Rubber Sheets

Chapter 1: Principle of Rubber Sheets

This chapter will discuss what rubber sheets are, their manufacturing, and how they are used.

What is a Rubber Sheet?

Rubber sheets are basically sheets made of rubber or cloth that are coated with rubber to improve the mechanical properties of rubber sheeting such as increased tensile strength and reduced elongation.

High-grade general-purpose material with broad applications in oil and petrol service such as fuel pumps, control valves, and other similar uses. They are products that are used in many areas and can be used flexibly for different purposes.

Manufacturing Rubber Sheet

The manufacture of rubber sheets depends on length, width, thickness, color, size, and what it needs to be used for.

Materials Used to Make Rubber Sheets

Recycled rubber, reclaimed rubber, synthetic rubber, and natural rubber are the main broad groups derived from rubber sheets. Recycled material is generally made from recycled tire crumb that has been put back together using an adhesive. This should not be confused with reclaimed material, which is also made from recycled tires and some other discarded parts. Reclaimed rubber is de-vulcanized, which matches through vulcanization again to make a rubber sheet.

Synthetic material is mainly derived from butadiene and styrene, which are by-products of petroleum. These gasses can be combined with many other different materials to produce sheets with different chemical and physical properties; producing a material ideal for many applications. Lastly, there is a natural rubber material harvested from the Hevea Brasiliensis tree in South America, Africa, and Asia. Through this extraction process, sap or latex is taken out of the tree and left to heal to supply more rubber known as natural rubber.

Thermoplastic Polymers

These are polymers that are used on their own, that is, Polystyrene, Polyethylene, Polypropylene. They are relatively easy to reuse and recycling can be repeatedly done a number of times without unnecessary deterioration.

Compounded and blended polymers are less stable and are more likely to deteriorate during recycling, that is P.V.C. and A.B.S. Additions can be made to polymers to enhance their stability, but they are expensive and sparingly used.

Thermoset Polymers

As their name states thermoset plastics are differentiated by the fact that when processing they are ‘cured’ and form a 3-dimensional network. Whilst thermoplastics can be described as long thin molecules laid out in a random manner but without any firm bond between the molecules. The curing process changes the thermoset plastics from a similar state to that of thermoplastics into a state where permanent bonds are formed at various points in the chain with some other molecules.

Rubbers in Rubber Sheets

These are taken as a particular class of Thermosets in various respects. Until they are vulcanized, the material is mostly thermoplastic in that it can be physically reshaped without losing most of its properties. Vulcanising is similarly described as a molecular cross-linking but the process of de-vulcanization is yet to be discovered. Because of the particular many unknown peculiarities of the rubber industry, most of the attention has been given to the recycling problem. Two forms of the product are Reclaim and marketed-Crumb. However, the issue of de-vulcanization remains unknown and unsolved.

Compounding Ingredients

Particular optimum characteristics of various polymers are only acquired by compounding both with mineral fillers and other polymers. A wide range of properties can thus be obtained by the incorporation of a number of various compounding ingredients. This is mostly true of rubbers, including thermoset plastics and of PVC which falls in the thermoplastics category.

Choosing Material to Make Rubber Sheets

The various considerations in selecting material to use in making rubber sheets include:

