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Colloid Polym Sci 274:599-611 (1996)
9 SteinkopffVerlag 1996
B.K. Kim
queou s po lyurethan e dispersions
Received: 17 October 1995
Accepted: 25 January 1996
Dr. B.K. Kim ([E~)
Department of Polymer Science
and Engineering
Pusan National University
Pusan 609-735, Korea
Abstract This review describes basic
chemistry, preparation process, and
physical properties of aqueous
polyurethane dispersions and the
derived films, along with the methods
of post treatment to modify the
properties. Basic way to render
a polyurethane water dispersible
without external emulsifier has been
described. Regarding the methods of
preparation, four major processes are
described and compared. Methods to
improve the relatively poor water and
solvent resistance of aqueous polyure-
thane dispersions which is introduced
by the hydrophilicity and linear
structure of polyurethane have been
discussed with an emphasis on
acrylate incorporations.
n t r o d u c t i o n
An aqueous polyurethane (PU) dispersion is a binary
colloid system in which PU particles are dispersed in a
continuous aqueous rnedium. PU dispersions have been on
the market since the late 1960s and have been commercial-
ly important since the early 1970s. One technical advan-
tage of aqueous dispersion is tha t the viscosity of disper-
sion is normally independent of the molecular weight
of the polymer. Thus, PU dispersion can be prepared
at a high solid content with a molecular weight high
enough to form films with excellent performance solely
by physical drying. This means that film formation can
occur by simple evaporation of water even at ambient
temperature.
Aqueous PU dispersions can be formulated into coat-
ings and adhesives containing little or no cosolvent, and
hence they are nontoxic, nonflammable, and do not pol-
lute the air. Such environmental advantages coupled with
increasing solvent price and the quality of those materials
have steadily expanded their usages in textile coatings,
fiber sizings, and adhesives for a number of polymeric
materials and glass surfaces. Over 1000 patents have been
issued in the last ca. 50 years of their existence. These are
well reviewed and documented in Dieterich [1] and Ros-
thauser and Nachtkamp [-21. A major portion of the newer
patents centers on the modification and tailormaking of
aqueous PU for specific end uses. A variety of aqueous PU
dispersions has in fact been commercialized in many coun-
tries. They are predominantly linear polymers [3]. Modifi-
cation is mainly directed to improve the water and solvent
resistance properties of the derived films, which on the
other hand, is introduced by the very nature of dispersion,
i.e., hydrophilicity and linearity of the waterborne PUs.
Improvements in these properties have more or less been
achieved by grafting of other polymers [4, 5], external and
internal crosslinkings [6, 7], and blending or interpene-
trating polymer network formations [8, 9].
This review describes the methods of preparation of
aqueous PU dispersions, physical properties of disper-
sions and dispersion derived films, and important
methods to modify the film properties together with the
applications.
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600 Colloid & Polymer Science, Vol. 274, No. 7 (1996)
9 SteinkopffVerlag1996
h e m is t ry a n d d i s p e r s i o n s
Basic polyurethane chemistry
Polyadd ition reactions between molar excess of diisocyan-
ates and polyols lead to the forma tion of NCO-terminated
polyurethane prepolymers, which are chain extended with
diamines to form polyurea groups. Therefore, typical seg-
mented polyurethanes are polyurethane-polyurea. Linear
products are obtained if the reactants are bifunctional but
higher functionality leads to the formation of branched
or crosslinked materials. Unlike polycondensation, the
urethane forming reactions normally evolve no by-prod-
ucts that require removal as the macromolecules are built
up.
Since PU dispersions are formed in water, the reaction
of diisocyanate with water should be minimized. Water
hydrolyzes the isocyanate groups with the evolution of
carbon dioxide resulting in severe foaming, which is detri-
menta l in bubble-free castings and continuous surface
coatings [10, 11].
~NCO + H20 ~ ~NH-COOH ~ ~NH2 + CO2
The amino groups can react with the remaining NCO
groups yielding urea linkages, which can contr ibute to the
chain extension and physical properties of high perfor-
mance PU. However, aqueous PU dispersions which have
been extended predominantly with water have inferior
properties to those extended with polyamines. This is
simply because two NCO groups yield only one urea
group in water extension, whereas each NCO group pro-
duces one urea group upon amine extension.
~NH2 + ~NCO --+ ~N H- CO -N H~
Additional reaction of isocyanate with urea, urethane
and anaide groups is also possible. Then chain branching
or crosslinking occurs due to the formation of acyl urea,
biuret and allophanate links onto the main chain. In order
to make a linear product, water must be strictly excluded
from the reaction.
RNCO + ~ R' NHCOR' -~ R' NCOR' - acylurea
CONHR-
- RNCO + R' NHCON HR'- ~ -R '- N- CO NH R' - biuret
C O N H R -
- RNCO + R'NHCOOR' - -~ - R' -N -COOR'-allophanate
CONHR-
Regarding the raw materials suitable for the prepara-
tion of aqueous PU dispersion, nearly any known type of
polyisocyanates, polyols, and polyamines have been de-
scribed. However, on isocyanate side, aliphatic isocyanates
such as 4,4'-dicyclohexylmethane diisocyanate (H12MDI),
isophorone diisocyanate (IPDI), and 1,6-hexamethylene-
diisocyanate (HDI) are preferred because of the lower
reactivity of their NCO with water. Among cycloaliphatic
isocyanates, aqueous PUs from H~2MDI generally give
finer dispersion and higher mechanical properties than
those from IPDI [12]. Closer solubility parameter of
H12MDI (17.75(cal cm-3) 1/2) to water (24.41) over the
IPDI (13.27) and the enhanced ordering of hard segment
domains due to the more symmetric structure of H~2MDI
should respectively account for the finer dispersion and
better mechanical properties of HlaMDI based PU. The
use of mixed isocyanates is likely to disturb the hard
segments and contribute to the soft segment-hard segment
phase mixings [13]. Aromatic polyisocyanates can also be
used if suitable preparation processes are followed.
Regarding the type of polyols, wide range of linear
polyether, polyester and polycarbonate polyols can be
used [2]. Typically polytetramethy lene ether glycol
(PTMG), poly(tetramethylene adipate) glycol (PTAd), and
polycapro lactone glycol (PCL) are often used. Short chain
diols, typically 1,4-butane diols are also used to control the
hard segment content. Basic design variables are type and
molecular weight. Generally, polyester gives better phy-
sical properties due to the stronger hydrogen bondings
within the soft segment domains. In addition hydrogen
bonding formations between ester groups of soft segment
and urethane groups of hard domains contribute to the
soft segment-hard segment phase mixings. On the other
hand, polyether shows relatively poor physical properties
and greater hydrolytic stability. Polyca rbonat e combines
these two properties properly [10]. Soft segments from
polyesters and polycarbonates having sufficiently high
molecular weight (> 1500) can be crystallized when the
soft segment fraction is high enough [14].
Regarding the chain extender, the aliphatic or cyclo-
aliphatic di- or triamines react with the isocyanate groups
order of magnitude faster than water does. Therefore it is
possible to extend the NCO-te rminated prepo lymer in the
form of dispersion. Chain extension with polyamines such
as diethylene triamine (DETA) and triethylene tetramine
(TETA) leads to the internal crosslinkings [6, 15, 16]. With
increasing extender functionality, modulus, strength and
thermal stability as well as the water and solvent resistance
of the dispersion cast films are generally increased.
Conventional polyurethane, like most polymeric ma-
terials are immiscible with water. NCO-terminated hydro-
phobic PU prepolymers can be dispersed or emulsified
with suitable external emulsifier and strong shear forces.
