Indian Journal of Chemistry Vol. 43A, August 2004, pp. 1696- 1700

Preparation, molecular weight determ ination and structure elucidation of melamine-urea­

formaldehyde, melamine-methylurea­forma ldehyde and melamine-dimethylurea­

forma ldehyde polymer resins with IR spectroscopy

Man Singh

Chem istry Research Lab, Deshbandhu College, Uni versity of Delhi , New Delhi 11 0019, India

Received 5 Decell/ber 2003; revised 12 May 2004

Melami ne-urea-formJldehyde (M UF), melami ne- methy lurea­formaldehyde (MMUF) and melall1ine-dimethylurea-forlllal­de hyde (MDUF) resins/po lymers ha ve been prepared by conden­sation po lymeri zati on. Average molecular weights (M w) have been determined wi th intrinsic viscosity and gel permeation chromatography (GPe) with known molecular weights of polyvi­ny l alcohol as standard marker for ca li br~ll i o n . Hydrogen atom substilliti on by methy l group in urea increases Mw and IR spectra mark the presence of several oligomer units with methy lene and ether linkages. Disappearance of 3734 cm·1 band from spectra of uncured MUF on curing from 140 to 200°C has been noticed with narrowness in bands and multip le spl its between 3451 and 3296 at 140°,3453 and 3328 at 160° and 334 1 and 3234 CIll ·

1 doublet at

200°e.

IPC Code: Inl. CI. 7 C 08 G 12/ 12

Melamine has been used to prepare ami noplastic res­inst with urea and forma ldehyde forming severa l oli­gomers. Preparat ion and characterizat ion of melamine and urea formaldehyde polycondensate res ins have been of industri al interes t. The methy lolurea and melaminemethylol oligomers with methylene and di­methylene ether bridge linkages2

.3 have been identi­fied. Spectroscopic investigati ons using IR, H t NMR and elJ NMR have provided insight in to the matri x microstructure-l-7 . Molecular weight determination w ith GPC

g has also been reported . The stud ies on

curing are significan t as they provide a deeper under­standing of thermal stab ilitl of the res ins. A seri es of

-methyl substituted ureas had been chosen for studying the structure of 01 igomers to and thei I' re­sponse towards IR spectroscopi t. Apart from MUF, less is known about the MM UF and MDUF res ins, however, recently some of the physico-chemical properties of MMUF have been studied tz. The prepa-

rati on and spectroscopic studi es are reported herein on MUF, MMUF and MOUF to identify the oligomers of the res ins. The MUF behaves as sel f-hardening resin and this feature has ini tiated the studies on MMUF and MDUF resins.

Experimenta l Melamine, urea, methy lurea ",nd dimethy lurea (AR,

BOH) and 40% IV/W formalin so lution (E. Merck)

were used. M elamine (mp-300DC) was separately mi xed with urea (mp-1 32DC), methy lurea (mp-95DC)

and dimethylurea (mp- 105DC), each hea ted overnight in oven and kept in P20 S filled vacuum des iccater for 24 hI' for dry ing and was used for resin making. Melamine, ureaJmethylureaJdimethy lurea and formal­dehyde in I :5: I mol ratio were polymerized by heat­ing to 80±1 DC for about 20 min in a 100 ml round bottom f lask. The react ion in each case was stopped by quenching the reaction by placing the fl ask in an ice co ld bath and thereafter material was brought to normal temperature for measuremenrs . The fo llowing condensation react ion forms the res ins:

C}N(iH(i (melamine)+CHz(OH)z (form aldehyde)+ H2NCONH2 (urea) ~

/ '. (-21-1 20) =C-NH--{~C I-I 2'~'- - - - HNCO H--- (MUF)

M ethylene li nkage =C-NH---CH2---NCON(CH})--- MMUF

=C-NH---C I-Iz---HNCON(CH1h--- MOUF

Molecular weight determination

Since the viscosity (17) is fu nction of the size and the shape of the molecule, it is lIsed for molecular weight determination of polymer resins. The fl ow time for equal volume of each of the so lvent and res in so lutions were measured through the capill ary of the low shear Ubbelohde vi scometer (0.5 mm internal diameter) under constant experi mental condit ions. The va lve of 17 was ca lculated from a re lation 17/ 170 =

17, = 1/loP/Po where 17 and 170 viscos ity , I and 10 flow ti mes and P and Po density of res in sol tion and water respecti ve ly. Triply distilled, deion ized and degassed

NOTES 1697

water was used as solvent. 17,- = 17/170 is relative vis­cosity .

