REINFORCED PLASTICS SYMPOSIUM
ORGANIC PEROXIDES ORVILLE L. MAGELI J A M E S R . KOLCZYNSKI
Thermal decomposition of organic peroxides provides f y e e radicals that cross-link Polymers in reinforced plastics bv establishing nem sites therein
hermal decomposition of organic peroxides usually Tleads to free radicals which can initiate polymerization of vinyl monomers or induce cross-linking of a substrate by the formation of free radical sites on a polymer. These sites can subsequently combine to form new bonds or cross-links. Cross-linking may be effected also by homopolymerization of di- or polyfunctional monomers, or by copolymerization of an unsaturated polymer with a vinyl monomer. The major use of peroxides for reinforced plastics is in polyester resin curing, where copolymerization is used to form the cross-linked network. Several reviews have been published on the chemistry of organic peroxides (27, 44, 87, 777). These compounds may be considered as analogs of hydrogen peroxide in which one or both of the hydrogens are replaced by an organic radical. The peroxide linkage is thermally unstable, the degree of instability depending to a large extent upon the organic substituents as well as on the nature of its environment. Reinforced plastics may be considered to embrace all types of organic polymer products in which reinforcing agents are used. This would include elastomers such as silicones, urethanes, and ethylene-propylene copolymers, as well as polyethylene, polyacrylates, polyesters, and other polymers and copolymers. Reinforcing agents are carbon black ( 7 7 6 ) , glass fibers, steel or other metal fibers (46),silicates and carbonates (84,as well as mixtures of fillers (72). I n this review, the emphasis is on the use of organic peroxides in rigid reinforced polymers, primarily peroxide curing of reinforced polyester resins. I t has been estimated that this rigid reinforced plastic market exceeded 340 million pounds in 1965, of which about 90% is polyester resin (86, 87).
Use of Organic Peroxides in Polyesters
Reinforced polyester resins became important in World War 11, when the U. S. Army Air Corps developed a critical need for a protective shield for selfsealing gas tanks (60, 776). The United States Rubber Co. discovered that laminates made from polymerized allyl diglycol carbonate and glass cloth prevented torn metal edges from blowing into the rubber tank lining of self-sealing gas tanks. Because of a shortage of this allyl carbonate and its slow curing properties, it was soon replaced by an unsaturated polyester resin of the type used today (70, 23, 97). This type of unsaturated polyester resin was rapidly cured by organic peroxides. Dibenzoyl peroxide, known for more than 100 years, was in commercial production in the United States in 1927. I n 1937 dilauroyl peroxide became commercial, and in 1938 tert-butyl hydroperoxide was synthesized. Before the end of World War 11, tert-butyl hydroperoxide, di-tert-butyl peroxide, tert-butyl peroxybenzoate, acetyl peroxide, and cyclohexanone peroxides were available commercially. The major uses for organic peroxides were in the synthesis of styrene-butadiene rubber and in curing polyester resins. With the termination of the war, some of the volume uses of organic peroxides were gone, and several years elapsed before volume increased in applications for civilian use. Selection of Organic Peroxides
Organic peroxides can be classified according to the functional group or radical with which the active oxygen occurs. The table beginning on this page presents a list of organic peroxides, many of which are currently used in curing reinforced plastics. Organic peroxides act as initiators in polymerization reactions because they undergo homolytic cleavage of the 0-0 bond to form free radicals: R0:OR
-+
RO.
+
*
OR
When these free radicals encounter unsaturation in the polyester or in the monomer used to dilute the polyester resin, active sites are formed which unite or copolymerize to form a cross-link. As more and more cross-links are formed, the copolymer chains become tangled and their motion is restricted. When the system can no longer flow, it is said to be gelled. Heat is generated during the polymerization, thus accelerating the reactions. VOL. 5 0
NO. 3
M A R C H 1966
25
Cross-linking continues causing further restriction of polymer motion until the entire polyester mass is solid or cured. The time to gel and cure a polyester system is a function of the concentration of free radicals produced by the peroxide. The rate of peroxide decomposition into free radicals is controlled by temperature and by the use of selected promotors. The selection of the proper peroxide initiator (78, 72, 74-76, 109, 770) will depend primarily upon the desired cure temperature and rate of cure, as well as
T A B L E 1.
I
O R G A N I C P E R O X I D E S U S E D T O D A Y FOR C U R I N G R E I N F O R C E D
DIACYL PEROXIDES 0 0
!I
l
CH3-C-O-O-C-CH 3 Acetyl peroxide in dimethyl phthalate
0
upon auxiliary factors such as shelf-life of precatalyzed polyester, color of resulting piece, and whether a promotor is used. The most important of these factors is the temperature at which the cure is to be carried out. A peroxide is characterized by the temperature at which it decomposes. Because peroxides decompose over a range of temperatures, with their rate of decomposition increasing with increasing temperature, they must be compared in either one of two ways : by the temperature at which a prescribed amount decomposes or by the
Half-Life at Selected __ Tern$.m a C. Hr. 50 158 70 8.0 85 1.1
0
y e ,
C. 69
2
K E T O N E PEROXIDES Structural formulas for methyl ethyl ketone peroxide found in most commercial formulations in dimethyl phthalate CH3 OOH C \'
0
~ c - o ll- o - f i - - @ j
70-Hr. Half-
70
13.0
100 85
2 0 . 14
72
CZH!
O ' OH H 0 CH3O
0
50 70 CH~(CH~)~-C-O-O-C(CHZ)~CH~ 85 Caprylyl peroxide in mineral oil
I1
II
/
63
56 3.3 0.5
75
C2H5
C2H5
H 0 CH3 0
CH3
0
CH~(CHz)8-C-O-O-C( CHz)aCH3 Decanoyl peroxide, granular or flaked solid
0 -O;-"O-O-"
0
II
/
62
50 70 85
18 1.4 0.25
60 70
CHI
62
0
II
0 0 50 71 /I 1; 70 4.5 CH3-CHz-C-O-O-C-CHz-CHa 85 0.7 Propionyl peroxide in high boiling hydrocarbon solvent a
C2H6
eo-o%
3
85 100 115
20 3.8 1.0
91
85 100 115 130
88.0 12.5 1.9 0.33
102
100 115 130
18 3.1 0.55
105
PEROXYES T E R S
7
64
CHa-C-O-O-C( CH3)3 tert-Butyl peroxyacetate in benzene
0
@jI!-0-0 -c (CH3)s
All hav-lqe determinations were made in benzene at concentrations of
0.7 to 0.2iM.
