UV adhesive, also known as no-shadow adhesive, photosensitive adhesive, or ultraviolet-curing adhesive, refers to a class of adhesives that can only cure when exposed to ultraviolet (UV) light. It can be used both as an adhesive itself and as a binder in paints, coatings, inks, and other similar products. "UV" is the abbreviation for "Ultraviolet Rays," which refers to ultraviolet light. Ultraviolet (UV) radiation is invisible to the naked eye and represents a portion of the electromagnetic spectrum located beyond the visible light range, with wavelengths ranging from 10 to 400 nanometers. The curing mechanism of UV adhesives relies on photoinitiators (or photosensitizers) contained within the UV-curable material. When these photoinitiators absorb UV light, they generate active free radicals or cations, which in turn trigger polymerization, crosslinking, and branching chemical reactions. As a result, the adhesive transforms from a liquid state into a solid state within just a few seconds.

I. Encyclopedia of UV Adhesives

1. Definition

(1) Definition: Also known as shadowless glue, photosensitive glue, or UV-curable glue.

(2) Curing Principle: Under ultraviolet irradiation, the photoinitiator (or photosensitizer) in UV-curable materials absorbs UV light and generates active free radicals or cations, which initiate polymerization and crosslinking reactions of monomers, causing the adhesive to transform from a liquid state to a solid state within just a few seconds.

2. Main ingredients

(1) UV resins: epoxy acrylates, polyurethane acrylates, polyether acrylates, polyester acrylates, acrylic resins, etc.;

(2) Monomers: 40-60% monofunctional (e.g., IBOA, IBOMA, HEMA), difunctional (e.g., TPGDA, HDDA, DEGDA, NPGDA), trifunctional, and polyfunctional (e.g., TMPTA, PETA);

(3) Photoinitiators: 1–6% of 1173, 184, 907, benzophenone, etc.;

(4) Additive: 0.2–1%; optional.

3. Common Applications

(1) Bonding of plastics to plastics, plastics to glass, and plastics to metals; (2) Primarily focused on self-adhesion and mutual adhesion of plastics in the crafts industry, as well as in the furniture industry—for example, bonding coffee table glass to steel frames and bonding glass aquariums; (3) Thermoplastic plastics such as PMMA acrylic (organic glass), PC, ABS, PVC, and PS.

4. Product Features

(1) Wide range of applications; (2) High bonding strength; (3) Instant positioning within seconds; (4) Maximum strength achieved in one minute; (5) Completely transparent; (6) Long-term resistance to yellowing; (7) Resistant to environmental cycling; (8) Excellent flexibility; (9) Does not turn white; (10) Easy to use.

5. Advantages

(1) Environment/Safety

a. No VOC emissions, causing no air pollution; b. Relatively few adhesive components are restricted or prohibited under environmental regulations; c. Solvent-free and low flammability.

(2) Economic viability

a. Fast curing speed facilitates automated production lines and boosts labor productivity; b. After curing, the material can be immediately inspected and handled, saving space; c. Curing at room temperature saves energy; d. The curing equipment is simple—requiring only lamps or a conveyor belt—thus saving space; e. It’s a single-component system that doesn’t need mixing, making it easy to use.

(3) Compatibility

a. Suitable for materials sensitive to temperature, solvents, and moisture; b. Curing can be controlled, with adjustable waiting times and degrees of cure; c. Gluing can be repeated multiple times for subsequent curing; d. UV lamps can be easily installed on existing production lines without requiring major modifications.

6. Disadvantages

(1) High costs: High raw material costs—no low-cost solvents or fillers are used—and high adhesive prices;

(2) Curing limitations: Ultraviolet light has limited penetration through certain plastics or translucent materials, resulting in a limited curing depth. This restricts the geometric shapes of products that can be cured. Areas that are opaque and dead spots that UV light cannot reach are difficult to cure.

(3) Material limitations: Typically, UV adhesives can only bond translucent materials. Bonding opaque materials requires the use of complementary technologies, such as light-delayed (cationic) curing, photo-thermal dual curing, and photo-humidity dual curing.

