Preparation and properties of silica sol/melamine glyoxal resin

ABSTRACT

In this study, a composite modifier for wood impregnation is prepared, which is functional and environmentally friendly. The surface of silica sol was modified by using KH-560. The modified silica sol, melamine, and glyoxal were used as raw materials. The silica sol/melamine glyoxal resin (from now on referred to as Silica sol/MG) composite modifier was prepared based on in-situ polymerization. The physicochemical properties (viscosity, solid content and etc.) of the composite modifier were evaluated. The structural and thermal properties were characterized and analyzed by FTIR spectroscopy, particle size distribution, TG and DSC. The results showed that the viscosity and solid content of the composite modifier decreased with the increase in the mass of the silica sol. The FTIR spectroscopy showed peaks at 473 cm⁻¹ and 1101 cm⁻¹, which were assigned to bending and stretching vibrations of the Si-O-Si bond, respectively, indicating that the modified silica sol was successfully introduced into the MG resin. When the modified silica sol mass fraction was 30%-40%, the particle size distribution of the composite modifier was relatively uniform. TG analysis found that the thermal stability of the composite modifier was significantly improved compared with the unmodified resin. DSC analysis showed that adding the modified silica sol reduced the curing temperature of the modifier from 115.5 °C to 107.9 °C.

KEYWORDS:

• Silica sol
• melamine glyoxal resin
• modification
• properties

1. Introduction

Melamine formaldehyde resin has excellent water and heat resistance, high hardness, and good wear resistance [1]. It is now widely used in many fields, such as coatings [2,3], impregnated adhesive paper [4,5], and wood adhesives [6]. However, some problems are associated with melamine-formaldehyde, such as the release of free formaldehyde in its use, which is hazardous to human health. Therefore, the green MG resin was prepared using low volatile and non-toxic glyoxal instead of formaldehyde in this study. However, the properties of MG resin have been put forward to higher requirements in different fields, like improving toughness, increasing strength, and reducing the brittleness of the resin have become the focused issues.

Nanoparticles have been widely used to prepare polymer nanocomposites, which can impart composites of excellent optical properties and mechanical and thermal stability [7,8]. SiO₂ particles have high strength and toughness, which, combined with organic resins, can improve the strength, toughness, and ductility of resin materials [9]. Silica sol is a colloidal solution formed by the diffusion of nanoscale SiO₂ particles in water or solvents, with a large number of hydroxyl groups on the surface, high specific surface area, and specific surface energy [10,11]. But it has poor compatibility and dispersion with polymers [12], due to which silane coupling agents were used to modify the surface of nanoparticles in this research. It can achieve compatibility between organic resins and inorganic nanoparticles and make nanoparticles dispersed in polymers uniformly [13,14]. When silica sol was introduced into the resin, the dispersion and thermal properties of the resin were improved. Guan Hui used silica sol to compound with epoxy resin and discussed the influence of the mass fraction of silica sol on the stability of epoxy resin [15]. The results showed that with the increase of the mass fraction of silica sol, the particle size gradually increased, and the thermal stability was also significantly improved. Moreover, the residual weight at 500 °C rose from 8.96% to 17.19% with the increase in the mass fraction of silica sol.

In this study, the surface of silica sol was modified using KH-560. The silica sol/MG resin was prepared based on in-situ polymerization under weak acid conditions by using melamine, glyoxal, and modified silica sol as raw materials and NaOH as a catalyst. The effect of the amount of modified silica sol on the physicochemical properties of the Silica sol/MG resin was investigated. The particle size distribution, chemical structure, and thermal properties of the Silica sol/MG resin were also characterized by using particle size distribution, Fourier transform infrared spectroscopy (FTIR), thermogravimetric (TG), and differential scanning calorimetry (DSC), aiming to provide the necessary technical support, scientific theoretical guidance for developing composite modifiers and subsequent impregnation of modified wood.

2. Experimental

2.1 Materials and instruments

Melamine was purchased from Tianjin Guangfu Fine Chemical Research Institute. Glyoxal was purchased from Tianjin Comio Chemical Reagent Co. NaOH, and anhydrous ethanol was purchased from Tianjin Damao Chemical Reagent Factory. All the above materials were analytically pure.

Silica sol was purchased from Jinan Yinfeng silica Products Co., Ltd. with an average particle size of 8 to 15 nm, solid content of 30%, and pH equal to 9.3. Silane coupling agent KH-560 (γ-glycidoxypropyltrimethoxysilane) with a solid content of 97% was purchased from Shandong Yousu Chemical Technology Co., Ltd. An NDJ-1 Rotary viscometer was purchased from the Shanghai Institute of Geological Instruments. A WQF-510A Fourier transform infrared spectrometer was purchased from Beijing Ruili Analytical Instruments. A TG209F3 Thermogravimetric Analyzer was purchased from Tianjin Tester Instruments Co., Ltd. A Mastersizer 300 Laser Particle Size Analyzer was purchased from Malvern Instruments Co., Ltd, UK. A DSC4000 Differential Scanning Calorimeter was purchased from Platinum Elmer.

