Vulcanization of chlorobutyl rubber. II. A revised cationic mechanism of ZnO/ZnCl2 initiated crosslinking

Data relating to the ZnO/ZnCl₂‐accelerated vulcanization of chlorinated poly(isoprene‐coisobutylene) (CIIR or chloro‐butyl) is examined. ZnCl₂ and conjugated diene butyl units on the polymer chain are both precursors to crosslinking, and a revised cationic mechanism is proposed to account for crosslinking, taking into account the involvement of conjugated diene butyl in the process. It is demonstrated that Zn₂OCl₂ will catalyze dehydrohalogenation, and the formation of catalytic amounts of Zn₂OCl₂ by the reaction of ZnCl with ZnO, followed by H⁺ abstraction to give Zn₂OCl₂ and HCl, is essential in the overall crosslinking reaction sequence. The HCl is trapped by ZnO as ZnCl₂. It is proposed that the abstraction by Zn₂OCl₂ of HCl in a concerted reaction leads to Zn(OH)Cl and ZnCl₂. Zn(OH)Cl remains in the polymer as an unextractable salt, while 50% of the chlorine in the rubber is extracted as ZnCl₂ when compounds reach their equilibrium crosslink density. ZnCl₂ initiates crosslinking by the abstraction of chlorine from the chain, but a crosslink will only result when a carbocation on a dechlorinated isoprenoid unit is close to a conjugated diene butyl on an adjacent chain; if not, dehydrohalogenation will result in the formation of a further conjugated diene butyl unit at that point in the chain. The maximum crosslink density achieved is only 1/4 that theoretically possible, as crosslinking restricts chain movement and limits the number of chance meetings between carbocations on the polymer and conjugated diene butyl units. Zinc stearate promotes dehydrohalogenation, ZnCl₂ being the only chloro‐zinc salt formed. Reversion occurs in compounds where there is insufficient ZnO to trap all of the chlorine present in the rubber. HCl per se does not attack the polymer, but promotes reversion only in the presence of carbocations on the chain, i.e., during the crosslinking process. Trapping of HCl by ZnO prevents reversion. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2302–2310, 2000     

Crystallization of vulcanizates. II. Low‐temperature crystallization as a function of extent of cure for polybutadiene vulcanized with dicumyl peroxide,tetramethylthiuram disulfide, 2‐bisbenzothiazole‐2,2′‐disulfide,and zinc–accelerator complexes

Polybutadiene compounds, vulcanized to various degrees of cure, were crystallized in a density column at ?16°C. The percentage crystallinity of vulcanizates was also determined by differential scanning calorimetry where samples, precooled at a programmed rate, were reheated. Curing with peroxides has little effect on either the rate or the extent of crystallization, except at very high crosslink densities, although the induction period prior to crystallization increases progressively with increased crosslink density. Tetramethylthiuram disulfide (TMTD)/sulfur and 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS)/sulfur vulcanizates, cured for progressively longer periods, were found to have lower densities, a result attributed to an increase in free volume occasioned by the formation of accelerator‐terminated pendent groups on the polymer chain. The induction period for crystallization increases and both the rate and the extent of crystallization decrease with extent of cure. These changes are more marked for MBTS vulcanizates that do not crystallize once a gel has formed. Formulations with zinc stearate develop higher crosslink densities and crystallize to a greater extent on cooling, showing the effect of zinc stearate in the crosslinking of pendent groups. The densities of both zinc dimethyldithiocarbamate [Zn₂(dmtc)₄]– and zinc mercaptobenzothiazole [Zn(mbt)₂]–accelerated sulfur vulcanizates increase with cure time, a result attributed to the formation of ZnS in the compounds. Zn₂(dmtc)₄ compounds crystallize extensively on cooling, pointing to limited main‐chain modification. It is suggested that main‐chain modification in these vulcanizates may comprise cyclic sulfide formation. Zn(mbt)₂ compounds crystallize less readily than Zn₂(dmtc)₄ compounds, but to a greater extent than MBTS/sulfur compounds. The crystallization of the vulcanizates is discussed in terms of vulcanization reactions that give rise to crosslinking with the different formulations used. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2573–2586, 2001     

Vulcanization of chlorobutyl rubber. I. The identification of crosslink precursors in compounds containing ZnO/ZnCl2