However the dispersions obtained in this way are coarse
and storage unstable. Therefore certain type of hydrophilic
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B.K. Kim 601
Aqueous po lyurethane dispersions
modi f ica t ion i s necessary befo re d i spers ion in aqueous
med ia i s poss ib le . Th is i s no rmal ly done by incorpora t ing
ion ic g roups [17 ] o r hydroph i l i c po lye ther cha in segmen ts
i n t o t h e P U s t ru c t u re [1 8 ] . P o l y u re t h a n e s c o n t a i n i n g
i o n i c g ro u p i n t h e i r b a c k b o n e i s c a l l e d i o n o m e r . B o t h
z wi t t e r a n i o n i c [1 9 ,2 0 ] a n d c a t i o n i c [2 1 ] g ro u p s h a v e
b e e n u t i li z e d in t h e p r e p a ra t i o n o f P U i o n o m e r . T h e i o n i c
g roups a re hydroph i l i c in na tu re and func t ion as in te rna l
emuls i f i e r . Therefo re PU ionomers a re se l f -d i spers ib le
under mi ld cond i t ions .
In te rna l emuls i f i e r
The m ain a dva ntag es o f th is self-emulsificat ion are: i) d is-
pers ing p rocesses d o no t re qu i re s t rong shear fo rce , i i) fine
par t i c les wi th improved d i spers ion s tab i l i ty a re ob ta ined ,
i ii ) wa ter sens i t iv i ty o f the f i lms a f te r ev apora t ion o f water
i s reduced , and iv ) res i s tance to nonpo lar agen ts i s h igh .
onomer
P U i o n o m e rs a r e s t ru c t u ra l l y m u c h m o re s u i t a b l e fo r t h e
p re p a ra t i o n o f d i s pe r s io n s . T h o u g h P U i o n o m e r c a n b e
prep ared in th ree types , i . e ., an ionom er , ca t ionom er , and
z wi t t e r i o n o m e r , a n i o n o m e r a n d c a t i o n o m e r a r e o f t e n e n -
c o u n t e r e d i n t h e p r e p a ra t i o n o f a q u e o u s P U . T a b l e 1 g i ve s
t y p ic a l p o t e n t i a l a n i o n i c a n d c a t i o ni c c o m p o u n d s . P o l y -
u re t h a n e c a t i o n o m e rs u s e d i n a q u e o u s d i s p e r s i o n a r e
genera l ly p repa red by incorpora t ing te r t i a ry amine func-
t iona l i ty in to the backbone [21 -23]~ Some ca t ion ic PUs
s h o w u n u s u a l l y g o o d a d h e s i o n t o v a r i o u s i o n i c m o s t l y
an ion ic-subs t ra tes such as g lass and lea ther . They a l so
have been p roven usefu l in spec ia l app l ica t ions , i .e ., add i -
t iv e s i n t h e c o a g u l a t i o n p ro c e s s u s e d t o m a k e p o ro m e r i c s
[1 0 ] . P U c a t i o n o m e r b l e n d s w i t h p o l y a c ry l o n i t r i l e s c o n -
ta in ing subs tan t ia l am oun t o f an ion ic spec ies show ed s ig -
n i f i c a n t l y i m p ro v e d m o rp h o l o g y a n d p h y s i c a l p ro p e r t i e s
a s c o m p a r e d w i t h P U a n i o n o m e r a n d n o n i o n o m e r b l e nd s
[24] . Never the less , ca t ion ic d i spers ions a re no t u sed wide-
ly a t p resen t [3 ] .
An i o n i c d i s p e r s i o n s a r e c o m m e rc i a l l y p r e d o m i n a n t .
S u l fo n at e s a n d c a rb o x y l a t e s g ro u p s a r e m o s t o f t en i n c o r-
p o ra t e d i n P U a n i o n o m e rs . T h e a c i d g ro u p is s u b s q u e n t l y
neu tral ize d w ith a base, typica l ly t rie , hylam ine, resu l t ing
i n th e fo rm a t i o n o f p o l y u re t h a n e a n i o n o m e r . P o l y c a r -
b o x y l a t e s p ro v i d e g o o d h y d ro p h o b i c c h a ra c t e r wh i l e p o l y -
su l fona tes g ive d i spers ions wi th exce l len t s tab i l i ty even
u n d e r u n fa v o ra b l e c o n d i t i o n s [1, 2 ] . Am o n g d i h y d ro x y l
carbox y l ic ac ids, ~ , ~-d imethy lo l p rop ion ic ac id DM PA )
is mos t o f ten encoun tered in the l i t e ra tu re as a po ten t ia l
ion ic cen ter . The adva n tage o f D M P A l ies in the ste r ic
Table I Ion ic internal emulsifiers
Sulfonate types
H2N- CH 2 CHz- N H- CH J , : S O3Na
x = 2 o r 3
H O - C H 2 C H z - C H C H 2 -O H
SO3Na
arboxylate types
CH 3
I
HO C Hz-C CH2-OH + Base
I
OzH
H2N- CH2)4-CH-NH 2 + Base
r
CO2H
H 2 N - C H 2 C H z - N H
CHzCH z C02N a
ationic Types
HO- CH 2CH 2- N- CH2C H2- OH + Ac id /Alkylat ing Agent
I
R
C2H5
|
H O - C H z - N - C H z - O H
[
CH2-N- CH3)2
+ Acid/Alkylating Agent
h i n d e ra n c e o f t h e C O O H g ro u p wh i c h p re v e n ts i t f r o m
reac t ing wi th the i socyanate g roups . Th is s ide reac t ion i s
n o t d e s i ra b l e b e c a u s e i t wo u l d c o n s u m e t h e p o t e n t i a l i o n i c
g roup s and cause h igh v i scos it i es due to b ra nch ing o f the
PU . R ecen t ly , the e ffect o f coun terca t io ns h ave a l so been
repor ted , where ammonia , t r imethy lamine , t r i e thy lamine ,
L i O H , N a O H , a n d K O H w e r e u s e d t o n eu t ra li z e th e
C O O H g r o u p [ 2 0 ] .
Nonionomer
An o t h e r a p p ro a c h t o o b t a i n i n t e rn a l l y e m u l s i f i a b l e P U
e m p l o y s p o l y e t h e r , p r e d o m i n a n t l y f ro m e t h y l e n e o x i d e ,
cha in segmen t [18 ] . Though i t appears conven ien t to
r e p l a c e s o m e o f th e h y d ro p h o b i c p o l y o l s b y p o l y e t h y l e n e
oxide PE O) , th is techn ique is no t very effective. I t is
n e c e s s a ry t o b u i l d a h i g h n u m b e r o f h y d ro p h i l i c p o l y e t h e r
segmen ts in to the PU to ob ta in s tab le d i spers ions . Th is
mak es the f i lm very w ater sensi tive . The d r ied coa t ings a re
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602 Colloid & Polymer Science, Vol. 274, No. 7 (1996)
9 SteinkopffVerlag1996
often water swellable or even water soluble. For most
coatings applications, water sensitivity should be mini-
mized. A balance between dispersibility and water
resistance of the dried products can be achieved by incor-
porating PE O segments into lateral or terminal positions
of the PU chains [25-27]. This is possibly done by using
either modified diols or diisocyanates as building blocks or
by employing monofunct ional PEO as such. However the
dispersions obtained by this way have generally poor
storage stability [28]. Another distinctive feature of
nonionic dispersion is that thermocoagulation is possible
due to the negative temperature gradient of solubility of
hydrophilic polyether segments in water [1]. This often
poses problems during the process of emulsification [28].
ombination o f ionic groups and hydrophilic segm ents
Since the incorporation of ionic centers and hydrophilic
PEO segments leads to a different stabilization mecha-
nism, a considerable difference in marcroscopic properties
is expected. Advantages of one type of dispersion may be
offset by disadvantages.
Nonionic dispersions have technical advantages over
the ionic ones in terms of stability against electrolytes,
freezing, and strong shear forces. Among these, stability
against the electrolytes and additives are especially impor-
tant since many of the practical formulations include pig-
ments, fillers, thickner, and other additives. On the other
hand, nonionomer dispersion is unstable to heating due to
the decrease of polyether solubility in water with increas-
ing temperature.
