Chapter II
Department of Materials Science 49
2.1 INTRODUCTION
The blending of polymers is now a days the most versatile method
for producing unique combination of properties such as mechanical,
thermal, flame retardant, etc. Literature survey reveals that considerable
work has been done on the blends of epoxies and polyurethanes [1-3]. In
most of these cases attempts have been made to improve physical and
mechanical properties of the epoxies but very little work has been done so
far to obtain flame retardant polyurethane/epoxy blend. However,
sufficient literatures are available where researchers have tried to prepare
flame retardant polyurethane and epoxy blends by mixing these polymers
individually with other polymer components rather than combining
polyurethane with epoxies. Discussed here are few recently developed
flame retardant systems.
Organic polyphosphazenes are used [4] in polymer blends and
interpenetrating polymer networks (IPNs) to impart flame retardancy.
Due to the presence of alternating phosphorous, nitrogen moiety in the
backbone, they have good thermo-oxidative stability. Hundreds of
polyphosphazenes [5] are reported with different side groups. But due to
their high cost, use is only limited to military applications.
Pin-Sheng Wang and his group have prepared polyurethane blends
with poly (bis-propoxy phosphazene) [6]. The blends follow two-step
degradation process and also have higher char yield at 550 OC. Their LOI
values are also higher than the neat polyurethanes.
Xelei Chen and his research team have reported UV-curable
blends of polyurethane acrylate modified with a phosphorous monomer
[7]. Combustion and thermal behaviour of the blends have been found
Chapter II
Department of Materials Science 50
out. Results show that the blends have good flame retardancy and burning
process can be explained on basis of a condensed phase mechanism.
Gouri et al [8] have synthesized hexaglycidyl cyclotriphosphazene
(HGCP) and used as a reactive flame retardant to blend with
commercially available epoxy resin (DGEBA). The blends of HGCP with
DGEBA improve the thermal stability at elevated temperature with
higher char yields compare to neat DGEBA thermoset.
2.2 AIM OF THE PRESENT WORK
At the present work, attempts have been made to develop certain
phosphorous containing flame retardant polymer blends based on
polyurethanes and epoxies. These polyurethanes, epoxies and their
respective monomers have been characterized by various chemical and
instrumental analysis techniques. These polymers have also been used to
prepare polymer nanocomposites. Polymer blends have been used for the
casting of films and coating for different substrates. Thermal, mechanical
and flame retardant properties of the neat as well as blends have been
found out.
This chapter deals with the synthesis and characterization of
monomers and polymers. The entire work has been divided into
following parts:
1. Synthesis and characterization of phosphorous containing
monomers: diols and polyols have been synthesized in the
laboratory
2. Synthesis of phosphorous containing flame retardant polyurethane
resins by reacting the phosphorous containing diols or polyols with
different diisocyanates
Chapter II
Department of Materials Science 51
3. Synthesis of phosphorous containing epoxy resins by reacting
phosphorous containing triols with epichlorohydrine
4. Characterization of the polyurethanes and epoxy resins using
chemical and instrumental analysis techniques
2.3 METHODS USED FOR THE CHARACTERIZATION OF
MONOMERS AND POLYMERS
The synthesized monomers and polymers are characterized by
various chemical analysis and instrumental analysis techniques which are
described below.
2.3.1 Chemicals analysis methods
(i) Determination of number of hydroxyl groups
Number of hydroxyl groups in monomers was determined using
acetylation method [9]. This method involves replacement of the
hydrogen on a hydroxyl group by acetyl group. The reagent used was
acetic anhydride in pyridine and was used in excess. Following reactions
occur
R(OH) n(CH3CO)
2O R(OCOCH
3) nCH
3COOH (CH
3CO)
2O
(CH3CO)
2O CH
3COOH
+ + + (Excess)nn
(Excess) + H2O 2
Addition of water converts excess of acetic anhydride into acetic
acid. The total free acid was determined by titrating with standard sodium
hydroxide solution.
A blank experiment was performed simultaneously identical as the
above in absence of sample. The difference in the volume of sodium
hydroxide solution required in the two experiments is equivalent to the
difference in the amount of acetic acid formed. By knowing the molecular
Chapter II
Department of Materials Science 52
weight of the compound, number of hydroxyl group can be calculated
using the following equation
Number of hydroxyl group = 1000 W
weightMolecular N )V-(V 12
Where, 42.02 =
W = Weight of sample in gm.
V2 = Volume of sodium hydroxide used in blank
V1 = Volume of sodium hydroxide used in sample
N = Normality of sodium hydroxide used in estimation
56.1 X 1000
Hydroxy Equivalent Weight = -------------------------
Number of OH group
Where, 56.1 = Equivalent Weight of KOH
(ii) Determination of percentage of isocyanates
The percentage of free isocyanate groups was determined by
reacting the sample containing isocyanate groups with excess of standard
n-butylamine in dioxane reagent [10, 11].
R–N=C=O + C4H9NH2 RNHCONHC4H9 + C4H9NH2
(Excess)
The excess n-butylamine was titrated with standard acid. A blank
experiment has also been performed in absence of sample.
Percentage of isocyanates group was calculated using the following
equation
% NCO = 1000 W
100 42.02 N )V-(V 12
Chapter II
Department of Materials Science 53
W = Weight of sample in gm
V2 = Volume of acid used in blank
V1 = Volume of acid used in sample
N = Normality of acid used in estimation
(iii) Determination of epoxy equivalent weight (EEW)
The epoxy equivalent weight was determined by reacting the
sample containing epoxy groups with excess of standard HCl in dioxane
reagent [12, 13].
R CH2
CH CH2
O
ClH R CH2
CH CH2
OH
Cl ClH+ + (Excess)
The excess HCl was titrated with standard alcoholic NaOH
solution. A blank experiment has also been carried out in absence of
sample. Epoxy equivalent weight of the sample was calculated using the
following equation.
Epoxy Equivalent Weight (EEW) =1000 W
(V2-V1) N
W = Weight of the sample in gm
V1 = Volume of alc. NaOH used in sample
V2 = Volume of alc. NaOH used in blank
N = Normality of alc. NaOH used in estimation
Chapter II
Department of Materials Science 54
2.3.2 Instrumental analysis techniques
(i) Measurement of Intrinsic Viscosity (η)
Intrinsic viscosity of the synthesized polymers was determined by
Ubbelohde Suspended Level Viscometer (USLV) in methyl ethyl ketone
(MEK) solvent at the temperature of 30 OC.
