I. Introduction With the iteration of consumer electronics towards thinner, lighter, curved, and higher-definition designs, and the rapid popularization of automotive displays and smart wearable devices, the market has placed higher demands on the appearance precision, mechanical strength, and surface performance of glass covers. Glass covers are mostly made of silicate glass or lithium-based glass, which undergoes strengthening treatment to form a stress layer, improving impact resistance. However, their surface is extremely susceptible to damage during processing—scratches from cutting processes, the high temperatures of strengthening processes, chemical corrosion from cleaning processes, and ink contamination from screen printing processes all contribute to increased product scrap rates.

As a "temporary protective barrier" during the glass cover processing, the protective film must not only possess basic scratch-proof, dust-proof, and stain-proof functions, but also adapt to the special environments of each processing step, such as high temperatures, chemical reagents, and mechanical friction. Simultaneously, it must meet core requirements such as leaving no adhesive residue after peeling and not affecting subsequent processing (such as ink printing and OCA lamination). Currently, protective films for glass cover processing have formed a product system primarily based on PET materials, supplemented by PE materials, with adhesive types covering silicone, acrylic, and PU adhesives. However, the selection of protective films for different processing scenarios still suffers from a lack of direction, and some protective films have low processing yields due to substandard performance. Therefore, a systematic study of the requirements for protective films in glass cover processing and the clarification of the core compatibility indicators for each process are of significant practical importance for improving the quality of glass cover processing and reducing production costs.

II. Core Processing Steps of Glass Covers and Requirements for Protective Films The glass cover processing flow is characterized by complex procedures and harsh environments. The processing principles and damage risks of different steps vary significantly, and the performance requirements for protective films also differ. The following section analyzes the specific requirements for protective films in detail, focusing on the core processing steps.

2.1 Cutting Process: Scratch-resistant, chip-resistant, and easy to peel off

Cutting is the first core step in glass cover processing, mainly using CNC cutting or laser cutting to cut the glass substrate into the required size and shape. The core risk of damage in this process is that glass shards generated during cutting adhere to the surface, causing scratches. Simultaneously, the mechanical pressure during cutting may damage or detach the protective film.

For the cutting process, the protective film must meet the following requirements: First, it must possess excellent surface hardness, typically reaching 3H or higher (pencil hardness), capable of resisting scratches from glass shards and preventing the exposed glass surface after the protective film breaks. PET protective films, due to their dense molecular structure and high surface hardness, are the preferred choice for this process, while PE materials, due to their lower hardness, are only suitable for low-precision cutting scenarios. Second, it must possess good adhesion, adhering tightly to the glass surface without air bubbles or curling edges, preventing cutting debris from entering between the protective film and the glass and causing secondary scratches. The adhesive layer must have moderate initial tack to ensure a firm adhesion. First, the protective film must be prevented from shifting during cutting. Second, the peel strength must be stable; after cutting, the protective film must be peeled off without any residue or tearing marks, and the peel strength must be controlled within a reasonable range, generally 0.5-5g/25mm, to facilitate automated robotic operation. Solvent-free UV-cured low-peel-strength protective films can effectively meet this requirement. Third, it must have a certain degree of tear resistance, able to withstand mechanical pulling during the cutting process, preventing the protective film from breaking and causing protective failure. The tensile strength of PET protective film can reach 40-50MPa, far superior to the 10-20MPa of PE protective film, making it more suitable for the needs of the cutting process.

2.2 Strengthening Process: High Temperature Resistance, Non-Volatile, and Stable Adhesion

Strengthening treatment is a key process for improving the mechanical strength of glass covers. It mainly uses chemical strengthening methods, placing the glass substrate in a silicate-containing strengthening solution and achieving ion exchange through high-temperature heating (usually 380-420℃), forming a stress layer on the glass surface. Silicon atoms from the silicate will enter the glass surface, increasing the surface silicon atom content. The core risk of this process is that high temperatures can cause the protective film to soften, deform, or volatilize, or leave adhesive residue during peeling. Furthermore, if the protective film components volatilize, they may contaminate the strengthening solution, affecting the strengthening effect.

