Types and uses of plastic films used in food packaging bags, worth collecting!

In recent years, the global demand for plastic film has shown a continuous upward trend, especially with packaging forms constantly shifting from rigid packaging to flexible packaging, which is also a major factor driving the growth in demand for film materials. So, do you know all about the types and uses of plastic films?

High-barrier plastic packaging materials

Refers to the barrier capability of plastic products against small molecule gases, liquids, water vapor, flavors, etc., which plays a role in preserving food quality, freshness, flavor, and extending shelf life. In the packaging industry, especially in food and pharmaceutical packaging, the use of barrier flexible packaging materials is increasingly emphasized. Due to their functionality, light weight, and ease of processing and transportation, barrier plastic packaging materials have developed rapidly in recent years.

The barrier performance of common monolayer films is shown in the table below:

 

 

 

 

The barrier performance of monolayer films is generally common. For most food packaging, further processing such as lamination is required.

With the increase in market packaging functions and heightened environmental awareness, the cost-effectiveness of products has become increasingly important. Flexible plastic packaging has evolved from single-layer films to multi-variety, multi-functional composite packaging films.

 

 

 

 

Coated films for food packaging

People often want to see the food inside the packaging to know with confidence what they are buying; this is a developing trend. Some foods require packaging that protects against light and UV rays to achieve food preservation. These light-sensitive foods can be packaged with metallized and aluminum foil composite films to form a barrier against oxygen and moisture. Some foods do not require light barrier, and to promote product sales, manufacturers will try to use transparent barrier film packaging.

The most common way to achieve transparent barrier food packaging film on the market is by using coated films, especially in the lightweight food packaging market for items such as biscuits, chocolates, and cheese. The global capacity has reached 200,000 tons and continues to show an upward trend.

High-barrier coated film

Coating is the process of applying a special functional layer to the surface of a substrate. Through coating, films can be endowed with new functions that the original substrate lacks or has poorly, such as barrier properties, aroma retention, heat-sealing performance, and better printability. At the same time, it can meet packaging requirements such as resistance to pasteurization and boiling.

Common substrates include: BOPP, BOPET, and BOPA

Currently, the most widely used barrier-coated food packaging film on the market is PVDC coated film (referred to as K-film below). PVDC's high barrier to oxygen makes it ideal for packaging meat and fish products; its barrier performance against water vapor allows it to maintain the fresh and crispy characteristics of food when packaging biscuit products; PVDC's barrier performance against odors keeps food taste and aroma unchanged, extending shelf life. Furthermore, PVDC complies with FDA regulations and can be in direct contact with food.

 

 

 

 

PVDC coated film (K-film) can be laminated with various materials. Suitable materials can be chosen based on packaging requirements:

 

 

 

 

Structure of coated products

 

 

 

 

Production process of coated products

 

 

 

 

For some lightweight food barrier packaging, the heat seal strength only needs to be 2-4 N/15mm. If packaged with a composite heat-seal layer of PE/CPP (>10N/15mm), it would result in packaging waste. By using double-sided coated film, both barrier properties and heat-sealing properties can be achieved, replacing barrier composite packaging and saving costs.

Since coated film manufacturers produce roll films, the biggest problem for double-sided coated films is the adhesion of the double-sided polymer coatings during transportation, as well as uneven running during subsequent application. Therefore, during the coating process, slip agents and anti-blocking agents are added to the coating material. These additives will slowly migrate to the surface of the coating, achieving the purpose of anti-adhesion and smooth running.

The barrier performance of common market barrier food packaging materials is shown in the table below:

 

 

 

 

The advantages and disadvantages of common market barrier food packaging materials are shown in the table below:

 

 

 

 

Trends in food packaging

1. Transparency

2. Safe and environmentally friendly

3. Meet food shelf life requirements

Therefore, for packaging materials, it is necessary to find environmentally friendly, more functional, and cost-effective high-barrier materials.

Environmentally friendly high-barrier water-based emulsion --- Polyvinyl alcohol barrier coating

Structural formula: －[CH2CH(OH)]n－

Advantages: High molecular structure compactness, its barrier performance is superior to PVDC

Disadvantages: Due to the presence of a large number of hydroxyl groups, PVOH coatings are highly water-absorbent in the environment, limiting their application.

To allow PVOH coatings to be directly applied in coating, existing market technology uses additives to shield some hydroxyl groups on its polymer chain, then adds nanomaterials to the coating to reduce its susceptibility to environmental humidity. The coating technology is then used to produce coated film, and subsequently, printing and lamination are performed on the PVOH coating, generally applied to high-sugar, high-fat, and nut foods.

 

 

 

 

Comparison of common packaging structures in the nut market is shown in the table below:

 

 

 

 

Common high-barrier materials

Currently, the commonly used barrier materials in polymer materials mainly include the following:

1. Polyvinylidene chloride (PVDC)

PVDC has excellent barrier properties against oxygen and water vapor.

PVDC's high crystallinity, high density, and the presence of hydrophobic groups result in extremely low oxygen and water vapor transmission rates, thus giving PVDC excellent gas barrier properties. Compared to other materials, it can better extend the shelf life of packaged items. Coupled with its good printability and ease of heat-sealing, it is widely used in the food and pharmaceutical packaging fields.

2. Ethylene-vinyl alcohol copolymer (EVOH)

EVOH is a copolymer of ethylene and vinyl alcohol, with excellent barrier properties. This is because the EVOH molecular chain contains hydroxyl groups, and the hydroxyl groups on the molecular chain are prone to forming hydrogen bonds, strengthening the intermolecular forces, making the molecular chain packing tighter, resulting in higher crystallinity of EVOH, thus exhibiting excellent barrier properties.

