Barrikade - tried and tested...now a commercial reality
TWI Bulletin, September/October 2003
John Fernie studied for a BSc in Metallurgy and Microstructural Engineering, followed by a PhD in Physics. He has more than 15 years' experience in ceramics and ceramic joining. A European Engineer, John is manager of the Ceramics Group at TWI and is also Programme Manager for Renewable and Advanced Energy Systems. He is currently engaged in the development and commercialisation of a number of ceramic-based innovations.
Paul Jackson joined TWI as a project leader in October 2000. Previously he studied for a BSc in Chemistry and Molecular Physics at Nottingham University followed by a PhD in Powder Metallurgy. Paul is currently involved in a number of ceramic joining technologies including brazing and diffusion bonding. Also, he is working on the development of several TWI innovations such as Barrikade, sol-gel coating technology and high temperature materials for applications in the power generation sector.
Barrikade ® is the generic term used to describe a new family of materials developed at TWI. The Barrikade system is a low density, low cost, fire resistant, thermally insulating material; with the added advantage of being bio-inert, even at high temperatures.
Since being conceived and patented at TWI in the late 1990s, considerable effort has been spent on understanding and refining Barrikade in terms of its properties and, in particular, its thermal performance. It is currently being commercialised as an alternative material for the construction and thermal insulation industries. John Fernie and Paul Jackson take up the story and describe Barrikade, its chemistry, properties, applications and potential future advances.
Barrikade was invented at TWI in 1997 and subsequently patented; this platform technology has now evolved to cover a number of products, namely:
Barrikade Loose-fill
The first stage insulation product is an exfoliated vermiculite (typical particle size 2-5mm) coated with Barrikade Binder.
Barrikade Binder*
Any inorganic material, glass or ceramic can be used, but a silicate-based glass ( eg water glass) is most common. The binder can also be used as an adhesive.
*As indentified by TWI as suitable for the manufacture of Barrikade products.
Barrikade Core
The second stage insulation product, for example, a shaped product or panel.
Barrikade Skin
A higher density product usually fabricated as a skin to the Barrikade Core.
The individual nature of these materials will be described later.
In terms of its primary performance characteristics, Barrikade is a low density, thermally insulating material, which does not produce harmful emissions on heating. It has many potential uses in sandwich structures, where traditional materials such as fibreboard are currently in favour. In several markets, Barrikade can be considered as an alternative to such fibrous materials, which are currently under scrutiny for their association with potential health risks. For example, Barrikade is being assessed as a fire door material; sandwiched between two panels, where its main function is to provide fire thermal protection.
Barrikade Loose-fill
The preparation of Barrikade Loose-fill entails coating exfoliated, or expanded vermiculite ( Fig.1) in a binder and then thermally curing between 100-300°C. The coating process is illustrated in Fig.2, with Fig.3 showing scanning electron microscope (SEM) images of the uncoated, exfoliated vermiculite and the Barrikade Loose-fill material. The platelet structure of exfoliated vermiculite is clearly shown in Fig.3a and the coating of the vermiculite with a binder does not significantly degrade the important structure, Fig.3b. The coating is present to control the infiltration of the binder during manufacture (and hence the final density).
Fig.1. Exfoliated, or expanded vermiculite
Fig.2. The production of Barrikade Loose-fill
Fig.3. Scanning electron micrographs: Fig.3a) Uncoated vermiculite; and
Fig.3b) Coated vermiculite, ie Barrikade Loose-fill
Although exfoliated vermiculite is available in a range of particle sizes, a range of 2-5mm is recommended for Barrikade applications.
Barrikade binders
Any glass or ceramic material may be used as a Barrikade Binder and some of the alternatives are described later. The binders are selected in terms of their fitness for purpose,
eg cost effectiveness against performance. However, the majority of the work to date has used a sodium-silicate based water glass.
Water glasses comprise relatively simple mixtures of silica and alkali metal oxides such as Na 2O and K 2O, mixed with water. (The relative amounts of these additives in the water glass must be controlled in order to retain a stable solution). The water glass is then set via the removal of the water, which produces a solid glass.
It is important to understand the nature and role of the binder. Initial work has used sodium-silicate based water glass and has produced a system with many attractive qualities. However, the interaction of water glass with water, ie dissolution, has proved to be a particular issue, with both advantages and disadvantages. Methods of controlling the properties of water glass, and/or even using other binders, are described later.
Barrikade core
Barrikade Core is prepared by bonding together quantities of Barrikade Loose-fill with further quantities of Barrikade Binder, moulding to the desired shape and then applying a final heat cure in a porous mould at a temperature above 100°C. This is illustrated in
Fig.4.
