Do you know why the touch screen fails underwater?

      Do you know why ordinary touch screens are not easy to use underwater? If there is water on the surface of an ordinary screen, water, as a conductor, will change the capacitance value, resulting in false touches or failure to recognize. Therefore, the touch screen used underwater needs materials that can resist the interference of water while maintaining touch sensitivity. When using the touch screen underwater, due to factors such as the conductivity of water, changes in dielectric constants and surface tension, ordinary capacitive screens are prone to false touches, signal interference or failure to work properly. Therefore, the materials of underwater touch screens need to be specially designed for waterproofness, anti-interference, corrosion resistance and optical properties. The following is a detailed explanation from the two aspects of the core material layer and auxiliary protective materials:

A. Core functional layer material requirements

1. Cover layer (cover material)

The cover layer is the interface that directly contacts water and fingers, and must meet the requirements of hydrophobicity, high light transmittance and mechanical strength at the same time.

· Material selection:

· Super-hydrophobic glass/plastic: Super-hydrophobic properties (contact angle > 150°) are achieved through surface nano-coating (such as fluorosilane, silica microspheres), so that water droplets quickly condense into balls and roll down, reducing the water film coverage area and avoiding capacitance interference caused by uniform spreading of the water layer.

· Strengthened glass (such as Gorilla Glass): After ion exchange strengthening, high-aluminum silicate glass has a surface compressive stress > 900MPa, strong scratch and impact resistance, and is suitable for high-frequency contact scenarios underwater.

· Transparent plastic (such as PET, PC): It needs to be combined with a hardening coating (such as UV-curing coating) to improve hardness and hydrophobicity, suitable for flexible or low-cost equipment (such as underwater cameras, diving watches).

· Key indicators:

· Light transmittance > 92% (close to ordinary glass) to avoid affecting the display effect;

· Surface energy < 20mN/m (super-hydrophobic threshold) to ensure that water droplets cannot spread;

· Salt spray corrosion resistance (such as 5% NaCl solution immersion for 500 hours without abnormalities).

2. Touch sensor layer (electrode material)

The ITO (indium tin oxide) film of the traditional capacitive screen is highly brittle and has poor corrosion resistance (easily oxidized by water/electrolyte), so it needs to be replaced with a more stable material for underwater scenes:

· Nanosilver wire (AgNW):

· Advantages: conductivity (conductivity ≈ 6×10⁷ S/m, close to ITO), flexibility (bendable), corrosion resistance (silver is stable in an inert environment, and the gap between nanowires is small and not easily penetrated by electrolytes);

· Application: Transparent electrodes are prepared by coating process, suitable for flexible underwater screens (such as diving glove integrated screens).

· Graphene film:

· Advantages: single atomic layer structure, transmittance > 97% (nearly unobstructed), excellent conductivity (conductivity ≈ 10⁶ S/m), extremely high chemical stability (acid and alkali corrosion resistance);

· Challenges: large-scale preparation costs are high, and it is currently mostly used in high-end underwater equipment (such as waterproof flat panels for scientific research).

· Metal grid (Cu/Cr):

· Advantages: copper has low cost and good conductivity (conductivity ≈ 5.96×10⁷ S/m), and high transmittance is achieved through micro-machining (line width < 5μm);

· Improvements: nickel/gold plating on the surface prevents oxidation and improves corrosion resistance, suitable for mid- and low-end underwater equipment (such as waterproof mobile phones).

· Self-capacitance vs mutual capacitance solution:

Self-capacitance solution (detecting the capacitance change between the electrode and the ground) is more recommended underwater, because mutual capacitance (detecting the capacitance between two electrodes) is easily interfered by the dielectric constant of water (the relative dielectric constant of water is ≈80, which is much higher than 1 of air), resulting in signal drift.

3. Substrate material (support layer)

The substrate needs to meet the requirements of insulation, water resistance and bonding with the sensor at the same time:

· Polyethylene terephthalate (PET): low cost, good flexibility (rollable), but average temperature resistance (<80℃), suitable for consumer-grade underwater equipment;

· Polyimide (PI): high temperature resistance (>300℃), chemical corrosion resistance, suitable for industrial-grade or deep-sea high-pressure scenarios (such as underwater robots);

· Glass fiber reinforced epoxy resin (FR-4): high mechanical strength, used for thick-screen devices that require rigid support (such as waterproof notebooks).

B. Requirements for auxiliary protective materials

1. Sealing and bonding materials

Underwater equipment must reach IP68/IP69K protection level, the key lies in edge sealing and interface bonding:

· Silicone sealant: high elasticity, aging resistance (-50℃~200℃), can fill the tiny gap between the screen and the shell to prevent water penetration;

· Polyurethane glue (PU): good hydrolysis resistance, suitable for long-term immersion scenarios (such as diving equipment);

· Optical grade OCA glue: used to fit the cover layer and the sensor layer, it must meet both high light transmittance (＞99%) and waterproofness (water absorption rate ＜0.1%).

2. Anti-electrolysis and anti-corrosion materials

Water (especially salt water) contains electrolytes, which can easily cause corrosion of metal parts or short circuit of sensors:

· Insulation coating: Coating polytetrafluoroethylene (PTFE) or ceramic coating on the surface of metal frames or structural parts to block electrolyte contact;

· Stainless steel/titanium alloy: used for internal structural parts (such as cable interfaces), stainless steel (316L) is resistant to chloride ion corrosion, and titanium alloy has high strength and good biocompatibility (suitable for diving medical equipment).

3. Water pressure resistant materials (deep sea scenes)

The deep sea (>100 meters) needs to withstand high pressure (every 10 meters ≈ 1 atmosphere), and the material needs to have deformation resistance:

· Tempered glass + PI substrate: The high hardness of glass can resist water pressure deformation, and the flexibility of PI substrate avoids stress cracking;

· Composite structure design: Adopting the "glass-elastomer-metal" multilayer structure, the elastomer (such as silicone rubber) absorbs water pressure deformation and protects the internal circuit.

       The material design of underwater touch screens must focus on the three core goals of "waterproof and water-proof, corrosion-resistant and non-failure, and touch without misjudgment". The super-hydrophobic cover layer is used to reduce water interference, corrosion-resistant conductive materials replace traditional ITO, and precision sealing structures block water penetration. In addition, the appropriate material combination is selected in combination with scene requirements (such as consumer grade/industrial grade/deep sea grade). Shenzhen Hongjia Technology can cooperate with customers to customize capacitive touch screens for underwater use. We have 12 years of industry experience and welcome customers to email us for consultation.