Curing mechanisms and mechanical properties of cured epoxy resins

Curing Mechanisms and Mechanical Properties of Cured Epoxy Resins Takashi Kamon Kyoto Municipal Research Institute of Industry Hitoshi Furukawa Sanyo Giken (Sanyo Technical Research), Limited 1-2 Minamida-cho, Nishikujo, Minami-ku, Kyoto 601/Japan Epoxy resins can be cured with various kinds of hardeners, which results in many types of epoxy resins with different structures. Here, the relationships between the structures and mechanical properties, particularly dynamic mechanical properties of those resins are reviewed. The structures hardly affect the mechanical properties in the glassy region. The mechanical properties drop rapidly at temperatures higher than the glass transition temperature (T,). The relationships between the resin structures and Tg are discussed. The factors affecting Tg such as the type of networks, the molecular structure of the initial starting raw materials, and the density of crosslinking (number of funetional groups), are considered here. Furthermore, mechanical properties of some cured resins havin 9 structures which cannot be presumed from initial resins are also discussed. Among the mechanical properties, impact strength is related to [~ dispersion at around --50 °C rather than to dispersion ( Tg). 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 2 The Curing Mechanism of Epoxy Resins and the Structure of Cured Resins t74 3 Types of Curing Agents and the Dynamic Mechanical Properties of Cured Resins 177 3.1 Dynamic Mechanical Properties of Cured Resins with Different Curing Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . 179 3.2 Structures and Dynamic Mechanical Properties of Epoxy Resins Cured with Polyamines . . . . . . . . . . . . . . . . . . . . . . . . 180 3.3 Dynamic Mechanical Properties of Resins Cured with Carboxylic Hydrazides . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 3.4 Dynamic Mechanical Properties of Epoxy Resins Cured with Carboxylic Anhydride . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 3.5 Dynamic Mechanical Properties of Mercaptan-cured Resin . . . . . . 188 4 Dynamic Mechanical Properties of Resins Cured with Different Hardeners. . 190 4.1 Dicyandiamide . . . . . . . . . . . . . . . . . . . . . . . . . t90 4.2 Resins Cured with Isocyanates . . . . . . . . . . . . . . . . . . 192 4.3 Other Examples . . . . . . . . . . . . . . . . . . . . . . . . . 193 5 Mechanical Properties of Cured Epoxy Resins in the Glassy State . . . . . 194 6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 7 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 Advances in Polymer Science 80 © Springer-Verlag Berlin Heidelberg 1986 174 1 Introduction T. Kamon and H. Furukawa Epoxy resins are superior in heat resistance, adhesion, corrosion resistance and also mechanical properties among thermosetting resins and are widely used for coatings, adhesives, electric insulating materials and matrices for FRP in areas such as aircrafts, electronics, electric power, and building and civil engineering. However, epoxy resins now in use are not of the same structures as thermoplastic resins. The factors determining the structure of cured resins and affecting the physical and mechanical properties are as follows: 1. Curing mechanism: kind of functional groups of hardeners. 2. Number of functional groups in resins and hardeners: density of crosstinking. 3. Molecular structure of bridges between functional groups in resins and hardeners. 4. Molar ratio of resin and hardener: density of crosslinking. 