Doctor of Philosophy / Department of Grain Science and Industry / Xiuzhi Susan Sun / Soy protein is one of the most promising bio-degradable adhesives, with great
potential as alternatives synthetic petroleum based adhesives for wood composite industries.
However, its intrinsic drawbacks such as low water resistance, high viscosity, and short shelf
life still limit its broad application. In this research, soy protein was further modified and
characterized to improve adhesion properties, flow-ability, water resistance, and long shelf
life, which could facilitate the industrialization of soy protein based adhesives.
In this study, we exploited the in situ sodium bisulfite (NaHSO3) modification on soy
protein in soy flour-water extracts, and then the modified soy protein was obtained through
acid precipitation. First, different concentrations of NaHSO3 were used to modify soy flour
slurry, then glycinin-rich and β-conglycinin-rich fractions were precipitated at pH 5.4 (SP 5.4)
and pH 4.5 (SP 4.5), respectively. Unmodified sample SP 5.4 and SP 4.5 showed clay-like
properties and viscoelastic properties, respectively; whereas with addition of NaHSO3 in
range of 2-8 g/L, both SP 5.4 and SP 4.5 had the viscous cohesive phase with good handability
and flow-ability. The overall adhesion performance of SP 4.5 was better than SP 5.4;
the wet strength of these two fractions was in the range of 2.5-3.2 MPa compared to 1.6 MPa
of control soy protein isolate.
Then soy protein with various β-conglycinin/glycinin (7S/11S) ratios were extracted
from soy flour slurry and characterized for adhesion properties based on the different
solubility of 7S and 11S globulins. Seven glycinin-rich soy protein fractions and six β-
conglycinin-rich soy protein fractions were obtained. According to the morphology, viscosity,
and particle size results, we proposed that proper protein-protein interaction, hydration
capacity (glycinin-rich fractions), and certain 7S/11S ratios (β-conglycinin-rich fractions) in
modified soy protein are crucial to continuous protein phase formation. The viscous cohesive
samples were stable for up to several months without phase separation at room temperature,
with the wet adhesion strength of 2.0-2.8 MPa.
The soy protein modified with NaHSO3 showed good compatibility with commercial
glues applied on plywood and paper labeling fields. The modified soy protein made some
functional groups, carboxylic (-COOH), hydroxyl (-OH) and amino groups (-NH2) available,
which cross-linked with hydroxymethyl groups (-CH2-OH) from urea formaldehyde (UF)
wood glue. The modified soy protein (MSP) with pH 4.8 also acted as an acidic catalyst for
the self-polymerization of UF based resin. The wet adhesion strength of MSP/UF blends
(40/60) was 6.4 MPa with 100% wood cohesive failure, as compared to 4.66 MPa of UF. As
to the paper labeling application, peel strength of MSP on glass substrate increased rapidly,
with curing time much shorter than commercial polyvinyl acetate based adhesives (PVAc).
And the MSP/ PVAc blends showed shorter curing time, higher water resistance and lower
viscosity than pure PVAc.
Chemical modification could also enhance the adhesion strength of MSP. 2-octen-1-
ylsuccinic anhydride (OSA) was proved to be grafted on soy protein through reaction
between amine, hydroxyl groups of protein and anhydride groups. The oily nature and
hydrophobic long alkyl chains of OSA mainly contributed to the significant water resistance
improvement of MSP.
| Identifer | oai:union.ndltd.org:KSU/oai:krex.k-state.edu:2097/16178 |
| Date | January 1900 |
| Creators | Qi, Guangyan |
| Publisher | Kansas State University |
| Source Sets | K-State Research Exchange |
| Language | en_US |
| Detected Language | English |
| Type | Dissertation |
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