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Effect of surface properties on the adhesion between PVC and wood veneer laminates.

INTRODUCTION

Since the last decade, composites consisting of lignocellulosic fibers and synthetic thermoplastics have received substantial attention in scientific literature as well as in industry (1-14). The use of cellulosic fibers in plastic composites is of particular interest because such fibers can serve as a good reinforcer and/or filler for synthetic polymers to enhance certain properties while reducing material costs (1, 3, 4, 9). Despite these advantages, the nature of adhesion between hydrophilic, polar cellulosic fibers and hydrophobic, nonpolar thermoplastics is crucial for both the processing and performance of these materials. In general, the addition of cellulosic fibers in a plastic matrix leads to poor interfacial adhesion between components, and as a consequence, strength properties deteriorate. An understanding of the adhesion mechanism between plastic and cellulosic fibers seems to be a requisite for predicting and improving the strength.

A great deal of research has been conducted on adhesion (15-22). At least seven theories of adhesion are currently in use. Each theory has its merits, but none is universally applicable. According to these theories, strong adhesion between materials is governed by two criteria: an intimate molecular contact closer than 9 [Angstrom] (necessary condition), and a maximum attractive force with a minimum potential energy (sufficient condition) (23).

The former criterion includes adsorption, diffusion, and mechanical interlocking. based on most of those theories, strong interfacial adhesion cannot exist without effective wetting of the matrix on a cellulosic fiber. A general condition that must be fulfilled for wetting is that the surface tension of the polymer melt must be less than that of the filler (3, 24). However, exceptions to this rule have led to the introduction of new models, such as the interfacial defect model and the fracture energy model of adhesion (20). According to these models, optimum conditions for wetting exist when the polarities of the two phases match each other. The interfacial tension, which depends on the polarity match, has also been used as the wettability criterion (19). On the other hand, the maximum attractive force criterion includes chemical bonding, acid-based, electrostatic, and weak boundary theories.

The intimate molecular contact criterion has been largely used to study and elucidate the mechanism of adhesion between cellulosic fiber and plastic. This large body of studies has led to the identification of suitable coupling agents for polyolefin (i.e., PE or PP) and cellulosic fiber composites. As a result, polyolefin/cellulosic Fiber composites with a superior strength and a higher modulus compared to the neat polymers have been produced (1-11, 14). On the other hand, polyvinyl chloride {PVC)/cellulosic fiber composites have not been studied extensively, although the annual production rate of PVC is the highest of all polymers in North America (25). The strength of PVC/cellulosic fiber composites has been reported to be lower than that of the neat polymer because an appropriate adhesion promoter and/or its form of application for PVC/cellulosic fiber composites has not been identified (26-31).

In this context, this study examines the adhesion between PVC and wood in order to identify a suitable coupling agent for PVC/cellulosic fiber composites. The adhesion is investigated by measuring the force required to detach two wood veneers laminated with a PVC film in a single-lap shear test. This test was selected to minimize the effect of the uneven coverage of the coupling agent on the fiber surface because of a large surface area of particulate fibers. The effectiveness of the surface modification of wood was investigated using X-ray photoelectron spectroscopy (XPS) and contact angle measurements. Particular emphasis was placed on investigating the role of surface tension on the adhesion between PVC and wood veneers in order to gain a better understanding of the surface modifications responsible for the improvement of adhesion, as well as elucidating the mechanism of adhesion.

EXPERIMENTAL

Materials and Sample Preparation

Wood veneers of approximately 50.8 mm x 25.4 mm (2 in. x 1 in.) in cross section and 3.2 mm (1/8 in.) in thickness were cut from a single board of yellow poplar (Liriodendron tulifera). [Gamma]-aminopropyltriethoxysilane (A-1100, Union Carbide Corporation), dichlorodiethylsilane (DCS, Dow Chemical Company), phthalic anhydride (PA, Aldrich Chemical Company), and maleated polypropylene (Epolene Wax E-43, Eastman Chemical) were used as a coupling agent. Sodium carbonate ([Na.sub.2]C[O.sub.3] anhydrous, J. T. Baker Chemical Co.) was used as a catalyst. A thin film of plasticized PVC with a thickness of 0.5 mm was used as an adhesive. This film was manufactured in our laboratory and the details of this film manufacturing have been described elsewhere (32). Glycerol (Fischer Scientific) was used as a wetting liquid for contact angle measurements.

