Chandrasekaran Essential Rubber Formulary: Formulas for Practitioners

1. Auflage 2007
ISBN: 978-0-8155-1709-2
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 205 Seiten

Reihe: Plastics Design Library

The author, a seasoned rubber technologist of four decades, provides more than 180 essential rubber formularies, some of which have never been published, that are used by practitioners the world over on a frequent basis. A special feature of the formulations is that they are designed for factory scale applications.
The opening chapter of this indispensable book gives practical information on compounding techniques, coloring, ingredients, as well as a whole section on typical rubber testing methods. The book concludes with appendices useful for the technologist that include seven conversion tables and three tables on scorching of rubber, specific gravity and volume cost, equivalent chemical names for trade names.
Designing a rubber formula on the factory floor demands knowledge of the whole undertaking, such as the physical nature of ingredients, the interaction of additives and the base rubber during compounding and processing, as well as making sure that the finished product conforms to specification and requirements. This book provides all the necessary knowledge for practitioners and students alike.

Autoren/Hrsg.

Chandrasekaran, Chellappa

Leseproben

7 Typical Rubber Testing Methods 7.1 Prelude
The relationship between the stress applied to rubber and the resultant strain was an inspiring subject in the early studies of rubber, and soon after scientists discovered the unusual elastic behavior of vulcanized rubber. But they were unable to pursue their studies satisfactorily because of the lack of knowledge on rubber and its composition and the nonavailability of suitable devices to test their investigations. In many cases, when investigating the stress–strain relationships, the composition of rubber was not defined. With all this, the study of tension stress was later extended to the study of compression stress too. Later in the 19th century, a systematic study of the physical properties of rubber, after adding certain compounding ingredients to rubber, was carried out by Pahl and his co-workers. Through their efforts and those of many later investigators, a foundation was laid for a better knowledge of the fundamental elastic properties of different types of vulcanizates. Many research works were focused on measurement of physical properties rather than direct evaluation of the performance characteristics of the rubber products in service. However, this led to the development of techniques for testing and the introduction of new methods and devices, accelerated ageing tests, testing under simulated service conditions, etc., which were hitherto being continuously improved, simultaneously creating the need for testing of rubber before processing, during processing, and after vulcanization. Manufacturers of rubber products understood that without any set norms for testing procedures, they were bound to lose their ground against the end users who in turn were actively involved in drafting specifications for rubber products, such as hardness, tensile strength, resilience, elongation, modulus, and many functional properties such as oil and fuel resistance, flame resistance, and chemical resistance. In the rubber industry the quality control aspects broadly covered the following tests: 1. Test on unvulcanized rubber stocks and 2. Tests on vulcanized rubbers Plasticity, scorch time, cure rate, and plasticity retention index (PRI) tests were generally done on the compounded unvulcanized rubber. They are called tests for processibility. 7.2 Tests on Unvulcanized Rubber Stocks
7.2.1 Plasticity
Standard plasticity tests use either the rotation or the compression principle. The rotation-based test consists of determining the torque necessary to rotate a disk in a cylindrical chamber filled with unvulcanized rubber under specified conditions of speed and temperature namely 2 rpm and 100°C, respectively. A number proportional to this torque is taken as an index of the viscosity of the rubber. The most popular testing device is called the Mooney Viscometer discovered by the American physicist Melvin Mooney nearly a century ago. The compression test called the Williams Plasticity Test conducted in a Wallace Rapid Plastimeter consists of compressing a cylindrical unvulcanized rubber test piece axially between parallel plates under specified conditions and measuring the compressed height. 7.2.2 Scorch Time and Rate of Cure
The Mooney Viscometer runs continuously as the test temperature rises and gives a curve. “Scorch time” is the time required for the reading to rise usually by 3 or 5 units above the minimum while the upward slope indicates the rate of cure. By convention, “cure rate” is expressed as the time required for the reading to rise from 5 or 35 units above the minimum. 7.2.3 Plasticity Retention Index
This test measures the resistance of raw natural rubber to oxidation (ISO 2930, ASTM 3194). It is an ageing test in which a small disk of rubber is heated in air at 140°F and its plasticity number is compared with that of an unaged disk. 7.3 Tests on Vulcanized Rubbers
