BENZOXAZINE CHEMISTRY: A NEW MATERIAL TO MEET FIRE RETARDANT CHALLENGES OF AEROSPACE INTERIORS APPLICATIONS

Transcription

1 BEZXAZIE CHEMISTY: A EW MATEIAL T MEET FIE ETADAT CHALLEGES F AESPACE ITEIS APPLICATIS Panayota Tsotra, Frans Setiabudi, Ulrich Weidmann Huntsman Advanced Material GmbH, CH-4057, Basel Switzerland ABSTACT In the current paper, a new class of polymers is presented as an alternative to currently commercially available polymers for the aerospace industry and more specifically for interior applications. The requirements in order to compete with the current systems used for such kind of applications are the fire resistance properties and the impovement processability over existing polymers through the alternative chemistry. 1. ITDUCTI The trend to migrate from brominated F-4 to halogen free systems in printed circuit boards (PCB) for environmental reasons is gaining considerable attentions especially in Japan and Europe where electronic wastes are typically incinerated due to landfill restriction. The generation of Dioxin, possibly resulting from inappropriate incineration conditions, is still remaining a major concern. Different types of flame retardant chemistry such as phosphorous, nitrogen, metal hydrate /oxide have been explored to substitute halogen. ecently benzoxazine chemistry has been investigated extensively and successfully introduced into the halogen free PCB industry. Although benzoxazines were first synthesized in 1940s [1], their potential has been recognised only recently [2]. s are the reaction products of amine, phenol and formaldehyde (Figure 1). It is widely known that the benzoxazine ring is stable at room temperature for a long period of time, starts to homopolymerize to form a high molecular weight oligomer or polymer at elevated temperature, and generates both hydroxyl and tetriary amine groups, as shown in Figure 2. Depending upon the backbone chemistry these systems can provide polymers with high glass transition temperature (Tg), low water absorption, excellent physical electrical performance along with excellent fire resistance properties. Phenol H H 2 Formaldehyde H H 2 Amine Figure 1: Example reaction scheme

2 ` ` ` ` H H ` H H n ` Figure 2: Curing mechanism of benzoxazine. ` The flammability characteristics of benzoxazines are very similar to the phenolic resins due to the formation of a comparable network created after polymerization. chemistry is a candidate of choice for industries interested in replacing phenolic resins because of toxicological issues in the workshops before or upon curing along with very good fire retardant properties. Aerospace industry uses huge quantities of phenolic pepregs to manufacture semi-structural interior parts which must satisfy the more stringent fire smoke and toxicity (FST) regulations. This work gives an overview of the recent developments in benzoxazine chemistry at Huntsman. The physical properties of the different building blocks are reviewed and their advantages and limitations are discussed. Moreover, preliminary results comparing the FST properties of a benzoxazine prepreg to an aerospace qualified, stateof-the-art, phenolic prepreg are presented. 2. EXPEIMETAL In Table 1 the available types of benzonazines resins in the Huntsman portfolio are presented. All of them are solid resins with melting points staring at C. XU3560 CH is a bisphenol A benzoxazine resin with a low menting point and viscosity of mpa.s at 125 C. LMB6493 is a bisphenol F benzoxazine resin which exhibits as well low melting point and viscosity at 125 C. The third type is a phenolphthaleine benzoxazine (patented by Hunstman, W A1) and has a higher melting point and viscosity compared to the two previous types of benzoxazines. In the following paragraphs the properties of some of these products are presented as neat resins and also as blends with epoxy resins.

3 Table 1: resins (Huntsman s product range). Product XU3560 CH LMB6493 LMB6490 Bis A Benzoxanzine Bis F Benzoxanzine Phenolphthaleine Appearance Yellow chunks Yellow chunks Yellow chunks Softening point C C C Melting Point C C C 125 C, cps CH 3 CH 3 C H 2 4. ESULTS AD DISCUSSI In Table 2 the properties of the cured bisphenol A-benzoxazine resin, XU3560 CH, are presented. The curing cycle followed was: 2hr at 180 C and 2hr at 200 C. The homopolymerization of the XU3560 CH leads to a thermoset system with high dry and wet Tg. This chracteristic is very important for the aerospace indursty where the requirement for systems with high wet Tg has become very important. Moreover, the resin shows high mechanical performance in flexural, tensile and compression loading conditions. For comparisson, the tensile modulus of a multifunctional resin cured with 4.4 Diaminodiphenyl sulphone, DDS (e.g. Araldite MY721/Aradur 976-1, Huntsman) is in the range of 3.5 GPa. s can be also used as blends with epoxy resins. The oligomer, formed during the homopolymerization reactions, can act as a hardener for epoxy resins to form a thermosetting resin without releasing volatiles, as shown in Figure 3. In Table 3 the properties of a multifuctional resin (Araldite MY720) cured with 4.4 Diaminodiphenyl sulphone, DDS (Aradur 976-1) are compared with the properties of a benzoxazine (XU3560 CH)/epoxy resin (Araldite CY179) blend. All the materials are manufactured by Hunstman. Both systems exhibit high Tg (> 200ºC) and a decrease of 24 and 20% of this property, respectively, in wet conditions. The benzoxazine/epoxy blend shows a significantly lower water absorption.

