Glass-Fiber Composites from Polyurethane and Epoxy

Downloaded by UNIV OF GUELPH LIBRARY on September 9, 2012 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0239.ch021

21 Glass-Fiber Composites from Polyurethane and Epoxy Interpenetrating Polymer Networks K. H. Hsieh , S. T. Lee , D. C. Liao , D. W. Wu , and C. C. M. Ma 1

1

1

1

2

Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 300, Taiwan 1

2

Unidirectional glass-fiber-reinforced composites of polyurethane -cross-linked epoxy and polyurethane-epoxy interpenetrating polymer network with polyurethane (PU) grafted on epoxy (graft-IPN) were prepared and their mechanical properties were investigated. The tensile strength of the PU-cross-linked epoxy or the PU-epoxy graft-IPN was significantly increased in the longitudinal direction but decreased in the transverse direction. The effect of PU content in the matrix on the tensile strength of the composites is not significant. The Izod impact strength of these composites was greatly improved by the addition of glassfiberin either the transverse or longitudinal direction. The values of impact strength strongly increased with increasing PU content, especially for transverse direction properties.

THE USE OF FIBER-REINFORCED POLYMERIC COMPOSITES for engineering applications has grown rapidly over the past few decades. Many signs indicate that this trend will continue. The elements that influence the mechanical properties of composites are the fibers and their orientation, the resin, and the interface adhesion between the fibers and the resin. Epoxy resins, which are associated with high modulus and strength, have been employed in 0065-2393/94/0239-0427$06.00/0 © 1994 American Chemical Society

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


428

N ITERPENETRATN IG POLYMER NETWORKS

high-performance structural composites. However, such resins suffer from low fracture strength and brittle behavior (J); therefore, applications are often limited by the low mechanical properties. To meet required end-use performance, the epoxy resin must be modi fied with either thermoplastics (2, 3) or conventional rubber or elastomers. Only recently, interpenetrating polymer networks (IPNs) have been consid ered for such application. The I P N structure is generally a network form with both polymer components cross-linked (4-6). The permanent entanglements and interlocking between the networks reduces the phase separation (for two incompatible polymers) and enhances the mechanical properties and compat ibility (7). In material technology, the term "compatibility" often is used to describe whether desirable or beneficial properties occur when polymer mixtures are blended (8). The two glass transition temperatures of two polymers shift to the intermediate range if the mixture polymers are compati ble (9). In our previous work (10-13), polyurethane-cross-linked epoxy resin based on diglycidyl ether of bisphenol A with epoxy equivalent weight ( E E W ) of 186 (epoxy) and graft-interpenetrating polymer networks of various types of polyurethane (PU) and epoxy were synthesized and their characteristics were investigated. The results showed that epoxy was significantly toughened and exhibits maximum values of tensile strength. The intensity of the loss modulus (E") peaks of PU-cross-linked epoxy increased when the polyestertype PU(PU(PBA)) [where P B A is poly (butylène adipate) glycol] was em ployed. Single-phase morphology was formed in the PU(PBA)-cross-linked epoxy, whereas the epoxy cross-linked with the polyether-type PU(PU(PPG)) [where P P G is poly(propylene oxide) glycol] exhibited a two-phase morphol ogy with the P U particles dispersed in the epoxy matrix. Two distinct E" peaks were observed: one at a high temperature of 107 °C (alpha transition) and the other at a low temperature of — 73 °C (beta transition) for the pure epoxy. For the graft-IPNs based on P U (PBA), the alpha transition peaks shifted to lower temperatures and the intensity of the beta transition peaks decreased. Furthermore, the P U (PBA)-based graft-IPNs showed lower alpha transition temperatures as compared with the P U (PPG)-based graft-IPNs. This difference suggests that the PU(PBA) has better compatibil ity with the alpha transition domain of the epoxy than does the PU(PPG). The morphological micrographs of the PU(PBA)-epoxy graft-IPNs showed a homogeneous system (one phase), whereas the PU(PPG)-epoxy showed a heterogeneous morphology (phase separation) with rubber particles dispersed in the matrix. Interpenetrating polymer networks now are considered for use in composites because of their outstanding matrix toughness. In the present work, the polyurethane-modified epoxy system is em ployed as the composite matrix. The polyurethane-cross-hnked (PU-crosslinked) epoxy and PU-grafted epoxy I P N (graft-IPN) were prepared and


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HSIEH ET AL. Glass-Fiber Composites from IPNs

used as the matrix for the unidirectional glass-fiber composite. The effect of various matrix properties on the mechanical properties of the composites will be investigated and discussed.


