Selective wetting and dispersion of filler in rubber composites under influence of processing and curing additives

The present work highlighted the effect of commonly used processing and curing additives on the wetting and dispersion kinetics of filler like silica and carbon black (CB) in some examples using the methods like the wetting concept and online measured electrical conductance. The adsorption of additives and mono-functional silane on silica surface increases the wetting speed of silica in single compound of nitrile butadiene rubber (NBR), natural rubber (NR) and styrene butadiene rubber (SBR) compounds. In rubber blend, for instance NBR/NR, the extent of filler surface fraction wetted by each blend component is strongly dependent on the additive/silica and silane/silica ratio r. A model based on the surface tension data of rubber components and filler (Z-model) was used for prediction of the selective filler wetting at a thermodynamic equilibrium state. By combining the experimental results from the wetting concept and theoretical prediction from the Z-model the silica surface tension changed during mixing can be characterized. It quantitatively describes the deactivation of the silanol groups on the silica surface by adsorbed additives. The effect of adsorption of additives on filler dispersion was exemplarily demonstrated on CB filled SBR compounds by means of the method of online measured electrical conductance. The influence of additives on the CB dispersion in low styrene-content SBR mixtures is much more pronounced than that in high styrene-content SBR mixtures.     

Largely restricted nucleation effect of carbon nanotubes in a miscible poly(vinylidene fluoride)/poly(butylene succinate) blend

The crystallization behaviors of miscible poly(vinylidene fluoride)/poly(butylene succinate) (PVDF/PBS) and its blend composites with carbon nanotubes (CNTs) during non‐isothermal and isothermal processes were investigated. The results showed that CNTs acted as heterogeneous nucleation agents and further improved the nucleation ability of PBS and PVDF in blends. However, compared with the nucleation effects of CNTs in PBS/CNT or PVDF/CNT binary composites, the nucleation effect of CNTs in miscible PVDF/PBS was largely restricted and nucleation efficiency was lowered. A reasonable explanation about the restricted nucleation ability of CNTs was studied from the viewpoint of interfacial interactions between polymer components and CNTs, in which a preferential affinity of CNTs to PBS was found. Further combined with the preparation method, it is proposed that PVDF chains adsorbed on the CNT surface in the master batch were peeled off from the CNTs by incorporated PBS chains, due to the better interaction between PBS and CNTs. Finally, the PVDF chains at the interface were diluted by PBS, and most of the CNT surface was covered by PBS chains, giving rise to the nucleation of PBS on the CNTs. On the other hand, unremoved PVDF still adsorbed on the CNT surface and crystallized. Compared with PVDF/CNT and PBS/CNT binary composites, the nucleation density in the ternary composites was greatly lowered, resulting in restricted nucleation effects of CNTs. On the other hand, the preferable adsorption of PBS on CNTs induced an apparent phase fluctuation in the PVDF/PBS blend composites, which also reflected the selective adsorption of PBS on the CNT surface. © 2016 Society of Chemical Industry     

Location of dispersing agent in rubber nanocomposites during mixing process

In the present work, the development of morphology and selective wetting of nanoclay and carbon nanotubes (CNTs) in rubber nanocomposites were characterized qualitatively by means of the optical microscopy, TEM and AFM and quantitatively by means of the wetting concept. Carboxylated hydrogenated nitrile butadiene rubber (XHNBR), ionic liquid and ethanol were used as dispersing agent and they show very good effect on the macro- and microdispersion of nanofillers in different rubbers. It was found that the selective wetting of filler surface by the dispersing agent and rubber matrix is controlled by thermodynamic and kinetic factors. A model basing on surface energy data of polymer components (rubber and dispersing agent) and filler was introduced in order to determine the thermodynamic equilibrium state of filler wetting, which is found to be simultaneously determined by the filler–polymer affinity and the rubber/dispersing agent mass ratio. During the mixing process a replacement process of bound polymer components takes place on the filler surface until the predicted state is reached.     

