Fire-Retardant Synthetic-Resin Paints A R T H U R V A N KLEECK
Forest Products Laboratory1, Madison, Wis.
I
N ADDITION to the work previously re ported on borax-linseed oil prepara tions (2), the Forest Products Labora tory has considered bases other than lin seed oil to which fire retardants might be added. Among the m o s t promising for good fire-retardant coatings are the syn thetic resins. This laboratory's experimentation has been limited principally t o two types of preparations—dicyandiamide-formaldehyde and, especially, urea-acetaldehydeformaldehyde resins. Coatings of these resins, t o which either phosphoric acid or ammonium phosphate has been added, give exceptionally good protection to wood against flame spread as measured by the fire-tube method (5). Protection is apparently due t o the frothy char pro duced by the intumescence of the resin, which serves to insulate the wood, and to the presence of the phosphate, which ren ders the char difficultly combustible. However, a practical coating should have, in addition to good fire retardance, the other properties of a good paint, var nish, or lacquer, such a s stability in the container, good moisture resistance, rea sonable freedom from checking, chalking, and brittleness, and sufficiently high vis cosity so that coatings thick enough for good fire retardance can be obtained with a reasonable number of coats. T h e syn thetic-resin coatings thus far prepared have been deficient in some of these prop erties. Following is a review of the properties of preparations investigated and formulation modifications made in a n effort to improve the properties of both sirups and coatings. This review indicates some lines along which additional work is necessary to re move objectionable features of the resins. The results obtained m a y b e of interest t o those familiar with or engaged in research o n other resins. The initial experimental work was done with dicyandiamide-formaldehyde resins containing monoammonium phosphate. Preparations with these constituents have been patented (3, 4). These coatings, when applied at the rate of about 30 or more grams of dry coating per square foot, possessed unusually good fire retard ance. A number of fire-tube specimens showed losses in weight as low as 6 to 15 ι Maintained in cooperation with the University of 'Wisconsin. Acknowledgment is made to Gordon Leader, formerly engaged in these studies at the Forest Products Laboratory, who de veloped many of the formulations used.
per cent. In the fire-tube test, an uncoated specimen loses approximately 80 per cent of its weight, pieces of unconsumed charcoal only remaining after the test. A percentage loss in weight of less than 25 is indicative of excellent protec tion, and of 25 to 35 of reasonably good protection. Although these coatings had excellent fire resistance, a serious weakness was their poor moisture resistance. Tackiness and even dripping of the coating occurred upon exposure to a relative hu midity of 90 per cent or more. Tests made on a few urea-formaldehyde and ethylidene urea—formaldehyde (1) resins t o which monoammonium phosphate was added showed them t o have good moisture resistance and excellent fire re tardance, although not so good as the best dicyandiamide preparations. Their weak nesses were lack of stability of the sirup and brittleness and checking of the coating. Since defects of the dicyandiamide and urea or ethylidene urea preparations were dissimilar, a number of combinations of the two resins were tried, but the prepara tions failed to show the desired improve ment. Most of them were poor in mois ture resistance. A study was made of the effect on the stability of the ethylidene urea resin sirup and the checking properties of the coatings by: Modification of some of the conditions that enter into the preparation of simple ethylidene urea-formaldehyde resins, such as temperature, period of digestion, and proportions of ethylidene urea and form aldehyde. Presence of additional materials, such as fire retardants, gel preventives, and plasticizers.
Preparation The ethylidene urea was prepared by re acting urea and acetaldehyde in the pres ence of dilute hydrochloric acid, accord ing to the directions of Barsky and Wohnsiedler (1). The ethylidene urea was as sumed to be uniform from lot to lot, which may or m a y not have been a correct as sumption, as it was noticed that the time for precipitation of ethylidene urea to oc cur varied somewhat from batch to batch. After washing and drying, the ethylidene urea was ground to pass through a 30-mesh screen. T h e resin sirups-were made by reacting ethylidene urea with a 37 per cent formaldehyde solution, usually in the pres
626
ence of varied amounts of either monoammonium phosphate or phosphoric acid. The reaction was carried out in a threenecked flask, provided with water con denser, stirrer, and thermometer, heated on a steam bath a t a maximum tempera ture of 92° C. Additional materials, such as diluents, gel preventives, and plasticizers, were added before, during, or after digestion of the basic constituents. A small quantity of wetting agent, such as Dreft or Aerosol, was added to improve wetting properties. The period of diges tion varied considerably with different preparations, depending upon the amount of catalyst and composition of the sirup.
