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An innovative phosphorus rich intumescent oligomer enabling commercially competitive high performance halogen free fire protection of polypropylene

Final Report Summary - PP-MIPS (An innovative phosphorus rich intumescent oligomer enabling commercially competitive high performance halogen free fire protection of polypropylene.)

Executive Summary:

‘Mixed intumescent polymeric salts’ (MIPS) are a novel cost efficient high performance intumescent FR additive. Utilising phosphorous rich oligomers synthesised from readily available raw materials, the MIPS chemistry integrates the essential components for intumescence (catalyst, spumescent and carbonific) within one molecule, locked in a reaction state immediately preceding intumescence.

The PP-MIPS project will further advance ‘MIPS’ as an intumescent fire protection chemistry for polypropylene, targeting commercial competitiveness with halogen based FR systems and enabling their replacement for high performance applications. The PP-MIPS compounds will be demonstrated for two high profile case study applications: 1) halogen free UL94-V2 rated polypropylene fibres; and 2) NFPA-262 rated polypropylene cable insulation jackets.

The technical work programme over the period 1st November 2010 to 31st October 2012 has involved the following:-
Work Package 1: MIPS Development & Characterisation
Work Package 2: MIPS Synergists
Work Package 3: PP-MIPS Processability
Work Package 4: Case Study 1: PP-MIPS Fibres
Work Package 5: Case Study 2: PP-MIPS Cable Insulation Jackets

The version of MIPS taken as the starting point for the project is prepared by the reaction between 1,4-butane diol, phosphorus pentoxide and pentaerythritol, followed by neutralisation with melamine and tetraethylenepentamine. (TEPA). This is designated MIPS 1BX TEPA. Compounded into polypropylene, this grade shows good fire retardant behaviour, provides high levels of expansion and effectively reduces the heat release rate of polypropylene.

With the aim of creating a version with improved resistance to decomposition and with higher activation temperatures for intumescence alternative MIPS formulations were synthesised. Thermogravimetric analysis shows the MIPS grade synthesised with 1,3-propane diol and diethylenetriamine (DETA) to have the highest decomposition and intumescence temperatures. Due to health and safety aspects of handling DETA, the Consortium continued with the second most stable variant MIPS 1PX TEPA. At 25% addition of MIPS 1PX TEPA into polypropylene, a UL94 classification of V0 at 3.2mm is achieved.

To further enhance the fire retardant properties a range of synergists were combined with MIPS. Antimony Trioxide, Molybdenum Trioxide, Tungsten Trioxide, Lanthanum Oxide, Yttrium Oxide, Zeolite and 1,3,5 - tris hydroxyl ethyl isocyanurate replacing the pentaerythritol core within the partial phosphate ester were all found to provide a synergistic effect in the MIPS fire retardant system.

In order to produce the larger volumes of MIPS required for the project, modifications to the pilot plant were carried out. 250kg of MIPS 1PX TEPA were successfully produced and supplied for polypropylene compounding trials to support production of material for the 2 case study applications: 1) halogen free UL94-V2 rated PP fibres; and 2) NFPA-262 rated cable insulation jackets.

Monofilament spinning trials were performed to evaluate the effective spinnability of a 25% MIPS 1PX TEPA filled compound, but at this high loading a reduction in mechanical properties occurred and it proved difficult to obtain a continuous filament. Multifilament extrusion trials were then performed to establish the maximum achievable MIPS content without affecting mechanical properties. This appeared to be just 5%. Polypropylene knitted and non-woven fabrics were prepared and tested according to UNI EN ISO 15025:2003 “Protective clothing – Protection against heat and flame – Method to test for limited flame spread” (with modifications). As expected considering the low amount of MIPS in the tested samples, the results show that the materials do not meet the requirements for protective clothing fabrics

For the cable insulation jacket application, process optimisation trials were performed on a crosshead extruder and co-axial and unipolar cables were successfully produced. However, to produce a stable compound it was necessary to reduce the MIPS content to 15%, but when tested to IEC 60332-1-2:2004 ‘Test on Electric & Optical Fibre Cables Under Fire Conditions Part 1-2: Test for Vertical Flame Propagation for a Single Insulated Wire or Cable – Procedure for 1kW pre-mixed flame’, the PP-MIPS co-axial cable did not meet the recommended performance requirements.

