Instrumentation concerns similar to those of a century ago.

Instrumentation concerns similar to those of a century ago

This article reports on a search through the extensive literature of instruments in the rubber industry, and endeavors to provide a perspective on their numerous applications. Even one hundred years ago, pioneers had similar philosophical concerns as their modern counterparts. Namely, what instrument is needed, what accuracy, sensitivity, range and reproducibility are required, and how to interpret the results in relation to first principles and fundamental problems?

We cannot look over a century of technical work without marveling at the uniqueness and durability of rubber, as well as the talent and insight possessed by early workers. A tire is expected to perform perfectly during a midwinter trip from Minneapolis to Miami where the temperature difference could approach 80[degrees] C. We read that a rubber gasket was removed from a sewer in London one hundred years after installation and was still performing its job. Laboratory measurements made by an early worker on natural rubber were reportedly found to be correct within a few percent when repeated a century later with modern instrumentation.

Mechanical properties

The history of the evolution of physical testing is inseparable from instrumentation. Even the first few issues of Rubber World contain articles on "The Royle Tubing Machine," "Methods of Gauging Sheet Rubber" and "A New Damper Regulator." The design of the Royle tubing machine bears a striking resemblance to its modern counterpart. The author, concerned with caliper and micrometer gaging, stated, "Perfect control of the calender demands a quick and accurate method of gauging, free from all errors of judgement on the part of the operator." The damper regulator provided a proportioning control of air to the boiler in an effort to maintain a constant steam pressure for plant operations. Thus the problems and goals of these pioneers are very similar to those in modern rubber literature, even though the techniques and tools available have undergone quantum jumps of evolution.

One authority in this field recently observed that among the earliest test instruments for rubber was the plastometer developed about the turn of the century. Articles in Rubber World and other sources show that tensile properties were used to characterize rubber in the first decade of the 20th century. Certainly the organization of ASTM D-11 on Rubber in 1912 constituted a milestone in terms of bringing together an industry-wide effort to identify standard instruments for properties widely run by producers and consumers alike. The work of Melvin Mooney in the 1930s, culminating in the ubiquitous Mooney test, was a development which meets the most important criterion of a good research project; namely, it raised more questions than it answered and started controversies which still rage today, especially between production-minded and research-minded people.

The development of electronic testing equipment based upon strain gages (Instron) revolutionized the measurement of mechanical properties in the 1950s. Response of a stress to an imposed strain or a strain to an imposed stress became very sensitive, instantaneous and capable of magnification over wide ranges, yet presented a minimum of mechanical inertia.

The development of curemeters in the 1960s, with its aggressive marketing by Monsanto, revolutionized cure testing in several major respects. The test was much more sensitive and discerning than the method in wide use at the time. The latter consisted of curing a tensile sheet at each of three times at a standard temperature, and subsequently determining tensile strength, ultimate elongation and stress at 300% strain on a Scott Tester. Further, the curemeter was much faster, opening the trend for its use in the plant. More recently activity has been directed toward refining the curemeter by a rotorless design to lessen friction, and also to hasten temperature equilibration by minimizing the mass of the specimen.

The rubber industry has greatly benefited from the determined efforts of technologists and their employers who have been willing to meet and support the discipline of establishing standard methods and instruments widely used. These provide a common language and arsenal of tools which enables our industry to fine tune its ability to compete with other industries. The Organization for International Standardization (ISO) performs this vital function on an international scale. The American Society for Testing and Materials (ASTM) is familiar to Americans, and has counterparts in many countries making excellent contributions to the international effort. The ASTM standards are widely available and familiar in our industry and list the equivalency to ISO procedures and instruments.

Research and development instruments have also been used to make very fundamental contributions to the rubber industry, especially in harnessing the unique and exciting properties of rubber for ever widening areas of application. Rubber groups and the Rubber Division of the American Chemical Society (ACS) sponsor multi-day programs and seminars aimed at improving the awareness of engineers and technologists of the usefulness of proper engineering design with rubber. Techniques and subjects include viscoelasticity, static and dynamic load deformation, finite element analysis (FEA), overall design criteria, heat build-up, fatigue, fracture and aging. Modern electronics have been applied successfully to study the intricacies of flow of plastics and rubbers at all applicable rates of shear, temperatures and conditions found in the latest production equipment. Instrumentation for electrical testing has kept pace with the incredible range of approximately 20 decades of volume resistivity available for exploitation in current polymers and composites. Automotive engineers and suppliers to their industry have developed a vast array of devices to measure the dynamic properties of rubber in virtually all conceivable sizes, geometries, rates of strain, rates of stress, temperatures, and configurations in end-use. Information and insight gleaned has been used all the way from the most theoretical research purposes to the most practical marketing and production goals. A recent review has emphasized the great advantages of using the FEA method in designing many of the approximately 600 rubber components of the modern automobile, many of which are critical and load-bearing.

