New UV-curable prepolymer, 2–hydroxy–3–[p–(1–{2–hydroxy–3–(isoprophenylcarbonyl- oxypropoxy)phenyl}methylcyclohexyl)phenoxy]propyl acrylate (BISCHDA) was synthesized by the reaction of p–[1–(p–hydroxyphenyl) methylcyclohexyl] phenol with methylcyclohexanone, epoxidation with epichlorohydrin followed by addition reaction with acrylic acid. All synthesized compounds were characterized by 1H-NMR,
13C-NMR and FT-IR spectroscopy. Several UV-curable formulated solutions were prepared separately by mixing the prepolymer, BISCHDA (75%), diluent, tri (ethylene glycol) diacrylate (TEGDA) (25%) and different concentrations of a photoinitiator, 2–benzyl–2–dimethylamino–1–(4–morpholinophenyl) butan–1–one (Irgacure 369/ BDMB) and irradiated at 365 nm. The effect of intensity of lights and various difunctional diluents on the kinetics of the photopolymerization, degree of photocuring (DC %) and rate of photocuring (Rp) was studied by FT-IR spectroscopy. The results show that, increasing the concentration of Irgacure 369 and intensity of light, increase DC (%) whereas, Rp increase suddenly and then decreases. The thermal behaviour of the UV-cured polymers by TGA and DSC techniques.
Keywords
UV-curable prepolymer, irradiation, FT-IR, diluents, degree of photocuring, rate of photocuring, TGA, DSC.
1. Introduction
The technology of photopolymerization is widely applied in several industrial fields, microlithography [1-3], printing material [4], liquid crystalline [5-9] and nonlinear optical materials [10-15]. An important advantages offered by this technology are high speed, reduced release of volatile compounds since no solvent are used and low cost of equipment. Also kinetics of photocuring, mechanical properties, inertness to solvents, resistance to abrasion depend on the degree of photocuring and rate of photocuring of final photocured polymer produced [16]. The UV-curable compositions are basically constituted from prepolymers, diluents and photoinitiators. Prepolymers are low molecular weight polymers, which contain at least two unsaturated groups such as urethane acrylates, epoxy acrylates and polyester acrylates [16, 17]. The commonly used prepolymer in coating industry is an epoxy acrylate. An epoxy acrylates are suitable to use in UV curing system than the any other acrylates are due to non-yellowing, good hardness and excellent characteristics to moisture [17]. The epoxy backbone in bisphenol epoxy acrylates promotes toughness and good chemical resistance of photocured films are due to carbon-carbon and ether bonds present in the prepolymer structure [18]. Monomers usually called reactive diluents. In addition to the viscosity regulations they affect the kinetics of photocuring and physical characteristics of the polymer formed [19]. Although the final properties of the cured polymers such as adhesion, shrinkage, surface energy, hardness, gel content, solvent and chemical resistance [16, 20, 21] depends basically on the structure and the type of monomer used [22]. The effect of various difunctional diluents and intensity of the lights on the kinetics of photopolymerization such as the degree of photocuring (DC %) and rate of photocuring (Rp) of the photocrosslinked polymers were studied by FT-IR spectroscopy.
In many applications, the important parameters of the photocrosslinked polymers are the maximum extent of reaction, rate of polymerization, dimensional and thermal stability, scratch resistance, low shrinkage, and low moisture absorption [23]. All of these parameters depend on the irradiation time, reaction temperature, the viscosity of the system, diluent structure, oligomers structure, photoinitiator concentration and the photoinitiator used [24]. In many of the research work, commercial prepolymer, bisphenol–a–glycerolate(1-glycerol/phenol) diacrylate (BISGA) is used to study the kinetics of photopolymerization.
In this compound, ethyl group present in between the bisphenol epoxy acrylate. The curing speed is one of the very important parameters in the kinetics of photopolymerization [25].
