Chemical Bonds

A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction.

Classification of Bonds

Chemical bond can be broadly classified as follows,

· Intrapolymer bonds

· Interpolymer bonds

Intrapolymer Bonds

These are bonds that are responsible for holding the atoms together to make up the fibre polymer. They are predominantly composed of carbon and hydrogen atoms. They can be taken as the bonds that are present within the fibre polymer. The major bonds that are used for intermolecular bonding are as follow,

· Covalent bonds

· Amide or peptide group

· Benzene ring

· Ether linkages

· Ester groups

· Hydroxyl group

· Nitrile group

Covalent bonds:

Covalent bonding is a common type of bonding, in which the electro negativity difference between the bonded atoms is small or nonexistent. The stability and unreactive nature of the single covalent bond joining the atoms of fibre polymers may be expressed objectively. They are practically indestructible when it occurs between carbon and carbon atoms, carbon and hydrogen, carbon and oxygen, carbon and chlorine, carbon and fluorine atoms. Their bond energy or bond strength is between 330 and 420 kilojoules.

The amide or peptide group:

In chemistry, an amide is an organic compound that contains the functional group consisting of a carbonyl group (R-C=O) linked to a nitrogen atom (N). When present in nylon polymers it is called the amide group. It is also present in silk, wool, mohair and all other animal or protein fibres and then it is called peptide group.

Benzene rings:

The benzene ring occurs at regular intervals in textile polymers like poly ethylene terephthalate. They are sometimes referred to as the aromatic radical. It is a hexagon shaped molecule composed of mainly carbon and hydrogen.

Ether linkages:

The ether linkages may be found in polymers such as cellulose, elastomeric, ester-cellulose and polyesters. It exists between carbon and oxygen atoms. Ethers are chemically unreactive. One reason for this is the great chemical stability of the carbon-oxygen linkages found in every ether molecule.

Ester groups:

They are formed by replacing the hydrogen of an acid with an organic radical. In fibre polymers they are usually the reactions between:

· A carboxyl group (-COOH), also called carboxylic acid
                And

· A hydroxyl group (-OH)

Hydroxyl group:

The hydroxyl group is the univalent OH group. The presence of OH groups on the fire polymers is of two fold significance:

· The OH groups are polar and will therefore attract water molecules, which are also polar. Thereby the OH groups are responsible for the moisture absorbancy of the fibre.

· The polarity of OH groups will give rise to the formation of hydrogen bonds. The formation of such hydrogen bonds will contribute significantly towards the coherence of the fibres polymer system.

The presence of such groups would increase the hydrophilicity of the synthetic fibres for which efforts are being made.

Nitrile group:

They are basically present in side groups of acrylic and modacrylic fibres (-CN). These groups do not react with acids or break down in acidic condition.

Inter polymer bonds

In basic senses these bonds are responsible for holding the polymers together for the formation of a fibre. The major bonds used for interpolymer bonding are as follows,

· Van der Waals forces

· Hydrogen bonds

· Salt linkages

· Cross links

Van der Waals forces:

They are weak forces which exist in the interpolymer forces of attraction when the atoms come close to one another. They are formed between atoms along the length of adjacent polymers when these are less than 0.3 nm apart but no closer than about 0.2 nm. They occur between all fibre polymer system and their bond energy in 8.4 KJ.

Hydrogen bonds:

They are formed between hydrogen and oxygen atoms, and hydrogen and nitrogen atoms on adjacent polymers when these are less than 0.5 nm apart. They occur within the natural polymers, regenerated cellulose polymers, nylon polymers, polyvinyl alcohol, polyester polymers, protein and secondary cellulose acetate fibres. Their bond energy is 20.9 KJ. The hydrogen bonds are mainly responsible for the tenacity and the elastic-plastic nature of the natural, regenerated cellulose, nylon, PVA and protein fibres.

Salt linkages:

They are formed between the carboxyl radical on one polymer and the positively charged amino group on an adjacent polymer. They exist mainly in the protein and nylon fibre polymers. Their bond energy is 54.4 KJ. They are responsible for the attraction of the water molecules and they too contribute to the strength of the fibre. The presence of salt linkages is necessary for dye absorption

Cross links:

They can be classified as the strongest bonds and exist in all protein fibres except silk. Their bond energy is 245.3 KJ. They contribute to the elastic nature of the fibre. The cross links are capable of breaking and reforming with adjacent atoms.