Rheological, Interfacial and Thermal Control of Polymer Adhesion. I. Isothermal Theoryf

We have developed an isothermal theory of separation in polymer-solid adhering systems. The model used is based on the (observed) drawing of filaments between a bulk polymer and a solid. In the isothermal theory, a criterion is set up, demarcating filament elongation vs. detachment of the filament base from the solid. It employs a dimensionless parameter, ω, that relates free energy of adhesion, elongational viscosity or yield strength of the polymer, and filament size, to adhesive performance. The isothermal theory can be applied directly to the separation processes that occur with pressure-sensitive adhesives. Certain observations by Aubrey and Sherriff, by Gardon and by Kaelble are explained. The validity of the demarcation is believed to extend beyond pressure-sensitive systems, to all thermoplastic adhesives and/or coatings.     

The Effect of Low Power Nitrogen Plasma Treatment of Carbon Fibres on the Interfacial Shear Strength of Carbon Fibre/Epoxy Composites

Type II (high strength) carbon fibres have been given a low power nitrogen plasma treatment. It is shown that this plasma treatment has no effect on the fibre diameter, no detrimental effect on fibre strength and can significantly improve fibre/resin adhesion. It is proposed that this improvement is due to chemical interaction via amine/epoxy bonding at the edge sites together with the interaction of the epoxy with activated basal planes present on the fibre surface. This improvement is only achieved if the fibres are immersed in resin before being exposed to air. Exposing the treated fibres to air drastically reduces fibre/adhesion due to the adsorption of moisture from the environment. Heating these latter fibres in a vacuum at 130°C for one hour allows some recovery of the interfacial strength. It is also demonstrated that the interfacial shear strength falls dramatically when the nitrogen-containing functional groups are completely removed from the fibre surface.     

The Effect of Low Power Nitrogen Plasma Treatment of Carbon Fibres on the Interfacial Shear Strength of Carbon Fibre/Epoxy Composites

Type II (high strength) carbon fibres have been given a low power nitrogen plasma treatment. It is shown that this plasma treatment has no effect on the fibre diameter, no detrimental effect on fibre strength and can significantly improve fibre/resin adhesion. It is proposed that this improvement is due to chemical interaction via amine/epoxy bonding at the edge sites together with the interaction of the epoxy with activated basal planes present on the fibre surface. This improvement is only achieved if the fibres are immersed in resin before being exposed to air. Exposing the treated fibres to air drastically reduces fibre/adhesion due to the adsorption of moisture from the environment. Heating these latter fibres in a vacuum at 130°C for one hour allows some recovery of the interfacial strength. It is also demonstrated that the interfacial shear strength falls dramatically when the nitrogen-containing functional groups are completely removed from the fibre surface.     

Interfacial tension from the height and diameter of sessile drops and captive bubbles with an arbitrary contact angle

A method has been developed to calculate the interfacial tension of sessile drops and captive bubbles of arbitrary contact angle by measuring the drop diameter and vertical distance to the apex at arbitrary horizontal planes within the drop. The procedure works in theory for any contact angle with an accuracy on the order of 0.1%. However, practical limitations reduce the range of angles to roughly 50°–180° but do not restrict the range of interfacial tensions (at least 0.01 mJ/m² to 72.0 mJ/m²). The optimal strategy is to use the method at several points on a single drop and to calculate the mean and standard deviation of the resulting interfacial tensions.     

Ultrasonic Determination of the Cohesive Properties of Bonded Joints by Measurement of Reflection Coefficient and Bondline Transit Time

An ultrasonic nondestructive technique for the quantitative determination of the cohesive properties of adhesive joints based on the measurement of the reflection coefficient from the top adhesive/adherend interface and the bondline transit time has been developed. The method requires access to only one side of the joint and, for joints with typical aerospace geometries, it can be implemented using a single transducer with a centre frequency below 50 MHz. The technique has been used to determine the longitudinal bulk wave velocity in aluminium-epoxy-aluminium joints to within ±6% of the nominal values determined from bulk samples. The bondline thickness of the samples tested was evaluated to within micrometer accuracy, and thickness variations within the scan area were detected to much better than micrometer accuracy. The method has been tested successfully on joints made with two-part epoxies and with film adhesives containing a “scrim” carrier, and it has also been shown that the different standard adherend preparation procedures have a negligible effect on the results. The method, therefore, promises to provide a reliable, nondestructive means of measuring the cohesive properties of a bonded joint and represents a significant advance on the currently available technology.     

