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Reactive acrylic liquid rubber with terminal and pendant carboxyl groups as a modifier for epoxy resin.

INTRODUCTION

Epoxy resins are a class of versatile thermosetting polymers, which are widely used in structural adhesives, surface coatings, and composites [1]. This is because of their high strength, low creep, very low cure shrinkage, excellent resistance to corrosion, and good adhesion to many substrates [1, 2]. A major drawback, which inhibits further proliferation of epoxy resins into various industrial applications, is that in the cured state they are brittle materials having fracture energy of about two orders of magnitude lower than engineering thermoplastics and three orders lower than metals [3]. Hence, the modification of epoxy resins to impart fracture toughness has been the subject of intense investigation throughout the world [4-12].

Toughness implies energy absorption and is achieved through various deformation mechanisms before failure occurs and during the crack propagation [4]. Toughening can be achieved by reduction of crosslink density or use of plasticizers, which lead to an increased plastic deformation. However, this approach may seriously affect the modulus and thermal properties of the material for only a modest increase in toughness. The most effective approach is the introduction of a second component, which is capable of phase separation such as reactive liquid rubbers [5, 6], engineering thermoplastics [7, 8], or core-shell particles [9, 10]. An attraction of liquid rubber like carboxyl-terminated copolymer of butadiene and acrylonitrile (CTBN) as a modifier is their solubility in the base epoxy with the formation of initially a homogeneous solution. As the curing reaction proceeds, the molecular weight increases and the phase separation occurs at some stage, leading to the formation of a two-phase morphology [11, 12]. Such a two-phase system having a small amount of rubber (10-15 wt %) often shows an outstanding fracture property as the rubber particles dispersed and bonded to the epoxy matrix act as centers for the dissipation of mechanical energy by cavitation and shear yielding [6, 12]. The improvement in fracture toughness is generally achieved without a significant reduction in thermal and mechanical properties of the crosslinked epoxy resin [5, 6, 11, 12].

However, the main deficiency of CTBN is the high level of unsaturation in their structure, which provides sites for degradation reaction in oxidative and high temperature environment [13]. The presence of double bonds in the chain can cause oxidation reaction or further crosslinking with the loss of elastomeric properties and ductility of the precipitated particles [14]. Second, there remains a possibility that traces of free acrylonitrile, which is carcinogenic, might exist and limit the use of these materials [15]. The saturated liquid rubbers such as siloxane [16], polyurethane [17], acrylates [18] etc. have been reported as an alternative to CTBN. Recently, Ratna et al. [19, 20] have shown that poly(2-ethyl hexyl acrylate) (CTPEHA) liquid rubber with terminal carboxyl groups ([M.sub.n] = 3600, f = 1.9) can be used as an effective impact modifier for epoxy resin cured with an ambient temperature hardeners. Ambient curing saves energy and is advantageous for surface coating applications and adhesives to the intricate structures.

Acrylate rubbers with terminal carboxyl groups have already been investigated [21, 22]. In the present work, acrylic rubbers with both terminal and pendent groups were investigated. The increase in functionality will increase the interfacial adhesion between the rubber particle and the epoxy matrix. Though the effect of other molecular and morphological parameters such as particle size, particle size distribution, and volume fraction of rubber on toughening have been extensively studied for rubber modified systems [23-26], the interfacial adhesion through the chemical bonding related studies are less [21, 22]. In this paper, synthesis, characterization of liquid rubbers with the terminal and pendant groups, and evaluation of the rubber modified epoxy networks will be discussed.

[FIGURE 1 OMITTED]

EXPERIMENTAL

Materials

The monomer 2-ethyl hexyl acrylate (EHA; Fluka) was purified by washing twice with sodium hydroxide solution (5% w/v) to remove the inhibitors and then repeatedly washed with distilled water. It was then dried over anhydrous calcium chloride for 48 h. Acrylic acid (AA) was obtained from Fluka and was used without further purification. 4,4'-Azobis(4-cyanovaleric acid) (ABCVA) (Aldrich) was recrystallized from ethanol before using as a free radical initiator. Dithiodiglycolic acid (DTDGA; Aldrich) was used as a chain transfer agent without further purification. Triphenyl phosphine (TPP; SISCO, India) was used as received. Solvents like toluene, dioxan, methanol, etc. were of analytical grade (BDH chem., India).

