EP0216145B1

EP0216145B1 - Heat treatment of paper products having milk and other additives

Info

Publication number: EP0216145B1
Application number: EP86111506A
Authority: EP
Inventors: Dinkar G. Wagle; Vacheslav M. Yasnovsky; Jeffery R. Jones
Original assignee: International Paper Co
Current assignee: International Paper Co
Priority date: 1985-08-23
Filing date: 1986-08-20
Publication date: 1991-01-30
Anticipated expiration: 2006-08-20
Also published as: BR8604012A; EP0216145A3; EP0216145A2; KR870002330A; FI863422A0; JPS6285097A; FI863422A; DE3677301D1

Description

This invention relates to the art of papermaking, particularly to treating paper products having additives such as protein, carboxy methyl cellulose, latex, milk, or starch with heat and then immediately rewetting the product to improve its properties, including dry and wet stiffness, wet tensile strength and opacity.
In the art of papermaking, it is customary to subject felted fibers to wet pressing, to consolidate the web, and then to drying on heated rolls.
There is currently considerable interest in improving various properties of paper and boards. Quantifiable paper properties include: dry and wet tensile strength, folding endurance, stiffness, compressive strength, and opacity, among others. Which qualities should desirably be enhanced depends upon the intended application of the product. In the case of milk carton board, for example, stiffness is of utmost importance. Linerboard has three aualities of particular interest, namely wet strength, folding endurance, and high humidity compression strength.
All of these properties can be measured by well-known standard tests. As used herein, then, "wet strength" means wet tensile strength as measured by American Society for Testing and Materials (ASTM) Standard D829-48. "Folding endurance" is defined as the number of times a board can be folded in two directions without breaking, under conditions specified in Standard D2176-69. "Stiffness" is defined as flexural rigidity and is determined by the bending moment in g-cm. "Linerboard", as used herein, is a medium-weight paper product used as the facing material in corrugated carton construction. Kraft linerboard is linerboard made according to the kraft process, and is well known in the industry. Folding carton board is a medium to heavy weight paper product made of unbleached and/or bleached pulps of basis weights from 40-350 g/m^2.
V. Vulchev et al. describes in 1004 Abstract Bulletin 56 (1986) May, No. 11, Appleton, Wisconsin, USA, the effect of drying temperature on surface-treated kraft liner properties.
US-A-2,116,544 relates to a method of enhancing the wet-strength of varios kinds of papers including papers impregnated with colloidal binders which, upon drying of the paper are more or less irreversibly set to impart water-resistivity or wet-strength to the paper.
Prior workers in this field have recognized that high-temperature treatment of paperboard can imrpove its wet strength. See, for example E. Back, "Wet stiffness by heat treatment of the running web", Pulp & Paper Canada, Vol. 77, No. 12, pp. 97-106 (Dec. 1976). This increase has been attributed to the development and cross-linking of naturally occurring lignins and other polymers, which phenomenon may be sufficient to preserve product wet strength even where conventional synthetic resins or other binders are entirely omitted.
It is noteworthy that wet strength improvement by heat curing has previously been thought attainable only at the price of increased brittleness (i.e., reduced folding endurance). Embrittled board is not acceptable for many applications involving subsequent deformation, and therefore heat treatment alone, to develop the wet strength of paperboard and carton board, has not gained widespread acceptance. As Dr. Back has pointed out in the article cited above, "the heat treatment conditions must be selected to balance the desirable increase in wet stiffness against the simultaneous embrittlement in dry climates". Also, in U.S.-A-3,875,680, Dr. Back has disclosed a process for heat treating already manufactured corrugated board to set previously placed resins, wherein the specific purpose is to avoid running embrittled material through a corrugator.
It is a plain that improved stiffness and wet strength, on one hand, and improved folding endurance, on the other, were previously thought to be incompatible results.
It is, therefore, an object of the invention to produce paperboard having both improved stiffness and wet strength, and improved folding endurance.
With a view to the foregoing, a process has been developed which dramatically and unexpectedly increases not only the stiffness and wet strength of various paperboards, but also preserves their folding endurance. In its broadest sense, the invention comprises steps of 1) applying one or more additives selected from the group including protein, latex, milk or starch to paperboard; 2) heating the paperboard so treated to an internal temperature of at least 400°F (205°C) and 3) rewetting the board immediately after the heat treatment to at least 1 % moisture by weight without allowing the product to cool substantially before the water application. These steps are followed by conventional drying and/or conditioning of the treated board. It is to be understood that steps 2 and 3 can be repeated several times.
This method produces a product having folding endurance greatly exceeding that of similar board whose stiffness and wet strength have been increased by heat alone. This is clearly shown by our tests exemplified below.
Of course, those skilled in the art will recognize the necessity of the product conditioning to a normal moisture content after this very hot treatment. See, for example, U.S.-A-3,395,219. A certain amount of rewetting is normally done, and in fact product properties are never even tested prior to conditioning. However, conventional rehumidification is done after the product has substantially cooled.
Our rewetting treatment differs from conventional conditioning in that we add water, by spraying or otherwise, to a very hot and dry paper or board at the very end of the heat treahment without allowing the product to cool substantially before the water application. It is critical that water be applied to the product while it is still hot, certainly above 50°C (122°F), and preferably above 205°C (400°F). Another heat treatment or drying step may follow rewetting, on or off the machine, during a subsequent operation such as sizing, coating or calendering.
We prefer to raise the internal temperature of the board to at least 450°F (232°C) during the heat treating step, as greater stiffness and wet strength are then achieved. This may be because at higher temperatures, shorter step duration is necessary to develop bonding, and there is consequently less time for fiber degradation to occur. Also, shorter durations enable one to achieve higher production speeds.
While the invention may be practiced over a range of temperatures, pressures and duration, these factors are interrelated. For example, the use of higher temperatures requires a heat treating step of shorter duration, and vice-versa. For example, at 550°F (289°C), a duration of 2 seconds has been found sufficient to obtain the desired improvements, while at 420°F (215°C), considerably longer is required.
The invention can be carried out either on a conventional papermaking machine or off the machine in an oven after a size-press; for high speed production, a papermaking machine is preferred.
In either event, the paper product is first treated by adding it to one or more of the following additives: protein, starch, latex or milk. The additive may be mixed with the pulp prior to sheet forming, or it may be added to a formed sheet by spraying or other means. Following wet pressing, the paper product is heat treated.
In the heat treatment step, the initial water content of the web must be in the range of 1-40% by weight and preferably to within the 10-15% range. Sufficient heat is then applied to the board to achieve an internal paper temperature of at least 400°F (205°C). The heat can be applied in the form of hot air, superheated steam, heated drying cylinders, infrared heaters, or by other means. Alternatively, the invention may be practiced by heating paper product in an oven after a size-press. The internal temperature of the baord should be brought to at least 400°F for at least 10 sec. Again, the nature of the heat source is not important.
The paper is immediately rewetted following the heat treating step, and while the paper is still hot. Water may be applied by spraying or other means. Even though one effect of the water application is to cool the paper, it is important that the paper not be allowed to cool substantially before the water application.
The heat treated and rewetted paper is then cooled, conditioned, and calendered according to conventional procedure.
The invention has been practiced as described in the following examples. The improvement in board quality will be apparent from an examination of the test results listed in the tables below.

