EP3303701B1 - Fibrous support material for the production of a porous covering raw paper or preimpregnate, and process for its manufacture - Google Patents

Description

Field of the invention

The present invention relates to a fibrous carrier material according to the preamble of claim 1 and to a method for its production. Furthermore, the invention relates to a coating base paper or prepreg formed from the novel carrier material. The products according to the invention are intended for the production of coating materials for furniture surfaces and furniture foils, but also for walls, floors and ceilings.

Background of the invention

The main objectives in the production of such papers are the qualitative properties in terms of strength, impregnation, paintability and printability, which are necessary for further processing, as well as the optical goals to achieve the required and specified color. In all cases, the paper must be through-dyed. Coating base papers are produced in all colors / saturations / brightnesses, which can be measured from the entire color spectrum.

Coating base papers, sometimes also referred to as decorative base papers, are highly technical special papers which are printed with aqueous or solvent-based color systems or are printed unprinted and in monochrome. This applies to all conventional printing processes such as gravure printing, offset printing, flexographic printing, screen printing, but also all non-impact printing processes such as digital printing systems. The further processing is essentially subdivided into the processes of impregnation, painting, pressing on wood-based materials or lamination on wood-based materials or other plate-like materials.

Wood-based materials are chipboard, fiberboard, medium-density fibreboard (MDF) and high-density fibreboard. However, plates may also be coated or laminated, which may be made of a variety of materials such as in particular mineral materials, plastics or metals.

Further processing of these papers is the production of decorative laminates, which are pressed from impregnated, printed and / or through-colored coating base papers and core papers to a homogeneous plate or produced in an endless process [1].

Coating base papers must be produced in all the colors of the color spectrum that can be perceived by the human eye, including the highest brightness (white) and the highest darkness level (black). In order to achieve a specific color and the specified color location and the physical properties, organic and inorganic pigments of different particle size are used in different mixing and concentration. Fillers are also used to meet and meet all physical conditions and requirements.

An important pigment used to improve the brightness and opacity of the paper is titanium dioxide (TiO ₂ ). Typically, titanium dioxide is added to the fibrous paper in a wet end process (see, for example WO 2013/109441 A1 ).

Coating base paper as a fibrous substrate is the most economical, flexible and functional solution for displaying designed and styled surfaces for a variety of applications such as furniture for living and sleeping areas, kitchens, offices, bathrooms, floors, interiors for large objects such as airports, hotels, office buildings, buildings of the public interest such as museums, galleries (see for example WO 2013/109441 A1 ). Coating raw paper requires a very high, possibly up to 100% opacity. The covering capacity against the background, ie the color of the carrier material, must be ensured without loss of color impression. Crucial for this are the content (amount) and the distribution of pigments and fillers in the paper body. The limit amount is dictated by the requirement for strength of the paper.

The limit amount can be increased in a conventional manner by increasing the basis weight of the paper. Thus, if the basis weight of the paper is high enough, the desired 100% opacity can be approached. The current state of the art places economic limits on the sensible use of pigments and fillers.

The most commonly used pigments, white (titanium dioxide) and colored (iron oxides), represent a high value and are subject to immense, cyclical price fluctuations. Therefore, maximum yield is very important. This, in turn, means that the pigments / fillers in the paper body must have maximum particle distribution to achieve the best possible opacity and hiding power. Until today it has not been possible to reach this standard. The pigments / fillers are present in the paper body as agglomerates. The light-scattering layers overlap and reduce the opacity effects and form a different color perception.

To reduce the agglomeration phenomena certain binders, fillers or dispersants are used, whereby an improvement of the light scattering efficiency is achieved [2]. However, in view of the increasing importance of environmental concerns and also because of the increasing costs of the raw material, new solutions are being worked out which, through the use of biomaterials, should lead to a reduction of the titanium dioxide demand.

Accordingly, it is an object of the present invention to provide a fibrous carrier material, in particular a coating base paper, which is distinguished by high quality, in particular by high opacity, lower need for pigments and good mechanical stability. A further object of the invention is to specify a method for producing the carrier material according to the invention. As a further object of the invention, a coating base paper or a prepreg having improved properties is provided.

Presentation of the invention

The abovementioned objects are achieved according to the invention by the fibrous carrier material according to claim 1, by the production method according to claim 5 and by the porous coating base paper or the prepreg according to claims 8 and 9, respectively.

Advantageous embodiments of the invention are defined in the dependent claims.

