I. The fundamental role of dimethyl silicone oil in silicone sealants
Silicone sealant is primarily made from polydimethylsiloxane (i.e., dimethyl silicone oil) as the base material, supplemented with appropriate amounts of fillers, plasticizers, crosslinking agents, coupling agents, and catalysts. At room temperature, the silicone sealant reacts with moisture in the air to form a crosslinked network structure, which then cures into an elastic silicone rubber. This sealant is widely used in various curtain walls and stone curtain walls for bonding and sealing, as well as for door and window caulking and interior decoration due to its ease of application, excellent UV resistance and high/low temperature resistance after curing, and good adhesion to most building substrates.
II. The effect of dimethyl silicone oil on the surface drying time of sealant
Surface drying time is a crucial indicator of the performance of silicone building sealants, directly impacting the required finishing time. According to GB/T 14683-2017, "Silicone and Modified Silicone Building Sealants," the surface drying time of silicone sealants under standard conditions should be within 180 minutes.
Studies show that increasing the dimethyl silicone oil content prolongs the surface drying time. This is because dimethyl silicone oil lacks reactive groups, does not participate in the curing and crosslinking reaction of the silicone sealant system, and exhibits some migration ability, migrating to the interface. This leads to a relatively lower concentration of crosslinking agents on the sealant surface, thus slowing down the crosslinking and curing speed.
When the dimethyl silicone oil content is low, its effect on the crosslinking agent concentration on the sealant surface is not significant; however, at higher content, the crosslinking agent concentration on the sealant surface decreases significantly, resulting in a markedly prolonged surface drying time. Further analysis revealed that the surface drying time of dealcohol-based silicone sealants is more easily affected by the dimethyl silicone oil content. Without dimethyl silicone oil, the surface drying time of the dealcohol-based sealant is approximately twice that of the oxime-based sealant; when the dimethyl silicone oil content increases to 50% of the system, the surface drying time of the dealcohol-based sealant is approximately three times that of the oxime-based sealant. This is likely due to the difference in the activity of the crosslinking agents used in the two sealants. The higher the hydrolytic activity of the crosslinking agent, the shorter the surface drying time and the faster the detackification of the sealant. The hydrolytic activity of the crosslinking agent in the dealcohol-based sealant is much lower than that in the oxime-based sealant, therefore its surface drying time is significantly longer, and the surface drying time is more likely to slow down with increasing dimethyl silicone oil content.
III. The Influence of Dimethyl Silicone Oil on the Sagging of Sealants
Sagging is an important indicator of the rheological properties of sealants, characterizing the anti-sagging performance of the sealant joint. The lower the sag, the less likely the sealant joint will deform and flow after application, which is especially important for vertical joints or inverted horizontal joints.
Studies show that adding an appropriate amount of dimethyl silicone oil to the sealant system does not affect its sag; however, when the dimethyl silicone oil content increases to over 30%, the sealant sag increases significantly, especially in vertically tested joints. Furthermore, compared to de-alcoholized and deoxime-based sealants, dimethyl silicone oil has no significant difference in its effect on sag.
