Progress in Organic Coatings 51 (2004) 267272

Weathering study of epoxWagchnolo

d 8 July

Abstract

The weath phenolF (DGEBXF h varioto accelerate using Dbetter chalk ed witThe methyl fone grdegradation . Whepaints showe th poorelationship ly twoequal to 1 da a nearl 2004 Pub

Keywords: Epoxy resins; Curing agents; Paint; Accelerated weathering; Environmental weathering; Chalk resistance

1. Introdu

Epoxy rfield becausatility [1].generally thence of UVfairly goodepoxy paindiscoloratiocern for theoutdoor ap

Kellehehigh molecfound thatas gel forming occurrebased polyresin led to

CorrespoE-mail a

0300-9440/$doi:10.1016/jction

esins have tremendous applications in the polymerse of their range of attainable properties and ver-However, due to the presence of aromatic moietyey absorb at about 300 nm and degrade in the pres-light and humidity. Weathering of epoxy resins isfor castings and laminates but for thin films and

ts, it is generally regarded only as fair, because ofn and chalking. This is the major cause of con-polymer chemists limiting the use of epoxies for

plications.r and Gesner [2] have studied the degradation ofular weight epoxy polymer of bisphenol A. Theythere was a decrease in intrinsic viscosity as wellation, indicating that chain scission and crosslink-d simultaneously. The irradiation of bisphenol Amers viz., polysulfone, polycarbonate and epoxy

chain scission as indicated by the reduction in

nding author.ddress: vcmalshe@hotmail.com (V.C. Malshe).

intrinsic viscosity. However, of the three, epoxy was the onlymaterial that also showed gel formation along with chain scis-sion. The singularity of this effect excluded the participationof the bisphenol portion of the chain in the gel forming reac-tion. Furthermore, irradiation sufficient to cause a productionof 25% gel in the epoxy was ineffective in forming gel in acompletely acetylated polymer. This clearly showed that thehydroxyl groups generated by the ring opening of epoxidewere responsible for the crosslinking reaction [3]. Riveton etal. [4,5] also carried out the photo-oxidation of epoxy resin atlong and short wavelengths and found that the main reactionsinvolve the methylene groups in the -position to the ethergroups formed due to the dehydration of secondary hydroxylgroups.

Silicone compounds are well known blocking agentsfor the hydroxyl groups [6,7]. The effect of aminosiliconecompounds as curing agents for epoxy resins have beenreported and found to improve their water resistant properties[8]. Ouyang et al. have also explored the effectiveness ofthin films of SiOx coatings in protecting polypropyleneand polyurethane polymers against photo-oxidation [9].Also the direct photolysis of the Si O bond is rather

see front matter 2004 Published by Elsevier B.V..porgcoat.2004.07.007V.C. Malshe, GulzarPaints and Polymer Division, University Institute of Chemical Te

Received 26 March 2004; accepte

ering characteristics of TiO2-based paints of diglycidyl ethers of bis), bis-2,6-xylenol S (DGEBXS) and bis-2,6-xylenol (DGEBX) witd and environmental weathering. Of all the epoxy resins, the paintresistance, optimum yellowing and good gloss retention when cursubstitutions on aromatic ring and presence of methylene and sulof epoxy resins as evidenced by their faster chalking on exposured excellent improvement in chalk resistance but were associated wibetween the two types of weathering was found to be approximatey in environmental weathering in Mumbai (India), thus leading to

lished by Elsevier B.V.y paintshoogy, Matunga, Mumbai 400019, India

2004

A (DGEBPA), bisphenol F (DGEBPF), bis-2,6-xylenolus curing agents have been studied by subjecting themGEBPA gave the best weathering characteristics with

h amine terminated dimer fatty acid-based polyamide.oups between the phenolic moieties caused enhancedn cured with an aminosilane compound all the epoxyr initial gloss and more yellowing during exposure. Theand a half hours exposure in accelerated ageing beingy 10-fold acceleration of weathering.

