http://dentalbooks-drbassam.blogspot.com/ 4Dent International Community www.4dent.net Applied Dental Materials http://dentalbooks-drbassam.blogspot.com/ Applied Dental Materials Ninth Edition John F. McCabe BSc, PhD, DSc Professor of Dental Materials Science Newcastle University Angus W.G. Walls BDS, PhD, FDSRCS Professor of Restorative Dentistry Newcastle University © 2008 by Blackwell Publishing Ltd Blackwell Publishing editorial offi ces: Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK Tel: +44 (0)1865 776868 Blackwell Publishing Professional, 2121 State Avenue, Ames, Iowa 50014-8300, USA Tel: +1 515 292 0140 Blackwell Publishing Asia Pty Ltd, 550 Swanston Street, Carlton, Victoria 3053, Australia Tel: +61 (0)3 8359 1011 The right of the Author to be identifi ed as the Author of this Work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The Publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the Publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. First published 1956. Second edition 1961. Third edition 1967. Fourth edition 1972. Fifth edition 1976. Sixth edition 1985. Seventh edition 1990. Eighth edition 1998. Ninth edition published 2008 by Blackwell Publishing Ltd ISBN-13: 978-1-4051-3961-8 Library of Congress Cataloging-in-Publication Data McCabe, J.F. (John F.) Applied dental materials. – 9th ed. / J.F. McCabe, A.W.G. Walls. p. ; cm. Includes bibliographical references and index. ISBN: 978-1-4051-3961-8 (pbk. : alk. paper) 1. Dental materials. I. Walls, Angus. II. Title. [DNLM: 1. Dental Materials. WU 190 M477a 2008] RK652.5.A55 2008 617.6′95–dc22 2007044377 A catalogue record for this title is available from the British Library Set in 9.5/11.5 pt Sabon by SNP Best-set Typesetter Ltd., Hong Kong Printed and bound in Singapore by Fabulous Printers Pte Ltd The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp processed using acid-free and elementary chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. For further information on Blackwell Publishing, visit our website: www.blackwellpublishing.com Contents Preface, viii 1 Science of Dental Materials, 1 1.1 Introduction, 1; 1.2 Selection of dental materials, 1; 1.3 Evaluation of dental materials, 3 2 Properties used to Characterise Materials, 4 2.1 Introduction, 4; 2.2 Mechanical properties, 6; 2.3 Rheological properties, 18; 2.4 Thermal properties, 21; 2.5 Adhesion, 23; 2.6 Miscellaneous physical properties, 26; 2.7 Chemical properties, 27; 2.8 Biological properties, 30; 2.9 Suggested further reading, 31 3 Gypsum Products for Dental Casts, 32 3.1 Introduction, 32; 3.2 Requirements of dental cast materials, 32; 3.3 Composition, 33; 3.4 Manipulation and setting characteristics, 33 3.5 Properties of the set material, 37; 3.6 Applications, 38; 3.7 Advantages and disadvantages, 38; 3.8 Suggested further reading, 39 4 Waxes, 40 4.1 Introduction, 40; 4.2 Requirements of wax- pattern materials, 40; 4.3 Composition of waxes, 40; 4.4 Properties of dental waxes, 41; 4.5 Applications, 42; 4.6 Suggested further reading, 45 5 Investments and Refractory Dies, 46 5.1 Introduction, 46; 5.2 Requirements of investments for alloy casting procedures, 46; 5.3 Available materials, 47; 5.4 Properties of investment materials, 49; 5.5 Applications, 51; 5.6 Suggested further reading, 52 6 Metals and Alloys, 53 6.1 Introduction, 53; 6.2 Structure and properties of metals, 53; 6.3 Structure and properties of alloys, 56; 6.4 Cooling curves, 57; 6.5 Phase diagrams, 57; 6.6 Suggested further reading, 61 7 Gold and Alloys of Noble Metals, 62 7.1 Introduction, 62; 7.2 Pure gold fi llings (cohesive gold), 62; 7.3 Traditional casting gold alloys, 63; 7.4 Hardening heat treatments (theoretical considerations), 65; 7.5 Heat treatments (practical considerations), 67; 7.6 Alloys with noble metal content of at least 25% but less than 75%, 68; 7.7 Soldering and brazing materials for noble metals, 69; 7.8 Noble alloys for metal-bonded ceramic restorations, 70; 7.9 Biocompatibility, 70; 7.10 Suggested further reading, 70 8 Base Metal Casting Alloys, 71 8.1 Introduction, 71; 8.2 Composition, 71; 8.3 Manipulation of base metal casting alloys, 72; 8.4 Properties, 73; 8.5 Comparison with casting gold alloys, 74; 8.6 Biocompatibility, 76; 8.7 Metals and alloys for implants, 77; 8.8 Suggested further reading, 79 9 Casting, 80 9.1 Introduction, 80; 9.2 Investment mould, 80; 9.3 Casting machines, 81; 9.4 Faults in castings, 81; 9.5 Suggested further reading, 83 10 Steel and Wrought Alloys, 84 10.1 Introduction, 84; 10.2 Steel, 84; 10.3 Stainless steel, 85; 10.4 Stainless