Anti-oxidant

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Test for the Presence of Antioxidants in Plastics
by Thin Layer Chromatography
Introduction
Many polymers are subject to oxidation (e.g. when exposed to high temperatures, air
or sunlight) during manufacturing, processing and in their end use. For example, in
the case of poly(propene), the tertiary carbon atoms are known to provide a site for
oxidative degradation.
.
Antioxidants (a kind of stabilizer) are added to poly(propene) to impact processing
stability and long-range thermal / photo oxidative endurance. The antioxidants used
for poly(propene) are mainly butylated hydroxytoluene (BHT) and butylated
hydroxyanisole (BHA).
C(CH3 )3
CH3
( CH
CH2 )
n
CH3
OH
C(CH3 )3
Poly(propene)
BHT
OH
CH 3 O
C(CH3 )3
BHA
In this experiment, the antioxidants in some poly(propene) samples are extracted by
refluxing with the solvent 1,1,1-trichloroethane. After concentration of the extract, the
presence of the antioxidants BHT and BHA is identified by thin-layer
chromatographic technique.
Procedure
1. Collect and cut 3-5 g of a poly(propene) sample (see Remarks below) into samll
pieces. Place the pieces into a 50 cm3 pear-shaped flask fitted for reflux.
2. Add 25 cm3 of 1,1,1-trichloroethane and some pieces of pumice stones. Refulx for
1 hour with a small steady flame.
3. Cool the mixture. Concentrate the extract by distillation until about 5 cm3 of the
extract is left. Transfer the extract into a test-tube with a dropper.
4. Mark a pencil line about 2 cm from the end of a precoated thin layer plate. Use a
capillary tube to spot the extract onto the line and repeat this several times at the
same spot.
5. Spot authentic solutions of BHT and BHA in 1,1,1-trichloroethane separately to
other positions on the line.
6. Place the thin-layer plate, with the spotted end down, into the chromatographic
tank which is filled with a solvent (1 part ethyl ethanoate to 4 parts petroleum
ether by volume) to about 1 cm in depth. Cover the tank and allow the solvent to
go up the plate.
7. When the solvent reaches the top of the plate, take out the plate and allow the
solvent to evaporate. (Hazard warning: This solvent is flammable.)
8. Put the plate into a gas jar containing some iodine crystals for one minute.
(Hazard warning: Iodine vapour is harmful. The gas jar should be covered with a
glass plate and placed inside the fume cupboard.)
9. When the positions of the antioxidants are located as brown spots, remove the
plate from the jar. Allow the residual iodine to vaporize in the fume cupboard.
10. Identify the antioxidants in the poly(propene) sample by comparing the positions
of the spots for the sample to those of the authenic BHT and BHA standards.
Remarks
A good plastic sample for this experiment can be obtained from feeding bottles.
Feeding bottles are usually made of poly(propene). They usually contain a relatively
higher content of antioxidants as the bottles require repeated sterizilization at the
temperature of steam.
Source: ED, Journal of Chemical Education, Vol.66, No.2, Feb. 1989(p.172-173)
Results
Distance travelled by the authentic BHT standard: __________ cm
Distance travelled by the authentic BHA standard: __________ cm
Distance travelled by spot 1 of the sample: __________ cm
Distance travelled by spot 1 of the sample: __________ cm
Conclusion:
Questions for discussion
1.Give reasons that antioxidant(s) should be added to plastic.
2.What structural feature(s) do you note that it (they) account(s) for their ability of
anti-oxidizing.
3.Why was the plastic sample cut into pieces (step 1)?
4.What was the purpose of adding 1,1,1-trichloroethane and some pumice stones?
(step 2)
5.Why was the extract concentrated? (step 3)
6.How can you identify BHT in this experiment?
7.Explain the formation of brown spot (step 9)
8.Write a short essay to explain how do BHT and BHA work as antioxidants.
End
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