Waterborne Polyurethane Adhesives Research Articles

Influence of the Chemical Structure of Urethane-Based Thickeners on the Properties of Waterborne Polyurethane Adhesives

To analyse the parameters that influence the thickening mechanism of waterborne polyurethane adhesives, different hydrophobically modified ethoxylated polyurethane based thickeners (HEUR) were used. The thickeners were characterized by proton nuclear magnetic resonance (1H-NMR) and gel permeation chromatography (GPC). The thickened adhesive solutions were characterized by flow rheology, pH, particle size measurements, solids content, and confocal microscopy. The thickened solid adhesive films were characterized by ATR-IR spectroscopy, parallel plate rheology, dynamic mechanical thermal analysis (DMTA), and differential scanning calorimetry (DSC). The adhesion was measured by a T-peel test of leather/polyurethane adhesive/SBR rubber joints. The addition of the different HEUR thickeners increased the viscosity of the polyurethane dispersion to different degrees. Furthermore, whereas the un-thickened polyurethane dispersion showed a Newtonian rheological behaviour, a shear-thinning rheological behaviour was observed in the thickened dispersions due to thickener-thickener and polyurethane-thickener interactions. The viscosity of the thickened polyurethane adhesive solutions increased with the degree of hydrophobicity and the molecular weight of the thickener. The addition of different thickeners increased the pH values due to the ionic adsorption, which is one of the interactions that contribute to the thickening mechanism of the polyurethane thickeners, besides hydrogen bonding and van der Waals hydrophobic interactions (micelles). The entanglement of the thickened polyurethane adhesives was studied by confocal microscopy. Although the addition of the thickeners did not affect the thermal properties of the polyurethanes, the T-peel strength of leather/adhesive/SBR rubber joints was influenced by the rheological properties of the thickened adhesives.

Properties of isocyanate‐reactive waterborne polyurethane adhesives: Effect of cure reaction with various polyol and chain extender content

AbstractThree series of isocyanate‐reactive waterborne polyurethane adhesives were prepared with various contents of chain extender (4.25/8.25/12.50 mol %) and polyol (20.75/16.75/12.50 mol %). Each series had a fixed amount of excess (residual) NCO group (0.50–2.00 mol %). FTIR and 1H‐NMR spectroscopy identified the formation of urea crosslink structure mainly above 80°C of various cure temperatures (20–120°C) with excess diisocyanate. The molecular weight, tensile strength, Young's modulus, and adhesive strength depend on excess NCO content and cure temperature and also varied with polyol and chain extender content. The optimum cure temperature was 100°C for all the samples. The tensile strength, Young's modulus, and adhesive strength increased with increasing cure temperature above 60°C up to the optimum temperature) (100°C) and then almost leveled off. Among all the samples, the maximum values of tensile strength, Young's modulus, and adhesive strength were found with 63.22 wt % polyol, 0.93 wt % chain extender, and 1.50 mol % excess (residual) NCO content at 100°C optimum cure temperature. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Effect of annealing on the properties of waterborne polyurethane adhesive containing urethane-based thickener

Different amounts of hydrophobically modified ethoxylated urethane-based thickener (HEUR) were added to improve the rheology of waterborne polyurethane adhesives. The thickened solid adhesive films were thermally annealed and characterised by IR spectroscopy, plate–plate rheology, dynamic mechanical thermal analysis (DMTA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Hydrogen bonds played a key role in the thickening mechanism of polyurethane dispersions containing urethane-based thickener, along with ionic adsorption and micelles formation between hydrophobic groups. The adhesion properties were measured from T-peel test of leather/polyurethane adhesive/vulcanised styrene-butadiene rubber joints. Optimal results were obtained when water in the waterborne polyurethane adhesive (just before joint formation) was removed under open air, while forced air drying impeded the complete removal of water. On the other hand, the thermal annealing did not greatly affect the rheological and thermal properties of the thickened polyurethanes.