steel denture bases, 86; 10.5 Wires, 86; 10.6 Suggested further reading, 88 11 Ceramics and Porcelain Fused to Metal (PFM), 89 11.1 Introduction, 89; 11.2 Composition of traditional dental porcelain, 89; 11.3 Compaction and fi ring, 90; 11.4 Properties of porcelain, 91; 11.5 Alumina inserts and aluminous porcelain, 92; 11.6 Sintered alumina core ceramics, 93; 11.7 Injection moulded and pressed ceramics, 94; 11.8 Cast glass and polycrystalline ceramics, 94; 11.9 CAD-CAM restorations, 95; 11.10 Porcelain veneers, 97; 11.11 Porcelain fused to metal (PFM), 97; 11.12 Capillary technology, 100; 11.13 Bonded platinum foil, 100; 11.14 Suggested further reading, 100 v 12 Synthetic Polymers, 101 12.1 Introduction, 101; 12.2 Polymerisation, 101; 12.3 Physical changes occurring during polymerisation, 105; 12.4 Structure and properties, 107; 12.5 Methods of fabricating polymers, 109; 12.6 Suggested further reading, 109 13 Denture Base Polymers, 110 13.1 Introduction, 110; 13.2 Requirements of denture base polymers, 110; 13.3 Acrylic denture base materials, 112; 13.4 Modifi ed acrylic materials, 121; 13.5 Alternative polymers, 123; 13.6 Suggested further reading, 123 14 Denture Lining Materials, 124 14.1 Introduction, 124; 14.2 Hard reline materials, 124; 14.3 Tissue conditioners, 126; 14.4 Temporary soft lining materials, 128; 14.5 Permanent soft lining materials, 128; 14.6 Self-administered relining materials, 132; 14.7 Suggested further reading, 132 15 Artificial Teeth, 133 15.1 Introduction, 133; 15.2 Requirements, 133; 15.3 Available materials, 133; 15.4 Properties, 133; 15.5 Suggested further reading, 135 16 Impression Materials: Classification and Requirements, 136 16.1 Introduction, 136; 16.2 Classifi cation of impression materials, 136; 16.3 Requirements, 137; 16.4 Clinical considerations, 142; 16.5 Suggested further reading, 146 17 Non-elastic Impression Materials, 147 17.1 Introduction, 147; 17.2. Impression plaster, 147; 17.3 Impression compound, 148; 17.4 Impression waxes, 151; 17.5 Zinc oxide/eugenol impression pastes, 151 18 Elastic Impression Materials: Hydrocolloids, 154 18.1 Introduction, 154; 18.2 Reversible hydrocolloids (agar), 154; 18.3 Irreversible hydrocolloids (alginates), 158; 18.4 Combined reversible/irreversible techniques, 161; 18.5 Modifi ed alginates, 161; 18.6 Suggested further reading, 162 19 Elastic Impression Materials: Synthetic Elastomers, 163 19.1 Introduction, 163; 19.2 Polysulphides, 163; 19.3 Silicone rubbers (condensation curing), 167; 19.4 Silicone rubbers (addition curing), 168; 19.5 Polyethers, 172; 19.6 Comparison of the properties of elastomers, 175; 19.7 Suggested further reading, 177 20 Requirements of Direct Filling Materials and Historical Perspectives, 178 20.1 Introduction, 178; 20.2 Appearance, 178; 20.3 Rheological properties and setting characteristics, 178; 20.4 Chemical properties, 179; 20.5 Thermal properties, 179; 20.6 Mechanical properties, 179; 20.7 Adhesion, 179; 20.8 Biological properties, 179; 20.9 Historical, 180 21 Dental Amalgam, 181 21.1 Introduction, 181; 21.2 Composition, 181; 21.3 Setting reactions, 183; 21.4 Properties, 184; 21.5 Clinical handling notes for dental amalgam, 189; 21.6 Manipulative variables, 191; 21.7 Suggested further reading, 194 22 Resin-based Filling Materials, 195 22.1 Introduction, 195; 22.2 Acrylic resins, 195; 22.3 Composite materials – introduction, 196; 22.4 Classifi cation and composition of composites, 197; 22.5 Properties of composites, 202; 22.6 Fibre reinforcement of composite structures, 213; 22.7 Clinical handling notes for composites, 216; 22.8 Applications of composites 223; 22.9 Suggested further reading, 224 23 Adhesive Restorative Materials: Bonding of Resin-based Materials, 225 23.1 Introduction, 225; 23.2 Acid-etch systems for bonding to enamel, 225; 23.3 Applications of the acid-etch technique, 227; 23.4 Bonding to dentine – background, 229; 23.5 Dentine conditioning – the smear layer, 233; 23.6 Priming and bonding, 234; 23.7 Current concepts in dentine bonding – the hybrid layer, 236; 23.8 Classifi cation of dentine bonding systems, 237; 23.9 Bonding to alloys, amalgam and ceramics, 238; 23.10 Bond strength and leakage measurements, 240; 23.11 Polymerizable luting agents, 242; 23.12 Suggested further reading, 244 24 Glass Ionomer Restorative Materials (Polyalkenoates), 245 24.1 Introduction, 245; 24.2 Composition, 245; 24.3 Setting reaction, 247; 24.4 Properties, 248; 24.5 Cermets, 252; 24.6 Applications and clinical handling notes, 252; 24.7 Suggested further reading, 256 25 Resin-modified Glass