Fortunately, combination of ionic and nonionic hy-
drophilic segments in the same PU has been attempted
and desirable synergistic effects in terms of dispersion
stability and fine particle size at an overall reduced hy-
drophilic group content have been reported [25-27]. For
example, in PT Ad- IPDI -DMPA system, when 20% of the
PTAd was replaced by PEO, particle size decreased from
0.3 to 0.1/~m [29].
Dispersion mechanism
PU ionomer solution in polar solvent such as acetone and
methyl ethyl ketone spontaneously form dispersion when
water is stirred in. The transformation of an organic solu-
tion into an aqueous dispersion takes place in several
steps. Following Dieterich [1], the initial addition of water
results in a sharp drop of viscosity owing to the reduction
of ionic associations. The ionic associations, formed upon
the neutralization of potential ionic centers, are a revers-
ible process, and any water present reduces the interchain
ionic associations. As more water is added, the solvation
sheath of the hydrophob ic chain segments decreases owing
to the decrease in acetone concentration, and the viscosity
increases via the hydrophobic interac tions induced by the
alignment of hydrophobic chain segments. Further addi-
tion of water produces a turbidity, indicating the begin-
ning of a dispersed phase formation, and subsequently
turbidity increases and the viscosity drops owing to the rear-
rangements of the agglomerates to microspheres where
the ionic groups are situated on the particle surfaces.
Recent studies on dispersion suggested, based on the
viscosity and conductivity measurements, that water is
first adsorbed on the surface of the hard segments micro-
ionic lattices, and then enters successively into the dis-
ordered and ordered hard domains [21]. Dispersion can
disrupt the order of hard domains, and hence the phase
separations between the soft and hard segments.
Particle stabilization
The stabilizing mechanisms of ionomer dispersion and
nonionomer dispersion are different [1,2]. The ionomer
dispersion is stabilized by the formation of electrical
double layers between the ionic constituents which are
chemically boun d to the PU, and their counterions which
migrate into the water phase around the particle. The inter-
ference of the electrical double layers of different particles
results in particle repulsion, leading to the stabilization
mechanism of dispersion. Addition of inert electrolytes
to the ionomer dispersion provides additional ions in the
water phase which reduces the range of double layer repul-
sion, and induces coagulation.
In nonionomer dispersion, the hydrophilic PEO seg-
ments are anchored on the surface of the particle, stretch-
ing into the water phase. The stabilization mechanism for
this type of particle structure can be explained in terms of
entropic repulsion. When the particles approach closely,
the freedom of motion of PEO chains in water phase is
restricted, leading to a reduction of entropy. Therefore
repulsion between particles is driven spontaneously.
r e p a r a ti o n p r o c e s s
The earliest process to prepare the aqueous PU dispersion
was the acetone process which still remains technically
important [10]. During the last couple of decades several
new processes have been developed. A common fea ture to
these processes is however that the first step is to prepare
the NCO-terminated P U prepolymers of medium molecu-
lar weight. The critically different step among the various
processes lies in the chain extension step which is normally
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B.K. Kim 603
Aqueous polyurethane dispersions
p e r f o r m e d u s i n g a m i n e s [ 2 ] . I n c h a i n e x t e n s i o n s te p , c o n -
t ro l o f e x t re m e l y f as t N C O - N H r e a c ti o n a c c o m p a n i e d b y
t h e v i sc o s i ty ri se is t h e m o s t i m p o r t a n t p a r a m e t e r t o
c o n t r o l .
A c e t o n e p r o c e s s
N C O - t e r m i n a t e d P U p r e p o l y m e r s a r e c h a i n e x te n d e d
w i t h d i a m i n e i n a n o r g a n i c s o l v e n t s u c h a s a c e to n e , m e t h y l
e t h y l k e t o n e , o r t e t r a h y d r o f u r a n , f o l l o w e d b y m i x i n g w i t h
w a t e r t o f o r m d i s p e rs i o n , a n d r e m o v a l o f t h e s o l v e n t b y
d i s t i l l a t i o n . T h i s w e l l e s t a b l i s h e d p r o c e s s u s e s a n o r g a n i c
s o l v e n t to c o n t r o l t h e v i s c o si t y d u r i n g t h e c h a i n e x t e n s i o n
s t e p. A c e t o n e i s e s p e c i a l l y s u i ta b l e b e c a u s e i t i s i n e r t w i t h
t h e P U f o r m i n g r e a c t i o n s , m i s c i b l e w i t h w a t e r , a n d h a s
a l o w b o i l i n g p o i n t . I n a d d i t i o n , i t r e d u c e s t h e h i g h
N C O - N H r e ac t iv i ty t h r o u g h r ev e rs ib l e k e t im i n e f o rm a -
t i o n. T h e a d v a n t a g e s o f a c e t o n e p r o c e s s i n c l u d e s w i d e
s c o p e o f v a r i a t i o n i n s t r u c t u r e a n d e m u l s i o n , h ig h q u a l i t y
o f e n d p r o d u c t s , a n d r e l ia b l e r e p ro d u c i b i l i t y . T h e s e a d -
v a n t a g e s a r e m a i n l y d u e t o t h e f a c t t h a t t h e p o l y m e r
f o r m a t i o n i s a c c o m p l i s h e d i n a h o m o g e n e o u s s o l u t i o n .
H o w e v e r , P U o b t a i n e d i n a c e t o n e p r o c e s s i s p r e d o m i -
n a n t l y l i n e a r a n d s o l u b l e in a c e t o n e s i nc e t h e c h a i n e x t e n -
s i o n is c a r r i e d o u t i n a c e t o n e . D i s t i ll a t io n o f la r g e a m o u n t
o f a c e t o n e m a k e s t h e p r o c e s s e c o n o m i c a l l y u n f a v o r a b l e .
T h i s p r o c e s s a l s o s u f f e r s f r o m a l o w r e a c t o r v o l u m e y i e l d
d u e t o t h e l a r g e q u a n t i t i e s o f s o l v e n t u s e d .
P r e p o l y m e r m i x i n g p r o c e s s
T h e p r e p o l y m e r m i x i n g p r o c e s s a v o i d s t h e u s e o f l a r g e
a m o u n t s o f s o l v en t . H y d r o p h i l i c a l l y m o d i f i e d N C O - t e r -
m i n a t e d P U p r e p o l y m e r s a r e m i x e d w i t h w a t e r t o f o r m
e m u l s i o n . T h e v i s c o s it y o f p r e p o l y m e r i s c ri t ic a l a n d m u s t
b e l i m i t e d o r t h e d i s p e r s i o n s t e p w i l l b e d i f fi c u lt . T h e r e f o r e
t h i s p r o c e s s i s s u i t a b l e f o r l o w v i s c o s i t y p r e p o l y m e r .
A s m a ll a m o u n t o f s o l v e n t c a n b e a d d e d t o t h e p r e p o l y m e r
t o r e d u c e t h e v i s c o s i t y . C h a i n e x t e n s i o n i s a c c o m p l i s h e d
b y t h e a d d i t i o n o f d i - o r p o l y a m i n e s t o t h e a q u e o u s d i s p e r -
s i o n . D i s p e r s i o n s t e p h a s t o b e c a r r i e d a t l o w e n o u g h
t e m p e r a t u r e s o t h a t t h e N C O - w a t e r r e a c t i o n i s i n s i g n i f i -
c a n t . F o r t h i s r e a s o n , c y c l o a l i p h a t i c d i i s o c y a n a t e s a r e
m o s t o f t e n u s e d d u e t o t h e i r l o w r e a c t i v i t y w i t h w a t e r .
S i n c e t h e c h a i n e x t e n s i o n s a r e p e r f o r m e d i n a h e t e r o -
g e n e o u s p h a s e , t h e y d o n o t p r o c e e d i n a q u a n t i t a t i v e w a y .
