A new family of UV curing Aerobic Acrylic adhesives has been developed and positioned specifically for optical assembly applications. The development of this new optical adhesives line is the result of a strong response to and use of industrial grades of Aerobic Acrylic adhesives by various companies in the optical industry. The purpose of this paper is to introduce the new product line to the optical assembly industry and describe the features and benefits it has to offer. The paper will discuss significant benefits of the new optical adhesives including: Higher strength bonds – comparable to epoxies; Faster cures than typical optical grade UV curing mercaptoesters; Low stress, high strength bonds lead to superior durability; Where they may be used to replace slower curing adhesives; and How Aerobic Acrylic formulations achieve these benefits.
Optical Adhesive
Adhesive
UV Cure
Light Cure
Acrylate
Bond
Cure
Assembly
Dymax
Cementing
A new family of UV curing Aerobic Acrylic adhesives has been developed and positioned specifically for optical assembly applications. The development of this new optomechanical line is the result of a strong response to, and use of industrial grades of Aerobic Acrylic adhesives by various companies in the optical industry. The purpose of this paper is to introduce the new product line to the optical assembly industry and describe the features and benefits it has to offer.
UV curing is almost as old as civilization itself. Certain dyes used to preserve Egyptian mummies had to be dried (cured) in sunlight. UV curing resins found their first commercial use during World War II to protect acrylic windows, nose cones and firing “bubbles” for military aircraft. The late 1970’s saw large scale acceptance and use of UV curing acrylic resins by the ink industry as an alternative to solvent based inks. In the early 1980’s UV curing Aerobic Acrylic Adhesives (AAA’s) were introduced to the assembly industry. Aerobic Acrylic Adhesives have the ability to cure between two surfaces regardless of the presence or absence of air. The absence of hydrocarbon-based solvents makes the choice of AAA’s much more environmental safe than conventional adhesives. Recent improvements in AAA’s have yielded performance properties that equal or exceed epoxies.

Tensile Strength per ASTM D1002,
DSTM 250, DSTM
251
Table 1. Adhesion of Various UV Resins
Steel
Glass
Plastic
The principal advantage of AAA’s include:
•
Lower costs
•
Faster cure times
•
Long shelf life
•
Room temperature cures
•
Less material waste
•
Higher production yields
•
Elimination of complex jigs and fixtures
•
Simpler application
•
Environmentally safe
•
Overall reduction in production times
Table 2. Typical Applications
Typical Applications for Aerobic Acrylics
Glass and plastic headlamp assembly
Electrical sealing and tacking
Optical component assembly
Microelectronic assembly
Dome coatings
Conformal coatings
Medical Disposable Device assembly
Coil terminating
Tamper proofing
Fingernail coating
Formed-In-place gaskets
Figure 1. Headlamp Lens
For all of these reasons and the strength and durability of the bonds formed, AAA’s began replacing heat curing and two part epoxies, urethanes and silicones in most applications where light could reach them to effect cure .
Since the early 1980’s, use and acceptance of these products has cut across the industrial spectrum to include uses in automotive motor and lighting assembly, aerospace, optical, military and industrial electronic assembly, appliance, medical disposable device assembly and electrical and architectural applications, to name a few. In addition, AAA’s are able to operate over a wide temperature range, from –60 o
C to 200 o
C, depending on the adhesive. Table 2 lists some typical applications for which AAA’s have been used.
Optical Engineers have relied on adhesives for many applications. In most situations it is the only reasonable method of mounting. Adhesive use is a reliable approach which is cost effective and process-controlled. Often specifications of adhesives in a design leads to production problems. These problems include but are not limited to: material handling (shelf life / proper mix ratio / pot life); long cure times often at high temperature; application
(tooling / training); and environmental concerns. UV and UV/visible light curing adhesives are more conducive to production applications. With improved performance these newer AAA’s are replacing many traditional adhesives. Table 3 provides a quick comparison of typical adhesives used in the Optics Industry.

