ROOM TEMPERATU RE-CU RING POLYURETHANE CASTING COMPOUNDS L . M 0 N T E S A N 0 , Bell Telephone Loborotorier, lm.,
Murray Hill,
N. J.
Room temperature-curing polyurethane elastomers made practical through the addition of triethylenediamine can be prepared in unlimited numbers. Polyurethane compounds have excellent plugging and sealing properties and con b e used to pot pressure-, moisture-, and temperature-sensitive electrical components in a manner unequaled b y other room temperature-curing casting compounds. These compounds are stable on aging, noncorrosive to copper, electron irradiation-resistant,'and low in water absorption and shrinkage, ond con be made nonflammable and fungus-resistant. Physical ana electrical properties can b e voried through materials selection. Room temperature-curing polyurethanes are of definite value and will grow in importance in the field of room temperature-curing liquid thermosetcng casting compounds. N THE
casting resin field there is a need for materials with low
I shrinkage, law water absorption, aging stability, and good
electrical properties for use in potting apd encapsulating pressure-sensitive electrical components. Room temperaturecuring polyurethane compounds have these properties. Polyurethane compounds appear to be of value in plugging telephone cables containipg paper- or polyethylenelinsulating conductors, as cable sheathing repair materials, and as protective coatings. Room temperatureemring polyurethanes are stable on heat aging, noncorrosive to copper, have low cure shrinkage, low water absorption (Figure l ) , high insulation resistance, dielectric properties equal to and in some cases better than epoxy and styrene-polyester casting compounds, and can he made flame-retardant and fungus-resistant (Figure 2). Compounds exppsed in the Van de Graaff generator remained relatively unchanged after irradiation equaling 10 years in the Van Allen belt. Viscosity of 'room temperature-curing polyurethanes increases gradually prior to setting up, unlike most epoxy compounds, which drop 'in viscosity as they exptherm.
HO-R-OH (Palyal)
+ 2 (OCN-R'-NCO) (Diisacyanate)
-
O
O H
I I1
I1 I
H
OCN-R'-N-C-0-R-0-C-N-R'NCO
(1)
(Prepolymer) Prepolymer
+ HO-R-OH
+
(Polyol)
I I
I
l
I
l
_
room temperature-cured urethane elastomers
CHs CH, C H ,
(Triethylenediamine)
Experirnenld
The liquid imcyanate-terminated prepnlymers are prepared hy reaction of excess diisacyanate with selected polyals as in Equation 1. The resultant viscous liquids are subsequently combined a t room temperature with more poly01 and, in the presence of a catalyst as in Equation 2, yield cured elastomers.
EPOXY-POLYAMIDE
--
EPOXY
-POLYSULFIDE
x3-
Figure 2. Effects of composition on fungus resistance o f polyurethane compounds Determined by ASTM D 1924-61 T. EPOXY- ANHYDRIDE
COMPOUND ( 2 , TABLE TI
upper left. Polycin U-56. 5 0 parts DB oil. 5 0 port5 Dabco. 0.25 part lower left.
Figure 1.
Water absorption at
23" C.
Polydn U-56. 73.5 ports Pblvcin 12. 26.5 a orli Pblycin port, I~ Dabco. 0 063 port 0.063
25X
Upper right. Polycin U-56. 69.5 parts Poiydn 51. 31.5 parts Dabco. 0.125 p a r t
Lower right. Polycin U-63 74 ports Polycin 12. 27 ports Aroclar 1254. 25 .arts ports Dabco. 0.063 port VOL. 3
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Table 1. Material
Polycin U-56
Descriptions and Suppliers of Materials Descrijtion Supplier
Polyester-base prepolymer derived from castor oil. Eauiv. w't. per NCO group 420, NCO content
Baker Castor Oil Co.. Bayonne, N. J.
