Feature

Next Generation Chemical Tanks

Posted on 27 January, 2010 | Tags: Anticorrosive Linings, Chemical Tank Linings, CPVC, Floropolymers, test

This paper covers the general properties and chemical uses of PVDF with a discussion of processing these special rotomolding grades and show some examples of recent industrial applications for this material

Polyvinylidene Fluoride (PVDF) has been in use for difficult chemical applications since 1964. Traditional applications have been extruded pipe and molded components like valves, fittings and pump parts. Another large market exists where the powder or pellet form of PVDF is solution cast into microporous membranes used in filtration and purification for the water industry.
While PVDF has been offered for rotomolding applications for over 20 years, it was not until recently that an improved material has been offered that has a good combination of surface smoothness and impact resistance. Kynar Flex® 3200 is a specially designed copolymer formulation that has a unique balance of strength, high temperature rating, impact resistance and ease of processing.
Apart from this traditional monolayer tanks made out of PVDF material, a second grade, Kynar® RDG, can be used for the inner lining of plastic and metal tanks for chemical Containment. Multi-layer yanks can also be made using special grades of PolyEthylene (PE). Kynar® RDG has very good direct adhesion with plastics and metals and offers various other advantages that include cost saving, good chemical resistance, high temperature rating and ease of processing.

Handling Difficult Chemical Containments
PVDF resin has a combination of performance properties that make it the material of choice often for handling difficult engineering problems in chemical containment applications.  To handle difficult chemicals, many potential choices of materials are available. The goal should be to consider structural performance, cost, longeivity and offset conditions that could occur and need to be considered in possible operating conditions. Finally special conditions that need to be considered include FDA listing, flame and smoke ratings (UL Ratings), abrasion or impact abuse at various temperatures, thermal cycling and sunlight exposure.

After all the factors mentioned above are considered, there are common applications where PVDF is preferred chemical material for

contact, such as

•  Halogen Containment - Chlorine and Chlorinated Compounds, Bromine and Brominated Compounds and Iodine solutions.
• Strong Acids - Bromic, Boric, Chromic, Citric, Hydrochloric, Hydrofluoric, Methane
• Sulfonic, Nitric, Phosphoric, Salicyclic, Sulfamic, Sulfuric, Tannic acids and others
• Deionized water (DI)
• Fuel Mixtures - Biodiesel, Aromatic Hydrocarbons, Aliphatic Hydrocarbons and Alcohols
• Waste Mixtures - Acids with varying concentrations, Chlorine and Bromine Compounds, Bases, Oxidants, Oils, Biowaste, Salts and Alcohols
• Radioactive Waste
• High Temperatures - short-term 150C and continuous to 140C (length of service depends on contact time and chemical mixtures involved). This would include team-cleaning cycles.

PVDF can give long life service of greater than 25 years for the applications that could cause even the highest quality metals to corrode or high performance polymers to soften or stress crack quickly.  Kynar PVDF complete fluid handling systems consisting of extruded and molded pipe and pumps, valves, nozzles and tanks are readily available from many suppliers. With further advancements in PVDF resin and processing technologies, many of the standard components can be made available using rotomolding as opposed to traditional fabrication methods.

Tracing the History of PVDF Rotomolding
Prior to the development of Kynar® RG 3200,manufacturing process of rotomolding resins involved taking standard grades of PVDF resins, reducing molecular weight grinding them to powder and offering them for rotomolding where niche manufacturers were able to find suitable use for items like holding tanks and plastic lined steel fittings.
These materials exhibited good performance for the users that could make them work, but manufacturers outside of these niche applications often complained about poor surface finish, lack of impact resistance for the final parts if the process was not done under strict processing conditions, and low yields in manufacturing as a result of rejected parts due to inconsistent flow pattern in the molds. Cost was another factor versus other options.
Due to the cost of making a mold for an item that may only need 1 - 10 identical pieces, many manufacturers choose to make PVDF tanks and PVDF lined pipes by welding together existing extruded and machined stock shapes such as rods, sheets, and pipes, or transfer molding right into the metal casing.

