Tuesday, August 6, 2013


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= (α1 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:

Τmax = (α1 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 α1 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.

Sunday, October 21, 2012

Transparent Polyamide sheet compared to Polycarbonate Sheet

Transparent Armor Window
Many people are aware of Polycarbonate sheet, PETG sheet and Acrylic sheet, but there is now a new material that has entered the transparent sheet market - Polyamide.

Polyamide is certainly higher in price than other transparent sheets, but it has an interesting range of properties that make it suitable for some technically demanding applications.

Like Polycarbonate, Polyamide can be used to make very high quality optical grade clear sheet.  In fact the light transmission is 90% compared to 89% for Polycarbonate.  Sheets of the same size as Polycarbonate can be easily manufactured.  Also, Polyamide is almost as unbreakable at Polycarbonate which means it can withstand environments that would damage polymer sheets made out  of Acrylic.

Where Polyamide really excels is in properties such as the heat distortion temperature, tensile modulus, density and solvent resistance.  We will look at each of these in turn and compare them to the other Polymers.

Heat distortion temperature.


Material                            Heat distortion temperature (C)     Glass transition temperature (C)
Acrylic                                         95                                               110
Polycarbonate                            137                                             148
Polyamide                                   180                                             190

These figures show that Polyamide can withstand much higher temperatures than both Acrylic and Polycarbonate before they will start to distort under a load.  In many applications, the heat distortion temperature is not an issue, but occasionally the part must be able to operate in higher temperature environments without distorting.  In these applications, Polyamide is an excellent option.

Tensile modulus

Material                           Tensile modulus (psi)
Polycarbonate                     348,000
Polyamide                            232,000

Polycarbonate is a very flexible material, much more so than Acrylic which is rigid by comparison.  This flexibility makes it very useful in applications such as security glazing and transparent armor.   While Polyamide grades are available with a number of different Tensile modulus characteristics, the one that HighLine Polycarbonate LLC is currently using to make sheet has a lower tensile modulus than even Polycarbonate, making the Polyamide sheet more flexible than Polycarbonate sheet.  This property means that in certain configurations, it can be a better material for bullet resistant applications than Polycarbonate.  Of course there is a price-performance balance that means that Polyamide is not suitable for all applications.
Where flexibility is needed, Polyamide is certainly an option to be considered.

Density

Polycarbonate and Acrylic have similar densities of around 1200 kg/m3.  Polyamide has a density of around 1060 kg/m3.  This means that Polyamide is about 10% lighter.  Where weight is important, Polyamide is an option.

Solvent resistance

Polycarbonate, while very resistant to a number of chemicals, does not perform well when exposed to certain other chemicals such as Ketones.  One of the strengths of Polyamide is that it has exceptional solvent resistance performance.  As well as being resistant to Ketones, Polyamide is not attacked by most fuels, oils and lubricants.   This chemical resistant makes it a good choice in transportation and aerospace applications.

As far as we know, HighLine Polycarbonate is the only manufacturer to currently offer clear Polyamide sheet to the market.  If you would like more information on this unique material, please contact us.


Thursday, May 10, 2012

Anti Glare Coatings explained

Anti-glare coatings are different to anti-reflective coatings.  Anti-glare coatings are generally produced using an abrasion resistant hard coat with small particles in the coating to give a matte surface.  This matte surface stops light being reflected from the sheet surface back to the viewer so that the user's view is not obscured by glare from lighting or the sun.
One down side to the matte surface is that the light transmission of the sheet is lowered and the view through the sheet is hazy.  The more of the matte agent that is put into the sheet the more the glare is reduce, but also the sheet becomes more hazy and the view more obstructed.

To illustrate the effect of an anti-glare coating we have taken three pictures of an anti-glare sheet with a 40% gloss level.  The 40% gloss is quite a high level of matte agent - we commonly supply product with gloss levels of 60% and as high as 80%.  The 80% gloss level is much more transparent but does not reduce the glare as much as the 40% gloss level material.

We are often asked how much does the reduction in gloss level obscure the view through the sheet?  The answer depends on what you are trying to view.  If you are trying to view something that is a long way away through the sheet, the object is still able to be seen but the view is very blurred.  To show this effect, we positioned a typed page only 15" behind the anti-glare sheet.  The page is visible but the details are not.

