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Synthesis and properties of peroxy derivatives of epoxy resins based on bisphenol A. 1. Effects of the presence of inorganic bases.

1. INTRODUCTION

Synthesis of polymeric systems typically proceeds in the presence of compounds that can serve as sources of free radicals under given temperature conditions. Among such compounds those containing unstable -O-O- bonds are quite important. If those compounds also contain other functional groups such as epoxy or hydroxy, three-dimensional polymer networks can be formed. Known peroxides (POs) include monomeric, oligomeric, and polymeric ones. The peroxides have current and potential practical applications as initiators of radical polymerization, crosslinking agents, and in synthesis of block copolymers. Thus, Geuskens and Kanda (11 used photo-generated hydroperoxides for grafting water-soluble monomers onto surfaces of block copolymer films. A successful use of an organic peroxide for curing unsaturated polyester resins for making polymer concrete has been reported by Abdel-Azim (2).

On the other hand, composites based on epoxy resins are already well established in a number of industries, as maintenance, marine, pipe, can, and drum coatings, electrical laminates, adhesives, and more (3-7). Carbon fiber + epoxy composites are extensively investigated from the point of view of interface properties and potential applications (8-11). The use of epoxies for flooring, paving, and in aggregates is expected to increase. Epoxidation of alkenes by alkyl hydroperoxides was performed by Stamenova and her colleagues (12). However, little attention has been paid to using PO compounds for synthesis of peroxy oligomers from epoxies, although an exception to that rule can be found (13). This situation prevails in spite of the fact that epoxy resins are characterized by high reactivity relatively to compounds containing mobile hydrogens. Bratychak and Hrynishyn (14) have obtained resins with terminal epoxy groups by polymerization of the (C.sub.8] - [C.sub.9] fraction from hydrocarbon pyrolysis with diepoxy derivatives of 4,4[prime]-azo-bis-4-cyanopentanoic acid. Under the circumstances, we have decided to use epoxy resins based on bisphenol A for obtaining oligomers containing -O-O- bonds, with aliphatic and aromatic hydroperoxides as mobile hydrogen carriers:

[Mathematical Expression Omitted]

where

R = structural fragment of an epoxy resin on the basis of Bisphenol A

[Mathematical Expression Omitted]

[Mathematical Expression Omitted]

X = -H, -C [(CH.sub.3]).sub.2][C.sub.6][H.sub.4] C [([CH.sub.3]).sub.2] OOH (1)

The X group, whether H only or the entire -[(CH.sub.3]).sub.2] [C.sub.6][H.sub.4]C[(CH.sub.3]).sub.2]OOH group, causes opening of the epoxy ring and formation of the hydroxyl group. Inorganic bases were used as catalysts for reaction (1) and the effects of their presence and concentration on the kinetics of PO formation investigated.

2. EXPERIMENTAL PART

2.1 Starting Reagents and Their Purifification

The epoxy resin used in this work, henceforth to be called ED-20, was synthesized by condensation of Bisphenol A with epichlorohydrin in the presence of NaOH at 70-80 [degrees] C during 4 h. The resulting number-average molecular mass Mn = 390 and the epoxy number e.n. = 20.7%.

Diglycidyl ether (DGEDPhP) of bisphenol A was obtained by standard procedures and purified by rectification, with the boiling temperature [T.sub.b] = 422 K under the pressure P = 7.5 x [10.sup.-3] Torr (= [10.sup.-6] J[center dot][cm.sup.-3] = 1 Pa). The distillation product had the refractive index [[n.sub.D].sup.20] = 1.5690 and e.n. = 25.30%, both values the same as reported in the literature.

Tert-butylhydroperoxide (TBHP) was also obtained by standard procedures and purified by rectification. The sample boiling between 308-310 under P [approximately equal to] 0.10 Torr was collected and had [[n.sub.D].sup.20] = 1.4006 and the density [[d.sub.4].sup.20] = 0.8961, in good agreement with the literature values.

Tert-amylhydroperoxide (TAHP) was synthesized from 1.1-dimethyl-propanol and hydrogen peroxide, lt was identified by the boiling point, the refractive index and density.

