The Importance of Cell Structure for Viscoelastic Foams

R. LANDERS*, R. HUBEL*, R. BORGOGELLI#

* Degussa GmbH,

Goldschmidt GmbH

Goldschmidtstr. 100

45127 Essen,

Germany

# Goldschmidt Chemical Corporation

914 East Randolph Road

Hopewell VA,

23860

USA

ABSTRACT

The influence of different effects for the delayed recovery of viscoelastic foam types is demonstrated by key experiments

and the possibility to produce viscoelastic foams just based on one of the effects is illustrated by test foams. The importance

of the adhesion for some viscoelastic foam types is highlighted. The role of the cell structure – cell fineness and porosity –

turns out to be of significance for viscoelastic foams relying on the pneumatic as well as the adhesion effect. Careful control

of cell structure is therefore one of the main challenges in production of viscoelastic foam.

INTRODUCTION

Viscoelastic polyurethane foams attract an increasing interest in PU foam industry [1-2]. The main application is the high

quality mattress and pillow segment, where viscoelastic foams distribute the body weight over a larger area. Furthermore,

vibration damping and sound absorption are technical applications of viscoelastic foams, but count for a lower amount. The

viscoelasticity is visible by two major properties: low resiliency and slow recovery after compression. Both properties are

related to delayed response of the foam towards deformation. The optical appearance is a little bit like a gel, which needs

time to flow back into the original shape. This observation caused the term viscoelastic foam, but several other expressions

like memory foam, slow recovery foam, controlled recovery or low resilience foam exist too. It is essential for establishing

viscoelastic foam formulations to understand the reasons for the unusual behavior. Some general ideas are published in an

article published 2002 [3]. However, the understanding of the reasons remains still a little bit vague.

OBJECTIVE

One of the main differences of viscoelastic foam towards conventional flexible foam is the slow recovery after a

deformation. Conventional flexible polyurethane foam almost immediately tries to get back its original shape. The different

involved forces could be divided into two groups:

Forces related to the macroscopic structure of the foam and forces related to the molecular level of the polyurethane network.

Figure 1 displays schematically the situation if polyurethane foam is compressed. A deformation of the foam structure is

visible on the macroscopic level. Struts and membranes are being bent, air needs to escape from the foam and membranes and

struts get in touch with each other. On the molecular level, the deformation of the polyurethane network is of importance. The

network tries to get back to the original entropy-favored configuration and produces the resilient force. This is well known in

the theory of rubber elasticity.

compressed structure

relaxed foam structure

macroscopic level molecular level

Figure 1: Schematic drawing of the changes occurring on the macroscopic and on the molecular level in flexible polyurethane foam under

compression.

The most often mentioned reason for viscoelastic foam properties is the glass transition temperature, which is typically

slightly below or roughly at room temperature. Elevated temperatures (above usual ambient conditions) reduce the

viscoelastic behavior making the foam feel more like a soft conventional foam. On the other hand, lower temperatures causes

the viscoelastic foam to turn quite hard. The glass transition temperature of conventional ether foam is far below ambient

temperature. At the glass transition temperature, polymer chains or (in case of a network) polymer network segments start to

freeze. This means that their mobility is drastically reduced. Nevertheless, a higher state of three dimensional orientation (like

crystallization) is not obtained. The glass transition takes place across a temperature range and is not limited to a specific

temperature because the length of the network segments follows a certain distribution. Shorter chain segments have higher

glass transition temperatures than longer ones. Overall, glass transition temperatures can be adjusted by changing the mobility

of chain segments (for instance by increasing cross-linking) or by changing the chemical nature of the network. For

viscoelastic foams at room temperature, at least some of the chain segments are still in a frozen state causing resistance

against fast deformation. Slow movement is possible if the immobile chain segments take up energy from surrounding areas

to become mobile. But this takes some time, delaying rapid movements. This is effective for any change of the foam

structure, both in compression as well as re-expansion, and is fully related to the polyurethane matrix on a molecular level.

A second important factor, already mentioned earlier, is the pneumatic effect, which is most relevant if the foam cell structure

is relatively closed. In this case, air needs time to leave the foam or (in case of re-expansion) to penetrate back into the foam.

This effect is only related to the foam structure and is clearly a macroscopic effect. It delays compression as well as re-

expansion. Figure 2 summarizes the relevant effects so far.

