Chromatographic studies of some thermosetting resins

Aldersley et al. : Chromatographic Studies of some Thermosetting Resins 101

CHROMATOGRAPHIC STUDIES OF SOME THERMOSETTING RESINS*

By J. W. ALDERSI,EY, V. M. R. BERTRAM, G. R. HARPER and B. P. STARK

The molecular weight distributions of resins produced by reaction of formaldehyde with phenols, melamine, or urea have been studied by gel permeation chromatography.

Introduction During the last few years, the technique of gel permeation

chromatography (g.p.c.) has become established as a major method for the fractionation of polymers and also for the separation of mixtures of compounds sf lower molecular weight. Compared with traditional methods such as precipitation and turbidimetry it shows a savirg in both time and labour and, in addition, the results obtained are often superior.

The fact that molecules could be separated by virtue of their size was first noticed in experiments on electrophoretic and ion-exchange separations, over fifteen years ago.*,2 The numerous developments and applications of the technique have been summarised in several reviews’,4 and in a comprehensive textbook by Determann.j

Widespread use of g.p.c. came with the commercial avail- ability of the hydrophilic Sephadex gels ti These were developed by Porath & Flodin and consist of dextrans crosslinked with epichlorohydrin. Unfortunately, Sc:phadex gels can be swollen only in water and a small numb1:r of polar solvents; this factor has limited their use mainly to the biochemical field.

More recently, several synthetic gels have been developed which can be used with a variety of solvents. The initial work was carried out by Moore and his co-worE.ers7 who synthesised crosslinked polystyrenes with varying porosities. The term ‘gel permeation chromatography’ was used by Moore to describe the separation process on the polystyrenes and this has superseded the essentially equivalent term of ‘gel filtra- tion’. These types of gels are now commercially available and are used widely where separation of different-sized molecules is required. Several other materials have been introduced as separating media, including polyacrylamides, polymeth- acrylates and, more interesting, glass beads with controlled pore sizes. This last material, although s .ill only at a development stage, has certain advantages owing to its rigidity and non-degradable properties.

In this paper, separations of three irtdustrially important types of resin based on formaldehyde arc: described: phenol-, melamine- and urea-formaldehyde condensates. The first of these classes includes resins based 011 various substituted phenols.

The phenolic and methylated melamine-formaldehyde resins were fractionated using polystyrene gels with tetrahydrofuran (THF) as solvent. THF is an ideal mobile phase for these systems in many respects. It swells the polystyrene matrix to a high degree, has a low refra4:tive index, has good ultra-violet light transmission characleristics, has a low viscosity and is a very good solvent for t i e two types of resins mentioned above. Both of these systems were monitored by continuous ultra-violet absorption measurements, using the melamine absorption peak at 237 nrn and the phenolic absorption peak at 280 nm.

Urea-formaldehyde resins created many more problems than the other two resin types, because clf solvent and column

effluent monitoring difficulties. A compromise system has been designed (Zahir, S. A., unpublished), which uses Sephadex G-series gels with water-lithium chloride as solvent and in which the intensity of the colour developed when the fractionated resin is continuously degraded with sulphuric acid plus chromotropic acid is measured. For a number of reasons to be described in a later section, this system has certain limitations.

Principles and Theory Under ideal conditions, the basis of 8.p.c. is the separation

of molecules solely by virtue of their size. Briefly, the gels, which are in bead form, consist of three-dimensional crosslinked polymers which can be swollen in solvent. The gel beads contain pores, the size of which is determined by the degree of crosslinking. (Glass beads consist of a rigid crosslinked structure unaffected by solvent.) When solvent is passed through a column of gel, it is able to permeate freely through both the void volume between the gel particles and the pores inside the gel. Although polymer molecules can pass freely through the whole of the void volume, their passage through the gel pores is usually limited. In practice, a gel or series of gels are chosen, which have been crosslinked to give a range of pore sizes corresponding roughly to the sizes of molecules to be fractionated.

As the separation process can be regarded as a partition of solute between solvent inside and outside the gel, the nomen- clature of partition chromatography has been applied to g.p.c.

The relationship between the various volumes is : Vt = Vo + Vi + Vm . . . . (1)

where Vt = total volume, V,, = void volume or volume of solvent outside

Vi = the solvent volume inside the gel beads, V, = volume of gel matrix.

the gel beads,

The partition coefficient, Kd, of a particular solute molecule is defined as the ratio of the concentration of solute inside the gel pores to the concentration in the void volume. Thus, the elution volume, Ve, of a molecule is given by the equation:

. . . . (2) From this definition, Kd has values between zero and unity,

being zero for large molecules which are excluded from even the largest gel pores, and unity for molecules which penetrate all the pores.

