epoxy composites

Interfacial strength measurements in poly(p-phenylene benzobisthiazole)/epoxy composites J. Scherf*, Y. Cohen t and H.D. Wagner* (*The Wetzmann Institute of Sclence/*Technion, Israel) Received 20 May 1 992; accepted in rewsed form 11 September 1992

A mlcromechanical study of the behaviour of poly(p-phenylene benzobisthiazole) (PBT)/ epoxy interfaces is performed using two testing procedures, the droplet micropull-out test, and a version of the fragmentation test in which the stress and strain are continuously monitored by optical microscopy and video recording. Since very few mterfacial strength data are currently available for PBT/epoxy composites, it is the purpose of our study to generate such data for this system. The fragmentation phenomenon in PBT/epoxy Is found to be complex and more difficult to interpret than in brittle fibre composite systems, due to the fibrillatton failure mode of the PBT fibre. The interracial shear strength value based on the fragmentation test is 17.3 MPa, approximately twice the value measured with the droplet micropull-out test.

Key words: adheston; mterfacml shear strength; droplet m~cropull-out test; fragmentation test; poly(p-phenylene benzobisthiazole) fibres; epoxy resin

The importance of the chemical nature and of the resulting micromechanical performance of interfaces m fibre-reinforced composites has been stressed repeatedly over the years. Moreover, novel interfaces are being created by the mere combination of newly developed fibres and matrices As an example, the stress transfer charactenst~cs of the mterfacml zone between poly(p-phenylene benzobisthiazole) (PBT), a rigid-rod polymer with repeat unit as in Fig. 1, and epoxy resins has not yet been the subject of pubhshed research, to the best of our knowledge*. High modulus, high strength, thermally stable PBT fibres may be prepared from amsotroplc solutions of lyotropic liquid crystalline PaT by a dry-.let wet spinning process 1-3 Heat treatment under tension provides significant increases in the longitudinal stiffness and strength The structural and mechanical properties of as-spun (no heat treatment) and heat-treated PBT fibres and films • A recent conference paper on th~s sublect was presented by R.J_ Young and P P. Ang =n Internattonal Phenomena sn Compostte Materials "91 ed=ted by I Verpoest and F R. Jones (Burterworth-Hememann. Oxford. 1991) p 49

have been extensively stud~ed 4-~3 In this article we intend to present prehminary data on the micromechamcal behaviour of the PaT/epoxy interface using two types of testing configuration, the droplet micropull-out test and an in-house version of the single-fibre fragmentation test

Experimental details A F T E C H - I I PBT fibres, obtained from the Polymer Branch of the Wright Research and Development Center ('USA), were characterized by spectroscopic and mechanical means as follows X-ray diffraction data were produced using a Philips dfffractometer wath monochromated CuK radiation The fibres were cut into lengths of about 15 m m and carefully disposed parallel to each other on glass microscope shdes previously coated with vaseline The diffraction patterns were found to be similar to those found by other workers 5 for PBT fibres, i e., the spacings of the major equatorial diffracuon peaks were 0.587, 0.356

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251

B

Table 1. Measured data for the tensile properties of single PBT fibres {at gauge length of 2 0 mm) Young's modulus, Er(GPa) --

Fig. 1

Strength (Weibull parameters) Scale, Shape, ~(GPa) fl

f/

Repeat umt of the PBT molecule

157.7 Present work: L = 2 0 mm (CV = 0 12)

and 0 319 nm, thus very close to the values of 0.594, 0.354 and 0.320 nm in Reference 5 Tensile modulus and strength of single PBT fibres were measured at an elongation rate of 0.05 m m rain -1. which corresponds to a strain rate of (about) 0 004% s The modulus was calculated from the overall displacement of the single fibres. The fibre diameters were measured by optical microscopy prior to testing. The nominal fibre gauge length was 20 m m m all cases. The fibres were glued to paper frames with a room-temperature-curing epoxy adhesive and mounted m an Instron universal testing instrument (floor model "IT). The edges of the frames were cut just before starting the test. Twenty seven samples were tested and the results for the strength were analysed using the Weibull distribution. A m a x i m u m likelihood (ML) estimation procedure t4-~6 was used to determine the parameters of the Weibull distribution from the strength data The results for the strength and the modulus are summarized in Fig 2 and Table 1

