Fibrous cellulose composite resin and production method therefor, and resin reinforcing material

A method for producing a fibrous cellulose composite resin, comprising:

• defibrating a cellulose raw material into microfiber cellulose and kneading the microfiber cellulose with a resin to obtain the fibrous cellulose composite resin, wherein the defibrating is performed so that the microfiber cellulose has an average fiber width of 0.1 μm or larger, an average fiber length of 0.02 to 2.0 mm, wherein 20% or more of the microfiber cellulose has a fiber length of 0.2 mm or shorter, and a percentage of fibrillation of 1.0% to 30.0%,
• the method further comprising:
• prior to or following the defibrating, modifying the cellulose fibers by substituting part or all of hydroxyl groups of the cellulose fibers with carbamate groups, and
• prior to or during the kneading, adding an acid-modified resin and a dispersant, wherein the acid-modified resin has a molecular weight that is higher than a molecular weight of the dispersant.

A reinforcing material for thermoplastic or thermosetting resins, comprising:

• fibrous cellulose and an acid-modified resin,
• wherein part or all of the fibrous cellulose is microfiber cellulose,
• the microfiber cellulose has an average fiber width of 0.1 μm or larger, an average fiber length of 0.02 to 2.0 mm, wherein 20% or more of the microfiber cellulose has a fiber length of 0.2 mm or shorter, and a percentage of fibrillation of 1.0% to 30.0%, part or all of its hydroxyl groups being substituted with carbamate groups, and
• part or all of the carbamate groups are ionically bonded to acidic groups of the acid-modified resin.