Thermoplastic Adhesives Research Articles

Innovative Simplification of Bonding and Joining Processes with Novel Film-Type Non-Crystalline Thermoplastic Adhesives

Innovative Simplification of Bonding and Joining Processes with Novel Film-Type Non-Crystalline Thermoplastic Adhesives

Six-membered ring-reinforced flexible high-elongation block polyamide as strong and multi-reusable hot melt adhesive

This study presented the synthesis and characterization of polyester block polyamide hot melt adhesives (PPHMAs) with high adhesion strength and toughness. The PPHMAs demonstrated an adhesion strength of 11.04 MPa and toughness of 22.54 MJ·m−3, making them suitable for various substrates such as aluminum, copper, and stainless steel. The polyamide segments synthesized from dimer acid and 2-methyl-1,5-diaminopentane exhibited characteristics of softness and high elongation. Using isophorone diamine as a reinforcing agent introduced a stable six-membered ring structure, which enhanced the polymer's rigidity and strength, thus meeting the requirements for polyamide segments to function as the hard segments in block polymers. A trace amount of Tris (2-aminoethyl) amine provided the cross-linking sites which are similar to those in neuronal structures. The polyester segments, which were synthesized from poly (propylene glycol) diglycidyl ether and sebacic acid, served as the soft segments in the block polymer and were connected to the hard segment via amide bonds. The addition of the polyester segments enhanced the non-covalent molecular interactions within PPHMAs, leading to dynamic crosslinking properties. The adhesive maintained stable adhesion even after six cycles, which showcased its durability. Furthermore, the PPHMAs exhibited notable chemical resistance. It retained over 60 % adhesion strength after exposure to some chemical solvents including artificial seawater, 5 % NaOH solution, 5 % HCl solution, and DMSO for 24 h. These findings offered a novel approach to enhancing the adhesion strength and toughness of conventional thermoplastic hot melt adhesives.

Innovations in Isocyanate Synthesis for a Sustainable Future

Isocyanates play a crucial role as key building blocks in the production of thermoplastic foams, elastomers, adhesives, agrochemicals, and pharmaceuticals. These compounds are essential in the manufacture of various polymeric...

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.

Rapid Reassembly, Biomass-Derived Adhesive Based on Soybean Oil and Diels-Alder Bonds.

Synthetic adhesives play a crucial role in holding together solid materials through interfacial interactions. Thermoplastic and thermosetting adhesives are important types of synthetic adhesives, with thermoplastic adhesives being reassemblable and thermosetting adhesives exhibiting high adhesive strength and creep resistance. However, there is a need to combine the advantages of both types and develop high bonding strength, reassemblable adhesives. Here, epoxidized soybean oil (ESO) was used to prepare adhesive networks and Diels-Alder bonds were incorporated to enhance reassembly ability. The ESO was functionalized with furyl groups and cross-linked via the reaction between furyl and imide groups to involve the Diels-Alder bonds. The resulting adhesive exhibited good solvent resistance and mechanical properties, which could be regulated by adjusting the quantity of cross-linker. The prepared adhesives also demonstrated self-healing capabilities, as the scratch on the surface gradually diminished with heating. Additionally, the adhesives showed the ability to undergo recycling without significant changes in properties. The prepared adhesives exhibited hydrophilicity and the flow characteristics during reassembly were characterized by a decrease in torque. This study provides a promising approach for the development of synthetic adhesives with reassembly ability, which has important implications for the field of bonding.

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.

Experimental study of adhesive joints with mechanical interlocking of acrylonitrile butadiene styrene (ABS) parts fabricated for fused filament (FFF)

