrous material when it has imbibed said alum in a quantity within the range of from about $0% to about 220% of the weight of said fibrous material before impregnation.

2. A process according to claim 1 , wherein the specific gravity of the impregnating bath is maintained within the range of 1 .325 to 1.375 when ammonium alum is used and is maintained within the range from 1.315 to 1 .360 when potassium 81561-2 I alum is used, wherein the temperature of said impregnating bath is maintained within the range from 9l °C. to 99 ° C. when ammonium alum is used and within the range from 93° C. to 97°G. when potassium alum is used, and wherein the treating of the fibrous material is terminated when it has imbibed said alum in a quantity within the range of from about 100% to about 220 of the weight of said fibrous material before impregnation.

3. A process according to claim 1 or 2, in which the fibrous material immediately prior to being treated in the bath is showered with steam on all of its surfaces.

4. I.. A process according to claim 1 or 2, in which the fibrous material is showered with steam on all of its surfaces after it has been treated in the impregnating bath but before the molten alum imbibed by the fibrous material has hardened.

5. A process according to claim 1 or 2, in which the impregnating bath is a continuous body of liquid and in which the treating step and the terminating step are performed respectively by dipping the fibrous material into and removing the fibrous material from said body of liquid through a surface of said body, and in which said surface of said body is continuously showered with steam to cause local dilution of those portions of said body of liquid which are adjacent said surface.

6. A process for producing a strengthened impregnated rodent-resistant fibrous material substantially as hereinbefore described.

7. . A rodent-resistant fibrous material whenever produced by the process according to any one of the preceding claims . por ttie /kp?iic3nts