FI61220C - ETT TAETT PAPPER OCH DESS FRAMSTAELLNINGSFOERFARANDE - Google Patents

Description

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C Patent granted on 10 06 1932 (* 5) patent meddelat ^ ^ (51) Ky.ik? /Fcw.a3 D 21 H 3/00, 3/38, 3/78 FINLAND —FINLAND (21) P »t.nttlhik .rm, .- Pw.nt »eknlnl 763012 (22) H» k * mi «p * ivt - Aiwetcnlnpdtf 21.10.76 * * (23) Alkuptlvl - Glltlghetadkg 21.10.76

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National Board of Patents and Registration .....

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Patent · och rejisteratyrelsen '' Anotun uthgd och uti.tkrtftM puMkend 26.02.82 (32) (33) (31) Privilege claimed —BegM priortt * (71) (72) William Gordon Louden, Ervinna, Bucks County, Pennsylvania, USA ( The present invention relates to dense paper and a process for its production, in particular to a dense paper having good oil and solvent resistance and consisting of a cellulosic fibrous web containing a polymer. and a filler.

Papers with different densities have been made for different purposes. Low density papers are soft and porous and also have high absorbency unless treated to reduce absorbency. Moderately dense papers include papers for writing, printing, and wrapping, as well as pouches and liners. Examples of high density papers are e.g. thin parchment, parchment, parchment paper, vulcanized fiber paper and supercalendered paper.

An example of a dense writing paper is described in U.S. Patent 3,839,144. Although this paper is dense and has excellent oil barrier properties, i. scratchability, among other features, it has certain limitations. For example, this paper is not cheap to make because it is made from a pulp slurry obtained by grinding the pulp to a high, pre-determined Schopper-Riegler grinding rate and adding small amounts of unground pulp, or by combining alpha or wood wool pulps with ordinary pulps and grinding the combined pulp well before the pulp slurry is deposited on the paper machine wire.

It has been found that water drains relatively slowly away from such highly ground pulp slurries, so that it is very difficult to produce the desired dense paper at high machine speeds. Thus, it is more expensive to manufacture than other papers. The maximum thickness of such paper is limited, and the forced grinding of the pulp greatly increases the cost of manufacturing the paper as well.

Dense paper has some desirable properties, e.g. good tensile and puncture resistance (Mullen), improved flexural strength, improved interfiber bonding and delamination resistance, solvent and oil penetration resistance and stiffness. On the other hand, dense papers have low tear strength, are brittle, have poor dimensional stability and aging properties, and have high manufacturing costs. For example, parchment papers and papers made by so-called volcanic fiber processes, although dense, have relatively poor tear strengths, as do dense papers made by supercalendering paper webs of moderate density.

In papermaking technology, papers with a relatively low density are impregnated with polymeric resins. However, these papers usually have a density before impregnation of 0.023 to 0.027 g / cm 2 (0.38 to 0.45 g / cm 2, where cc is cubic inch) and even as low as 0.020 g / cm 3 (0.32 g / cm 3). cc). After impregnation, the papers are still relatively porous, even if more than 50% of the dry solids weight of the paper has been used for impregnation. Due to its porosity and low density, such paper is not suitable for use as a scratch-off typewriter paper and does not have solvent resistance.

The polymeric resins used as coatings and impregnants for low density papers have been relatively soft and flexible in nature, as opposed to hard and inelastic polymeric resins. The thermal transition temperature (Tg) of glass is a measure of the rigidity of a polymer-resin, i.e. the film stiffness. This is the temperature at which the resin forms a continuous film. For example, the glass transition temperature of relatively soft and flexible polymeric resins is less than about 0 ° C. The glass transition temperatures of rigid and inelastic polymers, on the other hand, are higher than ‘about 15 ° C.

3 61220

Papers having a density of 0.031 g / an3 (0.51 g / cc) or more have been impregnated with rigid polymeric materials and have been found to have certain advantageous properties. For example, these papers have improved tensile and puncture resistance, abrasion resistance, delamination resistance, solvent and grease penetration resistance, and good refractive power. On the other hand, these papers have some poor properties that make them unsuitable for use as typing papers or in applications that require dense papers. These bad features include e.g. deteriorated tear strength, poor writing properties and increased brittleness. Because dense paper webs do not absorb as much impregnant as porous paper webs, it is generally believed that impregnation can provide improvements in the physical properties of paper only when the web is porous and the resin content of the finished paper exceeds about 50% by weight.