PurposeAdditive manufacturing has disadvantages, such as the maximum part size being limited by the machine’s working volume. Therefore, if a part more considerable than the working volume is required, the part is produced in parts and joined together. Among the many methods of joining thermoplastic parts, adhesives and mechanical interlocking are considered. This study aims to characterize and optimize mechanically stressed adhesive joints combined with female and male mechanical interlocking on acrylonitrile butadiene styrene (ABS) specimens made with fused filament fabrication (FFF) so that the joint strength is as close as possible to the strength of the base material.Design/methodology/approachThis study characterized the subject’s state of the art to justify the decisions regarding the experimental design planned in this research. Subsequently, this study designed, executed and analyzed the experiment using a statistical analysis of variance. The output variables were yield strength and tensile strength. The input variables were two different cyanoacrylate adhesives, two different types of mechanical interlock (truncated pyramid and cylindrical pin) and the dimensions of each type of mechanical interlock. This study used simple and factorial experiments to select the best adhesive and interlocking to be optimized using the response surface and the steep ascent method.FindingsThe two adhesives have no statistical difference, but they show different data dispersion. The design or yield stress was a determining factor for selecting the optimal specimen, with cylindrical geometry exhibiting higher resistance at initial failure. Geometry type is crucial due to the presence of stress concentrators. The cylindrical geometry with fewer stress concentrators demonstrated better tensile strength. Ultimately, the specimen with a mechanically reinforced joint featuring a cylindrical pin of radius 5.45 mm and height of 4.6 mm exhibited the maximum tensile and yield strength.Originality/valuePrevious research suggests that a research opportunity is the combination of bonding methods in FFF or fused deposition modeling, which is not a frequent topic, and this research to enrich that topic combines the adhesive with mechanically interlocked joints and studies it experimentally for FFF materials, to provide unpublished information of the performance of the adhesive joint with mechanical interlocking, to designers and manufacturers of this technology.

In-situ aligning magnetic nanoparticles in thermoplastic adhesives for contactless rapid joining of composite structures

Magnetic nanoparticles of high magnetic susceptibility, such as magnetite (Fe3O4), have been used for wireless heating of adhesives and composites through the magnetic hysteresis loss mechanism, but the high concentrations of nanoparticles needed to meet heating performance targets can degrade mechanical properties. Herein, we present an in-situ aligning method to enhance the heating efficiency of magnetite nanoparticles in a nylon thermoplastic matrix without adversely affecting its mechanical strength. A composite adhesive was made by dispersing Fe3O4 nanoparticles in a nylon matrix followed by hot melting. Experimental results show that by subjecting the adhesive to an alternating magnetic field during the hot-melt process, its heating rate can be improved by 200% compared to that without applying the magnetic field. The improvement in the heating performance has been identified to stem from the alignment of the ease axis of the magnetic nanoparticles. This in-situ aligning technique enables better induction heating performance with the same amount of Fe3O4 nanoparticles, avoiding the agglomeration problem of high nanoparticle concentrations. Moreover, this technique makes it possible to develop high-performance self-heating thermoplastic adhesive for reversible bonding and self-healing solution with a wide range of applications, such as bonding and debonding of composites, temporary attachment of systems, and recyclable bonded structures.

Analysis of Wood Bonding Failures that Initiated Before Adhesive Solidification: Air Fingers and Cavitation

Bond line openings while an adhesive is still viscous (liquid or paste-like) leave characteristic marks. Air fingers and cavitation develop on the surface of the adhesive when joint parts are fully or partially separated before solidifying of the adhesive. Therefore, the observation of air fingers or cavitation provides important clues on the root cause of the failure of bond lines. There is still limited knowledge about the factors that lead to the formation of either air fingers or cavitation. Additionally, a resoftening of thermoplastic adhesives by high temperatures may be confused with air fingers or cavitation that developed before the initial curing. To improve the understanding of the adhesive structures, we assemble some experiments on this phenomenon. Air fingers dominate when air easily penetrates the adhesive, the adhesive thickness is high, and the adhesive has a low viscosity (early in the process). Cavitation dominates when the adhesive layer is thin, when the viscosity is high (late in the process) or when the air ingress is restricted.

Prediction of the Strength of Oakwood Adhesive Joints Bonded with Thermoplastic Polyvinyl Acetate Adhesives

Among the several kinds of thermoplastic adhesives, structured and non-structured polyvinyl acetate (PVA) adhesives have a rather wide application and are used currently for forming adhesive joints from different wood species, especially oakwood. To ensure proper conditions of oakwood adhesive joints use, it is important to have fast and accurate methods of predicting their strength and durability. The strength changes of the oakwood adhesive joints bonded with structured and non-structured PVA adhesives have been investigated by conducting long-term experiments. Based on the generalization of experimental data and theoretical predictions regarding the mechanism of the adhesive seam formation, equations that allow calculating theoretically the strength of oakwood adhesive joints bonded with non-structured and structured PVA adhesives have been proposed. The pro-posed equations reproduce experimental data with suffi-cient accuracy of ±3.5 % within the temperature range from 251 K to 306 K and humidity range from 40 % to 100 %, and therefore, are recommended for practical use.