The densities were measured with two-limb pyk­nomtere under similar conditions. The solutions were separately filled in the pyknometer and thermally equilibrated for about 30 min . Thereafter, the pyk­nomter was taken out and dried with tissue paper and weighed with 0.0 I mg Ohona balance model 100 OS. The density (p) was calculated from eq uation given below:

where Po is dens ity of water and 0.00 12( I- (W- WoIWo­We)) is buoyancy correcti on for air, We, Wo and Ware weights of empty , so lvent and solution fill ed pyk­nometer respecti vel y.

The absolute di stributi ons can be obtained with end group analysis IJ and osmometry. Nevertheless, i ntri n­sic viscosity, [17] , has been considered an effective technique for M il' determination . The res ins were found to be water soluble, so 17,- was measured for 0.005 to 0.1540 g perlOO ml solutions at 25±0.01 °C. The [17] derived from (17,) were fitted to Mark­Houwink equat ion'4 for MI " the viscos ity average molecul ar weights. 17,,1' is specific viscos ity and cal­culated from relation, 17.11' = (17,--1) .

The reduced viscos ity (17,-"d = 17lpic = (17,-- j )/c) com­putes the [17] va lues from eq uation:

17m! = 17"pic = [17] + Dc

Intercept of 17spic vs c plot gives the ([ 17] = 17st/C, c~ 0) value, c is compos ition in gil 00 ml and 0 is the slope. Several so lutions of different compositi ons of PVOH of different molecular weights were pre­pared for the viscosity data. The [17] was evaluated from 17ljc vs c plots of each so lution of polyvinyl al­coho ls (PVOH ) of 45000, 50000, 65000 and 70000 g mort molecular weights, for calibrat ion. The PVOH is feebly hydro lyzed and develops hydrogen bonding with water, thereby its aqueous solution behaves as standard for [17] vs c va lues . Likewise the [17] values of res ins were calcu lated and fitted on the curve and 34 159. 36634 and 54078 g mort molecular weights of ML F, \ 1MUF and MOUF were est imated.

Gel permeation chromatography CG Pe)

GPC for PVOH solutions was run in 45x3 cm boros il glass column with packing of fine glass woo l at its lower end and silica (Si02) gel slurry in water as stat ionary phase. Gel phase was maintained com-

pletely wet. Firstly , 0.005 to 0.1 SO g per 100 ml solu­tion of each PVOH of known molecular weight was run @ I ml min- ' followed by elution with water. The 17, of each elution volume (Ve) was measured to de­termine [17] and plotted against Ve to determine con­stants k and a as

log [17] = log k + a log M (Mark Houwink equ at ion)

M is the molecul ar weight, the experimental values of k and a for PVOH were calculated to be 1.987x 10-4

and 0.7606 respectivel y against the literature va lues of 2x 10-4 and 0.76' 5. After washing the column with water, each res in so lution was run at the rate at whi ch PVOH so lution was run , the [17] was determined for each Ve and put on the calibration curve. A perpen­dicular on its abscissa gave the values 32990, 35352 and 52185 g mort for MUF, MMUF and MOUF; these were found about 3.5 % lower than those est i­mated with [17] method.

Results and discussion Viscos ity values and average molecular weights are

given in Table I. It is noti ced that bands between 3000 to 3600 cm-' are narrowed down on curing at 200°C which probably shows elimination of a hydro­gen bond due to =N-H stretching and free -O-H

Table 1-[/71 in ml g-Iobtained from plots or I).,'/c vs c data and filled to [1)1 = k M" or log [17] = log [k] + a log [M,,·l equation for determination of constan ts k and a

M olecular weights w ilh [ '1] M arkers M ol. wI. , LogM [111 , ml g-I log [n] V,., 1111

g mol-I

PVOH 45000 4 .6532 1 0.6877 -0.1626 220 PVOH 50000 4.69897 0.745 1 -0. 1278 180 PVO H 65000 4.8 129 1 0.9095 -0.04 12 75 PVOH 70000 4.845 10 0.9625 -0.0 166 50

Constants for PVOH ' a =0.7606, k = 1.987' Polymcr resins

MUF 34 149 4.53338 0.56 12 -0.25088 29 1.75 MMUF 36634 4.56388 0.5920 -0.22768 274.67 MDUF 54078 4.73302 0 .796 1 -0.09903 154.77

Constants for resins ' a = 0.7606, k =2.00' Molecular weights with GPC

Res ins PVOH Ve,1ll 1 [/11. M ol. wI. Ve. 1ll1 [I)], 1111 g -I Mol. wI.

