Post vulcanization stabilizer in compounding for improved durability.General compound properties of post vulcanization vulcanization (vŭl'kənəzā`shən), treatment of rubber to give it certain qualities, e.g., strength, elasticity, and resistance to solvents, and to render it impervious to moderate heat and cold. stabilizer stabilizer: see airplane. Over the years, the Years, The the seven decades of Eleanor Pargiter’s life. [Br. Lit.: Benét, 1109] See : Time rubber industry has developed two compounding approaches to address the changes in crosslink structure during thermal aging. These are the use of high accelerator/low sulfur ratios or sulfur donors, both of which provide a lower sulfur content in the crosslinks found daring vulcanization and thus improved thermal stability. As with many formulation changes in rubber compounding, there is a compromise that must he made when attempting to improve one performance characteristic. Improving the thermal stability of vulcanized vul·ca·nize tr.v. vul·ca·nized, vul·ca·niz·ing, vul·ca·niz·es To improve the strength, resiliency, and freedom from stickiness and odor of (rubber, for example) by combining with sulfur or other additives in the presence of heat natural rubber compounds by reducing the sulfur content of the crosslink through the use of a more efficient vulcanization system will reduce dynamic performance properties such as fatigue resistance, as indicated in figure 1. [FIGURE 1 OMITTED] The challenge became to define a way of improving thermal stability while maintaining performance characteristics. Work initiated at our laboratories to address this issue had led to the development and commercialization of disodium hexamethylene-1,6-bis-thiosulfate dihydrate (DHTS DHTS Delphi Harrison Thermal Systems or HTS HTS Heights HTS Harmonized Tariff System HTS High Throughput Screening (biomolecular assay screening) HTS High-Throughput Screening (Pharmaceutical Industry) HTS Harmonized Tariff Schedule ), a rubber chemical that promotes the formation of flexible hybrid crosslinks (figure 2). [FIGURE 2 OMITTED] Disodium hexamethylene-1,6-bis-thiosulfate dihydrate (DHTS) is a crosslinking agent which generates hybrid crosslinks, containing both sulfur and carbon atoms Noun 1. carbon atom - an atom of carbon atom - (physics and chemistry) the smallest component of an element having the chemical properties of the element interposing a hexamethylene 1,6 dithiyl group within the polysulfidic crosslinks during vulcanization. The generation of such hybrid crosslinks increases the resistance to changes in crosslink structure and density encountered with overcure, high temperature cures and anaerobic anaerobic /an·aer·o·bic/ (an?ah-ro´bik) 1. lacking molecular oxygen. 2. growing, living, or occurring in the absence of molecular oxygen; pertaining to an anaerobe. aging. This, in turn, reduces deterioration in physical and dynamic properties associated with reversion reversion: see atavism. . By reducing the average sulfur chain length at the points of attachment to the polymer backbone, thermal stability is improved. Maintaining a long chain within the crosslink structure provides enhanced flexibility under dynamic conditions. The use of DHTS eliminates the compromise between thermal aging and dynamic properties, as illustrated in table 1. Figure 3 shows that the DHTS compound exhibits reversion resistance which is similar to the semi-EV system, but with fatigue resistance comparable to that of the control system using conventional sulfur levels. [FIGURE 3 OMITTED] Mechanistic mech·a·nis·tic adj. 1. Mechanically determined. 2. Of or relating to the philosophy of mechanism, especially one that tends to explain phenomena only by reference to physical or biological causes. discussion The overall reaction product of DHTS in a sulfur cure can be visualized as depicted in figure 2. However, the step-wise sequence of reactions leading to a vulcanizate crosslinked in this manner is not well understood. Moreover, the chemical nature of DHTS does not lend itself well to analysis, and much of the mechanistic evidence gathered to date is of an indirect nature. A substantial amount of work has been done to establish how this post vulcanization stabilizer functions during and after sulfur vulcanization (ref. 1). As a bis-Bunte salt, disodium hexamethylene-1,6-bis-thiosulfate dihydrate suffers from insolubility in·sol·u·ble adj. 1. That cannot be dissolved: insoluble matter. 