Di octadecyl phthalate is primarily used as a plasticizer in the production of flexible PVC materials. It enhances the flexibility, durability, and workability of PVC products, making it suitable for applications such as cables, wires, and flooring. Additionally, it is utilized in the manufacturing of coatings and adhesives, where it improves the material's resistance to heat and weathering. Its stability and compatibility with various polymers also make it a valuable component in the formulation of inks and sealants.