Before the surface treatment, wood veneers were Soxhlet-extracted with acetone for 24 hours to remove contaminants or impurities on the surface of wood. After the extraction, wood veneers were air-dried for at least two weeks (moisture content of approximately 10%).

Treatment With Silanes

Silanes are applied using either an aqueous solution or dry blending with an organic solution depending on the type of reinforcement. An aqueous treatment is the usual method for fibrous materials, while dry blending with an organic solution of silane is the preferred method for particulate fillers because of the difficulty of drying the aqueous slurry. A hard cake formed after drying requires an additional effort to finely crush the aggregated particles (33). By either method, silane should first hydrolyze and form an organosilanol, which reacts with the hydroxyl groups of cellulosic fibers to form a silane layer on the fiber surface. Many factors, such as hydrolysis time, organofunctionality of silane, temperature, pH of the solution and so on, influence the formation of silanol groups during hydrolysis, and subsequently affect the effectiveness of coupling agent (34, 35). In addition, special attention should be paid to the concentration of silane in solution so that monomeric silane dominates, since oligomeric silanes are known to be less effective as a coupling agent (36). Ishida et al. (37) reported that the concentration of silane in water should be in the range of 0.01% to 2% by weight in order to ensure dominancy of monomeric silane.

Based on this information, in this study the treatment of wood veneers was carried out in a solvent-free system (dry blending) by spraying 0.1% of silanes (A-1100 and DCS) (based on the weight of water in wood) over the area to be bonded. The treated wood veneers were air-dried for 30 minutes and then oven-dried at 60 [degrees] C for 24 hours. Drying was followed by Soxhlet-extraction with acetone for at least 24 hours to remove the silane that was not chemically bonded to wood. After this extraction, the treated samples were air-dried for 24 hours and then dried at 60 [degrees] C in an oven with circulating air until a constant weight was achieved.

Treatment With Phthalic Anhydride (PA) and Maleated Polypropylene (E-43)

Wood veneer samples were immersed in a toluene solution containing 2 moles of PA at 100 [degrees] C for 20 minutes. [Na.sub.2]C[O.sub.3] by weight of wood) was used as a catalyst. The treatment of wood veneers with E-43 was also performed in a solvent system. The veneers were immersed in a 100 ml toluene solution containing 3% of E-43 (by weight of the wood veneer) at 100 [degrees] C for 20 minutes. Thee treatments were followed by Soxhlet-extraction of treated wood veneers with toluene for at least 24 hours, air-drying for 24 hours and finally oven-drying at 60 [degrees] C until a constant weight was achieved.

X-Ray Photoelectron Spectroscopic Studies

X-ray photoelectron spectra of both unseated and treated wood veneers were recorded by using an X-ray Photoelectron Spectrometer (Leybold MAX-200. Cologne, Germany) with an unmonochromated magnesium [K.sub.[Alpha]] source (1253.6 eV). The magnesium [K.sub.[Alpha]] source was operated at 15 kV and 20 mA. The samples were mounted on a holder with a double sided adhesive tape and placed in a vacuum chamber in the range [10.sup.-8]-[10.sup.-7] torr. the analyzed sample area was approximately 4 mm x 7 mm. For all samples, a low-resolution survey run (-1000 to 0 eV, pass energy = 192 eV) was performed. The atomic percentages of the elements present were derived from the spectra run of the region of interest also in a low-resolution mode (pass energy = 192 eV). The sensitivity factors used ([O.sub.1s] = 0.78. [C.sub.1s] = 0.34 [N.sub.1s] = 0.54, [C1.sub.2p] = 1.08, [Si.sub.2p] = 0.40) were empirically derived by the Leybold for the spectra normalized to unit transmission of the electron spectrometer (38). In order to obtain more information on the nature of surface, these spectral regions were also run in a high-resolution mode (pass energy = 48 eV). The energy scale of the spectrometer was calibrated to the Ag [3d.sub.5/2] and Cu [2p.sub.3/2] at 368.3 eV and 932.7 eV, respectively (39). The binding energy scale was then shifted to place the main hydrocarbon [C.sub.1s] feature present at 285.0 eV. The samples were analyzed at a takeoff angle of 90[degrees] relative to the electron detector and the spectra were deconvoluted using a curve fitting program (ESCA Tools version 4.2, Surface/Interface, Mountain View, California). The atomic ration of oxygen-to-carbon (O/C) was calculated from their normalized peak areas as

