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Rheological properties of thermoplastic polyurethane adhesive solutions containing fumed silicas of different surface areas
Abstract
Fumed silicas of different surface areas (90–380m2/g) and primary particle size (20–7nm, respectively) have been added to solvent-based thermoplastic polyurethane (PU) adhesives. Addition of silica increased the viscosity, imparted pseudoplasticity, and thixotropy or rheopexy to PU adhesive solutions. Rheological data of adhesive solutions fitted well the Casson model. Fumed silicas with intermediate surface areas (130–200m2/g) provided the best rheological performance to PU adhesive solutions. The increase of time after preparation of PU adhesive solutions produced an increase in viscosity, more marked pseudoplasticity and enhanced thixotropy (rheopexy was not exhibited). The reduced rheological properties of PU solutions containing Aerosil 380 have been ascribed to the relatively large size of the clusters produced by aggregation of small primary fumed silica particles, which under the experimental conditions used in this study were not fully dispersed in the PU solution. Contact angles obtained by placing adhesive drops on rubber substrates increased when the adhesive contains silica, the trend obtained was similar to that exhibited by the flow curves. This may indicate that a relationship between wettability and rheology of PU adhesive solutions may exists. Green (immediate) strength of roughened R1 rubber/PU adhesive joints was improved, if the adhesive contained fumed silica and did not vary with the surface area of the fumed silica, but the strength for (roughened+chlorinated) R2 rubber/PU adhesive joints was similar if the adhesive contained or not fumed silica, and independently of the surface area.
... Baron et al. [13] investigated the rheological and adhesive features of pressure-sensitive adhesives with PU graft copolymers. Jauregui-Beloqui et al. [14] reported the effect of silica particles with different surface areas on the rheological properties of PU adhesives. The desirable rheological features generally enable adhesives to have a high enough viscosity during storage to minimize settling phenomena. ...
... An MCR-301 rheometer (Anton Paar, Austria) equipped with a CTD 450 heat chamber was employed for these tests in the 25mm parallel-plate mode with a 0.5mm gap at room temperature except during the thermal curing test. To observe the thixotropic hysteresis loop, shear viscosity data of adhesive samples were measured [9][10][11]14]; e.g., the shear rate was increased from 0.1 to 100 s −1 in the first-stage and then decreased from 100 to 0.1 s −1 in the secondstage for adhesives with a single PU. Thixotropic indices of the adhesives were evaluated from viscosity data within a given shear rate range. ...
Rheological and adhesive properties of automotive structural adhesives including polyurethane (PU) toughening agents have been investigated by adjusting the molar ratio of polytetrahydrofuran (polyTHF) and hexamethylene diisocyanate (HMDI) for the control of molecular weight of a PU prepolymer. Thixotropic behavior, crosslinking characteristics, and lap-shear and T-peel adhesion strengths for various adhesives were significantly affected by single or binary-mixed PUs with different molecular weights. Thixotropic hysteresis loop of shear viscosity along with shear rate of an adhesive became larger with increasing PU molecular weight, exhibiting favorable flowability, however, the adhesion strengths were not satisfactory for injection and coating applications. We found that both rheological and adhesive properties of adhesives could be interestingly tuned by combining PU components with different molecular weights.
... Equally, these equations are graphed in contour and surface plots in Fig. 5. From Fig. 5(a), it can be observed the relation between the factors silicon dioxide (C) and Latex type ratio (D) added to WPCA's formulation and its viscosity. Silicon dioxide acts as a viscosity modifier, this component is able to increase up to 80% the viscosity values [26][27][28], as can been seen at contour graphic. This viscosity increase is created by roughly spherical particles with a diameter of 100 nm, these particles form aggregates, which in turn are grouped to make larger agglomerates, which form a gel-type three-dimensional structure when a dispersion of them is allowed to stand [29]. ...
... Silicon dioxide (C) and latex type ratio (D) are again the factors that have the major influence. These components tend to increase the cohesive forces of the adhesive, modifying its surface energy and viscosity, and affecting the capacity of the adhesive to wet well the substrate [27]. Together these components may increase up to 26.31% contact angle values, as shown in the contour graphic. ...
Waterborne adhesives for rubber to metal bonding have been available since 1990. However, published information about their formulation has been limited, as proprietary restrictions are exercised by companies. As a consequence, the way these adhesives interact with substrates has not been studied extensively. With the aim of investigating the effect the components of a waterborne adhesive have on rubber to metal bonding, fractional factorial and surface response methodologies of design of experiments were employed in this study. Twenty six formulations were prepared with a polychloroprene latex as the adhesive polymer. Viscosity, wettability and non-volatile solids content were measured with each liquid adhesive, while the mechanical strength was evaluated by applying a tensile mechanical stress over cured solid adhesive films. Adhesion properties were evaluated by using a single lap-shear test on metal to metal joints and a pull-out test on rubber to metal joints. The results showed that the components with the largest relative influence on cohesive and adhesives forces were tackifier resin, silicon dioxide and polychloroprene latex type. In order to better understand the contributions of these variables, mathematical models correlating them with the response variables were obtained. This study is valuable in explaining how, through statistical methods, a waterborne adhesive for rubber to metal bonding can be formulated with a reasonably low number of experiments.
... Fumed silica can be defined as finely divided amorphous silicon dioxide particles produced by high temperature oxidation of silicon tetrachloride in an oxygen-hydrogen flame [16,17]. Approximately every second silicon atom on the fumed silica surface bears a silanol group, and a large number of silanols are not hydrogen-bonded but are isolated and distributed over the surface [16,18,19]. For some polymer systems, it has been reported that the addition of highly dispersed fumed silica can give considerable improvements in the polymer thermal properties [17,18]. ...
... In general, the higher decreases of Mn as compared to Mw observed for all materials can be associated to higher hydrolysis rate of the low molecular weight PCL molecules as compared to high molecular ones, as it could be expected if hydrolysis is catalyzed by hydroxyl groups [17,19,27,43] and considering that the degradation of PCL has been reported to proceed in the vicinity of polymer chain-ends [4,47]. ...
PLA and PCL based nanocomposites prepared by adding three different types of fumed silica were obtained by melt blending. Materials were characterized by means of Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA) and Dynamic–Mechanical Thermal Analysis (DMTA).A good distribution of the fumed silica into both polymer matrices was observed. The highest thermo-mechanical improvements were reached by addition of the fumed silica with higher surface area. PLA and its nanocomposites were degraded in compost at 58 °C; at this temperature all samples presented a significant level of polymer degradation, but a certain protection action of silica towards PLA degradation was observed, whereas the addition of fumed silica did not show considerable influence on the degradation trend of PCL. These dissimilarities were attributed to the different degradation mechanism of the two polymers.Graphical abstract
... In addition, the SEM fracture of the ceramic fiber in Figure 4f seemed to have a certain incomplete molten sheet of TPU-95A#. This indicated that with the increase in the mass ratio of ceramic powder, the powder was not only not able to be completely wrapped by the melting TPU, but also easily led to insufficient TPU melting [36,37]. The prepared ceramic fiber was not as high in quality as the quality in Figure 4e. ...
