Is Cork Flammable?

In this, article we will answer the following: “Is Cork Flammable?”, and also other important questions about the nature of cork, where it comes from, how it looks closely, and some properties.

Is Cork Flammable?

It depends. Cork can ignite but is considered a fire-resistant material. Chemical, physical, and manufacturing processes explain how they can both work as insulator but still catch fire under specific conditions.

What is Cork?

Cork is the bark of the oak tree. It is periodically harvested from its surface every 10 years, more or less.

Cork production doesn’t require that the tree is chopped down and is considered an environmentally friendly practice.

The tree (named Quercus suber L) has slowly growing, and flourish in some specific areas of the Western Mediterranean and China. It requires a good amount of sunlight, low rainfall, and high humidity.

Cork stoppers are used to close wine bottles due to their impermeability to water and air, compressibility, resilience, and chemical inertness.

It is also used to produce insulation materials.

Portugal is the major cork producer in the world.

Oak characteristics

The cork tree has an interesting capacity of creating a voluminous structure. It is formed in its inner bark by the action of a tissue called phellogen, which is capable of forming new cells.

We can compare this tissue with our own biology. Cork would be something like our own epidermis, like the dead skin cells we need to scrub to get rid of. 

Cork produces this suberose tissue in basically three different ways: virgin cork, reproduction cork from the second stripping, and from subsequent strippings.

Cork is therefore a protective layer made of dead cells from the phellogen tissue. This is capable of good cell generation, so the new cells keep being added to the skin underneath the dead cell layer.

Cork serves as a barrier that keeps the atmosphere and the cortex of the stem apart, but it also has pores that allow the transference of water and gases along the tree, called lenticular channels.

Therefore, cork extractions are not harmless to the tree because cork serves purposes for the plant. Good extraction procedures, although, always aim to reduce the impacts and enhance productivity in the long term.

Cork structure and chemistry

Cork can be described as a homogeneous tissue made of thin-walled cells, analogous to a honeycomb, without empty spaces between its contiguous cells.

It’s made of lignin, suberin, wax, cellulose, hemicellulose, and other minor constituents.

The cells can be described as rectangular prisms aligned to each other. Specific cells arrangement can vary from tree to tree.

Cells are normally closed and hollow, and the air within is presumed to be similar to air.

Source: Silva et al, 2005.

The same tree can present different cell arrangements depending on the season and the time that it was allowed to grow.

Cork’s chemical composition depends on several factors, especially the geographic region where the tree is based, the climate and soil condition, its genetics, dimensions, age, and growth conditions.

Cork consists of lignin, suberin, wax, cellulose, hemicellulose, and other minor constituents.

Insulation Corkboard

Cork is used for insulation applications because of its airy nature.

It’s also resistant to chemical and biological agents.

It has 3 basic applications: thermal insulation, acoustic absorption, and vibration damping. These make it suitable for several applications

Corkboard is made of agglomerates of granules of cork, it’s manufactured using specific machinery called autoclave at a temperature of 300°C, and without the use of adhesives. 

A thermochemical degradation of the cork cell wall is induced, with the previous expansion of these granules. Once the degradation takes place, the byproducts bond themselves like natural adhesives.

The cell walls suffer many changes during the process. As a result, it loses weight due to the loss of extractable compounds like cellulose. The suberin and wax compounds then deposit on the cork granule surfaces.

Average property values of insulation cork aggregates:

  • Density (kg m–3): 100–130 {low}
  • Working temperature (°C): -180 to 110 {wide}
  • Thermal conductivity (20°C, kJ m–1 s–1 K–1 ): 4.1×10-5 {low}
  • Specific heat (20°C, kJ kg–1 K–1): 1.7 –2.1 
  • Thermal expansion coefficient: 40×10–6 {low}
  • Permeability to steam (kg Pa–1 s–1 m–1): 4.2×10–12 to 12×10–12 {low}
  • Tensile strength(MPa): 0.05 
  • Compressive strength at 10%, (MPa): 0.25
  • Poisson’s ratio: almost zero {good}
  • Bending tension (kN m–2): 1. 6×10–4

Cork Flammability

So Cork is widely used as an insulator, to prevent firings from happening. But can it catch fire?

Industrial transformation of cork normally generates many cork residues, and they are used to feed kilns due to their high flammability.

So yes, cork materials can ignite. But is also a fire-resistant material.

They just won’t do it normally because the heat takes a long time to spread in cork materials, due to its low thermal conductivity. For a big piece of cork to ignite it requires a big fire with direct heat.

Small cork pieces or powder, although, are flammable.

So, the low flammability of cork is due to its manufacturing process, rather than its composition.

A corkboard insulation material would still be capable of burning, but it would happen slowly. Many fire-resistant materials work like that.

If under fire, Cork doesn’t liberate harmful gases or vapors, unless it is not made of cork alone.

Ignition

It’s hard to say something that works for all cork products, but overall cork can ignite if it reaches a temperature around 150ºC / 300°F.

But cork scraps could burn much earlier.

In order for the cork to ignite it requires a direct source of intense heat or fire.

As explained before, cork can ignite much easier if it’s in the form of scraps or powder, but even these must be maintained close, like in a pile, and direct heat must be provided. Only then ignition would have a chance of taking place.

Specific products

In this section, we will present some flammability information about specific products made of cork, as well as the link for their safety data sheets.

Flooring material: according to this, the material can catch fire but doesn’t possess a fire hazard.

Cork Floating Floors: has low concentration of volatile compounds. Dust of fines eventually produced may present a fire hazard.

530 Eurosafe Cork: no applicable flammability properties according to the fabricant.

VersaCork: autoignition at 175°C. Dust clouds from wood and cork constituents of this product can present a strong explosion hazard.

Granulated Cork: used for spill absorption. Autoignition point above 300ºC.

Cork Expansion Joint: no specific flammability hazard was presented.

But all the above are only possible if conditions that allow fire to happen are provided, and if the specific instruction from the fabricants are not followed.

Conclusion

Cork can be considered a fire-resistant but flammable material. As long as it is kept away from intense heat or fire sources in your home or office, it is safe.

Frequently Asked Questions (FAQS): Is Cork Flammable?

Is cork sustainable?

Yes, it is considered sustainable since the Oak trees used to produce it are not chopped down.

But there are standard extraction procedures that should be followed so the tree is not harmed.

Will cork burn in the oven?

Yes. Cork will burn in the oven. The stove would create an atmosphere that’s very dangerous. More heat could arise from the burning of cork and an accidental fire could happen.

Is cork toxic?

No. Cork doesn’t contain any materials that are toxic to us. Even after a burning (or attempt of burning) toxic vapor and gases would not emerge, not more than the usual burning of wood.

Is cork waterproof?

It presents some waterproofing capabilities but overall, is not waterproof. The bark’s structure presents small gaps that could serve as an entrance for water, and leaking could appear.

Citations

Silva, S. P., Sabino, M. A., Fernandes, E. M., Correlo, V. M., Boesel, L. F., & Reis, R. L. (2005). Cork: properties, capabilities and applications. International Materials Reviews, 50(6), 345–365. doi:10.1179/174328005×41168 

https://en.wikipedia.org/wiki/Cork_(material)
https://en.wikipedia.org/wiki/Cork_cambium

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