For convenience, in this work phenol has been regarded as the standard small molecule and Kd* has been defined by the equation:

where Vphenol is the elution volume for phenol. In order to compare the separating efficiency of different

gels, the number of theoretical plates, N, is found using

Ve = vo + Kd vi

ve = vo + Kd* (Vphenol - Vo) . . . . (3)

*Presented at a symposiurn on ‘Structural Analysis of Polymers’, organised by the Plastics and Polymer Group of the Society of Chemical Industry, at Churchill College, Cambridge on 11-13 September, 1968.

Br. Polym. J., 1969, Vol. 1, May 1

102 Aldersley et al. : Chromatographic Studies of some Thermosetting Resins

columns of identical size. This parameter is obtained using Glueckauf's formula : 8

N = 8 (T)2 . . . . (4)

This formula assumes that the concentration of the eluting species follows a Gaussian distribution, being the width of the curve at l /e of its maximum height. If the length of the column is known, the height equivalent of a theoretical plate ( H E W ) can easily be found.

Several simplified modelss *lo have been advanced in order to attempt an explanation of the separating principle. These all gave a relationship between the elution volume and the molecular weight, of the form:

Kd = A - BlOgM . . . . ( 5 ) where M is the molecular weight and A and B are constants.

Over small ranges of M this relationship has been followed for oligomers of the same type, but for different types of polymers A and B were not universal constants.ll The use of mean end-to-end chain length and radius of gyration as operating parameters has led to improved orr relations.^^^^^ The most promising work to date is that of Grubisic et aL1* who found a universal linear relationship between the elution volume (or K d value) and the hydrodynamic volumes of the species in solution. This equation was of the form:

where [T] is the intrinsic viscosity and M is the molecular weight of the species.

This relationship probably would not hold for small molecules, of molecular weight less than 1000, and in the absence of direct information concerning the volume of the oligomers in THF, the simplified approach of Equation ( 5 ) has been adopted. The examination of molecular models showed that the use of end-to-end distances would not give significantly better results.

log[~]M = A' - B'Ve . . . . (6)

Experimental Phenol-formaldehyde resins

Novolacs and resols derived from several substituted phenols were fractionated on the same gel/solvent systems. The gels used were crosslinked polystyrenes (supplied by Bio-Rad Laboratories, Richmond, California), and designated Bio-Beads S-X2 (- exclusion limit 2,700), S-X4 (1,700) and S-X8 (1,OOO). The gels which are supplied dry, were swollen for 24 h in THF (from B.D.H., but distilled to remove the hydroquinone stabiliser), which was also used as solvent for the resins. The majority of this work was conducted with the S-X2 gel, although some of the resols showed improved fractionation on the S-X4 and S-X8 gels.

The columns used were made to specification (by T. W. Wingent Ltd., Cambridge) and a typical arrangement is shown in Fig. 1. Gyrolok couplings (from Hoke Manu- facturing Co., Tenafly, New Jersey) facilitated the joining of columns in series when it was necessary to use more than one gel to fractionate resins of wide molecular weight range. The columns were maintained at 30" (&0.5") with a water jacket as shown, and external lagging. The column effluent was monitored by ultra-violet absorption at 280 nm. The spectrophotometer used was a Unicam SP 500 Series I1 incorporating a variable path length flow cell consisting of flat silica windows separated by polytetrafluoroethylene (PTFE) spacers to give the required path length. The volume of the cell was 0.06 ml/mm of path length. The voltage output from the spectrophotometer was taken to a 0.5 decade Unicam SP 22 logarith- mic recorder which gives (with certain reservations to be discussed later) a trace directly proportional to the molecular weight distribution of the resin.

.. .. ..

Fig. 1 . Column used for phenol-formaldehyde resin fractionation, complete with Gyrolok couplings

A, silicone rubber septum; B, b in 0.d. polyethylene tubing (Portex): C, no. 1 porosity sintered glass disc; D. space filled with smallglass beads (0.744 mm dia.) to reduce 'dead' volume; E, & in 0.d. polyethylene tubing (L.K.B. 2.0 mm 0.d.;

0.85 mm i.d.)

Solvent was passed through the system via a sintered glass filter with the aid of a Hughes Series 11 micropump. This was fitted with a 5:l reduction gearbox and twin size 2 short-stroke heads which deliver together a maximum flow of 96 ml/h. All parts of the pump in contact with the solvent were made of either PTFE or stainless steel. In order to account for any variations in the flow rate, the effluent from the flow cell was collected in a calibrated syphon. Upon emptying, the syphon was made to activate a relay, which then allowed a condenser to discharge directly, to the recorder input, producing a small mark on the trace. This work was done using an approximate flow rate of 0.5 ml/min which gave a total analysis time of about 2fhIresin. This flow rate has been shown by Boni et a l . I5 to be the optimum rate for the minimisation of volume errors when using THF in an open system at low temperatures.

The spectrophotometer has several advantages over the differential refractometer when used in the analysis of U.V. absorbing species. In particular, it is not necessary to in- corporate a pulsation damper in the solvent line between the pump and the column, nor is it necessary to run a reference column in parallel with the 'active' column. Temperature control need not be as critical as with a refractometer, and furthermore it is not essential (although desirable), to degas the solvent when it is used at 30".

