SE461596B - PROCEDURE FOR MANUFACTURE OF LIGHT STABLE PAPER AND PAPER RECEIVED THROUGH THE PROCEDURE - Google Patents

Description

461 596 cause much of the yellowing of the paper. One has found that a blockade of these phenolic units by acetylation or reduction of the carbohydrate contained in the lignin nyl and olefinic groups with i.a. sodium borohydride significantly reduces yellowing. Other important steps are reduction of certain aldehyde and quinoid groups, to counteract chromophore formation, ie. formation of substances that can discolor the paper.

There is today a fairly large base, in terms of research the mechanisms surrounding the post-yellowing of paper made of mechanical pulp.

Because there are evidently many pathways, which for to form chromophoric compounds, the yellowing is chemically, a very complex problem. To only by chemical treatment achieve good color stability (implied whiteness) of paper made from mechanical pulp requires a rather costly process technology and a relatively large chemical effort, which makes the treatment out of financial, and also out environmental point of view, less attractive.

In order to achieve the maximum degree of brightness of high-performance lots of manufactured paper have different in the paper industry methods used, i.a. 1) choice of high-grade white paper raw material, 2) chemical treatment (bleaching of raw materials), 3) blending of white fillers and optical brighteners in the pulp, and 4) coating the paper with high-grade white coating pulp containing fillers and optical brighteners.

Diversity has long been used in the paper industry 461 596 procedures for surface treatment of different types of paper of lower quality. The surface treatment has the task here of something way to increase the quality of the paper, especially the printability.

Bl.a. the paper's sometimes quite poor brightness and light stability. However, this requires relatively thick coatings. layer.

For surface treatment of paper, a coating is usually used. composition, containing fillers (often kaolin), a binders and various additives (auxiliary chemicals).

Numerous paper coating compositions are available described in the literature, i.a. in - Coating formulations A.H. Nadelman, Lockwood Trade Journal, New York 1966 - Pulp and Paper, J.P. Casey, vol. 3, Interscience Publ. 1961

When using coating compositions, these are applied on the paper to modify the surface properties of the paper such as brightness, opacity, printability, mechanical strength, the water retention of the paper, etc. The imposition of a coating mass can take place by means of rollers in a so-called coating works, or by means of razors against special consideration rollers or against drying cylinders in a fourdrinier machine. One coating compound may, in addition to one or more fillers and one or more binders, contain emulsifiers, polysaocarides, dispersants, UV absorbers, optical brighteners, defoamers, fungicides, etc. as described in SE-B-7502846-4 (= FR-B-7408729). Other coatings compounds are described in EP-A1-0015517, SE-B-350083, SE B-3S3358 and SE-B-7908927-2. The three later publications preferably has the grain size distribution as a patent goal. 461 596 In addition to the risks of introducing a number of different in a manufacturing process, and the consequent danger of spills, which cannot be completely disposed of, must also be fillers of the compounds are considered as a contaminant. Also if the binders used in the context are relatively low vapor pressure at work occurring during papermaking temperatures, it must not be overlooked that vapors thereof exist in the workplaces for the production of paper.

The property of the fillers (pigments) of contamination comes in the day on which the paper fibers are to be recovered from recycled paper. Although such recycled paper is a raw material for the intermediate layers in carton or for fluting, where, for example, clay can drawn, the filler will cause interference to others other uses of paper fibers. The clay can largely kept suspended in paper mass, thereby not causing deposition in masses, but will wear especially on pumps and valves below the path of the mass to the paper machine. In this context, keep in mind that the coating mass is up to 20% of the weight of the recycled paper, and this presents problems, if not others recycled fibers are available for admixture / dilution of mass. In the manufacture of toilet and kitchen paper, which to 100% is made from recycled paper, there is no financial possibility to manually sort the recycled paper, for to limit the supply of coating compound. The massacre buffer volume is the only aid for diluting the coating composition, if a lot of coated paper were to be included in a certain batch of return material.

