DMTA specialists, rheology services, rheological sample testing, rheometers

We have been involved with the Polymer Laboratories DMTA since 1984 and now service all models from the original MK I through to the latest MK V.

We invite you to share your technical knowledge and experiences with other DMTA users and ourselves.

Below you will find some technical advice for common questions; we always enjoy hearing from you so give us some feedback!

ACCURATE MODULUS MEASUREMENTS ON THE DMTA

A frequently asked question by DMTA users concerns accurate modulus measurement. The DMTA is capable of accurate modulus measurement if the user is careful in sample loading and does not exceed sample dimensions relative to the sample frame size. The user can choose between 3 measuring frames and basically 3 types of drive shaft clamps. A common error is choosing the wrong drive clamp when using single or dual cantilever measurements. The user must remember to use the dual cantilever drive shaft clamp (Clamp A) only with dual cantilever measurements and the single cantilever drive shaft clamp (Clamp B & C) only with single cantilever measurements. These clamps are very different and will result in inaccurate modulus measurements if the wrong clamps are used. Please refer to your user manual for more details.

Length to Thickness Ratios

Another source of error can be sample free length. If the free length is not properly measured this will result in an error in calculating the geometry constant. Sample free length is defined as the distance between the sample clamps. However, when measuring a large sample or a high modulus sample there maybe movement underneath the clamp faces, if this is the case then the free length can not be defined as the distance between the clamps. This movement of the sample under the clamp faces can be limited in a number of ways. Generally the longer the free length the less important this source of error and it is useful to consider the ratio of length divided by the thickness of the sample. Ratios between 5 and 7 are considered to be ideal and the higher the modulus the greater this number should be.

Sample Torque

The clamping pressure used on the sample is critical for accurate sample modulus. There exists an ideal torque for each sample based on the clamping arrangement and modulus of the sample being measured. An experienced operator will develop a "feel" for this after a while, however, for most users we recommend the use of a torque wrench. To determine the correct amount of torque begin by loading the sample and using a small amount of torque to "square" the sample up, turn the DMTA to run and make a note of the Log modulus. Turn the DMTA to hold and increase the torque by a nominal amount ( try 5 cNm ), turn the DMTA to run and note the new modulus. Continue this process, increasing the torque by small amounts until there is no further increase in the measured modulus. Care should be taken not to over torque as this will compress and distort the sample.

Conclusion

Accurate modulus measurements are possible if the user is careful when selecting the frame and drive clamps, choosing an appropriate length to thickness ratio and of course the correct torque for the clamping nuts.

TEMPERATURE SENSING CONSIDERATIONS

"The PRT"

All DMTA's from the MK1 to the most recent MKV use a PRT to sense temperature in the oven. The PRT (platinum resistance thermometer) is of the PT100 variety using 2-wire construction. It is an excellent choice for the temperature sensing because of its high sensitivity and its fast reaction time.

In order to measure the temperature of the sample, it makes sense to keep the PRT as close as is practical to the sample itself. The bare wires of the PRT allow for it to be move d in several directions so as to accommodate the different measuring geometry's available on the DMTA. It is very important to bend the PRT carefully, as it's a delicate device. Too much bending will cause the wires to "work harden" and then to finally break.

"The Oven"

The construction of the oven has changed over the years but the basics have remained the same. The early MKI and II instruments used a detachable oven while the later MK III, IV and V have used an oven lid hinged at one end. The basics, however, have not changed, that being that the oven is heated by radiant heat from the heater coil windings within the oven. While this is an efficient method of heating, it does have some shortcomings. The biggest problem is that of temperature gradients which cause uneven heating within the oven. While this problem has been largely alleviated in the MKV, it remains a noticeable problem in the earlier models. The problem becomes more pronounced as ramp rates are increased. For this reason ramp rates above 3 degrees per minute are not recommended; in fact, the slower the ramp rate, the less thermal lag there will be. One way to improve thermal conditions inside the oven is to use the purge port that is available on all DMTA's. This is the purge port that is available on the back of the measuring head. By introducing a gas flow of say, Nitrogen into the oven, it can improve thermal conditions by combining radiant and convection heating. The result is less thermal gradients and better sample heating. One should keep in mind that polymer samples are very poor conductors of heat and that the use of stainless steel sample clamps makes heat conduction even worse, as it is well known that the stainless steel is a poor conductor of heat. For this reason the Aluminum clamps are preferable, however, their life expectancy is shorter than the stainless steel clamps.

PRT Replacement

Generally the PRT is very reliable however, it is fragile and can easily be contaminated. For these reasons we recommend changing the PRT once a year. This is easily performed simply by removing the instrument cover and unscrewing the connectors in the PRT terminal block. The PRT and wires can be withdrawn through the front by pulling it forward out of the ceramic tube. On the MK V the PRT is a plug-in device so there is no screw to loosen.

Conclusion

For accurate temperature sensing keep your temperature ramp rate low and keep the PRT close to the sample. Consider using a small amount of purge gas to assist in the heating of the oven. Always keep a spare PRT and remember to keep it clean. Consider replacing it once a year.

Maple Instruments Ltd.
116 Johnston Ave.
Toronto, Ontario M2N 1H2 Canada
Tel:416-590-7446 | Email: maple.instruments@sympatico.ca