Nanoclay/Polymer Composite Powders for Use in Laser Sintering Applications: Effects of Nanoclay Plasma Treatment
Nanoclay/Polymer Composite Powders for Use in Laser Sintering Applications: Effects of Nanoclay Plasma Treatment
ALAA ALMANSOORI 0
CANDICE MAJEWSKI 0
CORNELIA RODENBURG 0
0 1.-Department of Material Science and Engineering, University of Sheffield , Sheffield , UK 2.-Southern Technical University , Basra , Iraq. 3.-Department of Mechanical Engineering, University of Sheffield , Sheffield, UK. 4.-
Plasma-etched nanoclay-reinforced Polyamide 12 (PA12) powder is prepared with its intended use in selective laser sintering (LS) applications. To replicate the LS process we present a downward heat sintering (DHS) process, carried out in a hot press, to fabricate tensile test specimens from the composite powders. The DHS parameters are optimized through hot stage microscopy, which reveal that the etched clay (EC)-based PA12 (EC/PA12) nanocomposite powder melts at a temperature 2 C higher than that of neat PA12, and 1-3 C lower than that of the nonetched clay-based nanocompsite (NEC/PA12 composite). We show that these temperature differences are critical to successful LS. The distribution of EC and NEC onto PA12 is investigated by scanning electron microscopy (SEM). SEM images show clearly that the plasma treatment prevents the micron-scale aggregation of the nanoclay, resulting in an improved elastic modulus of EC/PA12 when compared with neat PA12 and NEC/PA12. Moreover, the reduction in elongation at break for EC/PA12 is less pronounced than for NEC/PA12.
Clay nanocomposites have gained much attention
in recent decades. When made through
melt-compounding processes, via extrusion or injection
molding, enhancement of the properties of the
meltcompounded objects have been reported.1–3 However,
challenges involved in the fabrication of complex
geometries have also been recorded.4 Compared to
the conventional techniques mentioned above, laser
sintering (LS) can create highly complex geometrical
parts and does not require any post-machining.5,6
Unlike other methods, LS, as an additive
manufacturing technique, uses 3D CAD from a computer
connected to the machine, to form three-dimensional
parts in a layer-by-layer process.7,8 In LS processing,
laser power and powder bed temperature have to be
carefully adjusted. The powder bed temperature is
held below the powder melting temperature,6 and is
used to preheat the powder, whereas the laser is used
to fuse polymer particles together.5,6,9 Preheating is
essential to reduce the thermal gradient between the
sintered and non-sintered powder and to reduce the
laser power needed to melt the powder.
Although many studies on LS have focused on
thermoplastic polymers, particularly
semi-crystalline thermoplastics, due to their low melting
temperature such as polyamides (nylons),7,8,10 few
studies have been conducted on the reinforcement of
polymers with nanofillers for LS in order to improve
the mechanical properties of neat polymers by
creating polymer nanocomposites. Polymers have been
filled with different types of nanomaterials such as
carbon nanotubes11,12 or carbon nanofibers.13 Among
all the nanofillers, nanoclay (mostly
montmorillonite) is the most commonly used because of the
remarkable changes exhibited by the polymer after
adding a small amount of nanoclay.5,14
Montmorillonite (MMT) is an inorganic, layered
silicate and the hydrophilic clay interacts only
weakly with organic polymers (typically
hydrophobic ones); it tends to aggregate to form large
agglomerations in the matrix. Therefore, very few
studies have investigated polymers filled with
pristine MMT (nontreated).2,15 Chemical
modification of the pristine MMT via surfactants is mostly
used to change the hydrophilic MMT to organophilic
by exchanging the interlayer cations with organic
cations (different kinds of surfactants were
used).1,2,16,17 Although surface modification of the
MMT has improved the interaction between clay
and polymer, chemical modification has also been
reported15 to be expensive; hence, alternative
processes are of interest.