The significance of the present work is summarized as follows: The production cost of the synthesized acrylate prepolymers are one third of the actual cost of production of the other commercial prepolymers [25]. The synthetic procedure is very easy [26]. iii) The reactivity of the synthesized acrylate and methacrylate prepolymers are high when compared to the commercial prepolymer, BISGA [25, 26]. The unique properties of the synthesized acrylate prepolymer is the DC (%), Rp and hardness are higher and also curing is also very fast when compared to the commercial prepolymer, BISGA [26].
To study the influence of photoinitiator concentration, various difunctional diluents, various intensity of lights on the kinetics of photopolymerization such as DC (%) and Rp are studied with the synthesized prepolymer. Several formulations have been prepared from synthesized prepolymers, diluents and photoinitiators separately [25, 26].
2. Experimental
2.1. Materials
Phenol, acetone, epichlorohydrin, acetic acid, hydrochloric acid (HCl), sulphuric acid (H2SO4), isopropanol, p-methylcyclohexanone, chloroform, methanol, 1 ,4-dioxane, benzene, sodium hydroxide (NaOH), potassium hydroxide (KOH) and hydroquinone were purchased from SRL (India) and used without further purification. Acrylic acid (≥ 99.0, SRL, India) was used after removal of inhibitor for acrylation reactions. The reagent, triethylamine (SRL, India) was heated under reflux over the sodium wire for 8 h, distilled and finally collected over fresh sodium wire. Ethyl alcohol was refluxed with calcium oxide for 6 h, allowed to stand overnight and distilled. The photoinitiator, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one (Irgacure 369/BDMB), difunctional diluents, 1, 6-hexanediol diacrylate (HDDA), di (ethylene glycol) diacrylate (DEGDA), bisphenol-A-ethoxylate (2EO) diacrylate (BISEODA), bisphenol-A-propoxylate (2PO) diacrylate (BISPODMA) tri(ethylene glycol) diacrylate (TEGDA) were purchased from Aldrich (USA) and used without further purification.
2.2. Synthesis of p-[1-(p-Hydroxyphenyl)Methylcyclohexyl]Phenol (HPMCP)
1.0 mol of phenol was mixed with the solution containing 150 mL of acetic acid in 300 mL
of concentrated hydrochloric acid into a 1000 mL five–necked round bottom flask fitted with a heating mantle, funnel, thermometer, mechanical stirrer and reflux condenser. The contents of the flask were stirred for a period of 15 min, and then the solution was heated to 40 ,
0.5 mol p-methylcyclohexanone was introduced drop wise to the solution for about 3 h. Then the mixture was poured into 60 mL boiling water containing 2N potassium hydroxide.
The solid product was separated and acidified with dilute sulphuric acid, then washed with distilled water and dried at 45 for 12 h. Recrystallization from benzene and the methanol – water system gave light yellow crystals. The synthesis of the HPMCP is shown in Fig. 1. Yield 88%.
2.3. Synthesis of 2-Oxiranyl-[p-(1-{p-[(2-Oxiranyl)Methoxy]Phenyl}Methylcyclohexyl) phenoxy]Methane (OOMPM)
A 1000 mL five–necked round bottom flask was fitted with a heating mantle, funnel, thermometer, reflux condenser and mechanical stirrer. Then 0.5 mol of HPMCP, 1.1 mol of epichlorohydrin and 250 mL of isopropanol were added to the flask. After thoroughly mixed, then the contents were heated at 45oC for 20 min. Subsequently a solution of 1.25 mol of sodium hydroxide in 500 mL water was added drop wise to the solution and then stirred continuously at The temperature of the system was maintained at 45oC for 5 h. The clear aqueous upper layer was carefully removed and the resin was washed with water. Finally, the product in the flask was washed several times with deionised boiled water to remove the remaining sodium chloride and excess of epichlorohydrin was evaporated and dried under vacuum to give a solid product of OOMPM. It was soluble in aromatic hydrocarbons, chloroform and acetone. The synthesis of OOMPM is shown in Fig. 1. Yield 85%.
