Why was silicone elastomer chosen as the material to support the TFM?

This literature review looks at the reasons why a silicone elastomer was chosen as the material to support the TFM (modified PTFE) facing and allowing the possibility of achieving the objective of improving diaphragm life exposed to steam at high temperatures. The reviewed work will look at the silicone compound and aspects such as different cure systems within the compound formulation and to see how different alternatives worked best to improve performance. As well as improving diaphragm life expectancy the review will also look at processing methods of the silicone compound which includes the advantage of post curing the finished product and the benefit this had on product performance. Keeping within the limits of the project a review will also be taken on how a silicone elastomer can also offer an advantage over previous offerings like an EPM product with regard to stress relaxation. This will include looking at specific problems encountered with compression and how a silicone elastomer displays improved compression set properties.
2.1 The Evolution of Materials and Diaphragm Design Concepts
The diaphragm has come a long way since its initial conception, literature produced
by Century Instruments, details how the first diaphragm was made in ancient Roman
and Greek times from leather. “It was used to control the water and temperature of the hot baths. With a crude leather diaphragm, which was manually closed over a weir, it was a primitive but effective control valve.”[1] The modem diaphragm valve as the world knows it today was, conceived in the early 1900’s by a South African mining engineer called P.K. Saunders. He utilised the ancient Roman and Greek concept, to develop the first modem diaphragm valve. [1] Information from the Saunders web site confirms that the diaphragm was invented in 1928. [2]
The importance of the diaphragm valve’s design cannot be underestimated. Its
simplistic weir design is intrinsic to its success. It has many advantages over other
current valve designs available, the introduction of a weir style design meant the
diaphragm valve can lend itself to be used in a wide range of applications. This was a view shared in the technical literature obtained from a book on, Process Piping Systems. “A number of diaphragm-valve body designs are available; they fit two general classifications-the weir (or Saunders-patent, F/15), and the straight-through type. The Saunders valve is the most widely used because it offers both tight shutoff and short stroke that permits the use of harder (and less flexible) diaphragm materials such as Teflon. The straight-through pattern is limited because there are few elastomers (and even fewer plastics) available that are sufficiently flexible to tolerate the long stroke that is needed. The diaphragm valve is good for viscous flowing media, slurries or corrosive fluids. Many solutions or slurries that would clog, corrode or gum up the moving parts of other valves pass through a diaphragm valve with no problems.” [3]
Another report produced by Gerald L. Gatcomb, was found to be of use in tracing the history of diaphragm design from its beginning as a component constructed from natural materials to present day, where diaphragms are now being skilfully
manufactured from expensive, technically advanced materials. Gatcomb’s paper
focuses more on the diaphragm’s inclusion in the automotive and aircraft engineering industries, where the diaphragm proved to be an important factor in helping to improve the safety and efficiency of all motorised vehicles. “The use of diaphragms in control and regulating devices has become a well-established in the automotive and aircraft industry over the last fifty years. From a rather simple beginning, diaphragm technology has advanced to its present state of sophistication and reliability.” [4] Gatcomb cites the fact that the incorporation of the diaphragm valve into, automobiles and aircraft, only happened during the late 1920’s, which correlates to the invention of the diaphragm valve, as stated previously. The article goes on to explain how this does not suggest that this was the beginning of use in automobiles and aircraft merely that previous designs cannot be traced or have not been recorded. The report details how the earliest recorded diaphragm was used in diaphragm equipped fuel pumps, “These early pumps had diaphragms made of multi-ply cotton fabrics coated with linseed and rapeseed oil”. [4] Such an invention was a revolution compared to the early gravity fed fuel systems, which had inherent problems when encountering any steep inclines during travel.
As mentioned earlier it is the diaphragm valves short stroke design, which allows it
to use a wider variety of more technically advanced elastomers. Development of
diaphragm, with regards to materials and design, began to increase, as the importance of the diaphragm, as the key component in the valve, was realised. “The diaphragm controls the flow through the valve, from 100 to complete leak-tight shut-off, it also protects all working parts of the valve by isolating them from the line media and this performance is required to be maintained for 1000’s of operations. This is testimony to the design of the diaphragm, which has been developed over the last 80 or so years”. [3].
