Indian Journal of .Fibre & Textile Research
Vol. 36, March 2011, pp. 35-41
Electroless nickel plated composite textile materials for electromagnet
compatibility
R Perumalraja
Bannari Amman Institute of Technology, Sathyamangalam 638 401, India
and
B S Dasaradan
Department of Textile Technology, P.S.G. College of Technology, Peelamedu, Coimbatore 641 004, India
Received 16 April 2010; revised received and accepted 8 June 2010
A study on electromagnetic shielding effectiveness of electroless nickel plated copper core with polyester sheath yarn
composite fabrics through Taguchi design and ANOVA has been reported. The electromagnetic shielding effectiveness of these
conductive composite fabrics has been measured in the frequency range 200 - 1000 MHz using network analyzer equipment. It
is observed that with an increase in palladium chloride and nickel (II) sulphate concentrations, the shielding effectiveness
increases. The ANOVA results show that the time and temperature are negligible factors as compared to other factors.
Keywords: Composite fabric, Core yarn, Copper, Dref II Spinning, Electromagnetic wave, Nickel plating, Polyester, Shielding
1 Introduction
Electromagnetic shielding provides protection by
reducing signals to levels at which they no longer
affect equipment or can no longer be received1. This
is achieved by reflecting and absorbing the radiation.
Polymeric composites are extensively used as passive
and active elements in some electrical circuit
components in different technological applications 2, 3
due to their light weight, easy process ability,
flexibility, corrosion resistance and cost-effectiveness.
But elastomers and plastics contain very low
concentrations of free charge carriers. They are
electrically non-conductive and transparent to
electromagnetic radiation. To provide conductivity
and shielding from Electromagnetic Interference
(EMI), incorporation of fillers of high intrinsic
conductivity such as particulate carbon blacks, carbon
and graphite fibres, or metal powder to the polymer
matrix is required4,5. The amount of electrically
conductive filler required to impart high electrical
conductivity to an insulating polymer can be
dramatically decreased by the selective localization of
the filler in one phase, or best at the interface of a
continuous two-phase polymer blend6-11. Unfortunately,
——————
a
To whom all the correspondence should be addressed.
E-mail: [email protected]
most polymer composites used for household
electrical and electronic devices are electrically
insulating and transparent to electromagnetic radiation
and electrostatic discharge (ESD). The potential
health hazards (e.g. cancer) associated with exposure
to electromagnetic fields12-17 are also the matter of
concerned. To shield and limit against EMI and ESD,
conductive polymer composites started replacing
coated materials for various shielding applications in
the electrical and electronic industries, especially for
electronic household materials. This trend has been
driven mainly because of the better characteristics of
these polymers in terms of ESD, shielding from EMI,
thermal expansion, density, and chemical (corrosion
and oxidation resistance) properties18-22. However,
conductive polymers have rigid characteristics owing
to their chemical conformation of benzene rings,
making their formation difficult. A few studies have
reported conductive knitted fabrics reinforced
composites as electromagnetic shielding effectiveness
(EMSE) and ESD materials. Previously, Cheng et al. 23-26
fabricated
some
knitted
fabric
reinforced
polypropylene composites for use as EMSE and ESD
materials, and indicated that the knitted fabric
reinforced polymer composites are suitable for
making complex shaped components and application
in electromagnetic shielding
36
INDIAN J. FIBRE TEXT. RES., MARCH 2011
The coating of fabrics is a textile finishing process
that adds to the value and improves functions 27. With
the advent of synthetic fibres, there are now less
expensive ways to make metallic fabrics in the
twenty-first century. Metallized fabric, in particular,
has an identity and a quality of brightness that can
create beautifully reflected and lustrous images. The
reflective surfaces can offer a unique appearance as a
result of this technical process and metalized fabrics
are very popular in both the contemporary consumer
market and in technical applications. Metalized fabric
has been produced by a laminating process between
metal powder or foil and fabric with binder28.
Vacuum deposition and sputtering coating processes
have been applied to polyester fabric with aluminium,
titanium and stainless steel 29 and a silver-plated
product was usually produced by electroplating on the
surface of conductive materials 30,31.
Electroless plating is a chemical reduction process
which depends upon the catalytic reduction of a
metallic ion in an aqueous solution containing a
reducing agent, and the subsequent deposition of the
metal without the use of electrical energy. The metals
capable of being deposited by electroless plating
include nickel, cobalt, copper, gold, palladium
and silver.
