Mechanical performance of polyvinyl acetate (PVA)-based biocomposites

Mechanical performance of polyvinyl acetate (PVA)-based biocomposites

13

A. Kaboorani, B. Riedl, Université Laval, Québec, QC, Canada

13.1 Introduction 13.1.1 Green chemistry Green chemistry, also known as sustainable chemistry is, among other things, the design of chemical products and processes to prevent potential pollution sources and use renewable materials. This also includes avoiding potential environmental contamination via spills, leaks, and other types of accidents. By using environmentally friendly materials, the risk for contaminating the environment is substantially reduced if an accident should occur. It is for this reason that it is important to develop a green ideology and methodology when pursuing the synthesis of compounds (Lech, 2008). Wood transformation is inherently applying several principles of green chemistry and green engineering, one of which is the use of renewable rather than depleting processes. Material and energy inputs should be renewable rather than depleting. Dealing with a renewable material such as wood, industry is applying this principle to a large extent and it should secure this position in the future. Although wood is known as a sustainable material, adhesives and coatings that are used with wood are not sustainable and are a source of concern. In this chapter, principles of green chemistry and green engineering are considered in developing polyvinyl acetate (PVA) nanocomposites with an application in wood adhesives.

13.1.2 A brief history of PVA PVA is a colorless, usually nontoxic thermoplastic adhesive prepared by the polymerization of vinyl acetate. PVA was discovered in 1912 by Dr. Fritz Klatte in Germany. It is one of the most widely used water-dispersed adhesives. PVA is made up of a water-based emulsion of a widely used type of glue, referred to variously as wood glue, white glue, carpenter's glue, school glue, or PVA glue. PVA is largely used in glass fiber-­ reinforced plastics to improve the stress and antishrink properties. It is also used in automobile headlights to promote their gloss performance. In addition, PVA may be added to cement/concrete where it can improve the water-resistance properties. Because PVA is a emulsion, not a true water solution, once the film is dry, it is rather hydrophobic. PVA setting and curing is accomplished by the removal of water due to evaporation or absorption into a substrate. PVA adhesives produce clear, hard films Biocomposites: Design and Mechanical Performance. http://dx.doi.org/10.1016/B978-1-78242-373-7.00009-3 Copyright © 2015 Elsevier Ltd. All rights reserved.

348

Biocomposites: Design and Mechanical Performance

that have good weather resistance and withstand water, grease, oil, and petroleum fuels. Additional properties are high initial tack, almost invisible bond line, softening at 30–45 °C, good biodegradation resistance, poor resistance to creep under load, and low cost. PVA adhesives and copolymers are also used as hot-melt adhesives, sealants, in fabric finishing, plastic wood, and inks.

13.1.3 Case study: PVA as a wood adhesive Recently there has been a strong tendency to use so-called green materials for industry. The tendency has forced industry to look for renewable materials with the least impact on the environment. One of the industries that can benefit from this trend and subsequently experience significant and constant growth is the wood industry. Wood can provide the market with the properties that cannot be offered by other materials. Wood has a great potential to become the dominant material for the construction industry. As a material for construction, wood has unique properties, such as high strength, flexibility, fire resistance, durability, insulation, and a low carbon footprint. Introduction and development of engineered wood products (EWPs), which can be produced in different shapes and sizes, has opened new markets for the wood industry and has expanded current markets. Adhesives play an important role in the performance of EWPs. Production of EWPs with high performance needs durable adhesives. Commercial durable wood adhesives contain formaldehyde, causing concerns about health during the production and service of EWPs. The wood industry is under increasing pressure to eliminate formaldehyde from its products; hence, the wood industry is looking for alternatives. PVA is a good alternative to replace some wood adhesives containing formaldehyde. PVA is a linear and thermoplastic polymer. It is water-soluble, biodegradable with excellent chemical resistance, and has no toxic effect on humans. As a wood adhesive, utilization of PVA is very simple and its curing does not need high temperatures. PVA has three main drawbacks: low resistance to water and humidity, poor performance at elevated temperatures, and high susceptibility to creep. The drawbacks stem from the structure of PVA adhesive. PVA is a linear amorphous polymer with a weak polar interaction among the molecular chains, which results in a relatively low glass transition temperature (Tg). Some research projects have been targeted at modifications of PVA in order to improve its performance. The modifications can be divided in two main groups: (1) Copolymerizing vinyl acetate with more hydrophobic monomers or functional monomers (Zhou, 1991; Cai, 1997; Chen, 1996) and (2) blending PVA with other adhesives or hardeners (Lu, 1996; Comyn, 1997; Wang, 1999; Qiao et al., 2000; Huang et al., 2002; López-Suevos and Frazier, 2006; Kim and Kim, 2005a,b). Using the abovementioned strategies could increase some properties of PVA at the expense of reducing some other properties. Moreover, some additives are so acidic that they can damage wood subtracts, finally affecting the overall performance of wood joints. For instance, copolymers of vinyl acetate and butyl acrylate or ethylene can increase water resistance and toughness of the adhesive, but will reduce its tensile modulus and stiffness, especially at elevated temperatures (Qiao and Easteal, 2001).