A renewable lignin-based thermoplastic adhesive for steel joining

Adhesive bonding of metals has become increasingly relevant in recent years due to the demand for reducing weight and improving performance in structural applications such as automobiles and aerospace. We developed renewable thermoplastic adhesives from technical organosolv lignin isolated from hardwood biomass and acrylonitrile butadiene co-polymer rubber (NBR) for joining steel substrates. NBR33, NBR41 and NBR51 with acrylonitrile molar ratios of 33, 41 and 51%, respectively, were blended with lignin to form two-phase thermoplastic adhesives, and their adhesion, viscoelastic and surface properties were measured. Lignin content in the compositions were varied, ranging from 40% to 80% (w/w), to alter toughness, stiffness, and surface energy characteristics of the material. Better interaction or reactivity between the lignin and NBR phases was observed with greater nitrile content in NBR, leading to greater modulus and stiffness of the adhesive. Simultaneously, increasing the proportion of lignin reduced toughness and improved stiffness, with the highest adhesive strength of 13.1 MPa measured in a 60% lignin loading ratio with NBR51. Surface energy measurements revealed that total surface energy (sum of polar and dispersive surface energy) raised with lignin loading, suggesting that both surface energy and matrix strength play a critical role in the adhesive properties of the synthesized materials. A finite element-based cohesive zone model (CZM) was developed and implemented to study the failure strength of the adhesively bonded joint. This study demonstrates the viability of lignin as a valuable building block for adhesives, not only due to its inherent chemical structure and rigidity, but also for its surface energy characteristics.

Gecko-Inspired Adhesive Mechanisms and Adhesives for Robots—A Review

Small living organisms such as lizards possess naturally built functional surface textures that enable them to walk or climb on versatile surface topographies. Bio-mimicking the surface characteristics of these geckos has enormous potential to improve the accessibility of modern robotics. Therefore, gecko-inspired adhesives have significant industrial applications, including robotic endoscopy, bio-medical cleaning, medical bandage tapes, rock climbing adhesives, tissue adhesives, etc. As a result, synthetic adhesives have been developed by researchers, in addition to dry fibrillary adhesives, elastomeric adhesives, electrostatic adhesives, and thermoplastic adhesives. All these adhesives represent significant contributions towards robotic grippers and gloves, depending on the nature of the application. However, these adhesives often exhibit limitations in the form of fouling, wear, and tear, which restrict their functionalities and load-carrying capabilities in the natural environment. Therefore, it is essential to summarize the state of the art attributes of contemporary studies to extend the ongoing work in this field. This review summarizes different adhesion mechanisms involving gecko-inspired adhesives and attempts to explain the parameters and limitations which have impacts on adhesion. Additionally, different novel adhesive fabrication techniques such as replica molding, 3D direct laser writing, dip transfer processing, fused deposition modeling, and digital light processing are encapsulated.

Electrically Conductive Adhesive Based on Thermoplastic Hot Melt Copolyamide and Multi-Walled Carbon Nanotubes.

For the bonding of the lightweight composite parts, it is desired to apply electrically conductive adhesive to maintain the ability to shield electromagnetic interference. Among various solvent-based adhesives, there is a new group of thermoplastic hot melt adhesives that are easy to use, solidify quickly, and are environment-friendly. To make them electrically conductive, a copolyamide-based hot melt adhesive was mixed with 5 and 10 wt% of carbon nanotubes using a melt-blending process. Well-dispersed nanotubes, observed by a high-resolution scanning microscope, led to the formation of a percolated network at both concentrations. It resulted in the electrical conductivity of 3.38 S/m achieved for 10 wt% with a bonding strength of 4.8 MPa examined by a lap shear test. Compared to neat copolyamide, Young’s modulus increased up to 0.6 GPa and tensile strength up to 30.4 MPa. The carbon nanotubes improved the thermal stability of 20 °C and shifted the glass transition of 10 °C to a higher value. The very low viscosity of the neat adhesive increased about 5–6 orders of magnitude at both concentrations, even at elevated temperatures. With a simultaneous growth in storage and loss modulus this indicates the strong interactions between polymer and carbon nanotubes.

