Structure,composition, and adhesion properties of thermoplastic polyurethane adhesives

The composition and hard segment content of 13 commercial thermoplastic polyurethane elastomers (TPUs) were obtained using 1H-nuclear magnetic resonance (¹H-NMR). The properties of the TPUs were studied using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), dynamic mechanical thermal analysis (DMTA), and contact angle measurements. Solventbased adhesives were prepared by dissolving the TPUs in 2-butanone. Films of the TPUs were obtained by solvent evaporation, and their properties were studied. Adhesion properties were determined from T-peel tests on solvent-wiped poly(vinyl chloride) (PVC)/polyurethane adhesive joints. The influence of the segmented structure on the properties of the TPUs was assessed. The increase in the hard segment content in TPUs favoured the incompatibility (i.e. reduced phase separation) between hard and soft domains. TPUs with a high hard segment content had a low crystallinity, a low wettability, and a high joint strength. The storage and loss moduli obtained using DMTA decreased as the hard segment content in the TPUs increased. Furthermore, the TPUs prepared using ε-polycaprolactone as the macroglycol had a slower crystallization rate than those prepared using the polyadipate of 1,4-butanediol or the polyadipate of 1,6-hexanediol. The increase in the length of the hydrocarbon chain of the macroglycol improved both the rheological and the thermal properties of the TPUs. Finally, TPUs prepared using MDI as the isocyanate showed a higher crystallinity and a higher degree of crosslinking than those prepared using TDI.     

Characterization of thermoplastic polyurethane adhesives with different hard/soft segment ratios containing rosin as an internal tackifier

Three thermoplastic polyurethanes (TPUs) containing different hard/soft (h/s) segment ratios (1.05-1.4) were prepared using the prepolymer method. MDI (diphenylmethane-4,4′diisocyanate) and polyadipate of 1,4-butanediol (M _w = 2440) were allowed to react to produce the prepolymer. To provide the polyurethanes with high immediate adhesion to different substrates, a rosin + 1,4-butanediol mixture (1 : 1 equivalent%) was used as chain extender (TPU-Rs). These TPU-Rs had two types of hard segments: (i) Urethane hard segments, produced by reaction of the isocyanate and the 1,4-butanediol, and (ii) Urethan-amide hard segments, produced by reaction of the isocyanate and the carboxylic acid functionality of the rosin. The TPUs and TPU-Rs were characterized using FTIR spectroscopy, gel permeation chromatography, differential scanning calorimetry, stress-controlled plate-plate rheology, stress-strain measurements, and Brookfield viscosity. The TPUs and TPU-Rs were used as raw materials to prepare solvent-based polyurethane adhesives, the adhesion properties of which were obtained from T-peel tests on PVC/polyurethane adhesive/PVC joints. The addition of rosin as an internal tackifier increased the average molecular weight, more markedly in the TPU-Rs containing higher hard/soft segment ratios, but the elastic and viscous moduli decreased. An increase in the hard/soft segment ratio of the TPU-Rs retarded the kinetics of crystallization (which was determined by the soft segment content in the polyurethane), and increased the immediate T-peel strength in PVC/polyurethane adhesive/PVC joints (which was determined by the urethan-amide hard segments). Furthermore, addition of rosin to the polyurethanes decreased the final adhesion, although always reasonably high peel strength values were obtained.     

Synthesis and characterization of new thermoplastic polyurethane adhesives containing rosin resin as an internal tackifier

