13
CHAPTER 2
OVERVIEW OF THE PROCESSES AND LITERATURE
REVIEW
2.1 PLASTICS AND POLYMERS. 15
2.2 OVERVIEW OF COMPOSITES 16
2.2.1 Classification of Composites 17
2.2.1.1 Polymer Matrix Composites 18
2.2.1.2 Factors Affecting Properties of PMCs 19
2.2.1.2.1 Interfacial Adhesion 19
2.2.1.2.2 Shape and Orientation of Dispersed Phase
Inclusions (Particles, Flakes, Fibres, and
Laminates) 20
2.2.1.2.3 Properties of the Matrix Properties 20
2.2.1.3 Fabrication of Composites 23
2.2.1.4 Effect and Functions of Fillers 26
2.2.2 ABS – Poly (Acrylonitrile, Butadiene, Styrene) 28
2.2.3 Nylon (Polyamides) 29
2.2.4 Wollastonite / Calcium Meta Silicate 36
2.2.5 Water Absorption Studies on Polyamide Composites 40
2.2.6 Mechanical, Electrical and Tribological Properties of
Polyamide-6 Composite Materials 44
2.3 METALLIZATION OF PLASTICS 50
2.3.1 Overview 50
2.3.2 Plating of plastics 52
2.3.2.1. Electroless plating 52
2.3.2.1.1 Composition of Electroless Plating Bath
53
2.3.2.1.2 Advantages of Electroless Plating 54
2.3.2.1.3 Applications of Electroless Plating 55
2.3.2.1.4 Electroless Plating of Copper 55
2.3.2.1.5 Electroless Nickel Plating 57
2.3.2.2 Electroplating 59
14
2.3.2.2.1 Principles of Electrodeposition 60
2.3.2.2.2 Mechanism of Electrodeposition 61
2.3.2.3 Effects of Plating Variables 63
2.3.2.4 Components of Electroplating 69
2.3.2.5 Plating Process 71
2.3.2.6 Electroplating of Chromium 74
2.3.2.7 Electroplating of Copper and Nickel 76
2.3.2.8 Distinction between Electroplating and Electroless
Plating 78
2.3.2.9 Conductive Paints 78
15
CHAPTER 2
OVERVIEW OF THE PROCESSES AND LITERATURE
REVIEW
2.1 PLASTICS AND POLYMERS
Plastics play a very significant role in our everyday life; many
vital products such as knee joints, disposable food utensils,
automotive components, etc. are fabricated with plastics. Since early
times, the polymeric materials have been used in various forms,
although there are no records to provide the evidence of their exact
usage. For instance, an observation about the use of natural rubber
balls for playing by the inhabitants of Haiti has been recorded by
Christopher Columbus in 1400s. In early 1900s, synthetic polymers
came into existence and were prepared using phenol and
formaldehyde to form resins known as Baekeland‟s Bakelite [6].
Even after the development of synthetic polymers, scientists
were ignorant of the true nature of the materials they had prepared.
Scientists for many years assumed that they were colloids, but it was
in 1920s that Herman Staudinger explained that polymers were
macromolecules. Later in 1928, Carothers developed PAs.
Subsequently, Ziegler and Natta‟s work in 1950s, led to the
development of high-density, linear polyethylene, polypropylene, and
other stereo specific polymers [6].
Plastics exhibit a remarkable range of properties like lower
densities, thermal conductivities, moduli, etc. The properties of
16
plastics can be customized by blending them with various additives
like fillers, fibres, plasticizers, etc. With the use of these additives the
compounder will be able to develop materials for specific applications.
Polymeric materials with lower densities benefit the applications
which require lighter weights. Conducting fillers, added into the
matrix of plastics, transform insulators into conductors and as a
result, they can be used in EMI/RFI applications. Plastics in
automotive area are used in fabricating interior and exterior
components. For instance, a swift glance inside an automotive vehicle
gives us an idea of the use of plastics. Components such as side view
mirror, headlight, bumpers, doors, wheel covers, trunk lids, hoods
and grilles.
Many varieties of thermoplastics are used in packaging
applications which include carbonated beverage bottles to plastic
wrap. The requirements of plastics are different for different
applications, but fortunately, plastic materials can be modified to
meet up with the varied service conditions. The part designer has to
carefully select from the array of accessible thermoplastic materials to
cope up with the essential demands [6].
2.2 OVERVIEW OF COMPOSITES
Composite materials or composition materials are resources
prepared from two or more essential materials with unusual
physical or chemical properties. The materials when combined,
creates a new material with characteristics divergent from the
individual components. This combination material may be chosen for
17
many reasons due to the benefits it offers like low weight, corrosion
resistance, high fatigue strength and faster assembly. These materials
are extensively used in building aircraft structures, electronic
packaging, medical equipments and space vehicles [7, 8].
The major demarcation between blends and composites is that
the two main constituents of the composites remain identifiable,
although these may not be decipherable in blends. Wood, ceramics,
concrete are some of the valuable materials used in our everyday life.
Surprisingly, the nature itself has gifted us with many polymeric
composites and these can be called as natural composites. For
example: The connective tissues found in mammals are a composite
material [8].
Composite materials are made of two or more distinct phases
known as a matrix and dispersed phase. The matrix phase is a
primary phase, which has a continuous character. It is more ductile
and is a less hard phase. Dispersed phase is a reinforcing phase,
usually stronger than matrix and is rooted in the matrix in a
discontinuous form [8].
2.2.1 Classification of Composites
Depending on the matrix phase, composites are classified as
“metal matrix composites (MMCs), ceramic matrix composites (CMCs),
and polymer matrix composites (PMCs) (fig. 2.1)” [7, 9].
According to the type of reinforcement, composites are classified
as either, particulate composites which are composed of particles,
fibrous composites which are composed of fibres and laminate
18
composites which are composed of laminates. Fibrous composites are
further divided as natural / biofibre or synthetic fibre [7, 10]. Of late,
the market is concentrating on Bio-based composites which are
normally referred to as “Green composites” made from natural /
biofibre and biodegradable polymers. Green composites are further
divided into two types, viz., hybrid and textile composites [7, 8].
Fig. 2.1: Classification of composites [11]
2.2.1.1 Polymer Matrix Composites
Polymer matrix composites (PMCs) are prepared by mixing a
range of short and continuous fibres / whiskers / particles, etc. with
the organic polymers. The reinforcements added into the matrix of
PMCs offer high strength and stiffness to the material. These
reinforcements actually support the structure that is subjected to
mechanical loads [12]. The only difficulty with PMCs is its low thermal
19
resistance and high coefficient of thermal expansion [7]. But
otherwise, put simply, they are exceptional materials with excellent
properties as listed in the ensuing literature.
Many of the commercially produced composites uses polymer
matrix material. There are numerous polymers available today
depending upon the raw materials. The most commonly known
polymers include polyester, polyamide, vinyl ester, epoxy,
polypropylene phenolic, polyimide, polyether ether ketone (PEEK) and
others. The reinforcement materials can be fibres, and can also be
minerals like calcium silicate, Calcium carbonate, graphite, etc. [7, 8].
In recent times, PMCs are gaining much importance due to their
low cost and uncomplicated fabrication techniques. The usage of
polymers in their unreinforced stage is limited due to the low-level of
mechanical properties. Polymers reinforced with fibres / additives are
characterized by the following properties: [7]
a) High specific
strength
b) High specific
stiffness
c) High fracture
resistance
d) Good abrasion
resistance
e) Good impact
resistance
f) Good corrosion
resistance
g) Good fatigue
resistance h) Low cost
2.2.1.2 Factors Affecting Properties of PMCs
2.2.1.2.1 Interfacial Adhesion
Composite material‟s behaviour is generally explained based on
the collective behaviour of the polymer matrix, the reinforcing
materials, and the fibre / matrix interface. “Superior mechanical
20
properties” are attained if “the interfacial adhesion is strong”. The
degree of interfacial adhesion is determined by anchoring the matrix
molecules to the fibre surface by chemical reaction or adsorption.
Recent developments in atomic force microscopy (AFM) and nano
indentation devices have assisted in the investigation of the interface
[7].
2.2.1.2.2 Shape and Orientation of Dispersed Phase Inclusions
(Particles, Flakes, Fibres, and Laminates)
Bednarcyk [13] states that particles are mainly used to improve
properties and also to lower the cost of isotropic materials. Particles
have no preferred directions. Josmin P Jose et al. [7] and Tabiei [14]
explains that the shape of the reinforcing particles can be either
spherical, cubic, platelet, regular or irregular geometry and the
dimensions of particulate reinforcements are more or less same in all
directions. Particle-reinforced composites are divided into large
particle and dispersion-strengthened composites. Laminar composites
are made from two dimensional panels / sheets that are stacked and
subsequently bonded together to form high strength materials.
2.2.1.2.3 Properties of the Matrix Properties
Huang [15] explains that the matrix properties of different
polymers will decide the application to which it is suitable. The
principal advantages of polymers as matrix are low cost, easy
processability, good chemical resistance and low specific gravity.
Conversely, low strength, low modulus, and low operating
temperatures limit their usage. Varieties of polymers for composites
21
are (a) Elastomers (b) Thermosetting polymers (c) Thermoplastic
polymers and their blends [7].
