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Industrial Crops & Products xxx (2018) xxx-xxx

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Industrial Crops & Products

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Exterior grade plywood adhesives based on pine bark polyphenols and hexamine Jorge Santosa⁠ ,⁠ ⁎⁠ , Nacarid Delgadoa⁠ , José Fuentesa⁠ , Cecilia Fuentealbaa⁠ , Johana Vega-Laraa⁠ , Danny E. Garcíab⁠ ,⁠ c⁠ ,⁠ d⁠ a

Área de Bioproductos, Unidad de Desarrollo Tecnológico (UDT), Universidad de Concepción (UdeC), Coronel, Biobío, Chile Laboratorio de Fitoquímica, Departamento de Química Ambiental, Facultad de Ciencias, Universidad Católica de la Santísima Concepción (UCSC), Concepción, Biobío, Chile Centro de Investigación en Biodiversidad y Ambientes Sustentables (CIBAS), UCSC, Biobío, Chile d Investigador Asociado Área de Bioproductos, UDT, UdeC, Concepción, Biobío, Chile b c

ABSTRACT

Keywords: Pine bark Adhesives Polyphenols Hexamethylenetetramine Exterior grade Plywood

The aim of this work was to formulate environmental friendly adhesives for plywood production. This adhesive is based on polyphenol extracts from Pinus radiata D. Don bark, the most important by-product from the Chilean forest industry. We focused on a bio-based formulation free of formaldehyde, phenol, and isocyanates. Hexamethylenetetramine was used as hardener. The influence of pH on gel time, rheological behaviour, and pot life was evaluated. The curing process was evaluated by differential scanning calorimetry, Fourier transform infrared spectroscopy, and automatic bonding evaluation system (ABES). For the first time, exterior grade plywood (EN 314) was manufactured with pine bark extracts obtained at pilot scale. Two alternatives for adhesive application were studied: (1) wood veneers activation using a diluted solution of the bio-adhesive, and (2) pine bark extract application as dry powder on the wood veneers. Both alternatives were not reported before and demonstrated an increase in the final plywood quality, as measured by the internal bond strength (IB). On basis of this research, pine bark from Chilean forest industry is an interesting polyphenolic source for the development of adhesives free of formaldehyde and phenol.

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ARTICLE INFO

1. Introduction

Nowadays, wood panels are primarily manufactured with thermosetting synthetic resins, most of them based on formaldehyde. However, environmental and health considerations are leading to increasingly severe standards regarding the maximum formaldehyde emission from wood boards. In addition, the increasingly high cost of synthetic resins based on petrochemicals has intensified the search for alternative resins based on natural materials for the formulation of wood adhesives (Norström et al., 2018; He, 2017; Hemmilä et al., 2017). The use of natural extracts with high content of polyphenols combined with alternative hardeners such as hexamethylenetetramine is a convenient alternative for reducing the use of formaldehyde (Böhm et al., 2016; Santos et al., 2017), aiding the design of wood panels with low emission (Vázquez et al., 2012; Santos et al., 2017). Polyphenols were industrially extracted from pine bark (Pinus radiate D. Don) in Chile and New Zealand during the 1990s. However, production has since ended. Such natural extracts were mostly used in adhesive formulations for particleboards and fiberboards (Zhang et al., 2017; Niro et al., 2016; Ghahri and Pizzi, 2018) as a partial component of phenol-formaldehyde resins (Li et al., 2018; Feng et al., 2016), or



Corresponding author. Email address: [email protected] (J. Santos)

https://doi.org/10.1016/j.indcrop.2018.05.082 Received 10 November 2017; Received in revised form 24 April 2018; Accepted 31 May 2018 Available online xxx 0926-6690/ © 2018.

