Download Comparison of Compression Behaviour of PU Foam and 3D

Proceedings of the SEM Annual Conference
June 1-4, 2009 Albuquerque New Mexico USA
©2009 Society for Experimental Mechanics Inc.
Comparison of Compression Behaviour of PU Foam and 3D Nonwoven
Raphaël L. Dupuisa, Nicole Njeugnab, Dominique C. Adolpheb,
Laurence Schacherb, Evelyne Aubrya, Jean-Baptiste Schaffhauserc, Patrick Strehlec
a
ENSISA, Modélisation Intelligence Processus Systèmes (MIPS), EA 2332 UHA,
12, rue des frères Lumière, 68093 Mulhouse Cedex France, [email protected]
b
ENSISA, Laboratoire de Physique et Mécanique Textiles (LPMT), UMR 7189 CNRS UHA,
11, rue Alfred Werner, 68093 Mulhouse Cedex France
c
N. Schlumberger, 170 rue de la République, 68502 Guebwiller Cedex France
ABSTRACT
This work has been done in the frame of recyclability of flexible cellular materials usually used in automotive
interior applications. This paper will present new textiles materials as 3D nonwoven which could be used to
substitute the polyurethane (PU) foams used in the automotive industry. The developed methods for the physical
and compression characterizations of the PU foams and the new 3D nonwovens will be detailed. The obtained
results and a study of the compression behaviour of those products will be discussed.
INTRODUCTION
Our work has been carried out in order to propose an alternative to recycling soft cellular solids used in the
automotive interior. Some new European Directives such as 2000/53/CE [1] imposes new rules concerning the
recycling with an aim of 85% of recyclable products and 95% re useable products at January 2015. Due to these
new constraints, car manufacturers and their partners have to review the automotive specifications for the
materials used in the automotive interior as the insulation material, the loudspeaker housing, the headliner facings,
car upholsteries, etc… The materials currently used as seat upholsteries are complex materials generally
composed of three layers (Figure 1). The first layer is a knitted or woven fabric, for the appearance, generally
made of polyester (PET). The second layer is polyurethane foam between 2 mm and 8 mm thickness. The third
layer is usually a knitted polyamide (PA) or polyester fabric. The use of polyurethane will bring comfort in the case
of flexible foams and excellent thermal insulation in the case of rigid foam. [2]
Polyester
Decorative fabric
Polyurethan foam
Foam
Polyester or
Polyamide
Knitted structure
Figure 1. Automotive complex composition.
The realization of automotive complex fabrics is usually performed by a lamination done by flame treatment. The
melt foam obtained after the surface flaming acts as a binder. This operation produces the emission of dangerous
toxic gases. [2] In addition to these toxic fumes, the recyclability of this complex is far from optimal because of its
composition. Indeed, the delaminating process of these fabrics is very complex because traces of foam remain in
the polyester or polyamide after separation of the different layers and pollute the extracted components. An
alternative to polyurethane foam may be the development of a single component material (100% polyester p.e.),
this solution will lead to an easier recyclability. This alternative material must have the same mechanical
characteristics, particularly in terms of compression, than the replaced foam. Moreover, it must meet the
requirements of the automotive industry in terms of weight, implementation and cost. [3]
In this paper, an alternative material to polyurethane foam has been studied. The new proposed product is a
polyester 3D nonwoven.
The methods of physical and mechanical characterization of foams and 3D nonwovens will be presented. First,
the characterization methods and compression tests will be detailed, then the results will be analyzed and a
critical analysis of the compression behavior of these materials will be carried out.
TESTED MATERIALS
The study focused on polyurethane foams commonly used in automotive trim and a new 3D nonwoven polyester.
The 3D structure is obtained thanks to pleats thermoformed. This process has been patented by the N.
Schlumberger Company [4]. The foam is soft cellular structure (Figure 2a). The cells can be modeled
geometrically by a pentagon. A method of characterization [5] adapted from the VISIOCELL® method has been
used to evaluate the average diameter of foam cells. The tested foams A and B were respectively open and
closed cells. The pleated structure of the 3D nonwoven (Figure 2b) has been modeled geometrically by an
isosceles triangle. This characterization method has been developed to assess the physical parameters of the 3D
nonwoven structure. The characteristics of the pleats are the pleat’s angle, the pleat’s hypotenuse and the
number of pleats per centimeter. The structure irregularity and the rate of pleat’s condensation have been also
measured.
All the parameters issued of these characterization methods are shown in table 1

e0
(a)
(b)
Figure 2. Polyurethane foam structure (a) [6] and 3D nonwoven pleat (b) [7].
Table 1 presents the main physical characteristics of foams and 3D nonwowen (NW) studied.