Chlorinated poly(isoprene‐coisobutylene) (CIIR or chloro‐butyl) was characterized by Nuclear Magnetic Resonance. Compounds with ZnO, ZnCl₂, and ZnO/ZnCl₂ were vulcanized in a DSC at a programmed heating rate and isothermally at 150°C in a press. Crosslink densities were determined by swelling and extracted ZnCl₂ analyzed by atomic absorption. ZnCl₂ formation precedes crosslinking, and at equilibrium only 50% of the chlorine in the rubber is extractable as ZnCl₂. ZnCl₂ promotes crosslinking, and its addition to formulations decreases but does not eliminate the induction period prior to crosslinking. Dehydrohalogenation, which is catalyzed by ZnCl₂ in an autocatalytic process, is accompanied by the formation of conjugated diene butyl on the polymer chain. It is demonstrated that both species, not only ZnCl₂, are necessary precursors to crosslink formation. Moist ZnCl₂ catalyzes dehydrohalogenation but not crosslinking, while ZnO does not promote either reaction. CIIR can be crosslinked to polybutadiene and to polyisoprene, no induction period applying when ZnCl₂ is present in the formulation, and high crosslink densities develop in blends precipitated from solution. Severe reversion occurs in formulations where there is insufficient ZnO to trap all of the HCl evolved. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2290–2301, 2000     

Effect of water and hydrogen sulfide in zinc dimethyldithiocarbamate accelerated sulfur vulcanization

Rubber and the model compound 2,3‐dimethyl‐2‐butene (TME) are vulcanized with zinc dimethyldithiocarbamate [Zn₂(dmtc)₄] accelerated sulfur formulations. When heated in dry nitrogen, Zn₂(dmtc)₄ is stable at vulcanization temperatures. However, it shows a mass increase when heated in moist nitrogen, indicating strong coordination with water; in a nitrogen/H₂S atmosphere rapid degradation to dimethyldithiocarbamic acid (Hdmtc) and ZnS occurs. Model compound studies show that crosslinked sulfides are essentially bis(alkenyl) and confirm the absence of accelerator terminated pendant groups in the vulcanizates, while the ease with which rubber vulcanizates crystallize on cooling in a density column also suggests that pendant groups are largely absent. However, the rates of crystallization, measured as the time for the crystallization process to go to 50% completion, are slower in lightly crosslinked gels than in peroxide cures of similar crosslink density, particularly in the vulcanizates cured in a vacuum; this is interpreted as an indication that some residual pendant groups are present in Zn₂(dmtc)₄ vulcanizates. Water promotes the rate of crosslink formation in both rubber and TME systems, and it is suggested that the strong coordination of water with zinc in Zn₂(dmtc)₄ promotes its reactivity. The H₂S liberated in the vulcanization process promotes decomposition of Zn₂(dmtc)₄ to Hdmtc, and this reaction makes an important contribution to the amount of Hdmtc that is formed in situ. The importance of Hdmtc as an accelerator and its role in providing alternative routes to crosslink formation in Zn₂(dmtc)₄ accelerated sulfur vulcanization are discussed. It is suggested that water, which is liberated when Hdmtc reacts with ZnO to form Zn₂(dmtc)₄, activates newly formed Zn₂(dmtc)₄ molecules; and this accounts for the beneficial influence of ZnO in Zn₂(dmtc)₄ formulations. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1516–1531, 2002     

Dimethylammonium dimethyldithiocarbamate‐accelerated sulfur vulcanization. II. Vulcanization of rubbers and model compound 2,3‐dimethyl‐2‐butene

Rubber and model compound 2,3‐dimethyl‐2‐butene were vulcanized for various times with dimethylammonium dimethyldithiocarbamate [(dma)dmtc]‐accelerated sulfur formulations in the absence of ZnO. Model compound systems were analyzed by HPLC, and no reaction intermediates containing pendent groups were found. Crosslinked sulfides, characterized by ¹H‐NMR, were found to be essentially bis(alkenyl). Residual curatives were extracted from rubber compounds vulcanized for various times and analyzed by HPLC. Compounds, cured to various crosslink densities, were found to crystallize readily in a density column at subambient temperatures. This supports evidence from model compound systems that pendent groups are largely absent from vulcanizates. It is suggested that a reaction mechanism, similar to that applicable to zinc dimethyldithiocarbamate‐accelerated sulfur vulcanization, may be applicable with (dma)dmtc accelerated formulations. Very limited crosslinking occurred on heating compounds under vacuum, and this can be attributed largely to the rapid loss of (dma)dmtc from rubber at elevated temperatures. However, the slower rate of crystallization on cooling of the gels, compared to the rate in press‐cured vulcanizates of similar crosslink density, was interpreted as evidence that some pendent groups did form during heating with (dma)dmtc/sulfur. Crosslinking of such pendent groups may be inhibited by the loss of (dma)dmtc, that, like zinc dimethyldithiocarbamate, may catalyze their crosslinking, and/or to the loss under vacuum of dimethyldithiocarbamic acid that would form thiol pendent groups that would rapidly crosslink with thiuram pendent groups. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3074–3083, 2001     