2.4. Synthesis of 2-Hydroxy-3-[p-(1-{p-[2-Hydroxy-3-(Vinylcarbonyloxy)Propoxy] Phenyl}Methylcyclohexyl)Phenoxy]Propyl Acrylate (HVMPA)
A 1000 mL five–necked round bottom flask fitted with a thermometer, funnel, reflux condenser and mechanical stirrer. Then 0.5 mol of OOMPM, and 300 mL of 1, 3-dioxane was added to the round bottom flask. . The contents of the flask were stirred, heated at 50oC followed by adding 1.0 mol acrylic acid, 7 mL triethylamine and 0.05 wt% hydroquinone at
5 h, until the acid number was less than 4 mg KOH/g. The standard alkali solution titration method was used to determine the acid content of the prepolymer. The synthesis of HVMPA is shown in Fig.1. Yield 91%. 2.5. Characterization Methods
1H–NMR and 13C–NMR spectra of HPMCP, OOMPM and HVMPA were taken in CDCl3 on Bruker DPX–300 MHz spectrometer using tetramethylsilane (TMS) as an internal standard. FT–IR spectra of the synthesized compounds, HPMCP, OOMPM, HVMPA and photocured polymers were recorded on a Perkin–Elmer spectrum one (7800–350 cm–1) spectrometer, using KBr pellets. All irradiations were carried out medium pressure mercury vapour lamp to the power output of 100 W/cm2. The influence of intensity of UV-radiation, 5, 14, 55, 87 and 117 mW/cm2 on the kinetics of photocuring, degree of photocuring
(DC %) and rate of photocuring (Rp) was used. The hardness of photocured polymer films was determined by A–Type JIS K6301 hardness tester (Germany). Differential scanning calorimetry analysis (DSC) was performed by Mettler DSC 30 Instrument, calibrated with indium. The UV-cured samples were cooled down to −150oC and were immediately heated at a heating rate of 10–150oC/min. The Tg was determined as the middle point in the transition. The thermal degradation behaviour of the UV-cured polymers were studied by a Perkin Elmer-6 Thermogravimetric Analyzer (TGA) at temperatures ranging from the ambient to 1000oC under a nitrogen atmosphere at the heating rate of 10oC/min. The measurements were conducted with about 6–10 mg of samples and weight-loss/temperature thermograms were recorded
2.6. UV-curable Formulation and Photocuring
UV-curable formulation containing the prepolymer, HVMPA (75%), diluent, TEGDA (25%) and the photoinitiator, Irgacure-369 (0.50%) were coated on a thin glass slide for 20 mm thickness, and then exposed to the medium pressure mercury vapour lamp with intensity of 117 mW/cm2 at a distance of 2.5 cm from the source point. All irradiations were carried out by varying the Irgacure-369 concentration at various time intervals. The effect of BDMB concentration on degree of photocuring (DC %), rate of photocuring (Rp), gel content (%) and hardness were studied [25, 26]. Simultaneously FTIR will be recorded for all UV-cured polymers. The decrease in the IR absorbance at 810-812 cm−1 was monitored [25, 26].
2.7. Gel Content (%)
The Gel content (%) of the photocured polymers was performed in a Soxhlet extractor using acetone as solvent refluxing at 50oC for 72 h. The swollen sample was isolated and subjected to vacuum drying for sufficiently long time to ensure complete drying. The gel content (%) was determined from the weight difference between the initial (Wo) and final weight (Wf) of sample [25, 26].
2.8. Relative Hardness Testing (Degree Shore)
The relative hardness (degree shore) of each of the photocured polymer films was determined by A–Type JIS K–6301 hardness tester. The photocured polymer films of more than 15 mm thickness were used for the study. The readings were taken in several places on the same photocured polymer film by pressing the needle of the tester. The average value was noted for hardness (degree shore) of the sample under test [25, 26].
2.8. Thermogravimetric Analysis
The thermal degradation behaviour of the UV-cured polymers were studied by a Perkin Elmer-6 Thermogravimetric analyzer (TGA) at temperatures ranging from the ambient to 1000oC under nitrogen atmosphere at the heating rate of 10oC/ minutes. The measurements were conducted with about 6–10 mg of samples and weight-loss/temperature thermograms were recorded
2.9. Differential Scanning Calorimetry
Differential scanning calorimetry analysis (DSC) was performed by Mettler DSC 30 Instrument, calibrated with indium. The UV-cured samples were cooled down to −150oC and were immediately heated at a heating rate of 10–150oC/minutes. The Tg was determined as the middle point in the transition.
