Polyurethane Adhesives Research Articles

Characterization of wood-adhesive bonds in wet conditions by meansof nanoindentation and tensile shear strength

The performance of adhesive-hardwood bonds can often be sensitive tohumidity and temperature variation. Therefore, it is frequently challenging to achievestandard requirements for structural applications. To gain a better understanding of thewood-adhesive bond, the properties of the individual constituents as well as the localinterface of European beech (Fagus sylvatica L.) woodcell walls in contact with structural adhesives were analyzed by means ofnanoindentation. These results are compared to classical lap-shear strength. Asadhesives two different one-component polyurethane adhesives (1C PUR) and a phenolresorcinol formaldehyde adhesive (PRF) were used. In one case, the beech wood wasadditionally pre-treated with an adhesion-promoting agent (primer) prior to bonding with1C PUR. Beech wood joints were analyzed subsequent to several treatments, namelystandard climate, after wet storage and in re-dried conditions. In addition, theinfluence of the primer on the hydroxyl accessibility of beech wood was investigatedwith dynamic vapor sorption (DVS). The lap-shear strength revealed good performance indry and re-dried conditions for all adhesives on beech. Both polyurethane adhesivesobtained deficits when tested in wet conditions. The use of a primer significantlyimproved the PUR performance in wet condition. DVS experiment demonstrated a decrease inhydroxyl group accessibility when using a high primer concentration. As novelty,nanoindentation was used for the first time to characterize the localwood–adhesive-interface properties in wet conditions. Nanoindentation showed that alltested 1C PUR perform quite similar in room climate, while PRF achieves considerablehigher values for reduced E-modulus and hardness. Wet storage led to a considerablereduction in mechanical properties for all adhesives, while the highest relative changewas observed for PRF. After re-drying, the adhesives re-gained a large part of theiroriginal mechanical properties in room climate. No distinct effect of the primer on thelocal micromechanical properties could be detected with nanoindentation in terms ofspecific work of indentation.

Preparations and properties of polyurethane adhesives modified by corn straw lignin

The conversion of lignin into valuable products has attracted the interest of researchers. A series of modified polyurethane adhesives were prepared by blending corn straw enzymatic hydrolyzed lignin with polyester polyol and tolylene-2,4-diisocyanate. Mechanical properties, chemical structures, and thermal stability of the adhesives were characterized by mechanical properties tests, Fourier transform infrared spectrometry (FTIR), and thermogravimetric analysis (TGA). The results of shear strength test under room temperature and high temperature showed that the shear strength for modified polyurethane adhesives was improved by introduction of lignin. The introduction of lignin also improved the heat resistance of polyurethane adhesive. The TGA analysis results showed there were two stages in the thermal decomposition of the lignin blend modified polyurethane adhesive, and the maximum decomposition temperature of the first stage increased with the increase of lignin content, while the maximum decomposition temperature of the second stage decreased with the increase of lignin content. The TGA-FTIR combination analysis studied the main gas generated in the two decomposition stage’s peak times, of which CO2 was produced in the first stage, and CH4 was created in the second stage, indicating that the molecular chain fracture process of the two kinds of adhesives was similar in the whole decomposition process.

Monocomponent Non-isocyanate Polyurethane Adhesives Based on a Sol–Gel Process

Non-isocyanate polyurethanes (NIPUs) based on five-membered cyclic carbonates have emerged as the most promising alternative to replace more toxic, conventional polyurethanes. However, the low reac...

Effects of wood species, number of teeth, and adhesive type on moment capacities of box-joints

Effects of the wood species, number of teeth, and adhesive type were studied relative to the moment capacities of box joints under tension and compression loadings, which is commonly used in case type solid wood furniture. For this purpose, L-type specimens were prepared from Scotch pine (Pinus sylvestris L.), poplar (Populus sp.), and fir (Abies sp.). Polyvinyl acetate (PVAc) and polyurethane (PU) adhesives were used in the preparation of joints. Tension and compression tests with 360 samples were performed under static loading. According to the results, the highest moment capacities were obtained for Scotch pine under tension, and poplar specimens under compression. With respect to the number of teeth, the highest moment capacities were obtained with 12-tooth joints under both tension and compression loadings. For adhesive types, the specimens glued with PVAc gave better results in both tension and compression. In manufacturing of solid wood based case-type furniture, the higher number of teeth resulted in a slightly better performance; however, the results with the 4-tooth structure was not too far from 8-tooth. Furthermore, it was concluded that Scotch pine as a substance and PVAc as adhesive could be recommended.