Three thermoplastic polyurethane elastomers (TPUs) were prepared using the prepolymer method. MDI (diphenylmethane-4,4′-diisocyanate) and the polyadipate of 1,4-butanediol (M_w = 2400) were reacted to produce a prepolymer containing unreacted isocyanate groups; chain extenders were different mixtures of 1,4-butanediol and a rosin resin (0-50%). The specific feature of this procedure was the introduction of a rosin resin as an internal tackifier to provide higher immediate adhesion to the TPUs. The new TPUs were characterized using gel permeation chromatography, wideangle X-ray diffraction, differential scanning calorimetry, stress-controlled rheology, and stress-strain measurements. The TPUs were used as raw materials to prepare solvent-based polyurethane adhesives, the adhesion properties of which were obtained from T-peel tests on PVC/polyurethane adhesive/PVC and leather/polyurethane adhesive/PVC joints. The addition of rosin resin as an internal tackifier contributed to the production of two types of hard segments, which affected the properties of the TPUs. Therefore, rosin resin as an internal tackifier produced an increase in the average molecular weight, an increase in the viscosity, and improved the rheological properties. The glass transtition temperature decreased if the TPUs contained rosin resin, due to a greater degree of incompatibility between the hard and soft segments. Consequently, slower kinetics of crystallization was obtained in the TPUs containing rosin resin. Depending on the amount of rosin resin in the TPU, different structures and properties were obtained. On the other hand, the immediate T-peel strength in all joints was improved if the TPU contained rosin resin.     

Solid-state NMR evaluation of the silane structure on nanoporous silica fillers

Coupling agents play a critical role in the function of organic/inorganic composite materials. However, the structure of these compounds as they reside on the filler surface is not well understood. Solid-state CP-MAS ^T13C and ²⁹Si FT-NMR were used to study the coupling of γ-methacryloxypropyltrimethoxysilane (MPS) on experimental nanoporous silica fillers following exposure to different coupling environments. The progressive consumption of Si OH species by the coupling reaction was clearly detected. Ethanol, a widely utilized solvent in coupling reactions, competes with the coupling agent for the surface. Different degrees of coupling apparently produce surface structures that vary from isolated MPS molecules to heavily condensed films. The use of CP-MAS ¹³C-NMR presents some advantages, as it detects uncondensed MPS molecules in the most condensed film. These results show that NMR can provide information that is otherwise unavailable on the attachment and structure of silane coupling agents resident on the surface of porous silica fillers.     

Effect of Surface Roughness on the Adhesive Strength of the Heat-Resistant Adhesive RTV88

Heat-resistant adhesive RTV88 is a hyper-elastic material and so far there have been little research on using RTV88 in adhesive joints. In this study, the effect of surface roughness on the adhesive strength of RTV88 was examined. Aluminum adherends were first sandblasted in order to generate rough surfaces, and then tensile–shear tests on Al/RTV88 single lap joints were performed. The shear strength was shown to be influenced by surface roughness. Peel failure was dominant when the surface roughness was at a low level. However, cohesive failure was the major type of failure when the surface roughness was at a high level. Effective area, peel failure area, and cohesive failure area were introduced to explain the effects of surface roughness on the adhesive strength. An empirical relation for the failure force was proposed, based on these parameters. Tensile tests of the RTV88 bonding was performed in order to obtain the necessary data. Finally, the empirical relation for the failure force was verified by tensile–shear test results.     

Effect of phenolic resin structure on the bonding properties of nitrile rubber-based adhesives

Three phenolic resins were used to study the effect of resin structure and its percentage content on the bonding properties of nitrile rubber-based adhesives. The phenolic resins studied are derived from phenol, resorcinol and para-t-butyl phenol, and are of novalac type. These resins were characterized by infra-red and thermal studies. Several formulations were prepared by varying the phenolic resin/nitrile rubber ratio and these formulations were tested for viscosity and bonding properties. At a given resin-to-rubber ratio, the viscosity for resorcinol-formaldehyde resin was higher than phenol-formaldehyde resin followed by para-t-butyl phenol-formaldehyde resin. The peel strength of nitrile rubber-based adhesive prepared from resorcinol-formaldehyde resin was higher than adhesives that from para-t-butyl phenol-formaldehyde resin followed by phenol-formaldehyde resin. The lap shear strength of nitrile rubber-based adhesive containing para-t-butyl phenol-formaldehyde resin was higher than of adhesives containing the other two resins. The trend observed in peel strength could be explained on the basis of hydrogen bonding between hydroxyl groups of phenolic resin (adhesive) and hydroxyl groups of the starch-coated canvas cloth (adherend), whereas the trend observed in lap shear strength could be explained on the basis of extent of linearity in the resin structure.     

