The Toughening of Epoxy Resins with Reactive Polybutadienes

3 The Toughening of Epoxy Resins with Reactive Polybutadienes K A R E L DUŠEK , FRANTIŠEK LEDNICKÝ , STANISLAV LUŇÁK , M I L O S L A V MACH , and D A G M A R DUSKOVÁ 1

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Downloaded by STANFORD UNIV GREEN LIBR on March 17, 2013 | http://pubs.acs.org Publication Date: December 5, 1984 | doi: 10.1021/ba-1984-0208.ch003

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Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 162 06 Prague 6, Czechoslovakia Research Institute for Synthetic Resins and Lacquers, Pardubice, Czechoslovakia Research Institute of Production Cooperatives, Prague 5, Czechoslovakia

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The modification of epoxy resins with carboxyl-terminated and hydroxyl-terminated liquid polybutadienes is described. The effect of this modification on the toughening of cured resins is compared with the effect of carboxyl-terminated butadiene-acrylonitrile copolymers (CTBN). Carboxyl-terminated polybutadienes (CTPB) are less compatible with the resin than CTBNs; resins modified with a prereacted CTPB of molecular weight ~2000 separate into two phases at room temperature, but may form a homogeneous solution at the elevated temperatures used in curing with anhydrides. The miscibility of the polybutadienes (PBD) was increased by attaching to the hydroxyl-terminated polybutadiene (HTPB) or CTPB polyester end-blocks formed by an in situ reaction of tetrahydrophthalic anhydride and phenyl glycidyl ether. The toughness of cured resins modified with prereacted CTPBs or PBDs with polyester end-blocks was comparable to those modified with CTBNs, although their morphology was usually much finer and had diffuse phase-separated regions.

OARBOXYL-TERMINATED BUTADIENE—ACRYLONITRILE ( C T B N ) Copolymers have been used most often for improving fracture properties of epoxy resins (1—4). T h e existence of phase-separated rubber particles 1 0 10 n m i n diameter is believed to be the necessary condition for a substantial increase i n fracture energy. T h e modified resins should be stable prior to curing over an extended period of time. T h e twophase structure should develop during polymerization at a time w h e n the system is viscous enough to prevent macroscopic phase separa -tion, b u t still below the gel point. If these conditions are not met, the chemical network w o u l d prevent the rubber phase from forming particles of a desirable size (5). 2

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0065-2393/84/0208-0027/$06.00/0 © 1984 American Chemical Society

In Rubber-Modified Thermoset Resins; Riew, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.


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RUBBER-MODIFIED THERMOSET RESINS

F o r some purposes, however, the acrylonitrile (AN) copolymers are not suitable. M o n o m e r i c A N is carcinogenic and butadiene-ac r y l o n i t r i l e ( B - A N ) c o p o l y m e r s always c o n t a i n traces of the A N monomer. Existing regulations do not permit the use of this material in products coming into direct or indirect contact with the human body. Also, when making l i q u i d rubbers by anionic polymerization, A N cannot be used as a comonomer. F o r these reasons, an investi gation of a modification using telechelic P B D s is of interest. N o sys tematic study of this modification has been carried out as yet, ob viously because of the general lack of compatibility of P B D and epoxy resins. In this chapter, we report our results on the modification of epoxy resins of diglycidyl ether of bisphenol A ( D G E B A ) with liquid P B D s . Their miscibility with the resins was increased (a) by prereaction of the carboxyl groups with epoxy groups of the resins and (b) by at taching to the O H or C O O H groups of the P B D s a polyester block formed by an i n situ reaction of monoepoxide and cyclic anhydride. Experimental Resins and Rubbers. Several commerical low molecular weight epoxy resins of the D G E B A type containing 5.07-5.28 mmol/g epoxy groups and 0.150.40% CI were used. The HYCARs (PBD) are commercial products of The BFGoodrieh Company. The development samples of HTPB (BH) were prepared by free radical polymerization using functional initiators. The development sam ples of CTPB (LBH) and HTPB (LBC), research products of the Research In stitute for Synthetic Rubber (Kralupy, Czechoslovakia), were prepared by an ionic polymerization. Their characteristics are given in Table I. Polybutadienes with Polyester End-blocks. HTPB or CTPB was reacted with hexahydrophthalic anhydride (HHPA) and phenyl glycidyl ether (PGE) under specific catalysis with Cr(III) complexes (see Reactions 1 and 2).

