POLY UR ET H A N E CO A T l N 0 S NCO/OH
INCO-3
NCO-2
NCO-I
1.2l1.0
\
-28
0,
a v) 60
A
In
H
-26
I
-2.
e
‘ ..
I
-30
70
\I
I 40
-20
y
-18
I-
.
6-30
\
-22
50
a
I
1
E
-14
30
-12
- 10
20
-2s
P
5--a
W
In
a
t
I
NC0.3
NCO.2
NCO-1
Pp ’ 30
40
#
-20
In
I
C
2
3--18
2-14
IC
AVERAGE
EQUIVALENT OF
WEIGHT
ISOCYANATE
PER
NCO-GROUP
INTERMEDIATE
NCO/OH
= 1.2/1.0
400
300
Properties of isocyanate intermediates cured with intermediate OH-1 not containing tertiary amine nitrogen
AVERAGE
600
500
EQUIVALENT WEIGHT PER NCO-GROUP OF ISOCYANATE INTERMEDIATE
Properties of isocyanate intermediates cured with intermediate OH-3 containing tertiary amine nitrogen NCO/OH = 1.2/1.0
rials used were related to resulting coating properties. Moisture-Cured Coatings were harder and less flexible than polyol or hydroxyl-terminated intermediate-cured ones. Their solvent resistance was excellent. Flexibility of moisture-cured coatings increased \\vith the length of the chain-extending diol. Poly01 Cured Coatings. Polyols having equivalent weights between about 50 and 160 gave acceptable drying and curing times of the coatings. Polyols of higher equivalent weight yielded softer and more flexible cured films. Polyols of lower equivalent weight provided harder and more brittle cured films. Triols and tetraols gave a shorter pot life than diols of the same equivalent weight. Coating formulations made from polyols containing tertiary amine nitrogen had a shorter pot life than those where the amine nitrogen was absent. The former polyols also contributed to the hardest coatings with the most rapid curing rate and best solvent resistance of those tested. Coatings Cured with Hydroxyl-TerHydroxylminated Intermediates. terminated intermediates (equivalent weight between 150 to 350 for those without tertiary amine nitrogen, and from 200 to 400 for those with tertiary amine nitrogen) cured isocyanate-terminated intermediates effectively. They imparted antisagging properties to the coatings. The intermediates containing amine nitrogen produced harder, more solventresistant, and faster curing films than intermediates where nitrogen was absent. The length of the diol used for chain ex-
tension of each intermediate defined the degree of film flexibility. Abrasion loss values were very low. After 1000 hours of Weatherometer exposure, no loss of gloss was observed. literature Cited (1) Bailey, M. E., SPE Journal 14, 41 ( 1958). (2) Brushwell, W., Am. Paint J.41, No. 44, 107(1957). (3) Hudson, G. A., Heiss, M. L., Saunders, J. H., Division of Paint, Plastics, and Printing Ink Chemistry, 130th Meeting .4CS, Atlantic City, N. J., September 1956. (4) Mobay Chemical Co., “L’rethane Surface Coatings.”
( 5 ) Owen, G. E., Jr., .4they, R. J., Remington, W.J., Elastomer Chemicals Dept., E. I. du Pont de Nemours 8r Co., Paint Bull. PB-4. ( 6 ) Pansing. H. E., 086. Digest Federaiion Paint & Varnish Production Clubs 30, 37 11958). ( 7 ) Rkmington, UT. J., Athey, R. J., Ibid., 31, 612-23 (1959). (8) Remington, W. J., Lorenz, J. C., Elastomer Chemical Dept., E. I. du Pont de Nemours & Co.. Paint Bull. PB-2 and PB-3. ADOLFAS DAMUSIS, J. M. McCLELLAN, Jr., and K. C. FRISCH Wyandotte Chemicals Corp., Wyandotte, Mich.
Stable Pigmented Coatings Systems
POLYURETHANE
coatings vehicles show substantial promise in many applicatioris where satisfactory coatings are not a t present available. ‘One factor which has impeded their progress has been a difficulty in pigmenting many of the systems. This investigation indicates that the instability of pigmented polyurethane prepolymers is caused primarily by adsorbed moisture and other reactive contaminants present in the pigments. A promising method for overcoming many of these problems consists of prereacting these impurities with iFocyanate by slurry grinding in a ball mill prior to addition of the prepolymer. Such a procedure utilizes conventional manufacturing techniques and equipment and should be practical for volume production.
For the purpose of this discussion, polyurethane vehicles may be divided into four categories.
