6 Best Food-Grade Rubbers, Their Uses & The Most Common Additives
Polymers used in food contact rubbers
Different rubber products require different rubber types. One of the most important criteria used in selecting a particular rubber for a specific use case is its temperature resistance, although other properties such as chemical inertness, the physical properties of the resulting product (e.g., tensile strength, abrasion and tear resistance) and additives are also considered.
Food-grade rubbers – rubbers that come in contact with consumables – must follow strict standards and officially be approved by the FDA, assuring chemical compatibility and non-toxicity. Food-grade rubbers used in food processing equipment must also withstand speed and abrasion demands during manufacturing.
In this article, our goal is to provide an overview on the most important classes of rubber used in the food industry for various use cases such as manufacturing dairy products, soft drinks, in breweries, abattoirs and meat processors, vending machine dispensers, for food canning and packaging.
Natural rubber – for aqueous foods at low temperatures
Food-grade natural rubber compounds are mainly used for
However, it is coming under pressure from synthetic polyisoprene and other rubbers (e.g., thermoplastic rubbers and silicones) due to the increasing incidence of protein allergies. In food processing equipment natural rubber products will be found in
belting and hosing products,
sometimes in blends with other rubbers such as styrene-butadiene rubber (SBR).
These rubbers are typically used with aqueous foods under flow or short term static conditions at low temperatures (<40 °C). The maximum temperature limit for the prolonged use of these products is around 80 °C.
Nitrile rubber – better aging up to 120°
Nitrile rubber is widely used in compounds designed for
in hoses for both aqueous and fatty foods.
In particular, dairy hosing and milk liners are normally manufactured in nitrile rubber or nitrile rubber blends (e.g., with SBR).
Nitrile rubber withstands heat ageing better than natural rubber and so the maximum continuous use temperature is higher at 120 °C. In practice, most applications involve flow or short term static conditions at temperatures below 40 °C.
Ethylene-propylene rubber – for aqueous foods at higher temperatures
The principal use of ethylene-propylene rubbers (EPDM or EPM types) is in the manufacturing of heat exchanger gaskets. When cured using peroxides, these materials can be used for extended periods at up to 150°C.
Normal conditions of service are high temperatures (<130 °C) and flow or static exposure to aqueous food products (e.g., beer).
Fluorocarbon rubber – for prolonged use at high temperatures
There are a number of sub-grades of fluorocarbon rubber (copolymers, terpolymers and tetrapolymers) and they are used in applications where the temperatures would degrade ethylene-propylene rubber products.
They are able to withstand prolonged use at temperatures up to 200 °C. Typical conditions are high temperature (<150 °C) gaskets under flow or static conditions, in contact with aqueous or fatty foods (including oils).
Silicone rubber – for both high and low temperatures
Most silicone rubbers used in the food industry are based on polydimethyl vinyl silicone and these materials have very good high and low temperature properties.
It is their high temperature resistance that enables them to be used for
For example, silicone rubber is used in beverage vending machines up to 100 °C. Cold cured silicones are used as release coatings on items such as food transportation belts and for sweet moulds.
There are a number of different thermoplastic elastomers, but they are all cross linked at room temperature. Usually due to the presence of ‘physical crosslinks’ formed by part of the matrix being below its glass transition or crystalline melting temperature, but become thermoplastic at processing temperatures (e.g., >150 °C) and can therefore be processed in the same way as plastics.
The fact that they are not thermoset materials affects their working temperature range and restricts it to less than 70 °C. Thermoplastic elastomers (TPEs) are used in a variety of food contact products, for example
flexible lids (styrenics, e.g., SBS or SIS),
tubing (particularly polyurethane types).
Other types of TPEs that include olefinic blends of polypropylene and ethylene propylene rubber, polyamides and polyesters.
Other types of rubbers used in the food industry
In addition to the main groups of rubbers covered above, there are also a number of other types that are used in the food industry. These include:
Butyl rubber – used for articles such as stoppers and seals in contact with aqueous foods
Polychloroprene rubber – used in articles such as conveyor belts for food transportation
Acrylic and Hydrin rubbers – specialty materials chosen when the food/contact conditions combination requires their specific properties (e.g., chemical inertness combined with relatively good heat stability).
Finding the right compound is critical for food industry applications
Additives used in food contact rubbers
Additives need to be incorporated into the compound to achieve the desired processing and final properties.
Plasticisers/Process Oils and Fillers
For plasticisers and process oils – to e.g., reduce hardness and tensile strength, and increase elongation – there is a wide range of substances to choose from, among other:
Fillers can be regarded in many ways as having the opposite effect to plasticisers; they increase compound viscosity and hardness and reduce elongation. Principal filler for rubber is carbon black, other fillers such as silica and the silicates also improve properties, but a number (e.g., calcium carbonate) are mainly used to just adjust hardness and reduce cost.
Curatives and Antidegradants
There are two main classes of curative used in rubber – elemental sulfur and peroxides. Other types of curative are used in rubber, e.g., sulfur donors, amines, metal oxides. All these types can be used with food contact rubbers.
A number of compounds (called accelerators) are used to modify the chemistry of a curing reaction to ensure that a rubber achieves a good state of cure in a reasonable time at a convenient temperature. Associated with sulfur cures, e.g.,
Typical cure co-agents:
zinc oxide and stearic acid for sulfur systems,
triallyl cyanurate for peroxides.
Antidegradants: three main classes of antidegradants are used in rubber – antioxidants, antiozonants and UV stabilisers. There are a number of antidegradants available, the two main classes being amines (staining) and phenolics (non-staining), and a mixture of two or more is often used to confer maximum protection. Other types such as thioesters (mainly used to stabilise the base rubber), phosphates and micro-crystalline waxes can also be used.
Prevulcanisation inhibitors are used to reduce the possibility of cure occurring during the mixing and forming stages.
Coupling agents help to promote filler-rubber interactions, deactivators (e.g., polyethylene glycol) to stop accelerators becoming adsorbed onto the surface of polar fillers such as silica.
Bonding agents assist fabric to rubber interactions in composite products.
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