26
\
Cyclohexanone peroxides, crystalline solid or paste form in dibutyl phthalate
Lauroyl peroxide, granular or flaked solid
I1
H 0 0 CH3
S"Io-oEQ
50 49 70 3.2 CH3(CH2)TC-O-O-C( CHz),CH3 85 0.5 Pelargonyl peroxide, solid below 50' F. (IO0 C.)
CH~(CH~)~OC-O-O-C(CH~)IOCH~ 100
0
C2Hs
I
54
62
11
\
I I/ c-00-c
13 -3.4 0.5
0
I/
i c-oo-c-oo-
C2Hj
c1 c1 2,4-Dichlorobenzoyl peroxide with dibutyl phthalate
II
H 0 0 CHI
C2H5
c1
0
1
/
\ I
13 3.4 0.5
\
C2Hj CZH6 H 0 CH3 0 CH3 \ I C-00-A-00-c
p-Chlorobenzoyl peroxide in dibutyl phthalate
60 70 85
Commercial formulations are mixtures of several peroxide and hydroperoxide structures having different half-lives
'I c-ooc I /
Benzoyl peroxide, granular solid or paste forms
0
H 0 0 CH3
105
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
tert-Butyl peroxybenzoate
amount decomposed at a given temperature. Half-life is a convenient means of expressing the rate of decomposition at a particular temperature. I t is defined as the time required for one half of the peroxide originally present to decompose. Doehnert and Mageli (26) and Mageli, Bukata, and Bolton (67) have evaluated commercially available peroxides with respect to their half-lives. Table I lists the commercially available organic peroxides with their respective half-lives at selected temperatures, and their 10-hour half-lives (the
temperature at which 10 hours are required to decompose half the peroxide initially present). These data, together with activation energies calculated from them, enable the selection of a peroxide for situations where cure temperature is limited. They are also useful when restrictions are placed on the temperature range over which a peroxide must decompose, when we must bear in mind that peroxides with high activation energies will decompose over a narrower temperature range than those of low activation energy.
P L A S T I C S M A Y BE C A T E G O R I Z E D I N T O F I V E P R I M A R Y GROUPS 0
II
C4Hg-CH-C-O-O-C(
CH3)3
I
70 85 100
13 2.2 0.4
72
I1
(CH3)2CH-C-O-O-C( CH3)3 tert-Butyl peroxyisobutyrate in benzene
ALKYL PEROXIDES C(CHa)3
100 115 130
I
0 0
CnHs tert-Butyl peroxy( 2-ethylhexanoate), liquid
0
4
33 5.4 0.7
108
100 115 130 115 130 145
218 34 6.4 12 1.8 0.3
126
115 130 145
17 2.8 0.4
119
0
I I/ CH~-C-CH~-CHZ-C-O-C~H~ 70 85 100
0
85 100 115 (CH3)3COOCOCH(CH3)n tert-Butyl peroxyisopropyl carbonate, liquid
I1
29 3.6 0.5
57 8.7 1.4
0
50 20 70 1.6 /I 85 0.25 (CHx)xC-C-O-O-C( CH3)3 . .,tert-Butyl peroxypivalate, liquid in mineral spirits
79
0 -
0 I
98
&CH& n-Butyl-4,4-bis( tert-butylperoxy )valerate, liquid (CH3)3C-O-O-C( CH3)3 Di-tert-butyl peroxide, liquid
CH3
55
CH3
@&-o-o-&a I I CH3
117
CH3
Dicumyl peroxide, crystalline solid
CH3
CH3 I CH3-C-CHzCHz-CI -CH3 I I
100 115 130
10 1.5 0.27
100
CH3
CH3
I I CHa-C-CH2-CH2-C-CH3 I I
0 0 0 0 C(CH3)3 C(CHa)3 2,5-Dimethyl-2,5-bis( tert-butylperoxy)hexane, liquid CH3
I I
CH3-C--C=C-C-CH,
2,5-Dimethylhexyl-2,5-di(peroxybenzoate), crystalline solid CH3
CH3
I I CH~-C-CH~-CHz-C-CH~ I I 0 0 I 0 0 I I c=o o=c I I CH-CZH~ CnHj-CH I I
70 80 100
6.4 2.0 0.2
66
@IIII -0-C-0 C
0-C 0-C
(CHs)3 (CHs)3
I
I
5
0
Di-tert-butyl diperoxyphthalate in dibutylphthalate
18 2.5 0.4
128
HYDROPEROXIDES
@+-0-0-.
100 115 130
49 8.2 1.7 0.3
0 0 0 0 C(CH3)3 C(CHs)a 2,5-Dimethyl-2,5-bis(tert-butylperoxy)hexyne-3,liquid
( CH3)&-O-O-H tert-Butyl hydroperoxide, liquid
C4H9 C4H9 2,5-Dimethylhexyl-2,5-di(peroxy 2-ethylhexanoate), liquid
0
115 130 145 160
CH3
130 145 160
520 120 29
172
115 130 145 160
470 113 29 9
158
CH3 or-Cumyl hydroperoxide, liquid 105
CH3 CH3 CH3-C-CH2-CHz-&-CH3 I
I
I
0
0
130 145 160
67 19 6.1
154
0 0 H H 2,5-Dimethylhexane-2,5-dihydroperoxide,solid
VOL. 5 0
NO. 3
MARCH 1966
27
Another convenient means of characterizing peroxide activity is by comparing the temperature at which various peroxides gel an unsaturated polyester. Mageli, Stengel, and Doehnert (77) have compared the relative activities of commercial peroxides in a standard unsaturated polyester resin. Listed are the temperatures at which the catalysts provide a 15-minute gel time in the resin system, The standard SPI Exotherm procedure was used ( 7 75). Peroxides can be used in pairs for specific applications. When a polyester resin containing a ketone peroxide, cobalt activator, and benzoyl peroxide is impregnated into glass cloth, the resin can be gelled at room temperature to give a nontacky material which can be stored for more than two weeks, and then finally cured at elevated temperature (49). Interactions between two- and three-component peroxide mixtures cause both synergistic and inhibitive activity in curing polyesters (43). The effects of residual peroxide ( 2 ) and peroxide diluent (37) on the cured piece have been investigated.