Improving the adhesion strength of polymers: effect of surface treatments

To improve their adhesion strength, polymeric surfaces are usually modified through different treatments. This study investigates the effect of mechanical, chemical, and energetic treatments on the bonding strength of ethylene propylene diene methylene (EPDM), polyvinyl chloride (PVC), and acrylonitrile butadiene styrene (ABS) materials. Three adhesives based on different chemical compositions, namely silicone, polyurethane, and modified-silane (MS) polymer, were considered. Results show that the effect of the applied treatments on the adhesion strength of EPDM surfaces is insignificant. Only a slight improvement is obtained in the case of polyurethane-based adhesive, while the failure modes remained adhesive. As for PVC, most treatments were effective in the case of the silicone-based adhesive, especially grit blasting, primer, and UV/ozone treatments. Only UV/ozone treatment improved the adhesion strength and altered the failure mechanisms of this material when polyurethane and MS-based adhesives are used. The adhesion of ABS increased and the failure modes changed from adhesive to cohesive for most treatments. Particularly, a significant improvement is obtained when primer coating and UV/ozone radiation are applied. This comparative study paves the way for the design of polymeric joints with highly enhanced adhesion performance.

High-value utilization of hydroxymethylated lignin in polyurethane adhesives

Lignin is a good candidate for the polymerization and chemical modification to prepare sustainable chemicals and materials, but a relatively low hydroxyl content becomes an obstacle for the preparation of lignin-based polyurethane (PU) adhesives. In order to improve its reactivity, the acetic acid lignin (AAL) was hydroxymethylated before copolymerized with isocyanate during the preparation of PU adhesives. The hydroxymethylation was carried out in an alkaline formaldehyde solution and it was found that 85 °C is the optimal temperature. On that condition, the free formaldehyde content of the corresponding product HL-6 was as low as 0.32%, while the hydroxymethyl was increased by 189.11% compared with original AAL and reached 2.92 mmol/g. In the polymerization of PU adhesives, the hydroxymethylated lignin with a higher aliphatic hydroxyl content formed a more compact three-dimensional urethane cross-linking network with isocyanate. The mechanical properties and thermal stability of the lignin-based PU adhesive were improved by 15–30 wt% in HL-6, and particularly the tensile strength was increased by 21–41 MPa, which indicated that the hydroxymethylation is an efficient way to enrich the hydroxyl in lignin, and the modified lignin is adequate to partially replace petroleum-based polyols for the preparation of PU adhesives with excellent properties.

Recycling of rigid polyurethane foam: Micro‐milled powder used as active filler in polyurethane adhesives

AbstractRigid polyurethane (PUR) foams are widely used as heat insulation material in construction industry or for electronic appliances manufacturing. After finishing their life‐time, it is necessary to eliminate foam wastes. The aim of this study was to prepare a pair of industrial PUR adhesives of medium viscosity containing recycled rigid PUR foam. Three methods of milling were tested: knife‐milling, two‐roll milling, and ball‐milling. Only two‐roll milling gives the PUR micro‐powder usable for following adhesives modification. The micro‐powder was used as passive filler in PUR adhesives and potential reactivity for polyol pack replacement was studied. Hydroxyl and amine numbers were determined in mixture with virgin polyol. One‐component PUR prepolymer adhesive was prepared using various dosages of the micro powder and the tensile strength of bound wood was measured. As additional parameters, also free film adhesive mechanical parameters were tested and particle size distribution of the micro powder was analyzed. Two adhesive formulations were prepared for independent evaluation of the micro powder reactivity. The results showed growing of the mechanical strength of wood bonding with growing dosage of the micro powder.

