Chemistry · Organic chemistry · Year 3

Chemistry of polymer materials

Why does a PVC pipe turn yellow in the sun, and how does sulfur turn tyre rubber into a real elastic material?

The situation

Pipes, window frames and linoleum are made from the polyvinyl chloride that Navoiyazot produces; but after years in sun and heat a white pipe turns yellow and brittle. A car tyre is not made of raw rubber: a little sulfur is added and it is baked — vulcanised. An ion exchanger for purifying water is made from nitron fibre, and water-soluble polyvinyl alcohol from polyvinyl acetate glue. How can a finished polymer be chemically modified, what damages it, and how is it protected?

Topic

Chemical modification of polymers and polymer materials. Navoiyazot PVC pipes age in sun and heat, while a car tyre is vulcanised rubber; an anion exchanger for water purification is made from nitron fibre. In the laboratory…

Key ideas

Polymer-analogous reactions
Vulcanisation
Degradation and stabilisation

Formulas

θ_{\infty} = e^{- 2}, X_{m a x} = 1 - e^{- 2} = 0.865

If neighbouring OH pairs link up at random (polyvinyl formal), ≈ 13.5 % of the OH groups are left isolated (Flory).

G = ν R T, σ = G (λ - \frac{1}{λ^{2}})

High elasticity: the shear modulus is proportional to the number of network chains ( $ν$ , mol/m³).

A = \frac{(V_{0} - V) c \cdot 4.3045}{m}, AD = \frac{162.14 A}{4304.5 - 42.04 A}

Cellulose acetate: acetyl content (%) and degree of substitution.

Q = \frac{c_{N a O H} V_{N a O H}}{m}

Capacity of an ion exchanger, mmol/g: the H⁺ released is titrated with alkali.

T = \frac{Q_{m}}{v λ} \cdot 1000 tex, d = \sqrt{\frac{4 T_{f}}{π ρ}}

The linear density of a fibre (g/km) and the diameter of a single filament.

Theory

Polymer-analogous and intramolecular transformations

Polymer-analogous reactions keep the chain length and change only the side groups: polyvinyl acetate → polyvinyl alcohol (PVA; the vinyl alcohol monomer is unstable, and PVA can be obtained only this way) → polyvinyl formal or polyvinyl butyral (the film in safety car glass), the hydrolysis of polyacrylamide and polyacrylonitrile, the acetylation and nitration of cellulose, the chlorination of PVC. Intramolecular reactions take place within one chain: on heating, PAN forms cyclic structures (the route to carbon fibre); PVC loses HCl and double bonds appear. The reactivity of functional groups differs from that in small molecules: the neighbouring-group effect — in the alcoholysis of PVAc with NaOH, the OH groups formed activate the neighbouring ester groups (an S-shaped curve), while in the hydrolysis of polyacrylamide the −COO⁻ groups formed do not let the OH⁻ ion approach the neighbouring amide groups — the electrostatic effect: the reaction almost stops at ≈ 60–70 %, and continues if salt (NaCl) screens the charges; the acrylamide monomer, however, is hydrolysed completely. Concentration effect: inside a coiled chain the local concentration of groups is greater than the average of the solution. Statistical effect: PVA with formaldehyde joins neighbouring pairs of OH groups; because the pairs are chosen at random, isolated OH groups are left — the limiting degree of conversion is $1 - e^{- 2}$ ≈ 86.5 % (P. Flory). A styrene–maleic anhydride copolymer binds 2 mol of NaOH per unit in alkali; nitron fibre is partly hydrolysed in alkali to a cation-exchange fibre with carboxyl groups, with amines to an anion-exchange fibre, and with hydroxylamine to a fibre with amidoxime groups (which complex metals). In cellulose acetates the acetyl content is determined by saponification and back-titration: triacetate — 44.8 %, secondary acetate (DS ≈ 2.4) — ≈ 39–40 %.

