Industry Focus: Overview of Specialty Inks (5)

Second, copy ink

     Some people once classified Xeroprinting ( Xerography) in the category of lithography, which is certainly far-fetched. In fact, this method is conceptually very different from the above-mentioned several printing methods. XerograPhy is from Xerosgraphos , Greece, which means " dry writing " , sometimes also called " inkless printing. "

     According to our habits, this method is called xerography (France). Its imaging is carried out using electrostatic principles. Mainly used to print documents, forms, etc. ( Printed lines can also be used to print this method ) . Due to its ease of use, the electrostatic copying method has become more common in China. There are more and more units producing this type of ink (in which the dry-type electrostatic toner is used in China, whether it is dry or wet, and the English name is almost collectively referred to as Toner) .

     The principle of electrostatic copying is to use photosensitive semiconductor materials ( such as selenium, sulfides, oxides, etc. ) in the dark as a highly resistive material - non-conductor, and under the strong light irradiation, the resistance drops rapidly and becomes the photoelectric properties of the conductor. of.

     Electrostatic photocopying can be divided into dry and wet methods. This refers to the state of the ink. The so-called dry method is a dry powder electrostatic copying method for copying. Wet method is a method of copying using a liquid electrophotographic ink.

     The following describes the more commonly used selenium electrostatic copying method: This method can be basically divided into (1) charge ( ie, with a charge electrode - corona generator to make the selenium semiconductor material with a positive charge ) ; (2) exposure ( with the original exposure The local charge on the selenium semiconductor material that is irradiated by light disappears, and the non-illuminated place becomes a positive latent image. ) (3) Developing ( makes the invisible latent image visible) The image may be developed by a dumping method, such as carrier development, magnetic brush development, spray development, and liquid ink development, etc. The toner cartridge method is mostly developed by a dumping method carrier ) ; (4) Transfer ( developing material that will be adsorbed on the selenium semiconductor material - Dry electrophotographic powder or wet electrophotographic ink is transferred to the paper. The paper is charged positively by the charging electrode - corona generator ) ; (5) Fixing ( solubilization of the developing material with heat or solvent fumigation ) ); (6) Finally, the "power consumption" (to remove residual charges), "clean" (wipe off residual developing material) and fully eliminate the residual charge on the exposed semiconductor material selenium.

     When liquid electrophotographic ink is used, the fixing process can be omitted, and the solvent in the ink printed on the paper can be blown with hot air and dried.

     The principle of the drum ( tube ) method is that a cylinder with a light guide surface (the cylinder is generally made of aluminum material. China's commonly known as a cartridge ) is charged with a high voltage electrostatic charge by the discharge of a charged corona generator.

     The image is projected on the cylinder by the scanning of the prism (the moving speed of the document is equal to the moving speed of the cylinder). Light on the non-graphic part of the manuscript is transmitted to the corresponding area on the barrel, so that the electrostatic charge on the corresponding area of ​​the barrel disappears. The facsimile image of the manuscript remains on the cylinder in the form of an electrostatic latent image.

     The container containing the electrophotographic toner ( also referred to as developer powder ) is self-opened, and the developer is spread on the surface of the cylinder and the image is displayed by the developer powder.

     A piece of paper is brought into contact with the cylinder, and the displayed image is transferred from the cylinder to the paper due to the electrostatic force of the transfer corona generator.

     The paper passes through a melting system. Due to the effect of infrared heat, the electrophotographic toner is melted and fixed on the paper. During the operation, a piece of cloth constantly wipes away the electrophotographic toner remaining on the cylinder. At the same time there is a set of unloading lamps to remove the residual charge.

     The principle of the method of zinc oxide paper ( photosensitive paper coated with zinc oxide on a paper substrate ) is that when the paper passes through an electric field, the paper coated with zinc oxide light guides is loaded - negatively charged. After the exposure, the exposed part of the charge disappears on its own, and is then developed with a positively charged electrophotographic toner, and then the zinc oxide paper with the electrostatic copy powder is passed through a hot pressure roller to press-fix the copy powder.

