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Improvement of Fracture Toughness and Glass Transition Temperature of DGEBA-Based Epoxy Systems Using Toughening and Crosslinking Modifiers.

INTRODUCTION

Due to their outstanding thermal, electrical and mechanical properties epoxy resins have a wide spectrum of applications as adhesives, casting resins, coatings and matrix resins in composites or as surface coatings [1]. One of the most important epoxy resins is diglycidyl ether of bisphenol A (DGEBA) [2]. There are two major ways of curing an epoxy resin: curing with coreactive agents (hardeners) as well as curing using initiators. Coreactive curing hardeners, typically primary and secondary amines, phenols, thiols and carboxylic acids, act as comonomers [3,4]. They become part of the network structure and affect the viscosity and the reactivity of the formulation. Via curing with initiators the homopolymerization of the resin is effectuated [1, 5-7], The type of curing agent defines the kind of chemical bonds, which are formed, and therefore the basic structure and the functionality of the cross-linked matrix.

A high crosslinking degree of the epoxy resin matrix and a high network density result in a number of advantageous material properties, for instance in high glass transition temperatures ([T.sub.g]), but may simultaneously lead to brittleness of the material. For this reason, the epoxy resins need to be toughened for various applications. The toughening modifiers create a heterogeneous system through the distribution of particles or through the separation of a second phase. The typical energy absorbing mechanisms in an epoxy resin are crack deflection or pinning, crack branching, particle bridging or debonding, microcracking, crazing or elastic deformation of the matrix. The toughening effect is dependent on the particle size of the modifier, the particle size distribution, the distance between the particles, the volume fraction and the interaction between the matrix and the modifier [8-15]. For example, the viscosity distribution during the curing process has an effect on the morphology and may be essential whether the modifier forms a separate phase [16]. Therefore, the choice of the optimal toughener is depending on the type of epoxy resin and curing agent as well as the curing conditions. There is a variety of modifiers for epoxy resins, which are described within the section state of the art.

However, the usage of toughening modifiers may lead to a decrease of the [T.sub.g]. Therefore, the modification of a resin system should consider not only the mechanical but also thermal properties. Phosphorous compounds like phosphites are known to increase the [T.sub.g] of epoxy resins. Dimethyl methylphosphonate (DMMP) was found to increase the [T.sub.g] through filling the free volume of the epoxy matrix and increasing the network density by building hydrogen bonds with the epoxy resin [17]. Furthermore, it is possible to increase the [T.sub.g] as well as the Young's modulus through subsequent crosslinking [18]. This postcrosslinking effect was found for phosphites like dimethylphosphite (DMP) and diethylphosphite (DEP). The formation of additional crosslinks is effected by transesterification reactions between hydroxyl groups of the epoxy matrix and methoxy or ethoxy groups of the phosphites whereby methanol or ethanol, respectively, are released [19].

This study deals with the effect of toughening and [20] postcrosslinking agents on the mechanical and thermal properties of two DGEBA-based epoxy resin systems. These investigations aimed to the development of resin systems with considerably improved [K.sub.IC] value which do not show a drop of the [T.sub.g]. The first resin investigated was neat DGEBA that was anionically homopolymerized using the initiator EMIM Ac. The second one consisted of DGEBA and the cycloaliphatic diamine IPD that acts as coreactive hardener.

Toughening agents of different classes were applied and their different effects were compared among each other in both resin systems. Combinations of toughening agents were also tested. The goal of these investigations was to discover basic information how structural patterns of the tougheners influence the toughening effect and the [T.sub.g] in both resin systems. The influence of the resin matrix on the toughening properties was investigated as well. The neat resins were investigated for comparison.

In addition, combinations of toughening and postcrosslinking modifiers were successfully investigated. Firstly, the modifiers were applied as simple mixtures. Then, novel systems with chemical linked post-crosslinking and toughening modifiers were tested. These investigations pursued the goal to develop optimized resin systems which show both improved [K.sub.IC] value as well as increased [T.sub.g].

STATE OF THE ART

Important types of tougheners suitable for epoxy resins and their special features are specified in the following.

Rubber-based modifiers are ranked among the most important toughener types because they provide a good miscibility with the uncured resin and therefore a good processability. Besides unreactive rubbers, there are reactive ones which are able to react with the epoxy resin during the curing process. The formation of chemical bonds leads to a better interaction with the epoxy matrix. A typical reactive rubber is a carboxyterminated butadiene acrylonitrile copolymer (CTBN). A common way of toughening is a combination of CTBN with glass particles to obtain a higher modulus. Despite their widespread applications as toughener, rubbers suffer from several disadvantages, like the dependence of the phase separation on the curing cycle and lowering of the network density what results in a deterioration of the thermal properties [20-27].

A possibility to improve the mechanical properties without a decrease of the [T.sub.g] is the use of core-shell-rubbers (CSR). CSR consist of soft particles (silicon rubber, SBR-rubber of acrylate rubber) whose softening properties are masked through a shell based on poly(methyl methacrylate) or styrene. The round shape of the particles leads to a heterogenic system with a toughening effect. No phase separation during the curing process is necessary. The toughening effect is consequently not depending on the curing conditions. However, the core-shell tougheners impair the processability of the resin formulation: The particles can be filtered out during the composite production and they cause an increase of viscosity [9, 28-30].

