GPC Results

Although GPC is a quick method to determine the molecular weight distribution of polymers it suffers from batch to batch variability. The normal practice was, therefore, to include a standard sample of clear sheet which could be used as a

reference. The results for the two GPC analyses carried out are given in table 14.

Despite there being a large scatter in the insults it would seem reasonable to assume that the I.C.I. materials, both

sheet and rod, are based on the same polymer. The clear weld rod would seem to be based on a different polymer but one with a similar molecular weight distribution.

The GPC analyses of the heat treated samples would indicate that they have been little affected, with respect to molecular weight distributions, by being kept at elevated temperatures for a time similar to that experienced during welding.

4.2.2. Determination of T_g

The DTA work was not easily reproducible and as sample preparation was time consuming it was decided to abandon this method at an early stage, in favour of .DSC.

The DSC method normally gave reproducible results, of a standard form, for the sheet material examined in the range 30 - 100°C, see fig, 52. Spurious peaks and slight variations were accounted for by either sample movement within the pans 198]

or erratic purge gas flow. Cycling of the sheet in the range 30 - 100°C produced no changes in the output thermogram.

However, the clear rod thermograms showed an endothermal peak over the glass transition region during the first heating run. Cycling of the rod produced thermograms of the standard form, see fig. 53.

The glass transition temperatures were determined using the Tcrossed-line’ method recommended by Griffiths and "Maisey [175], see fig. 54.

The Tg of the sheet was found to be 67.0- 1.5°C, with no change with cycling. The clear rod T was 64.9- 1,9°C for the initial heating but dropped to 58,6- 1.1°C after cycling. This large difference was due to the presence of the endothermal peak over the T region during the first heating.

8

The endothermal peak was, at first, thought to be due to the presence of a lubricant additive, Wax E, a Hoechst product, which has been observed to cause problems in interpreting the

glass transition curve for PVC. This lubricant has a melting

peak in the region of the polymer T . This peak was observed_S

to decrease in size and the polymer was found to decrease

by several degrees when the sample was cycled 1176] .

That the endothermal peak would arise as a product of the heating rate used was originally thought unlikely since to produce the peak a slow cooling and fast heating cycle is required [81]. It was thought that in the extrusion process the extruded rod would be rapidly quenched into a cold water bath during the haul-off. However, it is a common practice to extrude into a warm water bath, to prevent the formation of bubbles within the rod. This may be sufficient, combined with a long storage time, to give rise to the endothermal peak and to account for its removal by cycling.

The study of thermal behaviour of the sheet and rod over the temperature range -100°C to 0°C was of little use as the curves were masked by excessive noise. This seemed to be a problem associated with using the thermal analyser on its most sensitive setting and at low temperatures.

The high temperature study, 30°C to 350°C, was limited to two runs to prevent any damage to the DSC cell, caused by the hydrogen chloride given off by the PVC degrading. The thermograms for sheet and rod are given in fig. 55, The main difference

appears to be that the exothermic reaction associated with degradation starts at 170°C for the rod and at 195°C for the sheet. This suggests that during hot gas welding the rod is more likely to degrade than the sheet. Considerably more work

is required before anything more definite could be concluded but this technique might prove useful in comparing the sheet and rod in any further study.

4.2.3. Thermally Induced Shrinkage

The BS shrinkage test [172] for sheet, indicated that the sheet shrank less than 1% with only a small degree of orientation being left in the sheet after processing.

The shrinkages measured using the square block method rose to 3%, for heat treatments of 170°C for 1 hour. See table 15 for details of the measured shrinkages resulting

from various heat treatments. The scatter in results prevented any detailed analysis relating to frozen-in orientation.

It was estimated that when measuring the blocks the error was up to 0.5% of the measured shrinkage which would mask

any small effects due to orientation indicated by the results. However, any orientation would appear to cause less than 5% difference in the shrinkages measured parallel and perpendicular to the sheet length. It is assumed that the maximum

difference would be observed when measured in these directions. In all cases the sheet thickness was observed to increase

approximately 5% with the first 10 minutes of the heat treatment and remain constant thereafter.

The rod was found to shrink up to 25% along the length of the rod when heated to 180°C for 15 minutes, see fig. 56. The rod did adhere to some of the surfaces used for supporting the rods during the heat treatment and this may explain some of the scatter in the results. See table 16 for the measured shrinkages resulting from various thermal treatments to the rods. Measurement of the shrinkage was made using a 150mm steel rule since the heat treated rods were rarely straight enough to justify the use of a travelling microscope. A feature of all the rods was their tendency to become curved during cooling. Some scatter in the results for the shrinkages may have been induced by the method of straightening the rods as they cooled. The diameter of the heat treated rods was generally found to have increased, after allowing for the variations previously discussedin the diameter of rods prior to heat treatment. The rod diameter increase appeared consistent with the rod maintaining a constant volume during the shrinkage, although the measurements are not accurate enough to be conclusive. v Typically the rod diameter increased 10 - 15% for the rods

showing the maximum shrinkage.