However, EVOH's structure contains a large number of hydrophilic hydroxyl groups, making EVOH prone to moisture absorption, which greatly reduces its barrier properties; in addition, the large cohesive force and high crystallinity within and between molecules lead to poor heat sealing performance.

3. Polyamide (PA)

Generally speaking, nylon has good gas barrier properties, but poor water vapor barrier properties, strong water absorption, and swelling with increasing water absorption, causing a sharp decrease in gas and moisture barrier properties, and its strength and packaging dimensional stability will also be affected.

In addition, nylon has excellent mechanical properties, toughness, wear resistance, good cold and heat resistance, good chemical stability, easy processing, and good printability, but poor heat sealing.

PA resin has certain barrier properties, but its high moisture absorption rate affects its barrier properties, so it generally cannot be used as the outer layer.

4. Polyesters (PET, PEN)

The most common and widely used barrier material among polyesters is PET. Due to its symmetrical chemical structure, PET has good molecular chain planarity, tight molecular chain packing, and easy crystal orientation, which gives it excellent barrier properties.

PEN, whose application has developed rapidly in recent years, also has good hydrolysis resistance, chemical resistance, and UV resistance. PEN has a similar structure to PET, except that PET's main chain contains a benzene ring, while PEN's main chain contains a naphthalene ring.

Because the naphthalene ring has a larger conjugation effect than the benzene ring, the molecular chain is more rigid and the structure is more planar, so PEN has better overall performance than PET.

 

 

 

 

Barrier Technology for High-Barrier Materials

In order to improve the barrier properties of barrier materials, the commonly used technical methods are mainly as follows:

1. Multilayer Composite

Multilayer composite refers to the process of combining two or more films with different barrier properties together through a certain process. In this way, the permeating molecules have to pass through several layers of film to reach the inside of the package, which is equivalent to extending the permeation path, thereby improving the barrier properties.

This method combines the advantages of various films to prepare a composite film with excellent comprehensive performance, and the process is simple.

However, compared with intrinsic high-barrier materials, the films prepared by this method are thicker, and problems such as bubbles, cracks, or wrinkles that affect barrier properties are prone to occur, and the equipment requirements are relatively complex, and the cost is higher.

2. Surface Coating

Surface coating uses physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), molecular layer deposition (MLD), layer-by-layer self-assembly (LBL), or magnetron sputtering deposition to deposit metal oxides or nitrides on the polymer surface, thereby forming a dense and excellent barrier coating on the film surface.

However, these methods have problems such as time-consuming processes, expensive equipment, and complex processes, and pinholes and cracks may occur in the coating during service.

3. Nanocomposite Materials

Nanocomposite materials are prepared by using impermeable flaky nanoparticles with a large aspect ratio through intercalation, in-situ polymerization, or sol-gel methods. The addition of flaky nanoparticles not only reduces the volume fraction of the polymer matrix in the system to reduce the solubility of permeating molecules, but also extends the permeation path of permeating molecules and reduces the diffusion rate of permeating molecules, improving barrier properties.

4. Surface Modification

Because the polymer surface is often in contact with the external environment, it is easy to affect the surface adsorption, barrier properties, and printing of the polymer.

In order for the polymer to be better applied to daily life, the surface of the polymer is usually treated. This mainly includes surface chemical treatment, surface grafting modification, and plasma surface treatment.

This type of method has easily satisfied technical conditions, simple equipment, and low one-time investment cost, but it cannot achieve long-term stable effects. Once the surface is damaged, the barrier properties will be seriously affected.

5. Biaxial Stretching

Biaxial stretching can make the polymer film oriented in both longitudinal and transverse directions, increasing the degree of order of molecular chain arrangement and making the packing tighter, making it more difficult for small molecules to pass through, thereby improving barrier properties. This method complicates the preparation process of intrinsic high-barrier polymer films, and it is also difficult to significantly improve barrier properties.

General-purpose Plastics

1.PE

 

 

 

 

2.PP

 

 

 

 

3.PVC

 

 

 

 

4.PS

 

 

 

 

5.ABS

 

 

 

 

Engineering Plastics

1. PC - Polycarbonate (bulletproof glass)

2. PA Polyamide

 

 

 

 

3. POM Polyformaldehyde

 

 

 

 

4. Saturated Polyester

 

 

 

 

5. Polyphenylene Ether

 

 

 

 

Special Plastics

1. Heat-resistant Plastics

1) High-temperature Nylon

 

 

 

 

2) Polyphenylene sulfide - PPS

3) Polysulfone

 

 

 

 

4) Polyimide

 

 

 

 

5) Polyaryletherketone

 

 

 

 

6) Liquid crystal polymer - LCP (domestic production by Watter in 2015)

Barrier Plastics

 

 

 

 

Corrosion-resistant Plastics

 

 

 

 

Photoelectric Resin

 

 

 

 

Degradable Plastics

1.Polylactic Acid

 

 

 

 

2.Polyhydroxyalkanoates

 

 

 

 

3.Polyaliphatic Esters

 

 

 

 

4.Polycaprolactone

 

 

 

 

5.Carbon Dioxide Copolymer (APC)

 

 

 

 

Chemical Fiber

1.Polyamides

 

 

 

 

2.Polyesters

 

 

 

 

3.Polyacrylonitrile

 

 

 

 

4.Polypropylene

 

 

 

 

5.Polyurethane