We then moved the page to 5" behind the sheet.  Again the page is visible and you can even start to make out the detail of some of the larger font.  48 Point font is clearly legible, even 28 Point font is just visible, while smaller font can be seen but not read.

We then moved the typed page to immediately behind the sheet and the page was even touching the sheet.  Nearly all of the font, even the smallest can be clearly read.  

When choosing an anti-glare gloss level it is important to test it in your application.  The questions that need to be answered are how much do you need to reduce glare and how much haze can you accept.  The answers to these questions depend on what environment you are you using the sheet in and what do you need to see through the sheet.    


 
 Photo 1 - Typed page 15" behind the anti-glare sheet

 Photo 2 - Typed page 5" behind the anti-glare sheet

Photo 3 - Typed page immediately behind the anti-glare sheet (touching)

Tuesday, May 8, 2012

Clearfix - Repairing Polycarbonate sheet scratches



The above video shows how scratches in both uncoated and abrasion resistant Polycarbonate sheet can be easily repaired using a product developed by 3M and Clearfix Aerospace.  The product was initially developed to repair military helicopter windows; however, HighLine Polycarbonate has worked with 3M and Clearfix Aerospace to evaluate and test the product on Polycarbonate sheet used on transparent armor laminates as well as other applications.

The product works equally well on repairing scratches and other damage on both coated and uncoated Polycarbonate sheet.  Not only can the product be used to repair scratches on in service vehicles but it can also be used to repair scratches on production damaged laminates.  Laminates that would otherwise need to be scrapped can now be repaired allowing manufacturers and users to significantly reduce costs.

The product can be purchased from HighLine Polycarbonate LLC as we are now a primary distributor of Clearfix.  Potential users should contact us to schedule a demonstration at their facility.

Tuesday, April 17, 2012

LEDs in Polycarbonate

video

This video shows a new process we are developing to laminate ultra thin LEDs between sheets of Polycarbonate. The first sheet in the video shows the LEDs between two sheets of 0.118" thick clear Polycarbonate. The second sheet shows the LEDs between a mirrored piece of 0.177" Polycarbonate and a piece of clear 0.118" Polycarbonate - the LEDs are only visible when they are lit as normally they are hidden by the mirror.

The next production trial will use a piece of light diffusing Polycarbonate as the front face in order to diffuse the LED light and prevent "hotspots". We also plan to increase the density of the LEDs. We also plan to use thinner Polycarbonate to make the whole structure 0.118" thick in total.

Conventional lamination methods would damage LEDs, but a new technique that we are working on is making these type of products possible. The technique would allow very bright, low weight signs or lighting powered by only a 9 volt supply.

Monday, April 2, 2012

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.

Tuesday, March 20, 2012

Drying Polycarbonate sheet

Polycarbonate sheet readily absorbs moisture from the air. Eventually the water content will reach 0.2% by weight.
In most applications this water content is not a problem, however, in applications where you need to process the sheet above a temperature of 250F, this water can vaporize within the sheet during processing and lead to small bubbles forming. As little as 0.05% water can cause these bubbles.

Two processes that require the sheet to be heated above 250F are lamination of the Polycarbonate and thermoforming of the Polycarbonate. Both of these processes can have problems with bubbles if the sheet is not dried correctly.

To dry the sheet there is a wide range of recommendations that have been published. To dry 0.118" thick sheet it is normally recommended to use an oven set at 250F. The drying time suggestions can range from 6 to 12 hours. We would suggest that the longer time the sheet is dried the better and we would use 12 hours. Other recommendations suggest that a lower oven temperature of 180F can be used but the time must be increased to 24 hours. If your oven is only capable of reaching 180F rather than 250F, you could certainly try this method - however, we are very skeptical of this approach as to drive the water off effectively you really need to be above the boiling point of water.
As the thickness of the sheet increases, the drying time will increase significantly as the water needs to be removed from the center of the sheet. For 0.236" thick sheet we would recommend 30 hours of drying at 250F and for 0.375" thick sheet we would recommend 40 hours.

