Mechanism of action of benzoin as a degassing agent in powder coatings

Progress in Organic Coatings 48 (2003) 183–193

Mechanism of action of benzoin as a degassing agentin powder coatings

Shahab Jahromia,∗, Ben Mosterta, Andreas Derksb, Fokelien Koldijkb

a DSM Research, P.O. Box 18, 6160 MD Geleen, The Netherlandsb DSM Coating Resins, P.O. Box 615, 8000 AP Zwolle, The Netherlands

Abstract

The mechanism of action of benzoin as a degassing agent in powder coatings has been analyzed. The gas bubble shrinkage was monitoredusing a light microscope equipped with a hot stage. In the absence of benzoin, the air bubbles start to shrink very slowly as a result of adiffusion-controlled process. Because of the continuing cross-linking reaction and increase in the viscosity, the bubble shrinkage halts at el-evated temperatures. Quite remarkably, we observed that in the presence of benzoin the process of bubble shrinkage is accelerated to such anextent that most air bubbles disappear before any significant increase in the viscosity occurs. This suggests that benzoin functions by acceler-ating the rate of bubble shrinkage. To analyze the mechanism of action of benzoin in detail, we studied the coating formulations using varioustechniques. X-ray diffraction in combination with deuterium NMR using labeled benzoin indicated that benzoin dissolves on a molecularlevel in polyester resin and becomes mobile above the glass transition temperature of the matrix. Mass spectroscopy experiments revealed thatbenzoin, in its oxidized form (benzil), starts to evaporate above 100◦C. At 200◦C more than 90% of the initial concentration of benzoin hasevaporated from the coating. As was to be expected, the conversion of benzoin to benzil halted when the experiments were carried out in nitro-gen. We postulated that the action of benzoin as a degassing agent is somehow related to its ability to oxidize in situ. This claim is substantiatedby the results of bubble dissolution experiments using different gases such as oxygen and nitrogen. It is shown that in the presence of benzoinoxygen bubbles shrink much faster than air bubbles. On the other hand, the shrinkage of nitrogen bubbles is not affected by benzoin. Basedon the above results, a two-step mechanism is proposed for the action of benzoin as a degassing agent. This mechanism has been successfullyused as a guideline to identify alternative degassing agents with higher efficiency and fewer side-effects, such as less severe yellowing.© 2003 Elsevier B.V. All rights reserved.

Keywords: Benzoin; Oxidize; Degassing agents; Powder coatings

1. Introduction

In spite of continuous efforts to replace benzoin as thedegassing agent in powder coating formulations, this com-pound is still the additive of choice for facilitating air bubbleremoval in powder coating formulations. The availability ofalternative degassing agents is desirable on account of thenegative side-effects of benzoin, such as yellowing[1]. Thesearch for suitable degassing agents has hitherto been ham-pered primarily by lack of understanding of the mechanismof action of benzoin. Although several studies, for exampleby Uhlmann and coworkers[2],1 have focused on this issue,

∗ Corresponding author.E-mail address: [email protected] (S. Jahromi).

1 The reliability of the results regarding the effect of benzoin on surfacetension reported by Uhlmann and coworkers is doubtful. This is relatedto the fact that the surface tension measurements in the presence ofbenzoin were carried out above the volatilization temperature of benzoin,i.e. 120◦C. Therefore it is to be expected that bezoin will evaporate fromthe resin solution during long-term surface tension measurements, whichwill adversely influence the results of the experiments.

the mode of action of benzoin has so far remained largely amystery.

We have investigated the action of benzoin in a typical,polyester/epoxy based, powder coating. Both the chemicaland physical processes which take place during degassingin the presence and absence of benzoin have been studiedin detail using various techniques.