     From the above, it can be seen that since the developing method is different, the required electrophotographic material ( developing material ) and carrier are also different. Here, we will introduce the dry developer material used in the selenium electrostatic copying method, an electrophotographic toner ( hereinafter referred to as toner ) and a carrier.

     We know that two different substances rubbing against each other will generate static electricity, but the polarity of the static electricity they carry is opposite, that is, one is positively charged and the other is negatively charged. Because of the different structure of the material, some are easy to positively charge and some are easy to negatively charge, and the strength of charging is also different. If we rub various substances against each other and compare the positive and negative values ​​of their charged polarity, and arrange them into a sequence of positive to negative, the most in front of them is most likely to be positively charged, and then the last one is the most Easy to negative charge. Such a sequence is called a " friction charge sequence. " In general, the more distant the two substances in the sequence rub against each other, the more pronounced and intense their tendency to charge. However, in reality, the polarity and strength of the electrification are also affected by the size, shape, surface state, surface contamination, or adsorbate, etc., so sometimes these effects can completely reverse the sequence.

     Here is a related sequence for reference.

     Silica elastomer with silicon filler, glass beads, polyamide 11 , nylon 66 , wool, silicon, silk, ethyl fiber, cotton, polyurethane elastomer, polystyrene, flame polishing silicon, polyethylene glycol tetraphenyl Acid ester, phenolic resin, epoxy resin, rubber ( natural ) , sulfur, polyethylene, chlorinated polyethylene, polytetrafluoroethylene - .

     Based on the above principles, it seems that as long as it is found that two kinds of suitable substances can play a certain degree of frictional electrification when mixed together, it can be used as an electrostatic developer. However, its polarity and strength should be compatible with photoconductors used for photocopying, and should be compatible with the development method.

     It can be seen that the toner and the carrier should have the following basic requirements: (1) The toner should be easy to frictionally electrify, have the same polarity, and have a suitable charge strength, and should mainly be an insulator. The finer the particles, the better. The average fineness can be about 5 to 10 microns. No cohesion and caking. Hygroscopicity is small, moisture absorption will reduce the electrostatic properties. The heat is melted and fixed on the paper, and the melting point should be below 160°C . The color should be as dark as possible, and it has a certain resistance to light, heat and chemicals. Non-electrostatic adhesion is as small as possible, so as not to cause bottom ash and reduce the transfer performance; (2) The carrier used for the dumping method is a spherical shape, easy to roll and dredged, and the wear of the selenium layer with very low hardness is small. The proportion is 2 . About 5 , generally with glass beads, diameter of 0 . 4-0 . 8 mm. With steel balls, the diameter should be smaller. It should be easy to generate electricity by friction. Mixing with the toner can generate frictional static electricity and can attract toner. Hygroscopicity is low so as not to reduce the electrostatic properties. In addition to electrostatic adsorption, the non-electrostatic adsorption between the carrier and the toner should be as small as possible, otherwise the toner may not be well transferred to the selenium layer during development. According to actual needs, the carrier has two kinds of positive and negative.

     Toner ( or colored powder ) can be made from many different types of thermoplastic polymers such as acrylics, polyamides, hydrocarbons, epoxies, and the like. This depends on the softening point and hardness and their relative position in the " friction charge sequence " . They are then colored with carbon black ( or colored pigments ) and the colorant must be carefully selected to ensure that the correct charge is obtained. Because the charge is mainly determined by the pigments - dyes and resins. The following colorants will produce relatively positive developer powders: nigrosine black, porphyrin, anthraquinone, and phthalocyanine pigments, etc.; the following colorants will produce a relatively negative developer powder: chromium complexes, Metal diazonium salts and so on.

     The table below indicates a toner formula that can be positively charged and negatively charged.

     The process is to stir and mix all the raw materials and put them on a two roll machine heated with steam ( about 70° C.) to roll all the materials into thin sheets. First with a hammer mill coarse smash, fineness of about 0 . 5 mm. Supermicro crusher is used to perform ultrafine grinding to 5 to 10 microns.

     The following table shows the formula of the copolymer:

     The simple process is as follows: Non-ionized water ( or distilled water ) is charged into a polymerization vessel, magnesium carbonate diluted with a part of water is charged, and stirring is started. Styrene and butyl methacrylate were charged into another vessel, dibenzoyl peroxide was dissolved therein, and then charged into the polymerization vessel.