Effect of Nanoclay Incorporation on the Impact Properties of Adhesively Bonded Composite Structures

During manufacturing or service conditions, adhesively bonded composites are often subjected to impact. This impact may result in a reduction in strength and structural integrity of engineering components that are composed of adhesively bonded composite structures. The investigation of the degradation of strength of structural joints is, therefore, of paramount importance for their successful performance. Impact resistance of bondline in adhesively joined composites can be altered by the addition of nanoclay in the adhesive during fabrication of adhesive joints. In this study, impact test was carried out on graphite–epoxy composite panels bonded with nanoclay adhesive at different impact energies using drop-weight impact test equipment. Adhesive joints were fabricated by adding nanoclay in volume fractions of 1, 2 and 5% in the adhesive bondline. For comparison, plain adhesive joints were fabricated without nanoclay incorporation in the bondline. Impact testing was performed on these joints at 5, 10 and 20 J, to study the effect of inclusion of nanoclay in the epoxy adhesive. In order to determine the flexural load bearing capacity and stiffness reduction after impact, a three-point bending test was conducted on unimpacted and impacted specimens. The results showed that there was an improvement in impact capacity, however there was a reduction in flexural strength due to nanoclay incorporation.     

The effect of silica (SiO2) nanoparticles and ammonia/ethylene plasma treatment on the interfacial and mechanical properties of carbon-fiber-reinforced epoxy composites

A nanoparticle dispersion is known to enhance the mechanical properties of a variety of polymers and resins. In this work, the effects of silica (SiO₂) nanoparticle loading (0–2 wt%) and ammonia/ethylene plasma-treated fibers on the interfacial and mechanical properties of carbon fiber–epoxy composites were characterized. Single fiber composite (SFC) tests were performed to determine the fiber/resin interfacial shear strength (IFSS). Tensile tests on pure epoxy resin specimens were also performed to quantify mechanical property changes with silica content. The results indicated that up to 2% SiO₂ nanoparticle loading had only a little effect on the mechanical properties. For untreated fibers, the IFSS was comparable for all epoxy resins. With ethylene/ammonia plasma treated fibers, specimens exhibited a substantial increase in IFSS by 2 to 3 times, independent of SiO₂ loading. The highest IFSS value obtained was 146 MPa for plasma-treated fibers. Interaction between the fiber sizing and plasma treatment may be a critical factor in this IFSS increase. The results suggest that the fiber/epoxy interface is not affected by the incorporation of up to 2% SiO₂ nanoparticles. Furthermore, the fiber surface modification through plasma treatment is an effective method to improve and control adhesion between fiber and resin.     

Effect of fumed silica properties on the thermal insulation performance of fibrous compact

The particle properties play an important role in developing advanced thermal insulation board made by fumed silica. In this study, the effect of hydrophilic or hydrophobic on the thermal conductivity and fracture strength of the resultant insulation board was investigated. A higher thermal conductivity and higher strength were obtained in using hydrophilic particles. Very low thermal conductivity, below 0.02 W/m K, was achieved in using hydrophobic particles. The difference of thermal conductivity was mainly dominated by solid part. The decrease of thermal conductivity and increase of fracture strength were related to the bonding condition between particles in the boards.     

