PROCESS FOR TREATING AND SIZING PAPER SUBSTRATES

Abstract: Process for strengthening and sizing paper with isocyanate characterized in that the isocyanate does not contain water and the isocyanate is applied to the paper by means of a linear electrostatic spraying device. The process is an efficient way to apply water-free isocyanate to paper at low percentage levels in an environmentally safe manner.

Description: PROCESS FOR TREATING AND SIZING PAPER SUBSTRATES Field of Invention The present invention is concerned with a pro¬ cess for treating and sizing paper substrates with an isocyanate. In particular, 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 spray¬ ing device. Background of Invention The treatment of paper with isocyanateε has been disclosed. For example, a process for treating paper with isocyanates is disclosed in EP 140537. In United States Patent No. 4,505,778, a process for applying a size of an aqueous emulsion of an aromat¬ ic isocyanate to a wet paper web is disclosed. However, neither patent discloses a process in which isocyanates are electrostatically sprayed onto pa¬ per. Isocyanates emulsified with water have been applied to paper using a size press and unemulsified isocyanates have been applied using a rotogravure coating machine. When conventional means of spray¬ ing isocyanates are used, they are characterized by low transfer efficiencies. As a result, significant amounts of isocyanate are released into the atmo¬ sphere, thereby creating potential health problems. In addition, when the isocyanate that is applied to paper is not sprayed, solvent diluted, or emulsi¬ fied, 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. However, 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 isocy¬ anates to paper substrates at levels below 5 percent by weight based upon the weight of the paper sub¬ strates and a process that does not utilize emulsi¬ fied isocyanates, although emulsifiable isocyanates may be used. Surprisingly it has been found that these needs can be met by using an electrostatic spraying device to apply isocyanates to paper. By using this elec¬ trostatic spraying device, 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/m 2 . However, in commercial produc¬ tion settings, the amount of isocyanate needed is generally about 1.0 to 1.5 g/m 2 . Electrostatic spraying devices as such are known. See, for example, United States Patent Nos. 4854506 and 4846407 and EPA-193348. The electro¬ static spraying of various polymeric materials onto paper has also been disclosed in United States Pat¬ ent Nos. 3930614, 4609686 and 4837057. However, none of these patents disclose the electrostatic spraying of isocyanates onto paper. It is an object of this invention to provide a means for efficiently applying an isocyanate to a paper substrate. -2- It is a further object to provide a means for improving the crush strength, water resistance and wet strength of paper products. It is yet another object to provide a means of applying a uniform coating of isocyanate onto a paper substrate. It is also an object to apply an isocyanate to paper in an environmentally safe manner. These and other objects are obtained by the process of this invention. Summary of Invention The process of this invention is a process for treating a paper substrate with an isocyanate com¬ prising the step of applying an isocyanate which does not contain water to the paper substrate by means of an electrostatic spraying device. In a preferred embodiment, the process for treating a paper substrate w-ith an isocyanate com¬ prises 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 ori¬ fice, means for applying a first electrical poten¬ tial 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 be¬ tween 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 -3- material contained in a tubular sheath, character¬ ized 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 10 11 to 5 x 10 n ohm cms. The process of this invention provides light weight, uniform coatings of isocyanates on paper substrates and improves some of the physical proper¬ ties of coated paper substrates, such as water re- sistance, wet strength and crush strength. Detailed Description of Invention The process involves the electrostatic spraying of isocyanates onto paper substrates. The electro¬ static spraying device employed to spray isocyanates onto paper substrates preferably comprises an elec¬ trostatic 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 elec¬ trode 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 sprayh¬ ead's orifice, and means for applying a second elec¬ trical potential to the electrode such that an in¬ tense 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 10" to 5 x 10 13 ohm cms. Such a device and its operating parameters have been fully described in United States Patent -4- No. 4854506, which is incorporated herein by refer¬ ence in its entirety. Generally, as the