Comparing curing systems: peroxide-co-agent versus sulfur-accelerator in polyisoprene.The rubber industry has utilized sulfur and organic accelerators for over 70 years as the primary method of curing elastomers. Other methods of crosslinking, such as peroxides and amines amines (  mēnz´),n.pl organic compounds that contain nitrogen. , have filled various niches in specialty polymers and high performance applications. Until recently, peroxides and the commonly used co-agents (acrylic acrylic, artificial fiber made from a special group of vinyl compounds, primarily acrylonitrile. Acrylic fibers are thermoplastic (i.e., soften when heated, reharden upon cooling), have low moisture regain, are low in density, and can be made into bulky fabrics. monomers, allyl allyl /al·lyl/ (al´il) a univalent radical, —CH2dbondCHCH2. al·lyl n. The univalent, unsaturated organic radical C3H5. cyanurates, low molecular weight polybutadienes, and phenylene phen·yl·ene n. A bivalent organic radical, C6H4, derived from benzene by removal of two hydrogen atoms. phenylene The radical C6H4 dimaleimides), have produced vulcanizates with application-limiting original physical properties such as high modulus See modulo. , low tensile strength tensile strength Ratio of the maximum load a material can support without fracture when being stretched to the original area of a cross section of the material. When stresses less than the tensile strength are removed, a material completely or partially returns to its , higher hardness and low compression set. Of course, their heat aging properties were much more superior to sulfur cure systems, making them very attractive in rigid specification compounds. In this article, a new generation of peroxide-initiated solid functional coagents is introduced to cure synthetic polyisoprene elastomers. The resulting physical properties are similar to the classic sulfur-accelerator cure systems. In addition, the flex properties obtained by using the new system greatly surpass those of the sulfur system, while still maintaining the superior aging characteristics of a peroxide-coagent system. This unique combination of properties opens up a wide range of applications previously unavailable to peroxide-coagent curing of elastomers. Peroxide-coagent cure mechanisms Crosslinking with peroxide peroxide (pərŏk`sīd), chemical compound containing two oxygen atoms, each of which is bonded to the other and to a radical or some element other than oxygen; e.g. alone results in the formation of a covalent bond covalent bond (kō'vā`lənt): see chemical bond. covalent bond Force holding atoms in a molecule together as a specific, separate entity (as opposed to, e.g., colloidal aggregates; see bonding). , as shown in figure 1. This carbon-carbon bond A carbon-carbon bond is a covalent bond between two carbon atoms. The most common form is the single bond – a bond composed of two electrons, one from each of the two atoms. is quite rigid and stable (343.2 kJ bond energy), compared to the carbon-sulfur and sulfur-sulfur bonds in sulfur cured systems (C-S C-S Civil-Structural C-S Cheek-Shoulder (ASL) , 276.2 kJ bond energy; S-S S-S Surface-to-Surface S-S Space to Space , 205.1 kJ bond energy). The excellent heat stability of this carbon-carbon covalent bond explains the superior heat aging characteristics of peroxide cured systems. [FIGURE 1 OMITTED] In contrast, although polysulfide pol·y·sul·fide n. A sulfide compound containing at least two sulfur atoms per molecule. crosslinks formed in sulfur cures are thermally weak and can slip/break along the hydrocarbon hydrocarbon (hī'drōkär`bən), any organic compound composed solely of the elements hydrogen and carbon. The hydrocarbons differ both in the total number of carbon and hydrogen atoms in their molecules and in the proportion of