The use of epoxy resins is widespread, owing to their outstanding properties such as, for example, high impact strength and abrasion resistance and good chemical stability, and finds use in numerous sectors. Epoxy resins exhibit outstanding adhesiveness and electrical insulation capacity. They serve, for example, as a matrix for fiber composites, in the context, for example, of the building of wind power installations, and as structural components in the air travel sector. In electronics they are employed as electrical laminates in printed circuit boards. Furthermore, they are widespread in use as structural adhesives, as casting varnishes, and as powder coating resins.
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The curing of epoxy resins proceeds in accordance with a variety of mechanisms. Besides curing with phenols or anhydrides, curing with amines is described very frequently for the crosslinking of the epoxide groups. The stoichiometric amount of hydrogen atoms is added, as may be supplied, for example, by bifunctional amines. A further mechanism describes the reaction of an initiator or accelerator with epoxide groups, forming a highly reactive intermediate which is able to react with further epoxide groups without the need for further crosslinkers. The initiators may also lower the activation energy of the reaction of crosslinker or hardener molecules, so that the curing temperatures are lowered considerably. Compounds which have these properties are, in general, tertiary amines, imidazoles or else substituted ureas, which have the ability, for example, to reduce the cure temperature of dicyandiamide.
Usually the individual components of epoxy resin formulations are not mixed together until immediately before curing and heating, in order to prevent premature reaction. In this case the resin and, separately therefrom, a mixture of hardener and accelerator are combined and then reacted by heating. A disadvantage of these two component mixtures is a relatively short pot life, i.e., a relatively short time within which the mixture can be processed. Likewise, errors in mixing may lead to inhomogeneous products and hence to unsatisfactory results. One-component mixtures include, besides resin and further constituents (such as fillers, thixotroping agents, pigments, etc.), a hardener which is latent at room temperature, and they have a significantly longer pot life and require, for their curing, elevated temperatures, in particular above 1 00° C, and usually longer cure times. A typical example of a latent hardener is dicyandiamide (cf. EP 148 365 A1, U.S. Pat. No. 2,637,715 B1). In order to overcome these disadvantages, chemically latent accelerators are added to such one-component mixtures, with reductions in storage stability and processing time being accepted, in order to lower the temperature of curing. Examples of latent accelerators of this kind include, in particular, urons, such as 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) (cf. GB 1,153,639 A1, GB 1,293,142 A1, U.S. Pat. No. 3,386,956 B1, U.S. Pat. No. 6,231,959 B1). These compounds are usually 1,1-dialkyl-3-arylureas, where the aromatic may be substituted or unsubstituted, or else is hydrogenated. At elevated temperatures these compounds release dimethylamine and the aryl isocyanate, which synergistically accelerate the curing reaction with dicyandiamide. Hence it is possible to effect curing at significantly lower temperatures. The temperature at which this dissociation of the uron begins, and hence at which the crosslinking reaction commences, depends on the nature of the substituents. At the same time it is found that, the lower the temperature at which curing commences, the lower, too, is the stability of such a mixture at temperatures below the cure temperature.
The aryl-substituted urons employed to date have only a limited stability in the mixture; in other words, there continues to be a need for new accelerators which have the capacity for long storage stability and processing stability in combination with high reactivity. Additionally the mechanical properties of the cured polymer ought not to be substantially impaired as a result of the addition of the accelerator.
Many of the compounds employed as latent accelerators exhibit inadequate solubility in common solvents, thereby significantly reducing their spectrum of application, particularly in sectors in which a uniform reaction is needed. Some of the uron accelerators employed are halogen-substituted, which also limits their use in the electronics sector.
It was an object of the present invention, therefore, to provide latent accelerators for epoxy resin systems that do not have the stated disadvantages of the prior art. By latent accelerators are meant additives to a resin/hardener mixture that as far as possible do not lower the pot life, i.e., the time within which the mixture can be processed, and at the same time accelerate the reactivity, i.e., the crosslinking at elevated temperature. Compounds are desired, therefore, which allow a processing duration which is as long as possible. The accelerators of the invention for epoxy resin systems ought, consequently, to possess a high reactivity and very good storage stability at room temperature and at temperatures below the cure temperatures and ought, furthermore, as far as possible to be halogen-free and toxicologically unobjectionable.
This object has been achieved in accordance with the invention by using as accelerators asymmetrically substituted urea derivatives of the general formula (I)where Rand Reach independently are a linear or branched aliphatic hydrocarbon radical having 1 to 4 carbon atoms.
