A decrease in overall demand for gypsum wallboard and strong competition among producers over raw materials is pushing the plasterboard industry towards rapid innovation. This innovation is focused in two directions: the first is product innovation, with the appearance of new products like the ultra-lightweight boards, the requirements posed on dimensional stability and creep resistance and the growing concerns about sustainability and environmental compatibility of the manufacturing process and the final product. The other direction is rigid optimisation of the manufacturing process, with a streamlining of operations, a reduction of energy consumption and a better use of raw materials.
Introduction
Both product and process innovation require a thorough reconsideration of the wallboard production equipment and the process. The gains that can be obtained by an equipment upgrade are immediately apparent when we consider that a rebuild and revamping of the dryer may end up with a 25% energy consumption decrease, an increased flexibility to adapt to different raw material sources and an increased quality (e.g. higher flexural strengths) in the finished product.
The same is true for the stucco cooling system, which may result in a more consistent product and a faster setting, and for the use of thermal solar energy, which not only contributes to energy savings but also to the sustainability of the process. This important consideration is more and more frequently demanded by the shareholders, who ask for socially responsible investments.
Equally important, if not even more stringent, must be scrutiny of the production process. Wallboard production uses a great number of admixtures to control the technological parameters within narrow acceptance bands. The most important admixtures, which are always used in all production plants, are the liquefier and the foaming agent. These two admixtures interact with each other and with all other admixtures. They must be carefully chosen in order to maximise the productivity of the wallboard plant.
Process requirements
The formation of the gypsum core in wallboard manufacture is an astonishingly simple process:
plaster (calcium sulphate hemihydrate CaSO4∙½H2O) is mixed in water and dissolves according to its solubility constant. Gypsum, being less soluble than plaster, precipitates out of the solution forming a network of interpenetrating crystals which develop mechanical properties on setting. The process is summarised in the concentration plot Figure 1, in which point X marks the concentration of the solution during the dissolution/precipitation process, being undersaturated with respect to plaster and oversaturated with respect to gypsum.
However in wallboard manufacture there are a number of technological requirements which make the control of the process quite sharp, to cite only a few:
- The setting of gypsum must occur within three to four minutes from mixing with water and no flash setting shall occur in the mixer,
- Good adhesion must be assured between the lining paper and the gypsum core,
- The mechanical properties of the board, flexural strength and creep compliance, must satisfy prescribed standards and brittle edges must be avoided,
- The weight of the board must be reduced as much as possible, still complying with mechanical requirements,
- Porosity of the board shall be such to assure a fast and uniform drying in the dryer and the risk of over-drying shall be limited.
To satisfy all of these requirements, a number of admixtures are used in the process. Each of these admixtures has its own role, but it also interacts with all the others making the control of the process quite complicated and an art to be mastered by trial and error. The purpose of the following is to give some general guidelines on how to approach process optimisation.
Dispersing admixture – liquefier
The stoichiometric amount of water needed to completely hydrate plaster to gypsum results in a water-to-plaster weight ratio (w/p) of 0.19. However the requirement of obtaining a flowable slurry which homogeneously fills the gap between the top and bottom liners in a gypsum wallboard imposes the use of a much larger water amount, typically around w/p = 0.7.
The excess water remains in the board after setting and must be evaporated in the dryer, resulting in porosity in the final board. This porosity is welcome in wallboard, as it contributes to weight reduction. As such, the excess water is not harmful to the board quality.
However, a modern plasterboard line, running typically at 100m/min, will have to evaporate about 500kg/min of water, with an energy consumption in excess of 1.2 million kJ/min. Consequently a reduction of the water demand to obtain the prescribed flowability will result in a significant economic advantage in the plant operation. This is the reason why dispersing agents, also called liquefiers, are always used in gypsum wallboard production.
However the w/p ratio shall not be reduced below a limit usually set around 0.55-0.60 because otherwise excessive set retardation will occur. This is due to the fact that plaster hydration is an exothermic process and the temperature of the slurry will increase as the w/p ratio decreases. This is shown in Figure 2 for a plaster composed by 100% β-hemihydrate at 80°C, resulting from the cooling after calcination, and water at 15°C.
A higher slurry temperature will result in a reduced solubility difference between plaster and gypsum, as shown in Figure 3, and thus in a reduced driving force for gypsum precipitation and in a retarded setting. A better cooling of the plaster will allow reduction of the slurry temperature even when using less water and will result in an economic benefit, in addition to delivering a better plaster in terms of phase composition, with less residual gypsum and less soluble anhydrite.
Interaction with accelerators
The retardation can in principle be counteracted by the use of potash, K2SO4, which has the effect of increasing the solubility difference between plaster and gypsum. However, an excessive amount of alkali metals in solution will lead to poor bonding between the paper liner and the gypsum core.
