NO144223B - PROCEDURE FOR THE MANUFACTURING OF EXTENSIVE PAPER - Google Patents

Description

The present invention relates to a method for the production of stretchable paper made from fibers that have been mechanically compressed or packed together without crimping.

Stretchable (compressed) paper is produced, e.g. as described in U.S. patent No. 2,624,245, and it exhibits definite, widely recognized advantages and commercial applications. Paper of this nature, while in a partially moist to-

condition, subjected to compressive compaction or compression in the direction of movement of the paper web (machine direction or

MD) in a pinch so that the fibers are packed and driven together, whereby you get stretchability without crimping. Compressed paper has improved deformation work (TEA), cracking and tearing properties which are very desirable for e.g. manufacture of paper sacks. However, stretchable paper has reduced tensile strength and stiffness, and the reduction of stiffness to very small values increases the difficulty in producing sacks from said paper.

The purpose of the present invention is to improve the tensile strength and stiffness of compressed, stretchable paper without sacrificing the tear strength, and furthermore a radical improvement in TEA and tear strength is to be achieved. The method according to the invention involves a selective impregnation using starch as external gluing, particularly in a way that will not affect the stretchability of the paper.

External gluing of non-stretchable paper is well known

and has largely been used to improve the printing properties of good paper by applying to one or both sides a coating of starch, clay, polyvinyl alcohol or the like. In an article entitled "On Machine Surface Sizing Trials with Acid-Modified Wheat Flour" in Vol

53 no. 8 by Tappi, from August 1970, it is pointed out that gluing kraft paper with the application of an acid-modified wheat flour or a hypochlorite-oxidized starch led to an increase in tear resistance, tensile strength and stretchability. U.S. Patent No. 3,362,869 describes the application of a starch coating to one side of a moist compressed paper prior to drying the paper to give it a glossy surface. The information contained in this patent that normal smoothing of bundled paper can be carried out by re-introducing sufficient moisture for this purpose into the web after it has been compressed in the machine retnina, would go beyond much of the stretchability of the paper web and the advantages this entails. For this reason, to avoid unnecessary wetting of the paper, a coating of starch is applied to one side of a still damp paper web after the compression and just before the paper is transferred to a drying roller,

which helps to reduce the increase in moisture content in the paper web, while the starch glues the paper web to the drying roller and while the paper web is held against this under pressure, whereby the starch-coated side of the paper web becomes slippery.

The purpose of the present invention is to treat

a compressed paper web with a starch solution in a special way so that one overcomes or can disregard the adverse effect of introducing moisture into the compressed paper web.

According to the invention, the paper web is dried

so that after the compression, but before the final drying, it contains 1-15% moisture, after which both sides of the compressed paper are impregnated under pressure with a solution of starch under regulated conditions so that the moisture content in the paper does not exceed 45% by weight -% after the impregnation with the starch solution and before the final drying of the paper web, and adjusted so that the paper web contains 1-10% by weight of starch after it has been completely dried.

Under these conditions, it has been shown that the tensile strength of the paper web increases by 150%, the tear strength by 400%, the deformation work (TEA) by 300% and the stiffness by 100%.

As a result of these drastic and unexpected improvements

is it possible, when applying the invention, to use certain types of wood that were previously thought to be complete

useless for the production of paper for sacks, and as an alternative it is now possible to use smaller amounts of ordinary raw pulp without sacrificing any of the strength requirements.

The invention is characterized by those in the claims re-

given features and will be explained in more detail in the following with reference to the drawings where: Fig. 1 schematically shows a packaging device for the production of stretchable paper,

fig. 2 schematically shows a drying section which receives the bundled paper after treatment in the device of fig. 1

and a sizing press close to the drying section for applying a starch solution to the paper web,

fig. 3 schematically shows an alternative embodiment

for the invention, where the bundled web of material is fed directly to a glue press and from there to a drying section, and

fig. 4-9 graphically show the effects of different percentages of starch in relation to different strength factors and applied to bundled and unbundled paper.

