In recent decades, plastics have been used extensively in multilayer structures, especially for packaging applications. Multilayer structures can be used when one layer does not provide the appropriate performance characteristics for the intended application. The most common example of multilayer structures is in food packaging, where properties such as moisture retention, aroma resistance, heat resistance, and oxygen barrier properties are among the many important properties required. Since it is difficult to provide such performance requirements in one layer, multilayer structures become necessary. Depending on the desired end physical properties, the structure can have multiple layers (3, 5, 7, 9 or 11). The most common polyolefin used for multilayer structures is the binding layer resin. Adhesive resins modified with acids or anhydrides are often called binding layer resins.
Multilayer structures comes up with a bonding process. When two layers are similar in nature, "polar/polar" or "non-polar/non-polar",(Polarity & Non-polarity: In chemistry, polarity refers to the inhomogeneity of the charge distribution in a covalent bond or a covalent molecule.
Uniformity. If the charge is not uniformly distributed, the bond or molecule is said to be polar; if it is uniform, it is called nonpolar.) bonding is usually achieved by physical force.
However, when bonding between two layers with different properties (e.g. "polar/non-polar" substrates) is required, the bonding method requires one of the following methods:
- Thermal evaporation of one of the layers;
- Surface treatment of the substrate to induce polarity and thus improve bonding properties;
- Use of "extrudable" adhesives, such as bonding layer resins, ethyl acrylate (EAA), methyl acrylate (EMA), etc.
Polyolefins will adhere to barrier resins, with some exceptions. Exceptions are EVOH, nylon, polyester and a few other resins. The degree of adhesion of barrier resins to polyolefins varies. The following show the bonding properties between the different materials.
LDPE and EVA copolymers are cost-effective bonding adhesive resins. If these two resins do not provide the required adhesion and desired properties in a cost effective manner, then a bonding layer resin should be used. The primary purpose of the bonding layer resin is to act as an adhesive layer to bond two different substrates together, i.e. a "polar" substrate and a "non-polar" substrate, to meet the technical requirements of the end application.
Prior to the advent of bonding layer hot melt resins, the technology for manufacturing multilayer structures involved a "lamination" process. The earliest lamination processes were performed to combine various film structures. Film lamination techniques often used adhesives, which resulted in uneven coverage and, more importantly, odor problems that led to material waste. This resulted in plastic packaging structures that were not as cost effective as paper for packaging applications.
With the advent of "co-extrusion" technology, the problems associated with laminating technology have been solved. Co-extrusion is a method that allows two or more resins to be combined in a single extrusion operation to produce a single layer. Although co-extrusion is more efficient than lamination, it has its own disadvantages, such as:
- The bond strength varies with the film structure;
- In some cases, the polymers do not bond to each other.
These drawbacks led to the introduction of an intermediate "adhesive bonding layer", which combines the different layers together. Bonding layer resins are used as part of various packaging structures in both flexible and rigid applications.
Bonding layer resins were originally introduced for flexible packaging applications (e.g. cheese packaging, meat packaging, etc.). With the advent of EVOH, the use of bond layer resins in rigid packaging applications (e.g., multi-layer bottles, barrier sheets, etc.) began. Binding layer resins are now used in automotive fuel tanks to provide adhesion between HDPE and EVOH layers. Other applications where bonding layer resins are important are nylon modification, coupling agents for PP, pipe coating applications, wood plastic lamination and others.
ADI Chemical Market Resources (ADI CMR) has extensive experience in (1) polyolefins, (2) maleic anhydride and (3) olefin blends and has conducted this comprehensive global strategic business/technical analysis for the "Bonding Layer Resins" industry.
This report will be beneficial to (1) current and future players in the binding layer resins market, (2) polyolefin producers, compounders, (3) packaging industry players, and (4) end-users, entrepreneurs, and organizations trying to understand these complex issues and capture the future growth of the market. The future article updates will have deep discussion for the adhesive resin.