Polyurethane Adhesives Research Articles

Hydrophilic competent and enhanced wet-bond strength castor oil-based bioadhesive for bone repair

Bone adhesive has been proved to be a promising alternative in the clinical treatment of bone repairs. However, the problems of unsatisfying bone-bonding strength, especially the bonding of cortical bone in vivo, and blocked bone tissue recovery remain barriers to clinical reparation. Benefit from dopamine-modified castor oil synthesized by an epoxy-modification method, a porous and two-component polyurethane adhesive (PUA) was prepared to overcome the current challenges encountered. The tailored surface morphology and open porosity of the adhesive layer can be obtained to meet the requirements of bone repair by tuning the fraction of the formulation. Furthermore, the incorporation of nano-hydroxyapatite improved the mechanical properties and osteocompatibility of the material. Compared with PUA without catechol groups, the introduction of catechol groups not only increased the adhesive strength from 0.28 ± 0.05 MPa to 0.58 ± 0.06 MPa under wet conditions but also enabled the enrichment of Ca2+ on the adhesive surface to promote bone regeneration. Besides, the cell culture experiments also indicated that PUAs show good biocompatibility and excellent adhesion to stem cells. Given its excellent wet adhesive strength and biocompatibility, this system demonstrated potential applications in orthopedic treatment.

Evaluation of crack growth behaviors under Mode I static loading for two-part polyurethane adhesives

Structural polyurethane adhesives have been developed in recent years. Thin steel adherends bonded with polyurethane adhesives plastically deform in double cantilever beam tests, an energy release rate is affected by plastic deformation of adherends. This study suggests a loading-unloading test to evaluate crack growth behaviors for two types and two adhesive thicknesses of polyurethane adhesives under Mode I static loading even when plastic deformation occurs in the adherends. A load, in which the crack growth is stopped during test pausing after loading in the loading-unloading test, is defined as an arrest load. The arrest load governs crack arrest and crack regrowth of adhesives, and is not affected by plastic deformation of adherends. Thus, an energy release rate by the arrest load defines as critical separation energy. The critical separation energy for polyurethane adhesives is higher for higher tensile modulus and adhesive strength, and a thicker adhesive. Analytical results based on the triangle cohesive zone model using zero-thickness cohesive elements are agreed with the experimental results.

Effects of surface preparation on the adhesion of UV-aged polyurethane coatings

This paper focuses on the manufacturing and quality evaluation of polyurethane coatings deposited onto composite substrates. Various methods of surface preparation and roughness development were used to enhance adhesion: degreasing with acetone, abrasive blasting with F30 alumina, abrasive blasting and degreasing with acetone, and no preparation. As well as the influence of substrate preparation on the pull-off strength of the coatings, the influence of ageing on the coating adhesion was also investigated. This research shows that the choice of preparation of the laminate substrate has a significant impact on its surface roughness, which directly affects the strength of adhesion of polyurethane coatings to the composite substrate. The most effective method of surface modification turned out to be the use of abrasive blasting for coatings applied with the traditional method (increasing the strength by 41%). Coatings subjected to the process of accelerated ageing through ultraviolet radiation are characterised by a significant reduction in the adhesion force to the composite substrate in comparison to unaged coatings. The photodestruction of UV-aged coatings was documented by the characteristics obtained during tests carried out with differential scanning calorimetry and Fourier-transform infrared spectroscopy (in the attenuated total reflectance mode). Thermal analysis (thermogravimetric analysis/dynamic thermogravimetry) also was used to study the effect of ultraviolet radiation on the coatings.

Designing New Sustainable Polyurethane Adhesives: Influence of the Nature and Content of Diels-Alder Adducts on Their Thermoreversible Behavior.

With a view to the development of new sustainable and functional adhesives, two Diels–Alder (DA) adducts are incorporated as a third component into the curing process of solvent-based and solvent-free polyurethanes in this study. The influence of the nature and content of the DA molecules on the retro-DA (rDA) reaction and its reversibility and cyclability is investigated. It is demonstrated that the bonding/debonding properties of the adhesives are mainly controlled by the concentration of the DA adducts, with a minimum thermoreversible bond (TB) content required that depends on the system and the total ratio between all the diols in the formulation. For the solvent-based system, rDA/DA reversibility can be repeated up to ~20 times without deterioration, in contrast to the solvent-free system where a gradual loss in the DA network reconstruction efficiency is observed. Despite this limitation, the solvent-free system presents clear advantages from an environmental point of view. The changes observed in the physical properties of these new thermoreversible adhesives are of great relevance for recycling strategies and, in particular, their potential for separating multilayered film packaging materials in order to recycle the individual polymer films involved.