were combined with p-MDI (4,4′-methylene diphenyl diisocyanate) (Valenzuela et al., 2012). The use of isocyanates in adhesive formulations offer several advantages. However, isocyanate-based adhesives show high toxicity (Carré et al., 2016). The application of radiata pine bark extracts in plywood adhesives have been studied previously (Zhou and Pizzi, 2014; Ghahri et al., 2018; Rhazi et al., 2017). However, good results were obtained only when pine bark extract was added in a 20–35% proportion to either phenol formaldehyde, urea formaldehyde resin, or to pMDI. The greatest drawbacks that inhibit the use of polyphenols in wood adhesives are their high viscosities and the short pot life, particularly in pine bark extract (Vázquez et al., 2005). In addition, the current industry standards for adhesive formulations require low viscosity ( 0.99); PB: pine bark extract.

pH 8 (7 and 10% HEX) and pH 7 (at any HEX concentration) applicable in a practical process. In such conditions, the pot life values are feasible for industrial applications.

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polyflavonoids. This is crucial for gaining better control over the storage temperature of the extract solution. In view of the results, the extract solution at 40% solids content was only applicable at pH 7. In contrast, the solution pot life was reduced at pH 8 due to the apparently fast viscosity increase, which limits its application in adhesive formulation. In a second stage, the pH (7–8) and HEX content (5–10% on extract weight) influence on the rheological behavior and pot life of adhesives prepared with PB extract and HEX hardener at 40% solids content was analyzed (Table 2). In this case, the HEX was used as a powder and was dissolved in sodium hydroxide. Under such conditions, HEX provided a great reduction in the apparent viscosity and in the consistency index (K). The aforementioned effect was maintained over time, especially at pH 8. The highest decrease in apparent viscosity was found at pH 8 and when the HEX content increased from 5% to 7%. HEX dissolved in an alkaline solution seemed to break hydrogen bonding, resulting in the observed viscosity decrease (Vázquez et al., 2001; Pizzi, 1994). Adhesive formulations with HEX showed high pseudoplastic behavior. The result was significant since the adhesive was subjected to stress during application, and the apparent viscosity decrease favors flow. The influence of the HEX hardener made the adhesive formulation at

3.3. DSC analysis

The influence of the pH of the PB extracts on self-condensation of the extract solution was evaluated. Subsequently, the influence of the HEX concentration (5–7%) and pH (7, 8) on the chemical curing reaction of the PB+ HEX adhesives was analyzed. The thermograms corresponding to the curing process are showed (Fig. 2a). After calibration, the area enclosed by the curve can be integrated to give the overall curing enthalpy ΔH in J/g (Vázquez et al., 2012). The curing energy was calculated by this value, regardless of the method for interpreting the curing process. The corresponding values are shown in Table 3. The reaction temperatures of the self-condensation process increased with pH, which is consistent with the gel time values (Fig. 1). The only exception is at pH 9. It is known that polyphenol activation increases at higher pH, but in this case the methanol extract was obtained from an industrial by-product that contained a mixture of bark and wood. In this case, the reac

Table 2 Rheological behavior of PB+ HEX adhesives at 40% as a function of pH and HEX content. pH

HEX concentration (% on extract weight)

Time since mixing (min)

Apparent viscosity (mPa s)

K (Pa sn⁠ )

n

7

5

60 120 300 60 120 300 60 120 300 60 120 300 60 120 300 60 120 300

356 141 164 115 141 164 73 65 70 6737 2169 1017 893 412 174 1232 460 134

1.03 0.54 0.66 0.44 0.16 0.21 0.28 0.17 0.07 1.79 1.47 1.28 1.24 1.03 0.72 1.26 0.93 0.61

0.58 0.75 0.70 0.76 0.86 0.83 0.74 0.74 0.85 0.60 0.62 0.53 0.57 0.58 0.64 0.67 0.73 0.68

7

10 8

5 7

10

*not measurable; HEX: hexamethylenetetramine; K: flow consistency index; n: flow behavior index; K and n were calculated from the power law by linear regression (R2⁠ > 0.99); PB: pine bark extract. 4

