The Effects of Cure Conditions on the Relaxation Behavior of Thermosetting Adhesives

The ability to dissipate mechanical energy by plastic (viscous) deformation is an important strength consideration for structural adhesives. The bulk solid viscosity parameter, μ, used in spring-dashpot mechanical-model characterization of polymeric materials, provides a convenient measure of this capability. The viscosity parameter, in turn, can be related to the relaxation time for the material with the use of the elastic modulus.

This paper presents a study of the effects of cure temperature and time on the stress relaxation behavior of Metlbond 1113 modified-epoxy adhesive with carrier cloth. Short term (t=1/2 minute) relaxation data are successfully fitted with the modified Bingham mechanical model to obtain relaxation time and bulk solid viscosity parameter values corresponding to different cure conditions. Experimental results indicate that both the relaxation time and the bulk viscosity values behave in a bell-shaped increasing-decreasing manner with respect to the cure temperature for all cure time values. This behavior is similar to that of adhesive ultimate strength and elastic modulus as observed by us earlier.     

A Survey of Rheological Properties of One-Component Epoxy Adhesives

Flow characteristics of seven commercially available one-component epoxy adhesive pastes were measured using a controlled shear stress rheometer and a controlled shear rate rheometer over a temperature range from 5°C to 60°C. Combining data obtained from both controlled rate and controlled stress experiments over a wide range of shear rates, we observed Newtonian flow (shear stress proportional to shear rate) at very low shear rates, a plateau “shear thinning” region at intermediate shear rates, and a second region of linear dependence of shear stress on shear rate at high shear rates. The adhesive pastes exhibited a very broad range of rheological behavior. Two flow parameters important to adhesive application technology, the plastic viscosity and the apparent yield stress, were measured for each adhesive. The plastic viscosity ranged from 11.6 to 329.5 Pa. s; the apparent yield stress ranged from 56.2 to 413 Pa. The temperature dependence of the rheological parameters of the epoxy adhesive pastes was also determined. The results are reported as the activation energies, E_η and E_σ, of plastic viscosity and apparent yield stress, respectively. The apparent yield stress of each adhesive paste was much less sensitive to changes in temperature than was the plastic viscosity. This suggests that the processing characteristics are likely to show qualitative as well as quantitative changes with temperature.     

聚氨酯胶粘剂的动态流变行为研究

采用HAAKE旋转流变仪及旋辑黏度计研究了聚氨酯(PU)胶粘剂的非牛顿性、黏流活化能及松弛时间谱。结果表明:PU胶粘剂属于假塑性流体;PU胶粘剂预聚物的黏度及其温度敏感性是与聚合物的氢键密度相关的,氢键密度越高,黏流活化能越大,温度敏感性越强;探讨了PU胶粘剂预聚物的松弛时间谱及其影响因素,其测试结果对掌握复膜过程中胶液的流动性具有理论指导意义。     

Mechanical Properties of Acrylic Pressure Sensitive Adhesives and Their Relationships to Industry Standard Testing

Dynamic mechanical and tensile stress-strain properties were measured for four sets of acrylic pressure sensitive adhesives, and were compared to industry standard “applications” peel and shear properties. Correlations were established showing that more than half of the range of performance shown by commercial PSA's is controlled by the bulk mechanical properties of the adhesive polymer. A few exceptions stand out clearly. Also, room temperature performance properties were found to correlate better with DMA at higher temperatures than with room temperature DMA. The contribution of tensile properties to peel strength and failure mode is discussed. The results can be used to relate PSA performance to well-known concepts in other areas of materials science, e.g. fracture toughness, rubber elasticity, and rheology, and to key variables in the adhesive formulation or selection process.     

The Effects of Cure Conditions on the Relaxation Behavior of Thermosetting Adhesives

The ability to dissipate mechanical energy by plastic (viscous) deformation is an important strength consideration for structural adhesives. The bulk solid viscosity parameter, μ, used in spring-dashpot mechanical-model characterization of polymeric materials, provides a convenient measure of this capability. The viscosity parameter, in turn, can be related to the relaxation time for the material with the use of the elastic modulus.