(a) Elastomers:
Polymers which possess the property of viscoelasticity are
known as Elastomers and these are amorphous polymers that exist
above their glass transition temperature. Elastomers remarkably have
low elastic modulus and high yield strain in comparison with other
materials. A classical example for this is a „rubber‟. Rubbers are
reasonably softer and deformable at room temperatures. The young‟s
modulus of rubber is approximately 3 MPa at room temperature and
rubbers are usually used in preparing seals, adhesives and moulded
flexible parts. Polybutadiene, butyl rubber, chloroprene rubber,
ethylene propylene rubber, natural rubber etc., are other examples of
elastomers [7].
(b) Thermosetting polymers:
According to Josmin P Jose et al. [7] thermosets are those
polymers that do not soften but decompose on heating and cannot be
restructured after solidification. Epoxies, silicone, polyesters, urea,
etc., are some examples of thermosetting polymers.
The viscosity of the thermosetting resins is usually low.
Nevertheless, these resins go through chemical reactions and finally
form a three dimensional network. These three-dimensional
structures have higher dimensional stability, superior resistance to
solvents and high-temperature resistance. In recent times, significant
22
improvement has been made in enhancing the toughness and
maximum operating temperatures of thermosets [12].
(c) Thermoplastic polymers:
Thermoplastics are those polymers that can be restructured by
the application of heat and pressure [12]. Today, 85% of the polymers
made globally are thermoplastics [16]. Nylons, polyacetals,
polypropylene, polyvinyl Chloride (PVC), polyamideimide (PAI), etc. are
some examples of thermoplastic polymers [7, 12]. Some unique
polymers such as PEEK, polyimides, fluoropolymers and liquid-crystal
polymers are used in more sophisticated and high performance
application areas due to their high glass transition temperature „Tg‟ or
melting temperature „Tm‟ (290–350 oC).
Depending on the type of their distinctive transition
temperature, thermoplastics are classified as either crystalline or
amorphous. Amorphous thermoplastics are distinguished from
crystalline thermoplastics by their „Tg‟. The glass transition
temperature is the temperature beyond which the modulus of the
polymer reduces quickly and displays liquid-like properties;
amorphous thermoplastics are usually worked at temperatures well
beyond their Tg. For instance, the Tg for PVC may be as low as 65 oC
and for PAI, as high as 295 oC [16].
Crystalline thermoplastics or, put simply, semicrystalline
thermoplastics are usually processed or worked beyond the melting
temperatures „Tm‟ of thermoplastic polymers. The degree of
crystallinity ranges between 20 – 90 %. Melting temperatures for
23
polymers such as low-density polyethylene (LDPE) may be as low as
110 oC and for polyetherketone (PEK) may be as high as 365 oC. [16]
Thermoplastics have high resistance towards cracking and
impact damage. Some of the recently developed thermoplastics like
PEEK, which are semicrystalline in nature, demonstrate exceptional
high temperature strength and solvent resistance. The time involved
in heating and cooling of thermoplastics is faster and hence the curing
time in comparison with thermosets is low. This is very much
essential for automotive industries that need materials fabricated at a
faster rate to reduce the production time and maximize the profits.
Thermoplastics reinforced with discontinuous fibres are in high
demand these days and there is also scope for thermoplastics
reinforced continuous fibres and thus, these plastics will proffer a
huge scope in the future from the manufacturing and design
viewpoint. As an example, the thermoplastic composites could replace
thermosetting composites (epoxies) in many of the fighter aircrafts
[12].
2.2.1.3 Fabrication of Composites
There are various methods available for fabricating composite
components. Some of the methods were developed to meet specific
design and manufacturing needs and some were borrowed. However,
selection of a particular method depends on the materials, the part
design, and the end-user applications [17].
24
Some of the important processing methods are (a) Hand lay-up
(b) Bag moulding process (c) Filament winding (d) Pultrusion (e) Resin
transfer moulding (f) Injection moulding [7].
(a) Hand Lay-Up
Hand lay-up technique has been used in the manufacture of
reinforced (small and large) products. It is one of the simplest and
normally used techniques for fabricating laminate composites. This
method uses plastics, wood, metal or a combination of these to
fabricate materials [7].
(b) Bag Moulding Process
According to Josmin P Jose et al. [7] and Tay [18], the bag
moulding process is a very flexible type of composite fabrication
technique known to the mankind till date. It is a process wherein the
laminates are laid up or stacked up in a mould and resin is spread in
between them. A flexible diaphragm is spread over the stacked layers
and cured by applying heat and pressure. Once, the curing cycle is
complete, the materials take the shape of the mould itself. There are
three basic moulding method involved viz., pressure bag moulding,
vacuum bag moulding and autoclave moulding.
(c) Pultrusion
Josmin P Jose et al. [7] describes pultrusion as an automatic
process of fabricating composite materials into unbroken, regular
cross-section profiles. Pultrusion involves pulling the product from the
die rather than being forced out by pressure as in the extrusion. Using
25
appropriate dies, materials in the form of rods and tubes are
produced.
(d) Filament Winding
Filament winding is a method of creating surfaces of revolution
such as pipes, tubes, and cylinders. The method is normally used in
the construction of large tanks, pipes and tubes for industrial
purposes. The filament winding method is based on the principle of
laying down continuous reinforcements at high speeds in pre-specified
patterns [7].
(e) Resin Transfer Moulding
Resin Transfer Moulding (RTM) involves fabrication of large,
high performance products. In this process, a tool is manufactured
either from metals or composites and the resin is injected into the
tool. The reinforcements are also placed in the tool and due to the
injection the resin will come in contact with the reinforcements. RTM
can offer distinct advantages, particularly if the temperature
controlled tooling is used. The advantages include increased
productivity through the use of higher injection pressures and heated
moulds, improved quality, improved product consistency due to better
process control and improved dimensional tolerances [7].
(f) Injection Moulding (IM)
Wakeman et al. [19] describes IM as a method of manufacturing
both thermoplastic and thermosetting plastic materials. Materials
(matrix and reinforcement) are fed into a heated barrel, churned and
pushed into a mould cavity where it cools and hardens and finally
26
takes the shape of the mould cavity. Through IM, many materials
such as mobile phone cases, spike busters, internet dongles, etc., are
produced today. IM method is suitable for producing same objects in
larger quantities, since the cost involved in manufacturing dies is
more.
Josmin P Jose et al. [7] lists some of the pros and cons of IM
such as high production rates, repeatability, high tolerance, knack to
use a broad range of materials, low labour costs and minimal scrap
losses, but the tools and equipments involved are expensive; the need
to design mouldable parts and potentially high running costs makes it
suitable for producing articles in larger volumes.
2.2.1.4 Effect and Functions of Fillers
The solid additives normally added into the matrix of polymers
are termed as fillers [20]. Fillers can be classified into two types viz.,
Inert and Reinforcing fillers. Inert fillers are added from the
economical point of view. Even though termed inert, inert fillers can
nevertheless have influence on other properties of the composite,
besides cost. Adding them into the matrix may increase the density of
the prepared material, improves hardness and reduces shrinkage.
China clay (kaolin), talc, and calcium carbonate are some examples of
inert fillers. On the other hand, reinforcing fillers are added from the
strength point of view. They actually increase the mechanical
properties of the compound, increase heat deflection temperature,
reduce shrinkage of the composite, and bring improvements in creep
behaviour.
27
The properties of the composites are altered and improved by
the reinforcing fillers using several mechanisms. A bond (chemical) is
formed linking the polymer and the filler, in some cases and in other
cases, the properties of the thermoplastics is affected by the volume
occupied by the fillers. Consequently, interaction amongst the filler
and the thermoplastic and the surface properties are of greater
significance. The particle shape and size, the surface chemistry of the
particles also administrates the behaviour of composites. The smaller
particles lead to the greater improvements in the mechanical strength.
Particles of larger size may offer reduced properties in comparison
with virgin thermoplastics. It has also been observed that the shape of
the particles has a larger influence on the properties, for instance,
fibrous particles or plate-like particles results in anisotropic properties
with proper orientation of particles during processing. Calcium
carbonate is sometimes used with thermoplastics for strength
enhancement. Calcium carbonate is a natural product and has a
particle size of 1 μm [20]. Glass spheres, Talc, Kaolin, and Mica are
used as fillers in thermoplastics.
Glass spheres can be either solid or hollow, and talc, kaolin,
and mica usually have lamellar shapes [20]. Other fillers include
calcium meta silicate, barium sulphate, metal powders and silica.
Rubber industry usually uses carbon black as filler, but
thermoplastics use carbon black for conductivity, protection from
ultra violet radiations, and also as a pigment.
28
Thermoplastics often use fibres as fillers. Carbon, wood flour,
cotton, and fibreglass are some types of fibres used in thermoplastics.
Table 2.1 presents some typical fillers and their forms. Table 2.5
presents a summary of some characteristic fillers, and their effect on
properties of composites. Significant research interest is shown
towards inclusion of nanoscale fillers into polymers [6].
