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.527

Holzforschung48 (1994) 527-532

Effects of Reaction pH on Properties and Performanceof Urea-Formaldehyde ResinsBy Chung- Yun Hsel. :lJ1i- Yuan Xia2 and Bunichiro Tomita]I USDA Forest Service. Southern Forest Experiment Station, Pineville. Louisiana. USA

2lnstitute of Wood Industry, Chinese Academy of Forestry. Beijing. China]lnstitute of Agricultural and Forest Engineering, University of Tsukuba. Tsukuba. Japan

KeywontsUrea fomlaldehyde resinReaction pHMolar ratioMolecular weightI t NMR spectra

Bond strengthFonnaldehyde emission

SummaryUrea formaldehyde resins were formulated with combination variables of three reaction pH (1.0. 4.8.and 8.0) and four molar ratios of formaldehyde to u~ (2.5, 3.0. 3.5. and 4.0). The resins were preparedby placing all formaJdetlyde and water in reaction kettle and pH was adjusted with sulfuric acid andsodium hydroxide. respectively. U~ was added in 15 equal pans at I-minute intervals.The proportion of high molecular weight products in the resin increased substantially as the reactionpH decreased. Furthermore. the F/U molar interacted with reaction pH to effect resin molecular weight.At acidic pH, the high molecular weight products in~ as F/U ratio decresed; while. at alkalinepH. little difference was evident between the high or low molecular weight products at various F/Uratios. The formation of a high percentage of uron derivatives under strong acidic conditions alsoindicated these resins differed considerable from conventional UF resins formulated in the past.Panels bonded with resins catalyzed at strong acidic conditions resulted in lowest formaldehydeemission but slightly lower bonding strength. Of the three pH conditions evaluated in the study. bothweak and strong acid catalysts systems are fIO( commonly used in conventional UF resin formulation.Based on the bond strength and formaldehyde emission data. however, the weak acid catalysts seemsto provide the best compromise between the strong acid and the conventional alkaline-acid catalystsystem currently used for formulating UF resin wood adhesives.

a new resin fonnulation (greatly different from two-stage alkaline-acid catalysts system) was introduced.A significant development was the fonnulation of UFresins under weakly acidic catalysts without firstusing an alkaline catalyst (Lambuth 1987). The weakacidic catalyst was thought to promote higher degreesof condensation and to reduce formaldehyde emissionof the resin without changing the working conditionsof the resins. More recently, UF resins catalyzed bystrong acidic conditions have been introduced to fonnextremely stable UF prepolymers (Williams 1983).These developments have greatly expanded the rangeof potential effects in OF resin formulation. Theobjective of this study is to evaluate these newdevelopments and to examine the effects of wideranges of pH on properties and performance of UPresins.

Introduction

Low cost and proven performance have made urea-formaldehyde resins (UF) the most important of woodadhesives for interior applications. Up to the middleof the 1960's, most of these resins were formulatedusing two stage reactions. Initially, the resin wasreacted with an alkaline catalyst to initiate the addi-tion reaction and, at the appropriate reaction time, theresin solution was converted to the acid side topromote condensation reactions. These formulationshave long been used successfully for UF resins in F/Uratio between 1.45 to 1.65 (Rayner 1965).

However, during the late 60's and early 70's, both a)the formaldehyde emission from particleboard. and b)weakening glue bond caused by the hydrolytic degra-dation of UF polymers have increasingly become aproblem of major concern in the industry. New resinswith much lower formaldehyde emission were formu-lated mainly by decreasing F/U ratio (i. e. down to 1.1to 1.2). Lower F/U ratios, while yielding substantiallylower formaldehyde emission. resulted in longer curetime and lower IB strength due to insufficient formal-dehyde to complete the condensation reaction. Thus.

~ureResin preparation

All UF resins were IX'epared in the laboratory. Each resin prepara-tion was replicated one ti~. To JX'epare eacII resin. all f~hyde and water were pIKed in a reaction kettle and pH wasadjusted with sulfuric acid and sodium hydroxide. respectively.

