Bismaleimide Modified Epoxy-Diallylbisphenol System Effect of Bismaleimide Nature on Properties

Transcription

1 Bismaleimide Modified Epoxy-Diallylbisphenol System Effect of Bismaleimide Nature on Properties K. Ambika Devi, C.P. Reghunadhan Nair* and K.N. Ninan Propellants, Polymers, Chemicals and Materials Entity, Vikram Sarabhai Space Centre, Trivandrum , India Received: 20 December 2007, Accepted: 14 October 2008 SUMMARY The properties of the ternary matrix system (EPB), obtained by the reactive blending of epoxy resin - diallyl bisphenol and a bismaleimide, depend on the structure and properties of its components. The influence of the structural variations of bismaleimide on the thermal, physical and mechanical properties of the ternary blend was examined. EPB compositions were prepared using novolac epoxy, diallyl phenol A and three different bismaleimide systems viz. 2, 2 bis 4-(4 maleimidophenoxy) phenyl propane-bmip, bis (4-maleimidophenyl) methane-bmim and bis (4-maleimido phenyl) ether-bmie. The EPB systems with different types of bismaleimides were processed and characterized for their thermal, physical and rheological properties. EPB-glass composites processed using these matrix systems were characterized for their thermo-physical and mechanical properties. High temperature properties of these systems were assessed by their adhesive strength retention at different temperatures. The EPB system formed by the reactive blending of epoxy- phenol system with BMIP was found to yield the blend with improved mechanical performance at ambient conditions, while that with BMIM proved to be the best for high temperature application. INTRODUCTION The versatility in form, modification, properties, hardening methods, cure conditions and application are probably the most outstanding characteristic of epoxy resin. Its utility can be further widened by the matrix modification through blending with thermosetting resins to tailor its properties to suit the different applications. In earlier studies, we have observed that the reactive blending of the epoxy-allyl phenol resin system with bismaleimides improved the high temperature performance of the epoxyphenol system 1. The structural changes and the resulting property variations of the BMI systems can influence the performance of the ternary EPB blend formed by the reaction among epoxy-allylphenol and bismaleimide. *Author for correspondence, cprnair@hotmail.com Smithers Rapra Technology, 2009 Changes in the basic structure of the backbone linking the two maleimide end groups causes changes in its toughness and other physical properties. Generally, the aromatic linkages have higher rigidity than the aliphatic ones. This rigidity normally contributes to high melting point, glass transition temperature, thermooxidative resistance, decomposition temperature and modulus of a polymeric material. However, the processability and fracture resistance of aromatic BMI resins are drastically reduced by the increased molecular rigidity. The studies relating to the influence of BMI nature on the adhesive and thermo- mechanical characteristics of o, o -diallylbisphenol A (DABA) BMI system showed that the presence of polar groups such as sulphone and ether resulted in enhanced cohesive strength of the matrix 2. In an earlier study 3, it was observed that in diallyl bisphenol novolac (ABPF) cured using structurally different bismaleimides (BMIs), the adhesive properties are not significantly affected by the structural variations of the bismaleimides. The reported general trend in adhesive properties of the different bismaleimides in this study was in the order BMIS> BMIE >BMIP >BMIM. Where BMIS stands for bis (4-maleimidophenyl) sulphone. BMI resins are reactive towards many reactive species and curing reactions such as thermal polymerization, addition reactions etc. Diels-Alder reactions, have been developed to increase their application performance. The type and extent of each reaction depend on the chemical and molecular characteristics of the bismaleimide. Maleimides react with allyl phenols through Alder-ene reaction 4,5 to give rise to cyclic network structures with improved high temperature properties 6. The applicability of BMI-allyl phenol resins as matrices 141