[]

N = 27 o. = 2 8 GPo

13 = 6 0

0

i

o

5-2

0

-4

-2 Fig 2

252

-I

0 Ln (strength/GPo)

I

Weibull plot for (non heat-Created) PBT fibres

INT.J.ADHESION AND ADHESIVES OCTOBER 1 9 9 2

Reference 17. L = 10 mm L = 100 mm

2 76

6 01

2.12 1.95

8.59 6.35

CV ts the coeff=ment of varlatton 100 samples were tested at each length m Reference 17, and 27 samples were tested m the present work

Strength data are found to compare satxsfactonly with unpublished results by Netraval117 Two m~cromechanical methods were used to measure the resistance in shear of the PBX/epoxy interface. (1) the fibre/droplet composite pull-out test~S: and (2) a continuously monitored version of the single fibre composite (SFC) test (which we term CM-SFC), recently developed ~n our laboratory The advantage of the latter test is the wealth of information that is made available, especially when the test is performed In continuous mode using video microscopy ~9-2~ or acoustic emission 22- 23 The fibre/droplet composite pull-out test was performed as follows A (100-250pro long) epoxy droplet was carefully spread, and cured, onto single PBT fibres which had previously been put under a pretension of I-3% of the average fibre stren~h Roughened aluminium tabs were fastened at one end of the fibre/droplet assembly, and clamped in a mini tensile testing apparatus 09-20 The pull-out test was then performed by pulling the fibre through a 30pro circular hole punched (by laser drilling) through a metal disc The reason for using a circular hole rather than parallel plates (a more common loading configuration) was that, based on recent work bv H a a k s m a and Cehelnlk 24, a circular hole might'apply more uniform loads on to the epoxy droplet than parallel plates Results currently produced in our laboratory should shed more light on this issue The force applied by the disc on the droplet increased until debonding between the droplet and the fibre occurred, accompanied by a sudden j u m p of the droplet along the fibre and a corresponding drop m the force/displacement trace. The non-zero, quasiconstant force measured beyond the drop corresponds to post-debond frictional forces at the fibre/droplet interface The CM-$FC tests were conducted as follows. Epoxy films containing a single fibre were prepared according to a procedure which is now routinely used in our laboratory and described in our previous works 19-2t The Young's modulus of the fibre was 158 GPa, according to Table 1, and its nominal diameter was 16.1 pro. The matrix was D E R 331 from Dow Chemical Company. a bisphenoI-A based (DGEBA) liquid epoxy resin with an average epoxy equivalent weight (EEW) of 186-192, mixed with the cunng agent D E H 26.

t e t r a e t h y l e n e p e n t a m l n e (TEPA). with an a m i n e h}drogen eqm,,alent ',`.mght of 27 0 The Young's m o d u l u s of the cured matrix ns 1 25 G P a Prior to e m b e d m e n t w~thm the epox3 matrix, each stngle PBT fibre was put u n d e r a small, controlled, pre-tensson for a d e q u a t e straightening, by fastening notched fishermens" lead beads at both ends of the fibre The pre-tensmn was a p p r o x i m a t e l y I 5% o f the average ultimate fibre strength. C u n n g . film forming a n d s a m p l e cutting was p e r f o r m e d a c c o r d i n g to the p r o c e d u r e d e s c n b e d prevlousl} 21 The r e s u h m g single fibre c o m p o s i t e samples had typical cross-sectional d i m e n s i o n s of 3 1 X 0 100 mm-. and a gauge length of 20 m m M e c h a n i c a l testing was carried out using a c u s t o m - m a d e , c o m p u t e r - c o n t r o l l e d , tensile tester fitted to a stereozoom m~croscope possessing video-recording capabnhD' The d e f o r m a t i o n rate was 30-40,um mLn -1 Fracture events were followed a n d c o u n t e d from re:drime pictures on the colour video m o n i t o r The rising stress/strain curve, at `.,,ell as a magnified digital version of both the strcs,, a n d tile strain, also a p p e a r e d on the screen