I-4nl7f~h"no I Vnl ~ I IQQ4 INn"

C.-Y. HIe et al.: Effects of Reaction pH on UF Resins528

Urea was added in 15 equal parts at I-minute intervals. To initiatethe reaction. the mixture was heated and maintained at 50°C forthe first 30 minutes; thereafter, temperature was increased to 80°Cwith the exception of the resin made at strong acidic conditionthat was reacted at 60°C. When the viscosity reached Gardner-Holt viscosity A, the reaction was stopped by rapidly cooling themixture to 25 °C. Gel time. pH, solid content. specific gravity. andfree fonnalddlyde content were detennined. The variables forresin preparation were:

I. Reaction pHa. Strong acid - pH 1.0b. Weak *=id - pH 4.8c. Alkaline-acid - pH 8.0 for 30 minutes and then adjusted to

5.0.

2. Molar ratio of formaldehyde to urea (FlU)a.2.5b.3.0c.3.5d.4.0

+4 mesh: 1.2"-4 mesh. + 8 mesh: 2O.S'"-8 mesh. +20 mesh: SS.9'"

-20 mesh, +32 mesh: 12.1"-32 mesh. +48 mesh: 4.9"-48 mesh: 5.4"

The ~sins we~ adjusted to FlU ratio of 1.1/1 by adding urea onenight bef~ blending. Resin was awlied to the wood panicles bysprayer in a rotating drum with air-atomizing nozzles. The genera]conditions for board manufacture were:

Panel size: 20 x 24 irx:hesPanicle moisture content: 4.5 %Resin content: 4.5 ,.,Hot-press temperatu~: 37S ofHot-press time: 4 minutes

Bending strength (MOR and MOE) and internal bond (18) wereperflH'lned in accordance with ASTM standards for evaluating theproperties of wood-base fiber and particle panel materials (D-

1037-72).

Results and Discussion

Resin propertiesAverage physical and chemical properties of UFresins as affected by reaction pH and molar ratio ofFlU are summarized as follows:

Gel-penneat;on chromatographyA Water associates GPC with four Shodex GPC-AD-802S col-umns was used with dimethylforamide solvent at flow rate ofI mUminute. This gel is a polystyrene-divinyl benzene copolymerthat has an exclusion limit of 8,000 (polystyrene molecularweight). Sample injection volume was lOOuL at concentration ofI percent (wt/v).

NMR measunment

Each resin was diluted with 2- to 3-volumes of deuterium oxideand a 1:tc-NMR spectrum was obtained using a FX-IOO spec-trometer (Japan Electron Optics Lab. Co.) at a frequency of25.0 MHz. A 7.O-sec pulse delay time and a °45 pulse were usedwith the gated decoupling to remove N.O.E. Chemical shifts weredetermined using internal methanol at 50.0ppm. Quantitative ana-lyses were based on the signal intensities.

Viscosity Specificgravity

F~HCHO

%

F/Uratio

Solidcontent

%

ReactionpH

~

(gelled)80302S

1106S4030

4S3S2S25

J..O 2..5

3.0

33

4.0

2..5

3.0

3..5

4.0

23

3.0

3..5

4.0

1.2551.2501.250

1.2571.2551.2551.258

1.2551.2461.2501.245

6.3S6.607.0.5

6.3\6.466.576.86

S.\\5.70S.806.21

494543

54514846

SS514846

Free formaldehyde deteml;nation

A slightly modified sodium sulfite method was used for determi-nation of free fonnaldehyde in the resins. Fifty mL of a I-molarsolution of sodium sulfite and three drops of thymophthaleinindicator solution were placed in a 2SOmL Erlenmeyer nask andcarefully neutralized by titration within hydrochloric acid until theblue color of the indicator disappeared. An accurately measured.neutralized resin sample was then ~ to the sodium sulfitesolution and the temperature was kept at 4 °C to minimize hydro-lysis of the resin. The resulting mixture was titrated with the 0.1 NHCL to complete decolOration. The pen:ent of free formaldehydewas determined as:

4.8

8.G-S.o

In general, the solids content and viscosity of theresin decreased as the F/U molar ratio increased.Differences in resin specific gravity were not signi-ficant. The high free-formaldehyde content for theseresins was expected due to the high F/U molar ratioin the resin preparation. When compared among thesame reaction pH conditions, the free-formaldehydeincreased as the F/U ratio increased. It is noted alsothat, within the same F/U ratio, the free-formalde-hyde decreased as the reaction pH increased. Thisresult agreed with the general belief that the additionof formaldehyde is enhanced and the methylol com-pounds obtained are relatively stable under alkalineconditjons as compared to that of acidic conditions.

