Fire and Polymers IV - American Chemical Society

Chapter 15

Navy R&D Programs for Improving the Fire Safety of Composite Materials 1

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: November 24, 2005 | doi: 10.1021/bk-2006-0922.ch015

Usman Sorathia and Ignacio Perez

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Naval Surface Warfare Center, Carderock Division, 9500 MacArthur Boulevard, West Bethesda, M D 20817 Office of Naval Research, 800 North Quincy Street, Arlington, V A 22217

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Due to their inherent characteristics, fiber reinforced plastics (FRP), also referred to as polymer matrix based composite materials (PMC), have been making steady inroads into naval military systems for the past 10-15 years. Chief among their characteristics is the stiffness to weight ratio (much better than steel or aluminum), and the resistance to chemical attack (e.g. corrosion resistance). US Navy is currently using sandwich composites in most surface ship topside applications. The sandwich composite consists of brominated vinyl ester resin with glass or carbon reinforcement and balsa wood core. The unprotected vinyl ester based sandwich composite does not meet all o f the Navy's fire performance goals for interior applications. In order to use such composites inside the ship for manned spaces, it must be protected with either passive (fire insulation) and/or active (water mist) fire protection systems. Such fire protection adds weight and cost. Navy has invested over $10M over the last 5 yrs in SBIR (Small Business Innovative Research) and STTR (Small Business Technology Transfer) programs to develop flame resistant polymers suitable for room or low temperature processing by Vacuum Assisted Resin Transfer Molding ( V A R T M ) . Such flame resistant resins could then be used to produce sandwich composites that would meet the Navy's fire growth requirements without the need for passive fire protection. In this paper, we have presented summary of some of the R & D

© 2006 American Chemical Society

In Fire and Polymers IV; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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186 programs that the Navy is pursuing to address this issue. Selected low cost screening test procedures to facilitate such development are also discussed.


Introduction The 1975 collision involving the USS KENNEDY (CV-67) and the USS BELKNAP (CG 26), and resultant fire, influenced the Navy to improve the survivability of aluminum structures through mineral wool fire insulation. Principal use of steel instead of aluminum in the deckhouse design started with theDDG51 class ship. During the past 10-15 years, Navy has experienced a resurgence of interest in the development and application of composites for both primary and secondary load-bearing structures. This growing interest in composite materials is driven by the fleet needs to reduce maintenance, save weight, increase covertness and provide affordable alternatives to metallic components with lower life cycle costs. Polymer composites are engineered materials in which the major component is fiber reinforcement (typically a fiber made out of carbon, glass or Kevlar) and the minor component is an organic resin binder (such as vinyl ester, epoxy or phenolic resin). Currently, structural composites for U.S. Navy surface ship applications are typically glass reinforced with brominated vinyl ester and balsa wood core. Some recent notable large composite applications are the Advanced Enclosed Mast/Sensor (AEM/S) System for amphibious transport dock ship LPD-17 shown in Figure 1, and topside deckhouse for multi mission surface combatant DD(X). A significant concern in shipboard application of organic matrix based composites is the possibility that an accidental (or deliberate) fire may ignite the composite material. This may result in the spread of flame on the composite surface, and also release heat and generate potentially toxic smoke. Thus, the localized incidental fire may cause a larger structural fire involving the composite, which now becomes the fuel for the growing fire. In enclosed and confined spaces, such as in ships, the growing fire can lead to a "flashover" condition. "Flashover" is a term that is used to indicate the point during a fire when the internal temperatures in the upper regions of the compartment have increased to the point where the radiant energy from the hot upper layer spontaneously ignites all combustible materials within the compartment. Typically this is on the order of 600°C. If the affected composite component is part of a primary critical structure, the structure may collapse when exposed to


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fire. Recent fire incident on the Norwegian Orkla minesweeper, where the fire started in the lift fan room, reinforces the need to select resin and core materials for composite structures with improved flammability characteristics.

Figure 1: Composite Mast-LPD-17

Fire Performance Requirements Fire Performance Requirements for Submarines. Fire aboard a submarine threatens the platform itself and must be fought independently with limited onboard resources. The use of structural applications in U.S. Navy submarines is covered by MIL-STD-2031 [1]. Fire performance goals for use of composites in submarines are based on assumptions that a fire should be extinguished or brought under control within 5 minutes. This military standard contains requirements for limiting oxygen index (ASTM D 2863), flame spread index (ASTM E 162), heat release rates (ASTM E 1354), smoke generation (ASTM E 662), fire gas toxicity, quarter and large scale fire tests. For the purpose of this paper, a summarized version of cone calorimeter part of the MIL-STD-2031 acceptance criteria is at 25 kW/m , Time to ignition, 300 s; PkHRR, 50 kW/m ; AHRR, 50 kW/m . At 50 kW/m , 150 s, 65 and 50 kW/m . At 75 kW/m , 90 s, 100 and 100 kW/m . At 100 kW/m ,60 s, 150 and 120 kW/m . Fire Performance Requirements for Surface Ships. Egress and fire fighting situations onboard surface ships are different than those onboard submarines. In general, fire performance goals for the use of composites in surface ships are based on assumptions that typical time available for fire fighting operations is about 30 minutes. Design Data Sheet DDS-078-1 [2] covers the fire performance requirements for the use of composites in the topside of surface ship. In most cases, fire performance goals are based on fiill-scale fire tests as shown in Table I. 2

