New Synthetic Nitrogen Fertilizer Preparation and Properties of Urea-form K. G. CLARK, J. Y. YEE,
AND
K. S. LOVE
Bureau of Plant I n d u s t r y , Soils, and Agricultural Engineering, U . S . D e p a r t m e n t of Agriculture, Beltsville, iMd. Urea-formaldehyde reaction products suitable for fertilizer use (urea-form), that contain 36 or more % nitrogen and have varying rates of avaiIability, have been produced from both dilute and concentrated solutions having urea-formaldehyde mole ratios in the range 0.75 to 7.5 under various conditions of acidity, reaction temperature, and time. The availability patterns of such products are largely determined by their mole ratios and nitrogen solubility indexes. The lower the mole ratio and index, the lower is the rate of availability. It has been demonstrated that relatively simple and highly efficient processes may be devised for the rapid production of urea-form having predetermined nitrogen solubility and rate of availability to plants.
Reactions with U / F 7 1: NHCHzOH
I
co I
KHz
I
qo
CHzO
+
(2
SHCH~OH
NCHz
T
terials of low solubility for fertilizer use has engaged the attention of research workers for a number of years. The advantages to be gained are improved physical condition of the product, lower solubility and consequent reduced tendency to leach from the soil during periods of heavy rainfall, reduction of burning tendency, and availability of the nitrogen more in accordance with the demands of long season crops. Among the suggestions for accomplishing these objectives have been the ammoniation of peat or other natural organic materials (2, 6 ) and the ammoniation of superphosphate and superphosphate mixtures with solutions containing ammonia, urea, and formaldehyde (9). More recently. (after the work reported here was well advanced) a patent was granted to Rohner and Wood (11) for the production of insoluble nitrogen fertilizer by the reaction of urea and formaldehyde. Urea-form is the name recently applied (14) to a class of ureaformaldehyde reaction products which is suitable for fertilizer use. Such products are formed under conditions that will ensure the reaction of more than one molecular equivalent of urea per mole of formaldehyde. They usually contain 36 to 38% nitrogen and about 4% moisture, and are much less soluble than the chemical nitrogen fertilizers in present-day use. By contrast urea-formaldehyde reaction products formed under alkaline conditions such that the product contains less than one molecular equivalent of urea per mole of formaldehyde are readily soluble in water, and are used in the production of ureaformaldehyde resins. The distinction between urea-formaldehyde products suitable for fertilizer use and those suitable for resin formation is shown clearly in Figure 1, in which the possible urea-formaldehyde (U/F) mole ratios of the reaction products are plotted against the minimum number of moles of urea per mole of product. Dimethylol urea and monomethylol urea, which were first prepared and described by Einhorn ( 5 ) ,limit the resin field between U / F = 0.5 and U/F = 1.0. These methylol compounds are precipitated slowly under alkaline conditions from solutions containing formaldehyde in excess of urea. Under acidic conditions the corresponding methylene ureas are obtained.
.
1178
+
do NHz
HE problem of providing synthetic chemical nitrogen ma-
-I+
I co I
NH,
>L Hae
CHeO
+
-----f 2CH20
HzO
(3)
+ 2Hz0
(4)
NCHZ
I co
Acid
I
NCHa Dimethyl01 urea (Reaction 2) is readily prepared (6)and serGes as the starting material for the production of urea resins. When mixed with various acidic catalysts, plasticizers, and fillers or extenders, and subjected t o heat and pressure dimethyl01 urea is converted to insoluble condensation products or resins of high molecular weight, with the loss of water and some formaldehyde. The urea-formaldehyde mole ratio is increased by this treatment, but the h a 1 product always contains a greater molecular proportion of formaldehyde than urea-ie., U/F1, which is bounded by monomethyleneurea and methylenediurea. The probable reactions in this field are indicated below. Reactions with U/F
> 1:
"2
2
do + CHlO + I "2
Acid
N H * CH2 * N H
I co
NH2 I
I
CO NHz I
+
Hz0
(5)
mixture reacting a t 30 O C. was deterhined a t intervals throughout a 48-hour period.
