pMDI Emulsion Adhesive on Bonding Wood

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Materials and Interfaces

Behavior of water/pMDI Emulsion Adhesive on Bonding Wood Substrate with Varied Surface Properties Che Zhang, Li Yu, Fatemeh Ferdosian, Sucharita Vijayaraghavan, Julien Mesnager, Veronique Jollet, and Boxin Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03485 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 12, 2018

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Industrial & Engineering Chemistry Research

Behavior of water/pMDI Emulsion Adhesive on Bonding Wood Substrate with Varied Surface Properties Che Zhang1, Li Yu1, Fatemeh Ferdosian1, Sucharita Vijayaraghavan1, Julien Mesnager2, Veronique Jollet2, Boxin Zhao1* 1Department

of Chemical Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada 2 EcoSynthetix,

3365 Mainway, Burlington, ON L7M 1A6, Canada

KEYWORDS: pMDI, emulsions, wood adhesives, bonding strengths, penetration depths

ABSTRACT

Wood-based structure panels have been widely used in building construction, aerospace and furniture industries, where adhesives bonding plays an essential role in the manufacturing processes. Formaldehyde-free adhesive resins are in high demand to meet the increasingly stringent environmental regulations and have sparked enormous research interests. This work reports a systematic investigation of bonding behavior of a pMDI adhesive delivered as a water/pMDI emulsion and used as a model system to investigate the adhesive penetration depth and resulting bonding strength when used as an adhesive to bond wood plies. The approach of

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water/pMDI emulsion can avoid water distribution problem which has to be considered in traditional procedure for bonding wood materials. The stability and phase separation of the water/pMDI emulsion system were first examined by image analysis and viscosity measurements as well as the effects of the emulsion composition on the bonding strength of the adhesive. By both the pull-off and lap-shear tests, it was determined that a water content around 30% by weight of the emulsion was providing an optimum strength while yielding emulsions stable enough to be practically applied on wood surfaces. After this optimization, the adhesive penetration depth was altered by treating the wood surface with a silane agent (APTES) so as to improve the bonding strength. The results show that a moderate penetration helps enhance bonding strength, providing insights into the non-proportional relationship between adhesive penetration and bonding strength.

1. Introduction In the wood panel industry, the petroleum-derived amino and phenol thermosetting resins (e.g. urea-formaldehyde, melamine-urea-formaldehyde and phenol-formaldehyde) are very predominant adhesives especially for interior grades wood panels because of their advantages as low-cost, easy manipulation and good bonding properties.1 Nowadays, the increasing concern about the formaldehyde emission from the formaldehyde-base wood composites are demanding the use of formaldehyde free adhesives or apply formaldehyde scavengers to eliminate the formaldehyde emission.2 Polymeric 4,4-diphenylmethane diisocyanate (pMDI) is a formaldehyde free resin and has already been employed in producing oriented strand board (OSB) and in some cases medium density fiberboard (MDF).3 pMDI resins can provide the required level of bonding strength, water resistance properties, dimensional stability and load bearing for structural and exterior applications; it slowly became the predominant adhesive technology for OSB manufacturing. It also has several manufacture advantages such as appropriate curing speed, low


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loading quantity and high efficiency.2,4 The conventional methods in OSB manufacture for utilizing pMDI is applying the resin directly to the moisturized wood strands with wax in a tumbling blender using spray nozzles or spinning discs;5 in MDF processes, the resin is usually injected under pressure through a nozzle in a blow-line blender where the fibers travel at high velocity in a semi wet state depending upon the point of injection.6 pMDI resins are not commonly used in Plywood as their viscosity, tackiness and penetration is not appropriate to pick-up and spread the resin onto plies or keep the plies bonded before pressing. Herein, we explored an alternative approach to applying pMDI resin, in which the water/pMDI emulsions were first prepared and then applied to bond wood. To date, extensive efforts have been devoted to understanding the curing mechanism of pMDI resin and modulate factors that determine the bonding strength of the pMDI wood adhesives. Steiner et al. analyzed the wood-pMDI reaction by differential scanning calorimetry (DSC). They found the 2-stage reaction between poly-isocyanate and wood was influenced by time, temperature and the presence of water. The initial reaction takes place at 25-50 oC and a further cross-linking reaction occurs near 200 oC.7 Weaver and Owen used infrared spectroscopy to investigate the reactions of glucose, cellulose, lignin, and wood between phenyl isocyanate and 4,4'diphenylmethane diisocyanate (MDI).8 Their results show that the isocyanate can react with all of those hydroxyl-containing sugar derivatives while lignin has the highest reactivity. When water is present in this system, the water-isocyanate reaction dominates. The effect of wood moisture content on pMDI resin bonding performance was investigated by Gruver and Brown;9 their results indicate that increasing wood moisture content can lead to higher shear stress which the resin can bear.