Experiments have shown that the density of the paper web before impregnation and the amount of resin absorbed during impregnation into the web contribute to the decrease in tear strength associated with impregnation of the paper with a rigid polymer. For example, base paper sheets with varying initial densities were impregnated with an aqueous dispersion of a rigid homopolymeric polyvinyl acetate resin (PVAC) sold by VINAC 880 to Air Products and Chemical Co., Allentown, Pa. The dispersion contained 40% by weight of VINAC 880. Impregnation was performed by immersing the sheets in this aqueous dispersion and then passing the sheets between press rolls to remove excess impregnant. The sheets were dried for 4 minutes each, in a Williams paper sheet dryer at 104 ° C, for 2 minutes on each side. After several days of conditioning, the basis weight and thickness of each sheet were measured, and the tear strength of each sheet before and after impregnation was determined. The results are shown in the following table: Sample Initial density PVAC sheet Density impregnated Tear strength (%) change in g / cm g / cc g / cm3 g / cc (%)

A 0.021 0.35 48.5% 0.035 0.58 rise in 20 S

B 0.032 0.52 41.5% 0.050 0.82 decrease 33% C 0.034 0.56 38.6% 0.047 0.77 decrease 43% D 0.037 0.60 42.1% 0.054 0.88 decrease 37% E 0.038 0.63 30.1% 0.052 0.85 decrease 25% tn

It appears from the above that when dense papers are impregnated with rigid polymeric materials, their tear strength decreases significantly. This is unfortunate because other properties of the paper such as tensile strength, abrasion resistance and delamination resistance are at a maximum when the paper is dense.

U.S. Patent No. 3,634,298 discloses a paper coating composition. The mixture comprises a rigid polymeric material (having a glass transition temperature (Tg) in the range of about 29-43 ° C) mixed with a clay slurry. The mixture is deposited as a coating on a paper web to produce high quality paper.

In view of the above, the main object of the present invention is to provide a new type of paper which has the advantageous physical properties of dense papers, but not their poor properties.

It is a further object of the invention to provide dense papers which can be produced economically at relatively high paper machine speeds.

It is a further object of the invention to provide a novel type of dense paper which is oil and solvent resistant and which preferably has good tear strength, refractive power and abrasion resistance.

It is a further object of the invention to provide a peculiar paper which is useful e.g. as a book cover or as a carrier for emission covers.

It is a further object of the invention to provide an even better method of making compact papers.

It is a more specific object of the present invention to provide an inexpensive sealing paper impregnated with a sufficient amount of a rigid polymeric material to make it oil and solvent resistant, preferably without significantly impairing its tear strength and abrasion resistance.

According to the present invention, most of the disadvantages associated with impregnating a paper web with a rigid polymer are avoided, and a dense paper having good oil and solvent penetration resistance and generally also good 61220 tear strength, refractive strength and abrasion resistance can be produced. According to the invention, it has been found that these properties are achieved with paper characterized in that the polymer and filler are distributed throughout the cellulosic fibrous web in the form of a impregnating agent dispersion with 8.5-50% by weight of finished paper and 35-90% by weight of impregnating agent. , having a glass transition temperature of 15 ° -60 ° C, and 10 to 65% by weight of a compatible inert filler based on the weight of the impregnating agent.

The papers of the present invention having a final 3 uncalendered density of preferably 0.042-0.055 g / cm (0.69-0.90 g / cc) (weight of 500 sheets of 61 x 91 cm sheets per mm of sheet thickness) have some properties , which are unexpected on impregnated, dense paper. For example, the paper of the invention impregnated with an extended rigid polymer has a much better refractive power than a paper impregnated with a rigid polymer alone. Impregnation of the dense base paper with a rigid polymer would normally significantly reduce the tear strength of the resulting paper. However, the base paper impregnated by the preferred method of the invention surprisingly retains a significant amount of tear strength while having excellent oil barrier properties. By using an impregnating agent containing significant amounts of filler, the cost of making the paper is reduced due to the reduction in the total amount of polymer required to achieve the desired properties, as the more expensive polymer is replaced by cheaper fillers. Although paper may be inexpensive to manufacture, it nevertheless has the advantageous properties of dense papers, which are more expensive to manufacture.

It is preferred that the paper web be impregnated as it progresses in a papermaking process, such as a flat wire machine, in which a cellulosic fibrous pulp slurry is deposited on a running wire and formed into a web before being separated from the wire and dried. The impregnation step should take place after the web has formed and become cohesive and at least partially dried, and the web can be impregnated after the paper has been completely dried and rolled, for example at a later stage after manufacture. Preferably, the paper web is impregnated in a conventional paper machine gluing press. It is desirable to adjust the density of the paper web in a conventional manner so that its dry, unconditioned density before impregnation is between 0.027 and 0.043 g / cm (0.45-0.70 g / cc) and preferably between 0.033 and 0.042 g / cm 3. cm-2 (0.54-0.69 g / cc). The web is passed through an aqueous dispersion containing 12.5 to 60%, preferably 12.5 to 40% by weight of impregnating agent, and after impregnation, the web is heated to melt the impregnating agent in the web. The impregnating agent can also be mixed into the paper pulp wet end of the machine before webbing.