3. Results and Discussion
The structural conformation of synthesized compounds, HPMCP, OOMPM, and HVMPA are carried out by FT–IR, 1H–NMR and 13C–NMR spectroscopy. The FT–IR spectral data of HPMCP, OOMPM, and HVMPA are given as follows: HPMCP (KBr, cm–1): 3418
(–O–H str.); 3066 (Ar–C–H str.); 2971 (–C–H asym. str.); 2989 (–C–H sym. str.); 1620
(Ar–C=C– str.); 1454 (–C–H def str.); 1235 (methyl–C–H str.); 1233 (–C–O str.); 1176
(Ar-C-H def). OOMPM (KBr, cm–1): 3137 (Ar–C–H str.); 2927 (–C–H asym. str.); 2863
(–C–H sym. str.); 1637 (cyclic epoxy –C–H str.); 1630 (Ar–C=C str.); 1455 (methylene
–C–H str.); 1289 (methyl –C–H str.); 1232 (epoxy ring half ring str.); 1216 (–C–O str.); 1151 (–C–O–C–str.); 825 (epoxy whole ring str.). HVMPA (KBr, cm–1): 3414 (–O–H str.); 3125 (Ar–C–H str.); 3042 (alkenes =C–H str.); 2935 (–C–H asym. str,); 2863 (–C–H sym. str.); 1765 (C=O str.); 1648 (alkenes –C=C– str.); 1615 (Ar–C=C–str.); 1457 (–C–H bending str.), 1366 (–C–H rock str.); 1290 (methyl C–H str.); 1221 (–C–O str.); 1136 (–C–O–C–str.); 810 (vinyl group –C–H bending str.); 720 (vinyl group C–H rock str.).
1H-NMR (δ (ppm), CDCl3, 300 MHz) spectral data of HPMCP, OOMPM, and HVMPA are given as follows: HPMCP: 0.831 (d, 1H, –CH–CH3 cyclic), 1.08 (d, 1H, γ-CH2 cyclic), 1.31 (dd, 4H, β-CH2 cyclic), 1.55 (dd, 4H, α-CH2 cyclic), 4.67 (s, 2H, Ar–OH), 6.77–7.15 (d, 4H, Ar–H), 7.43–7.70 (d, 4H, Ar–H). OOMPM: 0.833 (d, 1H, –CH–CH3 cyclic), 1.11 (d, 1H, γ-CH2 cyclic), 1.33 (dd, 4H, β-CH2 cyclic), 1.57 (dd, 4H, α-CH2 cyclic), 2.28 (dd, 4H, –O–CH2–), 2.64 (dt, 4H, Ar–O–CH2–), 3.73 (m, 2H, –CH–O –), 6.79–7.16
(d, 4H, Ar–H), 7.41–7.72 (d, 4H, Ar–H). HVMPA: 0.835 (d, 1H, –CH–CH3 cyclic),
1.14 (d, 1H, γ-CH2 cyclic), 1.35 (t, 4H, β-CH2 cyclic), 2.15 (t, 4H, α-CH2 cyclic), 2.69
(dt, 4H, -CH2–O-), 3.25 (d, 2H, -OH-), 2.24 (dd, 2H, –CH–OH–), 4.03 (m, 4H, Ar–O–CH2–), 4.25 (m, 2H, CH2–CH–OH), 4.57 (m, 4H, –CH–OH–CH2–), 5.10 (d, 2H, –COO–CH), 5.58 (d, 2H, –CH=CH2 trans), 6.11 (d, 2H, –CH=CH2 cis), 6.29 (t, 2H, –CH=CH2), 6.58–7.11
(d, 4H, Ar–H), 7.37–7.67 (d, 1H, Ar–H), 7.40–7.57 (d, 1H, Ar–H).
