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Compounding with ammonium polyphosphate-based flame retardants 

Plastics Additives & Compounding Apr02

Like other flame retardants, systems based on phosphorus compounds are particularly sensitive to processing conditions and exhibit a limited processing window. Dr. Ottmar Schacker and Dr. Wolfgang Wanzke of Clariant GmbH describe the processing properties of ammonium polyphosphate (APP) based flame retardants and how polymer compounding can be optimized for these additives.

An optimized compounding procedure can be key to a satisfying property profile of the material produced and can make the whole process much more efficient. Compounding flame retardants in the thermoplastics industry often requires modified equipment or special processing conditions compared to standard compounding procedures. There are a number of reasons for this. For example, flame retardant additives often need high loadings in the polymer for a good performance, as well as being partially filler type materials. They sometimes start to decompose during processing and usually have an impact on mechanical properties

The problem of premature decomposition during compounding or other processing steps is a general one for flame retardants. It is practically impossible for a flame retardant to be completely stable within the whole processing range, but also start its decomposition exactly at the temperature it should for a perfect flame retardant effect. There are many chemical reactions involved in the fire protection mechanism; both with halogenated and non-halogenated flame retardants. Therefore there will generally be traces of decomposition products even at the processing temperature of plastics.

Taking temperature as the only variable (which is a particularly simplified scenario), Figure 1 illustrates how narrow a processing window can be between insufficient dispersion and throughput on the low temperature side and premature decomposition at the high temperature end.

Figure 1. The Processing window

Intumescent systems

There is a special form of flame retardants with phosphoric compounds, known as intumescent systems. Through the exact interaction of three main components, this system leads to an expansion process in which a large volume high-carbon protective layer is built up that protects the substrate below from the attack of heat.

These systems, which are used primarily as fire-proof coatings for steel and wooden materials, can now be successfully incorporated as flame retardant systems into plastics such as polyolefins.

Ammonium polyphosphate is the main constituent of many intumescent flame retardant systems in coatings as well as polyolefin plastics. It initiates the intumescent process functioning as an acid source when thermal decomposition starts. The mechanism of decomposition has been investigated in order to establish a model for intumescent systems.1,2,3

As well as ammonium polyphosphate being an acid source, intumescent flame retardant systems for plastics consist of two further main components - a carbon source and an expansion agent (Table 1). Other substances are also often added, such as auxiliary processing agents for plastics as well as anti-dripping agents that positively influence the drip off behaviour - an important criterion of some fire testing methods.

Table 1 :Typical composition of an intumescent fire protection system

Carbon supplier   Polyols as starch, pentaerythritol
Acid source 	  Ammonium polyphosphate
Expanding agent   Melamine

Typical intumescent process

I. Softening of the binding agent /polymer (polypropylene)
2. Release of an inorganic acid (ammonium polyphosphate)
3. Carbonization of polyols
4. Gas production through expanding agent (melamine)
5. Swelling out of the mixture
6. Solidification through crosslinking reactions

In an ideal situation, the volume can increase to 100 times the original in an intumescence process. This requires the precise matching of various processes, which is achieved through careful selection of the formulation components (see box).

Figure 2. The intumescent effect

Intumescence = swelling, foaming

The carbon rich foam layer formed

  • has a heat insulating effect

  • reduces further oxygen access

  • reduces fuel transport into the flame

  • prevents dripping

The substrate (binding agent of a coating and/or polyolefin), thermally softened, makes up the reaction medium. The thermolysis of the acid source generally used in this system, ammonium polyphosphate, leads to the release of polyphosphoric acid. This dehydrates the carbon- supplying compound (pentaerythritol, for example) leading to a polymer-like structure. Simultaneously, the expansion agent produces large quantities of gaseous products that swell out the high-carbon reaction mass formed. The onset of crosslinking reactions leads initially to increased viscosity that prevents the gases escaping and subsequently to a solidification of the formed foam.

Long chain APP of crystallinic form II is used in intumescent systems for polyolefins and starts to decompose slowly in the 240-280°C temperature range. The release of ammonia forms P-O-H functions within the polyphosphate chain, which significantly increases the acidity of the system. However, there is another important reaction taking place at the same time leading to the formation of matrix structures and water3. Once the water has been released it will partially undergo further reactions within this temperature range, especially the hydrolysis of APP chains leading to more acidic polyphosphate and shorter chain products with phosphoric acid terminal groups.

Figure 3 shows typical curves for the components (Exolit(r) AP 422, Synergist B) of an intumescent flame retardant and the ready mixed flame retardant (Exolit AP 750) itself. The results shown in Figure 3 demonstrate that the single components APP and synergist B are reasonably stable up to 250ºC, while the blend of the two products, the intumescent flame retardant Exolit AP 750 behaves differently. It shows increasing water release at higher temperatures starting at approximately 180°C.

Figure 3. Water release

This kind of interaction can be found with several synergists in APP based systems, particularly with polyol components. It is the effect of the decomposition that started earlier in connection with water release that makes the intumescent blends more sensitive in processing than the individual components. If the intumescent flame retardant is dispersed in a polyolefin compound then the effect is much less drastic than with the pure powder blend. However, the more decomposition takes place during processing, the more negative effects will be observed.