268 V.C. Malshe, G. Waghoo / Progress in Organic Coatings 51 (2004) 267272

impossible because the dissociation energy of the Si Obond is 185 kcal/mol [10].

In the present work, the chalking characteristics of thepaints madits chemicaare reportediglycidyl

The poshypothesizisopropylidgroup and

To studynamely, digcidyl etherbis-2,6-xylxylenol S ((DGEBX)group betwthe compaDGEBPA.matic ringtution on tDGEBX wwere alsodroxyl groing agent oepoxy whilgenerated dtion was stthalic anhyhydride. Sithe epoxy rcuring agening them tochalking, g

2. Experim

2.1. Mater

DGEBP180 was su(LAPOX XLtd. Valsaxylenol andcals Ltd., a

(FPA), with amine value 200 by Uniform Paints and-(aminoethyl)aminopropylmethyldimethoxysilane (AS) byReliance Silicones (India) Ltd. Epichlorohydrin, xylene,

ol, trA), dllitic

A), cgrad

Prepa

GEBXBX wmethlenolstio inanicahe mi% aquover a

o 110h, cooated ared cruepichly resinmple syleneill byS we

PA, Tn in th) of th

% and

Evalu

e paie 75 mrm coof acc

e weahis evight ae from the conventional liquid epoxy resin andlly modified forms with different curing agents

d. The chemical structure of typical amine curedether of bisphenol A is

sible weak links in the amine cured epoxy can beed as indicated by dotted lines in the structure: (A)ene group, (B) aromatic group, (C) alkyl hydroxy

(D) N-alkyl group.the effect of individual group, five epoxy resinslycidyl ether of bisphenol A (DGEBPA), digly-of bisphenol F (DGEBPF), diglycidyl ether of

enol F (DGEBXF), diglycidyl ether of bis-2,6-DGEBXS) and diglycidyl ether of bis-2,6-xylenolwere considered. DGEBPF, containing methyleneeen the two phenolic moieties was chosen for

rison with the isopropylidene group present inDGEBXF, having 2,6-dimethyl substituted aro-was taken to check the effect of methyl substi-he aromatic ring. DGEBXS having sulfone andithout any group between the phenolic moietiestaken for the study. The role of secondary hy-ups was evaluated by using an aminosilane cur-f which amine part was responsible for curing ofe the alkoxy portion silylated the hydroxyl groupuring the curing stage. The effect of N-alkyl por-

udied by using non-amine curing agents like ph-dride, trimellitic anhydride and pyromellitic dian-mple TiO2-based white paints were made from allesins and cured on aluminum panels with severalts. These panels were then evaluated by subject-accelerated and environmental weathering for theloss and yellowing properties.

ental

ials

A (GY250) with epoxy equivalent weight (EEW)pplied by Ciba Speciality Chemicals, DGEBPFR40) with epoxy equivalent weight 185 by Atul

d, bis-2,6-xylenol F, bis-2,6-xylenol S, bis-2,6-rutile grade TiO2 by Filtra Catalysts & Chemi-

mine terminated dimer fatty acid based polyamide

butan(TETtrime

(PMDtheticLtd.

2.2.

DDGEdardbisxylar ramechter. Tof 40wiserise tfor 1separsintecess

epoxSi

and xball mand ACA,ratio(PVCat 17

2.3.

Thof sizunifomentto thFor tUV liphenyl phosphine (TPP), triethylene tetramineicyandiamide (DICY), phthalic anhydride (PA),anhydride (TMA), pyromellitic dianhydride

yanuric acid (CA), sodium hydroxide, all of syn-e, were procured from Ranbaxy Fine Chemicals

ration

F with EEW 195, DGEBXS with EEW 245 andith EEW 190 were synthesized using the stan-

od of epoxy preparation. In a typical method,and epichlorohydrin were charged in 1:10 mo-

a four neck cylindrical glass vessel fitted with al stirrer, addition funnel, condenser and thermome-xture was heated to about 70 C and then 2.1 moleous sodium hydroxide solution was added dropperiod of 1 h, the temperature being allowed to

