Polyurethane Adhesive Joints Research Articles

Flexible Polyurethane Adhesives: Predictive Numerical Model Calibration through Experimental Testing at Elevated Temperature

The thermo-mechanical behavior of polyurethane adhesive joints in wood structures is a crucial aspect that needs to be understood to ensure the durability and safety of timber structures, especially in seismic regions. As mass timber, particularly cross-laminated timber, continues to gain popularity as a building material, it is important to pay special attention to the behavior of connections between the timber elements. The use of flexible polyurethane adhesives presents a promising alternative to conventional mechanical connections in seismic-resistant timber structures. This research highlights the potential of polyurethane-based joints at elevated service temperature, offers a promising alternative to traditional wood joints, and suggests viability for post-fire restoration of wood structures. The response at the interface between wood and polyurethane under flexural stresses is also evaluated, underscoring the broader application possibilities of flexible adhesives in wood construction for mechanical and physical improvements.

Robotised atmospheric plasma treatment to improve the adhesion of vulcanised and thermoplastic rubber materials for a more sustainable footwear

Surface treatment is essential to ensure the resistance and durability of adhesive upper-to-sole joints for footwear. This study investigates robotised atmospheric pressure plasma technology to increase adhesion on styrene-butadiene vulcanised (SBR) and styrene-butadiene-styrene thermoplastic (SBS) rubbers, which constitutes a more sustainable surface treatment as an alternative to solvent-based chemical treatments (i.e., halogenation). In this work, the different parameters such as application speed and distance of the nozzle of the atmospheric plasma system have been optimised and the effects of the plasma treatment on the physicochemical properties in the materials’ surface, roughness, wettability, as well as the adhesion properties have been evaluated by means of different experimental techniques: Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Photoelectron Spectroscopy (XPS), Field Emission Scanning Electron (FESEM), Contact Angles (CA) and Surface Free Energy (SFE) measurements and T-peel strength tests. The results show that the values obtained for the T-peel strength of the reactive hot melt polyurethane adhesive (HMPUR) joints studied exceed the minimum quality requirements for footwear according to standardised tests. In consequence, the optimisation and validation of operating plasma conditions and adhesion properties of rubbers could consolidate atmospheric plasma as a solvent-free alternative for the preparation of footwear soling materials. The great advantage of this technology is its introduction into the production line by fully automating the bonding process with the use of robots.

Effect of temperature and humidity on the creep and aging behavior of adhesive joints under static loads

ABSTRACT To study the creep and aging behavior under the combined effect of temperature, humidity and static load, polyurethane adhesive was selected to make butt joints. The creep properties of the joint were analyzed experimentally under different static loads in hygrothermal environment. At the same time, an aging test under the combined effect of temperature, humidity and static load was carried out, and the variation law of the residual strength of the joint with the load and loading time was obtained. The complex fracture behavior of polyurethane adhesive joints was studied from macro to microscale. The results show that the hygrothermal environment had a significant effect on the creep properties of polyurethane adhesive joints. The creep strain rate of the joint was significantly higher, and the failure fracture time was shorter in the high temperature and humidity environment. When the temperature, humidity and static load were coupled, the joint failure strength decreased with time. With increasing load, the decline rate of the failure strength increased significantly. The interface failure of the joints and the hydrolysis reaction of the polyurethane adhesive were the main reasons for reducing the failure strength.

Cyclic fatigue testing: Assessment of polyurethane adhesive joints' durability for bus structures' aluminium assembly

Over the years, using adhesive joints in structural applications widespread all industrial domain, achieving an amazing current usage. This is due to the benefits that this technology is capable of providing to complex-shaped structures, both in aerospace and automotive applications. Indeed, new polyurethane adhesives are a recently fastening method. This newest adhesive types bid great advantages especially on damping, impact and fatigue which are critical characteristics in the transportation industry. Besides, the current bus manufactures use this adhesive kind to join doors' structures aluminium assembly. As like, the Tunisian automotive industry ICAR uses a one-component polyurethane for doors' structure assembly. Crack problems due to fatigue are evident in those areas since they are continuously stressed. Thereby, working on more understanding polyurethane fatigue performance we investigate over this work, the response of the assembly aluminium-alloy polyurethane adhesive joints using SLJ with two design's parameters sorted from a static study under five distinct load levels. To achieve this purpose, substrates' surfaces roughness with an arithmetic average of surface heights Ra ≈ 0.6 µm, was used. Bonded specimens with 1 mm adhesive thicknesses were manufactured and examined using cyclic fatigue testing. Furthermore, the obtained results are also compared with others from literature using epoxy adhesives. Worthy results are obtained rising the effectiveness of this adhesive's type in the transport industry.

