FreeShip- Polypropylene Fiber, Resin Thickener, Thixotropic Agent- (Prompt rebate on orders with 3 or more FreeShip items!)

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Polypropylene fiber has very short, very thin, and very light weight fibers. They vary in length from less than 1/8" (2.5 mm) down to about 1 mm. They impart surprisingly good thixotropic properties to liquids they're added to (usually resins). Polypropylene (PP) fiber is a softer, more flexible, lighter, amorphous, opaque, and "clumpier" fiber than the nylon we have had in the past.

If you look at this PP's fibers under a microscope they are a tangled, varying diameter and length, bent fiber with small fibers branching off the thicker fibers (like "fuzz"), They don't slide past each other at all, they are fixed in place. You can "move" them only by tearing them into smaller clumps (which is easy to do). They vary in thickness by factors of 3 to 4. If you can follow a single fiber, it will have gradual but dramatic changes in diameter. If you want a fibrous material that will thicken and extend your resin this is one of the most effective. It's relatively inexpensive and a little will go a long way.
Polypropylene is related to polyethylene (PE) in that they are both olefins and are the most commonly used olefins. They are sometimes confused with with each other because they look similar, but there are quite a few physical properties. If you compare standard PE and PP (excluding ultra high molecular weight polyethylene (UHMW PE) is in a class of its own). One good way to tell them apart (if you have samples of similar thickness) is to bend them, PE is more flexible. PE is also softer, has a lower melting temperature, and is more translucent. It also has higher chemical resistance.

I was testing polymer fibers that would work well in resins like epoxy, acting as a reinforcing fiber at low loading percentages. I also wanted to be able to increase viscosity and thixotropy by adding more fibers while not turning it into a "stringy" paste as most longer and thicker fibers do when added to resins.
What I found (using epoxy) is that the PP fibers act as a thixotropic agent (when in motion, like pouring or being vibrated, it flows easily; when at rest it does not flow) at much lower loadings (by weight) than rayon without being "stringy". I'm now thinking it's the straightness and length of fibers which contribute to the aggravating condition of having the resin/fiber mix not lay flat but tending to have fiber clumps popping up out of whatever you're laying the mix onto.
You can add much lower percentages of PP fiber than other polymeric fibers like nylon and rayon to get the same viscosity. That's what would be expected given the physical form (tangled) of these fibers.
The percentage of PP added to epoxy was only 0.2% to get a similar viscosity as 5% rayon (by weight to total epoxy). That's a huge difference. Part of it is due to rayon's high density (it weighs more) compared with the low density of PP. I poured it out onto a ziplock bag in a strip of variable thickness. After being fully cured at 100F, it was stronger and more flexible, especially in thinner sections.

Considering the above information, the advantage of polypropylene as a fibrous reinforcement for liquid resins is the low loading necessary for a given viscosity. Mixing is easier and PP fiber reduces the cost of the fiber since PP is several times less expensive than rayon (and nylon, although nylon is less expensive than rayon). Viewed from another angle, a formulation with a high loading of PP fiber used with a low cost resin to make a paste would have even more cost advantages and the paste should have good controllable working properties due to the nature of the fiber, with good final strength.

The Wikipedia article on polypropylene is here, with some excerpts:
{ https://en.wikipedia.org/wiki/Rayon } :
"Polypropylene is the second-most widely produced commodity plastic (after polyethylene) and it is often used in packaging and labeling.....The density of (PP) is between 0.895 and 0.92 g/cm³. Therefore, PP is the commodity plastic with the lowest density. With lower density, moldings parts with lower weight and more parts of a certain mass of plastic can be produced. Unlike polyethylene, crystalline and amorphous regions differ only slightly in their density. However, the density of polyethylene can significantly change with fillers.....Polypropylene is normally tough and flexible, especially when copolymerized with ethylene. This allows polypropylene to be used as an engineering plastic, competing with materials such as acrylonitrile butadiene styrene (ABS). Polypropylene is reasonably economical."

Polypropylene is not considered toxic, hazardous, or flammable. It is combustible (it takes a sustained higher heat source to flame; flammable means it can ignite and burn at lower temperatures which do not have to be sustained).