Experimental Details Materials. The materials required and their designations are listed in Table I. Polypropylene oxide) glycol with molecular weights 400 (PPG400) and 1000 (PPG1000), and poly(butylene adipate) glycol with molecular weight 1000 (PBA1000) were used as polyols; trimethylolpropane (TMP) and 1,4-butanediol (1,4-BD) were used as the cross-linking agent and chain extender, respectively. 4,4'-Diphenylmethane diisocyanate (MDI) was used as the isocyanate for P U preparation. Diglycidyl ether of bisphenol A with epoxy equivalent weight (EEW) of 186 (epoxy) was employed for this study, and 2,4,6-trisCdimethylaminomethyl)phenol (TDMP) was used as catalyst for the epoxy. Polyols, T M P , 1,4-BD, and epoxy were continuously stirred and degassed overnight at 60 °C in a flask before use. M D I and catalyst T D M P were employed as received. Preparation of Polyurethane Prepolymers. To prepare P U prepolymers, two equivalents of M D I were charged into a reaction kettle and heated until molten at ~55 °C. Then one equivalent of polyol was added and mixed with the molten M D I . The mixture was vigorously agitated by a mechanical stirrer. The reaction took place under dry nitrogen atmosphere at a temperature of 68 °C. When the — N C O content, which was determined by the di-n-butylamine titration method (14), reached the theoretical value, the reaction was stopped.

Table I. Materials Designation PPG400 PPG1000 PBA1000 MDI 1,4-BD TMP Epoxy TDMP Glass fiber DBTDL

Description Poly(propylene oxide) glycol; M W = 400 Polyvpropylene oxide) glycol; M W = 1000 Polytbutylene adipate) glycol; M W = 400 4,4'-Diphenylmethane diisocyanate 1,4-Butanediol Trimethylolpropane Diglycidyl ether of bisphenol A; E E W = 186 2,4,6-Tris(dimethylaminomethyl)phenol Unidirectional Ε-glass fiber Dibutyltin dilaurate

Source Chiun Glong Co. Chiun Glong Co. Tai Chin Chem. Industry Co. Ltd. Bayer Chemical Co. Hayashi Pure Chem. Hayashi Pure Chem. Dow Chemical Co. Merck Chemical Co. Kyntex Inc. Merck Chemical Co.


430


Preparation of PU-Cross-Linked Epoxy. Epoxy was put into a reaction kettle. Dried nitrogen gas was blown into the reaction kettle to remove both the air and the moisture present in the kettle. A suitable amount of P U prepolymer was then poured into the kettle to mix with epoxy. The temperature was maintained at 68 °C. Several drops of dibutyltin dilaurate (DBTDL), approxi mately 0.02 wt% based on the weight of the P U prepolymer, was added and vigorously agitated with the mixture to accelerate the reaction for several hours. Afterward, a sample was taken to detect the — N C O group at 2270 c m " in the infrared (IR) spectrum until the — N C O absorption peak disappeared. The PU-eross-linked epoxy has the following reaction: Downloaded by UNIV OF GUELPH LIBRARY on September 9, 2012 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0239.ch021

1

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JLA

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[PU prepolymer)

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LA

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0

0

LA

LA

NH NH

0

0

C=0 Ô

OH

V PU -crosslinked epoxy resin

TDMP

PU - crosslinked epoxy

To cure the PU-cross-linked epoxy, a suitable amount of PU-crosslinked epoxy was placed in a cup into which 3 phr (parts per hundred parts of resin by weight) of curing agent (TDMP) was added, vigorously stirred, and drafted for several minutes to eliminate all the bubbles produced in the course of agitation. After the bubbles were removed, the reaction mixture was poured into a mold for the preparation of composites. Preparation of PU-Epoxy Graft IPNs. The preparation of PU-epoxy graft-IPNs was carried out in two steps. The first step involved the reaction of an excess of P U prepolymer with epoxy resin in the reaction kettle. Because the concentration of the — N C O groups exceeds the equivalent — O H groups in the


431

HSIEH ET AL.