Nw 1 Nw 2 Nw 3 Nw 4 Foam A
Thickness (mm)
6
6
6
6
4
Density (kg/m3)
68
75
73
75
42
Horizontale cell mean size (μm)
544
Vertical cell mean size (μm)
446
Number of pleats/cm
2,1
2,2
2,2
2,2
Pleat’s angle (degree)
43
42
41
39
Rate of pleats’ condensation (%) 63
65
65
67
Table 1. Physical characteristics of tested samples.
Foam B
4
42
687
562
-
In order to develop compression behavior analysis methods, we reviewed the typical approaches commonly
found in the literature. The literature on foam is quite large, composed as well as published paper than books. The
reference is obviously the work of Gibson and Ashby [8]. Thus, they have shown that the characteristics of foams
depend on various parameters such as cell morphology, size, arrangement, connections between them… Static
mechanical behavior of foams has been well described by Gibson and Ashby [8], particularly in the effect of a
compressive force. On the stress- strain curves of polyurethane foam, three main parts can be highlighted. First a
linear elasticity at low stress followed by a long collapse plateau, and finished by a densification of the material in
which the stress rises steeply.
Belouettar et al. [9] has suggested for the description of the constitutive behavior of foam, three types of approach.
The first is a complete experimental one. It consists in describing the behavior by unidirectional relations between
stress and strain. This method is highly recommended in ISO 3386 / 1: 1986 [10].
The second type of analysis is based on micro-macromechanical modeling. The aim is to obtain the global stress
strain behavior, after a homogeneous process, from modeling cells as elementary elements. The part of linear
elasticity of open-cells foam has been well described by Zhu et al. [11] and Li et al. [12]. The analysis of high
strain compression was conducted by Zhu et al. [13] and by Mills et al. [14]. The cells have been modeled in order
to simulate the buckling of elastic cell edges, and the predictive deformation has been compared to experimental
values of polyurethane foams. At least, Wang et al. [15] has introduced a micromechanical hyperelastic
formulation for foams which take in account the non linear effects at material and kinematics level.
The last method is to consider the foam as a homogeneous material, and to determine the strain energy.
Belouettar et al. [9] has developed a global model based on Ogden theory [16]. The parameters of this model
have been determined thanks to experimentations, and a model of car seat using finite elements modeling has
been proposed.
The 3D nonwovens and the foams have been characterized using a method based on the automotive standard
ISO 3386/1:1986 [10] and a method [17] developed on the Kawabata Evalution System and using the
recommendations of this device [18]. This paper will present the compression testing method developed and will
analysis of stress-strain curves. No modeling has been conducted this first approach.
EXPERIMENTAL METHODOLOGY
All the tests have been carried out on a universal screw driven testing machine (INSTRON 33R4204) (Figure 3)
fitted with 5kN load cell. The solicitation speed has been chosen at 0.05mm/s in accordance to the Kawabata
recommendations [18]. The test performed is a compression one and the machine is equipped by 2 strictly
parallel plates with a diameter of 150mm and a smooth surface (Figure 3). The test procedure is the following: the
sample is first compressed up to 50% of its initial thickness then decompressed at the same speed until the plates
are back to their initial locations, then 10s rest is given to the sample in order to recover. This cycle is repeated 5
times (Figure 4).
Figure 3. Universal testing machine INSTRON 33R4204
During the test, the time, the displacement and the applied force to the plate are measured. Using these data, we
can calculate the stress, the strain (Figure 4), the energy of the hysteresis, the maximum stress at 50% of initial
thickness deformation, the value of the first peak, the initial slope and the slope of the observed plateau can be
calculated
The sample size is a square of 10 x10 cm².
In this paper, we will only present the energy of the hysteresis and the maximum stress at 50% of initial thickness
deformation.
-55
Displacement (% of initial thickness)
-45
-35
-25
-15
-5
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
-0,005
Stress (Mpa)
0
-0,01
Figure 4. Classical stress – strain curve of polyurethane foam.
RESULTS AND DISCUSSIONS
The average values of maximal stress for a 50% of initial thickness deformation are calculated for each cycle.
This value, we call “lift”, is the compressive strength of material. It is commonly accepted [19] that greater the
strength necessary to compress the foam is, the greater the foam is called “lifting”, and as the notion of “lift” is
correlated to the comfort, the more it will be comfortable [19].
The results are represented as histograms (Figure 5). The statistical study of comparison of averages between
the different cycles shows that the 3D nonwovens 1 and 2 begin to stabilize their resilient behavior after the 3rd
cycle. The resilient behavior of the 3D nonwovens 3 and 4 stabilize at the 5th cycle while the foam A stabilizes
after the 4th cycle. Foam B has the same resilient comportment whatever the number of cycle is. Overall the 3D
nonwovens present stress values for a 50% strain 2 to 3 times higher than those for tested foams. We can
conclude that they are more resilient in compression and thus more comfortable than foam. The observed
dispersions are larger in the case of 3D nonwovens than for foams. This shows that the foams have a more
reproducible behavior. The low reproducibility of the behavior of 3D nonwovens can be explained by the
significant dispersion of the physical properties of this new material.
