How to avoid ozone cracking -- a solution for white and colored rubber goods.The demand for white and colored rubber compounds is growing, particularly for the production of car and bicycle tire components, a wide range of leisure goods and cables. Most of the above-mentioned rubber articles are manufactured using elastomers containing olefinic double bonds that are sensitive to ozone. When a rubber article is not under strain, the surface reacts with ozone, resulting in a dull surface. This surface degradation is called frosting. The typical, more severe ozone cracks, which can destroy a rubber article, appear on the surface of rubber articles under strain. The cracks grow at right angles to the direction of strain. Rubber articles can be protected by waxes which form a mechanical barrier against ozone attack after migration to the surface. In most cases, waxes are used in combination with antiozonants. Antiozonants are substances which also migrate to the rubber surface and they are more reactive to ozone than the olefinic double bonds in the elastomer. An experiment in which polychloroprene and a cyclic acetal 1. any of a class of organic compounds formed by combination of an aldehyde molecule and two alcohol molecules. 2. CH3CH(OC2H5)2, a colorless, volatile liquid used as a solvent and in cosmetics. ac·e·tal (, both dissolved carbon tetrachloride, were exposed to ozone showed that the double bonds of the elastomer were not attacked before approximately 90% of the double bonds of the antiozonant had reacted (figure 1) (ref. 1). [Figure 1 ILLUSTRATION OMITTED] The most common antiozonants in the rubber industry are p-phenylenediamines, such as IPPD IPPD - Integrated Product and Process Design IPPD - Integrated Product and Process Development, 6PPD and 77PD. However, these materials cannot be used in light-colored compounds due to their discoloring effect. Where contact staining has to be avoided, the ppd-antiozonants have to be replaced by non-staining materials in black-filled compounds also. Few non-discoloring antiozonants are available for use in white and colored rubber articles. Examples are a cyclic acetal named Vulkazon AFS and an enol e·nol ether named Vulkazon AFD AFD - A Few Days ic ( -n l k) adj.AFD - Abbreviated Functional Description AFD - Accelerated Freeze-Drying (food processing) AFD - Accident Free Discount (insurance) AFD - Acid Fractionator Distillate AFD - Acoustic Flat Diaphragm (electronics) AFD - Acrofacial Dysostosis AFD - Acrofacial Dysostosis, Catania Type AFD - Active Format Descriptor AFD - Adaptive Flexible Defense AFD - Adaptive Flight Display AFD - Adjustable Frequency Drive. This study shows the effectiveness of the cyclic acetal in compounds based on various elastomers. Experimental The experiments were carried out with white and colored test compounds suitable for various applications and based on nitrile rubber (NBR) and nitrile rubber/polyvinyl chloride (NBR/PVC), blends of butadiene rubber (BR) with natural rubber (NR) and styrene-butadiene rubber (SBR), and chlorinated polyethylene (CM). Due to the superior effectiveness of the cyclic acetal in halogenated rubbers and in butyl rubbers butyl rubber: see rubber., a comparison with a p-phenylenediamine was carried out in black-filled chloroprene chloroprene (klōr`əprēn') or 2-chloro-1,3-butadiene, colorless liquid organic compound used in the synthesis of neoprene and certain other rubbers. The structure of the chloroprene molecule is very similar to that of isoprene; the molecule contains two double bonds and is readily polymerized. rubber (CR), butyl rubber (IIR), bromobutyl rubber (BIIR BIIR - Basic Imagery Interpretation Report BIIR - Baylor Institute for Immunology Research (Dallas, Texas) BIIR - Brominated Isobutylene-Isoprene Rubber) and chlorobutyl rubber (CIIR CIIR - Canadian International Information Resource CIIR - Catholic Institute for International Relations CIIR - Center for Intelligent Information Retrieval CIIR - counterintelligence information report (US DoD)) compounds. The test recipes are shown in tables 1-11. Table 1 - CR test recipe (phr) Compound no. 1 CR 100.0 MgO 4.0 Stearic acid 2.0 ZnO 5.0 Black N774 75.0 Aromatic oil 25.0 ODPA 5.0 Microcrystalline wax 0.7 IPPD varied Cyclic acetal varied ETU 0.5 Table 2 - CR test recipe (phr) Compound no. 2A 2B 2C CR (S-grade) 100 100 100 MgO 4 4 4 Stearic acid 1 1 1 ZnO 5 5 5 Black N 550 20 20 20 Black N 990 93 93 93 Black N 330 12 12 12 Factice 23 23 23 Aromatic oil 13 13 13 IPPD 0 1 0 Cyclic acetal 0 0 1 MBTS 0.8 0.8 0.8 Table 3 - IIR test recipes (tube) Compound no. 3A 3B 3C IIR 100.0 100.0 100.0 Stearic acid 1.0 1.0 1.0 ZnO 5.0 5.0 5.0 Black N 660 65.0 65.0 65.0 Paraffinic oil 22.0 22.0 22.0 Cyclic acetal 0 0.5 1.0 MBT 1.0 1.0 1.0 TMTD 0.5 0.5 0.5 Sulfur 1.6 1.6 1.6 Table 4. IIR and HIIR test recipes
IIR and HIIR
test recipes
Compound no. 4/4 4B 4C
IIR 100
CIIR 100
BIIR 100
NR 0 0 0
Stearic acid 1 1 1
ZnO 5 5 5
MgO 0 0 0.5
Black N 660 65 65 65
Naphthenic oil 8 8 8
6PPD varied
Cyclic acetal varied
Microcrystalline wax varied
MBTS 1 1.5 1.5
TMTD 1 0.2 0.2
Ethyl tellurac 0.5 0 0
Sulfur 1.8 2 1
IRR and HIIR
blends with NR
Compound no. 4D 4E 4F 4G
IIR 60
CIIR 60 40
BIIR 60
NR 40 40 40 60
Stearic acid 1 1 1 1
ZnO 5 5 5 5
MgO 0 0 0 0
Black N 660 65 65 65 65
Naphthenic oil 8 8 8 8
6PPD varied
Cyclic acetal varied
Microcrystalline wax 0 0 0 0
MBTS 1.0 1.5 1.5 1.5
TMTD 1.0 0.2 0.2 0.2
Ethyl tellurac 0.5 0 0 0
Sulfur 1.8 1 2 2
Table 5 - SBR/BR test recipe Compound no. 5A 5B 5C SSBR (25% styrene) 60.0 60.0 60.0 High cis Nd-BR 40.0 40.0 40.0 Stearic acid 1.0 1.0 1.0 ZnO 5.0 5.0 5.0 Precipitated silica 50.0 50.0 50.0 Diethylene glycol 2.0 2.0 2.0 Pigment 3.0 3.0 3.0 Microcrystalline wax 0 3.0 3.0 Cyclic acetal 0 0 2.0 CBS 1.0 1.0 1.0 Sulfur 1.5 1.5 1.5 Table 6 - NR/BR test recipes Compound no. 6A 6B 6C 6D 6E NR (pale crepe) 60 60 60 40 40 High cis Nd-BR 40 40 40 60 60 Stearic acid 1 1 1 1 1 ZnO 5 5 5 5 5 Percipitated silica 50 50 50 50 50 Diethylene glycol 2 2 2 2 2 Pigment 3 3 3 3 3 Microcrystalline wax 0 3 0 3 3 Cyclic acetal 0 0 2 0 2 CBS 1 1 1 1 1 Sulfur 1.5 1.5 1.5 1.5 1.5 Table 7 - NR test recipes (peroxide cured) Compound no. 7A 7B 7C 7D NR (pale crepe) 100 100 100 100 Stearic acid 1 1 1 1 ZnO 5 5 5 5 Black N339 40 40 40 40 Paraffinic oil 8 8 8 8 TMQ 2 2 2 2 MMBI 2 2 2 2 Microcrystalline wax 0 2 2 2 Cyclic acetal 0 0 2 0 6PPD 0 0 0 2 ZBEC 1 1 1 1 Dicumylperoxide (40%) 2.5 2.5 2.5 2.5 TAC 1.5 1.5 1.5 1.5 Table 8 - NBR test recipes Compound no. 8A 8B 8C 8D NBR (34% ACN) 100 100 100 100 Stearic acid 1 1 1 1 ZnO 5 5 5 5 Black N550 55 5 5 55 Plasticizer 10 10 10 10 TMQ 1.5 1.5 1.5 1.5 MMBI 1.8 1.8 1.8 1.8 Microcrystalline wax 0 3 3 3 Cyclic acetal 0 0 3 0 6PPD 0 0 0 3 Insoluble Sulfur 0.3 0.3 0.3 0.3 CBS 2 2 2 2 TMTD 2.5 2.5 2.5 2.5 Retarder 1.0 1.0 1.0 1.0 Table 9 - NBR/PVC test recipes Compound no. 9A 9B 9C 9D 9E NBR/PVC (70/30) 100 Sulfur 0.2 Stearic acid 1 ZnO 5 Talc 80 Black N550 1 Plasticizer 20 [Sb.sub.2][O.sub.3] 10 TMQ 1.5 MMBI 1.5 Microcrystalline wax 0 2 5 0 0 Cyclic acetal 0 0 0 2 4 CBS 2 TMTD 2.5 Retarder 1.0 Table 10 - NBR/PVC test recipes Compound no. 10A 10B 10C 10D 10E NBR/PVC (70/30) 100 Stearic acid 0.7 ZnO 5 Talc 80 Black N550 1 Plasticizer 20 [Sb.sub.2][O.sub.3] 10 TMQ 1.5 MMBI 1.5 Microcrystalline wax 0 2 5 0 0 Cyclic acetal 0 0 0 2 4 Peroxide 4 Coagent 2 Table 11 - CM test recipes Compound no. 11A 11B CM 100 100 MgO 5 5 Hard clay 20 20 Silica 40 40 Plasticizer 8 8 Ti[O.sub.2] 5 5 Vinylsilane 1 1 TMQ 0.2 0.2 Paraffin wax 8 8 Microcrystalline wax 3 3 Cyclic acetal 0 1 Dicumylperoxide (40%) 6 6 TRIM 3 3 In most cases, the cyclic acetal has no effect on the