To meet the fire test criteria for both the fibre and cable jacket case studies it is necessary to increase the MIPS 1PX TEPA content to around 25%. This requires further processing optimisation as mechanical properties are very much affected at these MIPS loadings.
This can be achieved by the inclusion of phosphate ester plasticisers or compatibilisers such as maleic anhydride-grafted polypropylene (PP-g-MA). Earlier in the project, a 25% 1BX TEPA filled polypropylene processed well and gave good mechanical properties, but due to fume problems this product was not selected at the time. With extraction measures in place, the Consortium recommends revisiting this MIPS version.

Project Context and Objectives:
Offering good performance at low cost, polypropylene (PP) is a popular commodity plastic within many market sectors. However, due to its high flammability and low charring nature, PP is difficult to fire protected. Whist brominated flame retardants (FRs) are able to achieve excellent FR performance at moderate cost and with little impact on polymer properties, they release toxic and corrosive smoke during fire, are potentially harmful to the environment and restrict PP recyclability. There is thus increased legislative and political pressure for a reduction in halogen FR use. Existing halogen free PP flame retardants are based on low performance & low cost mineral FR additives or high performance & high cost intumescent systems, both requiring high to moderate loadings thereby impacting on polymer properties and restricting use for high performance applications.

‘Mixed intumescent polymeric salts’ (MIPS) are a novel cost efficient high performance intumescent FR additive. Utilising phosphorous rich oligomers synthesised from readily available raw materials, the MIPS chemistry integrates the essential components for intumescence within one molecule, locked in a reaction state immediately preceding intumescence.

The PP-MIPS project will further advance ‘MIPS’ as an intumescent fire protection chemistry for PP, targeting commercial competitiveness with halogen based FR systems and enabling their replacement for high performance applications. The PP-MIPS compounds will be demonstrated for two high profile case study applications:
1) halogen free UL94-V2 rated PP fibres; and 2) NFPA-262 rated cable insulation jackets.


To enable this, the specific scientific objectives of our work are:
1) MIPS Chemistry: To further understand the relationships between the MIPS chemical structure, the mechanism and properties of intumescence (MIPS activation temperature; carbonised layer formation, structure, durability, stability and decomposition; & phosphorous compounds volatilisation and gas phase activity); and the polypropylene grade and associated material physical and mechanical properties; and the fire protection properties (flame and fire retardancy, heat and smoke release & thermal insulation).
2) Synergism: To investigate and understanding the relationships between different condensed and gas phase synergists and char enhancing polymer blends; their mechanism of interaction and their impact on the resulting material fire protection and physical & mechanical properties.
3) Processability: To understand and characterise the processability of the MIPS additive within various polypropylene grades and to investigate to achieve optimised material properties and minimal decomposition of the MIPS additive.

Project Results:
The technical work programme over the period 1st November 2010 to 31st October 2012 has involved the following:-
Work Package 1: MIPS Development & Characterisation
Work Package 2: MIPS Synergists
Work Package 3: PP-MIPS Processability
Work Package 4: Case Study 1: PP-MIPS Fibres
Work Package 5: Case Study 2: PP-MIPS Cable Insulation Jackets


MIPS Development & Characterisation and Synergists
The version of MIPS taken as the starting point for the project is prepared by the reaction between 1,4-butane diol, phosphorus pentoxide and pentaerythritol, followed by neutralisation with melamine and tetraethylenepentamine. (TEPA). This is designated MIPS 1BX TEPA. On pilot plant facilities MIPS 1BX and variants with 1,3-propane diol and diethylenetriamine (DETA) were manufactured and supplied in dry ground form for compounding into polypropylene.

