Properties of Epoxy Resins Cured with Ring-Alkylated m

Chapter

14

Properties of Epoxy Resins Cured with Ring-Alkylated m-Phenylene Diamines

Downloaded by CORNELL UNIV on September 1, 2016 | http://pubs.acs.org Publication Date: April 18, 1988 | doi: 10.1021/bk-1988-0367.ch014

Robson F. Storey, Sudhakar Dantiki, and J . Patrick Adams Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406

Diglycidyl ether of bisphenol-A resin was cured, individually, with m-phenylene diamine (MPD) and ring-alkylated MPD's, i . e . , toluene diamine, diamino ethyl benzene, diamino isopropyl benzene and diamino tert-butyl benzene. For each diamine, glass transition temperature (DSC) of the cured resin was studied as a function of epoxide/amine stoichiometry. Room temperature tensile properties of resins cured at the stoichiometric ratio, using two cure cycles, were determined. Alkyl substitution generally decreased tensile strength relative to MPD-cured resins. Thermogravimetric analysis indicated s l i g h t l y higher thermal stability for resins cured with ring-alkylated MPD's compared to MPD. Dynamic mechanical analysis was used to detect the molecular relaxations in the resins cured with ring-alkylated MPD's. The concept of nodular morphology was invoked to explain the shift in the maximum in the beta relaxation observed for resins cured with ring-alkylatedMPD's. Aromatic diamines were introduced into epoxy resin curing technology to improve heat and chemical resistance over that a t t a i n e d with aliphatic diamines. They have been used successfully i n laminating applications and to a certain extent i n casting and adhesive applications. The most widely used aromatic diamines are m-phenylene diamine (MPD), methylene dianiline (MDA) and diaminodiphenyl sulfone (DADS). DADS i s generally viewed to be separate from the other two, being a higher priced, high-performance diamine yielding an elevated glass transition temperature (Tg) of the cured resin. Due to i t s low basicity DADS requires higher temperature cure schedules than MPD/MDA. Thus, MPD and MDA have for many years been the workhorses in the aromatic diamine-cured epoxy industry. However MDA, and to a lesser extent MPD, have recently come under 0097-6156/88/0367-0182$06.00/0 © 1988 American Chemical Society Dickie et al.; Cross-Linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


14.

STOREY ET AL.

Epoxy Resins Cured with m-Phenylene Diamines

increasing attack due to their high toxicity. In part, this i s due to the recent finding that MDA causes cancer in laboratory rats (1). Since liquid curing agents are desirable, MPD and MDA are most commonly sold as proprietary eutectic blends containing lesser amounts of other diamines. Thus, there i s a need for new aromatic diamines which can serve as replacements for MDA and/or MPD/MDA eutectic solutions. As we reported previously (2), ring-alkylated MPD's appear promising as candidate replacements for MDA. Certain of these compounds are neat liquids at room temperature, and others form eutectic liquids with MPD. It i s also thought that the presence of the alkyl group on the ring might produce a less toxic, more easily metabolized compound. We have undertaken a systematic investigation of the effect on cured resin physical properties of various alkyl groups on the MPD aromatic ring. In this report we present static and dynamic mechanical properties, density measurements, and glass transition temperature (Tg) measurements of diglycidyl ether of bisphenol-A (DGEBA) cured with various alkylated MPD's. In a thorough study on the temperature dependence of mechanical properties of MPD-cured epoxy resins, Gupta et a l . ( 3 ) concluded that in the glassy state, hiç£i strain properties such as tensile strength, elongation, and toughness are affected by intermolecular packing, molecular architecture, and molecular weight between crosslinks (^). In the rubbery state, crosslink density was reported to be the important factor. Gupta et a l . undertook these i n v e s t i g a t i o n s because of c o n f l i c t i n g observations reported i n the l i t e r a t u r e on the effect of stoichiometry, and thus crosslink density, on room temperature mechanical properties of MPD-cured epoxy resins. We were also concerned about the effects of stoichiometry on the optimization of properties using ring-alkylated MPD's, and we have viewed our results in terms of the steric effect of large alkyl groups within the crosslinked structure. In addition, we were interested i n the effect of phasemorphology of the crosslinked structure on the ultimate mechanical properties of epoxy resins. A stimulating controversy e x i s t s regarding the possible micro-phase separation of crossl inked epoxy networks (4). Several studies which focus on this point have used aliphatic amine curing agents (5-8). These studies have concluded that cured epoxy resins consist of higher crosslink density nodules imbedded i n a lower crosslink density matrix. The concept of nodular morphology was applied in the present investigation to explain the differences i n the dynamic mechanical relaxations of epoxy resins cured with stoichiometric amounts of different alkylated aromatic diamines. EXPERIMENTAL Materials. MPD was obtained from Aldrich Chem. Co. and used as received. The various alkylated diamines were experimental quantities kindly supplied by Dr. Arthur Bayer of First Chemical Corp. The various diamines studied are indicated below:

Dickie et al.; Cross-Linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

183

CROSS-LINKED POLYMERS

184

R = -H, m-phenylene diamine (MPD) = -CH3, toluene diamine (TDA) = -CH2CH3, diaminoethyl benzene (DAEB) = -CH(CH3) # diaminoisopropyl benzene (DAIPB) = -0(013)3, diagano-tert-butyl benzene (DATBB)


2

The epoxy resin, diglycidyl ether of bisphenol A (DGEBA) (DER 332, Dow Chemical Co.), was used without purification. According to the manufacturer, i t i s pure DGEBA without appreciable amounts of higher molecular weight oligomers and has the following structure:

Procedures. The equivalent weight of DGEBA was determined to be 172.1, by t i t r a t i n g epoxide groups using the hydrogen bromide/acetic acid method (9). The procedure for curing epoxy resins i n aluminum molds was as follows: An excess amount of DGEBA was heated i n a paper cup using a circulating a i r oven at 85°C. Meanwhile, an excess amount of the diamine was heated just to melting i n a test tube using a hot o i l bath. As the size of the alkyl substituent increases, the melting point of the compound decreases such that DAIPB i s a wet solid and DATBB i s a liquid at room temperature. Appropriate amounts of epoxy resin and liquified amine were mixed together and evacuated at 60°C i n a vacuum oven for ten minutes to remove a i r bubbles. The reaction mixture was poured into a hot (85°C) aluminum mold which was prepared i n advance by lightly spraying mold release agent (MR 515, Green Chem. Products Inc.) to the inner surfaces of the mold. Samples were cured two hours at 85°C followed by two hours at 150°C (standard cure). In some cases samples were subjected to an alternate, high temperature cure of two hours at 85°C and two hours at 175°C (HT cure). After curing, samples were stored i n a desiccator until use. Tensile properties were determined according to ASTM D638 using an Instron tensile tester equipped with a 500 kg load c e l l . Glass transition temperatures (Tg's) were determined using a Dupont DSC 910 attached to a 9900 data analysis system. For offstoichiometric studies, epoxy resin and diamine were cured i n situ within a hermetically sealed DSC pan (sample taken from 25°C - 300°C at 10°C/min), then cooled rapidly back to 25°C, and finally scanned from 40°C - 220°C to record the Tg. A l l samples were scanned under nitrogen atmosphere at a rate of 10°C/min.


14. STOREY ET AL.

Epoxy Resins Cured with m-Phenylene Diamines

The dynamic mechanical spectra were recorded using a Dupont 982 dynamic mechanical analyzer attached to a 9900 data analysis system. The amplitude was held constant at 0.2mm. A l l thermal scans were obtained at a scanning rate of 5°C/min under nitrogen atmosphere. Molecular weight between crosslinks, Mc, was calculated from the rubbery modulus, G , i n the dynamic mechanical spectrum i n the region where G i s independent of temperature. The following empirical equation given by Nielsen (10) was used to compute the Mc values: 1

1


log

1 0

G' = 7 + 293 p/Mc

where ρ i s the d e n s i t y a t 293°K. Densities of cured resins were obtained by accurately weighing rectangular solids which were precision^nachined from expended tensile specimens. Approximate dimensions of the specimens were 1 cm χ 1 cm χ 0.32 cm; accurate dimensional measurements were obtained using a micrometer. RESULTS AND DISCUSSION In our i n i t i a l studies (11), we examined several series of cured resins using ortho and meta isomers of phenylene diamine. p-Phenylenediamine was not considered for study due to i t s known carcinogenic nature. As shown i n Table I, p-phenylenediamine (0PD) when used alone was found to impart inferior properties to the crossl inked epoxy resin. We suspect the poor performance of 0PD i s attributable to intramolecular hydrogen bonding, as shown below: H

H

0PD i s the only isomer which can form such a structure, and i t s presence may interfere with complete curing. Interesting results were obtained when MPD/0PD blends were used as curing agents. When an 80/20 (wt/wt) MPD/0PD blend was used i n the standard cure procedure, a significantly higher glass transition temperature (Tg) was observed compared to resins cured with MPD alone. However, for samples cured with the HT cure schedule, MPD and 80/20 MPD/0PD yielded resins of essentially the same Tg. The increased Tg observed i n the case of epoxy resins cured with MPD/0PD blends under standard cure conditions may be due to the molecular architecture and/or intermolecular packing, but at this time we do not have a satisfactory explanation for the observed Tg's of resins cured with these blends. Thermal stability was evaluated for resins cured using standard cure. The temperature at which 5% weight loss occurred (Td) was taken as the onset temperature of decomposition. There was no appreciable difference i n the thermal stability of the resins cured with MPD/OPD blends compared to MPD alone.


