UV-curing adhesives can help get manufacturers out of sticky situations.

Walter Brenner
Technical Director
Master Bond Inc.
Hackensack, N.J.

A technician uses a UV curable adhesive for perimeter bonding an optic component to a metallic part.

A technician uses a UV curable adhesive for perimeter bonding an optic component to a metallic part.


Organic adhesives usually contain solvents and various diluents or are composed of two components which must be carefully weighed and mixed prior to use. After these adhesives are applied, it takes heat to drive off solvents and other volatiles and speed the cure or hardening, especially for two-component mixtures. This thermal curing puts as many as 6 gallons of solvent into the air for every gallon of solid adhesive left. Such pollution is regulated by the EPA and companies are often forced to buy expensive recovery systems with afterburners.

UV-curing adhesives and compounds, on the other hand, avoid most of these problems while providing top-notch bonding and performance. For example, UV adhesives use only one component, so there's no weighing or mixing before use. This simplifies production schedules by eliminating pot-life problems. UV adhesives are also 100% reactive, so there are no volatile losses during curing and the process is nonpolluting.

UV adhesives remain stable and usable — even after storage at ambient temperatures for six months — until activated by UV light. Activation occurs only when exposed to UV light of 250 to 350 nm, and then the materials cure on demand, letting companies get rid of lengthy ambient-temperature cures or ovens needed for elevated temperature cures.

HOW THEY WORK
UV adhesives absorb radiant energy from a UV light source and convert it to chemical energy so quickly that curing is practically instantaneous. So quick, in fact, that substrates experience only a brief, superficial temperature change. This lets companies use UV adhesives on heat-sensitive materials including plastic films, moldings, and synthetic fibers, as well as elastomers and paper products.

The lack of heat is particularly valuable in electrical and electronic industries where transient thermal changes can degrade a component's performance. In addition, the low heat reduces substrate shrinkage and warpage, and permits additional on-line processing and off-line handling. This eliminates having to move parts to cooling racks before they go down the line, along with the space and labor requirements.

Completely reactive UV adhesives are not oxygen-inhibited and have fast curing rates at ambient temperatures and atmospheres, though curing is faster if heat is applied. This eliminates the need for atmospheric control — a nitrogen atmosphere, for example — to get tack-free cures. Equally important, cure continues in the dark after UV exposure until all UV reacting species are consumed, thus making economical use of UV energy.

UV compounds can cure despite cross-section thicknesses up to 0.5 in. and more for specific formulations. Maximum dimensional accuracy is assured because the compounds cure with minimum shrinkage.

UV-cured bonds remain intact over temperatures ranging from –80 to 350°F. The bonds also stay intact for a long time because they resist most chemicals even in the presence of moisture and heat.

For best adhesion, substrates must be carefully cleaned of oils, greases, release agents, dirt, and other contaminants. In many cases, such as with metals and other inorganics, a simple test determines if the surface is clean. The test involves spreading a few drops of cool water on the surface. If water spreads over the area with a continuous film, parts are clean enough. But if water beads or stays in puddles, degrease the surface with an EPA-acceptable solvents such as IPA or acetone. Repeat the water test before applying UV-cure adhesives. Polyolefins, such as polyethylene and polypropylene, as well as fluorocarbon polymers such as polytetrafluoroethylene and various chlorinated fluorocarbon resins, require special surface treatments for adequate adhesion.

UV Adhesives

Grade

Viscosity RT (cps)

Color

Hardness (Shore D)

Temperature range (°F)

Application

UV10

300 to 400

Light amber/clear

60 to 65

-60 to 250

Low-viscosity, general-purpose adhesive, sealant, coating, and encapsulant. Cures rigid and up to 0.125-in. deep

UV10FL

600 to 700

Light amber/clear

45 to 50

-60 to 250

Flexible version of UV10. Resists shock and vibration. Good thermal cycling properties

UV10LV

150

Light amber/clear

75

-60 to 250

Ultralow viscosity. Good electrical properties. Ideal for conformal coatings

UV10MED

1,200 to 1,500

Light amber/clear

60 to 65

-60 to 250

Medical version of UV10. USP Class IV approved. Resists sterilants.

UV10PSA

16,000 & 18,000

Transparent

N/A

-60 to 250

Strong, fast-tacking pressure-sensitive adhesive. Available in two viscosities for flexible application.

UV10TK

30,000 to 40,000

Light amber/clear

70 to 75

-60 to 300

Higher viscosity version of UV10. Enhanced chemical and temperature resistance. Good dimensional stability, low shrinkage

UV14-3

8,000

Transparent

30

-60 to 250

Flexible adhesive. Removes easily with conventional solvents. Low refraction index (1.477)

UV14X-2TK

Thixotropic

Transparent

60

-60 to 250

Semiflexible adhesive. Durable. Good electrical properties and low shrinkage.

UV15

120 to 150

Slight amber/clear

Below 75

-60 to 300

Very-low viscosity, good temperature stability, resists chemicals and shrinkage.

UV15-42C

Paste

Translucent

Below 50

-60 to 250

Fast curing, good dimensional stability, and little shrinkage.

UV15-7

1,400 to1,800

Transparent

70

-60 to 300

Good adhesion and nonyellowing.

UV15-7DC

1,500 to 2,500

Transparent

Below 70

-60 to 300

Dual-cure version of UV15-7. Cures in shadowed out areas by adding heat (250°).

UV15-LRI

6,000 to 10,000

Transparent

50

-60 to 250

Low refraction index (1.481)

UV15-7SP4

800 to 1,500

Transparent

35

-80 to 250

Flexible version of UV15-7

UV15-7SP4DC

800 to 1,500

Transparent

35

-80 to 250

Dual-cure version of UV15-7SP4

UV15-7TK1A

Paste

Translucent

65

-60 to 300

Paste version of UV15-7

UV15FL

200 to 300

Light amber/clear

60

-60 to 250

Flexible version of UV15. Improved peel strength

UV15TK

8,000 to 10,000

Slight amber

Below 75

-60 to 350

High viscosity version of UV15

UV15X-2

6,000 to 8,000

Transparent

65

-80 to 250

Semiflexible adhesive

UB15X-2GT

Paste

Translucent

65

-80 to 250

Resists heat and moisture

UV15X-5

120,000

Transparent

35 to 40

-80 to 250

Flexible with good peel strength and abrasion resistance

UV15X-6 MED

24,000

Transparent

25 to 30

-80 to 250

Medical grade of UV15X-5. USP Class IV approved.

UV16

120 to 150

Slight amber/clear

Below 75

-60 to 300

Resists temperature and chemicals. Little shrinkage.

UV18 MED

1,800 to 2,000

Transparent

55 to 60

-60 to 250

Class VI medical adhesive. Resists sterilants

UV18S

1,800 to 2,000

Slight amber/clear

55 to 60

-60 to 250

Resists acids, bases, and solvents.

UV19

300

Transparent

15 (Shore A)

-60 to 250

Ultraflexible and soft curing.

UV21

32,000 to 36,000

Transparent

20 to 25

-60 to 250

Flexible and adheres to acrylics, glass, polycarbonates, and other optical-type substrates

UV22

4,000

Transparent

Below 85

-60 to 300

Nanoparticle-reinforced for strength and low shrinkage

The table lists various properties of UV-curing adhesives from Master Bond. Those with n/a under hardness are so listed because Shore-D hardness cannot be measured on these formulations.