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Besides the wood moisture content, the bonding strength of adhesive resin is also influenced by wood parameters (e.g. species and anatomy), adhesive properties (e.g. viscosity, pH) and bonding process (e.g. assembly time, temperature and pressure).10 The penetration depth of the adhesive could also be an effective factor to the adhesion strength of the bonded wood. In the literature, the penetration of the adhesive is defined as the movement of liquid adhesive into the porous wood tissues.11,12 In general, it can be divided into two levels: micrometer level (gross penetration) and nanometer level (cell wall penetration). The gross penetration is the result of the capillary flow of the low viscosity resin into the porous structure of the wood. In contrast, the cell wall penetration represents the diffusion of the resin into the cell walls, which can only be observed when the resin component has small-molecular weight.4,12,13 Different methods have been employed to examine the resin penetration, as summarized by Ferdosian et al.4 and Qin et al.13: fluorescence microscopy (FM), light microscopy (LM) and scanning electron microscopy (SEM) for characterizing the gross penetration of resin; UV-microscopy, transmission electron microscopy - electron energy loss spectroscopy (TEM-EELS), chemical-state X-ray microscopy, confocal laser scanning microscopy (CLSM), and scanning thermal microscopy (SThM) for the cell wall penetration. Because of the low viscosity, low molecular weight and low surface tension, the pMDI resin can readily wet the wood surface and penetrate deeply into the wood structure.14,15 This situation might not be good for remaining a sufficient amount of pMDI at interface to form the adhesive bonds. To the best of our knowledge, there is no clear correlation between the penetration depth and the final bonding strength. In this work, we first investigated the optimum content of water in the formulated water/pMDI adhesive for the best adhesion bonding. After that, wood substrates were modified by (3aminopropyl) triethoxysilane (APTES) to introduce amine group onto the wood surface and


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improve the interaction between the pMDI and wood substrates. The penetration depth of adhesive in the wood substrate was also investigated to explain the decline of bonding strength on high concentration APTES treated wood substrates.

2. Experimental section 2.1 Materials Polymeric diphenylmethane diisocyanate (pMDI) utilized in this work was supplied by BASF (LUPRANATE® M20 Isocyanate). pMDI is a dark amber liquid mixture containing pMDI (40% to 70%), diphenylmethane-4,4’-diisocyanate (4,4’-MDI, 30% to 60%), and methylenediphenyl diisocyanate (less than 5%). The initial viscosity of this pMDI liquid is 630 cP. The average molar mass is 360 g/mol and the average functionality is 2.7. All other chemicals, including (3aminopropyl) triethoxysilane (APTES), the silane agent; oil red O, the red dye for pMDI; and ethanol, the solvent, were purchased from Sigma-Aldrich and used as received. Pine wood substrate, which were utilized as the substrate for the contact angle measurement and bonding strength test, were supplied by First Voice (Tongue depressor, No-sterile, Model: First Voice TS4312-500). 2.2 Water/pMDI Adhesive Emulsion To prepared the water/pMDI emulsion, various weight ratios of water (from 5 to 50wt%) was added into pMDI; a planetary centrifugal THINKY Mixer (ARE-310, in STD mode), was used for mixing process (3 min at 2000 rpm) and followed by defoaming process (2 min at 2200 rpm). The bubbles are mainly air bubbles generated by the mixing of pMDI and H2O. Few are CO2 bubbles due to the slow reaction between pMDI and H2O at room temperature. The mixture was further homogenized using a Vortex Mixer (Fisherbrand™ Analog) for 10 min at 3200 rpm to obtain a uniform emulsion.


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2.3 Wood Surface Modification In order to further improve the bonding strength of the adhesive, the wood substrates were pre-treated by APTES to introduce amine groups onto the wood surface. Following the protocol in literature,16 two APTES solutions (1 and 3wt%) in ethanol/water (80/20 by weight/weight) were prepared by vigorously stirring for 2 h at room temperature. Wood specimen were then immersed in each APTES solution for 30 minutes and dried at 120 oC for 1 h. The samples were washed by water and ethanol for several times after the modification, so that self-binding parts of APTES will not “clog” the pores in the wood substrate. After washing, the samples were further dried at 120 oC

to remove all the solvent.