Electroless nickel (EN) is an alloy of 88-99%
nickel and the balance with phosphorous, boron, or a
few other possible elements. EN coatings, therefore,
can be tailored to meet the specific requirements of an
application with the proper selection of the nickel's
alloying element(s) and their respective percentages
in the plated layer. All varieties of EN, including
composite EN coatings, can be applied to numerous
substrates including metals, alloys, and nonconductors
with outstanding uniformity of coating thickness to
complex geometries.
Diligent control of the solution's stabilizer content,
pH, temperature, tank maintenance, loading, and
freedom from contamination are essential for its
reliable operation. EN solutions are highly surface
area dependent. Surface areas are introduced to the
solution by the tank itself, in-tank equipment,
immersed substrates, and contaminants. Continuous
filtration, often sub-micron, of the solution at a rate of
atleast ten turnovers per hour is always recommended
to avoid particle contamination which could lead to
solution decomposition or imperfections in the plated
layer. In this study, attempts have been made to
optimize the electroless nickel plated process
parameters to improve the EMSE of copper core yarn
with polyester sheath composite fabric through
Taguchi design and ANOVA.
2 Materials and Methods
The fabrics from complex copper core spun yarns
(179 tex) with polyester sheath (copper wire diameter,
0.12mm and core: sheath ratio of copper and
polyester, 23 : 77) were produced on Dref-II friction
spinning machines. These copper core / polyester
sheath yarns were converted into plain woven fabric
and the fabric was then treated with electroless nickel
plating as per Taguchi design to improve the EMSE
of composite fabric. In the EN plating process, the
driving force for the reduction of nickel metal ions
and their deposition is supplied by a chemical
reducing agent in solution. This driving potential is
essentially constant at all points of the surface of the
component, provided the agitation is sufficient to
ensure a uniform concentration of metal ions and
reducing agents. The EN plating process was
basically divided into five main stages, namely precleaning, sensitization, activation, electroless nickel
deposition and post-treatment. In the electroless
nickel plating process, unless otherwise stated, all
chemicals used are of A R grade.
2.1 Pre-treatment
To facilitate the plating of metal particles on the
fabric surface, pre-treatment was carried out. The
processes of pre-treatment consist of pre-cleaning,
sensitization and activation.
Pre-cleaning
Pre-cleaning is done for operating the electroless
nickel solution so that required properties are obtained
in the fabric. Additives and/or contaminants can have
a profound effect on performance, as will operate
conditions. In this process, all fabric samples were
pre-cleaned in a 2% non-ionic detergent at pH 7 and
temperature 40°C for 20 min. Deionised water was
then used to rinse the pre-cleaned samples. Triton X100 as a non-ionic detergent was used.
Sensitisation
Sensitisation is a process of catalyzing the nonconductive substrate. This enables the fabric surface
to act as a catalyst for the deposition of nickel. The
auto-catalysation is obtained through this process. In
the case of sensitizing fabric surfaces, the cleaned
fabric samples were subjected to the surface sensitizer
PERUMALRAJ & DASARADAN: ELECTROLESS NICKEL PLATED COMPOSITE TEXTILE MATERIALS
37
(mixture of 5 g/L stannous chloride and 5M/L
hydrochloric acid) with slow agitation for 10 min at
25°C and 1 pH. In sensitization, non-conductive
substrate absorbs the stannous Sn2+ ions from the
stannous solution.
Activation
Activation is carried out for activating the fabric
surface for the nickel deposition. The objective of this
process is same as that of sensitization. In this
process, a activator solution is used. The sensitized
fabrics were rinsed with deionised water subsequently
and then immersed in the activator solution [palladium
(II) chloride, hydrochloric acid (conc. 37%) and boric
acid] at pH 2 and 25°C for 5 min in order to achieve
surface activation. The activated fabrics were rinsed
with deionised water afterwards.
In activation, nucleation effect is performed on the
sensitized substrate in the acid palladium chloride
solution (i.e. nucleate solution), where the sensitized
substrate carries stannous (Sn2+) ions and reacts with
palladium (Pd2+) ions. The divalent tin on substrate
surface is eliminated due to the fact that the sensitized
tin on substrate surface reacts with the palladium salt
in the activation solution. Figure 1 clearly illustrates
that the stannous chloride layers envelope the
palladium metal clusters, which is lifted up on
substrate surface.
solutions results in electroless nickel plating as
demonstrated. After this, nickel particles are adhered
on the substrate surface.
