Two-Phase Interpenetrating Epoxy Thermosets That Contain

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26 Two-Phase Interpenetrating Epoxy Thermosets That Contain Epoxidized Triglyceride Oils Part II. Applications Isabelle Frischinger , Patrick Muturi , and Stoil Dirlikov 1

2

1

Eastern Michigan University, Coatings Research Institute, 122 Sill Hall, Ypsilanti, MI 48197 Kenya Industrial Research and Development Institute, Nairobi, Kenya 1

2

Two potential applications of two-phase interpenetrating epoxy thermosets based on diglycidyl ether of bisphenol A (DGEBA) epoxy resin, commercial diamine, and epoxidized vegetable oils are described. The applications are toughening commercial epoxy thermosets and preparation of crack-resistant coatings. Thermosets with DGEBA matrix and small "vegetable" rubbery particles have excellent toughness and other physicomechanical properties. Two-phase epoxies, especially those with continuous "vegetable" rubbery phase and small rigid DGEBA particles, form stress- and crack-resistant coatings. This in situ approach for preparation of two-phase coatings is an attractive alternative to the hard core-soft shell and soft core—hard shell reactive latex technology. Epoxidized soybean oil, which is commercially available at a low price, is especially suitable for these applications.

T W O - P H A S E INTERPENETRATING EPOXY THERMOSETS are formed, under cer

tain conditions, from homogeneous mixtures of diglycidyl ether of bisphenol A ( D G E B A ) epoxy resins and commercial diamines that contain epoxidized vegetable oils (1). Morphology, particle size, and phase inversion depend on the nature of the epoxidized vegetable oil and the diamine used for prepara tion. The two-phase epoxy thermosets, especially those based on the commer cially available inexpensive epoxidized soybean oil ($0.53 per pound), are very 0065-2393/94/0239-0539$06.00/0 © 1994 American Chemical Society

In Interpenetrating Polymer Networks; Klempner, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

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N ITERPENETRATN IG POLYMER NETWORKS

attractive for different industrial applications. In the present chapter, two potential applications—toughening commercial epoxy thermosets and prepa ration of crack-resistant epoxy coatings—are described.


Experimental Details The initial materials as well as the preparation of liquid rubbers, based on epoxidized vegetable oils and final two-phase interpenetrating thermosets, have been described in detail in the previous chapter (I). The epoxy morphology and transition temperatures were determined by scanning electron microscopy (SEM) and dynamic mechanical analysis (DMA), respectively, as also described in the previous chapter (I). Fracture Toughness. Single-edge-notched (SEN) specimens with approx imate dimensions of 100 X 12.7 X 6.7 mm were machined from castings (2). A sharp crack was introduced into the specimen by the strike of a razor blade (previously chilled in liquid nitrogen) with a rubber mallet. The tests were carried out with a three-point bending assembly, which was monitored by a servohydraulic materials testing machine (Instron 1331) with a span of 50.8 mm and a piston rate of 2.54 mm/s. A computer interface controlled the machine and recorded the data. A computer (Hewlett-Packard model 310) was programmed to calculate the critical stress intensity factor, K , using the relation (3) I c

2PS\/^

where Ρ is the critical load for crack propagation (newtons), S is the length of the span (millimeters), a is the crack length (millimeters), w is the width (millime ters), t is the thickness (millimeters), and Y is the nondimensional shape factor given by Y = 1.9 - 3.07U/u>) + 14.53(a/wf

- 25.1l(a/wf

+ 25.80(a»

4

The following relationship, which holds in case of linear-elastic-fracture mechanics ( L E F M ) under plane strain conditions, was used for determination of the fracture energy, G : I c

(1 -

v XK f 2

lc

where ν is the Poisson ratio and Ε is the elastic or Young's modulus. At least eight specimens of each formulation were used for determination of average fracture energy G . I c

Uniaxial Tensile Test. Dog bone bars with dimensions of 6 X 0.5 X 1/8 in. (152 X 12.7 X 3.5 mm) were cut with a high speed router and their external surface was polished with very fine (220 grit) aluminum oxide sandpaper (3M). A