Looking into the Future of PVDF Rotomolding
Up to 2009, even with the introduction of Kynar® RG 3200, when designers talked about PVDF rotomolded chemical handling items they most often spoke of 100 percent PVDF material mass in the production piece. To get into larger markets where a supplier would want to supply tens and hundreds of tanks while at the same time bringing the cost down would seem to be a major benefit.
In this case, users who would like the extra safety of having a PVDF chemical contact component rather than a polyolefin component, but do not want to pay 10 times greater value for the product, can use another technology that allows the combination of these two rotomolding materials.
Now this two layer technology is available where KYNAR RDG rotomolding grade can be comined in the production sequence with a PE RDG rotomolding material and the resultant adhesion of the PVDF layer and PE layer is greater than 10 N/cm.  
This concept offers superior chemical resistance, internal flame retardancy and higher contact thermal resistance of Kynar® PVDF with the structural strength of the outer PE shell. Additionally such a combination can add impact resistance to the item by allowing it to be thicker, while reducing cost, and potentially the overall weight of the structure. PVDF is a very high performance material. Case applications of underground composite fuel pipes, fabric-backed tanks and trailer linings, and liquid or powder coated metal parts have shown that a PVDF lining of as low as 0.5 mm - 1.0 mm can provide long term chemical permeation resistance even in very harsh chemical containment service.
Keeping this point under consideration, the concept of dual layers of PE and KYNAR  PVDF is taking shape since KYNAR and PVDF do not bond substantially. A thin layer of Kynar® 3200 RG can be rotomolded along with a second layer of polyolefin to have the combination of a chemically and thermally resistant PVDF contact area with the structural support of a less expensive material. If there is no vacuum concern or if disbondment would not create a failure in a holding vessel, the idea of a two- layer, non-bonding rotomolding 'tank within a tank' structure is a potential manufacturing cost savings technique.

PVDF Multi-Layer Tanks and Metal Tank Linings
Kynar® RDG is a specially designed product and can be used for metal tank inner surface lining. It can be incorporated with special grades of PE to produce multi-layer tanks. This special grade adheres very well to stainless steel, cast iron, mild steel and special RDG PE. Depending upon the end use of application one can opt for two or three layered structure in case of multiplayer tanks. However it is important to keep the other factors, pressure, temperature and chemicals under consideration to which tank will be exposed to. Applications, which require high purity chemical containment (eg Reagents for semiconductor fabrication, pharmaceutical intermediates etc) RDG PE - Kynar® RDG -Kynar® 3200 RG structure is recommended. It is necessary to provide minimum thickness of 0.2 mm made out of Kynar® 3200 RG on the inner most surface in this structure. But one can opt for twin layer structure of RDG PE or metal with a KYNAR RDG inner layer for applications, which do not require high purity. Using these types of multi- layer structures, one can get benefits like cost saving, lightweight structures, ease of production and transportation. With metal lining one can get value addition to present metal tanks in terms of prolonged life of the structure, better chemical resistance, improved corrosion resistance and cost effectiveness.

Processing Information
PVDF is considered in the family of fluoropolymers including PFA, PTFE, ECTFE, ETFE, and FEP. What sets PVDF apart from the other fluoropolymers is the fact that it can be designed to process at much lower temperatures than other fluoropolymers in this same family. Thus machines and molds designed to process Nylons and polyolefins can be used to process Kynar® 3200 RG without thermal modification.

For monolayer processing, many component manufacturers will readily see that common equipment is capable of utilizing Kynar® PVDF rotomolding grades
Oven Temperature: 285 - 325 degrees C
Peak Internal Air Temperature (PIAT): 225 - 250 degrees C
Processing time: Typically 20 minutes, but depends on part thickness and design
Shrinkage: 1.8 - 3.0 percent depending on size, thickness, and processing parameters
Mold Release: can be recommended for processing a Kynar® PVDF / PE two layer construction, be mindful that the optimum molding condition of the Kynar® PVDF and PE will be somewhat different.
An example of conditions to be used for this technique would be
Polyethylene Cycle
Oven Temperature: 300 - 315 degrees Celsius
Part Temperature: 183 degrees Celsius
Approximate time: 12 minutes - pull out mold from the oven and add Kynar® 3200 RG.
Kynar® RG 3200 Cycle
Oven Temperature: 315 degrees Celsius
Part Temperature: 232 Celsius
Approximate time: 20 minutes
Cooling cycle: Pull combined structure mold out and air-cool for at least 20 minutes.

Conclusion
Use of PVDF in the chemical processing industry has continued to grow as designers search for the products that can handle combinations of harsh chemicals and elevated temperatures in their plants and facilities. Kynar® 3200 RG, a new version of PVDF has been designed specially for rotomolding applications where a 100 percent structure of PVDF is desired. This polymer processes at very similar conditions to polyolefin and can be run on the same equipment with little or no modification. As compared to the earlier PVDF rotomolding grades, the result is smoother surface, better impact resistance and wider processing window. Innovative design professionals have further revealed that significant cost reductions can be achieved by using PVDF on the inside of a rotomolded tank and polyolefin as the structural support material. Kynar® RDG and PE RDG have been developed for applications that require strong bond, which helps marry a high performance fluoroploymer to a lower cost structural polyolefin.

Dr Lonnie Bryant received his Master of Science in Chemistry and his Doctor of Philosophy in Chemistry from the University of Tennessee, Knoxville (USA). Dr. Bryant has been involved in polymer synthesis, analysis, and characterization since the mid-1980. He has been responsible for business development of fluoropolymers throughout Asia for past six years.