J. ZACH, V. NOVÁK: POLYMER-MATRIX-BONDED POLYESTER FIBERS AS A SUBSTITUTE ...
511–514
POLYMER-MATRIX-BONDED POLYESTER FIBERS AS A
SUBSTITUTE FOR MATERIALS USED FOR THE CORES OF
VACUUM INSULATION PANELS
POLIESTRSKA VLAKNA, VEZANA V POLIMERNI MATRICI, KOT
NADOMESTEK ZA MATERIALE, UPORABLJANE ZA JEDRA
VAKUUMSKIH IZOLACIJSKIH PANELOV
Jiøí Zach, Vítìzslav Novák
*
Brno University of Technology, Faculty of Civil Engineering, Admas Centre, Veveøí 331/95, 602 00 Brno, Czech Republic
Prejem rokopisa – received: 2018-09-25; sprejem za objavo – accepted for publication: 2019-01-30
doi:10.17222/mit.2018.208
The paper discusses the use of alternative raw materials for the manufacturing of VIPs (vacuum insulation panels). VIPs are
thermally insulating boards with a fibrous core at an extremely low pressure. The research is focused on substituting glass fibers
with polyester fibers in VIP cores. Polyester fibers are thermally bonded using BiCo fibers, which, once they have cooled down,
form the matrix of a composite VIP core. It is proved that polyester fibers bonded with BiCo fibers show great promise, most
notably in terms of their excellent thermal-insulation properties, which they retain for a long time.
Keywords: vacuum insulation panels, polyester fibers, fibrous insulation materials, thermal conductivity
Avtorji razpravljajo o uporabi alternativnih surovin za izdelavo vakuumskih izolacijskih panelov (VIP). To so termi~ne
izolacijske plo{~e z jedrom iz vlaken pri ekstremno nizkem tlaku. Raziskava je bila osredoto~ena na zamenjavo steklenih vlaken
s poliestrskimi vlakni v VIP jedrih. Poliestrska vlakna so med seboj termi~no vezali z BiCo vlakni, ki po ohladitvi tvorijo
matrico kompozitnega VIP jedra. Avtorji so dokazali, da so poliestrska vlakna, vezana z BiCo vlakni, odli~ni in ~asovno stabilni
toplotni izolatorji in so zaradi teh lastnosti iperspektiven material za panele.
Klju~ne besede: vakuumski izolacijski paneli, poliestrska vlakna, vlaknasti izolacijski materiali, toplotna prevodnost
1 INTRODUCTION
The development of advanced thermal-insulation
materials is an important area, especially in the context
of limiting the amount of energy consumed by buildings
and reducing CO
2
emissions (see, e.g., the Kyoto Pro-
tocol). This relates mainly to the development of new
thermal-insulation materials that improve the energy
performance of buildings, meeting the constantly in-
creasing requirements (in Europe regulated by Directive
2010/31/EU on the energy performance of buildings)
without resorting to overly thick layers of thermal in-
sulation in order to deliver the desired thermal resistance
of the structure.
Current demands require seeking solutions among
advanced and smart materials, i.e., the ones that combine
several positive properties. Among thermal insulators,
there have been, for a number of years, some interesting
materials made of renewable raw materials and
by-products. Some countries (e.g., in Northern Europe)
are making an effort to use such materials as a replace-
ment for mineral wool or expanded polystyrene. How-
ever, insulations made with renewable and secondary
materials often suffer from poorer thermal-insulation
properties when compared to the conventional ones, and
are often sensitive to moisture.
1
Research shows that secondary and renewable ma-
terials can be used to manufacture an advanced insula-
tion system such as vacuum insulation panels (VIPs).
2
VIPs seal the insulation away from the outside environ-
ment, in an airtight envelope that eliminates the danger
of moisture sensitivity. On the other hand, only some
secondary and renewable materials can be used for VIPs
because they need to meet certain limits of the particle
size/fiber length and thickness, and need to be chemi-
cally pure enough to avoid low-pressure sublimation of
unstable compounds (outgassing), which causes ther-
mal-insulation properties to deteriorate after vacuum
sealing.
3
Research has found that fibers obtained by pulping
waste textile produced by both textile manufacturing and
recycling have very interesting properties and are, in
most cases, well suited for use in various thermal- and
acoustic-insulation materials. In addition, polymer fibers
can be made into a matrix, suitable for VIPs.
