During the last few years, many articles have appeared in GALILEO on sandwich panels with a PUR core. For the first time, this successful series will now be expanded with an article on the theme of “ Sandwich panels with a non-combustible rock wool core”. The proportion of rock wool used as core material in sandwich panels has increased to almost 10% during the last few years, and has become a fixed constituent in building. The use of mineral wool elements is expedient in all cases where particularly high requirements are made on sound insulation and fire protection.

1. Diversity, functionality and quality promote the sandwich market

For more than 30 years, sandwich panels have been successfully used for facades, roofs and ceilings, in Germany and beyond. During this period, manufacturers have developed many extremely diverse surface profiles, coatings and colours, to meet changing market requirements. Today in particular, planners, architects, building owners and processors value the ease of handling of sandwich panels, in conjunction with their outstanding building physics properties. The market for metal sandwich products in Germany exceeded a record 13,000,000 m² at the turn of the millennium. Almost 1 million m² of this consisted of rock wool sandwich panels. Due to their ease of handling and large number of positive product characteristics, sandwich panels will remain an essential element on building sites in the future.

 
 

Fig. 3.3.1 Sandwich panels will remain an essential element on building sites in the future - source [1]

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What makes the sandwich panel so successful?

Firstly, it must be said that, in the bulk of sandwich panels produced today, materials are used which have been proven over decades. Single-layer steel sheets, galvanised, or surface-finished with organic coatings and paints, can be found in any industrial façade today.

The insulating materials used, polyurethane and mineral wool, have been used successfully for decades. To ensure a durable and secure bond between steel sheet and insulating material core, the good adhesive properties of polyurethane are used as foam or as foaming adhesive. Similar bonding agents are also used in automotive body manufacture and in aircraft construction, as an alternative to the traditional metal welding process. This builds confidence.

 

Fig. 3.3.2 Sandwich panels have been successfully used for facades, roofs and ceilings for more than 30 years - source: [1]

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2. An embarrassment of choice

The range is now huge, offering every planner, architect or building owner a multitude of creative sandwich solutions. Whether with a smooth, light or heavy profile, with diverse coatings or paints in RAL or special colours. No limit is set to creativity. Even colour designers such as E-F. von Garnier have successfully taken up this theme.

However, in addition to the "visible components", sandwich panels also differ in function and field of use. The product range extends from wall, roof and ceiling panels through acoustic panels to special panels for shipbuilding or clean room technologies.

During the last 5-10 years in particular, the product range has been extended by the increased development of attractive rock wool sandwich panels. Originally developed and tested for use in fireproof structures, perforated rock wool sandwich panels are now also used increasingly to meet sound insulation requirements. Rock wool sandwich panels with heavier surface profiles have been available on the market since 2003.

 

Fig. 3.3.3 Variable surface structures of sandwich panels - source [1]

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Fig. 3.3.4 Design of micro sandwich panels: Standard, invisible fixing, different surface forms - sources: left [3], right: [4]

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3. Volcanic rock forms the basis

Rock wool has proven itself in fire protection, thermal insulation and sound proofing over many decades. Rock wool is enormously flexible and is marketed in all conceivable forms. As panels or rolls, raw shells for chimney construction, fire door inserts, preforms as fillers, pressed preforms for effective thermal insulation and sound proofing in automotive construction.

The high thermal loadability of the rock fibres that are extracted from volcanic rocks is utilised in almost all fields of application, with a melting point of over 1000°C.

You may already know that similar rock fibres can even occur under natural conditions. These natural fibre products can be found e.g. in active volcanoes (see figure 3.3.5).

Here, molten rock masses are blown out of the eruption crater at such high speed that a fibre forms from small lava droplets.

For almost 100 years, this effect has been used to extract rock wool fibres artificially by hand.

Today, the following principle is used (see figure 3.3.6 - Rock wool spinning chamber - bottom right, and figure 3.3.7 - Schematic representation of rock wool manufacture, on the next page.

 

Fig. 3.3.5 Natural mineral fibre formation from lava droplets during volcanic eruption - source [2]

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Fig. 3.3.6 Technical production of mineral fibres in rock wool spinning chamber - source [2]

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Fig. 3.3.7 Schematic representation of rock wool manufacture - source [2]

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Volcanic rock, lime, mineral aggregates and rock wool recycling material are heated in special ovens (cupola furnaces) by burning coke to more than 1500°C. This process produces a liquid molten rock mass, which is guided onto rotating “spinning wheels” and thrown off at high speed, during which process it is spun. After mixing with an organic binding agent, this wool is pressed in a continuous furnace and cured using a hot-air current. The final result is an endless rock wool strand, which is divided into customer-specific dimensions. The packed products are usually loaded directly into HGVs.