1111 g-I g 11101-1 g 1110 1' 1

176 .8 0.7697 52 185 50 0.9625 70000 283.5 0.5726 35352 75 0.9095 65000 299.7 0.5432 32990 180 0.745 I 50000

220 0.6878 45000

1698 INDIAN J CHEM, SEC A, AUGUST 2004

groups. Likewise a band at 3734 cm'l stretching de­pic ts the ex istcnce of free -O-H and =N-H in uncured MUF perhaps due to an excess of CH2(OH)1, along wi th thi s a broad band betwcen 3656 and 32 11 cm'l is

seen which indicates bonded primary -N-H with - 0 -H th rough hyd rogcn bonding. The disappearance of 3734 cm' l band on curing to 200°C confirms the pres­

ence of frce - O-H groups, which may be due to for­mati on of melam inemcthylol and urcamcth y lol oli ­gomcr units. Bands bctween 2828 and 2775 em' l infer prominen t C- H strctching in MUF but such bands are absent in MM UF and MDUF spcc tra . Probably tcrmi ­nal -CH, in I-substitutcd ureas in terfercs with stretching of -CH2 (meth y lenc) in chain. A lso a nega­ti vc chargc on oxygen of >C=O is cnhanced by prcs­ence of electron donati ng - CH, group that pumps clectrons to C of >C+-O'. The 167 1 cm' l band of un­cured and 167 1, 1672 and 1703 cm'l bands of cured MUF at 140, 160 and 200°C, also 164 1 and 1625 cm'l bands of MMUF and MDUF respecti ve ly are ob­served prov ing th at N-substituti on decreascs >C=O stretching frequencics . Noti ceably it inhibi ts split of bands duc to - N-H, wherc shoulder around 3225±4 em'l could find stretchi ng of bonded -N-H in amide and triazine (melamine). A comb ination of vibrati ons in vari ous oligomers of =C-N and - H2 of amide and melamine appear at about 1560±4 cm'l . Likewise band at 1440±4 cm,l characteri zcs stretchi ng vibra­ti ons of am ide of MUF and 8 15 and 770 cm,l bands belong to tri azinc. Band at 102 1 cm'l indicates the prcsencc of mcthy lol group and at about 11 40 cm' l a partial cross- linking between mclamine and urea meth y lol oli gomcr units is indicated. The band at 1652 cn,- I identifi es stretching of >C=O of amide

group of methy l urea (- CONHCH, ) and >C=N stretching vibrations of tri az ine.

Bands around 3330 and 3 130 cm,l seem due to su­

perimpositi on of - O-H and - N-H stretching vibra­ti ons of am ino groups of amide and triazine. A lso, the bands around 3470 and 34 19 cm'l belong to -N-H stretching vibrations of primary amino groups of tri­azine. Notably disappearance of 3734 cm' l band on curing to 140°C narrows broad band between 345 I to 3 132 cm' l with split and con f irms the presence of - CON H- and -N- H in MUF and curing to 160°C gen­crates va ri ous bands bctween 3562 to 3297 cm'l. The spl it of bands could be attributed to Fcrmi resonance phenomenon. And curing to 200°C reduces bands to 334 1 and 3234 cm'l with sli ghtly more broadness.

which perhaps proves the presence of ol igomer units with methy lene, ether linkages and intermolecular hydrogen bonding.

MMUF shows bands at 3453 , 3406 and 3328 cn,- I, the former two frequencies confirm superimposition of - O-H and - N-H and latter one the presencc of - CON(CI-h ) stretching with -CH1 and - CH2-0-CH2' linkages. For MDUF, thi s regi on becomes narrower with bands at 3422, 3375 and 3328 cm' l, which sig­nals out the absence of hydrogen and intramolecular bonding. The bands at 2852 cm'l of uncured, 2968 and 2828 cm' l of MUF cured to 140°C are found whi ch disappeal at 160°C but reappear at 2828 cm'l at 200°C. It seems that -C-I-I stret ching con firms transi­ti on state of cross- linking in oli gomer units and band at 2953 cm'l remains unchanged for MMUF and MDUF, which is attributed to the grcater stability of - C-I-I stretching.