185


186

CROSS-LINKED POLYMERS Tensile strength of the MPD cured resins decreased with a change to HT cure, i n spite of the higher Tg. Gupta et a l . (3) have observed similar behavior i n MPD cured epoxy resins and attribute this to an increase i n free volume of the sample. The increase i n crosslink density i s apparently less significant i n this case. HT cure increases the tensile modulus i n MPD cured samples; whereas i t did not show much impact i n epoxies cured with MPD/OPD blends. This confirms that the intermolecular packing i s different i n epoxies cured with different curing systems. In a l l cases, cured products with excess epoxy have higher modulus than the stoichiometric products. This increased stiffness results because fewer diamine molecules are involved i n the final network structure. Further efforts involved a study of the effect of alkyl substituants on the MPD aromatic ring. We have chosen methyl, ethyl, isopropyl, and tert-butvl substituents. Not only are the branched alkyl derivatives easier to synthesize, but also i t was felt that straight-chain substituents, e.g., η-butyl, might lower the Tg of the cured epoxy resin unfavorably. It i s known that with longer n-alkyl groups, the Tg of methacrylate homopolymers is lowered substantially due to internal plasticization (12); however i t has been reported that i n both free radically (13) and anionically (14) prepared poly (alkyl methacrylates), replacing the n-alkyl group with the corresponding iso or tert-alkyl group shifted the glass transition back to higher temperatures. Because stoichiometry plays an important role i n the network formation of epoxy resins, there was some concern that large alkyl groups on the ring might prevent f u l l reaction of the diamine with four equivalents of epoxy. The presence of bulky groups i n the v i c i n i t y of the amine group are known to preferentially slow the reaction rate of the secondary amine hydrogen r e l a t i v e to the primary amine hydrogen. In their molecular model studies, Morgan et a l (15). have found that the methyl group adjacent to the amine group i n an aliphatic polyether triamine would produce sufficient steric interference to significantly and preferentially slow the rate of reaction of the secondary amine with epoxide. Thus i t appeared possible that a theoretically stoichiometrically balanced system might i n fact be deficient i n amine, and the highest crosslink density might only be obtained f o r amine-rich systems. To test this possibility, epoxy resin was cured with varying amounts of amine, on either side of stoichiometry, and the Tg (DSC) of the cured resin was taken to be a direct measure of crosslink density. Figure 1 shows the effect of varying stoichiometry on the Tg of cured resins. Note that except for TDA, a l l of the diamines show a maximum crosslink density at 1:1 stoichiometry. Thus i t i s felt that steric bulk of the ring substituent has very l i t t l e effect on the f i n a l extent of cure obtained under these conditions. As w i l l be discussed below, TDA tends to also yield anomalous behavior i n other respects. Although Tg i s an acceptable indicator of crosslink density when compared using the same curing agent, i t may be unwise to attempt to correlate Tg and crosslink density among several different curing agents. The trend of the data i n Figure 1 i s apparent, however. DATBB yields the highest Tg resins, followed


14.

STOREY ET AL.

Epoxy Resins Cured with m-Phenylene Diamines Table I

PROPERTIES OF EPOXY RESINS CURED WITH MPD/QPD BLENDS


Curing Agent

Epoxy: Amine

DSC T&,°C

TGA Td,°C

T.S. ρβΙ,χΙΟ"

146 112 160

376 375 378

11.9 8.2 8.8

5.74 2.84 4.02

2.8 3.4 2.9

93

-

10.3

3.3

3.4

1 1 1 :1

168 170

--

10.9 8.8

3.9 2.9

3.8 3.4

1 :1

167

-

8.8

4.2

2.9

1:1

168

-

8.3

3.7

3.2

1:1

167

-

5.2

1.9

3.0

1:1

152

-

7.6

3.3

3.0

STANDARD CURE 1:1 MPD 1:0.75 MPD 1:1 MPD/OPD 80/20 MPD/OPD 1 0.75 80/20 HT CURE MPD MPD/OPD 80/20 MPD/OPD 80/30 MPD/OPD 50/50 MPD/OPD 20/80 OPD

Figure 1 - Effect temperature (DSC).

3

of stoichiometry

Elonga- Modulus tion,* ρβΙ,χΙΟ '

on glass

transition


187

188

CROSS-LINKED POLYMERS


by MPD itself and then DAIPB > DAEB > TDA. Except for TDA, which shows anomalous behavior, this ranking of Tg's i s perfectly c o n s i s t e n t with, f o r example, the family of poly (4alkylstyrenes) (16) as shown i n Table I I . Thus i t appears that alkyl groups on the aromatic ring create two conflicting effects. The group increases the bulk of the aromatic ring and thus decreases the mobility of that chain segment to which i t i s attached. This increases Tg and i s the predominate effect for the rigid tert-tautyl group. The presence of the alkyl group also creates free volume by decreasing the packing efficiency of neighboring molecules. This effect decreases Tg and appears to be the predominate effect for the less balky isopropyl and ethyl substituants. The anomalous behavior of methyl-substituted TDA escapes satisfactory explanation at this time.

TABLE II Glass Transition Températures of Poly (4-Alkyl Styrenes)

-(CH-CH)2

R

R -tert-butyl -H -CH3

-iso-propyl -CH2CH3

403 373 366, 374 (conflicting data) 360 300,

Properties of Epoxy Resins Cured with Ring-Alkylated m

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