FI57799B - FOERFARANDE FOER FAERDIGGOERNING AV EN PAPPERSBANA - Google Patents

Description

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Patent · oeh regItterttyrelMn '' Aiwakan uttagd och uti. »Krifwn puMic« r »d 30.06.80 (32) (33) (31) Requested utuoUcuw-Bugird prtortm 12.02.73 USA (US) 331815 (71) Scott Paper Company, Industrial Highway at Tinicum Island Road,

Pennsylvania, USA (72) Jay Henry Vreeland, Yarmouth, Maine, USA (7U) Oy Kolster Ab (5U) Method for finishing a paper web - Förfaremde för Färdig-görning av en pappersbana

The invention relates to a process for the preparation of highly glossy coated papers by hot xantheration and the use of coating compositions consisting of pigments and synthetic polymeric latices.

High gloss coated papers can be prepared using an aqueous coating composition containing a pigment and a thermoplastic binder, after which the gloss is provided to the coated paper by hot calendering, such as supercalendering or glosscalendering. The super calender consists of a group of steel calendering rollers and flexible counter-rolls in turn, as well as devices for heating the steel rolls or for heating under the influence of advanced friction. Most often, the paper web is run through the series so as to cover a considerable portion of the heated roll with respect to the roll circumference. Alternatively, the paper web can be guided directly into the gap formed by the heated roll and the counter roll by means of guide rollers. Supercalendering requires the use of relatively high line pressures between the rolls, generally 179-358 kp / cm and typically 215-286 kp / cm. Temperatures for steel finishing rolls are generally 6o-82 ° C. While supercalendering has the disadvantage of compressing and compacting the coated paper web during calendering, it has the advantage of higher speed and lower equipment costs compared to gloss scalarization, which is invariably done immediately upon coating delivery.

The method of polishing paper coatings by hot calendering, known in the papermaking industry as "gloss scalarization", involves providing a glossy surface on paper or similar webs by contacting the surface of the coated substrate with a polished cylinder compaction. This larger bulk, compared to that obtained by supercalendering, increases the whiteness and opacity, which are desirable properties in coated printing papers and make it possible to use paper with a lighter basis weight to achieve a certain thickness. When glossing the paper, an aqueous coating composition is applied to the paper web, the web is calendered in a nip formed between the gloss scalarization and the flexible counter-cylinder, and the paper web is removed from contact with the cylinder upon entry from the nip. The pressure conditions in the gloscalanter nip are generally lower, e.g.

90-161 kp / cm than in the supercalender and the temperature conditions are typically higher than 135-177 ° C,

Aqueous paper coating compositions generally contain a mineral component, which is preferably predominantly clay, but may also contain other mineral pigments, such as titanium dioxide, zinc sulfide or calcium carbonate, and a thermoplastic binder. It has recently been proposed (U.S. Pat. No. 3,583,881) to use so-called "hard" polymers as a binder in a paper coating composition to provide high gloss to the coated paper product. By "hard" is meant that the thermoplastic polymer has a relatively high apparent second transition temperature, more commonly referred to in the papermaking industry as the glass point (T) obtained by drawing 6

Yong's modulus as a function of temperature resistance.

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Each polymer has its own "glass point" (T). This concept is well known in the art 6 and is commonly used to define or describe the temperature above which the polymer has received sufficient thermal energy to cause molecular rotation or substantial torsional vibration in most of the backbone bonds. This treatment is also used to define the "minimum film formation temperature" of a polymer latex above which polymer particles are able to form a film upon evaporation of water by surface tension. In practice, the term "glass point" or "minimum film-forming temperature" describes a kind of internal "melting point" of polymer latices, but not phase change, whereby the polymer retains its externally visible solidity but behaves more like a viscous liquid capable of plastic flow and elastic deformation. For purposes of this invention, the term "glass dot" may be used interchangeably and as defined, such as the "minimum film-forming temperature" of a polymer latex. In fact, this "glass point" comprises a narrow temperature range rather than an exact point.

