ORGANIC PEROXIDES
a
Diisopropvl Peroxydicarbonate W.A. STRONG
carbonate esters are unusual peroxy compounds a wide range of properties. The first material representative of this type was reported by Wieland in 1925. A series of esters described by Strain and coworkers (72) included diiropropyl, dicyclohexyl, and di(2-nitro-2-methylpropyl)peroxydicarbonates. These dialkyl members are prepared by careful reaction of the appropriate chloroformate with aqueous sodium peroxide at low temperature (7, 70, 72) by the reaction:
p". wth
0
II
2ROCC1
+ NalOa
-
0
II
0
II
ROCO-0-COR
+ 2NacL
9
Diisopropyl peroxydicarbonate (IPP) is commercially available in over 99% purity and has the followiug properties :
* Molecular weight (CsH,rOs) Melting point Refractive index, nDw Specific gravity, 15.514' C. Solubility in water at 25' C., % Solubility in organic solvents
206.18 &loo C. (46-50' F.) 1.4034 1.080 0.04 Migdhlc with aliphatic and aromatichydrocarbons,esters, ethers, chlorinated hydrocarbons
Diisopropyl peroxydicarbonate (IPP) has an active oxygen content of l.Syocompared to 6.6% for benzoyl peroxide. It liberates iodine from acidified solutions of potassium iodide, providing the basis for an analytical method for determining the assay. When warmed above its melting point, to about 14 to 18' C., IPP undergoes decomposition as evidenced by a
slow bubbling. Shortly afterward, this temperature produces an auto-accelerative decomposition which may be hazardous if the material is confined. The final stage of the decomposition as ordinarily observed is a sudden effervescence which takes place in only a few seconds time and produces volatile products which are flammable. I n the presence of amines or aqueous alkali metal hydroxides, decomposition is accelerated. IPP decomposes on contact with concentrated sulfuric acid. It deflagrates on contact with flame although ignition is somewhat slower than with some other widely used peroxy compounds. When pure IPP is decomposed thermally by passing it through as team-heated tube, the products include (in moles per mole of IPP) carbon d i o x i d e l . 6 1 , isopropyl alcohol-0.91, acetone4.42, acetaldehyde4.45, and e t h a n d . 4 1 (72). Under these conditions, the isopropoxy radicals formed evidently attack the undecomposed IPP or further decompose to acetaldehyde and methyl radicals. Ethane is formed by dimerization of the latter. In the thermal decomposition of 20 to 400/, solutions of IPP in toluene, these primary products account for most of the material, with the isopropyl alcohol-acetone ratio about 2.5: 1. In dilute solutions of a solvent resistant to hydrogen abstraction the proportion of isopropyl alcohol to acetone is about equal to the theoretical (1 :1). For other solvents, higher ratios are observed. There is evidence of combination of alkyl carbonate radicals with fragments arising from solvents susceptible to hydrogen abstraction, resulting in formation of carbonate esters with consequent decreased evolution of carbon dioxide. VOL. 5 6
NO. 1 2
DECEMBER 1964
33
With proper precautions, IPP has been used safely over a long period Toxicity
The general toxicity of peroxides has been reviewed by Noller and Bolton (6). The toxicity of IPP has been carefully studied, perhaps as much as any peroxide, because of its unusual structure and properties as compared with other peroxy catalysts (7). Because of its low temperature of decomposition, testing required special techniques in some cases. Animal studies by independent biological laboratories indicate a moderate toxicity and moderate activity as a skin irritant, the hazard being reduced by the low vapor pressure of IPP. Prolonged exposure may cause eye irritation and lung edema. For IPP alone the acute oral toxicity in rats for single dosage is LD50 of 2.1 grams per kilogram, corresponding to slight toxicity. With rabbits, IPP gives a primary skin irritation index of 2.6, and thus can be classed as a moderate skin irritant. Rats confined in cages containing a supply of I P P for eight hours showed no symptoms of abnormality or distress. Examination of their lungs immediately after exposure and after one week revealed no significant pathological effects. Toxicity tests were also made on solutions, and IPP concentrations of 45y0in two solvents gave the following results :
Acute Oral LDsa (Rats), g./kg.