4 Table 2: Properties of the cured bisphenol A-benzoxazine resin, XU3560 CH. Tg [ C], DSC 171 Dry Wet* Tg [ C], DMA Flexural Modulus at T [MPa] 4602 Flexural Strength at T [MPa] 132 Tensile Modulus at T [MPa] 5334 Tensile Strength at T [MPa] 31 Elongation [%] 1.2 Compression Modulus at T [MPa] 3505 Compression Strength at T [MPa] 228 Elongation [%] 8.3 Fracture Toughness, K 1c [MPa.m 1/2 ] 0.94 Fracture Energy, G 1c [J/m 2 ] 168 * 48hr in boiling water H H Cat. H H H H n 1 " H 1 " H 1 " n Thermosetting resin Figure 3: eaction scheme of benzoxazines with epoxy resins.

5 Table 3: Comparisson of the properties of a cured multifuctional epoxy resin based system with the properties of a cured benzoxazine/epoxy system. Araldite MY 720 / Aradur XU3560 CH / Araldite CY179 Mixing ratio per weight: 75/25 Curing Cycle 2hr at 100ºC, 6hr at 177ºC, 5hr at 200ºC 2hr at 180ºC, 2hr at 200ºC Dry Wet* Dry Wet* Tg [ C], DMA Water pick-up [%] 48hr in boiling water Flexular, Modulus [MPa] Strength [MPa] Elongation [%] Tensile, Modulus [MPa] Strength [MPa] Elongation [%] Compression, Modulus [MPa] Strength [MPa] Elongation,% Fracture Energy, G 1c [J/m 2 ] * 48hr in boiling water ne of the main disadvantages of the benzoxazines is the high temperature needed for their polymerization. Systematic studies are made currently at Hunstman in order to determine the appropriate catalysts for decreasing the curing temperatures of the benzoxazines. In Figure 4, the significant decrease in the pick temperature of exothermic reaction of a benzoxazine/epoxy blend is shown by the use of catalyst (from 250ºC to around 215ºC).

6 35 30 b) Heat Flow [W/g] c) a) Temperature [ C] Figure 4: DSC curves showing the reactivity of a) a benzoxazine/epoxy blend, b) multifuctional/dds and c) a catalysed benzoxazine/epoxy blend. As it was mentioned in the previous paragraphs, benzoxazines are of great interest for the aerospace industry due to their inherent flame retardant properties. The flame, smoke and toxicity (FST) properties of two benzoxazine systems developed at Huntsman were tested at Airbus Bremen and compared to the state-of-the-art phenolic resins used currently for interior parts. LMB6498 is based on the phenolphthaleine benzoxazine LMB6490 in solvent, while LMB6531 consists of the same benzoxazine resin in combination with a catalyst. ne layer glass fiber prepregs were prepared with each product and the results of the FTS testing are summarised in Table 4. For the flammability tests the values reported show the burn length in mm, the after flame time in seconds and the after flame time of chips in seconds. After the ignition, for 12 and 60s, the flame was removed and the self-extinguishing and dripping characteristics were recorded. LMB6498 shows flammability performance within the specifications for both 12 and 60s test. The smoke density, the toxicity and the heat release rate for both benzoxazine prepregs are below the limits.

7 Table 4: FST properties of benzoxazine based prepregs compared with state-of-the-art phenolics. Burning behaviour Test methods Units Phenolic Prepreg, 296 g/m² E-glass, 40% resin, specification LMB6498 LMB6531 Laminate Laminate Laminate 1 layer 1 layer 1 layer Flammability (12s): AITM B mm/s/s - 128/0/0 142/2/0 Flammability (60s) AITM A mm/s/s 60/0/0 144/0/0 167/0/0 Smoke Density (Flaming mode) AITM Ds ppm HC ppm C Toxicity (Flaming mode) Heat elease & ate (H / H): esin content (cured) Curing conditions AITM AITM ppm x ppm S ppm HF ppm HCl kw/m² kw*min/m² 65/40 58/34 63/33 % C, 120min, 1.0bar 150 C, 120min, 1.0bar 6. CCLUSIS The present paper gave an overview of the recent developments in benzoxazine chemistry at Huntsman. The available systems show high Tg and superior mechanical properties as neat resins or as blends with epoxy resins. A main disadvantage of the existing benzoxazines is the high curing temperature. It was demonstrated that the use of catalyst can decrease the curing temperature by 50 o C with minor effect on the final Tg. Moreover, by the flame, smoke and toxicity tests performed on benzoxazine based prepregs, it was shown that benzoxazines are very promising candidates for interior applications in the aerospace industry. Their performance is comparable to the currently used phenolic resins while their halogen-free nature gives them great advantages for this kind of applications. EFEECES 1- Holly, F.W., Cope, A.C., Condensation products of aldehydes and ketones with o-aminobezyl alcohol and o-hydroxybenzylamine, Journal of the Americal Chemistry Society, 1944;66: Liu, J., Ishida, H. In: Salamone J.C., editor A new class of phenolic resins with ring-opening polymerisation. The polymeric material encyclopedia, Frorida: CC Press;1996: