Evaluation of the Performance of Bio-Based Rigid Polyurethane Foam with High Amounts of Sunflower Press Cake Particles - MDPI
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materials
Article
Evaluation of the Performance of Bio-Based Rigid Polyurethane
Foam with High Amounts of Sunflower Press Cake Particles
Agnė Kairytė 1, * , Sylwia Członka 2 , Renata Boris 3 and Sigitas Vėjelis 1
1 Laboratory of Thermal Insulating Materials and Acoustics, Faculty of Civil Engineering, Institute of Building
Materials, Vilnius Gediminas Technical University, Linkmenu Str. 28, LT-08217 Vilnius, Lithuania;
sigitas.vejelis@vilniustech.lt
2 Institute of Polymer & Dye Technology, Lodz University of Technology, 90-924 Lodz, Poland;
sylwia.czlonka@dokt.p.lodz.pl
3 Laboratory of Composite Materials, Faculty of Civil Engineering, Institute of Building Materials, Vilnius
Gediminas Technical University, Linkmenu Str. 28, LT-08217 Vilnius, Lithuania; renata.boris@vilniustech.lt
* Correspondence: agne.kairyte@vilniustech.lt
Abstract: In the current study, rigid polyurethane foam (PUR) was modified with 10–30 wt.%
sunflower press cake (SFP) filler, and its effect on performance characteristics—i.e., rheology, char-
acteristic foaming times, apparent density, thermal conductivity, compressive strength parallel and
perpendicular to the foaming directions, tensile strength, and short-term water absorption by partial
immersion—was evaluated. Microstructural and statistical analyses were implemented as well.
During the study, it was determined that 10–20 wt.% SFP filler showed the greatest positive impact.
For instance, the thermal conductivity value improved by 9% and 17%, respectively, while mechanical
performance, i.e., compressive strength, increased by 11% and 28% in the perpendicular direction and
by 43% and 67% in the parallel direction. Moreover, tensile strength showed 49% and 61% increments,
respectively, at 10 wt.% and 20 wt.% SFP filler. Most importantly, SFP filler-modified PUR foams
Citation: Kairytė, A.; Członka, S.;
were characterised by two times lower water absorption values and improved microstructures with a
Boris, R.; Vėjelis, S. Evaluation of the
reduced average cell size and increased content in closed cells.
Performance of Bio-Based Rigid
Polyurethane Foam with High
Amounts of Sunflower Press Cake
Keywords: sunflower press cake; polyurethane foam; thermal insulation; sustainability; mechanical
Particles. Materials 2021, 14, 5475. performance
https://doi.org/10.3390/ma14195475
Academic Editor: Andrea Sorrentino
1. Introduction
Received: 4 August 2021 It is estimated that within 10 years, global energy consumption will increase by
Accepted: 20 September 2021
~50% [1]. In order to reduce energy demands and greenhouse gas emissions, the easiest
Published: 22 September 2021
way to do so is through thermal insulation of buildings. If suitably selected and installed,
thermal insulating layers can save energy for heating in winter and cooling in summer.
Publisher’s Note: MDPI stays neutral
Rigid polyurethane foams are characterised by the lowest thermal conductivity, which
with regard to jurisdictional claims in
varies from 0.018 W/(m·K) to 0.025 W/(m·K) when the traditional blowing agents are used,
published maps and institutional affil-
and from 0.033 W/(m·K) to 0.040 W/(m·K) when water is used as a chemical blowing
iations.
agent [2]. Therefore, this building material has becomes an extremely important cellular
plastic due to its versatility and the possibility of its application in various industries such
as those concerned with thermal insulating layers, bedding, furniture, automotive seats
and everywhere where other materials might not be suitable [3].
Copyright: © 2021 by the authors.
Polyurethane foams can be produced from mass polymerisation, which is the easiest
Licensee MDPI, Basel, Switzerland.
technique for polyurethane foam synthesis and solely uses monomers such as polyols,
This article is an open access article
isocyanates and physical or chemical blowing agents [4]. Currently, these main raw ma-
distributed under the terms and
terials have become unpopular because they are based on petrochemical resources and
conditions of the Creative Commons
this has prompted the polyurethane industry and scientific community to develop greener
Attribution (CC BY) license (https://
raw materials such as polyols from various natural or waste plant oils [5–7], and iso-
creativecommons.org/licenses/by/
4.0/).
cyanates from fatty or amino acids [8], while ozone depletion and global warming have
Materials 2021, 14, 5475. https://doi.org/10.3390/ma14195475 https://www.mdpi.com/journal/materials
Materials 2021, 14, 5475 2 of 12
caused the development of blowing agents based on hydrofluoroolefins and water [9,10]
and the implementation of natural resources as fillers in the form of fibres [11,12] or par-
ticulates [13,14]. The application of these sustainable raw materials contributes to the
reduction of greenhouse gas emissions, increased environmental conservation, stability
and sustainability, and promotes convenient and cost-efficient ways to produce bio-based
polyurethane composites.
