Enhanced Polyisocyanurate Foams for Metal Faced Panels

Transcription

1 Enhanced Polyisocyanurate Foams for Metal Faced Panels GIUSEPPE VAIRO Dow Italia Srl Via Carpi, Correggio - ITALY LUIGI BERTUCELLI Dow Italia Srl Via Carpi, Correggio - ITALY LUIGI PELLACANI Dow Italia Srl Via Carpi, Correggio - ITALY PAOLO GOLINI Dow Italia Srl Via Carpi, Correggio - ITALY LUCA LOTTI Dow Italia Srl Via Carpi, Correggio - ITALY ABSTRACT Rigid Faced Double Belt Lamination (RF-DBL) is a term used to identify a continuous manufacturing process for producing steel faced foam cored sandwich panels for use as insulating elements in building applications. This type of construction product may need to be in compliance with stringent combustibility standards, which is why polyisocyanurate modified polyurethane foams are increasingly preferred to their polyurethane based counterparts. To meet this growing demand, Dow has developed and is introducing in the market a family of polyisocyanurate foam systems under the VORATHERM trademark, that are designed to meet specific customer requirements in terms of foam physical properties, processability, and flammability, working with different blowing agents. In this study, we discuss some of the characterizing structure-property relationship features in polyisocyanurate sandwich technology, including fire resistance. The latter is typically measuring the duration for which a passive fire protection system can withstand a standard fire endurance test. Polyisocyanurate foam, thanks to its intrinsically higher thermal stability, is particularly suitable to meeting fire resistance requirements: we report here some results obtained on panels featuring Dow VORATHERM polyisocyanurate foam in the core layer. INTRODUCTION Polyisocyanurate (PIR) foams represent the system of choice to produce metal-faced sandwich panels accordingly to Rigid Faced Double Belt Lamination (RF-DBL) process, providing energy efficiency and compliance to severe fire safety requirements. Such panels are used for large industrial buildings for production and storage, refrigerated buildings and food processing factories, office blocks, schools, etc. [1] PIR can deliver the same or better thermal insulation (R value) than polyurethane (PUR) foams, with better thermal stability and fire behavior. In fact PUR and PIR are organics and therefore combustible materials, and there may be some concerns in the case of fire. PIR foams exhibit better fire properties than PUR ones, due to the presence of isocyanurate rings, coming from the cyclotrimerization reaction of isocyanates. The higher the isocyanate excess (expressed as isocyanate index), the higher the relative concentration of isocyanurate rings to urethane and/or urea bonds in the polymeric foam backbone, the better the fire performance will be. This is the practical outcome of the higher bond energy associated to quasi-aromatic isocyanurate trimer structure vs. urethane one. Thermal stability of isocyanurate structure vs. urethane linkage has been evaluated in many fundamental studies; worth to note among the others a study from Kordomenos reported in a review by Chattopadhyay and

2 Webster [2], where decomposition of model compounds demonstrated that isocyanurates decompose in the range C, while urethanes start to decompose at 260 C. In previous publications [2, 3] it was outlined the versatility of PIR systems to work with a broad spectrum of blowing agents, by delivering the expected properties thanks to proper re-formulation development. The impact of the blowing agent on processing parameters, such as reactivity profile and free rise density is negligible, as well as the difference obtained in terms of mechanical properties and k-factor. This paper describes the range of properties achievable with the VORATHERM family of isocyanurate-modified polyurethane, showcasing reaction-to-fire and resistance-to-fire, mechanical and thermal insulation properties, as well as processing features. Fire reaction and fire resistance are the two base concepts for passive prevention systems to protect people and goods from the effects of a fire in a building. The requirement depends on building type and specific end use application. Fire reaction is about products behavior in the early stages of a fire, from ignition to flashover; it considers ignitability, spread of flame, heat release (total and rate), generation of smoke and the formation of burning droplets or particles. Fire resistance is about the capability of a building element (e.g. wall assembly of sandwich panels) to keep its characteristics of structural stability, integrity and thermal protection. Table 1. List of main fire reaction and fire resistance tests Test Type ASTM E 84 Fire reaction Euroclass SBI Fire reaction FM 4880 Fire reaction LPS 1181 Fire reaction EN /4/5 Fire resistance LPS 1208 Fire resistance A fire resistance rating typically means the duration for which a passive fire protection systems or elements can withstand a standardized fire endurance test. Insulated metal panels are typically tested as passive protection systems as walls, ceiling and roofs. In order to guide the development towards improving the overall fire performance behavior of a metal faced panel, a number of characterizing methodologies and lab tests have been combined such as cone calorimeter, smoke chamber, thermal gravimetric analysis (TGA) and fire resistance orientation tests. The final fire performance has been assessed using the test methods prescribed for compliance with regulations and/or insurance companies standards. EXPERIMENTAL Sample Preparation Samples were obtained either from industrial production of RF-DBL panel either from lab-scale prototyping experiments. Laboratory samples were produced by high pressure impingement mixing type of foam machinery, operating at constant processing parameters. Reacting mixture was poured into horizontal 70x40x10 cm mold, heated at C, with steel facers attached to the bottom and the lid. Table 2. Reaction characteristics and process parameters of VORATHERM PIR systems Property Low Index Medium High Index Index Isocyanate Index >3 Average trimer concentration (%) >42 Cream Time (s) 5-10 Gel Time (s) Typical molded density (kg/m 3 ) Temperature of the conveyor ( C)

