The key problem of dry compound milk powder film processing
ã€Abstract】 This article briefly introduces and analyzes the PE film selection, compound strength and odor control in the dry composite milk powder film process .
Keywords: milk powder film  Three-layer coextrusion  Compound strength  Odor
I. Introduction
In recent years, with the improvement of people's living standards and the world's leading food companies continuously entering the Chinese market, China's dairy industry has shown a vigorous development trend, in which the consumption of high-quality, highly nutritious milk powder is increasing. Corresponding to this, dry composite milk powder film packaging, which has a tendency to replace iron cans, has gradually gained a higher and higher market share. This article briefly introduces and analyzes several common problems in the current market milk powder film composite packaging.
2. Craft route
(A) Material Design: PET12 μ m // AL7 μ m // PE70 μ m.
In this structure, PET belongs to a stressed layer and has high tensile strength. At the same time, its excellent transparency can fully display the exquisite printing. AL foil is a barrier layer of light, oxygen and water, which plays a key role in the quality and fragrance of milk powder. The inner PE film is a heat seal layer and a material that directly contacts the milk powder.
(Ii) Process: PET printing - and the surface of the composite light AL (again complex) - and PE film (composite two times) - aged - slitting - Factory
3. Several key issues
( 1 ) Selection of PE film
The widespread use of high-speed automatic packaging machines has put forward higher requirements for milk powder packaging films, especially the heat sealing performance, pollution resistance performance and coefficient of friction (COF) of PE films . Using three-layer co-extrusion technology to design different extruded layers can achieve the purpose of not only ensuring product performance requirements, but also effectively controlling costs. Can be designed as follows:
It should be pointed out that metallocene polyethylene (mPE) has undergone tremendous development after nearly 10 years of production and application and research and development by companies such as EXXON and DOW . Metallocene polyethylene (of mPE) relatively narrow molecular weight distribution, a weight-average molecular weight (M W) and number average molecular weight (M S) ratio of 2, is generally much lower than the PE 3-5, with Excellent low temperature heat sealing performance and high heat sealing strength. DOW also uses INSTTE technology to introduce a long branched structure in mPE , which greatly improves the processing performance, so that it has been widely used in high-end flexible plastic packaging products. The PE design in the above table can meet user requirements. .
( 2 ) Compound strength
In the PET12 μ m // AL7 μ m // PE70 μ m structure, the peeling force of each layer directly affects the sealing strength of the composite film. As mentioned above , after using the above PE film, as long as the corona of the PE film is guaranteed The strength is between 38-40dyn / cm , the sizing amount is 2.5-3.5g / m 2 , the curing temperature is 45-50 ℃, and the curing time is 48 hours, it can ensure that the composite strength between the AL layer and PE is no problem. However , the composite strength between PET and AL is a weak link. Experiments have shown that under normal process conditions, if the corona strength of PET is only 45dyn / cm , the ink layer will transfer to the AL surface, and the peeling force is extremely low. To this end, increase the PET corona strength to 50dyn / cm , can avoid the appearance of ink wind, the test result is not peelable (PET tear ) . In addition, the state of the adhesive layer cannot be ignored. The curing agent in the adhesive is mostly polyisocyanate, and its active group -NCO will react with -OH , -OOOH , -NH 2 , -RNH and other groups, thus affecting the curing effect of the adhesive. Therefore, it is necessary to strictly control the moisture content in the ethyl ester solvent and the cleaning of the container to prevent the adverse effects of the substances containing the above-mentioned active groups.
( 3 ) Odor
Milk powder products are very sensitive to the odor of packaging materials. Serious foreign odor will change the flavor of milk powder and even endanger the health of consumers. There are two main sources of milk powder odor: one is residual solvent, and the other is PE resin. The two issues are explained below.
1. Control of residual solvents
Residual solvents come from printing and compounding processes. At present, gravure printing basically uses benzene ink, and the solvents are toluene, ethyl ester and methyl ethyl ketone. In practice, we control from the following aspects and the effect is better.
1) Due to the heavy odor of methyl ethyl ketone, ink suppliers may be required to ban it.
2) Reduce the addition of slow drying solvents. The higher the ambient temperature, the faster the solvent volatilization rate. In order to prevent blockage, xylene, isopropyl alcohol and other solvents with slow volatilization rate are often added. Install air conditioners in the production workshop to control the ambient temperature between 20--30 ℃, basically no need to add such solvents.
3) Reasonably set the temperature and exhaust volume of the drying oven. The gradation version is set at 55--60 ℃, and the large-area full-page printing is set at 70-85 ℃. The inlet air pressure and exhaust air pressure are not as large as possible ( excessively causing ink layer scratches or broken film ) , but to balance the two. The general inlet air pressure and exhaust air pressure are set at 80-90mmHg.
4) Since the full version of the ink is often printed in the last unit, the number of drying ovens passing through the previous level is less, so under the permitted conditions, run 1-2 units and rewind after emptying to minimize the residual solvent . In the printing process, the total amount of residual solvent should be controlled below 3 mg / m 2 , with toluene below 2 mg / m 2 .
The compounding process generally uses a two-component polyurethane adhesive, and the solvent is ethyl ester. The method of residual solvent control is:
1) In the case of meeting the requirements, through the comparative test, select the adhesive with good release of ethyl acetate solvent, and then determine the supplier. This point is often overlooked and should attract the attention of printing manufacturers.
2) Control the amount of glue. Under the premise that the composite strength and performance meet the requirements, the amount of glue should be reduced. Compared with adding ethyl ester to dilute the adhesive, it is more effective to choose an appropriate screen roller to adjust the amount of glue.
3) Three-stage drying: the temperature from the inlet to the outlet of the drying box is set to three gradients of 60 , 70 , and 80 , and gradually increased. If the temperature of the first stage is too high, the surface layer of the adhesive will be dried to form a film, which is not conducive to the release of deep solvent.
4) Increase the exhaust air volume as much as possible.
5) Aging: This is not only the curing process of the adhesive, but also the process of reducing the residual solvent to a certain extent.
6) Packaging method: film roll packaging can use paper instead of the usual plastic. Due to the very high air permeability of the paper, the residual solvent will be continuously released during the transportation and storage process before use.
2, PE resin odor
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