Problems with odors and residual solvents in gravure and flexo printing

This article will focus on: identification of odors and thresholds; causes of odors from packaging materials; optimization and measurement of residual solvents and estimation of residual solvents.

Today's food packaging mostly uses soft packaging materials. These materials are generally in close proximity to or even in direct contact with food, which may affect the quality of the food. Therefore, it is required that food packaging manufacturers must provide assurance on the quality of food.

Materials and commodities that have the following characteristics cannot be used for food packaging:
1. Endanger human health;
2. Causes changes in the sensory properties of foods such as internal components or color, smell, and taste.
As we all know, odor and food packaging is a very complex field, this article will focus on: odor identification and threshold; odor caused by the packaging material; residual solvent optimization and measurement methods and residual solvent estimates.

First, will the transfer of a particular compound on the packaging material to the packaged food impair the organoleptic properties of the food? This depends not only on the nature of the compound, but also on the ratio of the packaging material to the surface area of ​​the food, and even on the type of food. This means that solid, rough, solid foods will certainly not feel lighter than powdery or liquid foods. Second, consider the solubility of the transfer material for specialty foods. If a food contains fat, it will react with fat-soluble transfer material. Finally, consider the "adaptability" of food and transfer materials. In the fruit packaging process, there will be many different esters, such as ethyl acetaldehyde, so the transfer of residual acetaldehyde will not cause damage on the sensory characteristics of the fruit. But if the same material is used for the packaging of chocolate, the transfer of ethyl acetaldehyde will have serious consequences.

one. Odor discrimination and threshold

Odor is a subjective organoleptic sensation that cannot be directly measured by analytical methods. A common method of testing packaging materials is to place the quantified sample in a closed wide-necked flask and place it in a 40°C environment for two hours. The temperature was then returned to room temperature, at which point the sample's odor was judged. Because odors are very subjective, it is necessary to develop a standard set of measurement methods and train a group of dedicated personnel (odors) to judge odors. These people must use the same vocabulary to describe odors, and they need regular tests to test and adjust their abilities. Although gas chromatography helps to identify volatile compounds that can produce odors, we must be aware that any method of analysis cannot fully discriminate odors, including gas chromatography.

The threshold is a parameter that accurately describes the odor of a chemical substance. It describes the lowest concentration of gaseous compounds that people can feel. The following actual examples can give you a general impression of the concept of the threshold:
(1) Even if there are only 2 kilograms of paprika in the 50 trains, it can be felt;
(2) Even if there is only 1 gram of grapefruit thiol in the swimming pool, we will smell the taste of grapefruit;
(3) The wine cork has the smell of anisole trichloride, so even if only 33 grams of cork is placed in Lake Geneva, the whole lake will have the smell of anisole trichloride.

These examples show that, so far, human sensory organs are more sensitive than any analysis method. Therefore, we need the nose of a specially trained odor to distinguish the odor.

two. The smell of packaging materials

The odor of a defined packaging material is determined jointly by the components, which include:
* Printing materials and composites * Inks, primers, and spray paints * Layered or cold-packed adhesives

Finally, we also need to know that the odor will change due to the chemical reaction between the different compounds inside the packaging material and/or the packaged food.

Here are some examples of selected odour issues related to packaging materials:

1. Printing materials and composite materials

(1) Maggi flavor In order for the ink or adhesive to have better adhesion, different plastic films must be corona or flame treated. During the processing of polyolefin films (PE or PP), the film surface tension becomes stronger due to the oxidation reaction. A group of substances (hydroxy, aldehyde, ketone and carboxyl groups) produced during the treatment will be used in the subsequent coating. If the strength of the treatment 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 produce a typical Maggi taste.

(2) The film production of sulfur-based film is based on a "xanthate process". In this process, cellulose is converted by carbon disulfide into a water-soluble substance in an alkaline environment. In an acidic environment, water-soluble substances are converted back to cellulose while producing an odoriferous sulfur compound. Under inappropriate conditions, this sulphur odor is present in the final fiber sheet.

(3) Aging polyolefin films (PE, PP, OPP) will age during storage and release odorous compounds. A well-known aging process is a secondary-bound oxidation process that occurs in the base polymer and a common binder such as a complex of unsaturated fatty acids. Oxidative degradation also produces volatile aldehydes, ketones or small molecules of carbonic acid. In general, the use of antioxidants will prevent the occurrence of this chemical reaction and thus stabilize the film. However, if the wrong antioxidants are used, these antioxidants will produce odors themselves and the situation will become very complicated.

(4) Acetaldehyde odor of paper Due to the action of the compound, the paper or cardboard becomes dirty, resulting in a rancid odor. In the mixture of these odorous compounds, acetaldehyde (a volatile aldehyde) can be easily distinguished by analytical methods. These odorous compounds are the products of the oxidation of unsaturated fatty acids with microorganisms.

(5) Adhesives and Additives for Coated Films Many films used for packaging and printing are improved by coating to obtain printability, sealability, and barrier properties. These binders and additives are also one of the causes of the odor of the packaging material.

