The addition effects of hybrid copolymer between an urethane prepolymer and solid epoxy resin

329 Journal of Chemistry, Vol. 45 (6A), P. 329 - 333, 2007 THE ADDITION EFFECTS OF HYBRID COPOLYMER BETWEEN AN URETHANE PREPOLYMER AND SOLID EPOXY RESIN Received 15 October 2007 Do Truong Thien, Duong Anh Vu, Nguyen Tien An, Tran Thi Y Nhi Natural polymer laboratory - Institute of Chemistry - VAST summary Epoxy resin networks have been modified with hybrid copolymer consisting of an urethane prepolymer and a solid bisphenol A diglycidyl ether. Hybrid copolymer prepared from isocyanate-terminated urethane (NCOPUER) resulted in cured transparent epoxy networks with no discernible phase-separated morphology, as indicated by scanning electron microscopy. The different morphological characteristics observed in this modified system were attributed to the different structure of the block copolymer. The visual homogeneity of the NCOPUER block copolymer-modified network can be attributed, since the glass transition temperature of the epoxy matrix (determined from differential scaning calorimetry analysis) has not been substantially influenced by the presence of this block copolymer. The epoxy resin modified with the different block copolymer presented an improved toughness and chemical resistance. The best mechanical performance in terms of flexural and tensile properties was achieved with the block copolymer derived from isocyanate -terminated polyurethane, whereas a more flexible material has been obtained with NCOPUER hybrid copolymer-modified network. Keywords: Bisphenol A, diglycidyl ether epoxy resin, urethane, impact modifier. I - INTRODUCTION Epoxy resin is one of the most important thermosetting polymers. This resin exhibits many desirable properties, such as high thermal stability and excellent electric properties; however epoxy resin is generally rather brittle. The brittleness of material is not only depending on the chemical structure ratio of aromatic/aliphatic substance but also the crosslink. Polyurethane is another class of favored polymeric modifier for material applications owning to its superior resistance to UV light, abrasion resistance. Many efforts have been made to improve the brittleness and chemical resistance of the cured epoxy resin including modification of this resin with polyurethane. The effect of blending polyurethane with thermosetting epoxy resin improve the compatibitization, corrosion resistance and chemical properties compared to unmodified resins. In this article we reports on the addition effects on hybrid copolymer between an urethane prepolymer and solid epoxy resin. II - EXPERIMETAL 1. Procedure The epoxy resin used in this work was a commercial grade of glycidyl ether of bisphenol A (DER 662E), with an epoxy equivalent weight of 590 - 630 g/eq. The soften point was 87- 93oC, n = 2.4. 330 CH2 CH O CH2 O C CH3 CH3 O CH2 CH CH2 OH O C CH3 CH3 O n CH2 CH CH2 O The curing agent was DEH 84, a phenolic hardener based on an unmodified solid reaction product consisting of liquid epoxy resin and bisphenol A, containing about 2% of curing accelerator (2-metyl imidazol), its hydroxyl equivalent weight corresponding to 240 - 270 g/eq. Both the epoxy resin and the curing agent were produced by Dow Chemical Co., Ltd. All polymers were dried under vacuum in 10 hours at room temperature before using. The urethane prepolymer in this work was polyether with an excess amount of an isocyanate group, supplied by Erapol Co.,Ltd (Australia) containing two or more molecular terminals composed of free isocynate groups. The NCO number herein refers to an average number per molecular. It is preferred that NCO number per molecule of the urethane prepolymer is 4.2. ET90A was produced by reaction between a polyol compound and an excess amount of an isocyanate compound. CH3 NCO CH3 NCO NHCOO (CH2)3 O OCHN_ _ _ n Po ly u reth an e p rep o ly mer The urethane prepolymer was pre-reacted with a large excess of epoxy oligomer (20 g urethane prepolymer per 100g epoxy oligomer) at 110oC. The mixture was gently stirred for about 150 minutes to ensure proper dispersion of the prepolymer. Then the block copolymer was cooled immediately to room temperature . Then the amount of pre-heated curing agent was added to the block copolymer at 110oC for 5 minutes. The mixture was then poured in metal mould which was pre-heated at 180°C. The material were cured at 180oC for 20 minutes. 