Epoxy resin was a liquid diglycidyl ether of bisphenol A (Ciba Geigy, Araldite LY 556) with an equivalent weight per epoxide group of 195 [+ or -] 5. The ambient temperature hardener used was an aliphatic polyamine (Ciba Geigy, HY 951). The chemical structures of the epoxy resin and the hardener are given in Fig. 1. The curing starts with hardener at room temperature; however, post curing at higher temperature is necessary for completion of reaction due slowing down of diffusion process.

Synthesis and Characterization of Liquid Rubbers

CTPEHA oligomers with different molecular weights were synthesized by bulk polymerization using ABCVA as a free radical initiator and by varying the concentration of a chain transfer agent, DTDGA. The reaction was carried out in a three-necked reaction flask (500 mL) fitted with a stirrer, a thermometer pocket, and a gas inlet. Approximately, 100 g of inhibitor free EHA and AA monomer mixture (100 g, 0.54 mol) was taken into the reaction flask and was rapidly brought to the desired temperature. After the system was well purged with nitrogen gas, 3.0 g of ABCVA (2 mol%) and the required amount of DTDGA (2.3-11.3 g, 2-10 mol%) were added and the reaction was allowed to occur for 1 h with stirring. The mixture was then diluted with toluene (200 mL) and cooled to room temperature and kept for overnight. The unreacted ABCVA and DTDGA precipitated out and were removed by filtration. Unreacted monomer and the solvents were removed under vacuum on a rotary evaporator until a constant weight was obtained.

The carboxyl content of the CTPEHA oligomer was determined by titration with a methanolic solution of 0.10 N KOH using phenolphthalein as an indicator.

The viscosity was measured using a Haake Rotoviscometer (Haake RV III) at a shear rate range of 0-100 [s.sup.-1] at 27[degrees]C using an MV III head having a clearance of 0.96 mm between the concentric cylinders of the viscometer.

The number-average molecular weights (([M.sub.n]) of all the liquid rubbers were determined using a Knauer Vapor Pressure Osmometer (VPO), with toluene as the solvent and benzil as a standard. Molecular weights and the molecular weight distributions were determined from Gel Permeation Chromatography (GPC) with an interphase module system from Water Associates. Microstyragel was used as the column material and THF was the eluting solvent. The molecular weights were determined using polystyrene standard. The functionality (f) of the liquid rubber was calculated by multiplying the carboxyl content, expressed in equiv/g, with the number-average molecular weight. The functionality is expressed as equiv/mol.

Solubility parameters were determined by Hansen's iteration method from the three-dimensional solubility parameters of the solvents in which the polymer is miscible [27a, 27b]. In this method, first solubility of the polymer in various solvent was examined. The plots of the three-dimensional solubility parameters of the solvents (available in the literature [28]) give a three-dimensional spherical space, called Hansen's space. Now if the distance between any two points is measured by a computation method then the straight line connecting the two points situated at the longest distance will represent the diameter of the sphere and the center of the sphere will represent the three-dimensional solubility parameter of the polymer.

Modification of Epoxy and Curing

Epoxy resin (100 parts) was prereacted with 10 parts of each liquid rubber using TPP as a catalyst. The reaction was carried out at 80[degrees]C under nitrogen atmosphere until all the carboxyl groups were completely reacted. The modified epoxy networks were made by curing all the formulations with HY 951.