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tracts apparently comprise oligomer polyphenols (polyflavonoids) with linked carbohydrates, high molecular weight polyflavonoids (low mobility), carbohydrates, and stilbenes moieties (Santos et al., 2017). Results showing the influence of pH (7, 8) and HEX concentration (5–7%) on the curing reaction (Table 4 and Fig. 2b and c). One can see that a pH increase causes activation of the phenolate polyphenol groups, thus enabling the polyphenol reaction at lower temperatures with a high HEX concentration. When the polyphenol activation was high (pH 8) at the lowest HEX concentration (5%), the reaction with the less reactive extract fraction becomes the limiting reaction. The reaction temperature increases to higher values as a result. The opposite effect was observed at a low pH value (7). When the HEX concentration increased, polyphenols are less active, and the low reactivity of polyphenols at high HEX concentration shift the curing reaction to higher temperatures. The formulations that provided the lowest reaction temperature and higher enthalpy values where pH 7 (5% HEX) and pH 8 (7% HEX). 3.4. FTIR essay

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Fig. 3 shows the FTIR spectra of the dry uncured extract, as well as the extracts solutions at pH 7 and pH 8 cured at 160 °C and 120 s curing time. FTIR band assignment was based on previous studies (Kim and Kim, 2003; Pichelin et al., 1999; Lee et al., 2002). The polymerization process can be studied from the bands areas of the different polyphenol groups for the cured (Ac⁠ ) and uncured (Au⁠ c) extracts: α=100 x (Au⁠ c-AC⁠ )/Au⁠ c, is the partial polymerization degree as a function of the band studied. The changes in the main bands following the polymerization process are shown in Table 5. ⁠ 1) is associated with OH stretchThe broad band (3000–3700 cm− ing of the methylol group in polyphenol and the unreactive OH from polyphenols. Usually the most important reaction in the adhesive curing process was methylol condensation (-CH2⁠ OH) with the free active positions of other phenolic rings. This results in a relative intensity reduction in this band as the adhesive reaches a higher polymerization degree (Kim and Kim, 2003). The deformation vibration of the carbon–carbon bonds in the pheno⁠ 1 region. This peak showed a lic groups absorbs in the 1400–1500 cm− gradual decrease as polymerization progressed due to contraction in the volume system. ⁠ 1 is associated with the C The band around 1200 cm− O stretching of polyphenols. The band is reduced during the polymerization process. The deformation vibrations of the C H bond in benzene are also detected. This group disappears during polymerization. The aforementioned insight can be used as an index of the degree of polymerization. Regarding the partial polymerization degree, we can conclude that the pH influence (7, 8) in the polyphenol self-condensation process was negligible. This is correlates with the results obtained by gel time analysis and the valuable insights provided by the chemical curing study using DSC.

Fig. 2. (a) DSC curves of PB extracts at pH 7–9, (b) PB + HEX adhesives at 5–7% HEX pH 7, and (c) pH 8 at a heating rate of 10 °C/min. Table 3 Enthalpy and maximum temperature for PB (40%) solutions, measured at 10 °C/min. pH

Temp. Max. (°C)

Temp. Onset/Enset(°C)

ΔH (J/g)*⁠

7 8 9

110/127 114/132/136 130/141/152

107/127 111/156 126/160

1056 ± 22 1192 ± 12 1021 ± 15

*

calculated as the area over the curve. n = 3; PB: pine bark extract.

Table 4 Enthalpy and maximum temperature for PB + HEX adhesives (40%) measured at 10 °C/ min.

tion improved at pH 7–8, and higher pH values shift the reactivity towards competitive reactions that occur at higher temperatures. This justifies the similar enthalpy values obtained at pH 7 and 9. Regarding the PB extract solution thermograms, the effect of pH on self-condensation reaction can be observed. The first peak that appears in all the thermograms (105–115 °C) was due to the favorable self-condensation of polyphenols. The aforementioned peak, usually associated to polyphenol self-condensation shifted to lower temperatures as pH increased (Pizzi, 2016). Nevertheless, at all the pH values use during testing, other peaks occurred at higher temperatures due to competitive reactions that were especially activated at higher pH values. These ex

HEX concentration (% on extract weight) 5 7

*

5

pH

Temp. Max. (°C)

Temp. Onset/ Enset(°C)

ΔH (J/g)*⁠

7 8 7 8

137 167 158 139

114/168 144/192 125/186 105/173

1240 ± 11 925 ± 19 1125 ± 7 1225 ± 9

calculated as the area over the curve. n = 3; PB: pine bark extract.