From this definition, it can be seen that if the temperature T is taken as room temperature (20-25 ° C) it is any polymer whose T is substantially large. . ® rempx than T, e.g. 32 ° C, non-film-forming in T, while any polymer whose T is substantially below T, e.g. 16 ° C, β is a relatively good film-forming in T. As used in accordance with this invention, the term "hard" refers to binders having a T substantially higher than room temperature and generally a T greater than 30 ° C.

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In the aforementioned U.S. Patent 3,583,881, it is stated that in order to prevent the coating from adhering to the hot steel cylinder used in the gloss scaler, the average T 1 of all polymer components of the binder must be at least 1 + 3 ° C. It has further been found that the drying of the coating is carried out at a temperature which ensures the fusion of the polymer therein, the temperature being about 20-60 ° C higher than that of the polymer T added to the coating.

Another U.S. patent for the production of high gloss papers by coating papers with a combination comprising a pigment and a "hard" binder, 3,632-298, provides a theoretical explanation of why "hard" binders improve gloss. During the drying step, the pigment is oriented under the influence of surface tension forces, which bring the pigment matrix to its minimum volume. This is also the point where orientation and gloss in the unfinished state are at their highest. However, the binders solidify the pigment arbitrarily oriented as soon as the pigment is bound, which prevents maximum orientation and gloss before calendering. This binding effect begins when the emulsion has disintegrated during the drying step and film formation occurs. As the minimum film formation time increases, the time before fusion in drying increases, thus increasing the time for orientation to occur. Based on this, the gloss can be considered to be due to the glass point.

As will be seen below, such a conclusion is only partially correct. Both of the above patents further disclose that the binder is fused during the drying step to hold the pigment in place. U.S. Patents 3,583,881, column U. rows 18-22; 3,631 * 298, Sara ke 1, lines 69-72.

In the present invention, it has been found that without taking into account previous teachings in the art, namely that the binder should be fused or combined during the drying step and instead gently drying the coated paper web below T temperature (thus preventing bonding), much better gloss in post-calendering.

According to the present invention, the paper web is coated with a composition containing a "hard" polymeric binder, the coating is dried at a temperature lower than where the binder fuses, and the coating is calendered at a temperature higher than the fusing temperature. Because the binder in the coating is unfused or only partially fused, the coating pigments are unbound and the surface is therefore much more malleable and can give a much higher degree of gloss than if it had been heated above the T temperature before the edge entered the calendering cylinders.

Another important finding of the invention when using hard binders is the relationship between hardness and temperature. The shape of the curve obtained by plotting hardness as a function of temperature has been found to play a role in the ease and efficiency with which hard binders improve gloss.

Figure 1 is a graphical representation of the shape of a curve for a representative hard binder of the present invention obtained by expressing shore hardness A as a function of temperature; Figure 2 shows the shape of a curve for a representative soft binder when shore hardness A is expressed as a function of temperature; Figure 3 is a graphical representation of the shape of the curve for the second hard binder obtained when the shore hardness A is expressed as a function of temperature.

Referring to the drawings, it should be noted in Figure 1 that the area T is very short and the slope of the curve in transition from stiffness to softness is very steep. Accordingly, polymers having the hardness / temperature properties of the curved form shown in Figure 1 are preferred for use in the practice of this invention and are exemplified in the following Examples 1, 2, h and 5. · Such polymers are preferred because they become faster flowing. upon contact with the heated polishing surface and also solidifies more rapidly as the web cools after leaving the polishing surface because the transition back to hard occurs over a very narrow temperature range. This rapid recovery to hardness makes it possible for the coating to retain the high gloss it has received in the roll slot.

The curve shown in Figure 2 is typical of a soft polymer such as that described in Example 3 below and below the county temperature range shown in the figure. In the temperature range shown, the curve shows only the slope, which is characteristic of the hardness / temperature ratio of the polymers in the range which is quite below the T of the polymer. It can be seen that Figure 1

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the hardness of the binder shown at a sufficiently high temperature actually falls below the soft binder shown in Figure 2 at the same temperature. Thus, it has been considered that the better gloss achieved with hard binders is due in part to the fact that these hard binders at temperature T are in fact softer than soft binders and therefore give better results even if fused before polishing.

