Detailed Explanation of Food Packaging Composite Materials

Single-layer plastic films often cannot fully meet the requirements for protecting and beautifying goods, as well as adapting to processing. This led to the development of more ideal packaging materials composed of two or three or more layers of the same or different types of packaging materials laminated together. This composite material overcomes the shortcomings of single materials and achieves excellent properties that single materials do not possess. Currently, composite materials are in Food Packaging Bags a dominant position in food packaging.

 

 

I. Basic Structure of Composite Materials

Generally, the outer layer material of composite packaging materials should have a high melting point, good heat resistance, be scratch-resistant and not easily roughened, have good printing performance, and good optical properties. Commonly used materials include paper, aluminum foil, cellophane, polycarbonate, nylon, polyester, and polypropylene.

The inner layer material should have good heat-sealability, adhesion, be odorless, non-toxic, oil-resistant, water-resistant, and chemical-resistant. Examples include heat-resistant plastic materials such as polypropylene, polyethylene, and polyvinylidene chloride.

The middle layer of composite materials with three or more layers typically uses materials with good gas and moisture barrier properties, light-blocking properties, and high mechanical strength, such as aluminum foil, polyvinylidene chloride, cellophane, paper, and polyethylene.

Adhesives are coated between layers for bonding. Generally, solvent-based thermosetting polyurethane adhesives are used between the outer and middle layers, and modified polypropylene emulsion adhesives or specially modified carbonyl-containing acrylic co-resins are used between the inner and middle layers. Adhesives for the outer layer require high bonding strength, simple process, and low cost; adhesives for the inner layer require high temperature resistance, high peel strength, non-toxicity, no odor, no impact on the nutritional components of food, and the ability to maintain the food's color, aroma, and taste well.

In recent years, foreign countries have developed methods to uniformly coat a layer of coating agent on the surface of substrate films to improve the gas tightness, oil resistance, and chemical stability of packaging materials, making them suitable for various food packaging applications. Commonly used coating agents include polyvinylidene chloride resin and polypropylene-polyvinylidene chloride resin. Coating methods include single-sided coating and double-sided coating.

II. Commonly Used Food Packaging Composite Materials

 

 

Double-layer Composite

E.g., Cellophane/Polyethylene, Paper/Aluminum Foil, Paper/Polyethylene, Polyester/Polyethylene, PVC/Polyethylene, Nylon/PVDC, Polyethylene/PVDC, Polypropylene/PVDC, etc.

Three-layer Composite

E.g., Oriented Polypropylene/Polyethylene/Unstretched Polypropylene, Polyester/PVDC/Polyethylene, Polyester/Cellophane/Polyethylene, Cellophane/Aluminum Foil/Polyethylene, Wax/Paper/Polyethylene, etc.

(For details, refer to the specific comparison table in the PVC heat shrink cold expansion article published concurrently)

Four-layer Composite

E.g., Cellophane/Polyethylene/Oriented Polypropylene/Polyethylene, PVDC/Cellophane/PVDC/Polyethylene, Paper/Aluminum Foil/Paper/Polyethylene, etc.

Five-layer Composite

E.g., PVDC/Cellophane/Polyethylene/Aluminum Foil/Polyethylene, etc.

Six-layer Composite

E.g., Polyethylene/Paper/Polyethylene/Aluminum Foil/Polyethylene/Polyethylene, etc.

Chlorinated Polyethylene (CPE) is a saturated polymer material, appearing as a white powder, non-toxic and odorless. It possesses excellent weather resistance, ozone resistance, chemical resistance, and aging resistance, along with good oil resistance, flame retardancy, and coloring properties. It has good toughness (flexible even at -30°C), good compatibility with other polymer materials, and a relatively high decomposition temperature. Decomposition produces HCl, which can catalyze the de-chlorination reaction of CPE. Chlorinated polyethylene is a polymer material produced from high-density polyethylene (HDPE) through a chlorination substitution reaction. According to their structure and use, chlorinated polyethylene can be divided into two main categories: resin-type chlorinated polyethylene (CPE) and elastomer-type chlorinated polyethylene (CM). Besides being used alone, thermoplastic resins can also be blended with resins such as polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), polystyrene (PS), ABS, and even polyurethane (PU).

 

 

 

 

Retort Pouch

A retort pouch is a composite film food packaging bag that can be sterilized at high temperatures, originally developed by the United States to meet the needs of astronauts. In the 1960s, Toyo Seikan Co., Ltd. of Japan obtained patent licenses and achieved mass production. Currently, Japan alone consumes up to 500 million retort pouches annually. Countries like the United States and Germany are also developing at an annual growth rate of 10%-20%.

Compared to glass and metal packaging containers, retort pouches can not only seal and sterilize but also have strong seals, are easy to open, have aesthetic decoration, are lightweight, have good impact resistance, and are convenient for transportation and carrying.

According to sterilization temperature and shelf life, retort pouches are divided into two types: ordinary sterilization pouches and ultra-high temperature sterilization pouches.

