Applied Polymer Science - American Chemical Society

41 Urethane Coatings K U R T C. FRISCH1 and PANOS K O R D O M E N O S 2 Polymer Institute, University of Detroit, Detroit, MI 48221 Ford Motor Company, Paint Research Center, Mt. Clemens, MI 48043

1

Downloaded by GEORGETOWN UNIV on August 18, 2015 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch041

2

Raw Materials for Urethane Coatings Isocyanate Components Di- and Polyhydroxy Components Chain Extenders and Cross-linkers Catalysts Other Urethane Coating Components Types of Urethane Coatings One-Package Urethane Alkyd (Oil-Modified Urethanes) Coatings (ASTM Type 1) One-Package Moisture-Cure Urethane Coatings (ASTM Type 2) Single-Package Blocked Adduct Urethane Coatings (ASTM Type 3) Two-Package Catalyst Urethane Coatings (ASTM Type 4) Two-Package Polyol Urethane Coatings (ASTM Type 5) Urethane Lacquers Two-Package Coatings: Isocyanate-Terminated Prepolymers Cured with Polyamines Two Package Coatings: Isocyanate-Terminated Prepolymers Cured with Ketimines Two-Package Coatings with Aminoformaldehyde Cure Aqueous Urethane Coating Systems One Hundred Percent Solids Coating Radiation-Cured Urethane Coatings High Temperature Resistant Urethane and Other Isocyanate-Based Coatings Miscellaneous Urethane Coating Systems Coatings from Urethane Interpenetrating Polymer Networks (IPNs) Urethane coatings were f i r s t developed by Otto Bayer and his coworkers in the laboratories of the I. G. Farbenindustry, today's Farbenfabriken Bayer, in Leverkusen, West Germany. Although 0097-6156/85/0285~0985$12.25/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

APPLIED POLYMER SCIENCE

986


urethanes are characterized by the linkage -NH-C(=O)-O-, they may contain other functional groups such as ester, ether, urea, amide, and other groups. There are a number of syntheses leading to the formation of urethane polymers. However, the most important commercial route is the isocyanate addition polymerization, the reaction between di- and polyfunctional hydroxyl compounds such as hydroxy-terminated polyethers or polyesters and d i - or polyisocyanates. When difunctional reactants are being used, linear polyurethanes are produced and the reaction can be schematically represented as follows:

If the functionality of the hydroxyl or isocyanate component is increased to three or more, branched or cross-linked polymers are formed. The earliest commercial urethane coatings were based on polyester-polyisocyanate systems that exhibited excellent abrasion resistance, toughness, and a wide range of mechanical strength properties. Most urethane coating systems in this country were first based on tolylene diisocyanate (TDI), while in Europe many systems based on 4,4 -methylene bis(phenyl isocyanate) (MDI) were developed. In order to avoid the use of free TDI, adducts of polyols such as trimethyolpropane or 1,2,6-hexanetriol with TDI were introduced, particularly for two-component coatings (Ί, 2). Other types of isocyanate derivatives that find increasing application in the urethane coating industry are isocyanatecontaining isocyanurates, which include also aliphatic, cycloaliphatic, and aromatic derivatives. One-component urethane coatings with "blocked" isocyanate groups were developed by Bayer (_1, 3^) and Petersen (4^). Application of heat to these "splitters" with regeneration of free isocyanate groups and fast curing has led to the acceptance of these coatings as wire enamels in the electrical industry, as well as for coatings in other industries. "Urethane oils" or "uralkyds," one-component urethane coatings based on reaction products of drying oils with isocyanates, were early developments in West Germany, as well as in England and the United States (3-7). Because of low costs and ease of pigmentation, this type of coating has been extensively used in the United States. Closely related to uralkyds are urethane coatings, based on castor oil and castor oil derivatives, which have found use for maintenance and protective coatings (8-11). In more recent years one- and two-component urethane coatings based on polyethers have made a significant penetration of the urethane c o a t i n g s market. Due t o low c o s t and wide l a t i t u d e i n p h y s i c a l p r o p e r t i e s r a n g i n g from v e r y f l e x i b l e t o h a r d , tough and s o l v e n t f



41.

FRISCH AND KORDOMENOS

Urethane Coatings

987

resistant coatings, these materials have found acceptance for flexible substrates such as leather, textile, paper, and rubber and for hard substrates, such as wood (e.g., bowling a l l e y s and gymnasium floors), concrete, metals, and "seamless flooring" (12). An important application of urethane coatings is in the fabric coatings field. The term "wet look" was very fashionable for some time and was applied to highly glossy urethane coatings and was later succeeded by the "suede look." These types of coatings were applied to apparel but were also employed in furniture fabrics. Due to environmental regulations, a considerable amount of effort i s being devoted to the reduction or elimination of the solvent content through the development of high-solids or 100% s o l i d s coatings, as well as water-based coatings, for example, urethane latices. Another solventless approach to this problem is urethane powder coating systems. Raw Materials for Urethane Coatings The principal components of commercially available urethane coatings are d i - or polyisocyanates and d i - or polyhydroxy compounds. Active hydrogen-containing compounds, e s p e c i a l l y diols and diamines, as well as alkanolamines, are also employed as chain extenders. In addition, various cross-linking agents such as neutral or tertiary amine based t r i o l s or tetrols are also used. Isocyanate Components. Aromatic Di- and Polyisocyanates. The most important monomeric aromatic diisocyanates used for coatings are t o l y l e n e d i i s o c y a n a t e (TDI) and 4,4f-methylene bis(phenyl isocyanates) (MDI). Tolylene diisocyanate, a colorless liquid (bp 120 °C at 10 mmHg), i s generally used in 80/20 blends of the 2,4and 2,6-isomers. Pure 2,4-TDI isomer has also been employed for coatings. The NC0 group in the 4-position is about 8 times as reactive as the NC0 group in the 2-position at room temperature (25 ° C ) . With an increase of temperature, the a c t i v i t y of the ortho NC0 group increases at a greater rate than that of the para NC0 group until at 100 °C the ortho and para NC0 groups are similar in r e a c t i v i t y . This disparity in activity at low temperatures can be conveniently used in the preparation of isocyanate-terminated prepolymers. Low molecular weight isocyanates such as TDI have a high vapor pressure (vapor pressure of TDI = 2.3 χ 10*2 mmHg at 25 °C) and are respiratory irritants and lachrymators. Prolonged inhalation of TDI vapors may lead to symptoms resembling asthma. The maximum allowable vapor concentration has been reduced to 0.005 ppm. Good ventilation should be provided in working areas handling TDI and other isocyanates, and suitable gas masks should be used where the concentration of TDI exceeds the safe limit, especially for spray applications (13). 4,4'-Methylene bis(phenyl isocyanate) contains two equally reactive isocyanate groups: OCN-C6H4-CH2-C6H4NCO. MDI, a s o l i d , melting at 37 ° C , has a tendency to dimerize at room temperature. Storage at 0 °C or lower minimizes dimerization as does storage at 40-50 °C in the liquid state. MDI has a lower vapor pressure (10~~> mmHg at 25 ° C ) than TDI and hence i s l e s s of an i r r i t a n t . Nevertheless, contact with skin or eyes or breathing of vapor or