Polyvinyl acetate (PVA)-based biocomposites

349

As the cost of adhesives covers a substantial part of the total production cost of plants producing finger-jointed lumber, any attempt to boost performance of relatively cheap PVA adhesive at high temperatures could enhance the use of wood components in buildings. The properties of adhesives can be modified or adjusted by adding several compounds. The inclusion of tackifiers, adhesion promoters, and fillers to adhesives is commonly done to alter the performance of the adhesives with respect to specific applications (FerrandizGomez et al., 1996; Pastor-Sempere et al., 1995; Poh and Firdaus, 2010; Macia-Agullo et al., 1992). Conventional compounds sometimes modify or adjust some properties of the adhesives at the expense of the other properties. Meanwhile the extent of improvements achieved by conventional compounds is limited in some cases. The introduction of nanotechnology has opened an opportunity for the adhesives industry to develop a new generation of adhesives. The incorporation of nanoparticles into a polymer matrix can lead to a simultaneous improvement of different material properties (Wetzel et al., 2003; Cho et al., 2006). The development of nanoparticle-­ reinforced adhesive materials is presently one of the most explored areas in materials science and engineering. The exceptional properties of nanoparticles have led to widespread research in this area. Nanofillers provide many advantages over classical microreinforcements for adhesive materials; for example, they allow thin bond lines and consequently lower the risk of embrittlement within the bulk adhesive material, resulting in improved adhesive tensile strength (FerrandizGomez et al., 1996). Nanocomposites, with dispersed nanoparticles, have been studied extensively due to their capability to improve mechanical, physical, thermal, and barrier properties with very low nanoparticles loading of 1–5 wt.% (Ji et al., 2003; Zhang and Archer, 2004; Ash et al., 2004). For such composites, montmorillonite (MMT) and other clay nanoparticles have been used by many researchers (Pegoretti et al., 2004; Lee and Han, 2003; Chang et al., 2002; Ma et al., 2004; Fornes and Paul, 2004).With a structure of stacked platelets and one dimension of the platelet in the nanometer scale, MMT has a high aspect ratio and specific surface when exfoliated. If the platelets are dispersed properly, its nanosize can provide a significant amount of interface between the clay and the matrix resin with only a small weight percentage of MMT, thus contributing to the excellent mechanical and physical properties of the nanocomposites. A new generation of polymeric-based materials have been introduced by using nanotechnology. During the infancy period of nanotechnology, inorganic nanomaterials like nanoclay and metal oxide nanoparticles (ZnO, ZrO2, TiO2, SiO2, and Al2O3) were used. As the renewability and, more importantly, the health risk of these materials were questioned, a great deal of research was directed toward exploring the use of alternate organic nanoparticles. The result of the research was the introduction of nanocelluloses. Cellulose has a natural nanostructure. Different methods are used to explore the nanostructure of cellulose, leading to the introduction of various nanocelluloses; namely, nanocrystalline cellulose (NCC) and microfibrillated cellulose (MFC). NCCs, also known as whiskers, consist of rod-like cellulose crystals with widths of 5–70 nm and lengths between 100 nm and several micrometers. NCC is obtained by the acid hydrolysis of cellulose under conditions where the amorphous regions are selectively

350

Biocomposites: Design and Mechanical Performance

hydrolyzed. NCC is abundant, renewable, and nontoxic. Compared to mineral fillers, NCC has a lower density, a high form factor of about 70 and a high specific area of 150 m2/g. Among all advantages, surface reactivity of NCC provides a chance to graft a wide variety of chemical functions to the surface of NCC, resulting in NCC with different properties. In this chapter, the feasibility of improvement in bonding strength of PVA in dry and wet states, and also at elevated temperatures through using different nanomaterials are discussed. Nanomaterials include nanoclay and NCC. Moreover, the effects of dispersion methods on mechanical properties and the morphology of PVA nanocomposites are studied.

13.2 Experimental analysis of PVA based bio-composites 13.2.1 Materials A commercial PVA was received in liquid form. Two types of hydrophilic nanoclay, namely Nanofil® 116 and Lit.G-105 (polymer grade (PG) MMT) were supplied by Southern Clay Products, Inc., Gonzales, TX, and Nanocor, Inc., Arlington Heights, IL, respectively. Nanoaluminum oxide particles (Sasol Dispal 25 F4) were provided by Sasol Germany GmbH. NCC, provided kindly by Forest Products Laboratory in Madison, WI, was used as reinforcing material (in 5.5% suspension). Two wood species were used to make adhesive joints—sugar maple (Acer saccharum) and black spruce (Picea mariana)—which were obtained from trees grown in Quebec, Canada.

13.2.2 Methods Various blends of PVA with nanoparticles were prepared. Two methods were used to mix PVA with nanoparticle: high-speed mixer and ultrasonication. A high-speed mixer, Gast MFG Corp. 4AMFRV13C, with the addition of small glass beads was used to mix PVA with nanoclay. The formulations having different amounts of nanoclay were mixed at 1500 rpm for 15 min. For the ultrasonication method, a high-intensity ultrasonic probe (60 kHz, maximum amplitude 100 nm from tip to tip, Branson PG) was used to disperse the nanoparticles. The amounts of nanoparticles were 1%, 2%, and 4% wt/wt.% (solid content). The nanoparticles were added to PVA on a base of solid content of PVA in the emulsion of PVA adhesive. As ultrasonication generates a lot of energy in the form of heat and shear, the temperature of the solution increases rapidly, causing some interference with the results. In order to keep the temperature of solution under control (< 50 °C), the vessel containing the solution was cooled by means of a recirculating ethylene glycol bath. Sonication experiments were carried out with 50% amplitude and a volume of 20 ml of the nanoparticle–water mixture. During ultrasonication, the sonication power was gradually raised while maintaining the temperature of the mixture below 50 °C. The ultrasonicated solution was added to PVA. New formulated blends of PVA and nanoparticles were mixed for 30 min. For NCC, a simple mixer was used to blend PVA with NCC, as NCC had been already dispersed in water by the supplier.