Properties of crosslinked waterborne polyurethane adhesives with modified melamine: Effect of curing time, temperature, and HMMM content

A series of crosslinked waterborne polyurethane (WBPU) adhesives were prepared by prepolymer synthesis. Modified melamine, hexamethoxymethyl melamine (HMMM) was used as the crosslinking agent. It was elucidated by FTIR and 1H-NMR spectroscopy that the crosslinking reaction occurred between the polyurethane carboxyl acid salt groups and the HMMM methoxy groups. The hydrophobicity of the WBPU films increased after HMMM crosslinking. As the HMMM content was increased (increasing mole ratio of HMMM/dimethylol propionic acid (DMPA)), the water uptake (%) of the film decreased, and the water contact angle increased. The thermal stability, tensile strength, and initial modulus increased with increasing HMMM content up to an optimum value (mole ratio of HMMM/DMPA=0.5) at which point the maximum thermal stability, tensile strength, and initial modulus were recorded. The adhesive strength was found to be dependent on HMMM content, curing time, and temperature. The adhesive strength of crosslinked WBPU in the case of optimum HMMM content (8.46 wt%) was only slightly affected after immersing adhesive bonded nylon fabrics in water (for up to 2 days).

Properties of Waterborne Polyurethane Adhesives: Effect of Chain Extender and Polyol Content

A series of waterborne polyurethane (WBPU) adhesives were prepared with various ratios of polyol, poly(tetramethylene oxide glycol) (PTMG), and chain extender, ethylene diamine (EDA), at a fixed content of diisocyanate, 4,4-dicyclohexylmethane diisocyanate (H12MDI) and hydrophilic agent, 2,2-dimethylol propionic acid (DMPA). WBPU adhesives were characterized by IR and 1H-NMR spectroscopies, X-ray diffraction (XRD) and gel permeation chromatography (GPC). It was found that the extent of hydrogen bonds between hard–hard segment (i.e., hydrogen bonds between the NH and carbonyl groups) increased with increasing chain extender content (decreasing polyol content). Moreover, the disordered hydrogen bond of carbonyl group (hydrogen bond of urethane groups in the interfacial region) increased with increasing chain extender content (decreasing polyol content). The cyclic urea and allophanate group, which are attributed to the side reaction and cross-linking reaction, respectively, were found above a molar ratio 0.17 of chain extender to diisocyanate. The adhesive strength was maximum with 0.95 wt% and 63.10 wt% chain extender and soft segment (PTMG), respectively (H2 sample) at room temperature for the WBPU adhesive. However, with increasing application temperature the adhesive strength decreased for all samples.

Addition of Urethane-Based Thickener to Waterborne Polyurethane Adhesives Having Different NCO/OH Ratios and Ionic Groups Contents

Several waterborne polyurethane adhesives containing different hard-to-soft segment ratios and ionic groups were prepared by using the acetone process. To improve the rheological properties, a 5 wt% of hydrophobically-modified ethoxylated urethane-based thickener (HEUR) was added. The adhesives were characterized by shear rate-controlled rheology, pH, particle size measurements, solids content and laser confocal microscopy. The adhesive films were characterized by plate–plate rheology, dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC). The adhesion properties were measured using T-peel tests of leather/thickened polyurethane adhesive/SBR rubber joints. The addition of the HEUR thickener produced an improvement in rheological properties of polyurethane adhesive dispersions as a result of the physical interactions between the polyurethane particles and the thickener. The addition of the HEUR thickener markedly increased the viscosity of the polyurethane adhesives, as the hard-to-soft segments ratio decreased and the ionic groups content in the polyurethane increased. As the hard segment content of the thickened polyurethane adhesive decreased, the kinetics of crystallization was favoured as a result of stronger polyurethane/thickener interactions. As a result, an improvement in the adhesive strength in the leather/thickened polyurethane adhesive/SBR rubber joints was obtained.

