Response surface methodology was used to optimize the preparation conditions of soy-based adhesives (SBAs) in this work. The parameters such as the effects and interactions of waterborne polyurethane (WPU) addition level (
Formaldehyde-based adhesives such as urea-formaldehyde (UF), phenol-formaldehyde (PF), and melamine-formaldehyde (MF) are widely applied in wood industry due to their excellent adhesion performance and low cost [
Therefore, the need to find an environment-friendly and sustainable substitute is urgent at present. Great efforts have been made to develop formaldehyde-free adhesives from natural biomaterials, especially soy flour. Soy flour is one of the research focuses because not only it has high content of soy protein, but also it is abundant, affordable, and readily available [
In the past years, many attempts have been carried out to improve water resistance of soy-based adhesives. Most efforts have focused on unfolding soy proteins initially, exposing hydrophobic subunits and then employing methods such as enzymatic modification [
Considering both modification effect and cost, waterborne polyurethane (WPU) resins are chosen as cross-linking agent in this paper. WPU resins not only have good mechanical properties and low water absorption that can meet the requirements of modification of soy-based adhesives, but also are an environmental-friendly cross-linking agent that can resolve the concerns in formaldehyde emission and environment contamination. In addition, WPU resins are widely applied in many fields of industrialized production, including adhesives, films, foams, plastics, coating, and so on. Few reports have been reports that used WPU to modify soy protein to prepare films and foams [
Response surface methodology (RSM) is an effective experimental design methodology, which can explore the interactions between independent variables and one or more dependent variables and predict their responses under specified sets of conditions [
Therefore, the objective of this study was to research the interactions between independent variables (WPU addition level
Soy flour (protein: 52%; moisture: 10%) purchased from Xianglin Food Co., Ltd, China, was used as received. WPU was a commercial product and purchased from Guanzhi New Material Technology Co., Ltd, with the name of AH-1610 (solid content about 38%, pH 7–9, and viscosity 100–200 MPa·s). All other chemicals were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd, China. Poplar wood veneers with the dimensions of 600 mm × 600 mm × 1.6 mm (width × length × thickness) were provided by Zhensheng Wood Industry Co., Ltd, China. The moisture content of the veneer samples was no more than 10%.
Fourier transform infrared spectroscopic (FTIR) data of WPU, soy flour, and SBA were recorded with a NICOLET-is 5 spectrometer (Thermo Fisher, USA). The samples of WPU and SBA were kept into the oven for 2 h at 140°C. The cured samples were ground into fine powder. A total of 16 scans were performed with a resolution of 4 cm−1 in the range 650–4000 cm−1, using an attenuated total reflectance accessory (ATR).
A 500 ml three-neck flask equipped with a mechanic stirrer, a condenser, and a thermometer was charged with distilled water (190.5 g), ethylene glycol (1.5 g), sodium hydroxide (7.5 g), and urea (1.5 g). The solutions were blended together, and then, soy flour (99 g) was slowly added to the flask with rapid agitation. The mixture was heated to 80°C in 15–20 min and held for another 60 min. After addition of different amount of WPU into the SBA slurry, it was stirred for different times at different temperatures. The slurry was cooled to room temperature and adjusted to a pH value between 8.0 and 9.0 with 10 wt.% ammonium chloride solution.
All obtained SBAs were applied to prepare duplicate samples of three-layer plywood panels by coating 350 g/m2 of the adhesives on both sides of veneer. Then, the coated veneer was stacked between two uncoated ones with the grain directions of two adjacent veneers perpendicular to each other. These assembled plywood samples were cold-pressed at 0.8 MPa for 60 min at room temperature and then hot-pressed at 1.2 MPa and 85 s/mm at 120°C. After hot press, the panels were stored at ambient temperature for at least 24 h before tests.
Test samples shown in Figure
Test sample size of plywood.
A single-factor experiment was carried out to determine the preliminary range of wet shear strength including WPU addition level (2∼14 wt.%), reaction temperature (60∼80°C), and reaction time (20∼140 min). The wet shear strength was the dependent variable.
The software Design Expert (Trial Version 10.0.1.0, Stat-Ease Inc., Minneapolis, USA) was employed for experimental design. Based on the previous single-factor experiment, the standard RSM design called BBD (Box–Behnken Design) was applied to research the influence of three independent variables s (
Variables and levels used in RSM design.
Coded and uncoded variables | Levels | ||
---|---|---|---|
Factors | −1 | 0 | 1 |
WPU addition level ( |
8 | 10 | 12 |
Temperature ( |
60 | 70 | 80 |
Time ( |
80 | 100 | 120 |
Experimental scheme and results.
