Here Stefan Reichle, Market Unit Manager for Parker Engineered Materials Group, Prädifa Technology Division ponders sealing materials for kitchen use.
It is a requirement of most electrical kitchen appliances that the seals they employ are able to prevent the ingress of food residues, water and other contaminants such as flour and dust, and make for easy cleaning. As a result, ongoing research into materials and sealing solutions at the leading technology providers in this segment helps kitchen appliance OEMs meet relevant purity and stability requirements, as well as ensure long life and reliable equipment performance.
Driven by a growth in the number of TV chefs, cookbooks and online recipe channels, the world has witnessed a huge groundswell of interest in cookery. Today, many more people are prepared to ‘have a go’ in the kitchen, albeit with help from the latest appliances.
The modern kitchen is positively alive with technology, from hand-held blenders, mixers and steamers, through to the electric choppers and the latest-generation, fully automatic, app-controlled food processors that are able to almost single-handedly prepare a finished meal. Irrespective of the appliance, they all require seals made of suitable materials.
Take blenders, for example, which are typically used to process nuts, vegetables and fruit, and often help cooks make sauces, milk shakes or smoothies. Here, appropriate seals provide vital protection for the blade units against the blended liquids and solids. These special rotary seals effectively guard the drive shaft, gearbox and electrical system, ensuring appliance safety and longevity.
So what type of rotary sealing solutions are available? Well, appliance designers can choose from simple PTFE washers and rings that press-fit into the housing, which gives the final shape required for their function, to complex sealing solutions made of PTFE, rubber or other polymers with metal housings and additional ‘energising’ elements such as metallic springs.
While good sealing properties and long life are key selection criteria, in a highly competitive market, cost is almost always an additional primary factor. Typically, radial shaft seals with an all-rubber body or a metallic back-up element and spring offer the best price-performance ratio for budget-conscious designers.
Due to rigorous application requirements, seals for electrical kitchen appliances demand the use of rubber compounds with high wear resistance. After all, the blade units of most modern hand-held blenders rotate at up to 10,000 rpm without additional lubrication, while the foodstuffs to be blended also impart an unwanted mechanical influence, particularly ice cubes and nuts for instance, which can lead to the severe damage of incorrectly specified seals.
Thermal loading is a further issue: frictional heat caused by the rotational speed of the blade unit is compounded by warm or hot food such as soup or boiling water that results in an additional thermal load of up to 100°C.
Nitrile rubber
Among the most notable solutions for such demanding applications is a new formulation of NBR (nitrile rubber). NBR compounds are renowned for their mechanical strength, wide chemical resistance and attractive price-to-performance ratio. For example, the new N9400 material from Parker Prädifa, with its intelligent formulation, is one of the purest in its class and meets the requirements of food industry applications.
Important here is the absence of polycyclic aromatic hydrocarbons (PAH) and nitro-samines. Moreover, aside from the prescribed FDA extraction tests in water and n-hexane, the material passes many other extraction tests in media used commonly in the food and beverage industry. In migration tests in ethanol, acetic acid and MPPO (modified polyphylene oxide), the compound also complies with limits that accord to EU Regulation 10/2011 (which focuses on safety in the food supply chain).
Ultimately, seals for these applications must demonstrate good ageing resistance and compatibility with water. NBR seals are normally heat-resistant up to 100°C (short term up to 120°C) and exhibit good chemical resistance, particularly in mineral oils and greases, as well as animal and vegetable oils. They can also be used in cold conditions, typically between -20 and -55°C, depending on the composition of the specific NBR compound.
The need for FKM
In today’s kitchens, however, there are certain applications where even higher temperature and chemical resistance are required. Many of the latest blenders, for example, can effectively be labelled as pots or steamers with a blade unit, offering the capability to prepare complete meals. Some are even app-controlled, facilitated by a built-in heating system. Such appliances clearly make higher demands on the seal in terms of temperature resistance which cannot be met by even the latest NBR compounds.
Instead, appliance designers should look to FKM, a type of fluoroelastomer compound. This material not only offers extremely high temperature resistance and good wear resistance, but provides effective protection against caustic cleaning solutions found in the food industry.
Among the latest solutions is Parker Prädifa’s V9196, a pure material for hot applications. Also free of PAH and nitro-samines, the compound demonstrates the same extraction and migration test performance as N9400. Crucially, however, it is also characterised by good superheated steam resistance, while its high purity renders it suitable for applications in baby food production without any concerns.
In short, wherever seals have to deliver both excellent temperature and chemical resistance, fluoroelastomers are the material of choice, ranking them among the most important sealing compounds of all. Additionally, with FKM elastomers there is normally no shrinkage following exposure to media. Even in the case of heavily swelling mineral oils, any volumetric increase will only be moderate.
As with sealing solutions for all applications in the electrical kitchen appliance sector, making the right choices in material and seal design are imperative. Opting to work with a proven technology developer in this area will certainly pay dividends, as only this can ensure that appliances offer the reliable performance and long life expected by today’s consumers. Ease-of-cleaning is an important secondary benefit that promotes positive brand image in what has become a fiercely competitive marketplace.