YAG ceramic fiber, with its high thermal conductivity and easy to achieve limit size, provides design flexibility as a laser gain medium. Its mainstream forming method was mainly high-pressure extrusion, but there were disadvantages, such as lack of flexibility. In this work, the flexible green body of YAG ceramic fiber was prepared by melt spinning. The melting characteristics of TPU with four different Shore hardnesses were systematically investigated. The microstructure, element homogeneity of the surface and fracture SEM images of the prepared ceramic fiber were also analyzed in detail. The optimized process parameters of YAG ceramic fiber preparation were as follows: the melting temperature was 220 °C, the screw feed rate of the double-cone screw extruder was F = 15.0 mm/min and the TPU-95A# was used. The ceramic fiber with the mass ratio of TPU-95A# to ceramic powder = 4:6 had the best microstructure quality. It had good flexibility and could be knotted with a bending radius of about 2.5 mm, and the tensile strength reached approximately 20 MPa. These results are crucial for advancing YAG ceramic fiber applications.
... Incorporation of fumed silica in chlorinated rubber adhesive remarkably enhances its peel strength. [6][7][8] Nanosilica can be incorporated into the polymer matrix either by blending or by polymerization technique. Melt blending and solution blending are two widely used processes to incorporate nanosilica in the polymer matrix. ...
This investigation reports the preparation and properties of thermoplastic polyurethane/silica nanocomposite prepared via melt mixing process. In this case, nanosilica at different loading was mixed with a polyester-based thermoplastic polyurethane (TPU). The dispersion of nanofiller was studied by the SEM and TEM analyses. The nanocomposite with optimal dispersion of nanofiller has better filler-polymer interaction that has been confirmed by the FT-IR study. TPU with 1 phr of nanosilica loading showed better tensile properties. DSC study revealed that incorporation of fumed silica in the TPU matrix does not substantially affect the refinement of TPU crystallite, and thus resulted in an insignificant change in glass transition temperature (Tg) and crystalline melting temperature of the nanocomposite. TGA study also confirmed enhanced onset degradation temperature and maximum degradation temperature (Tmax) for optimal nanosilica composite. The final degradation temperature and char content also increased with an increase in fumed silica content. Water contact angle (WCA) study revealed that hydrophilicity of the nanocomposite increased with the incorporation of fumed silica.
... Another advantage of these nanoparticles is their easy surface modification through ion exchange, which can enhance their polymer compatibility. Considerable enhancements in the thermal stability [28][29][30], barrier properties [31][32][33], mechanical strength [34][35][36], elastic properties [37,38], biodegradation, and high energy shielding [39,40] were observed in nanocomposites using nanoclay as the filler [41]. Tuning of the morphology and mechanical properties of the polymer and its nanocomposites in the presence of nanoclay was also reported [9,42]. ...
This chapter describes the influence of different type of nanoparticles such as metals and metals oxides, ceramics, natural clay, various forms of carbon and zeolites on the properties of polyurethanes with special emphasis on the dimensionality of the nanoparticles. First, classification of the nanoparticles is made with typical examples. Distribution of the particles in polyurethane matrix and their interaction with the polymer chain alters the morphology of polyurethane both at the surface as well as in the bulk which in turn responsible for the dramatic changes in the properties of the composites. Organic modification on the surface of the nanoparticles make significant changes to the overall properties of the composites. Self-assembly phenomena in polyurethanes are illustrated where hydrogen bonded molecular layers stack together forming bigger clusters observable in optical microscope. The impact of layer by layer self-assembly on drug delivery and other properties are discussed. Improvement in the thermal, mechanical properties and flame retardancy of the composites with different type of nanoparticles are presented in detail with suitable examples. Polyurethanes are widely used in biomedical fields and the present chapter elaborates the effectiveness of the composites towards biocompatibility including in-vivo use vis-à-vis pure polyurethanes. It is suggested that polyurethane nanocomposites have great potential in biomedical applications.
... Nano-sized fumed silica has been widely used in many fields, such as cosmetics [1,2], CMP and electronics [3], sealants and adhesives [4][5][6], paints and coatings [7][8][9][10], silicone rubber [11], toners [12], etc., to improve their functionalities. For these applications, dispersion of the fumed silica in the desired medium, such as organic solvents, polymers, water, etc., is required. ...
The effects of a solvent on the surface modification progress of fumed silica were investigated. Hexane, toluene, and acetone as the reacting solvent and hexyltriethoxysilane as the surface modifier were used. After a 1 h reaction at 50 °C, the surface modifier density was calculated from the weight loss of the reacted modifier at the silica surface. With a decrease in the solvent polarity (in the order of acetone, toluene, and hexane), the surface modifier density increased. Hexane and toluene have similar solvent polarities. However, behavior of the surface modifier density as a function of the modifier amount was totally different. By considering the solubility of the modifier in the solvents, hexane has a higher solute ability for the modifier than toluene. The reaction between the modifier and silica was promoted in hexane, but not promoted in toluene. In addition to the solvent property, adsorbed water on the silica surface is important for the reaction. In hexane and toluene, the amounts of adsorbed water at the silica surface showed the maximum surface modifier density of about 30 mg and 50 mg, respectively. It can be postulated that the adsorbed water layer could be fixed in hexane while diffused in toluene. Based on these results, it can be concluded that the solvent polarity, solubility of the modifier, and adsorbed water amount at the silica surface should be considered in order to achieve the desired surface modification.
... Fumed silicas (nanosilicas) are fillers commonly added to improve the thermal, rheological and mechanical properties of TPU´s [2][3][4][5][6][7]. This improvement in properties has been previously ascribed to the creation of hydrogen bonds between the hydroxyl groups on the nanosilica surface and the soft segments of the polyurethane, favoring the degree of phase separation [8][9][10][11]. ...
Abstract
Nanosilicas with different silanol contents were obtained by treatment of hydrophilic fumed
silica with dimethyldichlorosilane. This treatment reduced the silanol content and produced
the particle agglomeration of the nanosilicas. Thermoplastic polyurethane (TPU) adhesives
containing nanosilicas were prepared and characterized by FTIR spectroscopy, differential
scanning calorimetry (DSC), plate–plate rheology, dynamic mechanical thermal analysis
(DMTA) and transmission electron microscopy (TEM).
It was demonstrated that addition of hydrophilic nanosilicas favored the degree of phase
separation between the hard (i.e. isocyanate + chain extender) and soft (i.e. polyol)
segments in the TPUs; the higher the silanol content on the surface of silica, the higher the
degree of phase separation, and the crystallinity of the polyurethane (due to the soft
segments) was also increased. Hydrogen bonds between the ester carbonyl groups in the
TPU and the silanol groups on the silica surface were created and more favored by
increasing the silanol content.
