Effects of Aging of Polyurethane Foams in the Context of Furniture Design

An ergonomic seat or bed must be capable of supporting optimally and evenly the user’s body for sustained periods of time. The objective of the performed investigations was to ascertain the impact of natural aging on the stiffness of polyurethane foams in dwelling apartment conditions and, additionally, to determine regression equations describing this dependence. Seven types of furniture foams, differing with respect to their apparent density and stiffness, were selected for the experiment. Experimental foams were exposed to aging for the period of 730 days (two years) in climatic conditions typical for dwelling facilities. Foam stiffness was determined in a uniaxial compression test determining strain characteristics in the function of deformations. The performed experiments made it possible to establish percentage changes of stiffness of some foams as well as the time required for those changes to assume a significant character.


INTRODUCTION 1. UVOD
Polyurethane foams are plastics consisting of polyetheretherketone skeleton surrounded by gaseous bubbles, most frequently of carbon dioxide.At the present time, they constitute a key constructional material applied in upholstered furniture, including wheel-chairs, various vehicles or aircraft improving comfort of their use.An ergonomic seat or bed must optimally and evenly support the user's body for sustained periods of time and, hence, it is important to recognize the influence of aging of polyurethane foams used in seats and/or beds on their physical-mechanical properties.However, the available literature on the subject is dominated by articles dealing, primarily, with issues con-  (Ostrogorsky et al., 1986) or foam cell structure (Dementyev et al., 1999).What is missing is a more comprehensive discussion on the effect of aging on stiffness of furniture foams.Only few articles (Garber et al., 1982;Noble et al., 1984) deal with changes in foam hardness as a result of aging.

DRVNA
The aim of this study was to determine the impact of natural aging on the stiffness of polyurethane foams in dwelling conditions and, additionally, to elaborate the regression equation describing this dependence.

METHODS AND MATERIALS 2. METODE I MATERIJALI
Seven types of polyurethane foams commonly used in furniture design were selected for the presented investigations.The experimental foams differed from one another in their density and stiffness.Major technical properties of these foams provided by the Polish manufacturer are given in Table 1.In the designation of foam types, the first two digits give information about the mean apparent density of a foam expressed in kg/ m 3 and the second two digits give information about foam mean stiffness expressed in 10 -1 kPa at the deformation of ε=0.4.Ten cubical samples were prepared for each type of experimental foams with the side of H=100 mm obtained from different places of a commercial block measuring 1.2 x 1.2 x 2 m, as shown in  As mentioned in the Introduction, in furniture design practice, stiffness is the most important criterion for selecting the foams.In accordance with the PN-EN ISO 3386-1:2000 standard, this stiffness is determined during the uniaxial compression test (Fig. 2) applying pressure beams in the form of drilled plates (Fig. 3).Individual experimental foams were compressed on a ZWICK 1445 testing machine recording force P with 0.01 N accuracy and dH displacements with 0.02 mm accuracy.The loading was terminated once the compressed sampled achieved the height of H' = 0. this period, the values of relative humidity and air temperatures were recorded.The above parameters were recorded with the assistance of a Datalogger AZ 8829 Bacto Laboratories Pty Ltd taking measurements with up to 0.1 °C and 0.1 % accuracy.The results of measurements were collected and presented on a single diagram indicating days on which investigations of the selected foams were carried out.The significance of the impact of aging on foam stiffness was evaluated by the t-test for dependent samples using the Statistica 9.1 StatSoft.Inc. statistical package.

REZULTATI I RASPRAVA
Figure 4 presents the changes in relative humidity and air temperature during the period of 730 days.On the day of the first measurement (Aug.20,2009), the recorded air temperature reached 27.9 °C, and relative air humidity -37.1 %.During autumn (until Dec. 12, 2009), air temperature dropped from 27.9 °C to 17.0 °C, while air humidity ranged between 55.5 % and 15.9 %.During winter, i.e. until April 20, 2010, as a result of the operation of the central heating system, air temperature ranged between 17.0 °C and 23.0 °C, while air humidity increased from 9.3 % to 28 %.During summer, i.e. until Aug. 20, 2010, the temperature increased significantly and ranged between 22.7 and 33.8 °C and air humidity increased from 32.5 % to 57.8 %.In the following year, until Aug. 20, 2001, the measured values were slightly different, but their trends were similar to the trends in humidity and temperature changes from the previous year.Changes in the stiffness of the selected foams were observed against this background.On the first day of investigation, a σ=f(ε) dependence was determined for each experimental foam as shown in Figure 5.
As evident from Figure 5, the dependence of strain on deformation for all the tested types of foam is of non-linear nature.During the initial stage of com-

