Introduction:
Organic materials are those materials that are derived directly from carbon. They consist of carbon chemically combined with hydrogen, oxygen or any other non-metallic substances, their structures, in most cases, fairly complex since the carbon shares the valance electrons very readily. The covalent or Homo-polar bond is the most usual type of bond in Organic materials. The compounds, which essentially contain carbon atoms along with other non-metallic atoms such as hydrogen, nitrogen, oxygen, chloride, etc in each molecule, are known as “organic compounds”. Important organic compounds are saturated organic compounds, unsaturated organic compounds and aromatic organic compound.
Different Organic Materials Used in Engineering:
Natural Organic materials: They include wood, cotton, natural rubber, coal, petroleum, and food products etc.
Synthetic Organic materials: They include synthetic rubber, plastics, and lubricants. soap oils, synthetic fibers, etc.
Polymers
A polymer is a large molecule built up by the repetition of small, simple chemical units. In some cases the repetition is linear, as a chain built up from its links. In other cases the chains are branched or inter connected to form three-dimensional networks. The repeat unit (small molecules) of the polymer is usually equivalent or nearly equivalent to the monomer, or the starting material from which the polymer is formed. For example, the repeat unit of polyethylene (-CH2-CH2-) is ethylene i.e. CH2 = CH2. Polymers are generally non-crystalline solids at ordinary temperature, but pass through a viscous stage in course of their formation when shaping is readily carried out. Polymers are not only made from carbon, but also from inorganic chemicals such as silicates and silicon’s.
Some terms regarding organic polymers are:-
Degree of polymerization:
It is the number of repetitive units present in one molecule of a polymer.
Molecular weight of a polymer
Degree of polymerization= ---------------------------------------------
Molecular weight of single monomer
Linear polymer:
It is a polymer, which is obtained by simply adding the monomers together to form long chains.
Co-polymer:
It is a polymer, which is obtained by adding different types of monomers.
Cross-linked polymer:
It is a polymer, which is obtained by connecting long chains through a covalent bond.
Thermo plastics:
These are linear polymers whose plasticity increases with rise in temperature.
Thermoset plastics:
These are the polymers whose plasticity does not change with rise in temperature.
These are two types of polymers.
Natural polymers: This includes protein, cellulose, resins, starch, shellac and lignin. They are commonly found in leather, fur, wool, cotton, silk, rubber, rope, wood and many others.
Synthetic polymers: This includes ploy-ethylene, ploy-styrene, nylon, teriline, dacron, etc. They are commonly found in plastics, fibers, and elastomers. The properties of synthetic polymers are superior to those found in natural polymers.
Mechanism of polymerization:
Polymerization is nothing but the process of forming a polymer. In this process, the monomers are linked together to form a polymer. The need to start with the process of polymerization lies on the necessity of breaking the double bonds (C=C) of monomers. This requires considerable energy and is equal to 612kJ/mole. However 348kJ/mole are released every time a single C-C is formed. The linking of monomers will continue till the end of availability of molecules in the immediate ends of chains.
Monomers should also have two or more reaction sides at which junction may be made to form a chain of monomers. The example above shown has two reaction sides and is called as Bi-functional monomer. Other molecules with three or four reactions sides are called A Tri and Tetra functional monomer respectively. Still higher functionality, while theoretically possible does not generally occur in practice because of space limitations.
15.4 Types of polymerization:
There are two types of polymerization, which results in the formation of a certain type of polymeric structure.
Addition polymerization
Condensation polymerization
Addition polymerization:
It is a polymerization process, where the polymer has the same composition as monomer or monomers from which it is formed. It is also called chain polymerization. It takes place in unsaturated organic compounds. These compounds are relatively unstable as compared to saturated organic compound. It has been observed that under suitable conditions, such as high pressure, temperature, &presence of catalysts, the unsaturated organic compound reacts in a manner to form long chains. In this process, their double covalent bond is broken &single bonds are formed in its place.
Addition polymerization process
Temperature, Pressure, catalyst
Monomer + Monomer------------------------------------------------------------ → Polymers
Co- polymerization is another kind of addition polymerization in which two or more chemically different monomers are polymerized to form long chains molecules. For example:
Three steps in polymerization
- Initiation: When free radicals (having odd number of electrons) are generated in the presence of a vinyl monomer, the radicals add to the double bond with regeneration of another radical. The regeneration of radicals is characteristics of chain reaction.
- Propagation: The chain radical formed in the initiation step is capable of adding successive monomer to propagate the chain. Propagation would continue until the supply of monomer was exhausted.
- Termination: It takes place in two ways namely combination or coupling or disproporstionation, in which hydrogen transfer result in the formation of two molecules with one saturated and one un saturated end group.
Condensation polymerization:
In this case the polymer formed does not have the same composition as the monomer or monomers from which it is formed. The polymerization is accompanied by the elimination of small molecule like H2O, HCl, etc. Depending on the type of monomer, the kind of polymer formed maybe linear branched or cross-linked polymer. It takes place in unsaturated organic compounds and requires suitable conditions, such as high pressure, temperature, and presence of catalyst in the same way as that of addition polymerization.