2 K. Ambika Devi, C.P. Reghunadhan Nair and K.N. Ninan for advanced composites have been studied extensively 7. This study summarises the influence of bismaleimide nature on the properties of the EPB resin system and their glass composites. This paper details the results of these studies. MATERIALS AND METHODS Materials The materials used in the study constitute novolac epoxy, diallyl bisphenol A (DABA), triphenyl phosphine (TPP), three different bismaleimides viz. 2,2-bis 4-(4 maleimidophenoxy) phenyl propane- BMIP, bis (4-maleimidophenyl) methane- BMIM and bis (4-maleimido phenyl) ether-bmie. Sources of epoxy resin, bismaleimides, DABA and TPP are given in Table 1. The characteristics of different bismaleimides are given in Table 2. Table 1. Sources of raw materials of EPB systems Materials Sources Novolac epoxy (EPN) Supplied by M/S Ciba Geigy, Mumbai. DABA Synthesized from bisphenol A by a known procedure 8 TPP Supplied by M/S E-Merck, and used as received BMIP (BMI-1) Synthesized in VSSC by a reported procedure 9 BMIM (BMI-2) Supplied by M/s Sigma Aldrich, USA BMIE (BMI-3) Synthesized in PSCD,VSSC by a standard procedure 10 Glass cloth Plain weave, silane treated E-glass fabric of thickness 0.25mm from M/S Unnathi Corporation, India Table 2. Characteristics of raw materials of EPB systems Materials Structure Equivalent weight Novolac epoxy (EPN) DABA TPP BMIP (BMI-1) BMIM (BMI-2) BMIE (BMI-3) Characterisation of the Raw Materials Characterisation of Epoxy Resin The molecular characteristics of the epoxy resins were evaluated by determining their molecular weight distribution using the gel permeation chromatographic technique. The IR spectra were recorded following the analysis conditions mentioned in the section to follow. Characterisation of Bismaleimides The three bismaleimides used for the study were characterized for their thermal behaviour by recording their DSC thermograms at a heating rate of 10 C/min in nitrogen atmosphere. Their IR spectra were also recorded to observe the difference in their structural characteristics. Preparation of Ternary Blend Epoxy resin and diallyl bisphenol A were weighed in a 100 ml reaction bottle so that they are in their stochiometric equivalent ratio. Known weight (0.5 weight%) of TPP was added to this flask and the content was mixed well to get the epoxy phenol (EP) system. For preparing the EPB system, calculated amounts of bismaleimide was added to the EP system so that the quantities of epoxy, allyl phenol and bismaleimide are in their stochiometric equivalent ratio (1:1:1). The resin blends required for analysis were prepared by dissolving them in AR acetone, heating to 60 C for complete dissolution and removal of acetone by evaporation in a water bath at 60 C. Complete removal of solvent was achieved by heating them in a vacuum oven at 70 C. A set of 142

3 three EPB samples was prepared using the same epoxy (EPN) and diallyl bisphenol A and different types of bismaleimides (BMI-1, BMI-2 and BMI-3) to get EPB-B1, EPB-B2 and EPB-B3 respectively. The components of the ternary blends are given in Table 3. Table 3. Identification of different EPB systems Sl. No Identification of EPB systems Components of the ternary blend 1 EPB-B1 EPN, DABA, BMI-1 2 EPB-B2 EPN, DABA, BMI-2 3 EPB-B3 EPN, DABA, BMI-3 Characterization of the Ternary Blend Spectroscopic Analysis Fourier Transform Infrared Spectroscopy (FTIR) was employed for the cure characterization of the matrix system. Samples were scanned for characteristic functional group absorptions in a Perkin Elmer Spectrum GX A FTIR spectrophotometer for a wave number range of cm -1. Rheological Characterisation The rheological analysis was done with a Reological Stress Tech Rheometer. The instrument was used in the oscillation mode using a parallel plate assembly (20 mm diameter) with a gap of 0.5 mm in a controlled strain mode. The storage shear modulus G, Loss shear modulus G, and complex viscosity η* were measured as a function of temperature. Isothermal rheological studies were also carried out by recording the above parameters as a function of time at the required temperatures. Curing of Neat Resin The EPB systems with different concentrations of bismaleimide were cured in a vacuum oven, following the optimized cure conditions given below. The cure conditions were fixed based on the DSC and rheological test results. The final curing was done by heating the same at 250 C for five hours as per the time- temperature schedule given below. Temperature ( C) Hold time (Min) Characterisation of Cured Ternary Blend Spectroscopic Analysis Fourier Transform Infrared Spectrum was recorded for the cured EPB matrix for a wave number range of cm -1 for confirming the cure completion. Thermogravimetric Analysis Non-isothermal thermo gravimetric analysis was performed on cured, neat samples in nitrogen atmosphere at a heating rate of 10 C/min from ambient to 700 C using a DuPont 951 thermal analyzer. The initial decomposition temperature (T