The present chapter deals with synthesis of Polyurethane acrylate Oligomers.

Transcription

1 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 allowed 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 (like 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 probability of cross linking. Department of Chemistry, S.P. University Page 55

2 Department of Chemistry, S.P. University Page 56

3 Department of Chemistry, S.P. University Page 57

4 2.2 Experimental: 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. Coconut Oil based Alkyd Resin -OH Value Acid Value % Non Viscosity (mg KOH/gm) (mg KOH/gm) Volatiles 25 o C) ± 5 70 ± Scheme 2.1: General structure of hydroxyl terminated Alkyd resin derived from coconut oil. Department of Chemistry, S.P. University Page 58

5 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 IPDI TDI HMDI MDI Physical appearance Colourless, Liquid Colourless to Pale yellow, Liquid Colourless, Liquid Pale Yellow, Liquid Density g/cm 20 o C Boiling Point ( o C) OCN NCO Isophorone Diisocyanate Hexamethylene Diisocyanate Department of Chemistry, S.P. University Page 59

6 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 2-HEMA Physical appearance Colourless, Liquid Density Boiling Point 25 o C ( o C) Structure of Hydroxyethyle methacrylate. Department of Chemistry, S.P. University Page 60

7 Dibutyltindilurate (DBTDL) was used as catalyst was procured from Himedia 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 DBTDL Physical appearance Pale Yellow, Liquid Density Boiling Point 25 o C ( o C) Structure of Dibutyltindilurate. All these materials were of commercial grades and used as such without any further purification 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]). Department of Chemistry, S.P. University Page 61

8 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 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: % NCO = (B-S) N W B & S = Volumes (ml) of the hydrochloric acid consumed in the titration of the blank And Sample respectively. Department of Chemistry, S.P. University Page 62

9 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 120 o C 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 cm -1. The liquid sample was taken on cell directly and run the instrument. The IR spectra of the oligomers are shown in Figures 2.1 to 2.4. Department of Chemistry, S.P. University Page 63

10 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-volatiles 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 liquids. 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 oligomers 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 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 Department of Chemistry, S.P. University Page 64

11 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. Department of Chemistry, S.P. University Page 65

12 Scheme 2.3: Synthesis of UV-curable urethane acrylates oligomer. Department of Chemistry, S.P. University Page 66

13 Table 2.5: Urethane acrylates oligomers based on IPDI. Sr. No. HTAR IPDI TDI MDI HMDI NCO/OH ratio HEMA Sample code PUA-1I PUA-2I PUA-3I PUA-4I Table 2.6: Urethane acrylates oligomers based on TDI. Sr. HTAR IPDI TDI MDI HMDI NCO/OH HEMA Sample No. ratio code PUA-1T PUA-2T PUA-3T PUA-4T Table 2.7: Urethane acrylates oligomers based on MDI. Sr. HTAR IPDI TDI MDI HMDI NCO/OH HEMA Sample No ratio code PUA-1M PUA-2M PUA-3M PUA-4M Department of Chemistry, S.P. University Page 67

14 Table 2.8: Urethane acrylates oligomers based on HMDI. Sr. HTAR IPDI TDI MDI HMDI NCO/OH HEMA Sample No ratio code PUA-1H PUA-2H PUA-3H PUA-4H Table 2.9: Physical properties of Urethane Acrylate Oligomers. Sr. No. Sample code. % NCO % NVM Clarity Color 1 PUA-1I Clear Colorless 2 PUA-2I Clear Colorless 3 PUA-3I Clear Colorless 4 PUA-4I Clear Colorless 5 PUA-1T Clear Light yellow 6 PUA-2T Clear Light yellow 7 PUA-3T Clear Light yellow 8 PUA-4T Clear Light yellow 9 PUA-1M Clear Reddish yellow 10 PUA-2M Clear Reddish yellow 11 PUA-3M Clear Reddish yellow 12 PUA-4M Clear Reddish yellow 13 PUA-1H Clear Colorless 14 PUA-2H Clear Colorless 15 PUA-3H Clear Colorless 16 PUA-4H clear Colorless Department of Chemistry, S.P. University Page 68

15 Figure 2.1: IR Spectrum of IPDI based Urethane Acrylate Oligomer (PUA-2I). Department of Chemistry, S.P. University Page 69

16 Figure 2.2: IR Spectrum of TDI based Urethane Acrylate Oligmer (PUA-2T). Department of Chemistry, S.P. University Page 70

17 Figure 2.3: IR Spectrum of MDI based Urethane Acrylate Oligomer (PUA-2M). Department of Chemistry, S.P. University Page 71

18 Figure 2.4: IR Spectrum of HMDI based Urethane Acrylate Oligomer (PUA-2H). Department of Chemistry, S.P. University Page 72

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20 14. Krishnamurti N. Prog. Org. Coat., Vol. 11(2), pp. 167 (1983). 15. Australian Colour Chemist s Association, Surface Coatings: Raw materials and their Usage, Vol. I Champman and Hall, London, pp. 265, (1963). Department of Chemistry, S.P. University Page 74