Heat Shrinkable Polymer Blends
as Effected by Processing

P.K. Patra and C.K. Das
Materials Science Centre
I.I.T. Kharagpur

Introduction :-
The concept of blending two or more existing polymers to obtain materials with specific chemical and physical properties is gaining wide spread attention because of technicaland economical consideration. The significant advancement in polymer blend technology enables one on tailor and optimize the properties of polymeric material to the specific need. Blending also offers better processability of difficult-to-form high performance polymers. The achieved improvement in processability results in better product uniformity. It is now a common knowledge that nearly all bends are comprised of one polymer domain dispersed in the matrices of other polymers.
The aim of the present investigation is to study the blends of polyethylenes and some elastomers with reference to their heat shrinkability. Here we have chosen five elastomers namely chlorosulfonated polyethene, (CSM) chlorinated polyethylene, (CM) silicon rubber (PVMQ), EPDM and bromobutyl (BB) and the plastic phase used are HDPE, LDPE and LLDPE.

Experiment :-
HDPE used is F46003 having MFI 0.30 and density 0.946 gm/cc. LLDPE is F19010 having MFI 0.90 and density 0.918 gm/cc. LDPE is 22FA002 with MFI 0.2 density 0.912 gm/cc. CSM is hypalam 40 CM is (CM 40), silicone rubber is PVMQ, EPDM is NORDEL 1040 and BB is (X2 variety). Blends of polyethylenes/elastomers were prepared by gradual replacement of polyethylenes with the corresponding elastomers in Brabender plasticorder (PLE 330) type internal mixer for constant period of time and speed. The temperature of mixing was 1500C for HDPE/elastomers and 1150C for LLDPE/elastomers and LDPE/elastomers respectively. Suitable curative system were chosen for specific polyethylenes/elastomer blending.
The blends thus prepared were allowed to cure in a hot press in a mould at 1500C for 5,10,15,20 and 25 minutes. This operation enabled the rubber phase phase to crosslink with plastic phase having partial influence on certain blend system. We have studied the length wise shrinkage and shrinkage was measured under the conditions below.
a) Shrinkage of above vulcanizates was measured at 1500C.
b) Above vulcanizates were given stretching at ambient and at 1500C and then the shrinkage of the stretched samples was measured.
c) Vulcanizates were stretched at 1500C and cured under stretching conditions and then the shrinkage was measured at 1500C.

Shrinkage (%) was measured as per formulation given below:

Sn (%)= (Lstr -Ls)/Lstr x100
Lstr =length of the sample after stretching.
Ls =length of the sample after it is shrunk.

Results and Discussion:

A. Blends of polyethylenes and Chlorosulfoned polyethylene
(CSM)
With increase in cure time the heat shrinkability of polyethylenes/CSM blends increases as revealed in fig 1 which may be due to more crosslinking of elastomer phase. High temperature (H-T) stretched samples are accompanied by higher shrinkage followed by room temperature (R-D) stretched samples. LLDPE/CSM blends show the higher shrinkage in comparison with LDPE/CSM and HDPE/CSM blends under both the conditions of stretching as mentioned above. Shrinkability of the blends increase with increase in elastomer loading. Here again LLDPE/CSM blends get an edge over the other two so far as shrinkability of the blends are concerned. The geater extent ofshrinkability of H-T stretched blends than the R-T counterpart may be attributed to increased elastomer phase which are crosslinked.

B. Polyethylenes and Chlorinated Polyethylenes (CM) Blends
These blends show the same trend in variation of shrinkability of polyethylenes/CM blends increases. LLDPE/CM blends show maximum shrinkability for both H-T and R-T stretched samples (as shown in fig.2).

C. Blends of Polyethylenes and Silicone Rubber
For both the H-T and R-T stretched samples shrinkability of all the blends increases with cure time and elastomer content (as shown in Fig.3). Compared to the earlier blend system as discussed in sections A and B, a difference is observed, where LDOE silicone containing blends get an edge over the LLDPE-silicone rubber and HDPE-silicone rubber so far as their shrinkage is concerned.

D. Blends of Polyethylenes and EPDM
Effect of two cure systems on the shrinkability of polyethylenes/EPDM blends have been studied as a function of cure time and elastomer content (as shown in Fig. 4). Shrinkability of the blends increases with both the cure time and elastomer content. Samples stretched under high temperature (H-T) show higher shrinkability than the R-T stretched samples. DCP (dicumylperoxide) is more effective curing agent than the sulfur to make a particular set of polyethylene/EPDM blend more heat shrinkable.

E. Blends of Polyethylenes and Bromobutyl Rubber
. Variation of shrinkability with the cure time reveals that increase in shrinkability is more in case of DCP cure blends than the sulfur cure samples (as shown in Fig.5). Higher heat shrinkability is observed in case of H-P stretched samples than the R-T stretched one. Effect of elastomer (BB) content is also proportional to the shrinkability of polymer blends. In any case LLDPE/BB blends show maximum shrinkability.

Conclusions :-

There is mutual correlation between the processing parameters and the shrinkability of the blends. H-T stretching is accompanied by higher heat shrinkage. Incompatible blends have an edge over compatible one so far as shrinkage is concerned. Amongst the polyethylenes used LLDPE/elastomer blends show maximum shrinkage except in case of silicone elastomer blended samples where LDPE takes over. Processability parameters which result in extension of elastomeric phase to rod like structure is always accompanied by high shrinkage at elevated temperature. Higher elastomers content and higher cure time increase the shrinkability of the blends.