i ), temperature of maximum thermal decomposition (T m ) and the temperature of completion of thermal decomposition (T ) as well as c the char-yield were obtained from the TGA thermograms. Determination of Glass Transition Temperature The non-isothermal DSC analysis was performed on the cured polymer for the determination of its glass transition temperature. A preliminary DSC analysis was done in nitrogen atmosphere using a sample mass of 15 mg at a heating rate of 20 C/min. The sample was cooled to 50 C. The heating and cooling was repeated twice and the final analysis was done at a heating rate of 2 C/min. The temperature corresponding to the midpoint of the shifted base line in the DSC curve is taken as the glass transition temperature. Evaluation of Adhesive Properties The high temperature performance of the EP and EPB matrix systems was evaluated by determining their adhesive properties at ambient and elevated temperatures (80, 100 and 120 C). The adhesive properties of the matrix system were evaluated by determining their lap shear strength as per ASTM D Chromic acid etched, B-51- SWP aluminium alloy strips of 25 mm x 150 mm were used as substrates. A solution of the TPP catalysed EPB blend in acetone (80% by weight) was applied as a thin layer over the aluminium substrate. The solvent was allowed to evaporate by keeping the specimen in hot air oven at C for two hours. They were then allowed to cool to room temperature and assembled for curing at desired temperature in an air oven. The assembled specimens were subjected to a pressure of 0.5 MPa using a lever press assembly. The curing of the polymer blend was achieved by following the time-temperature cure schedule optimised for the EPB system. The bonded specimens were tested in Universal Testing Machine Instron model 4202, at a crosshead speed of 10 mm/min. The adhesive properties were evaluated at higher temperatures by soaking the samples at the desired temperature in the Instron climatic chamber for 10 min prior to testing. Preparation of Laminates Stochiometric blends of DABA, EPN and BMI (1:1:1) with different types of bismaleimides were dissolved in sufficient quantity of acetone (~15 times the weight of the matrix blend). Glass fabric pre-pregs were prepared by dipping the fabric in this solution and drying for 18 h at room temperature. They were cut into pieces (12 cm x 10 cm), stacked and moulded in a hydraulic press between 143

4 K. Ambika Devi, C.P. Reghunadhan Nair and K.N. Ninan thick metallic plates to achieve desired thickness. The processing conditions were optimized based on the IR, DSC and rheological characterisation of the matrix system as elaborated in our earlier study on the EPB neat system 1. The following heating schedule was adopted for the composite fabrication. A pressure of 3.5 MPa was applied after the gelation of the resin at 150 C. Figure 1. IR spectrum of novolac epoxy Temperature ( o C) Hold Time (min) Characterisation of Composites The composites were characterised for their mechanical properties such as flexural strength (FS), compressive strength (CS), interlaminar shear strength (ILSS) and interfacial shear strength (IFSS) and thermal and physical properties such as linear expansion, density, resin-content, voidcontent and water absorption following the respective ASTM procedures. Figure 2. SEC trace of novolac epoxy RESULTS AND DISCUSSION Characterisation of Raw Materials Characterisation of Epoxy Resin The epoxy resin is characterized by FT- IR and size exclusion chromatography (SEC) analyses. The FT-IR spectrum of the epoxy resin is given in Figure 1. The characteristic epoxy absorption at 915 cm -1 was observed in the IR spectrum of the epoxy resin. The IR spectrum shows that EPN contains some OH groups. The size exclusion chromatography (SEC) trace of the epoxy resin is given in Figure 2. Figure 3. IR spectra of Bismaleimides (BMI-1, BMI-2 and BMI-3) The molecular weight distribution pattern obtained for the epoxy resin shows that it is constituted by a mixture of oligomers. Characterization of Bismaleimides The FT-IR and DSC techniques were used for the characterization of three bismaleimides - BMI-1, 2 and 3. The IR spectra of the three bismaleimides are given in Figure 3. All bismaleimides showed the characteristic C=O absorption at 1710 cm -1. The aromatic C=C absorption appeared at 1500 cm -1. The peaks at 830 and 690 cm -1 are characteristic of the C-H bending vibration of the maleimide groups. The cure characteristics of the three bismaleimides were compared using their DSC curves given in Figure 4. The peak temperatures of the melting 144