Promotors and Inhibitors
Promotors are added to the resin system to accelerate the decomposition of the organic peroxide into free radicals at room teniperature or at temperatures apprcciably lower than those at which the peroxide normally decomposes. Promotors are specific for the type of peroxide catalyst. They include metallic salts such as cobalt naphthenate, the tertiary amines (50), quaternary amines, and mercaptans. Typical accelerators and the organic peroxides with which they are active are listed (97). A combination of two activators with a peroxide can be used to provide a wide time lag between gel and cure in a manner similar to the use of two peroxides of different activity. Fibrous supports impregnated with resin containing benzoyl peroxide and accelerators of limited activity merely gel but do not cure the resin. When the gelled piece is later treated with dimethylaniline, the piece becomes fully cured ( 7 78). Inhibitors are added to an unsaturated polyester resin to enhance its'storage stability. They react with growing free radicals and prevent them from further reaction with the unsaturation in the resin or monomer, thus preventing pregelation. Before a peroxide-initiated polymerization can gel a resin, the inhibitors present in the resin must be consumed by some of the free radicals initially produced. Besides providing added storage stability, inhibitors must not cause any undesirable change in the physical properties of the polyester resin, or change the rate of the peroxide cure. A large number of inhibitors have been screened to determine which best meet these requirements (99). Gel and cure characteristics of a number of organic peroxide-accelerator-inhibitor systems have been investigated at both elevated and room temperatures (96). 28
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
PEROXIDES USED TODAY: Diacyl Peroxides
Benzoyl Peroxide. This peroxide is probably the most widely used peroxide in polyester applications. It is used for every fabrication technique except in hand-lay-up operations. Dry benzoyl peroxide is low cost but is shock sensitive, flammable, and relatively insoluble, so that it must first be dissolved in styrene before it is added to the resin. Because of these drawbacks the paste formulations of benzoyl peroxide are very widely used, being safer and easier to handle than the dry compounds, and readily dispersible in polyester resin. Several different paste forms are available: a 50YG dispersion in tricresyl phosphate which contains no thixotropic or suspending agents to cause cloudiness or haziness so it can be used to cure clear laminates or castings. Both 50Yc and 55YGnonseparating dispersions which are used where paste separation is a problem. The dispersing medium is generally a phthalate ester containing various proprietary additives which contribute toward maintaining a homogeneous product of consistent viscosity. Some organic peroxide manufacturers also offer fire-resistant paste formulations which are virtually nonflammable, can be shipped by parcel post, and do not require an ICC cautionary label. These paste formulations are most useful in tube machines for filling squeeze bottles and for tubes in auto body repair kits. I n applications where water is not objectionable, wet benzoyl peroxide containing about 30Yc water is available. This material is a granular solid. I t exhibits none of the hazardous characteristics, yet retains the desirable cure properties of dry benzoyl peroxide. Benzoyl peroxide and benzoyl peroxide pastes are used in the cure temperature range of about 180-2-50O F. without promotion, or at room temperature when promoted with the amine accelerators. 2,4-Dichlorobenzoyl Peroxide. This peroxide is more active than benzo)-l peroxide; it decomposes at a faster rate and is used in the medium temperature range (150-180O F.) where promotion is not desirable and where color integrity is required. I t is sold commercially as a 50YG paste dispersion with dibutyl phthalate or silicone fluid. p-Chlorobenzoyl Peroxide. This peroxide is more stable than benzoyl peroxide and is used at higher temperatures. I t is most useful in the production of clear transparent castings where a lower exotherm than that provided by benzoyl peroxide is required. I t is available also as a 50Y0paste with dibutyl phthalate. Acetyl Peroxide. This peroxide is an effective initiator in the low-temperature polymerization of some monomers and polyester resins. I t is often preferred over other diacyl peroxides because of its solubility, freedom from nonvolatile decomposition products, and the aliphatic rather than aromatic nature of its decomposition products. I t is available commercially as a 25YGsolution in dimethyl phthalate. Propionyl Peroxide. This catalyst is more reactive than acetyl peroxide. I t is available commercially as a 25YGsolution in high boiling hydrocarbon solvent and
finds greatest applications in the high pressure polymerization of ethylene. Lauroyl Peroxide, This is the peroxide used in medium temperature molding where discoloration must be minimized and where low peak exotherms are required. I n promoted systems, with tertiary amines, it will cure at about 110-125" F. It is available as a pure flaked or granular solid.
Decanoyl, Pelargonyl, and Caprylyl Peroxides. These catalysts are similar to lauroyl peroxides, varying only slightly in activity, and may be used wherever lauroyl peroxide is used. They differ in solubility, with pelargonyl peroxide being the most soluble. Decanoyl peroxide is available as a flaked or granular solid; caprylyl peroxide, in solution in mineral oil; and pelargonyl peroxide, as a solid below 50" F.
