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3.6.8 Additives for Specialty Papers

[13, 14] Specific functional chemicals are indispensable in the production of specialty pa¬per grades. Such grades are expected to display a very heterogeneous range of properties. Specialty paper grades account for only ca. 4 % of worldwide paper production, but the proportion of chemicals applied for their production is mostly significantly higher than for conventional papers. A number of specialty papers with their required properties and applied additives are described below .

3.6.8.1 Photographic Base Paper
Paper for photographs must carry a uniform emulsion coating, it must resist the development solution in the development bath, it must be perfectly clean for a clean image, and it cannot contain any inhibitors to the photochemical process like iron, copper, or sulfur. It must even be free of radioactive traces, which cause photographic reactions and spots in the image. The paper, which must have a stable white color, is made of clean bleached pulp. The necessary high dry and wet opacity will be obtained with titanium dioxide (TiO2), chalk and low molecular weight polymers (e. g. polyacrylamides). In order to obtain resistance against the reagents and rinsing water, including edge and dimensional stability, the base paper needs a strong stock-sizing with behenyl diketene and high wet-strength with polyamine-polyamide-epichlorohydrin resin. Additional the paper web is dip sized with gelatin, polyvinyl alcohol (PVA), polyacryl amide (PAM), and modified starch before calendering. Most photographic papers for color prints are extrusion coated with an opaque plastic film (e. g. polyethylene) to improve the impermeability.

3.6.8.2 Banknote Papers
These papers have to avoid forgery, must be durable and resistant to wetting, folding and aging. Therefore they are produced under alkaline conditions with high wet and dry strength. To achieve these properties, polyamide-epichlorohydrin resins, polyacrylamides and/or aminoplast resins are used, together with strong stock sizing with AKD (alkyl keten dimer) and surface sizing with proteins plus crosslinking agent (glutaraldehyde). For security reasons these papers will be marked by mingle colored fibers with the paper stock and/or by using uncolored reactive dyes, which create a certain color when they react with an acid or an alkali.

3.6.8.3 Laminate Papers: Décor Paper, Pre-impregnated Foils
Décor paper is made for white or colored décor, often imitating wood finishes. It needs a high wet and dry opacity and very high lightfastness, which is obtained by using titanium dioxide (TiO2) plus low molar mass polymers (polyamine-poly-amide-epichlorohydrin resins, polyacrylamides). Also high wet strength without loss of absorbency is demanded and achieved by relatively high addition rates of polyamine-polyamide-epichlorohydrin resins. Colored décor papers with very high fastnesses are produced with inorganic and organic colored pigments.
Pre-impregnated foils are used where the surface requirements for resistance and closedness do not require more expensive high-density laminates. The im¬pregnation will be made on the paper machine, using a modified size press. Typ¬ical resins for this application are UF (urea formaldehyde) or MF (melamine for¬maldehyde) resins with a very low content of free formaldehyde. To achieve good flexibility and printability of these foils, additional polymer dispersions (e. g. sty¬rene acrylates) are used.

3.6.8.4 Filter Papers
There is a wide range of filter papers for various purposes. The use of porous paper for filtering and separation ranges from the use of filter cartridges for engine protection to dust pouches for vacuum cleaners and air conditioning, and from tea bags and coffee filters to reagent carriers and filter disks for laboratory use. Their common denominator is the requirement for a controlled, high porosity. Depend¬ing on the intended use, additional required characteristics are the resistance to different media and/or temperatures, stiffness, and cleanliness. Specific character¬istics of filter papers are their resistance to flow, their filtration efficiency, and their dust-holding capacity.
The controlled porosity together with very high wet and dry strength is obtained by using polyamine-polyamide-epichlorohydrin resins together with a low molar mass polyacrylamide or polyethyleneimine, polyisocyanate, or polyvinylamine.

3.6.8.5 Imitation Parchment (Food Packaging)

Some of these papers have to be water-resistant and pore free, which is why micro¬crystalline wax, cationic styrene-acrylic emulsions, alkenyl succinic aldehyde (ASA) plus cationic starch, and styrene-maleic-anhydride (SMA) are used. To achieve resistance to oil and fat mainly perfluorinated alkyl acids, carboximethyl¬cellulose (CMC), alginates and stearyl-melamines are applied.

3.6.8.6 Aquarelle Board
These boards must be resistant to fading, neutral, and stable. The finish varies from a coarse surface for painting to a very smooth surface for graphic work. High opacity and uniform surface structure will be achieved by using chalk (calcium carbonate) together with a low-molar-mass polymer e. g. modified polyethylenei¬mine, polyamine, polyvinylamine. Cationic styrene-acrylic emulsions plus carboxy methyl cellulose (CMC), starch and/or low molar mass polyacrylamides or poly¬vinyl formamides lead to controlled uptake of water and oil as well as high stiffness and rattle. For rub out resistance, surface sizing with starch and styrene-acrylic emulsion is applied.

3.6.8.7 Carbonless Copying Paper
The dominating principle for carbonless copy is that an emulsion of a specific oil
e. g. diisopropylnaphthalene (DIPN) together with color formers (reactive dyes) is encapsulated in microcapsules (e. g. gelatine, aminoplast resins) applied as a coat¬ing on the backside of the copying paper (CB-coated back). Through the pressure of writing, the microcapsules are broken, and the color former solution flows to wet the front side coating of a receiving sheet (CF-coated front). The front side coating reacts with the color former, forming an image. The pressure sensitive coloring side (CB) has to be resistant to abrasion. Microcapsules from gelatine or polyurea or melamine-formaldehyde condensation products contain solvent e. g. diisopropylnaphthaline (DIPN), or isopropyl-butylbiphenyl, or phenylmethane¬ethane plus reactive dyes (color former). Polymer dispersions of styrene-butadiene are used as binder for the surface application of the microcapsules. The coating formulation of the reactive receiving side (CF) consists of activated bentonite or phenolic resin or zinc salicylate plus styrene-butadiene binder.

