How to Reduce Odor from Plastic Film Used in Packaging Bags?
The odor from packaging bags not only affects the contents but can also pose a potential health risk to customers, making it a critical product safety issue.
Reduce odor in composite packaging bags has become a fundamental requirement in the packaging industry.
Where does plastic film’s odor come from?
The odor in films mainly comes from two sources: additives and low-molecular substances in the resin itself, and odors that arise during the soft packaging process.
Films, various inks, adhesives, and organic solvents can all contribute to different odors.
The odors produced by packaging materials are diverse and complex, including sweet smells, sour or rancid odors, sulfurous smells, medicinal odors, burnt smells, waxy odors, petroleum-like smells, ink odors, soap smells, tar-like odors, and solvent odors, among others.
These odors are often caused by a mixture of substances rather than a single one, making it difficult to describe them accurately.
Even minor changes in the composition of the mixture can lead to differences in the odor.
Influence of the Resin Itself
Films are generally made from polymer resins, which consist of molecules of various sizes, and the molecular weight distribution varies between different resins, as does the content of low-molecular substances.
These low-molecular substances can easily oxidize and volatilize during the extrusion process, producing odors.
The resin synthesis process can be categorized into two methods: the solution method and the gas-phase method. These two methods result in different odors.
In the solution method, many low-molecular substances are removed through a post-synthesis removal process, theoretically resulting in a better odor profile compared to the gas-phase method.
Resins also require numerous additives to improve performance, and these additives are the main source of odors.
For example:
- slip agents are typically amide compounds with a slightly sweet smell.
- Antistatic agents, on the other hand, have a very strong and pungent odor and should not be used in low-odor packaging.
- Anti-block agents are usually inorganic, such as calcium carbonate and silica.
- While silica is much more effective than calcium carbonate for anti-blocking, calcium carbonate has a better odor profile.
- Antioxidants are often phenolic compounds, which also have a certain odor.
- Linear molecules typically require the addition of antioxidants due to catalyst residue issues.
- Common white masterbatch often contains waxes and stearates as dispersants and lubricants.
These low-molecular substances can separate during processing, generating strong odors, so it’s best to place them in the middle layer in multi-layer coextrusion.
Odors Arising During the Production of Composite Packaging Bags
1, Odors from Thermal Decomposition
The substances that cause odors from polyethylene decomposition can be broadly divided into two categories: aliphatic hydrocarbons and aromatic hydrocarbons.
Research shows that thermal decomposition produces saturated or unsaturated hydrocarbons ranging from three to twenty-eight carbons in length.
Of these, the compounds with three to six carbons, especially short-branched chains, have the greatest impact on the packaged contents.
Due to their low volatility and weak polarity, these substances can remain in the film for a long time.
2, Odors from Thermal Oxidation
During plastic processing under high temperatures and high shear forces, in addition to thermal decomposition, thermal oxidation reactions occur when the plastic exits the extrusion die and comes into contact with oxygen in the air.
The substances produced in this process can also result in odors.
3, Odors from Surface Corona Treatment
To improve the adhesion of inks or adhesives, various plastic films must undergo corona or flame treatment.
The oxidation reactions during this process increase the surface tension of the film.
During subsequent coating applications, the treated surfaces encounter a set of substances (hydroxyls, aldehydes, ketones, and carboxyl groups) generated during the treatment process.
If the treatment intensity increases, it not only raises the surface tension but also increases the degree of polymer degradation, resulting in volatile small molecules (aldehydes and ketones).
These volatile aldehydes and ketones contribute to the odor.
4, Odors Produced During Heat Sealing
During the heat sealing of composite films at high temperatures, additives in the material can decompose and volatilize, and various components in the inks and adhesives can evaporate and break down, leading to the production of odors.
The additives in plastic are a major factor contributing to these odors.
During the plastic processing and forming stages, various additives are used to improve processability or enhance the properties of the finished product.
These additives vary widely in type and composition and are one of the main sources of odors.
Examples include plasticizers, heat stabilizers, antioxidants, UV stabilizers, lubricants, anti-block agents, antistatic agents, and anti-fog agents.
Additionally, inorganic functional fillers such as calcium carbonate, kaolin, talc, and silica may be added, which also bring in numerous dispersants and coupling agents.
Organic pigments, masterbatches, and titanium dioxide are also often added, and during heating, these components release volatile substances.
If the added colorants or other substances promote polymer degradation, severe odors can be generated during heat sealing.
5, Residual Solvents from Inks and Adhesives
Residual solvents from inks and adhesives are one of the main factors contributing to odors in laminate flexible packaging materials.
