27 Jun 2003, 04:44, Charles Angelich (1:106/2000), wrote to Phil Marlowe:

Hi Charles.

 CA> I had mentioned this aging process quite some time ago for ALL
 CA> plastics and everyone (then) insisted I was wrong. 

 CA> One of my former employers specialization was plastics
 CA> engineering. He told me that all plastic continually shrinks
 CA> although at a much decelerated rate after the first few weeks
 CA> and will return to various carbon compounds and crumble. As I
 CA> remember it he mentioned 10-15 years. I have managed to keep
 CA> various plastic objects for that long and they do crumble if
 CA> you try to disassemble them. 

Shrink it does!  Back in the late 1970s I bought several reels of surplus
ribbon cable for something like $7.50 per 100 foot roll.  The supplier
clearly identified the product as having "some shrinkage", but
the point went over my head until I received it and tried making computer
cables from it.  That shrinkage referred to width-wise shrinkage of the
insulation.  When I tried making 50-pin cables with crimp-on connectors, I
found that when wire #1 was aligned with pin #1, wire #50 was over pin #48!
 Finally ended up slitting the ribbon after every fifth lead to make up for
the shrinkage.

 CA> I don't think sunlight is a factor but heat definitely is a
 CA> catalyst. 

Different plastics react differently with sunlight.  The polypropylene used
in gallon milk bottles will crumble to a gritty 'sand' in about three years
of sunlight, while the flexible polycarbonate used in two-liter cola
bottles seems to be totally unaffected by sunlight.  The rigid version of
that polycarbonate is used for CDs, and it has an opposite reaction with
sunlight than the flexible version.

One plastic that is immune to reasonable amounts of heat is Bakelite(TM),
which is most often found on electrical switches, outlets, toaster bases,
stove knobs, cookware handles, etc.

More recently, we are seeing a flood of ABS plastics in computer parts,
floppy drive faceplates, CD drive faceplates, monitor cases, etc.  Only a
few years ago (maybe 20 or so), ABS was only available in black, and was
used for sewerage pipes because of its chemical resistance.  The 'ivory' or
white versions used on computers most definitely yellows in sunlight.  I
believe (but could be mistaken) that ABS is a coal plastic rather than the
more common petroleum plastics.

Enough rant...


     Good luck...  M.

--- Msged/386 TE 06 (pre)