polyurethanes from novolac resins and polyols...
TRANSCRIPT
CHAPTER 3
POLYURETHANES FROM NOVOLAC RESINS AND
POLYOLS
75
3.1 INTRODUCTION
Novolac resins and polyols prepared were used as co-reactant
in the synthesis of polyurethanes. A variety of polyurethanes having
varying mechanical properties, fire-resistant capacity, mouldability,
adhesive character and laminates have been developed by many
researchers. Polyols of phenol-formaldehyde resins self curable at
ambient temperature or curable by amines has been reported'.
Synthesis and curing behaviour of a crosslinkable polymer2 , polyether3,
polyester 4, from cashew nut shell liquid and monomer from cardanol5
have been reported. Phenolic novolac resins mixed with
diphenylmethane diisocyanate (MDI) (7) as hardener for heat and
moisture resistant epoxy composites has been developed 6 . Polyurethane
foams have been developed from the benzylic ether of phenol-
formaldehyde resins and polyisocyanates made from diphenylmethane
diisocyanate 7 . Fire resistant phenol-formaldehyde, modified
polyurethane foam as thermal insulators have also been developed8'9
Phenol-formaldehyde resole resins have been used in developing ink
compositions for waterless lithography' 0 . Bisphenol-A (35) based
phenolic resin, with polyisocyanate in the presence of an amine was
used in inorganic mouldings 11-14
HOCOH--0- 1 _(:^_CH3
(35)
76
Resistance to fatigue and thermal stability of Phosphorylated cashew nut
shell liquid (PCNSL)-modified natural rubber vulcanizate has been
reported 15 . Natural rubber modified with CNSL and CNSL-formaldehyde
resins have also been reported 16 . CNSL resins are added to laminates
based on phenol-formaldehyde and epoxy resins to reduce brittleness
and to improve bonding to laminate substrate ' 7 . Bulk polymerised
prepolymers for easy dispensability, non-wicking and good dispersion in
potting of hollow fibre has been studied by Jayabalan et al' 8 . New
ferrocene polyurethane block copolymers based on diphenylmethane
diisocyanate for use as fire retardant have been synthesised' 9 . Phenolic
urethane- resins have-also beer studied for use as foundry binders20'21.
Urethane pre-polymer blowed with substituted phenol was used
as flock adhesive 22 . Sitaraman and Chaterjee prepared 23 pressure
sensitive adhesives from 3-pentadecyl phenol (hydrogenated cardanol).
Reaction of polyisocyanates with a mixture of benzyl ether type phenolic
resin (novolac) and hydroxyl terminated polyesters in the presence of
blowing agents and catalysts to form heat decomposable fire-resistant
phenolic urethane foam having good mouldability and mechanical
properties has been developed 24. Varnish for fire-resistant laminate was
developed from phenol-formaldehyde resin, ammonia and toluene
dilsocyanate, (TDI) (5)-based polyurethane s 25 . Hiroshi et at prepared 26
varnish from CNSL - formaldehyde resin using an isocyanate
crosslinking agent (Burnock D 750). Auto-oxidation polymerization of
Polyurethane films based on cardanol, glycol and toluene diisocyanate
77
catalyzed by cobalt salt have also been studied 27 . Sailan has prepared28
coatings based on epoxy resin modified with liquid in the presence of
phosphoric acid followed by treating with toluene diisocyanate and
phenolic resin. Speciality coatings based on cardanol-formaldehyde
resins copolymerized with toluene diisocyanate has been reported by
Hu et a1 29 . Processes for the development of high adhesion coatings,
varnishes, sealing compounds and fire-resistant foam materials have
also been developed 3034 . Polyurethanes containing unsaturated esters
curable by radicals for use as sealing compounds for anchoring rods
have been prepared from novolac-diphenylmethane diisocyanate and
benzoyl peroxide 35 . Interpenetrating polymer networks derived from
soybean oil-based polyurethanes and cardanol-m-aminophenol dye has
also been reported 36 . Recently the synthesis and characterisation of
polyurethanes based on cardanol-formaldehyde resins using dicyclohexyl
methane diisocyanate (SMDI) has also been reported 37 . However
synthesis and characterisation of polyurethane sheets based on novolac
resins and hydroxyalkylated cardanol -formaldehyde resins (synthesised
polyols) with diphenylmethane diisocyanate and toluene diisocyanate
have not been reported earlier. Hence, in the present investigation, a
systematic synthesis and characterisation of hard and soft segment
polyurethanes based on the cardanot-formaldehyde novolac resins and
the synthesised polyols have been undertaken.
78
32 EXPERIMENTAL
32.1 Synthesis of polyurethanes based on novolac resins
Polyurethanes were prepared using diphenylmethane
diisocyanate and toluene diisocyanate. Novolac resins were vacuum-
dried before use. Dibutyltin dilaurate was used as catalyst. Commercial
polyol, poly propylene glycol-2000 (PPG-2000) was dried in vacuum
oyen. The chemicals used for the synthesis and their source are given in
Table 3.1.
Table 3.1
Chemicals used for polyurethane synthesis
No Chemicals used Source
1. Di isocyanate:(I) Diphenylmethane diisocyanate (MDI) Fluka chernie AL(R) Toluene diisocyan .ate(TDI) Fluka. chemie AL UK
2. Polyol: polypropylene oxide glycol-2000 Aldrich chemicals(PPG-2000) USA
3. Catalyst: Dibutyltin dilaurate (DBTDL) Fluka chemie A.G. UK
The mole ratios used in the preparation of polyurethanes based
only on the novolac resin and the diisocyanate, diphenylmethane
diisocyanate or toluene diisocyanate producing hard segment
polyurethanes and also the polyurethanes based on novolac resin, the
diisocyanate, diphenylmethane dilsocyanate or toluene diisocyanate and
the commercial polyol, PPG-2000 producing soft segment polyurethanes
are presented in Table 3.2. In both the cases, the isocyanate index
(NCO/OH mole ratio) is kept at 1.4.
79
The hard segment polyurethanes prepared from novolac resins
using diphenylmethane dilsocyanate are denoted as "CRM" and the
corresponding soft segment polyurethanes are denoted as "CRMP". The
hard segment polyurethanes prepared from novolac resins using toluene
diisocyanate are denoted as "CRT" and the corresponding soft segment
polyurethanes are denoted as "CRTP". The hard segment polyurethanes
prepared from synthesised polyol using diphenylmethane diisocyanate
are denoted as "CREHM" and the corresponding soft segment
polyurethane are denoted as "CREHMP". The hard segment
polyurethanes prepared from synthesised polyol using toluene
diisocyanate are denoted as "CREHT" and the corresponding soft
segment polyurethanes are denoted as "CREHTP".