A s a r e s u lt , t h e p r o p e r t i e s o f P U d i s p e r s i o n p r e p a r e d i n
t h i s p r o c e s s a r e g e n e r a l l y i n f e r i o r t o t h o s e f r o m a c e t o n e
pr oces s .
M e l t d i s p e r si o n p r o c e s s
P r o b l e m s r e l a t e d t o t h e a m i n e c h a i n e x t e n s i o n a r e a v o i d e d
i n t h is p ro c e s s. T h e N C O - t e r m i n a t e d P U p r e p o l y m e r s a re
r e a c t e d w i t h a n e x c e s s o f a m m o n i a o r u r e a , r e s u l t in g i n
a p r e p o l y m e r w i th t e r m i n a l u r e a o r b i u r e t g r o u p s. T h i s
c a p p e d o l i g o m e r c a n b e r e a d i l y d is p e r s ed i n w a t e r w i t h o u t
Fig. 1 Acetone process
n H 0 ~ ' ~ O H +2n 0 C N - R - N C 0
o o
I [ I I
O CN - R - N H CO ~ , ~ O C N H - R - N CO
Acetone
H2NCH2CH2NHCH CH2SO3 Na+
O O O O O O
II
I I I [ r l
I[
OCN H R - NHCNI-ICH2CH2NCNH-R -NH CO ~ OCNH - R - NHCO r
C H z C H 2 S O 3 - N a
Water
Dispersion of Polyurethane in Water A cetone
i Acetone Removal
Aqueous Dispersion of Polyurethane
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604 Colloid & Polym er Science, Vol. 274, No. 7 (1996)
,9 SteinkopffV erlag 1996
Fig. 2 Prepolymer mixing
process
2 n H O ~ O H +
n H O C H 2 - - IC - - C H 2 O H
C O 2 H
+ 4 n O C N - R - N C O
o c 3
II
i l I I I I
O C N R - N H C O w O C N H - R - N H C O - C H f f - C - C H f f - O C N I - I - R - N H C O v,, O C N H - R - N C O
C O 2 H
O O O C I H 3 O O O
I] ii I] II II II
O C N - R -- N H C O v ,, O C N H - R - N H C O - C H E - C - C H ~ O C N H - R - N t t C O v,, O C N H - - R - N C O
C O 2 H N + R 3
Hyd rophilic Isocyanate Terminated Prepolymer
[ W a t e r
~ H 2 N - R ' - N H 2
O O CH3 O O O O
I I I I I I[ II IL II
, , m O C N H - R - N H C O - C H U C - C H E - O C N H - R - N H C N H - - R ' - - N H C N H - R - N H C O
C O 2 H N + R 3
queous Dispe rsion of P olyurethane
a n y o r g a n i c c o s o l v e n t . T h e r e a c t i o n w i t h u r e a i s c a r r i e d
o u t a t h i g h t e m p e r a t u r e ( > 1 3 0~ a n d t he r e s u lt i n g
o l i g o m e r s a r e u s u a l l y d i s p e r s e d a t s u f f i c i e n tl y h i g h t e m p e r -
a t u re ( > 100 ~ t o r educe t he v i scos i t y . Ch ai n ex t e ns i on i s
a c c o m p l i s h e d b y m e t h y o l o l a t i o n o f t h e b i u r e t g r o u p s w i t h
f o r m a l d e h y d e .
K e t i m i n e a n d k e t a z in e p r o c e s se s
T h e k e t i m i n e a n d k e t a z i n e p r o c e s s e s a r e s i m i l a r t o t h e
p r e p o l y m e r m i x i n g p ro c e ss , w i t h o n e i m p o r t a n t d i ff e re n c e
t h a t a b l o c k e d d i a m i n e a n d a b l o c k e d h y d r a z i n e a r e r e -
s p e c t iv e l y u s e d a s a l a t e n t c h a i n e x t e n d e r . D i a m i n e s a n d
h y d r a z i n e s a r e r e a c t e d w i t h k e t o n e s t o y i e ld k e t im i n e s a n d
k e t a z i n e s , re s p e c t iv e l y . K e t i m i n e s a n d k e t a z i n e s a r e p r a c -
t i c a ll y i n e r t t o w a r d t h e i s o c y a n a t e s i n n o r m a l p r o c e s s i n g
c o n d i t io n s . T h e s e a r e m i x e d w i t h t h e N C O - t e r m i n a t e d
p r e p o l y m e r s c o n t a i n i n g i o n i c g r o u p s w i t h o u t p r e m a t u r e
e x t e n s i o n . C h a i n e x t e n s i o n o c c u r s s i m u l t a n e o u s l y w i t h
d i s p e r s i o n f o r m a t i o n . R e a c t i o n w i t h w a t e r l i b e r a t e s t h e
d i a m i n e o r h y d r a z i n e , w h i c h r e a c t s w i t h t h e p r e p o l y m e r .
D i s p e r s i o n s p r e p a r e d b y t h i s p r o c e s s y i e l d h i g h p e r f o r -
m a n c e c o a t in g s . P U d i s p e rs i o n f r o m a r o m a t i c i s o c y a n a te s
are eas i l y access i b l e i n t h i s p rocess .
h y s i c a i p r o p e r t i e s
I n t h i s s e c t i o n s o m e t y p i c a l p h y s i c a l p r o p e r t i e s o f P U
d i s p e r s i o n a l o n g w i t h t h o s e f o r d e r i v e d f i l m s a n d f i l m
fo rmi ng p roper t i es wi l l be d i scussed .
D i s p e r s i o n p r o p e r t i e s
U n l i k e t h e s o l v e n t - b o r n e P U , t h e p a r t i c l e s m u s t f i rs t r e j o i n
i n t o a c o n t i n u o u s o r g a n i c p h a s e b e f o r e t h e i n d i v i d u a l
p o l y m e r c h a i n s c a n e n t a n g l e a n d d e v e l o p t h e u l t i m a t e f il m .
P o o r c o a l e s c e n c e c a n r e s u l t i n l o w g l o s s fi lm s , a n d u s u a l l y
i n a r e d u c t i o n o f o v e r a l l p h y s i c a l p r o p e r t ie s . T h e p a r t i c le
s iz e o f a q u e o u s d i s p e r s io n s c a n b e v a r i e d f r o m a b o u t 0 . 01
t o 5 / 2 m a n d h a s a d i r e c t im p a c t o n t h e d i s p e r s i o n s t a b il i ty .
