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Thermoplastic Elastomers (TPE) vs. Liquid Silicone Rubber (LSR)

Keys to a Successful Prototype

The prototyping stage is one of the most important in the product development lifecycle. The decisions you make at this point will have a long-lasting impact on the ultimate success of your product. It’s important to work with a provider you can trust — and to know what to expect before you begin. In this article, we look at some keys to building a successful prototype.

Orientation and Crystallinity of Liquid Silicone Rubber

As has been explained several times in previous blogs, polymers like silicones are macromolecular structures which are created through polymerization processes such as addition (chain growth) or condensation (radical initiated) polymerization [1]. Those chains are held together through van der Waals forces. The lengths of the different chains are not the same, so it is possible to find very short and very long chains in the material. When the length of a group of chains is short, its arrangement is random; that is, the zone is not organized and it can be said that the zone is amorphous. However, when the chain length is long enough, organized zones can be created called crystalline zones. The process of obtaining those organized structures in polymers is not easy or simple, and it always depends on the time and temperature. First, the chains fold together to create an ordered region called lamellae, which are fine layers of the chains form the structure. When a group of lamellae is big enough, another morphological structure is created: a spherulite. This structure can be seen by optical microscopy and it directly affects the mechanical properties of the material. Also, another special crystalline arrangement is found at the same level as the spherulites. Shish-Kebab structures are formed by crystals in the shape of circular plates and whiskers. These appear when a shear deformation occurs during solidification. With that, the arrangement of the polymer structure, called the morphology, in a conventional polymer can include a highly crystalline structure found just next to an amorphous zone. Figure 1 depicts the possible structures in a polymer.

Curing Speeds in Silicone Rubber

Silicone rubber is an inorganic polymer formed by silicon (Si), oxygen (O), carbon (C) and hydrogen (H). The main chemical chain, called the backbone, is formed by silicon and oxygen, which is why it is called siloxane. Carbon and hydrogen can be found in lateral chains as methyl (-CH3), vinyl (-CH=CH2), phenyl (-C6H5, also known as a benzene ring) or other groups (Figure 1). As with all polymers, silicone rubber needs a polymerization process to create the real material from SiOH monomers, which react with Me3SiOSiMe3 (where Me is a metal catalyst) to increase the chain length and with Me3SiO2 to end the chain. As a result of this process, the uncured silicone rubber contains chains with different lengths [1].

LSR and LSR Multi-shot in Healthcare Applications

Silicone rubber is recognized for being inherently biocompatible with human tissues, resistant to bacteria, and it does not degrade in the presence of fluids like blood, saliva, and others. This material is used more and more in medical applications, but increasingly strict requirements make it necessary to modify the silicone in order to use it. Additives are a common option for increasing mechanical and physical properties, but silicone can also be combined with other materials such as thermoplastics, thermosets, thermoplastic elastomers, ceramics, and metals to comply with application requests. In the industrial processing of materials, the challenge is always to create high-performance products in the shortest time with the least amount of the components. There are two options for that: (i) select the best material that fits with all the requirements of the final application; in some cases, this can be almost impossible since it is common that application requests are opposing, for example, rigid in one area but flexible in another area of the part. There is also option (ii): design a single part made with multiple materials that fulfill the application requirements, which is manufactured using co-injection or over-molding, otherwise called two-shot or multi-shot injection molding; this process offers extended functionality, better appearance, and high quality of the product. In this special process, critical variables include mold temperature, shrinkage, and deformation of the second injected material, and they must be analyzed carefully because the right choice will determine the success of the product.