A novel click-chemistry approach to flame retardant polyurethanes

Article abstract of DOI:10.1016/j.reactfunctpolym.2013.06.003

The low thermo-oxidative properties of PU foams somewhat limits their practical application, particularly as heat sensitive materials. The introduction of a covalently linked flame retardant organophosphonate ester into the PU foam was achieved using the

Full text of DOI:10.1016/j.reactfunctpolym.2013.06.003

Accepted Manuscript
A novel click-chemistry approach to flame retardant polyurethanes
Ana M. Borreguero, Pallavi Sharma, Christian Spiteri, María M. Velencoso,
Manuel S. Carmona, John E. Moses, Juan F. Rodríguez
PII:
S1381-5148(13)00136-3
DOI:
http://dx.doi.org/10.1016/j.reactfunctpolym.2013.06.003
REACT 3245
Reference:
To appear in:
Reactive & Functional Polymers
Received Date:
Revised Date:
Accepted Date:
20 December 2012
4 April 2013
6 June 2013
Please cite this article as: A.M. Borreguero, P. Sharma, C. Spiteri, M.M. Velencoso, M.S. Carmona, J.E. Moses,
J.F. Rodríguez, A novel click-chemistry approach to flame retardant polyurethanes, Reactive & Functional
Polymers (2013), doi: http://dx.doi.org/10.1016/j.reactfunctpolym.2013.06.003
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A novel click-chemistry approach to flame retardant
polyurethanes
Ana M. Borreguero,a Pallavi Sharma,b Christian Spiteri,b María M. Velencoso,a
Manuel S. Carmona,a John E. Mosesb,* and Juan F. Rodrígueza,*
aDepartment of Chemical Engineering, University of Castilla-La Mancha, 13071,
Spain
bSchool of Chemistry, University of Nottingham, NG7 2RD, UK
Juan.RRomero@uclm.es
john.moses@nottingham.ac.uk
1

ABSTRACT
The low thermo-oxidative properties of PU foams somewhat limits their practical
application, particularly as heat sensitive materials. The introduction of a covalently
linked flame retardant organophosphonate ester into the PU foam was achieved using
the CuAAC ‘click’ reaction of an alkyne-polyol and azidoalkylmonophosphonate.
These functionalised materials were prepared in four steps: first, a number azidoalkyl
monophosphonate compounds were formed via nucleophilic substitution of
bromoalkylphosphonates with NaN3; next, polyols bearing terminal alkyne groups
were prepared by anionic ring opening copolymerization between propylene oxide
and glycidyl propargyl ether; followed by ‘clicking’ the azidoalkylphosphonate to the
polyol and finally, synthesis of the PU foam with 2.4wt% of “click-polyol”. The
functionalised PU foam demonstrated a well-formed polyhedral cell structure and an
increase in the fire resistance, according to the SEM and thermogravimetric analysis,
respectively. Even after thermal treatment at 400 ºC, the new PU foam material
displayed enhanced flame resistant properties by forming a char layer on the surface
of the polymer, which, maintaining its polyhedral structure.
Keywords: polyurethane, polyol, click reaction, flame retardant, phosphorus.
2

