Modification and Compatibility of Epoxy Resin With H y d r oxyl -Te r mi n a t ed or Ami ne-Te r minat ed

Polyurethanes

HUEI-HSIUNG WANG and JUNG-CHIEH CHEN

Graduate School of Textile Engineering Feng Chia University

Taichung, Taiwan, Republic of China

Phenolic hydroxyl-terminated (HTPU) and aromatic amine-terminated (ATPU) PU modifiers were prepared by reacting two different macroglycols (PTMG, poly- tetramethylene glycol, M,, = 2000, and PBA, polybutylene adipate, M, = 2000) with 4,4’-diphenylmethane diisocyanate (MDI), then further coupling with two different coupling agents, bisphenol A or 4.4’-diaminodiphenyl sulfone (DDS). These four types of PU prepolymers were used to modify the epoxy resin with 4.4‘-diamino diphenyl sulfone as a curing agent. From the experimental results, it was shown that the values of fracture energy, GI,, for PU-modified epoxy were dependent on the macroglycols and the coupling agents. Scanning electron microscopy (SEMI revealed that the ether type (FTMG) of PU-modified epoxy showed the presence of an aggregated separated phase, which vaned between 0.5 pm and 4 pm in the ATPU (FTMG) and between 1 pm and 1.5 pm in HTPU (FTMG) modified system. On the contrary, the ester type (PBA) PU-modified epoxy resin showed a homoge- neous morphology and consequently a much smaller effect on toughening for its good compatibility with the epoxy network. In addition, it was found that the hydroxyl-terminated bisphenol A as a coupling agent improved fracture toughness more than the amine-terminated DDS because of effective molecular weight buildup by a chain extension reaction. The glass transition temperature (T,) of modified epoxy resin as measured by dynamic mechanical analysis (DMA) was lower in PTMG-based PU than in a PBA-based PU series with the same weight of modifier.

INTRODUCTION there are intrinsically strong chemical bonds across

everal million pounds per year of epoxy polymers S are produced in the world. These are widely em- ployed as the resin for adhesive coatings or the ma- trix for glass-, polyamide-, or carbon-fiber compos- ites. They are amorphous, highly crosslinked poly- mers. This results in high strength properties, and chemical and solvent resistance, but leads to low toughness and poor crack resistance.

Various ways to enhance the fracture toughness of epoxy network have been investigated. The common widely used methods are addition of a reactive liquid

the rubbery phase/rksin matrix interface. In the present study, we attempted to toughen the

epoxy network using polyurethane (PU) as the modi- fier. The problem of chemically linking the modifier to the epoxy network was overcome by using bisphenol A (Bis-A) or 4.4‘-diaminodiphenyl sulfone (DDS) as a coupling agent between the PU and the epoxy oligomer. In addition, the effects of different diol types and their concentration in polyurethane on the me- chanical properties and the morphology of the modi- fied network will also be discussed.

rubber (ATBN or CTBN) to the epoxy resin (1, 2). Block copolymerization of epoxy with different EXPERIMENTAL. oligomers containing vinyl, amine, or hydroxyl termi- nal groups has been also investigated (3-6). The pri- mary factor of a rubber-modified matrix was a chemi- Polytetramethylene glycol (FTMG 2000) and poly- cal requirement since the rubber needs to react with hutylene adipate (PBA 2000) were supplied by Ever- the resin, not only to increase its molecular mass and more Chemical Industry Co. The compounds 4.4’-di- so lead to phase separation, but also to ensure that phenylmethane diisocyanate (MDI), hisphenol-A

Materials

1468 POLYMER ENGINEERING AND SCIENCE, SEPTEMBER 1995, Vol. 35, No. 78

Modfication and Compatibility of Epoxy Resin

(Bis-A) and 4.4’-diaminodiphenyl sulfone (DDS) were purchased from the Merck Co. All materials were used as received.

The bisphenol-A diglycidyl ether type epoxy resin (Epon 826) was obtained from the Shell Chemical Corp. (equivalent w t = 190 g/mol). The hardener di- aminodiphenyl sulfone (HT976) was obtained from the Ciba-Geigy Corp.

