Amrita Chatterjee‡
a,
Shashank Jha‡b,
Sushmit Sen
a,
Keshav Devc,
Chayan Das
b and
Pradip K. Maji
*a
aDepartment of Polymer and Process Engineering, Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur 247001, India. E-mail: pradip@pe.iitr.ac.in
bDepartment of Chemistry, Visvesvaraya National Institute of Technology, Nagpur, Maharashtra 440010, India
cDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee, 247667, India
First published on 8th March 2025
Acrylic-based wood adhesives are widely recognized for their durability, UV resistance, and rapid drying properties, traditionally achieved using isocyanate-based curing systems despite their inherent toxicity. In this study, the potential of polysilazane (PSZ) as an alternative cross-linker for functional acrylic polymers was evaluated, focusing on adhesive properties. The crosslinking interactions between the acrylic polymer and PSZ formed a highly stable Si–O reinforced three-dimensional network, as characterized by analytical techniques and further supported by enhanced thermal stability and adhesive performance. The PSZ-cured acrylic system exhibited a glass transition temperature (Tg) increase to 56 °C from 31 °C of the uncrosslinked copolymer. In the tested range of 10–25 wt% of PSZ, the formulation containing 20 wt% PSZ achieved a 97.9% degree of crosslinking. Compared to traditional diisocyanate-crosslinked systems, better adhesive performance was obtained with maximum tensile shear strength values of 4.4 MPa for wood substrates (substrate failure) and up to 4.0 MPa for aluminum substrates. These findings confirm that PSZ enhances the mechanical properties of acrylic adhesives, offering optimal performance and ease of application and underscoring their practical utility.
Acrylic adhesives are predominantly cured with isocyanates, which form robust cross-linked networks that significantly enhance mechanical properties such as strength and chemical resistance. These classes of adhesives can also be crosslinked with various crosslinkers7 to suit specific needs such as polyfunctional acrylates like TMPTA (Trimethylolpropane triacrylate) for UV resistance and strength,8 or epoxy resins to boost chemical and heat resistance,9 while aziridines ensure quick curing and durability.10 Recently amino silanes like APTES have been used to promote adhesion to inorganic surfaces.11 However, concerns regarding the health and environmental risks associated with the leading option isocyanates—such as the release of volatile organic compounds (VOCs) and respiratory hazards—have prompted the search for safer alternatives.12
Silicon-based modifications offer an effective strategy to enhance acrylic-based coatings and adhesives, leveraging silicon's inherent flexibility, weather resistance, and thermal stability.13,14 These enhancements improve resistance to UV radiation and moisture, while also expanding adhesion to non-polar and low-surface-energy substrates.15 The interaction and chemistry of the silane and acrylic components have been described in depth in a work by Andre et al.16 Amino silanes enhance adhesion between acrylate polymers and glass by forming chemical bonds. The silane's methoxy head groups bond with silanols on the glass, leaving amine groups to interact with carboxylic acid functionalities in the polymer, creating ionic bonds. This modification improves interfacial structure, promotes molecular ordering, and strengthens adhesion, making it valuable for applications requiring robust polymer-glass interfaces.17 In the work of Pan et al., a silicon-based adhesion promoter was synthesized and applied to addition-type silicone rubber, enhancing bonding performance and compatibility. At 2.0 phr, it achieved a shear strength of 1.33 MPa with copper while maintaining ∼90% transmittance and improving wettability (contact angle reduced from 52.1° to 48.2°). The modification effectively improved adhesion and preserved optical properties, making it suitable for silicone-modified acrylic adhesives.18
Keeping the chemistry and advantages in mind, another material, polysilazanes, characterized by their alternating Si–N backbone units, present a promising alternative to isocyanates for curing acrylic adhesives.19,20 These compounds react with the hydroxyl (–OH) groups in acrylic polymers, forming a robust three-dimensional network via Si–O–Si cross-links.21 Polysilazane-cured systems offer numerous benefits, including reduced toxicity, lower environmental impact, and enhanced thermal and chemical resistance compared to isocyanate-cured adhesives.22,23 Their inorganic Si–N structure further enhances stability under extreme conditions, positioning polysilazanes as a viable option for high-performance adhesive formulations.24 The study by Widyastuti et al. investigates the effects of particle size, YSZ content, and curing time and temperature on the thermal conductivity and adhesion strength of thermal barrier coatings (TBCs) made from YSZ/polysilazane. Despite their demonstrated utility in coatings, the application of polysilazanes in adhesive systems remains underexplored.25
This study investigates the potential of polysilazane as a cross-linking agent for thermoset film adhesives derived from hydroxyl-functional acrylic polymers. Thermal curing was employed to achieve cross-linking, and the resulting adhesive properties, thermal behavior, and cross-link density were systematically analyzed. The adhesives were tested on both wood and metal substrates, chosen for their relevance in industries where strong, durable bonding is required under various environmental conditions providing reliable bonding under varying conditions, such as moisture, temperature fluctuations, and mechanical stress. This exploration aims to establish polysilazane-based acrylic adhesives as an easy-to-apply, high-performance alternative for demanding applications across a range of materials.