• Thickness – The thickness of the sheet is very important depending on what it is required to be used for. For instance, it is very important for the sheet to be thick when choosing it for gaskets and oil seals
• Temperature – The rubber sheet will be exposed to specific temperatures; these temperatures matter the most. If the temperatures are too high for the rubber to handle, it will melt. On the opposite hand, if the temperatures are less than its minimum tolerance capacity, the rubber will begin cracking.
• Resistance to Abrasion - rubber’s resistance to rubbing, scraping, or erosion should be considered
• Resistance to Ozone and UV - When exposed to ozone and UV, many types of rubbers break down which needs to be checked.
• Resistance to Oil –Rubber is commonly affected by liquids such as oil, therefore when choosing a type of rubber sheet, consider their property to withstand oil.
• Resistance to Acid – Not every rubber can resist acids. If one is going to use rubber in acidic environments, Vitron is one of the simplest options.
• Hardness – The chemical structure of the elastomers provides for an inherent hardness that can be altered. The enhanced hardness is then measured in terms of durometer (Duro) on a Shore scale. Checking for the hardness of the rubber prior to selecting it should be done especially for sealing purposes. For instance, Shore A is used for rubbers that are soft to medium-hard. Solid rubber has a hardness of 40 duro, with a consistency of pencil erasers. By contrast, harder rubber, like that usually used in hockey pucks, contains 90 duro hardness.
• Tensile Strength - Tensile strength is the amount of force required to tear apart a rubber specimen until it breaks. It is also referred to as ultimate tensile strength and is measured in megapascals or pounds per square inch (psi) in line with the specification ASTM D412. The tensile strength is an important factor for designers and buyers as it highlights the point of failure resulting from the maximum stretching of rubber.
• Tensile Modulus - Tensile modulus is the stress or force needed to produce a strain or an elongation in a rubber sample. It may sound similar to tensile strength but the properties are very different. Hard rubber has a higher tensile modulus which results in it being more resilient. It is extrusion resistant.
• Elongation - is known as the percentage increase, or strain, in the rubber sample’s original length with the application of stress, or a tensile force. Some elastomers tend to stretch more as compared to others. For instance, natural rubber can stretch up to 700% before it reaches its ultimate elongation, and then it breaks. Fluoroelastomers can however only withstand 300% elongation.
• Compression Set – A compression set is an extent to which an elastomer refuses to return to its initial thickness after releasing a compressive load. Consecutive compression of a rubber seal over time results in stress relaxation progression. Compression set is the final result of a continuous decline in a sealing force.
• Specific Gravity - is the ratio of the weight of a material to the weight of the same volume of water at a specific temperature. This property makes it easy for chemists to identify the compounds. It is paramount for technical buyers and part designers to be aware that rubber that has a low specific gravity provides more square inches per pound of stock. By comparison, those with higher specific gravity have more merits when it comes to molding consistency.
• Abrasion Resistance - is the rubber’s resistance to abrasion by rubbing or scraping. Abrasion-resistant rubber can be used in industrial applications such as in pumps that handle slurries and conveyor belts that move coal.

Processes of Manufacturing Rubber Sheets

Four main methods of making rubber sheets are molding, extrusion, latex dipping, and calendaring.

Molding of Rubber Sheets

Molding is the process of forming a compound into a particular shape using a mold. This is a typically less expensive method for more sophisticated products. Since smaller footage can be supplied and the products can be made to order, this reduces the overall costs of otherwise expensive rubber goods. Nevertheless, it is not as popular as the method of extrusion.

Molding consists of three main manufacturing processes: compression molding, transfer molding, and injection molding. Compression molding is the oldest and less expensive method. With compression molding, a rubber compound is made into a blank (a chunk of rubber); the blank then gets placed into a mold cavity to be shaped. The heating time is slow, which results in a long curing time; the heating can vary from three minutes for thin walls to a few hours for thick walls.

Some advantages of this method include being suitable for rubber compounds with large surface areas and therefore the ability to be used for rubber compounds with high viscosity and poor flow properties. On the negative side, the process is time consuming with a low production rate. The compression molding process is utilized in the manufacture of products like O-rings, seals, silicone wristbands, and electrical insulators.

Transfer molding is a natural progression in development to reduce the disadvantages of compression molding. The process starts out with a blank being put in the chamber, which is then distributed into several cavities. In this initial stage, pre-heating takes place within the rubber, forcing the rubber to flow through channels. This pre-heating reduces the curing time and allows the rubber to flow easier and fill mold cavities efficiently and appropriately. However, the molds are even more expensive and complicated.

Extrusion of Rubber Sheets

Extrusion starts with an unvulcanized compound being put into an extruder. Once it is inside the extruder, it gets carried forward to a dye, which is a specialized manufacturing tool used to mold the rubber. When the compound reaches the dye, the pressure from the process forces it through the opening of the extruder. Then, the extruded product will need to be vulcanized before it can be deemed usable. It is crucial to remember that any rubber compound should have a “cure package” already blended in before vulcanization. During the vulcanization process, the rubber may swell or shrink, after vulcanization the length of the rubber extrusion could also be impacted. Extrusion manufacturing has the merit of being able to produce products with lower production costs and in high volumes. Some of the common rubber products produced from extrusion include profiles, cords, tubing, and gaskets.