21. Glass-Fiber Composites from IPNs epoxy resin, the grafting reaction took place at 68 °C as follows: 0

0

/ Ν

,

0

L A

OCN

OH

0

0

+ OCN

ΙΔ

NCO NCO

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(epoxy;) Downloaded by UNIV OF GUELPH LIBRARY on September 9, 2012 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0239.ch021

(

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PU/epoxy graft - IPNs

OH

A sample of the reacting mixture was taken every hour for IR analysis. The ratio of the absorption peak of the — N C O group (2270 c m ) to that of the epoxide group (920 c m " ) decreased during the reaction course and eventually became a constant value. This stabilization indicates that the pendant secondary hydroxyl groups i n the epoxy completely reacted with the — N C O group of the P U prepolymer. The use of excess P U prepolymer resulted in unreacted — N C O groups in the mixture. Thereafter, 2 wt% of T D M P based on the epoxy resin and a stoichiometric amount of 1,4-BD-TMP mixture at an equivalent ratio of 4:1 were added to the mixture. After mechanical agitation and degassing for 30 s, the reaction mixture was poured into a mold for the preparation of composites. - 1

1

Preparation of PU-Epoxy Composites. The composites reinforced by glass fiber were prepared by a hand lay-up fabrication technique in a mold. Eight layers of unidirectional glass fiber were placed in the mold; then the resin mixture was poured uniformly over the reinforcement. After degassing for 5 min, the resin mixture was pressed and cured at 80 °C and the molding pressure was increased to approximately 14 MPa for 2 h. Finally, the samples were postcured at 120 °C for 6 h. Testing Methods. Stress-strain and flexural strength were measured using a tensile tester (Tensilon mode T C F - R C , Yashima Works Ltd., Japan). The test procedure for stress-strain followed ASTM D-638 with a crosshead speed of 10 mrrymin, and at least five specimens were taken for the test. Flexural strength was measured by following A S T M D-790 with a crosshead speed of 1.3 mm/min and a sample size of 80 X 25 X 32 mm. The Izod impact strength of the composites was measured according to A S T M D-256. Dynamic mechanical analysis (DMA) was performed on a D M A unit (DuPont 983) with an operating temperature range from —100 °C to 250 °C. The heating rate was set as 5 K/min. The sample size was approximately 60 X 10 X 2 mm. The mechanical properties measured for all the composite samples in the longitudinal and transverse directions are listed in Tables II through VI.


432


Table II. Tensile Strength of PU-Cross-Linked Epoxy Composites

Sample


PU(PPG400)-epoxy

PU(PPG1000)-epoxy

PU(PBA1000)-epoxy

Tensile Strength (MPa)

(NCO:OH) Equivalent Ratio

PU (wt%)

Neat Resin

0:100 30:100 50:100 80:100 100:100 0:100 30:100 50:100 80:100 100:100 0:100 30:100 50:100 80:100 100:100

0 4 6 10 12 0 6 10 15 19 0 6 10 15 19

55 74 61 66 61 55 58 76 79 70 55 53 70 54 40

Transverse Longitudinal Direction Direction 412 340 337 331 414 412 323 409 313 305 412 394 414 441 443

26 17 29 27 47 26 34 27 25 27 26 27 31 35 40

Table III. Izod Impact Strength of PU-Cross-Linked Epoxy Composites

Sample PU(PPG400)-epoxy

PU(PPG1000)-epoxy

PU(PBA1000)-epoxy

Impact Strength (kj/m)


PU (wt%)

Neat Resin

0:100 30:100 50:100 80:100 100:100 0:100 30:100 50:100 80:100 100:100 0:100 30:100 50:100 80:100 100:100

0 4 6 10 12 0 6 10 15 19 0 6 10 15 19

0.92 1 0.86 0.87 0.9 0.92 0.96 0.92 0.88 0.85 0.92 1.47 1.61 1.54 1.51

Transverse Longitudinal Direction Direction 236 297 426 313 360 236 339 452 343 391 236 360 421 489 349