Preparation of Phenolic Hydroxyl-Terminated (HTPU) or Aromatic Axnine-Terminated (ATPU) PU Prepolymer

The NCO-terminated PU with various macroglycol diols (PTMG 2000, PBA 2000) was prepared in dimethyl formamide (DMF). Two equivalents of 4,4’- diphenylmethane diisocyanate were dissolved in dimethylformamide and heated to 70°C. Then, one equivalent of macroglycol was slowly added to the solution with vigorous stirring. The reaction was car- ried out under nitrogen at 70-75°C until the theoreti- cal isocyanate content (determined by the di-n- butyl-amine titration method) was reached.

Two equivalents of the coupling agent (Bis-A or DDS) were dissolved in dimethyl formamide, and the solution of one equivalent of PU oligomer was added slowly at 80°C with good stirring for 180 min. The reaction products were dried for 48 h in a vacuum oven at 80°C. The synthesis scheme was shown as follows:

826 resin at 140°C (the amine-terminated PU at 120T) and prereacted for 30 min in a mechanical stirrer. The stoichiometric quantity of the hardener DDS was then added, and the system was main- tained at 140°C until a homogeneous clear solution was obtained. The resin was degassed at 140°C for 15 min, poured into a 180°C preheated polytetrafluo- roethylene mold, cured at 140°C for 2 h followed by 2 h at 200°C in an oven. The cured resin was slowly cooled to room temperature.

PU Prepolymer Characterization

Thermal analysis was performed with a DuPont 9 10 differential scanning calorimeter (DSC) at a heat- ing rate of 5”C/min from - 120°C to 220°C. Nitrogen gas was used to purge the sample chamber of the DSC. Infrared spectra of PU prepolymers were ob- tained with a Hitachi Model 26050, over a range of 250 cm-’ to 4000 cm-’.

Analysis of Curing Reaction Kinetics

A Rheovibron DDV-I1 was used to measure directly the time required for macroscopic gelation during isothermal cure at different temperatures at a fre- quency of 3.5 Hz. A heat-cleaned glass fiber was coated with the solution of the reaction mixture and mounted in the specimen chamber at room tempera- ture. The temperature was raised at 15”C/min to the

step 1. cure temperature and held at that temperature until the peak of tan 6 was clearly observed.

OCN-(MD1)-NCO + HO-(macroglycol)-OH - OCN-(MD1)- NHCO- (macroglycol)- OCHN =(MDI)- NCO

II 0

II 0

PU oligomer step 2.

OCN-[PU oligomerl-NCO + 2 HO- 0 -C(CH3),- 0 -0H- 0 0 CH 0 0 CH3

Hydroxyl-terminated PU prepolymer or

OCN-[PU oligomerl-NCO + 2 H,N- 0 -SO,- 0 -NH,- 0 0 0 -SO,- 0 -NH,

0 0 0 0 Amine-terminated PU prepolymer

Modification and Curing Procedures Using the Phenolic Hydroxyl- (or Aromatic Amine)- Terminated PU Prepolymer

The phenolic hydroxyl-terminated or aromatic amine-terminated PU prepolymer was added to Epon

POLYMER ENGINEERING AND SCIENCE, SEPTEMBER 1995, Vol. 35, No. 18 1469

Huei-Hsiung Wang and Jung-Chieh Chen

Cured Epoxy Resin Characterization

Dynamic mechanical measurements were per- formed in a DuPont DMA810 Module connected to 9900 Thermal Analyzer. All tests were run at an oscillation amplitude of 0.2 mm peak-to-peak and a heating rate of 5"C/min.

The critical stress intensity factor, K,, , and frac- ture energy, G,,, were determined using a compact tension (CT) sample according to a modified ASTM E399-83 procedure for one-crack propagation (7). The precracks of the compact tension sample were formed above Tg by an insertion technique (8). The crosshead speed was 1 mm/min for all sample tests.