2. The experiments

2.1. Powder coating formulations and materials

A typical formulation consists of 100 g of a Uralac®

polyester resin (acidity number of 82) containing 0.4% cat-alyst (triphenyl methyl phosphonium bromide), 100 g of anepoxy resin (Araldite® GT 7004; epoxy number= 725,Ciba), 3 g of a flow agent (resiflow®-PV5, Worleé) and anappropriate amount of a degassing agent as indicated inthe text. These formulations are designated clear coatingsin the text. The formulations referred to as white also con-

0300-9440/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0300-9440(03)00096-1

184 S. Jahromi et al. / Progress in Organic Coatings 48 (2003) 183–193

tain 100 g of a titanium dioxide pigment of type Kronos2310.

All the components were first blended in a Braun mixer(MX-40). The mixtures were then extruded through aPRISM TCE 16 PC at a temperature of approximately100◦C. The extruded material was ground and classified toa particle size lower than 90�m. The finished powders wereelectrostatically sprayed onto test panels (Q-Panel S-46).The coated panels were cured for 10 min at 200◦C in a hotair oven (Hereaus UT 6120). The color was measured usingthe standard DIN 6174. The measurements were conductedat a layer thickness of 60�m with the aid of a Dr. Lange ap-paratus type LMG140 equipped with a measuring head witha diameter of 2 cm. The degassing level was determinedby measuring the layer thickness using Fischer equipmenttype D211D, which reveals the first signs of pin holing.The deuterated benzoin (Benzoin-D10) was purchased fromCAMPRO Scientific (Veenendaal, The Netherlands).

2.2. Synthesis of benzoin derivatives

Benzoin derivatives were synthesized according to theclassical method as explained in[3].

2.3. Optical microscopy

These experiments were carried out using clear coatingformulations and a JANA PAL microscope equipped withhot stage and videotext units of the type LIN KHN. Theshrinkage of gas bubbles was recorded using this equipmentat various isothermal temperatures. Care was taken to ensurethat all the selected air bubbles were initially of a comparablesize. The size of the gas bubbles, displayed on a TV screen,was measured using a ruler and the data were corrected forthe right magnification.

2.4. Scanning electron microscopy (SEM)

The SEM experiments were carried out using a PhilipsCP SEM XL30 operating at 15 kV.

2.5. Mass spectroscopy experiments

The pyrolysis–gas chromatography–mass spectroscopy(Py–GC–MS) experiments were carried out using an ATASOPTIC2 (programmable injector) coupled to a MicromassAutoSpecE mass spectrometer. The thermal desorption(TD)–GC–MS experiments were conducted with the aid ofa Gerstel TDS (ThermoDesorption System) coupled to aHP5973 GC-MSD.

2.6. Rheological experiments

These measurements were carried out using an ARES/LSinstrument (Rheometric Scientific) operating in a frequencysweep mode.

2.7. 2H NMR experiments

2H NMR spectra were recorded using aVarianInova-400 MHz wide bore NMR spectrometer operating ata 2H frequency of 61.4 MHz with the aid of a wide-lineprobe. The 90◦ pulse width was 2.5�s. Wide-line2H spec-tra were obtained via Fourier transform of the free inductiondecay (FID) that was measured using the solid-echo pulsesequence withtse = 20�s. The recycling delay time wasgreater than the relaxation timeT1 by a factor of 5. About0.035 g of resin containing deuterated benzoin-D10 wasused for the measurements, which for each spectrum tookabout 2.5 days at 22–60◦C, 40 min at 80◦C and 10 minabove 80◦C.

2.8. X-ray diffraction experiments

These measurements were performed using powderdiffractometers PW1050 and PW1820 in Bragg Brentanogeometry with fixed slits (1 divergency slit; 0.2 mm en-trance slit). Cu K� radiation was used (40 kV, 50 mA) witha monochromator in the diffracted beam.

2.9. Thermogravimetric analysis (TGA)

These experiments were carried out with the aid of aPerkin Elmer TGS-2.

