When choosing an alternative to a down blanket, consider the fiber the blanket is made of. Synthetic fibers are polyester, acrylic, fleece and vellux. Polyester is a man-made material that is strong, durable, does not stretch, shrink or wrinkle and makes a good insulation in blankets because it is warm. Acrylic is inexpensive hypoallergenic, soft and warm. Fleece is made of acrylic but processed to be very soft. Vellux is made of nylon and foam fibers bonded together; the result is a soft, thick, well-insulated blanket.
Wool is one of the warmest natural fibers available. Most people do not like the scratchy feel of wool fibers. You can make an alternative down blanket by placing a wool blanket inside a duvet cover. A duvet cover is made of two large cotton, or silk sheets sewn together on three sides. Stuff a wool blanket inside the duvet cover. A wool blanket is heavier than a down blanket, but with the duvet cover over it, it will feel comfortable and as warm as a down blanket.
Another aspect to look for in an alternative to a down blanket is the manner in which the blanket is constructed. Down blankets are usually put together with two pieces of lightweight, cotton or silk sheet outer materials, sewn together and filled with down feathers. The blanket is usually quilted as well. Look for a similar type blanket, filled with a polyester microfiber, acrylic or vellux. Polyester microfiber is the most common down alternative.