Thermoset Adhesives Research Articles

Ultraviolet Irradiation Surface Treatment to Enhance the Bonding Strength of Polyamide-Based Carbon Fiber-Reinforced Thermoplastic Polymers.

Adhesive bonding is a suitable joining method to satisfy the increasing industrial demand for carbon fiber-reinforced polymers without the need for a machining process. However, thermoplastic-based carbon fiber-reinforced polymers have small adhesive strength with structural thermoset adhesives. In this study, an ultraviolet irradiation surface treatment was developed to improve the adhesive bonding strength for polyamide-based carbon fiber-reinforced polymer. The type of ultraviolet wavelength, irradiation distance and irradiation time were optimized. Surface treatment with simultaneous UV irradiation of 185 nm and 254 nm wavelength generated unbound N-H stretching that was capable of chemical bonding with epoxy adhesives through a photo-scission reaction of the amide bond of polyamide matrix. Therefore, ultraviolet irradiation treatment improved the wettability and functional groups of the polyamide-based carbon fiber-reinforced polymers for adhesive bonding. As a result, the adhesive strength of the polyamide-based carbon fiber-reinforced polymers was increased by more than 230%.

Controlling Hybrid Polyhydroxyurethane Adhesive and Rheological Properties by Partial Carbonation of Biobased Epoxy Monomer.

Controlling hybrid material properties by simple monomer design offers an elegant pathway to prepare thermoset adhesives with tunable properties. Herein, biobased hybrid polyhydroxyurethane/polyepoxy is prepared starting from partially carbonated cashew nut shell epoxy derivatives (NC514) and m-xylene diamine (MXDA). The curing reactions, that is, epoxy-amine and cyclic carbonate aminolysis, monitored by ATR-IR spectroscopy at 50°C are found to be concomitant yielding highly homogeneous materials. Hybrid networks are extensively characterized by swelling tests, TGA, DMA, DSC, tensile tests, rheology, and lap-shear-test on aluminum substrates. The introduction of hydroxyurethane moieties within the epoxy-amine networks enhanced the adhesion properties (up to 20% compare to neat epoxy resins) by combining hydrogen bonding capability and vitrimeric properties (thermoset able to flow). Rheological characterizations and reprocessing tests demonstrated that hybrid adhesives with up to 47 mol% of cyclic carbonate groups are capable of covalent exchange (internally catalyzed by tertiary amine) while keeping similar thermomechanical properties and enhanced adhesion strength compare to the permanent epoxy network.

Chemical and Physical Debonding-on-Demand of Poly(urethane urea) Thermoset Adhesives to Facilitate the Recycling of Engineered Wooden Products

Chemical and Physical Debonding-on-Demand of Poly(urethane urea) Thermoset Adhesives to Facilitate the Recycling of Engineered Wooden Products

Modeling of curing and post-curing kinetics for a thermoset adhesive

The properties of adhesives cured at low temperatures are often significantly affected by post-curing effects. This paper introduces a method to model both the curing and post-curing kinetics for thermoset adhesives. The total and residual enthalpies and the glass-transition temperature (Tg) of the adhesive were measured via differential scanning calorimetry. The obtained degree of cure (DOC) and Tg were then fitted to determine their functional relationship. Low-temperature curing experiments were conducted, and an nth-order curing kinetics equation was developed through the fitting of data. Post-curing experiments were performed on samples with varying initial DOCs, and the resulting post-curing data were normalized and fitted using an nth-order model. The relationships between the kinetic parameters and the initial DOC, post-curing temperature and time were analyzed, and a comprehensive kinetics model crossing the curing and post-curing stages was successfully established. Model validation was carried out, and the results demonstrated a good predictive capability.

Acrylonitrile‐butadiene‐lignin thermoplastic rubber adhesive for enhanced metal‐to‐metal joining