Performance of one-component polyurethane (1C-PUR) adhesives in the production of finger-joints fabricated from four planted forest species

Performance of one-component polyurethane (1C-PUR) adhesives in the production of finger-joints fabricated from four planted forest species

The effect of adhesive amount and type on failure mode and shear strength of glued timber-concrete joints

Wet and dry bonding techniques were used to bond the spruce timber and concrete with the adhesive as a timber-concrete joint (TCJ) in this study. This paper discusses the effect of adhesive amount and type, i.e., polyurethane and epoxy, on the bond strength of wet and dry TCJs. Adhesive amounts (i.e., quantified with equivalent thicknesses) for the polyurethane were 0.25-, 1- and 3-mm, and for epoxy 1-, 2- and 3-mm. For the wet TCJ, the hardener and resin of two-component polyurethane and epoxy were blended and applied to the spruce timber. Afterward, cast-in-place concrete was cast on the timber substrates with the delayed time (waiting time) of T0, TGEL2, and TGEL. According to the oscillation test, the gel time of adhesive (TGEL) was 72 and 221 min for the polyurethane and epoxy, respectively. A double lap shear test was run to the wet and dry timber-concrete joints to analyze the influence of adhesive amount and type on their failure modes and bond strength. Experiments indicated that increasing the adhesive thickness (or amount) of wet TCJs resulted in larger shear strength of the joints. A longer delayed time improved the bond strength of wet TCJ glued with 1- and 3-mm polyurethane equivalent thicknesses. However, with a similar amount of adhesive, the bond strength of wet TCJ glued with epoxy decreased with a longer delayed time. This behavior was attributed to differences in shear storage modulus of used polyurethane (G’=1.2 kPa) and epoxy (G’=229.7 kPa) adhesives at T0 delayed time. It was found that the wet bonding technique used to produce TCJs tended to have higher shear strength than the dry bonding technique for the given same adhesive amount and type.

Durability of epoxy and polyurethane bonded timber-hybrid FRP joints under hygrothermal and weathering conditions

This paper investigates the bond between laminated veneer lumber (LVL) and flax-glass hybrid fibre reinforced polymer (HFRP) with different fabric stacking sequences in the LVL-HFRP joints exposed to (1) room condition at the temperature of 20 °C and relative humidity (RH) of 57% (as reference condition), (2) hygrothermal condition (50 °C and 95% RH), and (3) weathering environment (a dry-wet-ultraviolet radiation cyclic condition). Epoxy (EP) and polyurethane (PUR) adhesives were used to manufacture the HFRP composites and bond them to the LVL as LVL-HFRP joints. The joints were tested under block shear tests to evaluate the bond behaviour. The LVL block shear specimens were also tested as the reference for the shear strength change of the LVL in the joints. Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) tests were conducted to study the influence of the hygrothermal and weathering conditions on the functional groups and the glass transition temperature (Tg) of the LVL-HFRP joints. After the hygrothermal and weathering exposures, the results showed that the Tg of the EP-based and PUR-based HFRP in the LVL-HFRP joints increased due to the post-curing and the arrangement of the disordered urea groups. The average shear strength values of the LVL-HFRP joints bonded with the EP adhesive decreased by 4.6–35.2% and the reduction of the shear strength of the LVL part in the joints accounted for the decreased shear strength at the joint level. The average shear strength values of the LVL-HFRP joints bonded with the PUR adhesive decreased by 27.9–57.0% and the reduction of the shear strength at the joint level was controlled by the glass fibre layer of the PUR-based HFRP, regardless of the fabric stacking sequence in the PUR-based HFRP.

