US3723164A - Mottle preventing treatment for paperboard - Google Patents

Abstract

PAPER AND PAPERBOARD, FOR USE IN PACKAGING DETERGENTS CONTAINING A PERSALT SUCH AS SODIUM PERBORATE, ARE TREATED WITH A WATER-SOLUBLE INORGANIC ALKALI SALT SUCH AS SODIUM NITRITE AND A BUFFERING AGENT SELECTED FROM THE GROUP CONSISTING OF CALCIUM CARBONATE, MAGNESIUM CARBONATE AND SODIUM SILICATE, TO STABILIZE THE SODIUM NITRITE IN THE PAPERBOARD, AND PREVENT THE PAPER AND PAPERBOARD FROM BECOMING MOTTLED IN THE PRESENCE OF THE PERBORATE CONTAINING DETERGENT UNDER EXTREMELY ADVERSE CLIMATIC CONDITIONS DUE TO OXIDATIVE DEGRADATION OF CARBOHYDRATE AND PROTEIN MATERIALS IN AND ON THE PAPER OR PAPERBOARD.

Description

United States Patent M 3,723,164 MOTTLE PREVENTING TREATMENT FOR PAPERBOARD Joseph Marton and Terezia Marton, Silver Spring, Md., assignors to Westvaco Corporation, New York, N.Y. No Drawing. Filed Apr. 29, 1971, Ser. No. 138,809

Int. Cl. B44d l/14 U.S. Cl. 117-76 P 4 Claims ABSTRACT OF THE DISCLOSURE SUMMARY OF INVENTION The present invention relates generally to the treat-- ment of paperboard with materials to inhibit the discoloration of the paperboard when used in containers to package detergents containing persalts. More specifically, this invention pertains to an improved process based on the disclosure in U.S. Letters Patent No. 3,536,578 assigned to the present assignee.

In the above-identified patent the use of persalts as bleaching agents in detergent formulations was recognized. Moreover, it was pointed out in the same patent that persalts, and especially sodium perborate, have been getting increased attention as bleaching agents in detergent compositions.

However, the use of sodium perborate in detergents has had certain disadvantages. Because of its high oxidative efficiency, paper or paperboard containers for the detergents become discolored during storage and are no longer esthetically appealing. The discoloration occurs on both the inside and the outside surfaces of containers in which the sodium perborate containing detergents are packaged. Generally, the inside surfaces have spots of brown color and in the most severe cases, the inside surfaces assume a substantially uniform brown color, whereas the outside surfaces of the containers may have a mottled appearance, or a non-uniform browning consisting of dark brown areas adjacent to light yellow areas.

The prior patent hypothesized that the discoloration of the paperboard Was due to the oxidative degradation of carbohydrate materials, and protein materials present in the board. It was suggested that during storage, and, especially in hot and humid areas, the perborate material in the detergent became active, thereby releasing hydrogen peroxide which oxidatively reacted with carbohydrate materials such as the cellulose in the paperboard and starch materials in and on the paperboard.

In terms of the container in which the perborate containing detergent is packaged, these reactions generally mean that the browning of the container on the inside surface involves the discoloration of the cellulose of the paperboard and the starch in the sizing, and the discoloration and mottling on the outside surface of the container is due to the oxidation products of cellulose, starch in the sizing and coating, and protein in the coating. The mottling on the outside surfaces of the containers is beleived to be caused by borate materials which migrate through the paperboard.

3,723,164 Patented Mar. 27, 1973 As noted in the prior patent however, it was found that the mottle or discoloration problem could be eliminated by manipulating the surface pH of the paperboard. This was done by treating the paperboard with chemical materials to provide the paperboard with an alkaline surface pH within the range of from 8.0 to 10.5, with a preferred range between about 9.0 to 10.0. Tests illustrated that the surface pH was the controlling factor and not the pH of the substrates beneath the surface layers which in most cases continued to have an acid pH after application of the alkali salts to the paperboard surface. The prior patent thus described and claimed a process for manufacturing paper and paperboard which included a sizing step wherein the paperboard was treated with an aqueous solution of an inorganic alkali salt to provide the paperboard with a surface pH of about 8.0 to 10.5. Specific claims in the prior patent indicated sodium nitrite to be preferred salt solution. Further studies of the mottling phenomena of perborate containing detergent con tainers formed from paperboard treated as described in the above-noted patent illustrated that while sodium nitrite was an efiective weapon against the occurrence of mottle, its protective ability was time-limited and dependent on humidity and temperature. I