Currently, there are several studies on the use of natural fillers in polyurethane foam
composites. For instance, the authors of [15] investigated the effect of coconut husk and
corn cob fillers on the apparent density, mechanical performance, and thermal conductivity
of polyurethane foam. To demonstrate the impact of these fillers, they varied the amount
of fillers from 5% to 25% and noted that the apparent density, thermal conductivity, and
compression set improved, while tensile strength and elongation at break had significant
reductions. However, another natural filler—nutmeg particles at amounts of (1–5%) incor-
porated into polyurethane foam—revealed that mechanical performance increased only up
to a certain limit, i.e., up to 2% of nutmeg filler, and thermal conductivity deteriorated at all
amounts of filler, although fire resistance, and antibacterial and anti-ageing properties of
modified polyurethane foams were improved [16]. Disregarding the positive or negative
impacts on apparent density and compression set or compressive strength of natural filler-
reinforced polyurethane foams, the study [17] showed that the addition of hazelnut and
walnut shells improved the thermal stability of the resulting products, while [18] revealed
that chemically modified lignin used as a natural filler in polyurethane foams decreased
thermal degradation at low temperatures but increased it at higher ones. Moreover, [19]
highlighted that 2% of sunflower press cake particles (SFP) improved the thermal stability
of polyurethane foam at all degradation stages but increased thermal conductivity by 4%
and compressive strength and flexural strength by 8% and 3%, respectively. The authors
thought it would be an interesting idea for press cakes to be used as natural fillers for
the modification of polyurethane foams, since SFP is a by-product of sunflower oil pro-
duction and it contains components which are insoluble in water. According to Ancuţa
and Sonia [20], press cakes can be used to feed animals because they are rich in proteins,
cellulose, and hemicellulose. Regrettably, due to antinutritional factors such as phytic
acid and trypsin inhibitors, press cakes may negatively affect the health of animals and
humans. Another problem is landfilling, during which waste decomposes, thus inducing
the production of methane and the pollution of water. Furthermore, press cakes can be
converted into energy through incineration. However, it is not cost-effective and negatively
impacts environmental emissions.
The aim of this study is to investigate and evaluate the impact of high amounts of
crushed SFP on rigid polyurethane (PUR) foam obtained from rapeseed oil- and sucrose-
based polyols and water as a blowing agent. Moreover, rheological and structural character-
isation, foaming process evaluation, and testing of thermal insulation, water resistance and
mechanical performance were conducted, and the statistical interpretation of the results
was implemented.
2. Materials and Methods
Polyol BioPolyol RD was obtained from rapeseed oil (SIA PolyLabs, Riga, Latvia) and
Petol PZ 400-4G from sucrose (Oltchim, Râmnicu Vâlcea, Romania). They were used as
the main components for the production of PUR foam and SFP filler-modified PUR foams.
Polyol from rapeseed oil has the hydroxyl value of 350 mg KOH/g, while the sucrose-based
oil has a value of 421 mg KOH/g. Water contents of each polyol were
factant ST-52 (Shijiazhuang Chuanghong Technology Co., Ltd., Shijiazhuang, China) was
added. The SFP filler was supplied by a local company (Vilnius, Lithuania).
The selected amounts of raw materials used for the preparation of PUR foam and SFP
filler-modified PUR foams are presented in Table 1 and the graphical preparation scheme
in Figure 1. The PUR foam and SFP filler-modified PUR foams were produced by a two-
Materials 2021, 14, 5475 3 of 12
component system which had an equivalent ratio of isocyanate and hydroxyl groups
equal to 1.25:1. As 100% rapeseed oil-based polyol does not produce dimensionally stable
foams, its amount was reduced to 40%. In order to obtain stable PUR foams, 60% of su-
crose-based polyol
(Shijiazhuang was incorporated.
Chuanghong Technology Co., Ltd., Shijiazhuang, China) was added. The
SFP filler was supplied by a local company (Vilnius, Lithuania).
TableThe
1. Formulations of PUR foam
selected amounts andmaterials
of raw SFP filler-modified
used forPUR
the foams.
preparation of PUR foam and
SFP filler-modified
Raw MaterialPUR foams are presented in Table 1 and the graphical
Content, Pbw 1 preparation
scheme in Figure 1. The PUR foam and SFP filler-modified PUR foams were produced
BioPolyol RD 40
by a two-component system which had an equivalent ratio of isocyanate and hydroxyl
Petol PZ 400-4G 60
groups equal to 1.25:1. As 100% rapeseed oil-based polyol does not produce dimensionally
Water (distilled) 2.7
stable foams, its amount was reduced to 40%. In order to obtain stable PUR foams, 60% of
Polycat 9 (catalyst) 1
sucrose-based polyol was incorporated.
ST-52 (surfactant) 3
SFP filler, wt.% 0;
Table 1. Formulations of PUR foam and SFP filler-modified PUR foams. 10; 20; 30
Isocyanate index 1.25
1 Raw of
Parts by total weight Material
polyols. Content, Pbw 1
BioPolyol RD 40
The system of two
Petol polyols was mixed with water, catalyst, and
PZ 400-4G 60surfactant, and thor-
oughly mixed Water at a speed of 1800 rpm for 1 min. Before the addition
(distilled) 2.7 of SFP filler, it was
Polycatwith
crushed and milled 9 (catalyst) 1
a laboratory shredder for 1 min and thermally treated at a tem-
perature of 110ST-52 (surfactant)
°C for 3
24 h to remove the excess of moisture. After that, 10 wt.%, 20 wt.%,
SFP filler, wt.%
and 30 wt.% of SFP filler were added into the prepared mixture.0;The
10; 20; 30
obtained premix was
Isocyanate index 1.25
thoroughly mixed with isocyanate
1 Parts by total weight of polyols.
for 10 s at 1800 rpm. The obtained viscous mass was
immediately poured into moulds and left to cure at a temperature of (23 ± 2) °C.
Figure 1.Figure 1. Graphical
Graphical representation
representation of the preparation
of the preparation schemescheme of (a)
of: (a) SFP SFPand
filler filler-modified (b) PUR foams.