Ionomers and Related Materials, 257 25.1 Introduction, 257; 25.2 Composition and classifi cation, 257; 25.3 Setting characteristics, 260; 25.4 Dimensional change and dimensional stability, 261; 25.5 Mechanical properties, 262; 25.6 Adhesive characteristics, 262; 25.7 Fluoride release, 263; 25.8 Clinical handling notes, 264; 25.9 Suggested further reading, 264 vi Contents 26 Temporary Crown and Bridge Resins, 265 26.1 Introduction, 265; 26.2 Requirements, 265; 26.3 Available materials, 265; 26.4 Properties, 266 27 Requirements of Dental Cements for Lining, Base and Luting Applications, 267 27.1 Introduction, 267; 27.2 Requirements of cavity lining materials, 267; 27.3 Requirements of luting materials, 270; 27.4 Requirements of endodontic cements, 271; 27.5 Requirements of orthodontic cements, 272; 27.6 Suggested further reading, 272 28 Cements Based on Phosphoric Acid, 273 28.1 Introduction, 273; 28.2 Zinc phosphate cements, 273; 28.3 Silicophosphate cements, 277; 28.4 Copper cements, 277; 28.5 Suggested further reading, 277 29 Cements Based on Organometallic Chelate Compounds, 278 29.1 Introduction, 278; 29.2 Zinc oxide/eugenol cements, 278; 29.3 Ortho-ethoxybenzoic acid (EBA) cements, 280; 29.4 Calcium hydroxide cements, 280; 29.5 Suggested further reading, 283 30 Polycarboxylates, Glass Ionomers and Resin-modified Glass Ionomers for Luting and Lining, 284 30.1 Introduction, 284; 30.2 Polycarboxylate cements, 284; 30.3 Glass ionomer cements, 285; 30.4 Resin-modifi ed glass ionomers and compomers, 286; 30.5 Suggested further reading, 288 31 Endodontic Materials, 289 31.1 Introduction, 289; 31.2 Irrigants and lubricants, 289; 31.3 Intra-canal medicaments, 290; 31.4 Endodontic obturation materials, 291; 31.5 Historical materials, 291; 31.6 Contemporary materials, 292; 31.7 Clinical handling, 294; 31.8 Suggested further reading, 296 Appendix 1, 297 Index, 299 Contents vii Preface In producing this ninth edition of Applied Dental Materials we have updated the text in both content and appearance. We hope that the book will remain helpful to students and teachers of the science of dental materials alike by remaining true to the core principles and developing them into a more comprehensive text which covers a wide spectrum of materials falling within the scope of ‘dental materials’. Hence, there are more refer- ences to practical issues like handling and clinical performance of materials, whilst maintaining a strong link to quality issues encompassed within newly developed ISO standards. Some areas, such as adhesion, ceramics, light activation technology, CAD-CAM have been developed and expanded signifi cantly from the eighth edition, refl ecting sig- nifi cant changes or developments in these areas. There is a new chapter on endodontic materials. All areas have benefi ted from a fresh approach to the use of drawings and photographs including the use of colour for the fi rst time. John McCabe Angus Walls viii Chapter 1 Science of Dental Materials whilst the intake of acid fruit juices or alkaline medicaments can extend this range from pH 2 to pH 11. The load on 1 mm2 of tooth or restorative mate- rial can reach levels as high as many kilograms indicating the demanding mechanical property requirements of some materials. Many products, for example direct fi lling mate- rials, are handled entirely by the dentist and their chairside assistant and are rarely encountered by the dental technician. Other materials are gener- ally associated with the work of the dental laboratory and in this case both technician and dentist require a thorough knowledge of the materials in order that they may communicate about selection, manipulation and any problems which arise. A third group of materials link the dental surgery and the laboratory. The most obvious example of such products is the impres- sion materials. Whilst the latter are under the direct control of the dentist it is essential that the dental technician also has a sound knowledge of such materials. 