The selection of monomers was guided by their specific contributions to the copolymer's properties. MMA, with its short chain length, imparts hardness to the resin, while SMA contributes flexibility due to its longer chain length. HEA, a functional monomer, provides plasticity and serves as a cross-linking site for the copolymer. Three initial batches of the copolymer were synthesized, each with a different monomer ratio, as summarized in Fig. 1c. The copolymer with a composition of MMA:
SMA
:
HEA = 40
:
40
:
20 wt% exhibited the optimal viscosity for adhesive applications without requiring additional solvent (Fig. 1S†). The polymerizations were conducted in a 500 mL four-necked, jacketed glass reactor equipped with a reflux condenser, an efficient stirrer, a dropping funnel, and a nitrogen inlet. Initially, half the quantity of AIBN and solvent was charged into the reactor and heated to 80 °C. Once the reflux began, the monomer mixture, the remaining solvent, and the initiator solution were gradually added via a syringe over two hours while maintaining constant stirring at 80 °C. The reaction proceeded until the viscosity of the mixture increased significantly, indicating polymerization.
Following copolymer formation, a portion of the resin was mixed with a calculated amount of polysilazane as a cross-linker (Table 2). Due to the evolution of ammonia (Fig. 1b), effervescence was observed during the mixing process, suggesting an active reaction between the copolymer and the curing agent. The amine-containing moieties (–NH) and –OH of the copolymer undergo hydrolysis and condensation reaction, releasing ammonia forming the Si–O bonds. After thorough mixing, the adhesive was applied uniformly to the substrate. The coated panels were oven-cured at 80 °C for 1 hour and then allowed to cool to room temperature before undergoing performance testing.
The thermal characteristic of the material was examined using a HITACHI TG/DTA 7200 thermogravimeter and differential thermogravimeter. The resins were heated at 10 °C per minute to a temperature range of 30 °C to 600 °C while contained in a Platinum sample pan.
To calculate the molecular weight, 10 mg of polymer sample was placed in a 100 mL glass beaker and 20 ml of tetrahydrofuran was added and stirred for 10 min. To guarantee full dissolution in THF, the samples were held for two to three hours.
Following that, the samples were passed through a 0.45 μm syringe filter. For analysis, 50 μL of samples were put into a GPC column.
The solid content of the adhesive was measured using the oven-drying method.26 About 3 g (initial weight, A) of the adhesive was placed in an oven and dried at 105 °C for several hours until a constant weight (B) was achieved. The solid content was then calculated using a specific formula. The average solid content was determined based on three parallel samples.
![]() | (1) |
The acrylic-PSZ film sample, with a known initial weight, was submerged in methyl ethyl ketone (MEK) for 24 hours. After the immersion period, the sample was removed, dried in an oven until a constant weight was reached, and subsequently used to calculate the gel content of the film, following eqn (2):27
Gel content (wt%) = Ww/Wd × 100 | (2) |
Ww is the dried film's weight, and Wd represents the initial weight.
Adhesive samples were heated in an oven at 80 °C until a constant weight (Wc) was achieved. The cured adhesives were then immersed in tap water at room temperature for 24 hours, followed by oven-drying at 100 °C for 5 hours until a stable weight (Ws) was obtained. The residual rate was calculated as the ratio of Ws to Wc, as expressed in eqn (3). The average residual rate was determined using three parallel samples.26
Residual rate (wt%) = Ws/Wc × 100 | (3) |
The produced films’ hardness levels were determined using ASTM D3363. Lead pencils with a wide variety of hardness levels were utilized in this experiment. The reported pencil hardness is the pencil's number that prevents visible scratches from appearing on the film's surface.
Setting time is another important parameter to keep in mind for adhesive's usability. A small amount of glue was placed onto a wood substrate with the use of a thin spatula to calculate the setting time. The spatula was used to pull threads from the resin repeatedly until thread formation became infeasible. The set-in period was considered to have passed when no thread creation was seen.
The tensile shear strength of the adhesives was studied using a universal testing machine. One end of the wood piece was coated with a precisely measured quantity of glue in such a way that the surface was appropriately moistened. After that, the adhesive-coated wood pieces were put together such that their grains ran parallel to one another. Following the clamping time, and drying in an oven at 80 °C for an hour, the panels were cooled down to room temperature, before being taken for testing. The sample was pushed apart at a regulated rate of 10 mm min−1 while being held by vice grips at each end. The force used was proportionate to the total adhesive surface area.