Table 3.2
Formulation of polyurethanes based on novolac resin.
Polyurethanes
CR1M
CR2M
CR3M
CR1MP
CR2MP
CR3MP
CR1T
CR2T
CR3T
CR1TP
CR2TP
CR3TP
Diisocyanate
- Concentration(mole xfunctionality)
- 0.70
- 0.56
- 0.42
- 0.98
- 0.84
- 0.70
- 0.70
- 0.56
- 0.42
- 0.98
- 0.84
- 0.70
Novolac resin
ConcentrationMole (mole x
functionality)
0.1 0.5
0.1 0.4
0.1 0.3
0.1 0.5
0.1 0.4
0.1 0.3
0.1 0.5
0.1 0.4
0.1 0.3
0.1 0.5
0.1 0.4
0.1 0.3
PPG-2000
ConcentrationMole (mole x
functionality)
0.0 0.0
0.0 0.0
0.0 0.0
0.1 0.2
0.1 0.2
0.1 0.2
0.0 0.0
0.0 0.0
0.0 0.0
0.1 0.2
0.1 0.2
0.1 0.2
Mole
0.35
0.28
0.21
0.49
0.42
0.35
0.35
0.28
0.21
0.49
0.42
0.35
E-4111
Vacuum-dried novolac resin and vacuum-dried diisocyanate
were added and mixed up in a cup at room temperature. It was then
stirred with a glass rod gently. Dibutyltin ditaurate (0.12 %) catalyst was
then added and mixed gently. The mixture was left undisturbed for
10-15 min,for the air bubbles to settle. It now looked like a warm
honey. Then it was transferred to the mould by pouring on the outer
edge first and then inside. No upper or tower miniscus was left. It was
then allowed to cure for 24 h without any disturbance. After 24 h, a
razor blade was inserted on the outer periphery and the polyurethane
sheet was removed gently from the mould. The percentage of hard
segment was also calculated.
3.2.2 Synthesis of polyurethanes based on synthesised polyols
The polyurethanes based on commercial polyol (PPG, 2000),
diphenytmethane diisocyanate or toluene diisocyanate, and synthesised
polyots CR 1 EH, CR2 EH, and CR3 EH were also prepared as discussed in
section 3.2.1. The isocyanate index in all these cases was also kept at
1.4. The mole ratios of the reactants are presented in Table 3.3. The
percentage of hard segment was also calculated.
Polyurethanes IDiisocyanate
CR1EHM
CR2EHM
CR3EHM
CR1EHMP
CR2EHMP
CR3EHMP
CR1EHT
CR 2 EHT
CR3EHT
CR1EHTP
CR2EHTP
CR3EHTP
Concentration
(mole x
functionality)
1.400
1.120
0.840
1.260
0.952
0. 588
1.400
1.120
0.840
1.260
0.952
0.588
Mole
0.700
0.560
0.420
0.680
0.476
0.294
0.700
0.560
0.420
0.680
0.476
0.294
Table 3.3
Formulation of polyurethanes based on polyols
81
Synthesised polyols
Concentration
Mole (mole x
functiortality)
0.10 1.00
(1.10 G80
0.10 0.60
0.07 0.70
0.06 0.48
0.05 0.30
0.10 1.00
0.10 0.80
0.10 0.60
0.07 0.70
0.06 0.48
0.05 0.30
Commercial Polyol(PPG-2000)
Concentration
Mole (mole x
functionality)
0.0 0.0
0.0 0-0
0.0 0.0
0.1 0.2
0.1 0.2
0.1 0.2
0.0 0.0
0.0 0.0
0.0 0.0
0.1 0.2
0.1
0.2
0.1
0.2
3.2.3 Spectral studies
Infrared spectral analysis was carried out for polyurethanes by
KBr pellet method using 3ASCO FT infrared spectrophotometer-410,
Japan. Pure and dry samples were used for recording the spectrum.
32.3.1 Determination of frequency shift values
The frequency shift, "&i" which has been measured as strength
of hydrogen bQnO is calculated using individual JR spectrum of
polyurethane by using the expression,
AV = 11 f + l)
82
where t-' = frequency of maximum absorption for the
free -NH groups
V b = frequency of maximum absorption for the
hydrogen bonded -NH groups.
32.4 Determination of crosslink density and molecular weightbetween crosslinks
The density of the polyurethanes was determined as per ASTM
D 792. The crosslink density (y) of the polyurethane was determined
from solubility parameter of the polyurethane. The solubility parameter
was determined by conducting swelling experiments using small
rectangular specimens in seven different solvents, starting from
n-hexane to glycerol having solubility parameters 7.3 to 16.5
[(cal cm 3 ) 112 ] respectively (Table 3.4).
The swelling coefficient 'Q' was calculated using the formula,
Weight of solvent in swelled polymer x dQ= ------------------------------------------------------------
Weight of the polymer subjected to swelling x dr
Where d 5 = density of solvent
dr = density of polymer.
83
Table 3.4
Solubility parameters of solvents used for determination of
solubility parameter of polyurethanes.
SolubilitySLN0. Solvent parameter
[(cal CM 3)1/2]
1 Hexane 7.3
2 Benzene 9.2
3 Acetone 9.9
4 Dimethyl acetamide 10.85 Dimethyl formam[de 12.1
6 Ethylene glycol 14.6
- 7 Glycerol S 16.5
The crosslink density or effective number of moles of
crosslinked unit per gram of polyurethane was determined using
modified Flory Rehner equation, 38
Vr + X Vr2 + In (lVr)y= -- -----------------------------=
drVü (Vr 1 " 3 - Vr12) M
Where Vr Volume fraction of polyurethane in swollen polymer
i.e V = 1/1+Q, where Q = swelling coefficient.
X = Polymer - Solvent interaction parameter
V0 = Molar volume of the solvent.
M = Molecular weight between two crosslinks.
dr = Density of the polyurethane.
IwIl
3.2.5 Solvent absorptivity percentage
Swelling behaviour of polyurethanes was also studied. Each
polyurethane sheet was put in 3 ml of different solvents for 16 h.