Adhesive
Cure
Mechanism
Typical
Properties
Time To Full
Cure
Odor
Storage
Table 3. Adhesives for Optical Applications
UV/Visible
Urethane
Acrylate
UV Or Visible
Light
Resilient - Hard
To Very Flexible
Tack In
Seconds
Full Cure In
Minutes
Low
1 Year, Room
Temperature
UV Mercaptoester
UV Light
Resilient – Hard To
Very Flexible
Pre-Cure In Seconds
Post Cure For
Minutes
Post Bake
Pungent
4-6 Months
Refrigerated
Epoxy
2 Part Mix
Or Frozen
Very Hard
And Brittle
Minutes To
Hours
Low
Various
RTV Silicone Cyanoacrylate
Moisture
Very Flexible
And Soft
Hours
Low
1 Year
Moisture
Hard And Brittle
Seconds
Pungent
4-6 Months
Refrigerated
UV Curing Aerobics - Offering A Wide Range Of Options
A tough, resilient urethane polymer backbone gives Aerobic Acrylics high strength, toughness, and resiliency as well as a range of properties from rigid to flexible; from very hard to very soft, depending on the formulation.
Formulations can come in a range of viscosities, from thin wicking grades to thixotropic gels. These adhesives cure “on demand”, allowing the operator to position the lens before affecting cure. UV curable resins can be cured with UV light only. Special grades of UV/visible light curable Aerobic Acrylics (called “Ultra Fast
™
) offer the benefit of being able to cure through even UV blocked plastic, such as of polycarbonate, polystyrene, and polymethylmethacrylate (acrylic). Multi-Cure
®
formulations
3
offer the ability to cure with heat as a secondary cure method, allowing for cure in a shadowed area.
UV/visible acrylates cure by a free radical polymerization. Cure is generally complete within a few seconds though optimum glass adhesion may take longer due to the slower reaction rate of adhesion promoting additives.
Figure 2. Free Radical Polymerization
• à
•
•
à
•
n
n
UV Curing Mercaptoester
Long a “standard” in Optomechanical assembly, thiol-ene polymer (aka mercaptoesters) systems can yield sealants or adhesives that range from hard to soft. Some formulations contain solvents and may be flammable.
Initiated by UV light thiol-enes cure by an addition mechanism. Most commercial literature indicates that a significant amount of time is required for cured properties to fully develop. Post cures of at least 1 week or 50ºC
(122ºF) are frequently required according to conventional literature.

Figure 3. Mercaptoester Polymerization
à
à
n
n
Both the AAA’s and the Mercaptoesters cure upon exposure to UV light though the cure chemistries are different.
Does this difference in composition then, result in a difference in performance? Laboratory testing of these two types of UV curing products in conjunction with a review of published literature has shown some significant distinguishing features. Figure 4 below shows a significant increase in both the speed of cure and strength of the
AAA’s compared with the Mercaptoester based adhesives.
Figure 4. Compression Strength Versus Time Following UV Cure
Dymax OP-29 *
UV Light Curing (RT)
Hour
o
o
o
120 o
C
o
o
*

During UV Cure, adhesion promoters become part of the polymer matrix, which after cure, can react with the glass surface to show a 10-200% increase in compressive strength. This increase is due to a secondary cure mechanism which forms strong covalent bonds directly to the silicone atoms in the glass. See figure 7 for more information.
Epoxies
Typically used when induced stress or speed of cure are not important, epoxies can provide durable, rigid structures for high temperature applications. There are a wide variety of systems. Typically, epoxies fall into one of three categories; two-component systems, one-component heat cure systems, and one-component frozen systems. However, all present processing problems, ranging from limited pot life and slow cure to special storage. Two-part epoxies typically require 24 hours for full cure. Heat curing types, whether premixed, frozen or a true single component grade, usually requires exposure to 120º - 150ºC heat.
UV Curing Epoxies
While similar in properties to heat curing epoxies, UV curing epoxies are typically lower in adhesive strength, especially to glass and metal. Older types of UV epoxies bond parts together by efficiently forming a vacuum between closely matched surfaces. Their greatest advantage is low shrinkage and good thermal stability. UV curing epoxies cure as fast or faster than other UV curing adhesives in very thin layers. However, their depth of cure is very limited and that coupled with lower adhesion has tended to limit their use in many assembly applications. Lower durability is frequently observed.
Figure 5. Epoxy and UV Epoxy Polymerization
Speed of Cure to 0.125”
Time to reach full strength
Pot Life
Table 4. Cure Time Comparisons Between AAA and Epoxy Systems
UV Aerobic
Acrylic adhesives
15-30 seconds
20 minutes
Unlimited
Conventional
UV Epoxy
120 minutes or longer
24 hours
Unlimited
2 part
Epoxy
10-50 minutes
24 hours
5-60 minutes
1 Part Thermal
Cure Epoxy
15 min - 2 hours @
100 -150 o
C.
24 hours
Days @ Room
Temperature
Frozen 1 part Epoxy
3 hour thaw;
10-50 minutes
24 hours
10-50 minutes @ RT
Silicones
Silicones have a highly flexible polymer backbone and are usually used in gasketing, and sealing applications.
Two part mix silicones cure in a few minutes while one part “RTV” types need controlled humidity and many hours to many days to cure fully. There are a few types of UV curing silicones. Most are used as release agents. Other types on the market are simply mixtures of UV curing acrylics and RTV silicones. As with other
RTV types, full cures require long exposures to humidity. Most RTV silicones give off other chemicals on cure
(outgassing) which may fog or even damage precision optical components.

