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Enhancement of mechanical strength of particleboard using environmentally friendly pine (Pinus pinaster L.) tannin adhesives with cellulose nanofibers

Abstract

Context

Condensed tannins have been successfully used as substitutes for phenol in the production of resins for wood products. However, the enhancement of the properties of tannin-based resins with nontoxic and cost effective additives is of great interest.

Methods

In the present work, the performance enhancement of tannin-based particleboards with cellulose nanofibers was investigated.

Results

In presence of 2 % of cellulose nanofibers, the viscosity of tannin-based adhesives and the internal bonding strength (IB) of the corresponding particleboards were increased from 350 to 5,462 mPa·s and from 0.85 to 0.98 MPa respectively. The modulus of elastic (MOE) and modulus of rupture (MOR) of the resins were also notably increased while thickness swelling (TS) of the panels was not affected.

Conclusion

The addition of cellulose nanofibers to tannin adhesives is an effective method for the production of high performance particleboards. Tannin-based adhesive containing 2 % of cellulose nanofibers exhibits the best mechanical strength.

Key message

The addition of cellulose nanofibers into tannin-based adhesives notably enhanced the mechanical properties of the wood particleboards produced. The formulation containing 2 % (w/w) of cellulose nanofibers exhibited the best mechanical strength.

1 Introduction

Wood adhesives play a key role for the production of wood-based panels such as plywood, fiberboard, particleboard, and related structural wood composite because of high-efficient utilization of wood and agri-foresty residues. The quality of bonding and enhancement of the properties of wood-based panels mainly depends on adhesives qualities (Takemura 2010; Stoeckel et al. 2013). Currently, formaldehyde-based synthetic adhesives such as urea-formaldehyde resin (UF), phenol-formaldehyde resin (PF), and melamine formaldehyde (MF) are the predominant adhesives for the manufacture of various wood panels. Because of formaldehyde emissions, it has been demonstrated that formaldehyde-based adhesives are not environmentally friendly products. Formaldehyde emissions are due to residual unreacted formaldehyde and to slow adhesive hydrolysis under hot/humid conditions during the production and use of panel (Han et al. 2008; Sari et al. 2010; He et al. 2012). In the USA, interior wood-based panels have been adapted to enacted regulations by the California Air Resources Board (CARB) that regulates formaldehyde emissions from wood-based panels. Various green building rating systems have addressed “low-emitting” and “renewable materials” for the building materials and adhesives (Ruffing et al. 2010). Therefore, environmentally friendly or green wood adhesives (Pizzi 2000; Imam et al. 2001; Ibrahim et al. 2008; Lei et al. 2008; Kajaks et al. 2012) derived from renewable resources (i.e., tannins, soy protein, lignin adhesives, etc.) have been a topic of considerable interest for many years and constitute an area of active research.

Condensed tannins are generally extracted from the bark of various trees like mimosa (Mimosa pigra), quebracho (Schinopsis lorentzii), or pine (Pinus). It was demonstrated that condensed tannins are both chemically and economically interesting for the preparation of adhesives and that they could be successfully used as substitutes for phenol in the production of resins (Pizzi et al. 1993a, 1993b; Li et al. 2004; Pichelin et al. 2006; Kim 2009; Moubarik et al. 2010; Patel et al. 2012). In the last decade, several approaches have been developed to produce wood panels with low formaldehyde emissions using tannin-based adhesives. Recently, it has been reported that grape pomace from wine industry is also an important source of tannins for the production of environmentally friendly tannin adhesives (Ping et al. 2011, 2012). At present, pine tannin and pecan nut pith tannin adhesives fortified with a synthetic resin such as polymeric 4,4′-diphenyl methane diisocyanate (MDI) are used commercially for wood-based panel (Pizzi et al. 1993a). However, the cost and the toxicity of MDI limit its broad utilization.

Cellulose nanofibers (CNF), because of their renewability, nanoscale dimensions, high surface area, and good mechanical response to stress, attracted substantial attention for the production of nanocomposites with enhanced properties (Bhatnagar and Sain 2005; Habibi et al. 2010). The significant enhancement of the mechanical properties of a composite reinforced with CNF can be justified by the high value of Young’s modulus of pure crystalline cellulose domains (about 150 GPa). While a considerable number of publications regarding the reinforcement of polymers with nanocellulose can be found, literature on cellulose-reinforced adhesives suitable for wood bonding is quite scarce. Veigel et al. (2012) described the production of particleboards prepared with nanocellulose-reinforced urea-formaldehyde and melamine-urea-formaldehyde adhesives. Richter et al. (2009) discussed the application of CNF for reinforcing polyurethane and polyvinyl acetate latex wood adhesives. However, to the best of our knowledge, the reinforcement of a tannin-based adhesive has never been reported in the literature.