... Fumed silicas (nanosilicas) are fillers commonly added to improve the thermal, rheological and mechanical properties of TPU´s [2][3][4][5][6][7]. This improvement in properties has been previously ascribed to the creation of hydrogen bonds between the hydroxyl groups on the nanosilica surface and the soft segments of the polyurethane, favoring the degree of phase separation [8][9][10][11]. ...
Thermoplastic polyurethanes (TPUs) are multi-phase segmented polymers that exhibit a two-phase microstructure (phase separation), which arises from the incompatibility between the soft and the hard segments. The hard rigid segment segregates into a glassy or semicrystalline domain, and the polyol soft segments form amorphous or rubbery matrices in which the hard segments are dispersed [1]. Fumed silicas (nanosilicas) are fillers commonly added to improve the thermal, rheological and mechanical properties of TPU´s [2–7]. This improvement in properties has been previously ascribed to the creation of hydrogen bonds between the hydroxyl groups on the nanosilica surface and the soft segments of the polyurethane, favoring the degree of phase separation [8–11]. Previous experimental evidence [12, 16-20] has corroborated the formation of hydrogen bonds between the nanosilica and the polyurethane. To analyze in depth the creation of hydrogen bonds between the nanosilica and the polyurethane and its incidence in the structure and properties of TPU´s, nanosilicas with different silanol contents were added to thermoplastic polyurethanes. Specific surface area is inversely related to the primary particle size of nanosilica, and the greater the specific surface area the greater the silanol content. If hydrogen bond formation is responsible for improving the properties of nanosilica–polyurethane mixtures, it can be expected that the greater the silanol content in the nanosilica, the more noticeable the extent of phase separation in the polyurethane should be; therefore, 2wt% of different nanosilicas was added to a TPU and the mixtures obtained were characterized by FTIR spectroscopy, DSC, X-ray diffraction, plate–plate rheometry, adhesion tests, DMTA and TEM.
... As the UPR contains polar groups, these groups may interact with the silanol groups on the nanosilica particles surface creating a polymer network. Because of the addition of nanosilica to the UPR matrix creates hydrogen bonds between the silanols groups of the nanosilica surface and the C@O and CAO groups in the UPR and/or the styrene, the formation of a three-dimensional physical structure (agglomerates) in the UPR solution is produced [15]. The number of the UPR-nanosilica aggregates increases by increasing the nanosilica content leading to an increase in viscosity of the UPR solution. ...
In this study, an unsaturated polyester resin (UPR) thin coating containing styrene monomer has been used to consolidate Marrón emperador marble pieces. Different amounts (0.5–3wt.%) of nanosilica was added to improve several properties, particularly the mechanical properties. The uncured UPR-nanosilica hybrids were characterized by rotational rheology, the gelation was monitored by means of a texture analyser and the viscoelastic and thermal properties were determined in the cured UPR-nanosilica films. Transmission electron microscopy (TEM) was used to establish the degree of dispersion of the nanosilica in the UPR-nanosilica composites. The mechanical performance of the UPR-nanosilica thin coated marble pieces was measured by means of 3-points bending and impact strength tests.Addition of nanosilica imparted pseudoplasticity and thixotropy to the UPR resin and an increase in viscosity was also produced. The rheology of the UPR-nanosilica solutions was fitted to the Casson’s model and an increase in yield stress was obtained. The gel time of the UPR resin was significantly decreased by adding small amounts of nanosilica due to the interactions between the filler and the styrene in the UPR resin. In the cured composites, improved thermal properties in UPR were reached by adding nanosilica due to the creation of a network between the filler and the polymer matrix. Bundle nanometric size nanosilica agglomerates were observed which affected the glass transition temperature and the viscoelasticity of the UPR-nanosilica composites. Finally, the improved properties in UPR obtained by adding nanosilica produced enhanced impact resistance to coated marble pieces, as both stiffness and toughness were improved by nanosilica addition.
... The time-dependent behavior of fumed silica suspensions is well known. Therefore, applying a preshear stage in order to erase this preshear history-dependent behavior is a common practice (Khan et al. 1994;Raghavan et al. 1998;Nguyen et al. 1998;Jaúregi-Beloqui et al. 1999;Torró-Palau et al. 2001). So, to ensure that the results were reproducible, before each experiment, a steady preshear was applied up to an equilibrium state dτ (t) dt ≤ 0.01 Pa/s . ...
We have found in this study, by means of steady and dynamic rheometry, that Aerosil® R816 particles, in which hydroxyl groups
have been mostly substituted by alkyls groups, form nonflocculated suspensions in polypropylene glycol, in comparison to what
was expected from previous studies. Steady flow curve shows shear-thickening behavior between two shear-thinning regions.
The transient rheological response has been analyzed using a protocol proposed a long time ago by Cheng (Rheol Acta 25:542–554,
1986). It has been found that, within the reversible shear-thickening region, all the constant structure curves overlap, which
suggests that the response at a certain shear rate does not depend significantly on the previous state. As a consequence,
this protocol is proposed as an alternative technique for distinction between flocculated and nonflocculated suspensions.
As an interdisciplinary field of science, nanotechnology is applied also in wood processing. Synthetic silica (nano-SiO2) is one of the most versatile and multipurpose nanomaterials: when introduced into a polymer matrix, it improves considerably the strength and heat-resistance of resulting nanocomposites. The main objective of this research work was to determine the effect of nano-SiO2 addition to urea-formaldehyde (UF) and phenol-formaldehyde (PF) resins on the properties of plywood and particleboards produced using them, and on this basis to propose new solutions. The purpose is to reduce the amount of binders required in the production of plywood and to improve properties of particleboards produced with the particles of nonwood raw lignocellulosic materials. Considering the fact that nanosilica shows a significant tendency to agglomerate, the goal of the work was also to determine the optimal degree of silica surface modification with a selected silane coupling agent, facilitating manufacture of boards with the best physico-mechanical properties. 3-Aminopropyl-triethoxysilane (APTES) was used as an adhesion promoter and surface modifier of inorganic fillers in many types of polymer systems, including phenolic, melamine and urea-formaldehyde systems. Such modification is applied mainly in the manufacturing of polymer composites, comprising various nanofillers. The scope of works included the preparation of plywood specimens bonded with nanocomposite adhesive resins applied at different loads per unit of veneer surface, manufacture of particleboards glued with a UF resin with various amounts of nano-SiO2 added, and preparation of plywood and particleboard specimens bonded with UF and PF resins with an addition of silica modified with various amounts of APTES. Taking into consideration an increasing interest in the use of alternative materials in the wood-based materials industry, especially in particleboard manufacturing, a part of the research work was devoted to particleboards supplemented with rape straw particles bonded with UF/nano-SiO2 resin. All the particleboards made were tested for their physico-chemical and hygienic properties according to the respective technical standards. The studies demonstrated that the use of nano-SiO2 as a filler for UF and PF resins in plywood manufacturing facilitated a considerable reduction in the adhesive resin load in the veneer bonding process. Even though the consumption of the nanocomposite adhesive resins was by 33% and 25% lower for the UF and PF resins, respectively, the obtained plywood specimens showed the required bonding quality. Moreover, formaldehyde emissions from the UF resins were also markedly reduced. Given the high cost of nanosilica production, such measures are of great economic importance and they are in line with the concept of sustainable development, promoting reduced consumption of raw materials, especially those posing environmental hazards. Moreover, modification of the UF resin with a properly adjusted amount of silica improves physico-mechanical and hygienic properties of particleboards: in those enriched with annual plant waste the use of nano-SiO2 in the UF resins faciliated a higher substitution of wood chips with straw particles. Particleboards with a 50% content of rapeseed straw particles and bonded with UF/nano-SiO2 resins met the requirements for type P2 particleboards. Bonding quality as well as strength of both plywood and particleboards bonded with nanocomposite adhesive resins can be further improved by modification of nano-SiO2 with the suitable aminosilane – APTES. An interesting observation made in the research work is also that some of the combustion properties of particleboards were also improved, mainly due to high barrier properties of nano-SiO2. As indicated by the preliminary flammability tests, introduction of small amounts of nano-SiO2 into the adhesive resin provides particleboards with greater fire resistance, which is manifested e.g. in lengthening board ignition time, limiting the burned area and increasing the oxygen index.