Excess of the solvent present on the surface of the polyurethane sheet
was removed in folds of a filter paper. Then it was weighed and the
solvent absorptivity percentage (SA %) was calculated using the
following equation,
(W2 - W1)SA% x 10
Wi
Where W 1 = Weight of the dry sample
W 2 = Weight of the sample after absorption of the solvent.
3.2.6 Thermal properties
The thermal properties of the new polyurethanes were
determined by Differential thermal analysis (DTA) and Thermo
gravimetric analysis (TGA). For the thermal analysis, a Dupont 2100
(USA), Shimatzu DT40 (Japan) and Mettler Toledo (Germany) thermal
analyzers were used. The sample was heated in a DTA analyzer from
ambient to 500°C at the heating rate of 10 O C/min under atmospheric
condition. The appearance of peaks for glass transition temperature,
softening temperature and decomposition temperature was checked for
all the polyurethanes. -
The thermogravimetric analyses (TGA) were carried out at the
heating rate of 10°C/mm. The samples were heated from ambient to
85
800 0 C under nitrogen atmosphere. The weight loss was noted for all the
polyurethanes.
3.2.7 Mechanical properties
The tear strength of the polyurethanes was determined as per
ASTM D 624-86 using unnicked 90 0 angle test specimens which were
punched out from cast sheets. A Zwick universal testing machine
(1435 Model-Germany) was used. Indentation hardness (shore A) was
determined as per ASTM D 2240-86. Polyurethane sheets were piled
together to get a thickness of 5 mm and used for hardness
measurement. A hardness tester (Durometer) was used. Tensile
strength of polyurethane sheets was determined using dumb-bell
specimens punched out from cast polyurethane sheets according to
ASTM D 412-87. The gauge length was fixed at 3 cm in each test. The
chart speed and cross head speed were 100 mm/mm. The tensile
strength and percentage elongation were calculated using standard
formulations. An average value of six test data was calculated and
presented. The average value lies within the standard deviation of 5%.
L.T[S1SI
3.3 RESULTS AND DISCUSSION
3.3.1 Synthesis of polyurethanes
The condensation reaction between novolac resins / synthesised
polyols and dilsocyanate can lead to the formation of stable urethane
linkages. The condensation reaction was found to be exothermic in both
types of resins. The reaction was carried out in the presence of
dibutyltin dilaurate catalyst and was completed within 15 mm. It was
found earlier that in the presence of organo tin catalyst and the absence
of water the reaction between the hydroxyl groups and isocyanate
groups gives urethanes at temperature below 100°C 3942 . Dusek43
proposed that the formation of urethane is the fastest reaction. Tin
compounds with shorter alkyl groups have higher reactivity than those
with longer groups. Diethyltin is the most effective catalyst, but due to
toxicity considerations, alkyl groups shorter than butyl are rarely used
commercially43.
The mechanism of catalytic behaviour of dibutyltin dilaurate has
been published already 44 . The reaction rate depends on the reactivity of
isocyanate groups and polyols. In the case of polypropylene glycol
(PPG) one end has secondary hydroxyl group and the other end has
primary hydroxyl group. The primary hydroxyl group is more reactive
than the secondary hydroxyl group which makes the polypropylene
glycol relatively lesser reactive than Polytetramethylene glycol (PTMG).
87
However difference in reactivity between PTMG and PPG polyol is less
pronounced".
Second order kinetics for the condensation between isocyanate
and hydroxyl group has been suggested by some investigators4647.
Pseudo first order kinetics was also suggested for the reaction of
aliphatic isocyanate with excess alcohol 48 . The most acceptable
mechanism for the polyurethane formation has been proposed by
Robin S49.
The ratio of isocyanate hydroxyl group, (1.4:1) is chosen in
the present synthesis of polyurethanes, so that excess of isocyanate
present leads to the formation of terminal isocyanate groups which has
been indicated by (Eq.13).
(n+1)OCN-R-NHCOO OCO NH - R - NCO + n HO - R'-OH
OCN (RNH COO OCONHRNHCOOR'OCONH) RNHCOO OCONHR - NCO
(Eq. 13)
The final curing of the reaction product leads to the formation of
allophanate linkages with the reaction involving terminal isocyanate
group with active hydrogen groups present in urethane groups of the
polymer (Eq.14)
YY'RNHC0O w'- OCO-NHR'-OCONH-R NCO
GO1H -Rw\-
wR-NHC00 'w OCO-NH-R' - OCO-N-R -w-
(Eq. 14)
The branching and the crosslinking in the present polyurethane
are possible due to the higher isocyanate index 1.4 and multifunctional
hydroxyalkylated resins. The present polyurethanes are composed of
variety of groups in the polymer chain including urethane, ether,
allophanate, hydrocarbon, aromatic in addition to unreacted hydroxyl
groups. Moreover the geometry and molecular weight of the hydroxy-
alkylated resin, polarity and molecular weight of the polyol are the other
factors which could influence the ultimate properties of the
polyurethanes.
3.31.1 Percentage of hard segments in the polyurethanes
based on novolac resins/synthesised polyols
The formulation of hard segment polyurethanes and the
commercial polyol-added polyurethanes based on novolac resins are
presented in Table 3.5.
r.x.reINI
Table 33
Percentage of hard segments in polyurethanes
based on novolac resins.
Polyurethane Hard Segment (%)CR 1 NI 100.0CR 2 M 100.0
CR3 M 100.0
CR I MP 71.4
CR2 MP 66.7
CR3I4P 60.0
CR 1 T 100.0
CR 2T 100.G
CR 3T 100.0
CR 1 TP 71.4
CR2TP 66.7
CR3TP 60.0
The hard segment content in CR 1 M, CR2 M, CR3 M, CR 1T, CR2T
and CR3T is l00%. Since these resins are multifunctional, they react
with bifunctional toluene diisocyanate or diphenylmethane diisocyanate
to give completely crosslinked structure with urethane and allophanate
structures. The hard segment content for the polyurethanes prepared
with the addition of commercial polyol is reduced and it ranges from1hc. cu-
71% to 60% in the polyol-added polyurethanes. Si-rni1ar4s the +estdts in
the case of synthesised polyols also (Table 3.6).
all
Table 3.6
Percentage of hard segments in polyurethanes
based on synthesised polyols
t
urethane Hard Segment (%)R 1 EHM 100.0
CR 2 EHM 100.0
CR3 EHM 100.0
CR 1 EHMP 77.8
CR2 EHMP 70.6
CR3 EHP4P 60.0
CR 1 EHT 100.0
CR2 EHT 100.0
CR3 EHT 100.0
CR 1 EHTP 77.8
CR2 EHTP 70.6
CR3 EHTP 60.0I
The formation of crosslinked product of hard segment
polyurethanes clearly indicates the completion of condensation reaction
leading to stable products. The addition of commercial polyol influences
the properties of the final product.
