By: Buffalo Manufacturing Works Team | September 24th, 2020

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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