Viscometry and kinetics of initial stages of curing polyurethanes

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A. YA. MALKIN et al.

10. Ya. M. SLOBODIN and A. P. KHITROV, U.S.S.1L Pat. 490790, Byull izobr., No. 41, 1975 l l . Ye. N. GUR'YANOVA, I. P. GOL'DSHTEIN and I. P. ROMM, Donortm-aktseptornaya svyaz' (Donor-Accepter Bond). p. 79, Khimiya, Moscow, 1973 12. L.M. SVERDLOV, M. A. KOZNER and Ye. P. KRAINOV, Kolebatel'nyye spektry nmogoatomnykh m<>lektll (Vibratory Spectra of Polyatomlc Molecules). p. 66, N~ltlk~l, Moscox',', 1970

:PolymerScienceU.S.8.]I.Vol. 25, No. 9, pp. 2270-2275,1983 Printed in Poland

0032-3950/83 $10.00+.00 © 1984 PergamonPress Ltd.

VISCOMETRY AND KINETICS OF INITIAL STAGES OF CURING POLYURETHANES* A . YA. MALKIN, Y. P. BEGISHEV, S. G. KULIOItIKI~IIN a n d V. A. KozI~I~A Scientific Indnstrml AssociAtion "Plastics"

(Recei~'ed 27 April 1982) Various methods were used to study kiuetics of initial stages of curing polyurethanes by using a biflmctional curing agent. It: was shown that under reaction conditions practically excluding biuret formation already in the initial stages threedimensional structures are formed with network units formed bY specific physical interactions.

TtIE chemistry of pol3alrethane formation has been studied in many papers, a general view exists therefore about this process [1]. The variation of rheological properties during the formation of polyurethanes h~ts been studied to a much lower extent [2, 3], although this process is both of general the:)retical and technical interest. The process of forming three-dimensional crosslinked structures i n the course of forming polyur(,thanes (PU) from p()lyfunctional oligomcrs may be regarded as taking place in two stages [2-4]. First viscosity increases vigorously as a conse(luenee of elongatiou of chains and the formation of bra~mhes while retaining the fluidity of the reaction mass, then in the critical point ("gel-point"), when a continuous three-dimensional network is formed of chemical bonds and fluidity is lost. From the point of view of evaluating technical properties ("workability") the first stage is of most interest, when the cured PU can be converted into products. There are several description,s in the literature regarding kinetics of formation of PU, using various methods (IR spectroscopy, differential-scanning calorimetry, torsion ax~alys::;, etc.) [1]. However, the viscometric method gives most direct information about the effect of th0 * Vysok¢)mol. s()yed. A25: No. 9, 1948-1952, 1983.

2271

Kinetics of initial stages of curing polyurethanes

chemical reaction ell properties of PU fi)rmed; this method enables also a certain opinion to be formed about macrokinetics of the formation of polymers, since a link may be established between this process and rheokinetics of polymerization, or polycondensation [5]. This stu~v therefore seeks to examine mechanisms of viscosity increase during curing P U in connectipn with a kinetic study of the initial stage of the process. A stud), was made of macrodi-isoeyanate, synthesized fi'om polytetramethyleneglycol and 2,4-toluylenedi-isocyanate in a molar ratio of 1 : 2. The initial polytetramethylene glycol contained 3.5O/o hydroxyl groups. )9L,/37L, determined by a gel-chromatographic method was 1.7; 3~¢,~=1020 (ebullioscopy). Before synthesis 2,4-toluylenedi-isocyanate was distilled in v a c u u m (1.33 kPa) at 120 °. The macrodi-isocyanate synthesized was analysed for NCO group content by well-known methods. 3,3'-Dichloro-4,4'-diaminodiphenyhnethane twice recrystallized from heptane was the curing agent. Rheokinetic investigations were carried out using a "Rheotest-2" rotary viscometer with a cone-plane operating unit at low rates of displacement. To obtain the reaction mixture, macrodi-isoeyanate was mixed with the requisite amount of previously melted 3,3'-dichloro4,4'-diaminodiphenyhnethane (m.p. 103.2 °) in a rapid mixing device at room temperature. The reaction mixture ( ~ 0.1 ml) was then placed iu /:he operating unit of the viscometer heated to experimental temperature. Special experiments indicate that as a consequence of the small vohune ef the sample and intense heat-exchange, curing conditions are close to isothermal. Deviations of temperature fi'om the value given did not exceed 1°. I n parallel, under conditions which we tried to make adequate to a inaximum extent to those used in the rheokinetic experiment, the reaction system was cured outside the viscometer, in order to evahmte process kinetics: therefore, at given time intervals NCO-group content was determined. The error of viscosity measurement did not exceed 6°/O, NCO-group coneent rat ion, 5°/o. The third (in addition to viscometric and chemical) ittdependent method of investigating kinetics of curing was calorimetry, which wa,s applied to determine the, rate of heat liberation during curing.

