ABSTRACTphysical blend of the epoxy resins and the flame retardants [6]. A free epoxy is used as the...

STUDY OF HYDROXYL TERMINATED ORGANOPHOSPHORUS CURING

AGENTS

T.LAKSHMIKANDHAN1, V.L.CHANDRA BOSS

2

1,2 Associate Professor,

Department of Chemistry,

BIST, BIHER, Bharath University,Chennai-73 [email protected], [email protected],

ABSTRACT

A novel hydroxyl-terminated hyperbranched polyphosphate (HHPP) was synthesized

by employing an A2 + B3 polycondensation with high functionality as a curing agent

and characterized by FTIR, 1H NMR and

31P NMR. The formation of the product was

monitored by thin layer chromatography, FT-IR and the evolution of HCl was

detected by using litmus paper. The distinctive absorption around 1300cm-1

corresponding to the stretching vibration of P=O in phosphorus oxychloride decreases

gradually until disappeared completely after 24 hours reaction. 1H NMR supports the

product formation. 31

P NMR Spectrum shows the incorporation of phosphorus in

hyperbranched (3-hydroxyphenyl) phosphate.

1. Introduction

Curing agents plays an important role in epoxy composites; it is also called as

“Hardener”. In chemistry[1-6], epoxy or polyepoxide is a thermosetting epoxide

polymer that cures (polymerizes and cross links) when mixed with a catalyzing agent

or hardener. The broad interest in epoxy resins originates from the extremely wide

variety of chemical reactions that can be used for the curing[7-12]. Increasing

requirements for thermally stable polymers have promoted research on the

modification of epoxies in particular certain organophosphorus compounds have

received considerable attention as possible candidates for making high-temperature

polymers. Various acid anhydrides are also used to crosslink the epoxy resins[13-21].

International Journal of Pure and Applied MathematicsVolume 119 No. 12 2018, 6173-6209ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

6173

1.1 Applications of epoxy composites

Epoxy resins are a most important class of thermosetting resins and it has

many engineering applications because of their high strength and stiffness, good

dielectric behavior, resistance to chemicals, corrosion1. Epoxy composites have a very

good chemical and fatigue resistance[22-26], thus epoxy resins replace the less costly

unsaturated polyester resins in many applications. Reinforced epoxy resins are very

strong and they have good dimensional stability and service temperatures as high as

3150C. Preimpregnated reinforcing materials are used to produce products by hand

layup, vacumm-bag, or filament winding processes[27-32]. Epoxy-glass laminates

find many uses because they have high strength-to-mass ratio, superior adhesion to all

materials and compatibility make epoxy resins desirable. Filled epoxy resins are

commonly used for special castings. These strong compounds may be used for low-

cost tooling many different types of filler are used in caulking and patching. Epoxy

composites are outstanding in protecting electronic parts from moisture, heat, and

corrosive chemicals. They are molded into small electrical items and appliance parts

and have many modular uses. Epoxy coating have replaced glass enamel finishes for

tank car and other container linings that need to resist chemicals. The flexibility of

many coating makes them popular for post forming coated metal parts. For example

sheets of metal are coated while flat[33-36].

1.2 Disadvantages of epoxy composites

1. Poor oxidative stability

2. Thermal stability limited to 350-450 0F [178-232

0C]

3. Many grades are expensive [2].

1.3 Properties of curing agent

The term “curing” is used to describe the process by which one or more kinds of

reactants (i.e., an epoxide and a curing agent) are transformed from low-molecular-

International Journal of Pure and Applied Mathematics Special Issue

6174

weight materials to a highly cross-linked network.. The network is composed of

segments involving only the epoxide or both the epoxide and the crosslinking. The

chemistry of epoxy-resin curing is that most of its reaction is ionic[37-41], that is a

bond is formed by the curing-agent species donating and the epoxide accepting the

necessary electron pair symbolized as follows

A: + B A: B

Where A: is the electron-donating species (in case of amine curing agent the

nucleophilic species ) and B is able to accept an electron pair (i.e., it is an

electrophilic species) It is used for epoxy, fusion bonded epoxy coating, paint, tanning

concrete, food preservation, building insulation materials[42-45].