Mechanical and physical characterization of natural and synthetic consolidants

The mechanical behavior of adhesives is strongly influenced by a large number of variables, relating to a complex interaction of mechanical-physical-chemical factors, such as its loading direction (shear, peel), the temperature and the environmental relative humidity (RH). These variables can have a large influence on the durability of restored art objects where thermoplastic adhesives have been used as a consolidant. This study aims to characterise the mechanical and physical behavior of some adhesives commonly used polymers by conservators as consolidants to restore cultural objects such as canvas paintings or historic wooden furniture. Twelve commercially available natural and synthetic adhesive materials were tested. The influence of RH at room temperature on the mechanical and physical properties of the adhesives was investigated. Shear and peel experiments were performed on adhesively bonded wood and canvas coupon to establish mechanical characterisation. The physical properties of the adhesives were determined by performing moisture adsorption measurements and Differential Scanning Calorimetry (DSC). The results of this study demonstrate that synthetic adhesive products are able to resist higher shear and peel loads than natural types. Moreover, the influence of important changes in RH on the mechanical properties of the adhesives was demonstrated. Reflecting on the combined data derived from shear and peel tests with the adhesive's sensitivity to moisture will help conservators to select the most suitable adhesives for their applications to achieve optimal durability and the best mechanical performance in versatile environmental conditions.

Sequence Control from Mixtures: Switchable Polymerization Catalysis and Future Materials Applications.

There is an ever-increasing demand for higher-performing polymeric materials counterbalanced by the need for sustainability throughout the life cycle. Copolymers comprising ester, carbonate, or ether linkages could fulfill some of this demand as their monomer–polymer chemistry is closer to equilibrium, facilitating (bio)degradation and recycling; many monomers are or could be sourced from renewables or waste. Here, an efficient and broadly applicable route to make such copolymers is discussed, a form of switchable polymerization catalysis which exploits a single catalyst, switched between different catalytic cycles, to prepare block sequence selective copolymers from monomer mixtures. This perspective presents the principles of this catalysis, catalyst design criteria, the selectivity and structural copolymer characterization tools, and the properties of the resulting copolymers. Uses as thermoplastic elastomers, toughened plastics, adhesives, and self-assembled nanostructures, and for programmed degradation, among others, are discussed. The state-of-the-art research into both catalysis and products, as well as future challenges and directions, are presented.

Wood Adhesives Based on Natural Resources: A Critical Review: Part IV. Special Topics

Various naturally-based chemicals can be used directly as wood adhesives or are precursors for the synthesis of adhesive resins. Liquefaction and pyrolysis of wood yield various smaller chemicals derived from the different wood components, which then are used in the preparation of adhesives by replacing mainly phenol as raw material. The possible replacement of formaldehyde in aminoplastic and phenolic resins would solve the question of the subsequent formaldehyde emission.<br/> The multiple unsaturations of the triglycerides in vegetable oils enable polymerization for the direct synthesis of thermosets, as well as bases for polyfunctionalization and crosslinking.<br/> Natural polymers, such as poly(lactic acid)s (PLAs), natural rubber, or poly(hyhydroxyalkanoate)s (PHAs) are thermoplastics and can be used for various special applications in wood bonding, in case they can also be crosslinked. For other thermoplastic wood adhesives, such as PUR or PA, chemicals based on natural resources can at least replace a part or even all synthetic raw materials (monomers); these monomers derive from targeted decomposition of the wood material in biorefineries.<br/> Cellulose nanofibrils (CNFs) can be used as as sole adhesives or as components of adhesives. Hydrogen bonding has a key function in binder applications related to adhesion between cellulose nanoparticles and other materials. CNFs are able to establish strong bonding between wood particles/fibres through flexible and strong films by a simple drying process.<br/> Cashew nut shell liquid (CNSL) is a by-product of the cashew nut processing with cardanol (CD) as main component. CD-formaldehyde resins show improved flexibility compared to phenol-formaldehyde (PF) resins; CD can replace up to 40% of the phenol.

Hot-Melt Adhesive Based on Dynamic Oxime–Carbamate Bonds

Polyurethane hot-melt adhesives featured with high adhesion strength and environmental friendliness possess broad applications in many fields. However, their practical application is hindered by existing drawbacks such as low adhesion strength and poor recyclability for thermoplastic polyurethane and reactive polyurethane hot-melt adhesives, respectively. Achieving high adhesion strength and efficient recycling for hot-melt adhesives simultaneously is challenging. Herein, reversible covalent bonds (oxime–carbamate) were introduced to construct a three-dimensional dynamically cross-linked polyurethane system which has the merits of favorably reworkable thermoplastic polyurethane hot-melt adhesives and high adhesive reactive polyurethane hot-melt adhesives to accomplish efficient bonding–debonding conversion on demand. Such a structure provided a high cross-linked covalent network and a fast reversible adhesive property, and hence improved the overall performance of holt-melt adhesives. As a result, the adhesive exhibited an impressive initial and ultimate adhesion strength of 5.96 ± 0.61 MPa (5 min curing) and 9.02 ± 1.36 MPa (10 days curing), respectively. More remarkably, favorable repeated adhesive capability was realized even after it was completely broken four times, while still retaining all above 4.40 ± 0.92 MPa. In addition, this adhesive also possessed excellent solvent resistance and good durability. Our work emphasized the importance of molecular design and a general method for strong and efficient adhesive systems.

