Multi-layer laminates of microporous films

A method of making multi-layer, microporous polyolefin sheet materials comprising the steps of

• (a) forming at least two microporous polyolefin layers by melt blending the polyolefin with an additive which is miscible with the polymer at the melt temperature of the blend but which phase separates on cooling, forming a layer from the solution, cooling the layer at a rate and to a temperature to initiate non-equilibrium phase separation, and cooling to solidify the layer;
• (b) extracting the additive from each layer with a solvent capable of swelling the polyolefin and contacting the solvent wet layers or contacting the solvent wet layers and extracting the additives from the contacting layers;
• and
• (c) drying the contacting layers to remove the solvent and effect bonding between the contacting layers.