(ii) Determination of average molecular weight of polymers
Number average molecular weight of the polyurethanes and
epoxies were determined by Gel Permeation Chromatographic (GPC)
technique using Perkin Elmer USA (Model Series 200).
(iii) Elemental analysis
Elemental analysis of the monomers and polyurethanes was carried
out using PERKIN ELMER USA (Model 2400 SERIES II). Theoretical
and experimental values in terms of percentage have been given in the
Tables.
(iv) Fourier transform infra-red (FTIR) analysis
The fourier transform infra-red spectra of the monomers and
polymers were recorded in KBr pallet on Perkin Elmer USA (Model
SPECTRUM GX, FT-IR spectrophotometer).
(v) Nuclear magnetic resonance (NMR) spectroscopy
1H NMR spectrum of the certain monomers was taken in DMSO
solvent using tetramethylsilane as reference on Bruker, Switzerland
Model AVANCE 400.
Chapter II
Department of Materials Science 55
2.4 EXPERIMENTAL
Different polyurethanes and epoxies synthesized in the laboratory
are given below.
2.4.1 Phosphorous based flame retardant polyurethanes
Phosphorous containing polyurethanes synthesized in the
laboratory are divided into groups as follows
(i) Mono phosphorous based polyurethanes
[a] Polyurethanes based on tris (m-hydroxy phenyl) phosphate
[b] Polyurethanes based on tris (bisphenol-A) monophosphate
[c] Polyurethanes based on bis (m-hydroxy phenyl) methyl
phosphine oxide
[d] Polyurethanes based on ester exchange reaction product of castor
oil and tris (m-hydroxy phenyl) phosphate
(ii) Diphosphorous based polyurethanes
[a] Polyurethanes based on resorcinol bis (hydroxy phenyl
phosphonate)
[b] Polyurethanes based on bisphenol-A bis (hydroxy phenyl
phosphonate)
2.4.2 Phosphorous based flame retardant epoxies
Phosphorous containing epoxy resins synthesized in the laboratory
are
[a] Triglycidyl ether of tris (bisphenol-A) monophosphate
(TGETBAMP)
[b] Triglycidyl ether of tris (m-hydroxy phenyl) phosphate
(TGETHPP)
2.4.3 Non-phosphorous based liquid epoxy resin
Diglycidyl ether of bisphenol – A (DGEBA) was procured from
local market and used for the study.
Chapter II
Department of Materials Science 56
The graphical presentation of the polymers used for the present
study is shown below.
Figure 2.1: Method of synthesis and their characterization
2.4.1 Phosphorous based flame retardant polyurethanes
(i) Synthesis of mono phosphorous based polyurethanes
[a] Polyurethanes based on tris (m-hydroxy phenyl) phosphate
(i) Synthesis of the monomer: Tris (m-hydroxy phenyl) phosphate
(THPP)
In a 250 ml round bottom flask equipped with a condenser,
phosphorous oxychloride (POCl3: 153 gm, 1 mole) and resorcinol (330
gm, 3.0 mole) were heated at 110 OC for 2.0 hours in presence of N, N-
dimethylaniline (PhNMe2) (3.4 ml, 0.027 mole). The reaction mixture
was further heated at 110-115 0C for 1.5 hours to get the title product.
Chapter II
Department of Materials Science 57
Product was purified by washing and neutralized to pH 7 to obtain solid
product. Crystallization was done in methanol [14].
Characterization:
(i) Melting point: 214-216 OC
(ii) Percentage yield: 75 %
(iii) Number of hydroxyl groups: 2.91
(iv) Elemental analysis (%)
C H
59.17
(57.4)
3.72
(4.01) (Calculated values are listed in parenthesis)
(ii) Infra-red analysis
IR spectrum of the monomer (THPP) shows broad absorption band
at 3388 cm-1
due to hydrogen bonded hydroxyl species, the band at 1261
cm-1
corresponds to Ar-OH symmetric stretching frequency, C=C
stretching of aromatic ring is obtained at 1494 cm-1
, the bands at 1252
cm-1
and 991 cm-1
are due to the P=O and P-O-C stretching frequencies
respectively. The aromatic C-H stretch appears at 3000 cm-1
.
Figure 2.2: FTIR spectrum of tris-(m-hydroxy phenyl)phosphate (THPP)
Chapter II
Department of Materials Science 58
(ii) Synthesis of polyurethanes
Polyurethanes were synthesized by reacting tris (m-hydroxy
phenyl) phosphate with different diisocyanates as shown in the reaction
scheme-1 [15]. To synthesize the PU-1, tris-(m-hydroxy phenyl)
phosphate (THPP, 1.0 mole) and solvent methyl ethyl ketone (MEK)
were charged into a three necked flask equipped with dropping funnel,
reflux condenser and over head stirrer. Toluene diisocyanate (TDI, 2.4
mole) was added in a dropwise manner into the reaction mixture over a
period of 1 hour with constant stirring at 60 OC. The molar ratio of OH to
NCO was kept as 1:1.6 to produce NCO terminated polyurethane. After
the completion of addition, heating was continued for further 2 hours to
complete the reaction. After vacuum distillation pure NCO terminated
polyurethane (PU-1) was obtained.
Other two polyurethanes PU-2 and PU-3 were also synthesized by
reacting the THPP with another two diisocyanates, isophoron
diisocyanate (IPDI) and hexamethylene diisocyanate (HMDI)
respectively in the same manner. Reaction conditions and other
particulars are given in the Table 2.4.1.
Chapter II
Department of Materials Science 59
Reaction Scheme 1:
OH
OH
P ClCl
Cl
O
OCN R NCO
O
OH
O
O
OHO
P
OH
O
O CO NH R NH CO NCO
O
O
PO
CH3 CH
3
CH3
CH3
CH3
CH2 (CH
2)6
+110 0C, 2 hrs
N,N-dimethyl aniline
60 0C, 3.0 hrs
ResorcinolPhosphorous oxychloride
Tris (m-hydroxy phenyl) phosphate (THPP)
Diisocyanates
Polyurethane based on tris (m-hydroxy phenyl) phosphate
110-115 0C, 1.5 hrs
n
Where R =
PU-1 PU-2 PU-3
Chapter II
Department of Materials Science 60
Characterization
Table 2.1
(i) Reaction Conditions and Specification of the Polyurethanes
(ii) Elemental analysis (%)
System C H N
PU-1 60.76
(60.07)
4.23
(4.00)
9.11
(9.34)
PU-2 60.84
(60.96)
6.70
(6.87)
8.46
(8.37)
PU-3 57.69
(57.21)
6.45
(6.13)
9.45
(9.53) (Calculated values are listed in parenthesis)
(iii) Infra-red analysis
In IR spectra, the broad bands around 3327-3389 cm-1
attribute to N-
H band, the bands at 1607-1677 cm-1
are for -NHCOO group. The
absorption bands around 2272-2281 cm-1
are due to free NCO groups in
polyurethane which are absent in the monomer. Bands around 1220-1243
cm-1
are observed for P= O group. The bands at 985 -995 cm-1
are due to
the presence of P-O-C frequency. The bands around 3000 – 3100 cm-1
are
due to the stretching frequency of aromatic C-H bond.
