Crosslinking system effect on processing behavior and performance profile of HNBR.Crosslinking system effect on processing behavior and performance profile of HNBR HNBR Hydrogenated Acrylonitrile-Butadiene Rubber Therban, the first representative of the new elastomer elastomer (ĭlăs`təmər), substance having to some extent the elastic properties of natural rubber. The term is sometimes used technically to distinguish synthetic rubbers and rubberlike plastics from natural rubber. class known as hydrogenated nitrile rubber Nitrile rubber, or Buna-N,is a synthetic rubber copolymer of acrylonitrile (ACN) and butadiene. Some trade names are: Nipol, Krynac and Europrene. (HNBR) is a heat, oil and wear resistant specialty rubber obtained by selective hydrogenation hydrogenation (hīdrôj`ənā'shən, hī'drəjənā`shən), chemical reaction of a substance with molecular hydrogen, usually in the presence of a catalyst. of the carbon-carbon double bonds of nitrile rubber (NBR NBR Number NBR Nightly Business Report (PBS show) NBR National Business Review (New Zealand weekly business newspaper) NBR National Bureau of Asian Research NBR National Board of Review ) (refs. 1 and 2). It is distinguished by a technically unique combination of high mechanical and chemical resistance, with such properties as: * outstanding mechanical values, even at elevated temperatures * very good wear resistance * excellent resistance to hot air, even at temperatures up to 175 [degrees] C for limited periods (beneficial in this context are special mineral fillers) * very good resistance to many technical oils and fluids containing chemically aggressive additives and to fuels, including "sour gasoline." This combination of desirable properties has resulted within a relatively short time in the successful use of Therban for numerous rubber goods - oil-resistant seals, o-rings, hoses, diaphragms, synchronous belts, pump linings, roll coverings, cable jackets and others (ref. 3). The principle on which elastomers of this class are produced is selective hydrogenation of the double bonds of NBR. It is thus clear that complete hydrogenation would lead to saturated products crosslinkable only with radical donors, such as peroxides, or by high energy radiation. Incomplete hydrogenation gives partly unsaturated unsaturated /un·sat·u·rat·ed/ (un-sach´ur-at?ed) 1. not holding all of a solute which can be held in solution by the solvent. 2. denoting compounds in which two or more atoms are united by double or triple bonds. polymers which are suitable for crosslinking with sulfur, thus offering more scope for variation. (Partly unsaturated HNBR is characterized by the content of residual double bonds. This is expressed as a percentage, calculated on the original double bond content of the NBR.) The advantages of sulfur crosslinking over 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. cure include higher 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. at break, greater tear strength and better dynamic behavior. Sulfur crosslinking also permits 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. at lower temperatures and to use processes, common for fabric or metal reinforced rubber goods and for extrudates. In the case of sulfur crosslinking numerous vulcanization systems are in practical use to obtain highly specific vulcanizate properties. The degree of crosslinking of partly unsaturated HNBR can be additionally controlled via the level of hydrogenation. Thus a higher content of residual double bonds - corresponding to a lower level of hydrogenation - raises the degree of crosslinking, though it naturally increases the sensitivity to hot air and aggressive media. The best possible balance regarding the level of hydrogenation, the particular crosslinking system and the resulting resistance to aging must therefore be sought with respect to the required property profile. To investigate the mutual interactions we studied the effects of different types of crosslinking systems on the processing behavior and property pattern of saturated and partly unsaturated HNBR as represented by two grades of Therban. Description of test and polymer properties For a comparison of the compound and vulcanizate properties of saturated and partly unsaturated HNBR the use of peroxide crosslinking is inevitable. For the sulfur crosslinking of the partly unsaturated grade, 1707 S, we used two sulfur systems which give similar degrees of crosslinking. Both were efficient vulcanization (EV) systems. Both of them, moreover, were thiuram (TMTD TMTD tetramethylthiuram disulfide. ) systems. The first (EV 1) consisted of TMTD, 0.5 phr sulfur and sulfenamide (CBS (Cell Broadcast Service) See cell broadcast. ). The second (EV 2) consisted of TMTD and a second sulfur donor (dithiodicaprolactam). EV 1 was