3.3.2 Spectral studies
Infrared spectral studies have been used in the present
investigation mainly to investigate the degree of hydrogen bonding,
which has greater influence on properties.
91
Infrared spectrum of the hard segment polyurethanes are
presented in Fig. 3.1 - 3.12. The JR spectral assignment for the
polyurethanes is presented in Table 3.7.
Table 3.7
IR Spectral assignments of polyurethane
Frequency of peak (cm')
3420-344534003315-33352955-30252855-29051720-17351635-165015401020-1110
AssignmentN-H Stretching (Free)0-H StretchingN-H Stretching (Bonded of polyurethane)Aromatic C-H StretchingC-H Stretching of methylene or alkylC=0 stretching (Free) in urethaneC=0 stretching (bonded) in urethaneN-H bending in urethaneC-0-C ether linkage.
The spectral data of the polyurethanes clearly indicates the
disappearance of peak due to isocyanate group at 2265 cm'. Similarly
no residual isocyanate was detected in any of the present polyurethanes.
The Hydrogen bonding was found in all the polyurethanes as shown in
Chart 3.1. The peak at 3400 cm' indicates the presence of 0-H
stretching which has been noticed for all the polyurethanes. The peak at
3425-3445 cm' indicates free N-H stretching frequency and the peak at
3315-3335 cm' indicates bonded N-H stretching frequency.
92
N
H Urethane
o- N —R---'--
HUrethane
-'''--R'---- N
0C
_/V\A R—O
R iMv\
N -
Allophanate
ifC—N---R!_/W\
HUrethane
Chart 3.1 Hydrogen bonding in polyurethanes
95
or)
go
4000 3000 2000 1000 4i;
Wavenumber[cm-1 I
Fig. 3.1 IR spectrum of CR1M
IN
1o!
35 3000 2000 1000 400
/be4fll.1]
Fig. 3.2 IR spectrum of CR2M
ico
1i]
oil H60'
I
4000 3000 2000 1000 400
Wavenwnbei[cinl]
Fig. 3.3 IR spectrum of CR3M
100
80
60
kP
40
20
0 11 I I I I I I I
4000 3000 2000 1000 40(
Wwenumber[cm- 1]
Fig. 3.4 IR spectrum of CR1T
too
go
204000
[71000 4003000 2000
Wawnib(cm.1J
Fig. 3.5 IR spectrum of CR2T
so
rl,
40
b
4000
3000 2800
1000 400
Wabfm.1J
Fig. 3.6 IR spectrum of CR3T
100
%T
1
80
%T
60
401 I
4000 3000 2000 1000 400
Wamimbecm1J
Fig. 3.7 IR spectrum of CR1EHM
110
80
70 L-
4000
3000 1000 1000 400
Wanumbe41)
Fig. 3.8 IR spectrum of CR2EHM
80
60
100
'
40 I I I I
4000 3000 2000 1000 400
WanumbcT(cm-1)
Fig. 3.9 IR spectrum of CR3EHM
100----.-
---.'
80-
60H
jI
20-1
tj
L
0 ---------------- I
I 4000 3000
2000 1000 40(
Wavenumber[cm- 1]
Fig. 310 IR spectrum of CR1EHT
20
10
4000
RA
80
60
40
3000
2000 1000 400
Wamnnbefcm.1J
Fig. 3.11 IR spectrum of CR2EHT
IN
80
20L
ON
3000 low low 40
Fig. 3.12 IR spectrum of CR3EHT
93
3.3.2.1 Frequency shift values
The frequency shift, "&.i " calculated from the individual IR
spectrum of all polyurethanes is presented in Table 3.8. The frequency
shift in the polyurethanes ranges from 110-152 cm'. In the cases of
diphenylmethane diisocyanate added soft segment polyurethanes, the
frequency shift is 110 cm -1 . But in the case of toluene diisocyanate
added soft segment polyuretharies, the frequency shift ranges from
126-138 cm' (Table 3.8).
Table 38
Frequency shift values of polyurethanes based on
novolac resins and synthesised polyols.
-Polyurethanes Au
CR 1 MP 110CR2 MP - 110CR4P -- 110CR ITP 152CR2TP 14&CR 3TP 148 -CR1 0EHMP 110CR2 CEHMP 110CR 3 CEF-tt4P 110CR I CEHTP 126CR2 CEHTP 132CR3 CEHTP 138
The frequency shift values indicate that these
polyurethanes are hydrogen bonded and crosslinked. The IR spectra of
some representative PPG-2000-added polyurethane are presented in
Fig. 3.13 - 3.16.
170
150
50
390 3000 2800 Iwo 400
Wathfcni1]
Fig. 3.13 IR spectrum of CR1MP
100
/
Q1\ 0%
_
1
/i
I j
601 1 / (
\ I i
/ ' I (VI V1i
0 I I I01 1 I
(Vj
IjJ
I I I IiI I I I Il!
I I I II I/If I 1I 1111 1
20 1
\/ I! I
Iiq
0.. ...__.
4000 3000 2000 1000 400
Wavenumberlern-11
Fig. 3.14 IR spectrum of CR1TP
200079' I
4000 3000 1000 400
IN
100
Vw
-11 V-/-,—
P
El
venumiIcm1]
Fig. 315 IR spectrum of CR1EHMP
4 300 0 20 1
nbc4n1J
Fig. 3.16 IR spectrum of CR1EHTP
94
3.3.3 Crosslink density and molecular weight between crosslinks
The density of the polyurethane prepared with higher mole ratio
of cardanol: formaldehyde is found to possess higher density in
comparison with that of other resins having lesser mole ratio.
The swelling coefficients of representative cases of
polyurethanes CR 1 M, CR 1 T, CR I MP and CR 1TP in the seven solvent
systems studied are presented in Table 3.9.