Polyvinyl acetate (PVA)-based biocomposites

351

For transmission electron microscopy (TEM) and atomic force microscopy (AFM), samples of nanocomposites were prepared by casting the NCC–PVA composites on Teflon sheets. Prior to further analyses, the sheets of nanocomposites were allowed to dry at room temperature for at least 2 weeks and conditioned to about 60% RH.

13.2.2.1 Fabrications and tests of wood joints Mixed solutions of PVA and different nanomaterials were used to bond wood samples. Prior to gluing, the moisture content of wood was fixed at 12% by conditioning wood to 20 °C and 60% relative humidity for two months. After applying glue on the surface of the wood, the samples were pressed in an MTS hydraulic test machine with 50 kN capacity at 2.46 kg/cm2 pressure for 2 h. Before testing, glued samples were conditioned to 20 °C and 60% relative humidity for 2 weeks. Twenty samples were tested for each set of formulation. To evaluate the impact of nanoparticles on the performance of wood joints, the shear strength of wood joints was measured in dry and wet states, and at an elevated temperature. An MTS hydraulic test machine with 50 kN capacity was used for load application, and the data was acquired by a computer. Wood failure and maximum load were recorded for each test. The block shear tests were carried out according to ASTM D905-98. The sizes of samples for “wet state” tests were the same as those for dry state tests. For “wet state” tests, the samples were taken directly out of the water after being immersed in water for 24 h. Before the tests, excess water was wiped off the samples. During the water immersion period, temperature of water was maintained at 23 ± 1 °C. Block shear tests at the elevated temperature were carried out according to ASTM 7247-07. Samples made of black spruce were heated in an oven, having a temperature controller with an integral and derivative (PID) control algorithm, until the temperature of the middle of the samples reached 100 °C. On average, it took 30 min to reach 100 °C in the middle of the samples. After reaching 100 °C in the middle of the samples, the samples were kept at 100 °C for 15 min more, followed by immediate block shear tests. The shear strength of the sample was measured by an MTS hydraulic test machine.

13.2.2.2 Transmission electron microscopy TEM allows a qualitative understanding of the internal structure, spatial distribution, and dispersion of the nanoparticles within the polymer matrix, and views of the defect structure through direct visualization. Analyses were performed on a JEOL JEM1230, transmission electron microscope at 80 kV. TEM specimens, having 50–70 nm thickness, were prepared by ultramicrotoming the nanocomposite samples encapsulated in an epoxy matrix.

13.2.2.3 Atomic force microscopy AFM observations were carried out using a NanoScope IIIa, an atomic force microscope (Veeco Instruments Inc.). AFM measurements were done under ambient air conditions in tapping mode. The sensitivity of the tip deviation and the ­scanner

352

Biocomposites: Design and Mechanical Performance

r­esolution was 0.1 nm. The resolution was set to 512 lines by 512 pixels for all observations. Two topographic and phase images were obtained within each sample for scan areas of 10 × 10 and 50 × 50 μm. Surface roughness was calculated in 50 ×50 μm scan areas, using the classical mean surface roughness parameters Ra and Rq (RMS). The parameters were calculated by the Nanoscope 5.30r3sr3 software.

13.3 Results of adding nanoclay and NCC to PVA based bio-composites The results of adding nanoclay and NCC to the PVA to bonding strength of PVA are presented in two separate sections.

13.3.1 Nanoclay Two types of nanoclay (at different loading levels of 1%, 2%, and 4%), coming from different companies (Southern Clay Products, Inc., Gonzales, TX, and Nanocor, Inc., Arlington Heights, IL), were first mixed with PVA by using a high-speed mixer. In the following, the effects of adding nanoclay on the bonding strength measured on sugar maple are discussed. The values of bonding strength for PVA and its nanocomposites prepared by a highspeed mixer are given in Figure 13.1. Adding nanoclay to PVA clearly did not increase significantly shear strength of joints bonded by PVA, although all values of strength of formulations with added nanoparticles were slightly above those of pure PVA. Optimal loading level of nanoclay depended on the type of nanoclay. Adding only

Shear sterngth (MPa)

25 20

Dry

15

Wet

10

100 °C

5 0

re

A PV

)

%

Pu

il

a

f no

N

1 6(

)

11

il

N

a

f no

)

11

il

N

a

f no

4 6(

11

)

1 5(

2 5(

-1

G

t. Li

)

%

4 5(

0

0

0

-1

G

t. Li

)

%

%

%

%

2 6(

-1

G

t. Li

Blend name

Figure 13.1  Values of bonding strength for PVA/nanoclay nanocomposites (mixed by a highspeed mixer) in dry and wet states, and at the elevated temperature.