Hygrothermal aging, fatigue and dynamic mechanical behavior of cellulosic particles reinforced one-component moisture curable polyurethane adhesive joints

Microcrystalline cellulose (MCC) and sawdust (SD) green fillers have great potential to act as moisture reservoirs and reactive fillers. In the present study, a one-component moisture curable polyurethane (PU) adhesive was reinforced with MCC and SD fillers. The durability of aluminum/steel joints produced using unreinforced and reinforced PU adhesives was studied using dynamic mechanical analysis (DMA), fatigue, and hygrothermal aging experiments. Lap-shear tests were conducted to evaluate the effects of aging on joints strength. Fourier-transform infrared spectroscopy (FTIR) experiments were performed on adhesive joints after hygrothermal aging to understand the changes in chemical structure due to aging, and the influence of cellulosic fillers on the aging phenomena. DMA results showed that the glass transition temperature of the reinforced PU was similar to that of the unreinforced PU. Fatigue performance of MCC and SD reinforced PU adhesive joints was found to be better than unreinforced PU joints. Further, MCC reinforced PU adhesive joints were found to be more resistant to hygrothermal aging than SD reinforced and unreinforced PU joints. Overall, MCC reinforced PU adhesive joints exhibited good fatigue and superior long-term properties.

Effect of moisture and temperature on thermal and mechanical properties of structural polyurethane adhesive joints

The aim of this study was to evaluate the effect of humidity and temperature in the mechanical and thermal properties of two polyurethane adhesives joints. Single and double component adhesives were selected, in order to compare their different properties to join dissimilar materials (steel and carbon fiber composite). By means of two devices specially developed for the study, durability and heat-aging tests were performed. The ability of the adhesives to cope with temperature (72 °C) and humidity (82%), and facing temperature variations (from 26 °C to 72 °C) was studied. Shear tests and Shore D hardness measurements were carried out after the specimens were subjected to different exposure times and adverse environments. Joint reliability was studied by means of a simplified Weibull model. Besides, differential scanning calorimetry and infrared spectroscopy were made to understand the changes produced in the adhesives. Different behaviors were found for each adhesive, but they showed good characteristics to be used in structural applications under adverse environments.

Characterization of aluminium one-component polyurethane adhesive joints as a function of bond thickness for the automotive industry: Fracture analysis and behavior

The automotive industry is increasing, nowadays, its use of high performance structural adhesives in order to reduce vehicle weight and increase the crash resistance of automotive structures. One-component polyurethane adhesives are attracting rising interest as an alternative to conventional rigid structural adhesives. This is due to several benefits such as the excellent impact resistance they provide. This paper investigates experimentally the mechanical behavior of one-component polyurethane adhesive joints and specifically the effect of the adhesive layer to be used in several parts for buses structure. Furthermore, it characterizes the fracture behavior of the adhesive layer with various bond’s thicknesses using mode-I and mode-II fracture testing. The fracture toughness under both modes is determined using Double Cantilever Beam (DCB) and End-Notched Flexure (ENF) tests, respectively. Assessment of the various fracture tests indicated that DCB and ENF provide the same shape of evolution of the fracture energy versus adhesive thickness. Moreover, the test results were able to fully mechanically characterize the adhesive and demonstrate that the adhesive has not only high mechanical strength but there is an optimal adhesive thickness, which allows a high toughness for bonded joints. This result proves that it is worthy to use the one-component polyurethane adhesive in the automotive industry.

Effect of adhesive thickness and surface roughness on the shear strength of aluminium one-component polyurethane adhesive single-lap joints for automotive applications

Adhesively bonded technology is now widely accepted as a valuable tool in mechanical design, allowing the production of connections with a very good strength‐to‐weight ratio. The bonding may be made between metal–metal, metal–composite or composite–composite. In the automotive industry, elastomeric adhesives such as polyurethanes are used in structural applications such as windshield bonding because they present important advantages in terms of damping, impact, fatigue and safety, which are critical factors. For efficient designs of adhesively bonded structures, the knowledge of the relationship between substrates and the adhesive layer is essential. The aim of this work, via an experimental study, is to carry out and quantify the various variables affecting the strength of single-lap joints (SLJs), especially the effect of the surface preparation and adhesive thickness. Aluminium SLJs were fabricated and tested to assess the adhesive performance in a joint. The effect of the bondline thickness on the lap-shear strength of the adhesives was studied. A decrease in surface roughness was found to increase the shear strength of the SLJs. Experimental results showed that rougher surfaces have less wettability which is coherent with shear strength tests. However, increasing the adhesive thickness decreased the shear strength of SLJs. Indeed, a numerical model was developed to search the impact of increasing adhesive thickness on the interface of the adherend.