This paper presents a study of the effects of cure temperature and time on the stress relaxation behavior of Metlbond 1113 modified-epoxy adhesive with carrier cloth. Short term (t=1/2 minute) relaxation data are successfully fitted with the modified Bingham mechanical model to obtain relaxation time and bulk solid viscosity parameter values corresponding to different cure conditions. Experimental results indicate that both the relaxation time and the bulk viscosity values behave in a bell-shaped increasing-decreasing manner with respect to the cure temperature for all cure time values. This behavior is similar to that of adhesive ultimate strength and elastic modulus as observed by us earlier.     

Morphology and Viscoelastic Behavior of Styrene-Diene Block Copolymers in Pressure Sensitive Adhesives

Linear (SDS) and radial (SD)_x block copolymers of styrene (S) and dienes (D=butadiene or isoprene), varying in composition and molecular weight, were formulated as pressure sensitive adhesives. The morphology of these compositions was determined by electron microscopy of ultra-thin sections and dynamic viscoelastic measurements were made at 35 Hz between -90° and + 140°C or higher. Pressure sensitive tack and holding power were determined and interpreted in terms of morphological and rheological properties.

A high degree of tack resulted only when the tackifying resin was compatible with the polydiene segments of the block polymer and incompatible with the polystyrene segments, provided also that the polydiene-tackifier phase was the continuum with the polystyrene phase forming spherical domains. All effective tackifying resins raised the glass transition temperature (T_g) of the rubbery phase, but plasticized the polymer at temperatures well above T_g Polystyrene domain connectivity was found to lead to diminished tack in block polymers containing more than 30% styrene, a result of decreased creep compliance on the time scale of the bonding process and failure to achieve full contact with the substrate. For adhesives not limited by contact, tack increased with the loss modulus of the adhesive on the time scale of the debonding process. Holding power (shear resistance) increased with polymer styrene content and molecular weight, the polystyrene domain structure effectively inhibiting viscous flow at temperatures sufficiently below T_g of the styrene blocks.     

Analytical modelling of the automated dispensing of adhesive materials

The integrity of joints is a critical issue in structural bonding and the shape of adhesive beads needs to be consistent. The right amount of adhesive must be dispensed and to ensure this, the dispensing process must be automated. To have a fully automated dispensing cell, the use of a knowledge-based expert system for set-up and diagnosis is appropriate. To achieve this, the modelling of dispensing flow of adhesive material is essential to ensure that the correct sized bead is dispensed at a variety of robot speeds. For a given adhesive, it is often necessary to perform many ad hoc adjustments and tests to produce optimal adhesive beads at different operational and environmental conditions. To avoid this, the rheological behaviour of the adhesives is analysed, to find a constitutive equation for representing an adhesive model which best describes the way that different types of adhesives flow under a variety of conditions. Modelling the material properties provides an initial approximation of the flow rate for dispensing the adhesive. The automated dispensing cell can then adapt to the operational conditions where the model also provides for its initial set-up and diagnosis. This modelling method can also be applied to similar adhesives with known rheological data and nozzle flow rate.     

单组分常温固化EP胶粘剂的研制

以环氧树脂(EP)为基体树脂、有机硅为偶联剂,成功研制出一种常温固化的建筑用EP/有机硅单组分胶粘剂,并对其力学性能、耐老化性能、耐盐雾性能和耐介质性能等进行了测定。结果表明:该胶粘剂固化前呈黏稠状液体,可常温固化,施工性能较好;其拉伸强度超过15MPa,剪切强度超过10MPa,并具有较好的耐介质性能;该胶粘剂避免了传统EP胶粘剂在施工方面的诸多不足之处,具有良好的应用前景。     

ON NONISOTHERMAL FLOWS OF BINGHAM PLASTICS

A numerical scheme based on the Galerkin's finite element method is used to analyze nonisothermal flows of Bingham plastics. These fluids exhibit yield stresses, below which the material does not flow. This condition is enforced by introducing the bi-viscosity model in which the plug behaviour is approximated by a highly viscous fluid. The bi-viscosity model approaches the ideal Bingham plastic model if the pre-yield viscosity becomes large. This numerical method is applied to solve three problems namely the Graetz-Nusselt, recirculating flow, and sinusoidal channel flow problems for various values of the yield stress. The size of the plug as well as the velocity and temperature fields correspond closely with values available in literature.     