The fibrous carrier material according to the invention comprises, in a known manner, a two-dimensional structure of cellulose fibers which additionally contains at least one pigment species and optionally further paper-like additives. Furthermore, the cellulose fibers contain a proportion of 1 to 20 wt .-% of nanofibrillated cellulose, which is to be understood here as the percentage of the total weight of all cellulose fibers. As will be explained in more detail below, in the present context the term "nanofibrillated cellulose", also abbreviated here as "NFC", comprises cellulose fibers with a diameter of approximately 3 nm to approximately 200 nm and a length of at least 500 nm and an aspect ratio ( Length: diameter) of at least 100 to understand. According to the invention, the NFC has a specific surface area (SSA) of at least 125 m ² / g.

Typically, the NFC fibers have a diameter of 10 to 100 nm, an average of 50 nm and a length of at least a few microns, and the aspect ratio may also be 1,000 or more.

According to one embodiment of the invention (claim 2), the NFC content is 5 to 10 wt .-%.

Surprisingly, it has been found that the embedding of a proportion of NFC in the two-dimensional structure of cellulose fibers has various advantageous effects on a fibrous carrier material produced therewith, which is provided in particular for producing a porous coating base paper or prepreg.

Previously, it was known that the addition of NFC leads to a densification of the paper. This usually results in the air permeability becoming worse or the associated Gurley value becoming higher. Surprisingly, however, it has been found that, despite higher Gurley values or lower air permeability, the coating base paper produced according to the invention achieves a further very good resin impregnation, improved topography and printability.

It is already known that the addition of NFC can have beneficial effects on strength. For example, in the EP 1936032 A1 a method for making multi-layered paper products, in particular low density cardboards such as beverage cartons. The main objective is to lower the grammage or basis weight while maintaining the strength properties.

The WO 2014/033409 A1 relates to an opacifying layer which is provided as a cover layer for a backing paper. The said layer consists of a mixture of NFC and at least one granular pigment species.

In the context of the present invention, it has been found as a novel effect that the addition of NFC in the preparation of highly pigment-containing porous, absorbent coating papers or prepregs enables a significantly more homogeneous absorption of the pigment species in the fiber network, which has very advantageous effects. The immediate advantage is that a given pigment content results in a significantly higher opacity or that a given opacity can be achieved with a lower pigment content. This results in clear economic as well as ecological advantages. A directly apparent advantage follows from the saving of pigment material, with concomitant cost reduction, but also with reduced dust formation during processing. In addition, chemicals can advantageously be dispensed with or reduced in their required amount, which are currently used to improve pigment retention. Another very significant advantage of the lower pigment content, given a given opacity, is a further improvement in structural integrity, particularly tear strength of the fibrous support structure, i. of the coating base paper. This applies in all directions within the support structure and both in the dry and in the wet state.

Apparently there is a synergistic effect of the addition of NFC: on the one hand, it seems to bring about a better mechanical cohesion by the formation of additional hydrogen bonds, and on the other hand, it seems by the possibility of reducing the pigment content as well as by a more homogeneous distribution of the pigment in the form of relatively small agglomerates or the avoidance of larger lumps, to make an additional contribution to the mechanical cohesion. Larger agglomerates would act as weak points and reduce the tear strength of the fibrous carrier material.

Another surprising advantage of the inventive fibrous carrier material when used as a coating base paper results from an improvement in the surface topography, which leads to better printability and ink acceptance with associated savings for the commonly used printing inks.

Cellulose nanofibers (hereinafter abbreviated to "NFC") have been extensively studied and described in the literature over the past 20 years. Also in the field of general papermaking such nanofibers have been proposed as a possible "wet end" additive to improve certain properties of the paper. However, it is also known that the addition of significant amounts of NFC generally results in a loss of opacity [3], which is highly undesirable especially for coating base papers.

NFC is generally obtained by a mechanical comminution process of wood and other vegetable fibers; first descriptions go to Herrick et al. [4] and Turback et al. [5] returned in 1983. The new material was initially called microfibrillated cellulose (MFC). Today, however, besides the term MFC, different terms such as cellulose nanofibers (CNF), nanofibrillated cellulose (NFC) and cellulose nano- or microfibrils are commonly used. It is a semi-crystalline cellulosic material made of cellulose fibers with a high aspect ratio (= ratio of length to diameter), lower degree of polymerization compared with intact plant fibers and correspondingly increased surface area, which is obtained for example by a homogenization or grinding process [6].

