A standard question that many manufacturers ask our Technical staff is:

“How do your adhesives stand up to certain chemicals?”

Seems simple, right? Based on the chemical used, there should be a standard answer to follow. Well it turns out this question is far more complex than people think.

There are many factors that determine chemical resistance.  Once you understand these factors, a manufacturer can work closely with their customers to determine the best adhesive to recommend.  Factors to consider are:

1.   The chemical resistance must be tested in the actual bond configuration where the adhesive will be used.  Example:  Interfacial bonds will be more resistant than an adhesive exposed directly to the chemicals.

2.   The temperature of the chemicals.  Example:  A bath of sulfuric acid at 80°C is more aggressive than one at 25°C. 

3.   The concentration of the chemical solution.  Example:  A higher concentration of sulfuric acid is more aggressive than a lower one.

4.   The type and length of exposure.  Example:  A splash or wipe is less aggressive than a soak.  The length of exposure to the chemical will also yield different results.

5.   The adhesion of the specific adhesive to the substrates.  Example:  If you were to test the resistance of an adhesive to a specific chemical when bonded to polycarbonate it may pass.  If you take the same adhesive, same chemical and a different substrate, such as polyethylene, it may fail miserably in this example.

6.   The chemicals the adhesive will be exposed to.  Example:  A product may do better withstanding ammonia and water, than potassium hydroxide and water.

7.   The polarity of the chemicals.  Example:  Acrylated urethanes withstand non-polar materials better than polar materials.

In general, it is difficult to predict the outcome of an adhesive’s chemical resistance without specifically testing the adhesive on the customer’s parts, through their process, using their curing equipment.  However, by understanding the factors that may affect the adhesive’s resistance, can ultimately lead to a better adhesive recommendation for the customer.

Adhesives Chemical Resistance

Ultraviolet (UV) light is a form of electromagnetic energy invisible to humans. UV light falls below visible light on the electromagnetic spectrum so it does not trigger the natural defenses of the eyes, such as pupil dilation experienced with bright visible light.  For this reason, it is important to use personal protective equipment and not to disable any safety controls designed into the UV light source.  Many people often mistake the bright light coming off these systems as harmful, but in reality what they are seeing is harmless visible light. 

While all UV light has the potential to harm an employee when used carelessly, the shortwave UV energy (UV-C) poses the greatest risk to those using these light sources.  Most UV sources sold in the light-curable adhesive market incorporate the safer UV-A energy.  It is important to review the specifications for your own UV source before using it. 

Ultraviolet light exposure is the primary cause of melanoma.  Most cases of melanoma, however, are preventable by protecting yourself from effects of UV exposure. 

In industrial settings UV exposure is often misunderstood, but it is in these settings where manufacturers have the greatest control over a worker’s health and safety.

Industrial UV light-curing systems are often designed with safety or engineering controls built into them.  These controls, such as shielding, safety interlocks, intuitive design, and light-absorbing plastics, allow operators to use them without ever exposing themselves to harmful ultraviolet light. Teaching employees how to protect themselves from UV exposure and training them to work safely around these UV systems will minimize any potential risk of harm. 

A device called a radiometer can be used to demonstrate the amount of UV light an employee is exposed to while operating a UV curing system.   Taking the radiometer’s sensor and holding it near a person’s exposed skin while the unit is on, and then comparing this to what a person is exposed to outside on a sunny day, will show the individual is experiencing greater exposure from the sun.  It is important to match the radiometer to the UV wavelength being measured.

When used properly and in conjunction with personal protective equipment and training, industrial UV light sources are safe and easy to use.

Safety Curing Equipment, Safety

DYMAX recently had a webinar informing people how to save 30% on their assembly bonding process. We talked about real-life ways to optimize assembly processes with cure-on-demand UV/Visible light-curable adhesives. This archived webinar is now available for viewing.

A lot of questions were asked during the webinar session. I decided to post a few below. Some of you may have the same questions in mind.

Adhesives, Curing Equipment Adhesives, Benefits UV Curing, Curing Equipment, Epoxies, Potting Materials, Removing Adhesive, Savings, See-Cure, Spot Lamps, Webinar

I was recently asked to respond to a question that I thought would make a good blog post. The question was:

“In the past, we have seen that some light curing adhesives are impacted by some type of inhibition, leaving a tacky residue at the surface.  What causes this, and how can we avoid this?”

 Here’s how it works…

Light curing acrylate adhesives cure by exposing photoinitiators to certain wavelengths of light, which break apart the photoinitiators into radical species, which react with the oligomers to create long chains and crosslinks. (When you shine a light on the adhesive, it turns into a solid).  Usually, by picking a high intensity light of >1 W/cm2 (keep in mind the sun is 0.002-4 W/cm2), and across a broad spectrum range from 300-450 nm, there is so much energy that the adhesives crosslink extremely fast, leaving a firm, tack free surface.  However, once in a while, some monomers & oligomers, which are the building blocks of these adhesives, may be susceptible to “oxygen inhibition” during the cure process.  Oxygen inhibition occurs during the curing step, whereby if there is oxygen present at the surface, the oxygen can penetrate into the surface and interfere with the radical polymerization, leaving unreacted monomers and oligomers at the surface. This is the tack that some people feel as they rub a finger across the surface, and may get traces of wet residue on their gloved hand.

Many of the newer adhesives are designed to go tack free under medium & high intensity conditions and when using the proper light wavelength. Generally, the higher the energy of light (lower wavelength (200-300 nm)) the better surface cure you get, but you limit the depth of cure. The lower the energy of light (higher wavelength (400-500 nm)), you get great depth of cure, but may get more oxygen inhibition.  The UV/Visible light spectrum in the 300-450 nm range seems to have the best blend of surface and depth of cure.  Now you have to consider the intensity of the lamps.  Higher intensity lamp systems can put out massive amounts of intensity, measured in mW/cm2.  Some spot lamp systems emit up to 15-20 Watts/cm2 (as measured at 365 nm).  It will be very easy to cure most adhesives to a tack free state with such a high intensity light emitting both UV and visible light.

We have observed that most adhesives have a minimum intensity threshold where they will go tack free within a specific time, as well as a minimum total energy threshold to get full cure.  Let’s look at the equation ”Joules/cm2 = Watt/cm2 x seconds”.  You can vary the intensity in Watts against the time of exposure in seconds to get the same amount of Joules/cm2.  2 Joules/cm2 = 2 Watts/cm2 x 1 second, OR  2 Joules/cm2 = 0.02 Watts/cm2 x 100 seconds.  While it may seem like you get the same amount of total energy in both situations (high intensity for short time, or low intensity for long time), the minimum intensity threshold may show that if you fall below 0.1 Watts/cm2 - you will never get a tack free state. 

Sometimes, some people find it beneficial to tackle the problem a different way, and flood the curing area with nitrogen or argon gas during the cure.  The nitrogen molecules are larger, and are not able to penetrate into the surface as easily, therefore give a tack free surface even with low intensity light sources. This is helpful in situations where a substrate is very heat sensitive or UV sensitive.  

Adhesive selection also plays a part.  The black magic happening in the chemistry labs allows the chemists to create new adhesives that go tack free at lower intensities.  Many of these find roles in potting and coating applications, with all different properties.  It used to be that only high durometer materials went tack free (D60-80).  Now the chemists have found ways to get soft yet tack free adhesives with low durometer ranges down into the A40-60 range. 

It all boils down to:  With the proper adhesive, process, light wavelength, time, and lamp intensity, you should be able to get a tack free surface.

Adhesives Adhesives, Oxygen Inhibition, Tack-Free