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Bullet resistant laminates

Bullet resistant laminates

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.

 

 

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Bonding Polycarbonate Sheets

Bonding Polycarbonate Sheets

One question that we are often asked is how can two Polycarbonate sheets be bonded together?  At HighLine Polycarbonate we are mainly involved in producing Polycarbonate sheets with a wide range of high tech properties. We only engage in a limited amount of fabrication which includes routing of the sheets into finished part shapes.  We do not engage in fabrication that requires bonding polycarbonate sheets together. Some of our customers do engage in this type of fabrication and we will list some of the methods that we know about for joining two sheets of Polycarbonate together. We would be very interested to hear from our readers about other methods that they know about so that we can update the post with additional information.

We do not plan to cover physical methods of joining sheets together such as rivets, screws and tapes.
- The first method that we know about is using Methylene Chloride or a 60%/40% mixture of Methylene Chloride and Ethylene DiChloride. This solvent bonding technique is known to give a good bond strength and excellent optical clarity along with low capital investment. The mixture of Methylene Chloride and Ethylene DiChloride gives a slightly longer curing time than neat Methylene Chloride allowing more time to get the parts in the correct position; this is particularly important for larger parts. Suppliers of these chemicals can be found on Google. We recommend reading the Material Safety Data Sheet for information on safe handling and disposal before using any chemicals. We also recommend that you test any method on a small part before using on critical parts.
Before starting the solvent bonding process, both surfaces should be cleaned with warm water. If there are greasy areas, IsoPropanol (IPA) should be used to wipe the surfaces clean. Some fabricators recommend dissolving between 2% and 5% Polycarbonate saw dust in the Methylene Chloride or Methylene Chloride/Ethylene DiChloride solvents before use in order to give a stronger bond strength. We have yet to see any evidence that the saw dust improves the bond strength. In any case, if you choose to try this method, make sure that all of the saw dust is fully dissolved before use, because otherwise lumps of saw dust may prevent good surface contact between the two parts. Another recommendation that we have heard from fabricators is that in order to prevent whitening of the joint occurring, 10% Glacial Acetic Acid should be added to the solvents. Whitening does not always occur, so we would only recommend that you try this solution if you are having problems with whitening on your particular parts.

Having made up the solution, the solvent should be applied to one of the clean parts. The two parts should then be clamped together with several hundred psi pressure for about 5 minutes. The parts should then be allowed to cure in a well ventilated area at room temperature for between two and five days.

- The second method of bonding polycarbonate sheets is to use an adhesive; this is a cheaper solution than solvent bonding but we believe that the bond strength and the optical clarity are not as good. Many customers have had excellent results with products such as “Weld-on”. These products can easily be found using Google.
- Other methods such as vibration welding and ultrasonic welding have had varying degrees of success depending on the part shape and thickness. We would suggest that you contact manufacturers of the equipment to see if these options are suitable for your needs. These methods would require capital investment.
- The final option that we know about is to laminate the two parts together using an interlayer material such as transparent Polyurethane. This method is often used to manufacturer ballistics laminates where Polycarbonate layers are bonded to glass. This method requires a lot of specialist knowledge and equipment, such as an autoclave so it is unlikely to be viable for the majority of applications.

 

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