Synthesis of Reactive Polyurethane Adhesives and Studying the Effect of Ketonic Resins

In this study, the effects of cyclohexanone formaldehyde ketonic resin (CFR) with different montmorillonite content on the viscosity, initial adhesion strength and temperature resistance of polyurethane one component adhesives were investigated. Cyclohexanone formaldehyde resin (CFR) with varying montmorillonite (MM) content were incorporated into formulation with a specified weight ratio (rate of 10% by weight of polyether polyol.) One-shot technique was used for synthesis. Blank polyurethane was synthesized using polyether polyol and polymeric 4,4'-methylene diphenyl diisocyanate (p-MDI). In experimental studies, the ratio of the isocyanate groups (NCO) to the hydroxyl group (OH) was 4.84. Reaction completion was followed by FTIR spectroscopy. Choloroparaffine was used as a plasticizer to adjust the viscosity and to reduce the fragile structure of the product. The NCO % values of the synthesized adhesive PU were measured by titration. When TGA datas were examined, it was found that the heat resistance of the product increased with the addition of CFR and MM-CFR. As a result, it has been determined that the adhesion properties and thermal resistance were improved with the use of ketonic resins in one component reactive adhesives.

Young’s Modulus and Poisson’s Ratio of Polyurethane Adhesive in Lightweight Floor System

Monography [1] confirms that Lightweight Floor System with aluminum foil, abbreviated as LFS-L meets at least three assumed hypotheses...

Shear Strength Tests in a Lightweight Floor System with Cement and Polyurethane Adhesives

The purpose of testing the samples made in the form of layered lightweight floor system (LFS) composites was to determine...

Application of atmospheric pressure plasma for adhesion improvement in polyurethane coating on polypropylene fabrics

Atmospheric pressure plasma treatment modifies the surface of textile materials at nano level. The effectiveness of plasma surface modification is closely associated with fabric weave construction and surface structure. Adhesion properties of three different polypropylene (PP) fabrics with a polyurethane (PU) coating have been investigated with respect to different plasma treatment time and power. Peel off strength was measured as the separation force required to separate coating layer from fabric. Statistical analysis of the peel off strength was performed by ANOVA. A substantial increase in adhesion of PU on plasma-treated samples was observed. Various characterization techniques such as contact angle, X-ray photoelectron spectroscopy, scanning electron microscopy and tensile measurement were used to inspect hydrophilicity, surface chemistry, surface morphology and physical properties of PP fabrics, respectively. As evident from results captured using various characterization techniques, atmospheric pressure plasma treatment enhances the surface properties (hydprophilicity, surface morophology and surface chemistry) without affecting the bulk (tensile strength) properties. It is shown that the plasma treatment parameters as well as the textile structure play an important role in adhesion improvement. Further, it is observed that the effectiveness of the plasma treatment is closely linked to the structure of the textiles fabrics.

Research of Properties and Preparation of Waterborne Polyurethane Adhesives for Shoes

In this paper, a waterborne polyurethane adhesive for shoes has been prepared by a composite method, and a series of tests such as peel strength, heat resistance, yellowing resistance, and hydrolysis resistance were performed on the sample. The results show that the performance of the water-based polyurethane adhesive prepared by this method has reached the level of similar products in the market. It has great promotion significance in shoe factories.

Analysis of glue line and correlations between density and anatomical characteristics of Eucalyptus grandis × Eucalyptus urophylla glulam

This study aimed to analyze the glue line thickness effect and its relation with the density and anatomical characteristics of eucalyptus wood. Thus, the thickness of main and secondary glue lines was measured as well as their interaction with the apparent density of elements glued with resorcinol-formaldehyde (RF) and castor oil polyurethane (CP) adhesives. Anatomical wood characterization of Eucalyptus grandis × Eucalyptus urophylla was performed by correlating glue line thickness. According to normative instruction, specimens were produced for delamination tests. The experiment was conducted in a completely random 2 × 2 design factorial scheme (two classes of wood apparent density and two adhesives). Pearson correlation (t < 0,01) was performed among variables. It was found that there was adhesive penetration into vessels and rays of wood. Glue line thickness was higher in woods with density higher than to 580 kg/m3 glued with RF adhesive. There was a low correlation between wood density and vessel diameter to main and secondary glue lines (t < 0,01)

Mechanical Strength Indicators of Polyurethane Adhesive in Lightweight Floor Systems

