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Hardness testing of sofa cushions

Investigating test procedures used to assess the hardness and durability of these furniture products.

There are ways of measuring the 'hardness' (or softness) of a cushion, which is an important aspect of its performance. Another important aspect is how the cushion stands up to use. Does it retain its initial hardness quality, or is this lost over time? Such 'durability' can be assessed by measuring the initial hardness, then subjecting the cushion to a 'pounding' or 'fatigue' process, where actual use is simulated, and then re-measuring the hardness afterwards.

There are standards that can be used to assess the polyurethane (PU) foam material that is often used as a filling material in cushions. The tests involved are carried out on rectangular blocks of PU foam with specified standard dimensions. These standards classify the initial hardness of the PU foam material, and also classify the fatigue performance after the blocks have been pounded. On the basis of the loss in hardness after pounding, the fatigue classification may be 'light', 'average', 'severe', 'very severe' or 'extremely severe', indicating the service type for which the PU foam is suitable.

For a whole sofa cushion, however, there is no national, European or International standard that can be used. A typical sofa cushion may have a mixture of foams, loose fibres, or feathers used as the filling material. By 'sofa cushion', we here refer to a large cushion that the user sits on – not the smaller 'scatter cushions' which are not subjected to such large loads, and for which fatigue performance is not generally so important.

A test method is developed

In response to requests from customers, SATRA has developed a test method (SATRA TP28 – 'Hardness testing of sofa cushions') to measure the initial hardness of a large complete cushion and then subject it to a pounding procedure to simulate extended use. The final hardness is then measured to assess any loss in performance during use. No classification scheme is provided, but the measurements may be used to compare similar sized cushions with different constructions. SATRA TP28 is used to test seat cushions – a modified version of this test is available for the assessment of back cushions.

The method utilised to measure the initial hardness of the cushion is based on a European standard that is used to characterise mattresses. This method is EN 1957:2012 – 'Furniture. Beds and mattresses. Test methods for the determination of functional characteristics and assessment criteria'. The thickness of a large sofa cushion is similar to a typical mattress, and so the test method is transferable.

An indentometer is used to measure the hardness of the cushion (figure 1). In this method, the cushion is compressed using a loading pad under computer control, and data is collected that enables the load versus indentation characteristic to be plotted. From this plot, EN 1957 provides a mathematical calculation that gives a 'hardness value' (in N/mm), and also a 'firmness rating'. It is the firmness rating which is perhaps more useful. The firmness rating is a number on a scale between one and ten, which expresses the firmness of the cushion. A rating of one corresponds to a very firm cushion, and a rating of ten corresponds to a very soft cushion.

​ Figure 1: An indentometer being used to measure the hardness of a cushion

Following the principles used in EN 1957 when testing mattresses, the cushions are 'conditioned' in a controlled temperature and humidity environment before each measurement is taken.

EN 1957 does have a fatigue procedure, but this is a rolling action that is only suitable for mattresses. It is not an appropriate fatigue procedure to use on a sofa cushion.

Thousands of cycles

The pounding procedure which simulates actual use is based on a European standard that is used to test the durability of complete chairs. This method is EN 12520:2015 – 'Furniture. Strength, durability and safety. Requirements for domestic seating'. In clause 5.4.1 of this method, there is a 'seat and back fatigue test'. This simulates a user sitting down in a chair, and then standing up again – many thousands of times. When using this to test just a cushion, the item is placed on the platform of the chair test machine, and is then subjected to the pounding procedure that would normally be applied to the horizontal seat of a complete chair (figure 2). Taking account of the test method (EN ISO 3385:2014) that is used to subject blocks of PU foam to a pounding process, the number of cycles used to pound the complete cushion is extended from 25,000 to 80,000 cycles.

​ Figure 2: Testing a cushion as described in the EN 12520:2015 method

After pounding, the cushion is re-measured using the indentometer. By comparing the initial firmness rating measured before pounding with the final firmness rating measured after pounding, as assessment of the durability of the cushion can be made.

This method is often used to compare cushions with different constructions (filling arrangements). The initial firmness rating, and how this stands up to the pounding procedure can be compared between cushions. In this case, the overall size of the cushions should be similar to enable a fair comparison.

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