Determination of Urea-Formaldehyde Resins in Coatings via Urea Analysis M. H. SWANN and G. G. ESPOSITO Coating and Chemical Laboratory, Aberdeen Proving Ground,
Md.
method. Some analytical results obtained by both methods are shown in Tables I and 11.
The release of ammonia from urea-formaldehyde coating resins on treatment with alkali is characteristic. I t provides a rapid quantitative means of analyzing coatings for urea resin modification, or of determining urea in urea-formaldehyde resins.
PROCEDURE
-4 250-ml. Erlenmeyer flask with 24/40 ground joint is arranged on an electric heater having a variable transformer heat control. Two adapters with 24/40 joints (Ace Glass, Inc., Nos. 5190 and 5235) are attached to the flask and connected to two gas-washing bottles so that air drawn through the apparatus bubbles first through concentrated sulfuric acid, then over the sample, and finally through another washing bottle. The latter contains 100 ml. of standard 0.lX sulfuric acid and is provided with a fritted glass disk of medium porosity to subdivide the air into fine bubbles. A vacuum source is used to draw the air slowly through the apparatus.
A
LKYD resins used for baking finishes are frequently modified with 10 to 20y0 of a nitrogen resin such as urea- or melamine-formaldehyde. Class B enamels of Federal Specifications TT-E-489b and TT-E429 are of this type. For some time, the extent of this modification has been measured by determining total nitrogen, using the well-known Kjeldahl method. With resinous products the Kjeldahl method is time-consuming but highly accurate. Recently, an infrared spectrophotometric method was developed ( I ) for determining relative amounts of urea- and melamine-formaldehyde resins in alkyd blends. The method is rapid and applicable to mixtures of these nitrogen resins, but is not so accurate as the chemical method described here. I t is also subject to interference from other resins.
Table 11. Determination of Urea-Formaldehyde Resin in Alkyd Blends
(Nonvolatile basis) Nitrogen Resins Present, % -4lkyd Resin Melamine Urea Soybeanoil, highphthalio 0 0 19 7 13 1 0 0 10 6 0 0 9 8 50 0 50.0
Coconut oil, high phthalic Table I.
Determination of Total Nitrogen in VreaFormaldehyde Resins
Sample Uformite F-233 Uformite F-210 Uformite F-240 Beckamine 2530 Syn-U-Tex 400
(Solution basis) Nitrogen Found, 7 0 Kjeldahl Alkali digestion 10 10 11 10 10
i 8 8
9 7
10 10 10 10 10
i i
i
4 4
The urea resins appear to be unique among resins in their reaction with alkali under high-temperature saponification conditions. They release ammonia, which can be collected and measured in standard acid, giving results in good agreement with the nitrogen content of the resin as determined by the Kjeldahl method. Melamine-formaldehyde, polyamide, and butadiene-acrylonitrile copolymer resins do not interfere; isocyanate and polyacrylamide resins have not been tested. In principle, the urea-formaldehyde resin is dissolved in ethylene glycol containing potassium hydroxide. Dry air is d r a m through this reaction flask, then through standard sulfuric acid. The reaction medium is gradually heated to boiling and after 1 hour the ammonia absorbed by the acid is determined by titration with alkali. If the coating contains mixed urea- and melamine-formaldehyde resins, the latter can be measured by determining the difference in the total nitrogen and urea nitrogen content. The factor of 6.58, used to calculate the urea resin, is based on the average nitrogen content of a variety of resins analyzed by the Kjeldahl
None
10 6 31 5 11 1 191 108 50 0 50 0
Urea-Formaldehyde Found,
%
19.8 10.8 31 . O 11 3 18 6 10 7 49.4 48.3
-4 sample of resin solution containing 0.2 to 0.4 gram (nonvolatile basis) of urea-formaldehyde resin is weighed into the flask, and 25 ml. of 14% potassium hydroxide in ethylene glycol is added. The addition of a Berl saddle is recommended. The flow of air is started and the heat applied gradually so that gentle boiling is reached in not less than 30 minutes. Boiling is continued for 1 hour. A small Bunsen burner flame is played over the outer walls of the vertical adapter for approximately 1 minute. The upper joint is disconnected and washed with water on the inside in such a manner that the wash water flows into the washing bottle of standard acid. The acid is then titrated with O.1N sodium hydroxide to the methyl purple end point. A blank titration on 100 ml. of the acid is also made.
% urea-formaldehyde resin (V,
=
-
T I p )
X normality of alkali X 6.58 sample weight
or normality of alkali X 1.401 % nitrogen - ( V I - V Z )X sample weight where VI = ml. NaOH for blank. V2 = ml. NaOH for sample. LITERATURE CITED (1)
Miller, C. D., Shreve, O.D., ANAL.CHEM.28, 200 (1956).
RECEIVED for review June 6, 1956. .4ccepted September 5, 1956.
1984
