Surface properties and adhesion of wood fiber reinforced thermoplastic composites

Colloids and Surfaces A: Physicochem. Eng. Aspects 302 (2007) 388–395

Surface properties and adhesion of wood fiber reinforced thermoplastic composites Barun S. Gupta 1 , Isabela Reiniati, Marie-Pierre G. Laborie ∗ Washington State University, Wood Materials and Engineering Laboratory, Pullman, WA 99164-1806, USA Received 6 January 2007; received in revised form 28 February 2007; accepted 1 March 2007 Available online 6 March 2007

Abstract The surface chemistry, wettability and topography of wood fiber reinforced thermoplastic composites were evaluated using Fourier transform infrared spectroscopy, contact angle analysis and profilometry. Wood plastic composites have a low surface energy (31 mJ/m2 ), a large surface roughness R (rms) (3.5–7.4 ␮m) and a large wetting hysteresis (63–90 mJ/m2 ) that stems from surface roughness, chemical heterogeneity and viscoelastic energy dissipation. Using a 180◦ peel test the adhesion of an acrylic coating on wood plastic composites was evaluated and a positive linear correlation (R2 = 0.89) between peel load and wetting hysteresis indicated that the wetting hysteresis was a good predictor of adhesion. Furthermore, strong linear correlations were found between peel load and surface roughness for formulations using polypropylene (R2 = 0.79) and formulations using high density polyethylene (R2 = 0.97). Formulations with polypropylene, which had higher surface wood index, also developed higher peel loads. Similarly formulations that did not use a maleic anhydride polypropylene coupling agent, which had higher surface roughness and have higher molecular mobility, also developed higher peel loads. Consequently surface roughness, polarity and viscoelastic energy dissipation mechanisms were proposed to be critical factors of adhesion on wood–plastic composites explaining the parallel between wetting hysteresis and peel load. © 2007 Elsevier B.V. All rights reserved. Keywords: Wood–plastic composite; Wetting hysteresis; Surface roughness; Intrinsic adhesion; Viscoelastic dissipation

1. Introduction With an average annual growth rate of approximately 18% in Northern America and 14% in Europe, wood fiber reinforced thermoplastic composites (WPCs) are gaining increasing importance in many exterior applications including decking, fencing, railing, siding and paneling [1]. These applications would greatly benefit from the ability to bond or coat WPCs. To that end a fundamental understanding of the surface characteristics and the adhesion properties of WPCs is needed. In adhesive systems, the practical work of adhesion has contributions from the intrinsic adhesion but also from a viscoelastic

∗ Corresponding author at: Department of Civil and Environmental Engineering, Washington State University, Wood Materials and Engineering Laboratory, Pullman, WA 99164-1806, USA. Tel.: +1 509 335 8722; fax: +1 509 335 5077. E-mail addresses: [email protected] (B.S. Gupta), [email protected] (M.-P.G. Laborie). 1 Current address: Department of Forest Resources, Pennsylvania State University, State College, PA 16804, USA. Tel.: +1 814 865 3002; fax: +1 814 865 3725.

0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.03.002

energy dissipation factor. The intrinsic adhesion arises from interfacial forces between the adhesive and the adherent and therefore can be related to surface energies and wettability. Five adhesion mechanisms are generally proposed for intrinsic adhesion, including mechanical interlocking, diffusion theory, chemical bonding, electronic and adsorption theories [2]. For any adhesive bond then several adhesion mechanisms often contribute to the intrinsic adhesion and it is difficult to discern the respective contribution of each mechanism. Naturally, the structure and properties of both the adherent and the adhesive dictate the occurrence of each of these mechanisms. When WPCs are used as substrates, the adhesion mechanisms may be particularly intricate owing to the complex composition of WPCs. Wood, generally 60% (w/w) of WPC formulations, is a hydrophilic porous composite of cellulose, lignin and hemicellulose polymers that are rich in functional groups such as hydroxyls. Wood also has a critical surface energy in the 40–60 mJ/m2 range [3]. Mechanical interlocking, adsorption theory and chemical bonding are commonly proposed adhesion mechanisms on wood substrates. On the other hand polyolefins that constitute 30% (w/w) of WPCs, have