2. Measurement The morphology of fracture surfaces was observed by a scanning electron microscope (SEM- JSM 5300) at an accelerating voltage of 15 KV. The samples were handing fractured and the surface was dried under vaccum before analyzing. Fourier transform infrared (FTIR) analysis was performed on a Nicolas 2100 spectrormeter from 4000 cm-1 to 400 cm-1 in KBr pellet. Different scanning calorimetry (DSC) was performed using a Netzchsta 409 PC/PG equipment in dynamic mode at 10°C/min under nitrogen. The 1H-NMR spectra were recorded on a 500MHz Bruker Avance in CDCl3 as solvent at ambient temperature. III - RESULTS AND DISCUSSION 1. Morphology of urethane modified epoxy system Fig. 1 shows the morphology of the urethane (ET90A) modified with epoxy oligomer (DER662). When block copolymer was pre- reacted with DER662 whose molecular weight was increased, the finest urethane phase could be uniformly and stably dispersed in the epoxy matrix. This results are consistent with that reported by M.Ochi and K.Takemiya who showed that the change in copolymer architecture affects the efficiency of a compatibilizer. Fig.2 showed the changes in morphology of urethane modified epoxy system with the addition of urethane prepolymer formed blockcopolymer. In the system without added block copolymer (Fig.1b;2b), it is clearly observed smooth surface. In this work, urethane oligomer could be dispersed in the epoxy matrix, because the compatibility between the epoxy resin and the urethane prepolymer was improved by the pre-reaction described in the previous section. 331 10µm 10 µm 100µm (a) 100µm (b) 2. Formation of epoxy modified urethane prepolymer In the first step, the isocyanate group of urethane prepolymer reacts with secondary hydroxyl group of the epoxy resin. The disappearance of isocyanate peak at 2270 cm-1 and formation of C=O and N-H peak of urethane group at 1680 cm-1 and 1581 - 1518 cm-1 respectively were used to ascertain the completion of the reaction.The reaction between NCO groups in urethane prepolymer and OH groups in epoxy resin is confirmed by the absence of NCO peak at 2270 cm-1 and presence of amide group of urethane at 1685 - 1640 cm-1 (Fig. 3). Fig. 1: SEM of fracture of uncured epoxy resin modified with urethane prepolymer total 20 %wt No block copolymer DER662E (a); Block copolymer EPU-2XA (b) Fig. 2: SEM of fracture of cured epoxy resin modified with urethane prepolymer total 20 wt% (a) No block copolymer DER662E; (b) Block copolymer EPU-2XA The EPUs were characterized by conventional spectroscopic method. FTIR spectra of EPUs showed characteristic bands of urethane groups at 3337 - 3370 cm-1 (N-H stretching), 1717 - 1730 cm-1 (NHCOO stretching), 1505 - 1510 cm-1 (C-N stretching, combined with N-H out of plan bending). The peaks of epoxy group were appeared at 851 - 965 cm-1. Fig. 3d showed FTIR characteristic peaks of urethane carbonyl as well as etheric or esteric of hydroxyl groups. (a) (b) 10 µm 10 µm 332 Fig. 3: The FTIR spectra of urethane prepolymer (a), epoxy resin (b), mixture of urethane prepolymer and epoxy with time reaction for 5 min (c), of urethane prepolymer and epoxy with time reaction for 150 min (d) 3. 