Interfaces in Composites. Reinforcement of Elastomers by Carbon Black

Basic features of carbon black-aggregation of particles into structure, particle size and morphology, and surface activity-are reviewed. Carbon black reinforcement of vulcanizates is first examined in the example of tearing, and the influence of hysteresis is considered. The dynamic properties of vulcanizates containing two major types of reinforcing carbon black are compared.

While particle size gives the best correlation with tensile strength of vulcanizates, surface activity is shown to be the key to reinforcement. The role of these properties of carbon black in dissipating rupture energy is discussed.

The relation between work to tensile break and hysteresis to break in gum rubbers can be applied to black-reinforced vulcanizates by use of a strain amplification factor. The complication introduced by stress-softening is explained in terms of localized stress relaxation. Abrasion reinforcement can also be explained in terms of hysteresis.

The Flory-Rehner relationship of modulus of elasticity of swollen vulcanizates to physically-effective cross-linking applies to unswollen vulcanizates only after prestressing. Black-reinforced vulcanizates involve application of the strain amplification factor.

The concept of mobile linkages to rubber chains at the surface of black particles is related to the influence of strain magnification and strain rate magnification in the reinforcing mechanism. These linkages result in formation of “shell” rubber adjacent to carbon black particles. The slippage of rubber chains relative to carbon black aggregates allows stress-sharing by highly-stressed chains. Bound rubber results from reaction of elastomer free radicals generated during mastication with carbon black.

There is a relation between bound rubber and reinforcement which is fully developed only after vulcanization. Formation of bound rubber results from the surface activity of carbon black rather than its structure. Its contribution to reinforcement of the vulcanizate may be as important as cross-linking.     

The elasticity of nematic networks

Nematic rubbers are composed of crosslinked polymer chains with stiff rods either incorporated into their backbones or pendant as side chains. When nematic effects axe strong, such rubbers exhibit discontinuous stress-strain relationships and spontaneous shape changes. We model such a rubber using Gaussian elasticity theory, including the nematic interaction via a mean field. Results are presented for the cases of uniaxial extension and compression. Under uniaxial extension the rubber can undergo a first order phase transition to a uniaxial nematic phase. Under uniaxial compression first or second order transitions are possible to genuinely biaxial nematic states with biaxial strains. When nematic effects are very small (i.e. T >> T_c where T_c is the nematic-isotropic phase transition temperature of the rubber) we postulate that the model is a good approximation to a conventional, non-nematic elastomer, and fit our model to data from an isoprene rubber.     

Stress–strain behavior in extension of elastomer networks with crosslinks of different lenghts

Stress–strain behavior in extension and the swelling of polymer networks with different lenghts of crosslinks is reported. These networks were prepared by copolymerization of butyl acrylate with different amounts of various difunctional comonomers which yield crosslinks of 4, 7, 10, and 16 bonds in length. The efficiency of the comonomers in crosslinking is low, improving with increasing length of the chain between their unsaturated endgroups. Analysis of a large number of stress–strain data obtained at elongations between 2 and 8% elongation showed that in this deformation range the stress–strain relation based on the statistical theory of elasticity represents the data better than does Hooke's law or the Mooney-Rivlin relation. It was found that the relation between the modulus at small deformations and the swelling ratio of the various samples inindependent of the length of the crosslinks. Also the shapes of the Mooney-Rivlin curves are the same for all networks. Furthermore, the creep behavior of various networks with different crosslink lengths is the same for networks compared at the same elastically effective chain concentration. It is concluded that the lenght of the crosslinks, at least up to 16 bonds, does not affect the elastic response of polymer networks.     

