I.Definition of Flexible Packaging Film
In the national general terminology of packaging (GB4122-83), the definition of flexible packaging is: flexible packaging refers to the packaging that can change the shape of the container after filling or taking out the contents. All kinds of bags, boxes, sleeves, encapsulations, etc. made of paper, aluminum foil, fiber, plastic film and their compounds are flexible packaging. In general, sheet plastics with a thickness of 0.25 mm or less are called films. The plastic film is transparent, flexible, has good water resistance, moisture resistance and gas resistance, good mechanical strength, stable chemical properties, grease resistance, easy to print exquisite graphics and texts, and can be heat-sealed to make bags. It can meet the packaging requirements of various items, and is an ideal material for packaging easy-to-store and easy-to-put convenience foods, daily necessities, and small packaging goods in supermarkets.
Flexible packaging printing based on plastic film occupies an important position in packaging printing. According to statistics, since 1980, plastic packaging in some advanced countries in the world has accounted for 32.5%~44% of the entire packaging printing.
Generally speaking, because the protection of the contents of a single film material is not ideal, a composite film that combines more than two kinds of films into one layer is mostly used to meet the requirements of food preservation and aseptic packaging technology. The outer layer of the composite film is not easy to scratch, sanding, excellent optical properties, good printing performance, such as: paper, cellophane, stretched polypropylene, polyester, etc., the middle layer is a barrier polymer, such as: aluminum foil, evaporated aluminum, polyap diazoethylene electric inner layer materials are mostly non-toxic, odorless polyethylene and other thermoplastic resins.
II.Introduction of Plastic Permeability Technology
1. What is the impermeability of plastics?
Plastic products (containers, films) can shield small molecule gases, liquids, water vapor and odors.
  2. Transmittance coefficient?
 An indicator of the size of the plastic's permeability capacity.
Definition:
Plastic products of a certain thickness (1mm), under a certain pressure (1Mpa), a certain temperature (23 degrees), a certain humidity (65%), a unit time (1day=24 hours), a unit area (1m2), through the volume or weight of small molecule substances (O2, CO2, H2O). Expressed as (cm3), (g)
For gases:
The unit is cm3, mm/m2, d, mpa;
For liquids:
The units are g,mm/m2,d,mpa;
3.The permeability coefficient of commonly used medium and high resistance permeable：

plastics: plastic variety/coefficient O2 CO2 H2O	O2	CO2	H2O
EVOH（PE29%）	0.1	1.5	20-25
EVOH（PE38%）	0.4	6	40-70
PVDC	0.5-4	1.-2	0.2-6
MXD6	2-5	28	15-30
PAN	8	16	50
PEN	12-22	50	5-9
PA666	15-30	50-70	100
PA6	25-40	150-200	150
PET	49-90	180	180-300
HDPE	2500
PP	3000
LDPE	10000
EVA	18000

 
4. Explanation of terms
EVOH: (EVAL) ethylene-vinyl alcohol copolymer
PA6—666: Polyamide (Nylon)
MXD6: Aromatic nylon, m-phthalenedimethylamine and adipic acid segregator
PVDC: Polyvinylidene chloride
PAN: Polyacrylonitrile
PEN: polynaphthalene glycol
PET: polyethylene terephthalate
PP: Polypropylene
PE: Polyethylene
PS: Styrene
HDPE: High Density Polyethylene
LDPE：low-density polyethylene
LLDPE Linear Low Density Polyethylene
MLLDPE: metallocene linear low-density polyethylene
IONOMER: Ionic Bond Polymer
EVA: ethylene-vinyl acetate copolymer
TIE: Titanium, resin binder
AC: Polyethylenimine AC: Ethanol: Water =1:6:3
PU: Polyurethane PU: Curing agent: Acetone = 7:1:45
PVA: Polyvinyl alcohol
Among them: high resistance permeability materials: EVOH, PVDC, PAN, MXD6;
                      Medium resistance permeability materials: PA, PEN, PET

5. What is the plastic penetration method?
  There are two kinds of layered blending molding and composite molding resistance
Layered blending is BOPA, BOPET, etc.;
Composite molding penetration resistance is further divided into:
Dry composite penetration resistance and melt co-extrusion composite penetration resistance
Dry composite permeability resistance is typically laminated composite molding of paper, CPP, BOPP, PET, aluminum-platinum, or aluminized film, such as milk packaging roof boxes, pillow bags, etc.
Co-extrusion composite resistance is made of a variety of plastic materials melted and co-extruded compound.
6. Impermeability formula of multilayer composite materials:
  1/P = T1/P1 + T2/P2 +T3/P3......
  where: T1, T2 ----- the thickness of each composite layer
  P1, P2 ----- each transmittance coefficient
 P: The total transmittance coefficient of the composite material, the more layers and the thicker, the better the permeability resistance, and the good overall permeability.
7. How to multi-layer co-extrusion compounding?
  1] General resin and general resin composite ------ suitable for packaging of soy sauce, vinegar and other packaging with weak impermeability.
Commonly used resins are:
  LDPE, HDPE, LLDPE, PP, PS, PVC, etc.
2], the composite of general resin and medium resistance resin ****
The outer layer is general resin, the inner layer is medium permeability resistance resin, and 3-7 layers of composite permeability resistance.
  LLDPE, LDPE, HDPE, PP, PS, PVC, + PA, PET, EVA, etc
 3], the composite of medium resistance resin and medium resistance resin*****
  MXD6, PA6, PEN, PET, EVA, PA666, good compatibility does not require adhesive resin, usually 2-3 layer structure composite molding.
4], composite molding of general resin and high resistance permeability resin********
  Generally, the outer layer is: LDPE, HDPE, PP, PS, LLDPE, etc
  The inner layer is: EVOH, MXD6, PVDC, etc., which need to be added with adhesive resin.
 5], the composite molding of medium resistance permeability resin and high resistance permeability resin
    Medium resins are: PET, PA, EVA, etc
    High resistance resins: EVOH, MXD6, PVDC, PAN, etc. do not need to be bonded resins.
8. What are the more common permeability composite methods at home and abroad?
  abroad：PE//TIE//EVOH//TIE//PE
  PE（EVA）//TIE/PA/EVOH/PA/TIE/PE
  Domestic: PP/TIE/PA/TIE/PP，
LLDPE/TIE/PA/TIE/LDPE，
MLLDPE/TIE/EVOH/TIE/MLLDPE，
Mlip/Iron/Fear/Iron/Mlip and so on.
10. What are the standards for medium and high barrier composite materials?
High barrier standard: O2 permeability is less than 5 ml/m2, d, moisture permeability is 2g/m2 or less, d.
Medium and high resistance permeability composites are
O2：5-10ml/m2,d
H2O：10g/m2,d.