In addition to this, varying the solubility difference between plaster and gypsum affects the crystal dimensions of the precipitated gypsum: increasing the solubility difference increases the nucleation rate thereby decreasing the crystal dimensions, and vice versa. Modifying the crystal dimensions will alter both the mechanical strength of the board, the tendency to develop brittle edges and the quality of the bonding to the facing paper.
Interaction with foaming agents
Using a liquefier to reduce the w/p ratio will result in a less porous board, heavier and more expensive since it contains more gypsum per unit surface. To compensate for the reduction of water, the quantity of foam introduced into the mixer has to be increased.
This is a technological advantage, as the dimensions and arrangement of the pores introduced with the foam can be controlled, as we will see further on, while the porosity coming from the excess water cannot be adjusted. Of course a fundamental requirement of the liquefier is that it shall not interact with the foam, neither depressing it nor enhancing it, otherwise process control will become unstable.
Ca-polynaphthalene sulphonate as a dispersing agent
These conflicting requirements have resulted in the selection of Ca-polynaphthalene sulphonate (Ca-NSF) as the liquefier in the wallboard industry. It has several advantages, including:
- A limited influence on gypsum crystals shape and dimension,
- A high efficacy of water reduction,
- No interference with the foam,
- No contribution to alkali metal content and good paper liner bonding.
More effective dispersants than Ca-NSF do exist but their use strongly impairs process control. This is due to the higher water reduction causing a strong retardation, as explained earlier, and usually they interact positively with the foam, causing imbalance in the slurry density.
Moreover the behaviour of Ca-NSF is as good with natural gypsum as with synthetic, flue-gas desulphurisation (FGD) gypsum, which is not the case for alternative liquefiers. Moreover the efficacy of Ca-NSF can be controlled by varying the polymer molecular weight distribution, as shown in Figure 4. In Figure 4 the efficacies of a standard grade Ca-NSF and a high molecular weight Ca-NSF are compared to a Ca-lignosulfonate benchmark. It is thus possible to choose the optimal polymer for the specific plaster being used and the plant operating parameters.
Environmental issues
Ca-NSF has been put under scrutiny because it usually contains a small amount, 100-300ppm, of residual free formaldehyde from the polycondensation process. Formaldehyde is classified by the International Agency for Research on Cancer (IARC), as a 'known carcinogen to humans,' while the European Community classifies it as a 'possible carcinogenic with insufficient evidence'. The EU risk phrase is 'Toxic by inhalation, in contact with skin and if swallowed. Causes burns. Limited evidence of a carcinogenic effect. May cause sensitisation by skin contact.'
The World Health Organisation has recommended a guideline value for formaldehyde in ambient air of 100μg/m3 as a 30 minute average, while the EU project INDEX, through a critical review of the information available worldwide, has set a 'no observed adverse effect level' (NOAEL) of 30μg/m3.
Indoor formaldehyde levels are usually higher than outdoor levels, up to 16 times higher according to the Australian NICNAS, due to the release of formaldehyde from building materials and limited indoor ventilation rates. A correlation has been found between the age of the house and the formaldehyde concentration in the indoor air, by a survey of indoor air quality in England (BRE Environmental Consultancy, HealthyAir – The impact of building materials on indoor air quality) as shown in Figure 5.
Several building materials may be sources of indoor formaldehyde emissions, especially adhesives, particle board and MDF, while the contribution of gypsum board is minor. In Figure 6 the results of laboratory testing in an INDOOTRON 30m3 chamber, performed within the frame of the European BUMA project are reported.
To avoid any possible contribution of the liquefier to formaldehyde emission, either in the indoor environment due to the installed wallboards or in the working environment of the production line, a new Ca-NSF has been introduced which does not contain any detectable amount of free formaldehyde – Fluplast 40.
Foaming agent
To make boards lighter, easier to handle and to install, foam is introduced into the mixer. When a dispersant is used to reduce the w/p ratio, foam must be increased in order to compensate for the reduction of water.
In wallboards, more than 70% of the volume is constituted by air. The bubble size-distribution must be carefully chosen to preserve the board strength characteristics and the foam must be evenly distributed in the gypsum core, without any accumulation under the paper liner, which would cause poor bonding and delamination.
Foam generation and mixing in the slurry are crucial aspects of wallboard production. Foam is produced in a foam generator, where a surfactant solution in water is mixed with compressed air and it is sent to a pin mixer where it is added to the water/plaster slurry. The length and diameter of the line connecting the foam generator and the pin mixer is also important and may act as a foam improver.
