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STUDY ON THE PROCESSING AND THE PERFORMANCE OF CYANATE ESTER COMPOSITES TOWARDS THE OPTIMIZATION FOR HARSH SERVICE ENVIRONMENTS

Final Report Summary - SPECIMEN (STUDY ON THE PROCESSING AND THE PERFORMANCE OF CYANATE ESTER COMPOSITES TOWARDS THE OPTIMIZATION FOR HARSH SERVICE ENVIRONMENTS.)

Executive Summary:
SPECIMEN project aims to fill this missing knowledge gap and create specific technical guidelines for the use of cyanate ester composite material systems in long-term harsh environmental conditions. Taking into account the technical requirements provided by the topic manager, SPECIMEN project was designed to provide CSJU a complete study that will include a Material Screening Procedure, a Manufacturing Process Plan, the methodologies for studying ageing process using accelerated testing protocols, the durability performance characteristics of selected materials and a Compliance and Optimization Study for selected materials. SPECIMEN team was Applied Mechanics Laboratory of University of Patras (GR) and TECNALIA R&D center in San Sebastian (ES).
At the beginning the Technical Requirements from partners were gathered from the ITD members. Requirements were categorized in terms of material performance, manufacturing processes and other parameters (e.g cost). Also envisioned applications were communicated. The search of materials has been based on requirements specified by the ITD members. After analyzing the technical requirements, material properties and recommendations from suppliers, the materials were selected for further screening. Afterwards a test plan has been elaborated in order to define an initial ageing procedure and a test campaign, which will allow the verification of the compliance of the selected material systems with the given technical requirements and the specifications by both ITD members. Since screening testing decided to be performed in FRP level, considerable effort was given on manufacturing process investigations towards the production of materials required for specimens. Work involved process optimization of 3 different GFRP pre-preg material systems and 2 different RTM bulk resin material systems. When the manufacturing processes were conclusively defined, specimens were provided for the finalization of the testing of all different materials. The analysis and the review of the results provided the background for the selection of one material per route.
The process optimization work was continued on the selected materials in order to produce specimens needed for the degradation studies envisioned. Under this framework for the case of RTM route, SPECIMEN team worked heavily to optimize the cure cycle succeeding 51% reduction of the curing time and a 33% of energy saving. Also, with the new proposed optimized cure cycle, a higher degree of cure is obtained in the post-curing and consequently a higher final Tg. Moreover, in the first dwell, the reaction rate is lower than in the baseline one, reducing the risk of exo-therm. For the case of PREPREG route, the optimization of the process was kept limited focused on cure cycle and degree of curing control.
In parallel to the process optimization work, SPECIMEN team established a comprehensive methodology for Degradation Behavior Study using the data from screening testing. The comprehensive methodology for an ageing study was based on the accelerated methods principles in order to investigate and understand the various degradation mechanisms of the Cyanate ester matrix composites that occur at high service temperature environments. Two (2) different protocols, for long-term exposure study were elaborated one for elevated temperatures for CFRP materials produced by RTM technique and one for elevated temperatures in humid atmosphere for GFRP materials produced by PREPREG/AUTOCLAVE technique. The actual ageing and respective testing & analysis performed provided significant information on the activation and operation of all applicable degradation mechanisms. Finally for RTM process a manufacturing of optimized fibre reinforced composites was performed in order to perform a Life Cycle Analysis.
Taking into account the work performed during early screening stage, the main Degradation behavior study and the given technical requirements by the topic manager last part of the SPECIMEN activity was focused on defining and addressing application specific critical parameters (e.g severity categorization of Degradation Mechanisms, classification of operational conditions per criticality etc.). Moreover Time Temperature Superposition (TTS) was used in order to try to estimate the lifetime under specific conditions. Additionally three (3) simple & sort test protocols based on accelerated testing principles were produced for future use. SPECIMEN activity ended by providing a deeper understanding of the degradation in harsh environments using advanced material screening/testing procedures, accompanied with optimized manufacturing protocols.