AbstractWith the growing requirement for lightweight structural materials in automotive, aerospace, and infrastructure applications, multi‐material joints made with adhesive have attracted intense research interest. Commercial thermoset adhesives are one‐time cures, and difficult to disassemble the bonded components for repair and recycling. Our prior work with a thermoplastic acrylonitrile‐butadiene‐lignin rubber (ABL) addresses this sustainability/recycling challenge, but the adhesive exhibits deficient joining strength compared to standard thermosets. Here, we modify the ABL matrix by loading particulate fillers to enhance its modulus and toughness. The goal is to manufacture a cure‐free thermoplastic adhesive system with a simple dispensing protocol and characteristic ductility combined with a high yield stress for improved shear strength of a bonded joint. Fumed silica (FS) and epoxidized glass spheres (EGS) were used as fillers in the ABL to promote the dispersion of lignin particles that tailored the functionalities and free energy components of the adhesive surface. With optimal loading of FS (5 wt%) and EGS (30 wt%) in the ABL adhesive matrix, the lap‐shear strength of the bonded aluminum joint was elevated by 128%, compared to the neat ABL, reaching 21 MPa, which is 90% of the performance of a commercial epoxy‐based adhesive.Highlights A partly renewable filler‐toughened thermoplastic adhesive has been developed. This thermoplastic gives nearly equivalent performance of adhesively bonded aluminum joint compared to standard thermosets. Experimental and simulation data help understand the adhesive reinforcing mechanism by the fillers.

Modeling the curing kinetics of two thermoset adhesives under varying temperature conditions

AbstractThis paper presents detailed curing kinetics models for two thermoset adhesives. The cure kinetics were characterized using differential scanning calorimetry in both anisothermal and isothermal modes. The Sestak–Berggren autocatalytic model was applied to describe the anisothermal cure kinetics of the two adhesives with the Malek and undetermined coefficients methods determining their kinetic parameters. The Kamal autocatalytic model was adopted for the isothermal curing processes with the Kenny analytical‐graphical method determining the kinetic parameters. A modified Kamal model was developed by introducing a concept of the maximum degree of cure (DOC) and temperature‐depended kinetic parameters to describe the isothermal cure kinetics of the adhesive with a typical exothermic peak, and an extended Kamal model was further proposed by adding an initial‐phase‐control term to the modified Kamal model to describe the isothermal cure kinetics of the adhesive with two exothermic peaks. The results showed that the presented curing kinetics models with the determined parameters can precisely predict the evolutions of the DOC of the two thermoset adhesives in both anisothermal and isothermal modes.

Comparative analysis of thermoplastic and thermoset adhesives performance and the influence on failure analysis in jointed elium‐based composite structures

AbstractThe preference for thermoplastic composites over thermoset ones is steadily growing, especially in constructing resilient, lightweight, and fuel‐efficient structures in industrial settings due to their recyclability and superior energy absorption. Elium®, an acrylic‐based thermoplastic resin, offers enhanced stiffness and improved toughness at room temperature. Investigation into elium‐based composites with diverse fiber reinforcements has revealed elium's potential as an alternative to traditional thermoset epoxy resins. This study explores recyclable elium resin as an adhesive for creating curable and non‐permanent joints in various elium‐based composites. Various joining techniques, including adhesive bonding, mechanical joints, and hybrid bolted/bonded connections, were assessed, with hybrid bonded/bolted (HBB) joints emerging as a preferred solution for structures requiring higher load‐carrying capacity. The study's results indicate that, when using elium adhesive, elium‐hybrid bolted/bonded (HBB) joints demonstrate superior mechanical strength compared to their epoxy counterparts, resulting in improved toughness and stiffness values for each type of composite material. In addition, the research demonstrates superior load‐bearing strengths in bonded glass/elium composite joints using elium adhesive compared to bolted joints, with HBB joints featuring elium adhesive displaying even higher load‐bearing strengths. This research highlights the potential of elium‐based composites and adhesives in various industrial applications, enabling the production of lightweight, robust structures with exceptional damage tolerance.Highlights Investigates recyclable elium resin for curable and non‐permanent joints. Elium‐based composites offer lightweight, robust, and damage‐tolerant structures. Elium HBB joints demonstrate superior strength. FTIR analysis reveals distinct chemical signatures and bonding characteristics. Elium adhesive enhances bond strength.

Carbon fiber reinforced polyethylene composite adhesive at elevated temperatures

The development of multi-layer-multi-material thermoplastic structures necessitates the bonding of dissimilar materials with varying mechanical, chemical, barrier, and thermal properties. Thermoplastic adhesives, such as maleic anhydride grafted polyethylene (PE-MAH), are advantageous over thermoset adhesives given their similar manufacturability, and numerous benefits as sustainable materials in compliance with strict environmental regulation in construction, automotive, and energy industries. A key drawback is that PE-MAH demonstrates weak adhesive properties at elevated temperatures. In this study, the adhesion strength of carbon fiber (CF) reinforced PE-MAH at elevated temperatures were evaluated using a novel method of T-peel test under a controlled environment. Significant improvements of 71% and 295% increase in peel strength at 90 °C and 110 °C, respectively, were achieved with CF reinforcement over neat PE-MAH. The improving mechanisms were discussed on both macro-level, i.e., enhanced peel zone and eliminating crazing, and micro-level, i.e., transferred stress and dispersed energy into micro peel zones. This study demonstrates the efficacy of tailoring the adhesion strength of thermoplastic composite adhesive for significantly improved performance at elevated temperatures.

