Blends elastomers

Blends comprised of amorphous, low Tg polymers are of primary interest for elastomeric type applications, of which the large tire market commands considerable interest. This section will consider blends of elastomeric polymers, generally low Tg, amorphous blends. In specific cases, low modulus, crystalline polymer blends (such as ethylene copolymers) with other elastomeric materials will be included. Also blends containing crystalline polymer, where the primary component of the blend is the elastomeric component and the blend is considered an elastomeric material, will be discussed. Specifically, dynamic vulcanized blends such as polypropylene/ethylene-propylene rubber blends will be included in this section. 

The majority of elastomer blends are phase separated, but of interest, as crosslinking can achieve mechanical compatibilization due to crosslinking between the phases as noted in Chapter 3. This compatibilization method can lead to unique and useful blends with a compromise in properties, offering useful commercial products as well illustrated by the applications in tire construction. The unsaturated hydrocarbon elastomers without polar functional groups are rarely miscible with each other, because no specific interactions are present to achieve the necessary thermodynamic driving force for miscibility. The few miscible examples noted generally exhibit matched solubility parameters. 

Polybutadiene (PB) can exist in many forms (cis-1,4 trans-1,4 vinyl-1,2) with TgS varying between—110 °C and—25 °C. The phase behavior of high l,2PB(Tg = —25°C) (also referred to as poly(vinyl ethylene)) and cis-1,4 PI (polyisoprene) was noted to exhibit single Tg behavior 

Additional examples of miscible elastomer blends are given in Table 4.1. 

PB(cis-l,4) SBRQSRISOO) Miscibility observed by dynamic mechanical and dielectric loss measurements 31 

The extension of thermodynamics to a blend of elastomers has been discussed by Roland [4], Miscible blends are most commonly formed from elastomers with similar three-dimensional [7] solubility parameters. An example of this is blends from copolymer elastomers (e.g., ethylene-propylene or styrene-butadiene copolymers) from component polymers of different composition, microstructure, and molecular weights. When the forces between the components of the polymer blend are mostly entirely dispersive, miscibiUty is only achieved in neat polymers with a very close match in Hansen s three-dimensional solubility parameter [7]. 

Miscible blends of elastomers differ from corresponding blends of thermoplastics in two important areas. First, the need for elastic properties require elastomers to be high molecular weight polymers with a limited polydisper-sity. This reduces the miscibility of dissimilar elastomers by interdiffusion of the low molecular components of the blends. Second, elastomers are plasticized in the conventional compounding with process oils. The presence of plasticizers leads to a higher free volume for the blend components and stabilizes, to a small extent, blends of dissimilar elastomers. 

The formation of miscible rubber blends slows the rate of crystallization [8a, b] when one of the components is crystallizable. This phenomenon accounts for data that shows lower heats of fusion that correlate to the extent of phase homogeneity [9] in elastomer blends. Additionally, the melting behavior of a polymer can be changed in a miscible blend. The stability of the liquid state by formation of a miscible blend reduces the relative thermal stability of the crystalline state and lowers the equilibrium melting point [10a, b]. This depression in melting point is small for a miscible blend with only dispersive interactions between the components. 

The principal effect of miscibility of elastomer blends of dissimilar elastomers is alteration of the glass-transition temperature. Since miscible blends should have negligible changes in the conformation of the polymer chains, the entanglement density of miscible blends should be a compositionally weighted average of entanglement density of the pure components. 

Nuclear magnetic resonance (NMR) has been applied to the study of homogeneity in miscible polymer blends and has been reviewed by Cheng [11a] and Roland [11b]. When the components of a blend have different Tg s, proton NMR can be used to assess the phase structure of the blend by taking advantage of the rapid decrease of proton-proton coupling with nuclear separation [lie]. For blends containing elastomers of almost identical Tg, proton MAS NMR is applied to blends where one of the components is almost completely deuterated [12], Another technique is crosspolarization MAS NMR [13], The transfer of spin polarization from protons to the atoms of 

The Science and Technology of Rubber. http //dx.doLorg/10.1016/B978-0-12-5945 4-6.00012-l 2013 Dsevier Inc. AH rights reserved 

In addition to changes and variation in morphology, immiscible blends show additional, more complex changes due to different chemical properties of the two-component elastomer. This difference in the chemical structure manifests as three distinct but interrelated properties. First, the dissimilar elastomers differ in the retention of the fillers (e.g., carbon black) and plasticizers (e.g.. 

