PLASTICS AND RUBBERS
. This lesson will expose some of the uses of plastics and rubber to you.
OBJECTIVES:
By the end of this course unit the student should be able to:
• Identify plastics.
• State the types of plastics.
• State at least five uses of plastics.
State some properties of plastics
Plastics Processing.
•INTRODUCTION
The word plastic itself comes from the Greek word plastic, which means to be able to be shaped
or moulded by heat. As we will see, shaping plastics by using heat is a basic part of nearly all
plastics manufacturing processes
VARIOUS CATEGORIES OF PLASTICS.
Natural plastics - these are naturally occurring materials that can be said to be plastics because
they can be shaped and moulded by heat. An example of this is amber, which is a form of
fossilised pine tree resin and is often used in jewelry manufacture.
Semi synthetic plastics - these are made from naturally occurring materials that have been
modified or changed but mixing other materials with them. An example of this is cellulose
acetate, which is a reaction of cellulose fibre and acetic acid and is used to make cinema film.
Synthetic plastics - these are materials that are derived from breaking down, or ’cracking’
carbon based materials, usually crude oil, coal or gas, so that their molecular structure changes.
This is generally done in petrochemical refineries under heat and pressure, and is the first of the
manufacturing processes that is required to produce most of our present day, commonly
occurring plastics. Synthetic and semi synthetic plastics can be further divided into two other
categories. These two categories are defined by the ways in which different plastics react when
heated.
a. Thermoplastics
b. Thermosetting plastic
Thermoplastics - these are plastics that can be softened and formed using heat, and when cool,
will take up the shape that they have been formed into. But if heat is reapplied, they will soften
again. Examples of thermoplastics are acrylic and styrene, probably the most common plastics
found in school workshops.
, Thermosetting plastics - these are plastics that soften when heated, and can be moulded when
soft, and when cool they will set into the moulded shape. But if heat is reapplied, they will not
soften again, they are permanently in the shape that they have been moulded into. Why this
happens we will look at later. Examples of thermosetting plastics are polyester resins used in
glass reinforced plastics work, and melamine formaldehyde used in the manufacture of Formica
for kitchen work surfaces.
‘Polymers’
Is a general term for all plastic materials and means that they are organic, carbon based
compounds whose molecules are linked together in long chain patterns. When we talk about
plastics in general, we will call them polymers, and when we talk about specific
plastic .materials, we will give them their real names, such as nylon or polythene.
The structure of polymers
To understand how plastics are made, and why certain plastics are suitable for some uses, and
others not, you have to understand a little about the structure of polymers.
Polymers are large molecules made up of many smaller molecules.
'Poly' means many
'mer' means units.
These smaller units are called monomers (mono = one, mer = unit) and are joined together
through polymerization to form polymers. A polymer contains hundreds of thousands of
monomers.
Polymerization, which means the linking of monomers to form polymers results from two kinds
of chemical reaction called condensation and addition. Polymers fall into two distinct groups,
thermosetting plastics and thermoplastics. Thermosetting polymers are converted into their
final form by heat and once set cannot be softened by further heating. Thermoplastics however
are softened and become fused or 'plastic' by moderate heating and then harden again on cooling.
This process can be repeated many times without radically altering the thermoplastic properties.
In a monomer atom are joined by double bonds, and these must be broken and new bonds created
between adjacent atoms to form the long chain molecules of a polymer through polymerization.
. This lesson will expose some of the uses of plastics and rubber to you.
OBJECTIVES:
By the end of this course unit the student should be able to:
• Identify plastics.
• State the types of plastics.
• State at least five uses of plastics.
State some properties of plastics
Plastics Processing.
•INTRODUCTION
The word plastic itself comes from the Greek word plastic, which means to be able to be shaped
or moulded by heat. As we will see, shaping plastics by using heat is a basic part of nearly all
plastics manufacturing processes
VARIOUS CATEGORIES OF PLASTICS.
Natural plastics - these are naturally occurring materials that can be said to be plastics because
they can be shaped and moulded by heat. An example of this is amber, which is a form of
fossilised pine tree resin and is often used in jewelry manufacture.
Semi synthetic plastics - these are made from naturally occurring materials that have been
modified or changed but mixing other materials with them. An example of this is cellulose
acetate, which is a reaction of cellulose fibre and acetic acid and is used to make cinema film.
Synthetic plastics - these are materials that are derived from breaking down, or ’cracking’
carbon based materials, usually crude oil, coal or gas, so that their molecular structure changes.
This is generally done in petrochemical refineries under heat and pressure, and is the first of the
manufacturing processes that is required to produce most of our present day, commonly
occurring plastics. Synthetic and semi synthetic plastics can be further divided into two other
categories. These two categories are defined by the ways in which different plastics react when
heated.
a. Thermoplastics
b. Thermosetting plastic
Thermoplastics - these are plastics that can be softened and formed using heat, and when cool,
will take up the shape that they have been formed into. But if heat is reapplied, they will soften
again. Examples of thermoplastics are acrylic and styrene, probably the most common plastics
found in school workshops.
, Thermosetting plastics - these are plastics that soften when heated, and can be moulded when
soft, and when cool they will set into the moulded shape. But if heat is reapplied, they will not
soften again, they are permanently in the shape that they have been moulded into. Why this
happens we will look at later. Examples of thermosetting plastics are polyester resins used in
glass reinforced plastics work, and melamine formaldehyde used in the manufacture of Formica
for kitchen work surfaces.
‘Polymers’
Is a general term for all plastic materials and means that they are organic, carbon based
compounds whose molecules are linked together in long chain patterns. When we talk about
plastics in general, we will call them polymers, and when we talk about specific
plastic .materials, we will give them their real names, such as nylon or polythene.
The structure of polymers
To understand how plastics are made, and why certain plastics are suitable for some uses, and
others not, you have to understand a little about the structure of polymers.
Polymers are large molecules made up of many smaller molecules.
'Poly' means many
'mer' means units.
These smaller units are called monomers (mono = one, mer = unit) and are joined together
through polymerization to form polymers. A polymer contains hundreds of thousands of
monomers.
Polymerization, which means the linking of monomers to form polymers results from two kinds
of chemical reaction called condensation and addition. Polymers fall into two distinct groups,
thermosetting plastics and thermoplastics. Thermosetting polymers are converted into their
final form by heat and once set cannot be softened by further heating. Thermoplastics however
are softened and become fused or 'plastic' by moderate heating and then harden again on cooling.
This process can be repeated many times without radically altering the thermoplastic properties.
In a monomer atom are joined by double bonds, and these must be broken and new bonds created
between adjacent atoms to form the long chain molecules of a polymer through polymerization.