Thus, the preferred method of the invention is a method of making dense paper comprising the steps of: carrying a paper web having a dry, non-calendered density between its 2 opposite surfaces of about 0.027 g / cm (0.45 g / cc) -2 of about 0.043 g / cm (0.70 g / cc); the web is impregnated by introducing an excess of an aqueous dispersion containing a mixture of a rigid polymeric material and a compatible inert filler comprising from about 35% to about 90% polymeric material and from about 10% to about 65% inert filler, the percentages being by weight of the mixture; and having a glass transition temperature of the rigid polymeric material of about 15 to about 60 ° C, and passing the web between two opposing press rolls to ensure that the dispersion penetrates the web and removes excess dispersion therefrom; and heating the web after passing between said rolls and removing excess dispersion therefrom to melt the mixture into the web to amount to about 8.5 to about 50% by weight based on the dry weight of the web.

In making paper according to the present invention, the polymeric material has a predetermined minimum inflexibility, i. brittleness or film stiffness of at least about 15 ° C, expressed as its glass transition temperature (Tg). Rigid polymeric materials that can function satisfactorily include: polyvinyl acetate copolymer latexes such as RESYN 1105 and 1255, manufactured by National Starch and Chemical Corp., New York, N.Y., and VINAC 880, a homopolymer manufactured by 7,61220

Air Products and Chemicals Co., Allentown, Pa. Suitable polyacrylate materials include e.g. RHOPLEX AC 201 and TR 407, manufactured by Rohm and Haas Company, Philadelphia, Pa. A suitable polyvinyl chloride material is GEON 351, manufactured by B F Goodrich Chemical Company, Akron,

Ohio. All of the above polymer materials are of commercially available quality and are sold for use in paper industry applications. It should be noted that the polymeric materials may be copolymer or may contain certain amounts of other polymers or blends, with each other; however, as long as the glass transition temperature of the polymeric material is within the above-mentioned limits, satisfactory results are obtained.

The inert filler that is mixed with the rigid polymeric material to form an aqueous dispersion is preferably a finely divided mineral filler, some commercial grade, sold for use in paper industry applications. The most suitable particle size for the filler is 2-5. Examples of mineral fillers that have been tested and found satisfactory are kaolin clay, calcium carbonate, mica and talc.

The amount of impregnant contained in the final paper must be within predetermined limits. Thus, the impregnating agent must be present in the paper in an amount of 8.5 to 50.5% by weight of the final total dry dry weight of the paper. If the amount of impregnating agent is below said lower limit, the scratchability of the obtained paper is poor. On the other hand, due to the density limitations of the base paper, it is difficult to impregnate the paper beyond the above-mentioned upper limit. The impregnating agent is preferably about 15-40%. The final uncalendered density of the impregnated paper should preferably be in the range of about 0.042 to 0.055 g / cm 2 (0.69 to 0.90 g / cm) (500 sheets of 61 x 91 cm).

In order to obtain the desired physical properties on the paper according to the invention, the rigid polymeric material must be extended within predetermined limits by one of the above-mentioned mineral fillers or mixtures thereof. The filler should comprise from about 10% to about 65% by weight of the impregnant solids, and preferably the filler should be from about 20% to about 65% by weight of the impregnant. The remainder of the weight of the impregnating agent is formed by a rigid resin so that the resin is about 35-90%, and preferably about 35-80% by weight of the impregnating agent.

It has been found that as the proportion of filler decreases below the lower limit of its primary range, the tear strength of the paper deteriorates markedly. On the other hand, as the proportion of filler increases above the upper limit of its primary range, the oil barrier layer and solvent resistance properties of the resulting paper tend to deteriorate.

The paper of the present invention is produced on a conventional paper machine such as a flat wire machine. In such a machine, the paper pulp slurry is deposited on a running wire, and once the slurry has formed into a web, the web is removed from the wire and passed over a plurality of heated drying cylinders to dry it. It is customary for the web, when at least partially dried, to be subjected to further treatments, e.g. gluing from the drying cylinders with a downstream gluing press.

