Controllable induction period

The high permeability of PCL and its copolymers coupled with a controllable induction period prior to polymer weight loss (vide infra) lends itself to the development of delivery devices that are based on diffusion-controlled delivery of the drug during the induction period prior to weight loss. The subsequent biodegradation of the polymer serves the purpose of eliminating the need to recover the spent device. 

Ideal performance of biodegradable plastics in agriculture. Eb = elongation at break, IPa,b = time-controlled induction periods during which mechanical and chemical properties do not change. 

To assist in control of the onset of vulcanization, a retarder or prevulcanization iuhibiter (PVI) is used. Retardation of the onset of cure does not mean that the rate of cure is slowed, in fact cure rate may actually be increased. Rather, there is an induction period prior to cure. 

Pedersen on the effect of various chelating agents upon the autoxidation of petroleum (a useful model system) catalysed by various transition metals. The deactivating efficiency is the percentage restoration of the original induction period observed with a control sample of petroleum without metal or deactivator. Only iV" -tetrasalicylidenetetra(aminomethyl)methane was effective... 

Reactions of the general type A + B -> AB may proceed by a nucleation and diffusion-controlled growth process. Welch [111] discusses one possible mechanism whereby A is accepted as solid solution into crystalline B and reacts to precipitate AB product preferentially in the vicinity of the interface with A, since the concentration is expected to be greatest here. There may be an initial induction period during solid solution formation prior to the onset of product phase precipitation. Nuclei of AB are subsequently produced at surfaces of particles of B and growth may occur with or without maintained nucleation. 

The investigators studied various blends of the three polymers in order to control the rate of chain scission and thus influence the induction period and onset of drug release. None of the blends provided the desired 1-week zero-order kinetics. However, blends of different microsphere types did show promise in vitro (88). 

The same samples, after a pretreatment in flowing oxygen (10%) at 625 K, were used as catalysts for the oxidative dehydrogenation of ethanol and methanol in the same reactor. The reaction mixture consisted of O2 (3 or 5%), methanol vapor (3%) or ethanol vapor (5%) and He (balance), all delivered by Tylan mass flow controllers or vaporizer flow controllers. Products were analyzed by gas chromatography. The catalysts exhibited no induction period and their activities were stable over many days and over repeated temperature cycles. 

In the same context, but in opposite sense, the presence of inhibitors (negative catalysts, increasing energy of activation) may seriously interfere with the smooth progress of a reaction. An inhibitor may initiate an induction period which can lead to problems in establishing and controlling a desired reaction. For further... 

Factors of importance in preventing such thermal runaway reactions are mainly related to the control of reaction velocity and temperature within suitable limits. These may involve such considerations as adequate heating and particularly cooling capacity in both liquid and vapour phases of a reaction system proportions of reactants and rates of addition (allowing for an induction period) use of solvents as diluents and to reduce viscosity of the reaction medium adequate agitation and mixing in the reactor control of reaction or distillation pressure use of an inert atmosphere. 

Interaction in hexane to produce the 2,4-di-fert-butyl derivative often starts only after an induction period and may then proceed very violently. Careful temperature control is imperative. 

The earlier references, which state that this powerful oxidant is stable when pure, but explosive when formed as a layer on metallic potassium [1,2], are not wholly correct [3], because the superoxide is manufactured uneventfully by spraying the molten metal into air to effect oxidation [4], Previous incidents appear to have involved the explosive oxidation of unsuspected traces of mineral oil or solvents [3]. However, mixtures of the superoxide with liquid or solid potassium-sodimn alloys will ignite spontaneously after an induction period of 18 min, but combustion while violent is not explosive [3], The additional presence of water (which reduces the induction period) or hydrocarbon contaminant did produce explosion hazards under various circumstances [5], Contact of liquid potassium with the superoxide gives no obvious reaction below 117°C and a controlled reaction between 117 and 177°C, but an explosive reaction occurs above 177°C. Heating at 100°C/min from IT caused explosion at 208°C [6],... 

The checkers observed a significant induction period prior to onset of the exothermic reaction. Care must be taken to avoid addition of the nickel acetylacetonate too rapidly initially or temperature control becomes difficult. 

More rudimentary tests to identify the stability of powders are the so-called "hot plate" tests. Here, the substance is placed in a well-defined manner (circular, specified thickness of the layer, and so forth) on a hot plate with a controlled temperature. The temperature in the center of the layer is recorded continuously, and the progress of the self-heating is followed. Observations are made if self-heating of the substance occurs, the induction period of such self-heating, and the extent of the effect such as smoldering or spontaneous ignition. 

Preparation of Grignard reagents is frequently beset by practical difficulties in establishing the reaction (i.e. is usually subject to an induction period). Improved equipment and control methods for safer preparations have been described [2], Initiation of the Grignard reaction has been studied in a heat flow calorimeter [3],... 

A retrospective study of 136 BCME workers employed at least 5 years revealed 5 cases of lung cancer, which represented a ninefold increase in lung cancer risks 0.54 cases would have been expected to occur in the plant population. The predominant histologic type of carcinoma was small cell undifferentiated. Exposure ranged from 7.5 to 14 years, and the mean induction period was 15 years. In addition, abnormal sputum cytology was observed in 34% of 115 current workers with exposure to BCME for 5 or more years, as contrasted with 11 % in a control group. 

Dark Control (mg) Induction Period Beginning (mg) Dark Control (mg) Induction Period ... 

---