JP3176627B2 - Method of treating and sizing a paper substrate - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a paper substrate with isocyanate.
ate) and a method of sizing. In particular, the present invention provides that the isocyanate is free of water and is electrostatically sprayed (eled).
A method for strengthening and sizing paper with isocyanates, characterized in that it is applied to the paper by means of trostatic spraying equipment.

BACKGROUND OF THE INVENTION The treatment of paper with isocyanates has been disclosed. For example, treatment of paper with isocyanates is disclosed in EP 140537. U.S. Pat. No. 4,505,778 discloses a method for applying the size of an aqueous emulsion of an aromatic isocyanate to a wet paper web.
However, none of the patents discloses a method for spraying an isocyanate on paper in an antistatic manner.

The isocyanate emulsified by water is applied to paper using a size press, and the non-emulsified isocyanate is applied by a rotogravure coating device. When using the customary means of applying isocyanates, these means are characterized by a low transfer efficiency. As a result, significant amounts of isocyanates are released into the atmosphere, thereby creating possible health problems. Furthermore, if the isocyanate to be applied to the paper is not sprayed, diluted with a solvent, or not emulsified, the isocyanate may be applied at a level of less than 5% by weight to 10% by weight based on the weight of the paper. Very difficult. Low isocyanate levels can be achieved with emulsified isocyanates. However, when emulsifying the isocyanate, the water starts to react with the isocyanate group, so that a pot life problem due to the isocyanate may occur. Furthermore, depending on when the emulsion is added during the process, further drying may be required. Therefore, a method for effectively applying isocyanate to a paper substrate at a level of less than 5% by weight, depending on the weight of the paper substrate, and a method without emulsified isocyanates, if emulsifiable isocyanates can be used Is necessary.

It has been surprisingly discovered that these needs can be met by using an electrostatic spray device to apply the isocyanate to the paper. By using this electrostatic spraying device, a transfer efficiency of more than 90% can be achieved, and isocyanate can be added to a paper substrate by 5%.
It can be easily applied at a level well below the weight percent. The required amount of isocyanate according to the invention can be as low as 0.5 g / m ² . However, in commercial production settings, the isocyanate requirement is generally about 1.0-1.5 g /
a m ^2.

Electrostatic spraying devices themselves are known. For example, U.S. Pat.Nos. 4,854,506 and 4,846,407 and European Patent Publication No.
See 193348. Electrostatic spraying of various polymeric materials on paper is also disclosed in U.S. Patent Nos. 3,930,614, 4,609,686 and 4,837,057.

However, none of these patents disclose electrostatic spraying of the isocyanate on paper.

It is an object of the present invention to provide a means for effectively applying an isocyanate to a paper substrate.

It is another 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 for providing a homogeneous coating of isocyanate on a paper substrate.

It is also an object to apply the isocyanate to the paper in an environmentally safe manner.

These and other objects are achieved by the method of the present invention.

SUMMARY OF THE INVENTION The method of the present invention is a method of treating a paper substrate with an isocyanate, the method comprising applying a water-free isocyanate to the paper substrate using an electrostatic sprayer.

In a preferred embodiment, the method of treating a paper substrate with an isocyanate comprises an electrostatic spray head having a linear orifice, means for applying a first potential to the liquid isocyanate exiting the spray head, and two spaced apart isocyanates. , Consisting of linear electrode elements arranged in parallel, one electrode element being provided adjacent to one side of the spray head orifice and the other element being provided adjacent to the other side of the spray head orifice. An electrode, and means for applying a second potential to the electrode such that an electric field is generated between the discharged liquid and the electrode, wherein the intensity of the electric field is sufficient to atomize the discharged liquid; Comprises a core of a conductive or semiconductive substance contained in a tubular sheath, wherein said sheath has a wall and the volume resistivity of a section of said sheath wall having a length of 1 cm is 5 × 10 ¹¹ to 5 × 10 ¹³ a step of applying water-free isocyanate to a paper substrate using an electrostatic spraying device characterized in the range of ohm · cm.

The process of the present invention provides a lightweight, homogeneous coating of the isocyanate on the paper substrate and improves some of the physical properties of the coated paper substrate, such as, for example, water resistance, strength and crush strength.