In making the paper of the present invention, it is desirable for the impregnation step to occur after the web has become stable after being at least partially dried and substantially free of sizing. Preferably, the web is impregnated in a size press; however, the impregnation step may occur at a later stage in the papermaking process. As is well known to those skilled in the art, two types of sizing presses are widely used in the paper industry, and both types can be satisfactorily used to impregnate paper produced by the process of the present invention. These two types are the so-called horizontal gluing press and the so-called vertical gluing press. In a horizontal gluing press, the two opposite rollers are mounted to rotate about horizontally spaced centerlines and the paper travels vertically downwardly between the rollers as they apply pressure to opposite surfaces of the web. The impregnating agent forms a pond between each side of the paper web and the roll on that side. In a vertical gluing press, on the other hand, the rolls are mounted to rotate at a vertical distance around the center lines from each other and the paper web passes horizontally between the rollers. The sub-roll rotates in the tray, lifts the impregnating material therefrom and deposits it on the underside of the web, while causing the impregnating material to flow to the upper side of the paper web, for example by pumping it from the container. In molecular type devices, a pressure of between 9 and 95 kg per line-cm is applied to the web as the web passes between the rollers, and in co-operation the rollers force the impregnant inside the web while removing excess impregnant from the opposite surfaces of the web.

9 61220

Regardless of which stage of the process the impregnation takes place, it is important that the mixture of resin and filler be distributed over the entire thickness of the web in order to achieve the full advantages of the present invention. These advantages cannot be achieved if the mixture is merely deposited as a coating on the surface of the web, for example, by the immersion roll and pattern system described in U.S. Patent No. 3,634,298 for coating the web. In this system, the lower circumference of the roll rotates in the trough containing the coating and its upper circumference is in contact with the underside of the running web. Thus, the roll lifts the coating from the tray and layers it on the underside of the web. The thickness of the coating is adjusted by passing the coated web over a scraper blade downstream of the roll so that only a certain amount of coating can remain on the underside of the web.

The mixture of resin and filler can even be mixed with the pulp slurry before the pulp slurry is deposited on the wire.

To ensure the distribution of the mixture over the entire cross-section of the web, there are certain conditions which are preferably observed in the manufacturing process. For example, prior to impregnation, the dry, non-calendered density of the web should preferably be in the range of about 0.027 to 0.043 g / an (0.45 to 0.70 g / cc). The density can be adjusted by a variety of different techniques, all of which are known to those skilled in the art. Density control prior to impregnation is important because when the dry, non-calendered density is less than 0.027 g / m 2 (0.45 g / cc), the resulting paper may be too porous. On the other hand, when the dry non-calendered density is greater than 0.043 g / cm (0.70 g / cc), the web may be unable to absorb a sufficient amount of impregnant to achieve the desired properties.

Another important step in the manufacture of the paper of the present invention is to control the amount of solids contained in the aqueous dispersion through which the web is passed. This amount of solids, which includes the combined amount of rigid polymeric material and filler, should preferably be between about 12.5 and about 60% of the total weight of the dispersion.

If this total weight is below said lower limit, the webs in the above-mentioned density range of 0.027 to 0.043 g / cm 3 (0.45 to 0.70 g / cc) may not absorb a sufficient amount of impregnating agent to achieve the desired results. On the other hand, if the percentage is above said upper limit, the dispersion tends to become viscous and the mixture of resin and filler only tends to coat the surface of paper webs with densities close to the upper limit of 0.043 g / cm (0.70 g / cc), instead of they would saturate such webs.

After impregnation, the paper web undergoes a heating step to melt the impregnating agent into the paper. In a conventional papermaking process, the web is heated to a temperature of about 100 ° C to dry it so that rigid polymers having a glass transition temperature above this temperature do not give satisfactory results. The upper limit of the glass transition temperature of the rigid polymeric materials used according to the present invention is preferably 60 ° C.

According to this process, certain advantages are achieved in the production of paper. For example, by using a rigid polymer impregnant extensively extended with a filler, the stickiness of the filler is removed so that the drying cylinders are easier to clean.

The importance of the above factors in making the paper of the present invention is apparent from the following examples. Briefly, Examples I and II illustrate the most appropriate degree of extension of a rigid polymeric material with a mineral filler. Example III shows the importance of polymer stiffness. The types of excipients that are preferably used to achieve satisfactory results are exemplified in Example IV. The desirability of adjusting the density of the paper web prior to impregnation is shown in Example V. The amount of impregnation that is preferably used to achieve the desired properties is shown in Example VI. The types of rigid polymeric materials that are preferably used are shown in Example VII. Example VIII illustrates the abrasion resistance properties of a paper that can be made according to the invention. Example IX illustrates the properties of paper impregnated with the composition of the present invention compared to the properties of paper coated with the same composition.

