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Jun 08. As the demand for different armoring solutions grows, the usage of polyurethanes and polyureas is becoming more and more common. In the world of weight versus cost, polyurethanes are allowing some of those gaps to be filled. The physical properties that can be achieved are allowing different composite armor solutions to be utilized.
Polyurethane/polyurea/hybrids
Polyurea can be defined as the result of a chemical reaction between an isocyanate and an amine. In polyurea coating terms, this is generally a reaction between an MDI (methylene diisocyanate), IPDI (isophorone di isocyanate), HDI (hexamethylene diisocyanate), and/or TDI(toluene diisocyanate) pre-polymer with amine-terminated resins. Polyurea is generally used as an industrial coating where fast reactivity, high film built, and moisture insensitive characteristics are important. Polyuerea coatings are predominantly designed for severe environments with good chemical resistance, and most likely are formulated for a 1:1 mix ratio.
Polyurethane can be defined as the result of a chemical reaction between an isocyanate and a polyol. Polyurethane rate of reaction and cure rate can be adjusted with catalyst. Polyurethanes are generally used in slow reacting products, self leveling such as as casting materials and pourable foams. They show good longevity, flexibility in end-product physical properties and are relatively inexpensive.
Polyurea/Polyurethane hybrid formulations can be defined as the result of a chemical reaction between an isocyanate and a mixture of polyol and amine co-reactants. These formulations generally provide an “intermediate” polyurea that displays many of the same properties of a polyurea. However, hybrid formulations can also be formulated to display the combined advantages of polyurea and polyurethane chemistries. Polyurea/polyurethane hybrids are normally associated with use in service environments items that generally do not require immersion or extreme temperature conditions for application. Some examples of these applications include sprayed-in truck bed linings and other automotive applications.
The vast majority of materials used in armoring applications will be hybrids due to their wide blend of polyurea and polyurethane physical properties. What is critical to note is that different physical properties will provide significantly different protection levels in armor applications. In simple terms, all polyurethanes are not equal.
Since the formulations can vary so dramatically, it is imperative that any supplier can provide independent certification of its physical properties. Also, the raw materials used in formulating can vary dramatically. While many of the products may look the same, the ability to perform in severe service environments can be dramatically different.
Applications
Currently, polyurethanes are being incorporated in three main areas: blast mitigation, spall protection, and simple impact/abrasion protection.
Blast mitigation on buildings using polyurethanes has been tested since 1999 when the USAF performed initial testing in Florida. The initial positive results led to a more formal test performed by Technical Support Working Group (TSWG) in 2004. Twenty seven companies submitted samples in that round of testing. Based on that testing, PAXCON has been awarded numerous contracts for applications to US Government buildings and private facilities as well.
Polyurethane hybrids have also been used by various defense contractors for simple abrasion and impact protection. These applications range from flooring on personnel carriers, dashboards, equipment racking, etc. These are all basically generic applications with minimal specification requirements.
In the last few years, the demands for blast mitigation protection on vehicles have increased. The new versions of armored personnel carriers are all being manufactured with different packages to mitigate the effects of IED and other roadside bombs. Hobson Industries of Lincolnshire, England has been involved in many various vehicular tests for blast mitigation.
Spall Protection
Spalling, by definition, are flakes of a material that are broken off a larger solid body and can be produced by a variety of mechanisms, including as a result of projectile impact.
In anti-tank warfare, spalling through mechanical stress is an intended effect of high explosive squash head (HESH) anti-tank shells and many other munitions which may not be powerful enough to pierce the armor of a target. The relatively soft warhead, containing or made of plastic explosive, flattens against the armor plating on tanks and other armored fighting vehicles (AFVs) and explodes, creating a shock wave that travels through the armor and breaks the softer metal on the inside. The resulting spall is dangerous to crew and equipment, and may result in a partial or complete disablement of a vehicle. Many AFVs are equipped with spall liners inside their armor for protection.
Spalling frequently occurs with bullets as well. The rounds fragment upon impact of equipment, breastplates, etc. The fragment can then travel into the bodies of nearby troops causing death and injuries.
Recently, there have been many polyurethane materials advertised for spall protection. The only company that offers certified testing is PAXCON by Line-X Corporation. Their product, PX-2100, has been tested and certified by H.P. White labs in Maryland, USA. The product was tested in accordance to STANAG 4569 III and NIJ III. The results showed that PX-2100 applied over armored steel at a thickness of 6mm completely stopped any and all bullet spalling. On a test to failure at .30 caliber, the protected side application of PX-2100 stopped all spalling as well, even though the round passed through the steel. The results can be viewed by request at www.PAXCON.com. The company can also provide hi-speed digital photography of the events.
