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All About Cotton and Rayon String Wound Filter Media

Why Cellulose?

Cellulose – plant fiber  – is a popular filter material because it is wonderfully compatible with a wide range organic solvents and high temperatures, because it is available in relatively thin fibers that enable fine levels of filtration, and because filter media manufacturers know how to process it into great filter media.  Polypropylene, another popular material, does not fare well in many organic solvents ( for example, halogenated hydrocarbons) – especially at elevated temperatures –  and cellulose fiber based filter media are often excellent for those.  The affordability of cellulose, plus its ability to be incinerated, also helps differentiate cellulose-based materials from alternatives such as fluoropolymers (such as PTFE) and glass.  Another plus is that cellulose is “renewable”. Some negatives for cellulose are incompatibility with some chemicals such as many types of acids, and the fact that despite its excellent compatibility with water, it is subject to attack by some waterborne micro-organisms who use it as a food source – and this can be problematic in long term water use.

The Appeal of Cotton

One common form of cellulose in wound filters is cotton, and another form is rayon.  Cotton and rayon have basically the same chemical compatibility range because they are both “cellulose”. Some environmentalists tout cotton as being more ecofriendly than tree-derived cellulosic fibers. Because cotton is a natural fiber, its fiber size can vary a bit. Also, its price can vary based on crop yields and other market factors. In the USA, cotton prices are rather low compared to many other regions, so cotton is a more popular filter medium than rayon for wound filters in the USA.  We see evidence for the low cost of American cotton in that, Asia, a region otherwise known for low-cost manufacture, is one of Delta Pure Filtration’s important export markets for cotton wound filters.

What About Rayon?

Rayon is a sort of “synthetic cotton” that can be synthesized from plant matter such as bamboo or trees. A type of rayon is “viscose rayon” or simply “viscose.” In the USA, rayon materials are more expensive than cotton, not only because of the processing that goes into rayon, but because rayon materials tend to come from overseas. There are no domestic USA sources of rayon staple fiber, and the USA tariff structure actually penalizes a filter media manufacturer for importing the fibers and making a rayon nonwoven in the USA with American workers – so instead rayon nonwoven is usually imported as finished roll goods. [i] But rayon pricing can be more favorable overseas, and chemical processes established outside the USA may at times be transferred into the USA with rayon filter materials specified for use in the process.  Filter users in the USA wanting to save money, and willing to do the paperwork often associated with change, can often switch from rayon to cotton string wound filters.  At any rate, Delta Pure Filtration can readily provide string wound filters of cotton, rayon, and numerous other materials.

What is DREF Yarn?

Most of our filter yarns are DREF yarns, which for our purposes are yarns that have a core-sheath structure.  When we refer to the core and the sheath of a DREF yarn, we are not talking about the structure of an individual fiber, but rather the structure of the yarn which contains thousands of fibers in any given cross section. As an example, a yarn cross section might have almost 3,000 fibers, with roughly half the fibers in the sheath (outer section) and half in the core (inner section).

Now that you know about DREF yarn and the core sheath structure, we can explore the different types of cotton yarn available from Delta Pure Filtration.

Types of Cotton and Rayon Filter Media

Did you know that many cotton filters often have polyester in them? Think about the filter selection process – imagine that you have a “difficult fluid” – for example, a halogenated hydrocarbon – that needs filtering, and you consult a compatibility table and find that your first choice of polypropylene is not good. Then, you continue scanning the table and see a lot of materials that have poor compatibility, but you finally find that cotton is rated as “excellent”, and so you select (for your initial testing / qualification trials) a cotton filter with stainless steel hardware.  We worked with a customer who went through a similar process, and they were testing a competitor’s cotton filter and couldn’t figure out why it was not working when it was supposed to be “excellent”. We conjectured that their cotton might have polyester in it, and so we supplied them a Delta Pure Filtration precision wound filter containing a 100% cotton filter medium for their trials.  The precision wound filter from Delta Pure Filtration worked wonderfully and solved their compatibility problem – and they continue to be a great customer!

Please Note:  coarse “cellulosic core cover” nonwovens can often have a significant percentage of polyester as well!

We have several string wound filter media in the cotton-rayon family – here is a table to help with understanding the different types of materials.

Types of Cotton and Rayon String Wound Materials

Filter Medium CodeSummaryDescription
04Natural Cotton BlendThis is a very economical tan/brown cotton filter medium with a small percentage of polyester. The same blend is used in the core and the sheath.
05White CottonThis filter medium has white bleached cotton on the outside sheath section, and unbleached tan cotton in the inside (core) section. No polyester is added.
06FDA Bleached CottonThis filter medium is of 100% bleached cotton fibers – both in the core and sheath.  The U.S. Pharmacopeial Convention (USP) sets many standards for drugs, foods, supplements, and even for “Purified Cotton”! The Purified Cotton standards provide the guidelines for bleaching what filter manufacturers call “FDA Bleached Cotton”.
09RayonThis is a 100% rayon product, both in the core and the sheath. Some manufacturers call their rayon “FDA rayon.” We also have certification from our supplier that the filter medium is in compliance with standards set forth in sections of the US FDA Code of Federal Regulations (CFR).

Delta Pure Filtration Offers A Variety of Materials

Delta Pure Filtration manufactures filters with a wide variety of materials and formats to assist with your filtration applications – cotton string wound filter elements, rayon string wound filter elements, and a whole lot more. Other examples of materials that we work with are: polypropylene, glass, polyester, nylon, acrylic, PPS (polyphenylene sulfide or RYTON®), and fluropolymers such as PVDF (Kynar®). We also have a filter made from recycled materials that is used for waste water applications.

Contact Delta Pure for assistance in selecting the optimal filter material for your application.

RYTON is a trademark of Chevron Phillips Chemical Company

Kynar is a trademark of Arkema

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[1] Miscellaneous Tariff Bill Update,  Nonwovens Industry, October 2 2014, as viewed on http://www.nonwovens-industry.com/issues/2014-10-01/view_capitol-comments/miscellaneous-tariff-bill-update-565000/