A NEW GENERATION OF HIGH STIFFNESS ROTATIONAL New Generation... 3.1 The Rotational Moulding Process...
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A NEW GENERATION OF HIGH STIFFNESS
ROTATIONAL MOULDING MATERIALS
A thesis submitted in partial fulfilment of the
requirements of the Manchester
Metropolitan University for the degree of
Doctor of Philosophy
School of Healthcare Science
The Manchester Metropolitan University in
Collaboration with Rotomotive Ltd.
This PhD thesis contains confidential data belonging to Rotomotive Ltd and Manchester
Metropolitan University (MMU). It should only be made available to the supervisory team,
examiners and authorised members of the Graduate School at MMU. Any publication or
duplication of this thesis, in partial or complete form, is prohibited. In order for patent
applications to be made relating to inventions that may arise from the work in this thesis, a two
year embargo period has been granted by the Research Degrees Committee at MMU. Therefore,
any inspection of this thesis by third parties requires the expressed permission of Hashim Bhabha,
Rotomotive Ltd. and MMU.
Polyethylene (PE), particularly linear medium density PE (LMDPE), is the most widely used
thermoplastic in the rotational moulding (RM or rotomoulding) industry, possessing a balance
between melt flow characteristics and mechanical properties best suited to the RM process
relative to alternative thermoplastics. Reliance of the RM industry on LMDPE limits the application
envelope for manufacturers due to the inherently low modulus of the material; manufacturers
overcome this low modulus by increasing the wall thicknesses of their products which is costly
and energy intensive. The addition of filler particles to PE as a method of modulus enhancement
was considered a feasible alternative to increasing the wall thickness. The resulting composite
material could down gauge part thickness and potentially expand the application envelope of RM.
Phase 1 of this study observed the behaviour of RM grade PE’s with the introduction of filler
particles in order to double the modulus (namely garnet, sand, cenospheres or fly-ash and the
latter two combined). The PE/filler composites were mixed by dry blending or melt compounding,
moulded and mechanically tested in tensile, flexural and Charpy impact mode. The aim of
doubling the tensile modulus of rotomoulding grade PE was achieved by the melt compounded,
rotomoulded PE/fly-ash composites. The introduction of maleic anhydride grafted linear low
density polyethylene (MA-g-LLDPE) coupling agent also increased the modulus and tensile yield
stress of LMDPE with the addition of fly-ash. However, the beneficial melt flow rate and impact
toughness of PE decreased significantly with the addition of fly-ash. The latter was especially true
for rotomoulded samples.
As the RM industry typically uses finite element analysis (FEA) to numerically approximate the
stress or deflection of load-bearing parts, phase 2 of this study focused upon developing
numerical material properties for FEA of the new PE/fly-ash composites. Physical measurements
from compression tests on rotomoulded PE/fly-ash safety steps were close to FEA approximations
(confirming the practical value of the numerical materials data), except in the case of the unfilled
and highest filled PE samples. The significant differences observed between physical
measurements and FEA were probably due to complex factors such as the non-linear behaviour of
PE and the variation in wall thickness of rotomoulded parts, highlighting the importance of
properly understanding the finite element method (FEM) for RM.
1. Bhabha, H. Using Finite Element Analysis as a Design Tool for Rotomoulded Parts, Manchester
Metropolitan University, Manchester, UK, British Plastics Federation Rotamoulding Tooling
Seminar Presentation, April 2013.
2. Bhabha, H. Using Finite Element Analysis for the Analysis of Rotomoulded Parts, Affiliation of
Rotational Moulding Organisations Newsletter, December 2013.
3. Bhabha, H. Liauw, C.M. Henwood, N.G. Taylor, H. Condliffe, J. Critical Factors Affecting the Use
of Finite Element Analysis For Rotomoulded Parts, Manchester Metropolitan University,
Manchester, UK, Rotomotive Ltd., Northampton, UK, Society of Plastics Engineers ANTEC (Annual
Technical) Conference Proceedings, April 2014.
