Chemistry

Key unit competence

To be able to relate the types of polymers to their structural properties and uses

Learning objectives
At the end of this unit , students will be able to:

•    Define the terms monomer, polymer and polymerization;

•    Describe the formation of polymers;

•    Describe addition and condensation polymerization;

•    Explain the terms thermosetting and thermosoftening of the plastics;

•    Discuss the advantages and disadventages of both natural and synthetic polymers;

•    Explain the biodegradability property of polymers based on their chemical structure;

•    Use equations to distinguish between condensation and addition polymerization;

•    Prepare phenol-methanol polymer (Bakelite);

•    Relate the structure and properties of polymers to their uses in the plastic and textile industries;

•    Reduce polymer wastes by reusing, recycling and appropriate disposal;

Introductory activity

The images below show materials commonly used in our daily life.

Observe carefully the following images in pairs, and answer the questions below:

a. Give the name for each material and one of its uses
b. Search in library (textbooks) and on internet, the chemical nature of each of the each material mentioned above
c. Discuss the characteristics of the above materials and identify what do they have in common

Nowadays the materials made of plastics such as fibers, plastic and rubber materials, are all around us and are commonly called polymers by chemists. Polymers are commonly used in household utensils, automobiles, clothes, furniture, spaceaircraft, biomedical and surgical components. Polymeric materials are light weight but can possess excellent mechanical properties and can be easily processed by different methods. In this unit you will learn more about polymers, their types and some important-synthetic and natural polymers.

6.1. Definition of monomer, polymer and polymerization

Activity 6.1

1. Explain the following terms:

a. Polymer
b. Polymerization

2. Identify the products made of polymers that are used at your home and mention at least 3 materials.

6.1.1. Monomer
The term monomer comes from mono “one” and meros “part”,which expresses a single unit or a small molecular subunit that can be chemically bind to another identical or different molecule to form larger molecule (polymer).The monomer is repeated in the polymer chain and it is the basic unit which makes up the polymer. For instance in the large compound formed by nA → -A n-  where A is a monomer and the polymer is given by the repeated monomers in the chain; i.e. –A-A-A-A-A-A-. The larger molecules such as carbohydrates, lipids, nucleic acids and proteins are found in living systems, like our own bodies.

6.1.2. Polymer
A polymer is a large molecule (macromolecule or giant molecule) composed of smaller molecules (monomers) linked together by intermolecular covalent bonds. Polymers have a high molecular weight in the range of 103 to 107. A polymer can be represented as (–An–) or (– A – A – A – A – A-……) which is a polymer of the monomer A.

Table 6.1: Some examples of polymers and monomers

6.1.3. Polymerization

Polymerization is the process in which monomer units are linked by chemical reaction to form long chains (polymers).

For example, a gaseous compound, Butadiene, with a molecular weight of 54 g/ mole combines nearly 4000 times by polymerization and gives a polymer, known as polybutadiene.

Butadiene + butadiene + butadiene + … + butadiene → Polybutadiene

Note: 1.The degree of polymerization is defined as the number of monomeric units in a macromolecule or polymer or oligomer (a polymer consisting of few number of monomers units) molecule.
           2. A polymer formed by identical monomers is called homopolymrer whilea polymer formed by different monomers is a copolymer.

Preparation protocol of plastic from milk

Experimental activity for the preparation of plastic from Milk Materials

•    500ml 2% milk

•    60ml vinegar

•    One beaker of 100 mL

•    2 beakers of 1 L each

•    1 spoon

•    1 strainer

•    Aluminum foil

•    Thermometer

•    Hot plate or Bunsen burner and stand

•    Matches if using a Bunsen burner

Procedure
1. Assemble all materials and chemicals

2. Turn on the Bunsen burner or hotplate.

3. Place beaker of milk over heat.  Stir constantly.

4. Place thermometer into the milk.  Heat milk until it reaches 37ºC. 

5. Remove from heat and immediately add vinegar, stirring constantly.  Here, the teacher should ask the students what they think will happen when the vinegar is added.

