Preventing delamination in transparent armor.

Delamination of transparent armor is an ongoing problem.  This blog post aims to explore the subject using some technical theory, with the aim of presenting simple solutions to minimize the problem.  The proper design of the laminate, manufacturing of the laminate and selection of materials can all lead to a significant increase in the life of the laminate.  This post will explore some of the issues and provide recommendations.

To start looking at the problem of delamination we want to start with a mathematical analysis of the problem and we therefore used a simple formula to model the stresses that cause delamination.  The simplified formula is taken from the paper “Thermal Stress in Bonded Joints” by W.T.Chen and C.W.Nelson.  It examines the thermal stresses in a bonded joint between two materials using an adhesive interlayer.  The paper also gives a more complex formula for three layers instead of two; for readers who would like to examine the formula for three layers, the paper is easy to find by entering the title of the paper in Google.  For more complex structures involving more than three layers, the formulas can be derived using the same principles.   The paper shows how the following formula is derived, but for this blog post, we will just take the formula as given.  For those that prefer to see the derivation, the paper is available to read.  It is recognized that equating a laminate to a bonded joint is somewhat simplistic, but it does give a good starting point to analyze the problem.

The formula presented in the paper for calculating stresses in a two-layer joint is:

Τ= (α_{1 –} α₂) T G sinh (β x)

β η cosh (β L)

β² = G  [ (1/(E₁ t₁) + (1/(E₂ t₂) ]

        η

Τ         = Shear Stress (Pa)

α₁     = Thermal expansion coefficient of layer 1 (/C)

α₂     = Thermal expansion coefficient of layer 2 (/C)

T         = Temperature change (C)

x          = Distance from center of joint (mm)

L          = Distance from center of joint to end of joint (mm)

G         = Shear modulus of interlayer (Pa)

η          = Thickness of interlayer (mm)

E₁        = Elastic modulus of layer 1 (mm)

E₂        = Elastic modulus of layer 2 (mm)

t₁         = Thickness of layer 1 (mm)

t₂         = Thickness of layer 2 (mm)

The formula can be used to calculate the Shear stress at any point in the laminate from the center to the edge.  When x = L at the edge of the laminate, the shear stress will be maximum, and:

Τ_max = (α_{1 –} α₂)T G

                        βη

This formula is somewhat intuitive.  The stress will be greater if the difference in coefficient of thermal expansion of the two materials α_{1 –} α₂ is large.  The stress will also be greater as the Temperature change T increases.  Also if the interlayer is thicker (η), it allows the stresses caused by the expansion and contraction of the materials to be reduced.

The first thing to note is that transparent armor is often exposed to environmental temperature changes in military applications.  ATPD.2352 requires testing over a temperature range of -31 C to +60C or a 91 degree C temperature range.  Although the laminate will not see this range in temperature every day, it is certainly possible that it could experience these conditions during its life. 

It should be noted that if normal operating temperature is say 15 C, this is not the temperature that has zero stresses.  The temperature that has zero stresses is much closer to the temperature during fabrication the polyurethane sets and bonds to the glass and polycarbonate.  Depending upon the polyurethane, this temperature could be 80 C or higher.  Selection of the polyurethane therefore has some impact on the maximum stresses that a laminate will see.  A polyurethane that sets at 120 C will lead to much higher stresses than a polyurethane that sets up at 80C.

To illustrate this point, the maximum stress will occur in a laminate when the temperature of the laminate is the lowest, in the case of ATPD.2352 this will be -31C.  Using a polyurethane that sets up at 120C rather than 80C will give about (120 - -31) / (80 - - 31) = 151/ 111 or about 36% more maximum stress in the laminate at the interface.

It should be remembered in the selection of polyurethane, that choosing a low melting polyurethane to minimize stresses should be done with careful consideration of the operating and storage environment that the laminates will see.  It is extremely counterproductive to have solar heating leading to the melting of the polyurethane, as this will lead to melting delamination rather than thermal stress delamination.

This problem can be made even worse by poor laminating control.  Polycarbonate expands or contracts a lot more than glass.  If the laminate is not uniform in temperature throughout the entire thickness at the time the Polyurethane is setting up, it is possible that some of the polycarbonate could be at a higher temperature at its core at the time the surface is bonding to the polyurethane.  This increased core temperature can cause increase stresses at the interface of the polyurethane.  Proper manufacturing that allows the temperature of the laminate to stabilize throughout, just above the temperature where the polyurethane sets up can significantly reduce stresses.