![]() | (4) |
Thermal stability was further assessed through thermogravimetric analysis (TGA), as shown in Fig. 3a. The addition of PSZ as a cross-linker significantly enhances the thermal stability of the resin. As observed in the DSC results, thermal stability improves with increasing PSZ content. This improvement is attributed to the higher degree of cross-linking, which results in the formation of a dense three-dimensional network structure, as well as an increase in the silicon content within the system.24
![]() | ||
Fig. 5 Change of (a) solid contents, (b) degree of crosslinking, (c) residual rate, with different amounts of crosslinker. |
The degree of crosslinking in the cured resin films was assessed using a solvent immersion test, by determining the gel content. A sample of the cured film, with a known initial weight, was immersed in MEK for 24 hours. The resulting swollen gel was then dried at 100 °C, and the weight of the dried film was recorded to determine the weight loss. The acryl-PSZ-10 exhibited a 94.7% degree of crosslinking, while acryl-PSZ-20 demonstrated a higher extent of crosslinking at 97.9%.
Water resistance, measured by the residual rate, is a critical property of wood adhesives. The adhesive had a residual rate of 85.1 wt%, due to the presence of hydrophobic groups. Incorporating polysilazane further enhanced water resistance, with the acryl-PSZ-10 formulation achieving a 91.5 wt% residual rate and acryl-PSZ-20 up to 95.6 wt% due to cross-linking between hydroxyl groups and polysilazanes.22 Overall, low surface energy and less water absorption also provides longevity of the adhesive system on wood.32 All the data is represented below in Fig. 5a–c.
During the reaction with hydroxyl group of the acrylic, polysilazane (PSZ) chains undergo cleavage of Si–N bonds, resulting in shorter chain lengths and the formation of additional Si–O bonds.35 Cross-linking is primarily attributed to the tri-functional (–Si(H)-NH–) groups in PSZ, which facilitate condensation reactions with alcohol-containing species on hydroxyl-functional polymers. This mechanism highlights a novel application of polysilazanes as curing agents for adhesives.36 Additionally, the incorporation of silicon into the hybrid network is expected to enhance thermal properties, offering improved performance under challenging conditions. The cross-linking mechanisms are illustrated in Fig. 1b.
To achieve an adequately cured adhesive system, an excess of PSZ—ranging from 20 to 30 wt%—is typically required.36 In this study, the best performance was observed at 20 wt%. This off-stoichiometric requirement arises from the breakdown of PSZ chains and the concurrent release of ammonia (NH3), which hinders the efficient reaction between -OH functional groups and remaining -NH groups.22
Despite efforts to optimize the curing process, completely suppressing foaming effects proved challenging, even in dilute PSZ solutions. The use of high concentrations of PSZ inevitably generates significant ammonia gas during decomposition, leading to defects such as bubbles within the adhesive. These bubbles reduce the overall bond strength of the prepared adhesives, highlighting a trade-off between PSZ concentration and adhesive performance.
The tensile shear strength results, presented in Fig. 7a, b and Tables 1–3, highlight that the optimal adhesive performance was achieved with a resin-to-crosslinker ratio of 3:
0.6 by weight. While polysilazanes possess a relatively low molecular weight, resulting in a higher weight percentage compared to conventional crosslinkers, this does not imply an excessively high molar quantity of crosslinkers. Attempts to determine the molecular weight of polysilazanes via gel permeation chromatography (GPC) were inconclusive; thus, the adhesive formulations were defined based on weight given the novelty of using polysilazanes as crosslinkers, no precedent exists in the literature. Consequently, an iterative experimental approach, inspired by the bisection method in numerical analysis, was adopted. This method commenced with two initial resin-to-crosslinker formulations, followed by iterative refinement. Each subsequent formulation was designed to (a) represent a midpoint between the initial formulations and (b) refine the combination yielding superior tensile shear strength. This iterative process continued until a single formulation consistently demonstrated superior performance over successive iterations.