10%
Polycin U-63
Polyester-base prepolymer derived from castor oil. Equiv. wt. per NCO group 308, NCO content 13.6% Solithane 113 Polyester-base prepolymer derived from castor oil. Equiv. wt. per NCO group 420, NCO content
Thiokol Chemical Co., Trenton, N. J.
10%
Polycin 12
Polycin 51
Polycin 52
DB Oil
Dabco Antifoam .4 Aroclor 1254
Poly01 isocyanate. Baker Caster Oil Co , Equiv. wt. 150, funcBayonne, N. J. tionality 4.2, hydroxyl number 370 Poly01 isocyanate. Equiv. wt. 186, functionality 2.3, hydroxyl number 295 Poly01 isocyanate. Equiv. wt. 163, func: tionality 2.9, hydroxyl number 340 Urethane grade of castor oil. Isocyanate equiv. wt. 442, functionality 2.7, hydroxyl number 163 Triet hylenediamine Houdry Process Corp., Philadelphia, Pa. Dow Corning Corp., Silicone oil with silica filler Midland, Mich. Chlorinated biphenyl Monsanto Chemical Co., New York, N. Y.
Catalysts are required to make the polyolisocyanate reaction proceed rapidly a t room temperature, and of those evaluated to date, triethylenediamine (Dabco) appears most effective ( 7). Sufficient curing polyol is added to the prepolymer [stoichiometric amount required to react with the residual isocyanate (-XCO) groups of the prepolymer] to effect the curve. 'The triethylenediamine or other catalyst accelerates the rate a t which cure takes place, making room-temperature cures prac-
3.0
tical. This paper deals only with castor oil-base prepolymers cured with polyols and triethylenediamine. Polyether-base prepolymer-polyol-catalyst systems which cure a t room temperature have also been developed and are now being evaluated. I n evaluating polyurethanes where either stiffening or reversion of the specimens on heat aging is possible, it was decided to study the effects of heat aging on tensile strength, tensile elongation, stiffness in flexure, and Durometer hardness. The tensile values were run in the manner prescribed in A S T M D 638-58T,although the specimens used were straightstrip specimens rather than dumbbell specimens. The compounds were cast to a depth of ' 1 8 inch in flatbottomed aluminum dishes Z3/ 1 inches in diameter. Specimens were prepared from these cast disks. conditioned, and tested for original properties and properties after aging 2 months a t 225' F. Disks were tested for original dielectric constant, dissipation factor, and insulation resistance values. Insulation resistance values were also obtained after conditioning 28 days a t 95' F. and 90y0relative humidity. Disks were irradiated in the \.'an d e Graaff generator to determine the effects of radiation on the specimens. Tables I and I1 give information on the various ingredients and the formulations of the compounds evaluated. Results and Discussion
Tables 11, 111. IV, V. and VI show the test results on 14 polyurethane compounds. 'The compounds remained stable on heat aging, with the exception of those to which a diluent was added (compounds 12 through 14), and those did not change enough to eliminate them from consideration. Aroclor 1254 and Polycin 12 contribute toward fungus resistance. Aroclor 1254 reduces flammability. The irradiated compounds (Table V I ) exhibited onlb- discoloration and moderate hardening. Compound 7 proved to be particularly resistant to irradiation. 'The first column of Table I1 shows the compound formulations. Polycin L-56, Polycin U-63, and Solithane 113 are castor oil--base prepolymers. T h e polycin materials without the "C" are all polyols. A silicone defoamer was used as an aid in degassing of the compounds. Polyols contribute toward efficient degassing by lowering the mix viscosity. Triethylenediamine is a solid soluble in polyols a t 185' F. By dissolving the triethylenediamine in the polyol, a two-part
COMPOUND : CASTOR O I L BASE 74 PARTS PREPOLYMER - 27 PARTS POLYOL 79' E TEMPERATURE
-
2.5
RANGE FOR EPOXY-POLYSULFIDE -EPOXY-POLYAM I D E EPOXY -ANHYDRIDE
VI
4 2.0 8
's
TIME TO S E T UP
WORKING L I F E
1.5
F
1.0
El
0.5
0
L
Figure 3. Effect of triethylenediamine on working life and setup time of polyurethane compound 134
l&EC
PRODUCT RESEARCH A N D DEVELOPMENT
Y
ZERO REF. PT. I
-40 TRIETHYLENEDIAMINE CONCENTRATION
Y
I I
5
I
1
I
+27 +40 TEMPERATURE ( 'c]
0
I +l
Figure 4. Frequency change for potted network vs. tem perature
Table II.
