Westlake Chemical (WLK), Rockwell Holdings (ROC), and OM Group (OMG) continued the trend of chemical makers reporting mixed results when they announced fourth-quarter earnings early Tuesday. Westlake Chemical makes commodity petrochemicals and polymer materials. The company said Q4 earnings were $1.42 per share vs. 40 cents in 2011, beating analyst expectations of $1.23. Sales fell nearly 7% to $801 million due to lower sales of polyethylene and