Silicone rubbers are a type of thermosetting elastomer that maintain their chemical, mechanical and electrical properties over a wide range of temperatures. They are therefore a natural choice for applications where high accuracy, stability, reliability and purity is required for instance in medical devices as well as automotive and aerospace components. It is used for the production of seals in the automotive and aerospace industry, connectors and cables for appliances and telecommunications, implants and devices for medical purpose, and packaging and baking pans for the food industry. As it is very pure, liquid silicone rubber (LSR) has gained a significant place as a material for medical applications.
Liquid Silicone Rubber in Syringes
Liquid silicone rubber is a prime candidate as a replacement for other elastomers such as polyisoprene. LSR has until now not been used in the manufacture of rubber syringe tips or stoppers. One of the main issues in syringe performance is the reliability of the material, and consequently the magnitude and consistency of friction between the stopper and the polypropylene barrel of the syringe. The critical factors involved are the break-out force and the sustainable force during operation.
A study of the material behavior of liquid silicone rubber for injection molding syringe applications was performed at SIMTEC Silicone Parts, LLC, the University of Wisconsin-Madison and the University of Erlangen-Nuremberg. The theoretical and experimental research showed that liquid silicone rubber has many benefits over high cure rubber (HCR) and polyisoprene. In this study the friction characteristics of LSR were measured and compared to the friction properties of polyisoprene rubber. LSR proved superior in all levels to its isoprene counterpart. The friction and curing study findings are illustrated in this paper.
Silicone rubber is a family of thermoset elastomers that have a backbone of alternating silicone and oxygen atoms and methyl or vinyl side groups.
Some of the salient features of silicone rubber are listed below:
- Silicone rubbers consist mainly of silicone polymers and fillers. Silicone rubbers come in two distinct physical states: solid and liquid.
- Solid silicone rubbers exhibit a high viscosity hence they are commonly referred to as high consistency silicone rubbers (HCR) and are processed similar to normal organic rubbers.
- Solid silicone rubbers are formed using linear polymers with molecular weights between 400,000 and 600,000 g/mol. These polymers contain an average of 6,000 siloxy units and are water clear Newtonian liquids with viscosities between 15,000 and 30,000 Pa-s.
- Solid silicone rubbers are normally vulcanized using two kinds of peroxide catalyst: aroyl-peroxides and alkyl-peroxides
- Liquid silicone rubbers (LSR) have the same structure as solid silicone rubbers. However, the chain length of the polydimethylsiloxane used for LSR is lesser by a factor of about 6. Therefore, the viscosity of the polymer is decreased by a factor of about 1,000.
- Another advantage is that the vulcanization of liquid silicone rubber, carried out with a platinum-catalyzed hydrosilylation reaction does not generate by-products, and hence holds significance for medical applications.
- LSR has consistent electrical, mechanical, and chemical resistance properties. Additionally, the material is not inhibited by oxygen.
- Since LSR has low viscosity, these rubbers can easily be injection molded. During the process they are pumped through pipelines and hoses to the vulcanization equipment.
- During injection molding of liquid silicone rubber, loss of material in the feed lines is avoided using cold runners.
Cure Kinetics Experiments
Most curing polymers are two-component systems. In the case of liquid silicone rubbers, component A contains a platinum catalyst and component B contains methylhydrogensiloxane for cross-linking, as well as an alcohol inhibitor.
The steps followed while conducting the experiment are listed below:
- The A and B components of the liquid silicone rubbers were stored in a two component cartridge.
- Cartridges were placed at room temperature with room humidity of 35% in a dark cabinet for storage.
- The two components were mixed in a 1:1 ratio using a static mixer with 13 mixing elements.
- Five types of LSR, between Shore 20 and 60, and one type of HCR were studied via thermal analysis to ascertain the progression of the vulcanization processes.
- The specific properties being studied were heat of reaction, peak temperature, and extent of reaction.
- Differential scanning calorimeter equipment manufactured by Netzsch (Phox DSC 200 PC) was used to determine the heat of reaction for the samples.
- Sealed aluminum pans were used to analyze all reactions. The mass of the samples ranged from 10 mg to 30 mg. A sealed empty pan was used as a reference.
- The total heat of reaction was measured by a dynamic scan from 20°C to 150°C using heating rates of 1.0, 2.5, 5.0, and 10.0 K/min. Multiple scanning rates were used to obtain the impact of time and temperature on the vulcanization reaction.
- Repeatability of vulcanization was obtained for each liquid silicone rubber and heating rate.
- Solid silicone rubbers present lower levels of repeatability than liquid silicone rubber, because the material is pre-vulcanized up to a certain degree before storage. During storage, the rubber continues to cross-link at various proportions, leading to a slab with variable degrees of conversion.