For the strengthening process, the protective film must meet the following core requirements: First, excellent high-temperature resistance. Long-term operating temperature must be stable above 400℃, and short-term resistance to around 450℃ is required without softening, shrinkage, or decomposition. Silicone-based PET protective films have a temperature range of -40℃ to 200℃, and some specialized high-temperature protective films can withstand temperatures above 160℃, making them suitable for the high-temperature environment of the strengthening process. Second, no volatiles. The protective film must not release harmful gases or small molecules at high temperatures to avoid contaminating the strengthening solution and glass surface. Solvent-free protective films, with 100% solid content and no VOC emissions, effectively avoid this problem. Third, stable adhesion. It should not peel or curl at high temperatures, and should leave no residue upon cooling. Protective films made of fluorinated materials can form chemical bonds between fluorine atoms and silicon atoms on the glass surface, significantly improving adhesion and high-temperature resistance. Fourth, compatibility with the strengthening solution. It should not react with silicates or metal ions in the strengthening solution, thus not affecting the ion exchange process and ensuring the strengthening effect of the glass cover.

2.3 Grinding/Polishing Process: Abrasion Resistance, Dust Resistance, and Scratch Reduction The core purpose of the grinding and polishing process is to remove burrs and scratches from the glass cover surface, improving surface smoothness and gloss. This process generates a large amount of glass dust and involves continuous mechanical friction, placing extremely high demands on the abrasion resistance and dust resistance of the protective film.

The main requirements for the protective film in this process include: First, excellent wear resistance. After surface hardening treatment, the hardness can reach 9H, enabling it to withstand continuous mechanical friction without damage. Adding ceramic particles or a silicone-modified acrylic hardening coating can significantly improve the wear resistance of the protective film. Second, good dustproof performance. The protective film surface must have a certain degree of dust-repellent properties to reduce the adhesion of glass dust. It must also adhere tightly to prevent dust from entering between the protective film and the glass, causing scratches on the glass surface after polishing. Third, moderate flexibility to adapt to the curved shape of glass covers (such as folding screen glass and curved glass for automotive center consoles), without wrinkles during application and without displacement during polishing. Fourth, no powdering. The protective film must not produce debris during friction to avoid contaminating the glass surface and polishing equipment. Ordinary acrylic adhesive protective films are prone to powdering during die-cutting; therefore, a high-cohesion adhesive material must be selected.

2.4 Cleaning Process: Chemical Resistance, Easy Cleaning, and Residue-Free The cleaning process removes dust, oil, residual chemicals, and other impurities from the glass cover surface, preparing it for subsequent coating and screen printing processes. Cleaning methods include plasma cleaning and ultrasonic cleaning, and commonly used cleaning agents include hydrofluoric acid and alkaline solutions. The core risk of this process is that the protective film may be corroded by chemical reagents, or that residual chemicals may adhere to the protective film surface after cleaning, affecting subsequent processing.

For the cleaning process, the protective film must meet the following requirements: First, excellent chemical resistance, able to resist corrosion from cleaning agents such as hydrofluoric acid and alkaline solutions, without discoloration, damage, or peeling. Acid-resistant protective films are specifically designed for cleaning scenarios containing hydrofluoric acid and possess excellent adhesion stability. Second, easy surface cleaning; cleaning agents should quickly remove attached dust and oil without residue. The protective film surface must have a certain degree of hydrophilicity or oleophobicity to prevent water stains from forming due to chemical residue. Third, stable adhesion; ultrasonic vibration and water flow impact during the cleaning process should not cause the protective film to peel off, and there should be no adhesive residue upon peeling after cleaning, without affecting the cleanliness of the glass surface. Fourth, compatibility with plasma cleaning; after plasma bombardment, the protective film should not age or break, and should not affect the surface tension of the glass surface, ensuring the adhesion of subsequent coatings and screen printing.

