Long ago I realized I had a huge gap in my knowlege - that I really
knew nothing about plastics. I set out to cure that. Lately there was
a posting that talked about “plastic” seemingly as though it were
some single, simple thing, which it’s not. I’m no expert on the
subject - just the guy who’s writing about it.
I’d welcome corrections or elaborations. This posting is intended for
lay people - I may sacrifice absolute accuracy for
understandablility, at times. I just came to realize that many people
are like I was - “You know, plastics and stuff”.
Polymers are long chains of atoms - molecules. The ability of carbon
to bond with itself (through covalency and catenation) and form
complex molecules is the basis for organic chemistry and indeed life
itself. A polymer can be thought of as a string or a rope, and that
explains many of their properties. I thought of getting a bit deeper
into this, but realizing that polymers are long strings is enough,
I’d think, for the scope of this writing. All plastics are polymers,
but not all polymers are plastics. Hair, wool, chewing gum and
proteins are polymers, too. The most “famous” polymer of all is
probably DNA. Just like real strings, the polymer molecules can be
made into rope, netting (it can be “cross-linked”) and all sorts of
arrangements, which largely determines their physical properties. All
of this gets real complicated real fast when you look deeper into it,
but the analogy is pretty accurate.
There are a few divisions in the plastics industry: The first and
biggest is thermosetting or thermoforming. Thermosetting plastics
can’t be melted once they set - they will burn instead. Thermoforming
plastics (or “thermoplastics”) can be melted or at least softened,
and formed over and over again. Almost all of the common plastics are
thermoform plastics. Polymers are generally named by the main
“monomer” - the base molecule. Thus, polyethylene is a polymer of
ethylene, polystyrene is a polymer of styrene, etc.
There are thousands of plastics, but there are several main
families:
Polyethylene, or PE, is one of the most common plastics, and it
comes in far to many varieties to get deeply into here. There is
High-Density (HDPE), Low density, cross-linked (PEX) and several
others. It’s one of the most common plastics most people encounter:
plastic soda bottles, plastic wrap, food coverings and containers,
wire insulation, tubing, buckets and all sorts of containers are
made from it. It’s comparatively soft, has little chemical
resistance, and like polystyrene is used as an all around commodity
plastic, mostly. It has a few more technical uses, too. Polyester
thread and clothing is a PE polymer (PET). “Tyvek” is a high density
PE fiber that’s pretty interesting. It’s used in protective clothing
and tarps. It’s also the stuff of those slick, soft mailing packs
some vendors have been using recently, like the post office. You can
cut it, but try to tear or puncture it…
Polypropyline, or PP, is a bit harder and tougher than PE. While PE
has a molecule much like a string, PP is 3 dimensional, like rings
stacked on each other, giving it greater strength - also heat
resistance. Soda bottle caps are PP, as are microwave safe utensils,
Rubbermade products, disposable diapers. Also used in clothing. On a
Tic-Tac container, the lid is PP because of it’s fatigue resistance.
The living hinge on that top can be cycled thousands of times…
Polystyrene, or PS, is harder, brittler and cheap to produce.
Plastic model kits are almost always polystyrene, as are most
plastic cutlery, crystal cases for CDs and DVDs, licence plate
frames, utility lab plastic. If you bend it a bit and it snaps,
chances are it’s polystyrene. Foamed polystyrene is the well-known
styrofoam.
Acrylic is actually poly (methyl methacrylate) or PMMA. Marketed as
Plexiglass or Lucite in solid form, it’s also the acrylic in acrylic
paints. It has moderate properties - reasonable light transmission,
low impact resistance, very low chemical resistance. It’s cheap and
has a great many uses, first off as a substitute for glass when
impact resistance isn’t a factor… Clear plastic furniture or desk
accessories are usually acrylic…
ABS is a “copolymer”. Think of it as a blend… Tough, hard and
resilient, it’s used for drain pipes, auto bumpers and wheel covers,
protective headgear (football helmets and hardhats), plastic
clarinets and Lego blocks.
