Initial Stages Of Vulcanization Research Articles

シリカがジエン系ゴムの加硫反応に与える影響

Effects of silica on the vulcanization reaction by sulfur/N-cyclohexyl-2-benzothiazolylsulfenamide (CBS) in isoprene rubber were studied by FT-IR and ESR. It was found that the induction time of vulcanization reaction was shortened by the addition of silica. However, the efficiency of vulcanization at the final stage of reaction was lower for silica filled sample than for unfilled one. The structure of CBS in the vulcanization system was investigated by the FT-IR at a crosslinking temperature. It was found that by the addition of silica in the system new peaks appeared at 1541 and 1598cm-1,suggesting a decomposition of CBS by silica. Further, the concentration of sulfur radical during the vulcanization was decreased by the addition of silica. However, the decomposition was suppressed by the removal of water molecules absorbed directly on silica surface. These results indicated that at an initial stage of vulcanization, most of CBS was decomposed by silanol groups which were covered with the adsorbed water, leading to the fast vulcanization reaction by sulfur, however, the concentration of CBS dropped quickly at the initial stage of vulcanization, resulting in the low efficiency of vulcanization. These results were explained by the decomposition of CBS by acid catalyst ability of silanol groups.

Solid-State Carbon-13 Studies of Vulcanized Elastomers. VIII. Accelerated Sulfur-Vulcanized Butyl Rubber at 75.5 MHz

Abstract The chemical microstructural changes of an accelerated sulfur cure of IIR with TMTD/ ZnO/sulfur have been elucidated. No reaction occurs in the isobutylene units. Isomerization occurs at the initial stages of vulcanization and reaches an equilibrium at 30–35 min. Sulfurization initially results in III and VI and is polysulfidic in nature. At cure times greater than 75 min, desulfurization of III and VI occurs and mono- and poly-sulfidic sulfurization giving IV and VII occurs. The reaction rate of III and VI is much greater than that of IV and VII. Equilibrium swelling measurements have been shown to correlate with rheometry measurements for this system and have shown that polysulfldic III and VI correlate with an increase in properties, while mono- and poly-sulfldic IV and VII correlate with reversion. No migration of the double bond, saturation, internal cyclization, or sulfurization resulting in V and VIII were observed.

Sulfur vulcanization of natural rubber accelerated with 2‐mercaptobenzothiazole plus tetramethylthiuram disulfide

AbstractThe sulfur vulcanization of NR by a MBT‐TMTD mixed accelerator system both in the presence and absence of ZnO and stearic acid with or without DCP has been studied. The rate of decomposition of DCP in the presence of both MBT and TMTD is quite similar to that in the MBT system alone. The reduction in crosslinking due to DCP is dependent mainly on MBT. The decomposition product of TMTD may contribute to it only at the later stage of vulcanization. Though TMTD has no influence on the decomposition rate, it reacts with MBT at least in the initial stage of vulcanization and suppresses the retardation caused by MBT on DCP vulcanization in accordance with the free sulfur decrease, the nature of crosslinking formation both in the presence and absence of DCP, and the methyl iodide treatment of the vulcanizates. The vulcanization process of the MBT‐TMTD‐S‐NR system has been interpreted in terms of both free radical and polar mechanisms. The domination of either of these depends on the dominant amount of either MBT or TMTD in the acclerator ratio. According to the initial high rate of crosslink formation, free sulfur decrease and also the initial additiveness of crosslinking in stocks containing DCP, the vulcanization process of MBT‐TMTD‐ZnO‐St. acid‐S‐NR system has been explained in terms of an ionic mechanism. The pronounced synergistic nature of such systems has been interpreted by the enhanced activation of MBT‐S‐ZnO‐St. acid complex due to the dithiodicarbamate ion formed in the initial stages of vulcanization, and also by the activation of TMTD accelerated vulcanization due to the mercaptobenzothiazylion.

Chlorobutyl Covulcanization Chemistry and Interfacial Elastomer Bonding

Abstract A new analytical and diagnostic technique has been developed and applied to covulcanized Chlorobutyl—polydiene rubber blends that indicates the existence or absence of interfacial bonds. This technique of differential solvent swelling is based upon solvent—elastomer interaction parameters and draws upon the analogy to filler-adhesion analysis of vulcanized networks. Interfacial bonds between dispersed Chlorobutyl and polydiene rubber phases are first obtained with very active thiuram and thiuram tetrasulfide curative systems. Subsequently less active curatives utilizing bis-alkylphenol polysulfides as sulfur donors were also found to promote interfacial bonds. By means of mercaptan probes it was established that strong evidence of interfacial bonds in covulcanized networks was associated with a preponderance of monosulfidic crosslinks. Prolonged heating, which matures polysulfidic links to more thermally stable monosulfidic links, did not produce strong interfacial bonds where none existed after the initial vulcanization cycle. Thus, interfacial bonds must be formed with more thermally stable monosulfide links during the initial stages of vulcanization. Reinforcing carbon black has a minimal effect upon the acquisition of interfacial bonds or the sulfur-bond chemistry. The curative formulation remains the factor which governs production of the monosulfidic links required for strong interfacial bonding. A definite correlation was found between interfacial bonding and physical strength of fully compounded Chlorobutyl—SBR covulcanized blends. Carbon-black reinforced systems covulcanized with a sulfur donor, such as bis-alkylphenol polysulfide, as the sole source of sulfur, displayed tensile strengths and rupture energies considerably greater than the equivalent systems containing elemental sulfur.

