Polymers are used across almost every sector, including packaging, medical, textiles, consumer
products, transportation, electronics etc.
Polymers are unique materials with awesome properties such as lightweight, toughness,
flexibility, transparency, and they are cheap too.
But their mechanical properties are not adequate for many high-end applications such as load
bearing applications.
For example, for making aero-plane, automotive parts.
For those applications polymers alone cannot be used.
If we compare polymer with metals, polymer possess low stiffness and strength.
Their modulus is 100 times lower, and strength is 5 times lower than the metals, which limits
their application as high strength material.
It is found that if polymer matrix is filled with fibrous materials, the resulting material
possess very high strength and excellent mechanical properties.
These materials can be used for load bearing and other high-end applications.
Such materials are known as composites.
The idea of composite is not new.
In nature composites exist several years.
Wood is a composite.
It is made of cellulose and lignin.
The very first man made composite is Brick.
Brick is made of straw and mud.
Our teeth, skin, bones, stones, concreate all are example of natural composites.
Composite material is made by combining two or more materials and these two materials
often have quite different chemical and physical properties.
The two materials work together to give the composite superior properties.
The main component of the composite is the matrix.
Second component is reinforcement which is more often called as filler.
Matrix surrounds and binds together filler material which provides strength and stiffness
to the matrix material.
Polymer composites can be of three types.
They are classified in three categories based on the type of reinforcement that is added
in the polymer matrix.
First one is particulate composite.
In particulate composite size of the reinforcing filler is small and their shape is spherical.
Aspect ratio of these type of filler is close to 1.
Second type of polymer composite is discontinuous fiber composite.
In this case short fibers are used as reinforcing fillers.
Aspect ratio of short fiber is greater than 1 which is close to 10 in most of the cases.
Third type of composite is continuous fiber composite.
In this case the length of the reinforcing fiber is almost equal to the length of the
product, or we can say that length of the reinforcing filler is much-much greater than
1.
It can be close to 1000 or even more.
Now these continuous fibers can be oriented in different directions.
Depending upon the direction of load bearing requirement in the composite, orientation
of filler is decided.
Continuous fiber composites are much stronger than the other two types of composites because
load is directly hold out by the fiber whereas in other two cases load is primarily hold
out by polymer matrix and then the filler.
Main function of filler in case of particulate and discontinuous fiber composite is to improve
modulus of the composites.
Here strength of the composite will mainly depend on chemistry between the filler and
matrix.
If interfacial interaction between these two is good composite will show good strength
if not, then the modulus will be improved but not the strength.
Now let’s see what fillers are those fall under particulate, discontinuous and continuous
category.
Spherical and irregular shaped particles such as carbon black, fumed silica, talc etc. are
particulate type of fillers and these fillers are widely used in industry specially carbon
black and silica.
Glass, carbon, aramid fibers are used to make continuous composites.
These composites have very high strength and can replace metals.
When continuous fibers are chopped and added in the polymer matrix the resulting composite
is called discontinuous fiber composite.
Other than these fillers natural fibers such as cellulose fiber, coconut shell power, jute,
silk fiber are also added in the polymer matrix as reinforcement.
Some fillers such as CaCO3, talk are cheaper than base polymer matrix so these are added
in the polymer just to reduce cost.
Now the recent trend is to add nanofillers in the polymer matrix and such type of composites
are called as nanocomposites.
Cellulose nanocrystals/nanofibers, graphene, carbon nanotube/nanofiber, carbon black are
examples of nanofillers.
These fillers are called nanofillers because at least their one dimension is in nm.
E.g., Carbon nanotube’s diameter is 5-20 nm.
Because of their nano-size, surface area of these fillers is extremely high which is very
important to achieve good reinforcing properties.
As we know Polymers vary in their chemical structure and offer different properties;
therefore, selection of polymer matrix is made based on application and property requirement
Thermoplastics can be used to make composites.
Thermoplastic polymers are good heat resistant, get soft on heating therefor can be easily
molded in any shape.
Also, these polymers can be remolded as they do not crosslink.
Most consumer products are made of thermoplastic composites.
Thermosets are highly crosslinked materials so do not soften at high temperature.
Thermoset composites remain stable in high and dynamic temperature therefore are highly
suitable for applications where extremely high modulus and high temperature stability
is required.
Elastomers are flexible, low strength materials.