O/C = ([I.sub.o]/[I.sub.c]) [multiplied by] ([S.sub.c]/[S.sub.O]) (1)

where [I.sub.o] and [I.sub.c] are the normalized integrated area of the peaks for oxygen and carbon, respectively. [S.sub.c]/[S.sub.o] is the corrected term for the sensitivity factor.

Contact Angle and Surface Tension Measurements

In general, it is very difficult to measure the interfacial tension of a solid directly because of the absent mobility of the solid phase. However, various indirect methods are available for estimating the surface tension of a solid (40). It is generally agreed that the measurement of contact angles on a given solid surface is the most practical way to obtain the solid surface tension.

There are a variety of techniques available for the measurement of contact angles (41-44). In this study, the Axisymmetric Drop Shape Analysis-Contact Diameter (ADSA-CD) technique is employed to measure the contact angle of glycerol sessile drops on the untreated and treated wood veneer surfaces and on PVC. In this approach, the contact angle is determined from the contact diameter of the drop viewed from the above to overcome the subjective problem of placing a tangent on the drop profile at the point where it contacts the solid surface, the problem encountered in the direct technique for contact angle measurement (45, 46). This approach has been extensively used to measure the contact angles of hydrophilic materials such as cell layers and wood (47, 48). Contact angles with high accuracy (a standard error less than [+ or -] 1[degree]) were obtained through ADSA-CD on the surface of rough and heterogeneous cell layers (48). This low standard deviation was far better than those obtained by the direct techniques.

The detailed description of the experimental setup and procedure for the contact angle measurement are given elsewhere (45-47). The drop image viewed from the above was obtained using a stereomicroscope (Wild-Heerbrugg M7S). A cohu CCD video camera was used to transfer the video image signal to the digitizer. The contact angle of the drop was quickly measured at room temperature and at specified times after deposition of the drop on the wood surface. These specified times ranged from 0 to 60 seconds.

The solid surface tension, [[Gamma].sub.sv], can be calculated by combining the Young equation:

[[Gamma].sub.1v] cos[Theta] = [[Gamma].sub.sv] - [[Gamma].sub.s1] (2)(2)

and the Neumann's equation of state for interfacial tensions (40):

[[Gamma].sub.s1] = [[Gamma].sub.1v] + [[Gamma].sub.sv] - 2 [-square root of [[Gamma].sub.1v] [multiplied by] [[Gamma].sub.sv]] [multiplied by] [e.sup.-[Beta]([[Gamma].sub.1v] - [[Gamma].sub.sv]).sup.2] (3)

where [[Gamma].sub.1v] is the liquid surface tension, [[Gamma].sub.s1] the solid-liquid interfacial tension, and [Beta] = 0.000115 [([m.sub.2]/mJ).sup.2] the Neumann's constant (40). By eliminating [[Gamma].sub.s1] from these equations,

cos[Theta] = -1 + 2 [-square root of [[Gamma].sub.sv]/[[Gamma].sub.1v]] [e.sup.-[Beta]([[Gamma].sub.1v] - [[Gamma].sub.sv]).sup.2] (4)

The solid surface tension [[Gamma].sub.sv] has been calculated by Eq 4 based on the experimentally measured contact angle [Theta] and the known liquid surface tension of glycerol ([[Gamma].sub.1v] = 62.66 mJ/[m.sup.2]).

Manufacturing and Testing of Lap Joints

Two wood veneers were laminated together with a PVC film in a hot press (Model 50-1818-2tm, Wabash Metal Products Inc.) as shown in Fig. 1. The samples were compressed in the hydraulic press at 190 [degrees] C for 1 minute under a pressure of 0.2 MPa (30 psi). The joints had an overlap area of 12.7 mm x 25.4 mm and were conditioned before testing in a moisture dessicator at room temperature (21 [degrees] C) and 50% relative humidity for at least 7 days.

