Network Chain Density Research Articles

せん断流動場反応制御技術によるゴムの連続再生 （第３報） ＥＰＤＭにおける脱硫反応過程の解析

We studied devulcanization process by measuring rubber state, viscosity, gel fraction, its effective network chain density and average molecular weight of sol. When passing through grinding zone, vulcanized rubber was ground by screw rotation and its size became small. Next, devulcanization proceeded in the devulcanizing zone and viscosity of rubber decreased. Gel fraction and its effective network chain, density decreased too. Finally, gel fraction of devulcanized rubber was 45% and its effective network chain density was one twentieth of virgin vulcanized rubber. On the other hand, average molecular weight of sol part did not change and its value was nearly equal to virgin raw rubber. Devulcanization process depended on shear stress by screw rotation. Gel fraction and its effective network chain density decreased by increasing screw rotation. Devulcanization rate depended on reaction temperature and became high by raising temperature.

Solid-state13C-NMR and equilibrium-swelling studies of filled, TBBS-accelerated sulfur vulcanization of natural rubber

The effective network chain density (νe) and the density of the chains between sulfurizations (2[S]chem) for natural rubber vulcanized with N-t-butyl benzothiazole sulfenamide (TBBS) in different levels of HAF–HS (N347) carbon black were determined by equilibrium swelling and solid-state 13C-NMR measurements, respectively. The proportion of the intermolecular crosslinks over a whole range of sulfurized products (Ec) for the gum stock was estimated by the comparison of the chemical network chain density (νchem) calculated from νe using Mullins's approach with 2[S]chem. Ec increases with cure, reaches a maximum at an optimum cure state (trpar;, and decreases during an overcuring. It was observed from the NMR results that the total amount of chemical sulfurization reactions occurring in the rubber is almost independent of the level of carbon black loading. Based on the assumption that the amount of the intermolecular crosslinking reactions is also constant with the carbon black content, the amount of physical crosslinks (vent) in filled formulations was determined by subtracting the estimated νchem from the νe. The linear relationship obtained from the plot of the vent at the optimum cure state (t90) as a function of carbon black content postulates that the physical adsorption plays a major role in the polymer–filler interactions. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1391–1399, 1998

Solid‐state 13C‐NMR and equilibrium‐swelling studies of filled, TBBS‐accelerated sulfur vulcanization of natural rubber

The effective network chain density (νe) and the density of the chains between sulfurizations (2[S]chem) for natural rubber vulcanized with N-t-butyl benzothiazole sulfenamide (TBBS) in different levels of HAF–HS (N347) carbon black were determined by equilibrium swelling and solid-state 13C-NMR measurements, respectively. The proportion of the intermolecular crosslinks over a whole range of sulfurized products (Ec) for the gum stock was estimated by the comparison of the chemical network chain density (νchem) calculated from νe using Mullins's approach with 2[S]chem. Ec increases with cure, reaches a maximum at an optimum cure state (trpar;, and decreases during an overcuring. It was observed from the NMR results that the total amount of chemical sulfurization reactions occurring in the rubber is almost independent of the level of carbon black loading. Based on the assumption that the amount of the intermolecular crosslinking reactions is also constant with the carbon black content, the amount of physical crosslinks (vent) in filled formulations was determined by subtracting the estimated νchem from the νe. The linear relationship obtained from the plot of the vent at the optimum cure state (t90) as a function of carbon black content postulates that the physical adsorption plays a major role in the polymer–filler interactions. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1391–1399, 1998