2.2 Deposition
2.4 Experimental Design
Deposition involves reducing the nickel salt into
nickel, i.e. the metal ions are reduced as metals by the
reducing agent. In case of electroplating, electric
current is used for the reduction process. In this
deposition, sodium hypophosphite monohydrate is
used as reducing agent. During the deposition stage,
the nickel plating solution was employed in the
electroless nickel plating tank, which was composed
of nickel (II) sulphate hydrate, tri-sodium salt
dehydrate, ammonium chloride, a few drops of
sodium hydroxide (conc. 10%) and sodium
hypophosphite monohydrate. The activated fabrics
were immersed in the nickel plating solution at pH 9
with various temperature and time, right after the
metallising reaction of electroless nickel plating.
In deposition, nickel sulphate (metal salt), sodium
hypophosphite (reducing agent) and stabilizer in
strong alkaline comprise a complex nickel plating
bath. The autocatalytic reaction between the nickel
ions and the reducing agent in the nickel plating
Taguchi method32,33 was applied to identify the
optimum level of electroless nickel process
parameters
for
electromagnetic
shielding
effectiveness performance characteristic individually.
Taguchi method replicates each experiment with the
aid of an outer array that deliberately includes the
sources of variation that a product would come across
while in service. Such a design is called a minimum
sensitivity design or a robust design. The robust
design method is called Taguchi method.
To achieve the optimum design factor setting,
Taguchi advocated a combination of two stage
process. First step is related to the selection of
robustness seeking factors and the second step is
related to the selection of adjustment factors to
achieve the desired target performance. Various
stages in the experimental design have been dealt by
many researchers in the past. In other experimental
designs, uncontrollable factors are kept under
observation during experimentation, whereas Taguchi
Fig. 1—Palladium cluster enveloped by stannous chloride layers
2.3 Post-treatment
Post-treatment is done to achieve the required
properties on the fabric surface after deposition. In
this process, the cleaned nickel-plated fabrics were
cured by steaming machine directly at 150°C for 1
min. All electroless nickel-plated fabric samples were
conditioned under standard atmospheric pressure at
65% ± 2% relative humidity and 21°C ±1° C for at
least 24 h before further evaluation.
INDIAN J. FIBRE TEXT. RES., MARCH 2011
38
methods include those factors in the experimentation
to make the design a robust one in the form of signalto-noise ratios (S/N). In robust design, sensitivity to
noise is minimized by seeking combinations of design
parameter settings. The most appropriate S/N ratio
can be selected depending upon the properties of
interest (Table 1) for both scaling factors and
adjusting factors.
The experiments using the controlled and
uncontrolled variables at different levels (Table 2) were
carried out randomly to avoid the systematic errors.
The selection of appropriate orthogonal array (OA) is
a critical step in Taguchi’s experimental design. The
OA selected should satisfy the following criterion:
Degrees of freedom (DOF) of OA > Total DOF required
Therefore, L9 Taguchi orthogonal array and 3
levels were selected to assign various columns. The
Table 1 — Signal-to-noise (S/N) ratio and its significance 32, 33
Case
S/N ratio
Target is the best
Small-the-better
Larger-the-better
Binary scale (GO/NO-GO)
S/N (θ) = 10 log10 (τ2 / s2)
S/N (θ) = -10 log10 (yi2 / n)
S/N (θ) = 10 log10 [(1/yi2) / n]
S/N (θ) = 10 log10 (p/1-p)
p= proportion of good products
Notation
Controlled variable
Uncontrolled
variable
Level Level Level Noise Noise
1
2
3 level 1 level 2
Palladium chloride
grains/gal
Nickel (II) sulphate
grains/gal
Time, min
Temperature, 0C
29.2
A
17.5
40.9
-
-
B
642.6 759.4 876.2
-
-
C
D
5
30
10
40
20
50
Frequency, MHz
E
-
-
-
Aerial density
F
-
-
-
2.5 Electromagnetic Shielding Effectiveness Test
The EMSE of the conductive fabric was calculated
by following ASTM D4935 – 99 test methods. The
basic characteristic of the conductive fabric is its
attenuation
property.
Attenuation
of
the
electromagnetic energy is a result of the reflection,
absorption and multi-reflection losses caused by a
specific material inserted between the source and the
receptor of the radiated electromagnetic energy.
Attenuation caused by a material is characterized,
depending on the measuring method used, by the two
quantities, namely shielding effectiveness (SE) and
insertion loss (A).
2.5.1 Shielding Effectiveness
Table 2—Controlled and uncontrolled variables
Factor
experiments were conducted according to the trial
conditions specified in L9 OA. A total of 36
experiments (three repetitions at each trial condition)
was conducted.
