Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H23/00—Processes or apparatus for adding material to the pulp or to the paper
  • D21H23/02—Processes or apparatus for adding material to the pulp or to the paper characterised by the manner in which substances are added
  • D21H23/22—Addition to the formed paper
  • D21H23/50—Spraying or projecting
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H19/00—Coated paper; Coating material
  • D21H19/10—Coatings without pigments
  • D21H19/14—Coatings without pigments applied in a form other than the aqueous solution defined in group D21H19/12
  • D21H19/24—Coatings without pigments applied in a form other than the aqueous solution defined in group D21H19/12 comprising macromolecular compounds obtained otherwise than by reactions only involving carbon-to-carbon unsaturated bonds

Definitions

• the present invention is concerned with a process for treating and sizing paper substrates with an isocyanate.
• the present invention is concerned with a process for strengthening and sizing paper with an isocyanate characterized in that the isocyanate does not contain water and is applied to paper by means of an electrostatic spraying device.
• Isocyanates emulsified with water have been applied to paper using a size press and unemulsified isocyanates have been applied using a rotogravure coating machine.
• conventional means of spraying isocyanates are used, they are characterized by low transfer efficiencies. As a result, significant amounts of isocyanate are released into the atmosphere, thereby creating potential health problems.
• the isocyanate that is applied to paper is not sprayed, solvent diluted, or emulsified, it is very difficult to apply the isocyanate at levels below 5 to 10 percent by weight based upon the weight of the paper. Lower isocyanate levels can be achieved by using emulsified isocyanates.
• pot life problems with the isocyanate can arise when the isocyanates are emulsified because the water begins to react with the isocyanate groups. Furthermore, depending upon when in the process the emulsion is added, it can result in additional drying requirements. There is therefore a need for a process for efficiently applying isocyanates to paper substrates at levels below 5 percent by weight based upon the weight of the paper substrates and a process that does not utilize emulsified isocyanates, although emulsifiable isocyanates may be used.
• an electrostatic spraying device to apply isocyanates to paper.
• transfer efficiencies above 90 percent can be achieved and the isocyanate can easily be applied to paper substrates at levels well below 5 percent by weight.
• the amount of isocyanate needed according to the invention can be as low as 0.5 g/m2. However, in commercial production settings, the amount of isocyanate needed is generally about 1.0 to 1.5 g/m2.
• Electrostatic spraying devices as such are known. See, for example, US-A-4854506 and 4846407 and EP-A-193348. The electrostatic spraying of various polymeric materials onto paper has also been disclosed in US-A-3930614, 4609686 and 4837057.
• the process of this invention is a process for treating a paper substrate with an isocyanate comprising the step of applying an isocyanate which does not contain water to the paper substrate by means of an electrostatic spraying device.
• the process for treating a paper substrate with an isocyanate comprises the step of applying an isocyanate which does not contain water to the paper substrate by means of an electrostatic spraying device which is comprised of an electrostatic sprayhead having a linear orifice, means for applying a first electrical potential to liquid isocyanate which emerges from the sprayhead, an electrode comprised of two mutually spaced, parallel arranged linear electrode elements, with one electrode element being mounted adjacent to one side of the sprayhead's orifice and the other element being mounted adjacent to the other side of the sprayhead's orifice, and means for applying a second electrical potential to the electrode such that an intense electrical field is developed between the emerging liquid and the electrode, the intensity ofthe field being sufficient to cause atomization of the emerging liquid, the electrode comprising a core of conducting or semiconducting material contained in a tubular sheath, characterized in that the sheath has a wall and the volume resistivity of a section of the wall of said sheath which is 1 cm in length is within the range
• the process of this invention provides light weight, uniform coatings of isocyanates on paper substrates and improves some of the physical properties of coated paper substrates, such as water resistance, wet strength and crush strength.
• the process involves the electrostatic spraying of isocyanates onto paper substrates.
• the electrostatic spraying device employed to spray isocyanates onto paper substrates preferably comprises an electrostatic sprayhead having a linear orifice, means for applying a first electrical potential to liquid isocyanate which emerges from the sprayhead, an electrode comprised of two mutually spaced, parallel arranged linear electrode elements, with one electrode element being mounted adjacent to one side of the sprayhead's orifice and the other element being mounted adjacent to the other side of the sprayhead's orifice, and means for applying a second electrical potential to the electrode such that an intense electrical field is developed between the emerging liquid and the electrode, the intensity of the field being sufficient to cause atomization of the emerging liquid, the electrode comprising a core of conducting or semiconducting material contained in a tubular sheath, characterized in that the sheath has a wall and the volume resistivity of a section of the wall of said sheath which is 1 cm in length is within the range of 5 x 1011 to 5 x 1013