Cyanoacrylates (CA’s)
Rigid and brittle CA’s (superglues) cure in seconds upon contact with the moisture that is naturally absorbed on almost all surfaces. Some plastics require priming. Many CA’s effloresce on cure, resulting in a “frosting” effect that can effect both appearance and the clarity of optical components. Though depending on it for cure, CA bond lines are quite sensitive to and degrade with exposure to too much moisture. Most CA bonds are not shock resistant.
UV curable acrylates are comprised of oligomers, monomers, additives, and photoinitiators. These formulations can be tailored to provide optically clear resins with specific refractive indexes, adhesion, weatherability, resistance to UV light, etc. Oligomers are medium length polymer chains that contribute to tensile strength, modulus, and elongation. Monomers provide adhesion to different substrates. Additives can provide a wide range of viscosities to allow for easier dispensing, or can be incorporated as fillers for extremely low shrinkage.
Figure 6. Typical UV-Curing Urethane Acrylate Adhesive
P.I.
ADDITIVE for fine tuning
MONOMER for specific properties
OLIGOMER for basic properties
The UV adhesives cure upon exposure to UV light in the 300-400 nm range of the electromagnetic spectrum as shown below in Figure 8. UV/visible light curing adhesives cure in the 300-500 nm range, which allows for curing to deep sections, faster cures, or cure through UV blocked substrates. Using a visible light curing adhesive also allows for faster cures using a lower intensity, low power light source, an advantage when considering the cost of light sources.
All light curing formulations include, in addition to the basic resin chemical constituents, oligomers, monomers, thickeners, adhesion promoters, etc. Photoinitiators that react when exposed to light of the correct wavelengths cause polymerization of the resins. These photoinitiators are organic molecules which can absorb light and produce radical species upon splitting. It is these radicals that initiate the polymerization of the monomers and meth(acrylate) oligomers. See figure 2 for reaction mechanism. Adhesion is developed by secondary reactions with adhesion promoters directly to the Silicone atoms in the glass within 1-60 minutes to gain a 10-200% increase in compression strength.
Figure 7 - Free Radical Initiation, Propagation, And Termination/Adhesion to Glass surface

AAA’s are designed to be sensitive to those areas of the electromagnetic spectrum which allow the optimum combination of speed and depth of cure for assembly applications. Formulations that cure only with UV light are primarily triggered by longwave UVA spectrum. Formulations for bonding through UV inhibited surfaces, combine catalysts from both the UV and visible regions and generally product faster and deeper cures. Shown below in Figure 6 is lamps spectral output matched to an adhesive’s spectral absorbance, indicating a synergy between the curing lamps and the intensity and wavelength that the adhesive needs to cure.
UV-C Region UV-B UV-A Visible
25
20
15
10
5
Adh.
Absorption
Spectrum
Lamp
Emission
Spectrum
0
200 225 250 275 300 325 350 375 400 425 450
Wavelength, nm
As has been shown, AAA’s come from a technology of acrylated urethanes that have a long history of use in high performance applications and some demonstrated advantages of strength and cure speed over UV curing mercaptoesters as shown in Graph 1. Table 5 below is a summary chart of the property ranges of the New
Aerobic Acrylic Adhesives for Optical Assembly.
Table 5 Typical Properties for Dymax OptoMechanical Resins
Property of the cured resin Property Ranges
Normal cure depth
Cure speed
Viscosity
Adhesion to metals, plastic & glass
Clarity
Shrinkage
Surface dryness/slickness
Moisture resistance
1-500 mils
1 to several seconds cures “typical”
10 - 200,000 cP – non flowing paste
To 4,000 psi or substrate destruction
Water white to transparent light, straw
Low
Dry surface cures
Good to excellent
Hardness (Shore A&D) A 30 to D 80
Resistance to thermal shock under load Good to excellent
Solvent resistance Good to excellent

