chapter-2 literature review

Toughening Procedure, Processing and Performance of Bismaleimide Resin Composites

CHAPTER-2 LITERATURE REVIEW This part of the thesis contains the recent literature review of present investigation. The chapter includes the literature reported on bismaleimide chemistry and properties, blend study and modification of bismaleimide resin and its composites by thermoplastics, thermosets and various allyl compounds. Finally, at the end of the chapter, research gap, objective, methodology of the present work and the scope of the research are also described.

2.1 Bismaleimide Resin and its Chemistry Generally, bismaleimides (BMI) are synthesized by reacting diamines with maleic anhydride to form bismaleimic acid in the first step followed by the imidization in the second step. The first step reaction is fast and exothermic and the latter can be carried out either thermally or chemically. It is reported that the synthesis leads to several by products such as, isoimides, acetanilides, maleimides acetic acid adducts and products with mixed functionalities which yield >70 % [67-68]. The reaction mechanism of formation of bismaleiamic acid involves the nucleophilic attack of the amino group on the carbonyl carbon of the maleic anhydride group followed by the opening of the anhydride ring to form amic acid group [69-70]. The reactivity of diamines is dependent on its basicity and diamines of low basicity do not exhibit sufficient nucleophilic character to form the polymer with anhydride and therefore ideally, the diamine should have a basicity of 4.5-6 for the bismaleicamic formation [71]. The exothermicity of the reaction is due to the strong acid-base interaction between the amic acid and the amide solvent and is the most important driving force of the forward reaction. Therefore, more basic and more polar is the solvent the more is the rate of poly (amic acid) formation. The thermal imidization is the direct thermal cyclodehydration of the bismaleimic acid by heating the mixture to around 300 oC and in the chemical imidization, a dehydration agent is added followed by a thermal imidization. Thermal process is the most costeffective and practical approach for commercial applications [72-73].

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2.1.1 General Bismaleimide properties BMIs are normally pale yellow colour, but if made from pure materials they are almost colorless. It is the addition of an amine that makes the resin yellow, which is the characteristic colour of the resin synthesized. On the contrary, DABA are always darker in colour, being usually brownish, amber liquid. [74] The greatest advantage of BMI is its versatile reaction capability of the maleimide functionality, which offers the possibilities to modify BMIs with components such as, reactive diluents, additives, comonomers and viscosity modifiers so that tailor made BMI systems are possible [75-76]. BMI resins are relatively stable up to about 400°C. Above this temperature, they begin to char slowly, and at higher temperatures, charring is more rapid.

O

O

O

NH

O

+

H2N

O

R

NH2

R

NH

DMF

HO

OH

Toluene

O

O

Heat HOAc

Heat

DMF

O

O N

O

R

DMF, NaOAc

Ac2O

N O

Fig.2. 1: The typical synthesis scheme of BMI


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2.1.2. Modification approaches of BMIs Considerable efforts are being devoted to improve the processing ability of BMIs and decreasing the brittleness of the cured resins [77]. In view of this, the versatile reaction capability of the maleimide groups is utilized to form an array of high performance polymeric systems. Maleimides can undergo Michael addition reaction with nucleophiles such as, primary and secondary amines, phenols, thiols, etc [78]. BMI being a bisdienophile can undergo Diels-Alder reactions with dienes [79]. Allylphenol reacts with BMI via an 'ene' reaction [80]. Vinyl and allyl type ethylenic double bonds with maleimides are also reported [81]. Anionic polymerization of the maleimide double bond is facilitated by tertiary amines and imidazoles [82].Other approaches for improving the toughness include modification with high performance thermosets such as, epoxy resins,[83] cyanate esters,[84] benzoxazines[85] etc and incorporation of engineering thermoplastics [86]. The forth coming section details the thermoplastic toughening approaches of BMIs reported in literature.