Structural characterization of vulcanizates. Part VI. The 2-mercaptobenzothiazole-accelerated natural rubber–sulfur system

The crosslinking efficiency of sulfur in the vulcanization of natural rubber in the presence of 2-mercaptobenzothiazole, zinc oxide, and lauric acid has been determined as a function of cure time, cure temperature, and lauric acid concentration. With a low concentration of lauric acid structurally complex networks are formed, which contain 11–19 combined sulfur atoms per chemical crosslink present. The complexity increases with time of vulcanization. With a high lauric acid concentration much simpler networks are formed, which become progressively more simple as reaction proceeds (6 network-combined sulfur atoms per chemical crosslink, decreasing to 2.4 with time). Increasing the cure temperature from 100 to 140°C. reduces the efficiency of crosslinking in both cases. The changes in efficiency are attributed to the influences of the reaction variables (in particular, the concentration of rubber-soluble complexes of the zinc laurate with zinc benzothiazolyl mercaptide) on the structure and subsequent reactions of initially formed polysulfidic crosslinks.     

A comparison of tetraethylthiuram and tetramethylthiuram disulfide vulcanization. II. Reactions in rubber

The reactions of tetraethylthiuram disulfide (TETD) with polyisoprene were investigated under vulcanization conditions. Samples of polyisoprene compounded with various combinations of TETD, sulfur, and ZnO were heated in a differential scanning calorimeter to various degrees of vulcanization. The crosslink density of the compounds was determined by swelling, and the extractable residual curatives and reaction products were analyzed with high‐performance liquid chromatography. TETD caused crosslinking to occur in the absence of added sulfur, as did tetramethylthiuram disulfide (TMTD), both sulfur donors. In the presence of sulfur, the formation of TETD polysulfides occurred immediately before the crosslinking reaction started. The TETD polysulfides were the initial crosslinking agents. The ready reaction between TETD and zinc oxide to form zinc diethyldithiocarbamic acid resulted in considerably higher crosslink densities. This greater reactivity between TETD and zinc oxide, compared with that between TMTD and zinc oxide, did not lead to any noticeable differences in the vulcanizate. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1119–1127, 2002     

Sulfur vulcanization of polyisoprene accelerated by benzothiazole derivatives. II. Reaction of 2-mercaptobenzothiazole and its zinc salt with sulfur and ZnO in polyisoprene

Compounds of polyisoprene with sulfur and bis(2-mercaptobenzothiazole)zinc(II) (Zn(mbt)₂) or ZnO and 2-mercaptobenzothiazole (MBT) were vulcanized by heating in a differential scanning calorimeter. The reaction was stopped at points along the thermogram and the product analyzed. ZnO and MBT readily react, the reaction going to completion during compounding. The presence of Zn(mbt)₂ delays the onset of crosslinking compared to compounds without zinc. It is suggested that the induction period prior to crosslinking is occasioned by the inactivity of Zn(mbt)₂, which must breakdown to MBT before it can participate in the vulcanization process. Such decomposition results from attack by anions generated when polysulfidic crosslinks, formed in the unaccelerated sulfur that occurs in the early stages of crosslinking, undergo scission. The effect of MBT, not bound to zinc, on the mechanism of the reaction is discussed. © 1995 John Wiley & Sons, Inc.     

The role of thiol intermediates in 2‐mercaptobenzothiazole accelerated sulfur vulcanization of rubber model compounds

The model compound, 2,3‐dimethyl‐2‐butene (TME), was vulcanized using 2‐mercaptobenzothiazole (MBT) and sulfur. MBT was not consumed during the vulcanization reaction. The resultant crosslink products were bis(alkenyl) in nature. 2,3‐Dimethyl‐2‐buten‐1‐thiol (TME‐SH) was identified as being present in the vulcanization mixture by a postcolumn derivatization technique. The appearance of thiol was coincident with crosslinking. Polysulfanes (H₂S_n) were formed on crosslinking. Studies of the reaction of TME‐SH and sulfur indicated a rapid reaction to form crosslink products and polysulfanes. No monosulfidic crosslink species were formed in these reactions. Closer investigation revealed the presence of small quantities of what appeared to be highly reactive polysulfidic thiols. This is the first time that such species have been identified in vulcanization systems. Consequently, MBT‐accelerated vulcanization of TME is proposed to occur via the reaction of MBT and S₈ to form polysulfidic MBT, which then reacts with TME to form polysulfidic thiols. These thiols then rapidly react via a metathesis reaction pathway to provide crosslink products and polysulfanes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 47–54, 2003     