This chapter is one of the very few reviews in the existing literature on shoe bonding and is mainly focussed in providing an updated overview of the upper-to-sole bonding by means of adhesives. Because the previous chapter of this book on shoe bonding (Martín-Martínez, 2005 [1]) dated from 2005, it is timely to give an updated revision in which the most recent literature is compiled. The surface preparation of the rubber and polymeric soles and the formulations of the polyurethane and polychloroprene adhesives used in shoe bonding have been updated, and they will be revised separately in the following sections.
In this work, microcrystalline cellulose (MCC) and sawdust (SD) green fillers have been demonstrated as reactive fillers and moisture reservoirs for enhanced curing and adhesion strength of one-component moisture curable polyurethane (PU) adhesives. The chemical reaction between the MCC or SD fillers and isocyanates present in the PU adhesives was tracked using Fourier-transform infrared spectroscopy (FTIR), while the joint strength was determined using lap-shear tests. The results showed that the addition of MCC or SD leads to an accelerated curing rate through the early consumption of isocyanates. The lap-shear strength of unreinforced PU adhesive joints was achieved in 12 hours. In turn, the curing time in MCC and SD reinforced joints was achieved in 5 hours. The addition of MCC and SD fillers also enhanced the lap-shear strength of the joints under full cure conditions. An autocatalytic growth model was used to describe the lap-shear strength variation with respect to curing time.
An emulgel-based delivery system formulated with sweet fennel essential oil was developed for the encapsulation of β-carotene. Emulgels were stabilized by using a mixed emulsifier system consisting of fumed silica particles (Aerosil 200) and a green surfactant (Amidet-N). The influence of the microfluidization conditions on mean droplet sizes and the effect of the fumed silica concentration on the physical stability and rheological properties of emulgels were investigated. The emulgel that presented better physical stability was that formulated with 5 g/100 g fumed silica due to the role of Aerosil 200 as Pickering stabilizer and its enhanced rheological properties. The resulting β-carotene-loaded emulgels successfully protected the encapsulated bioactive ingredient throughout the storage period.
This chapter constitutes one of the very few reviews in the existing literature on shoe bonding, and it gives an updated overview of the upper-to-sole bonding by using adhesives. The surface preparation of rubber soles and the formulations of the solvent-borne and waterborne polyurethane and polychloroprene adhesives are described in more detail. The preparation of the adhesive joints and the specific adhesion tests in shoe bonding are also revised. Finally, the most recent developments dealing with shoe bonding are described. © Springer International Publishing AG, part of Springer Nature 2018. All rights are reserved.
Fumed silica is an important industrial material, which is widely used in medical, cosmetic, and electronic products. One important industrial application of the fumed silica powder is using it as fillers, which are key materials for reinforcing the high-performance/high-functional industrial products. However, the use of fumed silica as filler is limited to micrometer or millimeter sized particles, because nanometer sized silica particles tend to aggregate. In this study, the surface of fine silica powder is modified with n- and t-butyl alcohols, which exhibit different steric hindrance effects. The surface wettability of each modified silica powders is determined in two ways: macroscopic and microscopic wettability. Macroscopic wettability refers to preference dispersion test and microscopic wettability refers to evaluation of the molecular level by Fourier transform infrared spectroscopy (FT-IR). The modification ratio of each sample is confirmed by thermogravimetry-differential thermal analysis (TG-DTA), and the hydrophobicity of these modified silica powders is evaluated by the preference dispersion test. Molecular level evaluation of surface wettability by FT-IR confirms an obvious structural difference due to the steric hindrance of the n- and t-butoxy groups on the surface of silica. In addition, a correlation between macroscopic and microscopic evaluation results for the surface wettability of modified silica powders is confirmed.
In this study, the effect of nano-silica (nano-SiO2) and boron carbide (B4C) on the curing behavior of the resole resin was evaluated using differential scanning calorimetry (DSC). For the curing behavior study, DSC analysis, carried out at different heating rates, was employed to evaluate the effect of fillers on heat generated during curing and evaporation of volatile components during the curing process. A kinetic model was suggested, and its parameters were determined. The results indicated that the existence of fillers in the resin decreased the evaporation heat of volatile components; meanwhile, the incorporation of the fumed silica increased the curing heat of the resin. Also the plot of reaction rate versus conversion showed that the curing kinetic of filled resin followed the autocatalytic mechanism because of the catalytic effects of water and hydroxyl groups on the surface of nano-silica particles. Model-plotting indicated the excellent agreement between experimental and model data.
In this study we adapt a conventional rotational rheometer to perform PIV measurements on wide gap Couette flow. We measure the radial profile of the azimuthal velocity and the rheometric properties simultaneously. We use image deformation to convert the images from cylindrical coordinates to a rectangular frame, thus improving the PIV algorithm performance. This technique is validated with the Newtonian liquid phase of the rheological suspensions, showing an excellent agreement with the theoretical velocity profile. We also characterize the rheological properties of Aerosil® R816 Polypropylene Glycol suspensions, which show a shear-thickening behavior between two shear-thinning regions. The relation between the viscosity and the shear rate is computed in the whole radial domain between the inner and the outer cylinder. These Rheo-PIV measurements allow us to distinguish the onset of shear thickening and jamming, the former one being connected to a smooth change in the measured shear rate, whilst the latter corresponds to a sudden change in the measured shear stress value.
