With the development of the national economy and the improvement of people’s living standards, food packaging has gradually become an important part of commodities from being an accessory of commodities. Flexible packaging refers to packaging whose container shape can change after filling or taking out the contents. Various bags, boxes, sleeves, and wrappings made of paper, aluminum foil, fiber, plastic film, and their composites are all flexible packaging.
The selection of food flexible packaging materials is crucial. Among many food flexible packagings, plastic films and their composite materials play a decisive role, including single films and composite films. Single films include ordinary films, stretched films, heat-shrinkable films, etc.; composite films include plastic-plastic composite films, aluminum-plastic composite films, iron-plastic composite films, paper-plastic composite films, etc.
I. Plastics
At present, there are more than thirty kinds of plastics that can be used for food packaging. In China, there are about twenty kinds of plastics used for food flexible packaging, such as PE, PP, PS, PET, PA, PVDC, EVA, NY, PY4, EVOH, PVC, PT, ionomer resin, etc. Among them, those with high oxygen barrier properties are PVA, EVOH, PVDC, PET, PA, etc.; those with high moisture barrier properties are PVDC, PP, PE, etc.; those resistant to high-temperature sterilization are PET, PA, etc.; those with good mechanical properties are ionomer resin, PA, PET, etc.; those that are resistant to both high and low temperatures are also PET and PA. Commonly used food packaging plastics are PE, PP, PET, NY, PT, etc.
PE (polyethylene) is divided into low density (LDPE), linear low density (LLDPE), and high density (HDPE) varieties. LDPE film has good transparency and heat sealing property and can prevent water and moisture. There are several types such as blown film (IPE), cast film (CPE), and low-foaming film. The CPE film has uniform thickness, better surface gloss, transparency, and heat sealing property than IPE, but its mechanical strength and opening property are worse. IPE is mainly used as the inner layer of composite bags. LLDPE has better heat sealing property, tensile strength, and puncture resistance than LDPE; HDPE has higher mechanical strength, so it can reduce the film thickness and has good heat resistance, but its transparency and heat sealing property are worse than LDPE. In general, PE has good chemical stability, is low in price, easy to process, has relatively balanced performance, is resistant to freezing, and has good moisture resistance. It is common in the market and widely used. However, due to its poor oxygen barrier property, it is not very effective in food packaging and preservation, and the storage period should not be too long.
PP (polypropylene) film includes biaxially oriented film (BOPP), blown film (1PP), cast film (CPP), etc. BOPP has better printability, tensile strength, transparency, and barrier property than CPP, but its heat sealing property and tear resistance are poor. Biaxially oriented polypropylene pearlescent film is a new variety of BOPP. It is made by co-extruding two layers of copolymerized PP and a layer of homopolymerized PP containing CaCO3 masterbatch into a sheet and then stretching it nearly forty times. In addition to the general characteristics of BOPP, it has better gas and moisture barrier properties and is about 28% lower in density. It can be directly used for making ice cream and other cold drink bags. After being laminated with a layer of PE, it is widely used in packaging candy, biscuits, and other foods. In general, PP is low in price, easy to process, has balanced performance, is heat-resistant (the service temperature can reach 160°C), and has good moisture resistance. However, its oxygen barrier property is poor (slightly better than PE), and the preservation effect for food packaging is not very good.
PET (polyester) and other films are generally biaxially stretched. It has good heat resistance, high transparency and strength, and good moisture and oxygen barrier properties, but its heat sealing property is poor and the price is high. It is only used as a composite substrate. NY (nylon) film has high tensile strength, good oxygen barrier property, good puncture resistance, and good heat resistance (the maximum service temperature can reach 190°C), but its moisture barrier property is poor, the heat sealing temperature is high, and the price is expensive. PT (cellophane) has good transparency and rigidity, does not generate static electricity, but has poor tear strength and sealing resistance.
II. Composite materials
Composite materials are the most diverse and widely used flexible packaging materials. At present, there are hundreds of multi-layer composite materials containing plastics used for food packaging. Generally, composite materials use 2 to 6 layers, and for special needs, it can reach 10 layers or even more. Scientifically and reasonably combining or laminating substrates such as plastic, paper or thin cardboard, and aluminum foil can meet the packaging requirements of various foods.
PVDC (polyvinylidene chloride) coated composite film is made by coating PVDC latex on BOPP film and then laminating PE film. After curing, it is obtained. It is relatively cheap in price, has excellent oxygen and moisture barrier properties, can withstand 100°C hot water sterilization, has good heat sealing property, good chemical stability, and no hygiene problems, and can meet the packaging requirements of various foods.
PVA (polyvinyl alcohol) coated composite film is made by coating PVA solution on BOPP film and then laminating PE film after acetalization treatment at normal temperature. It is cheap in price but slightly worse in heat resistance and can be used for packaging various foods.
PFT/AlU/PE has good mechanical properties and heat sealing properties and is suitable for vacuum or modified atmosphere packaging bags and vertical packaging bags for juices and other beverages. PET/EVOH/PE has good barrier property and transparency and is suitable for vacuum packaging of ham and vertical packaging bags for juices and other beverages.
In short, for ordinary short-term sales or foods with less strict preservation requirements, such as candy, biscuits, bread, soy sauce, vinegar, vermicelli, and bean paste, low-cost PE film and PP film are mostly used. For foods with high requirements for preservation and quality, especially for modified atmosphere packaged foods, it is better to choose various composite films. For foods that need high-temperature cooking and sterilization for a long time, materials such as PVDC coated film and NY can be selected.
The printing and compounding of food flexible packaging materials also have their characteristics. The printing suitability of common food flexible packaging materials is different. PE film has poor ink adhesion and needs corona treatment. The tension control should be small and constant during printing to prevent the film from deforming due to heat and ink from sticking. PP film also has poor ink adhesion and needs corona treatment. The ink adhesion of PP film needs a certain time to show good performance after printing, and when combined with polyurethane adhesive, there may be a problem of low peel force. PET film is not easy to stretch and often uses reverse printing technology. It is easy to generate static electricity and needs an electrostatic elimination device. The drying temperature can be relatively high, but the film surface temperature should not exceed 85°C. PT film has good operability and ink adhesion, but it has strong moisture absorption. Attention should be paid to the environmental temperature to prevent the ink adhesion from decreasing and the edges from cracking. The drying temperature should not be too high, and attention should be paid to tension control.
Food flexible packaging printing is mainly gravure and flexographic printing. The reverse printing process can make the printed matter bright and beautiful, with vivid colors, no fading, moisture-proof and wear-resistant, and without polluting the packaging items. Flexible packaging materials are mainly composite films. The main compounding methods include wet compounding method, dry compounding method, extrusion compounding method, hot melt compounding method, and multi-layer extrusion compounding method. In the North American market, food flexible packaging is developing towards multi-layer co-extruded packaging materials. Paying attention to the packaging effect of food, that is, good barrier property, longer shelf life, and convenient use. This trend is worthy of the attention of China’s packaging industry.