Shear testing of the lap joints was carried out on Instron Universal Testing Machine Sintech (Model 20) equipped with Testwork program (Version 2.10, Sintech Inc.) for statistical calculations. The joints were tensile sheared at a crosshead displacement rate of 2.5 mm/min at room temperature. Force-displacement curves were recorded and the failure load of the joints was determined from these curves. The shear strength values reported in this paper are the average of at least 10 samples for each set of experimental conditions.

[TABULAR DATA FOR TABLE 1 OMITTED]

RESULTS AND DISCUSSION

Surface Studies Using XPS

XPS was used to examine the effectiveness of the surface of wood veneers treated with silanes (A-1100, DCS), PA, E-43, and plasticized PVC since it provides qualitative and quantitative information about chemical functional groups and their concentration at the surface of solid. The XPS survey spectra results of wood veneers (before and after treatment) and plasticized PVC are presented in Fig. 2 and the elemental compositions of all the studied spectra are summarized in Table 1.

The untreated wood veneer [ILLUSTRATION FOR FIGURE 2A OMITTED] mainly consists of carbon and oxygen atoms. The presence of some traces of silicon atom, probably originating from inorganic compounds such as ash in wood, was also detected. The observed silicon on the surface of untreated wood veneer may also be a result of organosilicon contamination from the storage atmosphere (49). On the other hand, the spectra of plasticized PVC [ILLUSTRATION FOR FIGURE 2F OMITTED] consists of carbon, oxygen, and chlorine atoms as expected. The presence of oxygen atoms was due to the plasticizer used in the formulation of PVC (31).

The elemental surface composition of wood has been changed after treatments, indicating that the chemical reactions took place between wood veneers and chemical reagents. For example, an additional peak feature for nitrogen ([N.sub.1s]) was observed, while there was a significant increase in the amount of silicon present in the spectra of silane-treated wood veneers. The [N.sub.1s] photopeak was curve-fit and the results are illustrated in Fig. 3. It consisted of two peaks at around 401.4 eV and 399.8 eV, corresponding to -N[H.sub.3]+ and -N[H.sub.2], respectively (50-52). It may be noted that only 16.2% of the amine was in the protonated form (N[[H.sub.3].sup.+]) while the remainder was in the free amine form (-N[H.sub.2]). This result is similar and consistent with previous work with silane-treated recycled newsprint-fibers (53), which also showed a significant increase in the amount of silicon present and the evidence of a doublet on the [N.sub.1s] peak in the spectra of silane-treated newsprint-fibers.

The high-resolution spectra of wood veneers recorded for carbon also provided an additional proof that the chemical reactions took place on the surface of wood veneers. The curve fit analysis of the [C.sub.1s] spectra is illustrated in Fig. 4 and peak assignments are listed in Table 1. The [C.sub.1s] spectra of untreated and treated wood veneers consisted of three major component peaks: (C1 (C-C/C-H), C2 (C-OH), and C3 (O-C-O, C = O), respectively (54-58). The [C.sub.1s] spectra of wood veneer treated with PA showed an additional component: C4 arising from O-C = O.

The C1 atomic ratio of untreated wood (60.1%) was higher than the theoretical C1 atomic ratio of pure cellulose (0%) from the formula [([C.sub.6][H.sub.10][O.sub.5]).sub.n], indicating the presence of unoxidized carbon (C1) at the surface of wood veneer originating from lignin and extractive substances (54-58) of wood. After the treatment with silanes, the C1 atomic ratio of wood veneers increased and the concentration of oxidized carbon (C2 + C3) decreased. As a result, the oxygen-to-carbon ratio (O/C) also decreased after the treatment as shown in Table 2. This decreased O/C atomic ratio (or increased concentration of unoxidized carbon) arose from the hydrocarbon chains [(-C[[H.sub.2].sup.-]).sub.n] of silanes. Similarly, the treatment with PA and E-43 caused an increase in the concentration of unoxidized carbon (C1), and thereby, a significant decrease of the O/C ratio, as expected from compounds rich in carbon-containing groups. The increased concentration of unoxidized carbon must have originated from the aliphatic carbons of polypropylene chains of E-43 and from the carbons of the aromatic ring of PA. The decrease in O/C ratio of wood veneers following the treatment with A-1100, DCS, PA and E-43 indicates that these chemical reagents were attached to the surface of the wood veneers after a reaction and that the treated wood veneers became more hydrophobic.