The effect of the soak temperature applied to the classification of the water resistance grade of wood adhesives

Classification of thermoplastic wood adhesives for non-structural applications in the durability class according to EN 204:2017 is significantly influenced by the water temperature used during the test. When using dispersion with durability class D3 and D4 the bonding strength will decrease as the temperature rises. The temperature selected within the water temperature range specified in the standard can reduce the strength value by nearly a third, which can modify the classification regarding the corresponding durability class. Inappropriate classification is misleading the users which can cause their application on an inadequate field. Therefore the modification of standard in this field is recommended.

Вплив навантажень на формостійкість тришарової паркетної дошки

Досліджено вплив навантажень на формостійкість тришарової паркетної дошки, склеєної термопластичними полівінілацетатними клеями, залежно від їх величини під час експлуатації. На основі побудованої математичної моделі для прогнозування формостійкості тришарової паркетної дошки залежно від дії зовнішніх факторів під час експлуатації з використанням імітаційного моделювання досліджено пружно-деформаційний стан тришарової паркетної дошки, склеєної термопластичними полівілацетатними клеями. Побудовано графічні залежності радіальних та тангентальних напружень у тришаровій паркетній дошці і проаналізовано їх вплив на паркетну дошку під час експлуатації залежно від навантажень. Під час математичного та імітаційного моделювання тришарову паркетну дошку розглянуто як клейову конструкцію, що складається із трьох шарів, а саме: верхнього, середнього та нижнього, склеєних між собою термопластичними полівінілацетатними клеями із ступенем навантаження 3D. Кожний шар конструкції виготовлений із різних матеріалів, а саме: верхній шар – із твердолистяної породи деревини дуба, середній – із хвойної породи сосни та нижній – із фанери. Оскільки для матеріалу, з якого виготовлена конструкція, характерні різні фізико-механічні властивості, то відповідно, це матиме вплив на пружно-деформаційні процеси тришарової паркетної дошки під час навантажень та визначатиме формостійкість виробу. Встановлено, що за дії на тришарову паркетну дошку, склеєну термопластичними полівінілацетатними клеми, фізичних навантажень, тангентальні напруження будуть зростати у середньому шару, який виготовлений із ламелей деревини сосни та розміщений перпендикулярно до верхнього лицевого та нижнього основного шару. Мінімальне значення тангентальних напружень спостерігається у середині верхнього лицевого та нижнього шарів. Такий розподіл навантажень у тришаровій конструкції паркетної дошки, досягається завдяки нижньому шару, який виготовлений із фанери, та еластичності клейового з'єднання, яке формується термопластичними полівінілацетатними клеями. Це дає змогу компенсувати напруження у внутрішньому шарі тришарової паркетної дошки під час її експлуатації, цим самим покращити її формостійкість.

Use of waste plastics for the preparation of adhesives for wood bonding

There is more and more plastic waste, so we try to find different ways to recycle and reuse it. Recycled thermoplastic polymers represent an interesting potential for the production of adhesives, so the aim of our preliminary investigations was to examine the suitability of waste polystyrene for the production of adhesives for wood bonding. The adhesive mixtures were prepared by cutting waste polystyrene cups (L) and coffee spoons (Ž) into smaller pieces and dissolving them separately in five different solvents: Butyl acetate (BA), Dimethylformamide (DMF), Tetrahydrofuran (THF), Limonene (L) and Acetone (AC). From these we prepared composite samples of two beech lamellae (Fagus sylvatica L.), which were pressed for 10 minutes in a hydraulic press at a temperature of 150 °C and a pressure of 12 bar. In order to determine the quality of the bonding, we carried out a shear test with a Zwick/Roell Z005 universal testing machine. We found that the test specimens bonded with the adhesive mixture Ž-DMF achieved an average shear strength of 6.5 N/mm2 and the test specimens bonded with L-DMF achieved 3.8 N/mm2. All other test specimens showed a lower strength of the adhesive bonds, or they mostly delaminated after hot pressing or later during sawing into test specimens. In further research work we will modify these promising adhesive mixtures and optimise the pressing parameters in order to achieve bonding quality that meets the standard requirements for thermoplastic adhesives for interior applications.