5. Degree of curing, or curing conditions. Most epoxy resin prepolymers now in industrial use are diglycidyl ethers having two epoxy groups per molecule; a few tens of other epoxy resin prepolymers are now also available on the market. On the other hand, more than one hundred hardeners are known. Therefore, the number of combinations of resins and hardeners is very high and, accordingly, it is hardly possible to describe all mechanical properties of the cured resins with many different structures - - hardness, tensile strength, elongation at break, impact strength, etc. within a wide range of temperatures. A number of papers deals with the dynamic mechanical properties of cured epoxy resins 1 -s~ and with the effect of the structure of basic resins and the degree of cross- linking 6-8~. In this review, the effect of the cured epoxy resins on the dynamic mechanical prop- erties of epoxy resins is discussed. 2 The Curing Mechanism of Epoxy Resins and the Structure of Cured Resins The mechanical properties and dynamic mechanical properties of the cured epoxy resins are governed by their structures. The curing mechanism of an epoxy resin or the type of functional group of a hardener is the most essential factor determining the structure of the cured resin. The well- known hardeners are polyamines, acid anhydrides, and polymerization catalysts. Among these hardeners, amines are the most versatile ones at room temperature as well as elevated curing temperature. The curing mechanisms with amines and the structures of the amine cured epoxy resins have been most sufficiently studied, and the systems of epoxy resins with amine hardeners are most extensively used in the practical industrial fields. Curing Mechanisms and Mechanical Properties of Cured Epoxy Resins 175 The curing mechanism of a resin with a primary amine is as follows 9-~z). R-C-C \ / 0 HX + H2N-- R' R -C -C + R--C--C--NH-R' O OH R-C-C- -NH- R' (t) I OH OH I R-C-C... - R' 12) R_C_C jN - I OH OH I R-C-C + R' " -C -C -O- n ~0 / R -C-C / l \ R /n OH (3} As shown in the above reaction scheme, the epoxy groups are successively opened by amine active hydrogens [Eqs. (1) and (2)]. In this reaction, the presence of active hydrogen compounds (HX), such as water and alcohols as impurities, is required, and alcohols produced by Eqs. (1) and (2) accelerate the curing reaction (cf., e.g., Smith 9) and Kakurai 11)). Different mech- anisms have been proposed by Smith 9), Tanaka H), and Bell 12), but they have not yet been confirmed. It has been shown, however, that curing proceeds as shown in Eqs. (1) and (2), and the structures shown in Fig. 1 a are finally formed. It is generally accepted that Eq. (3) hardly occurs in case of the stoichiometrically equivalent system or with an excess of amine 13) In addition of polyamines, acid anhydride hardeners are used but the structure of cured resins is less understood. I ~' - NeC H~- C H-~-C H 'CH~-( - N~ ; 6H OH CHB H(~-OH , He-OH CH2 ~--"~ ~N~ ~N~ 81. ,C HiCH fl E'--~CH ~-._- OH OH , 0 0 t ' ----~OffCH-CH~O-~#.2"_',-:'z~-O~- CHCH~O-"~ --~-O-I-C H-CH£OFCH-C H£O I '-~ CH~ ----- C H----- ~CH~ ~CH~ b e =epoxy rest, E~ =curing agent rest. Fig. 1 a-c. Schematic structure of fully cured epoxy resin, a amine cured resin; b anhydride cured resin; c catalyst cured resin 176 T. Kamon and H. Furukawa The anhydrides are sometimes used alone, but more frequently in combination with such basic compounds as tertiary amine catalysts. The suggested curing mechanism is roughly as follows 14-16). 