The chemical 1-(1-hydroperoxy-1-methylethyl)-4-isopropylbenzene (HPMEIPB), obtained again by standard procedures, had the melting temperature [T.sub.m] = 308 K at the atmospheric pressure. 1-(1-hydroperoxy-1-methylethyl)-4-(1-tert-butylperoxy-1-methylethyl)-benzene (HPMETBPB) had [T.sub.m] = 328.3 K at the atmospheric pressure. In both cases the [T.sub.m] values agree with the literature.

The polyester resin we have used is a product of polycondensation involving ethylene glycol (27.5 wt%), propylene glycol (20.7%), phthalic anhydride (40.5%) and maleic anhydride (11.3%). Commercially available from Lvivlakofarba, Lviv, under the trade name PE-246, it has [M.sub.n] [approximately equal to] 1400 and the acid number a.n. = 40 mg KOH/g. Analytically pure KOH and NaOH used in this work were from FOCh, Gliwice, Poland.

2.2 Analytical Methods

The number-average molecular masses Mn of the synthesized oligomers have been determined by cryometry using benzene as the solvent. The active oxygen content [[O].sub.act] for the POs was determined by iodometry.

2.3 Spectral Methods

Infrared spectra (IR) were obtained using a dispersive Perkin-Elmer apparatus, with the relevant absorption range in the 4000-400 [cm.sup.-1] region.

Proton magnetic resonance 1H-NMR spectra were recorded on the BS-487c spectrometer of Tesla, Brno, Czech Republic, at the frequency v = 80 MHz in carbon tetrachloride. Hexamethyldisiloxane was used as internal standart. The chemical displacements of groups signals were determined by evaluating positions of centers of symmetry of these signals.

2.4 Determiniation of Reaction Kinetics

The epoxy compound and appropriate solvent (0.3-0.4 g-equiv. of epoxy groups per 1 1 of the starting mixture) were charged into a three-necked flask reactor. 15-20 min were needed to achieve thermal equilibrium. A solution of potassium salt of TBHP had been prepared separately in the respective solvent and then added to the reactor. The reaction rate was controlled by varying the epoxy concentration in the reaction mixture. Sampling was performed in regular intervals of time. Concentration of epoxy groups was measured via hydrochloric acid amounts needed for the base neutralization with reflux titration of the acetic acid excess. More specifically, the concentration of the epoxy groups was calculated as

[x.sub.1] = 0.05 ([a- 5.5) - (b - n)] (2)

where a = the amount of NaOH solution in ml expended for titration of "idle" (unreacting) samples; b = an analogous quantity expended for titration of acetic acid; and n = an analogous quantity expended for titration of hydrochloric acid in the reacting samples.

The effective rate constants were calculated as [k.sub.eff] = tan [Phi]{[[[C.sub.o]].sub.2] - [[C.sub.o]].sub.1]}, where [Phi] is the slope of the straight line in the plots of log {[[[C.sub.o]].sub.2] - [x.sub.2]}/{[[[C.sub.o]].sub.1] - [x.sub.1]} vs. time t. Here [[[C.sub.o]].sub.2] is the initial concentration of the hydroperoxide in the mixture, [[[C.sub.o]].sub.1] is the initial concentration of the epoxy groups, [x.sub.1] is the current concentration of the epoxy groups at time t while [x.sub.2] an analogous quantity for the hydroperoxide groups. The effective activation energies were calculated as [E.sub.act] = R[center dot][Psi][center dot](tan [Phi]), where R is the gas constant and [Phi] is the straight line slope in the plots of log [k.sub.eff] vs. [T.sup.-1].