In contrast to the straightforward "cellulose whiskers", which are also referred to as "cellulose nanocrystals" and have a rod-like shape with a length of usually 100 to 500 nm (depending on the cellulose source, there are also up to 1 micron long crystals) , cellulose nanofibers are long and flexible. The The resulting NFC usually contains crystalline and amorphous domains and exhibits a network structure due to strong hydrogen bonds [7, 8, 9].

By "customary paper additives" are meant in particular fillers. The pigments and fillers contained in the novel support material are preferably selected from the group of metal oxides, oxides and / or mixed oxides of a semi-metal / semiconductor or mixtures thereof. Preferably, the pigments / fillers may be selected from, but not limited to, silicon, magnesium, calcium, aluminum, zinc, chromium, iron, copper, tin, lead or mixtures thereof.

Preferred pigments / fillers are silicas, aluminas, iron oxides, magnesium silicate, magnesium carbonate, titanium dioxide, tin oxide, aluminum silicate, calcium carbonate, talc, clay, silica, inorganic materials such as diatomite, organics such as e.g. Melamine-formaldehyde resins, urea-formaldehyde resins, acrylates, polyvinyl alcohol, modified polyvinyl alcohol, polyvinyl acrylates, polyacrylates, synthetic binders, natural origin binders such as starch, modified starch, carboxymethyl cellulose or mixtures thereof.

A particularly preferred pigment species for forming a white coloration is titanium dioxide (claim 3). Another pigment species used for some applications is the iron oxide (claim 4).

• Bereitstellen einer wässrigen Suspension, welche ein cellulosehaltiges Material sowie eine Beimengung der besagten Pigmentspezies und optional weitere papierübliche Zusätze enthält,
• Blattbildung,
• Trocknung,

wobei das cellulosehaltige Material einen Anteil von 1 bis 20 Gew.-% an NFC mit einer spezifischen Oberfläche (SSA) von mindestens 125 m²/g enthält.According to a further aspect (claim 5), a method for producing the carrier material according to the invention comprises the following steps:

• Providing an aqueous suspension which contains a cellulose-containing material and an admixture of the said pigment species and optionally further paper-like additives,
• Sheet formation,
• drying,

wherein the cellulose-containing material contains a proportion of 1 to 20 wt .-% of NFC having a specific surface area (SSA) of at least 125 m ² / g.

Generally, it has been found that using NFC with a specific surface area (SSA) of 100 m ² / g or less gives significantly worse results in terms of measurable surface topography, printability and retention for pigments such as titanium dioxide.

It is also noteworthy that the use of highly pure cellulose instead of NFC does not lead to the quality improvement according to the invention. Without wishing to be bound by any particular theory, this finding indicates that the advantages according to the invention can not be achieved simply by comminuting the cellulose into particles with dimensions in the nanometer range, but that for this purpose the formation of fibers with a diameter in the nanometer range and in the nanometer range Aspect ratio of at least 100 is required.

According to one embodiment of the method (claim 6), the NFC content is 5 to 10% by weight.

The NFC used for the above process should have a specific surface area (SSA) of at least 150 m ² / g, in particular of at least 175 m ² / g, preferably at least 225 m ² / g (claim 7).

Advantageously, the process according to the invention uses a papermaking process which is suitable and optimized for the production of coating base paper. Such methods are known in principle. In the context of the present invention, the process will be modified such that the cellulosic material either immediately before the formation of an aqueous suspension or subsequently the said proportion from 1 to 20% by weight of NFC is added. Again, this percentage refers to the total weight of all cellulosic fibers.

According to a further aspect (claim 8), a porous coating base paper is provided, which is characterized by an increased opacity at a given pigment content or by a lower pigment requirement for a given opacity and at the same time with commercially available methods such as WO 2013/109441 A1 described further processed.

According to yet another aspect (claim 9), a pre-impregnate is provided, wherein the inventive carrier material is impregnated with a suitable synthetic resin dispersion. Preimpregnates are prepared in a manner known per se by impregnating a fibrous carrier material with an impregnating resin solution (see, for example, US Pat EP 0648248 B1 ). This impregnation step already takes place in the paper machine. As a result, the pre-impregnates can still be provided with a print motif. The novel prepregs are distinguished by the advantages already mentioned in connection with the coating paper according to the invention.