System Types of
isocyanate
used
Reaction
conditions
%
NCO
content
Intrinsic
viscosity
η
dl/gm
GPC
Data
Temp. OC
Time
hrs. Mn
Mw
Mn
PU-1 TDI 60 3 1.6 0.61 2304 1.8
PU-2 IPDI 60 3 1.8 0.57 2530 2.2
PU-3 HMDI 60 3 1.8 0.52 2231 1.9
Chapter II
Department of Materials Science 61
Figure 2.3: FTIR spectra of polyurethanes based on tris-(m-hydroxy
Phenyl) phosphate (THPP) (PU-1 to PU- 3)
Chapter II
Department of Materials Science 62
[b] Polyurethanes based on Tris-(Bisphenol –A )Monophosphate
(i) Synthesis of monomer: Tris-(bisphenol –A) monophosphate
(TBAMP)
The monomer tris-(bisphenol –A) monophosphate was synthesized
by reacting bisphenol-A (684 gm, 3.0 mole) and phosphorous
oxychloride (93.3 ml, 1.0 mole) at the temperature of 130 OC for 12 hours
in presence of a catalyst N, N, dimethyl aniline (0.0132 moles). Final
product was obtained by dissolving the crude product in acetone and
reprecipitating from cold water. The product was dried and crystallized
from methanol [16].
Characterization
(i) Melting point: 165 – 167 OC
(ii) Percentage yield: 70-75 %
(ii) Number of hydroxyl groups: 3.0
(iii) Elemental analysis (%)
Calculated values are listed in parenthesis)
(iv) IR analysis
The IR Spectrum of the monomer shows a broad absorption band
around 3410 cm-1
due to the presence of H-bonded hydroxyl species, the
band at 1262 cm-1
corresponds to Ar-OH symmetric stretching
frequency, the band at 1235 cm-1
is due to the presence of P=O group and
band at 975 cm-1
is due to the P-O-C stretching frequencies. The
aromatic C-H stretch appears at 3000 cm-1
.
C H
74.17
(73.85)
6.18
(5.27)
Chapter II
Department of Materials Science 63
Figure 2.4: FTIR spectrum of tris-(bisphenol –A) monophosphate
(TBAMP)
(ii) Synthesis of polyurethanes
Polyurethanes were synthesized by reacting tris-(bisphenol-A)
monophosphate with different diisocyanates. To synthesize the PU-4, tris-
(bisphenol-A) monophosphate (TBAMP, 1.0 mole) and solvent methyl
ethyl ketone (MEK) were charged into a three necked flask equipped with
dropping funnel, reflux condenser and over head stirrer. Toluene
diisocyanate (TDI, 2.4 mole) was added in a dropwise manner into the
reaction mixture over a period of 1 hour with constant stirring at 60 OC.
The molar ratio of -OH to -NCO was kept 1:1.6 to produce NCO
terminated polyurethane. After the completion of addition, heating was
continued for further 2 hours to complete the reaction. After vacuum
distillation pure NCO terminated polyurethane (PU-4) was obtained
(Scheme 2).
Other diisocyanates such as isophoron diisocyanate (IPDI),
hexamethylene diisocyanate (HMDI) were reacted with TBAMP using
above method to produce PU-5 and PU-6 respectively. Reaction
conditions are given in the Table 2.4.2.
Chapter II
Department of Materials Science 64
Reaction Scheme 2:
Chapter II
Department of Materials Science 65
Characterization
Table 2.2
(i) Reaction Conditions and Specification of the Polyurethanes based on
tris-(bisphenol –A) monophosphate
(ii) Elemental analysis (%)
System C H N
PU-4 69.07
(68.96)
5.53
(5.27)
6.57
(6.70)
PU-5 70.11
(69.73)
7.59
(7.10)
5.91
(6.03)
PU-6 67.53
(67.04)
7.02
(6.80)
6.89
(6.81) (Calculated values are listed in parenthesis)
(iii) Infra-red analysis
The IR spectra of polyurethanes show absorption bands around 1581-
1649 cm-1
for OCONH group frequency. The absorption bands around
2251-2263 cm-1
are due to free NCO groups in polyurethane. The bands
for P=O group appears around 1220-1241 cm-1
. P-O-C stretching
frequency is observed around 967-979 cm-1
. The broad bands around
3301-3347 cm-1
attribute to N-H bands. The bands around 3000 – 3100
cm-1
are due to the C-H stretching frequency.
System Types of
isocyanate
used
Reaction
conditions
%
NCO
content
Intrinsic
viscosity
η
dl/gm
GPC
Data
Temp. OC
Time
hrs. Mn
Mw
Mn
PU-4 TDI 60 3 1.9 0.95 3289 1.6
PU-5 IPDI 60 3 1.8 0.91 3357 1.8
PU-6 HMDI 60 3 1.6 0.85 3171 2.1
Chapter II
Department of Materials Science 66
Figure 2.5: FTIR spectra of polyurethanes based on tris-(bisphenol –A)
monophosphate (PU- 4 to PU- 6)
Chapter II
Department of Materials Science 67
[c] Synthesis of polyurethanes based on bis-(m-hydroxy phenyl)
methyl phosphine oxide
(i) Synthesis of monomer: Bis-(m-hydroxy phenyl) methyl phosphine
oxide (BHPMPO)
The monomer was synthesized in four steps.
1. Synthesis of diphenyl methyl phosphine oxide from triphenyl
phosphine.
2. Synthesis of bis-(m-nitro phenyl) methyl phosphine oxide from
diphenyl methyl phosphine oxide.