2.2 Thermoplastic-Bismaleimide blends Thermoplastics blend with BMI, which enhances toughness of the BMI and its composites [87]. In this regard, researchers showed that polysulfone (PS)-BMI [88] and polycarbonate (PC)-BMI significantly reduced the glass transition temperature (Tg) from 380 to 190 C, hence the toughness increased Takao Iijima et al. Blends with three components alderene resin, composed of 4,4‘-bismaleimidodiphenylmethane (BMPM), and o,o‘-diallylbisphenol A (DABA), and o, o‘-dimethallyl bisphenol A (DMBA)(1.0 : 0.3 : 0.7) was modified with polyetheretherketone (PEEK). When 15 wt % of PEEK was incorporated (44 mol % TP, MW 23,400), the stress intensity factor (KIC) value for the modified resin increased to 30% without compromising in the mechanical and thermal properties. Also, the KIC increased to 95% when compared to the value for a commercial bismaleimide (Matrimid 5292

TM

) resin. In a similar work, BMPM / DABA system was

incorporated with amorphous thermoplastic bisphenolA polysulfone (PSF), polyether ketone (PEK-C), and polyether sulfone (PES-C) bearing a phthalidylidene group [89]. The absence of endothermic peak or the decrease in the area of the endothermic peak of University of Mysore

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thermoplastic components in the aged blends was attributed to the formation of semiinterpenetrating polymer networks restricting the segmental mobility of thermoplastic components. In another study, BMPM/DABA system was modified with 4, 4‘bismaleimide diphenyl ether of biphenyl A (MEBMI), Allyl phenol epoxy (APE), and thermoplastic-modified polyetherketone PEK-C [90]. The impact strength of the BMI resin depends strongly on the amount of PEK-C incorporated. Polyetherimide (PEI) synthesized from different diamines were also used to toughen BMI [91-92]. All the modified compositions exhibited two relaxation peaks or shoulders corresponding to the relaxation transition of the two blend components, whereas the unmodified BMI resin gave only one tan δ peak at 293 °C. In a PEI 15-phr- and 20-phr-modified BMI system the fracture energy (GIC) increased with PEI content, In the PEI 15-phr-modified system, the GIC value was three times greater (203)than that of the unmodified BMI resin. Toughening of commercial BMI resin with varying proportion of Poly (phthalazinone ether ketone) (PPEK) was also reported [93]. The morphology of the cured resin changed from a dispersed structure to a phase-inverted structure with the increase of PPEK content. Compared to the neat resin, the fracture toughness of the modified resin exhibits a moderate increase (108 to 120) when PPEK was incorporated. Maleimide functional novolac phenolic resin (PMF), self-cured and co-cured with a novolac epoxy resin, was modified [94] as a function of the varying concentrations of the additives, ranging from 10 to 30 parts per hundred parts (phr) of the base resin by three thermoplastic elastomers viz (1) two grades of carboxyl terminated butadiene acrylonitrile copolymer (CTBN) of different molecular weights, (2) a low molecular weight, epoxidized hydroxyl-terminated polybutadiene [EHTPB], and (3) a high molecular weight acrylate terpolymer containing pendant epoxy functionality [EPOBAN]. CTBN-S (high molecular weight Mn65000g/mol) was the most effective additive with a good phase-separated morphology. For the more rigid, less ductile, epoxy-cured PMF system, the adhesive properties were marginally improved by the high molecular weight CTBN, whereas the other elastomers were practically ineffective.


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2.3 Thermoset-Bismaleimide blend A novel approach of developing high performance thermoset polymer is to introduce a second thermoset in to the first polymer matrix phase by which, it has been possible to produce a variety of high performing matrices with favorable balance of both thermal and mechanical properties. BMI and Bisphenol A-based benzoxazine (Bz-A) was thermally polymerized [95]. The Tp, decreased for the blend to 211°C in comparison to those of BMI (270 °C) and Bz-A (218°C). Moreover, the final cure temperature (Tf) of the blend was 284°C compared to 339°C of BMI. A hybrid polymer network, based on DABA modified BMPM, and 1, 1‘-bis (4-cyanatophenyl) ethane (BCyPE) with excellent dielectric properties has been reported [96]. The three formulations [BMPM/DABA: BCyPE,70:30 (F1), BMPM/DABA: BCyPE,80:20 (F2), BMPM/DABA: BCyPE,90:10 (F3)] developed by the researchers have good processing characteristics such as, low viscosity (220–411 cps) at 90 °C, suitable pot life (>4 h), excellent glass transition temperature (Tg 300 °C) and good reactivity, rendering them suitable for RTM technique. The system possessed lower dielectric constant and dielectric loss than the present commercially available bismaleimide systems (3.5 - 4.5 x 105 Hz.). A novel allylphenoxytriazine monomer, 2, 4-di (2-allylphenoxy)-6-N, N-dimethylamino-1, 3, 5triazine (DAPDMT) (Fig. 2.2) was synthesized and reacted with BMI in different molar ratios [97]. Compared with the neat BMPM matrix, the DAPDMT/BMPM copolymer matrix could improve the impact strength by 7.3 times.