Densely crosslinked polycarbosiloxanes. II: Thermal and mechanical properties

The thermal and mechanical properties of two densely crosslinked polycarbosiloxane systems were investigated in relation to the molecular structure. The networks were prepared from functional branched prepolymers and crosslinked via a hydrosilylation curing reaction. The prepolymers having only vinyl functionalities (poly[phenylmethylvinyl]siloxanes) were crosslinked by using crosslinking agents with reactive silicon–hydrogen groups. In prepolymers having both silicon–vinyl and silicon–hydrogen groups (poly[phenylmethylvinylhydro)]siloxanes crosslinking took place intermolecularly. The thermal and mechanical properties of the polymer networks were found to be dependent on the phenyl SiO3/2 (branches) content in the prepolymer, the number of elastically effective crosslinks, the elastically effective network chain density and molecular weight between crosslinks, length of the chain segments introduced by the hydrosilylation crosslinking reaction, and the number of dangling ends. As a consequence of the dense crosslinking, the mechanical properties were also strongly dependent on the glass transition temperature. A tough–brittle transition was observed around the glass transition temperature of the polymer networks. The properties of the poly(phenylmethylvinylhydro)siloxane networks were found to be superior to those of the poly(phenylmethylvinyl)siloxane networks. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1311–1331, 1997

Influence on Friction Force of Adhesion Force between Vulcanizates and Sliders

Abstract The effects of vulcanizate and slider surface free energy—as well as the adhesion force (P) between them—on friction the force (F) and the coefficient of friction (μ) has been investigated. SBR and NBR vulcanizates were prepared using three molds differing in surface free energy. The mold with a high surface free energy gave a vulcanizate surface possessing polar groups. The mold with low surface energy gave a vulcanizate surface with many nonpolar groups. The coefficient of friction increased with the surface free energy of SBR and NBR vulcanizates. With SBR vulcanizate (surface free energy, 31.3 mJ·m−2) and teflon slider (surface free energy, 28.1 mJ·m−2) combination having the least surface free energy, the coefficient of friction was constant at greater than a 0.2N load. With vulcanizates and an aluminum slider with high surface free energy, friction force could be detected even at zero load because of the adhesion force at the interface. Friction force increased linearly with adhesion force between vulcanizates and sliders when the physical properties of the vulcanizates and net work chain density were constant. The present results clearly demonstrate the contribution of adhesion force to the friction of vulcanizates.

Network structure and mechanical properties of sulfur-cured rubbers

The effect of sulfur/accelerator amount (at fixed ratio) and accelerator type (n-cyclohexylbenzothiazole-2-sulfenimide, diphenylguanidine) on network parameters (network chain density, cross-linking density, tube diameter, trapping factor) of styrene-butadiene copolymer networks is investigated via uniaxial stress-strain measurements up to large extensions. The mechanical properties are discussed in the framework of a non-Gaussian network model. It includes topological constraints for the network chains and the network junctions (cross-link and trapped entanglements). The model assumes a trapped entanglement contribution to the stress that does not vanish in the infinite strain limit

Photocrosslinking of polyethylene. II. Properties of photocrosslinked polyethylene

AbstractSome properties of photocrosslinked high density polyethylene (2 mm thick sheets) have been studied. The homogeneity of the network is greatly improved by the application of triallylcyanurate (TAC) as crosslinking agent. The role of TAC in promoting the crosslinking is discussed. The crosslinked PE is found to be durable towards immersion in boiling water. Oxygen permeability increases while density and melting point decrease with increasing degree of crosslinking. The network chain density is obtained using conventional swelling method in boiling xylene, which together with data from melting point depression show that contribution of chain entanglement is insignificant to the gel formation of PE in this work.

Influence of physical relaxation on chemical stress–relaxation of natural rubber vulcanizates

Networks were prepared in the swollen and dry states to investigate the influence of physical relaxation on chemical stress–relaxation. The stress–relaxation behavior of solution-cured samples was different significantly from that of the samples crosslinked conventionally. The same result was also observed in the number of chain scission for both kinds of samples. On the other hand, the number of chain scission estimated by using the swelling method for samples crosslinked conventionally was in good agreement with that by the chemical stress–relaxation for solution-cured samples. It was found that there is little or no influence of the physical relaxation caused by entanglements, and no effect of dangling chains arising from scission in the equilibrium swelling. The relative change of network chain density determined by means of the swelling method was also consistent with that by sol fraction determination. These results indicate that the swelling method can be used as a measure of a degree of degradation on chemorheology. Taking into account the influence of physical relaxation on chemical stress–relaxation, a new relationship between the relative stress decay and the relative network chain density was experimentally proposed.