DETAILED DESCRIPTION OF THE INVENTION The method involves electrostatic spraying of an isocyanate onto a paper substrate. The electrostatic spraying device used for spraying isocyanate on a paper substrate is preferably an electrostatic spray head having a linear orifice, a means for applying a first potential to the liquid isocyanate exiting the spray head, Consists of spaced apart, parallel arranged linear electrode elements, one electrode element provided adjacent one side of the spray head orifice and the other element on the other side of the spray head orifice. An electrode provided adjacently, and means for applying a second potential to the electrode such that a strong electric field is generated between the discharge liquid and the electrode, wherein the strength of the electric field causes the discharge liquid to be atomized. Sufficient, the electrode comprises a core of a conductive or semiconductive substance contained in a tubular sheath, the sheath having a wall, the length of the wall of the sheath 1c
The volume resistivity of the m section is in the range of 5 × 10 ¹¹ to 5 × 10 ¹³ ohm · cm. Such an apparatus and its operating parameters are described in detail in US Pat. No. 4,854,506, which is hereby incorporated by reference.

Generally, as the liquid isocyanate passes through the linear orifice of the device of U.S. Pat. No. 4,854,506, the isocyanate is charged at 30-40 kV by means of applying a potential to the isocyanate. Preferably, the isocyanate is positively charged. Once charged, the liquid isocyanate is charged in an electric field formed between the charged liquid and an electrode consisting of two linear electrode elements, which is charged to the same polarity as the liquid, typically in the range of 10 kV to 25 kV. The isocyanate is broken down into droplets having a diameter typically in the range of 40-150 microns in diameter. Generally, the voltage difference between the liquid isocyanate and an electrode consisting of two linear electrode elements is between 15 and 30 kV. This voltage difference is called "stress
s) ".

The isocyanate used has a viscosity in the range of 1 to 750 mPa.s, preferably in the range of 1 to 300 mPa.s, and 1x10 ^{6 to} 1x10 ¹¹ o.
It should have a volume resistivity in the range of hm · cm, preferably in the range of 5 × 10 ⁶ to 5 × 10 ⁹ ohm · cm, most preferably in the range of 5 × 10 ⁷ to 5 × 10 ⁸ ohm · cm. In general, the higher the viscosity of the isocyanate, the more difficult it is to apply the isocyanate.

Any isocyanate having one or more isocyanato groups and a viscosity and resistivity within the above ranges can be used. Useful isocyanates include aliphatic, cycloaliphatic and aromatic isocyanates, especially isocyanates which are liquid at room temperature. Aromatic isocyanates, especially aromatic polyisocyanates, are preferred. Mixtures of isocyanates can be used, including urethanes, allophanates, ureas, biurets, carbodiimides, uretonimines (ur
It is also possible to use isocyanates modified by the introduction of etonimine) or isocyanurate residues.

Examples of suitable aromatic isocyanates include m- and p-phenylene diisocyanate, toluene-2,4 and 2,6-diisocyanate, diphenylmethane-4,4'-diisocyanate, diphenylmethane-2,4'-diisocyanate, chlorophenylene -2,4-diisocyanate, diphenylene-4,4'-diisocyanate, 4,4'-diisocyanato-3,3'-dimethyldiphenyl, 3-methyldiphenylmethane-4,4'-diisocyanate and diphenylether diisocyanate, 2,4, There are 6-triisocyanatotoluene and 2,4,4'-triisocyanatodiphenyl ether. Mixtures of isocyanates can also be present, for example, commercially available 2,4- and 2,6-
A mixture of toluene diisocyanate isomers, such as a mixture of isomers, and a mixture of di and higher isocyanates produced by phosgenation of an aniline / formaldehyde condensate may also be present. Such mixtures are well known in the art, and such mixtures include crude phosgenation products, including methylene-crosslinked polyphenyl polyisocyanate mixtures containing diisocyanates, triisocyanates and more isocyanates with phosgenation by-products. is there.

