Materials used for food flexible packaging must have appropriate barrier properties, such as grease foods require high oxygen barrier and oil resistance; dry foods require high moisture resistance; aromatic foods require high resistance odor; and fruit, vegetables and fresh foods It requires a certain degree of permeability of oxygen, carbon dioxide and water vapor. Soft packaging materials must have good mechanical properties and tensile strength, tear resistance, impact strength, etc.; also have good chemical stability, should not have any chemical reaction with the interior food to ensure food safety. In addition, it must have high temperature resistance, suitable for high temperature sterilization and low temperature storage of foods.
Selection of food flexible packaging materials
Plastic film and its composite materials play a decisive role in many food flexible packaging. They include a single film and a composite film. Single film can be divided into ordinary film, stretch film, heat shrink film, etc.; composite film can be divided into plastic composite film, aluminum-plastic composite film, iron-plastic composite film, paper-plastic composite film.
At present, there are more than 30 types of plastics that can be used for food packaging. There are also about 20 kinds of plastics used in food soft packaging in China, such as PE, PP, PS, PET, PA, PVDC, EVA, NY, PVA, EVOH, PVC, PT, and Ion. Bond resin and so on. High-impedance oxygen is PVA, EVOH, PVDC, PET, PA, etc.; High moisture resistance is PVDC, PP, PE, etc.; High temperature sterilization such as PET, PA, etc.; Good mechanical properties such as ionomer resin, PA , PET, etc., both high temperature and low temperature resistant, such as PET, PA and so on. Both high temperature and low temperature resistance such as PET, PA and so on. Commonly used food packaging plastics are PE, PP, PET, NY, PT and so on.
PE (Polyethylene) is divided into low density (LDPE), linear low density (LLDPE), high density (H DPE) and other varieties. LDPE film has good transparency and heat sealability and can be waterproof and moisture proof. According to different production processes, there are blow molding (IPE), flow film (CPE), low foaming film and so on. The thickness of the CPE film is uniform, and the surface gloss, transparency, and heat sealability are better than those of the IPE, but the mechanical strength and openability are inferior to those of the IPE. IPE is mainly used as the inner layer of composite bags. LLDPE's heat sealability, tensile strength and puncture resistance are better than LDPE; H DPE has higher mechanical strength, which can reduce the thickness of the film, heat resistance is also very good, but the transparency and heat sealability is worse than LDPE. In general, PE has good chemical stability, low price, easy processing, balanced performance, resistance to freezing and good moisture resistance, so it is very common in the market and widely used, but because of its poor oxygen barrier, it is used Food packaging preservation effect is not good, storage period should not be too long.
PP (polypropylene) film is divided into biaxially oriented film (BOPP), blown film (IPP), flow film (CPP) and several other. BOPP has better printability, tensile strength, transparency, and barrier properties than CPP, but has poor heat sealability and tear resistance. A new type of BOPP is a biaxially oriented polypropylene pearl film. It is obtained by co-extruding a homopolymer PP containing two layers of a masterbatch of CaCO3 with two layers of copolymerized PP, and then stretching the original film by nearly 40 times, except for the general characteristics of BOPP. Gas, moisture resistance performance to add good, the density is also low about 28%, so can be used directly for ice cream and other cold drinks bag, a layer of PE, widely used in candy, cookies, snacks, fast food packaging. In general, PP is low in price, easy to process, balanced in performance, heat-resistant (use temperature up to 160Â°C), good in moisture resistance, and widely used, but its oxygen-barrier property (slightly better than PE) is used in food packaging. The preservation effect is not good.
PET (polyester) and other films are generally biaxially stretched, and have good heat resistance, high transparency and strength, good moisture blocking and oxygen barrier properties, but poor heat sealability and high price, and are only used as composite substrates. NY (nylon) film has high tensile strength, good oxygen barrier property, good puncture resistance, and good heat resistance (up to 190Â°C), but it has poor moisture resistance, high sealing temperature, and high price. PE (glassine paper) has good transparency and firmness, does not generate static electricity, but has poor tear strength and poor heat sealability.
