Effect of magnesium hydroxide as cigarette paper filler to reduce cigarette smoke toxicity

The use of magnesium hydroxide (Mg(OH)2) was proposed as a filler to replace part of the calcium carbonate (CaCO3) in cigarette paper and reduce the toxicity of the smoke from cigarettes. Physical property changes and smoke reducing ability of this possible substitution were effectively studied. The results showed that adding 10% Mg(OH)2 could meet the requirements of the physical property of the cigarette paper. Moreover, with the addition of Mg(OH)2 as a filler in the cigarette paper, the pyrolysis temperature of the cigarette paper decreased, while the porosity and specific surface area increased. As a result, the main-stream smoke had a lower smoke total particle matter (STMP), tar, nicotine and carbon monoxide content, and the side-stream smoke also had a lower STMP.


INTRODUCTION
Main-stream smoke and side-stream smoke are two types of smokes generated in cigarette smoking, in which more than 4000 harmful and carcinogenic substances such as CO, aromatic hydrocarbon, nitrosamine and radicals are produced. 1,2Tobacco combustion is a complex physical and chemical reaction involving chemistry, hydrodynamics, heat transfer and thermodynamics. 3Modification of cigarette paper to reduce the toxicity of smoke produced from tobacco combustion has been receiving much attention. 4,5 garette paper is consisted of cellulose fibers, fillers and other additives. 6Particularly, the types and dosage of fillers have great influences on its appearance and physical properties of the paper. 7In this study, Mg(OH) 2 , a mineral and also known as innocuous flame retardant, was added to cigarette paper as a filler, to investigate its effect on the physical properties of the paper and on the reduction of toxic substances in tobacco combustion process.

Materials
The bleached softwood pulp for this study was obtained from the Hengfeng Paper Co. (Mudanjiang, China) with an original beating degree of 18-20°SR.Cut tobacco and CaCO 3 were provided by the Guangzhou Cigarette Co. (Guangzhou, China), and the Mg(OH) 2 was a reagent grade purchased from Guangzhou Chemical Reagent Co. (Guangzhou, China) .

Preparation of cigarette paper
After being immersed in 5 L water for at least 1 hour, 360 ± 5 g dry pulps were beaten to a degree of 75 °SR-78 °SR with a laboratory beater (23 L VALLEY beater, USA).Filler was then mixed with the pulp, and lab paper sheets (hand sheets) were prepared with the mixture on a standard sheet-former (TMI 73-60, Germany).The dosage of filler was as mass percentage of the filler based on the mass of dry pulp.

Smoke test
The cut tobacco and paper sheets were balanced for 48 hours at 23 ± 1˚C and relative humidity of 50 ± 2%, and then cigarettes were made with a length and circumference of 84 mm and 24.5 mm respectively, without an end filter.It should be noted that commercial cigarette papers are composed of pulp fibers, filler, combustion-supporting reagents and other additives.
A 20-channel rotary smoking machine (Borgwaldt, Germany) was applied to determine the nicotine, tar, carbon monoxide (CO) and smoke total particle matter (STMP) in the main-stream smoke.The side-stream smoke testing was performed using a modified equipment system (Fig. 1) according to a reference, 16 with the following operating parameters: smoking time of 2 seconds, smoking volume of 35 ± 5 mL, smoking interval of 60 seconds, rotameter flow of 3L/min and burning length of 55 ± 2 mm.Three cigarettes were used for each test.The STPM in the side-stream smoke was determined by measuring the weight increase of the filtering membrane.

Thermogravimetric analyses
After being conditioned at 22 ± 1˚C and relative humidity of 60 ± 2% for 48 hours, the paper sheets were cut to 1x1 mm and tested with a thermogravimetric differential scanning analyzer (Netzsch TG20, Germany).
A Laser-diffraction size distribution analyzer (Mastersizer 2000, England) and pore size analyzer (Poremaster 33, USA) were used to determine the size distribution and the pore size of the filler particles.
Fig. 1 The schemes of the apparatus for the test of side-stream smoke

Distribution of Particles and Pore Size of the Filler particles
As an important factor for paper properties, generally, it is necessary for the size of filler particles to be in the range of 2 to 8 μm to get proper distribution for high uniformity of air permeance and to use the fillers effectively.Table 1, and Figures 2 and 3 show the testing results of the particle size and pore size distributions of the CaCO 3 and Mg(OH) 2 through the size distribution analyzer and a pore size analyzer.The data indicated that with the increased specific surface area of particles, the D (0.5) of Mg(OH) 2 particles was slightly smaller than that of CaCO 3 , and the mean pore size of Mg(OH) 2 was much smaller than that for CaCO 3 , which is good for the adsorption of toxic substances in the smoke.

Influence of Dosage of CaCO 3 on Properties of Cigarette Paper
The dosage of CaCO 3 had a significant influence on the properties of cigarette paper, as the filler particles interfere with inter-fiber bonding.Previous research had shown that increasing the dosage of CaCO 3 filler in the cigarette paper improved the brightness, opacity and air permeance, but caused the tensile strength (tensile index or tensile breaking length) and tensile energy absorption (TEA) to decrease. 17Table 2 shows the physical properties of the cigarette paper with various dosages of CaCO 3 .It can be seen from Table 2 that the tensile index, breaking length and tensile energy absorption (TEA) of the cigarette paper decreased by about 40%, with the increase of the CaCO 3 filler dosage from 0 to 35%.Simultaneously, the brightness, opacity and air permeance and opacity increased.The stretch at break also increased slightly.For a good balance between the strength properties and the air permeability as well as the optical properties, the CaCO 3 filler dosage should be in the range of 20%-30% for the cigarette paper.

Influence of Mg(OH) 2 on Properties of Cigarette Paper
In Table 3, the CaCO 3 filler was partly substituted with Mg(OH) 2 , while the total filler dosage was fixed at 30%.As the Mg(OH) 2 substitution rate increased, the strength properties of the resulting paper decreased slightly, and so was the brightness, opacity and air permeability.The stretch at break and TEA increased first and then started to decrease with the increase of Mg(OH) 2 addition to replace the CaCO 3 .The decrease of the strength properties with Mg(OH) 2 substitution may be attributed to its smaller particles size, porous particle structure and high specific surface area, as shown in Figures 2 and 3.
The optical properties (brightness and opacity) of filled paper is largely affected by the optical properties of the filler particles, including brightness, light scattering and absorption coefficients. 9The brightness of the Mg(OH) 2 filler was lower than the CaCO 3 filler, and thus substituting CaCO 3 with Mg(OH) 2 caused the brightness to decrease.Overall, using 7-10% of Mg(OH) 2 to replace CaCO 3 as the filler could still meet the requirement of strength and optical properties and air permeability for cigarette paper.
To further test the effect of Mg(OH) 2 on the physical properties of paper, in Table 4, the Mg(OH) 2 dosage was varied from 5 to 15% with the CaCO 3 dosage fixed at 15%.The results show that as the Mg(OH) 2 dosage increased the tensile strength and air permeability decreased, while the stretch at break and opacity increased.It is interesting to note that the TEA remained almost unchanged when the Mg(OH) 2 dosage was increased from 5% to 10%, but decreased significantly when the dosage was further increased from 10% to 15%.Therefore, it is recommended that the Mg(OH) 2 dosage should not exceed 10%.

Effect of paper grammage on the main-stream smoke
The grammage of cigarette paper affect the air permeability and thus affect the combustion temperature of tobacco, as well as the chemical composition of the smoke.As seen in Table 5, as the grammage of the cigarette paper increased, the STPM, nicotine, tar and carbon monoxide contents in the main-stream smoke increased.Commercial cigarette paper normally has a grammage of 28-30 g/m 2 , which was used as the norm for this study.

Effect of air permeance on the main-stream smoke
The air permeance of cigarette paper is another important parameter for the combustion condition. 2,3High air permeance increases the cigarette combustion rate and thus decrease the smoking times and the inhalable smoke.In Table 6, the air permeance of the cigarette paper was varied from 1.4 to 3.0 μm/Pa•s to investigate its effect on the chemical composition of the main-stream smoke.It can be seen that the STPM, nicotine, tar and carbon monoxide contents in the main-stream smoke decreased as the air permeability of the cigarette paper increased.For example, when the air permeability was increased from 1.4 to 3.0 μm/Pa•s, the STPM and CO contents decreased by 11.07% and 19.09%, respectively, and the nicotine and tar contents also decreased by about 10%.However, excessively high air permeance would make the smoke too thin to satisfy a smoker.Normally, an air permeance of 1.8-3.0μm/Pa s is recommended for cigarette paper.

