Packaging, especially plastic packaging, can be seen everywhere in modern society. The rapid development of the packaging industry is due to its ability to protect and stabilize products for a long period of time, as well as to avoid damage, contamination and moisture in products. To achieve the best protection, high barrier materials are the inevitable choice.
From a broad perspective, barrier packages have been used for many years. Barrier packaging is designed to protect products and contents from contaminants, flavors, stains, odors, and sometimes to prevent leakage of contents.
In other cases, a multi-layer structure (laminate or co-extrusion) is needed to increase strength and barrier properties. Even for seemingly simple products such as fast foods, the packaging can be complicated. These multilayer structures often contain 7 or more layers of different plastics, each layer having a different structural barrier or adhesion.
The synthesis of a special barrier resin, polyvinylidene chloride (PVDC), was introduced in the 1950s. After 60 years of development, plastic barrier packaging has undergone tremendous development since then. Soon, ethylene/vinyl alcohol (EVOH) was put into commercial use in the 1970s.
The barrier packaging industry has changed continuously since then. Due to different requirements for different applications, the ideal polymer barrier material does not exist and may never appear. For example, in some cases, PVC films that do not have good oxygen barrier properties are commonly used in supermarkets as meat packaging displays, mainly because PVC films keep the meat red and attract customers during display time. However, if the meat needs to be transported or stored for a long period of time, it must have good oxygen barrier properties in order to prevent deterioration of the meat.
At present, barrier packaging plastics have indeed met the needs. However, the problem still exists, which restricts the application and development of barrier packaging plastics in many applications. These problems mainly include:
High cost, generally higher than simple single-layer plastic packaging, such as: LDPE and LLDPE.
When exposed to moisture, it is particularly susceptible to contamination and degradation. EVOH is the most suitable material to solve this problem, but the hydroxyl group provides excellent barrier properties while at the same time making the material easier to hydrolyze. Therefore, EVOH can only be used as an inner layer of a multilayer structure.
Handling and reuse issues. Most multi-layered packages contain multiple plastics (such as HDPE and PET), so they are not easy to blend and reuse.
Challenges from alternative materials. New materials such as glass and traditional materials such as vitreous silica veneers provide excellent barrier properties.
According to its oxygen and moisture permeability and air permeability, the barrier resin should have the following permeation characteristics:
1. Oxygen: OTR is less than 2 ml/mil thickness/100 in2 of resin for 24 hours at atmospheric pressure. Most of the oxygen transmission rates were measured at a temperature of 73[deg.] C. and a relative humidity determined according to specific conditions. Many conventional resins have an oxygen transmission rate of 5, while most newer resins are equal to or lower than 1. For example, the oxygen permeability of a standard metallized PET film does not exceed 0.3; materials with an oxygen permeability of less than 0.1 are listed as high barrier materials, including PVDC and EVOH. Other materials are classified as medium barrier materials.
2. Water vapor (moisture): Resin with a moisture vapor transmission rate (WVTR) of less than 0.1. We define and classify the moisture barrier polymer structure according to the classification of pharmaceutical blister packaging industry. In other words, a film with a moisture permeability higher than 0.1 is listed as an ultra-low barrier film, a film with a moisture content of 0.06-0.1 is a low-barrier film, and a film with a thickness of 0.03-0.06. Listed as medium barrier films, films equal to or lower than 0.03 are listed as high barrier films.
A variety of resin films with a moisture permeability of 1 have been on the market for many years. PTCFE is the latest best moisture-proof film, and its moisture permeability rate is lower than 0.3. It is considered as the only high moisture-proof film resin at present. The moisture vapor transmission rate is generally measured at a temperature of 100°C and a relative humidity of 90%.
Various Barrier Materials Introduced Polyoxymethylene (POM)
Polyoxymethylene is a multifunctional engineering resin. Its chemical structure, regular molecular structure and high crystallinity give it excellent resistance to moisture, gasoline, solvents and many other neutral chemicals.
POM can be injection molded and extruded. Specific uses include: air spray containers, fuel tank caps, chemical sprayers, soap dispensers, paint mixers, plumbing fittings, gears, creep-resistant housings, and wear-resistant surfaces.
The barrier test showed that the permeability of the unfilled Ticona Celcon polyoxymethylene and glass-reinforced grades of polyoxymethylene was approximately the same when the film thickness exceeded 10 mils. DuPont Delrin products have excellent permeability and strength properties for many substances, making them suitable as containers, especially for air spray containers. Ticona Celcon POM can withstand high humidity and high temperature water for a long period of time. Therefore, the material is widely used in water pipe related applications.