2.5 Synthesis of Peroxy Oligomers

The peroxy oligomer, which will be denoted by PO-I, was obtained as follows. 100 ml of 90%-aqueous solution of tert-butyl alcohol and 30 g (0.53 mol) of KOH (as the 40%-aqueous solution) were charged into a three-necked flask equipped with a mechanical stirrer, thermometer, reflux condenser and a funnel. The mixture was cooled to the temperature not exceeding 278 K and then 172 g (1.83 mol) of TBHP (96% solution) were added dropwise. The solution was heated to 403 K, then 123 g of the epoxy resin dissolved in 250 ml of tert-butyl alcohol (90%) was added dropwise, the addition taking 1.5 hours. Subsequently 200 ml of the mixture consisting of benzene and n-propyl alcohol (1:1 ratio) were added slowly to the reactive mass at 320 [+ or -] 2 K for 2.0-2.5 h and then the system cooled to the room temperature. The organic phase was washed with water until neutral reaction and then distilled off at 323-328 K and 15[center dot][10.sup.-3] Torr to the constant mass.

A second peroxy oligomer, to be called PO-II, was synthesized in an analogous way. In the starting materials the ratio was 3 mol. of TAHP and 0.8 mol. of KOH per I epoxy group. PO-III and PO-IV have been obtained from 1.5 mol. of HPMEIPB or HPMETBPB, respectively, and 0.8 mol of KOH per 1 epoxy group of epoxy resin. The characteristics of the resulting derivatives are provided in Table 1.
Table 1. Characteristics of the Peroxy Derivatives of the Epoxy
Resin based on Bisphenol A.

PO Characteristics
 Type of
Number Hydroperoxide [M.sub.n] [[O].sub.act]/% e.n./%

I TBHP 490 2.1 6.5
II TAHP 500 1.6 6.2
III HPMEIPB 720 1.3 8.3
IV HPMETBPB 680 5.5 9.4


[TABULAR DATA FOR TABLE 2 OMITTED]

2.6 Oligopolyesteric Materials

These have been prepared by mixing one of the PO described above with the epoxy resin ED-20 at (325 [+ or -] 2) K until a homogenous mass was obtained. The PO contents in the mixture was varied in the range from 10 to 50 wt%. The gel weight fractions in the crosslinked products were determined performing acetone extraction in a Soxhlet apparatus for 14 hours.

3. REACTION KINETICS

3.1 The Catalyst Effect

The reaction rate of TBHP with the epoxy resin at 313 K in the presence of catalytic quantities of NaOH is fairly low. The product is characterized by the oxygen content [[O].sub.act], WhiCh does not exceed 0.9. The respective epoxy numbers are in the range 15-17%. We have also conducted the same reaction at 323 K, but the changes in [[O].sub.act] and in the e.n. were insignificant. Then we tried much higher temperatures, but decomposition of the peroxy groups was taking place. Finally, we switched to KOH, and a considerable increase in the reaction rate was observed. Some results which show the influence of the catalyst and of the TBHP concentration on the peroxy and epoxy group contents are displayed in Table 2. The data pertain to the reaction temperature of 318 K; the solvent is the 90% aqueous solution of tert-butanol.

3.2 Kinetics of Reactions of TBHP With Epoxy Compounds

We have used the epoxy resin described in the beginning of Subsection 2.1 as well as DGEDPhP as starting epoxies in our kinetic investigations. A graphical representation of several series of experiments is shown in Fig. 1. We can see from that Figure that reaction between DGEDPhP and TBHP is of the second order. Reaction rates depend significantly upon the temperature and quantities of hydroperoxide and base.

The kinetic parameters for DGEDPhP and our epoxy resin are reasonably similar, although there are small differences in numerical values. We infer that the chemical mechanism of epoxy ring opening for both compounds is of the same type. The hydroxy groups contained in the epoxy resin have very little influence upon its reaction rate with TBHP. Therefore, we have decided to confine subsequent kinetic studies to the epoxy resin, varying the solvent. Thus, we have used as media acetone, methylethylketone, dioxane and toluene at 303-323 K for 3-4 h. In all these cases the reaction does not proceed. By contrast, the reaction does proceed in secondary and tertiary alcohols.

The results obtained using various solvents are summarized in Table 3. In each case the amount of TBHP [TABULAR DATA FOR TABLE 3 OMITTED] was 2.0 moles per g-equiv. of epoxy groups and 90% aqueous solution of tert-butyl alcohol was the solvent.