Intermacromolecular reactions: vulcanisation, block and graft copolymers

Cross-linking joins linear chains into a three-dimensional network. Vulcanisation: rubber is heated with sulfur at 140–170 °C; S forms mono-, di- and polysulfide bridges between the chains. Accelerators (sulfenamides, thiurams) + activators (ZnO, stearic acid) speed up the reaction and use the sulfur efficiently: without an accelerator, 40–55 S atoms are used per cross-link; in a conventional accelerated system, several times fewer (5 in the model), and in an efficient (EV) system ≈ 2 (mostly mono- and disulfide bridges). On the rheometer curve: the induction (scorch) period is the safe moulding time, then the torque rises; $t_{90}$ is optimal vulcanisation; on prolonged heating the polysulfide bridges break (reversion). Theory of high elasticity: $G = ν R T$ ( $ν$ — network chains, mol/m³), $σ = G (λ - λ^{- 2})$ . Soft rubber has 1–3 % S (few bridges, stretches a great deal); hardness grows with S, and at 30 % S and more most double bonds are saturated and the glass transition temperature rises above room temperature — hard, brittle ebonite. Unbound sulfur blooms to the surface. Block copolymers (SBS: polystyrene–polybutadiene–polystyrene) are thermoelastoplastics: the PS domains are physical “knots”, which melt on heating so the material can be reprocessed. Graft copolymers (styrene grafted onto polybutadiene — high-impact polystyrene, ABS) are made via macroradicals or macroinitiators. Other modifications: chlorosulfonation, epoxidation, radiation cross-linking (polyethylene for cables). School-level vulcanisation is in the “Polymers, rubber and fibres” experiment.

Degradation and ageing

Degradation is the breaking of a macromolecule or the destruction of its composition. Chemical agents: hydrolysis (polyamides, polyesters, cellulose: $- C O N H - + H_{2} O \to - COOH + H_{2} N -$ ), acidolysis, aminolysis, oxidation (PVA is cleaved by periodate only at 1,2-diols — units joined “head to head”; they make up ≈ 1–2 %). For random scission, $1 / {\overline{X}}_{n} = 1 / {\overline{X}}_{n, 0} + k t$ . Physical: thermal, mechanical (grinding, ultrasound, vigorous stirring — polyacrylamide solutions), photochemical (UV light), radiation. Mechanisms: random scission of the chain (polyethylene, polyamides); depolymerisation — “unzipping” to monomer (heated PMMA gives MMA — the ceiling temperature!, polytetrafluoroethylene); loss of side groups — HCl from PVC: $- CH_{2} - CHCl^{-} \overset{}{\to} - CH = CH - + HCl$ , noticeable from ≈ 140 °C, with the HCl released speeding up the reaction (autocatalysis); as the polyene sequences lengthen, PVC turns yellow, brown and black. Thermal oxidation is a radical chain process: $R^{∙} + O_{2} \to R O O^{∙}$ , $R O O^{∙} + R H \to R O O H + R^{∙}$ , with hydroperoxides decomposing and branching the chain: first an induction period, then O₂ uptake accelerates. Ageing is the long combined action of air, light, heat, moisture and stress: embrittlement, cracking, colour change. Ionising radiation cross-links some polymers (PE) and breaks others (PMMA, PTFE).

Stabilising polymers

Accelerators of degradation: heavy-metal ions (Cu, Fe, Mn — they decompose hydroperoxides), the HCl released, acids, UV light; sometimes pro-oxidants are added deliberately, for biodegradable or photodegradable films. Retarders — stabilisers: HCl acceptors (Ca/Zn stearates, organotin compounds; lead salts are toxic and have been phased out in many countries), which also replace the labile (allylic) chlorine in PVC; epoxidised soybean oil — a co-stabiliser that binds HCl; antioxidants — chain-breaking phenols (ionol) and aromatic amines, phosphites and thioesters that decompose hydroperoxides (synergism); light stabilisers — UV absorbers (benzophenones, benzotriazoles), carbon black, hindered amines (HALS); metal deactivators. Requirements for a stabiliser: effective (in small amounts), compatible with the polymer, non-volatile and non-leachable, colourless, non-toxic (approved for food and water pipes), stable at the processing temperature, cheap. Stability is tested by the Congo red test (the time at 180 °C until HCl turns the paper blue), by the oxidation induction time (OIT) and in artificial weathering chambers.

