Sealing Solutions for Process Industries: Small Parts with Big Responsibilities

Seals for process equipment and production lines in the chemical, food and beverage, pharmaceutical, and related industries typically operate in harsh environments characterised by the presence of aggressive media, high and/or partially cyclical application temperatures, and/or high pressures.

The overall similarities of operating conditions in processing industries are contrasted by a wide variety of specific demands to be met by all components including the related sealing solutions.

In addition, due to the generally critical nature of the products of process industries, particularly those that are either intended for human consumption or that may otherwise come into contact with humans, the seals used in those applications must satisfy special product safety demands including exacting hygienic process standards and harmlessness of the sealing compounds used in them (i.e., no migration of harmful compound ingredients into the process or product, etc.).

Finally, when it comes to processing or producing chemicals, “technical safety” is another key criterion to prevent any accidental release of harmful substances into the environment. In a nutshell, this brief introduction shows that seals for process industries must meet a wide variety of basic prerequisites as discussed in this article.

Legal and Regulatory Requirements for Sealing Solutions

The utilisation of elastomers in the food processing industry is subject to compliance with specific requirements. In addition to the selection of raw materials from standard white lists issued by national authorities, it must be ensured that no substances are extractable from the material.

In the case of seals used in beverage production equipment, for instance, it is of paramount importance that no chemicals migrate from the seal compound and, in the worst case, contaminate the product.

Compound Purity and Stability

Many substances used in the food and chemical process industries are identical, whether they’re of natural origin or synthetically produced. Regardless of the type and occurrence of those substances – be it in process media, in raw materials for products, or in finished products – the materials for seals and engineered components used in production equipment coming into contact with diverse chemical substances must meet specific purity requirements.

In addition, they must be resistant against chemicals under the given process conditions. Purity and stability are therefore basic prerequisites for materials to be suitable and approved (see details below) for use in the chemical process and the food industry.

Consumer Health and Safety

To ensure that products intended for human consumption or other contact with humans are as safe as possible, the production processes and equipment, including the sealing compounds used, must meet a wide range of national, European, and global regulations and standards.

They concern ingredients and additives, residues, contaminations, and permissible migration levels (migration and leaching) in foodstuffs, pharmaceuticals, and chemical products. The following is a list of key requirements to be met by sealing compounds used in process industries, particularly in food, beverage, and pharmaceutical settings.

FDA

FDA conformity is an important requirement for materials used in the food and pharmaceutical sectors. The U.S. Food and Drug Administration (FDA) has issued a substance white list for elastomers in Chapter 21 (CFR 177.2600 – Rubber Articles Intended for Repeated Use).

Only the polymers, fillers, crosslinking chemicals, etc. on this list may be used in the compound formulation. In addition, the FDA requires an extraction test in water and n-hexane. Global migration must not exceed a specified level.

If both criteria, i.e. white list conformant raw materials and extraction, are met, the material is FDA-conformant. It should be noted that the FDA does not issue any approvals for elastomers (as is commonly the case for plastics) so that elastomers can only be FDA-conformant, but not FDA-approved.

USP

Other requirements to be met are the current quality standards of the U.S. Pharmacopeial Convention (USP) for the use of elastomer components in pharmaceutical and biotechnology equipment. In addition, the classification of elastomer components for any type of hygienically designed equipment according to USP Class VI is highly useful.

Suitable elastomers are classified according to USP Class VI, Chapter 87 in (vitro) and Chapter 88 (in vivo), ensuring that there are no concerns regarding their use.

3-A

3-A Sanitary Standards Inc., a hygiene organisation of the U.S. food industry, defines specifications and recommendations for the development, installation, and utilisation of hygienic equipment for dairy products.

They prescribe tests with limits for volumetric and weight changes for elastomer seals. The tests include air ageing, storage in grease (butter oil or milk fat), in distilled water, 0.5 % nitric acid, 1 % sodium hydroxide solution and in a hypochlorite solution. Due to their sensitivity to fat, EPDM compounds are classified in Class II.

National and EU Regulations for Sealing Solutions

In addition, there are many other national and EU regulations. The German Federal Institute for Risk Assessment (BfR), for instance, like the U.S. FDA, has issued recommendations for elastomers (XXI. Commodities Based on Natural and Synthetic Rubber). However, it should be noted that the two lists do not match so that the formulation of a compound taking both into account poses a great challenge.

Free of Polycyclic Aromatic Hydrocarbons

Polycyclic Aromatic Hydrocarbons (PAHs) are risk-prone chemicals in many respects. Many PAHs have been rated by the European Union as carcinogenic, mutagenic, or toxic for reproduction. A prominent example is benzo[a]pyrene.

This substance is largely responsible for the carcinogenic effects of cigarette smoke. Additionally, PAHs can be toxic for humans or other organisms. If PAHs are released into the environment, they will accumulate in plants, soil, and the air for an indefinite period, resisting degradation. Due to the combination of toxicity and persistence, PAHs are substances which are harmful to the environment.

Therefore, limiting the emission of PAHs and minimising the use of PAH-containing chemicals is highly important and the utilisation of sealing materials with low or no PAH content, particularly in the food industry, is a crucial step in the right direction.

The PAH content of seals can be determined by means of analytical chemistry. Due to the large number and diversity of PAHs, the concentration of 18 chemical compounds is determined as a representative sampling.

The measurement method of choice is GC-MS and the test procedure may be performed according to AfPS GS 2014:01 PAK / QMA 1284. When the sum of all 18 PAHs is below 0.2 ppm (i.e. less than 0.2 mg/kg) the seals are PAH-free.

Concentrations of less than 10 ppm (10 mg/kg) are considered PAH-low. Additionally, since December 2015, a new EU Regulation (REACH VO (EC) 1907 Annex XVII, VO 1272/2013) has been in effect which provides for a limit of 1 mg/kg for 8 defined PAHs.

So far, the restrictions of the regulation apply only to articles that come into direct as well as prolonged or short-term repetitive contact with the human skin or the oral cavity under normal or reasonably foreseeable conditions of use.

In the future, further laws and regulations at country and EU levels can be expected to regulate the commercialisation of PAH-containing products. The aim, therefore, is to offer the process industries PAH-free compounds for sealing solutions even before new restrictions come into effect.

Wide Range of Specialty Seal Compounds and Designs

The wide variety of requirements found in the process industries can only be covered by an equally wide range of compounds specifically developed for or modified to meet the relevant requirements including the requisite approvals/conformities as discussed above.

The main polymer material families used for process industry applications are EPDM (ethylene propylene diene monomer rubber), NBR (nitrile butadiene rubber), FKM (fluorcarbon rubber), FFKM (perfluoro elastomers), PUR (polyurethane), and PTFE (polytetrafluoroethylene). For particularly harsh application conditions outside the application range of polymer materials, metal seals may provide viable alternatives.

Functional elements used in the food industry such as seals, wipers, and molded parts not only have to be resistant against the foods passing through a filling line, but must also exhibit extreme wear resistance, as most applications run with no additional lubrication.

Particularly suitable for those purposes are PTFE seals and moulded parts featuring special designs and materials specifically modified for applications in no-lube environments in the food industry. This ensures low wear and friction as well as long service life even in harsh conditions, which may, for instance, occur in shock-freezing or in industrial lines for baking products.

As far as seal designs for use in process industry equipment are concerned, they range from conventional, straight-forward O-rings to highly complex, sophisticated sealing solutions. Particularly relevant are seal designs specifically tailored to the exacting hygienic requirements in food and beverage production and packaging.

Due to their material properties, specialty PTFE seals without undercuts are not only resistant against all commonly used cleaning processes such as CIP (Clean in Place) or SIP (Sterilisation in Place), or non-standard cleaning agents like concentrated acids and alkaline solutions, but, due to their special design, enable easy cleaning in the area of the seal or wiper without the risk of contamination of the food products subsequently passing through the filling line.

Conclusion

The challenging demands made on seals and engineered components in process industries exhibit some similarities as well as differences. However, regardless of whether the processes relate to chemical, food and beverage, or pharmaceutical products, the purity and stability of the seal compounds used in processing equipment are of paramount importance to ensuring consumer health and safety.

Working with an experienced seal manufacturer drawing on extensive knowledge of the critical requirements in the various process industries and in-house compound development specifically tailored to those demands provides the best prerequisites for companies in that sector to ensure the legal and regulatory compliance and therefore the safety of their products.