Table 3.9
Swelling coefficient of CR 1 M, CR 1T, CRIMP and CR1TP
in different solvents
Swelling coefficient "Q'I Polyurethane Hexane T
ieflZAcetoneJbMA DMflEthvie Glycerolglycol
CR 1 M 0.34. 0.48 073 1.16 T 81o. 0.24 0.10
0.40 0.63 1.14 1 0.64 0.17
9L0.96J1.21.07 0.42
0.65 0.82 1.19 0.92 0.36_1..IIT
CR1T
0.30
CRIMP 0.51
CR1TP 0.43
0.08
0.18
A graph between the soiubUty parameters of solvents in the
x-axis and the swelling coefficient of 'Q' of the polyurethanes in the
y-axis was plotted (Fig. 3.17). The peak of the curve gives the solubility
parameter of polyurethane (Op). Among all the solvents used the
solubility parameter of dimethyl acetamide (ös) was found to be the
solubility parameter of polyurethanes as there was maximum swelling
1.2
1.
z
. 0.8
C)000)C
C,)
0.4
0.2
j -e--CR1T
1MH
1TP
iMP;
1.4 -.-.--.-.-- ...-
0,
6
8 10 12 14 16 18
Solubility parameter of solvents (Cal/cm3)12
Fig. 3.17 Swelling coefficient curves of Polyurethanes
95
only in this solvent The polymer - solvent interaction parameter (X) is
given by the equation,
X = 13 + (Vs/RT) (6s-6p)2
Where Vs = Molar volume of solvent
R = Gas Constant
Os = Solubility parameter of DMA
Op = Solubility parameter of polyurethane
T = Absolute temperature
13 = Lattice constant.
When Os = Op, the polymer solvent interaction parameter (X)
becomes equal to the lattice constant (13) . Using solvent interaction
parameter X, the crosslink density of the polyurethanes is determined.
Crosslink density plays an important role in determining the
properties of polyurethanes 50 . With amorphous polymers, large increase
of crosslink density increases the properties such as hardness, glass
transition temperature and softening temperature 50 . With crystalline
polymers, small increase of crosslink density, changes the polymer from
high melting, hard dense crystalline polymer to a more elastic, softer
amorphous polymer. However with higher increase of crosslink density,
the effect observed with amorphous polymer could be noticed in the
crystalline polymers. The molecular weight between crosslinks (Me)
indicates the degree of crosslinking. Higher the M, lower will be the
crosslink density. The effective crosslink density of polyurethane is the
96
sum total of physical and chemical crosslinks. These crosslinked
polymer will only swell in a non-reactive solvent and do not dissolve in a
non-reactive solvent. The degree of swelling in a non-reactive solvent
determines the degree of crosslinking and molecular weight between
crosslinks.
In the present investigation the polyurethanes prepared only
from novolac resins/synthesised polyols are found to possess hard
segments (100%). The crosslink density of these polyurethanes is
found to be higher in comparison with that of the polyurethanes
prepared with addition of commercial polyol, PPG-2000 (Table 3.10).
Accordingly the molecular weight between crosslinks, Mc is also found to
be minimum in these cases. The percentage of hard segments in the
commercial polyol-added polyurethanes ranges from 71.4 - 60. The
reduced percentage of hard segment resulted in the reduction of
crosslink density in this class of polyurethanes. The low crosslink
density of commercial polyol-added polyurethanes may also be due to
the steric hindrance of the pendant methyl groups of Polypropylene
glycol.
97
Table 3.10
Characterisation of networks of polyurethanes of novolac resins.
Polyurethane
CR1M
CR2M
CR3 M
CRIMP
CR2MP
CR3MP
CR1T
CR2T
CR3T
CR1TP
CR2TP
CR3TP
Density(g/cc)
1.13
1.11
1.09
1.11
1.08
1.05
1.14
1.13
1.11
1.12
1.10
1.07
Swellingcoefficientin DMA(Q)
1.16
1.19
1.23
1.22
1.24
1.28
1.14
1.18
1.20
1.19
1.22
1.24
Crosslinkdensity(xlO3)
1.5052
1.4742
1.4328
1.4304
1.4262
1.4112
1.5440
1.4776
1.4532
1.4668
1.4434
1.4379
Molecularweight
between crosslinks (mole-')
664.36
678.33
697.91
699.08
701.16
708.62
647.67
676.77
688.14
681.76
692.81
695.46
Polyurethanes prepared with hydroxyalkylated cardanol-
formaldehyde resins, (synthesised polyols) also exhibit a very similar
behaviour as observed in the case of novolac resins (Table 3.11).
However, the polyurethanes based on synthesised polyols show higher
molecular weight between crosslinks. This is attributed to the variation
in geometry, structure and number of hydroxyl groups present in the
synthesised polyols. It is concluded that all the polyurethanes studied in
the present investigation are crosslinked polymers.
I
Table 3.11
Characterisation of Networks of polyurethanes of
hydroxyalkylated Cardanol formaldehyde resins.
Density(g / cc)
1.16
1.14
1.12
1.12
1.11
1.10
1.17
1.15
1.13
1.15
1.13
1.11
Cross linkdensity(xlO3)
1.3178
1.2587
1.2076
1.2076
1.0919
0.9973
1.3537
1.2868
1. 23 18
1.2119
1.1313
1.0919
Molecularweight
betweencross links(mole')758.84
794.47
828.09
828.09
915.83
1002.70
738.72
777.12
811.82 -
825.15
883.74
915.83
Polyurethane
I CREHM
PCR2EHM
LCR3EHM
LC R 1 E H M P
C R2C E H M P
E H M P
CR I EHT
CR2 EHT
CR3 EHT
CR1EHTP
CR2EHTP
CR3EHTP
Swellingcoefficient in
DMA(Q)
1.25
1.30
1.35
1.35
1.44
1.53
1.22
1.27
1.32
1.32
1.39
1.44
3.3.4 Solvent absorptivity percentage
From the data of solvent absorptivity percentage (SA %)
furnished in Table 3.12 and 3.13, the following inferences can be drawn.
The solvent absorptivity percentage of all the polyurethanes prepared
from novolac resins increases from the non-polar to polar solvents
indicating the hydrophobic nature of these polyurethanes. Maximum
swelling is noticed for all the polyurethanes in polar aprotic solvents like
DMF and DMA.
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101
Diphenylmethane diisocyanate treated polyurethanes have
higher SA% in all the solvents when compared to that of toluene
diisocyanate treated polyurethanes indicating the more hydrophobicity in
the former case than the latter. Similar solvent absorptivity percentage
is also noticed in the case of polyurethanes prepared from synthesised
polyols.