Example I

To determine the maximum allowable extension of the resin with an inert filler, sheets of unglued paper made from a mixture of 50% bleached hardwood sulfate pulp and 50% bleached "Northern" sulfate pulp were used as base papers. The paper had a basis weight of 23.5 kg, which is a weight of 500 sheets measuring 61 x 91 cm. The thickness of one sheet of paper was 0.14 mm. Since the density of the paper can suitably be expressed as its weight per mm of its thickness, the density of the base paper was 23.5 kg / 0.14 mm, i.e. approximately 0.037 g / an3 (0.60 g / cc). The impregnation was prepared by dispersing finely ground calcium carbonate powder having a particle size of about 2 μm in water and stirring. A finely divided rigid polyvinyl acetate emulsion was mixed into this aqueous dispersion so that the combined solids content of the resin and filler was 40% by weight of the dispersion. The calcium carbonate used was sold under the tradename CAMEL WHITE, a company called Harry T Campbell Sons Co., Towson, Maryland. The polyvinyl acetate emulsion used is sold under the tradename VINAC 880 by Air Products and Chemical Company, Allentown, Pa.

The sheets of paper were immersed in the dispersion and removed, and then passed between rubber rolls where the excess was squeezed from the sheets. The impregnated sheets were then dried for 4 minutes at 104 ° C, 2 minutes on each side, in a Williams paper sheet dryer. The sheets were allowed to condition (cure) for several days before being tested. Tear-off, puncture, and folding tests were performed using the standard TAPPI methods listed in Table I. The anti-jerker properties were determined by typing a letter on paper with a commercial travel typewriter and observing the difficulty or ease with which the letter could be removed by rubbing it with a "pencil-pen". Because inks in conventional typewriter ribbons contain significant amounts of non-drying oils, there is a direct relationship between the scratchability of the paper and its anti-scratch properties. A rating of "excellent" means that the letter was substantially completely scratched with a few abrasions. A rating of "good" means that the letter remaining after a few rubs was noticeable, but not obvious, to the naked eye after another letter was typed over the scratched letter. A rating of "satisfactory" means that the scratch was acceptable. A rating of "poor" means an unsatisfactory scratching result.

The solvent resistance properties of the sheets were determined by placing a drop of colored (purple) toluene on the surface of each sheet and allowing it to contact a predetermined surface area for 30 seconds. The toluene drop was then wiped off with a paper towel and the surface was rubbed with another paper towel impregnated with unstained toluene. In this way, the color remaining on the surface is removed, so that the degree of color penetration into the paper can be determined by detecting the remaining purple dye. A rating of "good" means that intrusion had occurred at several points, but that the degree of staining was mild and less than about 50% of the test area. A rating of "satisfactory" means that slight staining had occurred on most of the test area. A rating of "poor" means that the entire test area was stained dark. A rating of "zero" was given if the stain completely penetrated all the way to the back surface of the sheet of paper.

The results of the tests are shown in Table I: It should be noted that the symbol XD means that the test was performed on paper in the cross-machine direction. The units of measurement and the identification numbers of the standard test methods used in the various examples are given in Table I. 1 13 61 220 tn 0)

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From the table above, it can be seen that the sheet of paper before treatment (Sample H) had a tear strength of 147 g. A similar sheet impregnated with 100% solution of polyvinyl acetate (sample A) had a tear strength of 80 g. It should be noted, however, that when the impregnation was extended with 10-70% calcium carbonate, as in samples B-G, the tear strengths of the sheets decreased, but not to the same extent as when 100% polyvinyl acetate was used as the impregnation. the oil barrier layer and solvent resistance properties are maintained even if the impregnant had been extended up to 70% by weight with calcium carbonate. In addition, it should be noted that the flexural strength of the paper impregnated with the extended resin was higher than that of the sheet impregnated with 100% polyvinyl acetate.

Example II

The test procedure described above in connection with Example I was repeated; however, kaolin clay was used instead of calcium carbonate. The clay used was sold under the tradename HYDRAPRINT by a company called J M Huber Corporation, Huber, Georgia. The test results are summarized in Table II.

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It can be seen from the above tests that kaolin clay and calcium carbonate, mixed with a rigid polymeric substance and impregnated into the base paper, have essentially the same effect as fillers.