Polyvinyl acetate (PVA)-based biocomposites

353

1% of Lit.G-105 enhanced shear strength by 10% and further increase did not change the strength remarkably. In contrast, a significant improvement on shear strength was achieved for Nanofil 116 as nanoclay loading increased. The highest gain in strength was obtained at 4% loading of Nanofil 116. The constant increase in the strength in the case of Nanofil 116 could be related to its compatibility with the matrix. Such constant increase was not seen in Lit.G-105, possibly because Lit.G-105 cannot be mixed well with PVA at higher loadings due to its incompatibility with the matrix. Further morphological study is required to draw any clear conclusion regarding this phenomenon. Results of shear strength measurements performed on samples after spending 24 h in water are presented in Figure 13.1. Effects of nanoclay on the strength were significant. Nanoclay enhanced the resistance of wood joints toward water. At 1% loading, nanoclay did not change the strength considerably. But as loading nanoclay was increased, marked improvements in shear strength were achieved; as high as 53%. Increase in the barrier properties of glue line against water could explain the increase. Lit.G-105 and Nanofil 116 improved the water resistance of PVA almost in the same degree and no significant difference was detected. Shear strength of samples after being exposed to elevated temperature is given in Figure 13.1. Joints bonded with pure PVA lost their integrity at a low load. Effects of adding nanoclay on shear strength was significant. One percent nanoclay improved shear strength of PVA by 83%. Such important improvement stems from the fact that nanoclay changed the response of the adhesives toward elevated temperatures. In terms of the model for the origin of properties, 2% exfoliated clay theoretically has enough surface area to interact with about 60% of the polymer chains, confining them to a lower free volume. This may account for the increase in cohesive strength of the adhesive. The other 40% of the polymer chains reside in a higher free volume state than do the bulk, unmodified polymer chains. These chains have less constricted motion and exhibit more rubbery behavior than in the unmodified polymer (Krywko et al., 2002; Giannelis et al., 1999). Effects of nanoclay loading on shear strength at the elevated temperature were significant. At 1% and 2%, the strength was improved as nanoclay loading increased in the formulations. But at 4% loading, a reduction in strength was detected in comparison with the joints bonded with 2% nanoclay. Difficulty involved in optimizing dispersion of nanoclay in the matrix at high loading could be a reason for the reduction. Lit.G-105 increased the shear strength of wood joints a little bit more than Nanofil 116. No wood failure occurred in the tests and all failure occurred in the glue line. As the high-speed mixer could not disperse well one type of nanoclay (Lit.G-105) in PVA, we decided to use ultrasonication to disperse Lit.G-105 in PVA. Such an application allowed us to measure the efficiency of ultrasonication in dispersing nanoclay in PVA. The results are presented in the following discussion. Results of measuring shear strength of wood joints in dry tests are shown in Figure 13.2. Although fluctuations caused by varying loading of nanoclay on bonding strength were not found significant, all joints with nanoclay in their formulations had improved shear strength. The increase in wood joints strength was between 2% and 7%. The strength of joints showed improvements as nanoclay content increased in the matrix. An increase in bonding strength of wood joints not only could be ­measured in

Biocomposites: Design and Mechanical Performance

Shear stress (MPa)

12

Stress

120

Wood failure

10

100

8

80

6

60

4

40

2

20

0

Pure PVA

1% Nanoclay 2% Nanoclay Blend name

4% Nanoclay

Wood failure (%)

354

0

Figure 13.2  Values of bonding strength for PVA/nanoclay (Lit.G-105) nanocomposites (mixed by ultrasonication technique) in dry state at room temperature (Kaboorani and Riedl, 2013).

terms of shear strength, but also in terms of wood failure under shear load. Inclusion of nanoclay to PVA increased wood failure of wood joints under shear load (Figure 13.2). Higher wood failure in joints having nanoclay in their adhesives means that nanoclay increased the strength of the glue line to a level that the strength of the glue line surpassed the strength of wood. Figure 13.3 presents the values of shear strength after 24-h exposure of wood joints to water. Water exposure decreased the shear strength of wood joints. The shear strength of joints made of pure PVA decreased drastically, dropping down to one-sixth of their values in a dry state. Adding nanoclay to PVA improved the resistance of the glue line to water. The extent of improvement was between 25% and 64%. Any increase in nanoclay loading in the matrix gave a boost to water resistance of wood joints (at all levels of loading). Positive effects of nanoclay on water resistance can be attributed to better barrier properties of the glue line. Past research shows that nanomaterials can improve the barrier properties of polymers (Nielsen, 1967; Bharadwaj, 2001; Falla et al., 1996; Plate and Yampol'skii, 1994; Thran et al., 1999; Pethrick, 1997; Wang et al., 2004; Becker et al., 2003; Winberg et al., 2005). The better barrier properties are associated with the fact that permeate molecules are forced to follow tortuous pathways, reducing the diffusion coefficients (Fischer, 2003). In addition, the inclusion of nanoclay with layered structures and their adhesion to the polymer generate additional free volume, more likely affecting the polymeric chains located near interfacial regions (Plate and Yampol'skii, 1994;

Polyvinyl acetate (PVA)-based biocomposites

355

8

100 Stress

7

90

Wood failure

80 6 5

60

4

50 40

3

Wood failure (%)

Shear stress (MPa)

70

30 2 20 1 0

10

Pure PVA

1% Nanoclay

2% Nanoclay

4% Nanoclay

0

Blend name

Figure 13.3  Values of bonding strength for PVA/nanoclay (Lit.G-105) nanocomposites (mixed by ultrasonication technique) after 24 h of water exposure (Kaboorani and Riedl, 2013).

