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Vol. 17 No. 3 LIU Ling

t

a/: The Mechanical Properties Of Polyurethane Elasto me r... 51

c h in e . Th e mic ms t r u c tu r e o f te s t s a m p le s wa s in v e s t ig a te d

b y a S c a n n in g E le c t r o n M ic r o s c o p e ( S EM ) . Th e s u r f a c e o f

Ca S O4 wh is k e r d r i e d in v a c u u m w a s d e te r min e d b y in -

f r a re d a b s o r p t io n s p e c t r o s c o p y .

3 R e s u l t s a n d D i s c u s s i o n

3 . 1 E f f e c t o f CAlSO w h i s k e r c o n t en t o n m e c h a n i c a l

p r o p e r t i e s o f p o l y u r e t h a n e e l a s t o m e r

T h e N C O / O H r a ti o ( R ) o f p o l y u re t h a ne e l as t o m e r

wa s t a k e n to b e 2 , 6 , 1 3 a s ty p ic a l v a lu e s d e te r min e d in

th e s tu d y . Th e m e c h a n ic a l p r o p e r t i e s o f Ca S O4 w h is k e r /

p o ly u r e th a n e e la s to me r s p r e p a r e d wi th d i f f e r e n t NCO/OH

a r e s h o wn in F ig s . 1 - 6 . As th e c o n te n t o f Ca S O4 wh is k e r

in c r e a s e d , th e h a r d n e s s a n d r e s i l i e n c e v a lu e s in c r e a s e d ,

then bas ica l ly main ta ined s ta b le . But the tens i le s t rength

and u l t imate e longat ion va lue s we nt through a max imum ,

th e n d e c r e a s e d . Th e s e t a f t e r b r e a k v a lu e o f c o mp o s i t e s

wa s h ig h e r th a n th a t o f th e ma t r ix , y e t in c r e a s e d s l ig h t ly .

Th e r e s u l t s c o u ld b e in te r p r e te d b y th e s t r u c tu r e o f

p o ly u r e th a n e e la s to me r . P o ly u r eth a n e e la s to me r wa s k n o w n

to der ive the ir proper t ies and per formance f rom a phase

s e p a r a te d mo r p ho log y o f h a r d a n d s o f t d o m a in s . Th e p o ly -

me r i t s e l f wa s ma d e u p o f h a r d a n d s o f t s e g me n ts . Ch e mi-

ca l ly , the sof t segments w ere formed by the p olyol or poly-

o l i s o c y a n a te s e q u e n c e s . Th e h a r d s e g me n ts we r e mo r e

r ig id s e q u e n c e s o f i s o c y a n a te a n d e x te n d e r c h a in . Th e s e

ty p e s o f p o ly me r s we r e u s u a l ly r e fe r r e d to a s [ A B] ~ b lo c k

50 - ~ --

---,,- R= 2

--*- R=6

80 - --*- R=13

~ R=I3(KH-550)

4 0 - ~ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ~ , ~ . ~ . _ _ . ~

3 5 -

_ _ . ._ . . . . . - - - - ~

3 0 , _ . . _ - - - - ~ :

-2 6 : 2 J , d ~ lb 1 '2 i4 1 '6

CaS04whiskercontents/%

c o p o ly me r s . Th e e la s to me r ie n a tu r e o f p o ly u r eth a n e e la s -

to me r c a me f r o m th e in c o mp a t ib i l i ty , r e s u lt in g in s e g r e g a -

t io n o f h a r d a n d s o f t s e g me n ts in to s e p a r a te d o ma in s . Th e

h a r d s e g me n t d o ma in s a c te d a s t i e p o in t s o r v i r tu a l

c r o s s l in k f o r th e f l e x ib le s e q u e n c e s to f o r m a n e la s to me r ic

ne twork . The ' hard segm ent domains leve l la rge ly de te r -

min e d th e me c h a n ic a l p r o p e r t i e s o f p o ly u r e th a n e e la s -

to me r . [4 ] Ad d in g Ca S O 4 wh is k e r w i th h ig h mo d u lu s a n d

high s t rength was equiva lent to increase the concentra t ion

o f t h e h a r d s e g m e n t a n d t h e d e n s i ty o f c r o s s - l i n k i n g

p o in t s , wh ic h ma d e th e e x te n t o f mic r o p h a s e s e p a r a t io n

b e in g imp r o v e d , s o th e me c h a n ic a l p r o p e r t i e s o f c o mp o s -

i tes in tend ed towards r is ing . Bu t the d ispers ion of CaSO4

wh is k e r a f f e c te d th e me c h a n ic a l p r o p e r t ie s o f c o mp o s i t e s ,

p a r t i c u la r ly th e s t r e n g th . I f th e c o n te n t o f Ca S O4 wh is k e r

wa s to o h ig h , wh is k e r s e a s i ly a g g r e g a te d , an d we r e n o t w e l l

d i s t r ib u te d . I n th i s c a s e , Ca S O4 wh is k e r c o u ld n o e f f e c t iv e -

ly t r ans fe r the in te rna l s t r ess loca l ly produ ced in the m a-

t r ix u n d e r a lo a d , a n d e a s i ly r e s u l te d in c r a c k o c c u r r in g s o

tha t the s t rength and e longa t ion va lue decreased a f te r go-

ing through a maximum. The resu l ts ind ica te tha t compos-

i t e s w i th 5 % - 1 0 % Ca S O 4 wh is k e r e x h ib i t th e b e s t me -

c h a n ic a l p r o p e r t i e s .