D i s p e r s i o n s w i t h r e l a t iv e l y la r g e a v e r a g e p a r t i c l e s iz e
( 1 /~ m < ) a r e g e n e r a l l y u n s t a b l e w i t h r e s p e c t t o s e d i m e n t a -
t i o n . D i s p e r s i o n s w i t h s m a l l e r a v e r a g e p a r t i c l e s i z e a r e
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B . K . K i m 6 0 5
A q u e o u s p o l y u r e t h a n e d i s p e r s io n s
F i g . 3 M e l t d i s p e r s i o n p r o c e ss
II I[ f II
O C N R - N HC O a~ ', 'v~ OCNrH - R - NHCOCH2CH2CHCH2OCNH - R - N CO
SO3-Na+
~
2 N C N H2
O O O O O O O O
l i i l i t I I I i i l [ i [I
H 2 N C N H CN H - R - N H C O ~ ' ~ O C N H - R - N H CO C H 2 CH 2 C IH C H 2 OC N H R - - N H C N H C N H 2
SO3Na +
Hy drop h i l i c B is - B i rue t
i Wate r
Formaldehyde
O O O
I I l i I[
r162O C N H - R- N H C N H C N H
1
c z
N H C N H C N H - R N H C O 4 ~
J l i E i l
O O O
A q u e o u s D i s p e r s io n o f P o l y u r e t h a n e
F i g . 4 K e t i m i n e a n d k e t a z in e
process
O O O CIH O
O C N - R - N H C O ~ w O C N H - -R - -N H C O C H 2 CC H 2 O C N H - R - N C O
CO 2- +
H N R 3
Hyd r o p h i l i c I so cya n a t e T e r m in a t e d P r e p o l ym e r
+
K e t im in e / K e t a z in e
W a te r C = N - R . .. . N = C
R R
t < I I
2 C = O + H 2 N - - N - - 2
R
O O O O C H 3 O O
H I i l r i F / I E II
~ a ~ w N H C N H - R - N H C O ~ v~ O C N H - R N H C O CH 2 CC H 2O C N H -R - N H C N H ~ w w
I +
C 0 2 H N R 3
A q u e o u s D i s p e r s io n o f P o l y u r e th a n e
m o r e u s e fu l s in c e s u ch d i s p e r s i o n s a r e s t o r a g e s t a b l e a n d
h a v e h i g h s u r f a c e e n e r g y , w h i c h r e s u l t s i n a s t r o n g d r i v i n g
f o r c e f o r f i l m f o r m a t i o n [ 3 ] . T h e r e f o r e a b i l i t y t o c o n t r o l
t h e p a r t i c l e s i ze is p r a c t i c a l l y i m p o r t a n t . S i n c e t h e p a r t i c l e
d i a m e t e r l ie s o v e r s e v e r a l o rd e r s o f m a g n i t u d e s t h e a p p e a r -
a n c e c a n v a r y fr o m a n o p a q u e t r a n s l u c e n t s o l t o a m i l k y
w h i t e d i s p e r s i o n [ 3 0 ] . I n f ac t , P U d i s p e r s i o n i s a g o o d
e x a m p l e i n t h a t t h e r e is n o d i s t i n c t b o a r d e r l i n e b e t w e e n
s o l u t i o n , c o l l o i d a l s o l, d i s p e r s i o n , s u s p e n s i o n a n d l a t e x
[ d
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606 Co l lo id & Pol ym e r Sc ie nc e , Vol . 274 , No . 7 (1996)
9 S t e i n k o p f f V e r l a g 1 9 96
The par t i c le s i ze o f PU d i spers ion i s mo re o r l es s
c o n t ro l l e d b y s u c h m i x in g c o n d i t i o n s a s r p m a n d t e m p e r -
a tu re . However , i t i s mos t ly governed by the overa l l hy -
d roph i l i c i ty o f the PU , i . e . , h igher hyd roph i l i c i ty o f PU
leads to smal le r pa r t i c le s ize [ -31] . Typ ica l exam ples a re
s h o w n i n F i g . 5 fo r P U c a t i o n o m e r d i s p e rs i o n s [2 2 ] . Th e
decrease o f average par t i c le s ize wi th increas ing N-m ethy l -
d ie thano lamine (MDEA) con ten t i s c lear ly seen .
Th e i o n o m e r d i s p e r si o n s s h o w a n a s y m p t o t i c d e c re a s e
o f par t i c le s ize wi th ion ic con ten t . Th is beha v io r i s due to
the du al effect of ionic centers . T hat is , part icle s ize is
decreased due to the increased hydroph i l i c i ty , and in -
creased by the increased water swel l , and an overa l l
asympto t ic behav io r i s expec ted .
Aqueous PU d i spers ion has re la t ive ly low v i scos i ty .
Ty p i c a l c o m m e rc i a l g r a d e s h a v e t h e i r r o o m t e m p e ra t u r e
viscosi ty , 10 700 cP, with typical sol id conte nt , 30 ~ 40 .
Typ ica l d i spers ion v i scos it i es a re a l so show n in F ig . 5
w h e re a d r a m a t i c v a r i a t i o n o f v i s c o si t y w i t h i o n i c c o n t e n t
is noted.
Dispe rs ion v i scos i ty (~) i s d i rec t ly p ropo r t iona l to the
ef fec tive vo lum e f rac t ion o f d i spersed ph ase (0 ) a t low
dispersed phase concen t ra t ion (0 < 0 .02 ) , and concen t ra -
t i o n d e p e n d e n c e b e c o m e s m o re p ro n o u n c e d a t h i g h ~ ,
genera l ly expressed as fo l lows [32 ] :
F] = 1 q- kl /p q- k21p 2 q- . . . ,
whe re l ?s i s the v i scos i ty o f med ium, and k ' s a re con s tan t .
S ince ne t so l id con ten t o f typ ica l PU d i spers ion i s over
30 wt , h igher o rder t e rms shou ld con t r ibu te s ign i f ican t ly
as shown in the figure. Swellabi l i ty , which general ly in-
creases wi th increas ing hy droph i l i c i ty o f raw m ater ia l s
a l so con t r ibu tes to the e f fec tive vo lu me f rac t ion o f the
Fig . 5 Av e ra ge pa r t i c le s iz e a nd d i spe r s ion v i sc os i ty as a func t io n of
c a t i o n i c c o n t e n t
d ispersed phase in the above equat ion . Elec t rov i scous e f -
fec t o f iono me r d i spers ion a l so increases the v i scos i ty .
F i l m p ro p e r t ie s
PU d i spers ions exh ib i t exce l len t f i lm-fo rming p roper t i es
e v e n a t l o w t e m p e ra t u r e . Th e y c a n b e i m p ro v e d b y
the add i t ion o f h igh bo i l ing coa lescen t so lven ts such as
N -m e t h y l p y r ro l i d i n o n e o r k e t o n e , o r b y a d d i t i o n o f
p las t i c izers . Coalescen t agen ts a re o f ten necessary wi th
cross l inked PU d i spers ions , s ince the f i lm fo rming ab i l i ty
decreases wi th increas ing c ross l ink dens i ty . F i lm fo rming
proper t i es a re a l so improved wi th h igher d ry ing temper-
ature.
G e n e ra l s t r u c t u r e -p ro p e r t y r e l a t io n s h i p o f d is p e r s io n
der ived f ilm i s es sen t ia lly the same w i th so lven t -borne PU .
The ov era l l f ilm p roper t i es a re de te rm ined b y the type and
a m o u n t o f s o f t a n d h a rd s e g m e n ts , s of t s e g m e n t -h a rd
segmen t phase separa t ion , c ross l ink ing dens i ty , e tc . [33 ] .
How ever , there a re cer ta in p roper t i es o f d i spers ions tha t
a r e g e n e ra ll y i n fe r io r t o t h e s o l v e n t -b o rn e t w o -c o m p o n e n t
PUs . These inc lude water swel l res i s tance , hydro ly t i c s t a -
b i l ity , and so lven t res is tance [1 ] . S ince these p roper t i es a re
re la ted to the very na tu re o f d i spers ion , the improve me n t
i s a l so l imi ted [2 ] . Water swel l res i s tance and hydro ly t i c
s tab i l i ty increases wi th decreas ing am oun ts o f ion ic
cen ters , in wh ich c ase fo rmat ion o f st ab le and f ine d i sper -
s ions becomes d i f f i cu l t . So lven t res i s tance cou ld be im-
proved wi th increas ing c ross l ink ing dens i ty , wh ich on the
o ther hand , makes the f i lm fo rmat ion d i f f i cu l t due to
increased g lass t rans i t ion t empera tu re and hence the f i lm-
fo rming tempera tu re . I t i s however poss ib le to min imize
these d i sadvan tages by us ing fo rmula t ions wh ich have
a carefu l ba lance in te rms o f the am oun t o f hydroph i l i c
g roups and the degree o f c ross link ing .
N
l Z O
8
4
I I I
M D E c o n t e n t (wt )
2
1
m
511
rtl
odif ica t ions
The ob jec t ive o f mod i f ica t ion is to im prove cer ta in p roper-
t i es such as w ater , so lven t , and genera l chem ica l res i s tance ,
and the m echan ica l p ro per t i es o f the f ilms. Amo ng o thers ,
g raf t ings , and c ross l ink ings a re mos t o f ten encoun tered
[2 , 30 ] . B lend ing and in te rp ene t ra t ing po ly me r ne two rk
are a l so sub jec t s o f recen t a t t em pts to m od i fy these p roper -
ties [8, 9].