In the case of the said type of paper, the filler should not constitute anything problems in the final product. However, the risk of allergic reactions in mucous membranes, which come in contact with the paper in question, not excluded, unless one through several steps clean the recycled paper of adhesive in the coating the mass. 461 596 The object of the invention is to provide a paper of sufficient quality for archive purposes and with good print contrast, but made of a relatively inexpensive starting material. The invention reduces the impact on the environment of coating chemicals used hitherto, which are mainly are imported into paper-producing countries.

The term archival purpose means that the paper may not be yellow or otherwise change to reduced contrast between paper and writing or other print thereon, either the paper is stored in the dark or in a natural or physical light can illuminate its surface. In this context, take into account that paper used in office copying machines down, in these are loaded with relatively much light energy. This has a wavelength composition so selected that the transmission of text or image goes fast, but the paper fibers do not skonas. the object which the invention is intended to meet is primarily that offer a paper which, over time, as may be taken by the human eye, remains white. It is however fully possible, that with the use of the new procedure produce paper, which the eye perceives as slightly toned in pink, green, blue or other color. In all cases preventing the natural yellowing of paper fibers, and that the paper surface reflects light within the desired wavelength range advise. This is accomplished in the manner described below.

In order to reduce the use of chemicals and improve the light stability of paper made of mechanical or thermomechanical pulp is proposed according to the invention, that this treated in a process which, according to specific chemical treatment of the mass directed at - - - the phenolic units of lignin 461 596 - - - conjugated alpha-carbonyl structures (reduction), - - - alpha-, beta-unsaturated side chains (hydration) is characterized by the following process steps: Coating the paper surface with at least two coats, Layer 1 (closest to the surface of the paper), called the protective layer, consists of those of inorganic pigments, which are partly UV-reflecting, partly UV absorbing, Layer 2 (on top of layer 1) constitutes a surface layer with a maximum brightness and consisting of optical brightener and inorganic fillers.

Additional features Derived from the following claims.

It has surprisingly been shown that the brightness of paper made of relatively dark mechanical masses significantly improved by the proposed surface treatment of the paper.

In addition, such paper exhibits a clearly improved color. stability, and insensitivity to both natural and physical light. layer 1 which is applied directly to the paper is a layer, which shall protect the UV - sensitive compounds of the paper, so that Undesirable photolytic reactions are avoided between high energy UV radiation and for example lignin structures containing ring-conjugated alpha-carbonyl-, alpha-beta-oleEin- or biphenyl bonds.

The protective layer is applied as a thin coating consisting of a pigment mixture, binder, dispersant and water. The following substances are useful in the pigment mixture: a / high purity titanium dioxide (TiO2) with a grain size of 0.1 - 1.0 Pm (preferably 0.3 - 0.6 Pm), _ ..__.._; -. _....._..._..._._......_ .. 461 596 b / high purity barium sulphate (BaSO4), grain size 0.1 - 0.8 μm (preferably 0.1 - 0.3 μm) c / high purity, amorphous silica (SiO2). a / serves as a UV absorber, b / and c / serves as a UV reflectors.

The mixing ratio calculated in parts by weight is 1-3 TiO2 + 4-8 BHSO4 + 1-5 S102, preferably 2 TiO 2 + 6 BaSO + 3 SiO. 4 2 shear weight 0.2 - 0.8 g / mz (preferably 0.3 - 0.6 g / mz).

Useful binders are styrene acrylate latex, methylcellulose or PEO (Polyethylene oxide).

As the dispersant, TU (nonyl-phenol-ethylene- oxide).

The protective layer suspension is made of 10 kg pigment mixture (Tioz + BaSO4 + SiO2) 10 - 20 kg binder 0.1 - 0.2 kg dispersant 985 kg (max) water, which may have a hardness of max 3 dH ° and an iron content of max. 0.02 ppm.

This protective layer suspension is useful on dry paper (less than 7% moisture). The suspension is prepared with the aid of one dispersing equipment, and kept under stirring until until it is used for application on the paper web, preferably by spraying on dry paper.