4
Experiments have shown that the limit thickness of
glass-wool fibers, the dominant insulation used for VIPs
today, is 10 μm. Thicker fibers are not good for VIPs be-
cause an insulation with thicker fibers will not be im-
proved so much by vacuum sealing and, moreover, any
Materiali in tehnologije / Materials and technology 53 (2019) 4, 511–514 511
UDK 620.1:677.494.674:621.315.6 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(4)511(2019)
*Corresponding author's e-mail:
novak.v@fce.vutbr.cz

slight pressure increase inside the insulation will cause a
significant detriment to its properties.
2 FIBRE SPECIMENS AND THEIR PROPERTIES
During the research, we selected several suitable
primary and secondary materials based on cotton and
polyester. The goal was to find fibers with a high purity,
low thickness and sufficient length so that they may be
made into mats by thermal bonding using BiCo fibers,
see Figure 1. The actual manufacturing was done with
the air-lay method using standard PES fibers with a
thickness of 15–20 μm.
We then microscopically examined these fibers to
determine their length, thickness, and to study their
shape. The following samples were selected for further
tests, see Table 1.
VIPs require the use of the materials with the finest
pore structure or the thinnest fibers.
2,3,5
The measure-
ments show that pure cotton, depending on the type and
the place of origin, can reach a thickness of up to 10 μm,
which is why these fibers can be considered suitable for
the VIP manufacturing. Recycled cotton fibers reach
thickness values of 12–13 μm. Both recycled-cotton
samples exhibited very similar fiber-thickness values.
This is due to the fact that textile manufacturing selects
cotton of carefully controlled quality. On the other hand,
PES fibers are a little different; there are rather substan-
tial differences between the thickness values of primary
and secondary fibers. The primary fibers ranged from
10 μm to 15 μm, whereas the recycled fibers were bet-
ween 11 μm and 21 μm. When using PES fibers for
VIPs, it is therefore important to choose fibers carefully.
3 MANUFACTURING VIP-CORE SPECIMENS
AND PREPARATION FOR VACUUM SEALING
The next stage of the research involved the manufact-
uring of test specimens from the samples of potentially
suitable fibers (see above). The following fibers were
used:
1 sample of cotton (Sample 2 – see above),
2 samples of recycled cotton (Samples 1 and2–see
above),
2 samples of primary PES fibers (Samples 2 and 3 –
see above),
2 samples of recycled PES fibers (Samples 2 and 3 –
see above).
The samples were thermally bonded with an addition
of 15 % of BiCo PES fibers (the air-lay method). With
this process, we produced mats of 1200 mm × 600 mm,
with the highest bulk density the method allowed. These
mats were then cut into individual specimens (see
below).
In VIPs, the insulation’s purity is very important.
Every insulation always contains some moisture, which
is why it is important to thoroughly dry every core
material before incorporating it into a panel. However, it
is rather difficult to completely remove all the moisture
from a material, which is why a separate part of this
research was devoted to drying. While the insulation
materials were being dried, they were also continually
weighed. It appears that finding an optimum drying
regime is not quite straightforward. VIP-core insulation
cannot be dried at the standard 105 °C because this
temperature does not remove all the bound H
2
O and,
thus, two negative effects may occur as a result:
1) Bound moisture is released during vacuum sealing,
which markedly increases the costs and interferes
with the production efficiency,
2) The moisture is released after the insulation is sealed,
which increases the internal pressure and reduces
thermal-insulation properties. This usually occurs
very shortly after vacuum sealing.
These detrimental effects can be minimized by
choosing the optimal drying time and temperature for
every type of insulation. The above-described insulation
materials and a reference glass-wool sample were being
experimentally dried at several different temperatures,
while weight changes were recorded at specific time
intervals. The drying took place at temperatures of (105,
140, 160 and 180) °C at intervals within 0–60 h.
