Abstract
Foam forming is an innovative process for papermaking that yields various paper products with excellent formability and porosity. The stability of the foam is a critical factor in foam forming technology. The effects of different surfactants and gases (N2 and CO2) on the ability of the foams to coalesce and the stability of the foams were studied. The properties of the liquid film were investigated via high-speed camera observation and infrared spectrum. The CO2 foam was less stable than the N2 foam under the same conditions, especially for the polyvinyl alcohol surfactant. The infrared spectra and high-speed camera observation showed that the main factor that resulted in CO2 foam instability was the bubble coalescence caused via the gas diffusion in the foam column, although the process of liquid film thinning was performed simultaneously. The greater the liquid film permeability coefficient of the foam, the easier the gas was able to spread throughout the liquid film. Foam forming technology will likely be employed in many potential pulp and papermaking mill processes.
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The Influence of Gas Diffusion Mechanisms on Foam Stability for Foam Forming of Paper Products
Wuling Zhao, Qiupeng Hou, and Xiwen Wang*
Foam forming is an innovative process for papermaking that yields various paper products with excellent formability and porosity. The stability of the foam is a critical factor in foam forming technology. The effects of different surfactants and gases (N2 and CO2) on the ability of the foams to coalesce and the stability of the foams were studied. The properties of the liquid film were investigated via high-speed camera observation and infrared spectrum. The CO2 foam was less stable than the N2 foam under the same conditions, especially for the polyvinyl alcohol surfactant. The infrared spectra and high-speed camera observation showed that the main factor that resulted in CO2 foam instability was the bubble coalescence caused via the gas diffusion in the foam column, although the process of liquid film thinning was performed simultaneously. The greater the liquid film permeability coefficient of the foam, the easier the gas was able to spread throughout the liquid film. Foam forming technology will likely be employed in many potential pulp and papermaking mill processes.
Keywords: Foam forming; Stability; Foam drainage; Gas diffusion
Contact information: School of Light Industry Science and Engineering, State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, China; *Corresponding author: wangxw@scut.edu.cn
INTRODUCTION
Foam forming technology has potential use in the pulp and papermaking industry. In this work, a methodology was developed to estimate the effects of gas diffusion on the stability of the foam in those applications. The methodology and estimates could potentially be used for future planning and research.
Foam forming is an innovative papermaking process that effectively prevents fiber flocculation for long and synthetic fibers, and its products are characterized by excellent uniformity of formation. In addition, this method can be utilized with renewable materials. Foam forming technology was first developed by Radvan and Gatward in the 1970’s (Gatward and Radvan 1973; Radvan et al. 1973). They found that fibers could be dispersed uniformly in a water-based foam, even with high concentrations of long fibers. At the same time, the Radfoam process was employed to manufacture paper sheets (Smith et al. 1974). In the pulp and papermaking industry, this method has been further improved to include many kinds of fibers (Rodman and Lessmann 1988; Mao et al. 2008; MacFarlane et al. 2012). Recently, foam forming technology has been applied to both paper and paperboard production, in realistic conditions at VTT’s pilot plant environment (Alimadadi and Uesaka 2016).
Foam has been applied in many industries, but due to its thermodynamically unstable nature, the stability of the foam is one of the most important features. Therefore, it is vital to effectively evaluate and understand the mechanisms of foam stability. There are two decay mechanisms of foam: foam drainage and gas diffusion. Both are related to the liquid film properties as well as the interactions between the liquid film and the Plateau boundary (Rao et al. 1982; Weaire and Hutzler 1999).
In general, foam stability is evaluated by parameters such as the height or volume of the foam column (Khristov et al. 2001; Stubenrauch and Khristov 2005; Ruízhenestrosa et al. 2008), and the half-life of the foam (Vilkova and Kruglyakov 2008). In addition, a variety of modern experimental techniques, including interfacial rheology (Monteux et al. 2004; Espinosa and Langevin 2009), resonance Raman scattering, and X-ray reflection, have been used to investigate foam stability mechanisms (Heydarifard et al. 2016). In addition, the Gibbs-Marangoni effect (Deshpande et al. 2000; Tan et al. 2005) is widely used to explain dynamic foam stability. Also, the viscoelasticity of liquid film is thought to be an important factor that affects the foam drainage process and the stability of liquid films (Kovalchuk et al. 2005). In this paper, the effects of different surfactants and gas (N2 and CO2) on the ability of the foams to coalesce and the stability of the foams were studied. Moreover, the properties of foam liquid film were investigated via high-speed camera observation and infrared spectrum (FT-IR).
EXPERIMENTAL
Materials
Polyvinyl alcohol (PVOH) was purchased from Kuraray Co., Ltd, Chiyoda, Tokyo, Japan. Sodium dodecyl sulfate (SDS) and Cetyl-trimethyl ammonium bromide (CTAB) were obtained from Sinopharm Chemical Reagent Co., Ltd, Shanghai, China, and were both dissolved in distilled water at a pH 7 without further purification. In addition, 95% purity CO2 and N2 gases were used.
Foam stability
The stability of the CO2 and N2 foams were evaluated via the Waring-Blender method. The foam stability was analyzed via a foam decay curve, which described the changes in volume of the foam column over time. The half-life time of the foam column was also recorded (Bhattacharyya et al. 2000; Koehler et al. 2000; Duan et al. 2004). The amount of surfactant solution used for each batch was 200 mL, and 1000 mL of CO2 or N2 gas was filled with a sealed cylinder via use of a mass flow meter. Then, an aqueous foam was generated from a mixture of distilled water (pH = 7) and either the PVOH, or one of the surfactants sodium dodecyl sulfate (SDS) or cetyl trimethylammonium bromide (CTAB) respectively, and then mechanically agitated for 5 min (1600 rpm) in a cylindrical vessel with a 10 cm diameter. For this experiment, the temperature was regulated at 25 °C.