Table 2.1 Forms of various fillers [6]
Spherical Lamellar Fibrous
Sand/quartz powder Mica Glass fibres
Silica Talc Asbestos
Glass spheres Graphite Wollastonite
Calcium Carbonate Kaolin Carbon fibres
Carbon black Whiskers
Metallic Oxides Cellulose
Synthetic fibres
2.2.2 ABS – Poly (Acrylonitrile, Butadiene, Styrene)
ABS is basically a terpolymer of Acrylonitrile, Butadiene
and Styrene [21]. A terpolymer is a copolymer consisting of three
distinct monomers. It is produced by making acrylonitrile and styrene
react with polybutadiene. Acrylonitrile is made from ammonia and
propylene; butadiene is produced by steam cracking; and styrene is
produced from ethyl benzene [22].
ABS plastic is used for its impact resistance and toughness and
these are considered as the most important mechanical properties.
Alterations with respect to impact resistance, toughness, and heat
29
resistance can be made by incorporating certain fillers and
reinforcements into the matrix of ABS. ABS polymers are known for
their resistance towards vegetable oils, hydrochloric acid, mineral and
animal oils, alcohols, etc. But show least resistance towards
concentrated nitric and sulphuric acids. Apart from being used in
mechanical related applications, ABS also possesses electrical
properties that are reasonably constant over a broad range of
frequencies. ABS at high temperatures could be flammable, and are
damaged when exposed to sunlight. Table 4.3 indicates the various
properties of ABS.
The light weight of ABS and its ability to get it injection moulded
makes it extremely handy in fabricating products such as kitchen
utensils, bathroom accessories and fittings, automobile components,
electronic and electrical assembly enclosures, domestic appliances,
toy manufacturing, and even in fabricating musical instruments [22].
Fig. 2.2: Molecular Structure of acrylonitrile-butadiene-styrene (ABS)
2.2.3 Nylon (Polyamides)
Nylon is one of various man-made materials for plastics. Plastic
materials are organic solids that are either synthetic / semi-synthetic
with a wide range of properties that can be moulded into a desired
30
shape and size. Plastics are either natural or man-made. Chemically,
plastics are composed of chain-like molecules of high molecular
weight, called polymers, which usually have been built up from
simpler chemicals called monomers. Various monomers and an
amalgamation of different monomers are used to produce a different
combination of polymers. Many different kinds of polymers are
available to us today. For example: PVC, PP, PEEK, PAs, epoxy, etc.
Undoubtedly, polymers have made a great impact on our
society. They are rapidly reinstating many metals for making objects.
Chemical Structure of Polymers
Fundamentally, a polymer is made up of many repeating
molecular units formed by sequential toting up of many monomer
molecules to each other. Many monomer molecules of A, say one
thousand to one million, link themselves mutually to form a new
gigantic polymeric molecule.
An example of this type of polymer is the polyethylene, as shown
in the ensuing literature. In this case the monomer is ethylene.
Countless numbers of these molecules connect themselves together
into a long chain polymeric molecule by breaking the double covalent
bond and creating new single covalent bonds between the monomers.
31
This reaction is promoted by heat, pressure, and a chemical catalyst.
Another type of polymer uses two different monomer molecules,
A and B, to produce a polymer of alternating A and B. Again, this
polymer may have one thousand to one million units linked together.
An example of this type of polymer is nylon.
In the above reaction, HCl is lost and a covalent bond is formed
in its place. The new substance is given the trade name nylon, but
chemically it is known as a polyamide because of the type of linkage
which occurs at regular intervals.
32
Nylon is prepared by mixing hexamethylenediamine, water,
sodium hydroxide, adipoyl chloride and cyclohexane [23].
P.A.Mahanwar and Suryakanti Bose [24] have discussed about
the usage of N6 / PAs in various fields ranging from automobiles to
consumer applications. They report that PAs possess excellent
mechanical properties and at the same time have certain limitations
such as low heat deflection temperature, high water absorption and
dimension instability which makes them less usable in applications
related to structural components. They also report that by adding
inorganic fillers such as aluminatrihydrate, montmorrilonite, clays,
talc, mica, silica, flyash, kaolin, etc., one can make them usable in
many areas such as automobile, electrical, electronic, packaging,
textiles, etc.
Several different types of nylons are produced and marketed
worldwide, of these, nylon 6 (Polyamide 6), nylon 6/6 nylon 6/10,
nylon 6-6/6, nylon 6/12, nylon 11 and nylon 12 are the most
common ones [25]. Nylon6 and nylon 6/6 are the ones that are being
used in most of the applications as discussed in the previous
literatures. The typical molecular structure of polyamide series is as
shown in fig. 2.3.
The numbers 6, 6/6, 11, 12, etc., refers to the number of
methyl units (-CH2-) occurring on each side of the nitrogen atoms
33
(amide groups). It is understood that these methyl units have a strong
influence on the properties of various PAs in use. For instance, it is
noted that nylon 6/12 has a lower melting point, lower strength, lower
hardness, lower modulus, lower thermal distortion than nylon 6/6.
But, one relationship which does not conform is price, i.e., nylon 6/12
is more costly than nylon 6/6. The property which gives nylon 6/12
its efficacy is moisture absorption, which is just about half of that of
nylon 6/6 [25].
Similarly, N6 has some advantages over N6,6. To begin with, the
synthesis of N6 is cheaper than N6,6 and secondly, N6 has a better
attraction towards acid dyes than N6,6 [26]. The properties that make
N6 superior over other types are,
It‟s resistance to a good number of organic acids like ethers,
benzene, chloroform, esters, acetone, etc.
Shows least resistance to cresol, string mineral acids and
phenol.
Excellent resistance towards alkalies.
Resistance to inorganic acids.
Very good physical properties
Very good heat resistance
Excellent chemical resistance
Excellent wear resistance
Moderate to high price
Fair to easy processing
34
Applications of Nylon6
Electrical connectors, gear, slide, cams and bearings, cable ties
and film packaging, fluid reservoirs, fishing line, brush bristles,
automotive oil pans, fabric, carpeting, sportswear, sports and
recreational equipments.
35
Fig. 2.3: Typical Molecular Structure of Polyamide Series. [27]
36
2.2.4 Wollastonite / Calcium Meta Silicate
Wollastonite, an industrial mineral named after the mineralogist
Wollaston W H. It is formed when high pressure and temperature acts
an limestone in the presence of silica bearing fluids. In general
wollastonite is white in colour and consists of manganese, iron and
magnesium in small traces. The composition of wollastonite has been
high lightened in the Table 2.2. [28].
Wollastonite is also known by the names Calcium silicate /
Calcium meta silicate and is commonly used as a functional filler in
thermoplastics, particularly in polyamides and polypropylene because
of its availability in grades with very fine particle size and high aspect
ratio, containing acicular or needle-like particles [29]. Wollastonite
(CaSiO3) is an industrial mineral with calcium, silicon, and oxygen
being its main constituents. Theoretically, CaSiO3 is made up of
48.3% of calcium oxide (CaO) and 51.7% silicon dioxide (SiO2). The
specific gravity of commercial grade wollastonite is 2.90 and the
typical chemical compositions of wollastonite grades for plastics are
listed in Table 2.2.
The wollastonite mineral has many advantages and due to
which it is being used in many areas. One of the main advantages of
CaSiO3 is its low moisture absorption properties that make it possible
for being used in applications, mainly industrial, as fillers in paints,
ceramics, wear resistant materials, etc. Other characteristics of
CaSiO3 include: high brightness, low volatile content, good thermal
37
stability, low thermal expansion coefficient, high dielectric strength,
and low loss on ignition [16, 30].
Table 2.3, based on references [31, 32] and suppliers‟ literature,
summarizes the physical and chemical properties of wollastonite.
Table 2.2: Typical chemical analyses of wollastonite filler grades. [16]
Composition wt%a) wt%b) wt%c)
CaO 47 44 47
SiO2 49.5 50 50
MgO 0.2 1.5 0.3
Al2O3 0.6 1.8 0.3
Fe2O3 0.43 0.3 1
TiO2 Traces Not reported 0.05
MnO 0.29 <0.1 0.1
Na2O 0.02 0.2 Not reported
K2O 0.11 Not reported 0.1
a) Wolkem India Ltd (KEMOLIT® grade).
b) R.T. Vanderbilt Co. Inc. (VANSIL grade).
c) NYCO Minerals Inc. (NYAD® grade).
38
Table 2.3: Typical properties of wollastonite [16]
Property
Colour White
Crystal System Triclinic
Specific gravity 2.8-2.9
Coefficient of thermal expansion 6.5 x 10-6
Specific Heat (J/Kg k) 1003
Melting point (0C) 1540
Transition temperature, 0C (to pseudo wollastonite) 1200
Hardness (Mohs) 4.5-5
Refractive index 1.63-1.67
pH (10 wt% slurry) 9.0-11
Loss on ignition, % (950 0C) 2.5
Dielectric constant (104 Hz) 6
In engineering, plastics (thermoplastics and thermoset) with the
inclusion of wollastonite, results in an overall development of the
mechanical properties, that is, enhanced flexural modulus, enhanced
tensile strength, enhanced heat deflection temperature (HDT) and
enhanced dimensional stability. It is considered that the needle-like or
acicular shape of the wollastonite is the key contributing factor for the
enhanced mechanical properties. More generally, wollastonite has
been used as a financially viable alternative / supplement to glass
fibres, either chopped or milled. Table 2.4 summarizes the economic
benefits of using wollastonite as functional filler. The surface
39
chemistry of wollastonite provides a particular affinity for surface
treatments such as silanes, titanates, polymeric esters and other
additives. The benefits of such treatments include lower compound
viscosity for improved melt flow, easier dispersion for better property
development and improved physical properties [16].
