Automated Swell Index Measurements of Films Using
THERMO MECHANICAL ANALYZER (TMA)

SWAYAJITH SAHADEVAN swayajith@gmail.com Abstract
Dimensional instability on finished plastics products due to changes in humidity is one of the leading causes of product failure. The test lead times associated with measuring changes in dimensions due to humidity changes are prohibitively longer & lot of machine time is consumed in the process. This can become a limiting scenario wherein multiple materials are scaled up for measurements or multiple formulation variables are being analyzed. In this paper, we propose to use the “Swell Index” as the prime factor to be used for evaluating the changes in dimensions. Swell Index of a material is defined as the ratio of difference in dimension to the original dimension as a function of temperature or humidity and is expressed as percent swell index (temperature) or percent swell index (Humidity) respectively. Swell Index is measured using TMA by measuring the dimensional changes as a function of time while cycling the environment of the test sample between dry Nitrogen (~0% RH) and wet nitrogen (100% RH). This analysis can be accomplished using few grams of samples without manual intervention, thereby leading to significant reduction in test lead times.
Key words: -Thermo Mechanical Analysis, Swell Index, Dimensional stability

Principle
When hygroscopic polymeric materials experience humidity, the dimension of the material changes .The dimension changes due to moisture absorption can be measured either by gravimetric techniques (TGA) or by mechanical techniques (TMA or DMTA). Here we discuss about the mechanical quantification and the behavior of the dimensional variation as a function of humidity. The standard instruments available in the market do not come equipped with humidity chamber. We used a lab made humidity chamber, which apply nearly zero humidity i.e. dry nitrogen for a specified time and 100% humid nitrogen to the sample in alternate succession.

Instrumentation
TMA measures the dimensional changes of a material as a function of temperature, time or force. These data provide us with information on the modulus, coefficient of thermal expansion, creep stress relaxation transition temperatures, softening points etc of polymeric materials. It is widely used in quality control as well as research environment.

Generally the TMA is used to measure CTE (Coefficient of Thermal Expansion) and other parameters like modulus, creep, stress relaxation, etc. These parameters are measured or calculated by employing different modes and clamps. For this study the TMA 2940 from TA Instruments as well as TMA 6 from Perkin Elmer were used.

Figure 2

[pic]
Figure 3

Schematic of the automated swell measurement set up

Sample Preparation
Polycarbonate was investigated for this study. The sample is made in the form of films from pellets using solvent casting. film sample was prepared by casting from solution of the copolymer in chloroform. This solution was poured in Petri dishes of identical sizes for having consistency in the thickness of films. After the evaporation of the solvent, the films were kept in vacuum oven at 60 deg C for a period of 12 hrs to remove the residual solvent. They were then kept in the desiccator in order to avoid moisture absorption before taking up for the swell measurement test. The experimental sample were prepared by cutting strips from the solvent casted film with dimensions 25 mm x 4mm x 0.1mm.
Instrument set up
The instrument was modified as described below. The furnace of the TMA was replaced with a humidity chamber with two inlets .One of the inlet carried dry nitrogen and the other one carried wet (humid) nitrogen. These two inlets were controlled using the vent port and instrument port of the Gas Switching Accessory supplied along with the instruments.

The humidity chamber placed in the instrument is shown in the figure 1.The instrument mode was changed to Film fiber mode and the standard sample holder was replaced with film fiber sample holder as shown in figure 2 . Sample cut from the film is loaded to the instrument after clamping the sample in a steel jaw fastened with screws The humidity chamber is placed without disturbing the sample .The sample length was measured using the instrument software. The Schematic of the Instrument set up is shown in figure 3

Experimental Procedure
A thermal procedure to control the gas switching and disable the furnace was generated using the software. The thermal program is used to disable the furnace and switch the wet and dry nitrogen at an interval of 60 minutes for four cycles. This makes the whole analysis un-attended with better precision in switching time. The experiment procedure was made to apply a constant force of 10 grams in film fiber mode to avoid buckling the sample.