Chemical Stress Relaxation of NR Networks Prepared in the Swollen State

Abstract From the experimental results of the present investigation, it is apparent that two kinds of networks which have a different three-dimensional network structure give quite different behavior of chemical stress relaxation, even if both networks have the same network chain density. The difference in three-dimensional network structure for the two kinds of rubber arises from the degree of entanglement, which changes with the concentration of the polymer chains prior to the crosslinking process. The direct cause of chemical relaxation is due to the scission of network chains by degradation, whereas the total relaxation is caused by the change of geometrical conformation of network chains. This then casts doubt on the basic concept of chemorheology which is represented by Equation 2.

Gamma-ray induced deterioration of natural rubber vulcanizates

The influence of 7-ray irradiation on polymers has been studied extensively [1 -4 ] , and natural rubber (NR) is not an exception. However, the sofar-reported results of NR were obtained on the NR vulcanizates of filled stock, i.e. containing carbon black or some other flUers [1-3, 5 -7 ] , though in some examples the formulation recipes were not described in detail. The effect of radiation can be influenced by the presence of filler. We would like here to report the effect of radiation on purified NR vulcanizates which do not contain any fillers. RSS No. 1 was used as natural rubber for vulcanizations. It was mixed on an open two-roll mill with reagents shown in Table I. In Table I $1 means the formulation to obtain the mainly monosulphide-crosslinked vutcanizate, S= means that of mainly polysulphide crosslink, and B the carbon black stock. The compounds were press-cured at 140 ° C. The resultant vulcanizates were subject to solvent (acetone/methanol/chloroform = 35/28/ 29 in volume ratio [8]) extraction for a week to purify them. Extracted vulcanizates were vacuumdried to a constant weight. The weight decrease by extraction was below 5%. The 7-ray irradiation upon the purified vulcanizates was carried out in air at room temperature using 6°Co (12 000 Curie) as a source. The dose rate was 1.88Gyh -1. From the swelling in benzene at 25 ° C, the networkchain density (v) was determined [9]. The tensile test was conducted on an Instron-type tensile tester at the strain rate of 50mm min -1. Attenuated total-reflection infrared (ATR-IR) spectra were measured on an IR-A302 of Japan Spectroscopic Co. Ltd. with the ATR-6 attachment. Fig. 1 shows the stress-strain curves of NR vulcanizates before and after 100 kGy irradiation. The stress was not much affected by the irradiation

Studies of the network structure of rubber vulcanizates by a cryoscopic method: 1

The freezing-point depression, ΔT, of swollen dicumene peroxide cured natural rubber in benzene has been measured. The parameter a H, introduced by Kuhn, which varies inversely as ΔT, should be regarded as an indication of the depression. The anomalous depression is considered to occur due to a non-frozen benzene area formed on the surrounding network chains. From comparison of the scaling law and the experimental results, it was clear that a H can reflect not only the quantity corresponding to network chain density determined by conventional methods but also the difference in three-dimensional network structure between degraded and undegraded rubber vulcanizates.

Structure and Properties of Extensively Degraded Rubber Vulcanizates

Chemical stress relaxation is a unique technique to study the degradation of vulcanized natural rubber (NR).It is well-known that the relationship between the initial stress, f(0), and the initial network chain density, n(0), is given by eq. (1).f(t)=n(t) RT(α-α-2) at t=0 (1)at t =t (2)Thus far, eq. (2) has been used for degraded vulcanized NR. However, we have assumed that eq. (2) cannot be applied in the case of extensive degradation, and introduced a new method relating the netwark chain density with the number of scission in thermo-oxidative degradation of NR.Then, we have compared the network chain density calculated from relative stress with that calculated from the number of scission based upon the thermo-oxidative degradation of uncured-NR under the same degradative condition for DCP cured-NR.The values of the network chain density determined by the above two methods agreed with each other only in the early stage of degradation. However, it was found that eq. (2) is not valid as the degradation proceeds extensively. Consequently, a correction of eq. (2) has been advocated experimentally which is represented by f(t)-C·exp{-D·f(t)}=n(t)RT(α-α-2)