Samples were injected with a Hamilton gas-tight syringe directly into the gel bed via the silicone rubber septum. The size of sample was variable and dependent on keeping the maximum optical density below a value of 0.5 (the limit of the recorder). For resins, typical amounts were in the range 15-60 pl of -10% (wt./vol.) solution. For resins based on p-phenylphenol (with a high extinction coefficient) and for model compounds, smaller quantities were used. It was found that use of small volumes of concentrated solutions gave results better than those obtained for more dilute solutions with consequently longer injection times.

The one major drawback in this system was the use of silicone rubber septa. These were considerably swollen by

Br. Polym. J., 1969, Vol. 1, May


THF, which also dissohed a U.V. absorbing species from the ubber. Consequently the septa had to be :-eplaced at regular ntervals (one per week was found to be satisfactory).

Melamine-formaldehyde resins For fully alkylated melamine resins, 2 lmost exactly the

lame procedure was used as for the phenolic resins, the only iifference being that the monitoring was carried out at 237 nm 2nd that smaller samples were used (lypical values are 2040 pl of 3-5 % (wt./vol.) solution). For partly alkylated and non-alkylated melamine resins, water-THF mixtures containing up to 50% (by vol.) water were used as solvent. These latter types of resin were not precipitated when diluted further with THF.

Urea-formaldehyde resins Owing to the lack of U.V. absorbing species and suitable

solvents for use with a differential refractometer, a colour- producing degradation process has been utilised in the fractionation of urea-formaldehyde condensates. When reacted with chromotropic acid in concentrated sulphuric acid at loo", formaldehyde, in both free and bound forms, reacts quantitatively producing a purple colour absorbing at 570 nm.16 Urea-formaldehyde resins are extensively hydrogen bonded, which severely limits the number of available solvents. In addition it is advisable to fractionate the resins at 25", on account of their high reactivity. Solutions of lithium chloride (a known hydrogen bond breaker) in either methanol or water (both 20% wt./vol.) have been used for evaporated and unevaporated resins, respectively. Other solvents include dimethyl sulphoxide and pyridine but these have disadvantages when used with the present technique.

Several hydrophilic gels have been tested (crosslinked dextrans, polyacrylamides and polystyrenes), the best results so far being obtained with Sephadex G-series gels. To cover a suitable molecular weight range, a multiple column assembly maintained at 25" (f0.1") with a water jacket, was set up. This contained four gels whose properties u e given in Table I.

Some of the later work was done using nedium grade G-25 and G-50 of particle size 50-150 pm, which have recently become available. Finer grades are available but these did not give practicable flow rates.

The order in which the columns are con iected is immaterial as far as fractionation is concerned, but it is preferable to run solvent upwards through the G-50 column because of the greater compressibility of this gel.

The columns were packed in the usual way under gravity with gel which had been swollen in water for 24 hours. Thermostated water (25" 0.1") was passed through the column assembly at an approximate rate of 0.7 ml/min using a peristaltic pump (Type 4912A, from L.13.K.-Produkter AB, Sweden).

The formaldehyde analyser is shown in Figs 2, 3(a) and 3(b). A stock solution of chromotropic acid was kept in the Mariotte bottle B (Fig. 2). This solution was made by dissolving the sodium salt of chromotropic acid (8 g A.R. grade, from B.D.H.) in distilled water (4 ml), the resultant

slurry being made up to 1 litre with A.R. 97 % sulphuric acid (from B.D.H.). The chromotropic acid is syphoned through the capillary C to the vessel F where it meets the eluant from the columns. Reaction occurs in the steam-heated coil G, and after dilution with water from the Mariotte bottle C, the resulting solution is cooled in the condenser K before passing to the flow cell, illustrated in Fig. 3(b).

The capillary sizes were chosen to achieve specific reactant ratios and residence times in the various coils. It has been shown17 that to obtain maximum colour development in accordance with Beer's Law the following conditions must be satisfied. First, the concentration of sulphuric acid in the reaction coils must be in excess of 86% (by vol.). Secondly, the residence time in the steam-heated coil should be greater than 7.5 min. Thirdly, to follow Beer's Law, the diluted solution should have an optical density of less than unity (extinction coefficient = 16.5 x lo3 l/mole/cm). These conditions have all been met in the design of this apparatus.

l l K /"

L

Fig. 2. Flow system used for the continuous analysis for total formaldehyde content