Ultrasonic Determination of the Cohesive Properties of Bonded Joints by Measurement of Reflection Coefficient and Bondline Transit Time

An ultrasonic nondestructive technique for the quantitative determination of the cohesive properties of adhesive joints based on the measurement of the reflection coefficient from the top adhesive/adherend interface and the bondline transit time has been developed. The method requires access to only one side of the joint and, for joints with typical aerospace geometries, it can be implemented using a single transducer with a centre frequency below 50 MHz. The technique has been used to determine the longitudinal bulk wave velocity in aluminium-epoxy-aluminium joints to within ±6% of the nominal values determined from bulk samples. The bondline thickness of the samples tested was evaluated to within micrometer accuracy, and thickness variations within the scan area were detected to much better than micrometer accuracy. The method has been tested successfully on joints made with two-part epoxies and with film adhesives containing a “scrim” carrier, and it has also been shown that the different standard adherend preparation procedures have a negligible effect on the results. The method, therefore, promises to provide a reliable, nondestructive means of measuring the cohesive properties of a bonded joint and represents a significant advance on the currently available technology.     

Interfacial adhesion and deformation of thermotropic liquid crystal polymers in engineering thermoplastics: Blends of a poly(ester amide) with Nylon 6 and a polyester with PBT

A new method was sought to produce in situ composites by the addition of a third component that would react with matrix polymers and thermotropic liquid crystal polymers to form graft copolymers, which act as a compatibilizer at the interface. Morphological observation reveals the significance of compatibilization in immiscible polymer blends. Good adhesion at the interface by the produced compatibilizer enabled the finely dispersed liquid crystal phase to be deformed in shear flow without elongation, even when the viscosity of the matrix polymer was lower than that of the liquid crystal polymer. This study provides a way to produce a strong and tough in situ composite. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 359–366, 1997     

The Effects of Laser Radiation at 248 nm on the Surface Characteristics and Joint Properties of Aluminum Adherends

Procedures presently used to prepare surfaces of aluminum adherends for bonding with structural adhesives entail the use of corrosive solutions that are environmentally hazardous.

As an alternative, we investigated whether eximer laser radiation can effectively be used to prepare the surfaces of aluminum adherends. The results indicate such a methodology to be very promising. Using a KrF laser, at a radiant intensity of 1.67 × 10¹³ W/m²/pulse, successive mm² regions of aluminum coupons were irradiated for fixed numbers of pulses/region. This resulted in changes to the topographies and oxidation states of the surfaces. Symmetric lap shear joints from coupons so treated had considerably increased strains at fracture and 24% greater joint strengths as compared with controls. The results further indicate that fracture toughness can be increased above that of presently used procedures. The topographies of the treated and control surfaces were characterized using a new topographic characterizing system. An elastic model is presented that relates failure characteristics to experimentally-determined topographic parameters. The results suggest that different mechanisms for joint enhancement are operative.     

Characterisation of GFRP surfaces amenable for bonding and their effect on the strength of co-cured vacuum resin infused single lap joints

The choice of a fabrication method is of primary importance in order to optimise and control the performance of composite materials. This becomes increasingly crucial when the fabrication method itself becomes in parallel an adhesive bonding co-cure technique. In this paper, the manufacturing process of the vacuum resin infusion jointing is introduced and specimens of co-cured single lap joints are fabricated and tested under a tensile load. The aim is to show the correlation between the surface characteristics of mechanically treated composite laminates and the adhesion performances of the corresponding surface assemblies using the vacuum resin infusion as a jointing procedure. The mechanisms that govern adhesion are investigated by measuring several physical and chemical parameters of the bonded surfaces with techniques such as surface profilometry, contact angle and surface energy measurements, X-ray photoelectron spectroscopy and scanning electron microscopy. These methods qualitatively and quantitatively measure the influence of surface characteristics towards adequate interfacial bond strength and reveal the importance of the mechanical interlocking, kinetics of wetting, chemical reactivity and intermolecular adhesion of the vacuum resin infusion joint interfaces. The results clearly demonstrate the major influence of the surface contamination and surface topography but also the role of the joining process itself, having no distinct adhesive layer, on the adhesion properties. It is shown that the interfacial adhesion qualities alter significantly the fragmentation process and the strength of the vacuum resin infused joints.     