3 . 2

C t ~ O 4

w h i s k e r r e i n f o r c i n g m e c h a n i s m

Th e r e in f o r c e me n t wa s d u e to th a t th e s t r a in r e s u l t e d

f rom the load could be t r ans fe r red f rom matr ix to whisker .

Be c a u s e th e wh is k e r wa s s t r o n g e r a n d h a d a h ig h e r mo d u -

46

4 s

8:

34~

3d

.~ 26

18

t ;

-2

6 : ~ , i ~ ~ f o 1 '2 1 4

1'6

CaSO4 whisker Contents/%

Fig. 1

~ 701

66:

62.

5 8

541

501

Effect of

CaSO

whisker content on P UE resilience

78 .78

741 ~ * ~ 17470 ~

J * R = 6 I

9 x R = 1 3 I

v R=13~q+550)

-2 6 ~ ~ d ~ lb 1~ 1'4 ;6

Ca,SO,whiskercontent/%

Fig. 3

3 5 0 1

250

Effect of CaSO4whisker content on PU E ten sile strength

Ir ' ~

66 ,~ ~ --~--R=2

| ~ - ' ~ R - -6

62 ~ ~ ~ R=13

~ 20 0t q '-R=13(KH-550)

58 ~ 150

154 ~ , ~ ~ ' - - ~

100] _

150 50

-2 d :2 ~, 6 8 10 1'2 1'4 16

CaS04whiskerContents/%

Fig. 2 Effect of CaSO4 whisker content on PU E hardness

Fig .4 Ef fec tof CaSO4 whisker content on PUE elongation at

break

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52 Journal of Wuhan University of Technology-Mater. Sci. Ed. Sept. 2002

60,

50

40.

30

20.

1o

CaSO4 whisker contents/

Fig. 5 Effec tof CaSO whisker content on PUE set after break

lus than the matrix, when a stress was applied to the com-

posites , the w hisker showed much lower strain ca pability,

so a much higher stress was induced in. the whisker than

in the matrix around it. The high stress areas at the tips of

the whiskers were reduced when they fell within the

bounds of the low strain areas of adjacent whisker s. [53

The additive strain reinforcement was devel oped, so the

overall e ffect of CaSO 4 whisker was overwhelmingly rein-

forcing.

3.3 CaSO4whisker toughening mechanism

The CaSO whisker toughening mechanism co uld be

directly investigated by SEM. Different directions of the

whisker toward the fracture surface of the matrix had dif-

ferent toughening mecha nisms. Whe n the angle between

the direction of the whisker and fracture plane was very

small or even paralle l, the toughening mechanism was

crack deflection. Furrows left by whiskers in the matrix

plane (F ig. 7) , which indicated the crack deflection. The

toughening contribution from the crack deflec tion was due

to whiskers altering the c rack propagation path to absorb

the fracture energy. Because whisker modulus was high,

the crack extended to the whisker where the crack must

_ ~ _Undeflected crack ro/~ .

Fig.7 Schematicrepresentation and SEM photograph of crack

deflection mechanism

3. 4 Effect of interracial bond state on mechanical

propert ies of polyurethane elas tomer

Mechanical performance of a filler-reinforced com-

posite was primarily depe ndent upon the effectiveness of

the bonding be tween matrix and filler in transferring stress

across the interface. Bad bonding was the weak link,

200.

180,

t6o.

140,

~ 120

~ 100,

80.

60

4 0

-2 0

-~- R=2

4 R = 6

~ R = t 3

R=13ffKI-I+550)

~ = -

, i , , i i , i i

2 4 6 8 10 12 14 16

CaSO4 whisker Contents/

Fig. 6 Effect of CaSO4 whisker content on PUE tea r strength

deflect so that the crack was redirected out of the former

fracture plane E6~ . The crack deflection increased the tortu-

osity of the crack path where the rod - like whisker pro-

vided an effective shape for deflecting the crack to expand

fracture energy. Because the deflection of the crack plane

was inclined at angles < 90 degrees to the applied tensile

stress axis, an increased applied stress had to be required

to make the crack tip stress intensity sufficient to cause

the crack growth.