When applying the protective layer in a paper machine, then the paper web has reached a dryness between 65 and 75%, is lowered the water content of the suspension below 900 kg when sprayed and even lower when coating the paper. 461 596 Layer 2, which is laid on top of the protective layer, consists of one or several white pigments, the whiteness of which is further enhanced by addition of one or more optical brighteners. The white pigment may consist of pure grades of titanium dioxide, barium sulphate, calcium carbonate or aluminum silicate in grain size smaller than 1.0 um. Depending on the end use of the paper area can be one of the listed substances, or a mixture of several of them, used in the pigment layer. Such as optical white matter, one or more moisture-resistant, fluorescent inorganic materials, so-called fluorescents. Especially used only in this context are blue reflective substances such as strontium pyrophosphate (Sr2P2O7: Eu) or zinc sulphide (ZnS: Ag).

By increasing the amount of these substances, the paper turns bluish.

Should there be a request for rose reflectors paper surface, yttria (Y2O3: Eu) or magnesium fluorine germanate can be used. Barium magnesium aluminate and zinc cadmium sulfide gives green tinted paper.

Fig. 1 shows excitation and reflection spectra of a blue luminous (optical brightener), namely zinc sulphide, and Fig. 2 shows the corresponding emission spectrum.

The term fluorescents here stands for powdery inorganic materials, which are special chemical compounds with distinct luminescence in crystalline states, achieved after a special annealing process. The light generation of these substances occurs through the interaction of larger groups of atoms or molecules cools, sometimes whole crystals, which are subject to an excitation energy absorption.

Inorganic fluids are not ideal crystals, but real ones crystals with lattice disturbances. The design of energy levels in these real crystals have the greatest significance for the generation of luminescence.

The production of fluorescents is characterized by reactions in 461 596 solid phase, which usually give non-stoichiometric compositions preparations of the basic material and additions of special substances, so-called activators. When these are built into the crystal structure, activator centers (luminescence centers) are formed in in the form of scattered activator ions, or ion groups, which then is responsible for the light emission.

The phosphors can be excited by different types of radiation, and emits light within different wavelength ranges. The exciting the radiation is often UV radiation (ultraviolet), or sometimes even short-wave visible radiation. Some fluorescents can require excitation of higher energy photons, e.g. pelvic X-rays.

Knowledge of the fluorescents' absorption, excitation and emission spectra, as well as their powder characteristics, ie. the particle size distribution and the particle shape, are of mental significance in the choice of fluorescent substance for different uses areas. Likewise, the light stability of the light element is a given system (area of use) a crucial parameter.

In order for a light substance to be suitable as an optical brightener in a paper coating compound the following properties are required: The excitation spectrum must be in the wavelength range 350-420 nm so that the light substance can be excited by incident light, 2. The emission spectrum must be in the wavelength range 400-500 nm, preferably 430-470 nm, with a maximum spectral band width of 40 nm ("blue sensor"), 3. The fluorescent material must be light-stable against incident radiation and long-term resistant to moisture and others in the coating composition constituent, 4. The maximum grain size distribution of the luminaire shall be 461 596 10 between 1 and 5 μm, preferably between 1 and 3 μm.

Fluorescent substances that meet these requirements for a satisfactory ways are i.a. special types of light powder of magnesium tungstate (MgWO 4: W) and zinc sulfide (2nS: Ag) or (ZnS: Ag + Co0.Al203). Mixtures of these substances can also be advantageous in some cases may be used, depending on the application of the paper.

Binders useful for the pigment layer are styrene acrylate latex, polyethylene oxide or other water-soluble binder, which may give the paper favorable properties for certain areas, in particular with regard to reflected light.

The binder and white pigment must not have strong absorption frequencies in the spectral range 280-400 nm, which may interfere the excitation of the activator centers of the optical brightener.