B.S. Gupta et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 302 (2007) 388–395

very low surface energy (30 mJ/m2 ), are hydrophobic, devoid of functional groups and develop smooth surfaces [4]. As a result polyolefins are notoriously difficult substrates to adhere to [4]. Additives such as lubricants and coupling agents, although present in small amount in WPCs, may further complicate the adhesion properties of WPCs as they are low molecular weight, mobile compounds likely to migrate to the surface. Owing to the heterogeneity of WPCs then, it is likely that adhesion mechanisms on WPCs are complex and difficult to unravel. Furthermore, studies on the surface and adhesion properties of WPCs are scarce. In a study of acrylic coatings on wood fiber reinforced polyethylene composites the temperature at which the WPCs are molded was found to govern the amount of exposed wood fiber and therefore the wettability and peel strength of the coating [5]. The coating adhesion on cold molded WPCs was then four-fold higher than that on hot-molded WPCs. For cold molded WPCs, corona treatment further increased the peel force by another four folds. The corona treatment acted by increasing the surface energy and improving the wettability of the substrate [5]. Another study evaluating surface activation with chromic acid, flame treatment and water immersion on WPCs reported that surface oxidation and wood swelling were favorable to the adhesion of an epoxy resin [6]. While these studies provide some insight and practical guidelines on the adhesion to WPCs, a comprehensive understanding of adhesion phenomena on WPC surfaces is still lacking. In this context, this paper aims at providing a fundamental understanding of the surface and adhesion properties of WPCs. In particular the adhesion strength of a water-based coating on a series of WPCs is evaluated in light of the surface properties of WPCs. Relationships between adhesion strength and surface properties then provide a framework upon which adhesion mechanisms are discussed. This knowledge is significant to understand the adhesion of complex heterogeneous multicomponent composites and also has direct applications for the WPC industry. 2. Materials and methods 2.1. WPC sample preparation Sixty mesh pine (Pinus spp.) and maple (Acer spp.) flour was obtained from the American Wood Fibers Association. High density polyethylene, HDPE, having a melt index of 0.40 g/10 min and density 0.95 g/ml, and isotactic polypropylene, PP, with a melt index of 4.0 g/10 min at 230 ◦ C were obtained from Innovene Inc. and Equistar, respectively. In addition, maleic anhydride functionalized polypropylene, MAPP, with a density of 0.93 g/ml and free maleic anhydride content <0.9% was provided by Honeywell. A commercial lubricant (OP100) and talc from Honeywell and Luzenac America Inc. respectively were used in all formulations. A 23 factorial design was utilized to design eight WPC formulations so that the impact of polymer selection (HDPE versus PP), wood species selection (pine versus maple) and coupling agent could be evaluated (Table 1). The materials were first dry blended. Extrusion was then conducted on a 35 mm intermeshing twin screw extruder (Cincinnati Milacron) operating at a 5–8 rpm screw

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Table 1 Design of wood plastic composites formulations Polyolefin (wt%)

Wood species (wt%)

Coupling agent (wt%)

HDPE (33.8) HDPE (33.8) PP (33.8) PP (33.8) HDPE (36.1) HDPE (36.1) PP (36.1) PP (36.1)

Pine (59) Maple (59) Pine (59) Maple (59) Pine (59) Maple (59) Pine (59) Maple (59)

MAPP (2.3) MAPP (2.3) MAPP (2.3) MAPP (2.3) – – – –

Lubricant (1 wt%) and talc (3.9%) were added in all the formulations.