2.5 Coating/Screen Printing Process: High Transmittance, No Pollution, and No Residue Coating and screen printing are key processes for enhancing the functionality and aesthetics of glass covers. Coating is mainly used to achieve functions such as fingerprint resistance, blue light protection, and increased transparency. Screen printing is used to print patterns, logos, or light-shielding borders (such as a circular ink layer). This process places extremely high demands on the light transmittance, surface cleanliness, and compatibility of the protective film. Any residue or contamination will directly affect the coating effect and screen printing accuracy.

The specific requirements for the protective film in this process are as follows: First, high light transmittance, with a transmittance of over 90%, free from haze and yellowing, to avoid affecting the optical performance after coating. Optical-grade PET substrate can meet this requirement, as it has high thickness uniformity and a smooth surface. Second, high surface cleanliness, free from impurities, particles, and adhesive residue, to avoid pinholes and defects during coating, or missed printing and blurring during screen printing. Third, no silicone contamination; the release film and adhesive layer of the protective film must not contain silicone components to prevent silicone transfer to the glass surface, which would reduce coating adhesion and cause screen printing ink to peel off. Non-silicone PET release film can effectively avoid silicone transfer problems. Fourth, no residue after peeling; after coating and screen printing, there should be no adhesive residue or traces when peeling off the protective film, without affecting the coating layer or ink layer on the glass surface. Low peel strength antistatic voltage-sensitive adhesive protective film can reduce electrostatic adsorption during peeling and prevent dust adhesion.

2.6 Shipping/Transportation Process: Impact Protection, Scratch Protection, and Moisture Protection

After the glass cover is processed, it needs to be protected with a protective film for shipment to prevent collisions, scratches, and moisture damage during transportation and storage. The core requirements for the protective film in this process are comprehensive protection and ease of operation.

Specific requirements include:

First, it must have sufficient cushioning performance to withstand minor collisions and impacts during transportation. PE protective film, due to its good extensibility and high elongation at break (300%-500%), has excellent cushioning and shock absorption effects, making it suitable for shipment protection.

Second, it must have excellent scratch resistance and high surface hardness to resist friction and scratches from foreign objects during transportation.

Third, it must have good moisture protection to prevent mold growth and surface fogging in humid environments.

Fourth, it must be easy to apply and peel off, allowing for automated application and peeling to meet large-scale shipping needs, while leaving no adhesive residue during peeling, thus not affecting the product's appearance.

III. Core Performance Indicators of Protective Films for Glass Cover Plate Processing Based on the requirements of the above processing steps, the core performance indicators of protective films for glass cover plate processing can be summarized into the following categories. These indicators directly determine the protective effect and processing compatibility of the film, and are also the core basis for selection and research and development.

3.1 Substrate Performance Indicators

The substrate is the foundation of the protective film, and its performance directly affects the overall protective effect. Key indicators include:

1. Material type: PET, due to its high hardness, good temperature resistance, and high light transmittance, is currently the mainstream substrate for glass cover processing, with a thickness range of 25-150µm. Thinner versions are suitable for curved glass, while thicker versions enhance impact resistance. PE, due to its good flexibility and low cost, is mainly used for temporary or shipping protection, with a thickness range of 0.025mm-0.1mm.

2. Thickness uniformity: Deviation must be controlled within ±5% to avoid uneven thickness leading to poor adhesion and localized protective failure.

3. Light transmittance: For processes that expose the glass surface (such as coating and screen printing), the protective film's light transmittance must be ≥90%, with no obvious haze.

4. Mechanical strength: Tensile strength ≥30MPa, tear strength ≥5N/mm, to prevent damage during processing.

3.2 Adhesive Layer Performance Indicators The adhesive layer is the connecting carrier between the protective film and the glass cover. Its performance directly affects the bonding stability and peeling effect. Key indicators include: 1. Tackiness: This needs to be precisely controlled according to the processing steps. Initial tackiness is 1-3000g (adapted to the scenario). Cutting and strengthening processes require low tackiness (0.5-5g/25mm), while shipping protection requires medium to high tackiness to ensure a firm bond and no adhesive residue. 2. Cohesive Strength: Good cohesive strength is required to prevent adhesive layer breakage and residue during peeling. Solvent-free UV-cured adhesive layers have significantly better cohesive strength than traditional solvent-based adhesive layers. 3. Temperature Resistance: The adhesive layer must match the temperature resistance of the substrate. High-temperature processes (such as strengthening) must withstand temperatures above 400℃ without softening or dripping. 4. Antistatic Properties: The antistatic value of the adhesive surface must reach 10^7-10Ω to prevent static electricity from attracting dust and affecting the cleanliness of the glass surface.