1. One-package urethanes containing no free isocyanate. 2. Semiprepolymer two-package systems consisting of a polyisocyanate or isocyanate prepolymer containing free NCO, designed to react with substantial quantities of a polyol shortly before use. 3. One-package prepolymers containing free isocyanate but capable of conversion to films in the presence of ambient moisture. 4. Two-package prepolymer systems, containing free NCO, which are converted by small amounts of catalysts added a t time of application. The systems of 1, containing no free NCO, can be pigmented by conventional VOL. 51. NO. 11
NOVEMBER 1959
1387
Experiments with vehicle l , rutile titanium dioxide-2070 PVC, show that isocyanate pretreatment improves stability
0
I
2
3 TI ME
-
methods. A degree of success can be achieved in the semiprepolymer systems of the second type by dispersing the pigment in the nonreactive poly01 component and adding the isocyanate containing semiprepolymer shortly before use. The third and fourth types of prepolymer systems present a more difficult problem. The pigments must be dispersed in the NCO-containing prepolymer, and at conventional pigment loadings the usual result is immediate thickening and gelation within a short time. Even if a degree of stability is obtained, puffiness, bubbling, pinholing, and nonuniformity are experienced. It is believed that adsorbed moisture, impurities, and surface treatment chemicals present in the pigments react with the free isocyanate groups of the prepolymer to cause this instability. This concept suggests that if these contaminants can be prereacted with isocyanate before introduction of the prepolymer they would be rendered incapable of reacting with the free NCO in the prepolymer and stability may be accomplished. This article describes the results obtained to date on what may be termed a “slurry grinding” technique for prereaction of the contaminants in a practical and convenient manner. Briefly, the pigments, enough solvent to make a slurry, and an isocyanate are charged to a
Table 1.
Vehicle 1 2 3 4
1 388
5
4
6
MONTHS
NCO-Containing Prepolymer Systems Type of Type of % UnOH-Bearing Diiso- reacted cyanate NCO Component TDI 7.5 Vegetable oil TDI 8.0 Vegetable oil -PolYglYcol MDI 8.5 Vegetable oil TDI 10.0 Polyglycols
Table II.
Typical Polyurethane Prepolymer Enamel
Oven weight loss of pigment, 0.45%. Stoichiometric (1/1) weight of TDI, 12.5 (assuming wt. loss as all moisture) A B C (Control) (1/1) (2/1) Parts by Weight Slurry grind Rutile titanium dioxide 288 288 288 Xylene 280 280 280 TDI 0 12.5 25 Tumble overnight in ball mill; add Polpurethane prepolymer 300 300 300 Grind 24 hr., adjust, and package pvc, % 20 20 20 Vehicle nonvolatile, 70 51.7 51.7 51.7
closed dispersion system such as a ball mill and intimately mixed until the residues have reacted with the isocyanate. The prepolymer is then introduced and the completed system is ground until dispersion is satisfactory. Four film-forming prepolymer systems of the third and fourth types containing different levels and types of isocyanate and varying hydroxyl-bearing components were studied. All these vehicles are stable under normal storage conditions when unpigmented but unstable when pigmented by conventional procedures. The first step was to determine as accurately as possible the amount of moisture and other reactive impurities introduced into the paint systems. Initial investigations indicated that xylene and toluene could be obtained essentially
INDUSTRIAL AND ENGINEERING CHEMISTRY
free of reactive contaminants, and the study was limited to these two solvents. No entirely satisfactory method was found for measuring all the reactive residues in the pigments; therefore, it was decided to use the oven weight loss determination (ASTM D 280-33). T o provide a starting point for calculations, the weight loss was assumed to be entirely moisture, although this is not necessarily true. The pigment weight loss was then converted to equivalents of water and the theoretical quantity of isocyanate required to react stoichiometrically lvith the moisture was calculated. Three paints were prepared for each pigment-vehicle combination under study: a control containing no isocyanate, and identical paints containing one and two times the theoretical equivalent weight of isocyanate required to react with the water in the pigment. Tolylene diisocyanate (TDI) was used for all the pretreatments described because of its availability and relatively low cost; however, other isocyanates such as phenyl isocyanate or diphenylmethane diisocyanate may be equally effective. The figure illustrates typical results obtained with and without isocyanate pretreatment using vehicle 1 in the above formulation. To date approximately 20 pigments have been studied with the vehicles shown in Table I ; in general, the stability curves of vehicles 1, 2, and 4 with inorganic pigments are similar to the one illustrated. Complete stability has not been accomplished as yet with vehicle 3 ; however, considerable improvements were obtained and it is possible that a higher level of T D I pretreatment would give still further stabilization. Organic pigments known to contain free groups in their molecule which are reactive with isocyanates have given anomalous results and require further study. I n general, the organic pigments studied were a t least as stable without the pretreatment. Film and application properties were questionable, with or without pretreatment. This slurry grinding technique should not be applied to pigments or other ingredients containing excessive amounts of moisture, because the carbon dioxide gas evolved from reaction with isocyanates could lead to dangerous pressure build-up within the mill. Provision for pressure relief devices on ball mills may prove advisable; however, for economic as well as safety considerations it is best to use materials with as low a moisture content as is practical.
R. A. and
BIENEMAN, E. J. BALDIN, M. K. MARKOFF
Spencer Kellogg and Sons, Inc., Buffalo 5, N. Y.