Tire testing is a vitally important area of involving mechanical properties, although most of it is probably not reported because of its proprietary nature. Force variation, X-rays, infrared, microwaves, ultrasonics and laser interferometry holography have been used to evaluate tires on a non-destructive basis. Non-destructive tests have the potential to aid in the understanding of performance without destroying the product, enhance product reliability and assist in the study of product performance. A speaker at the Rubber Division meeting in Cleveland in 1987 described the program and instrumentation in one company to automate tire testing in the laboratory and integrate it with vehicle testing to achieve greater productivity. Instrumented trailers have been used for some time to measure tread wear and tire characteristics as a function of systematic settings of slip angle, load, speed and under varying road conditions.

Chemical analysis

The history of chemical analysis is as inseparable from instrumentation as that of physical testing, although instruments for analytical purposes were developed later. Even in 1937 in Davis and Blake's classic "Chemistry and Technology of Rubber" the only analytical instruments mentioned were a pH meter, nephelometer and a specific gravity balance. A milestone in the field of chemical analysis in our industry occurred with the publication of the first issue of Rubber Chemistry and Technology in 1928. The contributions of dedicated technologists and scientists to attacking and in many cases solving complex problems originating in the plant or laboratory have been faithfully reported. The advent of an annual Review issue in 1957 has been especially valuable in providing comprehensive and authoritative treatment of specialized topics in a form readily accessible to anyone interested.

The first decade of Rubber Chemistry and Technology included articles on the subjects of X-rays, pH, refractive index, electron diffraction, infrared spectroscopy and even Raman spectroscopy. The introduction of analytical instruments into the rubber industry has its human interest side.

One scientist in Akron built his own infrared spectrophotometer before they were available or affordable. Another reportedly bought an optical microscope with his own money during the Depression in order to have a needed instrument with which to do research.

The use of instruments for chemical analysis has been a continuing adaptation of principles, tools and techniques initially in the realm of physicists. Virtually every region of the electromagnetic spectrum has been adapted to chemical instrumentation. Proceeding from the most energetic (shortest wave length) end of the spectrum are: gamma rays, X-rays, ultraviolet, visible, infrared, microwaves and radio waves, at least seven in number. Each technique has unique applications, advantages and sensitivities. Most share the common characteristics that they are expensive, and require highly dedicated and trained specialists to separate out the true response and meaning from the enormous universe of background noise.

The UV visible spectrum was an early region of the electromagnetic spectrum to be used in the rubber industry and is still included in standard tests for accelerators, antioxidants and certain metals. Infrared spectroscopy is one of the most widely used and has a rich data base probably unmatched by other analytical techniques. It provides information on functional groups, crystal forms, hydrogen bonding, orientation, conformation, molecular symmetry, and other characteristics of large and small molecules. More recently, Fourier Transform Infrared spectroscopy (FT-IR) has made possible wider applications including the study of surfaces.

Optical microscopy has long been used to study fillers and compounding ingredients of small particle size, as well as inspection and study of surfaces, and plant troubleshooting. The electron microscope was developed to look for more detail because of its higher resolving power and magnification; it is especially useful for examining carbon black structure and crystallization patterns in polymers. The scanning electron microscope makes possible the study of surfaces, even rough ones, since it provides a wide depth of field.

X-rays were used initially to identify a crystal pattern in stretched natural rubber, leading to a better understanding of its strength properties. X-ray diffraction of metal oxides found in ash of rubber is a useful method of identification. X-ray absorption is used, for example, to show the position of cords in cord-rubber composites undergoing deformation. X-ray fluorescence techniques are useful to determine rapidly the amount of most elements.

The term "chromatographic analysis" was introduced early in this century by Tswett who separated colored plant pigments with a column technique. Gas chromatography has revolutionized the separation, qualitative and quantitative analysis of suitably volatile chemicals. For example, gasoline has more than one hundred major peaks, each clearly separated and representing a different chemical molecule. The method has been applied to determining components pyrolyzed from solid rubber for purposes of identifying the polymers or components.

