Dr. Jayalakshmi C G, DRDO Scientist

Thesis Abstract:

Continuous fabric reinforced polymer matrix composites having high temperature operational

capabilities of more than 200°C, have immense applications in various engineering sectors,

particularly in defence. Aircraft components such as radomes, wing leading edges, stealth

structures, interior structures, and missile components are a few examples of military

applications of structural composites. Manufacturing processes usually used to realize these

composites are based on autoclave moulding with prepregs, which require complex

machineries, huge infrastructure, high capital investment etc. Hence, in this research,

attempts have been made to realise high temperature composites through cost-effective Outof-

Autoclave (OOA) fabrication processes viz. Resin Film Infusion (RFI) and Temperature

assisted vacuum forming under 1 bar pressure. High temperature thermoset composites based

on cyanate ester resin and its blends were realised using RFI, while thermoplastic composites

based on Glass/Polyphenylene sulfide (PPS) commingled fabrics were developed using

vacuum forming process. The functional attributes primarily considered are electromagnetic

transparency for radome applications, high dielectric properties for stealth applications, very

high toughness along with fire resistance for aircraft interior and armoured hull applications

etc. Raw materials such as polymer resin matrix, fiber reinforcement, functional fillers are

selected in such a way that they can perform their functions at elevated temperatures.

Cyanate esters (CE) are thermoset resins having high Tg in the range of 250 - 400°C with

stable electromagnetic characteristics over wide microwave frequency bands. They are

available in different physical forms and are reported as an ideal material for performing

these specified applications. Initially quartz fabric and carbon fabric reinforced composites

based on Bisphenol A Dicyanate ester (BADCy) having a glass transition temperature (Tg)

more than 310°C was developed through RFI. Rheological and thermal studies were

conducted to optimize the process parameters for the realization of laminates. Mechanical

properties such as tensile strength, tensile modulus, Poisson’s ratio, compressive strength,

compressive modulus, Interlaminar shear strength (ILSS) were evaluated on both CE/quartz

and CE/carbon composites. Thermal properties of cured and post cured cyanate ester resin

were evaluated using Dynamic Mechanical Analyser (DMA). Electromagnetic properties

such as dielectric constant (real permittivity (ε’), imaginary permittivity (ε”), loss tangent (tan

δ) and transmission loss of CE/quartz composites were also studied using free space

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measurement system in X band (8.2 – 12.4 GHz) microwave region to check their suitability

for electromagnetic applications.

Even though cyanate esters are known for their excellent dielectric and thermal properties,

because of the presence of bulky groups (steric effects) and high cross link density they are

intrinsically brittle. Hence, to enhance the toughness of CE resin without compromising their

intrinsic characteristics, toughening has been attempted by blending with Polyetherimide

(PEI). The resultant blend is a type of semi interpenetrating polymer network (semi – IPN).

Upon curing, their morphologies (example: particulate, co-continuous and phase-inverted)

formed via phase separation control their physical as well as mechanical properties. Hence,

from this study, high temperature capable (Tg: 280°C – 290°C), broadband radar transparent (

ε’: 3.3 – 3.9; ε”: 0.02 – 0.17; tan δ: 0.01 – 0.04 and transmission loss: -0.18 dB to -1.26 dB)

in C, X and Ku bands (5.4 – 18 GHz) composites were developed using Polyetherimide (PEI)

toughened Bisphenol E Cyanate ester (BEcy) resin and E glass fabrics through (RFI) at a

range of PEI weight fractions via solvent free method. Their pronounced increase in impact

resistance in terms of mode І inter laminar fracture toughness (GIc), superior post-impact

residual compressive strength and enhanced structural properties compared with control

laminates, are corroborated using surface morphology characterisation to arrive at a

relationship. It has been understood that PEI modified cyanate esters can find extensive

applications in impact resistant composite radomes for high-speed aircrafts.

Subsequently to develop high temperature composites that are further cost-effective, with

enhanced dielectric properties aimed for stealth applications; blending of cyanate ester with

epoxy resin have been attempted. High temperature capable and highly toughened cyanate

ester HTL 300 (dicyanate ester (4,4’-Isopropylidenediphenyl dicyanate) was used to blend

with Bisphenol-A epoxy resin (Epotec YD 901). The composition of these blends was

optimized considering their ease of being cast to thin films suitable for RFI, with appropriate

tackiness at ambient conditions. Rheology was used as a tool to determine the processing

conditions for subsequent composites manufacturing through RFI and their cross linking has

been verified with FTIR. These blends are having a Tg of 165 - 195°C, are thermally stable at

least up to 320°C and results in composites with desired structural characteristics.

Electromagnetic properties such as ε’: 2.8 - 3.0, ε”: 0.02 - 0.14, tan δ: 0.008 - 0.051 and

transmission loss of < -1.5 dB makes them ideal resins for wideband electromagnetic

applications. The CE/Epoxy blend with higher dielectric values was chosen as the matrix for

developing composites for stealth application after addition carbon-based radar absorbing

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materials. Like HTL 300 / YD 901 blends, another CE / Epoxy blend was also attempted for

realising composites with more than 250°C Tg based on Bisphenol A cyanate ester called

Primaset PT 30 (Tg > 400ºC) and a Diglycidyl ether of Bis phenol A based solid epoxy resin

called Araldite GT 6071 at various epoxy weight fractions through RFI. Composite laminates

were realized using E-glass (GFRP) fabric reinforcements. Evaluation of their mechanical

properties revealed that cyanate ester / epoxy blends resulted in composites with required

mechanical properties. Laminate using quartz fabrics (QFRP) was also realized for

electromagnetic measurements. The low transmission loss of (-0.70 dB to -1.3 dB) for GFRP

and -0.1 dB to -.4 dB for QFRP in L and S microwave bands illustrates that, this matrix

formulation can be used for composite radomes and encapsulating electronic circuitries.

The fourth research domain was to realise thermoplastic composites with continuous fiber

reinforcements in an energy efficient, greener, cleaner, and a quick OOA manufacturing route

called vacuum forming process. Hence, commingled fabrics of E-glass and Polyphenylene

sulphide (PPS) was used to produce composite laminates through this facile manufacturing

technique. The resultant composites possess better toughness in terms of high impact

resistance of more than ten times, and damage tolerance in comparison to their thermoset

counterparts. Laminates are fabricated and tested according to the requirements of ASTM

specifications. Their void content has been evaluated and interface morphology has been

studied using field emission scanning electron microscopy. Composite test specimens are

subjected to evaluation of tensile, flexural, compressive, inter laminar shear, in-plane shear,

and impact properties. Dielectric properties such as permittivity, dissipation factor and

transmission loss in 8.2 - 12.4 GHz frequency region has also been reported. The results are

compared with data of E-glass/PPS composites that are manufactured using mainstream

methods and found that this technique of manufacturing thermoplastic composites is simple

and energy efficient and in turn can replace thermoset composites in various sectors.

The key findings of this research indicate feasibility of developing high temperature capable

structural and functional composites through an Out-of-Autoclave process in a cost-effective

manner. The potential applications of materials developed as a part of this research are also

briefed at appropriate sections.