Sunday, November 24, 2013

Pthalate in Plastics

Researchers in the USA have found a link between premature births and the exposure of  pregnant women to a group of organic (carbon based) chemicals known as phthalates.

Phthalates have been used for more than 50 years as plasticizers, substances which are added to plastics to increase their flexibility, transparency, durability and longevity. For example, adding phthalates to polyvinyl chloride (PVC) makes the PVC soft and flexible, ideal for flexible plastic tubing. You will find phthalates in many products you come into contact with every day, from vinyl upholstery and floor tiles, to plastic food containers and wrappers, to personal care products like perfumes, moisturizer, and liquid soap.

Phthalates have the general structural formula shown below:

R and R' represent alkyl groups.
If R and R' are both CH3 (methyl) groups, the molecule is known as dimethyl phthalate (abbreviated as DMP).
If R and R' are both C2H5 (ethyl) groups, the molecule is known as diethyl phthalate (abbreviated DEP).
When R and R' are very small alkyl groups, like methyl or ethyl groups, the phthalates are colourless, odourless liquids at room temperature and pressure.
The phthalates are added to molten plastic, allowing the phthalates to mix with with the polymer. The polar C=O groups in the phthalate molecules interact with partial positive charges on atoms in the polymer chain, for example, the Cδ+ of the C-Cl covalent bond in a polyvinyl chloride polymer.
When the plastic is cooled, the interactions between the phthalate and the polymer chains remain, and the alkyl groups of the phthalate screen the polymer chains from interacting with each other so that the polymer chains can slide over each other. This results in a more flexible plastic.


Further reading:
Esters and Esterification
Bond Polarity
Intermolecular Forces

Suggested Study Questions:
  1. Draw the structural formula for
    • dimethyl phthalate
    • diethyl phthalate
  2. Give the molecular formula for
    • dimethyl phthalate
    • diethyl phthalate
  3. On the structural formula for dimethyl phthalate, circle a benzene ring.
  4. On the structural formala for diethyl phthalate, circle an ester functional group.
  5. Dimethyl phthalate can be prepared in an esterification reaction using phthalic acid and methanol. Draw the most likely structural formula for phthalic acid. 
  6. In chemistry, what is meant by a polar bond?
  7. On the structural formula of diethyl phthalate,  show the partial charges of the atoms in the polar areas of the molecule.
  8. Draw a diagram to show the interaction between dimethyl phthalate and a section of the polyvinyl chloride polymer.

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