From: "Barry L. Ornitz" <ornitz@dpnet.net>
Subject: Plastics
Date: 03 Jun 1999
Newsgroups: rec.radio.amateur.antenna

schoo@my-deja.com wrote in message <7j5tel$egs$1@nnrp1.deja.com>...

>I have a stupid question I'd like to pose to someone who works in the
>field. How do you determine what material a plastic case is made of and
>how do you decide what to do about problems?

This not a stupid question at all.  It is a difficult one to answer too.
Ed (ekefkcfd@ptbo.igs.net) did a very good job.  I'll try to add just a
little to his reply.

>For example you have a late 50's to early 60's table set with a
>discoloration on the top. You don't know what it is or how it got there.
>It looks like maybe a stain but it could be from years of tube heat.

Some of the modern plastics were developed during and soon after WWII.
Before this time period, Bakelite (phenol-formaldehyde) and cellulosics
were the mainstays.  In consumer radios, Bakelite continued in popularity,
although styrene polymers started being used.

Bakelite is a thermoset polymer.  It does not melt when touched by a
soldering iron, and it does not dissolve in solvents.  It emits an acrid
phenolic odor when heated.  This is the odor of a burnt resistor or a
shorted ballast in a fluorescent light fixture.  Take a scrap of phenolic
and hold it in a flame for a few seconds.  Take it out and smell it.  The
odor is quite distinctive.  Heat can discolor Bakelite and some materials
may stain it.  Since solvents do not hurt Bakelite, they may be used in
cleaning.  However, polishing with a very mild abrasive will often do a
better job.

Styrene-based polymers are very soluble in many solvents.  They are also
thermoplastics, meaning they soften when heated.  Try to avoid solvents
with these.  There have been cases where a solvent that did not dissolve
the plastic, but caused future damage by stress corrosion cracking.  I know
of one particularly nasty case where the solvent in Armor-All caused major
damage to ABS plastics.  If you polish or buff these plastics, be very
careful about overheating them.  This happens quickly so be very careful.

>Or say you have another radio that was obviously white when new. The
>inside of the cabinet is white but where light hit it, it has badly
>yellowed. Can you remove the yellowing with bleach? If you do will it
>somehow degrade the integrity of the plastic?

The yellowing may come from a past owner who was a smoker, but as you note,
ultraviolet degradation is the likely culprit.  The bleach will likely do
little as the yellowing comes from the breakdown of the polymer chain, and
it (the color) is trapped in the plastic.  The bleach can only work on the
surface.  When ultraviolet light damages a plastic in this way, the polymer
chain is weakened leading to increased brittleness as Ed noted.

>Another example. I have a white Princess telephone that has yellowed and
>is dull. This is one of the old kind with the permanently attached
>handset cord. Is there a way to whiten it up and gloss the finish without
>damaging the plastic?

In addition to Ed's answer, I will add that some telephone sets were made
from plasticized cellulosic polymers.  I am not sure if these were still
used in the early days of the Princess phone.  Both ABS and cellulosic
plastics are very sensitive to solvent attack.  If you should decide to
repaint, consider the solvent in the paint too.

In plasticized plastics (virtually all vinyl plastics, and some
cellulosics), a material called a plasticizer is added to the basic plastic
during compounding and molding.  Polyvinyl chloride is a hard, brittle
material (like the black PVC plastic drain pipe).  But when plasticizers
are added, the material can become soft and supple.  Various plasticizers
are used and some are more volatile than others.  Very soft vinyls can
contain as much as forty percent plasticizer or more.
Over time, the plasticizer gradually evaporates from the vinyl, leaving it
stiff at first, and eventually brittle.  A good example is the vinyl used
in automobile seats.  When you park in the hot sun, the plasticizer
gradually evaporates off.  Some settles on the windshield making a sticky
mess which is difficult to remove.  But notice how the seats gradually
harden and eventually crack - due to this loss of plasticizer.  Contrary to
some advertising hype, there is no practical way of adding plasticizers
back to the material.  The best you can do, with proper care, is to slow
down the rate of loss of the plasticizer.

Once you go past the 1960's, many new plastics were introduced.
Identifying them is not a simple task.  I once had a devil of a time
identifying some particular plastic used in a Tektronix oscilloscope.
Infrared spectroscopy finally identified it as polyacetal (Delrin).  It is
rarely ever worth this much trouble, however.  In this case I was looking
for a surface etching method and glue that would work without requiring new
parts to be molded.

        73,  Barry L. Ornitz     WA4VZQ     ornitz@dpnet.net

From: "Barry L. Ornitz" <ornitz@tricon.net>
Subject: Re: Ultrasonic Welding
Date: 05 Jun 1997
Newsgroups: sci.engr.joining.welding

Bruce Brodnax <bbrodnax@sun2.lib.uci.edu> wrote in article
<5n5dp9$am9@news.service.uci.edu>...
> In article <3395d8d4.14504090@news.mindspring.com>, mabry@mindspring.com
> wrote:
> >I know that plastics and metals can be joined using ultrasonic
> >welding.Could paper be joined-sealed using ultrasonics?
>
> No. Paper is best describe as an unwoven mat or felt of cellulose fibers,
> and cellulose doesn' melt, short of enzymatic reactions. Burns good, tho'!
> :-)

Paper can be manufactured, however, with thermoplastic fibers incorporated
in the web.  Tyvek disposable medical garments are an extreme case where
nonwoven thermoplastic fibers make up the entire web.  All sorts of blends
are available with compositions between pure cellulose and pure
thermoplastics.

Many of these blends ARE suitable for ultrasonic bonding as well as
conventional thermal and dielectric bonding.  Coated papers are often also
suitable for such bonding methods.

	Dr. Barry L. Ornitz   WA4VZQ   ornitz@tricon.net