8 7.85 8.18 8.99 8.32 8 6.26 8.2 11.7 8.57 8 10 10.1 10.2 8.1

Results and Discussion Stress-Strain Properties. As shown in Figures 1 and 2, the tensile strength changes along with increasing P U content in the PU-epoxy matrix. The broken marks on the curves are the PU-epoxy compositions where the pendant hydroxyl group of the epoxy is stoichiometrically equivalent to the


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HSIEH ET AL. Glass-Fiber Composites from IPNs Table IV. Flexural Strength of PU-Cross-Linked Epoxy Composites

Sample

PU (wt%)

Transverse Direction

Longitudinal Direction

0:100 30:100 50:100 80:100 100:100 0:100 30:100 50:100 80:100 100:100 0:100 30:100 50:100 80:100 100:100

0 4 6 10 12 0 6 10 15 19 0 6 10 15 19

513 642 658 564 750 513 481 611 519 526 513 513 731 705 794

52 54 53 48 51 52 48 66 57 58 52 51 56 62 52

PU(PPG400)-epoxy Downloaded by UNIV OF GUELPH LIBRARY on September 9, 2012 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0239.ch021

Flexural Strength (MPa)


PU(PPG1000)-epoxy

PU(PBA1000)-epoxy

Table V. Tensile Strength and Izod Impact Strength of PU-Epoxy Graft-IPN Composites PU Tensile Strength (MPa) Impact Strength (kj/m) Content Neat Transverse Longitudinal Neat Transverse Longitudinal prh wt% Resin Direction Direction Direction Resin Direction

Sample PPG400based

—

0

42

26

412

0.84

236

8

PPG1000based

5 10 15

—

16 19 22 0

72 80 80 42

37 44 38 26

336 341 380 412

1.02 1.09 1.18 0.84

420 418 409 236

8.6 8.6 7.1 8

PBA1000based

5 10 15

—

22 25 27 0

53 54 65 42

38 28 27 26

315 316 301 412

0.92 0.93 0.96 0.84

426 360 344 235

8.2 9 8.1 8

5 10 15

22 25 27

76 80 78

41 40 34

287 329 428

1.23 1.23 1.26

400 400 380

8.2 8.3 11


434


Table VI. Flexural Strength of PU-Epoxy Graft-IPN Composites PU Content phr wt%

Sample

Transverse Direction

Longitudinal Direction

PPG400-

—

0

513

52

PPG1000-

5 10 15 —

16 19 22 0

702 717 721 513

49 59 57 52

PBA1000-

5 10 15 —

22 25 27 0

549 458 444 513

50 40 39 52

5 10 15

22 25 27

747 639 641

60 56 48

based


Flexural Strength (MPa)

based

based

5

10

15

20

25 30

PU content, wt.% Figure 1. Effect of PU(PBAIOOO) content in PU-epoxy matrix on the tensile strength of composites. Key: A, PU(PBAlOOO)-epoxy composites; A, PU(PBAIOQO)-epoxy neat resin.



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5

Κ) 15 20 25 PU content, wt.%

30

Figure 2. Effect of PU(PPG) content in PU-epoxy matrix on the tensile strength of composites. Key: Q PU(PPG400)-epoxy composite; Δ, PU(PPGIOOO)-epoxy composites; A, PU(PPGIOOO)-epoxy neat resin. isocyanate group of the P U prepolymer. If a sufficiently strong fiber-matrix bonding exists for the unidirectional composites, the modulus equation of the composite from the rule of mixture can be derived. However, the fiber should possess a much higher modulus than the matrix and the fiber content is generally controlled at about 6 0 - 6 5 % (9). Therefore, when a longitudinal stress is apphed to a fiber-reinforced composite, the fibers bear the major part of the stress. Thus, the resin matrix transfers the stress and binding between the fiber and the matrix. This transference is well illustrated in Figures 1 and 2, where the longitudinal tensile strength of the composites is much higher than the tensile strength of the neat resin. The longitudinal tensile strength of the PU(PBA1000)-epoxy composites increases rapidly with an increase of P U content in the matrix for the PU-cross-linked epoxy composites and increases significantly for the PU-epoxy graft-IPN composites. This increase indicates that the contribu tion of matrix modification should not be neglected. The cross-hnking and interpenetrating structure in the resin matrix result in an increase of tensile strength of PU-cross-linked epoxy and PU-epoxy graft-IPN matrix as in the previous study (10-13). The cross-hnking reaction between the P U prepoly mer and the pendant hydroxyl group in the epoxy actually contributes to the