Eventually the use of more, harder, less elastic, more exotic elastomeric materials
such as PTFE (Figure 2.5.) and Fluoroelastomers would prove to be the lynchpin for
the diaphragm’s success, allowing the diaphragm valve to enter a whole New World
of different applications. But before the application of these materials, the most
important developments involved the invention of the fuel resistant elastomers.
“Neoprene was introduced in the early 1930’s and the first fuel resistant diaphragm
based on Neoprene was developed in 1935. This product was used in automotive and aircraft fuel systems.” [4] Gatcomb’s paper also recalls how the outbreak of World War II resulted in a technical explosion that has much to do with shaping the look of diaphragm design today. Butadiene/Acrylonitrile elastomers were synthesised next, allowing resistance to aromatic fuels and the discovery of Nylon fabric would prove to further enhance the diaphragms technical ability.
Also during the same period considerable effort was made to change the process of
design and manufacture. “Advances were made in the fabrication and utilization of
diaphragms. Until this point, most diaphragms were installed from flat die cut pieces,
which limited their travel and efficiency. In the early post war years, new techniques
for forming convolutions, deep-draws, beads and semi-cures were developed. These
techniques allowed the manufacture of diaphragms to perform functions that would
not have been possible earlier”. [4]
It is the constant development of the diaphragm that has enabled it to function and
carry on performing in ever demanding environments. More recently the diaphragm
has become an integral component in the growth market of Food and Biotechnology.
Both, aforementioned industries, are governed by strict regulations, and some of the
harshest working environments. The superior design of the diaphragm valve, and the
diaphragm’s, technically superior material compositions, has helped it become the component of choice. A paper written by, Kaporovskii, Azarkh and Yurtsev details
why diaphragm valves are chosen for use in this industry. “Diaphragm (membrane)
valves are used in low-pressure hydraulic and pneumatic systems (up to 0.6 Mpa).
The cut-off element in these systems is a thin-walled, normally spherical rubber or
rubber-fabric diaphragm. It divides the cavity with the working product from the drive
cavity and seals the joint between the valve body and cap. Owing to the absence of
friction pairs exposed to the working medium, and also the almost complete
elimination of stagnation zones, these valves are widely used in the food industry and biotechnology, where the diaphragms are exposed to high temperatures for sterilisation of the system. [5]
It is in these sectors of industry where some of the more recent developments in
diaphragm design have occurred. Food and Biotechnology processing plants often
have to steam sterilise at least once a day. The general system consists of a valve that is kept shut and then opened once a day, allowing steam through to clean the system. This proved to be problematic for the conventional diaphragm design. The diaphragm has to remain in the closed position for long periods of time, in a highly stressed state, whilst subjected to steam at a temperature of 140°C. This often caused premature failure, occurring after only one or two operations.
The problem is solved by tailoring the diaphragm’s configuration, to suit the
application i.e. manufacturing the diaphragm so it is formed in the closed position.
Thus, it will not be in a permanent state of stress for the majority of its service life,
helping to increase durability. This was also an area of interest detailed in the work
by, Kaporovskii, Azarkh and Yurtsev. “Diaphragm valves are produced in two forms:
normally closed and normally open. In the former case, to shut off the working
channel it is sufficient to press the diaphragm, without changing the curvature of its
spherical section, against the sealing belt of the body, which is positioned between the entry and exit channels. When the valve is opened, this section of the diaphragm is bent so that there is a change in sign of its curvature. In a completely open valve, the diaphragm is in the stressed state, the most stressed sections being areas adjoining the tie. In normally open valves the reverse picture is observed: in the open valve the diaphragm is in the unloaded state, but when it is closed the curvature of the spherical section changes, and the diaphragm material adjacent to the sealing belt is compressed. [5]
From the articles consulted, it appears as if the diaphragm is already the finished
article. Unfortunately for all manufacturers of diaphragms, there is still a gap in the market needs exploiting. This gap being the need to develop a material that can with-stand high end temperature applications for extended periods of time.
