Causes and solutions for off-odors from packaging bags, useful!

Off-odors in packaging bags not only affect the sensory characteristics of the packaged contents, such as color, aroma, and taste, but also pose a potential health risk to consumers, making it a critical issue for product safety. In severe cases, the packaging site will be filled with pungent odors emanating from the packaging bags, significantly deteriorating the operating environment. Of course, one of the main causes of off-odors in packaging bags is residual solvents. If within the standard range, they pose little threat to human health. However, the long-term accumulation of highly toxic organic substances is quite harmful to health.

I. Odor Sources

 

 

 

 

Films, various inks, adhesives, and organic solvents can all contribute to various odors. The odors produced by packaging materials are diverse and complex, including sweet, sour, sulfurous, medicinal, burnt, paraffin, petroleum, ink, soapy, tar, and solvent smells. Odors are not caused by a single substance but often by a mixture of several substances, making it particularly difficult to describe the odor, as even minor changes in the composition of the mixture can lead to differences in smell.

The odors in films mainly originate from these two parts: low molecular weight substances and various additives inherent in the resin itself; and odors that occur during flexible packaging processing.

II. Source Analysis

 

 

 

 

1. Influence of the Resin Itself

1. Films are generally made from polymer resins, which are composed of molecules of varying sizes, and different resins have different molecular weight distributions and different low molecular weight contents. Low molecular weight substances are easily oxidized and volatilized during extrusion processing, producing odors.

2. Resin synthesis processes are divided into solution methods and gas-phase methods. The two different synthesis methods themselves also differ in odor: the solution method, due to the removal of the solution after synthesis, removes many low molecular weight substances, so the odor is theoretically better than the gas-phase method.

3. Many additives are added to the resin to improve its performance, and these additives are the biggest source of odor:

Slip agents are generally amide compounds and have a somewhat sweet odor.

Antistatic agents have a very pungent odor and cannot be used in low-odor packaging.

Opening agents are generally inorganic substances, usually calcium carbonate and silicon dioxide. However, in terms of opening effect, silicon dioxide is far superior to calcium carbonate, but in terms of odor, calcium carbonate is better than silicon dioxide.

Antioxidants are usually phenolic substances and also have a certain odor. Linear molecules generally have the problem of catalyst residue, so antioxidants must be added.

Masterbatches: Commonly used white masterbatches generally add some wax and stearate salts as dispersants and lubricants. These low molecular weight substances will precipitate during processing, resulting in a strong odor, so they should be placed in the middle layer as much as possible in multilayer co-extrusion.

The heat-sealing layers PE film and CPP film commonly used in composite packaging bags are the most prone to odors. Detection method: Prepare 5 1L wide-mouth bottles, cut 0.1㎡ of BOPP, PE, CPP, PET film, and finished composite film, respectively, and bake them in a 60℃ oven for 30 minutes. Take them out and cool them for 30 minutes to determine the odor.

2. Odors Occurring During Flexible Packaging Processing

1. Thermal Decomposition

The odor-producing components of polyethylene decomposition can be roughly divided into two categories: aliphatic hydrocarbons and aromatic hydrocarbons. Studies have shown that thermal decomposition produces saturated or unsaturated hydrocarbons with three to twenty-eight carbons. Among them, the three-to-six-carbon compounds produced by short-chain branches have the greatest impact on the contents. Due to their low volatility and low polarity, these substances can be preserved in the film for a long time.

2. Thermal Oxidation Produces Odor

When plastics are processed under high temperature and high shear force, in addition to thermal decomposition, they also undergo a thermal oxidation reaction with oxygen in the air after exiting the extruder die, and the resulting substances can also cause odors.

Surface corona treatment produces odor. In order to make inks or adhesives have better adhesion, different plastic films must be corona or flame treated. Due to the oxidation reaction, the surface tension of the printing plate becomes stronger, and a set of substances (hydroxyl, aldehyde, ketone, and carboxyl groups) produced during the treatment process will be encountered in the subsequent coating. If the treatment intensity is increased, not only will the surface tension increase, but also the degree of polymer degradation will increase, resulting in volatile small molecules (aldehydes and ketones); these volatile aldehydes and ketones will produce odors.