Effect of sub-micron silica fillers on the mechanical performances of epoxy-based composites

Solid state thermo-mechanical properties, as well as low and large strain mechanical behaviour, of epoxy composites filled with sub-micron pyrogenic silica are discussed in this paper. The reinforcement mechanisms involved are investigated. Two distinct series of pyrogenic silica were used: hydrophilic silica with various specific surface areas and silica grafted with various organo-modifications. Furthermore, two series of networks, having either a high or low crosslink density, and resulting thus either in glassy or rubbery materials at room temperature, were considered. Dynamic mechanical analysis, uniaxial tensile tests and fracture mechanic tests were performed.All our results showed that pyrogenic silica leads to an improvement of network mechanical properties both in the glassy and rubbery states. The simultaneous increase of stiffness and toughness was observed, demonstrating the great potential of pyrogenic silica for the reinforcement of thermosetting systems. This exceptional behaviour has been interpreted in terms of the interactions and morphology developed.     

气相白炭黑的比表面积与表面特性对硅橡胶补强效果的影响

探讨了气相白炭黑的比表面积和表面特性对硅橡胶力学性能的影响,同时采用扫描电镜和溶胀法分别研究了不同表面特性白炭黑补强硅橡胶的拉伸断面形貌和交联密度。结果表明：随着疏水性白炭黑比表面积的增加,硅橡胶拉伸强度和断裂伸长率增加;对比发现,疏水性白炭黑的补强效果优于亲水性白炭黑,这是由于疏水性白炭黑在橡胶基体中分散更加均匀,而且疏水性白炭黑补强硅橡胶的交联密度更大。     

Effect of silica and rice husk ash fillers on the modulus of polysiloxane networks

The preparation of polymethylvinylsiloxane rubbers with silica (SC) and black rice husk ash (BRHA) as fillers is reported. We conducted stress–strain experiments on these vulcanized rubbers to study the reinforcement properties of the fillers. Curves showing the dependence of the stress on the reciprocal of the elongation ratio displayed, in most cases, a slow upturn starting at rather low values of the elongation ratio attributed to a rather wide distribution of chain lengths between crosslinking points. Physical crosslinks between the hydroxyl groups of SC fillers and the polymer matrix seemed to enhance the modulus. BRHA–polymer interactions were rather weak in comparison with those occurring between SC and the polymer, presumably as a consequence of the carbon coating of the surface of the former fillers. These interactions were even less important for calcined BRHA. The absence of voids in the polymer–filler interfaces was proven by the analysis of gas diffusion across the rubbers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 421–429, 2003     

Effect of loading chemically and mechanically pre-treated fumed silica as filler on an etch & rinse model dental adhesive

Fumed silica is one of the most commonly used fillers for dental resin adhesives. However, fumed silica produced by flame hydrolysis of chlorosilanes are typically agglomerated aggregates instead of isolated primary particles. The fumed silica aggregates may affect the dispersion of fillers in the resin matrix, thus affect the bonding performance of the dental adhesives. In this study, fumed silica fillers chemically treated with methacrylsilane were loaded into an etch & rinse model dental adhesive. Filler 1 was only chemically pre-treated, while Filler 2 was combined chemically and mechanically pre-treated to break the agglomerated aggregates. The experiment result indicated that Filler 1 has faster rheology control effect than Filler 2 on the handling property of the filled resin adhesive showing as the rapid increased viscosity, which may due to the fast inorganic phase growing in the Filler 1 filled resin matrix. As for the mechanical properties, neither Filler 1 nor Filler 2 has an positive effect on the micro-tensile bond strength in short term and long term, despite of the fact that lower weight percent of Filler 2 loading is favourable to the penetration of the experimental adhesive into dentin substrate. In conclusion, mechanically pre-treatment to break the agglomerated fumed silica is helpful to increase the limit of filler loading percent while maintaining appropriate handling property, but the chemically and mechanically pre-treatment cannot provide positive effect on bonding performance within the range of this study.     