To evaluate the thermal stability, carbonisation and intumescence coarse TGA tests were performed. The results indicated that by making the product with pentaerythritol and 1,3-propane diol and precipitating with DETA will make a powder more compatible with extrusion. All the formulations intumesced to give 80% to 100% expansion.
Water absorption & solubility tests were performed which showed the DETA variants to have the lowest hygroscopic properties <3%.

A literature review of the synergists that interact in the gas phase has been undertaken. On the basis of the well-known synergic properties of metal oxides in flame retardant systems, three compounds were initially selected as PP-MIPS synergists to evaluate their effect on material thermal behaviour:

• Antimony oxide (Sb2O3)
• Tungsten oxide (WO3)
• Molybdenum oxide (MoO3)

These metal oxides were combined with 1BX TEPA in its precipitated form, at a loading level of 5% by weight. The combinations were then dried and the powders ground as in the standard MIPS production method, and compounded into polypropylene. Specimens were characterised by differential scanning calorimetry, thermogravimetric analysis, and calorimetric cone analysis. Overlapping profiles of heat release rate (HRR) curves shows the effectiveness of MIPS to reduce HRR of polypropylene. Moreover, by comparing PP-MIPS samples dosed with metal oxides and pure polypropylene is possible to observe a slight shift of HRR peak to the right. This evidence indicates the ability of metal oxides to delay the maximum rate of heat release.


The results from the cone calorimeter however cannot be directly related to UL94 data.
Morgan and Bundy (Fire and Materials 2007;31:257) indicate that the best correlation between UL94 data and the cone calorimeter is a measure based on peak heat release/time to peak heat release. This measure is equivalent to the Fire Growth Rate index (FIGRA) measure used in EN 13823 and ASTM E2257-08.
This measure is not automatically recorded by the cone calorimeter software so was calculated from the plots of HRR. These results clearly showed that the 1PX versions are superior to the 1BX versions. At that time the Consortium agreed to reject the 1PX DETA variant due to the health and safety reclassification of DETA as a very toxic substance. This justified the selection of MIPS 1PX TEPA as the flame retardant of choice.
The FIGRA index also showed that all of the metal oxide synergists have beneficial effect, in the order antinomy, tungsten and molybdenum.


Additional synergists and char enhancing polymers were selected and were compounded into polypropylene. These were characterised by differential scanning calorimetry and thermogravimetric analysis to evaluate their thermal properties, and analysed by scanning electron microscopy for the blending efficiency evaluation. The results showed that the following synergists provide beneficial fire retardant properties when combined with PP-MIPS and should be investigated further:-
Lanthanum Oxide, Yttrium Oxide, Zeolite and 1,3,5 - tris hydroxyl ethyl isocyanurate replacing the pentaerythritol core within the partial phosphate ester.
The inclusion of the char enhancing polymers TPU and Polyamide 11 did not provide any additional fire retardancy.

PP-MIPS Processability
Main plant trials in Period 1 had focussed on the optimisation of compounding MIPS 1BX TEPA with polypropylene. At 25% addition the UL94 vertical flame test showed a V0 rating.
However, thermogravimetric analysis performed on MIPS 1BX TEPA shows that it would not be well suited to the processing temperatures typically used to process polypropylene materials (~220-230°C). The next stage of the project therefore involved the compound optimisation of the more stable form, MIPS 1PX TEPA. This gave a much lower smell at the die compared to 1BX TEPA and no additional fume extraction was required. The MIPS 1PX TEPA was however more difficult to process, but this was optimised to eliminate any degradation. Samples were tested to UL94 ; the 25% MIPS 1PX TEPA filled polypropylene gave a V0 rating at 3.2mm therefore better than the 1BX version.

To ensure no hazardous products are evolved from the MIPS 1PX TEPA during processing, a sample of the additive was analysed by a combination of pyrolysis and gas chromatography/mass spectrometry (GCMS). No major pyrolysis products attributable to the sample were observed. Based on this analysis, MIPS 1PX TEPA is considered to be a non-hazardous material.