3. Results and discussion

3.1. Problem analysis

In order to gain insight into the mode of action of benzoin,we studied the degassing process in the presence and absenceof bezoin using a light microscope. During the heating andfinal curing of powder particles a series of physical andchemical processes take place, which ultimately resultin film formation and hardening of the coating. Above60–70◦C, i.e. theTg of the polyester resin, the powderparticles start to soften. Following a drastic decrease in theviscosity, the particles coalesce to form a continuous coat-ing. During this process, which is completed below 100◦C,air bubbles become trapped. In the absence of a degassingagent, i.e. benzoin, the majority of these air bubbles becomefixed as a result of the cross-linking process and increasein viscosity above 140◦C. Fig. 1 shows the SEM imagesof cross-sections of a clear coating in the presence andabsence of benzoin.

As can be expected, air bubbles, or rather cavities, withdiameters of less than 500 nm can be clearly seen in thecoating without benzoin. The number of air bubbles in thecoating containing benzoin is much smaller. Apparently,benzoin somehow facilitates the removal of air bubbles.During the degassing process, air bubbles may rise to the airinterface and collapse. Simultaneously, bubbles may shrink

S. Jahromi et al. / Progress in Organic Coatings 48 (2003) 183–193 185

Fig. 1. SEM images of the cross-sections of a clear coating in the absence(a) and presence (b) of 0.3% (w/w) benzoin.

as a result of diffusion of air through the matrix. In order tostudy these processes in more detail, we examined a seriesof clear coatings using a light microscope. We monitoredthe size of the air bubbles monitored during isothermal heat-ing of a benzoin-free formulation at different temperatures(seeFig. 2).

The size of the air bubbles decreased continuously asa function of time. The bubble shrinkage proceeded fasterwhen the temperature was raised from 120 to 130◦C, sug-gesting a diffusion-controlled process[4]. During heating at140◦C, however, the process of bubble shrinkage halted af-ter a certain time. This is related to the increase in viscosityas a result of the cross-linking reaction (seeFig. 3), whichhampers and eventually stops further shrinkage of air bub-bles. As can be inferred from the viscosity changes duringthe reaction shown inFig. 3, the addition of benzoin has noeffect on the absolute viscosity value nor on its evolutionduring the cross-linking process.

Fig. 4 shows the shrinkage of air bubbles as a functionof time at 120◦C for a powder coating formulation in thepresence and absence of 0.3% (w/w) benzoin.

0 100 200 300 400 500 600 700 800 900 1000

Time (sec)

0

10

20

Ra

diu

s (

mic

rom

ete

r)

R (120°C) R (130°C) R (140°C)

Fig. 2. Diameter of air bubbles as a function of time during isothermalheating at three different temperatures. The measurements were conductedwith the aid of a light microscope using a clear benzoin-free powdercoating formulation.

It is clear that air bubbles shrink faster in the presence ofbenzoin; at 120◦C by a factor of almost 3. In fact, the rate ofbubble shrinkage is accelerated to a degree that even duringcuring at 130 and 140◦C complete gas dissolution may occur(seeFig. 5). As a result of the acceleration effect of benzoin,the bubble shrinkage may evidently be completed beforea significant rise in viscosity has set in. Furthermore, thebubble dissolution in the presence of benzoin appears tofollow a two-step process. We will return to this point below.

How does benzoin accelerate the rate of bubble shrinkage?According to a theoretical study by Zana and Leal focusingon the dissolution of gas bubbles in viscoelastic liquids, therate of bubble shrinkage can in principle be increased bychanging the surface tension[5]. However, to explain theacceleration effect of benzoin observed here, the surfacetension would have to be altered by at least a factor of 2,which is not realistic. Indeed no such an increase in surfacetension has ever been observed in the case of the systemsdiscussed here. In the following, other possible mechanismsthat may explain how benzoin increases the rate of bubbleshrinkage.

3.2. Solubility of benzoin in powder coating formulationstudied by means of X-ray diffraction and 2H NMR

Since benzoin is a crystalline compound, X-ray diffractioncan be deployed to determine the amount of crystalline ben-zoin in a coating formulation after all the ingredients havebeen mixed in an extruder.Fig. 6 shows the X-ray diffrac-tograms of pure benzoin and the diffractogram of a coatingformulation containing an excess of benzoin (10%, w/w).

