CA1193384A

CA1193384A - Epoxy resin curing with modified polyanhydride

Info

Publication number: CA1193384A
Application number: CA000388859A
Authority: CA
Inventors: Hans R. Falkenburg; Siegfried Krause; Robert C. Mcguiness
Original assignee: HERMANN WIEDERHOLD GmbH; Imperial Chemical Industries Ltd
Current assignee: HERMANN WIEDERHOLD GmbH; Imperial Chemical Industries Ltd
Priority date: 1980-10-27
Filing date: 1981-10-27
Publication date: 1985-09-10
Also published as: ZW25581A1; EP0050940B1; AU7672781A; ZA817409B; DE3173824D1; US4391965A; EP0050940A1; ATE18060T1; JPS57135825A; GB2087875B; NZ198706A; GB2087875A; BR8106910A

Abstract

ABSTRACT

A modified polycarboxylic acid polyanhydride curing agent suitable for curing epoxy resins which is prepared by reacting a polycarboxylic acid anhydride with a monohydroxy compound ROH; and coating compositions, comprising the modified curing agent and a solution of an epoxy resin to be cured, which have improved storage stability.

Description

1~9338'~

EPOXV E~:SIN CURING WITH MODIFIED POLVANHYD~ID~3 This invention relates to improved curing agents and to their use in coating compositions, more particularly to improved curing agents which are based on polycarboxylic acid polyanhydrides and to their use in coating compositions which comprise an epoxy resin.
Acid anhydrides are well known as curing agents (or "hardeners") for epoxy resins, particularly where stability of the product at high temperatures is re~uired. They have found important application in ~he ormulation of powder coatings and in 1. ;n~ting and other "buIk" polymerisation processes where it is not necessary for the reactants to be dissolved in an organic solvent. In general the acid anhydrides which are use-ful as curing agents for epoxy resins have poor solubility lS in those organic solvents in which the epoxy resins are soluble and in organic solvents which are suitable for example with respect to toxicity and cost, in formulating solvent-borne coating compositions. Therefore, the anhydrides ~hich are solids at ambient temperatures, as opposed to t:hose which are liquids, have found limited use hitherto in solvent-borne coating compositions.
Where it has been desired that an a~id anhydride curing agent shall produce a high level of crosslinking in a cured product, for example in a coating derived from a powder coating composition, there has been used a polyanhydride, for example a dianhydride such as a condensation product of tr mellitic anhydride and a poly-ol. The useful polyanhydrides, as indicated above, have ~3~

a low solubility in the solvents which are suitable in formulating coating compositions comprising an epox resin. Therefore, if they are to be used in solvent-borne coating compositions, tl1ey are frequently S present initially as a dispersion or "paste" in an organic liquid which is mixed with a solution of an epoxy resin in the same or a similar organic liquid some time before the resulting coating composition is applied to a substrate.
Disadvantages of the use of a dispersion of the polyanhydride are for example: (1) the coating compos-ition may be initially inhomogeneous and o~ gradually changing character; and (2) depending upon the par~icular polyanhydride, the mixture may remain inhomogeneous, making impossible the formulation o a completely soluble product.
Moreover, it is fo~md that in those cases where a solvent-soluble coating composition compxising a mixture of a solution of an epoxy resin and the dispersion of a polya~hydride in an organic liquld (as discussed above) can be made, this has a relatively short storage life; e.g. after 8 days at 40C one composition is u~suitable for practical use and after 17 days at 40C
the same composition has gelled.
~5 We have now found that a polyanhydride curing agent as herein defined can be modified, so as to produce an improved curing agent suitable for subsequent use with a solution of an epoxy resin to be cured, by taking a dellberate step to cause xeaction ketween the polyanhyd-ride and a monohydroxy compound ROH, also herein defined, and by producing a solution of the reaction product in said compound, and/or in one or more organic liquids 3 ~ L~