1
Soltrol 730
Cyclohexanebenzene
1
8.5
6.5
I
Skin penetration LDSO (rabbits), ml. /kg. Eye injury (rabbits)
1
1.7
4.0
Irritant
Irritant
Only the L D I of ~ 1.7 indicates a slight toxicity, the other LD50 values indicate virtual nontoxicity. Although severe conjunctivitis, marked iritis, and corneal ulcerations were suffered as a result of the eye test, the eyes were clear of damage after 7 to 14 days. When a 45y0 Soltrol solution was used for prolonged exposure tests more serious effects were produced. In tests with rats, inhalation of solution vapor that was free of mist and droplets a t concentrations of 9 to 13 parts per million during regular exposures over a threeweek period produced the typical physiological manifestations of a n irritant gas. Growth was repressed and one third of the animals died. Pulmonary injuries were found but survivors recovered substantially in one month with no significant hepatic or renal effects. Thus, while severe toxic effects can result from unusual intensified conditions, available data indicate that IPP does not represent a serious toxicity hazard under ordinary conditions of handling. No ill effects have been observed among laboratory or plant personnel, except for isolated individual cases of dermatitis and 34
INDUSTRIAL A N D ENGINEERING CHEMISTRY
sensitivity to the characteristic odor of IPP. It is recommended that adequate ventilation be provided and rubber gloves and goggles be worn. Any material contacting the skin should be washed off promptly with soap and water.
Safety Hazard Testing and Recommendations
Because of the physical nature of IPP and its low decomposition temperature, many of the usual tests for peroxide stability and sensitivity (6) will yield significant information only if these characteristics are taken into account. In tests designed to make comparisons with other peroxy compounds and to define the degree of hazard in the storage and handling of IPP, there has been no indication that its decomposition, when unconfined and under normal atmospheric pressures, will result in a detonation. The decomposition of IPP, however, may occur with explosive violence, releasing large quantities of heat which may promote easy ignition, especially through secondary sources which may have reached kindling temperatures or which will catalyze ignition. In controlled tests (such as those with blasting caps described below) such ignition has not been observed. I P P is flammable and its volatile decomposition products can be ignited in air. Comparative ignitibility tests showed, however, that I P P is much less easily ignited than black powder or benzoyl peroxide. When a quantity of IPP as large as 15 pounds in an open container is ignited it does not explode, but burns with an intense flame for several minutes until consumed. A frozen block of IPP, however, is only melted in the vicinity of a n embedded electric squib after firing. Under the influence of a No. 8 blasting cap and moderate confinement, as in the ballistic mortar test, IPP develops 64% of the energy of black powder, the weakest explosive used industrially. O n the other hand, when IPP is tightly confined in a steel pipe and fired by a No. 8 cap augmented by a tetryl booster, fragmentation of the container may take place. I P P in the frozen state is not sensitive to friction and is less sensitive to impact (concussion and/or percussion) than black powder and many times less sensitive than benzoyl peroxide. In fact, bullets from a 30-caliber rifle only occasionally caused a partial decomposition. Because of the 1-iolenceof decomposition under conditions of confinement, tests attempting to initiate explosions of IPP with blasting caps and by other means have been conducted on successively larger scales. At first, comparison was made with half-pound charges
W . Albert Strong was Senior Research Chemist in the Research Department of the Chemical Division of Pittsburgh Plate Glass Co., Barberton, Ohio. He is now Administratice Assistant to the Director of Research.