11. Physical properties and mechanical properties of aseptic packaging
1], the O2 transmittance is ≤1 ml
2], tensile strength of ≥30Mpa
3], the elongation at break is ≥40
4], the peeling force is ≥1
5], the heat sealing strength is ≥40
6], the light transmittance is ≤0.4
7], the pressure resistance of the bag is as shown in the table:

The capacity of the bag	Internal Pressure Hold (2mm)		Rupture pressure （K pa）
	Holding Pressure (kPa)	Requirements
≤25L	20	impermeability, non-rupture	≥25
≥25L	10	impermeability, non-rupture	≥15

 
8], the drop performance of the bag is as shown in the table:

The capacity of the bag	Drop height H	Requirements
≤5L	0.8	impermeability, non-rupture
≤25L	0.6	impermeability, non-rupture
≥25L	0.4	impermeability, non-rupture

III.Composite Packaging Materials
On the basis of the development of the packaging industry, the packaging of items has also been developed accordingly. From simple paper packaging, to single-layer plastic film packaging, to the widespread use of composite materials. The composite film can make the packaging contents have the characteristics of moisturizing, fragrance retention, beauty, freshness, light, anti-penetration, and extending the shelf life, so it has developed rapidly.   
Composite materials are two or more materials that are combined together through one or more composite processes to form a composite material with a certain function. Generally, it can be divided into base layer, functional layer and heat sealing layer. The base layer mainly plays the role of beauty, printing, and moisture resistance. Such as BOPP, BOPET, BOPA, MT, KOP, KPET, etc.; the functional layer mainly plays the role of blocking, light-proofing, etc., such as VMPET, AL, EVOH, PVDC, etc.; the heat sealing layer is in direct contact with the packaging items, and plays the functions of adaptability, permeation resistance, good heat sealability, and transparency, day opening, etc., such as LDPE, LLDPE, MLLDPE, CPP, VMCPP, EVA, EAA, E-MAA, EMA, EBA, etc. The following is a brief description of the development, development and current situation of the inner material of composite flexible packaging materials.                 
 LDPE, LLDPE resins and membranes
China's composite film started from the end of the seventies, and from the early to mid-eighties, China began to introduce some extruders, blown film machines and printing presses to produce simple two-layer or book-layer composite materials. Such as extrusion composite BOPP/PE, paper/PE, pp/PE, dry composite BOPP/PE, PET/PE, BOPP/AL/PE, PET/AL/PE, etc., in which LDPE resin and film are often blended with a certain proportion of LLDPE to enhance its strength and stiffness. It is mainly used in the packaging of instant noodles, biscuits, mustard and other foods. Generally, the coating grade LDPE resin is IC7A, L420, 19N430, 7500, etc., the blown film grade LDPE resin is Q200, Q281, F210-6, 0274, etc., and the LLDPE resin is 218w, 218F, FD21H, etc.
CPP membrane, CPE membrane
From the late eighties to the early nineties, with the introduction of a new generation of flexible packaging equipment and casting equipment, the scope of packaging contents has been further expanded, and the transparency requirements of some puffed food, cereal and other packaging bags are higher, and boiling, high-temperature sterilization products have been introduced to the market, and the requirements for packaging materials have also increased accordingly, and the inner materials based on LDPE and LLDPE have been unable to meet the requirements of the above products. It is produced by the casting method and has good heat sealability. CPP with oil resistance, transparency, fragrance retention, and special low-humidity heat sealability and high-temperature cooking properties are widely used in packaging. Aluminized CPP, which was developed on this basis, is rapidly and widely used due to its metallic luster, beauty, and barrier properties. and CPE films produced by the cast method, which are further used due to their unidirectional tearability, low-temperature heat sealing, and good transparency.
MLLDPE resin
With the continuous development and change of the packaging market, there are more and more special requirements for packaging. DOW Chemical Company of the United States produced metallocene polyethylene MLLDPE by metallocene catalytic polymerization. Such as APFINITY, POP1880, 1881, 1840, 1450 and other resins. And then Exxon in the United States. Japan's Mitsui and the U.S.'s Phillips also produce MLLDPE. Such as Exxon's EX-CEED350D60, 350D65, Mitsui Petrochemical's E-VOLVE SP0540, SP2520, Philips' MPACT D143, D139, etc. Due to the good blendability and easy processing of MLLDPE with LDPE and LLDPE, MLLDPE can be mixed in blown film or cast processing, and the mixing ratio can be from 20% to 70%. This kind of film has good tensile strength, impact strength, good transparency, good low-temperature heat sealing and anti-pollution, and is widely used in frozen, refrigerated food, shampoo, oil, vinegar, soy sauce, detergent, etc. It can solve the packaging speed, broken package, leakage and penetration of the above products in the process of packaging production and transportation.
The inner material of the lidium film
Jelly, juice, yogurt, fruit milk, soup and other liquid packaging cups. bottle, the main material of which is HDPE. PP, PS, etc. The lid film of this packaging should not only consider the shelf life, but also consider the heat sealing strength between the lid film and the cup, and also consider the convenience of consumers - easy tearability. To achieve this particularity, the inner layer of the material can only form an interfacial bond strength with the cup, but cannot completely penetrate and fuse together. Generally, modified EVA resin is used. Such as the DuPont APPEEL53007 in the United States. TOPCO L-3388 FROM TOYO JAPAN, LOTRYL 20MA 08 FROM FRANCE, WT231 FROM HIRO.DINE FROM JAPAN, ETC. Its structure can be: PET/PE/HM, BOPA/PE/HM, PET/VMPET/PE/HM, PET/AL/PE/HM, paper/PE/AL/PE/HM. For those who require the lid film and the bottom cup to be firmly covered and not torn, it is generally required that the material of the bottom cup is consistent with the inner material of the lid film, so that the two materials can be completely fused when they are heat sealed. For example, HDPE cups, the inner material of the cap film is: LDPE or EAA, PP cups, the inner layer of the cap film is CPP film, PET bottles, we have found a kind of coating modified PET film as a heat sealing layer, capping pesticide PET bottles, and achieved satisfactory results.
Co-extruded film
The film produced by single-layer LDPE or LDPE blended with other resins has a single performance and cannot meet the requirements of modern product development for packaging, so the co-extrusion film produced by co-extrusion blown film or co-extrusion casting equipment has improved its comprehensive performance. For example, the comprehensive properties of the film, such as mechanical strength, heat sealing performance, heat sealing temperature, barrier, opening, and pollution resistance, are improved, and its processing cost is reduced, and it is widely used. For example, the hot melt adhesive film of H-layer co-extrusion blown film: PE/HM, the low-temperature heat sealing film of cable film PE/EAA, MLLDPE, the cover film of EVA, the antistatic film, and the slip film. H-layer co-extrusion cast co-extrusion CPP, unmodified PP/heat-sealable PP, etc. Three-layer and five-layer nylon co-extrusion films, five-layer and seven-layer EVOH, and PVDC high-barrier films are also developing and widely used.
The development of the inner film of composite flexible packaging materials, from LDPE, LLDPE, CPP, MLLDPE, to the current mass use of co-extrusion film, basically realizes the functionalization and personalization of packaging, and meets the conditions of packaging content shelf, processing performance, transportation and storage. With the continuous introduction of new materials and the improvement of inner film production technology and equipment, the inner film of composite flexible packaging materials will surely develop rapidly and promote food.　　Miscellaneous With the progress of society, human demand continues to grow, and various functional and environmentally friendly packaging films continue to appear. For example, water-soluble polyvinyl alcohol films, which are environmentally friendly, safe, fully degraded, and have good heat sealing properties, are being developed as inner films in addition to single-layer packaging materials.