% HCHO = (acid liter) (nonnaliry of acid) (3.003)

weight of sample

FOmIDldehyde emission measurement

The detennination of formaldehyde absorbed in distilled water isbased on the specific reaction of formaldehyde with chromorropicacid-sulfuric acid solution, fonning a purple mono-cationic chrc>-mogen. The absorption of the purple solution was read using aspectrophotometer at 5SOnm. The test was performed in acc0rd-ance with NPA 2-hour desicator test.

Glu~ bond perfonllanc~

All panels were ~pared in the laboratory with dried southernpine wood particles. The wood particles were obtained at apanicleboard plant in Louisiana and used without additional prep-aration. The sieve analysis of particles showed:

C.- Y. Hse tt al.: Effects of Reaction pH on UF Resins 529

The resins catalyzed with strong acids (pH 1.0) re-sulted in gel times almost twice as long as thosecatalyzed with weak acids (pH 4.8) or alkaline con-ditions (pH 8.0). When compared at the same reac-tion pH, the gel time decreased as the F/U ratioincreased. It should also be noted that the gel timedecreased slightly as the level of NH£L additionincreased as well.

Most UF resins were cured with the addition ofcatalysts such as NH.Cl. The gel time was measuredafter the addition of two levels of NH.Cl catalyst andthe results are summarized as follows:

Effect of reaction pH on resin structureThe effect of reaction pH on resin structure was in-vestigated by '~-NMR spectroscopy. I~-NMR spec-tra of resins prepared at various pH conditions atformaldehyde to urea ratios of 3: I are shown inFigure I. Each signal observed in the '~-NMR spec-trum could be assigned according to chemica] shiftsreported earlier (Tomita and Hatano ] 978; Slonimet al. 1977; Ebdon and Heaton 1977). In the I~-NMR spectta of resins synthesized under strong acidconditions of pH 1.0, the signals attributed to thecarbonyl carbons of uron derivatives were evident at

4.304.354.50

4.554.354.053.95

4.504.454.304.IS

1.451.281.20

0.750.700.520.42

0.810.800.700.60

4..154.204.35

4.204.053.953.95

3.954.154.103.90

1.801.551.38

0.900.870.600.55

0.970.960.850.65

Reaction pH"...a, ,.0 ~

--- .-K.

pH 4.8pHIc E GAStructU~

0.66 (1'.8)0.120.370.17

0.67 (16.1)O.~

0.42

0

0.41 (12.7)0.140.18OJ7J

1.14 (3.s.2)O.~

0.36 (8.2)O.~0.140.13

1.29 (29.8)0.49

0.65 (£7.7)0.070.320.26

0.90 (24.8)0.190.130.320.26

0.11 (3.1)0.11

0.48 (13.3)0.180.30

I.SO (41.2)0.53

0.56 (13.2)O.OS0.290.22

0.94 (22.3)0.200.10O~~0.29

0.1 (2.3)0.10

0.48 (11.3)0.160.32

2.1~ (50.9)0.10

0.64 o.m

0

0.530.090.320.12

0.870.31

0..56

0.050.030.02

0.440.140.30

1.220.45

0

0.911O.~0.41

o.77~0.25

1.0210.490.53

1.69 ~0.53

0.70 (16.3)0.3\0.39

2.\~ (S\3)0.68

0.870.62 1.10 0.410.73 ;~

0.11

1.001.00

0.29

1.001.00

0.15

1.001.00

0.36

1.001.00

- -Total methylene 0.63 (20.4)

