Page 171 - Engineering Plastics Handbook

144 Engineering Plastics

TABLE 7.7 Properties of Flame-Retardant Glass-Fiber-Reinforced PBT
† ‡
Halogen FR* Nonhalogen Nonhalogen
Property Unit (GF 30%) (GF 30%) (GF 30%)
Specific gravity — 1.62 1.60 1.70
Tensile strength MPa 130 120 110
Elongation at break % 3 3 3
Flexural strength MPa 190 180 170
Flexural modulus MPa 8500 9500 1100
Izod impact strength J/m 80 70 60
(notched)
HDT (1.82 MPa) °C 210 210 200
HDT (0.45 MPa) °C 215 215 210
Flammability UL-94 V-0 V-0 V-0
* Lupox GP-2306F, LG Chem, Ltd.
†
Lupox GN-2306F (with red P), LG Chem, Ltd.
‡
Lupox GN-2308F (without red P), LG Chem, Ltd.
SOURCE: Lupox PBT brochure and data sheets [20].

Flame-retardant systems that combine nitrogen and phosphorus, such as
ammonium polyphosphate or melamine phosphate, do not have sufficient
thermal stability for melt processing of PBT resins. Phosphorus-based
systems are generally effective in imparting flame retardancy to amor-
phous thermoplastics such as PC/ABS blends and modified polyphenylene
oxide. In PBT resins, these ingredients must be added at such high levels
to obtain V-0 class that physical properties are negatively affected. Surface-
treated red phosphorus is successfully used as an effective halogen-free
flame retardant for FR PBT. Red phosphorus systems have very high
active P components but may have a color limitation. Only a few success-
ful developments have been carried out in this field. While halogen-free
flame-retardant polyamide products have already been used in the market,
corresponding PBT grades are still at the beginning stage. Table 7.7 shows
a summary of properties of some flame-retardant, glass-fiber-reinforced
PBT grades.

Processing of PBT
The melt of polybutylene terephthalate shows a low viscosity and very fast
crystallization, hence allowing for easy processing. PBT resins can be
processed by using conventional processes such as spinning, extrusion,
injection molding, and gas-assisted injection molding. Injection molding
is the processing method that is used most frequently for PBT compounds
and its alloys. Processing is simple, with good flow properties leading to
fast cycle times by using standard injection-molding machines [19].
Postmolding operations such as welding, fastening, and gluing are also
easy, as are printing, painting, and laser marking.