Comjound
1. Polycin U-56 Polycin 51 Triethylenediamine Antifoam .qd 2. Polycin U-56 Polyc1n 12 Triethylenediamine 3. Polycin U-56 Polycin 12 Triethylenediamine 4. Polycin U-56 Polycin 52 Triethylenediamine 5. Polycin C-56 DB Oil Triet hylenediamine 6. Polycin U-56 DB Oil Triethylenediamine 7. Polycin C-63 Polycin 52 Trit-thylenediamine 8. Polycin U-63 DB Oil Triethylenediamine 9. Solithane 113 Polycin 12 Triethylenediamine 10. Solithane 113 Polycin 12 Triet h ylenediamine 11. Solithane 113 Polycin 52 Trieth y lenediamine 12. Polycin U-63 Polycin 12 Aroclor 1254 Triethylenediamine 13. Polycin L-63 Polycin 12 Aroclor 1254 Triet hylenediamine 14. Polycin U-63 Polycin 12 Aroclor 1254 Triet hylenediamine a A S T M D 638-55T.
Parts by Weight
68.5 31.5 0.125 0.03 73,5 26.5 0.125 73.5 26.5 0.063 68 32 0.125 50 50 0.25 60 40 0.2 65 35 0,125 50
Setup at 80" F., .Win.
Physical Properties
Tensile Strength.a P.S.I. After 2 months at 2 2 5 " F. Orig.
Tensile Stiffness in Flexure.b P.S.I. Eloneation., " ,I ", -After 2 After 2 months at months at 225" F. Orig 225' F. Orig.
Jurometer Shore A & D Hardness Valuesc
Y
Orig. A 70
After 2 months i t 225' F.
80
400
310
330
305
645
620
A 83
22
1,000
1,260
130
135
4,670
9,850
D 50
D 54
55
1,440
1,400
110
115
10,500
14,100
D 50
D 60
54
230
280
140
230
795
700
A 80
A 75
53
80
60
45
45
445
385
A 50
A 48
60
170
180
75
115
775
600
A 60
A 60
39
2,050
2,270
125
195
43,500
59,900
D 65
D 60
56
240
190
140
135
790
545
A 65
A 60
1,320
1,180
120
150
7,550
10,800
D 55
D 55
1,370
1 ,440
120
125
11,600
15,600
D 55
D 55
340
330
175
235
690
640
A 80
A 85
235
577
64
120
886
1,685
D 35
D 40
740
1,769
120
100
5,192
9,122
D 60
D 60
3,022
1,607
25
100
10,249
11,194
D 53
D 60
50
0.25 73.5 22 26.5 0.125 73.5 57 26.5 0.063 68 57 32 0.125 60 42 40 25 0.063 42 60 35 15 0,063 74 30 27 25 0.063 ASTM D 747-58T.