Kinetics of the phase selective localization of silica in rubber blends

The Fourier transformed infrared (FTIR) spectroscopy on the rubber‐filler gel has been used as a tool for the quantitative characterization of the phase selective silica localization in styrene butadiene rubber (SBR)/natural rubber (NR) blends. The so‐called rubber‐layer L was introduced to describe the selective wetting behavior of the rubber phases to the filler. SBR/NR blends filled with silica were the focus of the experimental investigation. NR shows a higher wetting rate than SBR. Silane addition does not affect the wetting of NR but slowdowns the wetting of SBR. With increasing chamber temperature the value of the rubber‐layer L of all mixtures increases owing to the different thermal activated rubber‐filler bonding processes. Using the wetting concept the kinetics of silica localization in the phases of heterogeneous rubber blends was characterized. Because of the higher wetting rate of the NR component, in the first stage of mixing of NR/SBR blends more silica is found in the NR phase than in the SBR phase. In the next stage, silica is transferred from the NR phase to the SBR phase until the loosely bonded components of NR rubber‐layer are fully replaced by SBR molecules. POLYM. COMPOS., 31:1701–1711, 2010. © 2010 Society of Plastics Engineers.     

Filler Wetting in Miscible ESBR/SSBR Blends and Its Effect on Mechanical Properties

The selective wetting behavior of silica in emulsion styrene butadiene rubber (ESBR)/solution styrene butadiene rubber (SSBR) blends is characterized by the wetting concept, which is further developed for filled blends based on miscible rubbers. It is found that not only the chemical rubber–filler affinity but also the topology of the filler surface significantly influences the selective filler wetting in rubber blends. The nanopore structure of the silica surface has been recognized as the main reason for the difference in the wetting behavior of the branched ESBR molecules and linear SSBR molecules. However, the effect of nanopore structure becomes more significant in the presence of silane. It is discussed that the adsorption of silane on silica surface constricts the nanopore to some extent that hinders effectively the space filling of the nanopores by the branched ESBR molecules but not by the linear SSBR molecules. As a result, in silanized ESBR/SSBR blends the dominant wetting of silica surface by the tightly bonded layer of SSBR molecules causes a low‐energy dissipation in the rubber–filler interphase. That imparts the low rolling resistance to the blends similar to that of a silica‐filled SSBR compound, while the ESBR‐rich matrix warrants the good tensile behavior, i.e., good abrasion and wear resistance of the blends.

     

Slipping of carbon nanotubes in a rubber matrix

The interactions of carbon nanotubes (CNTs) and carbon black (CB) with rubber matrices are of great interest. Although both belong to the carbon filler family, their interactions are different. In this study the adhesion of CNTs, if any, with natural rubber (NR) was examined. Scanning electron microscopy examinations made on cryogenically fractured surfaces of a crosslinked NR sample containing 7% by weight of CNTs showed that the CNT bundles emerged from the side surface (narrowed by Poisson's ratio) and slowly slid back in when the deformation was removed. The protruded lengths were many times larger than the nanotube bundle diameters. This extensive slipping out of CNTs from the rubber matrix suggests that interfacial interactions between CNTs and NR are quite weak. In contrast, relatively strong interactions were found between CB and rubber, indicated by the large amount of bound rubber formation. Reinforcement of rubber by CNTs is therefore attributed to the large aspect ratio of CNT bundles. Physical entanglement with rubber molecules is then able to generate effective load transfer, replacing the strong adhesion found with CB. Copyright © 2010 Society of Chemical Industry     

Molecular structure and storage hardening of natural rubber: Insight into the reactions between hydroxylamine and phospholipids linked to natural rubber molecule