• the isocyanate is charged at 30 to 40 kV by the means for applying an electrical potential to the isocyanate.
• the isocyanate is positively charged.
• the liquid isocyanate breaks into droplets having diameters typically ranging from 40 to 150 microns in an electric field created between the charged liquid and the electrode comprised of the two linear electrode elements, which are usually charged between 10 kV and 25 kV and at the same polarity as the liquid.
• the voltage difference between the liquid isocyanate and the electrode comprised of the two linear electrode elements is between 15 and 30 kV. This voltage difference is called "stress.”
• the isocyanates that are employed should have a viscosity in the range of 1 to 750 mPa.s, preferably in the range of 1 to 300, and a volume resistivity in the range of 1 x 106 to 1 x 1011 ohm cms, preferably in the range of 5 x 106 to 5 x 109, and most preferably in the range of 5 x 107 to 5 x 108 ohm cms.
• the higher the viscosity of the isocyanate the more difficult it is to apply the isocyanate.
• isocyanate having one or more isocyanate groups and a viscosity and a resistivity between the above indicated limits can be employed.
• the isocyanates which may be used include aliphatic, cycloaliphatic, araliphatic and aromatic isocyanates, especially those that are liquid at room temperature.
• Aromatic isocyanates, especially aromatic polyisocyanates, are preferred.
• Mixtures of isocyanates can be used and also isocyanates which have been modified by the introduction of urethane, allophanate, urea, biuret, carbodiimide, uretonimine or isocyanurate residues.
• aromatic isocyanates examples include m- and p-phenylenediisocyanate, toluene-2,4-and 2,6-diisocyanates, diphenylmethane-4,4'diisocyanate, diphenylmethane-2,4'-diisocyanate, chlorophenylene-2,4-diisocyanate, diphenylene-4,4'-diisocyanate, 4,4'-diisocyanate-3,3'-dimethyldiphenyl, 3-methyldiphenylmethane-4,4'-diisocyanate and diphenyletherdiisocyanate and 2,4,6-triiso-cyanatotoluene and 2,4,4'-triisocyanatodiphenylether.
• mixtures of isocyanates far example a mixture of toluene diisocyanate isomers such as the commercially available mixtures of 2,4- and 2,6-isomers and also the mixture of di and higher isocyanates produced by phosgenation of aniline/formaldehyde condensates.
• mixtures are well known in the art and include the crude phosgenation products containing mixtures of methylene bridged polyphenylpolyisocyanates including diisocyanate, triisocyanate and higher polyisocyanates together with any phosgenation by-products.
• compositions of the present invention are those wherein the isocyanate is an aromatic diisocyanate or polyisocyanate of higher functionality in particular crude mixtures of methylene bridged polyphenylpolyisocyanates containing diisocyanate, triisocyanate and higher functionality polyisocyanates.
• the methylene bridged polyphenylpolyisocyanates are sometimes referred to as polymeric methylene polyphenyldiisocyanate (MDI).
• MDI polymeric methylene polyphenyldiisocyanate
• Polyphenylpolyisocyanates are well known in the art and usually have an isocyanate functionality ranging from 2.0 to 3.0. They are prepared by phosgenation of corresponding mixtures of polyamines obtained by condensation of aniline and formaldehyde.
• Isocyanate-terminated prepolymers may also be employed and are prepared by reacting an excess of polyisocyanate with polyols, including aminated polyols or imines/enamines thereof, or polyamines.
• Emulsifiable isocyanates may also be employed.
• An emulsifiable isocyanate is an isocyanate/isocyanate prepolymer blend which is made by incorporating into an isocyanate a prepolymer formed by reacting monoalkyl ethers of polyalkylene glycols or polyester polyether glycols with a polyisocyanate to form an isocyanate terminated urethane adduct.
• Such blends are well known to be emulsifiable in water. Suitable emulsifiable isocyanates and their preparation are described in US-A-3,996,154 and 4,505,778, which are incorporated herein by reference.
• emulsifiable MDI is the most preferred.
• the most preferred aromatic isocyanates are polymeric MDI, emulsifiable MDI, MDI variants, and mixtures thereof.
• Suitable MDI variants include compounds in which the MDI has been modified by the introduction of urethane, allophanate, urea, biuret, amide, carbodiimide, uretonimine and/or isocyanurate residues.
• Suitable aliphatic polyisocyanates include ethylene diisocyanate, 1,6-hexamethylene diisocyanate, isophorone diisocyanate, cyclohexane 1,4-diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, saturated analogues of the above mentioned aromatic isocyanates, mixtures thereof and the like.
• the isocyanate to be used is a solid at the temperature of spraying, which temperature generally is 10 to 30°C and preferably 20 to 25°C
• the isocyanate may be heated in order to liquefy it and the heated isocyanate can be sprayed.
• the isocyanate should generally not be heated to temperatures above 38°C before being sprayed.
• the isocyanate is liquid at the spraying temperature.
• the liquid isocyanate is placed into a vessel from which the isocyanate can be pumped into the sprayhead of the electrostatic sprayer.
• a hose connects the vessel and the electrostatic sprayer.
• the vessel is usually equipped with or connected to a means for pumping the isocyanate to the sprayhead at variable flowrates.