1.4 Types of curing agents

Epoxy resin are the classical matrix resins for composite .these can be cured

by different curing agent types namely by

1. Amine curing agent

u2. Anhydrides curing agents

3. Carboxylic acids curing agent

4. Phenolic curing agents

5. Organophosphorus curing agents

6. Lewis acid catalyst curing agent

1. Amine curing agent

These cross-link the resins either catalytically or establishing links across the

resin molecules. In general, the primary and the secondary amines act as reactive

hardeners while the tertiary amines act as catalytic hardeners. Depending upon the

type of hardener or curing agent, the resin would have different quantities added to it

and consequently, it would have different pot life. For instance, diethylenetriamine is


6175

more reactive than the triethylene tertramine and hence, while the former is added to

the extent of 9-10 phr (parts per hundred parts of resin), the latter is added to the

extent of 12-13 phr. The resulting resin has a pot life of less than one hour at room

temperature for the former case and a little above one hour for the latter. Aromatic

amines give higher thermal resistance but require a higher cure temperature. Aliphatic

amines lead to fast cure and are suitable to room temperature curing epoxy resins.

2. Anhydrides curing agent’s (acid hardeners)

Acid hardening systems have been in use since the mid-fifties. Acid hardeners

have a lower level of skin irritation and give lower exotherm on cure. But the amine –

cured resins show greater alkali resistance than the acid-hardened ones. While using

the acid hardeners, the anhydrides are preferred over the acid hardeners because they

release less water on cure. Greater release of water may lead to foaming of the

products. Also, anhydrides are more soluble in the resin than the acids. When an

anhydride is used, first it goes through a ring-opening reaction by the hydroxyl group

in the resin [4].

3. Organophosphorus Curing Agents:

Organophosphorus compounds have exhibited high flame retardant efficiency

for epoxy resins and also been found to generate less toxic gas and smoke than

halogen-containing compounds therefore; less destruction to the earth’s environment

is the networthy benefit of replacing halogen with phosphorus in flame retardant

epoxy resins and the flame retardancy on epoxy resins via phosphorylation.

Incorporating covalently boned phosphorus into epoxy resins could be achieved via

using phosphorus-containing oxirane compounds or curing agents [5]. The oxirane


6176

compound with phosphorus in the back bone of epoxy resins exhibits much better

than flame retardancy and over comes. Several drawbacks associated with the

physical blend of the epoxy resins and the flame retardants [6]. A free epoxy is used

as the control resin for comparison of curing properties with the phosphorus-

containing epoxy resins. Flame resistance microscale combustibility and fire behavior

were used to assess flammability of phosphorus containing epoxy and diamine

formulation. Phosphorus was introduced as either part of the diamine curing agent or

part of an epoxy compound in a typical aerospace epoxy. Flame combustion

efficiency was used as a global measure of gas phase activity [7].

A phosphorous-containing reactive bis (3-hydroxyphenyl) phosphate

(BHPP) was incorporated into epoxy resin and is expected to impart the required

flame retardancy, less fume and higher thermal stability than the conventional

bromine containing fire retardant system. Phosphorus containing flame retardants

influence the reaction taking place in the condensed phase. Their effectivity depends

on the chemical structure of the polymer, they are particularly effective in materials

with high oxygen content, like polyesters, polyurethanes, epoxies or cellulose. The

phosphorus flame retardant is converted by thermal decomposition to phosphoric

acid. Further thermal decomposition leads to the formation of poly-phosphoric acid.

The polyphosphoric acid esterifies and dehydrates the pyrolysing polymer [8].

The phosphorus moiety decomposes at low temperature relative to the

polymer matrix. Phosphorus-containing compounds for flame retardants are used

either by blending with polymers or by reacting onto polymers. It is believed that

phosphorus containing compounds can quench flammable particles like H*

and OH*

reduce the energy of the flame in the gas phase. Moreover in the solid phase, the

phosphorus-containing functional groups are converted by thermal decomposition to

phosphoric acid [9].


6177

Several investigations in this regard have been recently reported such as

phosphorylated diamines, hydroxyphenyl phosphate, and phosphorus-containing

bismaleimide. Furthermore, excellent flame retardancy is also achieved through the

introduction of phosphoruscontaining functional groups into the backbones of epoxy

compounds such as triphenyl phosphate and its analogues have received much

attention due to absence of toxic gases and smokes during combustion compared with

halogen type flame retardants. The action of phosphorus containing flame retardant

can occur predominantly through a condensed-phase mechanism, i.e. a mechanism in

which combustion of the outer layers of the polymeric article containing the flame-

retardant mixture leads to the production of an intumescent carbonaceous char which

protect the underlying polymeric material as a thermal barrier from further attack of

flame or heating [10] .

However, the use of additives as flame retardant has several disadvantages

including high loading required to achieve a sufficient level of flame retardation,

detrimental changes to the polymer’s physical and mechanical properties and leaching

of the additive during service. In order to overcome these problems phosphorus

containing monomers or oligomers can be chemically incorporated into the polymer

backbone linking airborne carbon fibers to any unusual health hazard [11]. The use of

phosphorus (P) as a flame retardant, particularly in epoxy resins, has been widely

improved fire resistance, reduced smoke and toxicity .This effort concentrated on the

use of bis (3-aminophenyl) - methylphosphine oxide as a curing agent for epoxies.