liquid isocyanate passes through the linear orifice of the device of U.S. Patent No. 4854506, the isocyanate is charged at 30 to 40 kV by the means for applying an electrical potential to the isocyanate. Preferably, the isocy¬ anate is positively charged. Once the liquid isocy¬ anate is charged, it 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 be¬ tween 10 kV and 25 kV and at the same polarity as the liquid. Generally, the voltage difference be¬ tween the liquid isocyanate and the electrode com¬ prised of the two linear electrode elements is be¬ tween 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 10 6 to l x 10 11 ohm cms, prefera¬ bly in the range of 5 x 10 6 to 5 x 10 9 , and most preferably in the range of 5 x 10 7 to 5 x 10 8 ohm cms. Generally, the higher the viscosity of the isocyanate, the more difficult it is to apply the isocyanate. Any isocyanate having one or more isocyanate groups and a viscosity and a resistivity between the above indicated limits can be employed. The isocya¬ nates which may be used include aliphatic, cycloali- phatic, araliphatic and aromatic isocyanates, espe¬ cially those that are liquid at room temperature. Aromatic isocyanates, especially aromatic polyisocy- anates, are preferred. Mixtures of isocyanates can be used and also isocyanates which have been modi- -5- fied by the introduction of urethane, allophanate, urea, biuret, carbodiimide, uretonimine or isocya- nurate residues. Examples of suitable aromatic isocyanates in¬ clude m- and p-phenylenediisocyanate, toluene-2,4- and 2,6-diisocyanates, diphenylmethane-4, 'diisocya¬ nate, diphenylmethane-2,4'-diisocyanate, chlorophen- ylene-2,4-diisocyanate, diphenylene-4,4'-diisocyan¬ ate, 4,4'-diisocyanate-3,3'-dimethyldiphenyl, 3- methyldiphenylmethane-4,4'-diisocyanate and di- phenyletherdiisocyanate and 2,4,6-triiso-cyanatotol- uene and 2,4,4'-triisocyanatodiphenylether. There may be present mixtures of isocyanates for example a mixture of toluene diisocyanate isomers such as the commercially available mixtures of 2,4- and 2,6-iso- mers and also the mixture of di and higher isocyana¬ tes produced by phosgenation of aniline/formaldehyde condensates. Such mixtures are well known in the art and include the crude phosgenation products containing mixtures of methylene bridged polyphe- nylpolyisocyanates including diisocyanate, triiεocy- anate and higher polyisocyanates together with any phosgenation by-products. Preferred compositions of the present invention •are those wherein the isocyanate is an aromatic diisocyanate or polyisocyanate of higher functional¬ ity in particular crude mixtures of methylene bridged polyphenylpolyisocyanates containing diiso¬ cyanate, triisocyanate and higher functionality polyisocyanates. The methylene bridged polyphenyl¬ polyisocyanates are sometimes referred to as poly¬ meric methylene polyphenyldiisocyanate (MDI) . Poly¬ phenylpolyisocyanates 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. -6- Iεocyanate-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/iεocyan- ate prepolymer blend which is made by incorporating into an isocyanate a prepolymer formed by reacting onoalkyl ethers of polyalkylene glycols or polyes¬ ter polyether glycols with a polyisocyanate to form an iεocyanate terminated urethane adduct. Such blends are well known to be emulsifiable in water. Suitable emulsifiable isocyanates and their prepara¬ tion are described in United States Patent Nos. 3,996,154 and 4,505,778, which are incorporated herein by reference. Of the emulsifiable isocyanat¬ es, emulsifiable MDI is the most preferred. The most preferred aromatic isocyanates are polymeric MDI, emulsifiable MDI, MDI variants, and mixtures thereof. Suitable MDI variantε include compounds in which the MDI has been modified by the introduction of urethane, allophanate, urea, biuret, amide, carbodiimide, uretonimine and/or isocyanurate residues. Examples of 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 men¬ tioned aromatic isocyanateε, mixtureε thereof and the like. If the isocyanate to be used iε a solid at the temperature of spraying, which temperature generally is 10 to 30°C and preferably 20 to 25°C, the iso¬ cyanate may be heated in order to liquefy it and the heated isocyanate can be sprayed. However, the isocyanate should generally not be heated to temper- -7- atures above 38°C before being sprayed. Preferably the isocyanate is liquid at the spraying tempera¬ ture. The liquid isocyanate is placed into a vessel from which the isocyanate can be pumped into the sprayhead of the electrostatic sprayer. Generally a hose connects the vessel and the electrostatic spra¬ yer. The vessel is usually equipped with or con¬ nected 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/min¬ ute/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 εubstrates can be trea¬ ted 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 eεpecially uεeful in strengt¬ hening and sizing