3. Synthesis of bis-(m-amino phenyl) methyl phosphine oxide from
bis-(m-nitro phenyl) methyl phosphine oxide.
4. Synthesis of bis-(m-hydroxy phenyl) methyl phosphine oxide from
bis-(m-amino phenyl) methyl phosphine oxide.
1. Synthesis of diphenyl methyl phosphine oxide from triphenyl
phosphine (DPMPO)
To synthesize diphenyl methyl phosphine oxide, triphenyl
phosphine was allowed to react with methyl iodide at room temperature
in presence of petroleum ether and chloroform as solvent. The mixture
was stirred overnight. The precipitated white solid was filtered, washed
with petroleum ether and dried under vacuum. The dried solid was
refluxed in a mixture of water and aqueous KOH solution (40%) for 2
hours. Benzene evolved during the reaction was collected using Dean and
Stark apparatus. The content left in the flask was extracted in toluene
using separating funnel. The toluene extracts were collected and kept
overnight as such to get fine crystals of DPMPO [17].
Chapter II
Department of Materials Science 68
Characterization
(i) Melting point: 110-112 OC
(ii) Percentage yield: 87- 90 %
(iii) Elemental analysis (%)
C H N
72.07
(72.22)
6.11
(6.02)
-----
(Calculated values are listed in parenthesis)
(v) Infra-red analysis
IR spectrum of diphenyl methyl phosphine oxide shows the
absorption band at 1494 cm-1
corresponds to C=C stretching of aromatic
ring and the band at 1200 cm-1
is due to the presence of P=O group.
Figure 2.6: FTIR spectrum of diphenyl methyl phosphine oxide
(DPMPO)
Chapter II
Department of Materials Science 69
2. Synthesis of bis-(m-nitro phenyl) methyl phosphine oxide
Nitration of DPMPO was carried out using the following
method. A mixture of conc. HNO3 and conc. H2SO4 mole ratio of 1:1
was charged [18] into a three necked flask. Diphenyl methyl phosphine
oxide (50 gm, 0.24 mole) was added in pinch by pinch manner at
10-15 OC. After the completion of addition the content was stirred for
further 30 minutes at the same temperature. The resultant mixture was
poured into ice-cold water to get the precipitates. Pure nitro compound
was obtained after washing thoroughly with water and crystallizing from
ethanol.
Characterization
(i) Melting point: 201 OC
(ii) Percentage yield: 80 –82 %
(iii) Elemental analysis (%)
C H N
51.08
(50.98)
3.47
(3.59)
9.11
(9.15) (Calculated values are listed in parenthesis)
(iv) Infra-red analysis
The IR spectrum shows the absorption band at 1510 cm-1
due to
NO2 group. The band at 1494 cm-1
corresponds to C=C stretching of
aromatic ring and the band at 1200 cm-1
is due to the presence of P=O
group. The aromatic C-H stretch appears at 3000 cm-1
.
Chapter II
Department of Materials Science 70
Figure 2.7: FTIR spectrum of bis-(m-nitro phenyl) methyl phosphine
oxide (BNPMPO)
3. Synthesis of bis-(m-amino phenyl) methyl phosphine oxide
Bis-(m-nitro phenyl) methyl phosphine oxide ( 7 gm, 0.022 mole),
alcohol (70 ml), water (15 ml) and conc. HCl (1 ml) were charged into a
250 ml three necked flask and heated in a boiling water bath. Iron powder
(15 gm) was added pinch by pinch during 1 hour with constant stirring
[19]. Stirring was continued under the same reaction conditions for 1.5
hours. The reaction mixture was then made alkaline with ammonia and
filtered hot. The filtrate was allowed to cool in an ice-bath. The solid
separated was filtered and washed with water and crystallized from
alcohol to get brown coloured crystals.
Chapter II
Department of Materials Science 71
Characterization
(i) Melting point: 155-156 OC
(ii) Percentage yield: 70-72 %
(iii) Elemental analysis (%)
C H N
63.33
(63.41)
6.23
(6.10)
11.19
(11.38) (Calculated values are listed in parenthesis)
(iv) Infra-red analysis
IR spectrum shows the band at 1550 cm-1
due to the presence of -NH2
group, the band at 1500 cm-1
corresponds to C=C stretching of aromatic
ring, the band at 1200 cm-1
is due to the presence of P=O group. The
aromatic C-H stretch appears at 3000 cm-1
.
Figure 2.8: FTIR spectrum of bis-(m-amino phenyl) methyl phosphine
oxide (BAPMPO)
Chapter II
Department of Materials Science 72
4. Synthesis of bis-(m-hydroxy phenyl) methyl phosphine oxide from
bis-(m-amino phenyl) methyl phosphine oxide
The diazotization of bis-(m-amino phenyl) methyl phosphine oxide
was carried out at 0-5 OC using sodium nitrate solution in presence of
sulphuric acid. The resultant solution was poured into water at 80 OC. On
cooling hydrolyzed product was precipitated out. After filtration and
crystallization from ethanol pure product was obtained [18].
Characterization
(i) Melting point: 140-142 OC
(ii) Percentage yield: 65-70 %
(iii) Number of hydroxyl group: 1.94
(iv) Elemental analysis (%)
C H N
62.81
(62.90)
5.19
(5.24)
-----
----- (Calculated values are listed in parenthesis)
(v) Infra-red analysis
The band at 3200 cm-1
is due to the presence of –OH group
frequency. The band at 1260 cm-1
is due to the presence of P=O group.
The band at 1280 cm-1
is for Ar-OH symmetric stretching frequency.
C=C stretching of aromatic ring is obtained at 1470 cm-1
.
Chapter II
Department of Materials Science 73
Figure 2.9: FTIR spectrum of bis-(m-hydroxy phenyl) methyl phosphine
oxide (BHPMPO)
(ii) Synthesis of polyurethanes
Bis-(m-hydroxy phenyl) methyl phosphine oxide (BHPMPO, 1.0
mole, monomer) was charged into a three necked flask along with the
solvent methyl ethyl ketone (MEK) attached with a reflux condenser and
mechanical stirrer. Temperature was maintained at 60 OC and toluene
diisocyanate (TDI, 3.2 moles) was added in a dropwise manner over a
period of 1hour with constant stirring. Heating was continued for a period
of 2 hours with constant stirring. The polyurethane (PU-7) thus obtained
was vacuum distilled at 50 OC (Scheme 3) to get pure polymer.