Previously, very few attempts have been made to
treat clays using a different method, i.e. plasma
treatment,18,19 and there have been only a few
attempts at using the plasma-treated clay to
prepare polymer nanocomposites.20,21 However, none of
those studies used the treated nanoclay to prepare
the polymer/nanoclay nanocomposite through a LS
Here, we describe and employ a downward heat
sintering (DHS) process using a hot press to process
small quantities of dry mixed clay/Polyamide 12
(PA12) powders into tensile test specimens after
optimization attempts based on differential
scanning calorimetry (DSC) and hot-stage microscopy
(HSM).22 We also demonstrate that DHS results can
be successfully applied to adjust the LS bed
temperature to allow the fabrication of clay/PA12
Tensile tests were used to determine the strength,
elastic modulus, and elongation at break,2–4 and
some of the published results related to the current
work, in comparison with our results, are
summarized in Table S1 (supporting information) and
discussed in this paper.
MATERIALS, PREPARATION METHODS,
AND EXPERIMENTAL WORK
Materials and Preparation Methods
An organically modified layered silicate nanoclay
used in the current study is known commercially as
Cloisite 30B (C30B). It was obtained from Southern
Clay Products. Virgin Polyamide 12 [trade name is
Nylon 12 (N12)], the matrix, was purchased from
EOS (e-Manufacturing Solution). However, the
polymer used in this study was not virgin, it had
previously been exposed to a high temperature in a
LS at least twice, but the powder was still good
quality and the same batch was used for all trials to
The materials (PA12 and C30B) were processed
together to make nanocomposites using simple, easy
and low-cost methods comprising three parts: clay
treatment and modification, dry mixing and finally
Clay treatment: Plasma treatment technique
The C30B powder was treated for 30 min before
being mixed with PA12 powder. Plasma treatment
was carried out in a Plasma Cleaner Zepto (from
Diener Electronic) with the following parameters:
max power: 100 W, pressure: 0.2–0.4 mbar, time
period: 1000 s for each session, and process gas: air.
Etched (EC) and nonetched (NEC) C30B were added
to the neat PA12 in small glass jars (50 ml) as per
the concentrations (3 and 5 wt.%) shown in Table I.
The composite powders were then stirred using a
magnetic stirrer for 30 min at 800 rpm and
sonicated for another 30 min using an ultrasonic bath.
The resulting powder was stored in a sealed glass
jar for <2 weeks.
Sample fabrication method: Downward Heat
The composite powders and the neat powder were
formed into tensile test specimens in a hot press,
which was used to mirror the laser sintering
process, and therefore no additional pressure was
applied during the sample fabrication. A stainless
steel hollow mold was used to make tensile test
samples according to the British Standard (BS ISO
527) and it was closed from one side by a removable
Neat PA12 and composite powders (weight ratios
are given in Table I), respectively, were placed in
the mold and then the mold and the powder were
placed in between the two parts of the hot press.
The powders were preheated by the lower part only,
which was at a temperature of 185 C for the neat
PA12, and 188 C for PA12 composites, before the
upper part (temperature is 190 C for the neat PA12
and 192–195 C for PA12 composites) was brought
aNEC is nonetched nanoclay.bEC is etched nanoclay.
Sample per session
down. From the point at which the upper part comes
into contact with the lower part, the preheated
powder will be in a closed heated chamber similar to
the laser sintering chamber. As a result, the powder
temperature will then rise to just above the melting
temperature until being fully melted, after which,
the two hot press parts will release. Finally, the
parts are removed from the mold using a stainless
steel spatula and left to cool to room temperature.
Times and temperatures of DHS are shown in
X-ray Diffraction (XRD) and Scanning
Electron Microscopy (SEM)
XRD of powder and solid samples was carried out on
a Siemens D5000 (Cu, GAXRD). X-ray scans were
obtained at room temperature from 2h = 2 –27 in
steps of 0.02o with a dwell time of 1 s per step. The
machine was operated at 40 kV and 40 mA. The
obtained data were analyzed using the
DIFFRAC.EVA application from Bruker.
Morphological investigations were conducted
using a Nova NanoSEM (Low-voltageSEM)). Two
different detectors were used: a through-lens
detector (TLD) for secondary electron imaging at low
magnification and a concentric back-scatter detector
(CBS) using back-scattered electrons to obtain
highmagnification images. The TLD is normally used for
topography imaging whereas the CBS is for
DSC and HSM
A DSC 8500 from Perkin Elmer and a HSM (BX50
light microscope from Olympus with temperature
controlled stage from Linkam attached) were used
to optimize the melting temperature of PA12 and its
nanocomposites. Melting and cooling curves were
collected using associated software (PyrisTM).
