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11.8.4 Nylon, PET and recycling

Nylon

Definition

Nylon

A synthetic polyamide made by condensation polymerisation of a dicarboxylic acid with a diamine.

  1. Nylon is a synthetic polyamide, made in factories rather than by living things.

  2. It is made by condensation polymerisation of a dicarboxylic acid with a diamine.

    1. Each -COOH\text{-COOH}-COOH group reacts with an -NH2\text{-NH}_2-NH2​ group to form an amide linkage, -CONH-\text{-CONH-}-CONH-.
    2. A molecule of water is given off each time an amide linkage forms.

  3. In the block diagram of nylon, the empty box stands for the carbon chain of the acid and the shaded box stands for the carbon chain of the amine.

  4. The structure of nylon is drawn as the repeat unit below.

    [OOHH∥∥∣∣−C−A−C−N−■−N−]n \left[ \begin{array}{ccccccccccccc} & \text{O} & & & & \text{O} & & \text{H} & & & & \text{H} & \\ & \| & & & & \| & & | & & & & | & \\ - & \text{C} & - & \boxed{\phantom{A}} & - & \text{C} & - & \text{N} & - & \boxed{\blacksquare} & - & \text{N} & - \end{array} \right]_n ​−​O∥C​−​A​​−​O∥C​−​H∣N​−​■​​−​H∣N​−​​n​

  5. Every repeat unit contains two amide linkages, each with a C=O\text{C=O}C=O group bonded to an N-H\text{N-H}N-H group.

    1. The acid block sits between two C=O\text{C=O}C=O groups, and the amine block sits between two N-H\text{N-H}N-H groups.

  6. A common form of nylon is made from a dicarboxylic acid and a diamine that each have six carbon atoms.

  7. Nylon chains are long and strong and can be pulled into fibres, so nylon is used for clothing, tights, ropes, fishing nets and carpets.

Example

Forming nylon

  • The two monomers are a dicarboxylic acid and a diamine, each drawn with a block for its carbon chain.
HOOC−A−COOHH2N−■−NH2 \text{HOOC}-\boxed{\phantom{A}}-\text{COOH} \qquad \text{H}_2\text{N}-\boxed{\blacksquare}-\text{NH}_2 HOOC−A​−COOHH2​N−■​−NH2​
  • The -OH\text{-OH}-OH from each acid group and an H\text{H}H from each amine group are lost as water.
  • The carbon of each C=O\text{C=O}C=O group bonds to the nitrogen of the amine, making the amide linkages.
n HOOC−A−COOH+n H2N−■−NH2→[−CO−A−CO−NH−■−NH−]n+2n H2O n\,\text{HOOC}-\boxed{\phantom{A}}-\text{COOH} + n\,\text{H}_2\text{N}-\boxed{\blacksquare}-\text{NH}_2 \rightarrow \left[-\text{CO}-\boxed{\phantom{A}}-\text{CO}-\text{NH}-\boxed{\blacksquare}-\text{NH}-\right]_n + 2n\,\text{H}_2\text{O} nHOOC−A​−COOH+nH2​N−■​−NH2​→[−CO−A​−CO−NH−■​−NH−]n​+2nH2​O

PET

Definition

PET

A synthetic polyester made by condensation polymerisation of a dicarboxylic acid with a diol.

  1. PET is a synthetic polyester.

  2. It is made by condensation polymerisation of a dicarboxylic acid with a diol.

    1. Each -COOH\text{-COOH}-COOH group reacts with an -OH\text{-OH}-OH group to form an ester linkage, -COO-\text{-COO-}-COO-.
    2. A molecule of water is given off each time an ester linkage forms.

  3. The acid used to make PET contains a benzene ring, a ring of six carbon atoms, so here the empty box stands for that ring.

  4. The structure of PET is drawn as the repeat unit below.

    [OO∥∥−C−A−C−O−■−O−]n \left[ \begin{array}{ccccccccccccc} & \text{O} & & & & \text{O} & & & & & & & \\ & \| & & & & \| & & & & & & & \\ - & \text{C} & - & \boxed{\phantom{A}} & - & \text{C} & - & \text{O} & - & \boxed{\blacksquare} & - & \text{O} & - \end{array} \right]_n ​−​O∥C​−​A​​−​O∥C​−​O​−​■​​−​O​−​​n​

  5. Every repeat unit contains two ester linkages, each with a C=O\text{C=O}C=O group bonded to an oxygen atom in the chain.

    1. The acid block sits between two C=O\text{C=O}C=O groups, and the diol block sits between two O\text{O}O atoms.

  6. PET is strong, light, clear and does not let gases through easily, so it is used for plastic drinks bottles and food trays.

