J. Cent. South Univ. Technol. (2007)06-0753-06
DOI: 10.1007/s11771-007-0143-8
Structural characteristics and properties of
PU-modified TDE-85/MeTHPA epoxy resin
LI Zhi-hua(李芝华), KE Yu-peng(柯于鹏), REN Dong-yan(任冬燕), ZHENG Zi-qiao(郑子樵)
(School of Materials Science and Engineering, Central South University, Changsha 410083, China )
Abstract: Diglycidyl-4,5-epoxycyclohexane-1,2-dicarboxylate (TDE-85)/methyl tetrahydro-phthalic anhydride (MeTHPA) epoxy resin was modified by polyurethane(PU), and its structural characteristics and properties were studied by infrared spectrum analysis (IR), scanning electronic microscopy (SEM), mechanics testing and thermogravimetric analysis (TG). The results indicate that epoxy polymeric network I and polyurethane polymeric network II are formed in the PU-modified TDE-85/MeTHPA epoxy resin. Meanwhile the PU-modified TDE-85/MeTHPA resins have heterogeneous structure. The miscibility between epoxy (EP) and polyurethane (PU) as well as the phase size are dominantly determined by the mass fraction of polyurethane prepolymer (PUP) in the EP/PU blends. With the increase of PUP mass fraction, the tensile strength, impact strength and thermal stability of the PU-modified TDE-85/MeTHPA epoxy resin all firstly exhibit increasing tendency, and decrease after successively reaching their maxima. When the number-average molecular mass of PPG is 1 000 and the mass fraction of PUP is 15%, the tensile strength, impact strength and thermal stability of materials obtained, compared with TDE-85/MeTHPA epoxy resin, are improved obviously.
Key words: TDE-85; polyurethane; interpenetrating polymeric network; mechanical property; thermal stability
1 Introduction
The eye-catching diglycidyl-4, 5-epoxy- cyclo- hexane-1, 2-dicarboxylate (TDE-85) epoxy resin has many advantages such as small viscosity, high activity, good thermal stability and high strength, which make it have superiority over other materials to be used in the fields as thermal resistant coatings, composites and adhesives[1-2]. However, cracking at low temperature and bad impacting resistance have inhibited it from application for higher materials requirement. Improving the toughness and low temperature work property of TDE-85 will be the key direction of future researches, which certainly has a brightly developing foreground. General toughness-increased modifications to epoxy resin (EP) are at the cost of sacrificing its stiffness and thermal stability. Interpenetrating polymeric networks (IPNs) technique, since its advantage, has been attracted wide recognition for their synergistic effect that results in superior tensile strength, impact strength and thermal stability than either of the separated component[3-6]. EP modified by PU can improve both strength and toughness because of the formation of IPNs, which possess two separated but interlocked networks under proper experimental conditions[7-10]. Therefore, the studying on the IPNs of PU-modified TDE-85/MeTHPA resin, along with their designing, is the development direction and significant means of TDE-85 modification. Initiating study on PU-modified TDE-85/MeTHPA resin could obtain theoretical supports for the design of IPNs in the resin system, which surely has especial significance. Up to the present, though significant progresses of the design, characterization, and application of the materials based on TDE-85 have been made[11-13], the literature covering on TDE-85/MeTHPA modified by PU has been rarely reported. Therefore a series of PU-modified TDE-85/MeTHPA resins were prepared from polypropylene glycol (PPG) with the number-average molecular mass of 1 000. And the structural characteristics and properties of the modified-systems and their modification laws were discussed.