Samples for both DSC and HSM were heated from
ambient to 250 C with a rate of 10 C/min.
FTIR and TGA
TGA and FTIR were used to investigate the effect of
plasma treatment on the nanoclay decomposition
process. FTIR analysis was carried out by recording
10 scans between 400 cm 1 and 4000 cm 1 using a
PerkinElmer Frontier spectrophotometer. TGA was
conducted by Pyris from PerkinElmer.
The tensile test was carried out to evaluate the
mechanical properties of DHS samples using a
Hounsfield Tensometer according to BS ISO 527.
The test parameters used were: load cell was
10,000 N, the speed of test was 5 mm/min and a
preload 5 N.
RESULTS AND DISCUSSION
Optimization of Processing Conditions by Hot
To determine the most suitable process
temperature for the fabrication of parts from the composite
powder, it is necessary to use a technique that is
Fig. 1. Melting temperature for PA12 and its composites measured by DSC. Inset (a) is a SEM image of a cross-section of a NEC/PA12 tensile
test sample with non-melted particles; sample was made at a temperature suitable for neat PA12. Inset (b) unsuccessful LS attempt for printing
NEC/PA12 at neat PA powder bed temperature. Inset (c) successful LS attempted for EC/PA12 composite at DHS adjusted powder bed
Fig. 2. HSM results for PA12 and its composites at different temperatures. Neat PA12 at temperatures (a) 185 C, (b) 190 C, and (c) 192 C, 3%
NEC/PA12 at temperatures (d) 192 C, (e) 195 C, and (f) 200 C, and 3% EC/PA12 at temperatures (g) 190 C, (h) 192 C, and (i) 195 C.
most similar to the melting process during
fabrication. Although the DSC is commonly used to
quantify the melting behavior of samples in both
melt processing1,24,25 and powder sintering,5,7,14,26
we found that the melting temperatures obtained
from the DSC did not result in fully melted powders
in HSM (see Fig. 1). The DSC results showed a
single endotherm peak for each sample with
different intensities. The average melting temperature at
peaks and the onset points for all samples are
almost the same, as shown in Fig. 1 (Neat PA12, 3%
NEC/PA12 and 3% EC/PA12 composites).
The single endotherm peak corresponds to the c
crystal form.7 The peak positions were just above
185 C with a variation <1 C. However, the melting
temperature observed during HSM was different,
revealing a much larger variation between neat
PA12 and the two different composite powders, as
shown in Fig. 2a–i.
A clear difference was observed between the
mixtures, whereby the initial and final melting
temperatures increased from neat PA12 to 3%NEC/
PA12 to 3%EC/PA12. For these three mixtures,
respectively, melting began with micron-size
particles at 185 C, 192 C and 190 C (Fig. 2a, d, and g),
larger particles were partially melted and necks
were formed between adjacent particles at 190 C,
195 C and 192 C (Fig. 2b, e, and h), and the melting
process was completed at 192 C, 200 C and 195 C
(Fig. 2c, f, and i). This is in stark contrast to the
DSC results that do not show such clear differences.
Compared to NEC, the EC composite powder
resembles more closely the processing conditions
for neat PA12, whereas the NEC composite powder
required substantially higher temperatures.
As mentioned previously, the aim is to replicate
the melt processing of powder in the hot press. In
the HSM, the powder is heated in an open
Fig. 3. Influence of plasma treatment on the nanoclay using different characterization techniques; FTIR (a) and inset i and ii with magnified scale,
XRD (b), SEM (inset i and ii in b) and TGA (inset iii in b).
Fig. 4. SEM images of 3%NEC/PA12 mixed powders (a) and 3%EC/PA12 mixed powders (b).
environment, similar to the initial stages of DHS,
whereas DSC takes place in a fully sealed
environment. That the HSM delivered more reliable input
for both the DHS process and the LS is evident in
Fig. 1(inset c), which shows successful LS
attempted for the EC/PA12 composite when the
powder bed temperature was increased by 2 C
compared to neat PA12.