Table 3 shows that the epoxy groups concentration changes more rapidly in the propyl alcohol medium than in isopropyl alcohol and tert-butanol. This result can be explained as follows. In the medium of propyl alcohol two reactions of TBHP with the epoxy resin proceed in parallel. On one hand, epoxy groups react with the hydroperoxide, resulting in the formation of a PO. On the other hand, the epoxy groups also react with propyl alcohol, so that formation of simple ether bonds takes place.

The lower values of [k.sub.eff] in isopropyl alcohol and in tert-butanol imply that the side reactions take a lower share of the available epoxy groups. However, The results in Table 3 alone do not completely exclude the occurrence of side reactions in these solvent either. Therefore, we have investigated separately the reaction of the epoxy resin with the three alcohols in question. The results are reported in Table 4. They pertain to the reaction temperature of 323 K; the KOH contents [TABULAR DATA FOR TABLE 4 OMITTED] was 0.6 moles per g-equiv. of epoxy groups. Given those results, we have henceforth confined ourselves to tert-butanol as the solvent.

Cured epoxy-based materials are well known for their affinity to water; in glass + epoxy composites water can play the role of a reversible plasticizer (15). Alcohols are known to be hygroscopic. With the starting epoxy material and the solvent now chosen, there were thus several reasons to study the effects of water concentration on the reaction kinetics. Results from this stage of our work are summarized in Table 5; the percentages pertain to volume parts. In each case the starting TBHP amount [[[C.sub.o]].sub.1] = 1.0 mole; the KOH contents was 0.2 moles per g-equiv. of epoxy groups.

As we can see in Table 5, dilution of alcohol with water up to 30 vol%. of the latter has a positive influence upon the rate of PO formation. However, a further increase of the water content slows down the reaction. We shall return to the topic of water effects at the end of Section 4 - after presenting a general reaction scheme in Fig. 4.

Narrowing the field still further, we have settled on the 90% aqueous solution of tert-butanol. The initial hydroperoxide concentration [[[C.sub.o]].sub.2] is one of the key variables. The results obtained pursuing this factor [TABULAR DATA FOR TABLE 5 OMITTED] are summarized in Fig. 2. There is a clear decrease of [k.sub.eff] along with the increase of [[[C.sub.o]].sub.2]. We infer that side reactions are taking place along with the main reaction of the PO formation.

In turn, we now consider the next player in the kinetics, namely the KOH concentration. The results are displayed in Table 6. We see that concentration goes symbatically with the rate constant [k.sub.eff]. However, and as also displayed in Table 6, [k.sub.eff] depends on both [C.sub.KOH] and [C.sub.2], that is the concentration of the hydroperoxide. We have seen in Fig. 2 how the latter reflects the main reaction as well as the side reactions. Side reactions are rather typical for curing step-growth polymers (16). The result in our case is the apparent order of reaction lower than unity. In fact, our [k.sub.eff] values can be well represented by

[k.sub.eff] = [K.sup.*] [multiplied by] [([C.sub.KOH]/[C.sub.2]).sub.1/2] (3)

Verification of Eq 3 is shown in Fig. 3 for several temperatures. We see that deviations of experimental points from the straight lines prescribed by Eq 3 are insignificant.

[TABULAR DATA FOR TABLE 6 OMITTED]

Figure 3 shows that the reaction does not proceed at all in the absence of KOH. The data displayed in that Figure have been also used to calculate the apparent activation energies [E.sub.act]; the results are listed in Table 7. In each case the concentration of TBHP is 2.0 moles and that of KOH is 0.8 mole, both per g-equiv, of the epoxy groups.

4. REACTION MECHANISMS

Mechanisms of reactions of TBHP with ethylene oxide, propylene oxide, isobutylene and also with epichlorohydrine have been known for decades (17). We know that opening of the oxirane ring is possible for various levels of substitution of the carbon atoms. However, our understanding of reactions of hydroperoxides with more complicated systems, our DGEDPhP and epoxy resin among them, is far from sufficient. We have found that reactions of hydroperoxides with epoxy compounds do not proceed in aprotonic and protophilic solvents. We assume, therefore, an ionic type of mechanism. More specifically, ROO- anions are the active (catalytic) agents which cause opening of the epoxy rings; [Mathematical Expression Omitted] anions play a secondary role in ring opening.

