Technique for selectively controlling the porosity of a polymeric material

#Significant efforts have been made to produce porous polymeric materials to improve the use of natural resources and reduction of the carbon footprint in finished products. A typical approach to initiating pore formation in polymeric materials is by foaming the polymer using physical or chemical blowing agents, which create gas-filled pores though the bulk. Chemical blowing agents are compounds that undergo chemical reaction liberating gas that creates the pore structure through the bulk of the polymer. Physical blowing agents are typically compressed gases that are dispersed in the polymer and expand creating the pores. Regardless, typical foaming processes induce low molecular orientation because the pore formation happens when the polymer is in the molten state. This reduces the melt strength, thus leading to breaks in high speed production processes with high deformation rates (e.g., fiber spinning, film formation, molding, etc.). Another problem with such materials is that, even if they can be successfully manufactured, they tend to be stiff and thus difficult to manipulate into different shapes during use of the material.

#As such, a need currently exists for an improved technique for creating pores in polymeric materials.

#In accordance with one embodiment of the present invention, a method for controlling the degree of porosity in a polymeric material is disclosed. The material is formed from a thermoplastic composition containing a microinclusion additive and nanoinclusion additive that are dispersed within a continuous phase of a matrix polymer in the form of discrete domains, and at least a portion of the polymeric material is strained so that a porous network is formed therein. The method comprises heat treating at least a portion of the strained polymeric material, wherein the heat treated material exhibits a lower pore volume than the material prior to heat treatment.

#A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, which makes reference to the appended figures in which:

#FIG. 1 schematically illustrates one embodiment of a polymeric material that may be formed in accordance with the present invention;

#FIG. 2 schematically illustrates another embodiment of a polymeric material that may be formed in accordance with the present invention;

#Repeat use of references characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention.

#Reference now will be made in detail to various embodiments of the invention, one or more examples of which are set forth below. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations may be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.

#Generally speaking, the present invention is directed to a technique for selectively controlling the porosity in a polymeric material. The polymeric material is formed from a thermoplastic composition containing a microinclusion additive and nanoinclusion additive dispersed within a continuous phase that includes a matrix polymer. To initiate pore formation, at least a portion of the polymeric material is initially strained (e.g., bending, stretching, twisting, etc.) to impart energy at the interface of the continuous phase and inclusion additives, which enables them to separate from the interface to create a porous network. The material may also be strained in a solid state in the sense that it is kept at a temperature (“straining temperature”) below the melting temperature of the matrix polymer. Among other things, this helps to ensure that the polymer chains are not altered to such an extent that the porous network becomes unstable. For example, the material may be strained at a temperature of from about −50° C. to about 125° C., in some embodiments from about −25° C. to about 100° C., and in some embodiments, from about −20° C. to about 50° C. The straining temperature may also be below the glass transition temperature of the component having the highest glass transition temperature (e.g., matrix polymer, microinclusion additive, etc.). For example, the straining temperature may be at least about 10° C., in some embodiments at least about 20° C., and in some embodiments, at least about 30° C. below the glass transition temperature of the matrix polymer and/or microinclusion additive.