which is/are miscible with e solvent for the epoxy resin ko be cured. It is believed (see Kirk-Othmer, Encyclo-padia of Polymer Chemistry. Epoxy Resin~, p.230 Acid Curing Agents) that~in the mechanism of curing an epoxy S resin by an acid anhydride, at elevated temperature the anhydride irst reacts with a hydroxyl group, which is very often present in the epoxy resin, to cause xing~
openiny of the anhydride; and that the resulting ~arboxyl group derived from the anhydride reacts with an epoxy 10 group to yield a new hydroxyl group. It is, therefore, very surprising - bearing in mind the fact that the anhydrides are more effective curing agents than are the free acids - that we ~ind advantage in prereacting the anhydride group before it is brought into contact with lS the epoxy resin.
Thus, according ~o one aspect of this invention we pxovide a n~w curing agent suitable for use in combin-ation wikh a solution of an epoxy resin to be cured comprising t~ie product which xesul~s by taking a deliberate 20 step to cause reaction be~ween an acid polyanhydride curing agent as herein de~ined and a monohydroxy compound ROH
as herein d~fin~d.
Preferably the new curing agent also comprises one or more orga m c liquids which are miscible with the 25 solution o~ an epoxy resin to be cured.
This invention also provides a coating compos-ition which comprises in admixture:
(a) the new curing agent which i~ defined above, and (b) a solution of an epoxy resin (as herei~
de f ined) which is to be cured.
This invention ur~her provides a coating on llD33~

a substrate w~ich is prepared by applying to the substra~
a co~ting composition as def:ined above and kherea:Eter heating the substrate to a temperature yreater than 100 C
By a polyanhydride curing agent we mean a substance which i~ kncwn conve~tionally a~ a curing agent for an epoxy resin and which compxises a compound having two or more dicarboxylic acid anhydride groupæ
O which each form part of a ~-ive-membered - C ring. There may be present i~ the sub-~ 0 stance more than one pure chemical com-- C \ pound, for example two or more compounds o each having two or more dicarboxylic acid anhydride groups; or a mixtuxe~o~ such a com~und or compounds together with compounds which do not contain two or more dicarboxylic anhydride groups, for example compounds which contain only one anhydride group, free carboxylic groups, ox ester groups.
Suitable polyanhydrides include the trimellitic anhydride adducts o~ the general formula:
~ CC ~ _ C - 0 - R - 0 ~ C ~ C0 ~

wherein R is a hydrocarbon radical, preferably having from

2 to 12 carbon atoms, and wherein the aromatic nuclei may be substituted with one or more halog0n atoms and/or one or more hydrocarbyl groups. Particularly suitable adducts are the trimellitic anhy~xide-polyhydric alcohol adducts, preferably an adduct of a glycol such as ethylene glycol, and e.specially the adduct ~ CO ~--C ~ O ~ C~2 - CH2 - 3 - C~@~CO ~
Other suitable polyaIihydrides include ~he benzophenone tetracarboxylic dianhydrides o~ the formula~
e f ~3~,~ C _~ CO ~, ~co x x co~
w~erein X xepresen~s hydrogen, halogen, -~O~,-COOH, o~ SO3H and may be the same or dif~erent radicals.
Examples of benzophenone tetraca~boxylic dianhydride~
i~clude 3,3',4,4' - benzophenone tetracarboxylic dianhydride: 2~bromo-3,3',4,4' - benzophenone tetra-c~rho~ylic dianhydride; and 5-nitro-3,3',4,4' - benzo-phenone tetracarbox~lic dianhydri.de.
Other polyanhydrides which may be used are the polyfunctional cyclic anhy~ride~. Among these, are cyclopentane tetracarboæylic acid dianhydride. diphenyl-ether tetracarboæylic acid dianhydride~ ~he hexacarbo~ylic acid tria~hydride~ of benzene, and o~ cyclo~ n~, and 1,2,3,4-butane tetracarboxylic dianhydr iae.
Some commercially available polyanhydride curing agents ~re mixtures o~ two or more compounds, which re~ult from the condensation of kwo or more comp~
ounds a~ laa~t one of w~ich i~ polyfunctional; ~or example, ~r~m the condensation o~ a diol or`~riol ..~g~
et~le~e glycol or glycerol with krimellitic ar~ydride.
By a con~pourld RO~I we mean a compound whi-ch will react with a dIcarboxylic acid anhydride group, as exemp-li~ied by ph~ anhydride to ~orm an ester~ ~he group R may b~ for exam~le C1_1g al~yl, substituted alXyl or aralkyl. Typical alkyl substituent~ include ether gxoups and esker g~oup~.
Ex~mple~ o ~uitable compounds RO~ include ~lk~ols such as butanol~ isobutanol, hexanol, octanol and dodecanol; e~her~lk~nol~ ~u~h as the monoethyl ethers o~ ethylen2 glycol; and hydroxy~sker~ such as butyl ~33~