A Mariotte tubes. B aspirator ( 5 1 capacity) for sulphuric acid/chromotropic acid sdlution. C, aspidto; (20 1 capacity) for distilled water. D capillary (10 cm x 1-2 mm i.d.) at 25% to maintain constant flow of acid; I!, c&llary (20 cm x 0.8 mm 1.d.) at 25Oc to maintain constant flow of water. F eluant/acid mixer. G reactor coil heated at 1W"c with steam; H, acid/wa(er 'mixer: K, cooling 'coii thermostated at 25"c; L, outlet to flow cell;X, pressure reliefoutlets led to waste Eoth the reactor and cooling coils are 40 cm long and consist of 3 4 mm glass

tubing close wound on a 19 mm former

TABLE I Properties of Sephadex Gseries gels

Fractionation Sephadex gel range Water regain, Bed volume, Particle size,

(for dextrans) g H,O/g dry gel ml/g dry gel Clm _____ -

G-10 < 700 1.0 * 0.1 2-3 40-1 20 G-15 < 1,500 1.5 & 0.1 2.5-3.5 40-1 20 G-25 (coarse) loo- 5,000 2 5 * 0.2 4-6 100-300 G-50 (coarse) ~'oo.-1o,oO0 5.0 & 0.3 9-1 1 100-300

Br. Polym. ~~~

J., 1969, Vol. 1, May

104 Aldersley et al. : Chromatographic Studies of some Thermosetting Resins

The optical system is self-explanatory (Fig. 3(a) ) and was constructed specifically for this apparatus. A constant voltage mains input was used where necessary. The output from the photocell is amplified and fed into a Vitatron log- arithmic recorder. By injecting known amounts of formaldehyde into the system it was shown that the responses of both the optical system and the recorder were linear over the range to be used.

Samples were injected in the same way as for the phenolic resins and were of the order of 200 p1 of 2 4 % (wt./vol.) resin solution. A check showed that all the resin injected into the column was eluted, meaning that no precipitation occurred on diluting the methanolic lithium chloride solution with water.

The column assembly was calibrated with a 250 pl sample of a mixture of 05% Blue Dextran 2000 (rnol. wt. = 2 x lo6 from Pharmacia AB., Sweden) and 2.5% lithium chloride. The former was completely excluded by the gel (Kd = 0) and the latter was suitably small to be given a nominal Kd value of unity. The lithium chloride was detected on passage through a conductivity cell connected to the column outlet. This was filled with glass beads to reduce the dead space and the conductivity was recorded (via a continuously recording meter, from Portland Electronics Ltd., Stalybridge, Cheshire). The dextran was detected by measuring its absorption at 265 nm. Since lithium chloride was present in all of the resin solutions, it was possible to make a Kd = 1 calibration on every g.p.c. run, so as to reduce any errors owing to fluctua- tions in the solvent flow rate.

By monitoring the weak carbonyl absorption at 205 nm, it was shown that passage through the reaction coils did not significantly broaden the molecular weight distribution obtained.

Results and Discussion Phenol-formaldehyde resins

Before a molecular weight distribution can be found by g.p.c., it is necessary to study the effect of molecular weight variations on (a) the peak absorption wavelength and (b) the extinction coefficient of a phenolic nucleus.

To examine the variation of peak absorption, a Unicam SP 800 spectrophotometer was used. The separations were carried out in the normal way, but as the peak concentration for each oligomer was reached, the flow was stopped and a full wavelength scan was recorded. For four types of novolacs, the shift in the wavelength for dimer and higher novolacs was negligible but the parent monomer showed a peak absorption at a slightly lower wavelength, as shown in Table 11.

For convenience, all g.p.c. runs of phenolic resins were run at 280 nm. Extinction coefficients for several model compounds at this wavelength are listed in Table III.

These results show three features: (i) the variations between dimers and trimers in the same series are small ( N 1 %); (ii) the variations between monomer and other oligomers in the same series are larger, (-25%) as expected; and (iii) the variation between parent monomers is of the same order ( N 25 %) except for conjugated phenols (e.g. p-phenylphenol) which have much larger extinction coefficients.

TABLE I1 Wavelength peaks for 4 types of novolac

Wavelength peak, nm Phenol type

Monomer Polymer Phenol 273.5* p-Chlorophenol 284

p-t-Octylphenol 279 p-tButylpheno1 278.5

28 1

280 285

280.5

'Second peak at 280 nm

TABLE I11 Extinction coefficients of some phenolic resins at 280 nm

Extinction coefficient/

l/mole/cm Compound phenolic nucleus,

A I /

Fig. 3(a). Detector optics for urea-formaldehyde analyser A. lamp 6 V 36 W vertical filament prefocus Mazda PI 5d base; B, lens (f = 5 cm 38 mm dia.). C liquid filter 10%aq.solutionCu(NO,),;D,flowcell-seeFig. 3(b); E, gelatin fiite;(Kodak Wratten 23A); F, variable aperture; G, shutter; H, photo-

cell

Fig. 3(b). Flow cell A,inlet from flow system to funnel;B, outletto waste;C,cell(l cmdia., 1 cmpath

length)

Phenol o-Hydroxymethylphenol 4,4'-Dihydroxydiphenylmethane p-Cresol (1) OI) p-Chlorophenol (m) W) p-t-Butylphenol p-t-Octylphenol

1730 1905 2090 2140 2660.2

1760 221OC 2225d 1860 1920

268ob

a contains 96% phenolic units as dimer (estimated by g.p.c.) b ,. 95% ,, 7 . I , 3 . 1, ,. I ,

c *, 78% ,. .. ,, ,, ,. 1, ,, d ,, 69% ,, ,, ,, ,. ,. 3. >.


