Pressure Sensitive Materials: Paint, Film and Polymers Overview

Posted by Steve Hanly on May 24, 2017

nasa pressure paint imagePressure sensors are nothing new.  But they are expensive and can only measure pressure in discrete locations.

I want to highlight a few pressure sensitive materials we've explored for various engineering applications as alternatives to the traditional pressure transducer:

  • Pressure sensitive paint
  • Pressure sensitive film
  • Strain reactive polymers

In this post I'll provide a brief overview of the three materials. Additional resources on where to buy these materials and find out more information is also included.  And as always, don't hesitate to reach out if you have any questions!


Pressure Sensitive Paint

Origin

Pressure sensitive paint (PSP) has been around for around 25 years. Pressure sensitive paint was primarily developed by and for NASA.  They recognized the difficulty of measuring drag loading on airfoils with traditional pressure sensors and looked for alternatives.  Now other aerospace applications concerned with testing drag (like automotive) are using pressure sensitive paint.

The Technology

The pressure sensitive paint is based off the sensitivity of certain luminescent molecules to the presence of oxygen.  Through a process called oxygen quenching, the fluorescence of the molecules change based upon oxygen pressure.

The system consists of an illumination source to excite the molecules, a photodetector, and then of course the paint.  Figure 1 provides an overview of the system.

How Pressure Sensitive Paint Work
Figure 1: Pressure sensitive paint setup requires an illumination source and photodetector. Image courtesy of Ohio State.

The Benefit & Applications

Pressure sensitive paint can save quite a bit of time and money in instrumentation compared to methods utilizing arrays of pressure taps.  Therefore its used primarily in wind tunnel testing even though the technology (oxygen sensitivity) was first explored for medicine.

There are a few drawbacks; the paint is quite susceptible to temperature and the photodector and illumination source can be expensive to initially setup.  NASA is working on a new version to increase its response time and reduce its temperature dependency.  Figure 2 provides an example model in an Air Force test facility with the paint applied.

Pressure Sensitive Paint Wind Tunnel Model
Figure 2: The pink paint shines when exposed to blue light, glowing brighter or dimmer depending on air pressure in the area. Image courtesy of US Air Force.

Additional Resources

We're by no means an expert in pressure sensitive paints; but we do find them interesting! Here are a couple resources that provide more information.


Pressure Sensitive Film

The Technology

Pressure sensitive film utilizes a fairly intuitive technology.  The film consists of a micro-encapsulated color layer and a color-developing layer.  When a defined pressure is exceeded these microcapsules break and react with the adjacent color-forming layer. The results are monochromatic (one color) but the shading of the color will indicate pressure level.

Pressure Sensitive Film Technology
Figure 3: Under pressure encapsulated microcapsules break and react with a color-forming bottom layerImage courtesy of Fujifilms.

The Benefits

We've used the Fujfilm Prescale products before to diagnose issues with our piezo production line.  We manufacture these piezo packs under a high load; and we make a "book" at a time so the pressure is not uniform (by design).  It's difficult to know exactly the pressure distribution though throughout the book due to the number of layers and of varying thickness (and not uniform) and stiffness.  Not to mention, these are made at an elevated temperature - so it's hard to model!

These films have become quite useful in diagnosing issues with our lamination process. They are used in similar applications like automotive (gasket assembly and balance), LCD manufacturing, semiconductor lamination, and printed circuit board solder squeeze determination.

Pressure Sensitive Film Application
Figure 4: The color distribution and intensity in the film is directly proportional to a pressure in that localized areaImage courtesy of Fujifilms.

They come in a variety of pressure ranges (0.05 MPa to 300 MPa) with a cost around $0.10 to $0.50 per square inch.  We've only used Fujifilms Prescale; but there are other options.


Strain Reactive Polymers

The last material on our list is the most obscure and it's coming out of Duke University. They have developed a polymer that releases a chemical signal (to change color) in response to strain.  The coolest thing about this elastomeric network configuration is that this process is completely reversible with full shape recovery.  There isn't a whole lot of information out there on the technology besides this paper they published in 2014.  We have been in contact with them though and they are very interested in exploring any commercial opportunities.

Strain Reactive Polymers
Figure 5: The film can be activated by simple tension (A), or by rolling a patterned object to generate circles (B), lines (C), or a cross-hatch pattern.  The same sample was used for all images highlighting the reversible characteristic of the materialImage courtesy of Duke University.

The most impressive nature of this material is its repeatability highlighted in Figure 6. The strain induced in the material leads to covalent activation of the spiropyran.

Strain Reactive Polymer Hysteresis
Figure 6: Relaxing the sample and illuminating with bright white light for 1 minute returns the sample to its colorless state. The stress-strain plot shows the hysteresis in the first curve, followed by nine reversible cycles before a final elongation to failureImage courtesy of Duke University.

One word of caution though is how the material is quite temperature dependent as shown in Figure 6.  Although some folks here at Mide highlighted how this behavior could be used to your advantage given the right application and design.

Strain Reactive Polymer Temperature Dependency
Figure 6: The strained films exhibit both a lower critical temperature for measurable activation at elevated temperaturesImage courtesy of Duke University.

Again, this material seems in its infancy. There's not much content and information out there about it (besides this paper they published in 2014). But the authors have expressed interested in exploring any commercial opportunities.


Stay in Touch

If you'd like to hear more about cool technology and materials we come across, subscribe to our engineering blog.  We've planned a blog series around sharing best practices, and additional resources to help fellow engineers.  And if you're interested in partnering with us on a development opportunity in the future, please don't hesitate to reach out and contact us!  Hopefully we get to work together on an exciting new technology soon, one with a large commercialization opportunity that we're both excited about!

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Steve Hanly
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Steve Hanly on May 24, 2017
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Topics: General Engineering


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