A further way to obtain a soft domain that is surrounded by a hard zone is the use of block copolymers which form a coreshell-like structure during the curing. Those block copolymers consist of toughening segments, like poly(butyl acrylates), with a low [T.sub.g]. Other segments based on poly(methyl methacrylate) are compatible with the matrix. During the curing, the block copolymers arrange themselves to build domains with a soft core and hard shell. The big advantage of block copolymers is a good compatibility with the liquid resin [31-34]. The amount of block copolymer has a strong influence on the phase separation morphology of the toughened epoxy resin and the resulting mechanical properties and the [T.sub.g] [35, 36],

Another toughening approach is the utilization of thermoplastic particles. Poly(ether sulfone), poly(ether ketone) or poly(ether imide) are used as modifiers without a detrimental influence on the thermal properties. They change the morphology with increasing modifier amount as follows: A single-phase microstructure is obtained at a low amount of toughener. When the concentration is increased, a particulate microstructure of thermoplastic particles in the epoxy matrix is formed. An even higher amount of the toughener gives rise to a co-continuous structure and eventually to the phase inverted form. The single phase morphology does not lead to any toughening effect. In case of the other morphologies, the fracture toughness increases linear to the amount of modifier used. It is possible to obtain a better compatibility with the resin through the functionalization of the thermoplastic toughener with reactive groups [37-40],

Micronsized inorganic fillers, like glass or ceramic particles with a size of 4-100 [micro]m, can be applied without a drop of the [T.sub.g]. The toughening effect of inorganic microparticles is based on deflection and pinning of the crack [41-45]. However, more a pronounced toughening effect on epoxy resins is described for surface modified silica nanoparticles (20 nm). The predominant toughening mechanisms for silica nanoparticles are debonding of particles and plastic void growth as well as shear band formation. Furthermore immobilization of the polymer matrix around the particle contributes to energy dissipation. Commonly, in low concentrations (up to 10 wt%) silica nanoparticles have just little effect on the [T.sub.g] [12, 41, 43, 46-49],

The use of linear polymeric modifiers can provoke an increase of the viscosity of liquid resin formulations what may deteriorate the processability. To avoid that problem, hyperbranched polymers can be used due to their Newtonian behavior and comparatively low viscosities. Hyper-branched polymers consist of dendritic, relatively compact macromolecules. Their molecular structure and reactivity can be adapted to each specific resin system through the introduction of suitable functional groups as well as the choice of appropriate dendritic basic structures. However, a relative high price has to be accepted [50-53].

Furthermore the approach of combining different tougheners in order to obtain synergistic effects is also discussed in literature. Especially the combination of silica nanoparticles and rubber based additives is a very promising approach in order to receive a tough epoxy system without sacrificing a high glass transition temperature [14, 15].

EXPERIMENTAL

Materials

The difunctional epoxy resin diglycidylether of bisphenol A (DGEBA; Baxxores ER 220, BASF SE, Germany) was used as received. The epoxy equivalent of the DGEBA resin was 180 g/ Eq. Isophorone diamine (IPD) (Merck, Germany) was used as coreactive curing agent and l-ethyl-3-methylimidazolium acetate (EMIM Ac) (Aldrich, Germany) as curing initiator. The phosphites dimethylphosphite (DMP) and diethylphosphite (DEP) were received from Aldrich, Germany.

Following tougheners were tested: Self-organized block copolymers containing poly(butyl acrylate) and two poly(methyl methacrylate) blocks (Nanostrength[R] M22N, Arkema, France); masterbatch of 40 wt% silica nanoparticles with an average diameter of 20 nm in DGEBA (Nanopox[R] F400, Evonik Hanse GmbH, Germany); masterbatch of 40 wt% reactive NBRelastomer modifier in DGEBA (Albipox[R] 1000, Evonik Hanse GmbH, Germany); masterbatch of 40 wt% core-shell particles with an elastomeric silica core and a reactive shell in DGEBA (core-shell Albidur[R] EP 2240A, Evonik Hanse GmbH, Germany); poly(tetrahydrofuran) of different molecular weights (PolyTHF[R] 650, PolyTHF[R] 1000 and PolyTHF[R] 2000, BASF SE, Germany); hyper-branched dendric polyesters with different hydroxyl numbers (Boltorn[TM] Perstop, Sweden). Following dendritic polymers with different hydroxyl numbers were tested: Boltorn[TM] P501 (690-750 mg KOH/g; number average molecular weight 1700 g/mol) and Boltorn[TM] P1000 (430-490 mg KOH/g; number average molecular weight 1500 g/mol). The latter is modified with unsaturated fatty acid. Additionally, silica based functional linear polymer (Genioperl[R] W35, Wacker Chemie AG, Germany); polyethersulfone (PES) with a hydroxyl number of 159 microequivalent/g and a particle size of 45 pm (Virantage[R] VW-10700-RFSP, Solvay, Belgium).

