J. Li, et al.
IndustrialCrops&Products140(2019)111725
food-grade soybean flour and a wet strength resin. In 2017, the issue of
the more server Chinese Standard GB18580-2017 about formaldehyde
release from wood composites promotes the increasing been application
of soybean-based adhesives in the wood industry in China for compo-
sites manufacturing (e.g., plywood, blockboard, particleboard, and fi-
berboard), due to the additional advantages of environmental safety
Commonly, commercial soybean-based adhesive is a two-compo-
nent formulation composed of SM powder and EMPA solution (as either
nents are stored independently and mechanically blended at room
temperature to formulate the adhesive just before use. However, com-
pared to adhesive from fresh EMPA solution and freshly ground SM
powder, the bond strength and water resistance of the plywood pre-
pared from fresh EMPA solution and stored SM powder (especially that
stored for more than 3 months) deteriorated significantly. This occur-
rence is probably attributed to the ambient aging of SM powder during
its storage. It is industrially important to understand what happens to
SM powder during storage to avoid the ineffectiveness and instability of
SM powder and SM-based adhesive.
ground to a fine powder that passed through a 160-mesh sieve using a
high-speed crusher. EMPA solution as crosslinker was synthesized from
diethylenetriamine, adipic acid, and epichlorohydrin in our lab, and
had a solid content of 13.4%, a pH value of 3.2, and a viscosity of
68 MPa s (25 °C). Urea and other chemicals at reagent grade were ob-
tained from Tianjin Komio Chemical Co. Ltd. (Tianjin, China). Birch
veneers with dimensional size of 420 mm × 420 mm, thickness of
1.6 mm and moisture content of 6–8% were provided by Weihe Fulin
Plywood Plant (Heilongjiang, China).
2.2. Ambient aging and thermal oxidative aging of SMs
The freshly ground SM powders were placed into an open enamel
container with a sample thickness of approximately 1 mm, and then
subjected to ambient aging under the following conditions: ambient
temperature (20–30 °C), relative humidity (50–70%), no sunshine, and
ventilation. The SM powder was sampled and ground monthly for six
consecutive months. Six ambient-aged samples, with aging times of 1,
2, 3, 4, 5, and 6 months (labeled as SM-1 m, SM-2 m, SM-3 m, SM-4 m,
SM-5 m and SM-6 m), were obtained. A freshly ground SM sample was
taken as the control for characterization.
Similar findings for the time-dependent physical and chemical
changes of biopolymers, such as protein and/or wood, under ambient
protein concentrate powder decreased while being stored at 25 °C for
14 weeks, due to the protein denaturation and intermolecular interac-
ported that the mechanical properties of medium-density fiberboard
were greatly decreased when made with ambient-aged wood fibers.
This is attributed to the poor wettability between adhesives and the
tional property changes of soybean protein concentrate films was stu-
died; the reorganization of protein secondary structures and the mole-
cular interactions via disulfide crosslinks and/or Maillard aggregates
ditionally, reports indicated that soybean protein isolate plastics had
gradually decreased tensile strengths over the course of storage for 180
These works confirmed that many types of biomass suffers ambient
aging during storage, which may significantly affect the service life and
quality stability of commercial biomass-based products, due to dete-
riorated performances.
However, to our knowledge, no study has yet been reported on the
ambient aging of SM powder and its effects on the properties of SM-
based adhesives. In the current study, freshly ground SM powder was
subjected to ambient aging for 6 months under the following condi-
tions: ambient temperature (20–30 °C), relative humidity (40–60%), no
direct sunshine, and ventilation. The physicochemical properties of SM
powder and SM-based adhesive with various aging times were sys-
temically evaluated by means of Fourier transform infrared analysis
(FTIR, both transmission and reflection modes), X-ray photoelectron
spectroscopic (XPS) analysis, gel permeation chromatographic (GPC)
analysis, thermogravimetric analysis (TGA), X-ray diffraction (XRD)
analysis, sol-gel testing, contact angle measurements, functional group
determination, and mechanical testing of the SM-based adhesive-
bonded plywood, which inform the mechanisms of the SM powders’
ambient aging and its effects on the SM-based adhesives for industrial
applications.
2.3. Preparation of SM-based adhesives
Prior to adding 35 g of SM powder (35 g), 89.55 g of EMPA solution
in beaker was blended with 10.45 g of deionized water to adjust its solid
content to 12.0 wt%. The above mixture was mechanically stirred at
room temperature and rotating speed of 200 rpm for 5 min to form a
nongranular SM-based adhesive with a solid content of 32.5 wt%. SM-
6 m, SM-5 m, SM-4 m, SM-3 m, SM-2 m, SM-1 m and fresh SM were used
to prepare the corresponding adhesive samples.
2.4. Fourier transform infrared (FTIR) analysis of SMs with transmission
and reflection modes
All SM samples were FTIR analyzed with transmission and reflection
modes to evaluate the chemical structures of SM samples on their sur-
faces and in bulk. Transmission-FTIR was performed after mixing the
SM sample with potassium bromide crystals at a mass ratio of ap-
proximately 1/150 and molding into a transparent FTIR disk. A re-
flecting accessory was equipped to obtain reflection-FTIR spectra that
detected the chemical structures of each sample at approximately 5 nm
thickness. The two modes of FTIR spectra were recorded using a Nicolet
7600 spectrometer (Nicolet Instrument Corp., Madison, WI) from 500
to 4000 cm−1 with a 4 cm−1 resolution and 32 scans. The relative IR
absorbance of the carbonyl group of SM with various aging times was
determined according to the Lambert-Beer law by taking the C–H peak
2.5. X-ray photoelectron spectroscopic (XPS) analysis of SMs
Each SM sample was powdered and analyzed by an ESCALAB250
XPS apparatus (Thermo Fisher Scientific) equipped with an Al-K source.
All XPS spectra were recorded with 0.1 eV step and 20 eV pass energy.
Curve-fitting analyses of the C 1s, N and O peaks were performed
1s
1s
with Casa XPS 2.3.16 software.
2.6. Gel permeation chromatographic (GPC) analysis of SMs
A reaction kettle equipped with mechanical stirrer and refluxing
2. Materials and methods
equipment was charged with 5 g of SM and 100 g of urea solution
(8 mol/L). The mixture was maintained at 50
2 °C for 2.5 h with
2.1. Materials
stirring to dissolve the SM protein completely, during which partial
urea might convert to ammonia by the urease in the SM without
deenzyming or thermal treatment; however the urea concentration
(8 mol/L) was sufficient for complete dissolution of SM sample. After
Soybean meal (SM) with 43.5% soy protein content was obtained
from Laihe Oil Pressing Factory (Shangdong, China). The SM was
2