(b) SFP filler-modified PUR foams.
The
The system
moisture ofcontent
two polyols was mixed
of particles with water,after
was determined catalyst,
dryingandthree
surfactant,
samplesand thor-
at (105
oughly mixed
± 2) °C until atchange
the a speedinofmass
1800between
rpm forthree
1 min. Before the
weighings made addition of SFP
24 h apart wasfiller,
lessitthan
was
crushed
0.1%, in and milled with the
accordance a laboratory shredder
requirements for by
outlined 1 min
[21].and thermally treated at a tem-
perature
The of 110 ◦ C for
structural 24 h toofremove
studies the excess
SFP filler, of moisture.
neat PUR foam and After
SFPthat, 10 wt.%, 20 PUR
filler-modified wt.%,
and 30 were
foams wt.% conducted
of SFP fillerusing
werescanning
added into the prepared
electron microscopymixture.
(SEM) The obtained
JEOL premix
SM–7600F (JEOLwas
thoroughly
Ltd., Tokyo,mixed
Japan).with isocyanate
Before for 10
the analysis ofsthe
at 1800 rpm.the
samples, The obtained
filler viscous
and foams weremass
coatedwas
immediately poured
with a gold layer into vacuum
under moulds andwithleft
thetoQUORUM
cure at a temperature
Q150R ES (Quorum 2) ◦ C.
of (23 ± Technologies
Ltd.,The moisture
Lewes, Unitedcontent
Kingdom) of apparatus.
particles wasThedetermined
obtained imagesafterwere
drying usedthree samples
to further ana-at
lyse ±
(105 2) ◦ C until
structural the change
parameters suchinasmass between
average three
cell size weighings
and the size of made
filler24 h apartwith
particles was
less thansoftware.
ImageJ 0.1%, in accordance with the requirements outlined by [21].
The structural studies of SFP filler, neat PUR foam and SFP filler-modified PUR foams
were conducted using scanning electron microscopy (SEM) JEOL SM–7600F (JEOL Ltd.,
Tokyo, Japan). Before the analysis of the samples, the filler and foams were coated with a
gold layer under vacuum with the QUORUM Q150R ES (Quorum Technologies Ltd., Lewes,
UK) apparatus. The obtained images were used to further analyse structural parameters
such as average cell size and the size of filler particles with ImageJ software.
The test of dynamic viscosity was made using an SV-10 viscometer (A&D Company
Ltd., Tokyo, Japan), which has an accuracy of 0.01 mPa·s and a measuring range of up to
1200 mPa·s.
Materials 2021, 14, 5475 4 of 12
Materials 2021, 14, 5475 The test of dynamic viscosity was made using an SV-10 viscometer (A&D Company 4 of 12
Ltd., Tokyo, Japan), which has an accuracy of 0.01 mPa·s and a measuring range of up to
1200 mPa·s.
The
The characteristic
characteristicfoaming
foamingtimestimesand andreaction
reactionprofile were
profile were determined
determined according
according to
[22], Annex E. Before the test, all components were conditioned at
to [22], Annex E. Before the test, all components were conditioned at a temperature of a temperature of (20 ±
1) °C. The◦ measurements were conducted using an electronic stopwatch
(20 ± 1) C. The measurements were conducted using an electronic stopwatch with an with an accuracy
of 0.5 s. of 0.5 s.
accuracy
The
The linear
lineardimensions
dimensionsofofthe thesamples
sampleswere weredetermined
determined based
based onon thethe
requirements
requirements of
[23], while the apparent density was determined according to [24]
of [23], while the apparent density was determined according to [24] for 50 × 50 × 50 mm-for 50 × 50 × 50 mm-
sized
sized samples.
samples.
Thermal
Thermal conductivity
conductivity measurements
measurements were conducted for
were conducted for 300
300 × × 300
300 × × 50 mm-sized
50 mm-sized
samples, according to [25], with a FOX 304 (TA Instruments, United
samples, according to [25], with a FOX 304 (TA Instruments, United States of America) States of America)
heat
heat
flowflow
meter,meter,
whichwhich
has an has an active
active edgeedge insulation.
insulation. DuringDuring the the
the test, test,direction
the direction
of heatof flow
heat
flow was upwards.
was upwards.
The
The determination
determination of the the volumetric
volumetric percentage of closed cells was carried out based
on [26], method
on [26], method 2 2 for 100 ×
× 30 ×3030mm-sized
30 × mm-sizedsamples.
samples.
To
To determine the impact of water on the neat
neat PUR
PUR foam
foam and
and SFPSFP filler-modified
filler-modified PUR
foam,
foam, short-term
short-term water
water absorption
absorption by partial immersion was chosen according according to to ([22],
[22],
Table 77)andandcarried
carriedoutout according
according to to
[27] (officially
[27] replacing
(officially [28])
replacing for for
[28]) samples having
samples havingthe
size of 200
the size 200×× 50
of×200 × 50After
200mm. mm.the test,the
After samples were drained
test, samples for 10 min
were drained forusing a drainage
10 min using a
drainage stove.
stove.
The compressive
The compressive strength
strength and tensile strength were obtained in accordance with [29]
and
and [30], respectively for the
the samples
samples having
havingthe thesize
sizeof 50×× 50 ×
of50 50 mm
× 50 mm withwith the
the H10KS
(Hounsfield, United Kingdom) universal testing machine.