In order to produce the larger volumes of MIPS required for the project, modifications to the drive and heater systems of the pilot plant were carried out. 250kg of MIPS 1PX TEPA were successfully produced and supplied for compounding trials.
The high water content and viscosity of MIPS slurry makes the dewatering process very difficult. An investigation into the suitability of microwave drying was conducted. A combination of microwave heating and ultrasound cavitation is proposed which could potentially achieve rapid and thorough drying of the MIPS slurry.

Compound optimisation trials were performed and material successfully manufactured. This was supplied for PP-MIPS profile extrusion and injection moulding, and fibre extrusion trials.

By optimising processing conditions, PP-MIPS pipe and sheet were extruded and injection mouldings produced.
Mechanical testing however showed that the addition of MIPS 1PX TEPA to polypropylene significantly reduces the yield stress and elongation at break. Properties were improved by the addition of further polyolefin, but this effectively reduces the MIPS content and potentially the fire performance. The test results indicate an incompatibility of MIPS 1PX TEPA and polypropylene. Greater dispersion of the MIPS can reduce the effect, as can the use of coupling agents or compatibilisers such as maleic anhydride-grafted polypropylene (PP-g-MA).

Case Study 1: PP-MIPS Fibres
Preliminary monofilament spinning trials were performed to evaluate the effective spinnability of the compound. The aim was to extrude the PP-MIPS compound into fine count (20 µm) monofilament. Several trials were made with different temperature profiles, in order to evaluate if spinning was possible, but it proved very difficult to obtain a continuous filament.
In order to optimise the melt spinning process for the PP + 1PX TEPA MIPS multifilament production, a new process was set-up, consisting in a subsequent increase of additive concentration with adjustment of process parameters to achieve appropriate mechanical properties for the weaving. The maximum achievable MIPS content without affecting mechanical properties was found to be just 5%.

Spinning of PP with 3 % and 5 % of MIPS was possible, but it was difficult to draw these yarns as the mechanical properties were quite weak. PP with 1 % MIPS (drawn) was used for the production of samples of knitted fabric, while PP with 3 % MIPS and PP with 5 % MIPS (both undrawn) were used to produce samples of non-woven fabrics.

Both types of fabric were tested according to UNI EN ISO 15025:2003 “Protective clothing – Protection against heat and flame – Method to test for limited flame spread” (with modifications). As expected considering the low amount of MIPS in the tested samples, the results show that the materials do not meet the requirements for protective clothing fabrics

A life cycle review was undertaken. On the basis of the data obtained by the Life Cycle Analysis, it is possible to conclude that the process that more affects the global impact is the spinning process. This evidence can be explained by the complexity of the process and the high amount of resources that it needs in comparison with the other studied processes.

Case Study 2: PP-MIPS Cable Insulation Jackets
Two PP-MIPS compound formulations were produced and 200kg supplied for cable extrusion trials on a crosshead extruder. At 25% MIPS content there were difficulties achieving a good quality extrudate even with modifications to the process temperature profile. To produce a stable compound it was necessary to reduce the MIPS content to 15%, but when tested to IEC 60332-1-2:2004 ‘Test on Electric & Optical Fibre Cables Under Fire Conditions Part 1-2: Test for Vertical Flame Propagation for a Single Insulated Wire or Cable – Procedure for 1kW pre-mixed flame’, the PP-MIPS co-axial cable did not meet the recommended performance requirements.

From data collated during the project, a Life Cycle Analysis (LCA) for the PP-MIPS cable product is reported.

Future Work
To meet the fire test criteria for both the fibre and cable jacket case studies it is necessary to increase the MIPS 1PX TEPA content to around 25%. This requires further processing optimisation as mechanical properties are very much affected at these MIPS loadings.
This can be achieved by the inclusion of phosphate ester plasticisers or compatibilisers such as maleic anhydride-grafted polypropylene (PP-g-MA). Earlier in the project, a 25% 1BX TEPA filled polypropylene processed well and gave good mechanical properties, but due to fume problems this product was not selected at the time. With extraction measures in place, the Consortium recommends revisiting this MIPS version.

