GITAM University (Dr Kasthuri Mahesh) Page 1 Phenolic Resins (or) Phenoplasts :
Phenolic resins (or) Phenoplasts are condensation polymerization products of phenolic derivatives (like phenol, resorcinol) with aldehydes (like formaldehyde, furfural). Most important member of this class is bakelite or phenol – formaldehyde resin.
Bakelite (or) Phenol – formaldehyde resin:
It is prepared by condensing phenol with formaldehyde in presence of acidic / alkaline catalyst. The initial reaction results in the formation of o – and p – hydroxyl methyl phenol, which reacts to form linear polymer, novolac. Thus;
Properties :
(1) It is set to rigid, hard, scratch- resistant to non – oxidizing acids, salts and many organic solvents, but are attacked by alkalis, because of the presence of free hydroxyl group in their structures.
(2) Bakelite possess excellent electrical insulating character.
(3) Phenolic resins withstand very high temperatures and hence are used in missile cones.
Uses :
(1) For making electrical insultator parts like switches, plugs, switch boards, heater – handles, etc. (2) For making moulded articles like telephone parts cabinets for radio and television
(3) For impregnating fabrics, wood and paper. (4) As adhesives (e.g. binder) for grinding wheels (5) In paints and varnishes.
(6) As hydrogen – exchanger resins in water softening.
(7) For making bearings, used in propeller shafts for paper industry and rolling mills
(8) Bakelite can withstand at very high temperatures and hence is used in missile nose cones.
Polyamides : [Nylons]
Polyamides are the polymers, which contain many repeating amide groups in the main polymer chain. All polyamides are known as nylons. Different types of nylons indicated by a numbering system. This number gives the no. of carbon atoms present in the monomer molecule.
Example (i) Nylon-6:6 [Monomers: Hexamethylyne diamine (C6) and adipic acid (C6)] (ii) Nylon-6 [Monomers: caprolactum it’s aminoacid (C6)]
GITAM University (Dr Kasthuri Mahesh) Page 3 Preparation of Nylon-6:6 starting from 1,3 butadiene
NC CN
H2N NH2 HOOC
COOH
N
H NH
O
O
n
Nylon-6:6
Reduction Hydrolysis
Polymerization
1,3-butadiene
Properties of Nylons :
(1) they are translucent, whitish, high melting (160 – 2640C) polymers. (2) They possess high temperature stability and good abrasion – resistance.
(3) They are insoluble in common organic solvents. (like methylated spurut, benzene and acetone) and soluble in phenol and formic acid.
(4) Their mouldings and extrusions have good physical strengths (especially high impact strength) and self – lubricating properties.
(5) They absorb little moisture; and are thus “drip – dry” in nature. (6) They are very flexible and retain original shape after use. Uses of Nylons :
(1) Nylon – 6;6 is primarily used for fibres, which find use in making socks, ladies hoses, under garments, dresses, carpets, etc.
(2) Nylon – 6 and Nylon – 11 are mainly used for moulding purposes for gears, bearings, electrical mountings, etc. Nylon bearings and gears work quietly without any lubrication. (3) They are also used for making filaments for ropes, bristles for tooth brushes and films, tyre –
cords, etc.
Polycarbonates (PC)
Prepared by interaction of Diphenyl carbonate with bisphenol-A [2,2-bis(4-hydroxyphenyl)propane]
Properties:
GITAM University (Dr Kasthuri Mahesh) Page 7 Uses:
i. For prepare moulded domestic ware, housing for apparatus, and electrical insulator in electronics and electrical industries
INORGANIC POLYMERS They are gaint-molecules composed of atoms, except carbon. Atoms in these polymers are linked together by covalent bonds. Example:
Silicones (Poly Silaxanes), Polyphosphazine, Polysulphur nitride (not in syllabus)
SILOCONES:
Silicone resins contain alterantive Silicon-Oxygen structure
Preparation:
Preparation of monomers: Monomers can be prepared as follows Reacting silicon with alkyl halide
The reaction product is fractionally distilled to get different Organo-Silon chlorides, which are then polymerized by controlled hydrolysis.