R1 and R2 may be, for example, methyl, ethyl, propyl and butyl. Examples of such urea derivatives are, for example, N,N-diethylurea, N,N-dipropylurea, N,N-ethyl-methylurea and N,N-dimethylurea. A preferred urea derivative is N,N-dimethylurea.
It has surprisingly been found that the accelerators proposed in accordance with the invention not only have a very good reactivity and storage stability but also exert no negative effect whatsoever on the mechanical properties of the cured material.
It is true that the use of dimethylurea as an accelerator in combination with dicyandiamide is recommended by JP-A 79-26000 for urethane-modified epoxy resin systems; however, the storage stabilities in those formulations are only comparable with those achieved using (1,1′-methylenedi-p-phenylene)bis(3,3-dimethylurea) (=MDI uron). Surprisingly, with the asymmetrically substituted urea derivatives in the epoxy resin systems claimed in accordance with the invention, it has been possible to obtain substantially better storage stabilities than is possible with MDI uron.
Additionally, JP-A 81-133856, which describes the combination of N,N-dimethylurea with phenol novolaks as hardeners for epoxy resin systems in the semiconductor systems sector, contains no indication of the influence of N,N-dimethylurea on the storage stability of the corresponding epoxy resin formulations.
In accordance with the invention accelerators used in combination with dicyandiamide as latent hardener are asymmetrically substituted urea derivatives of the general formula (I)where Rand Reach independently are a linear or branched aliphatic hydrocarbon radical having I to 4 carbon atoms. Suitable in this context are methyl, ethyl, propyl, and butyl radicals, which may be linear or else, where appropriate, may be branched. Examples of urea derivatives of the invention are, N,N-dimethylurea, N,N-diethylurea, N,N-dipropylurea, and N,N-ethylmethylurea. The urea derivative N,N-dimethylurea is used with preference.
It is regarded as being essential to the invention that the inventively proposed combination of asymmetrically substituted urea derivatives and dicyandiamide are used for the following epoxy resin systems: epoxy resins based on unhalogenated or halogenated bisphenols of type A or F and also based on resorcinol or tetrakisphenylolethane.
Epoxy resins based on bisphenol A and F are used predominantly in the sector of fiber composites, of adhesives, and also, in relatively high molecular mass form, as solid resins in powder coating materials.
In the sector of electrical laminates the cured epoxy resin is expected to exhibit particular flame retardance and high temperature stability. For this purpose use is made predominantly of halogenated systems of bisphenol A, examples being tetrabromobisphenol A derivatives or trifluoromethyl-substituted versions thereof.
Particularly flame-retardant composites are produced, for example with epoxy resins based on resorcinol and tetrakisphenylolethane.
The proportions of dicyandiamide and urea derivative relative to the corresponding epoxy resin may be varied within wide limits. It has, however, proven particularly advantageous to use the dicyandiamide in an amount of about 1% to 15%, preferably about 2% to 12%, more preferably about 2% to 8%, by weight based on the epoxy resin. The urea derivative is used in an amount of about 0.5% to 15%, preferably about 1% to 12%, by weight based on the epoxy resin. A particularly preferred amount is about 1% to 10% by weight based on the epoxy resin.
According to one preferred embodiment the urea derivative and the dicyandiamide are employed in a very finely divided form, the components having a preferred average particle size of about 0.5 to 100 μm, in particular about 10 to 50 μm, more preferably about 2 to 10 μm. The curing reaction of the inventively proposed accelerators and hardeners with the respective epoxy resins can be carried out in accordance with the customary methods, with curing being carried out at temperatures between about 70 and 220° C., in particular between about 80 and 160° C.
The inventively claimed combination of urea derivative as accelerator and dicyandiamide as latent hardener is outstandingly suitable, for example, for the hot curing of epoxy resin in the sector of fiber composites, powder coatings, electrical laminates and adhesives.
The advantages of the accelerator/hardener combination of the invention are the excellent reactivity and very good storage stability. Surprisingly, the mechanical properties of the resins cured accordingly, as well, are likewise outstanding and are comparable with those of the blocked accelerators UR 200 (diuron) and UR 300 (fenuron) which have already been employed.
On the basis of these very good performance properties and a low toxicity, the inventively proposed hardener/accelerator systems are outstandingly suitable for technical use.
The examples which follow are intended to illustrate the invention.
