1 Overview
Liquid silicone rubber (LSR, Liquid Silicone Rubber) is a new type of rubber developed on the basis of room temperature vulcanized silicone rubber (RTV). Its base material is liquid polysiloxane, which has the characteristics of small molecular weight, low viscosity, and convenient processing and molding. It can omit mixing, pre-forming, post-processing and other processes, easy to achieve automation, save energy and labor, and has a short production cycle. And high efficiency. Products made of liquid silicone rubber have the characteristics of high purity, good transparency, low volatile content, and good oil resistance, chemical resistance, aging resistance, light resistance, insulation and heat insulation, and color The selection range is wide.
2 Classification and its vulcanization mechanism
2.1 Addition LSR
The development of addition-molded LSR began in the late 1970s. At present, most liquid silicone rubbers are prepared by addition methods, that is, addition-molded liquid silicone rubber. It uses vinyl-containing polydimethylsiloxane as the base polymer, and low-molecular-weight hydrogen-containing silicone oil as the cross-linking agent, which is heated and cross-linked into a network structure in the presence of a platinum catalyst. Addition LSR has the following advantages: ①No by-products are produced during the vulcanization process; ②The shrinkage rate is very small; ③It can be deeply vulcanized.
2.2 Room temperature vulcanized silicone rubber (RTV)
RTV is a kind of silicone elastomer that came out in the 1960s. Its most notable feature is that it can be cured in place by pressing at room temperature and is extremely convenient to use. Its physical form is usually flowable fluid or viscous paste.
Room temperature vulcanized silicone rubber can be divided into single-component and two-component room temperature vulcanized silicone rubber according to its packaging method; according to the vulcanization mechanism, it can be divided into condensation type and addition molding. Therefore, according to the composition, vulcanization mechanism and use process, it can be divided into three categories, namely, one-component room temperature vulcanization silicone rubber, two-component condensation type room temperature vulcanization silicone rubber and two-component addition type room temperature vulcanization silicone rubber (including elastic silicone Gel and silicone rubber).
3 Injection molding liquid silicone rubber
To process products with addition-molded liquid silicone rubber, two molding processes can be used-molding process and injection molding process; among them, the former is a traditional process, suitable for various silicone rubber molding; while the latter began in the 1970s, with With people's in-depth research on LSR, it has only slowly developed since the 1990s. It is mainly used for the vulcanization of addition-molded liquid silicone rubber. The liquid silicone rubber used for injection molding products is also called injection-molded liquid silicone rubber.
4 Application of liquid silicone rubber in the medical and health and food fields
As one of the typical medical polymer materials, silicone rubber has been verified for its chemical properties, physical and mechanical properties, and biological safety in medical and health applications for about 50 years. In particular, addition-molded liquid silicone rubber, as a part of silicone rubber, also plays a role in the fields of medical and health, human beauty, health care and food.
4.1 Medical and device fields
Utilizing the characteristics of low linear shrinkage (0.3%), good elasticity, fast curing, and small plastic deformation of liquid silicone rubber, it can be used as a dental impression material. Liquid silicone rubber has the characteristics of different permeability to different gases, and can be used to make artificial gills and artificial lungs. Liquid silicone rubber can be used as a repair material for the acoustic lens of a B-ultrasound probe, and a bonding material between the acoustic lens layer and the matching layer.
4.2 Food-related fields
Addition LSR is safe, non-toxic, excellent in performance, chemically inert, has a high sanitation level, and does not contain harmful substances such as nitrosamines, making it the first choice for baby products. In addition, by taking advantage of its good air permeability, it can also be made into a fruit and vegetable fresh-keeping atmosphere-modified film. SilasticLC-45-2004, launched by Dow Corning in 2007, is a liquid silicone rubber with a hardness of ShA45. It has the advantages of high transparency, strong mechanical properties, and short production cycle. It is suitable for manufacturing baby bottle pacifiers.