IV.Multi-Layer Composite Technology
Multi-layer composite technology is to use materials with medium and high barrier properties to composite with other low-cost packaging materials, and integrate the high barrier properties of barrier materials with other materials such as cheap or special mechanical, thermal and other properties. Different combinations of multi-layer composite films can meet different requirements. Multi-layer lamination technology mainly includes multi-layer dry lamination and multi-layer co-extrusion lamination.
(1) Multi-layer dry composite
Multi-layer dry composite technology was first used in the production of retorted food packaging, such as HDPE (PP) / EVOH / HDPE (PP), its structure is often the outer layer of BOPP, BOPET, the middle barrier layer can be PA, PVDC, EVOH or aluminum foil, the inner heat sealing layer is generally chlorinated polypropylene (CPP), if there is no need for high temperature resistance, PE can also be used, and each other can be bonded with adhesives. Its barrier properties are mainly related to barrier films and adhesives.　　In recent years, many new barrier substrates, such as MXD6 special nylon film and silicon oxide coated film, have excellent barrier performance, and can be reversed printing with exquisite printing quality. However, due to the need for overmolding and the high cost of adhesives used, people are gradually tending to apply multi-layer co-extrusion compounds.
(2) Multi-layer co-extrusion compound
Multi-layer co-extrusion composite is to combine two or more materials in a molten state in a die head. The basic resins of co-extrusion composite are generally HDPE, PP and other resins, and the barrier resins are mainly PA, EVOH, PVDC, etc. Since the compatibility of barrier materials and heat sealing materials is generally poor, it is important to consider the selection of a good compatibilizer, such as acrylic copolymer resins. The barrier resin requires good processing performance to meet the needs of the co-extrusion composite head with good fluidity, and the fluidity is too poor or the fluidity difference between several resins is too large, which will reduce the barrier performance of the composite film due to the formation of laminar flow. Generally speaking, the co-extrusion composite is designed according to the symmetrical design of ABCBA five-layer and ABCDCBA seven-layer structure, and its barrier and composite strength are the best.　　　
Compared with dry compounding, multi-layer co-extrusion composite technology started late, but it has the advantages of saving raw materials, diversifying raw materials, adapting to environmental protection requirements, and not using toxic adhesives. And the barrier effect is very ideal, and the effect gets better as the number of composite layers increases. At present, the number of composite layers has developed to nine layers, or even eleven layers, which has developed rapidly and has been applied to packaging films and hollow containers. However, the co-extrusion composite method has very strict requirements for the process and equipment, requires high worker quality and more sophisticated machinery and equipment, and the equipment is expensive, and the waste recovery rate is low, so it greatly limits its large-scale use.
A. Classification according to composite process.
At present, composite materials for flexible packaging are classified according to processing methods, mainly as follows:　　
① Dry-coated organic solvent adhesive composite soft packaging material;　　　　
② Wet-coated inorganic adhesive composite soft packaging materials;　　　　
③ Extruded film laminated composite packaging materials;　　　　
④ Multi-layer co-extrusion (by more than two extrusion heads) composite soft packaging materials;　　　　
⑤ Hot melt coating composite soft packaging materials;　　　　
⑥Solvent-free (i.e., solid adhesive) composite soft packaging materials;　　　　
⑦ Physical vapor deposition composite soft packaging materials;　　　　
⑧ Physical vapor deposition composite soft packaging materials;　　　　
⑨ Mixed composite soft packaging materials;　　　　
⑩ The aforementioned methods combine composite soft packaging materials, etc.
B. Classification according to composite materials.
Keeling, which is used as a flexible packaging compound, is mainly plastic film, followed by paper, metal foils and composites.
They can be used to manufacture the following types of composite packaging materials:　　　　
① paper and paper;
② Paper and metal foil;　　　　
③ Paper and plastic film;
④ plastic film and plastic film;　　　　
⑤ Plastic film and metal foil;　　　　
⑥ Plastic film and metal film;　　　　
⑦ Plastic film and matrix material;　　　　
⑧ Plastic film and inorganic compound film;　　　　
⑨ plastic film and organic compound film;　　　　
⑩ Amorphous plastic film and plastic film;　　　　　　
(11) Substrate and nano-type ultra-fine particle composite;　　　　
(12) Special additives for substrate coating or impregnation;　　　
(13) Multi-layer composite of composite materials, etc.
C. Classification according to composite form.
In general, material compounding is mostly carried out evenly across the entire surface.
The composite forms of packaging materials mainly include the following categories:　　
① Large-area (uniform) compounding;　　　　
②Local compounding, such as compounding and strengthening only in the packaging sealing part or strengthening part or giving certain functions;　　　　
③ Composite pieces or composite bags, which integrate special functions in one place and play an overall function;　　　　
④Gradual lamination (or non-uniform lamination), changing the concentration, composition or thickness of the added material in the thickness direction (or length direction) of the composite film as needed;　　　　
⑤ Special composite, such as reticulated pattern composite, etc。
The 3rd, 4th, and 5th are green process technologies.
D. Classification according to composite functions.
For the packaging industry, due to the different requirements of various industrial sectors for product packaging, different systems have different classifications, and there is no unified and standardized terminology at present.
According to the new packaging system advocated by Europe and the United States recently, it can be divided into the following 10 categories.　　
① Enhanced composite packaging;　　　　
② High impermeability composite packaging (gas barrier, water barrier, oil barrier);　　　　
③ Anti-corrosion (anti-corrosion and anti-rust) composite packaging;　　　　
④Anti-electromagnetic field (dry worry) composite packaging;　　　　
⑤ Antistatic composite packaging;　　　　
⑥ Biological composite packaging (fruit heat promotion, fish preservation, insect prevention, mildew prevention);　　　　
⑦Fresh-keeping composite packaging (for fruits, vegetables and meat products);　　　　
⑧Cooking and composite packaging (such as steaming, microwave baking, etc.);　　　　
⑨ Intelligent composite packaging;
⑩ Ultra-micro and nano composite packaging.

V.Multi-Layer Co-Extrusion Composite High Barrier Film
At present, the co-extruded high-barrier blown film composed of PA, EVOH, PVDC and PE, EVA, PP and other resins is popular in the functional packaging film market because of its reasonable, economical and reliable performance. In particular, some asymmetrical structure of multi-layer co-extrusion blown film with excellent composite peel strength, outstanding barrier, superior environmental resistance and chemical resistance, cheap processability, suitable secondary processing to replace many dry composite as the main packaging market, or simplify the dry lamination process, multi-layer co-extrusion composite film is favored by the market because of its no residual solvent pollution.
In addition to the rapid development of resins, the technological progress of additive multi-layer co-extrusion dies and the continuous improvement of plastic molding processing technology have accelerated the pace of continuous development of the plastic composite film processing industry to adapt to the social environment and technological changes. And the co-extrusion composite film has been pushed to a new period of development, which provides a broad development space for the improvement of the quality level of composite packaging film, especially packaging substrate film.
