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. f o r 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 A g e 86, 639 (1960).
Conclusions
The 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 preparation 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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prepared by gas injection techniques. Greatly improved dielectric loss properties of the foamed insulation are anticipated because the materials used to produce the hot spots are effective a t extremely low concentrations. Background
The small cell structure and high efficiency required in the production of useful foamed insulation can be obtained by the proper use of chemical blowing agents. Earlier studies have shown that high efficiency and fine cell structure are obtained in a n extrusion process only when decomposition of a suitable blowing agent is not complete within the extruder and when the decomposition of a finite portion of the remainder of the suitable blowing agent continues a t a reasonable rate as the molten polymeric composition goes from a high pressure area example, when the extrudate to a low pressure area-for emerges from the die ( 7 ) . T h e solution of gas in polymer resulting from the decomposition of the major portion of the blowing agent inside the extruder becomes a supersaturated solution of gas in polymer as soon as the pressure falls below the critical pressure of the solution. For best cell structure, this should not occur until the composition emerges from the extruder. It is necessary to nucleate the supersaturated solution of dissolved gas in the polymer extrudate in order to obtain small cell size. Nucleation is accomplished by the continued decomposition of a finite portion of the suitable blowing agent. I t is believed that the number of cells which are produced is directly related to the number of particles or agglomerates of the suitable blouing agent which decompose in the short time interval during the formation of the supersaturated solution and before completion of the expansion process. \Vhen decomposition of the blowing agent is essentially complete within the extruder or if the blowing agent decomposes sluggishly a t the conditions required for the production of the foamed structure, large cells are formed and efficiency is low because much of the dissolved gas is dissipated by diffusion to external surfaces rather than to the surfaces of bubbles, since fewer bubbles are initiated in these cases. In each of these situations, decomposition rate studies indicate that there are insufficient nuclei in the extrudate. Bubbles can form spontaneously, particularly at high levels of supersaturation. but much larger cells are formed at lower efficiency, Efficiency increases as
the ratio of volume to surface becomes greater because of the increased impediment to diffusion of gas to external surfaces. Recent work has shown that this hypothesis correctly accounts for the behavior of a blowing agent in an extrusion process or similar time-temperature-dependent expansion process. The hypothesis is incomplete because a suitable blowing agent has not been defined. Modifled Mechanism of Bubble Nucleation
A simple mechanism of bubble nucleation which accounts for the fact that some blowing agents are better nucleators than others is now proposed. I t is highly likely that nucleation of fine cell structure from a supersaturated solution of gas in polymer is a result not of the continued decomposition of a suitable blowing agent per se but rather of the continued formation of localized hot spots by the continued decomposition of a blowing agent which evolves heat during its decomposition. Gas is driven out of solution more rapidly a t the sites of the localized hot spots than from a matrix of uniform temperature. A corollary of this modified mechanism is that the hotter the spot, the more effective the nucleation. Similar reasoning shows that an increase in the degree of supersaturation of the polymer solution will make it less stable and thus easier to nucleate. T h e insolubility of the nucleator. its particle size, and its particle size distribution are important because of their effect on the amount of heat generated at the hot spots, Hot spots can be created by a wide variety of techniques. Improved expanded polymeric compositions can be prepared by using small amounts of hot spot generators to nucleate fine cell structure from solutions of gases in polymers. The solutions of gas in polymer may be prepared by using a secondary (nonnucleating) blowing agent or, preferably, by direct gas injection. Gas Injection Extruder
A simple modification of a vented extruder resulted in the very useful gas injection extruder shown in Figure 1. T h e vacuum port of a vented extruder was replaced by a gas injection port, which is simply a hole in the barrel of the extruder, located near the region where there is a large pressure drop due to the design of the screw. Furthermore. the pressure drop in the gas injection zone makes it possible to add gas over a range of convenient low pressures. The higher pressures in the zones around the gas injection zone prevent PER CENT BY WEIGHT OF Ti09
PORT
I I
G A S INJECTION EXTRUDE
I 1 I ZONE1
ZONE
I I
3 1 2 t m
A
AZODICARBONAMIDE PLUS INJECTED NITROGEN
B
TLO, PLUS INJECTED
c
INJECTED NITROGEN ONLY
NITROGEN
a
0.001 SCHEMATIC PRESSURE P R O F I L E
Figure 1. Gas injection extruder and schematic pressure profile showing low pressure trough a t gas injection zone which allows easy entry of gas 138
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
0.01 0.1 PER C E N T BY WEIGHT OF AZODICARBONAMIDE
3
Figure 2. Effect of nucleator concentration on relative bubble size Hot spot nucleators such as azodicarbonamide ore effective; materials such as Ti02 are not
possive
gas loss through the hopper of the extruder and assure the formation of a gas solution before the mixture enters the head of the extruder. Pressure in the head of the extruder was found to decrease because of the plasticizing effect of the dissolved gas. A variety of gases could be dissolved rapidly and completely in a variety of polymers and the solutions of gas in polymer could be used conveniently for the study of the nucleation of directly injected gases.