Preferred compositions of the present invention are those wherein the isocyanate is an aromatic diisocyanate or a higher functional
y) polyisocyanates, in particular diisocyanates,
A composition that is a crude mixture of a methylene-crosslinked polyphenyl polyisocyanate including a triisocyanate and a highly functional polyisocyanate. Methylene cross-linked polyphenyl polyisocyanate is sometimes referred to as polymeric methylene polyphenyl diisocyanate (MDI). Polyphenyl polyisocyanates are well known in the art and are usually 2.
Has an isocyanate functionality ranging from 0 to 3.0. These are prepared by phosgenation of a corresponding mixture of polyamines obtained by the condensation of aniline with formaldehyde.

Isocyanato-terminated prepolymers can also be used, and are prepared by reacting excess polyisocyanate with an aminated polyol or a polyol, including its imine / enamine, or polyamine.

Emulsifiable isocyanates can also be used. Emulsifiable isocyanates are produced by introducing a prepolymer formed by reacting a monoalkyl ether of a polyalkylene glycol or a polyester polyether glycol with a polyisocyanate into the isocyanate to form an isocyanato-terminated urethane product. Isocyanate / isocyanate prepolymer blend. It is well known that such blends are emulsifiable in water. Suitable emulsifiable isocyanates and their preparation are disclosed in U.S. Patent Nos. 3,996,154 and 4,505,778.
And these patents are hereby incorporated by reference. Among emulsifiable isocyanates, emulsifiable MDI is most preferred.

The most preferred aromatic isocyanates are MDI polymers,
Emulsifiable MDI, MDI variants, and mixtures thereof. Suitable MDI variants include compounds where MDI has been modified by the introduction of urethane, allophanate, urea, biuret, carbodiimide, uretonimine and / or isocyanate residues.

Examples of suitable aliphatic polyisocyanates include ethylene diisocyanate, 1,6-hexamethylene diisocyanate, isophorone diisocyanate, cyclohexane 1,
Examples include 4-diisocyanate, 4,4'-dicyclohexanemethane diisocyanate, saturated analogs of the above aromatic isocyanates and mixtures thereof.

If the isocyanate used is solid at the spraying temperature, which is generally between 10 and 30 ° C., preferably between 20 and 25 ° C., it is heated to liquefy and the heated isocyanate is sprayed on be able to. However, in general, the isocyanate should not be heated to a temperature above 38 ° C. before spraying. Preferably, the isocyanate is liquid at the spraying temperature.

Liquid isocyanate can be placed in a container from which the isocyanate can be supplied to the spray head of an electrostatic spraying device. Generally, a hose connects the container to the electrostatic spraying device. The container usually comprises or is connected to means for supplying the isocyanate at a variable flow rate to the spray head. The flow rate can be in the range of 0.5-150 g / min / cm of nozzle width, preferably in the range of 1.5-75 g / min / cm of nozzle width. Selection of the flow rate, for example,
It depends on factors such as the amount of isocyanate to be applied to the paper substrate and the speed at which the paper substrate passes below the nozzles of the spray device.

A wide variety of paper substrates can be treated by the method of the present invention. This method can be used to strengthen and size paper, cardboard containers, paper labels and paperboard. This method is particularly useful for strengthening and sizing cardboard containers, for example, containers that are prone to contact with moisture.

When spray coating a paper substrate, it is necessary to contact the paper substrate with a conductive surface in order to protect the paper substrate from charge build-up due to charged isocyanate particles adhering to the surface of the paper substrate. If charge accumulates on the surface of the paper substrate, the charge repels the charged isocyanate particles, thereby reducing transfer efficiency and resulting in a non-uniform coating of isocyanate on the paper substrate. In practice, the paper substrate usually contacts a metal roller over which the paper substrate passes. However, during the processing of the individual paper products, these paper products can pass under the nozzle, in contact with a flat metal plate or sheet, for example aluminum foil.

The nozzle is preferably wider than the width of the paper substrate to be spray painted so that the entire surface of the substrate is completely covered by the isocyanate. The distance from the surface of the paper substrate to the nozzle should be in the range of 7 to 23 cm. If the nozzles are too close, a spark can occur between the nozzles and the conductive surface in contact with the paper substrate. Also,
A stripe spray pattern can be formed on the substrate. On the other hand, if the distance exceeds 23 cm, the charged isocyanate particles tend to disperse and migrate, thereby reducing transfer efficiency and making it difficult to form a uniform coating on the paper substrate. is there.

