Effect of Reactive Diluents and Kaolin on the Mechanical Properties of Epoxy Resin

Transcription

1 Effect of Reactive Diluents and Kaolin on the Mechanical Properties of Epoxy Resin Effect of Reactive Diluents and Kaolin on the Mechanical Properties of Epoxy Resin Bakar M.*, Szymańska J., Rudecka J. and Fitas J. Plastics Department, Radom University of Technology, Chrobrego27, Radom, Poland Received: 26 October 2009, Accepted: 22 July 2010 SUMMARY Epoxy resin was modified using phenyl diglycidyl ether (PGE), xylene and kaolin. The impact strength (IS), critical stress intensity factor (K C ), flexural strength and glass transition temperature (Tg) were evaluated as functions of the modifier content. It was found that the addition of diluents led to a significant reduction of both viscosity and Tg. A three-fold increase of IS was obtained with the addition of 2.5% PGE. Moreover, the addition of 15% PGE resulted in about 145% enhancement of the K C parameter. However, the addition of xylene had no significant effect on these properties. Furthermore, the addition of 10% kaolin to epoxy resin gave compositions with the highest fracture toughness. Maximum K C values were obtained with epoxy hybrid compositions containing 10% kaolin and 2.5% PGE and 10% kaolin and 5% PGE respectively. All tested compositions had higher energy at break than nonmodified epoxy resin. SEM micrographs of fractured surfaces of modified epoxy compositions revealed the presence of plastic deformations, which can be considered as the main source of the mechanical properties enhancement. 1. INTRODUCTION Epoxy resins are widely used in different applications ranging from adhesives to matrices for high performance composites used in aerospace and automotive industry. For these and other applications, epoxy resins offer high stiffness, high specific strength, excellent dimensional stability as well as very good chemical resistance. However, the successful application of such polymeric materials requires a proper balance between the crosslink density controlled by the curing conditions and the processability characteristics of the resin 1. The different processing techniques of epoxy resin often require compositions with lower viscosities, particularly when applied in protecting varnish layers or during the jointing of elements with large surfaces. A low viscosity is also necessary in case of highly filled polymers production. Corresponding author: mbakar@wp.pl Smithers Rapra Technology, 2010 Different works have been devoted to the investigation of the effects of diluents on the viscosity, the mechanical properties and curing behaviour of epoxy resins 2-6. A specific flexibility and consequently welldefined mechanical properties of the resin can be easily reached by properly choosing the appropriate diluents and their optimum concentration. Alvey was the first to characterize the selective reaction between bisphenol-a diglycidyl ether and bisphenol-a as reactive diluent 2. Urbaczewski et al. used diglycidyl ether of 1,4-butandiol (DGEBD) to modify the most important epoxy resin, diglycidyl ether of bisphenol-a (DGEBA) 3. The analysis of the cure kinetics of epoxy/cycloaliphatic diamine formulations by means of size exclusion chromatography showed that the specific reaction rate constant of DGEBD increased while that of DGEBA decreased. In another work, Sautereau and coworkers studied the effect of chain flexibility of an aliphatic diepoxy diluent on the mechanical properties of DGEBA crosslinked with a cycloaliphatic diamine 4. Their results confirmed that the increase of the diluent content leads to an increase in the chain flexibility as well as the crosslink density. They further observed a decrease of Young s modulus and the yield stress. However, Ishida and Smith analyzed the reaction kinetics of diglycidyl ether of bisphenol-a modified bisphenol-a using FTIR technique 5. The reaction followed first order kinetics for reaction temperatures of 130 C and above through 95% consumption of the epoxide. The formed linear phenoxy backbone chains within the network structure were considered as thermoplastic polymer which could contribute to the enhancement in the fracture toughness of DGEBA. Scherzer for example used linear aliphatic diols to toughen epoxy resin 6. Polymers & Polymer Composites, Vol. 18, No. 9,

