Download - Bf 02838539
-
8/11/2019 Bf 02838539
1/4
-
8/11/2019 Bf 02838539
2/4
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
-
8/11/2019 Bf 02838539
3/4
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
-
8/11/2019 Bf 02838539
4/4
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