Investigations into the Influence of Temperature on the Tensile Shear Strength of Various Adhesives.

The temperature resistance of glued timber, which is crucial for glued wood construction, represents a significant assessment criterion. To gain insights into this aspect, this study utilized methods such as a shear strength test in accordance with EN 302-1:2013-06 under thermal loading (from 20 °C to 200 °C), and Differential Scanning Calorimetry (DSC) to determine the glass transition temperature (Tg). An increase in thermal load resulted in a decrease in shear strength and an increase in wood breakage. A hierarchy of adhesive groups was established based on strength performance and wood failure percentage (WFP) at 200 °C. Thermoset adhesives (MF: Melamine Formaldehyde, PRF: Phenol Resorcinol Formaldehyde) led the ranking, followed by elastomer adhesives (1C-PUR: One-Component Polyurethane, EPI: Emulsion Polymer Isocyanate), with thermoplastic adhesive (PVAc: Polyvinyl Acetate) last. Thermoset adhesives further cured under heat. PUR adhesives exhibited higher strength performance at 150 °C and lower temperatures.

Novel axial dynamic mechanical analysis setup for thermo-analytical study and curing kinetics optimization of thermoset adhesives.

Thermo-analytical studies of thermoset adhesives, either during research and development or in quality assurance activities, involve the application of various analytical equipment for adhesive characterization, from initial mixing to final product decomposition. Gelation is usually measured with rheometers or dynamic mechanical analyzers (DMAs); curing, post-curing, and curing kinetics are often studied using differential scanning calorimetry (DSC). Glass transition temperature (Tg) is measured via DSC or DMA, and finally, thermal decomposition measurements are done using thermal gravimetric analysis. Here, we present a new curing kinetics optimization module (C-KOM), an extension to an axial DMA, and a protocol for its usage, which combines elements from all of the above thermo-analytical techniques into one tool. As a case study, we apply C-KOM to investigate the effect of the curing temperature on the physical properties of an epoxy adhesive including gelation and end of cure points as well as its Tg. The data collected via C-KOM were used to extract the adhesive's curing reaction rates and its activation energy. Our research allowed us to compare and evaluate previously suggested curing procedures and assess their validity. As a final step, the thermal decomposition temperature of the epoxy adhesive was also identified via C-KOM. The newly suggested C-KOM setup provides a fast path toward characterization and optimization of the curing processes of thermoset materials in a way that was not available before.

Low-Temperature Processing of Electronic Materials Using Uniform Microwave Fields

This article presents the recent literature on the unique and surprising capability of microwave energy to effect chemical reactions at lower temperatures. These lower processing temperatures provide improved thermomechanical properties of organic and inorganic material combinations used in microelectronic assemblies. Variable-frequency microwave (VFM) technology eliminates arcing and produces the large and highly uniform fields necessary for use in high-volume manufacturing. Wafer coatings of polyimide thermoplastics can be fully polymerized 100-200°C lower with VFM than with thermal ovens, which lowers stress and warpage. VFM curing of thermoset adhesives for underfill, bonding, and package assembly can be done well below the usually inviolate “ultimate” glass transition temperature without vitrification and in less time. The unique kinetics and thermodynamics of microwave energy produce high chemical reactivity even after the high-viscosity thermoset gelation point. VFM-cured thermosets are highly uniform with less stress and warpage even on large substrates and panels. Field uniformity and penetration depth produce better adhesion and higher fracture strength. The lower process temperatures inherently produce more stable and reliable final products and allow for less costly low-temperature build materials and temperature-sensitive device designs. The anneal and activation process temperatures of implanted ions in silicon can also be reduced by half with VFM, which results in nearly zero dopant diffusion and high-quality SPE regrowth and low numbers of defect sites.