1.2 Selection of dental materials The process of materials selection should ideally follow a logical sequence involving (1) analysis of the problem, (2) consideration of requirements, (3) consideration of available materials and their properties, leading to (4) choice of material. Eval- uation of the success or failure of a material may be used to infl uence future decisions on materials’ selection. This selection process is illustrated in Fig. 1.2. Many experienced practitioners carry out this sequence with no apparent effort since they are able to call upon a wealth of clinical experi- ence. However, when presented with new or modi- fi ed materials even the most experienced dentist 1.1 Introduction The science of dental materials involves a study of the composition and properties of materials and the way in which they interact with the environ- ment in which they are placed. The selection of materials for any given application can thus be undertaken with confi dence and sound judgement. The dentist spends much of his professional career handling materials and the success or failure of many forms of treatment depends upon the correct selection of materials possessing adequate properties, combined with careful manipulation. It is no exaggeration to state that the dentist and dental technician have a wider variety of materials at their disposal than any other profession. Rigid polymers, elastomers, metals, alloys, ceramics, inorganic salts and composite materials are all commonly encountered. Some examples are given in Fig. 1.1 along with some of their uses in dentistry. This classifi cation of materials embodies an enormous variation in material properties from hard, rigid materials at one extreme to soft, fl exi- ble products at the other. Many dental materials are fi xed permanently into the patient’s mouth or are removed only intermittently for cleaning. Such materials have to withstand the effects of a most hazardous environ- ment. Temperature variations, wide variations in acidity or alkalinity and high stresses all have an effect on the durability of materials. Normal temperature variations in the oral cavity lie between 32ºC and 37ºC depending on whether the mouth is open or closed. The ingestion of hot or cold food or drink however, extends this tem- perature range from 0ºC up to 70ºC. The acidity or alkalinity of fl uids in the oral cavity as mea- sured by pH varies from around pH 4 to pH 8.5, 1 2 Chapter 1 should return to a more formal type of selection process based on the criteria mentioned. Analysis: The analysis of the situation requiring selection of a material may seem obvious but it is of paramount importance in some circumstances. An incorrect decision may cause failure of the restoration or appliance. For example, when con- sidering the selection of a fi lling material it is important to decide whether the restoration is to be placed in an area of high stress. Will it be visible when the patient smiles? Is the cavity deep or shallow? These factors and many more must be evaluated before attempting materials’ selection. Requirements: Having completed a thorough analysis of the situation it is possible to develop a list of requirements for a material to meet the needs of that situation. For the example men- tioned in the previous section, it may be decided that a fi lling material which matches tooth colour and is able to withstand moderately high stresses without fracture is required. Some tooth cavities are caused by toothbrush/toothpaste abrasion. In this special case the restorative material used should naturally possess adequate resistance to dentifrice abrasion. Hence, it is possible to build Fig. 1.1 Diagram indicating the wide variety of materials used in dentistry and some of their applications. Fig. 1.2 Flow chart indicating a logical method of material selection. Science of Dental Materials 3 a profi le of the ideal properties required for the application being considered. Available materials: The consideration of avail- able materials, their properties and how these compare with the requirements is carried out at two levels. The dentist, faced with the immediate problem of restoring the tooth of a patient in his surgery, must choose from those materials on hand at the time. Previous experience with materi- als in similar circumstances will be a major factor which infl uences selection. On a wider scale, the practitioner is able to consider the use of alterna- tive materials or newly developed products where these appear to offer a solution to cases which have proved diffi cult with his existing armoury of products. It is of paramount importance that the practitioner keeps up to date with developments in materials whilst taking a conservative approach towards adopting new products for regular use in his surgery until they are properly tested. Choice of material: Having compared the proper- ties of the available materials with the require- ment, it is possible to narrow the choice to a given generic group of products. The fi nal choice of material brand is often a matter of personal pre- ference on the part of the dentist. Factors such as ease of handling, availability and cost may play a part at this stage of the selection process. 