Mixed Cross-Link Systems in Elastomers

Rubbers are long-chain molecules which are plastic in nature in the raw or unvulcanized state. Vulcanization is an irreversible process by which the predominantly plastic rubber is converted to predominantly elastic and a three-dimensional network structure through the anchoring between two polymer chains. Chemically, this is done by an intermolecular cross-linking reaction. Cross-linking increases the retractive force and reduces the amount of permanent deformation remaining after removal of the deforming force. These links between polymer chains may be chains of sulfur atom or atoms, carbon-carbon bonds, polyvalent metal ions, etc., depending on the nature of the vulcanizing system. Properties of elastomers depend on how efficiently this cross-linking has been achieved and also which types of cross-linking agents are used. For example, the retractive force resisting a deformation is proportional to the number of network supporting polymer chains per unit volume of elastomers [1]. Again the strength of elastomers depends on the stress relaxation mechanism in the cross-linked material [2]. The modulus of a vulcanizate is proportional to the number of cross-links formed, while the tensile strength normally passes through a maximum with an increase in the number of cross-links. The resilience, heat build-up, and fatigue properties depend on the chemical nature of the cross-links and on the chemical structure of the base polymer [3]. Different types of cross-links have both advantages and disadvantages with respect to technical properties. To cite one example, a sulfur cross-linking system produces good tensile strength but poor aging properties. On the other hand, carbon-carbon cross-links produce good aging properties but poor tensile properties. Thus there are reasons for using a mixed cross-link system in order to obtain the right compromise.     

Relation of elastic modulus to crosslink and entanglement concentrations in rubber networks

The theory of rubber elasticity relates the elastic modulus of unfilled amorphous rubber to the concentration of elastically effective strands. A theoretical relation between this concentration and the concentrations of potential entanglements, random tetrafunctional crosslinks, and chain ends was proposed recently. In the present work, the new relation was combined with the theory of rubber elasticity and verified experimentally. Polydimethylsiloxane samples were cured by ⁶⁰Co irradiation and were extensively extracted to determine gel fraction, which was used to calculate concentrations of crosslinking and scission due to irradiation. Equilibrium modulus values determined from creep tests were in excellent agreement with those calculated using the new relation if the average spacing between potential entanglements is 116 (CH₃)₂SiO units. Thus, in typical commercial silicone rubbers, the contribution to the modulus from trapped entanglements is greater than the direct contribution from crosslinks. The new relation allows the calculation of crosslink concentrations from modulus measurements on other unfilled rubbers once the potential entanglement spacing of the polymer is determined.     

Thermal stability and flammability of butadiene- acrylonitrile rubber cross-linked with iodoform

Summary The paper discusses the results of thermal analysis and flammability of butadiene-acrylonitrile rubber, Perbunan NT 1845 of Bayer, cross-linked with iodoform. The properties of the iodoform vulcanizate have been compared with those of peroxide vulcanizate. The thermal analysis has been performed in air with use of a derivatograph under air and nitrogen atmosphere as well as dynamic scanning calorimetry (DSC). The flammability of vulcanizates has been determined by the method of oxygen index and in air. The toxicity of the thermal decomposition and combustion products of the vulcanizates under investigation has been also determined. Based on complementary examinations, DTA and DSC curves have been interpreted from the point of view of thermal transitions of the conventionally and non-conventionally cross-linked nitrile rubbers. The glass transition temperature of the cross-linked polymer both in cooling and heating has been determined.     

The role of the orientation tensor in the rheology of flexible polymers

The stress-strain relations for transversely isotropic deformations of linear and branched polymer melts as well as of (crosslinked) rubbers are discussed in terms of the orientation tensor. It is shown that orientation and network strand extension are decoupled, and that the relative tube diameter and its inverse, the molecular stress function f, can be extracted directly from experimental data, if the effect of network orientation is accounted for by the order parameter. The tension of the average network strand increases with increasing deformation. This is caused by an increasing restriction of lateral movement of polymer chains due to deformation. At small strains, f² is found to be linear in the average stretch for melts as well as for rubbers, which corresponds to a constant volume of the tube. At large strains, melts show maximum molecular tension, depending on the degree of longchain branching, while rubbers show maximum extensibility.     