7.3.1 Hardness
The most widely used device is a shore hardness meter. The hardness is measured by the depth of indentation caused by a rigid ball under a spring load or dead load, the indentation being converted to hardness degrees on a scale ranging from 0 to 100. The reading from a dead load hardness meter is called International Rubber Hardness Degrees (IRHD). The spring-loaded meter gives Shore A values. The hardness scale from 0 to 100 is chosen such that ‘0’ represents a rubber having an elastic modulus of zero and ‘100’ represents a rubber having infinite elastic modulus. The IRHD test consists of measuring the difference between the depths of penetration of the ball into the rubber relative to a ‘foot’ resting on the surface of the rubber under an initial load of 3 grams and a final load of 570 grams. The IRHD test is for all practical purposes the same as the Shore A scale. Shore A conforms to the rubbery stage and Shore D conforms to hard rubbers (ASTM 2240, DIN 53505) namely ebonites. 7.3.2 Tensile Test
The tensile test consists of stretching rubber samples at a uniform speed in a tensile tester and recording the values of stress on the samples and the resulting elongation at more or less regular time intervals. The curve drawn with the elongation on the abscissa and the stresses on the ordinate axis is called the tensile curve. The tensile stress is the ratio of the total force acting on the sample to the initial cross section of the sample. The tensile stress at the breaking point of the rubber sample is called tensile strength. It is defined as the force per unit area of the original cross section, which is applied when the specimen is ruptured. The maximum elongation is called “elongation at break” or “ultimate elongation.” With lowering of the temperature, the maximum elongation of the sample is reduced. The rate of stretching affects both the value of the tensile strength and that of the elongation. At varying rates of stretching, it was found that the higher the rate of stretching the greater will be the values of tensile strength and elongation. Compounding ingredients such as fillers, sulfur, accelerators, and plasticizers have a great influence on the tensile curves of different rubber compounds. The load per unit area of the original cross section at a given elongation value is called modulus. A rubber compound in some cases may be characterized by the values of tensile strength and ultimate elongation. It may also be characterized by its modulus, for example, at 100%, 200%, 300%, or 500% elongation. The value of the modulus can be calculated directly from the stress–strain curve. This modulus has nothing to do with what is commonly known as the modulus of elasticity. 7.3.3 Resilience
Rebound resilience is a very basic form of a dynamic test in which the test piece is subjected to one half cycle of deformation only. It is defined as the ratio of energy of an indentor after impact to its energy before impact expressed as a percentage, and hence in the case where the indentor falls under gravity it is equal to the ratio of the rebound height to the drop height. Resilience can also be measured using a falling weight. A well-known instrument is the shore scleroscope which has a hemispheri-cally headed striker. A number of designs of falling ball apparatus have been used (ASTM D2632, DIN 53512). 7.3.4 Specific Gravity
This is measured in relation to that of water assuming the density of water to be 1 kg/liter. The method involves weighing a test piece in air and water (ISO 2781: BS 903 Part AI). 7.3.5 Abrasion Resistance
The volume of rubber abraded from a specified test piece when subjected to abrasive wear under specified conditions is called abrasion loss. The reciprocal of abrasion loss is called abrasion resistance. There are many methods for determining the abrasion resistance of a rubber product. The ratio of the abrasion resistance of the rubber under test to that of a standard rubber expressed as a percentage is called the abrasion resistance index. For intra-laboratory comparison standardized abrasion test methods are available, such as Method A Dupont Machine (for tire treads, soles, and heals), Method B Dupont Machine with constant torque modification, and Method C Akron Machine (for rubbers with hardness ranging between 55°A to 80° A). The Method C Dunlop Machine is suitable for testing tire treads, conveyor belt cover compounds, etc. 7.3.6 Spark Testing
In rubber lining, spark testing is carried out with a high frequency spark tester to check the continuity of the rubber lining. Pinholes in a rubber-lined tank will expose the underlying metal surface to corrosive chemicals handled in the tank. Therefore, the continuity of the rubber lining is checked with a high frequency spark tester with a voltage varying from 6000 to 20,000 V depending on the thickness of rubber and also on the conductive property of the lining compound. While using the spark tester, it is necessary to ensure...