DETAILED DESCRIPTION As noted hereinbefore, it was found that perborate containing detergents caused the paperboard from which detergent containers were made to become mottled. The invention described in the prior U.S. Pat. No. 3,536,578 showed that the application of sodium nitrite, a potent oxygen acceptor, to the surface of the paperboard in a surface sizing treatment to raise the surface pH to around -10, proved to be an effective supressant to the formation of mottle. Later observations indicated, however, that under extremely adverse climatic conditions typical of the Southern and Southeastern United States, the sodium nitrite became unstable after a few months storage. Detergent containers collected from supermarkets located in Miami, Fla., and, Houston, TeX., contained only a trace of nitrite after six months storage; and these containers also became mottle prone. Therefore, it became clear that the ability of the sodium nitrite to protect the paperboard surfaces from ultimately becoming mottled was limited to the time during which the sodium nitrite remained in the board. Moreover, the disappearance of the nitrite from the board was surprising since it was known that nitrite solutions are normally fairly stable in the neutral or alkaline region, that is, above a pH of 5-6.

It was suggested that an application of excessive sodium nitrite to the paperboard would prolong the presence of nitrite in the paperboard. However, under the extremely adverse climatic conditions encountered, it was discovered that the excess alkali tended to promote the degradation of starch and cellulose yielding color reversion in the board along with an acidification of the board which was accelerated in the presence of heat and humidity. Moreover, it was also demonstrated with experiments that heating of the moistened paperboard containing sodium nitrite, without detergent, at 50 degrees C. for 45 hours, removed almost all of the nitrite content. Thus it was decided that the stability of the sodium nitrite in the paperboard was directly afiected by a decreasing of the pH of the paperboard due to an acidification of the paperboard in the presence of increased moisture and temperature. In addition, it was discovered that in the detergent containers, the perborate from the detergent would penetrate the board and react with the sodium nitrite under these adverse conditions of humidity and temperature. Thus it became important to develop some means whereby the stability of the sodium nitrite in the board could be improved to keep the paperboard substantially mottle free under the most adverse conditions.

It was obvious that increased doses of sodium nitrite to the board to maintain a continuously high surface pH was not the single answer, so, the activity of the perborate containing detergent was studied.

It was found that perborate was unstable in the detergent itself, and its decomposition was shown to increase with increases in moisture and temperature. Under normal conditions, at room temperature, perborate in typical detergents lose their activity very slowly with about half of the activity remaining at the end of several years storage. At 60 degrees C., the time to lose half activity is reduced to 12 days, and at 110 degrees C., to 42 minutes. The calculated activation energy of perborate was determined to be around 30 KcaL/ mole.

Sodium nitrite on the other hand is a salt of nitrous acid. When sodium nitrite is hydroylzed by water:

Nitrous acid is unstable when heated in a dilute solution and easily decomposes:

Further, while a strong oxidizing agent will convert the nitrite to nitrate, sodium nitrite can also act as an oxidant:

N gas is oxidized by air to N0 which in turn, gives nitric acid with water:

Nitrous acid can act as an oxygen scavenger, but it may also contribute to oxidation of the carbohydrates or other substances in the board.

Therefore, one of the probable roles of sodium nitrite is to react with the perborate and deprive it of the obnoxious oxygen. Studies indicated that the rate of oxidation of sodium nitrite to sodium nitrate by perborate was quite rapid at first, but increased at a slower rate with increasing temperature than, for instance, the rate of perborate decomposition. The role of the sodium nitrite in the board is, of course, to conteract the penetrating perborate solution and to react with the perborate before it gets a chance to oxidize the surfaces of the paperboard.

In addition, in dilute solutions, the stability of sodium nitrite was found to be strongly pH dependent. At 50 degrees C. and at a pH greater than 5.5, the sodium nitrite was stable, whereas at a pH of 4.5, there was a loss after 50 hours and a loss after 100 hours. If perborate was present, the reaction product was determined to be nitrate. At 50 degrees C. and at a pH greater than 5.5 in the presence of perborate, there was a 6 conversion of the sodium nitrate after 50 hours. With the same solution at a pH of 4.5, and under identical temperature conditions, there was an 85% conversion of sodium nitrite to sodium nitrate after 50 hours. And, of course, the sodium nitrate is completely ineffective as a means for preventing mottle in paperboard.

On the other hand, when a solution containing between 02-05% sodium nitrite with about 10% moisture was applied to paperboard in the absence of perborate, and the board was placed in a closed container at 50 degrees C. for 45 hours, at least 85 to 95% of the nitrite content was lost. This loss was due partly to an oxidation in the form of nitrate and partly in other ways. Hence, these experiments illustrated that even through the pathway of nitrite degradation in the board was complicated and not completely understood, certainly the propensity of the board to mottle could be arrested if some means were discovered for stabilizing the sodium nitrite in the board. Further experiments showed that the nitrite could in fact be stabilized in the board by the application of a butfering agent. The preferred treatment was found to comprise the use of magnesium carbonate, but, both calcium carbonate and sodium. si icate were alsQ found to gi sa i a t y results in retaining a part of the nitrite in the paperboard even after aging of the board under extremely adverse conditions.