Adhesive Evaluation for New Forms of LARCTM-TPI

LARC^TM-TPI is a linear aromatic polyimide that was developed at NASA Langley Research Center in the 1970's and subsequently licensed to Mitsui Toatsu Chemicals, Inc., (MTC) in Japan. This company has made it easier to process for use in application as a structural adhesive or as a composite matrix resin. The present forms that exist are (1) high melt viscosity or Low Flow Grade (LFG); (2) medium melt viscosity or Medium Flow Grade (MFG); and (3) low melt viscosity or High Flow Grade (HFG). As expected, the low melt viscosity material is the easiest to process but has poor toughness; the high melt viscosity material is very tough but is more difficult to process. Because of these two extreme situations we have worked closely with MTC to develop an optimized system. This work has resulted in the medium melt viscosity material as well as two other modified or blended medium-flow variations.

These novel forms of LARC^TM-TPI have resulted in adhesives that can be melt processed at pressures as low as 0.01 MPa (15 psi) at temperatures between 343-371°C (650-700°F). Evaluation of adhesive performance has been accomplished using lap shear specimens and evaluating flow, wet out and shear strength. Initial strengths for these optimized materials range from 20.7-41.4 MPa (3000-6000 psi) at room temperature and 13.8-20.7 MPa (2000-3000 psi) at elevated test temperatures.     

Dispensing of rheologically complex fluids: The map of misery

Rheological effects may complicate the dispensing of complex fluids, when compared to their Newtonian counterparts. In this work, fluids with tailored rheological properties have been studied using high‐speed video‐microscopy. The level of viscosity, the degree of shear thinning, and the elasticity have been varied independently. At low‐flow rates, droplets are formed that pinch off. The drop volumes, breakup mechanisms, and times have been identified. At higher‐flow rates, a continuous jet is observed, with the transition depending on the rheology of the dispensed fluid. The relevant nondimensional groups are the Ohnesorge, Deborah, and elasto‐capillary number, for when viscosity, inertia, or elastic forces dominate flow. In each of these cases, the transition between dripping and jetting dispensing occurs, controlled by a critical Weber, capillary, and Weissenberg number, respectively. This set of six nondimensional groups can be used to construct an operating space and map out areas of potential problems. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3242–3255, 2012     

单组分室温硫化硅橡胶的配制(十四)

介绍了脱丙酮型单组分室温硫化(RTV-1)硅橡胶的特点,以及低挥发性、抗黄变性、耐油性和低模量脱丙酮型RTV-1硅橡胶的配制.     

单组分室温硫化硅橡胶的配制(十五)

介绍了脱酰胺、脱羟胺型单组分室温硫化(RTV-1)硅橡胶的特点，以及脱酰胺／羟胺型(包括普通型、高粘接性型、耐水性型、超低模量型)、脱羟胺型(包括用含S-H基的硅氧烷或异氰酸酯作羟胺清除剂)、脱酰胺型(快干型、自流平型)及脱酰胺／酮肟型低模量RTV-1硅橡胶的配制。     

Polyetherimide Adhesive

High-temperature adhesives which can be adhered at adhesive temperatures lower than those of conventional polyimide adhesives were investigated. Polyetherimide (PEI), developed by General Electric Co., is one such promising low curing temperature adhesive because it melts at temperatures lower than those used for conventional polyimides. Lap shear adhesive strength was investigated in a 75 μm-thick PEI film using steel test pieces. 350 kgf/cm² was achieved after curing for 1 hour at 270°C and 150 kgf/cm² was achieved at the test temperature of 200°C. PEI adhesive dissolved by N, N-dimethylformamide exhibited a high adhesive strength of 240 kgf/cm² after curing for 2 hours at 200°C. In addition, it was found that PEI could be used at much lower adhesive pressures than those of conventional polyimide adhesives.     