2.2 Preparation of silica sol/MG resin composite modifier

According to the molar ratio M:G of 1:7, a certain amount of melamine and glyoxal was added to the beaker, and modified silica sol with different mass fractions (calculated by the percentage of the MG resin mass) was added. The mixture of melamine, glyoxal and modified silica sol was ultrasonically shaken for 15–30 mins to make the modified silica sol evenly dispersed in the whole solution. The reaction mixture was poured into a three-necked flask and was adjusted to be weakly acidic with 30% NaOH by mass. The reaction temperature was set at 70 °C and maintained for 120 min. The Silica sol/MG resin composite modifier was obtained after cooling to room temperature.

2.3 Testing and characterization

The appearance, viscosity, and solid content of the composite modifier were assessed in accordance with GB/T14074-2017, ‘Test Methods for Adhesives and their Resins for the Wood Industry.’

Particle size distribution

The modifier was diluted to a 10% mass fraction concentration with deionized water. The samples were ultrasonically dispersed for 10 min before being examined by a laser particle size analyzer.

Fourier transform infrared spectroscopy (FTIR) analysis

The appropriate amount of MG resin and Silica sol/MG resin were taken, dried, and ground into powder. The resulting powder was mixed with potassium bromide and pressed into tablets, which were tested and characterized by Fourier transform infrared spectrometer (WQF-510A).

Thermogravimetric analysis (TGA)

The dried MG resin with Silica sol/MG resin was tested using a TG209F3 thermogravimetric analyzer with a temperature increase of 10 °C/min. A scanning temperature range of 30–800 °C, an atmosphere purged with nitrogen, and a sample size of 5–10 mg were used for TGA.

Differential scanning calorimetry (DSC) analysis

A PerkinElmer analyzer (DSC4000) was used to test the MG resin with the Silica sol/MG resin. The DSC scans were recorded at a heating rate of 10°C/min under a nitrogen atmosphere with a flow rate of 20 ml/min. To determine the curing temperature of the resins, about 3–5 mg of the sample was added to the crucible. The samples were then heated from ambient temperature (25°C) to 300°C under a nitrogen atmosphere.

3. Results and analysis

3.1 Effect of the mass fraction of modified silica sol on the physicochemical properties of composite modifiers

The light-yellow liquid composite modifier was synthesized slightly acid condition with a pH of 5–6, and the properties are summarized in Table 1.

Table 1. Properties of MG resin modified with Silica sol.

Processing	Appearance	Viscosity/(MPa·s)	Solid content/%	Stability/d
MG	Light yellow transparent liquid	210.0	45.42	> 30
10%Silica sol/MG	Light yellow transparent liquid	58.2	42.40	> 30
20%Silica sol/MG	Light yellow transparent liquid	37.8	41.60	> 30
30%Silica sol/MG	Light yellow transparent liquid	33.6	40.24	> 30
40%Silica sol/MG	Light yellow transparent liquid	25.5	39.87	> 30
50%Silica sol/MG	Cloudy liquid	20.7	37.59	< 1

As shown in Table 1, the solid content and viscosity of the composite modifier show a decreased trend with the increase in the modified silica sol mass fraction. It is because adding modified silica sol significantly influences the polymerization reaction of MG resin, which makes the reaction slow. With the progress of the chemical reaction, water content increases, resulting in a decrease in viscosity and solid content of the resin. When the modified silica sol mass fraction was 50%, the solution appeared cloudy and less stable, with short storage stability. The solution used for wood impregnation needs to be homogeneous and stable, so the subsequent tests were carried out on the composite modifier with modified silica sol mass fraction in the range of 10% – 40%.

3.2 Effect of the mass fraction of modified silica sol on the particle size distribution of the composite modifier

The particle size distribution of MG resin and modified Silica sol/MG resin was tested using a laser particle size analyzer. The results are shown in Figure 1. It can be seen that when the modified silica sol was not added, the MG resin distributed more uniformly, with the particle size distributed mainly between 1 and 10 nm. The average particle size of the Silica sol/MG resin composite modifier increased when the KH-560 modified silica sol was added. The small distribution around 1–10 μm may be due to the presence of some nano-SiO₂ particles in the system that have not been successfully modified by KH-560, and the surface polarity is strong, which is easy to agglomerate and form agglomerates [16]. As the mass fraction of modified silica sol increases, the distribution of the composite modifier becomes more uniform, which can be attributed to the chemical bonding between modified silica sol and MG resin. In addition, it is also possible that KH-560 forms an organic adsorption layer on the surface of the silica sol particles, thus enabling the modified silica sol to be dispersed uniformly in the resin matrix [17]. Only one peak was observed when the modified silica sol mass fraction was 30%-40%. No large particle size was involved, and the average particle size of the composite modifier with 40% modified silica sol was 33 nm. The peak is narrow, and it can be seen that the particle size distribution is more uniform and optimal.