7. Operation Instructions

(1) One of the two objects to be bonded must be transparent, with a surface that has been thoroughly cleaned, dried, and free of any grease.

(2) Apply UV-curable adhesive to one of the surfaces, then bring the two surfaces together and cure them with light.

(3) It is recommended that, during the initial positioning stage lasting about 6 seconds of illumination, you remove any residual adhesive from the workpiece and then re-illuminate it until fully cured.

(4) The curing time should vary depending on the different preparatory materials,胶 thickness, and ultraviolet intensity.

(5) The temperature also has a slight effect on the adhesive’s activity; when the temperature is low, the curing time should be appropriately extended.

(6) During operation, do not apply excessive pressure or repeatedly rub the materials to be bonded; it is recommended to use fixing tools.

(7) When bonding plastics, the content of UV absorbers in the plastic should be taken into account. A high content of UV absorbers will significantly impair the transmission of ultraviolet light, thereby markedly affecting the curing efficiency of the adhesive—and may even prevent the adhesive from curing altogether.

(8) When bonding large areas, it is recommended to use a low-viscosity product. If conditions permit, it’s best to acquire vacuum equipment and perform the bonding in a vacuum environment, which helps eliminate bubbles and improve the yield of finished products.

(9) The above instructions are for reference only; customers should make appropriate adjustments based on their individual circumstances when carrying out the actual operations.

II. Application Fields

1. Processed glass

(1) Bonding of glass products, glass furniture, and electronic scales; (2) Fixing and setting of crystal jewelry crafts; (3) Bonding of transparent plastic crafts made of PMMA/PS.

2. Electronics and electrical appliances

(1) Coating and sealing of terminal blocks, relays, capacitors, and microswitches; (2) Mounting surface-mount components onto printed circuit boards (PCBs); (3) Bonding integrated circuit chips onto printed circuit boards; (4) Securing coil wire terminals and bonding components; (5) Fixing cable harness coverings to connectors or circuit boards.

3. Optics field

Optical fiber bonding, fiber coating and protection, and splice repair.

4. Digital disc

(1) In the manufacturing of CDs, CD-Rs, and CD-RWs, UV-curable coatings are primarily used for the deposition of reflective and protective layers. (2) UV-curable adhesives are also used for bonding DVD substrates and for sealing covers in DVD packaging.

5. Medical devices

(1) Use of subcutaneous injection needles and syringes, intravenous catheter adhesives, urinary catheters, and medical filters; (2) Medical devices such as respiratory systems, butterfly-type devices and masks, intravenous catheter sets, oxygenators, water reservoirs, and electronic diagnostic devices.

6. Other uses

(1) Micro-motors: Fixed and bonded to the circuit board using adhesive wires; (2) Optical drives: The laser head is fixed, the lens is fixed, and the circuit board is fixed; (3) Transfer processes: Transfer adhesive and surface protective adhesive; (4) Bonding of automotive components: Bonding for automotive lighting assembly, rearview mirrors and airbag components, as well as fuel injection systems.

III. Frequently Asked Questions

1. Common Question 1: Why does UV glue turn white after curing?

This phenomenon is commonly observed in the glass industry. The whitening effect is actually caused by tiny air bubbles that form within the adhesive layer itself. During the curing process, adhesives tend to shrink. If the adhesive layer is unevenly thick or has excessive hardness, the internal stresses generated by this shrinkage cannot be relieved. Over time, these stresses lead to the formation of minute air bubbles—precisely what we see as the whitening effect—eventually resulting in the detachment of the bonded materials. To address this issue, there are three effective solutions: First, choose a UV adhesive with a flexible formulation; second, ensure that the adhesive layer is applied uniformly during bonding; and third, use a low-power UV lamp during the initial curing stage to slow down the adhesive's curing rate. After positioning the parts, then switch to a high-power UV device for thorough, deep curing. This is because an excessively rapid curing speed can increase the adhesive’s shrinkage rate.

