the cross - linking agents produced according to the present invention can be used in water reducible systems and as a result , have the following outstanding attributes . these cross - linking agents permit one to achieve a good cure of a coating resin at low temperatures and show improved humidity resistance as compared to those fully alkylated and fully methylolated melamine reaction product compounds such as the hexakismethoxymethylmelamine . when compared to partially alkylated and partially methylolated melamine resins , the cross - linking agents of the present invention display improved stability , improved humidity resistance and reduced weight loss on curing . in order that the concept of the present invention may be more completely understood , the following examples are set forth in which all parts are parts by weight unless otherwise indicated . these examples are set forth primarily for the purpose of illustration and any specific enumeration of detail contained therein should not be interpreted as a limitation on the case except as indicated in the appended claims . into a suitable reaction vessel equipped with a stirrer , reflux condenser and thermometer there is introduced 800 parts of methanol , 2 . 2 parts of sodium bicarbonate and 742 parts of paraformaldehyde ( 91 % ch 2 o ). the mixture is heated to about 60 °- 62 ° c . in order to dissolve the paraformaldehyde . thereupon 630 parts of melamine are added to the reaction vessel . at this point , the mole ratio of the reactants is m / f / me = 1 . 0 / 4 . 5 / 5 . 0 . the mixture is then heated to reflux ( 75 °- 80 ° c .) and held for one hour . the heat is turned off and 1600 parts of methanol is then added to the reaction vessel . at this stage , the mole ratio of the reactants is m / f / me = 1 . 0 / 4 . 5 / 15 . 0 respectively . the mixture is then cooled to 50 ° c . and there is added 158 parts of a 70 % nitric acid solution in water . when the milky resin clears , the charge is cooled to about 30 °- 35 ° c . and held at that temperature for about 30 minutes . thereupon , there is added 138 parts of a 50 % caustic solution . vacuum distillation is then begun in order to remove the volatiles . thereupon , there is charged 2400 parts of methanol and the batch is cooled to about 30 ° c . there is then added an additional 150 parts of a 70 % nitric acid solution and the batch is held at about 30 °- 35 ° c . for about one hour . thereupon , there is charged 138 parts of a 50 % aqueous caustic solution and the volatiles are then removed with vacuum concentration and the solids content is adjusted to about 88 ± 2 % by the addition of a 1 / 1 blend , by weight , of isopropanol / isobutanol . the composition of this resin is of mf 4 . 2 me 3 . 8 as established by analysis . the average degree of polymerization as calculated from gel phase analysis and molecular weight determinations is about 3 . 0 . this means on the average 3 . 0 triazine moieties are linked together . the methylol content of this product is 1 . weight percent or about 0 . 12 mole of methylol groups per triazine ring . the calculated nh content is about 1 . 1 moles nh per triazine ring . this product will be identified hereinbelow as cross - linking agent a . the process of example 1 is repeated in all essential details except that the formaldehyde charge was reduced from 4 . 5 to 4 . 0 moles per mole of melamine . the resulting product had a combined m / f / me ratio of 1 . 0 / 4 . 0 / 3 . 7 . the average degree of polymerization was about 4 . 0 and the calculated nh content was 1 . 25 . the free methylol content was about 0 . 2 to 0 . 3 mole per triazine moiety . this product is identified hereinbelow as cross - linking agent b . the process of example 1 is repeated in all essential details except that the methanol charged in the first and second methylation steps was reduced to 10 moles of methanol per mole of melamine . the alkylation temperature was also reduced to 35 ° c . in both alkylation steps and the acid charge was reduced by one - half . the resulting product had a viscosity of x - y when measured on an 80 % solids solution in an isopropanol / isobutanol mixed solvent at a solvent ratio of 1 / 1 . the combination m / f / me ratio was 1 / 4 . 2 / 3 . 7 . ( nuclear magnetic resonance measurements give a methanol ratio of 3 . 2 . the 3 . 7 value was obtained by standard analytical techniques ). the final product has a high monomer content as determined by gel phase analysis and a calculated average degree of polymerization of 2 . 25 . the methylol content is about 0 . 35 to 0 . 4 per mole of triazine . the calculated nh content is 1 . 24 . this product is identified as cross - linking agent c . this cross - linking agent is a commercially available methylated melamine - formaldehyde cross - linking agent and is set forth herein for comparative purposes . this cross - linking agent identified as cross - linking agent d , has an average composition of m / f / me of 1 / 5 . 2 / 3 . 5 and a methylol content of 1 . 1 / 1 . 2 per mole of triazine moiety and an average degree of polymerization of 3 . 03 . this polymeric cross - linking agent also has a calculated nh content of 0 . 13 mole of nh per mole of triazine . the cross - linking agent in this example ( agent e ) is a commercially available fully methylated melamine - formaldehyde composition with an average melamine / formaldehyde / methanol mole ratio combined of 1 / 6 . 0 / 5 . 2 and a methylol content of 0 . 1 - 0 . 2 mole per mole of triazine and an average degree of polymerization of 1 . 67 and a calculated nh content of about 0 - 0 . 