All the formulations were analyzed for their epoxy content by standard titration [29] with hydrogen bromide in acetic acid. Accordingly, a stoichiometric amount (26 g of HY 951 for one equivalent of epoxy) of hardener was added and thoroughly mixed. The mixture was cast into a Teflon mold and cured at room temperature (RT) for 2 days. The samples were post cured at 80[degrees]C for 2 h.

Characterization of Modified Networks

A DSC instrument (DuPont 910) was used for the curing study and determination of glass transition temperature ([T.sub.g]). A heating rate of 10[degrees]C/min, sample weight of about 20 mg, and a nitrogen flow of 60 ml/min were maintained for all the experiments.

Dynamic mechanical analysis (DMA) were carried out for cured epoxy samples by a Dynamic Mechanical Thermal Analyzer (DMTA MK III, Rheometric Scientific) at a fixed frequency of 1 Hz with 3[degrees]C/min heating rate using liquid nitrogen for subambient region. Dynamic moduli and loss factors were obtained in a dual cantilever mode for the sample of size 14 X 10 X 2 [mm.sup.3].

The Izod impact test was carried out according to ASTM D-256 using an impact tester (Tinius Olsen, Model 892 T). The impact test was carried out at room temperature and impact energy was reported in J/m. The quoted result is the average of the determinations on six samples.

Fracture Surface Analysis

A low voltage scanning electron microscope (SEM) (JEOL, JSM-840) was used to examine the fracture surfaces of the toughened epoxy samples. A thin section of the fracture surface was cut and mounted on an aluminum stub using a conductive (silver) paint and was sputter coated with gold prior to the fractographic examination. SEM photo micrographs were obtained under conventional secondary electron imaging conditions, with an accelerating voltage of 20 kV.

RESULTS AND DISCUSSION

Liquid rubbers with the terminal and pendent carboxyl groups were synthesized by bulk polymerization of EHA and AA mixture using ABCVA as an initiator and DTDGA as a chain transfer agent at 120[degrees]C temperature. In our previous work [19, 20] we have shown that polymerization of EHA with carboxyl-ended initiator and chain transfer agent resulted in carboxyl-terminated poly (2-ethyl hexyl acrylate) (CTPEHA) with carboxyl functionality in the range of 1.7-1.9 equiv/mol, which is close to the theoretical value (2 equiv/mol). The deviation from the theoretical value had been attributed to the fact that some polymer chains are terminated by monomer chain transfer reaction or by disproportionation [20, 30]. Use of AA as a comonomer will produce pendant carboxyl groups in the liquid rubbers, resulting in a higher carboxyl functionality.

A series of liquid rubbers having different molecular weight and functionality were synthesized using various concentration of DTDGA and AA. The functionality of the liquid rubber increases with the incorporation of AA. It was also observed that molecular weight of EHA/AA copolymer increases with increase in AA concentration due to the higher reactivity of AA [31]. The liquid rubbers with molecular weight in the range 6800-7500 g/mol having varying functionality from 1.8 to 6.5 equiv/mol were made by adjusting the DTDGA concentration. Physicochemical properties of the liquid rubbers are shown in Table 1. The number-average molecular weights measured by VPO are higher than those measured by GPC. This can be attributed to the presence of carboxyl groups, which cause specific intermolecular interactions inside the GPC columns. This interaction prolongs the retention time, which is inversely related to [bar.M.sub.n]. All the liquid rubbers show broad molecular weight distribution. This might be due to the high reaction rate of the bulk polymerization of EHA, so that the reaction heat could not be evolved fast enough.

The liquid rubbers were characterized by FTIR spectroscopic analysis. Figure 2 shows the representative spectrum. From the figure it is clear that it displays two peaks in the carbonyl region, one at 1715 [cm.sup.-1] and other at 1740 [cm.sup.-1]. The former peak can be attributed to the carboxylic acid group and the latter is due to ester group. The [.sup.13.C] NMR spectrum for CTPEHA is shown in Fig. 3. The NMR spectrum clearly shows two peaks for carboxyl and ester carbon. The peaks at 173 and 167 ppm can be attributed to the presence of the ester and the carboxylic acid carbon, respectively. The peak at 77 ppm is due to the solvent CD[Cl.sub.3]. The peak at 65 ppm is due to the carbon atom attached to oxygen atom.