We have a relatively high concentration of KOH in our reactive medium. Given this fact we assume that the [ROO.sup.-] anion largely exists in the form of a weakly dissociated ROOK salt, and that the salt does not act as a catalyst. All processes which take place in the reaction of TBHP with the epoxy resin can thus be represented schematically by two empty routes I and II and one nonempty route III [ILLUSTRATION FOR FIGURE 4 OMITTED].

Using Fig. 4 as the basis, we first spell out the reactions that are taking place:

[Mathematical Expression Omitted] (4)

The tert-butylperoxide anion, which is formed in the presence of the base, reacts further with the epoxy group; this results in the formation of the alkoxide anion:

[Mathematical Expression Omitted] (5)

[TABULAR DATA FOR TABLE 7 OMITTED]

Further, the alkoxide anion reacts with TBHP since the latter is in excess, and the tert-butylperoxide anion is detached:

[Mathematical Expression Omitted] (6)

In Equations 4-6 we use the square brackets for the intermediate substances; substances without brackets are the reactive ones, The total combination of both stages of Route III corresponds to the stoichiometric equation for reaction of TBHP with the epoxy resin. The element P in Fig. 4 corresponds to the intermediate substance [Mathematical Expression Omitted].

Given the experimental results represented above by Tables 3-6 and Fig. 4, the rate r of the reaction between TBHP and the resin in the presence of KOH can be represented as

r = k [multiplied by] [[[C.sub.o]].sub.1] [multiplied by] [C.sub.2] [multiplied by] [([C.sub.KOH]/[C.sub.2]).sup.1/2] = k [multiplied by] [[[C.sub.o].sub.1] [multiplied by] [([C.sub.KOH] [multiplied by] [C.sub.2]).sup.1/2] (7)

Given these results, we can now return to the role of water, mentioned already in Section 3. Water exercises its influence upon the reaction rate in several different ways. The addition of water to tert-butanol should increase the dissociation of the potassium salt of the hydroperoxide. Although water and alcohol are both amphiprotonic solvents, the autoprotolysis occurs to a higher extent in water than in tert-butanol. From this point of view, water should increase also the reaction rate; this is what we observe going from 10 to 30 vol% water. To consider the second factor, we follow Route 1 in the schematic in Fig. 4, and we infer that water should decrease the rate of the process. This is in fact what we observe in the concentration range between 30% and 54% water (Table 5). Finally, we also have to take into account the solubility of the starting epoxy resin - which decreases when the concentration of water increases. The maximum recorded in Table 5 is thus explained by the domination of the first of the factors named at low water concentrations, and of the second and third at higher water contents.

5. SYNTHESIS OF PEROXY DERIVATIVES OF EPOXY RESINS

5.1 Starting Materials

The results reported in the preceding Sections made possible the development of a successful procedure of synthesis of peroxy derivatives of epoxy resins. Aliphatic (TBHP, TAHP) and also aromatic (HPMEIPB, HPMETBPB) compounds were used as the starting hydroperoxides. HPMETBPB contains peroxy groups in addition to hydroperoxy groups; this is why this compound has approximately twice as many unstable -O-O- bonds as the remaining compounds; see again Table 1. All the peroxides are viscous light-yellow products, stable at room temperature and soluble in organic solvents.

5.2 Spectral Characteristics of Peroxy Oligomers

Spectroscopic experiments were performed as described in Subsection 2.3. We discuss the IR results first. Spectra of the starting epoxy resin have been compared to those of the POs we have synthesized; the differences are significant. Weak vibration bands in the range 878-870 [cm.sup.-1] are found in the IR spectra of all compounds I-IV listed in Table 1. It is well known from the literature that these are the valency vibrations of the -O-O- bond (18). Moreover, a doublet of symmetric stretching (sometimes called haeme-dimethylic) vibrations at 1398-1378 and 1363-1343 [cm.sup.-1] characteristic for the [([CH.sub.3]).sub.3]C- group is present in the spectra of the POs. That group appears as the result of addition of the hydroperoxide to the epoxy groups. At the same time, we observe lowering of intensifies at the 920-907, 1292-1240 and 3024-3000 [cm.sup.-1] ranges, which are characteristic for the epoxy ring. This confirms the addition of the hydroperoxide to the epoxy group. Still further, we note that the intensity of the vibration band at 3585-3400 [cm.sup.-1] is higher in the POs than in the starting epoxy; this corresponds to a secondary hydroxy group formed at the opening of epoxy group and as a result of addition to it of a hydroperoxide.

