Recycled Plastic Compounds
Sieve testing is a means to determine the particle size distribution in powdered or granular materials. The method uses standard sieves (screens) which are arrayed in a stack of progressively finer screens, with a pan at the bottom. The material is placed into the top sieve and shaken in a specific way, and then the amount of material resting on each screen is measured, thereby determining the particle size distribution. The amount passing the finest screen is shown as "pan". The test method is described in ASTM Standard D-1921.
Mesh refers to the number of openings per linear inch in a screen. Generally, "35 mesh powder" would all pass through a 35 mesh screen. However, a more exacting specification that covers the distribution of the various particle sizes is normal in many applications, such as in rotational molding. This table provides the screen opening of each mesh in inches and microns.
Melt index refers to a specific test that measures the rate of extrusion of a thermoplastic, in grams per 10 minutes, through a specified orifice under specific temperature and pressure conditions. In layman's terms, the test measures the viscosity of the melted plastic, which is directly related to its molecular weight. The test is performed on a specialized apparatus called an extrusion plastometer or less formally, a melt indexer. The test is described in ASTM Standard D-1238.
Density, as the term is used with polyethylene (PE), actually refers to specific gravity. Specific gravity is the ratio of the weight of a given volume of material to the weight of an equal volume of water. Thus, PE with a density of 0.935 weighs 93.5% as much as an equal volume of water; i.e. it floats. Density in PE tells a great deal about the physical properties that the material exhibits. A fairly accurate measure is easy to perform using alcohol water solutions (for PE and PP) and a hydrometer. More accurate testing is more involved. There are several methods described in ASTM Standards D-2839, D-792, and D-1505.
Bulk density is the weight of a given volume of material, including the void spaces inherent in it. Scientists call this "apparent density." This is an important quality in bulk material handling applications, such as pneumatic conveying. It is also very important in rotational molding powders, since bulk density directly indicates the fluffiness of a powder. When pulverized, the particles of PE tend to form hairs. Under a microscope, they can look more like dust-bunnies than like grains of sand. This will effect how they mold in rotational molding, since rotational molding is (normally) done at ambient pressure, and the flow of the particles inside the mold is critical. ASTM Standard D-1895 describes the test method.
Pourability describes the same fuzziness property of plastic powder that bulk density does, determined by a different method. It is commonly used with rotational molding powders. A given quantity is allowed to pour from the bottom of a standard funnel and the result reported in seconds to empty the funnel. Generally, a faster time means greater bulk density.
Some other sources of plastics reference information are:
American Society for Testing and Materials (ASTM), 100 Barr Harbor Drive, West Conshohocken, PA 19248 610-832-9500. Promulgate (now there's a $5 word if there ever was one) worldwide materials testing standards.
Society of Plastics Engineers (SPE), 14 Fairfield Drive, PO Box 403, Brookfield, CT 06804-0403, 203-775-0471. A wealth of plastics information.
Association of Rotational Molders, 2000 Spring Road, Suite 511, Oak Brook, IL 60523, 603-571-0611. Rotomolding trade organization.