3.6.8.8 Ink-jet Papers
Ink-jet is a noncontact printing method, since no part of the printing device other than ink contacts the paper at the moment of ink transfer. A sharp, detailed printed image (no wicking and bleeding), high color density and no strike-through will be obtained by a hydrophilic paper surface using a coating color with silica gel, polyvinylalcohol (PVA) and/or carboxy methyl cellulose (CMC) plus low-molar-mass cationic polymer e. g. modified polyethyleneimine, polyvinylamine, conden¬sation product of organic amides with formaldehyde, polyacrylamide. The base paper should be hard-sized with alkyl ketene dimer (AKD), or alkenyl succinic aldehyde (ASA), or rosin size.

3.6.8.9 Fire-resistant Papers
Papers with fire-resistant properties are used for wallpaper, decoration paper, Chi-nese/Japanese lamps and partition walls. Flame retardants are added either at the wet end or by surface treatment in the paper production process. They either release incombustible gases on heating, which prevent the entry of atmospheric oxygen, or when heated produce a nonflammable melt that surrounds the paper. Chemicals for this purpose include calcium chloride, magnesium chloride, dia¬mmonium ethyl phosphate, and mixtures of zinc borates, antimony oxides, and organic haloid salts as well as inorganic bromides and oxybromides.

3.6.8.10 Anticorrosion Papers
These papers prevent the rusting of iron parts and the tarnishing of silver, alumi¬num, and copper. Generally they have to be produced in alkaline wet-end condi¬tions in the absence of any acid and alum. Additionally the paper has to be impreg¬nated or coated with chemicals that inhibit corrosion, e. g., sodium nitrite or so¬dium benzoate. The paper is coated by deposition of the chemicals from the vapor phase.

3.6.8.11 Abrasive Base Papers
Abrasive papers which are coated with an abrasive grit in a binder are used for belts in heavy grinding machines, as sheets for grinding by hand, in vibrating hand-held grinding machines, or disks in rotary machines. There are specific grades for wet grinding and for dry finishing by hand. The base paper must be strong enough to resist the forces in use, give a good anchoring of the grit, and suit the coating operation. High amounts of anionic and cationic starches, often to¬gether with carboxy methyl cellulose are used to obtain high strength properties. Additional polymer dispersions based on styrene acrylates or styrene butadiene are applied at the wet end of the paper machine and/or by on-machine or off-machine impregnation. With wet-end addition of these polymers effective fixation and re¬tention in the paper stock with alum and/or cationic polymers e. g. polyethyleni¬mines, polyamines, polyvinylamines are necessary.

3.6.8.12 Papers with Barrier Properties
Paper or board have almost no barrier properties against penetrants like moisture, water vapor, oxygen and other gases, aroma, grease and fat. To provide protection against outside influences as well as protection against loss of features from in¬side, a specific barrier coating is required. To make paper and board suitable as a barrier packaging material, the barrier layer has to be applied either by wax im¬pregnation, lamination with films e. g. PE (polyethylene) or aluminum foil or the extrusion of molten polymers, The most favorable and economic method is the on-or off-machine coating of paper and board with an aqueous system, e. g. an aque¬ous polymer dispersion. According to the laws of Henry and Fick, polymers are needed whose chemical nature is quite the opposite of the penetrant. So the most hydrophobic polymers suit as a barrier against hydrophilic penetrants like mois¬ture vapor whereas the most hydrophilic polymers protect against hydrophobic penetrants like oxygen or some solvents.

Suitable polymer dispersions are based on vinylidene chloride, acrylic esters, styrene-butadiene, polyurethane, polyethyl-ene-acrylic acid, or acrylic acid-acrylonitrile. A minimum of water vapor and oxy¬gen transmission can be reached with polyvinylidene chloride. Acrylics offer an excellent barrier against aromas like terpenes and hydrocarbons as well as fat and a moderate oxygen barrier combined with good water resistance. As a barrier poly¬mer, styrene-butadiene (S/B) offers moderate moisture and vapor protection as well as good water repellence. The aroma barrier to fruity esters is quite good. The S/B coatings are readily sealable with high sealing strength, correlating with the sealing temperature.
New in the market are hydrophobically modified styrene-butadiene dispersions which show an outstanding moisture vapor barrier, very similar to the moisture vapor barrier of a polyethylene film. The water resistance and the aroma barrier to esters are also very good

• The sealability of the modified S/B coat is better than the unmodified one. Polyurethane (PUR) coatings have very high permeation resis¬tance to moisture and vapor. For aroma, they offer a moderate or even good barrier against fruity esters and terpenes and an excellent barrier against hydrocarbons. The fat and oil resistance is good. PUR has extraordinary sealing properties and thermoactivability, which means, after thermal activation, the coating is cold seal¬able for about half a minute and afterwards no agglutination will occur. Polyethyl-ene-acrylic acid is also a coating with excellent sealing properties and, in contrast to PUR, offers a very good moisture and vapor barrier and an excellent aroma barrier against fruity esters. The surface is highly water repellent.

The aqueous polymer solution based on polycarboxylic acid derivative is an excellent oxygen barrier; it exceeds polvinyl dichloride (PVdC), but the coating is sensitive to water and moisture. The aroma barrier is also very good as long as the coating is dry.
Very high grease and oil resistance together with water and alcohol repellency are obtained by fluorinated acrylic copolymers which are completely miscible in water. These products can be applied either by surface or by internal application.

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