Language
- Chinese

language

Tel

Tel

+8618142863185
Follow us

Official Accounts

Official Accounts
Top

News

Liquid Optical Adhesive (LOCA)

Release time:

2016-09-07 15:24

I. Introduction to Liquid Optical Adhesives

Liquid Optical Clear Adhesive (LOCA) is a specialized adhesive used for bonding transparent optical components. It is colorless and transparent, with a light transmittance of over 98%. It exhibits excellent bonding strength and can cure under ambient or moderate temperatures. Additionally, it features low curing shrinkage and excellent resistance to yellowing.

Lamination of capacitive touch screens

II. Introduction to OCA Adhesive

OCA’s English name is “Optics Clarity Adhesive,” or optical transparent adhesive. It is a type of double-sided adhesive tape made by forming optical acrylic into a substrate-free structure, with a release film laminated onto both the top and bottom surfaces. This adhesive tape features a substrate-free material construction.

III. Comparison Between OCA and LOCA

Compared to traditional OCA tapes, LOCA offers particular advantages in certain application areas.

Can address the following limitations faced by OCA adhesive tapes:

	OCA tape	LOCA Liquid Optical Adhesive
Inventory Management	Corresponding die-cutting blades need to be provided for different customer products.	A product can be applied to multiple models of customer products.
Gap and Irregular Surface Filling	Only gaps less than 1/10 of the tape's thickness can be filled.	Can bond to irregular surfaces and surfaces with significant height differences.
Process	Rolling/Vacuum Lamination + Defoaming	Dispensing + Pressing + UV Curing
Total material cost	Thanks to the reduced die-cutting costs and waste associated with OCA tape, liquid optical adhesive offers a significant cost reduction compared to tape.
Application	For irregular and uneven surfaces, as well as for hard-to-bond hard-to-hard and soft-to-hard adhesions, liquid optical adhesives can overcome some of the limitations inherent in tapes themselves.

For bonding to uneven surfaces, the adhesive has excellent filling performance.

4. The Role of LOCA Adhesive

1. Bonding the touch screen to the LCD module

2. Fill the gap between the ITO glass and the cover glass to enhance brightness.

V. Introduction to Lenses and ITO

As a crucial component of mobile phones, the lens performs several vital functions: protecting the LCD, providing excellent light transmission, and serving as an aesthetic decorative element.

LENS Universal Material:

1. PMMA: Excellent light transmittance ≥91%, high surface hardness; after surface hardening treatment (hard coating), it can achieve a hardness of 3H or higher.

2. PC: Due to its surface hardness failing to meet requirements and its inferior light transmittance compared to PMMA, it is rarely used in mobile phones.

3. Glass: It has excellent light transmittance and high surface hardness—reaching above 7H—and can be made very thin (0.4mm). However, its manufacturing process is complex, and the price is slightly higher.

ITO, or indium tin oxide, is a transparent conductive material. By adjusting the ratio of indium to tin, as well as the degree of oxidation and the size of the crystal grains, the properties of this material can be tailored. It is used as the sensing material in resistive and capacitive touchscreens.

6. Key Parameters of LOCA Adhesive

1. Viscosity: The application viscosity of LOCA ranges from 1,000 to 4,500 cps.

2. Refractive Index: The ratio of the speed of light in air to the speed of light in the material. After using LOCA, the overall refractive index of the display area should be substantially uniform to minimize chromatic dispersion. Typically, the refractive index of glass is approximately 1.5, and a qualified LOCA has a refractive index of approximately 1.5 as well.

3. Elongation at Break: The longer the elongation at break after curing, the better the toughness (elasticity) of the LOCA.

4. Dielectric constant: Refers to a material's ability to hold electric charge, also known as the capacitance. The higher the value, the better the conductivity.

Typically, the dielectric constant of air at room temperature is approximately 1. The dielectric constant of LOCA usually ranges between 1.5 and 3; therefore, capacitive touchscreens using LOCA are more sensitive. The dielectric constant of glass is around 4 to 11.

5. Acidity: pH = 6.5–7.2, neutral, with virtually no corrosive effect on ITO and the materials themselves;

6. Light Transmittance: The percentage of luminous flux that passes through a transparent or translucent material relative to the incident luminous flux. The higher the transmittance, the clearer the image will be.

7. Curing Shrinkage Rate: The lower the curing shrinkage rate required for LOCA, the better—this reduces the likelihood of issues such as gel shrinkage, delamination, and discoloration. Industry standards specify a shrinkage rate of less than 2.5%; in recent years, the curing shrinkage rate of domestic products has typically been around 5%.

8. Adhesion strength: The cohesive force of LOCA after it has fully cured—derived from the intrinsic strength of the material itself.