Example III

To demonstrate the importance of impregnating a paper web with a polymer having a predetermined stiffness, the base paper of Example I was impregnated with a series of mixtures of polymer and filler that differed only in the glass transition temperature (Tg) of the polymer. As previously noted, Tg is a measure of the inflexibility, or film stiffness, of a polymeric material. In the example, the polymer was extended with calcium carbonate in a ratio of 60% polymer and 40% calcium carbonate. The results are shown in Table III below.

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61220 BC

The results shown in the tables show that a sheet of paper impregnated with polyvinyl acetate having a Tg of 16 ° C has moderate oil barrier properties and moderate solvent resistance. On the other hand, a paper sheet impregnated with a polyacrylate having a Tg of 101 ° C run at a standard papermaking process temperature has no oil barrier properties and no solvent resistance. Thus, it should be clear that the rigidity of a satisfactory polymeric material, i. The Tg should be in this range, from about 15 to about 100 ° C, and preferably not more than about 60 ° C (see Table VII for polyvinyl chloride having a Tg of 60 ° C) if satisfactory results are to be ensured. The most suitable Tg range is between 22 ° C and 44 ° C to ensure that the polymer melts at conventional temperatures in the papermaking process.

Example IV

The type of inert fillers used as a filler is important for the properties of the impregnated paper. This will be clear from the present example, in which the base paper of Example I was impregnated with polyvinyl acetate mixed with a series of different mineral fillers. Each mixture comprised 40% filler and 60% polyvinyl acetate based on dry weight of solids. Calcium carbonate and clay were the same species used in the previous examples; the talc used was sold by the United Sierra Division, Cypress Mines, Trenton, New Jersey, under the trade name MISTRON VAPOR; mica is sold under the trade name DAVENITE MICA P-12 by a company called Hayden Mica Co ·, Wilmington, Massachusetts; and diatomaceous earth is sold under the tradename CELLITE under the trade name Johns-Manvilie Corporation, New York, New York. Each of the above fillers is of a commercial grade normally used in paper industry applications and has particle sizes ranging from 2 to 5 microns. The results of the experiments are shown in Table IV.

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20 6 1 2 2 0

It can be seen from the above table that calcium carbonate, clay, talc and mica are satisfactory fillers; on the other hand, the diatomaceous earth is unsatisfactory because the anti-slip properties and solvent resistance of the obtained paper are poor.

It is likely that the unsatisfactory results obtained with diatomaceous earth may be due to its extremely high specific surface area, which makes it incompatible with a rigid polymer. Suitable fillers for the purpose of the invention are referred to herein as "compatible" fillers, i.e. fillers which allow a satisfactory coating of the base paper with a rigid polymer and thus give a satisfactory resistance to oil and solvent penetration.

Example V

The effect of using paper webs with different pre-impregnation densities is illustrated in this example. An aqueous dispersion of polyvinyl acetate and calcium carbonate was prepared as in Example I. The mixture of polymer and filler was 40% by weight of the dispersion and the ratio of polymer to filler was 60/40 by weight. A series of non-adhesive towers of different densities were dipped in the dispersion and excess impregnation was removed from the sheets by passing them between rubber rollers and drying the surface of the sheets with paper towels to ensure removal of excess impregnation from the sheet surfaces. The sheets were then dried for 4 minutes at 104 ° C, conditioned for several days and tested as described above. For comparison, a dispersion * containing 40% by weight of polyvinyl acetate alone was prepared, and another batch of base papers was impregnated in the same manner. As another comparison, paper that had not been impregnated was tested.

The results of the tests are shown in Table V below.

> 1 21 612 20

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It can be seen from the table above that the density of the non-calendered paper web before impregnation must be at least 0.027 g / cm (0.44 g / cc) as in sample B. The non-calendered density before impregnation must not exceed the same 0.043 g / cm 2 (0 , 70 g / cc) as in sample I. It should be noted that although sample B contained 48.5% impregnant, it was unsatisfactory in terms of its anti-slip properties. Although Sample J had excellent oil barrier layer properties, it absorbed 9.1% impregnant, but its tear strength was inherently poor. It should also be noted that all papers that were satisfactory 3 had a final uncalendered density above 0.042 g / cm (0.69 g / cc) 3 and below about 0.055 g / cm (0.90 g / cc).

Example VI

The amount of polymer and filler impregnant that is preferably used to obtain a satisfactory paper is exemplified in this example using a base paper made from bleached "Northern" sulfate pulp and containing approximately 5% titanium dioxide. Although the paper was non-adhesive, it had a slight sizing effect due to the remaining resin; however, it is thought that this was not sufficient to prevent impregnation from penetrating the inside of the sheet. The surface area of the paper weighed 15.7 kg (61 x 91 cm to 500 sheets). Samples were prepared as described in Example V; however, the dry matter content of the impregnant was varied from 10% to 40%.

