Category Archives: Transparent Armor
HighLine Polycarbonate has developed a multilayer structure designed to be used by laminators in conjunction with glass or transparent ceramics to extend the life of transparent armor manufactured to ATPD.2352 standards.
The product will improve operational availability of vehicle platforms such as the MRAP and JLTV.
The HighLine product extends the life of transparent armor in three important ways:
Prevents de-lamination by increased bond strength
Over the life of transparent armor, the bond between the polycarbonate and polyurethane weakens. The transparent armor often can delaminate at the polycarbonate–polyurethene interface. Using proprietory technology, we have modified the bonding between the polycarbonate and polyurethane to significantly increase the strength of the bond. This advanced technology significantly extends the life of transparent armor, increases vehicle operational availability and reduces field maintenance and repair costs.
Prevents de-laination due to reduced thermal stresses
The design of the inner polycarbonate layers of the transparent armor can impact the level of thermal stresses experienced. These stresses are caused by the different thermal expansion rates of glass and polycarbonate. The design of the polycarbonate and polyurethane layers have been optimized by mathematical modelling to reduce the thermal stresses in the finished transparent armor. The optimization of the design, in conjunction with the increased bond strength, significantly extends the life of transparent armor systems.
Prevents scratching of inside surface using advanced coating
One of the inherent weaknesses of polycarbonate compared to glass is the tendency of polycarbonate to scratch. To reduce this issue, hard coats have traditionally been applied to the polycarbonate surface. Even with the hard coats, the polycarbonate inside surface of the transparent armor is still prone to damage from the vehicle occupants, reducing the life of the transparent armor. Our advanced coating increases the scatch and abrasion resistance of the polycarbonate to a level similar to glass. The advanced coating will pass the ATPD.2352 specification for the exterior surface.
Structure of HighLine Laminate
One question that we are often asked by customers is “can we use the old sheet that we have in stock and not have problems?”
In general, the answer to this question is yes, as long as the sheet is still masked and has been stored in good conditions. The properties of the sheet will not deteriorate over time. The only thing that you may need to do, depending upon the application, is dry the sheet before laminating or thermoforming (and we would recommend that you dry even new sheets for these applications).
However, we recently came across a very specific problem that one of our customers was having while using old sheet. The customer laminates polycarbonate sheet to glass using transparent polyurethane. There was some old sheet that the customer wanted to use in this application, the sheet had been produced about three years ago, but was fully masked and had been stored carefully. When the polycarbonate was bond to the polyurethane, the bond strength was measured and was found to be about 10% lower than when using new sheet. In ballistics laminates, this lower bond strength can potentially lead to some longer term problems with ballistics performance and de-lamination. An extensive study was therefore carried out to identify the cause of this lower bond strength.
The conclusion of the study was that over time, plasticizers from the masking had migrated to the surface of the polycarbonate sheet. These plasticizers then weakened the bonding of the polycarbonate to the polyurethane. As a result of these studies we are now recommending that polycarbonate used for ballistics laminates is used within 12 months of production.
In theory, this recommendation sounds simple to achieve. The customer would just need to look at the production date on the packaging and confirm that it is less than 12 months old. However, the customer needs to understand that the production date on the packaging and/or masking is probably the date that the hard coat was put onto the sheet. In practice, the actual production of the original sheet could be many months or even years before this date. The plasticizers would have started migrating to the surface of the sheet ever since the original production date of the sheet and the coating process would not have removed them.
Fortunately, in a world where most produces have quality systems in place, traceability of material is nearly always available. We would now recommend that all laminators of ballistics material ask their polycarbonate supplier to send them the material lot number, the coating date of the sheet (if coated) and the production date of the base sheet. If the production date of the base sheet is over 12 months old, it is likely to have lower bonding strength which may decrease the product performance. If the sheet supplier is unable or unwilling to supply these dates, we would recommend that adhesion testing is carried out or material is purchased from another supplier.
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? The answer lies in the physics concept of kinetic energy
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.
Calculating the kinetic energy
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
Military transparent armor
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 material 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.