Figure 5. Maximal stress for a 50% initial thickness deformation.
Concerning the foam, we can notice that the foam A is more resilient than foam B, so foam A is more comfortable
than foam B. We can also conclude that the nonwovens 2 and 3 are the most resilient, they are more comfortable
than nonwovens 1 and 4. These results can be explained thanks to the analysis of their physical characteristics.
Regarding the 3D nonwoven 1, indeed, it presents the least pleated structure (the rate of condensation is the
lowest) and open (highest pleat’s angle). Similarly, 2 and 3 nonwovens show better physical characteristics than
the nonwoven 1. Regarding the nonwoven 4, in spite of better physical characteristics, its poor strength could be
explained by poor structure’s regularity. In this study, the samples were produced with a verticalisation process
that still under development and the regularity aspect is, up to now, not fully controlled on this prototype. This
process particularity must be taken into account when analyzing the results.
The energy dissipated during the compression is calculated from the area of the hysteresis. The results are
represented as histogram (Figure 6). Whatever the material is, we observe that the energy dissipated during the
1st cycle is twice larger than that dissipated during the 2nd cycle. This phenomena highlights a reorganization
process of the internal structure within the material during the 1st cycle. Overall the 3D nonwovens dissipate
almost twice more energy than the foams. The statistical study, comparison of averages between the different
cycles shows that the reorganization of structure of the 3D nonwoven 1 and 3 is settled after the 3th cycle. The
stabilization of this behavior could be observed at the 4th cycle in the case of 3D nonwoven 4. 5 cycles are needed
in the case of nonwoven 2 to obtain the same result. The collapse of cells in the expulsion of air of the foam A
stabilizes the deformation energy at the 4th cycle. This could be explained by the structure of closed cellular foam
A. The foam B does not present any stabilization behavior even after 5 cycles of compression. This could be
explained by its open cell structure. A large dispersion of results has been found for 3D nonwovens, this fact was
still observed for the maximal stress for 50% initial thickness deformation. The foams seem to have a more
reproducible compression behavior compared to 3D nonwovens.
Regarding the foams, we can notice that the foam A dissipate more energy than foam B. This might be explained
by its closed cellular structure. In terms of 3D nonwoven, nearly the same behavior as described previously
(resilience tests) can be observed. The difference in value between the 1st and 2nd cycle is hugely marked. This
discrepancy can be explained by the reorganization of the fibrous structure in the 3D nonwoven and the energy
lost in the inter fiber friction. From the 2nd cycle of compression, the energy tends to stabilize and this stabilization
is clearly observable from the 4th cycle. Depending of their internal fibrous structures, the nonwoven will stabilize
sooner or later, the following ranking could be done in terms of stabilization, 3D nonwoven 1 following by 3D
nonwoven 2, 3 and 4. is beginning to stabilize from the 3rd cycle, while it was not until the 4th round to observe a
clear stabilization of other nonwovens. In terms of energy dissipation, the following ranking could be extracted, 3D
nonwovens 2 and 3 following by nonwovens 4 and 1. Behavioral differences between the nonwovens can be
explained, as for the maximum stress, by the difference in the structure of the different samples of 3D nonwovens
with always the same problem of heterogeneity for the 3D nonwoven 4.
Figure 6. Energy dissipated during compression.
CONCLUSIONS
In this study, a method for compression testing for a new 3D nonwoven material based on the specifications of
the automotive world as been developed and detailed. This method has been used to test the polyurethane foams
and alternative proposed materials, the 3D nonwovens. The results showed that the 3D nonwovens are more
resilient than tested polyurethane foams. Indeed, this structure enables a vertical orientation of fibers, this vertical
orientation of fibers has the effect to provide maximum resilience to the material. On the other hand, it has been
shown that the 3D nonwovens dissipate more energy in compression than the foam. This result has demonstrated
the ability of the fibrous structure to be reorganized under compression stress. It also highlights the relationship
between the compressive mechanical behavior of these materials and their physical characteristics (values and
dispersion). All these results underline the ability of this new pleated structure material to replace polyurethane
foam in sustainability purpose. This is, obviously, the first step of this study, the characterization of these new
products. A study of dynamic loading and a modeling approach of these new structures are in progress. All these
tools and methods will allow us to fully characterize this new material and to answer to the initial question “Can we
replace the current polyurethane foams by these new 3D nonwoven?“.
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