A S T M D 7706-67.
casting compound is formed, part A being the prepolymer and part B being the polyol and triethylenediamine. Triethylenediamine has remained in polyol solutions for one year without separating and is still as active as when first mixed. Other catalysts, both liquid and solid, may also be used. T h e third column of 'Table I1 shows the pot lives of the compounds. which vary anywhere from 22 to 80 minutes. T h e pot lives may be shortened or lengthened by increasing or decreasing the quantity of triethylenediamine. This is shown by a comparison of compounds 9 and 10 (see also Figure 3 ) . I n compound 10 all ingredients are identical to compound 9 except that the amount of triethylenediamine has been halved, increasing the pot life from 22 to 57 minutes. Changes in concentration of catalyst d o not affect the aging properties of the compounds. Polycin U-56 and Solithane 113 prepolymers yield similar results, shown by a comparison of compounds 3 and 10. in
Used in all compounds
which the only difference is in the prepolymer used. T h e polives of compounds 3 and 10 as well as their physical propert ties are almost identical. Polycin 12 is the most reactive of the polyols used, having a functionality of 4.2 and hydroxyl number of 370. This is shown by comparing mixes 10 and 11 where, with less triethylenediamine, mix 10 containing Polycin 12 sets u p in the same time as mix 11 which contains Polycin 52. Compounds containing Polycin 12 also have better physical properties than compounds containing the other polyols in Table I1 (compare compounds 1 \vith 2 a n d 3 with 4 ) . Polycin 52 solidified a t room temperature in about one month. I t can be easily returned to its liquid form by slight warming. This polyol would be impractical in kits for field use. Compounds 5, 6, and 8: which contain a urethane grade of castor oil, have the poorest physical properties but are extremely resilient. These compounds are inexpensive. as the VOL. 3
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JUNE
1964
135
Table 111.
Compounda
Electrical Properties
Insulation Resistance,~Ohms Alter 28 days Dielectric at 95" F. and Constant,c Original 90% R H lo00 Kc.
1 >2 x 10'2 3 x 2 > 2 x 1012 8 X 4 >2 x 1012 6 X 5 > 2 x 1012 4 X 6 >2 x 10'2 2 x 12 >2 x 1012 >2 x 13 > 2 x 1012 >2 x 14 > 2 x 10'2 >2 x a Refer to Table II for formulations. D 1531-59T.
109 10" lO1o 10'0 109 10'2
3.4 3.6 3.6 3.8 3.4 3 7 2.6 1012 1012 3.2 A S T M D 257.
Table V.
Compound 1 2 3 4
0.02 0.03 0.03 0.04 0.03 0.03 0.01 0.02 ASTM
castor oil is low in cost and represents u p to one half of the total mix. Further price reductions are possible through the addition of diluents or fillers. Castor oil-base polyurethane casting compounds are lower in cost than epoxy compounds. T h e U63 prepolymer-polyol compounds have the best physical properties, as shown by comparing compound 4 with compound 7. Both compounds use the same polyol, yet compound 7 has physical properties far superior to those of compound 4. This is attributable to the higher N C O content of U-63, 13.6% us. 10% for U-56. Applications
The compounds listed in Table I1 set u p rapidly a t room temperature. Compound 3 has been used for potting pressuresensitive time-division networks. This compound is desirable because it is resilient, low in shrinkage, less expensive than an epoxy-polysulfide-tertiary amine compound previously used, less toxic, and relatively odorless. The strong odor of the polysulfide-tertiary amine combination is very objectionable, particularly in a plant operation. The dielectric and insulation resistance properties are superior to those of the polysulfide compound. There is no significant inductance change nor degradation of the frequency response in the network as a result of potting with the polyurethane compound. An oscillator network potted with compound 3 maintained a flat frequency response between 0' and 40' C., and its frequency response was similar to that of an unpotted network at -40' and +70° C. Networks potted with three different epoxy compounds did. not have this flat frequency response between 0' and 40' C. and varied more in frequency a t -40' and a t +70' C. (see Figure 4). The resonant frequencies of toroidal coils potted in a polyurethane compound remained constant during 45 days' exposure to an atmosphere of 95y0relative humidity at 100' F. The same coils were then heated a t 150' F. in an oven for 50 days without showing a change in resonant frequency. Shrinkage on curing is low (Table IV, compounds 12 and 14) ; therefore electrical properties of pressure-sensitive components are relatively unaffected. Adhesion to aluminum foil casting dishes is so great that the dishes have to be torn away from the castings. Table IV. Compounda 2 5 12 14
Corrosion of Copper at 95" F. and 96% R H under 45-Volt Potential No corrosion after 28 days No corrosion after 28 days
a Refer to Table I1 f o r Formulations. Brookjeld viscometer R V F .
