LIGHT-CURE ADHESIVES

June 24, 2014


The following post is taken from a training module written by this author and published through PDHonline.org.   PDHonline is a web site offering continuing education units (CEUs) for professional engineers. 

INTRODUCTION:

At the present time, adhesive manufacturers offer products classified as Cyanoacrylates, Epoxies, Hot Melts, Silicones, Urethanes, Acrylics (one-part and two-part) and Light-cures.  These classifications provide products with specific characteristics that allow for bonding, gasketing, potting and encapsulating, retaining, thread-locking and thread-sealing.  This post provides information on one very specific and very special adhesive category— LIGHT-CURE.

Light-cure adhesive technology offers a new approach to bonding similar or dissimilar substrates by using either ultraviolet light (UV) or light within the visible spectrum.  Extremely rapid cure times, superior depth of cure, (up to four inches) and easy dispensability are only three of the benefits when using these adhesives combined with the appropriate processes.   The newer visible light-cure materials can offer adhesion comparable to most commercially available UV adhesives, with particularly high adhesion on polycarbonate and polyvinylchloride (PVC) materials.  All equate to lower cost of assembly, more freedom when designing components and products and the saving of valuable production time.  This method of adhesion is extremely valuable when bonding thin films, needing heightened safety relative to skin and eyes and when bonding heat sensitive materials.  This process can lessen, or eliminate, the need for costly and harmful chemicals from the workplace and can be solvent-free and non-hazardous.  The use of light-cure adhesives will result in a very clean and “friendly” worker environment with no significant material disposal costs.  There is no need to mix, prime or rush to apply the adhesive due to minimal time to dispense.

HISTORY:

Approximately forty (40) years ago, the adhesive industry introduced an acrylic-based adhesive that cured or solidified upon exposure to ultraviolet light.  This was a tremendous breakthrough for the manufacturers and within a short period of time these adhesives became commercially available.  This offered distinct advantages over traditional adhesives categories such as cyanoacrylates (CAs) and epoxies.  Rapid cure times, adhesion to a variety of substrates and the ability to fill large gaps; i.e. 0.030 to 0.050 inches, were real winners with designers and assembly “shops”.  It allowed for greater flexibility in design and assembly.   Recent developments have produced adhesives that will cure using light within the visible spectrum.  This offers great possibilities over adhesives previously requiring UV cure.  These adhesives, UV/V (Ultraviolet/ Visible) were introduced in the 1990’s and involve employment of existing broadband-emitting UV light sources able to utilize an enlarged portion of the light spectrum.  The mechanism by which this happens is the introduction of photo initiators that react exclusively with light in the visible wavelengths; i.e. those which exceed 425 nm.  Development, as you might suspect, is still occurring and each year materials with improved mechanical characteristics and ease in application are being introduced into the commercial marketplace.

 

TYPICAL APPLICATIONS:

When we discuss applications, we find they generally fall into one of several basic categories; i.e. 1.) Bonding,  2.) Sealing,  3.) Cured-In-Place Gaskets,  4.) Potting and  5.) Coating.  With this in mind, we can see the following product applications now using the light-cure technology:

1.)Musical instruments

2.) Toys

3.) Sporting equipment

4.) Jewelry

5.) Optics (eye glasses )

6.) Needles

7.) Syringes

8.) Lighting

9.) Electronic Asms.

10.) Appliance assembly (refrigeration, laundry, etc.)

11.) Strain relief for wires and cord sets

12.) Conformal coating for PC boards

13.) Parts tacking

14.) Coil terminating

15.) Tamper-proofing

The development of light-curing adhesives has been enhanced by the latest generation of curing equipment.  This equipment includes both flood and point source configurations using bulb or lamp based systems.  In addition, equipment utilizing LED technology is now available for use with these adhesives.  The benefit here is that LEDs generate focused wavelengths that create appreciably tighter output range relative to regular visible lamp technologies.   Furthermore, because superfluous light and heat are not emitted, LED technology has proven to be both highly efficient and highly cost effective.  As might be expected, as a result of their small size, LED curing systems provide an LED light source that is perfect for curing tiny component parts.   The photographs and graphics below will give you an idea as to what types of products are now being produced using light-cure technology.

Figure 1

FRACTIONAL HP MOTOR ASSEMBLIES

Figure 2

 

OUTDOOR WINDOW ASSEMBLIES

Figure 3

OUTDOOR LIGHTING ASSEMBLIES

ADVANTAGES AND DISADVANTAGES:

Let us list now the relative advantages and disadvantages of using UV and V light-curing adhesives.

ADVANTAGES:

1.)     Reduced labor costs

2.)    Simplified automation when automation is used

3.)    Easier alignment of parts before cure

4.)    Improved in-line inspection

5.)    Reduced work in-process

6.)    Shorter cycle times due to rapid curing of components

7.)    Shorter lead times to customer possibly leading to reduced inventories

8.)    Fewer assembly stations required due to rapid cure times

9.)    No racking during cure

10.)No mixing generally required

11.)No pot life issues meaning generally much less waste of materials

12.)Reduced dispensing costs

13.)No hazardous waste due to purging or poor mixing

14.)No static mixers

15.)Easier to operate and maintain dispensing systems

16.)Better work acceptance

17.)No explosion proof equipment required

18.)Reduced health issues

19.)Reduced regulatory costs; i.e. reduced restrictions on volatile organic compounds

20.)Reduced disposal costs

21.) Very fast cure times

22.) Ideal for heat sensitive films and thin components

23.)Lower energy consumption required during processing of adhesive systems

24.)Visible light-cure adhesives cure through colored or tinted substrates

25.)Allows for miniaturization of component parts needing bonding or potting

26.)Improved manufacturing yield, quality and reliability

27.)Low odor

28.)RoHS compliant

29.)UL recognized materials available

30.)Low entrainment of moisture due to rapid cure times

31.)Solvent free

32.)Reduced material and process costs

DISADVANTAGES:

As with any process or adhesive material, there are several disadvantages. These are as follows:

1.)    Expenditure for curing equipment is necessary

2.)    Shielding when UV light is used may be necessary

3.)    UV blocking eye protection may be necessary depending upon the processing equipment

4.)    A radiometer may be necessary to measure the intensity of the UV light

5.)    When using UV light, the light source MUST reach the bond line if complete cure is to be had.  This means that transmission of light through at least one substrate is crucial.  Some substrates have UV inhibitors to lessen or eliminate degradation of the component.  These inhibitors will inhibit the penetration and lessen adhesion necessitating another method of bonding.  ( This is by far the biggest disadvantage for UV curing. )  A graphic depiction is given below that illustrates the principal.

As you can see, there are very specific uses for light-cure adhesives and engineers and engineering managers would be well-served to explore the possibilities.  I welcome your comments.

 

 

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