Resin taken (g) | Type of crosslinker used (5 wt% of the resin) | Avg. strength (MPa) |
---|---|---|
3 | TDI based crosslinker | 0.9 |
3 | Polysilazane | 1.1 |
Amount of resin (g) | Amount of polysilazane (g) | PSZ amount in wt% of the resin | Strength average (MPa) | Type of failure |
---|---|---|---|---|
3 | 1.5 | 50 | 1.7 | Adhesive |
3 | 1 | 30 | 2.5 | Adhesive |
3 | 0.6 | 20 | 4.4 | Substrate |
3 | 0.5 | 16 | 2.2 | Adhesive |
3 | 0.3 | 10 | 1.4 | Adhesive |
Amount of Resin (g) | Amount of polysilazane (g) | Avg. Strength (MPa) |
---|---|---|
3 | 0.3 | 1.5 |
3 | 0.6 | 4.0 |
3 | 1.5 | 0.9 |
Inadequate crosslinking, as observed in formulations such as 3:
0.3 and 3
:
0.5, led to reduced tensile shear strength and adhesive failure (Table 2 and Fig. 7e(iv)). Conversely, excessive polysilazane content, as in the 3
:
1 and 3
:
1.5 formulations, caused substantial NH3 released during curing. This resulted in the formation of a brittle, foam-like structure (Fig. 7e(v), inset picture), compromising adhesive integrity.36–38 However, the optimal formulation of 3
:
0.6 by weight demonstrated the highest tensile shear strength, leading to substrate failure rather than adhesive failure (Fig. 7e(v)). At this ratio, polysilazanes formed a robust, dense network with acrylic resin, providing exceptional mechanical strength. The results, summarized in the accompanying table, indicate that the use of 20 wt% polysilazane achieves the optimal balance between crosslinking and minimizing foaming.
The molecular weight of all the copolymers was also analyzed to evaluate their effect on adhesive properties. The low molecular weight copolymer exhibited reduced adhesion strength (∼3.0 MPa), attributed to insufficient molecular chain interactions and lower MMA content. In contrast, the high molecular weight copolymer demonstrated adhesion performance (∼3.7 MPa) comparable to the standard formulation (MMA:
SMA = 40%
:
40%). (Fig. 2S†) However, its significantly higher viscosity posed challenges during application, leading to uneven substrate coverage. These findings highlight the critical balance between molecular weight and resin viscosity, without having to add any external solvent, in achieving optimal adhesion and application performance, with the sample used in this work.
Polysilazanes, with their backbone of –Si–NH–, react with the –OH groups of the acrylic copolymer via hydrolysis, forming a dense –Si–O–Si– network that is critical for providing high adhesive strength.11,38 However, the final strength of the crosslinked adhesive is governed by two competing factors: (a) the density and extent of crosslinking through the –Si–O–Si– network and (b) the formation and release of ammonia during hydrolysis. The evolution of ammonia introduces a foamy structure into the adhesive, leading to defects that reduce its overall strength. Optimizing the balance between maximizing crosslink density and minimizing foaming is crucial. Extensive experimentation revealed that a polymer-to-crosslinker weight ratio of 3:
0.6 provides the best performance, achieving the highest tensile shear strength while minimizing defects caused by foaming.
Additionally, the molecular weight of the polymer significantly influences adhesive performance. A low molecular weight polymer lacks sufficient viscosity to remain on the substrate surface, leading to excessive penetration into porous materials like wood and inadequate interaction with polysilazanes for effective crosslinking. This also results in cohesive failure due to the inherently lower strength of the polymer. Conversely, a high molecular weight polymer exhibits excessive viscosity, hindering proper surface wetting and causing easy debonding from the substrate.
The optimal molecular weight balances these factors, ensuring adequate surface coverage and crosslinking efficiency without compromising substrate adhesion. Our polymer, with an Mn of 2.4 × 103 g mol−1, was found to exhibit the best performance as a base material for adhesives. When combined with polysilazanes in the optimized ratio, it formed a robust adhesive system with minimal foaming and exceptional mechanical strength, leading to substrate failure rather than adhesive failure during tensile shear testing. These findings demonstrate the importance of fine-tuning both molecular weight and crosslinker ratio to achieve high-performance adhesive formulations.
The data presented in the tables above detail the tensile shear strengths of the adhesive formulations tested with both wood and aluminum substrates. Corresponding adhesive assemblies and the observed failure modes are illustrated in Fig. 7e(iv and v). From the results, it is evident that polysilazanes can serve effectively as crosslinkers in acrylic-based formulations, enabling durable and high-performance adhesive applications.
The best results were achieved with an optimal polysilazane content of 20 wt%, which significantly improved the strength of the acrylic adhesive, yielding a bond strength of 4.4 MPa. This demonstrates that polysilazane is an excellent crosslinker for acrylic wood adhesives. Additionally, the same adhesive formulation was tested on aluminum substrates, where it achieved a bond strength of 4.0 MPa. Overall, it can be concluded that polysilazane-crosslinked acrylic adhesives are well-suited for commercialization as a two-component adhesive system, offering ease of application and excellent strength.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4lp00354c |
‡ Equal Contributions (These authors have contributed equally to this work). |
This journal is © The Royal Society of Chemistry 2025 |