“High Barrier” is a highly desirable attribute, one that is required for many polymeric packaging materials. High barrier in technical terms refers to very low permeability to low molecular weight chemicals such as gases and organic compounds. High-barrier packaging materials can effectively maintain the original performance of the product and prolong the product life.
Common high barrier materials
At present, the commonly used barrier materials in polymer materials mainly include the following:
- Polyvinylidene Chloride (PVDC)
PVDC has excellent barrier properties to oxygen and water vapor.
The high crystallinity, high density and the existence of hydrophobic groups of PVDC make its oxygen permeability and water vapor permeability extremely low, so that PVDC has excellent gas barrier properties, which can better extend the shelf life of packaged items compared with other materials. In addition, it has good printing adaptability and is easy to be heat-sealed, so it is widely used in the field of food and pharmaceutical packaging.
- Ethylene-vinyl alcohol copolymer (EVOH)
EVOH is a copolymer of ethylene and vinyl alcohol with very good barrier properties. This is because the molecular chain of EVOH contains hydroxyl groups, and hydrogen bonds are easily formed between the hydroxyl groups on the molecular chain, which strengthens the intermolecular force and makes the molecular chains more tightly packed, so that the crystallinity of EVOH is higher, so it has excellent barrier properties. performance.
However, the EVOH structure contains a large number of hydrophilic hydroxyl groups, which makes EVOH easy to absorb moisture, thereby greatly reducing the barrier properties. In addition, the strong intramolecular and intermolecular cohesion and high crystallinity lead to poor heat sealing performance.
- Polyamide (PA)
Generally speaking, nylon has good gas barrier properties, but poor water vapor barrier properties and strong water absorption. It swells with the increase of water absorption, so that the gas and moisture barrier properties drop sharply, and its strength and package size stability will also be affected.
In addition, nylon has excellent mechanical properties, toughness and wear resistance, good cold resistance and heat resistance, good chemical stability, easy processing, good printability, but poor heat sealing.
PA resin has certain barrier properties, but the moisture absorption rate is large, which affects its barrier properties, so it is generally not used as an outer layer.
- Polyester (PET, PEN)
The most common and widely used barrier material among polyesters is PET. PET has excellent barrier properties due to its symmetrical chemical structure, good molecular chain planarity, tight molecular chain stacking, and easy crystallization orientation.
In recent years, the application of PEN has developed rapidly, and it has good hydrolysis resistance, chemical resistance and UV resistance. The structure of PEN is similar to that of PET, except that the main chain of PET contains a benzene ring, while the main chain of PEN is 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 comprehensive properties than PET.
Barrier Technology for High Barrier Materials
In order to improve the barrier properties of barrier materials, the following technical means are commonly used:
- Multilayer composite
Multilayer composite refers to the composite of two or more films with different barrier properties through a certain process. In this way, the permeable molecules have to pass through several layers of membranes to reach the interior of the package, which is equivalent to prolonging the permeation path, thereby improving the barrier performance.
The method is a composite film with excellent comprehensive properties prepared by synthesizing the advantages of various films, and the process is simple.
However, compared with the intrinsic high-barrier material, the film prepared by this method is thicker, and is prone to problems such as bubbles or cracks that affect the barrier performance, and the equipment requirements are relatively complex and the cost is high.
- Surface coating
Surface coating is the use of 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. Materials such as metal oxides or nitrides are deposited on the surface of the film to form a dense coating with excellent barrier properties on the surface of the film.
However, these methods have problems such as time-consuming process, expensive equipment and complicated process, and defects such as pinholes and cracks may occur in the coating during service.
Nanocomposite materials are nanocomposite materials prepared by intercalation composite method, in situ polymerization method or sol-gel method using sheet nanoparticles that are impermeable and have a large aspect ratio. The addition of flake nanoparticles can not only reduce the volume fraction of the polymer matrix in the system to reduce the solubility of permeable molecules, but also prolong the permeation path of permeable molecules, reduce the diffusion rate of permeable molecules, and improve the barrier properties.
- 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 to make the polymer better used in daily life, the surface of the polymer is usually treated. Mainly include: surface chemical treatment, surface graft modification and plasma surface treatment.
The technical requirements of this method are easy to meet, the equipment is relatively simple, and the one-time investment cost is low, but the long-term stable effect cannot be achieved. Once the surface is damaged, the barrier performance will be seriously affected.
- Biaxial stretching
Through biaxial stretching, the polymer film can be oriented in the longitudinal and transverse directions, so that the order of the molecular chain arrangement is improved, and the stacking is tighter, so that it is more difficult for small molecules to pass through, thereby improving the barrier properties. The preparation process of the characteristic high-barrier polymer film is complicated, and it is difficult to significantly improve the barrier properties.
Application of High Barrier Materials
In fact, high-barrier films have already appeared in daily life. At present, high-barrier polymer materials are mainly used in food and drug packaging, electronic device packaging, solar cell packaging, and OLED packaging.
- Food and pharmaceutical packaging
Food and pharmaceutical packaging is currently the most widely used field of high barrier materials. The main purpose is to prevent oxygen and water vapor in the air from entering the packaging to deteriorate food and medicines, and greatly reduce their shelf life.
For food and drug packaging, the barrier requirements are generally not particularly high, and the water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) of the material requiring barrier should be lower than 10g/m2/day and 100cm3/m2/day, respectively.
- Electronic device packaging
With the rapid development of modern electronic information, people put forward higher requirements for electronic components, and develop towards portability and multi-function. This puts forward higher requirements for electronic device packaging materials, which must not only have good insulation, but also be able to protect them from being corroded by external oxygen and water vapor, and must also have a certain strength, which requires the use of to polymer barrier materials.
Generally, the barrier properties of packaging materials for electronic devices are that the water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) should be lower than 10-1g/m2/day and 1cm3/m2/day, respectively.
- Solar cell packaging
Since the solar energy is exposed to the air all the year round, the oxygen and water vapor in the air can easily corrode the metallization layer outside the solar cell, which seriously affects the use of the solar cell. Therefore, it is necessary to encapsulate the solar cell components with high barrier materials, which not only ensures the service life of the solar cells, but also enhances the resistance strength of the cells.
The barrier properties of solar cells for packaging materials are that the water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) should be lower than 10-2g/m2/day and 10-1cm3/m2/day, respectively.
- OLED packaging
OLED has been entrusted with the responsibility of the next generation display since the early stage of development, but the short lifespan has always been a major problem restricting its commercial application. They are all very sensitive and easily contaminated, resulting in the degradation of device performance, thereby reducing the luminous efficiency and shortening the service life.
In order to ensure the luminous efficiency of the product and prolong its service life, the device must be isolated from oxygen and water during packaging.
And in order to ensure that the service life of the flexible OLED display is greater than 10000h, the water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) of the blocking material must be lower than 10-6g/m2/day and 10-5cm3/day, respectively. m2/day, the standard is far higher than the requirements for barrier properties in the fields of organic photovoltaics, solar cell packaging, and packaging technology for food, medicine and electronic devices. Therefore, flexible substrate materials with excellent barrier properties must be used to package devices. , in order to meet the strict requirements of product life.