Sample Preparation

DGEBA was degassed at 120[degrees]C and 50 mbar for 2 h. Then, the modifiers were added to the DGEBA resin while stirring. 10 wt% of each toughener were added unless another concentration was recommended by the suppliers. In these cases the producer's recommendations were followed. In case of using postcrosslinking modifiers (DEP and DMP), the content of phosphorus was 0.3 and 0.6 wt% relating to DGEBA. The amount of PolyTHF2000[(DMP).sub.2] was 10 wt% with a phosphorous content of 0.3 wt%. Fluid modifiers were dissolved by stirring at 2000 rpm and 10 mbar for 5 min using a Speedmixer (DAC 400.1 VAC-P). Solid or hardly soluble modifiers were incorporated into the resin with a dissolver stirrer (DISPERMAT[R]) at 4000 rpm and 90[degrees]C-120[degrees]C for 30 min, whereby reduced pressure was applied. If necessary the mixture was stirred for additional time until it was degassed completely and the additives were evenly dispersed in the resin. After cooling down to room temperature the curing agent was added and the mixture was stirred at 2500 rpm for 5 min. IPD was used in a stoichiometric ratio to the epoxy resin (DGEBA/IPD; 2:1) and EMIM Ac in the ratio 90:5 parts by weight (DGEBA: EMIM Ac). The resin formulations were poured into preheated and coated (ACMOS coat) aluminum moulds and cured for 30 min at 80[degrees]C, 30 min at 120[degrees]C, I h at 160[degrees]C and 1 h for 200[degrees]C (DGEBA/IPD) or for 30 min at 110[degrees]C and 3 h at 160[degrees]C (DGEBA/EMIM Ac). Epoxy resin sheets with a thickness of 4 mm as well as sheets with a thickness of 2 mm were manufactured. Control samples of DGEBA/IPD and DGEBA/EMIM Ac without toughener were prepared in the same way.

Synthesis of PolyTHF2000[(DMP).sub.2]: 33.0 g (300 mmol) dimethylphosphite and 60.0 g (30 mmol). PolyTHF 2000 were charged in a three-necked flask with an attached reflux condenser, a mechanical stirrer, a thermometer and a nitrogen inlet. The mixture was stirred at 120[degrees]C for 24 h under nitrogen atmosphere. The excess of dimethylphosphite was removed at 80[degrees]C under reduced pressure. The product PolyTHF2000[(DMP).sub.2] was obtained as a viscous, colorless liquid.

Test Methods

The glass transition temperature was determined by dynamic mechanical thermal analysis (DMTA). Test specimens of 30 x 10 x 2 [mm.sup.3] were cut out of the sheets using a band-saw. DMTA was carried out on an Exstar 6000 (Seiko Instruments, Germany). The single-cantilever mode at an oscillation frequency of 1 Hz was applied from 30[degrees]C to 200[degrees]C with a heating rate of 2 K/min. The glass transition temperature was chosen as the temperature at the maximum of the loss factor tan [delta].

The compact tension (CT) test specimens (width w = 33 mm x 33 mm, thickness d = 4 mm) were used to determine the critical stress intensity factor ([K.sub.IC]). The CT-specimens were cut out of the resin sheets using a molding cutter. A sharp crack was generated using a sharp razor blade. The [K.sub.IC] values were determined on a universal tensile testing machine (Zwicki-Line 2.5 kN, Zwick Roell, Germany) according to the standard ISO 13586. Six specimens were tested for each set of data.

Scanning electron microscope (SEM) micrographs were recorded using a SM-300 (Topcon Co., lapan). The cross sections of resin samples were sputtered with gold using a SCD 005 Sputter Coater (BAL-TEC, Germany).

RESULTS AND DISCUSSION

Influence of Different Toughening Agents

The impact of the different types of modifiers on the critical stress intensity factor, [K.sub.IC], and the glass transition temperature was examined. The [K.sub.IC] and [T.sub.g] (max. tan[delta]) values obtained for IPD-cured and homopolymerized DGEBA-resin samples are Listed in Table 1 and 2, respectively.

Cured samples of the unmodified DGEBA/IPD system showed a [T.sub.g] (max. tan[delta]) of 164[degrees]C and [K.sub.IC] value of 0.72 [MPam.sup.1/2]. The [T.sub.g] of the neat resin DGEBA/EMIM Ac achieved almost the level of the IPD-cured one, but its critical stress intensity factor was found to be considerably lower ([T.sub.g] = 159[degrees]C; [K.sub.IC] = 0.44 [MPam.sup.1/2]).

The unreactive rubber (Albipox[R] 1000, 7.5 wt%) showed similar modifying effects in both epoxy systems investigated. It increases the critical stress intensity factor of DGEBA/IPD and DGEBA/EMIM Ac from 0.72 to 0.97 [MPam.sup.1/2] (+35%) and from 0.44 to 0.55 [MPam.sup.1/2] (+25%). However, it simultaneously decreases the [T.sub.g] values moderately (from 164[degrees]C to 156[degrees]C and 159[degrees]C to 153[degrees]C, respectively) (Fig. 1 and 2).