Mechanical behavior of polyurethane adhesive joints used in laminated materials for marine structures

One of the most relevant applications of adhesives in shipbuilding is the manufacturing of structural laminated materials. In this study, a new sandwich structure with a polyurethane adhesive joint, composite material and steel is evaluated. The use of this material in a marine environment for long periods of time and under adverse environmental conditions leads to accelerated degradation of the polyurethane adhesive. Therefore, to guarantee suitable functioning of the adhesive joint over a long period of time under adverse conditions and variable stress modes, a specific experimental procedure is required. For this purpose, we report a new experimental procedure for investigating the mechanical behavior of polyurethane adhesive joints.Test results have led to the conclusion that steel–polyurethane–steel adhesive joints immersed in sea water exhibit the best behavior in service when subjected to shear stress. In addition, the comparative analysis of the results obtained for the fracture energy and cohesive laws indicated that immersion in distilled water has a favorable effect on the behavior of the joint when subjected to peel stress and shear stress. Similarly, the low values of tangential and normal stress in the joints that were immersed in seawater indicate that this joint undergoes a progressive degradation process.

Technical Evaluation of Structural Adhesive Joints under Adverse Operation Conditions

This work analyzes the degradation of aluminum-composite adhesive joints under the action of water and motor oil. For this purpose, we have performed an accelerated ageing of the adhesive joint by immersion in water and oil. Likewise, we have evaluated the loss of mechanical properties that aging causes into adhesive joint. First, tests have been performed with bulk adhesive specimens (epoxy and polyurethane) immersed in water and motor oil during independent periods of time (1 at 128 days) in order to study the diffusion of water and motor oil into the adhesive. Second, we have evaluated the loss of mechanical properties that aging causes in the adhesive joint. Tests conclusions show that water degrades the adhesive more than motor oil. Additionally and under the action of water and motor oil, polyurethane adhesive joints retain their mechanical properties better than epoxy adhesive joints.

Fracture toughness improvement of polyurethane adhesive joints with chopped glass fibers at cryogenic temperatures

Polyurethane (PU) adhesive is widely used for bonding the secondary barrier of cryogenic containment systems (CCS) of liquefied natural gas (LNG) ships due to its ductility at room temperature, although it can become brittle at the cryogenic temperature below its glass transition temperature.To evaluate the reliability of PU adhesive at the cryogenic temperature of −150°C, the fracture toughness of double-cantilever-beam (DCB) adhesive joints composed of stainless steel and aluminum adherends was measured. Because the fracture toughness of the PU adhesive was found to be very low at the cryogenic temperature, the adhesive was reinforced with chopped glass fibers. The fracture toughness with respect to the length and volume fraction of the chopped glass fibers was then measured. Based on the experimental results, an optimal reinforcement method is suggested for the reliable application of PU adhesive at cryogenic temperatures.

Chemical nature of adhesion: Plasma modified styrene–butadiene elastomer and polyurethane adhesive joints

A study of the mechanism of gluing between plasma-modified styrene–butadiene elastomer (SBS) surfaces and polyurethane (PU) adhesives is presented in this paper. The plasma treatment was carried out employing low-pressure O 2 plasma generated by an RF discharge (13.56 MHz). FTIR–ATR spectroscopy and peel tests were utilized to characterize the SBS surfaces before and after the plasma treatment. It was found that hydroxyl groups are mainly created on the SBS surface by the plasma treatment and their concentration reveals very good linear correlation with the peel strength of the SBS–PU joints. The grafting of phenyl isocyanate (analog of the PU curing agent) into the plasma-treated SBS surface was also investigated. The process was performed at 353 K, that is at temperature in which the adhesive-bonded joints were made. It was shown that all hydroxyl groups on the SBS surface react with isocyanate groups forming carbamate bonds. The results obtained in this work strongly confirm the conception of the adhesion by chemical bonding between SBS and PU.