curing behavior and the vulcanizate properties of the compounds. Even peroxide curing is not affected by the addition of this material. As the physical properties of rubber compounds are not affected by the cyclic acetal, the investigations of this study were therefore focused predominantly on the ozone resistance. The ozone resistance was investigated by using the loop method (internal Bayer method). The test specimen, a rubber sheet of 55 x 45 x 4 mm, is mounted on a slotted board. The specimens can be subjected to strains of between 10 and 60% at the surface by bending them to different degrees (figure 2). An ozone concentration between 50 and 1,000 pphm was chosen according to the sensitivity of the different elastomers being investigated. The relative humidity was kept constant at a level of 55% for all the tests and a test temperature between 23 and 40 [degrees] C was chosen. [Figure 2 ILLUSTRATION OMITTED] The ozone tests with NBR/PVC cable jacket compounds were carried out according to VDE 0472 (805). Test results and discussion CR and CR blends with NR and SBR The cyclic acetal is very effective in chloroprene rubber and can be used without adding waxes. Here it is much more effective than the p-phenylenediamines (PPD). In blends with other diene rubbers, such as NR or SBR, this is also the case as long as the CR content in the polymer blend exceeds 30%. In CR compounds, a further advantage of the cyclic acetal in comparison to the PPDs is that there is no influence on the scorch stability and curing behavior. Figure 3 shows the outstanding performance of the cyclic acetal in a CR compound (table 1) in comparison to a control compound without any antiozonant and a compound with IPPD. [Figure 3 ILLUSTRATION OMITTED] In comparison to p-phenylenediamines, the cyclic acetal remains considerably more active if a rubber article is exposed to water. Figure 4 shows the ozone resistance of a CR compound (table 2) after immersion in seawater for one, two and three weeks. The compound with IPPD loses its ozone resistance after immersion in seawater, presumably due to leaching out of the antiozonant, while the cyclic acetal remains very effective after this treatment. [Figure 4 ILLUSTRATION OMITTED] IIR, BIIR, CIIR and their blends with NR In butyl rubber, cyclic acetals are also remarkably more effective than PPDs. As with chloroprene rubber, excellent ozone protection can be obtained with the addition of low quantities of the antiozonant without using waxes (figure 5). This is demonstrated here in a typical tube compound (table 3). [Figure 5 ILLUSTRATION OMITTED] Comparable ozone protection can only be obtained if a PPD is used in combination with high quantities of wax (figure 6). The test compounds for this comparison are shown in table 4. [Figure 6 ILLUSTRATION OMITTED] In BIIR and CIIR, aminic substances such as PPDs are known to act as crosslinkers and their use in these elastomers will lead to scorch problems while the cyclic acetal has no effect on the curing behavior here (figure 7). [Figure 7 ILLUSTRATION OMITTED] In blends of IIR with NR (table 4D-G), the cyclic acetal is much less effective than 6PPD, while blends with halobutyl rubber can be well protected as long as the proportion of NR does not exceed 40% (figure 8). [Figure 8 ILLUSTRATION OMITTED] General purpose rubbers (BR/NR/BR/SBR) NR, SBR and BR, with their high number of olefinic double bonds, are also very sensitive to ozone. Articles produced from these rubbers, such as bicycle tires, require effective ozone protection. We investigated the ozone resistance of a colored SBR/ BR compound without antiozonants and wax, with 3 phr of wax and with a combination of the cyclic acetal and wax (table 5). Figure 9 shows SBR/BR test pieces without antiozonants and with a combination of the cyclic acetal and