1 nh per triazine ring . a water - dispersible acrylic terpolymer having a composition of butylacrylate / styrene / acrylic acid in a weight ratio of 55 / 30 / 15 respectively is blended separately in three different batches with cross - linking agent a , cross - linking agent d and cross - linking agent e . in the first two blends the resin to cross - linking agent ratio is 60 / 40 whereas in the cross - linking agent e blend the resin to cross - linking agent ratio is 80 / 20 because a larger amount of cross - linking agent e inhibits the cure of the coating composition . the blends of the acrylic terpolymer with these cross - linking agents are pigmented with titanium dioxide in a pigment to binder ratio of 0 . 8 / 1 . 0 respectively . films were drawn down on bonderite 1000 steel panels from each of these three blends of the acrylic terpolymer and the respective cross - linking agents in a film thickness of 1 . 0 mil and the films were cured at 300 ° f . ( 149 ° c .) for 20 minutes . the cured films were subjected to exposure in a cleveland humidity test chamber for a plurality of days and the thus tested panels were inspected for gloss , 60 ° and blistering . the results of these tests are shown hereinbelow in table i . table i______________________________________gloss , 60 °: cla a cla d cla e______________________________________initial 95 93 99after 3 days 94 79 95after 6 days 89 67 91after 9 days 82 -- 56after 14 days 67 -- 25after 17 days 63 -- -- after 20 days 52 -- --* blistering : after 3 days 10 8 md 10after 6 days 9 m 7 d 10after 9 days 9 m -- 6 mdafter 14 days 8 md -- 4 dafter 17 days 8 md -- -- after 20 days 8 d -- -- ______________________________________ * blistering : m = medium md = medium - dense d = dense 10 = no blister 9 = very fine blisters 8 = slightly larger blisters 6 = larger blisters 4 = very large blisters it can be seen from the results shown in table i hereinabove that the cross - linking agent a containing coating composition of the present invention gives cured films with significantly improved humidity resistance when compared with the comparative cross - linking agents d and e containing coating compositions . a commercially available solvent borne acrylic polymer , identified as rohm and haas at - 56 , having a hydroxyl number of 88 and an acid number of 30 is pigmented with titanium dioxide and is then blended separately with cross - linking agents a , b , c , and d . the resin to cross - linking agent ratio is 70 / 30 respectively and the pigment / binder ratio is 80 / 100 respectively . the theoretical non - volatile content is adjusted to 65 . 2 weight percent . this non - volatile content is confirmed by carefully determining the solids by a foil method for 45 minutes at 45 ° c . this solids method prevents any decomposition of the melamine compound and prevents cross - linking reaction . films drawn down from each of these four blended coating compositions , after baking under regular baking conditions , showed increased weight loss or lower weight solids . this is due to the cross - linking reaction of the amino compound . because of ecological , economical and health reasons , it is desired to have the least weight loss of the amino resin during the baking process . cross - linking agents of the present invention have significantly reduced weight loss as shown in table ii hereinbelow . table ii______________________________________ cla a cla b cla c cla d______________________________________enamel non - volatile , % 65 . 23 65 . 23 65 . 23 65 . 27amino resin , % 10 . 81 10 . 80 10 . 81 10 . 85non - volatile , %, 20 &# 39 ; at 149 ° c . 64 . 00 63 . 95 63 . 90 62 . 50weight loss , % 1 . 23 1 . 28 1 . 33 2 . 77weight loss based 11 . 40 11 . 90 13 . 30 25 . 50on amino resin , % ______________________________________ as can be seen from table ii , the amino compositions of the present invention have about half of the weight loss during the baking cycle as is shown in cross - linking agents a , b and c when compared with the commercially available amino cross - linking agent d . the nh content of a melamine / formaldehyde reaction product is normally difficult to measure . titration with an acid gives some indication of nh content . the nh content can also be calculated using the following equation : dp = average degree of polymerization x = moles of combined formaldehyde per melamine the average degree of polymerization can be obtained by gel phase analysis or by average molecular weight determinations . the combined formaldehyde can be determined by the standard published techniques . the methylated , methylolated melamine compositions of the present invention have mainly methoxymethyl groups as functional sites and it is therefore surprising that coating compositions containing these melamine compositions cure at the same speed as certain commercially available partially methylated , partially methylolated melamine cross - linking agents in the absence of a strong acid catalyst . commercially available fully methylated , fully methylolated melamine cross - linking agents do require a strong acid catalyst in order to cure at a reasonable speed . in water - borne paint formulations , it has been found that the cross - linking agents of the present invention give superior humidity resistance under marginal cure conditions when compared with the commercially available melamine cross - linking agents . the cross - linking agents of the present invention also have the advantage of containing lower amounts of free formaldehyde which can cause occasional odor problems .