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23 January 2021 | New Scientist | 55

Tom Gauld

for New Scientist

Answers

Plastic pong

If plastic is inert, why does it
have a taste and smell? Plastic
bin bags have a strong smell
when first opened and water
left in plastic bottles can acquire
a distinct taste.

Greg Cash
Goodna, Queensland, Australia
Plastics in everyday use are usually
made of artificial polymers that
comprise a string of monomers
derived from oil. The polymers
consist of numerous, very long
chains of atoms. In thermoplastics,
these chain-like molecules aren’t
chemically bonded to each other.
In thermoset plastics, the polymer
strands are cross-linked.
Any smell won’t come from
these polymers, but must be from
small molecules that are volatile at
room temperatures. These could
be additives, left over monomers
from the polymerisation process
or breakdown products.
Non-rigid PVC (polyvinyl
chloride) contains plasticisers
such as phthalates which have

noticeable odours. They

are added to make a rigid

polymer flexible.

As a retired polymer chemist,

I can smell the phthalates in the

oxygen masks when I have minor

procedures at my hospital.

The water in clear drinks

bottles made of PET (polyethylene

terephthalate) that are left in the

sun can become contaminated

with some of the starting products

used to make the plastic of the

bottle. One of these is bisphenol A

and could be the source of the

taste in the water.

I have a number of tubs at

home made from LDPE (low

density polyethylene) in which

I store my PET bottles for

recycling. Over time, these

have developed an odour that

I know as polyethylene oxide and

which signifies the breakdown

of the polymer by free radicals.

They will soon split and I will

then need to put them in the

rubbish bin. Sending them for

recycling could contaminate

clean polyethylene and cause

it to degrade prematurely.

A fourth example I recalled after

a trip to the dentist is acrylic. This

is a clear rigid plastic with lots of

applications. It is mainly PMMA

(polymethyl methacrylate), but if

you cut it with a saw you will get

the sharp smell of methyl

methacrylate (the starting

material) which my dentist uses

as part of the preparation for

modern dental “fillings”.

Talia Morris

Cape Tribulation,

Queensland, Australia

Plastics have a taste and smell

because they aren’t as inert as we

would like to think they are.

Common plastics exude their

more-volatile components –

plasticisers such as bisphenols,

for example. These can be smelled

and tasted by us, and can also

mimic oestrogen. ❚

Quick quiz #85

Answers

1 Quasicrystals

2 The frontal lobe

3 26

4 Asclepius (also known

as Hepius)

5 Fringing reefs

Quick Crossword

#74 Answers

ACROSS 9 Upper, 10 North Pole,

11 Jan Oort, 12 Nominal,

13  Knees, 15 Nth, 16 Era,

17  Awl, 19 Eidetic, 20 KHz,

23  Vet, 24 Bus, 25 Lit up,

27  New idea, 29 Aspirin,

32  Gyrfalcon, 33 Twins

DOWN 1 Burj, 2 Sponge, 3 Arco,

4 Knot, 5 Bronchitis, 6 Chem,

7  Moonwalk, 8 Reflex, 13 Kea,

14 Sweat, 15 Nudibranch,

16  Excel, 18 Live wire, 21 Zip,

22 Ginkgo, 26 Tar pit, 28 Drag,

29 Acne, 30 Path, 31 Nash

#96 Inside the box^

Solution

Start at X and work outwards.

The red lines that link nine dots

arranged in a square follow the

path of the classic four line/nine

dot “out of the box” solution.

“ Water in clear drinks

bottles left in the sun

can get contaminated

with the products used

to make the plastic”