 
 1. The status quo and requirements of multi-layer co-extrusion composite films
目At present, the domestic production of multi-layer composite films usually adopts the method of composite of plastic films with different functions through bonding. Generally speaking, each additional function requires the addition of a film substrate and two additional production steps. During this period, not only the cost is increased, but also the contamination (in the case of lamination with solvent-based adhesives) and even the overfunction of the film due to the limitations of the production process. As a result, production enterprises have paid a heavy price for the increase in production costs, the decline in profits, and the weakening of the competitiveness of the product market.
The diversification of packaging functions, the rationalization of packaging structure, and the maximization of packaging benefits are the topics of research and development of composite film manufacturers, especially composite film substrates, in recent years.
1) Diversification of packaging functions　　　　
With the increasing demand for packaging functions in the market, the functions of multi-layer composite films are becoming more comprehensive, and asymmetric multi-layer co-extrusion blown films are affected by their fatal stress warping and difficult to improve the optical properties of the films, which affects the further development of their diversified functions. Especially in China, the existing secondary processing equipment on the market has special requirements for the asymmetry of the material, therefore, it cannot effectively improve the stress warpage and optical properties, which has reached the point that hinders the rapid development of multi-layer co-extrusion blown film.
2) Rationalization of packaging structure　　　　
The environment and safety are changing and guiding the development of the market in many ways, and at the same time, in order to protect the earth's resources and ensure the safety of people's lives. This requires a more reasonable packaging structure, which is embodied in the requirement that the composite film has a better structure (protection function), simpler (easy to use, easy to transport), safer (hygiene, environmental adaptability), and higher social and environmental adaptability (waste recycling, recyclability).
3) Maximize the benefits of packaging　　　
The maximization of packaging benefits is the goal pursued by composite film manufacturers in close cooperation with customers. Comprehensively improve product quality, maximize benefits through the increasing quantification of composite film functions, reduction of production processes, thinning of thickness, and lower cost (high productivity, saving manpower, saving resources, and saving energy).　　　
  From a technical point of view, what can meet the above three requirements at the same time and have formed industrial production is the development and application of co-extrusion technology. The development of co-extruded multilayer composite film is inseparable from the close cooperation and interaction of equipment, raw materials and processing technology. Only when the technology of processing equipment, processing raw materials, and processing technology reaches the best state through the cooperation, can the most ideal product be obtained.　　
 Compared with single-layer plastic substrate films, multi-layer co-extrusion composite films greatly simplify the production process of dry composite films, increase functions, and can be quantified through effective adjustment of thickness, convenient and flexible structural combination, and a wide range of materials. The comprehensive performance is full of functions and flexible changes, low cost and high quality level, high added value and strong market adaptability. Therefore, multi-layer co-extrusion composite films have been widely used in packaging.
 
2. The structure and development trend of multi-layer co-extrusion composite film　　　　
In general, the structure of a multilayer co-extruded composite film depends on the functional requirements of the film. Under the premise of meeting the process requirements, through the combination of different polymers, the functional requirements of packaging materials such as barrier, heat sealing, body strength, puncture resistance, environmental adaptability, secondary processing characteristics, and extended storage and shelf life are met. From the analysis of functional requirements, the combination of five polymers is sufficient. However, the application of seven-layer, nine-layer, ten-layer or even more layers of co-extrusion composite film has begun to be applied in the market, making it a trend and developing rapidly. The structural design of co-extruded composite film is gradually required to systematically achieve the ideal realm of integrating function, technology, cost, environmental protection, safety and secondary processing.
1], barrier
(1) Replacing a single layer of polymer with multiple layers of the same polymer in the barrier layer can improve the stability of the barrier layer. For example, if the PA material is set as a barrier layer, the oxygen barrier rate is 40 units. In order to ensure the stability of its oxygen resistance, usually the set value of its thickness is the theoretical calculated value of the material + the negative error value of the equipment + the safety factor. When we use multiple layers of the same polymer instead, the negative error value of the equipment of its thickness is significantly reduced, and the safety factor is significantly improved.　　　
(2) When we confirm that the thickness of the barrier layer is sufficient to meet the barrier requirements, we inject multiple layers of the same polymer into the barrier layer instead of a single layer of polymer, which can reduce the cost of high value-added raw materials for the barrier film.
(3) Replacing a single polymer with two different polymers in the barrier layer can significantly improve the barrier property of the film. For example, combining the EVOH layer with a typical PA layer not only protects the puncture resistance of the PA, but also increases the strength of the EVOH and improves the crack resistance of the EVOH. In order to prevent the outer barrier film from being susceptible to external force damage and resulting in the decline of the barrier effect, the method of increasing the PA thickness is usually used to make up for it, resulting in an increase in cost.
2], others
(1) The heat sealing layer and the composite layer are divided into two groups, one of which replaces the more expensive polymer with a cheaper polymer, so as to reduce the cost of the film while maintaining the strength of the film. The other group uses functional polymers that meet their functional requirements. Make more economical composite films with multi-layer concepts. For example, when two 1KB films are compared, the material cost of a five-layer film is about 19% higher than that of a seven-layer film.　　　　
(2) The performance of PA co-extruded films with less than five layers can be improved by using co-extruded films with more layers. For example, the use of an additional adhesive layer can improve the barrier properties of the film by increasing the film's water vapor barrier. Another advantage obtained at the same time is that the film can be made softer, feel good and have good crack resistance.　　
(3) The use of co-extruded films with more layers can improve the stress warpage resistance of PA co-extruded films with less than five layers. At the same time, it meets the needs of secondary processing such as bag making.
(4) It integrates other functions of dry composite film except lining, so that the composite process is simple, the composite structure tends to be flexible, the functions tend to be diverse, the cost is significantly reduced, it is safer, it is more in line with the requirements of health and environmental protection, and the social and economic benefits are more significant.　　　　
With the continuous progress of polymer synthesis technology, new polymers with unique physical and mechanical properties can be widely used to meet the needs of packaging. The function and structure of multi-layer co-extruded composite substrate films will be more flexible and economical. Through the application and improvement of molding equipment and technology, combined with the unique and effective design of composite structure, film manufacturers will have a revolutionary effect on the pursuit of concepts and ways of thinking such as diversification of packaging functions, rationalization of packaging structure, and maximization of packaging benefits.。
However, the scientific use of raw materials, the design of a reasonable product structure and the close cooperation with the processing technology are never-ending challenging topics in front of each of our composite film manufacturers. Because only when the technical resources of processing equipment, processing raw materials, and structural design (plus technology) are fully utilized and reach the best state, can the packaging substrate appear in the most economical, reasonable, adequate and cheapest form in its final product and meet the needs of the market.

VI.Packaging of Various Foods
Natural juices such as oranges and apples
There are cans, bottles and paper containers for sale in the market, among which the natural beverages in paper containers have the best flavor and the waste paper can be recycled and reused. The juice with aseptic packaging is the most popular on the market, the juice pH is below 4.5, and the sterilization temperature does not exceed 100 °C, such as Wenzhou mandarin juice, which is sterilized at 90 °C ~ 93 °C for a few seconds and then quenched to 20 °C, and the paper container is aseptically sterilized first, and then aseptically packaged. The packaging material of fruit juice drink is composed of PE/AL/PE/paper/PES layers, and aluminum foil can isolate the ingress of oxygen and prevent oxidation of fruit juice.