speed and at 3.45–5.52 MPa melt pressure and equipped with a water-spray cooler. The barrel and die temperatures were 163 ◦ C and 171 ◦ C for HDPE formulations and 185–193 ◦ C and 185 ◦ C for PP formulations, respectively. Rectangular sections (10 mm × 38 mm) were thus extruded and small specimens (1 mm × 9 mm × 36 mm) were sliced and milled from the center of the WPC cross-sections. In other words, the 1 mm thick WPC specimens had faces (9 mm × 36 mm) that came from the bulk WPC lumber. In machining specimens from the bulk lumber, it was intended to sample uniform, homogeneous surfaces whose properties would be independent of processing. For each specimen, the 9 mm × 36 mm surfaces were refreshed and cleaned according to ASTM D2093 prior to surface characterization and adhesion test [7]. Specifically, sanding with 320 grit sandpaper was followed by wiping with lint free cotton cloth and washing in acetone, after which the samples were dried for 1 h period at 40 ◦ C and stored in a desiccator with calcium sulfate overnight. All the surface characterizations were performed the day after sample conditioning ensuring that no significant aging would take place for any formulations. For each formulation four specimens were prepared for the series of surface characterization and another four specimens were prepared for acrylic coating application and peel adhesion test. For both surface characterization and the adhesion test, neat polyolefin and neat wood specimens were prepared similarly and evaluated for comparison. 2.2. Surface characterization techniques Surface chemistry was first characterized using attenuated total reflection Fourier transform infrared spectroscopy (ATRFTIR). A Thermo Nicolet Continuum model, equipped with a MCT-A detector, ZnSe IRE and using an incident angle of 45 ± 5◦ was used. Five hundred and sixty scans were acquired for each specimen in the IR region with a 4 cm−1 resolution. The spectra were analyzed with the Omnic 5.0 software. In particular, an index for surface cellulosic hydroxyl groups was obtained by normalizing the cellulosic hydroxyl peak intensity at 1023 cm−1 to the polyolefinic νC–H stretching peak intensity at 2912 cm−1 [8]. This method provides an approximate measurement of the surface wood content (1–2 ␮m deep) that may relate to the surface polarity and also surface hydrophilicity [8]. As such the surface wood index may be an important parameter of the adhesion with a polar, water-based acrylic coating.

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Immediately after ATR-FTIR measurements, the specimens were characterized by dynamic contact angle analysis (DCA) for wettability. A Cahn 322 model was used and the probe liquid was deionized water. The DCA stage speed was 194 ␮m/s. Using the Whilelmy plate principle (Eq. (1)) the advancing and receding water contact angles, θ a and θ r , were calculated by linear extrapolation of the advancing and receding force–displacement curves: F = γl p cos θ

(1)

In Eq. (1), F is the force at zero immersion, γ l the surface tension of the probe liquid, 72.8 mJ/m2 for water, p the wetted sample perimeter of sample and θ is the contact angle. The wetting hysteresis, W, was also computed from the advancing and receding contact angles, θ a and θ r : W = γl (cos θr − cos θa )

(2)

For some selected WPC formulations, critical surface tension (γ c ) was also determined with Zisman plots. In this case, a series of probe liquids consisting of 40%, 50%, 60% and 80% (w/w) acetic acid solutions in water were prepared to span a liquid surface energy from approximately 32 mJ/m2 to 42 mJ/m2 as measured with a glass slide [3]. For each probe liquid, a linear regression of the measured cos θ a against the liquid surface tension was extrapolated to zero contact angle at which γ c was defined [9]. Finally, surface roughness was evaluated on a SPN Technologies stylus profilometer. Scans, 20 mm long, were conducted at a speed of 20 mm/min and the root mean square roughness, R (rms), was recorded. 2.3. Adhesion test A separate set of four replicate specimens per formulation was prepared for the adhesion test. Adhesion was evaluated on a water-based white acrylic coating formulated by Drew Paints, Portland, OR. The acrylic coating was applied on the specimens

using a wire wound draw down bar (#32). A strip of cotton bandage, 9 mm wide, was then placed on the wet coated surfaces according to the procedure described in ASTM D6083 [10]. The assembly was then cured at room temperature (23 ◦ C) for 1 h, after which a second layer of coating was applied and cured for 48 h. The free end of the bandage was wrapped with a mask tape and a 180◦ peel test was conducted at a crosshead speed of 20 mm/min by peeling the bandage from the surface [11]. The peel test was performed on an Instron testing machine, model 4426, using a tensile grip. Peel load (N) was reported against 103 mm specimen width. 2.4. Statistical analyses An analysis of variance, ANOVA, was conducted at an α level of 0.1 to evaluate the effect of polymer selection, wood species selection and coupling agent on all the properties measured namely wood index, advancing contact angle, receding contact angle, wetting hysteresis, surface roughness and peel load. 3. Results and discussion 3.1. Surface properties of WPCs Table 2 summarizes the wood index (OH/CH), dynamic wettability (θ a , θ r , and wetting hysteresis), surface roughness of WPCs and the adhesion strength (peel load) of the water-based acrylic coating to WPCs. Data for the neat components [12–14] are also included in the table along with the formulation parameters that significantly affected the measured property. The surface wood index varies greatly among WPCs between 1.20 ± 0.24 and 2.80 ± 0.01 and it depends on the formulation. Specifically, PP formulations have on average a significantly higher surface wood index (2.37 ± 0.81) than HDPE formulations (1.83 ± 0.65). So do formulations with maple (2.41 ± 0.83) compared to those with pine (1.81 ± 0.60). These differences