  7. PET can also be spun into fibres, which are sold as polyester for clothing and fleeces.

Example

Forming PET

  • The two monomers are a dicarboxylic acid and a diol.
HOOC−A−COOHHO−■−OH \text{HOOC}-\boxed{\phantom{A}}-\text{COOH} \qquad \text{HO}-\boxed{\blacksquare}-\text{OH} HOOC−A​−COOHHO−■​−OH
  • The -OH\text{-OH}-OH from each acid group and the H\text{H}H from each alcohol group are lost as water.
  • The carbon of each C=O\text{C=O}C=O group bonds to the oxygen of the diol, making the ester linkages.
n HOOC−A−COOH+n HO−■−OH→[−CO−A−CO−O−■−O−]n+2n H2O n\,\text{HOOC}-\boxed{\phantom{A}}-\text{COOH} + n\,\text{HO}-\boxed{\blacksquare}-\text{OH} \rightarrow \left[-\text{CO}-\boxed{\phantom{A}}-\text{CO}-\text{O}-\boxed{\blacksquare}-\text{O}-\right]_n + 2n\,\text{H}_2\text{O} nHOOC−A​−COOH+nHO−■​−OH→[−CO−A​−CO−O−■​−O−]n​+2nH2​O

Recycling PET

  1. Used PET can be converted back into its monomers, the dicarboxylic acid and the diol.
    1. The ester linkages are broken, which reverses the condensation reaction.
    2. This can be done by heating the PET with water or another small molecule such as methanol, often with a catalyst.
  2. The monomers are separated and purified, then re-polymerised by condensation polymerisation to make new PET.
  3. The new PET is as good as PET made from fresh monomers, so a bottle can be turned into new bottles again and again.
  4. Recycling PET in this way has clear benefits.
    1. Fewer raw materials from crude oil are needed to make new monomers.
    2. Less plastic waste goes to landfill or ends up in the oceans.
    3. Less plastic is burned, so fewer toxic gases are released.
Example

Breaking PET into its monomers

  • Adding water across each ester linkage puts back the -OH\text{-OH}-OH on the acid end and the H\text{H}H on the diol end.
[−CO−A−CO−O−■−O−]n+2n H2O→n HOOC−A−COOH+n HO−■−OH \left[-\text{CO}-\boxed{\phantom{A}}-\text{CO}-\text{O}-\boxed{\blacksquare}-\text{O}-\right]_n + 2n\,\text{H}_2\text{O} \rightarrow n\,\text{HOOC}-\boxed{\phantom{A}}-\text{COOH} + n\,\text{HO}-\boxed{\blacksquare}-\text{OH} [−CO−A​−CO−O−■​−O−]n​+2nH2​O→nHOOC−A​−COOH+nHO−■​−OH
  • This is the reverse of the equation for making PET, so the same monomers come out.
  • The recovered monomers are then polymerised again to make new PET.

Telling nylon and PET apart

  1. Both nylon and PET are condensation polymers made from two different monomers, with water lost as each linkage forms.
  2. Look at the atom next to each C=O\text{C=O}C=O group in the chain.
    1. If it is N-H\text{N-H}N-H, the linkage is an amide linkage and the polymer is a polyamide such as nylon.
    2. If it is an oxygen atom, the linkage is an ester linkage and the polymer is a polyester such as PET.
  3. The monomers follow from the linkage: nylon needs a dicarboxylic acid and a diamine, while PET needs a dicarboxylic acid and a diol.
Common Mistake
  • Do not draw the N-H\text{N-H}N-H groups in PET, because PET is a polyester and contains no nitrogen.
  • Do not miss the H\text{H}H on each nitrogen atom in nylon, because each nitrogen in an amide linkage has three bonds.
  • Do not draw the C=O\text{C=O}C=O as C-O\text{C-O}C-O, because the double bond to oxygen is part of both linkages.
  • Do not say PET is recycled by melting it down into monomers, because melting only softens the polymer and the ester linkages have to be broken chemically.
Exam technique

Drawing nylon and PET

  • Show at least one complete linkage with the C=O\text{C=O}C=O drawn as a double bond and, for nylon, the H\text{H}H on the nitrogen.
  • Use boxes for the carbon chains unless the question gives you the monomers in full, and keep the two kinds of box different.
  • Finish with continuation bonds at both ends, because a structure with closed ends is a single molecule, not a polymer.
  • For recycling, write that PET is converted back into monomers and then re-polymerised, since both steps are needed for the mark.
Self review
  • Which two types of monomer are used to make nylon?
  • What type of linkage is found in PET?
  • Which small molecule is given off when nylon and PET are made?
  • Describe how PET can be recycled into new PET.
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11.8.4 Nylon, PET and recycling Revision Guide

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Revision notes for CIE IGCSE Chemistry 11.8.4 Nylon, PET and recycling: explanations and worked examples.

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