In Situ High Pressure Hydrogen Tribological Testing of Common Polymer Materials Used in the Hydrogen Delivery Infrastructure

Hydrogen gas is a promising potential fuel for use in near zero emissions vehicles. In order to realize the hydrogen economy, a refueling infrastructure is needed. Typical components within infrastructure include compressors, actuators, valves, regulators and hoses.

Many of which include some combination of metal and polymer materials. While the majority of metals used in these components have been heavily studied for their compatibility and high pressure hydrogen environments, there has been relatively little work studying the impact of high pressure hydrogen on polymer compatibility. Friction and wear properties of polymers under high pressure hydrogen are potentially very important to understand when designing dynamic seals and components.

Effects from hydrogen may be greater than those in metals due to a polymer's lower density and susceptibility to small molecule diffusion. In this video we will describe an in situ test methodology that we have developed to measure the wear and friction properties of polymers under a high pressure hydrogen gas environment up to 4000 psi. A note of warning.

This experiment requires the use of hydrogen gas under high pressures and requires extreme caution. Hydrogen gas is odorless, colorless and thus undetectable by human senses. Hydrogen is highly flammable and burns with a nearly invisible blue flame and can form explosive mixtures in the presence of oxygen.

High pressures in excess of 1, 000 psi add additional explosion hazards that must be appropriately planned for in preparation for any testing. This amount of stored energy represents a serious safety hazard. Therefore, due diligence, planning and safety evaluation must be performed before starting such an experiment to ensure that these hazards are mitigated.

The experiment presented here is performed in accordance with the appropriate safety precautions in an ASME-certified pressure vessel with a burst disk set to 5, 000 psi with proper ventilation. In order to analyze the friction and wear of the elastomers within a high pressure hydrogen environment we have designed an unique in situ tribometer operable in a sealed pressure vessel at 4, 000 psi. The tribometer measures the in situ frictional force required to move the elastomer sample in a linear reciprocating fashion against a vertically loaded steel pin in contact with the sample.

Additionally, a linear variable differential transformer or LVDT, measures the extent of the in situ indentation of the steel ball into the polymer surface during testing. To prepare the elastomer samples, we clean the stock polymer sheet by removing oils and talc powder applied during the manufacturing process. And then wash the polymer sheet with soap and water using a non-abrasive sponge.

To drive off any remaining water from the washing process dry the polymer strip in a drying oven set to approximately 85%of the material's maximum working temperature for approximately 72 hours. Then allow the sample to cool to room temperature. Store the washed and dried polymer strips in a humidity controlled environment near 25%relative humidity prior to tribological testing.

Now that the polymer strip is prepared, while wearing powder-free gloves mark one end of the polymer strip with a one-sided arrow. In a space about an inch from the first mark make a second arrow in the same direction as the first. These marks will ensure consistent sample orientation during the mounting process.

Now place an 7/8 inch circular dye around the second arrow and use the dye to cut a sample coupon from the polymer strip. Loosen the hex cap screws, securing the sample clamp on the in situ tribometer. Remove the screw and precision spring from the near corner of the sample clamp.

Taking care to ensure the arrow marked on the sample coupon is pointing to the back of the in situ tribometer slide the sample coupon into the sample clamp. Reinsert the precision spring and hex cap screw to the near corner slot. Insert a 0.113 inch gauge block in the gap between the top and bottom of the sample clamp and tighten the hex cap screws until the clamp is tightened around the gauge block, ensuring a 10%compression of the sample with the gauge block.

After ensuring power to the tribometer is off, to set the initial position of the sample stage, place a 0.95 inch gauge block along the wall of the in situ tribometer under the drive screw. Using the drive chain, back the sample stage along the drive screw until the stage is snug with the gauge block. To prepare the in situ tribometer for the experiment, first clean the surface of the steel ball with a soft cloth and an appropriate and an appropriate solvent, in this case acetone, to remove any potential polymer transfer film from the ball.