Table 2 Surface and adhesion properties of eight WPC formulations Formulation

OH/CH

θ a (◦ )

θ r (◦ )

HDPE/pine/MAPP HDPE/maple/MAPP PP/pine/MAPP PP/maple/MAPP HDPE/pine HDPE/maple PP/pine PP/maple Maple Pine PP HDPE

2.32 ± 0.15 1.52 ± 0.05 1.90 ± 0.03 2.56 ± 0.08 1.20 ± 0.24 2.75 ± 0.04 2.18 ± 0.26 2.80 ± 0.01 3.88 ± 0.77 2.45 ± 0.05 0.0 0.0

95 ± 5 95 ± 5 102 ± 6 101 ± 4 99 ± 3 98 ± 3 99 ± 2 105 ± 1 75 – 95a 87b

41 ± 35 ± 29.5 ± 24 ± 22.4 ± 20 ± 24 ± 18 ± – – – –

Significant factors (level giving the highest value is noted)

PP, Maple

PP

MAPP, HDPE

a b

From Ref. [14]. From Ref. [12].

3 7.5 9 8 7 2 13 9

Wetting hysteresis (mJ/m2 )

Roughness RMS (␮m)

Peel Load (N/m)

67 ± 63 ± 82 ± 80 ± 79 ± 78 ± 85 ± 90 ± – – – –

5.8 (±1.7) 5.1 (±2.0) 3.5 (±1.1) 4.5 (±1.6) 6.4 (±0.9) 7.4 (±2.2) 5.8 (±1.7) 7.3 (±1.2) – – – –