wax after 168 hours ozonization (50 pphm, 55% relative humidity, 40 [degrees] C). [Figure 9 ILLUSTRATION OMITTED] It can be seen that the compound with the combination of cyclic acetal and wax can avoid crack formation for more than nine weeks while with wax alone, the first cracks occur after two weeks, and, without any additives, crack formation occurs within a few hours (figure 10). [Figure 10 ILLUSTRATION OMITTED] The same is valid for NR compounds (table 6) and NR/BR blends. Here also a combination of the cyclic acetal with a microcrystalline wax is effective. (The ozone resistance improves slightly with an increase in BR content) (figure 11). [Figure 11 ILLUSTRATION OMITTED] The material can also be used in peroxide-cured compounds, which is not possible with most of the other antiozonants. 6PPD was used in a peroxide-cured NR compound. Table 7 shows that the tensile properties and compression set are not at all affected by the cyclic acetal, while 6PPD has a negative effect here as expected (figures 12 and 13). [Figures 12-13 ILLUSTRATION OMITTED] NBR and NBR/PVC Nitrile rubbers are very sensitive to ozone. All types of antiozonants have to be used in combination with waxes. In comparison to the NR/BR and SBR/BR compounds, the effectiveness of the cyclic acetal in NBR compounds (table 8) is weaker (only slightly better than a compound with wax alone) (figure 14). [Figure 14 ILLUSTRATION OMITTED] For outdoor applications, blends of NBR with PVC are often used to achieve sufficient weathering resistance. Addition of the cyclic acetal to NBR/PVC blends (tables 9 and 10) can further improve the ozone resistance (figures 15 and 16). [Figures 15-16 ILLUSTRATION OMITTED] CM Saturated elastomers, such as CM or CSM, which are often used in colored cable jacket compounds, are very resistant to ozone, and antiozonants are not normally necessary. Nevertheless, in some cases typical ozone cracks (at right angle to the direction of strain) also appeared on the surface of CM cable jackets (table 11). It can happen that at high cure temperatures during peroxide curing, double bonds are formed in CM vulcanizates (ref. 2) which can be attacked by ozone later. Figure 17 shows that the addition of a cyclic acetal to a CM compound can improve the ozone resistance considerably. [Figure 17 ILLUSTRATION OMITTED] Summary In summary * Vulkazon AFS provides outstanding ozone protection for CR, IIR, CIIR, BIIR and NBR/PVC blends without causing any staining or discoloration, even in the absence of waxes. * NR blends with CIIR and BIIR can be protected against ozone attack with cyclic acetal without using waxes, as long as the proportion of NR does not exceed 40%. In NR/IIR blends, the cyclic acetal has to be used in combination with wax. * Diene rubbers, such as NR, SBR, BR and NBR, can be protected against ozone by the cyclic acetal in combination with a microcrystalline wax. * There is no negative effect on the compound's scorch or storage stability as is observed when using p-phenylenediamine antiozonants. * The cyclic acetal does not affect vulcanization with peroxides. * Even the ozone resistance of saturated, peroxide-cured vulcanizates can be improved (e.g., CM cable jackets). * The material does not function as an antioxidant. References (1.) D. Bruck, Konigshofen, Ruetz, Wirkmechanismus von Ozonschutzmitteln in Kautschuk, Kautschuk und Gummi, Kunststoffe (38) 5/85, S. 372-376. (2.) Schafers, Dickmann, Zur Ozonbestandigkeit von CM Vulkanisaten, Kautschuk, Kautschuk und Gummi, Kunststoffe (40) 11/87, S. 1,038-1,039. Winfried Jeske is Rubber Chemicals Manager for Bayer AG's Rubber Business Group. He joined Bayer in 1975 and has worked in various positions in the rubber business. He was named to his current position in 1996. |
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