Consider now the proton magnetic resonance 1HNMR spectra, again comparing the starting epoxy with the POs. We observe in the latter new signals at 1.14-1.15 ppm. These correspond to protons in the [([CH.sub.3]).sub.3]COO- group, thus confirming independently the presence of peroxy groups in our oligomers. The presence of hydroxy groups in the POs is confirmed by signals of their protons in the range 3.85-4.00 ppm. Raising the temperature to 323 K results in displacement of these signals to 3.75 ppm. Thus, the spectra show that the POs we have synthesized contain peroxy, epoxy and hydroxy groups. We have achieved the desired multifunctional oligomers.

5.3 Network Formation

The presence of peroxy groups in the synthesized POs makes possible the use of these materials as curing agents for compounds containing unsaturated double bonds. Results of the crosslinking reactions at 423 K are summarized in Fig. 5 as curves of the gel weight fraction vs. time for the PO-I peroxy oligomer and the PE-246 polyester resin. The gel fraction were determined as described in Subsection 2.6. Other peroxy oligomers give similar curves, hence these results have been omitted for brevity.

Figure 5 shows that after 3 h the plateau region begins, and a further time lapse affects the gel fraction only slightly. This pertains to all PO-I concentrations. It is in the ascending parts of all curves that we see clear effects of the initial PO concentration. Going from 10 wt% of the peroxy oligomer to 25% increases significantly the rate of gelation as well as the fin gel weight fraction. The gel fraction after 5 h for 10% PO-I is 68% while for 25% PO-I it is 89%. However, a further increase of the peroxy oligomer concentration to 50% affects only little the gelation kinetics and the final gel fraction. For this last concentration, after 5 hours, we have the gel weight fraction = 93%. In other words, doubling the peroxy oligomer concentration has increased the final gel weight fraction by 4% only; there is little point in using the PO for crosslinking in concentrations exceeding 25%.

As expected, the rate of formation of three-dimentional networks depends significantly upon temperature. The process proceeds quite slowly at 383 K but at reasonable rates at 403 K, depending again on the initial peroxide oligomer concentration. We have seen that materials with high content of unsoluble products can be obtained at 423 K.

Kim and Robertson (19) reported possible phase-transformation toughening of epoxy resins by the incorporation of poly(butylene terephthalate). Karger-Kocsis explained such toughening processes in terms of enlargement of the plastic zone (20), possibly accompanied by other processes. Our peroxy-modified epoxies deserve to be investigated from the point of view of possible toughening transformations; we hope to report on this topic in a later paper.

ACKNOWLEDGMENTS

We appreciate discussions with Jozsef Karger-Kocsis, University of Kaiserslautern, with Kevin P. Menard, Perkin-Elmer Corp., and detailed discussions with Michael Hess of the University of North Texas and the Gerhard Mercator University of Duisburg. Useful comments on the manuscript were provided by the referees. This work was supported in part by the North Atlantic Treaty Organization, Brussels (Award # HTECH.LG 960084) and the State of Texas Advanced Technology Program, Austin (Award # 003594-077).

REFERENCES

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12. R. T. Stamenova, C. B. Tsvetanov, K. G. Vassilev, S. K. Tanielyan, and S. K. Ivanov, J. Appl. Polym., Sci., 42, 807 (1991).

13. V. I. Galibey, T. A. Tolpygina, and S. S. Ivanchev, Zh. Org. Him., 6, 1585 (1970).

14. M. Bratychak and O. Hrynishyn, Ukr. Him Zh., 63, 132 (1997).

15. J. Klotz, W. Brostow, M. Hess, and W. S. Veeman, Polym. Eng. Sci., 36, 1129 (1996).

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Date:Aug 1, 1999
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