177 ± 168 ± 232 ± 249 ± 218 ± 217 ± 290 ± 309 ± 524 ± – 126 ± 48 ±

No MAPP

No MAPP, PP

15 9 16 8 2 3 12 2

No MAPP, PP

21 13 9 9 16 23 24 20 64 35 1

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may be explained by the fact that maple has a higher OH/CH ratio than pine (Table 2) and also it may be that HDPE coats wood particles better than PP, leaving less wood exposed. The more flexible HDPE may crystallize on wood fibers more easily than PP thereby wrapping wood particles better and leaving less exposed wood on the surface. In any case, the ATR-FTIR results suggest that WPCs display some hydrophilicity within the 1–2 ␮m of the surface. Surface wettability, on the other hand, points to WPC surface hydrophobicity, with a high θ a (95 ± 5◦ to 105 ± 1◦ ), similar to that of plastics [14] and higher than that of wood [15]. The γ c obtained from the Zisman plot confirms that WPCs are low surface energy materials (31.5 mJ/m2 ), much similar to polyolefins (31.0 mJ/m2 ) [4] and distinct from maple (46.6 mJ/m2 ) or pine (58.5 mJ/m2 ) [15]. A practically small but significant difference in θ a is also observed among formulations (Table 2); PP formulations have a slightly higher θ a (103 ± 6◦ ) than HDPE formulations (97 ± 6◦ ), which likely reflects differences between neat PP (95◦ ) and HDPE (87◦ ) [12,14]. On the other hand the θ r for WPCs is low (18 ± 9◦ to 41 ± 3◦ ) at least compared to that for neat polyolefins (89◦ ) [13] pointing to the hydrophilicity of the surface upon dewetting. This is expected since for composite surfaces, θ a is dominated by the hydrophobic constituent, while θ r reflects the polarity or hydrophilicity of the material [16]. Accordingly, PP formulations, which were found to have a higher surface wood index and surface hydrophilicity than HDPE formulations, display a lower θ r (24 ± 9◦ ) than HDPE formulations (30 ± 5◦ ) (Table 2). Furthermore, formulations without MAPP (21 ± 8◦ ) were found to have a significantly lower θ r than those with MAPP (32.4 ± 7◦ ), which apparently is not caused by a difference in surface polarity (Table 2). The water receding angle, as it reflects the hydrophilicity of heterogeneous surfaces, is also greatly affected by the surface roughness [14]. Upon dewetting on a rough surface, water may be easily trapped in the surface asperities resulting in an apparently more hydrophilic surface and therefore a lower receding contact angle. Accordingly, the lower receding contact angle observed in MAPP-devoid formulations may simply arise from higher surface roughness. The surface roughness data support this hypothesis (Table 2). WPC surfaces are rough with an RMS roughness between 3.5 ± 1.1 ␮m and 7.4 ± 2.2 ␮m depending on the formulation. Formulations without MAPP have higher surface roughness (6.7 ± 1.5 ␮m) than those with MAPP (4.7 ± 1.6 ␮m). In the absence of coupling agent between wood and plastic, the lower interfacial adhesion may result in removal of larger material chunks upon sanding and therefore higher surface roughness. Higher roughness in MAPP-devoid formulations is therefore in line with the lower receding contact angle measured for these formulations. As a reflection of surface chemical and topographical heterogeneity, the wetting hysteresis of WPCs is large and also varies significantly among formulations from 63 ± 9 mJ/m2 to 90 ± 2 mJ/m2 (Table 2). Selection of the polymer matrix and presence of MAPP both significantly impact the wetting hysteresis. Namely, formulations without MAPP exhibit higher wetting hysteresis (83 ± 8 mJ/m2 ) than those with

391

Fig. 1. Relationship between surface OH/CH ratio and water wetting hysteresis of WPCs.

MAPP (73 ± 14 mJ/m2 ). Similarly PP formulations exhibit higher hysteresis (85 ± 11 mJ/m2 ) than HDPE formulations (72 ± 11 mJ/m2 ). As wetting hysteresis is generally ascribed to chemical and physical heterogeneities [17], both observations can be rationalized by recalling the impact of plastic selection and MAPP presence on the surface OH/CH ratio and the surface roughness respectively. First, PP formulations were found to have higher OH/CH ratio, i.e. higher wood fiber exposed on the surface and also lower receding contact angle than HDPE formulations (Table 2). With PP formulations having higher surface polarity and thus chemical heterogeneity the wetting hysteresis in these formulations is expected to be greater. Second, formulations without MAPP were found to have higher surface roughness and lower receding contact angle, which is in line with the larger wetting hysteresis observed for these formulations (Table 2). These observations warranted the need to directly probe the impact of surface chemistry and surface roughness on wetting hysteresis. When wetting hysteresis was evaluated against OH/CH ratio, no clear relationship could be detected albeit a weak ascending trend may have been present (Fig. 1). This suggested that in WPCs surface polarity may contribute little if at all to the wetting hysteresis. To further probe the relationships between surface chemistry and wettability, a fractional coverage of hydrophobic material over a hydrophilic surface was computed for each formulation from the OH/CH values obtained for WPCs and its respective wood (hydrophilic) and plastic (hydrophobic) constituents. This is of particular interest because the estimated fractional coverage-contact angle datapoints can then be compared with the prediction of the theoretical Chappuis model [18,19]. While wetting models are well established for rough surfaces [20,21], the Chappuis model is relevant for WPCs because it focuses on the effect of chemical heterogeneity in hydrophobic/hydrophilic materials, a likely model surface for wood (hydrophilic) plastic (hydrophobic) composites [16,18]. Namely, the Chappuis model (Eq. (4)) describes the dynamic contact angles of chemically heterogeneous surfaces in terms of the advancing and receding contact angles for the hydrophilic material (θ a1 and θ r1 ) and for the hydrophobic material (θ a2 and

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Fig. 2. Experimental and theoretically predicted dynamic contact angles of WPCs as a function of surface fractional coverage of the hydrophobic (plastic) component.