The effect of hydroxylamine on the molecular structure and storage hardening of natural rubber (NR) was investigated by the treatment of deproteinized NR (DPNR) latex with hydroxylamine. The hydroxylamine treatment decreased the content of long‐chain fatty acid ester groups in DPNR from about 2–0.7 mol per rubber molecule. The molecular weight and molecular weight distribution changed apparently after treatment with hydroxylamine. The relative intensity of the ¹H NMR signals corresponding to phospholipids at the α‐terminal group decreased after the hydroxylamine treatment. The Huggins ‘k’ constant of hydroxylamine‐treated DPNR showed the liberation of linear rubber molecules caused by decomposition of branch points derived from phospholipids. The absence of storage hardening in hydroxylamine‐treated DPNR was observed to be caused by not only the reaction of hydroxylamine and aldehyde groups but also the removal of phospholipids as well as the breakdown of phospholipid aggregations as a result of hydroxylamime, contributing to the establishment of a newly proposed mechanism of hydroxylamine on the inhibition of storage hardening in NR. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43753.     

Reducing and stabilizing the viscosity of natural rubber by using sugars: Interference of the Maillard reaction between proteins and sugars

This work is an extension of previous work elucidating the reduction and stabilization of solid natural rubber (NR) viscosity by using sugars. Various amounts of glucose, fructose, sucrose, and maltose were incorporated into fresh NR (FNR), deproteinized NR (DPNR), and synthetic polyisoprene (IR) latexes. The results revealed that all sugars cannot decrease the Mooney viscosity of FNR, while only monosaccharides, that is, glucose and fructose, can significantly decrease the Mooney viscosity of both DPNR and IR by way of a lubrication mechanism. The proteins in FNR can diminish the capability of glucose and fructose to decrease the Mooney viscosity. Furthermore, glucose was found to reduce the occurrence of storage hardening in DPNR by interacting with polar groups of phospholipids at the rubber chain ends. Measurements of browning intensity as well as analysis of Maillard reaction products together with the NR protein–glucose model compound were utilized to confirm that the reduction and stabilization of the viscosity of NR using monosaccharides were interfered by the Maillard reaction between the proteins in NR and the monosaccharides.     

Kinetics of filler wetting and dispersion in carbon nanotube/rubber composites

The effects of the surface modification of multi-walled carbon nanotubes (MWCNTs) by an ionic liquid, 1-butyl 3-methyl imidazolium bis(trifluoromethyl-sulphonyl)imide (BMI) on the kinetics of filler wetting and dispersion as well as resulting electrical conductivity of polychloroprene (CR) composites were studied. Two different MWCNTs were used, Baytubes and Nanocyl, which differ in their structure, purity and compatibility to CR and BMI. The results showed that BMI can significantly improve the macrodispersion of Baytubes, and increases the electrical conductivity of the uncured BMI–Baytube/CR composites up to five orders of magnitude. In contrast, the use of BMI slows the dispersion process and the development of conductivity of BMI–Nanocyl/CR composites. Our wetting concept was further developed for the quantification of the bound polymer on the CNT surface. We found that the bonded BMI on the CNT surface is replaced by the CR molecules during mixing as a result of the concentration compensation effect. The de- and re-agglomeration processes of CNTs taking place during the subsequent curing process can increase or decrease the electrical conductivity significantly. The extent of the conductivity changes is strongly determined by the composition of the bound polymer and the curing technique used.     

Quantitative analysis for reaction between epoxidized natural rubber and poly (L‐lactide) through 1H‐NMR spectroscopy

Reaction between epoxidized natural rubber and poly(L ‐lactide) (PLLA) was investigated quantitatively in terms of conversion of the epoxidized natural rubber. The epoxidized natural rubber was prepared by epoxidation of high ammonia natural rubber (HA‐NR) or deproteinized natural rubber (DPNR) with peracetic acid followed by depolymerization with ammonium persulfate. The resulting liquid HA‐NR having epoxy group (LENR) or liquid DPNR having epoxy group (LEDPNR) were subjected to heating at 473 K for 20 min, after blending with PLLA. The products were characterized through morphology observation, DSC measurement, and ¹H‐NMR spectroscopy. The conversions of the rubbers were estimated from intensity ratio of signals in ¹H‐NMR spectrum for the products after removing unreacted rubber with toluene. Difference in the estimated conversion between the LENR/PLLA and LEDPNR/PLLA blends was interpreted in relation to proteins present in the rubber. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Novel thermoplastic natural rubber based on thermoplastic polyurethane blends: influence of modified natural rubbers on properties of the blends