Time-, Temperature- and Frequency-Dependent Viscoelastic Properties of a Curing Epoxy Adhesive

Thermoset adhesives convert from liquid to solid due to chemical reactions. Once cured, these adhesives carry the potential to create strong load-bearing joints, resisting even severe detrimental service conditions. In the progress of curing of a thermoset adhesive the viscoelastic properties of the resin and hardener formulation change as the chemical reaction proceeds. Gelation occurs once a continuous 3-dimensional network of polymer chains has been created. After gelation, the microstructure of the resin is fixed and further cure is affected by diffusion limitations [1]. Mastering of the curing kinetics and the physicochemical changes in the transition from the liquid to the solid-state is essential to reliably process adhesives in industrial applications. Rheological experiments in parallel plate configuration have become a well-established practice in investigating viscoelastic properties in the progress of curing. In practice, it has shown to be challenging to access the full range of viscoelastic parameters of thermoset resins with a low initial viscosity from the very beginning of the curing reaction to the post-cure consolidation of the vitrified polymer. This paper will discuss experimental methods and criteria for the viscoelastic analysis of curing thermoset adhesives and present experimental data of the time-, temperature-, and frequency-dependent viscoelastic properties of a curing thermoset epoxy in relation to the features of its time-temperature-transformation-diagram.

Local heating and curing of carbon nanocomposite adhesives using radio frequencies

Abstract Carbon nanoparticles are known to produce a strong, rapid heating response in the presence of a radio-frequency (RF) electromagnetic field. Here we show that this property can be used to selectively cure nanocomposite thermoset adhesives in plastic bonding applications without deforming the surrounding plastic components. A non-contact RF applicator was used to generate an electric field through a carbon-black-loaded adhesive, which then volumetrically heats and cures the adhesive that bonds two plastic substrates. Because this heat was selectively generated in the adhesive, plastic substrates maintained temperatures lower than the curing temperature of the adhesive. Substrate temperatures as low as 53 °C were observed while the adhesive cured at a target temperature of 85 °C; in contrast, substrate temperatures exceed the adhesive temperatures when using conventional heating methods, such as IR lamps or ovens. The strength of cure was confirmed by mechanical testing; lap shear strengths of RF-cured adhesive joints were 3.26 MPa on average, well above the generally accepted green strength value of 1 MPa required for handling. This RF heating technology offers unique opportunities in industrial manufacturing applications, particularly in the automotive sector where thermoset adhesive bonding is ubiquitous.

A simple characterization methodology for the identification of the visco-elastic behavior of thermoset adhesives during cure

The identification of the mechanical behavior of adhesives is necessary to describe the development of residual stresses during their curing, which might impact their mechanical strength and lead to early failure of the bonding. A simple characterization and modeling approach is therefore developed and presented, which permits to monitor and identify the mechanical behavior of a thermosetting adhesive during the whole curing process. The test method is based on a compressive test, which consists in applying a periodic displacement and recording the subsequent load variations during cure. The test set-up relies on a simple apparatus as it consists in steel cylinders mounted on a standard tensile test machine. The analysis of the mechanical behavior is based on an analytical description of a visco-elastic constitutive law following the Maxwell model, leading to the identification of the material apparent viscosity and Young’s modulus. This characterization methodology is applied to an epoxy adhesive during cure. The obtained mechanical properties are in good agreement with values provided by the material supplier, which permits to validate the developed methodology.

Reversible adhesives based on acrylate copolymer modified by caffeic acid containing boroxin

ABSTRACT4‐formyl‐phenylboronic acid (FBA) and caffeic acid (CA) can combine to obtain a novel vinyl monomer containing boroxin, which was named as FBCA. Having been reduced the inhibition of phenolic hydroxyl groups, FBCA could directly copolymerize with acrylate (for instance, isobornyl acrylate (IBOA), butyl acrylate (BA), and hydroxypropyl methacrylate (HPMA)) by free‐radical polymerization to produce modified acrylate copolymer adhesive (FBCA‐IBH). Compared with sole acrylate copolymer adhesive (IBH), the FBCA‐IBH adhesive presents a higher thermal stability, cohesive strength, creep resistance, strain recovery, and adhesion strength to different substrate (such as carbon steel, polypropylene (PP), and polytetrafluoroethylene (PTFE)). According to the results of TGA, the initial degradation temperature of FBCA‐IBH is increased by 50 °C. The creep resistance of FBCA‐IBH is improved and its holding power to carbon steel is increased from 19 min to 3 h. It is eye catching that the shear strength to carbon steel could reach 9.37 MPa. Besides, this shear strength can be maintained even if after 18 cycle times, even the maximum value can achieve 14.36 MPa. The adhesion of thermoplastic adhesives to carbon steel exhibits adhesion strength comparable to that of thermoset adhesives. The reversible adhesive presents bonding strength in macro‐field, not just limited in micro‐field. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 137, 48703.

