Some records are made of styrene, a cheap and very easily degradable through "normal" wear, plastic. Since I'm a layman and not a chemist (or something smarty pants!?), read this internet nerd quote (from this site) 'cause it explains how styrene was used to make phonograph records:
Styrene (properly, Polystyrene). Hard, relatively inflexible plastic used to press records, mainly 7-inch singles, mainly using the Injection Moulding process. The material is heated to a liquid form and is then squirted or injected into the closed stampers in the press. This requires that the labels be either glued or painted on after the record leaves the press. The cost savings to the manufacturer comes from the extended life of the stampers because of the lack of a heating cycle to the stampers. The material can also be reused without noticeable change to its moulding properties. Styrene records will therefore usually have very quiet surfaces when found in an UNPLAYED Mint condition, but unfortunately they will wear to a noisy condition rapidly, especially if played with a bad stylus or an improperly tracking tonearm. They also are more prone to Cue Burn. The Columbia Records Pittman, New Jersey pressing plant was once the major source of Injection Moulded Styrene pressings, and pressings from this plant are found on MANY small labels. Look for the glued-on labels. Painted-on labels can be found on records from the Amy/Bell/Mala group.
Styrene was used to make records for years. Even here in the NW it was popular...wonder why your Sonics 45 looks great but kinda sounds...dirty? Pressed on STYRENE. Wonder why the title sticker on your Golden Crest label Wailers 45 just FELL the fuck OFF? Pressed. On. STYRENE!! The material is so poor an unplayed record might have inherent issues. It's obvious when crisp highs/or an "S" sound might playback as "ssshhhh" or the loud bits sound distorted. So, if you hear an occasional odd rattling/buzz over top the music on that "dead stock/unplayed/M-" $300 funk 45 you just got? Aw shit...STYRENE!!! Even with a nice (expensive) stylus there can be problems getting good playback...sometimes it can make a styrene 45 sound better, sometimes it doesn't. And don't ever add tracking weight to the tone arm if you hope to get the stylus deeper in the groove, that could make things worse! Styrene is SOFT and you might effectively sandpaper out the grooves, degrading the sound and causing hiss. Old turntables, Dansettes and portables, even home hi-fi/stereos, had tone arms that could weigh TEN GRAMS, so if vintage is your style you might NOT wanna play your (depending on how much you paid for 'em, I reckon) old 45s on that old gear. Styrene is also fragile. A couple times when cleaning a styrene 45 it BROKE IN MY HAND, cracked from the edge to the spindle hole. HAVE I MADE MY POINT YET?
Now, with all that said, as so many labels used styrene, if you buy old 45s, it is impossible to avoid. If you care (provided you're still reading!! HA!), you can kinda tell the difference without even playing the record, a styrene 45 weighs less than vinyl 45s, and, when tapped on the edge, say with a ring, it "ticks"...vinyl "tocks."
Okay...styrene is EVIL, but...I DO have plenty of styrene 45s that play great. Of course, I dont' play em out, not if they cost much, or play them often, but I can burn 'em, right, I'll listen to 'em that way. If I wannna play a styrene 45 out, if its a cheapy, I'll buy a double and play that copy. And if you care, as a fellow record nerd, just make sure your gear is set up properly AND you have good gear, not even GREAT gear, but gear that RESPECTS the medium. Um...I know that sounds goofy, but REALLY, it counts.
There you go, your record nerd/collector lesson for the day.
(This post is regarding comments in THIS post, BTW.)