The sealing ring made of addition-molded liquid silicone rubber will not suffer from the deterioration of the sealing performance caused by heating, so that the food safety is guaranteed, and the durability is also very excellent. Therefore, it can be used to manufacture the sealing ring of edible products. Used in bottles and canned food (such as bottles with strawberry jam, bottles with kimchi), tableware, etc.
There are also food-grade special mold silicone, chocolate mold special food mold silicone, etc. They are room temperature vulcanized silicone rubber (RTV), mainly used for food compound molds, candy molds, cake molds and equipment with stable size requirements. The raw materials of this product have passed the SGS environmental protection material certification. The product is translucent in color, cured at room temperature, smooth in color, non-toxic, odorless, has a certain degree of hardness, good elasticity, good tensile and tear resistance, and many times of mold turning. It is widely used in the production of various food molds.
4.3 The field of medical electronic products
The electronic cochlea (C I) has been developed for more than 50 years. The basic principle is to use acoustic-electric transducers to replace the loss-of-function sensory cells in the human cochlea, and to directly stimulate the remaining auditory nerve fibers with electrical signals to make the deaf people hear. It usually consists of a microphone, voice processor, transmission circuit, implanted stimulation circuit, electrode sequence, etc. The implanted electrode is an important part of the electronic cochlea, and its effect plays a decisive role.
The Silastic MDX2-4210 LSR launched by Dow Corning has the advantages of being curable at room temperature without shrinkage deformation, good insulation, and a wide range of colors. It can be made into an electronic cochlea. The electronic cochlear electrode made of it has good stimulation effect, accurate size and beautiful appearance. The LSR has the following advantages: ①Pass ISO 10993-1 and meet the following acceptance criteria: cytotoxicity, sensitization, irritation, subcutaneous reaction, systemic toxicity (acute) and subacute toxicity (through implantation test); ②Conform to the United States Pharmacopoeia (USP) grade VI and European Pharmacopoeia (Ph. Eur./EP).
4.4 Plastic surgery and repair
The application of liquid silicone rubber in cosmetic surgery and repair includes rhinoplasty, breast augmentation and other fields. Among them, the implantable artificial prosthesis used for breast augmentation is filled with silicone gel. In 1992, all silicone materials, including silicone capsule prostheses, were banned by the US FDA for breast augmentation. By 2006, due to the improvement of silicone prosthesis, the US FDA lifted the ban on silicone materials, but there have been some controversies in the use of silicone materials. Although there are different opinions, the market share of various types of silicone medical aesthetic products is still expanding and developing rapidly.
4.5 Application of Liquid Silicone Rubber in Drug Sustained Release System
The application products of silicone rubber as a carrier in drug slow-release systems mainly include liquid silicone rubber that can be made into microspheres. Through this drug release carrier, drugs (such as various steroids, prostaglandins, antibiotics, sedatives, sleeping pills, Anticancer drugs, anesthetics, etc.) slowly spread into the body.
4.6 Other applications of liquid silicone rubber
Certain special medical liquid injection-molded silicone rubbers have suitable softness and feel like skin, dry to the touch and non-oily, and can be used to produce prostheses. Of course, thanks to its good biocompatibility and safety, liquid silicone rubber can also be used as a coating material for contact lenses and pacemaker leads.
5 Outlook
Liquid silicone rubber is non-toxic and tasteless, and it is one of the high-grade silicone products that has developed rapidly in recent years. This kind of silicone rubber has low viscosity (it has certain fluidity and indeterminate shape before vulcanization). It can be fed by metering and mixing equipment, and the two components are mixed and injected with special injection molding equipment, and then quickly vulcanized to form a product . Compared with traditional materials, the advantages of injection molded liquid silicone rubber are: short molding cycle, high degree of automation, low vulcanization temperature, no by-products in the vulcanization process, high product precision, no trimming, and long product life. Therefore, high-quality liquid silicone rubber will receive more and more attention and applications in the food industry, maternal and child products, medical treatment and equipment and other fields.
