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Melt flow and surface stability effects on polymer flammability: A study on polystyrene, flexible polyurethane foam and polyolefin composites
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Our ability to accurately determine the flammability properties of polymers is hampered bythe erratic effects of deformation and liquid flow that occur during polymer combustion.This study aims to improve the understanding of such effects at several levels:

The often obvious impact of melt flow on standardized flammability test methods forelectrical appliances is described to identify the need and to motivate others to worktowards an improved material characterization.

The production of a burning degraded melt results from both, melting anddecomposition of thermoplastic polymers. The degradation of the polymer chains yieldsboth combustible volatiles that feed the flame and smaller molecules with a low viscositythat might spread to a burning pool. The molar mass is the property of the material that linksboth processes. Changes in molar mass at the surface of specimens exposed to fire‐like heatfluxes were analyzed, using polystyrene as a model thermoplastic material. Analyticalexperiments were combined with bench‐scale gasification and gasification/melt flowexperiments, in order to determine the extent of viscosity reduction by melting and bypolymer decomposition.

A strong volume contraction to a low viscous liquid is characteristic for the combustionof flexible polyurethane foam, a low density cellular material used for cushions in furnitureand mattresses. Foam decomposition is initiated at low temperature (< 300 °C), yieldingignitable gases and leading to a fast flame spread that results in the formation of a burningliquid pool. The foam morphology changes rapidly during combustion and the impact ofthese changes on the heat release rate was analyzed. The distribution of flame heat transferled to strong variations in the burning rate, which shaped the specimen surface. The shapeof the surface affected in turn the distribution of flame heat flux, leading to an interlockingprocess. A custom‐made vertical test arrangement gave the ability to study the effect of heatfeedback from the burning liquid. Foam composites with carbon nanofibers and organicallymodified clay were prepared with the aim to eliminate liquefaction. Incorporation of anetwork of carbon nanofibers in the foam struts is demonstrated to prevent foamliquefaction and collapse.

An approach to prevent melt‐dripping is the use of high filling levels of inorganicparticles in polyolefin cable materials and to promote reactions between the filler and thepolymer. The transition from a polymer melt to an immobile inorganic residue was studiedfor blends of acrylic copolymers of ethylene with chalk and silicone. The mechanical stabilityof the inorganic surface layer that formed during combustion was decisive for the materialsflammability properties. In‐situ thermocouple measurements and ex‐situ analysis of partiallyburned specimen surfaces were used to explain the coupling between the degradationmechanism of the polymer and the heat shield effect of the inorganic residue layer.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , 60 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2009:5
Keyword [en]
Melt, flow, dripping, collapse, combustion, pool fire, feed‐back, gasification, heat release, flammability, nanocomposites, molar mass, intumescence, polyurethane
National Category
Polymer Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-10079ISBN: 978-91-7415-239-5 (print)OAI: oai:DiVA.org:kth-10079DiVA: diva2:207437
Public defence
2009-03-20, F3, KTH, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100726Available from: 2009-03-11 Created: 2009-03-11 Last updated: 2010-07-26Bibliographically approved
List of papers
1. The role of depolymerization in simultaneous gasification and melt flow of polystyrene.
Open this publication in new window or tab >>The role of depolymerization in simultaneous gasification and melt flow of polystyrene.
(English)Manuscript (preprint) (Other academic)
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-14215 (URN)
Note
QC 20100726Available from: 2010-07-26 Created: 2010-07-26 Last updated: 2012-04-23Bibliographically approved
2. Heat release and structural collapse of flexible polyurethane foam
Open this publication in new window or tab >>Heat release and structural collapse of flexible polyurethane foam
2010 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 95, no 6, 1115-1122 p.Article in journal (Refereed) Published
Abstract [en]

Flexible polyurethane foam used in upholstered furniture remains one of the major fire hazards to date. The heat release rate of burning items made of foam depends strongly on the foam’s physical behavior, notably its collapse to a burning liquid that can result in a pool fire. In this contribution, the cone calorimeter was used to study the physical processes and to determine their influence on foam combustion over a range of external heat fluxes. The initial stage of foam collapse can be described as the propagation of a liquid pyrolysis layer through the foam sample. The rate of propagation of the liquid layer was found to depend strongly on the convective heat transfer from the flame, which simultaneously defined and depended on the sample shape. The effective heat of combustion during foam collapse and pool fire was matched to the heat release potential of the components of the foam formulation to deduce which are consumed. The proposed analysis can serve to clarify the mechanism of flame retardant action, as demonstrated for a commercial brominated-phosphorous compound.