Three different forms of natural rubber: maleated natural rubber (MNR), epoxidized natural rubber (ENR) and natural rubber-graft-poly(methyl methacrylate) (NR-g-PMMA) were prepared. Degree of functional groups in rubber molecules was quantified using the integrated peak areas of ¹H NMR. It was found that the modified rubbers with similar level of functionality had been successfully prepared. Thermoplastic natural rubber (TPNR) based on blending of thermoplastic polyurethane (TPU) and various forms of rubber were then prepared using melt blending method. The properties of the blends were studied and compared together in relation to different types of natural rubbers prepared (i.e., unmodified NR, MNR, ENR and NR-g-PMMA). It was found that the blends with modified NR exhibited superior stiffness, entropy effect and damping factor compared to other blends with unmodified NR. This is attributed to the chemical interaction between the functional groups of modified NR molecules and polar functional groups in TPU molecules which facilitated higher interfacial adhesion between both phases. The chemical interaction was verified by ATR-FTIR and TSSR techniques. It was also found that the MNR/TPU blend showed the highest tensile modulus, mechanical and elastic properties with smallest and finer grain dispersion of co-continuous phase compared to ENR/TPU, NR-g-PMMA/TPU and unmodified NR/TPU blends, respectively. This might be due to higher chemical interactions between MNR and TPU phases. Furthermore, the incorporation of rubber did reduce hardness (i.e., <60 Shore A) with improvement of elasticity of the blends compared with the original TPU (i.e., ~85 Shore A).     

Thermally induced phase separation in PαMSAN/PMMA blends in presence of functionalized multiwall carbon nanotubes: Rheology, morphology and electrical conductivity

The focus of this work is the evaluation and analysis of the state of dispersion of functionalized multiwall carbon nanotubes (CNTs), within different morphologies formed, in a model LCST blend (poly[(α-methylstyrene)-co-(acrylonitrile)]/poly(methyl-methacrylate), PαMSAN/PMMA). Blend compositions that are expected to yield droplet-matrix (85/15 PαMSAN/PMMA and 15/85 PαMSAN/PMMA, wt/wt) and co-continuous morphologies (60/40 PαMSAN/PMMA, wt/wt) upon phase separation have been combined with two types of CNTs; carboxylic acid functionalized (CNTCOOH) and polyethylene modified (CNTPE) up to 2 wt%. Thermally induced phase separation in the blends has been studied in-situ by rheology and dielectric (conductivity) spectroscopy in terms of morphological evolution and CNT percolation. The state of dispersion of CNTs has been evaluated by transmission electron microscopy. The experimental results indicate that the final blend morphology and the surface functionalization of CNT are the main factors that govern percolation. In presence of either of the CNTs, 60/40 PαMSAN/PMMA blends yield a droplet-matrix morphology rather than co-continuous and do not show any percolation. On the other hand, both 85/15 PαMSAN/PMMA and 15/85 PαMSAN/PMMA blends containing CNTPEs show percolation in the rheological and electrical properties. Interestingly, the conductivity spectroscopy measurements demonstrate that the 15/85 PαMSAN/PMMA blends with CNTPEs that show insulating properties at room temperature for the miscible blends reveal highly conducting properties in the phase separated blends (melt state) as a result of phase separation. By quenching this morphology, the conductivity can be retained in the blends even in the solid state.     