988

A P P L I E D P O L Y M E R SCIENCE

dust should be avoided. In addition to the "pure" MDI, a l i q u i d form of MDI (isomer mixture) is commercially available as well as a special grade of MDI containing carbodiimide groups (e.g., Isonate 143-L, Upjohn Co.). A number of "crude" isocyanates (polymeric isocyanates), u n d i s t i l l e d grades of MDI and TDI, are a v a i l a b l e in the market. Some of these products, such as various forms of crude MDI, have a functionality varying between 2 and 3. They have a lower reactivity and a lower vapor pressure than the corresponding pure isocyanate. They have found extensive use in one-shot r i g i d foams. However, they are a l s o employed i n c o a t i n g , s e a l a n t , and adhesive applications. The Upjohn Chemical Division has published an e x c e l l e n t b u l l e t i n on the use and precautions in handling isocyanates, polyurethanes, and related materials (13A). Both MDI and TDI, being aromatic diisocyanates, yield urethane polymers that tend to yellow on prolonged exposure to sunlight, presumably due to oxidation of some terminal aromatic amine (derived from these isocyanates). MDI also possesses a methylene group that i s susceptible to oxidation via a proton abstraction mechanism i n v o l v i n g autoxidation of the aromatic urethane groups to a quinoneimide structure as proposed by Schollenberger et a l . (24, 25). Beachell and Ngoc Son (16) have shown that the urethane group can be s t a b i l i z e d against thermal degradation and yellowing by substitution of the l a b i l e hydrogen atom by groups such as the methyl and benzyl groups. S i m i l a r l y , Wilson (17) reported that urethanes, treated with monoisocyanates, ketene, ethylene oxide, and acetic anhydride, can be s t a b i l i z e d to a large extent against yellowing. Indications are that both oxidation and u l t r a v i o l e t radiant energy may be involved in the processes responsible for yellowing of urethane coatings based on aromatic isocyanates. Although p-phenylene diisocyanate (PPDI) has been synthesized as early as 1913 (18), the Akzo Corp. has developed more recently a modified Hofmann process for the preparation of PPDI (19). A comparison of the reactivity of PPDI in comparison with MDI and NDI (1,5-naphtha1ene diisocyanate) as w e l l as with a l i p h a t i c diisocyanates (H12MDI, IPDI, and CHDI—see Aliphatic Isocyanates) was reported by Wong and Frisch (20). A l i p h a t i c Isocyanates. In recent years major emphasis has been placed upon efforts to develop aliphatic isocyanates to impart light stability as well as improved hydrolytic and thermal stability. The f i r s t commercial a l i p h a t i c diisocyanate used was 1,6hexamethylene diisocyanate (HDI), OCN (CH2)6 NCO, a c o l o r l e s s l i q u i d , b o i l i n g at 127 °C at 10 mmHg; i t is less reactive than either TDI or MDI in the absence of catalysts. However, certain metal catalysts, such as t i n , lead, bismuth, zinc, iron, cobalt, etc., activate the aliphatic isocyanate groups (21). HDI produces urethane coatings with better resistance to discoloration, hydrolysis, and heat degradation than TDI (22, 23). However, due to the high vapor pressure of HDI and i t s inherent danger to human exposure, a higher molecular weight modification of HDI was developed by Bayer A. G. by reacting 3 mol of HDI with 1 mol of water, forming an isocyanate-terminated biuret:


41.

FRISCH A N D KORDOMENOS

989

Urethane Coatings

NH-(CH2)6-NCO NCO

I

(CH2)6 + H20 NCO

C=0 > N-(CH2)6-NC0 C=0


NH-(Ch2)6-NCO This triisocyanate has been reported to impart good l i g h t stability and weather resistance when used in urethane coatings and i s among the most widely used aliphatic isocyanates (23, 24). More recently Bayer A. G. introduced another product based on HDI. By p a r t i a l trimerization of HDI in the presence of a catalyst, an isocyanurate ring containing isocyanate i s formed that exhibits superior thermooxidative stability and weatherability: NCO

I

(CH 2 ) 6

I

NCO

Ν

trimerization cat.

I

(ÇH2)6

/

\

o=c

c=o

N-(CH2)6NC0

OCN-(CH2)6N

NCO C

II

0

A number of other aliphatic diisocyanates that impart excellent color stability to urethane coatings have been introduced into the U.S. market. Foremost among these are 4,4,-methylene bis(cyclohexyl isocyanate) (Hi2MDI) and isophorone diisocyanate (25, 26). An isocyanurate ring containing isocyanate produced by trimerization of IPDI is also commercially available (T-1890). Hi2MDI exists in three isomeric forms, the trans-trans, transc i s , and c i s - c i s forms. Due to the fact the c i s isomer i s more s t e r i c a l l y hindered, the trans isomer reacts considerably faster with a hydroxyl group than the cis isomer (27). As with other aliphatic isocyanates, catalysis (generally organotin catalysts) is usually used for both the prepolymer preparation and the cure reaction (cross-linking). Other aliphatic diisocyanates that are being used commercially i n urethane c o a t i n g s are 3 - ( i s o c y a n a t o m e t h y 1 ) - 3 , 5 , 5 trimethylcyclohexyl isocyanate (Veba-Chemie A.G.) (28), "dimeryl" diisocyanate derived from dimerized l i n o l e i c acid (Henkel Corp.) (29), and xylylene diisocyanate (XDI) (Takeda Chemical Co.) (30). It i s interesting to note that no catalysts are required for the reaction of XDI with hydroxyl compounds and that its reactivity is similar to that of TDI. In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