Waterborne Polyurethane/Acrylic Adhesive Blends from Physaria fendleri Oil for Food Packaging Applications

Environmental concerns and the diminishing acceptability of using petrochemical polymers require innovative synthetic approaches to materials for essential polymeric technologies such as adhesives. Biobased plant oils have been suggested as replacements for petrochemical monomers in polyurethane formulations. A variety of seed oil extracts from plants contain naturally occurring functional groups such as hydroxyl and glycidyl ether, which can be utilized in polyurethane synthesis. Most studies of bioderived polyurethane adhesives occur in solventborne systems and with chemically modified oils. However, rising concerns and manufacturing limitations of volatile organic compounds in solventborne systems warrant investigation into more sustainable and alternatives that are easier to handle. In this work, we synthesized waterborne polyurethanes comprised of oil derived from Physaria fendleri seed (naturally occurring hydroxyl functionality), hexamethylene diisocyanate, toluene diisocyanate, and dimethyl propionic acid. Acrylate copolymers were synthesized via emulsion polymerization comprised of different butyl and methylmethacrylate monomer ratios. These polymers were formulated into waterborne polyurethane/acrylic adhesive blends. The resulting formulations possess a commercially comparable peel strength of >6 N and are suggested for use in resealable food packaging applications. This study demonstrates the utility of oil derived from Physaria fendleri seeds in waterborne adhesive applications, adding value with bioderived materials and increasing sustainability of polyurethane adhesives.

Impact of the processing method on the properties of castor oil/cellulose acetate polyurethane adhesives for bonding wood

During the last decades, the replacement of the traditionally petro-based raw materials by eco-friendly substances in polyurethane production has become a great challenge. The synthesis of polyurethanes is mainly conducted by means of a selective reaction between active isocyanate and hydroxyl groups, yielding a characteristic segmented chemical structure. In this study, the traditional solvent-based polycondensation reaction for the synthesis of bio-based polyurethane adhesives, involving first cellulose acetate modification with 1,6-hexamethylene diisocyanate and subsequent crosslinking with castor oil, was evaluated and compared with a simpler solvent-free single-step protocol. The impact of the preparation procedure and reagent proportions, on the chemical structure, thermo-rheological behaviour and ultimate adhesion performance were evaluated. The rheological response and adhesion performance of these bio-inspired polyurethanes were also compared to those found in some commercial adhesives. This research highlights the reinforcing and stabilizing effect of cellulose acetate on bio-polyurethane performance. Particularly, the inclusion of cellulose acetate into the adhesive manufacture allowed a reduction in the maximum curing time under room conditions to almost one week, while yielding a higher thermal stability and a fourfold increase in elastic modulus in comparison to the cellulose acetate-free counterpart. Moreover, the proposed solvent-free synthetic route enhanced the microphase mixing of the ensuing bio-based polyurethanes, exhibiting an adhesion performance comparable to the considered commercial benchmarks, but with failure occurring primarily within the substrate material – the preferred locus of failure.

Deformation Behavior of Polyurethane Adhesive in the Single-Lap Joint Based on the Microbeam X-ray Scattering Method

Deformation Behavior of Polyurethane Adhesive in the Single-Lap Joint Based on the Microbeam X-ray Scattering Method

Enhanced adhesion of EVA and NBR soles with polyurethane adhesives by cold atmospheric pressure plasmas

The enhanced adhesion of polyurethane (PU) adhesives with ethylene-vinyl acetate (EVA) midsole and vulcanized acrylonitrile-butadiene rubber (NBR) outsole using cold atmospheric pressure plasmas (CAPPs) was investigated at short exposure durations of 27–64 s. A 100% of failure at EVA cohesive, after T-peel test of CAPPs-treated EVA/PU adhesive/CAPPs-treated NBR, proves that N2-air CAPPs can efficiently work as a surface modification on EVA and NBR soles, compared with 100% of failure at PU adhesive/NBR interface after T-peel test of bonded as-received EVA/PU adhesive/as-received NBR. This study demonstrates that N2-air CAPPs significantly affect the surface polar properties and the enhanced adhesive strengths of EVA and NBR soles. Hydrophobic surfaces of EVA and NBR with small polar surface free energies (PSFEs) of 2.4 and 1.7 mN/m could be turned into highly polar surfaces as the values of PSFEs are respectively raised to 39.9 and 35.2 mN/m after the surfaces were treated by N2-air CAPPs. Increased compositions of O and N atoms for EVAs and NBRs by N2-air CAPPs have been identified to bond well with PU adhesives. For further insight into the treatment reaction on the surfaces of EVA and NBR using CAPPs, plasma active species of N2-air CAPPs were detected with optical emission spectroscopy.

