One way to improve the strength of plastic composite parts is to polmerize the plastic right in your injection or compression mold.
Crazy? Not at all.
The concept of in-situ polymerization of engineering thermoplastics goes back 30 or more years with concepts trialed by GE Plastics (now SABIC Innovative Plastics) and Dow Automotive. The concept never caught on commercially, but growing efforts to light weight cars with composites is triggering new efforts.
Two papers at the Society of Plastics Engineers Automotive Composites Conference & Exhibition outlined a promising new project using PBT-type polyester.
“Most reinforced thermoplastics are produced from fully polymerized resins which are then introduced to the reinforcement in a compounding extruder or a coating extruder line,” said Jim Mihalich, CEO of Cyclics Corp. (Schenectady, NY). “This creates a dilemma in selecting the appropriate molecular weight and viscosity of the matrix resin.”
Use of a low molecular weight matrix resin allows the reinforcing filler to be completely wetted out by the resin. Improved wetting and dispersion is an important benefit. Another is the reduction of thermal degradation when working the reinforcing material into the polymer. That means better color steadfastness of the final part. Reinforcement loadings in continuous fiber parts can be increased; another important advantage that increases mechanical properties.
There are seven steps in the polymerization of polyester oligomers.
1. Drying. Water inhibits the polymerization reaction.
2. Melting. Polyester oligomers are solids at room temperature and they are completely melted at 195C
3. Introduction of catalysts. The research focuses on tin-based chemicals now, but faster options are available.
4. Infusing. Proper development of the molecular weight of the polymer after infusing is a critical step.
In a separate presentation, Victor Bravo, representing the Magna-NRC Composites Centre of Excellence, described results of a test on the process using a mold for a large automotive storage bucket. Results were said to show the feasibility of the technology, but mechanicals on the tub were not great because of the size of the part. Results also seemed to indicate that improved thermal controls will be necessary. The tests were performed with cyclics polymerized in an extrusion barrel.