The following products and materials were used in the examples:
Epoxy Resins:
Epikote 828 (Resolution): bisphenol A resin, EEW 185
DER 664 UE (Dow): solid resin, EEW 910 (resin)
Hardener:
Dyhard 100 S (Degussa): micronized dicyandiamide, particle size 98% <10 μm, 50% approx. 2.5 μm (Dyh 100 S)
Accelerators:
Dyhard UR 200 (Degussa): micronized diuron or 3-(3,4-dichlorophenyl)-1,1-dimethylurea, particle size 98% <10 μm, 50% approx. 2.5 μm (UR 200)
Dyhard UR 300 (Degussa): micronized fenuron or 3-phenyl-1,1-dimethylurea, particle size 98% <10 μm, 50% approx. 2.5 μm (UR 300)
Dyhard UR 500 (Degussa): micronized TID uron or toluylbis-1,1-dimethylurea, particle size 98% <10 μm, 50% approx. 2.5 μm (UR 500)
Dyhard MIA 5 (Degussa): micronized adduct of methylimidazole with bisphenol A resin (Epikote 828), particle size 98% <70 μm
N,N-dimethylurea or 1,1-dimethylurea (Merck): ground in the laboratory, particle size 98% <10 μm, 50% approx. 2.5 μm (1,1-DMH)
N,N-diethylurea or 1,1-diethylurea (Merck): ground in the laboratory, particle size 98% <10 μm, 50% approx. 2.5 μm (1,1-DER)
MDI uron, (1,1′-methylenedi-p-phenylene)bis(3,3-dimethylurea), was prepared by known methods from MDI (1,1′-methylenedi-p-phenylene) diisocyanate and dimethylamine (e.g., EP 402 020 A1, CS 233 068 B1) and subsequently ground in the laboratory, particle size 98% <10 μm, 50% approx. 2.5 μm
Additive:
Lanco Wax TPS 040 (Lubrizol), micronized in the laboratory 98% <80 μm
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5 g in each case of a formulation, corresponding to the composition in the second column from the left in Table I (“Components”), made up of bisphenol A resin (Epikote 828, EEW 185), Dyhard 100 S as hardener, and inventive accelerator 1,1-dimethylurea (1,1-DMH) or 1,1-diethylurea (1,1-DEH), and also as a comparison thereto, formulations which correspond to the compositions of the second column from the left in Table 2 (“Components”) and which include the noninventive standard uron accelerators Dyhard UR 200 (diuron) and UR 300 (fenuron), were produced. A measurement was made in each case of the gel time at the stated temperature and the reactivity was determined by means of DSC.
As the temperature program for determining the peak temperature (DSC peak), heating took place at a rate of 10° C./min from 30 to 350° C. The onset of reaction (DSC onset) was determined from the same measurement by applying the tangent to the reaction peak.
For determining the glass transition temperature (Tg) the material from the gel time determination at 120° C. was employed. The formulation was fully cured by heating to 200° C. (temperature program: 30 to 200° C., heating rate 20° C./min) and maintaining this temperature for 30 minutes. After cooling to room temperature (RT) the sample was heated from 30 to 200° C. with a heating rate of 10° C./min, and the Tg determined therefrom.
Comparing the two Tables 1 and 2 it is apparent that the reactivity of the 1,1-dimethylurea acting as accelerator is entirely comparable with that of the standard accelerators of the uron series. This is also true, to a somewhat lesser extent, for the 1,1-diethylurea. The glass transition temperature of the material cured with dialkylurea accelerators, as well, is within the range of the values achievable with the standard accelerators Dyhard UR 200 and UR 300. Particularly when relatively large amounts of accelerator are added, the tendency toward Tg reduction in the case of the materials of the invention is less strongly pronounced.
Latency Experiments:
A formulation of 100 parts by weight of bisphenol A epoxy resin (Epikote 828, EEW 185) and 6.5 parts by weight of Dyhard 100 S was admixed in each case with the amounts of latent accelerators indicated in Tables 3 and 4. After the stated storage period at the respective temperature (40° C. or 23° C.) a measurement was made in each case of the viscosity, using a Haake viscometer. The viscosity values are shown in columns 3-8 of Tables 3 and 4.
As is clearly apparent from Table 3, the formulations of the invention have considerably better properties with regard to latency: while a doubling of the viscosity occurs in formulations with MDI uron at 40° C. after only 15 days, with 1,1-dimethylurea this is the case only after approximately 40 days. For MDI uron the processability of the formulation is below 25 days, while for formulations with 1,1-dimethylurea it is more than twice as high (more than 50 days).
The processability of the formulations comprising 1,1-dimethylurea is likewise considerably higher at room temperature than in formulations with MDI uron.