3. Odor Generation During Heat Sealing

When composite films are heat-sealed at high temperatures, the materials will thermally decompose and additives will precipitate, and various components in the inks and adhesives will volatilize and decompose, producing odors. Odors from plastic additives. During the processing and molding of plastics, various auxiliaries are added to improve processability or improve the properties of the products. These auxiliaries are diverse in variety and composition and are one of the main factors causing odors. Such as plasticizers, heat stabilizers, antioxidants, UV stabilizers, lubricants, opening agents, antistatic agents, antifogging agents, etc.; in addition, functional inorganic fillers such as calcium carbonate, kaolin, talc, and silicon dioxide are added, which will bring in a large number of dispersants and coupling agents; and various organic pigments, titanium white masterbatches, etc., will produce many volatile substances when heated. If the added pigments, etc., can promote polymer decomposition, severe odors will occur during heat sealing.

4. Residual Solvents in Inks and Adhesives

The odor caused by residual solvents in inks and adhesives is the main factor causing odor in composite flexible packaging materials, especially in dry lamination, residual solvents are unavoidable. Currently, international standards require residual solvents to be 5mg/㎡, and benzene should not be detected.

III. Measures to Reduce Odors

 

 

 

 

For the production control of composite packaging bags, the most important thing is to take measures to control the following factors with the greatest impact: solvent residue in printing and lamination, resin decomposition, and additive leaching.

1. Control of residual solvents

Residual solvents are introduced by inks and adhesives. Common solvents include ethyl acetate, toluene, xylene, ethanol, butanone, isopropanol, butanol, etc. Actual product test data show that ethyl acetate and toluene account for a larger proportion of the total residual amount and are therefore the key control targets. This is mainly due to the fact that the vast majority of flexible packaging products in China use intaglio printing inks and dry lamination production.

Printing substrate: Different printing substrates have different adsorption capacities and adsorption tendencies for the solvents used in printing. BOPP has a greater adsorption capacity for solvents than PET and nylon, which should be considered in product design.

Product structure: For product structures with poor barrier properties, such as BOPP/PE, a considerable amount of residual solvent can escape through processes such as oven curing and slitting and rewinding, especially easily volatile ethyl acetate. For product structures with a good barrier outer layer, the residual solvent is higher. For products with a double barrier structure, the residue is the most difficult to control and the most difficult to remedy afterwards.

Printed image: Products with multiple printing colors, thick ink layers, and large ink areas are prone to residual solvents. When inks are used, a large amount of mixed solvents, such as toluene, xylene, butanone, and ethyl acetate, are added; xylene has a high boiling point and is easily retained, and butanone has a strong residual odor.

1. Volatility and drying of solvents

Different solvents have different volatilization rates. For the same type of solvent, the lower the boiling point, the faster the volatilization rate. However, for different types of solvents, the volatilization rates are not necessarily close even if the boiling points are the same. The volatilization rate of the solvent is also related to the external pressure, temperature, and air volume. The greater the external pressure, the higher the temperature, and the greater the air volume, the faster the volatilization rate of the solvent.

Actual ink volatile substances are generally not single-component solvents. In this case, the highly volatile components escape first, while the less volatile components remain, changing the composition of the solvent. It is impossible to obtain a single volatilization rate at a constant boiling point like a single solvent, and the volatilization of the solvent gradually slows down. Generally, it is required that the boiling point range of the ink solvent should be as narrow as possible; otherwise, some high-boiling fractions are easily retained in the ink film.

2. Solvent purity

Low solvent purity and high content of high-boiling point components are one of the important factors leading to odor. In printing, if there are many high-boiling point components, the highly volatile solvents will volatilize first, leaving the less volatile ones behind, and the volatility of the solvent will gradually decrease, eventually resulting in a large amount of residual solvent. In addition, solvents are easily contaminated during storage and transfer.

3. Influence of ink

Using alcohol-soluble inks, benzene-free inks, and water-based inks can reduce odor generation at the source. The surface properties and concentration of pigments will affect the volatilization of solvents. Pigments with low density and small particles have less solvent volatilization per unit area and time.