Tensile tests were conducted in a Material Test System (MTS) 9 10 Module at a crosshead speed of 5 mm/min. Specimens were dogbone-shaped, as de- scribed in ASTM D-638.

Scanning electron microscopy (SEMI, Cambridge Steroscan-BOO, was used to study the fracture sur- faces of compact tension specimens in the modified networks.

RESULTS AND DISCUSSION

Analysis of PU Prepolymer

The transmission IR spectra of PU prepolymer are shown in Fig. 1. The IR spectrum of HTPU (FTMG) prepolymer (Fig. 1 a) was mainly characterized at 3400 cm- (- NH- stretch vibration), 1700 cm ~- ' (carbonyl), 2940 cm- (- CH- stretch vibration) and 1560 cm - ( - NH - deformation) absorption. The characteristic IR spectrum of ATPU (PTMG) pre- polymer ( R g . lb ) was similar except for a smaller

n A-

C 0 v) u)

E v) C a k w

W

.4

.3

CI c a, u k a, a

Fig. I . IR spectra showing the NH and carbonyl regions for (d HTPU (PTMG) (b) ATPU (FTMG) (c) HTPU (PBA) (4 ATPU (PBA) .

peak at 1700 cm-' (carbonyl). In Fig. Id, the ure- thane and ester carbonyl absorptions overlapped at 1720 cm I .

Table 1 shows the thermal transition behavior of PU modifiers with different macroglycols and cou- pling agents. The glass transition temperature of soft segment of HTPU (PTMG), ATPU (PTMG), HTPU (PBA) and ATPU (PBA) modifier was found to be - 80, - 78, - 48, and - 45°C. respectively.

Kinetics of Curing Reaction

From Fig. 2, the tan S peak designates the transi- tion time for liquid-to-rubber transformation (gela- tion), i.e, the time to attain insolubility. Since gelation represents a specific extent of reaction, the tempera- ture dependence of the time to gel can be described (9) by the Arrhenius equation:

In( k ) = - Ha/RT + C

where k is the rate constant varied with temperature and the activation energy, Ha The rate constant is inversely proportional to the time of gelation, which is independent of the order of the reaction. From the plot of the In( t ) vs. reciprocal temperature, the activa- tion energies were calculated and are shown in Fig. 3 and Table 3. It was apparent that there are significant differences in the reactivities of different macrogylcols and the coupling agent.

These results show that the activation energies of the crosslinking reaction between the epoxy and the hardener were increased by the addition of 10 phr of PU prepolymer, indicating that the presence of PU prepolymer slowed the curing process. Additionally,

Table 1. Composition of PU Prepolymer.

Tg ("C) Formulation Abbreviation Macroglycol Coupling Agent (by DSC)

HTPU (PTMG) PTMG 2000 Bis-A - 80 ATPU (PTMG) PTMG 2000 DDS - 78 HTPU (PBA) PBA 2000 Bis-A - 43 ATPU (PBA) PBA 2000 DDS - 40

0.30

0.25

Q 0.20 C a

0.15

0.10

0 140 "C - 160 "C 180 "C

10

Fig. 2. Tangent 6 at 3.5 H z us. logarithmic timefor DGEBA- DDS control.

1470 POLYMER ENGINEERING AND SCIENCE, SEPTEMBER 1995, Vol. 35, No. 18

ModiJcation and Compatibility of Epoxy Resin

Fg. 3. Arrhenius plots for the determination of activation energies of the curing reaction.

Table 2. Sample Description.

Description PU Modifier Content of PU of Sample Type Modifier (phr)

0 - E(O) MI(10) HTPU (PTMG) 10 M2(10) ATPU (PTMG) 10 Bl(10) HTPU (PBA) 10 BZ(10) ATPU (PBA) 10

the values obtained from the PBA-based PU-modified epoxy system were higher than those in PTMG-based PU system. This might result from the interaction between the ester segment of PBA based-PU and the epoxide of epoxy in the crosslinking reaction.