Foam coalescence
The coalescence behavior of the air bubbles was studied in a defined amount (100 mL) of surfactant solution. The bubbles were grown on two parallel facing stainless steel capillaries with a 1.0 mm inner diameter. Gastight precision syringes were used as an air reservoir, to ensure the initial bubble size was always adjusted to the same volume. Then, the bubbles were brought into contact with each other by moving the syringe piston (Duerr-Auster et al. 2009). The stability and coalescence structure of the two facing bubbles were observed with a high-speed camera operated at 40 fps (frames per second), and the effects of gas diffusion on the stability of the foam was investigated by liquid film.
Foam film
The properties of the foam film were measured by Fourier Transform Infrared Spectroscopy (FT-IR; Hyperion, Tensor27). In the quartz tube (90 mm length, 8 mm width), a single layer of liquid film was formed via a capillary blowing method. The quartz tube was placed in an infrared light path, scanned at regular intervals, and recorded the infrared absorption spectrum. The spectral resolution was set to 1 cm-1, the scanning spectral range from 4000 cm-1 to 400 cm-1 and scanned 32 times.
RESULTS AND DISCUSSION
Foam Stability
Table 1 summarizes the observations on the stability of the CO2 and N2 foams. Under the same conditions, the half-life of the CO2 foam was much shorter than the N2 foam, which indicated that the CO2 foam was less stable than the N2 foam. In addition, when the concentration of the PVOH was increased from 0.3% to 1.0%, the half-life of the CO2 foam barely changed. However, the half-life of N2 foam increased with an increase in PVOH solution concentration, showing improved foam stability. This is because, as far as foam stability is concerned, the concentration of surfactant depends on the adsorption of surfactant molecules on the gas-liquid interface and their diffusion in surfactant solution (Chistyakov et al. 2001; Bailon-Moreno et al. 2005). In this experiment, the stability of the CO2 foam was not affected by the concentration of PVOH, indicating that the PVOH concentration was not the main factor responsible for the stability of the CO2 foam.
Table 1. Foam Stability of CO2 Foam and N2 Foam in Different Foaming Systems
The decay curve of the general foam is generally divided into three stages (Deshpande and Barigou 1999); I. Drainage stage: the foam drains under the condition of gravity and capillary forces, there is essentially no bubble burst, and the foam volume only decreases slightly, II. Foam rupture stage: the foam continues to drain, but there was some bursting of bubbles, and the volume of foam column rapidly decreases, and III. Foam metastable stage: the foam drainage has essentially stopped, and the liquid film gradually forms a Newtonian black film (NBF), which remained stable in the absence of external interference.
As shown in Fig. 1, all three of the additives (the PVOH and the surfactants) used in the experiment showed that the decay curve of the N2 foam was consistent with the decay curve of the general foam. The stability of the CO2 foam was less stable than the N2 foam under the same conditions, especially for the PVOH. Figure 2 shows that, for the 0.7 % m/v PVOH foaming system, the volume of the N2 foam slowly decreased from 480 mL to 295 mL after the foam foamed. The diffusion rate of N2 through the liquid film was slow, approximately 0.62 mL/s. The liquid film thinning process caused by gravity was the main factor for the foam instability. However, the decay curve of the CO2 foam dropped rapidly, and the both the bubble size and the foam volume rapidly decreased. The volume of the CO2 foam decreased from 480 mL to 0 mL after the foam foamed, and the diffusion rate of CO2 through the liquid film was 1.6 mL/s under the same conditions. Because the CO2 molecule had relatively high water solubility, the gas diffusion rate of CO2 foam was obvious. The rapid gas diffusion rate increased the foam decay process, which resulted in foam instability. In a parallel comparison experiment, the authors found that the impact of the gas diffusion rate on the foam stability in PVOH systems was notable. Therefore, the differences between the decay curve of N2 foam and CO2 foam was the largest in PVOH systems, followed by SDS surfactant systems, and then CTAB surfactant systems under the same conditions.
Fig. 1. The drainage process of N2 foam and CO2 foam
Foam coalescence
As shown in Fig. 3, the foam coalescence phenomenon increased when the CO2 foam bubbles were in contact with each other. A large bubble was formed via the coalescence of two small bubbles, which then rose above the gas-liquid interface until they ruptured. When the N2 foam sources contacted, the two small bubbles dropped below the gas-liquid interface and formed a larger bubble until it ruptured.
Fig. 2. Foam decay curves of PVOH, SDS, and CTAB in CO2 and N2 systems
The theoretical mechanism of foam decay and its factors that affected the stability of the foam can be explained as follows. In a water-based foam system, the CO2 molecules competed with the polar groups of the surfactant for water molecules and then reduced the surfactant hydrophilicity, which resulted in the aggregation of surfactat molecules on the liquid film. In addition, due to the formed gas-permeable channel, the CO2 molecules easily diffused through the liquid film, which caused an increased rate of foam coalescence and much less stable foam.
Fig. 3. Photography of CO2 foam and N2 foam coalescence
Foam film
As shown in Fig. 4, the location of the 3400 cm-1 band represented the vibration absorption peak of an OH group, and the other bands were characteristic absorption peaks of the PVOH or surfactants used.