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541


screw-driven tensile instrument (Instron model 1185), equipped with an extensometer for determination of the longitudinal strain and a computer interface type 4500 series, was used at a stroke of 60 mm/min. At least 10 specimens of each formulation were used for determination of their average tensile properties at room temperature. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) measurements were carried out on a thermal analyzer (DuPont 2100 instrument with DSC model 2910) over the temperature range from —130 to + 200 °C at a heating rate of 2 °C/min. Water Absorption. Maximum water absorption was determined on rect angular specimens with approximate dimensions oi 45 X 7.8 X 3.5 mm that were predried in a vacuum oven at 60 °C to constant weight and then kept in boiling water for 3 to 4 weeks until saturation (i.e., to constant weight). Dielectric Properties. Dielectric constant and dissipation factor were measured at 1 MHz and room temperature on specimens with 3 X 3 X 1/8 in. dimensions on an instrument for electrical measurements (Genrad Digibridge model 1687 Β ) equipped with an L D - 3 cell and by using the two-fluid method [air and DC-200, 1 cS (centistoke)]. Sodium and Chlorine Content. Sodium and chlorine content were deter mined by Galbraith Analytical Laboratories, Inc. (Knoxville, TN). Paint Research Associates (Ypsilanti, M l ) confirmed sodium content by analysis with an atomic absorption spectrophotometer (Perkin-Elmer 2380). Coatings Preparation and Characterization. The coatings were applied as 3-mil-thick wet films on cold rolled steel panels and baked in an oven, first at 75 °C for 4 h and then at 150 °C for 2 h. Coatings properties were characterized at room temperature by A S T M testing methods as follows: adhesion (D3359-87), pencil hardness (D3363-74), rocker hardness (D2134-66), flexibility (D4145-83), and impact strength (D2794-84). Coatings that failed in reverse impact strength tests were used as S E M specimens. Broken pieces of these coatings were mounted on a holder and the fracture surface of their cross-sections was observed via scanning electron microscopy (SEM).

Procedures and Formulations The initial materials—DGEBA, 4,4'-diaminodiphenylmethane ( D D M ) , 4,4'diaminodiphenylsulfone (DDS), vernonia oil, and epoxidized soybean o i l — were described in detail in the previous chapter (1), together with the procedures for preparation of hquid rubbers and the final cure of two-phase thermosets. Three types of formulations have been evaluated in the present study: D G E B A - D D M - V R ( x ) , D G E B A - D D M - E S R ( x ) , and D G E B A - D D S ESR(x). Vernonia (VR) and epoxidized soybean (ESR) liquid rubbers were prepared from D D M and the corresponding oils. The D G E B A ~ D D M - V R ( x ) thermosets were obtained from initial homogeneous stoichiometric mixtures


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N I TERPENETRATING POLYMER NETWORKS

of D G E B A and D D M modified by vernonia rubber. The content of the liquid rubber (x) was varied between 0 and 100 wt%: χ = 0 corresponds to the neat D G E B A - D D M thermoset, whereas χ = 100 is pure vernonia rubber. Analogous abbreviations are used for the other two types of formula tions.


