An Introduction to Wax Emulsions

August 14 1 © 2010 • H&R InternationalAn Introduction to Wax Emulsions Version 1.1 August 14 2 © 2010 • H&R International Contents – What is an emulsion? – Emulsifier systems – Manufacture – Emulsion stability – Applications • Wood Panel board • Gypsum plasterboard – Handling and storage instructions August 14 3 © 2010 • H&R International What is an emulsion? • An emulsion is a mixture of two or more immiscible substances • Emulsions are made up of a dispersed phase and a continuous phase • For an oil-in-water emulsion the continuous phase is water and the dispersed phase is oil or wax. The reverse is true for a water-in-oil emulsion • Strictly speaking, an emulsion refers to a system where the dispersed phase is a liquid • A dispersion is a solid dispersed phase in a liquid continuous phase Continuous phase Dispersed phase August 14 4 © 2010 • H&R International Emulsifier system • Generally, an emulsifying agent lowers the interfacial tension of the two phases preventing the wax particles from readily separating from the water, and agglomeration of the wax particles • Emulsifiers reduce the mechanical work required to form an emulsion • Emulsifiers typically consist of a hydrophilic group and a long hydrophobic chain Hydrophilic group Hydrophobic chain • Some emulsifier systems impart a charge to the surface of the wax particle leading to repulsion of like charges preventing agglomeration • Different emulsifier systems are more resistant or susceptible to certain conditions August 14 5 © 2010 • H&R International Anionic emulsions • Anionic emulsifiers impart a negative charge to the surface of the wax particles • A common example of an anionic emulsifier system is fatty acid + base • The base reacts with the hydrophilic group of the fatty acid to form a negative charge • The hydrophilic group of the fatty acid is attracted to the water • The hydrophobic chain of the fatty acid is repelled by water but attracted to the wax particle so the emulsifier remains between the interface of the wax particle and the water • Anionic emulsions are typically only stable in alkaline pH conditions, and are often instable in hard water and acidic pH conditions August 14 6 © 2010 • H&R International Cationic emulsions • Cationic emulsifiers impart a positive charge to the surface of the wax particles • A common example of an cationic emulsifier system is an acid with amine groups • The amine reacts with the hydrophilic group of the acid to form a positive charge • The hydrophilic group of the acid is attracted to the water • The hydrophobic chain of the acid is repelled by water but attracted to the wax particle so the emulsifier remains between the interface of the wax particle and the water • Cationic emulsions are typically resistant to hard water but only stable in acidic pH conditions and are incompatible with other emulsion types August 14 7 © 2010 • H&R International Nonionic emulsions • Nonionic emulsifiers prevent agglomeration of the wax particles by steric hindrance • A nonionic emulsifier has a non-polar hydrophobic chain and some polar groups to provide the necessary water solubility • The hydrophobic chain is repelled by water but attracted to the wax particle • The polar groups are hydrophilic which lowers the interfacial tension of the wax and water phases • Nonionic emulsions are typically resistant to hard water, stable in most pH conditions, and offer good stability to shear August 14 8 © 2010 • H&R International Vivashield emulsifier system (Europe) • Vivashield products sold in Europe all use the same combination of anionic and nonionic emulsifiers • Vivashield products sold in Europe are named based on 3 variables • The first number indicates the oil-in-wax content – 8YZZ contains 15% oil-in-wax for higher shear stability – 9YZZ contains 7% oil-in-wax • The second number indicates the solids content – X5ZZ is a 50% solids product – X6ZZ is a 60% solids product • The last two numbers indicates the wax type – XY52 is based on 150 slackwax (Vivamelt S52) – XY62 is based on 400 slackwax (Vivamelt S62) • The 8 products currently sold are 8552, 8562, 8652, 8662, 9552, 9562, 9652, 9662 August 14 9 © 2010 • H&R International Vivashield emulsifier system (Europe) • Vivashield sold in Europe all use the same combination of anionic and nonionic emulsifiers • This allows