|MadSci Network: Physics|
Chris, Density is not the main property that determines the suitability of a material to be used for a container to hold another material. A container material must have, among others, the following properties 1. The container and its contents must be mutually insoluble. 2. They must not react chemically. 3. The actual molecules of the container must be large and the overall structure can't contain macroscopic or molecular sized voids. Some engineering considerations are: 4. The material must be moldable, machinable or capable of being assembled into a monolithic shape that can be capped or sealed. 5. The bottle etc must be of sufficient strength to contain the contents physically. 6. Reasonable cost. 7. Stability over expected product life. 8. Ability to be recycled or biodegradability. There are many others. Some common material choices for water based contents are glass, hydrocarbon based polymers such as polyethylene, polypropylene and polyethylene terephthlate[PET]; glass, and metal cans. The latter are usually given a protective coating to slow corrosion. You can find examples of the above [check the recycling codes for identity] and maybe figure out why one is used over another for a particular use. Many composite materials are used such as waxed paper milk cartons mostly replaced by plastic coated paper cartons, aluminum backed paper The aluminum is a liquid-vapor seal. Paper by itself is poor for water because of its fibrous nature and because it tends to dissolve water. Paper based materials such as cellulose acetate[cellophane] are semipermeable and pass water while slowing the flow of ions and other dissolved materials. Examples of naturally occurring composites are cell membranes. skin, frog's skin. egg shells [a specialized cell membrane] Indeed any material becomes permeable to something when made thin enough. Thickness is important to maintaining a container's usefulness. There is often a tradeoff between useful thickness and cost and strength that sometimes is not gotten right and the container fails in use. When is density important? When examining properties of very similar materials. Polyethylene[PE] is a good example. Paraffin wax is essentially a very low molecular weight and low density PE. It is water insoluble and wax coated cardboard makes good containers for milk and fruit juices. However, paraffin dissolves in gasoline so waxed paper fails as a container for organic solvents. High density PE,especially lightly crosslinked, has molecules much too large to dissolve in gasoline and intermolecular spacing too small to dissolve gasoline molecules so it is suitable for large organic molecules. Some smaller molecules such as benzene can cause PE to crack because it fits between the molecules. Another example is wood. A boat of balsa wood would absorb water and not be as water tight as one of more dense spruce or mahogany. Another example of density importance is in hydrophilic [water loving] soft contact lenses. These are water soluble polymers that are prevented from dissolving by control of molecular weight and by the correct amount of crosslinking, usually less than 0.5%. This controls the strength and softness of the material and to some extent the permeabiliy of the lens to dissolved materials. The pores are usually large enough to be open to water and saline but the material can be semipermeable to other materials A search on Google or MSN search for "permeation of liquids" gives too much info; see what helps. Likewise a search on "bottles". Here is one hit on permeation. http://www.diffusion-polymers.com/ Plastics or polymers are widely applied as elastomers, fibres, coatings, adhesives and structural composites in the chemical, construction, food and pharmaceutical industries. This because of their chemical resistance, low weight and mechanical strength. However, polymers are not completely chemical tight. Although the polymer or composite seems to resist chemicals, they move through. This so called diffusion, permeation or migration is key to chemical degradation, reduces the mechanical performance of the plastic and affects the material or environment that a plastic must protect. Diffusion-polymers.com supports you with diffusion and chemical resistance of polymers. We share our knowledge by an interactive forum, tabled diffusion figures, solubility parameters and experimental information. Request professional diffusion support when you need additional knowledge, experiments or specialized contract research. Your inquiry may concern any application in which the rate, amount and related effects of diffusion on material and environme.nt has to be determined As you can see this is an area of study and commercial interest. When you pick up a bottle think about why the material was chosen both from an aesthetic and engineering-chemical viewpoint. Regards, JimG
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