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Cl

Cl N Cl

o-allylphenol (2 eqiv.)

N N

Cl

N

acetone/ H2 O, aq.NaOH

N N

O

O

Me NH 2 acetone / H O 2 aq.NaOH

H3C

N

N O

CH3 N

N

O

Fig.2.2: Synthesis of allylphenoxytriazine monomer Researchers have developed novel allyl functionalized dicyanate ester resin bearing sulfoxide linkage and were co-reacted with bismaleimide at various ratios in the uncatalyzed condition [98]. From DSC, it was observed that the pure allyl cyanate gives two distinct exothermic peaks and up on incorporation of BMI only a single exothermic peak was observed at 260-280 °C due to the Alder ene reaction. The kinetic data reveal that a considerable extent of cure takes place in about 200 min at 190°C and the cure attains near completion in about 3 h at this temperature. A high performance matrix blend was developed using bisphenol A dicyanate (BADCy), BMI, diallyl phthalate (DAP), and Cobalt (III) acetylacetonate/ nonyl phenol (NP) as a complex catalyst system [99]. A single compound maleimide epoxy system containing both a maleimide unit and an allyl ether group has been synthesized using (N-(p-carboxyphenyl) maleimide (CPMI) and allyl glycidyl ether (AGE) [100]. CPMI-AGE underwent an initial weight loss at 205 °C and the 5% weight loss was observed at 375 °C. The exothermic transition in the DSC scan of the CPMI-AGE adduct beginning at 200 °C is comparable to the cure exotherm seen in the Matrimid 5292TM system. In another study, new co-monomers were synthesized by reacting epoxy resin with 2-allyl-4-methylphenol (AE) and reacted with BMI [101]. The catalyst used to accelerate the difficult reaction between phenolic hydroxyl and the epoxy ring was Mg (BF4)2. The moisture absorption of the polymers University of Mysore

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after ageing for 100 hrs at around 250 °C ranged between 3.2 to 3.8%. The mechanical properties such as, SBS and flexural strength were also retained at 75 and 60 % of the original value when tested at 200 and 230 °C. Epoxy acrylate resins and allylepoxy resin were co-cured with BMPM/DABA [102].

2.3.1 Phenol-formaldehyde (Novolac)-BMI blends The properties of the novolac modified BMI resultant matrix depend on the molecular structure and relative ratio of the two reactants, and the extent of cure [103].Novolac was allylated by reacting with allyl halide and then co-reacted with BMI. An allyl content of 50% showed increased thermal stability (372 °C to 400 °C) [104-105]. The maximum thermal and mechanical properties with 48% allylated novolac-BMI system (Tg: 274 °C and modulus: 3.53 GPa at RT). The higher allyl content caused a decrease in the thermal properties and mechanical properties of the complex resin as expected. Nair et al [106107] synthesized Phenolic novolac resins, bearing maleimide groups. The allyl and maleimide

incorporated

resin

systems

envisaged

include

(i)

Phenol–

Hydroxyphenyl(maleimide)– Formaldehyde (PMF) Resin, (ii) Phenol–Allylphenol– Formaldehyde

(PAF)

Resin

and

(iii)

Phenol–Maleimidophenol–Allylphenol–

Formaldehyde (PMAF) Resin. Increasing the allylphenol content decreased the crosslinking in the cured matrix, leading to enhanced toughness of the resultant silica laminate composites. Good mechanical performance was observed at an optimum allyl phenol: maleimide molar ratio of 1:3 when a partially allylated novolac was cured with BMI in the presence of allyl phenyl ether diluent [108]. The system retained a flexural strength and flexural modulus of 65% and 78% at 300 °C and 200 °C respectively. From TGA, the residual weight at 500°C and 700°C was 80 %, and 51% respectively. A resin system with molar ratio of allyl to maleimide groups of 100:15 (BMAN15) was found to be appropriate for RTM [109]. The cured BMAN15 resin showed a very high Tg at 418°C, and the modulus retention was over 90% up to 350°C. By virtue of the high Tg of the matrix resin, the composite exhibited a modulus retention rate around 89 % and a strength retention rate of 46.4% at 350°C. Yet another work in the same area [110], allyl novolac (AN) was co-polymerized with BMPM in different weight ratios (1:1, 3:2 and 7:3). The modified BMI resin was stable up to 484 °C. After ageing for I00h, water University of Mysore