There are several aspects that make this operation highly difficult to control. First of all foams are out-of-equilibrium materials, subject to drainage, coarsening and bubble coalescence via film rupture. In Figure 7a the bubble growth with time is reported for an experimental foam with an initial gas volume fraction φ = 0.95. In Figure 7b the drainage profiles for the same foam measured at different heights are reported.
The properties of foams are a function of the gas volume fraction, which typically ranges around φ = 0.92 for the foams produced for inclusion in the gypsum slurry. These foams approach the dry limit and, although formed from the simple ingredients of air and a surfactant solution, exhibit many unusual properties, such as the appearance of a yield stress, usually associated with solids.
Yield stress is a function of the gas volume fraction, surface tension and bubble size and as such varies with foam evolution. The yield stress and more generally the rheological properties of the foam must match with the water/plaster slurry in order to ensure am homogeneous mixing, without shear banding and foam separation. An example of this is shown in Figure 8.
There is a complex and highly non-linear relationship between bubble size distribution, foam density, surface tension and viscosity of the water/plaster slurry, which makes any adjustment an art rather than precise science. The situation is further complicated by the fact that the surfactant solution may include a high molecular weight polymer to control foam yield stress and a film-forming polymer to increase the set board drying rate, thus decreasing energy consumption and cost.
It appears that the choice of the liquefier and of the foaming agent is rigidly coupled, as any change may induce unexpected results due to the lack of control of the many interacting parameters. On the other hand a correct choice of the coupled liquefier/foaming agent may result in important savings, associated with a reduced amount of water to be evaporated, easier drying due to the porous boards and the lighter boards produced, which can contain up to 12% less gypsum.
To take advantage of the possible benefits arising from the presence of foam in the wallboard the correct choice of bubble size distribution has to be selected. To achieve this bubble size distribution a combination of surfactants is usually used, resulting in the presence of both small bubbles, which form the texture of the gypsum core, and a smaller number of large isolated bubbles, fewer in number but comprising a high volume fraction. This decreases the density of the board without impairing its mechanical properties and flexural strength.
Interaction with waterproofing admixtures
As a final remark on foaming agents it must be said that incompatibilities have been observed between surfactants containing non-ionic polyglycol units, such as alkyl-ether sulphates and poly-methyl-hydrogen-siloxanes (PMHS). Large air blisters develop under the paper liner during the drying stage, which interfere mechanically with the board transport and blocking the dryer, with significant downtimes.
Even though such incompatibilities are not yet entirely understood, it seems likely that they come from an interference of the surfactant with the initial PMHS reaction of hydration at the hydrogen site to generate a hydroxyl group with resulting hydrogen gas
generation. This reaction does not occur in the initial slurry formation, but is delayed until after setting towards the end of the line, resulting in blister formation due to the gas evolution.
Admixtures for creep mitigation
There is a need for greater dimensional stability of gypsum wallboard. Under certain conditions, especially high humidity and temperature, the board has a tendency to deform permanently under its own weight. To partially overcome this problem, boric acid has been used in small amounts, around 0.1% by weight of plaster, to eliminate sagging. Boric acid in gypsum slurry promotes the formation of large bulky crystals as opposed to long needle-like ones.
The bulky gypsum crystals impart a more rigid character to the board and in the presence of foam the effect of boric acid is to increase paper bond, through interaction with the starch. This prevents the formation of brittle edges.
A much more important effect of strength improvement and creep mitigation is obtained by the use of phosphorous containing admixtures, either sodium trimetaphosphate (STMP) or phosphonates like hexamethyl-
ene-diamine-tetramethylene-phosphonic acid (HMDTMP). These admixtures also interact with the foaming agent through modifications to the texture of the gypsum core. Usually, less foaming agent is needed to get the desired porosity level when such admixtures are used. The effect of such admixtures is to modify the solubility of the gypsum crystals, in particular to modify the influence of pressure on solubility, thus greatly reducing the flow mediated by transport in the liquid phase.
Conclusion
A great number of different admixtures are used in the production of gypsum wallboards in order to satisfy all the technological requirements posed by the fast production rate used in modern installations and to meet all the required specifications for strength, weight and resistance to environmental conditions. Moreover the production line must work in the most energy- and material-efficient way available at each production site.
Each admixture has its own specific role, but it necessarily interacts with all others and with the technological process since the crystallisation of gypsum is influenced by any product added to the slurry.
Both the rate of precipitation, and the crystal shape and dimensions will vary, with significant fallouts on process and performance. Moreover the environmental compatibility of the admixtures both in the working ambient and in the final indoor use of the boards is emerging as a fundamental prerequisite.
Owing to this complexity, and sometimes conflicting requirements, the admixtures should be chosen to be compatible with each other. Changing just one admixture may have unpredictable consequences on the rest of the production process and the final wallboard produced.
