High-specific-gravity thermoplastics open new design windows

May 3, 2001
Advanced materials and processes continue to push the performance limits of engineered plastics. Here are some recent developments and a few design tips to get the most from that plastic part.

Thermoplastics have been used to decrease part weight and part count or both. Once a base resin has been selected, its physical, mechanical, and electrical properties can be enhanced or manipulated with the addition of carefully selected fillers and reinfor

cements. Examples include carbon-black fillers for electrical conductivity, glass fiber to boost strength, and rubber tougheners for increased flexibility.

Now, a new class of heavily filled thermoplastics with specific gravities between 1.8 and 10 have entered the mix. While retaining the ability to tailor properties and consolidate parts to lower costs, highspecific-gravity (HSG) thermoplastic compounds are providing designers with an additional and powerful tool — the ability to "dialin" a specific part weight across a density range that, at the high end, approaches lead and its alloys. Furthermore, the fillers used in formulating HSG compounds can also impart highly specialized properties including X-ray shielding, X-ray opacity, "soft" magnetic properties, frangibility, and noiseattenuation. In metal-replacement applications, HSG compounds are also injection moldable, which reduces or eliminates secondary machining or assembly operations and cuts cost.

HSG compounds are created by homogeneously combining metallic or mineral fillers with thermoplastics. However, because filler loading is so high, it's critical to select a resin that can accept such loadings without loss to mechanical strength and processibility.

Base resins are typically crystalline thermoplastics because they have better flow properties than amorphous resins. Some of the more common base resins include polyesters (PE), polyamides (PA), polypropylene (PP), and polyurethane (PU) elastomers. The particle size distribution and surface chemistry of the fillers, along with lubricant technology, also play a role in maintaining processability.

Weighing in
A wide range of properties are available across three different weight classes. "Lightweight" HSG compounds have specific gravities in the range of 1.8 to 4. They are typically used as weight replacements for glass, aluminum, and clay. Lightweight HSG compounds also are used in medical devices. Extruded medical tubing, for example, is X-ray visible so that it can't inadvertently be left behind during certain procedures. Injectionmolded poker and other gaming chips also come from lightweight HSG compounds.

Some lightweight HSG materials incorporate reinforcements to increase strength while others add impact modifiers to improve toughness. Because mineral fillers are generally used for this weight range, most lightweight HSG compounds are readily pigmented from light/bright to dark tones.

Specific gravities for the "Middleweight" HSG compounds range from four to seven. They are in the same weight class as die-cast zinc. In several cases, these materials have been designed to conduct magnetic flux (become magnetic). Automotive safety-restraint components come from middleweight HSG compounds. Resin systems such as PP, ABS, PU and nylon 6, for example, have demonstrated no significant loss to physical properties with the high metallic-filler loadings. Middleweight HSG parts can be colored in darker hues.

The "heavyweight" HSG compounds have specific gravities between 7 and 10. They have equivalent weights to stainless steel, bronze, and lead. The heavyweight HSG compounds are an effective lead-free alternative for shielding X-rays in diagnostic medical applications. Target shooters also benefit from the lead-free compounds. Training rounds made from the material withstand muzzle pressures as unjacketed projectiles, yet disintegrate on impact. Heavyweight HSG materials can also be used to damp vibration such as in an automobile steering columns.

Information for this article provided by LNP Engineering Plastics Inc., 475 Creamery Way, Exton PA 19341, (800)532-2567, www.LNP.com

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