Alan T. Riga

Thermal stability of folic acid

Abstract This study attempts to identify the degradative process which folic acid undergoes under thermal stress. In order to facilitate the process, the various pieces of the chemical structure, namely, p -amino benzoic acid (PABA), pterin and glutamic acid as both its d - and l -isomers were investigated as separate entities. These structured pieces were then compared to the composite folic acid degradative thermogram in order to identify the peaks seen and provide direction for the interpolation of the degradative mechanism [Thermal stability of folic acid and associated excipients, M.Sc. thesis, 2001]. It was observed that none of the structural pieces could be superimposed as assumed earlier, and hence, an attempt was made to identify the decomposition products using various analytical techniques such as infrared (IR) spectroscopy, mass spectroscopy (MS) and X-ray diffraction (XRD) which suggested that the glutamic acid fragment is lost first as evidenced by acid loss and amide enhancement in the IR spectra. The vitamin was ultimately degraded to carbon fragments and that further identification was not necessary.

Machinability of steels and titanium alloys under lubrication

Abstract The madiinability of AISI 304 stainless steel and Ti-6Al-4V alloy was investigated using a tapping tester under lubrication. Three overbased sulfonates with/without sulfurized olefin were used as lubricants for the tests and compared with a reference lubricant to calculate the tapping efficiencies. Tapping test results showed that sodium sulfonate and calcium sulfonate gave better performance than magnesium sulfonate, which was related to the presence of carbonate and their reactivity with metals at high temperatures. It was also observed that the addition of sulfurized olefin to the overbased sulfonates enhanced tapping efficiencies. Surface analysis showed that the improved efficiency and surface finish of stainless steels tested with a combination of overbased sulfonates and sulfurized olefin is due to the presence of a binding oxide layer underneath the mixed sulfide/oxide layer and an increased total film thickness. For Ti-6Al-4V alloy, the presence of titanium transfer was observed. It is suggested from this study that the tapping efficiency is inversely proportional to the degree of titanium transfer from the Ti-6Al-4V plate to the high strength steel tap.

Oxidative behavior of polymers by thermogravimetric analysis, differential thermal analysis and pressure differential scanning calorimetry

Abstract Oxidative behavior of commercial engineering plastics, polyolefins and elastomers has been evaluated by simultaneous thermogravimetric analysis (TGA)–differential thermal analysis (DTA) and pressure differential scanning calorimetry (PDSC). There is a good correlation between the measured stability by PDSC in oxygen and DTA/TGA in air and nitrogen for olefin polymers. The thermo-oxidative properties of engineering plastics, polyolefins and elastomers are reliable and precise. Glass-filled Nylon 66 and reinforced polypropylene were studied in an air oven at 300°C as a function of time by Fourier transform infrared (FTIR) spectroscopy. The mechanism for thermal oxidation was different for these two polymers. Polyolefin oxidation was delineated by rapid carbonyl formation, while Nylon 66 showed minimum carbonyl in the FTIR. An interesting outcome of this study is that polyethylene and polypropylene have significantly high heats of combustion. At high temperatures and pressures, it appears that these polymers can be used as sources of fuel.

The erosion-corrosion of nickel-base diesel engine exhaust valves

Abstract There are many variables that can influence the failure of nickel-base, two-cycle diesel engine exhaust valves, including engine design characteristics and operating conditions, as well as lubricant additives and resulting deposits. The entire system must be considered to understand fully the failure mechanism. However, in this preliminary investigation the emphasis was on modeling exhaust valve failure as an erosion-corrosion wear mode. This study includes (a) the effect of lubricant additives on the formation of exhaust valve deposits and (b) the effect of valve deposit morphology and composition on the erosion-corrosion of valve seat surfaces. In order to study the potential effects of oil ash chemistry, experimental all-calcium and all-magnesium formulations were explored. Testing with a calcium-containing engine oil resulted in the formation of deposits rich in calcium sulfate (CaSO 4 ), and catastrophic failure of several exhaust valves. Although a heavier accumulation of deposits was observed on valves tested with a magnesium-containing oil, erosion-corrosion of valve seat surfaces did not occur. Deposits from the latter test contained magnesium zinc phosphate (MgZn 2 (PO 4 ) 2 ) as the principal constituent. This study provides some initial evidence that the erosion-corrosion of exhaust valves was not exclusively related to the thickness of valve seat deposits. Heat transfer and thermal expansion characteristics of deposits, as influenced by oil additive metals, may have played a role in valve failure. Spalling of calcium-sulfate-containing deposits and accelerated oxidation of the valve alloy resulted in the leakage of hot exhaust gases and subsequently, severe erosion of valve seat surfaces.

Thermal Stability Of Folic Acid in the solid-state

This study attempts to identify the degradative process which folic acid undergoes in the solid-state under thermal stress. In order to facilitate the process, the various pieces of the chemical structure, namely, p-amino benzoic acid, pterin and glutamic acid as both its d- and l-isomers were investigated as separate entities. These structured solid-state pieces were then compared to the composite solid state folic acid degradative curves in order to identify the peaks seen and provide direction for the interpolation of the degradative mechanism. It was observed that none of the structural pieces could be superimposed as assumed earlier and hence an attempt was made to identify the decomposition products using various analytical techniques such as infrared spectroscopy, mass spectroscopy and X-ray diffraction which suggested that the glutamic acid fragment is lost first as evidenced by acid loss and amide enhancement in the IR spectra. The vitamin was ultimately degrading to carbon fragments and that further identification was not necessary.