3.3 Surface Performance Indicators The surface performance of the protective film directly affects the protective effect and processing compatibility. Key indicators include: 1) Surface hardness: pencil hardness ≥3H, and 9H after grinding and polishing to resist scratches and friction; 2) Dust and oil repellency: surface contact angle ≥90° to reduce dust and oil adhesion and facilitate cleaning; 3) No silicon contamination: no residual silicon components on the surface to avoid affecting subsequent coating and screen printing processes; 4) No particulate impurities: surface particle diameter ≤5μm, and particle count ≤3 particles/㎡ to avoid scratching the glass surface.

3.4 Environmental Adaptability Indicators The glass cover processing environment is complex, requiring the protective film to possess excellent environmental adaptability. Key indicators include: 1) Chemical resistance: resisting corrosion from common reagents such as hydrofluoric acid, alkaline solutions, and alcohol without damage or discoloration; 2) Temperature resistance: no shrinkage, embrittlement, or decomposition under high and low temperature environments (-40℃~450℃); 3) Moisture resistance: no fogging or mold growth after 72 hours in an environment with 85% RH and 40℃; 4) Weather resistance: no yellowing or aging after long-term exposure to light and temperature changes, ensuring stable protective performance.

IV. Existing Problems with Protective Films for Glass Cover Processing Although the technology for protective films in glass cover processing is relatively mature, several problems still exist in practical applications, leading to decreased processing yield and increased production costs. These problems mainly focus on the following aspects:

First, there is insufficient process adaptability. Some protective films use a universal design without performance optimization for specific processing steps. For example, protective films used in the strengthening process lack sufficient temperature resistance, leading to softening and residue at high temperatures; protective films used in the screen printing process exhibit silicone contamination, affecting ink adhesion. Furthermore, switching between protective films between different processes is cumbersome, and some protective films require additional cleaning of residual adhesive after peeling, increasing processing steps and costs.

Second, the problem of residual adhesive is prominent. Some protective films have insufficient cohesive strength in the adhesive layer or improper tack control. After high-temperature, long-term bonding, residual adhesive easily appears upon peeling, especially after strengthening and coating processes, where the residue is difficult to clean, directly leading to product scrap. Traditional solvent-based adhesives (such as ordinary acrylic adhesives and polyurethane adhesives) show a significant increase in peel strength after high temperatures, making them more prone to residual adhesive problems.

Third, it's difficult to balance temperature resistance and abrasion resistance. Some high-temperature resistant protective films have low surface hardness, making them unsuitable for high-intensity friction processes like grinding and polishing; while high-abrasion resistant protective films lack sufficient temperature resistance, making them unsuitable for high-temperature processes like strengthening. This necessitates companies stocking multiple types of protective films, increasing inventory costs.

Fourth, environmental friendliness needs improvement. Traditional protective films often use solvent-based coatings for their adhesive layers, with a solid content of only about 20%. The large amount of organic solvent evaporation generates VOC emissions, failing to meet environmental policy requirements. Furthermore, solvent residue may affect the cleanliness of the glass surface.

Fifth, poor compatibility with curved glass. With the increasing prevalence of curved glass covers, traditional flat protective films are prone to wrinkles, bubbles, and edge lifting during application, resulting in poor protective performance. Moreover, peeling them can easily damage the glass edges, affecting the product's appearance.

V. Optimization Directions for Protective Films in Glass Cover Processing Addressing the above-mentioned problems and considering the development trends in glass cover processing, the following optimization directions are proposed from the aspects of material research and development, performance optimization, and process adaptation to promote precise compatibility between protective films and glass cover processing.

First, we developed process-specific protective films, optimizing the material and adhesive formulation to meet the core needs of different processing steps.