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188 Material fire performance goals should be incorporated in conjunction with existing or additional detection, suppression, and fire-fighting systems. Fire Performance of Current Composite Material System. The US Navy is currently using sandwich composites in most surface ship topside applications. The sandwich composite consists of brominated vinyl ester resin with glass or carbon reinforcement and balsa wood core as shown in Figure 2. Vinyl ester resins are mixtures of styrene and methacrylated epoxy. Styrene has one reactive vinyl group while the vinyl ester monomer has several reactive vinyl end groups. These end groups provide cross-linking capacity and branching while styrene provides linear chain extension. The polymerization reaction proceeds by free radical chain growth. The brominated vinyl ester resin, shown in Figure 3, was selected based on its fire retardant properties, chemical resistance, cost, room temperature curing properties, and ease in large scale processing methods such as Vacuum Assisted Resin Transfer Molding (VARTM). The cone calorimeter data for sandwich composite and its components is shown in Table II. The unprotected vinyl ester based sandwich composite does not meet all of the Navy fire performance goals for interior applications. For example, in mock up room comer fire tests, the unprotected sandwich composite (critical heat flux for ignition * 15 kw/m , ignition temperature « 657K) ignites in less than 120 seconds, delaminates from balsa core at approximately 660 seconds, and exhibits total heat release rates of close to 1.0 MW shortly after the burner heat release rate was increased to 300 kW [3]. The combustible nature of vinyl ester based sandwich composite with balsa core, and its propensity to be driven to flashover by small to intermediate fires, is the major difference between the metallic (steel) and polymer matrix based composite structures. On the other hand, balsa core sandwich construction provides superior fire resistance. Fire resistance is the ability of building structures to limit the fire spread from room of fire origin to adjoining spaces, such as bulkheads and overheads, by preventing ignition of items on the non-fire side of the bulkhead (backside). 2

Figure 2: Sketch of a sandwich

composite


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Figure 3: Brominated bisphenol A epoxy vinyl ester resin

New Resin And Core Materials. The US Navy has long recognized the need for the development of new low cost resins and core materials which can be processed by VARTM and which have the mechanical characteristics comparable to vinyl esters and balsa core, but have the superior flammability characteristics comparable to phenolics. To this end, US Navy has invested over $10M towards the development of new fire restricting resins and foams over the last 5 years. Most of this investment is made through ONR SBIR/STTR as well as internal research programs. The SBIR/STTR program is designed to provide funding to stimulate technological innovation in small business to meet DoD research and development needs. This investment includes the modification of vinyl ester with fire retardants (Marquette University), development of products such as phthalonitrile by Naval Research Laboratory (NRL), modified phenolics by Texas Research Institute (TRI), epoxy and cyanate ester resins based on bisphenol-C (Shade Inc.), polyhedral oligomeric silesquioxane (POSS) based resins (Hybrid Plastics), nanoclay reinforced vinyl esters (Hydrosize Inc.), improved phenolic foams (University of Southern California, University of North Carolina), and carbon foam (Touchstone Research Laboratory) materials. A more detailed description of some of these research efforts is given in the following sections. Small Scale Screening Methodology. Most of the tests required to qualify the fire performance of composite systems for naval applications are large-scale tests. When developing new resins and core materials, it is expensive to repeatedly conduct these tests to determine the performance of the most recent design. Instead, more cost-effective small-scale testing is preferable to intermittently evaluate performance. The U.S. Navy sandwich composite uses balsa core (3.0 inch). The low density core (9.5 pcf) provides superior resistance to heat transmission which yields low temperature rise on the unexposed side during fire resistance tests using UL-1709 fire exposure curve [4]. As such, meeting the requirements for fire resistance (heat transmission) is not an issue with such sandwich composites. It is, however, the combustibility or the high heat release rates of sandwich composite and its components which result in the failure of unprotected composite system in the room comer fire test (ISO 9705). This fire test (100 kW for 10 minutes, 300 kW for 10 minutes) requires that three


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Table I. Summary of Fire Performance Goals for Composite Topside Structure Category Surface Flammability

Test Method ASTM E-84, ASTM E84, "Standard Test Method for Surface Burning Characteristics of Building Materials.

Fire Growth

ISO 9705 "Full-scale room test for surface products" Annex A, standard ignition source fire ISO 9705 "Full-scale room test for surface products" Annex A, standard ignition source fire ASTM E662, ASTM E662, Specific Optical Density of Smoke Generated by Solid Materials. Navy modified: UL 1709 fire curve for 30 minutes using IMO A.754 (18) test procedures IMO App. A. Ill &A.IV apply Maximum fire test load

Smoke Production

Smoke Toxicity

Fire Resistance and Structural Integrity Under Fire Bulkheads/ Overheads/ Decks/Doors/

Criteria Interior applications: Flame spread index : 25 max Smoke developed index: 15 max Exterior applications: Flame spread index: 25 max Smoke data for review by NAVSEA 05P4 Net Peak heat release rate less than 500 kW Net Average heat release rate less than 100 kW Peak smoke production rate less than 8.3 m /s Test average smoke production rate less than 1.4 m /s 2

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CO: 350 ppm (max); HC1: 30 ppm (max); HCN: 30 ppm (max) Fire Gas IDLH Index, IIDLH

Fire and Polymers IV - American Chemical Society

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