3. Results
3. Results and
and Discussion
Discussion
Figure 22 presents
Figure presents the
the external
external appearance
appearance of of SFP
SFP filler.
filler. The
The microscopic
microscopic investigation
investigation
revealed the presence of micron-sized particles with a wide granulometric distribution
revealed the presence of micron-sized particles with a wide granulometric distribution of of
2–695 µm. It can also be observed that the filler particles are not homogeneous;
2–695 μm. It can also be observed that the filler particles are not homogeneous; some of some of
them are
them are agglomerated
agglomerated and and have
have aa rough
rough surface,
surface, which
which inin turn
turn may
may positively
positively affect
affect the
the
adhesion ability between the SFP filler and polyurethane
adhesion ability between the SFP filler and polyurethane matrix. matrix.
Figure2.2.SFP
Figure SFPfiller:
filler: (a)
(a) at
at ×
×40
40 magnification
magnification and
and (b)
(b) at
at ×60
×60magnification.
magnification.
The
The moisture
moisture content
content of of fillers
fillers in
in PUR
PUR foam
foam compositions
compositions is is of
of great
great importance
importance due
due
to the mixture’s
to the mixture’s sensitivity to water and/or moisture during reaction
and/or moisture during reaction with isocyanates. The
parameter
parameter is is too
too important
important to to neglect
neglect its impact
impact on foaming and apparent density of the
final products. Therefore,
final products. Therefore,the theexperimental
experimentalinvestigation
investigation showed
showed that
that thethe original
original mois-
moisture
ture content
content of SFPof SFP
fillerfiller particles
particles waswas8.4 ±8.40.3
± 0.3 wt.%
wt.% andand it confirms
it confirms thethe results
results obtained
obtained in
in other
other studies
studies [31,32],
[31,32], i.e.,i.e.,
upup to 11to wt.%.
11 wt.%. However,
However, after
after the the thermal
thermal treatment
treatment ◦ C,
at 110
at 110
the moisture content was reduced to 1.1 ± 0.2 wt.%. According to the results presented
in Table 2, the addition of SFP filler greatly impacts the characteristic foaming times of
polyurethane mixtures.
Materials 2021, 14, 5475 5 of 12
Table 2. Parameters of the foaming mixtures.
SFP Filler, wt.%
Parameter
0 10 20 30
Dynamic viscosity mPa·s 115 ± 5 130 ± 3 275 ± 4 400 ± 6
Cream time, s 25 ± 2 31 ± 3 40 ± 4 53 ± 3
Gel time, s 53 ± 3 63 ± 2 72 ± 3 89 ± 3
Tack-free time, s 76 ± 2 96 ± 3 112 ± 2 136 ± 3
Compared to the neat PUR foam, the cream time increases by 24%, 60% and 112%,
gel time by 19%, 36% and 68%, and tack-free time by 26%, 47% and 79%, all at 10 wt.%,
20 wt.% and 30 wt.% SFP filler, respectively. It was previously observed by [19] that the
presence of hydroxyl groups of SFP filler can interrupt the stoichiometry of synthesis due
to the reaction between OH groups of SFP filler and NCO groups of isocyanate. As a
result, the higher content of NCO groups is consumed and a reduced amount of CO2 is
generated, thus slowing down the foaming process. The changes in characteristic foaming
times can also be attributed to a higher dynamic viscosity of PUR foam mixtures with a
higher content of SFP filler because it impedes the expansion and formation of larger cells.
Very similar trends were observed in [33]’s study, where the authors incorporated silanised
and non-treated plum stone filler into PUR foam compositions. Although, some natural
raw materials such as natural cork have the opposite effect and reduce cream and gel
times by 28% and 20%, respectively, and this phenomenon is attributed to the low thermal
conductivity of cork agglomerates [34].
Compared to PUR foam without the addition of SFP filler, the incorporation of
10–30 wt.% of SFP increases the dynamic viscosity of the polyol premixes. The highest
increase has been noticed in the case of premixes containing 30 wt.% SFP filler. The param-
eter increases rapidly from 115 mPa·s to 400 mPa·s. The result may be explained by the
fact that SFP filler particles tend to agglomerate and form coarse aggregates.
The addition of SFP filler to PUR foams resulted in an apparent density increase
(Figure 3a). From an industrial point of view, modified PUR foams at 10 wt.% SFP filler
content have a favourable apparent density value, which is 5% higher compared to the neat
PUR foam. However, 20–30 wt.% SFP filler contents lead to a 69% and 120% increase in
apparent density. The growth of apparent density values is evident in almost all inorganic
and organic filler-modified PUR foams. For instance, [35] reported that the addition of
eucalyptus fibre in a range from 8 wt.% to 16 wt.% in PUR foams led to a 10–28% increase in
apparent density value. Moreover, [36] obtained a 78% increase in apparent density of PUR
foams modified with 20 wt.% of ground tire rubber compared to the neat PUR foam. The
magnitude of the increase in apparent density is basically related to filler characteristics
and mixture formulations, i.e., the bulk density of the filler, filler content in the premixes,
moisture content in the filler and, of course, whether the moisture content is considered
while preparing premixes or not, as the extra moisture/water will cause an additional
expansion of the foaming mixture.
The change in apparent density of SFP filler-modified PUR foams may also be described
by the regression equation (Equation (1)) with determination coefficient R2SFP = 0.973 and
standard deviation SSFP = 3.46 kg/m3 .
ρSFP = 37.105 + 0.3115 · mSFP + 0.04525 · m2SFP (1)
where ρSFP —apparent density of SFP filler-modified PUR foams, kg/m3 , and mSFP —SFP
filler content, wt.%.