1H-NMR analysis The 1H-NMR method for determining branch point relies on rapid reaction between isocyanate and hydroxyl groups incorporated in the epoxy resin. After the reaction, the absorption of carbinol methane proton moves downfield from about 4.2 to 5.5 ppm. In such a way is possible to estimate the number of branch points in molecules. The 1H-NMR spectra are recorded twice before and after the reaction of hydroxyl with isocyanate of urethanane prepolymer. These spectra are shown in Fig. 4. Fig. 4: The 1H-NMR spectra of (a)EPU-2XA, (b) DER662 1H-NMR spectra of EPU-2XA and DER662 were also in accordance with the proposed structure. Peaks due to oxirane groups were observed at about 2.61 - 3.17 ppm. NH groups were presented at about 7.3 ppm, CH2 groups attached to urethane oxygen atoms were presented at about 3.59 - 4.1 ppm and CH2 groups attached to urethane nitrogen atoms were presented at about 2.71 - 2.92 ppm. Metylen groups attached to epoxy resin and urethane oxygen groups were observed at about 4.3 - 4.5 ppm. The CH groups linked with OH groups of epoxy shifted from 3.8 ÷4.4 ppm to 5.4 ppm indicating the formation of urethane linkage. (a) (b) (c) (d) (b) (a) 333 From the results it is obviously that the NCO groups reacted with the hydroxyl groups of epoxy resin to form urethane crosslink, epoxy groups did not take part in the reaction. 4 Differential scanning calorimetry Error! 50 100 150 200 250 300 350 400 450 500 0 0.5 1.0 1.5 2.0 DSC /(mW/mg) Onset: Mid: Inflection: End: Delta Cp*: 72.5233 84.2153 84.6793 95.9073 0.077 J/(g*K) 386.772 Onset: Mid: Inflection: End: Delta Cp*: 129.199 143.702 138.269 158.204 0.894 J/(g*K) Onset: Mid: Inflection: End: Delta Cp*: 126.07 137.248 128.318 148.425 0.680 J/(g*K) [1] [2] [3]  exo [#] Instrument [1] NETZSCH STA 409 PC/PG [2] NETZSCH STA 409 PC/PG [3] NETZSCH STA 409 PC/PG File DER 662.dsv EPU 2XC.dsv ER 2C.dsv Identity 34 2007 34 2007 34 2007 Sample DER 662 EPU 2XC ER 2C Date 9/11/2007 2:45:09 PM 9/11/2007 9:58:59 AM 9/11/2007 4:58:59 PM Mass 13.400 mg 6.800 mg 7.100 mg Segment 1/1 1/1 1/1 Range 30/10.00(K/min)/500 30/10.00(K/min)/500 30/10.00(K/min)/500 Atmosphere O2/0 / N2/30 O2/0 / N2/30 O2/0 / N2/30 Correction DSC:020/TG:020 DSC:020/TG:020 DSC:020/TG:020 Administrator 13-09-2007 14:55 Fig 5: Variation of Tg for samples. (1) uncured epoxy (DER662) , (2) EPU-2XA, (3) cured epoxy (ER-2C) Form of NCO between urethane prepolymer and epoxy resin could be demonstrated by the presence of a broad melting endotherm between 70°C-160°C. The Tg values of samples with 0 and 20% of urethane prepolymer epoxy (cured with DEH84) are 128°C and 139°C, respectively. The low temperature transition is related to the glass transition (Tg). The high temperature transition is attributed to the melting of crystalline phase structure of soft segment (Tg). It could be concluded that the EPU-2XA and ER-2C are semi-crystalline compounds. The Tg of EPU-2XA is higher than ER2C. The investigation of Fig.5 showed that for sample EPU-2XA could be formed hybrid copolymer crosslink. IV - CONCLUSION Epoxy terminated urethane resin based on epoxy resin and urethane prepolymer were combined to form mixtures and cured with equivalent amount of DEH84 curing agent. In the system with the added hybrid copolymer that has a good balance between the affinity for epoxy matrix and urethane prepolymer, it was clearly shown by the SEM observations that the urethane prepolymer is well dispersed in to epoxy matrix. A disappered peak near 2270 cm-1 was observed by FTIR for EPU-2XA. This vibration is assigned to NCO groups added OH groups of epoxy resin. Upon reaction of these NCO groups with OH groups, two vibration at lower wavernumber shifts appeared at 1505 cm-1 and 1730 cm-1. The hybrid copolymer between urethane prepolymer and epoxy resin were developed and thermal characteristics studied. The incorporation of urethane moiety further increases the Tg value of hybrid copolymer material. REFERENCES 1. L. B. Fabio, P. A. Thiago. Polymer, 44, 5811 - 5819 (2003). 2. M. Ochi, K. Takemiya. Polymer, 41, 195 - 201 (2000). 3. AAnand Prabu. Progress in Organic coatings, 49, 236 - 243 (2004). 4. Dariusz. B. Jaroslaw, G. Polymer, 44, 7795 - 7780 (2003). 5. Y. Hamid, M. L. Moslem. European Polymer Journal, 41, 2370 - 2379 (2005). 334 NH HNG C A PH N NG CNG HP COPOLYME LAI TO GIA PREPOLYME URETAN VI EPOXY RN