Toughening Commercial Epoxy Resins A small amount of discrete rubbery particles with an average size of several micrometers and randomly distributed in a glassy, brittle epoxy thermoset is known to dissipate a part of the impact energy. This energy dissipation improves crack and impact resistance without significant deterioration of other properties (4, 5). Epoxy toughness is usually achieved by separation of a rubbery phase with a unimodal particle size distribution from the matrix during the curing process. Different reactive liquid rubbers, based on low molecular weight carboxy- or amino-terminated oligomers of butadiene and acrylonitrile ( C T B N and A T B N ) , are usually used for the formation of the rubbery phase. Low molecular weight amino-terminated (methyl) siloxanes offer other alterna tives. However, some of these oligomers are quite expensive. Epoxidized vegetable oils, such as vernonia, epoxidized soybean, and linseed oils, provide new opportunities. Epoxidized vegetable oils react with commercial diamines to form epoxy resins that are elastomers at room temperature with low glass-transition temperatures in the range of — 70 to 0 °C, dependent on the nature of the amine used for curing. Epoxidized soybean rubber, therefore, was evaluated for toughening commercial epoxy resins. For this purpose, D G E B A - D D M and D G E B A - D D S formulations were toughened by the addition of 10, 15, 20, and 30% epoxidized soybean liquid rubber. The resulting thermosets consist of a rigid D G E B A matrix and small "soybean" rubbery particles (Table I). The morphology of these ther mosets was discussed in detail in the previous chapter (1). Glass-Transition Temperature. The glass-transition temperatures (T ) were determined by dynamic mechanical analysis ( D M A ) from the temperature dependence (maximum) of their tan delta values (Table II). The glass-transition temperatures of the neat D G E B A - D D M and D G E B A - D D S thermosets are observed at 190 and 185 °C, respectively. Both thermosets also exhibit beta relaxations at lower temperatures of —30 and —35 °C, respectively. The pure epoxidized soybean rubber (ESR), cured under the same conditions in the absence of D G E B A - d i a m i n e component, shows a glass-transition temperature of —25 °C by D S C . The two-phase D G E B A - D D M - E S R and D G E B A - D D S - E S R formula tions exhibit two transitions at higher and lower temperatures. The higher temperature transitions in the range of 145-190 °C have been assigned to the T of their D G E B A matrices. Τ gradually decreases with an increasing g

g


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Table I. Average Range of "Soybean" Particle Size Distribution of the Rubber-Modified DGEBA-DDM-ESR and DGEBA-DDS-ESR Thermosets


Thermoset Formulation

Particle Size ( μητ)

DGEBA- DDM-ESR(IO) DGEBA--DDM-ESRU5) DGEBA- DDM-ESR(20) DGEBA- DDM-ESR(30) D G E B A - DDS-ESR(10) D G E B A - DDS-ESR(20) D G E B A - -DDS-ESR(30) DGEBA- -DDS-ESRGO)^ F L

a

no phase separation 0.1-0.5 0.1-0.4 0.1-0.8 1-2 1-5 1-17 5-10, 100-200

Tensile specimens. Fracture toughness specimens.

Table II. DMA Lower and Higher Temperature Transitions of Pure DGEBA-DDM and DGEBA-DDS Thermosets and Their Rubber-Modified DGEBA-DDM-ESR and DGEBA-DDS-ESR Formulations Thermoset Formulation DGEBA-DDM DGEBA-DDM-ESR(15) D G E B A - D D M - E S R(20) DGEBA-DDM-ESR(30) DGEBA-DDS DGEBA-DDS-ESR(IO) DGEBA-DDS-ESR(20) DGEBA-DDS-ESR(30) ESR C

Transition (° C) Overlap

0

-30 -35 -33 -39 -35 -45 -43 -30 -25

DGEBA

b

190 161 165 145 185 168 166 161

The lower transition temperature corresponds to an overlap in the glass-transition temperature of the rubbery particles and the beta transition of the D G E B A matrix. Glass-transition temperature of the D G E B A matrix. Determined by DSC.

a

c

amount of soybean liquid rubber due to plasticization of the D G E B A rigid matrix. As previously discussed (1, 2), the nonpolar soybean rubber has higher solubility in less polar D G E B A - D D M than in the more polar D G E B A - D D S matrix. The plasticization phenomenon, therefore, is more pronounced for the D G E B A - D D M - E S R formulations. The depression of the glass-transition temperature of their matrices is about 25 °C at 15-20% soybean rubber content, and almost 45 °C at 30% loading. The less miscible D G E B A - D D S - E S R formulations exhibit smaller depressions: 15 °C at 10% soybean hquid rubber to approximately 25 °C at 30% loading.


