Kurt Richard of Germany's FarT>worke Hoechst doubts t h e full correlation of the Bell test in applications where mechanical stresses remain constant— such as polyethylene pipes under inside pressure. H e notes that when a second sample has ten times tfie experimental stress-crack resistance of the first, it does not follow that the comparable pipe will have ten times the life. Basically, creep and stress cracking hamper the exact determination o»f polyethylene's maximum stress. According to Richard, the testing must be extended over several y e a r s and performed with p i p e under a constant inside pressure. This tells the influence of time, shape, and stress conditions, a s well as of the ambient medium Some people say that West German custom demanding a 50-year guarantee for high pressure potable w a t e r pipe stems from past fierce competition among steel manufacturers. In underground applications, wintertime temperatures average about 46° 3\ in GerLack of polyethylene pipe accelerated test methods h a s created problems. many, according to Richard. Spring Plastics must b e tested in pipe form under pressure t o include multiaxial stress's a n d fall increase to about 50° F . , while influence. H e r e a Hoechst ( G e r m a n y ) technician adjusts pipe for testing in summer a short duration increase t o 60° F. occurs. Effective average temp e r a t u r e is therefore around 50° F \ Usual Ziegler-type polyethylene has a t least a 30-year lifespan at that temperature a n d 710 p.s.i., which Richard conFormulation, fabrication techniques help siders quite favorable. However, i n Germany, coming applications will b e eliminate Ziegler polymer's major fault more a n d more surface pipe, required to stand averages of 60° F. lriairitairiing T h e elastomer achieves this the 50-year life at 710 p.s.i. ACS Reports of en- ance. NATIONAL vironmental stress- result by lowering stresses within the By increasing molecular weight and MEETING cracking almost polymer. Addition of the r u b b e r has using "some additional means" Richard gave Ziegler low- interesting side-effects. The compound claims to have developed a low-presPaint/ Plastics, p r e s s u r e-p r o c ess is softer a n d tougher, modulus a n d ten- sure-process polyethylene resin which & Printing Ink polyethylene an in- sile strength are lower, and the rubber he has pipe-tested satisfactorily for ;:: Chemistry::: surmountable b a d acts as a plasticizer. 25,000 hours at room temperatvxre. H e McTigue notes the correlation be- feels this is equivalent to an. actual n a m e for pipe. Developments in t h e past year have been keyed to improving t w e e n molecular weight and stress- service life of more than 5 0 years the resistance of ethylene polymers to cracking resistance. T h e Bell Tele- at 68° F. a n d a maximum stress of 710 this defect. Three techniques provide p h o n e Laboratories test method ( u n d e r p.s.i. The pressure allows a safety facthe solution: consideration by ASTM as a proposed tor of 1.3 against any pipe "bursting standard test since 1955) shows poly- after 5 0 years. In practice, actual • Synthesis of increased molecular mers with molecular weights of 115,000 safety factors would be considerably weight resins t o 250,000 had 50r'< failures after 20 higher, as piping is rarely u n d e r con• Stress reduction, by better molding t o 120 test hours. On the other hand, tinuous stress at full pressure. methods polymers with molecular weights of Richard feels t h a t molecular orienta• Compounding with elastomeric ad- 4 5 0 , 0 0 0 to 570,000 provide stress-crack tion h a s not been sufficiently considditives resistance well beyond 2000 test hours. ered a s a possibility for increasing pipe Hercules researcher, F. H. McTigue, I n some cases where tests w e r e allowed strength. Processing methods definitely points to the use of up to 20r