Comparison of N,N-dimethylurea With Various Standard Accelerators (MDI Uron, UR 300 and UR 500):
Formulations are produced which are composed in each case of 100 parts by weight of bisphenol A epoxy resin (Epikote 828, EEW 185), 6.5 parts by weight of Dyhard 100 S, and the amount of the respective accelerator indicated in Table 5. After the storage period at 40° C. indicated in the second column, the viscosity was determined in each case, using a Haake viscometer. The viscosity values are shown in columns 3-6 of Table 5.
In comparison with standard accelerators of the uron series the advantage of using 1,1-dimethylurea in one-component mixtures becomes even more distinct: while the standard products UR 300 and UR 500 can be processed only for up to I week at 40° C., a formulation with MDI uron can be processed for at least 3 weeks. The formulation comprising dimethylurea, indeed, can be processed for 7 to 8 weeks.
Powder Coating Examples:
Formulations A, B, C and D below, consisting of the components indicated in Table 6, were compared with one another:
The formulations were each extruded at 95° C.
For the production of the corresponding powder coating materials, the raw materials in powder form were each premixed, extruded for better homogenization at 95° C., then ground, and subsequently applied by spray gun to steel plates in film thicknesses of between 60 to 80 μm and cured or crosslinked at two different temperatures (180 and 200° C.). The results of the tests on the cured powder coating formulations are depicted in Table 7.
The mechanical properties of the accelerators of the invention in powder coating formulations are absolutely comparable with those of the prior-art methylimidazole adduct (Dyhard MIA 5), with at the same time a lower yellowing tendency and better leveling properties.
One of the most important resin parameters affecting the processing of wood-based panels is the resin gel time. The gel time is the time from when the material begins to soften to when gelation occurs, where gelation is the irreversible transformation from a viscous liquid to an elastic gel. The effect of the acidic ionic liquid content on gel time is shown in Figure 3 . It can be seen that the addition of ILs from 1% to 3% significantly reduces the gel time of UG resins. The faster gel time of catalyzed UG resins indicates that their cross-linking rate becomes faster by adding [HNMP] HSO. The shortening of the resin gel time with the addition of the catalyst is surely related to the decreasing pH value. The advantages of shorter gel times of thermoset resins with the addition of catalysts have been reported by several researchers [ 15 17 ].
According to Figure 2 a, the curve of the UG resin containing 0%, 1%, 2% and 3% catalyst shows remarkable differences in pH values. This seems to contradict the previous findings about the influence of NHCl on UF resins. Previous studies have indicated that the pH value of UF resin decreases with catalyst addition very quickly at the beginning, but the changes in the pH value become smaller with increases in the catalyst content [ 4 ]. This finding indicates that, compared to NHCl, the use of acidic ionic liquid as a catalyst can decrease the pH value uniformly and continuously. Additionally, Figure 2 b indicated that increasing ILs from 0% to 3% decreased the pH of resin at a hot temperature. This finding gives important information about the behavior of UG resin under a hot pressing condition for wood panel manufacturing. According to Figure 2 b, the pH of the resin is still higher than 3.8, even with the addition of 3% catalyst. This is comparable to the pH obtained by catalyzed urea-formaldehyde resins, and it ensures that the pH reached on curing by the UG resin catalyzed with IL is not damaging to the lignocellulose of the wood substrate.
The pH value of the UG resin decreased with increasing [HNMP] HSOcontent from 0% to 3%, as shown in Figure 2 Figure 2 a showed that decrease in the pH value was initially very quick, and the slope of curve then became less sharp with increasing catalyst content. The effect of [HNMP] HSOon UG-resin curing is to release Hand HSO. With an increasing HSOconcentration in the system, the rate of HSOrelease is retarded. Thus, the pH decreases very quickly in the beginning and then more slowly ( Figure 2 a).
The influence of ILs on thermal curing behavior of UG resins was investigated by DSC. The DSC curves of the UG resin without catalyst and UG resin with 2% ILs are shown in Figure 4 . The curing of the UG resin is an exothermic reaction similar to other formaldehyde-based thermoset resins. The exothermic peak of the UG resin is affected by the presence of the ionic liquid. Adding the IL caused the peak of temperature () to be reached much earlier and at a lower peak temperature. For the pure UG resin, the averagewas about 103 °C. Adding the ILs did change the peak temperature to 77 °C. The decrease in peak temperature of the UG resin containing the IL catalyst indicated that ILs accelerates the hardening of a UG resin. The DSC analysis also showed thatincreased from 35 to 45 °C with the addition of 2% ionic liquid. The clear indications of all these are the addition of IL to the UG resin decreasing the energy of activation of the curing reaction, rendering possible cross-linking. This was confirmed by the DSC results showing that the enthalpy of the cure reaction (Δ) (the area under the DSC exotherm curves) of the UG resin containing ILs was lower than that of pure UG, indicating also that low heat was generated by this mix.