Dairy product　　　　
There are two types of cream produced in Japan: regular cream and soft cream, and cheese is fresh cheese and processed cheese, and there are two types of cheese: hot and cold packaging.　　
Typical packaging materials and microbial control methods for cream and cheese are as follows:　　　　
Cream, packaging materials are metal cans, sulfuric acid paper and aluminum foil film, low temperature circulation to prevent microbial recontamination.　　　　
Soft cream, packaged in aseptic plastic, circulated at low temperatures.　　　　
Fresh cheese, aseptically packed in aluminium foil and plastic film, aseptically packed in polystyrene containers, circulated at low temperatures.　　Carton cheese, packaging material is sulfuric acid paper, plastic packaging material and carton, aluminum foil film, heat treatment to prevent microbial recontamination.　　　　
Stick cheese, packaged in PVDC film, heat treated to prevent microbial recontamination.
Flake cheese, filled with PVDC, wrapped in cellophane PET, wrapped in PET/PVDC/PE.KOP/PE, can be inflated to prevent microbial recontamination.
Meat products　　　
Processed meat products (ham, sausages) are required to be low in salt and calorie, and are packaged in aseptic packaging and gas filling. The microorganisms that grow on the raw meat of ham and sausage include streptococcus spp., Pseudomonas, Cocci spp., Leuconostoc spp., Lactobacillus spp., and highly heat-resistant bacteria such as Clostridium botulinum toxin.　　　　　
The packaging, packaging material composition and characteristics of meat products are as follows:　　　
Aseptic tank: Most of them are used in the United States to package beef stew, meat paste, broth, etc., which are sterilized by high temperature steam.　　　　
Inflatable packaging: packaging material PET/PFOPP/PE, cheap gas, packaging sausages, beef, mixed sausages, etc.　　　　　
Sealed inert gas packaging: used for high-grade sausages, high-grade beef, mixed sausages, etc., the packaging material composition is KM/PE.KOP/CPP, OPP/QS/CPP, and the gas and water vapor barrier is good.　　　　　
Surface packaging method packaging: used for packaging bacon slices (a meat-eating processing name), sausage slices, ham slices, sausages, etc. The packaging materials are composed of IONO MER/E-VAL/EVA, IONOMER/PVDC/EVA, EVAL and PVDC for gas barrier, and E-VA for heat adhesion.　　　
Deep pressure vacuum packaging: used to pack ham slices, sausage slices, roast pork. The packaging material is made of Len/PE, Len/IONOMER, and the cover material: OPP/PE, PET/PE or ILNOM ER-NY/PE or IONOMER.　　　
Sequential tight packaging: packaging ham slices, sausage slices, sausages, packaging materials for soft materials PVC/PVDC, PVC/PVDC/EVA pressed film, hard materials for PVC film.　　　　
Vacuum shrink wrapping: whole or half sausages packed whole or half pieces of ham, the packaging material is EVA/PVDC/EVD, and the hot scalding shrinks after vacuum packaging.　　　　
Jets: packaging hams, sausages, packaging materials PVDC.　　　
Packing retort pouches: Packing hamburgers, beef patties, meatballs, etc. Packaging materials PET/PVDC/CPP, PET/AL/CPP.　　　
Processed meat products are protein foods, there are microbial contamination and the problem of changing the color of meat, bagged products are generally aseptic at 118 °C for 35~40 minutes, and jet packaged products are sterilized at 120 °C in the distillation kettle for 10~27 minutes, which can completely sterilize the microorganisms that make the meat processed products spoil, and stored at room temperature for 3 months.　。　　
In order to preserve ham and sausage products for a long time, the following packaging bags and preservative methods are adopted: make the processed meat products almost frozen before slicing, and prevent recontamination of knives. Encapsulated with 100% nitrogen, or carbon dioxide that lowers the pH, prevents microbial growth. The packaging material is a multilayer film composed of EVA or polyvinylene dichloride with good insulation. Gas-filled packaging products are sold below 10°C.
Vegetables-based foods　　
Foods centered on salads (Western-style salad) are being developed. The raw materials of the salad are carrots, cucumbers, onions, potatoes, macaroni, vermicelli, etc. Microbial control in raw material handling and packaging, with an eye to reducing pH, most of the bacterial contamination of salads comes from the process of heat treatment and cutting and mixing of raw materials. In order to prolong the shelf life of non-staple foods such as vegetables, it is necessary to pay attention to the disinfection and sterilization of raw materials, machinery and utensils, and containers.
For example, cucumbers are soaked in acetic acid solution with available chlorine of 200~300ppm and pH 4.0 and stirred for 60 minutes, then washed, passed through a water applicator for 20~30 seconds, and then sliced with an aseptic knife after treatment with hot water above 90 °C containing 200~300ppm of available chlorine and 50~100ppm iodoform.
With the development of the economy and the change of social life, rapid production and then large-scale production have become the main factors for various commodities to reduce manufacturing costs and survive in the highly competitive market. In the process of food packaging, except for a few manufacturers who still use bag packaging, most manufacturers set up large-scale fast packaging machines on the food production line, so rolls of flexible color art packaging plastic film are widely used. However, with the market consumption of personalization, specificity and the needs of the world's environmental protection trend environment, a variety of different flexible packaging bags have regained vitality, and new flexible packaging bags have become popular.

In recent years, the prosperity of instant noodles has made the best explanation and annotation for the packaging of "high-temperature sterilization room temperature conditioning bag". Among them, in order to increase the variability and diversification of instant noodles, traditional flavored foods such as stir-fried pork with bamboo shoots, roasted beef with green onions, angelica mutton, curry chicken and other seasoning ingredients have also been made into delicious conditioning bags through advanced food processing and packaging technology, with noodles, enriching the types of instant noodles, activating the instant cooked food industry, and adapting to the needs of people's fast-paced life.　　　　
Foods that have been sterilized at high temperature are prone to changes in color and taste, especially vegetables, such as brightly colored green peppers, tomatoes, carrots, etc., once conditioned by high temperature, in addition to the serious loss of vitamins, the color also changes, and the appearance appears lifeless. In order to overcome these shortcomings, some Japanese food preparation machinery manufacturers have begun to promote the use of conditioning and storage (MAP) conditioning machines for the conditioning and packaging of fresh food and meat in their own and Taiwan markets. Its main principle of action is to use soft materials with high resistance gas (nitrogen, carbon dioxide, oxygen) to package food, and adjust the composition of the gas in the packaging bag to reduce the respiration rate of the contents and inhibit the growth of microorganisms, so as to achieve the purpose of prolonging the shelf life of food. Its main material structure is special polyester film/ONY/R-CPP, which has excellent barrier function for gas with special polyester film to meet the needs of rapid penetration of content gas. However, due to the high price of machinery and equipment and the limited application field, it is not widely used at present, and it is still a product field to be developed.　　