1.3 Evaluation of materials As the number of available materials increases, it becomes more and more important for the dentist to be protected from unsuitable products or mate- rials which have not been thoroughly evaluated. It should be emphasized, however, that most man- ufacturers of dental materials operate an extensive quality assurance programme and materials are thoroughly tested before being released to the general practitioner. Standard specifi cations: Many standard specifi ca- tion tests, of both national and international stan- dards organizations, are now available which effectively maintain quality levels for some dental materials. Such specifi cations normally give details for the testing of certain products, the method of calculating the results of the minimum permissible result which is acceptable. Although such specifi - cations play a useful part they should not be seen as indicating total suitability since the tests carried out often do not cover critical aspects of the use of a material. For example, many materials fail by a fatigue mechanism in practice, but few specifi ca- tions involve fatigue testing. Laboratory evaluations: Laboratory tests, some of which are used in standard specifi cations, can be used to indicate the suitability of certain materials. For example, a simple solubility test can indicate the stability of a material in aqueous media – a very important property for fi lling materials. It is important that methods used to evaluate materials in the laboratory give results which can be correlated with clinical experience. For example, when upper dentures fracture along the midline they do so through bending. Hence a bending or transverse strength test is far more meaningful for denture base materials than a compression test. Clinical trials: Although laboratory tests can provide important and useful data on materials the ultimate test is the randomised controlled clinical trial and the verdict of practitioners after a period of use in general practice. Many materi- als produce good results in the laboratory, only to be found lacking when subjected to clinical use. The majority of manufacturers carry out extensive clinical trials of new materials, normally in co-operation with a university or hospital department prior to releasing a product for use by general practitioners. Chapter 2 Properties used to Characterise Materials and as a result may perform poorly. Such materi- als are said to have limited shelf life. Some materi- als have an extended shelf life if refrigerated during storage. One technique commonly employed to predict stability is to carry out accel- erated ageing by storing samples at elevated tem- perature, commonly 60ºC, followed by evaluation of material properties. Containers used for materials generally have a batch number stamped or printed onto them from which the date of manufacture can be obtained. Thus, for materials with limited shelf life it is pos- sible to ascertain the date at which one would expect the properties to deteriorate. Properties of materials during mixing, manipula- tion and setting: Properties of materials during mixing, manipulation and setting are considered together since they mainly involve a consideration of rheological properties and the way in which these change as a function of time during setting. For materials of two or more components which set by a chemical reaction, thorough mixing is essential in order to achieve homogeneous distri- bution of properties throughout the material. The ease of mixing depends on factors such as the chemical affi nity of the components, the viscosity, both of the components and the mixed material, the ambient temperature, the method of dispensa- tion and the method of mixing. Several methods of dispensation exist among materials used in dentistry. Some involve the mixing of powder and liquid components, others the mixing of two pastes, while others involve paste and liquid components. When the mixing of two pastes is required, the manufacturer often gives a good colour contrast between the two pastes. The achievement of a thorough mix of the two components can be judged by the attainment 2.1 Introduction Many factors must be taken into account when considering which properties are relevant to the successful performance of a material used in den- tistry. The situation in which the material is to be used and the recommended technique for its manipulation defi ne the properties which