2. Difficult or impossible to solve or explain; insolvable: insoluble riddles. in non-polar solvents and is very reactive with chemical nucleophiles. The net result is that, once DHTS is incorporated in rubber, any remaining DHTS and its reaction products are not easily detected. Nonetheless, significant chemistry of its reactions with and without rubber additives has been elucidated and is shared here. Conventional and semi-EV cures of natural rubber compounds containing disodium hexamethylene-l,6-bis-thiosulfate exhibit improved tear resistance, flex fatigue and dynamic properties associated with the retention of a polysulfidic network on aging. These performance characteristics of rubber cured with DHTS suggest the formation of flexible crosslinks which compensate for the normal decrease in sulfur rank of the initial polysulfide pol·y·sul·fide n. A sulfide compound containing at least two sulfur atoms per molecule. network during the anaerobic aging process. This observation for DHTS cures is in accord with the work of others (refs. 2 and 3) who reported similar properties in rubber containing hybrid crosslinks. That work provided the incentive to explore possible chemistries that would also indicate the formation of hybrid crosslinks whenever DHTS is used in rubber. In 1994, Nordsiek and Wolpers reported (ref. 3) that 1,2-dithiacyclooctane (DTCO DTCO Digital Tachograph ), when used in combination with rubber accelerators and low levels of sulfur, effectively crosslinks olefinic polymer chains to give heat stable vulcanizates. Indeed, compelling evidence was presented for a stable modulus See modulo. at curing temperatures as high as 180[degrees]C. Using differential scanning calorimetry Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature. (DSC (1) (Digital Signal Controller) A microcontroller and DSP combined on the same chip. It adds the interrupt-driven capabilities normally associated with a microcontroller to a DSP, which typically functions as a continuous process. See microcontroller and DSP. ) followed by HPLC HPLC high-performance liquid chromatography. HPLC high performance liquid chromatography. HPLC High-performance liquid chromatography Lab instrumentation A highly sensitive analytic method in which analytes are placed analyses of the residue, we have observed efficient formation of a hybrid crosslinker (1) from DTCO and MBTS MBTS 2-Mercaptobenzothiazyl Disulfide MBTS Missile Bit Test Set MBTS Missile Bench Test Set , as shown in equation 1. [ILLUSTRATION OMITTED] No evidence of the crosslink precursor (I) was discernable under the same conditions when MBT MBT Minimum (Spark Advance For) Best Torque MBT Masai Barefoot Technology MBT Main Battle Tank MBT Mechanical Biological Treatment (waste treatment) MBT Mercaptobenzothiazole MBT Master of Business Taxation was substituted for MBTS, and only modest concentrations of I formed in the presence of the sulfenamide, TBBS TBBS The Bread Board System TBBS The Big Blue Sky (website) . Furthermore, when we added DTCO to a conventional sulfenamide cure of NR, we observed little reversion and excellent dynamic properties with low heat build-up build·up also build-up n. 1. The act or process of amassing or increasing: a military buildup; a buildup of tension during the strike. 2. . When DHTS (3 phr) was added to the NR compound (table 1 master) and heated in the absence of sulfur and accelerator, significant concentrations of DTCO were generated and measured by GC-MS GC-MS Gas chromatography-mass spectroscopy. See there. . Similarly, on heating DHTS in the presence of TBBS and sulfur via the DSC technique, lower quantities of DTCO could be measured. This work corroborates that of Moniotte (ref. 4) and shows that DHTS or its reaction products not only generate DTCO on heating in rubber, but that the cyclic cyclic /cyc·lic/ (sik´lik) pertaining to or occurring in a cycle or cycles; applied to chemical compounds containing a ring of atoms in the nucleus. cy·clic or cy·cli·cal adj. 1. disulfide di·sul·fide n. A chemical compound containing two sulfur atoms combined