Covulcanisation of natural rubber and chlorobutyl rubber. Effect of curative diffusion between phases and ZnCl2 addition

Abstract

Natural rubber (NR) and chlorobutyl rubber (CIIR) were compounded with various formulations containing tetramethylthiuram disulphide (TMTD), sulphur, and ZnO, and masterbatches of these compounds were blended in a 70 : 30 NR/CIIR ratio and vulcanised in a press at 150°C. Crosslink densities of vulcanisates were determined by swelling and tensile properties measured. In formulations in which the concentration gradient permitted the diffusion of TMTD and sulphur to a CIIR phase containing ZnO, tensile strengths were slightly better than 70% of the values of NR compounds of similar crosslink densities. In formulations in which TMTD and sulphur diffused to the faster curing NR phase, blend properties were also better than 70% of those of NR compounds, but at higher crosslink densities, tensile strengths and elongation at break decreased in parallel. This was attributed to failure in a layer of more highly crosslinked material formed within the NR phase close to the interface. Although ZnCl₂ can crosslink CIIR to NR, thus ensuring good interfacial bonding, the addition of ZnCl₂ to the CIIR masterbatch led to attack on the TMTD accelerator and a significant reduction in crosslink density and tensile properties of blends.     

Vulcanization of polychloroprene rubber. I. A revised cationic mechanism for ZnO crosslinking

Data relating to the vulcanization of mercaptan‐grade polychloroprene (CR) by ZnO and MgO (alone or in combination) are examined. Compounds were vulcanized through the isothermal heating of samples at 140°C in a laboratory press and at programmed rates in a differential scanning calorimeter. The reaction was stopped at various points during the heating process. The crosslink densities were determined via swelling. Extractable ZnCl₂ and MgCl₂ were analyzed by atomic absorption spectrometry. Three different crosslinking processes were identified. The first crosslinking process involved the activation of the highly reactive tertiary allylic 1,2‐units along the polymer chain, whereas the second and third crosslinking processes were attributed to the activation toward crosslinking of 3,4‐ and 1,4‐units, respectively. The crosslinking reactions of the 1,2‐units comprised three distinct steps: isomerization (promoted by ZnO), dechlorination, and crosslinking. ZnCl₂ (which formed during compounding and upon crosslinking) promoted crosslinking, and its addition to formulations decreased but did not eliminate the induction period before crosslinking. MgO retarded the crosslinking process by limiting the formation of ZnCl₂ during mixing. The results of the CR/ZnO system are discussed, and a modified cationic mechanism for crosslinking is proposed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

N,N′‐Pentamethylenethiuram disulfide and N,N′‐pentamethylenethiuram hexasulfide accelerated sulfur vulcanization. III. Vulcanization of polyisoprene and 2,3‐dimethyl‐2‐butene in the absence of ZnO

Polyisoprene and model compound, 2,3‐dimethyl‐2‐butene, were vulcanized with N,N′‐dipentamethylenethiuram disulfide (CPTD), CPTD/sulfur and N,N′‐dipentamethylenethiuram hexasulfide (CPTP6) in the absence of ZnO and residual extractable curatives and reaction intermediates analyzed by HPLC at various stages of the reaction. Accelerator polysulfides, required for the formation of accelerator‐terminated polysulfidic pendent groups, form rapidly, or are present from the outset in the case of CPTP6. Model compounds confirm the formation of thiuram‐terminated polysulfidic pendent groups as intermediates in the vulcanization process. Removal of pentamethylenedithiocarbamic acid (Hpmtc) from the system during heating delays the onset of vulcanization and leads to very low crosslink densities. Rubbers heated under vacuum can subsequently be crosslinked by the addition of zinc stearate, pointing to the presence in the compound of thiuram‐terminated pendent groups. However, such pendent groups do not readily crosslink on their own, and hydrogen‐terminated polysulfidic pendent groups, formed by the reaction of sulfurated Hpmtc with the polymer, are suggested to be involved in the crosslink formation. N,N′‐Pentamethylenethiurea forms in the vulcanizate, but is not as product of crosslinking reactions, rather of CPTD degradation. The data are discussed with respect to mechanisms proposed in the literature for crosslinking, and it is concluded that the data support recently formulated mechanisms in which crosslinking involves reaction between thiuram and thiol‐terminated pendent groups, with Hmptc playing and essential role in the overall process. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1100–1111, 2000     

