61  US 9,958,775 B2  62  hydroxymethyl groups or alkoxymethyl groups in total (has  more preferably two or more alkoxymethyl groups), and has  a molecular weight of 1,200 or less.  The alkoxymethyl group is preferably a methoxymethyl  group or an ethoxymethyl group.  Among the cross-linking agents, the phenol derivative  having a hydroxymethyl group can be obtained by allowing  a corresponding phenol compound which does not have a  hydroxymethyl group and formaldehyde to react in the  presence of a base catalyst. Furthermore, the phenol deriva› tive having an alkoxymethyl group can be obtained by  allowing a corresponding phenol derivative having a  hydroxymethyl group and an alcohol to react in the presence  of an acid catalyst.  Other preferred examples of the cross-linking agent can  further include a compound having a N-hydroxymethyl  group or a N-alkoxymethyl group, such as an alkoxymeth› ylated melamine-based compound, alkoxymethyl glycoluril- based compounds, and an alkoxymethylated urea-based  compound.  Examples of these compounds include hexamethoxym› ethyl melamine, hexaethoxymethyl melamine, tetram› ethoxymethyl  glycoluril,  1,3-bismethoxymethyl-4,5-bis- methoxyethylene urea, bismethoxymethyl urea, and the like,  and these are disclosed in EP 0,133,216 A, German Patent  3,634,671, German Patent 3,711,264, and EP 0, 212, 482 A.  Particularly preferred examples among these cross-link- ing agents will be shown below.  [Chem. 35]  OH  OH  OH  OH  HO  OH  OH  OH  HO  OH  OH  OH  Ls  OH  OH  OH  OH  OH  Ls  OH  -continued 