Cigarette manufacturing is a high-speed process which puts certain demands on the mechanical properties of the cigarette paper. The structure of the cigarette paper also affects the combustion mechanisms that occur during both smouldering and puffing. This, in turn, can significantly affect the smoke chemistry and other performance characteristics of the cigarette.
The principal ingredient of cigarette paper is cellulose fibre, derived mainly from either wood pulp or flax. It usually also contains about 20% to 30% calcium carbonate as filler pigment and usually up to about 2% of burn additives - salts like potassium or sodium citrate that affect the burn rate of the paper.
The combination of the filler and the burn additives gives a white, coherent paper ash after the paper is burnt.
Fibre processing during paper manufacture gives the paper its inherent or natural porosity - influencing the burn rate of the cigarette and consequently the mainstream and sidestream smoke yields.
In addition, burn additives that promote faster burning may result in lower machine measured mainstream ‘tar’ yields of the cigarette by reducing the puff number. ‘Tar’ yield per puff is not increased by burn additives.
The scanning electron micrograph (SEM) images below show a typical structure of the cigarette paper.
Air permeability and natural porosity are the two parameters used to describe and characterise the porous structure of cigarette paper.
Permeability is measured using an internationally defined standard, e.g., CORESTA Method No. 40 whilst porosity is usually measured by a technique called mercury porosimetry.
Porosity is more closely associated with the diffusion capacity of the cigarette paper than air permeability. This is due to the physics that controls air flow through pores of different sizes.
Electrical or laser perforation is sometimes introduced to further enhance the permeability of the cigarette paper and to offer increased air dilution during smoking. In contrast to natural permeability, electrical perforation has a minor effect on oxygen diffusion through the paper and consequently does not change the burn rate of the cigarette paper.
The phenomenon becomes more complex as the porosity (and permeability) of the paper changes when the paper is being heated during smoking.
Below is a box-plot of a statistically based central composite design involving three cigarette paper parameters, the amount of fibre, amount of filler and the permeability.
The study found that all three parameters could have an impact on smoke chemistry. Data analysis indicated that it is possible to predict the effect of paper physical characteristics on machine measured mainstream smoke yields of tar, nicotine and CO within the experimental space, for a given blend.
Several countries (including Canada and Australia) have introduced regulations requiring cigarettes to pass a test for ignition propensity, and there are a variety of scientific challenges in developing and assessing such products.
Additional alginate, starch or cellulose fibre may be applied to the surface of cigarette paper as bands to reduce its air porosity or permeability. This technique is widely used to control a cigarette’s ability to self-extinguish during the ‘ignition propensity’ tests (ASTM 2187-04 Standard Test Method for Measuring the Ignition Strength of Cigarettes).
These infrared images show a progressively upward bending of the smouldering coal and the curved paper char lines.
This is caused by the lower region of the smouldering cigarette (the “coal”) losing heat to the substrate and therefore burning at a lower temperature - visible in the images. A physical gap is created between the coal and the substrate allowing the natural air flow - or buoyancy - around the periphery of the coal.
These images provide detailed insights into the interaction between a burning cigarette and a cellulose substrate as set out in the ASTM 2187-04 test method.