Fumed silica suspensions in low molecular weight liquids are used in many photonic and microelectronic applications, playing its rheology a major role in the effectiveness of their usage. Particle-particle and particle-liquid medium interactions of suspensions of hydrophilic fumed silica in low molecular weight polar media, polypropylene glycol of 400 and 750 g/mol, concretely, have been already investigated. There, the affinity between polar solvent molecules and fumed silica particles prevents the formation of a 3D gel network. In this work it has been found that fumed silica can develop a flocculated suspension when it is dispersed in polypropylene glycol with a molecular weight of 2000 g/mol. Besides, it has been found that this suspension exhibits time dependent behaviour within its reversible shear thinning region, which is related to thixoelasticity. The experimental method, proposed theoretically by Cheng in 1986 to obtain the dynamic yield stress in thixotropic systems has been here extended successfully to a thixoelastic system.
This chapter constitutes one of the very few reviews in the existing literature on shoe bonding, and it gives an updated overview of the upper to sole bonding by means of adhesives. The surface preparation of rubber soles and both the formulations of polyurethane and polychloroprene adhesives are described in more detail. The preparation of adhesive joints and adhesion tests are also revised. Finally, the most recent development and technology in shoe bonding is described.
We report the use of fumed silica (hydrophilic colloidal silica particles) to generate triglyceride solvent-based soft matter systems (organogels and bigels). Interestingly, the bigels showed a better gel strength compared to organogels while showing a comparatively weaker thixotropic recovery. Electron microscopy and energy dispersive X-ray spectroscopy were used to understand the microstructure of these new thixotropic molecular gel systems with respect to the fractal-like aggregation of silica particles as well as the percolating network of organic-aqueous phases.
Polyvinyl alcohol (PVA)/fumed silica/clay nanocomposites are prepared via solution intercalation by exploiting phase separation based on the bridging of particles by polymer chains. PVA/fumed silica/clay nanocomposites are characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy, and thermogravimetric analysis. Mechanical properties are determined by universal testing machine. From FTIR results, it indicates that IR spectrum for PVA/fumed silica/clay nanocomposites, especially PVA/fumed silica/clay (1.30E) nanocomposites, is much broader than pure PVA and other clay nanocomposites. The better interfacial bonding between PVA/fumed silica/clay (1.30E) nanocomposites are reflected in the improvement of the mechanical properties as well as thermal stability. The surface area analysis result proves that the PVA/fumed silica/clay (1.30E) nanocomposites have higher surface area and pore volume with less pore size. With the addition of 1.30E clay to the composite system, the tensile strength and modulus had shown the highest values as well as higher activation energy for thermal decomposition. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41843.
In this study, the effect of nanosilica and boron carbide on the adhesion strength of a phenolic resin for graphite bonding was studied. The adhered specimens were cured at 250 degrees C and then heat treated to examine the thermal resistance in the range 200-1000 degrees C. Then, the adhesion strength of specimens was examined by a tensile method. The chemical structure of bonding and possible carbonization was examined using Fourier transform infrared spectroscopy (FTIR). Thermogravimetric analysis (TGA) was employed to determine the char yield, and X-ray diffraction (XRD) was used to study crystalline phases at different temperatures. The elemental analysis and morphology of the adhesive bond were investigated using energy-dispersive Xray spectroscopy (EDS) and scanning electron microscopy (SEM), respectively. The results showed that sintering of reformed boron carbide and amorphous carbon above 800 degrees C resulted in a significant increasing in adhesion strength at 1000 degrees C.
—Both fumed silica and sepiolite have been used as a filler of polyurethane (PU) adhesives. Although effective, the small particle size and the relative high cost of fumed silica are limitations in some applications. Sepiolite is cheaper than fumed silica, but its relatively large particle size facilitates its settling from the adhesive solutions. In this study, the usefulness of using sepiolite + fumed silica mixtures as a filler in solvent-based PU adhesives is demonstrated. The rheological and adhesion properties of the PU adhesive solutions and the rheological and mechanical properties of the PU films (without solvent) were studied. SEM micrographs of PU films showed the morphology and compatibility of the fillers with the PU matrix. The use of sepiolite + fumed silica mixtures inhibited the settlement of the filler from the PU adhesive solutions, increased both the storage and the loss moduli, and improved the rheological and mechanical properties of the PU. On the other hand, the green (immediate) T-peel strengths of roughened styrene-butadiene rubber/PU adhesive joints and plasticized PVC/PU adhesive joints were greatly improved in filled PU adhesives. The effects produced by using fumed silica alone or sepiolite + fumed silica mixtures were very similar, although in general, somewhat more marked in fumed silica-filled PU.
Thermoplastic polyurethane (TPU) adhesives containing nanosilicas with different specific surface area and silanol group content were prepared and characterized by FTIR spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TGA), X-ray diffraction, plate-plate rheology, dynamical–mechanical–thermal analysis (DMTA), transmission electron microscopy (TEM), and strain–stress test. Adhesive strength was obtained from T-peel tests of PVC/polyurethane adhesive joints.Formation of agglomerates of nanosilica particles within the polyurethane matrix were favoured by increasing the silanol content likely due to stronger hydrogen bond interactions between the silanol groups on the nanosilica over those between the polyurethane and the nanosilica. As a consequence, inter-urethane bonds formation rather than ester-urethane bonds were favoured, leaving the soft segment chains more free to interact between them. Thus, addition of nanosilica favoured the phase segregation in the thermoplastic polyurethane. The increase in specific surface area and silanol content in the nanosilica, generally enhanced the degree of phase separation in the polyurethane, being less marked for nanosilicas with more than 200m2/g and 0.60mmol SiOH/gsilica. On the other hand, the addition of the nanosilica improved the tensile strength and elongation at break, and the viscoelastic properties of the polyurethane. The immediate adhesive strength of PVC/polyurethane adhesive joints increased in the filled adhesives and it was determined by the rheological properties of the polyurethane–nanosilica mixtures. By increasing the time after joint formation, the crystallization of the polyurethane was produced giving higher adhesive strength and although a cohesive failure in the PVC was always obtained, a slight though progressive increase in joint strength was found with the passage of time with the ordering of the three systems (PU-0.45, PU-0.60 and PU-0.90) remaining unchanged with the PU-0.60 system the stronger and the PU-0.90 system the weaker. This is in agreement with the trends in the viscoelastic and mechanical properties of the filled adhesives.
The presence of hydrogen bonds in the chemical structure of polymers promotes and stabilises the crystalline phase. For liquid crystalline (LC) polymers, the side insertion of aliphatic units to the mesogenic unit is a suitable artifice to decrease the crystalline stability, without significantly affecting the stability of the LC phases. Here, we report on the synthesis of a LC homo-polyurethane with high hydrogen bond concentration along the chain and bearing an n-pentyl side-chain. Rheological behaviour, thermal analysis, and X-ray diffraction show that the stable LC phase is the nematic.
