By: Buffalo Manufacturing Works Team | November 5th, 2020

A polymer can act rubbery or glassy depending on its
temperature or rate of deformation, such as from an impact. This is important
for medtech devices that are stored at low temperature, vibrate, or are
required to pass drop testing. In general, the quicker the polymer is impacted,
the more brittle it acts and the greater the chance it will crack. Let’s examine
why this phenomenon occurs, ways to measure it, and how to use material
selection to avoid failures.

Some real-world examples are:

• An autoinjector drug-delivery device cracks
  after a larger drive spring is implemented. The larger drive spring releases
  faster which decreases impact time, thus causing the polymer to act more
  brittle.
• An ultrasound probe used to visualize a fetus in
  utero operates at 5 MHz and cracks a durable polymer housing with its high
  frequency signal.

The rubbery to glassy transition occurs at the polymer’s
glass transition temperature (T_g). At temperatures lower than the T_g,
the polymer acts glassy, whereas above it, the polymer acts rubbery. The
transition is based on the amount of thermal movement that the polymer chains
possess. Like a cold-blooded animal, at low temperatures the chains have
slower, more limited mobility than at higher temperatures. Along the same
lines, when impacted at low temperature the chains do not have time to
rearrange and accept the stress, so the polymer responds plastically by cracking
(glassy). At higher temperatures, the stress can be absorbed by fast chain
movement and the polymer will respond elastically by absorbing the energy (rubbery).

This phenomenon also applies to the polymer’s behavior when
impacted at different frequencies, such as spring size in an autoinjector. Frequency
and impact time are inversely proportional (frequency = 1 / impact time), so a
larger spring contacts the polymer with lower impact time, inducing higher
frequency.  Consider a polymer like
polypropylene (PP) or silicone that is rubbery at room temperature. If the
spring impacts the polymer faster than the chains can rearrange, then it will
act glassy even though the temperature is above the T_g. The high
frequency shifts the T_g to a higher temperature than its stationary
T_g, making the polymer more susceptible to cracking.

The frequency induced shift in T_g can be
experimentally determined using a dynamic mechanical analyzer (DMA). For
frequencies less than 100 Hz this test is simple to run as temperature sweeps
at multiple frequencies. The DMA oscillates a polymer, with small amplitude,
and measures the in-phase component (storage modulus) and out-of-phase component
(loss modulus). Tan Delta, the damping behavior, is the ratio of the loss to
the storage moduli. A maximum in Tan Delta is a good indicator of the polymer’s
T_g. As displayed in the figure, the T_g of polyethylene
terephthalate (PET) shifts 10°C when the frequency is increased from 0.1 to 10
Hz.

Figure 1. Polyethylene terephthalate (PET) frequency induced T_g shift (plot reproduced from TA Instruments Thermal Analysis-2010 data)

Higher frequencies are common in medical applications and can cause large T_g shifts to occur. Consider the ultrasound probe that operates at 5 MHz; it can increase the T_g of a polymer by tens of degrees. This is enough to shift the T_g of PP from -10°C to ambient temperature. This shift can be estimated using the DMA by running frequency sweeps at multiple temperatures. Then, using time-temperature superposition (TTS) techniques, the data can be shifted out to higher frequencies to create a master curve in order to find T_g values at high frequencies. (For more background on TTS, see my earlier post on Long-term Polymer Properties). In essence, the lowest temperature data gets shifted to the highest frequency. This technique has been used to shift data orders of magnitude into the megahertz range and can explain how a silicone damping material can shatter when in use with a 5 MHz oscillating sensor.

When performing material selection to reduce the risk of
product failure, there are many factors that must be considered. A polymer like
PP might be chosen because it is inexpensive, easy to mold, and the data sheet
indicates it has a high enough impact strength to pass a drop test at room
temperature. However, when it is put into use in a high frequency device, it
ultimately fails by brittle fracture. The T_g, how it will shift at
use frequencies, and the product’s use temperature range should be considered
in this scenario. The T_g is not always listed on the polymer’s
technical data sheet, but a range for a class of polymers can be found in various
technical resources. A rule of thumb is to choose a polymer that has a T_g
at least 10°C away from the expected temperature range the product will
encounter. For products that vibrate at high frequency or experience impacts
like released springs or drops, a larger difference from the T_g
should be implemented to avoid product cracking.

Unexpected failures can occur during prototyping if the dynamic polymer properties are not considered early in the design cycle. Once the injection molding tools have been machined, a material change can be costly and time consuming. Material choice and proper characterization of expected behavior across the product’s frequency and temperature use ranges will mitigate risk of failure.

If you would like to learn more about how to avoid these issues, contact Jeff Ellis at [email protected] or 614.688.5114.

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To read more articles about avoiding material failure in plastic products by Jeff click here.

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