Thermosetting synthetic resin, known as a lignin-phenol-formaldehyde resin, or LPF resin, which is obtained by polycondensation of formaldehyde, phenol and lignin in the presence of a basic or acidic catalyst, characterized in that the lignin is a lignin that is not chemically modified at the available functional groups, and in that said available functional groups are chosen from the group comprising aliphatic hydroxyls and phenolic hydroxyls, wherein the resin comprises a degree of substitution by weight of phenol with the said lignin which is between 50 and 70%; and in that the said lignin that is not chemically modified at the available functional groups is produced according to a process for producing paper pulp, lignins, sugars and acetic acid, characterized in that it comprises the following successive steps consisting in: (i) placing the annual or perennial plants, which are used in total or partly, and which constitute the starting lignocellulosic raw material, in the presence of a mixture of formic acid containing at least 5% by weight of acetic acid which is brought to a reaction temperature of between 50°C and 115°C; (ii) next, separating at atmospheric pressure the solid fraction constituting the paper pulp from the organic phase; the said process also comprising a preliminary step consisting in (iii) performing a preliminary impregnation of the plant matter with the mixture of formic acid containing at least 5% by weight of acetic acid, at atmospheric pressure and at a temperature at least 30°C below the reaction temperature. wherein the resin comprises a degree of substitution by weight of phenol with the said lignin which is between 50 and 70%; and in that the said lignin that is not chemically modified at the available functional groups is produced according to a process for producing paper pulp, lignins, sugars and acetic acid, characterized in that it comprises the following successive steps consisting in: (i) placing the annual or perennial plants, which are used in total or partly, and which constitute the starting lignocellulosic raw material, in the presence of a mixture of formic acid containing at least 5% by weight of acetic acid which is brought to a reaction temperature of between 50°C and 115°C; (ii) next, separating at atmospheric pressure the solid fraction constituting the paper pulp from the organic phase; the said process also comprising a preliminary step consisting in (iii) performing a preliminary impregnation of the plant matter with the mixture of formic acid containing at least 5% by weight of acetic acid, at atmospheric pressure and at a temperature at least 30°C below the reaction temperature. (i) placing the annual or perennial plants, which are used in total or partly, and which constitute the starting lignocellulosic raw material, in the presence of a mixture of formic acid containing at least 5% by weight of acetic acid which is brought to a reaction temperature of between 50°C and 115°C; (ii) next, separating at atmospheric pressure the solid fraction constituting the paper pulp from the organic phase; the said process also comprising a preliminary step consisting in (iii) performing a preliminary impregnation of the plant matter with the mixture of formic acid containing at least 5% by weight of acetic acid, at atmospheric pressure and at a temperature at least 30°C below the reaction temperature.