In the case of the binder shown in Fig. 3, while the T-region is very short and the slope of the curve in the transition region is very steep, the change in hardness is smaller than in the case illustrated in Fig. 1. The curve in Figure 3 shows that even when this polymer is in its "soft state", it is not as easily moldable as the polymer represented in Figure 1. Thus, a material of the type represented in Figure 3 is less preferred for use in this invention than shown in Figure 1 because the material is harder at surface finish temperature. and thus more difficult to shape.

While the curves shown in Figures 1 and 3 depict the polymer itself, the hardness / temperature properties of the combination containing pigment and binder can also be measured. The shape of the resulting curve is correlated with the gloss obtained using the coating composition.

At the heart of this invention is that the effectiveness of a hard binder to impart high gloss when used in paper coating compositions is due not only to the chemical composition of the binder or its glass point, but primarily to the sharpness and magnitude of hardness change at T, glass point. In this 6 invention, the preferred binders are hard at temperatures below T and 6 very soft at temperatures above T.

According to the present invention, an aqueous coating composition comprising a paper coating pigment in a binder consisting essentially of 6,57799 comprises a polymer latex having a T greater than 30 ° C and dried 6 under conditions such that the surface temperature of the coating remains below T and the coating is the paper is surface finished, at a temperature above T β. As can be seen from this, the air used to dry the coating can be heated to a temperature substantially higher than T. Although-

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If the phenomenon has not been generally studied, the actual surface temperature of the paper web, for example in a fast-acting air with a damp s a, can be observed to be about 38 ° C lower than in the air circulating in the dryer. The explanation for this temperature difference is that the coating is cooled by the evaporation of water therefrom (evaporative cooling). The surface temperature of the web does not reach the temperature of the circulating air until substantially all of the moisture has evaporated from the coated web. As will be appreciated by those skilled in the art, the moisture content of the coated web need not be reduced to such a degree of drying for calendering. Thus, it can be seen that rapid drying can be achieved using very hot air, but drying can be performed in accordance with this invention, for example at a temperature below T 1, by removing the coated web from the dryer before the cooling evaporation is complete.

The term "consisting essentially", as used herein, indicates that the binder is entirely of a hard, thermoplastic type, and the binder component may contain a minor amount of a non-hard thermoplastic type, provided that this amount is less than that required to bind the pigment. this necessary amount is less than 5% per 100 parts of pigment, but may vary depending on the amount of binder adsorbed on the paper web.If a soft polymer is used in an amount sufficient to bind the pigment, a musk gloss will not be obtained.

A further aspect of the invention is that a number of unique and desirable properties are developed in the finished coating. It is believed that the final film structure obtained by the method of the invention is different from that obtained with ordinary soft binders. The hot surface finish does not quite completely fuse the latex particles together into a solid impermeable film, only forming a film that could be characterized as "microporous" and in which the latex particles are fused together but form a semi-uniform structure.

Such a film, in addition to certainly binding the pigment, gives the paper coatings certain highly desirable properties. One use of coated paper is to have a flat surface for subsequent gloss printing. An important feature of such a coating, among other things, is that it has a fine porous structure, which causes the ink to adhere due to the capillary effect of the coating, even in several layers of printing ink. This feature reduces the tendency of printed images to move or "stamp" on the back surface of the next 7,57799 sheets when the printing sheets are assembled with a press. As a result of the present invention, it is possible to provide a coated paper having excellent printing ink absorption properties compared to conventional coated papers.

Another advantage of such a structure is that papers are obtained for web-transfer printing. For the paper to be sufficiently resistant to bubbling, its porosity must be sufficient to remove the sheet moisture removed by the high temperatures of the oven dryers. The method according to the invention practically gives a product which has excellent resistance to bubbling compared to similar coated papers made using conventional soft binders.