Polyvinyl chloride, or PVC, is the third most widely used plastic,
after PE and PP. It comes in many forms depending on what additives
and processes are used to make it. It is tough, cheap, and used
everywhere. All of the above mentioned plastics are “vinyl”, but if
something is called vinyl, it’s PVC. Piping, vinyl floor tiles,
vinyl siding, wire insulation, commercial signage (road signs),
clothing, children’s toys, waterbeds, pool toys, inflatable
structures are just a few of it’s uses. It is tough, chemically
resistant and fairly heat resistant.
Nylon is a Polyamide, which means it’s the same chemical family as
hair, proteins and wool. It’s properties are likely well known by
most readers -it is strong, it can be made into thread and woven or
cast into gears and rollers. It also has low friction properties,
which comes in handy with plastic gears.
Kevlar is a unique plastic that’s recent. Some things are just trade
names, which indeed “Kevlar” is. But it’s not just another name for
nylon, either. It’s a para-aramid synthetic fiber, for those who
want to know…
Epoxy is a generic term for thermosetting resins that are
copolymers. The “resin” is one monomer and the “hardener” is the
other. When mixed they form a new polymer which is hard and strong.
Epoxy’s properties are also pretty well understood by most readers,
I’d think.
Polycarbonates are so named because the strings are linked by
carbonates.
Also called PC. Crystal clear and hard as nails, this is the stuff
of safety glasses and bullet proof (resistant) glass. Also compact
disks (CDs and DVDs), drinking glasses, airplane cockpits and
windows. Reasonably chemically resistant, too. CR-39 is related
though not the same, and it’s the stuff of almost all plastic
eyeglass lenses.
And Teflon, which is actually a trade name for PTFE (not gonna spell
it out…) There are actually three of them with slightly
different properties. It’s a flouropolymer, which means it contains
fluorine (fluoride).
The fluorine makes it chemically impervious to almost all things.
It’s not just non-stick - nothing will stick to it. Put a rod in a
glass of water or grease and it will come out dry as a bone… It’s
first commercial use was as a pipe liner for uranium hexafluoride in
the Manhattan Project.
Aside from lining pots and pans it has more uses than there is space
here to enumerate. It’s also the “secret” ingedient in Gore Tex
fabrics - a layer of woven teflon sandwiched between other plastic
fibers.
There are others - the list goes on - polybutadiene is in tires,
polyurethanes are mostly made into foams and wood finishes, on and
on.
The other major family of plastics I’ll just mention: Elastomers.
Elastomers are elastic, and often called synthetic rubbers. They’re
actually just stretchy polymers, but they have such unique
properties they get their own category. Butyl rubber, nitrile
rubber, neoprene, others. I don’t know a lot about them, plus I
don’t want to take the space here to get into what is another
article.
Likewise Silicones, which are essentially polymers with silicon in
place of the carbon chain - oversimplified, but largely true. They’re
not much used in jewelry and also a long story. Plus silicones are
almost always used in liquid form, gels, foams or greases.
And there are thousands of additives and processes that give
plastics different properties - plain old PVC is hard and tough. Put
in “plasticizers” and it becomes soft and supple, but it’s still
PVC… Finally, I’ll add the resin codes here. Supposedly everything
plastic has the resin code stamped on it these days, to aid in
recycling. It’s the number inside the triangular arrows, and tells
you what plastic it’s made from:
1 - PET polyethylene terephthalate - the most common everyday
plastic.
2- HDPE High Density polyethylene - grocery bags, bottles…
3- PVC
4- LDPE Low Density polyethylene “Six pack rings” among a great many
other things
5- PP Polypropylene Auto parts, food containers, dishware
6- PS Polystyrene
7- Other…
Just enough so that people will understand some things… the whole
field takes years to grasp…