6. The solution will rapidly separate into curds suspended in a clear yellow liquid.

7. Strain the solution through the strainer into the empty 1L beaker.  Hold up the beaker to see the clear liquid.

8. Scoop the curd onto a large piece of aluminum foil and press out into a thin layer.  Pass the aluminum foil around the class. Then dry especially overnight

Milk is a colloid, which is defined as a suspension of large molecules of proteins in water.  Essentially, milk is a suspension of protein globules in water. 

These proteins can undergo polymerization to create a natural plastic, as the casein molecules are associated together in long chains.  Proteins are generally unstable and are prone to unfolding, which changes the natural state of the protein.  This process is called denaturing.  The addition of acid, in this case vinegar, causes the casein protein to unfold and rearrange into the long chains of a polymer.  The process then causes the casein to precipitate out of the milk, leaving a clear watery substance behind.  The casein can then be formed into various shapes before drying.  In our experiment, thermal energy in the form of heat was applied to speed the process and cause a more complete separation.

There two types of polymerization reaction: Addition polymerization and condensation polymerization.

6.2.1. Addition polymerization

Addition polymerization is a process where monomers are linked together to form a polymer, without the loss of atoms from the molecules. When the monomer molecules add up to form the polymer, the process is called “Addition polymerization”.

This involves the combination together of monomer units to give new product (polymer ) having the same empirical formula to the monomer but having relative molecular weight.The monomer units are usually unsaturated compounds.

Some examples of addition polymers are polyethylene, polypropylene, polystyrene, polyvinyl acetate, polyvinyl chloride (PVC), rubber, polytetrafluoroethylene (Teflon), etc.

Example 2: Formation of PVC (polyvinylchloride)

PVC (polyvinyl chloride) is found in plastic wrap, simulated leather, water pipes, and garden hoses, it is formed from vinyl chloride (H₂C=CHCl).

Vinyl is a common name for ethylene, PVC is formed from the following reaction:

Example 3: Formation of rubber: Rubber can be natural or synthetic

Rubber is a natural polymer of isoprene (2-methyl-1,3-butadiene). It is formed by a linear 1,4- addition polymer of isoprene.

Natural rubber has elastic properties because it has the ability to return to its original shape after being stretched or deformed. Therefore it is known as Elastomer. Natural rubber is prepared from latex which is a colloidal solution of rubber in water.

 Addition polymerization takes place in three steps like initiation, propagation and termination reading to the formation of polymer.

i. Chain initiation

A peroxide molecule breaks up into two reactive free radicals. Light or heat can provide the energy needed for this process. The chain is initiated by free radicals produced by reaction between some of the ethene and the oxygen initiator from peroxide.

The process of initiation involves two parts: generation of initiator free radical and initiation of polymerization reaction.

The second part of initiation occurs when the free radical initiator attacks and attaches to a monomer molecule. This forms a new free radical, which is called the activated monomer as indicated in the chemical equation below:

ii. Chain propagation

During this step, there is successive addition of large number of monomer molecules to form polymer free radical chain.In the propagation phase, the newly-formed activated monomer attacks and attaches to the double bond of another monomer molecule. This addition occurs again and again to make the long polymer chain.

iii. Chain termination

Termination step involves the coupling of two free radicals in order to produce a final molecule. The process is a termination step because no new free radicals are formed.

6.2.2. Condensation polymerization

Condensation polymerization is a process where two or more monomers chemically combine to form a polymer with elimination of simple molecules like water, ammonia, hydrogen chloride, alcohol, etc.

In this type of reaction, each monomer generally contains two functional groups for the condensation reaction to take place.

There are two main types:  polyamides which are formed between a diol with a dicarboxylic acid and polyesters which are formed when a dicarboxylic acid and a diamine react to form nylons.The condensation polymers include Nylons, Terylene, Kevlar polymer, proteins, cellulose, Bakelite, Dacron, etc.

Example 1: FORMATION OF POLYAMIDES

This type of polymers is formed by the result of generation of amide bonds in the polymerization reaction.

i. Formation of Nylon

Nylon 6 and nylon- 6, 6 are important examples for this type of polymers nylon 6 is synthesized from e-caprolactam, which on heating decomposes into 6-aminohexanoic acid that polymerizes into nylon 6. Here the number 6 represents the number of carbon atoms present in the monomer unit.