Thran et al., 1999; Pethrick, 1997). Wood failure of joints was affected by adding nanoclay as well. Wood joints bonded with PVA and 2% nanoclay showed a small percentage of wood failure under the shear load. The wood failure could be related to good dispersion of nanoclay in PVA matrix. Values of shear strength of wood joints at 100 ºC are given in Figure 13.4. Nanoclay had a positive effect on heat resistance of wood joints. The effects were more pronounced at 1% loading (resulting in a 96% increase in shear strength at 100 ºC). As nanoclay content in the PVA matrix increased, shear strength of wood joints improved at the elevated temperature. The extent of improvement was very significant at 4% nanoclay loading. The PVA with 4% nanoclay had 1.40 times higher shear strength. In our past experience (Kaboorani and Riedl, 2011) with a high-shear mixer, a decrease in shear strength of wood occurred at 4% nanoclay loading because of poor dispersion of nanoclay in the PVA matrix. Such a reduction did not occur in this study where an ultrasonic technique was used to disperse nanoclay particles in PVA. Thus, the results showed that using ultrasonic technique to disperse nanoclay particles in the matrix can enhance the effectiveness of nanoclay particles in the matrix. No wood failure was observed in the joints at the elevated temperature.

13.3.1.1 Transmission electron microscopy Figure 13.5 shows the TEM images for nanocomposites containing nanoclay. Adding nanoclay at 1% and 2% loadings led to good dispersion of nanoclay in

4

100 Stress

Wood failure

90 80

3

60 50

2

40

Wood failure (%)

Shear stress (MPa)

70

30 1 20 10 0

0

0

0

0

Pure PVA

1% Nanoclay

2% Nanoclay

4% Nanoclay

0

Blend name

Figure 13.4  Values of bonding strength for PVA/nanoclay (Lit.G-105) nanocomposites (mixed by ultrasonication technique) at elevated temperature (100 °C) (Kaboorani and Riedl, 2013).

100 nm

100 nm

(a)

(b)

100 nm

(c) Figure 13.5  Images of PVA/nanoclay composites with different nanoclay (Lit.G-105) loadings: (a) 1%, (b) 2%, and (c) 4% (Kaboorani and Riedl, 2013).

Polyvinyl acetate (PVA)-based biocomposites

357

matrix. The distance between nanoclay platelets was increased and the polymer chains entered between the platelets space. This nanostructure is referred to as exfoliated structure, giving superior properties to nanocomposites. As can be seen in Figure 13.5c, nanocomposite with 4% is composed mostly of intercalated structure with very large aggregates or tactoids in the order of several tens of silicate layers. Intercalated structure is not considered an ideal structure for a nanocomposite and it does not grant the nanocomposites the superior properties that exfoliated structure does. As observed in measuring bonding strength (in dry condition and at elevated temperature), adding nanoclay at 1% and 2% loadings gave a significant boost to the properties, but improvement of the properties at 4% content was not much different than with nanocomposites with 1% and 2% nanoclay. This phenomenon should be related to difficulty of dispersing nanoclay in matrix at high loadings. Past research has shown that there is a direct linkage between properties of nanocomposites and quality of nanoclay dispersion (Macia-Agullo et al., 1992; Teresa et al., 2008; Mirzataheri et al., 2010; Zilg et al., 1999; Ranade et al., 2002; Fu and Qutubuddin, 2001). In fact, the extent of improvement, as a result of adding nanoclay, cannot be solely proportional to nanoclay loading because of the difficulty of obtaining good dispersion at high loading.

13.3.1.2 Atomic force microscopy Although TEM is used to study the quality of nanoclay dispersion in the polymer, subjectivity of TEM observations raises some questions regarding the results obtained by TEM. In order to draw a firm conclusion on the structure of nanocomposites a quantitative technique should be used. In this study, AFM was used to determine the effects of adding nanoclay on PVA surface structure. The AFM images for dried surfaces of pure PVA and its composites are presented in Figure 13.6. Pure PVA had a smooth surface as it had low roughness values. As nanoclay was added to the matrix, a reorganized surface was observed. The reorganization became more notable as nanoclay loading in the matrix increased. At 4% nanoclay loading, the surface of PVA film totally reorganized as a big increase in roughness values was detected, although this roughness is still too small to be detected by the human eye or resulted from light diffusion. The results of this study show that the ultrasonication technique is an effective way to disperse nanoclay in the PVA matrix. The extent of improvement on PVA performance as a wood adhesive was superior or at least similar to that obtained by high-speed mixing. When a high-speed mixer was used to disperse nanoclay at a high loading (4%) in the matrix, a remarkable reduction was observed in the improvement gained by adding nanoclay. Such a reduction was not observed in the case of the ultrasonic technique. Although adding 4% nanoclay to the PVA matrix resulted in an intercalated structure, nanoclay did increase the shear strength of wood joints in humid conditions and at the elevated temperature. The results show that the ultrasonic technique is very efficient in dispersing nanoclay, especially at high loadings, in contrast to the high-shear speed mixer. High-speed mixing could disperse nanoclay in the PVA only at low loadings and increased bonding strength of PVA in ­different

358

Biocomposites: Design and Mechanical Performance

2

2 4

4 6

6 8

(a)

µm

x 2.000 µm/div z 450.000 nm/div

8

(b)

µm

x 2.000 µm/div z 450.000 nm/div

µm

x 10.000 µm/div z 450.000 nm/div

2 4

10 6 8

(c)

20 µm

x 2.000 µm/div z 450.000 nm/div

(d)

Figure 13.6  AFM images of pure PVA and its composites with different nanoclay (Lit.G-105) loadings: (a) pure PVA, (b) 1% nanoclay, (c) 2% nanoclay, and (d) 4% nanoclay (Kaboorani and Riedl, 2013).

c­onditions. High-speed mixing has some disadvantages: possible damage to PVA emulsion ­(because of strong shear force used during the mixing), high cost, and high energy consumption. By contrast, the ultrasonication technique has minimum negative impact on PVA emulsion. Moreover, the ultrasonication technique is economical, as ultrasonic mixing could take place before production of PVA and the solution containing nanoclay can be added to PVA during the production process. By considering the results obtained from this research and our previous work (Kaboorani and Riedl, 2011), and the advantages of the ultrasonication technique over high-speed mixing, adding nanoclay to PVA in an industrial scale seems feasible and can be recommended to wood adhesive manufacturers.

