Environmentally friendly ink in green printing
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As environmental protection calls are growing, as a major component of green printing, environmental requirements for inks are increasing. It can be said that environmentally friendly inks are the first considerations for ink development in the future.
1. The harm of ink to the environment
Ink is currently the largest source of pollution in the printing industry, with an annual global ink production of 3 million tons. The global emissions of organic volatiles (VOC) caused by inks have reached several hundred thousand tons per year. These organic volatiles can form a more serious greenhouse effect than carbon dioxide, and will form oxides and photochemical smog under the sunlight, which will seriously pollute the atmosphere and affect people's health. In addition, food, toys and other packaging and printing of ordinary inks, such as heavy metals, harmful components of the human body will directly harm the health of the consumer.
(1) Impact on human health
The printing ink is composed of a pigment, a linking agent, a solvent, and an auxiliary agent. Among them, organic solvents and heavy metal elements are seriously harmful to the human body. There are two kinds of pigments in the ink - inorganic and organic, both of which are insoluble in water and other media, and have vivid color and stability. Some inorganic pigments contain heavy metals such as lead, chromium, copper, mercury, etc., which have certain toxicity and cannot be used for printing food packaging and children's toys; some organic pigments contain biphenyl gum, which has carcinogenic ingredients and should not be used. Organic solvents can dissolve many natural resins and synthetic resins. They are important components of various inks, but some of them can damage the human body and subcutaneous fat. Long-term contact can cause dry and rough skin. If it penetrates into skin or blood vessels, it will endanger people with blood. Blood cells and hematopoietic function; absorbed into the airway, bronchus, lungs or through blood vessels, lymphatic vessels to other organs, and may even cause chronic poisoning of the body. Some inks have the toxicity problem of heavy metal ions, and pigments and dyes contain carcinogenic components, which are harmful to human health. Composite packaging materials use a large amount of ink, organic solvents and adhesives in printing. These accessories are not in direct contact with food. However, during the packaging and storage of food, some toxic substances will migrate into the food and endanger people's health. Organic solvents such as ethanol, isopropanol, butanol, propanol, methyl ethyl ketone, ethyl acetate, butyl acetate, toluene, and xylene are often used in printing inks. Although these solvents can be removed by drying, the residual solvent migrates into the food and harms the human body. The solvent used in the gravure ink is generally butanone, xylene, toluene, butanol or the like. In particular, methyl ethyl ketone has a strong residual odor. Since the pigment particles in the ink are small and have strong adsorption force, although they are heated and dried during printing, they are often incompletely dried due to short time and high speed, especially for printed matter having a large inking area and a thick ink layer. There are many residual solvents. These residual solvents are brought to the compounding process and are more difficult to run off after compounding, which will slowly migrate and penetrate, so solvent residues must be kept to a minimum.
(2) Impact on environmental quality
The problem of atmospheric pollution caused by solvent volatilization in the ink is that the gravure ink is the most serious because it uses more VOC as a solvent. Long-term high concentration of VOC will cause great damage to the human body, especially the nervous system. VOC will also photochemically react with nitrogen oxides in the air to produce ozone and smoke, causing air pollution. To reduce VOC, there are two solutions for recycling and burning exhaust solvents, but there are many gravure presses that are not equipped with such devices. In addition, water and offset fountain solutions that drain aqueous inks can contaminate water. The disposal of industrial waste such as printed matter and ink containers is also very difficult.
2. At present, the main environmentally friendly inks
To meet the environmental requirements of the ink, the ink composition should be changed first, that is, the environmentally friendly material is used to prepare the new ink. At present, environmentally friendly inks mainly include water-based inks, UV inks, water-based UV inks, and some alcohol-soluble inks.
(1) Water-based ink
The biggest difference between water-based ink and solvent-based ink is that the solvent used is water instead of organic solvent, which significantly reduces VOC emissions, prevents air pollution, does not affect human health, is not easy to burn, is stable in ink, bright in color, and does not corrode. The plate is simple in operation, cheap in price, good in adhesion after printing, strong in water resistance and quick in drying. Therefore, it is especially suitable for packaging printed matter such as food, beverage, medicine, etc. It is recognized as an environmentally friendly printing material in the world and is also currently used in all printing inks. The only ink approved by the American Food and Drug Association. At present, 40% of plastic printing in the United States uses water-based inks. In other developed countries (such as Japan, Germany, France, etc.), the use of water-based inks in plastic film printing is increasing. In June this year, the United Kingdom enacted legislation to ban solvent-based inks from printing food films. According to relevant information, in the United States, the VOC-compliant water-based ink must reach the state of use of the ink composition, the proportion of volatile components is less than 25%, the moisture content is above 75%; the ink with less moisture is in use, non-volatile components These two standards are above 60%. Water-based inks are commonly used in the printing of paper-based products, including paper-plastic composite products. The characteristic is that it can satisfy the ink absorption of paper printing, and make the printed matter full of color. What is more rare is that the solvent is water and ethanol, which is less polluting to the environment, and is called environmentally friendly green ink.