It is important to ensure that the objects surrounding the nozzle of the spray device are not too close to the nozzle. The reason for this is that surrounding objects that are too close to the nozzle compete with the paper substrate for charged isocyanate particles, reducing transfer efficiency. As a result, the surrounding object should generally be maintained at a distance from the nozzle that is at least four times the distance between the nozzle and the paper substrate.

The paper substrate can be treated on one or both sides. When treating both surfaces, it is preferable to treat the second surface after the isocyanate coating film on the first treated surface is cured.

Because the isocyanate can be applied to the paper substrate without emulsification, it is not necessary to expose the substrate to a heat treatment to drive off the water used to emulsify the isocyanate. Nevertheless, heating the paper substrate after application of the isocyanate promotes curing and can beneficially affect some of the physical properties of the coated paper substrate, for example, crush strength, Preferably, the method of the present invention includes a heat treatment step. If a heating step is included, the paper substrate is exposed to a temperature typically in the range of 65-205C for about 1-30 seconds. Heat treatment of the paper substrate is usually performed in an oven, and the paper substrate is passed through the oven.

Although it is preferred to apply the neat isocyanate to the paper substrate, additives compatible with the isocyanate and free of 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 adjust the viscosity of the isocyanate.
However, too much propylene carbonate should not be added, as propylene carbonate tends to reduce the isocyanate resistivity.

By treating a paper substrate using the method of the present invention, the physical properties of the substrate can be improved. For example, the water resistance, wet strength, and crush strength of a substrate can be enhanced by using the methods of the present invention. This method also results in a homogeneous distribution of the isocyanate on the paper substrate. Because charged particles are used in this method and these particles need to be ground instead of floating in the atmosphere, this method has a higher level of isocyanate in the atmosphere than the usual method of applying isocyanate to a paper substrate. Will be greatly reduced. Since conventional spraying methods release too much isocyanate into the atmosphere and so much isocyanate, the process must be surrounded by a high degree of air extraction.

The present invention is not limited by the following examples.
explain.

Example 1 The electrostatic spraying device used was a device according to U.S. Pat. No. 4,854,506 having a linear orifice and the following features: This device was a linear nozzle spraying having a width of about 50 cm (20 inches). It had a blade.

-This device consisted of two semiconducting rods, with two electric field adjusting electrodes arranged in parallel on both sides of the linear nozzle orifice.

Rubinate XI-241, a polymer MDI,
Available from ICI Americas Inc. or Rubicon Inc., having a viscosity of 200 mPa.s, a volume resistivity of 1 × 10 ⁸ ohm.cm, and a liquid at room temperature. This was placed in a pressure vessel connected to an electrostatic spraying device. Liquid isocyanate was supplied to the apparatus from a pressure vessel using about 80 pounds per square inch (5.62 kg / cm ² ) of air. The air pressure was adjusted by a pressure regulator to produce a flow rate of 40 g / min or 0.8 g / min per cm nozzle width. The pressure was about 12 pounds / square inch (0.84 kg / cm ² ). If the linear nozzle spray blade is completely wetted by the isocyanate and the isocyanate begins to fall off the blade, the nozzle is adjusted so that a charge of -38 kV on the nozzle and a charge of -13 kV on the field control electrode are created. Electric power was applied to the electric field adjustment electrode.

^{Then, 40lb / 1000ft 2 (0.195kg /} m 2) of the liner board
A 60 × 60 cm (2 × 2 ft) section was manually passed under the nozzle of the electrostatic sprayer. The nozzle was approximately 12.5 cm (5 inches) above the paper and perpendicular to the paper. The paper contacted the steel plate as it passed under the nozzle, simulating paper traveling on rollers. After coating on one side, the paper was cured for 24 hours at room temperature. The paper was then passed under the nozzle to cover the other side of the paper. The amount of isocyanate applied to the paper was calculated based on the paper weight.
It was 6% by weight.

The physical properties of the treated paper were tested and are shown in Table 1 below. The treated paper had better water resistance and wet strength than untreated sections of paper, and also had improved crush strength.

