Boiled bags with BOPA // LDPE structure are widely used in the packaging of pickles and bamboo shoots. BOPA // LDPE boiled bags actually have higher technical requirements. Although a certain scale of soft bag companies can make boiled bags, the quality is also uneven, and some will have more batch quality problem. Here, this article analyzes the key points in the production process of BOPA // LDPE boiled bags.

 

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1. Material selection

 

1. The choice of BOPA film

①The bow phenomenon of nylon film

BOPA film can be produced by tube film stretching method, or can also be produced by plane biaxial stretching method. The biaxially stretched nylon films produced by different methods have different bow effects, which have a significant impact on the film's printing overprint accuracy and the flatness of the packaging bag (including the appearance of the bag surface before and after cooking).

 

The specific method for detecting the bow effect of nylon film is to measure the thermal shrinkage of the diagonal. The detection condition of heat shrinkage in the national standard is that we can also detect the wet heat shrinkage rate of nylon film according to the actual use conditions of boiled bags (such as 100 ℃ 30min), the smaller the difference of the diagonal heat shrinkage rate, indicating The better the balance of the product; the difference is less than 1.5%, and there will be no warp angle during bag making.

 

② Market supply varieties

BOPA film is divided into two types: printing grade and composite grade. The printing grade can be used for printing and composite processes. The composite grade is only recommended for composite processes that do not require printing. The thickness is generally available in 12μm, 15μm and 25μm specifications. 15μm for flexible packaging composite film, 25μm for cold-formed aluminum pharmaceutical packaging. When used for intermediate layer compounding, and when used for boiling and cooking, double-sided corona film must be used.

 

③Key quality requirements of BOPA film

a. If the flatness is high, the synchronous stretch film with small bow effect should be selected.

b. The surface tension of the film is ≥50mN / m to ensure the fastness of ink adhesion. The processing value is not as large as possible.

c. Choose a film with good adaptability to relative humidity to ensure overprint accuracy.

d. Choose the film type with small heat shrinkage rate (wet heat shrinkage rate).

 

2. Selection of heat sealing layer PE

Boiled bag PE is different from ordinary PE: ① better heat seal strength; ② good heat sealability of inclusions; ③ better heat adhesion strength (not to break the bag when boiled); ④ good thickness uniformity → The quality of heat sealing is stable; ⑤Good transparency, no obvious water streaks; ⑥No fish eyes, impurities, crystal points that affect the use → Appearance bubbles, even piercing the PA film → Barrier decreases, or oils appear Leakage phenomenon. The quality characteristics of the first three points are mainly determined by the pellet formulation of each layer of the PE film during blow molding.

 

3. Selection of printing ink

Nylon film printing generally uses polyurethane special inks: ① no benzene and ketone series; ② no benzene and ketone series.

 

Note when choosing printing ink:

①Choose the resistance of different color models, such as F1200 red, 1500 red, F1150 red, F2610 gold red, F3700 orange, F4700 medium yellow and other color inks of a polyurethane ink, it is indicated in the manual that it cannot be used for BOPA / PE The structure of the boiled film printing, some colors are not resistant to boiling, when the water is boiled, it is easy to appear color bleeding.

 

②Gold and silver ink should be used with caution. For gold and silver inks, ink factories are not recommended for boiling purposes in the manual, but some of the boiled bags on the market use gold and silver colors. The general practice is to consult the ink factory for formula design before application. Note that large color blocks cannot be printed.

 

③ The nylon film should have good ink adhesion fastness, so as to ensure the final peel strength of the ink part.

 

4. The choice of adhesive

Choose an adhesive that is resistant to boiling and ensure the degree of crosslinking and curing after compounding. In addition, the old glue should be used with caution (generally should be avoided), because the effective ratio of the main agent and the curing agent group in the glue during the old glue has been out of balance, and the glue layer is prone to dry.

 

5. Quality requirements of ethyl acetate

China's "Industrial Ethyl Acetate" GB / T 3728-91 stipulates that the moisture content of premium products is less than 1000ppm, the moisture content of first-class products is less than 2000ppm, and the moisture content of qualified products is less than 4000ppm.

 

Moisture and alcohols in ethyl acetate (not only ethanol, but also the content of methanol and isopropanol should be controlled) will react with the curing agent in the glue and consume the curing agent, causing the glue layer to dry out. One of the main reasons for the wrinkle of the bag rubber layer.

 

 

Second, the gravure printing process

1. Selection of specific ink models

It is produced according to the type of ink specified in the process. As mentioned above, some inks of other colors are not suitable for BOPA // PE printing.

 

2. When reusing the old ink, more than 50% of the new ink needs to be added, and the modified ink should not be used.

 

3. If necessary, a certain proportion of curing agent can be added to the white ink

The purpose of adding a certain proportion of curing agent to the white ink is twofold: one is to improve the heat resistance of the ink; the other is to offset the consumption of the curing agent by the resin groups in the ink to avoid the incomplete curing of the adhesive layer in summer.