Nanostrength"' M22N block copolymer and core-shell particles (Albidur[R] EP 2240A) induced pronounced toughening effects in both systems without nameable impact on the [T.sub.g]. Both modifiers increased the Kic of DGEBA/IPD samples by about 30% and showed comparable toughening effect in DGEBA/EMIM Ac. The SEM micrographs revealed the formation of two phases in both resin systems as expected from the observations described in literature (Fig. 3) [35, 36, 54-56].

Inorganic particles (nanosilicates Nanopox[R] F400) induced only a marginal toughening effect (K]c +11%) on the DGEBA/ IPD resin and do not alter its [T.sub.g]. However, as described in literature, the toughening effect of silica nanoparticles is more pronounced in combination with polymeric toughener [14, 15]. Therefore, silica nanoparticles were applied in combination with Nanostrength M22N block copolymer M22N and with Albidur core-shell particles (EP2240A) as well. In fact, the Nanostrength M22N Nanopox F400 mixture (5 + 5 wt%) showed the best toughening properties of all modifiers investigated in the aminecured system: a remarkably high stress intensity factor of 1.15 [Mpam.sup.1/2] ([K.sub.IC] +60%) was achieved, but the [T.sub.g] was found to be moderately decreased by 10 K. Compared to the Nanostrength M22N/Nanopox F400 mixture the Nanopox F400/Albidur EP2240A combination is less effective in the IPD-cured resin. The addition of Nanopox F400 particles did not lead to a further increase of the [K.sub.IC] above the level achieved with samples only toughened with Albidur EP 2240A (Fig. 1 and 2).

Contrary to the unreactive rubber and the block copolymer inorganic particles (Nanopox F400) showed quite different toughening properties in the homopolymerized system DGEBA/ EMIM Ac. Here, they effectuate an increase of the critical stress intensity factor of approx. 30% without any drop of the [T.sub.g] value. However, combinations of inorganic particles with block copolymers or core-shell particles, respectively, did not lead to further improved results in DGEBA/EMIM Ac compared to modified systems without the particles (Table 2 and Fig. 2). The results confirm the high potential of hybrid resin systems containing inorganic silica nanoparticles and rubber based toughening agent in terms of a pronounced toughening effect without sacrificing a high glass transition temperature. However, further investigations regarding the microstructure of these hybrid systems should be carried out focusing on the effect of silica nanoparticles on the phase separation of block copolymer toughened systems, considering the type of hardener used. Especially for the homopolymerized system DGEBA/EMIM Ac it would be interesting weather the nanoparticles are still homogeneously dispersed and evenly distributed in both phases or if aggregation occurs respectively one phase, e.g. the epoxy rich phase, shows a higher nanoparticle concentration. As described by Sprenger et al. the presence of CTBNs could also cause nanoparticle aggregation during phase separation and thus might reduce the toughening effect [14].

Substantial differences between the toughening properties in both epoxy systems were also found for functionalized silica polymer (Genioperl[R] W35) as well as for (poly(ether sulfone) (PES, Virantage[R] VW-10700-RFSP). Whereas the silica polymer strongly improves the critical stress intensity factor of DGEBA/IPD ([K.sub.IC]= 1.0 [MPam.sup.1/2], approx. + 40%) without a drop of [T.sub.g], it showed the lowest increase of the Kic value of all tested tougheners in DGEBA/EMIM Ac resin and deteriorates the [T.sub.g]. PES (Virantage VW-10700-RFSP) also provided contrary results in both resin systems. It does not act as toughener in DGEBA/IPD because it provokes a significant decrease of the [K.sub.IC]- PES showed a moderate increase of the [K.sub.IC] of the homopolymerized system along with slight decrease of [T.sub.g]. SEM provided the explanation of this behavior: the micrograph of a DGEBA/IPD sample containing PES revealed an incompatibility between the resin matrix and PES (Fig. 3).

As it is shown in Fig. 1 the addition of all toughening agents investigated is followed by a decrease of [T.sub.g]. Just the modification with PES (Virantage) and silica nanoparticles has little effect on [T.sub.g]. However, remarkable toughening was achieved with 5 wt% Nanostrength, especially in combination with 5 wt% nanosilica in the hybrid system.

On the contrary the approach of hybrid toughening was not successful in the homopolymerized DGEBA/EMIM Ac system. Moreover the [K.sub.IC] of the hybrid toughened systems is lower as compared to the solely rubber toughened systems (Fig. 2). Toughening without sacrificing the [T.sub.g] was achieved for the DGEBA/EMIM Ac system by adding the 5 wt% of Nanostrength M22N. However it was not possible to improve both the [K.sub.IC] and the [T.sub.g].

