Q.Write examples of thermosetting plastic articles.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Classification of Polymers
Classification of Polymers – A First Look
You already know what a polymer is: a long chain made of many small repeating units (monomers). But not all polymers are the same. Think about the difference between a rubber band, a plastic water bottle, and a cotton T-shirt. They feel different, stretch differently, and are made from different starting materials. That's why we classify polymers — to understand their behaviour and how they're made.
The classification is done along four main axes: source, structure, mode of polymerization, and intermolecular forces. Each gives you a different lens to look at a polymer.
1. Classification by Source
This is the simplest: where does the polymer come from?
- Natural polymers – found in nature. Examples: cellulose (cotton, wood), proteins (wool, silk), natural rubber, starch, DNA.
- Synthetic polymers – man-made in a lab or factory. Examples: nylon, polyester, polythene, PVC, Teflon.
- Semi-synthetic polymers – natural polymers chemically modified. Examples: cellulose acetate (rayon), vulcanized rubber.
Semi-synthetic polymers are a middle ground. Rayon is made from natural cellulose but treated with chemicals — it's not fully natural, not fully synthetic.
2. Classification by Structure
This is about how the polymer chains are arranged. Imagine a pile of spaghetti versus a ladder versus a tree branch.
- Linear polymers – long, straight chains. They pack closely, so they have high density, high melting points, and are strong. Example: high-density polythene (HDPE).
- Branched polymers – chains with side branches. They pack less tightly, so they are less dense and melt at lower temperatures. Example: low-density polythene (LDPE).
- Cross-linked polymers – chains connected by covalent bonds in a 3D network. They are rigid, hard, and do not melt on heating (thermosetting). Example: bakelite, melamine.
Don't confuse "branched" with "cross-linked". Branched chains are still separate; cross-linked chains are chemically bonded together into one giant molecule.
3. Classification by Mode of Polymerization
This is about the chemical reaction that builds the polymer. There are two fundamentally different mechanisms.
Addition polymerization – monomers add to each other without losing any small molecule. The monomer usually has a double bond that opens up. The polymer has the same empirical formula as the monomer.
Example: ethene (CH2=CH2) → polythene (−CH2−CH2−)n
Condensation polymerization – monomers join by eliminating a small molecule like water, ammonia, or HCl. The polymer has a different formula from the monomer (because something is lost). These polymers usually have functional groups like ester, amide, or ether links.
Example: hexamethylenediamine + adipic acid → nylon-6,6 + water
| Feature | Addition | Condensation |
|---------|----------|--------------|
| By-product | None | Small molecule (H₂O, NH₃, etc.) |
| Monomer | Contains C=C double bond | Two different functional groups |
| Example | Polythene, PVC, Teflon | Nylon, polyester, bakelite |
4. Classification by Intermolecular Forces
This is the most practical classification for everyday use. The forces between polymer chains determine whether the material is stretchy, stiff, or fibre-like.
The strength of intermolecular forces increases in this order:
Elastomers < Plastics (Thermoplastics < Thermosetting) < Fibres
Let's unpack that.
Elastomers – weak van der Waals forces between chains, but the chains are lightly cross-linked. They can stretch a lot and snap back. Example: natural rubber, neoprene.
Fibres – strong intermolecular forces (hydrogen bonds, dipole-dipole) plus linear, closely packed chains. They are strong, tough, and have high tensile strength. Example: nylon, polyester, silk.
Plastics – intermediate forces. They are further divided into:
- Thermoplastics – linear or branched chains with moderate forces. They soften on heating and harden on cooling (reversible). Example: polythene, PVC, polystyrene. …
Thermosetting plastics set permanently on heating. …
Step 1. A thermosetting plastic cures irreversibly into a cross-linked network on first heating and cannot be softened again.
Step 2. Familiar articles: bakelite switches, plugs, handles of cookware and switchboards; melamine cups and plates; urea-formaldehyde lami …
Think of articles that must withstand heat without deforming — they ar …
- Citing polythene bottles or PVC pipes — those are thermoplastics, …
- CBSE 2022Set HE2181 markMCQQ.Nylon is an example of -(a) Polyamide(b) Polythene(c) Polyester(d) Polysaccharide
›Reveal solutionSolution
Nylon-6,6 is made by condensation polymerisation of a diamine and a diacid, joined by repeating amide (-CONH-) bonds, which is why it is classed as a polyamide.
