Date

1-1-2020

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Polymer Science and Engineering

First Adviser

James F. Gilchrist

Abstract

The morphology of polyethylene (PE) depends on physical and chemical influences affecting the size and distribution of the crystalline domains or spherulites that develop upon cooling from the melt state. Optimizing the final crystal morphology in PE can improve and change physical properties such as water vapor transmission, optical clarity, shrinkage and electrical properties such as direct current breakdown (DCBD) strength. Effective techniques to modify the growth and morphology of the spherulites include controlling the rate and method of cooling from the melt state and incorporating nucleating agents (NAs) in the PE formulation.

Nucleating agents are more widely developed and utilized for polypropylene (PP) than PE as its slower crystallization rate allows greater control in achieving property improvements. Nevertheless, NAs are used commercially in PE and a host of NAs have been studied and results reported in many publications. Most studies have focused on mechanical property changes or improvements and less on changes relating to dielectric properties. A limited number of studies reported significant changes in dielectric properties in low density polyethylene (LDPE) containing NAs. The research described in this dissertation focuses on understanding how NAs affect morphology and resulting dielectric properties in LDPE resin typically used as insulation in power cable applications. Results of this research may be utilized to gain a more in-depth understanding of how to formulate insulation compounds with improved dielectric properties.

The first part of this dissertation describes the effects of several different types of NAs on DCBD in LDPE and linear low density polyethylene (LLDPE). The selected NAs were surface treated nanosilica, calcium 1,2-cyclohexanedicarboxylate (CDA) and 1,3:2,4-bis(dimethylbenzylidene) sorbitol (DMDBS) and each had a significant effect on the morphology and crystallization kinetics in both LDPE and LLDPE. The NAs produced significantly reduced spherulite sizes, higher crystallization temperatures and faster crystallization rates consistent with effective NA activity. The LDPE and LLDPE samples with CDA had the highest increase in crystallization temperature, indicating stronger NA performance. Excluding a few outliers, the NAs had higher DCBD compared to the neat LDPE. Results showed that smaller spherulite size correlated with higher DCBD and lower conduction current.

The second part of this dissertation focuses on the effects the same NAs used in the previous study have on a broad range of dielectric properties including alternating current breakdown (ACBD) strength, dissipation factor, dielectric constant, conductivity, water tree growth and DCBD as a function of NA concentration in LDPE. In addition, polyethylene glycol (PEG) was evaluated as an NA in LDPE, which was not previously recognized for its NA effects in LDPE. Crystallization studies confirmed that all the selected NAs including PEG acted as NAs in LDPE.

The addition of NAs within specific concentration ranges improved DCBD strength in LDPE, compared to neat LDPE. The DCBD results showed a linear correlation to the Avrami exponent and crystallization temperature. The NAs did not increase ACBD strength in the nucleated LDPE. Dissipation factors increased significantly with temperature and electrical stress in both sorbitol and CDA composite samples whereas the nanosilica composite samples maintained low DF under the same conditions similar to neat LDPE.

In addition, conduction currents were measured to assess electrical property parameters such as direct current steady state decay constants, charge mobility, current-voltage power law dependence, and activation energy. The nanosilica and CDA composite samples showed significantly lower conduction current compared to neat LDPE due to a higher density of deep traps. However both the sorbitol and PEG composites had higher conduction current attributed to increased shallow trap density. The nucleated composite samples showed higher charge mobilities compared to neat LDPE. Results of decay constant and power law dependence of the current-voltage characteristics were consistent with the space charge limited conduction (SCLC) model. The changes in the dielectric properties were attributed to the morphology in the nucleated samples which resulted in an increased amorphous-crystalline interfacial area leading to increased trap density.

Available for download on Saturday, January 29, 2022

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