Phosphorus when incorporated in polymers as an additive or reactive comonomer is

known to impart fire retardation by condensed phase and gas phase mechanisms. In

the condensed phase P catalyzes char formation which protects the underlying

material from heat and acts a barrier to the release of fuel gases from the surface.

When acting in the condensed phase as a char catalyst, P retards the spread of fire


6178

with minimal release of toxic gases. In the gas phase P acts as a flame poison with PO

species participating in a kinetic mechanism that is analogous to that of halogens in

flames. Gas phase activity is indicated by low heats of flaming combustion, the

production of visible smoke and mineral acids (halogens), and high yields of carbon

monoxide as consequence of the incomplete combustion of the fuel gases in the

flame. Phosphorus has been incorporated into polymeric materials both as an additive

and as part of the polymeric chain. Additives are normally more economical but tend

to leach out, and have a negative impact on processability and mechanical properties.

Cured epoxy resins have a high concentration of hydroxyl (OH) groups and therefore,

P-containing flame retardant compounds are particularly effective because P tends to

react with OH groups [7]

Curing characteristics of epoxy resin-hardener systems

Useful guides to the handling properties of an epoxy resin-hardener system are

the pot life and exothermic .the pot life is the time during which the epoxy resin-

hardener mixture remains usable and is generally regarded as the time required for a

system to become gelled. Tests intended to classify the curing behavior are commonly

carried out under orbitrary conditions. Pot life can simply be determined by trial

stirring. However gel timers are commercially available. Available also is apparatus’

that allows simultaneous measurement of viscosity and temperature.

The exotherm generally understood as the maximum temperature evolved by a

system, depends strongly on the mass of the casting and its shape. It can be

determined by temperature measurement with a thermocouple, the tests conducted

under a given set of experimental circumstances .in order to obtain complete

information a curve of temperature versus time should be plotted.


6179

The cure time required at a fixed cure temperature to bring about thorough

cross linking for each application by development of the optimum value of some

important property.dannenbery and harp dived cure into two components

“conversion” related to the extent of chemical reaction, and “cross linking”, which

considers the three-dimensional aspect of the cure process. The degree of cross

linking, which depends on chemical conversion and on the functionality of the

compounds entering the cure reaction


6180

2. Aim & Objectives

A review of recent literature reveals that there is no much report on hydroxyl

terminated phosphorus containing curing agents. Hence the aim of the present

investigation is to synthesize Hydroxyl-terminated hyperbranched polyphosphate

(HHPP).It was synthesized by employing an A2 + B3 polycondensation of various

dihydroxy compounds as an A2 monomer and phosphorus oxychloride as a B3

monomer.

A2 – Dihydroxy benzene, substituted dihydroxy benzene and Bisphenol

A.

B3 – Phosphorus Oxychloride

1. Synthesis of Hydroxyl Terminated Organophosphorus Curing Agents

2. The products were characterized by FT-IR, 1H NMR and

31P NMR.


6181

3. Experimental

3.1 Materials

Resorcinol, Hydroquinone, bisphenol-A were kindly purchased from Sisco

Research Laboratories. Phosphorus Oxychloride, ter-Butyl catechol, Catechol,

Acetone, Pet. ether, Ethyl acetate, DMSO, Triethylamine, Ethanol supplied by sdfine

chem. Ltd. Solvents were purified by using standard procedures prior to use.

3.2 Measurements

The Fourier transform infrared (FTIR) spectra were recorded with a Thermo

Nuclear330 instrument. The 1H NMR spectrum was recorded with AMX-400

spectrometer using tetramethylsilane as an internal reference and CD3COCD3 as a

solvent. The 31

P NMR spectrum was recorded with a JEOL GSX-400 NB multi

nuclei FT NMR spectrometer using tetramethylsilane as an internal reference and

CD3COCD3 as a solvent.

3.3 Synthesis of hyper branched (3-hydroxy phenyl) phosphate (HHPP)

6.988g (0.0636mol) of Dihydroxy benzene (DHB) was taken in a 500ml three

necked RB flask equipped with a reflux condenser and an over head mechanical

stirrer, and heated to 120oC under a dry nitrogen atmosphere. Then 3.889g

(0.0254mol) phosphorus oxychloride was added dropwise into the flask after DHB

was melted. The evolution of HCl was detected immediately by litmus paper. The

reaction mixture was further heated to 135oC and the temperature maintained for 24

hour with continuous stirring. Finally ethyl alcohol was added into resultant and then

the freshly distilled triethylamine was added dropwise to neutralize the residual HCl

until no white smoke was observed. The formed triethylamine hydrochloride salt was

removed by simple filtration, then the solvent was distilled off and the obtained


6182

product was washed with water until the peaks around 2700 cm-1

in the FT-IR

spectrum disappeared.

