CM8001

This happens because the gas of the foaming agent has been removed, allowing the styrofoam to become solid and much denser.

a. Branched LDPE cannot be used in this application because it does not have the required strength.

b. The arrangement of the molecules of HDPE provides the required strength and a thin liner of HDPE allows sufficient flexibility.

The same thing does not happen when a piece of paper is being pulled because the molecules in paper are held more rigidly and are not free to become aligned whereas the molecules in plastic can be aligned parallel to each other and in the direction of the pull – allowing the plastic to be stretched.

a.
1. Non-toxic – to eliminate foreign body response
2. Easily sterilised
3. Excellent chemical resistance
4. Acceptable shelf life

b. Polymethyl methacrylate (PMMA) – non-toxic, permeable to oxygen, inexpensive
Silicon Hydrogel – extremely high oxygen permeability, high level of comfort

c. hard contact lenses are more durable as they are more chemically inert whereas soft contact lenses are more flexible and more comfortable.

The rigidity of PMMA allows hard lenses to retain the shape – offering clearer vision therefore providing better comfort.