I&EC REPORTS & COMMENTS Whiskers for reinforcing high temperature plastics Less power for making acetylene From plasma jets N e w method for predicting polymer stability
n
WHISKERS AS REINFORCE M E N T MATERIALS Present use of short asbestos fibers suggests how whiskers can be used for laminated and wound structures Stronger and more resistant plastics are certainly destined for the future, and reinforcement of thtse materials with whiskers offers a wide area for profitable exploitation, especially for laminated and wound structures. This was the subject of a paper presented recently by T. V. Milewski and J. J. Shyne of Thermokinetic Fibers, Inc., before the 20th Annual Technical Conference of the Society of Plastics Engineers. Any material which forms crystals can grow whiskers. For example, if sodium chloride is allowed to stand for several days in a polyefiylene bag, whiskers of sodium chloride will form on the outside. These whiskers, and those from other materials as well, are usually single crystals with the growth axis parallel to a prominent crystal direction.
Their cross sections are usually polygonal with low index planes defined in the faces, and they occur in a variety of sizes and shapes, usually with diameters of 1 to 50 microns and lengths up to several centimeters. An essential feature is that one dimension must be large compared to the others. Whiskers are strong because they are essentially perfect crystals-their extremely small dimensions allow little room for defects. Microndiameter sapphire whiskers can have tensile strengths greater than 3.5 million p.s.i. Another outstanding property is elastic behavior-e.g., iron whiskers can be strained elastically up to 5% without permanent deformation, but bulk iron sets at 0.2% strain. Metal oxide whiskers can be deformed 3 to 4Y0 without permanent set. Metal and metal oxide whiskers are grown by four principal processes: high stress; chemical re-
Sapphire whiskers can be grown in several :onzgurations.
duction of halides; electrolytic deposition; and vapor deposition. For example, sapphire (aluminum oxide) whiskers are grown by passing hydrogen containing a small amount of water over molten aluminum a t 2300' to 2900' F. Within to 2 hours, a growth of sapphire whiskers appears over the aluminum, ranging from a cotton-like wool to single needles up to 2 inches in length and 0.005 inch in diameter. Until now, according to Milewski and Shyne, investigations in uses of whiskers for reinforcing composites have been confined to metal and ceramic matrices. Little work has been done in plastics, but more is expected in the future largely because sapphire whiskers, grown in pilot plant quantities today, are entirely satisfactory as reinforcement materials. Asbestos laminates, 457, filled, are now being made with a tensile strength up to 50,000 p.s.i. from fibers having a tensile strength
Shown below from left to right are needles, cluster ball wool, and bayonet blades
VOL. 5 6
NO. 4
APRIL 1964
9
I&EC REPORTS
FUNCTION AND P O T E N T I A L USES OF SAPPHIRE WHISKERS Whiskers Type
I
yo
1
Function
Afifilzcatzon
-
Potential
PLASTICS Cluster mix Chopped needle wool
30-60
Filler in epoxy and Novalac resins
Ablation throat inserts
1-5
Addition to phenolic molding compd. to enhance strength High strength sheet structur6s laminates with phenolic and epoxy resin Composites with high strength, high temp. new polymers Sprinkle additions to tape windings in highly stressed areas
Minature rockets; fuse parts; high strength molded itemsHigh strength bread board structures; computer systems
Needle mat
30-60
Xeedles
10-50
Fine needles
5-20
Wool and ne ed 1es
3-10
800' F. higher service than silica fibers 20-507, strength increase 500,000 p.s.i laminates
Super high temp. molding plastic
100,000 p.s.i. a t 1000"-1500°F.
Flanges and bosses of filament wound structures
Local reinforcement of highly stressed areas to prevent crazing
METALS
1-5
Fine needles Mixed needles
20-50
Filler in powder metallurgy composites with Mo, wo Filler in castings
Rocket nozzles ; gears
Reinforcing unidirectional members
Rods, wires, beams, etc., for space frames
Valves; pumps; nozzle components
Double strength at 3000O F. Reduce brittleness; increase toughness Unidirectional structural beams, 1,000,000 p.s.i.
CERAMICS 1-5
Fine needles
Mixed needles and cluster balls Wool
Wool
5-25 100
90-95
Additive to slip castings
Additive to hydraulic setting
Cast-in-place assemblies
Thermal insulation
High temperature heat shielding rockets ; plasma j furnaces Super high strength, high temp. ceramic
Fine needle wool ground into 1OOyo whisker slip
of 200,000 to 300,000 p.s.i. Hence, it is reasonable to conclude that a laminate 10 times stronger could be produced from sapphire whiskers because their tensile strength is 10 times greater than asbestos-2 to 3 million p.s.i. Also, silicon carbide and graphite look especially promising. I n general, epoxy and phenolic resins wet sapphire whiskers without the pretreatment coating normally required for glass fibers. Shear strengths of 4000 p.s.i. and higher 10
Thin walled high dielectric insulator; high temp motors: plasma guns
Multiple increase in green strength; enhance thermal shock resistance Super bonding cement 3000 '-3500 service
O
F.
1,000,000 p.s.i. ceramic
have been observed between epoxy resins and sapphire whiskers with no pretreatment of fiber surface. Consequently fiber as short as 0.015 inch will provide reinforcement. I n plastics, the straight fiber and wool whiskers can be used in a manner similar to that now used in making asbestos-reinforced composites (nonwoven felts, paper, and molding compounds). But more can be accomplished with whiskers because they can be grown in a variety of
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
shapes such as needles, needle wool, bayonet blades, and clusters of crystals resembling somewhat a ball of loosely packed wool. The important point is that the high strength in the fiber is obtained by having at least one very small cross-sectional dimension. As the technology of using whiskers for reinforcement materials progresses, specific shapes for specific applications will probably be favorede g . , one dimensional fibers for unidirectional reinforced composites such as structural rods and beams, and plastic wire. T h e two dimensional bayonet blades and ribbons will be used in bidirectional laminates for electronic breadboards and component boxes. The tridirectional whiskers such as woollike clusters may be ideally suited for high strength molding compounds. E . KELLER
ACETYLENE FROM PLASMA JETS
Lower energy requirements of improuedyet designs should make filasrna converters more attractive to industry The high power requirements indicated in studies of the conversion of methane to acetvlene by E. N. Eremin in 1958 and 1960 were apparently peculiar to the apparatus and not necessarily indicative of eventual requirements. Soviet investigators at the Institute of Petroleum Chemical Synthesis have studied the problem further [Gulyaev, G. V., Kozlov, G. I., Polak, L. S., Khitrin, L. N., Khudyakov, G. N., Dokl. Akad. Nauk SSSR, Khim. Tekh. 148, No. 3, 641 (1963)], using a 15-kw. single-stage plasmatron with an argon-stabilized arc, and injecting methane through the jet wall downstream from the arc. They found that acetylene yield depends on methane flow rate, arc power consumption, and nozzle design. (Contznued on page 72)