-NH.Q!zNH- 0.10-N(CH,-)CH2-hNH- 0.30-N(CH,-)CH~(CH,-)- 0.23

Total methylol 0.93 (30.0)-NHm..oH 0.19-NHCH~HPH 0.14-N(.Q!z-)CHPH 0.39-N(Q!2-)CHPCHPH 0.21

Total methylated methylol 0.10 (3.2)-NHQ!...QCH) 0.10-N (CH,-)CH PCH2 -

Total dimethylene ether 0.52 (17.0)-NHCH~H~ 0.13-N(CH,-)CHPCH2= 0.39

Total free formaldehyde 0.92 (30.0)HOCHPH 0.43

HOCH~-H...oHENCHzOgiPH 0.43

HOC-1!JOCH)-CH...oQ:I...oCH) 0.06

Total carbonyl carbon 1.00-NCON= 0.7SURON 0.2SH;zNCONHCHPH -H2NCONH2 -

Total formaldehyde 3.1 (100)

CombinM fonnaldd1yde 2.18 (70.3)-- ~

Note: Amount of formaldehyde is baled on the molar ratio to urea residue (FlU). The value in parentheses is pen:entage (%) to total

formaldehyde.

0.24

1.000.820.18

0.30

1.000.8S0.1'

3.2 (100)

2.4 (76.1)

4.3 (100)

2.7 (61.2)

4.1 (100)

2.4 (48.9)

-3.6 (100)

2.IS (.58.8)

4.2 (100)

2.1 (49.7)

3.1 (100)

2.1 (60.7)

Holzfonchung I ~. 48 I 1994 I No.6

(16.9)

(28.3)

(f.s)

(14.0)

(39.3)

[21.1)

123.9)

[23..5)

;31,1)

C.-Y. Hse ~t aI.: Effects of Reaction pH on UP ResinsS30

155-156ppm as well as those of urea residues atabout 160ppm.

The results of quantitative measurements on sevenresins are summarized in Table I. The high freeformaldehyde content in all resins, caused by thehigh molar ratio of FlU, was confirmed also withfree-fonnaldehyde determination by sodium sulfitemethod. When compared among resins made withthe same molar ratios, the free formaldehyde contentdecreased with increased reaction pH implying thatcombined fonnaldehyde increased with increased re-action pH. A large amount of hemiformal methylolgroup was evident in resins prepared with higherfree-formaldehyde. Methylene linkages between urearesidues increased as the pH was lowered, whiledimethylene ether linkages increased with increasedreaction pH.At the strongly acidic condition of pH 1.0, however,the greater part of dimethylene ether group wasconfirmed to be involved in the formation of uronrings. For example, in the resin with a FlU molarratio of 3 : I, the formation of uron ring amounted to25 % of the total urea. Since two methylene carbonsare joined with a carbonyl carbon in the fonnationof the uron ring, the result indicated that the quan-tity of methylene carbon incorporated in uron wasFlU = 0.5. On the other hand, the quantity ofdimethylene ether as determined in Table I wasFlU = 0.52, which was only slightly higher than thevalue of 0.5. Since the carbonyl carbon in the uronring could not be detected in resins made at higherpH, the formation of uron derivatives was indicativeof reactions made under strong acid conditions.

l'ig.1. IJc-NMR spectra of UF resins reacted at various pHconditions at FlU ratio of 3/1.

DH 8.0-5.0

pH 1.0

» .40 .so aso 20 oW~ . 40

Elution Volume (mL)

Fta. 2. OPC chromatograms of UF resins reacted at diffe~nt pH and F/U raI.iOI.