In order to assist those skilled in the art, the following procedures are presented to illustrate the practice of this invention. The base material or fibrous cellulosic substrate to be coated according to the invention can be one of many different types depending on the intended use of the product. It can be a pulp glued or surface glued product and can vary in weight from lightweight paper, such as magazine paper, to paperboard. As is well known to those skilled in the art, in order to achieve a high level of surface finish, it is desirable to pre-coat the surface to be finished. The composition of such a precoat, and not even its presence, has not been considered critical to this invention.

While this description mentions coating one surface of the paper, both surfaces may be so coated, if desired. "Rii-si", as used herein, means 310 m. All weights are calculated on a dry matter basis unless otherwise stated. All gloss readings reported here were obtained at 75 ° according to TAPPI standard T-U80 ts-65.

The principles, features and advantages of the invention will become more apparent from the following specific examples.

Example 1

The pulp-bonded raw paper was precoated on both sides with a conventional paper coating mixture comprising starch and clay of 3.8 g / m 2 per side.

A coating composition was prepared having the following composition:

Parts by weight

Mineral pigment 100 tetrasodium pyrophosphate (dispersant) 0.15 polyvinyl oxide emulsion ^ 20 T = k9 ° c e

Sufficient water to give a coating mixture having a solids content of 65% ^ "Gelva E-900" is produced by Monsanto, 3,57799 and brushed at a speed of 7> 6 m / s on one side of the precoated web with a wide flow coater at a speed to give a coating strength of m. Immediately after application of the coating, it was dried by passing it through an air hood containing rapidly circulating circulating air heated to about 113 ° C under conditions such that the temperature of the coating did not exceed kO ° C due to evaporative cooling.

The dried sheet was passed through four successive Gloss calender nips using guide rollers used to guide the web to the nips. The temperature of the fish sharpening cylinder was 160 ° C and the line pressure of the rolls was about 89.5 kp / cm. The gloss of the surface coating after four cylinder spacing in the Gloss calender was 77 "78.

If the coated paper of this example is dried at a temperature higher than T 1, for example, 150 ° C aerated at a relative humidity and post-treated as above, the gloss reading is about 10 units lower.

If the finishing treatment of the coated and dried paper web of this example is performed by supercalendering, instead of Gloss calendering, the gloss of the polished coating is 80. If the coated paper of this example is dried at a temperature higher than T, e.g., 150 ° C, δ conditioned at U5 # in moisture and polished by supercalendering, has a gloss reading of about 7 ^.

Example 2

A coating composition was prepared having the following composition:

Parts by weight

Mineral pigment 100 tetrasodium pyrophosphate 0,15 (2) s thyrene-ls opreem -s ecapolymer emulsion ι ot a 20 T = 50 ° C δ water sufficient to give a coating mixture with a solids content of 65% (2) "XD 3709,08" produced by Dow Chemical Company as UU - # and applied to the precoated raw paper of Example 1 in the same manner and weight as in Example 1. After application of the coating, the paper was dried and finished as in Example 1. After calendering, the coating had a surface gloss of 76.

If the coated paper of this example is dried at a temperature higher than T, for example 150 ° C, aerated and finished as above, the gloss reading is about 10 units lower.

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Example 3

A coating composition having the following composition was prepared

Parts by weight

Mineral pigment 100 tetrasodium pyrophosphate 0.15 styrene-butadiene latex

T ° C ° S

sufficient water to give a coating mixture having a solids content of 6 2% v 620 "produced by the Dow Chemical Company and applied to the precoated raw paper of Example 1 in the same manner and weight as in Example 1. After application of the coating, the paper was dried (above temperature T) and finished as in Example 1. Gloss calendered paper had a gloss of 50.

As mentioned above, the present invention provides coated papers having excellent printing ink absorption properties as compared with conventionally coated papers. This ink absorbency can be described by a test that is not a standardized test, but is nevertheless useful for comparison purposes, and can be described as follows. A predetermined amount of specific commercial printing ink is applied as a flat film to the paper to be tested. At regular intervals after the ink is applied to the paper, the ink-treated sample is pressed against the surface (the composition of which remains unchanged each time) under uniform conditions. The density of the printing ink transferred to the surface is measured optically and represents the amount of absorbency. The end point of the test occurs at the time when the density of the transferred printing ink reaches a predetermined value. The time taken from the application of the ink film on the paper to be tested until the absorption density of the muffle reaches an acceptable value is called the "ink absorption time".