0 0 I I I I C ~"~''~ C -- 0 e tt II 0 0 (4) 0 [~C + CH2--- CH- NR3 c -o o \ / 11 0 0 0 ~ C4-NR3 C-O-CH2--CH-- II I 0 0 e (5) 0 I I II I 0 0 e 0 I I II 1 0 O-C :O I 0 II o II 0 0 It (~ ~ C ~ NR3 C-O-CH2-CH- 0 ° I I O--C=O C=O 0 e ii + OCH2CIH-O--C- O- (6) 0 0 I I I I I ,,, [~C- -O- - CH2--CH-O- C- C -O-CH2- CH- II I 0 O-C--O I + NR3 (7) A different initiating mechanism than that given by Eq. (4) has been proposed 15-17) The curing reaction generally proceeds as shown in Eq. (6), resulting in the alternating copolymerization of epoxy groups and acid anhydrides. The structure of the cured resin is shown in Fig. 1 b. It is, however, well known that some epoxy groups may react with other epoxy groups. Another curing mechanism is operative in the curing reaction initiated by a poly- merization catalyst for epoxy ring opening, where anionic catalysts such as tertiary amines 18.19) and imidazols 2o,2t) and cationic catalysts such as amine complexes of Curing Mechanisms and Mechanical Properties of Cured Epoxy Resins 177 Lewis acids 22.23) are used. Among these curing agents, many show a particular curing behavior; for example, the BF3-amine complex 22,23) is an excellent latent curing agent, and diaryliodonium salts 24) are known as agents to initiate UV curing 22-24) Some of the initiating mechanisms are not yet clear. In general, polyethers are produced through the ring-opening polymerization of epoxy groups as shown in Eq. (8): n R -C-C cat . - . - f -C -C -O- -~C-C-O . . . . (8, The structure of the finally cured resin is shown in Fig. 1 c. 3 Types of Curing Agents and the Dynamic Mechanical Properties of Cured Resins Murayama 8) studied a series of resins from diglycidyl ether of Bisphenol A (DGEBA) cured with varying quantities of diaminodiphenylmethane (DDM). These cured resins had the same main chain structure, but differed in the degree of crosslinking. Izumo 25) performed similar studies by using diethylenetriamine (DETA) as a 10 Io ~ 10 ~ -m i0 s I I [ I I I I 20 100 200 Temperature (*C) T 0.I 0.01 Fig. 2. Dynamic modulus and loss tangent vs. temperature for EDA cured epoxy resins 26L Mol ratio (amine/epoxy); 1: i.4, 2: 1.2, 3: 1.0, 4: 0.8, 5:0.6 178 T, Kamon and H. Furukawa 10 ~° 1o9 10 8 n /- -'---_,,_ - . / / , , l s t , , ] , , f l l , 20 100 Temperature (°C) 200 1 c i J I 0.01 Fig. 3. Dynamic modulus and loss tangent vs. temperature for Me HHPA cured epoxy resins 27) Mol ratio (anhydride/epoxy); 1: 1,0, 2: 0.9, 3: 0.8, 4:0.7 curing agent, and Kamon 26) used ethylenediamine (EDA). An example of the results is shown in Fig. 2. The dynamic mechanical properties of a series of resins cured with varying quantities of an acid anhydride are shown in Fig. 3 z7). The temperature dispersion of the dynamic mechanical properties of all these cured resins indicates three regions: (1) the glassy region where the dynamic Young's modulus (E') is about t0 l° dyne/cm 2 which changes little at lower temperature, (2) the transition region where E' is changed from 10 I° to 10 8 dyne/cm 2 in a narrow temperature range and (3) the rubbery region where E' becomes equal to about 10 8 dyne/cm 2 and is constant at higher temperatures. The temperature indicating the maximum tan 6 is taken as the glass transition temperature Tg. The value of E' in the rubbery region was theoretically and experimentally studied and it was found to be proportional to the crosslinking density (Q) or inverse to the molecular weight between linkings (Me). Tobolsky 1,2s), used Eq. (9) to demonstrate the interrelationship : E' = 3q) dRT/Mc (9) If the effect of dangling is neglected, then: M~ = d/~ Curing Mechanisms and Mechanical Properties of Cured Epoxy Resins 179 Thus, Eq. (9) changes to: E '= 3(0QRT (10) Here, E' is the Young modulus in the rubbery state, d the density, R the gas constant, T the temperature in K, Mc the molecular weight between crosslinks, O the cross- linking density, and q) the front factor. The value of q) is close to unity. 