corrugated paper containers, such as containerε that are likely to come into contact with moisture. The paper subεtrate, when being sprayed, needs to be in contact with a conducting surface in order to prevent the paper subεtrate 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 sub¬ strate, the charge will repel charged isocyanate particleε thereby leading to reduced transfer effi- ciencieε and a non-uniform coating of isocyanate on the paper substrate. In practice the paper sub¬ strate is usually in contact with a metal roller over which the paper substrate passes. However, -8- when individual paper articles are being treated, they can be passed under the nozzle while in contact with a flat metal plate or sheet, such as a sheet of aluminum foil. Preferably, 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 sub¬ εtrate. Alεo, a εtriped spray pattern may be pro¬ duced on the subεtrate. On the other hand, if the distance is greater than 23 cm, the charged isocyan¬ ate particles tend to spread out and wander thereby lowering the transfer efficiency and making it dif¬ ficult to have a uniform coating on the paper sub¬ strate. It is important to insure that objects sur¬ rounding the nozzle of the spraying device not be too close to the nozzle. The reason is that sur¬ rounding 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 noz¬ zle 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 sideε, it iε preferred for the isocyanate coating of the first treated side to be cured before treating the second side. Since the isocyanates can be applied to paper substrates without being emulsified, it is not nec¬ essary to expose the substrate to a heat treatment -9- 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 physi¬ cal properties of the coated paper substrate, such as crush strength. When a heat step is included, the paper substrate is usually exposed to a tempera- ture in the range of 65 to 205°C for about 1 to 30 εeconds. The heat treatment of the paper substrate usually takes place in an oven through which the substrate is passed. Although it is preferred to apply neat isocyan¬ ate to the paper substrates, 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. For example, propylene carbonate can be added to the isocyanate to modify the viscoεity of the isocyan- ate. However, too much propylene carbonate should not be added because the propylene carbonate tends to lower the resistivity of the isocyanate. By using the process of this invention to treat paper substrateε with isocyanates, 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 in¬ creased by using the process of this invention. The procesε also results in a uniform distribution of isocyanate on the paper substrates. Since charged particles are employed in the process and the parti¬ cles seek to ground themselves instead of floating around in the atmosphere, the process results in greatly reduced levels of isocyanate in the atmo¬ sphere compared to conventional methods of applying isocyanates to paper substrates. Conventional spray -10- techniques release so much isocyanate into the atmo¬ sphere that the procesε muεt be encloεed with high air extraction. The invention iε illuεtrated, but not limited, by the following examples. Examples 1. The electrostatic spraying device used was a device according to United States Patent No. 4854- 506 having a linear orifice and the following char¬ acteristics: the device was equipped with a linear noz¬ zle spraying blade having a width of about 50 cm (20 inches) the device was equipped with two field adjusting electrodes, which were two semi-conducting rods and which were placed parallel to the linear nozzle orifice at both sides. Rubinate XI-241, which iε polymeric MDI, iε available from ICI Americas Inc or Rubicon Inc, has a viscoεity of 200 mPa.ε, haε a volume reεistivity of 1 x 10 8 ohm cms and is liquid at room temperature, was placed into a pressure vessel which was connect¬ ed to the electroεtatic spraying device. The liquid isocyanate was delivered to the device from the pressure vessel using air at about 80 pounds/inch 2 . The air pressure was regulated by means of a pressure regulator so as to give a flow rate of 40 g/min or 0.8 g/minute/cm of nozzle width.