The composite packaging bag has an odor, affecting the product. What should I do?

The odor in packaging bags not only affects the packaged contents but also poses a potential health risk to consumers, making it a critical product safety issue. Reducing odors in composite packaging bags has become a fundamental requirement in the packaging industry. Below, a professional flexible packaging manufacturer will explain the causes of odors in packaging bags and their solutions based on its rich experience in composite packaging bag production.

I. Odor Sources

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

Films, various inks, adhesives, and organic solvents can all bring 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 are often caused by mixtures of several substances. Describing the odor can be particularly difficult because even minor changes in the composition of the mixture can lead to differences in smell.

II. Source Analysis

1. Influence of Resin Itself

Films are generally made from polymer resins. Resins 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.

There are two methods for resin synthesis: solution method and gas-phase method. The two different synthesis methods 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.

Many additives are added to the resin to improve its performance. These additives are the main source of odor. Slip agents are generally amide compounds with a slightly 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 better than 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.

Masterbatch: 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.

2. Composite packaging bag production process Odors that appear

1. Thermal decomposition

The components of the odorous substances produced by the decomposition of polyethylene 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, a thermal oxidation reaction occurs with oxygen in the air after the extruder die, and the resulting substances will also cause odor.

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 odor.

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. The odor of 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 odor. 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, serious odor will occur during heat sealing.

4. Residual solvent of ink and adhesive

The odor caused by the residual solvent of ink and adhesive. In composite flexible packaging materials, the main factor causing odor comes from the residual solvent of ink and adhesive, especially in dry lamination, residual solvent is unavoidable. Currently, the international requirement is that the residual solvent is 5mg/㎡, and benzene is not detectable.

III. Measures to Reduce Odor

For composite packaging bag production control, 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 precipitation.

1. Control of residual solvent

Residual solvents are introduced by inks and adhesives. Common solvents include ethyl acetate, toluene, xylene, ethanol, methyl ethyl ketone, isopropyl alcohol, and butanol. Actual product test data show that ethyl acetate and toluene account for a large 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 inside printing composite inks and dry lamination production.

Printing substrate: Different printing substrates have different adsorption capacities and tendencies for the solvents used in printing. BOPP adsorbs more solvent than PET and nylon, which should be considered during product design.

Product structure: For products 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 volatile ethyl acetate. Products with a good barrier outer layer have higher residual solvent levels. Products with a double barrier structure are the most difficult to control residual solvent, and are also the most difficult to remedy afterwards.

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

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

The higher the drying oven temperature, the faster the solvent evaporates, but excessively high temperatures can cause the printing substrate to stretch and deform. Therefore, the temperatures discussed here are relatively low and high temperatures, i.e., low-temperature drying and high-temperature drying under conditions that meet printing quality.

3. Relationship between ink viscosity and residual solvent

To understand the influence 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 residual solvent.

4. Control of residual solvent by 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 residual solvent; on the other hand, the high temperature and large air volume in the dry lamination process will further remove the residual solvent in the printing. If solvent-free lamination is used, the influence of residual solvent in lamination will not exist.

5. Influence of adhesives

Experienced companies have the following understanding: When using dry lamination, the residual solvent of 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, whose molecular bonds contain active hydrogen hydroxyl groups, will form hydrogen bonds with ethyl acetate, greatly restricting the volatilization of ethyl acetate. Changes 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 result in different residual solvent results. If solvent-free lamination is used, special attention should be paid to the detection of incoming material odor, as well as the castor oil content and odor, and 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 laminated 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 barrier structure product, longer baking will not help.

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