3.1 Dynamic Mechanical Properties of Cured Resins with Different Curing Mechanisms Although the cured epoxy resins produced by the three typical curing mechanisms have different structure, the temperature dependence of the dynamic mechanical dispersion is similar as shown in Figs. 2 and 3, but Tg is different. Therefore, in order to check the relationship between the structure and Tg, Q should be identical. The relationship between Tg and Q(E') (Q(E') has been calculated from Eq. (9) for q~ = 1) of a series of the cured resins with different Q are shown in Fig. 4. The curing agents used are DETA 25), EDA 26), and DDM s) as a diamine, succinic acid anhydride (Suc A) 27) as an acid anhydride and 2-ethyl-4-methyl imidazole (Im) 21) as a catalytic curing agent. The relationships between Tg and Q have been studied in a number of papers 29-31). Shibayama studied theoretically the relationships based on the volume shrinkage due to crosslinking, and suggested that Eq. (11) was applicable to many crosslinked high polymers 32) Tg = Kl log K2Q (1 1) E o 100 2oo_ Im lt~D M - - Suc A ~- . so ~ I I , , ~1 , 2.0 2.5 3.0 - log ~¢E'~ I I I I Fig. 4. Glass transition temperature (Tg) vs. crosslinking density (O(~'r) for resins cured with various curing agents 180 T. Kamon and H. Furukawa TaMe 1. K 1 and log K z in Eq. (11) for epoxy resins cured with various curing agents Curing agent K~ log K 2 EDA 112.7 3.86 DETA 96.3 3.84 DDM 59.3 5.5 AADH 51.0 5.92 SucA 50.4 3.68 Me-HHPA 84.8 4.56 PnEThGE 47.3 4.61 TETA-MeDPTA 107 3.82 EDA-HMDA 216 3.23 AADH-SL-20DH 248 3.36 Polyanhydride 227 3.21 Imidazole 78.6 4.27 Resin: DGEBA Here, K1 and K2 are constants for the same groups of polymers; K2 depends on the strength of interaction and the rigidity of the main chain, and K 1 is related to the volume shrinkage due to crosslinking and characterizes the degree of restraint of the molecular motion near the crosslink (K1 is about 100 for no restraint, and approaches zero as the restraint increases). The dependence predicted by Eq. (11) is obeyed for a number of epoxy resin-curing agent systems, as shown in Fig. 4. The values of K~ and log K2 shown in Table 1. The values of log K2 are nearly the same, which indicates that the resins are almost the same in physical properties, although their main chain structures are different (Fig. 1). K1 is nearly 100 for EDA and DETA since there is no restraint in the molecules of the curing agents; Q~E') is higher and Tg is slightly elevated in Im and Suc A since the restraint is high. This is due to increasing flexibility of the main chain in the series: hydrocarbon < polyether < polyester. For DGEBA resins, Tg increases in the series: amine- > catalyst- > acid anhydride, for the same value of Q~E'). If in the [] segment in Fig. 1 the alkyl group is replaced by an aryl group, e.g., EDA by DDM, or Suc A by methylhexahydrophthalic anhydride (MeHHPA), K 2 becomes higher, the rigidity of the main chain increases, and Tg becomes higher (Table 1). 3.2 Structures and Dynamic Mechanical Properties of Epoxy Resins Cured with Polyamines Amines are the most frequent curing agents, and the curing mechanism is determined by a simple addition reaction of active hydrogens of polyamines to epoxy groups. Consequently, the structures of the cured resins are not much different from the ideal structure shown in Fig. 1 a. Some examples of polyamines are shown in Table 2. The structures of the cured resins obtained from DGEBA and stoichiometrically equivalent amounts of polyamines are shown in Fig. 1 a, and differ in the [] segment. Curing Mechanisms and Mechanical Properties of Cured Epoxy Resins Table 2. Structure and boiling point of polyamine curing agents 181 Code Structure b.p. (*C) E DA H2N (CH2)zNH2 117 DETA H2N(CH2)zNH(CH2)2NHz 208 TETA H2N(CH2)2NH(CH2)2 157°Ct20mm NH(CH2)zNH2 PDA H2N(CHz)3NH2 140 DPTA H2N(CH2)3NH(CH2)3NH2 241 MeDPTA H2N(CH2)3NI (CH2)3NH 2 234 CH3 HMDA H2N(CH2)6NH2 205(m.p. 42°C1 TMAH H2N(CH2)3CH(CH2)3NH2 160 °C/10 mm / CH2NH2 MXDA H2NCHz ~ CH2NH2 245 °C v H3C. ,CH3 LARO H2N- - (~ CH2 ~ - NH2 200"212"0/ 20 mm Hg DDM H2N-~ CH2- -~"- NH2 {m.p. 90*0) The temperature dependence of the dynamic mechanical properties of the cured resins is similar as in Fig. 5 33). This shows that