G ra f t i n g a n d b l o c k c o p o l y m e r i z a t i o n
Graf t ing i s u sua lly accompl i shed in the aque ous phase us ing
t h e c o n v e n t i o n a l e m u l s i o n p o l y m e r i z a t i o n t e c h n i q u e s .
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B.K . Kim 607
Aqueous polyurethane dispersions
Fig. 6 Grafting of acrylic
monom er onto unsaturated
polyester polyvol (a) and
propylene glycol (b) segm ent of
polyurethane ionomers
o o o
[ I J l I 1
, ,v w ~ N H C O a ,w w ~ O C ~ _ _ / C O ~ , ~
Drier i 02
O O 0
I I I I U
~ N H C O ~ O C C O ~
~ 'r ~ OC C O ~
I f H I~
o o 0
R
Acryl ic
x idat ive
D r y i n g
a )
Acrylic
I I I I
~ N H C O ~ v ~ O C
0 ~ , ~
Acrylic
0 c n 3
I F ' R
~ w ~ N - C - O - C - C H 2 - - O ~ ' ~
t I Acryl ic
H H
b )
O C H 3
I r J
~ v , N - C - O - C - - C H 2 - - O ~ w
A c ry l i c
M o n o m e r a n d f r e e r a d i c a l i n i t i a t o r a r e a d d e d t o
t h e a q u e o u s p h a s e [ 5 ] . B o t h a n i o n i c a n d c a t i o n i c P U s
c o n t a i n i n g u n s a t u r a t e d p o l y e s t e r p o l y o l s ( F ig . 6 a ) a n d
p o l y p r o p y l e n e g l y c o l ( F i g . 6 b ) i n t h e i r b a c k b o n e h a v e o f -
t e n b e e n u s e d to g r a f t a c ry l a t e m o n o m e r s o n t o t h e m a i n
c h a i n [ 2 ]. A n i o n i c P U s c o n t a i n i n g c a r b o x y l a t e s a lt c a n b e
f u r t h e r c r o s s l i n k e d w i t h e p o x y r e s i n s .
T h o u g h g r a f ti n g is a u s e f u l t e c h n i q u e t o m o d i f y t h e P U
i o n o m e r , i t i s g e n e r a l l y l i m i t e d t o P U c o n t a i n i n g u n -
s a t u r a t e d p o l y e s t e r a n d p o l y p r o p y l e n e g l y c o l s e g m e n t s .
P U i o n o m e r s c a n b e m o d i f i e d i n a s im i l a r w a y t o t h e u l t r a
v i o le t ( U V ) c u r in g . N C O - t e r m i n a t e d P U p r e p o l y m e r s c o n -
t a i n i n g p o t e n t i a l i o n i c g r o u p s a r e f i r s t c a p p e d w i t h a r e -
a c t i v e d i l u e n t s u c h a s 2 - h y d r o x y e t h y l a c r y l a t e ( H E A ) v i a
p o l y a d d i t i o n . T h e n t h e p o t e n t i a l i o n i c g r o u p s a r e n e u -
t r a l i z e d a n d a c r y l a t e m o n o m e r s p o l y m e r i z e d a t t h e H E A
t e r m i n i v ia a r a d i ca l p o l y m e r i z a t i o n m e c h a n i s m . T h e
so lu t i on i s d i sper sed wi th wat er s t i r r ed i n . As a r esu l t ,
b l o c k c o p o l y m e r s o f P U i o n o m e r s a n d p o l y a c r y l a te s ar e
o b t a i n e d . V a r i o u s t y p e s o f a c r y l a t e s s u c h a s m e t h y l
m e t h a c r y l a t e ( M M A ) , e t h y l m e t h a c r y l a t e ( E M A ) , t - b u t y l
m e t h a c r y l a t e ( t- B M A ) , c y c l o h e x y l m e t h a c r y l a t e ( C H M A ) ,
t r i p r o p y l e n e g l y c o l d i a c r y l a t e ( T P G D A ) , a n d t r i m e t h y l
p r o p a n e t r ia c r y l a t e ( T M P T A ) h a v e b e e n i n c o r p o r a t e d i n
P U i o no m e r s f ro m H 1 2 M D I - P C L D M P A [ 4] . W i t h
M M A i n c o r p o r a t i o n , s w e l l i n w a t e r i s s i g n if i c a n t ly d e -
c r e a s e d , t o g e t h e r w i t h t h e i n c r e a s e i n c o n t a c t a n g l e w i t h
2
8 5
9
16
I 2
8 0
7 5
9
r
7 0
r
6 5
0 1 0 2 0 5 4
M MA e o n t a n t ( w t )
Fig. 7 Effect of MM A con tent on wate r swell and contact angle of
PU acrylate fi lms ( based on solid)
w a t e r ( F ig . 7 ). W i t h a c r y l a t e i n c o r p o r a t i o n , h a r d n e s s ,
m o d u l u s , a n d e l o n g a t i o n a t b r e a k a r e s i g n i f i c a n t l y
increased .
R e g a r d i n g t h e m i c r o s t r u c t u r e , a c r y l a t e s c o n t r i b u t e t o
t h e h a r d s e g m e n t d o m a i n s s i n c e t h e y a r e m o r e l i k e l y
m i s c i b l e w i t h t h e h a r d s e g m e n t o f P U . A s a r e s u lt , s o ft
s e g m e n t c h a r a c t e r i s t i c s d i s a p p e a r . T h e n w i t h i n c r e a s e d
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608 Colloid & Polym er Science, Vol. 274, No. 7 (1996)
9 SteinkopffV erlag 1996
e~
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0 .0
9 MMA20 "
o M M A 3 0 ~ ' o k c~
; 9 o
m o ~ " n o
a m o o o 9
t 2. ,o
e d d~
0 i 6
50 0 50 1 O0 150
T e m p e r a t u r e ( ~
Fig. 8 The tan c5 curve of PU acrylate films as a func tions of MM A
content
1 2 9
1 1
=
5q
l 0
9
8 0
B o :
9
7
7 0
M M A E M A
t - B M A C H M A T P G D A T M P T A
F i g . 9 E f f e c t o f a c r y l a t e t y p e o n w a t e r s w e l l a n d c o n t a c t a n g l e o f P U
a c r y l a t e f i l m s
h a r d d o m a i n f r a c t i o n , t h e s o f t s e g m e n t g l a s s t r a n s i t i o n
d i s a p p e a r s a n d h a r d s e g m e n t g l a s s t r a n s i t i o n m o v e s t o -
w a r d t h e h i g h e r t e m p e r a t u r e ( F i g . 8 ) .
R e g a r d i n g t h e e f f e c t o f a c r y l a t e t y p e c o n t a c t a n g l e w i t h
w a t e r i n c r e a s e s w i t h i n c r e a s i n g a c r y l a t e f u n c t i o n a l i t y , d u e
p r o b a b l y t o t h e i n c r e a s e d c r o s s l i n k i n g d e n s i t y o f t h e P U
a c r y l a t e s , e s p e c i a ll y in t h e i r h a r d d o m a i n s ( F ig . 9). A m o n g
m o n o a c ry l a te s s u ch a s M M A a n d E M A m o n o m e r w i th
a l a r g e s u b s t i t u e n t g r o u p g e n e r a l l y s h o w s r e l a t iv e l y li t tl e
s w e l l a n d l a r g e c o n t a c t a n g l e , d u e p r e s u m a b l y t o t h e
h y d r o p h o b i c i t y o f l a r g e a l k y l s u b s t it u e n t s . I t s e em s t h a t
m e c h a n i c a l p r o p e r t ie s a s w e l l a s w a t e r r e s i s t a n c e o f P U
i o n o m e r s c a n b e p r o p e r l y i m p r o v e d b y p r o p e r s e le c t io n o f
a c r y l a t e m o n o m e r t o i n c o r p o r a t e i n P U .
o
II t f
CH3CH3C (- CH 20 - C - CH2CH - -N ,~ )3
+
C O 2 " H N E t 3
- - N E t 3
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C = O
I
o
t
C H 2 - - C H 2 -- O H
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C = O
I
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C H 2 - - C H 2 - - N I - I R
F i g . 1 0 C r o s s l i n k i n g w i t h p o l y f u n c t i o n a l a z i r i d in e
E x t e r n a l c r o s s l i n k i n g
A q u e o u s P U d i s p e r s i o n c a n b e c r o s s l i n k e d i n m u c h t h e
s a m e w a y a s t h e o t h e r w a t e r - b o r n e p o l y m e r s . M o s t r e a c -
t i o n s c e n t e r o n t h e c a r b o x y l i c a c i d g r o u p s [ 3 4 ] .
