Additives are one of the most vital components of a polymeric product.
It has played very important role in the growth of plastic industry.
A typical polymer product contains several types of additives like filler, plasticizer,
colorant, stabilizers and so on.
In this video we will try to explain why additives are used in plastics and also what are the
functions of these additives.
So let’s start with first question, why additives are used in plastic products?
We know polymers are formed by joining several monomers like ethylene, propylene, styrene etc.
The monomers in the polymers are mainly connected by carbon-carbon, amide or ester bonds.
These bonds are not very strong and in presence of extreme condition like high temperature,
UV light, oxygen or moisture, these can easily break.
Also except thermoset and rubber, majority of the polymers are used in uncrosslinked
form so that they can be recycled and processed multiple times.
therefore majority of polymers are inherently weak which makes them susceptible to degradation
under extreme conditions.
The first step where polymer experience extreme conditions is during extrusion process.
Extrusion is performed well above the melting temperature of polymer, at a very high pressure
and in air which contains oxidizing agent like oxygen and metal.
At such a high temperature and pressure the bond connecting the monomers are highly susceptible
to oxidation by oxygen and metal.
And if it is not controlled, polymers can degrade severally which will make final product
very weak and brittle.
Polymeric products also have to go through rough conditions even when they are being
used for different applications like when products are used for outdoor application,
the product can be in direct sunlight which contains UV radiation and also generate high
amount of heat.
Since both UV light and heat can causes degradation of polymer chain, both of these factors must
be taken into account.
Different degradation mechanisms of the polymeric materials are shown here.
We will not discuss these mechanisms in detail but in general we can say that heat, uv light
and moisture lead to degradation of polymeric materials.
And as polymers are expected to survive several years in different applications like in automotive
application, polymeric material is expected to retain its mechanical property and appearance
for more than 15 years, whereas when polymeric materials is used in building, it is expected
to survive for more than 50 years.
Therefore, if polymeric materials are not protected from the harmful effect of heat,
uv light and moisture, polymeric materials cannot be used for these applications.
And here is the answer to our first question, not all but several additives are used to
protect the polymeric materials from the harmful effect of heat, oxidizing agent, UV light
and moisture to retain their physical properties and appearance.
And as a certain level of strength and flexibility are required in the final product, appropriate
additives are used to improve strength and flexibility of the product.
Now we can say that additives are used to facilitate processing, achieve desired properties
and maintain appearance.
Now, let’s discuss about the different additives and their function in plastics.
There are several steps in which polymers are converted into different product.
Those steps can be represented by two basic steps.
In the first step, polymers are mixed with additive package.
The additive package is the name of the mixture of different additives which are added to
plastic to achieve desired performance.
Then the mixture of polymer and additive package is extruded to prepare a formulated resin,
which is then molded to form desired product.
Additional additives can be added during molding process or post production process but for
simplicity let’s assume that all the additives are added through additive package.
Now let’s start with the additives used to achieve desired performance in the product,
and then we will discuss the additives for molding and extrusion process too.
So first thing that we see in a product is its color, colorants are one type of additive
which are added to achieve the desired color.
The colorant can be in the form of dye or pigment.
Dyes are organic chemicals which can absorb light and pigments are particles which can
also absorb light of different color.
Dyes are used for bright color that faded with time and pigment are used for long lasting
color so depending upon the use and need, either dye or pigment are used as colorant
to achieve desired color in final product.
Next, let’s discuss additive used for physical property which is probably the most important
property of the product.
The physical properties basically represent the three different properties of the product.
First is impact strength, in case of chair we can think like this, when we sit on the
chair, we are applying an instantaneous load on the chair so it need to have good impact
strength.
Impact modifiers are added to achieve desired impact strength.
The impact modifiers are mostly functionalized rubbers like butadiene rubber, acrylic block
copolymers such as SBS, NBR, and EPDM.
Second property is tensile strength or in general strength of the materials, again,
when we are sitting on a chain, we are applying a constant load on the chair so in absence
of sufficient strength, the chair will break after some time; therefore reinforcing fillers
are added to achieve desired strength.