Project Context and Objectives:
Cyanate esters are a relatively new generation of thermosetting resins that exhibit considerable tolerance in high service temperatures where the effects of moisture and micro-cracking due to thermal cycling loading are important. They exhibit low moisture absorbency, whilst retaining useful processing characteristics, e.g. cure temperatures, handling and composite processing by pre-preging or by resin transfer molding. In particular, the improvements over epoxies are good toughness, lower moisture absorption, higher glass transition temperatures at lower cure temperatures and increased resistance to micro-cracking. Consequently, cyanate ester based composites are exceptional candidate materials for parts used in aircraft or rotorcraft components that are subjected to a complex history of temperature, moisture and other environmental conditions. Nevertheless, the long-term exposure of cyanate ester based composite materials to those harsh environmental conditions and the respective degradation of their properties defined as material ageing process is not yet fully studied and understood. This aspect limits the further utilization of such materials in aeronautical applications. SPECIMEN project aims to fill this missing knowledge gap and create specific technical guidelines. Taking into account the technical requirements provided by the topic manager, SPECIMEN project will provide CSJU a complete study that will include a Material Screening Procedure, a Manufacturing Process Plan, the methodologies for studying ageing process using accelerated testing protocols, the durability performance characteristics of selected materials and a Compliance and Optimization Study for selected materials. The deeper understanding of the degradation in harsh environments using advanced material screening/testing procedures, accompanied with optimized manufacturing protocols will clarify issues concerning the inputs, outputs and nuisances during all three phases of aircraft life.

Project Results:
WP2 Materials selection and screening
Task 2.1 Materials Benchmarking and Selection
Technical Requirements from partners were gathered from the ITD members (Eurocopter, EADS and Liebherr). Requirements were categorized in terms of material performance, manufacturing processes and other (e.g cost). Also envisioned applications were communicated. The search of materials has been based on some of the requirements specified by the ITD members. After analyzing the technical requirements, material properties and recommendations from suppliers, the materials selected for further screening were:
• For LTS-GFRP-AUTOCLAVE route 3 prepreg materials were selected.
• For EC-CFRP-RTM route 2 resin systems.
Task 2.2 Materials Screening Test Methodology
Within Task 2.2 a test plan has been elaborated in order to define an initial ageing procedure and a test campaign that will allow the verification of the compliance of the selected material systems in the task 2.1 with the given technical requirements and the specifications by both partners (LTS and EC). It should be noted it was proposed by ITDs that the material screening should be performed in FRP level rather than in polymer (as described in Annex I) and include a comprehensive ageing response study apart from basic tests envisioned in polymer level. Based on the fact that the current project focused on the study of the ageing response of FRPs systems, a comprehensive screening methodology has to take into account except from its performance under specified conditions, the processability and the acceptability of the material according to the supplier's data, too. So the materials' screening methodology defined is composed of
• A test matrix for material acceptance/processability control
• An ageing procedures list (6 procedures)with respective test matrix for the material performance control.
Task 2.3 Preliminary Testing of materials
Due to the fact that the screening was decided to be performed in FRP level, task 2.3 was linked with Task 3.1 which focused on establishing and optimized both AUTOCLAVE and RTM composite production routes. This delayed the start of the testing till CFRP (RTM) and GFRP (AUTOCLAVE) composite material specimens to be produced.
TECNALIA concentrated its effort on for material acceptance/processability control via DSC and DMA for CFRP-RTM route and contributed via TGA tests to Ageing Procedures for both routes. (CFRP-RTM, GFRP-AUTOCLAVE).
UOP worked for material acceptance/processability control via DSC for GFRP-AUTOCLAVE route and performed all ageing procedures and respective tests.
When the manufacturing processes were conclusively defined & the testing was finalized for all different materials. The analysis and the review of the results provided the background for the selection of one material per route.
• For LTS-GFRP-AUTOCLAVE route 1 prepreg material was selected.
• For EC-CFRP-RTM route 1 resin system.


