In a widely reported local regulatory filing Lotte said it had made a proposal to the Atlanta company on June 3 but named no price.
The two companies are together building a 1 million tonne a year ethane-based ethanol plant in Lake Charles, La.
The South Korean company's intervention comes as Westlake's $3.1 billion hostile bid will effectively get put to a vote at Axiall's annual shareholders' meeting on June 17, where Westlake wants to replace the board with its own nominees.
Axiall in April said it received and rejected a revised Westlake offer worth $14.00 cash and 0.1967 of a Westlake share per Axiall share after extensive discussions with the Houston company.
Axiall later requested a revised takeover proposal by June 3 but Westlake demurred.
Axiall has said it's exploring various strategic alternatives and had already planned to sell its building products division, which Westlake opposes.
Westlake shares closed up 73 cents, of 1.6%, at $45.64 on Monday. Axiall shares closed up 58 cents, or 2.6%, at $23.30.
Based on Westlake's close, its offer for Axiall is worth $27.98 per share, or 20% more than its closing price.