3 Isocyanate excess is expresses as isocyanate index: index>1 corresponds to NCO/OH ratio higher than stoichiometric. Table 2 describes the reaction characteristics and process parameters of three families of VORATHERM isocyanurate modified foams: low, medium and high index. Trimer concentration is a calculated value expressed as % weight of PIR modified foam on the total, considering the excess of isocyanate will quantitatively lead to isocyanurate. Reactivity parameters (cream and gel time), as well as applied density are indicative values and refer to full scale production lines. All the systems referenced in this paper use hydrocarbons as physical blowing agent. Test Methods Insulated metal panels, for use as building elements, must comply with industry standards. For the purpose of this study a number of mechanical and thermal properties have been evaluated as well as reaction to fire and fire resistance tests. The resistance to fire is evaluated by submitting the insulated metal panels, to a temperature-time curve, simulating a full scale fire in a room. Criteria to be met are the following: - load-bearing capacity, identified with letter R - integrity, identified as E - insulation (temperature rise on cold side of the panel), identified as I Non load-bearing elements have only the EI rating. As reported in Table 1, fire resistance can be assessed accordingly to different standards. The European norm EN 1363 test equipment consists in a 3x3 m furnace, in which thermocouples are placed in order to have a controlled temperature vs. time curve (max 1100 C), as stated by the norm [5]. Furnace Temperature vs. Time T ( C) T = log 10 (8 t+1) t (min) Figure 1 Furnace temperature vs. time curve, accordingly to EN 1363 fire resistance test. Steel faced sandwich panels are placed on the furnace opening, and thermocouples are located on the non-exposed facing. Typically three panels need to be jointed together to cover the 3x3 meter surface. Failure criteria for insulation (I index) is that average temperature of the non exposed facing will not exceed an increase of 140 C over room temperature. As far as concern integrity (E index), the criteria are: no persistent flames on external facing, no cracks greater than a certain dimension, no ignition of a cotton layer in contact with the non exposed facing. Orientation tests have been carried out on smaller panels (50x50 cm) placed on a multi-opening frame, using a standard furnace and standard temperature-time profile. Results of fire resistance obtained in orientation tests or full scale are clearly indicated in the figure and/or tables. Mechanical and thermal insulation properties have been tested accordingly to the standards methods reported in Table 3.