Effect of Mg(OH) 2 dosage on the main-stream smoke
Table 7 shows the influence of Mg(OH) 2 dosage on the main-stream smoke chemical composition, with the CaCO 3 filler dosage fixed at 20%.With the increase of the Mg(OH) 2 dosage from 0% to 11 %, the STPM, nicotine, CO and tar concentration in the main-stream smoke decreased significantly.With 8% Mg(OH) 2 , these harmful substances in the main-stream smoke were reduced by about 5 to 10%.The observed reduction of the toxic substances in the main-stream smoke with the Mg(OH) 2 addition may be attributed the fire retardant property of magnesium hydroxide.Mg(OH) 2 is a well-known fire retardant which can decompose into MgO and H 2 O at 332°C.The heat absorbed by the reaction will decrease the combustion temperature of tobacco, which in turn decrease the formation of the harmful substances in the main-stream smoke.The water released from the decomposition reaction also dilutes the combustible gases and inhibits oxygen from aiding the combustion.As a result, the combustion temperature of a tobacco cigarette decreases.It is well known that the generation of the harmful substances in the main-stream smoke is strongly associated with the combustion temperature of tobacco in cigarette smoking.The higher the combustion temperature, the more toxic substances are produced in the smoke.For example, K. Torikai reported that a 100°C increase in burning temperature would produce thousands of more additional chemical substances in the combustion of tobacco. 4igure 4 shows the TG analyses of the cigarette paper.Compared to the control cigarette paper with only CaCO 3 as the filler, the cigarette paper with Mg(OH) 2 added had lower decomposition temperature (318.5 vs 338.3°C).The decrease of decomposition temperature of the cigarette paper can be explained by the fact that the decomposition temperature of the Mg(OH) 2 filler is much lower than that for the CaCO 3 , as shown in Fig. 5.This is in agreement with an earlier study that found using the cigarette paper with Mg(OH) 2 could decrease the harmful substance in the main-stream smoke.The cigarette paper had a grammage of 28.6 g/m 2 , air permeance of 1.9 μm/Pa•s, and CaCO 3 content of 21.8%

Effect of Mg(OH) 2 addition on the side-stream smoke
The influence of adding Mg(OH) 2 to the cigarette paper on the STMP concentration of the side-stream smoke are shown in Table 8.The amounts of nicotine, tar and CO released from the side-stream smoke were too low to test.As the Mg(OH) 2 dosage increased, the STMP concentration in the side-stream smoke decreased proportionally, which can be attributed to the large specific surface area and flame retardant properties of magnesium hydroxide.As shown in Table 1 and Fig 6, the particle size and pore size of Mg(OH) 2 were smaller than those for CaCO 3 , so the cigarette paper with Mg(OH) 2 had smaller pores and more specific surface area for the adsorption of STMP.The presence of magnesium hydroxide could lower the combustion temperature of tobacco and reduced the amount of harmful substances released along with the side-stream smoke.

CONCLUSIONS
To reduce the amount of the most harmful substances in the smoke from cigarette smoking, it was feasible to substitute about one-third of the CaCO 3 filler in cigarette paper with Mg(OH) 2 without affecting the strength properties and air permeability of the cigarette paper to large degrees.It was found that as the Mg(OH) 2 dosage increased, the most toxic STPM, nicotine, tar and CO contents in the main-stream smoke decreased significantly.However, as the dosage of Mg(OH) 2 increased, the tensile index and air permeance of the paper decreased.A balanced filler composition was found to be 8% Mg(OH) 2 and 20% CaCO 3 , under which conditions the most harmful substances in the main-stream smoke were reduced by about 5 to 10%.The addition of Mg(OH)2 in cigarette paper also decreased the STMP in the side-stream smoke.The observed reduction of the most harmful substances in the smoke was attributed to the larger specific surface area flame retardant properties of Mg(OH) 2 .It was proposed that the presence of magnesium hydroxide in the cigarette paper could lower the combustion temperature of tobacco and thus reduced the amount of harmful substances released along with the smoke.

Fig. 4 Fig. 6
Fig.4 The TG analysis of cigarette paper with different dosage of filler

Table 1
Size and pore distribution of CaCO 3 and Mg(OH) 2 particles

Table 2
Influences of dosage of CaCO 3 on the properties of cigarette paper

Table 3
Influence of Mg(OH) 2 instead of CaCO 3 on the properties of cigarette paper

Table 4
Influence of Mg(OH) 2 dosage on the properties of cigarette paper with CaCO 3

Table 5
The influence of grammage of cigarette paper on the main-stream smoke

Table 6
The influence of air permeance of cigarette paper on the main-stream smoke

Table 7
The influence of dosage of Mg(OH) 2 on the main-stream smoke

Table 8
Influence of Mg(OH) 2 addition on STPM in side-stream smoke