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Improvement Effect of Montmorillonite on Resistance-Humidity Relationship of MF Resin

Melamine formaldehyde resin (MF resin) is a well‑known thermosetting polymer used widely in coatings, laminates, and composite materials. Less‑known is its inherent humidity‑sensitive electrical property: a pure MF film changes electrical resistance as ambient relative humidity shifts. However, unmodified melamine‑formaldehyde suffers from long response‑recovery time, poor linearity, and large humidity hysteresis, limiting its practical deployment as a humidity‑sensing layer. Introducing montmorillonite (MMT), a natural layered clay mineral, to create a composite drastically improves the resistance‑humidity relationship, producing a promising low‑cost thermoset‑based humidity‑sensing film.

Challenges of Pure MF resin as Humidity‑Sensing Material

Cured MF resin contains polar groups that absorb water molecules, so its resistance varies with surrounding humidity. Nevertheless, neat MF shows obvious technical drawbacks for sensor fabrication:

Slow dynamic response: response time 125 s and recovery time 25 s, too sluggish for real‑time monitoring.
Low linear correlation: the linear coefficient |R| reaches only 0.970, introducing significant measurement deviation.
Large humidity hysteresis up to 12.3 %RH, inconsistent readings during adsorption‑desorption cycles.
High base resistance, limiting measuring-circuit compatibility.

These constraints prevent direct manufacture of pure MF resin into qualified resistive humidity sensors. Montmorillonite (MMT) modification addresses most of these bottlenecks through its layered nanostructure, high specific surface area, and intrinsic cation‑exchange capacity.

MF resin and Montmorillonite Composite Preparation Workflow

MF resin and Montmorillonite composite material is synthesized via pre‑polymer mixing, ultrasonic dispersion, dip‑coating, and thermal curing, without complex high‑pressure equipment.

1. Synthesize MF prepolymer: melamine reacts with formaldehyde solution under controlled conditions; adjust solid content to 30 % or 60 % and add glycerol as an auxiliary agent.

2. Add weighted montmorillonite powder to the MF prepolymer solution; shake, then ultrasonicate to achieve homogeneous MMT dispersion. Two MMT loading levels (2 and 4 parts) are studied in lab trials.

3. Dip‑coat the mixed MF‑MMT dispersion onto interdigital electrode substrates.

4. Dry at 90 °C, then cure at 130 °C to form a cross‑linked thermoset MF@MMT sensing film.

Among the tested formulations, MF60@MMT4 (60 %‑solid MF prepolymer with 4‑part MMT additive) delivers the best overall sensing performance and becomes the preferred sample for comprehensive characterization.

Material Characterization of MF resin and MMT Composite

SEM, AFM, FT‑IR, and TGA tests confirm successful compounding of MF resin and montmorillonite.

Morphology (SEM & AFM): The pure MF surface is smooth. After MMT incorporation, surface average roughness increases to 21.7 nm. Moderate roughness facilitates water‑molecule adsorption and diffusion across the sensing layer. MMT sheets are uniformly wrapped inside the MF matrix without massive agglomeration.
FT‑IR spectroscopy: Characteristic absorption peaks of both melamine‑formaldehyde resin and montmorillonite co‑exist in MF@MMT spectra, confirming physical‑chemical composite formation.
Thermal stability (TGA‑N₂): MF@MMT shows thermal‑decomposition behavior similar to the original MF resin. The composite remains structurally stable up to around 300 °C, suitable for high‑temperature sensing scenarios.
EDS elemental analysis: MMT contains abundant Mg, Al, and trace Ca cations, which play a critical role in ion‑conduction under humid conditions.

Resistance‑Humidity Sensing Performance of MF resin and MMT Composite

Measurements are carried out across 30 %RH ~ 90 %RH at room‑temperature, AC 1 V / 1 kHz test condition. Compared with neat MF, MF60@MMT4 shows remarkable performance improvements:

Performance parameter	Pure MF	MF60@MMT4 (optimised)
Resistance range (30‑90 %RH)	1‑17 MΩ	1.53‑5.24 MΩ
Response / Recovery time	125 s / 25 s	28 s / 5 s
Linear coefficient \|R\|	0.970	0.997
Humidity hysteresis	12.3 %	5.39 %
Temperature coefficient (10‑70 °C)	Higher drift	0.0159 (very low thermal drift)

Sensing Mechanism

Impedance analysis reveals dual conduction modes inside MF@MMT:

Low‑humidity environment: Intrinsic electronic conduction mainly dominates the composite impedance.
High‑humidity environment: Water molecules intercalate into MMT interlayer spaces. Exchangeable Mg² and Ca²⁺ cations from montmorillonite are activated, and ion conduction becomes the primary charge‑transport pathway, which lowers overall film resistance as relative humidity rises.

The layered MMT structure provides abundant adsorption sites for water vapor, speeds up molecule diffusion, shortens response‑recovery time, and greatly reduces hysteresis error. Meanwhile, higher surface roughness promotes fast water‑vapour uptake and desorption.

Thermal and Long‑Term Stability

MF resin and Montmorillonite composite demonstrates outstanding anti‑interference capability against temperature variation from 10 °C to 70 °C. MF60@MMT4 achieves a minimal temperature coefficient of 0.0159, much better than many conventional polymer‑based humidity‑sensitive films. In continuous 30‑day aging tests under cyclic humidity conditions, MF@MMT maintains stable resistance output without obvious signal drift, demonstrating reliable long‑term service life for resistive humidity‑sensor devices.

Key Formulation Influencing Factors

Solid content of MF prepolymer: 60 % solid‑content MF combined with appropriate MMT loading yields superior sensing performance. Excessive viscosity impairs coating uniformity on interdigital electrodes; too-low solid content reduces final film thickness and signal amplitude.
Montmorillonite addition amount: Within the test scope, higher MMT loading improves linearity and reduces hysteresis. Excess MMT triggers particle aggregation and degrades film‑forming quality.
Curing temperature: Proper 130 °C curing completes MF cross‑linking, obtaining insoluble thermoset composite films for stable sensor operation.

Main Application Potentials of MF resin and Montmorillonite Sensing Material

Benefiting from its thermoset nature, high thermal tolerance, low hysteresis, and long‑term stability, MF resin and Montmorillonite opens new application directions for melamine‑formaldehyde functional materials:

Resistive‑type humidity‑sensing elements for industrial environmental monitoring.
High‑temperature‑tolerant humidity‑detecting layers working under elevated‑temperature conditions.
Cost‑effective sensing material for IoT humidity‑monitoring nodes.
Functional modification research platform for other thermosetting resin‑based sensor development.

conclusion-Improvement Effect of Montmorillonite on Resistance-Humidity Relationship of MF Resin

Montmorillonite modification effectively optimizes the resistance‑humidity relationship of melamine‑formaldehyde resin. The optimized MF60@MMT4 composite film achieves high linearity (|R|=0.997), low humidity hysteresis (5.39 %), fast response‑recovery (28 s/5 s), low temperature‑drift coefficient, and 30‑day long‑term stability in the 30 %RH‑90 %RH testing range.

Montmorillonite layered structure and cation‑exchange capability introduce ion‑conduction channels and accelerate water‑molecule adsorption‑desorption processes. This research expands melamine‑formaldehyde resin from traditional coating‑laminate use to functional electronic sensing materials. It provides a feasible route to develop low‑cost thermoset‑based resistive humidity‑sensor devices.