Materials 2021, 14, 5475 6 of 12
Materials 2021, 14, 5475 6 of 12
Dependence of
Figure 3. Dependence of properties
properties on
on the
the amount
amount ofof SFP
SFP filler:
filler: (a)
(a) apparent
apparent density
densityand
and(b)
(b) thermal
thermal conductivity. •—
conductivity. •—
individual
individual values
values of
of thermal
thermal conductivity
conductivity atat specified amount of SFP filler; Continuous line—dependence line; Dotted
line—upper
line—upper limit
limit at
at 90%
90% confidence
confidence level.
level.
Thermal
Thermal conductivity
conductivity is is one
one of of the
the most
most important
important parameters
parameters for for thermal
thermal insulating
insulating
materialsand
materials andisismainly
mainly determined
determined byby morphological
morphological features
features andand apparent
apparent densitydensity
(Ta-
(Table
ble 3). Therefore,
3). Therefore, it great
it is of is of great
interest interest
to gaintoknowledge
gain knowledge on howon how natural
natural resources resources
impact
impact
the the mentioned
mentioned parameter. parameter.
It has been It has been reported
reported that the incorporation
that the incorporation of variousof various
cellulosic
cellulosic fillers in PUR foams increases thermal conductivity
fillers in PUR foams increases thermal conductivity [37,38]. However, different observa- [37,38]. However, different
observations
tions were conducted
were conducted with SFP with SFP filler-modified
filler-modified PUR foams PUR (Figure
foams (Figure
3b). The 3b). The average
average value
value
of of thermal
thermal conductivity
conductivity for thefor neatthePURneatfoam
PURisfoam0.0354is 0.0354
W/(m·K),W/(m ·K), is
which which
similaris similar
to tra-
to traditional
ditional thermalthermal insulating
insulating materials
materials such such as expanded
as expanded polystyrene,
polystyrene, stonestone
wool, wool, or
or ex-
extruded
truded polystyrene.
polystyrene.
The advantage of SFP incorporation can be noticed at 10 wt.% and 20 wt.% contents
Structural
Table 3.reduce
which parameters
thermal of SFP filler-modified
conductivity values by 9%PUR andfoams.
17%, respectively. Such a result may
be explained by the increasing content of closed cells and reduced average cell size (Table
Amount of SFP Filler, wt.%
3). Even though
Parameter SFP filler-modified water-blown PUR foams have higher values of ther-
0 10 20 30
mal conductivity compared to traditional hydrofluorocarbons-blown PUR foams, it is
Average cell size, µm 566 ± 12 516 ± 24
worth mentioning that the use of plant-based fillers and polyols increases the renewable 364 ± 15 272 ± 22
Closedofcell
content thecontent,
resultant vol.%products81while ± 2 water, which87 ± 2 is used as 90 ±1
a blowing agent,85does± 4 not
contribute to the depletion of the ozone layer. However, further studies on alternative
blowingThe agents,
advantage suchofas SFPfourth generation
incorporation olefin-based
can be noticed at ones, may and
10 wt.% result
20 in even
wt.% lower
contents
thermal conductivity values.
which reduce thermal conductivity values by 9% and 17%, respectively. Such a result
mayFigure 4b andby
be explained Figure 4c show that
the increasing the addition
content of closed of 10–20
cells andwt.%
reduced SFPaverage
filler leads
cell to
sizea
more
(Tableuniform
3). Evenstructure;
though SFP on filler-modified
average, it increases the content
water-blown PURof closed
foams havecells from values
higher 81 vol.% of
to 87 vol.%,
thermal respectively,
conductivity and reduces
compared the cellhydrofluorocarbons-blown
to traditional size by 9% and 36% compared PUR to the neat
foams, it is
PUR
worth foam. A higher
mentioning content
that the use of of
closed cells in fillers
plant-based the case andofpolyols
SFP filler-modified
increases thePUR foams
renewable
content
can of the resultant
be associated products while
with sufficient water,
interfacial which isbetween
adhesion used as athe blowing agent, does
filler surface and thenot
contribute to the depletion of the ozone layer. However, further
polymer matrix. In addition, the authors of [35] and [39] have concluded that average cell studies on alternative
blowing
size has a agents,
great impactsuch on as fourth
the totalgeneration olefin-basedvalue
thermal conductivity ones,ofmay result materials,
composite in even lower and
thermal conductivity
alteration values.
in the cell morphology can also be related to the ability of fillers to act as gas
Figures
nucleation 4b during
sites and 4c foaming
show that andthetheir
addition
abilityofto10–20 wt.% to
contribute SFP thefiller leads to
formation ofanucle-
more
uniform
ating structure;
centres for the on average,
gaseous it increases the content of closed cells from 81 vol.% to
phase.
87 vol.%, respectively, and reduces the cell size by 9% and 36% compared to the neat PUR
foam. A higher content of closed cells in the case of SFP filler-modified PUR foams can be
associated with sufficient interfacial adhesion between the filler surface and the polymer
matrix. In addition, the authors of [35] and [39] have concluded that average cell size has a
great impact on the total thermal conductivity value of composite materials, and alteration
in the cell morphology can also be related to the ability of fillers to act as gas nucleation
became similar to the one at 10 wt.% SFP filler content. Even though the average cell size
at such a filler content was reduced by 52% compared to the neat PUR foam, the micro-
structure (Figure 4d) became non-uniform, and large voids and destruction of the cellular
structure were evident. According to [40], this is very common in composite foams be-
Materials 2021, 14, 5475 cause of the increase in viscosity in the prepolymerisation stage, which promotes the 7 ofco-
12
hesion of cells during curing and leads to an increase in thermal conductivity value com-
pared to modified PUR foams with 10–20 wt.% SFP filler contents. The obtained thermal
conductivity, closed cell content, and average cell size results show that in all cases SFP
sites during foaming and their ability to contribute to the formation of nucleating centres
filler improves the structure, its parameters, and the overall thermal performance of mod-
for the gaseous phase.
ified PUR foams.