Aldersley et al.: Chromatographic Studies of some Thermosetting Resins 105

OH OH OH

A further part of this examination involved analysis by g.p.c. of a novolac based on p-cresol. The trace obtained was analysed graphically into a series of approximately Gaussian shaped peaks corresponding to each oligomer. Well-defined peaks were obtained for oligomers containing up to six p-cresol units and the positions of the other peaks (up to 20 p-cresol units) were estimated by extrapolation of the log (mol. wt.) - elution volume graph. (In terms of U.V. absorption of individual p-cresol groups, oligoiners containing six or less p-cresol units comprised about 83 % of the total, and oligomers containing more than twelve units comprised - 2.6%.) In view of the small differences in the extinction coefficients of the dimer and trimer (see Table 111), separate values were assigned to the two constituent groups (V) and (VI).

OH I

Calculation of the extinction coefficients and weights of all the oligomers enabled values to be found for the number- average and weight-average molecular weights, the average weight of an individual p-cresol nucleus and the average extinction coefficient. The latter quantity was also found by direct measurement. A combination of the various results led to assignments of 2670 and 2780 l/mole/cm for the extinction coefficients of the mono- and di.substituted p-cresol units. This is an extreme variation of only 4 % and probably compares very favourably with refractive index changes in low molecular weight materials. (The only comparable results available are due to Ziegler et a!,ls who found that for substituted polyphenylenes, there was a 100 % variation in refractive index between dimer and octamer with a levelling out at molecular weights in excess of 1000.) The number- average molecular weight calculated from the g.p.c. trace was 340; a value of 361 was obtained using a vapour pressure osmometer.

Novolacs based on six different phenols have been analysed along with several model compounds. The six phenols were phenol, p-cresol, and phenols substituted in the para position by chlorine and the phenyl, p-t-butyl- andp-t-octyl- [-C(CH,), -CH,-C(CH3)3] groups. Fig. 4 shows the separation of a typical novolac based onp-t-butylphenol. Distinct peakscorre- sponding to seven oligomers can be seen and this is typical for all the phenols examined. The component of lowest molecular weight has not yet been characterised although examination of an evaporated sample of a presumably similar compound derived from a p-chlorophenol novolac showed it to be non-hydroxylic.

Detailed Kd* values for the components of all the phenols studied are given in Table IV.

Log (mol. wt.) vs. Kd* plots have been made and the results for four novolacs are shown in Fig. 5. The weights were calculated with the assumption that each hydroxyl group is hydrogen bonded to a THF molecule - as was reported by Gardikes 8c Konrad.'* The existence of this hydrogen bonding was confirmed by finding the elution volumes of non- hydroxylic species (benzene, anisole and diphenylether). The Kd* values of these and other model compounds are given in Table V.

When these Kd* values were plotted against log (mol. wt.), it was apparent that all types of active hydrogen (phenolic, alcoholic and amino) atoms exhibit solvent hydrogen bonding to some extent. It is evident from Fig. 5 and Table V that oligomers derived from the trifunctional phenol molecule show different elution properties from similar oligomers derived from bifunctional substituted phenols and also from compounds VII and VIII, the latter showing properties similar to the corresponding p-cresol oligomers. These variations in hydrodynamic volume can arise from two possible sources. First, it is well known that whereas particular oligomers derived from bifunctional phenols can assume only one structure, those derived from trifunctional phenols have a

- MOLECULAR WEIGHT

Fig. 4. Distribution of a typical novolac based on p-t-butylphenol

TABLE IV Kd* values For phenolic oligomers (based on a value of Kd = 1 for phenol)

No. of p-chloro- p-t-butyl- p-t-octyl- p-phenylphenol units Phenol phenol phenol phenol p-cresol phenol

1 l m o t 0.9477 08777 0.797.1 0976t o.919t 2 0.776 0.724t 0.660 0.574t 0.807t 0.690 3 0.619 0.5891. 0522 0.434t 0.6881. 0.547 4 0.510 0.481 0.414 0.330 0592 0.438 5 0.421 0.394 0.330 0.251 031 3 0.358 6 0.345 0.323 0.262 0.188 0.446 - 7 Unknown 1.131 1.071 0.984 0.885 1.106 1.131

____- - -

- - - - 0.214 -

tKd* value checked against pure compound


106

1000

I- f

I?

