CM8001

Acetone dissolves the polymer, which allows the gas of the foaming agent to escape, causing the polymer the collapse on itself and become more dense.

a. LDPE cannot be used in this application because it does not have the required strength.
b. To have the strength in this application, the HDPE molecules must line up in a way that produces the required strength, which the linear molecules allows the sufficient flexibility.

When plastic is stretched, the molecules become aligned parallel to each other and in the direction of the pull. This change in its structure is irreversible and if the pulling continues the plastic will break. When paper is pulled, the paper tears instead of stretching. The cellulose molecules in paper are held rigidly in place, thus they are not free to become aligned as in plastic.

a. The four properties are:
(1) stable over time of intended use
(2) non-toxic.
(3) Lack of solubility in body fluids
(4) Lack of reactivity in body fluids
b. Several types of polymers are commonly used. Polymethyl methacrylate (PMMA), fluoro-silicone-acrylate polymers and fluoro-silicones. Polymacon (38% water) is typical of the polymers used for soft lenses and is a polymer of 2-hydroxyethyl methacrylate (HEMA). Other methacrylates include hioxifilcon
(48% water) and methafilcon (55% water) or even lidofilcon (70% water).
Desirable properties include being nontoxic, permeable to oxygen, comfortable to wear, and inexpensive. Also desirable is the ability to conform to the shape of the eye and to be easily cleaned (if not disposable).
c. Hard contact lenses are typically made of PMMA, a rigid non-gas permeable plastic. The soft contact lenses that replaced them are made of
silicone, which is flexible and allows oxygen to reach the eye. Because of these properties, the soft lenses tend to be more comfortable.