Prediction Models for Distortions and Residual Stresses in Thermoset Polymer Laminates: An Overview

This paper reviews predictive models developed for the development of residual stresses and shape distortions during the manufacturing of thermoset polymeric/composite materials. Different sources that produce residual stresses and shape changes in the laminated panels are described and reviewed. An overview is presented on the characterisation and predictions of the phenomena resulting in residual stresses. The focus will be on the models accounting for the parameters during the cure cycle of the thermoset composite materials published in the literature from 2005 until 2018. The material types covered here range from thermoset adhesives, full composites, and fibre metal laminates. Furthermore, selected works are reviewed on the reduction of the shape changes and residual stresses of composites and fibre metal laminates consisting of thermoset polymers.

Introducing flexibility in urea–formaldehyde resins: Copolymerization with polyetheramines

ABSTRACTAdhesives obtained by copolymerizing urea, formaldehyde, and difunctional polyetheramine with different molecular weights (230, 600, 900, and 2000 g mol−1) are presented as a more resilient alternative to conventional urea–formaldehyde resins. Urea and polyetheramine contents were varied and the resulting resins characterized by FTIR, 13C‐NMR, and TGA. These resins were used for production of agglomerated cork panels, an application that demands that the binder system is flexible. Polyetheramine with molecular weight 900 g mol−1 yielded the most promising agglomerated cork panel, with remarkable flexibility, good tensile strength, and with the E1 formaldehyde content specification for wood‐based panels used in construction, according to European Standard EN 12460‐5. These new thermoset adhesives have demonstrated to be capable of being used in systems where conventional formaldehyde‐based resins do not perform well due to inherent high rigidity. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 1834–1843

Synchronous curable deoxidizing capability of epoxy–anhydride adhesive: Deoxidation quantification via spectroscopic analysis

ABSTRACTThermoset resins have been extensively utilized as an adhesive for electronic devices. In particular, semiconductor packaging materials between chips/substrates require electrical interconnections by using deoxidizing agents along with thermoset adhesives. In this study, a facile combination of hydroxyl‐functionalized epoxy and cyclohexane‐structured cyclic anhydrides produced curable acidic moieties, thereby allowing synchronous deoxidizing and curing without the use of a deoxidizing agent. By contrast, unitary cyclic anhydrides rarely presented acidic moieties, probably due to their rapid networkable reactivity caused by structural flexibility once opened, which was detected and quantified by spectroscopic analyses. This binary system in the absence of additional deoxidizing agents achieved the complete wetting of common Sn‐based solders via chip bonding systems due to the generated acidic moieties. The binary mixture also showed an increased glass transition temperature of 110 °C and enhanced Young's modulus of 1.6 GPa, relative to the ternary mixture (85 °C and 0.8 GPa) comprising a deoxidizing agent. This discovery is beneficial in terms of the simplification of composition design and mechanical robustness of solderable adhesives. Quantification via spectroscopic studies can also help anticipate the deoxidizing capability of binary and ternary systems at different temperatures for various bondable and solderable applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46639.

Characterization of the chemical structure of vinyl ester resin in a climate chamber under different conditions of degradation

Due to the good strength and similar toughness of epoxy resins, vinyl ester resins are widely used as thermoset adhesives in structural adhesive joints and as composites for different industrial applications. However, vinyl ester adhesives are difficult to cure completely under environmental conditions, even after long periods of time because of gel formation slows the necessary diffusion of the catalyst across the polymer network.Several studies have used weathering chambers to investigate the degradation mechanisms of vinyl ester adhesives. However, a review of the scientific literature revealed both a wide variety of aging processes and several ambiguities between the recorded experimental results.In this work, post-cured vinyl ester resins at different aging cycles were aged under high temperature and relative humidity, and the changes in their structure, mechanical and adhesion properties were studied. Chemical and structural changes were observed in the vinyl ester resins after aging in a climatic chamber.

Recycling of carbon fiber-reinforced thermoplastic composite wastes from the aerospace industry

Post-industrial trimmings and off-cuts of carbon fiber/polyether ether ketone composite were successfully recycled into new composite products. The original composites were thermally characterized by dynamic thermomechanical analysis, differential scanning calorimetry, and thermogravimetric analysis. Melt-bonding and thermoset adhesives were used to bond the carbon fiber/polyether ether ketone. Performance of the bond was evaluated through double lap-shear tests. The carbon fiber/polyether ether ketone scraps were mechanically refined to a variety of elemental sizes, subsequently subjected to high-temperature hot pressing to form panel composites. The influences of element size and processing temperature were evaluated through mechanical testing.