Michele Modesti

Expandable graphite as an intumescent flame retardant in polyisocyanurate-polyurethane foams

Abstract Flame retarded polyisocyanurate–polyurethane (FR PIR–PUR) foams have been synthesized by the use of a new flame retardant, expandable graphite (EG foams), never used on industrial scale in polyurethane rigid foams, and a mixture of expandable graphite and triethylphosphate (EG–TEP foams). FR PIR–PUR foams, blown with n -pentane, have been prepared with a constant NCO index (250) in order to achieve greater thermal stability and better fire behaviour than polyurethane foams, without significantly decreasing physical–mechanical properties. It was then investigated, as the presence of the flame retardants influences both physical–mechanical properties and fire performance. In particular, fire behaviour of the foams has been studied through cone calorimeter analysis, by the oxygen index and by the DIN 4102-B2 test. The results show that an increase in expandable graphite amount in EG foams or in triethylphosphate content in EG–TEP foams does not worsen mechanical properties. Increasing the amount of triethylphosphate does not influence the thermal conductivity, but an increase in the quantity of expandable graphite causes a worsening of insulating properties, probably due to the bigger dimensions of the foam cells. The presence of expandable graphite alone or in mixture with triethylphosphate brings an overall improvement in fire behaviour. In particular, the oxygen index (OI) increases in a linear way and the highest OI values are obtained for EG–TEP foams. The results from the cone calorimeter are in agreement with those of OI; EG and EG–TEP filled foams show a considerable decrease of rate of heat release (RHR) with respect to unfilled foams. In particular, for EG–TEP foams, the higher the triethylphosphate content the higher the RHR decrease. The only hazard observed is an increase of CO/CO 2 weight ratio in the presence of very high content (25%) of expandable graphite; this effect is not shown when increasing the TEP amount.

Halogen-free flame retardants for polymeric foams

The effectiveness of some mixtures of halogen-free flame retardants (i.e. expandable graphite, triethylphosphate and red phosphorus) in flame retardancy of polyisocyanurate-polyurethane (PIR-PUR) foams, blown with n-pentane, has been investigated by means of DIN 4102-B2 and oxygen index tests. The thermal stability and mechanical properties of PIR-PUR filled foams have also been considered. The results showed that the introduction of increasing amounts of expandable graphite, into foams containing triethylphosphate or red phosphorus, causes a significant worsening of physical-mechanical properties. The fire behaviour characterisation has demonstrated that the introduction of such flame retardants as fillers leads to a great improvement particularly for foams filled with expandable graphite and triethylphosphate. Also significant improvement has been observed in thermal stability due to the presence of flame retardants.

Improvement on fire behaviour of water blown PIR–PUR foams: use of an halogen-free flame retardant

Abstract A new flame retardant, i.e. expandable graphite (EG), has been used in polyisocyanurate–polyurethane (PIR–PUR) foams in order to improve fire behaviour of such foams. In order to obtain a completely halogen-free material, water-blown PIR–PUR foams have been prepared thus avoiding the use of hydrochlorofluorocarbons or hydrofluorocarbons. The influence of several EG amounts on physical–mechanical properties and fire performances of such foams has been analysed. The results obtained show that the use of EG affects significantly physical and mechanical properties, such as compression strength and thermal conductivity, particularly at very high EG content (25 wt.%), as it often happens in presence of fillers. The fire performances have been investigated by mean of cone calorimeter apparatus and oxygen index test; the results obtained show that the fire behaviour of PIR–PUR foams could be significantly improved by use of EG; in particular it has observed a dramatically decrease of rate of heat release even for relative low EG amount (15%).

Flame retardancy of polyisocyanurate–polyurethane foams: use of different charring agents

Abstract The influence of different charring agents on physical-mechanical properties and fire behaviour of polyisocyanurate–polyurethane (PIR–PUR) foams has been investigated. In particular the use of varying amounts of ammonium polyphosphate, melamine cyanurate and expandable graphite has been analysed; all, when involved in fire, lead to the formation of a char layer on the polymer surface, but their ways of flame retardancy are different. The results obtained show that the higher the filler content the lower the compression strength; in particular the worst results have been obtained in the presence of melamine cyanurate. Moreover, the presence of ammonium polyphosphate or melamine cyanurate causes any significant worsening on thermal conductivity, while expandable graphite leads to a quite marked increase of the thermal conductivity. The fire behaviour has been studied by means of cone calorimeter apparatus and oxygen index test; it has been observed that the best results, i.e. the lowest rate of heat release and the highest oxygen index, are achieved with expandable graphite. Also the ammonium polyphosphate brings slight improvement in fire behaviour, whereas the effect of melamine cyanurate is negligible.