Figure 1. Particle size distribution of MG resin and modified silica sol/MG resin.

(a-0%; b-10%; c-20%; d-30%; e-40%)

3.3 FTIR analysis

To analyze the interaction between the modified silica sol and MG resin, FTIR spectroscopy was conducted on silica sol and KH-560 modified silica sol, MG and Silica sol/MG resins.

From the FTIR spectra of silica sol and KH-560 modified silica sol in Figure 2 and Table 2, 1101 and 796 cm⁻¹ are the antisymmetric and symmetric stretching vibration peaks of Si-O-Si. The absorption peaks at 3449 and 1641 cm⁻¹ are the stretching vibration peaks of -OH on the surface of silica sol, which are weakened by the reduction of -OH on the surface after modification. Two new absorption peaks appear at around 2936 and 2850 cm⁻¹, which are characteristic peaks of -CH₃ and -CH₂. These results suggest that KH-560 was successfully grafted onto the surface of silica sol.

Figure 2. Infrared spectra of silica sol and KH-560 modified silica sol.

Figure 3 and Table 2 show the IR absorption spectrum of MG resin and modified Silica sol/MG resin. The broad and intense absorption peaks at 3500 cm⁻¹ to 3100 cm⁻¹ are mainly attributed to the overlap of the stretching vibrations of N-H and O-H. The absorption peak near 1651 cm⁻¹ is due to the active amine (primary amine N-H bond) in melamine. The disappearance of this absorption peak at 1651 cm⁻¹ is evidence of the chemical reaction of melamine with glyoxal [18,19]. The absorption peak at 1565 cm⁻¹ is caused by the stretching vibration of the C = N of the triazine ring in melamine. The absorption peak at 1242 cm⁻¹ is because of the stretching of the C-O-C bond of the ether. The absorption peak at 811 cm⁻¹ is due to the out-of-plane bending vibration of the melamine skeleton. The absorption peaks at 1565 cm⁻¹, 1465 cm⁻¹ and 811 cm⁻¹ indicate that the triazine ring structure of melamine has not been disrupted in the synthetic resin [20].

Figure 3. Infrared spectra of MG resin and modified Silica sol/MG resin.

Table 2. IR spectral data of silica sol, KH-560 modified silica sol, MG resin and silica sol/MG resin.

Group	Vibration	Silica sol (cm^−1)	Modified silica sol	MG resin	Silica sol/MG resin
O-H	Stretching	3449, 1641		3415
N-H	Stretching			3415, 1651
CH3	Asymmetric stretching		2936
CH2	Symmetric stretching		2850
C = N	Stretching			1565
C-O-C	Stretching			1242
Si-O-Si	Asymmetric stretching	1101	1101		1101
Si-OH	Bending				948
Triazine ring	Skeleton vibration			811
Si-O-Si	Symmetric stretching	796	796		811
Si-O-Si	Bending				473

When different mass fractions of modified silica sols were added to MG resin, the absorption peaks at 1565 cm⁻¹ and 811 cm⁻¹ were significantly enhanced. Also, new absorption peaks appeared at 1101 cm⁻¹ and 473 cm⁻¹ compared to the unmodified MG resin. 811 cm⁻¹ and 473 cm⁻¹ were attributed to the symmetric stretching and bending vibrations of the Si-O-Si bond. Similarly, 948 cm⁻¹ was due to the bending vibration of Si-OH. Additionally, the absorption peak at 1101 cm⁻¹ was because of the antisymmetric stretching vibration of the Si-O-Si bond, indicating that the modified silica sol was successfully introduced into the MG resin [21,22].

(curves a, b, c, d, and e are modified silica sol mass fractions of 0%, 10%, 20%, 30%, and 40%, respectively)

3.4 TG analysis

The thermal properties of MG and modified Silica sol/MG resins were characterized using a thermogravimetric analyzer. The results are shown in Figure 4. The TG curve can be divided into three distinct phases. The first stage (at room temperature of 170 °C) is mainly water evaporation from the sample, with a maximum mass loss of 6.0% approximately. The second stage (at temperatures between 170 °C and 300 °C) is mainly the decomposition of the MG resin and modified Silica sol/MG resin, with the fastest decomposition rate around 175 °C. It is primarily caused by the decomposition of the functional groups of the resin network structure after cross-linking and the removal of volatile components, with a weight loss of about 31%. In the third stage (at a temperature of 300 °C and beyond), the resin undergoes intense pyrolysis, with the carbonization of the resin macromolecules, volatilization of gases such as CO₂, and a weight loss of 33% approximately. At around 800 °C, the amount of residual carbon in the MG resin increases from 29.4% to 35.8%, indicating significant improvement in the heat resistance of the composite modifier.