I
A AZODICARBONAMIDE ONLY
B AZODICARBONAM IDE PLUS DISSOLVED NITROGEN
C
AZODICARBONAMIDE PLUS ADDITIONAL DISSOLVED NITROGEN
Pressures as low as 50 and as high as 2200 p.s.i. were used, depending on the expansion desired and the conditions of extrusion (die size, throughput, etc.). The die most commonly used was a 100- by 250-mil ribbon die which afforded samples which were easily collected and compared, but a number of experiments were performed in which 19-gage copper wire was coated with about 10 mils of expanded insulation. Proof of Hot Spot Mechanism
The validity of the hot spot theory was confirmed by successful nucleation of fine cell structure from solutions of gas in polymer by hot spots created by both physical and chemical methods. In the physical method, hot spots were created by the interaction of externally applied energy with particulate matter in the polymeric mixture. For example, 1000 grams of low density polyethylene and 5 grams of a coarse carbon black (average particle size about 400 mg) were admixed in a paint shaker. The resultant mixture was extruded through the gas injection extruder a t an applied pressure of 1000 p.s.i. of nitrogen. Large, randomly distributed bubbles (5 to 9 per inch), identical to those obtained in the absence of carbon black, were obtained. However, when four 150-watt projection lamps were used to irradiate the extrudates as they emerged from the die of the extruder, an expanded polymer having a uniform distribution of finer cells (50 to 80 bubbles per inch) was obtained from the composition containing carbon black, although there was no change in the number or size of the bubbles formed in the extrudate in the absence of carbon black. Hot spots created by the interaction of radiant energy from the projection lamps with carbon black particles or agglomerates caused the nucleation of many more cells than were observed in the absence of hot spots. It was possible to nucleate bubbles from supersaturated solutions of gases in polymers by creating hot spots by chemical decomposition or rearrangement of many unstable compounds. Most of these nucleators also evolved gas during their decomposition, although gas evolution was not a prerequisite for formation of fine cell structure. In general, those materials evolving about 100 or more calories per gram during decomposition or rearrangement were potentially useful nucleators. Rate of decomposition or rearrangement (because of rate of heat evolution) and other factors such a3 solubility, melting point, particle size, and particle size distribution all affected ultimate efficiency of nucleation. Nucleation with Azodicarbonamide
The decomposition of azodicarbonamide is highly exothermal. Since it is not very soluble in polyolefins and since it decomposes a t a rate which is compatible with the extrusion process, azodicarbonamide has been described as a suitable blowing agent for the extrusion of expanded polyolefins ( 7 , 2). It is suitable because it creates many localized hot spots during its continued decomposition while the solution of gas in polymer formed by its partial decomposition emerges from the extruder. The many localized hot spots initiate many bubbles, and fine cell structure and high efficiency are observed. It is reasonable to assume that azodicarbonamide should also be able to nucleate fine cell structure from solutions of gas in polymer which have been prepared by direct gas injection,
I0 )
I
I
Figure 3. Relationship between nucleator concentration and number of bubbles formed at several levels of supersaturation Ease of nucleation increases as supersaturation increases
The bubble size studies in Figure 2 show that azodicarbonamide can effectively nucleate injected gas. In these experiments the extruder was charged with mixtures of low density polyethylene and azodicarbonamide and 200 p.s.i. of nitrogen was applied a t the gas injection port. Nucleation was observed a t concentrations a t least as low as 0.001 weight yGbut was more effective a t slightly higher concentrations. For comparison, the effect of other insoluble materials which might function as nucleators by acting as “boiling chips” was also studied. Polyethylene containing several of these finely divided solids in several concentrations was extruded at the same conditions as for azodicarbonamide (Figure 2) and