436


soft characteristics of the P U as well as to the cross-linking structure of the epoxy. The tensile strength can be increased by the cross-hnking structure (cross-linking effect) and decreased by the introduction of more or longer soft segments of the P U (softening effect). Compared to the tensile strength of the neat resin with its corresponding composites, the longitudinal tensile strength of the PU(PBA1000)-epoxy composite jumps to a high value of 400 M P a when the glass fiber is introduced and then gradually increases, similar to the neat resin, with an increase of P U content in the matrix for the PU-cross-linked epoxy ( P U content below the broken mark) composite as shown in Figure 1. For the PU(PBA)-epoxy matrix, the cross-hnking effect of the epoxy matrix cross-hnked by the PU(PBA) on the longitudinal tensile strength of the composite plays a more important role than the softening effect of the PU(PBA) soft segment. The higher cross-hnking density of the matrix pro vides a stronger tensile strength and adhesion property with the glass fiber of the PU(PBA)-epoxy composite. O n the other hand, as shown in Figure 2, the longitudinal tensile strength decreases with an increase of PU(PPG) content in the matrix for the PU(PPG)-cross-linked epoxy composite, although the higher cross-linking density of the PU(PPG400)-cross-linked epoxy was in the matrix. The morphology from scanning electronic microscopy ( S E M ) in previous work (13) showed that the PU(PPG)-epoxy is a two-phase system with P U ( P P G ) particles dispersed in the epoxy matrix. This observation implies that the morphology of the matrix can significantly change the longitudinal tensile strength of the corresponding glass-fiber composite. A l though the two-phase morphologies are toughened for the neat resin, they tend to decrease the longitudinal tensile strength of the resultant composites. As discussed before, the PU(PBA)-epoxy matrix is a one-phase morphology in S E M (13) and, therefore, results in a great increase of composite strength as expected. The irregular changes in the longitudinal tensile strength of the compos ites as shown in Figure 2 for the PU(PPG)-cross-linked epoxy composites are probably a result of the phase-separated matrix, which might easily interfere with the fibers during preparation. Furthermore, the longitudinal tensile strength of the PU(PPG1000)-epoxy graft-IPN composite is lower than that of the PU(PPG400)-epoxy graft-IPN composite. The longitudinal tensile strength of the PU(PPG1000)-epoxy graft-IPN composites decreases, but that of the PU(PPG400~epoxy graft-IPN composites increases as the P U content increases. This observation points out that the long soft segment of the P U in the polymeric networks will reduce the longitudinal tensile strength of the composite; that is to say, the softened matrix has a lower ability to transfer stress from one fiber to another. The foregoing observation also shows that the lower the molecular weight of polyol in P U , the higher the compatibility between the P U and epoxy. The observed tensile strength in the transverse direction of the PU-cross-linked epoxy or the PU-epoxy graft-IPN composites was lower



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than the tensile strength of the neat resin. Unlike the longitudinal tensile strength, there is no simple relation such as the rule of mixture for predicting the transverse strength of the composites. Generally speaking, many factors affect the transverse tensile strength of composites: the presence and distri bution of voids, the interfacial bonding strength, and the properties of the matrix and the fiber. Reduction of the transverse tensile strength of the composites is probably due to the lesser content of resin in the sample as well as the foregoing factors. As a result, the changing curves of the transverse tensile strength of the composites are dissimilar to the curves of the neat resin as P U content increases in the system. There is no significant change in the transverse tensile strength of the composite as the P U content increases in the matrix for both PU(PBA) and PU(PPG)-epoxy composites (Figures 1 and 2, respectively). Izod Impact Strength. The impact strength of the PU(PBA)-epoxy composites is shown i n Figure 3. The longitudinal impact strength of the composites increases by about 1 order of magnitude, reaches a maximum value in the PU(PBA)-cross-linked region, and increases with increasing PU(PBA) content in the PU(PBA)-epoxy graft-IPN region. The increase of impact strength with increasing P U content is similar to that of the neat resin for the P U content below the break mark. Therefore, the longitudinal impact strength of the composites might result mainly from the bridging effects of the fibers and partially from the resin itself. In the PU(PBA1000)-epoxy