~"I..flWCrh"no I Vnl 4R I 1994 I No. 6

531C.-Y. Hse er al.: Effects of Reaction pH on UF Resins

FlU ratio Low molecular High molecularweight products weight products

% %

Rain pH

54.778.288.8

70.579.888.292.3

94.896.297.096.1

4S.321.811.2

29.S20.211.87.7

5.2.3.83.03.9

1.01.01.0

4.84.84.84.8

8.6-S.08.6-S.08.6-S.08.6-S.0

3.0

3.5

4.0

2.5

3.0

3.5

4.0

2.5

3.0

3.5

4.0

ABC

DEF0

HIJK

20 30 40 50Elution Volume (mL)

Fig. 3. GPC chromatogram of UF resins reacted at different pHconditions at FlU ratio of 3/1. The proportion of high molecular weight products

increased substantially as the reaction pH decreased.Furthermore, the FlU molar ratios interacted withreaction pH to effect resin molecular weight. Atacidic pH the high molecular weight products in-creased as FlU ratio decreased; while, at alkaline pH,little difference was evident between the high orlow molecular weight products at various FlU ratios.It should be noted, however, the molecular weightbuildup was so fast for the resin made with a FlUratio of 2.5 at strong acidic condition (i. e., pH I) thatan insoluble gel formed early so control of thereaction was not practical.

Mechanical properties and formaldehyde emission

Average mechanical properties and the formaldehyderelease of the particleboards are summarized as fol-lows:

Bending Strength FormaIde-MOR MOE hyde

Emission

MPa MPa ppm

Resin pH FlU InternalRatio Bond

Smngthpsi MPa

0.560.570-'-'

0.700.690.710.71

0.740.670.620.68

II.~11.9311.52

11.7212.~12.6712.05

12.6012.5'12.1112.SI

24S9~2241

252327722613~

268927792S792717

1.4

2.1

2.3

1.6

2.0

2..5

3.0

1.9

2.3

3.4

3.8

1.01.01.0

4.84.84.84.8

8.0-5.08.0-5.08.0-5.08.0-5.0

3.03.54.0

2.s3.03.54.0

2.s3.03.54.0

ABC

DEF0

HIJK

Panels bonded with resins catalyzed at strong acidconditions (A-C) resulted in consistently lower in-ternal bond strength and bending strength. On theother hand, resins catalyzed with weak acid (D-G)and alkaline (H - K) conditions produced panels with

Gel permeation chromatographyFigure 2 shows the GPC chromatograms of UFresins reacted at different pH and F/U ratios. Thesignificant effect of reaction pH on molecularweight distribution is illustrated in Figure 3. Thelarge fraction of sample eluted between 21 and29mL (upper chromatogram) indicated that strongacidic conditions favored condensation. On the otherhand, the strong signal between 30 and 39 mL(lower chromatogram) showed that alkaline condi-tions enhanced methylol formatjon. The high methy-101 content in resins made under alkaline conditionsas compared to acidjc conditions was also detectedby '~-NMR analysis (Table I). In general, GPC canbe divided into two groups of peaks. The peaksbetween 39 and 40 mL elutjon volume are those ofurea or mono-methylol urea. the first negative peakbetween 41 and 43 mL is due to formaldehyde andwater in the sample. These identifications weremade by comparing GPC chromatograms of UFresins before and after addition of urea, formalde-hyde, and water to the sample. Other peaks evidentat early stages of reaction (i. e., those between elu-tion volumes 30 and 39mL) may be dimers, trimersand tetramers of polymethylol urea as indicated byprevious workers (Dankelman et al. 1976; Dunkyand Lederer 1982; Matsuzaki et al. 1980; Tsuge1980). These peaks can be integrated together(group A) as an estimate of the amount of lowmolecular wejght products. The peaks at low elutionvolumes (i.e., 21 to 29mL) can be integratedtogether (group B) to represent a measure of thehigh molecular weight products obtained from con-densation reactions.

The composition of low and high molecular weightproducts are summarized as follows:

Holzforschung I Vol. 48 I 1994 I No.6

532 C.-Y. Hse ~t al.: Effects of Reaction pH on UF Resins

lyst system currently used for formulating UF resinwood adhesives.

References

Dankelman, W.. J.M.H.Daemen. A.J.J.de Breet. J.L.Mulder.W.G.B. Huysmans and J. de Wit. 1976. Modem medIods forthe analysis of urea formaldehyde resins. Angew. Makromo.Chemie 54: 187-201.

Dunky, M. and K.~. 1982. Studies of the molecular weightdisuibution of urea formalddlyde resins. Angew. Makromo.Chemie.102: 199-213.