• the flowrate can vary from 0.5 to 150 g/minute/cm of nozzle width, and preferably is in the range of 1.5 to 75 g/minute/cm of nozzle width.
• the selected flowrate will depend upon factors such as how much isocyanate is to be applied to the paper substrate and the speed at which the paper substrate is passing below the nozzle of the sprayer.
• a wide variety of paper substrates can be treated according to the process of this invention.
• the process can be used to strengthen and size paper, corrugated paper containers, paper labels and paper-board.
• the process is especially useful in strengthening and sizing corrugated paper containers, such as containers that are likely to come into contact with moisture.
• the paper substrate when being sprayed, needs to be in contact with a conducting surface in order to prevent the paper substrate from building up a charge due to the deposition of charged isocyanate particles on the surface of the paper substrate. If a charge builds up on the surface of the paper substrate, the charge will repel charged isocyanate particles thereby leading to reduced transfer efficiencies and a non-uniform coating of isocyanate on the paper substrate.
• the paper substrate is usually in contact with a metal roller over which the paper substrate passes.
• a flat metal plate or sheet such as a sheet of aluminum foil.
• the nozzle is wider than the width of the paper substrate being sprayed so as to insure that the entire surface of the substrate is coated with isocyanate.
• the distance of the nozzle from the surface of the paper substrate should be in the range of 7 to 23 cm. If the nozzle is too close, there can be sparking between the nozzle and the conducting surface in contact with the paper substrate. Also, a striped spray pattern may be produced on the substrate. On the other hand, if the distance is greater than 23 cm, the charged isocyanate particles tend to spread out and wander thereby lowering the transfer efficiency and making it difficult to have a uniform coating on the paper substrate.
• surrounding objects surrounding the nozzle of the spraying device It is important to insure that objects surrounding the nozzle of the spraying device not be too close to the nozzle. The reason is that surrounding objects that are too close to the nozzle will compete with the paper substrate for the charged isocyanate particles and thereby reduce transfer efficiencies. As a result, surrounding objects should generally be kept away from the nozzle at a distance at least four times the distance between the nozzle and the paper substrate.
• the paper substrate can be treated on one side or both sides. If treated on both sides, it is preferred for the isocyanate coating of the first treated side to be cured before treating the second side.
• the isocyanates can be applied to paper substrates without being emulsified, it is not necessary to expose the substrate to a heat treatment step in order to drive off water that is used to emulsify isocyanates. Nevertheless, it is preferred to include a heat treatment step in the process of this invention because heating the paper substrate after the isocyanate is applied promotes curing and can have a beneficial effect upon some of the physical properties of the coated paper substrate, such as crush strength.
• a heat step is included, the paper substrate is usually exposed to a temperature in the range of 65 to 205°C for about 1 to 30 seconds. The heat treatment of the paper substrate usually takes place in an oven through which the substrate is passed.
• additives which are compatible with the isocyanate and do not contain water can be mixed with the isocyanate prior to applying the isocyanate to the paper substrate.
• propylene carbonate can be added to the isocyanate to modify the viscosity of the isocyanate.
• too much propylene carbonate should not be added because the propylene carbonate tends to lower the resistivity of the isocyanate.
• the physical properties of the substrates can be improved. For example, the water resistance, the wet strength, and the crush strength of the substrates can be increased by using the process of this invention.
• the process also results in a uniform distribution of isocyanate on the paper substrates. Since charged particles are employed in the process and the particles seek to ground themselves instead of floating around in the atmosphere, the process results in greatly reduced levels of isocyanate in the atmosphere compared to conventional methods of applying isocyanates to paper substrates. Conventional spray techniques release so much isocyanate into the atmosphere that the process must be enclosed with high air extraction.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Paper (AREA)
• Application Of Or Painting With Fluid Materials (AREA)

Applications Claiming Priority (3)

Publications (2)

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• 1993
  • 1993-02-01 DE DE69301029T patent/DE69301029T2/de not_active Expired - Fee Related
  • 1993-02-01 EP EP93905232A patent/EP0628108B1/en not_active Expired - Lifetime
  • 1993-02-01 ES ES93905232T patent/ES2080613T3/es not_active Expired - Lifetime
  • 1993-02-01 DK DK93905232T patent/DK0628108T3/da active
  • 1993-02-01 CA CA 2130922 patent/CA2130922A1/en not_active Abandoned
  • 1993-02-01 JP JP51527593A patent/JP3176627B2/ja not_active Expired - Fee Related
  • 1993-02-01 AU AU48079/93A patent/AU668758B2/en not_active Ceased
  • 1993-02-01 WO PCT/EP1993/000229 patent/WO1993018228A1/en not_active Ceased
• 1996
  • 1996-10-17 HK HK191196A patent/HK191196A/en not_active IP Right Cessation

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