Surface Free Energy Theory for Evaluating Moisture Damage in Expandable Polyurethane Grouting Materials

The resistance properties of the interface between polyurethane(PU) grouting materials and rocks subjected to water erosion are the theoretical basis for evaluating the long‐term performance, which, however, is not clear yet. Herein, surface free energy (SFE) theory is used to analyze the mechanism of water damage at the interface between the polyurethane and rock. The surface energy parameters of various PU grouting materials with different densities, limestone, and granite rock materials are tested. The adhesion and peeling work indices are calculated by the SFE theory. The results show that water brings little influence on PU grouting materials. The lower the density of PU is, the better the hydrophobicity is. The increase in the density of PU promotes adhesion between the PU and rock with and without water. Moreover, density increases the difference between SFEs of PU–rock and water–rock, decreasing the detachment tendency of PU over the rocks. Lithology also affects the adhesion of PU and rocks. PU–granite composite exhibits the largest adhesion work and smallest peeling work, which indicates that it has stronger water damage resistance. Finally, the validity of the surface energy theory is proved by mechanical, bond tests and the application.

The effects of the addition of surfactants and the machining method on the adhesive bond quality of spotted gum glue-laminated beams

The effects of adding surfactants to polyurethane and resorcinol formaldehyde adhesives were tested relative to the gluability of spotted gum timber for structural glue-laminated beams (Glulam). While previous attempts to improve the bond performance of this very difficult to glue timber have focused primarily on timber surface preparations, this study concentrated on lowering the adhesive surface tension through added surfactants to improve the adhesive-timber surface wetting. Accordingly, 44 glulam samples were manufactured using polyurethane and resorcinol formaldehyde adhesives, with eight surfactant formulations and two different pre-gluing surface machining methods, i.e., face milling and planing. Although the surfactants were successful in drastically lowering the adhesive surface tension and improving adhesive spreading, none of the surfactant formulations tested were successful in improving the glulam adhesive bond qualities. Overall, the surfactant formulations produced considerably higher delamination, lower shear strength, and lower wood fibre failure compared to the control samples; therefore, they are not a viable solution to improve the gluing of spotted gum. The resorcinol formaldehyde adhesive and face milling produced considerably better results compared to the polyurethane adhesives and conventional planing.

Experimental Analysis on the Curing and Adhesive Behaviour of Standard Moisture-cured and Fast-cured Polyurethanes Used in Automotive Industry

In this paper, the adhesive behaviour of different polyurethane glues was studied and compared. The study was inspired by real conditions in the production of car windows, and it is divided into several parts, which deal with three types of polyurethane adhesives. The first type is a standard one-component polyurethane adhesive, which cures with air humidity. The second type is a one-component polyurethane adhesive with the addition of a curing accelerator. And the third type is a two-component polyurethane adhesive. Adhesive blocks of defined dimensions and plastic parts glued to the glass were used for this experiment to simulate a real product. The influence of temperature and humidity on the curing process was evaluated. The test included measuring the force required to tear the parts from the glass and assessing the type of failure. Based on the achieved results, the adhesives were evaluated regarding their applicability in the real automotive industry and the key properties necessary to maintain the safety and quality of the glued part were determined

Influence of Manufacturing Environment on Delamination of Mixed Cross Laminated Timber Using Polyurethane Adhesive

To investigate the influence of manufacturing environment on bonding performance of mixed cross laminated wood (CLT) using polyurethane (PUR) adhesive, a boiling water soak delamination test according to the temperature and relative humidity was conducted. The 5-ply mixed CLT consisted of Japanese Larch for external and middle layer and yellow poplar for internal layer. The PUR adhesives with different opening times of 10 and 30 minutes were used. The mixed CLT was manufactured according to pressing times of PUR and manufacturing environments of summer and winter. In case of summer environment, the delamination rate of the mixed CLT with pressing time of 4 hours using a PUR adhesive with open time of 10 minutes met the requirements of KS F 2081. In case of winter environment, the delamination rate of the mixed CLT didn’t meet the requirements of KS standard. However, it was possible to confirm the effect of improving the adhesive performance by adjusting the pressing time according to the open time of the adhesive under the manufacturing conditions. The delamination rate of CLT with open time 30 minutes PUR, manufactured by indirect moisture supply methods was 11.2% better than direct moisture supply methods. As a result of delamination test in the same condition of relative humidity and adhesive, it was found that the temperature of manufacturing environment influences the adhesive performance.