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2272

A. YA. MALKI:~et al.

molecular chains is reflected by kinetics of concentration reduction of functional end groups. A variation in the concentration of NCO end groups before the formation of an insoluble fraction observed in the solution, is shown by Fig. la. Kinetics of polycondensation of two bifunctional monomers present in equimolecular ratio are usually described by a second order equation using the concentration of functional groups, integration of which gives the following well-known formula for number-average degree of polycondensation of the polymer formed: N - - 1 -~xokt, where N is the degree of polycondensation; x 0 is the initial concentration of functional groups, k is the reaction rate constant; t is time.

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Results of analysing experimental data using this formula are shown in Fig. lb. However, experimental results in Fig. l b are not approximated with one straight line, two linear sections with different angular coefficients. Changing the gradient of straight line in Fig. lb may be evidence either of a changed rate constant (i.e. in this case an acceleration of the reaction at the Second stage), or a change in the reaction mechanism after several initial stages of condensation. Analysis of results of isothermal calorimetry also lead to similar conclusions which confirm the difficulty of describing even initial stages of curing with one rate constant. Assuming that the reaction is of second order and the rate of transformtion fi is proportional to heat liberation q, the dependence of q (fl) takes the form 1

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(1)

Kinetics uf initial st,ages of curing polyurethanes

2273

where c is the normalizing instrumental coustant, which links /? and q and is proportional to the overall heat effect of the reaction. Constant c should be selected so as to approximate in an optimum way the experimental dependence of q (t) yzith the help of function (1). This has been done in Fig. 2 shich shows that if such an approximation is completely satisfactory for a large part of the process at the second (conclusive) stage, it is unsuitable for the first stage. The converse is also correct. This means that constant k cannot be constant for the entire process. I t is typical that the point of divergence of experimental and calculated curves in Fig. 2 cohlcides with the breaking point in Fig. lb. l~esults obtained, i.e. a variation of gradient of the straight line in Fig. lb and the divergence of curves in Fig. 2 prove that in the case examined the form of the process changes which is reflected b y kinetics. We try and understand the cause of this effect b y rheokinetic analysis. r] , Pu.sec

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If the polymer is formed b y a mechanism of linear polycondensation, it should be expected that the viscosity of the reaction system varies as follows [4, 6]: vl~K(xokt) a, where K is the constant dependent on temperature and the type of polymer, i.e. the dependence of log ~ on log t is linear with an angular coefficient of 3.4. Corresponding results are shown in Fig. 3a. Comparison of results in Figs. 1 and 3 enables the dependence of the viscosity of P U on molecular weight to be plotted, which is shown by Fig. 4. The dependence obtained is described by a step function of the form ~/--_hTa, where exponent a varies between 1-0 and 4.6. If the value of a---- 1.O is typical of polymers with low MW, the value of 4.6 exceeds the "universal" value of this expommt, which is 3.4. Sm-h an excessive v,flue of this exponent follows also from Fig. 3.