the product to be resistant to hard water, stable in most pH conditions, and have good stability to shear • The anionic system is derived from stearic acid and monoethanolamine • The nonionic system arises from stearyl alcohol which increases shear stability Stearic acid Carboxylate formed with reaction of Stearic acid and MEA Stearyl alcohol August 14 10 © 2010 • H&R International Aqualite emulsifier system • Aqualite is based on an anionic emulsifier system • Aqualite 70 is a 40% solids product • Aqualite 72 is a 50% solids product • The emulsifier system arises from the carboxylic acid and ester groups of the montan wax reacting with the potassium hydroxide Carboxylic acid R2 = H, Ester R2 = alkyl chain • Polyvinyl alcohol is used in Aqualite primarily as a waterproofing additive, and partly as an emulsifier • Lignosulfonate can be used in Aqualite primarily to help reduce viscosity when emulsion is added to the gypsum slurry, and partly as an emulsifier C O O R 2 R 1 K d+ d- OH + C - O O R 2 R 1 K + - OH + August 14 11 © 2010 • H&R International Manufacturing emulsions – An overview • It is possible to prepare the raw materials in a single kettle when producing Aqualite or Vivashield • In Aqualite, PVOH must be dissolved in hot water before the addition of wax • After this step all remaining raw materials can be added to the vessel • All raw materials are heated to above the melting point of the wax phase, so a minimum of 70°C for Vivashield, >85°C for Aqualite • All of the raw materials are mixed prior to the homogeniser to form a ‘pre-emulsion’ • The hot blend is passed through the homogeniser, which provides a constant flow of material through a highly pressurised chamber, reducing the wax particle size to approximately 1 micron • The blend is then shock-cooled to below 35°C in seconds, which cools the wax particle below its’ melting point ensuring a stable emulsion is formed August 14 12 © 2010 • H&R International Manufacturing emulsions – An overview Plant setup in Nuth melts Montan wax in separate vessel prior to addition to paraffin wax in order to reduce melt time August 14 13 © 2010 • H&R International Emulsion Stability – Shelf life • The stability of an emulsion is critical to the customers’ process, storage and handling • The emulsion stability (shelf life) is the rate of separation of the dispersed phase from the continuous phase • The mechanisms of destabilisation are i) Creaming ii) Sedimentation iii) Flocculation iv) Coalescence • The rate of each mechanism will be different for different emulsions • Due to the lighter density, waxes will tend to cream • Additives that are readily soluble in water will tend to sediment • In some emulsions, flocculation can be reversible if the emulsion is sufficiently mixed • However, coalescence is generally irreversible in wax emulsions • Frequent agitation of the emulsion can help reduce the amount of material that has creamed or sedimented, and can help reverse flocculation, but increases the shear the emulsion is exposed to August 14 14 © 2010 • H&R International Emulsion Stability – Shear stability • Shear stability is another important property of the emulsion • Emulsions can be exposed to shear by any form of agitation or movement, e.g. mixing, pumping, transferring • During shear, the wax particles can begin to coalesce due to the emulsifiers being removed from the surface of the wax particle • Shear stability can be measured using: – Silverson method: An individual times how long it takes a high shear mixer to break the emulsion. – Rheometer method: A constant high shear rate shears the emulsion between a cone and plate and measures the point in time at which the emulsion breaks. • The emulsifying system employed is to ensure the wax emulsion remains stable until it reaches the injection point in the customers’ process • Shear stability is achieved through selection of emulsifiers, e.g. stearyl alcohol, and/or a higher oil-in-wax content August 14 15 © 2010 • H&R International Panel board applications • Wax emulsions are used in wood panel boards for either waterproofing or to act as a binder and lubricant during the manufacture of the board • There are 3 main types of wood panel boards – MDF, OSB, chipboard • Panelboard processes… MDF OSB Chipboard Types of boards Flooring Function of wax Waterproofing Binder August 14 16 © 2010 • H&R International The Role of Wax • In wood panelboard, the binding performance and the waterproofing performance are dependant on the wax • In panelboard, literature suggests that n-paraffins are better at waterproofing • n-paraffins are not branched so better coverage on the wood fibres or chips is achieved compared to iso-paraffins • In chipboard the most waterproofing molecules are found to be C26 – 31 • In MDF, the most waterproofing molecules are found to be higher • For binding performance a higher oil-in-wax content is desirable August 14 17 © 2010 • H&R International Moisture Resistant Gypsum Plasterboard • For high humidity applications there are moisture resistant gypsum wallboard • The two major commercial methods to make gypsum wallboard moisture resistant are by incorporation of hydrocarbon wax or siloxane in to the gypsum slurry during board manufacture • The desired effect of the water resistance treatment is to lower the two hour water uptake of the gypsum board from over 40% to the ASTM specifications of less that 10% for Gypsum sheathing and less than 5% for Water Resistant Gypsum Backing Board • The manufacture of wallboard is a continuous process • Gypsum is dried and then a flowable slurry of gypsum, water, wax emulsion and other additives is rapidly prepared • The slurry is pumped on to a paper line and then a second paper lining sandwiches the gypsum slurry • The gypsum sets within a few minutes whilst travelling horizontally down a continuous line • Boards are cut from the solid gypsum and then passed through ovens with controlled temperature profiles to remove water • Dried boards are then cut to size more accurately and stacked on pallets August 14 18 © 2010 • H&R International Plasterboard Plant August 14 19 © 2010 • H&R International Gypsum Cycle Calcium Sulfate Dihydrate CaSO 4 .2H 2 O Calcium Sulfate Hemihydrate CaSO 4 .½H 2 O Water, Additives, wax/ siloxane, accelerators, retarders, etc. Flowable slurry Add Heat Loses Water •Gypsum is calcium sulphate dihydrate with an average of two molecules of water per molecule of calcium sulphate (CaSO4.2H2O) •Gypsum originates from 2 major sources: •Natural: Limestone (calcium carbonate) mines where over time, pressure and sulfur addition the carbonate is oxidised to calcium sulphate •Synthetic: Limestone is injected into power plant stack discharge gases to form calcium sulphite, which is later oxidised and recrystallised to form calcium sulphate •For wallboard, gypsum is heated to remove some of the water to generate calcium sulphate hemihydrate (CaSO4.½H2O) •On the addition of water, a flowable slurry is formed which quickly sets as the dihydrate form crystallises •Once cut, the board is passed through the ovens to remove water to produce the hemihydrate form August 14 20 © 2010 • H&R International Moisture Resistant Gypsum plasterboard applications • For the wax emulsion to have the greatest beneficial effect, the emulsion must maintain integrity until the stucco is set • As water is removed during drying the wax emulsion is broken and allows the wax particle to become available • As the drying temperature is above the melting point of the wax fractions, the wax melts and flows to increase coverage • Waxes give moisture resistance due to the their hydrophobocity and distribution in the wallboard matrix • It is believed that the water resistance mechanism of the wax is blocking of the pores in the gypsum matrix that protect the dried board from liquid water intrusion Wax August 14 21 © 2010 • H&R International The Role of Wax, and Gypsum Slurry Additives • For maximum waterproofing performance the following paraffin wax properties are desirable: – Low oil-in-wax content (<1%) – High n-paraffin content – C27 - 29 • Montan wax and PVOH also provide waterproofing • Other additives that are added to the gypsum slurry during production of the board: – Water reducing agents: Lignosulfonates, naphthenaline sulfonates, PCEs. All to reduce the amount of water that is needed to maintain a gypsum slurry with the correct flow characteristics, saving energy in the drying stages – Retarders: To increase the set time of the gypsum slurry – Accelerators: To decrease the set time of the gypsum slurry • Variances between and in gypsum sources cause the efficiency of these additives, including waterproofing additives, to vary significantly