The above same procedure was followed to prepare various hyperbranched

poly phosphate esters by various dihydroxy compounds, substituted dihydroxy

compound and Bisphenol A used as A2 monomer with phosphorus oxychloride. Name

of the reactants and product codes were given in Table 1


6183

General Reaction scheme:

OHHO

Resorcinol

P

O

Cl

Cl

Cl135oC

N2

O P

O

O

O

O

O

P

P

O

O

O

O

OO

HO

O

HO

O P

O

O

O

HO

O P

O

O

O

HO

O

O


6184

Table 1 Product codes

S.No Dihydroxy compounds Product Code

1 Resorcinol (HHPP) RES

2 Catechol (HHPP) CAT

3 Bisphenol A (HHPP) BPL

4 4-ter Butyl Catechol (HHPP) TBC

5 Hydroquinone (HHPP) HRQ


6185

4. Results and Discussion

Synthesis of Hyperbranched (3-hydroxyl phenyl) phosphate (HHPP)

4.1 FT-IR Studies

In this study, the synthesis of HHPP was performed by starting from DHB as

A2 monomer and phosphorus oxychloride as B3 monomer. The reaction process was

monitored by FT-IR as the reaction between phosphoryl oxychloride and DHB

proceeded, the distinctive absorption around 1300cm-1

corresponding to the stretching

vibration of P=O in phosphoryl oxychloride decreases gradually until disappeared

completely after 24h reaction. The formation of the product was confirmed by FT-IR

spectrum by the formation of P-O-Ph bond, P=O shifted from 1300 cm-1

to around

1270 cm-1

which is a characteristic of phosphate compounds.

Figures 1-5 and Table 2 shows the FT-IR spectrum of HHPP, characteristic

peaks at around 1080 and 910-988 cm-1

are attributed to the stretching frequency of P-

O-Ph stretching frequency. The peak at around 1260 cm-1

indicates that the P=O of

the compound. The peaks at around 3450 cm-1 show the Ph-OH groups of the

compound. Around 2960cm-1

indicates the presence of methyl C-H stretching

frequency.

Table 2 Characteristic stretching frequency of HHPP

Characteristic

Stretching

frequency

RES

cm-1

BPL

cm-1

CAT

cm-1

TBC

cm-1

HRQ

cm-1

Ph-OH 3413 3434 3479 3429 3451

P-O-Ph 1081, 915 1091, 906 1091, 988 1066, 964 1088, 992

P=O 1242 1233 1254 1283 1279

CH3 - 2962 - 2954 -


6186

Figure 1 FT-IR of HHPP (CAT)

CA T -B

40

8.1

4

55

9.9

7

68

4.7

8

77

4.3

5

98

8.1

210

91

.12

11

28

.72

12

54

.02

13

84

.26

14

78

.00

15

97

.72

34

79

.52

37

41

.20

37

93

.66

38

50

.40

2 5

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

%T

ran

sm

itta

nce

5 00 1000 1500 2000 2500 3000 3500

W avenum bers (c m-1 )


6187

Figure 2 FT-IR of HHPP (TBC)

18

T B C

43

3.9

1

54

8.4

8

64

4.9

8

81

5.2

68

56

.53

96

4.4

4

10

66

.11

12

83

.30

13

84

.38

14

55

.47

15

05

.76

15

99

.93

17

31

.78

29

54

.62

34

29

.33

38

86

.54

2 5

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

%T

ran

sm

itta

nce

5 00 1000 1500 2000 2500 3000 3500



6188

16

Figure 3 FT-IR of HHPP (RES)

RE ST

40

7.1

7

53

4.0

4

68

4.6

577

6.3

38

13

.84

86

2.6

0

91

5.4

6

98

8.3

71

01

1.6

2

10

81

.58

11

27

.31

11

62

.60

12

42

.34

13

84

.10

14

79

.63

15

05

.79

15

39

.03

15

58

.23

15

99

.26

16

52

.18

16

83

.66

16

99

.31

34

13

.29

7 6

78

80

82

84

86

88

90

92

94

96

98

100

%T

ran

smit

tan

ce

5 00 1000 1500 2000 2500 3000 3500



6189

17

Figure 4 FT-IR of HHPP (HRQ)

