научная статья по теме GAS CHROMATOGRAPHY: A NEW METHOD OF NON CONTACT NDE FOR CURE MONITORING OF CARBON-PHENOLIC COMPOSITES FOR DETERMINATION OF THE GELATION REGION FOR PRESSURE APPLICATION Общие и комплексные проблемы технических и прикладных наук и отраслей народного хозяйства

Текст научной статьи на тему «GAS CHROMATOGRAPHY: A NEW METHOD OF NON CONTACT NDE FOR CURE MONITORING OF CARBON-PHENOLIC COMPOSITES FOR DETERMINATION OF THE GELATION REGION FOR PRESSURE APPLICATION»

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УДК 620.179

GAS CHROMATOGRAPHY: A NEW METHOD OF NON CONTACT NDE FOR CURE MONITORING OF CARBON-PHENOLIC COMPOSITES FOR DETERMINATION OF THE GELATION REGION FOR PRESSURE APPLICATION

K. Veera Brahmam *, V. Venkateswara Rao

Advanced Systems Laboratory, DRDO, Kanchanbagh, Hyderabad,

Andhra Pradesh-500 058, INDIA *Correspondence author: E-mail: kveerabrahmam@rediffmail.com Phone: 040-24583682, 09441426860. Fax: 040-24346062

Abstract. We report the use of gas chromatography as a non contact Non destructive method for on-line cure monitoring of the carbon-phenolic composites during autoclave curing process to identify the gelation region for pressure application. Experimental trials are made on the laminates prepared by hand lay-up process in autoclave. During curing process methylol-phenol (M-phenol) and water are evolved as reaction by-products and the concentrations of the evolved byproducts are monitored by on-line gas chromatograph connected to autoclave facility. Experimental parameters like sample injection method, sample volume and time of injection have been optimized and M-phenol and water evolution concentrations are monitored as a function of component temperature. M-phenol evolution is more consistent compared to water evolution and therefore based on the falling trend of the methylol phenol concentration a broad region of gelation for pressure application is identified. The region thus identified is further narrowed down based on the diminishing trend of the M-phenol concentration and the experimental analysis of the laminates focusing on resin content and void content. Finally the on-line pressure application criterion was established based on the resin content and void content data.

Key words: polymer matrix composites (PMC), gas chromatography, cure monitoring.

ГАЗОВАЯ ХРОМАТОГРАФИЯ: НОВЫЙ БЕСКОНТАКТНЫЙ МЕТОД НЕРАЗРУШАЮЩЕГО КОНТРОЛЯ ДЛЯ МОНИТОРИНГА

ВУЛКАНИЗАЦИИ УГЛЕРОД-ФЕНОЛЬНЫХ КОМПОЗИТОВ

С ЦЕЛЬЮ ОПРЕДЕЛЕНИЯ ОБЛАСТИ ЖЕЛИРОВАНИЯ ПРИ ПРИЛОЖЕНИИ ДАВЛЕНИЯ

К. Веера Брахмам, В. Венкатесвара Рао Лаборатория передовых систем, ДРДО, Хидерабад, Индия

Показано использование газовой хроматографии как бесконтактный неразрушающий метод для мониторинга в режиме реального времени углерод-фенольных композитов в процессе вулканизации в автоклаве с целью определения области желирования при приложении давления. Эксперименты проводили на слоистых пластиках, приготовленных ручной укладкой в автоклав. Во время процесса вулканизации метилол-фенол (М-фенол) и вода выделяются как побочные продукты реакции, их концентрации проверяют в режиме реального времени с помощью газового хроматографа, подсоединенного к автоклаву. Экспериментальные параметры (метод и время впрыскивания образца, его объем) были оптимизированы. Концентрации выделений М-фенола и воды инспектирвоали в зависимости от температуры компонент. Выделение М-фенола более плотное по сравнению с выделением воды, поэтому на основе тенденции уменьшения концентрации метилол-фенола была определена широкая область гелеобразования при приложении давления. Основываясь на тенденции уменьшения концентрации М-фенола и анализе экспериментальных данных о слоях, сфокусированных на содержании смолы и пористости, было установлено, что обнаруженная область дополнительно сужается. В итоге критерий приложения давления в режиме реального времени был создан на основе данных о содержании смолы и пористости.

Ключевые слова: композиты с полимерной матрицей (ПМК), газовая хроматография, мониторинг вулканизации.