2. Composite materials
Composite materials are the most versatile and widely used type of flexible packaging material. At present, there are hundreds of multi-layered composite materials containing plastics for food packaging. Composite materials generally use 2-6 layers, but special needs can reach 10 layers or more. The use of plastic, paper or paperboard, aluminum foil and other substrates, combined with scientific and rational compounding or laminating, can almost meet the requirements for various foods for packaging.
The PVDC (polyvinylidene chloride) coated composite film was prepared by coating PVDC latex on a BOPP film and then compounding the PE film on the coating and curing it at a certain temperature for 48 hours. The price is relatively cheap, but the oxygen resistance, moisture resistance, excellent, resistant to 100 Â°C hot water sterilization, thermal performance, good chemical stability, no health problems, to meet the packaging requirements of various foods.
PVA (polyvinyl alcohol) coating composite film is prepared by coating PVA solution on BOPP film, and then performing warm acetalization and then compounding PE film on the coating. It is cheap, but the heat resistance is slightly poor. . It can be used as a packaging material for a variety of pickled products, pickle products, soy products, tea, sweets, cakes, and crisp foods.
PET / AL / PE, mechanical properties are very good, good sealing performance, suitable for vacuum or modified atmosphere bags, suitable for a variety of fruit juices and other drinks vertical bags. PET / EVOH / PE, good barrier properties, good transparency, suitable for ham vacuum packaging and a variety of juice and other beverage vertical packaging bags.
In short, ordinary short-term sales or foods that are less stringent for keeping fresh, such as sweets, biscuits, bread, soy sauce, vinegar, vermicelli, bean paste, etc., use lower-cost PE and PP films, and most Foods such as cooked meat, dairy products, pickles, fried foods, tea, etc., have higher requirements for preservation and shelf life. In particular, modified atmosphere packaging foods need excellent barrier properties and have other excellent properties. It is better to use a variety of composite membranes. For example, milk packaging made of multi-layer composite materials such as plastic/cardboard/aluminum foil/plastic, the shelf life can be up to six months to one year, while the shelf life of some high barrier soft packaging cans can reach three years. For foods that need to be steamed and sterilized after bagging and have a longer sterilization time, PVDC coating film, NY, etc. should be used.
Printing and compounding of flexible packaging materials
1, the printability of common food flexible packaging materials
PE film inks are poorly adherent and require power handling to improve. Wetness tension is an indicator used to identify the quality of corona treatment. General LDPE, LLDPE film in 38-42mN / m, H DPE film in the 40-44mN / m is better, commonly used polyamide ink printing. The LDPE film has a low expansion strength, a large tensile elongation, and it is difficult to set the tone. Therefore, when printing, the tension control should be small and the tension should be constant everywhere. If multi-color printing, in order to ensure the accuracy of overprinting, it is best to use a large roller type printing press. A small amount of LDPE film will be softened by heat, and it is easily stretched and deformed. For this reason, the surface temperature of the film should not exceed 55Â°C when the printing is dry. To prevent ink sticking, the temperature of the wound film should be cooled to below 35Â°C.
Like PE, PP film has poor adhesion to ink and corona treatment before printing. Its wetting tension is preferably 40-44 mN/m. Commonly used polyamide ink, chlorinated polypropylene ink (composite) printing. The use of polyamide inks to print PP films has low ink adhesion, and after about one day of aging, it shows good adhesion, while the use of polyurethane adhesive composite film. There will be a failure of small peel force.
PET film is not easy to stretch, the tension control requirements lower than the PE film, often printed in the India process, and CPP complex cooking bag. The PET film has good insulation properties, and is prone to static electricity during the printing process and is not easily lost. In severe cases, there is also a film sucked on the roller and printing cannot be performed. Therefore, in high-speed printing, static elimination devices are needed. The drying temperature of the PET film can be controlled high, but its film surface temperature should not exceed 85Â°C. When winding up, the film should be cooled to room temperature or slightly above room temperature.
