Why does PVC hose become hard and yellow?

Spiral Hose

Hose Suppliers

oil discharge hose

Layflat pvc hose for irrigation

SAE 100 R6 Textile Reinforced Hose

Air Compressor Hose

PVC Steel Wire Reinforced Hose vs. Polyurethane Hose

Clear PVC Hose

Heavy Duty Layflat Hose