over a range of other conditions with the same results. The addition of finely divided T i 0 2 (300 A . , essentially anatase) actually appeared to inhibit the self-nucleation of bubbles in polyethylene (curves B and C, Figure 2), because the bubbles produced in the presence of T i 0 2were larger than those formed in its absence. A number of other additives, including a sample of sodium aluminum silicate (Molecular Sieve), SiOl, FerOl, rutile TiOz, and carbon black and pigments in a range of particle sizes And concentrations, were similarly ineffective. Increasing the gas pressure to 400 and to 800 p.s.i. resulted in slightly smaller cell structure, even though the cells were more numerous, which indicates that more highly supersaturated solutions are more readily nucleated. There appeared to be no significant difference in cell size when gases such as nitrogen, argon, helium, carbon dioxide, or air were studied. The effect of nucleator concentration and degree of supersaturation is shown in Figure 3. When azodicarbonamide was admixed with low density polyethylene and extruded (curve A ) , no cells were observed in experiments where the concentration of azodicarbonamide was less than 0.10 weight YG. However, a concentration of 0.05 weight 7G is more than adequate to nucleate dissolved gas obtained by the decomposition of 0.2 weight of benzazimide, a poor blowing agent (curve B ) . or dissolved gas VOL. 3
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139
DAYS IN DISTILLED WATER
(23.C)
Figure 4. Effect of residue from decomposition of ozodicarbonomide on dielectric properties of unfoomed polyethylene Residue content of polyethylene shown as arodicarbonomide content. Azodicorbonarnide deemposed b y heating under pressure to prevent foerning of polymer
which was directly injected by application of 1500 p.s.i. of nitrogen (curve C). Cell size and resultant density for most of these experiments are listed in Table I. T h e data in Figure 3 and Table I also show that more highly supersaturated solutions are more easily nucleated. Furthermore, cell size remains fairly large until about 100,000 bubbles per cubic inch are produced. At about this point cell size begins to decrease and nucleation begins to occur much more rapidly. As nucleator concentration is increased, efficiency of expansion and cell size are both improved, the former more rapidly than the latter a t low nucleator concentrations. At higher concentrations, cell size decreases more rapidly than efficiency increases. Nucleation of bubbles from solutions of directly injected gas in a variety of polymers by the exothermal decomposition
Figure 5 . Effect of nucleator concentration a t essentially constant levels of supersaturotion Smallert bubbles and greatest number of bubbler formed a t highest ""Cleator soncentratipnr
of azodicarbonamide is observed a t concentrations of the latter of about 0.0005 weight %. Finer cell structure and higher efficiency are obtained by the use of slightly greater amounts of azodihrbonbmide and expanded structures are obtained which have improved dielectric properties because the amount of nucleator used is only a fraction of the concentration of azadicarbonamide normally used to expand the polymer. T h e adverse effect of residues remaining after decomposition of varying amounts of azodicarbonamide on the dielectric properties of unfoamed polyethylene is shown in Figure 4. Fine cell structure was obtained by the use of other gasevolving nucleators. A number of blowing agents which decompose exothermally at rates compatible with thc extrusion of foamed polymers were found to be useful nucleators. Ma-
Tabla l. Nucleator Concentration and Expansion Efficiency Relolive
Ejicimcy
% '424-
dicorbonamidc Urcd 0 0.001 0.002 0.005 0.01 0.02 0.03 0.05 0.1 0.15 0.2 0.25 7 . Arodrcarbonamrde
140
No. of Bubblerper Cu. Znch 7-
2
... ... ... ...
180 1,100 3,600 6,700 24,000 65,000 257,000 2,450,000 16,000,000
0
n 0 20,000 110,000 271,000 552,000
only
... ... ...
Diameter of Bubbler, Mils 7 2 17 ...
i5
... ... 3.5 4
3.7 3
2 Arodtcnrbonnmtdc plus 0 2% beroarmtdc
l & E C P R O D U C T RESEARCH A N D DEVELOPMENT
15 15 13 12 9 5 3.5
...
Dcnrily of Foorn, Gram Per Cc.
o/ Gar ~ t & ~ ~ t i ~
7
2
2
... ... ... ...
0.92 0 924 0 925 0 913 0 898 0 871 0 811 0 173 0 728 ...
1 3.3 13.6 19.7 56.1 113 178 263 476
...