Ketone Peroxides
This group includes the methyl ethyl ketone peroxides, cyclohexanone peroxides, and other ketone peroxides, all of which, with activation, operate in the room temperature range of polyester curing. Methyl Ethyl Ketone Peroxides. Methyl ethyl ketone peroxides encompass a wide area of chemical activity. Nearly all of the formulations will cure the same resins : they differ with regard to the rate at which they gel and cure a resin system. They are most widely used with metal or metal/amine promotors to provide room temperature cures in hand-lay-up and gel coat operations. Methyl ethyl ketone peroxides are all derivatives of methyl ethyl ketone and hydrogen peroxide; but, because a number of peroxide structures can be formed when these two reagents are brought together, most commercial formulations are mixtures of various peroxides, hydroperoxides, and usually some hydrogen peroxide. Since these different species have different activities, commercial formulations may differ slightly depending on the relative amounts of each species present. Table I illustrates the empirical formulas for the four common methyl ethyl ketone peroxide species found in commercial formulations. Many other structures (83) are possible and may exist to some minor extent, particularly in formulations which have decomposed significantly after long storage. No two manufacturers of methyl ethyl ketone peroxide formulations make exactly identical products, although in general the same type of formulation will cure the same resins. T h e fabricator's choice of catalyst should be dictated by conditions peculiar to his curing system and is determined by his own experimentation or the recommendations of the catalyst supplier. T h e most common and most widely used methyl ethyl ketone peroxide formulation is a clear, odorless solution of ketone peroxides in dimethyl phthalate containing 11% active oxygen. I t finds widest application in handlay-up fabrications. Most manufacturers of methyl ethyl ketone peroxides offer, in addition, two similar formulations, each slightly more active than the former to allow greater flexibility
for the fabricator. The activities of these formulations are controlled by the relative concentrations of the various peroxide species present. Special methyl ethyl ketone peroxide formulations are also available for specific applications. Flame retardant formulations have recently found favor due to their inherent safety features and because users are able to reduce fire insurance costs. A low assay (30y0 methyl ethyl ketone peroxide) formulation is also available, diluted with diallyl phthalate monomer. I t is particularly applicable for sprayup applications because it is more accurately metered, has lower volatility than the 6OOj, active formulations or those diluted with flammable solvents, thus is safer and leaves no pinholes in the cured piece. Diallyl phthalate has the added advantage of being a cross-linkable monomer so that it improves the physical properties of the cured piece. Methyl ethyl ketone peroxide formulations specifically designed for molding polyester joints in clay tile are also available. These formulations exhibit extremely fast gel times (1-2 minutes) and ensure rapid hardness development. Bis(1-hydroxycyclohexyl) Peroxide. This compound is a crystalline cyclohexanone peroxide having several distinctive properties. It provides a low peak exotherm so it can be used for thick castings and large pieces-Le., bowling balls; it is very sensitive to cobaltactivation, and rapid cures can be obtained with little or none of the color development characteristic of cobaltaccelerated cures-Le., buttons. This particular peroxide is useful where little or no plasticizer is required for reasons of clarity of the cured piece.
1-Hydroxyperoxy-1'-hydroxycyclohexyl Peroxide. This cyclohexanone peroxide is usually compounded with dibutyl phthalate in paste form. It imparts advantages similar to those of bis(1 -hydroxycyclohexyl)peroxide but is used in areas where plasticizer does not pose a disadvantage regarding clarity of the cured piece. It is readily dispersable.
Peroxyesters
tert-Butylperoxybenzoate. This peroxide is the most commonly used catalyst for high temperature molding. It is used in the temperature range around 280' F. in premix (95) and prepreg and offers the advantage of long precatalyzed resin shelf-life at room temperature. I t is offered commercially as a pure liquid. Z,!i-Dirnethylhexane 2,5-Diperoxybenzoate. This peroxide is the difunctional analog of tert-butylperoxybenzoate. I t is a solid crystalline material which provides curves in the same temperature range but offers the added advantages of somewhat greater precatalyzed resin shelf-life and in many instances imparts superior surface finish to the cured piece than its monofunctional counterpart. Peroxyoctoates. tert-Butyl peroxyoctoate and its difunctional analog, 2'5-dirnethyl hexane-2,5-diperoxyoctoate (U. S. Peroxygen-245), are relatively new to the VOL. 5 0
NO. 3 M A R C H 1 9 6 6
29
reinforced plastics industry. They are active in the same temperature range as benzoyl peroxide but less hazardous and more efficient. Because they are liquids, they dissolve more easily in monomers and resins and can be metered or pumped, thus enabling automated catalyst addition to the polyester resin system. Di-tert-butyl Diperoxyphthalate. This peroxide is used in the intermediate temperature range (about 280" F.). Its activity is similar to that of tert-butyl peroxybenzoate. I t is used primarily in polyester molding for electrical parts, providing better electrical properties than other peroxides.
