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POLYPROPYLENE GUIDE - Part I

- Prepared by S. Rajendran

Typical Applications

Structural tanks and covers

Ducts and hoods

Tank linings

Vacuum formed trays

Plating barrels and tanks

Orthopedic devices

dishwasher-safe food containers

Molecular formula

Resin identification code :

SUMMARY OF PROPERTIES

Chemical name Poly(1-methylethylene)

Chemical formula (C3H6)x

Monomer Propylene (Propane)

Density

(Atactic) Amorphous: 0.85 g/cm3

(Isotactic) Crystalline: 0.95 g/cm3

Melting point 173 C (Isotactic)

Degradation point 286 C

Glass transition temperature -17 C

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Introduction:

Polypropylene or polypropene (PP) is a thermoplastic polymer, used in a widevariety of applications, including food packaging, textiles, laboratory equipment,

loudspeakers, automotive components, and etc. Polypropylene is one of those rather

versatile polymers, popularly known by the abbreviation PP. It is a member of the

Polyolefin family of pure hydrocarbon plastics. This is an addition polymer made fromthe monomer propylene

Fig.1. Molecular formula of PP

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This highly crystalline thermoplastic resin is built up by the chain-growth

polymerization of propylene (CH2=CHCH3) monomer.Only the isotactic form of polypropylene is marketed in significant quantities. (In isotactic polypropylene, all the

methyl [CH3] groups are arranged along the same side of the polymer chain.) It is

produced at low temperatures and pressures using Ziegler-Natta catalysts.

It serves both as a plastic and as a fiber. As a plastic, polypropylene is blow-molded into

bottles for foods, shampoos, and other household liquids. It is also injection-molded intomany products, such as appliance housings, dishwasher-proof food containers, toys,

automobile battery casings, and outdoor furniture. When a thin section of molded

polypropylene is flexed repeatedly, a molecular structure is formed that is capable of

withstanding much additional flexing without failing. This fatigue resistance has led to

the design of polypropylene boxes and other containers with self-hinged covers.

Polypropylene is often used for food containers, particularly in dishwasher safe

containers. The melting point of polypropylene is high compared to many other plastics,at 320F (160C), which means that the hot water used when washing dishes will not

cause polypropylene dishware to warp.. Polypropylene is also very easy to add dyes to,

and is often used as a fiber in carpeting, which needs to be rugged and durable, like thecarpet one finds around swimming pools. Unlike nylon, which is also often used as a

fiber for rugged carpeting, polypropylene doesn't soak up water, making it ideal for uses

where it will be constantly subject to moisture.

Where do we use PP?

Although PP has almost half the strength of PVC (with a design stress of 70Kg/cm2 or1000 psi at 22 deg C / 73 deg F), it has a good chemical resistance and used in industrial

piping. We need to consider the fact that there are no known solvents for PP. As a result,

it is a material of choice for industrial chemicals like concentrated acetic acid, alkalis,salts, solvents, hydroxides etc. However with strong oxidizers like hypochlorites,

sulphuric, nitric and hydrofluoric acids environmental stress cracking is likely to result

over a period of time depending on the concentration of oxidizers involved.

It doesn't melt below 160oC (320oF). In fiber form, polypropylene is used to make

indoor-outdoor carpeting, used around swimming pools and mini golf courses. For

outdoor carpet, it is easy to make colored polypropylene, and doesn't absorb water.

Polypropylene shares some of the properties of polyethylene, but it is stiffer, has a highermelting temperature, and is slightly more oxidation-sensitive

What a designer should know about PP ?

1) Polypropylene is a translucent material.2) PP is semi-crystalline and if properly molded will be 50-60% crystalline.3) PP is inert and porous.

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4) PP is an ideal material for integral injection molded hinges. Theoretically thehinges can be as low as 0.1mm thick but preferable to have a product thickness

of more than 0.38 mm. The length of hinge can be at least 1.5 mm for better flex

life.

5) Average density of PP is 0.903 g /cu.cm.6)

The melt flow index range of PP is approx 1 to 35 g/10 min.7) Below a certain thickness, PP molecules orients in the direction of flow. But,Perpendicular direction to flow will be stronger in flexing.

8) Molding tolerances achievable using PP are between 0.07 mm and 0.18 mm9) Production of fibres and film are the two largest markets for PP.10)There are no known solvents for PP11)With strong oxidizers like hypochlorites, sulphuric, nitric and hydrofluoric acids

Environmental stress cracking is likely to result

12)Water absorption is negligible.13)Stiffer, has a higher melting temperature, and is slightly more oxidation-

sensitive than HDPE.

14) Has enough chemical resistance to be used in industrial piping. Althoughlargely unreactive chemically, the presence of the methyl groups makes PP

slightly more susceptible to attack by strong oxidizing agents.15) A major advantage is Polypropylene's higher temperature resistance makes it

particularly suitable for items such as trays, funnels, pails, bottles and

instrument jars that have to be sterilized frequently for use in clinical

environments.

16)Clamping forcesbetween 1 and 5 Tons/square inch can be used as aguideline. Low viscosity (high melt flow rate) resins generally require lower

tonnages than high viscosity resins.