Synthesis of Zn/Mg oxide nanoparticles and its influence on sulfur vulcanization

To reduce the ZnO levels in rubber compounds, mixed metal oxide nanoparticles of zinc and magnesium (Zn_1−xMg_xO) have been synthesized and used as activator. The aim is to obtain better curing properties due to its nanosize and to take advantage of the behavior of both ZnO and MgO in sulfur vulcanization. The model compound vulcanization approach with squalene as a model molecule for NR and CBS as accelerator has been used to study the role of the mixed metal oxide along the reaction. The results found show that with Zn_1–xMg_xO nanoparticles the reaction of CBS becomes faster, higher amounts of MBT are formed at shorter reaction times, and the consumption of sulfur occurs faster in comparison with standard ZnO. Furthermore and more important, an increased crosslink degree calculated as the total amount of crosslinked squalene is obtained. All these findings indicate that Zn_1−xMg_xO is a promising candidate to reduce the ZnO levels in rubber compounds. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Structural characterization of vulcanizates. Part VII. The N-cyclohexylbenzothiazole-2-sulfenamide-accelerated natural rubber–sulfur system

The crosslinking efficiency of sulfur in the vulcanization system comprising (in parts by weight) natural rubber (100), sulfur (1.5), N-cyclohexylbenzothiazole-2-sulfenamide (2.37), zinc oxide (5), and lauric acid (1–10) is relatively insensitive to the lauric acid concentration and to the temperature of vulcanization (between 100 and 140°C.). The networks formed contain initially 8–11 combined sulfur atoms per chemical crosslink present, but this number falls progressively to about 4 as the reaction proceeds. The results are consistent with the intermediate formation of a rubber-soluble complex of cyclohexylamine with zinc benzothiazolyl mercaptide. This complex is believed to be responsible also for the further slow crosslinking which the vulcanizates undergo on standing at room temperature.     

Study of curative interactions in cis-1,4-polyisoprene. VI. Interaction of some combinations of sulfur,tetramethylthiuram disulfide,and ZnO

The interaction of curatives in the systems cis-1,4-polyisoprene (IR)–sulfur, IR–sulfur–ZnO, IR–tetramethylthiuram disulphide (TMTD), and IR–sulfur–TMTD were studied. Thermal events observed in the differential scanning calorimetry curing curves characteristic of these systems were explained in terms of the melting/liquefaction of compounds, the evaporation of gases, and the vulcanization process itself. The similarity of the IR–sulfur and IR–sulfur–ZnO curing curves suggested that sulfur and ZnO were unreactive during vulcanization. On heating the IR–TMTD and IR–sulfur–TMTD systems, gases such as Me₂NH and CS₂ formed easily. Although the maximum crosslink densities in the latter systems were low, the crosslink formation was found to be strongly exothermic. The sulfur efficiency parameter E was estimated for the IR–sulfur–TMTD system and decreased steeply from 37.5 (at 143.2°C) to 16.6 (at 151.0°C). This was taken as evidence that much of the bound sulfur was initially combined in pendent groups. Then E increased dramatically toward the advanced stages of cure, emphasizing the extraordinary inefficient manner in which sulfur was utilized to form crosslinks.     

Thiuram-accelerated sulfur vulcanization. I. The formation of the active sulfurating agent

Mixtures of tetramethylthiuram disulfide (TMTD)/sulfur/ZnO were heated in a DSC to various temperatures. Zinc dimethyldithiocarbamate (Zn₂(dmtc)₄ formed only in undried TMTD/ZnO mixes, the reaction being catalyzed by water on the ZnO surface. The presence of ZnO delays the decomposition of TMTD by adsorbing thiuram sulfenyl radicals, which are needed to initiate tetramethylthiuram monosulfide (TMTM) and tetramethylthiuram polysulfide (TMTP) formation. Increased amounts of TMTM are formed in mixes where ZnO is present, and TMTP are detected prior to TMTM formation. Zn₂(dmtc)₄ does not react with sulfur under conditions where labile hydrogen atoms are not available. © 1996 John Wiley & Sons, Inc.     

