MadSci Network: Physics

Re: If plastic is less dense than water, then why doesn't water

Date: Sun Aug 14 22:31:38 2005
Posted By: James Griepenburg, , Chemical consultant, Chemmet Services
Area of science: Physics
ID: 1119670950.Ph


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 
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.

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. supports you with diffusion and chemical 
resistance of polymers. We share our knowledge by an interactive forum, 
tabled diffusion figures, solubility parameters and experimental 

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.


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