Therefore, for the present invention, a buffering agent was added to the paperboard in addition to the sodium nitrite treatment, to stabilize the sodium nitrite in the paperboard with satisfactory results. The paperboard was made substantially as before, as taught in US. Pat. No. 3,536,578, except that at the size press, where normally a solution of clay and starch is applied, in two cases a part of the clay was replaced with either calcium carbonate or magnesium carbonate, and in another case, sodium silicate was added to the size press mixture in addition to the normal clay and starch solution.

As shown in Table I, the paperboard for the present invention was made in a conventional manner and then applied with a sizing treatment on both sides of between 1 and 2 pounds per side. The sizing treatment varied, comprising a conventional sizing with and without sodium nitrite, and with varying amounts of clay and different buffering agents. After drying, the sized paperboard was then coated on one side (wire side) with a conventional coating composition comprising adhesive and pigment. The paperboard was then used to construct detergent containers which were tested both empty and containing detergent.

It should be appreciated however, that the sodium nitrite treatment could be made at the waterbox on a calender after the sizing treatment instead of simultaneously with the sizing if desired. The prior US. Pat. No. 3,536,578 clearly illustrated that the place of application of the sodium nitrite to the paperboard was equally effective both at the size press or at the waterbox. In addition, the sodium silicate buffering treatment could also be successfully applied at the waterbox if desired with equally satisfactory results. Of course, when either calcium carbonate or magnesium carbonate were used as the buffering agent, the application would have to be in the form of a size press treatment.

In the examples which follow, the size press mixtures each consisted of certain standard ingredients with other ingredients added as shown by Table I.

TABLE I Preparation of Size Press Mixtures Standard Ingredients: (percent solids 13-16) Starch PG 230: (450 gr.) Starch PG 260: (150 gr.) 50% NaOH: (20 gr.) Carbowax 3000: (15 gr.) Calgon: (10 gr.)

DC A: (40 gr.)

Additional size press ingredients (grams) No. 2 Parez Ex. clay 613 CaC O3 MgCOa NazSlO; NaNOz Water 850 (Control) 11, 000 850 200 10, 500 425 200 12, 000 425 425 200 11, 000 800 200 200 10, 000

As shown in Table I, the first example was the control and consisted only of the standard size press ingredient plus clay and Parez 613 a melamine formaldehyde resin. The board in Example 2 was applied with a standard size press treatment including the addition of sodium nitrite as disclosed in the prior US. Pat. No. 3,536,578. The remaining three examples were treated at the size press equal parts of clay and calcium carbonate (Example 3); equal parts of clay and magnesium carbonate (Example 4); and, with clay plus sodium silicate (Example 5 The examples noted above were then put through two tests, an accelerated aging test to determine the retention of the nitrite in the paperboard under adverse conditions, and, a simulated field test where after 4 weeks of aging, visual observations were made a d recorded to show the TABLE II Accelerated Aging Test 50 degrees 0., 46 hours (+10% moisture) Initial After aging NOz-N NOz-N Percent Examples (p.p.m.) (p.p.m.) retention TABLE III Aging of Paperboard with Bold Detergent 50 degrees C.4 weeks Initial N O 2-N (p.p.m.) Visual observations No gottle felt side-Heavy mottle outside.

Examples 2. 290 o. 3 255 No mottle felt side-Very light outside. 4- 390 Do.

In another example using paperboard produced at Westvacos Covington, Va. paper mill, the paperboard was subjected to the same tests described above for the paperboard produced on a laboratory paper machine. In 3 this latter example, the paperboard was made substantially as described in prior US. Pat. No. 3,536,578 except that the sodium nitrite treatment was applied at a waterbox on the calender and calcium carbonate was substituted for one-half the clay at the size press. For this example, the percent retention of nitrite in the board after the accelerated aging test at 50 degrees C. for 45 hours (with 10% moisture), illustrated that 28% of the nitrite remained in the board. In the visual test for the determination of mottle after aging the paperboard containers packaged with Bold detergent at 50 degrees C. for 4 weeks, the container showed no mottle. Thus it can be shown by this example that it is not critical where the nitrite is applied, only that it is critical whether or not the buffering agent is added.

In the accelerated aging test (Table II), the percent retention'of nitrite in the paperboard was measured after the board underwent the following test. Detergent containers were fabricated from the paperboard and a known amount of water (approximately 5-8 percent) was applied to the surface of the paperboard to simulate the high humidity conditions which were found to be troublesome. The tightly closed containers without detergent were then placed in an oven at 50 degrees C. and left for 45 hours to accelerate the aging of the board before measuring the nitrite retained.

