Slow crack growth behavior of medium‐density polyethylene pipe samples has been investigated by applying standard and nonstandard fracture mechanics test methods. Slow crack growth rate varied by several orders of magnitude among the PE2306 materials from different manufacturers. Slow crack generated surfaces exhibit fibrous textures of varying degrees of coarseness when examined under the scanning electron microscope. When the measured fiber heights, δ/2, of these samples were plotted against the applied stress intensity factor, KI, they showed a simple power law relationship (of exponent 2). Assuming that the pulled‐out fibers transmit almost uniform tensile stresses across the crazed zone at the crack front, the Dugdale‐Barenblatt model was introduced to predict the fiber height. Although the model needs further modification, it provides a general power law relationship between the fiber height and the applied stress intensity factor.

Environmental stress cracking (ESC) has been investigated as a tool for evaluating the relative quality of piping materials used for distributing fuel gas. Heretofore unexplained differences in failure times of pipe extruded by different manufacturers from the same medium-density polyethylene resin have been observed. Lot-to-lot variations have also been noted for which no plausible cause could be found. Scanning electron microscopy has revealed that surface features of the pipe can contribute to premature failure in polyethylene piping material. EXPERIMENTALTwo lots of 1-in.-diam (2.54 cm) polyethylene pipe corresponding to ASTM designation P E 2306,' both made from the same material but processed by different extruders, were tested in the compressed-ring constant strain test. The test was first described by Rader2 and is being balloted by ASTM for possible adoption as a recommended procedure. In effect, the test approximates the stress configuration in the more common bent strip test developed by Bell Laboratorie~.~The compressed-ring method (for 1-in. pipe) consists of cutting a 12.7-mm-wide ring out of the pipe and pressing a razor blade 0.64 mm deep into the outside wall a t the center of the ring, parallel to the edge. The notch is 19 mm long and is consistently located in the region of minimum wall thickness.Eight of these specimens are mounted between two compression plates with the notched area positioned parallel to the direction of compression. The specimens are compressed so that the plates are 9.14 mm apart and are then placed in a 25% solution of the stress cracking agent a t 50OC. The times for crack initiation are recorded. A picture of the compressed ring fixture with specimens prior t o immersion in Igepal are shown in Figure 1.The stress cracking agent used in this test, Igepal CO-630 (GAF Corp.), is a highly polar, nonionic surfactant with the following structure:

SynopsisA process has been developed by which very high-modulus semicrystalline polymer films can be extruded continuously from a melt. This is accomplished by controlled cooling of the melt in a two-stage flow channel. A temperature gradient along the flow channel quenches the melt prior to an area reduction in which the polymer undergoes solid-state orientation. Analysis of high-density polyethylene tapes extruded by this process shows that they have properties similar to samples hydrostatically extruded a t 120°C. Infrared analysis was used to determine both the degree of crystallinity and degree of orientation in these tapes as well as previously prepared hydrostatically extruded samples.

A simple algebraic model of wave motion arising in the adiabatic fixed-desiccant bed dehumidification of an air stream is applied to the prediction of the performance potential of a desiccant air conditioning system. The model is used to explain the increase in cooling system performance that has been realized through the use of mixed inert-desiccant material adsorption beds. The response of cooling system performance to changes in external process conditions is examined and conclusions are drawn relative to optimization of system characteristics.

Calendering of molten thermoplastics is frequently used to produce sheet materials of uniform thickness. The mechanical and chemical properties of such sheets are those inherent to the basic isotropic material. If semicrystalline polymers are rolled into sheets at temperatures below their melting point, significant molecular orientation takes place. Mechanical property enhancement and improved chemical stability then accompany good thickness control in the oriented sheet. In a series of experiments with high-density polyethylene and polypropylene, tensile moduli and tensile strengths as high as 8.7 GPa and 300 MPa, respectively, have been obtained. Output speeds as high as 20 m/min are easily reached. Chemical and dimensional stability in hot polybutene oil has been demonstrated. Less than 2 percent change in weight, thickness, or length of samples has been observed after 5000 hr exposure in the oil at 100°C. The most critical factors in obtaining and controlling these properties are roll temperature and degree of thickness reduction obtained during rolling. Property control and reproducibility have been shown to be good.