Effect of protic solvents on the physical properties of the ambient pressure dried hydrophobic silica aerogels using sodium silicate precursor

Considering the importance of the highly porous, low density, transparent and nanostructured hydrophobic silica aerogels in the scientific and industrial applications, the experiments have been carried out to prepare the low density silica aerogels using the 1.12 specific gravity water glass (sodium silicate, Na₂SiO₃) precursor, ammonium hydroxide (NH₄OH) catalyst, trimethylchlorosilane (TMCS) silylating agent, various first exchanging protic solvents and hexane as a second exchanging aprotic solvent. The first exchanging solvents used were: methanol, ethanol, propanol, isopropanol, butanol, isobutanol and hexanol. The molar ratio of the Na₂SiO₃:H₂O:NH₄OH:TMCS was kept constant at 1:56:0.02:0.4 respectively. The ambient pressure dried method was used for the preparation of hydrophobic silica aerogels. The effect of the exchanging protic solvents on the physical properties of the aerogels such as density, % of volume shrinkage, % of porosity, % of optical transmission, thermal conductivity, thermal stability and contact angle of the aerogels with water, were studied. FTIR studies were carried out to confirm the silylation of the aerogel samples. It was found that the exchanging protic solvents have profound effect on the physical and hydrophobic properties of the aerogels. Low density (0.07 g/cm³), high porosity (96.6 %), low thermal conductivity (0.091 W/mK), high contact angle (166°) silica aerogels could be prepared by using the isopropanol first exchanging solvent followed by the hexane as the second exchanging solvent along with the TMCS silylating agent with sodium silicate precursor.     

Effect of isocyanate‐modified fumed silica on the properties of poly(butylene succinate) nanocomposites

Using the grafting method on a silica surface with PBS molecules, we prepared novel poly(butylene succinate) (PBS)/silica nanocomposites to enhance dispersibility and interfacial adhesion between silica particles and the PBS matrix, and also investigated the effects of silica‐g‐PBS on the PBS matrix using differential scanning calorimetry, thermogravimetric analysis, transmission electron microscopy, a tensile testing machine, and rheometry. The thermal stability, mechanical properties, and rheological properties of PBS nanocomposites containing silica‐g‐PBS was remarkably improved because of the surface characteristics of the silica grafted with PBS molecules, which provided good compatibility and dispersion. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effect of alkali and alkaline earth metal ion as glass modifiers on the spectroscopic characteristics of Er3+-ion doped lead silicate glasses

The spectroscopic characteristics of Er-doped lead silicate glasses were investigated with respect to the effects of glass modifiers (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺) with various optical basicities. Using the absorption spectra of the glasses, the Judd–Ofelt parameters of the glasses were calculated and examined, with an emphasis on the glass emission intensity ratio at 1572 nm. The spectra of the samples at low temperatures were examined, and the Stark splitting of Er3+ was investigated. The McCumber method was used to determine the emission cross sections of glasses. The SPM glass exhibited high values of full width at half maximum (51.24 nm) and the emission cross section at 1572 nm (1.908 × 10⁻²¹ cm²), with potential applications for guiding component design of 1.5-μm fiber lasers and amplifiers.     

Effect of Ti3C2Tx/Ag MXene fillers on the electrical conductivity of Ag-coated Cu conductive adhesives

In this paper, we prepared ECAs (electrical conductive adhesives) with high electrical conductivity by using Ag-coated copper powder and M − II (Ti₃C₂T_x/Ag powder) as conductive filler. M-Ⅰ (Ti₃C₂T_x) and M-Ⅱ were prepared by acid etching and hydrothermal methods, respectively, and the electrochemical properties of M-Ⅱ and the effects of different contents of M-Ⅱ on the conductivity of ECAs were investigated. M − II has a unique reticular structure which helps to create conductive pathways, and its high specific surface area provides a large number of active sites for charge storage, resulting in increased electrical conductivity. The experimental results show that the best conductivity is achieved when the M-Ⅱ powder content is 1.2%, with a volume resistivity of 4.84 × 10⁻⁶ Ω m, at which time the Ag-coated Cu powder content is 69.8%. It has been shown in many studies that only when Ag-coated copper powder content is 70%–80%, the resistivity reaches about 10⁻⁶ Ω m. This work showed that adding a small amount of M − II and reducing the amount of Ag-coated copper powder could maintain the high electrical conductivity of ECAs.     

