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The Mechanical Properties of a Security Glazing Interlayer Film from its Product Data Sheet

Posted by Manuel Garcia

04-jul-2016 9:10:36

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In order to get the “finger print” of any manufactured product the technical people should look at its Product or Technical Data Sheet (PDS or TDS).

The PDS documents show the more relevant Technical parameters. In the PDS the Manufacturer provides the properties and their values: Mechanical, Physical, Chemical and Optical critical parameters. These parameters are used to identify the product, to describe their properties and in some cases to point out how to process it. 

All properties are measured according to International Standard Procedures (ISO, ASTM, UNE). The use of standard and usual data is useful when a comparison of products is needed. But to get some parameters there is no normalized procedure available. In those cases the Manufacturer is forced to describe its internal measurement method.

 

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Depending on the final use of the product the parameters should be interpreted accordingly. The interpretation of the data is not so clear and straightforward for non technical byased people. With this paper we want to provide some help to get a more clear interpretation of the parameters.  

As an example of how to interpret a technical PDS of one optical film that is being used as an interlayer to built security glass products, we want to take a close look at the PDS of the NovoGlass SF 1959 optical film.

See below the PDS of the NovoGlass SF 1959.

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If you like you can get a pdf copy of the latest version of this document by just download it.

 

The film identified by this PDS is a transparent and adhesive element of security glazing products. It is an aliphatic polyether TPU, made of combinations of diisocyanates, polyols and short length diols. It is extremely good film for this application because of its excellent transparency (mínimum haze), adhesion (durability vs de-lamination) and processability (easy to process). Moreover it is also very well appreciated because of its outstanding mechanical properties.

To know its mechanical properties deeper, let’s go through each one of them as they are referred in its Technical Document:

 

Specific Gravity. The density or specific gravity is a bulky physical property that points out towards the product mechanical properties. It is referring the mass of material that can be arranged in a specific volume. This parameter tells us how to turn Kilograms into Liters or Pounds into Cubic Feet.

This parameter does not help us to discriminate between similar TPU (ThermoPlastic polyUrethanes) materials, but it is good to distinguish between completely different types of materials like for example TPUs and EVAs  (1,08 vs 0.95 g/cm3).

It is measured following Standard Norm ASTM D-792. In our film, the measured value is 1.077 gr/cm3


Shore Hardness. The Shore Hardness values refer to the abrasion resistance of materials when they are exposed to mechanical erosion. It is well accepted that in general stiff materials are more resistant to abrasion than soft ones.

For laminated structures where the film is used as an interlayers element, the Shore hardness is not relevant. We should remember that this parameter is telling us how resistant is the TPU film to abrasion. TPU is intended to be used between glass-glass sheets or glass-PC sheets then... does it really matter how resistant it is to abrasion? The answer is “no”. This parameter it is not relevant to the application of security glazing.

Nevertheless in spite that the Shore Hardness is a meaningless parameter for security glazing laminated products, there is an approach to make use of it in this type of products. Since Shore Hardness values provide us with the idea of how much stiff or soft the materials are in comparative terms, we might grow up a “mental picture” of how it would be adapted to difficult surface forms, such as the curved or spherical surfaces of aerodynamic structures. 

The Shore Hardness is measured following Standard Norm ASTM D-2240. The measured Shore Hardness of the NovoGlass SF 1959 is 64-A. This is a very low value in terms of abrasion resistant. But in terms of hardness and adaptabilitiy, this lower value becomes very appreciated when highly curved or spherical glass structures want to be laminated, like the one that you can see in the picture above.

 

Tear resistance is another parameter described wich representes the resistance to growth of cut when under tension. This parameter might be someway valuable when a bullet breaks the laminated structures, but just when glass and PC sheets have vanished because of impact energy.

This parameter is telling us how difficult it is to open a gap among TPU molecules in the film. If you try to make a hole/puncture onto a TPU film using a needle or a nail you will see how difficult it is. Despite TPU films are between glass and PC sheets it is sensible thinking that a bullet can find more resistance to trespass a TPU with higher tear resistance through bullets are not so sharpen!

It is measured according Standard Norm ASTM D-624. The measured value is 52.90 N/mm

 

This polyurethane is a Thermoplastic material with plastic and elastic behaviors. The main Plastic and Elastic Mechanical properties of materials are obtained from Stress/Strain measurements. The following graph represents the general strain behavior of all plastic materials when they are submitted to stress. On the same graph we have also referred to the most important parameters determined from this type of test: the tensile strength, the modulus, the elastic limit and the elongation at break.

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This graph is a very useful tool to understand how this product behaves as an interlayer film during the impacts. We should consider that in some way, a bullet impact can be considered as “hard Stress/Strain test”. So by extrapolation we might consider that during the (bullet) impact the elongation of TPU film goes through two stages:

 

Elastic elongation stage: the film absorbs energy as it expands according to its tensile strength. Depending on the TPU film the elastic range of elongation can be lower or higher, but always within the level of elongation at break. The amount of energy absorbed during this stage is easy to predict since we are in a reversible phase of elongation.

 

Plastic elongation stage: in case the impact is powerful enough the TPU foil keeps expanding “plastically” beyond its “elastic limit” until the elongation at break… at this stage the energy absorption is typically lower and less predictable than at the elastic stage, since we are now in a non-reversible phase, when the TPU structure is dramatically altered by the impact energy. 

 

Therefore the importance of the parameter “elongation at break” is to be considered together with the tensile strength, as the total absorption of (impact) energy is represented by the Area beyond the curve.

 

But, what do the elements of the curve mean? Let's go through them all.

 

Tensile Strength. What does it mean? Does it really matters to the application? In this case the answer is “yes”, since tensile strength is telling us how much force must be applied to a given section of TPU to provoke its elongation (strain) in a direction perpendicular to the given section. In other words it represents the amount of energy that a TPU film can absorb when elongating because of a (bullet) impact. As long as a security glass is engineered to minimize damage to the “secured customer” it is easy to understand that when comparing data the higher tensile strength the better. 

From the graph above we might see how tensile strength gives the slope of the linear part of the curve, just at the beginning of elongation (when the laminated structures are still in place..).

It is measured according Standard Norm ASTM D-712. The measured value in our film is 53.8 MPa.


The elastic modulus: 100% modulus and 300% modulus are also common parameters given in data sheets. In some cases also the modulus at 200% of elongation is also reported. They give us a “pressure value” generated by the TPU when strained until a given level of elongation. For example, 4,98 MPa @ 100% means that one “bar” of TPU with a cross section of 1 cm2, if elongated until two times its original length will require a force of 498 Kg !!!. It looks evident that regarding this parameters, the higher the better, as we can say that the amount of energy potentially absorbed by a TPU film in case of impact is higher as these values increase.

It is measured according Standard Norm ASTM D-712.  The measured values in our film are 4.98 MPa for 100% modulus, and 23.10 MPa for 300% modulus.

 

Elongation at break is another commonly referred mechanical parameter in data sheets. When considering it, what is it better?. It represents the level of elongation at which the TPU film breaks, so it seems logical that regarding this parameter. The higher the value, the better. The area below the curve depends on it.

It is measured according Standard Norm ASTM D-712. The measured value of elongation for our film is 379%

 

 

We hope that you find this information helpful for your purposes, anyhow your comments and questions are Welcome!. 

If you are interested in getting a comparison of different Security glazing TPU films, in terms of their Mechanical Properties, pls let us know....  

Yes, I am interested

 

Topics: Security Glass