Curing Behaviors and Thermal Properties of Dimer Fatty Acid(DFA) Modified Multifunctional Epoxy Resin

YU Ming-ming(),SUN Jin-liang(),REN Mu-su(),LI Ai-jun()*,BAI Rui-cheng(),HU He-feng()

1 Research Center for Composite Materials,Shanghai University,Shanghai 200072,China

2 Department of Polymer Materials,Shanghai University,Shanghai 201800,China

Introduction

As one of the most important high performance thermosetting polymers,amine based multifunctional epoxy resins are widely used in many fields since they can rapidly cure,resulting in products with outstanding thermal properties[1-5].In recent years,many multifunctional epoxy resins have been reported[6-11].However,the pure cured resins trend to be too brittle to be applied due to their rigid backbone structure and high cross-linking density.Many toughening modifiers have been used to improve the toughness of these epoxy resins,including liquid elastomers,nano-particles,hyper-branched polymers,high performance thermoplastics,and thermo-tropic liquid crystal polymers[12-13].The modification by using elastomers is mainly applied in the epoxy resins with lower cross-linking density since the commonly used liquid elastomers (e.g.,carboxyl-terminated butadieneacrylonitrile copolymer(CTBN),amino-terminated butadieneacrylonitrile copolymer(ATBN))will influence the thermal property of the cured resin[14],and organosilicon elastomers (e.g.,polydimethylsiloxane) generally have poor compatibility with epoxies due to their difference in solubility parameter[15].It also has been proved that the thermal properties of the cured resin were influenced when they were toughened with hyper-branched polymers and nano-particles[16-17].High performance thermoplastic polymers are considered as the most appropriate toughening modifier of the multifunctional epoxy resins since they have flexible molecular chain with good thermal stability[18].Some high performance thermoplastics such as poly(ether imide) (PEI),poly(ether sulfone) (PES),and poly(ether ketone) (PEK) have been employed to modify multifunctional epoxy resins[19-23],the results showed the toughness of the cured resins improved with the increasing thermoplastic content and their thermal properties were not influenced.Some thermo-tropic liquid crystal polymers are also used as the toughness modifier,and the toughness could be greatly improved without decrease of thermal property when the epoxy was toughened with these compounds[24-27].But these methods are only used in some special area since all these thermo-tropic liquid crystal polymers and high performance thermoplastics are very expensive.In the present work,a commercial product with lower price,a dimer carboxylic acid named dimer fatty acid (DFA) was used as the toughening modifier of the amine based multifunctional epoxy since it contained both flexible aliphatic main chain and hexatomic rings with good thermal stability.On the other hand,the mobility of the molecular chain would also influence the curing reaction and the thermal properties of the cured resins.In the present work,a dimer acid,DFA,was used to modify a amine based tetrafunctional epoxy,N,N,N′,N′-tetraglycidyl-2,2-bis[4-(4-aminophenoxy)phenyl]propane (TGBAPP),then they were cured with methyl nadic anhydride (MNA).The influence of the modification on the curing behaviors were studied by differential scanning calorimetry (DSC) method and the thermal properties of the cured epoxy resins were investigated with the thermo-gravimetric analysis (TGA) and dynamic mechanical analysis (DMA).Besides,the toughness of the cured resins was characterized with the impact resistance tested by charpy impact testing.

1 Experimental

1.1 Materials

MNA was purchased from Aldrich.Dimer acid (DFA,PRIPOL 1013) 195 mg KOH/g was obtained from Croda,Holland.TGBAPP with epoxy equivalent weight (EEW) 172 g/mol was synthesized by ourselves.All chemical agents were used without further purification.

1.2 Characterization of TGBAPP

Fig.1 FTIR spectrum of TGBAPP

Fig.2 1H NMR spectra of TGBAPP

Table 1 The integral peak area of TGBAPP

1.3 Preparation of DFA-TGBAPP

DFA-TGBAPP was prepared as the formula shown in Table 2.The mixture of DFA and TGBAPP were added to a three-necked flask,and then they were stirred at 115℃ for 40 min.

Table 2 The formula of DFA-TGBAPP

1.4 Preparation of the cured epoxy resins

The modified epoxy was mixed with a stoichiometric amount of the curing agent (MNA) and heated to 65℃ under vacuum to remove air bubbles and moisture.Subsequently,the mixture was cured at 120℃ for 1 h,followed by curing at 180 ℃ for 2 h,and finally post cured at 200℃ for 4 h.

