Many of us were taught that if more than one variable were changed at a time, the cause of some end result could never be determined. This is certainly true if the experiment is not planned to accommodate analysis of the data. The complex problems faced by modem scientists and engineers can not be solved efficiently using one-at-a-time methods. Good experimental design promotes broader exploration of the variables studied, produces more results from fewer experiments, and validates results by comparing them against measured experimental error. This is done by deliberately varying all variables from run-to-run in systematic patterns. Timely product development and problems solving bears directly on profitability. A systematic approach can pay handsome dividends. The basis of good experimental design and several simple designs for everyday use are presented.
An earlier paper ( 1 ) established the need for new compounding technology to deal with changes in melt flow resulting from the postchlorination of PVC resins. The present work attacks the more-than-doubled melt viscosity of chlorinated PVC as measured in stabilized resin. The potential for marked reduction in viscosity, improvement in processing stability, and retention of marketable product properties is explored. The development process including the definition of performance requirements, test methods used, and resulting balance of properties is described.
he evaluation of both initial and long-term properties T of rigid plastic materials for outdoor use must be based on the requirements of specific applications. Plastic parts for architectural use frequently serve decorative or protective functions requiring no structural strength beyond self support. Such parts must withstand abuse in the form of impact from hail, ladders, car doors, and various types of assault by children and lawn mowers.It must not be assumed that a stable color and surface appearance indicate retention of impact or abuse resistance. Many plastic materials suffer gradual embrittlement when exposed to weathering conditions. This embrittlement appears to proceed much faster in thin sections than many common test methods would indicate. Materials with very high initial impact strength do not always remain more durable after years of ex-I C MANDREL SPEC1 PEDESTAL 4 I Figure 1. Variable height impact tester. 124 posure than materials of low-to-moderate initial impact strength.Preliminary work indicated that a variable height impact test, such as the Gardner Tester, performed on samples of extruded products yielded more meaningful weatherability data than did a variety of test methods previously employed. Specimen GeometryA test which will measure properties of a specimen representative of the end products involved is obviously desirable. The influence of ultra-violet excitation, moisture, and constantly varying temperature will be much more profound on a thin sample with its large surface area per unit volume than on a thick sample. Weatherability evaluations should be made with specimen thick. nesses typical of the application considered.The necessity for die cutting or machining specimens introduces errors regardless of project design. Machining prior to exposure introduces surfaces and surface characteristics to exposure which are not present in product usage. Machining after exposure allows error through heat build up in the sample and variations due to wear of the cutting tools. The most desirable specimens will require little or no special preparation.
The behavior of PVC resins has been shown to change dramatically when postchlorinated from 57 to 70 percent chlorine. Many of these changes are beneficial: heat resistance, thermofonnability, and combustion properties are improved to an extent that opens new market opportunities for properly formulated chlorinated PVC (CPVC) materials. Sharply increasing melt viscosity also results from increasing chlorine content, creating the need for compounding and processing expertise differing from that for rigid PVC. The fact that such expertise has been developed, and is constantly being improved, is expanding the use of CPVC.
This is the second in a series of reports on outdoor performance of 20 plastics. Computer-aided analysis of tensile and flexural strength data is presented for 24-months exposure in Arizona, Florida and Washington, D. C. Ultimate elongation (tensile) and 5% stress (flexural) showed the greatest change of 11 parameters investigated. These changes characterized loss of elasticity and flexibility. Because of the exponential nature of the deterioration, an objective critical fai lure-point was defined, by which most of the change had occurred. This is reached at 36.87, retention of initial property-value. Classification systems were useful for quantitatively distinguishing physical performance. Generally, samples performed worst in Arizona and best in Washington, D. C. Actinic radiation and heat thus appear to be primary agents causing physical degradation.
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