Commonly encountered problems with nylon film packaging bags

Polyamide film: PA—A linear high polymer with many repeating amide groups (-CO-NH-) in the molecular chain segment, with a specific gravity of 1.14g/cm3. Polyamide (PA) refers to a polymer with amide groups in the main chain, commonly known as nylon (Nylon).

Currently, the nylon films used in China are all made of nylon 6 obtained by ring-opening polymerization of caprolactam, and belong to oriented films.

Nylon films are produced by different methods: cast film (CPA), blown film (IPA), and biaxially oriented film (BOPA). The stretching method is further divided into synchronous stretching and asynchronous stretching.

Nylon film is an oriented film, and the performance characteristics of oriented film are:

A. After longitudinal and transverse stretching, the polymer molecules are highly oriented, thus losing their heat-sealability, such as BOPP, PET, BOPA.

B. Increased crystallinity, greatly improved physical and mechanical properties, and improved transparency.

C. The initial tear strength is very high, but the secondary tear strength is greatly reduced.

1. Why is nylon film easy to absorb moisture?

Nylon is mainly made from dicarboxylic acids, diamines, or amino acids through polycondensation.

Because the polyamide molecule contains highly polar amide groups, but not all molecules in the polyamide can crystallize and coordinate molecules during production, and there are also some non-crystalline polyamide polar genes. This part of the non-crystalline polyamide molecular chain The amide groups can coordinate with water, that is, they have water absorption capacity and can absorb highly polar water molecules. In severe cases, a water film will be formed on the surface of the film, greatly reducing the barrier and airtightness of the nylon film.

2. Why does color migration and color penetration sometimes occur after printing on nylon film?

Color migration and color penetration are two different concepts:

1. Color migration is mainly due to the vigorous Brownian motion of small molecules of pigments in the ink layer under certain temperature conditions, thus detaching from the lattice formed by weak intermolecular forces, and migrating and dispersing colors together with other organic molecules.

2. Color penetration refers to the overlapping of plastic printing substrates, and the surface of the lower substrate is overprinted with the pattern printed on the upper substrate.

The main reason is: in the polymer of plastic film with printed ink film, the macromolecules move violently, forming porous gaps. When the temperature rises, the pores also expand and become larger.

At this time, the pigment molecules also undergo vigorous Brownian motion, and they will diffuse and penetrate through the pores. In particular, cyan, pink, etc. are more prone to color penetration.

The diffusion and penetration of pigment molecules mainly occur in the amorphous region during the thermal motion of the polymer chains of the plastic film. Nylon film and non-linear polyethylene films belong to amorphous plastics. Due to their low crystallinity and large intermolecular gaps, especially in high-temperature and high-humidity environments, they are prone to thermal deformation, high water absorption, and hydrolysis when encountering hot water.

Therefore, this type of plastic film is more prone to penetration of certain colors.

3. The peeling strength of nylon film laminated with PE is acceptable when it comes off the machine, but the reason for peeling and tearing after curing for 16h?

There are the following phenomena:

1. Whether the corona treatment of raw materials (printing materials and heat-sealing materials) meets the standard (PA—52dny, PET—48dny, PE—40dny).

2. Too many additives (auxiliaries) in printing and heat-sealing materials float up after heating, causing the composite layer to degrade and easily peel off.

3. Insufficient adhesive application.

Generally, for small bags below 200g, there should be 2.5g/㎡, ordinary bags should reach 3g/㎡, boiled bags should reach 4.5g/㎡, and steamed bags should be greater than 5g/㎡.

(Refers to the amount of dry adhesive) Refer to GB/T10004---2008 standard.

4. The amount of release agent in the PE film is too high, that is, the additive content is above 500PPm, and the film thickness is too thick (above 60um), and too much additive is added, resulting in low peeling strength.

5. The curing conditions do not meet the requirements, or the curing time is too short.

The complete crosslinking of various different adhesives is different. To ensure the complete crosslinking of the adhesive, different curing processes should be used according to different grades of adhesive and different uses.

6. Climatic conditions.

Generally, humidity exceeding 80% is very unfavorable for dry lamination, and it should generally be controlled at 23℃±3℃ and humidity 65%±5%.