developed for rapid vulcanization in conjunction with adequate processing safety, while EV 2 was developed for a particularly high degree of processing safety. Similar EV systems are also used for other rubbers, e.g. NBR, if low compression set is desired in addition to the best possible heat resistance. In HNBR these EV systems improve the compression set behavior, compared to standard sulfur/accelerator systems, only slightly. Unpublished investigations had shown that this behavior is improved considerably if the 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. is replaced by zinc peroxide Zinc peroxide (ZnO2) is a chemical compound used as a bleaching and curing agent. Perhaps its most important use is to promote cross-linking in carboxylated nitrile rubber and other elastomers. (ref. 4). Correspondingly modified EV systems, representing a third type of crosslinking system were therefore included in the study in addition to the peroxide system and the conventional EV systems. Two HNBR grades having the same acrylonitrile acrylonitrile /ac·ry·lo·ni·trile/ (ak?ri-lo-ni´tril) a colorless halogenated hydrocarbon used in the making of plastics and as a pesticide; its vapors are irritant to the respiratory tract and eyes, may cause systemic poisoning, and are content, and differing only in residual double bond content, were chosen from the Therban range for the study. The raw material properties of these polymers are shown in figure 1. 1707 represents the practically completely hydrogenated HNBR with very low double bond content, 1707 S is a partially hydrogenated HNBR grade. Processing behavior Recipes and mixing procedure The compound recipes are given in table 2. To make the compounds and vulcanizates comparable, substantial uniformity was maintained with regard to those compounding ingredients which do not affect the crosslinking. Uniformity was achieved most easily in the case of the filler fill·er 1 n. One that fills, as: a. Something added to augment weight or size or fill space. b. A composition, especially a semisolid that hardens on drying, used to fill pores, cracks, or holes in wood, plaster, loading (FEF FEF forced expiratory flow. FEF abbr. forced expiratory flow FEF forced expiratory flow rate. black was used), of course. In the case of the antioxidants Antioxidants Substances that reduce the damage of the highly reactive free radicals that are the byproducts of the cells. Mentioned in: Aging, Nutritional Supplements antioxidants, n. a higher dosage dosage /dos·age/ (do´saj) the determination and regulation of the size, frequency, and number of doses. dos·age n. 1. Administration of a therapeutic agent in prescribed amounts. was used for the sulfur-cured compounds, as they do not impair im·pair tr.v. im·paired, im·pair·ing, im·pairs To cause to diminish, as in strength, value, or quality: an injury that impaired my hearing; a severe storm impairing communications. sulfur crosslinking - in contrast to peroxide crosslinking. From the EV compounds containing zinc peroxide we also omitted not only the zinc oxide, but also the magnesium oxide magnesium oxide: see magnesia. , because this was found to increase the degree of crosslinking. The compounds were mixed in a double stage procedure. First stage in a laboratory internal mixer under conditions similar to those used for NBR, the cooling water being adjusted to a temperature of 50 [degrees] C. After the compounds had been dumped they were cooled down on a laboratory mixing mill and the appropriate crosslinking system was incorporated. Compound properties The uncured compound properties are shown in table 3. The compounds with the EV 2 system had lower Mooney viscosity values both at 100 [degrees] C and at 120 [degrees] C. All compounds showed a substantial drop in viscosity with increased temperature. Scorch testing and cure metering were done at higher temperatures for the peroxide compounds, as usual for this curing system. Long scorch times, and hence safe processing, were obtained not only by the peroxide cure but also by the EV 2 system containing TMTD and dithiodicaprolactam, irrespective of irrespective of prep. Without consideration of; regardless of. irrespective of preposition despite whether this was activated with zinc oxide or with zinc peroxide. The EV 1 system comprising 2 phr TMTD, 0.5 phr CBS and 0.5 phr sulfur gives a considerably higher rate of vulcanization and shorter scorch and flow time, which could impair the processing safety at elevated temperatures. The addition of zinc peroxide results in a 10-20 percent improvement in this respect. Compound properties after storage Table 4 shows that after storage at 40 [degrees] C the Mooney viscosity of the EV 1 compounds rose appreciably ap·pre·cia·ble adj. Possible to estimate, measure, or perceive: appreciable changes in temperature. See Synonyms at perceptible. within seven days, whereas the EV 2 system and the peroxide system still showed no variation from the original values. Where scorch is concerned, the relationships are similar. The EV 2 and peroxide compounds are very stable in storage and have high processing safety, whereas the EV 1 system tends to give considerably shorter scorch times. Under critical processing conditions preference should therefore be given to the EV 2 system. Physical properties of the vulcanizates Mechanical properties In comparison with the peroxide-cured vulcanizates, which, in accordance with the normal practice for peroxide cure, were additionally post-cured, all sulfur-cured vulcanizates showed higher 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 and considerably higher elongation at break values. (It must be emphasized, that for most requirements peroxide cured Therban needs no post cure to obtain a high level of mechanical properties and aging resistance.) In both EV systems elongation was somewhat decreased by zinc peroxide activation. The modulus See modulo. values were highest with peroxide cure. In the case of the EV vulcanizates, however, an increase was given by the substitution of zinc peroxide for zinc oxide. In Shore hardness all the crosslinking systems gave similar results. On the other hand, higher rebound resilience and substantially higher tear strength were obtained by the EV systems. Compression set Figure 5 shows the compression set curves 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 D 395-85 for the peroxide-cured samples based on grades 1707 and 1707 S in the temperature range -10 to 150 [degrees] C (test time 70 hours/25% deformation deformation /de·for·ma·tion/ (de?for-ma´shun) 1. in dysmorphology, a type of structural defect characterized by the abnormal form or position of a body part, caused by a nondisruptive mechanical force. 2. ). At temperatures below 23 [degrees] C the measurement was performed at the storage temperature. The values obtained for the two vulcanizates are similar except that the curve for the partly unsaturated polymer (1707 S) is slightly more favorable in the low-temperature range. The behavior of the sulfur-cured vulcanizates was different. At elevated temperatures it was less favorable than that of the peroxide-cured vulcanizates, according to expectation. With both EV systems, 1 and 2, however, a marked improvement was obtained when zinc peroxide was substituted for zinc oxide. Still more favorable behavior is obtainable if the vulcanizates are post-cured four hours at 150 [degrees] C. Low-temperature behavior Low-temperature properties Table 5 shows the effects of the various crosslinking systems on low-temperature behavior as determined by a variety of test methods. The compilation shows that an influence of the crosslinking system is observed only in the TR test according to ISO (1) See ISO speed. (2) (International Organization for Standardization, Geneva, Switzerland, www.iso.ch) An organization that sets international standards, founded in 1946. The U.S. member body is ANSI. 2921 and that differences in crosslink density are not reflected by the results of any other test method used, such as ISO 2856 for the glass transition temperature The glass transition temperature is the temperature below which the physical properties of amorphous materials vary in a manner similar to those of a solid phase (glassy state), and above which amorphous materials behave like liquids (rubbery state). testing, the torsion torsion, stress on a body when external forces tend to twist it about an axis. See strength of materials. pendulum test according to ISO 4663, the Gehman test (ISO 1432) or the brittleness point test (ISO 812). TR test according to ISO 2921 From figure 2 it is evident that in the TR test of peroxide cured samples the choice of elongation parameters decisively affects the magnitude of the values. This is a consequence of elongation crystallization Crystallization The formation of a solid from a solution, melt, vapor, or a different solid phase. Crystallization from solution is an important industrial operation because of the large number of materials marketed as crystalline particles. ; the TR values measured at 50% elongation are thus shifted towards lower temperatures compared to those measured at 150% elongation. The results also show that the TR values of peroxide vulcanizates based on 1707 S - i.e. partly unsaturated Therban - are lower, at a given elongation, than those of corresponding vulcanizates based on the saturated grade, 1707, indicating reduced crystallinity Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. In a gas, the relative positions of the atoms or molecules are completely random. of 1707 S. Figure 3 shows the influence of the crosslinking system on the TR curves of vulcanizates based on 1707 S at 50% elongation. The tighter peroxide crosslinking gives somewhat lower TR values than the EV 1 and EV 2 systems. The difference is substantially eliminated and the TR values are shifted towards lower temperatures if zinc peroxide is substituted for zinc oxide. Summarizing these remarks on low-temperature behavior, one can say that according to the conclusions drawn from the results of the TR test HNBR vulcanizates evidently remain serviceable ser·vice·a·ble adj. 1. Ready for service; usable: serviceable equipment. 