23 Controlled Release from Hollow Fibers E. ASHARE, T. W. BROOKS, and D. W. SWENSON
Downloaded by INDIANA UNIV BLOOMINGTON on March 24, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0033.ch023
CONREL, An Albany International Co., 735 Providence Highway, Norwood, Mass. 02062
In recent years controlled release of active materials has emerged as a distinct technology in answer to a great many end-useneeds i n medicine, agriculture, forestry and the home. The subject has gained enough prominence to deserve a book (1) and two international symposia (2). A growing number of commercial products are based on controlled release formulations including such familiar items as time release oral medications, fragrance dispensing wicks and gels, and impregnated plastic pesticide strips or animal c o l l a r s . Modern controlled release devices and systems such as impregnated plastic or rubber matrices, membrane envelopes, laminated poromerics and microcapsules testify to the growing level of sophistication being demanded of this technology to satisfy increasingly complex end-use requirements. A novel controlled release device for vaporizable materials has been developed, consisting of hollow fibers which function both as a reservoir and as a means of control over dissemination of vapors from the open end of the fibers. There i s great f l e x i bility in the use of this system in that it i s capable of dispensing any vaporizable material at a release rate which i s dependent on the inside diameter and number of fibers and for an effective l i f e dependent on fiber length. This system shows promise for use with insect pheromones, insecticides, animal repellents, and fragrances. Mass Transport Theory The mechanism for dispensing of volatile materials from capillary channels consists of three steps: a. evaporation at the liquid-vapor interface; b. diffusion from the liquid-vapor interface to the open end of the hollow fiber; c. convection away from the open end. Generally, the diffusion step w i l l be the rate-controlling factor. The rate of diffusion can be predicted by use of the transport equations. 273
Paul and Harris; Controlled Release Polymeric Formulations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
274
CONTROLLED
The molar flux (Fig. 1), N
RELEASE
POLYMERIC
FORMULATIONS
i s given by (3, p. 522):
a
N « - cD_ ^ a (1) 1-x dz where x i s the mole fraction of the volatile material, c i t s molar density, and D the diffusion coefficient. A mass balance, dN /dz«0, combined with Eq. (1) and assuming ideal gas behavior gives: a
a
â
a
-In
