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STEPWISE ISOTHERMAL SEGREGATION TECHNIQUE (SIST) FOR PREDICTING EARLY BRITTLE FAILURE IN LLDPE PIPE GRADE MATERIAL

IP.com Disclosure Number: IPCOM000239637D
Publication Date: 2014-Nov-20
Document File: 3 page(s) / 119K

Publishing Venue

The IP.com Prior Art Database

Abstract

Stepwise isothermal segregation technique (SIST) is performed on a conventional differential scanning calorimeter (DSC), which has shown to be able to detect differences in the chain crystallization behavior of LLDPE resins used in pipe applications. These differences in the chain crystallizability are attributed to the co-monomer distribution on the polymer chains, and appear to be impacting the performance of the material when transformed into pipes, in particular, the hydrostatic resistance of pipes at high temperatures. Thermal fractionation techniques have been used to determine co-monomer distribution and short chain branching distribution . SIST was used in this study to predict polymer behavior.

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 Stepwise Isothermal Segregation Technique (SIST) for predicting early brittle failure in LLDPE pipe grade material

Stepwise isothermal segregation technique (SIST) is performed on a conventional differential scanning calorimeter (DSC), which has shown to be able to detect differences in the chain crystallization behavior of LLDPE resins used in pipe applications. These differences in the chain crystallizability are attributed to the co-monomer distribution on the polymer chains, and appear to be impacting the performance of the material when transformed into pipes, in particular, the hydrostatic resistance of pipes at high temperatures. Thermal fractionation techniques have been used to determine co-monomer distribution and short chain branching distribution1. SIST was used in this study to predict polymer behavior.

The optimized SIST allows material screening in 2 hours, instead of several hours needed in the hoop test, ASTM D 1598. Note that the hoop test requires extrusion of the resin into pipes to detect possible failures. The appearance of early brittle failures found in the hoop stress test was correlated with a critical parameter identified based on SIST data of a vast amount of LLDPE samples. This parameter predicted correctly the likelihood of early brittle failures observed in hoop stress testing for majority of the studied samples.

The method comprises melting the polymer to remove any thermal history using a DSC instrument, then cooling the melted polymer to -30°C in such a manner that isothermal crystallization of the polymer is allowed to take place at stepwise intervals of 10K, then raising the temperature to melt the crystals and determining peak areas of melt fractions and melting peak temperatures. This method allows material screening in 18hrs. The partial areas of the melt fractions are determined using perpendicular drops to the baseline between the initial and the final melting temperature on the DSC heating thermogram, as shown in Figure 1. The partial areas are expressed as amount of molten material in percent using equation 1.

Partial area = (HiSL / HtotalSL) * 100 % (1) HiSL is theheat of fusion of fraction i, while HtotalSL is the total heat of fusion of the material.

The peak area of the highest melt fraction was found to correlate with hoop str...