In fig. 1 generally shows a packaging device 10 for the production of non-crimped, stretchable paper 11. The device 10 has an endless elastomeric belt 12 and a rotating cylinder 13 with a smooth (chrome-plated) exterior onto which the material web 11 is guided. Several of the guide rollers 14 , 15,

16 and 17 are placed inside the belt loop 12 where the guide roller 14

has the purpose of pressing the strip 12 against the cylinder 13 in a press nip N, where the strip 11 is temporarily reduced in thickness when it passes through the press nip. A section of the guide rollers 15 is located at a distance from the first guide roller 14 and holds a part 12a of the belt in contact with the cylinder 13 on the discharge side of the pinch N and belt guide means 18, 19 feed the tape onto the cylinder 13 for movement through the pinch N. The device 10 is shown placed in a paper machine between the second and third drying sections. A drying roller is shown as a broken piece at 20 and represents the last roller in the drying section, while the roller 33, of which a broken piece is also shown, represents the first roller in the third drying section 30. The paper web is led from the drying roller 20 under the paper roller 19 and over a tension roller 18 and then into the nip N. The paper web 11 comes into contact with the surface of the chrome-plated roller 13 before it runs into

pinch N. The paper web 11 is then held in contact with the roller 13 over a significant part of the roller's circumference by means of the belt part 12a, after which the paper web is separated both from the belt 12 and the surface of the roller 13 and is led around a paper guide roller 22 and over a further tension roller 23, and further to the drying roller 33.

The paper web 11 that comes from the dryer 20 has the optimum moisture content that is necessary for the packaging treatment. The optimal value can vary between 20

and 50% moisture content when the paper web 11 enters the device 10.

As previously explained, the roller 13 has a glazed surface which is preferably chrome-plated. The roller 13 is rotatably stored in the bearing 13a and is driven by a suitable power source, e.g. a motor which is shown schematically at M. In reality, the motor M provides driving force for the entire device 10. In this way, the roller 13 drives the belt 12 mainly by pressure at the nip N so that the tension in the belt 12 is reduced in the nip N.

As will be seen, a certain force is required to drive the belt 12 around the various guide rollers and rollers such as rollers 14, 15, 16 and 17, and the point where the belt 12 is driven (which in this case is pinched N) is the point where the stretch in the band is reduced. As the tape has elastomeric properties, a slight reduction in the stretch of the tape will result from the reduction in tensile stress, which causes the tape 12 to pack the paper web 11 in the direction of movement when the paper web 11 passes through the pinch and immediately after. The smooth surface of the roller 13 allows for some sliding between the paper web 11 and the roller. The paper web 11, which is held in fixed frictional contact with the belt, is thereby packed together in the machine direction without the paper web being crimped. A lubricant shower 25 is arranged just below the paper web 11 on the inlet side of the pinch N. The lubricant shower 25 is preferably a silicone shower, but other release agents are also appropriate when, applied to the relatively small amounts of liquid, they form a film on the surface of the roller 13.

The roller 13 is heated as shown schematically, by means of a suitable steam source S which leads steam axially into the roller 13 in the same way as steam is supplied to ordinary drying cylinders or rollers in paper machines. The steam has a relatively high temperature of between 100°C and 180°C at the surface of the roller 13 to create a very thin steam film or cushion between the paper web 11 and the roller 13. This steam cushion also enables the relative sliding between the paper web 11 and the roller 13, (which is necessary to allow folding of the paper web by the ribbon) and will soften certain of the constituents of certain paper types.

The device that has been described so far is well known and forms no part of the present invention beyond illustrating the production of bundled or stretchable paper which is later to be processed according to the invention.

The following part of the description concerns the treatment of bundled paper, that is stretchable paper according to the method according to the invention. After it leaves the compaction device 10, the paper web 11 is taken to the drying section 30 which includes ordinary drying rollers 31-36, where the paper web first passes over the drying roller 33, then the roller 34 etc. as shown. Felt drying bands 37, 38 are laid around guide rollers 39 and are in contact with the respective drying rollers 31-36 to ensure good contact between the paper web 11 and the respective drying rollers.

After it leaves the compaction device 10, the moisture content of the paper web will be 35-45% by weight. After passing the drying section 30, the moisture content of the paper web 11 will be between 1 and 15% by weight.

The reference numeral 40 denotes a glue press for processing the dried, bundled paper web 11 in accordance with the present invention. The paper web 11 first passes over guide rollers 41, 42 and continues from there through a nip formed by two oppositely rotating glue press rollers 4 3 and 44. The bottom piece 46, 47 controls metering valves 48, 49 for starch in lines 50, 51 so that a metered amount of starch solution 52 to flow into the nip between the glue press rollers 43, 44. Edge collection pans 53 collect excess starch solution at each end of the nip. The starch solution is supplied from a starch dissolution tank 54 with a stirrer 56, and the starch is taken from the tank 54 through the line 57 to the pump 58 which feeds the measuring valves 48 and 49. A return pump 60 for starch sits in the line 61 from the collection pans 53 and returns starch solution via line 62 to tank 54.