The products according to the invention are used as surface layers for a very wide variety of plate-shaped materials, in particular laminate materials. Such laminates are known in particular as "high pressure laminates (HPL)" and "low pressure laminates". This could be used indoors for floors, walls and ceilings and all furniture surfaces. It is understood that depending on the application, the surface layer is still provided with an additional protective layer (overlay) or lacquered.

Literature:

1. 1. Istek, A .; Aydemir, D .; Asku, S. The effect of décor paper and resin type on the physical, mechanical and surface properties of particleboards coated with impregnated décor papers. Bioresources 2010, 5, 1074-1083 ,
2. Second Bardet, R .; Belgacem, MN; Bras, J. Different strategies for obtaining high opacity films of MFC with TiO2 pigment. Cellulose 2013, 20, 3025-3037 ,
3. Third Herrick, FW; Casebier, RL; Hamilton, JK; Sandberg, KR Microfibrillated cellulose: Morphology and accessibility. J. Appl. Polym. Sci. Appl. Polym. Symp. 1983, 37, 797-813 ,
4. 4th Turbak, AF; Snyder, FW; Sandberg, KR Microfibrillated cellulose, a new cellulose product: Properties, uses, and commercial potential. J. Appl. Polym. Sci. Appl. Polym. Symp. 1983, 37, 815-827 ,
5. 5th Nakagaito, AN; Yano, H. Novel high-strength biocomposites based on microfibrillated cellulose having nano-order unit web-like network structure. Appl. Phys. A-Mat. Sci. Process. 2005, 80, 155-159 ,
6. 6th Andresen, M .; Johansson, LS; Tanem, BS; Stenius, P. Properties and characterization of hydrophobized microfibrillated cellulose. Cellulose 2006, 13, 665-677 ,
7. 7th Lu, J .; Askeland, P .; Drzal, LT Surface modification of microfibrillated cellulose for epoxy composite applications. Polymer 2008, 49, 1285-1298 ,
8. 8th. Zimmermann, T .; Pöhler, E .; Geiger, T. Cellulose fibrils for polymer reinforcement. Adv. Eng. Mat. 2004, 6, 754-761 ,
9. 9th Iwamoto, S .; Kai, W .; Isogai, A .; Iwata, T. Elastic modulus of single cellulose microfibrils from tunicate measured by atomic force microscopy. Biomacromolecules 2009, 10, 2571-2576 ,

Brief description of the drawings

Fig. 1

die spezifische Oberfläche SSA (engl. "Specific Surface Area") in m²/g von NFC-haltiger Cellulose als Funktion des Gewichtsanteils an NFC; und

Fig. 2

die Lichtreflexion (Mittelwert im Band 360 bis 740 nm) auf schwarzem Hintergrund als Funktion des TiO₂-Gehaltes in Gew.-%, für Verpressungen mit Papieren ohne NFC (Dreiecke) sowie mit Papieren mit 5 Gew.-% NFC (Quadrate).

Exemplary embodiments of the invention will be described in greater detail below with reference to the drawings, in which:

Fig. 1

the specific surface area SSA in m ² / g of NFC-containing cellulose as a function of the weight fraction of NFC; and

Fig. 2

the light reflection (mean in the band 360 to 740 nm) on a black background as a function of the TiO ₂ content in wt .-%, for pressings with papers without NFC (triangles) and with papers with 5 wt .-% NFC (squares).

Ways to carry out the invention Example 1

Like from the Fig. 1 As can be seen, the specific surface area (SSA) in m ² / g of NFC-containing cellulose increases linearly as a function of the weight fraction of NFC. While it is only around 75 m ² / g in the case of conventional cellulose without NFC addition in the example shown, it has values of around 225 m ² / g at 100% NFC values; see more: Josset, S. et al. Energy consumption of the nanofibrillation of bleached pulp. Nordic Pulp & Paper Research Journal 29, 167-175 (2014 ).

For the comparative evaluation of the properties of conventional coating base papers without NFC and those with NFC, paper blanks having a constant pulp density of 50 g / m ² and increasing TiO ₂ contents were produced by means of a sheet former (Estanit, Mülheim an der Ruhr, Germany, based on DIN EN ISO 5269-2 - DIN 54358).

Bleached wood fiber pulp was ground by a standard method to a Schopper-Riegler value of 35 SR °.

A first 1% by weight suspension of this pulp was prepared to prepare standard paper blanks.

A second 1 wt% pulp suspension with 5 wt% NFC (based on total pulp amount) was prepared to make modified paper blanks. The NFC from softwood fibers (ECF, Stendal, D) was produced according to the process described in the following reference: Josset, S. et al. Energy consumption of the nanofibrillation of bleached pulp. Nordic Pulp & Paper Research Journal 29, 167-175 (2014 ).