First and foremost I would like to express my utmost thanks to my supervisory team, namely my
director of studies Dr. Chris Liauw, academic supervisor Dr. Howard Taylor and industrial
supervisor Dr. Nick Henwood. Without the guidance of my supervisory team, this PhD study
would not have been possible.
Furthermore, I am grateful to Mike Green of the polymer processing workshop at Manchester
Metropolitan University (MMU) and my industrial mentor Peter Luxford of Rotomotive Ltd for
training on various machinery such as the twin screw extruder, tensometer and water jet cutter.
I would also like to thank Gary Miller and Elaine Howarth of the analytical sciences department at
MMU who conducted scanning electron microscopy (SEM), energy dispersive x-ray (EDX) and
differential scanning calorimetry (DSC) on my behalf.
Moreover, I am thankful to the following people for assistance during the course of my studies:
Steve Davies of Crossfield Excalibur Ltd. for providing computer aided design (CAD) files of
the rotomoulded safety step.
David White of Excelsior Ltd. for providing the rotomoulded safety steps and access to
Mark and Karen Drinkwater of JSC Rotational Ltd. for introducing me to numerous
members of the rotomoulding industry, including members of the British Plastics
All of the technical staff members in the Faculty of Science and Engineering also deserve great
recognition for their help.
Finally, I would like to express the most heart-felt appreciation to my family and friends for
providing their moral support.
I, Hashim Bhabha, hereby confirm that this work has not been and will not be submitted for any
other award apart from the award of Doctor of Philosophy from the Manchester Metropolitan
University. I also confirm that where other sources of information have been included, they have
been acknowledged correctly.
“Bismillah ir-rahman ir-rahim”
In the name of God, the most beneficent, the most merciful.
To Umi Jaan (Mother), Abba Jaan (Father) and all my family, with love.
Abbreviation Description Units (where applicable)
PE Polyethylene -
LMDPE Linear medium density polyethylene -
HDPE High density polyethylene -
XLPE Cross-linked polyethylene -
PP Polypropylene -
PA Polyamide -
PC Polycarbonate -
MA-g-LLDPE Maleic anhydride grafted linear low density
TSE Twin screw extruder/twin screw extrusion -
SEM Scanning electron microscopy/ Scanning
electron microscope -
RM Rotational moulding -
EDX Energy dispersive x-ray -
DSC Differential scanning calorimetry -
MFR Melt flow rate dg min-1
MPF Maximum packing fraction Value between 0-1
PSD Particle size distribution -
FE Finite element
FEA Finite element analysis -
FEM Finite element method -
CAD Computer aided design -
CAE Computer aided engineering
CNC Computer numerical control -
1. Introduction and Problem Statement 1 1.1 Aims and Objectives of the Study 2
2. Thesis Overview 4
3. Literature Review 6 3.1 The Rotational Moulding Process 6
3.2 Polymers Used for Rotational Moulding 8
3.2.1 Polyethylene 8
188.8.131.52 Polymerisation of ethylene 9
184.108.40.206 Development of chain branches in polyethylene 11
3.3 Particulate-Filled Polymers Used for Rotational Moulding 13
3.3.1 Particulate-filled polymer composite theory 14
220.127.116.11 Modulus of particulate-filled polymer composites 15
18.104.22.168 Yield stress of particulate-filled polymer composites 16
3.3.2 Origins of the filler particles 16
22.214.171.124 Silica sand 16
126.96.36.199 Almandine garnet 17
188.8.131.52 Fly-ash and cenospheres 17
3.3.3 Effects of filler reinforcements on polymer properties 17
3.3.4 Filler-matrix interaction (interfacial regions) 19
3.3.5 Filler-matrix coupling agents and filler particle surface treatments 19
3.4 Introduction to Finite Element Analysis 21
3.4.1 Partial differential equations 22
3.4.2 History of the finite element method 23
3.4.3 Theory and Requirements of the Finite Element Method 24
3.4.4 Finite element model development 25