When the angle between the directions of the whisker

and fracture plane was large, the toughening mechan ism

was whisker pullout. Fig. 8a shows the transverse section

of a fractured whisker. The Fig. 8b shows leaving pore af-

ter whisker pullout, whisker pullout resulted when the ten-

sile stress transferred to the whisker in the wake of the

matrix crack was less than the fracture strength of the

whisker and the shear stress generated was greater than

the interfacial shear strength of the whisker-matrix inter-

face . The whisker pullout from the matrix expen ded the

crack propagation energy and, therefore, increase the

toughness of the composite.

str ss

n te f f ac ia l

shearstress

Fig. 8 Schematic representation and SEM photograph of

whisker pullout mechanism

which was the limiting factor for composite mechan ical

properties. The data from Fig. 9 show obvious differences

between perfect bending and poor bonding at the inter-

face. As the content of CaSO4 whisker increased, the tear

strength value of the composite with a low NC O/ OH ratio

(R = 2) incre ased,b ut the value of the composite with a

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Vo l.17 No.3 L1U Ling et a/:T he Mechanical Properties Of Polyurethane Elastomer... 53

t

' \ i . I J

Fig.9 SEM photographs of fractured surface

a) R= 2 b) R=13

high NCO/O H ratio (R = 13) decreased. Fig. lOa shows

holes left by pulled whiskers on fracture surface of com-

posite with low NC O/OH ratio ( R = 2) , which belonged

to toughness fracture feature, while Fig. 10b showed brit-

tle fracture feature for the composite with a high NCO/OH

ratio (R = 13). The reason was that the matrix with low

NCO/OH ratio was tough, and could be well bonded to

the CaSO4 whisker, so a load could b e effectively transfer

red from matrix to whisker through the interface to rein-

force and toughen. But the hard segment concentration of

the polyuretha ne incr eased , due to inversion or mixing of

the hard and soft phases, the polyurethanes varied from a

tough elastomefic material to a more brit tle, high-modulus

plastic at about 6 0% wt . hard segment content ~7] . Bec ause

the matrix with a high NCO/OH ratio was brittle (the sys-

tem with R = 13 being approximately 68 % w t. hard seg-

ment content), it couldn't be well bonded to the CaSO4

whisker. In this cas e, the interface turned into weak link

where fracture was easily initiated, when a load was ap-

plied to the composite. After the surface of the CaS04

whisker being treated by KH - 550, a good bonding was

made between whisker and matrix so that stress could be

effectively transferred in all parts of the comp osite from

whisker to whisker across the matrix - whisker interface.

Infra-red spectrum of CaSO4 whisker showes hydroxyl

groups were present on CaSO4 whisker surface. (F ig. 10)

When the whisker surface was treated by silane coupling

agent, the alkoxy groups of silane coupling agent could re-

act c hemically with hydroxyl groups on CaSO4 whisker s ur-

face to make attachment to the whisker by covalent bond.

The coupling agent, in addition, contained other functional

groups, which could core act with the matrix during the

curing process. In this way, the coupling agent acted as

molecular bridge to achieve a measure of chemical

bonding between :whisker and m atrix. The surface treat-

men t cont ribut ed to the fairly improved strength a nd

toughness. Fig. 6 shows the tear strength valu e was in-

crease d, the elongation value had been alm ost doubled

through the surface treatment. Because of the higher elas-

57

5

I I I

4 31 22 13 4

v era )

Fig. 10 Infra-red spectrum of CaSO4whisker

teric modulus of the matrix, the improvement on toughness

was more outstanding than rcinforcement. The great im-

provement in composite properties obtained by modifying

the interface with a trace of coupling agent suggests that

load transferring was the mainly reinforce mechanism.

Conclusions

The reinforcing and toughening of composite were

relative with the dispersity of CaSO4 whisker and the de-

gree of the interfacial interaction. The results indica te that

composites with 5% - 10% CaSO4 whisker exhibited the

best mechanical properties. Good bonding interface was

formed between whisker and matrix after the surface of

CaSO4 whisker being treated by silane coupling agent. The

strength and toughness of comIx~ite were fairly improved

through CaSO4 whisker surf ace treatment The m icroanaly-

ses show that CaS O4 whisker reinforcing mechanism for

polyurethane elastomer mainly was load transferring and

its toughening mechanism involved crack deflection and

whisker pullout.

References

1 Joh n V Milewski.Whiskers and Short Fiber Technology. Po/yer

Compos/tes, 1992,13(3) :223 236

2 Sa lih B, 1L'llkuY, Fikret P, Saim . Effect of Fillers on Thermal

and Mechanical Properties of Polyurethane Elastomer. J App/

Polym

Sc/,1998, 68: 1057- 1065

3 N Yin, M Q Kang, L M Zhang, Y L Feng, X K Wang. Reinforce-

ment o f Carbon Fiber on Polyurethane Elastomer. Syn

Rubber

lnd, 1998,21( 1 ) :4 2 - 44

4 S L Cooper, A V Tobolsky. Properties of Linear Elastomeric

Polyurethanes. J. App/.

Polym.

Sc /., 1966,10:1837 - 1844

5 D M Schuster, E Scala. The Mechanical Interaction of Sapphire

whiskers with a B i- Refringent Matrix. Report, 1963,166:4 - 5

6 K S Mazdiyasni.

Fiber Reinforced Ceramic Composites

USA:

Noyes Publications,1990,321

7 R J Zdrahala, R M Gerkin, S L Hager and F E Critchfield.

Polyether - Based Thermoplastic Polyurethanes( I )Effect of the

Ha rd - Segment Content. J Appl. Polym. Sci. 1979,24:2041

-2050