Polyblends System of Poly Urethane and H-NBR (Peroxide cure)

M. Maity, B.B. Khatua, S.R. Chowdhury & C.K. Das
Materials Science Centre
Indian Institute of Technology
Kharagpur-721 302, India

Introduction

Polymer blender are of growing industrial importance. Blending of polymers is an attractive and echonomical method to create and develop new polymeric materials with enhanced performance characteristics and improved properties than existing polymers. The applications of poly urethane (PU) elastomers are in the wide varieties of products. Millable branches of industry, including mechanical engineering, textile and petroleum industry and motor vehicle constructions. The combintion of extellent mechanical strength and abrasion resistance makes the PU industrically important. On the other hand, H-NBR possesses a high resistance to oxidative and ozone degradation, excellent tensile strength, and very good abrasion resistance properties.

Experiments

Blends were prepared in an internal mixer at a constant speed for fixed period of time by gradual replacement of polyurethane with H-NBR in the entire composition range using DCP as the curative. Then the mixture were sheeted in an open two roll mixer. Blends were allowed to cure in a hot press at temperature 1500C upto optimum cure under constant pressure. With the blends, following tests were conducted. Physical Properties of the blends like hardness, modulus, tensile strength, elongation at break etc. Were measured on cured sheets both before and after ageing. Measurement of cure characteristics were carried out at 1500 C in a Monsanto Rheometer (R-100) in order to study the processibility, cure mechanism and activation energy of curing etc. DTA/TGA of the blends were studied by Shimadzu Thermal Analyser in air at the heating rate of 100 C/min within the temperature range from Room Temperature to 6000 C. IR spectral studies were also carried out to establish the interchain crosslinking if any. The phase morophology was studied for extracted samples with the help of SEM.

Results and Discussion

Effect of H-NBR on the cure characteristics of the blends:

Polyurethane rich blend shows the maximum state of cure than that of the H-NBR rich blend (as shown in figure 1). Modulus and Hardness are in line with the state of cure as above. The tensile strength changed on ageing and the percentage change increased with increasing H-NBR content. But elongation at break was lost on ageing. The percent change decreased with increasing H-NBR content.

IR Spectral Analysis:

The IR spectra of thin films of PU, H-NBR and PU/H-NBR (50:50) blend (without any curatives) heated at 1500 C for 30 min are shown in the figure 2. PU shows a broad peak at 1716 cm-1 for amide >C=0 stretching and at 3510 cm-1 for N-H stretchings. In H-NBR there was a sharp peak at 2230 cm-1 for –CN group. The blend showed were also peak at 2290 cm-1 for –CN group which clearly indicates tha –CN group were not takes part in the reaction. The peak at 3510 cm-1 for N-H stretching of PU was absent in the blend, suggesting the crosslinking. Occurs through ‘N’ in PU with H-NBR. This also support from the presence of one additional peak at 1520cm-1 in the blend (this peak probably due to the crosslinked C-N stretching). Figure 3 shows a plausible mechanism for the interchain crosslinking reaction between PU and H-NBR.

Thermal Analysis:

The degradation pattern of the PU/H-NBR blends has been studied. The DTA/TGA plots are shown in figure 4 for PU/H-NBR blends.
From the DTA/TGA plots it was observed that the degradation occurs in three steps. In the case of the blend with lower amount of H-NBR early degradation occurs starting at 4100C. The second step starts step at 4900C whereas for the blend with higher amount of H-NBR, the degradation is delayed and starts at 4500C. The second step, however, starts at 5000C.

SEM Study:

The Samples were DMSO extracted to remove the Phase and then the morphologies were studied by SEM. It appears from the micrograph that PU has the tendency to form continuous phase even at relatively lower percent content. At the middle region, however, appearance of discrete particles are observed. Interchain crosslinking enhances the phase adhesion.

Conclusion

The incorporation of H-NBR improves the ageing properties and heat resistance properties of the blends. The blend properties largely on the blend ratio and in the blending techniques. Heat treatment of the blends before adding curatives enhances the properties due to the interaction crosslinking. Preheating is more effective in case of Pu/H-NBR blends when peroxide curing system in used.

Reference

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4. Singh Roy S.K. and Das C.K., Polymers and Polymer Composites, Vol.3 No.6, (1995)
5. Tripathy A.r., Ghost M.K. and Das C.K., Intern. J, Polymeric Matter, 17 (2), 77 (1992)