Occidental Chemical Corp.'s planned acquisition of two polyvinyl chloride plants from Tenneco Polymers Inc. was blocked last night by a federal judge here only hours before the deal was to be completed in Houston.

U.S. District Judge Harold H. Greene, acting at the request of the Federal Trade Commission, issued a temporary restraining order halting the $70 million sale on the grounds that it was "not in the public interest."

Greene said that absorption of Tenneco's two plants, in Pasadena, Tex., and Burlington, N.J., in Occidental Chemical's operations would lessen competition in the PVC market and that probably would raise prices on such items as water pipes and phonograph records.

Ronald B. Rowe, assistant director of the FTC's Bureau of Competition, told Greene that the commission voted earlier in the day to ask for the court order after several days of meeting with both companies. He said the commission had found that the loss of competition would be "significant . . . and imminent" if the sale went through.

But attorneys for both companies questioned the FTC's timing in making the request less than nine hours before the sale was to be completed.

"All the parties are gathering down in Houston. . . . The money has been taken to settlement," said William C. Pelster, an attorney for Oxychem's parent company, Occidental Petroleum Corp. He said the merger was announced Dec. 9 and that the companies had made most of their filings with the FTC by Jan. 3.

Theodore Weiss, an attorney for Tenneco Polymer's parent firm, Tenneco Inc., told Greene that his company, which loses money on the two PVC plants, had been trying to sell them for five years and that "harm is being done as we sit here today."

Weiss argued that further delays in completing the sale would drive away customers and employes.

But in granting the 10-day temporary restraining order, Greene said none of the harm that Tenneco might suffer was "enough to outweigh the public interest" in blocking the sale.