990


An i n v e s t i g a t i o n by F r i s c h et a l . (31) has shown that combinations of aliphatic and aromatic diisocyanates (e.g., XDI and TDI) impart good light stability to urethane coatings. More r e c e n t l y , Takeda Chemical Co. has introduced 1,3bis(isocyanatomethyl)cyclohexane (H5XDI) for coating applications. Both one- and two-component coatings made from H5XDI as well as from adducts of H5XDI and trimethy l o i propane were reported by Tanaka and Nasu (32). S i m i l a r l y , excellent two-component urethane coatings were obtained from the adduct of trimethylolpropane and 4-bis(isocyanatomethyl)cyclohexane (BDI), which was developed by Suntech (Sun O i l Co.) (33). An interesting process for the preparation of polymeric isocyanates was reported by Wright and Harwell (34). It consists of the Hofmann degradation from the copolymers of methacrylamide with either butyl acrylate or styrene: ~*

CH 1

--CH2-C —

— --CHo-CH 1 — 1

c=o J

NaOCl

χ

—

R

>

CH3 1 -CH - 2-J — — -CH?-CH— II NCO R

-

y

—X

-

y

where R = C6H5- or -C-OC4H9

These isocyanates were used in coating applications although the r e a c t i v i t y of the isocyanate groups i s very low, even i n the presence of tin catalysts. Similarly to PPDI, trans-1,4-cyclohexyl diisocyanate (CHDI), was prepared by the process described by Zengel (19). Both PPDI and CHDI are currently available from Akzo in developmental quantities. The Dow Chemical Co. has recently been offering isocyanatoethyl methacrylate (IEM): CH3

I

0

il

CH2=C - C-0-CH2-CH -NC0 2

IEM can be used to prepare urethane prepolymers containing terminal or pendent methacrylie groups that can be cured either by means of free r a d i c a l i n i t i a t o r s or by radiation using UV or electron beam radiation. Alternately, IEM can be copolymerized with other acrylic or vinyl monomers and then cured via the isocyanate group (e.g., by moisture cure or by reaction with active hydrogen compounds). IEM can be used for the preparation of one- and twocomponent urethane coatings and adhesives as well as for crosslinking of other types of coatings containing active hydrogencontaining resin intermediates (35, 36). New types of aralkyl diisocyanates have been reported by Arendt et a l . (37). These are m- and p-tetramethylxylene diisocyanates:


41.

FRÏSCH A N D KORDOMENOS

Urethane Coatings


NCO

991

NCO

m-TMXDI p-TMXDI Similar to other aliphatic isocyanate groups, tin catalysts such as dimethyl t i n dilaurate and tetrabutyldiacetyldistannoxane and lead octoate were found to be effective for reaction of these isocyanates with hydroxyl compounds. Isocyanate Dimers and Trimers. to dimerize readily

Aromatic isocyanates have a tendency 0

2ArNC0

0 The rate of self-polymerization of isocyanates to dimers (uretidine diones) depends upon the electronic or steric influences of ring substituents. Ortho substitution greatly retards dimerization of the NCO groups, with the ortho NCO slower to dimerize. This dimerization is catalyzed strongly by t r i a l k y l phosphines (38, 39) and more mildly by tertiary amines, such as pyridine (40, 41). MDI dimerizes slowly on standing at room temperature even without catalysts but is stable at low or at slightly elevated temperatures (40-50 ° C ) . Isocyanate d i m e r i z a t i o n i s an e q u i l i b r i u m r e a c t i o n . Dissociation of the dimer occurs only at elevated temperatures (42). In the absence of catalysts, temperatures as high as 175 °C are required to completely dissociate the dimer of 2,4-TDI, although i n i t i a l dissociation was observed to occur at 150 °C (43). The preparation of aliphatic isocyanate dimers has also been reported (44)· Both aliphatic and aromatic isocyanates can trimerize to yield isocyanates:


992

A P P L I E D POLYMER SCIENCE

0

3R-NC0

> R-N/ 0=C

C

\ N-R

/

\

C=0

N


R In contrast to dimer formation, trimerization is not an equilibrium reaction since trimers are stable in the range of 150-200 °C. Ortho substitution of an aromatic isocyanate greatly reduces the ease of trimerization. Many c a t a l y s t s have been reported for the trimerization of aliphatic and aromatic isocyanates (39, 45-57). Nicholas and Gmitter (58) studied the relative effectiveness of a number of trimerization catalysts (see Table I). Isocyanurate coatings based on TDI and HDI have been reported by Kubitza (59). In addition, Sandler (60) prepared polyisocyanurate adhesives from isocyanate-terminated prepolymers employing calcium naphthenate as a trimerization catalyst. Isocyanate-Terminated Adducts and Polymers. Monomeric diisocyanates such as TDI, MDI, or H12MDI, because of t h e i r i r r i t a n t characteristics, are seldom used in an unreacted form for the compounding of urethane coatings. They are normally converted into isocyanate-terminated polymers or adducts of polyols such as hydroxy1-terminated polyesters and polyethers, castor o i l , etc. Adducts of TDI to monomeric polyols such as trimethylolpropane are frequently used. When reacted at a NC0:0H ratio of 2:1, the following product is formed: C2H5C[CH20C0NHC6H3(CH3)NC0]3. Because of its low vapor pressure, this triisocyanate adduct does not have the irritant properties of TDI and can be handled relatively safely in coating formulations. Other types of adducts are formed by reacting hydroxylterminated polyethers or polyesters with TDI. When these polyols are reacted at an NC0:0H ratio of 2:1, these adducts, also referred to as prepolymers, have the schematic structure, depending upon the functionality of the polyol, shown in Figure 1. The diisocyanate and p o l y o l s can be reacted i n various combinations by using diols, t r i o l s , and tetrols, or combinations of these in which the NC0:0H ratio is lower than 2. Typical idealized structures are shown in Figure 2. The adducts and polymers can be produced with a r e l a t i v e l y low content of free unreacted disocyanate. Because of the v o l a t i l i t y and irritant characteristics of diisocyanates such as TDI, i t is important to keep the unreacted TDI to a minimum, p a r t i c u l a r l y for spray applications. A vacuum stripping step is often recommended to eliminate this hazard. Blocked Isocyanates. For certain applications, such as wire enamels, c o i l coatings, fabric coatings, powder coatings, cationic electrodeposition coatings, etc., "blocked" isocyanates are being



41.


Table I.

993

Urethane Coatings

Activity of Catalysts in Trimerization of Phenyl Isocyanate

Catalyst 2,4,6-Tris[(dimethylamino)methyl]phenol3 2,4,6-Tris[(dimethylamino)methy1]phenol: diglycidyl ether of bisphenol A (1:1) Mixture of o- and p-(dimethylamino)methylphenylb Ν,N ,NM-Tris[(dimethylamino)propyl]sym-hexahydrotriazine Ν,Ν' ,ATff-Tris[ (dimethylamino)propyl]sym-hexahydrotriazine:diglycidyl ether of bisphenol A (1:1) Benzyltrimethylammonium hydroxide in dimethyl sulfoxide (as 25% solution) Benzyltrimethylammonium methoxide Sodium methoxide in dimethyl formamide (as saturated solution)

Relative Reactivity 1.0 1.2 1.5

%

2.4 7.7 8.5 4.0 25.0

DMP-30 (Rohm & Haas). DMP-10 (Rohm & Haas).

a

b


994


(j)NCO Diol

Triol

"Ο NCO

OCNO

Difunctional

OCN O

-Ο NCO

Trifunctional


>NCO Tetrol

ΟNCO

OCNQ-

ΟNCO = Urethane group l i n k e d t o an aromatic or a l i p h a t i c isocyanate

ONCO Tetrafunctional Figure 1.