3.3.5 Thermal studies
Crosslink density, molecular weight between crosslinks,
percentage hard segment, percentage soft segment largely influence the
thermal properties of the polyurethanes 51 . Presence of long alkyl side
chain at the meta position and the hard segment existing between
urethane linkages largely influence the thermal properties of hard
segment polyurethanes based on diphenylmethane diisocyanate or
toluene diisocyanate. In the case of soft segment polyurethanes based
on diphenylmethane dilsocyanate or toluene diisocyanate, apart from
the above reasons, presence of flexible polyether polyol segment also
accounts for the thermal behaviour.
3.3.5.1 Differential thermal analysis
Differential thermal analysis of some diphenylmethane
dilsocyanate or toluene dilsocyanate treated hard segment
polyurethanes and the commercial polyol added soft segment
polyurethanes of novolac resins are presented in Fig.3.18-3.22. No
endothermic peak has been noticed in all these polyurethanes
(Table 3.13). However, both the hard and soft segment polyurethanes
18
14
12
CtO
0>
BaC.U
BX
100
90
so
, 70CSCi 60
Sa.
40
30
20
to
0
2
0
-e
80
60 -
40 -J
20
Deg C
Fig. 3.18 TGA and DTA curves of CR1M
---- --- -------------OH
100..
U
0.4
U
0-O
- I
-.--- ..- --0.
200 400 600 800
To91joratu'0 (C)
Fig. 3.19 TGA and DTA curves of CRIMP
04---0
r.
C
:00 00 130 :00 :)0C bOO
P.0
00 -
80-1
•1
(30 --
.0
Fig. 3.20 TGA and DTA curves of CR3MP
Fig. 3.21 TGA and DTA curves of CR3T
o H
//
1
'I100 200 300 400 5C0 600 700
0 0 40 50 60 70
Fig. 3.22 TGA and DTA curves of CR1TP
102
show two exotherms, a weak one around 300 0 C, and a strong one above
380 0C, ranging from 380-645 0C. The weak exotherm is due to the
cleavage of meta-substituted alkyl side chain in the phenyl ring. This is
well in conformity with the results reported earlier 51-53
Table 3.13
Differential Thermal Analysis Data of
polyurethanes of Novolac resins
Polyurethane Exotherm (°C)
First
Second
CR1M
301
645CR2M
295
630CR3M
290
600CR1M
300
400CR2M
280
390CR3M
275
380CR1T
315
330CR2T
310
325CR3T
300
320CR1TP
300
390CR2TP
295
393CR3TP
285
380
Highest exotherm 645 0C is noticed in the case of
diphenylmethane dilsocyanate treated hard segment polyurethane,
CR 1 M. The corresponding soft segment polyurethane namely, CRiMP
shows the second exotherm only at 400 0C. In the case of toluene
diisocyanate treated polyurethanes also, the second exotherm is found
to be maximum in the hard segment case than the corresponding soft
segment.
103
There is a gradual decrease in ithe second exotherm when we
move from the higher mole ratio of cardanol : formaldehyde
polyurethanes to that of lower mole ratio of cardanol : formaldehyde
polyurethanes.
The differential thermal analysis very clearly indicates the
thermal stability of diphenylmethane dilsocyanate treated polyurethanes
when compared to toluene dilsocyanate treated polyurethanes.
In the case of synthesised hydroxyalkylated cardanol-
formaldehyde resins based polyurethanes with diphenylmethane
diisocyanate or toluene diisocyanate (Fig. 3.23-3.28) also exhibit a
similar behaviour. The commercial polyol treated polyurethanes also
exhibit a very similar trend (Table 3.14).
Table 3.14
Differential Thermal Analysis Data of
Polyurethanes of synthesised polyols
Polyurethane Exotherm (°C)
First Second
CR 1 EHM 377 616CR 2 EHM 350 540CR3 EHM 329 453CR1EHMP 303 366CR2 EHMP -- 350 390CR3 EHMP 280 380CR1EHT 295 395CR2 EHT 290 390CR3 EHT 280 385CR 1 EHTP 370 416CR2 EHTP - 360 410CR3EHTP 350 - 407
/
U
100
90
80
70
60
50
40
30-
20
10
00
Fig. 3.23 TGA and DTA curves of CR1EHM
05
0,15-1
Cl
I\ /7
-005
, \\
0 0100 200 300 400 500 600
Temperature (°C)
Fig. 324 TGA and DTA curves of CR3EHM
tQ -iITB
toB
B
7B
0>
U.4
4 x
a
a
I
0
-t
100
go.
' B0rB
70•EL -
60
go
40
30
100 200 300 400 500Temp erature (°C)
6
-2
210
too l--
.lJ
0
4
U0
4-IC
2C,
>
LC,
0
P00
Fig. 3.25 TGA and DTA curves of CR1EHMP
Fig. 3.26 TGA and DTA curves of CR3EHMP
18
£8
£4
1243
10
4
2
0
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- ----- ------- ----- -- -----.-- -v------,--
'S\
1 'S
\ I
5 0.5 II
n n-1 I I
\1
100 200 300 403 500 600 700
I I I . I I I I I I II I I I0 10 20 30 40 50 60 70 mi n
Fig. 3.27 TGA and DTA curves of CR1EHT
0
100
So -
so
, 70
80 60C.0a. 80•
30-
20-
£0 -
0-
Dog C
Fig. 3.28 TGA and DTA curves of CR3EHTP
104
In the hydroxyalkylated cardanol-formaldehyde resins also,
diphenylmethane diisocyanate treated hard segment polyurethanes of
higher mole ratio is exhibiting the highest second exotherm, indicating
its maximum thermal stability.
3.3.5.2 Thermo gravimetric studies
Thermograms of some representative polyurethanes are
presented in Fig.3.29-3.33. The data showing the percentage weight
loss at various temperature ranges are furnished in Table 3.15 and
Table 3.16.
