POLYURETHANE COATINGS gelation should not have occurred in any of the systems used. As gelation did occur, it appears that undesirable side reactions were taking place. The most probable causes of these reactions are adventitious impurities: including water. The water present could have
reacted with tw’o isocyanate groups to form a urea linkage, which in turn could have reacted with an isocyanate group to form a biuret, and so forth. Thus a very small amount of water left in the resin after drying might eventually have lead to gelation.
S. N. GLASBRENNER Armstrong Cork Corp., Lancaster, Pa.
BRAGE and L, c, CASE School o f Chemical Engineering, Purdue University, Lafaye++e,Ind.
Baked Polyurea Coatings polyurea coatings considered here are one-component coatings that can be applied from either a water or a solvent system. The coatings are formed bv the reaction of urethanes with amine-bearing resins, and are cured by baking at 350” to 400’ C. for one half hour. In order to be stable at room temperature, the urethane portion of the svstem must be based on the reaction product of aliphatic alcohols with polyisocvanates. Urethanes based on isocyanates and phenolic-hydroxyls will react with aminebeariqg resins at room temperature. T h e mechanism proposed for the formation of polyurea coatings is shown in Equation 1. Properties of polyurea coatings may be varied over a wide range by varving the nature of the reactants. In general, polvurea coatings have good resistance to acids, alkalies, and water-immersion exposures. They also have good flexural T H E
Urea
A4kohol
and hardness properties and show excellent adhesion to metal. Polyurea coatings should find use as industrial primers o r as protective coatings. Their ability to be applied from a water-thinned system is an especially attractive feature.
Co.’s experimental product Mondur E-92 and General Mills’ Versamid resins of the 100 series.
The raw materials used in formulating polyurea coatings are Mobay Chemical
M o b a y Chemical Co., New Martinsville, W. Va.
’.
G. A. HUDSON, J. C. HIXENBAUGH, E. R* WELLS, and E. E* HARDY
Urethane Coatings from Castor Polyols applicability of castor polyols to the preparation of urethane coatings has been extensively reported by Metz, Ehrlich, Smith, and Patton ( I ) , Jvho systematically evaluated 17 castor poly01s. The screening work reported established that excellent urethane films could be prepared from castor polyolurethane systems. Exploratory work has been extended in a more detailed investigation of three new promising castor polyols to give a refining of the preparation methods and formulating variables. Because glyceryl monoricinoleate, of the three previously tested monoester castor polyols, had exhibited an optimum balance of over-all properties, it was selected as most suitable for more detailed investigation. Modifications of this basic chemical type were accordingly prepared and evaluated in terms of ultimate performance in urethane vehicles. I t was found that a lower prepolymer reaction temperature (50” C.) compared with that previously used (70’ C.) was desirable from a production standpoint, as better control of the reaction as well T H E
as improved aging properties was obtained. Despite a higher cost factor, it was decided to raise the NC0,’OH equivalent ratio from a previously recommended 1.75/1 to values in the range of 2.1/1 to 2.5/1, because this increase markedly improved the shelf and pot life stability of the urethane vehicle while retaining excellent coating performance. Of the monoglyceride variants prepared (using different ratios of castor oil to glycerol) and tested, one of medium monoglyceride content (Polycin 61 containing 14% monoglyceride) was selected as giving a best compromise of cost and ultimate performance properties in urethane coatings. From an economic standpoint, the reduced isocyanate demand of the Polycin 61 is definitely advantageous, for the diisocyanate chemical is by far the more expensive ingredient. To a large extent, the higher isocyanate equivalent weight of Polycin 61 compensates (costwise) for the increased NCO/OH ratio used in preparing the urethane vehicle. An analysis of performance data for
the two glyceryl monoricinoleate-urethane vehicles reveals a markedly improved shelf and pot life obtained by using the higher NCO/OH ratio. Despite some sacrifice in hardness, there is a significant gain in solvent and chemical resistance. In the prior investigation, attempts had been made to improve the over-all cost of urethane vehicles by designing castor polyols with high isocyanate equivalents. LTnfortunately, the few castor polyol derivatives developed at the time with isocyanate values higher than castor oil exhibited incompatibility characteristics that discouraged further work along such lines. Subsequently, however, a series of polymerized castor oils (urethane grade blown oils) having isocyanate equivalent weights in the range of 360 =t 15, has shown very acceptable properties. Low viscosity blown castor oils were found suitable for preparing either a prepolymer or polyisocyanate type of urethane vehicle. The high viscosity blown castor oils lent themselves only to VOL. 51, NO. 1 1
0
NOVEMBER 1959
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the preparation of urethane vehicles of the latter type, A comparison of the performance properties of the two blown castor oilpolyisocyanate clear coatings with the two control coatings (castor oil and ethylene glycol monoricinoleate based, respectively) reveals that the blown castor vehicles exhibit markedly reduced dry times and, in general, excellent overall performance. Furthermore, they match castor oil coatings costs. Adhesion of Urethane Coatings to Substrates
Work to improve the adhesion of urethane coatings to metal and wood has led to the following suggestions: for
clear coatings applied to metal, that the metal surface be acid treated or sand blasted; for pigmented coatings on metal, that a poly(viny1 butyral) wash primer be used, followed by sufficient coats of the urethane top coat to give the desired film thickness (intercoat adhesion of urethanes is excellent). For clear coatings on wood, normal oil-base or water-base wood fillers are not recommended for use with these urethane coatings because they act to reduce adhesion. Instead, a novel wood filler using AA or DB castor oil as the vehicle and fillers such as silica, talc, and desired tinting pigments is recommended. This wood filler is easily- applied, slow drying, and inexpensive. It can be wiped
off immediately after applying, or an hour or so later. With the castor oil filler, no drying time is required before the urethane coating is applied. The castor oil reacts with the isocyanate in the top coat to give excellent adhesion.