Hyperbranched Dendritic Polymers and Poly(THFs)

Another approach investigated is the use of hyperbranched dentritic polymers and poly(THF) as toughening modifiers. As the dendritic polymer with the highest hydroxyl number, Boltorn P501 (10 wt%) increases the [K.sub.IC] by approx. 90% (DGEBA/ IPD) and 60% (DGEBA/EMIM Ac). Suchlike distinct toughening effects could not be reached with any other modifier tested in both resin systems investigated (see Tab. 1 and 2). However, Boltorn P501 suffers from a serious disadvantage: it causes considerable deterioration of the thermal properties of the epoxy resins. The [T.sub.g] was lowered by 35 K in the case of the IPDcured DGEBA resin and even a decrease of approx. 50 K was found for the homopolymerized system (Fig. 4 and 5).

Boltorn P1000 causes a similar effect on the homopolymerized system DGEBA/EMIM Ac [19], but it did not show any toughening effect on the amine-cured resin DGEBA/IPD.

The resin system DGEBA/IPD was modified with poly(tetrahydrofuranes) (PolyTHF[R]) with number average molecular weights of 650, 1000 and 2000 g/mol. The samples containing PolyTHFs of low molecular weight (650 and 1000 g/mol) were transparent indicating complete homogeneity of the material. SEM investigations confirmed that these modifiers did not cause any phase separation (Fig. 6). Due to their solubility in the resin matrix PolyTHFs of low molecular weight act as plasticizer and therefore they decrease the [T.sub.g]. The incorporation of PolyTHF of higher molecular weight (PolyTHF2000) into DGEBA/IPD led to non-transparent opaque resin specimens, which is a clue of a phase separation. In fact, the SEM micrograph unambiguously shows that a separation of a second phase occurred (Fig. 6). As to be expected PolyTHF2000 induces a toughening effect on the DGEBA/IPD resin. The application of 10 wt% of PolyTHF2000 increased the [K.sub.IC] of DGEBA/IPD from 0.72 to 0.92 [MPam.sup.1/2] accompanied with just a little drop of the [T.sub.g]. Therefore an additional crosslinking was performed for this resin system with 10 wt% poly THF in order to increase the [T.sub.g] as described in the following section.

Contrary to their behavior in the IPD-cured resin, all PolyTHFs had a toughening effect in the resin system DGEBA/ EMIM Ac (Table 2) [19], Among the PolyTHFs investigated the one with the lowest molecular weight showed the most pronounced increase of KiC that gained from 0.44 to 0.62 [MPam.sup.1/2] (approx. + 40%), but it simultaneously provoked the most serious decrease of the [T.sub.g] that is lowered from 159[degrees]C to 123[degrees]C. The effects on [K.sub.IC] and [T.sub.g] were found to be weakened with increasing molecular weight of PolyTHFs.

The main reason for the different modifying effects of PolyTHFs in the resin systems DGEBA/IPD and DGEBA/ EMIMA Ac is that different curing conditions were applied. Whereas the maximum temperature during the homopolymerization of DGEBA was 160[degrees]C, the DGEBA/IPD system was subjected to temperatures up to 200[degrees]C (post-curing step). The SEM micrographs revealed that PolyTHFs with low molecular weights induces a phase separation at moderate curing temperatures also in the case of DGEBA/IPD, but the initially two phased systems becomes homogenous at the high temperatures of the post-curing.

Combinations of a Post-Crosslinking Modifier and a Toughening Modifier

In order to improve the glass transition temperature of the resin system DGEBA/IPD toughened with 10% of poly THF 2000, the resin system was modified with combinations of the toughener PolyTHF 2000 (10 wt%) with dimethylphosphite (DMP) and diethylphosphite (DEP), respectively. DMP and DEP are known to increase the glass transition temperature by acting as post-crosslinking modifier in epoxy resins [19, 57]. The amount of toughening agent was kept constant, while the phosphites were applied in different concentrations to get a system with optimal thermal properties and fracture toughness. The obtained [K.sub.IC] and [T.sub.g] values are summarized in Table 3.

The influence of PolyTHF 2000 (10 wt%) and the phosphites in different concentrations on the [K.sub.IC] and the [T.sub.g] value of the DGEBA/IPD resin is pictured in Figure 7. As previously shown the addition of 10 wt% polyTHF increased the fracture toughness of the DGEBA/IPD System. However, toughening is accompanied with a slight drop in [T.sub.g]. On the other hand the addition of just the crosslinking modifier DMP (0.3 wt%) increases the glass transition temperature as compared with the neat DGEBA/IPD resin, while the fracture toughness is strongly deteriorated. Combining these two kinds of modifier reveals a very promising approach to increase fracture toughness and glass transition temperature simultaneously. The synergistic effects with DMP (0.3 and 0.6 wt% P) confirm the approach. However the effect on [T.sub.g] of DEP as compared to DMP is more pronounced. The addition of at least 0.3 wt% P (DMP) to the DGEBA/IPD resin system with 10% polyTHF 2000 led to an increase of 10 K in [T.sub.g] while the [K.sub.IC] remains almost unaffected. The investigations revealed an optimal modifying effect of a combination of 10 wt% PolyTHF 2000 with 2.7 wt% DEP (0.6 wt% P). A high [T.sub.g] value of 180[degrees]C (+16 K) and a [K.sub.IC] of 0.86 [MPam.sup.1/2] (+0.14 [MPam.sup.1/2]) were achieved by applying this combination as compared to the neat DGEBA/IPD resin system. That means a system with improved thermal as well as mechanical properties could be obtained by using a combination of a post-crosslinking and a toughening modifier.