Network Effect

deformed PUR networkproduces the resilient force

Pneumatic Effect

air needs time to flowback into the foam

Relaxation Effect

relaxation of PUR networksegments takes time, if themobility of at least a part of the segments is restricted.

air

air

Figure 2: Schematic drawing of the different forces involved in the behavior of viscoelastic polyurethane foams.

Compressed viscoelastic foam tries to re-expand because of the resilient force. The relaxation effect as well as the

pneumatic effect delays this re-expansion. Methods to separate the different effects would be beneficial for a better

understanding.

EXPERIMENTAL

Six different foam samples were analyzed within this study. Five were viscoelastic foams, one was a conventional ether

foam. The viscoelastic foams were either taken from commercial sources (4 foams) or made in the laboratory by using well

established formulations (1 foam). These foams were selected to illustrate the broad range of viscoelastic foam types

available on the market. The details of the foams are listed below:

1. Laboratory viscoelastic TDI 80 based foam

This foam was produced in a small box in the laboratory using a well known TDI 80 based formulation.

30 pphp DOW Voranol®

CP 3322, 70 pphp DOW Voranol®

CP 755, 7 pphp DOW Voranol®

CP 1421, 1.95 pphp water, 0.3

pphp TEGOAMIN®

33 (triethylene diamine 33 % solution) 0.2 pphp TEGOAMIN®

BDE (bis(dimethylaminoethyl)ether), 0.2

pphp TEGOAMIN®

DMEA (dimethylethanol amine), 0.07 pphp KOSMOS® 29 (stannous octoate), 1.2 pphp ORTEGOL®

75 and 40.3 pphp TDI 80 for an index of <85> (45.0 pphp for <95> and 49.8 pphp for <105>). Polyols, amines, water and the

stannous octoate were weighed into a beaker and stirred for 60 s at 1000 rpm. The isocyanate was added and the mixture

again stirred for 7 s at 1500 rpm. The whole mixture was poured into an insulated foaming box with a size of 20 x 20 cm and

25 cm in height. The formulation was also used for obtaining the swelling results displayed in Figure 7. The different cell

sizes displayed in Figure 10 were obtained by different research versions of ORTEGOL 75 during the development phase of

this product. The foam produced by using the above mentioned formulation has a density of 41 kg/m! and a cell number of 9

cells / cm.

2. Commercial TDI 80 foam

This is a commercial TDI 80 based viscoelastic foam from the North American Market. It has a density of 65 kg/m! and a cell

number of 15 cells / cm.

3. Commercial MDI viscoelastic foam

This is a commercial MDI viscoelastic foam of European Origin. It has a density of 39 kg/m! and a cell number of 13 cells /

cm.

4. Commercial MDI reticulated foam

This is a commercial MDI viscoelastic foam of Asian origin, which was reticulated after production to enhance air

permeability. It has a density of 41 kg/m! and a cell number of 9 cells / cm.

5. MDI molded foam

This foam is the only molded foam in this study. It was made by using a well established MDI based system. A step mould

was used to produce the foam sample. A 10 cm foam cube was cut from the molded foam, with two sides of the cube still

having the original skin. The foam has a density of 76 kg/m! and a cell number of 20 cells / cm.

6. Conventional ether foam

The conventional ether foam is the reference foam. It has been produced in the laboratory by using the following

formulation.100 pphp polyol Voranol®

CP3322, 4.0 pphp water, 1.0 pphp TEGOSTAB BF 2470, 0.15 pphp TEGOAMIN 33,

0.16 pphp KOSMOS 29, 52.9 pphp TDI 80 <index 115>.

The foam has a density of 24 kg/m! and a cell number of 13 cells / cm. This formulation was also used to obtain the results

displayed in Figure 13 and 19. The different cell numbers in Figure 13 were obtained by changing the stirrer speed.

Glass transition temperatures were measured by a standard DSC (QS 1000 from Thermo) with a heating /cooling rate of 20

K/min and a temperature range of -80°C up to +50°C. Cell numbers were analyzed by electronic cell structure analysis using

a flat bed scanner and an image analysis software (a4i from Soft Imaging Systems).

Porosity measurements were made by using the back pressure method. A constant air stream of 8 l/min is forced to flow

through a 5 cm thick sheet of the foam, cut perpendicular to the rise direction. The resulting back pressure is measured in mm

water column. High values indicate a very tight cell structure. The range is from 1 mm water column (very open) to > 300

mm water column (very closed).