Mechanisms of Adhesion in Surface Chlorinated Thermoplastic Rubber/Thermoplastic Polyurethane Adhesive Joints

Abstract Chlorination of a thermoplastic styrene-butadiene-styrene rubber (S0) with a solution of 2 wt% trichloroisocyanuric acid (TCI) in ethyl acetate improved its adhesion to polyurethane adhesives. The analysis of the failed surfaces (obtained after T-peel test of S0/PU joints) showed that the locus of failure in the rubber/polyurethane joints progresses from adhesion (in non chlorinated-S0/PU joint) to cohesion in the chlorinated layer (for chlorinated-S0/PU joint). The composition of this chlorinated layer differed from the composition of the non bonded-chlorinated rubber, i.e. the failure of the joint was located in a chlorinated layer with a distinctive chemistry. On the other hand, the analysis of the chlorinated rubber/PU adhesive interface showed that chlorination with TCI produces a crosslinking of the rubber surface as well as strong interactions between the uppermost-chlorinated layer and the PU adhesive.

Durability of carbon fibre reinforced polyetheretherketone–polyurethane adhesive joints in aqueous environments

As part of a project to develop a biomimetic artificial joint, it was necessary to bond carbon fibre reinforced polyetheretherketone (APC2) to thermoplastic polyurethane. As no qualified adhesives were available, adhesive bonding using hot compression moulding was the chosen methodology. The human body is one of the most aggressive environments that an adhesive bond has to endure. The durability of these APC2–polyurethane joints has been investigated in various aqueous environments including Ringer's solution and distilled water. Prior to bonding, the APC2 specimens were exposed to plasma etching and oxidising flame surface treatments to improve adhesion properties. Peel testing revealed that strong and durable bond strengths were recorded and no aging effects were observed when the adhesive joints were stored in a dry atmosphere for 1 year. However, when stored in the aqueous media the bonds rapidly deteriorated to unacceptable levels after 7 days. It was found using a variety of techniques including mass uptake analysis, surface force microscopy, and X-ray photoelectron spectroscopy, that the primary mechanism of bond degradation was due to ingression of water by wicking and diffusion.

Influence of the composition on the crystallinity and adhesion properties of thermoplastic polyurethane elastomers

Crystallinity and adhesion properties of nine thermoplastic polyurethanes (TPUs) prepared by varying the hard-to-soft segments ratio, the molecular weight of the macroglycol (related to the size of the soft segments) and the chain extender size and nature (i.e. hard segments length and morphology) were studied. TPUs structure was characterised using 1 H Nuclear Magnetic Resonance ( 1 H NMR), and their properties were studied using Differential Scanning Calorimetry (DSC) and Wide Angle X-ray Diffraction (WAXD). TPUs were used as raw materials of solvent-based adhesives. The adhesion properties were measured from T-peel tests of solvent-wiped polyvinyl chloride (PVC)/polyurethane adhesive joints. It was found that the composition of the TPUs influenced their structure and properties. An increase in the hard segments content in the TPUs reduced the degree of phase separation and the organisation of their structure, resulting in less crystalline polymers with worse adhesion properties.

Structure, composition, and adhesion properties of thermoplastic polyurethane adhesives

The composition and hard segment content of 13 commercial thermoplastic polyurethane elastomers (TPUs) were obtained using 1H-nuclear magnetic resonance (1H-NMR). The properties of the TPUs were studied using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), dynamic mechanical thermal analysis (DMTA), and contact angle measurements. Solventbased adhesives were prepared by dissolving the TPUs in 2-butanone. Films of the TPUs were obtained by solvent evaporation, and their properties were studied. Adhesion properties were determined from T-peel tests on solvent-wiped poly(vinyl chloride) (PVC)/polyurethane adhesive joints. The influence of the segmented structure on the properties of the TPUs was assessed. The increase in the hard segment content in TPUs favoured the incompatibility (i.e. reduced phase separation) between hard and soft domains. TPUs with a high hard segment content had a low crystallinity, a low wettability, and a high joint strength. The storage and loss moduli obtained using DMTA decreased as the hard segment content in the TPUs increased. Furthermore, the TPUs prepared using ε-polycaprolactone as the macroglycol had a slower crystallization rate than those prepared using the polyadipate of 1,4-butanediol or the polyadipate of 1,6-hexanediol. The increase in the length of the hydrocarbon chain of the macroglycol improved both the rheological and the thermal properties of the TPUs. Finally, TPUs prepared using MDI as the isocyanate showed a higher crystallinity and a higher degree of crosslinking than those prepared using TDI.