One question that we are asked is "Do you need to remove the masking before drying?" In general the Polyethylene or paper masking is not a very good moisture barrier, so it should not hinder drying very much. There is generally more risk of damage to the sheet if the masking is removed prior to drying, so unless you have good handling conditions to prevent damage, the marginal improvement in drying time is usually not worth the risk.

Once the sheet has been dried, it should either be used immediately or stored in a dehumidified area with a dew point less than 10F. To illustrate why this is the case, on a hot, humid day a dried sheet can absorb 0.5% water within 3 to 4 hours and at this level, bubbles could occur during processing.

Monday, February 6, 2012

Printing with Polycarbonate


video

We recently came across a very interesting Blog post by a company called ProtoParadigm. They can be found at http://protoparadigm.com We encourage you to check them out.
The blog post covers a topic that we have not seen before - 3D Printing with Polycarbonate. This technique can be used to construct proto-types from Polycarbonate. The profiles in the photo and video were made by printing with Polycarbonate. With the permission of ProtoParadigm, we have included a video and picture from their blog as well as the text below. Please watch the video, it is an amazing use of technology. Their article also clearly identifies the importance of drying Polycarbonate. As users of Polycarbonate sheet know, Polycarbonate absorbs a lot of moisture and should be dried before thermoforming otherwise a lot of bubbles can form in the finished part.

Here is their post:

"We tracked down a number of material samples from our supplier and a little gem called Polycarbonate (PC) caught our eye. Having seen the success of Richrap printing with Polycarbonate we were anxious to work with it. Polycarbonate (wiki) is a strong thermoplastic with high optical clarity and (relatively) high melting temperature. Unlike PLA with a fast transition temperature, PC slowly softens when heated allowing successful (if not slow) extrusion at lower than processing temperatures. This is useful when switching from a plastic with a lower extrusion temperature as you can slowly start pushing PC through at the temperature of your previous plastic until you clear the hotend. It is important to purge ALL of the previous plastic before raising the printing temperature as ABS puts off some dreadfully nasty fumes at 260C.

The sample we received was extruded to 1/8″ diameter and we had let it sit out in open air for a good while before getting to it. Initial purging at 260C (Modified Makergear Stepstruder) showed extrudate that was bubbly and white; a big red flag that this plastic needed to be dried. 10 hours at 160F in an old food dehydrator showed filament that was noticeably clearer and extruded a smooth clear thread from the nozzle. Setting extruder to 260C and the Polyimide Tape covered heated bed to 120C we repurposed an ABS printing profile for PC and started printing; once flow-rate was dialed in we tried printing our Plastic T-Slot. It was by far the strongest beam we had printed and clear enough that looking straight through it you could make out objects on the other side.

It’s worth noting that adjusting temperature is similar to PLA, printing at higher flow-rates will require higher extruder temperatures for a consistent melt. An indication the flow-rate is to high or temperature to low is stripping or skipping at the filament driver. Those with Bowden style extruders will need to watch for signs of excessive force where the Bowden tube meets the filament driver and hotend. For the Ultimaker I’m using this thing to keep everything secure. If you print PC near the high end of your firmwares temperature limit, PID fluctuations can send it hot enough to force a shutdown of the hotend; temperatures drop, nozzles clog, filaments strip, things get ugly. Also, for hotends that use PTFE (teflon) insulators there is the concern of dangerous fumes when temperatures approach 300C (see Polymer Fume Fever for example.) Care should be taken to avoid inhalation of dangerous fumes or, better yet, to avoid creating them.

", it’s finally time to share what we’ve learned about printing with Polycarbonate. As we recently announced, pre-orders for Polycarbonate in both 3mm and 1.75mm diameters are available. It took us awhile to get the details sorted out, but we’ set for a ship date of January 30, 2012. There’s a whole world of materials out there for that hungry printer on your desk, and we plan to dish up a feast.

Larger prints were prone to peeling off the print-bed if they contained too many long aligned traces; examining the datasheet revealed that this PC had a mold release additive, great for injection molding, not so great for us (the PC available for pre-order does NOT have this additive and should stick easier to print beds). Small objects printed fine with no warping but we needed to find a way to keep large prints held down; enter ABS Glue. Painting a thin coat of that on the bed before printing completely eliminated peeling and warping, we could even print without the heated bed and maybe see only the smallest of curling on the corners of large prints.