Three block polyurethane samples [differing in hard (urethane) to soft (macroglycol) segments ratio] dissolved in 2-butanone, are investigated. Rheological results obtained in the range 10-25°C led to the hypothesis of an order-disorder transition associated with H bonds being discarded. The critical concentration and the characteristic molecular weight for entanglements, Mc, are estimated approximately from linear viscosity results. Mc decreases as the percent of hard segments in the polyurethane increases, a result which is correlated with short-distance chain parameters such as length and molecular weight per bond and characteristic ratio. Annealing clear solutions at temperatures around 0°C causes haziness; on heating, a clearing temperature is detected at Tc ≈ 15°C. This transition coincides with a maximum observed in complex viscosity η* versus temperature curves. These results are probably due to incipient crystallization of soft segments domains. This hypothesis is compatible with differential scanning calorimetry results. Opaque solutions give rise to thermoreversible gels: we assume that the network branch points necessary for gelation involve microcrystalline domains which result from crystallization of soft segments from solution.
Two fumed silicas, one hydrophilic and another hydrophobic, were added to a two-component polyurethane (PU) adhesive and their properties compared. The filled polyurethanes were characterized by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and water and diiodomethane contact angle measurements. The adhesive strength was evaluated from single lap-shear tests of solvent wiped stainless steel/polyurethane adhesive joints. The fumed silicas were well-dispersed in the polyurethane matrix as bundles of nanometric spherical silica particles. However, some micron size agglomerates of fumed silica appeared in the filled polyurethane. The addition of fumed silica favoured the degree of phase separation between the hard and the soft segments in the polyurethane. The polyurethanes were not fully cured under the conditions used in this study and the addition of fumed silica inhibited the curing reaction; the extent of the curing reaction was not affected by the hydrophilic or hydrophobic nature of the fumed silica. The filled polyurethanes were further cured during DSC experiments and this was noticed by the appearance of the melting of the hard segments and the displacement of the second glass transition to a higher temperature. On the other hand, the addition of fumed silica increased the wettability and the surface energy of the polyurethane; this increase was mainly due to an increase in the polar component of the surface energy. The increase in surface energy is somewhat more marked in the case of the hydrophilic silica filled polyurethane, indicating that the polarity of the fumed silica affected the surface properties of the polyurethane. Finally, the addition of the hydrophilic fumed silica increased the adhesion of the filled polyurethane adhesive to stainless steel which was in agreement with the higher surface energy of the hydrophilic fumed silica filled polyurethane.
Three nanosilicas with different silanol contents were prepared by treatment of hydrophilic fumed silica with dimethyldichlorosilane. This treatment reduced the silanol content and produced the particle agglomeration of the nanosilicas. Thermoplastic polyurethane (TPU) adhesives containing nanosilicas were prepared and characterized by FTIR spectroscopy, differential scanning calorimetry (DSC), plate–plate rheology, dynamic mechanical thermal analysis (DMTA), transmission electron microscopy (TEM) and stress–strain testing. Adhesive strength was obtained from T-peel tests of PVC/polyurethane adhesive joints.The addition of hydrophilic nanosilicas favoured the degree of phase separation between the hard (i.e. isocyanate+chain extender) and soft (i.e. polyol) segments in the TPUs; the higher the silanol content on the surface of silica, the higher the degree of phase separation, and the crystallinity of the polyurethane (due to the soft segments) was also increased. Hydrogen bonds between the ester carbonyl groups in the TPU and the silanol groups on the silica surface were created and more favoured by increasing the silanol content. The tensile strength increased and the elongation at break of the polyurethane decreased by increasing the silanol content of the nanosilica. Addition of nanosilica increased the immediate adhesion of the polyurethane adhesives to PVC, irrespective of the silanol content on the nanosilica. The higher the mechanical and the rheological properties of the polyurethanes containing nanosilicas with different silanol content, the higher the final adhesive strength.
In this article, the influence of fumed silica nanofiller on the structure and properties of segmented polyurethane elastomer (PUR) was investigated. In order to investigate the interactions at the filler–matrix interface, nonmodified and commercially modified fillers (with methacrylsilane and octylsilane) were used. The PUR com-posites with 1.0, 2.0, 4.0, 6.0, and 9.0 vol % of all fillers were prepared by solution casting method. Surface free energy of the fillers and polymer matrix was determined using contact angle measurements with different testing liquids. Change in morphology was analyzed using optical polarization microscopy and distribution of the filler in polymer matrix using scanning electron microscopy. The influences of silica fillers on mechanical and thermal prop-erties of PUR were investigated. Results showed that sur-face treatment of silica filler with methacrylsilane and octylsilane reduces the agglomeration of particles that improves dispersion at microlevel. Addition of all fillers disrupts spherulite morphology and decreases crystallinity of the PUR matrix. Nonmodified silica nanofiller has the least pronounced influence on spherulite morphology and the lowest influence on polyurethane crystallinity and thus the best mechanical properties. Surface modification of silica with octylsilane has less influence on polyurethane crystallinity and on decreasing of mechanical properties than modification with methacrylsilane. V C 2011 Wiley Peri-odicals, Inc. J Appl Polym Sci 125: E181–E190, 2012
To analyse the parameters that influence the thickening mechanism of waterborne polyurethane adhesives, different hydrophobically modified ethoxylated polyurethane based thickeners (HEUR) were used. The thickeners were characterized by proton nuclear magnetic resonance (1H-NMR) and gel permeation chromatography (GPC). The thickened adhesive solutions were characterized by flow rheology, pH, particle size measurements, solids content, and confocal microscopy. The thickened solid adhesive films were characterized by ATR-IR spectroscopy, parallel plate rheology, dynamic mechanical thermal analysis (DMTA), and differential scanning calorimetry (DSC). The adhesion was measured by a T-peel test of leather/polyurethane adhesive/SBR rubber joints. The addition of the different HEUR thickeners increased the viscosity of the polyurethane dispersion to different degrees. Furthermore, whereas the un-thickened polyurethane dispersion showed a Newtonian rheological behaviour, a shear-thinning rheological behaviour was observed in the thickened dispersions due to thickener-thickener and polyurethane-thickener interactions. The viscosity of the thickened polyurethane adhesive solutions increased with the degree of hydrophobicity and the molecular weight of the thickener. The addition of different thickeners increased the pH values due to the ionic adsorption, which is one of the interactions that contribute to the thickening mechanism of the polyurethane thickeners, besides hydrogen bonding and van der Waals hydrophobic interactions (micelles). The entanglement of the thickened polyurethane adhesives was studied by confocal microscopy. Although the addition of the thickeners did not affect the thermal properties of the polyurethanes, the T-peel strength of leather/adhesive/SBR rubber joints was influenced by the rheological properties of the thickened adhesives.