4. Influence of printing drying on residual solvent

The solvents contained in the printed ink layer are mixed solvents, and each solvent has a different volatilization rate. In this case, the volatilization of the solvent changes, the highly volatile solvent escapes first, and the less volatile components remain, changing the composition of the solvent. It is impossible to obtain a single volatilization rate at a constant rate like a single solvent, and the volatilization of the solvent will gradually slow down. If the purity of the solvent does not meet the requirements (e.g., too many high-boiling point components) or too much slow-drying solvent is used, serious residual solvent problems will occur under normal conditions.

In addition, because the pigment particles in the ink are very small, the specific surface area is very large, and the adsorption capacity is very strong, although it has been heated and dried during printing, due to the short time and fast speed, it is often not completely dried, and there is too much residual solvent, especially in large-area multi-color printing. These residual solvents are brought into the lamination process, and it is more difficult to remove them after lamination, so they can only slowly migrate and penetrate.

When printing overlapping colors, because the ink layers of the first and second layers are not completely dry, it is easy to cause color pulling, color bleeding, and odor. Because after the ink in the intaglio printing plate is printed on the film, the organic matter in the ink vaporizes under the action of temperature, and its movement is bidirectional. On the one hand, it moves to the surface of the ink and is drawn away by the exhaust fan, while on the other hand, it moves to the depth of the ink layer and the overlapping ink layer. If the previous ink layer is not completely dry, it will absorb and cause odor. The key is that the ink layer must be completely dry. Some ink layers seem to be dry, but this is only surface drying, not completely dry. Often, these incompletely dried ink layers contain high-boiling point solvents, causing problems such as subsequent sticking and odor. For printing drying, the key is to maintain the printing drying equipment, control the printing temperature, printing speed, and printing environment, and these factors are often interrelated. Mainly:

5. Regularly maintain and clean the drying device to prevent blockage. It is best to have a double drying device.

6. If conditions permit, print one or two blank colors after printing to further volatilize the solvent.

7. Increase the drying temperature of the last color and the humidity and organic solvent content of the drying medium (air).

2. Relationship between residual solvent and printing speed and drying temperature

The higher the drying oven temperature, the faster the solvent volatilizes, but too high a temperature will cause the printing substrate to stretch and deform. Therefore, the temperature discussed here is relatively low and high temperature, that is, low-temperature drying and high-temperature drying under the condition of meeting printing quality.

3. Relationship between ink viscosity and residual solvent

To understand the impact of ink viscosity on residual solvent, it is necessary to study the relationship between ink dilution rate and ink printing viscosity, ink application amount (dry weight, the same below), and residual solvent. Using a printing press (or proofing press) to simulate printing for qualitative analysis is a relatively suitable method. By testing the relationship between different dilution rates and the three factors, a foundation can be laid for setting the printing ink viscosity and controlling the residual solvent.

4. Control of residual solvent in dry lamination

The dry lamination process is a key process affecting residual solvent. On the one hand, the ester-soluble adhesive used in dry lamination will introduce ethyl acetate, and the alcohol-soluble adhesive will introduce ethanol, etc., increasing the residual solvent; on the other hand, the dry lamination process has a high temperature and large air volume, which will further remove the residual solvent in printing. If solvent-free lamination is used, the influence of residual solvent in lamination will not exist.

5. Influence of adhesives

Experienced companies have this understanding: when using dry lamination, the residual solvent in composite films produced using adhesives from different manufacturers under the same materials and processes is different. This is because the main agent of polyurethane adhesive, its molecular bonds contain active hydrogen hydroxyl groups, which will form hydrogen bonds with ethyl acetate, greatly restricting the volatilization of ethyl acetate. The change in the main agent structure will affect the strength of this hydrogen bond. The stronger this hydrogen bond, the more difficult it is to release ethyl acetate, and therefore different main agents will cause different residual solvent results. If solvent-free lamination is used, special attention should be paid to detecting the smell of incoming materials, and paying attention to the castor oil content and smell problems. Accurate proportioning is essential during the process.

6. Influence of curing

The hot air in the curing chamber should enter from the bottom and exit from the top. If ethyl acetate is found to exceed the standard after lamination, the composite film can be baked in the curing chamber for a longer time. The time should not be too long, otherwise the additives in the inner layer material will precipitate, which will increase the odor. If it is a double-layer barrier structure product, longer baking will not help.

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