Re: [APD] Silicone that really sticks

["S. Hieber" <shieber at yahoo_com>]

Yes, and sort of no.

Technically, the automobile windshield glass is merely
annealed but also heat treated (heated and more or less
rapidly cooled to make it brittle), so that it breaks into
many small pieces rather than fewer large ones. You can see
evidence on the glass of the cooling drafts of air by
viewing the glass in sunlight while wearing polarized lense
-- evident will be a distinct pattern where the cooling was
slightly uneven depending on how the drafts passed over the
glass. It is not the same process as full glass-tempering.
If you've ever seen a hose window pane break, you know what
I mean -- a 2'x3' pane might break into only two or three
pieces. If the windshiled broke into large pieces (say,
cracked in half), the large pieces would be heavy
sharpe-edge "guillotine" blades of glass. Rather, with an
automobile windshieled, if it gives, it is into many
thousand small pieces. Photographs of broken windshield
reveal this. The layer of plastic (polyvinyl butyral --
PVB) also helps to cause breaks to go into many small
pieces rathn than roughly linear splits. So it is not
tempered glass in the proper sense, but it is not ordinary
glass ike you might find in a house window.

Does road blindness happen if something hits it while going
down the road? You bet. It happended to my wife just last
February while driving down the highway in her two-week old
vehicle! It was more than a stone that hit the windshield
but instant road-blindness was the result. Still, that was
preferrable to a 2' by 3' sharp-edged piece of glass coming
at her at 40 mph. The thin layer of polyvinyl butyral
inbetween the glass layers held most of the glass pieces in
place. However, months later, and after repeated and
rigorous cleanings, we still occassionally find tiny bits
of glass here and there.

True "tempered" glass is about 5 times stronger than glass
that is merely annealed. The outer layer is compressed and
the tension between the surface and the glass underneath
stores a lot of energy -- so tempered glass sort of breaks
from the inside out, which can, in some circumstances yield
clustering or larger pieces (or collections of pieces) of
debris than annealed and layered glass.

Another kind of saftey glass, more expensive than the
others is a type that has a plastic layer on the side
facing vehicle occupants. The plastic film holds the layers
together even if the glass breaks and more, it acts as a
barrier against glass from the outer layer entering the
vehicle. Regulations do not require this type of glass on
automobiles.


Scott H.
--- Vaughn Hopkins <hoppy1 at surewest_net> wrote:

> Automobile windshield glass can't be tempered.  Imagine
> the effect of 
> driving along at 70 mph and a small stone hits the
> windshield!  Instant 
> blindness!  Windshields are just laminated with a thin
> plastic layer in 
> between two ordinary pieces of glass to hold the pieces
> in place if it 
> breaks.  And, the breaks are all into large pieces, not
> the tiny pieces 
> that tempering causes.
> Vaughn H.
> 
> On Saturday, September 10, 2005, at 06:37 PM, Liz Wilhite
> wrote:
> 
> >  You sure about the layering? I thought there was
> tempered glass that 
> > is a
> > single layer used in window panes and for side and rear
> windows in 
> > cars and
> > laminated safety glass that is 3 layers, also known as
> auto glass, 
> > that is
> > used for windshields among other things. Could easily
> be wrong, but if 
> > fully
> > tempered glass is laminated it is news to me.
> >  Liz
> 
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> 


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