Ordinary sterilization pouches are generally heated and sterilized below 121°C and are roughly made of two to three layers of composite materials. Ultra-high temperature sterilization pouches are generally sterilized at 135°C, and the pouch materials are often more than three layers, with aluminum foil sandwiched in between, having a shelf life of 1-2 years. Some also use four or five layers of composite materials.

The outer layer material of retort pouches mostly uses polyester film, with a thickness of 10-16 micrometers, serving for reinforcement and high-temperature resistance; the middle layer material is 11-12 micrometer aluminum foil, primarily for gas and moisture barrier and light shielding; the inner layer material directly contacts food and forms the heat seal in the retort pouch, so in addition to meeting packaging hygiene standards, it also requires chemical stability, typically using non-toxic polypropylene with a thickness of 70-80 micrometers, or other polyolefin films like HDPE.

Currently, retort pouches are mainly used for packaging canned food, and in Japan and Western Europe, they are also used for packaging beverages and juices.

III. Several Major Factors Affecting Food Packaging Safety

 

 

Food packaging is called a "special food additive." It is the final process in modern food industry. To a certain extent, food packaging has become an integral part of food. However, the situation facing China's food packaging industry is not optimistic. Packaging, as the "close-fitting garment" of food, its safety in terms of raw materials, auxiliary materials, and processes will directly affect food quality and subsequently affect human health. Currently, most of the materials used to package food are plastic products. Under certain medium environments and temperature conditions, the polymer monomers and some additives in the plastic will leach out and transfer to food and drugs in extremely small amounts, thus causing health hazards to humans.

1. Packaging Materials

The residual harmful substances in food packaging mainly come from packaging materials, especially the inks and solvents used in the packaging printing process, which use harmful chemical materials such as benzene, n-hexane, and halogenated hydrocarbons as main raw materials. Moreover, such inks and solvents rich in harmful substances will also cause acute and chronic poisoning of operators during production, affecting both labor-capital relations and seriously affecting social stability.

The main food soft packaging materials include high-polymer materials such as polyethylene, polypropylene, polyester, and polyamide. These packaging materials show significant differences due to their inherent molecular structure, molding process, and added auxiliaries. Therefore, it is particularly important for food manufacturers to choose packaging materials suitable for their products, otherwise food safety problems will occur. For example, due to poor material barrier properties, the shelf life of liquid milk will be shortened or even deteriorate in a short time, while for preservation film, if there is no appropriate amount of air permeability, the freshness of vegetables cannot be guaranteed. The potential harm of PVC preservation film to the human body mainly comes from two aspects: one is that the residual amount of vinyl chloride monomer in PVC preservation film exceeds the standard, and the other is that DEHA plasticizer is used in the addition process of PVC preservation film. When encountering oil or heating, DEHA is easily released and enters the human body with food, which is harmful to health.

Bisphenol A is a chemical substance commonly used in plastic food packaging materials, and it is also used in tin can coatings and adhesives. Bisphenol A in plastic food packaging can be incorporated into food after heating, and it has estrogenic effects. Not long ago, American researchers found through animal experiments that bisphenol A may increase the risk of breast cancer in women.

2. Printing Ink

In addition to general bonding strength and abrasion resistance with the substrate, food packaging films also require resistance to sterilization and boiling treatment, as well as freeze resistance and heat resistance, to ensure that ink shedding and coagulation do not occur during transportation and storage.

After sampling and testing, relevant Italian authorities found trace amounts of photosensitive chemicals—isopropyl thioxanthone—in Nestle baby milk. This substance originally existed in the printing ink of baby milk packaging boxes, and it may be that a small amount of ink seeped into the baby milk. Similar incidents of printing ink contaminating food have also occurred in China. In 2005, a food factory in Gansu Province found that the potato chips it produced had a strong strange smell, and the factory immediately recalled all more than 600 boxes of products that had been distributed to the market. After testing by the chemistry laboratory of Lanzhou University, it was determined that the strange smell came from benzene in the printing ink of the food packaging bag, and its content was about 3 times the national allowable limit. If the residual amount of benzene exceeds the standard, it may cause cancer and blood system diseases.

4. Printing Process

Currently, food packaging bags are mainly gravure printing. Various food packaging bags seen in supermarkets, including biscuits, pastries, and milk powder packaging, are mostly printed with chlorinated polypropylene inks. However, countries such as Europe and the United States mainly use flexographic printing. Flexographic printing is slightly inferior to gravure printing in terms of dot reproduction and printing quality, but it has a significant advantage in environmental protection.

IV. Factors Affecting Food Safety

 

1. Biological Hazards

1) Bacteria

2) Fungi

3) Viruses

4) Parasites

2. Nutrition-Related Diseases

1) Over-nutrition

2) Malnutrition

3. Trace Chemical Substances

1) Naturally occurring

2) Contaminated by toxic substances

3) Substances intentionally or unintentionally added to food

4. Food Additives

5. Physical Hazards

6. Newly developed foods and new process products

7. Food processing, storage, and packaging processes

8. Counterfeit and substandard products

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