hydrogen chain, they have the ability to reform. This mobility and breaking explains their superior tensile strength and tear properties. The solid functional coagent-peroxide crosslink bond is ionic i·on·ic adj. Of, containing, or involving an ion or ions. ionic pertaining to an ion or ions. ionic medication iontophoresis. , as shown in figure 2. The technology and characteristics of this ionic bond ionic bond: see chemical bond. ionic bond Electrostatic attraction between oppositely charged ions in a chemical compound. Such a bond forms when one or more electrons are transferred from one neutral atom (typically a metal, which becomes a cation) have been detailed in the literature on commercial ionomers. These ionic bonds exhibit both good heat aged stability and the ability to slip along the hydrocarbon chain, even break and reform. Thus, this unique system embodies the best characteristics of both the peroxide and sulfur cure systems: High tensile strength, high tear strength, high modulus, and outstanding flex and heat aged properties. [FIGURE 2 OMITTED] Experimental Materials A masterbatch containing 100 phr Natsyn 2200 (Goodyear), 50 phr N660 carbon black, 5 phr zinc oxide zinc oxide, chemical compound, ZnO, that is nearly insoluble in water but soluble in acids or alkalies. It occurs as white hexagonal crystals or a white powder commonly known as zinc white. , 1 phr stearic acid stearic acid /ste·a·ric ac·id/ (ste-ar´ik) a saturated 18-carbon fatty acid occurring in most fats and oils, particularly of tropical plants and land animals; used pharmaceutically as a tablet and capsule lubricant and as an emulsifying and 1 phr Agerite Resin D was used in all experiments related to natural rubber (synthetic polyisoprene). The solid functional coagent used in this experiment was supplied by Sartomer. Difficulties in mixing were initially encountered when the solid functional coagent was used in its traditional powder form. The product is now being offered as a pelletized 75% active dispersion dispersion, in chemistry dispersion, in chemistry, mixture in which fine particles of one substance are scattered throughout another substance. A dispersion is classed as a suspension, colloid, or solution. in ethylene-propylene copolymer copolymer: see polymer. (EPM EPM equine protozoal myeloencephalitis. ). Alternate co-polymer binders are in development. This form provides the usual improved mixing benefits of faster incorporation, more uniform dispersion and lower scorch times. This form is also dust-free, leading to a safer and healthier work place. Formulations Rubber compounds represented in table 1 were mixed in a Brabender prep mixer mixer, either of two electronic devices in which two or more signals are combined. In the type of mixer used in radio receivers, radar receivers, and similar systems, a signal is translated upward or downward in frequency. . Rubber compounds represented in tables two through five were mixed in a lab-scale internal mixer. All samples were compression molded 21-23 minutes at 320 [degrees] F, except for the sulfur samples, which were cured for ten minutes. For DeMattia flex testing, cure times were 32 minutes for peroxide samples and 20 minutes for sulfur samples. Measurement Cure characteristics which include scorch time, cure rate and torque values were measured over a thirty-minute period at 160 [degrees] C and 3 [degrees] are using an oscillating-disk rheometer rhe·om·e·ter n. An instrument for measuring the flow of viscous liquids, such as blood. according to according to prep. 1. As stated or indicated by; on the authority of: according to historians. 2. In keeping with: according to instructions. 3. ASTM ASTM abbr. American Society for Testing and Materials method D-2084-95. Compression set was determined by compressing com·press tr.v. com·pressed, com·press·ing, com·press·es 1. To press together: compressed her lips. 2. To make more compact by or as if by pressing. 