Figure 4.
Figure 4. Microstructure of SFP
Microstructure of SFP filler
filler modified
modified PUR
PUR foams
foams((×50
×50 magnification):
magnification): (a)
(a) at
at 00 wt.%;
wt.%; (b)
(b) 10
10 wt.%;
wt.%; (c) 20 wt.%
(c) 20 wt.% and
and
(d) 30 wt.%.
(d) 30 wt.%.
Furthermore,
However, 30 wt.%it is evident that
SFP filler the results
started of thermal
to increase conductivity
the thermal of modified
conductivity valuePUR
and
foams is dependent
it became on the
similar to the onecontent
at 10 wt.%of SFP
SFPfiller.
filler Therefore, it isthough
content. Even of greatthe
importance to
average cell
evaluate the strength
size at such of theirwas
a filler content impact on theby
reduced final
52%PUR foams. Test
compared results
to the neatindicate that the
PUR foam, the
microstructure (Figure 4d) became non-uniform, and large voids and destruction of the
cellular structure were evident. According to [40], this is very common in composite foams
because of the increase in viscosity in the prepolymerisation stage, which promotes the
cohesion of cells during curing and leads to an increase in thermal conductivity value
compared to modified PUR foams with 10–20 wt.% SFP filler contents. The obtained
thermal conductivity, closed cell content, and average cell size results show that in all cases
SFP filler improves the structure, its parameters, and the overall thermal performance of
modified PUR foams.
Furthermore, it is evident that the results of thermal conductivity of modified PUR
foams is dependent on the content of SFP filler. Therefore, it is of great importance to
evaluate the strength of their impact on the final PUR foams. Test results indicate that
the thermal conductivity of SFP filler-modified PUR foams may be approximated by the
regression equation (Equation (2)) presented below.
λSFP = 0.035360 − 0.000178 · mSFP − 0.000022 · m2SFP + 0.000001 · m3SFP (2)
thermal conductivity of SFP filler-modified PUR foams may be approximated by the re-
gression equation (Equation (2)) presented below.
2 3
Materials 2021, 14, 5475 SFP 0.035360 0.000178 mSFP 0.000022 mSFP 0.000001 mSFP 8 of(2)
12
where SFP —thermal conductivity of SFP filler-modified PUR foams, W/(m·K), and
mSFP —SFP filler content, wt.%.
where λSFP
The —thermal
resulting dataconductivity
show that of theSFP filler-modified
determination PUR foams,
coefficients forW/(m ·K), and mequa-
the presented SFP —
SFP filler content,
2 wt.%.
tion are R SFP 0.936 , the standard deviation— S 0.000615 , and Student’s criterion
The resulting data show that the determination coefficients for the presented equation
SFP
t R12.325=and
are SFP
the
0.936, 0.90standard
. deviation—SλSFP = 0.000615, and Student’s criterion
tα = The
1.325obtained
and α = determination
0.90. coefficients indicate the suitability of the regression
model Theforobtained
the proper descriptioncoefficients
determination of thermalindicate
conductivity values of
the suitability oftheSFP filler-modified
regression model
PUR
for thefoams.
proper description of thermal conductivity values of SFP filler-modified PUR foams.
The mechanical
The mechanical performance
performance of of polymeric
polymeric porous porous materials
materials is is influenced
influenced by by a few
factors such as closed
factors closed cellcellcontent,
content,apparent
apparentdensitydensityofofthe thefinal product,
final product, and andfiller topology,
filler topol-
i.e., its
ogy, ability
i.e., to effectively
its ability adhere
to effectively to theto
adhere polymer matrix.
the polymer Compared
matrix. Comparedto neattoPUR neatfoam,
PUR
SFP filler-modified PUR foams showed a noticeable improvement
foam, SFP filler-modified PUR foams showed a noticeable improvement in compressive in compressive strength
in the perpendicular
strength and parallel
in the perpendicular anddirections (Figure 5).(Figure
parallel directions When it 5).came
When to it10came
wt.%toSFP, there
10 wt.%
was athere
SFP, slight wasincrease
a slightinincrease
the compressive strength in
in the compressive the perpendicular
strength direction,direction,
in the perpendicular i.e., 11%,
while
i.e., improvement
11%, while improvementin the parallel
in thedirection was 43% was
parallel direction compared to unfilled
43% compared to PIR foam.
unfilled A
PIR
furtherAincrease
foam. in SFP filler
further increase in SFP content
fillerto 20 wt.%
content to strengthened PUR foams
20 wt.% strengthened PUR byfoams
28% and 67%
by 28%
in the
and 67%perpendicular and parallel
in the perpendicular anddirections, respectively.respectively.
parallel directions, Some studies Some[41–43] have[41–43]
studies shown
that up to 20 wt.% of certain fillers in polyurethane foam systems
have shown that up to 20 wt.% of certain fillers in polyurethane foam systems is the con- is the content limit at
which the mechanical performance of filled PUR foams improves,
tent limit at which the mechanical performance of filled PUR foams improves, and further and further addition of
filler is not
addition favourable.