2.4 Characterization 2.4.1 Viscosity The viscosity of the water/pMDI emulsion was measured by a viscometer (Brookfield CAP 2000+). Each measurement was carried out using a cone spindle (Model: No. 1) at room temperature (23 oC) and constant shear rate (1333 1/s). For every data point, three independent measurements were carried out to calculate the average viscosity. The viscosity of each formulated emulsion was measured every hour (5 hours in total) to monitor the viscosity variation over time. 2.4.2 Scanning Electron Microscopy - energy dispersive X-ray spectroscopy To validate the bonding of the APTES to the wood surface, the neat wood and APTES treated wood specimens were viewed under the ZEISS Scanning Electron Microscopy and the energy dispersive X-ray analysis (EDX) was applied to identify the elements on the surface. 2.4.3 Contact Angle The modification of wood substrate by APTES was further demonstrated by static contact angle measurement at room temperature (23 oC) and relative humidity 30%. NE-1010 Higher


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Pressure Syringe Pump was used to pump out a 5 L DI water droplet or pMDI droplet onto the neat or pre-treated wood surface. The wood substrates for this measurement were pre-dried in oven at 120 oC until mass constant and placed properly to insure the fiber direction of the wood is kept constant (perpendicular to the camera). An in-house custom-made contact angle goniometer was utilized to record the contact angle and the pictures of contact angle were analyzed by the scientific image analysis software ImageJ. 2.4.4 Pull-off Test To determine the bonding strength of the water/pMDI adhesive, pull-off test was conducted based on ASTM D4541−17 standard 17 to measure the stress required to detach the stub from the wood substrate (shown as Figure 1a). To simulate the application of pMDI adhesive in wood industry, wood on wood pull-off test was performed by attaching a circular shape wood piece (0.5inch diameter) onto a same size pin stub with epoxy adhesive. Afterward, 0.03 g/cm2 formulated water/pMDI adhesive was applied uniformly on the neat or APTES treated wood specimen, and then the coated wood specimen was put gently on the top of the wood bonded pin stub and cured in a preheated oven at 160 oC for 5 h under 50 kPa pressure. Six samples were prepared for each formulated adhesive and all of the wood substrate used in the test were pre-dried in oven at 120 oC until mass constant. The cured samples were cooled down and stored in the sealed zip-lock bag for 3 days. After 3 days, the prepared samples were examined by a universal materials tester (UMT, UNMT-2MT) with constant loading rate (0.1 mm/s) at room temperature (23 oC) and relative humidity 30%. 2.4.5 Single Lap-shear Test Single lap-shear test was another test method utilized to determine the bonding strength of the water/pMDI adhesive. It was conducted based on ASTM D1002-10 standard 18 to measure the


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required shear stress to detach the bonded wood samples. To fabricate the samples for single lapshear test, the pre-dried neat or APTES treated wood substrate was cut into a rectangle shape (0.5 in × 3 in). A small square wood (0.5 in × 0.5 in) was glued on one side of the cut wood substrate by epoxy resin to make a holder for the shear test sample. On the other side of the cut wood substrate, 0.03 g/cm2 formulated water/pMDI emulsion was applied to a small surface (0.5 in × 0.5 in). Then, the wood substrate was put onto another one with the adhesive-coated surface overlapped (Figure 1b). Afterward, the sample was cured in a preheated oven at 160 ºC for 5 h under 50 kPa pressure. Five samples were prepared for each wood adhesive formulation. After storing the samples at room temperature for 3 days, the single lap-shear test was performed by a tensile testing machine (INSTRO 4465) with a constant speed of 3mm/min at room temperature (23 oC) and relative humidity 30%. 2.4.6 Penetration Depth of pMDI To evaluate the penetration depth of pMDI in the neat or APTES treated wood substrate, differential staining method was applied by coloring the pMDI specifically with oil red O. Certain amount of oil red O (1 mg/g of pMDI) was firstly dissolve in pMDI, and then 30wt% of water was added into the colored pMDI. Following the same procedure of preparing adhesive emulsion, the formulated adhesive was applied onto two wood substrates homogeneously, and then the coated specimens were put together and cured at 160 oC for 5 h at 50 kPa pressure. After cooled to the room temperature, the cross-section of the cured sample was cut and observed by a digital microscope (Dino-Lite Premier AM3113T 2.0). The penetration depth was determined by analyzing the sample cross-section image with ImageJ. The penetrated line of pMDI in the crosssection is described as the deepest straight-line which is parallel to the bonding interface and goes through at least five detected pMDI objects (red spots inside the wood structure), and the