Recovery times were measured using a special self made device, which is displayed in Figure 3. Two wood plates are fixed

9 cm away from each other. A third wood plate in between is able to move up and down. In the lowest position, the middle

plate is 3 cm away from the fixed bottom plate. A foam cube with 10 cm side length is inserted below the middle plate and

compressed. The compression is 70 %. The middle plate is fixed in the lower position by wrapping a cotton string around the

middle plate and the bottom plate. The cotton string is cut by an electrically heated metal resistance wire. The electrical

energy is supplied by two insulated copper wires. The re-expanding foam lifts the middle plate to the top plate. The recovery

time is measured from the moment of cutting until the full contact of the middle plate to the top plate (10 % compression).

The main advantage of the whole assembly is that it could be placed in a vacuum chamber or a heating oven. The cutting

could be initiated from outside by switching the electrical power supply on. The vacuum chamber with the device is shown in

Figure 4. The recovery time is measured manually.

Figure 3: The apparatus to determine the recovery time in vacuum. The two electrical wires to supply electrical energy are visible at the

bottom as well as the cotton string. The picture shows the foam in a compressed state.

Figure 4: Photograph of the heating oven used for vacuum experiments. The electrical wires to start the expansion experiment are visible at

the bottom.

Surface treatment for elimination of the adhesion influence was done with AEROSIL®

200, a nanometer scaled pyrogenic

silica. The foam sample and the powder (5 g) were placed in a plastic bag and the bag was sealed. The bag was shaken and

compressed at least 20 times to allow the powder to penetrate into the foam. Finally, the foam was removed and excess

powder was removed by carefully shaking the sample.

The calculation of the cell size influence towards the internal surface was done using a simple model. A cube with side

length L is filled with spheres of radius r by using a hexagonal dense packing. The number of spheres within the cube is

calculated by using equation 1:

2

22

3

30cos432 !!

"

#$$%

&

°'((((

=

rrr

Lnumberspheres

Equation 1

Using the number of spheres it is possible to calculate the surface of all spheres (equation 2):

spheresnumberrsurface !!!=2

4 " Equation 2

The cell number could be determined from the radius of the spheres (equation 3):

r

Lnumbercell

!=2

Equation 3

Equations 1-3 have been used to calculate Figure 11.

RESULTS

The first important point to be studied is the relaxation related effect of the glass transition temperature. This is evaluated

by changing the foam temperature or by altering the glass transition temperature. First method is easily performed by putting

the foam in heating oven. Figure 5 illustrates the influence of the temperature on the recovery time of a typical commercial

viscoelastic foam. The viscoelastic foam has a glass transition temperature of -15 °C, determined by Differential Scanning

Calorimetry (DSC). A Dynamic Mechanical Analysis (DMA) experiment results in a higher value of 5°C.

0

2

4

6

8

10

12

-25 -5 15 35 55

temperature [°C]

rec

ov

ery

tim

e [

s]

Figure 5: Influence of the temperature on the recovery time of a commercial TDI 80 viscoelastic foam.

It is clear, that an increase in temperature results in a much lower recovery time. The viscoelastic foam turns from

viscoelastic foam into very soft conventional foam. Six foams samples were analyzed with respect to the recovery time at

21°C and at 50°C. Five of them are viscoelastic foams (commercial as well as experimental types), and one is a conventional

flexible foam. Figure 6 displays the results.

0,1

1

10

100

lab.

TDI v

isco

com

mer

c. T

DI

com

mer

c. M

DI

com

mer

c. M

DI r

etic.

MDI m

olde

d

conv

entio

nal e

ther

rec

ov

ery

tim

e [

s]

21°C

50°C

Figure 6: Recovery time at different temperatures. The conventional flexible foam has in both cases a recovery time of 0.1s.

It is surprising that the impact of the temperature increase is different for the viscoelastic foams. Some are very sensitive,

others are less. Obviously, either the glass transition temperature range is broader or effects other than the relaxation effect of

the glass transition temperature also influence the results. The impact of temperature change is especially low for the

commercial TDI based viscoelastic foam as well as for the MDI based molded foam.