In addition, the refractive index of the white pigment and binder should differ as much as possible from each other, to the maximum coverage must be achieved. Pigment layer coverage ability is directly proportional to the relative refraction the index. This is defined by the formula _'nP rel _ nm np = refractive index of pigment nm = refractive index of ambient medium (binder).

In addition to the binder, the surrounding medium (m) may be other additives (filler, extender) that are optically smaller active than the pigment. This means a lower refractive index index, which results in a higher value ° of relative refraction the index.

To suppress the "yellow stick" in the paper surface should the coating composition (layer 2) consist of pigment and possibly extender, which are fine-grained, i.e. has a grain size below 1.0 um. Through this fine grain, the remission in it is strengthened blue wavelength range compared to the remission in it yellow wavelength range. Because the remission is the diffuse one 1: ' 461 596 11 the reflection of incident light, the eye perceives it referred the light as the color of the viewed surface has. This is mainly due to the remission consists of non-parallel radiation. The color impression from the paper surface in question will be the same, regardless of the clock what angle the paper is viewed, i.e. regardless of that incident reflection angle of light. The remission increases with decreasing wavelength. Thus, the remission becomes greater for blue than for red light, measured at constant grain size at the pigments in the paper surface. This has resulted in the invention the advantage that the short-wave blue light, which constitutes ment color to yellow light, is reflected to a greater degree than anything else light. This reinforces the impression of another blue and white white paper. Thus, the paper testifies with time, because despite the protective layer the underlying wood fibers will yellow slightly, thus complementing the initial blue tone.

The protective layer's mixture of UV-absorbing and UV-reflecting substances determine the balance point, whenever they can selected so that the reflected wavelength of 185 nm becomes nerande. UV light of this wavelength is significantly more energy-rich than yellow light, and can turn off one on remission part of the yellow issue.

In the manufacture of the coating compound must be a good dispersion of the primary grains is achieved. Any reagglomerate- ringing of the pigments reduces the brightness and opacity of the pulp, and creates an uneven paper surface, which is perceived as a matte surface. An efficiently operating dispersing equipment, and addition of auxiliary chemicals, i.e. surfactants, are necessary.

Four different compositions of coatings have tried according to the following recipe.

A. 60.0 kg Baso4 2.0 kg optical brightener (MgW04: W) 5.0 kg styrene acrylate latex (binder) 0.1 kg of surfactant 32.9 kg water an ° = max 3 iron content: max 0.02 ppm 461 596 B. so, o z fl o S.ø 0.1 32.9 C. 60.0 2.0 510 0.1 32.9 D. 60.0 2.0 5.0 0.1 32.9 kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg 12 TiO2 + BaSO4 (1 + 1 parts by weight) optical brightener (MgWO4: W) - styrene acrylate latex surfactant water dH °: max 3 iron content: max 0.02 ppm TiO2 + BaSO4 (1 + 1) optical brightener (ZnS: Ag) styrene acrylate latex surfactant water dH °: max 3 iron content: max 0.02 ppm ZnO optical brightener (ZnS: Ag) styrene acrylate latex surfactant water dH °: max 3 iron content max 0.02 ppm Brightness measurements have been made with spectrophotometers in wavelength range 380-800 nm. The relative lightness of the paper has calculated from measured spectral data according to the method specified in Journal of Color and Appearance vol. 1 No. 5 1972 page 341.

Rapid aging of the paper has been performed using the xenon method.

The results are reported in the tables below.