θ r2 ) as: θa = θ2a ,   cos θr = 2(1 − α) cos θ1r + (1 − 2(1 − α) cos θ2r )

(4)

The Chappuis model thus predicts that the receding contact angle increases significantly with higher hydrophobic fractional coverage while the advancing contact angle remains constant near that of the hydrophobic material [16]. In Fig. 2, advancing and receding contact angles for WPC surfaces of various fractional coverages of hydrophobic material are compared to the predictions of the Chappuis model using θ 1r = 10◦ and θ 2a = θ 2r = 90◦ for the wood and the plastic, respectively [3,13]. In Fig. 2, the advancing and receding contact angles predicted with the Chappuis model have been adjusted by +10◦ and −10◦ respectively throughout the compositional range to further account for the effect of roughness [14]. While arbitrary in magnitude, these adjustments were deemed reasonable for the only purpose of evaluating trends in surface composition-contact angle data. Irrespective of the estimated fractional coverage of hydrophobic, the advancing contact angle remains rather constant at 100 ± 5◦ , whereas the receding contact angle tends to increase with fractional coverage spanning a range from 18◦ to 41◦ (Fig. 2). As a result, wetting hysteresis appears higher for low fractional coverage of hydrophobic material corresponding to high exposed wood content. Although there is some scatter in the data, probably owing to confounding effects of factors such as roughness, these general trends appear to be consistent with the prediction of the Chappuis model (Fig. 2). Consequently, chemical heterogeneity may indeed contribute to the variations in the dynamic contact angles and therefore wetting hysteresis that are observed among formulations. While surface roughness is also likely to contribute to the greater wetting hysteresis in MAPP-devoid formulations, no clear trend could be detected between the two properties in the series of WPCs, suggesting again that the effect of surface roughness is obscured by other factors. Other possible factors of the wetting hysteresis include molecular rearrangements dur-

ing the measurement [22] and viscoelastic dissipation in the wetting ridge [23,24]. Molecular rearrangements are unlikely in this case considering the briefness of the DCA experiment. On the other hand, viscoelastic dissipation at the wetting ridge may well contribute to the wetting hysteresis [23,24]. It is indeed well established for soft substrates such as plastics that viscoelastic dissipation at the wetting ridge and wetting hysteresis occur all the more that the material has a high damping ability or loss modulus [23,24]. The addition of the MAPP coupling agent in WPC formulations is also known to decrease the loss modulus and damping of the composite [25]. The contribution of viscoelastic dissipation to the wetting hysteresis is then consistent with the observation of greater wetting hysteresis in MAPP-devoid formulations. Altogether WPC surfaces can be described as chemically and topographically heterogeneous surfaces, which are highly hydrophobic upon wetting with water and hydrophilic upon dewetting. WPCs therefore exhibit a large wetting hysteresis which results from both surface chemical and topographical heterogeneity and has also contribution from viscoelastic energy dissipation mechanisms. The choice of the plastic matrix, PP or HDPE, influences all of the surface properties evaluated, surface chemistry, wettability and roughness. With PP as the plastic matrix, WPCs display higher wood index as measured on a 1–2 ␮m depth and higher wetting hysteresis. Adding MAPP as a coupling agent in WPCs decreases the surface roughness and wetting hysteresis. With this understanding of the surface properties of WPCs, the adhesion of the acrylic coating on WPCs can be discussed next. 3.2. Adhesion of the acrylic coating to WPCs With peel loads ranging from 168 ± 13 N/m to 309 ± 20 N/m, the adhesion of the water-based acrylic coating on WPCs is intermediate to that measured on neat polyolefins (48 ± 1 N/m) and on wood (524 ± 64 N/m) (Table 2). Furthermore, as previously observed for the receding contact angle, surface roughness and wetting hysteresis, both the selection of the polymer and the presence of MAPP have a significant impact on the coating adhesion (Table 2). Higher peel loads (270 ± 45 N/m) are measured on PP formulations than on HDPE formulations (195 ± 37 N/m). Also, higher peel loads are measured on MAPP-devoid formulations (258 ± 54 N/m) than on formulations containing MAPP (207 ± 44 N/m). To gain an understanding of the adhesion mechanisms occurring between WPC surfaces and the acrylic coating, relationships between surface chemistry, wettability and coating adhesion were examined. While wetting hysteresis has long been anticipated to relate to adhesion [26,27], several studies have demonstrated the correlation between wetting hysteresis and adhesion in some bonded systems [13,28,29]. Such a correlation is expected because wetting hysteresis not only portrays intermolecular forces and chemical restructurations at the liquid/solid interface [30] but also surface roughness and viscoelastic energy dissipation [23,24], all of which directly impact the practical adhesion. For the WPC surfaces and the water-based acrylic coating used in this study a strong linear correlation (R2 = 0.89) between water