of filler However, contrary
is not favourable. However,observations were conducted
contrary observations at 30 wt.%atSFP,
were conducted 30
which significantly increased the compressive strength
wt.% SFP, which significantly increased the compressive strength in both directions, in both directions, i.e., 114%i.e.,in
the perpendicular
114% in the perpendicularand 71% in 71%
and the parallel directions.
in the parallel The obtained
directions. resultsresults
The obtained indicate great
indicate
compatibility between SFP filler and PUR matrix. As [44] reported,
great compatibility between SFP filler and PUR matrix. As [44] reported, the addition of the addition of natural
fillers impacts
natural the crosslink
fillers impacts densitydensity
the crosslink of PURoffoams due todue
PUR foams hydroxyl groups
to hydroxyl of lignin
groups and
of lignin
cellulose. They react with isocyanate groups by forming a more
and cellulose. They react with isocyanate groups by forming a more compact structure in compact structure in PUR
foamsfoams
PUR and increasing the number
and increasing of urethane
the number groups, groups,
of urethane which provide
which additional crosslink
provide additional
sites and increase the number of hard segments. Additionally, the more compact structure
crosslink sites and increase the number of hard segments. Additionally, the more compact
between SFP filler and PUR foam matrix assures an efficient stress transfer from matrix to
structure between SFP filler and PUR foam matrix assures an efficient stress transfer from
filler during compression.
matrix to filler during compression.
Figure 5. Compressive strength
strength of
of SFP
SFPfiller-modified
filler-modifiedPUR
PURfoams:
foams:(a)
(a)perpendicular
perpendiculartotofoaming direction
foaming and
direction (b)(b)
and parallel to
parallel
to foaming
foaming direction.
direction.
The tensile
The tensilestrength
strengthtest
testofofthe
theSFP
SFP filled
filled PUR
PUR foams
foams was
was conducted
conducted to investigate
to investigate the
the PUR-SFP interaction. The interface between filler and matrix plays an important
PUR-SFP interaction. The interface between filler and matrix plays an important role role
in
in the mechanical properties of all composite materials because the load transfer to the
dispersed phase occurs at the interface [45]. The results of SFP filler-modified PUR foams
can be observed from Figure 6. It can be clearly seen that the addition of up to 30 wt.% SFP
improves the tensile strength of modified PUR foams. The incorporation of 10 wt.% SFP
shows a 49% higher improvement of tensile strength compared to neat PUR foam while at
20 wt.% and 30 wt.%, and the tensile strength increased by 61% and 89%, respectively. It isthe
themechanical
mechanicalproperties
properties of ofall
allcomposite
compositematerials
materialsbecause
becausethe theload
loadtransfer
transferto tothe
thedis-
dis-
persed
persedphasephaseoccurs
occursat atthe
theinterface
interface[45].
[45].The
Theresults
resultsofofSFP
SFPfiller-modified
filler-modifiedPUR PURfoams
foamscancan
be
be observed
observed from
from Figure
Figure 6.6. ItIt can
can be
be clearly
clearly seen
seen that
that the
the addition
addition of of up
up toto 30
30 wt.%
wt.% SFP
SFP
improves
improvesthe thetensile
tensile strength
strengthof ofmodified
modified PURPURfoams.
foams. TheTheincorporation
incorporationof of1010 wt.%
wt.%9 SFP
SFP
Materials 2021, 14, 5475 of 12
shows a 49% higher improvement of tensile strength compared
shows a 49% higher improvement of tensile strength compared to neat PUR foam while to neat PUR foam while
at
at20 20wt.%
wt.%and
and30 30wt.%,
wt.%,and andthethetensile
tensilestrength
strengthincreased
increasedby by61%
61%andand89%,89%,respectively.
respectively.
ItItisisclearly
clearlyvisible
visiblethat
thatSFP
SFPfiller
fillerparticles
particlesare
areattached
attachedto tothe
thecell
cellwalls.
walls.Some
Someof ofthem
themareare
clearly
also
also visible that
detectable
detectable in SFP
in the
the filler
cell
cell voidparticles
void and are attached
and aa coarse
coarse surface
surfacetocan
the be
can cell walls.
be seen
seen in Some
in the ofstruts
the cell
cell them (Figure
struts are also
(Figure
detectable in the cell void and a coarse surface can be seen in the cell struts (Figure 6b).
6b).
6b).
Figure
Figure6.6.Tensile
Tensile strength of SFP filler-modified PUR foams and SFP filler interaction with polyurethane foam matrix: (a)
Figure 6. Tensile strength
strengthof ofSFP
SFPfiller-modified
filler-modifiedPURPURfoams
foamsandandSFP
SFPfiller interaction
filler with
interaction polyurethane
with foam
polyurethane matrix:
foam (a)
matrix:
tensile
tensilestrength;
strength;(b)
(b)SFP
SFPfiller
fillerparticle
particleafter
aftertensile
tensilestrength
strengthtest
test(×200
(×200magnification).
magnification).
(a) tensile strength; (b) SFP filler particle after tensile strength test (×200 magnification).
As
Asaaaresult,
As result,the
result, theimprovements
the improvementsare
improvements areattributed
are attributedto
attributed tothree
to threefundamental
three fundamentalreasons
fundamental reasonsconcluded
reasons concluded
concluded
inin the study conducted by [46]. The first reason is that, during tensile loading, the
in the
the study
study conducted
conducted by
by [46].