Methods of Test FIRE
RETARDANCE.
AS
previously
stated, the fire-tube method (5), developed at this laboratory, was used to deter mine fire-retardant performance. In this method, specimens measuring 48 X 3/4 X 3/8 inches are coated with the preparation under test. The amount applied is deter mined by weights taken before and after application, and coatings of various weights are normally applied to determine the minimum coating weight necessary for the desired protection. With the ethyli dene urea resins approximately 20 to 30 grams of dry coating per fire-tube speci men or 26 to 40 grams per square foot of wood surface were usually required, at tained by applying three or four coats of a fairly viscous sirup. STABILITY, CHECKING, A N D MOISTURE
RESISTANCE. These properties were de termined qualitatively. Small portions of each sirup prepared were set aside in stop pered glass bottles. Gelling was consid ered to have occurred when no flow took place on inversion of the bottle. Checking and moisture resistance were determined by observing the properties of coatings applied t o 4 X 8-inch wood panels which were exposed to conditions of 30 and 90 per cent relative humidity at 80° F.. respectively.
Effect of Variables and Modifying Agents on Resin Properties The large number of variables entering into preparation of these resins make difficult the determination of the effect of change i n any o n e variable with concomi tant changes in other variables. Obvi ously, a n enormous number of tests would be necessary for such determinations. I t was hoped that preparation of a number of formulations of somewhat different com positions prepared under different condi tions would reveal certain trends. This hope has been only partially realized, be cause one of the most serious problems has been the difficulty of obtaining satisfactory duplication of results, probably because of the failure to recognize the importance of and control adequately some variable or variables.
June 10, 1941 Ethylidene Urea—Formaldehyde Ratio During the subsequent discussion, quan tities of ethylidene urea (E. U.) and form aldehyde are expressed in terms of molar concentration. Thus an E.U.-HCHO ratio of 1:1 means 86 grams of ethylidene urea and 30 grams of formaldehyde (81 grams of a 37 per cent solution). With only ethylidene urea, formalde hyde, and fire-retardant chemical present, the E . U . - H C H O ratio had a decided effect on the stability of the sirup. When phosphoric acid in the proportion of 10 per cent by weight of the E. U.-HCHO sirup was used as fire retardant, preparations with an E. U . - H C H O ratio of 1:1.3 and 1:1.9 gelled within a few days; some of those with an E. U . - H C H O ratio of 1:2.6 were stable from one to two months. Gel ling was ordinarily much slower when the phosphoric acid was added to the sirup after cooling. If added to the preparation during or prior t o digestion, gelling was likely to occur in the digestion flask, es pecially with preparations of lower form aldehyde content. When monoammonium phosphate in the proportion of 10 per cent by weight of the E. U . - H C H O sirup was used as the fire re tardant, the sirups with an E. U . - H C H O ratio of 1:2.6 gelled much more quickly than those containing a smaller proportion of formaldehyde.
NEWS
EDITIΟΝ
taining diammonium phosphate was poor, but that of coatings with monoammonium phosphate or phosphoric acid was usually good. Checking properties of prepara tions containing phosphoric acid were somewhat better than those containing monoammonium phosphate.
Temperature and Period of Digestion Most of the preparations were made at temperatures of 90° to 92° C , and the effect of time of digestion on the stability was influenced b y other factors, such as the quantity and kind of fire retardant and the E . U . - H C H O ratio. The effect of time of digestion on stability can be stated qualitatively only ; under conditions favor able to gelling, increasing the period of digestion accelerated gelling. A few preparations were made at tem peratures lower than 92° C , and the time required to effect reaction at the lower temperature was considerably longer. N o decided difference in properties was no ticed between a sirup digested for 25 min utes at 78° C. and one digested for 3 m inutes at 92° C. Kind and Quantity of Fire Retardant The pH of the sirup could be controlled very easily by using the basic diammonium phosphate, neutral mixtures of di- and monoammonium phosphate, and the acidic monoammonium, phosphate or phosphoric acid. The basic sirups were the most stable; such preparations apparently re main liquid indefinitely. Those contain ing phosphoric acid were least stable; gelling was difficult or impossible to pre vent in sirups of the viscosity desirable for fire-retardant coating application. Preparations containing monoammonium phosphate were intermediate. The moisture resistance of coatings con-
a b c d Fire-tube specimens, (a) Uncoated speci men before fire test, (b) Uncoated specimen after test, (c and d) Speci mens coated with 32 grams per square foot of ethylidene urea resin containing ammonium phosphate. Loss in weight in fire tube (b)—81 per cent; (c)—16 per cent; (d)—18 per cent. These specimens represent different types of oehavior when coatings are exposed to heat. In tumescence occurred on (c) over the entire area, on (d) only on area ex posed to hottest portion of the tube.