Bands from 345 I to 32 18 cm'l o f cured M UF at 140°C match the bands at 3453 and 3328 cm 'l of MMUF confirming the presence of - N-H stretch ing. A si ngle broad band of dimcthy lurea at 3375 cm,l is detained in the spcc tra of MDUF. At 1656, 1625 and 1578 cm' l bands appcar in spectra of meth ylurea while they appear at 164 1 and 1570 cm' l for dimeth­

y lurea. The -CH, in - CON(CH)) reduces stretching of >C=O which is also found in the spectra of MMUF and MDU F.

Bands at 2852 cm'l belong to -C-H stretch ing of -

CH2- and 2656 cm'l to - C- H stretching due to Ve1l2. Bands between 1450 to 850 cm'l remain almost unaf­fected on curi ng at 140°C and from 2650 to 1850 cm' l

variation is found to be the least around 140 to 160°C.

The C-H stretching in MUF spectra is found at 2852 to 2 195 cm,l and of > C=O at 1952 cm,l but gets eliminated for MMUF. Shifts in C- H stretching fre­quency near N- F-I with narrow bands indi cate wcak­ening of hydrogen bond and seems that - CH, of N­substituted ureas increases narrowness in bands around 32 I 8 to 342 I cm'l . It confirms that hydrogen atom of N substituted urea is replaced by -CH3 so an avai lability of sites for hydrogen bonds is reduced to few therefore narrowness matches the fact that hyd ro­gen bonding in oligomers or res ins of methy l and di­methy lureas is inhibited. Further fundamental fre­quencies of N- H, O-H , >C=O, and C-H vary due to the presence of different linkagcs \\'hich affect over­tones and combi nati on tones of frequcncics wi th several bands.

NOTES 1699

The >C=O due to dipole moment shows asymmet­ri c stretching at 2350 cm-I, and separation in con­secutive >C=O groups by C (O=C-C-C=O). It is seen to multiply an effect on spectra i.e. >C=O band is at 171 5 cm-I, N-H from 1497 to 3077 cm-I in -NH2, -CONH2 and C-H is from 3000 to 2760 cm-I in asso­ciation of -CH2 and -CH3. In general , 3800 to 1300 cm-I regions represent fun ctional groups, while 1653 to 1436 cm-I charac terizes melamine where -NH2 shows doublets at 3469 and 3419 em-I. Also C-H stretching and bending frequencies show characteri s­tic vibrations which result in appreciable shifts and attachment of CH). CH2 or >C=O or aromatic ring to C-H also affects i.e. C-H bending at 1467 and 1450 cm-I. For example, CH3 of MMUF and MDUF does affect C-H stretchi ng due to VasymmclricCH) ( vasCH3)

and VsymmelricCH3 (vs CH3) which are found at 2953 and 2844 cm-I respecti vely.

H H

t t C ---'H and C -H

I ~ H H

Asymmetri c sy mmetri c stretching

Likewise CH 2 shows VassymclricCH2 and VsymclricCH2 at 2926 and 2853 cm-I res pectively, and VsCH3 and VasCH3 in and out of phase bending at 1375 and 1450 cm-I respectively. Scissoring bend (OSYJlllllClricCH2) of CH2 at 1465 and rocking vibrati on (pCl-h ) of CH2 a ~

720 cm-I i.e . the CH2 rock in phase for straight chains. Here, the CH2 twisting and wagging vibrations are fo und at 1350 and 1150 cm-I respec ti ve ly (in - CHr CONH-). The 3584 and 3656 cm-I bands predict the ex istence of primary and secondary - NH with meth­ylols and 3550 to 3200 cm-I frequencies predict in­termolecular hydrogen bond among oli gomer units of uncured MUF, here these bands could transpire about the stability of oli gomers. The O-H bending from 1420 Lo 1330 cm-I seems LO couple furth er with C-H wagging vibrati ons, whi ch produces two bends at 1420 and 1330 cm-I. Also the vibrations from 1150 to 1085 r i~l - 1 show coupling of -H2C-O-CH2- (ether link­ag~ ) st ,·etching with other.

A decrease in frequency of spectra of melamine occurs fro m 3469 and 34 19 to 3422, and 3375 cm-I

for MDUF, 3453 and 3406 cm-I fo r MMUF and 344 : to 3234 cm-I from the substituti on of -NH2. In gen­eral, the decrease is from 1348 to 11 4 1cm-1 at 140°,

1348 to 1138 cm-I at 160°, and 1359 to 1161 cm-I at 200°e. Notably 1030 cm-I band is found in the spectra of uncured MUF and also in that of res in cured at 140° and 160°C, but it di sappears at 200°C, whi ch predicts the removal of as(asymmetric) and s (sym­metric stretching)stretching at 200°e.

