Chemistry · Organic chemistry · Grade 10

Polymers, rubber and fibres

How are a polyethylene bag, a tyre and a cotton fibre built, and where do they end up?

The situation

The bag brought back from the market holds a polyethylene bag, a piece of PVC pipe, a polystyrene cup and a scarf woven from cotton. An old car tyre stands in the yard. All of them are polymers: giant molecules formed when thousands of small molecules (monomers) join into a chain. Why does the bag soften when heated while the tyre does not melt? Why does burning cotton smell of burnt paper and burning wool of burnt hair? Where do used plastic and paper go, and what else can be made from them?

Topic

Polymers, rubber and fibres; recycling of waste. Polyethylene bag, PVC pipe, car tyre and cotton fibre: how polymers form and where they end up

Key ideas

Degree of polymerisation n
Vulcanisation
Fibres and processing

Formulas

M = n \cdot M_{monomer}

Average molar mass of a polymer: $n$ is the degree of polymerisation (the number of repeat units in the chain).

n CH_{2} = CH_{2} \overset{\overset{}{t, p, cat \cdot}}{\to} (- CH_{2} - CH_{2} -)_{n}

Polymerisation: the double bonds open and the monomers join into a chain.

n CH_{2} = C (CH_{3}) - CH = CH_{2} \overset{}{\to} (- CH_{2} - C (CH_{3}) = CH - CH_{2} -)_{n}

Natural rubber is cis-1,4-polyisoprene; vulcanised with sulfur, it becomes vulcanised rubber.

n H_{2} N - (CH_{2})_{5} - COOH \overset{}{\to} [- NH - (CH_{2})_{5} - CO^{-}]_{n} + n H_{2} O

Polycondensation (nylon): water is released as the chain grows.

C_{6} H_{12} O_{6} \overset{\overset{}{bacteria}}{\to} 3 CH_{4} + 3 CO_{2}

Biogas from organic waste: bacteria produce methane without air.

Theory

Polymer, monomer, degree of polymerisation

A polymer is a high-molecular-mass compound built from identical pieces repeated many times (repeat units); the small molecule that forms it is the monomer. The number of repeat units in a chain, $n$ , is the degree of polymerisation, $M = n \cdot M_{monomer}$ . The chains in one sample have different lengths, so $n$ and $M$ are average values. Polymerisation is the joining of unsaturated monomers into a chain as their double bonds open (ethene → polyethylene, propene → polypropylene, vinyl chloride → PVC, styrene → polystyrene); in polycondensation a small molecule such as water is released as the chain grows (nylon, lavsan, phenol–formaldehyde resins). By origin, polymers are natural (starch, cellulose, proteins, natural rubber), artificial (made from a natural polymer by chemical modification: viscose, acetate) and synthetic; by structure they are linear, branched and network (cross-linked). Linear and branched polymers soften when heated and can be reshaped (thermoplastics: PE, PP, PVC, PS), while network polymers do not melt (rubber, ebonite, phenolic plastics).

Rubber (2.12)

Natural rubber is obtained from the sap (latex) of the Brazilian rubber tree, Hevea; it is cis-1,4-polyisoprene ${(- CH_{2} - C (CH_{3}) = CH - CH_{2}^{-})}_{n}$ : the chains are coiled up and straighten when stretched. Raw rubber is sticky when hot and brittle when cold, and when it is stretched the chains slide over one another and the shape does not recover. In 1839 C. Goodyear discovered vulcanisation by heating rubber with sulfur (patent — 1844): through the double bonds, sulfur “bridges” form between the chains — a network structure. A few per cent of sulfur gives elastic rubber (tyres, rubber goods), 25–50 % gives hard ebonite. Synthetic rubbers: butadiene rubber by S.V. Lebedev's method (1932), isoprene rubber, butadiene–styrene rubbers. Carbon black (technical carbon) is added to tyre rubber to make it stronger. Trans-1,4-polyisoprene is gutta-percha: it is not elastic.