Dynamic Mechanical Properties

Figures 4, 5, and 6 show the temperature depen- dence of loss tangent of HTPU (PBA), HTPU (PTMG) and ATPU (PTMG) modified cured epoxy resins, re- spectively. The primary dispersions ( a-relaxation) of the cured resin with 0, 5, and 10 phr of HTPU (PBA), were observed as 221, 208.2, and 195.6"C, respec- tively (Fig. 4). The dispersion temperature went down and became broader in the presence of HTPU (PBA). This indicated that the HTPU (PBA)-modified epoxy network had some degree of homogeneity at the molecular level. Moreover, as a result of the increase of activation energy of the crosslinking reaction upon addition of the PU modifier (see Table 31, the curing rate was substantially slowed by the presence of HTPU (PBA) in the resin. This might also result in a lower- ing of glass-rubber transition temperature by effec- tively hindering the chemical reactions between the epoxide and the hardener.

Figure 5 shows the tan8 curves for the HTPU (PTMG)-modified epoxy system. The peak of a-relaxa- tion for the modified epoxy with 5 phr of HTPU (PTMG) was decreased from 221.0 to 218.1"C with respect to

Lo O . ' 1

Temperature ( O C 1 R g . 4. Temperature dependence of the Tan 6 for DGEBA-DDS control and HTPU (PBA)-modi$es epoxy resin.

Temperature ( O C 1 Rg. 5. Temperature dependence of the Tan 6 for DGEBA-DDS control and HTPU PTMG)-modiJed epoxy resin.

Q

C cd +

- 0 100 200 300 o . o o l ~ o o ' ' ' ' ' '

-100

Temperature ( O C 1 FUJ. 6. Temperature dependence of the Tan 6 for DGEBA-DDS control and ATPU(PTMG)-modi$ed epoxy resin.

POLYMER ENGINEERING AND SCIENCE, SEPTEMBER 1995, Yo/. 35, No. 18 1471

Table 3. Chemical Activation Energy of the Curing Reaction for DGEBA-DDS Control and PU-Modifier Epoxy Resin.

Sample E(0) MI(10) M2(10) BI(10) B2(10)

AE (J/mol) 65.0 70.6 71.8 74.6 75.7

the unmodified epoxy resin. Also, the peak position of a-relaxation was shifted slightly toward a lower tem- perature and became broader, as the HTPU (FTMG) concentration increased. The decrease in the a-re- laxation temperature in HTPU (PTMG)-modified epoxy was less than that of the HTPU (PBA)-modified sys- tem with the same weight content, which suggests that HTPU (PBA) exhibited greater compatibility than HTPU (PTMG) in the cured epoxy matrix.

The cured epoxy resins are known to have a local transition ( p-relaxation) in the low-temperature re- gions. The p-relaxation of cured epoxy resins has been ascribed to the crankshaft motion of their hy- droxyl ether portion (- CH ,CH(OH)CH ,O- ) ( 10). From Fig. 4, the amplitude of p-relaxation was a p parently enhanced and its peak temperature shifted slightly to a lower temperature upon the addition of HTPU (PBA).

In the ATPU (FTMG)-modified resin system (Fig. 61, the p-relaxation peak of tan 6 in M2( 15) decreased toward a lower temperature and had increased ampli- tude as compared with the control. This phenomenon was observed in many rubber-toughened resins. It is well known that the p-relaxation corresponds to the toughness of a matrix (11). In particular, in the M1 series, upon addition of the HTPU (PTMG), there was a sub-T, relaxation peak of Ml(15) shown at - 78°C. This suggests that a more clear two-phase separation existed in HTPU (PTMG) than in ATPU (PTMG)-mod- ified epoxy. It was also observed that the magnitude of the sub-T, relaxation peak became more pro- nounced with increasing HTPU (PTMG) content, which signifies that more separation exists between the HTPU (PTMG) modifier and the cured epoxy resin.