A second experiment used water as a solvent to enhance the mobility of hydrophilic chain segments. The water lowers the

glass transition temperature. The same foam as in Fig. 7 was dipped into water and compressed on paper to remove the

excess water. The foam takes up 151 % water compared to the original foam weight. Also the size is increased due to

swelling. Figure 7 shows a photograph of both foams. The strong water uptake is due to the presence of hydrophilic polyols

in the formulation and the relatively low network density.

Figure 7: Photographs of laboratory made viscoelastic TDI 80 foam in a dry state (right) and swollen (left).

The recovery time decreases by the water uptake from 3 s down to less than 0.2 s, which is a typical value for a

conventional flexible polyurethane foam in this test. Both experiments make it reasonable to pronounce the dominant role of

the glass transition temperature for at least some viscoelastic foam types. Nevertheless, the influence of the glass transition

temperature seems to be not the only factor.

Viscoelastic foam properties are also influenced by the pneumatic effect, which is related to the porosity of the foam. The

same set of foam samples were tested again with respect to porosity. Measurements were taken using the back pressure

method. Fig. 8 shows the results, noting specifically that the cell structure of the moulded foam sample is much more closed

than the other foams.

0

50

100

150

200

250

300

350

lab. TDI visc

o

com

merc

. TDI

com

merc

. M

DI

com

merc

. M

DI re

tic.

MDI m

olded

conve

ntio

nal e

ther

back p

ressu

re [

mm

wate

r]

Figure 8: Porosity of the test foams determined by the back pressure method. High values indicate tight foam structures.

In a first test, the pneumatic effect can easily be checked by measuring the recovery time of the foam samples both in

vacuum and under normal atmospheric pressure. No air flow from or into the foam is required under vacuum conditions. A

strong influence of the air flow would result in shorter recovery times under vacuum conditions. Figure 9 displays the

observed results.

0,1

1

10

100

1000

lab.

TDI v

isco

com

mer

c. T

DI

com

mer

c. M

DI

com

mer

c. M

DI r

etic.

MDI m

olde

d

conv

entio

nal e

ther

rec

ov

ery

tim

e [

s]

1013 mbar

20 mbar

Figure 9: The influence of vacuum towards the recovery time of viscoelastic foams. The value for the conventional foam is 0.1 s under

both conditions.

The opposite of the expected outcome was detected. For most foam samples, the recovery time was longer in a vacuum.

Only the MDI molded foam had a shorter recovery time under reduced pressure. This is the foam with the lowest air

permeability. This foam shows the expected influence of the pneumatic effect towards the recovery time. All other foam

samples exhibit a longer recovery time in vacuum. The reason is perhaps a vacuum induced drying of the foam influencing

the glass transition temperature. The conventional ether foam had a very fast recovery time under both conditions.

So far it has been demonstrated that the glass transition temperature related relaxation force as well as sometimes the

pneumatic effect have some influence towards the performance of viscoelastic foams. Nevertheless, some results from

previous published experiments [4] are quite difficult to explain using only the above mentioned effects. A series of

viscoelastic foams with the same formulation but with different cell sizes and roughly similar porosity were made and the

recovery time was measured. The result is displayed in Figure 10:

0

50

100

150

200

250

300

350

0,00 5,00 10,00 15,00 20,00

mean cell number [1/ cm]

rec

ov

ery

tim

e [

s]

Figure 10: The influence of the cell structure on the recovery time. Displayed are the test foams with an index of <85> and constant

porosities.

It is obvious in this series that the recovery time depends on the cell size of the foam. Viscoelastic properties become very

strong, if the foam is very fine. No recovery at all can be observed above a certain limit value. This result cannot be explained

by the relaxation effect (always the same foam formulation) nor by the pneumatic effect (porosity was constant). A third

effect, strongly related to the cell size seems to be involved. This effect can be described as an adhesion effect. It summarizes

all attractive and adhesive forces between struts and membranes when foam is compressed and foam structure elements get in

touch. Membranes are expected to play the dominant role due to their large surface. The effect is proportional to the

stickiness of the foam and to the internal surface of the foam. The internal surface strongly depends on the cell size. Figure 11

analyses the influence of the cell size by a simple model. A cube with 10 cm side length is filled by a dense hexagonal

package of spheres, with just one diameter. The diameter of the spheres is varied. The total surface of all spheres is shown in

the diagram. It is evident, that the overall sphere surface grows linear with the number of cells per cm.

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

cell number [1/cm]

su

rface [

m_]

Figure 11: The increase of the internal surface with the cell number. Data are generated from a simple geometrical model with

monodisperse spheres.