Investigation of chemical transformations of thiophenylglycoside of muramyl dipeptide on the fumed silica surface using TPD-MS,FTIR spectroscopy and ES IT MS

In this study, chemical transformations of benzyl ester of О-(phenyl-2-acetamido-2,3-dideoxy-1-thio-β-d-glucopyranoside-3-yl)-d-lactoyl-l-alanyl-d-isoglutamine (SPhMDPOBn) on the fumed silica surface were examined, and the surface complex structure was characterized by temperature-programmed desorption mass spectrometry (TPD-MS), infrared spectroscopy (FTIR) and electrospray ion trap mass spectrometry (ES IT MS). Stages of pyrolysis of SPhMDPOBn in pristine state and on the silica surface have been determined. Probably, hydrogen-bonded complex forms between silanol surface groups and the C = O group of the acetamide moiety NH-(CH₃)-C = O…H-O-Si≡. The thermal transformations of such hydrogen-bonded complex result in pyrolysis of SPhMDPOBn immobilized on the silica surface under TPD-MS conditions. The shifts ∆ν of amide I band (measured from 1,626 to 1,639 cm^−l for SPhMDPOBn in pristine state) of 33 and 35 cm^−l which occurred when SPhMDPOBn was immobilized on the silica surface may be caused by a weakening of the intramolecular hydrogen bonding of the SPhMDPOBn because the interaction with the silica surface as hydrogen bond with silanol groups is weaker than that in associates.     

Effect of calcium to silica ratio on the synthesis of calcium silicate hydrate in high alkaline desilication solution

High alkaline desilication solution (DSS), a high volume byproduct from the pretreatment of high-alumina fly ash, was used as low-cost mother liquor for the synthesis of calcium silicate hydrate (C-S-H). Through the combined analysis of X-ray diffraction, thermogravimetric analysis, X-ray fluorescence, ²⁹Si MAS NMR, and Brunauer-Emmett-Teller, the relationship between chemical composition and structure of C-S-H synthesized under Ca/Si of 0.83:1 to 2.0:1 was investigated. Silicon conversion and yield of product have a positive correlation with Ca/Si. Sodium uptake in C-S-H is inhibited as Ca/Si increases. The formation of sodium in C-S-H transfers from “bound Na” to “mobile Na” and aluminum from tetrahedrally coordinated Al (IV) to octahedrally coordinated Al (VI). The increase of Ca/Si leads to shortening of silicate chain and formation of more dimers, which causes more water bound in C-S-H. The mechanism of calcium addition on silicate chain obtained from DFT calculation primarily results from more interlayer calcium occurrence to affect bridging tetrahedron and cationic bounding states reorganization. Reasonable control for Ca/Si momentously contributes to the adjustment for composition and structure of C-S-H synthesized in DSS.     

Effect of aging temperature on the porosity characteristics of subcritically dried silica aerogels

Silica aerogels were synthesised by subcritical drying technique which involves controlled solvent exchange and aging of the wet gel in silane solution followed by drying under controlled conditions. Effect of temperature of aging in silane solution on the porosity characteristics of silica aerogels and the thermal pore stability of the resultant gels were investigated. Aging in silane solution leads to an increased degree of condensation reactions, siloxane crosslinking and the dissolution and reprecipitation of silica monomers to the gel structure and enhances the total strength of the gel. Thermal aging of the wet gel have a pronounced effect on bulk density, linear drying shrinkage, surface area and pore volume. As the temperature of aging increases the bulk density decreases whereas the surface area and pore volume were found to increase. We could achieve a surface area of 1040 m²/g, pore volume 1.2 cc/g and an average pore size of 49 Å corresponding to an aging temperature of 70 °C. Thermal pore stability of the gel was found to be up to 700 °C above which densification of SiO₂ gel starts. The novel findings will help in tailoring the process parameters to prepare mesoporous oxides from sol–gel precursors with specific pore features.