Table I. Characteristics of Liquid Rubbers

Rubber HYCAR C T B N 1300X8 L B C 264 L B C 266 L B C 272E HYCAR HTB 2000X166 BH L B H 283E

Reactive Groups

AN (%)

COOH COOH COOH COOH

19 0 0 0 0 0 0

OH OH OH

c (mmol/g)

Microstructure (%) M

f

1.4

1.2

0.53 1.17 0.79 0.91

3200 1550 2200 1980

1.7 1.9 1.7 1.8

88.0 31.2 33.9 38.6

12.0 68.8 66.1 61.4

0.41 1.03 0.77

3800 2200 2150

1.6 2.3 1.7

79.0 44.0

—


—

21.0 56.0

3.

DUSEK ET AL.

Toughening of Epoxy Resins

AA/vPB/vv\OH

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+ (x+1)

OCH2CHCH2

—

\ / O


AAAPB /WNQ

CO

COOCH2CHO

OCH

COOH

2

Reaction 1 A M P B /WVCOOH

+

OCH CHCH

(^j)

2

x HHPA + (x-l)PGE

AMPB AMCOOCH2CHCH I OH /VVlPB

OCH CHOCO

/VNACO

2

2

OCH

CO

OH

2

Reaction 2 Typically, the HTPBs were stirred with HHPA and the Cr catalyst ATC-3 (Cor dova Chemical, Sacramento, CA), and the temperature was raised to 60-80 °C. After 20 min, P G E was added, and the mixture was kept at 80 °C for 4 h. The CTPBs were first reacted with an equimolar quantity of P G E (catalyst AMC-2) at 80 °C for 2 h. HHPA and possibly P G E were then added, and the mixture was stirred at 80 C for another 2 h. The block copolymers are listed in Table II. The copolymers were diluted with the epoxy resin, and the mixture was stirred at 80 °C for 2 h; the HYCAR HTB 2000X166 was stirred at 100 °C. Prereaction. All carboxyl-terminated rubbers were prereacted with the epoxy resins either in the absence of a catalyst or in the presence of Cr(III) catalyst AMC-2. In the absence of the catalyst, the components were reacted for 2 h at 150 °C. (6). In the presence of the Cr(III) catalyst, the reaction time was 2 h at 80 °C. The rubbers with polyester end-blocks terminated with a carboxyl group were also prereacted with the resin. The prereaction was cata lyzed with the C r complexes already present in the system to catalyze the formation of polyester as described in the previous section. Curing and Testing of Cured Resins. Bulk samples were prepared from resins cured with HHPA in presence of 0.5% benzyldimethylamine at 100 °C Q


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Table II. Polybutadiene-Polyester Block Copolymers


Sample Designation BH/P-1 BH/P-2 BH/P-3 HY/P LH/P LC/P

Moles per Group Parent Rubber BH BH BH HYCAR HTB L B H 283E L B C 272E

PGE 0 1 2 2 1 1

HHPA 1 2 3 3 2 1

wt% PBD in Copolymer 88 68 57 77 74 78

for 2 h and at 130 °C for 16 h in Teflon-plated molds. Standard methods were used for determination of thermomechanical properties. Fracture toughness was measured in three-point bend tests on single-notched specimens (7, 8). Curing with diethylenetriamine at room temperature was used for deter mination of lap shear strength. The surfaces of an aluminium alloy (Dural) were first degreased with acetone and then pre-etched with acid chromate solution. The curing took place at room temperature for 5 days. Results and