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penetration depth is defined as the distance between the interface of two bonded wood pieces and the penetrated line of pMDI.11 Eight samples were prepared for each specific wood (neat wood, 1wt% APTES treated wood and 3wt% APTES treated wood) for the final result.

Figure 1. (a) Sample preparation and testing procedure for pull-off adhesion test, (b) Form and dimensions of lap-shear test specimen (Unit: inch).

3. Results and discussion In this work, the water/pMDI emulsions were prepared and then applied on to dry wood substrates. This approach is different from the common application of directly applying pMDI resin on wood materials that contains a finite amount of moisture.15,19,20 The advantage of our approach is that it allows a localized addition of water within the pMDI to provide sufficient reagent for curing. It also provides the opportunity to modulate adhesive viscosity by forming water/pMDI emulsion. Our first objective was to determine the optimal water content in the emulsion in terms of emulsion stability and the pull-off adhesion tests and lap-shear adhesion tests.


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Figure 2. (a) Photographs of water/pMDI emulsions of different constitutions. (b) Chemical structure of the dye. (c) The solubility of the dye in water and pMDI. (d) Thermogravimetric analysis curve of the dye. (e) Optical microscope images of water/pMDI emulsions of different constitutions (Note: the dye is in pMDI phase). Figure 2 shows the stability of the water/pMDI emulsions. Macroscopically, water/pMDI emulsions can keep uniform and stable for approximate 1 h when the water concentration is less than 35wt%. Phase separation of water and pMDI is observed when the water concentration is over 40wt% (Figure 2a). Note that it usually takes 1 h to prepare and coat the emulsion on the wood substrates before the curing process; there are no noticeable gas bubbles generated within


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this time frame. To characterize the microstructure of the emulsions and penetration of the emulsions on the wood substrate, a thermally stable dye was doped into water/pMDI emulsions; this dye has a good solubility in pMDI and but is insoluble in water (Figure 2b-d). It can be seen that the microscopic water droplet disperses in pMDI suggesting this is a water in pMDI emulsion; which means water and pMDI act as dispersed phase and continuous phase, respectively (Figure 2e). For water concentration less than 35wt%, the distribution of the size of water drops is narrow; but the size of water drops gradually increases with the increasing concentration of water. For the water concentration greater than 40wt%, both the size and distribution of water droplets greatly increase. Water/pMDI emulsions appear to transform from uniform state to phase separation at the concentration of water is over 40wt%.

Figure 3. (a) Dependence of the viscosity of water/pMDI emulsions on the water concentration and storage time. (b) The mechanism for the reaction between pMDI and water to generate the urea linkage. (c) Photographs of the water/pMDI emulsions in sealed syringe at initial state and storage for 12 h at room temperature. The viscosity of the water/pMDI emulsions was measured over a certain period of storage time (Figure 3a). As expected, the viscosity of the emulsions increases with time and water


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concentration, suggesting that some curing reaction leading to an increase of molecular weight takes place once water and pMDI are mixed even at room temperature. pMDI curing is composed of two consecutive steps (Figure 3b) in which the reaction between pMDI and water molecules produces carbon dioxide and an amine by decomposition of a carbamic acid, followed very quickly by the reaction of the resulting amine with another isocyanate function to form a urea linkage.15,19,21 It is also possible that there is some contribution of foaming due to the entrapment of CO2 resulting in the apparent increase of viscosity of the overall system. The release of carbon dioxide over time was verified by sealing some emulsions in a syringe to observe the volume expansion over time (Figure 3c). To ensure wetting and spreading of the water/emulsion onto the wood so as to form good adhesive bonds, the emulsions were applied on the surface of the wood pieces in a timely manner within 1 hour after mixing to avoid high viscosity and be consistent as some pre-curing may happen. For the bonding process, the emulsions were first coated on the surface of the wood piece; the coated wood specimen was put gently on the top of an array of wood bonded pin stubs (for pull-off test), or another piece of wood (for lap-shear test) in the second step. The bonded samples were cured in a preheated oven at 160 oC for 5 h under 50 kPa pressure. The 160 oC curing temperature is to accelerate the reaction between pMDI, water as well as wood stricture and the 5 hours curing time is to make sure the reaction is completed.