2 Bakar M., Szymańska J., Rudecka J. and Fitas J. The obtained results showed that networks with low crosslink density were formed rapidly when diols having longer chain length were used, whereas the addition of shorter diols resulted in slow network formation with a higher crosslink density. The glass transition temperature of the obtained compositions confirmed the proposed mechanism. Unfortunately, the mechanical properties were not assessed in that work. The effect of butanediol diglycidyl ether was also studied by Cizravi as a reactive diluent for bisphenol A diglycidyl ether-amine propoxylate systems 7. The obtained results showed that the elongation at break increased with increasing reactive diluent amount, while the impact strength was maximally enhanced with 38.1%wt. of diluent. On the other hand, Sautereau et al. have investigated the effect of adding an aliphatic diepoxy diluent (diglycidyl ether of 1,4-butanediol) on the mechanical behaviour, especially the flexibility of epoxy-based networks 8. The network crosslink density was changed using an aliphatic monoamine. As expected, it was found that the elastic modulus decreased with increasing reactive diluent content. Tänzer et al. found that only a small fraction of the hydroxyl groups were linked to the network 9. They found that in the networks containing 1,4-butanediol, the primary hydroxyl groups were detected only after their concentration reached half that of the epoxy groups. On the other side, the study of the reaction of phenyl glycidyl ether with aliphatic alcohols in the presence of benzyl dimethylamine at temperatures above 100 C showed that the chain length of the oligomers formed decreased with increasing temperature and the number of branched products terminated with the non-reactive groups increased 10. The aim of the present work was to investigate the effect of reactive diluents on the mechanical and rheological properties of epoxy resin without and with kaolin. The synergism effect was also checked for hybrid compositions containing both reactive diluents and kaolin. 2. EXPERIMENTAL 2.1 Materials The following ingredients were used for the compositions preparation. - Epoxy resin: Epidian 5 produced by Organika-Sarzyna, Nowa Sarzyna, Poland. Epidian 5 has an epoxy number of eq/ 100 g and a viscosity of mpa.s at 25 C. - Triethylene tetramine (trade name Z1) from Organika -Sarzyna was used as hardener. - Two diluents were used: xylene and diglycidyl phenyl ether. - Kaolin, Polarite 102A from Imerys with a surface area of 7 m 2 /g. 2.2 Preparation of the Formulations For the diluted compositions, first a specific amount (2.5 15%) of diluent was added to the epoxy resin and mixed for 5 min., then the hardener was added and mixing continued for an additional 5 min. In the case of filled compositions, dried kaolin was first added to epoxy resin and mixed for 5 min before the hardener was incorporated. The prepared epoxy resin compositions contained 5, 10 and 15 wt.% of kaolin. Two types of hybrid compositions were prepared using epoxy resin modified with 2.5 wt.% diglycidyl phenyl ether (PGE) or 5% PGE and different amount of kaolin. First, the diluent was mixed with epoxy resin, then kaolin was added to the mixture, and finally the curing agent while mixing for 5 min. The obtained compositions were poured into teflon coated plates with adequate sample geometries (60 mm x 10 mm x 4 mm). Curing was carried out at room temperature for 24 h, followed by postcuring at 60 C for 6 hours prior to solid state mechanical properties evaluation. All compositions were based on 100 parts of epoxy resin and 12 parts of hardener. 2.3 Properties Evaluation of Epoxy Compositions The impact strength was evaluated on samples having 8 cm in length, 1 cm in width, 4 mm in thickness and 1 mm in notch length using Zwick 5102 apparatus according to Charpy method and ISO 179. The critical stress intensity factor (K C ) was measured in a three point bending mode on single edge-notched specimens (8 cm x 1 cm x 4 mm) using the following equation 11 : K C = 3 P L a1/2 2 t w 2 Y (1) P is the critical load for crack propagation, L the distance between the spans, a the crack length, w and t the specimen width and thickness, Y a geometrical factor expressed by the following formula: Y =1,93 3,07 ( a w )+14,53 ( a w )2 25,11 ( a w )3 + 25,80 ( a w )4 (2) The flexural tests were carried out on unnotched samples with the same dimensions and under similar testing conditions as for K C parameter by means of ISO 178. The tests were performed at room temperature using an Instron 5566 tensile machine with a strain rate of 5 mm/min. Five samples were tested for each data point. The viscosity was measured at room temperature by means a Brookfield viscometer according to ISO Polymers & Polymer Composites, Vol. 18, No. 9, 2010