Even at a loading of 10% (w/w), no crystalline benzoinwas detected in the coating formulation. This indicates thatduring the extrusion step benzoin dissolves in the polyesterresin at least up to a concentration of 10% (w/w), which is


50.0 80.0 110.0 140.0 170.0 200.0

101

102

103

104

105

106

107

Temp [°C]

η*

()

[P

a-s

]

With Benzoin

Without Benzoin

Fig. 3. Complex viscosity (η∗) as a function of temperature of a white powder coating formulation in the presence and absence of 1% (w/w) benzoin.The viscosity initially drops because of the temperature effect and then starts to rise again at about 140◦C due to the cross-linking reaction.

well above the common loading of benzoin as a degassingagent (<1%, w/w).

In order to obtain more detailed information on the localmobility of benzoin in the polyester formulation, a series of2H NMR experiments using a powder coating formulationcontaining deuterated benzoin (seeScheme 1).

By conducting2H NMR experiments as a function of tem-perature, the mobility of deuterated benzoin (benzoin-D10)molecules can be monitored especially during film formationand air bubble dissolution.Fig. 7shows the2H NMR spectra

Fig. 4. Diameter of air bubbles to the power of 3 during heating at 120◦Cof a coating formulation with and without 0.3% (w/w) benzoin. In contrastto the coating without benzoin, the bubble shrinkage in the presence ofbenzoin appears to proceed faster and follow a two-step process.

of a coating formulation containing 1% (w/w) benzoin-D10measured at different temperatures up to 120◦C.

The spectrum of crystalline benzoin shows a typicalquadrupolar splitting of deuterium in a rigid environment,i.e. the crystalline phase. The spectrum of benzoin-D10 inthe coating formulation at 22◦C is quite broad, indicatingthat the mobility of benzoin is still highly restricted. The re-striction of the mobility may be due either to the crystallinenature of benzoin or to the fact that benzoin resides in theglassy regions of polyester resin. Which is the case is diffi-cult to infer from this spectrum. The temperature-dependentexperiments however revealed a drastic line narrowing es-pecially upon heating above the glass transition temperatureof the polyester resin, i.e. 60◦C. This proves that the mo-bility of the benzoin is closely coupled to the mobility ofpolyester resin, indicating that benzoin indeed dissolves ona molecular scale in the polyester resin.

3.3. Chemical transformation of benzoin during thedissolution process

Benzoin is a volatile compound, which starts to sublimeabove 120◦C, i.e. its melting point. This can be clearly

DD

DD

C C

OH O

D D

D

D D

D

Scheme 1. Chemical structure of deuterated benzoin.


Fig. 5. Diameter of air bubbles to the power of 3 as a function of time at 120, 130 and 140◦C of clear powder coating formulations containing 0.3%(w/w) benzoin.

inferred from the TGA thermogram of benzoin shown inFig. 8.

Some data in the literature suggest that benzoin can evap-orate from coatings in significant amounts during curing ofdifferent types of powder formulations[6]. In order to studythis phenomenon in some detail with respect to the afore-mentioned formulations, we conducted a series of GC–MSexperiments using coatings cured in an inert atmosphere, i.e.helium, and in air. The samples were isothermally heated for60 min at different temperatures and the volatile compoundswere collected in a cold trap. After a desorption step, theevaporated compounds were then characterized by meansof GC–MS. The results of these investigations are shown inFig. 9.

During the curing in an inert atmosphere, benzoin startedto evaporate above 100◦C. After heating at 200◦C, 0.2%(w/w) benzoin was released, which more or less correspondsto the initial loading of benzoin. Similar trends, thoughseemingly in somewhat lower concentrations, were observedwhen the curing was carried out in air. But in this case notbenzoin, but its oxidation product, i.e. benzil, was obtained(seeScheme 2).