According to Robinson et al. [33], functional fillers such as mica
and talc, having similar Young‟s moduli, but platy shape of these
materials can offer enhanced overall stiffness in different directions
due to increased dissipation of stress across a wider flat plane. Impact
strength may also be improved by the use of platy minerals. However,
with proper selection of median diameter and control of top size of the
wollastonite particles, the notched impact strength performance in
certain resins can be improved over the unfilled polymer and over
other fillers.
With a thorough literature survey, and knowing the benefits of
incorporating the wollastonite, it is decided to utilize CaSiO3 as the
reinforcing filler in the present research. The following are the
improvements expected:
Table 2.4: Economic benefits of using wollastonite as functional filler. [16]
Enhanced:
Stiffness and
flexural strength
Impact
resistance
Dimensional
stability
Wear / sliding
resistance
Surface
appearance
Melting
Temperature
Electrical
properties
Features Advantages Benefits
Low cost reinforcing filler Less expensive than Better economics for
40
milled or chopped
glass fibres
filled plastics while
giving strength improvements
Binary reinforcement
synergy
Cost effective
combination with treated calcined clays
Better balance of
stiffness, impact, and electrical
properties in
engineering resins
Moderate specific gravity Ease of volume substitution
Smoother reformulation
transition and
improved cost
Lower cost than
polymers
Can replace more
costly resins
Maintain or improve
desired properties
over unfilled systems
High melting temperature
Contributes to high processing
temperatures
Allows improvement of melt flow rates
and yield by
assisting lower melt temperature
polymers to be
processed at higher temperatures
Less costly than certain
chemical additives
For impact
modification can
replace certain chemical modifiers
Better impact at
lower cost
2.2.5 Water Absorption Studies on Polyamide Composites
As it is already known that, PAs/nylons are high recital
engineering thermoplastics, that are semi-crystalline in nature, with
striking physical and mechanical properties that offers a broad range
of significant end-use presentations in numerous industrial
applications [34]. In general, all PAs are hygroscopic [35] i.e., they
absorb water from both air and liquids. Hygroscopic property of
polymers becomes an essential factor while selecting materials,
designing parts, mechanical performance and optimization. It is
estimated that the equilibrium moisture content of N6 is around 2.5
41
wt% at 23°C, 50% RH and 9 wt% at 23°C, 100% relative humidity (RH)
[36].
Generally, it is noted that the moisture content in nylon is a
prime factor affecting polymerization, compounding and moulding. It
is seen affecting the mechanical, dimensional, and surface appearance
when used in end-user products [37]. Dimensional changes of 0.7%
can result in nylon parts from the "as-moulded" state to equilibrium at
50% RH environments. This change occurs in approximately 150 days
for a 0.060 inch (1.5 mm) thick part [25]. The absorbed water in the
layers of polymer works as a plasticizer. Since Nylon being
semicrystalline material has both crystalline and amorphous phases.
The moisture actually penetrates into the amorphous region, although
small, water molecules take up space and displace the nylon
molecules. This results in the nylon molecular matrix swelling [25].
The water molecules tries to establish hydrogen-bonds with the amide
group and enhance the molecular chain mobility [37], which in turn
affects material properties such as modulus, yield stress, toughness,
etc. [34]. The water absorption by plastics also results in alterations of
dimensions and mass due to the stress and swelling that causes
serious damage to the whole structure. Being dimensionally stable is
very important for components used with narrow tolerances and
intricate shapes [38]. The moisture absorption by polyamide is a
totally reversible physical reaction. The moisture in the PAs can be
driven off by drying them in ovens. The absorption rate varies with
type of nylon as well as depends on temperature and RH.
42
The problem of water absorption in plastics relates itself to a
number of problems, both in the use and processing of
thermoplastics. There are many ways by which water absorption in
plastics can be reduced. One of the methods is to reinforce them with
fillers. There are varieties of fillers available and some of them have
been listed in the Table 2.5. The absorption effects can also be
reduced by storing and drying them in a suitable atmosphere [38].
Knowing the concept of absorption, one should also be able to
differentiate between absorption, adsorption and hygroscopicity; as
these terms are often confusing and needs to be understood
thoroughly. Most researchers use the terms absorption and
adsorption interchangeably. Nevertheless, it should be noted that the
adsorption refers to the absorption by the material, both by gases /
vapors and liquids and absorption refers to the absorption of liquids,
whereas hydroscopicity is the propensity of a material to absorb water
[39, 40] and on the other hand, the material moisture content refers
to the percentage of water contained in it [38].
J Sukumaran et al. [41] have discussed issues related to
tribological properties of polymers subjected to water lubrication in
their review article on, “A Review on Water Lubrication of Polymers”.
Their observations say that water can act as a good lubricant for the
polymers, subjected to wear studies and can provide an economical
solution for lubrication related issues. They further report that,
polymers subjected to water lubrication have shown positive frictional
character in comparison with polymers subjected to oil and air
43
lubrication, wherein wear behaviour was uncertain. Therefore,
understanding the Tribo-mechanical characteristic in wet state abets
to select suitable material for water lubricated applications. Some of
the regularly used engineering plastics such as Polyamide 6 (PA6),
Polyoxymethylene (POM) and Polytetrafluoroethylene (PTFE) have been
tested for their tribological performance under water lubrication. The
outcome of the tests indicates that the PTFE has better frictional traits
on comparing with POM and Ultra High Molecular Polyethylene (UHM-
PE) [42].
Water absorption can be a major problem and a key parameter
for polymeric materials like coatings and electrical insulations, which
are immersed in water for shorter or longer durations at some stage in
the application. Water absorption by polymeric materials actually
increases dielectric constant and on the other hand decreases volume
resistivity [43].
Pawlikowski and Gregory [44], in their work on “Effects of
Polymer Material Variations on High Frequency Dielectric Properties”,
have discussed issues related to PAs and say that Water can set
plasticization in the PAs, and due to which PAs exhibit variations in
the dielectric properties.
44
2.2.6 Mechanical, Electrical and Tribological Properties of
Polyamide-6 Composite Materials
PAs are one of the most frequently used polymers used in many
engineering applications, but inherently PAs have certain
disadvantages like high water absorption, low impact strength, certain
fragility, etc. and therefore additional functional materials are added
to PAs to improve the mechanical properties in a cost effective manner
such that they can be suited for end-use applications. But, on the
other hand, PAs are characterized by their possessing of high tensile
strength, elasticity, tenacity and resistance to abrasion. These
properties of PAs make it usable for short term applications [45-47].
Over the years, several types of fillers have been in use with
PA‟s. Biobased fibres such as coconut fibre, wood flour, sisal, wheat
straw, kenaf, jute and minerals such as carbon fibres, mica, talc,
calcium carbonate, glass fibres and wollastonite have been favourably
used with PA either as fillers or as reinforcements to provide better
strength and modulus to the composite, and sometimes PAs are also
mixed with other polymers to bring in changes with respect to the
mechanical, electrical and thermal properties [46, 47]. Compounding
or blending of polymers with fillers is fairly an easy way to create new
polymeric materials with preferred properties. These functional
materials are classified into different groups, namely: modifiers,
property extenders and processing aids. Antioxidants, light stabilizers,
UV absorbers, peroxide decomposers, nucleating agents, flame
retardants, colourants, lubricants, and blowing agents come under a
45
class of additives which are dispersed in the PA resin during several
stages of processing to provide resistance and also to stabilize the
base material. They also contribute by improving the mechanical,
thermal and electrical properties to a certain extent that simple
polymers do not [32, 48-51].
Most of the fillers are polar in nature, so there arises a problem
of reduced adhesion between the non polar matrix and the polar filler
which can affect the final properties of the composite. As a solution,
several types of coupling agents or compatibilizers were developed
with the aim of improving the bonding. Literature studies have shown
that wollastonite reinforced polymer composites have been used in
most of the end user applications [52-54].
N6 and High Density Polyethylene (HDPE) materials have been
used in a large number of industrial applications and many a times,
they have also been used in combination for specific applications [55,
56].
A large variety of filled PA composites are found in the vehicle‟s
interior, exterior and under-the-bonnet applications. Long glass fibre
reinforced PA have found its use as bumper beams. Talc filled impact
copolymer composites are used in door and instrument panels [57].
Low cost, ease of processing and recyclability of PP composites are
paving way to the rapid market growth in the automotive sector.
Huseyin Unal et al. [58], have made a study on the mechanical,
tribological and electrical properties of N6 composites by using
graphite as filler. According to them, composites by IM method
46
showed enhanced properties when the percentage of graphite varied
from 5 to 15 wt%. They have identified that the young‟s modulus,
dielectric losses, dielectric permittivity increases when the graphite
content increases.
Ozcan Koysuren et al. [59] have discussed about Nylon
6/carbon black conductive composites. They have made an in-depth
study on the distribution of agglomerates of carbon black in the
matrix of N6 and in turn, its effect on the electrical and mechanical
properties. The electrical resistivity, impact strength, and elongation
at fracture of the prepared composites decreases and on the contrary
tensile modulus increases with the increase in carbon black content
in the composites.