Nylon -6,6 is produced by the condensation reaction between two monomer units adipic acid and 1,6-hexanediamine in the presence of heat. This is formed from a sixcarbon diacid and a six-carbon diamine as shown below.

ii. Formation of Kevlar

Kevlar is formed from the polymerization of benzene -1,4-diamine and benzene-1,4dioic acid as follows:

In that polymerization –NH₂ group of hexamethylenediamine reacts with –COOH group of adipic acid to form –NH-CO- amide linkage with elimination of H₂O.

Polyamides such as nylon-6,6 and Kevlar are widely used in clothing. Kevlar has some remarkable properties,including fire resistance and higher strength than steel. It is used to make protective clothing-for example for fighters,bulletproof vests and helmets.

Example 2: FORMATION OF POLYESTERS

i. Formation of Terylene

ii. Formation of Dacron

It is made from dimethyl -1,4-benzene dicarboxylate and 1,2-ethane diol: Dimethyl -1,4-benzene dicarboxylate + 1,2-ethane diol → Dacron + methanol

Other polymer formation:

Formation of bakelite:

Bakelite    (polyoxybenzylmethyleneglycolanhydride)    is    the    oldest    synthetic    polymer.    It    is    formed    from    phenol    and    formaldehyde    in    the    presence    of    either    an    acid    or    a    base    catalyst.    The    initial    product    could    be    Novalac,    then    novalac    on    heating    with    formaldehyde    undergoes    cross    linking    to    form    bakelite.

Experiment on the preparation of phenol formaldehyde resin (bakelite) Chemicals

•    Glacial acetic acid,

•    40% formaldehyde solution,

•    Phenol, conc. H₂SO₄

Apparatus Required:
•    Glass rod,

•    beakers,

•    funnel,

•    measuring cylinder,

•    dropper

•    Filter paper

Procedure:
1. Place 5ml of glacial acetic acid and 2.5ml of 40% formaldehyde solution in a 500ml beaker and add 2 grams of phenol.

2. Add few ml of conc. Sulphuric acid into the mixture carefully. Within 5 min, a large mass of plastic is formed

3. The residue obtained is washed several times with distilled water, and filtered product is dried and yield is calculated.

Conclusion:

A mixture of phenol and formaldehyde are allowed to react in the presence of a catalyst. The process involves formation of methylene bridges in ortho, para or both ortho and para positions. This results first in the formation of linear polymer (Called NOVALAC) and then in to cross-linked polymer called phenol-formaldehyde resin or bakelite.

Protein formation:

Polymerization of amino acids:

•    Two amino acids can be linked by a condensation reaction (by removing of water molecule)

•    Peptide bond is formed between the carbon atom in the acid group and the nitrogen atom in the amine group.

•    The result molecule is called a dipeptide

•    A chain of amino acids can be built up in this way and is called polypeptide

•    A protein may just contain one polypeptide or may have two or more chains that interact

The polyester Dacron and the polyamide Nylon-6,6, are two examples of synthetic condensation polymers. Some differences between addition polymerization and condensation polymerization are summarized in Table 6.2.

6.3.1 Classes of polymers

In general, polymers are classified into two classes such as natural polymers and synthetic polymers.

6.3.1.1 Natural polymers

Natural polymers are those that are obtained from natural sources. They are made naturally and are found in plants and animals or other living organisms. For examples: cotton, silk, wool, natural rubber, cellulose and proteins.

a. Silk

Silk is a fine continuous protein fiber produced by various insects’ larvae usually for cocoons.It is mainly a lustrous elastic fiber produced by silkworms and used for textiles.

For example, orb-weaving spiders produce a variety of different silks with diverse properties, each tailored to achieve a certain task. Most arthropod species produce silks used for building structures to capture prey and protect their offspring against environmental hazards.

b. Cotton

Cotton is soft, fluffy staple fiber that grows in a boll, or protective case, around the seeds of the cotton plants. Cotton is natural cellulosic fiber.

c. Proteins

Proteins are highly complex substance made up of hundreds of thousands of smaller units called amino acids which are attached to one another to make along chain. There are around 20 different amino acids that occur naturally in proteins.  Many proteins act as enzymes that catalyze biochemical reactions and are essential to metabolic functions.

d. Natural rubber

Natural rubber is an elastic material obtained from the latex sap of trees that can be vulcanized and finished into a variety of products. It is a hydrocarbon polymer of isoprene (2-methylbuta-1,3-diene) obtained from latex. Latex is an emulsion of rubber particles in water that coagulate by addition of ethanoic acid.

e. Cellulose

Cellulose is an insoluble substance which is the main constituent of plant cell walls and the vegetable fibres such as cotton. It is a polysaccharide consisting of chains of glucose monomers.