Polymer materials, fibres and ion exchangers

Structural plastics: polyethylene, polypropylene, PVC (pipes, profiles), polystyrene, polyamides, polycarbonate, PET. Rubber and fibres: natural (cotton, wool), man-made (from a natural polymer: viscose — cellulose is converted to xanthate with NaOH and CS₂ and regenerated as cellulose in an H₂SO₄ bath: $C_{6} H_{10} O_{5} + NaOH + CS_{2} \overset{}{\to} C_{7} H_{9} O_{5} S_{2} Na + H_{2} O$ ; acetate — “dry” spun in hot air from a solution in acetone of the secondary acetate (DS ≈ 2.4) obtained by partial hydrolysis of triacetate) and synthetic (polyamide, polyester, nitron — from melt or solution). The linear density of a fibre is measured in tex (g/km); drawing orders the macromolecules along the axis and increases strength. Polymer blends (ABS, PC/ABS, high-impact PS) and composites (glass and carbon fibres + epoxy or polyester resin; filled plastics) combine properties. Functional polymers: ion exchangers — a sulfonated polystyrene–divinylbenzene cation exchanger (−SO₃H, capacity in dry resin ≈ 4–5 mmol/g), an anion exchanger with quaternary ammonium groups; water softening and desalination (the “Water purification and softening” experiment), separation of uranium and gold, regeneration with acid/alkali; polymers for electrical conduction, membranes, drug carriers and nanomaterials (nanocomposites, dendrimers). Processing: extrusion (pipe, profile, film — a screw pushes the melt through a die), injection moulding (articles), calendering (film and linoleum between rolls), pressing (thermosets), blow moulding (containers). In chemical technology polymer materials start from synthesis gas and ethylene: $CO + 2 H_{2} \overset{}{\to} CH_{3} OH$ (Cu/ZnO, 5–10 MPa), ethanol by hydration of ethylene or by fermentation; since 2019 Navoiyazot has produced PVC, caustic soda and methanol. Synthesis of polymers in bulk, solution, suspension and emulsion is in the “Polymer synthesis and kinetics” experiment, and methanol and plant equipment in the “Chemical plant: reactors and workshops” experiment.

Lab work, safety and the model

19 pieces of work are in the “Lab work” list (High-molecular-weight compounds L28–L44, Kimtex L21–L22). On the bench: a flask with a reflux condenser (alcoholysis, acetal formation, hydrolyses), titration (ester value, acetyl, NH₃, ion-exchanger capacity), a hot press and a stretching machine, PVC test tubes in an aluminium block and Congo red paper, an ion-exchange column, a spinning line and an extruder. Safety: methanol is poisonous (drinking it causes blindness and death); formalin, acetic anhydride, acrylamide (a neurotoxin — do not handle the powder), hydrazine and hydroxylamine — in a fume cupboard, with gloves and safety goggles; if alkali gets in the eyes, rinse with plenty of water; PVC releases HCl when heated — only in a fume cupboard, and never burn PVC; do not touch the hot press or the 180 °C block; CS₂ (viscose) ignites very easily. Do not taste anything. Model: the kinetic constants, neighbouring-group coefficients (1 : 0.25 : 0.04), stabiliser capacities, vulcanisation efficiency and rheometer curve, OIT and the capacity of the ion-exchanger sample are model values of the order found in the literature; high elasticity uses the affine model ( $G = ν R T$ , +0.2 MPa for entanglements); the modulus of ebonite is semi-empirical, dependent on $T_{g}$ ; the chain and network models are schematic.

Did you know?

Charles Goodyear discovered vulcanisation by heating rubber with sulfur in 1839 and patented it in 1844.
Polyvinyl chloride is one of the most widely produced synthetic plastics in the world — in about third place after polyethylene and polypropylene.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Carbonyl compounds and synthesis — Year 3
Heterocycles and DNA — Year 3
Amino acids and proteins — Year 3
Physics of macromolecules — Year 3
Polymer synthesis and kinetics — Year 3