Fig.4. Production of Barrikade Core
Figure 5 shows a scanning electron micrograph of the bond between two particles of Barrikade Loose-fill after the fabrication process. The binder (water glass in this example) has wetted and flowed along the boundary between the particles, bonding them together at the points of contact.
Fig.5. SEM image showing the bond between two particles of Barrikade Loose-fill
In addition, initial studies have shown that Barrikade Loose-fill can be sprayed in conjunction with Binder, with panels of up to 20mm in thickness deposited on to a range of substrates.
Barrikade skin
The surface of Barrikade Core material has a rough and particulate appearance. When Barrikade is used as a component part of a system, a wood or metal laminate may be bonded to the surface using a Barrikade binder.
In some applications it may be preferable to use a non-combustible, low thermal conductivity skin material. For this specification, Barrikade skin was developed comprising fine vermiculite flour (sub-mm) and Barrikade Binder. These materials are mixed into a slurry and then dried into sheets of up to 5mm in thickness and cured as described above. The skin material can then be attached to the Barrikade Core surface using the Barrikade Binder in a similar manner to other laminate materials.
Water glass bonded Barrikade core
The Table shows some typical properties of Barrikade Core (with water glass as the binder) and how these compare with other thermally insulating materials. The data is comparative rather than exhaustive - there are many available materials.
Table: Comparison of various thermal insulation systems
| Thermo-mechanical | Property | Barrikade ® | Ceramic foam | PU foam |
| Density, (kg/m 3) | 250-350 | 130-250 | 40-150 |
| Thermal condition, (W/m.k) | 0.10 | 0.06 | 0.02 |
| Flex strength, (MPa) | 0.3 | 0.31 | 0.35 |
| Compressive strength, (MPa) | 0.2-0.3 | 0.10 | 0.15-0.50 |
| Application specific | Impact resistance | Good | Good | Good |
| Fire protection | Excellent | Excellent | Poor |
| Health & safety concerns | Low | Medium | High |
Relative cost
20mm thickness panels | Low | Medium | Low |
In terms of thermo-mechanical performance, the table shows that all three 'families' are similar. Unless ultra-low density or thermal conductivity are of paramount concern, there is very little to differentiate these materials.
However, Barrikade Core comes into its own when its techno-economic performance is taken into account. At a relatively low cost, it offers very good fire protection (with no emissions on heating). In addition, the health and safety issues arising from the use of Barrikade are lower than either ceramic or polyurethane foams ie there are no concerns regarding particulate inhalation during installation and decommissioning, or toxic emissions above 200°C.
Like all thermal insulation materials, Barrikade Core has modest mechanical strength. Hence, it will most likely be used as part of a system (sandwich structure) where it provides the thermal insulation and another material provides the mechanical support.
Thermal performance
It is important to understand why water glass-bonded Barrikade Core is not only a good thermal insulator, but how it performs in a range of environments. The thermal performance of Barrikade has been measured using two techniques; thermal gradient and isothermal tests.
The thermal gradient test is shown in Fig.6. Barrikade Core is placed in the entrance of a furnace and the temperature held at 925°C, which is similar to the temperature used for the SOLAS (safety of life at sea) fire protocol. The back face is exposed to air and the temperature of this face recorded using several thermocouples.
Fig.6. Thermal gradient test
A typical thermal trace is given in Fig.7 where it can be seen that the cold face temperature reached a steady state of 200°C over a thickness of 45mm. Macrographs of the hot and cold surfaces of the Barrikade material are shown in Fig.8. This shows that the surface of the hot face has degraded only slightly during the test.
Fig.7. Typical result from a thermal insulation test
Fig.8. Results from thermal gradient test showing effects on the hot and cold faces
The isothermal test involved placing small samples of the same Barrikade Core material (50mm cubes) into a furnace and raising the temperature of the whole piece of material. Isothermal tests were carried out over the temperature range 700-850°C to investigate the effect on Barrikade. Macrographs of these test results are shown in Fig.9. The figure shows that when the Barrikade material is held at ~700°C the material is stable and unchanged in appearance. However, when the temperature is increased towards and then beyond the glass transition temperature (T g), of the water glass binder (~700°C), the Barrikade sample begins to slump. This is shown clearly in a sample heated to 950°C ( Fig.9).
Fig.9. Isothermal test results
The slumping can be avoided by replacing the water-glass with another inorganic material such as metal phosphate. The use of such alternative binders is discussed later.