It is well known that many viscoelastic foams feel somehow “wet” and sticky. The introduction of such an adhesive force

could simply explain the curve in Figure 10. If the resilient force is in a first assumption considered to be constant in respect

to the cell size, and the adhesion force is growing with finer cell size, it might happen that the adhesion exceeds the resilient

force at a certain cell number. No recovery at all would be visible under these circumstances. This is illustrated in Figure 12.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

cell number [1/cm]

F a

dh

esiv

e [

N] Resilient force

Adhesive force

Figure 12: Schematic illustration of the balance between the resilient force and the adhesive force in respect of the cell number of a sticky

viscoelastic foam. The crossing point is marked by a black line. Above this point remains the foam compressed. Below the recovery time is

becoming shorter the coarser the foam is.

In reality the situation is more complex, because also the resilient force depends on the cell size. Higher cell numbers have

typically slightly lower resilient forces. This is visible in the drop of hardness for fine celled foams. Typical results for

conventional flexible foam are displayed in Figure 13.

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

0 2 4 6 8 10 12 14 16 18 20

cells per cm [1/cm]

CL

D 4

0 %

[kP

a]

Figure 13: The influence of the cell number on the CLD 40 % values of a conventional flexible foam.

In viscoelastic foams, this additional effect results in an even more pronounced influence of the cell number towards

adhesion as it shifts the crossing point in Figure 11 to lower cell numbers.

The adhesion between foam structure elements may have several reasons. Mechanical entanglements of struts may be one

aspect, liquid surface films a second, hydrogen bridges and van-der-Waals bonds a third. Molecular entanglements of the

polymeric network segments may also be factor as well as reactive linkages, although unlikely. Within the scope of this paper

it is not possible to differentiate between these sources of attractive forces. A schematic drawing of molecular chain

entanglements as an example is displayed in Figure 14.

Figure 14: Schematic drawing of adhesion forces generated by entanglements of network segments and loose ends of the polyurethane

network.

Whatever the reason, the key is that membranes stick together and delay the re-expansion. Each glued membrane pair

separates at an individual point of time when the foam re-expands. If the foam is coarse enough, it is sometimes also possible

to “hear” the separation of the membranes, causing a typical sound.

The adhesion effect is not only related to the foam structure (macroscopic level), but also to the molecular level associated

with the stickiness of the foam. In contrast to the other effects, the adhesion effect is not a factor for both compression and re-

expansion. It only affects the re-expansion of foam. To evaluate the extent by which this adhesion forces influence the

behavior of viscoelastic foam types, a simple experiment was developed. A foam sample is put into a plastic bag, a very fine

nanometer scaled powder (pyrogenic silica) is added, the bag is sealed and the foam is compressed many times under

continuous shaking. The fine powder sticks to all surfaces and acts as a release agent. By repeating the compression and

shaking of the bag the powder is able to enter also inner surfaces of the foam. Figure 15 displays a photograph of the foam

sample in the bag.

Figure 15: Powder treatment of a viscoelastic foam in a plastic bag.

The pneumatic as well as the relaxation effect are not affected by this surface treatment. The influence of the surface

treatment towards the recovery times under atmospheric pressure are shown in Figure 16.

0,1

1

10

100

lab.

TDI v

isco

com

mer

c. T

DI

com

mer

c. M

DI

com

mer

c. M

DI r

etic.

MDI m

olde

d

conv

entio

nal e

ther

reco

very

tim

e s

tan

dard

[s]

standard

after surface

treatment

Figure 16: The influence of a post treatment with fine powder on the recovery time of viscoelastic foams and conventional polyurethane

flexible foam. The conventional foam in both cases has a recovery time of 0.1 s.

The relatively fine celled laboratory foam exhibits the strongest shift. Coarser commercial foam grades are also affected but

to a lower extent. The coarse reticulated MDI viscoelastic foam does not exhibit any shift of the recovery time. This

experiment clearly demonstrates the adhesion effect as a function of cell size. For some viscoelastic foams, this adhesion

effect is very significant.

Network Effectdeformed PUR network

produces the resilient force

Pneumatic Effectair needs time to flow

back into the foam

Relaxation Effectrelaxation of PUR networksegments takes time, if themobility of at least a part of the segments is restricted

air

air

Adhesive Effectadhesive forcesfrom stickysurfaces

Figure 17: Schematic drawing of the different forces involved in the behavior of viscoelastic polyurethane flexible foams.

