General Properties of Silicone Rubber
– High purity
– Thermal stability
– Resistant to high and low temperatures (-50 to +180C)
– Insulating (heat and electrical)
– Resists natural UV radiation, weathering, ozone and moisture
– Ignites at 430C, low toxicity smoke, silicon dioxide ash.
– High permeability
– Available in Hardness range 10-85 Shore A in general, but with care can be sxpanded to both softer and harder materials.
– Hydrophobic – water repellent surface
– Can be repeatedly steam sterilized at 120-130oC
– Sterilization by Gamma and Beta radiation does not impair quality
– High transparency, easily pigmented
– Good compression set
– Good mechanical properties
– Good release properties
– Good Grip/Anti-slip
– Ease of processing
Silicone Rubber is frequently selected because of the unique properties that can be achieved for specific application by careful compound design.
Physically two main groups
HCR – High Consistency Rubbers-ie. The unvulcanised material being a soft, almost putty-like formable solid compound
Liquid Silicone Rubber- Highly flowable, high viscosity fluids. These are essentially two-part systems, supplied deaerated ready for use often in pre-metered equipment. Low injection pressures and low-pressure forming techniques are sufficient. They cure after mixing the two separate portions. Curing is often complete in as little as a few seconds at temperatures of about 200°C and post-curing is not usually required.
Vulcanisation also named by characteristics eg:
High Temperature Vulcanisation. Usually describing process activated by heat to enable cross-linking eg. via peroxide imitiation and platinum catalysed addition with both routes amenable to control via appropriate compounding to match processing requirements. Compounds cure- a generic word covering the change from plastic to elastic during the forming process- on the application of heat, typically between 100oC to 185oC with cycle times tailored to achieve specific process requirements, significant variations and control can be achieved.
RTV – Room Temperature Vulcanisation. Compound cure at room temperature with cure times measured in minutes (Platinum cure) or days (Moisture activated). Often 2 part materials.
Silicone Rubber can be designed to enhance many properties:
– Mineral oil resistance
– Low flammability
– Metal detectable
– Electrical and heat conductive
– Radio opaque
– High temperatures (up to 300C) Steam Resistant
– Self-lubricating (oil bleeding)
– Low Friction
– Many colours available
Processing of Silicone Rubber can include, Mixing, Moulding, Extrusion, Straining/Pre-form, Calendaring, Fabric Coating, Rotocuring, Rapid prototyping and Post Curing.
The conversion of silicone rubber compounds can involve many processes to produce the finished product. Our wide experience of these varying processes enables us to understand the particular requirements in most cases- discussion is vital.
Our ‘customised silicone rubber approach’ enables us to combine materials and experience to cover a wide range of both processing (conversion to finished product) needs and performance in use. We see our customers finished products performing well in many areas from medical to Mechanical.
When it comes to the production of silicone compounds in the UK, A Littlejohn have a wealth of experience and knowledge. Below is a brief outline of the areas covered; please do not hesitate to contact us for more information.
Our ‘Specialist Silicone Compounding Service’ would typically operate within the following areas:-
– Medical – i.a.w. European / United States Pharmacopoeia e.g.: USP Class VI
– Food Contact Material – i.a.w. Food and Drugs Administration
– Aero Space
– Roller Industries
And the application/services we supply:
– Addition Cure Systems
– Moulding and Extrusion – ‘Lightning fast’ or ‘Slow, let it flow’
– Specialist / Specialised Applications
– Liquid Silicone Rubber
– Silicone Adhesives
– Base Rubber
– Flame Retardant
– Low Smoke / Low Toxicity
– Metal Detectable
It will be appreciated that, because of the range and scope of our expertise in silicone custom compounding, the above can only give an indication of the diversity of our ‘product range’.
There is no substitute to speaking directly with our Technical team to discuss any potential needs. Contact Us
Typically, needs might include: ‘problem solving’; ‘project / product development’; supply of ‘base materials’ etc. and ‘Custom Compounding’, where our expertise in this field is well known.
We are proud of our continuing efforts to maintain a positive response to enquiries and a listening approach to customers needs and aims.
Communicating your requirements:
– It is good for design and applicational engineers to talk needs through thoroughly with compounders and convertors where necessary.
– A wide and open discussion will lead to a good optimisation of compound design. NB. A ‘Literature-only’ approach can often lead to a an over-engineered material that is not cost efficient overall. Open discussion can reveal ‘hidden’ details that will need to be considered.
– The environment in which the finished is produced can be hugely influential on the compound, so all factors need to be considered – improving production, costs and the quality and suitability of the finished product.
– There are compromises that have to be made with all materials and often, with full understanding, compromise can be made that balance all requirements effectively.
Custom compounds can include the following:
– Oil leeching
– Heat stabilised
– Heat resistant
– Abrasion resistant
– Colour changing
– Vibration dampening
– Metal detectable
– Low compression set (Good deformation recovery)
– Flame resistant
– High resilience
– High tear
– Low temperature
– Xray opaque
– Steam resistant
– Colour matching
– Specific finishes; eg grippy or high gloss
And also combinations of the above, the possibilities for Silicone Rubber Compound are endless…….
As a distributor of Wacker product, we are also able to supply base materials in smaller quantities. From 1kg up
High Consistency Rubber (HCR)
General Purpose (R401 Grade)
High Green Strength (R402 Grade)
High Tear (R420 Grade)
Electrically Conductive (R570 Grade)
Oil Resistant (R701 Grade)
Flame Retardant (R770 Grade)
Media Resistant/Low Compression Set (R861 Grade)
Rebound Resilience and Minimal Compression Set (R865 Grade)
Liquid Silicone Rubber (LSR)
General Purpose (LR3003 Grade)