The proton decoupled 13C–NMR (δ (ppm), CDCl3, 300 MHz) spectral value of HPMCP, OOMPM, and HVMPA are given bellow: HPMCP: 168.34, 140.13, 134.21, 126.13, 114.16, (Ar–C–), 155.31 (Ar–C–O–), 41.12 (Ar–C–CH3), 31.12 (Ar–C–CH3), OOMPM: 168.43, 141.30, 135.17, 126.31, 114.24, (Ar–C–), 123.32 (Ar–C–O–CH2–), 72.61 (Ar–C–O-CH2–), 50.53 (–O–CH2–), 47.34 (–CH–CH2–O–), 45.27 (–CH–CH2–O–), 41.12 (Ar–C–CH3–), 31.10 (Ar–C–CH3). HVMPA: 167.52 (–CO–O–CH2–),160.34, 140.31, 134.78, 126.21, 112.41 (Ar–C–), 132.30 (–CH=CH2–), 127.22 (–CH=CH2), 122.32 (Ar–C–O–CH2–), 72.41 (Ar–C–O-CH2–), 69.34 (–CH–OH–), 50.34 (–CH–CH2–O–),44.72 (–CH–CH2–O–), 41.05 (Ar-C-CH3), 31.03 (Ar-C-CH3).
3.1. Degree of Photocuring
The degree of photocuring (DC %) reacted is calculated by using the following equation:
DC (%)=(((A_acrylate )_(t_2-) (A_acrylate )_(t_1 ))/(A_acrylate )_(t_0 ) ) × 100 (1)
Where, (Aacrylate) t2, (Aacrylate) t1 and (Aacrylate) t0 are carbon-carbon double bond absorbance at t2, t1 and t0 time respectively. The decrease in the intensity of acrylated carbon–carbon double bond absorbance (Aacrylate) at 810 cm–1 was monitored. To study the influence of the photoinitiator concentration, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl) butan-1-one (Irgacure 369/BDMB) on the kinetics of photopolymerization, the prepolymer, 2-hydroxy-3-[p-(1-{2-hydroxy-3-(vinylcarbonyloxy)propoxy]phenyl}methylcyclohexyl)phenoxy]propyl acrylate (HVMPA) (75%) and the diluent, 1, 6-hexanediol diacrylate (HDDA) (25%) were mixed separately with the various concentration of the photoinitiator, Irgacure-369, 0.5, 1.0, 1.5, 2.0 and 2.5%. The polymerizations were carried out at the five time intervals such as, 1, 2, 3, 4 and 5 minutes by medium pressure mercury vapour lamp at 117 mW/cm2 (Heber Scientific, India, Chennai) in the presence of air. The kinetics of photopolymerization such as, degree of photocuring (DC %) and rate of photocuring (Rp). The effect of Irgacure-369 concentration on DC (%) is given in Fig. 2. The DC (%) increases with an increase in the Irgacure-369 concentration. At 5 min irradiation time, keeping Irgacure-369 and HDDA concentration as constant by increasing the Irgacure-369 concentration from 0.5 to 2.5%, the DC (%) values increased from 50 to 74%. This may be due to the formation of excess of free radicals during irradiation and participated in photocuring reaction. By keeping HVMPA, HDDA and Irgacure-369 (0.50%) concentration as constant by increasing the irradiation time from 1 to 5 min, the DC (%) values are 19 and 50% respectively. This shows that an increase in the irradiation time results in increase of DC (%) value which may be due to the availability of more photocuring energy resulting in the formation of extra free radicals, which involved in photocrosslinking polymerization [25, 26].