Factors affecting oxidation properties in differential scanning calorimetric studies

Abstract Fundamental knowledge of the oxidative properties of commercial oils is necessary to predict the stability of these fluids. There is an industry-wide need for a hydrocarbon reference fluid that can be used to establish oxidation properties of motor oils, greases, diesel oils, transmission fluids and vegetable oils. A primary tool to determine the oxidation of oils is differential scanning calorimetry (DSC) or pressure DSC. A diluted passenger-car motor oil was used to establish an experimentally designed relationship between the variables and the oxidation induction time (OIT) by DSC or PDSC. The variables used for developing this protocol were temperature, pressure, heating rate, sample mass, gas flow rate, and gas type, air or oxygen. The DSC pan metallurgy played a significant role in the measured OIT. Statistical quality control charting of the oxidation properties focused on a special cause, out-of-control problem that was related to the impurities in the aluminum pans. Iron impurities caused a decrease in OIT of the reference oil. Increasing concentrations of chromium, probably as chromium oxide, stabilized the oxidation process.

Characterization of the ash deposits from AFBC system using thermal techniques

Deposit formation is considered as an important problem challenging the improvement of coal-combustion performance. Deposits usually result from the thermal decomposition of coal-borne mineral matter followed by impact and adhesion along the hot gas pathway. Several ash deposit samples collected from the heat exchange tube in the Atmospheric Fluidized Bed Combustor (AFBC) at Western Kentucky University, were used in the present study. The SDT-MS and XRD techniques were employed to examine the sample composition and the mechanism responsible for the deposit formation. This work especially concentrated on the investigation of the chlorine containing species and the behavior of chlorine during the heat treatment. It was proved that the alkali or alkali earth chloride would vaporize significantly under the tested condition, and these compounds were likely responsible for the deposit formation.

Oxidative behavior of materials by thermal analytical techniques

The symposium and this subsequent publication examine the new thermal analytical techniques describing the physical properties and oxidative degradative behavior of polymers, lubricants, and petrochemicals. Historical reviews, oxidation mechanisms, new test methods, unique techniques, robotic methods, new reference standards, and bias considerations form the basis of this publication. Papers have been processed separately for inclusion on the database.

Transcleral delivery of triamcinolone acetonide and ranibizumab to retinal tissues using macroesis

Aim To determine the feasibility of macroesis for the delivery of ranibizumab and triamcinolone acetonide via a transcleral route. Methods Macroesis is a non-invasive method of drug delivery that uses alternating current (AC) to deliver drugs to target tissues. Two preclinical models of drug delivery were used for feasibility studies of delivering ranibizumab and triamcinolone acetonide to ocular tissues. In the first model, full-thickness sections of rabbit ocular tissue (conjunctiva to retina) were placed on an interdigitated electrode platform, and the drug was placed on the surface of the tissue. A non-uniform electrical field was applied to the ocular tissue, and electrical conductivity, a measurement of drug delivery, was monitored during the course of the experiment. In a second model, termed a ‘simulated vitreous model,’ the same full-thickness sections of rabbit ocular tissue were mounted below the electrode device, and the test compounds were placed on the electrodes. The fluid below the tissue, which simulated the vitreous cavity, was analysed using UV spectroscopy at the end of the study for the presence of drug. Results In the electrical conductivity studies, the electric characteristics of the tissue–drug system clearly showed movement of the drug through the tissue to the dielectric sensor based on changes in the electrical conductivity of the tissue sample with triamcinolone. No change in tissue conductivity was observed when no drug was placed. No heat generation occurred during the course of the study; nor was any gross tissue destruction noted. In the simulated vitreous model, studies using triamcinolone yielded concentrations ranging from 0.280 to 0.970 mg/ml, depending on the voltage, frequency and time applied. In as little as 6.7 min, clinically efficacious doses could be obtained in the preclinical system. Studies using ranibizumab yielded concentrations of 0.070–0.171 mg/ml, depending on the voltage, frequency, and time applied. In as little at 6.7 min, 92.8% throughput could be achieved. Conclusion Successful delivery of ranibizumab and triamcinolone acetonide can be achieved with macroesis in preclinical studies.

Determination of Crystalline Content Gradients in Cold-Drawn Poly-L-Lactic Acid Films by DSC

Poly-L-lactic acid (PLLA) is a semi-crystalline, optically active, biodegradable, and biocompatible polymer that has been utilized extensively in biomedical applications as an implantable artificial cell scaffold material. In its crystalline form, PLLA is piezoelectric and it has been implicated in the enhancement of electromechanically induced osteogenesis in vivo. In its amorphous state, however, PLLA does not exhibit piezoelectricity. By uniaxially cold-drawing the polymer, PLLA can be endowed with varying degrees of piezoelectricity. It is important to understand the crystalline architecture of drawn PLLA so that the osteogenic potential imparted by piezoelectricity, if any, can be differentiated from the effects of sample crystallinity. In our work we investigate the induced crystallinity for samples of drawn PLLA at draw ratios between 1.0 and 5.5 by differential scanning calorimetry (DSC). As long-range molecular ordering occurs along the draw axis, we observe an increase in the average percent crystallinity up to a draw ratio of 5.0 and a slight decrease at a draw ratio of 5.5. More importantly, we observe significant heterogeneity in the crystalline content along the draw axis of standard dumbbells cut from PLLA and cold-drawn to representative draw ratios of 2.5 and 4.0. On average, the highest percent crystallinity occurs nearest the dumbbell center, but the maximum crystallinity is independent of draw ratio. Therefore, the draw ratio should not be considered a semi-quantitative estimate of localized PLLA crystallinity and point-to-point analysis of crystallinity in PLLA samples is required for constructing scaffolds with enhanced cell growth properties.