Figure 3. Effect of PU(PBAIOOO) content in PU-epoxy matrix on the impact strength of composites. Key: A, PU(PBA1000)-epoxy composites; A, PU(PBA1000)-epoxy neat resin.



438


graft-IPN region, the longitudinal impact strength increases with increasing PU(PBA) content, especially when the P U content is at 27 wt%. This increase indicates that an interpenetrating network structure with a one-phase matrix actually does improve the longitudinal impact strength of the compos ite. As shown in Figure 4, the longitudinal impact strength of the PU(PPG)-epoxy composites also reaches a maximum value in the PU-crosslinked epoxy region. For the P U content beyond the break mark in PU-epoxy graft-IPN composites, the longitudinal impact strength also increases to a maximum value along with the increase of P U content. This increase might be caused by a better interpénétration network with the soft P U in the matrix as shown in Figures 9 and 12. O n the other hand, for the PU(PPG)-epoxy composites, the short soft segments of PU(PPG) in the networks make a more positive contribution than the long soft segments do. As expected, the impact strength in the transverse direction increases with increasing P U content in the matrix and reaches a maximum value for either PU(PBA)-cross-linked epoxy or PU(PPG)-cross-linked epoxy compos ites. Thus, the toughened matrix resin also evidently leads to composite toughening in the transverse direction. Moreover, the toughening effect on composites from either the one-phase PU(PBA)-epoxy or the less-compatible PU(PPG)-epoxy matrix is more significant than on the neat resin. The transverse Izod impact strength decreases, in some cases, with increasing P U content in the PU(PBA)-epoxy graft-IPN and PU(PPG)-epoxy graft-IPN

•neat resin

5

10

15

20

25

30

PL) content ,ννί.7ο Figure 4. Effect of PU(PPG) content in PU-epoxy matrix on the impact strength of composites. Key: Q PU(PPG400)-epoxy composite; Δ, PU(PPG1000)-epoxy composites; •, PU(PPG1000)-epoxy neat resin.


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systems. This decrease is probably the result of the dominancy of the softening effect over the interpenetrating effect as more P U is introduced into the matrix. Some irregular changes in transverse impact strength of the PU(PPG)-epoxy composites observed in Figure 4 are probably due to the imperfect structure formed from the interference of fibers with the less-com patible PU(PPG)-epoxy matrix during the preparation. Flexural Strength. When flexural strength is measured, the inner surface of the sample is loaded with a compression force and a tension force is placed on the outer surface of the sample. The compression force loaded on the inner surface of the sample results in buckling of the fibers and local enrichment of the resin. Therefore, the flexural strength of the composites is affected by many complicated factors. As shown in Figures 5 and 6, the flexural strength of the composites greatly improved for both the PU(PBA)-cross-linked epoxy and PU(PPG)-cross-linked epoxy systems with the increase of P U content in the matrix. The improvement in the flexural strength of the composite is more effective for the one-phase PU(PBA)-cross-linked epoxy composite as can be seen in Figure 5. The transverse flexural strength increases significantly with an increase of the P U content in the PU(PBA)-cross-linked epoxy composite. As shown in Figure 6,

5

10 15 20 25 30 PU content, wt.%

Figure 5. Effect of PU(PBAIOOO) content in PU-epoxy matrix on the flexural strength of composites.