2. Common Question 2: Why does the glue remain sticky on the surface even after being left to dry for a long time?

After the adhesive has cured, the surface of the gel comes into contact with air. Due to the inhibiting effect of the air, the gel surface cannot release its adhesion, resulting in the phenomenon of sticking to your hands.

The solutions to this problem include: (1) using a higher UV intensity; (2) using a higher photoinitiator dose; (3) using a faster-reacting photoinitiator; (4) using a UV light source with a wavelength of 365 nm.

3. Common Question 3: Is UV glue toxic? Why does using UV glue cause redness and itching on the skin?

UV glue is an environmentally friendly, green, and economically viable chemical product that contains no organic solvents and has a 100% solid content. It poses no toxic, carcinogenic, or mutagenic hazards to human health. The main components of UV glue are oligomers from the acrylate series and, to a lesser extent, monomers from the same series. Acrylate monomers have some volatility and can be mildly irritating to the skin. In the initial stages of contact, if proper protective measures are not taken and the user comes into direct contact with the glue or is exposed for prolonged periods in environments with high concentrations, it may cause skin allergies, such as redness and swelling of the eyes, rashes, and itching on the body. However, after the body has adjusted to the product over time, these allergic reactions—such as skin rashes—will typically no longer occur.

4. Common Question 4: Is the curing speed of UV glue related to the quality of the glue itself?

The curing speed and, in particular, the positioning speed of UV-curable adhesives have always been key factors that consumers pay close attention to. Assessing the quality of UV-curable adhesives involves multiple aspects: positioning time, curing depth, bond strength, flexibility of the adhesive film, and more. We believe that the faster the positioning speed, the greater the internal stress generated during the adhesive’s curing process. Generally, a positioning speed between 6 and 10 seconds is ideal. Therefore, it would be incorrect to judge the quality of a UV-curable adhesive solely based on its curing speed.

5. Common Question 5: Is it better to apply more adhesive when bonding?

When using cyanoacrylate adhesive, applying a larger amount of glue does not necessarily lead to better bonding performance. Experiments have shown that the thinner the adhesive layer, the higher the bond strength. Generally speaking, it’s best if the adhesive film thickness does not exceed 0.2 micrometers.

6. Frequently Asked Question No. 6: What is the viscosity of cyanoacrylate adhesive? Does lower viscosity mean lower strength?

The measure of the frictional resistance that arises between molecules when a liquid moves under the influence of an external force is called viscosity. The greater the frictional resistance, the higher the viscosity; the smaller the frictional resistance, the lower the viscosity. In the International System of Units, the unit of viscosity is mPa·s. The viscosity of water is 1.14 mPa·s.

The viscosity of cyanoacrylate adhesive has no direct relationship with its strength. It would be incorrect—and a common misconception—to equate viscosity with strength.

7. Frequently Asked Question 7: What’s the difference between invisible glue and other types of glue?

Shadow-free glue begins to set within 1–5 seconds under UV irradiation and completes bonding in just 20–30 seconds. After irradiation, it quickly attains high strength, meeting the demands of automated production line rhythms. Acrylate structural adhesive sets initially within 1–10 seconds and reaches its maximum strength after 24 hours. Room-temperature-curing epoxy structural adhesive sets initially within 10–120 minutes and achieves its maximum strength after 7 hours.

8. Frequently Asked Question 8: Why does UV adhesive used for bonding glass to metal tend to peel off easily in summer?

Summer is the peak season for UV adhesive failures. During rainy or humid summer weather, glass surfaces easily absorb moisture. Many customers find that simply wiping with a cloth isn't enough to remove the water film, leading to a gradual loss of adhesion and eventually to delamination. To ensure long-lasting bonding, it’s essential to dry the glass thoroughly using a hairdryer.