5 Figure 4. DSC thermograms of bismaleimides (BM-1, BMI-2 and BMI-3) endotherms of the three bismaleimides were found to be 72 C, C and C for BMI-1, BMI-2 and BMI-3 respectively. The cure reaction initiated more or less at the same temperature for BMI-2 and BMI-3, while for BMI- 2 it was initiated comparatively at a lower temperature. The corresponding exothermic peak temperatures were in the order BMI-1>BMI-3 >BMI-2. The difference in temperature between the melting and decomposition is largest in the case of BMI-1, which allows a wider processing window for the system, while it was minimum for BMI-2. Figure 5. IR spectrum of the EPB-B1 system before and after curing Cure Characterisation of the Ternary Polymer Blend IR Spectroscopy: The IR spectra of the cured and uncured EPB systems (EPB-B1, EPB-B2 and EPB-B3) with different bismaleimides in combination with novolac epoxy and DABA are given in Figures 5, 6 and 7. The characteristic absorption due to allyl at 917 cm -1 and maleimide at 690 cm -1 (=C-H bond) disappeared in the cured resin. The intensity of the absorption at 830 cm -1 diminished. In fact, this absorption is a combination of =C-H (maleimide) and C-H (aromatic) bending vibrations. The former only disappears on curing. Figure 6. IR spectrum of the EPB-B2 system before and after curing Differential Scanning Calorimetry The influence of the bismaleimide structural variations on the cure reaction of the EPB system was studied using three bismaleimides BMI-1, BMI-2 and BMI-3. The DSC curves of the EPB systems are shown in Figure 8. The patterns of DSC cure curves were found to be different. The phenol-epoxy, Ene, Wagner-Jauregg and Diels-Alder reactions of the EPB blends occurred almost at the same temperature range in all the three cases. However, there is a difference in the relative enthalpies of different reactions in the three EPB systems, particularly for EPB-B2. In 145

6 K. Ambika Devi, C.P. Reghunadhan Nair and K.N. Ninan Figure 7. IR spectrum of the EPB-B3 system before and after curing Figure 8. DSC cure thermograms of EPB systems with different bimaleimides (TPP- 0.5%, H.R-10 C/ min) (T) using the rheometer. The variation of G with respect to temperature for these EPB systems is given in Figure 9. The non-isothermal rheograms of these EPB systems gave a better insight into the variation in the temperature dependence of their cure reaction with structural changes in bismaleimide. The cure reaction was monitored as a function of time under isothermal condition to optimize the processing conditions of their composites. In systems with varying BMI nature, the onset of gelation is facilitated by the molecular mobility. In BMI-1 with flexible spacers, the ene reaction is facilitated more than in the other two. Between EPB-B2 and B3, BMI-3 having a flexible ether group, facilitated this reaction. However, the difference in the temperature scale is subtle. Interestingly, the modulus build up is maximum for BMI-1. This is due to the fact that the absolute concentration of BMI is more in this ternary blend. It appears that all the unsaturation groups in BMI-2 and BMI-3 are not consumed in crosslinking due probably to their structural rigidity in comparison to BMI-1. As a result, the curing of unused groups takes place at a higher temperature (~280 C). Further, the increase in modulus due to residual unsaturation in polymerization is insignificant in EPB-B2 and B3 in comparison to EPB-B1. this case, the relative exothermicity of phenol epoxy reaction was maximum and it occurred at a slightly higher temperature and that for Ene-reaction was minimum among the three systems studied. This is only apparent as the absolute concentration of DABA and EPN is less in BMIP when compared to the other two, because of the higher molecular weight of the particular BMI in the stoichiometrically equivalent blend. A weak exotherm at 250 C in EPB-B3 can be attributed to the self polymerization of BMI-3 that might not have been incorporated in the matrix by the Alder-ene reaction, at lower temperature. Rheological Cure Characterization of the Blends The rheological cure characterisation of the different EPB blends was carried out to get a better insight in to their cure profile. The complex viscosity (η*), storage shear modulus (G ) and loss shear modulus (G ) of the ternary blends with different bismaleimides (EPB-B1, EPB-B2 and EPB-B3) were monitored as a function of temperature The isothermal rheograms of these matrix systems, recorded at 250 C revealed that the storage shear modulus levels off after about five hours at this temperature, indicating the cure completion of the system. The rheograms supplemented the DSC observation of the cure reaction. A typical isothermal rheogram of EPB-B1 is given in Figure 10. Characterisation of the Cured EPB System Thermogravimetric Analysis The variation in TG profiles among the EPB systems with different BMIs 146