One elegant solution to the problem is to use radio frequency lamination to lower the temperatures of the polycarbonate and glass at the time of lamination.  This type of lamination heats only the polyurethane interlayer and can therefore reduce the zero stress temperature well below the temperature achieved by conventional autoclaves.  We can provide laminators with information on this process if requested.

The other item to note from the formula is that the thickness of the polyurethane is important.  Using a thicker polyurethane can allow the stresses to be significantly reduced.  If we consider that case where 6mm glass is bonded to 6mm polycarbonate, using the above formula the stresses can be reduced from 13.7 MPa to 6.9 MPa if using 0.075mm polyurethane rather than 0.025mm polyurethane with a temperature swing of 111 degrees C.

Decreasing the amount of thermal stress generated will significantly affect the life of the laminate.  Halving the stress, as in the above example, could be the difference between delamination and no delamination.  The other factor that affects delamination is the adhesion between the polyurethane and the other materials – glass and polycarbonate.  Delamination will occur at the weakest of these joints, which is typically the polycarbonate, polyurethane interface.  Delamination will occur when the forces due to the thermal stresses are stronger than the adhesion of the polyurethane to the polycarbonate or glass. 

One area where we have started to have some positive effects in reducing delamination in high-end laminates is increasing the bonding between the polyurethane and the polycarbonate.  We have been tackling this area in two ways, firstly by correct selection of the polyurethane and secondly by modifying the chemistry of the polycarbonate.  We have recently made available an enhanced grade of polycarbonate that has significantly higher bond strength to polyurethane.

The next area that should be considered is the area of laminate design.  In some cases laminates are configured only to pass ballistics specifications and little consideration is give to how the configuration may affect stresses and delamination.  To illustrate this point we will use the three-layer formula developed in the paper that we discussed earlier.  Due to the formula’s length, we will not present it here, but again the paper can easily be found.

In the first case we will consider a two-layer laminate consisting of 6mm Polycarbonate bonded to 6mm Glass using a 0.025 mm polyurethane.  The change in temperature that the laminate will be exposed to will be considered to be 100 degrees C.  We have calculated that the maximum stress will be 12.3 MPa.

If we then change the laminate configuration, with the aim of keeping the total thickness the same, to 3mm Polycarbonate, 3mm Polycarbonate and 6mm Glass, the total amount of polycarbonate and glass will remain the same. In this configuration the maximum stress between the glass and the polycarbonate will be 11.70 MPa.  Although the difference may not seem to be much, it is a 5% reduction in the stress.  In a laminate that is close to the point of delamination, reducing the stresses by 5% could be enough to significantly increase the life of the laminate or even prevent delamination occurring.  Reducing thermal stresses, particularly when done in conjunction with increasing the bond strength between the polyurethane and the polycarbonate, can be very effective in decreasing delamination and increasing laminate life.

Other factors do affect delamination including edge seals, chemical attack and edge finishing, but the aim of this article is mainly to look at some of the factors associated with delamination caused by thermal stresses.

The key points to minimize thermal stresses and reduce delamination are:

·      Select the correct thickness of polyurethane to minimize thermal stresses

·      Select the correct type of polyurethane to minimize stresses and increase bonding, while also considering environmental conditions that the laminate will be exposed to.

·      Optimize autoclave conditions to reduce thermal stresses.

·      Improve the bond strength between the polycarbonate and the polyurethane by using an enhanced polycarbonate designed to increase bond strength in transparent armor.

·      Design the laminate configuration to minimize stresses in addition to achieve ballistics requirements.

Is Polycarbonate Bullet Resistant?

We recently came across this video on YouTube. It is certainly one of the more interesting and better produced of the videos about Polycarbonate and bullet resistance.



We will concentrate our discussion to the first two rounds fired, the 0.22LR and the 9mm round.

Most ballistics certifications for bullet resistant glass constructions, such as UL.752, start their testing with a 9mm Full Metal copper Jacket with a lead core. This bullet weighs 8 grams and has a test velocity of 358 m/s. The 0.22LR in the video has a weight of about a third of this at 2.6 grams and a velocity of around 290 ms.
Using our Kinetic Energy formula of Energy = 0.5 x Mass x Velocity x Velocity, the 9mm round has about 4.7 times the energy of the 0.22LR round.
For the UL.752 Level 1 test three shots of a 9mm FMJ must be fired at a 12" x 12" target and the shots must land within a 4" triangle area. To pass the test no bullets must pass through the material and no pieces of the material must come off the back with sufficient velocity to damage a cardboard witness plate located a short distance behind the sample.
The 12" x 12" test piece is fully supported and will not move during the testing.