Nylon-6,6 is manufactured by heating hexamethylenediamine [H2N-(CH2)6-NH2] with adipic acid [HOOC-(CH2)4-COOH] under high pressure and temperature. Each condensation step eliminates a water molecule and forms an amide (-CO-NH-) linkage between the -COOH of one monomer and the -NH2 of the other:
n H2N(CH2)6NH2 + n HOOC(CH2)4COOH -> [-NH(CH2)6NH-CO(CH2)4CO-]n + 2n H2O
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- CBSE 2020Set HE8221 markQ.Fill in the blank: Neoprene is a ______ rubber.
›Reveal solutionSolution
Neoprene is a man-made (synthetic) rubber, an elastomer formed by free-radical addition polymerisation of chloroprene (2-chloro-1,3-butadiene).
Rubbers (elastomers) are classified as natural (e.g. natural rubber, obtained from the latex of Hevea brasiliensis by polymerisation of isoprene units) or synthetic (man-made, produced industrially by polymerising suitable monomers).
…
- CBSE 2020Set ANNUAL1 markMCQQ.Identify synthetic polymer amongst the following:(a) Linen(b) Jute(c) Silk(d) Terylene
›Reveal solutionSolution
Terylene is the man-made polyester; the other three are natural fibres.
Linen (from flax stem), jute (from the jute plant) and silk (secreted by silkworms) are all natural fibres of plant or animal origin. Terylene, on the other hand, is polyethylene terephthalate (PET) — a synthetic polyester manufactured industrially by condensation polymerisation of ethylene glycol with terephthalic acid (or its di …
- CBSE 2019Set HE1 markQ.Match the following. Column A term: backelite. Column B options to match from:(a) buna-rubber(b) thermosetting plastic(c) Isoprene(d) CaOCl2(e) lyophilic colloid(f) NaCl
›Reveal solutionSolution
Bakelite is a phenol-formaldehyde resin formed by the condensation polymerisation of phenol and formaldehyde; it sets into a hard, infusible, cross-linked (thermosetting) solid on heating/moulding, and cannot be remoulded/softened again.
Bakelite is prepared by the condensation of phenol with formaldehyde (using either an acid or a base catalyst), forming a highly cross-linked, three-dimensional network polymer once cured. Because of this rigid cross-linked network, it is classified as a thermosetting plastic (as opposed to thermoplastics, which soften on heating and c …
- CBSE 2019Set ANNUAL1 markMCQQ.Which of the following is a copolymer?(a) Buna-S(b) PAN(c) PVC(d) Polythene
›Reveal solutionSolution
Buna-S is a copolymer of two different monomers (1,3-butadiene and styrene); the other three options are homopolymers.
A copolymer is formed from two or more different monomer units, while a homopolymer is built from only one type of monomer.
- Buna-S: a copolymer of buta-1,3-diene and styrene ("Bu" for butadiene, "Na" for sodium catalyst, "S" for styrene) — used in making synthetic rubber/tyres.
- PAN (polyacrylonitrile): a homopolymer of a single monomer, acrylonitrile (CH2=CH−CN). …
- CBSE 2018Set ANNUAL1 markMCQQ.In Buna-S, letter S corresponds to:(a) sodium(b) sulphur(c) styrene(d) ethene
›Reveal solutionSolution
Buna-S is a copolymer of butadiene and styrene; "Bu" = butadiene, "Na" = sodium (the polymerisation catalyst), "S" = styrene.
Buna-S is a synthetic rubber made by co-polymerising 1,3-butadiene with styrene in the presence of sodium as a catalyst. Its name is built from: Bu (1,3-Butadiene) + Na (sodium catalyst) + S (St …
- CBSE 2017Set ANNUAL1 markQ.What are thermoplastics? Give one example.
›Reveal solutionSolution
Thermoplastics have linear/slightly-branched chains held only by weak intermolecular forces, so heating lets the chains slide past each other and re-set into a new shape.
Thermoplastics are polymers made of linear or slightly branched long-chain molecules that are held together only by weak intermolecular forces (van der Waals forces, not covalent cross-links). On heating, these weak forces are overcome, the chains can move/flow past one another, and the polymer softens; on cooling it hardens again in its new shape. Because no permanent chemical bonds are broken or formed, this softening–hardening cycle can be repeated many times, allowing the plastic to be remoulded/recycled.
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