Since the strength requirement of household stuff like chair will be less compared to
polymer composites used in car or airplane application so reinforcing agent are also
different for these products.
When high strength is required glass fiber, carbon fiber or Kevlar fibers are used as
reinforcing fillers.
In case of moderate strength, talc, clay or mica are used as reinforcing fillers as well
as cost saving filler.
Next property is flexibility, again, when we are sitting on the chair, the chair need
to have sufficient flexibility otherwise it will crack like a glass even at slight bending
and also the chair won’t be comfortable to sit.
So to achieve desired flexibility plasticizers are used.
Plasticizers are one of the most used additives in plastics and they vary significantly in
terms of their chemical nature and use.
So let’s discuss plasticizers in more detail.
As I mentioned; plasticizers improve the flexibility and durability of plastics.
Plasticizers molecules break intermolecular forces between polymer chains and thus give
greater freedom of movement to each molecular chain.
Since plasticizers are blended into the polymer in large amounts, their compatibility with
the polymer must be good.
Plasticizers can be broadly classified into two main categories, internal and external
plasticizer.
Internal plasticizers are chemically modified polymers.
Basically these are copolymers of the monomers of the desired polymer and of a polymer with
lower glass transition temperature.
The most common monomers which can act as internal plasticizers are vinly acetate and
vinylidene chloride.
These plasticizers are in limited use due to their limited availability.
Next type of plasticizer is called external plasticizers since these are not connected
polymeric chains.
This type of plasticizer can be further divided into two category; primary and secondary plasticizers.
Primary plasticizers are chemicals and shows significantly high compatibility with the
polymer resin as a result they can be used at very high loading.
Primary plasticizers can provide desired flexibility by itself whereas secondary plasticizers cannot
be used alone since they have limited compatibility with polymer so cannot be used at very high
loading.
The most common primary plasticizers are phthalate compounds and the secondary plasticizers are
sebacate, polyester and epoxidised oil.
Extenders are a subset of secondary plasticizers.
They are commonly employed with primary plasticizers to reduce cost in general purpose flexible
PVC.
They are mostly low cost oils having limited compatibility in PVC.
They are added to reduce cost and in some cases to improve fire resistance.
Examples of extenders include naphthenic hydrocarbons, aliphatic hydrocarbons, chlorinated paraffins
and others.
Now coming back to the properties of final product.
As many plastic products are used outdoor which means under the sunlight.
As sunlight contains high energy UV radiation, it can break the polymer chains and lower
the strength and deteriorate the appearance of the product.
So to protect the product from harmful effect of UV light, UV stabilizers are added.
There are two types of UV stabilizers; UV absorbers which preferentially absorbs UV
light and HALS which quenches the free radical and thus provides indirect protection.
As I mentioned previously, depending upon the performance requirement of final product
and environmental condition during use, different additives are added to achieve the performance
and also protect the product from the harmful factors.
Now, let’s talk about the additives which are used to protect polymer and facilitate
molding process.
The molding process is one of the process by which formulated resins are converted to
different products.
Injection molding is one such process.
To know more about injection molding process, please watch our video on that topic.
In injection molding process, molten polymer is pushed inside a mold cavity which is of
the shape of the final product and when the polymer melt cools, product is released from
the mold.
So the two main requirements during molding process is mold filling and easy release of
the product so that product will not have surface defects and also the process can be
continuous.
To facilitate mold filing, a lubricant is added.
These type of lubricant is called internal lubricants because it reduces the friction
between the polymer chains and thus facilitate flow of polymer inside the mold cavity.
The common example of internal lubricants are fatty ester, EVA, waxes and oxidized polyethylene.
For mold release and surface finish, mold release agents are added, it reduces the adhesion
between polymer and mold surface.
The common example of mold release agents are silicone oils, graphite, zinc stearate
and molybdenum disulfide.
Use of these additives helps to improve the molding processing and also reduce cost by
minimizing defects and stoppage of the equipment.