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absorption and heat deflection temperature (HDT) were 3.2% and 277 °C, respectively. In the case of Short Beam Shear (SBS) strength of the composites, 86 % and 78 % of the original room temperature value was retained during testing at 230 °C and 250 °C, respectively. Similarly, flexural strength also (85% and 72%) retained during testing at 230 °C and 250 °C. Allyl-functional novolac (AN) resins with varying degree allylation, (from 32.4 to 114.6%) were blended with BMPM at a weight ratio of 2.50: 1 to form Alder-ene resins [111]. As allylation degree increased, thermal stability of cured resins showed an enhancing trend because of increase in crosslinking density, but char yield of the above resins at 800°C decreased in turn. When allylated PDMS of the numberaverage molecular weight 1000 was used to modify BMI, in 5 wt. %, the impact strength increased by over three times that of the parent AN/BMPM resin (2.38 kJ/m2) [112-113]. The same research group has investigated the curing behavior of the system in the presence of PDMS. Incorporation of PDMS into the backbone of AN was found to favour Claisen rearrangement reaction during curing. Acetyl-Capped Paraformaldehyde (ACPF) - BMI-AN resin system has been developed [114]. The viscosity time–temperature profile of the 0.5% ACPF incorporated system after 4 h at 100°C showed a viscosity of 300°C). Such high water absorption was attributed to more polar spirodilactam moeities, high crosslink density, and the high free volume associated with the high Tg. The flexural modulus increased from 3.93 to 4.48 GPa when the spirodilactam bisallylether content was increased from 20 to 50 mole %.Modified BMI resin matrices with enhanced processing characteristics were realized from BMPM and allyl phenyl compounds, allyl epoxy resins and epoxy acrylate resins [125]. The modification improved the impact strength of the systems by 200% (5.8 -15.2 k J / m2). After 100h in boiling water, water absorption of composites is less than 1.1% and the flexural strength and shear strength properties were retained by more than 85% and 90%, respectively. Effects of vacuum thermal cycling on unidirectional carbon Fiber (T 300) reinforced BMPM/DABA composites increased the Tg from 233.54oC to 250.55o C [126]. The transverse tensile strength of the composite fell by approximately 9%, and the flattened off after 95 cycles. The variations of the flexural strength and the inter laminar shear strength(ILSS) were similar in tendency, increasing firstly due to the thermal cycling induced crosslinking

and then falling back to a plateau value after 198 cycles.

Compared with conventional Alder ene resin (BMPM/DABA) composite, the glass transition temperature of BAPOFP-BMI [127] composite is higher (322°C and 339°C), while the storage modulus was lower. The BAPOFP-BMI composites possessed much lower dielectric constant and dissipation factors than that of BMPM/DABA/glass composites. The dielectric constant of BAPOFP-BMI composites was 3.06 at 1 MHz and 2.99 at 1 GHz, respectively while that of BMPM/DABA was 4.01 and 3.91. A dissipation factor of BAPOFP-BMI composites is 0.009 at 1 MHz and 0.005 at 1 GHz, and that of BMPM/DABA composites is 0.012 and 0.007. The reason being, Fluorine substitution lowers the K value by decreasing the polarizability and the moisture absorption and by increasing the free volume. Three novel allyl–maleimide single component monomers namely A2B, AB and AB2 were developed [128]. Polymers of A2B and AB2 showed high glass transition temperatures (Tg> 270 °C) and their corresponding composites showed high bending modulus (E‘ > 1900 MPa). Char yield of the cured allyl–maleimide resins at 700 °C was in the range of 30–60% under Nitrogen atmosphere. Modification of bisphenol-A based bismaleimide resin (BMIP) with an allyl-terminated hyperbranched polyimide (AT-PAEKI) was reported [129]. Polyaralkyl-phenolic resin (Xylok) was University of Mysore