INTERPENETRATING POLYMER NETWORKS


544

The lower temperature transitions of the D G E B A - D D M - E S R and D G E B A - D D S - E S R formulations, observed in the range of - 3 0 to - 4 5 °C, are assigned to an overlap of the glass-transition temperatures of the E S R rubbery particles and the beta transition of their matrix (2). In general, the glass-transition temperature of the rubbery particles i n the two-phase ther mosets corresponds to the glass-transition temperature of the pure E S R rubber. This result indicates that the rubber phase in these two-phase thermosets does not contain a significant amount of the D G E B A component and is formed by practically pure soybean rubber. Fracture Toughness. The fracture toughness, in terms of stress intensity factor, K , and fracture energy, G , is given in Table III. The fracture surfaces of both formulations D G E B A - D D M - E S R and D G E B A D D S - E S R show extensive particle cavitation (I), which is the main toughen ing mechanism. A better toughening effect, which gradually increases at a higher soybean rubber content, is observed for the D G E B A - D D M - E S R formulations. This effect is probably due to the plasticization phenomenon that occurs on a larger scale for these formulations (I). The soybean rubber, which acts as a matrix plasticizer, improves toughness. A smaller improvement of the fracture toughness and an optimum toughening effect at 2 0 % soybean rubber is observed for the D G E B A - D D S - E S R formulations. The D G E B A - D D S - E S R ( 3 0 ) specimens at 30% soybean liquid rubber, which were used here for determination of I c

I c

Table III. Stress Intensity Factor, K and Fracture Energy, G , of Pure DGEBA-DDM and DGEBA-DDS Thermosets and Their Rubber-Modified DGEBA-DDM-ESR and DGEBA-DDS-ESR Formulations lc9

I c


K (MPa · m )

(J/m )

0.75

175

DGEBA-DDM-ESR(15)

1.24

564

DGEBA-DDM-ESR(20)

1.33

802

DGEBA-DDM-ESR(30)

1.40

1008

DGEBA-DDS

0.75

DGEBA-DDS-ESR(10)

1.09

145 374

DGEBA-DDS-ESR(20)

1.19

596

DGEBA-DDS-ESR(30)

0.54

145

Ic

1/2

D G E B A - D D M DGEBA-DDM-ESR(10)

DGEBA-DDS

£*lc

2

A

162

F O

DGEBA-DDS-CTBN(10)

È

242

This formulation does not phase separate due to better miscibility to its two phases (I). Its fracture toughness has not been determined. Reference 6.

a



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Epoxy Thermosets: Part II

545

fracture toughness, have a bimodal particle size distribution that is character istic for the beginning of phase inversion phenomena (I). At this point the physicomechanical properties start to deteriorate rapidly and lower fracture toughness is observed. In summary, the soybean liquid rubber significantly improves the frac ture toughness of commercial highly cross-linked, brittle D G E B A - D D S and D G E B A - D D M epoxy thermosets. This toughening effect is due to both rubber toughening as a result of particle formation and enhanced ductility of the matrix through plasticization (I). Tensile Properties. Young's moduli were determined from the cor responding tensile stress-strain curves as an average value of several inde pendent measurements (Table IV). In contrast to fracture toughness, where D G E B A - D D S - E S R ( 3 0 ) specimens with a bimodal particle size distribution were used, all tensile specimens used in this study had a unimodal distribu tion (Table I). Reproduction of the exact morphology of D G E B A D D S - E S R ( 3 0 ) is difficult because this formulation is very close to its phase inversion point and small deviation of the experimental conditions leads to different morphologies. As expected, the elastic moduli gradually decreased with increasing soybean fraction. Linear relationships can approximately fit for both types of D G E B A - D D M - E S R and D G E B A - D D S - E S R formulations, and a slight deviation is observed only at a higher content of soybean rubber. The tensile properties of the D G E B A - D D S - E S R formulations, however, decrease faster with increasing soybean rubber content than do the tensile properties of DGEBA-DDM-ESR. Table IV. Young's Modulus, E, of Pure DGEBA-DDM and DGEBA-DDS Thermosets and Their Rubber-Modified DGEBA-DDM-ESR and DGEBA-DDS-ESR Formulations Ε (MPa)