0.92 0.92 0.92 0.918 0.915 0.852 0.784
...
terials such as 4.4'-oxybis (benzenesulfonylhydrazide) and .V,S'-dinitrosopentamethylenetetramine are typical of those blowing agents which, like azodicarbonamide, were also useful nucleators at concentrations below which no cell structure was observed in the absence ofinjected gas. Hot Spot Formation without Concomitant Gas Evolution
An unusual type of nucleator which has been discovered functions without evolution of gas and thus further confirms the hot spot theory of bubble nucleation. S-Aminophthalimide. for example: melts at about 200' C. and then rearranges in the mrlt to give o-phthalhydrazide Lvithout evolution of gas. 0
0
II
I1
II
II
0
0
N -AMINOPHTHALIMIDE
0
- PHTHALHYDRAZIDE
'l'he o-phthalhvdrazide formed by this rearrangement is a stable compound which normally melts a t about 350' C. Since it is formed as a liquid from molten .V-aminophthalimide at about 200' C.. it is highly supercooled and crystallizes, giving off heat of crystallization. The rate of heat evolution from this crystallization process is compatible with the extrusion of polyolefins. for .Y-aminophthalimide and its homologs have been used to nucleate fine cell structure from solutions of gases in polyolefins.
production of foamed polymers by nucleation of these solutions by localized hot spots is described. The hot spots may be created by physical or chemical techniques. Materials which produce the greatest amount of heat are generally the most effective bubble nucleators. Concentration of the bubble initiator, since it need not act as a source of gas, can be several orders of magnitude less than the concentrations at which blowing agents are used. Dielectric performance of the resultant expanded polymer is greatly improved because of the small amount of nucleator required for the formation of fine
eel+
Ultimate cell size and efficiency of expansion depend upon a number of factors, such as the extent of supersaturation of the gas in the molten polymer, the number of hot spots produced (Figure 5). the amount of heat produced a t the hot spot. and the ratio of external surface area to the internal surface area of the nucleated bubbles during their growth period. A simple theory which explains the observed results is proposed. Acknowledgment
The authors thank Thomas DeBenedictis and Harold M . Gilroy for their assistance in some of the experiments and David B. Herrmann for the dielectric data shown in Figure 4. Literature Cited (1) Hansen, R. H., SPE J . 18, 77 (1962). (2) Hansen, R. H., DeBenedictis, T., Am. Chem. SOC.,Div. Org. Coatings Plastic Chem., Preprints 21, No. 2, 32 (1961).
RECEIVED for review February 17, 1964 ACCEPTEDApril 2, 1964
Conclusions
.4 simple extruder modification for preparing solutions of gases in polymers is suggested and a novel method for the
20th Annual Technical Conference, Society of Plastics Engineers, Inc., Atlantic City, N. J., January 1964.
PHOSPHORUS-CONTAINING U N S A T U R A T E D POLYESTER R E S I N S FROM T R I A L K Y L PHOSPHITES R A Y M O N D R . H I N D E R S I N N AND NICODEMUS E . BOYER' Hooker Chemical Gorp., Siagara Falls, IV Y .
Trialkyl phosphites have been shown to react smoothly and exothermally with monoalkyl maleates under The corresponding controlled reaction conditions i o yield predominantly tetraalkylphosphonosuccinates. dialkyl fumarate and carbon dioxide are by-products of the reaction. The reaction has been used to prepare fire-resistant polyester resins from glyceryl tris(alky1ene glycol) hexamaleates which have unusually good light stability and weather resistance, and which can b e formulated to have a very low smoke content on burning. The reaction of triallyl phosphite with the same hexamaleate resins yields unsaturated resins with unusually high heat distortion points. The reaction of triallyl phosphite with monoallyl maleate results in a colorless liquid composed largely of tetraallylphosphonosuccinate. The crude product has been shown to yield hard, fire-resistant compositions with methyl methacrylate. H I S \voRh DESCRIBES the preparation of light-stable. fireTiesistant polyester resins with improved weather resistance. .4lthough chlorine-containing. fire-resiFtanr resins can be stabilized with various additives so that a fair degree of light stability can be obtained. the presence of large quantities of halogen necessary for suitable fire resistance makes the resin surceptible to yellowing and erosion when exposed to the elements. Because of these difficulties. it was decided that the Present address. Richardson Chemical Corp., Chicago, Ill.
use of phosphorus as the main fire retardant element might lead to fire-resistant materials of improved light stability. The susceptibility of many phosphorus esters to hydrolytic cleavage led to the decision to incorporate the phosphorus group into the polymer as a pendant group on the main chain rather than as a constituent of the polymer backbone. Phosphonate esters were chosen because of their better hydrolytic stability ( 7 . 2) and the relative ease with which they may be synthesized VOL. 3
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