AUTHOR A t the time this article was prepared,
i
and I P P was observed to have much less explosive power than benzoyl peroxide. More conclusive evidence for partial explosions of a low order was obtained with 15pound charges. Finally, refrigerators equipped with interior heating devices were loaded with 100-pound quantities of IPP. Vigorous decomposition soon occurred, accompanied by fumes and charring of the refrigerator but there were no actual explosions and no flames in the absence of a n external ignition source. It should be emphasized that such severe conditions are not likely to be encountered in actual practice of storage and handling. Since solutions of I P P in organic solvents are often stored and handled, the hazards involved here were also investigated. Concentrations of 50 to 90% of I P P in cyclohexane-benzene (1 : 1) have been tested from the explosion hazard standpoint. Thermal stability was determined by heating charges of eight grams of solution at the rate of 10' C. per minute in 80-ml. bombs equipped with thermocouples and with gages to measure rapid pressure changes. Thermal decomposition was found to be relatively slow, reaching full intensity in a few hundred milliseconds compared to only 1 to 10 milliseconds for detonations. The decomposition was not sensitized by nitrogen under pressure in the bomb, and no increase of the auto-ignition temperature (45 to 50' C.) was noted. Rather, the maximum temperature was depressed due to the heat absorbing capacity of the inert gas. These effects are illustrated for selected runs in Figure 1. The maximum rate of pressure increase (maximum Ap/At) varied from lo4 for the 50% solution to lo6 p.s.i. per second (near detonation rates) for the 90% solution, or a range of one hundred-fold. Qualitative demonstrations resulted in no explosions when one-pint bottles of 50 to 90% I P P solutions were dropped 20 feet onto a stone surface or when 30-caliber rifle bullets were fired into 35 to 90% solutions. Standard impact tests (each consisting of 20 drops with a five-kilogram weight) showed that shock sensitivity in-
creased with increasing IPP concentration. No explosions occurred a t drop heights of over 55 inches with 50y0 solution. Explosions did occur for 50% of the drops a t heights of 22 inches for 75% solution and 10 inches for 90% solution. These results are in essential agreement with those of Guillet and Meyer (4). Concentrations greater than 40-5070, therefore, should not be handled. Tests have been made with five-gallon quantities of I P P solutions under probable storage conditions. To show the effects of temperature, 45% concentrations in cyclohexane-benzene were stored and analyzed periodically. At -12' c. (10' F.) there is no appreciable loss of assay in glass, stainless steel, mild steel, or aluminum. From -10 to 0' C. (15-32' F.) there is a slow loss of assay. At a storage temperature of about 5' C. (42' F.) the solution temperature rises slowly a t first, then a t a n accelerated rate, and this culminates in explosive decomposition after about one day. Other storage tests were also made under adiabatic conditions on 10 to 3Oy0concentrations. These demonstrate that 10% solutions are safe in case of refrigeration failure a t a n ambient temperature maximum of 40' C. (105' F.). Higher concentrations result in excessive assay loss or gross decomposition. Here only five-gallon quantities have been tested and since a bulk or mass effect is known to operate for I P P solutions, larger quantities of dilute solution may exhibit exothermic self-sustaining decomposition a t lower temperatures. While emphasizing the additional precautions to be observed in the case of higher concentrations of solutions, storage recommendations should be aimed principally at ensuring protection against fire rather than explosion becabse of the hazard of flammable decomposition vapors or volatile solvents. For solid IPP, storage well below the freezing point is mandatory. As containers, enameled or aluminum trays with loose-fitting lids are preferred because of accessibility. Tests also show that the catalytic activity of I P P in polymerization of allyl resins is not impaired by storage in a plastic (polyethylene) container. For solutions, storage a t low temperature (-18' C. or 0' F.) is recommended, withefficient heat transfer and slow agitation to eliminate temperature gradients and hot spot formations as additional precautions. Precautions should be observed in the disposal of I P P and its solution. Small spills of IPP can be flooded with water and washed up with soap and water. Small quantities can be disposed of by cautious addition to 5y0 alcoholic potassium hydroxide. Larger quantities can be destroyed by scattering on the ground in a well ventilated disposal area free of combustible material and allowing the I P P to melt and gradually decompose. Solvent solutions can be disposed of in porous sandy soils with proper regard for solvent flammability. Sewering of I P P in any form is not advisable.