O Si CH3
CH3
O Si O
CH3 CH3 Si O CH3 CH3 Cl Si CH3 CH3
Cl HO Si
CH3 CH3 OH Si CH3 CH3 O Si CH3 CH3 O +H2O
-HCl
-H2O n n n n Unstable Umstable Ploymerization Dimethyl Silicon Dichloride
OR
Dimethyl silicon ploymer (Silicon Rubber)
2. Trimethyl silicon chloride: ‘monofunctional’ and hence chain stopper. This is used in limit the chain length
3. Monomethyl silicon chloride: Trifunctional; gives cross-linked polymer
CH3 CH3 CH3 CH3
| | | |
- O –Si - O –Si - O –Si - O –Si – O -
| | | |
O O O O
| | | |
- O –Si - O –Si - O –Si - O –Si – O -
| | | |
CH3 CH3 CH3 CH3
Characteristic of Silicones:
i. Depends on the various alkyl halides use Silicones are liquids, viscous liquids, rubber-like and solid in nature
GITAM University (Dr Kasthuri Mahesh) Page 9 iii. They are good water-resistance, good oxidation stability but their chemical resistance
lower than other plastics
iv. Their specific gravity ranges from 1.03 to 2.1
v. Their physical properties are much less effective by variation in temperature vi. They rae non-toxic in nature
Types of Silicon and their Uses:
1. Liquid Silicones or Silicon oils: Relatively low molecular weight silicones, generally of dimethyl silicones. They posses great wetting-power for metal, low surface tention and show very small change in viscosity with temperature
Uses: High temperature lubricants, antifoaming agents, water-repellent finishes for lather and textiles, heat transfer media. Also used in cosmetics and polishes
2. Silicon greases: Modified silicon oils, obtained by filler like silica, carbon black, lithium soap etc.
Uses: Used as lubricants in situation where very high and very low temperatures are encounter.
3. Silicon Rubbers: Obtained by mixing high molecular weight dimethyl silicon polymers with fillers (silicon dioxide) and peroxide containing curing agents. Peroxide cause the formation of dimethyl bridge (cross-link) between methyl groups of adjacent chains.
Properties:
i. They possess exceptional resistance to prolonged exposure to sunlight, weathering, most common oils, boiling water, dilute alkalis and acids.
aircrafts, since they prevent the damage on landing.
iii. Upon decomposition they converted into non-conducting silica (SiO2), instead of carbon tar Uses:
i. As a sealing materials in search-lights and in aircraft engines ii. Manufacturing of tyres for fighter aircrafts
iii. For insulating the electrical wiring in ships
iv. In making lubricants, paints and protective coatings and water-proofing
v. Adhesive in electronic industries, insulation for washing mechines, electric blankets for iron board covers, artificial heat valves and padding for plastic
vi. For making boots for use at very low temperature as they less affected by temperature variations: Neil Armstrong used silocone rubber boots when he walked on the moon
4. Solid silicon resins: are cross-linked silicons, obtained from b- and tri-functional silicon halides
GITAM University (Dr Kasthuri Mahesh) Page 11 Differences between Thermoplastics & Thermosettings
Thermoplastics Thermosetting
1. These resins become soft on heating and regid on cooling reversibly
2. The heating and cooling don’t alter the chemical nature of these resins because the changes involved are purely of physical nature.
3. These are formed by chain polymerization
4. They consist of long chain linear polymers with weak secondary – vander waal’s forces of attraction between
5. They are softened on heating readily because the secondary force of attraction between the individual chain can break easily by heat, pressure or both
6. They can be reshaped
7. These are soft, weak and less brittle
8. These resins are usually soluble in organic solvents
9. Polyethylene, polyvinyl chloride are the examples.
During fabrication process these resins are moulded once they are solidified they cannot be softened
These resins are permanent setting resins
These are formed by step polymerization
They have three dimensional network structure
They bonds retain their strength on heating hence they do not soften again on heating.