22~4

A. YA. MALKII~et at.

Let us examine also the temperature dependence of viscosity at various stages of polycondensation, which is important, since this dependence includes formal rheo]ogical constants of the material. The temperature dependence of the viscosity of the reaction system is actually determined by the activation energy of both the ohemical reaction U and by the activation energy of viscous flow E, and for polycondensation "effective" activation energies are expressed as follows [6]: Et~--E--aU; E , : U - - ( E / a ) , where Et and E, are "effective" activation energies reflecting temperature dependences of viscosity with constant reaction time E, or a time required for achieving a certain ~-iscosity level E,~. Results of analysing experimental data, according to these formulae are given in Fig. 5. Comparison of activation energies of condensation U determined from the dependence of k--~k0 exp( -- U/RT) and from Fig. 5 with a--4.6 and independent of the measured value of E : 4 1 . 6 kJ/mole gives practically the same value, which is 31.5 k J/mole. Therefore, results (compliance of reaction macrokinetics to a second order equation and rheokinetic ratios of condensation) prove that ia the case examined, i.e. during interaction of the bifunctional macrodi-isocyanate with diamine, polycondensation does indeed take place until an insoluble fraction is formed. As noted, however, two special features were observed: firstly, a variation in the gradient of curves in Fig. lb and secondly, an unusually high exponent a according to r/--fi;a which is 4"6. These special features may, apparently, be explained

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FI(~'. 5. Dependence of time dt, rillg which a viscosity level of 1.4 Pa.see (1) is achieved, of initial viscosity of the reaction mass (2) and tha rate constant of polycondensatio~l (3) on temperature. FIG. 6. Visc(~sity v~lriatioH (hlri~lg polyeox~d(~2isation of PU: 1--t)iflulctional curing agent (3,3'-diehloro-4,4'.diamit~odil)henylmethane); 2--trifunctiona] curing agent (aecor(liltg to results of an earlier paper [7l).

Kin,ot.ies of initial stages of euring poiyuretbmles

2275

b y the fact t h a t after the first stages of the reaction t m m c h e s a p p e a r in growing ehains which result in the t b r m a t i o n of micro-gel nuclei as a consequence of the a p p e a r a n c e of insoluble "fractions. This e s s u m p t i o n is confirmed b y results in Fig. 6, where a eomparisoT1 is made between curves of voscosity increase plotted in this s t u d y and previously [7]. A t.rifunetional curing agent is used in the latter ease for curing PIS, which presumes the f o r m a t i o n of b r a n c h e d chains starting from the first stages of the reaction. T h e similarity of dependences t/(t) and the equality of the index a = 4 . 6 in b o t h eases proves t h a t the physical p a t t e r n of processes c o m p a r e d is equivalent, i.e. confirms t h a t b r a n c h e d p r o d u c t s are formed in our ease as well. The existence o f ehemieal bonds forming b r a n c h e d maeromolecules by the i n t e r a c t i o n of two bifunctional monomers is difficult to assume under these react.ion conditions, which exclude the f o r m a t i o n of biurets. T h e effects observed may, most probably, be explained b y the feet. t h a t a f t e r the first stages of the reaction specific interactions of the t y p e of h y d r o g e n b o n d begin to have a significant role; these link u r e t h a n e and carbamide chain fragments, which tend to form h y d r o g e n bonds [8, 9] and t h e r m i n a l isoeyanate groups. This s o m e w h a t reduees the c o n c e n t r a t i o n of isocyanate groups d e t e r m i n e d b y chemical analysis, since the a p p a r e n t reduction of functional groups exeeeds even t h a t used up in the course of the ehemieal reaction. As a consequence art inflection is observed on the course showing the depe]ldence of the degree of polyeortdensation on t i m e (Fig. l b ) a n d therefore the dependence on the calculated value deviates using constants d e t e r m i n e d for the initial p a r t of the reaction (Fig. 2). This, u l t i m a t e l y , results in the a p p e a r a n c e of branches with n e t w o r k units f o r m e d b y speeific interactions and effects t h a t are similar to those observed during the f o r m a t i o n of a n e t w o r k of ehemieal bonds. Tra~slated by E. SE31[ERE REFERENCES

1. 8intez i fizikokhin~liya poliureta~tov (Synthesis and Physicochcmistry of Polyurethanes). Naukova dumka, Kiev, 1967 2. F. G. MUSSATI and C. W. MACOSKO, Polymer Engng. 8ci. 13: 3, 236, 1973 3. M. R. KAMAL, Polymer Engng Sei. 14: 4, 231, 1974 4. P. WRIGHT and A. CUMMING, Poliuretanovyye elast