Effect of Plasma Treatment on the Nanoclay
The FTIR spectra shown in Fig. 3a indicate the
presence of structural changes resulting from
subjecting C30B to 30 min of plasma treatment. A
significant decrease of the stretching vibration of
the Si–O-Si bonds (990 cm 1) and some reduction in
Fig. 5. (a) Elastic modulus and (b) tensile strength of EC/PA12 and
NEC/PA12 composites (each curve is almost the best in tensile
strength). (c) The elongation at break values of EC/PA12 and NEC/
PA12 composites, and the SEM image of 3%EC/PA12 composite
showing features of ductile fracture (inset i).
Si–O bonds (1116 cm 1) are observed in EC
compared to NEC. These reductions suggest the
introduction of lamellae disorder,21 which can also
explain some of the observed broadness of the
XRD peak of EC (Fig. 3b).21,27 The change in the
chemical structure was not limited to the silicate
band but also led to the formation of new hydroxyl
groups, as evidenced by a small increase in intensity
at 3623 cm 1 in the EC spectrum (Fig. 3a-ii). These
might be the reason for the new shoulder appearing
in the XRD pattern of EC in Fig. 3b. The FTIR also
showed a decrease at the peak 3360 cm 1 that
revealed a reduction in the adsorbed water. The
peaks associated with the organo-modifier were also
changed. The peak at 1640 cm 1 (the stretching of
the quaternary ammonium salt) was slightly
decreased, and a new peak was observed at wave
angle 1695 cm 1. The interpretation of this change
is the formation of carboxyl from the carbon of the
Figure 3b shows the XRD diffraction patterns for
both plasma-treated and non-treated nanoclays (EC
and NEC). At high angles, the XRD spectra of NEC
and EC exhibited two weak peaks at positions (2h of
19.7 and 2h of 19 ). While, at low angles, the XRD
patterns of NEC and EC are different, although
both exhibited the same characteristic basal
diffraction at 2h of 4.8 and the interlayer spacing
(dspacing) of those peaks was equal to 1.8 nm (001
crystal lattice). The peaks for NEC are in good
agreement with previous studies.5,14,28 The pattern
of EC shows a much broader diffraction peak,
consisting of a peak at 4.8 and shoulder (2h of 6 ).
The formation of the shoulder is probably
attributable to a breakdown of Si-O-Si bonds and a
formation of new hydroxyls (Si-OH).19
Moreover, the plasma treatment can induce
oxidation of the octahedral iron leading to the release
of interlayer cations.19 The effect of the oxidation in
EC can be observed by the color change of the
nanoclay particles from an off-white to a gray color.
The formation of the oxidative layer on the EC due
to plasma treatment might lead to less absorption of
moisture, which could result in an improvement of
the thermal stability, which can be tested by TGA.
The TGA results (Fig. 3b-iii) show that both EC and
NEC are degraded in four stages, i.e. desorption of
water, dehydration of hydrated cation, loss of
surfactant, and dihydroxylation.16 For a given
temperature, the weight loss of EC is always smaller
than in NEC. Hence, the EC is more stable than the
NEC, even at higher temperatures. In addition,
TGA results corroborate our FTIR results, which
indicated that plasma treatment releases some of
the free water.
It is noted that the platelets of untreated clay
were stuck together to form microsized
agglomerates, as shown in the SEM image in Fig. 3b-i. Such
agglomerations reduce the surface contact area
between the clay and polymers and can weaken
the composite.5 In contrast, the SEM image of the
EC (Fig. 3b-ii) reveals the separation into platelets
resulting in a much increased surface area.
Investigation of the Properties of the Etched
Nanoclay/Polymer Composites Powders
The composite powders that were made via dry
mixing were investigated by XRD and SEM. The
back-scattered (BSE) SEM images in Fig. 4a, and b
show the incorporation of platelet-shaped nanoclay
in the circular or potato-shaped PA12 particles.