In particular, residual solvents from inks and adhesives are a key cause of odors in dry lamination processes, where it is impossible to completely eliminate them.
Currently, international standards require residual solvent levels to be less than 5 mg/㎡, with no detectable traces of benzene compounds.
Methods to Reduce Odor
In controlling the production of composite packaging bags, the most important approach is to manage the following key factors: residual solvents from printing and lamination, resin decomposition, and additive volatilization.
1, Control of Residual Solvents
Residual solvents are introduced through inks and adhesives, with common solvents including ethyl acetate, toluene, xylene, ethanol, methyl ethyl ketone (MEK), isopropanol, and butanol.
Practical product testing data shows that ethyl acetate and toluene account for a large portion of the total residuals, making them key targets for control.
This is primarily because the majority of flexible packaging products in the domestic market are produced using reverse printing composite inks and dry lamination processes.
- Printing Substrate:
Different printing substrates have varying absorption capacities and tendencies for the solvents used.For example, BOPP has a greater solvent absorption capacity than PET or nylon, which should be considered during product design.
- Product Structure:
In structures with poor barrier properties, such as BOPP/PE, residual solvents can dissipate significantly through processes like curing in ovens and rewinding during slitting, especially volatile substances like ethyl acetate.However, in structures with good barrier layers, residual solvent levels tend to be higher.
For products with double barrier structures, controlling residuals is particularly difficult, and it’s hard to correct once the problem occurs.
- Printed Design:
Products with many color sequences, thick ink layers, and large ink coverage are prone to residual solvent issues.Mixed solvents such as toluene, xylene, MEK, and ethyl acetate are often added to the inks. Xylene, with its high boiling point, tends to leave higher residuals, and MEK can lead to severe residual odor problems.
2, Relationship Between Residual Solvents, Printing Speed, and Drying Temperature
Higher drying oven temperatures accelerate solvent evaporation.
However, excessive temperatures can cause the printing base film to stretch and deform.
The goal is to use relatively low and high temperatures under conditions that maintain printing quality, to balance solvent evaporation.
3, Ink Viscosity and Its Relationship to Residual Solvents
Understanding the influence of ink viscosity on solvent residuals involves studying the relationship between ink dilution rate, printing viscosity, ink application amount (dry weight), and residual solvents.
Simulated printing using a printing machine (or proofing machine) for qualitative analysis is an effective method.
By testing different dilution rates and their relation to these factors, a solid foundation can be established for setting ink viscosity and controlling solvent residuals.
4, Control of Residual Solvents in Dry Lamination
The dry lamination process is a key factor in controlling residual solvents.
On one hand, ester-based adhesives used in dry lamination introduce ethyl acetate, and alcohol-based adhesives bring in ethanol, increasing solvent residuals.
On the other hand, the high temperature and large airflow in the dry lamination process further help to reduce residual solvents from printing.
Using solvent-free lamination avoids the issue of residual solvents from lamination altogether.
5, Impact of Adhesives
Experienced companies have observed that different adhesives from various manufacturers, when used under the same materials and processes, result in different residual solvent levels in laminated films.
This is because the main agent in polyurethane adhesives contains hydroxyl groups that form hydrogen bonds with ethyl acetate, significantly restricting the evaporation of ethyl acetate.
Changes in the structure of the main agent affect the strength of these hydrogen bonds.
The stronger the hydrogen bond, the more difficult it is to release ethyl acetate, resulting in different residual solvent outcomes based on the main agent used.
When using solvent-free lamination, it is particularly important to monitor the smell of incoming materials and pay attention to the castor oil content and its odor.
Precise mixing ratios are critical for this process.
6, Impact of Curing
In the curing chamber, hot air should enter from the bottom and exit from the top.
If the composite film shows excess ethyl acetate, it can be further heated in the curing chamber.
However, the curing time should not be too long, as prolonged exposure can cause additives in the inner layer to migrate, increasing odors.
For double-barrier structures, prolonged heating is not beneficial.
What KDW does to reduce odor from plastic film
In addition to strictly controlling each step of the process to reduce odor emissions, Kinderway has introduced low-odor films that minimize the release of odors during use or application.
These low-odor films feature minimal additive emissions, high-purity resins, and unique barrier properties.
They are mainly used in the packaging of food, medical supplies, and consumer goods.
Low-odor films ensure product safety, making them essential in industries where cleanliness and freshness are critical.
Start With KDW
+86 13559233681(Wechat, Whatsapp)
No1, Anbian Rd, Torch High-Tech Zone (XiangAn), Xiamen, Fujian, China