3. samples at 25% deflection deflection /de·flec·tion/ (de-flek´shun) deviation or movement from a straight line or given course, such as from the baseline in electrocardiography. de·flec·tion n. 1. for 70 hours at 212 [degrees] F. Each sample was then removed and the permanent set measured as a percentage of original height according to ASTM standard D-395B. Durometer A hardness tests were determined for samples after molding using a hand-held durometer. Original physical properties were measured using die D dumbbells tested at 20 in./min. All samples were tested according to ASTM standard D 412-97, method A, 2240-97. Heat age testing was performed after 70 hours at 100 [degrees] C. All testing conformed to ASTM standard D 573-88 (94). Flexibility testing was measured according to DeMattia flexibility ASTM D-813-95. In this test, each specimen was pierced pierced adj. 1. Cut through with a sharp instrument; perforated. 2. Of or relating to a body part that has been perforated for the purpose of attaching a piece of jewelry. 3. 0.08 inches and tested at 300 cycles per minute. Cycles to failure were reported when the 0.08 piercing grew to 0.5 inches. Tan delta and complex modulus (E* 120 [degrees] C/E C/E Chief Engineer C/E Concurrent Engineering C/E Components/Equipment C/E Calculation Experiment C/E Calculated-to-Experimental (value or ratio) * 25 [degrees] C) were measured through dynamic mechanical analysis (DMA (1) (Digital Media Adapter) See digital media hub. (2) (Document Management Alliance) A specification that provides a common interface for accessing and searching document databases. ). DMA testing followed ASTM standard D 2231-94. Results and discussion The initial study conducted in polyisoprene (IR) was a comparison of three cure systems: sulfur, peroxide and a peroxide-solid coagent (Peroxide-S). Table 1 highlights the results. The high tensile tensile, adj having a degree of elasticity; having the ability to be extended or stretched. data and the five-fold increase in flex life, as shown, resulted in a much more extensive study of the new cure system in order to arrive at an optimum formulation. Polyisoprene was selected in this expanded study for two reasons: * It is more `pure' than natural rubber, i.e., less likely to interfere with the free radical mechanism (more consistent cures). * The large amount of historical flex data that are available for comparison with this polymer. Goals were set in this study to minimize the tan delta and to have the ratio of the complex modulus (E* 120 [degrees] C/E* 25 [degrees] C) approach 1.0 as closely as possible. The following fine-tuned study shows the effect on the above, plus original physicals, aged physical properties and flex data versus a sulfur-organic accelerator cured compound. In table 2, the effects of varying peroxide levels while keeping coagent levels constant were studied. Once again, a conventional sulfur cure control was included. As shown in table 2, an increase in the level of peroxide has the expected effect of increasing hardness and modulus, while overall tensile strength remains relatively constant. In table 3, the coagent level was varied with constant peroxide levels. Once again, a sulfur system was included. Surprisingly, there is little or no effect on the physical properties by increasing the coagent levels, by almost double. In the past, trends have shown that by increasing the levels of standard acrylic coagents (TMPTMA, EGDMA EGDMA Ethylene Glycol Dimethacrylate , etc.), properties such as cured hardness and modulus increase dramatically. Table 4 summarizes the dynamic mechanical analysis data for the compounds in tables 2 and 3. The E* ratio