3.2. Influence of Functionality of Diluents
To study the influence of the functionality of diluents on the kinetics of photopolymerization, several formulations have been prepared from the prepolymer, HVMPA (75%), and the monofunctional diluent, DEGEEA, (25%), difunctional diluent, TEGDA (25%) and, trifunctional diluent, TMPEOTA (25%) is mixed separately with the photoinitiator, DMPA (1.00%). The contents were exposed to the medium pressure mercury vapour lamp at 117 mW/cm2 (Heber Scientific, India, Chennai) at different time intervals in the presence of air [25, 26]. The effect of functionality of diluents on DC (%) and Rp is given in Fig. 3. The DC (%) increases with an increase in the functionality of diluents. At 5 min irradiation time, keeping Irgacure-369 and HVMPA concentration as constant by varying the functionality of diluents from mono, DI and tri functional diluents, the DC (%) values increase from 50, 62 and 74% respectively. This may be due to the increasing the reactivity of diluents from mono to tri functional diluents, results in increasing the reactivity of photocuring reaction.
3.4. Effect of Various Difunctional Diluents
To study the effect of various difunctional diluents on DC (%), various UV-curable formulated solutions were prepared by mixing separately with the difunctional diluents, 1, 4-butanediol diacrylate (BDDA), 1, 6-hexanediol diacrylate (HDDA), di (ethylene glycol) diacrylate (DEGDA), bisphenol-A-ethoxylate (2EO) diacrylate (BISEODA), bisphenol-A-propoxylate (2PO) diacrylate (BISPODA) tri (ethylene glycol) diacrylate (TEGDA) (50%), prepolymer (50%) and photoinitiator, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one (Irgacure 369 /BDMB) (1.0%). Then photocurable formulation was placed in thin glass slides to give 20 mm film thickness and cured under the medium pressure mercury vapour lamp at 117 mW/cm2 (Heber Scientific, India, Chennai) at different time intervals in the presence of air. The effect of various difunctional diluents on DC (%) is given in Fig. 4.
The DC (%) is increased with increase in the viscosity of diluents. At 5 min irradiation time, keeping Irgacure-369 and HVMPA concentration as constant by varying the diluents from BDDA, HDDA, DEGDA, BISEODA and BISPODA, DC (%) values increased from 90, 81, 73, 66 and 58% respectively. This may be due to the reduction in viscosity of system from BDDA to BISPODA more favourable for photocuring reaction.
3.5. Effect of Light intensity
To study the effect of light intensity on DC (%) and Rp, photocurable formulation was prepared by using the synthesized prepolymer, HVMPA (70%) and diluent, HDDA (30%) in the presence of the photoinitiator, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one (Irgacure 369/ BDMB) (1.0%) and cured under 5 mW/cm2 radiation at various time intervals. To study the intensity of UV-light intensities, 5, 14, 55 87 and 117 mW/cm2 were used to study the kinetics of photocuring reactions. Fig. 5 shows the effect of light intensity on DC (%) and Rp. The final DC (%) of photocured polymers produced from 5, 14, 55 87 and 117 mW/cm2 were found to be 47, 66, 74 and 80% respectively. This may be due to increase in intensity of UV-light, increases number free radicals produced and reacted in acrylate groups present in both prepolymer, HVMPA and diluents, HDDA in the phopolymerization reaction, which increases the DC (%).