Addition of different amounts of a urethane-based thickener to waterborne polyurethane adhesive

To adjust the rheology of waterborne polyurethane adhesives, different amounts of a hydrophobically modified ethoxylated polyurethane thickener (HEUR) were added. The thickened adhesive solutions were characterized by flow rheology, pH measurements, particle size, solids content and confocal microscopy. The thickened solid adhesive films were characterized by IR spectroscopy, plate-plate rheology, dynamic mechanical thermal analysis (DMTA), differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. The adhesion was measured from T-peel test of plasticized PVC/polyurethane adhesive/plasticized PVC and leather/polyurethane adhesive/SBR rubber joints. The addition of the HEUR thickener increased the viscosity of the polyurethane dispersion, and a shear-thinning behaviour was observed due to polyurethane–thickener interactions. The addition of thickener improved the rheological properties of the polyurethane, more noticeable as the content of the thickener increased. The crosslinking of the thickened polyurethane was studied by confocal microscopy. Although the addition of the thickener did not greatly affect the thermal properties of the polyurethane, an improvement in the adhesive strength of leather/adhesive/SBR rubber joints was obtained.

Synthesis and characterization of waterborne polyurethane adhesive from MDI and HDI

AbstractA series of waterborne polyurethane adhesives (WPUAs) were prepared from diphenylmethane‐4,4′‐diisocyanate (MDI), 1,6‐hexamethylene diisocyanate (HDI), poly(1,4‐butanediol adipate) diol (PBA), 1,4‐butanediol (BDO), and internal‐emulsifying agents by the prepolymer mixing method. The viscosity, mechanical properties, thermal properties, and adhesion strength of the samples were measured. The structure–property relationship was discussed primarily. The results indicated that the MDI/HDI and PBA/BDO molar ratio influenced these properties. The WPUA exhibited excellent T‐peel strength and mechanical properties at a suitable MDI/HDI (or PBA/BDO) molar ratio. Moreover, higher MDI/HDI (or PBA/BDO) molar ratio resulted in higher thermal stability. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Cross-linking reaction of waterborne polyurethane adhesives containing different amount of ionic groups with hexamethoxymethyl melamine

Two series of cross-linkable waterborne polyurethane (WBPU) adhesives were synthesized by a conventional prepolymer process using 2,2-bis(hydroxymethyl) propionic acid (DMPA) (13.85 and 25.00 mol%) and hexamethoxymethyl melamine (HMMM) as a hydrophilic agent and cross-linking agent, respectively. Using infrared (IR) and 1H NMR spectroscopy analysis, the cross-linking reaction between DMPA (carboxyl acid salt group) and HMMM (methoxy group) was identified, and it was found that two methoxy groups per HMMM molecule, containing six methoxy groups, participated in the cross-linking reaction. Adhesive strength significantly increased by cross-linking, up to the optimum content of HMMM (molar ratio of HMMM:DMPA=1:2) and then showed a small decrease with excess HMMM. Adhesive strengths of samples containing 25.00 mol% DMPA (i.e. S-series) were higher than those of samples containing 13.85 mol% DMPA (i.e. P-series). It was found that the optimum pressing conditions for joint values and optimum HMMM content gave the highest failure temperature and thermal stability of adhesives.

Characterization of waterborne polyurethane adhesives containing different soft segments

Waterborne polyurethane (WBPU) adhesives were prepared by the pre-polymer process using different polyester polyols (M n = 2000), namely poly(tetramethylene adipate glycol) (PTAd) and poly(caprolactone glycol) (PCL), and polyether polyols (M n = 2000), namely poly(tetramethylene oxide glycol) (PTMG) and poly(propylene glycol) (PPG), as soft segments. This study focused on the effect of various soft segments on the physical, mechanical and thermal properties, as well as adhesive strength of WBPUs. All of the WBPU dispersions showed small particle size and high viscosity. PTAd-based WBPU shows higher thermal stability, mechanical properties and adhesive strength than PCL-R, PTMG- and PPG-based WBPUs. During preparation of adhesive-bonded samples the optimum pressing temperature and pressure, which were the same for all samples, were found to be 100°C and 15 kg/cm2, respectively. However, it was found that only PTAd-based WBPU had good adhesive strength under moderately high temperature (100°C) compared to all other polyol (PCL/PTMG/PPG)-based WBPUs.