13.3.2 Nanocrystalline cellulose NCC was added to PVA at 1%, 2%, and 3%. Figure 13.7 shows shear stress at failure of wood joints bonded by pure PVA and its composites with NCC. Adding NCC to PVA did increase the stress but not in a significant way. Although not being significant

Polyvinyl acetate (PVA)-based biocomposites

359

10

100

8

80

6

60 Stress

4

Wood failure

2

40

Wood failure (%)

120

Shear stress (MPa)

12

20

0

0 Pure PVA

NCC 1%

NCC 2%

NCC 3%

Blend name

Figure 13.7  Values of bonding strength for PVA/NCC nanocomposites in the dry state.

in terms of the stress at failure, adding NCC to PVA significantly increased the wood failure, proving its positive effects on bonding strength of PVA glue line. Wood failure value increased from 59% for pure PVA to 84% for adhesive with 1% NCC and finally to 97% for adhesive with 3% NCC. Such an increase shows that NCC made the glue line of PVA stronger than wood, causing failure in the wood rather than in the glue line. The loading level of NCC affected the performance of PVA. Wood failure percentage increased as more NCC was added to PVA. The values of shear stress at failure of wood joints after spending 24 h in water are shown in Figure 13.8. Nanocellulose significantly affected shear strength of wood joints after spending 24 h in water. Adding only 1% NCC to PVA increased the strength by 63%. Further increase in NCC (from 1% to 2%) increased shear strength of wood joints but on a much smaller scale. An increase in NCC loading from 2% to 3% did not change the strength much. The improvement in water resistance of PVA was so high that some failure occurred in the glue line instead of the wood. As more NCC was inserted in PVA, the percentage of wood failure increased, reaching its maximum value at 3% NCC level (21%). Figure 13.9 shows the values of shear strength at 100 ºC. NCC significantly affected PVA performance at this high temperature. Adding NCC to PVA doubled the shear strength of PVA. There was no significant difference between different loading levels of NCC. Adding only 1% NCC to PVA resulted in a 100% increase in shear strength and further addition did not change the strength much. At 1% loading, NCC improved bonding strength of PVA significantly. But adding more NCC (2% and 3%) did not increase the properties beyond the extent obtained by 1% addition. This trend is related to the quality of NCC dispersion. At 1% NCC

360

Biocomposites: Design and Mechanical Performance 8

100 Stress

7

90

Wood failure

80 6 5

60 50

4

40

3

Wood failure (%)

Shear stress (MPa)

70

30 2 20 1

10

0

0 Pure PVA

NCC 1%

NCC 2%

NCC 3%

Blend name

Figure 13.8  Values of bonding strength for PVA/NCC nanocomposites after spending 24 h in water. 4

Shear sterss (MPa)

3

2

1

0 Pure PVA

NCC 1%

NCC 2%

NCC 3%

Blend name

Figure 13.9  Values of bonding strength for PVA/NCC nanocomposites at elevated temperature (100 °C).

Polyvinyl acetate (PVA)-based biocomposites

361

loading, significant improvement on bonding strength was achieved as the quality of NCC dispersion was good. At 2% and 3% NCC loadings, dispersion of NCC in PVA encountered difficulty, as shown by the AFM results (Kaboorani et al., 2012), and a poor dispersion of NCC in the matrix was obtained leading to a leveling-off of the improvement. In the context of the other studies conducted on the reinforcement of PVA with inorganic nanomaterials, the extent of improvement achievable by NCC is higher than that of inorganic nanomaterials; namely, nanoclay and aluminum oxide. The improvement is especially much higher for bonding strength in wet conditions. In this study, unmodified and hydrophilic NCC was used to reinforce PVA. Using such unmodified and hydrophilic NCC led to agglomeration of NCC and difficulty in dispersion of NCC in PVA at high loadings (2% and 3%). The agglomeration limits the extent of improvement that be achieved by NCC. Modifying the surface of NCC a little bit and imposing a slightly hydrophobic character on the surface can help to prevent agglomeration of NCCs and to have a better NCC dispersion in the matrix at a high loading. The modification of surface should be done in controlled conditions without making NCC too hydrophobic or degrading its reinforcing effects.

13.4 Conclusion The results of this study showed that ultrasonication technique is efficient in dispersing nanoclay in PVA at low (1% and 2%) and high (4%) loadings. Bonding strength of newly formulated adhesives measured on block shear samples increased in wet conditions and at the elevated temperature. In the dry state, the positive effects of nanoclay on strength of the glue line could be detected in terms of wood failure percentage. The strength of the glue line was so high that some failure occurred in the wood rather than in the glue line. Improved barrier properties strengthened the resistance of the glue line to water and, subsequently, a significant increase was observed in bonding strength in the wet state. In contrast to the results obtained from the dry condition, the extent of improvement on bonding strength in the wet state was proportional to nanoclay loading. Bonding strength of PVA at the elevated temperature was also affected by adding nanoclay. As nanoclay loading in the PVA matrix increased, the shear strength of wood joints at the elevated temperature improved, despite the fact that the biggest gain in the shear strength was observed at 1% nanoclay loading. The morphological studies of nanocomposites revealed that the fluctuations observed in bonding strength tests were related to dispersion quality of nanoclay in the matrix (PVA). AFM proved that it is a credible technique to examine the quality of dispersion, as the results of AFM fully conformed to the TEM observations. At low loadings (1% and 2%), an exfoliated structure is achieved, causing a significant improvement in PVA properties. At high loading (4%), a coexistence of exfoliated and intercalated was observed and so improvements on the shear strength of wood joints were achieved to an extent. NCC was found to be an effective nanoreinforcing material for PVA. Bonding strength of wood joints was improved significantly as NCC was inserted to PVA. The improvement was measurable in terms of wood failure percentage in the dry condition

362

Biocomposites: Design and Mechanical Performance

and in terms of shear strength in the wet condition and at the elevated temperature. Fluctuations in properties of PVA as a wood adhesive and a polymer at different loadings of NCC could be explained by the quality of NCC dispersion in the matrix.