The PT film has good operability and ink adhesion, does not carry static electricity, and does not easily absorb dust. PT has good hygroscopicity, and the elongation after moisture absorption becomes large, resulting in wrinkles or looseness, which affects the quality of printed graphics; in addition, ink adhesion may also decrease, so it is necessary to attach great importance to the ambient temperature of storage and printing. Its tear resistance is poor, if the edge has a crack, it is easy to tear along the opening, to prevent cutting, printing edge cracking. When it dries, it shrinks, and its strength decreases or becomes brittle. Therefore, the drying temperature should not be too high. In addition, pay attention to tension control to prevent tensile deformation and wrinkles.
2. Printing and compounding of food flexible packaging materials
Soft food packaging printing is mainly based on gravure and flexo printing, but there are many differences compared with general gravure and flexo printing used in publishing and commercial printing. For example: Flexible packaging printing is to print on the surface of the roll of the substrate. If it is a transparent film, the pattern can be seen from the back, and sometimes a layer of white paint or a printing process is needed.
Liyin refers to a special printing method that utilizes reverse-image-printing plates to transfer ink to the inside of a transparent printing material so that the front surface of the to-be-printed object behaves like a graphic. Printed in India is brighter and more beautiful than surface prints, with vivid colors, no fading, and moisture-proof and wear-resistant. Also, due to the composite processing after printing, the ink layer is sandwiched between two layers of film, so it does not contaminate the packaged items.
The printing color sequence of the Indian printing process is the opposite of the ordinary surface printing, for example, the ordinary printing first prints the background color, and the Indian printing is the last printing background color. Liyin is a unique process for plastic composite packaging and printing. It has become more diverse, multi-functional, and serialized in printing equipment. Such as the introduction of various models of plastic packaging gravure printing production line, has blown, printing, compounding, slitting, bag making and other multi-process linkage.
Followed by the use of flexible printing press printing plastic film, flexographic printing presses and dry laminating machine mostly composed of production lines.
Soft packaging materials generally do not use a single material, but mainly composite film. The main composite methods are the following:
Wet compound method. A layer of water-soluble adhesive is applied to the surface of the substrate (plastic film, aluminum foil), and is compounded with other materials (paper, cellophane) through a pressure roller, and then dried by a hot drying tunnel to form a composite film. This method is suitable for packaging dry foods.
Dry compound method. The solvent-based adhesive is first uniformly coated on the substrate, and then it enters the hot tunnel to fully volatilize the solvent and immediately combines with another film. Such as stretched polypropylene film (OPP), after printing is generally used dry composite process and other materials, the typical junction: biaxially oriented polypropylene film (BOPP, 12Î¼m), aluminum foil (AL, 9Î¼m) and single Stretched polypropylene film (CPP, 70 Î¼m). In the process, a solvent-based â€œdry bonding powderâ€ is uniformly coated on the base material by a roller coating device, and then it enters a hot drying tunnel so that the solvent is fully volatilized and then composited with the other thin film using the composite roll.
Extrusion compound method. The curtain-shaped molten polyethylene extruded from the slits of the T mold is pressed by a pinch roll, flows onto a paper or film, is coated with polyethylene, or other film is supplied from the second paper feeding portion, and polyethylene is used as the viscous material. Bonding layer for bonding.
Hot melt compound method. The polyethylene-acrylic acid ester copolymer, the ethylene acid-ethylene copolymer, and the paraffin wax are put together and heat-melted, and then they are coated on a substrate and immediately combined with other composite materials and cooled.
Multilayer extrusion composite method. A variety of different properties of the plastic resin through a plurality of extruders, squeeze into the mold composite film. This process does not use adhesives, organic solvents, no odor, no harmful solvent penetration, suitable for a longer shelf life