2 Experimental
2.1 Materials
Diglycidyl-4,5-epoxycyclohexane-1,2-dicarboxylate (TDE-85) was purchased from Tianjin Jindong Chemical Plant (Tianjin, China). Methyl tetrahydrophthalic anhydride (MeTHPA) was from Wenzhou Qingming Chemical Plant (Wenzhou, China). 2,4-toluene diisocyanate (TDI) was provided by Shanghai Shiyi Chemical Agent Co., Ltd. (Shanghai, China). Polypropylene glycol (PPG) with number-average molecular mass (Mn) of 1 000 was purchased from Tianjin No.3 Petrochemical Factory (Tianjin, China). 1,4-buranediol (1,4-BDO) and trimethylolpropane (TMP) were supplied by China Medication Shanghai Chemical Agent Co., Ltd. (Shanghai, China). 2,4,6-tris (dimethyl-aminomethyl) phenol（DMP30）was obtained from Changsha Chemical Research Institute (Changsha, China).
2.2 Experimental process
2.2.1 Synthesis of isocyanate-terminated PUP
PPG of proper quantity was filled into a clean and dried four-neck flask, and stirred at 120 ℃ for 1 h to dehydrate. Then it was cooled down to around 50 ℃ and stoichiometric quantity of TDI was added. The mixture under stirring was gradually heated to about 80 ℃ and maintained until the reaction was completed. Then isocyanate-terminated PUP was obtained.
2.2.2 Cast under vacuum
Moulds coated with antisticking agent were kept in the furnace at 80 ℃ for further use. TDE-85 and MeTHPA were separately dried under vacuum of 0.009 MPa at 80 ℃ for 2 h, then cooled to 60 ℃. TDE-85 and isocyanate-terminated PUP obtained, 1,4-BOD as chain-extended agent, TMP as cross-linked agent, MeTHPA as curing agent and DMP-30 as curing accelerant according to proper proportion were added into the vacuum casting machine. The mixture was stirred and heated for a certain time, then casted into the moulds. The resin cured after a series of processes as procedure heating solidification (120 ℃/2 h + 140 ℃/2 h + 160 ℃/2 h).
2.3 Characterization
The curing reaction was surveyed by Nicolet AVATAR360 type FT-IR spectrometer. Cured samples were analyzed using the KBr pellet technique and liquid samples were directly analyzed or after diluted by CCl4.
Scanning electronic microscopy (SEM) was done on a FEI Sirion200 type field emission instrument to detect the micromorphology. Fracture surface of fresh-broken sample was firstly etched by dimethylformamide, then took metal spraying. Then the sample was fixed on a metal substrate by conductive adhesive tape before examination.
Tensile strength was recorded on a computer- controlled electronic universal machine (SANA- CMT5105) according to GB 1040-79, and impact strength was performed by JB-5 type impact testing machine of Wuzhong Material Testing Machine Company according to GB 1043-79 (unnotched samples), respectively. After impact strength test fracture surface morphology was detected using SEM (KYKY-2800).
Thermal stability of EP was characterized by thermogravimetric analysis on an instrument (SDT Q600, USA). TG curves were recorded under the following operational conditions: temperatures range from 30 to 600 ℃, heating rate 10 ℃/min and in air.
3 Results and discussion
3.1 Infrared spectra
Infrared spectra of TDE-85, PUP and TDE- 85/MeTHPA resin samples are shown in Fig.1. The —NCO group characteristic absorption peak of PUP at about 2 270 cm-1 disappears in the infrared spectrum of PU- modified TDE-85/MeTHPA samples, so does the epoxy group of TDE-85 at 908 cm-1, which confirm that isocyanate groups of PUP and epoxy groups in TDE-85 resin have reacted completely.
The systematic study in Refs.[14-15] indicated that during the curing process of PU-modified TDE-85/MeTHPA epoxy resin (PU-modified EP), epoxy polymeric network I is formed through reaction between TDE-85 and MeTHPA, and PU polymeric network II is generated by the chain-extended and cross-linked reactions among 1,4-BDO, TMP and PU prepolymer.
Fig.1 IR spectra of TDE-85, PUP and PU-modified TDE-85/MeTHPA resins
3.2 Micro-morphology
Miscibility of the polymers and characteristics of morphology were surveyed by field emission SEM. Through SEM surface observation diphase distribution and interpenetrating extent between PU and EP were detected. PUP was synthesized from PPG (Mn=1 000) and 2, 4-TDI. Fig.2 shows SEM images of TDE-85/ MeTHPA epoxy resins modified with 0, 5%, 10%, 15%, 20%, 25%(mass fraction) of PUP, respectively.