Abdul Razak Rahmat et al. [27] and Akay et al. [60] have
reported that fibre reinforced thermoplastic compounds, produced by
IM method, offered an improvement in mechanical properties over
unreinforced ones. The composites so prepared have the ability to be
used in a variety of engineering applications because of their ease in
fabrication of materials. According to them, thermoplastic composites
containing short fibres have been used in many applications, due to
their ability to reduce the tribulations relating to defects like cracks
and voids in the materials that may be formed in the matrix or in the
fibre or at the matrix / fibre interface.
47
Amintowlieh [61] has discussed the use of wheat straw as a
reinforcing fibre for N6. He reports that wheat straw reinforcement
increased modulus by 26.9 % and decreased the strength by 9.9 %.
Patricia Alvarez de Arcaya et al. [62] have discussed the use of
high performance thermoplastic matrices such as PAs instead of the
commonly used polyolefins to develop natural fibre composites for
substituting glassfibres without renouncing to their mechanical
properties. For this purpose, they have used different natural fibres
such as flax, jute, pure cellulose, and wood pulps with PAs to analyse
the effect of fibre content on mechanical properties.
P.A.Mahanwar and Suryasarathi Bose [24] have discussed the
effects of fillers on the Properties of N6. They report that composites
showed improved mechanical, thermal, as well electrical properties on
addition of fillers. According to them, the size of filler has a greater
influence on the electrical properties of the composites. They also
stated that the dielectric strength increases with the increase in filler
concentration.
Junlong and Huo [63] have used Kevlar fibre as a reinforcement
material, embedded into the matrix of PA6. The resulting product was
a composite of PA6/kevlar. The composites, so produced, were
subjected to study the effects of Kevlar fibre content and wear
condition on the tribological properties of the composites. The worn
surface was investigated under a scanning electronic microscope. It
was found that Kevlar fibre had improved the wear resistance of PA6.
48
The observed values indicated that wear of PA6/Kevlar fibre composite
was lower than that of the PA6 and increased slowly, while its friction
coefficient decreased gradually with load.
Iztok S et al. [48] have discussed the effects of incorporating
wollastonite into the matrix of Isotactic polypropylene (iPP), and
according to them, wollastonite improves some of the mechanical
properties such as stiffness, hardness and strength of the ipp, but at
the same time reduces toughness.
Jin Tong et al. [64] have discussed the effects of the wollastonite
incorporation on the sliding wear behaviour of the Ultra-high
molecular weight polyethylene (UHMWPE) composites. They report
that with the incorporation of wollastonite, improved wear resistance,
tensile strength and the impact resistance of the UHMWPE composites
were observed.
49
Gla
ss F
ibre
Asbesto
s
Wollasto
nit
e
Carb
on
fib
re
Wh
iskers
Syn
theti
c f
ibre
s
Cellulo
se
Mic
a
Talc
Gra
ph
ite
San
d/quart
z
pow
der
Sil
ica
Kaoli
n
Gla
ss s
ph
ere
s
Calc
ium
carb
on
ate
Meta
llic
oxid
es
Carb
on
bla
ck
Tensile strength
×
Compressive
strength
Modulus of
elasticity
Impact strength
- - - -
- - - -
-
Reduced
shrinkage
Better thermal
conductivity
Electrical
conductivity
Electrical
resistance
Chemical
resistance
×
Better abrasion
behaviour
Large Influence, influence, × no influence, - negative influence
Table 2.5: Effect of filler type on properties [6, 65]
50
2.3 METALLIZATION OF PLASTICS
2.3.1 Overview
Metallization of polymer substrates is an important practice in the
electronics and automotive industries. It is used for various purposes,
such as EMI / RFI applications, reduction in weight, better impact
strength, and corrosion resistance. The techniques available for the
metallization of polymer substrates include chemical [66] and physical
vapor deposition [67], metal-powder coating [68] and electroless plating.
Metallization of plastics has gained much importance these days
due to its increasing use in EMI shielding applications [69]. Also,
metalized plastic parts can be used for decorative or functional purposes
[70]. Previously, POP / metallization of plastics were done to provide
decorative and aesthetic views to the plastics. In the early days of the
1960s, plating began on a commercial level and this was possible only
with the improvement in chemical processing techniques. Industries that
exploited this technique of plating apart from automotive and electronics
were plumbing and appliance related industries. The reasons of this
exploitation were mainly due to the reduced weight, lower cost, design
freedom, and no secondary operations with plated parts. Reduction in
weight of the parts came in handy when the gasoline crisis occurred in
the early 1970s. Polypropylene was one of the earliest plastics to be
plated and today there are many plastics that can be plated, viz, ABS,
Polysulfone, Polyethersulfone, Polyetherimide, Teflon, Polyarylether,
51
Polycarbonate, Polyphenylene oxide (modified), Polyacetal, Urea
formaldehyde, Diallyl phthalate, Mineral-reinforced nylon (MRN) and
Phenolic [71].
Plastics, being nonconductors, were difficult to plate materials and
thus various methods of metalizing plastics were explored like vacuum
metallization, plating, flame or arc spraying and painting, apart from the
ones discussed above. But, of all the processes, plating was the most
economical methods discovered till date. Plating of plastics includes
electroless and electroplating procedures, that gives the plastic parts a
glossy appearance, enhanced reflectivity, improved abrasion resistance,
high electrical conductivity, and / or to provide electromagnetic shielding
[70]. More about the plating has been discussed in the ensuing
paragraphs.
One important observation that is made during this literature
survey was that, the majority of the work on plating was done
considering the aesthetic and decorative applications only. Very few
literatures address the application of plating from the strength
enhancement point of view. Therefore the present work will be to discuss
the application of plating procedure from strength enhancement point of
view.
52
2.3.2 Plating of Plastics
2.3.2.1. Electroless Plating
Electroless (without the use of electricity) plating refers to the
deposition of metal layers over a suitably treated / activated surface in a
controlled manner so as to prepare the material / substrate for further
electroplating process [72].
Plating using electroless technique is more frequently used in POPs
due to its advantages like giving an unbroken uniform coating on
materials with greater complexity, its easiness in transforming
nonconductors into conductors, its inexpensiveness, etc., makes it an
affordable and reliable technique for POPs [73-74].
The reducing agent actually converts the metal ions (Mn+) to the
metal (M) which gets plated over a catalytic surface as shown in Eqn. 2.1.
𝑀𝑛+ + 𝑅𝑒𝑑𝑢𝑐𝑖𝑛𝑔 𝑎𝑔𝑒𝑛𝑡𝑐𝑎𝑡𝑎𝑙𝑦𝑡𝑖𝑐
𝑠𝑢𝑟𝑓𝑎𝑐𝑒→ 𝑀 + 𝑂𝑥𝑖𝑑𝑖𝑧𝑒𝑑 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 …2.1
The driving force in electroless plating is an autocatalytic redox
reaction on a pretreated active surface. An important aspect of
electroless plating is the preparation of the surface of the object such
that an active surface is obtained [72] and for that, the electroless plating
process involves degreasing, etching, surface seeding with a catalyst,
electroless plating and electroplating. [75]
(a) The degreasing process is usually a cleaning process that uses
alkaline or acid solutions containing surfactants to remove oils and
53
other organic chemicals and make the surface of the polymer dirt
free.
(b) Chromic acid and/or sulfuric acid are some of the strong oxidative
acids used for chemical etching [76, 77] or hydrogen peroxide, to
roughen the sample surface for adhesion enhancement [78, 79].
The roughening actually creates a bonding site for further plating
of samples.
(c) Surface seeding with a catalyst involves treating the surface of
polymers with stannous chloride and a palladium chloride solution
alternatively [72].
(d) In the electroless plating, the catalyst imbued polymer substrate is
dipped in a solution containing a metal salt, a reducer, a stabilizer
and a buffer system.
(e) Electroplating involves the deposition of a metal over the
electroless plated substrate, by electrolysis. This process actually
produces a dense, uniform and adherent coating, which may later
be used for decorative and/or protective purposes or for enhancing
the specific properties of the surface.
2.3.2.1.1 Composition of Electroless Plating Bath
A typical electroless plating bath consists of:
(a) A soluble electroactive metal salt (chloride or sulphate).
54
(b) Reducing agents like hypophosphites, formaldehyde, sodium
borohydride, hydrazine are added to improve the overall plating
process.
(c) To improve the plating quality, complexing agents like tartrate and
citrate are added into the plating bath.
(d) Exaltants like succinate / fluoride are added to the plating bath to
boost the plating rate.
(e) Stabilizers (prevents the instability of the bath) e.g. lead, calcium,
thallium ions, thiourea.
(f) Buffer is added to control the pH.
2.3.2.1.2 Advantages of Electroless Plating
Electroless plating has the following advantages over electroplating:
1. No usage of electrical power
2. Can be used for POPs and semiconductors
3. Due to the better throwing power of electroless baths, complex
shaped articles can be plated with ease.
4. Hydrogen gas liberated is not trapped in blind holes.
5. Coatings that are produced by electroless plating are harder.
6. Levelers are not required in electroless plating.
7. Deposits produced in electroless plating have exclusive
properties (magnetic, electrical, and mechanical).