6.3.1.2 Synthetic polymers

A part from the natural polymers, there are also synthetic polymers which are synthesized in the laboratory. They are manufactured from lower mass molecular compounds. Synthetic polymers (polymer made by two different monomers) are copolymers formed when many molecules of buta-1,3-diene or its derivatives chemical are joined with unsaturated compound. They can also be either thermosetting or thermoplastic polymers.

Examples: Polyethene is a polymer formed by linking together a large number of ethene molecules. PVC, Nylons, Terylene and Polystyrene.etc

6.3.2 Types of polymers
The polymers can be classified into three main different types such as

(i) plastics,

(ii) rubber, and

(iii) fibers.

a. Plastics
The plastics are types of polymers that are the most commonly used. Plastics are polymerized organic substances, solid of high molar mass, which at some time in its manufacture can be shaped by flow. They are electrical and thermal insulators. Their advantage is recycling and this allows them to be used many times. Plastics are materials that can be softened (melted) by heat and re-formed (molded) into another shape. The disadvantage of plastics is in their temperature resistance as they get quite fast soft and loose mechanical properties.

Examples: fibers, Polyethylene, Teflon, Plexiglas, PVC, etc.

b. Rubbers

Rubber is a tough elastic polymeric substance made from the latex of a tropical plant or synthetically made.  There are two types of rubbers; natural rubber and synthetic rubber.Rubbers are soft and springy and return to their original shape after being deformed.

•    Natural rubber

Natural rubber is an elastic material obtained from the latex sap of trees that can be vulcanized and finished into a variety of products. Natural rubber is extracted from rubber producing plants, most notably the tree “Hevea brasiliensis”, which originates from South America.

Natural rubber is a polymer of the monomer 2-methylbuta-1,3-diene (isoprene). Poly(2-methylbuta-1,3-diene) can exist in cis- and trans- isomeric forms. Natural rubber is the cis-form.

•    Synthetic rubber

A synthetic rubber is any artificial elastomer (man-made polymer having elastic properties)

There are several synthetic rubbers in production. These are produced in a similar way to plastics, by a chemical process known as polymerization. They include neoprene, Buna rubbers, and butyl rubber. Synthetic rubbers have usually been developed with specific properties for specialist applications. Synthetic rubber can be made from polymerizing buta -1,3-diene, CH₂=CH-CH=CH=CH₂.

The synthetic Rubber is an important addition of polymers that are obtained by polymerizing a mixture of two or more monomers. An example is styrene-butadiene rubber (SBR), a synthetic rubber formed by a mixture of 1,3-butadiene and styrene in a 3 to 1 ratio, respectively.

The combination of monomer units gives a new polymer product having the same empirical formula to the monomer but having a higher molecular weight. The monomer units are usually unsaturated compounds (i.e. alkenes and their derivatives).

Alkenes can be made to join together in the presence of high pressure and by adding a suitable catalyst. The π-bond breaks and the molecules are held together.

c. Fibers

Fiber (from Latin Fibra) is a natural or synthetic substance that is significantly longer. Fibers are often used in the manufacture of other materials. In manufacture of strongest engineering materials often fibers are incorporated, for example carbon fiber and ultra-high molecular-weight polyethylene.

Fibers are strong polymers that do not change shape easily. They are made into thin, strong threads which can be woven together, Nylon is an example.

•    Natural fiber

Natural fibers are substances produced by plants and animals that can be turned into filament, thread or rope and further be woven, woolen, matted or bound.

•    Synthetic fiber

A man made textile fibers including usually those made from natural materials such as rayon and acetate as well as fully synthetic fibers (such as nylon or acrylic fibers).

