Ali Utku Gökçe-030060060-10th week

Antioxidants


The combined action of oxygen and heat during the life cycle of polymers(manufacture, processing, storage and end use) causes thermooxidation of materials. This phenomena generate a change in molecular weight that lead to modificiations of physical and mechanical characteristics such as increase or decrease of melt flow, yellowing or dislocation, loss of gloss or transparency, chalking and superficial micro-cracks. After  the superficial changes in the degradation continues to greater extent thus decreasing the mechanical properties ( elongation at break, tensile strength, impact strength, flexural modulus). With ongoing aging the characteristics of the plastic artice are so modified that it loses its usefulness. The thermal oxidation reaction can be inhibited as follows;

-          -Using a primary antioxidant that inteferes with the chain propagation step

-          -Through a radical termination reaction

-          -Using a secondary antioxidant taht decomposes the hydroperoxides

-          -Primary antioxidants belong to the hindered phenols chemical family, called also “Radical Scavengers”. Secondary antioxidants belong to Phosphite/ Phhosphonite and Trioethers chemical family. The combination of both Antioxidant classes allows to achieve a synergistic effect.

(Ernest W. Flick, Plastic Additives, p. 62)

Light Stabilizers

In general, polymers deterioratein the presence of sunlight, which results in cracking, embrittlenement, chalkin, discoloration, or loss of mechanical properties, such as tensile strength, elongation, and impact strength. Photodegradation occurs as a result to exposure to ultraviolet ligth at wavelengths 290-400nm. Different wavelengths may produce different types of degredation, depending on the polymer. Specially chemicals, called light stabilizers or UV stabilizers, are used to interfere with the physical and chemical processes of light- induced polymer degradation. Stabilization of the polymer can occur by the use of additives that absorb UV radiation, preventing its absorption by the molecules of the polymers, by free radical scavengers, by the additives that decompose peroxides, or by quenchers that accept energy from the chromophore and convert it to heat.

(Krishna Seshan, Handbook Of Thin-Film Deposition Processes and Techniques, p. 13.14) 


Vitrification

The most significant steps in the cryopreservation of cattle embryos in more recent times inclued the aiblity to freeze and transfer embryos in straws without dilution and the development of the open-pulled-straw(OPS) method for efficient vitrification of embryos and ooctyes. Although vitrification appeared on the scene of embryo cryopreservation in the mid 1980s as an alteranticve to traditional slow-rate freezing of cattle embryos, its possible advantages( simplicity, speed, low cost) had little impact on commercial cattle Et operations and its application was generaly confined to research studies. In vitrification, ice-crystal formation is prevented by using high concentrations of cryoprotectants and high cooling and warming rates.

(Ian R. Gordon, Laboratory production of Cattle Embryos,p. 279)

Key Linkage

        With a certificate, key management concerns are the validity of the attribute/key linkage, the length of the root key, the length of individual keys, the number of roots, and the lifetime of the certificate. For a certificate to be valid, it has to have integrity, and authentication must be possible. A signature from the root or any trusted authority can provide both of these, given that the root key is secure and the information in the certificate is still valid at time of use.
         Processes are a key linkage between the enablers of planning(leadership driving policy and strategy, partnership and resources) through people into the performance(measured by people, society, customers and key outcomes)...

...new model for quality management, based on all the excellent work done during the last century, provides a simple framework for excellent performance, covering all angles and aspects of an organization and its operation.

Performance is achieved, using a ‘business excellence’ approach, and by planning the involvement of people in the improvement of processes. This has to include;

-Planning

- Performance

- Processes

- People

(John S. Oakland, Oakland on Quality Management, p.35-37)

(L. Jean Camp, Trust and Risk in Internet Commerce, p.86)