These new adhesives have been designed with the applications and concerns of the optical designer in mind.
We will now turn our discussion to these design considerations and associated concerns and how the new adhesives respond to these challenges.
It is frequently critical to design the optical component with the adhesive in mind. The properties of these adhesives cover a wide range for uses in differing applications. Two common areas for use are glass to glass and glass or plastic to metal. In glass to glass bonding such as doublet, prism , or fiber optic assembly, the adhesive transmission and index of refraction are critical. AAA’s can be developed with excellent lens bonding properties and with a wide range of refractive indices. The UV instant curing is well suited to doublet assembly.
Glass to metal or plastic to metal bonding, in applications such as lens, prism, mirror mounting, ferrule bonding, or fiber optic tacking, require other adhesive characteristics. Viscosity, shrinkage, speed of cure, flexibility, and tensile strength can be critical to the success of an application. A wide range of products may be utilized to match every application requirement. Adhesives such as the OP-24 may be utilized for mirror bonding as the multiple cure methods offer many advantages over conventional epoxies. Multicure adhesives can cure with
UV/visible light, activator for room temperature cold curing between metal and metalized glass surfaces, or heat for areas not able to receive UV light.
Low Shrinkage Applications - Pinning
Pinning applications require an extremely low shrinkage adhesive. As operators align optical components, the adhesive can already be in place. Once the proper alignment has been obtained, a spot curing system can focus a narrow beam of light to the adhesive with subsequent cure in seconds with very little movement upon cure. It is important to cure multiple bond sites simultaneously to avoid uneven shrinkage, even with the low shrinkage values. Adhesives like the Dymax OP-61 will maintain this position without moving over the life of the part, even at most operating temperatures and through thermal cycling due to the low coefficient of thermal expansion
(CTE) value of 58x10
-6
in/in o
C. While for most applications, the correct light source will play a significant role in the shrinkage of the adhesive, these special resins maintain low shrinkage over a wide light intensity range.
Figure 9 shows an approximate value of 0.3% linear shrinkage irregardless of the energy of the light source.
Figure 10 depicts a typical prism mount.
Figure 9. % Shrinkage vs Energy for OP-61 Figure 10. Typical Prism Mount
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
19.4
7.2
% Shrinkage vs Energy
OP-61
5.5
5.4
4.6
Energy (UV-A), J/cm2
2.8
2.1
Stress Protection Theory