500 !2 LL

3 0 W

_I 0 I

5

200

Aldersley et al. : Chromatographic Studies of some Thermosetting Resins

-

-

-

TABLE V

Kd* values of model compounds

K d *

Benzene 1.168 Diphenyl ether 1.048 Anisole 1.137 Benzyl alcohol 1.011 o-Hydroxymethylphenol 0.917 p-Hydroxymethylphenol 0.871 Catechol 0.899 Resorcinol 0.878 Hydroquinone 0.873 Pyrogallol 0.818 Phloroglucinol 0.781 o-Phenylphenol 0.937 4,4'-Dihydroxydiphenynylmethane 0.769

0.822 0.709 0738 Bisphenol A

Aniline 1.019

g:f,

R k a Im, R - H I

2oool------

I I 0.5 1.0

100~

Kd

Fig. 5 . Molecular weight vs. Kd plots for a number of novoiacs 0 phenol novolac; 0 p-octylphenol novolac; x p-phenylphenol novolac;

pcresyl novolac

number of isomers, this number increasing rapidly as tht number of phenol units in the oligomer increases. In additior there is also the possibility of chain branching when thc number of phenol units exceeds three. A comprehensive account involving the calculation of the numbers and type: of possible isomers has been given by Hollingdale & Megson.2'

Secondly, the active hydrogen atoms can exhibit intramolecular hydrogen bonding to phenolic oxygen atoms as opposed to intermolecular bonding to THF. This occurs to a large extent in oligomers derived from bifunctional phenols and compounds of type VII and VIII and has been confirmed by measurements of the dissociation constants for the active hydrogens.21

The extent of the contributions made by these two effects cannot be gauged with any certainty until further work with model compounds or different solvent systems has been carried out.

In the case of the isomeric hydroxymethyl phenols, the difference in elution volume is almost certainly caused by intramolecular hydrogen bonding in the ortho isomer.

Steric factors can also contribute, causing incomplete solvent hydrogen bonding and this probably explains the small differences found between the various isomers of the di- and tri-hydroxy benzenes (see Table V).

The analysis of novolacs based on mixtures of different phenols is more difficult. The number of distinct peaks is diminished, mainly because an increased number of constituent compounds are present, and the weight increments between them are consequently smaller. In addition, the molecular weight-elution volume relationships are different for the constituent phenol oligomers and for the mixed oligomers. In spite of these difficulties, useful information of a semi-quantitative nature can be obtained from a fractionation. The method can also be used for 'fingerprinting' novolacs based on mixtures of unknown phenols.

Although the hydrogen bonding to THF causes some difficulties in the interpretation of novolac fractionations, it can be an advantage in the analysis of resols. In these resins the weight increment for a methylol group is effectively increased from 30 to 102 because of hydrogen bonding to THF. Thus, constituent methylol compounds can be detected (Fig. 6). Improved resolution of these mononuclear methylol compounds can be achieved by use of the gel of smaller pore size, S-X8.

G.p.c. using the crosslinked polystyrene Bio-Beads S-X2 produces excellent fractionations of novolacs and resols of low molecular weight. Useful information such as an indica- tion of the amounts of residualmonomer or of high molecular weight material is easily obtained. Application of curve analysis techniques using a computer can quickly give values for both number- and weight-average molecular weights.

- MOLECULAR WEIGHT

Fig. 6. Distribution of a phenol-based resol A, p-hydroxybenzyl alcohol; B, o-hydroxybenzyl alcohol



Except for monomer, any changes in extinction coefficient can be ignored without introducing any significant error. The fact that these averages refer only to absorbing phenolic species has both obvious advantages and disadvantages compared with values obtained by other techniques.

Melamine-formaldehyde resins Most of the melamine resins studied were aqueous suspen-

sions of low molecular weight materials. Fig. 7 shows results from two resins produced under different conditions. Molecules containing different numbers of triazine nuclei are separated into distinct peaks, the substituted single melamine nucleus having an elution volume count of approximately 60 in the diagram. In a typical melamini: resin, three types of groups can be attached to an amino nitrogen. Thus for a mono-nuclear derivative (IX), R,-R, cart be any combination of the three groups -H,-CH,OH or -CH,OCH,.

- The three fully substituted melamines w th R, = R, . . . . - & have been eluted separately. Melamine and hexamethylol- melamine were dissolved in a mixture of THF and water, and the curves showed some ‘tailing’ owing to insolubility in the THF eluent. The elution volumes obtained for the compounds with R, = -H,-CH,OJI and -CH,OCH, were 61.5, 58.3 and 61.7 respectively with molecular weights 126, 306 and 390 (cf. cyanuric chloride, mol. wt. 184, elution volume approximately 69 counts). Values from partly substituted resins substantiated these resulfs and it has not been possible to resolve the peaks into any constituent parts, even

with the use of small pored S-X8 gel. These results support the view that THF is hydrogen bonded to both amino and methylol groups. The extent of this bonding cannot be gauged from the elution volumes and hence exact molecular weight data cannot be obtained. As the total proportions of methylol and methoxymethyl groups can be found by chemical methods, the fractionation obtained giving quantitative data on the degree of polymerisation is obviously invaluable.