1. INTRODUCTION
The unique structural and mechanical properties of polyurethane (PU) polymers have
led to their wide-spread application in material products such as: building insulation,
domestic mattresses, furniture, sealants, adhesives, leathers and various biomedical
products [1]. However, the low thermo-oxidative properties of PU-foams present a
serious draw-back, since these materials are readily flammable unless treated. To this
end, various modifications of PU foams have been introduced to improve their flame
resistance properties including: chemical modification of the surface and core of the
polymers, or more commonly, inclusion of an additive. For example, nitrogen and
phosphorus bearing compounds as in the phosphazine polymer [2,3], halogen-laced
substrates as radical quenchers [4], minerals such as aluminum trihydroxide,
magnesium hydroxide [4] or melamine derivatives [6] are just a few of the routinely
used additives.
Unfortunately, the compromise of structural and mechanical properties [1-7]
associated with most modification techniques, as well as the environmental and
human health hazards associated with bio-accumulation of halogenated additives,
warrants the development of new strategies.
Stabilised and erosion resistant PU materials can be achieved by the covalent
modification of the polymer back-bone with a suitable auxiliary. This approach has
several advantages over conventional flame retardant PU foam materials, , allowing
lower concentrations of the additive to be employed, with better mechanical
properties and resistance to leaching [8].
Organophosphonate esters are routinely employed as flame retardant additives owing
to their low toxicity and almost smoke-free burning. They are generally used in
combination with radical quenchers such as bromine or along with other phosphorous
and nitrogen based polymers (e.g. phosphazine) [8,9]. However, though the use of
phosphonate esters as flame retardants is widespread, few methods are reported for
their robust and reliable incorporation into PU foams [10].
Here we report the development of a new class of organophosphonate ester
functionalised PU-foam with flame retardant properties. The key to our approach was
the use of the copper catalysed azide-alkyne cycloaddition (CuAAC) reaction [11], or
3

‘click’-reaction for covalently introducing the organophosphonate into the PU-foam
[12-24].
In our design, we reasoned that in addition to the phosphorus functionality, the
presence of nitrogen in the triazole group could further improve the flame retardant
properties to the PU-foams. At elevated temperature the decomposition of triazole
could technically provide a blanket of nitrogen [25].
We have previously demonstrated the successful generation of alkyne-decorated
polyols (5), by anionic ring opening of propylene oxide (3) and glycidylpropargyl
ether (4), along with their copolymerization (Scheme 1) [26]. It was anticipated that
these alkyne-polyols would readily ‘click’ to an azide bearing organophosphonate
compound using the Cu(I) catalysed process.
2. EXPERIMENTAL
2.1. Materials
Sodium azide (>99.5%, Fischer Scientific), 1,3-dibromopropane (98%, Acros
Organics), 1,5-dirbomopentane (98%, Alfa Aesar), 1,10-dibromodecane (>97%,
Sigma-Aldrich), 1,4-bis(bromomethyl)benzene (>98%, Alfa Aesar), acetone (reagent
grade, Fischer Chemical). Copper (II) sulfate pentahydrate (>99.5%, PANREAC),
sodium ascorbate (>99%, PANREAC), tetrahydrofuran (THF) (reagent grade, Fischer
Chemical), ethanol (96%, Sigma-Aldrich), ethyl acetate (EtAc) (reagent grade,
Fischer Chemical), dichloromethame (DCM) (reagent grade, Fischer Chemical),
deionised water, diethyl ether (>99.8 %, Sigma Aldrich).
Glycerol (≥ 86%, Sigma-Aldrich), potassium hydroxide (KOH) (85%, PANREAC),
propylene oxide (OP) (100%, PRAXAIR) and glycidyl propargyl ether (GPE) (>90%,
Sigma-Aldrich) were used in the synthesis of the polyol as received. Methanol
according to Karl Fischer (99.5%, PANREAC) and ion exchange resin Amberlite 252
(Rohm and Hass) were used for catalyst removal from the “click polyols”.
For the PU foams synthesis, Alcupol R-458 from Repsol YPF S.A. and the
synthesised click polyol were used. As isocyanate, diphenylmethane-4,4’-
diisocyanate (MDI), supplied by Merck Group, was used. Catalyst Tegoamin ZEI and
4