[FIGURE 2 OMITTED]

The liquid rubbers were prereacted with the epoxy and the consistency/viscosity of the product was examined. The reaction is basically a carboxyl-epoxy esterification reaction as discussed for CTPEHA elsewhere [19, 20]. The viscosity and carboxyl content values after the prereaction are shown in Table 2. It was found that viscosity significantly increases after the prereaction due to the chain extension during the prereaction. It was also found that liquid rubbers having the carboxyl functionality higher than 5 equiv/mol (CTP-5, CTP-6) resulted in gelling and solidification in the prereaction stage and hence, they cannot be used as a toughening agent for epoxy. The liquid rubbers (CTP-1 to CTP-4) have molecular weight in the range of 5000 to 7000 g/mole. The carboxyl-terminated acrylate liquid rubbers, in this molecular weight range, behave almost equally in terms of toughening [30]. Hence the liquid rubbers were used to determine the effect of functionality on the thermomechanical properties of the modified networks.

[FIGURE 3 OMITTED]

DSC and DMA Studies

The experimental [T.sub.g] values obtained from DSC for the modified networks using liquid rubbers with varying carboxyl functionality are presented in Fig. 4. Except epoxy/CTP-4, all the other modified networks show [T.sub.g] much higher than that predicted from Fox equation. This indirectly indicates the phase separation. It was also found that [T.sub.g] decreased with increasing functionality of the liquid rubber used for the modification. The decrease in [T.sub.g] for the cured rubber-modified epoxy systems arises from the incomplete phase separation caused by the plasticization phenomenon that has been noted in varied rubber modified epoxy formulations [32, 33]. Hence the extent of the dissolved rubber increases with increase in the carboxyl functionality.

The rubber [T.sub.g] is not discernible in DSC. However, it is detected clearly by DMA, which is related to mechanical relaxation. The advantage of DMA over many other methods in determination of [T.sub.g] is that it is sensitive enough to detect even weak transitions. In fact all the properties measured by this technique generate strong well-defined signals that are not clouded by the background noise or other interferences.

[FIGURE 4 OMITTED]

All the modified networks were analyzed by DMTA from -100 to 160[degrees]C. The effects of CTPEHA concentration on low temperature and high temperature relaxation peaks are shown in Figs. 5 and 6, which represent the loss factor (tan [delta]) plots against temperature for the pure and the modified epoxy networks. From the figures it is clear that all the modified networks show two relaxation peaks, one at -40[degrees]C and other at 140[degrees]C. The former peak is attributed to the [alpha] relaxation of the acrylate rubber and latter peak is due to the [alpha] relaxation of the unmodified epoxy.

[FIGURE 5 OMITTED]

DGEBA/CTP-1 (f = 1.9) system shows tan [delta] peak temperature almost at same temperature as for the pure DGEBA network and the peak shifted towards lower temperature with increase in the functionality of the liquid rubber. This supports the DSC results. DGEBA/CTP-1 shows a sharp low temperature peak at -40[degrees]C and the height and broadness of the peak increase with increase in the carboxyl functionality of the rubber. This can be attributed to increase in the chemical interactions of the liquid rubber and the epoxy matrix as a result of an increase in the functionality of the liquid rubber. The absence of any low temperature peak in DGEBA/CTP-4 is a behavior typical of a miscible blend, indicating that CTP-4 forms a compatible blend with the epoxy.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

The thermal behavior of the blends as discussed earlier can be explained in terms of solubility parameter and chemical interaction considering the thermodynamics of reaction-induced phase separation. Combining the Flory-Huggins equation and the Hildebrand equation [34], the free energy of mixing can be expressed as

[DELTA][G.sub.m]/V = [[phi].sub.e][[phi].sub.r]([[delta].sub.e] - [[delta].sub.r])[.sup.2] + RT{[[[[phi].sub.e] ln [[phi].sub.e]]/[V.sub.e]] + [[[[phi].sub.r] ln [[phi].sub.e]]/[V.sub.r]]} (1)

where [[phi].sub.e], [[phi].sub.r] are the volume fractions, [[delta].sub.e], [[delta].sub.r] are the solubility parameters, and [V.sub.e] and [V.sub.r] are the molar volume of the epoxy and the rubber, respectively. The second term of the equation is always negative as [[phi].sub.e], [[phi].sub.r] are fractions. For a fixed epoxy/rubber composition, [DELTA][G.sub.m] at a constant temperature depends on [[delta].sub.r], i.e. the chemical nature of the rubber, and [V.sub.r], which is dependent on molecular weight of the rubber. The proximity of [[delta].sub.e] and [[delta].sub.r] and low molecular weight favor the mixing process. If these two parameters are controlled in such a way that [DELTA][G.sub.m] is marginally negative, then the rubber will be compatible with the epoxy before the curing, but with the advancement of curing reaction, [V.sub.e] and [V.sub.r] will increase due to an increase in molecular weight of the rubber and the epoxy, and at a certain stage [DELTA][G.sub.m] will become positive. At that point rubber starts undergoing phase separation, which is called cloud point.

According to the Eq. 1, if ([[delta].sub.e] - [[delta].sub.r]), i.e., the difference between the solubility parameters of the epoxy and the liquid rubber is very low, then [DELTA][G.sub.m] will be highly negative and the change in entropy factor due to the curing reaction will result in free energy change of mixing ([DELTA][G.sub.m]) positive prior to the gelation. In case of CTP-4, the solubility parameter difference ([[delta].sub.e] ~ [[delta].sub.r]) is less than 0.1, which explains the homogeneity of the epoxy/CTP-4 system. In addition, the liquid rubber having higher functionality undergoes extensive chemical reaction with the epoxy, which further makes the phase separation more difficult. This observation that the liquid rubber having higher functionality does not phase separate from the matrix is consistent with the experimental findings of Bell et al. [35] and Lee et al. [36].

[FIGURE 8 OMITTED]

Impact Property

Impact strength of the modified epoxy networks as a function of carboxyl functionality of the liquid rubber is shown in Fig. 7. It was found that impact strength increases with an increase in the carboxyl functionality, attains a maximum, and then decreases. It appears that increase in chemical interaction of the liquid rubber and the epoxy matrix leads to the initial increase in impact strength of the matrix. However, the increase in toughness cannot be attributed only to the increase in rubber particle to matrix adhesion, as the thermal analysis results indicates that the amount of dissolved rubber increases with an increase in the functionality of the rubber. The dissolved rubber increases the ductility of the epoxy matrix and which in turn may increase the effectiveness of the toughening process imparted by the rubber particles [37, 38]. Hence, both the increase in rubber particle to matrix adhesion and the flexibilizing effect of dissolved rubber contribute for the improvement in toughness. A decrease in toughness above an optimum value of the functionality of the liquid rubber can be explained in terms of the morphology examined by SEM.

Morphology

The SEM microphotographs for fracture surfaces of the unmodified epoxy and the liquid rubber modified epoxy networks are shown in Fig. 8. From the photograph (Fig. 8a) it can be seen a smooth glassy fractured surface with cracks in different planes, for the case of the unmodified epoxy. This indicates a brittle fracture of the unmodified epoxy, which accounts for its poor impact strength.

DGEBA/CTP-1 shows a two-phase morphology where globular rubber particles are dispersed in the epoxy matrix. With the increase in the functionality of the rubber, it was found that discrete nature of the rubber particles was reduced and a partial co-continuous nature was introduced. There are clear indications of higher matrix-particle adhesion, cavitation, and plastic deformation with increase in functionality of the rubber (Fig. 8c, DGEBA/CTP-2; Fig 8d, DGEBA/CTP-3). This explains an initial increase in toughness with increase in the functionality of the liquid rubber.

