The past few years witnessed the moderate development of resin ink industry. The use of resins in this field is estimated to grow in the coming years according to the market report. However, the exact role these resins play in the ink industry maybe not known to every ordinary people.
Modern ink, based on the version, can be divided into 4 categories: letterpress printing ink, planographic printing ink, gravure printing ink and screen printing ink. Although different in proportion, the main components are the same as follows:
Pigment consists of paint and dye. The colored material used in printing inks is usually pigment, but some dyes are also useful. Pigment is very fine colored material generally insoluble in water and binders but suspends in solution, whereas dye is generally soluble in binders. Pigment has a great impact on the performance of ink, for example, the relative density, transparency, heat resistance, light resistance and resistance to chemicals.
- Filling material
Filling material is a white solid that can evenly disperse in connection material. When the filling material is mixed and ground in the binders and becomes pulp, some of it is transparent and some of it is opaque. Commonly-used fillers are aluminum hydroxide, potassium sulfate, magnesium carbonate and calcium carbonate. Filling material in ink helps improve the consistency, viscosity liquidity and yield. On the other hand, it can also adjust the concentration of printing ink.
- Connection material
Connection material is a kind of vicious and sticky liquid that has many effects on different aspects. As carriers of pigment, it serves to connect the powder-shape pigment and other solid together to make sure that they can be attached to the printed material. Connection material, in addition to the effect of determining the fixation speed and drying type, gives to the ink a certain luster, frictional resistance and yellowing resistance. Other properties of ink, such as viscosity, yield value and liquidity are to some extent controlled by connection material.
Various resins are used as connection material in printing ink. It can be seen from here that ink resin as the carrier of pigment in ink, is the core material. The quality of these resins will directly affect the performance of the printing ink because the connection material to a large extent determines the important qualities of printing ink mentioned above. Thus, there is a word that the high quality of printing ink comes from high-quality connection material.
To be specific, the properties of printing ink controlled by resin are:
The adhesion of ink to printed material mainly includes chemical bond force, intermolecular force, interface electrostatic force and mechanical force. These forces are derived from the resin used in printing and the substrate material. For different materials, it is crucial to use different resins. Generally speaking, for PE, PP and other non-polar substrates, even by corona surface treatment, its surface tension can only reach 38 Dyne or so. Instead, it should use non-polar resin structure such as chlorinated polypropyl resin connection materials; For PET, PA and other surface polarity substrate, polyurethane resin connection material is better.
- Tinting ability
The inorganic pigment is the basic material to provide color for ink, but as the ink adhesion on the surface, there is always some deviation from the original color. The reason lies in printing ink. The pigment particles do not exist alone but are scattered in the connecting medium. On the micro-level, the best wetting dispersion state of a pigment particle is completely wrapped in the resin binder evenly and attached to the substrate surface. The display of color is realized by reflection: the light incident on the coated pigment surface goes through the resin connecting material film layer and is then transmitted to the consumer’s eyes. The transparency of the resin and the size of the color number directly affect the expression of different colors. At the same time, the sufficient dispersion of resin pigment directly affects the resin pigment uniform packaging and the quality of ink itself.
- Viscous resistance
Viscous resistance is an important index in ink evaluation. Gravure printing is a printing method with a high ink supply, good step reproducibility and fast printing speed. The inks used in it are mostly solvent volatile drying inks. After the printing ink plate transfers to the substrate, through the drying path, the organic solvent quickly volatilizes after drying. When going through the high-temperature drying road after rewinding, the surface temperature of the printing film is generally between 40℃~60℃, at the same time, due to the existence of rewinding pressure, if the ink itself does not have good anti-viscosity, it will lead to serious rewinding adhesion accident or quality problem.
In gravure printing, ink re-solubility is also a key indicator of ink performance. Especially in color ink printing, moderate solubility, not only can make the color show fully, but also can well solve the printing plate shallow screen block, overprint color problem.
- Storage stability
Ink, if being placed for a long time in the container, is prone to appear problems like uneven concentration, decreased viscosity, changed color etc. that might affect its use. Proper resin added will help solve this problem effectively.
Now, there are primary 4 kinds of resins modified in the market to meet the requirements of printing ink.
Acrylic resin is the general name of acrylic acid, methacrylic acid and its derivatives. Acrylic resin coatings are thermoplastic or thermosetting resin coatings, or acrylic radiation coatings, which are made by copolymerizing acrylic resin with methyl acrylate, styrene and other acrylates as the main bodies. The thermoplastic acrylic resin in the process of film formation does not have further crosslinking, so its relative molecular weight is relatively large, and it has good light and color retention, water and chemical resistance. It can also dry fast, construct conveniently, and be easy to repaint and rework.
- Epoxy resin
Epoxy resin refers to a class of polymers containing more than two epoxy groups. It is a condensation product of epichlorohydrin and bisphenol A or polyols. It is a thermosetting resin whose rings can be opened by a variety of compounds containing active hydrogen and cured by cross-linking to form a network structure. Bisphenol-A type epoxy resin not only has the largest yield and the most complete varieties and quality but also rich in new modified types that can meet the increasing demand in ink industry. Because of the unique advantages such as high insulation, high structural strength and good sealing property, epoxy resin has been widely used in electrical appliances and electronic components as well.
- Polyurethane resin
Polyurethane resin is a polymer whose backbone contains repeating — NHCOO — structural units. It is formed by the polymerization of isocyanates monomers and hydroxyl compounds. Due to the strong polarity carbamate group, it is insoluble in non-polar groups and has good oil resistance, toughness, wear resistance, aging resistance and adhesion. Using different raw materials can obtain several kinds of polyurethanes with different properties that are suitable for a wide temperature range (-50 ~ 150℃), including elastomers, thermoplastic resins and thermosetting resins.
Phenolic, also known as bakelite or bakelite powder, is originally colorless or tawny transparent solid made by the reaction of phenol and formaldehyde. Phenolic resin is used most intensively as thermoplastic resin in various industries. Phenolic resin is resistant to weak acid and weak base, while decompose and corrode in strong acid. The outstanding properties of the phenolic resin include good electrical properties, rigid set, excellent tensile strength and resistance to heat, water and organic solvents.
In printing ink, phenolic resin due to its role in the connection material brings high viscosity to plate offset printing. At the same time helps strengthen the ink body bone, improve the ink fixation speed and dry thoroughness. For example, rosin modified phenolic resin made of tall oil has the characteristics of high solubility, good pigment dispersibility and fluidity that is especially suitable for printing ink.
In conclusion, ink resin as part of the printing ink composition is crucial in its quality control and the market for ink resin is continually enlarging because of the increasing demand from all end used such as packaging, commercial publication, garment production etc. Now with more researches focusing on new modified rosins for printing ink, the ink industry has already entered the resin-based developing stage.