But the concentration of these additives are very carefully decided since it may also cause
some negative issue like flashing due to low melt viscosity and also blooming of the additives
on the surface of the final product.
Next, and actually the first step in which polymers are required to be protected is extrusion.
Therefore some specific additives are added to facilitate the extrusion process and protect
the polymer during the harsh conditions of extrusion.
During extrusion there are three main factors which can cause significant degradation of
polymer chains.
These factors are temperature, oxidizing agents, friction between polymer chains and screw
elements.
For temperature, we cannot do anything except properly controlling temperature so that it
should not shut up from the set value.
Because high temperature makes polymer chain much more susceptible to oxidizing agent as
the chemical reaction rate significantly increases at high temperature.
Therefore additives are added to protect the polymer from oxidizing agents.
These additives are called processing stabilizers.
There are different types of stabilizers like antioxidant, peroxide decomposers and chelating
agents.
Antioxidants helps to stop degradation caused by thermo-mechanical or thermo-oxidative conditions.
The way these antioxidants interrupt the degradation process varies according to their structure.
Some of the common antioxidants are amines, phenolic, phosphites, and thioesters.
These stabilizers are also called primary stabilizers.
Peroxide decomposers are secondary stabilizers which help to safely decompose peroxides.
Peroxides are very dangerous for polymer because radical formed due to decomposition of peroxide
are highly energetic and reacts with polymer chain to break them into two and in this process
additional peroxides are formed.
As a result peroxides starts an auto-degradation cycle.
Therefore additives are required to safely decompose peroxide so that it cannot react
with polymer chain and create more peroxides.
Chelating agents are also added to protect polymer from metals.
Some special stabilizers are added for halogenated polymer since their degradation mechanism
is somewhat different than normal polymer.
For the third factor which is friction between polymer and screw, lubricants are added.
This time these lubricants are called external lubricants since these additives reduce the
friction between polymer chain and metal surface.
The common external lubricants are metal soaps, fatty acids, paraffin’s and low molecular
weight polyethylene.
So we can see that to facilitate and protect polymer during processing, different types
of processing stabilizers and external lubricants are added.
Here again concentration is very important to achieve desired outcome otherwise the cost
and quality of the product can suffer.
In addition to additives mentioned previously, some other additives are also added for different
reasons like fillers, flame retardants, nucleating agents, blow agents, conductivity fillers
etc.
Many times fillers are added to reduce cost of the final product since the cost of some
fillers are significantly lower than base polymer.
Other times fillers help to achieve dimension stability and strength.
Some of the common fillers are listed here.
Flame retardants reduce the flammability of plastics.
They can be divided into inorganic and organic compounds, the most common inorganic flame
retardants are aluminum hydroxide; also known as aluminum trihydrate or ATH.
Other inorganic and organic flame retardants are listed in the slide.
The effectiveness of organic flame retardants like chlorinated and brominated compounds
increases with the halogen content.
Sometimes a combination of two flame retardants is used to increase their efficiency.
Nucleating agents are added to influence the rate of crystallization, mechanical and optical
property of the polymers.
Some of the common nucleating agents are listed in the slide.
To know more about nucleating agents watch our video on that topic.
Blowing agents are chemicals which are used to develop foamed morphology.
Isocyanate and water are used as blowing agent for polyurethane, azodicarbonamide for vinyl,
hydrazine and other nitrogen-based materials for thermoplastic and elastomeric foams, and
sodium bicarbonate for thermoplastic foams.
Conductive additives are special additives that help to make plastics thermally or electrically
conductive.
Some of the common conductive fillers are carbon black, graphite, and metal particles.
So from today’s discussion, it is clear now that additives are essential part of polymers.
Without these additives, application of polymers can be very limited because these additives
are not only helping it to achieve desired performance but also important for long term
use.
In absence of additives, polymers do not have required strength and durability to be used
in many applications.
With that I would like to end today’s video.
I hope it will a useful video for you all.