9. Haze: The cloudy or hazy appearance of the interior or surface of transparent or translucent materials caused by diffuse scattering of light. It is expressed as the percentage ratio of the flux of diffusely scattered light to the flux of light transmitted through the material.

10. Yellowing: The extent to which a colorless, transparent, semi-transparent, or nearly white polymeric material deviates from its original white color. Measured using a colorimeter after exposure in a UV aging chamber at 60℃ for 500 hours. When the polymer adhesive layer is subjected to prolonged UV radiation and the energy exceeds bond energy, on the one hand, active centers are easily generated along the molecular chains, triggering gradual degradation of the polymer and leading to yellowing; on the other hand, carbon-carbon bonds within the molecular chains undergo cleavage, also causing yellowing.

7. LOCA Gluing Process

Currently, the LOCA adhesive bonding technology controls the gap by precisely regulating the amount of adhesive applied. On the upper or lower substrate, a UV adhesive of a specific shape—typically resembling a “double Y-shaped dogbone”—is dispensed at designated points. The upper and lower substrates are then brought together at a controlled speed. Once the substrates have reached a certain height, the applied force is released, allowing the adhesive to naturally flow and fill the gap completely before being cured under UV light.

8. Common Solutions for LOCA Bonding Anomalies

1. Bubble issues

To address the bubble issue, apply a small drop of glue onto the upper sheet. Then, by flipping the sheet over, ensure that the glued surface faces downward, forming a droplet. The bottom surface of the droplet will come into contact with the lower sheet, expelling any trapped air. However, be sure to press down slowly.

2. Adhesive overflow issue

The colloid can simultaneously or nearly simultaneously overflow to all four corners of the product, and once it immediately solidifies, the issue of excess glue will naturally be resolved.

9. Reliability Analysis of LOCA Adhesive

1. Adhesion Strength Testing

Testing method:

(1) Substrate: 30×60×3 mm glass plate;

(2) Speed: 10 mm/minute;

(3) Adhesive thickness: 100 μm.

2. Light transmittance and haze

Testing method:

(1) Substrate: 50×60×1 mm glass plate;

(2) Adhesive thickness: 100 μm.

Transmittance = T2/T1 × 100%.

Haze = (T4/T2 - T3/T1) × 100%.

T1: Incident luminous flux.

T2: Transmitted luminous flux.

T3: The scattered light flux of the instrument.

T4: The flux of scattered light from the instrument and the sample.

3. ITO Compatibility Test

Testing method:

(1) Apply 100 μm of LOCA onto ITO glass with etched ITO conductive traces. After curing, measure the resistance value of the ITO.

(2) Place the sample in a 60℃, 90% RH aging chamber and test its resistance value after aging for 500 hours.

(3) Resistance change rate = (Resistance value before aging - Resistance value after aging) / Resistance value before aging.

4. Anti-UV Yellowing Test

Testing method:

(1) Sample Preparation: Fill a 100-μm-thick LOCA adhesive between two 1-mm-thick glass slides, and after curing, test the optical performance.

(2) Place the sample into a UV aging chamber and expose it to conditions of 60℃ and 0.86 W/cm² for 500 hours. Then, test the optical performance of the sample after aging.

5. Reliability Testing

Testing method:

(1) Sample Preparation: Fill a 100-μm-thick LOCA adhesive between two 1-mm-thick glass slides. After curing, test the optical performance.

(2) Place the sample into the aging chamber under the following aging conditions:

a. High-temperature test: 85℃, 500 hours;

b. High-temperature and high-humidity test: 60℃, 90% RH, 500 hours;

c. Thermal Cycling: -40℃ for 60 minutes, 85℃ for 60 minutes, 250 cycles.

(3) Optical performance of the samples after aging tests

6. Rework of uncured adhesive

(1) Wipe off the uncured adhesive using a lint-free cloth.

(2) Wipe the panel with a dust-free cloth dampened with ethyl acetate until it is completely clean.

(3) Reapply adhesive and proceed with bonding;

(4) Confirm that the panel has been thoroughly cleaned and the solvent has completely evaporated before proceeding with re-gluing.

7. Reworking of cured adhesives

(1) TP separation

a. Molybdenum wire cutting method;

b. Liquid nitrogen cryopreservation.

(2) Remove glue

Place two dust-free wipes soaked in solvent onto the panel, leave them in place for 30 minutes, and then begin removing the adhesive. Use a soft scraper to remove the adhesive in one consistent direction, followed by wiping with a solvent-soaked dust-free wipe, and then again with a clean dust-free wipe. Once the solvent has completely evaporated, you can reapply the adhesive.

Share to:

A Big Comparison of Material Performance for Phone Cases

Curing Mechanism of Epoxy Resins and Common Curing Agents