A z i r i d i n e s
P o l y a z i r i d i n e s , e s p e c i a l l y t r i f u n c t i o n a l a z i r i d i n e s h a v e
b e e n e x t e n s i v e l y u s e d f o r c r o s s l in k i n g a n i o n i c t y p e P U
d i s p e r si o n s . T h e a z i r i d in e m u s t b e a d d e d j u s t p r i o r t o t h e
a p p l i c a t i o n s i n ce i t c a n l o s e a c t i v i t y a f te r 2 d a y s ' s t o r a g e i n
w a t e r a t r o o m t e m p e r a t u r e [ 2 ] . A d e s i r a b l e f e a t u r e o f
p o l y a z i r i d i n e s is t h a t t h e c r o s s - l in k i n g r e a c t i o n p r o c e e d s a t
l o w t e m p e r a t u r e . A n e g a t i v e f e a t u r e i s t h a t t h e r e i s s o m e
d o u b t a b o u t t h e t o x i c i t y o f t h i s cl a s s o f p r o d u c t s .
B l o c k e d i s o c y a n a t e s
A b l o c k e d i s o c y a n a t e i s a n i s o c y a n a t e w h i c h h a s b e e n
r e a c t e d w i t h a m a t e r i a l w h i c h w i l l p r e v e n t i t s r e a c t i o n a t
r o o m t e m p e r a t u r e w i t h c o m p o u n d s t h a t c o n v e n t i o n a l l y
r e a c t w i t h i s o c y a n a t e s b u t w i l l p e r m i t t h a t r e a c t i o n t o
o c c u r a t h i g h e r t e m p e r a t u r e [ 3 5] . B l o c k e d i s o c y a n a t e s
h a v e b e e n w i d e l y u s e d b o t h i n c o n v e n t i o n a l a n d w a t e r -
b a s e d p o l y u r e t h a n e s [ 3 6] . E x a m p l e s o f b lo c k i n g a g e n t s
w h i c h h a v e b e e n c o m m e r c i a l ly u s e d a r e c a p r o l a c t a m a n d
m e t h y l e t h y l k e t o x in e . W a t e r - b a s e d b l o c k e d P U p r e p o l y -
m e r s c a n b e m a d e b y b l o c k i n g t h e N C O - t e r m i n a t e d
p r e p o l y m e r s w i t h a s u i t a b l e b l o c k i n g a g e n t , f o l l o w e d b y
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B . K . K i m 6 0 9
A q u e o u s p o l y u r e t h a n e d i s p e r s i o n s
F i g . 1 1 C r o s s l i n k i n g w i t h
b l o c k e d i s o c y a n a t e
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F i g . 1 2 C r o s s l i n k i n g w i t h
m e l a m i n e / f o r m a l d e h y d e r e s i n
2 , , ~ N H C N H r
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n e u t r a l i z a t i o n a n d e m u l s i f i c a t i o n i n t h e n o r m a l f a s h i o n .
D i s p e r s i o n s o f t h i s t y p e m a y e i t h e r b e u s e d a s t h e s o l e
c o m p o n e n t o f a c o a t i n g f o r m u l a t i o n o r a l t e rn a t i v e l y u s e d
a s c r o s s l i n k e r s f o r t h e l i n e a r t y p e s .
elamine~formaldehyde resin
T h e u s e o f m e l a m i n e / f o r m a l d e h y d e a s c r o s sl i n k e rs i s w e ll
e s t a b l i s h e d , e s p e c i a ll y i n c o n n e c t i o n w i t h a c r y l i c p o l y m e r s .
I n t h e c a s e o f p o l y u r e t h a n e d i s p e r s io n , t h e c r o s s li n k i n g
g e n e r a l l y o c c u r s a t e l e v a t e d t e m p e r a t u r e s , a n d p r o c e e d s
v i a e i t h e r t h e u r e t h a n e o r u r e a l i n k a g e s [ 2 ] .
Radiation induced crosslinkinc]
T h e U V o r e l e c t r o n b e a m c u r i n g s a v es e n e rg y , a n d r e d u c e s
o r e l i m i n a t e s s o l v e n t e m i s s i o n . T h e r e f o r e , th i s t e c h n i q u e
h a s b e c o m e c o m m e r c i a l l y i m p o r t a n t r a n g i n g f r o m p r o t e c -
t i v e c o a t i n g s t o p h o t o r e s i s t s f o r f a b r i c a t i o n o f m i c r o e l e c -
t r o n i c d e v i c e s [ -3 6 - 38 ] . M o s t o f t h e c o m m e r c i a l s y s t e m s
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610 Colloid & Polym er Science, Vol. 274, No. 7 (1996)
9 SteinkopffV erlag 1996
u s e a c r y l ic - b a s e d m o n o m e r s a n d r e s i n w h i c h c u r e b y a f re e
r a d i c a l m e c h a n i s m . R e c e n t l y , t h e r e h a s b e e n a g r o w i n g
t r e n d i n t h i s a r e a f o r th e u s e o f w a t e r - b a s e d r a d i a t i o n
c u r i n g [ 3 9 ] . T h e m a i n r e a s o n f o r t h is is t h a t t h e a m o u n t o f
a c r y l i c m o n o m e r u s e d a s r e a c t i v e d i l u e n t c a n e i t h e r b e
g r e a t l y r e d u c e d o r e v e n e l i m i n a t e d . T h i s im p l i e s t h a t u s u -
a l l y b e t t e r p h y s i c a l p r o p e r t i e s o f t h e d e r i v e d c u r e d s y s t e m
a r e a c h i e v e d . T h e l o w d i s p e r s i o n v i s c o s i t y i s a n o t h e r a d -
v a n t a g e c o m p a r e d w i t h t h e c o n v e n t i o n a l s o li d t y p e w h i c h
r e q u i r e s a v e r y h i g h p r o p o r t i o n o f r e a c t i v e d i lu e n t t o
r e d u c e w o r k a b l e v i s c o s i t i e s , w i t h c o r r e s p o n d i n g d e c r e a s e
in phys i ca l p roper t i es [21 .