with other elements or radicals. Also called bisulfide. is also produced more slowly in the presence of sulfur and accelerators. To the extent that MBTS is formed in benzothiazole cure packages, some of the DTCO likely is trapped before it can escape the rubber matrix. Thereby, one mode of action that DHTS can take is to form a simple hybrid crosslinker, I, in situ In place. When something is "in situ," it is in its original location. as shown above in equation 1. As Trivette (ref. 2) teaches, 1 readily inserts elemental elemental emanating from or pertaining to elements. elemental diet see elemental diet. sulfur and ultimately inserts hybrid crosslinks between polymer chains. However, this is not believed to be the predominant pathway of DHTS action in a vulcanizate. On heating DHTS and 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. via DSC, a broad exotherm or gas evolution is observed beginning at 130[degrees]C and ending at 165[degrees]C. In the absence of zinc oxide, only the DHTS waters of hydration hydration /hy·dra·tion/ (hi-dra´shun) the absorption of or combination with water. hy·dra·tion n. 1. The addition of water to a chemical molecule without hydrolysis. 2. are liberated lib·er·ate tr.v. lib·er·at·ed, lib·er·at·ing, lib·er·ates 1. To set free, as from oppression, confinement, or foreign control. 2. Chemistry To release (a gas, for example) from combination. prior to attaining 130[degrees]C under the same DSC conditions. It is believed that the primary reaction between zinc oxide and DHTS in rubber involves the formation of disulfidic oligomers of the type shown here where the ligands can be hydroxide hydroxide (hīdrŏk`sīd), chemical compound that contains the hydroxyl (−OH) radical. The term refers especially to inorganic compounds. and/or 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 :
[[Na[O.sub.3]S[(S[(C[H.sub.2]).sub.6]S).sub.n]].sub.2]Zn. Ligands
II
In this fashion, the derived zinc oligomers of the bis Second version. It means twice in Old Latin, or encore in French. Ter means three. For example, V.27bis and V.27ter are the second and third versions of the V.27 standard. bunte salt become soluble in the rubber matrix. When heated in the presence of sulfur and accelerator, complexes like II are believed to serve as precursors to oligomeric polysulfides and 1,2-dithiacyclooctane (DTCO) by interacting with accelerator-zinc complexes (ref. 5) present during and after vulcanization. The sulfur that is introduced in the productive stage of mixing inserts into the disulfide bond disulfide bond n. The covalent bond between sulfur atoms that binds two peptide chains or different parts of one peptide chain and is a structural determinant in many protein molecules. , forming a hybrid crosslinking reservoir capable of generating hybrid crosslinks in the presence of activating zinc complexes such as Zn[(MBT).sub.2][Ligand ligand (lĭg`ənd), charged or uncharged molecule with one or more unshared pairs of electrons that can attach to a central metallic atom or ion to form an aggregate known as a complex ion (see chemical bond). .sub.2]. Thereby, the hybrid crosslinks formed (figure 2) help to retain longer crosslinks during and after vulcanization and subsequently, realize physical properties associated with a polysulfidic network. When heat history causes this crosslink network to mature in the presence of zinc complexes, even though elemental sulfur is extruded from the crosslink, the methylene methylene /meth·y·lene/ (meth?i-len) the bivalent hydrocarbon radical —CH2— or CH2dbond. meth·yl·ene n. bridges serve to maintain more stable crosslinks between polymer chains. Under post cure aging conditions, the maturated DHTS network continues to sustain the physical properties normally attributed to a conventional polysulfidic network. Comparison to other thermal resistant vulcanization systems It is well known that thermal and thermal-oxidative aging resistance of natural rubber can be improved by the use of efficient vulcanization (EV) systems. EV systems are produced by the use of sulfur donors in place of elemental sulfur or by empolying very high ratios of accelerator or sulfur. The vulcanization systems produce vulcanizates in which a high portion of the crosslinks are monosulfidic