WP3 Manufacturing Process Development
Task 3.1 Preliminary investigations and Selection of the baseline process
Task 3.1 was mainly focused on process investigations towards the production of materials for Task 2.3. UOP worked with Glass fabric cyanate ester pre-pregs and established a production process using AUTOCLAVE technique. TECNALIA worked with Carbon fabrics and cyanate ester resin systems and established a production process using RTM technique. Work involved process optimization of 3 different material systems in the case of AUTOCLAVE (UOP) and 2 different material systems in the case of RTM (TEC). Task 3.1 finished with the delivery of test specimens (a) CFRP-RTM, (b) GFRP-AUTOCLAVE for Task 2.3 within reporting period
Task 3.2 Process optimization and Manufacturing of specimens for Degradation Studies
Objective of Task 3.2 was defined to be the optimization of both manufacturing process (GFRP-PREPREG/AUTOCLAVE, CFRP-CE RESIN/RTM) selected in order to produce the specimens need for the degradation studies envisioned in WP4. Under this framework TECNALIA worked heavily to optimize the cure cycle towards minimization of volatiles for CFRP-CE RESIN/RTM route based on task 3.1 conclusions. Based on the task 3.1 conclusions, a new baseline process that uses catalyst was established after experimentation using DSC, TGA, and Viscosity tests. Preliminary conclusions are the new baseline process that includes catalyst / cure cycle seem appropriate. Results showed that whereas with the addition of catalyst and the new baseline cure cycle proposed in task 3.1 the curing time was reduced 15% and there was an energy saving of 16%, thanks to the optimization of the cure cycle the curing time could be further reduced up to 51% and a 33% of energy saving. Also, with the new proposed optimized cure cycle, a higher degree of cure is obtained in the post-curing and consequently a higher final Tg. Moreover, in the first dwell, the reaction rate is lower than in the baseline one, reducing the risk of exo-therm.
For GFRP-PREPREG/AUTOCLAVE route the optimization of the process is limited due to the work performed in Task 3.1. UoP in Task 3.2 will be focused on cure cycle and degree of curing control in communication with Liebherr after the finish of Task 2.3 where the selection of one pre-preg material will take place.
Task 3.3 Testing and selection of the optimum process parameters
Objective of Task 3.3 was defined to define and recommend optimum processing methodologies for both production routes. Task 3.3 will be based on material selection in Task 2.3 and process development and optimization in Task 3.2. Focus will be given mainly to RTM production route due to its complexity. Taking into account a quality assurance based testing (eg. Micro-structural analysis, Fibre and void volume content, ILSS, Compression strength, DMA.) The optimum process parameters will be identified and document in D3.3 (Development of manufacturing process for composite specimens). Coupons have been manufactured with the selected optimized process parameters and tested. Thermal, mechanical, as well as micro-structural analysis results have allowed validating the optimized process. The laminates showed a good quality and no voids or porosity areas were observed. Mechanical properties were also equivalent to the baseline process.
Final degree of cure and Tg achieved where higher than the baseline process as expected and in accordance with the with the kinetic and Tg models predictions. Energy consumption was also measured during the process, confirming the 33% energy savings predictions.
WP4 Degradation Behavior Study
Task 4.1 Methodologies Development
Within this Task, UOP taking into account the operational service conditions of the communicated applications of the ITD members started the establishment of a comprehensive methodology for Degradation Behavior Study. The comprehensive methodology for an ageing study is based on the accelerated methods principles in order to investigate and understand the various degradation mechanisms of the cyanate ester matrix composites that occur at high service temperature environments.
The goal of an Accelerated Ageing Protocol Study (AAPS) is
• Determination of the dominant Environmental Degradation Factor (EDF) among the operating environmental degradation conditions of the application.
• Determination of the Critical Degradation Mechanisms (CDM) among the possible active degradation mechanisms under the operating degradation conditions.
Using data from testing in Task 2.3 the dominant environmental condition for both cases was determined with the data available up to that time:
• CFRP →thermo-oxidation
• GFRP →hydrothermal
Two (2) different protocols, for long-term exposure study were elaborated:
• At elevated temperatures for CFRP materials produced by RTM technique at TECNALIA and
• At elevated temperatures in humid atmosphere for GFRP materials produced by PREPREG/AUTOCLAVE technique.
Task 4.2 Testing and Reporting
In this task the actual ageing and respective testing & analysis was covered. The significant and significant observations of the 2 different protocols are presented below:
THERMO-OXIDATION-CFRP-RTM
• Until the 14days of exposure the material retains a stably descending degradation of its properties under both environments.
• The material is considered almost totally degraded after 21days at both environments.
• All the degradation mechanisms (DMs) have the same onset time at 7days. Nevertheless their contribution varies during exposure.
HYDRO-THERMAL-GFRP-PREPREG/AUTOCLAVE
• The PLASTICIZATION is activated at very early stages of ageing (~4days).
• The maintenance of ILSS, the stable behavior of the rest mechanical response and the slight decrease of Tg until 39days (absorption) in combination with slightly higher weight gain rate declare:
o The acceleration of diffusion rate due to higher temperature,
o The HYDROLYSIS events initiation at earlier stages and
o The onset of micro-cavities nucleation (OSMOSIS onset) formed by HYDROLYSIS products.
