Fluoropolymers are ideal for pharmaceutical and biopharmaceutical processing equipment because they have excellent chemical and thermal resistance. Their molecules have continuous non-reactive surfaces and are compatible with virtually all chemicals and solvents. They are far more resistant to chemical attack than conventional chlorinated and hydrocarbon polymers, and have far higher service temperatures.
There are a number of materials in the Fluoropolymer family. PTFE (Polytetrafluoroethylene) is the original Fluoropolymer, discovered by DuPont in 1938. From this original development there have been a number of other materials developed – trying to improve on the difficult processing characteristics of PTFE. These have been more or les successful, having to trade properties of PTFE in order to attain other characteristics. The principal materials used in the industries that we serve are:
PTFE is not melt-processable and therefore usually needs to be formed into the required shape prior to sintering.
PTFE comprises both carbon and fluorine atoms, as a straight chain molecule, the carbon backbone being protected by a helix of the fluorine atoms wrapped around it. This carbon fluorine bond is one of the strongest chemical bonds and gives the material its properties of corrosion resistance and non-stick.
Key Properties of PTFE
PTFE resists the most aggressive organic and inorganic chemicals and solvents over a broad temperature range. This includes:
- Strong mineral acids
- Inorganic bases
- Inorganic oxidising agents
- Salt solutions
- Organic Acids
The ability to resist an electrical charge is measured by surface and volume resistivity. In the case of PTFE these figures are colossal. Depending upon how the test work is performed, values in excess of 1017Ω cm are considered minimum. In the case of equipment lined for the chemical and pharmaceutical industry this property can be a nuisance as PTFE lined equipment may build up electrostatic charges and be unable to dissipate them. This is considered elsewhere in relation to static-dissipating materials.
PTFE has a very low coefficient of friction and for all calculation purposes it can be considered hydraulically smooth,
PTFE retains useful properties (i.e. not more than 15% loss of chemical resistance) at up to 200oC and sometimes beyond depending upon the application. In fact it has the highest retention of its chemical properties of any known plastic like material. (Please be wary of materials performance data stating that PTFE has an upper service limit of 260 oC. This is true in a laboratory situation, but in service most material properties such as mechanical strength will have been lost preventing it from performing a useful duty.)
PTFE Paste Extrusion
CRP uses the most expensive grades of PTFE. These are coagulated dispersions, often referred to as fine powders. These have a very close process control of grain size range and are uniquely capable of being sheared by lubricated paste extrusion into a coherent fibrous matrix with useful structural integrity. Pipe liners for the chemical and pharmaceutical industry are almost universally manufactured from PTFE. The paste extrusion process provides the highest level of surface finish, the highest resistance to permeation and the closest dimensional control of all of the methods of lining in PTFE. This serves to differentiate what we manufacture from processing technologies using lower quality and priced materials – being ram extrusion, mandrel wrapping and isostatic moulding.
Paste extrusion however is only suitable for making straight lengths of PTFE tubing colloquially known as pipe liners in our industry. It can be subsequently manipulated into bends and other simple forms, but is not capable of lining complex shapes without joints.
PFA (Perfluoroalkoxy) was developed in order to achieve a true melt-processable fluoropolymer. Its characteristics are such that in service it can be considered as interchangeable with PTFE in terms of its chemical service and temperature and pressure duty. It has the highest permeation performance of the fluoropolymers, exceeding that even of paste extruded PTFE. It also provides the smoothest and least wettable finish of all of the Fluoropolymers. Unfortunately the trade-off is cost, the material being more expensive than PTFE. CRP uses PFA in the production of most of its lined fittings and the Flowserve range of Atomac and Durco valves are available lined in PFA.
FEP (Fluorinated Ethylene Propylene) is another melt-processable Fluoropolymer. It does not have the almost universal chemical resistance of PTFE and PFA and its maximum operating temperature in service is 150oC. CRP uses the material occasionally for the manufacture of “sheet lined products”. This is essentially a lining made up of sheet and tube elements of FEP welded together in situ. Sheet lining allows one to line complex articles without the use of expensive moulding tools, to cope with the inevitable tolerance errors of welded fabricated steelwork and with the use of bonded linings to provide a degree of vacuum performance. Where the chemical performance and temperature duty are not an issue, the material provides a less expensive alternative to PFA. Flowserve manufacture the Atomac range of valves lined in FEP.
PVDF (Polyvinylidene difluoride) is an engineering fluoropolymer and has a more limited range of chemicals performance and an upper temperature limit of 120 oC. Although replaced in many applications by the improved performance of PTFE resins, it remains the material of choice for low temperature halogen applications – Bromine and Chlorine.
Chemraz® is a perfluoroelastomer and is a proprietary compound manufactured by Greene Tweed. It is used specifically in the manufacture of seals, combining the resilience and sealing force of an elastomer with chemical resistance approaching that of PTFE. CRP uses Chemraz 505 seals in the manufacture of sampling systems.
Glass / Alloys
Borosilicate glass has a very high resistance to attack from water, acids, salt solutions, halogens and organic solvents. It also has a moderate resistance to alkaline solutions. The chemical resistance of DURAN® exceeds that of most metals and other materials even where long exposure times and temperatures in excess of 100°C are involved. Exposure to water and acids only results in the leaching out of very small amounts of mainly univalent ions from the glass. The resultant very thin layer of silica with few pores in it that is formed on the surface inhibits further attack. DURAN® has very high resistance to attack by water, neutral and acid salt solutions, strong acids and mixtures thereof, and also chlorine, bromine, iodine and organic substances.
Hastelloy C276 alloy has excellent corrosion and erosion resistance to an extremely wide range of chemical process environments, including strong oxidizers such as ferric and cupric chlorides, hot contaminated media (organic and inorganic), chlorine, formic and acetic acids, acetic anhydride, and seawater and brine solutions. C276 alloy has excellent resistance to pitting and to stress-corrosion cracking. Hastelloy C276 alloy is a nickel-molybdenum-chromium wrought alloy that is a very versatile corrosion resistant alloy because of this versatility it is ideally used in multipurpose plants.
It is worth stating that CRP only use fluoropolymers from well respected global producers. This provides confidence in the raw materials in terms of their standards, both quality and consistency of materials and product traceability.
Pipe Spools and Fabricated Fittings
Pipe spools are manufactured using carbon steel rated for pressure containing purposes. These are either provided with integral stub ends behind which we mount a flange – referred to as Van Stone or Conrac spools, or a welded flange or stub end combined with a loose or rotating flange. Fabricated fittings are made from a combination of carbon steel pipe, forgings, flanges and stub ends. All welding activities are undertaken using “coded” welders.
Where CRP differs significantly from the competition is that the majority of pipe fittings are manufactured as investment cast carbon steel components. This has significant advantages over the use of fabrications or iron castings. The dimensional control of the component is extremely accurate. This in turn allows for accurate assembly on site and from a manufacturing perspective ensures a more consistent lining thickness improving product integrity. The lack of welding is an advantage – eliminating a potential source of weakness and the casting is an extremely strong construction in its own right. Not only are those essential elements of the casting machined, but attention to detail improves the product – the rear face of flanges is machined improving bolting and all holes and edges are chamfered for ease of product handling. An additional advantage of castings are that the manufacturers details, product identification and foundry markings are clearly shown.