2. Able to give long service; durable: a heavy, serviceable fabric. at lower temperatures than would be expected from the glass transition point. This appears to be particularly true where the deformation is small, as in the case of sealings and gaskets used under typical service conditions. Abrasion abrasion /abra·sion/ (ah-bra´zhun) 1. a rubbing or scraping off through unusual or abnormal action; see also planing. 2. a rubbed or scraped area on skin or mucous membrane. loss versus temperature By the method of DIN 53 516 it was not possible, either with 60-grade or 40-grade emery emery: see corundum. emery Granular rock consisting of a mixture of the mineral corundum (aluminum oxide, Al2O3) and iron oxides such as magnetite (Fe3O4) or hematite (Fe2O3). , to determine an influence of the crosslinking systems on the abrasion resistance of the vulcanizates, the scattering of the results obtained at room temperature evidently being greater than the effects of the individual systems. We therefore used a modified DIN abrasion method by which the abrasion loss is determined not only at room temperature but also at 150 [degrees] C. The results are shown in figure 4. It was found that standard EV 1 gave best abrasion resistance at both temperatures and that for both standard EV systems abrasion resistance is very little affected by temperature - contrary to the peroxide curing system. The EV 1 and EV 2 systems modified with zinc peroxide are intermediate in this respect. Ozone resistance As shown in figure 5, the ozone resistance test revealed some unexpected features. The samples based on the fully hydrogenated 1707 remained crackfree at 50 pphm ozone, 40 [degrees] C, 60% elongation until the end of the 168 hours test, as expected. The ozone resistance of the partly unsaturated 1707 S, however, depends substantially on the crosslinking system: Despite having lower modulus values the sulfur cured samples have remarkably less resistance to ozone than the peroxide cured. This is surprising, as with chlorinated chlorinated /chlo·ri·nat·ed/ (klor´i-nat?ed) treated or charged with chlorine. chlorinated charged with chlorine. chlorinated acids some, e.g. polyethylene or chlorosulfonated polyethylene just the opposite is observed - the material can become ozone sensitive by peroxide cure, whereas it is very ozone resistant with non-peroxide cure (refs. 5 and 6). It was found, unexpectedly, that zinc peroxide has a beneficial influence in this case also. It improved the ozone resistance of the sulfur-cured samples considerably - a trend that was confirmed by further tests performed under the same conditions and also by tests at a higher ozone concentration of 250 pphm. Hot air aging The hot air resistance of peroxide-cured HNBR vulcanizates was described in earlier studies (ref. 7) with reference to Arrhenius curves. The influence of the degree of saturation Degree of saturation is a physical property of soil indicating a degree of saturation of pores in a soil. It is defined as a ratio of total volume of water (liquid phase content) and total volume of voids (liquid and gas phase): on the aging resistance was described as well. The effects of crosslinking systems EV 1, EV 2 and their zinc-peroxide-modified forms on the hot air resistance of the vulcanizates after 28 days at 130 [degrees] C are shown in figure 6. The results clearly show the superiority of the EV 1 and EV 2 systems modified with zinc peroxide, whose vulcanizates still had, at the end of the test, 62 and 56% of their original elongation at break values, whereas the equivalent percentages for the conventional EV 1 and EV 2 systems were 43 and 39%. Even under relatively severe conditions, such as a test temperature of 150 [degrees] C, the systems containing zinc peroxide retain their superiority over the conventional systems. The hot air resistance is not equal, though, to that of the vulcanizates crosslinked with peroxide. Figure 7 shows the Arrhenius curves obtained for the hot air aging, of the initially mentioned mineral filled (A) and carbon black-containing peroxide vulcanizates (B) of the saturated grade, 1707, in comparison with the vulcanizates cured with the