The peak at 100 ppm belongs to the C linked to two urea molecules, as in the structure:
Previous studies have reported that the structure and property of thermoset resins vary as the synthesis conditions change [ 2 ]. Nuclear magnetic resonance spectroscopy (NMR) is one of the most effective methods of studying resin structure. So far,C NMR has been used to characterize the structures of various resins, such as UF, PF and PUF resins. The solid phase CP MASCNMR spectra of a hardened UG resin is shown in Figure 5 . The NMR of the hardened UG resin shows some features of interest. The shift at 163 ppm is indicative of the C=O of urea disubstituted for reaction with the glyoxal, and the shift at 154 ppm indicates a urea more multisubstituted, indicating tridimensional cross-linking of the hardened resin. The shifts at 87, 77 and 61 ppm are indicative of the carbon shifts of the following repeating unit,with the 87 ppm shift belonging to the C1 carbon directly linked to the –NH– group of urea, the 77 ppm shifts to the C3 not linked to the urea, and the wide but small 61–62 ppm peak belonging to the C2 and C4 of the repeating unit above.
The MALDI TOF spectrum in Figure 6 shows a number of peaks. There are clearly two repeating motives in the spectrum, one of 175 + 1 Da and a second one of 161 + 1 Da.
These are representative of the following two repeating units, namely for the 174–176 Da:
The second repeating mass interval at 162 Da is indicative of the following repeating unit:
The two units are forced to alternate to form a repeating unit that is:
This alternance give rise to the series of peaks at 699–728, 875, 1037 and 1192 Da.
2OH group and added to the Na+ used as a matrix enhancer. Thus, for example, the peak at 728 Da is the following molecule deprotonated and added to 23 Da of the Na+used as enhancer of the matrix
+.
The following one in the series is:and so on. The peak at 538 Da is derived from the above oligomer species by the loss of a –CHOH group and added to the Na+ used as a matrix enhancer. Thus, for example, the peak at 728 Da is the following molecule deprotonated and added to 23 Da of the Naused as enhancer of the matrix728 Da, deprotonated +Na
2OH and –OH group, respectively. The peak at 875 Da is derived from the addition of a 176 motive to the 713 peak to form an oligomer of the type+ used as a matrix enhancer, gives 875 Da. The following peak at 1037–1038 Da is the oligomer derived by adding a further 162 Da fragment to the 875 Da oligomer, giving the following type of oligomer:
The peaks at respectively 699 and 713 Da are obtained from the above molecule by the loss of an –CHOH and –OH group, respectively. The peak at 875 Da is derived from the addition of a 176 motive to the 713 peak to form an oligomer of the typewhich, added to the Naused as a matrix enhancer, gives 875 Da. The following peak at 1037–1038 Da is the oligomer derived by adding a further 162 Da fragment to the 875 Da oligomer, giving the following type of oligomer:
Three more findings result from the MALDI TOF analysis, the first of which being that the –CH–O–CH– ether bridges are formed by glyoxal resins just as easily as methylene ether bridges are formed in UF resins. These types of bridges, however, appear to be more stable than the methylene ether bridges of UF resins, where rearrangements with elimination of formaldehyde are rather common. However, rearrangements of such bridges with elimination of glyoxal cannot be excluded at this stage. These glyoxal ether bridges present in the 162-Da repeating unit alternate with definite –CH–CH– bridges characteristics of the 176-Da repeating unit. One last thing evident from the MALDI analysis is that the oligomers formed are mainly linear and appear to present little ramification, a fact that should definitely have a bearing on the gelling and curing time and behaviors of such adhesives. One of the bearings of these characteristics is most likely to be the higher energy activation for curing UG resins than that for UF resins. That is why ILs, and the lower pHs reached during hot-pressing, are necessary for proper curing.
As regards to the lower pH reached on curing, this has been shown to do little damage to the lignocellulosic substrate due to a very short period of time the acid is free and in contact with it, exclusively during the short hot pressing time, before being again neutralized on the cooling of the panel and the reforming of the IL in the cured resin [ 8 ]. Moreover, the pHs reached with the adhesive system UG+IL are not lower than what was achieved by a UF resin and a standard hardener.
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