7. The intrinsic quality of the adhesive.

First of all, the percentage content of the curing agent. Generally, if the percentage content of the curing agent is less than 2%, the viscosity of the water will be greatly reduced. Secondly, the proportion of the curing agent cannot be too large, otherwise it will cause the composite film to become brittle and tear.

4. Why does the finished product have sporadic bubbling when nylon film is compounded with CPP?

This situation is mostly caused by excessive adhesive bubbles or moisture absorption of the nylon film, as well as too much residual solvent in the composite layer. In the humid climate of southern China, this is a common problem in cooking packaging. At this time, the circulation of the adhesive in the adhesive tank should be increased to control the occurrence of bubbles, or the drying temperature of the printing and laminating machine should be increased, or less than 5% curing agent should be added to the adhesive. Moreover, when the weather is good (humidity is less than 70%), the printed nylon film should be laminated immediately, or wrapped with aluminum foil paper, and should not be left for too long (within 5 minutes) to prevent the nylon film from absorbing water and becoming damp, resulting in bubbles.

5. Reasons for delamination when using nylon film as the surface layer for composite materials after boiling?

Due to the water absorption of nylon, nylon film is more problematic for surface printing and composite boiling or steaming, seriously affecting the peel strength. Therefore, generally, water-boiled adhesives dare not use adhesives that are resistant to temperatures below 121℃.

In the BOPA/PE (115℃) and BOPA/CPP 121℃ structures, only boiling adhesives resistant to 135℃ can be used, that is, the adhesive temperature resistance is increased by one level, and the amount of adhesive is appropriately increased.

It is best to use a waterproof coating to prevent water from entering the nylon film. The nylon film turns slightly yellow after boiling, which may cause color difference in the packaging bag, so special attention should be paid.

6. Why do bubbles form in composite materials with BOPA film after a period of time, while BOPP does not?

BOPA is a material with good barrier properties. However, there is more residual solvent in the printing and compounding process. After curing, it cannot penetrate the film and volatilize, but remains in the film interlayer. This is because the residual water reacts with the isocyanate groups of the curing agent in the adhesive, generating gas mainly composed of carbon dioxide.

BOPP has weaker barrier properties. Through curing, some of the solvent remaining in the film interlayer volatilizes from the surface of the BOPP. Therefore, generally, less bubbles are produced after BOPP film composite curing.

7. Cracks or even breaks in the aluminum foil at the sealing area of OPA/AL/RCPP material combination?

BOPA film and aluminum foil are materials with certain ductility. However, cracks appear during the heat sealing moment when processing into packaging bags. Generally, the extensibility applied to the packaging material by the heat sealing knife has exceeded the material's tolerance, resulting in the heat sealing layer being broken.

If it is automatic packaging double-roller heat sealing, the concave-convex shape of the double rollers should also be considered. If it is too sharp, it may pierce and squeeze, causing the aluminum foil layer to crack into black lines.

8. Delamination is prone to occur at the heat-sealed edges when making bags using BOPA/AL/PE (15.07.60μm) composite materials?

Because the bag-making heat sealing temperature is high, and generally, a second heat sealing increases the heat-receiving temperature of the heat sealing area. The heat sealing position is in a blank transparent state. Low molecular weight substances (lubricants, etc.) in the film will penetrate and float to the surface at high temperatures, causing the adhesive to degrade and delaminate.

On the other hand, if the oil stains on the AL are not thoroughly cleaned during production, a large hot inter-peel distance will occur during heat sealing, resulting in delamination.

9. Why does the printing ink on the packaging bag containing pickles easily fade when using a composite material containing nylon film?

Material combination BOPA/AL/LDPE or PA/CPE. In fact, this phenomenon occurs with any film combination. It is mainly caused by the contents.

Pickles are foods containing various seasonings in a solid-liquid coexistence state. Their chemical components are as complex as those of pickled vegetables, and many are highly permeable chemical substances.

Therefore, LDPE or CPE cannot be used for the inner layer. It is best to use CCPP (flowable polypropylene) with good cold resistance, which can withstand -10 degrees without becoming brittle, so the probability of penetrating the ink layer and causing chemical changes is smaller.

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