The article describes the basic features of the lightweight floor system with heat-dissipating lamellas (LFS-L) that do not require screeds and are used in the design of radiant heating. It was assumed that reactive polyurethane adhesive constitute the connection layer between the ceramic tile floor and the thermal insulation substrate, which is covered with aluminium foil. This type of construction has not been fully tested for mechanical strength. To define it, for example, using the finite element method, strength indicators of the tested adhesives which were not used in any of the previous studies discussed in this paper should be determined, such as Young's modulus E, Poisson's ratio ν and linear thermal expansion coefficient 𝛼. This article presents research methods by which these data were determined. Module E and the ν ratio were determined in the compression strength test of cylindrical samples of polyurethane adhesive. Coefficient 𝛼 was determined by using digital image correlation in the Aramis system, placing the prepared adhesive samples in a thermal chamber.

Feasibility of polyamines and cyclic carbonate terminated prepolymers in polyurethane/polyhydroxyurethane synthesis

Polyurethanes are a well-established part of adhesive markets. However due to the toxicity of di-isocyanates used in the synthesis, finding an alternative route to synthetize polyurethanes is increasingly important. One strategy is to use cyclic carbonate terminated prepolymers with polyamines to yield polyhydroxyurethanes. This research highlights the possibility to use commercially available polyurethane prepolymers with different isocyanate chemistries for cyclic carbonate terminated prepolymer synthesis with the help of glycerol carbonate and a catalyst. These synthetized prepolymers were used in a screening study with different commercially available low toxic amines. It was observed that when secondary amines were used, the reaction advanced at room temperature with no further heating required. The development of lap shear strength over time on stainless steel, gel content, tensile strength and elongation were measured from room temperature cured polymers. The synthetized polyurethane/polyhydroxyurethane (PU/PHU) hybrid materials had high initial lap shear strength close to current industrial polyurethane adhesives. The strength development over time was negligible. Full conversions were seen within one month from the reaction. Tensile strength levels were slightly lower than typical industrial polyurethane adhesives. Even though the lap shear strength results and gel contents at room temperature were on a good level, curing at elevated temperature had a positive impact on them. The best performing combination was cyclic carbonate functionalised hexamethylene di-isocyanate prepolymer with multifunctional polyethyleneimine amine. In short, di-isocyanate free PU/PHU hybrid materials were successfully synthetized from commercial raw materials and their performance was comparable with current industrial polyurethane adhesives.

Migration test and safety assessment of polyurethane adhesives used for food-contact laminated films

Abstract Migration test and safety assessment of polyurethane adhesives commonly used for food-contact laminated films are experimentally studied. Six harmful migrants from isocyanate residues and additives in adhesives are identified by combined GC–MS with NIST index and retention time of standard products. A series of migration tests regarding “LDPE-adhesive-LDPE” sandwiching laminated films are carried out, aiming to unfold the effects of temperature-time conditions, cross section and nanomaterials on the migration of identified compounds into food simulant isooctane. The results reveal that three of the six identified compounds, i.e., KH-560, 2,4-DTBP and DMPA, may migrate into isooctane. Temperature is found to be the most important factor on the migration behavior. From the slight effect of cross section area on migration of compounds, it can be concluded that compounds easily diffuse into substrates and migrate into isooctane by penetrating through the substrates rather than cross section. We also found that adding graphene and functionalized graphene neither promote the migration nor inhibit. For safety assessment, threshold of toxicological concern method is alternatively used to evaluate the migration. The estimated daily intakes of KH-560 and DMPA in solvent-free polyurethane adhesive laminated films exceed the safety threshold recommended by threshold of toxicological concern, while those in solvent-based adhesive laminated films do not.

A comparative study of the microstructure and water permeability between flattened bamboo and bamboo culm

The objective of this study is to investigate the microstructure, water permeability and the adhesion of waterborne coating on the flattened bamboo. The flattened bamboo was obtained by softening bamboo culm at 180 °C followed by compression. The microstructure and chemical component of flattened bamboo were investigated by scanning electron microscopy, Fourier transforms infrared spectroscopy, and X-ray diffraction. The adhesion and interface structure of waterborne coating onto flattened bamboo surface were also examined. The result indicated that the parenchyma cells in flattened bamboo were compressed, and starch in the parenchyma cell was extracted during the softening and flattening process in which the main chemical component did not change significantly. The water permeability of both flattened bamboo and bamboo culm is dependent on the direction: longitudinal direction > tangential direction > radial direction. However, the water permeability in all three directions in flattened bamboo was higher than those in the untreated bamboo. In addition, alkali dye solution was found to more easily permeate through the flattened bamboo when compared to acid dye solution, and the permeability varied depending on alkali dye or acid dye concentration. The adhesion of water-based polyurethane coating on the flattened bamboo can reach the second level.