Quantitative characterization of interfaces in rubber–rubber blends by means of modulated‐temperature DSC

Rubber–rubber blends are used widely in industry, for example, in tire manufacture. It is often difficult to characterize interfaces in such rubber–rubber blends quantitatively because of the similarity in the chemical structure of the component rubbers. Here, a new method was suggested for the measurement of the weight fraction of the interface in rubber–rubber blends using modulated‐temperature differential scanning calorimetry (M‐TDSC). Quantitative analysis using the differential of the heat capacity, dCp/dT, versus the temperature signal from M‐TDSC allows the weight fraction of the interface to be calculated. As examples, polybutadiene rubber (BR)–natural rubber (NR), BR–styrene‐co‐butadiene rubber (SBR), SBR–NR, and nitrile rubber (NBR)–NR blend systems were analyzed. The interfacial content in these blends was obtained. SBR is partially miscible with BR. The cis‐structure content in BR has an obvious effect on the extent of mixing in the SBR–BR blends. With increasing styrene content in the SBR in the SBR–BR blends, the interface content decreases. NR is partially miscible with both BR and SBR. The NBR used in this research is essentially immiscible with NR. The maximum amount of interface was found to be at the 50:50 blend composition in BR–NR, SBR–BR, and SBR–NR systems. Quantitative analysis of interfaces in these blend systems is reported for the first time. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1791–1798, 2000     

Carbon nanotubes/carbon black synergistic reinforced natural rubber composites

Abstract

Carbon black (CB) and carbon nanotubes (CNTs) filled natural rubber (NR) composites were prepared. In order to overcome the dispersion of CNTs in rubber matrix, the surface modification of CNTs with bis-(γ-triethoxysilylpropyl)-tetrasulphide (Si-69) was undertaken, and a two-step mixing process, i.e. the use of twin roll mill followed by mixing in a Haake Banbury mixer (TR-THM) was used. The structure and mechanical properties were investigated. The results show that the Si-69 treated CNTs (S-CNTs) were dispersed in the rubber matrix uniformly. Compared with CB/NR composites without CNTs, the S-CNTs/CB/NR composites have better mechanical properties. When the ratio of S-CNTs/CB/NR was 5 : 20 : 100, the tear strength was improved by ～60%, and the mechanical properties reached a maximum. Dynamical mechanical analysis (DMA) reveals that with increasing content of CNT, the elastic modulus of composites at room temperature increases, and the maximum loss tangent and the corresponding glass transition temperature of composites decrease.     

Influence of rubber content on mechanical,thermal, and morphological behavior of natural rubber toughened poly(lactic acid)–multiwalled carbon nanotube nanocomposites

The effects of natural rubber (NR) on the mechanical, thermal, and morphological properties of multiwalled carbon nanotube (CNT) reinforced poly(lactic acid) (PLA) nanocomposites prepared by melt blending were investigated. A PLA/NR blend and PLA/CNT nanocomposites were also produced for comparison. The tensile strength and Young's modulus of PLA/CNT nanocomposites improved significantly, whereas the impact strength decreased compared to neat PLA. The incorporation of NR into PLA/CNT significantly improved the impact strength and elongation at break of the nanocomposites, which showed approximately 200% and 850% increases at 20 wt % NR, respectively. However, the tensile strength and Young's modulus of PLA/NR/CNT nanocomposites decreased compared to PLA/CNT nanocomposites. The morphology analysis showed the homogeneous dispersion of NR particles in PLA/NR/CNT nanocomposites, while CNTs preferentially reside in the NR phase rather than the PLA matrix. In addition, the incorporation of NR into PLA/CNT lowered the thermal stability and glass‐transition temperature of the nanocomposites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44344.     