Cannot be reshaped
These are hard, strong and more brittle
Due to strong bonds and cross links, they are insoluble in almost all organic solvents.
Bakelite, Nylons, etc, are the examples.
MOULDING CONSTITUTENTS OF A PLASTIC
It is unusual to have a finished high-polymer article made up of solely of high-polymer only. It always mixed with 4-10 ingredients.
Each of which
o either discharge a useful function during the moulding (or) o Import some useful function to finished article
Important compounding ingredients and their functions
1. Resin: It is binder, which holds the different constituents together.
2. Plasticizer are material that added to resins to increase their plasticity and flexibility. They neutralize a part of the intermolecular attractions of macromolecules of resins, thereby increasing flexibility and plasticity of resins, but at the same they reduce strength and chemical resistance of resins.
Examples: Vegetable oils, camphor, esters (steric or oleic or phthalic acids) and some phosphates (tricresyl phosphate, tributyl phosphate, tetrabutyl phosphate and triphenyl phosphate).
3. Fillers are added to give to the final plastic better hardness, tensile strength, opacity, finish and workability and reducing the cost, shrinkage on setting and brittleness.
Example: (i) carborandum, quartz and mica added to provide extra hardness (ii) barium salts are added to to make plastic impervious to X-rays (iii) addition of asbestos provides heat and corrosion resistance
Most commonly used filler are wood-flour, china clay, talk, gypsum, saw dust, ground cork, marble flour, paper pulp, corn husk, mica, carbon black, pumic powder, cotton fibre, metallic oxides (like ZnO, PbO) and metal powder (like Al, Cu, Pb)
4. Lubricants like waxes, oils, stearates and soaps are employed to make the moulding of plastic easier and impart a flawless, glossy finish to the product
5. Catalysts or accelerators are mainly used in thermosetting plastics moulding process. They accelerate polymerization of fusible resins into infusible resins during the moulding process. Examples: hydrogen peroxide, benzoylperoxide, metals like Ag, Cu, Pb, metallic oxides like ZnO.
6. Stabilizers: they are added to improve thermal stability during the moulding process of vinyl chloride and vunylidine polymers
Example: (i) Opaque moulding compound like salts of lead, white lead, lead chromate, red lead, lead silicate (ii) transparent moulding compounds like stearates of lead, cadmium and barium.
GITAM University (Dr Kasthuri Mahesh) Page 13 MOULDING OF PLASTICS INTO ARTICLES
Moulding is an important method to convert fabricating plastics into desired-shapped article and is done in several ways as given below.
A). Compression moulding B) Injection Moulding C) Transfer Moulding D) Extrusion Moulding.
A). Compression moulding : This method is applied to both thermoplastic and thermosetting resins. The predetermined – quantity of plastic ingredients in proper proportions get fill between the two half – pieces of mould, which are capable of being moved relative to each other.
Then high pressures and temperatures are applied to complete the curing process. The cavities get filled with fluidized plastic. Two halves are closed very slowly. Finally, curring is done either by heating (in case of thermosetting) or cooling (in case of thermoplastics). After curing the moulded article is taken out by opening the mould parts.
*Compressing moulding of plastics*
Advantages : This method is the most widely used for moulding of thermoplostics, because of high speed production, low mould cost (since fever cavities are needed), very low loss of material and low finishing cost.
GITAM University (Dr Kasthuri Mahesh) Page 15 Advantages : 1) Since the mix flows into the mould cavity in a highly plasiticized condition, so very delicate articles may be handled without distortion or displacement. Fine wires and glass fibers may be inserted into the mould.
2) Article produced is free from Flow Marks
3) Even Thick pieces curve almost completed and unifromly.
4) Owing to uniform and through cure, shrinkage and distortions are at a minimum and hence, the mechanical strength and density of fabricated pieces is higher
5) Finishing cost of Fabricated article is almost entirely eliminated 6) Mould cost is less, since it involves very low abrasive action.