For the visual observations to determine the amount of mottle protection afforded with the addition of buffering agents (Table III), both treated and untreated samples of the paperboard were used to construct detergent containers, and the containers were then filled with detergent which included sodium perborate as the bleaching agent. The coated or wire side of the paperboard was the outside surface of each detergent container in each example. The filled containers were then grouped in sets of two or three boxes and wrapped and sealed initially with kraft paper. A second double layer wrap of aluminum foil was then applied and all seams sealed prior to placing the packages in the oven. The packages were then placed in an oven at 50 degrees C. for 4 weeks, and afterwards I the center-board surfaces of each container were checked visually for mottle and with a reflectance instrument. The test was carefully designed to simulate as closely as possible typical containers on a supermarket shelf under the extremely harsh environmental conditions encountered in the problem areas of the country. Of course, as shown in Table III, the treated boards performed quite satisfactorily. In addition, both the accelerated aging test of the board to determine the retention of the sodium nitrite in the paperboard, and the extended aging test of the paperboard containers containing detergent have been shown to correlate well with actual field results.

From the above data, it can be seen that when sodium nitrite is applied either with a size press treatment or a waterbox treatment to paperboard, it can be made to remain in the paperboard with a suitable buffering agent which is also applied with the size press treatment. Moreover, the data also shows that when the nitrite is somehow fixed or retained in the paperboard by the buffering agent, a substantial reduction in the mottling tendency of the coated side of the paperboard can be achieved.

While the theory behind the effectiveness of providing the paperboard with the bufifering agents to fix the nitrite in the board is not fully understood, it is believed that without the addition of a buffering agent, the moist fiber surface accelerates the hydrolysis of nitrite thus:

And the nitrous acid formed undergoes several types of reactions (oxidation, nitration, decomposition) with each of these reactions resulting in the effective loss of the protective agent nitrite. The buifering agent tends to prevent this hydrolysis and thereby retains the nitrite in its substantially inert form. The buffering agent is not too eifective however, and still releases just enough nitrous acid to react with the perborate when it penetrates the board under the extremely severe conditions of temperature and humidity.

Since the protective action of the nitrite is based primarily on its reaction with perborate, too much alkalinity would render the oxygen acceptability of the nitrite minimal. The compromise of course is to keep the nitrite in its reactive state but still prevent it from being hydrolyzed. Apparently the selected buffering agents serve this purpose From the above example, it is apparent that the treatment of paperboard with sodium nitrite, as described in the prior US. Pat. No. 3,536,578, and, with a buffering agent as described herein, presents an effective deterrent to the discoloration and mottle problems under conditions of high humidity and temperature, that were found in paperboard containers for perborate containing detergents. Specifically, the treatment disclosed herein which involves only an addition of from 1025 pounds of buffering agent per ton of paper produced, satisfies the requirement of mottle-free paperboard in extremely adverse climatic conditions.

Obviously, however, since only several specific treatments have been fully disclosed herein, it is believed that other various changes may be made in the treatments described without departing from the spirit of the invention or the scope of the appended claims.

We claim:

1. Paperboard, suitable for use in containers to package detergents containing a persalt such as sodium perborate as a bleaching agent, bearing on at least one side thereof a coating containing starch and sodium nitrite, the improvement comprising an insoluble buffering agent in the coating selected from the group consisting of magnesium carbonate, calcium carbonate or sodium silicate for the purpose of stabilizing the sodium nitrite in the paperboard and preventing the discoloration of the carbohydrate and protein materials in and on the paperboard under adverse climatic conditions.

2. In the process of manufacturing paperboard which comprises the steps of sizing the paperboard with a sizing treatment containing starch and sodium nitrite, coating at least one surface of the paperboard with a coating composition that includes pigment and binder, and then drying the paperboard, the improvement for the prevention of mottling of the paperboard in the presence of perborate containing detergents, consisting of applying to the paperboard an insoluble buffering agent selected from the group consisting of magnesium carbonate, calcium carbonate or sodium silicate for the purpose of stabilizing the sodium nitrite in the paperboard under conditions of high humidity and temperature.

3. The process of claim 2 wherein the sodium nitrite and the buffering agent are applied to the paperboard simultaneously with the sizing treatment.

4. The process of claim 2 wherein the sodium nitrite and the buffering agent are applied to the paperboard in separate treatments.

References Cited UNITED STATES PATENTS Bowlby 11786 Yowtz 162160 Fleck.

Brundige 162206 Bolt 212.5

Baseman 212.5 Brundige et al. 117169 X WILLIAM D. MARTIN, Primary Examiner M. R. LUSIGNAN, Assistant Examiner US. Cl. X.R.