- This inconsistency in the HSR material makes it very challenging to take small samples of equal amount of initial cure for testing and processing. During processing this leads to parts that reach final cure at different times, introducing uncertainty when predicting the process time using predictive kinetic models such as the Kamal-Sourour model.
- The data of heating rate and the temperature at which the maximum rate of reaction occurs was plotted, and fitted to a linear model.
- The activation energy of each silicone rubber was calculated with the data from the four dynamic scanning rates experiments. The slope of the line corresponding to the negative ratio of the activation energy and the universal gas constant as can be seen for the LSR Shore 50 in shown in Figure 1.
Figure 1. Kissinger model for an LSR Shore 50.
The data obtained from the dynamic DSC scans was used to ascertain the kinetic constants for the Kamal-Sourour kinetic model. This process was performed by fitting the instantaneous vulcanization rate and the percentage of vulcanization at each specific temperature into a model that describes the reaction. The method uses one or more dynamic DSC scans to determine a set of kinetic parameters that model the vulcanization process for all heating rates tested. A graph of the experimental data and fitted Kamal-Sourour model for the Shore 50 liquid silicone rubber is shown in Figure 2. The fitted models are in good agreement with the DSC data.
Figure 2. Fitted model and experimental data for an LSR Shore 50.
The results of HCR and one type of LSR were also compared. Different material samples were taken from one HCR batch and tested. Due to the variation in the initial degree of cure between the samples, only one was chosen to fit the model. Figure 3 shows the fitted data and compares it to a HCR for similar applications. As can be seen, HCR vulcanizes at higher temperatures than LSR leading to larger cycle times and energy costs. In addition, the vulcanization rate of LSR is higher than that of HCR. A higher reaction rate will significantly reduce the cycle time.
The kinetic friction coefficient was measured between two varieties of liquid silicone rubber having 50 and 70 shore hardness, as well as a polyisoprene for comparison, and an unnucleated polypropylene .
Figure 3. Comparison of curing reactions for comparable LSR and HCR materials
The steps followed to measure the kinetic friction co-efficient are listed below:
- In order to measure the kinetic friction coefficient between the elastomers and the polypropylene, tiny 6mm diameter discs were cut from the 3 mm thick elastomer sheets and bonded into a 1mm deep indentation of the fixture.
- The rubber elements were pressed against a rotating 110 mm diameter polypropylene disc molded from an un-nucleated material from Basell. The experiments were performed at the Plastics Technology Institute at the University of Erlangen-Nuremberg.
- A cured material, and an elastomer were tested against a thermoplastic material. As a result, the tests had to be adjusted due to the large difference between elastomers and thermoplastics, such as the significantly larger compliance of elastomers.
Table 1 summarizes the experimental set-up and parameters. It should be noted that while the standard testing pressure is 1 N/mm2, a lower pressure of 0.5 N/mm2 had to be used for the polyisoprene sample.
Table 1. Friction Test Parameters
|Rubber sample size
6 mm diameter
|Polypropylene disc size
110 mm diameter
Table 2 shows the friction measurements results.
Table 2. Friction Test Results Between Rubber Samples and Polypropylene Disc Material Coefficient of Friction Standard Deviation
|Coefficient of Friction
LSR Shore 50
LSR Shore 70
As seen in the table, the Shore 50 LSR presented the lowest coefficient of friction; almost half the coefficient of friction between polyisoprene and polypropylene. All three tests are very repeatable, with relatively low standard deviations.
The major conclusion from this study are that LSR is a reliable material that renders a reproducible process and product. The precise metering and mixing that occurs during processing of LSR renders a product with consistent properties. This study demonstrated how the curing behaviour of LSR occurs at considerably lower temperatures than HCR, leading to lower energy consumption during processing. Furthermore, the reaction rates with LSR are much lower than with HCR, resulting in shorter cycle times, furthermore adding to energy and production costs. Most importantly for medical applications, specifically syringes, the studies show that the friction coefficient between LSR and polypropylene are considerably lower than the friction between polyisoprene (currently being used to manufacture syringe stoppers) and polypropylene. These findings open doors to a new application for LSR that will most probably render products of excellent quality with reproducible functionality. An additional benefit of employing LSR for syringe stoppers is the high purity of the material, which certainly broadens the application of such stoppers for syringes. Compatibility issues need, however, to be studied by the syringe manufacturers and suppliers.
SIMTEC is a research and technology driven company. Since their establishment in 2002 in Madison, Wisconsin, USA, they have been continuously developing Extraordinary Solutions™ for leading industries worldwide with services ranging from prototyping to serial production of high precision Liquid Silicone Rubber (LSR), overmolded and Two Shot (LSR/Thermoplastics) components.
This information has been sourced, reviewed and adapted from materials provided by Simtec.
For more information on this source, please visit Simtec.