In this paper, the production and the characterization of tannin-based adhesives reinforced with cellulose nanofibers are described. Particleboards with improved mechanical strength were produced.

2 Materials and methods

2.1 Materials

Commercial tannins from pine (Pinus pinaster L.) were used to prepare tannin adhesives. Cellulose nanofibers (KY100) were produced by Intelligent Chemicals Pty Ltd (China). Their solid content, diameter, and length are 25 %, 1050 nm, and 400600 um respectively. One to two percent (wt) suspension of CNF were prepared through ultrasonication treatment before mixing with tannins. Solid paraformaldehyde as hardener was purchased from Sigma-Aldrich Company.

2.2 Characterization of tannin adhesives with cellulose nanofibers

The viscosity of the resins was determined using a rotational viscometer (Brookfield DV-II + Viscometer) at 25 °C for tannin-based adhesives without and with CNF. About 10 g of a 30 % (w/w) lyophilised tannin solution, 5 % of powdered paraformaldehyde on a dry and solid tannin matter content basis were added to a test tube with or without CNF. The tube was placed in a water bath, which was maintained at boiling temperature (just below 100 °C) at normal atmospheric pressure. The time taken to reach the gel point was recorded during constant stirring with the aid of a wire spring and a stopwatch. The test was duplicated, and the average value is reported.

About 5 g of a 40 % (w/w) tannin solution, 5 % of powdered paraformaldehyde on a dry and solid tannin matter content basis were added and mixed together. Twenty-five milligrams of the mixture was coated in one side of two small wood chips (5 mm × 20 mm) for the measurement of the modulus of elasticity by on a thermal mechanical analyzer with a TA controller (Mettler TMA 40) equipped with a STARe data processing software. The experiments were conducted under the following operating parameters: a heating rate of 10 °C/min from 25 to 250 °C and three-point bending TMA mode.

2.3 Wood particleboard preparation and testing

Triplicates of single-layer particleboards of 340 × 300 × 14 mm3 dimension were prepared using particles of beech (Fagus sylvatica) and Norway spruce (Picea abies) at 28 kg/cm2 maximum pressure, 195 °C press temperature, and 7.5 min press time. Ten percent (w/w based on dry particles) of adhesive resin solid was loaded. All particleboards were tested to measure the internal bonding strength in the dry state according to the relevant international standard European Norm EN 312, wood particleboard-specifications. The modulus of elasticity and modulus of rupture were, respectively, determined according to the European Norm EN 310. The thickness swelling determined after 2-, 6-, and 24-h immersion in cold water was carried out according to the European Norm EN 317.

3 Results

3.1 Viscosity of tannin adhesives modified with adjunction of cellulose nanofibers

The viscosity of tannin adhesive in absence and presence of different amount of cellulose nanofibers is shown in Table 1. It is clearly seen that it strongly increased as the nanocellulose concentration increased. With 3 % of cellulose, a solid-like viscoelastic behavior was observed.

Table 1 Viscosity and gelation time of the adhesives and internal bonding strength of the particleboards prepared with and without cellulose nanofibers

3.2 Gelation time of tannin adhesives modified with adjunction of cellulose nanofibers

Gel time is defined as the point at which the polycondensates formed by the reaction of a tannin solution and formaldehyde become an elastic, rubbery solid. This is an indication of the reactivity of the sites of tannin molecules toward formaldehyde. The experimental results concerning the determination of gel time with and without addition of CNF are shown in Table 1. All formulations exhibited a similar gel time (near 340 s) and a slight increase in the presence of nanofibers corresponding to a decrease of the tannin reactivity.

3.3 Mechanical properties of particleboard glued with tannin adhesives modified with adjunction of cellulose nanofibers

The mechanical strength of bonded wood joints has been scanned by a thermomechanical analysis in bending according to a technique already reported in a previous work (Laigle et al. 1998). The variations of the modulus of elasticity as a function of an increasing temperature are given in Fig. 1. Interestingly, addition of nanofibers in the resins yielded a noticeable improvement in the maximum value of the modulus of elasticity. It appears from Fig. 1 that 2 % of CNF gives the best performance with a maximum modulus of elasticity value of 4,100 MPa. A further increase in the CNF content led to a slight decrease in the performance of the resin. Tannin-based adhesives containing 0–3 % of CNF were also tested for application to wood particleboard. Table 1 also gives the internal bonding strength of panels which are a direct measurement of the performance of the adhesive. The data showed that the addition of CNF in the resin formulation greatly improved the panel strength; the addition of 2 % of CNF yielded the highest internal bonding strength value (̴ 1 MPa). It clearly appears from Fig. 2 that the modulus of elasticity and the modulus of rupture values of the particleboards slightly increased with the addition of low amounts of CNF (1–2 %) but dramatically decreased for a higher nanofibers content (3 %). All these results are in accordance with the thermomechanical analysis experiments previously exposed.