Reclamation of vulcanized rubbers by chemical degradation. V. Degradation of vulcanized synthetic isoprene rubber by the phenylhydrazine–ferrous chloride system

AbstractThe changes of effective network chain density and soluble fraction were measured for gum vulcanizates of synthetic isoprene rubber (IR) during the degradation by the phenylhydrazine–ferrous chloride system. From the decrease of effective network chain density and the increase of soluble fraction, the network chain was concluded to undergo a severance reaction by the reagents in the following order of ease: polysulfide linkage > monosulfide linkage > carbon–carbon crosslink. Analysis of the relation of soluble fraction with decrease of effective network chain density according to the theory of Horikx indicated that the severance of crosslinks predominated over main‐chain scission in the degradation of sulfur vulcanizate by the reagents. The experiments of the degradation of sulfur vulcanizate in sealed vessels in the absence of oxygen indicated that atmospheric oxygen is necessary for the degradation reaction.

Diisocyanate Vulcanization of 1-Chlorobutadiene-Butadiene Rubber

Abstract TDI-d was found to react quantitatively with hydroxyl group in CB-BR under relatively mild curing conditions to give CB-BR vulcanizates, while AEP afforded vulcanizates having a certain amount of pendent AEP. The latter could be used to increase the network-chain density by postcuring with TDI. This crosslink formation was found to proceed almost quantitatively through the reaction of pendent AEP and hydroxyl groups with equimolar amounts of TDI in CB-BR under suitable curing conditions.

Kinetics of thiuram–accelerated sulfur vulcanization

On the basis of continuous measurements in a Vuremo curemeter at 145°C, kinetics of tetramethylthiuram disulfide (TMTD)-accelerated sulfur vulcanization of natural rubber has been investigated. It was found that the cure rates increase with increasing TMTD concentration, the sulfur content being kept constant, up to a TMTD:S weight ratio of 2:1. Beyond this value, the cure rates again decrease. This TMTD:S ratio corresponds to 3.8 gram atoms of sulfur per mole TMTD, and it is in good agreement with findings that in TMTD-accelerated sulfur vulcanization systems the peak value of zinc dimethyldithiocarbamate (ZnDMDC) formation reaches an endvalue when the stocks contain 4 gram atoms of sulfur per mole TMTD. These facts lead us to suppose that ZnDMDC is the actual accelerator in TMTD-accelerated sulfur systems. Support for this view derives from our experiments with model curing systems as well as from the generally known practical experience that dithiocarbamates are faster accelerators than thiuram disulfides. For the reasons described above and for the finding that the dependences of the ultimate extent of vulcanization (network chain density) on the concentration of TMTD in the absence and in the presence of elemental sulfur are analogous, the mechanism of thiuram-accelerated sulfur vulcanization is very probably similar to that of sulfur-free thiuram vulcanization.

The Determination of Network Chain Density and the Chemical Stress Relaxation of Crosslinked Polymers

Abstract Both initial network chain densities nM(0) and nS(0) of dicumyl peroxide- cured natural rubbers were determined from the tensile stress and swelling method, respectively. The difference between nM(0) and nS(0) was usually constant, independent of the magnitude of network chain density. That is, it was found that the number of entanglement network chains in the crosslinked natural rubber was usually constant, independent of network chain density. The entanglement network chain density nII(0) was 0.7×10−4 mole/cc. This led to the supposition that the molecular weight between entanglement points Me would be about 9000. Although this value is far from exact, it does not differ too greatly from the value found for noncrosslinked natural rubber. Next, in order to calculate the number of main-chain scissions of crosslinked polymers from their chemical stress relaxation, we proposed our modification of Tobolsky's equation. Using our equation, it was found that the scission of dicumyl peroxide-cured natural rubber occurred in the main chain only. Furthermore, this value agreed with the one obtained from the oxidation of toluene solution of noncrosslinked rubber under the same conditions.