3 Effect of Reactive Diluents and Kaolin on the Mechanical Properties of Epoxy Resin The coefficient of linear thermal expansion was measured using a Ceast dilatometer and ISO The glass transition temperatures were evaluated as the intercept of the two straight lines obtained from the linear thermal coefficient temperature relationship. 2.4 Morphology Characterization Scanning electron microscopy was used to examine the fracture surfaces of selected compositions for further insight into the properties enhancement mechanisms. The specimens obtained from the impact tests were prepared and examined using a JSM-6490LV Scanning Electron Microscope. 3. RESULTS AND DISCUSSION In Table 1 are shown the values respectively of the glass transition temperatures (Tg) and viscosity of epoxy compositions containing different amounts of diluent and kaolin. As expected, we observe that Tg significantly decreases with the addition of 2.5% of either modifier. However the change in Tg is more pronounced with PGE or kaolin (75% decrease of Tg as compared with that of neat epoxy resin). Moreover, it seems that kaolin gives similar results to PGE. One can explain the depression of the glass transition by the additional free volume resulting from the diluent incorporation. Moreover, it can be indicated that the viscosity decreases as the diluent concentration increases. It decreased from about mpa.s (viscosity of virgin Epidian 5) to mpa.s and mpa.s with the addition of respectively 2.5% xylene or 2.5% phenyl diglycidyl ether (PGE). The decrease represents approximately 65% and 50% compared to that of virgin epoxy resin. The decrease is more significant and attains respectively 95% and 85% when the addition of either diluent is 10%. From Figure 1, representing the impact strength (IS) results, it can be noted that IS increases and then decreases as PGE content increases, reaching a maximum value at 2.5% PGE. The IS improvement represents approximately 300% in comparison with that of neat epoxy resin. Moreover IS of all epoxy compositions containing PGE as diluent is higher than that of neat epoxy resin. However, the addition of xylene did not influence significantly IS of the tested epoxy resin. The improvement of the impact strength induced by the addition of diluents can be partly explained by the high flexibility of the obtained compositions. Similar results were obtained with formulations of Figure 1. Impact strength (IS) as function of diluents content unsaturated polyester resins containing plasticizers 12. Maximum values of impact strength were also observed with other modifiers. Figure 2 represents the effect of diluents content on the critical stress intensity factor (K C ). It can be observed that, similarly to impact strength results (Figure 1), the addition of xylene did not affect significantly the resistance to crack propagation as expressed by K C parameter. Nevertheless, with phenyl glycidyl ether, the increase of K C is almost linear and attains approximately 145% increase (in relation with pristine epoxy samples) with the addition of 15% of reactive diluent. The stress at break under three-point bending of epoxy resin as function of xylene and PGE is shown in Figure 3. As the amount of diluents increases, the flexural strength increases prior to Table 1. Glass transition temperatures as function of diluents and kaolin content Diluent and Temperature ( o C) Diluent Viscosity (mpa.s) kaolin content PGE xylene kaolin content (%) (%) xylene PGE Polymers & Polymer Composites, Vol. 18, No. 9,