1. INTRODUCTION

Aerospace structures are made by embedding high strength fibers in a light weight thermoset plastic by adding hardener or supplying heat energy. In this

process, addition or condensation polymerization type of thermoset resins are used. Epoxy resins used in structural applications undergo addition polymerization without volatiles evolution and hence control of curing process is easy. In case of resins undergoing addition polymerization, the degree of cure is evaluated by conventional methods like differential scanning calorimetry (DSC) or dielectric methods [1—6]. Carbon-phenolic(C-P) composites are used as thermal protection layer in aerospace structures as ablative liners [7—9]. Phenolic resins undergo condensation polymerization and produce water and methylol phenol (M-phenol) by-products during curing. These by-products come out as volatiles/ vapors during curing process and form porosity in the component. The curing reaction is a function of temperature, time and pressure. Therefore to control the curing process the component is cured in autoclave by keeping in vacuum bag. Volatile management with selection of vacuum levels, rate of heating and gelation region for pressure application are crucial parameters to minimize the porosity and better consolidation among the fabric layers. Among the above parameters identification of gelation region for pressure application is most sensitive parameter which depends on the advancement of resin/prepreg properties. Due to above criticalities, an on-line cure monitoring system for selection of on-line pressure application point is most essential.

Cure monitoring is required for tracking the real-time changes in a chemical reaction that occurs during advancement of the resin [8]. In case of epoxy resins, during curing process reactions have been studied by many researchers with the help of (1) modeling the cure process [2, 6] (2) in-situ monitoring and control of curing during the actual manufacturing of composite products. Many authors reported the cure monitoring techniques by measuring the specific property of the material [2—6] by embedding the sensors in the component. But in case of C-P composites, the embedded sensors act as hot spots, generate high temperature under thermal environment and the structure fails easily. Therefore a contact and embedding sensor type of cure monitoring method is not suitable. In view of the above circumstances, the author developed a low cost, non-contact and non embedding sensor type of non destructive technique by cure monitoring using on-line gas chromatograph.

Phenolic resin is a thermoset type of resin with aromatic network and is obtained by condensation of phenol with formaldehyde as shown in Fig.1. In the first step of curing reaction, the M-phenol interacts with phenol and forms a polymer chain with methylene-bridge. In the second step, the M-phenol reacts with M-phenol and forms a polymer chain with ketone-bridge. Further continuous supply of heat energy forms a 3D-network of cured solid. Therefore the liquid phenolic resin initially in the low molecular weight monomeric stage undergoes long chain pre-polymer formation and subsequent gelation (rubbery state) to the final stage of chemical cross linking (solid glassy state).

Stage-2

Ct , OH OH

Stage-1

M-phenol

OH OH —> (§TCH ~(§) + H2O

As + (¿Г OH OH

гл1 hcho —► ^ ^^ . .

H2O

Phenol Formaldehyde (Methylol-phenol) ^^ "^'(Water)

Cross-linked polymer

Fig. 1. Condensation reaction in phenolic resin.

In the present technique the evolved gaseous by-products as a mixture (with methylol phenol and water) are injected into the gas chromatograph and separated

64

K. Веера EpaxMaM, B. BeHKaTecBapa Pao

as individual components and their concentrations are determined. The concentrations of M-phenol and water are monitored as a function of component temperature by connecting gas chromatograph (G.C) to autoclave facility. Finally based on the falling trend of M-phenol concentration and post analysis of laminate properties, the criterion for pressure application is established.

2. EXPERIMENTAL 2.1. Sample Preparation

Laminates were made by hand layup process with 45 layers and subsequently cured in autoclave by keeping the component in a vacuum bag. Teflon treated releasing fabric is used as a separator ply followed by nine layers of bleeder material. The bleeder material is used to absorb excess resin and to provide path to volatiles at vacuum ports. The total lay-up was kept in a kapton made vacuum bag and cured in autoclave.

2.2. Coupling of gas chromatograph equipment with autoclave facility

Chromatography is an analytical technique, which separates the gas mixture in to individual components to identify and quantify the concentrations [10]. Fig. 2 shows the coupling of on-line gas chromatograph with autoclave facility. The total experimental setup contains four sub parts 1. Gas distribution panel 2. Sample connection line 3. Gas chromatograph 4. Electronic modules interfaced with P.C. In gas distribution panel the gases (nitrogen, oxygen and hydrogen) from different cylinders were purified by sending through the molecular sieves. The gas chromatograph is coupled to autoclave through a suction pump followed by a gas-sampling valve (GSV). Sample line was maintained at 140 °C to avoid condensation of gaseous sample mixture. The gas mixture from vacuum bag is injected into gas chromato-

Electronic Module interfaced with P.C Gas chromatograph

Fig. 2. On-line gas chromatograph coupled with autoclave facility.

graph every 4 minutes by operating GSV through electronic timer circuit module. The gaseous mixture form the component is separated into individual components using Porapak-Q column. The separated components of methylol-phenol (M-phenol) and water concentrations are determined at the other end of the column with thermal conductivity detector (TCD) and flame ionization detectors (FID) respectively. Chromatograms are obtained at every four minute interval of time and the concentrations of the M-phenol and water is determined from the area under the peaks.

The electronic data acquisition module converts the analog response into digital file format. The digital data is transferred to computer through RS-232 interfacing cables and stored with TCD and FID extension files. The process parameters are controlled through user-friendly software. On-line cure monitoring of the process is carried out an

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