T o p r e p a r e t h e w a t e r d i s p e r si b l e P U a c r y l a t e s , t h e
N C O - t e r m i n a t e d P U i o n o m e r p r io r t o e m u l s i f ic a t io n is
r e a c t e d w i t h a h y d r o x y a c r y l a t e [ 3 8, 3 9 ]. T h e n t h e m i x t u r e
o f P U a c r y l a t e s , r e a c t i v e d i l u e n t , a n d p h o t o i n i t i a t o r a r e
c a s t o n a g l a s s p l a t e , a n d i r r a d i a t e d . C o n s i d e r a b l e w o r k
h a s r e c e n t ly b e e n d e v o t e d t o d e v e l o p w a t e r s o l u b le p h o t o -
i n i t i a t o r s fo r t h i s sys t em.
p p l i c a t i o n s
A q u e o u s P U d i sp e r s io n s a r e p r e d o m i n a n t l y li n e a r p o l y -
m e r s a n d h a v e f o u n d a p p l i c a t i o n s i n a r e a p r e v i o u s l y
d o m i n a t e d b y c o n v e n t i o n a l s o l v e n t - b o r n e p o l y u r e t h a n e
c o a t i n g s . I n a d d i t i o n , a q u e o u s d i s p e r s i o n s f i n d t h e i r
a p p l i c a t i o n s a s a d h e s i v e s o n a v a r i e t y o f s u b s t r a t e s . S o m e
of these ap p l i ca t i ons a re l i s t ed be low [ -3 ]:
C o a t i n g s f o r f l e x ib l e s u b s t r a t e s
9 t ex t i l e 9 pap er
9 r u b b e r 9 l e a t h e r
9 v iny l 9 f i lm an d fo i l
C o a t i n g s f o r f i n i s h in g
9 m a c h i n e h o u s i n g
9 p l as t i c par t s
9 m e t a l p r i m e r
C o a t i n g f o r w o o d
9 f u r n i t u r e 9 p a n e l i n g
9 f l oo r ing 9 sea l e r s
T h e e a r l i e s t a p p l i c a t i o n o f a q u e o u s P U d i s p e r s io n s
was fo r t ex t i l e coa t i ng wh ich i s s t i l l one o f t he l a rges t
m a r k e t a r e a [ 40 , 4 1 ]. C o a g u l a t i o n o f f il m s o f a q u e o u s P U
d i s p e r s i o n a n d s u b s e q u e n t l a m i n a t i o n o n t o t e x t i l e r e s u l t s
i n p r o m e r i c s . P r o m e r i c s a r e m i c r o p o r o u s p l a s t i c s w h i c h
a r e p e r m e a b l e t o a i r a n d w a t e r v a p o r s , b u t i m p e r m e a b l e t o
w a t e r . P r o m e r i c s a r e u s e d a s c o a t i n g o n s p l i t l e a t h e r , a n d
e s p e c i a ll y in t h e p r e p a r a t i o n o f l e a t h e r s u b s t i tu t e s .
A q u e o u s P U s a r e u s e d a s s i z in g a g e n t f o r t e x ti l e f i be r s
t o p r o v i d e i n c r e a s e d t e n s i l e s t r e n g t h a n d r e s i s t a n c e t o d r y
c l e a n i n g s o l v e n t s a n d a q u e o u s d e t e r g e n t s o l u t i o n s
[ 41 , 4 2 ] . P a p e r f i b e r s a r e a l s o s i ze d w i t h a q u e o u s P U s t o
i n c r e a s e t h e s t r e n g t h o f p a p e r a s w e l l a s t o r e d u c e t h e
t e n d e n c y t o i n k t o r u n o n t h e s u r f a c e [4 2 ] . I s o c y a n a t e o r
b l o c k e d i s o c y a n a t e c o n t a i n i n g P U s s h o w e x c e l l e n t a d -
h e s i o n t o c e l l u l o u s m a t e r i a l s . W o o d c o a t i n g , e s p e c i a ll y f o r
f l o o r i n g , a r e m a d e u s i n g a q u e o u s P U a s s u c h o r w i t h
w a t e r b o r n e a c r y l i c s. P U c o a t i n g s p r o v i d e h i g h g l o ss , h i g h
a b r a s i o n r e s i s t a n c e , a n d h i g h f o u l i n g r e s i s t a n c e t o m o s t
h o u s e h o l d c l e a n s e r s . O r g a n i c s o l v e n t a g g r e s s i v e ly a t t a c k
m a n y p l a s t i c s u r f a c e s . A q u e o u s P U d i s p e r s i o n s h a v i n g
g o o d a d h e s i o n p r o p e r t i e s a r e u s e f u l f o r p l a s t ic c o a t i n g s .
F o r a d h e s i v e a p p l i c a t i o n s , a q u e o u s P U d i s p e r s i o n s
h a v e b e e n u s e d f o r a n u m b e r o f p o l y m e r s u b s tr a te s , u s u -
a l l y i n c o m b i n a t i o n w i t h o t h e r p o l y m e r t o l o w e r c o st .
P o l y u r e t h a n e io n o m e r s h a v e g o o d a d h e s i o n t o n a t u r a l
a n d s y n t h e t i c ru b b e r s u r f a c e s, a n d c a n b e u s e d i n t h e
m a n u f a c t u r e o f f o o t w a r e [ 42 , 4 3 ] .
P U i o n o m e r d i s p e r s i o n s h a v e b e e n u s e d a s m e d i a i n
w h i c h m o n o m e r s h a v e b e e n p o l y m e r i z e d . T h i s t e c h n i q u e is
k n o w n a s l a t e x i n te r p e n e t r a t i n g p o l y m e r n e t w o r k , a n d i s
u s e f u l t o c o n t r o l t h e p h a s e s e p a r a t i o n . A q u e o u s P U d i s -
p e r s i o n s a r e a l s o u s e d f o r m e t a l c o a t i n g s s u c h a s m e t a l
p r i m e r s , c o i l c o a t i n g r e s i n s , a n d a s w i r e c o a t i n g s . C a t i o n i c
e l e c t r o d e p o s i t i o n c o a t i n g s a r e w i d e l y u s e d f o r a u t o m o t i v e
m e t a l p r im e r s w h i c h p r o v i d e e x c e l l e n t c o r r o s i o n r e s i s t a n c e
t o c a r b o d i e s . M o r e d e t a i l s r e g a r d i n g t h e p a t e n t a p p l i c a -
t i ons a re found i n r e f . [2 ] .
C o n c l u s i o n s
T h e d e v e l o p m e n t o f a q u e o u s p o l y u r e t h a n e d i sp e r s io n s h a s
b e e n m o t i v a t e d p r i m a r i l y b y e n v i r o n m e n t a l c o n s i d e r -
a t i o n s . A v a r i e t y o f a q u e o u s p o l y u r e t h a n e d i s p e r s io n s
h a v e b e e n d e v e l o p e d a n d s u c c e s s f u ll y a p p l i e d f o r t e x t il e
c o a t i n g s , f i b e r s i z i n g s , a n d a d h e s i v e s f o r a n u m b e r o f
s u b s t r a t e s . T h e y a r e b a s i c a l l y o n e c o m p o n e n t f u l l y r e a c t e d
a n d p r e d o m i n a n t l y l i n e a r p o l y m e r s . T h e r e a r e c e r t a i n
t y p e s o f p r o p e r t i e s s u c h a s r e s i s ta n c e t o w a t e r a n d s o l v e n t
w h i c h a r e i n f e r i o r t o t h e c o n v e n t i o n a l s o l v e n t - b o r n e
p o l y u r e t h a n e s . T h e s e p r o p e r t i e s a r e r e l a t e d t o t h e v e r y
n a t u r e o f d i s p e rs i o n , v iz . h y d r o p h i l i c i t y a n d l i n e a r it y ,
w h i c h a p p a r e n t l y m a k e t h e i m p r o v e m e n t l i m i t e d . H o w -
e v e r , t h e s e p r o b l e m s h a v e p r o p e r l y b e e n r e d u c e d o r e v e n
r e s o l v e d b y i n t r o d u c i n g c r o s s l in k i n g s , g r a f t i n g s a n d o t h e r
m e t h o d s .
S i nc e th e p e r f o r m a n c e o f a q u e o u s p o l y u r e t h a n e
d i s p e r s i o n i s c o m p a r a b l e i n m a n y w a y s t o t h a t o b t a i n e d
f r o m s o l v e n t - b o r n e p o l y u r e t h a n e a n d a i r p o l l u ti o n r e g u l a -
t i o n b e c o m e s s t r i n g e n t i n m a n y c o u n t r i e s , t h e m a r k e t
f o r a q u e o u s p o l y u r e t h a n e d i s p e r s i o n s e e m s t o g r o w
c o n t i n u o u s l y .
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B.K. K im 611
A q u e o u s p o l y u r e t h a n e d i s p e r s io n s
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