Latex Dipping of Rubber Sheets

Latex dipping occurs when thin walled molds are dipped into latex compounds and then slowly removed. The thickness of the dipped product can easily be increased by simply re-dipping the product within the latex compound. After the dipping process occurs, the end result is finished by vulcanization. Depending on the finished dipped product, post-treatments may also be required. A proprietary dipping method to achieve the optimal overall product may be done, to produce the required finished product. The main merit of dip molding includes being able to dip latex products with thinner walls and to make complex shapes than extrusion. Some of the well-known dip molded products are grips, rubber gloves, balloons, bladders, tubing.

Calendering of Rubber Sheets

The last most popular manufacturing process is called calendaring, which works by forcing softened material into the center of counter-rotating rollers. Rollers compact the material and therefore the overall thickness of the product is determined by the gap distance between cylinders, which may be adjusted for varying product thicknesses. When the material passes through cooling rollers, it will be vulcanized. Some of the merits of calendaring include controlling the thickness of the product and the ability to produce parts that are thinner and wider than with the process extrusion. Whichever process, calendaring has high operating costs compared to other processes.

Rubber Sheet Joining

Rubber sheets can be joined as a cylinder to produce protective sleeves or cut to length or joined in a conical profile to make products for the marine industry, such as mast boots. Two methods are used: to glue or not. The first thing that can be said is that extremely good adhesives are available, and they offer strong bonds both to rubber-rubber and to some other materials. The question is, why would you consider vulcanizing a joint rather than simply gluing it? The answer lies in the environment together with the uses; adhesive joints will not actually withstand aggressive pressures and temperatures; they will break down when they come in contact with particular chemicals and are not always waterproof. Vulcanized joints can provide an improved aesthetic finish.

How can a vulcanized joint be formed? Firstly, there is a need to cut the profile of the rubber material so as to form the strongest bond – a perpendicular straight cut to the surface may fail to achieve this. The most effective solution may be the creation of a dovetail joint. Next, the material needs to be prepared – this usually involves a mechanical preparation, like buffing rubber in the provision for bonding.

In preparation for vulcanization, a liquid compound is then added to the joint that is also in preparation for bonding. A full mold too or a jig is then needed in order to hold the two parts in position during vulcanization. If it is a complex joint or profiled extrusion, the joint is then put under pressure and heat in order to cure the rubber and by vulcanization form a chemical bond. This method provides a different type of bond than when making use of an adhesive alone. The use should be known in order to determine the required type of bond. This method produces a good bond but is incapable of offering a dimensionally tight tolerance on finished dimensions as on a fully molded product. This saves costs and time, depending on the quantities involved and on the product. When bonding and joining an extruded product into a ring, other profiles are incapable of producing a usable and consistent product, if the profile is deep and the ID is particularly small.

Specifications for Rubber Sheets

The two main specifications include:

>ASTM D1056: For Flexible Cellular Materials – Expanded or Sponge Rubber

There are flexible cellular rubber products that are covered by this specification called sponge rubber and expanded rubber. However this type of rubber does not apply to ebonite cellular rubber or latex foam rubber. The base material for closed/open cellular merchandise may be made of natural, synthetic, a mixture, or reclaimed rubber, and may contain various polymers or chemicals, or both, which inorganic or organic additives can enhance.

MIL-R-3065: Rubber for Fabricated Products

For fabricated products, the specification requires synthetic rubber, vulcanized rubber, or rubber-like compositions individually or in combinations, together with the methodologies of the inspection of such products.

Chapter 2: Types Of Rubber Sheets

The different types of rubber sheets include:

Natural Rubber Sheets

These have great dynamic and rebound properties and are immune to tear. When used for packaging purposes they are used as anti-vibration pads. They comprise excellent dynamic, rebound, elasticity, and mechanical properties. These sheets are suitable to be used as general sealing materials. They are most widely used in the market, as they are suitable in any environment without specific requirements.

Neoprene Rubber Sheets

These are great for corrosion-resistant coatings and are used in power transformers and most other electric purposes. These sheets have the ability to withstand attacks from petroleum products, oil, flame, acids, and alkalis. In industrial applications, neoprene rubber sheets are used as padding in external metal cases for guarding the contents, corrosion-resistant coatings, noise isolation in the installation of power transformers, and any other electric applications.