55
2.3.2.1.3 Applications of Electroless Plating
Electroless baths have better throwing power. Therefore,
electroless plating is practiced on objects which have complex shapes.
The deposits are pore-free and have better corrosion resistance than
electroplated ones. They give rise to harder surfaces and as a result the
wear resistance also increases. Electroless Ni-P coatings are used in
electronic applications. Polymers such as ABS plastics, coated with
electroless Ni have decorative and functional applications. Thus, knobs
on hi-fi equipments, tops on perfume bottles, costume jewellery and car
trim make use of electroless plating. Plastic cabinets coated with
electroless Cu and Ni are used in digital and electronic instruments for
EMI shielding. The electroless technique is used where plating-through-
holes is required.
2.3.2.1.4 Electroless Plating of Copper
In case of metals, copper from its solution gets attracted towards
metals like gold, silver, platinum, palladium, rhodium, iron, cobalt and
nickel, and the deposition starts impulsively when it comes across these
reducing agents as discussed above, but in insulators like glass and
plastics, the substrates need to be activated before they are subjected to
electroless treatment (plating). A best example for electroless plating is in
case of PCB‟s, in PCB‟s the base object is a plastic material and upon
which a layer of Cu is deposited. The reactions occurring in the
electroless plating is as shown in the Eqns. 2.2-2.4
56
Pretreatment and Activation of the surface
The plastic substrate is first degreased, and then subjected to
etching treatment. The etched samples needs to be activated and this is
done by dipping the etched substrate in a solution made of stannous
chloride and HCl, the solution bath is maintained at room temperature,
say 250C followed by palladium chloride treatment.
Plating Bath Solution
Solution of copper sulphate (12 g/L), reducing agent –
formaldehyde (8 g/L) buffer, EDTA complexing agent and exaltant (20
g/L), sodium hydroxide (15 g/L) and Rochelle salt (14 g/L), pH – 11
Reactions
Cathode 𝐶𝑢2+ + 2𝑒 → 𝐶𝑢 …2.2
Anode 2𝐻𝐶𝐻𝑂 + 4𝑂𝐻− → 2𝐻𝐶𝑂𝑂− + 2𝐻2𝑂 + 𝐻2 + 2𝑒 …2.3
Overall reaction 𝐶𝑢2+ + 2𝐻𝐶𝐻𝑂 + 4𝑂𝐻− 𝐶𝑎𝑡𝑎𝑙𝑦𝑡𝑖𝑐
𝑆𝑢𝑟𝑓𝑎𝑐𝑒→ 𝐶𝑢 + 2𝐻𝐶𝑂𝑂− + 2𝐻2𝑂 +
𝐻2 …2.4
Plating Process
The plating is carried out at room temperature by dipping the
activated plastic surface in the bath solution. The temperature is
maintained at about 25 oC. The process is continued till a layer of 5 to
100 µm thickness is obtained. The rate of plating is maintained at 1 to 5
µm h-1.
57
2.3.2.1.5 Electroless Nickel Plating
Electroless plating of Ni on aluminium, iron, copper and brass can
be carried out directly in the presence of reducing agents without
activation. The stainless steel surface has to be activated by dipping in a
hot solution of 1:1 sulphuric acid. Magnesium alloys are given a thin
layer of zinc and copper. Insulators such as plastics are subjected to
electroless Ni plating by dipping them in solutions as depicted in Table
2.6. The chemical reactions in electroless Ni process are as shown in
Eqns. 2.5 - 2.7.
Reactions
Cathode 𝑁𝑖2+ + 2𝑒 → 𝑁𝑖 …2.5
Anode 𝐻2𝑃𝑂2− + 𝐻2𝑂 → 𝐻2𝑃𝑂3
− + 2𝐻+ + 2𝑒 …2.6
Overall reaction 𝑁𝑖2+ + 𝐻2𝑃𝑂2− + 𝐻2𝑂
𝐶𝑎𝑡𝑎𝑙𝑦𝑡𝑖𝑐
𝑆𝑢𝑟𝑓𝑎𝑐𝑒→ 𝑁𝑖 + 𝐻2𝑃𝑂3
− + 2𝐻+ …2.7
58
Table 2.6: Electroless plating (Nickel) procedure. [75]
Sl No. Process Chemicals Concentration
1
Cleaning /
degreasing
Sodium carbonate (Na2CO3) 50 g/L
Sodium dodecylbenzene
sulfonate (C18H29NaO3S)
35 g/L
Disodium metasilicate
(Na2SiO3)
3 g/L
2 Etching
Chromic acid (CrO3) 400 g/L
Sodium bisulfite (NaHSO3) ---
Sulfuric acid (H2SO4) 400 g/L
3
Surface
activation and
reduction.
Nickel(II) acetate
((CH3CO2)2Ni.4H2O) ---
Cobalt chloride (CoCl2) ---
Sodium borohydride (NaBH4) ---
Stannous chloride (SnCl2) ---
4
Electroless
Nickel plating
bath
Nickel sulphate (NiSO4.7H2O) 26 g/L
sodium acetate (CH3CO2Na) 16 g/L
sodium hypophosphite
(H2PO2Na)
38 g/L
sodium citrate (C6H5O7Na3) 46 g/L
ammonium hydroxide
(NH4OH) To adjust pH = 9
59
Plating process
The plastic substrate is first cleaned with solvents as indicated in
Table 2.6. The cleaned surfaces are given acid treatment, followed by
dipping of acid treated substrates in the electroless Ni plating bath. The
pH is maintained at around 9 and the plating is carried at 930C at a rate
of 20 µm per hour. During the reaction, H+ ions are released and this
reaction changes the value of the pH of the solution. This mostly affects
the quality and the rate of plating. The buffer controls the pH during the
plating. The Plating will start only when the temperature of the bath
reaches 700C. It is also observed that at 930C maximum rate of
deposition is achieved, say about; 20 µm h-1. A further rise in
temperature leads to the decomposition of the electroplating bath
solution.
2.3.2.2 Electroplating
Electroplating or electrodeposition is the process of deposition of a
layer of metal, over the surface of a substrate by electrolysis. The
substrate may be a conductor like metals, or nonconductors like
polymers, and composites [72]. Electrodeposition can be made on:
(i) Single metals like tin, copper, nickel, chromium, zinc,
cadmium, lead, silver, gold and platinum.
(ii) Alloys like Copper-Zinc, Copper-Tin, Lead-Tin, etc.
(iii) Composites
60
2.3.2.2.1 Principles of Electrodeposition
The process of electrodeposition is initiated by passing the current
through the suitable electrolyte placed in the tank. The tank composes of
cathode and anode. The cathode is the substrate and the anode is the
metal to be deposited on the cathode. With the passage of electricity, the
metal ions due to the anode in the bath get transferred onto the
substrate. The reactions in the plating bath of the electrolyte „MA‟ is
given by:
The ionization of electrolyte in aqueous solution
𝑀𝐴(𝑎𝑞) ↔ 𝑀(𝑎𝑞 )+ + 𝐴(𝑎𝑞 )
− …2.8
Anode dissolves in the electrolyte to give metal ions
𝑀 → 𝑀(𝑎𝑞 )+ + 𝑒 …2.9
Cathode takes the reduced metal ions from the electrolyte
𝑀+ + 𝑒 → 𝑀 …2.10
To maintain the concentration of M+ in the electrolyte of the tank, the
current efficiencies of reactions (2.9) and (2.10) are maintained equal. If
it is not maintained, then there are chances of oxygen evolution from the
anode as seen from the reaction 2.11.
𝐻2𝑂 →1
2𝑂2 ↑ +2𝐻+ + 2𝑒 …2.11
61
Under acid conditions a side reaction involving release of H2 may take
place as seen from reaction 2.12
𝐻+ + 𝑒 →1
2𝐻2 …2.12
Also, it becomes necessary to have a solution with high
conductivity and optimum current density to get a good deposit.
2.3.2.2.2 Mechanism of Electrodeposition
Deposition of metal and the crystal growth are fairly similar. The
deposition of metal on the substrate, usually happens in two stages:
(i) Nuclei formation takes place on the substrate. This growth of
nuclei is for a few atomic layers.
(ii) There will be further growth of these atomic layers, which may
grow up to 60 µm thick.
It is interesting to note that, for the nuclei formation it is always
necessary to have a higher overvoltage initially. As the formation begins,
they grow more swiftly at a lower overvoltage. The atoms so formed will
promptly occupy a place that helps in further development of atoms into
layers. These places / sites which the atoms occupy are called as kink
sites and here the atoms network themselves with the three
neighbouring atoms as shown in the fig. 2.4. There are also some places
like edge sites where the atoms reside, here it is believed that atoms
interact with two neighbouring atoms or sometimes even remain as
62
single atoms called adatoms which interact with only one neighbouring
atom. It has been identified by researchers that the phase growth
happens in successive stages, viz.
(1) Atoms start to deposit on the surface of the electrode due to the
mass transport of ions in the solution.
(2) Adatom formation.
(3) Movement of adatoms into the kink sites.
It is to be noted that the steps (2) and (3) play an important role in
the identification of plating deposits. High quality plating is always
exemplified by certain identification and remarkable qualities like (a)
adhesion (b) fine and equally grained structure (c) uniform thickness (d)
throwing power (e) covering power (f) brightness.
