Sunday, November 21, 2010

Process Making Fiber Cement old type and improvement

Process Making Fiber Cement old type and improvement

Asbestos fiber cement techhology about 120 years ago, Ludwig Hatschek made the first asbestos reinforced cement products, using a paper-making sieve cylinder machine on which a very dilute slurry of asbestos fibers (up to about 10% by weight of solids) and ordinary Portland cement (about 90% or more) was dewatered, in films of about 0.3 mm, which were then wound up to a desired thickness (typically 3-6 mm) on a roll, and the resultant cylindrical sheet was cut and flattened to form a flat laminated sheet, which was cut into rectangular pieces of the desired size.
For over 100 years, this form of fiber cement found extensive use for roofing products, pipe products, and walling products, both external siding (planks and panels), and wet-area lining boards.
Asbestos cement was also used in many applications requiring high fire resistance due to the great thermal stability of asbestos.The great advantage of all these products was that they were relative lightweight and that water affected them relatively little, since the high-density asbestos/cement composite is of low porosity and permeability.The disadvantage of these products was that the high-density matrix did not allow nailing, and methods of fixing involved pre-drilled holes.Although the original Hatschek process (a modified sieve cylinder paper making machine) dominated the bulk of asbestos cement products made, other processes were also used to make specialty products, such as thick sheets (say greater than about 10 mm which required about 30 films). These used the same mixture of asbestos fibers and cement as with the Hatschek process, although sometimes some process aid additives are used for other processes.
For example, fiber cement composites have been made by extrusion, injection molding, and filter press or flow-on machines.

Two developments occurred around the middle of the last century that had high significance to modern replacements of asbestos based cement composites.
The first was that some manufacturers realized that the curing cycle could be considerably reduced, and cost could be lowered, by autoclaving the products.
This allowed the replacement of much of the cement with fine ground silica, which reacted at autoclave temperatures with the excess lime in the cement to produce calcium silica hydrates similar to the normal cement matrix.
Since silica, even when ground, is much cheaper than cement, and since the autoclave curing time is much less than the air cured curing time, this became a common, but by no means universal manufacturing method.
A typical formulation would be about 5- 10% asbestos fibers, 30-50% cement, and about 40-60% silica.
The second development was to replace some of the asbestos reinforcing fibers with cellulose fibers from wood.
This was not widely adopted except for siding products and wet-area lining sheets.
The great advantage of this development was that cellulose fibers are hollow and soft, and the resultant products could be nailed rather than by fixing through pre-drilled holes.
The siding and lining products are used on vertical walls, which is a far less demanding environment than roofing.

However, cellulose reinforced cement products are more susceptible to water induced changes, compared to asbestos cement composite materials. A typical formulation would be about 3-4% cellulose, about 4-6% asbestos, and either about 90% cement for air cured products, or about 30-50% cement and about 40-60% silica for autoclaved products.

Asbestos fibers had several advantages. The sieve cylinder machines require fibers that form a network to catch the solid cement (or silica) particles, which are much too small to catch on the sieve itself. Asbestos, although it is an inorganic fiber, can be"refined"into Asbes, They are stable at high temperatures. They are stable against alkali attack under autoclave conditions. Hence, asbestos reinforced fiber cement products are themselves strong, stiff (also brittle), and could be used in many hostile environments, except highly acidic environments where the cement itself is rapidly attacked chemically. The wet/dry cycling that asbestos roofing products were subjected to, often caused a few problems, primarily efflorescence, caused by the dissolution of chemicals inside the products when wet, followed by the deposition of these chemicals on the surfaces of the products when dried. Efflorescence caused aesthetic degradation of roofing products in particular, and many attempts were made to reduce it. Because the matrix of asbestos reinforced roofing products was generally very dense (specific gravity about 1.7), the total amount of water entering the product even when saturated was relatively low, and the products generally had reasonable freeze thaw resistance. If the density was lowered, the products became more workable (for example they could be nailed) but the rate of saturation and the total water absorption increased and the freeze thaw performance decreased.
Alternative Fiber Cement Technologies In the early 1980's, the health hazards associated with mining, or being exposed to and inhaling, asbestos fibers started to become a major health concern. Alternatif Fiber Semen Technologies . Manufacturers of asbestos cement products in the USA, some of Western Europe, and Australia/New Zealand in particular, sought to find a substitute for asbestos fibers for the reinforcement of building and construction products, made on their installed manufacturing base, primarily Hatschek machines. Over a period of twenty years, two viable alternative technologies have emerged, although neither of these has been successful in the full range of asbestos applications.
In Western Europe, the most successful replacement for asbestos has been a combination of PVA fibers (about 2%) and cellulose fibers (about 5%) with primarily cement (about 80%), sometimes with inert fillers such as silica or limestone (about 10- 30%). This product is air-cured, since PVA fibers are, in general, not autoclave stable. It is generally made on a Hatschek machine, followed by a pressing step using a hydraulic press.
This compresses the cellulose fibers, and reduces the porosity of the matrix. Since PVA fibers can't be refined while cellulose can be, in this Western European technology the cellulose fiber functions as a process aid to form the network on the sieve that catches the solid particles in the dewatering step. This product is used primarily for roofing (slates and corrugates). It is usually (but not always) covered with thick organic coatings. The great disadvantage of these products is a very large increase in material and manufacturing process costs. While cellulose is currently a little more expensive than asbestos fibers at $500 a ton, PVA is about $4000 a ton. Thick organic coatings are also expensive, and hydraulic presses are a high cost manufacture step.

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