1.5 Characterization

FTIR spectrum was recorded on a Nicolet 380 infrared spectrometer in the range of 4 000-400 cm-1.

DSC was performed on a TA Q2000 (TA instruments,USA) with a constant nitrogen flow of 50 mL/min.About 5 mg of a sample (DFA-TGBAPP,TGBAPP/MNA,DFA-TGBAPP/MNA) was weighted and put into a hermetic aluminum sample pan at 25℃,which was then sealed,and the sample was tested immediately.The dynamic scanning experiment was ranged from 80 to 320 ℃ at a heating rate of 5 ℃ /min.

TGA was performed with a TA Q500 (TA instruments,USA) from 50 to 750℃ at a heating rate of 10 ℃/min in a nitrogen atmosphere,and the thermal stable residue was formed.About 10 mg of a sample (the cured epoxy resin),which had been cured and ground into a powder,was put into a ceramic cell and placed on a detector plate.

DMA was characterized with a TA Q800 (TA instruments,USA) in the air.The specimen of 60mm×10mm×3 mm was loaded in a three-point bending mode from 50 to 300 ℃ at a heating rate of 5 ℃ /min with a frequency of 1 Hz.

The charpy impact testing was determined with a ZBC 1400-2 pendulum impact testing machine from Shenzhen SANS Testing Machine Co.,Ltd.,China,according to GB/T 2567-2008.The specimens were 80×10×4 mm without notches.

2 Results and Discussion

2.1 Modification of TGBAPP with DFA

The modification of TGBAPP with DFA was through the reaction between the epoxy groups from TGBAPP and the carboxyl groups from DFA.The reaction process was characterized with a dynamic FTIR method and DSC method.Four samples were prepared,which were the system of TGBAPP mixing with DFA at room temperature (assuming unreacted,named 0 min),the system of TGBAPP reacting with DFA for 10 min at 115℃ (named 10 min),the system of TGBAPP reacting with DFA for 20 min at 115℃ (named 20 min) and the system of TGBAPP reacting with DFA for 40 min at 115℃ (named 40 min).

Fig.3 FTIR spectra of the modification process

It can be seen that the spectrum of carbonyl was shifted up with the reaction proceeding.It’s because the ester groups were being formed with the occurrence of the reaction between the carboxyl groups and the epoxy groups,and the spectrum of carbonyl from ester groups was higher than those from carboxyl groups due to their different electro-negativity.

Additionally,the thermal behaviors of the reactions were studied by DSC method,as shown in Fig.4.It can be seen that the exothermic reaction took place slowly and reached the maximum reaction rate at the peak temperature of about 165℃ which increased slightly with the reaction proceeding,and then the reaction slowed down because less unreacted material was available.Besides,the peak area of the DSC curves was used to evaluate the conversion,and the peak area of the sample of reacting for 0 min was assumed for unreacted and that of reacting for 40 min was assumed for reacted completely.So the conversion were 0%,13%,61%,and 100% for the samples of reacting for 0 min,reacting for 10 min,reacting for 20 min,and reacting for 40 min,respectively.The results indicated that the reaction was slowly in the first 10 min,and it would accelerate in the next 10 min.

Fig.4 DSC curves of DFA-TGBAPP systems at various reaction times,10℃/min

2.2 Curing behaviors of TGBAPP/MNA & DFA-TGBAPP/MNA systems

In order to investigate the influence of the modification on the curing reaction,the curing behaviors of TGBAPP/MNA and DFA-TGBAPP/MNA were studied with a non-isothermal DSC method,as shown in Fig.5.And the initial curing temperature (Ti),the peak curing temperature (Tp),the finishing temperature (Tf),and some typical data on these curing reactions were summarized in Table 3.The results indicated that the initial curing temperature (Ti) decreased with the increasing content of DFA,and the curing reaction enthalpy was increased when the epoxy was modified with DFA.In addition,the modification also led to the increase of the curing range and the curing duration.All of these can be attributed to two reasons.On one hand,the curing reaction based on epoxy/anhydride system was influenced with the content of hydroxyl groups,and these groups were increased with the increasing content of DFA when the epoxy was modified with DFA.On the other hand,the multifunctional epoxy/anhydride systems were too difficult to be cured fully due to their large steric hindrance.However,the modification could improve the post curing reactions since the aliphatic chain of DFA would improve the mobility of the cross-linking networks in the latter curing reaction stage.Therefore,the curing systems could be further cured when the epoxy was modified with DFA.