The cooled rock wool then has a typical melting point of over 1000°C, which meets even the highest fire stability requirements, e.g. in building (according to DIN 4102).

4. Symbiosis of steel and rock wool

Paired with fire-proof steel sheets, which have a similar melting point, steel sandwich panels with a rock wool core of upright fibres form a well-coordinated base for stable, fire-retardant (F30, T30, L30, EI30 etc.) and fire-proof lightweight structures (F90, EI 90).

 

Fig. 3.3.8 Diagram: A comparison of melting points of different materials - source [2] Schmelztemperatur in °C - Melting temperature in °C Kunstoffe - Plastics Aluminium - Aluminium Gläser - Glass Kupfer - Copper Steinwolle - Rock wool Eisen - Iron

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Fig. 3.3.9 Diagram: Temperature variation during fire testing to DIN 4102 - source [2]

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5. What happens during fire testing?

The diagram in fig. 3.3.9 shows the temperature developments of rock wool sandwich panels during a 90 minute fire test.

Over a period of at times more than 90 minutes, the rock wool provides almost sole protection against the acting fire load. In hardly any other construction is the insulating material subjected to such prolonged and heavy stress. Usually, in traditional wall systems, thick wall construction materials such as stone, concrete, plaster, cement or derived timber panels provide additional protection for the insulating material.

The outstanding thermal insulation characteristics at high temperatures and the high heat accumulation mass mean that the heat only passes slowly through the insulating material core. Even tests of 150 minutes and more were successfully performed in this way with rock wool sandwich panels. Fire protection can therefore be easily planned with sandwich panels. In “F” classification fire protection tests, both sides are rigorously tested in accordance with the standard curve (see red curve in fig. 3.3.9), while in a “W” test (e.g. testing of bay walls), one side is tested up to 1000°C, and the second side is tested according to the reduced temperature curve at max. 659°C. “F” solutions are therefore a higher quality alternative to “W” constructions, and can be successfully used not only in facades but also in partitions or in room-in-room systems with a fire risk occurring on both sides.

6. Exemplary fire protection with rock wool steel sandwich elements

Fires at several locations in Europe have since many years confirmed the fire-resistant and fire-proofing effect of sandwich panels with rock wool core. Despite intensive flame application, the panels do not contribute to spreading or intensification of the fire, and only release small quantities of smoke. These characteristics are now also reflected in testing to European standards.

Sandwich panels that are classified in building materials class A2-s1d0 throughout Europe have been on the market since 2003. s1 and d0 are additional classifications, and document the smoke behaviour (s) and melting behaviour under flame application (d).

Only the combination s1 and d0 is permitted for non-combustible building materials. Any higher assessment of “s” and “d” results in grading in at least European building material class B and then means e.g. flame-resistant.

The protective action against massive fire has been documented to particularly striking effect after the almost complete burn-up of a food operation in England. In the event of fire, a module that was built around the deep-frying plant was supposed to prevent the fire spreading to the building. The complete opposite happened. The building, together with the stored goods, burned down, while the sandwich cell with its rock wool core remained standing. Sandwich panels with a rock wool core are therefore being used more frequently, e.g. in the internal refurbishment of industrial buildings. A further advantage: the insurance industry also rewards the installation of non-combustible building materials in susceptible buildings with more favourable premiums.

 

Fig. 3.3.10 Table: Meaning of additional class in accordance with prEn 13501, the red-marked areas are permitted for non-combustible products - source [2]

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Fig. 3.3.11 Herten-Forum: F90 façade with Hoesch isorock® sandwich panels; planning: Röhm architecture firm, Münster; installation: Henneck & Grabow - source [3]

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7. Together they are strong

Non-combustible sandwich panels with an A1 rock wool core make sense in cases where traditional PUR sandwich panels can or may no longer be used. Typical examples of use occur in the following sectors:

• Cold store construction with fire protection zones
• Interior refurbishment of existing buildings with fire protection requirements
• Walls for closely developed property boundaries (<5 m distance)
• Partitions in accordance with the Shop Ordinance
• Fire walls in accordance with the new Industrial Construction Directive
• Requirements on building components in accordance with the premium guidelines of insurance companies
• Airport buildings etc.

Sandwich panels with a rock wool core and sandwich panels with a PUR core can also be combined as required, thus ensuring a functional and visually sophisticated façade design, which does not present any thermal bridges in the area. In addition, the products are available with facings in aluminium and glass-fibre reinforced plastics for special applications in the food industry and in clean room technology. Virtually an Eldorado for planners and processors in the light construction industry!