 

Adding method: first dilute with solvent, then slowly add it into the ink while stirring until it is evenly mixed.

 

Wrong method: add the curing agent directly into the ink, or add it to the ink tray, so that the mixing will be uneven, but it will not have the effect of adding the curing agent.

 

In addition, attention should be paid to the timeliness: the timeliness of the addition is generally 12 hours, and the overnight ink curing agent has failed, or a certain amount of curing agent should be added when it is used again.

 

4. Moisture-proof management of nylon membrane

Nylon absorbs moisture, so it is easy to appear lotus leaf, edging, stripes, coloring is not easy, and color registration is not accurate when printing.

 

It is best to control the temperature and humidity of the production workshop when printing. When the humidity of the production workshop exceeds 80%, the nylon film is easy to absorb moisture and rise, causing a series of quality problems in printed products.

 

Specifically, pay attention to the following issues: ① Do not open the package prematurely before use. ② Try to use it all at once, and the rest of the film should be wrapped with good barrier material. ③ The first color group is not pre-dried when printing, and it is pre-dried. ④Ensure reasonable temperature (25 ℃ ± 2 ℃) and humidity (≤80% RH) in the production workshop. ⑤ The printed nylon film should be packed with moisture-proof film.

 

 

Three, dry composite process

1. Selection of the amount of glue

Standard sizing amount range: 2.8 ～ 3.2gsm. Excessive sizing amount has no meaning for peel strength, but increases drying load. For compound equipment with insufficient drying capacity, it increases the probability of delamination and bag breaking after cooking.

 

When testing the amount of glue, pay special attention to the change in water content before and after the nylon film passes through the drying tunnel, which affects the accuracy of the detection of the amount of glue!

 

When we produce boiled bags, we can not only look at the amount of glue, but also pay attention to the microscopic uniformity of the adhesive coating. The parameters of the screen roller will directly affect the microscopic uniformity of the adhesive coating.

 

2. Moisture requirements of ethyl acetate

GB / T3728-2007 has clear requirements for the moisture content and ethanol content of ethyl acetate. The unqualified quality of ethyl acetate (such as moisture content and alcohol exceeding the standard) often leads to wrinkle quality accidents of the composite film.

 

Table 1 is the ethyl acetate moisture content test data of a flexible packaging factory. Among the 20 batches of incoming inspection, 4 batches exceed 500PPM (excellent products), one of which is unqualified, and the batch is in barrels (distributed Distributors) solvents, other batches are tank truck solvents. Most small flexible packaging companies purchase drummed solvents. The moisture content in ethyl acetate is likely to exceed the requirements of qualified products.

 

Table 1: The detection data of ethyl acetate water content is: PPM

 

(Note: Click on the picture to see a clear picture)

The alcohol content in ethyl acetate often does not attract the attention of most flexible packaging companies. One flexible packaging company's ethyl ester detection data (barreled solvent) found that only one superior product and two first-class products in 14 batches, overall The quality is poor and the soft pack factory should pay attention.

 

3. Ensure that the adhesive is dry enough

We usually only pay attention to the amount of compound glue. In fact, insufficient drying is often the most direct cause of incomplete curing of the adhesive (insufficient heat resistance of the adhesive layer), delamination and wrinkling quality of the packaging bag when it is boiled. There is a small amount of water and alcohol impurities in the prepared glue solution. Good drying property can make the water and other impurities in the glue layer volatilize as much as possible, reducing the consumption of the proportion of curing agent in the glue layer. In order to ensure that the adhesive is fully dried during dry compounding, pay attention to the following points:

 

(1) The drying performance of the equipment itself, such as the air intake and exhaust volume of the equipment and the length of the oven
 

(2) Setting of drying temperature.

①Set the drying temperature in the first zone. The drying medium in the first zone has the highest ethyl ester concentration, so the drying temperature in the first zone cannot be set too high (generally not higher than 65 ° C). Because the temperature is too high, the solvent on the surface of the adhesive layer volatilizes quickly and the skin is instead inhibited the escape of the inner layer solvent in the drying section of the latter areas.

②Dry temperature gradient setting. The temperature of the oven should be set according to the law of the gradual increase of the gradient, the purpose is to accelerate the diffusion and volatilization of the solvent of the adhesive layer in the hardening zone and the elimination of odor zone, and reduce the solvent residue in the film.

 

(3) Adjustment of air intake and exhaust.

① In the drying process, the intake and exhaust air volume valve should be opened to the maximum, and the return air valve should be closed.

②In the dry hardened area and odor elimination area, the return air volume can be increased appropriately, which can save part of the energy consumption.