Oxidation of benzoin to benzil is a very common reactionin organic chemistry[7], which has been carried out usingdifferent types of catalysts[8]. As far as the chemical trans-formation in the coating is concerned, it is difficult to de-

C C

OH OO2 H2O2

C

O

C

O

Benzoin (white) Benzil (yellow)

Scheme 2. Chemical transformation of benzoin to benzil. Benzoin forms white crystals, whereas benzil is a yellow crystalline compound.

termine whether benzoin is formed in the condensed phase,i.e. before evaporation, and/or in the gas phase. This mostcertainly depends on factors such as the curing temperatureand the oxidation/sublimation rates of benzoin and benzil.As can be seen inFig. 8, TGA traces of benzil and bezoin arequite comparable, suggesting that benzil and benzoin evap-orate from the coating in a similar manner. It should also benoted that, because of its conjugated structure, benzil is ayellow compound. This suggests that the yellowing side ef-fects of benzoin on powder coatings may be associated withthe formation of benzil.

In short, demonstrated that during curing of powder coat-ing formulations under atmospheric conditions, benzointransforms to benzil, which oxidation product evaporatesfrom the coating. Interestingly, both the oxidation and theevaporation process occur at temperatures at which benzoinbecomes active as a degassing agent, i.e. 100–120◦C. Inthe following we will examine whether there is indeed arelation between the oxidation/volatilization and the actionof benzoin.

3.4. Bubble dissolution experiments under variousconditions

We now have some indications that oxidation of benzoinmay indeed play a role in the degassing process. In order to


Fig. 6. X-ray diffraction of pure benzoin and a powder coating formulation containing 10% (w/w) of benzoin. The peaks observable in the diffractogramof the powder coating formulation derive from the pigment, i.e. TiO2.

study this in more detail, the rate of shrinkage of bubblesconsisting of various types of gases. The experiments werecarried out by simply curing the clear coating formulationsin the presence of different gases so that the bubbles formedduring film formation would consist of the correspondinggases.Fig. 10 shows the rate of shrinkage of different gasbubbles formed during isothermal heating of clear coatingformulations at 120◦C.

In the presence of benzoin, oxygen bubbles shrink muchfaster than air bubbles. The rate of shrinkage of nitrogenbubbles is low and comparable with the dissolution rate of airbubbles in benzoin-free formulations. In fact, in the absenceof benzoin, the shrinkage of all three gases in the polyesterresin was roughly the same (data are not shown). This islogical in view of the fact that the van der Waals volumesof O2 and N2 are similar[9].

From the above results we can conclude that benzoin isapparently inactive as a degassing agent when curing is car-

ried out in N2. Benzoin becomes active in the presence ofair and is most active when pure O2 is used. This is a veryimportant conclusion, again pointing to the fact that the oxi-dation of benzoin to benzil indeed plays a crucial role in thedegassing process. Based on the results presented here, wepropose the following mechanism for the action of benzoin.

3.5. Proposed mechanism for the action of benzoin

Air consists primarily of nitrogen (mass percentage of dryair at sea level: 75.5% N2, 23.2% O2 and 1.3% Ar)[10].Therefore, the decrease in the volume of air bubbles shouldnot exceed 25% solely as a result of the consumption ofoxygen during the formation of benzil. It is possible thatdegassing under the influence of benzoin occurs in two steps,with the first process being controlled by the oxidation ofbenzoin and the second by the diffusion rate of N2 in thematrix. Careful analysis of all the shrinkage data suggests


Fig. 7. 2H NMR spectra of a coating formulation containing 1% (w/w) deuterated benzoin (benzoin-D10) at different temperatures. For comparison the2H NMR spectrum of crystalline benzoin-D10 is also shown.

that the dissolution of air bubbles under the influence ofbenzoin indeed occurs in two steps (see, for example,Fig. 4).The first process is fast and accounts for 20–25% of thetotal volume shrinkage. This value agrees well with the masspercentage of O2 in air. The second process is somewhatslower, but the shrinkage rate is still high when comparedwith the dissolution rates of pure N2 in polyester resin; at120◦C the shrinkage rate was almost three times higher. Thechemical scavenging of O2 by benzoin leads to a reductionin bubble diameter, which in turn results in an increase in the

Laplace pressure inside the bubble, as dictated by the lawsof surface tension. The increase in pressure forces nitrogento dissolve faster in the polymer, which further reduces thesize of the bubbles. The removal of air bubbles mediatedby chemical reactions has been proposed before, but neverproven, for example for the degassing processes in the glassmelt and refining[11].

