Polyamide film: a linear polymer containing many repeating amide groups (- CO-NH -) in the PA molecular chain segment, with a specific gravity of 1.14 g / cm3. Polyamide (PA) refers to the polymer with amide group in the main chain, which is usually called nylon.
At present, nylon films used in China are all made of nylon 6 obtained by ring opening polymerization of caprolactam. And nylon film on the production and molding process is divided into: salivation method (CPA), blowing tube membrane method (IPA), biaxial stretching method (BOPA). Drawing method can be divided into synchronous drawing and asynchronous drawing.
Nylon film is a kind of directional film, and its properties are as follows:
A. After longitudinal and transverse stretching, the polymer molecules have a high degree of orientation, so they lose the heat sealing property, such as BOPP, pet, BOPA, etc;
B. The crystallinity is increased, the physical and mechanical properties are greatly improved, and the transparency is also improved;
C. The initiation tear strength is large, but the secondary tear strength is greatly reduced.
1. Why is nylon film easy to absorb moisture?
Nylon is mainly made of dibasic acid, diamine or amino acid group by condensation polymerization. Because polyamide contains a wide range of amide groups, but in the production process, not all the molecules in the polyamide can crystallize and coordinate with molecules, and there are some amorphous polyamide polar genes. The amide groups in the amorphous polyamide molecular chain can coordinate with water, that is to say, it has water absorption, and can absorb water molecules with strong polarity. In serious cases, it will form a The barrier and air tightness of nylon film are greatly reduced by water film.
2. Why does nylon film sometimes produce color migration and color penetration after printing?
Color transfer and color penetration are two different concepts
1) The color transfer is mainly that the small molecules of pigment in the ink layer do intense Brownian motion under the action of a certain temperature, thus breaking away from the lattice formed by the weak intermolecular force, and following other organic molecules to do the migration movement together to disperse the color;
2) Color penetration refers to that the plastic printing substrate is superimposed on top and bottom, and the pattern printed on the upper substrate is printed on the surface of the lower substrate.
The main reason is: in the plastic film polymer printed with ink film, the polymer moves violently to form interstitial pores. When the temperature rises, the pores also expand and enlarge. At this time, pigment molecules also do intense Brownian motion, and they will diffuse and penetrate through the pores. In particular, blue lotus and peach red are more prone to color penetration.
The diffusion and penetration of pigment molecules mainly occurs in the amorphous region of the polymer chain of plastic film during thermal movement. Nylon film, non-linear polyethylene and other types of films belong to amorphous plastics. Due to their low crystallinity and large molecular gap, they are easy to thermal deformation, absorb large amount of water and hydrolyze when heated, especially in high temperature and humidity environment. Therefore, this type of plastic film is more prone to some color penetration phenomenon.
3. The peeling strength of the composite PE is good when it is off the machine, but it is easy to peel and tear after curing for 16 h?
There are several phenomena as follows:
1) Whether the corona surface treatment degree of raw materials (printing materials and heat sealing materials) reaches the standard (pa-52dny, pet-48dny, pe-40dny);
2) There are too many additives (additives) in printing and heat sealing materials, which make the composite layer degraded and easy to peel off;
3) Not enough glue. Generally, the blank small bag should be 2.5g/m2 below 200g, the ordinary bag should be 3G / m2, the boiling bag should be 4.5g/m2, and the cooking bag should be more than 5g / m2. Refer to GB / t10004-2008 standard;
4) The smooth dosage of PE film is too high, that is, the additive content is more than 500ppm, and the thickness of PE film is too thick (more than 60um). Too much additive addition causes low peeling degree;
5) The curing conditions did not meet the requirements or the curing time was too short. The complete crosslinking of different adhesives is different. In order to ensure the complete crosslinking of glue, different curing processes should be adopted according to different brands of glue and different uses;
6) Climate conditions. Generally, the humidity over 80% is very unfavorable for dry lamination, which should be controlled at 23 ℃± 3 ℃ and 65% ± 5% humidity;
7) The intrinsic quality of the adhesive. The first is the percentage content of curing agent. Generally, the percentage content of curing agent is less than 2%. The viscosity of prepared water will be greatly reduced. Secondly, the proportion of curing agent should not be too large. If it is too large, it will cause embrittlement and tear of the composite film.