The situation displayed in Fig. 2 has to be modified to consider these new findings. Fig. 17 includes also this adhesive

effect. The goal in achieving the desired viscoelastic properties of the polyurethane foam is to balance the resilient force from

the network with the three opposite effects. The important aspects of the different forces are summarized in Fig. 18.

yes, strongyesnoMacroscopic +

Molecular

Adhesion Effect

nonoyesMacroscopicPneumatic Effect

noyesyesMolecularRelaxation Effect

yes, weakyes, weakyesMolecularNetwork

Resilient Effect

Cell Size

Influence

Temperature

Influence

Applies to both

Compression

and

Re-Expansion

Macroscopic /

Molecular

Figure 18: Characterization of the different forces involved in the viscoelastic behavior of polyurethane viscoelastic foams.

It is an important feature of the adhesive forces that they are only detectable after compression and of the pneumatic effect

that it is not influenced by temperature.

Additionally, foams with a delayed recovery, but just based on one effect have been prepared to demonstrate, that it is

possible to play with all three effects to generate a viscoelastic behavior. Foams with just the relaxation effect are well known

(for example the commercial reticulated MDI viscoelastic foam included in this study). Foams with a closed cell structure

delaying re-expansion are also well known to polyurethane foam industry. Foams with delayed recovery just based on

adhesive forces have been prepared by post treatment of conventional ether foam. The conventional flexible foam has been

dipped into aqueous solution of bean honey. The honey remained on the foam surface after evaporation of the water and

drying of polyurethane matrix, causing stickiness, which gave the desired viscoelastic effect. The recovery time was 4 s at

atmospheric pressure and 21 °C. Porosity as well as matrix properties remained unchanged by the sticky coating. All three

effects could therefore in theory be used alone to get the desired delayed recovery; typical viscoelastic foams exhibit all three

effects to some extent. But the contribution of the different effects varies. Typically slab stock viscoelastic foams rely more

on the relaxation effect. The adhesive effect is especially important for fine celled viscoelastic slab stock foams. Molded

viscoelastic foams in contrast are often more focused on the pneumatic effect. The pneumatic effect of molded foams is

promoted by the dense foam skin. The air permeability of the skin is in this case an important issue putting the release agent

and the molding conditions into focus. Overall, it is the balance between delaying effects and the resiliency of the network,

which controls the viscoelastic behavior. The tailoring of the delaying effects is typically done by choosing special polyols

and additives. Many different tricks exist to address the fine tuning of viscoelastic properties. But the resilient force often

needs to be adjusted as well. Conventional flexible foams are typically optimized to have strong resilient forces (high CLD

values). The above mentioned delaying effects are in many cases not strong enough. It is therefore helpful to reduce the

resilient force by lowering the cross linking density. This could be done by lowering the index. The influence of the index

towards the resilient force is displayed in Figure 19.

0

1

2

3

4

5

6

80 90 100 110 120 130

index [ ]

CL

D 4

0 %

[kP

a]

Figure 19: The influence of the index towards the CLD 40 %.

Which conclusions can be drawn for optimizing viscoelastic foam production? Viscoelastic slab stock foams need a careful

control of the cell size. This is possible by adjusting the formulation and by adjustment of processing conditions (air

injection, stirrer speed). A very constant performance of all raw materials is beneficial for a reproducible viscoelastic slab

stock production. For molded viscoelastic foams the situation is much more complicated. The pneumatic effect has a much

stronger contribution. The exact control of the air permeability is in this case absolutely essential. Slight changes of the cell

size and porosity result in different viscoelastic properties. Many molded viscoelastic foam formulations are so sensitive with

respect to cell structure, that it is advisable to contact the suppliers of all raw materials influencing porosity and cell structure

in advance. The supplier needs to know about the sensitivity of the viscoelastic application to ensure special tailor made

quality control tests.

