Sizing composition for mineral fibers and resulting products

A method of manufacturing an insulation product, comprising:

• sizing mineral fibers with a sizing composition comprising a liquid phenolic resin and an extender, the liquid phenolic resin having a free formaldehyde content, expressed with respect to the total weight of liquid, of not more than 0.1%,
• (a) said liquid phenolic resin being obtained by a first process comprising
• mixing phenol and formaldehyde in an aqueous solution in a presence of a basic catalyst to form a formaldehyde/phenol mixture that has a formaldehyde/phenol molar ratio between 2 and 4,
• heating the formaldehyde/phenol mixture to a temperature between 60° C. and 75° C.,
• after said heating, cooling the formaldehyde/phenol mixture while introducing an alkanolamine to trigger the Mannich reaction and form a reaction mixture that consists essentially of phenol-formaldehyde and phenol-formaldehyde-amine condensates, and
• neutralizing the reaction mixture with an acid consisting essentially of sulfamic acid to stop condensation reactions until a pH of the neutralized reaction mixture in aqueous form is from 4 to 5.5,
• (b) said sizing composition that is applied onto said mineral fibers being subsequently obtained after preparing said liquid phenolic resin by a second process comprising mixing at least said liquid phenolic resin with said extender.