4 RESULTS
It was found that the release of moisture from all the
materials depends on the temperature. The residual
moisture content ranged within 0.1–2.5 % in all the
J. ZACH, V. NOVÁK: POLYMER-MATRIX-BONDED POLYESTER FIBERS AS A SUBSTITUTE ...
512 Materiali in tehnologije / Materials and technology 53 (2019) 4, 511–514
Table 1: Properties of cotton and PES fibers
Fibers Cotton PES
Type Primary Secondary Primary Secondary
Sample 121212312345
Thickness [μm] 12.74 10.80 12.61 12.54 15.09 10.01 12.35 12.41 12.26 11.49 19.97 21.39
Length [mm] 26.60 26.10 22.40 23.50 47.30 85.60 72.60 49.20 35.50 48.91 61.15 76.20
Figure 1: Diagram of a BiCo fiber cross-section

cases. Cotton samples had a higher moisture content than
PES samples, see Figure 2. Drying at 105 °C was not
sufficient for VIPs in any of the cases. Sufficient drying
was achieved with temperatures of 160 °C or higher, see
Figure 3. A suitable drying time appears to be 3–5 h de-
pending on the temperature and type of insulation.
During the next step of the experiment, the mats were
cut into specimens with dimensions of 200 mm × 200
mm. The following measurements were made: linear
dimensions according to EN 822
6
/EN 12085
7
, thickness
according to EN 823,
8
and bulk density according to EN
1602.
9
The specimens were also tested for thermal conduc-
tivity according to EN 12667
10
and ISO 8301
11
at the
normal and reduced pressure (5 Pa) to assess their
overall potential for use in VIPs, see Table 2. The
measurements were made using a FOX 200 Vacuum
from TA Instruments at a mean temperature of 10 °C and
temperature gradient of 10 °C.
5 DISCUSSION
The results show that recycled textile fibers can be a
suitable feedstock for the VIP manufacturing. However,
it is important to choose the fibers with the correct shape
and thickness. Cotton fibers are particularly problematic
because they are not completely straight, thus affecting
the final properties at the normal pressure as well as the
vacuum.
It is also necessary to choose the correct drying re-
gime with regard to the type of fibers, residual moisture
and the overall behavior during drying. The nature of the
fibers does not allow them to be dried at very high
temperatures. It was found that the best temperature is at
least 160 °C (depending on the type of fibers) and the
best drying time is at least 5 h. It was found that the core
insulation based on alternative fibers is comparable to
the behavior of glass-wool materials. This is evidenced
by the results shown in Table 2. In the framework of
further research, we will predominantly verify the use of
getters to reduce the effect of drying on the properties
and durability of VIPs based on this alternative insu-
lation.
6 CONCLUSIONS
The results of the thermal-conductivity tests show
that at both the normal pressure and vacuum, PES sam-
ples perform better than cotton samples of the same
thickness, due to a generally better structure of PES
materials made with the air-lay method (the difference is
a result of the shape of the fibers – see above). However,
the results for both fiber types are comparable. As for
secondary fibers, they had similar properties to those of
primary materials (in both PES and cotton); however, it
is always important to choose the correct type.
J. ZACH, V. NOVÁK: POLYMER-MATRIX-BONDED POLYESTER FIBERS AS A SUBSTITUTE ...
Materiali in tehnologije / Materials and technology 53 (2019) 4, 511–514 513
Figure 4: Dependence of thermal conductivity on the pressure for
primary-PES Sample no. 2
Figure 2: Residual moisture content depending on the temperature of
recycled-cotton Sample no. 1
Figure 3: Moisture losses over time at a temperature of 180 °C for
recycled-cotton Sample no. 1
Table 2: Values of thermal conductivity at normal pressure and at 5 Pa
(vacuum)
Fibers Type Sample
Normal
pressure
Vacuum
Cotton Primary 1 0.03275 0.00333
Cotton Primary 2 0.03112 0.00264
Cotton Secondary 1 0.03558 0.00385
Cotton Secondary 2 0.03734 0.00515
PES Primary 1 0.03325 0.00374
PES Primary 2 0.02967 0.00221
PES Primary 3 0.03175 0.00253
PES Secondary 1 0.03197 0.00276
PES Secondary 2 0.03602 0.00583
PES Secondary 3 0.03526 0.00395
PES Secondary 4 0.03859 0.00552
PES Secondary 5 0.03912 0.00611

Acknowledgment
This research was carried out under projects
GA17-00243S, Study of the behavior of insulating
materials under extremely low pressure, and FV20127,
Research and development of advanced thermal and
acoustic insulating materials based on waste textile and
natural fibers.
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J. ZACH, V. NOVÁK: POLYMER-MATRIX-BONDED POLYESTER FIBERS AS A SUBSTITUTE ...
514 Materiali in tehnologije / Materials and technology 53 (2019) 4, 511–514