PVC is manufactured predominantly through a process known as suspension PVC or, more generally, suspension polymerization. In this process, free-radical initiators react with vinyl–chloride monomers (VCMs) after they are dispersed in water. This forms small droplets that contain the polymerization reactions. These droplets then continually aggregate to form fused agglomerates that produce a polyvinyl chloride resin grain.
It is crucial for PVC manufacturers to control VCM porosity and bulk density of the resultant PVC resin grain. These properties are directly related to droplet size and are important when producing higher-purity PVC or determining if flexible or rigid PVC will be made. The porosity of polyvinyl chloride resin grains influences the ease of removing unreacted VCM and the absorbance of additives such as plasticizers, stabilizers, pigments, and impact modifiers. On the other hand, bulk density plays a significant role when considering material storage, flow, and handling of resin. The diagram below illustrates the mechanism that takes place for suspension polyvinyl chloride polymerization.
Manufacturers of polyvinyl chloride must often balance bulk density and porosity to create a resin with the necessary properties suited for specific end-use applications. It is critical to use an appropriate suspending agent and understand how it influences the PVC manufacturing process, as suspending agents have the largest influence on suspension PVC droplet size. This can impact PVC particle morphology and, therefore, also influence porosity and bulk density.
METHOCEL™ Suspending Agents
METHOCEL™ suspending agents are nonionic, water-soluble cellulose ether polymers that are naturally derived. Two functional properties of METHOCEL™ make it effective at suspension. First, it facilitates better dispersion and creates smaller droplet sizes by reducing the interfacial tension (IFT) of suspension PVC droplets. This leads to resins with higher porosity and less bulk density. Second, METHOCEL™ keeps vinyl–chloride droplets in contact with the growing PVC polymer chain by forming a protective colloid. This protective colloid helps prevent the coalescence of the droplets and keeps droplet size more uniform during the “sticky” aggregation phases of the polymerization process.
Selecting the Right METHOCEL™
Suspending Agent There are a variety of METHOCEL™ grades with different substitution levels for methyl and hydroxypropyl groups attached to the cellulose backbone. In the diagram below, a letter is used in each product to denote the specific degree of substitution of both methoxy and hydroxypropyl groups.
The following questions can aid in selecting a METHOCEL™ grade for your specific application.
- Are lower-VCM residuals needed?
- Is a lower or higher bulk density of resin preferred?
- Is a less porous or more porous PVC resin grain needed?
Generally, METHOCEL™ grades that provide lower IFT will also have less vinyl–chloride residuals, higher porosity, lower bulk density, and more uniform PVC grain size distribution. Typically, as the degree of substitution of METHOCEL™ increases, IFT also increases. Therefore, METHOCEL™ is ranked in the following order by the letter indicating the degree of substitution:J < E < F < A < K. Specific METHOCEL™ grades can be chosen depending on the desired porosity and bulk density necessary for each end-use application.
Though the METHOCEL™ portfolio has many different grades, lower-viscosity grades are the most compatible for PVC resin polymerization. The grades below are the best-performing options for PVC manufacturers and their specific applications.
Grade
|
IFT (dyne/cm),
in 1,2-Dichloroethane
|
Viscosity (mPa·s)
|
Application/Result
|
METHOCEL™ K100LV
|
8.66
|
100
|
Rigid PVC, low porosity, high bulk density
|
|
|
7.36
|
15
|
Rigid PVC, low porosity, high bulk density
|
|
|
5.17
|
50
|
General-purpose suspending agent
|
|
|
4.34
|
15
|
Flexible PVC, high porosity, low VCM residuals
|
|
|
4.03
|
50
|
Flexible PVC, high porosity, low VCM residuals
|
METHOCEL™ Uses for Suspension PVC Polymerization
Size of Reactor
|
Speed of Agitator (rpm)
|
METHOCEL™ Content (%, based on monomer weight)
|
0.5-gallon reactor
|
150–170
|
0.2–0.3
|
1-to-2-gallon reactor
|
150–170
|
0.2–0.3
|
20-gallon reactor
|
150–170
|
0.15–0.18
|
50-gallon reactor
|
150
|
0.08
|
500-gallon reactor
|
120
|
0.06
|
3,000-gallon reactor
|
120
|
0.05
|
Based on the reactor size, the above chart can be used as a starting point for the percent METHOCEL™ needed for use. In general, METHOCEL™ is added at a range of 0.05–0.30% of VCM weight for large-scale operations of suspension polyvinyl chloride. Most of the variability in this range depends on the reactor size being used, where smaller reactors require a higher concentration of METHOCEL™ to gain better efficacy.
Furthermore, higher concentrations of METHOCEL™ will result in more effective surface coverage of monomer droplets and film formation, reducing the tendency for droplet aggregation.
Prior to adding the VCMs, it is recommended to charge the reactor with water and a METHOCEL™ solution to avoid the formation of coarse resin particles and resin buildup on the reactor walls. Other types of suspending agents, such as hydroxypropyl cellulose and polyvinyl alcohol, can be used with METHOCEL™ to further regulate grain size over a broader range of shear forms. Instructions on how to produce solutions of METHOCEL™ for your application can be viewed in this
video.
Overview
Suspension PVC manufacturers must have strict control over properties such as porosity, grain size, 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 options that permit customization of PVC resin properties and is ideal for suspension PVC manufacturers to use. To request a sample or get in touch with one of our experts regarding your PVC manufacturing process, contact us below.
