Determination of % polyethylene in polyethylene/polypropylene blends by FTIR

Polyethylene (PE) is the most common group of thermoplastic polymers due to its low cost and versatile physical properties. PE is blended with polypropylene (PP) to improve physical properties, such as low temperature impact performance. The composition of these blends is important with regard to performance, and the correct mixing of the pure homopolymers (PE and PP) can eliminate the need for costly synthesis of new block copolymers. Knowing the composition of these blends is also critical to the recycling and regeneration of polyolefins in waste and scraps.


The calibration plot of PE/PP blends prepared as cast films, and analyzed using the TumblIR or DialPath on the Agilent Cary 630 FTIR. The same calibration with traditional transmission compartment (film cast in salt plate) yields similar calibration results. The calibration uses the ratio of the PP band at 1376 cm-1 to that of the 1462 cm-1 band in both PE and PP.

The FTIR analysis of 35–85% PE concentrations in PE/ PP blends is now easier than ever using the versatile Agilent Cary 630 FTIR spectrometer. An excellent calibration was developed using the same cast film technique and IR peaks as the ASTM D3900 PE/ PP copolymer method. The Cary 630 FTIR standard transmission compartment is used for the measurement of these cast polymer films on salt plates.

A second, novel method has been developed using the Cary 630 FTIR DialPath transmission accessory, which is easier and more versatile, because larger pieces of self-supporting films can be analyzed in a short amount of time. The polymer films can be repositioned and measured in multiple regions without opening the cell, thus allowing the analyst to find the ideal film thickness for the measurement. Both methods yielded the same excellent calibration and identical R2 value. The PE:PP calibration is now part of a method that has been added to the Cary 630 FTIR software, allowing the polymer ratio in unknown samples to be instantly calculated and displayed.



Posted on

18 December, 2016