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provided valuable insight regarding the curing process. Table 6 shows the influence of HEX in the main bands involved in the curing process. In all cases, HEX increased the polymerization degree. However, the ⁠ 1) could not be recognized. In addiN H wagging peak (700–750 cm− ⁠ 1 tion, the N H stretch band (3280–3320 cm− ) overlapped with the OH ⁠ 1 signals (3000–3700 cm− ), which explains the significant performance at increased HEX concentration. Regarding the polymerization degree, it seems that HEX used at pH 7 resulted in the highest crosslinking. Moreover, an increasing the hardener content from 5% to 7% also causes a slight increase in the final polymerization degree. 3.5. Mechanical cure (ABES)

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The curing behavior can be evaluated by ABES. The aforementioned method evaluates how fast the bond strength develops under controlled hot pressing conditions. The effects of press time (15–120 s) and temperature (120, 160, and 190 °C) on the mechanical cure were analyzed (Fig. 4). The resulting shear strength was higher at temperatures 160 and 200 °C. The behavior was similar at this temperatures, showing light fluctuations as the pressing time increases. The maximum shear strength was 6.89 MPa at 160 °C (90 s pressing time) and 6.62 MPa at 200 °C (120 s pressing time). At 120 °C, the minimum shear strength was observed at low HEX concentration (5%) and high pH (8). On the other hand, the behavior of the curves at 7% HEX and neutral pH was similar, showing insignificant differences in the shear strength after 120 s hot pressing time. At pH 7 an increased HEX concentration reduced the curing rate and resulted in a low maximum shear strength. Nevertheless, at pH 8 the HEX increase causes the opposite effect in term of the curing rate and the maximum shear strength. The results are in accordance with the DSC results regarding the chemical cure study, considering that the adhesives formulated with 5% HEX (pH 7) and 7% HEX (pH 8) showed the lowest reaction temperature and the highest curing enthalpy values. At 160 °C, the maximum shear strength was achieved at pH 7 at the highest HEX content, while the opposite effect was observed at pH 8. These results show the same correlation as those obtained with FTIR (ATR) for the adhesives formulated with HEX and cured at 160 °C dur Table 6 FTIR band assignments for the PB + HEX adhesives cured at 160 °C for 120 s.

Fig. 3. FTIR spectra for (a) dry PB extracts, (b) cured PB extract solutions, and (c) PB + HEX adhesives. Table 5 FTIR band assignments for the PB extracts solutions cured at 160 °C during 120 s. Wavenumber (cm-1) 3000–3700 1400–1500 1200 740–910

Type of bond

OH stretching

C

C deformation

C

O stretching

C

H deformation

pH

α (%)

7 8 7 8 7 8 7 8

9.84 9.59 6.53 5.96 13.32 13.14 1.87 1.95

Wavenumber ⁠ 1) (cm−

Type of bond

3000-3700

OH stretching

1400–1500

PB: pine bark extract α: partial polymerization degree calculated from the bands areas.

1200

Fig. 3c shows the FTIR spectra of the adhesives at pH 7 and pH 8 with 5% and 7% HEX after curing at 160 °C and 120 s. The pine bark polyphenols-based adhesive in the presence of HEX produces methylene bridges and a very high proportion of benzylamine bridges during the cross-linking process. A comparison between the band areas of the polyphenol extract with HEX (uncured and cured)

740–910

C C deformation

C O stretching

C H deformation

pH

HEX concentration (% on extract weight)

α (%)

7

5

17.08

7 8 8 7

7 5 7 5

16.68 16.97 16.55 14.07

7 8 8 7

7 5 7 5

14.36 13.88 13.84 23.84

7 8 8 7

7 5 7 5

24.15 23.06 23.08 3.71

7 8 8

7 5 7

3.81 3.50 3.65

PB: pine bark extract; α: partial polymerization degree calculated from the band areas. 6

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J. Santos et al.

