Failure analysis of LLDPE based materials

Krige, Ben Hendrik (2018-03)

Thesis (MSc)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: In this study the environmental stress cracking resistance (ESCR) of various low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) materials are evaluated. The main purpose was to correlate micro molecular structure, such as molecular weight (MW), molecular weight distribution (MWD), short chain branches (SCB) and crystallinity to the ESCR of these materials. Initially a suitable method for evaluating environmental stress cracking (ESC) was found. The ESCR test for ethylene based plastics was first used on a LLDPE film. A more fundamental approach was attempted with the modified bending test, adapted from the bell telephone test for the purpose of this study. Several stress crack agents (SCAs) were used with varying hansen solubility parameters (HSPs). The materials were characterised by DSC, FTIR and high temperature GPC. Scanning electron microscopy (SEM) was used to analyse the morphology of the cracked and delaminated surfaces. Extractable fractions from the bulk were also analysed with DSC and HTGPC. Tensile testing was done on films exposed to SCAs as well as on cracked materials obtained from the modified bending tests. It was found that additives are easily removed from the material surface during exposure to alcohols and aliphatic solvents. The modified bending test was able to simulate cracking conditions and when a certain polymer solvent combination did result in crack propagation that same combination allowed the extraction of low molecular mass branched polyethylene from the bulk to the solvent. It was found that little change in film properties (thermal and mechanical) were observed after exposure to various SCA’a, although heterogeneous layered films did show a diminishing effect (in terms of material performance) when a force was applied on the material. Heterogeneous layered films also showed incompatibility by a delamination effect. When applying a force on a PE film when exposed to light hydrocarbons did show to decrease the yield strength causing the material to deform inelastically, which was attributed to the penetration of these hydrocarbons followed by the untangling PE molecules in the amorphous phase.

AFRIKAANSE OPSOMMING: In hierdie studie word die omgewingsstreskraakweerstand van verskeie lae digtheid poliëtileen (LDPE) en lineêre lae digtheid poliëtileen (LLDPE) materiale geëvalueer. Die hoofdoel was om mikromolekulêre struktuur, soos molekulêre gewig (MW), molekulêre gewigverdelin , kortkettingtakke (SCB) en kristalliniteit aan die omgewingsstreskraakweerstand van hierdie materiale te korreleer. Aanvanklik is 'n geskikte metode vir die evaluering van omgewingsstres krake gevind. Die omgewingsstreskraakweerstand -toets vir etileen-gebaseerde plastiek is eers op 'n LLDPE-film gebruik. ‘n Meer fundamentele benadering is gepoog met die gewysigde buigingstoets, aangepas uit die kloktelefoontoets vir die doel van hierdie studie. Verskeie stres kraakmiddels is gebruik met verskillende Hans oplosbaarheid parameters . Die materiale is gekenmerk deur DSC, FTIR en hoë temperatuur GPC. Skandeer-elektronmikroskopie (SEM) is gebruik om die morfologie van die gekraakte en gesegmenteerde oppervlaktes te analiseer. Uittrekselbare breuke uit die massa is ook geanaliseer met DSC en HTGPC. Spanningstoetsing is gedoen op films wat aan SCA's blootgestel is, sowel as op gekraakte materiaal verkry uit die gewysigde buig toetse. Daar is gevind dat bymiddels maklik uit die materiaaloppervlak verwyder word tydens blootstelling aan alkohole en alifatiese oplosmiddels. Die gewysigde buig toets was in staat om kraakomstandighede te simuleer en wanneer 'n sekere polimeer oplosmiddel kombinasie tot kraakvervorming gelei het, het dieselfde kombinasie die onttrekking van lae molekulêre massa vertakte poliëtileen vanaf die massa tot die oplosmiddel toegelaat. Daar is bevind dat klein veranderinge in film eienskappe (termies en meganies) waargeneem word na blootstelling aan verskeie SCA'a, hoewel heterogene lae films 'n afnemende effek (in terme van materiaalprestasie) toon toe 'n krag op die materiaal toegedien is. Heterogene gelaagde films het ook 'n delaminerende effek onverenigbaar getoon. Wanneer 'n krag op 'n PE-film toegedien word wanneer dit aan ligte koolwaterstowwe blootgestel word, het dit getoon dat die opbrengs sterkte veroorsaak dat die materiaal onelasties vervorm word, wat toegeskryf word aan die penetrasie van hierdie koolwaterstowwe, gevolg deur die onophoudelike PE molekules in die amorfe fase.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103525
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