Natural and artificial fibres (3.22)

Natural fibres: plant fibres — cotton and flax (mainly cellulose; about 90–96 % of dry cotton fibre), animal fibres — wool and natural silk (proteins). Chemical fibres: artificial — made by processing a natural polymer: viscose (cellulose is dissolved and precipitated again), acetate (cellulose acetate); synthetic — from synthetic polymers: nylon (polyamide, nylon-6), lavsan (polyester — polyethylene terephthalate, PET; drink bottles are also made from it), nitron (acrylic). Synthetic fibres are strong, do not rot and are not eaten by moths, but they let air and moisture through poorly. Uzbekistan is a major cotton-growing country; thread and fabric are made from cotton fibre, and artificial fibres and gunpowder from cotton cellulose.

Identifying fibres by how they burn (practical)

Take a few fibres from a sample with tongs, hold them in the flame of a spirit lamp and watch. Cotton, viscose: burn quickly, keep burning when removed from the fire, smell of burnt paper, soft grey ash. Wool, silk: burn slowly, sputter, go out when removed from the fire, smell of burnt hair (feathers), a black brittle bead that crumbles between the fingers (protein). Acetate: melts and burns, smell of vinegar, hard brown bead. Nylon: first melts and drips, leaving a hard shiny bead. Lavsan: melts, burns with a smoky flame, hard black bead. Safety: carry out the experiment in a fume cupboard or a ventilated place, with a small sample, held with tongs; drops of molten synthetic fibre stick to the skin and burn it — put a ceramic tile under the sample; waft the smell towards you with your hand.

Organic waste and recycling (4.2–4.3)

Plastics break down in nature very slowly — over tens and hundreds of years. That is why they are sorted: the number inside the triangle on an object shows the type of polymer (1 — PET, 2 — PE-HD, 3 — PVC, 4 — PE-LD, 5 — PP, 6 — PS, 7 — others). Mechanical recycling: washed, shredded, melted and made into granules (PET, PE and PP are the easiest to recycle). Chemical recycling: pyrolysis or depolymerisation — monomers or fuel are obtained again. PVC gives off $HCl$ when burned, so it cannot be burned in the ordinary way. Paper (practical 4.3): waste paper is soaked in water, turned into pulp, the fibres are caught on a sieve frame, pressed and dried; each time the fibres get a little shorter — in practice paper can be recycled about 5–7 times, after which new fibre is added. Food and plant waste is made into compost or fermented without air to give biogas (mainly methane).

Limits of the model

The molecules have real bond lengths and angles (1 Å ≈ 4 cm); a real chain has thousands of repeat units, here there are three. The chain-length distribution is taken to be the Flory distribution (when the chain is terminated by disproportionation in radical polymerisation). The rubber network is schematic: the chains are in a bead model, the volume is conserved on stretching, the stress is $σ = G (λ - 1 / λ^{2})$ , and the dependence of $G$ on the sulfur fraction and the breaking points are approximate. The burning of fibres is shown according to observations in textbooks. The decrease in fibre length with each recycling cycle (12 %) is a schematic value.

Did you know?

Charles Goodyear is said to have discovered vulcanisation in 1839 when he accidentally dropped a mixture of rubber and sulfur onto a hot stove; he received a US patent for the method in 1844. The name “vulcanisation” comes from Vulcan, the ancient Roman god of fire.
Cotton fibre is almost pure cellulose: about 90–96 % of the dry fibre is cellulose. A cellulose molecule is a linear chain built from thousands of β-glucose residues.

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.

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Organic chemistry: structure — Grade 10
Isomers and nomenclature — Grade 10
Alkanes and cycloalkanes — Grade 10
Alkenes and alkadienes — Grade 10
Alkynes, benzene and styrene — Grade 10
Natural gas, oil and coal — Grade 10
Alcohols and ethers — Grade 10
Phenol, aldehydes and ketones — Grade 10
Acids, esters and soap — Grade 10