HRQ

69

5.5

2

82

8.9

4

99

2.5

9

12

79

.61

13

72

.43

14

55

.47

15

05

.46

16

00

.75

16

44

.65

16

98

.671

87

3.1

1

20

94

.81

34

51

.88

37

53

.30

38

02

.91

38

22

.60

38

45

.57

39

06

.09

2 5

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

%T

ran

sm

itta

nce

5 00 1000 1500 2000 2500 3000 3500



6190

19

Figure 5 FT-IR of HHPP (BPL)

BP L-B

40

3.6

8

58

3.1

86

37

.86

69

3.7

975

7.6

1

83

6.6

3

90

6.5

9

98

3.1

01

01

5.6

7

10

87

.89

11

65

.70

12

33

.00

13

83

.99

14

46

.39

14

78

.91

15

08

.78

15

47

.08

16

04

.29

16

38

.02

17

09

.51

29

62

.64

34

34

.42

8 8

89

90

91

92

93

94

95

96

97

98

99

100

%T

ran

sm

itta

nce

5 00 1000 1500 2000 2500 3000 3500



6191

4.2 NMR Studies

The 1H NMR spectra of HHPP are given in figures 6-10 and data given in

Table 3. It shows that all the distinct peaks can be assigned to the hydrogen in HHPP

and further confirms the reaction of phosphoryl chloride with dihydroxy compounds.

Aromatic protons comes around 6.5 – 8 ppm, hydroxyl protons at 3 – 3.6 ppm and

methyl protons shows the peaks at around 1.3 -1.6ppm.

Table 3 Proton NMR chemical shifts

Chemical shift BPL (ppm) CAT (ppm) TBC (ppm) HRQ (ppm) RES (ppm)

Aromatic

protons

7-8 6.8-7 6.6-7 6.6-7.4 6.5-7.5

-OH proton 3.5 3.2 3.0 3.0 3.6

Methyl protons 1.6 - 1.3 - -

The outcomes obtained from 31

P NMR spectra demonstrated the reaction

mechanism. Figures 11-15 and in Table 4 displays the 31

P NMR spectrums of a

typical HHPP prepared via A2 + B3 route with 3:1 molar ratio of dihydroxy compound

to POCl3. Three Signals at obtained for HHPP (RES), -6.5, -12.5 and -17.7 ppm are

assigned to terminal, linear and dendritic units, respectively. Signals at -12.3 and -

17.4ppm are assigned to linear and dendritic units, respectively for HHPP (BPL).

Table 4 Phosphorus NMR chemical shifts

Chemical

shift RES(ppm) BPL(ppm) CAT(ppm) HRQ(ppm) TBC(ppm)

Terminal -6.5 -- 0.2 - 0.8

Linear -12.5 -12.3 -2.1 -11.8 -1.5

Dendritic -17.7 -17.4 -11 -16.3 -3.2

Three Signals at obtained for HHPP (CAT), 0.2, 2.1 and -11 ppm are assigned

to terminal, linear and dendritic units, respectively. Signals at -11.8 and -16.3 ppm are


6192

assigned to linear and dendritic units, respectively for HHPP (HRQ). Three Signals at

obtained for HHPP (TBC), 0.8, -1.5 and -3.2 ppm are assigned to terminal, linear and

dendritic units, respectively.


6193

22

Figure 6 1H NMR of HHPP (CAT)


6194

25

Figure 7 1H NMR of HHPP (TBC)


6195

23

Figure 8 1H NMR of HHPP (RES)


6196

24

Figure 9 1H NMR of HHPP (HRQ)


6197

26

Figure 10 1H NMR of HHPP (BPL)


6198

2

7

Figure 11 31

P NMR of HHPP (CAT)


6199

Figure 12 31

P NMR of HHPP (TBC)


6200

Figure 13 31

P NMR of HHPP (RES)


6201

30 Figure 14 31

P NMR of HHPP (HRQ)


6202

32

Figure 15 31

P NMR of HHPP (BPL)


6203

CONCLUSIONS

A novel hydroxyl-terminated hyperbranched polyphosphate (HHPP) was

synthesized by employing an A2 + B3 polycondensation with high functionality as a

curing agent and the products were characterized by FT-IR, 1H NMR and

31P NMR..

1H NMR supports the product formation.

31P NMR Spectrum shows the incorporation

of phosphorus in hyperbranched (3-hydroxyphenyl) phosphate.


6204

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ABSTRACTphysical blend of the epoxy resins and the flame retardants [6]. A free epoxy is used as the...

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