In many applications, rigid epoxies often create stress relative to coefficient of thermal expansion differences.
Stress in optical components creates stress birefringence and optical distortion. Birefringence stress creates polarization sensitive changes, which in many cases affects performance. High temperature curing of many epoxies creates severe stress conditions at room (and lower) temperatures. Solutions to avoid stresses have often led to flexible adhesive use with both lower strengths and higher outgassing problems. New AAA’s have combined the higher strength with the ability to reduce stress due to the very low modulus inherent in these new materials.
Low modulus adhesives produce less stress on substrates than do high modulus materials. AAA’s are available with very low modulus. Equation (1)
5
describes the shear stress produced by thermal excursions between the substrate and the adhesive bond as follows:
= (
adh
-
sub
) *
T * E avg where,
σ
α
α adh sub
∆
T
E avg
=
=
=
=
=
Shear stress due to adhesive bond
Coefficient of Thermal Expansion of Adhesive
Coefficient of Thermal Expansion of Substrate
Temperature Range
(1)
Average Modulus of the adhesive over the temperature range
When choosing a resin system based on the relative shear stress that might be introduced, it is important to weigh the processing cost and cure speed with the shear stress.
Table 6. Relative Shear Stress Due To Thermal Expansion From -55/125 o
C For Rigid
And Flexible Resins Bonding To Aluminum 6061-T6.
Liquid Adhesive Resins
2 Part Silicone
UV Aerobic Acrylic Urethane (OP-30)
Frozen Filled Epoxy
UV Mercaptoester
2 Part Epoxy Resin
Relative Shear Stress -
55/125 o
C, (psi)
14
95
810
4300
6345
Cure Speed
8 hours
10 sec
3-4 hours
10 sec
1-3 hours
Coefficient Thermal
Expansion, x 10
-6 in/in/ o
C
340
200
20
200
70
Modulus of
Elasticity (psi)
250
3,000
1,500,000
150,000
750,000
Some resins contain high levels of mineral filler in an attempt to lower the measured CTE and increase the
Glass transition temperature (Tg), thereby increasing the temperature at which a dimensional change will take place to above the operating temperature of the device. However, by choosing a low modulus material, the overall stress imparted upon an object will be minimized irregardless of the CTE and Tg.
Figure 11 shows that high modulus resins such as epoxies characteristically develop high stress rapidly with only small physical changes. Stress is much lower, and builds more slowly, with dimensional changes in flexible resins. Figure 12 typifies doublet assembly.

Figure 11. Stress/Strain Relationships
Figure 12. Doublet Assembly
Lens Mounting
As lenses get smaller and bonds more delicate, the strategies to overcome the stresses inherent in the materials and between adjacent bonded surfaces have become more exacting in order to survive the environmental rigors that the device will be exposed to. Choosing a material such as the OP-29 or OP-30 that is resilient and strong with low shrinkage characteristics may be ideal for an application of this type.
Optimization of the adhesive in the bond line can be done with some calculations such as this simple model for calculating adhesive thickness for a lens mount. A preferred bond line might range from 0.002” to 0.005”, but can vary from 0.005” to 0.25” or more, with special applications requiring greater than a 0.25” bond line.
Figure 13. Simple Lens Potting Model
4
.
For minimized stress on the lens: hr = dg(
α
2(
α m
- r
-
α
α g m
)
)
Where: hr Required Elastomer Thickness dg
α
α m g
-
-
Lens Outer Diameter
Cell thermal coefficient of expansion
Lens thermal coefficient of expansion

α r
-
Adhesive thermal coefficient of expansion
Choosing the right curing lamp for the application can be critical. The light source should be selected based on whether or not the light frequency and intensity matches the curing chemistry of the adhesive being used. One need to also consider is the throughput needed, matching the shape of the lamp to the area of curing, whether the manufacturing process is going to be a batch process or continuous, and the speed of cure needed to achieve a full cure. The parameters that define a successful cure are entirely dependent on the end-product requirements. In other words, the cure is only successful if the desired properties of the end product are achieved with ease, consistency and cost effectiveness.
When setting up the process, it is important to determine the minimum light intensity and time of exposure needed to create a fully cured sample. Measuring the light energy required to achieve full cures easily does this. Simply increasing or decreasing either the intensity or the time of exposure, an acceptable cure range can be determined.
This information is necessary in order to establish the most cost effective curing process. If the sample does not receive sufficient light intensity, then the bonds might not form fully, resulting in poor adhesion, or uncured adhesive.
Thick layer industrial applications have given rise to a UV equipment industry all it’s own. This has been most noticeable in the last ten years with the development of high intensity spot, or wand style curing lamps designed to cure small, relatively thick spots of adhesive. Advancements in equipment technology have been directly attributed to the rapid growth in end use applications and the need for custom manufacturing solutions (rather than just new adhesives or sealants) to meet customer needs in the assembly of their products. Such responsiveness to individual end user manufacturing needs through the engineering of equipment systems that take full advantage of UV technology will continue to support application growth in the future.
Many applications require only lower intensity curing lamps. Bonding between surfaces, one of which allows light to pass, is one of the applications where resin may be effectively cured with lower intensity, lower cost lamps.
Most UV formulations are capable of curing within 1 to 30 seconds through clear glass or plastic parts under lower intensity lamps. Because the inhibiting effect of oxygen is not present, cures are relatively fast. Bond lines are usually fairly thin - 0.001 to 0.125 inches.
The optimum shape or pattern of light emitted by a lamp for any specific application is called its “footprint”, and depends upon the size and geometry of the bond area and other process recommendations.
6
•
Spot - Spot source lamps with light guides are typically used for curing small areas, or areas where light needs to reach partially obstructed areas that can not be reached by a flood type system.
•
Flood – Flood sources offer a large cure area but generally lower light intensity. The emit less heat as well, and are usually the product of choice for bonding heat sensitive plastic parts. The large area of flooded light also is useful for curing many parts at once. Flood lights may be fitted over a conveyor or turntable for continuous product assembly.
•
Focused beam – Focus beam lights may also be used over a conveyor, and focus the energy of light into a higher intensity 1”x6” beam.
The lamps shown below represent a range of equipment available for Optical Adhesive applications.