A modified clay was used to prepare waterborne polyurethane (WPU)/clay nanocomposite dispersions in this study. XRD and TEM examinations indicate that the clay platelets are exfoliated. Moreover, AFM phase image of the WPU/clay film shows that the isolated clay-rich phases are surrounded by a continuous WPU-rich phase. Clay does not influence the location and peak broadness of the Tg of soft segment domains in the WPU/clay films. However, clay raises the Tg of hard segment domains, which is probably due to the ionic interactions between the clay and hard segments of WPU. These ionic interactions could be evidenced by IR spectra of the WPU/clay nanocomposites. The WPU/clay dispersion with higher clay content exhibits a larger average particle size, a less negative zeta potential, and a higher viscosity, whereas the corresponding cast film possesses lower volumetric and surface electrical resistances. The volumetric and surface electrical resistances of the WPU/clay nanocomposite film are nearly identical. This indicates that clay platelets do not migrate to the surface regions of WPU/clay films because of the above-mentioned ionic interactions. In addition, the incorporation of clay is also capable of enhancing the thermal resistance and tensile properties of WPUs dramatically.
Hard segment hydrogen bonding interactions in a particular polyurethane segmented copolymer have been analysed through a viscometric approach to determine the behaviour of polyurethane solutions with different solvent compositions. Analysis of log η sp versus log c [ η ] master curves (solely comprising dilute and semidilute regimes) showed that these systems could be differentiated by the slope of the first part of the curve (dilute regime), the characteristic reduced concentration c *[ η ], and the slope of the second part of the curve (semidilute regime), differences which were related to a possible occurrence of polymer aggregates. Experiments with solutions in the concentrated regime, at different temperatures, were used to relate non‐Newtonian behaviour to hydrogen bonding.
© 2001 Society of Chemical Industry
0.5–3 wt% nanosilica was added to an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) and cured at 25, 40 or 60 °C using isophoronediamine (IPDA) as hardener. Aggregates of nanosilica were properly dispersed into the DGEBA-IPDA resin and agglomerates formation was avoided. Addition of nanosilica increased the storage modulus E′ and the area and height of the tan δ curve of DGEBA-IPDA resin cured at 25 °C, but no significant differences were found by curing at higher temperature. Gel time measurements and the results obtained by applying the Kamal model to isotherm DSC curing of DGEBA-IPDA-nanosilica revealed that nanosilica catalysed the curing reaction between DGEBA and IPDA, in less extent by increasing the curing temperature.
The effects of ionic interactions between clay and waterborne polyurethanes on the structure and physical properties of their nanocomposite dispersions are analyzed. The performance of solvent-based PUs or WPUs can be enhanced by the incorporation of nanosized layered silicates. Most PU/clay nanocomposite have been prepared in various organic solvents, so a lot of organic solvent needs to be removed. The WPU/clay nanocomposites are prepared either by a modified acetone process or by the simple mixing of a PU latex with an aqueous silica dispersion. The viscosity of the neat WPU dispersion is 5 cps, which is not viscous. The viscosities of the WPU/clay dispersions with clay concentrations of 1 and 5 wt % are 6 and 72 cps, respectively. XRD and TEM examinations of the WPU/clay nanocomposites indicate that most clay platelets are exfoliated. The ionic interactions are evidenced by IR spectra of the WPU/clay nanocomposites. The WPU/clay dispersion with a higher clay content exhibits a higher viscosity.
The objective of this paper is to present the investigations of the synthesis of polyurethane (PU) spinning solutions in dimethylformamide (DMF) controlled by means of the power consumption. Power input to the mixed bulk for the two-type synthesis of polymerization process was measured in a reactor composed of multi-ribbon agitators. For these cases, the mixing power, speed rotation, temperature and mass of the reactor bulk were measured. The resultant density of mixer bulk at time duration of polymerization process was calculated using the mass and volume of reactionary mass. Based on the experimental data for the different Newtonian and non-Newtonian fluids, the analytical relationship was proposed for determination of relation between power number and Reynolds number. The flow curves of the obtained polyurethanes (PUs) from the synthesis were determined using a rotational stress-controlled rheometer and the obtained results compared with a earlier experimental data given in reference Lobanova et al. [G.A. Lobanova, V.Z. Volkov, A.I. Sokolova, Fibre Chemistry 13 (6) (1982) 399–401]. Temporal changes of the apparent viscosity of reactor bulk were obtained by shifting the power number results to the power characteristic curve and calculated from the appropriate Reynolds number. It is observed that the relationship between these dimensionless numbers may be presented in the simulation form of non-stationary power characteristics.
To adjust the rheology of waterborne polyurethane adhesives, different amounts of a hydrophobically modified ethoxylated polyurethane thickener (HEUR) were added. The thickened adhesive solutions were characterized by flow rheology, pH measurements, particle size, solids content and confocal microscopy. The thickened solid adhesive films were characterized by IR spectroscopy, plate-plate rheology, dynamic mechanical thermal analysis (DMTA), differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. The adhesion was measured from T-peel test of plasticized PVC/polyurethane adhesive/plasticized PVC and leather/polyurethane adhesive/SBR rubber joints. The addition of the HEUR thickener increased the viscosity of the polyurethane dispersion, and a shear-thinning behaviour was observed due to polyurethane–thickener interactions. The addition of thickener improved the rheological properties of the polyurethane, more noticeable as the content of the thickener increased. The crosslinking of the thickened polyurethane was studied by confocal microscopy. Although the addition of the thickener did not greatly affect the thermal properties of the polyurethane, an improvement in the adhesive strength of leather/adhesive/SBR rubber joints was obtained.
This study combines the properties of siloxanes and lignin polymers to produce hybrid functional polymers that can be used as adhesives, coating materials, and/or multifunctionalized thin-coating films. Lignin-silica hybrid copolymers were synthesized by using a sol-gel process. Laccases from Trametes hirsuta were used to oxidize lignosulphonates to enhance their reactivity towards siloxanes and then were incorporated into siloxane precursors undergoing a sol-gel process. In vitro copolymerization studies using pure lignin monomers with aminosilanes or ethoxytrimethylsilane and analysis by ²⁹Si NMR spectroscopy revealed hybrid products. Except for kraft lignin, an increase in lignin concentration positively affected the tensile strength in all samples. Similarly, the viscosity generally increased in all samples with increasing lignin concentration and also affected the curing time.
Poly(vinylpyrrolidone) (PVP) hydrogels were crosslinked by gamma irradiation to add structure and rigidity, and then rheological and mucoadhesive properties were evaluated. The effects of PVP concentration, radiation dose, and additives, such as poly(ethylene glycol) (PEG) and glycerol, on rheological properties were investigated. In an oscillatory analysis, an increase in polymer concentrations increased the storage modulus (G') and the loss modulus (G″) but decreased the loss tangent (tan δ < 1). The relationships between G'or G″ and the frequency levelled off at higher frequencies, which is indicative of polymer chain entanglement and network formation. Each of the 6% PVP hydrogels exhibited plastic flow with rheopectic behavior. PVP concentration, radiation dose, and the presence of PEG or glycerol influenced the rheological and mucoadhesive properties of the hydrogels. However, adding acyclovir to the formulation did not have a profound effect on the rheological behavior of the hydrogels. The results suggest that a 3% PVP hydrogel with 1% PEG crosslinked with 20 kGy is the most appropriate hydrogel. The results demonstrated the successful complementary application of oscillatory and flow rheometry to characterize and develop a hydrogel for mucosal drug administration.
