Using Barrikade® - the practical approach

TWI Bulletin, July/August 2000

Paul Burling obtained his Higher National Certificate in Production Engineering from Cambridge College of Art and Technology. He has extensive experience in the project management of large commercial projects worldwide. These include in depth knowledge of composite material for military and commercial applications.

Sandwich structures for the engineering and construction industry have been used for many years. The sandwich structure consists of a laminated material in which the outer skins are relatively thin, strong and dense, whereas the cores are usually weaker than the skin material. As Paul Burling explains the object is to obtain a structure which is strong and stiff as a result of its design and construction, but at the same time is light in weight, and has other desirable attributes which are not achievable when using the materials separately.

There has been a growing interest in the opportunities of lightweight composite sandwich structures for many applications. Architects have always been creative, looking for new ideas and materials, always wanting to produce structures that challenge the current materials available.

Sandwich structures, such as plywood and ply metal have been used extensively, therefore, the belief is that Barrikade fabrication should not present too much of a problem for the construction industry.

This article addresses the fabrication and joining methods that could be used when using Barrikade core, board and panel products.

Background

Barrikade® is a low density, fire resistant inorganic material, consisting of vermiculite particles (aluminium iron-magnesium silicate resembling mica in appearance) and a sodium silicate blend binder. (In principle any inorganic binder could be used).

In its unconsolidated form Barrikade is pre-coated vermiculite granules/particles, Fig.1.

To understand the make up of Barrikade it is important to explain what vermiculite is and the typical binder.

Vermiculite

Vermiculite is a mineral similar to mica that exfoliates or expands when heated. Vermiculite is the geological name given to a group of hydrated lamina minerals.

Crude vermiculite is found in various parts of the world, and in its commercial form consists of thin golden brown flakes classified into grades. Within the internal structure of vermiculite lie water molecules, which when rapidly heated to high temperature, transform into steam causing the vermiculite particles to increase volume. This process of thermal exfoliation produces a lightweight product, which finds uses in various construction products, agriculture, horticulture and other industrial applications. In its exfoliated form it has a low density and the biological inertness of expanded perlite, while maintaining a chemically active surface.

The term exfoliated is used when layers within the vermiculite structure are separated from each other in a direction roughly perpendicular to the silicate sheets. This term is used in preference to the non-directional behaviour described by expansion (such as perlite). The physical thickness can be altered by both thermal exfoliation and chemical exfoliation.

Commercial vermiculite is generally marked as concentrates of size particles, with an average grade of purity of 90% by weight. Deposits of vermiculite may sometimes contain zones of almost pure vermiculite but vermiculite ore is normally a mixture of vermiculite and gangue minerals. Common gangue minerals include pyroxenes, magnetite, feldspar and aptite.

The structure of vermiculite is formed by the surface weathering of rocks containing large crystals of mica known as iron-bearing phlogopite and biotite. The difference between iron bearing phlogopite and biotite is simply the amount of iron substituting magnesium in the phlogopite structure.

Exfoliated vermiculite is ultra lightweight, inert, non-combustible, insoluble in water and all organic solvents. Vermiculite deposits are to be found in many parts of the world; South Africa, Australia, China, India, Russia, Morocco, North America, Canada to name a few.

From the current active vermiculite mines alone there are sufficient reserves to last more than 50 years. The unexploited reserves are vast, exceeding 200 million tonnes.

All these deposits, although very similar, do have slightly different chemical composition and physical properties and a range of grade sizes is available.

Barrikade binder

The Barrikade binder is essential if Barrikade core material is to be manufactured. Any silicate binder or other amorphous inorganic binder could be used in principle. The Barrikade binder consists of a number of silicates blended together to provide the bonding performance required for both loose and consolidated Barrikade products.

The Barrikade binder can also be used as an adhesive to adhere machined sections of Barrikade core together.

Barrikade high temperature adhesives

Using the Barrikade binder in conjunction with various inorganic fillers a high temperature adhesive can be produced. The fillers act as a toughening agent and help elevate the operating temperature. There are a number of variations depending on what needs to be bonded. Barrikade high temperature adhesives cover the temperature spectrum of 350-1400°C.

Barrikade high temperature coatings

The mixture between fillers and the Barrikade binder can be modified to produce a coating which can be plastered onto the surface of the Barrikade core. The greater the amount of filler, the lighter the coating, but, the surface can be coarse. This surface can be machined or rubbed down using abrasive paper to produce a smooth finish. Alternatively a laminate can be produced separately and adhered to the surface of the core.

The process in brief

Barrikade can be processed in various ways including, pressing, moulding (and potentially spraying), allowing both cores and protective skins to be manufactured. Before any machinable material can be used the core product needs to be made.

Producing the core is a two-stage process. The first stage consists of coating exfoliated vermiculite with Barrikade binder. This can be achieved in a variety of ways but the preferred method involves applying the Barrikade binder in the correct quantities to the exfoliated vermiculite using a mixer. (This can be carried out using a cement mixer if required). The coated exfoliated vermiculite is then cured, typically by placing in an oven at 100°C for approximately one hour.

Once the coating has cured, the exfoliated vermiculite can then be collected and bagged.

The pre-coated exfoliated vermiculite has a hard texture and is golden brown in colour. This is the most basic form of Barrikade and is now available from a manufacturer bagged and ready for use.

To produce a core product additional Barrikade binder is added to the pre-coated vermiculite. The mix ratio of Barrikade binder and vermiculite will determine the overall density of the core and its strength.