Run number | WPU addition level ( |
Reaction temperature ( |
Reaction time ( |
Wet shear strength ( |
---|---|---|---|---|
1 | 10.00 | 80.00 | 120.00 | 0.97 |
2 | 8.00 | 70.00 | 80.00 | 1.03 |
3 | 10.00 | 60.00 | 120.00 | 1.02 |
4 | 10.00 | 70.00 | 100.00 | 1.18 |
5 | 10.00 | 70.00 | 100.00 | 1.13 |
6 | 12.00 | 70.00 | 80.00 | 1.06 |
7 | 8.00 | 80.00 | 100.00 | 0.98 |
8 | 10.00 | 70.00 | 100.00 | 1.14 |
9 | 8.00 | 60.00 | 100.00 | 1.09 |
10 | 10.00 | 70.00 | 100.00 | 1.12 |
11 | 12.00 | 70.00 | 120.00 | 1.12 |
12 | 12.00 | 60.00 | 100.00 | 0.97 |
13 | 12.00 | 80.00 | 100.00 | 1.14 |
14 | 8.00 | 70.00 | 120.00 | 0.93 |
15 | 10.00 | 60.00 | 80.00 | 1 |
16 | 10.00 | 70.00 | 100.00 | 1.13 |
17 | 10.00 | 80.00 | 80.00 | 1.05 |
The following second-order polynomial model was used to describe the relationship between the dependent variable and the independent process factors [
The quality of the fit of the polynomial model equation was assessed by the coefficient of determination (
The effect of WPU addition level on wet shear strength is shown in Figure
Effects of WPU addition level on wet shear strength.
The reaction temperature was an important factor to fabricate soy-based adhesive. Thus, the effects of reaction temperature ranging from 30°C to 90°C were investigated under WPU addition level of 10 wt.% and reaction time of 120 min. As shown in Figure
Effects of reaction temperature on wet shear strength.
By fixing the WPU addition level of 10 wt.% and reaction temperature of 70°C, respectively, the effects of reaction time ranging from 20 to 140 min were studied. Figure
Effects of reaction time on wet shear strength.
The important independent variables (
In order to determine whether the second-order polynomial model was significant, it was necessary to apply ANOVA. The results of the analysis of variance and fitness of the model are presented in Table
Variables and levels used in RSM design.
Source | SS |
|
MS |
|
|
Significance |
---|---|---|---|---|---|---|
Model | 0.084 | 9 | 9.328 × 10−3 | 9.67 | 0.0034 |
|
|
8.45 × 10−3 | 1 | 8.45 × 10−3 | 8.76 | 0.0211 |
|
|
4.5 × 10−4 | 1 | 4.5 × 10−4 | 0.47 | 0.5165 | — |
|
1.25 × 10−3 | 1 | 1.25 × 10−3 | 1.30 | 0.2924 | — |
|
0.02 | 1 | 0.02 | 20.33 | 0.0028 |
|
|
6.4 × 10−3 | 1 | 6.4 × 10−3 | 6.64 | 0.0367 |
|
|
2.5 × 10−3 | 1 | 2.5 × 10−3 | 2.59 | 0.1514 | — |
|
5.158 × 10−3 | 1 | 5.158 × 10−3 | 5.35 | 0.0540 | — |
|
0.015 | 1 | 0.015 | 15.72 | 0.0054 |
|
|
0.021 | 1 | 0.021 | 21.40 | 0.0024 |
|
Residual | 6.75 × 10−3 | 7 | 9.643 × 10−4 | — | — | — |
Lack of fit | 4.55 × 10−3 | 3 | 1.517 × 10−3 | 2.76 | 0.176 | — |
Pure error | 2.2 × 10−3 | 4 | 5.5 × 10−4 | — | — | — |
Correlation total | 0.091 | 16 | 9.328 × 10−3 | — | — | — |
|
0.9256 | — | — | — | — | — |
The lack of fit was used to evaluate the validity of the model. In this model, the
The WPU addition level (
By employing RSM, the influences of the
Response surface and contour plots for effect of temperature and WPU addition level on wet shear strength.
Response surface and contour plots for effect of time and WPU addition level on wet shear strength.
Response surface and contour plots for effect of time and temperature on wet shear strength.
However, the influences of the
The predicted optimal conditions derived from surface response experiments were obtained by using (
Figure
FTIR spectra of WPU, soy flour, and SBA with 12 wt.% WPU addition.
In this research, SBA with high water resistance was prepared and the preparation conditions were optimized by applying RSM. The statistical analysis illustrated a significantly good fit for the model that could be used to navigate the experimental design space. The regression model for SBA preparation was significant (
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work was supported by the grant from Forestry Science and Technology Demonstration Project Funds of Central Finance (2016[XT] 006) and Major Project of Science and Technology of Hunan Province (2016NK1001-3).