4 Bakar M., Szymańska J., Rudecka J. and Fitas J. Figure 2. Effect of diluents content on the critical stress intensity factor (K C ) of epoxy resin Figure 3. Effect of diluents content on the flexural strength of epoxy resin Figure 4. Effect of diluents content on the strain at break under flexure of epoxy resin decreasing. Maximum flexural strength is reached with 2.5% PGE and 7.5% xylene, representing respectively 90% and 120% property enhancements. From Figure 4 representing the effect of diluents content on the strain at break under three point bending, we observe a steady increase in the strain at break as the amount of xylene increases. Furthermore, the strain at break reaches maximum value at 2.5% PGE representing approximately 35% increase in comparison with the strain at break of nonmodified epoxy resin. Such an increase might be explained by the additional free volume provided by the diluents incorporation in the epoxy mixture. Similar results were obtained with plasticizers and already reported in the literature by other researchers. The values of impact strength (IS), critical stress intensity factor (K C ), fracture energy, stress at break, strain at break and flexural energy to break of epoxy resin are shown as functions of kaolin content in Table 2. The fracture energy was measured from the area under the load-displacement curve during K C parameter evaluation, while the flexural energy was obtained from the stress-strain relationship during the bending test. The usage of kaolin is motivated by the positive results reported in our previous work indicating an improvement of epoxy resin mechanical properties due to filler loading 13. It can be noted that the addition of 10% kaolin gave maximum impact strength and critical stress intensity factor values which correspond respectively to 250% and 75% enhancement in comparison with nonmodified epoxy resin. The flexural properties also increased, with 10% kaolin reaching more than 50%, 360% and approximately 45% for the flexural strength, strain at break and energy to break in comparison with pristine epoxy samples. The significant improvement of epoxy resin impact strength and other mechanical properties with kaolin 506 Polymers & Polymer Composites, Vol. 18, No. 9, 2010

5 Effect of Reactive Diluents and Kaolin on the Mechanical Properties of Epoxy Resin Table 2. Mechanical properties of epoxy compositions containing different amount of kaolin Kaolin content (%) IS (kj/m 2 ) K C (MPa.m 0.5 ) Fracture Energy (kj/m 2 ) Stress (MPa) Strain (%) Flexural energy (kj/m 2 ) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.3 can be attributed most probably to the reactions which may take place between hydroxyl groups on the surface of kaolin particles and the epoxy resin according to the following scheme, as reported in our previous work 13, see Scheme 1. The effect of kaolin on the critical stress intensity factor (K C ) of epoxy resin modified with 2.5% and 5% PGE is illustrated in Figure 5. One can observe that the addition of 5% and 10% of kaolin to epoxy resin modified with 2.5% PGE yields compositions having K C values higher by respectively about 65% and 70% than that of neat epoxy resin. However, the increase is more pronounced (i.e. attaining even 75%) with the addition of only 10% kaolin. The compositions containing higher amounts of diluents do not show higher K C values. Furthermore, it has to be mentioned that no synergic effect was observed with such obtained hybrid compositions, but all tested compositions had higher K C values that neat epoxy resin and seem very attractive from an economic point of view. Figure 6 shows the effect of kaolin on the fracture energy of epoxy resin modified with 2.5% and 5% of phenyl glycidyl ether (PGE). It can noted that the addition of 5% or 10% kaolin to epoxy resin gives composition with the highest fracture toughness. The fracture energy of compositions with 10% kaolin is 180% higher than that of neat epoxy resin. However, the hybrid compositions based on 5% kaolin/2.5%pge and 10% kaolin/2.5%pge have the same fracture energy enhancement equating to 110%. Scheme 1. Figure 5. Effect of kaolin content on the critical stress intensity factor (K C ) of epoxy compositions modified with 2.5% and 5% of phenyl glycidyl ether (PGE) Figure 6. Effect of kaolin content on the fracture energy of epoxy resin modified with 2.5% and 5% of phenyl glycidyl ether (PGE) Polymers & Polymer Composites, Vol. 18, No. 9,

