Hard Rubber and Soft Rubber Linings Compared
As the Blair Rubber Co. increases its global presence, it has become apparent that hard natural rubber lining is common throughout the world. This is not surprising, since hard rubber has withstood the test of time as a protective membrane for metal vessels handling chemically aggressive materials.
This is due in large part to the inertness of hard rubber created by consumption of virtually all of the carbon-carbon unsaturation sites in the polymer during the vulcanization process. The majority of failures in tank lining, particularly natural rubber, are due to chemical and physical attack at these sites. It is also important to note that natural rubber was the only polymer suitable and readily available for the purpose of corrosion resistant lining worldwide until the 1940’s.
The terms hard natural rubber, hard rubber and Ebonite all refer to high sulfur content natural rubber compounds with a typical Shore D hardness range of 38-80.
This review will address both the pros and cons of using both hard and soft rubber linings as protective membranes.
Hard Rubber Lining Advantages:
1- The overall inertness of vulcanized hard rubber linings is by far the biggest advantage of their use. They are resistant to many oxidative chemical systems simply because there is nothing to be attacked by these chemicals.
2- These linings are very resistant to penetration and swelling by organic liquids such as lubricating oils. This is a result of extremely high crosslinking or crosslink density, effectively leaving no room between molecules for anything else to enter the polymer network.
3- Since hard rubber linings contain no accelerators, their comparative storage life is quite lengthy.
4- Since hard rubber has had such a long history of suitability as a protective membrane, it has been the only specified lining for many applications. This means that any potential replacements will have to be evaluated to determine their suitability. Due to the expected service life of tank lining products, this can be very time consuming.
Hard Rubber Lining Disadvantages:
1- Due to the lack of rubber accelerators, hard rubber lining require much longer curing times at much higher temperatures. Exhaust steam curing is not possible.
2- Hard rubber, when cured, has very low elongation. This means that if the exterior of a vessel lined with hard rubber is exposed to higher temperatures or a temperature differential, the lining will not be able to stretch to accommodate the resulting expansion and thus it will tend to crack.
3- Hard rubber lining has poor impact resistance and if any tool or the like is dropped onto the lining or if excessive vibration is encountered, such as during transport, cracking will result.
4- Hard rubber has very poor compressibility so if it is used as a flange gasket it is likely to crack upon tightening.
5- Hard rubber lining is generally butt-seamed which means the seam will be a weak point in any lining project since the adhesive used to join the seams most likely will not have the chemical resistance of the lining.
Soft Rubber Lining Advantages:
1- Soft rubber linings may be vulcanized at comparatively low temperatures, such as exhaust steam conditions, without the need for a pressurized autoclave.
2- Soft rubber linings also may be vulcanized under steam autoclave conditions. When this method is chosen the temperatures are considerably lower than those for hard rubber and the necessary time for effective cure is lower.
3- Many soft rubber linings are available as chemical cure versions. These require no added heat and are very effective for field applications or where steam is not available.
4- Additionally, some soft rubber linings are available in pre-cured form. These only require adhesion to the desired substrate and a short wait for the cold-bond adhesive system to cure completely.
5- Soft rubber linings are available in many synthetic and natural rubber versions as well as blends thereof. This makes it possible to select rubber lining for very specific service conditions; chlorobutyl for impermeability, polychloroprene for abrasion, nitrile for hydrocarbons and EPDM for ozone, to name a few.
6- Soft rubber, when cured, has high % elongation. This means that if the exterior of a vessel lined with soft rubber is exposed to higher temperatures or a temperature differential, the lining will be able to stretch to accommodate the resulting expansion and as such, will not crack.
7- Soft rubber lining has high impact resistance and if any tool or the like is dropped onto the lining or if excessive vibration is encountered, such as during transport, cracking will not result.
8- Soft rubber has excellent compressibility, if it is used as a flange gasket and the proper tightening procedures are followed, it will seal well and not crack.
9- Soft rubber linings are generally lap-seamed which means the seam will have a 2-inch overlap allowing a very secure attachment far greater than a butt-seam where the thickness of the lining is the only security.
Soft Rubber Lining Disadvantages:
1- Since soft rubber linings are often designed for specific service conditions, switching services may not be possible.
2- Lengthy and extensive testing is often required to show soft rubber lining will outperform hard rubber in similar service conditions.
3- Soft rubber linings require some cooling to obtain suitable storage life in the uncured state.
The success of hard rubber lining as a protective membrane in chemical service cannot be denied. It has been and continues to be a very effective product. However, as the use of hard rubber in such products as battery cases, bowling balls, combs, dentures, etc. has been replaced by newer and more effective polymer compounds over the years, one must consider the possibility of replacing hard rubber tank linings with newer and more effective soft rubber linings.
We hope that the above listing of the advantages and disadvantages of both hard and soft rubber linings will be useful to those interested in making changes or additions to their current lining choices.