7 was very benign. Though the initial decomposition temperature is a few degrees lower for EPB-B1 system visa-vis the rest, this has got a reduced rate of decomposition at higher temperature regime. The thermal stability of EPB-B2 was comparatively higher and that for EPB-B1 (containing BMI- 1) was the minimum. The thermal stabilities of the EPB systems with bismaleimide structural variation indicated that their thermal stabilities are in the order EPB-B2> EPB-B3 > EPB-B1. The relevant thermograms and temperature data are given in Figure 11 and Table 4 respectively. The maximum thermal stability obtained for the EPB-B2 system may be due to its higher crosslink density resulting from the shorter distance between the maleimide groups in BMI-2. EPB-B3 has more or less the same environment as EPB-B2 and the difference between the two is within experimental scatter. Even though the presence of more aliphatic groups in BMI-1 was expected to contribute to its lower thermal stability in comparison to the systems containing BMI-2 and BMI-3, this is offset by the high concentration by weight of BMI in the ternary blend. This is also reflected in the comparatively better char residue of the system. Figure 9. Dependence of storage shear modulus of EPB systems with nature of bismaleimides Figure 10. Isothermal rheogram of EPB-B1, 250 C Evaluation of Adhesive Strength The adhesive properties of EPB systems with different bismaleimides (EPB-B1, EPB-B2 & EPB-B3) were evaluated by determining their lap shear strength. The material performance evaluated at different climatic conditions is given in Table 5. Figure 11. The TGA thermograms of cured EPB blends with bismaleimide structural variation (H.R 10 C/min, N 2 ) For the system with bismaleimide variation, the result indicated an improvement in the adhesive strength of the material (for EPB-B2) up to 120 o C and thereafter it showed a reducing tendency. At 150 C, the strength retention is comparatively poor for EPB system with BMI-1, as in this case, the T g is lower than the other two cases. The marginally enhanced polarity contributed by the ether group 147

8 K. Ambika Devi, C.P. Reghunadhan Nair and K.N. Ninan Table 4. Thermal decomposition characteristics of EPB systems with bismaleimide structural variation Reference Temperature EPB-B1 EPB-B2 EPB-B3 T i ( C) T m ( C) T s ( C) Residue at 800 C (%) Table 5. Adhesive properties of EPB systems with bismaleimide structural variations Reference Lap shear strength (kg/cm 2 ) Temperature ( C) RT EPB-B EPB-B EPB-B Table 6. Mechanical characteristics of EPB composites with different bismaleimides Property EPB-B1 EPB-B2 EPB-B3 ILSS (kg/cm 2 ) IFSS (kg/cm 2 ) Compressive strength (kg/cm 2 ) Flexural strength (kg/cm 2 ) interfacial shear strength of EPB-B1 and EPB-B3 were comparable, while the increase in flexural strength of EPB-B1 was much higher compared to the other two systems. In the case of BMI-1, its moderate crosslink density (large spacing between imide groups) and better compatibility with diallyl bisphenol (similarity in structure) could furnish a good matrix system with better flexural characteristics. Thermomechanical Analysis The thermomechanical analysis of the different EPB composite systems were carried out to determine their linear expansion coefficient ( ). The linear expansions of the samples were in the range 3.1 x 10-5 and 3.4 x 10-5 for the systems with different bismaleimides. The variation in T g of the resin systems with different bismaleimides obtained from their DSC analysis was found to range from 178 to 193 C. As expected, the T g values of the different EPB systems were in proportion to the crosslink density of the BMI in the respective system. in BMI-3 is reflected in its better LSS in comparison to BMI-2. The high temperature (150 C) performance of EPB-B2 was found to be the best among the three. This might have been contributed by the reduced distance between the maleimide groups in BMI- 2. In the case of EPB-B3 even though the distance between the maleimide groups in BMI-3 is practically the same as that in BMI-2, the presence of flexible ether linkage in its structure might have contributed to its inferior strength retention in comparison to EPB-B2. Characterisation of EPB-glass Composites The glass laminates prepared using the three component EPB resin systems