2. Example 1 was repeated except that 10% by weight of propylene carbonate was added to the polyisocyanate. The amount of the composition applied to paper was 3.5% by weight, calculated based on the weight of the paper. The physical properties of the paper were tested and the results are shown in Table 1 below.

3. The paper was treated with a conventional paper coating machine using the electrostatic spraying device according to claim 1. The Carrier Ross roll coater was equipped with an electrostatic spraying device as claimed in claim 1 so that the paper was spray coated before entering the oven. The nozzle was directly 6 inches (15.24 cm) above the metal roller so that the paper contacted the metal roller where the isocyanate was sprayed on the paper. 65 lb / 3000ft ² (0.106kg /
m ² ) A bag paper roll is placed on a carrier loss machine and 230 feet / minute (7
0.1 m / min). Isocyanate spray-painted on paper is Rubinate XI, an MDI that can be emulsified with water.
-242. It is available from ICI Americas and Rubicon, has a viscosity of 250 mPa.s, and 5x10 ⁷ ohm
Has a volume resistivity of cm. The charge on the isocyanate was -37.1 kV, and the charge on the field control electrode was -18.1 kV. The flow rate of isocyanate is 40g / cm per nozzle width.
Min or 0.8 g / min. After spray coating with the isocyanate, the paper was passed through a 121 ° C. oven to dry the isocyanate. The amount of isocyanate applied to the paper was about 1% by weight based on the weight of the paper.

After treatment with the isocyanate, the physical properties of the paper were tested. The paper had improved crush strength and exhibited an unexpected enhancement of water resistance and wet strength as compared to untreated sections of the paper.

The above examples demonstrate that paper treatment with isocyanates results in a distinct improvement in the water resistance and wet strength of the paper substrate as well as an improvement in the crush strength.

────────────────────────────────────────────────── (5) References JP-A-61-216759 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) D21H 11/00-27/42 B05D 1 / 04

Claims (11)

(57) [Claims] 1. A method of treating a paper substrate with an isocyanate, comprising the step of applying water-free isocyanate to the paper substrate with an electrostatic spraying device having a linear orifice. 2. A method of treating a paper substrate with an isocyanate, comprising: an electrostatic spray head having a linear orifice;
Means for applying a potential and two mutually spaced, parallel, linear electrode elements, one electrode element being provided adjacent one side of the orifice of the spray head and the other element Comprises an electrode provided adjacent to the other side of the orifice of the spray head, and means for applying a second potential to the electrode so that a strong electric field is generated between the discharge liquid and the electrode. The strength is sufficient to nebulize the release liquid, the electrode comprises a core of a conductive or semiconductive substance contained in a tubular sheath, the sheath having walls and a length of 1 cm of the sheath; The step of applying a water-free isocyanate to a paper substrate using an electrostatic spraying device characterized in that the volume resistivity of the wall section is in a range of 5 × 10 ¹¹ to 5 × 10 ¹³ ohmcm. Method. 3. The process according to claim 1, wherein the isocyanate has a viscosity in the range from 1 to 750 mPa.s and a volume resistivity in the range from 1 × 10 ^{6 to} 1 × 10 ¹¹ ohm.cm. 4. The method 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 × 10 ^{6 to} 5 × 10 ⁹ . 5. The method according to claim 4, wherein the isocyanate has a volume resistivity in the range of 5 × 10 ⁷ to 5 × 10 ⁸ . 6. An isocyanate of 0.5 to 1 cm per nozzle width.
The method according to claim 1 or 2, wherein the method is applied to a paper substrate at a flow rate of 150 g / min. 7. The method of claim 6, wherein the flow rate is in the range of 1.5 to 75 g / min / cm nozzle width. 8. The method according to claim 1, wherein the isocyanate is an aromatic isocyanate. 9. The method according to claim 8, wherein the isocyanate is an MDI polymer, an emulsifiable MDI, an MDI variant or a mixture thereof. 10. The method according to claim 9, wherein the isocyanate is an emulsifiable MDI. 11. The method according to claim 1, wherein after the isocyanate is applied to the paper substrate, the paper substrate is heated to a temperature in the range of 65 to 205 ° C.