4 Table 3. Physical-mechanical properties and test methods for the characterization of the panels and polyisocyanurate foams Property Test Method Density EN 1602 k-factor EN Compressive Strength ISO 604 Tensile Bond Strength Internal Dow Method Tensile bond strength measures the bond between the foam and metal facing and represents a very important parameter to predict the integrity and durability of the panel once in use. The fire reaction properties and thermal stability have been evaluated by characterizing the foams with cone calorimeter, smoke chamber, and TGA (see Table 4). The cone calorimeter equipment and methodology are described in ISO 5660: a foam specimen, 100 x 100 mm (50 mm thick), is exposed at constant radiant flux of 50 kw/m 2 and a spark plug positioned over the specimen ignites flammable gases produced. The effluent is collected in a hood and then through a duct, where smoke and oxygen consumption are measured. In addition, the weight of the specimen is recorded vs. time. The outcome of the test is a measurement of time to ignition, heat release, smoke production and weight loss. Table 4. Techniques and test methods used for the thermal characterization of the polyisocyanurate foams TGA Lyon method Cone Calorimeter ISO Smoke Chamber ASTM E-662 TGA experiments have been performed to characterize foams according to Lyon method. Lyon developed a model for charring efficiency from TGA data [6]. The char is the carbonaceous material formed during thermal degradation or pyrolysis. It is common opinion that high char yield obtained during TGA experiments is a desirable attribute for improved fire behavior. In fact char provides a thermally insulating layer or barrier at the surface of the burning polymer, reduces heat and mass transmission into the material and provides good insulation to the underlying combustible polymer from the heat and fire. The char formation at the surface often produces a foamed carbonaceous matrix in a process known as intumescence. Modeling of TGA experiments allows the determination of kinetic parameters of charring: 1 Y Ag E ln ln Yc A c Ec R T c g 1 (1) Where Y c is the yield of charring, A g and A c are respectively the pre-exponential factor of gas and char formation, and similarly E g and E c are the activation energies. An important parameter obtainable from the model is the so called cross-over temperature: the temperature at which the gas formation and charring rate are equal; at higher T the gas will be formed quickly than char. Isothermal TGA experiments have been conducted at with a TGA Q500 instrument (TA Instruments-Waters SpA), according to a methodology Heat and Hold, under nitrogen stream, with initial heating at 200 C/min and at 4 different temperatures ( C). Specimens have been taken from the core of the foam. RESULTS AND DISCUSSIONS As pointed out in the Introduction section, isocyanurate concentration is a key parameter for improving the inherent thermal stability and fire performance of an isocyanate based polymer. High isocyanate index formulations, leading to high trimer concentrations are the preferred route to the development of foams suitable to comply with severe fire coding, either reaction-to-fire (Euroclass B, FM 4880, LPS 1181) either resistance-to-fire (e.g. EN ).

5 It is important to be noticed that a sandwich panel, and more specially a sandwich panel assembly, is a structure made where the insulating foam in only one of the components. Considering that when panel assembly is tested for fire resistance, three panels are jointed together, the joint design can play an important role. Even if optimization of joints is outside the purpose of this paper, it is worth to outline what is the extent of joint effect on fire resistance test, when comparing panels obtained with the same insulation foam. Figure 2 shows data from an orientation test on model systems, where panels of same thickness and prepared with the same PIR foam formulation lead to a different performance depending on the joint type. Insulation Endurance vs. Joint Type min Type 1 Type 2 Type 3 Non Jointed Joint Type Figure 2 Fire resistance insulation (I) endurance vs. joint type. Data obtained from orientation test, with the same PIR model foam

6 Insulation Endurance vs. Panel Thickness min Thickness (mm) Figure 3 Fire resistance insulation (I) endurance vs. panel thickness. Data obtained from full scale test, with the same joint type. Considering how insulation and integrity are measured during the fire resistance test, it is intuitive that the performance of the panel will be proportional to the panel thickness. Experimental results obtained either in orientation tests either in real scale ones confirm this statement. For example, Figure 3 shows data obtained in real scale fire resistance test, on panel with variable thickness and same joint type and insulation foam. Researchers have since long tried to correlate polymer chemical and physical structure with combustion behavior, in other words the intrinsic properties to extrinsic behavior; even a more complex task is to develop tools to predict the behavior of composite structure such as metal faced sandwich panels. Reaction-to-fire As above mentioned fire reaction is about products behavior in the early stages of a fire, from ignition to flashover. Lab test methods, commonly used to guide the development, are the cone calorimeter and NBS smoke chamber. In consideration of the measurement of heat release in high ventilation conditions, the cone calorimeter has been found very useful to support the development towards severe Euroclass classifications [7], such as Bs2d0. The NBS smoke chamber is a key tool for screening formulations aiming to match the smoke density level needed for class I according to ASTM E 84. Table 5 shows cone calorimeter data of foams taken from real scale panels referenced in Table 2, selected across a broad range of isocyanate index. Table 5. Cone calorimeter and Smoke Chamber characterization of some VORATHERM PIR foams Property Low Index Medium Index High Index Isocyanate Index Cone Calorimeter Heat release, total (MJ/m 2 ) Heat release, rate peak (kww/m 2 ) Total smoke produced in the first 200 s (m 2 ) NBS Smoke Chamber Avg D s max Avg wt loss (%)