In the three-side sealing (three-side) bag mouth that has been used for many years, add a clamp chain or add a bottom, so that the capacity of the bag increases and can be standed, and can also avoid the food in the bag from deteriorating or loosening, falling, and the bag can only be tied with a rubber band after opening the bag before. The introduction of zipper bags is a representative work of food flexible packaging manufacturers, material suppliers, and machinery manufacturers to exert their wisdom and improve the quality of life, and is generally used for food packaging that must be reused many times, such as dried tofu, beef jerky, pork jerky and other snack foods or pet feed. In recent years, there have been breakthrough applications in the consumer market, such as changing the tea bags originally packaged in ordinary packaging cartons to sandwich gas chain stand-up pouches to make them present a charming and elegant unique style, which can be said to give full play to the unique functions of this type of packaging.　
In the past, after consumers bought traditional canned food such as juices, alcohol, salad dressings, sauces, etc., most of them were accustomed to pouring the contents into fixed containers and discarding the original packaging, which increased the burden on the cleaning department and brought a considerable amount of environmental pollution problems. At present, a number of manufacturers in Taiwan have launched environmentally friendly stand-up pouches that can replace blow molding bottles, and the characteristics of stand-up pouches can also achieve the effect of upright placement, and can reduce the cost of being passed on. In addition to food and beverage applications, there has been an increasing trend in the proportion of packaged goods in this form on the shelves of some shopping malls. Some manufacturers have also invented the stand-up pouch for environmental refills, that is, a rotary plug is installed at the corner of the stand-up pouch, which allows consumers to confirm the integrity of the product before use, and whether someone has opened it in advance. In addition, the packaging makes it easy to dump the contents into the container bottle and roll the bag into a tube and dispose of it in the trash can when it is finished, taking up very little space. Generally, stand-up pouches or environmentally friendly supplementary stand-up pouches mostly use thick material structures to meet the special requirements of the product and achieve the display effect of standing upright.
According to the prediction of international packaging market experts, the development trend of food packaging is as follows:
1. Aseptic packaging will adopt high-tech and new materials.
2. Non-toxic packaging materials are safer.
3. Large plastic containers and small and medium-sized plastic bottles will gradually replace glass products.
4. The composite flexible packaging made of paper, aluminum foil, plastic film, etc., will be more high-grade and multi-functional.
5. Apply petrochemical products to manufacture auxiliary packaging materials for new packaging raw materials.
6. The development potential of special trays for microwave ovens is great.
7. Fast food packaging will be further developed.
8. Rationalization of packaging design and overall lightness of packaging.
9. Improve the performance and function of packaging materials, reduce the amount of packaging materials, and reduce packaging costs.
10. "Cramming" stuffing packaging will continue to be promoted.
Food Packaging (II)
Food packaging occupies a very important position in China's packaging industry and is the pillar industry of the packaging industry. And the competition in the food market in this century will largely depend on the competition of packaging. According to the World Packaging Organization, the global packaging industry has a turnover of more than $500 billion, accounting for 1%-2% of global GDP, and is composed of: paper and cardboard 32 per cent, plastics 28 per cent, metal 24 per cent, glass 6 per cent, packaging machinery 5 per cent and other 5 per cent. This paper will discuss the current situation and development trend of food packaging in China from the four main areas of food packaging: plastic and composite packaging, metal packaging, glass packaging and paper packaging.
Plastic and Composite Packaging　　　
Plastics and composite materials are lightweight, beautiful, low cost, and the empty bags of flexible packaging take up little space. With low processing energy consumption and flexible production methods, it can be made into various forms of packaging and containers, such as bags, bottles, cans, hoses, casings, heat shrinkable films, stretch films, special functional films, foam plastic containers, sheet thermoformed cups, plates, basins, skin packaging containers and blister packaging containers.　　
It is precisely because plastics have the advantages that other packaging plastics do not have, so plastics and plastic-containing composites are developing rapidly. From the mid-90s of the 20th century to 2000, the average annual growth rate of synthetic resins for packaging in the world was about twice the growth rate of the packaging industry. The growth rate of plastic packaging materials has been significantly faster than that of other packaging materials. In the U.S. flexible packaging market, plastics account for about three-quarters, with an average annual growth rate of 6%, twice the GDP growth rate, while paper and metal foil grew at only 1.0% and 1.5% respectively in the same period.
VII.Commonly Used Barrier Materials
In the past 10 years, the variety of plastic packaging materials in China has been increasing, and the annual output of packaging materials has increased by more than 10%, and it is estimated that the plastic packaging market in the 21st century will grow by 7%-9%. The plastics used for flexible packaging are mainly polyethylene, accounting for about 80% of the plastics used in the flexible packaging market; HDPE accounts for the largest share of the plastic rigid packaging market, accounting for about 45% of the resin used in rigid packaging, and has maintained an average annual growth rate of about 7.3% in recent years. PET bottles have entered the new generation of beverages, hot-filled bottles (90°C cans), juices and drinking water markets, and are developing into the liquor and beer markets.　　　
However, plastics also have great weaknesses, such as pollution problems to the environment, and their temperature resistance and barrier properties are generally inferior to metal and glass containers. High barrier plastics Internationally the polymer with a transmission rate to oxygen less than 3.8Cm3.mm/24b.m2.MPa is called a barrier polymer. High barrier plastic materials have the characteristics of oxygen barrier, water vapor resistance, oil resistance, and transparency, which can effectively maintain the original taste and odor of the food in the container and packaging, prevent quality deterioration, and prolong the life and shelf life of the food. In the international food packaging industry, the application of barrier plastics in packaging is increasingly emphasized. The consumption of barrier plastic packaging in the United States increased from 1.04 million tons in 1995 to 1.99 million tons in 2000, an annual increase of 13.6%, while the development of barrier plastic packaging materials in China is still in its infancy.　　　　
At present, the industrialized barrier plastic packaging materials mainly include EVOH, nitrile-based resin, PVDC and PEN. All kinds of plastics can be further enhanced by blending, surface plating, surface coating, multi-layer composite, tensile orientation and other methods according to the specific situation.
1.1. EVOH is an ethylene-vinyl alcohol copolymer, its most significant feature is that it has excellent gas barrier, which can effectively block the penetration of oxygen, carbon dioxide and other gases, and it also has good transparency, gloss, mechanical strength and thermal stability. It is used as an intermediate layer of the composite film to make rigid or flexible containers, which can be used for the packaging of sauces, tomato juices, juices, meats, cheeses and fruits, etc.
1.2 PVDC (polyvinylidene chloride) PVDC has a high degree of crystallinity, and its biggest feature is excellent comprehensive barrier performance, but due to its hard texture, high softening point, and thermal instability, it is quite difficult to process and form. If its monomer VDC (vinylidene chloride) is copolymerized with other monomers such as vinyl chloride, methyl acrylate, acrylonitrile, etc., the copolymer can better solve the above problems, so the products are usually in the form of copolymers. At present, PVDC copolymers have been widely used in food packaging. For example, the single-layer PVDC film is made by double-stretch blow molding, which has shrinkage and barrier properties, and does not decompose under microwave heating conditions, and is widely used for household preservation. The co-extrusion composite film of PVDC and PS, HIPS, PP and other resins can be used for vacuum packaging of dairy products, jams, etc.  