627 The effect of the amount of fire retardant in each formulation upon the re sistance to fire spread was not checked, but in the best preparations the fire re tardant usually represented from 15 to 30 per cent of the weight of the dry coatings. Increasing the quantity of phosphoric acid from 15 to 35 per cent of the dry coat im proved checking characteristics in several cases, but in a few cases weakened mois ture resistance. As a result of the different characteris tics produced with different phosphates the following technique was used for a number of preparations. T h e ethylidene urea and formaldehyde were digested with a very small quantity of phosphoric acid as catalyst (one or t w o drops for a 200gram lot). After digestion, the sirup was made alkaline with ammonium or sodium hydroxide. Immediately before applica tion, the desired amount of phosphoric acid was mixed into the sirup. Advantages of this procedure were that the alkaline sirup could be kept indefinitely without gelling so far as is known, and from a can-cor rosion standpoint such sirups are much more satisfactory than those containing the acidic monoammonium phosphate or phosphoric acid. G E L PREVENTIVES. Ethylene chlorohydrin was very effective for preventing gelling. Methyl and ethyl alcohol were also useful for this purpose but not so ef fective as ethylene chlorohydrin. The quantity of gel preventive used varied con siderably with composition of the sirup. The range was ordinarily between 20 and 40 per cent of the combined weight of ethylidene urea and formaldehyde. Use of these materials delayed or prevented gelling of sirups which would normally gel in a comparatively short time because of the specific ratio of ethylidene urea to formaldehyde used in compounding or because of the presence of fire-retardant chemical. PLASTICIZERS. Efforts t o produce more flexible coatings with plasticizers have not proved very satisfactory. Certain ma terials improved flexibility of the coatings but at the expense of other necessary properties, such as fire retardance and moisture resistance. Materials tried for this purpose were aluminum and calcium stéarates, ethylene glycol, and ethyl lactate. The stéarates, used in concentrations of 2.5 to 7.5 per cent of ethylidene urea-formaldehyde sirup, were good plasticizers, but the fire retardance of preparations containing them was very poor. Ethylene glycol improved checking properties somewhat but weakened moisture resistance. Ethyl lactate had a good initial plasticizing effect, but it was not permanent. OTHER ADDITION AGENTS.
Phenol
ap-
peared to improve moisture resistance of the coating. When used it was digested with the ethylidene urea and formaldehyde, and it is possible that it was present, in part at least, as a phenol-formaldehyde resin.
628
ΝEW
S
EDΙΤΙΟN
FIRE RESISTANCE:
Grain» D i c y a n d i amide Formaldehyde (37%) Acetaldehyde Water M o n o a m m o n i u m phosphate
45 Ï 50 | 15! 55 63 J
E t h y l i d e n e urea Formaldehyde (37%) D i c y a n d i amide Monoammonium phosphate Water
241 36 |
E t h y l i d e n e urea Formaldehyde (37%) Phenol Monoammonium phosphate E t h y l e n e chlorohydrin ( 4 0 % ) E t h y l i d e n e urea Formaldehyde (37%) P h o s p h o r i c acid
MOISTURE RESISTANCE
SULFAGUANIDINE, newest of the sulfadrugs, has been made available to the medical profession generally by Lederle Laboratories, Inc., 30 Rockefeller Plaza, N e w York, following extensive clinical tests. Made by the Calco Chemical Division of American Cyanamid Co., sulfaguanidine when administered b y mouth remains largely in the intestinal tract, whereas other sulfadrugs are readily absorbed into the blood stream. Thus t h e new drug is able to destroy or prevent growth of cer tain bacteria in the alimentary canal. Of the sulfonamide compounds only three—sulfanilamide, sulfapyridine, and sulfathiazole—have previously been gen erally accepted by the medical profession. Sulfaguanidine now widens the sphere of action of chemotherapy.
FROM CHECKING
% loss in weight (fire tube)
Poor
Fair to goo