Using Taguchi analysis and analysis of variance
(ANOVA), the optimal performance parameters were
determined and the optimal values were predicted.
The average values of performance characteristics at
each level and against each parameter were calculated
and are given in Table 3.
N1
N2
(High) (Low)
N4
N3
(High) (Low)
Shielding effectiveness (SE) is defined as the ratio
of electromagnetic field strength (E0) measured with
and without the tested material (E1) when it separates
the field source and the receptor, as shown below34:
SE = E0 / E1
In decibels,
SEdB = 20log E0 / E1
This depends on the distance between the source
and the receptor of electromagnetic energy. In the far
field zone, it characterizes the attenuation of the
electromagnetic wave. The measurement carried out
Table 3 — Experimental test setup for optimization of electromagnetic shielding effectiveness
Expt. No. Palladium chloride Nickel (II) sulphate
grains/gal
grains/gal
1
2
3
4
5
6
7
8
9
17.5
17.5
17.5
29.2
29.2
29.2
40.9
40.9
40.9
642.6
759.4
876.2
642.6
759.4
876.2
642.6
759.4
876.2
Time
min
Temperature
0
C
N1
N3
N2
N3
N1
N4
N2
N4
S/N
ratio
5
10
20
10
20
5
20
5
10
30
40
50
50
30
40
40
50
30
28
30
34
36
37
41
45
42
44
25
26
30
33
34
38
42
39
40
26
27
31
34
35
39
43
40
41
27
28
32
35
36
40
44
41
43
28.44
28.83
30.01
30.74
30.99
31.92
32.76
32.14
32.45
PERUMALRAJ & DASARADAN: ELECTROLESS NICKEL PLATED COMPOSITE TEXTILE MATERIALS
39
in the near field zone characterizes the attenuation
effectiveness for the electric or magnetic field
component only.
2.5.2 Insertion Loss
Insertion loss (A) is a measure of the losses (or
attenuation) in a transmitted signal caused by the
tested material being inserted into the measuring
channel. The insertion loss can be measured using the
following relationship34:
A = U0 / U1
In decibels,
AdB = 20 log (U0 / U1)
where U0 is the channel output voltage without the
tested material; and U1, the same voltage with the
tested material.
A spectrum analyzer with coaxial transmission
equipment was used to measure the EMSE of
the copper core conductive fabric at frequency ranges
20-200MHz, 200 MHz - 1GHz and 1-18GHz.
2.6 Test Procedure
The tests were carried out in an anechoic chamber.
Anechoic chamber is a room in which no acoustical
reflections or echoes exist. The floor, walls and ceilings
of these rooms are lined with a metallic substance to
prevent the passage of electromagnetic waves. Radio
frequency signal was generated by a signal generator
and it was transmitted through an antenna outside the
chamber. Signal from the signal generator was
measured by spectrum analyzer with antenna inside the
chamber. The first measurement (calibration) was
carried out without the test fabric. The tests in the
different frequency ranges were conducted.
3 Results and Discussion
Higher shielding effectiveness value indicates the
better electromagnetic shielding effectiveness from
nickel plated copper core fabric and hence larger - the
- better S/N is employed to optimize the process
parameter of electroless nickel plating. Table 3 shows
the orthogonal array of the experimental setup
followed by shielding effectiveness and the relevant
S/N ratio. Figure 2 shows the effects of electroless
nickel plating process parameter on shielding
effectiveness values in terms of S/N ratio. The
efficiency of electromagnetic shielding depends on
the amount of nickel deposited over the fabric surface.
This amount of deposited nickel depends an surface
activation. The activation of the fabric surface is
Fig 2—Effects of control factors
carried by palladium chloride in combination with
HCl and boric acid. Hence, the main control factors
involved in this process are palladium chloride, nickel
sulphate, time and temperature.
3.1 Effects of Palladium Chloride Concentration on EMSE
The palladium has a dominant influence on EMSE
of the electroless nickel plated copper core yarn
composite fabrics. Figure 2 shows the effects of
palladium chloride on copper core/ polyester sheath
yarn composite fabric. With an increase in
concentration of palladium chloride, a general
increase in EMSE is observed due to nickel chemical
deposition. Electroless nickel plating is the process of
depositing metallic nickel by controlled chemical
reactions catalyzed by the metallic nickel being
deposited. It produces a film of uniform thickness
with very good conductivity. The amount of nickel
deposition improves the absorption and reflection of
electromagnetic wave. The absorption loss (dB) is
proportional to the square root of the product of the
permeability and the conductivity of the electroless
nickel plated copper core yarn composite fabric. If a
magnetic material is used in place of a good
conductor, there is an increase in the permeability and
a decrease in the conductivity. It increases the
absorption loss, since the permeability increases more
than the conductivity decrease for the reflections loss.