One statement that we often hear is that Polycarbonate is “bullet-proof”. There are two problems with this statement; the first is that a single Polycarbonate sheet by itself should not be used to stop bullets as it really offers very little protection. The second problem is subtle, materials constructed from Polycarbonate are not bullet-proof but rather bullet-resistant; fire enough shots of high enough caliber and velocity and they will eventually fail. There has been a need in both the civilian and military sectors to develop glazing materials with bullet-resistance. There are a number of ways of achieving this bullet resistance depending on the required stopping power, cost and weight restrictions.While this article cannot cover all of the options in detail, we will try to give an overview of the options for producing bullet resistant laminates and transparent armor:
1) Perhaps the easiest to make and the cheapest product to buy is specially designed Acrylic sheet that has been specifically tested for bullet resistance. Typically a 1.25” thick Acrylic sheet such as Plexiglas SBAR will stop a 9mm bullet as tested by UL.752 Level 1 test. To get increased stopping power, it is necessary to increase the thickness to 1.375”. At this thickness Plexiglas SBAR product will stop a 0.357” shell as tested by UL.752 Level 2 test. The limitation of this technology is the thickness required to achieve greater stopping power becomes difficult to produce and difficult to install due to the size and the weight.
2) The next option is to use a combination of Acrylic and Polycarbonate. This method is used by Sheffield Plastics, amongst others, in their Hygard BR bullet resistant laminates range. The Acrylic and Polycarbonate are laminated together in various configurations in a vacuum chamber using an interlayer to bond the sheets together. The 9mm UL.752 Level 1 protection is achieved by laminating a ½” acrylic sheet between two layers of 1/8” Polycarbonate. The acrylic sheet absorbs the energy while the more flexible Polycarbonate holds the structure together and prevents shards of Acrylic breaking off and injuring the person behind the window. It can be seen that this type of structure is only 0.75” thick to achieve the Level 1 protection compared to 1.25” for the SBAR product. The 0.357 Level 2 protection is achieved by sandwiching two 3/8” Acrylic sheets between two outer 1/8” Polycarbonate sheets giving a total thickness of 1.0”. A UL.752 Level 3 protection, which uses a Magnum 0.44” has a similar construction that is 1.25” thick. These multiple layer plastic constructions offer greater protection from a thinner material, but at the downside of a greater cost.
3) The next option is to introduce glass. Different companies use different options for the configuration, but nearly all of them use glass bonded to Polycarbonate using inter-layers. Typically one or two sheets of 1/8” Polycarbonate are used. The glass absorbs the energy of the ballistics material and the Polycarbonate holds the material together. Often a sheet of Polycarbonate is put on the inside surface to act as a “spall” layer. This layer prevents shards of glass breaking off and injuring the person behind the glass. These types of bullet resistant laminates are often used in armoring commercial automobiles for VIPs or diplomats. Using the glass gives additional stopping power, but at the expense of cost and additional weight.
4) The next option moves from the area of commercial ballistics laminates to military transparent armor. These laminates often use multiple layers of glass and multiple layers of Polycarbonate – both as spall shields and internal structures. The completed laminates are often many inches thick and can stop a wide range of military projectiles. Often several different types of glass can be used in a single window to give different properties, the hardness of the glass and the energy absorption of the glass are two such properties.Many of the configurations used by different companies are confidential.The performance of these materials is excellent but they are costly and extremely heavy.
5) The final option is to use advanced materials for the construction of the bullet resistant laminates and transparent armor. These materials include ALON, Sapphire and Spinel.Details of these materials can be found on the websites of their manufacturers. While these materials offer exceptional protection they are extremely expensive and often the production process can only produce small parts.
At HighLine Polycarbonate we have a great deal of experience in transparent armor.We have developed a Polycarbonate grade that gives increased performance and stopping power in military laminates compared to other commercial grades of Polycarbonate. We have also developed an advanced thermoplastic sheet, which is more flexible than Polycarbonate and gives a significant improvement in performance when used as a spall shield. The material is lighter than Polycarbonate and is resistant to a wide range of chemicals and solvents, making it ideally suited to use in military transparent armor.
At HighLine Polycarbonate we also are able to include EMI/RFI shielding meshes, transparent conductive heaters, self-repairing coatings, anti-fog coatings, super abrasion resistant coatings, IR shielding and anti-microbial properties – all of which enable our products to be used in the harshest of military environments.