Bubreak siloxane technology: the key to profitable pulping

Glenn Mudaly, R & D Chemist, Buckman Laboratories (Pty) Ltd

ABSTRACT

"I'm forever blowing bubbles" is an unwanted theme song in a variety of industries. Foam generates widespread problems in all too many manufacturing processes, and can impose severe limitations on the efficient use of equipment.

Additionally, the problems are aggravated in some cases by the development of new, high-speed processes. The need for maximum production at times of peak demand can often be frustrated by a disproportionate generation of foam. If left unattended this can lead to serious loss in output and quality of the finished product.

Up to now, the most widely used defoamers are organic, mineral oil-based products. To render them more effective, they contain hydrophobic substances such as silica, waxes and varying amounts of dispersing agents, depending on the intended application.

Such products had been introduced some years ago and were looked upon as innovative compared with pure petroleum products such as kerosene, which were in use at that time. However, these product types did not meet the latest technical standards.

Following intensive research and development work, Buckman Laboratories have now introduced a technologically advanced range of siloxane based products which are highly effective and will be particularly welcome in pulp production, since they can help make the process more economic and environmentally friendly.

INTRODUCTION

The pulp industry can look back on a long history rich in tradition, in which many revolutionary inventions have formed the basis for the present state of technology. Fundamental innovations in industrial manufacturing processes became established towards the end of the 19th century and are still felt even today.

The pulp industry is constantly on the lookout for new ways of solving problems that arise during the various stages of the manufacturing process. This industry has historically been a centre for product innovation, due to the large volumes of FCA's (foam control agents) consumed annually, high production speeds, and stringent end product quality requirements. This is considered to have been the "birth-place" of the now widely accepted silica/wax based products. Considerable investment has also been undertaken to achieve the so called ultimate "water-based" product. "Water extended" and the conventional silicone water based products have been available for some time. The former type was an attempt to cheapen existing oil based materials, and provide more tolerance to overdosing. Whilst in theory there should be a reduced tendency for contamination, the agglomeration of silica and wax is still possible, but more important is the disastrous consequence of emulsion inversion on the paper machine.

These materials are dispersions of waxes and/or silicas, with dispersants, emulsifiers and a variety of materials to modify the surface activity of the defoamers and/or the process liquors in which they may function. The unfortunate side effects of these chemicals become apparent as they are pushed to their maximum. "Pitch" or dirt deposits in paper are found to contain large amounts of high molecular weight fractions of the oils, or agglomerated particulates of the insoluble amide waxes, as are compounds fouling wires and felts in subsequent paper making operations. The products are found to contain greater or lesser amounts of dioxin and furan precursors contributing to the safety concerns surrounding goods manufactured from bleached pulps containing residual defoamers. Buckman Laboratories has now addressed these, and many other mill specific concerns and process chemical requirements. The result is Buckman's Bubreak Siloxane Technology; a highly proprietary foam control agents for the pulp industry of today, based on the "cutting" edge of organosiloxane chemistry.

THE THEORY OF FOAM FORMATION

Pure liquids do not foam. Only in the presence of surface active agents will the heterogeneous system of a gas (inner phase) in a liquid (outer phase) be stabilized by a thin film (lamella).

Figure 1Surface active agents accumulate on the gas/liquid interface and reduce the surface tension of the liquid phase. At the same time the surface viscosity and elasticity increases. High surface viscosity and elasticity make the foam lamella stable against mechanical force. Foam changes character during the time from micro-foam to polyhedral foam and a  so-called plateau border is formed between the air bubbles. Thus, liquid films become thinner by drainage due to gravity.

High temperature reduces the liquid viscosity and as a result drainage from the lamellas increases. The electrostatic force, or the Maragoni -Gibbs effect protects the thin foam lamella against mechanical forces. The Maragoni effect tends to repair the spontaneous changes on the lamella interface by removing surface active agents to the affected area . In practice there is usually no time for the foam to collapse naturally, which is why defoamers are needed.