glycolate. ~he choice o~ a su.itable com~ound P.O~ will of course depend upon the nature, or example khe reactivity, of the polyanhydride with which it is to be specifically react~d. Preferably the compound RO~ is a liquidO
Examples of organic liquids which are suitable soLvents or epoxy resins when used in coating composit-ions and which may therefore be used, optionally in co~junction with the compo~nd ROH,to produce a solution of the curing agent o this invention include ester solv~nts such as ~-ethoxyethyl acetate, 2-methoxyekhyl ace$ate, 2~isopropoxyethyl acetate, butyl acetate' ketone solvents such as methyl ethyl ketone, methyl isobutyl ketone; e~her sol~e~ts~and aromatic solve~ts such as xyleneO

The new curing agent of this invention is prepared by taking a deliberate step to cause reaction be~ween the polyanhydride and the compound RO~. Prefer-ably there is taken the deliberate step o.f promoting the reaction at an elevated temperature, for example at a temperature greater than am~ient and more preerably at greàter than 60 C, ~or such a period of time that a substantial proportion o~ the polyanhydrid~ groups are reacted with the compound ROH. There i9 ~mployed ~rom 0.5 to 5.0, and more preferably 0.5 to 2.0, equi~alents of ROH for each a~hydride group of the polyanhy~ride.
~e believe, but the scope of our invention is not to be intQrpreted or limited i~ the light of this belief, that there is produced as th~ novel curing agent a partial ester o~ the polyanhydride and the compound RO~
and that it is essential or complete success that at leas~ 50% and pre~rably at lëast 75~/0 of the anhydride

3~

groups of the polyanhydride are reac~d with the compound ROH. This is to be clistinguished from any other circumstances where, ~or example at ambient temperature~
the polyanhydride is merely mixed with a compound ROH as a solvent and esterification either does not take place or is of li~t~e conse~ue~e. We believe that a partial and - deliberate esterification usi.ng the compound ROH may be responsible ~or the improved stability of a mixture of a solution of the curing agent of this invention together with a solutio~ of an epoxy resin, as compared with the relative instability of a mixture of the polyanhydride precursor of our curing agent together with a solution of the epoxy resin.
The coating compositions of this invention may co~tain any of the conventional additives ~uch as flow agents, accelerators and pigments.
The invention is illustrated by the following Example~ in which parts are by weight.

E~MP~E 1 Th.is Example illustrate~ the advantageous e~ect on storage ~tability (as indicated by the variation of vi~cosity with time) o~ deliberately reacting with a conventional commercially available polyanhydride curing agent a monohydroxy compound ROH, butyl glycolate, before it is mixed with a solution of an epoxy resin, A commercially available polyanhydride curing agent, "Epikure" F~SO ("Epikure" is a Registered Trad~
Mark o~ Shell~ believed to be a condensation product of trimellitic anhydride and a diol~ was mixed with an organic liquid or liquids (including ROH) as indicated in Table I to give a product 'X'.

