Understanding Functional Silica Additives for HDPE Composite Materials

The primary aim of this project was to study the impact of nanoscale pyrogenic silica additives on high-density polyethylene (HDPE) for the potential use as a liner material against corrosion in challenging environments such as oil and gas pipelines. The main body of work involves the analysis of the chemistry of Aerosil® grade pyrogenic silicas mainly A200, R8200, R9200, and R812S. Then, the work focused on the assessment of the processing characteristics, properties, and performance of the HDPE upon incorporation of the silicas. Thermogravimetric analysis (TGA), ¹H liquid nuclear magnetic resonance (NMR), and ²⁹Si high-power decoupling (HPDEC) magic-angle spinning (MAS) NMR methods were employed in an attempt to quantify the silanol density of the pyrogenic silicas. These techniques were supplemented by the use of ²⁹Si and ¹³C solid state NMR to characterise the functional groups on the surface of the different grades of silica. The estimate of silanol density for the unfunctionalized A200 using TGA methods was in good agreement with the established literature. These findings were further supported by the ¹H NMR data which was also consistent and in broad agreement with the widely accepted literature values following deuterium exchange protocols. The methodology was validated against previous studies used on hydrophilic silica and was found to be consistent. The technique was extended to characterize hydrophobic silica, thus advancing the quantification of silanols in a novel approach. The type of functional group on the surface of the silica particle has a clear influence on the silanol density due to considerations of steric hindrance. TGA, differential scanning calorimetry (DSC), dynamic rheology, and dynamic mechanical analysis (DMA) experiments were undertaken to investigate the influence of the pyrogenic silica on the resulting HDPE-silica nanocomposites. The results demonstrated a high level of reproducibility and a uniform dispersion/distribution of all pyrogenic silicas with the HDPE matrix. The TGA and DSC revealed a key finding where the chemical compatibility concern of high loading levels of silica and the unfunctionalized silica was overcome by the processing routes and conditions used. There were subtle increases in the degree of crystallinity. However, the data demonstrates a significant scatter based on the silica type and loading making it challenging to deconvolute. The relative viscosity of the HDPE-silica nanocomposites was estimated using the Batchelor model. The model was well defined in both high and low frequency regions. The most compatible pyrogenic silica with HDPE was found to be R9200 which contains the lowest silanol density values, thus, less likely to have particle-particle interactions that can result in a network structure. The widely accepted view in the literature is that a more chemically compatible additive would improve material properties. However, the unfunctionalized hydrophilic silica which is the most chemically different from HDPE and the R9200 exhibited similar impacts on the processing characteristics and properties. ¹H MRI was used to study the solvent ingress into HDPE and HDPE-silica nanocomposites. The solvent identification was based on the Hansen solubility parameters and the findings correlate with the solubility predictions. The incorporation of all types of pyrogenic silica were seen to facilitate solvent ingress into HDPE at low and high loading levels. The findings in this work collectively contribute to the understanding of the impact of silica incorporation into HDPE. Furthermore, it outlines critical assessments of the HDPE-silica nanocomposite properties and performance providing a framework for material evaluation. Lastly, the work highlights areas that can have potential improvements, making it a useful resource for advancing in the nanocomposites field.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Alghfeli, Mohammad

Advisor dc:contributor.advisor

• Mantle, Michael

• HDPE nanocomposites
• Material properties
• Silica

Licence

• https://www.repository.cam.ac.uk/bitstreams/3845e0e4-7e20-4d20-b66c-28b5fdc01352/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.112674

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/374714