Mechanical characteristics of hydrogenated natural rubber vulcanizates

Mechanical properties of partially hydrogenated natural rubber (HNR) vulcanizates were evaluated regarding their chemical structure and crystallizable nature of HNR, and are reported here, to the best of our knowledge, for the first time. HNRs of three levels of hydrogenation (20.6, 29.0, and 40.6 mol%) were successfully prepared by the chemical modification of natural rubber (NR) latex using N₂H₄ and H₂O₂ as reagents, in a sufficient amount for preparing sulfur‐crosslinked samples to be subjected to mechanical and structural measurements. The three HNR vulcanizates were found to be crystallizable upon stretching; it is noted that even 40.6 mol% hydrogenation did not prevent HNR vulcanizates from crystallization upon stretching, while their onset strain of crystallization was higher than that of NR vulcanizate. The hysteresis loss and residual strain up to a stretching ratio of 2 for the HNR vulcanizates tended to become larger with the increase in the degree of the hydrogenation. Tensile and dynamic mechanical properties of 20.6 mol% hydrogenated HNR vulcanizate were comparable to those of NR vulcanizate. From differential scanning calorimetry and temperature dispersion of dynamic modulus or loss, the glass transition temperatures of HNR vulcanizates were found to be almost the same as that of NR vulcanizate, which is also notable. The thermal stability of HNR vulcanizates was better than that of NR vulcanizate. Thus, this chemical modification seems to give a promising NR derivative whose properties can be equivalent or even better than the mother polymer. Copyright © 2008 John Wiley & Sons, Ltd.     

Elastic behavior of single polymer chains adsorbed on rough surfaces

In this paper, elastic behaviors of single polymer chains adsorbed on the rough surfaces with a substrate and some periodically tactic pillars are investigated by the pruned-enriched-Rosenbluth method (PERM). In our simulation, a single polymer chain is firstly adsorbed on the substrate and then pulled along the z-axis direction, which is vertical to the substrate. We investigate the chain size and shape of polymer chains, such as mean-square radii of gyration per bond 〈S²〉_xy/N, 〈S²〉_z/N and shape factor 〈δ^∗〉 in order to show how the size and shape of adsorbed polymer chains change during the desorption process. Due to the occurrences of separation of the chains from the substrate, farther adsorption on the upper surfaces of pillars and complete separation from the whole rough surfaces in the elastic process, the changes of 〈S²〉_xy/N, 〈S²〉_z/N and 〈δ^∗〉 during the process are complicated. On the other hand, some thermodynamic properties such as average energy per bond, average Helmholtz free energy per bond, elastic force f are investigated, and our aim is to study the elastic behaviors of polymer chains adsorbed on the rough surface during the elasticity process. Elastic force f has some plateaus during the desorption process for strong adsorption interaction. If there is no adsorption interaction, the chains can get away from the rough surfaces spontaneously. These investigations can provide some insights into the elastic behaviors of polymer chains adsorbed on the rough surface.     

Elastic properties and network structure in silicone gum vulcanizates

A model of a rubber network formed from finite, linear polymer chains is treated, using expressions from accepted elastic theory as well as relationships that have been less generally used or only implied. The resultant empirical mathematical expressions conform very closely to the predictions of the simple kinetic theory of rubber elasticity and are mathematically consistent, in that the major properties and measurements are interrelated, and apparent inconsistencies in the literature are reconciled. As a consequence, an empirical equation is developed for the relationship between elastic properties and network structure in silicone gum vulcanizates. This equation is then applied to the estimation of peroxide crosslinking efficiencies.     

A new attempt to reconcile the statistical and phenomenological theories of rubber elasticity

The statistical and phenomenological theories of rubber elasticity are reviewed briefly. Combining recent concepts proposed by Yeoh and Gent, a new theory is proposed. The proposed constitutive model for rubber vulcanizates invokes two mechanisms; one influences behavior at small strains while the other dominates behavior at large strains. Network flaws, such as entanglements, are suspected to be responsible for the first mechanism. Finite extensibility of network chains is identified as the cause of the second. Thus, macroscopic behavior is directly linked to molecular concepts. The proposed theory allows prediction of the stress–strain behavior of a family of four rubber vulcanizates in different modes of deformation (simple extension, compression, and simple shear) from regression analysis of tensile data alone from just one member. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1919–1931, 1997     

The effect of the nature of cross-links on stress-relaxation and elastic deformations of α-methyl styrene-butadiene vulcanizates