Acknowledgments The authors acknowledge financial support from the Canadian Forest Nano Products Network (ArboraNano). The authors would like to express their thanks to Forest Products LaboratoryUSDA Forest Service for so kindly providing nanocrystalline cellulose (NCC) for this research.

References Ash, B.J., Siegel, R.W., Schadler, L.S., 2004. Mechanical behavior of alumina/poly(methyl methacrylate) nanocomposites. Macromolecules 37 (4), 1358–1369. Becker, O., Cheng, Y.B., Varley, R.J., Simon, G.P., 2003. Layered silicate nanocomposites based on various high-functionality epoxy resins: the influence of cure temperature on morphology, mechanical properties, and free volume. Macromolecules 36 (5), 1616–1625. Bharadwaj, R.K., 2001. Modeling the barrier properties of polymer-layered silicate nanocomposites. Macromolecules 34 (26), 9189–9192. Cai, Y., 1997. Synthesis of melamine-formaldehyde resin modified poly(VAc-MMA) emulsion. China Adhes. 6 (2), 43–45. Chang, J.H., Seo, B.S., Hwang, D.H., 2002. An exfoliation of organoclay in thermotropic liquid crystalline polyester nanocomposites. Polymer 43 (10), 2969–2974. Chen, Y., 1996. Researches of wood adhesives with high strength, water and creep resistance. Nianjie 17 (1), 25–27. Cho, J., Joshi, M.S., Sun, C.T., 2006. Effect of inclusion size on mechanical properties of polymeric composites with micro and nano particles. Compos. Sci. Technol. 66 (13), 1941–1952. Comyn, J., 1997. Adhesion Science. The Royal Society of Chemistry, London, pp. 36–39. Falla, W.R., Mulski, M., Cussler, E.L., 1996. Estimating diffusion through flake-filled membranes. J. Membr. Sci. 119 (1–2), 129–138. FerrandizGomez, T.D., FernandezGarcia, J.C., OrgilesBarcelo, A.C., MartinMartinez, J.M., 1996. Effects of hydrocarbon tackifiers on the adhesive properties of contact adhesives based on polychloroprene. I. Influence of the amount of hydrocarbon tackifiers. J. Adhes. Sci. Technol. 11 (10), 1303–1319. Fischer, H., 2003. Polymer nanocomposites: from fundamental research to specific applications. Mater. Sci. Eng. C 23 (6–8), 763–772. Fornes, T.D., Paul, D.R., 2004. Structure and properties of nanocomposites based on nylon-11 and -12 compared with those based on nylon-6. Macromolecules 37 (20), 7698–7709. Fu, X., Qutubuddin, S., 2001. Polymer–clay composite: exfoliation of organophilic montmorillonite nanolayers in polystyrene. Polymer 42 (2), 807–813. Giannelis, E.P., Krishnamoorti, R., Manias, E., 1999. Polymer-silicate nanocomposites: model systems for confined polymers and polymer brushes. Adv. Polym. Sci. 138, 107–147. Huang, M., Kuo, S., Wu, H., Chang, F., Fang, S., 2002. Miscibility and hydrogen bonding in blends of poly(vinyl acetate) with phenolic resin. Polymer 43 (8), 2479–2487.