conventional EV-systems (D) and the zinc peroxide modified EV-systems (C), based on the partly unsaturated 1707 S. It can be seen that one indicator for the performance limit (50% retention of initial elongation) of the vulcanizates cured with the modified EV systems is roughly half-way between those of the vulcanizates cured with the conventional EV systems and those of the vulcanizate cured with peroxide. The service temperature for 1,000 hours life time, e.g. is shifted approximately 13 [degrees] C upwards. This means a substantial upgrading of sulfur cured HNBR vulcanizates. Dynamic behavior Heat-build-up (Goodrich Flexometer) The fatigue resistance was determined with a Goodrich Flexometer at 100 [degrees] C. Figure 8 shows the temperature increases of the individual vulcanizates in relation to the crosslink system before and after hot air aging for seven days at 150 [degrees] C. In the unaged state the specimens crosslinked with EV 1 and EV 2 do not pass the 25-minute test at the ambient temperature Outside temperature at any given altitude, preferably expressed in degrees centigrade. of 100 [degrees] C. But the modification with zinc peroxide makes the heat-build-up comparable with that of the peroxide-cured vulcanizates in the case of EV 1, and even superior to it in the case of EV 2. After aging all the specimens pass the 25-minute test, but with different results. The vulcanizates cured with peroxide do so most satisfactorily, followed by those cured with the systems containing zinc peroxide. In contrast, the specimens cured with the unmodified Adj. 1. unmodified - not changed in form or character unqualified - not limited or restricted; "an unqualified denial" modified - changed in form or character; "their modified stand made the issue more acceptable"; "the performance of the modified aircraft EV 1 and EV 2 systems show very big increases in temperature. DeMattia flexing test Figure 9 shows how the crosslinking systems affect the crack formation as determined by the DeMattia flexing test ISO 132. It will be seen that the values obtained for the unaged vulcanizates are most favorable in the case of the EV 1 system and the saturated Therban. After hot air aging the results obtained for the EV/zinc peroxide systems are, surprisingly, much improved and likewise equal to the value obtained for the saturated Therban. In addition, the EV 1 vulcanizate behaves considerably better than the EV 2 vulcanizate. The peroxide-cured, unsaturated Therban behaves less well than the saturated Therban, and this behavior deteriorates further after aging. Adhesion The adhesion of HNBR to fabrics, cords and metals is closely connected with the crosslinking system. Preliminary investigations have confirmed that in the case of HNBR, also, sulfur crosslinking gives better adhesion values than peroxide crosslinking, in addition to offering more scope for variation in manufacturing processes and compound formulation. Extensive adhesion studies using modifications of the crosslinking systems described here have been initiated. Summary To fully exploit the many valuable properties of HNBR, use can be made of basically two crosslinking systems - peroxide systems and systems based on sulfur. The purpose of this investigation was to determine the effects of the different crosslinking systems on the processing behavior and properties of compounds and vulcanizates so that ways of optimizing compound formulations could be recommended for practical use. It has been found that the substitution of zinc peroxide for zinc oxide and magnesium oxide in sulfur systems improves the performance profile of the vulcanizates. Thus, in comparison with standard EV systems, EV systems modified with zinc peroxide give * more favorable (lower) compound viscosity * similar or longer scorch times * higher modulus * lower compression set * improved ozone resistance * considerably better hot air resistance, and * improved resistance to dynamic stresses. As compared with the use of peroxides, the modified systems give: * higher tensile strength * considerably greater elongation at break * higher rebound resilience * substantially higher tear strength, and * better wear resistance at elevated temperatures. Owing to owing to prep. Because of; on account of: I couldn't attend, owing to illness. owing to prep → debido a, por causa de its outstanding properties Therban has established itself in the manufacture of oil and heat resistant seals, gaskets, hoses, synchronous belts, pump linings, roll coverings, cable jackets and other goods. [Tabular tab·u·lar adj. 1. Having a plane surface; flat. 2. Organized as a table or list. 3. Calculated by means of a table. tabular resembling a table. Data 1 to 5 Omitted] [Figure 1 to 9 Omitted] |
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