Packaging bag odor? How to solve it?

Odor from packaging bags can directly cause the contents to acquire off-flavors, and in severe cases, it can affect product quality or even consumer health.

At the same time, it can degrade the operating environment and cause discomfort to operators. In recent years, the issue of odor from packaging bags has increasingly gained attention from relevant enterprises, and much progress has been made.

The main factors contributing to odor in packaging bags include residual solvents, odors inherent in raw and auxiliary materials, odors generated during processing, and environmental pollution.

Below ShunXingYuan Packaging, a professional flexible packaging manufacturer will briefly analyze the above factors separately.

1. Residual Solvents

After printing ink is transferred to the film, solvents on the surface of the printed ink film evaporate first, while solvents inside the printed ink film must first diffuse and permeate to the film surface before they can retrogradely evaporate and dry.

When drying enters its final stage, the surface layer of the printed ink film has already solidified, which restricts the diffusion of internal solvents, thereby causing residual solvent issues.

Residual solvents contained in printed matter are the main cause of odor in plastic gravure flexible packaging.

Their residue amount can be easily measured using a chromatograph, allowing for qualitative or quantitative management. When abnormal values appear, problematic printed materials can be collected and identified by sensory evaluation (smell).

Most of the organic solvents in the printing ink layer instantly dry within the drying oven, and the remaining trace amounts of solvents solidify and are encapsulated within the ink film, leading to a residual state.

1. Solvent release characteristics of binder resins

We know that, to meet printing needs or for economic considerations, mixed solvents are often added to ink during printing.

Each binder has its own true solvent, co-solvent, and non-solvent. The better the solubility, the greater the affinity between the binder resin and solvent molecules; the better the solvent's solubility for the resin, the worse the resin's release of the solvent, and the worse the release, the more likely it is to cause residual solvent issues.

For a certain type of ink, solvent release is largely its inherent property, so before starting to use an ink, it is necessary to ascertain its residual solvent tendency from the ink manufacturer in advance to make appropriate adjustments.

The solvents contained in the printed ink layer are mixed solvents, and each solvent has a different evaporation rate: in this situation, the evaporation of solvents changes, with highly volatile solvents escaping first, and less volatile components remaining, leading to a change in the solvent composition. It is impossible to achieve a single evaporation rate at a constant speed like a single solvent, and thus solvent evaporation gradually slows down.

If the purity of the solvent does not meet requirements (e.g., if it contains too many high-boiling point components) or if too much slow-drying solvent is used, then severe residual solvent problems will arise under normal conditions.

2. Drying Conditions

Drying conditions include drying air temperature, air volume (air speed), and drying equipment structure, which determine drying efficiency. Poor drying will increase the amount of residual solvent, so the best possible drying conditions should be created.

Increasing air temperature and air volume (air speed) can undoubtedly strengthen drying conditions, but it should be noted that when the printed ink layer is thick, if drying is too fast, the surface of the ink layer will rapidly form a film, preventing internal solvents from escaping, which in turn will lead to an increase in residual solvents and the generation of odor.

3. Printing Speed

The printing speed determines the length of the drying time. Therefore, the machine speed should be gradually increased while ensuring that the printed ink layer is fully dry.

If the speed is too high, sufficient drying after film printing cannot be guaranteed, especially when the distance between the last color print and the winding unit is short (and often involves large ink coverage), which can easily lead to excessive residual solvents.

4. Humidity of Drying Medium (Air) and Organic Solvent Concentration

The humidity of the drying medium (air) has a significant impact on residual amounts.

Firstly, moisture in the air enters the ink, causing a decrease in the overall volatility of the solvent. Secondly, the significant presence of moisture in the drying medium (air) also inhibits solvent evaporation. If humidity doubles, the drying speed of the ink will slow down by nearly twofold.

Therefore, under high humidity conditions, printing speed should be slowed down as much as possible, and contact between ink and air should be reduced to ensure thorough drying of the ink. At the same time, the solvent formulation should be adjusted immediately to create a special formula suitable for such climate.