Checking up 6.3

1. Polymers found in natural materials can be formed by the reaction between amino acids

a. Deduce the formula of the product formed when two molecules of alanine, CH3CH(NH2)COOH react together and deduce the name of linkage present in the product.
b. Give the name of the type of naturally occurring polymer containing this linkage.

2. Write the structural formula of;

a. Polypropylene (PP)
b. Polyvinyl chloride (PVC)
c. Polystyrene (PS)
d.  Nylon 6, nylon-6,6
e. Teflon (polytetrafluoroethylene

Polymers or in general plastics have different properties depending on their nature. Among the properties, they can be thermosetting or thermosoftening whereas on the other side they can be biodegradable or non-biodegradable.

6.4.1. Thermosoftening and thermosetting properties

Thermosoftening (thermoplastics) and thermosetting (thermosets) are properties of polymers on how they soften on heating and harden on cooling.

a. Properties of Thermosoftening

polymers Thermosoftening polymers have weak intermolecular forces and they can be remolded into new shapes. They can be softened between 65 oC and 200oC and can be returned to their original state by heating.

At higher temperatures thermoplastic becomes liquid and suitable for injection molding. After cooling, melt harden and keep a given shape. Disadvantage of thermoplastic is in their temperature resistance, meaning that they get quite fast soft and loose mechanical properties. Thermoplastic have linear and complex molecules.

Thermosoftening is a property by which some polymers can be softened on heating. This allow them to cool and harden, and then can be resoftened many times.The

For the thermosoftening polymers, the Van der Waal’s forces between the chains are often very strong and the polymers have relatively high melting and boiling points.  Due to their variable chain length, most polymers have different Van der Waal’s forces and these polymers tend to melt gradually over a range of temperatures rather than sharply at a fixed temperature. As the chains are not rigidly held in place by each other, polymers tend to be reasonably soft.

The density and strength of addition polymers varies widely and they depend to a certain extent on the length of the hydrocarbon chain, but depend much more strongly on the nature and extent of the branching on the chain.

Polymers which have very few branches are very compact and the chains can thus pack together very efficiently.examples P.V.C, Dacron, Polypropene. Etc

b. Properties of Thermosetting polymers

Thermosetting Polymers are some polymers which cannot be reshaped once heated as they are completely decomposed. Thermosetting polymers include phenol-formaldehyde, urea-aldehyde, silicones and allyls. Thermosetting Polymers have cross-links (covalent bonds between chains) that do not break on heating and they comprise three dimensional network structure. The greater the degree of crosslinking makes the polymer more rigid. These polymers are generally insoluble in solvents and have good heat resistance quality.

Thermosetting polymers are generally stronger than thermoplastic polymers due to strong covalent linkage between polymer chains.
When thermosetting plastics or thermosets are molded, covalent bonds are formed between the chains. They are more brittle in nature and their shape is permanent. Once they are softened, they cannot be returned to their original state by heating. Thermosetting polymers include phenol-formaldehyde, urea-aldehyde, silicones and allyls.The following is the structure of thermosetting polymers;

Advantages and disadvantages of the above mentioned properties:

. Advantages

- Thermoplastics are convenient for manufacturers to use and they are not expensive. They are even recyclable.
- Thermosets retain their strength and shape even when heated, they have high heat resistance and structural integrity. .   

Disadvantages

- Thermoplasts melt and some degrade in direct sunlight or under high U.V light levels. Many suffer from creep, a relaxation of the material under long term loading. They tend to fracture rather than deform under high stress.
- Thermosets absorb moisture and toxicity easily.
- They are not recyclable.

6.4.2. Biodegradable and non-biodegradable properties

These are the properties of polymers depending on how they react overtime as a result of biological activity especially to be broken by microorganisms.
If they do not respond on the degradation or decomposition, they are said to be non-biodegradable polymers

a. Biodegradable polymers

Biodegradable polymers are the polymers that are fully decomposed into carbon dioxide, methane, water, biomass and inorganic compounds under aerobic or anaerobic conditions and the action of living organisms. Therefore the biodegradable properties are all characteristics of some polymers to be decomposed under aerobic or anaerobic conditions and action of living organisms.