Thermoset Formulation DGEBA-DDM DGEBA-DDM-ESR(10) DGEBA-DDM-ESRQ5) DGEBA-DDM-ESR(20) DGEBA-DDM-ESR(30) DGEBA-DDS DGEBA-DDS-ESR(IO) DGEBA-DDS-ESR(20) DGEBA-DDS-ESR(30) DGEBA-DDS DGEBA-DDS-CTBN(10)

2840

A

F E

F C

2410 1950 1720 3420 2810 2100 1780 3360 3000

This formulation does not phase separate and its tensile properties have not been determined. Reference 6.

a



546


Water Absorption. High water absorption is another major disad vantage of commercial epoxy resins in addition to their poor fracture tough ness. Maximum water absorption was determined by placing predried sam ples in boiling water until constant weight was achieved. Although the results are somewhat lower than expected, the water absorption of both formulations decreases gradually and linearly with increasing soybean content (Table V). Obviously, the highly hydrophobic long fatty chains of the epoxidized soybean oil reduce the water absorption of its epoxy thermosets. As expected, the D G E B A - D D S - E S R formulations at different soybean liquid rubber con tents have a higher water absorption than the corresponding D G E B A D D M - E S R formulations, due to the more polar character of D D S in comparison to D D M . Dielectric Properties. The introduction of epoxidized soybean oil, with its long aliphatic chains, was expected to lower simultaneously the dielectric constant and dissipation factor of the commercial epoxy resins, despite the formation of free hydroxyl groups by the opening of its epoxy rings. Surprisingly, the dielectric properties of both types of formulations do not change with increasing content of soybean liquid rubber (Table VI). Sodium and Chlorine Content. A requirement for the application of epoxy resins in the electronics industry is low ionic content (often below 10 ppm), especially for sodium and chlorine ions. Electronics-grade epoxy resins are more expensive due to the additional purification procedure needed to lower their ionic content. The epoxidized soybean oil without any purification has a very low (below 5 ppm) content of sodium and chlorine (Table VII) due to its hydrophobic nonpolar molecular structure that lacks free hydroxyl or carboxyl grcaips. Table V. Maximum Water Absorption of Pure DGEBA-DDM and DGEBA-DDS Thermosets and Their Rubber-Modified DGEBA-DDM-ESR and DGEBA-DDS-ESR Formulations Thermoset Formulation DGEBA-DDM DGEBA-DDM-ESRQ5) DGEBA-DDM-ESR(20) DGEBA-DDM-ESR(30) DGEBA-DDS DGEBA-DDS-ESR(10) DGEBA-DDS-ESR(20) DGEBA-DDS-ESR(30)

Water Absorption (%)

2.42 2.22 2.00 1.62 3.61 3.33 3.13 2.80


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Table VI. Dielectrical Properties of Pure DGEBA-DDM and DGEBA-DDS Thermosets and Their Rubber-Modified DGEBA-DDM-ESR and DGEBA-DDS-ESR Formulations



Dissipation Factor

Dielectrical Constant

0.033 0.035 0.036 0.037 0.025 0.033

3.78 3.78 3.77 3.77 3.95 3.88

DGEBA-DDM DGEBA-DDM-ESR(IS) DGEBA-DDM-ESR(20) DGEBA-DDM-ESR(30) DGEBA-DDS DGEBA-DDS-ESR(20)

Table VII. Ionic Content and Bulk Price of Epoxidized Soybean Oil (ESO), Vernonia Oil (VO), and CTBN

Sodium (ppm) Chlorine (ppm) Price ($/lb) a

ESO

VO

4.2 < 5.0 0.53

4.3 906 b

CTBN

a

380 952 2-2.50

Hycar (CTBN) 1300 X 16 (BFGoodrich Chemical Co.). Not industrially produced.

