1 Adhesives (how they work) and Friction
First let's see how smooth a material really is by looking at some material
under an electron microscope in the picture below. The mild steel shown
would look quite bright and smooth to the naked eye.
Click
picture to enlarge
So if these materials that are normally considered smooth to the naked eye
can look like this other materials not so shiny must be rough places indeed.
So how do adhesives work?
They work by moulding themselves into the comparatively rough surface of the
material they are forced into contact with. This is why if you apply
sticky tape to a surface with light pressure it is relatively easy to peel off,
if you place the tape under force the tape will take a lot more force to remove.
Similarly adhesive tape applied to glass is fairly easy to remove while the same
tape applied to paper will either remove the surface of the paper when you try
to peel it away or the paper may
tear, glass being very smooth and paper rough
because it is made up of fibres. Temperature has an effect because the
adhesive becomes softer and more pliable as the temperature increases so it is
able to conform to the surfaces more thoroughly.
The second way adhesives work is by suction. If the adhesive material
fits all of the contours of the surfaces it becomes in effect an infinite number
of tiny suction pads, of course this is not applicable if the surface is porous.
Thirdly adhesives can work by one or both of the above but instead of the
adhesive staying pliable as with surgical or skin tape they cure to become rigid
or semi rigid. Examples of adhesives that set are putty which contains
linseed oil which when exposed to the air oxidises and dries combined with the
filler to become rock hard and sticks to not only the frame but to the glass as
well. Old style paints work on oxidisation of the oil content hence the
old reference to 'oil paints',
Adhesive tape used on the skin are very special as they have specific uses,
the old zinc oxide fabric plasters that leave a mess on the skin if left on too
long and the hypoallergenic tapes with backing of paper, plastic, fabric or
special non tear wove style plastics. The adhesives can be made to stick
on damp skin, be designed for constant long term use on the same patch of skin,
for frequent apply and remove applications or of course any combination.
Most specialist tapes are designed to leave the minimum residue of adhesive on
the skin and be easily removed without harm to the skin beneath it.
Whatever the claims of individual types/brands all properties are comparative
and often trial and error has to be the end resort for particular and demanding
applications.
Friction
When two surfaces are in contact with each other there is friction between
them even if they are at rest. The amount of friction depend upon the
roughness of the two surfaces in contact with each other. When the high
spots on one material settle into the low spots in another there is a grip or
friction between them. look at the picture above to see that if two
surfaces that are relatively rough are together for one to slip over the other
the high spots of one surface have to be lifted out of the low spots of the
other which takes energy or force.
For any two materials there exists what is known as the coefficient of
friction which is constant. This constant value say wood on steel may be
0.5 if you need more friction then you must increase the force pushing the two
materials together so the force required to lift the high spots out of the lows
has to be greater. As the coefficient of friction is a constant if you
cannot increase the force on the two materials then the only option for
increased friction (perhaps termed as grip in our application) is to change one
or both of the materials for a higher combined coefficient of friction or make
the surfaces rougher.
Adhesives have a very high coefficient of friction but because in the case of
adhesive tapes the sticky surface is soft it can fail by deforming to the point
of rolling over the surface.
One most important point is that friction decreases at the point where
the two surfaces just begin to move. Have you ever thought why tape
or other holding devices are very effective perhaps for many hours and then all
of a sudden you feel a slipping sensation? Once this happens everything
speeds up and in a flash the tape has peeled off, the weight drops to your toes
or your testicles are twanged by the sudden release of the elastic strap.
There is no mystery why this occurs, it is simply because once two surfaces
start to move friction takes a plunge and as the tension or weight is constant
the forces speed up the movement to exacerbate the problem, failure is
inevitable and quick.
Šelks2002
Page last updated 19th March 2008