FDA and NSF Standard 51 grades and UV absorbers

Polycarbonate sheet is widely understood to block UV wavelengths below 385-390 nm. What is not so well known is it is not the Polycarbonate that blocks these wavelengths, but rather the UV absorbers that are added to the Polycarbonate that block the UV light.

Polycarbonate sheet that has no UV absorbers will only block wavelengths below 290 nm. Unfortunately wavelengths below 385 nm will cause the Polycarbonate to weather and become brittle and yellow. Manufacturers therefore add UV absorbers to the Polycarbonate resin to give it some protection against the UV light. Some outdoor grades of Polycarbonate also have an additional cap layer or coating heavily loaded with additional UV absorbers to further protect the sheet against the affect of UV light.

There are some grades of Polycarbonate, that are often known as FDA or NSF Standard 51 compliant grades that have no UV absorbers. The reason that no UV absorbers are added is that these grades are designed to be used in the Food Processing environment and the UV absorbers are not approved by the FDA to be used in Food Processing areas. The manufacturers therefore produce grades without the UV absorbers. Because these FDA grades of Polycarbonate sheet have no UV absorbers, they should not be used outside as they will yellow very quickly.

One question that we are often asked is are the FDA approved grades safe to be used in food contact applications? The FDA grades of Polycarbonate sheet do not have UV absorbers in them because they are not approved for materials used in Food Processing environments. However, the Polycarbonate itself does still have Bisphenol A or BPA in it and there is currently a great deal of debate about whether BPA is safe in food contact applications such as baby feeding bottles. As a result of this debate, at HighLine Polycarbonate we do not sell any Polycarbonate sheet that will be used in applications where it comes into regular, direct contact with food. However, FDA grades of Polycarbonate sheet can be used as machine guards to protect operators on food packaging lines when the machine guards do not come into contact with food that will be eaten.

One un-intended market for FDA approved grades of Polycarbonate sheet is to customers who bond Polycarbonate sheet to other materials using a UV cured adhesive. The adhesive requires light from a UV lamp to pass through the sheet in order to bond it to another material. The UV absorbers in Standard Polycarbonate sheet block the UV light from the lamp preventing the adhesive from curing. By using an FDA grade of Polycarbonate sheet, the adhesive is able to be cured effectively. After bonding, the sheet can be protected against UV light by adding a coating with UV absorbers.

Kinetic Energy of Ballistics rounds and transparent armor

We are often asked about the difference between bullet resistant windows installed in 24hrs stores or banks and the transparent armor used by the military.

The bullet resistant windows in convenience stores and banks are often made of cell cast acrylic sheet or a combination of acrylic and Polycarbonate. They are often about 1.25" to 1.375" thick and are designed to protect against threats that are likely to be encountered in that environment. Typical bullet resistant ratings of UL.752 Level 1 to Level 3 are encountered. But what does a UL.752 Level 1, Level 2 or Level 3 mean and how does it compare to the transparent armor of military applications?

A UL.752 Level 1 material is designed to stop 9mm FMCJ rounds weighing 8.0 grams traveling at a velocity of up to 394 meters/second.

A UL.752 Level 2 material is designed to stop 0.357 Magnum JSP rounds weighing 10.2 grams traveling at a velocity of up to 419 meters/second.

A UL.752 Level 3 material is designed to stop 0.44 Magnum rounds weighing 15.6 grams traveling at a velocity of up to 453 meters/second.

But what does this mean? One of the most important factors in determining whether a bullet resistant structure will stop a ballistics round is how much Kinetic Energy does the ballistics round have.

Using the equation for Kinetic Energy:

Kinetic Energy (Joules) = 1/2 x Mass (Kilograms) x Velocity (meters/second)^2

Calculating the Kinetic Energy for the UL.752 Level 1 ballistics round we find:

Kinetic Energy = 1/2 x 0.008 x 394 x 394 = 620 Joules

For the three UL.752 Levels we get:

Level 1 620 Joules

Level 2 895 Joules

Level 3 1600 Joules

We can see as the weight and the velocity of the round increase the Kinetic Energy of the round increases. The bullet resistant material needs to be able to resist a larger amount of Kinetic Energy.

We can now look at the military grades to compare the amount of Kinetic Energy they are designed to stop. Military grades of transparent armor are composed of multiple layers of glass and polycarbonate. The glass can be of various types. In some cases advanced materials such as Spinel and ALON are also used. Often the structures can be many inches thick.

For US military grades a standard known as ATPD.2352 is used. The different rounds that the materials must stop is listed but the velocities are classified. The fact that the velocities are classified makes it difficult to calculate the required Kinetic Energy that must be absorbed; it would be possible to take an educated guess at the velocities, but for the purposes of this blog post, we do not need to do this is we can use the NATO standard AEP55 STANAG 4549 Volume 1.

STANAG 4549 has 5 protection levels for Light Armored Vehicles. For the purposes of the discussion on transparent armor we will just look at Levels 1 and 4.

Level 1 material is designed to stop a 7.62 mm x 51 NATO ball round weighing 9.65 grams traveling at 833 meters/second.

Level 4 material is designed to stop a 14.5 mm x 114 API/B32 round weighing 64 grams traveling at 911 meters/second.

A Level 1 round has a Kinetic Energy of 3,348 Joules

A Level 4 round has a Kinetic Energy of 26,557 Joules

You can see that the energy that a UL.752 Level 1 material needs to stop is over 40 times less than a STANAG 4549 Level 4 material. The reason for this difference is that the type of ballistics rounds likely to be encountered at a convenience store are likely to be very different from those encountered by the military. Indeed the deterrence factor of bullet resistance glass in commercial applications should not be underestimated.

It should be noted that this discussion is very much a simplification and is only meant to compare the Kinetic Energy of the different rounds used for the different tests. There are a number of parameters that have not been discussed in this blog post such as the multi shot spacing and the shape of the round.