Condensation polymerization process
Example:
Temperature, Pressure, catalyst
Monomer + Monomer------------------------------------------------------- →Polymer + Byproduct
NH2- (CH2) 6-NH2+HOOC- (CH2)-COOH (NH2)(CH2) 6NHCO(CH2) 4COOH + H2O
(HEXA METHYLENE DIA AMIME) + ADIPIC ACID NYLON
Condensation polymerization is a stepwise regular process where there is steady increase in molecular weight. The monomer disappears in the early stage of the reaction and polymer is formed. The catalyst used in the condensation process is a true catalyst remaining unchanged. A condensation product may be either a thermo set or thermoplastic. Polyesters, Amino plastics, polyamides etc are some of the condensation products.
ADDITIVES TO POLYMERS:
Most polymers are seldom used as pure organic resins, but they are modified and compounded with various additives to modify the mechanical, chemical and physical properties for particular uses. The various additives used for this purpose are (1) Plasticizers, (2) Fillers (3) Catalysts, (4) Initiators, (5) Pigments. One or more of these additives are added in different portions to obtain the desired properties of the polymers.
1. Plasticizers:
They are added to improve the flow and hence improves prossesability and to reduce the brittleness of the product. Plasticizers are compounds of low molecular weight and are oily in nature. These act as internal lubricants and prevent crystallization by keeping the chains separated from one another.
The effect of plasticizers on the thermo plastic materials is to give them a more flexible or rubber like nature. The pure polyvinyl chloride is hard substance at room temperature, but when it is plasticized with tricresyl phosphate; it becomes flexible.
Sl #
|
Type of plasticers
| Uses |
01
|
Phthalate esters
|
Accounting for over half of the total volume of plasticizers used.
|
02
|
Phosphate esters
|
Flame proofing.
|
03
|
Adiphates, azelates, oleates sebacates
|
Used in vinyl resins for improving low temperature flexibility
|
04
|
Fatty acid esters, hydrocarbons.
|
Secondary plasticizers
|
2. Filler materials:
These are most often added polymers to improve tensile and compressive strength, abrasion resistance, toughness, dimensional and thermal stability and other properties. Filler materials include finely powdered saw dust, silica flour and sand, glass, talc, lime stone, etc. These inexpensive filler materials replace some volume of the expensive polymer, hence cost is reduced.
In resent years the use of the fillers has increased markedly with emphasis on calcium carbonate, which dominates, the market. The custom compounding companies now offer’s filler concentrates much like colour concentrates, to provide an easy way for small processes to incorporate fillers into their product.
3. Catalyst:
These are added to expedite and complete the polymerization reaction. The catalysts are also called as accelerators and hardners.
Following table gives some examples of catalysts and their uses in polymerization:
S.No.
|
Catalyst
|
Polymer to be produced
|
1.
|
Boron tri-fluride, Friedel crafts
|
Isobutylene.
|
2.
|
BF’s and its complexes (Lewis acids)
|
Epoxy resins.
|
3
|
Tertiary amines or stannous soaps
|
Polyurethane elastomers.
|
4
|
Peroxide
|
Diallylpathalate and diallisophthalate
|
5
|
ZiIegler-Natta or Ziegler catalyst
|
Ploy propylene.
|
4. Initiator:
These are added to initiate to the reaction among the monomers and to stabilize the reaction of the molecular chain. The hydrogen per oxide(H2O2 )is a common initiator. Initiator relatively unstable material which will decompose and form a free radical. Further the free radical promotes the chain reaction in polymerization process.
In many instants, the species that initiates the ionic polymerization is produced by ionization of the added initiator. For example H+ from BF3H2O or C5H11 from potassium amyl.
5. Dyes and Pigments:
These are to impart the desired color to the finished polymers. Organic pigment such as Phthalocyanies lead to transparent color. Inorganic pigments impart opacity to the plastics. Common color (dyes) are titanium dioxide and barium sulphate (white), thylocynan blue and green, ultramarine blue, chrome green, quinocridone reds and magentas, molybdate oranges, cadmium reds and yellows, iron oxide and chrome yellow, carbon blacks, flake aluminium for silver metallic and lead carbonate mica for pearlesence.
Classification of polymer structure:
According to the shape of the macromolecules, polymer structure may be broadly classified as
- Linear and frame work structure.
- Branched-chain structure.
- Cross linked chain structure.
- Crystalline structure.
1.Linear and frame work structure:
Linear polymer structure consists of long chains in which individual long chain molecules are quite separate from one another. Only van-der wall’s bond hold adjacent chains together, this makes it possible to dissolve or melt them by mating and produces slip between molecules. Linear polymers commonly have a tangled structure which gets some of their mechanical strength from the inter wining of molecules. A bifunctional monomer such as ethylene produces a linear polymer. Tri functional and tetra functional monomers form network or frame work polymers in which individual molecular chains are difficult to distinguish. These framework structure polymers have distinctive mechanical and thermal properties.
Some of the common polymers that from with linear structures are polyethylene polyvinyl chloride, polystyrenes, polymethyl meyhacrylete, nylon, and the fluoro carbons. Phenol formaldehyde is one of the polymers, which is having framework structure.