A further increase of the P-content up to 1 wt% P shows no further improvement in [T.sub.g]. Moreover decreasing resin properties were observed.

In addition, it was investigated whether combinations of postcrosslinking and toughening modifier that are linked by chemical bonds result in even better thermal and mechanical properties of the DGEBA/IPD resin. PolyTHF2000[(DMP).sub.2] as a suchlike novel system with chemical linkages between both modifiers (PolyTHF 2000 and DMP) was synthesized and tested. PolyTHF2000[(DMP).sub.2] was obtained by transesterification of PolyTHF2000 with DMP. However, the modifying effect of that system was found to be slightly inferior compared to the corresponding effect of a simple mixture of PolyTHF2000 and DMP (Tab. 3 and Fig. 7).

CONCLUSIONS

The impact of different types of modifying agents on the [K.sub.IC] and the [T.sub.g] values of a homopolymerized and an amine-cured DGEBA-based epoxy resin was examined in detail. The investigations revealed that the toughening effects are not only dependent on the structural features of the modifiers, but are also influenced by the resin system to be modified, its viscosity and the curing conditions applied. Therefore, most of the modifiers induced different effects in both epoxy systems investigated. Despite the different modifying effects observed in both resin system the following tendency was discovered: A strong increase of the Kic is associated with a serious drop of the [T.sub.g] in many cases. Amongst others, this dependency was found to be valid for the dendritic polymers and the PolyTHFs. Nevertheless, it was found to be possible to obtain resin systems with considerably increased [K.sub.IC] value and a [T.sub.g] in the same order of the nonmodified resin. In particular, Nanostrength M22N blockcopolymers exhibited this desirable behavior in both resin systems. Linear THF polymers showed different modifying effects in the amine-cured and the homopolymerized resin, but the toughening was found to be dependent on the molecular weight in both systems. In the case of DGEBA/EMIM Ac resin the toughening effect decreases with increasing molecular weight of PolyTHF. Only PolyTHF with the highest molecular weight of 2000 g/mol worked as toughener for the DGEBA/IPD resin. No toughening effect could be effectuated with PolyTHF of lower molecular weight (PolyTHF 650 and PolyTHF 1000). The reason of this different behavior is that a phase separation of the DGEBA/IPD resin after the postcuring at 200[degrees]C does only retain with the use of PolyTHF 2000, whereas it does not with PolyTHF650 and PolyTHF 1000.

In the case of DGEBA/EMIM Ac, the most distinct toughening effects were achieved with Boltorn 501, Boltorn 1000 and PolyTHF 650, which however simultaneously provoke a decrease of the [T.sub.g]. Moderate toughening effects without a drop of the [T.sub.g] could be achieved with Nanopox F400, Albidur EP 2240A and Virantage VW-10700-RFSP. However, the modification of the homopolymerized DGEBA resin did not result in a system that reaches the level of non-modified DGEBA/IPD or even of the best toughened DGEBA/IPD systems.

Combinations of different modifying agents were successfully tested in DGEBA/IPD. DGEBA/IPD samples containing a Nanostrength M22N/Nanopox F400 combination achieved superior [K.sub.IC] value ([K.sub.IC] =1.15 [MPam.sup.1/2]) while the [T.sub.g] was only marginally decreased.

In addition, combinations of toughening and postcrosslinking modifiers were successfully tested in the DGEBA/ IPD resin. DGEBA/IPD samples which exhibited both improved thermal and mechanical properties could be obtained this way. The modification of DGEBA/IPD with PolyTHF 2000 (10 wt%) and DEP (2.6 wt% DEP, 0.6 wt% P) led to following properties: [T.sub.g] (max. tan[delta]) = 180[degrees]C and [K.sub.IC] = 0.86 [MPam.sup.1/2]. Novel systems with chemical linked post-crosslinking and toughening modifiers were tested as well. These modifiers improved the properties of the DGEBA/IPD resin but exhibited less effect compared to combinations of separated modifiers. Therefore, they showed no advantages over optimized mixtures of modifiers.

The results of the study revealed that crucial thermal and mechanical properties of epoxy resins can be considerably improved by the application of suitable combinations of modifiers. However, an optimal modifier or modifying system has to be chosen for each particular resin system.