C. The mixture was continued to reflux at 110 Cled and allowed to separate. The organic layer wasfter washing with water and then filtered using acible. The filtrate was distilled to remove the ex-

orohydrin to get a clear pale yellow viscous liquid.olvent-based white paints using epoxy resin, TiO2butanol (1:1) solvent mixture were prepared in

milling for 24 h. The curing agents FPA, TETAre added at the time of application whereas DICY,MA and PMDA were mixed during paint prepa-e ball mill. The pigment volume concentratione paints including the curing agent was optimizedthe total solid content was 60%.

ation

nts were applied on 1 mm thick aluminum panelsm 150 mm by flow coating method to have a

ating thickness of about 5080m. In the assess-elerated weatherability, the panels were exposed

thering in QUV apparatus (Model QUV/Basic).aluation the panels were exposed to the cycle ofnd humidity. In a typical cycle, exposure to UV

V.C. Malshe, G. Waghoo / Progress in Organic Coatings 51 (2004) 267272 269

light was at 50 C for 4 h and exposure to humid conditionswas at 45 C for 4 h as per ASTM D4587. The panels weresubjected to ageing for different time intervals up to 1000 h.The environmental weathering was done on the top of a threestorey building in UICT, Matunga, located in Central Mum-bai (latitude 1901.6) with annual rainfall of about 2200 mm,relative humidity ranging from 61 to 87% the latter being thehighest in the monsoon period and mean daily temperatureranging from 24 to 33 C. For the assessment of environmen-tal weatherability, the panels were directly exposed to theweathering in an unbacked rack inclined at 52 facing south.The evaluations were done at intervals of 15 days for a periodof 12 months.

The gloss of the panels were measured at 60 angle ofincidence (ISO 2813) using BYK Glossometer. Yellownessindices (ASTM D1925) of the panels were determined byGretag Macbeth make Spectrophotometer CE 7000A (Op-tiview propas the yelltions of thetaken. To eMethod Brecommendfingertip, hchalking. Sproximatelpressure. Tfinger on a bwas obtain

In case oweighed bebalance wiloss of wei

The permula:

% weight lo

where x isy the weighbeing equaand the bla

Table 1Chalking char ering

Curing agents E

D

B

FPA (50 phr, 2 1TETA (13 phr 1DICY (10 phrPA (60 phr, 18TMA (32 phr,PMDA (30 phCA (25 phr, 1AS (30phr, 27

3. Results and discussions

3.1. Evaluation of chalking

Table 1 gives the chalking time of various epoxy paintsin hours and days exposed to accelerated and environmen-tal weathering, respectively. The amount of curing agent isexpressed in parts per hundred parts of epoxy resin (phr)and varied for different curing agents. For cyanuric acid as acuring agent, TPP (0.25% based on epoxy resin) was addedduring the paint making.

The order of chalking was found to be DGEBXS >DGEBX > DGEBXF > DGEBPF > DGEBPA. Replace-ment of isopropylidene group (DGEBPA) with the methylenegroup (DGEBPF) decreased the chalk resistance slightly. Allthe three resins with methyl substitutions on the aromaticgroup showed faster and more chalking than the ones with-

e subbsencties fu. Amothe bed CA

e andDICYmaini

t, therkablyand Dst foumum.e resutrendd withing evll thee durentalg agebetweximaalette 2.0e). The measurements of gloss as wellowness index were done at three different por-panel and the average of the three readings was

valuate the degree of chalking ASTM D4214 TestWet Finger Method was employed. The methoding one continuous rub on the surface with a wetowever, was not suitable for materials with lesso the fingertip was revolved for 25 times over ap-y 2 in. 2 in. of the exposed area with a mediumhe medium pressure was quantified by placing thealance and pressing downward until 2 kg pressure

ed.f accelerated weatherability, the panels were alsofore and after the exposures on a Sauter AR 1014

th a capacity to measure up to 100m to study theght during the weathering.centage weight loss was calculated using the for-

ss = x yx

100

the weight in grams of paint before exposure andt in grams of paint after exposure, the value of xl to the difference in weight between the coatednk aluminum panel.