TABLE X

Polyan- 2-methoxy ROH Prepar- Initial Example hydride ethyl (butyl ative appear-~o. curing acetate glycol- con~it- ance~
agent (parts) ate.) ionS for nature (parts) (parts) prod~ct uct '~' 1.1 40 60 - ) Mixed at Tnhomo-) ambientgeneous ~ temp.
1.2 40 30 30 ) (25C~. "
Mixed &
1.3 40 30 30 at 150CClear /1 hr.

29 parts o~ each o~ the three products 'X' of Table I were mixed with 150 parts of a solution of an epoxy resin solution 'Y' prepared by dissolving 40 parts o~ "Araldite" GT 6097 (a commercially available poly-epoxide of epoxide equivalent 1700-2500 ~ Araldite" i5 a Registered Trade Mark of Ciba-Geigy) in 60 parts of 2-ethoxy ethyl acetate to provide a lacquer.
The increase in viscosity with time of the lacquers X-Y prepared from the product 'X' and the solution 'Y' was determined according to Table 2.

Viscosity ~seconds DI~ 4 mm viscosity Example of lacquer X~ at 20C~
No.
InitialAftex 6 weeks at 40C
1~4 mixturemixture remains inhomogeneous inhomogeneous 1~5 92 371 1.6 99 154 The lacquer X-~ o~ Example 1.6 was spread onto tinplate by a roller coating method and cured in an oven for 12 minutes at 200 C to produce a dried film of 10 micron thicknes~.

Adhesion of the film to the tinplate wa~ shown to be excellent by the crosshatch/tape test, a test well sstablishea in the coatings industry.
The resistance o~ the film to solvents was good as indicated by its being able to withstand 40 double rubæ with methylethyl. ketoneO

EXAMPLÆ 2 This Example illu~trates the use of 2-ethoxy-ethanol as the compound R0~, instead o~ that used in lS Example 1, but the other components o~ the lacquer were the same.
Polyanhydride was mixed with organic liquids according to Table 3~ to give a product 'X'.

3~'~

Example Polyan- Organic Liquids Prepar- Initial ~o. hydride 2-meth- RO~ ative appear~
curing oxy 2-eth- condit_ ance/
agent ethyl oxy- ionS fo~ ~ature (parts) acetate ethanol product f (parts) (parts) 'X' pxoduct 2.1 40 60- ~ mixed at Inhomo-) ambient geneous 2.2 40 39.5 20.5 ) tempar-) ature ) (25~)-2.3 40 39~5 20.5 Mixed ~ Clear - ~eated soln.
at 150C
/2 hrs.

~ he inerease in viscosity with time of lacguers X-Y pxepared by m; x; ng the ~roducts 'X' with solutions 'Y' prepared a~ in ExampLe 1 was determined according to Table 4.

1 Viscosity (seconds DI~ 4 mm viscosit~
Examp e o~ lacquer X-Y cup at 20 C) Initial ¦ After 5 weeks at 40C
2.4 RPmA; n~ ; n~lOmOge~ 30US
2.5 79 ~01 2.6 94 99 The lac~uer X-Y of 2.6 was spread onto tin-plate by a roller coati~g method and cured in an oven ~or 12 mi~utes at 200C to produce a dried film of 10 micron thickness. Adhesion of the film to the substrate was excellent (crosshatch test~ and the film withstood 40 double rubs using methylethyl ketone.

33~

E~MPLE 3 This Example illustrakes the use o~ 2-ethyl-hexanol as another monohydroxy compound RO~. The other components r~m~; ned th~ ~ame a~ in Example 1.
Polyanhydride was mixed with organic liquids according~to Table 5.
TABLæ 5 r P~an- Organic liquids Prepar- I tial ~xample hydride 2-meth- ROH ative ance/
~o. curing oxy (2-ethyl condit- nature agent ethyl hexanol~ ions for 0~
(parts) acetate (parts) product product (parts) 'X' ,~, 3.1 40 60 - ) mixed at Inhomo-) ambient geneous 3~2 40 30.5 29.5 ) temper-) ature ) (25C) 3.3 40 30.S 29.5 Mixed &
Heated c,lear at 150C ~oln.
/2 hr~.