Effect of Surface Treatment on Contact Angle and Surface Tension

The contact angle was measured from the wetting experiments of glycerol sessile drops on the untreated and treated wood veneers and on PVC as shown in Fig. 5. Each point in this Figure represents the average of at least five sessile drops. The treated wood veneers showed higher contact angles than the untreated one regardless of the sessile drop exposure time, implying the modification made on the surface of wood veneer. The contact angles increased from 40 [degrees] (untreated) to 70 [degrees] or higher after 60 seconds. This significant increase in the contact angle after treatment showed that the wood veneers had become more hydrophobic, as revealed by the increase of C1 (or decrease of O/C atomic ratio) in the XPS analysis.

The surface tensions of wood veneers and PVC calculated from Eq 4 are shown in Table 2. Before the treatment, the surface tension was greater than 50 mJ/[m.sup.2]. With the treated wood samples, the surface tension was reduced to 40 mJ/[m.sup.2] or lower.

Effect of Surface Treatment on the Adhesion Between PVC and Wood Veneer

The compatibility of treated wood veneers with PVC was examined by measuring the shear strength of a single lap joint. The measured shear strengths are compared, together with the surface tensions for the untreated and treated wood veneers and PVC in Table 2. It was observed that the adhesion between PVC and wood veneer laminates (or the shear strength) was improved only when the surface of wood veneer was treated with amino-silane (A-1100). Table 2 also shows the mode of failure in the joints tested. If the failure is in the adhesive or adherent, the failure is considered cohesive. If the failure is in the interface, the failure is adhesive (59). Cohesive failure within the wood veneers occurred only in the joints where wood veneers were treated with amino-silane. The remainder of the failures were all adhesive in the interface between PVC and wood veneers. Because the amino-silane treated wood veneers/PVC joints failures were wood cohesive failures, the real shear strength value of PVC/amino-silane treated wood laminates must be greater than the reported value of 3.9 [+ or -] 0.3 MPa.

An attempt has been made to correlate the surface tension and the adhesion properties in order to elucidate the mechanism responsible for the improvement with the amino-silane treated wood/PVC system. It appeared that the known necessary condition for a good adhesion, i.e., the interfacial tension matching of the two phases (3, 19, 20, 24), does not seem to be effective for wood and PVC. The treatments with dichlorodiethylsilane, phthalic anhydride, and maleated polypropylene have significantly reduced the surface energy of wood veneers close to that of PVC without enhancing the shear strength of the laminated composites (Table 2). It should be noted that, for polyolefins and maleated polypropylene-treated cellulosic fiber systems, the matching of fiber and polymer surface [TABULAR DATA FOR TABLE 2 OMITTED] tensions has been found to be an effective criterion for a good adhesion (3, 4, 13).
Table 2. The Influence of Various Surface Treatments on the
Shear Strength of Wood Veneers/PVC Joints.

Wood Veneer [[Gamma].sub.sv] O/C [Lambda] Type of
Samples mJ/[m.sup.2] MPa Failure

Untreated 54.5 [+ or -] 2.4 0.49 2.2 [+ or -] 0.2 adhesive

Treated with

A-110 35.1 [+ or -] 1.2 0.42 3.9 [+ or -] 0.3 cohesive
DCS 39.9 [+ or -] 0.8 0.34 2.1 [+ or -] 0.4 adhesive
PA 24.3 [+ or -] 1.6 0.45 2.3 [+ or -] 0.3 adhesive
E-43 22.9 [+ or -] 2.2 0.29 2.0 [+ or -] 0.3 adhesive

The surface tension and the O/C ratio of plasticized PVC are 27.8
[+ or -] 2.1 mJ/[m.sup.2] and 0.19, respectively