FOAM OCCURANCE DURING PULPING PROCESS

The problems experienced by pulp manufacturers are high dirt count, pitch problems, high water consumption during the washing process, high energy costs in the evaporation plant, high consumption of defoamers and proper control of dosage. Due to the high mechanical forces involved in the process, air is always present, and this results in foaming. Foaming is most common in the pulp washing and screening processes, but can also sometimes be seen during cooking and bleaching stages. Often, foaming occurs during wastewater treatment.

The tendency towards foaming depends on the composition of the wood material, the type of washing process and other process parameters. In pulp production the presence of surface active agents such as resins and fatty acids derived from the wood causes the black liquor to foam. Foam lowers the pulp washing capacity and often adversely affects pulp quality by hindering the filtration through the pulp mat. Foaming problems can be seen at almost all pulp mills, but foaming conditions can vary greatly.

SILICONE EMULSION- A NEW GENERATION OF DEFOAMERS / DRAINAGE AIDS

A recent breakthrough in the search for effective water based Defoamers/Drainage aids for the pulping process has resulted in a completely different type of siloxane compound being formed. This product is more environmentally friendly and represents a completely new approach to the pulp industry's foaming problems.

This product was developed to not only control foaming during the pulping process but improve drainage on the washers or belt filters and to lead to profitable pulping. The Bubreak Siloxane Technology balances polydimethylsiloxane chemistry, silica chemistry, and organomodified siloxane chemistry with the chemistry and process conditions of the black liquors in brownstock washing. Unlike conventional silicone emulsion defoamers, Bubreak Siloxane Technology products are formulated to function at much lower dosages than conventional silicones, and to possess optimal foam and drainage control for specific liquors and processes. This is to eliminate any potential impact on product or process, and to utilise water as the carrier medium for the active ingredients rather than potentially detrimental mineral oils.

Polydimethylsiloxane chemistry and its derivatives are unique in that they possess the following properties that are advantageous in pulp production:

  • very low surface tension
  • excellent spreading characteristics
  • non-toxic and chemically inert
  • resistant to high temperatures
  • highly effective
  • long term efficiency in alkaline medium
  • low environmental hazard
  • organomodified siloxanes exhibit low dynamic and equilibrium surface tensions

Combination of the mentioned physical properties resulted in the development of highly efficient foam control agents and drainage aids.

These organomodified siloxanes are modified polydimethyl siloxanes that make them compatible with, or soluble in, aqueous and/or organic systems. This was achieved by incorporating various organic moieties. Furthermore, this type of modification makes it possible to produce specific physico-chemical interactions or chemical reactivity with, or in, a substrate phase. In addition to their pronounced surfactant properties in aqueous and organic systems, these organomodified siloxanes are primarily characterized by their multi-functionality. These products reduce surface tension much more than organic surfactants.

The general chemical structure of these organomodified siloxanes is shown below:

equation 1
Polydimethyl siloxane
Viscosity: 50 - 100,000cps

equation 2
Silicone polymer dimethyl polysiloxane

THE MECHANISM OF ACTION OF SILICONE DEFOAMERS

The surface tension of a defoamer has to be lower than that of the foaming liquid. Defoamers should not be soluble in the foaming system, but should disperse uniformly and rapidly. It is essential for a defoaming agent to have a positive entering and spreading coefficient.

E = F-A+F/A>0

S = F-A-F/A>0

E = entering coefficient
S = spreading coefficient
F = surface tension of the foaming medium
A = surface tension of the defoaming agent
F/A = surface tension in the interface of defoaming agent and foaming medium

The lower the surface tension of the defoaming agent, the more likely E and S are to be positive. Siloxane defoaming agents enter the foam lamellas by spreading over the bubble surface. The foam stabilizing surfactants are repelled by the defoaming agent. The surface active agents tend to move in the direction of higher surface tension. A weak area on the lamella is formed and foam rupture becomes possible.