Because of the detrimental role of the oxidation of ben-zoin, it is important to establish whether the degassingprocess benefits most from reaction in the condensed


Fig. 8. TGA trace of benzoin (dotted lines) and its oxidation product, i.e. benzil (solid lines).

Fig. 9. Characterization of volatile compounds released during isothermal curing of white powder coating formulations at different temperatures in airand in a helium atmosphere. The data were obtained by means of GC–MS. The concentration values are given as mass ratios relative to the total weightof each formulation, i.e. including pigment. The loading of benzoin in the formulations was 0.3% (w/w) relative to the binder (resin+ cross-linker) and0.2% (w/w) relative to the total mass including pigment.

phase or in the gas phase, i.e. in the bubble. In otherwords: is the volatilization of benzoin important for itsaction as a degassing agent? To answer this question,we chemically attached benzoin molecules to end groupsof polyethylene glycol. As a result, benzoin moieties

became immobilized without their ability to oxidize be-ing affected. These benzoin-functionalized polyethyleneglycol molecules were tested in powder coating formu-lations. The molecules were found to be inactive as adegassing agent, which indicates that the volatilization of


No Benz. (Air) Benz. (Air) Benz. (N2) Benz. (O2)

0

10

20

30

40

50

60

Sh

rin

kag

e r

ate

(cm

3/s

ec).

e-9

Fig. 10. Rate of shrinkage of different gas bubbles formed during isothermal heating of clear coating formulations (0.3%, w/w; benzoin) at 120◦C in acorresponding atmosphere. For comparison the rate of air bubble shrinkage of a benzoin-free formulation at 120◦C is also shown. The shrinkage rateswere determined via a linear fit through all the data points. The type of gas is shown between brackets.

benzoin is also an important feature (data are not shownhere).

Based on the proposed mechanism of action, we can for-mulate a set of conditions with which potential degassingagents must comply:

(1) To insure a homogeneous distribution, the degassingagent must be soluble (compatible) in the polyester resinformulations.

(2) The degassing agent must be volatile and start to evapo-rate preferably just above the temperature at which filmformation is complete and air bubbles start to form, i.e.90–110◦C.

(3) The volatilization should be accompanied by oxidationof the degassing agent in a temperature range similar tothat specified above.

Benzoin satisfies all the above conditions. This knowledgeof the mechanism of action of benzoin can be utilized todevelop more efficient degassing agents.

3.6. Development of alternative degassing agents

If the criteria given in the previous section are indeedcorrect, other molecules with chemical structures differingfrom that of benzoin should also be able to act as degassingagents in powder coatings. One of the commercially avail-able compounds at which we arrived using the three criteriais oxindole (seeScheme 3).

Like benzoin, oxindole can also oxidize. The oxidationprobably proceeds via H abstraction� to the carbonylgroup. As can be seen inFig. 11, the two compounds alsoshow very similar sublimation behavior. Testing of oxindolein powder coatings has indeed shown that this compound isan effective degassing agent.

N

H

O

Scheme 3. Chemical structure of oxindole.

Besides looking for completely different compoundsusing the criteria given in the previous section, anotherpossibility is to modify benzoin to obtain a more efficientdegassing agent with less severe yellowing side-effects. Forexample, since the oxidation of benzoin appears to play asignificant role in the rapid dissolution of air bubbles, in-creasing the rate of oxidation should in principle increase theefficiency of the degassing agent.2 It is well known that theoxidation of organic compounds can be affected by addingvarious groups[12]. If the activity of the degassing agentcan be increased it should be possible to lower the loading,which could in turn lead to less severe yellowing side-effectsof the benzoin derivatives.p-Chlorobenzoin is one of thederivatives that we have synthesized (seeScheme 4).