From the video of the 022LR it is clear that the 0.5" Polycarbonate does not allow the round to pass through. One concern that we would have is that the test piece was not supported, so some of the energy was absorbed by moving the piece when it was hit. That would not be realistic in real life where a window would be supported. Also the test in video did not consider multiple hits in a small area as in the UL.752 Level 1 test. However, it appears likely that Polycarbonate that is supported in a frame could stop 0.22LR rounds at a reasonable thickness - however, without testing in a controlled manner it is not possible to say whether the required thickness is 0.5" or greater.

From the video of the 9mm round, two 0.5" pieces of Polycarbonate were clamped together. This test was designed to see if 1.0" of Lexan could stop a 9mm round. We have some similar concerns as for the first test where the test sample was not supported. More importantly the pieces broke free from the clamp and it is not clear whether the second piece was hit straight on or whether the bullet glanced off the piece. We don't think that the video is claiming that a 1.0" piece of Lexan can stop one or more hits from a 9mm round but we would be concerned if someone inferred this from the video.

One thing that we do know is the a 0.75" construction made from 1/8" Polycarbonate - 1/2" Cell cast Acrylic - 1/8" Polycarbonate can be tested to UL.752 Level 1 with the 9mm threat and will pass. So a single 1,0" supported layer of Polycarbonate may or may not be effective for stopping 9mm rounds but there are potentially cheaper and lighter options available that will.

If you put thick enough piece of Polycarbonate in front of a 9mm round it will eventually stop the round. It just may not be the cheapest or lightest way of doing it, which is why Polycarbonate is not normally tested and approved as a bullet resistant material as a stand alone solution.

The video even states that their test is completely unscientific.
All of this does not make the video any less interesting or enjoyable. It is also very well produced.

How thick does Transparent Armor need to be?

A question that we are frequently asked is how thick is transparent armor made from glass and polycarbonate?

The answer to the question depends on what level of threat the armor needs to stop. As we discussed in a recent post, the Kinetic energy of a bullet can be calculated if the weight of the bullet and the speed of the bullet are known using the following formula:

Kinetic Energy (Joules) = 1/2 x Mass of bullet (grams) x [Velocity of bullet (m/s)]^2

The more Kinetic Energy the bullet has, the thicker and heavier the transparent armor needs to be. Of course there are many manufacturers of bullet resistant glass and transparent armor. Each of these manufacturers have their own knowledge of how to produce the lightest and thinest armor to stop a specific threat. However, if we look at the top military transparent armor producers, there is only limited variation in the performance of the products.

We recently compared data published on the internet from the top laminators to see how thick and how heavy their products are to stop a given threat. We compared products that were designed to stop rounds with between 650 Joules and 3500 Joules of Energy. Many of the manufacturers do not publish the data for rounds with Energy above 3500 Joules as much of the information is classified.

Within the energy range considered there was surprisingly little variation in the thickness and weight of products. We analyzed the data and carried out some linear regression and were able to obtain the following equations:

Thickness (mm) = [0.0085 x Energy (Joules)] + 10

Weight (kg/m^2) = [0.02 x Energy (Joules)] +20

Using these equations we can calculate that to stop a bullet weighing 9.45 g and traveling at 830 m/s the energy would be about 3255 Joules.

This would give a thickness of about 38 mm and a weight of about 85 kg/m2.

Of course, just making some transparent armor of this thickness and weight does not guarantee that it will stop this level of threat. The armor has to be properly designed and tested by a certified testing company. The figures do show what the main manufacturers are able to achieve.

It should also be remembered that the Kinetic Energy is not the only factor that needs to be considered - other factors such as the shape of the bullet need to be taken into account.

The above figures are based upon transparent armor solutions using Glass and Polycarbonate. A more expensive option is to use advanced materials in the construction such as transparent ceramics. The performance of these ceramics, while not available in detail, is discussed on some of the manufacturers websites and claims of 20% weight reduction and 10% thickness reduction are listed.

Bullet Resistant Laminates and Transparent Armor

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:

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 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. This type of option is 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 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.