and tan delta data for the peroxide-coagent systems are comparable to the data for a sulfur-accelerator system. The DeMattia flex results are outstanding for a peroxide-cured compound, with results 3-6 times greater than the sulfur cured compound. Optimization of peroxide/coagent levels Having studied a grid of peroxide levels from 2-4 phr and coagent levels from 5-9 phr, it seemed necessary to try to optimize the new coagent system (keeping in mind the pound volume cost of the cure system versus the sulfur cure system). Tables 2 and 3 suggest that lower peroxide and median coagent levels would be optimum. Table 5 is the result of that optimization. From table 5 it is concluded that compounds 7 and 8 exhibit the optimum cure levels, although each application will require its own optimization. Outstanding adhesion adhesion /ad·he·sion/ (ad-he´zhun) 1. the property of remaining in close proximity. 2. the stable joining of parts to one another, which may occur abnormally. 3. benefits In recent years, a great deal of research has been put into the evaluation of how solid functional coagents within rubber compounds positively affect adhesion to steel, brass, zinc, aluminum, polyester, rayon and other substrates. Technical papers that describe the outstanding adhesion promotion of these coagents have been published in Rubber World, Rubber & Plastics News and several ACS (Asynchronous Communications Server) See network access server. Rubber Division symposia sym·po·si·a n. A plural of symposium. . Selected literature references: Rubber World - November 1998; Rubber & Plastics News - April 17, 2000; Sartomer Application Bulletins #4550 and 4801. These selections focus primarily on EPDM EPDM Ethylene-Propylene-Diene-Monomer EPDM Enterprise Product Data Management EPDM Ethylene Propylene Dimonomer (industrial/commercial piping/plumbing components) EPDM Engineering Product Data Management elastomers. In order to show similar trends in polyisoprene elastomers, two additional formulations were created. The first was a sulfur accelerator based sample, and the second was a Peroxide-S coagent based system. Each sample was cured to approximately the same modulus (at 100% elongation elongation, in astronomy, the angular distance between two points in the sky as measured from a third point. The elongation of a planet is usually measured as the angular distance from the sun to the planet as measured from the earth. ). Lap shear adhesion was determined by testing a rubber specimen cured between two cold rolled steel coupons. This 0.030 inch specimen was cured at 160 [degrees] C for 40 minutes between 1 x 3 x 0.030 inch coupons overlapped one inch. A pressure of 30,000 psi PSI - Portable Scheme Interpreter was applied during curing. The results of this test are presented in figure 3. [FIGURE 3 OMITTED] Summary Previously, sulfur-accelerator and peroxide-coagent cure systems had their own domains of applications. Now, with the discovery of a new solid functional coagent-peroxide system, the two can be used interchangeably INTERCHANGEABLY. Formerly when deeds of land were made, where there Were covenants to be performed on both sides, it was usual to make two deeds exactly similar to each other, and to exchange them; in the attesting clause, the words, In witness whereof the parties have hereunto with regard to original physical properties. In addition to the joint synergy between the two systems, the peroxide-coagent system offers much improved aging characteristics, greatly enhanced flex properties, excellent compression set, and much improved adhesion to various metals and fabrics when compared to classic sulfur cured systems.
Table 1 - historical comparison
Peroxide Sulfur Peroxide-S
Natsyn 2200 IR 100 100 100
Cabot Sterling V 50 50 50
Zn0 5 5 5
Stearic acid 1 1 1
Agerite resin D 1 1 1