NHz (n
I
+ 1)
+ nCHzO
CO
I
d
Acid
NHz
+
-
NHZ(CO. N H C H Z - N H ) ~ C O N H ZnHzO
(n
+ 1)
60
Crystalline urea was dissolved in water to produce a 3 molal solution (180 grams per liter). This solution at 25 O C. was buffered at pH 3.6 by addition of the required quantities of citric acid and disodium h drogen phosphate dodecahydrate. This solution and a 37% grmaldehyde solution were mixed in the proportion of 1.625 moles of urea per mole of formaldehyde, and sufficient water was added to make the mixture 2.4 molal in urea (144 grams per liter). The pH of the mixture was checked and readjusted t o 3.6. The temperature of the mixture was allowed t o rise t o 30" C. and was maintained a t t h a t point by moderate cooling. Samples of the clear liquid phase adequate in amount for determination of the uncombined formaldehyde, total formaldehyde equivalent, and total nitrogen were obtained by filtration through an Alundum thimble immersed in the reaction mixture. Ali uots of the li uid phase sample were taken immediately for anaqysis. The t o t 3 formaldehyde and nitrogen aliquots were reserved for subsequent analysis, but those for uncombined formaldehyde were added to cold sodium sulfite solution and the sodium hydroxide liberated in accordance with Reaction 8 was titrated a t once (16).
(6)
+ nCHzO -
----f
4H2
Acid
I do
1I
I
NHz
2"
Marvel, Elliott, Boettner, and Yuska (10) consider that neither of the types of chaining indicated in Reactions 6 and 7 satisfactorily explains the structure of urea resins. Their conclusions are based largely on the fact that polymers have not been reported as a result of the reaction of formaldehyde with symmetrically substituted dialkylurea, RNHCONHR, corresponding to Reaction 6, or with unsymmetrically substituted dialkylurea, RzNCONH2, Reaction 7. The urea-formaldehyde mole ratio requirements for products obtained in the urea-form field, however, preclude any of the complex cross-linked ring structures that have been assigned the resins. They further limit the possible structures of urea-form to the indicated linear chains or their combinations. When n equals 1 both Reactions 6 and 7 are equivalent to Reaction 5, and the product is methylenediurea.
NazSOa
RATEOF REACTION.Urea-form materials may be prepared by reaction of 37% formaldehyde solution and aqueous urea solutions a t normal temperatures. In order to obtain information on the rate of reaction, the composition of the liquid phase of a
-
28
-
c
g 24-
0
K
3 20m!
f r
Data on the effect of varying the initial molecular proportions of urea to formaldehyde in reacting solutions are reported in Table I.
/:
Y)
le-
.s
I
I
.7 s
Ur
(8)
EFFECT OF MOLERATIO.
UR E A-FORM
16-
P
+ CHzO + HzO--+NaCHzOHSOs 3. NaOH
The results of these determinations are represented graphically in Figure 2, in which the percentages of the urea and formaldehyde reacted are plotted against time. The first traces of insoluble precipitate appeared in the mixture a t the end of 48 minutes and rapidly increased in volume for the &-stfew hours thereafter. A reaction between urea and formaldehyde to form soluble products was initiated almost immediately on mixing the reactants. At the end of 6 hours, 99% of the formaldehyde had reacted, yet only 45% was present in the solid phase. In 48 hours, 60.7y0 of the urea and 83.7% of the formaldehyde were in the solid phase and the urea-formaldehyde mole ratio of the liquid phase had increased from 1.625 t o 4.1. The reaction between formaldehyde and urea appears to yield a t first soluble methylol or methylene derivatives of urea, or both, which are converted later to more complex less soluble materials. The rate of formation of these complexes decreases appreciably after disappearance of the uncombined formaldehyde, . . although the equivalent mole ratio of urea to formaldehyde a t this point, 1.95, is considerably larger than the initial value. This would seem to indicate that the combined formaldehyde in solution a t this stage was p r p dominantly in a less reactive methylene form, possibly as methylenediurea.
REACTION IN DILUTE SOLUTION
32
1179
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1948
I
I.25 1.5 -formaldehyde mole ratio of product
I
1.75
Figure 1. Urea-Formaldehyde MoIe Ratio and Minimum Number of Moles of Urea in Product
I
2 .o '
The msterials were prepared in 25-gallon batchea from initial mixtures, 2.4 molal in urea and p H 3.6, reacting over a 48hour period a t approximately 30" C. At the end of the reaction period the product waa aeparated
1180
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 7
tng the effect of initial pH of the reacting solution on the character and yield of the products. The period of reFormaldehyde In sohd phase action (48 hours), reaction 0 temperature (30" C.), and initial urea concentration Urea in solid phose (2.4 molal) were the Same as before. With the smaller quantities, filtration and washing were readily acromplished with the use of a 5-inch basket centrifuge. As shown by the data in Table 11, the effect of increasing the pH of the initial I I I I I I I I I I I 4 8 12 I6 20 24 28 32 36 40 44 48 3olution for a given mole Reacllon tlrne, hours ratio of reactants was to d o Figure 2. ltelation between Urea-Formaldehyde Reaction and Time crease the yield and t o increase the solubility of the Initial conditions:
