Thermoset Adhesives Research Articles

Double cantilever beam tests on a viscoelastic adhesive: effects of the loading rate

Adhesive bonding technology is a promising alternative to traditional joining techniques. Indeed, bonded joint shows higher strength and fatigue life than bolted or riveted joints having identical weight. However, bonded joints are sometime reputed to be little reliable since significant dispersion could be observed while measuring their strength but also due to strong sensitivity to adhesion defect and poor surface preparation. Damage tolerance philosophy is now recommended for more reliable design of critical bonded parts by precise prediction of decohesion initiation and propagation along the bondline.Double cantilever beam (DCB) test is the most popular method to characterize the decohesion resistance of bonded interface by measuring their fracture energy or their R-curve in case significant nonlinear behaviour is observed. These past years, several efficient analysis techniques have been proposed to evaluate the fracture energy but also some optimization techniques to identify more complex interface behaviour. However, most of these techniques consider non time dependent behaviour while thermoset adhesives are known to be viscoelastic and in some condition can also show viscoplastic behaviour. Such effects are important to evaluate when bonded joint sustain stationary loads since they could lead to delayed fracture and slow crack growth.In the present work, we evidence some strain rate sensitivity at the bondline scale by performing DCB test under different opening rate conditions. At first, the viscoelastic behaviour of the adhesive is studied by performing creep test in a Dynamic Mechanical Analyser. The DCB tests results are interpreted with several methods including the Simple Beam Theory. It is shown that fracture energy is not an appropriate quantity to evaluate the crack propagation condition.

Experimental investigation on the effect of nanostructuration on the adherence properties of epoxy adhesives by a probe tack test

In this work, we aimed to characterize the energy of adherence of nanoscale structured epoxy adhesives Diglycidyl ether of bisphenol-A/Methylene–diethylaniline (DGEBA/MDEA) induced by phase separation triblock copolymers Poly (Methacrylate de Methyl)-b-Poly (Butyl Acrylate)-b-Poly (Methyl Methacrylate) (PMMA-b-PBA-b-PMMA) at gel state by a probe tack test, which is an original use of this kind of test for thermoset adhesives. For a set of mechanical parameters (probe’s roughness, contact time, contact pressure and debonding velocity), we measured the energy of adherence for both neat and filled adhesives. The probe tack test was performed at different steps of gelation. We compared the behavior of the adhesives and evaluated the dissipation contribution to the energy of adherence of the adhesives during the test. We finally discussed the nanoparticles׳ influence on the competition between cavitation and fibrillation. We report that the addition of nanoparticles leads to an overall improvement of the energy of adherence, with a significant increase of the dissipation contribution to the energy measured.

Carbon and Tungsten Disulfide Nanotubes and Fullerene-like Nanostructures in Thermoset Adhesives: A Critical Review

Thermoset adhesives such as epoxy exhibit high stiffness, strength and adhesion properties, but relatively poor toughness, elongation and conductivity. Nanofillers such as carbon (CNTs) and tungsten disulfide (WS2) nanotubes (INTs) and fullerene-like (IF) nanoparticles (NPs) possess excellent mechanical properties and also high electrical (thermal) conductivity. The quality of dispersion of the nanofillers in the thermoset adhesives has a major effect on the final properties of the nanocomposite adhesive. Dispersion techniques such as solution mixing, ultrasonication, three-roll milling, etc. have been used. Significant enhancements in stiffness, tensile strength, shear and peel strengths, fracture toughness, glass transition temperature, CLTE (coefficient of linear thermal expansion) among others, can be achieved at low CNTs concentrations (less than 1 wt.%), while at higher content of the CNTs, agglomeration reduces the toughness. Also for the WS2 nanoparticles, the lower concentrations (up to 1 wt.%) are preferable in order to improve the mechanical performance of the epoxy-based composites. Enhanced dispersion and interfacial interactions, resulting in improved mechanical performance of the nanocomposites containing CNTs, could be achieved by covalent and noncovalent modifications. The dispersion of WS2 nanostructures in the polymer medium was obtained using simple techniques like shear mixing and sonication without surface modifications. Moreover, it was found that the IF-NPs could covalently interact with the epoxy, without any surface modifications. The toughening mechanism of polymer matrices by the tubular nanofillers is mainly attributed to pull-out and crack bridging of the nanotubes. At the same time, inorganic nanocomposite adhesives exhibit promising results. IF structures of WS2 were found to exhibit crack deflection and bowing mechanisms. Electrically and thermally conductive adhesives have been obtained by adding CNTs, whereas conductive nanocomposites containing INT-WS2 have not been reported. In the case of CNTs the percolation path could be realized already at low levels (0.25 -1 wt.%). Furthermore, as the CNTs content increases, so does the conductivity of the composite material. In addition, the thermal conductivity of CNTs was found to have insignificant effect on the thermal conductivity of the nanoadhesives. Furthermore, the electrical and thermal conductive properties of most surface treated and functionalized CNTs were found to decrease, compared to untreated CNTs. The resulting nanocomposite adhesives containing carbon and tungsten disulfide nanotubes and fullerenes exhibit promising results in various applications.

Influence of post-curing temperature on the structure, properties, and adhesion of vinyl ester adhesive

Vinyl ester (VE) resins are widely used as thermoset adhesives in structural joints and composites, but complete curing under environmental conditions is not produced. The existing literature dealing with the effect of post-curing on the structure, viscoelastic, mechanical, and adhesion properties of VE resin is scarce. Therefore, in this study, VE resin was subjected to different post-curing temperatures (50–140 °C) for one hour, and the changes in structure and properties were assessed. The degree of cross-linking of the VE resin depended on the post-curing temperature and cure started to be completed above 100 °C, a temperature close to the glass transition temperature (115 °C) of the completely cross-linked polymer. Furthermore, gel formation in VE resin was evidenced for post-curing temperature below 100 °C. In order to fully cross-link the VE resin, post-curing at 140 °C for one hour was necessary, and it was evidenced by an increase in the glass transition temperature and in the mechanical properties; an increase in adhesion to cold rolled steel was obtained although the shear strength was lower than in the joint produced with the non-post-cured VE resin.

Permanent Wafer Bonding and Temporary Wafer Bonding / De-Bonding Technology Using Temperature Resistant Polymers

There can be two kinds of de-bonding options in the aspect of the releasing interface, one is carrier release and the other is device wafer release. Thermoplastic adhesives are used in most cases for carrier release because the materials can easily be cleaned by solvent. On the other hand, device wafer release is usually selected for thermoset adhesives, which is difficult to remove by solvent.

New adhesives: Bismaleimide liquid crystals containing polymethylene flexible groups and aramide–arylate mesogen groups

Four adhesives (one adhesive BEA 811 and three adhesives BENDA 811) containing new bismaleimide (BMI: B 8 and B 11) with aramide/arylate and polymethylene as mesogen and flexible groups, respectively were studied. Starting from the BMIs, eutectic mixture BE 811 was developed. It presents low melting temperatures (153.4°C) and crosslinks giving liquid crystal ordered state. The new adhesives have been prepared by mixing the original BMIs (eutectic mixture of B 8 and B 11: BE 811) with other compounds usually used in the formulations of composite materials and structural adhesives. Adhesive formulations of BEA 811, mixture BE 811 and DABA, and BENDA 811, mixture BE 811, DABA and NOVOLAC, crosslink generating an ordered state as well. The relation between structure, thermal properties and breaking force was investigated. Subsequently, these mixtures were tested as iron–iron adhesives. Finally, aging tests were achieved, in both laboratory and in-situ environments (in Vietnam and Morocco). The high performances of thermoset adhesives (BENDA 811 and BEA 811), compared to industrial polyimide adhesive (MATRIMIDE 5292), were attributed to molecular structures and their crosslinking ability in the liquid crystal state.

Thermosets as compatibilizers at the isotactic polypropylene film and thermomechanical pulp fiber interphase

The objective of this study was to improve interfacial adhesion properties at the interface of thermomechanical pulp (TMP) fiber and isotactic polypropylene (iPP) using thermoset adhesives such as phenol formaldehyde (PF) and urea formaldehyde (UF). This study also attempted to achieve fiber-to-fiber adhesion using thermoset adhesives before the molten iPP would flow into the fiber web. The fracture surfaces with thermoset adhesive showed identical differences in terms of fracture modes at the interface. An increased TMP fiber failure was observed with increased thermoset quantity at the interface. Using one percent resin content of weight fraction of TMP fiber handsheet, the tensile strength properties increased almost two fold higher than the strength of control samples. Additional adhesive contents of three and five percent showed gradual strength enhancement. However, the enhanced strength was statistically insignificant. UF resin showed slightly better strength performance over PF resin. This result may be caused by solid contents and additional pigments of resins.

Analysis of Back Injecting Real Wood Veneers using Engineering Polymers in an Injection Moulding Process

The current veneering process for interior trim components in the premium car segment involves the use of aluminium or polymeric substrates with thin wood veneers hot embossed to the surface using thermoset adhesives. The alternative process of back injecting real wood veneers with engineering polymer materials is considered here. The main aim of this work was to trial a number of engineering polymeric materials in a simple geometry component. In addition the basic adhesion and dimensional performance was also assessed. Five engineering polymers were selected and trialled for this process. A cross-section of the parts made were tested to an adhesion delamination test (DIN 53255), all of which passed the test successfully. Dimensional analysis of the mouldings show there is a distinct relationship between component distortion and the shrinkage value for that material, at both post injection moulding and post cooling. A relationship between the veneer and low polymer shrinkage was shown. The blend polymer, PA + ABS with 15% glass filler performed ideally with dimensional data. This material had the lowest measured shrinkage value of 0.15% and produced the flattest test samples. The worst dimensionally performing material with veneer was PA 66; which produced dimensional readings approximately three times worse than the PA + ABS 15% GF. This correlated with the polymer shrinkage values for this polymer being the highest recorded from this series of experiments.

Potential of tannin extracts as resorcinol replacements in cold cure thermoset adhesives

Blends of phenol resorcinol formaldehyde (PRF) resin with tannin obtained from bark offers the potential to minimise the quantity of resorcinol required in conventional two pot, cold set PRF wood adhesives. In the presence of ammonia, both quebracho and radiata pine bark extracts, resorcinolic and phloroglucinolic type tannins respectively, showed accelerated adhesive cure behaviour with formaldehyde, in a similar manner to PRF adhesive Greenweld technology for modifying glued timber processing rates. Segregation of pine bark components established that only tannin-containing fractions exhibited accelerated adhesive cure behaviour. Results from adhesive testing indicated adhesive blends of quebracho extract and PRF resin failed to attain satisfactory bond strength at full cure. Of the pine bark components, only a pure tannin fraction produced acceptable bond strength when blended with PRF (3:1). The study indicated pine bark extract can be substituted into PRF adhesives and adhesive cure rate accelerated using ammonia. However, extracts containing crude tannin or carbohydrate materials in adhesive blends led to inferior adhesives.

Novel adhesion promoters based on hyper-branched polymers

With the emergence of experimental and commercial highly branched polymers (HBPs) having two or three dimensional morphologies with high peripheral functionality, new opportunities have been created for formulating and tailoring new architectures for thermoset adhesives and related adhesion promoters. The authors studied a few adhesive systems previously, where the crosslinking agents had been either totally or partially replaced by the HBPs. In epoxy and polyurethane systems addition of poly-amido-amines (PAMAMs) HBPs, improved the mechanical and thermal properties of the adhesives due to increase of crosslinking density and formation of modified multiphase morphologies having high-energy absorption network. In the present study the objective was to investigate the effects of HBP PAMAM primers on interfacial characteristics of epoxy and polyurethane bonded joints, using various substrates (Aluminum, Magnesium, plastics and fibers) prior and after exposure to a combination of heat and humidity. It was shown that the interfacial adhesion and the durability increase significantly (100–250%) in all the studied joints where PAMAMs were used for surface treatment.

Pros and cons of thermoplastic and thermoset polymer adhesives in microelectronic assembly applications

Reviews the traditional use of thermoset (epoxy) adhesives for various bonding applications and highlights some limitations in today’s microelectronics arena. In particular, concerns for thermal and stress management associated with large area silicon bonded to a wide variety of substrate materials has led to an increasing interest in thermoplastic adhesive technology. Thermoplastics are not always the best solution for every application. This paper sets out to address the “pros and cons” of each polymer technology for different microelectronic applications taking into account some of the key physical properties such as Tg, TCE and modulus. In addition, practical issues such as handling, storage and processing are considered in detail.

A quantitative analysis of the stress relaxation effect of thermoplastics in multilayer substrates

All plastics can be divided into two groups called thermoplastics and thermosets, and each group exhibits very different behaviors. Unlike thermosets, thermoplastics are empirically known by the viscoelastic behavior of the thermoplastic layers; the stress generated by building subsequent layers can be significantly relieved in a multilayer structure. In this study, we quantitatively investigated the stress relaxation effect of thermoplastics in lamination-based multilayer substrates. As an increasing number of layers were laminated using either thermoplastic or thermoset adhesives, the maximum bow values were measured layer-by-layer using a laser profilometry during thermal cycling. When analyzed through an analytical model, the measured thermal behavior of thermoplastics exhibited as much as 70% stress relaxation in the multilayer structures, and highly contrasted with that of thermoset adhesives. The result provides an important design guideline that thermoplastics can be used as a stress relaxation layer in lamination-based multilayer substrates.

Physical modifications for improved peel strength in a high-temperature epoxy adhesive

The thermal performance of high-temperature thermoset adhesives is invariably gained at the expense of mechanical properties, particularly peel strength. A microscopic study of failure propagation in adhesive joints revealed that fibres within the bondline can affect the fracture path during peel failure in a commercially available modified epoxy adhesive, developed for high-temperature use. This adhesive has been further modified by the incorporation of polymer fibres and the resultant adhesive joints have shown significant increases in peel strength combined with both increases and decreases in lap-shear strength, depending on the fibre orientation. These effects were measured from −55°C to +180°C. Improvements were also seen when larger joints, more typical of aerospace structures, were tested.

Glue-line wood cell walls increased viscoelastic flow by high moisture tolerance thermoset adhesives

Glue-lines in boards made of wood particles using high moisture tolerance tannin-based adhesives were observed to present the unusual behaviour of producing a melting effect of the wood cell walls directly in contact with the adhesive. The high moisture content of up to 29% used in the preparation of these exterior grade particleboard and OSB industrial panels at standard pressing times causes considerable flow of the amorphous lignin and hemicelluloses in the wood directly in contact with the adhesive leading to a composite, compact interphase in which almost loose wood fibers are drowned in a hardened adhesive matrix. In this interphase the anatomical characteristics of the wood cell walls appear to be lost. This considerable flow is apparently due to the marked decrease of the glass transition temperature of hemicelluloses and lignin at the very high moisture contents used. This effect was replicated in boards prepared with a synthetic MUF adhesive using also a moisture content level much higher than what normally this resin is capable of tolerating by just lengthening considerably the pressing time used.

