The tie-resin should be selected based on the resins being bonded. For example, an anhydride-modified polyolefins (AMP) tie-resin is going to provide minimal benefit for adhering polyolefins to each other. Also, a tie-resin that performs well for one substrate may or may not perform well with a different substrate. The processing conditions, any secondary operations (e.g., orientation or thermoforming) and the end-use environment will all affect the performance of the tie-resin. It is imperative that the conditions most likely to cause adhesion failures are identified.
These conditions may occur during retort, orientation during processing, at a point of high stress in a thermoformed part or during use by the consumer. Processors often desire maximum adhesion in their multilayer structure, however, maximum adhesion means using a thicker tie-layer or a higher performance resin which are both costly alternatives. Therefore, the minimum adhesion should be determined by measuring adhesion under the most severe conditions and including a safety factor.
This information can then be used to select the most cost-effective tie-resin and layer thickness. Any regulatory requirements, such as those of the Food and Drug Administration (FDA), must also be identified. The performance requirements other than adhesion should also be considered. The tie-resin, which is often thought of as simply an adhesive, can effect machinability, clarity and physical properties, such as puncture, stiffness and moisture vapor transmission rate (MVTR).
Once this information has been collected, the proper Plexar® resin can be identified based on the resin type, melt index and cost. A grafted resin will have similar properties as the resin backbone, which can be useful in selecting a resin type. Key properties for polyolefins are shown below. For example, a HDPE tie-layer will have higher stiffness and lower MVTR than a comparable EVA but with lower clarity. The graph below shows a comparison of MVTR values for various tie-layer resins:
| Two key characteristics for each type of anhydride modified polyolefin resin: |
|
EVA |
Clarity |
Polarity for adhesion |
|
LDPE |
Process ability |
Clarity |
|
LLDPE |
Elongation |
Tensile strength |
|
HDPE |
MVTR |
Stiffness |
HDPE resins have high crystallization that gives good chemical resistance and better thermal resistance than LDPE. However, the high degree of crystallization results in higher shrinkage compared to other polyolefins that can lead to lower adhesion due to high stress at the interface. Therefore, HDPE tie-resins are typically used in combination with HDPE resins. For film applications, LLDPE tie-resins are increasingly popular due to a good combination of physical properties and processability.
Information that is needed to select a proper tie-layer resin:
- Adhesion strength
- Economics
- Conversion method(s)
- Substrates to be bonded
- End-use requirements
- Process parameters
- Physical requirements
- Optical requirements
- Regulatory requirements
The melt index should be selected so that the layers in contact have similar viscosities; otherwise, flow instabilities can lead to waviness or poor layer distribution. Furthermore, the interior layer should have the highest viscosity, while the skin layer should have the lowest viscosity. Simply matching the melt index may not always be sufficient but is a good starting point. For example, EVOH is not as shear thinning at PE, so at high shear rates it would be necessary to use resins with a different melt index.
Example: Cast film was being produced with the following structure: LDPE/tie/EVOH/tie/EVA and melt indices: 6/2/4/2/6. A flow instability was seen as non-uniform layer distribution and parabolic flow lines. By changing the melt index of the EVOH from 4 g/10 min to 2.4 g/10 min, the cast film quality was greatly improved.
A tie-resin should be selected that gives the overall best value to the processor. The processor who selects a tie-resin simply based on the lowest cost resin may be missing cost savings by not downgauging a higher performing, but more expensive, tie-resin or may not be achieving maximum product performance based on the properties of the tie-resin.