6 Bakar M., Szymańska J., Rudecka J. and Fitas J. All tested compositions exhibited higher fracture energy values compared to nonmodified epoxy resin. Furthermore, among the compositions containing 2.5% PGE as for K C values, Figure 7. SEM micrograph of unmodified epoxy resin the composition based on 10% kaolin showed the highest toughness. These results evidence that the addition of diluents leads to some flexibility in the compositions while inducing a decrease in their stiffness, which is usually expressed by a clear decrease of the composition hardness or modulus. Fracture Surface Analysis SEM micrographs obtained from fracture surfaces of pristine samples and hybrid compositions allow elucidation of the toughening mechanism induced by solid particles incorporation. The micrograph of unmodified epoxy composition fracture surface (more or less flat and glassy) presented in Figure 7, indicates a regular uninterrupted and elastic crack propagation path. Figure 8. SEM micrograph of epoxy resin modified with 2.5% of phenyl glycidyl ether Generally, optimally cured epoxy resin samples without modifier exhibits a glassy structure with no plastic deformation indicating a rather poor resistance to crack propagation (i.e. low values of impact strength and critical stress intensity factor). Figure 9. SEM micrograph of epoxy resin modified with 10% of xylene Figure 8 illustrates the fractured surface of epoxy resin modified with 2.5 wt.% phenyl glycidyl ether (PGE). The fractured surface is no more flat, but exhibits a more elongated structure due most probably to plastic yielding occurrence. The addition of PGE leads to a decrease in the degree of epoxy resin reticulation and thus the increase of free volume together with the formation of plastic yielding zones. This can be related to the observed impact strength increase. SEM micrograph of fractured surfaces of epoxy resin compositions modified with 10% of xylene is shown in Figure 9. A clear plastic yielding zone with some stratification of the material is present at the surface of the broken sample. This can explain the significant increase of the strain at break of the tested composition versus neat epoxy resin. The presence of the diluents in the composition under 508 Polymers & Polymer Composites, Vol. 18, No. 9, 2010

7 Effect of Reactive Diluents and Kaolin on the Mechanical Properties of Epoxy Resin study contributes to the increase of free volume, which in turn may facilitate the matrix chain movement hence, the enhanced flexibility exhibited by the obtained samples. The presence of intensive plastic yielding zones on the fracture surfaces of epoxy based samples containing phenyl glycidyl ether or xylene may also explain somehow the flexural strength improvement as shown in Figure 3. Moreover, it seems that kaolin shows good bonding with the polymer matrix, and the addition of liquid modifier contributes to the formation of an epoxy based structure with more elongated material. Figure 13 is an SEM micrograph of epoxy resin modified with 2.5% PGE and 10% kaolin. The fracture surface is rougher and the solid particles of kaolin seem to be well dispersed in the epoxy matrix, leading to some plastic deformation of the latter. Figure 10. SEM micrograph of epoxy resin modified with 5% of kaolin From Figure 10 showing the micrograph of epoxy resin modified with 5% of kaolin, it can be noted that the fractured surface is rough with a dispersed kaolin phase. One can further observe the presence of voids originated most probably from the delamination of epoxy resin. As the amount of kaolin is increased in epoxy composition from 5% (Figure 10) to 10% (Figure 11), the solid particles are irregularly dispersed within the polymer matrix. However, the sample containing 10% kaolin is more compact compared with that containing 5%. This might explain the high impact strength exhibited by the composition with high particles loadings. The use of higher amount of solid particles is interesting from an economic perspective as well as from the engineering side, since the performance properties are improved. The SEM micrograph of epoxy composition modified with 10% xylene and 5% kaolin is represented in Figure 12. One can observe a homogeneous structure with multilayered surfaces made of polymer domains, diluents and solid particles. Plastic yielding regions are observed on the fractured surfaces together with a uniform dispersion of kaolin particles in epoxy resin phase. These findings can explain the increase in some epoxy resin properties. Figure 11. SEM micrograph of epoxy resin modified with 10% of kaolin Figure 12. SEM micrograph of epoxy resin modified with 5% kaolin 10% xylene Polymers & Polymer Composites, Vol. 18, No. 9,