literature Cited (1) Metz, H. M., Ehrlich, A , , Smith, M. K., Patton, T. C., Paint, Oil Chem. Rev. 121, 6-12 (April 17, 1958).
T. C. PATTON and H. M. METZ Baker Castor Oil Co., Bayonne, N. J.
Diisocyanate Coatings Based on Castor Oil
DURING
their brief lifetime, the isocyanate-based coatings have developed much interest. One of the simplest urethane coating systems is an adduct of a diisocyanate and a poly01 prepared with an NCO/OH ratio in the order of 2. Thus, one NCO group reacts in the adduct and one is free to take part in cross-linking reactions after the film is laid down. These reactions, in which the water vapor in the air plays an important part, result in cure of the film. Generally, a catalyst is added prior to use. Any polyfunctional organic compound containing two or more reactive hydrogens should serve for the preparation of this type of urethane coating. Castor oil is an especially versatile starting material. Coatings with outstanding prop-
4
5
6 TIME
IN
7
8
DAYS
W a t e r vapor permeability of films (90' C. and 100% R. H.) Polyol modification A. None 6. Ethylene glycol C. Diethylene glycol D. Sorbitol P. Dipropylene glycol F. Propylene glycol G. Glycerol
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erties may be formulated from tolylene diisocyanate and castor oil or its derivatives ( 7 , 2). In a castor oil adduct type of coating the hydroxyl groups which provide the
erides. Castor oil was transesterified with several polyols. Some propertirs of the polymers obtained by polymerizing these transesters with tolylene diisocyanate are:
Properties of Castor Oil-Nacconatea 80" Films Polyol Present
Tensile Stzength, P.S.I.
Ethyl glycol Diethylene glycol Propylene glycol Dipropylene glycol Glycerol Pentaerythritol Sorbitol None
8200 7800 8430 8120 7880 6650 7790 3020
Elongation,
%
Dry Time,b Min.
HardnessC (Sward)
75 63 75 75 53 90 65 150
54 64 76 70 60 59 64 33
25 28
27 34
28 21
27 166
Mixture of approximately SO% 2,4-tolylene diisocyanate and 20% 2,6-tolylene diisocyanate. One-week cure a t ambient conditions. b
4% methyldiethanolamine catalyst based on nonvolatiles.
initial cross-linking sites are rather widely separated. If the molecule could be altered to bring the hydroxyls closer together, a tighter, more cross-linked polymer would be formed, resulting in a harder coating with greater chemical resistance. In an attempt to lower the average distance between cross-linking sites, a triol such as glycerol was cold-blended with castor oil before adduct formation with tolylene diisocyanate. Improved coatings did result. However, the coating solutions and resulting films were hazy or turbid, probably because of preferential reaction of the primary hydroxyl groups of the glycerol with the diisocyanate to form oil- and solventinscluble material. This difficulty should be resolved by using a standard alcoholysis reaction with castor oil and a polyol to give a mixture of essentially mono- and diglyc-
INDUSTRIAL AND ENGINEERING CHEMISTRY
In preliminary experiments using NCO/OH ratios of 1.7, 2.0, and 2.3 a ratio of 2.3 gave films with optimum flexibility, dry time, and tensile strength, and coatings with the longest pot life. The polyol-modified polymers showed a substantial increase in tensile strength (over double) with a similar decrease in elongation (but sufficient for normal use on many substrates). Improvement in dry time and hardness is substantial. One of the most interesting effects is the decrease in water vapor permeability of the urethane polymer. There is little difference between the monomer and dimer of the same glycol, whereas a significant difference is indicated between ethylene and propylene glycol. The glycerol modification shows the least permeability, probably because of the greater degree of cross linking derived from its trifunctionality. T h e sorbitolmodified coating did not show as good