Thus, epoxidized soybean rubber is especially suitable for toughening epoxy resins for electronics applications. Epoxidized Soybean L i q u i d Rubber versus C T B N . The effect of the epoxidized soybean liquid rubber was compared with the effect of C T B N , which is commonly used for epoxy toughening in commercial-scale applications. The introduction of 10% C T B N liquid rubber into a D G E B A - D D S epoxy resin produced similar two-phase thermosets with C T B N rubbery particles (about 5 urn). The fracture energy of the unmodified D G E B A - D D S thermoset increased from G = 162 J/m to G = 242 J/m in the presence of 10% C T B N with A G = 80 J/m , whereas the Young's modulus simultaneously decreased from Ε = 3360 M P a to Ε = 3000 MPa (6). Our unmodified D G E B A - D D S thermoset exhibited similar frac ture toughness ( G = 145 J/m ) and Young's modulus ( E = 3420 MPa) (Tables III and IV). The introduction of 10% soybean liquid rubber, how ever, more than doubled the fracture energy from G = 145 J/m to G = 374 J/m (Table III) with A G = 229 J/m , whereas the Young's modulus decreased from Ε = 3420 M P a to a slightly lower value of Ε = 2810 MPa (Table IV). The soybean liquid rubber appears to have a better toughening effect than C T B N . 2

I c

2

I c

2

I c

I c

2

I c

2

I c

I c

2


2


548


Another advantage of the epoxidized soybean rubber is its low sodium and chlorine content (an important characteristic for electronics applications) in contrast to the carboxy-terminated C T B N (Table VII). The epoxidized soybean oil does not have carbon-carbon double bonds, and IR epoxy thermosets are expected to have better weatherability and thermal stability than C T B N - m o d i f i e d epoxies. Each C T B N butadiene unit has one carbon-carbon double bond that spontaneously cross-links after prolonged storage and U V radiation or exposure to higher temperatures. Finally, epoxidized soybean oil is industrially produced and available at about $0.50 per pound, a price below that of C T B N (in the range of $2.00-2.50 per pound). The price difference makes the toughening of commercial epoxy resins by epoxidized soybean oil very attractive for com mercial apphcations on a large scale.

Stress-Resistant Two-Phase Epoxy Coatings Two-phase epoxy thermosets are attractive for preparation of stress-resistant coatings as well. To evaluate the morphology and physicomechanical proper ties of two-phase thermosets, model two-phase interpenetrating epoxy coat ings based on a model D G E B A - D D M stoichiometric formulation modified by the addition of 5-, 10-, 15-, 20-, 30-, and 50-wt% vernonia liquid rubber were studied (Table VIII). Vernonia rubber was selected in this research because it contains carbon-carbon double bonds and stains with osmium tetroxide. This stamability enabled us to easily identify the vernonia phase in the final two-phase coatings. In contrast, soybean rubber does not have carbon-carbon double bonds and does not stain with O s 0 . The addition of 10 or 15% vernonia rubber [ D G E B A - D D M - V R ( 1 0 ) and D G E B A - D D M - V R ( 1 5 ) ] results in two-phase coatings that consist of a rigid D G E B A matrix and randomly distributed small vernonia rubber parti cles with a diameter ranging from 0.2 to 1.2 urn. The small particles are barely observed in the coatings cross section at low magnification (Figure 1), but are distinguished clearly at higher magnification (Figure 2), especially after staining with O s 0 (Figure 3). As expected these coatings exhibit improved impact strength and flexibility without significant deterioration of hardness and adhesion in comparison to the unmodified D G E B A - D D M coatings. At higher magnification, most of the rubbery particles of the D G E B A - D D M - V R ( I O ) coatings have cavitated (black holes in the particles, Figures 2 and 3), which is a major epoxy toughening mechanism. 4

4

In a way, these in situ two-phase epoxy coatings with lower content of vernonia rubber ( < 20%) resemble coatings obtained by hard shell-soft core reactive latex technology. In both cases, the coatings consist of a continuous hard rigid phase (matrix) and small soft rubbery particles.


DGEBA-DDM

Homogeneous 70

50

e

Ι Ο

«5 160 > 160

I

VR

xibility ο

D

Adhesii

FRISCHINGER ET AL.