Their strength can be improved by incorporating fillers like carbon clack, silica etc.
So, elastomers can be used as matrix material to develop polymer composites with flexibility
and strength.
Now the question is how to incorporate filler in the polymer matrix to develop composite.
So, there are three methods, melt mixing, solution blending and in-situ polymerization.
All three methods are used to make particulate and discontinuous fiber composites.
There is no synthesis method for developing continuous fiber composites.
Continuous fiber composites have direct product manufacturing process.
First one is Solution Blending: In this method first, polymer is dissolved in a suitable
solvent and filler is dispersed in the same solvent.
Then rigorous mixing is done by means of external mixing such as magnetic stirring, shear mixing,
reflux or most commonly, through ultrasonication which facilitates de-aggregation and dispersion
of filler.
Finally, the composite is recovered by precipitation or casting film.
Melt mixing is the simplest method to mix filler in the polymer.
In this method filler and polymer are taken together in an extruder and high shear is
applied by the screw of the extruder to disperse and distribute the filler particles in the
polymer matrix.
To know more about extrusion please watch our video on polymer extrusion.
In situ polymerization, this technique is particularly important for the preparation
of insoluble and thermally unstable polymers, which cannot be processed by solution or melt
processing.
In in-situ polymerization monomers are polymerized in presence of fillers.
Depending on required molecular weight and molecular weight distribution of polymers,
free radical, ionic, and ring-opening polymerizations can be used for in-situ polymerization.
So, these are three main methods to prepare composites.
But to achieve composite material with desired properties these two factors must be taken
care off.
First one is Dispersion and distribution of filler particles in the polymer matrix
So, what happen, when we add a filler in the polymer matrix it is always present in the
form of aggregates.
These aggregates are also called as agglomerates.
Agglomeration is a tendency of high surface area fillers where individual filler particles
join and form a big lump.
Due to high surface area of filler particles, surface contact area is very high, therefore
paticle-2 interaction is more and because of high surface area, filler particles like
to form aggregates.
So, if the filler is present in aggregated form, the composite will show poor performance.
Therefore, it is important to open these bundles into individual particles.
The process of opening or dispersing fillers in individual particles is called dispersion.
If the filler particles are individually present in the polymer matrix, we say dispersion is
good.
If the particles are present throughout the matrix, we say distribution is good.
For better performance of the composite material both dispersion and distribution should be
good.
Otherwise, we will have to compromise with properties and performance of composite material.
Second important factor is interfacial interaction between filler and the polymer matrix.
This depends on two factors.
surface to volume ratio or aspect ratio in case of nano fillers and chemistry between
the filler and the polymer matrix.
The surface to volume ratio is the amount of surface area per unit volume of an object
and is inversely proportional to the size.
So, the materials with a high surface area to volume ratio are those that are small in
diameter or very porous in structure.
These react at much faster rates than macro materials because more surface area is available
to interact with polymer.
Now, size of the filler is important but chemistry between the filler and polymer matrix more
important.
E.g., if we try to mix hydrophobic filler in hydrophilic polymer no matter how small
the size of the filler is, the resulting composite will have poor properties due poor interfacial
interaction.
So, to improve the adhesion, fillers are usually surface treated to improve compatibility which
in turn improves interfacial interaction between the two.
It also helps to reduce agglomeration of high surface area filler.
Other than surface treatment, high shear mixing, sonication, are also effective methods to
improve dispersion of fillers those can be applied during the synthesis method.
Surface modification of filler allows introduction of different functional groups on filler surface.
These surface groups can be either covalently attached to the filler or non-covalently where
fillers have adsorbed surface groups.
Function groups present on filler surface act as bridge between filler and the polymer
matrix, thus improve interfacial interaction.
These functional groups also increase electrostatic repulsion and steric repulsion among the filler
particles and help in de- agglomeration of fillers.
So, it is clear now, to achieve polymer composite with desired properties dispersion/distribution
of filler, their size and interfacial interaction between filler and matrix must be good.
By choosing an appropriate combination of reinforcing filler and matrix material, and
processing method we can produce polymer composite with desired properties that exactly fit the
requirement for a particular application.
And that is the reason demand of polymer composites with superior properties is growing and there
is constant quest of finding more advanced materials.
With that I would like to end today’s video.
I hope this video will help you to understand polymer composites.
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Thank you.