Six systems were employed to vulcanize α-methyl styrene-butadiene rubber SKMS-30, viz. sulphur-MBTS + DPG, sulphur-DEBTSA, sulphur-Zn dimethyl dithiocarbamate, sulphur-MBT, TMTD. and p-tert-butylphenol-formaldehyde resin-chlorosulphonated polyethylene. The resulting vulcanizates had good physicomechanical properties.The natures of the network structures formed by these vulcanizing systems were evaluated by stress-relaxation experiments at 130 in air. TMTD and phenolic resin form the most rigid networks. while sulphur-MBTS + DPG forms the most flexible one.Rubber samples vulcanized with different systems were subjected to a compression of 20% of the initial sample height for fixed periods at 100 and 130°. The instantaneous and retarded elastic recoveries were determined. The relation between the nature of the network structure and elastic recoveries of the rubber vulcanizate are discussed.     

Rheological properties of ethylene-propylene-diene elastomers

The processing of rubbers comprises a combination of complicated processes which realization almost always requires knowledge of their rheological behavior. The goal of the present work is to describe the rheological behavior of the EPDM vulcanizates via rotational rheometry. The effect of the cross-linking density and plasticizer content on the rheological behavior of the EPDM vulcanizates is described. The viscoelastic characteristics of the samples are defined from the frequency dependences of elasticity modulus G' and loss modulus G".     

Dynamic mechanical properties of cross-linked rubbers. V. An approximate analysis of the modulus contributions of trapped and untrapped entanglements

Using the criterion that a coupling entanglement in a polymer network is trapped if all four strands radiating from it terminate in chemical cross-links (an approximation to a more rigorous treatment of Langley), an equation is derived relating the equilibrium modulus to the magnitude of the compliance in the frequency region where all coupling entanglements, whether trapped or not, contribute to the elasticity. The latter value is estimated from the storage compliance at the frequency where the storage compliance of the uncross-linked polymer corresponds to the entanglement compliance derived from integration over the loss compliance. The theory agrees rather well with data on vulcanizates of natural rubber. For 1,4-polybutadiene and styrene–butadiene rubbers, the agreement is somewhat less satisfactory, but the results support the hypothesis that the low-frequency losses observed in lightly cross-linked rubbers are due to relaxation of untrapped entanglements.     

Large deformation rheology of gelatin gels

We studied the linear and the non-linear elastic behaviour, the breaking stress and breaking strain of gelatin gels as a function of a number of experimental conditions: gelatin concentration, gelatin bloom value, ageing time, ageing temperature, pH, NaCl and CaCl₂ concentration, whey protein concentration, the amount of pre-shearing, strain rate or compression speed, using both shear deformation and compression. We analyzed the stress-strain curves using the BST-equation (Blatz et al., Trans. Soc. Rheol. 18, (1974) 145) and extracted a parameter that characterizes the linear elastic behaviour at small deformations (the moduli E or G) and one that characterizes the non-linear elastic behaviour at large deformations (the elasticity parameter n). The phenomenological BST equation describes rheological experiments adequately both in shear deformation and in compression.We found that the modulus correlates with the breaking stress. For the non-linear elastic properties of gelatin, we found that the elasticity parameter n correlates with the breaking strain Qualitatively, the non-linear elastic properties can be explained by assuming that the gelatin chains are partially in a crystalline triple helix state (the cross-links) and partially in a random coil state (the network bonds): the more extensive the rigid cross-link regions, the shorter and more stretched the network bonds become as a result of an externally applied deformation. The network bonds behave as anharmonic springs under extreme extension.Manipulation of the breaking strain was attempted in two ways: (i) by changing the (non-linear) elasticity parameter of the gel: this is possible by using a gel that has been further aged; (ii) by adding defects to the gel structure: this is possible by either pre-shearing the gel or by adding whey protein particles. The pre-shearing gives rise to a temporary effect, the addition of whey protein particles to a permanent effect.     