Other polyurethanes (PU-8) and (PU-9) were also prepared in the
similar manner by reacting the monomer (BHPMPO) respectively with
isophoron diisocyanate (IPDI) and hexamethylene diisocyanate (HMDI).
Reaction conditions are given in the Table 2.4.3.
Chapter II
Department of Materials Science 74
Reaction Scheme 3:
CH3 CH
3
CH3
CH3
CH3
CH2 (CH
2)
P P
CH3
O
P
CH3
ONO
2O2N
P
CH3
ONH
2NH2
P
CH3
OOHOH
P
CH3
OOO CO NH R NH CO
OCN-R-NCO
CH3I
Where R =
PU-7 PU-8 PU-9
Triphenyl phosphine
Diphenyl methyl phosphine oxide
Bis-(m-nitro phenyl) methyl phosphine oxide
Bis-(m-amino phenyl) methyl phosphine oxide
Bis-(m-hydroxy phenyl) methyl phosphine oxide (Monomer)
n
Diisocyanate
60 0C
MEK
Polyurethane resin
(i) in Petroleum ether and CHCl324 hrs at room temp.
(ii) Reflux in mix. of H2O and
aqueous KOH solution (40%)
Conc. HNO3
Conc. H2SO4
15 - 20 oC30 mon.
SnCl2.2H2O 90 - 95 oC1 hr
Conc. H2SO4
NaNO2
0 - 5 oC
6
Chapter II
Department of Materials Science 75
Characterization
Table 2.3
(i) Reaction Conditions and Specification of the Polyurethanes
(ii) Elemental analysis (%)
System C H N
PU-7 62.08
(62.21)
4.47
(4.53)
9.23
(9.36)
PU-8 63.57
(63.25)
8.31
(8.40)
7.86
(7.98)
PU-9 59.47
(59.39)
6.54
(6.67)
9.32
(9.56) (Calculated values are listed in parenthesis)
(iii) Infra-red analysis
IR-spectra of polyurethanes (PU-7 to PU-9) show the broad bands
around 3287-3388 cm-1
which attribute to >N-H bonds. The bands around
1502-1504 cm-1
and 1600-1679 cm-1
are due to –NHCOO groups. The
absorption bands around 2261-2290 cm-1
are due to free NCO groups in
the polyurethanes. The bands around 1056-1074 cm-1
and 1216-1220 cm-1
are due to P-O-C and P=O stretching frequencies respectively. The bands
around 2850 – 3000 cm-1
are due to the aromatic C-H stretching.
System Types of
isocyanate
used
Reaction
conditions
%
NCO
content
Intrinsic
viscosity
η
dl/gm
GPC
Data
Temp. OC
Time
hrs. Mn
Mw
Mn
PU-7 TDI 60 3 1.8 0.42 1822 1.5
PU-8 IPDI 60 3 1.7 0.39 1947 1.8
PU-9 HMDI 60 3 1.8 0.36 1628 1.6
Chapter II
Department of Materials Science 76
Figure 2.10: FTIR spectra of polyurethanes based on bis-(m-hydroxy
phenyl) methyl phosphine oxide (PU-7 to PU-9)
Castor oil has been used in making a variety of polyurethane
products, ranging from coatings to foams, and the use of castor oil
derivatives continues to be an area of active development.
Chapter II
Department of Materials Science 77
[d] Synthesis of polyurethanes based on ester exchange reaction
product (EERP) of castor oil and tris (m-hydroxy phenyl)
phosphate
(i) Synthesis of polyol
The ester exchange reaction product (EERP) was synthesized by
ester exchange reaction [20, 21] of castor oil with tris-(m-hydroxy
phenyl) phosphate (THPP). In this process castor oil and THPP were
mixed in equimolar proportion in a reaction flask equipped with a
thermometer, reflux condenser and overhead stirrer. The mixture was
refluxed for 4 hours at 140-150 OC followed by the heating for 6 hours at
170-180 OC to get the crude product which was vacuum distilled to obtain
high viscous polyol.
Characterization
(i) Percentage yield: 85-88 %
(ii) Number of hydroxyl group: 2.87
(iii) Elemental analysis (%)
C H N
70.77
(71.17)
9.43
(9.14) -----
----- (Calculated values are listed in parenthesis)
(iv) Infra-red analysis
The IR spectrum of the phosphorous containing polyol shows broad
absorption band at 3417 cm-1
due to OH stretching frequency. The strong
band at 1739 cm-1
clearly indicates C=O stretching frequency of ester
group in polyol. The bands at 1058 cm-1
and 1172 cm-1
are due to P-O-C
and P=O stretching frequency respectively. The bands at 1619 cm-1
and
1463 cm-1
are due to C=C stretching frequency in aliphatic chain and
aromatic ring respectively. The aromatic C-H stretch appears at 3000
cm-1
.
Chapter II
Department of Materials Science 78
Figure 2.11: FTIR spectrum of polyols based on ester exchange reaction
product of castor oil and tris (m-hydroxy phenyl) phosphate
(ii) Synthesis of polyurethanes
Polyurethanes were synthesized by reacting this novel polyol
(EERP) with various diisocyanates such as TDI, IPDI, HMDI, and MDI.
To synthesize the polyurethane (PU-10), EERP (1.0 mole) and solvent
MEK were charged in a three necked flask equipped with a dropping
funnel, reflux condenser and mechanical stirrer. TDI (2.0 moles) was
added in a dropwise manner over a period of 1.5 hours with constant
stirring at 60 OC. Heating was continued for further 2 hours to complete
the reaction. In this reaction the ratio of EERP to TDI was kept 1:2 to get
a NCO terminated polyurethane. After vacuum distillation pure product
was obtained which have ricinoleic moiety in the structure (Scheme 4).
Other polyurethanes PU-11, PU-12 and PU-13 were also
synthesized by reacting the EERP with the diisocyanates such as
isophorone diisocyanate, hexamethylene diisocyanate and methylene
diphenyl diisocyanate respectively according to the reaction conditions
mentioned in Table 2.4.4.
