File menu > Export > Wavefront Material (*.MTL)
The ASCII-based MTL format makes it possible to exchange graphical data between many applications. The MTL format uses floating-point numbers to define each material's ambient, diffuse, and specular colors, as well as alpha and shininess values. Optionally, the MTL files can also specify texture-map filenames.
Source—Choose whether to export only materials used in the current scene, or the entire material library.
Export maps—When on, any map files used in exported materials are referred to in the exported MTL file. File paths are not included, so map files must be available upon subsequent importing.
# of Digits—Specifies the number of decimal places for exporting color values such as ambient and diffuse in floating-point format.
Compress numbers—When on, trailing zeroes are eliminated from decimal fractions. This makes the exported file smaller without sacrificing any precision, but can cause compatibility issues when importing with certain programs. If you're unable to import a MTL file, make sure this option is off and export again.