Fracture Energy and Fractography

Effect of Reactive Functional Group of PU

Figure 7 shows the fracture energy ( Grc) of various PU-modified epoxy resins as a function of the PU concentration at 20°C. It is evident that the fracture energy of the cured epoxy resins was greatly in- creased by the PTMG-based PU modifier. The bisphe- no1 A coupling agent was more effective than diamin- odiphenyl sulfone in increasing toughness.

Figure 8a-c shows the post-failure appearance of CT specimens of HTPU (FTMG)- and ATPU (PTMG)- modified epoxy under the scanning electron micro- scope at low magnification (100 x ). With the pres- ence of FTMG-based PU particles, the stress-whiten- ing zones developed on the fracture surface of the modified epoxy could be clearly seen. This stress- whitening effect is related to the local plastic defor- mation at the crack tip. As the FTMG-based PU con- tent increased, the extent of the stress-whitening

PU content ( p h r )

Huei-Hsiung Wang and Jung-Chieh Chen

1472 POLYMER ENGINEERING AND SCIENCE, SEPTEMBER 7995, Vol. 35, No. 18

Fig. 7. Effect of PU concentration on the fracture energies GI,.

zone increased. The HTPU (FTMG) modifier was more significant than the ATPU (PTMG) at the same weight content. The increase in stress-whitening effect seemed to be related to the increase of fracture energy.

SEM micrographs at high magnification from the stress-whitened zone of M 1( lo), M2( lo), B 1( 1 O), are shown in Fig. 8d, e , and f , respectively. The cavities in these micrographs were due to the cavitation and fracture of the PTMG-based PU particles and growih of the resultant voids. From Fig. 9, the fracture sur- face of M2(10) sample showed a broad particle-size distribution, ranging from particles of 0.5 pm to above 4.0 pm in diameter. Nevertheless, the M1( 10) sample showed a uniform distribution of particles, with di- ameters of 1- 1.5 pm. The presence of large particles (> 4 pm) in the ATPU (FTMGI-modified epoxy indi- cates that microgelation might occur during the pre- reaction because of the presence of the amine-termi- nal group. Moreover, the hydroxyl group of HTPU (PTMG) in the end position produced more improve- ment in toughness because of the effective molecular weight buildup by a chain extension reaction.

Toughening by modification with elastomers can be attained by dissipating the fracture energy because of interaction between a particles-induced shear band near the crack and the crack tip. Such dissipation of fracture energy by plastic shear deformation of the matrix has been studied previously (12, 13). In the other words, an increase of fracture energies is mainly attributed to the cavitation and shear-yielding mech- anisms. Since the rubber cavitation mechanism had little effect in absorbing fracture energy, the main energy-absorbing mechanism was the yielding mech- anism. Also, the above results indicate that the con- tribution of the plastic deformation to the toughening of the epoxy resin was one of the most important factors in PTMG-based PU-modified epoxy resin, and the uniform distribution of particles, which formed a homogeneous yielding, was more effective in increas- ing toughness.

Modijkation and Compatibility of Epoxy Resin

Effect of Macroglycols It is concluded that the presence of microphase

The unmodified epoxy and PBA-based PU-modified epoxy were transparent and of only one phase. In contrast, the FTMG-based PU-modified epoxy was opaque in appearance, owing to the presence of the PU particles with a refractive index different from the matrix, causing light scattering.

The fracture energy (Glc) for the PBA-based PU- modified epoxy network was increased slightly with an increase of PBA-based PU. The addition of 10 phr of HTPU (PBA) (Bl(10)) resulted in a 170% fracture energy increase. Also, the PBA-based PU series was less efficient than the PTMG-based PU system. Apart from this observation, the fracture surface of PBA- based PU-modified epoxy was smooth and feature- less, similar to that of the unmodified epoxy resin. Figure 8J shows that the fracture surfaces of the PBA-based PU-modified epoxy network manifest river-like markings, tracks in the direction of crack propagation. The river markings were more prc- nounced as the HTPU (PBA) concentration increased. This suggests that there may be localized plastic de- formation (14) in the crack front with the addition of PBA-based PU.

separations is for the ether type (FTMG) of PU- toughened epoxy resin. The ester type (PBA) of PU was compatible with epoxy resin, which lowered the fracture energy. Therefore, the effectiveness of PU as a modifier depended on both of the macroglycol and the coupling agent.