440



800i

PU content, w t . % Figure 6. Effect of PU(PPG) content in PU-epoxy matrix on the flexur al strength of composites. Key: Q PU(PPG400)-epoxy composites; A, Ρ U(PPG1000) -epoxy composites. enhancement of the strength by the long chain segments of the PU(PPGIOOO) is not as significant as enhancement by the short chain segments of the PU(PPG400) in the PU(PPG)-epoxy composite. The flexural strength of the composites based on the PU(PBA1000)-epoxy graft-IPNs and the P U (PPG1000)-epoxy graft-IPNs decreases significantly as the P U content i n creases in the matrix as shown in Figures 5 and 6. For the composites based on PU(PPG400)-epoxy graft-IPNs, the flexural strength of the composite increases with an increase of PU(PPG400) content in the matrix. This behavior in the flexural strength shows that the softness of the matrix has a dominant effect on the flexural strength of the composites. The two-phase and softened PU(PPG)-epoxy matrix causes a small improvement in flexural strength of the composites. The changes of longitudinal flexural strength with increasing P U content are irregular and probably due to the interference of fibers with the PU-epoxy morphology during preparation. Dynamic Mechanical Properties. The effect of PU(PBA) content in the PU(PBA)-epoxy matrix on the dynamic mechanical properties of the composites is shown in Figures 7 and 8. As shown in Figure 7, the epoxy has transition peaks at 130 °C (alpha transition) and —73 °C (beta transition).


21.

441



1800

Figure 7. Temperature dependence of loss modulus (Έ") for the PU(PBA1000)-cross-linked epoxy composites with various PU(PBA) content (wt%) in the matrix: , 0; ---,6 wt%; ,10 wt%; —- -—- -, 19 wt%. 1800i

-100

-50

0

50

100

150

2 0 0

Temperature [X) Figure 8, Temperature dependence of loss modulus (E") for the PU(PBA1000)-epoxy graft-IPN composites with various PU(PBA) content (wt%) in the matrix: , 0; ---,22 wt%; ,25 wt%; —- -—- -, 27 wt%.



442


Because PU(PBAIOOO) is employed as a cross-linking agent to cross-link the epoxy, the alpha-transition peak shifts to a lower temperature and increases its intensity of loss modulus ( E " ) when the PU(PBAIOOO) content increases. On the other hand, the beta transition peak shifts to a higher temperature and rapidly reduces its E" intensity as the PU(PBAKX)O) cross-links the epoxy. This reaction indicates that the epoxy matrix (alpha-transition domain) is toughened and compatible with the cross-linked PU(PBAIOOO). Further more, the intensity of the beta-transition domain of the epoxy rapidly reduces as the pendant hydroxyl group of the epoxy is cross-linked by PU(PBAIOOO). This response indicates that the beta-transition domain is associated with the motion of the linear epoxy structure, which contains hydroxyl group. F o r the PU(PBA1000)-epoxy graft-IPNs as shown i n Figure 8, the alpha-transition peak and the PU(PBAIOOO) transition peak (at - 3 5 °C) gradually merge when the PU(PBAIOOO) content increases up to 27 w t % . This merger indicates that the structure of the interpénétration, in addition to the grafting, can enhance the compatibility of the two polymers. However, for the PU(PPG)-epoxy system as shown i n Figures 9 and 10, the short P U ( P P G -

1800

Temperature (°C) Figure 9. Temperature dependence of loss modulus (~E") for the PU(PPG400)-cross-linked epoxy composites with various PU(PPG400) content (wt%) in the matrix: , 0; ---,4 wt%; ,10 wt%; —- -—- -, 12 wt%.


21.


443


4800

Temperature (X) Figure 10. Temperature dependence of loss modulus (E") for the PU(PPG400)-epoxy graft-IPN composites with various PU(PPG400) content (wt%) in the matrix: , 0; ---,16 wt%; , 19 wt%; — - — -, 2 2 wt%.