Thin layer chromatography (TLC) is widely used to identify the antidegradants, accelerators (via residues), waxes, lubricants and other compounding ingredients extracted from a finished, vulcanized product. Some feel that the wide use of TLC, especially in conjunction with other techniques, has eliminated the secrecy that formerly prevailed regarding compound formulation. In other words, your competitor can find out the identity and amount of virtually any technologically significant ingredient in your formulation if the budget and determination permit. TLC has an advantage of being one of the least expensive analytical techniques with which to get started, but requires a highly talented scientist to make it effective, who must also know the technology thoroughly.

Liquid chromatography has found increased use in the rubber industry in recent years because it is more suitable for higher molecular weight materials, such as oligomers and certain compounding ingredients. Separation of the material being eluted occurs because of its varying chemical affinity for the column packing material, as in gas chromatography. The difference is that high pressures of 20-200 atmospheres or higher in a liquid medium are required, and UV, transport or other detectors are used. Size exclusion chromatography (SEC), sometimes called gel permeation chromatography (GPC), operates on the principle of mechanical separation. Large molecules are held the shortest times in the interstices of the column packing material, while the smallest molecules are held the longest. The technique is useful for molecular weight distribution and architecture of polymers.

Nuclear Magnetic Resonance (NMR) has become a powerful tool for characterizing the structures of rubbers and providing information about the arrangements of constituents along the chains and their compositions. Like other forms of spectroscopy, NMR requires a source of electro-magnetic radiation (in this case radiowaves), a sample of absorption and an electronic-detector-recorder system. The absorption of energy originates from the magnetic moment in the nuclei of the sample. Thus intimate structural details like sequence distribution, head-head vs. head-tail linkages, and stereochemical arrangements have been studied as related to polymerization mechanisms and desirable mechanical properties sought. One very practical application is that the rubber in a vulcanizate can be identified with necessity of separating out the carbon.

The main types of thermal analysis used in the rubber industry are thermo-gravimetric (TGA), Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC). They operate, respectively, on the basis of responding to changes in the specimen regarding mass, temperature and heat content. When a specimen of vulcanized rubber is put into a TGA apparatus and the temperature is increased, the most volatile components come off first, such as light oils. Next, the heavy oils come off, followed by pyrolysis of the polymers. Upon admission of an oxidizing atmosphere, carbon black burns off leaving ash. With DTA and DSC, the glass transition temperature can be identified, followed by thermal changes due to ingredients melting, loss of moisture, melting of crystallites, additional vulcanization and finally degradation of polymer. Commercial equipment is available aimed at research, development and plant use.

Atomic absorption and emission spectroscopy are used in the rubber industry to determine metals. When, for example, the ash of an important specimen is subjected to an intense source of energy, traditionally a flame or carbon arc, the metals give off characteristic wave lengths of radiation. The most familiar is the yellow color coming from sodium when water boils over on the stove. The lines can be separated and used to identify the metal or metals. Atomic absorption works on the principle of intentionally putting the exact wave length of the material being analyzed through the flame. Since most of the atoms are not incandescent or flaming, the absorption is used to measure the quantity. Standard methods and equipment are available making the technique attractive for many locations.

The concept of mass spectrometry consists of fragmenting molecules and creating ions in a gas phase under high vacuum, and then separating the fragments according to their mass-to-charge ratio. The technique requires a very small amount of sample, can elucidate structural information and determine molecular weight. Its combination with gas chromatography has proved a powerful tool for analyzing complex mixtures, such as gasoline. The dilemma exists that, although the goal of polymer chemists is to put the polymer in directly, the extreme non-volatility of the polymer makes this difficult. Accordingly, ingenious techniques have been developed for instantaneously and reproducibly volatilizing the molecule in the instrument.

Process control

Process control has been defined to include all actions taken to insure that the output of a given process is acceptable for operations downstream and ultimately to the consumer. If accepted, the definition applies equally to all conceivable operations in a rubber factory. These include mixing, calendering, extruding, molding, etc. In the broadest sense, the definition includes batch test/analysis and statistical practices which have been used over the years, no matter how slow, insensitive or variable they may have been. The trend is essentially a continual effort to install appropriate sensors, display crucial information, connect the components electronically, provide decision making capability, all to regulate suitably the enormously complex processes in spite of sporadic and disturbing influences.