Tensile Mechanical hoperties

Table 4 shows that the tensile properties of a cured epoxy resin network at room temperature vary with the weight percentage and type of PU modifier. The tensile modulus gradually decreased with increasing PU content in all modified samples. Vakil (15) and Nielsen (16) reported that the modulus of a network at room temperature did not exhibit any crosslink density dependence in the range of 300 to 1500 g/mol. Therefore, the decrease of modulus for the modified epoxy network might be due to the effect of the soft segment structure of the PU modifier.

On the other hand, the modified epoxy network with low content of PTMG-based PU modifier exhib ited better tensile strength, accompanied by an in- crease in the strain at break. Particularly, the Ml(10)

Fig. 8. Scanning electron micrographs of the fracture surfme of CT specimens of PU-modijied epoxy resin: la)(b) M I l l O ) : ic)(d) MZ(10): (el@ Bl(1O).

POLYMER ENGlNEERiNG AND SCIENCE, SEPTEMBER 1995, Vol. 35, No. 78 1473

Huei-Hsiung Wang and Jung-Chieh Chen

for M1 series and M2(10) for M2 series reached a maximum value at 10% modification. In contrast to the PTMG-based PU modifier, the PBA-based PU- modified network exhibited only a marginal increase in tensile strength in comparison with the unmodi- fied epoxy resin.

CONCLUSION

Epoxy resin was modified with a phenolic hydroxyl- terminated (HTPU) and aromatic amine-terminated

(ATPU) PU prepolymer and further cured with 4,4’- diaminodiphenyl sulfone. From the morphological features, it was demonstrated that the disperse-phase structure was observed in the ether type (PTMG)- based PU series. The PU particles with dimensions of a few microns (1- 1.5 pm) was obtained by the addi- tion of HTPU (PTMG) prepolymer, which resulted in a more effective increase in toughness than for the ATPU (PTMG)-modified epoxy system. The ester type (PBA)-based PU-modified epoxy series exhibited a ho- mogeneous morphology and consequently had little effect on toughness. Therefore, the values of fracture energy (G,J for the PU-modified epoxy network were primarily dependent upon the macroglycol and the coupling agent.

The glass transition temperature of modified epoxy networks showed lower values in the PBA-based PU series than in the PTMG-based PU series with the same weight of modifier. There was a continuous decrease in the tensile modulus of the networks with increasing PU content. However, the modified epoxy networks with low PU content exhibited better tensile strength, as well as an increase in the strain at break.

Fg. 8. Continued.

. . , I , ,

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Particle Diameter ( p m) Fig. 9. Distribution of PU-particle diameters for PTMG-based PU-modiJed epoxy resin.

Table 4. Dynamic Glass Transition Temperature and Tensile Mechanical Properties of Pu-Modified Epoxy Network.

T,(Y* Ti p E U r

Sample (“C) (“C) (GPa) (MPa) (%)

E(O) 221 - 21 2.60 70 2.5 Ml(5) 21 8 - 27 2.58 72 2.7 Ml(10) 21 2 - 34 2.38 76 3.0

M2(5) 21 4 - 32 2.57 74 2.6 M2(10) 210 - 34 2.42 79 2.9

BI(5) 208 - 30 2.60 73 2.67 Bl(10) 193 - 39 2.58 74 2.8 B2(5) 209 - 26 2.61 74 2.6 B2(10) 195 - 32 2.54 76 2.7

Ml(15) 198 - 38 2.18 68 3.5

M2(15) 196 - 43 2.25 76 3.3

* J , a , J , P obtained from DMA.

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Modflcation and Compatibility of Epoxy Resin

ACKNOWLEDGMENT and Brittleness o f Plastics, ACS Adv. C h e m Ser., 154

The authors would like to thank the National Sci- ence Council, R.O.C., for sponsoring this work under Contract No. NSC 83-0405-E035-009.

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