Fig. 2.4: Kink sites, edge sites and adatoms on electrodeposited
surface.
63
2.3.2.3 Effects of Plating Variables
The nature of electrodeposit is affected by several factors. The
important ones are discussed below.
(a) Current Density
The current density (c.d.) is the current per unit area normally
expressed in (mA / cm2) of the electrode surface. At low current densities,
diffusion of ions on the surface is fast compared to the electron transfer
and the adatoms move towards Kink sites that later result in excellent
deposition. Further, with the increase in the current density, the
diffusion of ions slows down and the adatoms may not be able to reach
the most favourable positions. But there will be an increase in nuclei
formation that causes disordered plating, also when the current densities
are higher; there are chances of hydrogen evolution along with the loose
bonding of particles / atoms to the surface of the material to be
electroplated. The hydrogen evolution results in formation of oxides and
hydroxides. Therefore, it becomes important to identify and use the best
possible current density for electroplating purpose.
(b) Plating Bath
The plating bath contains a solution of the metal salt and also in-
order to improve the plating process, the plating bath is added with
additives (organic), buffers, electrolytes, and complexing agents.
64
(i) Metal-ions: the optimum metal ion (either a simple ion or a
complex ion) concentration is normally 1-3 mol dm-3. Higher
concentrations increase the mass transfer leading to poor
deposit.
(ii) Other electrolytes: Other electrolytes are added to increase the
conductivity of the electroplating bath. These do not help the
plating bath in any having any reactions. These only help in
increasing the conductivity of the plating bath and also to
control the change in pH.
(iii) Complexing agents: These are added to enhance the adhesion
between the electroplated layers and the surface. These also
help in forming fine-grained layers. More often in electroplating
the Complex ions are used:
When the plating ion is known to react with the cathode
metal.
When plating is required at a lower potential.
To improve current efficiency.
To improve the throwing power of the electroplating bath.
Examples of complexing agents are cyanides, hydroxides and
sulphamates. Copper, silver, gold, zinc and cadmium are plated as
smooth deposits in the presence of cyanide ions.
65
(iv) Organic additives: Organic additives like levellers and
brighteners, wetting agents and structure modifiers are used to
alter the properties of electroplating tank and hence the plating
on surfaces.
Brighteners: These are basically used to give a lustrous
finish for the plated surfaces. These when added into the
tank gives fine deposits. Aromatic sulphones and molecules
containing 𝐶 ≡ 𝑁,𝑁 = 𝐶 = 𝑆, 𝑜𝑟 𝐶 = 𝑂 groups (e.g. thiourea and
coumarin) are used as brighteners.
Levellers: Plating happens swiftly in the regions of cracks or
dislocations. These are the areas that attract the ions and
results in producing non-uniform coating thickness on the
surface. Therefore, in-order to tackle with such issues,
levellers are added into the plating bath. Levellers get into
the regions that are prone to faster deposition, thereby,
reduce the electron transfer rate.
Structure modifiers: Commonly known as stress relievers
and have the capacity to change the properties of
electrodeposits. Saccharin is the most commonly used stress
reliever. Many a times electrodeposits may result in the
formation of residual stresses and due to which may result
in microcracking of the deposits
66
Wetting agents: During electroplating hydrogen is evolved
at the cathode and in-order to liberate hydrogen from getting
entrapped in the surface of the cathode, wetting agents are
added. These agents help the plating baths to acquire
properties such as reduced brittleness of the deposits, for
providing uniform levelling and homogenous plating,
Sodium lauryl sulphate are used as a wetting agent.
(v) pH: From the previous paragraph, it is understood that
hydrogen gas is released at the cathode and the reason for this
is due to the low pH that results in burnt deposits. If the pH of
the plating bath increases, then the possibilities of electrode
getting coated with insoluble hydroxides is more. For that
reason pH in the range of 4 – 8 is desirable. Buffers are added
during the plating process to control the pH of the plating
solution. Buffers like borate and citrate are used in nickel and
gold plating respectively.
(vi) Temperature: One of the other reasons for hydrogen liberation
at the cathode is the high temperature. The temperature rise of
the bath can lead to the corrosion of the plating tank. It has
also been observed that due to the high temperature the organic
additives have sometimes undergone decomposition. For good
deposits, it becomes important to have reasonable temperature
that enhances surface diffusion and reduce hydrogen evolution.
67
The optimum temperature for good deposits can be in the range
of 35 and 60 oC.
(vii) Throwing Power: Knowing the throwing power of the plating
bath / tank is particularly essential while plating substrates of
intricate shapes. The ability to completely plate the surface of
the object under consideration is called the throwing power of
the plating bath. Haring – Blum cell (fig. 2.5) is mainly used to
find out the throwing power of the plating tank. If uniform
distribution is observed on the substrate then it is said that the
throwing power is good.
The Haring-Blum cell is made up of (i) the electrolyte, (ii)
an anode, positioned at the exact centre of the plating tank, and
(iii) cathodes (two nos.). The cathodes are so positioned that
their distances measured from the anode are C1 and C2, where
C1 > C2 as shown in the fig. 2.5. Once the initial setup is
established, electroplating is performed on both the cathodes.
Later, the amount of metal deposited on both the cathodes is
recorded as w1 and w2 (which represent the masses). It will be
noticed that C1 will have a lesser deposit in comparison to C2.
This is due to the lower over-potential at C1 in comparison to
C2.
Equation 2.14 is used to calculate the throwing power of
the plating bath.
68
%𝑇𝑟𝑜𝑤𝑖𝑛𝑔 𝑝𝑜𝑤𝑒𝑟 =100(𝑋−𝑌)
(𝑋+𝑌−2) …2.14
Where X = c1/c2 (when c1 > c2) and Y= w2/w1
If the masses w1 and w2 are equal, irrespective of the
distances C1 and C2 of the electrodes, the throwing power is
then to be 100%. It should be remembered that throwing power
cannot be used as a tool to measure the type and character of
the plating. Factors like conductivity, anodes, complexing
agents, and hydrogen evolution are often expressed in terms of
throwing power.
Conductance of the solution: This is an important
factor to be considered for having a uniform rate of deposition
on any intricate substrates. If the plating bath has high
conductance, then the distribution of the current would be
uniform everywhere in the plating bath.
Anodes: Uniform deposition can be attained with the
identification of suitable and most favourable positions for the
anodes in the plating tank.
Complexing agents: addition of levellers and brighteners
into the plating bath improves the throwing power.
Hydrogen evolution: As already discussed hydrogen
evolution poses a major problem such as hydrogen
embrittlement in the cathodes.
69
Fig. 2.5: The Haring-Blum cell
2.3.2.4 Components of Electroplating
Commercial electroplating depends on the type of metal to be
electroplated, the size and type of substrate material and its number, the
objective of plating and the economics. Depending on the size and the
number of articles to be plated, plating tanks of various sizes and
fixtures are used. Some of the fixtures used are jig or rack mounting for
large scale routine jobs; barrel plating for routine processing of large
batches of small objects; individual mounting for large and single
specialized work pieces (e.g. motor car parts and computer frames) and
wire mounting. A description of a rectangular tank (vat) plating process
is as shown in the fig. 2.6.
70
Fig. 2.6: Electroplating process
(a) Electroplating tank: Electroplating is generally carried in a steel
tank (vat) lined with polymeric materials on the inner side of the
tank to provide thermal insulation.
(b) Heating Process: The electroplating bath is heated by either
heating coils or hot gases. To maintain uniform temperature of the
electrolyte air is passed through a low pressure compressor that
helps in setting a convection cycle in the plating bath.
(c) Electroplating bath: For electroplating the solution used should
be conducting. Sometimes electrolytes are mixed with certain
chemicals that ultimately increase the conductivity of the solution
and hence the throwing power. For maintaining pH the electrolytes
are added with buffers. Also to improve the deposition process
additives are added. It should also be ensured that tank has
71
sufficient electrolyte to see that the cathode and anode are
completely immersed in the tank.
(d) Electrical equipment: A DC voltage of 8 to 12 Volts is generally
used in the electroplating process. Also a current density of 1 and
200 mA / cm2 is desirable. This is achieved by using either a motor
generator or DC rectifier units.
(e) Filters used in electroplating tanks: Utmost care has to be
taken with respect to plating bath, as there are chances of metal
particles or sludge from the anode getting mixed with the
electrolyte and disturbing smooth deposition process. These
particles of the anode can be retained by covering it with cotton
bags.
(f) Electrodes - Anode and Cathode: The power (DC) to the
electroplating bath is supplied via bus bars or cables (brass,
aluminium or copper). The cathode and the anode are attached to
these cables and then dipped in the tank. Anodes are those
materials that form a deposit on the cathode. Anodes may be in the
form of plates / rods.
2.3.2.5 Plating Process
The plating process is carried out by pretreating the object followed
by electrolysis.
72
Pretreatment
The substrate surface should be free from impurities. These are
removed by pretreating the substrate as follows.