Moisture resistance
Barrikade Core comprises mixtures of exfoliated vermiculite and inorganic binder. When the binder is water glass, the moisture resistance of the material may be expected to be low, which is very useful for recycling. The Barrikade Core can be simply immersed in water, thus dissolving the binder allowing both this and the vermiculite to be separated, dried and reused.
However, there are certain applications when Barrikade must be moisture/water-resistant. For example, when the material is used in damp or humid environments, such as those in the marine industry. In many of these applications, the Barrikade Core will be used within a sealed system, preventing the moisture reaching the Barrikade. However, for situations where this is not possible or further resistance is warranted, several methods of improving moisture resistance have been evaluated. These are based on additions of insoluble metal salts to the binder and also the use of a carbon dioxide atmosphere during manufacture.
It is reasonably well documented that adding zinc oxide to water glass improves moisture resistance. Zinc oxide was added to the binder and the best results were obtained with an addition of 7wt%. This addition was incorporated into both the pre-coat and the moulding stages of the Barrikade process.
Carbon dioxide treatments were used during the curing of the final moulded panel only. Barrikade Core was held under a carbon dioxide atmosphere for approximately 15 minutes and then cured in the usual manner. In addition, Barrikade Core panels were fabricated using a combination of both the zinc oxide additions and carbon dioxide curing treatments.
Moisture resistance was assessed using several techniques including water immersion and humidity testing. The results from a one-week 45°C/98% humidity test are given in Fig.10a-d. These macrographs show that the unmodified Barrikade Core material has the lowest water resistance ( Fig.10a). The binder has dissolved leaving loose vermiculite particles. Figures 10b and 10c, respectively, show that using zinc oxide and carbon dioxide significantly improves the moisture resistance of Barrikade. The materials retain mechanical integrity. The use of zinc oxide and carbon dioxide together further improves the moisture resistance of Barrikade, Fig.10d.
Fig.10. Barrikade core after humidity testing: Fig.10a) Untreated;
Fig.10b) With ZnO treatment;
Fig.10c) With CO 2 treatment;
Fig.10d) With both ZnO and CO 2 treatments.
Analyses carried out jointly by TWI and the University of Cambridge indicate that the zinc oxide partly dissolves into the water glass and the effect of the CO 2 is to convert part of the glass into a more ceramic-like material based on sodium carbonate.
Alternative binders and modifications
The properties of Barrikade can be tailored through the use of different binders, altering particle size, addition of other materials and modifications to the processing route. For example, the role of T g has been discussed and there are various glasses each with their own specific T g, which can be applied as binders. There are also families of glasses, which can be heat treated to form crystalline materials; known as glass-ceramics. These materials offer a wide range of controllable properties such as thermal expansion and enhanced upper usage temperatures.
Another range of potential binders is based on phosphate ceramics; these materials are very refractory and retain their mechanical integrity above 1350°C - the approximate melting point of vermiculite.
Secondary and tertiary materials can also be added to alter the thermal expansion and/or mechanical performance (including strength and fracture toughness). A good example would be the addition of fibres to the mix. Dyes can also be added to either enhance the aesthetics of Barrikade or act as a quality control check.
Current and future applications of Barrikade
With its excellent thermal protection properties, Barrikade is initially being targeted at two specific markets.
Buildings
- fire doors
- interior and exterior panels
Transportation
However, there is a wide range of other potential applications. These include furnace construction, shielding and sound applications, as well as other miscellaneous thermal insulation systems.
Barrikade Core could be used in the furnace/heat treatment sector:
- Furnace linings for heat treatment
- Furnace linings for liquid metal containment
- Furnace/kiln furniture
Tailored properties, additives or modifications to Barrikade (Core, Loose-fill or Skin) could expand its uses for other functional performance:
- Acoustic properties/sound attenuation
- Electromagnetic shielding
Summary
Barrikade is now a tradename for a range of products covering the Binder, first and second stage insulation products (Loose-fill and Core respectively) and a skin material. Invented, patented and developed at TWI, it meets an industrial need for a low cost, lightweight thermally insulating material, with no health and safety issues. The properties can be tailored to meet a wide variety of applications.
Barrikade is now commercially available and is being licensed to a number of companies.
Acknowledgements
This article summarises the work of a number of people and we particularly wish to acknowledge the work of Paul Burling and Steve Mycock. We would also like to acknowledge the contribution of Russell Goodall and Professor Bill Clyne at the University of Cambridge, who are working with TWI in the characterisation and optimisation of Barrikade.
