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Influence of the catalyst content on the curing of the UF adhesive

Ammonium chloride (NH4Cl) is a commonly used catalyst for UF adhesives designed

for the production of particle-board or medium density fiber-board !n order to evaluate its

catalytic influence on the curing of UF adhesives" #$C scans %ith different amounts of

NH4Cl %ere run &hese scans %ere performed on adhesive %ithout the addition of %ood

flour &he #$C scans of the changes of the heat content as the curing reaction progresses at a

heating rate 'Cmin-' are given in Fig * &his figure further presents the influence of the

NH4Cl content on the curing reaction of the UF adhesive

Fig. 3: C curves for the curing of UF adhesives with different catalyst NH4Cl concentrations

obtained at a heating rate at 10 .in!1.

Acting as the catalyst in the UF adhesive system" NH4Cl reacts %ith free

formaldehyde (and to a lesser e+tent %ith methylol groups)" forming he+amethylene

tetramine" hydrochloric acid and %ater &he hydrochloric acid lo%ers the pH of the system

thus creating favorable acidic conditions for the polycondensation reaction and final curing of

the UF adhesive" %hich is significantly accelerated by heat !n the #$C scans (Fig *)" the

curing reaction of the UF adhesive is represented by an e+othermic region %ith a single pea,

As e+pected" the e+othermic pea, %as shifted to%ard lo%er temperatures %ith increasing of

NH4Cl concentration n the other hand" the catalytic effect of NH4Cl dramatically

decreased %hen its content reaches . / (based on the adhesive dry content)" especially at

levels of ' / and more &his is presumably because of the limited 0uantity of free

formaldehyde available for the cross-lin,ing reactions" in the ne% formulation of 1' class of

UF adhesive !n addition" %ith increasing NH4Cl content" the most reactive groups of the

adhesive %ere activated &his leads to the conclusion that after a certain level" further

addition of NH4Cl is not effective 2ased on the obtained thermograms" it %as concluded that

the critical content of NH4Cl catalyst is bet%een . / and ' /" %hich roughly concurs

%ith literature data for this ne% formulated type of adhesive (3ao et al 5) 6hen the

temperature reaches the region bet%een '*7 and '4C (above the e+othermic curing

process)" an endothermic reaction can be noticed &his may be assigned to a certain amountof degradation of the urea polymer" as this temperature level coincides %ith the melting

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temperature for urea" %hich occurs at a temperature of '*C As a mean of lo%ering

formaldehyde emission" urea itself is usually subse0uently added at the end of synthesis of

UF resins" thus acting as a scavenger for free formaldehyde Non-reacted" lo% molecular

%eight methylol derivatives might be the cause of the endothermic reaction" %hich" in this

particular case" seems to have no effect on the main curing reaction $ubse0uent rescanssho%ed no endothermic process" thus confirming former assumption &he NH4Cl addition

reduced the pea, temperature but only slightly reduced the reaction enthalpy ("H) &he

relevant results are given in &ab '

#ab. 1: $ata for the obtained %ea& te%eratures and reaction enthal%ies for adhesive curing

'without wood flour( in %resence of different content of NH4Cl catalyst.

NH4Cl content

(% based on the

dry adhesive

content)

Curing peak

teperatures

Tp(!C)

"nthalpy of

curing reaction

H (#$g&)

84. '7'

. 55. '49

' 59 '4'

'7 5.' '*8

579 '4'