During the passage of the paper web 11 through the nip between the press rollers 43 and 44, a starch solution is applied under pressure to both sides of the paper web in accordance

with the varying percentage contents which are reproduced in the following as special examples. The passage of the paper web 11 through the starch solution and between the rollers 43 and 44

would be sufficient and would be so adapted in terms of application speed and strength of the starch solution that one obtains an impregnation of both sides of the paper web 11 with starch according to a predetermined amount. The weight of starch that is impregnated in and deposited in the paper web will be from 1% by weight to 10% by weight based on the total weight of the paper web after drying, in order to obtain good results with optimal values for the different strength properties after starch impregnation, and one then takes the percentage values shown in fig. 4-7. During the treatment of the paper web, the moisture content in it will increase from 1 to 5% at the end of the drying six ion 30 to a value of 35-45%.

After applying and impregnating with starch on both sides of the paper web 11, it is guided around guide rollers 64 and then into the second drying section 66. The drying section 66

has a plurality of drying rollers 67, 68, 69, 70 etc. around which there are drying filters 71, 72, controlled by rollers 73. The drying section 66 corresponds in all respects to the drying section 30 and the paper web 11, which passes through the section, will emit the moisture introduced into the paper web by the glue press and the moisture the paper web has from the drying section 66 and arrive at

an approximate moisture content of 5-10% by weight.

Fig. 3 shows an alternative embodiment of the invention

where the same reference numbers are used for the same parts as can be found in fig. 2. In the embodiment of fig. 3, the paper web 11 is fed directly from the compacting device 10 directly without first passing through the drying section 30. As a result of this, the paper web 11 contains approximately 30-40% by weight of moisture before passing between the glue press rollers 43 and 44. It will be seen that because the paper web 11 contains this amount of moisture, it will be possible to impregnate the paper web with smaller amounts of starch solution,

but a sufficient amount of solution will impregnate the paper web so that one gets clear improvements in the results in terms of increasing tensile strength, tear strength and (TVA), and the improvement can be expected and is about 50% of the improvement achieved with the equipment shown on fig. 2. After passing through the sizing press 40, the paper web 11 is fed into the drying section 66 which reduces the moisture content of the paper web to a level of 5-10% by weight. The advantage of the device shown in fig. 3, is the use of a simple drying section for treating the paper web after it has passed the glue press, although the moisture content of the paper web will usually require a larger drying section to reduce the moisture content to values that can be accepted.

It should be noted in connection with both devices in fig. 2 and 3 that it is possible to slide the paper web 11 by feeding it on the downstream side of the glue press 40 around a simple drying roller on the condition that the starch solution when applied to the press 40 provides sufficient binding of the paper web to the cylinder. In order to achieve gliding, you must have a pressure contact against the drying cylinder as shown, e.g. in the U.S. patent No. 3,362,869. The difference between the device described above and that shown in patent no. 3,362,869 is that the starch applied in the aforementioned US patent is a surface coating used to create an adhesive bond between the paper web and the drying roller, while according to the present invention impregnates the paper web with starch on both sides to later obtain the aforementioned improved values for increased tensile strength, tear resistance and TEA. These additional effects are made possible by the use of the size press which includes size press rollers to drive the starch solution into both side surfaces of the paper web. After this description of the device according to the invention, a number of examples of how the invention can be implemented are given.

In the examples, the percentage contents of pulp and starch are calculated on the basis of the dry weight of both components (oven dried).

Example 1

Mass of unbleached 30% softwood and unbleached 70% hardwood.

Unrefined - (12 sec W)

Starch - (acid modified corn starch).

Example 2

Mass of unbleached 30% softwood and unbleached 70% hardwood. Refined (14 sec. Williams Freeness).

Starch (unmodified potato starch).

Example 3

Mass of unbleached 30% softwood and unbleached 70% hardwood. Refined (40 sec. Williams Freeness).

Starch (acid modified corn starch).

From the above examples, it appears that two factors have a major impact on the results. These were (1) degree of refining and (2) amount or type of starch added. Maize starch gave greater improvements in terms of tensile strength and stiffness than potato starch under otherwise similar conditions. These were the properties it was desirable to improve so that the paper would have better properties for further production. Furthermore, it can be concluded that even if the tear strength of bundled or stretchable paper was reduced with increasing refining and increased with starch levels, the effect of the degrees of freedom usually associated with sack production could not be large enough to outweigh other advantages of gluing. In general, the characteristics will level off between 3

and 4% added potato starch, as no marked changes were obtained above this level. In contrast to this, with added maize starch it will be possible to get increases beyond this point. Stickiness or brittleness could not be found in any of the tests carried out with starch content up to approximately 8%. Above this value, a certain brittleness could be observed.