Extra Soft Neoprene Rubber Sheets

A 40A Durometer Neoprene Rubber can withstand less force or pressure and is also easy to compress.

Soft Neoprene Rubber Sheets

A 50A Durometer Neoprene Rubber is able to withstand less force or pressure but is also easy to compress.

Medium Hardness Neoprene Rubber Sheets

A 60A Durometer Neoprene is the fixed hardness and is suitable for water, oil, beverage, and food whilst remaining weather resistant and tough.

Hard Neoprene Rubber Sheets

A 70A Durometer Neoprene sheet is harder and stronger to compress than the initial standard option.

Nitrile Rubber Sheets

They are specially designed to highly withstand attacks from oil and solvent products and therefore they are used in making electrical transformers, for sealing, in the food industry, and also in a special application for dielectric oils for electrical installations and equipment. Moreover, they are often used between flanges and alike joints subjected to fluid or atmospheric pressure.

Hydrogenated Nitrile (HNBR)

Hydrogenated nitrile rubber compounds have better chemical and oil resistance than nitrile rubbers. They can withstand much larger temperatures. HNBR promises exceptional resistance to fuels, oils, many chemicals, ozone, and steam. It also offers excellent tear and tensile strength, abrasion resistance, and elongation. On the other hand, HNBR is more expensive, has poor electrical insulation and offers limited flame resistance, and is incompatible with polar organic solvents and aromatic oils. HNBR is generally mostly used in the automotive industry for many components, including hoses, static seals, and belts to name but a few.

Ethylene Propylene Rubber (EPDM Rubber Sheets)

Being resistant to acids, alkali, and ketones, they are used for outdoor and high-zone environmental applications.

Silicone Rubber Sheets

These have low permeability, are resistant to high temperature, weather-related aging, ozone, and chemicals. The pharmaceutical industry is their well-known area of application.

Fabric Reinforced Rubber Sheets

This type of sheet is made of natural or synthetic rubber. Here one or more layers of fabric are reinforced to render strength against high pressure so that they can be used for various purposes.

Styrene-Butadiene Rubber (SBR) Sheet

Styrene-butadiene rubber is a less expensive synthetic rubber resistant to abrasion, outstanding impact strength, high tensile strength, and good resilience. On the other hand, SBR offers poor resistance to ozone, sunlight, oils, and steam. The main uses of styrene butadiene rubber are automotive parts, tires and tire products, and mechanical rubber goods.

Vition™ Sheets

Vition™ sheets are fluoroelastomer materials that are used for a wide variety of applications. This fluoropolymer elastomer and durable synthetic rubber has a temperature stability range from -20°C to 205°C. Vition™ can absorb solvents that are fluorinated, can fail if the wrong grade is used, and are expensive. With nitrile, it is one of the most commonly used elastomers for sealing applications, including gaskets, O-rings, and seals.

Vition™ fluoroelastomers were developed in the 1950s for use in the space program due to their resistance to the effects of high temperatures since they can withstand exposure to temperatures over 204°C for over 10,000 hours and temperatures of up to 325°C for shorter periods of time. In addition to its resistance to the effects of high temperatures, vition™ fluoroelastomers are resistant to the effects of most chemicals.

Butyl (IIR)

Butyl rubber is a great alternative for shock absorption. It offers excellent moisture and low gas permeability and outstanding resistance to aging, heat, ozone, weather, chemical attack, flexing, tearing, and abrasion. Butyl has resistance to ester-based phosphate hydraulic fluids and has exceptional electrical insulation properties. It has a tendency to blister, creep, and trap air during manufacture. Common uses include O-rings, sealants, and tank liners. It is used for seals in vacuum applications as it is impervious to gas.

Chapter 3: Applications and Benefits of Rubber Sheets

This chapter will discuss the applications and benefits of rubber sheets.

Applications of Rubber Sheets

Flooring and roofing applications are the most popular uses of rubber sheets in the industry. Rubber provides good resistance to substances like oil and petroleum, ultra violet lights, and oxidizing elements. Furthermore, it has the capacity to maintain flexibility even in cold temperatures. It is elastic and that even makes it suitable for various kinds of shock absorbers and for specialized machinery mountings designed to reduce vibration.