A number of synergists were found to provide beneficial fire retardant properties when combined with PP-MIPS. These require further evaluation.

Potential Impact:
Socio-economic Impact

The PP-MIPS project will further advance ‘Mixed Intumescent Polymeric Salts – MIPS’ as an intumescent fire protection chemistry for polypropylene, targeting commercial competitiveness with halogen based flame retardant systems and enabling their replacement for high performance applications.
The PP-MIPS compounds will be demonstrated for two high profile case study applications: 1) halogen free UL94-V2 rated PP fibres; and 2) NFPA-262 rated cable insulation jackets. Such applications are currently only possible using halogenated polymer compounds.

The PP-MIPS project will develop a high performance, low smoke and zero halogen fire protection chemistry that is applicable to most polymeric materials and fire regimes and is cost competitive with halogenated materials. The project will specifically advance the performance of fire & flame retarded PP, achieving fire protection properties (generating significantly reduced smoke), cost competitiveness and material physical and mechanical properties, in line with and beyond existing halogenated PP materials, thereby enabling:

• Improved PP in service performance (enabling new product design), recyclability and added value, thereby offering differentiation and competitive advantage;

•Utilisation of PP materials for new product applications targeting replacement of more expensive polymers (such as high impact polystyrene) or less desirable halogenated polymers (such as PVC):

The PP-MIPS project contributes to a range of community and societal objectives, including:
- Reduction in environmentally toxic chemicals: halogens (& their antinomy trioxide synergists) are known to be hazardous to the environment and thus there are numerous political / legislative, societal and industrial drivers for their reduced use / removal. The MIPS additive is synthesised from non- hazardous raw materials to produce an environmentally benign orthophosphate structure. The MIPS additive demonstrates cost & performance competitiveness with halogens and thus could substitute its use. The PP-MIPS project therefore demonstrate the potential to reduced halogen (& synergist) use by up to 67,725 tonnes (currently used for the flame retardancy of PP);
- Improved PP Recyclability: the use of halogens is a limitation for the recycling of PP, either via legislative restrictions preventing halogen containing material recycling (e.g. WEEE directive) or through the production of corrosive gases (e.g. during energy recovery); and consequently a significant proportion of halogen flame retarded PP is sent to landfill. The MIPS additive is entirely benign within all recycling & recovery operations, thereby enhancing FR-PP recyclability. Through replacement of halogen FRs, the PP-MIPS project therefore demonstrates the potential to ensure the recyclability of FR-PP thereby diverting up to 338,625 tonnes of PP from landfill for reuse, recycling or recovery.
Furthermore, MIPS do not release toxic or corrosive gases during fire thereby reducing the release of toxic species into the environment as a consequence of fires.

Main dissemination activities

A project website www.pp-mips.eu & www.pp-mips.com was set up at the start of the project to disseminate the project objectives and activities.

On 8th November 2011the UK Materials Technology Research Institute attended the ITF's (Industry Technology Facilitator) Annual Technology Showcase in Aberdeen, Scotland for oil and gas professionals.
On the UK MatRI Stand posters for the PP-MIPS project were exhibited.

AIS attended the following conferences:-
Offshore Technology Conference May 2012 in Houston

Offshore North Sea 2012 Exhibition & Conference in Norway in August 2012

Centrocot presented on the project at the FR Textile Conference & Milano Unica 2012 – 18th May 2012

Centrocot had an exhibition stand at Cinte Techtextil China 2012, 22 - 24 October 2012. Flyers and posters for the PP-MIPS project were produced.

List of Websites:

www.pp-mips.eu

For further details, contact:

Andrew Bennion
Advanced Insulation Systems Ltd
Quedgeley West Business Park
Bristol Road
Gloucester
GL2 4PA
Tel: +44 (0) 1452 880 880
andrew.bennion@aisplc.com