As an illustrative example, the Gloss calendered sheet described in Example 1 gave an ink absorption time of 275 seconds as measured by the above test, while the gloss calendered sheet of Example 3 gave a value of 1.075 seconds.

As stated above, at the same time, the present invention provides a product having excellent resistance to bubbling compared to papers coated with combinations containing conventional soft binders. Resistance to bubbling can be described by performing the following comparison. A uniform ink film is applied to each side of the test sheet, which is conditioned at standard relative humidity and 10,57799 temperature conditions. To simulate the conditions in commercial web offset presses, the ink-treated sheet is then immediately passed through a heated furnace to test its bubbling tendencies. The surface temperature of the sample is monitored and bubbling is assessed visually.

The experiment found that the gloss-calendered sheet of Example 1 thus tested proved to be bubble-free at 10 ° C, while the Gloss-calendered sheet of pre-sheet 3 showed clear bubbling at 1 ° C.

Example h

A coating composition was prepared having the following composition:

Parts by weight

Clay 100 tetrasodium pyrophosphate 0.15

(M

acrylic polymer emulsion 16

T = 68 ° C S

enough water to give a coating composition having a solids content of 63% ^ "B-83" is produced by Rohm & Haas Company with a solids content of kU # and applied to the precoated raw paper of Example 1 by dragging flow.

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with a coating scraper at a speed to give a coating thickness of 12 g / m ~. After application of the coating, the raw paper was divided into two parts. One portion was air dried at room temperature overnight and the other was oven dried for one minute at 150 ° C.

After drying, both parts were polished by Gloss calendering. Gloss calendering was accomplished by passing the dried, coated web through the slits of three consecutive pairs of Gloss calender rolls. The temperature of the calendering cylinder was 11 ° C and the line pressure between the cylinders was J.6.6 kp / cm. The gloss of the air-dried coating after calendering was 63 and the gloss of the oven-dried coating surface was 57 ·

Example 5

A coating composition was prepared having the following composition:

Parts by weight

Clay 100 tetrasodium pyrophosphate 0.05 acrylic polymer emulsion ^ 166 T = 103 ° C δ water sufficient to give a coating mixture having a solids content of 63% ^ "B-85" was produced by Rohm & Haas Company 11 57799 and applied to the same precoated raw paper of Example k. in the same way and with the same weight as in example ä. After application of the coating, the paper was then dried and finished as in Example 1.

The gloss of the surface of the air-dried coating after calendering was 6k and the gloss of the surface of the oven-dried coating was 56 ·

Claims (4)

12,57799 A method of finishing a paper web by coating at least one side of the paper web with an aqueous composition comprising a paper coating pigment and a binder consisting essentially of a polymer latex having a glass point T above 38 ° C by heating and hot calendering, characterized in that (1) the coating is dried, is lower than the glass transition point T 1 of the polymer, (2) the coating is finished by hot calendering in a manner known per se at a temperature higher than the top point T of the polymer. Process according to Claim 1, characterized in that a binder is used whose hardness / temperature ratio is as shown in Figure 1. * 3 * Process according to Claim 1 or 2, characterized in that the binder is vinyl acetate, styrene-isoprene - or acrylic polymer latex. Method according to one of the preceding claims, characterized in that the coating is finished by gloss scalaring. Method according to one of Claims 1 to 3, characterized in that the coating is finished by supercalendering. An aqueous paper coating composition useful in the method according to any one of claims 1 to 3, comprising a paper coating pigment and a binder, characterized in that the binder consists essentially of a polymer latex having a temperature above 38 ° C and a hardness / temperature ratio as shown in Figure 1. Composition according to Claim 6, characterized in that the binder is a vinyl acetate, styrene-isoprene or acrylic polymer latex.