To test the effect that leaving the PC out in open air was having we split up the sample; one went in the dehydrator, another into one lucky fellow’s home for a couple days. Printing with them revealed obvious difference. The dried sample printing clear and smooth without hiccups, the sample that had gone through a few days of home living printed white and would occasionally pop and bubble. Comparing prints side by side shows an obvious reduction in clarity and surface quality for the undried filament. While we haven’t done any numerical testing of compared strength, the moisture laden sample felt more brittle and prints made from it break much easier. Objects printed with the dried PC are clear and strong. Returning to the T-Slot it is clear to see the differences between dry filament and filament left where humidity is not controlled. Click the pictures below for high resolution to really see the differences.

All in all, a very simple material to start printing with. As long as it is kept relatively free of moisture and/or dried, printed objects turn out looking good, are well bonded and very strong. This is a plastic that can take a bit more of a beating and stand a little more heat, not bad if you need something close to you’re hotend such as a cooling duct. Printing parameters we’re using so far are:

Makerbot

  • Extruder – Makergear Plastruder (modified directing heat closer to nozzle and further away from insulator)
  • Extrusion Temperature – 260C (success at low and high flow rates)
  • Bed – Heated Polyimide Tape (aka Kapton) bed at 120C OR unheated bed with ABS Glue brushed down before hand

Ultimaker

  • Stock Extruder
  • Extrusion Temperature – 270C (evaluating how to safely go hotter for better inter-layer adhesion)
  • Bed – Unheated BlueTape or Polyimide Tape (recommended for keeping parts flat) bed with ABS Gluebrushed down before hand
  • Add-on Ultimate BowdenFeeder Repair Kit to keep Bowden assembly secure

We’ve got it on pre-order, prices include shipping within the USA, world wide shipping is available through ourinternational ordering form with an additional $9.00 to match the increased shipping cost of the flat rate mailers we are able to use. We have a scheduled ship date of January 30, 2012 after which the product can batch with other orders and the shipping cost will be subtracted back out of the product listing if we have any remaining inventory. Go on over and grab some in either 3mm or 1.75mm."

Tuesday, January 10, 2012

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.

Tuesday, November 15, 2011

Birefringence, Photoelasticity, Anisotropic Materials, Iridescence and the Rainbow Effect - Part 3

In the final blog post of this trilogy we will discuss Iridescence and how it can cause a rainbow effect on abrasion resistant coated Polycarbonate sheet. We will also discuss how the rainbow effect can be minimized.
Iridescence is the rainbow or oil slick type pattern that often appears on the surface of a Polycarbonate sheet particularly under artificial lighting conditions.

Coated Polycarbonate sheet has a thin film of coating on the surface of the sheet in order to protect the sheet against abrasion damage. It is this thin film of coating material that causes the problem, in much the same way that a thin film of oil on the surface of a pool of water exhibits the rainbow patterns. The effect is due to the process known as interference.

As discussed in previous blog posts, whenever light travels from a material with one refractive index to another material with a different refractive index, some of the light is reflected. In the case of the coated Polycarbonate sheet, when light moves from the air into the coating some of the light is reflected. Then, when the light moves from the coating into the actual Polycarbonate, some more of the light is reflected. When the light that is reflected from the first surface comes into contact with the light that is reflected from the second surface the light waves recombine.

Depending on how thick the coating layer is, the light waves may be in sync when they recombine or may be out of sync when they recombine. If they are in sync the two waves will added together and will have constructive interference. If they are out of sync the two waves will start to cancel each other out and will have destructive interference.

Since visible light has wavelengths of 380nm(violet) to 750nm (red) and a typical hard coat has a thickness of 4 to 7 microns [4000 to 7000 nm], the coating thickness is an order of magnitude thicker than the wavelengths of visible light. A small percentage variation in the coating thickness can therefore change whether the constructive interference or destructive interference occurs. If there is variation of coating thickness over a small area of sheet, even if the variation is only tens or hundreds of nanometers, then there will be areas of constructive interference and areas of destructive interference. This variation in the interference patterns is part of the cause of the iridescence or rainbow effect.