As shown in Fig.2, TDE-85/MeTHPA resin has a unilateral continuity morphology(Fig.2(a)), while PU-modified TDE-85/MeTHPA epoxy resins have diphase structure and exhibit obvious differences in conformity with various PUP mass fractions. Figs.2(b) and (c) illustrate that when the mass fraction of PUP is low, PU phase is uniformly dispersed in the continuous
Fig.2 SEM images of samples with different mass fractions of PUP
Mass fraction of PUP: (a) 0; (b) 5%; (c) 10%; (d) 15%; (e) 20%; (f) 25%
EP phase, appearing distinct “island” structure and phase interface. When mass fraction of PUP is 15%, as shown in Fig.2(d), there are almost no phase interfaces and phase separation is fairly obscure, suggesting that interpenetrating networks have been formed at the point of phase interface with high level of interpenetration and mutual tangling. When the mass fraction of PUP reaches 20%, as can be seen in Fig.2(e), phase separation degree and phase size increase slightly. When mass fraction of PUP reaches 25%, as shown in Fig.2(f), with the increment of mass fraction of PUP and its lager space continuity, the phase interface becomes clearer and phase size increases conspicuously, which means that the diphase miscibility decreases and phase separation evidently occurs.
The analysis above demonstrates that PU-modified TDE-85/MeTHPA resin has a diphase structure, and mass fraction of PUP is the main factor that influences the miscibility and phase size of PU/EP system. There are obvious differences in the micro-morphology of PU-modified TDE-85/MeTHPA resin when various mass fractions of PU are introduced into in system. With a proper mass fraction of PUP addition (accounting for gross mass of the system), there are high level of interpenetration and mutual tangling between PU and EP at the point of phase inversion in material obtained, which shows characteristic of EP/PU IPNs.
3.3 Mechanical properties
Variation tendency in the mechanical properties of TDE-85/MeTHPA epoxy resin modified by PU is presented in Fig.3 according to different PUP additions. As can be seen in Fig.3, when the mass fraction of PUP arrives at 15%, the tensile strength of material obtained is 69.39 MPa, exhibiting 48.0% increase, and impact strength is 23.56 kJ/m2 with 115％ increase. When the PUP addition is small, mechanical properties of PU-modified TDE-85/MeTHPA resin show an uprising tendency accompanied with increase of PUP addition. Tensile strength and impact strength reach their climaxes successively. Thereafter both of them decrease with further increasing of mass fraction of PUP.
Fig.3 Effect of mass fraction of PUP in EP/PU on tensile strength and impact strength of samples
Properties of materials are determined by their structures. For EP system, deformation occurs in EP etwork when stressed, without stress transfer between different networks. But for PU-modified EP it is in another case because of the formation of IPNs between PU and EP. Thereupon the stress can be detracted, and the external force and more external work are needed to damage the IPN structure, which exhibits the rising tendency of mechanical properties i.e. tensile strength and impact strength.
3.4 Fracture surface morphology analysis
Fig.4 shows the SEM fractographs for samples after impact tests. As can be seen in Fig.4, TDE-85/MeTHPA epoxy resin denoted as sample (a) has a basically plane, smooth and level fracture surface. The cracks linearly extend in the same direction, indicating little stress scattering and bad dynamic ductility. Samples from (b) to (f) are PU-modified TDE-85/MeTHPA resins. Their fracture surfaces are quite ragged, and breaches are relatively rounding off, and cracks are no more in the same direction but scattering, so it can be inferred that impact property has been improved.