- The pressure was about 12 pounds/inch 2 . Once the linear nozzle spraying blade was completely wetted with isocyanate and the isocyanate was dripping off the blade, the power to the nozzle and the field adjusting electrodes was turned on so as to give a charge of -38 kV on the nozzle and a charge of -13 kV on the field adjusting electrodes. -11- A 60 x 60 cm (2 2 ft) piece of 40 lb/1000ft 2 linerboard was then manually passed under the nozzle of the electrostatic spraying device. The nozzle was about 12.5 cm (5 inch) above and perpendicular to the paper. The paper was in contact with a steel plate while the paper passed beneath the nozzle to simulate paper going over a roller. After being coated on one side, the paper was allowed to cure for 24 hours at room temperature. It was then passed under the nozzle to coat the other side of the paper. The amount of isocyanate applied to the paper waε 3.6% by weight calculated on the paper weight. The physical properties of the treated paper were tested and are set forth in Table I below. The treated paper had excellent water reεistance and wet strength compared to an untreated piece of paper and had improved crush strength as well. 2. Example 1 was repeated except that 10% by weight of propylene carbonate was added to the poly¬ isocyanate. The amount of this composition applied to the paper calculated on the weight of paper was 3.5% by weight. The physical properties of the paper were tested and the results are set forth in Table I below. 3. The electrostatic spraying device of claim 1 waε used to treat paper on a conventional paper coating machine. A Carrier Ross roll coater machine was equipped with the electrostatic εpraying device of claim 1 so that the paper would be sprayed prior to entering an oven. The nozzle was situated six inches directly above a metal roller so that the paper would be in contact with the metal roller at the point where the isocyanate was sprayed onto the paper. A roll of 651b/3000ft 2 bag paper comprised of 20 percent recycled newspaper was placed on the Carrier Ross machine and fed underneath the nozzle -12- of the spraying device at a rate of 230 feet per minute. The isocyanate sprayed onto the paper was Rubi- nate XI-242, which is a wate_r emulsifiable MDI, is available from ICI Americas Inc. and Rubicon Inc. , has a viscosity of 250 mPa.s, and has a volume re¬ sistivity of 5 x 10 7 ohm cms. The charge on the isocyanate was -37.1 kV and the charge on the field adjusting electrodes was -18.1 kV. The flowrate of the isocyanate was 40 g/min or 0.8 g/minute/cm of nozzle width. After being sprayed with the isocyan¬ ate, the paper was run through a 121°C oven to dry the isocyanate. The amount of isocyanate applied to the paper was about l percent by weight based upon the weight of the paper. After being treated with the isocyanate, the physical properties of the paper were tested. The paper had improved crush strength and exhibited a dramatic increase in water resistance and wet strength compared to an untreated piece of paper. -13- 'MD = Machine Direction 2 CD = Cross Direction The examples demonstrate that treating paper sub¬ strates with isocyanateε leads to dramatic improve¬ ments in the water resistance and wet strength as well as improvements in the crush strength of the paper subεtrateε. -14-

Claims: CLAIMS What is claimed is: 1. 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 spray¬ ing device having a linear orifice. 2. 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 spray¬ ing device which iε comprised of an electrostatic sprayhead having a linear orifice, means for apply¬ ing a first electrical potential to liquid isocyana¬ te which emerges from the sprayhead, an electrode comprised of two mutually spaced, parallel arranged linear electrode elements, with one electrode ele¬ ment being mounted adjacent to one side of the sprayhead's orifice and the other element being mounted adjacent to the other side of the sprayh¬ ead's orifice, and means for applying a second elec¬ trical potential to the electrode such that an in¬ tense electrical field iε 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 10 11 to 5 x 10 13 ohm cms. -15- 3. The procesε according to claim 1 or 2 wherein the iεocyanate has a viscosity in the range of 1 to 750 mPa.s and a volume reεistivity in the range of 1 x 10 6 - 1 x 10 π ohm cms. 4. The procesε according to claim 3 wherein the isocyanate has a viscosity in the range of 1 to 300 and a volume resistivity in the range of 5 x 10 6 to 5 x 10 9 . 5. The process according to claim 4 wherein the isocyanate has a volume resistivity in the range of 5 x 10 7 to 5 x 10 8 . 6. The process according to claim 1 or 2 wherein the isocyanate is applied to the paper sub¬ strate at a flow-rate of 0.5 to 150 g/minute/cm of nozzle width. 7. The process according to claim 6 wherein the flowrate is in the range of 1.5 to 75 g/minute/- cm of nozzle width. 8. The process of claim 1 or 2 wherein the isocyanate is an aromatic isocyanate. 9. The process according to claim 8 wherein the isocyanate is a polymeric MDI, an emulsifiable MDI, an MDI variant or mixtures thereof. 10. The process according to claim 9 wherein the isocyanate is an emulsifiable MDI. 11. The process of claim 1 or 2 wherein after the isocyanate is applied to the paper substrate, the paper substrate is heated at a temperature in the range of 65 to 205°C. -16-

Inventors: COLEMAN PAUL DAVID, ROBERTSON JOHN RUSSELL, COLEMAN, PAUL, DAVID, ROBERTSON, JOHN, RUSSELL
Applicants: ICI PLC, IMPERIAL CHEMICAL INDUSTRIES PLC
Application year: 1993
Main classification: D21H19
IPC codes: B05D1, B05D7, D21H19, D21H23