High-heat nylon challenges PPS.

As an addition to its Zytel nylon line, DuPont Co., Wilmington, Del., is launching a new polymer chemistry that features significantly higher temperature performance and reduced moisture absorption. Three new injection molding grades of Zytel "HTN" (high-temperature nylon) contain glass fiber and are heat stabilized. Their melting points are 572 F, 68 [degrees] above that of nylon 66.

Aside from its enhanced performance profile, DuPont says the Zytel HTN series also offers more "friendly" processing and flow characteristics than either polyphenylene sulfide (PPS) or polyetherimide (GE's Ultem), as well as a pricing structure that the company says will give it a cost/performance advantage over these higher-end engineering thermoplastics in automotive and electronic applications.

Ash Sahi, Zytel HTN business development manager, describes the new resin as an aromatic, semicrystalline nylon copolymer. Its properties stem from a composition based on three nontraditional nylon monomers, all made by DuPont. The first two, aromatic terephthalic acid and aliphatic HMD (hexamethylene diamine), account for the high-temperature, low-moisture-absorption properties and improved chemical/fuel resistance. The third monomer, an aliphatic amine known as MPMD (sold under the DuPont trade name "Dytek" A), helps to enhance processability, offering cycle times and flow characteristics similar to standard nylon 66, DuPont says.

DuPONT'S NEW ZYTEL HTN NYLONS (DRY AS MOLDED)

                                51G45HSL    51G35HSL    FR51G35L
                                45% Glass   35% Glass   35% Glass

Tensile Strength, psi            35,500      31,000      24,000

Elongation, %

Flexural Modulus, kpsi           2000        1500        1800

Notched Izod, ft-lb/in.          2.2         1.8         1.6

Heat Deflection, F

Specific Gravity                 1.58        1.47        1.65

Moisture Absorption, %
@ 24 hr                          0.27        0.40        0.25
@ Saturation                     2.80        3.50        -

Mold Shrinkage, % (0.125-in.)
Flow Direction                   0.18        0.20        -
Transverse Direction             0.70        0.70        -

Dielectric Strength, v/mil,
1/16-in., Short Time             -           730         630

Properties of the three grades appear in the accompanying table. The flame-retardant grade (designated "FR") reportedly meets UL 94V-0 at 1/32 in. That grade also has a dielectric constant of 4.4 at 1000 Hz and dissipation factor of 0.03 at the same frequency. Its volume resistivity is [10.sup.15] ohm-cm and comparative tracking index (CTI) is 590. DuPont recommends a melt-temperature range of 600-625 F for these materials and mold temperature of 265-320 F.

The HTN line is designed to offer greater dimensional stability over a wider temperature range, owing to its lower moisture absorption. Zytel HTN has a glass-transition temperature of 257 F, compared with 122 F for standard nylon 66, and is expected to withstand long-term continuous use in the range of 320 F.

Sahi also says Zytel HTN will offer advantages in performance or pricing relative to other recently introduced partially aromatic polyamides, such as Amodel PPA from Amoco Performance Products Inc., Atlanta, or Stanyl nylon 46 from DSM Engineering Plastics North America, Evansville, Ind. (AlliedSignal Inc. introduced a lower-moisture-absorption nylon 6 at NPE, but has shelved plans for commercial development of its own higher-heat nylon.)

The new grades are priced in a range of $2.50 to $3.50/lb tl. Sahi says additional injection molding grades are slated for introduction in the near term.