Chapter II
Department of Materials Science 79
Reaction Scheme 4:
Chapter II
Department of Materials Science 80
Characterization
Table 2.4
(i) Reaction Conditions and Specification of the Polyurethanes
(ii) Elemental analysis (%)
System C H N
PU-10 68.11
(68.32)
7.34
(7.59)
4.39
(4.83)
PU-11 69.33
(68.94)
8.61
(8.78)
4.17
(4.47)
PU-12 65.09
(65.73)
8.22
(8.76)
4.89
(5.11)
PU-13 70.17
(70.51)
7.05
(7.31)
4.12
(4.45) (Calculated values are listed in parenthesis)
(iii) Infra-red analysis
IR-spectra of polyurethanes (PU-10 to PU-13) show the broad
bands around 3315-3421 cm-1
which attribute to >N-H bonds. The bands
around 1541-1576 cm-1
and 1600-1635 cm-1
are due to –NHCOO groups.
The absorption bands around 2266-2275 cm-1
are due to free NCO groups
in polyurethanes. The bands around 1028-1076 cm-1
and 1199-1249 cm-1
are due to P-O-C and P=O stretching frequencies respectively. The bands
around 2900 – 3000 cm-1
are due to the aromatic C-H stretching.
System Types of
isocyanate
used
Reaction
conditions
%
NCO
content
Intrinsic
viscosity
η
dl/gm
GPC
Data
Temp. OC
Time
hrs. Mn
Mw
Mn
PU-10 TDI 60 3.5 1.8 0.92 4571 2.1
PU-11 IPDI 60 4.0 2.2 0.86 4873 2.3
PU-12 HMDI 60 4.5 2.1 0.81 3812 2.2
PU-13 MDI 60 3.5 2.0 0.95 4953 2.4
Chapter II
Department of Materials Science 81
Figure 2.12: FTIR spectrum of polyurethanes based on ester exchange
reaction product (PU-10 to PU-13)
Chapter II
Department of Materials Science 82
(ii) Diphosphorous based polyurethanes
[a] Synthesis of polyurethanes based on resorcinol bis-(hydroxyl
phenyl phosphonate)
(i) Synthesis of monomer: Resorcinol Bis-(Hydroxyl Phenyl
Phosphonate) (RBHPP)
The monomer resorcinol bis (hydroxyl phenyl phosphate) was
synthesized [22] by reacting resorcinol (1 mole, 110 gm) with phenyl
phosphonic dichloride (2.0 moles, 390 gm). In this reaction, resorcinol
and phenyl phosphonic dichloride were mixed in a round bottom flask
equipped with a condenser and dropping funnel and heated at 120 OC in
the xylene solvent for 8 hours with constant stirring. Water (2.0 mole, 36
ml) was added to the reaction mixture at room temperature with constant
stirring. Then the mixture was heated at 60 OC with continuous stirring
for 3 hours. The xylene was removed by vacuum distillation. The crude
product was washed with hot water and neutralized by sodium carbonate
solution and vacuum distilled to get final product (scheme-2).
Characterization
(i) Melting point: 153 – 155 OC
(ii) Percentage yield: 80 - 82%
(iii) Number of hydroxyl groups: 1.92
(iv) Solubility: MEK, MeOH, THF, DMF, etc.
(v) Elemental analysis (%)
(Calculated values are listed in parenthesis)
C H
54.98
(55.38)
4.03
(4.10)
Chapter II
Department of Materials Science 83
(iv) Nuclear Magnetic Resonance (NMR) spectrum
The 1H – NMR signals of the monomer in DMSO give the value at
δ 7.30 – 7.40 ppm and δ 6.55 – 6.75 ppm for the aromatic protons of the
main chain while a broad multiplet around δ 6.75 – 7.30 ppm is
associated with the pendant phenyl rings. The broad band around δ 7.40 –
7.50 ppm is due to the presence of terminal hydroxyl groups.
Figure 2.13: 1H-NMR spectrum of resorcinol bis-(hydroxyl phenyl
phosphonate) (RBHPP)
(v) Infra-red spectrum
The IR spectrum of this diol (RBHPP) shows broad absorption
band at 3262 cm-1
due to OH stretching frequency. The bands at 982 cm-1
and 1133 cm-1
are due to P-O-C and P=O stretching frequencies
respectively. The band at 1479 cm-1
is due to C=C stretching frequency in
aromatic ring. The aromatic C-H stretch appears at 3000 cm-1
.
Chapter II
Department of Materials Science 84
Figure 2.14: FTIR spectrum of resorcinol bis-(hydroxyl phenyl
phosphonate) (RBHPP)
(ii) Synthesis of polyurethanes
Polyurethanes were synthesized by reacting the above diol with
various diisocyanates such as toluene diisocyanate (TDI), isophorone
diisocyanate (IPDI) and hexamethylene diisocyanate (HMDI). To
synthesize the polyurethane (PU-14), diol (RBHPP, 1.0 mole) and solvent
MEK were charged in a three necked flask equipped with dropping
funnel, reflux condenser and stirrer. TDI (3.2 moles) was added in a
dropwise manner over a period of 1 hour with constant stirring at 60 OC.
Heating was continued for further 2 hours to complete the reaction. The
molar ratio of diol to TDI was kept 1 : 1.6 to produce NCO terminated
polyurethane. After vacuum distillation pure product was obtained
(scheme-5).
Chapter II
Department of Materials Science 85
Other polyurethanes PU-15 and PU-16 were also synthesized by
reacting the diol with the diisocyanates, IPDI and HMDI respectively.
Reaction conditions are given in the Table 2.4.5.
Reaction Scheme 5:
OH
OH
P
O
ClCl
NCO R NCO
n
OOP
O
P
O
O C
O
NH
R NH
C
O
OCR NH
O
C
O
NH
NCO
OOP
O
P
O
OH OH
CH3 CH
3
CH3
CH3
CH3
CH2 (CH
2)6
OOP
O
P
O
Cl Cl
OH2
+
60 0C3.5 hrs
Where R =
Resorcinol Phenyl Phosphonic Dichloride
Resorcinol bis-(chloro phenyl phosphonate)
Resorcinol bis-(hydroxy phenyl phosphonate)
Polyurethane resin based on resorcinol bis-(hydroxy phenyl phosphonate)
PU-14 PU-15 PU-16
Diisocyanate
120 0C, 8 hrs
60 0C, 3 hrs
Chapter II
Department of Materials Science 86
Characterization
Table 2.5
(i) Reaction Conditions and Specification of the Polyurethanes
(ii) Elemental analysis (%)
System C H N
PU-14 58.01
(58.38)
3.97
(4.05)
7.21
(7.57)
PU-15 60.19
(60.43)
6.07
(6.24)
6.43
(6.72)
PU-16 62.59
(62.88)
3.83
(4.05)
6.49
(6.67) (Calculated values are listed in parenthesis)
(iii) Infra-red spectra
IR-spectra of polyurethanes (PU-14 to PU-16) show the broad
bands around 3278-3412 cm-1
which attribute to >N-H bonds. The bands
around 1503-1504 cm-1
and 1597-1600 cm-1
are due to –NHCOO-
groups. The absorption bands around 2263-2273 cm-1
are due to free
NCO groups in polyurethanes. The bands around 982-1068 cm-1
and
1171-1172 cm-1
are due to P-O-C and P=O stretching frequencies
respectively. The bands around 2900 – 3000 cm-1
are due to the aromatic
C-H bond stretching.