surfactant Tegostab B8404 were supplied by Evonik Degussa International AG.
Deionised water was used as blowing agent.
2.2. Physico-chemical characterisation
13
1H NMR and C NMR spectra were recorded on Bruker AV400 or Bruker DPX400
1
13
(400 MHz for H and 100 for C). Fourier Transform Infrared (FT-IR) spectra were
obtained on Bruker Tensor 27 spectrometer as a solution in chloroform.
Thermogravimetric analyses (TGA) were performed in TA Instruments SDT Q600
Simultaneous DSC-TGA at a heating rate of 10 ºC min−1 under nitrogen atmosphere
from room temperature to 700 ºC. Synthesised PU foams were depicted by scanning
electron microscope (SEM, Quanta, FEI Company) in order to observe the possible
changes in their cellular structure when the ‘click’ polyol was added in the recipe,
using a large field detector (LFD) with an accelerating voltage of 10 kV and with
magnification x 100. Heat treatment of the PU foams samples was carried out in a
muffle type furnace at 400 ºC for 15 minutes for further analysis of their char
residues, observing their structure by SEM analysis.
2.3. Synthesis of the diethyl azidoalkylphosphonates
The synthesis of the diethyl azidoalkylphosphonates was carried according to Scheme
2. The azidophosphonate compounds 8a-c was synthesised using Michaelis-Arbuzov
conditions [27]. Thus, the commercially available dibromo-alkanes (6a-c) were
readily transformed into the corresponding phosphonate compounds 7a-c in the
presence of triethylphosphite under elevated temperature and further purified by
means of flash silica gel chromatography (CH2Cl2) [28]. Nucleophilic substitution of
the remaining bromo-group with sodium azide rendered the azidoalkylphosphonates
8a-c in good to excellent yields (Scheme 3). The azioalkylphosphonate products were
separated from the excess of azide by cooling the suspension, filtering and
evaporating the solvent under vacuum.
5

2.4. Synthesis of diethyl-4-(azidomethyl)benzylphosphonate
The synthesis of the diethyl-4-(azidomethyl)benzylphosphonate was carried out
according to Scheme 3.
The procedure to obtain diethyl-4-(bromomethyl)benzylphosphonate (10) was similar
to that explained above for the diethylbromoalkylphosphonates. Next, the resulting
diethyl-4-(bromomethyl)benzylphosphonate
(1.0
equiv),
N,N’-
dimethylethylenediamine (0.15 equiv), CuI (0.1 equiv) and sodium ascorbate (0.5
equiv) were added to a mixture of EtOH/H2O [70/30]. The reaction flask was flushed
with argon, NaN3 added, and the reaction mixture was heated to reflux for 3h [29].
The product was obtained by removing the volatiles from the reaction mixture in
vacua, extracting the 4-azidobenzylphosphonate (11) with ethyl acetate, extracting the
organic layer with water and drying the organic layer over anhydrous MgSO4,
filtration and removing the solvent in vacua Finally, the product was purified by flash
silica gel column chromatography (CH2Cl2/EtAc, 10:1).
2.5. Click reaction
With the alkyne-decorated polyol (5) and azidophosphonate compounds (8a-c, 11) in
hand, the key ‘click’ reaction to render the triazole-polyols (phos-triz-polyol) were
performed. A solution of the respective azidophosphonate (1 equiv) and the alkyne-
polyether polyol (1 equiv of alkyne groups) in THF:Ethanol:H2O [1:2:2] were taken
in a round bottom flask [30]. Then, the catalyst mixtuire, copper sulphate
pentahydrate (0.01 equiv) in water and sodium ascorbate (0.1 equiv) in water were
added [31]. The reaction mixture was stirred under microwave irradiation for 1 h at 75
ºC (Scheme 4).
The synthesised phos-triz-polyol and the ion exchange resin Amberlite 252 were
stirred together at 100 ºC in order to eliminate the remaining catalyst [32].
2.6. Polyurethane foam synthesis
One of the functionalised phos-triz-polyol (12b, 5C-phos-triz-polyol) was next used
as reagent in the synthesis of PU foams in order to check the feasibility of using these
functionalized polyols as PU foams raw material. PU foams were synthesised by
mixing the required masses of the 5C-phos-triz-polyol and a commercial polyol
(Alcupol R-458), silicone Tegostab B8404, deionised water and amine Tegoamin ZEI
6