These concepts for formation of miscible blend of elastomers with similar or near equivalence of solubility parameters require the components to be similar in properties. Thus a wide variation in the properties of the elastomer blends by changing the relative amounts of the two elastomers is not typical since it is unlikely that, for example, a nonpolar polyolefin elastomer and a polar elastomer like acrylate would be similar in solubility parameters. This relative invariance in the properties of the blend compared to the components is an inherent limitation on the basic, economic, and technological need for elastomer blends, which is to generate new properties by blends of existing materials. Similar or near equivalence of solubility parameters can be difficult to predict from chemical structure. For example, chemically distinct 1,4-polyisoprene and 1,2-polybutadiene are miscible, but isomeric 1,2-polybutadiene and 1,4-polybutadiene are immiscible. It is illustrative of this concept that an apolar hydrocarbon elastomer and a highly polar elastomer such as an acrylate cannot have, under any practical structural manifestation for either, a similar solubility parameter and thus be miscible. 

Multiple notable reviews of elastomer blends exist. The first general treatment of the subject by Hess et al. (1993) reviews the applications, analysis, and the properties of the immiscible elastomer blends. Two related treatments by Roland (1989) and Ngai and Roland (2004) exist and describe the physics of mixing immiscible polymer blends and a more recent account of the analytical methods. Mangaraj (2002) has a more detailed review of elastomer blends. Other reviews by Corish (1978) and McDonel et al. (1978) deal with specific aspects of elastomer blends. A publication by Zhang (2009) on specific EPDM blends with NR/BR for tire sidewall approaches this area from the view of a specific application. Less comprehensive accounts of this area are also described for polyolefin elastomer blends by Slusarski et al. (2003) and Feldman (2005). 

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Ester plasticizers are used mainly in very polar elastomers, such as neoprene and nitrile mbber, to improve low or high temperature performance or impart particular oil or solvent resistance to a compound 5—40 parts are commonly used (see Plasticizers). Resins and tars are added to impart tack, soften the compound, improve flow, and in some cases improve filler wetting out, as is the case with organic resins in mineral-filled SBR. Resinous substances are also used as processing agents for homogenizing elastomer blends. 

Tackifying resins enhance the adhesion of non-polar elastomers by improving wettability, increasing polarity and altering the viscoelastic properties. Dahlquist [31 ] established the first evidence of the modification of the viscoelastic properties of an elastomer by adding resins, and demonstrated that the performance of pressure-sensitive adhesives was related to the creep compliance. Later, Aubrey and Sherriff [32] demonstrated that a relationship between peel strength and viscoelasticity in natural rubber-low molecular resins blends existed. Class and Chu [33] used the dynamic mechanical measurements to demonstrate that compatible resins with an elastomer produced a decrease in the elastic modulus at room temperature and an increase in the tan <5 peak (which indicated the glass transition temperature of the resin-elastomer blend). Resins which are incompatible with an elastomer caused an increase in the elastic modulus at room temperature and showed two distinct maxima in the tan <5 curve. 

All resin-elastomer blends show a similar variation in tack as a function... 

The chemical nature of the tackifier also affects the compatibility of resin-elastomer blends. For polychloroprene (a polar elastomer) higher tack is obtained with a polar resin (PF blend in Fig. 27) than with a non-polar resin (PA blend in Fig. 27). Further, the adhesion of resin-elastomer blends also decreases by increasing the aromatic content of the resin [29]. Fig. 28 shows a decrease in T-peel strength of styrene-butadiene rubber/polychloroprene-hydrocarbon resin blends by increasing the MMAP cloud point. Because the higher the MMAP... 

Addition of low molecular weight resins with narrow molecular weight distribution produces compatible resin-elastomer blends, while incompatible blends are obtained with resins having a wide molecular weight distribution. In a recent study... 

Another area of recent interest is covulcanization in block copolymers, thermoplastic rubbers, and elasto-plastic blends by developing an interpenetrating network (IPN). A classical example for IPN formation is in polyurethane elastomer blended acrylic copolymers [7]. 

A. Elastomer-Elastomer Blends with Dissimilar Cure Rate... 

The oldest technology involved in the elastomer blending and vulcanization process is essentially a temperature controlled two roll mill as well as internal mixers followed by an optimum degree of crosslinking in autoclave molds (compression, injection, etc.) in a batch process or in a continuous process such as continuously heated tube or radiated tubes. A few examples of laboratory scale preparation of special purpose elastomeric blends is cited here. 

Figure 11 Polarized microscope photographs ( x 800) (A) PBT-TLCP-elastomer blend (60 25 15 wt ratio). Dark phase is the TLCP phase. (B) Nylon 6-TLCP-elastomer blend (60 25 15 wt ratio). Dark phase is the TLCP phase and large white one is the elastomer phase. Source Ref. 56.

Figure 12 SEM photographs of fractured surfaces (X1000). (A) PBT-TLCP-elastomer blend. (B) Nylon 6-TLCP-elastomer blend. Source Ref. 56.