Signally improvement of polyurethane adhesive with hydroxy-enriched lignin from bagasse

Acetic acid lignin (AAL) obtained from Bagasse was used as a partial substitute of polyols for polyurethane (PU) adhesive due to the presence of reactive sites, i.e. aliphatic and aromatic hydroxyl groups, in its structure. To improve its reactivity, lignin is partly demethylated before the copolymerization with polyisocyanate. The demethylation, catalyzed by hydrobromic acid solution under heating conditions, resulted in a decrease of methoxy groups on the phenyl ring and the increase of phenolic hydroxyl groups. It is found that 120 ℃ is the preferred temperature for the demethylation resulted in a 36.5 % increase in hydroxyl content of lignin up to 5.25 mmol/g. Lignin-based polyurethane adhesives were prepared in two steps including preparing a prepolymer based on mixtures of toluene-2,4-diisocyanate (TDI) and unmodified or demethylated lignin, and then the polymerization was completed by adding polyethylene glycol (PEG). The mass ratio (1:1) of TDI: (lignin + PEG) in the compositions was here investigated. The results indicated that the presence of lignin (with and without demethylation) in polyurethane adhesives increased the glass transition temperature, Young’s modulus, and elongation at break. Particularly, the tensile strength was increased by 15–36 MPa with the addition of 10–30 wt% of lignin. Lignin with higher amount of hydroxyl groups prepared by partly demethylation is better integrated into the three-dimensional cross-linking networks of the covalent polymer as a substitute of polyols. Compared to those PU adhesives based on the raw AAL, the adhesives prepared by demethylated lignin have higher tensile strength and thermal stability. It indicates that the demethylation is effective and it is a good strategy to prepare lignin-based PU adhesive with favorable mechanical properties.

Fractography combined with unsupervised pattern recognition of acoustic emission signals for a better understanding of crack propagation in adhesively bonded wood

In this paper, acoustic emission (AE) signals obtained during quasi-static crack propagation in adhesively bonded beech wood were classified using an unsupervised pattern recognition method. Two ductile one-component polyurethane (1C-PUR) adhesives with the same formulation except for one system being reinforced with short polyamide (~ 1 mm long) fibers were compared to a relative brittle phenol–resorcinol–formaldehyde (PRF) adhesive. Using only localized AE signals, it was shown that the signals originating from the crack propagation could be classified into two different clusters. Comparing the AE signals with a new fractography method, it was estimated that different clusters are due to distinct failure mechanisms, with signals of cluster 1 being associated with wood failure and signals of cluster 2 with adhesive failure. The obtained results suggest that for the PRF adhesive the wood fibers help to slow down the crack propagation. A similar but lesser effect was noted for the polyamide fibers added to the 1C-PUR adhesive matrix.

CO2-based poly (propylene carbonate) with various carbonate linkage content for reactive hot-melt polyurethane adhesives

Reactive hot-melt polyurethane adhesives (RHMPA) are compelling green adhesives without any solvent. However, current RHMPAs are mainly made from non-renewable petroleum-based polyols. Poly (propylene carbonate) (PPC) derived from abundant, cheap and renewable greenhouse gas CO2 is a high-profile green renewable polyol. Here, a series of RHMPAs (PPC-RHMPAs) based on PPC with different carbonate linkage content were prepared. The difference of carbonate linkage content of PPC-RHMPAs was investigated by 1H NMR and FTIR. The effects of carbonate linkage content on the moisture-curing rate, crystallization behavior, thermal stability, mechanical properties and adhesion properties of the resulted PPC-RHMPAs were analyzed and discussed by real-time FTIR, DSC, TGA, tensile and adhesion tests. This work would pave a way for designing high-performance PPC-RHMPAs.