Suspension PVC and Process Considerations
PVC is predominantly manufactured through the process of suspension polymerization, where it is known as suspension PVC. In this process, vinyl-chloride monomers (VCMs) are dispersed in water and react with free-radical initiators to form small droplets that contain the polymerization reactions. These droplets then continually aggregate to form fused agglomerates that produce a PVC resin grain. Below is a diagram of the mechanism that is thought to take place for suspension PVC polymerization.
Control over VCM droplet size is crucial for PVC manufacturers, as both porosity and bulk density of the resultant PVC resin grain are directly related. The porosity of PVC resin grains influences the ease in removal of unreacted VCM and absorbance of additives like plasticizers, stabilizers, pigments, and impact modifiers. These properties are important, for example, to produce higher-purity PVC or determine if flexible or rigid PVC will be made. On the other hand, bulk density is of concern when considering material storage, flow, and handling of resin.
PVC manufacturers must often balance porosity and bulk density to create a resin with the necessary properties suited for specific end-use applications. Suspending agents have the largest influence on suspension PVC droplet size, ultimately impacting PVC particle morphology and, therefore, porosity and bulk density. Thus, it is critical to use an appropriate suspending agent and understand how the selection of a suspending agent influences the PVC manufacturing process.
METHOCEL™ Suspending Agents
METHOCEL™ suspending agents are naturally derived cellulose ether polymers that are nonionic and water-soluble. There are two functional properties of METHOCEL™ that make it an effective suspending agent. First, it reduces interfacial tension (IFT) of suspension PVC droplets to facilitate better dispersion and create smaller droplet sizes. These smaller droplet sizes will lead to resins with higher porosity and less bulk density. Second, METHOCEL™ serves as a protective colloid around VCM droplets while keeping it in contact with the growing PVC polymer chain. The protective colloid that METHOCEL™ forms helps to prevent coalescence of the droplets while keeping droplet size more uniform during the “sticky” aggregation phases of the polymerization process.
METHOCEL™ is available in a variety of grades with different substitution levels for methyl and hydroxypropyl groups attached to the cellulose backbone. A letter is used in each product to denote the specific degree of substitution of both methoxy and hydroxypropyl groups, as shown in the diagram below:
How to Select the Right METHOCEL™ Suspending Agent
Here are a few questions to help guide a METHOCEL™ grade selection:
- Is a more porous or less porous PVC resin grain desired?
- Is a higher or lower bulk density of resin preferred?
- Are lower VCM residuals needed?
With these in mind, as mentioned previously, a lower IFT will contribute to better dispersion and a smaller VCM droplet size, which will form more porous, smaller grain PVC resins. Therefore, METHOCEL™ grades that provide lower IFT will also have lower bulk density, lower VCM residuals, and a more uniform PVC grain size distribution. In general, IFT increases as the degree of substitution of METHOCEL™ increases. Therefore, IFT can be ranked in the following order by the letter indicating the degree of substitution: J < E < F < A < K.
Other factors, such as gel strength and organosolubility of individual METHOCEL™ grades, may also play a role in controlling droplet size but do not have an impact as significant as IFT. With all the above in consideration, specific METHOCEL™ grades can be selected, depending on the desired porosity and bulk density necessary for each end-use application.
|Grade||IFT (dyne/cm), in 1,2-Dichloroethane||Viscosity (mPa*s)||Application/Result|
|METHOCEL™ K100 LV||8.66||100||Rigid PVC, low porosity, high bulk density|
|METHOCEL™ A15 LV||7.36||15||Rigid PVC, low porosity, high bulk density|
|METHOCEL™ F50||5.17||50||General purpose suspending agent|
|METHOCEL™ E15 LV||
|15||Flexible PVC, high porosity, low VCM residuals|
|METHOCEL™ E50 LV||4.03||50||Flexible PVC, high porosity, low VCM residuals|
While dozens of products exist within the METHOCEL™ portfolio, lower-viscosity grades are the most suitable for suspension PVC polymerization. The grades above provide PVC manufacturers with the best-performing options.
How to Use METHOCEL™ for Suspension PVC Polymerization
Typically, for large-scale operations of suspension PVC manufacturing, METHOCEL™ is added at a range of 0.05–0.30% of VCM weight. Most of the variability within this range depends on the reactor size being used, where smaller reactors require a higher concentration of METHOCEL™ for better efficacy. Below is a chart that can be used as a starting guideline for the percent METHOCEL™ required, based on the reactor size.
|Size of Reactor||Speed of Agitator||% METHOCEL™ (based on monomer weight)|
|0.5-gallon reactor||150–170 rpm||0.2–0.3|
|1-2-gallon reactor||150–170 rpm||0.2–0.3|
|20-gallon reactor||150–170 rpm||0.15–0.18|
|50-gallon reactor||150 rpm||0.08|
|500-gallon reactor||120 rpm||0.06|
|3,000-gallon reactor||120 rpm||0.05|
It is worth noting that with each reactor size, increasing the concentration of METHOCEL™ has also been shown to reduce PVC grain size further. This is because higher concentrations of METHOCEL™ will result in better film formation and surface coverage of monomer droplets, reducing the tendency for droplet aggregation.
Procedurally, it is recommended to charge the reactor with water and a solution of METHOCEL™ prior to adding the VCMs to avoid the formation of coarse resin particles and build-up of resin on the reactor walls. For instructions on how to make solutions of METHOCEL™, please review the following video. METHOCEL™ may also be used with other suspending agents such as hydroxypropyl cellulose and polyvinyl alcohol to adjust further grain-size control over a wider range of shear profiles.
Manufacturers of suspension PVC require strict control over resin properties such as grain size, porosity, and bulk density to create the specific types of PVC resin needed for each end-use application. METHOCEL™ is a naturally derived suspending agent available in several different grades to permit customization of these resin properties. Contact us below to request a sample of METHOCEL™ to use in your suspension PVC manufacturing process.