The results of this test are shown in Table VI.

23 61 220 3

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24 612 20

From the above results, it appears that a significant improvement in oil barrier layer and flexural strength properties occurs when the polymer and filler impregnation makes up 5-10%, or about 8.5% by weight of the sheet (compare Sample B) and excellent properties result when the impregnant is 15 -25% by weight of the sheet as shown in samples D and E, and up to about 48% as shown in Example I.

Example VII

To represent the different types of rigid polymeric materials that can be satisfactorily used in the manufacture of the paper of the present invention, sheets of the base paper of Example I were impregnated with a polymer-filler dispersion having a solids content of 40% by weight and a polymer to filler weight ratio of 60/40. . The filler was calcium carbonate and the polymers were: RHOPLEX AC-201, a polyacrylate emulsion manufactured by Rohm & Haas Co., Philadelphia, Pa.y and GEON 351, a polyvinyl chloride emulsion manufactured by B F Goodrich Chemical Co., Akron, Ohio.

The test results are summarized in the following Table VII.

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26 61 2 20

The above results show that the impregnating agent consisting of a rigid polyacrylate material extended with calcium carbonate has essentially the same effect on the base paper as the impregnating agent consisting of rigid polyvinyl acetate and calcium carbonate (compare Tables I and VII). It should be noted that similar results are obtained when the impregnating agent consists of a mixture in which polyvinyl chloride is extended with calcium carbonate}. Thus, it will be apparent that advantageous results are obtained by preferably using certain types of rigid polymeric materials such as polyvinyl acetate, polyacrylate and polyvinyl chloride.

Example VIII

The paper of the present invention has good abrasion resistance even if the impregnation mixture is extended with significant amounts of filler and although it is possible that significantly less than half the weight of the paper is impregnating agent. To determine the abrasion resistance of the paper of the invention, sheets of the base paper of Example I were impregnated with various rigid polymers and mixtures of polymer and filler according to Example I. The polyvinyl acetate polymer was VINAC 880} and the polyacrylate polymer was RHOPLEX 407} and the polyvinyl chloride was GEON 351. The sheets were subjected to a Tabor abrasion test according to TAPPI standard T 476 ts-63. An H-18 rubbing disc was used in the test and the revolutions of the disc were counted until a hole had worn the sheet. The results are shown in Table VIII.

27 61220

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• rH -P -P (0 C -H (0 3 rH

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• Η Φ O ^ -H-HO fi Φ> to ww4J> H: 0!>> I (0 (0> i +>> · Λί ti γη * η ή η (0 ι-n · η ag β aHWH « J Ο (0 OHdg: (0> ιΟ> ι «ft Η ft> ιΟ <0» 0 0)> ιΟ> 1>> ι> ιΟ ιΗ +> ρ m en βίο wha em m «to MJ-HUriwaO ^ aO -HU-P: (0

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28 6 1 2 2 0

The above results show that the abrasion resistance of a sheet of paper impregnated with a mixture of rigid polyvinyl acetate and a mineral such as calcium carbonate is only slightly lower (530 revolutions) than a sheet of paper impregnated with polyvinyl acetate alone (600 revolutions). A sheet impregnated with polyvinyl acetate extended in clay in said proportions has an even greater abrasion resistance than a sheet impregnated with polyvinyl acetate alone. A sheet impregnated with a polyacrylate material extended with calcium carbonate also has a higher abrasion resistance than a sheet impregnated with polyacrylate alone. A sheet impregnated with a filler-extended mixture of polyvinyl chloride has only slightly higher abrasion resistance (400 revolutions) than a similar sheet impregnated with polyvinyl chloride alone.

Example IX

The paper of the present invention must be impregnated with the above-mentioned dispersion; it cannot be prepared by simply depositing a dispersion on it as a coating. To demonstrate this, sheets of base paper having a basis weight of 23.5 kg (500 sheets 61 x 91 cm) and a thickness of 0.14 mm were impregnated and coated with a mixture of polymer and filler. For example, one sheet of this base paper was impregnated with the mixture described in Example I and another sheet of base paper was coated with the same mixture as described in Example 1 of U.S. Pat. No. 3,634,298, except that VINAC-880 polyl was used instead of the polymer synthesized in Example 1 of that patent. with a Tg of 31 ° C. The dispersion contained 55% by weight of a mixture of clay and polymer, which in turn contained 83% clay and 17% polymer. The resulting dispersion was deposited on one side of the base paper as a coating using a Meyer bar as described in Example 1 of said patent. The coated paper was then dried for 1 minute at 149 ° C.