Fig. 1
figure 1

Thermomechanical analysis of tannins adhesives with and without cellulose nanofibers

Fig. 2
figure 2

Modulus of elasticity and modulus of rupture of particleboard glued with tannin-based adhesives with or without cellulose nanofibers

3.4 Thickness swelling of particleboard glued with tannin adhesives modified with adjunction of cellulose nanofibers

Finally, the water resistance of wood particleboards bonded with tannin-based resin reinforced with CNF was evaluated. The thickness swelling (%) after immersion of the wood panels (prepared with and without CNF) in cold water during 2, 6, and 24 h is displayed in Fig. 3. After 2 h of immersion, a dramatic swelling was observed with an increasing content in CNF (until 2 %). However, for longer immersion durations (6 and 24 h), the presence of CNF has a little or no effect on the swelling thickness.

Fig. 3
figure 3

Thickness swelling of tannin-based particleboards with or without cellulose nanofibers

4 Discussion

Viscosity is a measurement of the resistance to gradual deformation by shear stress, which largely influences at the first stage of the distribution of the adhesive on the wood particle surface. The high viscosity observed in presence of nanofibers may result from strong hydrogen bond network between CNF and from their interaction with tannin molecules. However, the gelation time results showed that the presence of nanofibers does not seem to have a significant impact on the tannin reactivity toward crosslinking reactions.

The results of thermomechanical analysis and mechanical strength of particleboard prepared with or without CNF confirm that (1) addition of CNF in a tannin-based adhesive formulation notably enhanced its mechanical properties and (2) the optimum CNF content was established at 2 %. Chun et al. (2011) described the preparation of ultrastrength nanopapers using cellulose nanofibrils. The authors explained the high tensile strength of the nanopapers observed by a three-dimension network between the CNF and the paper components (siloxane). Similar papers (Zimmermann et al. 2010; Eyholzer et al. 2012; Hrabalova et al. 2011) reported the utilization of nanocellulose for polymer matrix reinforcement. The dramatic increase of both viscosity and mechanical strength of the resin in presence of 2 % CNF can be rationalized by a strong H-bond network involving CNF and flavanol units (Fig. 4); this network should form a tight complex besides the high Young’s modulus of CNF in bond line. The observation of thickness swelling can probably be explained by the presence of a large number of hydroxyl groups on the surface of CNF, increasing the kinetic of the swelling of the material. Nevertheless, the actual water resistance of the panels seems not to be notably affected by the presence of CNF. Thus, it can be concluded that panels bonded with tannin adhesives including cellulose nanofibers could be used for interior applications. Utilization of a synthetic crosslinker such as polymeric 4,4′-diphenyl methane diisocyanate (pMDI) could be considered in future investigations for exterior purposes.

Fig. 4
figure 4

Schematic diagram of the H-bond network between CNF and flavanols moieties

5 Conclusions

The experimental results obtained in this research work have shown that the addition of cellulose nanofibers to tannin adhesives for the production of particleboards could notably enhance the mechanical properties of the panels produced. The presence of cellulose nanofibers in tannin adhesives does not largely influence the polycondensation reaction of tannins but remarkably increases the viscosity of the resin. Thermomechanical analysis results have demonstrated that tannin adhesives containing 2 % of cellulose nanofibers exhibited the best mechanical resistance. The measurements of the internal bonding strength, modulus of elasticity, and modulus of rupture of particleboards produced with these tannin adhesives confirmed the thermomechanical analysis. The water resistance of the panels seems not to be notably affected by the presence of cellulose nanofibers.

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Acknowledgments

The authors gratefully acknowledge financial support from Jiangsu Planned Projects for Postdoctoral Research Funds (1302042B), Cooperative Innovation Fund of Jiangsu Province (BY2013006-02), the National High Technology Research and Development Program of China (863 Program) (2012AA03A204), Forestry Industry Research Special Funds for Public Welfare Projects (201104042), and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The authors are grateful for the financial support of this research from CPER 2007–2013 “Structuration du Pôle de Compétitivité Fibres Grand’Est” and from the French National Research Agency through the Laboratory of Excellence ARBRE (ANR-12- LABXARBRE-01).

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Cui, J., Lu, X., Zhou, X. et al. Enhancement of mechanical strength of particleboard using environmentally friendly pine (Pinus pinaster L.) tannin adhesives with cellulose nanofibers. Annals of Forest Science 72, 27–32 (2015). https://doi.org/10.1007/s13595-014-0392-2

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  • DOI: https://doi.org/10.1007/s13595-014-0392-2

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