The kinetics of thermal degradation process on thermoplastic polyurethanes (TPUs) adhesives with hydrophilic nanosilicas was studied using isothermal thermogravimetric analysis within the temperature range from 360 degrees C to 460 degrees C in nitrogen. The effect of nanosilicas with different hydrophilic degree in the thermoplastic polyurethanes was investigated. It was found that the thermoxidative degradation of polyurethane-silica nanocomposites takes place in one step. An analysis of the isothermal methods to evaluate kinetic parameters of decomposition of solids from isothermal thermogravimetric data is presented. Patents WO07146353A2 and US20070292623A1 have some relevant information about the topic develop in this study, because the principle in both cases relies on the interactions between reactive groups in the polymer (TPUs) and the silanol in the silica nano-particles.
Flow can induce reversible and irreversible structural changes in dispersions. The analysis of flow effects on microstructure and rheology remains one of the challenging problems in colloid science. The rheological manifestation of flow-induced structural changes is a variable viscosity. If the changes are reversible and time dependent, the effect is called thixotropy. The basic elements of this concept are reviewed here, including its definition and the relation with nonlinear viscoelasticity. The omnipresence of thixotropy is illustrated with a wide range of examples from natural and manmade colloidal systems. Its various rheological manifestations are reviewed as well as possible measurement procedures. The microstructural changes due to flow are quite complex and not fully understood. Existing models for thixotropic suspension rheology are categorized and evaluated.
—Different amounts (5-20 wt% with respect to the solids content in the adhesive) of a hydrated magnesium/aluminum silicate (attapulgite) were added to solvent-based polyurethane (PU) adhesive formulations. The rheological, thermal, mechanical, surface, and adhesion properties of the filled PU adhesives obtained were measured. The addition of attapulgite provided an increase in viscosity, imparted thixotropy and pseudoplasticity to the PU adhesive solution, and produced an increase in storage and loss moduli. Increase in the amount of attapulgite enhanced these effects. Some interactions (not well defined in this paper) between the attapulgite, the polyurethane, and/or the solvent seemed to be responsible for the improved properties of the filled PU adhesives. Furthermore, the addition of attapulgite increased the mechanical properties, decreased the glass transition temperature, affected the crystallization rate and melting process, and modified the surface properties of the PU adhesive films (without solvent). On the other hand, the immediate (green) T-peel strength of roughened or (roughened + chlorinated with 1 wt% trichloroisocyanuric acid solutions in 2-butanone) styrene-butadiene rubber (SBR)/PU adhesive joints was greatly improved if the adhesive contained attapulgite. However, similar T-peel strength values in fully cured adhesive joints (measured 72 h after bond formation) were found irrespective of whether the PU adhesives contained attapulgite or not.
The influence of the addition of silica (Aerosil-200) (5-25 wt%) to polyurethane adhesives on their adhesion properties with non-chlorinated and surface-chlorinated rubbers has been studied. The chlorinating agent was Trichloroisocyanuric acid (TIC) in 2-butanone solution at a concentration of between 1 and 9 wt%. In general, silica produced an increase in the adhesive viscosity and an improvement of green (immediate) peel strength (especially with chlorinated rubber). The best results were obtained for a silica content of 10-20 wt%. However, the addition of silica did not improve the peel strength after a thermal ageing process. Polyurethane adhesives containing silica undergo an improvement in their resistance to degradation by chlorine on the rubber surface. On the other hand, the chlorination of silica produces the rupture of Si-O bonds and the formation of Si-H and Si-Cl groups. Furthermore, the stirring speed (directly related to the dispersion) of silica into the adhesive is an important parameter which affects the viscosity and peel strength. A stirring speed of 1000 rpm gives the best silica dispersion.
Fumed silica is a well-known mineral filler of epoxy and polyurethane adhesives. Although effective, the small particle size and the relative high cost of fumed silicas suggest the need for an alternative filler. In this study, the usefulness of adding a natural hydrated magnesium silicate (sepiolite) as a new filler in solvent-based polyurethane (PU) adhesive formulations has been demonstrated. The rheological and adhesion performance of the sepiolite-filled PU adhesive was compared with that in PU adhesives containing fumed silicas. The addition of a filler to PU adhesives provided an increase in viscosity, imparted thixotropy and pseudoplasticity to the adhesive solution, produced an increase in storage and loss moduli, and improved the rheology of the PU. The mechanical properties of adhesive films were increased by adding filler, mainly with fumed silica. On the other hand, the immediate T-peel strength of roughened or (roughened + chlorinated) styrene-butadiene rubber/PU adhesive joints was greatly improved in filled PU adhesives. The effects produced by adding sepiolite or fumed silica to the adhesives were very similar, although in general more noticeable in fumed silica filled PU due to the formation of hydrogen bonds between the filler and the solvent and/or the polyurethane (in sepiolite-filled adhesives, van der Waals forces seemed to be responsible for the interactions between the filler and the solvent and/or polyurethane).
A sepiolite silicate was heat-treated at 550 and 1000°C to modify its structure, and was used as a filler in a solvent-based polyurethane (PU) adhesive. The treated sepiolites were characterized by X-ray diffraction and infra-red spectroscopy, and it was observed that the water was irreversibly removed from the structure and pores of the sepiolite, changing the structure. The increase of temperature produced a collapse of the sepiolite structure. The rheological, mechanical, thermal and adhesion properties of the filled PU adhesives were measured. In general, the addition of treated sepiolite to PU adhesives resulted in a loss of adhesive properties with respect to the blank (PU adhesive with untreated sepiolite). The loss in properties was more noticeable as the treatment temperature increased. Thus the PU adhesives containing treated sepiolite had reduced rheological properties (lower viscosity, lower storage and loss moduli, and they did not provide thixotropy and pseudoplasticity to the solutions) with respect to the PU adhesive filled with untreated sepiolite. On the other hand, the addition of treated sepiolite decreased the mechanical and thermal mechanical properties of PU films. The T-peel strength of roughened and roughened + chlorinated (with 1 wt% trichloroisocyanuric acid in 2-butanone) styrene-butadiene rubber/PU adhesive joints was improved if the PU adhesive contained untreated sepiolite, but it decreased if the sepiolite was heat-treated. Interactions between the untreated sepiolite, the solvent and the polyurethane were responsible for the improved properties of PU adhesives. These interactions disappeared when the sepiolite was heat-treated, because of the destruction of the structure of the sepiolite and the removal of surface silanol groups.
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