peroxy)hexane, and 2,5-dimethyl-2,5-bis(tert-butylperoxy) hexyne-3 (8, 77, 68, 69, 77). The improvement of the phl-sical properties of crosslinked polyethylene, such as tensile strength, aging and weathering characteristics, dimensional stability, resistance to stress cracking, resistance to solvents, and improvement of hot strength. is discussed in several articles (3, 7 1 , 33). Carbon black reinforcement is most common (8, 7 7 , 33, 37, 707), but many nonblack fillers like clay (77), lead carbonate (29), chalk (33), and lithopone (705) are also used for wire and cable applications (24,56, 57), the major use for cross-linked polyethylene. Recently some work has been done in cross-linking high density Alkyl Peroxides polyethylene with organic peroxides (40). These catalysts are used primarily as high temperaPolyorganosiloxanes. The most useful organic perture initiators. oxides for curing reinforced silicone rubbers are benzoyl Di-tert-butyl Peroxide. This peroxide has a very peroxide, di-tert-butyl peroxide, dicumyl peroxide, 2,5uniform decomposition rate, is relatively unaffected by dimethyl-2,5-bis(tert-butylperoxy)hexane,and the chloimpurities. I t is stable at high temperatures so it can rinated derivatives of benzoyl peroxide : bis(2,4-dichlorobe used to catalyze prepolyiners that are in a solid state benzoy1)peroxide and bis(p-chlorobenzoy1)peroxide ( 79, at room temperature. At temperatures u p to about 25, 27, 708. 720-722). Some experimental work has 180 " F. it can be mixed into a prepolymer with little or also been done with other peroxides ( 4 ) . no decomposition. The prepolymer can then be cooled, Properties that are improved are tensile strength solidified, and maintained for subsequent molding. (80): shrinkage (722),compression set ( 7 79), tendency to Its response to promotion is only slight, tertiary amine harden (32), resistance to oxidation (707), low temperaactivators having only little effect in reducing its cure ture properties (702),and heat stability (53). temperature range. Silica and alumina are the most common fillers, Dicumyl Peroxide. This compound (65) and 2,5but carbon black can also be used (702). Glass fiber dimethyl-2,5-bis(tert-butylperoxy)hexane (42), 2,5diand polysiloxane varnishes have been press-molded to niethyl-2,5-bis(tert-butylperoxy)hexyne-3(82), and ngive high heat and electrical resistance (55). Finely butyl-4,4-bis(tert-butylperoxy)valerate are relatively ground powders of the metals, tin. lead, bismuth, zinc, stable peroxides, decomposing in the 300-350" F. range, cadmium, and antimony, or their alloys are used as hence find use only in more specific applications requirfillers to provide improved strength and heat stability ing high temperature cures. Cross-linking of an unto polysiloxanes (53). saturated polyester with epoxidized polybutadiene is Many patents illustrate the specific application of accomplished by 2,5-dimethyl-2,5-bis(tert-butylperoxyj- certain peroxides. Di-tert-butyl peroxide absorbed on hexane (39). This latter high temperature peroxide also a molecular sieve prior to incorporation into the mix finds specific application in curing allylic monomerprovides improved storage stability of the precured mix polyester systems (63). (28). Benzoyl peroxide, dicumyl peroxide, and di-tertbutyl peroxide have been used together with a blowing agent for curing filled silicone rubber to be used for Hydroperoxides insulating and packing materials of high heat stability This class of peroxides is highly affected by solvents (77). Blends of silicone rubber and poly-1,3-butadiene or the media in which they are placed. Consequently, and a filler cured by an organic peroxide provide a less they are used to a lesser extent and in specific areas in expensive formulation which still retains most of the which they have been found to be applicable ( 7 ) . Other desirable properties of silicone rubber ( 7 ? 7). Polyperoxides can also cure polyester resins-i.e., ethyl amyl siloxane filler compositions cured with di-tert-butyl ketone peroxide (22)-but have not been widely used to peroxide and 2,5-dimethyl-2,5-bis(tert-butylperoxy)date. hexane provide products with better heat stability than products cured with other peroxides (5, 25). Crosslink densities in peroxide-cured silicone rubbers have Organic Peroxide Use in Other Reinforced Plastics been calculated ( 9 ) . Polyethylene. T h e use of organic peroxides to upgrade the physical properties of reinforced low density AUTHORS Orville L. Magelz is Director of Research and polyethylene has been known for some time. The first James R. Kolczynski is Superoisor of Applications Research peroxide to be used for this purpose was dicumyl f o r the Lucidol Division of PVallace @ Tiernan, Inc. Both peroxide (20, 37, 38, 703-705, 707). Since then authors have published widely in the Jeld of reinforced plastics several other organic peroxides have become available and are actiae in the Reinforced Plastics Division of the Society and been found useful-i.e., tert-butyl cumyl, di-tert-butyl, tert-butyl peroxybenzoate, 2,5-dimethyl-2,5-bis(tert-butyl- of Plastics Industry. 30
