environmental stress cracking (ESC) problems. The maximum service temper-
ature is above 100˚C (110˚C/230˚F). The electrical insulation properties of PP
are good and a high gloss, scratch-resistant, surface is possible.
PP is extensively modified by the addition of glass fibers (GF),
mineral fillers, thermoplastic rubbers or a combination of these. For example,
talc filling improves the rigidity, hardness, creep resistance, dimensional sta-
bility and the HDT of PP. However, it worsens surface appearance and elonga-
tion at break. Calcium carbonate filling improves the surface hardness, elonga-
tion at break, and the notched impact strength, at lower cost, compared to
talc. However, a lower modulus, tensile strength and HDT, compared to talc,
result. Glass fiber (GF) filling improves the rigidity, stiffness, hardness, creep
resistance, dimensional stability and the HDT of PP but worsens the surface
appearance and elongation at break. There is a greater improvement in proper-
ties, if the fillers/fibers are coupled to the PP. Rubber modification, to produce
an elastomer modified grade, improves the low temperature impact strength
and the elongation at break (of both PP-H and PP-CO). There is some reduc-
tion in stiffness and an increase in the cost. The use of rubber helps to offset
the effect of filler addition. New polymerization technology now allows the pro-
duction of, directly polymerized, soft grades that compete with elastomer
modified PP. The flexural modulus of a directly polymerized, super-tough
material can be as much as 80 MPa.
PP producers continue to develop and to create new markets. This
is because of the incredible versatility of the material and the sophisticated
nature of the polymerization catalyst systems now employed. For instance,
two incompatible polymers may joined, forming what is called a graft copoly-
mer. To do this, one monomer is polymerized in the presence of a formed
polymer and some of the new polymer formed, chemically grafts, or joins,
onto the existing polymer to covalently bond them together. Thus, one can
produce a thermoplastic material, based on a polyolefin (PO), onto which is
bonded, or grafted, an amorphous material. Such a material may be referred
to as high value alloys (like Hivalloy from Montell). These alloys will compete
with materials such as nylon, acetal and polyphenylene oxide (modified). By
using different co-monomers it is possible to produce grades that have good
weatherability, or good dimensional stability and impact strength or another
that has good dimensional stability at high temperatures. A major advantage
of these copolymers is their low densities, which range from 0.93 to 0.96
g/cm
3
, regardless of the grafted comonomer.
Most of the chemical companies that produce polyolefins, are
investing in new technology, especially metallocene-based catalysts, to obtain
a wider range of products (high product flexibility), at comparatively low prod-
uct costs, from their production facilities. Because of such developments and
investments, homopolymers and copolymers based on olefins will increasingly
dominate the plastics industry. Metallocene catalyzed polypropylene (mPP) is
predominantly based on propylene and produced using a metallocene con-
strained geometry catalyst. The PP produced can be a homopolymer or a
copolymer and the process can produce both isotactic polypropylene (iPP or
miPP) and syndiotactic polypropylene (sPP or msPP). In general, the miPP is
claimed to have properties that are superior to other types of iPP, such as
improved toughness, better clarity, and lower heat seal temperatures. It can be
used in place of post-reactor peroxide-cracked polypropylene and medical
grade PP (mPP has lower extractables). Extra stiff grades with good clarity
and impact strength are now also available. At present, mPP costs more than
conventional PP, but it can be processed on similar equipment.
7. Flow Properties
PP extrusion resins can have melt flow rates (MFR) (230˚C/21.6 N or 2.16 kg)
between 0.4 and 8. The low MFR values refer to materials with high impact
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section 8: guides for the following materials