Slide the bronze ball carrier onto the rail perpendicular to the sample sled allowing the counter ball to slide into the keyhole and rest onto the polymer sample. Place the copper stationary weight on the bronze carrier. Using a hex key and two bronze screws, reattach the LVDT measurement arm to the bronze carrier, such that freely floating cylinder of the LVDT rests on the arm.

Adjust the clamp holding the LVDT in place, such that the LVDT is measuring near at zero point. Lower the tribometer assembly into the pressure vessel, ensuring that the thermal well on the top flange of the vessel will lower into the gap between the tribometer and the wall of the vessel. Wrap the hastelloy o-ring with two and a half layers of PTFE tape and place the o-ring in the groove in the lip of the pressure vessel.

Reconnect the five power wires for the tribometer motor, the four data wires for the load cell, and the five data wires for the LVDT. Now, to seal the pressure vessel, lower the top flange of the pressure vessel to close it, taking care to lower the top gently onto the PTFE-wrapped o-ring. While lowering, be aware of the position of the data and power wires, such that they are not pinched between the lid and the lens ring.

Insert the bolts into the numbered holes on the top flange in ascending order until they are finger tight. Using a hex key tort the flange bolts in ascending order to hand tight and repeat until the bolts can no longer be tightened. Starting at 90 foot pounds and increasing in thirty foot pound increments, use a torque wrench to torque the flange bolts in ascending order until they are torqued to 200 foot pounds.

This portion of the experiment requires the use of high pressure hydrogen. It is critical that the full safety evaluation and appropriate safety factors are considered when using flammable and high pressure gas. Connect the swagelok gas fittings to the autoclave lid and flush the pressure vessel with low pressure around 80 psi Argon gas for approximately an hour until the oxygen content of the vessel drops below 10 ppm.

Once the oxygen levels are safe, slowly, at less than 40 psi per second, flush the vessel with 99.995%purity hydrogen gas up to 1000 psi, then slowly vent the gas. Repeat the flushing process two more times. After flushing the pressure vessel, slowly fill the pressure vessel with hydrogen gas up to 2, 000 psi and allow the vessel to rest for 10 minutes, such that the temperature of the gas within the vessel equilibrates to room temperature.

Continue to fill the vessel with high pressure hydrogen gas, this time up to 3, 000 psi and then wait another 10 minutes. Finally bring the vessel up to the target 4, 000 psi with hydrogen gas and close off all valves. Allow the polymer sample to soak for approximately 12 hours in the hydrogen gas before starting the experiment.

The experiment is performed for one hour of linear motion, which translates to 120 linear reciprocating cycles of the sample stage. During the experiment, the load cell measures the amount of force required to slide the surface of the elastomer sample coupon across the loaded steel ball. Additionally, the LVDT measures the penetration depth of the steel ball into the elastomer sample as a function of time.

Once the experiment has completed, slowly depressurize the system at a rate below 50 psi per second to prevent possible explosive decompression and thereby damage to the polymer sample surface not representative of any friction or wear mechanisms. From the frictional force measured by the load cell called F of k, and the normal force exerted by the steel ball, called F of n, the kinetic coefficient of friction mu can be calculated. Using the LVDT penetration depth data x, and the pressure, volume and time, the wear factor, the material's resistance to wear, can be calculated.

These plots contain representative data gathered using the test methodology described in this video, while testing EPDM rubber and high pressure hydrogen gas, the blue data, and an ambient pressure air, the black data. In the data set shown here, the coefficient of friction for the steel ball on the EPDM polymer sample was increased in high pressure hydrogen gas as compared to ambient air. However, the wear resistance of the EPDM sample was lower in high pressure hydrogen than in ambient pressure air.

This is most likely due to the increased density of the pressurized samples. The hydrogen fuel delivery infrastructure is expanding quickly to meet the needs of new alternative fuel vehicles. The safety and reliability of this infrastructure will rely on understanding soft material's compatibility with high pressure hydrogen.

Here we have demonstrated a test methodology for measuring the tribological properties of polymer materials in a high pressure hydrogen environment using an unique in situ tribometer. This test methodology can be used to assess the fitness of polymer materials for service in the hydrogen infrastructure.