DGEBA-DDM-VR(5) — 70 40 DGEBA-DDM-VR(IO) 0.3-1.2 90 80 DGEBA-DDM-VR(15) 0.2-1.0 150 150 DGEBA-DDM-VR(20) 0.5-47^ 150 160 DGE Β A- DDM - VR(30) 1-27^ > 160 > 160 DGEBA-DDM-VR(SO) 0.4-1.0 » 160 » 160

(μπι)

5»

Hardness PartickSize Impact Resistance" Rocker Pencil Direct Reverse \

WD

Coatinr Formulation

1

.g

Table VIII. Morphology and Physicomechanical Properf iesof DGEBA--DDM- VU Coat


26.


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PQ Κ ffiffiffiffiffiffi

oo oo m in co ΛΛ

ο co in ο oo

r—4 r—ι CO ^ M CD

00


549


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INTERPENETRATING POLYMER NETWORKS

Figure 1. SEM micrograph of the fracture surface of a DGEBA-DDM-VR(lO) coating cross-section with lower and upper edges ( X1100).

Figure 2. SEM micrograph of the fracture surface of a coating cross-section ( X 3000).

DGEBA-DDM~VR(10)



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Figure 3. SEM micrograph using a backscattered electron imaging technique of the fracture surface of a DGEBA-DDM-VR(lO) coating cross-section stained with osmium tetroxide ( X 5900).

The addition of 20% vernonia rubber leads to the beginning of phase inversion observed in D G E B A - D D M - V R ( 2 0 ) . Figure 4 shows very large vernonia rubbery particles with diameters from 10 to 50 urn, which is comparable to the coating thickness (about 50 urn). These particles have a hghter shade after staining with O s 0 (Figure 5), and they contain smaller and darker rigid D G E B A particles with average diameters from 0.5 to 1.0 um. Further increase of rubber results in complete phase inversion, and coatings that contain more than 30% vernonia rubber consist of a continuous vernonia rubbery phase that contains small rigid D G E B A particles. The particle diameters of the D G E B A - D D M - V R ( 5 0 ) coatings vary from 0.4 to 1.0 um and it is difficult to distinguish them in the coatings cross section at low magnification (Figure 6). Coatings morphology is observed better at higher magnification (Figure 7). Coating failure occurs in the continuous rubbery phase and the rigid particles are not well distinguished; obviously, they are covered with vernonia rubber. The rigid particles, however, are clearly observed after staining with O s 0 using a back scattered electron imaging technique (Figure 8). These coatings have excellent flexibility, impact 4

4



552


Figure 4. SEM micrograph of the fracture surface of a DGEBA-DDM-VR(20) coating cross-section with lower and upper edges.

Figure 5. SEM micrograph using a backscattered electron imaging technique of the fracture surface of a DGEBA-DDM-VR(20) coating cross-section with lower and upper edges stained with osmium tetroxide.

strength, and adhesion due to the continuous rubbery phase (Table VIII), and they also have much better hardness, due to the rigid particles, in comparison to the homogeneous one-phase coatings with the same composition prepared directly from vernonia oil (unpublished results). Remember that the introduc tion of vernonia oil to D G E B A - D D M formulations leads to homogeneous



FRISCHINGER ET AL.


Figure 6. SEM micrograph of the fracture surface of a DGEBA~DDM~VR(50) coating cross-section with lower and upper edges ( X1000).

Figure 7. SEM micrograph of the fracture surface of a DGEBA-DDM ~VR(50) coating cross-section ( X 5000). In Interpenetrating Polymer Networks; Klempner, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

553



554

Figure 8. SEM micrograph using a backscattered electron imaging technique of the fracture surface of a DGEBA-DDM~VR(50) coating cross-section stained with osmium tetroxide ( X 5000). one-phase thermosets and coatings in contrast to vernonia rubber that produces two-phase materials (1). These stress- and crack-resistant two-phase coatings with a continuous rubbery phase are suitable for coil and especially can (container) coatings that undergo deformations after the coating application. The coatings with higher content of vernonia rubber ( > 30%) resemble coatings obtained by hard core-soft shell reactive latex technology. In both cases, the coatings consist of a continuous soft rubbery phase (matrix) and small hard particles. The neat D G E B A - D D M formulation and the neat vernonia rubber form homogeneous one-phase coatings. The properties of these one-phase coatings are given in Table VIII for comparison.