 

4. The influence of environmental temperature and humidity

The high-temperature and high-humidity season in summer is a frequent time period for the quality accident of the dry-composite double-layer polyurethane adhesive dry layer. According to an adhesive factory, 95% of the quality feedback received in summer is not dry with the adhesive layer related. Under the environment of high temperature and high humidity, the water content in the air is very high, and it is easy to consume the curing agent through the volatilization of acetic acid and acetic acid into the rubber tray, which makes the ratio of the main agent of the adhesive and the curing agent unbalanced, resulting in the compound after curing. The cross-linking and curing of the layer is not complete, and the phenomenon of delamination and wrinkling occurs when boiled.

 

Flexible packaging enterprises that do not have conditions to control the temperature and humidity of the workshop should pay attention to the following points in the high-temperature and high-humidity season in summer:

① By detecting the temperature and humidity of the environment and the temperature above the rubber tray and the solvent tank, the "dew point" phenomenon is avoided. Once the "dew point" appears, it means that a large amount of moisture in the air enters the rubber tray, and the glue layer is prone to dry.

② The boiled bag should be compounded as far as possible in the high humidity period during production arrangement.

③ The ethyl acetate barrel and glue circulation barrel used for compounding should be covered and sealed. If a semi-closed rubber plate is used, the influence of environmental humidity can be further reduced.

 

5. Aging process requirements

General curing conditions: temperature 50 ～ 55 ℃, 48 hours.

 

In addition, we should also pay attention to the uniformity of the entire film roll curing: ① Show whether the temperature is consistent with the actual value near the film roll (the actual temperature of the film roll up, down, left and right); ② Whether the air near the film roll can achieve effective convection; ③ The surface of the roll Effect of temperature on curing: There is a certain process of heat transfer, and whether the curing conditions of the wound core composite film meet the requirements. (This may be a major cause of unstable quality.)

 

 

4. Bag making process

The heat sealing strength of the boiled bag is better, and the most important thing is to require the entire batch to be stable, such as: ① no local sealing failure; ② no individual sealing failure in the entire batch.

 

Pay attention to the following points when making boiled bags:

① Set a slightly higher heat sealing temperature to ensure the appearance of the sealing, to avoid the phenomenon of unstable heat sealing quality caused by the thickness deviation of the composite film.

 

② During normal production, the edge sealing shall ensure three effective heat sealing times. When the machine is restarted after shutdown, the parts that have been hot pressed once or twice may have cooled down when the machine is restarted (the first hot press can only play a role of preheating), and the actual number of effective heat seals is only twice Therefore, it is also necessary to set a higher heat-sealing temperature (the heat-sealing can be done well after two heat presses) to avoid a small number of poor sealing phenomena when restarting after shutdown.

 

③Most of the boiled bags are liquid packaging, which has high requirements on the drop resistance of the packaging bag. Avoid the undercut of the heat sealing edge when making the bag, especially the edge of the heat sealing knife should not be too sharp, and should be chamfered or polished properly .

 

 

V. Testing requirements

1. Representativeness of sampling

① When confirming the first sample, the number of continuous sampling should cover the length of all sealing knives in order to effectively avoid the failure of partial sealing failure.

②Sampling is to take samples after normal commissioning, adjust the heat sealing temperature, pressure, machine speed, and re-confirm after replacing the film roll.

 

2. The effectiveness of the heat seal strength detection and determination method

① The correct method is to cut the heat-sealing edge of the bag into a narrow strip of 20-30mm and tear it in the direction perpendicular to the sealing line.

②There should be no phenomenon that the width of the sealing edge can be torn apart with a width of more than 2mm. Otherwise, the strength is qualified when tested on the machine, but the heat sealing layers are not fully integrated, resulting in a decrease in the sealing strength during boiling and the phenomenon of broken bags when boiled. When the bag is detached from the interface between the two inner PEs on the sealing edge after boiling, it is a problem that the heat sealing edge is not strong.

 

3. Key points of boiled test

(1) Sampling method

① After the boiled packaging bag test machine is normal, the inspector will randomly draw several sample bags from each row in the test machine bag (the sampling number is required to cover the length of the vertical sealing knife) and fill the container with water for boiling test.

② When sampling more than two bags, use a marker to clearly mark the bag and the left and right directions in order to accurately determine the position of the sealing knife where the heat seal is not strong.

③ When there are obvious changes in the normal production process conditions, such as machine speed and temperature adjustment, etc., the sample must be re-sampled for the boiling test.

④After each shift is transferred, the boiler should be re-sampled for boil performance test.

⑤ Isolate the nonconforming products found in the process in time.

 

(2) Test conditions

① Put 1/3 ～ 1/2 volume of water in the packaging bag and exhaust as much as possible when sealing. If too much air is enclosed, it will easily lead to misjudgment. Cover the boiling test to slightly increase the pressure inside the bag.

②The boiling time is subject to the customer's use conditions, or according to relevant testing standards.

 

(3) Test eligibility criteria

① There is no overall or partial wrinkling and delamination on the surface of the bag; feel the peel strength after boiling.

②The printing ink has no discoloration or bleeding;

③ No leakage of bags or bags; no obvious edge running phenomenon at the sealing edge (the edge width should be controlled within 2mm).