Especially silicone foam stabilizers are known to influence nucleation and cell opening to a very high extent and are key

raw materials for tailoring the viscoelastic foam properties. It is necessary to control the performance of such additives much

more precisely than usual in flexible foam production. Individual tailor made additives are often necessary. Several special

additives have therefore been developed in recent years. Detailed recommendations of stabilizers for viscoelastic foams are

not possible because of the very broad range of viscoelastic foam formulations. But dividing the formulations regarding the

type of isocyanate helps to get a rough overview. TDI 80 based slabstock formulations often have a very narrow processing

latitude and need critical control of foam porosity. Cell opening surfactants like ORTEGOL®

75 or cell openers like

ORTEGOL®

500 are helpful in this case. Sometimes also low potency standard surfactants are in use. TDI 65 based

formulations are less critical regarding porosity. Subsequently, it is possible to use standard ether surfactants. MDI based

formulations exist for slabstock as well as for molded applications, but the type of processing does not impact surfactant

selection. More important is the amount of physical stabilization. Formulations demanding strong stabilization are typically

produced with OH-functional comb-like polyether siloxane copolymers. Formulations which are almost self-stable are

typically produced with special siloxane oils that provide cell regulation with little added stability. Figure 20 summarizes the

overview.

Isocyanates for viscoelastic foams

TDI 80

Slabstock production

MDI TDI 65

Slabstock production Slabstock +

Molded production

Conventional

flexible foam

stabilizer

Cell opening surfactants:

ORTEGOL® 75

Cell openers:

ORTEGOL® 500

Low potency

standard surfactants

REQUIRED SURFACTANT TYPE

formulations requiring control

of cell opening and cell size,

but less stabilization:

Special siloxane oils

formulations requiring strong

stabilization:

OH-functional polyether

siloxanes

Figure 20: Surfactant types for viscoelastic polyurethane foam.

SUMMARY AND OUTLOOK

The influence of three different effects for the delayed recovery of viscoelastic foam types has been demonstrated by key

experiments. The importance of the adhesion for some viscoelastic foam types has been highlighted. It was possible to

produce viscoelastic foams just based on one of these effects. The role of the cell structure – cell fineness and porosity –

turned out to be of great importance for foam types associated with the pneumatic as well as the adhesion effect. Foam

stabilizers in general influence the cell structure and are subsequently crucial for the viscoelastic properties of many

viscoelastic foams. More sophisticated experiments using mechanical testing with different frequencies will be necessary to

quantify the different effects in future. A good correlation between the different effects and tailor made formulations is still

lacking, but this paper might be a good starting point to systematically investigate this opportunity for polyurethane flexible

foam industry.

ACKNOWLEDGEMENT

The authors thank Helmut Pelka for his laboratorial work.

REFERENCES

[1] S. Kintrup, J.P. Treboux, H. Mispreuve, "Low Resilience - High Performance Recent Advances in Viscoelastic Flexible

Slabstock Foam", Polyurethanes EXPO 2000

[2] S. Hager, R. Skorpenske, S. Triouleyre, F. Joulak, "New Technology for Viscoelastic Foam", Polyurethanes EXPO 2000

[3] Viscoelastic Foams, Urethanes Technology, vol. 18, no. 6, December 2001/ January 2002, p.22

[4] R. Landers, W. Knott, T. Boinowitz, New Cell Opening Strategies for TDI 80 Viscoelastic Foams by Additive Means,

Conference proceedings API 2005, October 17-19, Houston, Texas, p. 77-83

BIOGRAPHIES

Ruediger Landers

Dr. Ruediger Landers, born in 1973, studied Chemistry at the Technical University of Clausthal.

After receiving his PhD in Polymer Chemistry at the University of Freiburg he joined Degussa

Goldschmidt in 2004 as a technical manager for polyurethane additives. His main focus are

stabilizers for the polyurethane industry.

Roland Hubel

Dr. Roland Hubel, born in 1972, studied Chemistry at the Ludwig-Maximilians-University of

Munich. He received his PhD in Coordination- and Metal organic Chemistry. After working for one

year in the field of genetic engineering as civil servant he joined Degussa Goldschmidt

Polyurethane Additives as a research scientist in 2000. After that he was responsible for R&D of

polyethers and development and application technology of additives for microcellular foams. Today

he is globally responsible for development and application technology for Flexible Slabstock

Polyurethane Additives.

Rob Borgogelli

Rob Borgogelli received a BSc degree in Chemical Engineering from Queen’s University in

Kingston Ontario. In 1985, he joined Woodbridge Foam Corp. gaining experience in flexible

slabstock foam manufacturing and product development. He has been employed by Goldschmidt

Chemical Corporation since 1994, and is currently the Applied Technology Manager responsible for

polyurethane additives to the flexible slabstock industry in North America.