charac- terise the material. Laboratory tests used to evalu- ate materials often duplicate conditions which exist in situ. This is not always possible and some- times not desirable since one aim of in vitro testing is to predict in a rapid laboratory test what may happen in the mouth over a number of months or years. Many tests used to evaluate dental materi- als involve the measurement of simple properties such as compressive strength or hardness which have been shown to correlate with clinical performance. Many materials used in dentistry are supplied as two or more components which are mixed together and undergo a chemical reaction, during which the mechanical and physical properties may change dramatically. For example, many impres- sion materials are supplied as fl uid pastes which begin to set when mixed together. The set material may be a rigid solid or a fl exible rubber depending upon the chemical nature of the product. The acceptance of such a product by the dentist depends upon the properties of the unmixed paste, the properties during mixing and setting and the properties of the set material (Table 2.1). This classifi cation of properties applies to virtually all groups of materials. Properties of unmixed materials: Manufacturers formulate materials which give optimal perfor- mance as evaluated by their quality assurance pro- gramme and clinical trials. It is known however, that certain products deteriorate during storage 4 Properties used to Characterise Materials 5 of a homogeneous colour with no streaks. When powder and liquid or paste and liquid are mixed, the achievement of a thorough mix is less certain. The components are mixed for a recommended time and/or until a recommended consistency is reached. A growing number of materials are mixed mechanically. This method removes uncertainty and gives a more reproducible result. The use of encapsulated materials which are mixed mechanically is becoming very popular. These offer the dual advantages of easier and more reproducible mixing coupled with pre-set proportions of components within the capsules. Certain products have specifi ed manipulative requirements which will be referred to later. For many applications, materials should be in a rela- tively fl uid state at the time they are introduced into the patient’s mouth but should undergo rapid setting involving a change to a more rigid or rubbery form. From the commencement of mixing, two important times can be defi ned which have an important bearing on the acceptability of mate- rials. The fi rst is the working time, defi ned as the time available for mixing and manipulating a material. For example, an impression material should be seated in the mouth before the end of the working time otherwise setting will have pro- ceeded suffi ciently for the viscosity to have increased considerably. The other time which characterises setting is the setting time. This, like working time, is to some extent arbitrary since it is defi ned as the time taken for a material to have reached a certain level of rigidity or elasticity. It is known that many materials continue setting for a considerable time after the apparent setting and optimum properties may not be achieved until several hours later. Properties of the set material: The properties of the unmixed material and those during mixing and setting are important and may infl uence the practitioner’s selection. Generally, it is the proper- ties of the set material which indicate the suit- ability of a product for any application. For example, in the case of a fi lling material, the method of dispensation, viscosity of the mixed material, working time and setting time control the ease of handling of the product, but the dura- bility of the material in the oral environment depends on factors such as strength, solubility, abrasion resistance, etc. The properties of the set material can be conveniently divided into the fol- lowing categories: mechanical properties, thermal properties, chemical properties, biological proper- ties and miscellaneous other physical properties. Naturally, the properties relevant to any one material will depend on the application. Table 2.1 Illustrating the different requirements and associated tests used for materials at different stages during their storage and use. Stage of use Practical issues Tests required During storage Before use in surgery or laboratory. Require material to keep fresh and last a long time. Wastage is minimised and bulk purchases can be made. Shelf life, expiry date or date of manufacture given by