Compatibilization along with dynamic vulcanization techniques have been used in thermoplastic elastomer blends of poly(butylene terephthalate) and ethylene propylene diene rubber by Moffett and Dekkers [28]. In situ formation of graft copolymer can be obtained by the use of suitably functionalized rubbers. By the usage of conventional vulcanizing agents for EPDM, the dynamic vulcanization of the blend can be achieved. The optimum effect of compatibilization along with dynamic vulcanization can be obtained only when the compatibilization is done before the rubber phase is dispersed. 

Electron and optical microscopes are being used to see blend homogeneity. Elastomer-plastic blends are somewhat easier to identify than elastomer-elastomer blends because normal staining techniques, e.g., osmium tet-raoxide, can be used in the case of plastic-elastomer blends. Normally, there are two methods that are followed for examining the blend surface by electron microscopy. 

It is possible to distinguish between SBR and butyl rubber (BR), NR and isoprene rubber (IR) in a vulcan-izate by enthalpy determination. In plastic-elastomer blends, the existence of high Tg and low Tg components eases the problems of experimental differentiation by different types of thermal methods. For a compatible blend, even though the component polymers have different Tg values, sometimes a single Tg is observed, which may be verified with the help of the following equation ... 

The results obtained by Kuila et al. and Acharya et al. [63,64] from the EVA elastomer blended with lamellar-like Mg-Al layered double hydroxide (LDH) nanoparticles demonstrate that MH nanocrystals possess higher flame-retardant efficiency and mechanical reinforcing effect by comparison with common micrometer grade MH particles. Kar and Bhowmick [65] have developed MgO nanoparticles and have investigated their effect as cure activator for halogenated mbber. The results as shown in Table 4.2 are promising. 

Kresge E.N., Polyolefin thermoplastic elastomer blends. Rubber Chem. TechnoL, 64, 469, 1991. 

Since most polymers, including elastomers, are immiscible with each other, their blends undergo phase separation with poor adhesion between the matrix and dispersed phase. The properties of such blends are often poorer than the individual components. At the same time, it is often desired to combine the process and performance characteristics of two or more polymers, to develop industrially useful products. This is accomplished by compatibilizing the blend, either by adding a third component, called compatibilizer, or by chemically or mechanically enhancing the interaction of the two-component polymers. The ultimate objective is to develop a morphology that will allow smooth stress transfer from one phase to the other and allow the product to resist failure under multiple stresses. In case of elastomer blends, compatibilization is especially useful to aid uniform distribution of fillers, curatives, and plasticizers to obtain a morphologically and mechanically sound product. Compatibilization of elastomeric blends is accomplished in two ways, mechanically and chemically. 

Source Aijunan, P., Technological Compatibilization of Dissimilar Elastomer Blends Part 1. ... 

Elastomer blends in which the components react (reactive blending) with each other provide the best route to obtain a homogeneous product with improved physicals. The negative free energy... 

Research concerning nylon-elastomer blends has mostly focused on the improvement of mechanical and thermal properties. Their dynamic mechanical properties are quite important both for processing and engineering applications. Wang and Zheng have smdied the influence of grafting on the dynamic mechanical properties of a blend based on nylon 1212 and a graft... 

Aijunan, P. Technological Compatibilization of Dissimilar Elastomer Blends Part 1. Neoprene and Ethylene o-Propylene Rubber Blends for Power Transmission Belt Application. Rubber Division, Proceedings of the American Chemical Society, Nashville, TN, Sept. 29-Oct. 2, 1998, Paper No. 52, 1-28. 

Wang, W. and Zheng, Q. The Dynamic Rheological Behavior and Morphology of Nylon/Elastomers Blends, J. Mater. Set Lett. 40, 2005. 

In fact viscosity reduction of diene elastomer blends with QDI show an optimum based on mixer discharge temperature. Figure 16.13 shows the results of two experiments done on the 16 L scale. The viscosities represent the viscosity of the fourth stage of a multistage mixing experiment. These compounds were 60/40 blends of either BR or SBR with NR and contained 50 phr of N-234 carbon black. 

Galuska, A.A., Poulter, R.R., and McElrath, K.O., Eorce modulation AEM of elastomer blends Morphology, fillers and cross-hnking. Surf. Interface Anal., 25, 418, 1997. 

Silicones are frequently used in transdermal drug delivery. Recently, the use of loosely cross-linked silicone elastomer blends for this application was surveyed.537 The mechanisms of controlled drug release in the silicone-based systems have been studied,538 as silicones are evaluated for relatively new protein drug-delivery systems.5... 

Smith, J. M. Thomas, X. Gantner, D. C. Lin, Z. Lossely Cross-Linked Silicone Elastomer Blends and Topical Delivery. In Advances in Controlled Drug Delivery-, Dinh, S. M., Lin, P., Eds. ACS Symposium Series 846 American Chemical Society Washington, D.C, 2003 ... 

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