Figure 4. TG(a) and DTG(b) graphs of MG resin with modified silica sol/MG resin.

The thermal decomposition temperature of the Silica sol/MG resin was significantly increased by the introduction of silica sol, which is mainly due to the good thermal stability of the silica sol itself. Adding the silica sol to the MG resin increased the cross-linked chain length, which improved the MG resin’s thermal stability and increased the residual weight [23]. With the increase in the mass fraction of modified silica sol, the graph of the composite modifier gradually tends to flatten out. It indicates that the addition of silica sol helps to delay the decomposition of the composite modifier and increase the decomposition temperature of the composite modifier, which is especially obvious with the mass fraction of 30% modified silica sol. Silica sol achieves the modification of MG resin and effectively improves MG resin molecules’ dispersion and decomposition temperature. The decomposition temperature of modified silica sol with a mass fraction of 40% was lower than that of unmodified resin. It is probably due to the excessive addition of silica sol, which increased agglomeration and affected the structure of the MG resin [24].

The thermal weight-loss parameters of the composite modifiers are shown in Table 3. The temperatures of thermal weight loss at 5% and 10% degradation were 169.75 °C and 176.43 °C for MG resin, and the residual weight at 800 °C was 29.42%. The T_5% and T_10% of the silica sol modified MG resin showed a trend of increasing and decreasing relative to the MG resin. Moreover, the amount of residual weight increased with the mass fraction of the modified silica sol, indicating that adding silica sol can effectively improve the thermal stability of MG resin. The effect of thermal stability is most significant when MG resin is modified by adding the modified silica sol with a mass fraction of 30%.

Table 3. Thermogravimetric analysis of modified silica sol/MG resin.

Modified silica sol mass fraction/w%	T5%/°C	T10%/°C	Cwt/% (800 °C)
0	169.75	176.43	29.42
10	170.65	177.57	32.21
20	171.22	178.98	34.27
30	170.12	179.59	35.33
40	162.41	176.85	35.79

3.5 DSC analysis

DSC analysis was conducted to verify further the effect of modified silica sol on the thermal properties of MG resin. The MG resin and modified Silica sol/MG resin were examined with TG-DSC. As can be seen in Figure 5 that the exothermic peaks of MG resin and modified Silica sol/MG resin are shifted towards lower temperatures, indicating that the resins can be entirely cured at a lower temperature. Moreover, the curing speed is accelerated at the same rate [25–27]. The maximum curing temperature for MG resin is 115.5 °C, and it shifts from 115.5 °C to 107.9 °C towards lower temperatures after adding different mass fractions modified silica sol. It indicates that adding silica sol to MG resin can effectively promote the curing reaction of MG resin.

Figure 5. DSC curves of MG resin and modified silica sol/MG resin.

4. Conclusion

In this paper, Silica sol/MG resin composite modifier was prepared based on in-situ polymerization. The influence of the amount of modified silica sol on the physicochemical properties of the composite modifier was discussed. The structure of the modifier was characterized by using particle size distribution, FTIR spectroscopy, thermogravimetric analyzer, and differential scanning calorimetry.

The viscosity and solid content of the composite modifier tended to decrease as the modified silica sol mass fraction increased. When the mass fraction of modified silica sol was 40%, the particle size distribution of the composite modifier was more uniform.

FTIR analysis showed that peaks at 811 cm⁻¹ and 473 cm⁻¹ were due to the symmetric stretching and bending vibrations of the Si-O-Si bond. The bending vibration of Si-OH appeared at 948 cm⁻¹, and the anti-symmetric stretching vibration of the Si-O-Si bond appeared at 1101 cm⁻¹, indicating that the modified silica sol was successfully introduced into the MG resin.

TG and DTG analysis showed that the thermal stability of the Silica sol/MG resin was significantly improved compared with MG resin. The thermal stability was optimal when the modified silica sol mass fraction was 30%.

DSC curves showed that adding the modified silica sol reduced the curing temperature of the MG resin.

Acknowledgments

The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 31972947, 31400503 and 32171712), Technology Development Innovation Platform (Base) and Talent Project (Grant No. 20220508119RC) and Postgraduate Innovation Program of Beihua University (Beihua Research and Innovation Hezi [2021] 016). Central guidance for local science and technology development projects, Grant No. 202002003JC, is gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Ministry of Education National Natural Science Foundation (Grant No. 31972947, 31400503, 32171712), Technology Development Innovation Platform (Base) and Talent Project (Grant No. 20220508119RC) and Postgraduate Innovation Program of Beihua University (Beihua Research and Innovation Hezi [2021] 016).