Modification and compatibility of epoxy resin with hydroxyl-terminated or amine-terminated polyurethanes.
Several million pounds per year of epoxy polymers are produced in the world. These are widely employed as the resin for adhesive coatings or the matrix for glass-, polyamide-, or carbon-fiber composites. They are amorphous, highly crosslinked polymers. 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 rubber (ATBN or CTBN) to the epoxy resin (1, 2). Block copolymerization of epoxy with different oligomers containing vinyl, amine, or hydroxyl terminal groups has been also investigated (3-6). The primary factor of a rubber-modified matrix was a chemical requirement since the rubber needs to react with the resin, not only to increase its molecular mass and so lead to phase separation, but also to ensure that there are intrinsically strong chemical bonds across the rubbery phase/resin matrix interface.
In the present study, we attempted to toughen the epoxy network using polyurethane (PU) as the modifier. The problem of chemically linking the modifier to the epoxy network was overcome by using bisphenol A (Bis-A) or 4,4[prime]-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 mechanical properties and the morphology of the modified network will also be discussed.
Polytetramethylene glycol (PTMG 2000) and polybutylene adipate (PBA 2000) were supplied by Evermore Chemical Industry Co. The compounds 4,4[prime]-di-phenylmethane diisocyanate (MDI), bisphenol-A (Bis-A) and 4,4[prime]-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 wt = 190 g/mol). The hardener diaminodiphenyl sulfone (HT976) was obtained from the Ciba-Geigy Corp.
Preparation of Phenolic Hydroxyl-Terminated (HTPU) or Aromatic Amine-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[prime]-diphenylmethane diisocyanate were dissolved in dimethylformamide and heated to 70 [degrees] C. Then, one equivalent of macroglycol was slowly added to the solution with vigorous stirring. The reaction was carried out under nitrogen at 70-75 [degrees] C until the theoretical 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 [degrees] C with good stirring for 180 min. The reaction products were dried for 48 h in a vacuum oven at 80 [degrees] C. The synthesis scheme was shown as follows:
[Mathematical Expression Omitted]
[Mathematical Expression Omitted]
[Mathematical Expression Omitted]
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 826 resin at 140 [degrees] C (the amine-terminated PU at 120 [degrees] C) and prereacted for 30 min in a mechanical stirrer. The stoichiometric quantity of the hardener DDS was then added, and the system was maintained at 140 [degrees] C until a homogeneous clear solution was obtained. The resin was degassed at 140 [degrees] C for 15 min, poured into a 180 [degrees] C preheated polytetrafluoroethylene mold, cured at 140 [degrees] C for 2 h followed by 2 h at 200 [degrees] C in an oven. The cured resin was slowly cooled to room temperature.
PU Prepolymer Characterization
Thermal analysis was performed with a DuPont 910 differential scanning calorimeter (DSC) at a heating rate of 5 [degrees] C/min from - 120 [degrees] C to 220 [degrees] C. Nitrogen gas was used to purge the sample chamber of the DSC. Infrared spectra of PU prepolymers were obtained with a Hitachi Model 26050, over a range of 250 [cm.sup.-1] to 4000 [cm.sup.-1].
Analysis of Curing Reaction Kinetics
A Rheovibron DDV-II was used to measure directly the time required for macroscopic gelation during isothermal cure at different temperatures at a frequency 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 temperature. The temperature was raised at 15 [degrees] C/min to the cure temperature and held at that temperature until the peak of tan [Delta] was clearly observed.
Cured Epoxy Resin Characterization
Dynamic mechanical measurements were performed 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 [degrees] C/min.
The critical stress intensity factor, [K.sub.IC], and fracture energy, [G.sub.IC], 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 [T.sub.g] 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) 910 Module at a crosshead speed of 5 mm/min. Specimens were dogbone-shaped, as described in ASTM D-638.
Scanning electron microscopy (SEM), Cambridge Steroscan-600, was used to study the fracture surfaces 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 (PTMG) prepolymer [ILLUSTRATION FOR FIGURE 1A OMITTED] was mainly characterized at 3400 [cm.sup.-1] (- NH - stretch vibration), 1700 [cm.sup.-1] (carbonyl), 2940 [cm.sup.-1] (- CH - stretch vibration) and 1560 [cm.sup.-1] (- NH - deformation) absorption. The characteristic IR spectrum of ATPU (PTMG) pre-polymer [ILLUSTRATION FOR FIGURE 1B OMITTED] was similar except for a smaller peak at 1700 [cm.sup.-1] (carbonyl). In Fig. 1d, the urethane and ester carbonyl absorptions overlapped at 1720 [cm.sup.-1].
Table 1 shows the thermal transition behavior of PU modifiers with different macroglycols and coupling 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 [degrees] C, respectively.
Kinetics of Curing Reaction
From Fig. 2, the tan [Delta] peak designates the transition time for liquid-to-rubber transformation (gelation), i.e, the time to attain insolubility. Since gelation represents a specific extent of reaction, the temperature dependence of the time to gel can be described (9) by the Arrhenius equation:
ln([Kappa]) = -Ha/RT + C
where [Kappa] 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 ln(t) vs. reciprocal temperature, the activation 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, [TABULAR DATA FOR TABLE 1 OMITTED] 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.
Table 2. Sample Description.
