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The present chapter deals with synthesis of Polyurethane acrylate Oligomers.

2.1 Principle reactions in preparation of UV-curable polyurethane

acrylates oligomers

If polymer chain carries -NCO group as end group then it is

referred as –NCO terminated prepolymer and if it carries –OH

group as end group then as Hydroxy terminated prepolymer.

When –NCO terminated prepolymers are al lowed to react with

hydroxy alkyl acrylates, they form Urethane acrylates. Examples

of such hydroxyl alkyl acrylates are 2-Hydroxyethyl methacrylate,

2-Hydroxyethyl acrylate, Hydroxypropyl acrylate.etc. [1-6].

There are two possible methods of preparing the same

urethane acrylate when modifying hydroxy components are use

[7-11]. In the first method, the diisocyanate and the hydroxy

alkyl acrylate are reacted and the half adduct is then reacted

with the modifying hydroxy compound. In the second method, the

modifying hydroxy compound is reacted with the diisocyanate and

the resulting product is then reacted with the hydroxy acrylate.

Which are shown as below.

The incorporation of non-acrylate hydroxyl containing

component into urethane acrylate structures may vary. The order

of addition to the reaction mixture, the type of hydroxy

functionality, the difference in reactivity between isocyanate

group, and possible difference in reactivity of hydroxyl groups,

may all have different impact. For symmetrical diisocyanates, the

order of addition is unimportant but for asymmetric diisocyanate

(l ike 2, 4 TDI and IPDI and also Polyol carrying more than four

hydroxy group e.g., Glycerine, Trimethylol propane,

Pentaerythritol) the order of addition is strictly important due to

probabil i ty of cross l inking.

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Chapter 2

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Chapter 2

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2.2 Experimental:

2.2.1 Materials:

Coconut oil based alkyd resin as polyol was obtained from Reliable

Paints., Makarpura G.I.D.C., Vadodara. Its specifications are shown in Table

2.1 and it’s general structure are shown below in scheme 2.1.

Table 2.1: Specification of Coconut Oil based Alkyd Resin.

-OH Value

(mg KOH/gm)

Acid Value

(mg KOH/gm)

% Non

Volatiles

Viscosity

(Poise @ 25oC)

Coconut

Oil based

Alkyd

Resin

130 25 ± 5 70 ± 2 6-9

Scheme 2.1: General structure of hydroxyl terminated Alkyd resin

derived from coconut oil.

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Various aliphatic and aromatic diisocyanates like Isophorone

diisocyanate (IPDI), toluene diisocyanate (TDI), Hexamethylene diisocyanate

(HMDI), and Methylene diphenyle diisocyanate (MDI) are obtained as

technical grade. Their specifications are shown in Table. 2.2 and the

structure shown in Scheme 2.2.

Table 2.2: Specifications of Aliphatic and Aromatic Diisocyanates.

Mol. Wt.

g/mole

Physical

appearance

Density

g/cm3 @ 20oC

Boiling Point

(oC)

IPDI 222.3Colourless,

Liquid1.062 158

TDI 174.2

Colourless to

Pale yellow,

Liquid

1.214 251

HMDI 168.2Colourless,

Liquid1.047 255

MDI 250.25Pale Yellow,

Liquid1.203 314

NCOOCN

Isophorone Diisocyanate

Hexamethylene Diisocyanate

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Toluene Diisocyanate

Methylene diphenyl Diisocyanate

Scheme 2.2: Structures of Aliphatic and Aromatic diisocyantes.

Hydroxyethyle methacrylate (2-HEMA) was obtained from Aldrich

chemical Co. Its specifications are shown in Table. 2.3 and structure is

shown below.

Table 2.3: Specification of Hydroxyethyle methacrylate.

Mol. Wt.

g/mole

Physical

appearance

Density

g/ml @ 25oC

Boiling Point

(oC)

2-HEMA 130.14Colourless,

Liquid1.073 67

Structure of Hydroxyethyle methacrylate.

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Dibutyltindilurate (DBTDL) was used as catalyst was procured from Hi-

media Co. Its specifications are shown in Table. 2.4 and structure of

Dibutyltindilurate is shown below.

Table 2.4: Specification of Dibutyltindilurate.

Mol. Wt.

g/mole

Physical

appearance

Density

g/ml @ 25oC

Boiling Point

(oC)

DBTDL 631.56Pale Yellow,

Liquid1.066 -

Structure of Dibutyltindilurate.

All these materials were of commercial grades and used as such

without any further purification.

2.2.2 Preparation of Urethane Acrylate Oligomers

Step 1: Preparation of isocyanate terminated prepolymers.

The isocyanate prepolymer was synthesised as follows, HTAR (hydroxyl

terminated alkyd resin) as polyol and isophorone diisocyanate (IPDI) (1:1.25

mol/mol) were charged into a 250ml four necked round bottom flask with a

reflux condenser in a waterbath and equipped with mechanical stirrer, a

thermometer, a dropping funnel, under nitrogen atmosphere and were mixed

gently. 0.05% (w/v) DBTDL was added in the reaction mixture and then the

urethane-forming reaction proceeded at 65°C. The reaction further continued

till the desired % NCO value was obtained. (% NCO determine by

dibutylamine back titration method [12]).