containing different bismaleimides were characterized for their mechanical, thermomechanical and physical properties. Mechanical Properties The mechanical properties - compressive, flexural and interlaminar shear strength - of the EPB composites were found to vary with the nature and properties of the components of the EPB matrix system. The strength of these composites measured under different loading environments, such as tension, compression and flexure, are summarized in Table 6. The general trend in these properties was EPB-B1> EPB-B3 > EPB-B2 for the systems with different types of bismaleimides. When the structural dependence of BMI on the mechanical performance of the EPB system was examined, BMI-1 and BMI-3 with flexible spacers were found to have distinct advantage over BMI-2. The interlaminar shear strength, compressive strength and Physical Properties of EPB Composites The physical properties of the composites viz. density, water absorption, coefficient of linear expansion, resin/reinforcement content etc. give information regarding its quality and suitability for specific application. The properties evaluated for different EPB systems are given in Table 7. Based on physical parameters, the composites were found to be of good quality and we can infer that the change in mechanical properties described previously is not a consequence of the defects in composites. The resin content in these composites determined by matrix digestion showed a variation from 20.4 to 22.3 for those with different bismaleimides. The water absorption values determined by boiling water immersion (2 hrs.) were found to be within a narrow band. 148

9 Table 7. Properties of EPB composites with different bismaleimides Property EPB-B1 EPB-B2 EPB-B3 Density (g/cc) Water absorption (%) Resin content (%) α x 10-5 ( C) T g ( C) CONCLUSIONS The variation in the nature of BMI in the epoxy-allylphenol-bmi system has resulted in significant variation in the relative enthalpies of the different cure reactions observed in the DSC curve. The rheological behaviour of the BMI modified systems showed marginal shift in the different stages of reaction. Among the bismaleimide modified systems, thermal stability was found to be maximum for EPB-B2. The higher crosslink density resulting from the shorter distance between the maleimide groups in BMI-2 has contributed to its superior thermal stability. The adhesive strength and the strength retention at elevated temperature for the BMI modified systems showed that the EPB-B2 has got the best thermal characteristics. EPB-B2 and EPB-B3 were good in respect of adhesive strength and its high temperature retention. The marginally enhanced polarity of BMI-3 has reflected in its better LSS in comparison to BMI-2. But it was reversed for the high temperature retention, where the flexibility of the spacer group dictated the property. The high temperature performance of these systems followed the trend in their T g values. The glass transition temperature was maximum for BMI- 2, due to the higher cross link density resulting from the shorter spacing between the maleimide groups. The trend in the strength of the glass composites was in the order EPB-B1 > EPB-B3 > EPB-B2 for bismaleimide modified systems. When the structural dependence of BMI on the mechanical performance of the EPB system was examined, BMI-1 and BMI-3 with flexible and polar ether spacers exhibited distinct advantage over BMI-2. The EPB system formed by the reactive blending of novolac epoxydiallylbisphenol A with BMI-1 was found to yield a ternary blend with improved mechanical performance at ambient conditions, while that with BMI-2 was found to be the best with respect to high temperature performance. REFERENCES 1. Ambika Devi K., Reghunadhan Nair C.P., and Ninan K.N., Bismaleimide co-cured diallyl bisphenol A - epoxy novolac system - Cure and thermal properties, J. Appl. Polym. Sci., 106(2) (2007) Gouri C., Reghunadhan Nair C.P., and Ramaswamy R., Polymers and Polymer Composites, 11(4) (2003) Gouri C., Reghunadhan Nair C.P., and Ramaswamy R., Polymers and Polym. Composites, 11(4) (2003) Zahir S.A., Chaudhari N.A. and King J., Macromol. Chem Macromol. Symp., 25 (1989) Reyx D., Campistron L., Caillaud C., Villatte M. and Cavendon O., Macromol. Chem., 196 (1995) Morgan R.J., Shin E., Rosenberg B. and Jurek A., Polymer, 38 (1997) Liang G. and Gu A., Polymer Journal, 29(7) (1997) Gouri C., Nair C.P.R., and Ramaswamy R., Polym. Int., 50 (2001) Nair C.P.R. and Francis T., J. Appl. Polym. Sci., 74 (1999) Nair C.P.R., Sebastian T.V., Nema S.K., and Rao K.V.C., J. Polym. Sci. Chemistry, 24 (1986)