manufacturer. Test that certain key properties are within acceptable limits after a period of storage. During proportioning, mixing and manipulating. Easy and accurate to proportion, mix and use. Should not ‘drip’ off instruments. Should not stick to instruments. Test reproducibility of proportioning. Test effect of proportions on properties. Test speed and completeness of mix. Test rheological properties and ‘tackiness’. During setting. Material should have a convenient rate of set. Dimensional and temperature changes on setting should not cause problems with accuracy or irritation. Measure working time and setting time using meaningful tests. Measure dimensional change during setting. Measure heat absorbed or evolved using thermometry. Set material Material should have an acceptable appearance and suffi cient durability to serve its function. Should be safe and harmless. Measure mechanical properties such as strength, hardness, abrasion resistance. Evaluate resistance to fl uids such as saliva and dietary liquids. Evaluate colour, translucency and gloss. 6 Chapter 2 2.2 Mechanical properties Most applications of materials in dentistry have a minimum mechanical property requirement. For example, certain materials should be suffi cient- ly strong and tough to withstand biting forces without fracture. Others should be rigid enough to maintain their shape under load. Such proper- ties of materials are generally characterised by the stress–strain relationship which is readily obtained by using a testing machine of the type shown in Fig. 2.1. Before considering the various types of experi- ment which can be carried out and the relevance of the data obtained, it is necessary to defi ne the terms stress and strain. Stress: When an external force is applied to a body or specimen of material under test, an internal force, equal in magnitude but opposite in direc- tion, is set up in the body. For simple compression or tension the stress is given by the expression, Stress = F/A, where F is the applied force and A the cross-sectional area (Fig. 2.2). A stress resist- ing a compressive force is referred to as a com- pressive stress and that resisting a tensile force a tensile stress. Tensile and compressive stresses, along with shear, are the three simple examples of stress which form the basis of all other more complex stress patterns. The unit of stress is the pascal (Pa). This is the stress resulting from a force of 1 Newton (N) acting upon one square metre of surface. Whereas the tensile stress can be visual- ized as a purely uni-axial stress set up within a material, the compressive stress is more complex and comprises force vectors which act to intro- duce elements of shear within the specimen under compression. One test method commonly used for dental materials is the three-point bending test or trans- verse test (Fig. 2.3). When an external force is applied to the mid- point of the test beam the stresses can be resolved as shown. The numerical value of stress is given by the expression Stress = 3 2 2 FL bd where L is the distance between the supports, b is the width of the specimen and d its depth. When a cylinder of a brittle material is com- pressed across a diameter as shown in Fig. 2.4a, a tensile stress is set up in the specimen, the value of the stress being given by Stress at the axis of the cylinder = 2F DT π where F is the applied force, D the diameter of the cylinder and T the length of the cylinder. This type of test is referred to as a diametral compressive tensile test and is commonly used when conven- tional tensile testing is diffi cult to carry out due to the brittle nature of the test material. For non- brittle materials the equation used to calculate stress breaks down due to the increased area of contact between the testing machine platen and the material under test (Fig. 2.4b). (a) (b) Fig. 2.1 (a) Mechanical properties testing equipment. This shows the equipment used in determining mechanical properties of materials. The test being performed is a tensile test on a sample of impression material. Note the shape of the sample which is shown in the inset (b). This shape is referred to as a dumb bell shaped specimen and it is designed so that the specimen can be gripped at each end and stretched. The shape of the specimen will ensure that fracture occurs in the middle region and not at the ends of the specimen where the specimen is gripped. The results are given on the computer screen in the form of a plot of force against displacement, which can easily be converted into stress against strain.