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Step 2: Preparation of urethane acrylate oligomer.

The above reaction mixture was cooled to 45°C, and then HEMA

(hydroxyl ethyl methacrylate) and 0.05% (w/v) DBTDL were added dropwise

with continuous stirring for 60 minutes till the % NCO become almost zero.

Other poly(urethane acrylates) oligomers using different aliphatic and

aromatic diisocyanate with different compositions (NCO/OH mole ratio like

1.25, 1.50, 1.75 and 2.00 respectively) were synthesised by the similar

method and are summarize in Tables 2.5 to 2.8.

2.3 Measurements:

(i) Determination of % NCO Content (dibutylamine back titration

method)

Volumetric determination involves treatment of the sample with excess

amine and back titration of the unreacted amine with standard hydrochloric

acid solution. The most reactive amines are the dibutylamine, dimethylaniline,

and butyl amine. The titration can be carried out in a variety of the inert

solvents such as chlorobenzene, acetone, and dioxane [13].

Polyurethane (0.5 gm) was taken in a conical flask and 25 ml of dry

dioxane was used to dissolve the sample completely. The solution was then

treated with 25 ml of 0.1 N dibutylamine in dioxane, and shaken for 15

minute. To this isopropyl alcohol (100 ml) and 4-6 drops of 0.1 %

bromophenol blue as an indicator was added and then the solution titrated

against 0.1 N Hydrochloric acids. (S)

Similarly, a blank titration (B) was run independently,

% NCO was calculated by using the following formula:

4.202 (B-S) N

W

B & S = Volumes (ml) of the hydrochloric acid consumed in the

titration of the blank And Sample respectively.

% NCO =

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N = Normality of Hydrochloric acid solution.

W = Weight of sample in gm.

(ii) Non Volatile Contents (%Solids)

Non volatile contents [14] of any oligomer are evaluated to ascertain

the film build and the related characteristics. The solid content of each

oligomer was determined by weighing a small amount of oligomer in a Petri

dish and heating it in a vacuum oven at 120oC for 2 hr. to drive off the

volatile components, subsequent weighing gives the weight of the residual

non-volatile matter. The percentage of solid in the original sample can then

be calculated. The results are presented in Table 2.9.

(iii) Colour and Clarity

The urethane acrylates oligomers were taken in 100 ml glass cylinder.

The visual appearance of the viscous liquid was checked as a clear

(transparent), or translucent or opaque and accordingly reported. The colour

of urethane acrylate oligomers were reported in the Table 2.9.

(iv) IR-Spectroscopy

The IR-Spectra of polyurethane oligomers [15] were

scanned on ABB IR-spectrophoto-meter in the range of 4000–400

cm-1. The liquid sample was taken on cel l directly and run the

instrument. The IR spectra of the oligomers are shown in

Figures 2.1 to 2.4.

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2.4 Results and Discussion

All the above prepared urethane acrylate oligomers from coconut oil

based alkyd resin were found to be transparent viscous liquids. The

characterization was emphasized particularly on the properties, which have

direct relevance to their utilization in coating formulations.

(i) Non-Volatile Contents (% Solid)

The percentage non-volati les of all the experimental sets of

urethane acrylate oligomers derived from hydroxyl terminate

alkyd resin are found to be in the vicinity of the theoretical

values and the results are reported in the Table 2.9 this clearly

indicates less using of solvents during the reaction, absence of

any unreacted monomeric compounds as well as presence of low

molecular weight degradation products.

(ii) Colour and clarity

All the reported urethane acrylate oligomers based on

hydroxyl terminated coconut oil based alkyd resin are found to be

clear and transparent l iquids. But the colours of Urethane

acrylate oligomers specially derived using aromatic diisocyanate

in the present study are somewhat darker in colour. It could be

due to the color of diisocyanate monomers.

(iii) IR Spectroscopy

Typical IR spectra of different urethane acrylate ol igomers

are depicted in Figures 2.1 to 2.4 respectively. The spectral

analysis was mainly used to check the completion of the

polymerization reaction in terms of the disappearance of the NCO

band at 2265 cm-1 and the appearance of the N-H band at 3000-

3400 cm-1, which could be ascribed to the hydrogen bonding

between N-H and carbonyl groups. As shown in Figure, the

spectra of the two urethane acrylate oligomers did not show any

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detectable band at 2265 cm-1 but did show strong absorption

bands at 1724 cm-1 (amide I, stretching of the ester C=O bond).

Absorption at 1068 and 1192 cm-1 showed stretching frequencies

for C-O group of acrylic compound. IR spectra of polyurethane

acrylate oligomers showed additional absorption bands at 1740

and 3390 cm-1 corresponding to the presence of urethane

carbonyl (-C=O) and amide (-NH) stretching respectively. The

band at 1281 cm-1 is because of stretching of C-N in urethane.

The characteristic stretching band of –N=C=O at 2265 cm-1 is

absent that indicates all –NCO have reacted.