Humidity Vlaga
Temperature Temperatura pression, for 0<ε≤0.07,foam stiffness increased distinctly because the polyurethane skeleton transferred all the external loads.Within the range of deformations from 0.07<ε≤0.63,cell walls lost their stability and allowed large displacements under the influence of even small loads.Another increase of foam stiffness occurred for ε>0.63, i.e. after the concentration of the matter and crushing of cell walls.Furthermore, this drawing clearly shows that foam stiffness did not always depend on its density.For further analyses, Figure 6 collates foam stiffnesses determined at deformations of ε=0.4 at five consecutive measuring periods.It can be clearly seen that in the observed periods, individual foams differed considerably among themselves with respect to their stiffness.It is worth stressing that foam stiffnesses established on the first day of testing exceeded considerably maximum values given by the manufacturers in the product card.These differences are collated in Table 2.It is evident that stiffness of the manufactured foams exceeded by 110 to 217 % catalogue values provided by manufacturers.Simultaneously, it can also be noted from Table 2 that small values of standard deviations (0.08 to 1.00 kPa) as well as of coefficients of variability (1.31-13.78%) confirmed high uniformity of each group.Therefore, the applied number of samples, 10 for each type of foam, was adequate to obtain reliable research results.Figure 6 also illustrates that quality relationship between foam stiffnesses was maintained stable during the period of aging of 730 days.However, the process of foam natural aging caused stiffness depreciation meaning that stiffness of some of the tested foams was reduced to values comparable to the initial stiffness of other foams.This occurred in the case of T2538 and T3538 foams, which reached stiffness comparable or lower to the stiffness of the T1830 foam determined on the first day of testing after 366 and 730 days of aging.Table 3 presents the results of the t Test (p<0.05) for dependent samples indicating high probability of the lack of significance of differences between stiffnesses of some foams subjected to aging.It is evident that only the T2315 foam turned out to be completely insensitive to a change in its stiffness as shown by the result of aging.The stiffness of T 1823 and T3538 foams did not change during the first 123 days (4 months) of aging, the T3037 foam exhibited resistance to aging during the first 244 days (8 months) of this process, whereas the T3546 foam returned to its original stiffness after 366 and, then, after 730 days of aging.For the remaining foams, there is high probability of a significant impact of aging on their stiffness.This impact is shown in Figure 7-9.3, the T1823 foam was not sensitive to aging only during the first 123 days.After 730 days, the stiffness of this foam decreased with respect to the initial value by 26 %.The T1823 foam distinctly reduced its stiffness during the entire period of aging.The greatest decline of stiffness (23 %) occurred after 366 days of aging.After another year passed (730 days in total), foam stiffness increased slightly (by 4.5 %) and this difference was statistically significant.
The influence of aging on changes in the foam stiffness of 23 and 25 kg/m 3 apparent density is shown in Figure 8.This Figure corroborates information from Table 3 that the T2315 foam did not undergo statistically significant changes in its stiffness as a result of aging.On the other hand, the T2538 foam of only slightly higher density lost stiffness as a result of aging.After 366 days, the loss of stiffness in relation to the initial value amounted to 27 %.After the following 364 days of aging, the stiffness of this foam increased by 2.9 %, and however at p<0.05, this difference was not statistically significant.
In the case of the foam characterized by 30 kg/m 3 apparent density, statistically significant differences in their stiffness only took place after 366 days of aging and more (Fig. 9).This drop amounted to 14 % in relation to the initial stiffness.Continued aging lasting up to 730 days did not cause further changes in the foam stiffness, which remained at the same level of 5.4 kPa.A similar tendency was observed in the case of the  T3538 foam.Its stiffness decreased by 14% after 366 days of aging and remained at a constant level of 5 kPa for the consecutive 364 days.The T3546 foam turned out to be the most sensitive to aging and, after each period of aging, as well as quantitative and qualitative changes in its stiffness, changes were statistically significant at p<0.05.The greatest drop of stiffness of this foam amounting to 16 % occurred after 366 days of aging.After the second successive year of aging, the stiffness of this foam increased slightly so that the difference between the initial stiffness and the stiffness after 730 days of aging amounted to 10 %.
Quantitative differences in foam stiffnesses in relation to their initial stiffness are presented in Figure 10.It is evident that the greatest differences took place after 366 and 730 days of aging.In the case of foams characterized by apparent density of 18 kg/m 3 , similar differences in stiffness occurred after 730 days of aging.On the other hand, in the case of foams of 35 kg/ m 3 apparent density, changing differences of stiffness occurred during the entire period of aging indicating high sensitivity of this material to aging.
Contemporary offices involved in designing of daily necessities commonly apply computer engineering techniques (for example CAD or CAE), which constitute an element of rapid prototyping techniques.For simulation purposes of the impact of aging of foam materials on the functional properties of upholstered furniture, it is necessary to gain knowledge about stiffness of these materials as well as its variability under the influence of aging in dwelling conditions.The regression equations presented in Figure 7, 8 and 9, providing the correlation between foam stiffness and the number of days of the aging period, will make it possible to estimate future stiffness of foams used in dwelling facilities if climatic conditions are similar to those occurring in the described investigations.