136
b
5 6 7 8 9 10 11 12 13 14
Flexibility Rating
Good Fair Fair Good Excellent Excellent Good Excellent Fair Fair Good Good Good Good
Excellent Excellent Excellent Excellent Poor Poor Excellent Poor Excellent Excellent Excellent Excellent Excellent Excellent
Flexible Semirigid Semirigid Semirigid Resilient Resilient Semirigid Resilient Semirigid Semirigid Semirigid Semirigid Semirigid Semirigid
Because of the materials involved and the fact that only small concentrations of catalyst are necessary, toxicity should be less of a problem with the catalyst-polyol-cured compounds than with many of the epoxy compounds. However, it is advisable to exercise the usual safety precautions, including good ventilation in both mixing and curing areas. Shelf Stability
Tests conducted to determine the stability of various prepolymers on exposure to normal laboratory atmosphere have shown the polyurethane prepolymers to be affected by moisture in the air. Thick skins formed on the air surfaces of the polyether-base prepolymers in a matter of weeks. The castor oil-base prepolymers showed no skinning over in 1 month; a '/s-inch skin formed in 3 months. After 9 months the l / b i n c h skin on the castor oil-base prepolymers had not thickened, while the polyether prepolymers had solidified completely. After one year the skins on the castor oil-base prepolymers were pierced and the prepolymers under the skins found to be still liquid. For optimum shelf stability of the prepolymers their containers must be kept tightly sealed- to prevent moisture from reacting with the polymers. Castor oil-base prepolymers have remained stable for 3 years in sealed containers. Containers of prepolymer are best sealed under an atmosphere of dry air or inert gas to prevent contact with moist air. The polyol-catalyst part of the mix is not affected on shelf aging, but precautions should be taken to prevent moisture from being adsorbed by the ingredients. If proper packaging and storage procedures are followed, polyurethane compounds can be used, too, in the field in two-part kits. Compounds 12 and 14 have been used experimentally to plug gas-pressurized telephone cables. These plugged cables have been temperature-cycled extensively between - 40' and f140' F. without pressure failure of the polyurethane plugs. Compound 12 has also been used experimentally to sheath old cracked lead telephone cables to prevent ingress of moisture.
56 days
...
...
A S T M D 590-59aT.
Tear Strength Rating
Miscellaneous Properties
24 hours ...
...
Fluidity
Toxicity
% Water Absorptionb
0.15
...
No corrosion after 28 days
Qualitative Ratings
Based on visual and manual observations
Dissipation Factor,c 7OOO Kc.
...
volume ShrinkageC ...
...
2.1 3.2 Water displacement method.
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
0.8
...
Flammabilityd
Viscosity, Centipoises"
Self-extinguishing
... ...
...
Self-extinguishing 1928 Self-extinguishing 2500 A S T M D 635-56T. e ASTM D 7 8 2 4 - 6 1 T ;
Table VI. Effects of Irradiation on Polyurethane Compounds A S T M D 7706 59 T Duromzter
Color Compound"
1
Orig.
After irradiationb
Straw-yellow
Brown
~
Hardness Values After irradiaOrig. tion
A 70
D 65
Flexibility Rating
Aftel-.