The tear, puncture, and fold strengths of the coated and impregnated papers were subsequently determined, and the results of these tests are shown in Table IX.

29 61 220 a

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g rH <0 r- ~ r * i — i • H pj (N (N en dS nj m ro Section X

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3 I

* 11 ° a s | > U e

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a il iill ijjfl 'I

I II ma mt / 30 61 220

The table above shows that the impregnation reduced the tear strength by about 13%, while the coating increased the tear strength by about 12%. Impregnation more than doubled the puncture resistance while the coating improved the puncture resistance by only about 50%. In the folding test, the impregnation improved the refractive power considerably more than twice (even at a lower, 10-20% relative humidity), while the coating deteriorated the refractive power by as much as about 30%. The delamination resistance of the coated paper was the same as that of the base paper; the delamination resistance of the impregnated paper, on the other hand, was over 300 g / cm, at which point the paper is actually torn and not de-laminated. Thus, the above results indicate that the mixture of polymer and filler must be impregnated inside the paper and not merely deposited on its surface as a coating.

From the above description and examples, it should be apparent that the present invention provides a novel dense paper having the advantageous properties of dense papers without their disadvantageous properties, which makes the paper useful in many applications. In addition, the dense papers of the invention can be produced economically at relatively high paper machine speeds.

Accordingly, although one embodiment of the present invention has been described in detail above, many modifications and changes may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.

Claims (13)

31 61220 A dense paper having good oil and solvent tightness and consisting of a cellulosic fibrous web containing a polymer and a filler, characterized in that the polymer and the filler are distributed throughout the cellulosic fibrous web in the form of a impregnating dispersion with an impregnating agent of 8.5-50% based on the weight of the paper, and that the impregnating agent contains 35 to 90% by weight of a polymeric material having a glass transition temperature of 15 ° to 60 ° C, and 10 to 65% by weight of a compatible inert filler based on the weight of the impregnating agent. Paper according to claim 1, characterized in that the glass transition temperature is from about 22 to about 44 ° C. Paper according to Claim 1 or 2, characterized in that it contains an impregnating agent from about 15% to about 40% of the final weight of the paper. Paper according to one of the preceding claims, characterized in that it contains about 20% to about 65% by weight of filler. Paper according to one of the preceding claims, characterized in that the polymeric material is selected from the group of polymeric materials consisting of polyvinyl acetate, polyacrylate, polyvinyl chloride and mixtures thereof. Paper according to claim 5, characterized in that the polymeric material is selected from copolymers and homopolymers of said materials. Paper according to one of the preceding claims, characterized in that the filler is selected from the group of minerals consisting of clay, calcium carbonate, mica and talc and mixtures thereof. A method of making a dense paper according to any one of the preceding claims, characterized in that it comprises the following steps, performed in the following order: conveying a paper web having a dry, non-calendered density between its opposite surfaces of about 0.027 g / cm (0.45 g / cc) 2 to about 0.043 g / cm (0.70 g / cc); the web is impregnated by introducing an excess of an aqueous dispersion containing a mixture of a rigid polymeric material and a compatible inert filler, the mixture comprising from about 35% to about 90% polymeric material and from about 10% to about 65% inert filler, calculated as a percentage of the mixture; by weight, and having a glass transition temperature of the rigid polymeric material of about 15 to about 60 ° C, and passing the web between two opposing press rolls to ensure that the dispersion penetrates the web and removes excess dispersion therefrom; and heating the web after passing between said rolls and removing excess dispersion therefrom to melt the mixture into the web to amount to about 8.5 to about 50% by weight based on the dry weight of the web. Process according to Claim 8, characterized in that the polymeric substance of the mixture is selected from the group consisting of polyvinyl acetate, polyacrylate, polyvinyl chloride and homo- and copolymers thereof and mixtures thereof. Process according to Claim 8 or 9, characterized in that the inert filler is selected from the group consisting of clay, calcium carbonate, mica, talc and mixtures thereof. Process according to any one of claims 8 to 10, characterized in that the aqueous dispersion contains about 12.5 to 60% by weight of said mixture, based on the total weight of said dispersion. Process according to one of Claims 8 to 11, characterized in that the density of the paper web before it is impregnated with the mixture is in the range from about 0.033 g / cm 3 (0.54 g / cm 3) to about 0.042 g / cm 3 (0.69 g / cm 3). cc). Method according to one of Claims 8 to 12, characterized in that the web is substantially free of gluing before it is impregnated. • 1 S? '33 61 220