2. Printing inks and paints

The components of the paint include adhesives, additives, and solvents, and the ink contains more than one pigment. When choosing food packaging inks and lacquers, care should be taken to use odorless raw materials to ensure that compounds do not react chemically and produce harmful odors. In general, most of the ink solvents are volatile organic solvents. The most commonly used solvents in inks for gravure and flexo package printing are ethanol and ethane alkyl acetate. At the same time for special needs and adjust the special properties of the ink, such as drying speed, will add a small amount of ethoxy propanol, n-propanol, propanol, methyl ketone MEK), aliphatic hydrocarbons, pentyne-like or n- Pentylene and other substances.

Both the ink manufacturer and the printer must check all the solvents on a regular basis because even a slight amount of impurities can produce unacceptable odors. Printing workers often add a quantitative amount of solvent to the ink to change the composition of the ink so that printing tack and drying speed can be adjusted. The solvent used must meet the ink manufacturer's purity requirements.
Since the residual solvent of the packaging material is also one of the causes of the odor, it should be reduced as much as possible.

3. Adhesives

(1) Layered adhesives. The layered binders may be solvent-containing or solvent-free. Solvent-containing adhesives also have the problem of residual solvents. This determines whether the adhesive is the ink surface for the printing surface of the packaging material or the non-printing surface for the packaging material. If it is the former, the printed ink layer will chemically react with the wet adhesive. Moreover, the former will have more residual solvent than the latter.

(2) cold package adhesive agent. The most common cold sealant is a natural rubber with oxidizing properties, a liquid mixture of synthetic polymer dispersions and additives. However, copper or microbes can affect the oxidation properties. Therefore, it is necessary to use proper equipment and good management at the time of printing to avoid this.

three. remaining solvent

As mentioned earlier, residual solvents on the packaging material can produce unpleasant odors. Next, we will give a brief overview of the basic elements that affect residual solvents.

1. Basic elements

Assuming that the evaporation rate of diethyl ether is 1, the evaporation rate of the solvent at the same time relative to the diethyl ether is used as a reference value. The larger the value, the slower the evaporation. The evaporation rate of ethanol is 8.3, and the evaporation rate of ethyl acetate is 2.9. Seeing this set of data, some people may think that under the given ink composition, ethanol will produce less residual solvent than ethane-acetate. Practice has proved that this is not correct. Residual solvents are not only related to the composition of the ink, but also to the interaction between the primer, the ink and the paint, and even to the relationship between the solvent and the material. Please refer to the following table:
Residual solvent for ink (mg/m2)
material
EtOH
EtOH/EAC
Eac
Paper 35 35 20
Coex.BOPP 68 6
NC-Aluminim28 35 45
Polyamide 85 78 22
Cellophane (PVDC) 154752
(All data measured under laboratory conditions)

Regardless of the composition of the solvent, the residual solvent on Coex.BOPP is minimal. On NC-Aluminim (Nitro-cellulose-lacquered aluminum foil), the residual solvent of ethanol is less than that of ethane-acetate, because ethane-acetate is more decomposing to NC paint than ethanol; On paper and polyamide, ethanol has more residual solvents than ethane alkyl acetate. Also, in the above, the cellophane layer has a large amount of acetylacetonate, since ethanealkyl acetate is more easily soluble in cellophane coating. Because polyamide has a significant absorption effect on ethanol, there is a large amount of ethanol on the polyamide.

A lot of experience shows that residual solvents are closely related to the following processes and elements:
* The speed of solvent drying;
* Ink and paint composition;
* The interaction between primer, ink and spray paint;
* printing material;
* Printing viscosity;
* Printing speed;
* dry weight of the ink;
* Drying equipment (drying temperature, volume and disturbance of dry air)

2. Residual solvent control

Printers must use analytical methods (meteorological chromatography) to control residual solvents during the production of packaging materials. In order to meet customer quality requirements, it is often necessary to control the material of each reel. However, in practice, conventional CG UNIT cannot control each reel because there is usually not enough time to control each reel while printing. A new gas chromatography (GC) (for example, QFID100) has appeared on the market. It can achieve 8 measurements per hour, which greatly improves work efficiency.

four. Is residual solvent harmful to human health?

As mentioned at the beginning of this article, substances transferred from packaging materials to foods should not cause harm to human health. Below, we will illustrate this in detail through several examples.
A 1m2 wrapper can pack approximately 50g of 50 candy bars. If this wrapper contains 5 mg of residual ethanol and ethane alkyl acetate per square metre, and assuming 50% has been transferred to the food in the wrapper, then each candy bar will contain 0.05 mg of ethanol or B Alkyl acetate.

An apple contains 3-4 milligrams of organic esters, and it takes more than 60 candy bars to consume the same amount of ethyl acetate. If the old saying “One apple a day, healthy and healthy!” is correct, then even if a person eats a lot of candy bars every day, the residual solvents from the packaging paper will not affect health.

One piece of white bread (50 g) contained 100 mg of ethanol. If according to the previous algorithm, at least 2,000 candy bars must be eaten in order to consume the same amount of ethanol.