Biodegradation or biotic degradation is a specific property of certain plastic materials. Microorganisms (bacteria, fungi, algae) recognize polymers as a source of organic compounds (e.g. simple monosaccharides, amino acids, etc.) and energy that sustain them.

Biodegradable plastics are plastics that will fully decompose to carbon dioxide, methane, water, biomass and inorganic compounds under aerobic or anaerobic conditions and the action of living organisms. Plastics are typically composed of artificial synthetic polymers.

For biodegradation to happen there are two reactions that can allow it to proceed: biodegradation based on oxidation and the other based on hydrolysis.

Those reactions can occur either simultaneously or successively. The decomposition of condensation polymers (example: polyesters and polyamides) take place through hydrolysis, while polymers with carbon atoms only in main chain (example: polyvinyl alcohol, lignin) decompose by oxidation which may be followed by hydrolysis of products of oxidation.

The advantage of biodegradable plastics is that they decompose into natural substances and do not require separate collection, sorting, recycling or any other final waste solution (disposal at landfills or burning) as is the case with nonbiodegradable plastics.

The biodegradability of condensation polymers may compromise their effectiveness, since physical and chemical durability is one of the reasons for their widespread use. A balance must be struck between practical durability and long-term biodegradability. The degradable polymers are applied in many areas (Table 6.3).

b. Properties of non-biodegradable polymers

Most of the major synthesized polymers are non-biodegradable. All kind of plastics and synthetic fibres are non-biodegradable.They are polymers which are resistant to environmental degradation thus accumulate in form of waste. These polymers cannot be changed to a harmless natural state by the action of bacteria, and may therefore damage the environment.

Since the chains of non-biodegradable polymers are non-polar, addition polymers are insoluble in water. Their intermolecular forces are strong and the chains are often tangled, they are generally insoluble in non-polar solvents as well.
In fact the long saturated hydrocarbon chains result in polyalkenes being very unreactive generally, as they cannot react with electrophiles, nucleophiles or undergo addition reactions.

This results in their widespread use as inert materials; they are commonly used as insulators, packaging and in making containers.
However their low reactivity means that they are not easily decomposed in nature and as a result have a very long lifetime. Such substances are said to be nonbiodegradable, and constitute an environmental hazard as they are very persistent in nature and thus difficult to dispose of.

Checking up 6.4

1. Polyethene is a non-biodegradable plastic.

a. Explain the term bio-degradable
b. Give one environmental benefit of using biodegradable plastics.
c. Developing biodegradable plastics involves compromise. Suggest one factor that requires careful consideration and explain your choice. 

2. Multiple choice questions (choose the letter corresponding to the right answer);

A.The word ‘polymer’ meant for material made from ______________.

a. Single entity

b. Two entities

c. Multiple entities

d. Any entity

B. One of characteristic properties of polymer material __________ .

a. High temperature stability

b. High mechanical strength

c. High elongation

d. Low hardness

C. Polymers are ___________ in nature.

e. Organic

f. Inorganic

g. Both (a) and (b) h. None

D. These polymers cannot be recycled:

a. Thermoplasts

b. Thermosets

c. Elastomers

d. All polymers

E. In general, strongest polymer group is __________ .

a. Thermoplasts

b. Thermosets

c. Elastomers

d. All polymers

F. These polymers consist of coil-like polymer chains:

a. Thermoplasts

b. Thermosets

c. Elastomers

d. All polymers.

G. Strong covalent bonds exists between polymer chains in __________ .

a. Thermoplasts

b. Thermosets

c. Elastomers

d. All polymers

H. Following is the unique to polymeric materials:

a. Elasticity

b. Viscoelasticity

c. Plasticity

d. None.

I. Elastic deformation in polymers is due to _____________ .

a. Slight adjust of molecular chains

b. Slippage of molecular chains

c. Straightening of molecular chains d.

d. Severe of Covalent bonds

J. Kevlar is commercial name for ___________ .

a. Glass fibers

b. Carbon fibers

c. Aramid fibers d.

d) Cermets 

Vulcanization is process used to convert natural rubber or related polymers to improve its resilience, elasticity and durability by heating them with Sulphur or other equivalent curatives or accelerators.  During this process, the rubber undergoes a multiple series of chemical change

The vulcanization process was discovered in 1839 by Charles Goodyear in USA and Thomas Hancock in England. Both discovered the use of Sulfur and White Lead as a vulcanization system for Natural Rubber.