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Katja Utaloff, (1) Martin Heinz Kothmann, (2) Michael Ciesielski ((iD),1) Manfred Doring, (1) Thomas Neumeyer, (2) Volker Altstadt, (2) Irene Gorman, (3) Michael Henningsen (3)

(1) Fraunhofer Institute for Structural Durability and System Reliability LBF, Darmstadt, Germany

(2) Department of Polymer Engineering, University of Bayreuth, Germany

(3) BASF SE, Ludwigshafen, Germany

Correspondence to: M. Doring; e-mail: manfred.doering@lbf.fraunhofer.de

DOI 10.1002/pen.24870

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Toughening effects of Nanostrength M22N (5 wt%), Nanopox F400 (5 wt%), Genioperl W35 (5 wt%), Albidur EP 2240A (5 wt%), Albipox 1000 (7.5 wt%) and Virantage VW-10700-RFSP (10 wt%) on the resin systems DGEBA/IPD. The mixtures contain 5 + 5 wt%.

Caption: FIG. 2. Toughening effects of Nanostrength M22N (5 wt%), Nanopox F400 (5 wt%), Genioperl W35 (5 wt%), Albidur EP 2240A (5 wt%), Albipox 1000 (7.5 wt%) and Virantage VW-10700-RFSP (10 wt%) on the resin systems DGEB/EMIM Ac. The mixtures contain 5 + 5 wt%.

Caption: FIG. 3. SEM micrographs of DGEBA/IPD and DGEBA/EMIM Ac with Albidur EP 2240A (5 wt%) and Virantage VW-10700-RFSP (10 wt%) as modifiers (a) DGEBA/IPD/Albidur EP 2240A, (b) DGEBA/EMIM Ac/Albidur EP 2240A, (c) DGEBA/IPD/Virantage VW-10700-RFSP, and (d) DGEBA/EMIM Ac/Virantage VW-10700-RFSP.

Caption: FIG. 4. Toughening effects of PolyTHF 650, PolyTHF 1000, PolyTHF 2000 and Boltorn P501 and Boltom P1000 (10 wt%) on the resin system DGEBA/IPD.

Caption: FIG. 5. Toughening effects of PolyTHF 650, Poly THF 1000, PolyTHF 2000 and Boltorn P501 and Boltorn P1000 (10 wt%) on the resin system DGEBA/EMIM Ac.

Caption: FIG. 6. SEM micrographs (left) and photographs (right) of resin samples of DGEBA/IPD with (a) PolyTHF650 (10 wt%) and (b) PolyTHF2000 (10 wt%).

Caption: FIG. 7. Toughening effects of combinations of PolyTHF[R] 2000 and phosphites DMP and DEP on the resin system DGEBA/IPD. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Mechanical and thermal properties of DGEBA/IPD samples
containing different modifiers (curing: 0.5 h 80[degrees]C,
0.5 h 120[degrees], 1 h 160[degrees]C, 1 h 200[degrees]C).

                                       amount   [T.sub.g] (max.
                                       [wt%]      tan[delta])
DGEBA/IPD with modifiers                         [[degrees]C]

Neat resin                               0            164
Albipox[R] 1000 (unreactive rubber)     7.5           156
Nanostrength M22N [R] (block             5            161
copolymer)
Nanopox[R] F400 (inorganic fillers)      5            163
Nanostrength M22N [R] + Nanopox[R]     5 + 5          157
F400
Albidur[R] EP 2240A (core-shell          5            162
particles)
Albidui[R] EP 2240A and Nanopox[R]     5 + 5          161
F400
Virantage[R] (PES) VW-10700-RFSP         10           164
Genioperl[R] W35 (silica-based           5            161
functional polymer)
PolyTHF[R] 650                           10           128
PolyTHF[R] 1000                          10           132
PolyTHF[R] 2000                          10           162
Boltorn[TM] P501 (dendritic polymer)     10           136
Boltorn[TM] PI000                        10           132

                                          [K.sub.IC]
                                       [M[Pam.sup.1/2]]
DGEBA/IPD with modifiers

Neat resin                                   0.72
Albipox[R] 1000 (unreactive rubber)          0.97
Nanostrength M22N [R] (block                 0.96
copolymer)
Nanopox[R] F400 (inorganic fillers)          0.8
Nanostrength M22N [R] + Nanopox[R]           1.15
F400
Albidur[R] EP 2240A (core-shell              0.93
particles)
Albidui[R] EP 2240A and Nanopox[R]           0.99
F400
Virantage[R] (PES) VW-10700-RFSP             0.59
Genioperl[R] W35 (silica-based               1.0
functional polymer)
PolyTHF[R] 650                               0.61
PolyTHF[R] 1000                              0.6
PolyTHF[R] 2000                              0.92
Boltorn[TM] P501 (dendritic polymer)         1.39
Boltorn[TM] PI000                            0.7

DGEBA/IPD with modifiers

Neat resin                             [+ or -] 0.09
Albipox[R] 1000 (unreactive rubber)    [+ or -] 0.04
Nanostrength M22N [R] (block           [+ or -] 0.13
copolymer)
Nanopox[R] F400 (inorganic fillers)    [+ or -] 0.08
Nanostrength M22N [R] + Nanopox[R]     [+ or -] 0.03
F400
Albidur[R] EP 2240A (core-shell        [+ or -] 0.03
particles)
Albidui[R] EP 2240A and Nanopox[R]     [+ or -] 0.08
F400
Virantage[R] (PES) VW-10700-RFSP       [+ or -] 0.06
Genioperl[R] W35 (silica-based         [+ or -] 0.09
functional polymer)
PolyTHF[R] 650                         [+ or -] 0.07
PolyTHF[R] 1000                        [+ or -] 0.12
PolyTHF[R] 2000                        [+ or -] 0.13
Boltorn[TM] P501 (dendritic polymer)   [+ or -] 0.06
Boltorn[TM] PI000                      [+ or -] 0.05

IPD-cured and homopolymerized DGEBA-resin samples are
listed in Table 1 and 2, respectively.