Accelerator Activity of 2-Benzothizole Sulfenamide Derivatives and the Exchange of Benzthiazyl Thiyl Radicals with Radioactive 2-Benzthiazyl Disulfide

Abstract The authors obtained data characterizing the accelerating activity of sulfonamide derivatives of MBT. The reactions of isotope exchange between such derivatives and MBTS labeled with S35 in the disulfide bridge were investigated, using chromatography to separate the components of the reaction mixture. It is shown that between these compounds there takes place an exchange of benzthiazyl radicals. A direct dependence is established between the accelerator activity of the sulfenamide compounds and their capacity for exchange of benzthiazylthio groups. The easier this takes place, the higher the activity of the compounds in the initial stage of vulcanization. The observed reactions of exchange of benzthiazylthio radicals may be used for the synthesis of sulfenamide derivatives of MBT labelled with radioactive S35.

Accelerator Action of Certain Heterocyclic N-Thiocarbamylsulfendialkylamides

Abstract 1. The accelerator activities of heterocyclic N-thiocarbamylsulfendialkylamides based on piperidine, morpholine, and piperazine were investigated; it was found that compounds of this type are highly active accelerators of the vulcanization of natural and styrene butadiene rubber stocks, inducing rapid crosslinking and giving rubbers of higher modulus than the usual vulcanization accelerators. 2. The vulcanization kinetics in presence of these heterocyclic N-thiocarbamylsulfendialkylamides depends to a considerable extent on the nature and number of hetero atoms in the accelerator molecule. Morpholine and piperazine derivatives have a slower effect at the initial stage of vulcanization than the corresponding piperidine derivatives, and confer greater resistance to premature vulcanization of the rubber stocks.

Effect of Structurizing Agents on the Plasticization and Vulcanization of Butadiene-Styrene Rubber

Abstract Compounds of the type of p-anisidine and N,N′-diphenyl-p-phenylenediamine cause structural changes in SKS-30A rubber and its solutions, particularly during heating. Vulcanization accelerators, in particular, mercaptobenzothiazole, greatly accelerate these structural changes in rubber in the presence of p-anisidine or N,N′-diphenyl-p-phenylenediamine during both heat oxidative plasticization and vulcanization. The degree and nature of the structural changes in rubber vary according to the type of accelerator. The structurizing action of p-anisidine and of N,N′-diphenyl-p-phenylenediamine during vulcanization is manifested in the increase of the tensile strength, equilibrium modulus, and rebound resilience of vulcanizates, and the retention of high relative elongations. The agents cited accelerate somewhat the process of combination of sulfur with the rubber in the initial stage of vulcanization, without increase of combined sulfur content at the point of optimum vulcanization. The hypothesis is advanced that the influence of p-anisidine and N,N′-diphenyl-p-phenylenediamine on the observed structural changes in SKS-30A rubber during its heat plasticization and vulcanization is related directly to the character of the oxidation processes, and may be caused by the reaction of the oxidized segments of the rubber (peroxide radicals, stable peroxides, functional oxygen groups, etc.) In unvulcanized rubber, this leads to a loss of plasticity and solubility, and, by vulcanization, to the formation of additional cross-links, to an increase of tensile strength, and with retention of high elongation at rupture. Vulcanization accelerators, such as mercaptobenzothiazole, which activate the oxidation processes in the rubber, increase considerably the rate and extent of these structural changes.