SEM-BSE images reflect the average atomic
number. As nanoclay is largely a mineral material,
whereas PA12 is an organic material, the nanoclay
For the NEC, we find stacks of clay platelets
accumulated on the PA12 particle surface in some
areas, resulting in a non-homogeneous nanoclay
distribution. The EC-based composite exhibits less
accumulation and a much more homogeneous clay
Testing the Mechanical Properties
Comparisons made between neat PA12, NEC/
PA12, and EC/PA12 are summarized in Fig. 5a, and
b and all data are accessible in Ref. 29. Compared to
neat PA12, an improvement of the elastic modulus
and strength was found for both NEC/PA12 and EC/
PA12 at clay concentrations of 3% and 5%, whereas
at the same time a reduction in the elongation at
break was measured. Ultimately, a combination of
tensile modulus, tensile strength and elongation
must be considered.30 It was found in the current
study that the best combination of these properties
was obtained at 3% EC/PA12. As can be seen from
the table in Fig. 5a, adding the EC at a
concentration of 3% has increased the elastic modulus and
tensile strength by 19% and 9%, respectively
(compared with neat PA12), with a simultaneous
reduction in the elongation at break by 24%. Both
exceed the performance of clay/PA12 laser sintered
nanocomposites with the same clay loading reported
in Ref. 5. The elongation of EC/PA12 composite that
decreased by ( 24%) is smaller than that obtained
from the NEC/PA12, which is 52% (see Fig. 5c).
The SEM gave further evidence of the ductile
fracture for EC/PA12 as shown in Fig. 5c (inset i).
Incorporation of the rigid clay strengthens the
matrix polymer but it also leads to a reduced
ductility and brittle fracture, as expected.14 In
addition, the poor interaction between the
nonorganic clay and organic polymer is not enough to
resist the axial force. Micro-voids will be presented
as a result of the bad dispersion.17 The micro-voids
may develop to initiate a micro-crack and the
propagated cracks will lead to a brittle fracture.
Hence, our results suggest that the EC may have a
stronger interaction with PA12 than the NEC. A
notable result from the tensile testing is the
reduction in the variation of the elastic modulus results
between different specimens, but only in the case of
adding the EC to PA12, as shown in the tables of
Fig. 5. This is attributed to a more homogeneous
distribution and better dispersion of the EC within
the PA12 powders and ultimately the composite (as
evidenced by the SEM images of powders and
fracture surfaces, respectively.).
The incorporation of clay at high concentration
resulted in less strengthening5 and reduced
ductility.17 Similarly, our results at 5% concentration
showed that the strength and elastic modulus
hardly improved. Moreover, the elongation at break
was decreased dramatically by 62% (EC/PA12)
compared with PA12.
Figure S1(a and b) (supporting information) from
the SEM images at low magnifications show the
difference between two fracture surfaces: (1) the
NEC-based composite exhibits brittle fracture
areas, and (2) the EC fracture surface (second
fracture) shows a more ductile and uniform surface,
presumably due to the avoidance of micron-sized
agglomerates, which was the main aim of this work.
Further optimization of the plasma treatment
should focus on the nano-scale dispersion (e.g.,
exfoliation and intercalation), which will be
investigated in future work.
Hot-stage microscopy has been successfully used
to determine suitable processing temperatures to
fabricate nanoclay-Polyamide 12 composites, while
DSC has been shown to be less suited for process
optimization, as it did not clearly reveal the
difference in melting behavior for composite powders.
The nanoclay/Polyamide 12 composites obtained
with powders made in a dry mixing process of
plasma-treated nano-clay with PA12 through
downward heat sintering compare favorably to other
mixing processes described in the literature, and are
therefore encouraging for the use in laser sintering.
Downward heat sintering was used to predict a
suitable powder bed temperature, which was
successfully applied to the laser sintering of the
The current problem addressed is the avoidance
of the micron-scale aggregates, which has been
achieved using a plasma treatment technique. It
has been demonstrated that large clay aggregates
can be avoided through the use of plasma treatment
leading to smaller variations in mechanical
properties between different test specimens.
A. Almansoori thanks the Ministry of Higher
Education and Scientific Research (MOHESR) of
Iraq for financial support. C. Rodenburg thanks the
Engineering and Physical Sciences Research
Council (EPSRC) for funding under EP/N008065/1.
We also thank Dr. B. Chen, W. Birtwistle, Nicola A.
Stehling, and Rob Master and staff of the Sorby
Centre for Electron Microscopy and Microanalysis
and acknowledge their support.
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The online version of this article (doi:10.1007/
s11837-017-2408-5) contains supplementary
material, which is available to authorized users.
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