Effect of the spatial network structure and cross-link density of diene rubbers on their thermal stability and fire hazard

The paper presents the results of investigating the effect of the density and spatial network structure of diene rubbers (NBR and SBR) on their thermal properties and fire hazard. The rubbers were either conventionally cross-linked by means of sulfur and organic peroxide or nonconventionally with the use of iodoform (CH₃I). Based on thermo-kinetic analysis, the destruction activation energy of the elastomers investigated and their vulcanizates was determined under air and inert gas. The analysis of particular stages of their thermal decomposition was also presented. During the combustion of the elastomeric materials obtained, it has been found that their fire hazard depends not only on the elastomer chemical structure but also on the method of its cross-linking.     

Thermal stability of acrylonitrile/chlorosulphonated polyethylene rubber blend

The properties of chlorosulphonated polyethylene (CSM) rubber, acrylonitrile rubber (NBR) and their blend (50/50 w/w) were studied. Fourier transform infrared (FTIR) studies supported that CSM/NBR rubber blend is self curable, when cross-linking takes place between acrylonitrile groups of NBR and –SO₂Cl groups or in situ generated allyl chloride moieties of CSM. The thermal stability of vulcanizates was analyzed in nitrogen by thermogravimetry. It was found that the initial degradation temperature of elastomer based on CSM rubber is lower than of pure NBR rubber. By adding NBR to CSM rubbers, the degradation temperature of crosslinked material increased, indicating higher thermal stability. The activation energy for the degradation are determined using the Arrhenius equation The activation energies for the rubber blends are higher than for elastomers based on pure rubbers. It was found that the mass loss of the blends at any temperature was between those of the pure rubbers. The differential scanning calorimetry (DSC) was used for the glass transition temperature determination. It is estimated thermodynamic immiscibility of NBR/CSM blend based on noticed two different glass transition temperatures, corresponding to CSM and NBR rubbers.     

Effect oftrans-polyoctenamer on thermal stability of rubbers determined by thermal analysis

An effect of a cyclic low molecular-weight polymertrans-polyoctenamer rubber (TOR) on the thermal stability of diene rubbers and their vulcanizates was investigated. The investigation was carried out in the air and nitrogen atmospheres using thermogravimetry, DSC and simultaneous thermoanalytical methods. The thermal stability indexes:T ₅,T _max and activation energy of degradation (E), as well asT _g andT _m values, have been determined.It was found thattrans-polyoctenamer (TOR) increases of the thermal stability indices of raw diene rubbers and their vulcanizates. The results show that the thermal degradation of diene rubbers occurred at higher temperature if they were blended with TOR. In our opinion, intermolecular structures formed between the cyclic low-molecular weight polymer and some linear rubber molecules may be the reason for the higher thermal stability of these rubber blends.The work was supported by State Committee for Research, Poland. Grant No. 7.T08 E 032-08.     

Structural-phenomenological simulation of the mechanical behavior of rubbers

It is hypothesized that, during deformation of rubbers, polymer chains slip off the layers at filler particles into voids between inclusions and high-strength polymer fibers in the uniaxially oriented state are formed in the voids. As a result, the macroscopic strength of elastomers increases by an order of magnitude and the elongation at break simultaneously increases relative to the unfilled elastomer. Aggregates of carbon black particles that occur close to one another in the initial sample depart to very large distances upon stretching the material. The fibers that tie the aggregates must extend their length by a factor of a few tens in this case. A mathematical model that takes into account these processes is proposed. It was shown that the set of constitutive equations makes it possible to simulate with good accuracy both the viscoelastic behavior of rubbers and the Mullins softening effect under finite strain conditions.     

Sulphur and peroxide vulcanisation of rubber compounds — overview

Vulcanisation is a process of transforming a plastic rubber compound into a highly elastic product by forming a three-dimensional cross-linked network structure in the rubber matrix. Many systems have been developed to vulcanise rubber compounds, among which sulphur and peroxide curing systems remain the most desirable. The application of sulphur systems leads to the forming of sulphidic cross-links between elastomer chains, while carbon–carbon bonds are formed in peroxidecuring. Both vulcanisation systems provide certain benefits to the cross-linked rubber articles, but also some disadvantages. The present work seeks to provide an overview on both vulcanisation systems; their composition, possibilities of their application, reaction mechanisms, structure of the cross-links formed and the main feature of the final cross-linked materials — vulcanisates.