EoW July 2009

technical article

▲ ▲ Figure 1 : PVDF master curves – complex viscosity at 230ºC

▲ ▲ Figure 2 : Relaxation modulus of PVDF samples at 230ºC

to relax after the drawing operation. Conversely, the higher viscosity samples as well as the non-random samples will not have relaxed in this time period and it would be assumed that the majority of the elastic stresses will be frozen-in to the resultant jacket. With the knowledge of the rheological characteristics necessary to achieve low shrinkage properties, and analytical methods developed to screen new materials, efforts to further improve low shrinkage characteristics in PVDF have been initiated. PVDF structures already identified as having low shrinkage characteristics were modified for possible further improvement. Two existing commercial grades of PVDF identified as PVDF 1A and PVDF 2A were characterised for shrinkback by conducting a series of shrinkback experiments on cables jacketed with these products. Cable jackets were applied in-house using a small lab extrusion line consisting of a 1-inch Killion extruder outfitted with a BH-30 cross head and all the necessary downstream equipment. Conditions such as barrel temperature, water temperature, line speed and tank distance were all standardised to eliminate these as variables in the experiment. Cables were cut in 10-foot lengths and the jackets were removed by slitting the length of the cable. Shrinkback experi- ments were conducted by measuring the jacket length before and after a thermal exposure of 121°C for 1 hour. A 24-hour recovery period was allowed before taking the final measurement. Shrinkback experiments were repeated using similar PVDF products having some modifications to the structure with the goal of further reducing shrink- back characteristics. These samples are identified as PVDF 1B and PVDF 2B. A summary of the results from shrinkback experiments can be found in Table 3 .

For the purposes of this paper, the relaxation times for each PVDF sample to reach the arbitrary value of 100 Pa were examined. These values can be observed in Table 2 . As can be seen in Table 2 , the relaxation response for the K2500-10 is considerably faster than any of the other products tested. A significant amount of this behaviour can be attributed to the structure of this product. When comparing K2500-10 to K3120-10, it is noted that the relaxation response for the K2500-10 (random copolymer distribution) is signi- ficantly faster than for the K3120-10 (non-random copolymer distribution). The faster relaxation response was predicted from the master curve ( Figure 1 ), which showed the rheological differences of these two products in the low shear range. When interpreting the data, it is beneficial to have an understanding of the cooling environment typical for PVDF processing. In a standard jacketing operation producing a 0.020" jacket and run at 300 feet per minute with a 6" gap between the cooling tank and the die, it was estimated that the time from the end of drawing and before entering the tank will be 0.10 seconds, and the actual time to solidify the jacket will be approximately 0.42 seconds for a total cooling time of 0.52 seconds. Based on this approximation, the K2500-10 will have sufficient time

K3120-10 is a comparable sample with a low viscosity, but it differs from the K2500-10 by exhibiting a non-Newtonian response at low shear rates. It would be inferred that this rheological characteristic would delay relaxation of molecular orien- tation after completion of the drawing operation resulting in higher shrink- back. This has been confirmed through subsequent shrinkback experiments. To further understand the relationship between PVDF structure and shrinkback, stress relaxation experiments were performed. It can be reasoned that a polymer having the ability to relieve stresses quickly in the melt state will exhibit lower polymer orientation and consequently lower shrinkback. The stress relaxation experiments were conducted using the ARES-LS strain rheometer using 25mm parallel plate geometry. A step strain of 100% was applied to PVDF samples and the decay of modulus was recorded as a function of time. Results of these experiments can be seen in Figure 2 . As expected, the higher viscosity samples such as K2750-01 and K3120-50 show a relatively slow relaxation response whereas the lower viscosity samples such as K2500-10 and K3120-10 exhibit a fast relaxation response. The quick relaxation response for these low viscosity samples should result in less polymer alignment in the final product.

▼ ▼ Table 2 : Stress relaxation at 230°C and 100 Pa

Viscosity (Pa.s) @ 80 s –1

PVDF Sample ID

Time (s) @ 100 Pa

K2500-10 K3120-10 K2500-20 K3120-15 K2750-01 K3120-50

0.65

795 650

2.0 6.8

1460 1230 2290 2390

10.0 220 400

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EuroWire – July 2009