Results and discussion Droplet micropull-out tests Ten s a m p l e s were prepared antt tested a c c o r d i n g to the p r o c e d u r e described above_ The length of the droplets was m the range o f 300-450#m. and the load at which d e b o n d l n g of the droplet from the fibre occurred w,as wp~call} 15-20 g We a s s u m e d that the droplet length was large enough so that we could calculate the shear strength b} s i m p l y dt,,ldmg the load at the onset of d e b o n d l n g by the contact area Thus assumes that the lnterfacial shear stress ,s constant along the e m b e d d e d fibre length, although thus us p r o b a b l y not the case This a s s u m p t i o n is made here because it is difficult Ln practice to prec~sel.~ m a p the lnterfacual s h e a r stress for the d r o p l e t s c o n s u d e r e d in such tests We found t h a t t l l e a ; e r a g e shear strength was 8 10 MPa. with a coefficient o f , , a n a t m n of 13 5°. The results arc deta,led in Table 2

CM-SFC tests Tile Welbull shape and scale p a r a m e t e r s of the strength distribution of the PBT fibre were calculated from the CM-SFC tests using the p r o c e d u r e d e s c n b e d in our r e c e n t v, o r k i9 "_.i. rather than from extens,ve s~ngle fibre testing Key aspects o f this p r o c e d u r e are as follows The hnk betv, een the a p p l i e d stress o a (v~huch is the output Information in our e x p e n m e n t a l set-up) and the fibre stress c*t is a s s u m e d to be given by al = (El/Era)or,, (where Er a n d E m are the fibre a n d matrix moduh, respectively), which ns satnsfactoD' at small strains, and as long as strain c o n t i n u i t y at the fibre/matrix interface is m a i n t a i n e d At larger strains. the n u m b e r of fibre fragments increases significantly and the fragment length L is progressively reduced Thus. the effective fibre m o d u l u s us also reduced :s. but s,nce at larger strains the m o d u l u s o f the m a t n x us also progressnvel} reduced, due to inelastic behavlour, the rano Ef/Em ,.',as a s s u m e d not to vary significantly as the strain increased, and the above link between the

Table 2

Data from the droplet micropull-out test

Sample

Fibre diameter, Df (,urn)

Droplet length, L (um)

I nterfacnal shear strength, rd(MPa)

