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Polyurethane: general characteristics

Polyurethane plays an important role in several industrial sectors: the new generation of polyurethanes offers multiple opportunities for applications requiring differentiated features and exceptionally high performance. Continual research into new production technology with ever lower environmental impact combined with extreme versatility and easy recyclability assure the role of polyurethane as a modern and multifunctional material. Polyurethane insulation can increase the energy efficiency of buildings and of a large number of industrial products while simultaneously helping to reduce energy consumption and protecting the planet by eliminating the associated harmful emissions. Polyurethane’s exceptional resilience and versatility are highly valued throughout the packaging industry, while its properties of comfort and low weight make it ideal for automotive applications. In the bedding and upholstery sector, in addition to its technical features of low weight, hygiene and firmness values that can be differentiated to match the specific application, newly developed polyurethanes offer a heat sensitive effect that facilitates transpiration from the body during contact. Polyurethane’s open cell structure facilitates the passage of air to ensure the correct exchange rates to guarantee long-lasting resistance to mould formation, moisture accumulation and malodorous emissions. These characteristics greatly limit the creation of conditions favourable to the proliferation of dust mites, which are considered to be among the main causes of respiratory allergies. SITAB P.E. produces a range of expanded polyurethanes with different physical-mechanical characteristics: each product is the result of a painstaking R&D process designed to produce a comprehensive response to the needs of clients and consumers alike.

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Technical notes

A concise definition of several terms utilised to define the physical properties of flexible polyurethane foams.

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Density refers to weight per unit volume of the foam and is normally expressed in kilograms per cubic metre (kg/m3).
The most popular range of flexible polyurethane foams has a density of between 18 and 50 kg/m3.
Unlike in the case of latex foam, density is not a measure of firmness for polyurethane foam.

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Firmness is a measurement of load bearing capacity and it is expressed in Newton per 323 cm2 at a given foam compression percentage. To calculate the value a sample of foam is compressed with a 323 cm2 surface area disk (the sample surface area must be larger than that of the disk) to an established percentage of the original foam thickness. At this point the force applied to obtain the compression is measured. There are normally two firmness values corresponding to sample compression to 25% and 65% of its original thickness.
For example, a value of 130 Newton at 25% means that a force of 130 Newton is required to compress a foam sample by 25% of its original thickness (thickness reduced, for example, from 100 mm to 75 mm ).
It follows that the higher the firmness value the "harder" the foam, and vice versa.

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Like firmness, also compression load deflection measures the load bearing capacity and it is generally expressed in kilo Pascal at a given deflection of the foam (usually to 40% of its original thickness). In this case the surface of the sample must be smaller than the surface area of the pressure disk. The higher the compression load deflection the "harder" the foam, and vice versa.

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The foam sample is subjected to repeated compression and relaxation cycles (dynamic fatigue). At the end of the cycle the firmness test is repeated and will give a lower value than that measured before the cycle. The difference between the original firmness and the firmness measured after the fatigue test is expressed as a percentage. The lower the value, the higher the foam quality.

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Tensile strength measures the amount of force required to break the foam sample. This parameter is measured in Pascal. The higher the value, the stronger the foam The elongation test is performed at the same time as the tensile strength test. The result defines the maximum percentage elongation, with respect to the original size, reached by the foam before breaking. The higher the value, the higher the foam quality.

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Tear strength measures the force required before propagation of a cut made in the foam sample. This value is expressed in Newton per metre (N/m). This property is important when the foam is designed to be fixed to other supports (e.g. wood) by means of metal staples, etc. The higher the value, the higher the foam quality.

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Compression set measures the deformation of the foam following compression for a given period of time at a given temperature. In the test the sample is compressed to a given percentage of its original thickness. The percentage loss in height following this procedure is then measured. The lower the value, the higher the foam quality.

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Resilience measures the elasticity of the foam. The relative test involves dropping a steel ball onto a sample from a predetermined height and then measuring the rebound height attained by the ball. In supersoft foams this test can give a value below the effective resilience due to the surface collapse of the sample, which tends to exert lateral friction on the ball and thus reduces rebound height. The higher the value, the higher the foam quality.

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