Characterization of Thermoplastic Polyurethanes Prepared Using Different Macroglycols

Three polyurethane elastomers (PUs) were prepared using macroglycols of different nature (varepsilon-polycaprolactone, polyadipate of 1,6-hexanediol) and length of the hydrocarbon chain (polyadipate of 1,4-butanediol, polyadipate of 1,6-hexanediol). The PUs were characterized using Gel Permeation Chromatography, Differential Scanning Calorimetry, Wide X-ray angle Diffraction, Dynamic Thermal Mechanical Analysis, stress-controlled rheometry and stress-strain experiments. The surface properties were evaluated from contact angle measurements. The PUs were used as raw materials for solvent-based adhesives, whose adhesion properties were measured from T-peel strength of plasticized poly(vinyl chloride) (PVC)/polyurethane adhesive joints. The use of polyadipate of 1,6-hexanediol produced a polyurethane with high crystallinity (i.e. poor rheological and mechanical properties) and enhanced interactions between soft segments. Low adhesion was obtained in joints produced with this polyurethane and a cohesive failure of the adhesive was produced. The decrease in the polyadipate hydrocarbon chain length decreased the degree of crystallinity between polymer chains, therefore, no reduction in rheological and mechanical properties was obtained; a higher joint strength was also obtained. In this study the best performance was obtained with the polyurethane based on varepsilon-polycaprolactone, presumably because of its higher surface energy and reduced crystallinity. The properties of the polyurethanes prepared in this study were more affected by the characteristics of the macroglycol, and the crystallinity of the polyurethane had a more marked effect on the properties than the degree of phase separation.

Properties of Polyurethane Elastomers with Different Hard/Soft Segment Ratio

Three polyurethane elastomers (PU) containing different hard/soft (h/s) segment ratios were prepared. The PUs were characterized using Gel Permeation Chromatography (GPC), DSC, Wide Angle X-ray Diffraction (WAXD). Dynamic Mechanical Thermal Analysis (DMTA), and Stress-controlled rheometry. The surface properties were evaluated from contact angle measurements. The PUs were used as raw materials for solvent-based adhesives, whose adhesion properties were measured from T-peel tests of solvent-wiped polyvinyl chloride (PVC)/polyurethane adhesive joints. The increase in the amount of h/s segment ratio affected the structure and morphology of the PUs, reducing the degree of phase separation and the extent of the secondary interactions between polymer chains. The h/s segment ratio determined the thermal, mechanical, rheological and adhesion properties of the PUs.

Attapulgite as a filler for solvent-based polyurethane adhesives

—Different amounts (5-20 wt% with respect to the solids content in the adhesive) of a hydrated magnesium/aluminum silicate (attapulgite) were added to solvent-based polyurethane (PU) adhesive formulations. The rheological, thermal, mechanical, surface, and adhesion properties of the filled PU adhesives obtained were measured. The addition of attapulgite provided an increase in viscosity, imparted thixotropy and pseudoplasticity to the PU adhesive solution, and produced an increase in storage and loss moduli. Increase in the amount of attapulgite enhanced these effects. Some interactions (not well defined in this paper) between the attapulgite, the polyurethane, and/or the solvent seemed to be responsible for the improved properties of the filled PU adhesives. Furthermore, the addition of attapulgite increased the mechanical properties, decreased the glass transition temperature, affected the crystallization rate and melting process, and modified the surface properties of the PU adhesive films (without solvent). On the other hand, the immediate (green) T-peel strength of roughened or (roughened + chlorinated with 1 wt% trichloroisocyanuric acid solutions in 2-butanone) styrene-butadiene rubber (SBR)/PU adhesive joints was greatly improved if the adhesive contained attapulgite. However, similar T-peel strength values in fully cured adhesive joints (measured 72 h after bond formation) were found irrespective of whether the PU adhesives contained attapulgite or not.

Influence of the length of the chain extender on the properties of thermoplastic polyurethanes

Three polyurethane elastomers (PUs) were prepared using MDI (diphenylmethane-4,4'-diisocyanate), polyadipate of 1,4-butanediol, and three linear chain extenders with different lengths (ethylene glycol, 1,4-butanediol, and 1,6-hexanediol). The prepolymer procedure was used to prepare the PUs. The PUs were characterized using gel permeation chromatography, differential scanning calorimetry, wide angle X-ray diffraction, dynamic mechanical thermal analysis, and stress-controlled rheometry. The surface properties were evaluated from contact angle measurements. The PUs were used as raw materials for solvent-based adhesives, the adhesion properties of which were assessed from T-peel tests of solvent-wiped poly(vinyl chloride) (PVC)/polyurethane adhesive joints. The use of a short-chain extender produced phase separation, high crystallinity, and adequate rheological properties in the PUs. The length of the chain extender used in this study did not affect the surface properties of the PUs nor the T-peel strength of PVC/polyurethane adhesive joints but a change in the locus of failure was obtained: the decrease in the length of the chain extender, i.e. the improved crystallinity and the high degree of phase separation in the PUs, favoured the interfacial failure.