Figure 1 Figure 2 2 .
Figure 1 Commercial block of foam and places of sample collection (cm) Slika 1. Komercijalni blok pjene s označenim mjestima uzimanja uzoraka (cm) 3 H.The course of compression was illustrated in the form of the σ=f(ε) dependence assuming that: σ=P/H 2 , ε=dH/H and dH=H-H'.The stiffness of individual foams was determined on the basis of the σ=f(ε) dependence as the value of strain in kPa at the deformation of ε=0.4.Following the first stiffness evaluation of samples conducted on the 20 th of August 2009, they were exposed to climatic conditions natural for dwelling facilities.Stiffness tests of experimental foams were carried out at quarterly and annual intervals, namely: after 123 days (2009.12. 20), 244 days (2010.04.20), 366 days (2010.08.20) and 730 days (2011.08.20).Throughout

Figure 4
Figure 4 Changes in runs of temperature and relative air humidity in the laboratory facility Slika 4. Promjene temperature i relativne važnosti zraka u laboratoriju

Figure 7
Figure7presents the influence of aging on the stiffness of foams of 18 kg/m 3 density.As shown in Table3, the T1823 foam was not sensitive to aging only during the first 123 days.After 730 days, the stiffness of this foam decreased with respect to the initial value by 26 %.The T1823 foam distinctly reduced its stiffness during the entire period of aging.The greatest decline of stiffness (23 %) occurred after 366 days of aging.After another year passed (730 days in total), foam stiffness increased slightly (by 4.5 %) and this difference was statistically significant.The influence of aging on changes in the foam stiffness of 23 and 25 kg/m 3 apparent density is shown in Figure8.This Figure corroborates information from Table3that the T2315 foam did not undergo statistically significant changes in its stiffness as a result of aging.On the other hand, the T2538 foam of only slightly higher density lost stiffness as a result of aging.After 366 days, the loss of stiffness in relation to the initial value amounted to 27 %.After the following 364 days of aging, the stiffness of this foam increased by 2.9 %, and however at p<0.05, this difference was not statistically significant.In the case of the foam characterized by 30 kg/m 3 apparent density, statistically significant differences in their stiffness only took place after 366 days of aging and more (Fig.9).This drop amounted to 14 % in relation to the initial stiffness.Continued aging lasting up to 730 days did not cause further changes in the foam stiffness, which remained at the same level of 5.4 kPa.A similar tendency was observed in the case of the

Table 1
Technical properties of selected foams (according to manufacturer's data)

Table 2
Foam stiffness at ε=0.4 according to data provided by the manufacturer and obtained in the course of experimental studies Tablica 2. Krutosti pjena pri ε=0,4 prema podacima proizvođača i podacima dobivenim u ovom istraživanju