An experimental method for evaluating isocyanate conversion and trimer formation in polyisocyanate–polyurethane foams

Abstract In this paper, we have developed simple though efficient test method for evaluating both isocyanurate formation and isocyanate degree of conversion using FT-IR analyses and have applied them to polyisocyanurate–polyurethane foams to verify the reliability of the results obtained and evaluate the influence of isocyanate index on both these quantities. We have also characterised the foams from a physical–mechanical point of view and demonstrated that the dimensional stability and compression strength of polymers are closely related to isocyanurate content and therefore to isocyanate index. The results obtained have clearly revealed that an increase in isocyanate index leads to an increase in trimer content and consequently to an improvement of mechanical properties; on the other hand, an increase in the isocyanate index brings about a decrease in isocyanate conversion. Moreover, since isocyanurate content and free isocyanate amount affect the fire behaviour of the foam we will apply the procedure proposed in this article to analyse the influence of both factors on the fire behaviour of such foams; the results of our research will be published in a future work.

Ceramic Foams from a Preceramic Polymer and Polyurethanes: Preparation and Morphological Investigations

Open-cell ceramic foams were obtained from a preceramic polymer (silicone resin) and blown polyurethanes. The preceramic polymer, which is crosslinked by condensation of silanol groups, was dissolved in CH2Cl2 and added to a liquid polyol containing the surfactant and the amine catalyst. Isocyanate was then added to the mixture and the foam was obtained through a twofold blowing mechanism (physical and chemical blowing). The morphology of the expanded polyurethane, which can be flexible or semirigid, characterized the final structure of the ceramic foam. The materials obtained were pyrolyzed in a nitrogen flux at temperatures of 1000–1200°C, thus allowing for the polymer-to-ceramic transformation to occur in the preceramic polymer. The ceramic foams produced in this way consisted of an amorphous silicon oxycarbide ceramic (SiOC). They presented a density ranging from 0.1 to 0.3 g/cm3. The average pore diameter ranged from 200 to 400 μm and they possessed 80 to 90% open porosity.

Influence of different flame retardants on fire behaviour of modified PIR/PUR polymers

The influence of some halogen-free flame retardants (ammonium polyphosphate or ammonium polyphosphate+melamine cyanurate) on fire behaviour of modified polyisocyanurate/polyurethane (PIR/PUR) foams, with a constant isocyanate index equal to 200, have been studied by means of a cone calorimeter. Foams were prepared from isocyanates modified with adipic acid, in order to obtain amide-modified isocyanurate foams that exhibit better thermal stability performance than polyurethane and lower friability than unmodified polyisocyanurate foams. The results obtained demonstrate that an increasing amount of filler causes a slight worsening of physical and mechanical properties. On the other hand, the cone calorimeter results show that the presence of melamine, which causes a rapid decrease of rate of heat release (RHR) and rate of weight loss, considerably improves the fire behaviour of the foams.

Silicon Oxycarbide Foams from a Silicone Preceramic Polymer and Polyurethane

Ceramic open-cell foams were obtained from a preceramic polymer (a silicone resin) and blown polyurethanes, by pyrolysis at 1200°C in nitrogen. Silicon carbide submicron powders were also added to the silicone resin to give SiOC + SiC composite foams. The morphology of the foams was dependent on the architecture of the blown polyurethanes. The crushing strength as well as the elastic modulus increased with increasing relative density, reaching values as high as 14 and 450 MPa, respectively. Some of the foams displayed an excellent thermal stability (resistance to oxidation in air and decomposition in inert atmosphere) up to elevated temperatures.

Recycling of flexible polyurethane foams with a low aromatic amine content

Abstract The glycolysis process is the basis for a controlled degradation of crosslinked polyurethane for recycling. Flexible water-blown polyurethanes are polymers with repeating urethane and urea groups. When they undergo heating in presence of glycols and selective catalysts, the reaction of these groups leads to products that are liquid at room temperature. The transesterification reaction of the urethane groups, that leads to the formation of new carbamate is faster than that of the urea groups that leads to the formation of carbamates and amines. The carbamates in turn undergo aminolysis due to the amines formed in the glycolysis of the urea groups to give insoluble products. The possibility of transforming the free aromatic amines by means of a reaction with formaldehyde into products with methylol end groups has been studied. At the same time the formation of a solid phase, mainly of urea groups, has been avoided. The use of ethylene glycol (EG) allows the process to be carried out with high polymer/glycol ratio (up to 4:1) even if a biphase product is obtained. The top phase is mainly formed by the polyether polyol from the polymer, the bottom phase is formed by the solution of carbamates, ureas and amines in EG.

Electrospun scaffolds of self-assembling peptides with poly(ethylene oxide) for bone tissue engineering

Structural, mechanical and biochemical properties have to be considered when searching for suitable extracellular matrix substitutes. Fibrous structures of synthetic or natural polymers have received increasing interest as three-dimensional scaffolds for tissue engineering applications as they can be easily produced by electrospinning with different topographical features by changing the process parameters. On the other hand, the nanobiotechnology approach suggests mimicking molecular architectures in nature through self-assembly. In particular, self-assembling peptide-based biomaterials have been successfully used as scaffolds for cell growth. In order to amalgamate these two strategies nanofibrous electrospun scaffolds of hybrid polymer were designed and obtained by mixing poly(ethylene oxide) and self-assembling peptides in aqueous solution. The results of in vitro osteoblast adhesion and proliferation assays on the electrospun scaffolds obtained using different self-assembling peptide sequences are discussed.