7 Table 6. Reaction-to-fire and Resistance-to-fire certifications achieved with VORATHERM PIR foams Isocyanate Euroclass ASTM E-84 FM Index Bs2d0 Class I 4880 LPS 1181 EI 2.0 Pass 60 mm Pass Pass Pass 60 mm Pass Pass mm 200 mm Heat release data correlate well with isocyanate index, demonstrating the beneficial effect of isocyanurate stable structure for fire performance. In the case of smoke opacity, it seems that isocyanate index is not the only factor playing a role; other formulation variables (i.e. polyol backbone and fire retardant additives) have to be taken in consideration to minimize smoke density allowing matching with most severe rating. Table 6 reports general classification, reaction and resistance-to-fire, achievable with VORATHERM PIR low, medium and high index. Euroclass Bs2d0 is attainable across a broad range of isocyanate index. ASTM E-84 Class I and FM 4880 need an index in the range (in line with expectations from cone calorimeter and NBS smoke chamber reported in Table 5), while the more severe large scale test LPS 1181 can be best matched with very high isocyanate index formulations. Resistance-to-fire will be discussed in the next section. Resistance-to-fire In an effort to investigate laboratory tools to guide the development for resistance-to-fire, we explored the use of the Lyon method involving isothermal TGA performed at different temperatures. Tables and figures below report Lyon characterization referred to the same PIR foams referenced in Table 2. Figure 4 Example of a medium index VORATHERM PIR System: Isothermal TGA at different temperatures

8 Figure 5 Isothermal TGA at 280 C for different low, medium and high index VORATHERM PIR systems Figure 6 Isothermal TGA at 420 C for different low, medium and high index VORATHERM PIR systems Isothermal TGA curves for low and medium index systems seems to be quite similar at 280 C, while at 420 C, the differences among the two is amplified. On the other hand high isocyanate index foams show much slower decay than low and medium at both temperatures.

9 Table 7. Kinetic parameters from Lyon method Property Low Index Medium Index High Index Isocyanate Index k 420 C (1/s) Y 420 C T cross over ( C) Table 7 summarizes some of the kinetic parameters obtained from Lyon method modeling. k p is the kinetic constant of the pyrolysis reaction, calculated by the model: lower values are associated to better thermal stability. Y c is the char yield. Another important parameter obtainable from the model is the so called cross-over temperature: the temperature at which the gas formation and charring rate are equal; at higher T the gas will be formed faster than char. The higher the cross-over temperature, the higher the propensity of the polymer to decompose to a char: in general we can hypothesize that the higher cross-over temperature the higher the insulation and integrity parameters in a fire resistance test will be. The char yield and cross-over temperature are strongly affected by isocyanate index, indicating the important role played by isocyanurate concentration, leading to a distinct improvement for isocyanate index in the range above 3. This is aligned with the fire resistance results obtained on industrial panels, reported in Table 6. Mechanical Properties, Thermal Insulation and Process-ability All the data generated and summarized in tables 5-7 reinforce the known beneficial effects of isocyanate trimer content towards the increasing of the thermal stability and fire behavior of PIR-PUR foams. This study is also about showcasing the ability of VORATHERM foams to match the fire performance with mechanical, thermal insulation properties and processability. Table 8 reports examples of mechanical and thermal insulation properties of systems referenced in Table 2: proper formulations make it possible to keep properties suitable for the production of top quality RF-DBL panels, across the range of isocyanate index. The newly developed low index VORATHERM, has demonstrated to deliver severe reaction-to-fire classification, as well as EI 15 resistance-to-fire, not easily achievable with PUR. Moreover low index VORATHERM PIR systems can be processed at the typical conveyor temperature used for PUR panels manufacturing (Table 2), offering a valuable attribute for the panel manufacturers. In addition, the development of VORATHERM foams, and in particular the last studies made on the low isocyanate index family, led to minimization of halogenated flame retardants, indicating the route for complete elimination for the next step of the development. Table 8. Examples of properties achievable with VORATHERM PIR systems Medium High Property Low Index Index Index Index Core Density (kg/m 3 ) Compressive strength thickness (kpa) Tensile Bond Strength (kpa) C (W/m K)