Results of thermal analysis (Figs. 4 and 5) indicate that the amount of dissolved rubber increases with increase in the functionality. Hence the volume fraction of phase-separated rubber is also expected to decrease with an increase in the functionality of the rubber. However, the same was not observed, as with the increase in the functionality the molecular weight of the DGEB A-rubber adduct (rubber-rich phase) increases. When the functionality increases beyond 4, the rubber cannot undergo phase separation due to the proximity of solubility parameters and extensive chemical interactions leading to the formation of a single-phase morphology as indicated by DMA studies, discussed earlier. The SEM microphotograph for fracture surface of DGEBA/CTP-4 (Fig. 8d) clearly indicates a single-phase morphology. The toughening mechanisms such as rubber cavitation and shear shielding, which are responsible for the dramatic increase in toughness in the rubber-modified epoxy, cannot operate in DGEBA/CTP-4 network, due to the absence of phase-separated rubber particles. That is why the impact strength decreases beyond an optimum functionality. However, the CTP-4 modified epoxy shows higher impact strength compared to the unmodified epoxy due to the massive plastic deformation caused by the dissolved rubber as evident from the SEM photograph (Fig. 8d).

CONCLUSION

Investigation of toughening effect of the liquid acrylate rubbers with the terminal and pendent carboxyl groups on room temperature cured epoxy system has shown that the functionality of the liquid rubber plays an important role in determining the impact performance of the blends. The impact performance of epoxy/rubber blend improves with an increase in the functionality of the liquid rubber up to an optimum value (dependent on chemistry of the rubber) due to an increase in the interfacial adhesion. However, introduction of reactive groups into the rubber changes its chemistry (solubility parameter), which affects the dynamics of the reaction-induced phase separation and the final morphology of the blend. Beyond the optimum functionality, the solubility parameter of the rubber becomes so close to that of the epoxy that it cannot phase separate from the matrix and naturally the impact performance of the blend decreases as phase separation is the necessary condition for toughening. Because of the interdependent parameters it is difficult to precisely study the effect of interfacial adhesion alone. Further complexity arises due to the presence of the dissolved rubber, which increases the ductility of the matrix and thereby enhances the effectiveness of the toughening process.

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D. Ratna (1), A.K. Banthia (2)

(1) Naval Materials Research Laboratory, Shil-Badlapur Road, District Thane, Maharashtra 421 506, India

(2) Materials Science Center, I.I.T. Kharagpur, West Bengal 721302, India

Correspondence to: A. K. Banthia; e-mail: akb@matsci.iitkgp.ernet.in
TABLE 1. Physico-chemical properties of reactive acrylic liquid rubbers.

 Solubility
 Mol wt. parameter
Liquid DTDGA AA [bar.M.sub.n] Functionality (J/
rubber (wt%) (wt%) (g/mol) PDI (eq/mol) cc)[.sup.0.5]

CTP-1 4 0 5,600 6.2 1.8 20.0
CTP-2 6 2.5 5,800 5.4 2.5 20.3
CTP-3 8 5.0 4,900 7.2 3.2 20.5
CTP-4 8 7.5 6,400 6.5 4.4 20.5
CTP-5 10 10.0 6,800 7.9 5.3 21.1
CTP-6 10 12.5 7,500 8.5 6.5 21.1

TABLE 2. Prereaction of liquid rubbers with epoxy.

Liquid Consistency after Viscosity Carboxyl content
rubber precuring (Pa s) (mmol/g)

CTP-1 Liquid 29.4 Nil
CTP-2 Liquid 33.2 Nil
CTP-3 Liquid 44.5 Nil
CTP-4 Liquid 47.6 0.004
CTP-5 Gelling -- --
CTP-6 Gelling -- --
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Date:Jan 1, 2007
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