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allylated and co-polymerized with Bismaleimidodiphenyl methane by Aijuan Gu et al [130]. When the prepolymer is heated at 150 and 200°C, gelation occurred in 35 and 2 min, respectively. No weight loss was observed when heated to 490-500°C under nitrogen atmosphere. The system showed decomposition temperature of 530°C, and char yield of about 25% at 800°C. After ageing for 100 h, water absorption and heat deflection temperature (HDT) were 2.3% and 280°C, respectively. For the corresponding glass reinforced composites, the flexural strength, when tested at 200 and 250°C, 83 and 67% retention of the original room temperature value was observed [131].

2.4 Characterization of BMI The chemical structures of the monomers were confirmed by 1H, 13C NMR, FTIR, UVvisible spectroscopy, Raman spectroscopy and elemental analysis. Morgan [132] characterized BMPM/DABA system curing through FTIR and DSC as a function of isothermal exposures from 130 to 300°C in intervals from 1 to 14 h. The allyl, propenyl and maleimide double C=C bonds of the 'ene' adduct completely reacted after 3 h at 250°C as evidenced by FTIR. However, only 50% of the hydroxyl groups of the 'ene' adduct forms cross-links via dehydration at 250°C, over long durations, resulting in the increase of Tg. Further post-curing at 300°C for 9h results in a further ~10% increase in ether crosslinks, as monitored by the decrease in intensity of the hydroxyl band at 3473 cm- 1, and an associated increase in Tg from 280 °C to 350 °C. UV reflection spectra of BMI/DABPA resin before and after ―full cure‖ showed an overall decrease in the percentage reflectance near 230 nm and 295 nm with a simultaneous overall increase in the percentage reflectance near 266 nm [133]. Hamerton et al [134] have co-reacted propenyl –substituted cyanate ester with bismaleimide and modelled the reaction mechanism using simple model compounds (1cyanato-2-(2-propenyl)-4-tert-butylbenzene)

(geometric

isomers)

phenoxy)phenylmaleimide) to reduce the likelihood of crosslinking.

and

N-(4-

The thermally

initiated co-reaction between blends of the two model compounds when analyzed by Raman spectroscopy revealed that, as the thermal reaction proceeds, there is a University of Mysore

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pronounced decrease in the alkenyl C=C

stretch band at 1655 cm-1 which is

accompanied by a concomitant decrease in the vinylidene band at 3010 cm-1. Viorica Gaina et al. studied [135] new polymers containing alkylidenesuccinimide structures prepared from phosphorous ylides of BMI and aromatic dialdehydes by the Witting reaction, Elemental analysis data are in good agreement with the calculated values. The 1H-NMR spectrum of the polymer shows the disappearance of the signals due to the maleimide protons at 7.20 ppm and the appearance of the chemical shifts ascribed to the succinimide protons at 3.92 ppm and 7.65 ppm attributed to the benzylidene proton, respectively. Hong-qiang Yan et al [136]reported the novel interpenetrating network BMI–triazine polymers derived from BMI, viz. 2,7-bis(4-maleimidophenoxy)naphthalene (BMPN), and a dicyanate ester, viz. 2,7-dihydroxynaphthalene dicyanate (DNCY), possessing similar backbone structures. The cure process of the blends including of 0.11 mmol/mol Fe(AcAc)3 and 2% nonyl phenol is characterized by FTIR. In the FTIR spectra of cured resin, the absorption peak of the

bending vibrations decreases continuously until

the end of cure reaction after that of the OCN group, disappears mostly. FTIR confirmed this observation.

Three new allyl–maleimide monomers (i.e., A2B, AB and AB2) having good solubility were designed and synthesized by Haoyu Tang et al.[137] TGA investigations revealed that all the cured allyl–maleimide resins had high decomposition temperature (Td>380oC) air. PA2B and PAB2 had high glass transition temperature (Tg> 270oC), which was studied by DMA. The cured BMI resins (BR and BR–AB) showed good thermal stability (Td>400oC, both in nitrogen and in the air), high glass transition temperature (Tg > 320oC), good mechanical properties and low water uptake (