On the basis of the principles underlying the invention, it is proposed that slightly converted (plasticized) starch will be better than highly converted starches, particularly at high percentage values for the starch impregnation. A plasticized starch will have properties that match well with the tensile properties of bundled paper. However, it is not in any way suggested that the use of unconverted starch is undesirable, even though the examples described here were carried out with starch in a highly converted form.

As shown in fig. 4, a reduction in tears will take place for bundled paper with increasing starch content at 525 CF. (23.7 Schopper Riegler) . The work was carried out with an acid-modified maize starch (Eclipse N) produced by A.E. Staley. With a lower degree of refining, that is to say

730 CF. (14.3 Schopper Riegler) the tear strength increases markedly with increasing starch levels. At some target value of the refining, one can then expect to find small effects. That this is correct will be seen from the more extensive work that has been done with potato starch (see fig. 5), where the effect of increased starch content on tearing strength is in reality rather small. The tear strength seems to approach some fixed value which is probably dependent on variable values for wood mass.

The tearing strength of refined paper increases with the addition of starch due to the better fiber bond which requires greater forces to pull the fibers apart when tearing. In the case of refined pulp used at this stage, it is assumed that optimal bonding for development of tear strength has already been reached during refining so that extra bonding achieved with the starch leads to easier failure due to shear forces acting on the fibers and reduced tear strength. The tearing strength across the machine direction compared to in the machine direction is in all examples significantly better for bundled paper than for unbundled paper, and this can be explained by the fact that when tearing in the transverse direction, the break in the paper must propagate across bundled fibers, and bundled fibers with their lower modulus and greater elongation are more difficult to break by shear forces than non-compacted fibers. The first increase in tear strength across the machine direction as the starch content increases is due to greater bonding and greater difficulty in tearing up these bonds. However, further increases in the starch content will lead to an increase in the fiber's modulus or stiffness as well as further increases in the bond, favoring fiber failure due to shear stress and reduced tear strength.

As usual for stretchable paper, the tear strength in the machine direction is lower than the values across the machine direction. The refined pulp loses its tearing strength when starch is added as explained above (see Fig. 6).

In order to draw attention to properties that are improved by the addition of starch, i.e. tensile strength, stiffness and TEA, it is shown in fig. 7 shows tensile results in the machine direction for potato starch and corn starch in different amounts. Two different refinement levels were investigated in each case. It should be noted here that you get an increase in tensile strength with potato starch during the first 3% addition, while you get increased tensile strength with grain size first with a higher percentage for the addition. It will also be seen that the addition of 3% starch to the bundled paper brings the tensile strength up to the level of the untreated (not bundled) sample.

This can be understood if you look at the fact that the molecular structure of corn starch has a more linear orientation than potato starch and thus forms a film with greater tensile strength.

Application of high levels (5%) of corn starch doubled the stiffness of the bundled sheets and approached the value of untreated (non-bundled) material. In fig. 8 you can also see the results for potato starch where what is achieved is not very dramatic, especially not at average refining levels which are normal for paper sack production. At additions of 3% potato starch where it seems that the best results are obtained, an increase of approximately 40% in stiffness was achieved.

Another property that increases with the addition of starch is the work involved in breaking the paper in the machine direction (TEA), and a contributing factor here is the increase in tensile strength (see fig. 9). While an increase of 30% can be achieved with corn starch, only half of this can be achieved with the same content of potato starch. The usual 5-7 fold increase in TEA in the machine direction compared to unbundled paper is present whether the material is starched or not.

No differences in tear strength occurred between bundled and non-bundled sheets with increasing adhesive content. About 30% to 40% increase at the 3% level was achieved in both cases. This was to be expected because the tensile strength across the machine direction and the TEA properties affecting the tear strength were not significantly changed during the wrapping process. The results of the tear tests are therefore dependent on the starch level.

Claims (4)

1. Process for the production of stretchable paper made from fibers that have been mechanically compressed or packed together without crimping, characterized in that the paper web is dried until after the compression and before the final drying it contains 1-15% moisture, after which both sides of the compressed paper impregnated under pressure, with a solution of starch under regulated conditions, so that the moisture content in the paper does not exceed 4-5% by weight after the impregnation with the starch solution and before the final drying of the paper web, and adjusted so that the paper web contains 1-10% by weight of starch after it is completely dried. 2. Method as stated in claim 1, characterized in that the compressed paper is glued with resin before it is impregnated with starch, in order to regulate the moisture absorption in the compressed paper from the starch solution. 3. Method as stated in claim 1, characterized in that the impregnation with starch takes place with plasticized converted starch. 4. Method as stated in claim 1, characterized by the use of grain starch and impregnation of the paper web so that it contains between 3 and 10% by weight.