aqueous polyurethane dispersions

8/17/2019 Aqueous Polyurethane Dispersions

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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


6/13

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


7/13

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


8/13

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 .


9/13

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


10/13


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

..~ -

C = O

I

o

t

C H 2 - - C H 2 -- O H

R

I

+ N

C 0 2 H C H / ~ CH 2

C = O

I

o

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


11/13

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

2 @ - N C O ) 3 + 4 H O - B L

O

11

2 BL--O C - N H

~ - N C O

B L ~ C - - N H

O

O

~ O

CH

I

) H O C H 2 - f C H 2 O H

C O 2 H

2 ) N Et 3

o

CH 3 I!

B L - O C - - N H ii I

N H C O B L

, ~ - N - C - - 0 . C H ~ - C - - C H 2 - 0 - C - N "

" I " N H _ C O _ B L

B L O C - N H H C O O " H N + E t 3 II

O O

O

II

~ x

L e g a n d ' T ~ = ~ w ( C H E ) 6 - - N N - - ( C H 2 ) 6 "w ~

r

O ~ C \ N ~ C ' > o

CH2 6

C H 3

B L = v w , N C ~ C H 2 C H 3

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

CH2OR

+ R - N

c~2oR

o

- - R ' O H , C H 2 - - N C N H

R - N

C H 2 - - N C N H ~w~

I J

~ O C NH ~ , ~ , ~

9C H 2 O R '

+ R - N

C H 2 O R '

, C H 2 O R '

R - N =

C H 2 O R '

- - R ' O H

o

, C H 2 - - N C O ~

R - N

C H 2 N C O

N ( C H 2 O R ' ) 2

N ~ N

( R O C H 2 ) 2 N N N ( C H 2 O R ' ) 2

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


12/13


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 .


13/13

B.K. K im 611

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