MATERIAL MANUFACTURING DETAILS

The information in the following sections (3.2.1 to 3.2.3) relates to work undertaken by Bradford University to manufacture the samples used for testing and is included here for reference.

Originally, rigid, hollow tube samples were produced for ease of handling and use. It was then proposed to include smaller filament samples (either hollow or solid) within the tubes in an attempt to improve the shrinkage stress generated. These filaments were also manufactured and tested outside of tube samples. This resulted in the four sample types shown in Figure 3.1.

Figure 3.1: Manufactured PET samples: (a) tube with filaments, (b) tube, (c) solid filament, (d) filament tube.

3.2.1. PET solid filament and filament tube

For the PET solid filaments and filament tube, a commercial grade of PET (Dow Lighter C93), recommended for injection moulding, stretch blow moulding and thermoforming, was obtained in granular form. It was first made into fibres by melt extrusion. This was done using a

single-(a) (b) (c) (d)

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screw Killion S1748 25mm extruder operating at a screw speed of 15rpm and a screw pressure of 30bar. The maximum melt temperature in the extruder was 280°C and the die head was at 270°C; above the 260°C melting temperature of PET (Göschel, 1996), while avoiding thermal degradation of the material through excessive temperature (Sciences, 2011). The material was extruded through a circular die of 4mm internal diameter at a rate of 5m/min. On exit from the die it was cooled in a glycerol bath at room temperature. The final diameter of the cooled product was 1.8mm.

The second stage of the process was to introduce molecular orientation into the fibres. The technique of die-drawing was used, as described for polymers by Coates and Ward (1981). Fibre at room temperature was pulled through a 1.5m length fan-assisted oven with a controlled air temperature. On exit from the oven, it entered a converging conical die with cone angle 30° and final diameter 1mm, held at a constant temperature. After the die the fibre was gripped by a caterpillar-type haul-off device operating at a constant linear speed.

Since the highest levels of orientation are associated with the highest haul-off speeds and lowest temperatures, the caterpillar speed was set at its maximum 1m/min and both the oven and die temperatures were lowered in 1°C increments from 80°C until stress whitening and failure of the fibre were observed at 75°C. The die temperature was then raised incrementally until satisfactory fibre was produced; the final settings were thus 75°C in the oven and 80°C in the die.

After exiting the die, further drawing took place during cooling between the die and the caterpillar, resulting in a final diameter of 0.9mm, corresponding to a draw ratio of 4.0. At the start of the process the haul-off force was measured as 80N, and this reduced to a steady load of 50N as the process stabilised; these loads correspond to stresses of 99 and 62MPa respectively, to be compared with the yield strength of 55MPa according to the manufacturer's data sheet.

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3.2.2. PET tube

The outer cylinder was made from the same grade of PET as with the filaments described above.

Tubes were extruded using the same Killion extruder operating at a screw speed of 70rpm and a pressure of 200bar. The maximum temperature in the extruder was 280°C and the die head was at 260°C. Extrusion was through a circular section die of diameter 14mm, with a central internal pin of diameter 4mm to create the central hole. The haul-off speed was 500mm/min and final dimensions were outer diameter 13mm and inner diameter 6mm.

The tubes were then die-drawn from 1m lengths through a 30° conical die with an exit diameter of 7mm. Otherwise, the die-drawing equipment was as used for the drawing of the filaments described above. Both the die and oven air temperatures were set at 75°C. The initial haul-off speed was 40mm/min, and was increased during the die-drawing process to reach the maximum draw ratio. The tube’s final outer diameter was 6.61mm on average, corresponding to a draw ratio of 3.9.

3.2.3. PET tube with filaments

An alternative system was studied that consisted of PET filaments inside hollow PET tubes. This was assembled in its initial isotropic state (see configuration in Figure 3.2) and then the complete assembly was die-drawn.

Both the internal fibres and the tubes within this system were extruded from Tergal grade T74F9 PET. The source material was changed (for the solid filament, filament tube and tube samples) to Dow Lighter C93 for reasons of reliability of supply. Extruder and die temperatures were both 280°C, with extrusion through a 2.5mm spinneret at a screw speed of 5.4rpm to achieve a final fibre diameter of 1.2mm. The outer cylinder was manufactured following the same process described for the PET tube samples.

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Figure 3.2: Indicative cross-section of PET tube with filaments system before drawing. Dimensions in mm.

1m lengths of tube and filaments were assembled and then die-drawn through a 30° conical die with an exit diameter of 7mm, following the same procedure as for the PET tube samples. This resulted in a final external diameter of 6.54mm for the samples, with internal fibres of average diameter 0.57mm. This corresponds to a draw ratio of 4.0 for the tubes and 4.4 for the internal fibres.

3.2.4. PET strips (commercially available)

The PET strip specimens were commercially available under the name ‘shrink tite tape’ and were obtained from Cytec (CYTEC, 2016). This is the same tape used by Jefferson et al. (2010) in previous studies. These strips are made from oriented PET but the manufacturing details are not available.

3.2.5. Summary

Table 3.1 shows a summary of the properties of all samples used during testing.

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Table 3.1: Summary of properties of samples.

Designation Material Dimensions Draw ratio Draw rate

(mm/min)

Strips Oriented PET¹ 0.046mm thickness

32mm width unknown unknown

Solid filament Die-drawn PET 0.95mm dia. 4.0 1000

Filament tube Die-drawn PET 0.95mm dia. 4.0 1000

Tube Die-drawn PET 6.61mm external dia.

2.56mm internal dia. 3.9 40 (initial)

Tube with filaments Die-drawn PET 6.54mm dia. 4.0

4.4 40 (initial)

1Manufacturing details commercially sensitive