The bands from 1261 to 1250 cm -I for M U F and MMUF spectra except cured MUF at 200°C, 1281 and 12 19 cm- I for MDUF and from 1029 to 1020 cm-I fo r spectra of MMUF and MDUF predict presence of as (asymmetric) and s (symmetri c stretching). The posi­tions of bands for stretching frequency of >C=O groups in ureas are up to 1870 from 1540 cm-I. Stretching of primary N-H group of melamine does show 3469 and 3419 cm-I bands due to s and as whil e at the same time N-H (VN- H stretching) of secondary amide finds only one frequency. Frequency from pri­mary to secondary (N-H) amide is found to decrease due to hydrogen bonding and superimposition of N-H and O-H stretching, which makes it difficult to di stin­gui sh bending from 1650 to 1515 cm-I. Further, it ap­pears that the out of plane N-H wagging seems re­sponsible for broad band of medium in bending in frequency ranging from 800 to 666 cm-I. Dimeri zation of amide could also affect its frequencies as amido­imidol tautomeri sm makes C-N bond to acquire dou­ble bond character and leads to sharpening of the band of MUF spectra.

The note presents the molecul ar weights and IR studi es of melamine and N-substituted ureas using HCHO or CH2(OHh (formaldehyde) as binder. The melamine and ureas contain 3 and 2 amino (-NH 2) groups, I and 2 hydrogen atoms of urea are subst i­tuted by one and two -CH3 groups in methylurea and dimethylurea. Due to substitutable hydrogen atoms of melamine and N-ureas several oli go mer units like melaminemethylol and ureamethylols were observed. The oligomers form polymer water insoluble network at above 120°e. Intensely the reagents were polym­erized to 80°C and the prepared res ins of low polym­eri zati on were found water-soluble but the resins, prepared above 180°C were insoluble_ So, they repre­sent a very useful class of aminoplastic polymer res­ins. It is seen that curing inhibits number of free -O-H groups and multiple splits in bands.

Acknowledgement Author is thankful to Uni versity Grants Commis­

sion , New Delhi , for finan cial support. Mr. A.K. Agrawa l deserves special thanks fo r recording IR , at

1700 IND IAN J CHEM, SEC A, AUGUST 2004

TIT Delhi. The Auth or also appreciates the coopera­ti on ex tended by Dr. S.c. Gupta fo r fruit ful di scuss ion and Mr. Avtar Singh and l agtar Singh for recording lR , at RSIC, Punjab Uni versity , Chandigarh.

References I Eugster P & Zo rn nger H, /-Ielv Chi/II !\c/({, 1969 ( 1985) 52. 2 13 r~\lIn 0 & Bayersdorf F, Allgelv MakrOlllol Chelll, 85 ( 1980)

I. 3 13raun 0 & Bayersdorr F, AlIgel\l Makrolllo l Chelll , 83 ( 1979)

2 1. 4 13raun D & 13 ayersdorf F, II lIgew Makrolllol Chelll , 89 ( 1980)

183 . 5 Dunky M & Lederer K, ; llIgelv Makmlllol Chelll. 102 ( 1982'

199.

6 Meyer B & Nuli st R, PolYIII Prep.- Alii ChclII Suc Div POII'III Chelll , 22 ( 198 1) 130. .

7 Schirndlbauer H & Schuster J, Kllllslslo/le, 73 ( 1983) 325.

8 Janca J Ed, SIal ic £xc/ llsirm Liqllid Chrolllatography of PolYlll ers (Marce l Decker, Inc, New York'), 1984.

9 Brau'n 0 , AllgelV Makrolllol Chelll , 76177 ( 1979) 360.

10 Troger F, Gri gori ou A & Hey F, /-Iolz Roil- Werk.l/ , 35 ( 1989) 379.

II Pshen itsy na V P, Molotkova N N, Frenkel M D, Tikhomi-rova Ye & Kochnov 1M, PolYIII Sci USSR, 2 1 ( 1980) 2 145.

12 Man Si ngh, J Appl PO/yll/ Sci (2004) in press .

13 To mito B. J App/ PO/YIII Sci, Vol. 15 ( 1977) 2347.

14 Braun 0 & Gunther P, II lIgelV Macrolllo/ Chelll , 128 ( 1984) I.

15 Jah rom i S, PO/Ylll er , 40 ( 18) ( 1999) 51 03 .