Ebdon, J.R. and P.E. Heaaon. 1977. Characterization of urea-for-maldehyde abducts and resins by Carbon- 1 3 NMR spectro-scopy. Polymer 18: 971-974.

Lambuth, A. 1987. Private communication.Matsuzaki. T.. Y. Inoue. T. Ookubo. B. Tomita, and S. Mori. 1980.

Size exclusion ctlromatOIJapby of melamine-, urea-. and phe-nol-formaldehyde resins using N.N-dimethylformamide aseluent. J. Liquid Chrom. J (3): 3S3-365.

Meyer. B. 1979. Urea-Formaldehyde Resins. Addison-Wesley Pub.Co., Reading. MA.

Rayner. C.A.A. 1965. Synthetic organic adhesives. In: Adhesionand Adhesives. ~. R. Houwink and G. Salomon. ElseevierPub. Co. New York. NY. pp.I86-3S2.

Sionim. I. Ya., S. G. Alekseyeva. Ya. O. Urman, B. M. Arshava.B.Ya. AkseI'I1Xi. and L.N. Smimova. 1977. Determination ofthe structure of cyciocbain u~-formaldehyde resins by'Jc:NMR analysis. Polymer Sci. U.S.S.R./9 (4): 920-936.

Tomita. B. and S. Hatono. 1978. Urea-formaldehyde resins. III.Constitutional characterization by IJc fourier transform NMRspectroscopy. J. PoIym. Sci. 16: ~-2525.

Tsuge. M. 1980. Chemical stnactu~ and molecular species ana-lysis of formaklehyde condensate resins. J. Thermoset Resin(Japan) 1(2): 31-45.

Williams. J.H. 1983. Hydrolytically stable urea-formaldehyderesins and process for manufacturing them. U.S. Patent4.410,685.

Received August 31st, 1993

Dr. Bunichiro TomitaInstitute of Agricultural

and Forest EngineeringUnivenity of TsukubaTsukuba, Ibaraki 305Japan

Dr. Chung- Yun HseSouthern Forest Experiment Station2500 ShrevelKJft HighwayPineville. LA471~U.S.A.

slightly higher formaldehyde emission. When com-pared among the resins catalyzed with weak acid andalkaline conditions, little difference was evident inthe mechanical properties. However, alkaline cata-lyzed resins resulted in consistently higher formalde-hyde emission than that of weakly acidic conditions.

One of the interesting results of the study was theformation of a high percentage of uron derivativesunder strong acidic conditions. The formation ofthese cyclic derivatives was not detected in resinsmade using either weak acid or alkaline conditions.The results indicated that resins formulated understrong acidic conditions differed considerably fromconventional UF resins formulated in the past. Ex-periences with UF resins used in the textile and paperindustries has indicated that cyclic compounds aredesirable compounds as cross-linking agents (Meyer1979). This study also showed the resins catalyzedunder strong acidic conditions resulted in lowestformaldehyde emission. The result is particularlysignificant since the formaldehyde release from theglued wood products is the most pressing environ-mental problem concerning the wood industry. Itshould be mentioned, however, the bonding strengthwas slightly lower using resins synthesized understrong acidic conditions. Furthermore, it was alsonoted that the resins catalyzed at strong acidic con-ditions resulted in cure rates, as measured by geltime, almost twice as long as those of resins cata-lyzed with weakly acidic or alkaline conditions. Theslower cure rate was not expected because the strongacid was known to favor condensation as shown inGPC data. The result suggests that curing agentsother than conventional NH£1, such as mixed cata-lysts, should be evaluated in order to improve thecure rate. Development of highly reactive catalystsystem would be important economically and practi-cally for the UF resins.Of the three pH conditions evaluated in the study,both weak and strong acid catalysts systems are notcommonly used in present UF resin formulation.Based on the bond strength and formaldehydeemission data, however, the weak acid catalysisseems to provide the best compromise between thestrong acid and the conventional alkaline-acid cata-

Dr. Zhi- Yuan XiaInstitute of Wood IndustryChinese Academy of Forestry

BeijingChina

Holzforschun~ f Vol. 48 f 1994 f No.6