9. Frequently Asked Question 9: Why does the adhesive strength of some adhesives seem to decrease in winter?

When the temperature drops in winter, UV adhesives become harder and more brittle, leading to a decrease in tackiness. In cold regions, during winter, it’s necessary to adjust the adhesive formulation to enhance its low-temperature flexibility.

10. Frequently Asked Question 10: How long does it take for UV glue to fully cure?

Depending on the specific UV adhesive and the curing lamp used by the customer, it’s not possible to make a general statement—experimentation is required to determine the appropriate parameters. If the ultraviolet light intensity is high, a shorter irradiation time will suffice for curing; conversely, if the intensity is low, a longer irradiation time will be needed.

IV. Precautions

1. Precautions (Adhesive Storage)

(1) Protect from light. Visible light can also cause the glue to cure slowly, so it should not be exposed to air for extended periods.

(2) Moisture resistance: After absorbing moisture, the adhesive’s performance will deteriorate; therefore, do not leave the bottle cap open for extended periods.

(3) Room temperature. When the temperature exceeds 80 degrees, chemical reactions may be triggered; therefore, glue should not be left in high-temperature environments for extended periods.

2. Precautions (Before Applying Adhesive)

(1) The oil, moisture, impurities, and surface smoothness of the workpiece to be bonded will directly affect the bonding quality. Be sure to dry and clean the surfaces thoroughly, ensuring that the bonding surfaces are smooth and flat.

(2) When working in rainy or overcast weather, be sure to keep the bonded workpieces clean and dry. We strongly recommend using a dedicated shadow-free curing lamp for light-curing to ensure bonding quality.

(3) Colored or thicker glass can absorb ultraviolet or visible light, preventing certain wavelength bands from passing through or weakening them, thereby affecting the curing of UV-curing adhesives. Therefore, it is recommended to use a specialized adhesive or to confirm through experimentation the appropriate adhesive to use.

(4) The service life of UV lamps typically ranges from 800 to 1,000 hours. The intensity of the UV light emitted will gradually weaken as usage time increases, so the lamp tubes should be replaced regularly with new ones.

3. Precautions (during gluing)

(1) During the bonding process, be sure to eliminate any possible air bubbles. During the curing process under light irradiation, ensure that the bonded surfaces are fully exposed to UV light or sunlight. Be sure to press the bonded workpieces firmly together; the thinner the adhesive layer, the stronger the bond will be.

(2) Before the workpiece to be bonded is positioned, avoid moving it under UV light or sunlight at all costs, as this could cause whitening and affect its appearance. We recommend using clamps instead.

(3) The UV irradiation dose is crucial for the curing of UV adhesives, and it is determined by both the intensity of the light and the duration of exposure. For UV shadow-free adhesives cured using a dedicated shadow-free curing lamp, the positioning time typically takes just a few seconds, while full curing usually requires about 30 seconds. When curing with sunlight, the positioning time depends on the intensity of sunlight; generally, it takes several tens of seconds for positioning, and sufficient additional time must be allowed for complete curing.

(4) Regarding the issue of residual adhesive affecting product aesthetics, this can be addressed by precisely controlling the amount of adhesive applied, or by wiping away excess adhesive before light-curing and then proceeding with light curing.

(5) UV-curable adhesives are industrial chemicals. Before curing the adhesive under light, please take appropriate protective measures to avoid direct skin contact. If the adhesive accidentally gets on your skin, simply rinse it off thoroughly with plenty of water. Individuals with skin allergies should seek medical attention immediately; generally, these adhesives do not cause any adverse effects.

V. Selection of UV Resins

1. Selection Principles

(1) Good adhesion; (2) No yellowing; (3) Low shrinkage; (4) Low odor; (5) Good flexibility.

2. Resin Recommendations

(1) B-151; (2) B-205; (3) B-210GB; (4) B-268M; (5) B-295A; (6) B-361.

Disclaimer

The above-mentioned indicators and recommended performance levels are measured under fixed conditions and do not guarantee suitability for all intended applications. For different application purposes and conditions, users must independently verify the formulations to ensure compliance with requirements.