The graph shows an example of the molecular distribution of an unstripped polymer.
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The commercial production of silicone polymers involves what is called ring opening polymerization (ROP). The process begins by introducing a chemical initiator and endblocker to a silicone polymer cyclic. In organic chemistry, cyclics are compounds in which a series of carbon atoms connect to form a loop or ring. The initiator reacts with and combines with a cyclic molecule, causing the molecule to open into a linear configuration.

These linear chains can form longer and longer chains with other opened cyclics. As the reaction consumes available cyclics, the active chain reacts with the endblocker, which prevents additional siloxane units from connecting. After several hours, the process reaches equilibrium, the reaction is complete, and either high temperature or carbon dioxide is used to deactivate the initiators.

It is customary to test the polymer for volatile content to verify equilibrium. The expected percentages of linear chains and volatile components depend on the type of siloxane polymer being produced. The most common polymer, polydimethylsiloxane, comprises 85% linear chains and 15% volatile components.

Statistically speaking, the polymer chain length (or molecular weight) of the bulk of the polymer is normally distributed. Gel permeation chromatography shows a bimodal distribution (a probability distribution with two different modes) with a smaller, much lower molecular-weight peak (representing cyclics and short chains) and a larger peak representing polymers with larger molecular weights.

Volatile components

The volatile content produced during ROP can negatively affect both the material processing and the end product. Unstripped polymers, or those containing a significant volatile fraction, pose obstacles for manufacturing an elastomer. Why? Because this kind of manufacturing requires first building a base, which typically consists of polymer and silica. The silica is used to reinforce the polymer and help control its flow. When the polymer’s volatile component is not adequately removed, it can treat the surface of the silica, affecting uncured properties such as rheology and cured properties like hardness and tensile strength.

A gas chromatograph (GC) of an unstripped 3,000 DP polymer shows an example of the degree of polymerization (DP) or the number of repeating siloxane units of the polymeric chain. The sample was injected after being diluted with hexane (hexane peak is at far left). The taller series of peaks are cyclic molecules and the shorter series of peaks are trimethylsilyl endblocked linear molecules. This anlaysis shows D-4 cyclics = 5.9% and D-20 cyclics = 0.13% of polymer mass. The graph is attenuated for the best visual presentation.
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Of greater concern is the impact unstripped or poorly stripped polymer has on how finished products perform. As an example, consider platinum catalyzed liquid silicone rubbers (LSRs). They are typically built by synthesizing a polydimethylsiloxane polymer and then chemically endblocking the reaction with vinyl functional units. This turns the polymer into a base, which is typically divided into a Part A and Part B. Part A contains a platinum catalyst and Part B contains a short-chain functional copolymer called a “crosslinker.” When the two parts are mixed, the system catalyzes. Applying an elevated temperature makes the polymers rapidly crosslink, producing a cured elastomer.

Platinum systems generate no by-products during vulcanization and therefore mold predictably with minimal shrink. In fact, while the shrink associated with this kind of LSR is typically estimated to be about 2%, it’s mostly the result of the coefficient of thermal expansion of the silicone as opposed to actual mass lost. However, if such an LSR were to be built with a poorly stripped polymer, the shrink could increase, become unpredictable from lot-to-lot, and prove an unnecessary processing variable for molders and fabricators.

Such a scenario could also result in voids in the cured component — often referred to as “porosity.” Porosity happens when silicone cures around gaseous bubbles in the free space of the silicone material. Bubbles can be generated in the course of the chemical reaction and from volatilization when the silicone is raised to curing temperatures. At molding temperatures, short chains or residual cyclics rapidly volatilize. Volatile portions escaping the molded part cause shrinkage, while portions that are trapped before the component cures cause porosity. A poorly stripped polymer can result in dimensional failures and cause problems with appearance and mechanical integrity.

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