单组分湿固化聚氨酯胶粘剂的研究进展

介绍了单组分湿固化型PU(聚氨酯)胶粘剂的发展概况,讨论了该类胶粘剂预聚体的合成方法。重点介绍了两种新型单组分湿固化PU胶粘剂(硅氧烷封端湿固化PU胶粘剂和潜性湿固化PU胶粘剂),并展望了其发展前景。     

VISCOMETRIC MEASUREMENT OF CHROMIUM(III)-POLYACRYLAMIDE GELS†

Gelled polymers are being used increasingly to modify the movement of injected fluids in secondary and enhaced oil recovery processes. A common gelation process involves the reduction of Cr(VI) to Cr(III) in the presence of polyacrylamide. The Cr(III) reacts or interacts with the polymer to form a gel network. Although correlations of gelation time with principal process variables have been obtained, viscometric data have not been reported during or after gelation. These data are needed for fluid flow calculations in surface equipment and estimation of flow behaviour in reservoir rocks.

A Weissenberg Rheogoniometer, with cone and plate geometry, was used to obtain viscometric data for the gelation of polyacrylamide and chromium (III). Solutions consisting of polyacrylamide polymer, sodium dichromate-dihydrate and sodium bisulfite were gelled under a steady shear field at constant temperature. The shear stress versus time profile for the galation process was interpreted to define a gelation time and to determine the apparent viscosity of the gelled fluid. The gelation time decreased as the applied shear rate increased up to about 14.25 sec^-1 and was affected by shear rate history. Viscometric properties of the gelled solutions were determined. Apparent viscosity of the gelled solutions decreased as the shear rate under which they were formed increased.

Post gelation studies indicated that gels exhibited a residual stress at zero shear rate and behaved as Bingham plastics under steady shear. Gels formed at low shear rates were more viscous than gels formed at high shear rates. However, the structure of these gels was susceptible to shear degradation.     

Hygrothermal Properties of Epoxy Film Adhesives

The hygrothermal response of high performance epoxy film adhesives, in their bulk state, has been characterized over a wide range of temperatures, following exposure to a combination of humidity (95% R.H.) and heat (50°C).

Experimental results have indicated that the testing temperature has a pronounced effect on both tensile modulus and strength of the adhesives, while the effect of moisture content varies with respect to the adhesive type. The moduli of the film adhesives, which have a wide range of glass transition temperatures (T_g), have been related to both moisture level in the adhesive and testing temperature. This has been accomplished by employing a dimensionless temperature, which incorporates the wet and dry T_g and the testing, as well as a reference, temperature. The strength properties have shown a higher degree of scatter using the abovementioned dimensionless temperature.

Scanning electron microscopy of the fracture surfaces have shown a good agreement between the effects of moisture and the mechanical properties. Adhesives which exhibited good moisture resistance, as manifested by the stability in their tensile properties, showed minor changes in their fracture surfaces regardless of moisture conditioning. Distinctively, the effect on strength properties has been correlated with typical moisture-induced fracture mechanisms.     

HEAT TRANSFER TO NON-NEWTONIAN LIQUIDS FLOWING THROUGH HORIZONTAL TUBES

Experimental and analytical investigation of heat transfer to single phase flow of pseudoplastic fluids under constant heat flux condition was carried out. Experimental data for laminar flow heat transfer in the thermally developing region over a wide range of rheological properties ( n from 0.54 to 0.84 and K from 0.07 to 2.41 Nsⁿ/m²) in 0.028 and 0.057 m diameter pipes were collected. Enhancement in heat transfer rate due to natural convection and temperature dependent rheotogy was observed. The temperature dependent viscosity effect is a function of flow behavior index and it contributes more towards heat transfer enhancement than natural convection does. A correlation (confidence level ±15%) for the entire set of data is also presented.     

Thermoplastic-thermosetting hybrid polymer systems as gap-filling adhesives

The most important requirement for a hot melt adhesive to perform well in gap-filling applications is the ability to yield or flow under shrinkage tension which develops in the adhesives during cooling, while maintaining a satisfactory bond to the substrate. In addition, such an adhesive must have a high room temperature viscosity and high green strength to effect an acceptable bond before curing. We found that the required combination of properties for the gap-filling applications can be achieved with interpenetrating networks of selected thermoset-thermoplastic polymers. The best results were obtained with systems consisting of an amorphous polyester and an epoxy blend. The properties of these adhesives depend on numerous variables. This study concerns the three composition variables which appeared particularly important in the performance of these adhesives: (1) epoxy-polyester ratio, (2) molecular weight of the epoxy system, and (3) the quality of dispersion.