The materials sector, as represented by the XLB ETF, experienced a significant downturn, falling 2.00% in the after-hours session on top of a 2.3% decline during regular trading. This move was contrary to the broader market, which saw significant gains. The weakness in materials appears to be driven by sector-specific concerns, particularly after Graphic Packaging International lowered its full-year guidance, citing persistent inflation. This negative outlook overshadowed a strong earnings report from Westlake Corporation, another key company in the sector.