and disulfidic with minimal modification of the main chain by sulfurization. When compared with conventional high sulfur and low accelerator natural rubber compounds, efficiently vulcanized compounds exhibit excellent resistance to reversion and oxidative aging. However, these compounds give relative poor dynamic fatigue properties. Efforts made to overcome this deficiency have led to the so-called semi-efficient (semi-EV) vulcanization systems. The semi-EV systems are obtained by the use of intermediate sulfur to accelerator ratios, or by partial replacement of sulfur with a sulfur donor. When compared with the conventional high sulfur and low accelerator systems, the semi-EV systems also provide excellent resistance to thermal and thermal oxidative aging with much improved fatigue properties as compared to the EV systems. This section presents some laboratory data comparing the above four vulcanization systems to a number of semi-EV systems in natural rubber compounds in processing, curing and functional properties before and after aging. Results and discussion Thermal resistance (reversion) Reversion resistance is measured by the percent tensile tensile, adj having a degree of elasticity; having the ability to be extended or stretched. and 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. retentions on overcure (10 x 190 mins.) at 150[degrees]C (figure 4). As expected, all the cure systems with the low sulfur level of 1.5 phr provide improved reversion resistance as compared to the TBBS control compound with the conventional sulfur level of 2.5 phr. The cure systems based on TBSI TBSI Tert-Butyl-2-Benzothiazyl Sulfenimide (compound 2), DHTS and BCI-MX (compounds 7-9) also can give improved reversion resistance in spite of the fact that these systems use the conventional sulfur level of 2.5 phr. Among the improved reversion resistance cure systems, the following ranking in decreasing order is observed: [FIGURE 4 OMITTED] BCI-MX/DHTS > BCI-MX > ZBPD > DHTS > TMTD TMTD tetramethylthiuram disulfide. > semi-EV > DTDM DTDM Do The Damn Mission DTDM Deterministic Time Division Multiplexing > TBSI > TBBS Thermal oxidative resistance (hot air aging) Heat aging is conducted in a hot air oven at 100[degrees]C for 24 and 48 hours (figure 5). Aging resistance is measured by the percent of tensile retention of the test samples after exposure to these aging conditions. Again as expected, the low sulfur compounds provided the better heat aging resistance than the conventional sulfur compounds. There are no significant differences in this oxidative aging comparison among the cure systems within the two different sulfur level groups. [FIGURE 5 OMITTED] Flexometer heat build-up data Overall, the semi-EV cure systems based on TMTD and ZBPD give the lowest heat generation data (as indicated by the longer blowout time), followed by the BCI-MX and DHTS/BCI-MX conventional sulfur cures, which also exhibit excellent resistance to heat generation. These conventional sulfur cures outperform Outperform An analyst recommendation meaning a stock is expected to do slightly better than the market return. Notes: Exact definitions vary by brokerage, but in general this rating is better than neutral and worse than buy or strong buy. both the semi-EV cures based on the hi/lo and the DTDM systems. Fatigue-to-failure The best fatigue resistance cure system is the DHTS-based conventional cure, followed by the DHTS/BCI-MX system (figure 6). Next in line are the conventional sulfur cures based on TBBS, TBSI and BCI-MX. As expected, the semi-EV cure systems exhibit low fatigue properties. Among these systems, the hi/lo and DTDM cures give better fatigue properties than the TMTD and ZBPD systems. [FIGURE 6 OMITTED] Tear resistance Trouser tear data show that the DTDM semi-EV cure system give the best tear resistance properties especially under overcure environment. This is followed by the the two HTS based compounds. All the other cure systems exhibit similar tear properties. Processing/cure properties Both the hi/lo and DTDM semi-EV cure systems give long scorch time (Mooney scorch time t5) with fast rheometer rhe·om·e·ter n. An instrument for measuring the flow of viscous liquids, such as blood. cure rates as compared to the control cure