2.2 Silicone Elastomers
Silicone elastomers are elastic substances which contain linear silicone polymers crosslinked in a 3 – dimensional network. In most cases this network also contains a filler which acts as a reinforcing agent or as an additive for certain mechanical, chemical or physical properties. In general all silicones (polydimethyl siloxanes) are noted for their high thermal stability, biocompatibility, hydrophobic nature, electrical and release properties. When silicones are crosslinked to form a silicone rubber their characteristic properties are still prevalent. Hence silicone elastomers can be widely used in a great variety of applications. [6]
The siloxane bonds (-Si-O-Si-) that form the backbone of silicone are highly stable. At 433 kJ/mol, their binding energy is higher than that of carbon bonds (C-C), at 355 kJ/mol. Thus, compared to common organic polymers, silicone rubbers display improved properties with the mentioned higher heat resistance and chemical stability, and enhanced electrical insulation. Silicone rubber can be used indefinitely at 150°C with almost no change in its properties. It is known to withstand use even at 200°C for 10,000 hours or more, and some products can withstand heat of 350°C for short periods. Silicone rubber also has excellent resistance to cold temperatures. The embrittlement point of typical organic rubbers is between -200 and -30°C, compared to -600 to -70°C for silicone rubbers. Even at temperatures at which organic rubbers turn brittle, silicone rubber remains elastic. [7]
2.2.1 Silicone Types
Silicone elastomers are commercially available in two types: millable high consistency silicone rubber (HCR) and pumpable liquid silicone rubber (LCR). Silicone elastomers are proprietary compositions that contain silicone polymers, reinforcing and extending fillers, and cure ingredients. The polymers used in silicone elastomers are of the general structure depicted in Figure 2.1, where R represents -OH, -CH=CH2, -CH3, or another alkyl or aryl group, and the degree of polymerisation (DP) is the sum of subscripts x and y. For high consistency silicone rubber elastomers, the DP is typically in the range of 5000 to 10,000. Thus, the molecular weight of the polymers–generally called gums–used in the manufacture of high consistency silicone rubber elastomers ranges from 350,000 to 750,000 or greater. In liquid silicone rubber elastomers, the DP of the polymers used typically ranges from 10 to 1000, resulting in molecular weights ranging from 750 to 75,000. The polymer systems used in the formulation of these elastomers can be either a single polymer species or a blend of polymers containing different functionalities or molecular weights. The polymers are selected to impart specific performance attributes to the resultant elastomer products.
Figure 2.1 Two Roll Mill
2.2.2 Silicone Cure Systems and Post Curing Process
A paper titled “Elastomer Engineering Guide” written by James Walker, a global manufacturing organisation that supplies a vast range of products and services to elastomeric and plastic industries explains how “The basic properties of elastomeric products are highly dependent on the base polymers used in their manufacture”. (8) This paper also gives detail on the many additives and their uses that can also be added to the polymer(s) to tailor physical properties and final product performance. In particular interest is of different cure system types. As mentioned in a Journal titled “Crosslinking of Siloxane Elastomers” written by Jacob Heiner, Bengt Stenberg, and Maria Persson (9) explains how two major curing mechanisms are frequently used to crosslink silicone elastomers. These two mechanisms being free radical and addition curing, whereby, both processes transfer the polymer from the liquid state into an elastomer state. In this paper Heiner investigated the dependence of the elastomer properties on the crosslinking type system. With silicone elastomer materials widely used in applications relating to the Food and Drug Industry Heiner was interested in whether sterilisation had an effect on the properties of samples produced from both systems. The report concluded that an increase in modulus and hardness was found in silicone elastomers cured with peroxides. The 50% increase in modulus was deemed to be a change from the formation of additional crosslinking sites. The platinum cure samples displayed consistent physical properties after numerous sterilisation cycles (120°C for 30 minutes) compared to original. The additional