and further stirring the mixture for 1 min. Next, an adequate quantity of isocyanate
was added to the mixture and the resulting solution was stirred for just 5 seconds,
moment at which the foam started to grow up. Finally, the obtained foams were cured
at room temperature. Table 1 shows the foam synthesis recipe.
3. RESULTS AND DISCUSSION
3.1. Synthesis of the foam material
The syntheses of all the flame retardant components were achieved as described and
the strucutres confirmed using complementary spectroscopic techniques. The
presence of azide clearly apparent by the characteristic peaks at 2100 cm-1 in the IR..
The yields of the products obtained are shown in Table 2, with poorer conversions
achieved with increase in hydrocarbon chain complexity. To the best of our
knowledge, the synthesis of these compounds has not previously been reported.
3.2. Click reaction
The ‘click’ reaction was followed by IR and monitoring the decrease in azide peak (at
2100 cm-1) [21,33,34]. The corresponding flame retardant monomer was confirmed
1
by H NMR analysis in D2O, observing the triazole peak at 8.3 ppm (Figure 1)
[21,34].
3.3. Synthesis of PU foams
The addition of small amounts of phosphorous (1.0 wt%) into rigid polyurethane
foams produce an important improvement of the foams fire resistance [35]. In the
present work a 2.4 wt% of the functionalised 5C-phos-triz-polyol (12b) was
incorporated in PU foam during its formation (phos-triz-PU). For comparative
purposes, a PU foam was prepared without any covalent fire retardant.
Figures 2a and 2b show the cell structure of the pure PU and the phos-triz-PU foams,
respectively. Both materials exhibited polyhedral closed-cell structures with
pentagonal or hexagonal faces containing nodes formed by the junction of four struts,
but with higher sizes when adding the 5C-phos-triz-polyol. The increase in cell size
can be attributed to the larger volume of the 5C-phos-triz-polyol due to the pendant
triazole groups with the flame retardant compounds or because of the modification in
recipe for PU formation. It is known that unadjusted foam formulation can modify the
nucleation rate and the final cell size [36].
7

To test the improvement in the fire resistance of the phos-triz-PU foam compared to
the pure-PU, thermo gravimetric analyses (TGA) and thermal treatment in furnace
were performed. The flame retardant property of polymer is directly related the char
formation during the combustion process [37-39] and, the char percentage measured
by TGA is accepted as an indirect way of measuring this property [8,40].
Figure 3 compares the TG and DTG analyses of pure-PU and phos-triz-PU foams.
The thermal degradation of both samples took place in two main steps, the first one is
attributed to the crosslinked polymer degradation including its hard and soft segments
and the second is due to the stabilized structures that can be formed during the
previous degradation processes [8,41]. It can be observed that just a small addition of
flame retardant reduced the temperature at which the maximum degradation occurred
from 340 ºC to 324 ºC (Figure 3b) but increased the char yield an 18.2%, from 10.9
wt% to 12.9 wt% (Figure 3a), which confirms the enhancement of the fire resistance.
On the other hand, naked eye visualisation of photos of the char residues for the pure-
PU and phos-triz-PU foams after the thermal treatment in the furnace under 400 ºC in
air, showed black and brown porous solids for the pure-PU and the phos-triz-PU,
respectively (see Figures 4a and 4b). It was clear that the phos-triz-PU foam char
residue was covered by a black and bright char layer (See Figures 4c and 4d) whereas
no such appearance was recorded for pure-PU.
SEM analyses of these char residues showed that the cell structure of the pure-PU
foam was quite degraded, without a defined cell structure (Figure 5a), while the char
of the click-PU foam conserved a well defined cell structure with pentagonal or
hexagonal faces (Figure 5b). Hence, it can be concluded that the covering char layer
acted as protecting barrier, hampering the mass and heat transfer during the
combustion [40].
4. CONCLUSIONS
Different
diethyl
azidoalkylphosphonates
and
the
diethyl
4-
(azidomethyl)benzylphosphonate were successfully synthesized via a Michaelis-
Arbuzov reaction and linked to polyols containing terminal alkyne groups via Cu(I)-
catalysed Huisgen 1,3-dipolar cycloaddition. The functionalized polyol containing the
(5-azidopentyl) phosphonate was used as raw material for a PU foam synthesis (phos-
8