1.3 The difference between the molecular structure of PEN (polyethylene naphthalate ethylene glycol) PEN and PET is that the naphthalene ring is used instead of the benzene ring in the PET molecule, and the raw material monomer of the polymer is replaced by 2,6 naphthalene dicarboxylic acid, which is similar in composition to PET. As a result, PEN is the best polyester material, which outperforms PET in almost all respects, with excellent barrier properties, UV absorption, heat resistance and chemical stability. We can foresee the development prospects of PEN from the development potential of polyester bottles. Polyester bottles incorporated with PEN are beyond reproach for their heat resistance, barrier properties and other aspects that make them the protagonists of beverage packaging.
1.4 MXD6 aromatic nylon In China, among the four typical barrier materials, the development of EVOH, PEN and nitrile-based resins is still blank, and the application of PVDC in domestic food packaging has started, but it still cannot meet the competitive needs of the international and domestic food packaging market, and barrier plastic materials have broad development prospects in China's food packaging market.

 八、  VIII.Application Examples
Generally, it is recommended to use 5-layer and 7-layer co-extrusion equipment, among which the 5-layer structure is the most widely used, such as ABCBD and ABCBA. For example, the structure of the 3-layer co-extrusion layer is: PA/Ad/PE (PP), and the nylon is in the outer layer and is not protected. Nylon has poor impact resistance, easy to scratch, air leakage, hydrophilicity, easy to absorb moisture, and reduced oxygen permeability. In the case of a 5-layer structure, PP (PET)/PE, PA outside/Ad/PA/Ad/PP(PE). In this way, the nylon is protected from both sides and prevents the nylon from absorbing moisture. After experiments on 7 layers and 9 layers, if various resins are properly arranged, the shelf life can reach more than 1 year, almost reaching the tinplate canned effect. For example, PET/Ad/PA/EVOH/PA/Ad/PE (PP).
In multilayer films, the functions and roles of each layer are different.
Oxygen impermeable layer: according to the storage period of the packaging, determine the corresponding permeability material (PVDC, EVOH) and layer thickness, the thickness of nylon is generally 20%, EVOH is generally 10-15%, and the thickness deviation shall not exceed 10%, in this range, the impact on the storage of the packaging is very small.　　　
Adhesion layer (AD): Although nylon and EVOH have good extrusion formability, they have no knot ability with other layers (LDPE, PP, EVA) during co-extrusion, and special knot resin must be used as the knot layer to achieve high peel strength between layers. Therefore, the choice of layer material is a very important factor. The appropriate grade should be selected according to the structure, and the thickness is generally 5-15g/m2.　　
Inner and outer layer: nylon, EVOH although the oxygen permeability is very small, but the water vapor permeability is very large, therefore, when determining the inner and outer materials, the problem of water vapor permeability should be fully considered, and the inner layer of the material must take into account the heat sealing performance and heat sealing strength, such as LDPE, mlLDPE, EVA, PPO, etc., if necessary, can be modified with Surlyn, PRIMCOR, POP. When used as a composite substrate, the corona treatment strength of the outer surface should reach 38-42 dynes.
IX.Multi-Layer Co-Extrusion Composite Film Compared with Single-Layer Plastic Film
Compared with single-layer plastic substrate films, multi-layer co-extrusion composite films greatly simplify the production process of dry composite films, increase functions, and can be quantified through effective adjustment of thickness, convenient and flexible structural combination, and a wide range of materials. The comprehensive performance is full of functions and flexible changes, low cost and high quality level, high added value and strong market adaptability. Therefore, multi-layer co-extrusion composite films have been widely used in packaging.
Structure and Development Trend of Multi-Layer Co-Extrusion Composite Film
In general, the structure of a multilayer co-extruded composite film depends on the functional requirements of the film. Under the premise of meeting the process requirements, through the combination of different polymers, the functional requirements of packaging materials such as barrier, heat sealing, body strength, puncture resistance, environmental adaptability, secondary processing characteristics, and extended storage and shelf life are met. From the analysis of functional requirements, the combination of five polymers is sufficient. However, the application of seven-layer, nine-layer, ten-layer or even more layers of co-extrusion composite film has begun to be applied in the market, making it a trend and developing rapidly. The structural design of co-extruded composite film is gradually required to systematically achieve the ideal realm of integrating function, technology, cost, environmental protection, safety and secondary processing.
1. Barrier　　　　
(1) Replacing a single layer of polymer with multiple layers of the same polymer in the barrier layer can improve the stability of the barrier layer. For example, if the PA material is set as a barrier layer, the oxygen barrier rate is 40 units. In order to ensure the stability of its oxygen resistance, usually the set value of its thickness is the theoretical calculated value of the material + the negative error value of the equipment + the safety factor. When we use multiple layers of the same polymer instead, the negative error value of the equipment of its thickness is significantly reduced, and the safety factor is significantly improved.　　(2) When we confirm that the thickness of the barrier layer is sufficient to meet the barrier requirements, we inject multiple layers of the same polymer into the barrier layer instead of a single layer of polymer, which can reduce the cost of high value-added raw materials for the barrier film.　　(3) Replacing a single polymer with two different polymers in the barrier layer can significantly improve the barrier property of the film. For example, combining the EVOH layer with a typical PA layer not only protects the puncture resistance of the PA, but also increases the strength of the EVOH and improves the crack resistance of the EVOH. In order to prevent the outer barrier film from being susceptible to external force damage and resulting in the decline of the barrier effect, the method of increasing the PA thickness is usually used to make up for it, resulting in an increase in cost.
2. Miscellaneous　　　　
（(1) The heat sealing layer and the composite layer are divided into two groups, one of which replaces the more expensive polymer with a cheaper polymer, so as to reduce the cost of the film while maintaining the strength of the film. The other group uses functional polymers that meet their functional requirements. Make more economical composite films with multi-layer concepts. For example, when comparing two 1kg films, the material cost of a five-layer film is about 19% higher than that of a seven-layer film.　　　
（(2) The performance of PA co-extruded films with less than five layers can be improved by using co-extruded films with more layers. For example, the use of an additional adhesive layer can improve the barrier properties of the film by increasing the film's water vapor barrier. Another advantage obtained at the same time is that the film can be made softer, feel good and have good crack resistance.　　　　
(3) The use of co-extruded films with more layers can improve the stress warpage resistance of PA co-extruded films with less than five layers. At the same time, it meets the needs of secondary processing such as bag making.　　　　
(4) It integrates other functions of dry composite film except lining, so that the composite process is simple, the composite structure tends to be flexible, the functions tend to be diverse, the cost is significantly reduced, it is safer, it is more in line with the requirements of health and environmental protection, and the social and economic benefits are more significant.　　　
With the continuous progress of polymer synthesis technology, new polymers with unique physical and mechanical properties can be widely used to meet the needs of packaging. The function and structure of multi-layer co-extruded composite substrate films will be more flexible and economical. Through the application and improvement of molding equipment and technology, combined with the unique and effective design of composite structure, film manufacturers will have a revolutionary effect on the pursuit of concepts and ways of thinking such as diversification of packaging functions, rationalization of packaging structure, and maximization of packaging benefits. However, the scientific use of raw materials, the design of a reasonable product structure and the close cooperation with the processing technology are never-ending challenging topics in front of each of our composite film manufacturers. Because only when the technical resources of processing equipment, processing raw materials, and structural design (plus technology) are fully utilized and reach the best state, can the packaging substrate appear in the most economical, reasonable, adequate and cheapest form in its final product and meet the needs of the market.