acteristics of epoxy paints exposed to accelerated and environmental weath

Accelerated weathering (h)Diglycidyl ether of

BPA BPF BXF BXS BX

7 C) 250 200 100 100 100, 27 C) 250 150 50 50 50, 150 C) 100 100 100 100 1000 C) 150 100 100 100 100180 C) 100 100 100 100 100r, 180 C) 100 100 100 100 10050 C) 150 100 100 100 100C) 800 600 400 300 400

out thand amoiepaintgavePA anbrittlwiththe reagenrema

BXFalmomaxi800 h

Thsame

coatechalkthat ater thronm

curinshipappronvironmental weathering (days)iglycidyl ether of

PA BPF BXF BXS BX

50 90 45 45 4520 45 30 15 4560 45 45 45 4560 30 30 30 4545 30 30 30 3045 30 30 30 3090 60 60 60 45

270 210 120 180

stitutions. Presence of sulfone group (DGEBXS)e of any group (DGEBX) between the phenolicrther deteriorated the chalk resistance of epoxyngst the curing agent, use of FPA for DGEBPAst results with least chalking followed by TETA,but the films formed with CA as curing agent wereflaked off with longer exposure. The paints cured, TMA and PMDA gave much faster chalking thanng curing agents. When AS was used as a curing

chalk resistance of all the paint panels improved. The improvement was more prominent for DGE-GEBX which showed chalk resistance of 400 h,

r times of that with FPA. DGEBPA showed thechalk resistance and showed chalking only after

lts of the environmental weathering showed theas that of the accelerated weathering. The panelDGEBPA paint and cured with AS did not showen after 365 days. It was also interesting to noteAS cured paints showed faint chalking even af-

ation mentioned in accelerated as well as envi-weathering compared to samples cured with othernts which showed severe chalking. The relation-en the two types of weathering was found to betely two and a half hours exposure in accelerated

270 V.C. Malshe, G. Waghoo / Progress in Organic Coatings 51 (2004) 267272

Fig. 1. Variation of yellowness index of various epoxy paints exposed to accelerated weathering.

environment being equal to 24 h in environmental weather-ing for the climatic conditions prevailing in Mumbai, thusleading to a

3.2. Evalu

The varifor the paining reportechalking (For less theDGEBPF amore yelloples, the mexposure, ato the factformed a pr

degradation of paint polymer. In the accelerated tests, afterthe initial increase in yellowness index there was a gradual

ase inwed bashedrmedmore

Evalu

g. 3 gs cureeight

er 20ured pht loss

as obnearly 10-fold acceleration of weathering.

ation of yellowness index

ation of yellowness indices with the exposure timets cured with FPA and AS are the only ones be-

d since the other curing agents led to much fasterigs. 1 and 2). The yellowness index was moresame for all the paints, the maximum being fornd DGEBX. All the AS cured samples showed

wing than the FPA cured ones. For all the sam-ajor yellowing occurred in the initial duration offter which it showed a little change. This was duethat once chalking occurred, the chalked pigmentotective layer on the surface preventing the further

decrefollogot wconfiwith

3.3.

FipaintThe wof ovAS cweigloss wFig. 2. Variation of yellowness index of various epoxy paints exposits value with more exposure as UV cycle wasy condensation cycle where the chalked pigmentaway exposing fresh surface to the top. This was

by the decrease in the weight of the paint panelsand more exposure.

ation of weight loss

ive the percentage weight loss of the epoxy resind with FPA and AS in the accelerated weathering.loss was more prominent after the QUV exposure

0 h for FPA cured paints and over 500 h for theaints. Also due to chalking there was up to 45%with FPA curing agent whereas only 12% weightserved with AS. DGEBPA showed the minimumed to environmental weathering.

V.C. Malshe, G. Waghoo / Progress in Organic Coatings 51 (2004) 267272 271

Fig. 3. % Weight loss of various epoxy paints exposed to accelerated weathering.