Discussion Initial results show that two-phase interpenetrating epoxy coatings based on epoxidized soybean rubber can be obtained. Further research indicates that the introduction of vernonia oil in other formulations such as urethanes (a



26.

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mixture of methylenediphenylene diisocyanate and 1,10-decanediol) also re sults in two-phase thermosets under similar conditions (Dirlikov, S.; Muturi, P., unpublished results). In general, initial homogeneous formulations based on two components with different polarity and different reactivity and a suitable curing agent are expected to undergo phase separation by two-stage (condensation) polymer ization. In the foregoing formulations, the commercial epoxies (and urethanes) are polar components with very fast cure rates. As these materials cure at lower temperature (first stage), they form the rigid phase. At this stage, the hydrophobic epoxidized vegetable oil (or rubber), which is the nonpolar component and has a slow cure rate, phase separates and forms a liquid phase. The liquid phase cures later at a higher temperature (second stage) with the unreacted curing agent (diamine, etc.) to form the rubbery phase. Similar two-phase thermosets have been prepared via two-stage addition polymerization as reported recently by Derrough et al. (7) for two-compo nent formulations based on methyl methacrylate (or butyl acrylate) and diallyl carbonate of bisphenol A .

Summary and Conclusions Epoxidized vegetable oils and especially epoxidized soybean oil, which is commercially available at a low price ($0.50 per pound), are very attractive for preparation of two-phase interpenetrating epoxy thermosets. Two poten tial industrial applications have been evaluated: toughening commercial epoxy resins and preparation of stress-resistant coatings. Thermosets with D G E B A matrix and "soybean" rubbery particles have excellent toughness without deterioration of other physicomechanical proper ties. Epoxidized soybean liquid rubber has a better toughening effect, lower ionic content, and lower price than C T B N s . Stress- and crack-resistant two-phase epoxy coatings, especially those with a continuous rubbery phase and rigid D G E B A particles, are suitable for coil and container coatings. This in situ approach for preparation of coatings with two-phase morphology is an attractive alternative to hard core-soft shell and soft core-hard shell reactive latex technology.

Acknowledgment The authors thank South Coast Air Quality Management District and the U.S. Agency for International Development for financial support. Dielectric prop erties were studied by D . Shimp at Hi-Tek Polymers, Inc.


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References 1. Frischinger, I.; Dirlikov, S. This volume, Chapter 25. 2. Frischinger, I.; Dirlikov, S. In Toughened Plastics: Science and Engineering; Riew, K. C.; Kinloch, A. J., Eds.; Advances in Chemistry 233; American Chemical Society: Washington, DC, 1992; pp 451-489. 3. Brown, W. F. Srawley, J. E. ASTM STP 1965, 381, 13. 4. Riew, K. C.; Gillham, J. K., Eds.; Rubber-Modified Thermoset Resins; Advances in Chemistry 208; American Chemical Society: Washington, DC, 1984. 5. Riew, K. C., Ed.; Rubber-Toughened Plastics; Advances in Chemistry 222; Ameri can Chemical Society: Washington, DC, 1989. 6. Pearson, R. Α.; Yee, A. F. J. Mater. Sci. 1989, 24, 2571. 7. Derrough, S. N.; Rouf, C.; Widmaier, J. M.; Meyer, G. C. Polym. Mater. Sci. Eng. 1991, 65, 1-2.


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RECEIVED for review February ber 16, 1 9 9 2 .

11, 1992.

ACCEPTED revised manuscript Decem


Two-Phase Interpenetrating Epoxy Thermosets That Contain

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