Molecular Weight: Distribution

These were obtained via the RAPRA Service. Samples were sent to RAPRA where they were dissolved in tetrahydrofuran (THF) and analysed using gel permeation chromatography. The RAPRA service includes a computer analysis of the molecular weight distribution which gives the number,weight, viscosity and z averaged molecular weights. Samples sent for testing included sheet and clear rod,as-received,and sheet, clear rod and I.C.I. triangular rod both as-received and post a 10 minute, 160°C thermal treatment.

2.2. Glass Transition Temperature, T_{- g}

Initially, the T^ was studied using the technique cf

differential thermal analysis, (DTA) with a Du Pent 900 Thermal Analyser, set up with the DTA head. Samples were taken from granulated material. However, due to experimental requirements, only the very fine powder not the granules was used. Glass micro spheres were used as the reference material. A starting

temperature of 30°C and a heating rate of 10°C/min were used. After ten trial runs this method was dropped due to the poor reproducibility of the results. The T analysis was, then,

based on the technique of differential scanning calorimetry (DSC) again using the Du Pont 900 Thermal Analyser, this time fitted with the DSC head. Samples were taken either from the granulated

stock, this time using the granules, or from shavings taken directly from the materials.

Samples, approximately 15mg', were accurately weighed into aluminium pans which were then hermetically sealed. A pan containing a similar weight of glass microspheres was prepared as the reference.

For most of the study a temperature range from 30°C to J00°C was used, with a heating rate of !0°C/min. In certain cases

this range was extended down to -100°C, using the manufacturer;’s

cooling attachment, or up to 330°C. In these cases a heating

rate of 20°C/min. was used. Some samples were cycled, that is heated to 100°C, cooled, either in air or in the cell, back to 30°C and then x^ere retested.

It was found necessary to use the machine on its most sensitive range and spurious peaks x^ere sometimes observed due to fluctuations in the purging gas flox?. The heating rate was checked with a

stop-xtfatch and the temperature calibration checked using a polystyrene standard.

3.2.3. Thermally Induced Shrinkage

Due to differences in manufacturing processes, the sheet and weld rod may be expected to have different residual stresses in

their as-received form. To investigate this, their relaxation at elevated temperatures x-;ere studied.

The 10mm clear sheet was tested as required in BS3757 pt.1. [172]. For this, a 12inch square of material has a .lOinch diameter circle scribed about the centre of the square. The sheet is then placed in a pre-heated air oven set at 120°C, on a glass plate covered with cloth. After 45 minutes the sheet is withdraxm and allowed

to cool on the glass plate. Once cooled a second lOinch circle, is scribed about the centre and the difference between the

original and second circle is taken as a measure of the relieved internal strain.

A second test used 4 inch squares of sheet... About the centre of the square, a grid of four horizontal and four vertical lines,

spaced 20mm apart, were scribed, drawing out nine squares in a

square pattern. Each block x-jas labelled, and the position of fifteen of the points were measured x^ith respect to one corner point, using a travelling microscope. The thickness was

measured at each corner.

One block was then placed in an oven, set at either 110°C, 130°C, 150°C or 170°C, on a PTFE sheet. Then at specified times the block and base \jere withdraxra and allowed to cool. Once

cool, measurements were made of the grid and the thickness and the block then returned to the oven. At the higher temperatures the blocks were found to stick slightly to the PTFE sheet.

In the case of the weld rod, 150mm lengths were used.

Initially tests were done using a ’non-stick’ baking tray as the base, however this was later changed to a PTFE sheet and then later still to a bed of glass microspheres. These changes were made as the rod was found to stick to the first two base materials, at higher temperatures. Heat treatments used temperatures between 70°C and 180°C, for a range of times, up to several hours, except where limited by degradation of the rod. Once heated the rod usually became curved, which caused difficulties when measuring the length. To straighten them the rods were allowed to air cool until just flexible and were then held against a wood-former until rigid again. Measurements of the length were made with a

150mm steel rule and of the diameter with a micrometer. 3,2.4. Birefringence

The birefringences of both clear sheet and clear weld rod were determined using a calibrated quartz wedge attachment on a Vickers M55 Microscope. The stress-optic-coefficient of clear

sheet was determined using cantilever beam specimens loaded between the crossed polars of a circular polari.scope,