Vulcanization of rubbers by sulfur alone is an extremely slow and inefficient process. The chemical reaction between sulfur and the Rubber Hydrocarbon occurs mainly at the C = C (double bonds) and each cross-link requires 40 to 55 sulphur atoms (in the absence of accelerator.

Importance of vulcanization is that, it converts the raw rubber into more durable rubber with high tensile strength. It can withstand high temperature between 40 and 1000 centigrade.

By heating rubber with sulphur, sulphur atoms are introduced between the chains and improve its elasticity .The properties of rubber improved by vulcanization include tensile strength; elasticity; hardness; tear strength; abrasion resistance; and resistance to solvents.

Another important of vulcanization of rubber is the cross links that may be derived from carbon to carbon or through an oxygen atom or through sulphur atom, or through all the three, therefore, percentage elongation at break decreases. The vulcanization decreases the tendency of water absorption of rubber.

Checking up 6.5

1. a. The vulcanization is a process used to convert the natural rubber into more durable materials. Explain five important uses of vulcanization.   

b. Name two substances added to natural rubber during vulcanization process as: i. Accelerators ii. Fillers iii. Anti-oxidants

2. a.  What is vulcanized rubber?       

c. Give two useful items of vulcanized rubber

3. Describe briefly the process of vulcanization of rubber.

Polymers are widely used materials in our daily life. To date, the importance of polymers is highlighted in their applications in different areas of sciences, technologies and industry from basic uses to biopolymers and therapeutic polymers.

6.6.1. Uses of polymers

Polymers found many uses in our daily life:

Plastics are inexpensive, lightweight, strong, durable, corrosion-resistant materials, with high thermal and electrical insulation properties.
In general due to the properties of polymers, they are used to make a vast array of products that bring medical and technological advances, energy savings and numerous other societal benefits.
The following Table 6.4 shows the uses of the following commonly known polymers:

Table 6.4: Uses of common used polymers

6.6.2. Effects of polymers on environment

Polymers or polymeric materials have eased the life of people but there are dangerous to the environment.

However, concerns about usage and disposal are diverse and include accumulation of waste in landfills and in natural habitats, physical problems for wildlife resulting from ingestion or entanglement in plastic, the leaching of chemicals from plastic products and the potential for plastics to transfer chemicals to wildlife and humans. In the polymerization of compounds, some additive chemicals can be potentially toxic (for example lead and tributyl tin in polyvinyl chloride, PVC). Further substantial quantities of plastic have accumulated in the natural environment and in landfills.

The rural areas are more prone to this type of contamination and the related effects, as a majority of the people from these areas as there is over use of plastics on a large scale. Discarded plastic contaminates a wide range of natural terrestrial, freshwater and marine habitats.

When dumped in landfills, plastic materials interact with water and form hazardous chemicals which may be toxic to humans and other aquatic organisms. If these compounds seep down towards groundwater aquifers, they degrade the water quality, leading to groundwater pollution.

Many of plastics waste lead to the formation of persistent organic pollutants (POPs), compounds which are very dangerous to the whole environment. These compounds persist in the environment and due to their property of bio-accumulation; they have high levels of toxicity in the food chain. Blockage due to plastic accumulation may form breeding grounds for mosquitoes and other harmful vector insects, which might cause numerous diseases in humans.

Burning plastic leads to the contamination of the atmosphere, due to the release of other poisonous chemicals, leading to air pollution. Recycling them requires carefull attention as they lead to the development of skin and respiratory problems due to inhalation of toxic chemicals.

When plastic is burned they release toxic chemicals that are deposited in soil and surface water and on plants.
Non-biodegradable polymers or long term biodegradable materials, especially plastic bags when they in the soil, they do not allow rain water for penetration which causes soil erosion.

NB: Rwanda has taken a tremendous decision of stopping the use of plastic bags in the country, which has got promising result for the environment and the users in general.

