Heat Shrinkable Polymer blends Based on Ethylene Vinylacetate
(EVA) and Polyacrylic Rubber (AR-31)

S. Raychodhury, B.B. Khatua, M. Maity and C.K.Das
Materials Science Centre, I
IT, Kharagpur, India

Abstract

From the technological point of view the use the use of polymer blends and composites is very important. There is very little information regarding heat shrinkability of polymer blends. Here efforts have been put forward to develop blends based on EVA and AR-31 which are having considerable shrinkability. High elongation of EVA have led its use for the property concerned. It is observed that with increase in sure time and elastomer content the shrinkability of the blends increase H-T sample get an edge over R-T counterpart so far as shrinkability is concerned. Thermal studies like YGA/DTA will corroborate the higher stability of these polymer blends. Through XRD we have studied the change of crystallinity of polymer blends with increase in elastomer content and the cure time. These blends find their use in cable insulation as encapsultant and also in the field of telecommunication applications

Introduction

Because of their growing industrial importance polymer blends have become an important area of research. The blends prepared by meltimixing of elastomer and thermoplastic materials have gained appreciable attention {1,2} now-days. The incorporation of crosslinking agents improves the mechanical properties of the blends {3,4}. There is very few published systematic information concerning the shrinkability of elastomer blends. The use of plastic EVA in blends has an importance from the technological point of view. AR-31 shows excellent properties in terms of ozone resistance, high temperature (H-T) resistance. Recently Das et al. [5] have studied the heat shrinkability and flame resistance of some thermoplastic/elastomer blends. This paper correlate shrinkability with various processing parameters.

Experiment

The blends of EVA/AR-31 were prepared by gradual replacement of EVA by AR-31; -EVA/RA-31 = 90:10, 70:30, 60:40 and 40:60 in brabender plasticorder (PLE-30) for 7 min. at 20 rpm. The temperature of mixing was 1100 C, the curative system used is Pb3O4 (2 ph. The temperature of mixing was 1100 C, the curative system used is Pb3O4 (2 phr, based on elastomer), CBS (2 phr), S (1 phr). The blends prepared were taken to be cured in a mould in hot press at 1500 C, 4 T pressure for 10,15,20,25,30 min. BY this operation the rubber phase crosslinks but platic phase remains as it is. Lengthwise shrinkage was studied and under three different conditions shrinkage was measured.

(a) The vulcanizates were given elongation at ambient, then the shrinkage of the elongated sample was measured at 1500 C.
(b) The vulcanizates were elongated at 1200 C and 1500 C and shrinkage was measured at 1500 C.

Shrinkage (%) was measured as per formula given below:
Sh (%) = Lstr-Ls


Sh (%) = % shrinkage; Lstr = length of the sample after stretching;
Ls = length of the sample after it has relaxed.

Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were conduced using Shimadzu Thermal Analyser (DT-40) in air at a rate of 100 C/min within the temperature range of 25-6000 C. X-ray Diffractometer with cobalt target (CoK) at a speed of 0.050 2/sec chart speed 10 mm/2, CPS 5000 c/s and a slit of 0.2 mm applying 40 KV,20 mA current to get the idea of crystallinity of the thermoplastic as affected by the elastomer.

Results and Discussion

A. Effect of Cure Time and Elastomer Content on Shrinkability
The variation of shrinkability of the blends with cure time at constant elastomer content (EVA/AR-31  90:10) for both the higher temperature (H-T) stretched and Room Temperature (R-T) stretched sample are shown in Fig.1. The figure showsthat the shrinkability of the blends increase with cure time in case of all blends. H-T stretching shows higher shrinkability followed by R-T stretched samples, at 10 min cure time only is the exception. This may be because of the crosslinking of the elastomer. More the cure time more is the extent of crosslinking. In case of H-T stretching samples shrinkability is higher than that of R-T stretching samples. It may be due to the large extent of crosslinking of the former as it is kept in higher temperature for longer period of time.
The variation of shrinkability with elastomer content at constant cure time is shown in Fig.2. It is obvious from the figure that at constant cure time the increase of elastomer content increases the shrinkability of the blends for both R-T and H-T stretched samples. H-T shrinkability is getting edge R-T increase in shrinkability may be attributed to the increased elastomeric phase which is crosslinked. From the figure it is seen that for 10 min cure time the value of shrinkability in case of H-T stretched sample is lower than that of R-T stretched
Sample and also from figure 2 it is clear that for constant cure time the shrinkage value H-T stretched sample with 10% wt elastomer is lower than that of R-T stretched sample with equal composition. This anamoly may be because of lesser amount of elastomer and lower extent of crosslinking which is obtained in 10 min and also in case of H-T stretched sample the extent of stretching is higher than that for R-T stretched sample. Actually this feebleness of elastomer arises due to its inability to bring back the polymer blends in its normal positions. The inability is due to its smaller amount and lower extent of crosslinking.

B. Effect of Crosslinking on Stability of Polymer Blends
It is found from the TGA and DTA that the stability of polymer blends with constant elastomer content increases with increase in cure time. For 10% elastomer content the degradation temperature increases, with cure time. Degradation starts from 3400C, 3500C and 3600C for cure time 10 min, 15 min and 30 min respectively.

C. Effect of Cure Time and Elastomer Content on Crystallinity
Crystallinity is measured by X-ray diffraction. With increase in cure time 10 min to 20 min for a blend with fixed composition, the crystallinity increases, again from 20-30 mins the crystallinity of theblends decreases. But for the case of blend with higher elastomer content the crystallinity decreases slightly, in comparison with the blends cured in same time and of different composition. These can be explained in this way-for fixed elastomer content blend the crystallinity increases from 10 min to 20 min cure time, because elastomer phase gets crosslinked in higher extent. But from 20 min to30 min the crystallinity decreases, due to the higher extent crosslinking the orderness of EVA may be disturbed and consequently crystallinity goes down. But in case of higher elastomer content, cure time, remaining the same, due to greater effect of elastomer present orderness may be disturbed and crystallinity may decreases.


Conclusion

With increase in cure time and elastomer content shrinkability of blends increases both in case of H-T stretching and R-T stretching. H-T stretching sample generally shows higher shrinkability then R-T stretching sample. Thermal stability increasing in cure time of the blend. Cure time and elastomer content have profound effect on crystallinity.

References

1. Waler, B.M. (1979). Hand Book of Thermoplastic Elastomers, Van Nostrand Reinhold, New York.
2. Wheler, A and Lee, K.S. (1982). Developments in Rubber Technology Thermoplastic Rubbers, Applied Science, London.
3. Campbell, D.S., Elliot, D.J. and Wheelans, M.A (1978). NR Technol, 9, 21
4. Coran, A.Y and Pelet, R.P. (1980). Rubber Chem. Technol., 53, 781.
5. Patra, P.K. and Das, C.K. (1997). Intern. J. Polymeric Mater, 35, pp. 103-118.