Rheology of peroxide modified recycled high density polyethylene
Creators: name: Parmar, Harisinh
Publisher: name: RMIT University
Year: 2024

description: Consumption of plastics has increased exponentially, in line with the world's population. Not surprisingly this is reflected in enormous growth of the plastic industry especially during the last five decades. Commensurate with this, waste produced from plastics consumption has created a major environmental problem. Many types of waste disposal methods have been used all over the world so far, but all of them have disadvantages. Furthermore, some methods are responsible for the generation of green house gases and further contribution to global warming. Recently, reduction of green house gas emission has become a target of most industries. Plastic recycling and reuse breaks the cycle of endless production of virgin polymer and thus contributes to a net reduction of green house gas emission.<br><br>Recycling of plastics should produce materials with improved properties to replace virgin plastics for a variety of applications. Improvement in the properties of recycled plastics can be achieved by blending with other plastics, by filler addition and by modification using free radical initiators. Introduction of the free radical initiator (organic peroxide) during reprocessing of the recycled plastics has been found to offer significant property improvements to the recycled materials. Extremely small amounts of a free radical initiator (typically ranging between 0.01 wt% to 0.2 wt%) is capable of enhancing the properties of the recycled plastics to a great extent.<br><br>This project investigates the use of free radical initiators in the recycling of post consumer recycled high density polyethylene using reactive extrusion. Both molecular and rheological characterisation of recycled and reprocessed materials was carried out and this was followed by tensile testing of the modified materials to satisfy end use applications such as packaging and drainage piping.<br><br>Post consumer recycled high density polyethylene (R-HDPE) resin and virgin high density polyethylene (V-HDPE) were reactively extruded with low concentrations of dicumyl peroxide (DCP) and 1, 3 1, 4 Bis (tert- butylperoxyisopropyl) Benzene (OP2) respectively in a twin screw extruder in order to produce modified materials with varying composition (0.0 wt%, 0.02 wt%, 0.05 wt%, 0.07 wt%, 0.10 wt% and 0.15 wt%) of both organic peroxides. Morphological characterisation using modulated differential scanning calorimetry (MDSC) demonstrated that there is a decrease in the crystallinity level for all the modified samples.<br><br>Shear rheological tests were carried out to study the structure of the modified materials within the linear viscoelastic region. Viscoelastic parameters, such as storage modulus (G'), loss modulus (G") and complex viscosity (ç*) showed improvement at all frequencies tested with increase in both peroxides compositions. Higher improvements were experienced with OP2 modified V-HDPE and R-HDPE. R-HDPE showed greatest improvement in viscoelastic properties due to the inclusion of low level of carbon black as stabilizer. However, increase in peroxide loading causes appearance of the divergence of complex viscosity profile from Newtonian plateau to non-Newtonian slope analogous to finite yield stress region. Formation of long chain branches or extended chain mechanism causes disappearance of terminal zone and shear thinning like characteristics.<br><br>Extensional rheological study, using the melt strength tester, was carried out to evaluate the response of the modified material to extensional deformation. However, melt strength is not a well-defined rheological property because of non-uniform strain and temperature along a drawn filament. Melt strength of V-HDPE and R-HDPE was enhanced with peroxide modifications. Alignment of the long chain branched molecules in the uni-directional stretching requires more force to stretch the polymer strand leading to enhancement in the melt strength. On the other hand, these long chain branched molecules restrict motion of the chain due to the higher relaxation time, thus can not resist rupture of the strand at higher draw ratio. Thus, extensibility was found to decrease with increased melt strength. Instabilities like draw resonance and ductile fracture were experienced with peroxide loading leading to processing limitations. Extensional viscosity was found to increase with all modified samples. This enhancement was a lso experienced with recycled modified materials.<br><br>Molecular characterisation was done to assess the molecular level changes during reactive extrusion and peroxide modification process for all the modified materials. Gel Permeation Chromatography (GPC) of all the peroxide modified materials showed increase in weight average (Mw) and number average (Mn) molecular weight with respect to the peroxide loading. In addition, increments in Mw than Mn indicated increased branch length. Branching index (g') also showed lower values as branching mechanisms developed. Environmental stress cracking resistance was also measured to evaluate the suitability of the modified material in terms of the targeted pipe application. Resistance to stress crack growth increased with peroxide modifications for both virgin and R-HDPE. However, elongation properties were found to decrease with peroxide loading.<br>