Urea-formaldehyde resins In order to make direct comparisons between molecular

weight distributions, each curve was normalised to give both differential and integral formaldehyde distributions. The abscissa was converted from a time scale to a Kd scale (using the lithium chloride and dextran volume calibrations) and the ordinate from an optical density scale to a fractional weight percentage formaldehyde (differential plot) and a cumulative weight percentage formaldehyde (integral plot). This is illustrated in Fig. 8 for a representative resin. This procedure gives each differential curve a nominal area of 100 units. A, computer programme was written for this normalisation procedure to give results in the form of both graphs and tables of values.

Although the separations obtained are not as good as with the phenol and melamine condensates, useful information can often be obtained from the distributions regarding batch variations and the formation of high molecular weight material. Fig. 9 illustrates the distributions of samples taken

‘OJroo

- \

w 1. 2 0

I a: 0 U

Fig. 8. Typical integral and differential curves from urea-formaldehyde analyser

40 , I

Kd

Fig. 9. Distributions of samples taken during a urea-formaldehyde production run

- 1 ; - - - 2 ; . . . . 3 ; - x - 4 ; - - . - 5 ; - . . - 6 ; - xx-7correspondto figures in Fig. 10

108 Aldersley et al.: Chromatographic Studies of some Thermosetting Resins

at different stages during the production of a typical resin. This figure can be compared with Fig. 10 which shows the variation of labile formaldehyde (= free plus hydrolysable) in the same resin. Both of these figures indicate the signi- ficance of the reflw and acid addition stages, Fig. 9 showing the increase in the amount of high molecular weight material (0 < Kd < 0.5) and Fig. 10 showing the increase in the rate of conversion of formaldehyde from the labile to the bound state. Further main features of these distributions are the large amounts of material with Kd values in excess of unity, and the ‘tailing’ observed with samples 1 and 3. Such ‘tailing’ is caused by the presence of free formaldehyde and this effect is further illustrated in Fig. 11.

When the medium grades of G-25 and G-50 were substituted for the coarse grades, much sharper peaks were observed and the plate count increased from - 800 to N 1300.

Results from several low molecular weight textile resins are shown in Fig. 11. These are methylated urea-formaldehyde condensates and show a low molecular weight peak which is shifted to a lower Kd value (0.97) compared with the un- methylated resin shown in Fig. 9 (Kd 1.06). The behaviour of several model compounds was investigated and their Kd values are shown in Table VI.

All these compounds produced symmetrical peaks with the exception of formaldehyde (see Fig. 11). These K d values are based on a value of unity for lithium chloride, which seems reasonable for this column system, as the lithium chloride

I T I M E -

Fig. 10. Labile formaldehyde contents of samples taken during a urea-formaldehyde production run

R, reflux started; A, acid added

I I I I

K d

Fig. 11. Distributions of low molecular weight compounds ~ Bis(methoxymethyl)urea, - - - - formaldehyde, . . . . urea-formaldehyde textile resin

TABLE VI Kd values of several model compounds

~~

Compound Mol. wt. Kd

Formaldehyde (HOCH,OH) Urea* Monomethylolurea Dimethylolurea Methylenebisurea (X) Bis(methoxymethy1)urea (XI) (MI)

30 (48) 1.10 60 0.93 90 1.17

120 1.09 132 1.49 i 48 097 192 1.10 222 1.02

*Measured at 205 nm

concentration is in excess of those affected by the ion-exclusion effect.22 From this evidence it appears that certain compounds are adsorbing to the gel and hence have greater Kd values than predicted. Whilst it is difficult to interpret the combined separation by size and adsorption, several conclusions can be drawn : (i) fully methylated species, e.g. bis(methoxymethy1) urea show little or no adsorption; (ii) methylol groups and formaldehyde show moderate adsorption; and (iii) unsubsti- tuted urea aminogroupsshow strong adsorption. This statement is based only on the Kd value obtained for methylenebisurea. This value was confirmed on different samples of the compound, and the peak at Kd = 1.49 was shown actually to decrease in area on reaction with formaldehyde. With monomethylolurea, although the Kd value i s greater than in dimethylolurea, it is not as high as expected (but this may be due to intramolecular hydrogen bonding effects). Urea gave apparently anomalous results.

The low plate counts of the Sephadex columns coupled with the above adsorption effects means that the distribution obtained is both diffuse and difficult to interpret. Also the small increments in molecular weight between the different species means that discrete peaks cannot be obtained. Despite these difficulties, the fractionations have given information superior to that obtained using the standard precipitation techniques.

To overcome the inadequacies of the hydrophilic gels, the authors are considering mild chemical reaction of the methylol groups in the resin in order to render the resulting polymer soluble in organic solvents. The normal polystyrenes might then be used for fractionation together with the differential refractometer as a monitor.