Keyword
Cone calorimeter; Convective heat transfer; Effective heat of combustion; External heat flux; Flexible foams; Flexible Polyurethanes; Foam collapse; Foam formulation; Heat release; Heat release rates; Initial stages; Liquid layer; Physical behaviors; Physical process; Pool fires; Rate of propagation; Sample shape; Structural collapse; Upholstered furniture, Combustion; Flame retardants; Liquids; Phosphorus; Rigid foamed plastics; Thermochemistry, Fire hazards
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-14211 (URN)10.1016/j.polymdegradstab.2010.02.019 (DOI)000278750800027 ()2-s2.0-77953232908 (Scopus ID)
Funder
Formas, 243-2004-1748
Note
QC 20100726Available from: 2010-07-26 Created: 2010-07-26 Last updated: 2017-12-12Bibliographically approved
3. Flammability reduction of flexible polyurethane foams via carbon nanofiber network formation
Open this publication in new window or tab >>Flammability reduction of flexible polyurethane foams via carbon nanofiber network formation
Show others...
2008 (English)In: Polymers for Advanced Technologies, ISSN 1042-7147, E-ISSN 1099-1581, Vol. 19, no 6, 588-595 p.Article in journal (Refereed) Published
Abstract [en]

Untreated polyurethane flexible foams (PUFs) are prone to rapid fire growth due to their low density and low thermal conductivity. Furthermore, the low viscosity of the decomposition products generates severe dripping that increases the fire hazard related to the combustion of PUFs. In fact, this downward flow of flaming liquid often results in a pool-fire that promotes flame propagation and boosts the rate of heat release (HRR) due to a significant increase in the burning area and to feed-back between the flame on the pool-fire and the residual foam. In this work the effect of nartoparticles, i.e., clays and carbon nanofibers (CNFs), on the HRR is investigated with special attention given to melt dripping. A modified cone calorimeter test has been developed for this purpose. It is shown that CNFs form an entangled fiber network which eliminates melt dripping and decreases the HRR.

Keyword
nanocomposite, polyurethane, flammability, foam
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-14213 (URN)10.1002/pat.1111 (DOI)000257014100018 ()2-s2.0-47149101515 (Scopus ID)
Note
QC 20100726Available from: 2010-07-26 Created: 2010-07-26 Last updated: 2017-12-12Bibliographically approved
4. On the intumescence of ethylene-acrylate copolymers blended with chalk and silicone
Open this publication in new window or tab >>On the intumescence of ethylene-acrylate copolymers blended with chalk and silicone
2007 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 92, 1899-1910 p.Article in journal (Refereed) Published
Abstract [en]

The combustion and melt dripping of poly(ethylene-co-butyl acrylate) (EBA), EBA blended with polypropylene (EBA-PP) and poly(ethylene-co-methacrylic acid) (EMAA), each blended with calcium carbonate and polydimethylsiloxane, were studied. In situ measurement of the temperature gradient in the cone calorimeter were combined with infrared spectroscopy measurements on specimens withdrawn and quenched at different times of the experiment. The reactions that govern the degradation at the high heating rates met in the combustion could be determined and the gap to analytical techniques such as thermogravimetry bridged. The interplay of mechanical char integrity and heat feedback by the flame determined how much time the specimen dwells in temperature range of 300-420 °C where char expansion due to calcium salt formation is effective and thereby affects the heat release rate strongly. Vertical cone calorimeter and vertical flame testing were used to assess melt dripping and char stability under flaming combustion. Plate-plate rheological measurements proofed to be of limited use to compare the effect of different degradation atmospheres on the melt viscosity. The EMAA formulation had the most effective intumescent process with a low heat release rate and good char stability even in vertical configuration. Electron-beam irradiated EMAA specimens with different levels of cross-links were tested in the cone calorimeter in order to understand the role of cross-links for the intumescent process.

Keyword
Acrylics; Flame retardants; Heating rate; Infrared spectroscopy; Polydimethylsiloxane; Polymer blends; Thermal gradients, Calcium carbonate; Chalk; Intumescence, Copolymers
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-14212 (URN)10.1016/j.polymdegradstab.2007.06.014 (DOI)000250950300018 ()2-s2.0-34948885154 (Scopus ID)
Note
QC 20100726Available from: 2010-07-26 Created: 2010-07-26 Last updated: 2017-12-12Bibliographically approved
5. Degradation of poly (ethylene-co-methacrylic acid)-calcium carbonate nanocomposites
Open this publication in new window or tab >>Degradation of poly (ethylene-co-methacrylic acid)-calcium carbonate nanocomposites
2007 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 92, no 10, 1795-1802 p.Article in journal (Refereed) Published
Abstract [en]

Composites of poly(ethylene-co-methacrylic acid) with 5 mass fraction percent of precipitated calcium carbonate nanoparticles were prepared by melt extrusion on a miniature melt-blender and medium-scale production equipment. The composites consisted mostly of isolated particles. The ultimate mechanical properties of the nanocomposites were consequently largely superior to composites with micron-sized filler. The calcium carbonate particles were shown to offer a large surface area for calcium salt formation during the thermal degradation of the material. This imparted a stabilizing effect to the copolymer that was comparable to the neutralization of the methacrylic acid units with calcium ions. The rate of calcium salt formation was fast at temperatures above 350 degrees C. Stearic acid surface coatings did not interfere significantly with the calcium salt formation. The oxidative stability of the composites was further largely improved by the formation of a diffusion barrier.

Keyword
polyethylene-co-methacrylic acid), calcium carbonate, nanoparticles, degradation
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-14214 (URN)10.1016/j.polymdegradstab.2007.07.006 (DOI)000250950300006 ()2-s2.0-34948872071 (Scopus ID)
Note
QC 20100726Available from: 2010-07-26 Created: 2010-07-26 Last updated: 2017-12-12Bibliographically approved

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