The acronyms may be short in the chemical processing but they pose big sealing challenges. CIP (cleaning-in-place), uses chemicals lethal to the majority of elastomer seals, while SIP, (sterilisation-in-place) employs superheated steam up to 140°C with an even greater deteriorating affect. In addition to this, for some applications, the seals must meet the strict requirements for FDA, Food and Drugs Administration, approval.

The trend within the chemical industry is for increasingly aggressive chemicals and processes. To extend seal life and reduce downtime, they tested existing materials and developed new compounds to cost-effectively meet performance requirements and approvals. The main conclusion of Trelleborg Sealing Solutions research, the first of its kind within the industry, was that by matching the right sealing compound to a specific application, this could be achieved.

Specifying the right material when CIP and SIP is used is not a simple matter. In fact, test results proved that some elastomer types recommended in DIN 11864 and DIN 11851, applying to fittings for connections in process equipment usually subject to CIP and SIP, were unsuitable or not wholly suitable for sealing with these cleaning regimes. And FFKM, perfluoroelastomer, not given as a recommended material for sealing within the standards, was universally suitable even at elevated temperatures.

Automated CIP or SIP systems ensure safe and efficient processing, prevent toxic contamination of products and minimise re-contamination of the process. To achieve cleaning efficiency and effectiveness, a complex mixture of chemicals is required.

CIP cleaning media includes the following:

• Alkaline CIP mixtures based on sodium hydroxide and surfactants used to remove organic and fatty contamination, emulsions and gels.
• Acid CIP mixtures based on nitric acid and phosphoric acid or sulphuric acid used to remove inorganic sediments in tanks, tubes, fermentation or similar equipment.
• Alkaline CIP disinfecting mixtures based on sodium hypochlorite and active chlorine.
• Acid CIP disinfecting mixture based on per-acetic acid and hydrogen peroxide used on surfaces based on activated oxygen.
• Intensive disinfecting media based on hydrogen peroxide and surfactants added to alkaline CIP media to remove those residues requiring an oxidation stage.

These chemical media do not affect processing equipment manufactured from stainless steel but severe damage may be caused to elastomeric seals, especially in applications where thermal load and pressure are involved.

Seal materials must be chemically resistant to the product processed, CIP media used and also the harsh SIP sterilisation process using superheated steam up to 140°C. They need to withstand abrasion by any solids contained in the product and have a low level of extractables. Seal surfaces must be easy to clean and sterilise and if required, the material should have FDA approval.

Standards do not really help in deciding which materials to specify for seals. They only offer general information and refer just to groups of elastomeric materials and their compliance with FDA standard 21CFR177.2600. No specific characteristics are defined, such as vulcanising system or shore hardness, nor are indications given regarding chemical resistance to common CIP media or performance in steam sterilisation at temperatures up to 140°C.

The standards DIN11864 and DIN11851, covering fittings for the food, chemical and pharmaceutical industry on aseptic and stainless steel connections, recommend Ethylene Propylene Diene Rubber (EPDM), Hydrogenated Acrylonitrile Butadiene Rubber (HNBR), Silicone Rubber (VMQ) and Fluoroelastomer (FKM) as materials for elastomeric seals used in sterilising technology. For some applications, these materials must also comply with the relevant FDA regulation under 21CFR177.2600.

What follows is a summary of Trelleborg Sealing Solutions test results:

• Silicon rubber (VMQ) is not suitable for most CIP and SIP applications including alkaline media or under hot steam sterilisation conditions.
• Generally, HNBR is not suitable in any CIP media.
• The FDA compliant FKM materials showed significant weaknesses in alkaline-based media as well as acidic media, including the per-acetic acid and hydrogen peroxide mix due to their polar nature.
• EPDM was established as an excellent sealing material for aseptic technology but some formulations are less suited to ‘intensive’ CIP additives.
• FFKM, Perfluoroelastomer showed most resistance in CIP media, especially at elevated temperatures. It is universally suited for CIP applications and SIP sterilisation processes.

Performance in SIP processes

After CIP cleaning, many process plants employ a sterilisation stage with steam, typically in the range 125°C up to 140°C and for this reason, the steam resistance of the elastomers used must always be considered. The conclusions on performance of elastomer materials in steam were:

• At 125°C up to 140°C FKM and HNBR are already beyond their performance limits.
• In the presence of atmospheric oxygen and steam, EPDM is resistant up to 150°C whereas without oxygen, temperatures of up to 200°C are achievable.
• FFKM, Perfluoroelastomer offers the best temperature resistance with some grades capable of operating in temperatures up to 230°C.
• Steam temperatures are increasing up to and beyond 150°C. Under these operating conditions, EPDM may reach its natural limit and in these cases, perfluoroelastomer is the best material.

Selecting for chemical compatibility

Trelleborg Sealing Solutions publishes a chemical compatibility chart to help equipment manufacturers and processors select the right seal for an application. The following seal materials were tested by Trelleborg Sealing Solutions:

• Two types of peroxide cross linked Ethylene Propylene Diene Rubber (EPDM).
• Two types of Fluoroelastomer (FKM), cross-linked with bisphenol.
• A Hydrogenated Acrylonitrile Butadiene Rubber (HNBR), cross-linked with peroxide.
• A FFKM Isolast perfluoroelastomer, which conforms to FDA Guidelines 21CFR177.2400(d) and 21CFR177.2600(e,f).

Silicone rubber (VMQ) was not tested as initial experiments showed it not to be sufficiently resistant in alkaline media and under conditions of live steam sterilisation.

The material test specimens were immersed in the following CIP media concentrations, at temperatures given for one week in accordance to DIN53521:

• Alkaline CIP formulations based on caustic soda liquor and surfactants; concentration of 4percent, temperature 80°C.
• Acidic CIP formulations based on nitric acid or sulphuric acid; concentration of 2percent, temperature 80°C.
• Disinfectant alkaline CIP formulations based on sodium hypochlorite and active chlorine; concentration of 3percent, temperature 80°C.
• Disinfectant acidic CIP formulation based on peracetic acid and hydrogen peroxide; concentration of 2percent, temperature 80°C.
• Disinfectant spray injection media based on hydrogen peroxide with a concentration of 1percent, temperature 80°C.

After the test period the hardness (shore A) and per centage weight change were measured along with the most important parameter, volume change.

Recommendations for use dependent on volume change: less than 5percent – material suitable for sealing; more than 5percent but 10percent or less – material can only be used for sealing with reservations; more than 10percent – material not suitable for sealing.

Alkaline CIP formulations based on caustic soda liquor and surfactants; concentration of 4percent, temperature 80°C: EPDM1 Not recommended 8percent shrinkage due to surfactants, which extracted constituents from the seal. EPDM2 Strongly recommended Volume change minimal. FKM1&2 Unsuitable Destroyed by sodium hydroxide. HNBR With reserves 5percent volume change but other results in alkali show limited performance. FFKM Strongly recommended Best sealing material in this media on all three parameters – practically unchanged.

Acidic CIP formulations based on nitric acid or sulphuric acid; concentration of 2percent, temperature 80°C: EPDM1&2 Strongly recommended Practically no volume change. FKM1&2 Unsuitable Extreme volume increase. HNBR Unsuitable Swelled by almost 100percent. FFKM Strongly recommended Practically no volume change.

Disinfectant alkaline CIP formulations based on sodium hypochlorite and active chlorine; concentration of 3percent, temperature 80°C: EPDM1&2 Strongly recommended Minimal volume change. FKM1 With reserves Volume change of 6percent. FKM2 Strongly recommended Minimal volume change.

HNBR Unsuitable Volume change significantly higher than other materials. FFKM Strongly recommended Minimal volume change.

Disinfectant acidic CIP formulation based on peracetic acid and hydrogen peroxide; concentration of 2percent, temperature 80°C: EPDM1 Unsuitable 8 per cent volume increase. EPDM2 Strongly recommended Fully inert, the most cost-effective material for this media. FKM1&2 Unsuitable Volume change exceeded 65percent probably due to absorption of acetic acid. HNBR Unsuitable Significant volume change. FFKM Strongly recommended Minimal volume change.

Disinfectant spray injection media based on hydrogen peroxide with a concentration of 1percent, temperature 80°C. EPDM1&2 Unsuitable Volume increase of 20percent and surfaces became sooty. FKM1&2 Unsuitable Volume change of between 15 and 22percent. HNBR Unsuitable High volume increase. FFKM Strongly recommended Fully inert, the only sealing material suitable for this media.

Dr Burkhard Ledig is CPI Global Technical Manager at Trelleborg Sealing Solutions. www.trelleborg.com