0 OCN

Schematic structure of prepolymers.

N C 0

NCO/OH ONCO

0 -

Î

NCO

Ο NCO

1.6

ONCO

— o — ' — ο — ι — ο NCO

ι NCO OCN Ο

11 — Ο

OCNQ

Ο

Figure 2.

Ο-

Ο

î

1.55

NCO -ONCO

Ο

1.50

Ο

QNCO

Ι . 20

Typical idealized structures of diisocyante-polyol combinations in which the NC0:0H ratio is lower than 2.


41.


Urethane Coatings

995

used in order to provide one-component coatings with hydroxylcontaining or other active hydrogen-containing components. The "blocking" reaction is reversible according to the classic reaction of the adduct from trimethy l o i propane and TDI with phenol: C2H5C(CH20C0NH-R-NC0)3 + 3COH50H


C2H5C(CH20CONH-R-NHCOOC6H5)3 Application of heat ("150 °C) regenerates the free isocyanate, which is then capable of reacting with the hydroxyl component. Both a l i p h a t i c and aromatic isocyanates can be blocked by a variety of blocking agents. These include alcohols, phenols, oximes, lactams, β - d i c a r b o n y 1 compounds, hydroxamic acid esters, b i s u l f i t e addition compounds, hydroxylamines, and esters of phydroxybenzoic acid and s a l i c y l i c acid. Excellent reviews of blocked isocyanates have been written by Wicks (61). Perhaps the most widely used blocking agents at present are phenol, branched a l c o h o l s , 2-butanone oxime (methylethyl ketoxime), and ε caprolactam. The kinetics of the unblocking reaction of aliphatic and aromatic isocyanurate ring containing blocked isocyanates were studied by Kordomenos et a l . (61B) using isothermal thermogravimetric analysis. It should be pointed out that a number of blocking agents, notably 3-dicarbony1 compounds such as raalonic esters and acetoacetic esters, give abnormal products (62, 63), for example, mixtures of esters and amides, presumably via ketene intermediates. More recently, newer types of blocking agents have been reported such as AMiydroxyphthalimide and JV-hydroxysuccinimide (64), aromatic triazoles (J5_5 ), and s u b s t i t u t e d imidazolines and tetrahydropyrimidines (66). Ulrich et a l . (67-69) have demonstrated that macrocyclic ureas w i l l dissociate thermally to y i e l d amido isocyanates: (CH 2 ) n -

reflux N-C-C 6 H 5

> C6H5NHC0N(CH2)nNC0

0

It was found that the reaction proceeds when η = 4 or 5 but not when η = 2 or 3. Difunctional macrocyclic urea amides were made by reacting the macrocyclic urea with diacid chlorides such as isophthaloyl chloride. The u t i l i t y of these cross-linkers was proven in solvent coatings, acrylic powder coatings, and electro deposition coatings. Catalysis plays an important role in the deblocking or thermal dissociation of the blocked isocyanates. Notably organometal l i e compounds and tertiary amines are capable of lowering both the deblocking temperature and time as compared to the uncatalyzed systems. Wicks (61) has pointed out that since most of the deblocking reactions are carried out in the presence of hydroxyl