Example 1

A commercial bleached kraft board was sized with different starch protein (casein) mixtures. Both starch and casein solutions had the same concentration of 8% polymer by weight. The size press was adjusted to have a polymer add-on of 2.4% by weight. A part of the sample was conventionally dried (C) on Emerson speed drier, model 10 at 230°F (110°C). Another portion of the samples was heat treated (HT) at 400°F (205°C) for 30 seconds and rewetted immediately after heat treated. After conditioning for 48 hours under standard conditions (70°F, 21°C, 65% relative humidity), the samples were tested for stiffness (Table 1).

Example 2

Board as in Example 1 was sized with different starch-whole milk mixtures. Starch and milk concentrations were 8% and 4% by weight respectively. The size press pressure was adjusted to achieve an add-on value of 2.4% by weight. A part of the sized samples was conventionally dried on an Emerson speed drier, model 10 at 230°F (110°C). Another portion of the samples was heat treated at 400°F (205°C) for 30 seconds. All the samples were conditioned for 48 hours under standard conditions. The resultant properties are listed in Table II.

Example 3

Board as in Example 1 was treated with a 50:50 mixture of starch and acrylic latex (Rohm-Maas Rhoplex HA-16). The starch and latex concentrations were 8% and 50% respectively. The size press pressure was adjusted to achieve a polymer add-on of 10.5%. A portion of the samples was conventionally dried on Emerson Speed drier, model 10 at 230°F (110°C). Another portion of the samples was heat treated at 400°F (250°C) for 30 seconds. All the samples were conditioned for 48 hours under standard conditions. The resultant sample properties are listed in Table III.

Example 4

A commercial kraft linerboard having a kappa number of 105 and Canadian Standard Freeness of 720 mls was sized and treated as in Example 1. After conditioning for 48 hours under standard conditions, all the samples were tested for their properties (Tables IVa and IVb).

Example 5

The same board as in Example 4 was sized and treated as in Example 2. All the samples were conditioned for 48 hours under standard conditions. The resultant board properties are listed in Table V.

Example 6

The same board as in Example 4 was sized and treated as in Example 3. All the samples were conditioned for 48 hours under standard conditions. The resultant product properties are listed in Table VI.

Claims

1. A method of treating a kraft paper product by applying milk, latex, protein and/or starch to said product, then heat treating said product at an internal temperature of at least 205°C and rewetting the product immediately after said heat treatment without allowing the product to cool substantially before the water application.

2. The method of claim 1, characterized in that the product has an initial moisture content in the range of 1.0 to 40% of weight before said heat treating step.

3. The method of claim 1 or 2, characterized in that the moisture content of said product after rewetting is between 1.0 and 20% by weight.

4. The method of claim 1, characterized in that the additive is liquid milk.

5. The method of claim 1, characterized in that the temperature of the product before rewetting is above 205°C.