Introduction

Vietnam is located in a tropical climate zone where the relative humidity frequently exceeds 70 %. This creates favorable conditions for mold growth, causing corrosion to technical equipment and weaponry in general, and optical glass in particular. In island and coastal environments, salt mist accumulation on glass surfaces also causes corrosion, leading to glass clouding. Various preservation methods are applied to prevent mold and clouding on optical sights to mitigate these processes.

Siloxane-based materials have attracted significant attention for use as direct coatings on glass surfaces. These coatings are effective in resisting mold and corrosion caused by salt mist in Vietnam's sea and island environments [1] [2]. Scientists worldwide have conducted numerous studies on fabricating material systems based on organosilicon compounds to protect glass surfaces. Siloxane is a material with low surface energy, and artificial roughness can be created on its surface through physical or chemical methods to produce superhydrophobic materials capable of self-cleaning and anti-fouling [3].

This paper presents the synthesis process and key factors in creating a coating based on PMHS and TEOS compounds to resist corrosion by sea salt mist, thereby protecting optical glass surfaces from clouding.

Materials and methods

Material Synthesis

Chemicals: Polymethylhydrosiloxane (PMHS, 99 %), Tetraethoxysilane (TEOS, 99 %), Ethyl alcohol (anhydrous), Sodium hydroxide (NaOH, 99.9 %).

Optical Glass Substrate: K8 glass material, refractive index n _D = 1.51679.

Solution A: PMHS:C ₂ H ₅ OH volume ratio = 1:9.

Solution B: TEOS:C ₂ H ₅ OH volume ratio = 1:9.

Synthesis Process: The material synthesis based on PMHS compounds is calculated according to the volume ratio PMHS:C ₂ H ₅ OH = 1:9. The process is carried out as follows: add 1 part of PMHS (or TEOS) solution to 9 parts of C ₂ H ₅ OH sollution. The pH of the system is adjusted to 9–10 using 0.1 N NaOH solution in ethanol. The mixture is stirred continuously for 2 hours at room temperature.

The final synthesized material system is obtained by mixing Solution A and Solution B in corresponding volume ratios of PMHS:TEOS = 1:1, 1:2, and 2:1, under catalytic conditions of 0.1 N NaOH within a pH range of 9–10. The mixture is stirred for 2 hours at room temperature and aged for 10 hours at room temperature.

Testing Methods The synthesized material is tested directly on the optical glass surface. After coating, the samples undergo SEM imaging and contact angle measurement to evaluate material properties. Other tests include refractive index measurement, water droplet adhesion test, and resistance to mold growth in natural environments.

Contact Angle Measurement: Measured using a KSV contact angle and surface tension meter (Germany).

Optical Transmittance: Measured on a UV-2550 spectrophotometer (USA).

Mold Culturing: Performed in incubators and culture chambers. Mold morphology is determined using a Hitachi D4800 Scanning Electron Microscope (SEM) and monitored via an Olympus microscope (Japan).

Results and discussions

Synthesis of PMHS and TEOS Materials The organosilicon compounds used to synthesize the material samples are designated as L1, L2, and L3, corresponding to PMHS:TEOS volume ratios of 1:1, 1:2, and 2:1, respectively, with 0.1 N NaOH catalyst and pH in the range of 9–10.

Table 1

Water Contact Angle

The results show that the uncoated glass sample (L0) has a contact angle of 57.40°. The samples coated with synthesized materials L1, L2, and L3 (ratios 1:1, 1:2, 2:1) achieved contact angles of 102.6°, 94.38°, and 111.26°, respectively, after the coating dried completely for 2 hours.

The change in contact angle depends on the variation of PMHS and TEOS precursors. Increasing the PMHS content leads to a stronger increase in contact angle compared to increasing the TEOS content. This is because PMHS molecules in ethanol solvent tend to form a helical structure. The degree of coiling depends on the repulsive forces between molecules; low precursor concentration results in weak repulsion and long helices, while high concentration results in dense helices. Consequently, a large amount of -CH ₃ groups are positioned on the surface of the ring, significantly increasing the hydrophobicity of the coating.

In the presence of NaOH catalyst, TEOS hydrolysis occurs, generating -OH groups. A condensation reaction then takes place between these -OH groups, the -OH groups of the glass substrate, and the -OH groups of PMHS, forming a coating on the glass surface. Thus, TEOS plays a primary role in bonding the glass substrate with PMHS.

However, the survey shows that when the PMHS:TEOS volume ratio is 2:1, the film drying time is longer, and the adhesion of the film decreases compared to the 1:1 and 1:2 ratios.

Fig. 1.Water contact angles before coating (L0) and after applying protective coatings (L1, L2, L3), corresponding to PMHS:TEOS precursor ratios of 1:1, 1:2, and 2:1, respectively

Study on the Effect of Anti-fungal Additive and Coating Technique

To enhance the biological resistance of the coating under tropical conditions, the study investigated the incorporation of Trimethyltin chloride as an anti-fungal additive into the PMHS/TEOS system (1:1 ratio) at concentrations of 0.1 %, 0.3 %, 0.5 %, and 1.0 %. Additionally, the coating application method was optimized by comparing single-layer and double-layer brush coating techniques.

Regarding Coating Technique: The number of coating layers significantly influenced the surface morphology. The double-layer coating samples exhibited a more uniform, smoother surface with fewer defects compared to single-layer samples, thereby minimizing sites for fungal spore adhesion.

Regarding Anti-fungal Efficiency: The coating sample containing 0.1 % Trimethyltin chloride demonstrated superior mold inhibition capabilities. Under favorable incubation conditions, while the control sample (without additive) exhibited fungal hyphae growth after 4 weeks, the additive-modified sample extended the protection period, with slow fungal growth appearing only after 6 weeks. The addition of this agent did not significantly alter the optical properties of the glass.

Conclusion

In conclusion, the PMHS/TEOS coating at 1:1 ratio provides an optimal balance of hydrophobicity (102.6° contact angle), high optical transmittance (>92 %), and durability against salt mist and mold in tropical conditions. The addition of 0.1 % trimethyltin chloride combined with double-layer application significantly enhanced antifungal resistance up to 6 weeks without compromising optical clarity.

This work offers a simple and effective solution for protecting optical glass in Vietnam’s humid and coastal environments. Further field testing and mechanical improvement are recommended.

References:

1. Thanh, V. M., Anh, L. D., Tien, N. M., Long, D. V., Anh, D. T., Khuong, T., Anh, P. T., Dinh, T. D., & Cuc, T. T. T. «Determination of causes of optical glass clouding and composition of anti-clouding materials for optical sights in marine and island environments», Journal of Military Science and Technology, No27 (8/2013).
2. A Levkin, F Svec and J J M Frechet, Porous polymer coatings: a versatile approach to superhydropho bic surfaces, Advanced Functional Materials, Vol. 19, No. 12, 2009, pp. 1993–1998.
3. John A Glass Jr, Edward A, Wovchko, John T Yates, Reaction of atomic hydrogen with hydrogenated porous silicon-detection of precursor to silane formation, Surface science, 348 (3) (1996), 325–334.