crosslinking with the peroxide system was attributed to the post cure process carried out. This post curing process of 4 hours at 200 °C was required to remove any by products and ensure conformity to FDA regulations.” Any increase in the crosslink density due to residual peroxide would increase the crosslink density of the polymer and the residual peroxide in the elastomer might be activated during the sterilisation process” was concluded within the report. It was noted from the report that only a change to the modulus occurred after 75 sterilisation cycles. The modulus with the peroxide cured system remained quite linear to that point. Additives can be added to the system which aid crosslinking but there are also additives which prevent crosslinking at ambient temperatures which is important for platinum catalyst systems. A paper titled “Platinum Catalysts used in the Silicone Industry” [10] details the importance of an inhibitor when using platinum as a catalyst. The author, Larry Lewis, concluded how the incorporation of an inhibitor was a must for such Silicone types to prevent hydrosilyation (crosslinking) at low temperatures but permitting rapid reaction at elevated temperatures. “Dimethyl fumarate and Dimethyl maleate both succeeded in obtaining long working life at ambient temperatures but also allowing a formation of a complex with the catalyst when activated together at elevated temperatures” [10]
Gaining approval accreditations to these FDA regulations is an extremely important factor for a product to sell in the BioPharm industry so the Post Curing Process plays an important part in ensuring the removal of any by products. Shen Diao explains in a paper titled “Preparation and properties of heat curable silicone rubber through chloropropyl / amine crosslinking reactions” (11) the post curing process can also help in improving physical properties of a Silicone elastomer. Part of the work in this paper explores alternative post curing process conditions and the resultant effect on the physical properties achieved. Temperatures of 180°C, 200°C and 220°C all for 4 hours were evaluated. The data collected from the investigation concluded that the silicone rubber post cured at 200°C for 4 hours exhibited optimal mechanical properties. The tensile and tear strengths recorded figures of 10.25 MPa and 41.87 KN/m respectively. This was an improvement on the Non post cured figures of 9.81 MPa and 41.14 KN/m figures with optimum amount of cross linker added. The lower post cure temperature of 180°C did not complete the condensation reaction whilst the higher post cure temperature of 220°C actually decreased the properties due to “ageing” the compound (11).
2.2.3 Benefits of Fabric Reinforcement in Elastomers
Achieving desired physical properties in diaphragm compounds are not the only concern with diaphragm manufacture. The problem of visco-elastic deformation is a common factor most rubber engineering components will have to deal with if they are to be durable and have an acceptable service life. The most probable way of inhibiting such behaviour, involves the conception of some sort of composite design. Rubber composite design in an engineering component, usually involves some sort of reinforcement being incorporated into the body of the elastomer. A paper written by Long and Dill, on reinforcing materials for rubber products, illustrates the need for reinforcement and details some of the various types available for use in rubber products. “For most applications, vulcanised elastomeric material alone is too elastic and must be reinforced for strength improvement. A wide range of reinforcing materials is currently available to the product design engineers-cotton, rayon, nylon, polyester, fibreglass, steel and aramid, all of which possess different properties and subsequently different performance characteristics.” [12) What becomes apparent when looking into the subject area of reinforcement in rubber is that, “the rubber industry consumes a very large quantity of reinforcing material in the manufacture of its products.” [12] The addition of reinforcement is required to improve the overall mechanical properties of a rubber component, and the actual reinforcement type chosen has just as an important effect on the composite’s properties as the elastomer itself. Other factors effecting reinforcement choice depend largely on the application in which it is to be used and therefore requires a full understanding of the mechanics and environment before a composite design is chosen.