Laminating Barrikade core (sandwich structures)

The main advantage in laminating Barrikade core is that it enables performance to be achieved which would be impossible or at any rate very unreliable if carried out using the core product on its own. However it must be stated that machining the core is easy and units can be built up similar to the construction of the box, Fig.2. This box although self-supporting is vulnerable to damage and therefore needs reinforcing with a laminate or coating to protect it.

By using laminates, products can be built up in a convenient way which will provide maximum strength and be free from visible joints.

Laminates/skin materials

There are a wide variety of materials that can be used, aluminium steel, wood, plastics, composites, cloth, and also Barrikade laminates. The choice of laminate and skin materials, is dependent on the final product and its operating function. Using the Barrikade high temperature adhesives will allow bonding of all the materials stated above with varying performance characteristics. The coefficient of expansion needs to be understood if aluminium and steel are to be used in areas where the product will be subjected to high or fluctuating temperatures. To overcome this problem, a laminate can be manufactured using a Barrikade coating. This material is manufactured in a similar way to the core product but because the laminating material is more dense and consolidated together tightly it is more water-resistant.

Reinforcing the laminating material is possible using, glass, carbon, kevlar, cotton and other fibrous materials. Various thicknesses can be produced depending on the performance of the final sandwich structure, Fig.3.

Manufacturing of board materials

Caul laminating is where two laminates are prepared to bond to the Barrikade core using caul plates. Wood or aluminium caul plates are heated thoroughly right through, and are clamped down over the laminate so that the latter is pressed tightly down using a typical pressure of about 1bar. A sheet of paper placed beneath the caul prevents the latter from adhering. As the Barrikade adhesive must be forced from the centre outwards so that it can escape at the edges, either the caul is made a trifle round, or cross bearings slightly rounded in their length are clamped over it so that the pressure is applied at the centre first.

Presses

Only large workshops can install big presses, but there are smaller hand presses which are comparatively inexpensive. The choice is largely one of economics.

Hand presses

These would typically press small structures capable of being made up to boards of a half metre square. The principle is similar to that of the old fashioned wooden press for papers in which pressure is obtained by means of a large wooden screw. The device may be a press pure and simple, or it may have means of heating the plate or caul.

Machine presses

There are many types of these, all of which are capable of being controlled thermostatically to enable them to deal with the many bonding methods for attaching the skins.

Those intended for flat work are the multi plate type so that many panels can be pressed in one operation. Pressure is usually hydraulic, with heating by means of hot water or steam which can be circulated through the plates and chilled as required. When producing laminating boards it is important not to remove a laminated board from a hot press, this may cause bowing and delamination. In addition to this, a board needs to be balanced having laminates of equal thickness and density placed either side of the Barrikade core, although in some special cases one side laminating can be achieved.

To produce shaped work a vacuum press can be used. One advantage of this method is that no specially shaped caul is needed. Briefly, the core, with the adhesive and skin, is laid in position and is placed onto a bed. A stout rubber sheet is drawn over the whole surface, and an air pump withdraws the air from beneath so that atmospheric pressure forces down the rubber sheet (approx 1bar), thus pressing the laminate down onto the core. Assemblies made by this method can be placed in an oven to speed up the cure time or left at ambient temperature. The rubber must be used in conjunction with a breather fabric to absorb the moisture during curing.

Solid cauls

For comparatively small work complicated shaped solid cauls can be used. Using pre-shaped high density plaster, the laminates can be placed between the cauls and pressed into shape using clamps. A piece of felt and breather fabric will allow equal pressure throughout.

Fabrication using boards 'joints'

If aluminium or steel skins are to be used then standard equipment for folding the skins can be used to help in reducing tooling costs. An example could include traying up all four edges of the skin and then filling the tray with mixed uncured pre-coated Barrikade or alternatively 'bonding in' a cured Barrikade core using the Barrikade adhesive.

Post fabrication of a sandwich panel is possible using a brake press and a swan neck tool. Tight forming of flat panels can take place by removing sections of the back skin and Barrikade core. The actual dimensions will depend on how far the panel will be folded to allow clearance for the press brake tool. Once the material has been folded into shape, a doubler, manufactured from the back skin material can be bonded and riveted into place. Rivets are only used as a tooling aid. Internal and external folds can be achieved without causing any detrimental effect to the Barrikade sandwich panel.

Typical aluminium coated material used for cladding sandwich panels is aluminium with a 25µm polyvinyl fluoride PVF2 coating, Fig.6 and 7. This material can withstand a temperature of 130°C. The workability of the coated aluminium is sufficiently flexible to withstand a 2.5T bend radius (T being the thickness of the aluminium sheet), Fig.8 and 9.

Using other materials such as wood or Barrikade plaster/laminate as the skin, this post fabrication is not possible.

A feature common to laminated boards is that the edges are open and it is not always practical to cover these in the laminating process. Therefore suitable joining methods must be applied.

Fabrication of Barrikade laminated boards is straightforward, using many of the methods and techniques borrowed from the woodworking industry. Barrikade laminated boards are cut into finished panels with traditional tools such as saws, routers, drills. Fixing positions can be made by bonding and the use of inserts. Individual panels can be joined into assemblies using extrusions, or tongue-and-groove joints. Figures 10-12 show how edges can be closed and joints produced when using Barrikade laminated products.

Handling and safety precautions

When handling and fabricating Barrikade laminated boards it is advisable to wear disposable rubber gloves throughout the entire operation. This reduces any contamination to the work and any skin irritation. Avoid breathing the dust generated by cutting operations by always using a suitable breathing mask.