1. Jump to 0.00 °C 2. Force 0.100 N 3. Select gas: 1 4. Isothermal for 60.00 min 5. Select gas: 2 6. Isothermal for 60.00 min 7. Select gas: 1 8. Isothermal for 60.00 min 9. Select gas: 2 10. Isothermal for 60.00 min 11. Select gas: 1 12. Isothermal for 60.00 min 13. Select gas: 2 14. Isothermal for 60.00 min 15. Select gas: 1 16. Isothermal for 60.00 min 17. Select gas: 2 18. Isothermal for 60.00 min 19. Select gas: 1 20. Isothermal for 60.00 min Chart 1 [pic]

Figure 4

The data obtained from the experiment is shown in figure 4. The time of the experiment is around 550 minutes and the gases were switched at an interval of one hour. From this experiment the swell index in percentage is calculated using the equation [pic]

Thermal procedure for controlling the humid gas and dry gas on a prescribed interval of one hour is accomplished using the vent port and instrument port purge of the TA instrument Gas Switching Accessory and the instrument control software .The Thermal program and its explanation is given in the Chart 1.

Results
The percentage swell from the above experiment is calculated using the equation and shown here in the table 1. The first cycle shows a percentage swell of 0.4543 may be attributed to the initial slip and creep of the sample from sample holder and the next three cycles data shows a standard deviation of .007%

|Cycle Number |Swell Index in % |
|Cycle 2 |0.396 |
|Cycle 3 |0.387 |
|Cycle 4 |0.3821 |
|Standard Deviation |0.007 |

Table 1
Summary
The dimensional stability of a material as a function of humidity may be evaluated using this technique with very small sample quantity. This technique may be extended to the study the other mechanical properties like modulus, creep, and stress relaxation etc. as a function of humidity. This technique can provide valuable information to design engineers who can account for the dimensional changes in the design calculations. Application areas include mating plastic parts viz., gears, shafts, cams, etc.; parts functioning in cyclic humidity conditions viz., refineries, automotive, large appliances, etc

Acknowledgements
The author wishes to thank Dr Duraiswami srinivasan , Dr Gautam Chatterjee and Dr Ravi Ravikumar for their support and guidance in developing this technique and implementing it in the JFWTC Material Characterization Lab

References 1. Thermal analysis fundamentals and applications to polymer science Hatakeyama, T. 2. TA Instruments, TMA 2940 Thermomechanical Analyzer Operator’s Manual, 1999 3. ASTM Standard E 831, Standard test method for linear thermal expansion of solid materials by thermomechanical analysis, 2003 4. ISO Standard ISO 11359-1, Plastics-Thermomechanical analysis(TMA)—Part 1: General principles 5. ISO Standard ISO 11359-2, Plastics-Thermomechanical analysis(TMA)—Part 2: Determination of coefficient of linear thermal expansion and glass transition temperature
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Figure 1

FILM CLAMP
RELIEF NOTCH
AT TOP

STRAIGHT EDGE
AT BOTTOM

FILM SAMPLE

STRAIGHT EDGE
AT TOP

FILM CLAMP
RELIEF NOTCH
AT BOTTOM

Film Fiber

Tension

Switch of the Heater of the instrument
Applies a force of 10 grams
Dry nitrogen flows through the sample
The flow of nitrogen continues for 60 minutes
Wet nitrogen flows through the sample
The flow of nitrogen continues for 60 minutes
Dry nitrogen flows through the sample
The flow of nitrogen continues for 60 minutes
Wet nitrogen flows through the sample
The flow of nitrogen continues for 60 minutes
Dry nitrogen flows through the sample
The flow of nitrogen continues for 60 minutes
Wet nitrogen flows through the sample
The flow of nitrogen continues for 60 minutes
Dry nitrogen flows through the sample
The flow of nitrogen continues for 60 minutes
Wet nitrogen flows through the sample
The flow of nitrogen continues for 60 minutes
Dry nitrogen flows through the sample
The flow of nitrogen continues for 60 minutes

Swell Measurement Data in Automated System