The increase in viscosity with time o~ ].acquers X-Y prepared by m;~;ng products 'X' with solutions 'Y' prepared as in Example 1 was determined according to 10 Table 6.
~AB~E 6 Viscosity (seconds DI~ 4 mm cup at 20~C) ~xample of lacquer~ X-Y
Initial ¦ Aftex 5 weeks at 40~C
3.4 R~;nq ;n~mogeneou8 3.5 105 165 3.6 111 128 A ~ilm prepared ~rom the lacquer mixture X-~of 3.6 as descxibed in Example 1 had eæcellent adhesion and a solvent resistance of 50 double rubs using methyl-ethyl ketone.

This Example illustrates the use of another monohydroxy compound ~0~, n-butanol. The other compon ents remained the same as in Example lo Polyanhydride was mixed with organic liquids according to Table 7.
T~BLE 7 Organic Liquids Initial Example Polyan- Prepar- ~ppear-~o. hydhide 2~meth~ RO~ ative ance/
~uri~g oxy (n- condit~ Nature a~ent ethyl butanol) ions ~or o~
~part~) acetate (parts~ 'X' product (parts) 'X' lo 4.1 40 60 ~ ) mixed at Inhomo-) ambient geneous ) temper-

4.2 40 46.4 13.6 ) ature ) (25C) 4.3 40 46.4 13.6 Mixed ~ ~lear heated soln~
at 150C
/1 hr.

The increase in viscosity with time o~ lacquers X-~ prepared by mi~;n~ products 'X' with solutions 'Y' prepared as in Example 1 was determined according to Table 8 ~

3~

T~BLE 8 Vi.sco~ity (seconcls DI~ 4 mm cup a~ 20C3 Example o~ lacquer~ X-Y
Initial A:P.ter 5 weeks at 40C
4. 4 RemaiIls ; nhor ogen~ous 4.5 7g 1~0 4. 6 92 10~
A f ilm prepared from the lacquer mixture X-Y
oi~ 4. 3 as described in Example 1 had excellent adhesion and a solvent resistance of 70 double rubs using methyl-

5 ethyl ketone.
E2~D?LE 5 This Example illustrates the use o~ a poly-anhydride, benzop~enone tetraca:~boxylic acid diardlydride, which is different ~rom that used in Ex~mples 1 to 4, together with buty lglycolate as the monohydroæy compound RO~I.
Benæophenone tetracarboxylic acid dianhydride was mixed with organic liquids according to Table 9.

! Organic liquids Initial Example Polyan- 2 meth- ROH Prepar- appear-~o. hydride oxy (butyl~ ative ance/
cuxing ethyl glycol- condit- Nature agent acetate at~ ions of of pro-~parts) (parts~ (parts) 'X' duct.X
5.1 40 60 - ) mixed at Inhomo-an~ient geneous 5. 2 40 22 38 ) temper-) ature ) (25C) 5.3 40 22 38 EeatedClear at 150C soln.
/l hr.

The incxease in viscosity with time o~ lac~uer~
X Y prepared by mi~ g products 'X' with 501utions prepared as in Example 1 was determined according to Table 10 :

Viscosity (seconds DI~ 4 ~m cup at 20C~
Exampleo~ lacquers X-Y
'Initial ~ After 5 weeks at 40C
5 . ~R~; n9 ; n~mogeneous 5.5Remains ;nho~geneous 5.G124 ¦ 172 A film prepared from the lacquer X-Y of 5.6 as described in Example 1 had excellent adhesion in the crosshatch test and a solvent resistance of 40 double xub~ using methylethyl ketone.
E~A~PIE 6 This Example illustrates the u~e o~ a poly-anhydride, butane - 1,2,3,4, - tetracarboxylic dianhyd-ride, w~ich i~ different ~rom that used in Example~ 1 to 5, together with butyl glycolate as the compound R0~.
Butane ~ ,3,4 - tetracarboxylic dianhydride was mixed with organic liquids as described in Example 1 and according to Table 11 :