Checking up 6.6

1. Give one large scale use for polyester polymers and state the property of polyesters on which the use depends.

2. Enumerate three environmental problems caused by the widespread use of plastics as polymers.

Plastics have transformed everyday life; its usage is increasing gradually all over the world. It is evident that plastics bring many societal benefits and offer future technological and medical advances. However, concerns about usage and disposal are diverse and include accumulation of waste in landfills and in natural habitats, physical problems for wildlife resulting from ingestion or entanglement in plastic, the leaching of chemicals from plastic products and the potential for plastics to transfer chemicals to wildlife and humans.

The management of waste materials including polymers involves improvement of effects caused by these materials.  A major part of plastic produced is used to make some disposable items of packaging or other short-lived products that are discarded within a period of manufacture. Due to the durability of the polymers, we have seen that substantial quantities of discarded end-of-life plastics are accumulating as debris in landfills and in natural habitats worldwide. A number of waste prevention techniques such as reuse, recycling and disposal of plastics are discussed below.

a. Reuse

This is a process of using the polymers more than once. This encompasses the entire spectrum of used goods. Spanning from collectables, antiques and memorabilia to general used goods retail and wholesale. Dealing in secondhand items typically involves the salvage of used items and may dismantle into components. Beyond salvage and to enhance reuse the industry includes repair and refurbish, remanufacturing.

There is considerable scope for re-use of polymer materials for the transport of goods, and for potential re-use or re-manufacture from some plastic components in high-value consumer goods such as vehicles and electronic equipment.

b. Recycle

Recycling is one of the most important activities that can be applied to reduce the impacts of the plastics used in industry. Recycling is a waste-management strategy; it is a technique that can be used to reduce the environmental impact and resource depletion.

Recycling provides opportunities to reduce oil usage, carbon dioxide emissions and the quantities of waste requiring disposal. Today’s recycling industry has evolved largely into a service industry involved in the collection, sorting, processing and transportation of waste streams and by-products.

Different polymer materials need to be collected, separated and cleaned. Further, they are melted down before being changed into a new material. Some plastics cannot be melted – they burn or harden instead of melting. It is even more difficult to recycle these plastics as they can only be used in the same shape as they were originally cast.

The biodegradable materials can be recycled, broken down into their original components and reused, but they still need to be collected, separated and cleaned.

Plastic materials can be recycled in a variety of ways and the ease of recycling varies among polymer type, package design and product type.

c.  Disposal

Old polymers disposal is the action of getting rid of old polymers. The disposal of non-biodegradable polymers is a significant problem.

There are three options:

i. Burying in landfill sites

This is widespread in all developed countries but is a completely unsustainable practice, as each landfill site will eventually fill up. Landfill sites are also unsightly and unhygienic.

iv. Burning

This is a technique to dispose some materials. However, burning polymers releases greenhouses gases such as carbon dioxide and can also release toxic gases, depending on exactly type of polymer being burned.

Checking up 6.7

List the advantages and disadvantages for each of the method to deal with the old polymers

END UNIT ASSESSMENT

1. Explain the terms crosslinking and thermosetting with reference to condensation polymers.  

For what purposes are thermosetting polymers suitable?  

2.  a. How the chemical inertness of poly(ethene) arise?      

     b. How does it increase the usefulness of the material?
      c.  How does it affect the disposal of waste polyethene?

3. a. What type of functional group joins the repeating units in nylon?

         b.  In what way does the structure of nylon resemble that of a polypeptide?
          c.  What type of interaction takes place between polymer molecules which contain the functional group present in polypeptide?

4. Identify 3 examples of synthetic polymers

5. What is the benefit of cross-linking polymer chains?

6. a) Give one example for each of the following type of polymer. Write the structural formula of the polymer and monomer unit. Give at least one use of the polymer named

i. Natural addition polymer

ii. Synthetic addition polymer

iii. Natural condensation polymer

iv. Synthetic condensation polymer        

b. State the role of each of the following in the manufacture of plastics

i.  Fillers

ii.  Plasticizers

c. i.  what is meant by vulcanization?

 ii. Discuss the effect of vulcanization on rubber molecules and state how it affects the physical properties of rubber.
iii. Name the monomer units in natural rubber
iv. Name one commercial synthetic rubber. Write equation to show how it is formed and give one use of it