[46]. The
The first
first reason
reason is
is that,
that, during
during tensile
tensile loading,
loading, the filler
the filler
filler
particles in the PUR foams improve the restriction of the mobility
particles in the PUR foams improve the restriction of the mobility of polymeric chains [47].
particles in the PUR foams improve the restriction of the mobility of
ofpolymeric
polymeric chains
chains [47].
[47].
The
The second
The second reason
second reason isis
reason is that
that the
that the addition
the addition of
addition of SFP
of SFP filler
SFP filler to
filler to PUR
to PUR foams
PUR foams helps
foams helps to
helps to increase
to increase the
increase the
the
resistive
resistive surface
resistive surface area
surfacearea which,
areawhich, consequently,
which,consequently,
consequently, increases
increases
increases the capacity
thethe
capacity of tensile
of tensile
capacity loading
loading
of tensile in SFP-
in SFP-
loading in
filled
filled PUR
PURPUR
SFP-filled foams
foams [48]. The
Thethird
[48].[48].
foams reason
thirdthird
The reason isisthat
reason is at
that atlower
that lower filler contents,
fillerfiller
at lower contents, the
theinteraction
contents, interaction
the be-
interactionbe-
tween
tween
between PUR
PUR PUR foam
foam
foamand
andand SFP
SFPSFPfiller
filler isis more
filler more
is more effective,
effective, and
and it
effective, it forms
and forms aa high
it forms higha matrix-filler
matrix-filler
high rein-
rein-
matrix-filler
forcement
forcement
reinforcement to
toresist
resist loading
loading
to resist force [49].
[49].These
forceforce
loading These three
[49]. Thesethreereasons
reasons
three explain
explain
reasons the
theimprovement
explain improvement
the improvement which
which
iswhich
isshown
shown in
inFigure
is shownFigurein 6a.
6a.
Figure 6a.
Water
Water absorptionisis aa vital
absorption vital property
property for for materials
materials which
materials which will will have
have direct
direct ororindirect
or indirect
indirect
exposure
exposureto towater.
to water.Short-term
Short-termwater
Short-term water absorption
waterabsorption
absorptionby by partial
bypartial
partial immersion
immersion
immersion waswas
was chosen
chosen
chosen accord-
accord-
according
ing
ing to
to thetothe
therequirements
requirements
requirements of
of theofthe
theharmonised
harmonised
harmonised factory-made
factory-made
factory-made PUR PUR
PUR products
products
products standard[22][22]
standard
standard andand
[22] and
the
the
the results
are are
results
results arepresented
presented
presented in
inFigure
in Figure 7. 7.7.
Figure
Figure 7. Short-term water absorption by partial immersion of SFP filler-modified PUR foams.
As the results show, SFP filler considerably improves water absorption of the resulting
PUR foams. Interestingly, the reduction is two times higher compared to the neat PUR foam.
Firstly, as sunflower press cake is a concentrated fodder which is obtained by pressing
sunflower oil, the remaining oil (1.1–5.9 wt.% [50]) in SFP filler acts as a barrier for waterMaterials 2021, 14, 5475 10 of 12
to penetrate the particles. Secondly, the tensile strength results (Figure 6) show that the
filler-matrix interaction is sufficient and no voids or cracks form at the interface and around
the contact zone of the SFP filler and PUR foam matrix. Lastly, the reduction in water
absorption may be attributed to an increased closed cell content of SFP filler modified PUR
foams, as well (Table 3). The obtained short-term absorption values also indicate that SFP
filler-modified PUR foams are competitive with the ones that already exist in the market
(up to 0.3 kg/m2 ).
4. Conclusions
Polyurethane foam was modified with 10–30 wt.% sunflower press cake filler. The
impact of sunflower press cake filler on selected properties of PUR foams, such as rheology
(dynamic viscosity, characteristic foaming times), thermal insulating properties (thermal
conductivity), mechanical properties (tensile strength and compressive strength perpen-
dicular and parallel to the foaming directions) and water resistance (short-term water
absorption by partial immersion) were investigated. It was determined that the greatest
improvement was achieved for PUR foams with 20 wt.% SFP. The thermal conductivity was
reduced by 17% at an average apparent density value of 65 kg/m3 , while short-term water
absorption by partial immersion results showed that the parameter was twice reduced at
20 wt.% SFP. Mechanical performance was significantly improved; compressive strength
perpendicular to the foaming direction increased by 28%, compressive strength parallel
to foaming direction increased by 67%, and tensile strength increased by 61%. However,
among other properties, significant improvement in mechanical performance was achieved
for PUR foams with 30 wt.%; the compressive strength increased by 114% and 71% in the
perpendicular and parallel directions, respectively, and tensile strength by 89%.
In summarising the obtained results for SFP filler-modified PUR foams, it is worth
noting that such building materials are promising for future application as thermal in-
sulating layers in building envelopes. In addition, the replacement of petroleum-based
polyols with plant-based polyols, traditional blowing agents with water, and synthetic
fillers with natural or plant-based fillers will contribute to the achievement of sustainability
goals and the reduction of carbon dioxide emissions coming from the building industry.
However, further studies on plant-based PUR foams are needed in order to reduce thermal
conductivity values.
Author Contributions: Methodology, A.K., S.C., R.B., S.V.; investigation, A.K., S.C., R.B., S.V.; data
curation, A.K.; writing—original draft, A.K.; writing—review and editing, A.K.; visualization, A.K.
All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data sharing not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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