,, .
.
, ----TABLE ll Oxganic l.iquids Initlal Example Polyan- Prepar- appear-~o. hydride 2-meth~ ROH ative an~e/
curi~g oxy (butyl condit- ~ature agent e~hyl glycol- ions of of dis-(parts3 acetate ate ~ per~ed tparts) ~parts) product 'X'

6.1 19.380.7 - ) mixed at Inhomo-) ambient geneous ) temper-6~2 19.350.7 30 ) ature "
) (25C) 6.3 19.350.7 30 Mixed &
heated clear at 160C soln.
/2 hrs.
The increase in viscoqity with time o~
lacquers X~ prepared by mi~;n~ products 'X' with solutions Iyl prepared as in Example 1 was determined acccrding to Table 12 :

Example Viscosity ~second~ DI~ 4 mm cup at 20 C) Initial ¦ After 5 weeks at 40C
6.4 R~m~in.q inhomogenaous 6.5 R~m~;n~ i.nhomogeneou~
6.6 76 1 ~2 A film prepared from the lac~uer X-Y of 6.6 as described in Example 1 ~ad ex~ellent adhesion in the crosshatch test and a so7vent resistance of 40 double rubs using methylet~yl ketone.

~his Example illustrates the advantageous e~ects on the storage s~ability of lacquers prPpared 3~

according to ~he invention in Examples 1 to 4 even in the presence of hexabutoxymethylmelamine as an accelerator, which is described in our copending Canadian Application No. 388,849.
Experiments 1 to 4 were repeated but 0 D 5% by weight based on the weight of the total formulation o~ a commercially available hexabutoxymethylmelamine was added in each case:

Ex~?er iment ~o. Visco~ity (seconds DI~ 4 mm cup at 20C~
~referriny to previous Initial After 2 weeks at 40C
Example¦.

/ 1.4 Remains inho~ogeneous 1.5 91 1 256 1.6 95 1 16 2.4 Remains tn~t ~cgeneous 2.5 91 1 173 2.6 83 1 118 ~4 R~ ~;n.~ inhomogeneous 3.5 98 1 177 3.6 97 1 137 4.~ ~t~A i n ~ inhomogeneous .5 88 16~
.6 ~0 118 ~.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process of coating a substrate which comprises applying to the substrate a coating composition comprising in admixture to separately prepared solutions (a) a solution in a volatile organic solvent of an epoxy resin which is to be cured, and (b) a solution in a volatile organic solvent of a curing agent for the epoxy resin which has been prepared by heating an acid polyanhydride curing agent with a monohydroxy compound ROH, wherein R is C1-18 alkyl, substituted alkyl or aralkyl, to a temperature greater than 60°C for such a period of time that at least 50%
of the anhydride groups are reacted with ROH, and thereafter heating the substrate to a temperature greater than 100°C.

2. A process according to Claim 1, wherein a solvent for the epoxy resin and optionally also for the curing agent is selected from an ester solvent, a ketone solvent, an ether solvent and an aromatic solvent.

3. A process according to Claim 1 wherein the monohydroxy compound ROH is selected from an alkanol, an ether alkanol and a hydroxy ester.

4. A process according to Claims 1, 2 or 3, wherein at least 75% of the anhydride groups have been reacted with ROH.

5. A process according to Claim 1, wherein the polyanhydride is an adduct of trimellitic anhdride and a polyhydric alcohol.

6. A process according to Claim 5, wherein the polyol is ethylene glycol.

7. A cured coating on a substrate obtained by the process of Claim 1.