Looking at materials for Security and Ballistic Resistant Glass applications, we can say that today there is no single material that fits all properties needed for these specific applications.
We have still not found the perfect glass that fulfills all the resistance levels, at the time it is light, thin, and optically perfect.
New materials, and the possibility to combine them properly, offer some options and solutions these days.
Originally all the security constructions were made of a pile of glued glass layers. These structures were rigid, thick, heavy and in many cases with several type of optical defects.
New materials developments opened new possibilities and helped to address and change some of the limitations of the old times. That is Why a combination of glass and polymers in laminated constructions are often used as safety screens, bullet proof windows or windshields. New materials have been brought to this field coming from Polymer Materials Science.
From the Inorganic Materials Science, and in order to increase the impact resistance, different and new type of glass compositions have been used to improve their strength and impact resistance properties. Different type of glass compositions like soda lime glass or borosilicate type of glasses are used. More recently, some new technical advances are pointing towards the possibility to replace the silica oxides structures (glass types) by Aluminium-Magnesium oxides or Aluminium oxo Nitride (transparent ceramics). They will be new structures that will help to lighten and improve the mechanical properties of these inorganic components.
Other improvements have been addressed from Polymer Materials Science to enhance some key properties of these laminated structures, mainly referred to their weights. After time, some of these constructions have been reconfigured using other rigid, but less heavy materials, with also excellent optical properties and good impact resistant properties like polycarbonates (PC). In some cases, and for some special constructions the poly methyl methacrylate (PMMA), a less though polymer than PC has been used too. The possibility to use polymer materials provide new opportunities and open new construction challenges. In combination with glass, these rigid polymer materials help to reduce thickness and weight, two of the most important parameters, after optics, for the construction of windshields and side glasses used in the construction of security cars and armored vehicles.
For the construction of these type of structures, that combine inorganic and organic rigid components, another type of polymer materials are needed and used. We, in a very simply way, can refer to them as the “glues”. These “glues” are also polymer materials. But these materials are not rigid. By the contrary, they are soft and flexible, playing a key role in these structures. Having to be the glue of the rigid structures, these polymer materials have to fulfill special requirements. They have to link soft polymers with rigid glasses or ceramics. They have to be extremely transparent and also they have to have good mechanical properties sustaining all the parts and minimizing the impact damages. In reality, they are very sophisticated materials and critical elements for the construction of the security and bullet proof glass structures.
For this type of applications just three type of polymer materials are used: PVB (polyvinyl butyral), TPU (polyurethanes) or ACR (acrylates). The first two type of adhesives are flexible thermoplastics while normally the third ones are crosslinked polymers, with limited flexible mechanical properties. All of them have very good optical properties but they have mechanical and adhesion properties differences.
Besides the fact that the PVB has been the most popular “glue” material used for the lamination of Arquitectural and Automobile glass configurations, but it has some well known weak points. Due to these weak points, PVB is not the best material to be used for security and bullet proof applications.
Let’s point out the advantages and disadvantages of using PVB for laminates:
From one side we know that PVB:
- has excellent optical properties
- has good mechanical properties
- has all the “certification medals”. Easy task, if we realize that certification test have been made specially for PVB. That is why, it has been presented in all Arquitectural Schools as the standard and the best lamination material.
but, at the same time, we shouldn’t forget also that:
- PVB is a hygroscopic material (tends to absorb and trap water easily and keep in their molecular structure)
- that is Why it needs to be transported, stored and used before lamination under strict humidity and temperature control conditions
- that is Why it has very limited lifetime before lamination use
- PVB needs to be processed at high pressures (between 10 – 12 bar) because it has not chemical adhesion to glass and only can achieve the needed adhesion by using the pressure, that means that the laminates are sustained by physical adhesivity, which is weaker than a chemical adhesion.
- PVB has not adhesion at all to polycarbonate (PC) and polymethyl methacrylate (PMMA).
- PVB is very sensitive to the components of the edges sealing materials .
- PVB delaminates easily, normally from the edges by weathering attack (humidity, temperature, mechanical stress,…).
Let’s point out the advantages and disadvantages of using TPU for laminates.
From one side we know that:
- TPU has good optical properties
- TPU has excellent mechanical properties in terms of elongation and strength
- TPU if well formulated has excellent chemical adhesion to glass
- TPU if well formulated has excellent chemical adhesion to PC
- TPU if well formulated has good chemical adhesion to PMMA
- TPU due to its chemical bonding to all substrates (glass, PC, PMMA,…..) allows to be laminated at lower pressures
- TPU is less sensitive to components of the edges sealing materials
- TPU is not hygroscopic as PVB is, so is less sensitive to humidity. That means:
- TPU doesn’t need to be transported, stored and used before lamination under strict humidity and temperature conditions
- that TPU has a much longer life time at normal conditions of storage and use
but, at the same time, we have to consider that normally:
- TPU has higher price cost as a main disadvantage
- TPU, being a recently new material, has not as much certifications as PVB nowadays
These disadvantages can be easy overcome, since the properties of the TPU can help reduce the total cost of the laminates (materials + processing), increase the life time of the laminates and facilitate to fulfill all certifications requirements. Anyhow, “do you know any big fish with low weight?”.
If you need more details or have comments and experiences you would like to share, feel free to contact us.