Although I didn't deal with polymerizations, I think this is a word borrowed from statistics. Molar mass distributions of polymers are determined thermodynamically or kinetically by the synthesis conditions, and to a lesser extent, by degradations during the processing of plastics.
There are various: Schulz-Zimm, Kubin, Tung, mostly polydisperse, narrow monomodal or monodisperse Poisson, etc. distributions, discrete or continuous.
A bimodal distribution is sometimes obtained by the use of two different catalysts at the same time, resulting, as claimed by various patents, in enhanced properties of the polymer.
I would presume that 25362's interpretation is correct. Polymer science loves statistical distributions because they are required to describe polymer molecular weight. Here, I would take bimodal to mean two distinct molecular weights that each have a poisson-like distribution centered upon them.
Though this is the first time I've heard the term "bimodal resin".
Hi, 26362 is correct, a number of plastics (primarily, polyethylenes) are produced with a bimodal blend of different MW polymers for better properties. As can be seen at the 1st site below, the MW distribution doesn’t consist of 2 narrow peaks, but rather, one broad one with humps near each end. Here are some representative websites:
US Patent 5882750: Single reactor bimodal HMW-HDPE film resin with improved bubble stability
“Single reactor bimodal MWD high molecular weight polyethylene film resin has improved bubble stability. The bimodal molecular weight distribution resin has low resin elasticity. Preferably the resin is made catalytically in one reactor. The preferred catalyst appears to control the properties of bubble stability and elasticity.”
wherein the resin has a density of 0.89 to 0.97; has a Mw/Mn of 2.5 to 60, and has FI of 1 to 100, wherein said resin comprises two components of different molecular weight, wherein one of said two components is a high molecular weight component (HMW) and wherein another of said two components is a low molecular weight component (LMW) with a molecular weight less than that of HMW;
wherein the LMW component is produced by a metallocene transition metal catalyst component and has a narrower molecular weight distribution than the HMW component which is produced by a nonmetallocene transition metal catalyst component;
wherein the resin is formed in situ in a single reactor by a catalyst comprising a source of a metallocene transition metal…”
from
HDPE for Pressure PipeGains Wide Approvals - 06/02
A new HMW-HDPE copolymer with bimodal MW distribution has been approved for a wide range of high-pressure pipe applications by the Plastics Pipe Institute (PPI). Alathon XL5008 from Equistar Chemicals, LP, Houston, exceeds PPI’s PE3408 (E8) rating, which specifies 1600-psi Hydrostatic Design Basis (HDB) at 23 C. It also meets the PE3408 (E2) rating—1000-psi HDB at 60 C. As a result, it can now be used in pressure pipe for oil and gas production, industrial and mining pipe, and potable-water systems.
…”
“Polymer Technologies
1. HDPE (CX Process)
Bimodal slurry phase process to produce high quality HMW-HDPE for Ultra-thin film, Small & Large blow molding and Gas/Water pipes
2. m-LLDPE (Evolue Process)
Bimodal gas phase process utilizing Mitsui's proprietary Metallocene Catalyst to produce LLDPE having easy processing and excellent mechanical and optical properties “
