For thermoset polymer compounds, cross-linking between the growing polymer chains occurs during the polymerization reaction, so the production and processing steps need to be combined when working with these compounds. For some thermoplastic polymers, it is also more economical to combine the production and processing steps. Fiber spinning, extrusion, and injection molding are some of the most common forms of reactive polymer processing.
Polymer Product Manufacturing Steps
Reactive polymer processing is one of the ways you can process the polymer in the manufacture of polymer products:
Reaction Fiber Spinning
In reaction fiber spinning, monomers are continuously bonded and cross-linked together in simultaneous chain extension reactions as they are drawn into their fiber form. For more information on the processing of polymers into fibers and threads, see the Fiber Spinning page of the polymer processing section.
Reaction spinning is similar to dry spinning, where the material is dissolved and then extruded through a spinneret to create fine fibers, but with an additional polymerization step. During polymerization, the shorter monomer chains react and cross-link to create the new enhanced polymer fiber.
Reaction spinning is not very widely used for fiber spinning, but its most common product is spandex. Reaction spinning is used to crosslink a flexible and stiff pre-polymer. The resulting enhanced fiber is the strong, durable, stretchy, material we know as Spandex.
- Creates enhanced fibers with unique qualities.
- Limited by reaction conditions.
- Not commonly used in industry
- Generally more expensive than other reactive polymer processing methods.
Polymer products with viscosities of 10 to 1,000 poise can be formed using reaction extrusion, as the torque from the screw provides ample force to move the material through the barrel despite the high viscous resistance. Typically, twin-barrel extruders are used for polymerization reactions due to their excellent mixing abilities. The melt-state of the feed components is induced by the shearing action of the tightly-intermeshing twin screws fighting against viscous forces as well as by the heating action provided by heating elements outside the barrel. Read more on the Extruders page.
The feed monomer can enter the reactor in numerous phases, such as solid pellets or granules; dissolved in a volatile solvent; or as a liquid. In order for the reaction to occur, the feed must reach a fluid melt state. The barrel of the extruder is often broken up into hypothetical sections, each serving a specific purpose. The first portion of the barrel length is dedicated to heating the feed to the temperature required to achieve melt. The second section of the barrel allows for the complete mixing of all now-fluid components. After the feed has been melted and mixed, the polymerization reaction occurs in the third section. A devolatilization separation process typically occurs in the remaining length of the barrel that removes any volatile byproducts of the polymerization reaction.
Furthermore, it is possible to further designate sections of the barrel to allow for heating and cooling within different regions. This additional segmentation also allows for different zones of the barrel to be operated at different pressures. Sample ports can also be installed in these different barrel regions so polymer properties such as average molecular weight can be tested at different points in the reaction. The residence time, or the time a given quantity of material remains within the extruder, is dependent on the length of the barrel and the rotation speed of the screw. All things considered, higher residence times lead to longer chain lengths in the product polymer.
Reaction extruders are used to produce a majority of some thermoplastic polyurethanes. To do this, liquid reactants are fed in stoichiometric ratios to a twin-screw extruder. Constant mixing and kneading action is applied by the rotating screws. Any reaction heat can be dissipated through the walls of the barrel and removed using cooling water. The product is forced through a die and cooled and pelletized, to be further processed later. Thermoplastic polyurethane created in this fashion is used to make ski boots and medical hoses, among other products.
- Excellent control over residence time distribution by adjusting screw speed
- Allows for high-pressure reactions
- Can handle extremely viscous polymer products
- High level of radial mixing and dispersion
- High costs for longer reaction times
- Difficult to remove heat from polymerization without unwanted side effects
Reaction Injection Molding
Reaction injection molding (RIM) was developed in the late 1950s at General Motors Corporation for the production of the bumper cover for the Corvette. RIM is a batch process similar to injection molding, but the polymer itself is not directly injected into the mold. Instead, two or more monomers are injected into the mold and polymerized within the mold, so that the mold acts as a chemical reactor. To read more about injection molding, see the Injection Molding page in the polymer processing section.
The reactants are kept in overhead feed tanks. A loop that includes circulation pumps and heat exchangers ensures that the reactants are maintained near the reaction temperature. Melting pumps provide additional heating and deliver the reactants to the mixing head. There the reactants are mixed and pushed into the mold, where the mixture can react and be molded simultaneously.
The mixing head shown on the left below is typical of those used in reaction injection molding. The picture on the right shows a reaction injection process to make polyurethane roofing tile.
The following animation depicts the reaction injection molding process. The hot reactants are injected into the mixing head, where they are forced to mix by impingement. The monomer mixture is then injected into the mold. There it reacts to form a polymer while being molded into the desired shape. Once the product cools, the mold is emptied and the process is repeated.
Reaction injection molding is used to make products in a wide range of fields, from sports equipment to home constructions and biomedical applications, as well as the production of several kinds of structural foams in a process called Foam Injection Molding (FIM). RIM foams are produced by introducing a blowing agent, such as nitrogen gas, to the monomer mix. The blowing agent helps make the foam more flexible than other RIM plastics. The bike seat shown below is made of polyurethane integral skin foam, meaning it has a high-density outer skin and low-density inner core.
Structural reaction injection molding (SRIM) is similar to RIM except for the addition of reinforcement to the mold prior to monomer injection. The monomers are infused into the reinforcement, made of fabric, metal, or plastic, to produce a more rigid structure than those of RIM products. Shown below is a picture of a mold for a SRIM truck door panel.
SRIM was developed by General Motors Corporation and used to make spare tire well covers. SRIM has since been used to produce many other car parts, such as door panels, instrument panels, and sunshades. In addition to automotive parts, SRIM is commonly used to produce satellite dishes and door panels, such as the one pictured below.
- Lower energy requirements to pump monomers rather than polymers.
- Relatively inexpensive.
- Process can be repeated as many as 1 million times on the same mold.
- Can produce a variety of different products.
- Hardness and other physical properties can be adjusted.
- Produces low-weight products.
- RIM parts are more paintable than parts produced through other methods.
- Inconsistent moldings.
- Reactants must be fast reacting.
- Rates of reaction must be synchronized with molding times.
- Requires high pressures.
- Temperature must be precisely controlled throughout the process.
- Poor performance at high humidity.
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