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Mono-olefin Copolymers. Elastomers of ethylene, propylene, and butylene copolymers, particularly ethylene-propylene rubbers, have become important in the past five years. Because they contain no unsaturation, they must be cured by free radical initiators. T h e rubbers have been described and organic peroxide cure systems developed for them (6, 36, 88, 89-97, 706). The types of organic peroxides used in vulcanizing these reinforced copolymers are those which are active in the temperature range of about 300-400" F. These are primarily n-butyl-4,4-bis(tert-butylperoxy)valerate, 2,5-dimethyl-Z, 5-bis (tert-butylperoxy) hexane, 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexyne-3,dicumyl peroxide, tert-butylperoxybenzoate, and di-tert-butylperoxide (73, 35, 706), which are available commercially. Some experimental work has been done with other peroxides but they are not used in this country (700). Coagents-for example, sulfur, polynitroso compounds, acrylamides, and cross-linkable monomers and polymers-can be used with these peroxides to provide greater efficiency in cross-linking (35, 47, 48, 67, 772). Efficiency of such peroxide cross-linking has been discussed (64). Reinforcing agents in the saturated elastomers are primarily the different types of furnace, channel, and thermal blacks (35, 62, 706), but many applications require nonblack fillers, which include talc, silicas, clays, and calcium carbonate (35, 59, 706). Organic peroxides may also be used to cure ethylenepropylene terpolymers, which contain a minor amount of a nonconjugated diene (45, 48, 85) Primary areas of applications of these reinforced elastomers are electrical and outdoor uses (34)like window channel, garden hose, and automobile rubber parts. Miscellaneous Plastics. Other reinforced elastomeric or plastic materials can also be cured by organic peroxides. Carbon black reinforced polyurethanes are cured by the same list of peroxides as polyethylene and the monoolefin copolymers, with similar improvements in physical properties (47). Sixteen peroxides are compared in the vulcanization of natural rubber and styrenebutadiene rubber reinforced with carbon black and lightcolored fillers (30). Filled ethylene vinyl acetate copolymers can be vulcanized with organic perOKides to products of high strength and hardness (7). T h e products are highly resistant to oxygen, ozone, light, weathering, and steam. A new type of epoxy-acrylic polymer which combines the chemical and solvent resistance of epoxy resins with the rapid low temperature cure of unsaturated polyester resins is now available (78). It can be cured by benzoyl peroxide-dimethyl aniline or methyl ethyl ketone-cobalt naphthenate systems. Several patents illustrate the usefulness of peroxide curing of still other reinforced plastics. Polyvinylmethyl ethers, reinforced with high abrasive furnace black (HAF) and cured with dicumyl peroxide, find use in wire coatings and molded goods (52). Peroxide vulcanization of reinforced higher molecular weight poly(vinylalky1 ethers) has also been investigated (58). Polystyrene, rubber, and filler compositions are cured
with organic peroxides or peresters to produce materials with improved shock resistance ( I 73). Thermoplastic sheets are coated with a tetramethyleneglycol dimethacrylate which is cross-linked by peroxides to provide a mar-resistant coating (57). Glass-reinforced laminates containing graft copolymers of butadiene, sytrene, ahd a monomer can be peroxide-cured to give good flexural strength ( 7 74). Development Trends
I n 1961, an article in Chemzcal Week described the introduction of difunctional high temperature peroxides for use in cross-linking polyolefins, elastomers, and new vinyl resins (92). These high temperature curing agents are useful in the curing of unsaturated polyester and diallyl phthalate resins and have been described in previous sections of this paper. More recently, two articles on commercial organic peroxides appeared which emphasized the trend toward more active initiators for lower temperature polymerization ( 74, 75). The trend toward more active initiators or curing agents for the reinforced plastics field is now evident. As an example, the use of a solution of benzoyl peroxide in tert-butylperoxybenzoate is one approach to get more rapid cures in premix applications. New peroxy esters, active in the temperature range of benzoyl peroxide, are being offered (16). These peroxy esters-namely, tert-butylperoctoate(2-ethyl hexanoate) and 2,5-dimethylhexane-2,5-diperoxyoctoate-are liquid products which combine the high efficiency of these tertiary peresters with easy metering and ready miscibility. They also have higher relative activity on the weight basis and are less hazardous to handle than pure benzoyl peroxide. Safety in handling organic peroxides used in the reinforced plastics industry is especially important (73, 93, 98). T h e first industry approach to making safer peroxides was the preparation of benzoyl peroxide paste products. I n recent years the improvement of these paste products by addition of water has made them fireresistant. Very recently, the safety of all commercial organic peroxides has been reviewed, and a relative classification of hazards has been suggested (70, 94). This work led to the development by one peroxide manufacturer of much safer methyl ethyl ketone peroxide solutions (70). These latter methyl ethyl ketone peroxide formulations are the preferred initiators in much of the curing of reinforced polyester resins. A new, highly active ketone peroxide formulation which has exceptional safety characteristics was introduced at the beginning of 1965 (54). REFERENCES (1) Allied Chemical Corp ,Brit Patent 832,169 (April 6, 1960) (2) Alt,B ,Kunststofe52,394 (1962) (3) Anaconda Wire and Cable Co ,Brit Patent 876,011 (Appl Feb 8,1960) (4) Bailey, D L , Black, W. T I Dunham, M L (Union Carbide), Brit Patent 819,245 (Sept 2, 1959). (5) Bailey, D L , Black, W. T., Dunham, M L (Union Carbide), U S Patent 3,183,205 ( M a y l l , 1965). (6) Ballini, G Giandinoto, G V , Pantani, G , Portolani, A , Materte Plnsttche Elastomen 29 IS), 1201 (1963). (7) Barth, H , Peter, J (Farbenfabriken Bayer Akt -Ges , Leverkusen), Knutschuk Gummi 14, WT23-32 (1961) (8) Behr,E, Kunstslofe53 (8), 502-9 (1963). (9) Bobear,W J , Ind Eng. Chem Prod. Res Develop 3,277 (1964)