Kinetics of Sulfur-Free Thiuram Vulcanization

Abstract Sulfur-free thiuram vulcanization has been investigated at temperatures from 160° to 190° C over 0.5 to 600 min. Continuous measurements in a VUREMO curemeter were used to estimate the extent of crosslinking, which was plotted against cure time. Simultaneously the values of the network chain density were calculated from swelling measurements on the vulcanizates. The cure curves show clearly an induction period (ti), then fast crosslinking, a partial degradation, a “long-time” crosslinking, and finally a slow, limited degradation. Apart from the induction period, the kinetic graphs are satisfactory represented by a rate equation assuming three independent first-order reactions: fast crosslinking, degradation, and slow crosslinking. The rate equation contains seven kinetic parameters. Over the temperature range studied, there is no difference between the values of activation energy for fast crosslinking, for degradation, for slow crosslinking, and for ti−1. Due to the presence of thiourea, the values of the induction period, the rate constant, and the extent of slow crosslinking are decreased. Simultaneously the activation energies calculated from degradation and slow crosslinking are significantly increased. On the basis of the above results, the mechanism of the sulfur-free thiuram vulcanization, in which ionic and radical reactions take place, is discussed.

The determination of network chain density and the chemical stress relaxation of crosslinked polymers

AbstractBoth initial network chain densities nM(0) and ns(0) of dicumyl peroxide‐cured natural rubbers were determined from the tensile stress and swelling method, respectively. The difference between nM(0) and nS(0) was usually constant, independent of the magnitude of network chain density. That is, it was found that the number of entanglement network chains in the crosslinked natural rubber was usually constant, independent of network chain density. The entanglement network chain density nII(0) was 0.7 × 10−4 mole/cc. This led to the supposition that the molecular weight between entanglement points (Me) would be about 9000. Although this value is far from exact, it does not differ too greatly from the value found for noncrosslinked natural rubber. Next, in order to calculate the number of main‐chain scission of crosslinked polymers from their chemical stress relaxation, we proposed our modification of Tobolsky's equation. Using our equation, it was found that the scission of dicumyl peroxide‐cured natural rubber occurred in the main chain only. Furthermore, this value agreed with the one obtained from the oxidation of toluene solution of noncrosslinked rubber under the same conditions.

Overall Kinetics of Natural Rubber Vulcanization at High Temperatures I—review of Investigation Techniques, Methods and Previous Results of Kinetic Analysis

Abstract Analysis of previously published cure data illustrates the importance of correct determination of the ultimate level of network chain density in the absence of reversion, and the correction of each kinetic data point for “reversion contributions”. A graphical procedure is described for the determination of kinetic parameters which takes these precautions into account.

Kinetics of sulfur‐free thiuram vulcanization

AbstractSulfur‐free thiuram vulcanization has been investigated at temperatures from 160° to 190°C over 0.5 to 600 min. Continuous measurements in a VUREMO curemeter were used to estimate the extent of crosslinking, which was plotted against cure time. Simultaneously the values of the network chain density were calculated from swelling measurements on the vulcanizates. The cure curves show clearly an induction period (ti), then fast crosslinking, a partial degradation, a “long‐time” crosslinking, and finally a slow, limited degradation. Apart from the induction period, the kinetic graphs are satisfactory represented by a rate equation assuming three independent first‐order reactions: fast crosslinking, degradation, and slow crosslinking. The rate equation contains seven kinetic parameters. Over the temperature range studied, there is no difference between the values of activation energy for fast crosslinking, for degradation, for slow crosslinking, and for t. Due to the presence of thiourea, the values of the induction period, the rate constant, and the extent of slow crosslinking are decreased. Simultaneously the activation energies calculated from degradation and slow crosslinking are significantly increased. On the basis of the above results, the mechanism of the sulfur‐free thiuram vulcanization, in which ionic and radical reactions take place, is discussed.