Polyurethane adhesives from castor oil and modified lignin via reaction with propylene carbonate

AbstractOver the last several years, lignin hydroxypropylation with propylene oxide (PO) has been used as the main strategy to overcome lignin limitations, such as its low reactivity, poor dispersion, and high stiffness, in the context of polyurethane (PU) synthesis. However, PO is a flammable, toxic, and carcinogenic compound. Propylene carbonate (PC), a compound with low toxicity and biodegradability, has emerged as a feasible alternative to PO. Although this compound has potential for use in syntheses, lignin hydroxypropylation with PC has not yet been explored in PU synthesis. In this work, PU adhesives are synthesized using castor oil (CO) and hydroxypropylated lignin via the reaction with PC (HKL_PC) as a polyol. HKL_PC is incorporated in CO at different concentrations (10 wt%, 20 wt%, and 30 wt%). Curing kinetics is investigated with temperature‐modulated optical refractometry (TMOR). Dynamic mechanical analysis (DMA) demonstrates an increase in Tg after lignin addition. Furthermore, Young's modulus and practical adhesion are improved with increasing HKL_PC concentration. Results prove that hydroxypropylation with PC is a feasible method for lignin‐based PU synthesis and reveals the potential of this approach as an alternative for the PO‐based method.

Chemical surface treatment mechanisms involved in a flamed polypropylene/polyurethane bonded interface: Toward a treatment by nitrogen plasma

Flaming treatments have been used at the industry level since the 1950s to improve the chemical reactivity of polyolefins. This treatment allows the grafting of polar functional groups onto the substrate to improve its chemical affinity with paints and adhesives, which is important in fields such as the automotive industry. Therefore, the primary purpose of this study is to identify which groups grafted by flaming treatments are involved in the adhesion between polypropylene long glass fibers (PPGFL) and polyurethane (PU) adhesives, with or without surrounding water. The effect of an alternative low-pressure nitrogen plasma treatment on the PPGFL/PU interface was then studied.The main conclusions arising from the study of flaming are the following. X-ray photoelectron spectroscopy analyses revealed that the flaming treatment grafted four different chemical groups: hydroxyl, ether, ketone, and acid. Model reactions studied using nuclear magnetic resonance spectroscopy showed that only hydroxyl groups played a role in PPGFL/PU adhesion, as long as the environment was not saturated with water.On the other hand, the low-pressure nitrogen plasma study showed that adhesion could also be obtained through amine groups. Model reactions showed that these groups were very reactive with the isocyanates contained in the adhesive; the reaction was complete and immediate. In addition, this particular reaction produced urea groups that were only slightly parasitized by water, which made the process more robust than flaming.This study primarily aims to link the chemical mechanisms occurring at the interface with a macroscopic adhesion measurement of a bonded system. Further, kinetic monitoring allows the comparison of the reaction speed of each grafted group with the adhesive. Consequently, this makes it possible to determine the functional groups that are most important for adhesion and the most efficient and robust treatment method.