Rates of Decomposition in Solution
Figure 7 . Rate of pressure rise in the thermal decomposition solutions (solvent, cyclohexane-benzene)
of
IPP
I P P is usually employed in solution either in a nionomer or a n inert solvent. Stability of the catalyst in VOL. 5 6
NO. 1 2 D E C E M B E R 1 9 6 4
35
This unusual peroxide is useful as a source of solution is thus of importance. The ease of producing free radicals for use in polymerization is a b - related to the rate of decomposition in the medium being used. The rate of decomposition of an organic peroxide at a specified temperature in terms of its half-life provides a basis for comparison of different compounds and different solvents. Data for the half-life of IPP have been determined in a variety of solvents, particularly those applicable to the polymerization of ethylene and vinyl chloride (7, 7). Measurements have been generally limited to the range of one half-life due to the deviation from linearity of first-orderrates beyond that point. This deviation may be attributed to several factors, mainly the change in medium as soluble decomposition products increase in concentration. Rates of decomposition are summarized for higher concentrations in a variety of solvents (Table I), and for lower concentrations in selected solvents, to show the effect of temperature (Table 11). The decomposition rates are shown to be affected by the nature of the solvent. As a further example, 30% IPP in isopropyl alcohol at 25' C. is extremely unstable, resulting in violent ebullition in 25 minutes. In the measurement of very short half-lives by a different technique (S), the rate of decomposition of IPP was found to be independent of the concentration by determinations on 10 to 45% solutions in kerosene. The half-life values for pure IPP at different temperatures are: Temperature, Half-life, sec.
C.
121 10
135 1.5
143 1.0
Kinetic studies on the rate of decomposition as well as the effect of inhibitors and type of solvent have been made by Strain and coworkers (72). The kinetic data show that IPP decomposition in dilute solution, for example 3% in toluene, conforms quite closely to first order kinetics. The over-all activation energy for decomposition in dilute toluene solutions, 28.1 kilocalories per mole, is close to the values reported for benzoyl peroxide (29 to 30 kilocalories per mole) in solutions of similar concentration, indicating that the primary ratecontrolling step is the Same for both-homolytic cleavage of the peroxy oxygen bond. Evidence that chain reactions of the radical type are involved in the decomposition of IPP is provided by the observation that the tendency for auto-accelerated decomposition can be moderated or inhibited by small additions (1 to 5%) of a variety of compounds, the most effective being known inhibitors of chain reactions, including polyhydroxy- and nitroaromatic compounds, certain unsaturated compounds, oxygen (or air), and iodine. The decomposition of pure IPP and IPP in concentrated solutions is more rapid than in dilute solutions. This is attributed to reactions of higher than first order, 36
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
5 FRO)
,TION
TABLE I .
DECOMPOSIT
DECOMPOSITION OF IPP IN SOLVENTS Ciginol
Corn., Solvent
% __
Diethyl maleate Dibutyl phthalate Triuesyl phosphate Phillips Soltrol No. 130 Sohio Varnolcne 3039 V M P naphtha Sohio aliphatic solvent Tetralin Xylene Toluene Benzene Cyclohexane-benzene(1 :1 ) Cyclohexane-benzene(3: 1 ) Cyclohexane Diddoroethane Perchloroethylene
27.7 28.8 27.6 15.0 33.4 5.0 5.0 29.4 5.0 45 46.4 45.1 35.0 45.6 20.0 30.6 45.0
xol/-Li/8, Boys ot 35' C.
at 25" C.
12.5 4.0 10.1 12.4 1 .O (30') 3.9 8.7 4.9 14.6
... ...
0.78 0.91
... ...
4.5
-
...
I
... ... ... 0.6
...
...
... ...
... 1.1