In in 461 596 13 TABLE 1 Relative brightness (%) Relative brightness (%) (S) Spreadsheet initial aged Diff Newspaper (xenon test) paper. untreated 63 59 -6.3 surface treated layer 1 65 62 -4.6 (protective layer) layer 1 + layer 2 alt.A 84 81.5 -3.0 layer 1 + layer 2 alt.B 86 84 -2.3 layer 1 + layer 2 alt.C 89 87 -2.2 layer fold: layer 1 = 0.3 g / m2, layer 2 = 2.0 461 596 14 TABLE 2 Relative brightness (%) Relative brightness (%) (%) Measuring sheet initial aged Diff 55% abrasive mass I + 45% Barrsulfate- pulp untreated 70 65 -7.8 surface treated layer 1 72.5 69.5 -4.1 (protective layer) layer 1 + layer 2 alt.A 90 87.5 -2.8 layer 1 + layer 2 alt.B 91.5 89 -2.7 layer 1 + layer 2 alt.C 94 92 -2.1 layer 1 + layer 2 alt.D 91 89 -2.2 layer weight layer 1 = 0.4 g / m2, layer 2 = 1.5 g / m2 The tables clearly show that the protective layer improves the yellowing resistance of the paper, but by as on the test sheets add 1.0 g / m2 of the coating mass is achieved one paper, which practically does not age, ie. resists yellow and retains its brightness. By using protective layer, the pigment layer can be made much thinner than otherwise.

Experiments with only protective layers have shown that then these applied thicker, the paper becomes more yellowing resistant. Though li 461 596 15 has when weighing all factors with regard to paper use judged, that the combination very thin skydassxikt (0.3 - 0.4 g / m2) and thin pygmy: layer_ (1 - o - 2.0 g / m2) provides optimal quality. For gravure printing paper coating with up to 4.0 g / m2 pigment in is proposed the paper machine, and subsequent supercalendering.

The invented method can be practiced in two ways. The first is to spray in a paper machine on the protective layer, then the paper web has a dry matter content between 65 and 70%. Then intermediate paper is dried to between 80 and 90% dry matter, after the pigment layer, the paper web is applied bilaterally by means coating plant or squeegee coating.

The second loading procedure involves the paper delivered from a paper machine to a special coating machine. In this, the protective layer is applied, either by spraying, or by squeegee coating. After drying of the protective layer is applied to the pigment layer, either by rolling coating or by squeegee coating. Finally dried the paper before it is rolled up or cut.

Claims (9)

461 596 16 Patent claims A method of manufacturing light-stable paper of mechanical or thermomechanical pulp, comprising the two-sided coating of the paper web with two layers, characterized in that a first layer (protective layer) consisting of UV radiation absorbing and reflecting pigments in an aqueous suspension is applied to the paper, after which it is coated. with a white reflective pigment layer containing fluorescent, inorganic materials (light substances) and binders and surfactants. Method according to claim 1, characterized in that the protective layer is applied to the paper web as an aqueous suspension containing 5-3 parts by weight of titanium dioxide, 4-8 parts by weight of barium sulphate, 1-5 parts by weight of silica, binder and dispersant. Method according to claim 1 or 2, characterized in that each side of the paper web is applied to the protective layer suspension to a resulting dry content of 0.2 - 0.8 g / m 2, preferably 0.3 - 0.6 g / m 2. Process according to Claim 1, characterized in that the light substances contained in the pigment layer are selected from the groups magnesium tungstate and zinc sulphide. Process according to one of the preceding claims, characterized in that the pigment layer, in addition to light substances, consists of one or a mixture of several of the following substances: titanium dioxide, barium sulphate, calcium carbonate and aluminum silicate. Method according to one of the preceding claims, characterized in that the pigment layer is applied by means of a roller coating plant to the paper web as a binder containing aqueous suspension to a resulting dry content of 0.3 - 4.0 g / m 2, preferably 1.0 - 2.0 g / m 2. m2. Method according to one of the preceding claims, characterized in that the protective layer is applied by spraying to the paper web when it has passed a first calender and has a dry content of more than 65%, that the pigment layer is applied to the paper web when it has reached a dry content of about 85%. after which the paper web may pass a second calender. Process according to one of Claims 1 to 6, characterized in that it is applied to dry (92-93% dry matter) glossy paper, which is first spray-coated with a protective layer, then intermediate-dried and squeegee-coated with a pigment-layer suspension, and then finally dried. Fiber-containing paper produced according to the method of claim 1, characterized in that it is coated on both sides of the sheet with a UV radiation absorbing and reflective protective layer, on top of which there is a pigment layer containing fluorescent, inorganic substances.