Table 3.15
Thermo gravimetric analysis data of
Polyurethanes of novolac resins
In the case of polyurethanes prepared from cardanol-
formaldehyde novolac resins, thermal stability is found to be less when
compared to that of polyurethanes based on phenol-formaldehyde
resins51 . This may be attributed to the stereo chemical crowding of alkyl
Fig 3.29 TGA curve of CR3M
-- - -- --------.-------
00
70
IC
30
?0
olog
COO 'Co 200 XC lao 500 500 ICC 000 0053
Fig. 3.30 TGA curve of CR1T
120 --. •--.--- .--..-.-----
100
80-2' I
wI 60IC:0 I
C1 40
i 20
U r------r-
0 50 100 150 200 250 300 350 400 450
DegC
Fig. 3.31 TGA curve of CR3TP
120
100
801
60
401
2:1
100 200 300 400 500 600
Temperature (°C)
Fig. 3.32 TGA curve of CR3EHT
Fig. 3.33 TGA curve of CR1EHTP
105
side chain at the meta position of the cardanol, which may decrease the
case of crosslinking and also their higher thermal degrading aptitude as
compared to benzene nucleus.
Both the polyurethanes developed using cardanol-formaldehyde
novolac resins and the hydroxyalkylated cardanol-formaldehyde resins
(synthesised polyols) showed the following thermally induced
phenomena:
(i) A minor weight loss (v 2%) has been observed in the temperature
range of 0-200 0C, due to the moisture present in the sample.
(ii) A gradual weight loss which occurs in the temperature range of
2000 -3000C, may be due to the re-crosslinking or post curing
process. Re-crosslinking in these polyurethanes makes them more
rigid. The new crosslinks formed in these polyurethanes develop a
strain in the macro molecular chains. The small groups present
outside the macro molecular structure are released with a weight
loss of about 50%.
(iii) 85% weight loss occurs in the temperature range 3000-5000C,
which may be due to the segmental release of larger groups.
(iv) Pre polymeric part has been left as the char residue in the
temperature range of 5000-6000C.
In the present investigation (Table 3.15), it has also been found
that both the hard segment and soft segment polyurethanes derived
from novolac resins and diphenylmethane diisocyanate are found to
undergo no weight loss up to 1000C indicating the absence of moisture.
106
Even in the case of toluene dilsocyanate treated hard and soft segment
polyurethanes, a very small weight loss (within 1%) has been noticed.
In both the cases, hard segment polyurethanes are thermally stable
than soft segment polyurethanes.
Between the diphenylmethane dilsocyanate and toluene
diisocyanate treated polyurethanes, the former ones are found to be
thermally stable than the latter. In both the cases, the hard segment
polyurethanes are thermally stable up to 650 0C and the soft segment
polyurethanes are thermally stable up to 5000C.
The percentage weight loss, even at higher temperature, in the
case of higher mole ratio of cardanol formaldehyde hard segment
polyurethanes are comparatively lower than that of lower mole ratio of
cardanol : formaldehyde polyurethanes, indicating the higher stability of
these polyurethanes. The same trend has been noticed in the case of
soft segment polyurethanes also.
In the case of polyurethanes prepared from synthesized polyols
also (Table 3.16), a similar trend has been observed.
107
Table 3.16
Thermo gravimetric analysis data of
Polyurethanes of synthesised polyols
Polyurethane % Weight loss at various temperature °C
100 200 300 400 500
CR 1 EHM 1.0 5.5 16.0 47.0 63.0CR2 EHM 1.0 6.0 21.0 52.0 72.0CR3 EHM 1.0 6.0 25.5 55.0 78.5CR1EHMP 0 0 1.0 57.3 CR2 EHMP 0 0 2.0 60.0 CR3 EHMP 0.4 2.1 19.0 61.5 CR1EHT 0 1.7 15.0 51.0 77.0CR2 EHT 0 1.8 16.0 54.0 78.5CR3 EHT 0 2.0 21.0 60.0 80.0CR 1 EHTP 1.0 3.0 106 57.5 87.0CR2 EHTP 1.0 2.0 11.8 64.8 87.8CR3 EHTP 1.0 2.0 13.1 78.8 88.8
The thermal stability of diphenylmethane diisocyanate treated
polyurethanes has been reflected in the mechanical properties also.
3.3.6 Mechanical properties
The mechanical properties of the polyurethanes especially
tensile strength and tear strength are largely influenced by the presence
of aromatic groups, ether groups, long alkyl chain, dangling chains,
branching and crosslinking and also degree of secondary bonding forces
(Hydrogen bonding)50.
3.3.6.1 Tear characters
Tear test dataof the polyurethanes of the present investigation
are presented in Table 3.17 and Table 3.18.
108
Table 3.17
Tear characters of polyurethanes of novolac resins
Molecularweight Tear Elongation I Tear
Polyurethane between strength (%) Moduluscross links (kN/m) (kN/m)(mole')
CR 1 M 664.36 B B BCR2 M 678.33 - B B BCR3 M 697.91 B B BCRI MP 699.08 120.0 75.0 160.0CR2 MP 701.16 115.0 82.0 140.2CR3 MP 708.62 110.0 85.0 129.4CR 1 T 647.67 B B BCR2T 676.77 B B BCR3T 688.14 B B BCR 1 TP 681.76 70.0 70.0 100.0CR2TP 692.81 65.0 75.0 86.7CR3TP 695.46 60.0 80.0 75.0
B = Brittle
Both the hard segment polyurethanes prepared from
diphenylmethane diisocyanate and toluene diisocyanate, crumbles
during tear test indicating their brittleness. The poor tear characteristics
in these polyurethanes may be attributed to the higher crosslink density.
As the commercial polyol, PPG-2000 is being added, the percentage
elongation increases thereby indicating the increase in degree of
flexibility in these polyurethanes.
Tearstrength(kN/m)
BBB
115.0112.0100.0
BBB
95.090.085.0
Elongation(%)
111111]
S.
TearModulus(kN/m)
BBB
127.8112.0
90.9BBB
111.8100.089.5
109
Table 3.18
Tear properties of hydroxya!kylated cardanol formaldehyde
resins (synthesised polyols)
Polyurethane
CR1EHMCR2EHMCR3EHMCR1EHMPCR2EHMPCR3EHMPCR1EHTCR2 EHT --CR3EHTCR1EHTPCR2EHTPCR3 EHTP
B = Brittle
Molecularweight
betweencross links
(mole-")758.84 -794.47828.09828.09
- 915.831002.70738.72777.12811.82825.15883.74 -915.83
Similar is the trend noticed in the case of hydroxyalkylated
cardanol formaldehyde resins based polyurethanes. The present study
reveals that diphenylmethane diisocyanate treated polyurethanes are
mechanically stable than toluene diisocyanate treated polyurethanes.
110
3.3.6.2 Shore hardness
Shore hardness of polymers is defined as the resistance offered
by the polymeric material to the penetration of truncated cone
(shore 'A'). The shore hardness of the polyurethanes of the present
investigation is presented in Tables 3.19 and 3.20 respectively.
