Co injection dripping molding 1 - How long does silicone rubber take to cure? - ZSR

How long does silicone rubber take to cure?

In its uncured state, silicone rubber is a highly adhesive gel or liquid. To convert it to a solid, it must be cured, vulcanized, or catalyzed. This is normally carried out in a two-stage process at the point of manufacture into the desired shape, and then in a prolonged post-cure process. It can also be injection molded.

Silicone rubber may be cured by a platinum-catalyzed cure system, a condensation cure system, a peroxide cure system, or an oxime cure system. For the platinum-catalyzed cure system, the curing process can be accelerated by adding heat or pressure.

Platinum-based cure system

In a platinum-based silicone cure system, also called an addition system (because the key reaction-building polymer is an addition reaction), a hydride- and a vinyl-functional siloxane polymer react in the presence of a platinum complex catalyst, creating an ethyl bridge between the two.

The reaction has no byproducts. Such silicone rubbers cure quickly, though the rate of or even ability to cure is easily inhibited in the presence of elemental tin, sulfur, and many amine compounds. Maybe 1s or more.

Peroxide cure system

Peroxide curing is widely used for curing silicone rubber. The curing process leaves behind byproducts, which can be an issue in food contact and medical applications. However, these products are usually treated in a post-cure oven which greatly reduces the peroxide breakdown product content.

One of the two main organic peroxides used, dicumyl peroxide (compared to cumene hydroperoxide), has principal breakdown products of acetophenone and phenyl-2-propanol. The other is dichlorobenzoyl peroxide, whose principal breakdown products are dichlorobenzoic acid and dichlorobenzene. That cure needs 2-6 minutes according to the structure of the product.

Condensation cure system

Condensation curing systems can be one-part or two-part systems. In a one-part or RTV (room-temperature vulcanizing) system, a cross-linker exposed to ambient humidity (i.e., water) experiences a hydrolysis step and is left with a hydroxyl or silanol group.

The silanol condenses further with another hydrolyzable group on the polymer or cross-linker and continues until the system is fully cured. Such a system will cure on its own at room temperature and (unlike the platinum-based addition cure system) is not easily inhibited by contact with other chemicals, though the process may be affected by contact with some plastics or metals and may not take place at all if placed in contact with already-cured silicone compounds.

The crosslinkers used in condensation cure systems are typically alkoxy, acetoxy, ester, enoxy or oxime silanes such as methyl trimethoxy silane for alkoxy-curing systems and methyl triacetoxysilane for acetoxy-curing systems.

In many cases, an additional condensation catalyst is added to fully cure the RTV system and achieve a tack-free surface. Organotitanate catalysts such as tetraalkoxy titanates or chelated titanates are used in alkoxy-cured systems. Tin catalysts such as dibutyl tin dilaurate (DBTDL) can be used in oxime and acetoxy-cured systems.

Acetoxy tin condensation is one of the oldest cure chemistries used for curing silicone rubber and is the one used in household bathroom caulk. Depending on the type of detached molecule, it is possible to classify silicone systems as acidic, neutral or alkaline.

Two-part condensation systems package the cross-linker and condensation catalyst together in one part while the polymer and any fillers or pigments are in the second part. Mixing of the two parts causes the curing to take place. A typical filler is fumed silica, also known as pyrogenic silica, which used to control the flow properties of the sealant.

Once fully cured, condensation systems are effective as sealants and caulks in plumbing and building construction and as molds for casting polyurethane, epoxy and polyester resins, waxes, gypsum, and low-melting-temperature metals such as lead. They are typically very flexible and have a high tear strength. They do not require the use of a release agent since silicone have non-stick properties. That need 24 hours.

In ZSR Group. The silicone molding process that we have are Liquid Silicone Rubber molding. Solid Silicone
Compression Molding. And Co-injection molding (Epoxy molding). For all the curing, we used is Platinum-based cure system and Peroxides system(cold curing, heated curing, pressure curing). The time for these curing is from 1s to 10 minutes depending on the material and silicone rubber products structure.

About Author: Z.S.R International Group

Z.S.R International Group(Hong Kong) co., Limited,  is a one-stop supplier for molded silicone products and silicone products molding solution provider in the consumer products field. We offer OEM services from product design to Silicone products contact manufacturing. We have the capability for LSR molded silicone products, solid silicone molded products, molded silicone multi-colored products. We also can custom molded silicone, custom molded LSR, custom molded dripping injection dispensing (co-injection) silicone multi-colored products, custom tooling.