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Figure 4. Dependence of the (a) pull-off stress and (b) lap-shear stress on the concentration of the water in the emulsions. Photographs of the failure surface of the samples with different concentrations of water after (c) the pull-off and (d) lap-shear tests. Figure 4a shows the pull-off stress almost linearly increases with the increasing concentration of the water till a maximum value. The pull-off stress reaches the maximum value of 1.25 MPa as the water concentration is 30wt%. As the water concentration is over 30wt%, the pull-off stress slightly decreases. As shown in the lap-shear test result (Figure 4b), the changing trend of lap-shear stress as the concentration of water is similar to that of the pull-off test. The lapshear stress can be as high as 5 MPa. The dependence of the pull-off and lap-shear stress on the concentration of the water both illustrates that the bonding strength of wood adhesives is favorably enhanced with higher content and availability of the water for the bond line to cure; it also demonstrates that it can be a way to provide locally the necessary reagent for pMDI to cure more


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efficiently. The effect of the composition of emulsions on the bonding strength of wood adhesives can be more directly observed at the failure surface of the samples after the pull-off and lap-shear tests. At relatively lower water concentration (20wt%), more and more wood fibers can be seen at the failure surface indicating the failure occurs at the wood substrate, which is named as cohesive failure of the wood (Figure 4c and d).23 Both the pull-off and lap-shear tests suggested that around 25wt% to 30wt% water led to the highest bonding strength on the neat wood surface.


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Figure 5. SEM-EDX analysis spectroscopy of (a) pure wood specimen (b) APTES-treated wood specimen (c) Water or pMDI contact angle on the surface of pure wood specimen, 1wt% or 3wt% APTES modified wood specimen. In addition to the adhesive composition, we modulated the bonding strength through the modification of the wood substrate since both the adhesives and wood characteristics are important for the bonding strength. The silane agent APTES was utilized to modify the wood substrates and


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the penetration of pMDI. We use SEM-EDX to prove the bonding of APTES to wood (Figure 5a and b). In neat wood substrate, C and O elements are observed. After the modification, Si element belonging to APTES is discovered, suggesting the bonding of APTES to the wood substrate. The aliphatic chains of APTES exposed on the surface make wood substrates more hydrophobic, which is also reported in other work (Figure 5c).24,25 This APTES agent has been used to promote urea-formaldehyde resins and protein based resins in wood panels.26 We thus hypothesized that during the curing step, APTES might be capable of enhancing the interaction between the pMDI adhesive and the wood surface as an adhesion promoter since it contains amine groups susceptible to react with pMDI and silanol moiety that can adhere to surfaces with hydroxyl groups,27 including the wood surface. At the same time, it could act as a compound capable of modifying the surface energy of the wood and change the affinity of pMDI with the wood substrate, therefore changing its penetration. As shown in Figure 5c, water contact angles are larger on the APTES-treated wood specimen but pMDI contact angles are smaller on the APTES-treated wood, suggesting that the wood surface become more hydrophobic but more pMDI-philic. This would suggest that APTES molecules may orient in a way that would disfavor water/pMDI emulsion penetration.


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Figure 6. Dependence of the (b) pull-off stress and (c) lap-shear stress on the concentration of APTES for wood treatment. Herein, the adhesive with 30wt% water content was chosen to investigate the influence of the modification of the wood substrate on the bonding strength. Figure 6a and b show that the bonding strength first increases and then decreases with the increasing concentration of APTES. It seems to suggest that the bonding strength goes through an optimum based on the affinity and penetration of the pMDI within the wood surface. It can be hypothesized that a quenching effect of APTES by reaction of the amino group with isocyanate could alter the pMDI thermosetting reaction. However, we suspected that the bonding strength to be mainly related to the penetration of the adhesives.