X.Printing Technology of the Film 　(I), the Pre-Press Treatment of the Plastic Film
Plastic films generally use gravure printing, flexographic printing and screen printing and other processes, and are usually compounded after printing to achieve good use results. The surface properties of various plastics vary greatly due to their molecular structure, polar groups, crystallization degree and chemical stability. These factors have a great impact on the binding fastness of printing inks and the adhesive strength of composite films. Therefore, before printing or composite processing, the surface treatment should be carried out appropriately according to the surface properties of different plastics, so as to obtain excellent processing adaptability.　　
Theoretically, in order to obtain good printability on the substrate, the surface tension of the substrate should be higher than that of the printing ink. PP and PE film are non-polar polymer materials, with stable chemical properties and low surface tension, and it is difficult to bond with ink due to the influence of anti-opening agents, antistatic agents, and anti-aging agents added when synthesizing resins.　　　
In order to improve the surface tension of plastic films, improve their wetting properties and adhesive properties, and thus improve the fastness of printing inks, the commonly used surface treatment methods are as follows.　　　　
① Corona treatment method.
Using a high-frequency (medium-frequency) high-voltage power supply, a corona release phenomenon is generated in the gap between the discharge knife holder and the blade to surface treat the plastic film, which is called corona treatment, also known as electronic shock or electric spark treatment. Its treatment functions are:
a. Ozone is produced by ionizing oxygen between the poles by discharging. Ozone is a strong oxidizing agent that can immediately oxidize the surface molecules of plastic films, transforming them from non-polar to polar, and the surface tension increases.
b. After the electronic impact, the surface of the film will produce slightly concave dense holes, which will roughen the surface of the plastic and increase the surface activity.　　　　
② Chemical treatment.
The surface of the polyolefin film is treated with oxidant solutions such as potassium dichromate and sulfuric acid to generate polar groups such as hydroxyl and carbonyl groups, and at the same time, a certain degree of coarsening is obtained.　　　
③ Photochemical treatment. Ultraviolet light of the appropriate wavelength is selected to irradiate the polymer surface to cause chemical changes such as cleavage, cross-linking, and oxidation.　　　
 At present, the corona treatment method is mostly used in production. The chemical treatment method has a long treatment time, and the treatment liquid is chemically aggressive, so it is only applied when it is inconvenient to use other treatment methods, while the photochemical treatment method is not ideal enough, and the time is long and the cost is high.
2. Anti-static treatment　　　
The electrostatic formation of thin films is due to the excellent dielectric properties of PE and PP, high resistance and poor conductivity. In the process of extrusion and winding, static electricity is generated due to friction, which further generates and accumulates static electricity during the printing process, and is not easy to release, so that a large amount of electrostatic charge accumulates on the surface of the film. Static electricity in thin film printing can present a series of operational challenges that directly affect the quality of the print. Due to electrostatic adhesion, the film is in a state of lack of oxygen, which will hinder the curing process of the inner layer of the plastic surface; if it encounters high temperature and high humidity environment, it is easier to form ink layer adhesion, which will make the printing ink transfer dyeing, increase the difficulty of printing, slitting, finishing and other processes, and the film will be connected to each other and cannot be torn apart, resulting in the scrapping of the printed product.
XI.Heat Seal Ability of Plastics
The heat seal ability of plastic is a very important performance requirement for flexible plastic packaging materials, because for any soft plastic packaging products, it is necessary to make a pocket, and it is necessary to rely on hot melt and weld into a bag A2 type to package various commodities, and the pocket after packaging goods should also be sealed by heat sealing, it can be said that heat seal ability is the main characteristic requirement of flexible packaging, and there is no heat seal ability of plastic, and there is no flexible packaging.
According to the Japanese Industrial Standard ISZ1526-1976, the standard heat sealing strength is the heat sealing strength of the resin at a temperature of 130~140 °C, a pressing force of 1kgF/cm2 and a heat sealing time of 2~3s.　　　
The heat seal ability of a resin has the following requirements:　　　　
(1) Standard heat sealing strength　　
This is the heat-sealing strength of the resin for heat sealing under optimal heat-sealing conditions. Standard heat seal strength, for the same main substrate film, such as BOPA6, the standard heat seal strength is mainly affected by the extrusion thickness of the extrusion coating heat seal resin and the type of resin. For example: the main substrate film is BOPA6 15μm thick, with LDPE extrusion and composite, heat sealing at a temperature of 130~140 °C, 2kgF/cm2 pressure and 1s time, the relationship between the heat sealing strength kgF/15mm width and thickness is as follows: LDPE thickness μm 20, 30, 40, 50, 60, 70 heat sealing strength, kgF/15mm1.0, 1.5, 2.5, 3.5, 4.2, 5.5 The heat sealing strength is basically proportional to the thickness of the heat sealing resin.
  For the same LDPE, the standard heat sealing strength is related to the MI and density of the resin, for example: for PT300#/extruded 20μ LDPE, MI=3.7g/10miND=0.923g/cm, 1.25kgF/15mm, MI=3.0D=0.921 is 1.4 kgT/ 15mm;MI=7.0D=0.917, Heat sealing strength is 1.75kgF/15mm; MI=9.5,D=0.917. The heat seal strength is 2.0kgF/15mm. It can be seen that the sealing strength increases significantly with the increase of MI. Japan's Mitsui Petrochemical Co., Ltd. has launched new resin grades VL-LDPE and CS-C for heat sealing, which have improved the level of heat-sealing strength, and the heat-sealing strength can reach 5~6kgF/15mm at 150°C.
(II) Low Temperature Heat Seal Ability (Strength)
 The so-called low-temperature heat sealing strength means that the resin has a low heat-sealing temperature, and can obtain reliable heat-sealing strength at a lower heat-sealing temperature, which is suitable for the requirements of high-speed automatic filling and filling molding equipment. This resin has a low heat seal initiation temperature. For LDPE, its low-temperature heat seal ability is mainly affected by MI and density. High MI and low density LDPE, good low temperature heat seal ability, -5 ethylene phase copolymer resin low strength heat seal ability, the composition and content of co-monomer have a great relationship, such as: EVA resin, with the increase of VA content density increases, softening temperature and melting temperature decrease, EVA resin low temperature heat seal ability becomes better, EVA crystallinity decreases with the increase of VA content, when VA content exceeds 25% EVA becomes an amorphous polymer.　　　　
(III) Heat Seal Ability of Inclusions
 The heat seal ability of inclusions refers to the fact that the heat-sealing surface of the heat-sealing resin is infected with dust, greasy, dirty, and powder of the contents of the product, etc., and its heat seal ability is still good, which is called inclusion and heat seal ability. Inclusions and heat seal ability are particularly important when filling liquids, powdered rice foods or pesticides, and resins with better heat seal ability of inclusions are resins such as SurlYN (ionic resin), LLDPE, EMAA, EEA, etc.
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