Paint bottle caps cracking

Posted in Custom Workstation

Anybody ever have the white plastic bottle caps on Apple Barrel brand acrylic paints crack apart on them, for no apparent reason (a horizontal crack around the diameter of the cap)? It's happened to be twice now, with both the 8 ounce gloss red and blue (hasn't happened with any matte ones yet.) In both cases, the paint leaking out of the crack dried and hardened without making a huge mess (I generally don't make it a policy to store my paints upright, they're just as likely to end up on their sides or upside down when I'm done with them.) Any idea what's causing this? Humidity/temperature fluctuations, moisture (from making paint washes inside the cap), tightening the caps too much, build-up of dried paint inside the caps, etc? Those particular paints were admittedly pretty old, although still perfectly usable (used the blue one yesterday in fact, which is how I discovered the crack), but I have other colors that are just as ancient, and those bottles are fine.