(2) UV curing ink
Ultraviolet curing (UV) ink refers to an ink that forms and dries an ink by ultraviolet light of different wavelengths and energies under ultraviolet irradiation. Different UV spectra can be used to generate different energies to polymerize monomers in different ink binders into polymers, so UV inks have good mechanical and chemical properties. The main advantages of UV ink are: (1) no solvent; (2) fast drying speed and low energy consumption; (3) good gloss and bright color; (4) good resistance to water, solvent and abrasion. The photoinitiator in the UV ink is a compound that is easily excited by light. After absorbing light, it is excited into a radical, and energy is transferred to a photosensitive molecule or a photocrosslinker to cause a photocuring reaction of the UV ink. At present, UV ink has become a more mature ink technology, and its pollutant emissions are almost zero. According to statistics, the annual output of UV ink is about 16,000 tons in Japan, about 18,000 tons in Europe, and about 19,000 tons in North America. In addition to solvent-free, UV inks are also difficult to paste, clear dots, bright and bright ink, excellent chemical resistance, and low consumption. According to statistics, the US version of flexographic printing UV ink used in 2000 has reached 3,500 tons.
(3) Water-based UV ink
Water-based UV ink is a new direction in the field of UV ink research. Prepolymers in conventional UV inks are generally very viscous and need to be diluted with reactive diluents. While the currently used thinner acrylate compounds have varying degrees of skin irritation and toxicity, while developing low viscosity prepolymers and low toxicity reactive diluents, another development direction is to study aqueous UV inks, ie, water. And ethanol as a diluent. Currently, water-based UV inks have been successfully developed and used in some printing applications. In addition, the alcohol-soluble ink, which mainly plays a role in flexo printing, is also a kind of ink with little pollution, and is mainly used in food, medicine, beverage, tobacco and alcohol, and packaging and printing of daily necessities in contact with human body.
(4) Bio-ink
American scientists have long been working on a protein molecule called bacteriorhodopsin. Natural materials often exhibit complex functions that are not readily available for materials such as semiconductors. The genus Rhodopsin is derived from a strong purple cell membrane grown on the body of the salt marshes known as the "Salt Products Halophilicus". Illuminating its protein can trigger a photochemical reaction loop. The principle is that the photochemical reaction ring carries protons along the channel surrounding the cell membrane. The purple color of this cell membrane is derived from the bacteriophage component called "retina". The amine acid in the cell membrane channel is firmly bound to the "retina". The "retina" in the dissolved state does not bind to the amine acid and is pale yellow. By alternately irradiating two groups of lasers of different wavelengths onto the protein molecule, the protein molecule can be converted back and forth between the purple structure and the yellow structure. It is this habit that stimulates people to research and use it. It can be used as a photosensitive element in the analog retina, as a memory element in a ruthenium-based processing device or as a memory element in an optical computer, and its film can also change color according to the conversion of an external electric field. In conventional rhodopsin, low contrast color changes from purple to blue are formed under different electric fields. However, the rhodopsin structure produced by a certain mutant bacteria changes from blue to pale yellow under the action of a strong electric field. This protein film is sandwiched between transparent plates combined with a large number of electrodes to form a display plate. Applying an appropriate voltage to different parts of the film allows you to "write" a page of a document or "place" an image. Like the ink, the image on the Violet display can also be seen in normal light. Moreover, the color transition time can be accelerated to 200 μm. At present, the main difficulty faced by the entire development work is that it requires an electric field that consumes hundreds of volts to excite its color change. Researchers are working hard on this area and expect breakthrough progress.
(5) New environmentally friendly ink "Opis100"
Osaka Printing Ink Co., Ltd. recently launched a new environmentally friendly ink "Opis100", which is a sheetfed offset ink that meets the green label certification standards revised by the Japan Environmental Association at the end of last year. It has the following characteristics: (1) conforming to the newly revised green logo "lithographic" Ink V-2" standard; (2) accredited by the American Soybean Association; (3) meets the environmental standards of the Japan Printing Industry Federation; (4) meets the GPN ordering guidelines; (5) is 100% based on soybean oil Vegetable oil ink; (6) VOC content less than 1%; (7) good stability and transferability on the machine; (8) good abrasion resistance, gloss and good hair color; (9) good suitability of dampening solution; 10) It is easy to biodecompose and deink; (11) It has excellent fusion with UV-OP varnish and better gloss; (12) It also has excellent printing adaptability to double-sided 8-color machine.