system TBBS (figures 7 and 8). There is a slight reduction in scorch satiety satiety being in a state of satiation; in experimental animals used with reference to eating and drinking. satiety center located in the ventromedial hypothalamic nucleus. with the DHTS-based cure systems. Although giving very fast cure, the TMTD and ZBPD semi-EV cures also give the shortest scorch times. [FIGURES 7-8 OMITTED] Physical properties There is no significant difference in tensile and elongation among the cure systems evaluated. Although the semi-EV based cures tend to exhibit slightly higher 300% modulus. In summary, the proper selection of semi-efficient vulcanization systems has the potential of providing a good balance of properties which indicates that the choice of a particular system will depend on a number of factors which can include processing, curing, properties retention on aging, cost and other performance properties. Table 3 compares these various vulcanization systems. Effect of disodium hexamethylene-1,6-bis-thiosulfate (DHTS) on rubber to brassed steel adhesion As reported in an earlier publication (ref. 1), DHTS is a bonding promoter for natural rubber to brass-coated steel cord adhesion. It has been reported that polysulfides are involved in bonding with the copper subsulfide layer on the brass surface of the wire cord. During service, the polysulfidic crosslinks degrade TO DEGRADE, DEGRADING. To, sink or lower a person in the estimation of the public. 2. As a man's character is of great importance to him, and it is his interest to retain the good opinion of all mankind, when he is a witness, he cannot be compelled to disclose progressively and the sulfur liberated contributes to a further sulfidification of the copper subsulfide. Thus, bigger and more brittle copper sulfide Copper sulfides describe a family of chemical compounds and minerals with the formula CuxSy. Both minerals and synthetic materials comprise these compounds. Some copper sulfides are economically important ores. crystals are formed, thereby weakening the rubber-brass layer bond. During such a process, the zinc can also be sulfidized, further weakening the bond strength. In the presence of DHTS, hybrid crosslinks are formed in the vicinity of the rubber to metal interface. Because these hybrid crosslinks maintain their polysulfidicity longer than the classical polysulfidic crosslinks, the bonds formed between the rubber and the brass layer retain their strength longer during service. A second benefit of using DHTS is worth noting. With the classical formulation, it is difficult to maintain the best retention of the bond strength during both steam and saline solution saline solution n. A solution of any salt, usually an isotonic sodium chloride solution. Also called salt solution. Saline solution A solution of sterile water and salt used in a variety of medical procedures. aging conditions. Generally, an optimized formulation provides the bond retention during only one of these two aging conditions. The use of DHTS provides the best maintenance of the rubber-brass bond during both steam and saline solution aging tests as shown in table 4. Table 5 shows that sulfur loadings may be reduced in the presence of DHTS, without loss of adhesion, thus improving the compound stability. Conclusion Disodium hexamethylene-1,6-bis-thiosulfate dihydrate (DHTS) is a crosslinking agent which generates hybrid crosslinks containing both sulfur and carbon atoms. DHTS interposes a hexamethylene 1,6-dithiyl group within the polysulfidic crosslinks during vulcanization. The generation of such hybrid crosslinks increases the resistance to anaerobic aging of the rubber vulcanizates and improves the adhesive strength between rubber and brass-coated steel wire. Optimization of DHTS based cure systems can be achieved through statistically designed experiments, giving the compounder flexibility to select the vulcanization system which best fits its performance requirements.
Table 1 - effect of DHTS on thermal aging and
fatigue of natural rubber
Sulfur, phr 2.5 2.5 1.20
TBBS, phr 0.6 0.6 1.75
DHTS -- 2.0 --
Retained 300% modulus (%)
10 x t' (90) @ 144[degrees]C 75.0 91.0 105.0
5 x t' (90) @ 181[degrees]C 56.0 68.0 69.0
t' (90) @ 181[degrees]C 73.0 89.0 91.0
vs.