B.S. Gupta et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 302 (2007) 388–395

393

Fig. 5. Dependence of the coating adhesion strength and wetting hysteresis on receding contact angle. Fig. 3. Relationship between adhesion strength of an acrylic coating on WPCs and water wetting hysteresis of WPCs.

wetting hysteresis and peel adhesion is observed (Fig. 3). In other words, 89% of the variations in the adhesion strength of the coating can be explained by the variations in the wetting hysteresis of WPCs with water. The occurrence of a strong linear relationship between water wetting hysteresis and peel load has important implications for the development of WPCs with improved adhesion properties. First, wetting hysteresis can be used as a rapid predictor of coating adhesion. Second, in heterogeneous surfaces such as WPCs wetting hysteresis is a strong function of the details of the coverage rather than only fractional coverage [31]. Simple manipulations of the structure of the wood/plastic coverage on WPCs via changes in the component particle size for instance may then be effective at enhancing the adhesion properties of WPCs. Interestingly, a similar correlation between wetting hysteresis and peel load has been previously observed on aged polyolefins [13]. For aged polyolefins though, the positive correlation between contact angle hysteresis and peel adhesion was concurrent with a decrease in both advancing and receding contact angles and could therefore be ascribed to surface oxidation and improved wettability [13]. In the present study on WPCs, the correlation between wetting hysteresis and peel adhesion is concurrent with a decrease in receding contact angle but

Fig. 4. Dependence of the coating adhesion strength and wetting hysteresis on advancing contact angle.

also with an increase in advancing contact angle (Figs. 4 and 5). In fact, both wetting hysteresis and peel load increase linearly (R2 = 0.72 and R2 = 0.66) with θ a and also decrease linearly (R2 = 0.67 and R2 = 0.55) with θ r (Figs. 4 and 5). Such correlations can be understood when recalling that surface chemical heterogeneity, roughness and also viscoelastic dissipation are all contributing to the wetting hysteresis and could also impact the peel load. When evaluating the peel load as a function of surface roughness, the impact of surface roughness on adhesion becomes obvious (Fig. 6). Peel load increases linearly with surface roughness within series of WPCs with the same polymer matrix (R2 = 0.79 for HDPE and R2 = 0.97 for PP). This correlation is easily explained by the fact that surface roughness provides a greater area of contact for interfacial adhesion and may also facilitate adhesion mechanisms such as mechanical interlocking [2]. However surface roughness need not contribute to the intrinsic adhesion, it may also contribute to the dissipative term of the practical work of adhesion [32]. Indeed surface roughness also causes irregular stress concentrations, which could ease crack initiation and lead to lower adhesion strength. On the other hand, high stress concentration could also interfere with

Fig. 6. Dependence of the coating adhesion strength on surface roughness.

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4. Conclusions

Fig. 7. Dependence of coating adhesion strength.

