Chain microstructure comparison of two transparent polypropylene resins fractionated by temperature rising elution fractionation

Microstructure distribution in polyolefin resins is one of the key factors governing the macroscopic properties of the products. The chain microstructure of two transparent poly(propylene- co -1-butene) (PPB) resins is investigated in this study through temperature rising elution fractionation in the temperature range of 35 − 140 °C. Both resins contain small amount of room temperature soluble component, i.e. less than 3.80 wt% fractions eluted at 35 − 50 °C, implying that the two resins could be used as food containers or food packaging films safely with very low migration of soluble contents. The two resins exhibit remarkable differences in composition distributions as 41.24 wt% of resin PPB-1 was collected at 100 °C, whereas 58.24 wt% of resin PPB-2 was collected at 83 °C. The composition and sequence distribution in each fraction were analyzed by 13 C NMR spectroscopy. All the fractions are random propylene/1-butene copolymers containing 1-butene concentration in the range of 2.7 − 7.6 mol% for resin PPB-1 and 2.2 − 8.5 mol% for resin PPB-2. Individual 1-butene unit is mainly inserted into the polypropylene main chains since the average sequence length of 1-butene ( n B ) is really short (1.0 − 1.3). Resin PPB-1 has more fractions with high molecular weight and isotacticity collected above 105 °C than resin PPB-2, which contributes to a better stiffness of resin PPB-1. Besides, the difference in haze for the films produced from resins PPB-1 and PPB-2 is also demonstrated based on the chain microstructure results. It is found that resin PPB-2 has higher MFR and longer propylene sequences than resin PPB-1, resulting in a higher haze.