Chapter 2

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Scheme 2.3: Synthesis of UV-curable urethane acrylates

oligomer.

Chapter 2

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Table 2.5: Urethane acrylates oligomers based on IPDI.

Sr.

No.

HTAR

(g)

IPDI

(g)

TDI

(g)

MDI

(g)

HMDI

(g)

NCO/OH

ratio

HEMA

(g)

Sample

code.

1 7.15 2.3 - - - 1.25 0.53 PUA-1I

2 6.5 2.51 - - - 1.5 0.98 PUA-2I

3 5.96 2.68 - - - 1.75 1.34 PUA-3I

4 5.5 2.83 - - - 2 1.65 PUA-4I

Table 2.6: Urethane acrylates oligomers based on TDI.

Sr.

No.

HTAR

(g)

IPDI

(g)

TDI

(g)

MDI

(g)

HMDI

(g)

NCO/OH

ratio

HEMA

(g)

Sample

code.

1 7.53 - 1.9 - - 1.25 0.56 PUA-1T

2 6.88 - 2.08 - - 1.5 1.03 PUA-2T

3 6.33 - 2.23 - - 1.75 1.43 PUA-3T

4 5.86 - 2.36 - - 2 1.76 PUA-4T

Table 2.7: Urethane acrylates oligomers based on MDI.

Sr.

No

HTAR

(g)

IPDI

(g)

TDI

(g)

MDI

(g)

HMDI

(g)

NCO/OH

ratio

HEMA

(g)

Sample

code.

1 6.95 - - 2.51 - 1.25 0.52 PUA-1M

2 6.3 - - 2.74 - 1.5 0.95 PUA-2M

3 5.77 - - 2.92 - 1.75 1.3 PUA-3M

4 5.31 - - 3.08 - 2 1.6 PUA-4M

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Table 2.8: Urethane acrylates oligomers based on HMDI.

Sr.

No

HTAR

(g)

IPDI

(g)

TDI

(g)

MDI

(g)

HMDI

(g)

NCO/OH

ratio

HEMA

(g)

Sample

code.

1 7.58 - - - 1.84 1.25 0.57 PUA-1H

2 6.93 - - - 2.02 1.5 1.04 PUA-2H

3 6.38 - - - 2.17 1.75 1.44 PUA-3H

4 5.91 - - - 2.3 2 1.78 PUA-4H

Table 2.9: Physical properties of Urethane Acrylate Oligomers.

Sr.

No.

Sample

code.% NCO % NVM Clarity Color

1 PUA-1I 0.32 99.87 Clear Colorless

2 PUA-2I 0.22 99.84 Clear Colorless

3 PUA-3I 0.45 99.79 Clear Colorless

4 PUA-4I 0.27 99.71 Clear Colorless

5 PUA-1T 0.41 98.81 Clear Light yellow

6 PUA-2T 0.30 98.66 Clear Light yellow

7 PUA-3T 0.16 97.94 Clear Light yellow

8 PUA-4T 0.24 97.82 Clear Light yellow

9 PUA-1M 0.31 98.94 Clear Reddish yellow

10 PUA-2M 0.36 98.91 Clear Reddish yellow

11 PUA-3M 0.41 98.82 Clear Reddish yellow

12 PUA-4M 0.38 97.86 Clear Reddish yellow

13 PUA-1H 0.16 99.85 Clear Colorless

14 PUA-2H 0.21 99.81 Clear Colorless

15 PUA-3H 0.25 99.78 Clear Colorless

16 PUA-4H 0.29 99.74 clear Colorless

Department of Chemistry, S.P. University

Figure 2.1: IR Spectrum of IPDI based Urethane Acrylate Oligomer

Department of Chemistry, S.P. University

IR Spectrum of IPDI based Urethane Acrylate Oligomer

Chapter 2

Page 69

IR Spectrum of IPDI based Urethane Acrylate Oligomer (PUA-2I).

Department of Chemistry, S.P. University

Figure 2.2: IR Spectrum of TDI based Urethane Acrylate Oligmer

Department of Chemistry, S.P. University

IR Spectrum of TDI based Urethane Acrylate Oligmer

Chapter 2

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IR Spectrum of TDI based Urethane Acrylate Oligmer (PUA-2T).

Department of Chemistry, S.P. University

Figure 2.3: IR Spectrum of MDI based Urethane

Department of Chemistry, S.P. University

IR Spectrum of MDI based Urethane Acrylate Oligomer

Chapter 2

Page 71

Acrylate Oligomer (PUA-2M).

Department of Chemistry, S.P. University

Figure 2.4: IR Spectrum of HMDI based Urethane Acrylate Oligomer

Department of Chemistry, S.P. University

IR Spectrum of HMDI based Urethane Acrylate Oligomer

Chapter 2

Page 72

IR Spectrum of HMDI based Urethane Acrylate Oligomer (PUA-2H).

Chapter 2

Department of Chemistry, S.P. University Page 73

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1084 (1948).

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Department of Chemistry, S.P. University Page 74

14. Krishnamurti N. “Prog. Org. Coat.”, Vol. 11(2), pp. 167

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