Long and Dill list the basic functions required from an effective reinforcement. “(a)
Impart load-carrying capacity. (b) Serve as the medium for stress transmission. (c)
Provide dimensional characteristics. (d) Determine basic strength and durability.” [12]
Most applicable to this body of research, is the use of textile reinforcement, as it is without doubt the most suitable type of reinforcement for the mechanics encountered by a diaphragm. Textile reinforcement has the ability to provide much needed dimensional stability and resistance to extensibility, enabling greater pressure retention, without impairing flexibility, another property essential to the mechanics of a diaphragm. For this performance to be obtained the textile itself must have certain basic physical and chemical properties, such properties are listed in a paper titled, Essential Properties of a Textile Fibre for use in Rubber Industry, written by Ghosh.[13] Of particular interest is the information on nylon fibres, as it is the textile reinforcement of choice for this application. In general the properties of textile fibres can be separated into two different categories, properties, which are inherent to the structure of the material the fibre is made from and properties, which, come about as a result of the processes used during the manufacture of the textile.
Tenacity is a physical requirement mentioned and one that is inherent to the molecular structure of a fibre. Tenacity is governed by the amount of crystallinity present, the higher the crystalline content, the greater the strength due to strain crystallisation in tension, predominantly amorphous fibre structures will have a reduced tenacity .Modulus is another property governed by molecular structure and one where the balance needs to be correct if the reinforcement is to be effective. “Although resistance to stretch is a prime factor in a cord used in the rubber industry, certain extensibility is always desirable. Unless the cord has certain extensibility, it will not be able to withstand the growth of the product during making moulding and using and there is a possibility of premature failure due to bursting of the structure. The lower extensibility at load and higher at break is desirable in an industrial cord.” [13] Creep resistance is a property of the fibre and a property, which is desired to be as low as possible. “Cords must be capable of returning to its original position after use in order that it again will have the ability to absorb an equal amount of energy on subsequent uses.” [13) Nylon has better creep properties than most man-made fibres. Toughness and Impact Resistance can be important in applications where the composite could be subjected to high speed loading, as in aeroplane tyres. Impact resistance basically comes as a result of the fibre being able to absorb energy quickly. Again, nylon excels in both these properties as it has, “high strength and elongation. Its load-elongation area is more and so it is a very good energy absorber and has high impact resistance. In other words, it may be said that higher the load-elongation area of a fibre, better the impact resistance” [13]
Flexibility of the reinforcing fibre is of paramount importance, especially for composites in dynamic applications. Flexibility is another property inherent to the material and “depends on the elastic modular of the fibre and this modulus is controlled by the structure of the molecules i.e. the amount of crystallinity and amorphous regions.” [13]
Some fibre properties can be modified during manufacture, for instance, the twisting of a man-made fibre can affect the strength and flexibility of a cord. Increasing the degree of twist, results in an increase in flexibility, but a decrease in strength.
Fibres made from thermoplastic materials are heat stretched during manufacture. Heat stretching has the effect of stabilising the fibres. The process occurs in four stages, “Drying, Heat Stretching, Normalising and Cooling.” [13] Such a treatment reduces shrinkage and distortion during manufacture. Nylon fibres have especially high extensibility and require heat stretching to manipulate their modulus, reducing extensibility and growth during service. Another essential process in the manufacture of fibres, in particular man-made fibres, is their pre-treatment with adhesive bonding agents. The nylon reinforcement used in the diaphragm for this research, has a Resorcinol Formaldehyde Latex dip, the importance of which is detailed by both Ghosh and Long and Dill. “Man-made continuous fibres are very smooth and their bondage with rubber is very poor.” [13]
For this reason fibres are dipped in a rubber solution to achieve good bondage between rubber and textile. “Although the adhesive represents only a small percent of the reinforcement’s total weight, it performs an important function; that of stress transfer between the relatively low modulus rubber and the high modulus reinforcing material.” [12] Without this bond, a composite will fail prematurely due to separation between rubber and textile. In general the composition of the dip is chosen specifically to suit the fibre material and the rubber matrix it is to be bonded to.