(solventless compounding is very strict for curing process, to be discussed separately)

4. Why nylon film composite CPP, the finished product has sporadic foaming phenomenon?
This situation is mostly caused by glue bubble too high or nylon film damp, and the residual solvent in the composite layer is too much. In the humid climate of the south, this is a common problem in cooking and packaging. At this time, it is necessary to increase the circulation of glue in the glue tank to prevent the occurrence of bubbles, or to increase the drying temperature of printing and laminating machine, or to add no more than 5% curing agent in the glue. Moreover, when the weather is good (humidity is less than 70%), the printed nylon film should be compounded immediately or wrapped with aluminum foil, Do not use for too long (within 5min), in order to avoid the nylon membrane water absorption and moisture, and produce bubbles.
5. The reason why the nylon film is easy to peel off when it is used as the surface layer of composite water boiling?
Due to the water absorption of nylon, there are more problems in surface printing, composite boiling or cooking of nylon film, which seriously affects the peel strength. Therefore, the general water boiling adhesive dare not use the adhesive with resistance below 121 ℃. In the structure of BOPA / PE (115 ℃) and BOPA / CPP (21 ℃), only 135 ℃ cooking adhesive can be used, that is, the temperature resistance of the adhesive can be increased by one grade and the glue amount can be properly increased. It is better to use a waterproof coating to prevent moisture from entering the nylon membrane. Nylon film after cooking a small amount of yellowing phenomenon, will make the packaging bag color difference, should pay special attention to.
6. What are the reasons for the bubble formation after a period of time in combination with BOPA film but not in BOPP?
BOPA is a good barrier material, but in the process of printing and lamination, the residual solvent is too much, which can not be volatilized through the film after aging, but still remains in the film interlayer. This is due to the reaction of residual water with isocyanate group of curing agent in adhesive, resulting in the gas residue dominated by carbon dioxide. However, the barrier property of BOPP is weak, and some residual solvents in the film interlayer evaporate from the surface layer of BOPP by aging. Therefore, there are less bubbles in general BOPP film after compound aging.
7. The aluminum foil of OPA / Al / rcpp material combination cracks or even breaks at the sealing position?
Both BOPA film and aluminum foil are materials with certain ductility, but the phenomenon of cracking occurs at the moment of heat sealing when it is processed into packaging bag. Generally, the elongation of the package material imposed by the heat sealing knife has exceeded the acceptable level of the material, so the thermal seal layer is crushed. If it is automatic packaging double roller heat sealing, it should also consider whether the concave convex shape of the double roller is too sharp and piercing extrusion makes the aluminum foil layer break into black line cracking.
8. When BOPA / Al / PE (15.07.60um) composite is used, the edge of bag making heat sealing is easy to delamination?
Due to the high heat sealing temperature of bag making, the heating temperature of heat sealing position is generally increased by two-stage heat sealing. And in the heat sealing position is in the blank transparent state. Low molecular substances (such as smooth agent) in the film will permeate and float up to the surface at high temperature, which will degrade the adhesive and cause delamination. On the other hand, the oil contamination in the production process of Al is not completely removed, and a large heat separation distance occurs during heat sealing.
9. Why is there a nylon film combination, the printing ink containing jam bag is easy to fade?
Material combination b0pa / Al / LDPE or PA / CPE. In fact, no matter what kind of film combination, there are some phenomena. Discoloration caused mainly by contents. Pickle is a kind of food with various condiments in it. Its chemical composition is as complex as mustard bag, and many of them are high permeability chemicals. Therefore, LDPE or CPE should not be used in the inner layer. CCPP (salivating copolypropylene) with good cold resistance should be used, which can withstand 10 degrees below zero without embrittlement, so as to penetrate into the ink layer and have a smaller probability of chemical change.
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