Polymers are more alive than you may realize. There are molecules within them that wiggle, others that move from here to there, and finally some that permeate all the way through. Polymer properties change as all this motion is occurring. When this is not understood or anticipated, products fail. The good news is that if we consider the product’s environment, we can choose a material that will ensure your product lasts.
Small gas molecules, such as hydrogen, oxygen, and even water vapor are constantly diffusing through polymers. They are small enough fit in between the amorphous polymer backbone chains and can cause issues for real life products. Likewise, there are additives, such as plasticizers, that are mobile and move through polymer.
- Plasticizers diffuse out of the polymer and leave the material less elastic and more brittle. For example, that new car smell is plasticizers leaving your dashboard and making it more vulnerable to cracking.
- Oxygen permeates through packaging and reduces shelf life, thus spoiling food contained within. Oxygen can also leak through seals in packaging; an EWI spin-off, UltraThinSeal, has found that ultrasonic seals for potato chip bags allow less oxygen than heat seals, see Figure 1.
- Hydrogen absorbs into elastomers seals for pumps and storage containers which can cause catastrophic failure during decompression due to supersaturation and foaming.
- Water vapor (humidity) permeates into packaging and cakes dry powder products like sugar or changes the composition of fluids like brake fluid.
- Water vapor permeates out of packaging leaving liquid products with decreased volume and/or increased concentration in liquid medicines.
- Water vapor absorbs into a plastic that is ultrasonically welded leading to voids and low strength.
So, barrier properties are important for everyday applications – but how do these things happen? Gases move in or out of polymers based on pressure or concentration gradients. Polymers attract like gas molecules, so a polar group on polymer chain will absorb more water than a non-polar group. For example, Nylon has polar side groups and it absorbs a relatively high percentage of water humidity from the atmosphere, while high density polyethylene (HDPE) is non-polar and allows very little (0.01%). Therefore, HDPE has a lower water vapor transmission rate. Barrier properties can be tuned to meet the needs for the product.
Crystallinity and fillers in the polymer effect gas molecule permeability. Spherulites are crystalline arrangements of polymer backbone molecules that pack much tighter than the amorphous regions. The tight packing excludes gas molecules from entering. Similarly, fillers such as glass and calcium carbonate also block molecules. The gas molecules must find a circuitous path around the obstacles in the polymer to permeate through. So, polymers with high crystallinity or filler content typically absorb less gas and have lower permeation rates.
Permeation rates of gases through polymers can be measured. One of the most popular and accurate instruments is a permeation analyzer. For this test, a film of the material is clamped into the instrument. One side of the film is exposed to the gas of interest (e.g. oxygen or water) and the other side is flushed with an inert gas (nitrogen or argon). There is a detector on the flush side to determine when the gas of interest permeates through the material. Solubility, diffusivity, and permeability can all be determined from this measurement. The permeation rate is typically linearly dependent on the gas concentration and exponentially dependent on temperature.
The higher the gas solubility in the polymer, the more it increases the void space and thus allows greater polymer chain movement. This can relieve molded-in stress, and in extreme cases deform parts. Higher absorption also allows for molecules, such as plasticizers, to diffuse within the polymer, combine and form micro-regions with much different properties. These plasticizers can also diffuse all the way to the polymer surface and render it stiffer and more brittle.
Overall, barrier properties must be considered when choosing a material for a product. The product’s environment and expected lifetime are used along with its barrier properties to make predictions of how much gas will absorb and permeate through the polymer. This can give you a clear picture of the risk of product failure during its intended lifetime.
Are you experiencing barrier diffusion issues with your plastic products? EWI can help. Contact Jeff Ellis at [email protected] or 614.688.5114 to learn more.
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