Blended Fabrics are made up of blended yarns. Blended yarns contain fibers of different composition in fixed proportions. Thus a blended fabric may be made of polyester/cotton in 67:33 ratio in both warp and weft.
Union fabrics are the fabrics where in the fibre content of warp is different form that of weft. Thus a Silk/Viscose union fabric may have silk in the warp and viscose in the weft.
An excellent study on silk/viscose union fabrics can be found here.
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