     The polymerization pot was evacuated and then filled with nitrogen. The vacuum was repeated three times and nitrogen was supplied twice. The vacuum requirement is 500 mmHg and the nitrogen filling pressure is 0 . 2 kg / cm 2.

     Raise the temperature to 75°C±2°C within 15-30 minutes, keep warm for one hour, heat up to 85°C±2°C again , keep warm for more than seven hours till sampling 10 % sulfuric acid solution is not qualified. Then warm up to 95°C±2°C and incubate for two hours. Then vacuum was applied at 95°C for 15 minutes. Allow it to cool down and continue to vacuum for 15 minutes. Let it cool down to 40°C and discharge it into the dissolution pot. Slowly add 10% sulfuric acid solution to make the pH value reach 1-2 . Stir for 30 minutes and wash with water until the pH is around 7 .

     The copolymer was dried and dried below 50°C . The water content was below 2 %.

     The following table shows the formulation of positive carriers.

     The simple process is as follows: The ethanol and benzyl alcohol are charged into the pot. Take out a portion of the oil dissolved black dissolved. The blender is slowly charged with ethyl fiber, and if necessary, it can be suitably heated ( not more than 60°C) to accelerate the dissolution, and then the dissolved oil is dissolved in black.

     The glass beads were treated with a 3% dilute sulfuric acid solution to remove alkaline adherents and other impurities from the surface, rinsed to neutrality, and dried.

     Add the washed glass beads in the rotating pot, blow with hot air to heat, then spray the above solution, only spray a small amount at a time to avoid sticking into pieces, immediately blow dry, then spray, and then blow dry until the solution is sprayed Finished. Blow dry. Sieve to remove the glass beads that have stuck together.

     The following table shows the formulation of the load body.

     The preparation process is briefly described as follows: Acetone and cyclohexanone are charged into a pan and mixed well. (The quantity taken out / put in another container 3 is dissolved in a phenol resin). Then, under constant stirring, slowly spread the perchloroethylene, do not cause agglomeration, if necessary, heat ( but the temperature can not exceed 50 °C) to accelerate the dissolution. Then, the dissolved phenolic resin solution and the methyl violet ethanol solution are added, and the mixture is stirred uniformly, that is, the carrier coating solution.

     Other processes are similar to positive carriers.

     The above is some simple production process of electrostatic copy powder.

     When used, the toner can be used with a positive carrier, and the toner is negatively charged and can be used for the positive copy of a positive document. If the toner is matched with the load body, the toner is positively charged and can be used for positive copying of negative originals. If the toner is matched with a suitable iron powder carrier, it can also be used for magnetic brush development of selenium electrostatic copying or zinc oxide copying. This type of toner can still be used for electrostatic spray development of X- ray selenium electrostatic development, and can be charged positively or negatively during development.

     The principle of liquid development is to suspend the carbon black particles in a low dielectric constant aliphatic hydrocarbon. Due to the movement of the carbon black in the liquid, the particles obtain a surface charge (zeta potential ) ; and migrate toward the opposite charge direction. Therefore, if the photoconductor with the latent image pattern is passed through the suspension liquid, the colored particles will adhere to the latent image pattern by the liquid migration. Excess solvent can be squeezed out by the roller.

     Liquid type electrophotographic inks generally include a colorant ( for example, carbon black ), dodecyl methacrylate ( or tetradecyl ester ) and ethylene β -hydroxyethyl ( hereinafter, mainly used as a binder ), and an organic acid metal salt ( for example, naphthenic acid). Cobalt - as an electrical control material , etc.

     In production, a ball mill can be used to grind and disperse the carbon black with an appropriately charge oriented dye ( such as nigrosine black ) together with a soluble binder and an aliphatic hydrocarbon solvent having a low dielectric constant. The finished product has a particle size of 0 . 1 to 2 microns.

     When using the liquid type electrophotographic ink, can generally be diluted with a solvent to a light 200 is about 3/6 or 1000/1000 can.