Mastication and Rate of Set-Up. Part II

Abstract In the earlier communication it was shown that the length of time of milling of a mixed rubber exerted a marked effect on the rate at which it commenced to scorch. The longer the time of milling, the slower was found to be its rate of set-up. It is intended in this paper to give a brief account of some experiments which were undertaken with a view to deciding which factor or factors in the process of mastication are responsible for this retardation of the initial stages of vulcanization. During the milling of rubber only a limited number of factors can be considered as having any effect on its subsequent behavior as far as its rate of set-up is concerned. These are, decomposition of the accelerating components due to heat and pressure, softening of the rubber due to mechanical working, disaggregation or other causes, breakdown of the ultimate rubber particles, depolymerization, or, finally progressive oxidation of the rubber throughout the milling and mixing process. The possibility of decomposition of the accelerator during milling was examined by adding the accelerator to one batch at the beginning of the mixing process, adding it to another at the end and comparing the results of the rates of set-up on the two rubbers. Softening of the rubber without resorting to mechanical means was carried out by adding various softeners to the rubber while in solution and dispersing these by gentle stirring. It is of interest to note that the addition of softeners to the mixing without simultaneous mechanical working resulted in only a slight lowering of the plasticity reading (Y value), which seems to indicate that, for the effective development of softening action, mechanical working or the catalytic effects induced during mechanical working are essential.

Studies of Hard Rubber Reactions

Abstract As shown in Fig. 1, all the compounds containing 5 to 150 per cent of sulfur (based on the rubber) indicated a gradual loss of total sulfur, with a relatively rapid loss after about 90 minutes, which corresponds to the period of the hard rubber reaction. Some investigators hold the view that the loss of total sulfur is the result of liberation of hydrogen sulfide by the interaction of sulfur with rubber, resins, and proteins, and sublimation of sulfur. In the present experiments, the cause of the comparatively marked loss of total sulfur seems to be related to the hardening or polymerization reaction in hard rubber. No good mixture could be obtained from which the maximum loss of total sulfur was reached, with reference to the content of rubber, i. e., rubber resins, proteins, and that of sulfur. The loss of sulfur by sublimation was restricted as far as possible by enveloping the samples with cellophane. It appears from these facts that the depolymerization phenomena of soft or semi-hard rubber products may occur during the initial stage of vulcanization and those of the hard rubber after 90 minutes. Considering again the vulcanization curves in the previous report (Part IV), there is no loss of sulfur in the initial stage of vulcanization but considerable losses of sulfur are seen in the present curves of total sulfur; in other words, during the whole course of vulcanization the combined sulfur increases to a certain extent, accompanying the loss of sulfur on one side. In Fig. 2, the relationship between the loss of total sulfur and the tensile strength of the hard rubber product is shown. Of all the hard rubber compounds, the samples containing 70–80 per cent of sulfur showed the maximum loss of sulfur, coinciding with the maximum tensile strength after vulcanization for 90 minutes. It may be assumed that the polymerization and the depolymerization reactions occur simultaneously and are closely related.

Porosity in Vulcanized Rubber

Abstract A systematic investigation has been made of the factors likely to promote porosity in vulcanized rubber. Of the methods developed for assessing the degree of porosity, measurement of the percentage decrease in specific gravity is shown to be the most satisfactory. The permeability of the sheet to moisture, the acetone extraction of aged samples, and visual examination also give some indication of the degree of porosity. Severe over-mastication, particularly on cold rolls, plasticizes rubber and increases the entrapment of gases during mixing. If the pressure applied during vulcanization is low, the volume decrease, which occurs as the rubber vulcanizes, causes further lowering of pressure; some of the entrapped gases are liberated, and as the rubber is partially set up free escape is prevented and pores are developed. The effect of over-mastication can also be produced by heat or oxidation treatment of the raw rubber. Mechanical treatment of the mix, such as sheeting from the mill at friction speed, calendering to a thick gage, or extrusion of thick articles may increase the tendency for porosity under the appropriate conditions. Softeners added to the mix produce the same effect as over-mastication in increasing porosity, whereas stiffening fillers oppose its production. When no mechanical pressure is applied during vulcanization, porosity is likely to develop, but a slight pressure may cause even worse porosity through preventing the free escape of gases during the soft stage before vulcanization commences. The degree of porosity depends on the conditions prevailing during the initial stages of vulcanization and is, in fact, a useful indication of setting up when scorching tests are being carried out. It is not influenced by the action of heat, in the later stages of vulcanization on the rubber-sulfur or oxygenated rubber complexes. Moisture, derived from the ingredients of the mix, the processing or the methods of vulcanization, increases the porosity under appropriate conditions. The use of ultra-accelerators or high-curing temperatures, tends to increase porosity of mixes liable to this defect, as the escaping gases are more completely entrapped by a rapid setting up of the rubber. Some applications of the results to methods of overcoming porosity under works conditions are given.

Scorching, and Other Plasticity Changes in Rubber Compounds on Heating

Abstract A method is described which employs the Goodrich plastometer for detecting the initial stages of vulcanization of uncured rubber compounds. Reduction in plasticity, at standard room temperature, of test pieces previously heated for various intervals at selected temperatures is used to determine the degree of cure and thus to estimate safe operating temperatures and periods. Examples of the application of the method to a variety of compounds are presented. Other temperature effects, such as heat stiffening and softening, which have not previously been reported, are illustrated.