400) soft segment, which acts as a cross-linking agent in the PU(PPG400)-epoxy matrix, is miscible with the epoxy matrix. Smaller shifts in the alpha-transition peak, compared with those in Figure 7, indicate that the PU(PPG400)-eross-linked epoxy matrix is toughened less but has higher cross-hnking than the PU(PBA1000)-cross-linked epoxy matrix. For the PU(PPG400)-epoxy graft-IPN system (Figure 10), the alpha-transition peak of the epoxy shifts to a much lower temperature than the peak in the PU(PPG400)-eross-linked epoxy system. The peak broadens and reduces in intensity as the PU(PPG400) content increases. This phenomenon results from the matrix being grafted and interpenetrated by the cross-linked PU(PPG400) structure that has less compatibility with the epoxy. However, the compatibility is enhanced by the grafting and interpénétration network of the short PPG400 segments in the P U . For the longer PU(PPG1000) soft segment in the PU-epoxy matrix, two distinct transition peaks (alpha and beta) are observed as shown in Figures 11 and 12. The behavior is similar to the behavior of the neat resin in previous work (13). Thus, we believe that a


444



1800r

Temperature Ct) Figure 11. Temperature dependence of loss modulus (E ) for the PU(PPGIOOO)-cross-linked epoxy composites with various PU(PPGIOOO) content (wt%) in the matrix: , 0; ---,6 wt%; ,15 wt%; —- -—- -, 19 wt%. ,f

two-phase matrix is formed in the composite. The increase of PU(PPGIOOO) content in this composite system partially results in an increase in the number of dispersed P U particles in the matrix as found i n the previous study by S E M . Therefore, the small shift of the alpha-transition peak to low tempera tures is observed for the longer PU(PPGIOOO) soft segment introduced into the system (Figures 11 and 12). The foregoing observation implies that the shifts in alpha-transition peak are merely contributed to the epoxy matrix by the grafted structure. This contribution indicates enhanced compatability between the two polymers.

Conclusions The mechanical properties of fiber-reinforced composites can be significantly enhanced through modification of the matrix resin. Moreover, the effect of matrix resin modification on the toughness of the composites is more signifi cant than the effect of the neat resin.


21.

445



ΙΘΟΟι

-100

-50

0

50

100

150

200

Temperature (*t) Figure 12. Temperature dependence of loss modulus (E") for the PU(PPG1000)~epoxy graft-IPN composites with various PU(PPGIOOO) content (wt%) in the matrix: , 0; ---,22 wt%; ,25 wt%; —- -—- -, 27 wt%.

Acknowledgment The authors acknowledge with gratitude the financial support of the National Science Council, Taipei, Taiwan, Republic of China, through Grant NSC780405-E002-07.

References 1. Lee, H . ; Neville, K. Handbook of Epoxy Resin; McGraw-Hill: New York, 1978. 2. Ibrahim A. M . ; Quinlivan, T. J.; Seferis, J. C. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1985, 26, 277. 3. Bucknall, C. B.; Partridge, I. K. Polymer 1983, 24, 639. 4. Ika, P. V.; Frisch, H . L.; Frisch, K. C. J. Polym. Sci., Polym Chem. Ed. 1985, 23, 1163. 5. Kim, S. C.; Klempner, D.; Frisch, K. C.; Radigan, W.; Frisch, H . L . Macro molecules 1976, 9, 258. 6. Klempner, D. Angew. Chem. 1978, 17, 97. 7. Klempner, D; Frisch, K. C. Advances in Interpenetrating Polymer Network; Technomic: Lancaster, PA, 1989; Vol. 1. 8. Paul, D. R.; Newman, S. Polymer Blends; Academic: New York, 1978; Vol. 1, pp 17-18.


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9. Paul, D. R.; Newman, S. Polymer Blends; Academic: New York, 1978; Vol. 2, pp 21-22. 10. Hsieh, K. H.; Han, J. L. J. Polym. Sci., Polym. Phys. Ed. 1990, 28, 623. 11. Hsieh, Κ. H.; Han, J. L. J. Polym. Sci., Polym. Phys. Ed. 1990, 28, 783. 12. Han, J. L.; Tseng, S. M . ; Mai, J. H . ; Hsieh, Κ. H . Angew. Makromol. Chem. 1990, 182, 193. 13. Han, J. L.; Tseng, S. M . ; Mai, J. H . ; Hsieh, Κ. H . Angew. Makromol. Chem. 1990, 184, 89. 14. Kadurind, T. I.; Prokopenko, V. Α.; Omelchenko, S. I. Eur. Polym. J. 1986, 22, 865. RECEIVED for review November 11, 1991. ACCEPTED revised manuscript September 3, 1992.


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