Table 3.19
Hardness of polyurethanes of novolac resins
Hard Molecular weight HardnessPolyurethane between crossShore 'A'segment
links (mole')CR 1 M 100 664.36 88
CR2 M 100 678.33 85CR3 M 100 697.91 80
CR 1 MP 71.4 699.08 65CR2 MP 66.7 701.16 60CR3 MP 60.0 708.62 55CR 1T 100 647.67 89CR2T 100 676.77 85CR3T 100 688.14 80CR 1TP 71.4 681.76 60
CR2TP 66.7 692.81 55
CR3TP 60.0 695.46 50
In the present investigation the shore hardness of hard
segment polyurethanes are found to be more than that of the soft
segment polyurethanes. This may be attributed to the presence of hard
segment percentage in these polyurethanes. The higher shore hardness
in both the hard and soft segment polyurethanes of the higher mole
111
ratios supports the higher crosslink density and lower molecular weight
between the crosslinks.
Table 3.20
Hardness of polyurethanes of hydroxyalkylated cardanol
formaldehyde resins (synthesised polyols)
Hard Molecular weight HardnessPolyurethane between crossShore 'A'segment
links (mole')CR 1 EHM 100 758.84 97
CR2 EHM 100 794.47 96
CR3 EHM 100 - 828.09 92
CR 1 EHMP 77.8 828.09 67
CR2 EHMP 70.6 915.83 62
CR3 EHMP 60.0 1002.70 53
CR 1 EHT 100 738.72 97
CR2 EHT 100 777.12 95CR3 EHT 100 811.82 93
CR 1 EHTP 77.8 825.15 65
CR2 EHTP 70.6 883.74 60CR3EHTP 60.0 915.83 55
Similar trend has also been noticed in the case of polyurethanes
prepared from synthesised polyols (Table 3.20).
3.3.6.3 Tensile properties
The tensile properties of the polyurethanes based on novolac
resins and hydroxyalkylated cardanol formaldehyde resins (synthesised
polyols) are presented in Tables 3.21 and 3.22 respectively.
112
Table 3.21
Tensile properties of polyurethanes of novolac resins
Molecular
weight Tensile . TensilePolyurethane between strength Elongation
Oj Moduluscross links (MPa) (MPa)
(mole-'-)CR 1 M 664.36 - B -B-- BCR2 M 678.33 B -. B BCR3M 697.91 B B BCR 1 MP 699.08 -21.47 120 17.89CR2MP- 701.16 19.16 128 14.97CR3 MP - 708.62 18.43 135 13.65CR 1T 647.67 B -- B BCR2T 676.77 B B BCR3T 688.14 B B BCR 1 TP 681.76 20.46 - 110 18.60CR2TP 692.811 18.52 117 15.43CR3TP 695.46 17.21 - 125 13.77B = Brittle
Table 3.25
Tensile properties of polyurethanes of hydroxyalkylated cardanol
formaldehyde resins (polyols)
Molecular Tensile Tensile
weight Elongation ModulusPolyurethane between cross strength (%)links (mole-')
(MPa) (MPa)
CR1 EHM 758.84 B BCR2 EHM 794.47 B B BCR3 EHM 828.09 B B BCR 1 EHMP 828.09 28.82 150 19.21CR2 EHMP 915.83 26.64 - 155 17.19CR3 EHMP 1002.70 24.49 163 15.02CR 1 EHT 738.72 B B BCR2 EHT 777.12 B B - BCR3 EHT 811.82 B -- B BCR 1 EHTP 825.15 25.67 140 18.34CR2 EHTP 883.74 24.34 145 16.79CR3EHTP 915.83 } 23.18 150 15.45
B = Brittle
113
The tensile strength of polyurethanes prepared from novolac
resins varies from 17.21 MPa to 21.47 MPa. In the case of hard segment
polyurethanes prepared from both diphenylmethane diisocyanate and
toluene diisocyanate are found to be brittle. The tensile strength for
diphenylmethane diisocyanate treated soft segment polyurethanes are
found to be higher when compared to that of toluene diisocyanate
treated soft segment polyurethanes. Tensile properties of the prepared
polyurethanes very clearly support the higher crosslink density in these
polyurethanes.
Similar trend is also noticed in the case of polyurethanes
prepared from synthesised polyols. Tensile stress-elongation curves of
these polyurethanes are presented in Fig. 3.34-3.37. From the figure, it
can be inferred that the polyurethanes changes from rigid to tough
character.
16 -
14
12
(50
(n(8
U,C)
U,C 6C)I-
14 4
- 1' -
0
10
8
CC)I-6
4
2
0
—
-- - ---- -----
,- - ------
- —s-- _---
--- CR1 MP-g-CR2MP
- L CR3MP
10 20 30 40 50 60 70 80 90 100 110 120 130 140
Elongation (%)
Fig. 3.34 Tensile stress-elongation curves of MDI treated softsegment polyurethanes based on novolac resins
4
-R- CR2TP
:..____0 10 20 30 40 50 60 70 80 90 100 110 120 130 140
Elongation (%)
Fig. 3.35 Tensile stress-elongation curves of TDI treated softsegment polyurethanes based on novolac resins
--CR1EHMP
3 -D--CR2EHMP
-á-CR3EHMP
0 - - -0 20 40 60 80 100 120 140 160 180
Elongation (%)
Fig. 3.36 Tensile stress-elongation curves of MDI treated softsegment polyurethanes based on synthesised polyols
20
18
16
14
05 12U,U)
10U)
C)
C)I-
6
4
2
U ------,- -------.-0 20 40 60 80 100 120 140
Elongation (%)
Fig. 3.37 Tensile stress-elongation curves of TDI treated softsegment polyurethanes based on synthesised polyols
114
3.4 CONCLUSION
From the present study, it can be concluded that the hard
segment polyurethanes synthesised from cardanol-formaldehyde
novolac resins and hydroxyalkylated cardanol-formaldehyde resins
(synthesised polyols) are found to possess higher thermal stability than
the soft segment polyurethanes. Diphenylmethane diisocyanate treated
polyurethanes in both the cases are found to be mechanically and
thermally stable than the toluene diisocyanate treated polyurethanes.
The performance character also reflects the good thermal and
mechanical stability of diphenylmethane diisocyanate treated
polyurethanes.
115
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