Polyvinyl acetate (PVA)-based biocomposites

363

Ji, X.L., Hampsey, J.E., Hu, Q.Y., He, J.B., Yang, Z.Z., Lu, Y.F., 2003. Mesoporous silica-­ reinforced polymer nanocomposites. Chem. Mater. 15 (19), 3656–3662. Kaboorani, A., Riedl, B., 2011. Effects of addition of nano-clay on performance of polyvinyl acetate (PVA) as a wood adhesive. Compos. A: Appl. Sci. Manuf. 42, 1031–1039. Kaboorani, A., Riedl, B., Blanchet P., 2013. Ultrasonication technique: a method for dispersing nanoclay in wood adhesives. J. Nanomater. 2013, 1–9, http://dx.doi. org/10.1155/2013/341897. Kaboorani, A., Riedl, B., Blanchet, P., Fellin, M., Hosseinaei, O., Wang, S., 2012. Nanocrystalline cellulose (NCC): a renewable nano material for polyvinyl acetate (PVA) adhesive. Eur. Polym. J. 48 (11), 1829–1837. Kim, S., Kim, H., 2005a. Thermal stability and viscoelastic properties of MF/PVAc hybrid resins on the adhesion for engineered flooring in under heating system; ONDOL. Thermochim. Acta 444 (2), 134–140. Kim, S., Kim, H., 2005b. Effect of addition of polyvinyl acetate to melamineformaldehyde resin on the adhesion and formaldehyde mission in engineered flooring. Int. J. Adhes. Adhes. 25 (5), 456–461. Krywko, W.P., McAuley, K.B., Cunningham, M.F., 2002. Mathematical modeling of particle morphology development induces by radical concentration gradients in seeded styrene homopolymerization. Polym. React. Eng. 10 (3), 135–161. Lech, J., 2008. Green Chemistry: Nitration of Tyrosine. CHMC42: Synthetic Chemistry. University of Toronto Press, Toronto, Ontario. Lee, K.M., Han, C.D., 2003. Linear dynamic viscoelastic properties of functionalized block copolymer/organoclay nanocomposites. Macromolecules 36 (3), 804–815. López-Suevos, F., Frazier, C.E., 2006. Fracture cleavage analysis of PVAc latex adhesives: influence of phenolic additives. Holzforschung 60 (3), 313–317. Lu, G., 1996. Improvement of polyvinyl acetate emulsions. Huagong Shikan 10 (6), 17. Ma, J., Xiang, P., Mai, Y.W., Zhang, L.Q., 2004. A novel approach to high performance elastomer by using clay. Macromol. Rapid Commun. 25 (19), 1692–1696. Macia-Agullo, T.G., Fernindez-Garcia, J.C., Torro-Palau, A., Orgiles-Barcelo, A.C., MartinMartinez, J.M., 1992. Addition of silica to polyurethane adhesives. J. Adhes. 38 (1–2), 31–53. Mirzataheri, M., Atai, M., Mahdavian, A., 2010. Physical and mechanical properties of nanocomposite barrier film containing encapsulated nanoclay. J. Appl. Polym. Sci. 118 (6), 3284–3291. Nielsen, L., 1967. Models for the permeability of filled polymer systems. J. Macromol. Sci. (Chem.) 1 (5), 929–942. Pastor-Sempere, N., Fernandez-Garcia, J.C., Orgiles-Barcelo, A.C., Torregrosa-Macia, R., Martin-Martinez, J.M., 1995. Fumaric acid as a promoter of adhesion in vulcanized synthetic rubbers. J. Adhes. 50 (1), 25–42. Pegoretti, A., Kolarik, J., Peroni, C., Migliaresi, C., 2004. Recycled poly(ethylene terephthalate)/layered silicate nanocomposites: morphology and tensile mechanical properties. Polymer 45 (8), 2751–2759. Pethrick, R.A., 1997. Positron annihilation—a probe for nanoscale voids and free volume? Prog. Polym. Sci. 22, 1–47. Plate, N.A., Yampol'skii, Y.P., 1994. Relationship between structure and transport properties for high free volume polymeric materials. In: Paul, D.R., Yampol'skii, Y.P. (Eds.), Polymeric Gas Separation Membranes. CRC Press, Boca Raton, FL, pp. 155–207 (Chapter 5). Poh, P.T., Firdaus, S.Z., 2010. Effect of hybrid tackifiers on adhesion properties of epoxidized natural rubber-based pressure-sensitive adhesives. J. Polym. Environ. 18 (3), 335–338.

364

Biocomposites: Design and Mechanical Performance

Qiao, L., Easteal, A., 2001. Aspects of the performance of PVAc adhesives in wood joints. Pigm. Resin Technol. 30 (20), 79–87. Qiao, L., Easteal, A.J., Bolt, C.J., Coveny, P.K., Franich, A., 2000. Improvement of the water resistance of poly(vinyl acetate) emulsion wood adhesive. Pigm. Resin Technol. 29 (3), 152–158. Ranade, A., D'Souza, N., Gnade, B., 2002. Exfoliated and intercalated polyamide-imide composite with montmorillonite. Polymer 43 (13), 3759–3766. Teresa, V.G., Carlos, G.S., Javier, R.D.R., Virgilio, G.G., 2008. Nylon 6/organoclay nanocomposites by extrusion. J. Appl. Polym. Sci. 08 (5), 2923–2933. Thran, A., Kroll, G., Faupel, F., 1999. Correlation between fractional free volume and diffusivity of gas molecules in glassy polymers. J. Polym. Sci. B Polym. Phys. 37 (23), 3344–3358. Wang, J., 1999. Study on improving properties of polyvinyl acetate emulsion. Zhanjie 20 (3), 16–19. Wang, Y.Q., Wu, Y.P., Zhang, H.F., Zhang, L.Q., Wang, B., Wang, Z.F., 2004. Free volume of montmorillonite/styrene-butadiene rubber nanocomposites estimated by positron annihilation lifetime spectroscopy. Macromol. Rapid Commun. 25 (23), 1973–1978. Wetzel, B., Haupert, F., Zhang, M.Q., 2003. Epoxy nanocomposites with high mechanical and tribological performance. Compos. Sci. Technol. 63 (14), 2055–2067. Winberg, P., Eldrup, M., Pedersen, N.J., van Es, M.A., Maurer, F.J.H., 2005. Free volume sizes in intercalated polyamide clay nanocomposites. Polymer 46 (19), 8329–8349. Zhang, Q., Archer, L.A., 2004. Optical polarimetry and mechanical rheometry of poly(ethylene oxide)–silica dispersions. Macromolecules 37 (5), 1928–1936. Zhou, H., 1991. Research on the improvement of water resistance of polyvinyl acetate emulsion. Nianjie 12 (4), 11–12. Zilg, C., Mülhaupt, R., Finter, J., 1999. Morphology and toughness/stiffness balance of nanocomposites based upon anhydride-cured epoxy resins and layered silicates. Macromol. Chem. Phys. 200 (3), 661–670.