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Figure 7. (a) Sample preparation for penetration of pMDI characterization. (b) Photographs of the cross-section of un-bonded pure wood specimen. Photographs of the cross-section of (c) pure wood substrate and modified by (d) 1wt% and (e) 3wt% APTES bonded by dye doped adhesives. Table 1. The penetration of pMDI on treated wood substrates. Types of Wood

Depth of pMDI Penetration (μm)

Neat Wood

341 ± 31

1wt% APTES-Wood

281 ± 23

3wt% APTES-Wood

210 ± 29

To investigate the penetration of the adhesives, we doped the adhesives with Oil red O, which is solvable in pMDI but not in water, as pMDI is effective component of the adhesives while water works as curing agent (Figure 7a). Thus, we can easily characterize the penetration of the adhesives by observing the distribution of dye doped adhesives in the cross-section of the wood specimen using the microscope. To quantify the penetration behavior of the adhesives, we determined the penetration depth defined as the distance of the liquid adhesive moved into the porous wood structure 11. In the cross-section, the penetration depth is measured as the distance between the bonding line and the adhesive penetrated line. For reference, only porous structure was seen in virgin wood pieces (Figure 7b). In the cross-section of the adhesive bonded wood substrates, some red dots corresponding to the dye doped adhesives are clearly observed around the bonding line (Figure 7c-e). Although the wettability of pMDI on the wood surfaces enhanced by the modification of APTES (Figure 5c), the penetration depth decreases as the APTES concentration increases (Table 1). The explanation for this phenomenon is that the APTES


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modification increases the wettability of pMDI on the wood substrates and makes the pMDI more easily spread in the horizontal direction and penetrate less in vertical direction within the process. Combining with the penetration result, we were able to explain the effect of the modification of the wood specimen on the bonding strength. Without the modification of the wood, it shows the lowest bonding strength due to relatively weaker interaction between the adhesives and wood substrate. In addition, over penetration (penetration depth of 341±31 μm) results in a small portion of the adhesives remaining at the interface of two bonded wood pieces that cannot provide enough mechanical property of the adhesives. When the wood is modified by 1wt% APTES, it shows the highest bonding strength due to enhanced interaction between the adhesives and the substrate, and moderate penetration (penetration depth of 281±23 μm). The best affinity between the adhesives and the wood surface is achieved when the specimen is modified by 3wt% APTES, the isocyanate adhesive having the lowest pMDI contact angle. However, the bonding strength decreases at the same time than the penetration of the adhesive (penetration depth of 210±29 μm). APTES induces the pMDI adhesive to penetrate less inside the wood which would decrease the interlocking between the wood and the adhesive, leading to a decrease in strength.28 It is also possible that at high concentration the amine of the silane reacts furthermore with pMDI and disturbing the thermosetting reaction with water. Overall, the treatment of wood with APTES is a way to modulate the adhesive performance through various mechanism, including penetration depth.

4. Conclusions In summary, we investigated the behavior of pMDI adhesives delivered as water/pMDI water in oil emulsions and its bonding performance as a wood adhesive under laboratory conditions. A pMDI soluble but water insoluble dye was doped into water/pMDI emulsions in order to determine the microstructure of the emulsions and penetration behavior of the adhesives on the wood


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substrate. The results show that the emulsions consist of water droplets dispersed in continuous pMDI and are stable enough to provide a workable adhesive within a timeframe of 1h when water contents are less than 35wt%; phase separation is observed at higher water contents probably due to phase inversion or poor stability in the absence of surfactant. Both the pull-off and lap-shear tests suggested that 25wt% to 30wt% water led to the highest bonding strength on the neat wood surface. By the modification of the wood using silane agent APTES, we were able to modulate the interaction between the adhesives and wood, its wettability and penetration in the substrate. The results show that strong interactions between the adhesives and wood substrate and a certain level of penetration of the adhesives are required for good bonding performance where excessive penetration might be detrimental to the adhesive bonding strength. Future works should be extended to other reactive or non-reactive compounds for wood treatment applied by spraying rather than dipping and on exploring further wood surface modifiers for adhesive enhancement onto various wood substrates (chips, strands or plies).

AUTHOR INFORMATION Corresponding Author * Boxin Zhao, Email: [email protected]. Tel.: 519-888-4567 ext. 38666 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The authors would like to thank the financial support from the NSERC CRD project, thank Prof. Shoufa Lin for the assistance with the optical microscope and Mark Griffett for the assistance with lap-shear test and EcoSynthetix technical team Sabina Di Risio, Ted Van Egdom and Niels Smeets for comments, suggestions and discussions.


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SYNOPSIS


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pMDI Emulsion Adhesive on Bonding Wood

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