Bond Box II Flood Lamp PC-3D UV Spot and Dispenser Compact Cure Conveyor
Table 7
Typical Thick Layer Adhesive and Coating Cure Rates With A Range Of UV Curing Lamps
Lamp/Type
Moderate
Intensity UV
Flood
2000-EC
300-500
Higher
Intensity UV
Flood
5000-EC
200-500
High Intensity
UV Spot
PC-3Ultra
Very High
Intensity UV
Spot
3010-EC
200-500
Conveyorized
Beam
2 x 1200-EC
High Intensity
Spectral Output of Lamps
(nanometers)
Nominal Intensity (mW/cm2)
200-500 200-500
20-60 175-225 1000-2000
Typical Adhesive Cure Rate
1800-5000 225-275
Between Surface
Cures (Glass)
On Surface Cures*
1-4 sec
(UV/Visible Cure Adhesive)
1-3 sec <1-2 sec
≤
1 sec 3-5 feet/min
40-240 sec 10-40 sec 2-10 sec
(UV Cure Adhesive)
1-5 sec 1-3 feet/min
Between Surface Cures
(Glass)
2-6 sec 1-4 sec 1-3 sec
≤
2 sec 2-4 feet/min
On Surface Cures 30-600 sec 20-50 sec 3-5 sec 1-3 sec 1-2 feet/min
Ranges represent the fastest and slowest cure times of Dymax formulations under the stated lamps.
*Some formulations never achieve a dry surface cure, though most do. The time range stated represents the fastest to the slowest curing products.
Conveyorized
Electrodeless
Lamps
200-500
1700 - 2000
5-20 feet/min
3-10 feet/min
5-15 feet/min
1-10 feet/min
As with any technology, there are times when a photocuring process may not be the best choice. The primary one would be if some aspect of the assembly blocks resins from exposure to light. For most practical purposes, light curing adhesives and sealants require direct exposure to light in order to cure. There are however, exceptions to this rule. Multi-Cure
®
UV adhesives, for example also contain a secondary heat curing mechanisms.
Understanding the differences in different types of adhesives used in Optomechanical Assembly, and the differences between UV curable adhesives is useful for helping the optical engineer solve and conquer those difficult applications. This paper has evaluated the adhesive options available to the optical engineer with particular emphasis on UV curing resins. It has discussed the unique benefits of the new Aerobic Acrylic
Adhesives for optical assembly. It has attempted to show how the features of faster cure and higher bond strength, nevertheless yield low stress bonds. Additionally, it has attempted to address the major concerns of optical assemblers and show how this new line of adhesives may solve current problems and offer new solutions for designers.

References:
1. A. Bachmann, “Ultraviolet Light Curing ‘Aerobic’ Acrylic Adhesives”, Adhesives Age , December 1982
2. C. Bachmann, “Light Curing Assembly; A Review and Update”, Wavelength, May 1999, pp 6-10.
3. Registered Trademark, Dymax Corporation
4. M. Bayer, “Lens Barrell Optomechanical Design Principles”, Optical Engineering , 1981
5. J. Arnold, “Low Stress Aerobic Urethanes; Lower Costs for Microelectronic Encapsulation” SME Adhesives 1995, EE95-
263; September 1995.
6. C. Bachmann, “UV Light Selection Requires Fitting Lamp to Application”, Adhesives Age , April 1990, Pg. 19.
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