Production of a cellular structure in carbon nanotube/natural rubber composites revealed by nanomechanical mapping

Composites based on an elastomer, natural rubber (NR), and pristine multiwalled carbon nanotubes (MWCNTs) were prepared using a two-roll mill mixer. The high shear stress produced a homogeneous dispersion of CNTs in the NR matrix. Using an atomic force microscopy nanomechanical mapping technique, the structure and nanoscale mechanical properties of the resulting composites were investigated in term of CNT loading. Topography, elastic modulus, and adhesive energy distribution maps of the materials were obtained at the same point and at the same time in a single scan. Such maps were used to identify and characterize the CNT, NR, and mechanical interfacial regions formed around the CNTs. The mechanical interfacial regions together with the CNTs divide the elastomer matrix into small “cells”. We claim that the cellular structure is produced in the CNTs/NR composites.     

Effect of phenol functionalization of carbon nanotubes on properties of natural rubber nanocomposites

This paper reports the results of studies on the effect of phenol functionalization of carbon nanotubes (CNTs) on the mechanical and dynamic mechanical properties of natural rubber (NR) composites. Fourier transform infrared spectrometry (FTIR) indicates characteristic peaks for ether and aromatic rings in the case of phenol functionalized CNT. Although differential scanning calorimetric (DSC) studies show no changes in the glass‐rubber transition temperature (T_g) of NR in the nanocomposites due to surface modification of CNT, dynamic mechanical studies show marginal shifting of T_g to higher temperature, the effect being pronounced in the case of functionalized CNT. Stress‐strain plots suggest an optimum loading of 5 phr CNT in NR formulations and the phenolic functionalization of CNT does not affect significantly the stress‐strain properties of the NR nanocomposites. The storage moduli register an increase in the presence of CNT and this increase is greater in the case of functionalized CNT. Loss tangent showed a decrease in the presence of CNT, and the effect is more pronounced in the case of phenol functionalized CNT. Transmission electron microscopy (TEM) reveals that phenol functionalization causes improvement in dispersion of CNT in NR matrix. This is corroborated by the increase in electrical resistivity in the case of phenol functionalized CNT/NR composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Electrical and mechanical behaviors of carbon nanotube‐filled polymer blends

Four carbon nanotube (CNT)‐filled polymer blends, i.e., CNT‐filled polyethylene terephthalate (PET)/polyvinylidene fluoride, PET/nylon 6,6, PET/polypropylene, and PET/high‐density polyethylene blends, have been injection‐molded and characterized in terms of their microstructures, electrical conductivities, and mechanical properties. The distribution of CNTs in the polymer blends has been examined based on their wetting coefficients and minimization of the interfacial energy. The electrical conductivity and mechanical properties have been related to the cocontinuous polymer blends, the conductive path formed by CNTs, the CNT distribution, and the intrinsic properties of the constituent polymers. It is found that to obtain a CNT‐filled polymer composite with both high electrical conductivity and good mechanical properties, it is preferred that most CNTs distribute in one polymer phase, while the other polymer phase(s) remain neat. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 477–488, 2006     

The effect of surface functionalization of carbon nanotubes on properties of natural rubber/carbon nanotube composites

The objective of this study was to prepare natural rubber composites filled with carbon nanotubes (CNTs) that show an electrical percolation threshold at very low CNT concentrations. Therefore, two methods of surface functionalization of CNTs were investigated to enable an improved dispersion of CNTs and chemical interaction between CNTs and rubber matrix. On one hand, the CNTs have been functionalized ex situ by acid treatment and silanization reaction with bis(triethoxysilylpropyl) tetrasulfide before mixing with the rubber and otherwise in situ functionalization was directly carried out during the processing of the composites in the internal mixer. The grafting of silane molecules onto CNT surface was established by Fourier transform infrared spectroscopy and scanning electron microscopy. Tensile tests revealed the outstanding properties of composites prepared by in situ silanization method. The in situ silanization led to a better dispersion of the CNTs and the formation of chemical linkages between CNT surface and rubber and this became manifest in higher reinforcement of the rubber, higher crosslink densities, and a lower electrical percolation threshold. It was also shown that the in situ silanization is retarding the vulcanization reaction. POLYM. COMPOS., 36:2113–2122, 2015. © 2014 Society of Plastics Engineer