2.Branched chain structure:
Polymers may be synthesized in which side branch chains are connected to the main ones. The branches considered to be part of the main chains molecule result from side reactions that occur among the synthesis of the polymer. The number branches and the ratio of the length of main chain to that of the side chain may vary.
In this synthesis, branches of different monomers species can be add (grafted) on to the linear chain of macromolecule. Such branched polymers are referred to as “graft co-polymer”. The importance lies to branching is extensive; the polymer will become stronger and less plastic due to the simple interlocking actions of the branched chains with each other. The controlling of branching with in polymers is of great industrial importance.
3.Cross-linked chain structure:
In cross-linked polymers, adjacent linear chains are joined one to another at various positions by covalent bonds. The process of cross-linking is either during synthesis or by a nonreversible chemical reaction that is usually carried out at an elevated temperature. Often additive atoms or molecules that are covalently bonded to the chains accomplish this cross-linking.
Cross-linking may be deliberately to increase strength and reduce plasticity or it may occur naturally. Many of the rubber elastic materials are cross-linked. In rubber cross-linking is called vulcanization.
4.Crystalline structure:
The crystallinity of polymers is largely determined by the geometry of the polymer chains. The crystallinity totally depends upon the linear polymer chains. As the packing becomes more apart, due to irregularities, then the structure become amorphous. It has been observed that polythene and ploy-tetro-fluro-ethylene (Teflon) having the regions of about 10 nanometers possesses three-dimensional structure. In co-polymers, units of two or more monomers do not necessarily occur in a regular sequence, hence they are more likely to be non-crystalline.
The behavior of partially crystalline polymers is intermediate between those for crystalline and amorphous polymers. When flexibility is in a material is required, the crystallinity is undesirable when a polymer comprising linear chain is elongated, and there is an initial and almost linear elastic region. The elongation becomes constant at higher values of loads. The polymers, which pass this type of behaviour, are known as “elastomers”. The above results are shown in load-extensive curve. During extension of polymer, the molecular chain gets oriented in the direction of its elongation. This alignment gives them a crystalline structure.
S.No |
Polymer characteristics
|
Example
| Typical applications |
1
|
Some crystallinity, flexible chains
|
Poly ethylene
|
Pipes, thin films, food containers, buckets, etc.
.
|
Poly propylene
|
Fibers, rope, films, household’s wares, hot water tubing etc.
| ||
Poly vinyl chloride
|
Pipes toys, electrical conductors, seating, etc,
| ||
Nylon
|
Low strength Mechanical components, fabrics, stockings etc
| ||
2
|
Crystalline domains well developed
|
Terylene
(Dalron)
Cellulose acetate
|
Both used for fibers for clothing and for thin films etc.
|
3
|
Moderate cross linking with crystalline domains
|
Neoprene
Polyisoprene
|
Oil resistant synthetic rubber, high resilience synthetic rubber.
|
4
|
Cross- linked amorphous structure.
|
Phenol
Formaldehyde
Urea formaldehyde
|
Radio and TV cabinets, telephone cases, ash trays etc
Toys, buttons, light switches, waterproof glues for plywood, etc.
|
STRENTHENING OF POLYMERS:
Strengthening mechanisms are employed to the polymers to make them more rigid, more resistant to temperature, more resistant to corrosive chemicals.
Strengthening methods:
-Crystallization
-Cross-linking
-Chain-stiffening
CRYSTALLIZATION:
Crystallization in polymers gives a regular arrangement of molecules in their structure. The molecules are arranged in a regular manner by using the van-der waal’s force. Crystallization is more easily achieved in small molecules than in the larger molecules. The crystallization depends up on orientation of chain. It is not temperature dependent, &can be reversed with decomposition of the polymer. Crystalline polymer is usually stronger and more than that of the amorphous polymer. Crystallinity is an important factor to determine the stiffness and yield point for most crystalline plastics. As crystallinity decreases, both the stiffness and the yield stress decreases. As a result of the latter change, the chance of brittle failure is reduced.
Cross- linking
In cross-linking the adjacent linear molecular chains are joined at a various positions by covalent bonds. Cross-linking depends upon the chemical reaction, rather than on the chain orientation. It is strongly accelerated by the temperature, and is not reversible due to cross-linking. The movements between the chains are greatly restricted and mechanical properties are changed towards the advantages.
Chain- stiffening:
Chain stiffening of polymers can be produced by a number of methods. “Hang” is one of the methods where the atom on the chain is restricted from bending, but they can twist on its bonds. Some monomers have ring shafed groups and thus have inherit rigidity.
The kinds of stiffness (modulus- slope of the tensile stress- strain curve) of polymer materials is as shown in the table below.
Description of polymers | Stiffness | Yield stress | Ultimate strength | Elongation |
Soft, weak
Soft, tough
Hard, brittle
Hard, strong
Hard, tough
|
Low
Low
High
High
High
|
Low
Low
None
High
High
|
Low
Moderate
Moderate
High
High
|
Moderate
High
Low
Moderate
High
|
COMMENTS