~
Orig.
irradiation
Flexible; pliant
ObservationJ after Irradiationc
Change in color and increase in hardness. No adverse effects such as blistering, disintegration, or reversion D 50 D78 Semirigid Semirigid Discoloration and hardness increase, but ex2 Pale tan Brown cellent retention of physical characteristics 5 Straw-yellow Brown '4 50 D 62 Flexible; very Flexible semi- Discoloration and significant hardness increase resilient rigid from resilient flexible material to semirigid material 7 Yellow Brown D 65 D 63 Semirigid Semirigid Discoloration of specimen but otherwise excellent retention of physical characteristics Irradiation in nitrogen atmosphrrc at 1 m.e.a. for 5 hours. Total Pztx 2.9 X 70'6 dectrons p e r sq. cm. Temperaa I)esiqnation numbers inoTable II. All specimens held u p c e l l ; none shoremed signs of deterioration. ture ,,! Jppcimen surfaces 65 C. Irradiation dosage equal to 70 years in Vun Allen belt. Compounds 2 and 7 showed ixcclient stability after irradiation.
Semirigid
Compounds 1 and 5 showed significant changes in hardness but no other adverse effects.
Modifications have been made to adapt these basic compounds to particular cable applications. An aluminum powdei filler has been added to bring the thermal coefficient of expansion of the compound closer to that of lead. Milled glass fiber has been added for reinforcement. Fillers should be dried prior to use. Compounds have been applied as thixotropic pastes as well as in combination with various wrapping tapes. These sheathed cables have exceeded 400 of the aforementioned temperature cycles without kaking gas. Also, flexing of test cables for as many as 5600 times has resulted in no loss of gas pressure.
e.g., epoxy and styrene-polyester. These properties include intrinsic flexibility, self-extinguishment in flammability tests, radiation resistance. very low shrinkage after cure, very low water absorption, and good stability a t moderately elevated tempera tures. These polyurethanes have proved of definite value in potting of electrical components. cable sheathing, cable plugging, dip coating, and other applications.
literature Cited
(1) Patton, T. C., Ehrlich, A., Smith, M. K., Rubber Age 86, 639 (1960).
Conclusions
T h e room temperature--curing polyurethane compounds are a valuable addition to the casting compound field. They can be prepared in a n unlimited variety with properties superior to those found in other room temperature-curing compounds-
RECEIVED for review February 10, 1964 ACCEPTED April 13, 1964 Division of Organic Coatings and Plastics Chemistry, 147th Meeting, ACS, Philadelphia, Pa., April 1964.
N O V E L M E T H O D S FOR T H E PRODUCTION OF FOAMED P O L Y M E R S Nucleation of Dissolved G a s by Localized Hot Spots R A L p H H.
H
A NS EN A N
D W I L L I A M M. M A R T I N ,
BE11 Telephone Laboratories. Inc., ,%furry H d l ; h'i 3;
A highly effective technique for preparing foamed polymers b y nucleation of directly injected gases has been discovered.
Fine cell structure is obtained in an extrusion process, for example, if the extrudate, which
i s essentially a supersaturated solution of gas in polymer, contains an abundance of localized hot spots. Hot spots generated b y a variety of physical and chemical techniques were shown to b e capable of nucleating bubbles from solutions of gases in polymers.
Those materials which produced the greatest amount of heat
were generally the most effective bubble nucleators.
A simple extruder modification permits easy prepara-
tion of the solutions of gases in polymers. OLYOLEFINS are preferred for primary electrical insulaP t i o n because of their excellent dielectric properties. T h e dielectric constant can be further improved and the cost of the insulation can be lowered by foaming the polymer. However, even when expansion is accomplished by the use of the proper blowing agent under optimum conditions (7. Z ) , undesirable dielectric loss effects usually result from the presence of the residue remaining after decomposition of the blowing agent.
Best dielectric performance will be obtained from a foamed polyolefin which has been expanded b) a gas alone. Although expanded polymers have been prepared by direct gas injection, cell size is relatively large and the technique is of no value in the preparation of thin-walled structures required in primary electrical insulation. Now, hokvever. localized hot spots have been found to be highly effective nucleators for the production of fine cell structure from solutions of gas in polymer VOL. 3
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137
