To manufacture high quality flame retardant polypropylene sophisticated machinery and technology are required. Investment in machinery is very high even for medium sized manufacturing unit. These overheads, together with the high rate of power consumption per unit weight of material manufactured, substantially add to the cost of the finished product. However, this increase can be compensated by adding fillers, which increases the complexity of compounds. Higher safety standards required by applications normally justify higher cost of flame retardant materials.

UL Classification
Following is the basics of Underwriters Laboratory specifications:

This is a flammability test devised by Underwriters Laboratories for plastic materials used in electrical devices and appliances. In this test, specimens are exposed for two successive 10 seconds ignitions from 3/4" burner flame. They are classified according to the time it takes the flame to extinguish and the length of time any"after glow" persists.

Rating are as follows:
94 V-O
Extinguishment time 0.5 seconds
After glow time 0.30 seconds
94 V-1
Extinguishment time 6-25 seconds
After glow time
No. flaming drips 0-60 seconds
94 V-2
Extinguishment time 0.25 seconds
After glow time
Flaming drips permitted 0-62 seconds

Chlorine based systems:
This is the most economic way to make Flame Retardant Polypropylene although recently highly advanced and expensive systems are also available suitable not only for polypropylene but also for nylon, PBT etc.

Similarly Table 9 shows properties of flame retardant polypropylene copolymer. These are only typical examples of data sheet properties. However, enough scope exists to further modify the properties as per the demands of applications. It is also necessary to understand the limitations of this most economic system. The thermal stability and heat resistance of flame retardant poly propylene is greatly reduced. Flame retardants also plasticise polypropylene is mouldable at lower temperature. This reduces the heat distortion temperature and sets a limit to the applications which need higher performance temperature. Chlorine based systems find application in small iinjection moulded parts and is not recommended for any of the extrusion applications as it poses greater degree of damage to screw, barrel, dies etc.

Table 9 : Flame retardant polypropylene- copolymer chlorinated system

Bromine based system:
This is the best way of making propylene flame retardant. The end product has better thermal stability and hence can be processed by injection moulding and extrusion. Heat deflection temperature is also not adversely affected. Table 10 shows typical properties of polypropylene homopolymer based compounds.

Since this system is versatile and can be used in wider applications, it is extensively studies from the point of view of smoke and toxicity hazards.

Polypropylene compounds having required properties can be tailor-made for give application by incorporating various mineral fillers and glass fibres.

Table 11

Non-halogenated Flame retardant polypropylene-homopolymer

Non-halogen based systems
Smoke density and toxicity regulations limit the use of halogen based flame retardant polypropylene. Acidic halogen based gases evolved during burning also damage expensive electronic circuitary. The development of non-halogenated systems took place considering these aspects. However, it is the most expensive system today for injection moulding applications. It is widely used for extrusion applications mainly polyethylene based cables.

•  Elastomer Modified PP products
One of the most common reasons to utilize a rubber/elastomer "modifier" in PP is to improve its low temperature impact resistance. Originally, the frist impact PP products were formulated from blends of PP homopolymer with butyl rubber. Subsequently, it was discovered that ethylene-propylene (EPM) rubber offered greater toughening power and was easier to disperse and compound into PP. The industry then evolved towards fine tuning of the EPM materials with the closely related ethylene-propylene-diameter polymers (EPDM).

When EPM/EPDM rubbers are thoroughly blended with either PP homopolymer or copolymer to an extent of 10-40% by weight, a new family of thermoplastic materials called rubber or toughened PP is produced. They have flexural modulus of elasticity within 600-1200 MPa, together with an exceptional impact resistance down to - 50 deg. C according to the grade and percentage of EPDM used. Such a combination has been the main reason for the success of these materials in the automobile industry.

The three properties, elastic modulus in flexural mode, impact strength and brittle/tough transition temperature represent the most important indicators for end-use behaviour of elastomer modified PP technical articles.

A single rubber particle acts as a centre of absorption and elastic redistribution in the surrounding area of the shock wave caused in the material when a local impact is applied. The impact energy is, therefore distributed across a large volume of material, decreasing local intensity when the finely dispersed elastomeric particles are numerous. Table (12) lists some properties of EPDM modified PP vs EPDM content.

Table 12 Properties of Elastomer Modified Polypropylene:

Modification by Other Components
Elastomer modified PP grades require greater stiffness at elevated temperatures, better HDT and shape stability. Filler can be added to improve these properties. Talc is generally used in 15 to 40% by weight of finished products. EPDM-PP Talc compositions permit a wide range of possible proportions and corresponding broad performance area for the commercial products.

I II III IV V
PP: 50 60 70 80 90
EPDM: 50 40 30 20 10

Applications:

In the recent past, the most important uses of PP-EPDM blends has been in the automobile industry. Comparative lightness, low cost and a wide range of mechanical properties are the main factors responsible for the continuous growth of these materials since 1975. Their main use in car bumpers using either PP-EPDM flexible shapes mounted on a rigid steel framework or fabricated self-supporting finished articles such as complete car front masks.

Suitable blends contain from 10 to 35% by weight of EPDM, according to flexibility and imapct requirements. EPDM-PP talc ternary blends find their biggest outlet as interment panel dash boards in many models of low to medium cost cars. For this application stiffness and shape stability, coupled with an impact resistance high enough to avoid splintering failures or dangerous sharp fragments in medium speed crashes provide these blends with an ideal application.

•  Compounding Machinery
Compounding of PP with additiives, fillers and reinforcements can be achieved using various types of machines depending on cost and quality of mixing desired. A few of these are discussed in the section.

•  Compounding low add levels
PP is generally an easily compounded material and the compounding of low-add-level products usually requires nothing more than a dry solids mixer for preblending and a single-screw extruder for pelletizing. Extrusion temperatures will depend primarily on the viscosity or melt flow rate of the PP homopolymer or copolymer component., with melt temperatures typically falling between 190 deg. c. and 2245 deg. c. Depending on the equipment, PP may give lower output rates on extruders than other polymers as a result of both its low density and its rheological properties.

At high melt flow rates, pelletizing may present some problems owing to the low melt strength. With specialized equipment, melt flow rates as high as 1000 have been pelletized. Both hot-cut and cold-cut pellletizing systems are suitable for PP. The choice usually depends on the equipment available as well as downstream processing considerations. In some subsequent extrusion and injection moulding processes, the shape and size of the pellets will influence the products processing behaviour. For example, the flat, thin wafer configuration characteristic of the hot cut has been reported to cause slipping problem in the extrusion of thin film.