(i) Removal of organic impurities (greases): Organic solvents
remove organic impurities like greases from the surface. An
effective method is the vapour phase degreasing, wherein the
vapours of the solvent are allowed to condense on the substrate
surface. Trichloroethylene was previously used for removing
varnish or paint films and resins; perchloroethylene for removing
high melt waxes and 1,1,1-trichloroethane for cleaning PCBs and
other electronic components. These are banned and in their place
non-hazardous substances such as isopropyl alcohol are
recommended. The treatment (alkaline) given to the surface
transforms the surface into a cathode with the removal of
impurities and the treatment is carried in between 60 and 800C.
The current required for this treatment will be in the range of 30 to
80 mA/cm2. This procedure ensures that the impurities are
removed with liberation of hydrogen at the cathode.
(iii)Pickling: removal of oxide scale: after the removal of organic
impurities, the substrate is immersed in 10% sulphuric acid, to
remove the scales formed due to oxidation.
73
(iv) Polishing: electroplated substrates are further treated by either
chemical or mechanical means to get a polished surface.
(v) Rinsing and drying: The substrate is finally rinsed with hot
deionized water and dried.
Electrolysis
For the electrolysis process, the electricity is passed through the
electrodes that are dipped in the electrolyte. One of the electrodes will be
the substrate (cathode) on which the plating is to be carried out and the
other called anode dissolves in the electrolyte and helps in the deposition
on a substrate under study. The cathode is dipped in the electrolyte only
after it undergoes necessary pretreatment procedures. The plating bath
is maintained at the required temperature. The plating time depends on
the complexity of the substrate to be plated. Sometimes the plating time
varies from a few seconds to several minutes. The plating thickness can
vary from a few microns to 100 microns. There are various plating
processes known and practiced these days and few of them have been
discussed in the ensuing sections.
74
2.3.2.6 Electroplating of Chromium
Plating bath Chromic acid bath; chromic acid (CrO3) and H2SO4
in 100:1 proportion
Operating
temperature
45 - 60 0C
Current density 100 – 200 mA / cm2
Current efficiency 8 – 12%
Anode Insoluble anodes – Pb – Sb or Pb – Sn coated with
PbO2 or stainless steel.
Cathode Objected to be plated; pretreatment
Application Decorative and corrosion resistant finish.
The plating bath contains CrO3 in which Cr is in +6 oxidation
state. This is reduced to +3 oxidation state by a series of complex
reactions in the presence of SO42− furnished by H2SO4. Cr3+ ions are
reduced to elemental Cr which gets deposited on the substrate. The
reactions taking place during Cr plating are given below,
𝐶𝑟𝑂3 + 𝐻2𝑂 → 𝐻2𝐶𝑟𝑂4 → 𝐶𝑟𝑂42− + 2𝐻+ …2.15
2𝐻2𝐶𝑟𝑂4 → 𝐻2𝐶𝑟2𝑂7 + 𝐻2𝑂 …2.16
𝐻2𝐶𝑟2𝑂7 → 𝐶𝑟2𝑂72− + 2𝐻+ …2.17
75
𝐶𝑟2𝑂72− + 14𝐻+ + 6𝑒
𝑐𝑎𝑡 .(𝑆𝑂42−)
2𝐶𝑟3+ + 7𝐻2𝑂 …2.18
𝐶𝑟3+ + 3𝑒 → 𝐶𝑟 …2.19
2𝐶𝑟3+ + 3𝑂2
𝑃𝑏𝑂2 2𝐶𝑟𝑂3 + 6𝑒 …2.20
In order to get good deposits it becomes essential to restrict the
quantity of 𝐶𝑟3+ ions. To control the 𝐶𝑟3+ ions, anodes that are insoluble
are covered with lead oxide layer. This oxidizes 𝐶𝑟3+ to 𝐶𝑟6+ oxidation
state. Usually, anodes made of Cr are not employed in Cr plating and
this is due to the formation of black deposit of Cr obtained in the
presence of large concentration of 𝐶𝑟3+ ions.
76
2.3.2.7 Electroplating of Copper and Nickel
Electroplating of Copper
Sulphate bath Cyanide bath
Plating bath 200 - 250 g of CuSO4, 50 - 75 g H2SO4 per L of
bath solution
40 - 50 g of CuCN, 20 - 30 g of KCN,
10 g K2CO3 per L
Operating
temperature
20 - 400C 40 - 700C
Current density 20 – 50 mA / cm2 10 – 40 mA / cm2
Addition agents Gelatin, dextrin, sulphur containing brighteners,
sulphonic acids
Sodium thiosulphate
Current efficiency (%) 95 – 99 60 - 90
Anode Phosphorous containing rolled Cu O2 free high conductivity Cu
Cathode Object to be coated pretreated Object to be coated pretreated
Applications In printed circuit boards, low throwing power
(not suitable for iron and iron alloy plating)
As an undercoat for Cr plating, in
printed circuit boards, good throwing
power
(suitable for iron and iron alloy
plating)
77
Electroplating of Nickel
Sulphate bath Cyanide bath
Plating bath 250 g of NiSO4, 45 g of NiCl2, 30 g of boric acid 600 g of Ni sulphamate, 5 g of NiCl2,
40 g of boric acid per L, pH 4
Operating
temperature
40 - 700C 50 - 600C
Current density 20 – 50 mA / cm2 50 – 400 mA / cm2
Addition agents Coumarin, Saccharin, benzene sulphonamide Napthalene – 1,3, trisulphonic acid
Current efficiency (%) 95 98
Anode Ni pellets or pieces Ni pellets or pieces
Cathode Object to be coated pretreated Object to be coated pretreated
Applications As an undercoat for Cr plating medium throwing
power
Decorative to be coated pretreated,
Decorative, mirror finish at 400 mA /
cm2
78
2.3.2.8 Distinction between Electroplating and Electroless Plating
Electroplating Electroless
Driving force Current Autocatalytic redox
reaction
Anode Separate anode Catalytic surface of
substrate
Cathode Object to be plated (treated
to remove surface
impurities)
Object to be plated
(treated to make
surface catalytically
active)
Reducing
agent
Electrons Chemical reagents
Applicability Only to conductors Conductors and non
conductors
Nature of
deposit
Not satisfactory for intricate
parts
Satisfactory for all
parts.
2.3.2.9 Conductive Paints
Conductive Paints (CPs) have usually been employed to paint
plastics and to realize EMI shielding. This method / technique
requires the inclusion of a conductive filler(s) and pigment(s) into a
binder usually made of resin. Generally, present day CPs are based on
nickel, copper, graphite, or silver. The binders used for manufacturing
of CPs comprises of epoxies, vinyls, polyurethanes, and acrylics [69].
A large variety of Conductive Inks- Paste/ Paints are available in
the market. Some of them are air-drying/curing type, firing type,
79
thermally conducting and electrically insulating compounds and
grease, some used in PCB repair kits, etc. Conductive Inks- Paste/
Paints are prepared by using conductive metal powder(s) that includes
silver, gold, platinum, palladium, graphite and carbon. The particle
size of the metal powders range from 2 to 25 microns and will be 99.9
percent pure [80].
Conductive painting method offers a radically unusual method
of networking with electronics and its applications. The CPs can today
be used on textiles. These paints can be used as a substitute for acid
etching used in electroless procedure. Most of the CPs developed are
nontoxic and water-soluble, which can be used without gloves and
mask. These paints dry very rapidly / quickly at room temperature
[81].
In the present day, the CP technology that is used commercially
is based on copper and silver. But one disadvantage associated with
copper plating is that, it is prone to oxidation and hence needs a
protective coating upon Cu plating such as Ni coating. The benefits of
using Ni paints when compared with silver and copper CPs is the
permanence of Ni paints, but due to the problem with its conductivity
the use of Ni paints has been restricted from its use in many of the
present day electronic products. Among all the types of coats / paints,
a silver-based CP presents excellent conductivity. Conversely, the
inflated material costs linked with silver paint tend to confine its use
[69].
80
The Cu and silver paints are being used in EMI/RFI
applications [69] also coatings are used to give decorative surfaces
and also to impart other desirable characteristics such as abrasion
and corrosion resistance, thermal and impact resistance, electrical
and thermal conduction and thermal and optical reflectively [72].
CPs are applied by some usual techniques like High-volume,
low-pressure (HVLP) equipment. The HVLP equipment and robots are
employed for high-volume production; these are used to enhance the
constancy and homogeneity of the CPs during their applications [69].
Before selecting or choosing CPs, the relative cost associated
with it has to be inspected from the process point of view. Differences
in cost of CPs are based on applications, for instance, use of CPs for
certain applications may find it very useful and for some it may be
little more expensive and hence it becomes very important to identify
the variables / elements of the process cost that can play a significant
role in influencing the overall cost. In some cases, the cost associated
with the use of CP and electroless plating are comparable, for
example, applications that require Double-sided plating will involve
less cost in plating, but at the same time requires exterior painting for
the product. Therefore, in such cases CPs can be used to avoid the
plating procedure. Therefore, one has to be careful while choosing
CPs. Sometimes certain factors like fabrication losses, disposal of
waste, etc. needs to be considered [69].
To summarize, CP technology unrelentingly evolved itself with
the product design changes and the convolution of the present day
81
electronics. The evaluation of CP should be based on the ability of the
process to handle all the important variables / elements of product
design. In addition to that, cost is also an essential consideration that
one has to make in the selection process.