triz-PU foam). The phos-triz-PU foam showed an improved flame resistance property
when compared to pure-PU. TGA results showed that the addition of a small
percentage of phos-triz-polyol (2.4 wt%) increases an 18.4% in the char layer residue.
Finally, the residues obtained by firing the foams in a furnace allowed to conclude
that the resistance enhancement of the click PU foam was achieved by the formation
of a char layer that covered the rest of the PU foam, and prevented its degradation.
Therefore, the present design of flame retardant based on robust, easy to perform and
reliable click chemistry technology could be a real advancement in the field of
polyurethane foams synthesis with flame retardant properties.
9

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13

FIGURE CAPTIONS
1
Figure 1. H NMR analyses in D2O of the phos-triz-polyols derived from diethyl (3-
azidopropyl) phosphonate (a), diethyl (5-azidopropyl) phosphonate (b), diethyl (10-
azidopropyl) phosphonate (c) and diethyl 4-azidobenzylphosphonate (d).
Figure 2. SEM analysis of pure-PU (a) and phos-triz-PU (b) foams.
Figure 3. TG (a) and DTG (b) analyses of the pure-PU and the phos-triz-PU foams.
Figure 4. Photos of the char residues of: pure-PU; b) phos-triz-PU; c) phos-triz-PU
(covering layer, front view); d) phos-triz-PU (covering layer, side view).
Figure 5. SEM analysis of PU foams char residues of pure-PU (a) and phos-triz-PU
(b) foams.
14

FIGURES
O
N
N
N
Ha
O
P
OEt
EtO
Ha
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
Chemical Shift (ppm)
a) 1H NMR (400 MHz, D2O)
O
N
N
N
Ha
O
P
OEt
EtO
Ha
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
Chemical Shift (ppm)
b) 1H NMR (400 MHz, D2O)
15

O
N
N
N
Ha
O
P
EtO
OEt
Ha
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
Chemical Shift (ppm)
c) 1H NMR (400 MHz, D2O)
O
N
N
N
Ha
O
P
OEt
EtO
Ha
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
Chemical Shift (ppm)
d) 1H NMR (400 MHz, D2O)
FIGURE 1
16

a)
b)
FIGURE 2
17

100
80
60
40
20
0
Pure-PU
Phos-triz-PU
0
100
200
300
400
500
600
700
Temperature (ºC)
a)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Pure-PU
Phos-triz-PU
0
100
200
300
400
500
600
700
Temperature (ºC)
b)
FIGURE 3
18

a)
b)
c)
d)
FIGURE 4
19

a)
b)
FIGURE 5
20

SCHEMES
Scheme 1. Anionic ring opening copolymerisation between propylene oxide and
glycidylpropargyl ether
Scheme 2. Synthesis of diethyl azioalkylphosphonates
Scheme 3. Synthesis of diethyl 4-(azidomethyl)benzylphosphonate.
21

Scheme 4. Synthesis of the phos-triz-polyols.
22

TABLES
Table 1. Weight percentage of raw materials for PU foams synthesis.
Material
Alcupol R-458
5C-phos-triz-polyol
Water
Weight percentage
46.01
2.42
1.21
0.73
1.21
48.43
Silicone
Amine
MDI
Table 2. Yields of the flame retardant synthesis reactions.
Flame retardant
Yield (%)
90.4
Diethyl (3-azidopropyl) phosphonate
Diethyl (5-azidopentyl) phosphonate
Diethyl (10-azidodecanyl) phosphonate
Diethyl 4-(azidomethyl)benzylphosphonate
85.0
46.6
36.1
23