“Generally, adhesive consists of highly reactive chemicals, such as resorcinol formaldehyde resins and isocyanates, dispersed in a rubber latex medium which co-cures with the rubber during vulcanisation.” [12] During the manufacture of vulcanised rubber composites, it is always advisable to avoid the presence of moisture, as it can cause moulding defects such as, blisters and poor adhesion between reinforcement and rubber. To avoid such occurrences it is important for the fibre to be dried, especially when using fibres that have high moisture absorption.
When using woven fabric reinforcements in composites, it is possible to influence the end properties exhibited, through the weave or pattern of construction chosen. Long
and Dill explain how the textile fabrics used for reinforcement in belting, hose, gaskets, and expansion joints, have properties that are influenced by the weave construction applied. “There are four principal types of weave, namely plain weave, twill weave, basket weave and leno weave.” [12] Each of the aforementioned weave constructions have a specific load carrying ability and are therefore applied in different composite constructions or within the same composite constructions for a combined strengthening effect. The example below is taken from the paper by Long and Dill and illustrates the way in which construction can be used to engineer composite properties. “Most hose ducks are so-called square woven fabrics. The tensile strength of square woven fabric is equal in both directions. Because fabric is applied to hose on the 45 bias, the resultant force of the warp yam and the fill yam of equal strength will be on the longitudinal axis of the hose to reduce movement of the hose under internal pressure. Hose ducks are generally of soft open weave.” [12]
With the ability to tailor such effects to suit the end properties of a composite, then it is understandable why there are many papers studying the effects of reinforcement construction in more detail. A paper written by Morozov on the, Mechanics and
Analysis of Fabric Composites and Structures is, “devoted to the mechanics, modelling and analysis of fabric-reinforced composites and structural components.”
[14] Morozov illustrates how the strength of a fabric composite “depends not only on the yams and matrix properties, but on material structural parameters as well, i.e. on fabric count and weave.” [14]
2.2.3 Shrinkage in Moulded Silicone Products
Incorporation of fabric reinforcement in compression moulded diaphragms does not eliminate the need to allow for shrinkage allowances in the finished component. In general most types of rubbers exhibit the same shrinkage factor after moulding. A book titled “The Complete Book on Rubber Processing and Compounding Technology “[15] written by NIIR board of consultants and Engineers explain the shrinkage found with compression moulded Silicone Rubber products is approximately between 2 – 4% which is twice that of organic rubbers. Factors affecting this primarily are that the liner thermal expansion of Silicone rubber is between 17 – 20 times that of Steel. This, however, depends on a number of factors:
• Temperature at which the product is moulded
• Filler level within the compound composition – reducing the polymer content
• The Vinyl content –
o Increased in High strength bases, compared to general purpose grades
• Inclusion of low molecular …… – which exude/migrate to the surface, increasing the shrinkage rate
These factors also augmented by the release of volatiles during the curing and post curing process [15]. The effect of post curing on shrinkage in silicone products was also investigated in a book written by Brian Ellis and Ray Smith. Titled “Polymers a property Database” [16] the authors investigated a wide range of polymers and gathered information to create a comprehensive database on the physical properties of each polymer type and also slight variations in structure influenced by filler type for example. Ellis and Smith explain how silicone shrinkage can be reduced by including glass content filler in place of traditional fillers but the downside of not achieving FDA approvals will be affected. With this work also recognising the effect of post curing silicone compounds, alternative methods of reducing by-products from Silicone compounds was also explored. Solvent extraction using Methanol (MeOH) as a wash succeeded in removing volatile siloxanes from the compound and therefore reduced the need for post cure and the effect of further shrinkage. [16]
With the proposed aim of the project being to produce a diaphragm material that would sell many thousands of parts per year this work would not suit current manufacturing methods used in the manufacturing plant due to safety, industry approval and time purposes. The information gathered around the shrinkage factor related to moulded silicone products unfortunately deemed the necessity of the new design concept within this report to have its own dedicated compression mould tooling. The theories discussed in the aforementioned review led to the starting point of the new product Development Project.

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