the physical properties of cured resins are mostly characterized by the differences of Tg. The diamine curing agents H2N-(-CH~-)~,NH 2 (n = 2, 3, 4, 6) have been taken as an example (cf. Table 2). It can be assumed that Tg is lowered since M c becomes higher with increasing n 34) [Fig. 5, Eqs. (10) and (11)]. As n increases from 2 to 6, 0(~,) is lowered and the rigidity of the main chain decreases. It is suggested that the effect of this decrease on Tg is almost constant 34) The following example shows that Mc can be simply changed by changing the functionality of the hardeners which is 6, 5, and 4 for TETA, DPTA and MeDPTA, respectively. The relationships between Tg and log Q(E') are described by straight lines in Fig. 6, where K1 is 107 which is similar to the value found for EDA and DETA systems, and show clearly the effect of the phenyl groups on Tg (Table 1). The resins cured with EDA, DETA and TETA were already studied by Katz 35) In this system, the number of the functional groups of the polyamine is increased if 182 Y. Kamon and H. Furukawa 10 ~° E o Io 9 10 8 \ 4 3 2 1 .... I I , I , , , , I , , , , I , , , , I , , , 20 t00 200 Temperoture (*C) I 0.1 0.01 Fig, 5. Dynamic modulus and loss tangent vs. temperature for various diamine cured epoxy resins 33). 1: DDM, 2: EDA, 3: BDA, 4: HMDA EDA is replaced by TETA. Accordingly, despite the increase of Q(E'), Tg is slightly lowered 34) (cf. Fig. 6). TMAH is a diamine having a primary amine group in the center of the molecule, and the dynamic mechanical properties of the cured resin are interesting (cf. Table 3). Both Tg and Q(E;) are almost the same as for TETA, which suggests that such a slight Table 3. Dynamic mechanical properties and flexural strength of diamine cured epoxy resins Curing T~ E' x 10- lo Q(E') Flexural agent strength (°C) (dyne/cm 2) x 103 (mol/cm a) (Kg/cm 2) DDM 180 2.52 2.51 t230 LARO 180 1.70 2.34 1070 MPDA 185 1.86 2.40 1090 MXDA 135 2.01 1.71 1240 BDA 140 1.29 2.28 750 Curing Mechanisms and Mechanical Properties of Cured Epoxy Resins 183 160 - - 140 -HMDAo~/ i , - - / / l I / 100 - - • '/, M, D, PTAI , -3 .0 -2.5 tog ~(E'~ 120 / TMAH EDA I~[~TETA PDA,~,,. ?l:::lE TA /,'~DPTA I I I -2.0 Fig. 6. T 8 vs. log Q(w) for aliphatic polyamine cured epoxy resins s4) difference in molecular structure has almost no influence on physical properties a4) I f an alkyl chain (BDA) is substituted by m-phenylenediamine (MPDA), Tg is elevated by 45 °C (Table 3). However, almost the same effect is observed for DDM, while for MXDA having an alkyl-phenyl group, almost no effect on Tg is observed because the rigidity of the main chain is not increased enough. In both cases of cycloaliphatic and phenyl groups, nearly the same effects on the increase of Tg are observed. LARO and DDM can serve as example (cf. Table 3) 34). 3.3 Dynamic Mechanical Properties of Resins Cured with Carboxylic Hydrazides In Section 3.2, the effect of the segment structure and the number of functional groups on physical properties is discussed. There, it has been analyzed how the struc- ture of the crosslinks ( -N( ) influences the physical properties. Since carboxylic hydrazides can be easily synthesized from carboxylic acids, hardeners of various structures can be obtained. As the structure of the functional group (- -CONHNH2) is similar to that of amines, it is interesting to compare the properties of the cured resins. The hydrazides are powders with high melting points and are used as latent curing agents with a pot life of more than four months 36, 37) (Table 4). The curing mechanism of epoxy resins with carboxylic hydrazides has not been sufficiently revealed so far. However, Kamon a6) has shown that one hydrazide group reacts with two epoxy groups. This result suggests that the structure of the epoxy resin cured with hydrazide