As an example, the control system during injection molding must instantaneously monitor numerous inter-related and complex steps including the following:

* Machine sequence,

* Time per operation,

* Safety,

* Speed of sequence,

* Temperatures,

* Sub-programming (pressure, screw-recovery rate, etc.) and

* Diagnosis

Elements and circuits in the system can be mechanical, hydraulic, electrical, electronic or a combination. Each must function reliably, sensitively, safely and with a minimum of human involvement. Otherwise inconsistent, variable moldings are the result.

Visual display units (VDU) operated by microprocessors are favored for the main parts of the machine monitoring system, namely the transducer, transmitter and display. Thermocouples are widely used for temperature measurement; strain gages and transducers for pressure. Proportional-integral-differential (PID) controllers provide the most exact control of temperature compared to other types.

The Cavity Pressure Control (CPC) system provides better control of the process of filling the mold than less elaborate systems. It consists of a sensor, a device to interpret the signal and initiate control action, plus a means of electrically connecting the two. Advantages claimed are lowered inspection costs, cycle time reduction, automatic fault warning, more consistent ejection, better dimensional tolerance and material savings.

Computer-control of injection molding machines has become common-place. Systems are available which control 30 or more PID temperature control loops. Set-points can be entered digitally, read into the system and displayed. They are useful for trouble-shooting, quality control, collecting production information and assisting process optimization. Diagnostic routines can be programmed to give specific messages. Flexible manufacturing systems (FMS) are possible wherein a combination of mechanical engineering and microelectronics bring to batch work the economies of scale.

The process of extrusion consists of forcing a molten polymer through orifices forming desired shapes which may have varying cross-sections. The extrudate can then coat other substrates, undergo further shaping or deformation, or maintain its extruded shape. Earlier, Mooney and the Garvey Die Swell Tests were used to predict suitability. Recent literature shows a major concern for developing control tests which are rapid and reliable in the areas of carbon black dispersion, die swell testers and stress relaxation tests. Dark field reflected light (DFRL) microscopy has been developed to monitor carbon black dispersion. Die swell can be monitored by measuring the ratio of the velocity of the extrudate internal and external to the die. The most widely used is no doubt the Monsanto Processability Tester (MPT), which is a laser scanning system to measure the increase in thickness of the extrudate as it emerges from a specially designed capillary rheometer. The RAPRA Processability Tester measures the relaxation of an initial rapid compressive deformation. Other useful devices are on the market. Microprocessors are finding increased interest in application to control the extrusion process, because of low cost and great potential. Interaction with the operator, performing logical operations, receiving information from multiple sensors and adjusting control parameters are among the possibilities.

However, the literature warns that limiting factors will continue to include the difficulty of interpreting the claims of microprocessor manufacturers, the skill, judgement and/or dexterity required in many processes, and lack of a complete, quantitative model. The possible trap always exists of rapid obsolescence of components. Even so, confident predictions are made regarding continued activity in this area because of the substantial economic and technical rewards for those who solve the problem.

Control of processes has been broadened to include key elements and the whole production system in tire plants. A vendor recently reported on a non-contact laser measurement system for automation of tire tread extrusion lines. The goal was progress toward achieving zero conicity waste and perfectly symmetrical distribution of extruded material across the tire tread.

One authority in this field points out that there are dangers in installing a stand-alone computer. He prefers a hierarchical system where computers of lower level and more limited capacity unload data or summaries to computers of higher level capacity under pre-programmed conditions. Thus, routine and normal processing information can be put in the background or discarded, while exceptional or unusual conditions can be emphasized.

The tone of most articles is that suitable hardware and software are available commercially. The key is to design the system exactly ahead of time to meet all the demands that may be put upon it - hardly a simple task. The role of the automation engineer in the rubber industry is to prevent companies from suffering from technological malnutrition by using the instrumentation available to the fullest competitive advantage.

Summary

Thus we have seen that instrumentation has been involved in virtually every aspect of rubber science and technology. Instruments perform vital roles in production and QC, as well as providing data for R&D scientists and engineers to create new products and techniques to meet intense competition. Use of computers and microprocessors ranges from small dedicated units to large hierarchical systems and can only increase in the future. The trend will be that all operations will be subjected to increasing scrutiny to minimize adventitious and unwanted sources of variability. The consumer is demanding ever greater reliability and dependability of products reaching the marketplace.