Fine-pitch connection to rigid substrate using non-conductive epoxy adhesive

Journal of Electronics ManufacturingVol. 02, No. 01, pp. 7-12 (1992) PAPERSNo AccessFine-pitch connection to rigid substrate using non-conductive epoxy adhesiveHELGE KRISTIANSEN and ARE BJORNEKLETTHELGE KRISTIANSENCentre for Industrial Research, Forskningsveien 1, Mail Box 124 Blindern, 0314 Oslo 3, Norway Search for more papers by this author and ARE BJORNEKLETTCentre for Industrial Research, Forskningsveien 1, Mail Box 124 Blindern, 0314 Oslo 3, Norway Search for more papers by this author https://doi.org/10.1142/S0960313192000030Cited by:9 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail AbstractWe have carried out reliability testing of different interconnection technologies, mainly to connect driver electronics to liquid crystal displays. This work has involved connection between flexible circuits made of Kapton, and rigid glass substrates. One of the applications also involved connection between flexible circuits and FR-4 substrates. Tests have been performed both on test structures and on real displays. The two different interconnection technologies we have studied are anisotropic conductive films (ACF) and non-conductive epoxy adhesive (NCA). Anisotropic conductive film is at present extensively used in producing high-quality LCDs. The other technology is, as far as we know, not used in production at this moment. Anisotropic conductive films contain a small volume fraction of conductive particles to accomplish electrical conductivity between the two connected contact areas. With non-conductive adhesive we use the surface roughness on the individual contact areas to establish electrical contact. In comparison with conventional anisotropic conductive film (ACF), based upon thermoset adhesives, the use of non-conductive adhesive (NCA) has proved to be superior with respect to reliability. This result is verified by thermal cycling (−30°C to 90°C) and by humidity testing (85% RH/85°C), both on test structures and real displays.Keywords:Fine-pitch electrical connectionliquid crystal displaynon-conductive adhesiveanisotropic-conductive filmreliability FiguresReferencesRelatedDetailsCited By 9Conductive Adhesives for Flip-Chip ApplicationsDaoqiang Daniel Lu and C. P. Wong4 January 2013A Method to Connect Flexible Printed Circuits and Indium Tin Oxide Electrodes Using Non-Conductive Film and PlatingYohei Wakuda, Yoshiyuki Okura, Koichiro Kawate, Hideaki Yasui and Mitsuhiro Watanabe et al.1 Jan 2008 | Transactions of The Japan Institute of Electronics Packaging, Vol. 1, No. 1Koichiro KAWATE1 Jan 2007 | Journal of The Surface Finishing Society of Japan, Vol. 58, No. 12Processability and reliability of nonconductive adhesives (NCAs) in fine-pitch chip-on-flex applicationsW. K. Chiang, Y. C. Chan, Brian Ralph and Andrew Holland1 Mar 2006 | Journal of Electronic Materials, Vol. 35, No. 3Overview of conductive adhesive interconnection technologies for LCDsH. Kristiansen and J. Liu1 Jun 1998 | IEEE Transactions on Components, Packaging, and Manufacturing Technology: Part A, Vol. 21, No. 2Characterisation of electrical contacts made by non-conductive adhesiveH. Kristiansen, M. Gulliksen, H. Haugerud and R. FribergElectrical conductive characteristics of ACA bonding: a review of the literature, current challenges and future prospects Guangbin Dou, D.C. Whalley and Changqing LiuRecent advances in conductive adhesives for direct chip attach applicationsJ. LiuOverview of conductive adhesive interconnection technologies for LCD'sH. Kristiansen and J. Liu Recommended Vol. 02, No. 01 Metrics History Received 1 September 1991 Accepted 1 October 1991 KeywordsFine-pitch electrical connectionliquid crystal displaynon-conductive adhesiveanisotropic-conductive filmreliabilityPDF download

Bonding adhesive joints with radio-frequency dielectric heating

Abstract The feasibility of using radio-frequency (RF) dielectric heating to cure thermoset adhesives has been evaluated. Thermoset and thermoplastic polymer panels have been bonded to steel using conventional one-and two-part epoxy and two-part urethane adhesives. Process cycle times for adhesive cure using RF heating were about 20 to 60 seconds, compared with about 20 to 30 minutes for the same materials using conventional oven-cure methods. Thermoset substrates bonded included glass fibre-reinforced composite panels based on sheet moulding compound (SMC) and resin transfer moulding styrene-vinylester (RTM). Thermoplastics such as polycarbonate, polyarylate, Noryl, ABS and polymethylmethacrylate were also successfully bonded. RF bonding experiments were performed by preparing and testing lap-shear joints as well as by joining a large test structure with a 25 mm x 1.25 m bondline. Bonding of painted steel to SMC composite, SMC to SMC and steel to RTM composite were also accomplished using the RF dielectric heating process.

Preparation of metallic surfaces with microfibrous topography and their effect on adhesion of hot-melt and thermoset adhesives

Various chemical and electrochemical methods have been developed which produce acicular oxide or metal growths on the metal surface, and the adhesion of various polymers to such microfibrous surfaces has been studied. Polyethylene, which normally adheres poorly when applied to metals as a hot-melt coating unless conditions permit its oxidation, gives good adhesion to microfibrous surfaces in the presence of antioxidants. Similar surfaces give good adhesion with ethylene vinyl acetate. Comparative examination of the fracture surfaces enables the phenomenon to be related to theories of adhesion. The effect of microfibrous surfaces on steel, copper and zinc on the adhesion of epoxy resins has been studied using fracture mechanics tests giving values for G c fracture energy. In some cases these strong engineering adhesives give problems with the strength of bond between the metal and microfibrous surface layer. However, in many cases changing from a smooth to a microfibrous surface alters the mechanical response of the adhesive from a brittle to a more ductile one, thus increasing the value of joint toughness. With a rubber-toughened epoxy, the microfibrous surface enables much more of the potential toughness of the resin to be realized.