溴化丁基橡胶特性及其应用

溴化丁基橡胶(BIIR)是卤化丁基橡胶的一种,它既保持了丁基橡胶原有的特性,又增添了许多优异的性能,如硫化速度快,硫化方式多样等.本文主要论述了溴化丁基橡胶的结构与性能、硫化体系、生产现状及应用情况.在将溴化丁基橡胶与丁基橡胶(IIR)、氯化丁基橡胶(CIIR)比较的基础上,阐述溴化丁基橡胶的优异特性及广阔的应用前景.     

Sulfur vulcanization of polyisoprene accelerated by benzothiazole derivatives. III. The reaction of 2-bisbenzothiazole-2,2′-disulfide with sulfur and ZnO in polyisoprene

The sulfur vulcanization of polyisoprene accelerated by 2-bisbenzothiazole-2,2′-disulfide (MBTS) was investigated. Rubber compounds were heated in a DSC and removed at various temperatures along the DSC thermal curve. The rubber vulcanizate was analyzed for crosslink density and for residual reactants and extractable reaction products. MBTS reacts readily with sulfur, and the polysulfidic accelerator complexes react with the rubber chain to form pendent groups. Crosslinking results from hydrogen abstraction, by the benzothiazole pendent group, from a neighboring chain. 2-Mercaptobenzothiazole, a product of crosslinking, also acts as an accelerator in the later stages of the reaction. MBTS has been shown not to react with ZnO and the higher crosslink densities obtained when ZnO is present are attributed to ZnO aiding the abstraction of the benzothiazole pendent group to give zinc mercaptobenzothiazole. A mechanism for the MBTS acceleration of sulfur vulcanization is proposed. © 1996 John Wiley & Sons, Inc.     

Thermally stable bromobutyl rubber with a high crosslinking density based on a 4,4′‐bismaleimidodiphenylmethane curing agent

The development of thermally stable bromobutyl rubbers has been a challenge in rubber chemistry and engineering. In this circumstance, 4,4′‐bismaleimidodiphenylmethane (BMI) was newly applied as a novel crosslinking agent for thermally stable brominated isobutylene–isoprene rubber (BIIR) with a high crosslinking density. With oscillating disk rheometry and differential scanning calorimetry, the curing characteristics of BIIR were systematically investigated with respect to the content of BMI. We found that BMI alone could crosslink BIIR at higher temperature, and a corresponding possible chemical reaction mechanism was proposed. With the introduction of zinc oxide, the curing reaction of BIIR with BMI was significantly accelerated, and the resulting vulcanizate provided a higher state of curing with excellent overcure reversion stability even at a temperature of 190 °C for 2 h. The content of the dicumyl peroxide (DCP) reaction accelerator was also optimized to be BMI/DCP = 1:0.05 on the basis of considerations of the curing rate, scorch safety, maximum rheometric torque, and reversion resistance at 160 °C. Compared with the conventional sulfur‐cured BIIR, the BMI‐cured BIIR exhibited a higher crosslinking density with a superior low compression set property at elevated temperatures and an excellent thermal stability. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44092.     

Phase morphologies of vulcanized chlorobutyl rubber/polyamide 12 blends: The breakup of pre‐crosslinked CIIR phase

Dynamic vulcanization to prepare blended thermoplastic vulcanizates (TPV) is a kind of complicated blending technology, where the breakup of the rubber phase happens accompanying with the crosslinking of rubber. In this study, we aim to investigate the effect of crosslinking on the breakup of chlorobutyl rubber (CIIR) phase in thermoplastic polyamide 12 (PA 12) matrix by purposely using pre‐crosslinked CIIR with different crosslink degrees and plasticizer contents. Besides, the effect of blending conditions on the breakup of crosslinked CIIR phase was studied. The results show that a low crosslink degree, a high content of plasticizer, a low blending temperature and a morderate rotor speed of 70 rpm facilitate the breakup of pre‐crosslinked CIIR in PA 12 matrix. This is ascribed to the decrease in the modulus of pre‐crosslinked CIIR phase because of either a low crosslink degree or a high content of plasticizer, the increase in the molten viscosity of thermoplastic matrix because of a low blending temperature and a moderate rotor speed. It is indicated that the breakup of pre‐crosslinked rubber is mainly dominated by the modulus of crosslinked rubber phase as well as the molten viscosity of thermoplastic matrix and shear stress. This study will provide guidance for the preparation of CIIR/PA TPV. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40765.