3.6. Rate of Photocuring (Rp)
Rp can be calculated by using the following equation:
〖 R〗_p=[M_0 ]×[((A_810 ) t_1-(A_810 ) t_2)/((A_810 ) t_0×(t_2-t_1 ) )] (2)
Where [M0] is the monomer concentration before irradiation, (A810)t0, (A810)t1 and (A810)t2 which represent the absorption due to carbon–carbon double bond before and after the exposure during the time periods t1 and t2 respectively. The effect of Irgacure 369 concentration on Rp is given in Fig. 2. The Rp depends on the DC (%). At 1 min irradiation time, keeping HVMPA and HDDA concentration as constant by increasing the Irgacure 369 concentration from 0.50 to 2.50%. The Rp values increased from 5.19 10–2 to 7.37 10–2 mol l–1 sec–1. This may be due to the increase in the number of free radicals produced and subsequently participated in photopolymerization reaction. The effect of functionality of diluents on Rp is given in Fig. 3. The results show that at 1 min irradiation, increasing the functionality of diluents from mono to trifunctional diluents, Rp increases from 4.14 10–2 to 6.63 10–2 mol l–1sec–1. The increase in Rp may be due to the reduction in viscosity, which may increase the mobility of free radicals that react with acrylate. The effect of various difunctional diluents on Rp is given in Fig. 4. The Rp increases with an increase in the viscosity of diluents. At 5 min irradiation time, keeping Irgacure-369 and HVMPA concentration as constant by varying the diluents from BDDA, HDDA, DEGDA, BISEODA and BISPODA, Rp value increases from 3.42 10–2, 4.24 10–2, 5.65 10–2, 6.76 10–2 and 7.41 10– 2 mol l–1 sec–1 respectively. This may be due to the reduction in viscosity of system from BDDA to BISPODA more favourable for photocuring reaction. Fig. 5 shows the effect of light intensity on Rp. The (Rp) max of photocured polymers produced from 5, 14, 55 87 and 117 mW/cm2 light intensity were found to be 4.13 10–2, 5.41 10–2, 6.72 10–2, 7.32 10–2 and 8.72 10– 2 mol l–1 sec–1 respectively. This may be due to increase in intensity of UV-light, increases number free radicals produced and reacted in acrylate groups present in both prepolymer, HVMPA and diluents, HDDA in the phopolymerization reaction, which increases the (Rp) max.
3.7. Thermogravimetric Analysis
The Thermogravimetric analysis (TGA) of the cured polymers were performed in a nitrogen atmosphere at a heating rate 25°C/minute in order to calculate their relative thermal stabilities. The TGA curves of selected UV-crosslinked polymers are given in Fig. 6. The initial decomposition temperature (IDT) values, corresponding to the various weight-losses (%) is given in Table 1.The first significant break in the thermograms corresponds to IDT. It is indicated that the UV-crosslinked polymers are stable up to 248-318°C and degradation thereafter, indicating that the initial decomposition temperature of the UV-crosslinked polymers vary between 248-318°C. The IDT values depend upon the irradiation time. Comparing the IDT values of the UV-crosslinked polymers obtained from the prepolymer, HPEPA with DEGEEA, TEGDA and TMPEOTA at an initiation concentration of 1.25% for an irradiation time of 5 minutes, it is indicated that the IDT is higher for the UV-crosslinked polymers obtained from the prepolymer, HPEPA with trifunctional diluent, TMPEOTA. This study shows that the UV-crosslinked polymer obtained from the trifunctional diluent, TMPEOTA has shown a greater stability than the polymers obtained from the difunctional diluent, TEGDA and the monofunctional diluent, DEGEEA. This suggests that increasing the functionality of diluent increases the stability.
3.8. Differential Scanning Calorimetry (DSC)
DSC Thermograms of the UV-crosslinked polymers are shown in Fig. 7. Glass transition temperature (Tg) values for the UV-crosslinked polymers obtained from the prepolymer, HEPPA with monofunctional diluent, DEGEEA (160°C), difunctional diluent, TEGDA (170°C) and trifunctional diluent, TMPEOTA (200°C) in the presence of the photoinitiator, DMPA (1.25%) and irradiations time of 5 minutes. The result shows that UV-crosslinked polymers obtained from trifunctional diluent, TMPEOTA hasa higher Tg than difunctional diluent and trifunctional diluent. This indicates that Tg value depends upon the functionality of diluent.
4. Conclusions
The photocurable prepolymer, HVMPA was synthesized from HPMCP and OOMPM and confirmed by FT–IR, 1H–NMR and 13C–NMR spectroscopy. The Photocuring studies were carried out by irradiating the UV-curable formulation containing the HVMPA with various difunctional diluents mixed separately in the presence of various concentrations of 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one (Irgacure 369) using medium pressure mercury vapour lamp. It was found that DC (%) of the photocured polymers increased with increasing irradiation time and Irgacure 369 concentrations, whereas Rp increased rapidly, then decreased and became a consultant. The unique properties of the synthesized prepolymer are DC (%) and Rp are higher and curing is also very fast. The production cost of the synthesized prepolymer, HVMPA is one third of the actual cost of production of the other commercial prepolymer.
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