Investigation of the “Surface” and “Interphase” Composition of Adhesion Promoter/Thermoplastic Olefin Systems: The Effect of Adhesion Promoter Bake Temperature

The interfacial chemistry of model systems consisting of two adhesion promoting primers and a single Thermoplastic Olefin (TPO) substrate was examined. Two commercial adhesion promoter (AP) materials were applied to a commercially-available TPO material and either flash dried at room temperature or baked at 100°C. The surface composition of the AP films and TPO substrate, and the interfacial compositions of the AP/TPO systems were characterized using x-ray photoelectron spectroscopy (XPS). The AP films studied were based upon a chlorinated polyolefin (CPO). For one adhesion promoter film (AP-1), no chlorine was present at the surface suggesting a nonhomogeneous system. For the second adhesion promoter film (AP-2), the surface composition was about 15% CPO and 85% AP matrix. No changes in AP surface composition were evident for the different bake conditions for either AP. Interfacial compositions of the room temperature flashed materials were found to be very similar for both AP/TPO systems, with CPO being present for each and at similar concentrations. Interfacial compositions for the baked materials were also similar for the two systems, although the level of CPO at the interface increased for both the AP-1 and AP-2 relative to the unbaked materials. The relative increases observed were 46% and 41% for the AP-1 and AP-2 systems, respectively. The increase in the relative concentration of CPO at the interface with bake temperature suggests that there is a stronger interaction between the AP and TPO. The implication of these data is that a baked AP should result in a more robust paint system with respect to AP/TPO adhesion.     

The effect of photooxidation aging on the cohesion and interfacial adhesion behavior of epoxy asphalt with a composite acidic curing system based on molecular simulation

This paper investigated the cross-linking network structure of epoxy asphalt (EA) with the different ratio of composite curing agents, and further determined the effect of photooxidation aging on the tensile and interfacial adhesion behavior of EA with different cross-linking network structures through molecular simulation. Based on the molecular models of EA and the interface model between EA and aggregate, the crosslinking network structural characteristics and tensile mechanical behavior were determined. The interfacial adhesion behavior and the mechanization were further studied. The results indicate that 20-EA and 6-EA have an epoxy resin cross-linked network with uniform pore structure and phase distribution. 2-EA showed a denser cross-linking network and uneven aggregation phenomenon. Photooxidative aging alleviated the aggregation phenomenon. A dense cross-linking network improved the tensile strength and the ability of tensile performance to resist photooxidation aging. The interface of EA-quartz exhibited higher adhesion strength than EA-calcite due to the closer distance and stronger nonbonding interactions between EA and quartz. The low anhydride content and photooxidation aging made EA approach to the aggregate interface, increasing nonbonding interaction and interfacial adhesion strength. In addition, quartz aggregates were more suitable for application in EA mixtures due to the higher interfacial adhesion strength and lower water sensitivity.     

An Interphase with Changing Properties and the Mechanism of Deformation in Particulate-Filled Polymers

The comparison of the results of calculations based on experimental data with those derived from a simple, two phase, elastic model proved the existence of a hard interphase in particulate-filled composites. Moreover, beside elastic properties, also other mechanical characteristics of the interphase, including yield stress, are different from those of the components. An energy analysis showed that the relationship between the yield stress of the matrix and the debonding stress determines the mechanism of deformation. Strong adhesion leads to matrix yielding, while decreased interaction leads to debonding, with a corresponding dependence of composite yield stress on filler content. Particle size, interaction and interphase properties determine the stress necessary to separate the matrix/filler interface. The thickness of the interphase depends on the strength of the interaction; a linear correlation was found between the size of the interlayer and the reversible work of adhesion.     

The Effects of Coupling Agents on the Mechanical,Rheological and Thermal Properties of Calcium Carbonate-Filled LDPE Compatibilized with Maleic Anhydride-g-LDPE (Part II)

This study deals with the chemical modification of calcium carbonate (CaCO₃) by means of coupling agents and its effect on the mechanical, rheological, and thermal properties of the ternary composite of low density polyethylene (LDPE), calcium carbonate (CaCO₃), and maleic anhydride-grafted low density polyethylene (MAH-g-LDPE).

The variations in the properties were dependent on the treatment method and on the nature and extent of the interactions developed between the filler particles, the matrix and the compatibilizer.

The impact resistance results revealed a transition in the behavior of the composite from ductile to brittle. This embrittlement becomes more pronounced as the concentration of CaCO₃ and that of the coupling agents are increased.

The rheological characterization showed that the interactions developed at the interface greatly affected the flow of the material.

The differential scanning calorimetry results showed that even though the fusion temperature and the crystallization temperature were not influenced by the filler or by its treatment, crystallization was altered.

Finally, the thermogravimetric analysis showed that the dispersion and the adhesion of the filler with the matrix significantly affect the thermal stability.     

发泡剂分解残渣和填料pH值对PVC低发泡中螺杆腐蚀及表面堆积物的影响

分析比较了PVC低发泡中发泡剂AC，DDL-101，DDL-102，DDL-103分解残渣的酸碱性，并探讨此特性对金属腐蚀及物料稳定性的影响，分析了填料酸碱特性与表面堆积物的关系以及使用不同填料对金属表面堆积的影响。并提出了相应的对策。