Can someone explain why carseats expire

I know they do, but in cases where a seat hasn't been used or stored in extreme temperatures I don't understand why they do. I would never use an expired seat, but my sister thinks it's ok since she has kept it stored in her air conditioned home for the last 3 years. I want her to stop using it, so if someone could let me know why they expire even if not in use I will pass that on to her.

Here is what I tell people:

1. Plastic expands and contracts with heat and cold, weakening it. If you've ever seen a plastic toy left out in the yard that is white and brittle, it's the same thing, it just takes longer because it is thicker plastic. If you go to youtube and search for "10 year old Britax" you can see a crash test of a expired seat, the harness rips through the plastic shell and strangles the test dummy.

2. Parts go missing, labels fall off, companies may not carry the replacement parts any more.

3. New technology comes out to keep our children safer all the time, this phases the seats out that aren't up to the new standards.

I know the first one isn't what you were looking for, but the second 2 might help.

I'll add that light breaks down plastic, possibly even more than heat/cold cycling.

In addition to the fact that seats improve over time with knowledge, nonobvious but important parts may be missing & plastic degrades over time (more so in extreme weather climates, especially hot), I also explain the following. . . .

My family makes post-operative prosthetic sockets for amputees (instead of traditional casting after surgery, which takes 3X as long to heal from). We use the same plastic that carseats are made of. These prosthetic sockets are stamped/imbedded with a warning that they are only intended for use with 1 person & only for about a total 90 days' worth of constant rehabilitation. One item keeps the knee bent & another allowes the knee to bend (for more advanced physical therapy). The only legal liability case we've been involved with was when a patient wore the straight leg socket up & down his house stairs. The plastic gave out & he tumbled, requiring an above knee amputation (Judge ruled it was the patients fault for not listening to his practitioner & failing to read the warning on the item itself).

Expired carseats are even more risky because of the speed at which crashes occur. Even 30 MPH crashes are violent. A 30# child in a car that crashes @ 30 MPH = about 900 pounds of force on the carseat. You want to make sure the carseat isn't in any way compromised to ensure that the child won't be ejected. Ejected occupants are about 4 times as likely to die, but "death is merely the tip of the iceberg".

It's just all about following the directions. Read the manual, don't make up your own rules