The total loss through a nickel plated copper core/
polyester sheath yarn composite fabric is the sum of
loss due to absorption and that due to reflections by
the composite fabric. The ANOVA carried out for
EMSE of fabric shows the dominating effects on
palladium chloride concentration (Table 4).
3.2 Effect of Nickel (II) Sulphate on EMSE
The nickel (II) sulphate concentration has a
significant influence on EMSE of the electroless
INDIAN J. FIBRE TEXT. RES., MARCH 2011
40
Table 4 — ANOVA factors effects and F value of response variable for electromagnetic shielding
Control factor
Degree of
freedom
F before
pooling
2
2
2
2
44
3
1
1
Palladium chloride, grains/gal
Nickel (II) sulphate, grains/gal
Time, min
Temperature, 0 C
Empty or pooled F after
F=<1.5
pooling
nickel plated copper core / polyester sheath yarn
composite fabrics. Figure 2 shows the effect of nickel
(II) sulphate concentration on EMSE of composite
fabric with incident frequency from 200MHz to
1000MHz. The ANOVA carried out for EMSE of
fabric shows the significant effect of nickel (II)
sulphate concentration. Metal deposition reduces the
nickel salt into nickel. The nickel particles are adhered
on the substrate surface mainly by autocatalytic
reducing agent in the nickel plating solutions. The
absorption and reflection of electromagnetic waves are
improved by nickel particles.
3.3 Effect of Time on EMSE
The ANOVA carried out for EMSE of fabric shows
the neutral / negligible effect of time. Figure 2 shows
that the increase in the shielding effectiveness of the
fabric with the increase in time is due to metal
adhesive performance on fabric surface but it slightly
drops at 10 min and hence the processing time factor
is neutral/ negligible as compared to other factors.
3.4 Effects of Temperature on EMSE
The ANOVA carried out for EMSE of fabric shows
the neutral / negligible effect on temperature. It is
observed from Fig. 2 that the optimum performance
of EMSE occurs at 400 C due to the fact that the
optimized metal deposition occurs at this temperature
and the nickel particles move to distribute uniformally
on the composite fabrics.
ANOVA carried out for the signal-to-noise ratio of
various combinations of the design and noise factors
shows the predominant controlling nature of
palladium chloride, nickel (II) sulphate, time and
temperature (Table 4). ANOVA results also show the
dominating effects of palladium chloride as compared
to nickel (II) sulphate, time and temperature. Effect
of individual variables in terms of their average
S/N ratio along with the % factors effects and F
values (pooled and unpooled) further confirm the
dominant nature.
Confirmation tests in the Taguchi method
supplements assures the validity of the results
obtained in the experimental designs, orthogonal array
No
No
Pooled
Pooled
35
3
-
Nature
Dominant
Significant
Neutral/negligible
Neutral/negligible
Optimum % Factor
level
effects
0.7
15
-
88
7
3
2
selected in the study and various levels of the design
parameters and their interactions. Confirmation test
carried out with optimum parameters for the response
variable shows the closer results to that of original
results and does not show any significant difference at
95% confidence levels. The values obtained in
confirmation tests along with the original values for
response variables, considered in the study, are given
below:
Original result
: 45
Confirmation result
: 44
Significance at 95% confidential
level (Yes/No)
: No
Scaling factors
: A1 and B3
Adjustment factors
: C3
4 Conclusion
The electroless nickel plated copper core/ polyester
sheath yarn composite fabrics provide an attenuation
of 25-45 dB at the frequency range 200 - 1000 MHz.
Hence, these fabrics can be used to shield the household electrical and electronic applications, such as
FM/AM radio, wireless phone, cellular phone and
computer that operate up to 1000 MHz frequency
ranges. It is observed that
(i) with an increase in the palladium chloride and
nickel (II) sulphate, an increase in nickel deposition
and electromagnetic shielding has been observed in
200 - 1000 MHz frequency range.
(ii) the ANOVA carried out for EMSE of
electroless nickel plated copper core / polyester sheath
yarn composite fabric shows the neutral / negligible
effects on time and temperature factors.
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