Discussion

In this preliminary study, we have attempted to answer the questions:

following

1. C a n prereacted C T P B form stable solutions in the resin at room temperature? 2. If these solutions phase separate at room temperature, can they be used i n high-temperature curing? 3. H o w long should the polyester end-block be to secure the solubility of P B D in the resin? 4. H o w much does the toughening efficiency differ from that produced with C T B N ? 5. Is the morphology the same as for C T B N - m o d i f i e d and cured resins, and is the presence of segregated spherical rubber particles a necessary condition for toughening? The choice of functional rubbers as far as the variations in com position are concerned was rather limited. Samples with a sufficient range of molecular weights were represented by anionically poly merized P B D with a rather high vinyl content and a narrow molecular weight distribution. The samples prepared by free radical polymer ization were of different origin and/or the chain terminal units to which the O H or C O O H groups were attached were of different composition. In addition to the rubbers listed in Table I, H Y C A R 2000X162 (higher molecular weight and low vinyl content) and Nisso G - 3 0 0 0 (very high vinyl content) were modified, and their miscibility with the epoxv resin was tested. Stability of the M o d i f i e d Resins. Similarly to B - A N copoly mers (9), the P B D s also exhibit an upper critical solution tempera-



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DUSEK ETAL.

Toughening of Epoxy Resins

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ture, but the immiscibility gap is shifted to considerably higher tem peratures. A l l resins containing 5 - 2 0 % rubber not prereacted with the resin formed two-phase systems; the mixtures separated more or less rapidly into two bulk layers. The prereaction helps, but only the prereacted P B D with the lowest molecular weight (1550) was com patible with the resin at room temperature (Table III). For these anionically polymerized rubbers, the threshold molecular weight for the appearance of phase separation is estimated to be between 1600 and 1900. Attachment of the polyester block helps to increase the compat ibility as is demonstrated by the results of the modification of the B H polybutadiene (Table IV). The m i n i m u m length of the end-block required to produce a stable system is equal to two anhydride and one epoxide unit. The anionically polymerized L B H and L B C rub bers modified by the same procedure behave somewhat untypically in that they are slightly turbid after modification. The turbidity is stable, and no phase separation is observed. Also, the viscosity i n creases m u c h more than it does as a result of modification of the B H rubber. Originally, the turbidity was presumed a result of a high vinyl content. Experiments with the Nisso rubber (much higher molecular weight) disproved this assumption, however, because the modified r u b b e r was clear. T h e p o s s i b i l i t y of i m p u r i t i e s such as r e s i d u a l l i t h i u m that c o u l d catalyze the formation of unattached polyester could be ruled out because of the very low concentration of L i (sev eral parts per million). A reason still to be checked is the relatively narrow distribution of molecular weights. Although the long-term stability of the modified resins is an i m portant factor, its absence does not prevent the rubber from being used in the modification at higher curing temperatures at w h i c h the system is homogeneous again. The prereacted L B C rubbers of moTable III. Appearance and Morphology of Cured Modified Resins 10% Rubber Rubber HYCAR CTBN 1300X8 L B C 264 L B C 266 BH/P-2 BH/P-3 HY/P LC/P LH/P

mr c c t,s c c,h c t,s t,s

anc t,P c,S c,h,S-R c,S-R c,S-R c,S-R c,S-R

20% Rubber amc

—

c,S

—

h,S-R c,h,R c,R h

—

mr c c t,s c c c h,t h,t

amc

anc t,P c,S-R h,F c,S-R

—

c,R c,S-R c,S-R

h

— —

c,h,F c,h,F c h,R

—

Key to abbreviations: mr, modified resin; anc, anhydride cured; amc, amine cured; c, clear; t, turbid; h, hazy; s, separates upon storage into two layers, room temperature. Morphology—P, separated particles; S, smooth surface; F, pronounced fluctuations without a sharp boundary; R, rippled surface.


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Table IV. Effect of Length of the Polyester End-block on the Stability of Modified Resins and Lap Shear Strength (a ) of Resins Cured with DETA at 25 °C s


Rubber Type

Modified Resin Appearance

Unmodified Prereacted BH/P-1

clear turbid turbid

BH/P-2 BH/P-3

hazy clear

a

Stability

Viscosity

Epoxy (mmol/g)

Cured Resin

The Toughening of Epoxy Resins with Reactive Polybutadienes

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