As can be seen inFig. 12and as confirmed in the literature,p-chlorobenzoin oxidizes much faster than benzoin[13].

The degassing levels and color performance of ben-zoin and its derivative are shown inTable 1. The benzoinwas tested at 0.3% (w/w), whereas the loading used forp-chlorobenzoin is one-third of that value, i.e. 0.1% (w/w).

2 In principle, the dissolution rate should increase with the oxidationrate of benzoin above 120◦C. Some of our shrinkage rate data (notshown here) however suggest that the viscosity of the medium may havea limiting factor on the shrinkage rate, i.e. if the viscosity is too high,the shrinkage rate may not be influenced by the oxidation rate of thedegassing agent.


Fig. 11. TGA trace of oxindole compared with that of benzoin.

As can be seen inTable 1, p-chlorobenzoin is a moreeffective degassing agent than benzoin even at one-third ofits loading. Because of the lower loading, the yellowingside-effect is also less severe.

Another consequence of the use of more efficient com-pounds, besides from the possibility of using a smaller

Fig. 12. Oxygen uptake ofp-chlorobenzoin compared with that of benzoin. The measurements were carried out in diphenyl ether (DPE; 20%, m/m) at120◦C.

amount of degassing agent, is the possibility of the color ofthe coating being affected by the position of the substituentson the benzene ring. As mentioned above, the oxidationproduct of benzoin, i.e. benzil, has a yellow color becauseof the conjugated structure of the compound. Substitutingboth benzene rings at theortho position may interrupt this


C C

OH O

Cl

Scheme 4. Chemical structure ofp-chlorobenzoin. In principle, the halo-gen, i.e. Cl can be positioned at either aromatic ring. For simplicity, onlyone of the structures is shown here.

Table 1The performance ofp-chlorobenzoin as a degassing agent compared withthat of benzoin

Compound Degassing level (�m) b∗ a

Benzoin 102 3.5p-Chlorobenzoin 130 1.0

a Indication of the yellow color of the coating.

conjugation as a result of sterical hindrance. As a result,colorless benzil derivatives may be produced. This hasindeed been demonstrated by means of UV-spectroscopyusing differently substituted benzil derivatives[14]. Exper-iments withortho–ortho di-substituted benzoin derivativeshave indicated that when these compounds are used as de-gassing agents the yellowing side-effects are indeed lesssevere (data not shown here). Finally, we should stress thatbecause the action of benzoin is related to its oxidation, andoxidation processes are usually synonymous with yellowingeffects, finding degassing agents that do not adversely affectthe color of powder coatings would be a very challengingtask.

4. Conclusions

The results presented here indicate that benzoin facilitatesthe removal of gas bubbles by accelerating the rate of bub-ble shrinkage. During the degassing process, benzoin evapo-rates (into the gas bubbles) and oxidizes to form benzil. Thesize of the air bubble shrinks rapidly 20–25% because of theresulting oxygen depletion. The decrease in the bubble di-ameter leads to an increase in the pressure inside the bubble(as dictated by the laws of surface tension) and forces thenitrogen to dissolve in the system. Guided by this mecha-nism of action, we have been able to identify alternative de-gassing agents with structures completely different from that

of benzoin. It has also been shown that much more efficientdegassing agents with less severe yellowing side-effects canbe obtained through “smart” derivatization of benzoin basedon the elucidated mode of action.

Acknowledgements

The authors would like to thank the management ofDSM Coating Resins for granting permission to publishthe present work. V. Litvinov and G. Kwakkenbos (bothfrom DSM Research) carried out the NMR and massspectroscopy experiments, respectively. P. Gijsman and G.Meijer (both from DSM Research) conducted the oxidationexperiments.

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Mechanism of action of benzoin as a degassing agent in powder coatings