For the production of leaves, in each case 150 ml of a suspension were diluted to 4 l (corresponding to 50 m ² / g of pulp in the final paper). To this pulp TiO _{2 was added} in increasing amounts (0.1 g to 2.0 g, from a 10 wt .-% suspension). Each mixture was adjusted to a pH of about 6.3 by means of Al ₂ SO ₄ and treated by means of a homogenization system (Ultraturrax) for 30 seconds at 15,000 rpm. The leaves were then produced by vacuum filtration (according to DIN EN ISO 5269-2) and then vacuum-dried. From each leaf, a sample was taken to determine its TiO ₂ content by incineration (900 ° C, 10 min).

The remaining material was pressed on a black background with aqueous melamine resin impregnated overlay paper to a high gloss composite (60 bar, 2 min at 150 ° C, re-cooling: 5 min, to about 45 ° - 50C °). The average light reflection of these compressions was determined by means of a spectrophotometer (Konika Minolta, CM-2500D) between 360 and 740 nm.

Like from the Fig. 2 As can be seen, the addition of 5 wt% NFC causes a significant increase in light reflectance. For example, at a TiO ₂ content of about 17% by weight, the light reflection increases from about 49% (without NFC) to about 54% (with NFC). Especially noteworthy is the behavior in the flattening region of the curves at higher TiO ₂ contents. For example, to achieve a reflection of 54%, conventional paper requires a TiO ₂ content of about 22% by weight, which in the case of an addition of 5% by weight NFC to about 17% by weight. lower. This corresponds to over 22% savings in TiO ₂ .

Example 2

Several sections of monolayer fibrous substrate were made using NFC of different types, i. with different specific surface area (SSA) values prepared in the above-mentioned manner. The ash content in wt .-% was used as the usual measure of the retention capacity of the mineral components, in particular titanium dioxide. The following results are given as an average of 3 measurements.

An ash content of 30.8% by weight was found for the preparation considered as reference base without NFC.

When using an NFC with an SSA of about 95 m ² / g (prior art), the ash content was 32.6 wt .-%, which corresponds to an absolute increase of 1.8 wt .-% compared to the reference.

When using an NFC with an SSA of about 165 m ² / g (according to the invention), the ash content was 38.9 wt .-%, which corresponds to an absolute increase of 8.2 wt .-% compared to the reference.

When using an NFC with an SSA of about 225 m ² / g (according to the invention), the ash content was 43.5 wt .-%, which corresponds to an absolute increase of 12.7 wt .-% compared to the reference.

Claims (9)

1. A single-layered fibrous substrate material for producing a porous coating base paper or prepreg, comprising a planar impregnatable structure made of cellulose fibers, which contains at least one pigment species and optionally contains further additives conventional for paper, characterized in that the cellulose fibers contain a proportion of 1 to 20 wt.-% of nanofibrillated cellulose (NFC) in the form of cellulose fibers having a diameter of 3 nm to 200 nm, and a length of at least 500 nm, and an aspect ratio of at least 100, wherein the NFC has a specific surface area (SSA) of at least 125 m²/g.
2. The fibrous substrate material according to claim 1, wherein the NFC portion is 5 to 10 wt.-%.
3. The fibrous substrate material according to claim 1 or 2, wherein the said pigment species is titanium dioxide.
4. The fibrous substrate material according to claim 1 or 2, wherein the said pigment species is iron oxide.
5. A method for producing the fibrous substrate material according to claim 1, comprising the steps of:

   - providing an aqueous suspension containing a cellulose containing material and an admixture of said pigment species and, optionally, further additives conventional for paper,

   - sheet forming,

   - drying,

   characterized in that the cellulose containing material contains a proportion of 1 to 20 wt.-% of NFC with a specific surface (SSA) of at least 125 m²/g.
6. The method according to claim 5, wherein the NFC proportion is 5 to 10 wt.-%.
7. The method according to claim 5 or 6, wherein the NFC has a specific surface (SSA) of at least 150 m²/g, in particular at least 175 m²/g, preferably at least 225 m²/g.
8. A porous coating base paper produced from a fibrous substrate material according to one of claims 1 to 4.
9. A prepreg produced by impregnating a fibrous substrate material according to one of claims 1 to 4 with a synthetic resin dispersion.

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