JOURNAL OF MATERIALS SCIENCE 35 (2000)2115– 2120
Mechanical properties of HDPE/magnesium
hydroxide composites
S. ULUTAN
Department of Chemical Engineering, Faculty of Engineering, 35100,
Ege University, Bornova-Izmir, Turkey
M. GILBERT
Institute of Polymer Technology and Materials Engineering, Loughborough University,
Loughborough, Leicestershire, LE11 3TU, UK
E-mail: M. Gilbert@lboro.ac.uk
Fillers incorporated into polymers for flame retardancy can decrease their mechanical
strength. Coating of the filler can enhance the properties of polymer composites. A platy
magnesium hydroxide, uncoated, or coated with magnesium stearate or stearic was used
as filler in high density polyethylene composites. Tensile and flexural properties were
measured. Experimental results were compared with various existing models.
Experimental data for both tensile and flexural yield strength showed a good fit to the
Pukanszky model. Interfacial interaction was also evaluated through this model. Coating
modified tensile and flexural yield strength in different ways. Results were explained by the
effect of platelet alignment which was measured by X-ray diffraction. Flexural modulus
showed a good fit to the Halpin-Tsai equation, but tensile modulus increased less with filler
volume fraction, an effect also believed to relate to filler alignment. Elongation at yield
decreased with the addition of filler, more so when coatings were present. This property
seemed to be controlled mainly by filler dispersion.
C
2000
Kluwer Academic Publishers
1. Introduction
Flame retardant and smoke suppressant fillers are be-
coming of increasing importance, particularly for plas-
tics used in the cable industry, for example. There is a
developing interest in magnesium hydroxide as a flame
retardant filler that does not produce toxic and corro-
sive substances during combustion, and can be used at
higher processing temperatures than the more widely
used aluminium trihydrate [1]. However, fillers can
adversely affect some mechanical properties of plas-
tics, and often decrease their tensile strength [2]. The
mechanical strength of composites can be enhanced by
coating the filler [3]. Coating with fatty acids is widely
used, as they are cheap and easy to apply. They also
facilitate processing and lower the water adsorption of
the composites produced. Recent work has shown that
for magnesium hydroxide, stearates, which form ionic
rather than stronger covalent bonds with the filler, can
have beneficial effects [4, 5].
A number of workers have developed models for the
prediction of properties of filled plastics [1, 2, 6–14].
Properties predicted most successfully include modu-
lus, yield strength and elongation at break. The mod-
ulus evaluation is easier than that of tensile or yield
strength because while the first is a bulk property, the
second depends on local polymer-filler interactions [6].
Nielsen [2] stressed that factors affecting the proper-
ties of filled systems are often difficult to separate and
evaluate in a quantitative manner. The particle size and
shape, degree of dispersion, the strength of any aggre-
gates, particle matrix interaction, and the orientation of
theparticlesallaffect the mechanical and physical prop-
erties of composite materials. As the particle size de-
creases, the modulus and yield strength increase while
the elongation at yield decreases [2]. Composites with
geometrically anisotropic particles can behave either as
isotropic materils if the orientation of particles is ran-
dom or as an anisotropic body when the particles are
aligned in some manner [7].
Properties of composites containing magnesium hy-
droxide in polypropylenehave been modelledby Jancar
[7, 8] and Jancar and Kucera [1], who have studied the
effect of filler particle shape and filler matrix adhesion
on their yield behaviour and elastic modulus.
The objective of this paper is to investigate the ef-
fect that the type of coating on a magnesium hydroxide
filler has on the mechanical properties of the resulting
composite material. The results are analyzed in terms
of the volume fraction of filler using relevant models.
Although many equations are found in the literature
[2, 6] only those which best fit the experimental data
are presented here.
2. Experimental
2.1. Sample preparation
Magnesium hydroxide DP 393, supplied by Premier
Periclase (density 2360 kg m
−3
, surface area 13 m
2
g
−1
,
0022–2461
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2000 Kluwer Academic Publishers
2115