PBT-6 PBT-11L PBT-11R PBT-12L PBT-17 PBT-18 PBT-19 PBT-21 PBT-22 PBT-23

17 83 15 35 15 84 1584 16 83 15 84 16 34 14 85 15 84 14 85

439 433 390 311 488 396 432 439 311 494

7 66 8 82 9 35 834 7 60 7 15 9 49 6 42 6 98 9 23

Average CV

1594 006

4135 0 15

2 1 4 I 0 5 1 2 1 1

8 10 0 14

a p p h c d stress a n d the fibre stress ',',as c o n s i d e r e d to remaLn ,,alld at progre,,sixely s m a l l e r fragment lengths This allows tile Welbull s h a p e a n d scale p a r a m e t e r s lot strength,/3 and a to be calculated from a 'qngle f r a g m e n t a t m n test rather than froin the con~,entuonal. extensive, testing of _,,ln,.zle fibres at xau~ous ,.zauee lengths Is-~6 Indeed. Lf&e assume that. a t s m a l l ~ s t r a ] n s ( t h u s a~`.av from the s a t u r a n o n Intuit). the W e [ b u l l ' weakest link model apphe,~ to fragmer~ted fibres of v a d o u s l e n g t h s , then as shov, n prevLousl} t ' ~ : l a plot of I n ( / ) . where [ is the axeragc fragment length. against In(aa) },elds a straight hne ,alth slope equal to uranus tile fibre Welbull ,,hape p a r a m e t e r / 3 Tile fibre Welbull scale p a r a m e t e r o can /her1 be calculated from the value of the intercept, v, htch ,s equal to ,B[Ina + lnF(l + 1',8)]. v, here F( ) is the G a m m a fmlctlOll Fig 3 us a bpncal plot of the results from a single CM-SFC test Two regions are clearlx defined: (I) the linear range (symbol r--I) ~ h e r e the fragments are few and do not interact V,lth each other, and the fragmentatmn process us completely r a n d o m , a n d (2) the saturation range (s}mbol X). where manx fragments are present and new breaks c a n n o t occur wlthLI1 the VlClnlt', of e a r l i e r ones and thus the process us no longer r a n d o m The l i n e a r region only was used to calculate the We,bull s h a p e and scale parameters. using the p r o c e d u r e abo,.e This LS a valid p r o c e d u r e as long as one views the f r a g m e n t a t m n test as a chain o f nndeperldent tensile tests, v, hich is a fairly good a p p r o x i m a t i o n as long as the s a t u r a t i o n limit is not approached_ The h n e a r part of the plot in Fig 3 is therefore equivalent to classical linear In(strength) vs In(length) plots o b t a i n e d from fibre strength data at se`.'eral lengths, a n d described by Weibull/x~eakest hnk statustucs Once the fibre strength vs fragment length data are avadable, ut LS a s~mple matter to extrapolate the fibre strength at the s a t u r a n o n length, a n d then to calculate the interface s h e a r streneth by m e a n s of the K e l l y Tyson formula 27 rKT l~DroriL*)/2L*. `.~here K --" 0 75, D t zs the fibre diameter, o-r(L* ) is the stremzth of a fibre fragment at the s a t u r a t m n length L*. calculated from =

INT J ADHESION AND ADHESIVES OCTOBER 1992

253

4

PBT (NI } 3 -

?L-E .g;

E

E

,_

Fig 4 Scanning electron mlcrograph O+ fractured section of PST/el:x)x~ sample which reveals the ex~enstve hbrdla~lon ,nth4=flbr= X X X

xx -I

-2

70

I 75

I

I

80 B5 90 Ln ( hbre stress/MPa)

I

1

95

I0

F=g. 3 Ln-ln plot of the mean fragment length against fibre stress The least-squares regress=on line through zhe data described by an open square symbol gives r 2 = 0 9 4 6 !

ar = ctL(-i/l~)F(1 + ~)

(1)

using a and fl obtained as previously explained. Before presenting the results, it ts necessary to emphasize that the fragmentation phenomenon in PBT/ epoxy, as observed here by means of polarized light microscopy, is far from being unambiguous (unlike the situation for carbon/epoxy samples, for example) since the PBT fibre exhibits a non-brittle failure mode, characterized by extensive fibrillation (see Fig 4) The fragmentation of PBT fibres in epoxy is illustrated in Fig. 5, which shows atyptcal birefringence pattern at fibre breaks. As seen. it is difficult to exactly identi~ the fragment ends, although it appears quite clearly that there are strong zones of light around a relatively small number of sites and weaker (more 'wavy') zones of light around a much larger number of sites. The larger light patterns may probably be associated with fibre breaks whereas the weaker light areas probably result from stress concentrations at the fibre surface, reduced by fibrillation_ In view of this, m Table 3 we present the results for a,/3 and rKr based on the inclusion of all sites, on the one hand. and based on the breaks sites (from whtch only larger light zones emanated) on the other hand As expected the two calculation methods yield similar shape and scale parameters for the fibre strength, whereas the interface shear strength is much lower

254

INT.J.ADHESlON AND ADHESIVES OCTOBER 1992

b

".