Application Methods
Along with the varied formulations, there are also different application methods. Hi-pressure spray, low-pressure spray, and casting are the most common forms of applications.
Hi-pressure applications are the most common. Most are 1:1 ratios, mixed in the chamber of a triggered gun. Application temperatures are around 65C (150F), pressures around 2500psi. The material is delivered in heated hose and the material sets up in seconds.
Low-pressure applications are generally mixed in a static mixing tube outside of the gun. The delivery temperatures are generally ambient. There are variable ratios available. The material will generally set up in 15-45 seconds.
Casting is a unique way to deliver the material. There are thousands of cast polyurethane applications in the world, from bumpers to roller skate wheels. Casting of urethane molds can provide different shapes, sizes, etc. Many casting formulations are used with polyurethanes that contain solvents. The curing of the materials is affected by the off-gassing of the solvents which can last minutes to hours. Non-solvented polyurethanes are also available and cure in minutes.
Castable polyurethanes are currently being used in armor panel applications in conjunction with different ceramic layups. The Polyurethane will not stop the projectiles, but is terrific in keeping the ceramic from braking apart. Many breastplates are coated in polyurethanes. If applied correctly, this can allow the plates to be hit multiple times and still maintain physical integrity.
When NOT to use Polyurethanes
While polyurethanes are terrific solutions to various service environments, there are certain times they will not work. Polyurethanes will NOT stop bullets. Bullets will generally cut right through. Exposure to solvents. While there are polyurethanes that have very good chemical resistance, exposure to solvents like acetone, MEK, or NMP will soften and dissolve the solid product. Most suppliers can provide specific chemical resistance information.
Exposure to fire has to be looked at carefully as well. Standard polyurethanes will generally fail smoke density requirements. There are many different tests for fire exposure, so make sure any test results match the demand of the application. There have been many new additives that have allowed polyurethanes to perform better than in the past. Check with suppliers for the latest information.
Like many products, all polyurethanes are not the same. With the risks involved in protecting personnel and materiel, it is critical that suppliers are qualified and can provide the testing and backup to the claims that are made. But polyurethanes should be looked at as a viable option to achieve the many different protection levels that are being demanded.
Hobson Industries and Paxcon Line-X
Over the last 8 years Hobson Industries has been upgrading and re-armouring Land Rover steel armoured vehicles for the Police Service of Northern Ireland. Essentially this was a highly manoeuvrable vehicle suited to the streets of Belfast and other towns but with cross-country ability. It is a fully protected and fire-protected vehicle capable of withstanding high powered rifle round, fire-bombs and explosives. This work was carried out in a special build fabrication facility by welders holding the necessary welding qualifications in the special metals used. Testing and actual in-theatre targeting proved highly successful.
Through the experienced gained from this project we expanded our field to include less conspicuous vehicles such as Mitsubishi 4 by 4s and Skoda saloon cars. Hobsons has expanded this business to include the armouring of Land Rovers and other vehicles for UK police services and government offices.
Over the last few years we have been developing vehicle protection solutions suitable for the UK Ministry of Defence (MoD) Land Rover vehicles in theatre in Afghanistan and Iraq. Weight constraints of the Land Rover chassis have focused Hobson’s attentions on light weight protective armour for ballistic and mine blast situations. A strategic alliance was formed with Aigis who has developed a light weight energy absorbing material suitable to fix to the blast side of steel armour currently being tested to provide enhanced blast protection whilst allowing lighter steel to be used.
An exciting new development from the USA called Paxcon Line-X polyurethane coating has added the benefit of a light weight spall lining that can be applied directly to metal and non metal surfaces.
Line-X material has been subject to testing by the US Military where it provided the best anti spall properties of a batch of some 30 different products. Since those tests in the late1990’s Line-X have further developed their products to include fire retardant properties and are currently carrying out tests with the US Department of Justice to ascertain the improved capabilities of their products. Additionally the scientific branch of the UK MoD is currently carrying out tests on Line-X to determine defensive properties suitable for many applications.
Electro Magnetic Compatibility shielding quality and Infra Red Reflective protection is also being developed and tested for Line-X.
Through an agreement with Paxcom Line-X USA Hobson Industries have secured the UK rights for LINE-X for vehicle applications in defence.
Hobson Industries boasts a proud reputation of quality and reliability. Vehicle and power train remanufacturing have been carried out for the MoD and Land Rover UK for many years. Major parts supply contracts are in place with the UK MoD. Accreditation for Quality and Environmental Management Systems with the British Standards Institution are held and our thoroughly “Green” credentials can be assessed by viewing the attached press releases.
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