VOL. 5 8
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(10) Boenig. Herman V., “Unsaturated Polyesters; Structure and Properties,” Elsevier, Amsterdam, 1964.. (11) Boonstra, B. B. S. T. (Cabot), Ger. Patent 1,143,326 (Feb. 7, 1963). (12) Brown, A . W. (Owens Corning Fiberglas), Belg. Patent 618,688 (Sept. 28, 1962). (13) Bukata S. W. Zabrocki, L. L.. McLaughiin M . F., Kolczynski, J. R., Mageli, 0. L., Ind.’Eng. &ern. Prod. Rer.Deaelop. 3, 261 (i964). (14) Chem. Eng. Ne‘ews42 (E), 25 (1964). (15) Chern. Week 1965, April 24, p. 73. (16) ibid., July 3, p. 44. (17) Connecticut Hard Rubber, Brit. Patent 864,566 (April 6, 1961). ( 1 8 ) C3(-winski, ’. J. W‘., “The Role of Organic Peroxides in Curing Polyester Resins and heir Influence on the Physical Properties of Reinforced Plastics,” Novadel, Ltd., Lucidol Products Division, St. Ann’s Crescent, London, 1960. (19) D’Adolph, S., Rubber World 1 4 9 , 3 7 (1963). (20) Dannenberg, E. M.,Jordan, hl. E., Cole, H. hl., J. PoljmerSci. 31, 127 (1958). (21) Dauies, A. G., “Organic Peroxides,” Butterworths, London, 1961. (22) Davis, 0. L., Dorn, R. W.(Shell Oil), U. S. Patent 3,151,170 (Sept. 29, 1964). (23) DeDani, A,, ed., “Glass Fibre Reinforced Plastics,” J. W.Arrowsmitb, Great Britain, 1960. (24) DeMelio, F. A,, Rubber%ge96,713-17 (1965). (25) DiCerbo, P. M. (Gene;al Electric), Ger. Patent 1,007,057 (April 25, 1957). (26) Doebnert,D. F., hiageli, 0. L., .Mod. Plastics36 (6), 142 (1959). (27) Dunham, M. L., Bailey, D. L., bfixer, R. Y., IND.Exc. CHEM. 49, 1373 (1957). L., Jr. (Union Carbide), Brit.Patenr 827,402 (Feb. 3, 1960). (28) Dunham, 1%. (29) Eastman, T V . 0 . (General Electric), U. S. Patent3,039,989 (June 19, 1962). ( 3 0 ) Ecker, R. (Farbenfabriken Bayer Akt.-Ges.), Kautschuk Gummi 1 2 , WT351 (1959). (31) Esrrada, N. S., Malkemus, J. D., paper presented at the 20th Ann. Meeting of T h e Society ofThe Plastics Industry, Reinforced Plastics Div., Chicago, 1965. (32) Fekete, F. (Union Carbide), U. S. Patent 2,954,357 (Sept. 27, 1962). (33) Ferch, H., Kunststofe 52,326-31 (1962). (34) Fisher, W ,F., Staib, J. H., Wire @ WireProducts38 (2), 214-18 (1963). (35) Fuscn, J. V., Rubber W o r l d , 48-54 (February 1963). (36) Godfrey L. Cabot, Inc., Brit. Patent 852,035 (Oct. 19,1960). (37) Ibid., 856,833 (Dec. 21, 1960). (38) i b i d . , 863,681 (March22, 1961). (39) Greenspan, F. T., Riech, M. H. (FMC), Belg. Patent 618,066 (September 17, 1962). (40) Gregorian, R. S., Baffprd, R. A., Ind. En!. Chem. Prod. Res. Deuelop. 3, 267 (1964). (41) Gruber, E. E,, Keplinger, 0. C., INn. ESG. CHEM. 51,151 (1959). (42) Harrison, J. B., Mageli, 0 . L. (Wallace & Tiernan), U. S. Patent 3,117,166 (January 7, 1964). (43) Harrison, J. B., .Mageli, 0: L., Stengel, S. C., M o d . Plastic539 (S), 135 (1962). (44) Hawkins, E. G. E., “Organic Peroxides,” Van Nostrand, Princeton, 1961. (45) Haxo, H. E., Jr.: Bingham, W.R., Whitehouse. TV. G., Rubber Age 94 (2), 22567 (1963). (46) Hazel, F. R., Brit. Patent971,597 (Sept. 30, 1964). (47) Heuck C. !)’inter J., Stemmer, H . D., Schmidt, E. (Farbwerke Hoechst AvG.), Gkr.P’atent 1,1$6,979,NN.7 (1963). (48) Howarth, J. T., Cornell, J:A., Olson, L. R., Rubber W o r l d 148 (5), 69 (1963). (49) Hugova, E., Zvonar, V., Buchta, O., Ger.Patent 1,071,334 (Dec. 17, 1959). (50) Hurdis, E. C. (United States Rubber), U. S. Patent 2,480,928 (Sept. 6, 1949). (51) Idelson, E. M. (Polaroid), Ibid., 3,081,192 (March 12, 1963). (52) Kirk, D. C., Jr. (HerculesPowder),Ibid., 2,984,655 (May 16, 1961). (53) Kniege, 54. (Farbenfabriken Bayer A-G.), Ger. Patent 1,162,560 (Feb. 6, 1964). (54) Knlczynski, J. R., Kressin, D. hl., Wesrbrook, S.C., J r . , MMageli, 0. L., paper presented a t the 20th Ann. hleeting of The Society of T h e Plastics Industry, Reinforced Plastics Div., Chicago, 1965. (55) Kujoshi, Abe, Minoru, Toyoda, Yoshinao, Iida, Ball Inst. Chem. Res., Kyoto Univ., 22,87-8 (1950). 1 5 6 ) Lacombe. J.. !\’robel. LV., Wire Wire Prod. 38 (4) 508-9 (1963). Fbid., 38 (51,670-8 (1963); (58) La1 J. McGrath, J. E., presented a t the Meeting of Div. of Rubber Chemistry of the Lderican Chemical Society and Chemical Instirute of Canada, Toronto,
iGj
.-,* IY0.7.
(59) Lamar, R. S., Mulryan, H. T., Warner, M. F., Rubber World 147, 60-4 (1963). (60) Lawrense, J. R., “Polyester Resins,” Reinhold, New- York, 1960. (61) Lenas, L. P.,ind. Eng. Chem. Prod. Res. DerreIop. 2 (3), 202 (1963). (62) Ibid., 3,269 (1964). (63) Litwin, J., Beacham, H . H., paper presented at the 20th Ann. Meeting o f T h e Society of the Plastics Industry, Reinforced Plastics Div., Chicago, 1965. (64) Loan, L. D., J . PolymerSci. Pi. A 2 (7), 3053-66 (1964). (65) Lorand, E. J., Riese, J. E. (Hercules Powder), U. S. Patent 2,691,683 (October 12, 1954). (66) Lowell, A. I.: Mageli, 0. L. (Wallace 8; Tiernan), ibid., 3,121,705 (Feb. 18, iO < A’,. > (67) Mageli, 0. L., Bukata, S. D., Bolton, D. J.. Bull. 30.30, Lucidol Div., Wallace & Tiernan, Buflalo. (68! M@:g:?., Harrlson, J. B. (!Vallate 8; Tiernan), U. S. Patent 3,086,966 A,”