i

Fig 5 la) BIrefrlngence patterns ~n fragmented PBT/epoxy sample {b} Schematic dlustrat=on of the fragmentation pattern =n PBT/epoxy composites Two populations of defect occur, as reflected by the different polarized hght signatures- breaks through the hbre bulk produce larger hght zones (B) whereas stress Concentrations Jnduced by f]brJllatJon at the fibre surface produce smaller light spots (A)

when only the smaller number of large fragments is taken into account However. this ~s still well above the value of ra obtained from droplet micropull-out tests (Table 2) This discrepancy be~veen the results from both tests has been observed in the past in our laboratory with carbon/epox3 composites. Additional evidence of thts discrepancy is available from other sources 28. Contributing factors to the relatively lower numbers obtained with the droplet m~cropull-out test might be the following • in the fragmentation samples the polymenzmg matnx reduces more pressure (because of the much larger of amount of polymer m dogbone-shaped samples, or because of mechanical pressure apphed to the mould dunng curing of tbm samples -~°) than m the mlcropull-out samples.

Table 3.

Data from the C M - S F C test

Sample

Weibull parameters for fibre strength Scale, e(GPa)

Shape, ,B

PBT-C 1 PBT-D1 PBT-D2 PBT-K1 N PBT-M3N PBT-N 1 PBT-N2N

4.13 3.51 3.74 3.67 3.95 4.28 4.72

10 92 5 65 5.66 6.01 6.69 11.68 5.04

Average CV

4.00 0.10

7.38 0 37

PBT-C1B PBT-D1B PBT-G2B PBT-K1B PBT-M3B PBT-N1B

4 67 3 52 5.90 4 36 4.03 5 10

10.04 9.07 3 52 4.26 9.54 8 54

Average CV

4 60 0.18

7.50 0.38

Number of fragments at saturation, Nfs

Interracial shear strength, rKT(M Pa)

52 40 26" 38 29 44 35

69.4 44.0 29.3 40.8 33.7 57 2 49.3

37 7 0.24

46.24 0.30

23 14 12 14 14 10

30 7 13 5 16.6 14 4 15 4 13.1

14.5 0.31

17.30 0 39

All fragments

Large fragments only

*Sample that did not reach saturatton

• upon the apphcation of the stress the mfimte matrix

volume m the fragmentation test induces a stress field In the vicinity of the fibre/matrix interface that is close to pure shear, whereas the stress reduced m a droplet by the apphcation of force includes both shear and compression (due to Hertzian contact loads), • free surfaces, or three-phase contact lines, exzst m the mlcropull-out samples, that induce larger stress concentrations and lower the debonding forces Further work is necessary to resolve th~s issue For the znterpretat~on of results from micropull-out tests the reader is referred to in-depth analyses of Penn and Lee 29 and P~ggott3°. Note finally that Young and Ang (see footnote) found a value of 40 GPa for the mterfacial shear strength of as-spun PBT fibres in epoxy using R a m a n spectroscopy. This value hes between the values obtained here by fragmentauon (see Table 3) using either all fragments or the large fragments only However, Young and Ang used as-spun fibres, as opposed to the heat-treated A F T E C H - I I fibres used here

difficult to interpret than m bnttle fibre composite systems, due to the extensive fibrillation fadure of the PBT fibre. We found that even sf only the lengths of fibre fragments from whsch strong zones of polarized light emanated are included m the calculations, the interfacial shear strength value based on the fragmentation test is about twace the value found from the droplet micropull-out test.

References 1

2 3 4 5 6 7 8

Conclusions

A study of the micromechanical behavlour of PBT/ epoxy interfaces was conducted w~th the purpose of generating data for the interfacml shear strength of this system. Two methods were employed, the droplet m~cropull-out test and an m-house version of the fragmentatson test. We found that the fragmentation phenomenon in PBT/epoxy is much more complex and

9 10 11 12 13

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Authors J. Scherf and H.D. Wagner are in the Department of Materials and Interfaces, The Weizmann Institute of Science, Rehovot 76100, Israel Y. Cohen is in the Department of Chemical Engineering at Technlon - Israel Institute of Technology, Haifa 32000, Israel Enquiries should be directed to H D Wagner