System Type of
isocyanate
used
Reaction
conditions
%
NCO
content
Intrinsic
viscosity
η
dl/gm
GPC
Data
Temp. OC
Time
hrs. Mn
Mw
Mn
PU-14 TDI 60 3 1.4 0.081 2303 1.8
PU-15 IPDI 60 3 1.4 0.079 2659 1.6
PU-16 HMDI 60 3 1.5 0.074 1929 1.7
Chapter II
Department of Materials Science 87
Figure 2.15: FTIR spectra of polyurethanes based on resorcinol bis-
(hydroxyl phenyl phosphonate) (PU-14 to PU-16)
Chapter II
Department of Materials Science 88
[b] Synthesis of polyurethanes based on bisphenol-A bis (hydroxyl
phenyl phosphate)
(i) Synthesis of monomer: Bisphenol-A bis (hydroxyl phenyl
phosphate) (BABHPP)
The monomer bisphenol-A bis (hydroxyl phenyl phosphate) was
synthesized by reacting bisphenol-A (1 mole, 222 gm) with phenyl
phosphonic dichloride (2.0 moles, 390 gm). In this reaction, bisphenol-A
and phenyl phosphonic dichloride were mixed in a round bottom flask
equipped with condenser, dropping funnel and heated at 140 OC in the
xylene solvent for 10 hours with constant stirring. Water (2.0 moles, 36
ml) was added to the reaction mixture at room temperature with constant
stirring. Then the mixture was heated at 60 OC with continuous stirring
for 4 hours. The xylene was removed by vacuum distillation. The crude
product was washed with hot water and neutralized by sodium carbonate
solution and vacuum filtered to get final product.
Characterization
(i) Melting point: 137 – 139 O
C
(ii) Percentage yield: 84 - 86%
(iii) Number of hydroxyl group: 1.93
(iv) Solubility: MEK, MeOH, THF, DMF, etc.
(v) Elemental analysis (%)
C H N
63.56
(63.78)
4.97
(5.12)
----
---- (Calculated values are listed in parenthesis)
(iii) Infra-red spectra
The IR spectrum of phosphorous containing polyol shows broad
absorption at 3348 cm-1
due to OH stretching frequency. The bands at
Chapter II
Department of Materials Science 89
1041 cm-1
and 1132 cm-1
are due to P-O-C and P=O stretching
frequencies respectively. The band at 1438 cm-1
is due to C=C stretching
frequency in aromatic ring. The aromatic C-H stretch appears at 3000
cm-1
.
Figure 2.16: FTIR spectrum of bisphenol-A bis (hydroxyl phenyl
phosphate) (BABHPP)
(ii) Synthesis of polyurethanes
Polyurethanes were synthesized by reacting the above diol with
various diisocyanates such as toluene diisocyanate (TDI), isophorone
diisocyanate (IPDI) and hexamethylene diisocyanate (HMDI). To
synthesize the polyurethane (PU-17), diol (BABHPP, 1.0 mole) and
solvent MEK were charged in a three necked flask equipped with
dropping funnel, reflux condenser and stirrer. TDI (3.2 moles) was added
in a dropwise manner over a period of 1 hour with constant stirring at
60 OC. Heating was continued for further 2 hours to complete the
reaction. The molar ratio of diol to TDI was kept 1:1.6 to produce NCO
terminated polyurethane. After vacuum distillation pure product was
obtained (scheme-6).
Chapter II
Department of Materials Science 90
Other polyurethanes PU-18 and PU-19 were also synthesized by
reacting the diol with the diisocyanates, IPDI and HMDI respectively.
Reaction conditions are given in the Table 2.4.6.
Reaction Scheme 6:
P
O
ClCl
NCO R NCO
OP
O
OCC
O
NH
R NH
O
n
O P
O
O C
O
NH
R NH
C
O
C
CH3
CH3
C
CH3
CH3
OHOH
C
CH3
CH3
O P
O
ClOP
O
Cl
C
CH3
CH3
O P
O
OHOP
O
OH
CH3 CH
3
CH3
CH3
CH3
CH2 (CH
2)6
OH2
+
Diisocyanates
60 0C3.5 hrs
Phenyl phosphonic dichloride
Polyurethane resin
Bisphenol-A Bisphenol-A bis(chloro phenyl phosphonate)
Bisphenol-A bis(hydroxy phenyl phosphonate)
Where R =
PU-17 PU-18 PU-19
140 0C
10 hrs
60 0C, 4 hrs
Chapter II
Department of Materials Science 91
Characterization
Table 2.6
(i) Reaction Conditions and Specification of the polyurethanes
(ii) Elemental analysis (%)
System C H N
PU-17 62.38
(62.94)
4.16
(4.66)
6.03
(6.53)
PU-18 63.99
(64.29)
6.19
(6.51)
5.63
(5.88)
PU-19 66.05
(66.39)
4.27
(4.59)
5.48
(5.85) (Calculated values are listed in parenthesis)
(iii) Infra-red spectra
IR-spectra of polyurethanes (PU-17 to PU-19) show the broad
bands around 3432-3284 cm-1
which attribute to >N-H bonds. The bands
around 1503-1504 cm-1
and 1599-1639 cm-1
are due to –NHCOO-
groups. The absorption bands around 2262-2272 cm-1
are due to free
NCO groups in polyurethanes. The bands around 1058-1068 cm-1
and
1172-1199 cm-1
are due to P-O-C and P=O stretching frequencies
respectively. The bands around 2850 – 3000 cm-1
are due to the aromatic
C-H bond stretching.