Fig. 5. Plywood manufacturing strategy.

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this study, the influence of the time elapsed since adhesive preparation, the pH, and the adhesive solids content was studied. (2) The second set of panels were manufactured by applying the adhesive over the activated wood with a PB extract and 7% HEX diluted solution (10%) after 24 h. The influence of wood activation and adhesive pH was studied. (3) The third plywood set was manufactured via wood activation with an alkaline hexamine solution after applying the PB extract as a powder. In this case, the influence of pH and the alkaline hexamine solution was also analyzed. Plywood was manufactured with PB extracts and 7% of HEX at pH 7 and pH 8 after 2 h and 24 h following adhesive formulation. In addition, plywood was formed with a phenol-formaldehyde commercial resin under the same conditions in order to compare the results. The viscosity changes due to the reactivity of polyphenols, so the influence of the time elapsed following adhesive preparation was studied. Table 7 shows the results obtained for the internal bond strength and wood failure for dry boards and for boards immersed in cold water, boiling water, and cold water, with a traditional application method. The internal bond (IB) strength of a board is a direct measure of the performance of the adhesive. Comparing the IB values using the traditional application method of one-phase solution for dry boards, it was found that the pH, the solids content, and the time elapsed since adhesive preparation have a high influence on the board strength. As shown in Table 7, the time passed since component mixing improved the dry and wet IB results and the wet wood failure rate, except for the adhesive formulated with 40% solids at pH 8 and applied 24 h after the components were mixed. Such an adhesive slightly improved the dry IB and wet wood failure results, but the wet IB value decreased with the time passed since mixing. Regarding the influence of solids content, a significant influence on the dry IB strength was not observed at pH 8. However, an increased solids content affects the plywood water resistance by increasing the wet IB values with adhesive solids content. The behavior changes when the adhesive was applied 24 h after the preparation. The best results were obtained when the adhesive was prepared with the lowest solids content. In addition, the results show that the adhesive solids content has a considerable influence on the viscosity, wood wettability, and IB dry strength. However, the increased solid content is not directly related to better wet IB strength values, according to the requirements described in the industry standard.

Fig. 4. Development of tensile shear strength as a function of hot pressing time of PB + HEX adhesives at (a) 120 °C, (b) 160 °C, and (c) 200 °C measured by ABES.

ing 120 s. This shows the importance of spectroscopy in studying the curing reaction. At 160 °C, the adhesives increased the shear strength at pH 8 (5% HEX) and pH 7 (7% HEX). Apparently, the chemical cure process is dominated by the less reactive extract components. This explains why a reaction high temperature is required. The maximum shear strength at the highest temperature was similar for all formulations, except the adhesives formulated at higher pH values and HEX concentration. Such adhesives show smaller shear strength values, regardless of the cure times. However, our results show that strength values higher than 4 MPa could be obtained with our process. This value represents an important benchmark as this performance was obtained for phenol-formaldehyde industrial adhesives using the same technique as reported by Ghorbani et al. (2016). The curing reaction was very fast at all temperatures. In fact, the maximum shear strength was achieved in less than 60 s. 3.6. Internal bond strength evaluation

Tree methods were used for plywood manufacture using the strategy shown in Fig. 5:

(1) The first set of panels were manufactured by applying the adhesive in a single step, as is usually the case in the plywood industry. In 7

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Table 7 Results for plywood bonded with PB + 7% HEX adhesives. Time since mixing (h)

30 40 40 30 40 40 PF

2 2 2 24 24 24 –

pH

Dry IB strength (MPa)

Dry Wood Failure (%)

Wet IB strength after 24 h in cold water (MPa)

Wet Wood Failure (%)

6 h boil water + 1 h cold water IB strength (MPa)

Wet Wood Failure (%)

8 7 8 8 7 8 –

1.50 ± 0.13 1.75 ± 0.04 1.41 ± 0.13 2.34 ± 0.33 1.82 ± 0.06 1.62 ± 0.05 2.53 ± 0.11