Various types of rubber sheets are used for numerous purposes for example industrial buildings, agricultural machines, and Original Equipment Manufacturer parts. They also have commercial and residential usage in storerooms, gyms, dance studios, garages, under shower pans, drainage systems, washer and dryer pans, weather stripping, and in many other construction and remodeling uses.

Rubber sheets are used in every industry be it scientific or otherwise. Nevertheless, it is used in footwear, medicine, engineering, railways, automotive, defense, aeronautics, marine, and many others. For instance, they can be used as railroad tank cars and a lining for storage tanks, for making floor mats, gaskets, bearing pads, horse stall mats, sandblasting and orthopedic footwear, or other types of curtains capable of withstanding intense work conditions. Such sheets can even be laid under showerheads and outside washrooms to prevent slipping or tripping.

As rubber sheets are becoming more and more widely used their demand now and in the near future will increasingly grow. Above all, the rubber sheets are more cost-effective in comparison with their substitutes available in the market. The ability of rubber to combine itself easily with other products and its cost effectiveness makes it the most favored product in the industrial market.

Benefits of Rubber Sheets

The benefits of rubber sheets include:

Protection of Work Surface – Rubber in rubber sheets is very useful since it is a very good cushion or bump stop in many cases. Because it is elastic, it can protect several work surfaces in industrial environments from damage.

Safety – some of the benefits of rubber are that it is safe for everyone be it the user or the staff. It is safe to use in public areas and therefore very common.

Protection of Machines – Rubber is the ideal choice of material when it comes to protecting your equipment. Rubber is a shock absorber.

Anti-Vibration – Rubber can naturally stop vibrations from the machines. These vibrations can cause annoyance and the eventual wear and tear of machines due to excessive vibrations.

Resistance – Rubber is resistant to electricity. It is also resistant to discoloration and rusting.

Versatility - There are different types of materials that rubber sheets can be cut into. The following are examples of those materials: pads, washers, and even gaskets.

Availability- Available in a range of materials suitable for a wide variety of settings. Rubber sheets can be found in many different colors making them a suitable option for retail and commercial purposes. They can be cut into gaskets, washers, and pads.

Disadvantages of Rubber Sheets

Natural rubber has a low resistance to hydrocarbons, fats, oils, and greases, which is one of its disadvantages. If it comes in contact with these substances the rubber part may swell or dissolve. Ethylene propylene rubber (EPDM) sheets are not recommended for food use or exposure to aromatic hydrogen. Seals for high-temperature water vapor environment, Seals or parts for bathroom equipment, rubber parts in the brake system. Seals in radiators (automobile water tanks). Silicone rubber sheets are generally high in cost. Some of the rubber sheets can be affected by moisture in the air causing them not to last very long. It can expand due to changes in temperature conditions and therefore it should be used in places where this characteristic can be manipulated effectively.

Maintenance of Rubber Sheets

Rubber sheets can last long if they are taken care of properly. They have to be maintained. Maintenance is done by initial cleaning, regular maintenance, and application of rubber finishes. The use of doormats has been recommended strictly, at the main door. A wet vacuum is recommended once a day. This is recommended as it sucks the dirt particles in from the grooves of the sheet.

The chemicals should not be used over the rubber sheet as they may react. Rubber sheets used in hospitals for labor rooms as well as for neonates must be washed under running water and on top of that, an antiseptic solution must be used. After antiseptic treatment, it should be hung till dry and then rolled to keep on the shelf after application of powder. Therefore, the maintenance is easy but needs to be done regularly.

If the rubber sheet is found to have developed cracks, scratches, and thickness reduction, in that case, it may not be enough to ensure the insulation performance and should be replaced in a timely manner. Should avoid direct sunlight or sharp metal scratches, and should avoid being too close to heat sources (heating.) when storing to prevent exacerbation of aging deterioration, thereby reducing insulation performance. They also should be kept dry and clean, taking care to prevent contact with acids, alkalis, and various substances, in order to avoid corrosion after aging, cracking, or viscosity, thereby reducing insulation properties.

Conclusion

Therefore, rubber sheets are basically material that is made from rubber. They are cost effective and safe for use by the public. Manufacturing rubber sheets is a lucrative business as they have immense and various uses in every industry worldwide (chemical, biological, commercial, environmental, and physical). They can be molded into various shapes and in various colors.