• The deterioration of the mechanical properties and the Tg reduction at 61 days in combination with the formation of cavities and the absence of any superficial micro-cracking confirm:
• The superficial HYDROLYSIS events
• No osmotic- microcracking propagation.
WP5 Compliance & Optimization Study
Task 5.1 Compliance Study
Taking into account the work performed in WP2 & WP4 and given the technical requirements by the topic manager (EC & LTS) the aim of Task 5.1 was to try to define and address some critical parameters that play important role in degradation environments under study.
The main objective of EC application scenario (CFRP/RTM technique) was to answer the following questions:
• General observations about the DMs?
• Which is the most critical DM?
• Which of the operational conditions are the most critical? (determination of the dominant environmental degradation factor (EDF))
• Material behavior after the ageing? Durability/usability of the material during ageing?
• How the microstructure characteristics (manufacturing) of the material can affect its degradation?
Regarding LTS application scenario (GFRP/PREPREG/AUTOCLAVE technique) the following questions were set:
• General observations about the DMs?
• Which is the most critical DM?
• Which of the operational conditions are the most critical? (determination of the dominant EDF)
• Material behavior after the ageing? Durability / usability of the material during ageing?
• Is an assessment of life time of the part possible under the worse conditions?
• Is possible an estimation of material durability after long term exposure?
• How the microstructure characteristics (manufacturing) of the material can affect its degradation
Task 5.2 Material/ Process Optimization
The main goal of task 5.2 for EC application scenario was the investigation of the threshold temperature, where the CFRP material starts to degrade. The threshold temperature was determined after a comparison of the results obtained in WP2 and WP5.
In the case of GFRP material, after discussion with LTS three instead four initially planned routes were decided:
AGEING TESTING (UoP):
a) use Time Temperature Superposition (TTS) in order to try to estimate the life time under hydrothermal conditions.
b) draft a test protocol (short & simple) based on accelerated testing principles. Three different scenarios were proposed:
b1) cyclic test as WP2 by changing the frequency from 16h to 2h.
b2) mechanical fatigue combined with temperature & humidity.
b3) a simulation of the real case
MATERIAL (UoP): the main scope of this route was to proposed, based on the literature and our experience, one way to improve materials properties. In our case, we developed the advantages of using nanoclays.
Task 5.3: Manufacturing of final composites
Task 5.3 work included the manufacturing of optimized fibre reinforced composites by RTM (resin transfer moulding process) corresponding to the task T5.3 “Manufacturing of final composites”. It is also included the Life Cycle Analysis of the optimized RTM process.

Potential Impact:
SPECIMEN project addresses the topic (JTI-CS-2010-5-ECO-01-010) that belongs to the Area 01: EDA (Eco-Design for Airframe) of the Eco-Design Clean Sky Integrated Technology Demonstrator (ITD).
SPECIMEN project aims to provide CSJU the technical guidelines on the use of cyanate ester based composites in harsh environments such as elevated temperature and humidity. These guidelines shall be part of roadmap towards the replacement of aluminum alloys by continuous fiber reinforced polymers on parts used for aircraft and rotorcraft components subjected to elevated temperature and moisture atmosphere. SPECIMEN project in order to synthesize these guidelines will try to fill the missing gap of knowledge that concerns the effects of long term exposure at elevated temperature (200-250oC), as well as in a combined moisture/temperature atmosphere, on the cyanate ester matrix
composite properties.
SPECIMEN project results will give CSJU the tools for resolving the technical challenge of substitution of metallic parts with parts made by cyanate ester based composites for components subjected to harsh thermal or thermal/humid environments. The deeper understanding of the degradation cyanate ester based composites in harsh environments using advanced material screening/testing procedures and accompanied with optimized
manufacturing protocols will clarify issues concerning the inputs, outputs and nuisances during all three phases of aircraft life. (Design& Production, Use & Maintenance and Withdrawal)

List of Websites:
Name, title and organisation of the scientific representative of the project's coordinator :
Professor Vassilis Kostopoulos
Director of Applied Mechanics Laboratory
Dpt of Mechanical Engineering and Aeronautics
University of Patras-Greece
Tel: +302610969441
Fax:+302610969417
E-mail: kostopoulos@mech.upatras.gr