Conclusions G.P.c. has proved an effective technique for fractionating

three types of formaldehyde-based resins. In all the systems described, hydrogen bonding has led to some complications; for example, with phenol- and melamine-based resins, in addition to there being difficulties of interpretation, the range of molecules capable of being separated is decreased owing to the increase in the effective size of each molecule because of the bonded solvent.

Although refinements are necessary before the technique can be applied to kinetic work, the separations obtained are sufficiently resolved, especially for novolacs, to give reliable data on molecular weight distributions and degrees of reaction.


Aldersley et a].: Chromatographic Studies of some Thermosetting Resins I09

Acknowledgments The authors are indebted to Dr. W. Wilson who directed

this project throughout, and to Dr. S. A. Zahir (now with the Plastics Department of CIBA Ltd., Bask) who carried out the initial work on the gel permeation I:hromatography of urea-formaldehyde resins in these laboratories, and laid down the foundations for the work descrimi in this Paper.

Porath, J., & Flodin, P., Nature, Lond., 1959, 183, 1657 ’ Moore, J . C., J. Polym. Sci., 1964, A2, 835 8 Glueckauf, E,,

(London: Society of Chemical Industry) Porath, J., Pure appl. Chem., 1963, 6, 233

l o Ackers, G. K*, l1 Heitz, W., Platt, K. L., Ullner, H., & Winau, H., Makromolek.

l2 Moore, J. C., & Hendrickson, J . G., J . Polym. Sci., 1965, C8,233 l3 Moore, J. C., & Arrington, M. C., 3rd Int. g.p.c. seminar,

l4 Grubisic, Z . , Rempp, P., & Benoit, H., Polym. Lett., 1967, 5, 753 lQ Boni, K. A., Sliemers, F. A., & Stickney, P. B., J. Polym. SC;.,

I6 Bricker, C . E., &Johnson, H . R., Ind. Engng. Chem., analyr. Edn.,

exchange and its applications~, 1955, p. 34

N . y., 1964, 39 723

Chem., 1967, 102, 63

Geneva, May 19-20, 1966 CIBA (A.R.L.) Ltd., Duxford,

Cambridge. 1968, A2, 1567 Received 2 January, 1969

1945. 17. 400

References Siliprandi, D., & Siliprandi, N., Biochim. biophys. Acta, 1954, 14,

Wheaton, R. M., & Baumann, W. C., Ann. ~ V . Y. Acad. Sci., 1953,

Lab. Pract., 1967, 16, (7), 838-870 Cazes, J., J. chem. Educ., 1966,43, A567, A.625 Determann, H., ‘Gel Chromatography’, 15168 (Berlin: Springer-

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755

DISCUSSION

Mr. W. R. Cross (B.X.L. Ltd., Birmingham 11):-Can you obtain a sufficient quantity of different ~nolecular species to study by other methods such as i.r. or pyrolysis/gas chromatography from analytical g.p.c. even if it means re-running the sample several times ?

Author’s reply :-One can obtain quantities of the different compounds by repeated running of similar samples. Thus, we obtained sufficient quantities of the unknown compound of low molecular weight derived from a p-chlorophenol novolac by running ten 200 ~1 samples of a 20% solution of the resin. This gave sufficient material for an infra-red spectrum. Obviously separation of higher oligomers would require re-cycling procedures but this has not been attempted.

Dr. G. J. Reynolds (Vinyl Products Ltd., Carshalton) :-Some years ago we attempted to apply the g . x . technique using Sephadex columns and U.V. detection to the fractionation of non-ionic surfactants. We encountered ihe problem of non- reproducible retention volumes at times. At that stage the manufacturers of Sephadex recognised this problem and attributed it to varying bed volume, but could offer no solution. Have you had this difficulty, and if so have you managed to overcome it?

Author’s reply :-We have also encountered variations in the retention volumes. This was overcome by having two internal markers, as outlined in the text. Reproducible K d values could then be obtained by interpolation between the markers.

Dr. 1. Haslam (I.C.I. Ltd., Welwyn Garden City):-Did you ever find danger on recoveries, for i.r. examination, by evaporation of THF solutions, owing to the presence of peroxides ? Author’s reply:-I would agree that evaporation of un- stabilised THF is dangerous. However, as the volumes we used were small ( N 50 ml), the hazards were of a minor nature.

Dr. W. May (Shell Plastics Labs, Delft, Holland):-In the collection of 6.p.c. fractions for further identification caution is certainly required; the procedure has been carried out repeatedly at Shell Plastics Labs in Delft, and, after evaporation of the tetrahydrofuran, substances are detected by i.r. which should not be there and have not yet been identified. Author’s reply :-We have aIso detected several unknown compounds by ultra-violet in the normal running of resins. These have not been seriously investigated yet.


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Chromatographic studies of some thermosetting resins