System Type of
isocyanate
used
Reaction
conditions
%
NCO
content
Intrinsic
viscosity
η
dl/gm
GPC
Data
Temp. OC
Time
hrs. Mn
Mw
Mn
PU-17 TDI 60 3 1.9 0.084 3557 1.7
PU-18 IPDI 60 3 1.5 0.081 3812 1.9
PU-19 HMDI 60 3 1.7 0.079 2825 1.6
Chapter II
Department of Materials Science 92
Figure 2.17: FTIR spectra of polyurethanes based on bisphenol-A bis-
(hydroxyl phenyl phosphonate) (PU-17 to PU-19)
Chapter II
Department of Materials Science 93
2.4.2 Synthesis of phosphorous based flame retardant epoxies
[a] Synthesis of triglycidyl ether of tris-(m-hydroxy phenyl)
phosphate
Triglycidyl ether of tris-(m-hydroxy phenyl) phosphate was
prepared by the reported method [14]. Tris-(m-hydroxy phenyl)
phosphate (50 gm, 0.134 moles), epichlorohydrine (185 gm, 2.0 moles)
and water (1-2 ml) were charged into a three necked flask equipped with
overhead stirrer, reflux condenser and dropping funnel. The reaction
mixture was stirred at 120 OC followed by the addition of 10% NaOH
solution over a period of 1.5 hours. The reaction mixture was further
heated at the same temperature for 3.5 hours. After completion of
reaction vacuum distillation was carried out to remove excess
epichlorohydrine. Toluene was added to extract the epoxy resin followed
by the vacuum distillation to get pure epoxy resin (Scheme 7).
Reaction Scheme 7:
PO O
O
OH
OHOHO
CH2
CH CH2 Cl
O
PO O
O
OO
CH2
CH CH2
O
Tris (m-hydroxy phenyl) phosphate
+
Epichlorohydrine
120 0C, 5 hrs
10% NaOH
3
Triglycidyl ether of tris (m-hydroxy phenyl) phosphate (TGETHPP)
Chapter II
Department of Materials Science 94
Characterization
(i) Percentage yield: 80- 85 %
(ii) Epoxy equivalent weight (EEW): 202 gm/equiv.
(iii) Solubility: MEK, THF, DMF and Xylene.
(iv) Intrinsic viscosity (η) in MEK solvent: 0.0864 dl/gm
(v) Number average molecular weight ( Mn ): 1626
(vi) Polydispersity index (PDI, MnMw ): 1.6
(vii) Elemental analysis (%)
C H N
58.12
(59.70)
5.36
(4.90)
-----
----- (Calculated values are listed in parenthesis)
(viii) Infra-red analysis
IR spectrum of epoxy resin shows absorption bands of terminal
epoxy group at 1259 cm-1
, 905 cm-1
and 842 cm-1
. The bands at 1042 cm-1
and 1169 cm-1
are due to P-O-C and P=O stretching frequencies
respectively. C=C stretching of aromatic ring is observed at 1496 cm-1
.
The band at 3000 cm-1
is due to the aromatic C-H bond stretching.
Figure 2.18: FTIR spectrum of Triglycidyl Ether of Tris (m-Hydroxy
Phenyl) Phosphate (TGETHPP)
Chapter II
Department of Materials Science 95
[b] Synthesis of triglycidyl ether of tris-(bisphenol-A) monophosphate
Tris-(bisphenol-A) monophosphate (120 gm, 0.134 mole),
epichlorohydrine (185 gm, 2.0 moles) and water (1-2 ml) were charged
into a three necked flask equipped with overhead stirrer, reflux condenser
and dropping funnel. The reaction mixture was stirred at 120 OC followed
by the addition of 10% NaOH solution over a period of 1.5 hours. The
reaction mixture was further heated at the same temperature for 4 hours.
After completion of the reaction vacuum distillation was carried out to
remove excess epichlorohydrine. Toluene was added to extract the epoxy
resin followed by the vacuum distillation to get pure epoxy resin (Scheme
8).
Reaction Scheme 8:
CH2
CH CH2
Cl
O
PO O
O
C
CH3
CH3
O
O
CH2
CH CH2
O
HO OC
CH3
CH3
OHC
CH3
CH3
OH
CCH3
CH3
P O
O
O
Epichlorohydrine
120 0C, 5.5 hrs
10% NaOH
3
Triglycidyl ether of tris bisphenol-A mono phosphate (TGETBAMP)
+
Tris bisphenol-A monophosphate
Chapter II
Department of Materials Science 96
Characterization
(i) Percentage yield: 78- 81 %
(ii) Epoxy equivalent weight (EEW): 309 gm/equiv.
(iii) Solubility: MEK, THF, DMF and Xylene.
(iv) Intrinsic viscosity (η) in MEK solvent: 0.0971 dl/gm
(v) Number average molecular weight ( Mn ): 2384
(vi) Polydispersity index (PDI, MnMw ): 1.5
(vii) Elemental analysis (%)
C H N
72.23
(72.32)
6.19
(6.36)
-----
----- (Calculated values are listed in parenthesis)
(viii) Infra-red analysis
IR spectrum of epoxy resin shows absorption bands at 1220 cm-1
,
972 cm-1
and 830 cm-1
are due to terminal epoxy groups. The bands at
1011cm-1
and 1191cm-1
are due to P-O-C and P=O stretching
frequencies respectively. C=C stretching of aromatic ring is observed
at 1460 cm-1
. The aromatic C-H stretch appears at 3000 cm-1
.
Figure 2.19: FTIR spectrum of Triglycidyl Ether of Tris Bisphenol – A
Monophosphate (TGETBAMP)
Chapter II
Department of Materials Science 97
2.4.3 Non-phosphorous containing liquid epoxy resin
Diglycidyl ether of bisphenol – A (DGEBA)
Diglycidyl ether of bisphenol-A (DGEBA) is available
commercially as LAPOX. It is a product of Atul Limited located at
Valsad, Gujarat. The structure of liquid epoxy resin is given below.
Characterization
(i) Epoxy equivalent weight: 190 gm/equiv.
(ii) Solubility: MEK, THF, DMF and Xylene.
(iii) Viscosity: 11500-11600 c.p.
(iv) Intrinsic viscosity (η) in MEK solvent: 0.037 dl/gm.
(v) Elemental analysis (%)
C H N
75.32
(75.00)
6.94
(7.05)
-----
----- (Calculated values are listed in parenthesis)
These polymers have been used for the fabrication of casted films
and coatings which are discussed in the next chapter.
Chapter II
Department of Materials Science 98
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Chapter II
Department of Materials Science 99
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