Optimized conditions for compounding

A soapy surface on the polymer strands after running through a cooling bath and a high water content of the granules after compounding are good indicators for partial decomposition during processing. These effects can be avoided or minimized if a few compounding rules are observed. A typical corotating twin screw extruder for compounding polyolefins should be equipped in the following way:

Temperature and residence time are the most important factors regarding the degree of decomposition during processing. Consequently, temperature peaks caused by high shear elements like kneading blocks should be avoided and the residence time for the flame retardant kept short by downstream side feeding.

For example, Exolit AP 750 can be easily dispersed in the polymer melt and does not require intensive shear for that purpose. The good flowability of the resulting compounds means that a low temperature profile setting can be used (see Figure 4). The screw design shown for a 25 mm corotating twin screw extruder was successfully used for 30 % Exolit AP 750 in polypropylene.

Figure 4. Screw design for Exolit AP 750 (berstoff ZE 25/40D, corotating twin screw extruder)

An inverse temperature profile is recommended in order to keep the melt temperature as low as possible. The values shown in Figure 4 are for an injection moulding grade with a melt flow rate of 12 at 230ºC/2.16 kg. The low temperature profile applied for this compounding process does not necessarily mean that the output rate has to be low as well. Even for a temperature sensitive material the output rate can be optimized without exceeding a melt temperature that is considered to be critical. Figure 5 shows how this can be done4. The maximum output of an extruder is determined by its torque limit.

Figure 5. Optimizing output rate without exceeding melt temperature.

Therefore the best way to maximize the output rate is to increase feeding rates and keep the screw speed as low as necessary to stay within the temperature limit (in this case 210ºC). Increasing screw speed means increasing shear rate depending on the nip width between screws and barrel and as a consequence of that melt temperature is raised as well. Following the path of Figure 5 with 30 % of Exolit AP 750 in polypropylene, an output rate of 20 kg/h at 200 rpm could be achieved on the 25 mm twin screw extruder described in Figure 4, which is the limit for such a small machine. The good flow properties of Exolit AP 750 in polypropylene mean that the maximum torque of the machine is not the limiting factor, but the capacity of the screw that did not allow conveying more of the flame retardant because of its low bulk density.

Under these conditions a starting decomposition of the intumescent flame retardant could not be detected by the typical effects, such as soapiness of the polymer strands, increased water content of the granules or contamination of the cooling bath with degraded polyphosphates. A flame retarded polypropylene compound with suitable properties can be produced this way even on larger machines and is used in several applications, particularly in the electric and electronic industry.

Compound properties

By using the processing knowledge acquired for this special type of intumescent flame retardants, the mechanical properties of polypropylene compounds can be optimized and are comparable with those of commonly used products based on halogen flame retardants (see Table 2). The flow properties are particularly favourable.

Table 2: Properties of polypropylene compounds with Exolit AP 750

Property 			 Range
Impact strength (Charpy) 	 20-80 kj/m2
Notched impact strength (Charpy) 2- 12 kj/m2
Elastic modulus 		 1300-2200 MPa
Elongation at break 		 15-350%
Melt flow index (2.16kg,230°C)   5-20g/10 min
UL 94 classification 		 V-O (1.6 mm)

Compounders and end users are now able to choose flame retardants according the requirements of the end market - halogenated or non-halogenated. For example, this is of particular importance in transportation and electronic applications where smoke density and corrosive gases play an important role.

Outlook

In order to widen the processing window for this type of intumescent flame retardant and make it less sensitive to shear and temperature peaks, Clariant has developed a new formulation. Using special additives it exhibits a significantly higher thermal stability compared to the established Exolit products. Figure 6 shows the difference in decomposition behaviour between the modified blend and Exolit AP 750.

Figure 6. The difference in decomposition behavior between the modified blend and Exolit AP 750.

The starting point for flame retardant decomposition was changed to around 220°C for the modified system from approximately 180°C for Exolit AP 750. By widening the processing window by 30-40X, greater flexibility is offered for processing polypropylene flame retarded with this intumescent system.

Conclusions

The aim of the tests undertaken by the Business Line Flame Retardants at Clariant was to protect flame retardants in compounds from decomposition as early as the processing stage. The findings can be easily implemented by the processor. Compounding prior to injection moulding should be carried out with a soft screw configuration, with the lowest possible melt temperature and a high output rate with low screw speed. The processing recommendations for extrusion differ only with regard to the suitable screw configuration - in this case sharper screws may also be used. When these processing rules are observed, polyolefins can easily be given excellent fire protection.

References

1. G. Camino, N. Grassie and I. C. McNeill, J. Polym. Sci., Polym. Chem. Ed., 16, 95 (1978).

2. G. Camino, L. Costa and L. Trossarelli, Poly. Deg. and Stab. 6, 243, (1984).

3. G. Camino, L. Costa and L. Trossarelli, Poly. Deg. and Stab. 12, 203, (1985).

4. CPM GmbH, Georgsmarienhiitte/ Germany, Trainee course manual: ‘Compounding of Additives using Corotating Twin Screw Extruders’, 2000.

Contact: Dr. Ottmar Schacker Clariant GmbH Pigments &Additives Division, BU Additives Tel: +49 8214792763 Fm: + 49 8214792968 Email: ottmar.schacker@clariant.com

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