smooth crack propagation, leading to the deformation of larger volumes of material and increased energy loss in the case of plastic materials [33]. Observations on polyolefins support that the latter mechanism may also be occurring in WPCs [33–35]. Evaluating the adhesion of a series of ethylene vinyl acetate (EVA) copolymers to aluminum, Bistac et al. attributed the correlation between contact angle hysteresis and peel adhesion to surface roughness and viscoelastic dissipation [13,29]. On soft solid surfaces, viscoelastic dissipation may occur at the wetting ridge inducing some wetting hysteresis along with an increase in practical adhesion via the viscoelastic energy dissipation term [23,24]. For WPCs, the contribution of viscoelastic dissipation to the peel strength is further supported by the observation that for a selected polymer matrix, those formulations without MAPP, which have the highest peel load, also have the greatest molecular mobility as evidenced from their loss moduli [25]. It is therefore clear that the surface roughness and viscoelastic energy dissipation mechanisms play a strong role in the adhesion of an acrylic coating to WPCs. The surface chemistry and surface polarity may also contribute to the adhesion of the acrylic coating to WPCs. Note indeed in Fig. 6 that PP formulations all display higher peel load than HDPE formulations, which could potentially be traced back to the differences in wood index and thus the surface polarity between PP and HDPE formulations (Table 2). In Fig. 7 the peel load dependency on the surface OH/CH ratio or wood index is examined. While no clear relationship between the surface wood index and peel load exists, may be owing to the distinct sampling depths of the measurement techniques, there is an overall ascending trend. Surfaces with a high wood index are also those with a high peel load. Therefore, surface chemistry likely plays a role in the adhesion properties of WPC surfaces; however it is not as straightforward as that of surface roughness. Altogether surface roughness likely contributes greater interfacial area, mechanical interlocking and also viscoelastic dissipation mechanisms to the overall adhesion of the acrylic coating on WPCs. Surface polarity also contributes to the adhesion properties of WPCs with an acrylic coating, likely via polar interactions.

The surface chemistry, dynamic wettability and roughness of a series of wood thermoplastic polymer composites was evaluated and related to the adhesion with a water-based acrylic coating. Wood plastic composites were found to be highly hydrophobic, low surface energy materials similar to polyolefins albeit having a high level of heterogeneity as evidenced from the low receding water contact angle and the large wetting hysteresis. In particular formulations using polypropylene as the thermoplastic matrix and no maleic anhydride polypropylene coupling agent exhibited the greatest wetting hysteresis. Formulations without coupling agent also displayed a greater surface roughness further explaining the larger wetting hysteresis for these formulations. The peel strength of a water-based acrylic coating to WPCs was intermediate to that on neat polyolefins and on wood and was also higher for those formulations having polypropylene as the matrix and no coupling agent. A strong positive correlation was found between the wetting hysteresis of WPCs with water and the adhesion of the acrylic coating. Surface roughness, chemical heterogeneity and viscoelastic energy dissipation were likely the underlying parameters of both wetting hysteresis and the peel strength of the acrylic coating. Surface roughness was proposed to enhance intrinsic adhesion by providing greater interfacial area and some mechanical interlocking mechanism and by participating to the viscoelastic dissipation factor in the practical adhesion. Surface chemical heterogeneity as measured by the surface wood index was also an important factor of both the wetting hysteresis and the peel strength of the acrylic coating, suggesting the role of polar interactions. For wood plastic composites then, wetting hysteresis can be used as a good predictor of adhesion properties. The structure of the plastic/wood coverage and the surface roughness could be manipulated to optimize adhesion. Acknowledgements The authors would like to thank Kieffer Tarbell from Drew Paints, Portland, OR, for formulating and supplying the acrylic coating used in this study. This work was sponsored by the Office of Naval Research, under the direction of Mr. Ignacio Perez, under Grant N00014-03-1-0949. References [1] C. Clemons, Wood–plastic composites in the United States—the interfacing of two industries, Forest Prod. J. 52 (2002) 10–18. [2] A.J. Kinloch, Adhesion and Adhesives: Science and Technology, Chapman & Hall, London, New York, 1987. [3] D.J. Gardner, N.C. Generalla, D.W. Gunnells, M.P. Wolcott, Dynamic wettability of wood, Langmuir 7 (1991) 2498–2502. [4] R.A. Ryntz, Adhesion to Plastics: Molding and Paintability, Global Press, 1998 [S.l.]. [5] A. Akhtarkhavari, M.T. Kortschot, J.K. Spelt, Adhesion and durability of latex paint on wood fiber reinforced polyethylene, Prog. Org. Coat. 49 (2004) 33–41. [6] W.M. Gramlich, D.J. Gardner, D.J. Neivandt, Surface treatments of wood–plastic composites (WPCs) to improve adhesion, J. Adhes. Sci. Technol. 20 (2006) 1873–1887.

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