Dicumyl peroxide
(40% active) 4 -- 2
Saret -- 4
Sulfur -- 1.6 --
TBBS -- 1.6 --
Tensile strength 2,825 2,840 3,165
% elongation 375 355 380
[Modulus.sub.100] (PSI) 385 430 465
E* 120 [degrees] C/
E* 25 [degrees] C 1.017 1.122 .899
DeMattia flex
(cycles to failure) 33,000 30,000 156,000
Tan delta .22 .18 .19
Compression set % 16 54 36
(70 hrs. @212 [degrees] F)
Table 2
1 2 3 Sulfur
control
Natsyn 2200 100 100 100 100
Cabot Sterling V 50 50 50 50
Zinc oxide 5 5 5 5
Agerite resin D 1 1 1 1
Stearic acid 1 1 1 1
Peroxide S 5 5 5 --
Dicumyl peroxide
(40% active) 2 3 4 --
TBBS -- -- -- 1.6
Sulfur -- -- -- 1.6
Rheometer data; ASTM D 2084-95
Max. torque (in.-lbs.) 56.7 69.4 76.5 71.8
Scorch time
(Ts2, min.) 1.8 1.6 1.4 3.3
Cure time
(Tc90, min.) 17.0 17.5 17.1 6.1
Original physicals ASTM D 412-97 Method A; 2240-97
Tensile strength 3,380 3,350 3,340 2,840
300% modulus (psi) 1,810 2,260 2,450 1,720
Elongation, % 520 450 430 440
Durometer A hardness 58 60 61 63
Heat aged physicals ASTM D 573-88(94)
Tensile strength (psi)
% change -8.3 -2.9 -3.9 -20.4
300% modulus
% change -5.0 -1.8 -2.9 16.3
Elongation
% change -3.8 -6.7 -4.7 -25.0
Hardness change
Pts. +1 +2 +1 +3
Table 3
4 5 2 Sulfur
control
Natsyn 2200 100 100 100 100
Cabot Sterling V 50 50 50 50
Zinc oxide 5 5 5 5
Agerite resin D 1 1 1 1
Stearic acid 1 1 1 1
Peroxide S 9 7 5 --
Dicumyl peroxide
(40% active) 3 3 3 --
TBBS -- -- -- 1.6
Sulfur -- -- -- 1.6
Rheometer data; ASTM D 2084-95
Max. torque (in.-lbs.) 64.2 71.8 69.4 71.8
Scorch time
(Ts2, min.) 2.0 1.6 1.6 3.3
Cure time
(Tc90, min.) 15.4 16.1 17.5 6.1
Original physicals ASTM D 412 Method A; 2240-97
Tensile strength 3,220 3,590 3,450 2,840
300% modulus (psi) 2,180 2,360 2,260 1,720
Elongation, % 440 460 450 440
Durometer A hardness 61 62 60 63
Heat aged physicals ASTM D 573-88(94)
Tensile strength (psi)
% change -6.3 -8.6 -2.9 -20.4
300% modulus
% change -4.6 -6.4 -1.8 16.3
Elongation
% change 0.0 -4.3 -6.7 -25.0
Hardness change
Pts. +2 +1 +2 +3
Table 4
Peroxide 4 5 1
control
Natsyn 2200 100 100 100 100
Cabot Sterling V 50 50 50 50
Zinc oxide 5 5 5 5
Agerite resin D 1 1 1 1
Stearic acid 1 1 1 1
Peroxide S -- 9 7 5
Dicumyl peroxide
(40% active) 2 3 3 2
TBBS -- -- -- --
Sulfur -- -- -- --
DeMattia flexibility, ASTM D 813-95
Cycles to failure 33,000 140,000 110,000 240,000
Dynamic mechanical analysis (DMA)
ASTM D2231-94
E* 120 [degrees] C/
E* 25 [degrees] C 1.02 0.92 0.96 0.93
Tan delta
@25 [degrees] C 0.22 0.20 0.14 0.17
2 3 Sulfur
control
Natsyn 2200 100 100 100
Cabot Sterling V 50 50 50
Zinc oxide 5 5 5
Agerite resin D 1 1 1
Stearic acid 1 1 1
Peroxide S 5 5 --
Dicumyl peroxide
(40% active) 4 --
TBBS -- -- 1.6
Sulfur -- -- 1.6
DeMattia flexibility, ASTM D 813-95
Cycles to failure 140,000 80,000 30,000
Dynamic mechanical analysis (DMA)
ASTM D2231-94
E* 120 [degrees] C/
E* 25 [degrees] C 0.96 0.97 1.04
Tan delta
@25 [degrees] C 0.16 0.15 0.15
Table 5
6 7 8 Sulfur
control
Natsyn 2200 100 100 100 100
Cabot Sterling V 50 50 50 50
Zinc oxide 5 5 5 5
Stearic acid 1 1 1 1
Agerite resin D 1 1 1 1
Dicumyl peroxide
(40% active) 2 2 2 --
Saret -- 3 5 --
TBBS -- -- -- 1.6
Sulfur -- -- -- 1.6
Rheometer data; ASTM D 2084-95
Max torque (in-lbs) 39.4 47.8 51.7 75.1
Scorch time
(Ts2, min.) 1.8 1.8 2.0 2.8
Cure time
(Tc90, min.) 17.5 16.6 16.6 5.1
Original physicals ASTM D 412-97 Method A; 2240-97
Tensile strength (psi) 2,320 3,120 3,230 3,270
300% modulus (psi) 1,010 1,530 1,480 1,610
Elongation, % 550 530 540 530
Durometer A hardness 49 56 57 64
Heat aged physicals ASTM D 573-88(94)
Tensile strength (psi) 1,720 2,530 2,440 2,250
300% modulus (psi) 680 1,460 1,520 2,230
Elongation, % 540 470 440 300
Shore A hardness 45 54 56 66
Compression set, %
(70 hrs. @212 [degrees] F) 15 34 36 56
DeMattia flex 50,000 130,000 190,000 30,000
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