10 CONCLUSIONS The attempt of correlating intrinsic properties of PIR foam materials with extrinsic behavior of sandwich panels led to the confirmation of utility of using cone calorimeter and NBS smoke chamber as developmental tool for reaction-to-fire classifications. The isothermal TGA Lyon method seems to be a useful tool to indicate the direction of improving resistance-to-fire performance. A variety of VORATHERM PIR tailored formulations have been developed to match severe fire coding in compliance with regulations and insurance standards. EI 15, 30 and 60 rating have been achieved with respectively 60, 100 and 200 mm panel thickness, as well as demanding fire reaction certifications such as Euroclass Bs2d0, FM 4880 and LPS On the top of fire performance, Dow VORATHERM family of products exhibit balanced process-ability and good mechanical and thermal insulation properties. Within the family of VORATHERM PIR, the low index range is of particular interest, being developed with the scope of offering a PUR like processing, coupled with the possibility to achieve EI 15 fire resistance rating, even with minimizing the use of halogenated additives aiming to halogen free compositions. REFERENCES 1. ISOPA Fact Sheet 2. Chattopadhyay, D.K. Webster D.C.; Thermal stability and flame retardancy of polyurethanes - Progress in Polymer Science 34 (2009) Volz, T; Skowronski, M.; Comparison of Blowing Agent Performance in Polyisocyanurate Foams used in the Production of Rigid Faced Continuous Panels CPI Pellacani, L.; Pignagnoli, F. High Energy Saving PUR&PIR Solutions for Construction Metal-Faced Sandwich Insulated Panels, Meeting European Regulation Utech EN Fire Resistance Tests-Part 1: General Requirements 6. Lyon, RE; Pyrolysis kinetics of char forming polymers - Polymer Degradation and Stability 61 (1998) Golini, P. et al. - Advances in polyurethane and polyisocyanurate solutions for the metal faced insulating panels industry- Utech 2006 Giuseppe Vairo Giuseppe Vairo is Global Technology Leader for Polyurethane Rigid Foams of Dow Formulated Systems and he is located in Correggio, Italy. He joined Dow in 2001 through the acquisition of polyurethane division of EniChem, and before his current position worked as R&D specialist for footwear and rigid foam polyurethane technologies. Giuseppe holds a MS in Industrial Chemistry from University Federico II of Naples (Italy, 1996). Luigi Pellacani Luigi Pellacani is based in the R&D Center of Excellence of Dow Formulated Systems located in Correggio, Italy, where he is responsible for development activities in the Construction application segment. He joined Dow in 1987 in the Technical Service & Development department for flexible and SRIS PU foam applications. Before being appointed to his current position he held different roles in other Dow locations, namely Terneuzen (The Netherlands), Ahlen (Germany) and Erstein, France. Luigi holds a degree in Organic chemistry from the University of Modena.

11 Luigi Bertucelli Luigi Bertucelli is Research Leader for Dow Formulated Systems and is located in Correggio (Italy). He joined Corradini Poliuretani in 1981 and later on Dow in 1983, with the acquisition of the company. He has been working mainly in the developments of polyurethane rigid foams and composites. Luigi holds a degree in Industrial Chemistry from the University of Parma (Italy, 1978). Paolo Golini Paolo Golini is Senior Development Specialist for Polyurethane Rigid Foams of Dow Formulated Systems and he is located in Correggio, Italy. He started his career at EniChem in the corporate R&D, focusing on organic chemistry catalysis for the production of intermediates for different industrial fields. In 1994 he moved to the Polyurethane Division, in R&D and TSD positions for rigid foams and in 2001 joined Dow in the current position. Paolo holds a MS degree in Chemistry and Pharmaceutical Technology from University of Milan (Italy, 1985) Paolo is co-author of several patents and papers for rigid foams technologies regarding flammability behavior and blowing agents. Luca Lotti Luca Lotti is a Polymer Chemist in the Italian Systems House of The Dow Chemical Company located in Correggio (RE), Italy. He holds an Italian MS in Inorganic Chemistry from the University of Pisa, and a Ph.D. degree from the Scuola Normale Superiore of Pisa. He has authored several scientific publications and contributions to polymer-related congresses. His responsibilities include the development of innovative fire-rated rigid polyurethanepolyisocyanurate foams for the production of insulated metal panels..