The air humidity in a printing workshop generally should not exceed 70%, and certainly should not be too dry, otherwise it can easily cause static electricity problems.

5. Substrate Film

Films made of different materials have different selective absorption tendencies for solvents.

For example, films like aluminum foil and polyester are non-absorbent films, generally having less residual solvent; polypropylene films tend to retain hydrocarbon solvents, while absorbent films (such as nylon, cellophane) tend to retain alcohol solvents.

Additionally, additives in certain plastic films can affect solvent volatilization, thereby increasing the amount of residual solvent.

6. Others

1. Aging Treatment

Only after aging treatment allows residual solvents to fully volatilize and molecular structures to fully stabilize can the material proceed to the next process.

2. For multi-color printing, avoid overlapping colors when designing patterns; if possible, use spot colors to prevent localized ink layers from becoming too thick.

3. Use paper packaging to package products

If high-seal plastic film is used for packaging, it is equivalent to storing all the odors released by the product during storage. Once the packaging is opened, the odor will float out (the odor will significantly decrease after being placed for a while).

Therefore, using breathable paper packaging allows odor residues to fully volatilize, which also helps to reduce odor.

4. In case of defective prints, they can be remedied by ventilation and drying.

II. Odor from the raw and auxiliary materials used in plastic gravure flexible packaging

1. Solvent Selection

The purity of the solvents in the ink and the solvents used in ink adjustment, and the exceeding standard of non-volatile substances (high-boiling point solvents, etc.) are important factors leading to odor.

When using solvents, the amount of organic solvents toluene or xylene should be reduced as much as possible, and petroleum toluene should be selected. Coke plant toluene is refined from coal tar or crude toluene, and it is easy to produce a tar smell when used in plastic gravure flexible packaging, so coke toluene cannot be used.

In addition, some barrels of armored benzene and other solvents have been rusted extensively after long-term use, or sometimes the lids of used solvent barrels are not covered, which may cause foreign impurities or rainwater to enter.

When refilling solvents, due to factors such as the deterioration of solvent quality, the entire barrel of solvent is prone to odor over time.

2. Printing Ink

The odor of printed matter is not only caused by residual solvents, but sometimes also by the low molecular weight substances of the binder resin, auxiliaries, and pigment activators in the hand ink; this is an inherent property of this type of ink, and the ink supplier should be consulted or the ink should be re-selected if necessary.

Some companies use surface printing inks (the formula design does not consider the impact of lamination on residual solvents, and some resins themselves have odor) instead of composite intaglio inks, which not only affects the lamination strength but also easily causes odor problems.

3. Composite Substrate

Some composite substrates have a certain odor due to excessive corona treatment, improper selection of additives, etc., so they should be carefully checked before use.

III. Odor generated during the composite processing process (resin odor)

Low odor during resin processing is an important indicator for evaluating the thermal stability of the resin.

When extrusion laminating, polyethylene, polypropylene, and other resins and the added white masterbatch extruded from the T-die generally release resin odor due to oxidation at a high temperature of about 300 degrees Celsius.

It is difficult to completely remove the resin odor, but it is possible to reduce its generation.

1. To reduce odor

Lowering the processing temperature of the resin or shortening the distance from the T-die to the composite pressure roller is effective, where possible.

2. Different types of resins have different odor conditions

Polypropylene has a smaller odor, while polyethylene has a relatively larger odor, and EVA, due to its poor thermal stability, is more prone to odor problems.

3. Different grades of the same resin also have different odor conditions

They should be carefully compared before use.

IV. Environmental Pollution

If the concentration of organic solvents in the environment is high during the processing, use, and storage of plastic color printing composite substrates and products, etc., strong odor stimulants, then the plastic color printing composite products may be contaminated.

When selecting a storage location, pay special attention to the concentration of organic solvents and stay away from strong odor stimulants.

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