t' (90) @ 140[degrees]C
Fatigue-to-failure (KC) @ 100% strain
t' (90) @ 144[degrees]C 197.0 241.0 123.0
10 x t' (90) @ 144[degrees]C 127.0 169.0 90.0
Table 1 master Phr
SMR 5 100
N-330 black 50
Aromatic oil 5
Zinc oxide 5
Stearic acid 2
6 PPD 2
Table 2 - cure system comparison study in natural rubber *
Compound 1 2 3 4 5
Cure system I.D. Conv. TBSI Hi/Lo TMTD ZBPD
Sulfur 2.5 2.5 1.5 1.5 1.5
TBBS 0.6 -- 1.5 0.7 1.0
TBSI -- 0.6 -- -- --
TMTD -- -- -- 0.5 --
ZBPD -- -- -- -- 1.25
DTDM -- -- -- -- --
DHTS -- -- -- -- --
BCI-MX -- -- -- -- --
Compound 6 7 8 9
Cure system I.D. DTDM DHTS BCI- DHTS/
MX BCI-MX
Sulfur 1.5 2.5 2.5 2.5
TBBS 0.8 0.6 0.6 0.6
TBSI -- -- -- --
TMTD -- -- -- --
ZBPD -- -- -- --
DTDM 0.6 -- -- --
DHTS -- 2.0 -- 2.5
BCI-MX -- -- 0.75 1.0
* Table 1 master
Table 3 - comparison of natural rubber vulcanization systems
Cure system Semi- Conven- Conven-
EV tional tional
plus
DHTS
Processing/curing
properties
Scorch safety 100 -/=/+ -
Rheometer cure rate 100 -/=/+ -
Reversion resistance
Extended cure time 100 - +
High temperature cure 100 - +
Performance properties
Oxidative aging resistance 100 - =
Heat build-up resistance 100 - +
Fatigue resistance 100 + ++
Tear resistance 100 + +
Relative cost comparison 100 - -/=/+
Cure system Conven- Conventional
tional plus
plus DHTS/BCI-
BCI-MX MX
Processing/curing
properties
Scorch safety = =
Rheometer cure rate = =
Reversion resistance
Extended cure time ++ +++
High temperature cure ++ +++
Performance properties
Oxidative aging resistance = =
Heat build-up resistance ++ +++
Fatigue resistance = +
Tear resistance -/= +
Relative cost comparison + ++
Table 4 - DHTS vulcanization system in rubber to
brassed steel adhesion (effect of DHTS loading)
Insoluble sulfur (80%) 5.0 5.0 5.0
DCBS 1.75 1.75 1.75
DHTS 0 3 6
Wire adhesion test (ASTM 2229)
Unaged (N) 476 (9) * 494 (9) 478 (9)
Steam, 8 hrs./120[degrees]C (N) 466 (8) 417 (8) 407 (8)
Salt (5%), 48 hrs./90[degrees]C (N) 184 (0) 363 (4) 467 (5)
Aged tensile properties
(48 hrs. @ 100[degrees]C)
% Tensile retained 79 78 76
% Elongation retained 48 46 49
* Numbers in parentheses indicate wire coverage with
(0) = no coverage and (10) = full coverage.
Test compound Phr
SMR 5L 100
N-339 black 55
Aromatic oil 3
Zinc oxide 8
Stearic acid 0.5
6 PPD 2.0
TMQ 1.0
Cobalt borate 0.1
Table 5 - DHTS vulcanization system in rubber to
brassed steel adhesion (effect of reduced sulfur
loading)
Insoluble sulfur (80%) 5.0 3.27 6.73
DCBS 1.75 0.84 0.84
DHTS 0 1.27 1.27
Wire adhesion test (ASTM 2229)
Unaged (N) 476 (9) 522 (9) 494 (9)
Steam, 8 hrs./120[degrees]C (N) 466 (8) 439 (8) 470 (7)
Salt (5%), 48 hrs./90[degrees]C (N) 184 (0) 253 (2) 143 (2)
Aged tensile properties
(48 hrs. @ 100[degrees]C)
% Tensile retained 79 88 62
% Elongation retained 48 71 39
References (1.) Anthoine, G., Lynch, E.R., Mauer, D.E. and Moniotte, P.G., "A new concept to stabilize cured NR properties during thermal aging and improve adhesion to brass," Rubber Division Meeting, 1985. (2.) C.D. Trivette (to Monsanto Co.), U.S. Pat. 3,869,435 (1975). (3.) K.H. Nordsiek and J. Wolpers, KGK KGK Kesintisiz Güç Kaynaklari Kautschuk Gummi Kunststoffe 47, 319 (1994). (4.) PG. Moniotte, Internal Monsanto Report, September 21, 1992. (5.) J.I. Cunneen and R.M. Russell, Rubber Chemistry, and Technology, 43, 1,215 (1970). Byron H. To and Otto W. Maender, Flexsys America and Gilbert Anthoine, Flexsys N.V. |
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