Effect of polyisocyanate hardener on adhesive force of waterborne polyurethane adhesives

A series of waterborne polyurethane (WBPU)/hardener adhesives were obtained from mixing of WBPU containing different types of polyol as a soft segment with aliphatic and aromatic polyisocyanates hardeners. By characterization of allophanate and biuret bonds formed from the reaction of hardener NCO with urethane/urea groups of WBPU using 1HNMR spectroscopy. It was found that the optimum number ratio (molar ratio) of NCO group of hardener to urethane/urea group of WBPU that shows the highest adhesion force was depended on the type of hardener (aliphatic/aromatic polyisocyanate) and dimethylol propionic acid (DMPA) content (total content of urethane/urea groups); however independent of the type of soft segment (polyol) of WBPU. The optimum number ratio (molar ratio) of NCO group of aromatic polyisocyanate hardener to urethane/urea was higher than that of aliphatic hardener to achieve the highest adhesion force of WBPU. The adhesive force increased with increasing hardener content up to the optimum point and then decreased. Poly(tetramethylene adipate glycol) (PTAd) based WBPUs with aliphatic hardener show higher adhesive force than Poly(tetramethylene oxide glycol) (PTMG) and aliphatic hardener-based WBPUs at the optimum number ratio (molar ratio) of NCO group of hardener to urethane/urea group of WBPU. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3663–3669, 2007

Effect of polyisocyanate hardener on waterborne polyurethane adhesive containing different amounts of ionic groups

Waterborne polyurethane (WBPU) adhesive with varying amounts of dimethylol propionic acid (DMPA) was synthesized by prepolymer process and blended with polyisocyanate hardener. The mean particle size of the WBPU dispersion decreased with increasing DMPA content.1H NMR spectroscopy confirmed the formation of allophanate bonds and biuret bonds due to the reaction of hardener NCO with urethane/urea groups. The optimum NCO content with the greatest adhesive strength was dependent on the total content of urethane/urea groups in the WBPU molecules. The optimum NCO content increased with increasing number of urethane groups (DMPA content). The adhesion strength of WBPU adhesives was maximized at a molar ratio of hardener NCO to urethane/urea of about 0.28.

Synthesis and characterization of waterborne polyurethane adhesives containing different amount of ionic groups (I)

AbstractA series of waterborne polyurethanes (WBPU) containing different amount of 2,2‐bis(hydroxymethyl) propionic acid (DMPA) were synthesized using prepolymer mixing process. Relationships between the DMPA content and physical, mechanical, and thermal properties as well as adhesive behavior at different condition were investigated. Stable aqueous dispersions of WBPU were obtained when the DMPA content was more than 10 mol %. At higher DMPA content, the particle size of the WBPU dispersion was lower but the viscosity of the dispersion was higher. Water swelling and tensile strength of the films increased with increasing of DMPA content. The optimum adhesive strength of WBPU adhesives was found to be depended on the DMPA content, pressing temperature, and pressure on adhesion process. The adhesive strength of WBPU adhesives increased with increasing DMPA content. The optimum pressing temperature decreased with increasing DMPA content. The adhesive strength of WBPU adhesives increased with increasing pressure up to 15 kg f/cm2 and then leveled off. The optimum pressing temperature of WBPU adhesives samples containing 24.02, 22.05, and 17.05 mol % DMPA was about 100, 120, and 140°C, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5684–5691, 2006

Structure and properties of waterborne polyurethane adhesives obtained by different methods

The structure and properties of two aqueous polyurethane dispersions prepared by two different methods (acetone method and prepolymer mixing method) were compared. The aqueous polyurethane dispersion obtained by the acetone method (PU-AP) showed a narrower particle size distribution than that of the sample obtained by the prepolymer mixing method (PU-PMP), because the use of an organic solvent in the synthesis produced a lower prepolymer viscosity, thus facilitating dispersion in water. PU-PMP showed more crosslinked structure than PU-AP, and, consequently, PU-PMP had higher elastic (G') and viscous (G") moduli and also a higher cross-over temperature between these moduli in the rheological curves. On the other hand, the lower degree of crosslinking in PU-AP facilitated polyurethane crystallinity. Thermal degradation resistance was similar and both polyurethanes showed similar decomposition patterns. Immediate adhesion of the aqueous poly-urethane dispersions to PVC was low, although it was somewhat higher for the joint produced with PU-AP. The adhesive strength increased by increasing the time after joint formation, and after 72 h was similar for the two adhesives.