The question then becomes, how do we eliminate the variation in coating thickness? Abrasion resistant coatings are often added to sheet by a process known as flow coating. The sheet is hung vertically and coating solution is allowed to run down the surface of the sheet from top to bottom under gravity. The solvents are then allowed to evaporate. If the sheet is allowed to move before most of the solvents have evaporated, the coating surface can become uneven. However, we need to remember that the coating surface is not the only surface that we need to be concerned about - there is also the sheet surface that is reflecting light. The sheet is extruded between large chrome rolls which are powered by motors. If there is any variation in the motor speed of these motors or the motors pulling the sheet, there can be variation in the thickness of the sheet. While the variation in the thickness will be small, it only requires very small variation to cause iridescence.

The reality is that neither the sheet producers or the abrasion resistant coaters have the ability to control their processes to the level of 10-100nm thickness. If we look at sheet producers, many of them state that their thickness specification is plus or minus 10%. On a 0.118" thick sheet that corresponds to 300,000 nanometers. While this is an overall thickness tolerance and not a measure of local variation of thickness, it does give some idea of the magnitude of the problem. Using this information we can determine that the sheet producers and coaters cannot prevent the problem. In many cases sheet producers often blame the coaters for the problem and coaters often blame the sheet producers. This then leaves us with the question of how do we solve the problem?

To answer the question we will briefly move to another topic - different types of lights. Traditional incandescent light bulbs have a relatively smooth light spectrum across the visible region and are similar to sunlight in this respect. When sunlight is split into its component wavelengths (such as in a rainbow) there is a smooth transition from violet through the various colors to red. There are no wavelengths missing. An incandescent light bulb behaves in the same way (as do some full spectrum LED bulbs).
Fluorescent bulbs, mercury bulbs, sodium bulbs and non full spectrum LEDs are different. When the light is split into its component parts, there are peaks at some wavelengths and gaps at other wavelengths. For example, a low sodium bulb emits an almost monochromatic light source at 589.3nm and a standard fluorescent bulb has 22 peaks with the main four being Mercury at 437nm, Terbium at 543nm, Mercury at 547nm and Europium at 611nm. These wavelengths of a fluorescent bulb combine to yield a light that looks like natural light but has discrete wavelengths rather than the continual spectrum of natural light.

Having a light source composed of discrete wavelengths rather than a continuous spectrum is a major problem for iridescence; when the light is reflected from the two surfaces the discrete wavelengths make the problem much larger as there are no intermediate colors to cancel out the iridescent effect. In short, the light source can make the problem of iridescence much greater.

The best way to reduce the effect of iridescence is to change the lighting source to a full spectrum light source such as incandescent bulbs or full spectrum LEDs. If the only option is to use fluorescent bulbs, it is better to use a bulb with more emission peaks to more closely resemble full spectrum light.

Another option to completely resolve the problem is to use what is known as an index matched abrasion resistant coating. The Polycarbonate sheet has a refractive index of 1.585 and most coatings have a refractive index of 1.49. If an abrasion resistant coating with a refractive index of 1.585 is used, the light will treat the coated Polycarbonate sheet as a single layer material and the effect of iridescence will be completely eliminated. While this process sounds great (and HighLine Polycarbonate can offer index matched abrasion resistant coated products) there is a significant downside - index matched coatings are very expensive. In most applications it is better to install full spectrum bulbs to reduce the problem.

Finally, to illustrate the effect of lighting on the visual appearance of iridescence we will recount a case study about the problem. A rail car manufacturer was experiencing oil slick like patterns on the Polycarbonate windows of their railcars. The manufacturer of the windows was inspecting the windows prior to sending them to the rail car manufacturer to try and identify the problem. They were unable to detect the issue as their factory was lit with incandescent lights. When the windows were installed in the railcars, the oil slick appearance was easily visible because the internal lights on the rail car were fluorescent bulbs.
The most practical solution would have been to change the bulb type on the railcar, but unfortunately the window manufacturer did not understand the problem. They told the rail car manufacturer that the problem was due to birefringence, which, as anyone who has read these three blog posts knows, was not the cause of the problem. By understanding the cause of the problem it is easier to recommend a solution to the customer.