8 Bakar M., Szymańska J., Rudecka J. and Fitas J. Figure 13. SEM micrograph of epoxy resin modified with 2.5% phenyl glycidyl ether and 10% kaolin The features showing some plastic deformation might be attributed mainly to the matrix, which will be associated with higher energy absorption during the crack propagation processes (i.e., impact strength and critical stress intensity factor tests). Therefore, the significant shear yielding of the polymer matrix containing diluents and kaolin can be identified as the main fracture toughness improvement mechanism. It is indeed well accepted and documented in the literature that the formation of a second phase within polymer matrix is generally associated with higher energy absorption during the crack propagation processes either under low speed (K C ) or high speed (IS) conditions. 4. CONCLUSIONS Based on the obtained results one can make the following conclusions: The addition of diluents significantly reduced the viscosity and the glass transition temperature of epoxy based compositions. As expected, it was observed that T G was significantly decreased with the addition of 2.5% diluents or solid particles. The impact strength was significantly improved with the addition of 2.5% of phenyl glycidyl ether (three-fold increase in comparison with that of neat epoxy resin.). However, the addition of xylene had no significant effect on this property. The addition of 10% kaolin gave maximum improvement of the measured mechanical properties. The improvement of the impact strength induced by the addition of diluents can be explained by the high flexibility of the obtained compositions. However, the significant epoxy resin IS enhancement due to kaolin incorporation can be attributed to the crack-pinning mechanism and probably to the reaction which may take place between hydroxyl groups on the surface of kaolin particles and the epoxy resin. Maximum K C values improvement was obtained with epoxy hybrid compositions containing 10% kaolin and 2.5% PGE and that with the same amount of kaolin and 5% PGE. It can be further stated that the addition of 10% kaolin to epoxy resin gave compositions with the highest fracture toughness. The fracture energy of such composition was 180% higher than that of neat epoxy resin. However, the hybrid compositions based on 5% kaolin/2.5%pge and 10% kaolin/2.5%pge had the same fracture energy enhancement, equating to 110%. All tested compositions had higher energy at break than nonmodified epoxy resin. SEM micrographs of fractured surfaces of modified epoxy resin compositions as well as those of hybrid compositions revealed the presence of plastic deformations, which could be taken as the main source of the mechanical properties enhancement. Most probably, the improvement of mechanical properties of the epoxy resin induced by kaolin incorporation might be partly attributed to the reactions which may take place between hydroxyl groups on the surface of kaolin particles. REFERENCES 1. Young R.J., Beaumont P.W.R., J. Mater. Sci., 12 (1977) Alvey F.B., J. Appl. Polym. Sci., 13 (1969) Urbaczewski E., Pascault J.P., Sautereau H., Riccardi C.C., Moschiar S.S., Williams R.J.J., Macromol. Chem., 191 (1990) Urbaczewski-Espuche E., Galy J., Gerard J.F., Pascault J.P., Sautereau H., Polym. Eng. Sci., 31 (1991) Ishida H., Smith M.E., Polym. Eng. Sci., 32 (1992) Scherzer T., J. Appl. Polym. Sci., 51 (1994) Cizravi J.C., Subramaniam K., Bin Hussein J., Polym. Intern., 47 (1994) Espuche E., Galy J., Gerard J.F., Pascault J.P., Sautereau H., Rev Métall., 9 (1995) Wolf D., Schlothauer K., Tänzer W., Fedtke M., Spevacek J., Polymer, 32 (1991) Tänzer W., Reinhardt S., Fedtke M., Polymer, 34 (1993) Kinloch A.J., Young, R.J., Fracture Behaviour of Polymers, Applied Science Publishers: New York, (1983). 12. Bakar M., Djaider F., J. Thermopl. Compos. Mater., 20 (2007) Fellahi S., Chikhi N., Bakar M., J. Appl. Polym. Sci., 82 (2001) Polymers & Polymer Composites, Vol. 18, No. 9, 2010