996


compounds, the role of the catalysts in the deblocking reactions may i n v o l v e catalysis of the alcoholysis reaction between the a r y l urethane and the hydroxyl group of the second component. In addition to t i n compounds such as d i b u t y l t i n dilaurate and d i b u t y l t i n diacetate, various other metal compounds have been claimed to be e f f e c t i v e deblocking c a t a l y s t s such as zinc naphthenate (70), lead naphthenate, bismuth s a l t s , and titanates (71), and Ca, Mg, Sr, or Ba salts of hexanoic, octanoic, naphthenic, or l i n o l e n i c acid (72) and metal acetyl acetonates (73). Also combinations of organotin compounds and quaternary ammonium s a l t s have been claimed to have a synergistic effect in lowering the deblocking temperature (74). The use of blocked isocyanates for aqueous systems is of special interest. The sodium b i s u l f i t e b l o c k i n g of hexamethy1ene diisocyanate was already described in Petersen's classic paper on blocked isocyanates (4^). Its application in cross-linking paper (75) and acrylamide copolymers (76) has been described. The stability of bisulfite-blocked aromatic isocyanates in water can present a d i f f i c u l t y , and various s t a b i l i z e r systems have been proposed. Peters and Reddie (77) have made an extensive study of the various factors affecting stability and have found that optimum stability was obtained at pH of 2-3 in aqueous ethanol with excess hydrogen peroxide. Sodium b i s u l f ite-blocked isocyanate-terminated prepolymers are e x t e n s i v e l y used as s h r i n k - r e s i s t a n t f i n i s h e s for wool. Polycarbamoylsulfonates (PCS) are produced from NCO-terminated prepolymers with bisulfite salts in aqueous alcohol as the solvent, shown schematically as follows (78-80): H20 R(NC0)n + NaHS03 _ - _ - - > R(NHC0S03 Na+)n NCO-terminal prepolymer

PCS

The principal curing reaction of PCS is the hydrolysis to form urea cross-links: H20 2RNHC0S03-Na+

> RNHCONHR + C02 + 2NaHS03

Other water-soluble, blocked isocyanate cross-linkers have been prepared by reaction of diisocyanate with α,α-dimethylolpropionic acid followed by b l o c k i n g with caprolactam and subsequent s o l u b i l i z a t i o n in water by means of a tertiary amine (81) or 2ethylimidazole (82). Blocked or partially blocked isocyanates have been extensively employed in cationic electrodeposition. Polymeric compositions employed for cationic electrodepositable coatings have been reviewed by Kordomenos and Nordstrom (82A). Matsunaga et a l . (83) were the f i r s t to synthesize cationic thermosettable urethane resins. The system described was prepared from an isocyanato-termina ted polyurethane prepolymer by reacting the free isocyanato groups with a tertiary amine that had at least three hydroxyl groups. The


4L

997

Urethane Coatings



polyurethane prepolymer was prepared by reacting a diisocyanate with a d i o l at a ratio of NCO:OH = 1.5-2.0. One of the examples given was synthesized according to the following scheme:

^ 2 N-cCH 2 CH 2 OH) 3 CH o CH o 0H I 2 2

Ο «

N-C^CH^O-C-NH-T

CH 2 -CH 2 OH

,

Ο

CH3-^Q^NHCO—R

.

υ

-OC NH-^^V»CH, NH I C=0 0

/CH 2 CH 2 OH 1 CH2CH2NN CH 2 CH 2 OH R=

CH-> CHj 3 I 3 (CH o C-0) - C H 0 C z η ζ

Pampouchidis et a l . (83AtB) also synthesized urethane-based cationic electrodeposition resins. In one of the examples, 1 mol of isophorone-diisocyanate was reacted with 1 mol of dimethylpropanolamine and the remaining isocyanato groups were reacted with 0.5 mol of tripropylene glycol monomethacrylate and 0.5 mol of pentaerythritol trimethacrylate

a c c o r d i n g t o the scheme on the f o l l o w i n g page.

This reaction took place i n the presence of polymerization inhibitors. The majority of the currently used coating systems i s based on copolymers prepared from epoxy resins, polyfunctional amines, and p a r t i a l l y blocked isocyanates (84-87). The resins are generally solubilized by the formation of cationic derivatives of amines by aminolysis of the carbamate in the p a r t i a l l y blocked isocyanate. The principal applications of these cationic electrocoating systems are as automotive primers, in appliances, as primers and one-coat systems, and in various other i n d u s t r i a l applications. The main In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

998


CH CH'

NCO

CH 3

CH2NCO

CH CH Downloaded by GEORGETOWN UNIV on August 18, 2015 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch041

CH1V J>N-CH 9 CH 9 0H CH 3

+

NCO

CH-

ROH

CH„NHCOCH„CH„N< «

2

2

2

Ο

^CH\

CH-

Ο

rpf^\~

NHCOR

Cn

CH 3

NHCOCH2CH2N

Applied Polymer Science - American Chemical Society

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