TABLE 2. Mechanical and thermal properties of DGEBA/EMIM Ac
with different modifiers (curing: 30 min 110[degrees]C,
3h 160[degrees]C).

                                      amount   [T.sub.g](max.
                                      [wt%]      tan[delta])
DGEBA/EMIM Ac with modifiers                    [[degrees]C]

Neat resin                              --           159
Albipox[R] 1000 (unreactive rubber)    7.5           153
Nanostrength M22N [R] (block            5            159
copolymer)
Nanopox[R] F400 (inorganic fillers)     5            157
Nanostrength M22N [R] + Nanopox[R]    5 + 5          156
F400
Albidui[R] EP 2240A (core shell         5            155
particles)
Albidur[R] EP 2240A and Nanopox[R]    5 + 5          152
F400
Genioperl[R] W35 (silica based          5            146
functional polymer)
Virantage[R] (PES) VW-10700-RFSP        10           154
poly THF[R] 650                         10           123
polyTHF[R] 1000                         10           146
polyTHF[R] 2000                         10           144
Boltorn[TM] P501 (dendritic             10           110
polymer)
Boltora[TM] P1000                       10           117

                                         [K.sub.IC]
                                      [[MPam.sup.1/2]]
DGEBA/EMIM Ac with modifiers

Neat resin                                  0.44         [+ or -] 0.06
Albipox[R] 1000 (unreactive rubber)         0.55         [+ or -] 0.08
Nanostrength M22N [R] (block                0.61         [+ or -] 0.04
copolymer)
Nanopox[R] F400 (inorganic fillers)         0.57         [+ or -] 0.03
Nanostrength M22N [R] + Nanopox[R]          0.61         [+ or -] 0.03
F400
Albidui[R] EP 2240A (core shell             0.53         [+ or -] 0.03
particles)
Albidur[R] EP 2240A and Nanopox[R]          0.51         [+ or -] 0.04
F400
Genioperl[R] W35 (silica based              0.48         [+ or -] 0.04
functional polymer)
Virantage[R] (PES) VW-10700-RFSP            0.52         [+ or -] 0.04
poly THF[R] 650                             0.62              --
polyTHF[R] 1000                             0.54              --
polyTHF[R] 2000                             0.5          [+ or -] 0.05
Boltorn[TM] P501 (dendritic                 0.72              --
polymer)
Boltora[TM] P1000                           0.66              --

TABLE 3. [T.sub.g] and [K.sub.IC] values of DGEBA/IPD samples
which are modified with different amounts of PolyTHF 2000 and
phosphites DMP and DEP.

DGEBA/IPD with                Amount [wt%]       [T.sub.g]
modifiers                                          (max.
                                                tan[delta])
                                                1[degrees]C]

Neat resin                         --               164
PolyTHF[R] 2000                  10 wt%             162
DMP                            0.3 wt% P            166
PolyTHF[R] 2000 + DMP      10 wt% + 0.3 wt% P       170
PolyTHF[R] 2000 + DEP      10 wt% + 0.3 wt% P       172
PolyTHF[R] 2000 + DEP      10 wt% + 0.6 wt% P       180
PolyTHF[R] 2000 + DMP      10 wt% + 0.6 wt% P       167
PolyTHF[R] 2000 + DMP       10 wt% + 1 wt% P        173
PolyTHF2000[(DMP).sub.2]   10 wt% + 0.3 wt% P       163

DGEBA/IPD with              [K.sub.IC]
modifiers                  [MPam.sup.1/2]

Neat resin                     0.72        [+ or -] 0.09
PolyTHF[R] 2000                0.92        [+ or -] 0.07
DMP                            0.51        [+ or -] 0.02
PolyTHF[R] 2000 + DMP          0.85        [+ or -] 0.06
PolyTHF[R] 2000 + DEP          0.85        [+ or -] 0.17
PolyTHF[R] 2000 + DEP          0.86        [+ or -] 0.04
PolyTHF[R] 2000 + DMP          0.93        [+ or -] 0.08
PolyTHF[R] 2000 + DMP          0.79        [+ or -] 0.05
PolyTHF2000[(DMP).sub.2]       0.82        [+ or -] 0.08
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Author:Utaloff, Katja; Kothmann, Martin Heinz; Ciesielski, Michael; Doring, Manfred; Neumeyer, Thomas; Alts
Publication:Polymer Engineering and Science
Article Type:Report
Date:Jan 1, 2019
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