1© 2009 Ops A La Carte

Accelerated Life Testing of Thermoplastic Polyurethane

(TPU) Insulation

Problem Statement Consumer Products warehoused in Georgia experienced

insulation degradation Actual degradation, remaining insulation life if damaged

and expected life of undamaged insulation all needed evaluation

The product in question: a thermoplastic polyurethane (TPU) but unknown if it is ester or ether based.

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Sample Undamaged Specimen

Sample has visible surface oily residue

TPU wire is 1/8” ϕ Most samples are

cracked or cracks are being initiated

Millions at stake

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Properties of TPU Understand properties

helps to know how to accelerate testing

Unknown whether it is ether or ester based

Polyether based TPU resistant to hydrolysis

Polyester based TPU is resistant to temperature

Standard Properties

Range

Max Operating Temperature

-40C to 80C

Proper Storage Temperature

23 C / 74 F

Proper Storage Rel. Humidity

50%

Activation Energy

0.2-1.0 eV

Heat of Combustion

25,000-29,000 J/g

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Strength vs Weaknesses

Polyester Based: Highest heat resistance Highest mechanical

properties Highest resistance to

mineral oils

Polyether Based: Highest hydrolysis

resistance Best low-temperature

flexibility Resistance to

microbiological degradation

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Hydrolysis Effects on TPU

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Failure Parameters Based on material properties and trends, the failure

parameters need to be identified Analysis evaluates possible acceleration factors and will

provide some foundation for forensics Temperature, hydrolysis and other environmental factors

specific to storage location have to be isolated Eliminate the outliers such as biological factors, potential

chemical exposure and mechanical stresses

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Types of Reliability Testing

Types of reliability testing:1. HALT (AFR)2. ALT3. RDT

Choose Accelerated Life Testing because MTTF is desired

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Acceleration Factors, Af for Temperature and RH

Af refers to the amount of energy required to initiate an internal chemical change

As identified the activation energy for TPU is 0.2-1.0 eV

Af depends on choice for activation energy

Test Temp ©

Relative Humidity

Accel.Factor

20 75% 2.740 75% 12.160 75% 46.320 80% 3.240 80% 14.760 80% 56.220 85% 3.840 85% 17.760 85% 67.420 90% 4.640 90% 21.060 90% 80.020 95% 5.440 95% 24.760 95% 94.120 100% 6.340 100% 28.860 100% 109.8

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Forensic ALT

Objective is to determine the MTTF using major stressors, adequate sample size and confidence level

25 test samples 50, 100, 150 and 240

test hours 75%, 80%, and 90%

confidence levels Combined Af of 3, 15,

21

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MTTF Calculations

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Test Methodology

High confidence requirement means 90% level Combined stressors (temperature and humidity Not all TPU will have aged at the same rate Stepped temperature approach, incremental

changes allow for early failures of TPU Let the Customer decide the desired temperature,

suggested between 40°C-60°C

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Test Methodology (cont’d) 10°C increments for 10 minutes up to the upper

functional operating temperature limit. At the end of each temperature step perform a visual check

for cracks or crack initiation of the insulation. If a failure is visually found, note the failed temperature

and quantity of failure. Remove the sample and replace with another sample.

Add the relative humidity to 90% wait for temperature to stabilize, since there will be a drop with water vapor

The expected acceleration and hence degradation of the TPU insulation will have been reached.

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Visual Checks and Balances

The specimens were to be visually checked every hour over a normal working day.

Failure will likely be rapid with the onset of an initial crack and subsequent propagation.

After reaching the first 50 hours of testing, a MTTF can be determined from calc Table

Could continue to the next milestone of 100 hours and so on up to 200 hours

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Some Calculations

50 hours at 40 C / 80% RH we had seven (7) failed out of 25 specimens1. MTTF = 1593 @ 90 % confidence2. MTTF = 1832 @ 80 % confidence3. MTTF = 1936 @ 70 % confidence

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Conclusions Isolated failure parameters, identified two as possible

stressors and determined acceleration factors Provided MTTF table with acceleration factors for relative

humidity and temperature with sample size of 25 Customer decided on temperature 40C, relative humidity

of 80% giving us an acceleration factor of 15 7 of 25 samples failed after only 50 hours of testing ALT provided MTTF of 1593 at a confidence of 90% Forensically, testing confirmed stressors, failure

mechanisms (cracking and propagation through the TPU) Limited test time and cost (2.5 days of lab time)

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