Description PU Modifier Content of PU of Sample Type Modifier (phr)
E(0) - 0 M1(10) HTPU (PTMG) 10 M2(10) ATPU (PTMG) 10 B1 (10) HTPU (PBA) 10 B2(10) ATPU (PBA) 10
Dynamic Mechanical Properties
Figures 4, 5, and 6 show the temperature dependence of loss tangent of HTPU (PBA), HTPU (PTMG) and ATPU (PTMG) modified cured epoxy resins, respectively. The primary dispersions ([Alpha]-relaxation) of the cured resin with 0, 5, and 10 phr of HTPU (PBA), were observed as 221, 208.2, and 195.6 [degrees] C, respectively [ILLUSTRATION FOR FIGURE 4 OMITTED]. 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 3), the curing rate was substantially slowed by the presence of HTPU (PBA) in the resin. This might also result in a lowering of glass-rubber transition temperature by effectively hindering the chemical reactions between the epoxide and the hardener.
Figure 5 shows the tan [Delta] curves for the HTPU (PTMG)-modified epoxy system. The peak of [Alpha]-relaxation for the modified epoxy with 5 phr of HTPU (PTMG) was decreased from 221.0 to 218.1 [degrees] C with respect to the unmodified epoxy resin. Also, the peak position of [Alpha]-relaxation was shifted slightly toward a lower temperature and became broader, as the HTPU (PTMG) concentration increased. The decrease in the [Alpha]-relaxation temperature in HTPU (PTMG)-modified epoxy was less than that of the HTPU (PBA)-modified system with the same weight content, which suggests that HTPU (PBA) exhibited greater compatibility than HTPU (PTMG) in the cured epoxy matrix.
Table 3. Chemical Activation Energy of the Curing Reaction for DGEBA-DDS Control and PU-Modifier Epoxy Resin.
Sample E(0) M1(10) M2(10) B1(10) B2(10)
[Delta]E (J/mol) 65.0 70.6 71.8 74.6 75.1
The cured epoxy resins are known to have a local transition ([Beta]-relaxation) in the low-temperature regions. The [Beta]-relaxation of cured epoxy resins has been ascribed to the crankshaft motion of their hydroxyl ether portion (- C[H.sub.2]CH(OH)C[H.sub.2]O -) (10). From Fig. 4, the amplitude of [Beta]-relaxation was apparently enhanced and its peak temperature shifted slightly to a lower temperature upon the addition of HTPU (PBA).
In the ATPU (PTMG)-modified resin system [ILLUSTRATION FOR FIGURE 6 OMITTED], the [Beta]-relaxation peak of tan [Delta] in M2(15) decreased toward a lower temperature and had increased amplitude as compared with the control. This phenomenon was observed in many rubber-toughened resins. It is well known that the [Beta]-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.sub.g] relaxation peak of M1(15) shown at -78 [degrees] C. This suggests that a more clear two-phase separation existed in HTPU (PTMG) than in ATPU (PTMG)-modified epoxy. It was also observed that the magnitude of the sub-[T.sub.g] relaxation peak became more pronounced 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 ([G.sub.IC]) of various PU-modified epoxy resins as a function of the PU concentration at 20 [degrees] C. It is evident that the fracture energy of the cured epoxy resins was greatly increased by the PTMG-based PU modifier. The bisphenol A coupling agent was more effective than diaminodiphenyl sulfone in increasing toughness.
Figure 8a-c shows the post-failure appearance of CT specimens of HTPU (PTMG)- and ATPU (PTMG)modified epoxy under the scanning electron microscope at low magnification (100 x). With the presence of PTMG-based PU particles, the stress-whitening zones developed on the fracture surface of the modified epoxy could be clearly seen. This stress-whitening effect is related to the local plastic deformation at the crack tip. As the PTMG-based PU content increased, the extent of the stress-whitening zone increased. The HTPU (PTMG) 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 M1(10), M2(10), B1(10), 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 growth of the resultant voids. From Fig. 9, the fracture surface of M2(10) sample showed a broad particle-size distribution, ranging from particles of 0.5 [[micro]meter] to above 4.0 [[micro]meter] in diameter. Nevertheless, the M1(10) sample showed a uniform distribution of particles, with diameters of 1-1.5 [[micro]meter]. The presence of large particles ([greater than] 4 [[micro]meter]) in the ATPU (PTMG)-modified epoxy indicates that microgelation might occur during the pre-reaction because of the presence of the amine-terminal group. Moreover, the hydroxyl group of HTPU (PTMG) in the end position produced more improvement 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 mechanisms. Since the rubber cavitation mechanism had little effect in absorbing fracture energy, the main energy-absorbing mechanism was the yielding mechanism. Also, the above results indicate that the contribution 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 increasing toughness.
Effect of Macroglycols
The unmodified epoxy and PBA-based PU-modified epoxy were transparent and of only one phase. In contrast, the PTMG-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 ([G.sub.IC]) 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) (B1(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 featureless, similar to that of the unmodified epoxy resin. Figure 8f 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 pronounced as the HTPU (PBA) concentration increased. This suggests that there may be localized plastic deformation (14) in the crack front with the addition of PBA-based PU.
It is concluded that the presence of microphase separations is for the ether type (PTMG) 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 Properties
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 exhibited better tensile strength, accompanied by an increase in the strain at break. Particularly, the M1(10) 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 unmodified epoxy resin.
Epoxy resin was modified with a phenolic hydroxyl-terminated (HTPU) and aromatic amine-terminated (ATPU) PU prepolymer and further cured with 4,4[prime]-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 [[micro]meter]) was obtained by the addition 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 homogeneous morphology and consequently had little effect on toughness. Therefore, the values of fracture energy ([G.sub.IC]) 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.
The authors would like to thank the National Science Council, R.O.C., for sponsoring this work under Contract No. NSC 83-0405-E035-009.
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|Author:||Wang, Huei-Hsiung; Chen, Jung-Chieh|
|Publication:||Polymer Engineering and Science|
|Date:||Sep 1, 1995|
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