Fig. 4. Variation of % gloss of various epoxy paints cured with FPA exposed to accelerated weathering.

Fig. 5. Variation of % gloss of various epoxy paints cured with FPA exposed to environmental weathering.

272 V.C. Malshe, G. Waghoo / Progress in Organic Coatings 51 (2004) 267272

weight loss confirming least chalking while maximum weightloss was observed for DGEBXS.

3.4. Evaluation of gloss

The effect of exposure on the gloss of only FPA curedpanels is shown in Figs. 4 and 5. In both the accelerated aswell environmental weathering, all the paint panels showedmarked decrease in gloss. It was found that the initial glossof all aromatic methyl substituted epoxy paints were veryhigh with DGEBX having the highest gloss. The gloss ofDGEBPA and DGEBPF comparatively were less and thiswas also confirmed by taking a commercial white paint ofDGEBPA, which also showed poor initial gloss. It was inter-esting to note that though the loss of gloss as a percentagewas high for DGEBX, still its gloss after 150 h correspondedto the 0 h gloss for DGEBPA in the accelerated ageing. Agradual decrease for gloss of DGEBPA paint in environmen-tal weathering was observed due to less chalking. The initialgloss for all the AS cured paint samples were very low withgloss values less than 10%. So their further evaluation ofgloss after exposure was not done.

4. Conclu

The abosized:

(A) The ardationmethyincreawith D

(B) The suof any

DGEBPF between the two phenolic moieties cause moreand faster chalking than the isopropylidene in DGEBPA.

(C) The secondary hydroxyl group is the next contributingfactor to the degradation of epoxy paints after the aro-matic moiety as evidenced by the remarkable control ofchalking on silylation of the hydroxyl groups.

(D) Severe chalking with the anhydrides curing agent ex-cludes any substantial role of the N-alkyl portion indegradation.

Acknowledgement

The authors wish to thank Mr. T.N. Venkatesan, ManagingDirector, Filtra Catalysts & Chemicals Ltd., for the supportto this project.

References

[1] H. Lee, K. Neville, Handbook of Epoxy Resins, McGraw Hill BookCompany, 1967.

[2] P.G. Kelleher, B.D. Gesner, J. Appl. Polym. Sci. 13 (1969) 915.[3] B.D. Gesner, P.G. Kelleher, J. Appl. Polym. Sci. 13 (1969)

1832191.. Riva

1997) 3. Riva

1997) 3. Lalo.A. Olhemist. Mar

eries, A. Ouy

0 (2000. Ranb

tabilisatsions

ve study shows that of the weak links hypothe-

omatic moiety plays the major role in the degra-of epoxy resin. This is confirmed by the fact thatl substitution on aromatic part causes faster andsed chalking of the paint as observed in the caseGEBXF, DGEBXS and DGEBX.lfone group in DGEBXS, followed by absencegroup in DGEBX and the methylene bridge in

2[4] A

([5] A

([6] M[7] G

C[8] M

S[9] M

7[10] B

ston, L. Moreau, J.-L. Gardette, Polym. Degrad. Stab. 5821332.ton, L. Moreau, J.-L. Gardette, Polym. Degrad. Stab. 5833339.nde, T.H. Chan, Synthesis (1985) 817845.ah, G.K. Prakash, R. Krishnamurthy, Advances in Siliconry, JAI Press, Greenwich, 1991.kovitz, L.S. Kohn, Epoxy Resins, Advances in Chemistrymerican Chemical Society Publications, 1970.

ang, P.P. Klemchuk, J.T. Koberstein, Polym. Degrad. Stab.) 217228.

y, J.F. Rabek, Photodegradation, Photo-oxidation and Photo-ion of Polymers Principles and Applications, Wiley, 1975.

Weathering study of epoxy paintsIntroductionExperimentalMaterialsPreparationEvaluation

Results and discussionsEvaluation of chalkingEvaluation of yellowness indexEvaluation of weight lossEvaluation of gloss

ConclusionsAcknowledgementReferences