Microscopic ice friction of polymeric substrates

Interest in snow and ice friction comes from the need to understand and control phenomena of practical importance such as glacier and avalanche movement, traction of automobile tires, snow and ice sports. The need to minimize friction on ice and snow in competitive winter sports is the main motivation behind the present work. A novel tribometer was designed and utilized in conjunction with a conventional rheometer for measuring and understanding the mechanisms of ice friction over polymeric surfaces. Experiments were performed to measure friction between ultra-high molecular weight polyethylene, polytetrafluoroethylene and poly(methyl methacrylate), and ice as a function of sliding velocity, temperature, surface roughness and hydrophobicity. Various techniques were utilized to modify the properties and characteristics of the polymeric surfaces. Light microscopy and scanning electron microscopy as well as surface profilometry were utilized to perform surface analysis and characterize the surface. A goniometer set-up was used for the measurement of the water contact angle measurements and X-ray photoelectron spectroscopy for conducting the elemental analysis. Overall it was found that the magnitude of the sliding velocity and temperature play important roles in ice friction. The more hydrophobic polymers exhibit a lower coefficient of friction. Liquid fluorinated additives as well as a plasma enhanced chemical vapour deposition in a fluorinated gas can improve the hydrophobicity of a polymer and decrease its coefficient of friction over ice. These two concepts can directly be applied in snow winter sports and more specifically in ski and snowboard bases production and preparation where greater speeds, shorter times and therefore less friction are in high demand.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat

Rheology and processing of molten poly(methyl methacrylate) resins

The rate of production in many commercial polymer processing operations is limited by the appearance of flow instabilities. Flow instabilities might manifest themselves as fine and periodic distortions on the extrudate surface (sharkskin or surface melt fracture) or in the form of helix and/or chaotic distortions (gross melt fracture). To postpone such phenomena to higher shear rates processing aids are used. External processing aids, such as fluoropolymers and waxes, have been found to be effective in eliminating sharkskin. Internal processing aids, such as boron nitride and clays, have been found effective in postponing the onset of gross melt fracture. In addition, different polymers have also been used as processing aids in the processing of other polymers with both polymers to be immiscible. In many of these cases melt fracture phenomena have been eliminated. The first objective of this present work was to study the rheological behavior of poly(methyl methacrylate) resins under shear and extension. Several frequency sweep experiments were carried out using the concentric parallel plate rheometer at various temperatures. By applying time temperature superposition, master curves for the linear viscoelastic moduli were obtained and the activation energy of flow was found to be independent of molecular weight. Following, extensional measurements using the Sentmanat Extensional RJieometer (SER) were attempted. It was found that poly(methyl methacrylate) resins exhibit shear thickening effects only at high shear rates. The main objective of this work was to identify suitable processing aids for the extrusion of poly(methyl methacrylate). Three different molecular weight poly(methyl methacrylate)s were tested together with various processing aids through capillary rheometer studies. First, it was found that poly(methyl methacrylate) polymers exhibit spiral/helical type of distortions at a critical shear stress value (0.35 ± 0.03 MPa) independent of temperature and molecular weight. "Traditional" processing aids used mainly in the extrusion of polyolefins and some other commercial polymers were found ineffective in eliminating instabilities in the case of poly(methyl methacrylate) processing. On the other hand, mixing of poly(methyl methacrylate) with a proprietary blend of synthetic resins and fatty glycerides with modified organic fatty acids, MoldWiz® INT-35UDH, was able to reduce the extrusion pressure and postpone the onset of gross melt fracture to higher shear rates. Also, the addition of different polyethylenes (LLDPE, LDPE and HDPE) resulted into a significant pressure reduction along with significant postponement of gross melt fracture to higher shear rates.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat

Maxillary residual ridge resorption after the insertion of a complete denture

Introduction: Residual ridge resorption (RRR) is a serious oral disease with both social and financial consequences. Bone resorption is a chronic, progressive, not reversible procedure occurring in all human races, irrespectively of people’s sex, age and health status. Although the rate of bone resorption can differ from one person to the other, it has been demonstrated that immediately after teeth extraction proceed faster and it slows down thereafter. A comparison of the mean values of the height and width of the residual ridges in the canine, premolar and molar regions, have been performed in an attempt to answer some of the above mentioned hypotheses. The patients were separated in 3 groups: 1. Males-females, 2. Patients who used the completed dentures for 12 hours - Patients who used the completed dentures for 24 hours, 3. Patients who had a complete denture experience for 1st, or more for one time. Purpose: The purpose of this research project was to determine the height and the width of the Residual ridge of the maxilla, in the canine, premolar and molar regions after a period of 6 months after delivery of complete denture prostheses. Methods and materials: Completely edentulous patients treated in the Removable Prosthodontics Clinic of the Aristotle University’s School of Dentistry participated in present study. A maxillary and a mandibular impression were made for each patient and they were poured with dental stone (Cast A). Six months after, a second set of impressions and casts (Cast B) were made for comparison reasons. Each cast was sectioned in the canine, premolar and molar regions and the specimens were electronically scanned and evaluated to determine if there were any changes. Results: The comparison between casts A and B (in the above mentioned regions) did not reveal statistically importance changes, in height or width of the residual ridges. Conclusion: Patients to whom a new set of removable complete dentures were delivered, did not present statistically significant differences in the canine, premolar or molar regions. However, patients who were new complete denture wearers, presented a difference in both height and width in the premolar region and a difference in width in the left side of the molar region. As anticipated, all dimentions were smaller in casts B.Η Απορρόφηση της Φατνιακής Απόφυσης (ΑΦΑ), θεωρείται σήμερα, σοβαρό στοματικό νόσημα για την Οδοντιατρική επιστήμη με οικονομικές και κοινωνικές προεκτάσεις. Έχει χρόνια προοδευτική πορεία, η οποία είναι αμετάστρεπτη και αθροιστική. Αφορά όλες τις φυλές της γης, άνδρες και γυναίκες, νέους και γέρους, υγιείς και ασθενείς. Ο ρυθμός της διαφέρει από άτομο σε άτομο. Είναι ταχύτατη τους πρώτους μήνες μετά την εξαγωγή των δοντιών και ακολουθεί μία φθίνουσα πορεία στην υπόλοιπη ζωή. Στην προσπάθεια να δοθεί κάποια απάντηση σε ορισμένες από τις παραπάνω υποθέσεις έχει γίνει σύγκριση του μέσου όρου των διαστάσεων του ύψους και του εύρους στην κυνοδοντική προγομφιακή και γομφιακή περιοχή, μετά τον διαχωρισμό του δείγματος των ασθενών στην κατηγορία ανδρών και γυναικών, στην κατηγορία των ασθενών που χρησιμοποιούσαν 12 και 24 ώρες τις ΟΟ, και στην κατηγορία των ασθενών που δέχτηκαν για πρώτη φορά ή περισσότερες από μία φορά ΟΟ. Σκοπός. Η έρευνα είχε σαν σκοπό να προσδιορίσει την ΑΦΑ σε ύψος και σε εύρος, στην κυνοδοντική, προγομφιακή και γομφιακή περιοχή της άνω γνάθου μετά την τοποθέτηση μιας καινούργιας ολικής οδοντοστοιχίας, ύστερα από τους πρώτους 6 μήνες λειτουργίας της. Υλικά και μέθοδος. Σε ολικά νωδούς ασθενείς που προσήλθαν στο ΑΠΘ κατασκευάστηκαν Ολικές Οδοντοστοιχίες και την ημέρα λίγο πριν την τοποθέτησή τους, αποτυπώθηκε η άνω γνάθος και κατασκευάσθηκε το εκμαγείο Α και ύστερα από 6 μήνες, ακολουθώντας την ίδια διαδικασία κατασκευάσθηκε τα εκμαγείο Β. Με την βοήθεια συσκευής ανάρτησης και κοπής εκμαγείων δημιουργήθηκαν τα περιγράμματα των τομών στην κυνοδοντική, προγομφιακή και γομφιακή περιοχή, τα οποία μεταφέρθηκαν σε υπολογιστή για την μέτρηση και σύγκριση των ευρημάτων. Αποτελέσματα. Από την σύγκριση των εκμαγείων Α και Β στις υπό έλεγχο περιοχές δεν προέκυψαν στατιστικά σημαντικές διαφορές στο ύψος και στο εύρος των υπολειμματικών φατνιακών αποφύσεων. Συμπέρασμα. Οι ασθενείς του δείγματος που φόρεσαν καινούργιες ΟΟ, δεν εμφάνισαν στατιστικά σημαντική διαφορά στην κυνοδοντική , προγομφιακή , και γομφιακή περιοχή των υπολειμματικών φατνιακών αποφύσεων. Εξαίρεση απετέλεσαν οι ασθενείς που φόρεσαν για πρώτη φορά ΟΟ, στους οποίους το ύψος και το εύρος της προγομφιακής περιοχής και το αριστερό εύρος της γομφιακής περιοχής ήταν μικρότερα στα εκμαγεία Β