The experimental results suggest that other mechanisms such as acid/base interactions between aminosilane treated wood and PVC may be responsible for the improvement observed with amino-silane treatment. The improvement of adhesion may be due to the incorporation of the nitrogen-group on the surface of wood veneers that changed the electron donor/acceptor characteristics of wood (3). This was implied by the presence of the doublet in the high-resolution spectra of nitrogen (N[[H.sub.3].sup.+] and -N[H.sub.2]), as noted from the XPS high-resolution spectra of [N.sub.1s]. The aminosilane treated wood veneer would have more basic characteristics and behaved as an electron donor. The acid/base (or electron donor/acceptor) nature of amino-silane treated cellulose fibers has been characterized by inverse gas chromatography (IGC) (3). On the other hand, PVC has acidic (or electron acceptor) characteristics as determined by the IGC retention volumes of vapor probes (60). In this context, a chemical interaction might be formed at the wood/PVC interface according to Lewis acid-base theory (61): When exposed to chlorine atoms of PVC, the amino-silane treated wood veneer may chemically react and form an ionic bond with the matrix because of the highly electronegative nature of chlorine atoms of PVC (23). However, the acid/base approach that is the fundamental basis for the theory of surface tension components has been criticized as violating the thermodynamic phase rule (62, 63). Further research is required to clarify the fundamental mechanism of adhesion and the role of acid/base interactions in the PVC and wood system.

SUMMARY AND CONCLUSIONS

The effect of various surface treatments on the adhesion between wood veneers laminated with a thin PVC film has been investigated using XPS, contact angle and surface tension measurements, and lap shear test. Wood veneers were treated with silanes (amino and chloro), and phthalic anhydride, and maleated polypropylene for surface modification.

XPS results based on the elemental and functional compositions of wood veneers before and after the treatment indicated that modifications were made on the surface of wood veneer. The nature of the wood surfaces was changed from hydrophobicity to hydrophobicity as a consequence of the treatments. This was also detected by measuring the contact angle and surface tension of wood surfaces. The contact angle increased whereas the surface tension decreased after the treatment, confirming the modifications of the surface observed in the XPS analysis.

The lap shear test was found to be very effective in evaluating adhesion in the PVC/wood composites and in identifying a suitable adhesion promoter.

Amino-silane has been observed to be a suitable adhesion promoter for PVC/wood composites. The tensile shear strength of wood veneer laminated with PVC has significantly increased after this treatment. It was speculated that the incorporation of nitrogen atoms, which changed the electron donor/acceptor characteristics of wood. might be responsible for the enhancement of adhesion. Other treatments (dichlorodiethylsilane, phthalic anhydride, and maleated polypropylene) were found to be ineffective, giving similar bond strengths compared to untreated wood veneers with PVC adherent. Unlike polyolefin/wood composites, matching the surface tension of wood veneers to that of PVC has been found to be ineffective for a good adhesion between PVC and wood veneer.

ACKNOWLEDGMENTS

The authors are grateful for the generous donation of PVC materials and financial support provided by Royal Plastics Limited. Le Fonds pour la Formation de Chercheurs et l'Aide a la Recherche (Fonds FCAR) of Quebec and the University of Toronto are acknowledged for the scholarships awarded to one of the authors (L.M.M.). The authors would also like to thank Prof. A.W. Neumann and Dr. R. N. S. Sodhi for the use of the ADSA-CD and XPS equipment, as well as for their helpful discussions. Lastly, we are grateful to Ms. Z. Policova and Mr. J. McCarron for their useful technical advice during contact angle measurements and preparation of wood veneer samples, respectively.

NOMENCLATURE

C Carbon. [E.sub.b] Binding energy, eV. N Nitrogen. O Oxygen. O/C Oxygen-to-carbon atomic ratio, %. [Beta] Neumann's constant, [([m.sup.2]/mJ).sup.2]. [Theta] Contact angle, [degree]. [[Gamma].sub.1v] Liquid surface tension, mJ/[m.sup.2]. [[Gamma].sub.sv] Solid surface tension, mJ/[m.sup.2]. [[Gamma].sub.s1] Solid-liquid interfacial tension, mJ/[m.sup.2]. [Lambda] Shear strength, MPa.

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Author:Matuana, Laurent M.; Balatinecz, John J.; Park, Chul B.
Publication:Polymer Engineering and Science
Date:May 1, 1998
Words:5391
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