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The odor of recyclable flexible packaging products can directly cause absorption and cross contamination of packaging contents, such as food cross contamination in packaging. In severe cases, it can also affect the taste of food and even harm the health of consumers. At the same time, the deterioration of the production operating environment caused by odors has brought discomfort to operators and users. With the increasing demand in the upstream and downstream of the industry chain, the importance of odors is also increasing.
The main sources of odor in recyclable packaging are printing and lamination, with printing being the most likely. Due to the volatile drying method used in gravure printing ink, the amount of ink and solvent used is also very large. It can be said that odor is a significant industry characteristic of recyclable plastic flexible packaging gravure printing.
Due to the solvents, inks, resins, films, etc. used in gravure printing, there is a possibility of producing odors. The main sources of odors in printed materials are ink, solvent residues, odors carried by films during production, as well as production environment and processing processes.
There are 4 reasons of odor in printed recyclable pouches.
Gravure printing ink consists of four parts: binder (resin), colorant (pigment), solvent, and additive. The selection of these materials requires good comprehensive printing adaptability and controllable costs, but some ink manufacturers use low-priced and inferior materials to reduce costs, resulting in ink having an odor from the source.
The commonly used gravure printing inks are divided into benzene based and benzene free types. Non benzene based inks are further divided into polyurethane based and alcohol soluble inks. Different substrates use different inks, and each series of inks has different flavors due to the formulation of resins, pigments, solvents, and additives.
Therefore, choosing ink with low odor and good solvent release effect is particularly crucial in flexible packaging processing.
The selection of resin should not only have good printability and cost-effectiveness, but also should not have a significant odor. After film formation, it is required that there should be no odor, and there should be no odor generated during the mixed use of pigments, solvents, and additives.
High quality pigments with strong coloring power and high color stability are usually selected as pigments, and the structure and fineness of pigment particles need to be controlled and selected. Require pigments to maintain good solvent absorption and release properties, avoiding odor caused by residual solvents;
Organic solvents themselves have a certain odor, and their release and volatilization properties need to be fully considered when considering their ability to dry and dissolve. Including residual odors after solvent evaporation, such as coking toluene extracted from coal tar or crude toluene, even after ink evaporation and drying, it still produces a tar odor that is uncomfortable.
In addition to the functions of the additives themselves, it is advisable to avoid selecting additives that can easily cause odor residue after drying the ink film.
Due to the fact that solvents themselves are diluents with a certain odor, they may not completely evaporate during the printing process, resulting in solvent residue and causing the printed product to produce an odor, leading to solvent residue. Solvent residue is related to the following factors:
In order to achieve better printing adaptability, solvents are usually mixed to achieve the goal. The selection of mixed solvents is crucial, and it is necessary to evaluate the overall impact on the ink evaporation rate to ensure that there is no solvent residue or odor during drying; Moisture in the air enters the ink, causing a decrease in the overall volatility of the solvent.
The affinity of solvents for ink affects the volatility of solvents. Choosing the appropriate resin and matching it with the corresponding solvent ratio can ensure both printing performance and thorough solvent evaporation.
The heating temperature, wind pressure, wind speed, exhaust efficiency, and drying channel length of the equipment directly affect the drying performance of the ink and have a significant impact on the residual solvent content.
The solvent vapor concentration, drying medium (air), machine speed, ink layer thickness, film adsorption of solvents, and ink release of solvents in the drying oven can all affect the evaporation and drying of solvents, resulting in residual odors.
① The high concentration of solvent vapor in the oven inhibits the evaporation rate of the solvent, and the high concentration of solvent vapor can easily cause safety accidents such as flash fire and explosion.
② Air is the medium for solvent evaporation and drying, and excessive humidity leads to a large amount of moisture in the air, thereby suppressing the rate of solvent evaporation. Taking all factors into consideration, the most suitable printing humidity is between 45-60. Excessive humidity makes it difficult for solvents to evaporate and cause residue, while low humidity can easily generate static electricity.
③ Machine speed: The increase in printing machine speed makes the solvent drying time shorter and more prone to residue formation.
④ Ink layer thickness: The thickness of ink used in printing, especially in multi-color deep web overlay printing, increases the difficulty of ink drying and increases the risk of residue.
⑤ The strong molecular affinity between the solvent molecules of ink and the resin makes it difficult for the solvent to evaporate, resulting in residual solvent in the ink film. The better the solubility of the resin in the ink by the solvent, the poorer the release of the resin from the solvent.
⑥ The adsorption and affinity properties of different material films for solvents are different, so the difficulty of solvent drying is also different.
⑦ The release of solvents by ink itself. If the pigment is too fine and the surface is loose, it is easy to adsorb solvents, which can easily lead to solvent residue.
⑧ The printing film is wound too tightly, which is not conducive to the volatilization of residues.
The wind reduction and concentration enhancement measures required for environmental protection and energy management essentially involve recycling the hot air inside the drying oven, reducing the entry of fresh air while also increasing the solvent vapor concentration inside the drying oven. This facilitates end of pipe treatment but poses a risk of solvent residue.
The odor caused by the film can be divided into the film itself, surface treatment, and selective absorption of solvents by the film. The odor generated by the film can also cause the printed product to have an odor, which is difficult to eliminate.
The resin used to process the film itself has a certain odor, or high temperatures, additives, etc. during processing can cause the film to produce an unpleasant odor. The presence of vinyl chloride monomer in PVC film may still result in a slight odor.
Printing films may increase their odor during surface coating or corona treatment. The ozone generated by high-voltage electrodes during corona treatment gives people a serious discomfort and produces a strange odor as the film is wound up.
Thin films of different materials have different selective absorption tendencies towards solvents. Films such as aluminum foil and polyester are non absorbent and generally have less residual solvents; Polypropylene films are prone to residual hydrocarbon solvents, while water absorbing films (such as nylon and glass paper) are prone to residual alcohol solvents. The addition of certain additives to plastic films also affects the evaporation of solvents, increasing the production of residual solvents and odors.
4 Environmental problem
In the process of film production, printing, processing and storage, if the concentration of organic solvents in the environment is high, or there are drugs, essence, odor and other strong odor sources nearby, which lead to odor in the production, storage and transportation environment, then the printed matter may be polluted due to absorption of the odor of the environment.
When selecting production, storage, and transportation locations, attention should be paid to avoiding organic solvent concentrations and odor environments.
Longdapac as the leading 100% recyclable mono-material PE pouches manufacturer in China. We got rich experience in the odor problem in flexible packaging printing. We can guarantee that we will avoid this odor problem in all our packaging production.