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
(69) Ibid... 3.214.422 iOctober26. 1965) , , . (70j Mageli, 0. L., Noller, D. C., Mazurowski, S.J., Linden, G. F., deleeuw, F. J. G., paper presented a t the 20th Ann. hleeting of The Society of the Plastics Industry, Reinforced Plastics Div.. Chicago, 1965. (71) Mageli, 0.L., Stengel, S. D.. Doehnert,D. F., Mod. P/astics36,135 (1959) (72) Maira, S.: Wahl, A. A., M o d . Pfostics Encyclopedia 38,392 (1961). (73) Malkemus, J. D.! M o d . Plasiics41, 119 (1963) (74) Marks, 1 4 , E,, Mod. PlaslicsEncyclopedia 42,410-14 (1964). (75) Marks, h l . E., Plastics Techno/. 10,44 (1964). (76) Marks, Xl, E,, WesrernPlastics25 (July 1964). (77) Martens, S. C., Rothenberg, S. (General Electric), U. S. Patent 3,148,169 (Sept. 8, 1964). (78) Mav C. A,, N-ewey, H. A , , paper presented at the 20th Ann. Meeting of The Societ): bfThe Plastics Industry, Reinforced Plastics Division, Chicago, 1965. (79) Medalia, A. I. (Cabot), U. S. Patent 3,105,057 (Sept. 24, 1963). (80) Midland Silicones, Ltd., Brit.Patent 798,667 (July 23, 1958). (81) Milas, N. A., Encyclopedia of Chemical Technology, ed. by R. E. Kirk and D. Othmer, Interscience 10,58 (1953). (82) hlilas, S. A. (Research Gorp.), U. S.Patenr 2,670,384 (February 23, 1954). (83) Milas, N. A , , Golubovic, A,, J.A m . Chem. SOC.81,5824 (1959). (84) Minter, Herbert F., Foster, N. C. (to !Vestinghouse Electric), U S. Patent 2,871,420 (January27, 1959). (85) Mitchell, J. M., Barnhart, R. R., et a l . , Rubber World 148 ( j ) ,52 (1963). (86) M o d . Plasttcr42 ( j ) , 101 (1965). (87) Ibid., (6), p. 122. (88) Monrecarini Societa Generale, Ital. Patent 587,681 (Jan. 20, 1959). (89) Natta, G., Crespi, G., Valvassori, A . , Sartori. G., Polyolcfin Elastomers, Rubber Reviews, Rubber Chemistry and Technology, XXXVI, 1945 (1963). (90) S a t r a , G.: Sartori, G., Valvassori, A., Mazzanti, G., Crespi, G., Petrol. Refiner 41 ( 8 ) , 103-8 (1962). (91) Ibid., (9), 261-6. (92) New Payofl in Peroxide Research, Chem. M’eek 1961, July 22, p. 39. (93) Noller, D. C., Bolton, D. J., A n d . Chem. 35, 887 (1963). (94) Noller, D. C.?hlazurowski, S. J.. Linden, G. F., deleeuw, F. J. G., Mageli, 0 . L., INn. EbG. CHEM. 56 (12), 18 (1964). (95) Noller, D . C., Schack, H. N., D’.Amico, W.F., Duprez, H., paper presented at 17th Ann. Meeting of T h e Society of T h e Plastics Industry, Reinforced Plastics Div., Chicago, 1962. (96) Noller, D. C., Stengel, S. D., Mageli, 0. L., papcr prcsented at 16th Ann. Meeting of The Society of T h e Plastics Industry, Reinforced Plastics Div., Chicago, 1961, (97) Oleesky, Samucl S., Mohn, J. Gilbert. Handbook of Reinforced Plastics ofT h e Society of The Plastics Industry. Inc., Reinhold, New York, 1964. (98) Organic Peroxides-Their Safe Handling and Use, Buii. 30.40, Lucidol Dix-., Wallace 61 Tiernan, Buflalo. (99) Parker, E . E., I n d . Eng. Chem. Prod. Res. Develop. 2,102 (1963) (100) Pedretti, G . (Montecatini Societa Generale), Can. Patent 704,507 (Feb. 23, 1965). (101) Pike: R. 4 . (Union Carbide), U. S. Patent 2,953,543 (Sepr. 20,1960). (102) Pike, R. A , , Williams, T. C. (Union Carbide), Brit. Patent 939,572 (Oct. 16, 1963). (103) Precopio, F. M., Gilberr, A. R. (General Electric), U.S. Patent 2,888,422 (May 26,1939). (104) Ibid.,3,079,370 (Feb. 26, 1963). (105) Prober, M . , Gilbert, A. R., presented at the Meeting of Div. of Rubber Chemistry of the American Chemical Society and the Chemical Institute of Canada, Toronto, 1963. (106) Robinson, S. E., Strohmayer, H. F., Ziarnik, G . J., S. G. F. (Sveriges GummitekForen.), 1962,Publ. 19, 17pp. (107) Ropp, W. S.(HerculesPowder), U. S. Patent 3,036,981 (May 29, 1962). (108) Rousch, C. W., Kosmider, J., Jr., Benfer, R. L., Rubber Age 94,744 (1964). (109) Rybolt, C. H.: paper presented a t the 11th Ann. Meeting of T h e Society of T h e Plasrics Industry, Reinforced Plastics Div., Atlantic City, 1956. (110) Rybolt,C.H.,Swigert,T. C.,,Mod.Plastics26, 101 (1949). (111) Safford,pi. N., Myers, R . L. (General Electric), E. S.Patent 2,867,603 (Jan. 6, 1959). (112) Shell Internationale Research Maarschappij N. V., Brit. Patent 873,598 (July 26.1961). (113) Shell Inrernationale Research Maatschappij N. V., Belg. Patent 611,793 (June20, 1962). (114) Slotterbeck, 0. C., Lakritn, Jr. (Esso Research and Engineering), U. S. Paten 3,079,295 (Feb. 26, 1963). (115) SPI Procedure for Running Erotherm Curves in Polyester Resins, Method approbed by SPI Reinforced Plastics Div., Sept. 2, 1960, published a t 16th Ann. Conference Reinforced Plastics Div., 1961 (116) Sweitzer, C \V,, Lvon, F., Grabowski, T. S., IND.Eso. CHEM.47,2380 (1955). (117) Tobolsky, Arthur V., Mersrobian, R . B., “Organic Peroxides,” Interscience, New York. 1954. (118) Varlet, P. P. W.(Souete Anon. des Usines Chausson), Fr. Partent 1,258,405 (.Appl,March3, 1960). (119) IVarrick, E. L. (Dow- Corning), L.S . Patent 2,572,227 (Oct. 23, 1951) (120) ibid., Ger. Patent 940,187 (March 15, 1956). (121) \Tillis, \V, D. (Hercules Powder), U. S. Patent 2,816,089 (Dec. 10, 1957). (122) TVormuch, W.J. (General Electric), U. S. Patent 2,938,011 (May24, 1960). ,
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