Recently, Ninglu Zhang, Liangxian Liu, Yuyan Jiang, Haiyu Li, Na Wang, Ming Wei, Dexiu Min, Zhenqian Pang, Yanjun Xie, Jian Li, and Shaoliang Xiao have successfully developed a viscosity-tunable polyvinyl alcohol/tannic acid supramolecular adhesive (PTSA) based on a mussel-inspired biomimetic strategy!
This adhesive uses ethanol as a modulator of intermolecular physical interactions. Viscosity is dynamically adjustable by adjusting the ethanol/water ratio and temperature. It exhibits excellent water resistance, strong adhesion, and high processability, demonstrating outstanding performance in plywood, particleboard, and wood coatings. Its relevant properties meet corresponding national standards, and its formaldehyde emission meets the most stringent ENF level requirements, providing a new pathway for the multifunctional application of biomass-based supramolecular adhesives.
The relevant findings were published in the *Chemical Engineering Journal*.
With the depletion of petrochemical resources and the intensification of climate change, the utilization of sustainable resources has received widespread attention. Wood, as an abundant renewable resource, requires adhesives in panel processing. While traditional formaldehyde-based adhesives offer good bonding performance, the formaldehyde they release poses a health risk, thus the demand for green, environmentally friendly, formaldehyde-free wood adhesives continues to grow.
Polyvinyl alcohol (PVA), as a water-soluble polymer, has advantages such as being non-toxic and low-cost, but its water resistance is poor. Mussel-inspired adhesives have attracted attention due to their strong water resistance and wide-ranging adhesion. Tannic acid (TA), as a low-cost natural polyphenol, is a good alternative to dopamine. However, direct mixing of PVA and TA easily leads to agglomeration and precipitation, and viscosity is difficult to control. Existing related adhesives suffer from low bonding strength and limited application scenarios, restricting their application in wood bonding.
Based on this, the authors developed a polyvinyl alcohol/tannic acid supramolecular adhesive (PTSA) using ethanol as a modifier of intermolecular physical interactions. The viscosity of the adhesive was controlled by adjusting the ethanol/water ratio and temperature. The preparation process of the adhesive was systematically studied, and parameters such as reaction time, ethanol content, PVA/TA ratio and hot pressing conditions were optimized. Its application in plywood, particleboard preparation and wood coating was explored. The bonding performance, water resistance and thermal stability of the adhesive were analyzed by various testing and characterization methods. At the same time, the mechanism of ethanol in regulating viscosity was revealed by molecular dynamics simulation.
Experimental Materials
Experimental reagents included: Polyvinyl alcohol (PVA 1799, 99% hydrolyzed), tannic acid, ethanol, distilled water, poplar veneer, and poplar wood shavings.
Experimental Procedures
Preparation of PVA Adhesive: PVA powder was added to mixed solvents with different ethanol/water ratios and stirred at 90°C for 3 hours. An ethanol-free PVA adhesive was prepared simultaneously.
Preparation of PTSA: PVA and TA were added to mixed solvents with different ethanol/water ratios in appropriate proportions and stirred under sealed conditions at 90°C for 3 hours.
Preparation of H-PTSA: PTSA was cast into a film and heated in an oven at 150°C for 5 minutes.
Preparation of Three-Layer Plywood: PTSA was evenly coated on both sides of the middle layer of wood veneer. The three layers of veneer were assembled along the grain direction, cold-pressed at room temperature for 4 hours, and then hot-pressed at 170°C and 1 MPa for 5 minutes.
Preparation of Particleboard: Surface and core wood shavings are uniformly mixed with PTSA at 90℃. The mixture is then layered and pre-pressed in a mold, followed by hot pressing at 210℃ and 3 MPa for 5 minutes.
Preparation of Wood Coating: PVA and PTSA adhesives are uniformly coated onto the surface of the wood board. After drying at room temperature for one day, the boards are dried in an oven at 150℃ for 0, 5, 10, and 15 minutes respectively.
Testing and Characterization Methods
Rotational Viscometer: Tests adhesive viscosity
Scanning Electron Microscopy (SEM): Observes the cross-sectional morphology of plywood
Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA): Analyzes thermal properties
X-ray Diffraction (XRD): Characterizes crystal structure
X-ray Photoelectron Spectroscopy (XPS): Analyzes changes in chemical functional groups
Variable Temperature Fourier Transform Infrared Spectroscopy (FTIR) and Two-Dimensional Correlation Spectroscopy (2D-COS): Studies curing mechanisms
Universal Testing Machine: Tests mechanical properties
Contact Angle Meter: Tests coating contact angle
Simultaneously, according to relevant national standards, the following were tested:
Formaldehyde emission from plywood
Water absorption and thickness swelling rate of particleboard
Coating hardness and adhesion, etc.
Functions of Each Component
Polyvinyl Alcohol (PVA): Provides abundant hydroxyl groups, enhancing adhesive properties. It is the basic polymer matrix of adhesives, with advantages such as non-toxicity, low cost, and scalability.
Tannic acid (TA) is a natural polyphenol, rich in catechol and pyrogallol groups, which endow adhesives with excellent water resistance and bonding ability. It can form supramolecular hydrogen bonds with PVA, disrupting the ordered arrangement of PVA chains, reducing curing energy, and improving thermal stability.
Ethanol acts as a modulator of intermolecular physical interactions, weakening van der Waals forces within PVA chains and between PVA and TA, preventing agglomeration and precipitation, forming a homogeneous solution, and achieving adjustable viscosity. Simultaneously, the azeotropic effect of ethanol and water reduces curing temperature and energy consumption.
Water acts as a solvent, dissolving PVA and TA, providing a medium for the reaction.
Reasons for the excellent material performance
PTSA's excellent performance mainly stems from the following factors:
Synergistic effect of components: PVA provides abundant hydroxyl groups to form the bonding basis, while TA's polyphenolic structure endows it with water resistance and bonding activity. The two form a stable structure through supramolecular hydrogen bonds, while TA disrupts the ordered structure of PVA, reducing curing energy consumption.
Ethanol's Modulating Effect
Ethanol acts as a modulator of intermolecular interactions, weakening van der Waals forces, preventing PVA and TA aggregation, achieving adjustable viscosity, improving processing performance, and lowering curing temperature.
Stable Cross-linked Structure Formation After Curing
During hot pressing, PTSA forms a mechanical interlock with the wood, while simultaneously forming a cross-linked network of hydrogen and ether bonds through hydroxyl condensation, improving cohesive strength and water resistance.
Optimized Preparation Process
By controlling parameters such as the ethanol/water ratio, temperature, and PVA/TA ratio, the adhesive maintains optimal performance in different application scenarios.
Key Innovations of the Paper
The main innovations of this paper are:
A novel strategy using ethanol as a modulator of intermolecular physical interactions is proposed to solve the problems of easy agglomeration and difficult viscosity adjustment when mixing PVA and TA, achieving dynamic viscosity adjustment of the adhesive.
A mussel-inspired PVA/TA supramolecular system is constructed, synergistically improving bonding strength and water resistance through hydrogen bonding and ether bonding cross-linking networks.
The adhesive is applied in multiple fields, including plywood, particleboard, and wood coatings.
The product performance meets relevant national standards, with formaldehyde emission reaching ENF level.
The viscosity regulation mechanism and curing mechanism are systematically revealed through molecular dynamics simulations and various characterization methods, providing a theoretical basis for the design of biomass-based supramolecular adhesives.
Website: www.china-pva.net
Whatsapp: 0086-15380400285
E-mail: sales2@china-pva.net