Low-temperature curing of adhesives – Large-scale experiments

ABSTRACT In contrast to mechanical joining techniques, which can be manufactured under any ambient conditions, adhesively bonded joints always require some minimum temperatures – typically above 10 °C – in order to cure safely. Due to this limitation, either many technical applications exclude adhesives right from the start or practitioners at high costs are forced to adapt their production processes to the specific environmental requirements of the polymers to be cured. As a response to these manufacturing challenges, a new process for accelerated curing – induction heating with Curie particles (CP) – was studied to cure three different structural adhesives (two epoxy resins and one polyurethane) at low temperatures. The CP are added to the adhesives as a typical filler material and can only be heated by an externally applied electromagnetic field up to their material-specific Curie temperature T c. Thus, a self-regulating process is created, which effectively prevents the adhesives from overheating while simultaneously eliminating the need to control curing temperatures by external measures like IR cameras. Recently, the authors published a series of papers in which CP were used to cure at ambient temperatures (+23 °C); the results of which are now to be extended towards low-temperature curing. For that, an experimental campaign was carried out using the exemplary connection type of Glued-in Rods (GiR), which are frequently used in various timber engineering applications. In detail, the heating behaviour of inductively cured GiR conditioned at –10 °C and –30 °C was compared to GiR inductively cured starting from +23 °C. In addition, reference sets with and without added CP were produced, whereby all GiR were subjected to tensile tests in the following. Afterwards, the series were compared on the level of load–displacement curves, failure loads, as well as observed fracture patterns. The results proved that CP-curing is capable of curing adhesives in temperature ranges currently not intended for structural adhesives by technical regulations. When cured under low-temperature conditions, the GiR involving epoxy adhesives showed a fracture behaviour that proved to be equal to that of the references. The failure loads of the 2 K-PUR dropped to 50–75% of the reference values, which, however, is strongly assumed to be related to moisture on the cold adherends and not to a general unsuitability of polyurethane adhesives for the process – an assumption that will be validated by low-temperature experiments using single lap shear joints presented in a subsequent study.

Analysis of the adhesive spread and the thickness of the adhesive bonded joint depending on the compressive force applied to bonded materials with different surface structure

The thickness of the adhesive layer in a bonded joint is one of the important factors determining its strength and its recommended value varies depending on the type of the adhesive used and the type of load on the joint. Specimens for testing adhesively bonded joints with a specified thickness of the layer are usually produced under laboratory conditions using spacers of a specified thickness, and the remaining excess adhesive is removed from the specimen. In industrial conditions, this method of obtaining an appropriate thickness of the adhesive layer is not always possible and desirable. The method of adhesive spreading between the joined plates under the influence of pressure without spacers was analysed. For this purpose, 3 pairs of aluminium plates were used, differing in surface texture and roughness for the Ra parameter at 0,25 μm, above 3 μm, and above 5 μm. 0.5 ml of Bonapur RZ45 polyurethane adhesive was compressed between the plate pairs without additional force (except for the own weight of the plates) and with load of 50 N, 100 N, 250 N, and 500 N. The surface where the adhesive would be distributed was checked to estimate the resulting thickness of its layer. The size of the layer ranged from 0.09 (for the force 500 N) to 0.53 mm (without additional force, only plate). The values obtained of the layer thickness in the case of using additional force are lower than recommended in the literature (0.5–2 mm) for polyurethane adhesives. The measurements showed that the roughness of the surface does not significantly affect the differences in the resulting thickness of the adhesive layer, whereas the visible surface machining footprint causes changes in the way the adhesive spreads and impacts the shape of the adhesive on the plate surface.

Large-Scale Enclosure Fire Experiments Adopting CLT Slabs with Different Types of Polyurethane Adhesives: Genesis and Preliminary Findings

This paper provides understanding of the fire performance of exposed cross-laminated-timber (CLT) in large enclosures. An office-type configuration has been represented by a 3.75 by 7.6 by 2.4 m high enclosure constructed of non-combustible blockwork walls, with a large opening on one long face. Three experiments are described in which propane-fuelled burners created a line fire that impinged on different ceiling types. The first experiment had a non-combustible ceiling lining in which the burners were set to provide flames that extended approximately halfway along the underside of the ceiling. Two further experiments used exposed 160 mm thick (40-20-40-20-40 mm) loaded CLT panels with a standard polyurethane adhesive between lamella in one experiment and a modified polyurethane adhesive in the other. Measurements included radiative heat flux to the ceiling and the floor, temperatures within the depth of the CLT and the mass loss of the panels. Results show the initial peak rate of heat release with the exposed CLT was up to three times greater when compared with the non-combustible lining. As char formed, this stabilised at approximately one and a half times that of the non-combustible lining. Premature char fall-off (due to bond-line failure) was observed close to the burners in the CLT using standard polyurethane adhesive. However, both exposed CLT ceiling experiments underwent auto-extinction of flaming combustion once the burners were switched off.