Fig.4 SEM images of fracture surfaces of samples with different mass fractions of PUP
Mass fraction of PUP: (a) 0; (b) 5%; (c) 10%; (d) 15%; (e) 20%; (f) 25%
When mass fraction of PUP is small, as shown in Figs.4(b) and (c), the cracks change from straight lines to curves diverging to a series of directions, and the length of cracks increases accordingly. Meanwhile fracture surfaces in the two images become well-bedded, which also suggests improved toughness. With further addition of PUP, as shown in Figs.4(d) and (e), fracture surfaces become rougher and the cracks arrange irregularly, exhibiting paralleled and crossed. When the resin system is impacted, this kind of structure guarantees that more energy can be effectively absorbed, more stress can be detracted. Thus impact property is further upgraded. Compared with samples (d) and (e), uniformity of crack distribution in Fig.4(f) declines, so do the number of small cracks and roughness of the fracture surface. The effect of modification by PU on TDE-85 becomes worse.
From the analysis above, it is inferred that with the addition of PUP, the fracture surfaces of PU-modified TDE-85/MeTHPA resin change to be rough from smooth and the cracks turn to be complex curves from approximate straight lines, which show the characteristics of ductile rupture.
3.5 Thermal stability
TG curves of samples are shown in Fig.5. As can be seen in Fig.5, sample with a 15% mass fraction of PUP has the best thermal stability. The temperature of 1% mass loss of sample with 15% PUP is 300 ℃, which is 30 ℃ and 130 ℃ higher than that of sample without PUP and sample with 25% PUP, respectively. Half life temperature with 50% mass loss for sample with 15% PUP is 378 ℃, 367 ℃ for sample without PUP and 363 ℃ for sample with 25% PUP. So sample with 25% PUP has the worst thermal stability.
Fig.5 TG curves of samples with different mass fractions of PUP
(1)－0; (2)－15%; (3)－25%
The results above have intimate relations with the structure of the three samples. There are only epoxy networks in TDE-85/MeTHPA resin system, in contrast there are IPNs consisting of mutually interpenetrated PU and EP networks in PU-modified TDE-85/MeTHPA resin system. As analyzed previously, when mass fraction of PUP reaches 15%, there is good miscibility between PU and EP, and PU networks uniformly disperse in EP networks. Permanent diphase tangling spots are about to saturating, and cross-linking density approximately reaches its climax, therefore more energy is needed to damage the EP/PU INPs structure than to damage EP network. In this case, it is shown that sample with 15% PUP has a higher temperatures when the mass loss is 1% and 50%. But more addition of PUP results in the decrease of concentration of permanent tangling spots, phase separation becomes more obvious, and the miscibility between PU and EP begins to turn worse. Therefore thermal stability of sample with 25% PUP decreases dramatically compared with sample with 15% PUP, even worse than TDE-85/MeTHPA system.
4 Conclusions
1）PU networks are formed through the reactions among －NCO group terminated PUP, dibasic alcohol as chain-extending agent, and ternary alcohol as cross-linking agent. EP networks are produced by the curing process between TDE-85 and MeTHPA. The PU-modified TDE-85/MeTHPA resin designed has a diphase structure, and PU content is the main factor that influences the miscibility and phase size of PU/EP system. There are obvious differences in the micro-morphology of PU-modified TDE-85/MeTHPA resin when various mass fractions of PUP are used in system.
2) With the increasing of PUP addition, the tensile strength, impact strength and thermal stability of PU-modified TDE-85/MeTHPA resin system show rising tendency at the beginning, then successively reach their climaxes and thereafter decrease, respectively.
3) When a proper mass fraction of PUP, e.g. 15%, is introduced into the modification of TDE-85/MeTHPA resin, the PU-modified TDE-85/MeTHPA epoxy resin with IPNs can be obtained. Its tensile strength, impact strength, and thermal stability all exhibit remarkable improvement.
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(Edited by YANG Hua)
Received date: 2007-03-20; Accepted date: 2007-05-25
Corresponding author: LI Zhi-hua, PhD; Tel: +86-731-8830838; E-mail: ligfz@mail.csu.edu.cn
