To repair a cracked plastic seat on an outdoor bench after a few years in the sun, you first need to accept that the plastic has become brittle from UV exposure. A lasting repair needs to close the crack and reinforce the area so it does not reopen.
Start by moving the bench into the shade and cleaning the cracked area with warm soapy water, then rinse and let it dry completely. Lightly scuff the area around the crack with 120-grit sandpaper and wipe it with rubbing alcohol. If the crack has opened up, use a clamp or ratchet strap to gently pull the edges back together before applying anything.
For most outdoor bench plastics, a two-part plastic epoxy works better than super glue. Choose one labeled for polyethylene, polypropylene, or outdoor plastics. Mix it according to the instructions and press it deep into the crack with a small spreader or toothpick. If you can reach the underside of the seat, lay a fiberglass mesh tape or a thin piece of plastic over the crack while the epoxy is still wet. This spreads the load and makes the fix much stronger.
If the plastic is thick and you have a soldering iron or plastic welding kit, plastic welding is another solid option. Heat the crack edges just enough to melt them, then run a matching plastic filler rod along the joint. Zip ties can work as filler rod in a pinch, but they need to be the same type of plastic.
Let the repair cure for at least 24 hours before putting weight on the bench. Once hardened, sand the area lightly with fine sandpaper to smooth it. If the bench will stay in the sun, finish with a UV-resistant clear coat or spray paint made for plastic. A cushion or shade cover will also help prevent future sun damage.
If the plastic is chalky, crumbling, or has multiple cracks spreading from the same area, the seat is probably too far gone. In that case, replacing the seat or switching to a wooden top is the better long-term fix.