Characterization of waterborne polyurethane adhesives containing different amounts of ionic groups

Waterborne polyurethane adhesives are an interesting alternative to the current solvent-based polyurethane adhesives used in footwear industry. In this study, different aqueous polyurethane dispersions were prepared by using the prepolymer mixing process. The ionic groups content in the polyurethane ionomer structure was varied by changing the amount (5 and 8 wt% (with respect to the prepolymer weight) of the internal emulsifier—dimethylolpropionic acid (DMPA). The decrease in the DMPA content slightly increased the particle size in number due to the decrease in both the hydrophilicity of the polyurethane ionomer, and the electrolytic stability of the aqueous polyurethane dispersion. Furthermore, the lower the DMPA amount, the lower the hard segment content in the ionomer and the higher the crystallinity of the polyurethane; however, the resistance to flow under temperature (i.e. thermoplasticity) was reduced. The resistance to thermal degradation of the polyurethane ionomer increased by decreasing the DMPA content due to the lower hard segment content. Finally, high initial adhesive strength was obtained and the adhesion to PVC increased as the DMPA content in the polyurethane ionomer decreased.

Water-based chlorination treatment of SBS rubber soles to improve their adhesion to waterborne polyurethane adhesives in the footwear industry

Solutions of trichloroisocyanuric acid (TCI) in different organic solvents are commonly employed in the footwear industry to improve the adhesion of SBS (styrene-butadiene-styrene) rubber soles to polyurethane adhesive. To avoid the use of organic solvents in the chlorinating solutions, several water-based chlorinating treatments were investigated in this study: (i) inorganic chlorine compounds (HCl-acidified sodium hypochlorite solution; free active chlorine (FAC) = 47.8 g/l); (ii) organic chlorine donors (aqueous solution of 3 wt% TCI/H2O, and ethanol solutions of 3 wt% HD (1,3-dichloro-5,5-dimethylhydantoine), or NCS (N-chlorosuccinimide); (iii) organic chlorine donor salts (aqueous solutions containing 3 wt% DCI (sodium dichloro isocyanurate), CB (chloramine B, N-chloro-sodium-phenylsulphenamide), or CT (chloramine T, N-chloro-sodium-p-toluenesulphenamide). The surface modifications produced by treatment of SBS rubber with the aqueous chlorinating agents were compared with those obtained by using the current solvent-based chlorinating treatment (3 wt% TCI/MEK). The FAC concentration and the chlorine stability in the solutions were determined by iodine titration, and the SBS rubber surface pH was determined with a flat pH probe. The surface modifications on the SBS rubber were analyzed by ATR-IR spectroscopy, XPS, contact-angle measurements and SEM. The adhesion properties were evaluated by T-peel strength tests on treated SBS rubber/waterborne polyurethane adhesive/roughened leather joints. The failed surfaces obtained after peel tests were analyzed by ATR-IR spectroscopy to precisely assess the locus of failure of the adhesive joints. The nature of the modifications produced on the SBS rubber surface depended on the chlorinating system used, the SBS rubber surface pH value, and the free active chlorine concentration of the chlorinating solution. The most effective chlorinating agents were TCI/H2O and HD/EtOH, but they were not stable over time due to quick chlorine evolution. Treatment with NaClO/HCl and DCI/H2O provided acceptable adhesive strength values although there was fast chlorine evolution in the NaClO/HCl solution; the free active chlorine concentration in the DCI/H2O solution was stable for at least 4 days after preparation. Finally, the treatment with NCS/EtOH, CB/H2O and CT/H2O did not chemically modify the SBS rubber surface, so the adhesion to polyurethane adhesive was not improved.