Page 168 - Engineering Plastics Handbook

Polybutylene Terephthalate (PBT) 141

TABLE 7.5 Summary of PBT Blend Systems
Types PBT blend Property
Crystalline PBT/PET Surface gloss, trans reaction
Amorphous PBT/PC Impact strength, dimensional stability,
toughness, trans reaction
PBT/ABS Low warpage, impact strength
PBT/PPE Heat resistance, dimensional stability
PBT/PS Dimensional stability
Impact modifier PBT/TPU Impact strength, chemical resistance
PBT/elastomer Low-temperature impact strength

parts molded with semicrystalline polymers will be more chemically resist-
ant than amorphous polymers. The semicrystalline character and the
chemical structure of PBT ensure a good resistance to many chemical
substances. PBT thermoplastics are characterized by their excellent resist-
ance to organic solvents such as aliphatic hydrocarbons, petroleum, alco-
hols, oils, ether, detergents, and chlorinated hydrocarbons at room
temperature. The material is also resistant to weak acid, weak base, water,
and most aqueous salt solutions. However, PBT is not resistant to strong
acids and bases, phenols, or strong oxidizing acids. Above the glass tran-
sition temperature of PBT, the polymer becomes more sensitive to the
effects of aromatic hydrocarbons and ketones. However, PBT is resistant
to a typical automotive fluid like petroleum, greases, engine and trans-
former oils, or brake fluid, even at high temperatures. Table 7.6 shows an
overview of the resistance against various chemicals. Product design, pro-
cessing methods and equipment, and other parameters may affect the
actual chemical resistance of a molded part. The true chemical resistance
of specific applications should be tested in each specific use condition
before commercial production.

Hydrolysis resistance
Under some conditions, the interaction of PBT with water can cause
hydrolytic degradation and give loss of molecular weight. With loss of
molecular weight, the PBT thermoplastics will lose properties and become
brittle. The combination of high temperatures and high moisture content
affects the properties of PBT to a serious degree. The hydrolytic degrada-
tion can also be accelerated by acidic or basic conditions. The number of free
carboxylic end groups in the PBT molecule also influences the hydrolytic
resistance [14, 15]. There are two major classes of water exposure: One is
the presence of moisture in a molten state during melt processing, and the
other is exposure of a molded part to water during its actual use. During
melt processing at high temperature, moisture will react very quickly with