In order to be able to carry out non-resistance automatic packaging production and to prevent the packaging from slipping during storage, packaging products have special requirements on the coefficient of friction (COF). Control of COF is very important, but it is difficult to achieve during printing and processing.
The film utilizes various lubricants, such as fatty acid amides, to obtain the desired COF specification. . To obtain the desired COF, the lubricant used in printing inks can range from polyethylene microparticles to silicones and fatty acid amides.
The COF is defined as the necessary dynamic coefficient that overcomes the force between two relatively stationary objects to cause them to move (static friction coefficient) or to maintain movement (dynamic friction coefficient).
Although automatic packaging operations are more concerned with dynamic friction, the sliding tendency of packages is controlled by static friction. Even in the best environment, packaging products encounter the problems of increased and decreased friction coefficients.
When using inks or coatings on a film, the COF of the printed film is usually higher than it is. As the sliding agent spreads into the film layer and the ink layer, the COF of the printed film will decrease over time.
If there is residual solvent on the printing film, it will lead to an increase in COF; after the ink is dried, the COF will decrease. The presence of residual solvent also increases the diffusion of the sliding agent. In some cases, the sliding agent diffuses into the ink to soften the ink, thus resulting in an increase in the COF of the printing ink.
The COF of the printing ink is controlled by the added lubricant while being controlled by the resin and solvent used. The lubricant it uses can be fine particles of polyethylene, silicones, fatty acid amides, or other products that reduce COF.
The very important point of printing ink is that COF is the softening point Tg of the adhesive. In general, the higher the Tg value, the lower the COF value. The presence of plasticizers and high-boiling solvents will increase the COF value.
The increase in printed film COF is less common, but it does occur. The increase in the COF value over time usually implies the appearance of some kind of movement, perhaps a reduction of the lubricant on the surface of the ink layer, or a softening of the ink.
Chemical changes may also lead to an increase in COF. For example, corona treatment of a printed film will increase its COF value. Then, the corona-treated film will increase the COF value by chemically oxidizing the surface components and increasing the surface energy of the printed film. If you need to increase the COF beyond the specifications, this will help you.
The phenomenon of COF increase over time is rare. However, there are times when you may encounter problems with careful analysis of components that may move such as solvents and plasticizers.
The softening ink layer/film layer also improves the coefficient of static friction while improving the coefficient of kinetic friction. When the movement of the plasticizer affects the softening of the ink, the difference between the coefficient of static friction and the coefficient of kinetic friction becomes large and can be used to estimate the degree of such influence.
UV-curable inks can show differences in COF, for example, UV inks can be cured to varying degrees due to the different spectral absorption characteristics. Black UV inks have the lowest degree of cure and higher COF, while other colors of ink may not be affected.
The water-based ink consists of an alkali-soluble resin and an emulsion. Ink drying and film formation can lead to non-uniform "lubricant slip." This situation is further exacerbated when the ink is coated with multiple layers of water-based resin of different compositions.
The COF problems encountered by most packaging and processing plants are related to changes in processing methods. By controlling the corona treatment, solvent residues, and ink formulation, one can obtain a higher degree of COF consistency.
Source: Chinese and foreign printing and packaging network