All the results showed that the curing reactions could be improved when the multifunctional epoxy was modified with DFA,and the curing temperatures were decreased with the increasing content of DFA.

Fig.5 DSC curves of TGBAPP and DFA-TGBAPP curing with MNA,5℃/min

Table 3 Characteristics of the epoxy/anhydride curing systems,5℃/min

2.3 Thermal stability of the cured epoxy resins

The thermal stability of the cured epoxy resins were studied with TGA.Figure 6 showed the TGA curves of the cured epoxy resins at a heating rate of 10 ℃/min,and the most important parameters were shown in Table 4.It can be seen that the cured resins exhibited a reasonable thermal stability up to 330 ℃,which due to a well-developed networks caused by tetrafunctionality.Moreover,the initial decomposition temperature and the yield char of the cured resins were not influenced obviously after the modification.

To further investigate the thermal stability of the cured

epoxy resins,the heat resistance index was calculated by a statistical method as Eq.(1)[28].

Tc=0.49[T1+0.6(T2-T1)]，

(1)

whereTcis the value of the heat resistance index,T1andT2are the 5% and 30% weight loss temperature,respectively.The results were listed in Table 4.

All the results indicated that the thermal stabilities of the cured aromatic tetrafunctional epoxy resins were not influenced obviously after the modification.

Table 4 The decomposition data of cured epoxy resins at 10 ℃/min

Fig.6 TGA curves of cured epoxy resins at a heating rate of 10 ℃/min

2.4 DMA analysis of cured epoxy resins

More detailed information was obtained from the measurements of the dynamic mechanical behavior of the cured epoxy resin as a function of temperature as shown in Fig.7.As quantified mechanical performance at elevated temperatures,storage modulus (E′) was seemed as an important indicator of the performance of the cured resins.In addition,as the indicator of cross-linking density,the glass transition temperature (Tg) of TGBAPP/MNA,2% DFA-TGBAPP/MNA,10% DFA-TGBAPP/MNA,and 20% DFA-TGBAPP/MNA were 231℃,234℃,226℃ and 202℃,respectively,determined by the peak temperature of tanδcurves.The results indicated that the cross-linking density of the cured aromatic tetrafunctional epoxy resins could be slightly improved after the modification and the excess content of DFA would decrease the cross-linking density of the cured resins.It’s because the well developed cross-linking networks of the aromatic tetrafunctional epoxy resins were formed with the post curing reactions which were strongly influenced with the large steric hindrance and the

(a)

(b)

(c)

(d)

rigid molecular chain,and the reaction could be improved after the modification since the aliphatic chain from DFA would improve the mobility of the networks.Meanwhile,the excess aliphatic chain would decrease the cross-linking density of the cured resins.Therefore,the thermal properties of the cured resins were not influenced after the modification with appropriate content of DFA.

2.5 Impact strength of cured epoxy resins

As an important indicator of the toughness of the thermosetting polymers,impact strength of cured epoxy resins was tested by charpy impact testing and the results were listed in Table 5.It can be seen that the impact strength of the cured epoxy resins was significantly improved after the modification,and the value of 20% DFA-TGBAPP/MNA was over 55% higher than that of TGBAPP/MNA.It is obvious that the toughness of the cured aromatic tetrafunctional epoxy resins could be improved after the modification.Moreover,the toughness of the modified resins improved with the increasing content of DFA.

3 Conclusions

An amine based aromatic tetrafunctional epoxy,TGBAPP,was modified with DFA.The modification reaction was characterized with FTIR and the thermal behaviors of the reaction were studied by DSC method.The results indicated that the spectrum of carbonyl was shifted with the reaction proceeding,and the reaction happened after 40 min at 115 ℃.And then the modified epoxies were cured with MNA.The curing behaviors were studied by a non-isothermal DSC method,which suggested that the modification would improve the curing reactions,and the curing temperatures were decreased with the increasing content of DFA.Additionally,the TGA and DMA results indicated that the thermal properties of the cured resins were not influenced obviously when the epoxy was modified with appropriate content of DFA.Besides,the toughness of the cured resins could be improved after the modification and increased with the increasing content of DFA.

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