There appears to be several other factors that influence defoamer performance such as:

  • particle or droplet size of the defoamer
  • types of emulsifiers employed in its emulsion manufacture
  • concentration of emulsifiers in the defoamer emulsion
  • size, surface area and shape of the silica particle
  • hydrophobicity of the silica particle
  • molecular weight and molecular weight distribution of the siloxane polymer
  • nature of the pendant groups on the siloxane backbone
  • temperature of the foaming solution
  • presence of other materials in the defoamer which may offer synergy with the siloxane

DEFOAMER EVALUATIONS AND RESULTS

The foaming that occurs during the pulping process was simulated in the laboratory using the recirculation test method. These tests gave an indication of the actual process conditions, enabling selection of the most suitable defoamer for the plant trial. However, the most reliable test result can be seen in the pulp washing process itself.

Black liquor samples were taken from the places where foaming is greatest. The samples were evaluated at the process temperatures by the recirculation test method. The foam level in the cylinder was allowed to reach its maximum height and thereafter the defoamer was added. The initial knockdown efficiency of the defoamer was recorded in the first 30 seconds (foam dispersing property).  Thereafter, the foam height was recorded every minute for 5 minutes in order to determine the effectiveness over a period (longevity/foam inhibiting property).
The lower the foam height in the cylinder, the more efficient the defoamer. The graph below shows the foam dispersing and inhibiting properties of a mineral oil-based defoamer and conventional silicone defoamer compared to that of Bubreak Siloxane Technology.

Figure 2

For drainage evaluation, the following test was conducted:

400 mls of stock (before washing) at 80C was poured into a 1 litre beaker. A 0.02g aliquot of defoamer under evaluation was added to the stock via a hypodermic syringe. The stock was then mixed for 1 minute using a high shear mixer. The stock was filtered through a Whatman GFA filter paper using a Buchner Funnel subjected to a constant vacuum. The amount of filtrate obtained in the first 10 seconds was recorded.

Figure 3

DISCUSSION

Buckman's Bubreak Siloxane Technology was developed to meet the demands of the pulp industry. Compared to traditional defoamers the consumption of emulsions based on Bubreak Technology can be reduced by a factor of ten, depending on the concentration of the product. It is also possible to achieve additional benefits which greatly increases the profitability of pulp production.

The following are some of the added benefits based on this technology that leads to profitable pulping:

  • improved drainage -> leads to smoother production and therefore to higher pulp       production capacity
  • more efficient washing -> less water is used and, therefore, also less energy in evaporation
  • less bleaching chemicals are needed, so that there are economic savings and reduced environmental pollution
  • improved pulp quality
  • the greater effectiveness, better dispersibility and total absence of wax in these siloxane emulsions means greatly reduced pitch deposits and, therefore, improved pulp quality
  • siloxane defoamers do not form dioxins in the chlorine bleaching process, which could be possible with mineral oil-based products.

CONCLUSION

Quality is a concept that covers not only the properties of a product but everything to do with it - from "cradle to grave". The appeal of a product to the end user is determined by the way it fits into everyday life. Concern for the environment has become the order of the day and is now as important as product performance.

Buckman's Bubreak Siloxane Technology now offers the pulp industry the economic advantages and embraces all aspects of the environment.

This development has put us as a company in the forefront with regards to foam control in the pulp industry.

Bibliography

(1) McGee, J - " Water Based Brownstock Antifoams", 24th EUCEPA Conference Book of Papers, May 1990, Stockholm, Sweden.

(2) Clark, D - Foam Control Agents in paper making processes, Chemspec Europe 89 BACS Symposium, Manchester

(3) Wacker-Chemie GmBH - Technical brochure

(4) Outi R, Eero K, Risto H - A new generation of antifoam agents for pulp mills

(5) McGee, J.B - Silicones: The environmentally friendly drainage aids for brownstock washing, APPITA Conference, Melbourne, Australia, 1991

(6) Pelton, R - A review of brownstock defoamer fundamentals, Pulp and Paper Canada, Vol 90, 1989

(7) Mudaly G.D - Silicone Defoamers : The key to profitable pulping, Buckman Marketing Conference, April 1994