Wallboard Basics

:: Tuesday, October 30, 2018 :: Posted By Frank Wawrzos
Making wallboard appears simple, but it is actually quite technically demanding. The process starts by taking gypsum, chemically known as calcium sulfate dihydrate (CaSO4 2H2O) and calcining it to convert it to plaster (often referred to as “stucco,” but not the same material as used on the outside of buildings).

Calcium sulfate hemihydrate (CaSO4 ½H2O) is the chemical form of plaster (stucco). This material is mixed with water and various additives and squirted onto a moving sheet of paper. The slurry then passes through a roll and is covered with more paper. It travels further down rollers where it hardens, is cut into appropriate lengths, and goes through a dryer to remove excess water.

Gypsum, unlike cement, is a physical structure rather than a chemical structure. The needle-like gypsum crystals form a network to give the board its physical integrity.

Lightweight Benefits

The trend in board making in North America has been towards lighter and lighter boards. There are many reasons for this, but the two main reasons are:
  1. Ease of handling during installation
  2. Shipping costs
Shipping costs are quite significant for wallboard, as it is a high weight product with relatively low value.

Quality + Lightweight Is Not Easy

Lighter weight means less gypsum in the board as a whole—in particular less gypsum at the paper/core interface and at the edges and ends. The less gypsum at the interface, the easier it is to over-dry and calcine the surface. When the gypsum crystals at the surface are calcined (converted back into their hemihydrate form), they lose their strength and the bond between the gypsum and the paper can fail. This “splitting” is a serious quality issue. Over-drying of the ends (end burn) and edges can also be a problem, and can lead to crumbling during handling and nailing.

Humidified deflection of “sag” is another problem related to lightweight boards in ceiling applications—particularly in humid environments. When wet, the gypsum needles can become “lubricated” and slip over each other in response to gravity.

Other properties related to the perimeter and surface of the board are referred to as edge hardness and nail-pull resistance. These properties measure how robust the board is to handling both in the manufacturing process and during installation, and are related to the strength of the board near the surface, edges, and ends—where the board is most likely to be calcined during drying.

The Optibor TG Solution

Using Optibor® TG as an additive can address all of these problems related to lightweight boards. It effectively raises the calcination temperature for gypsum, thereby protecting the wallboard from the effects of over-drying. Water soluble Optibor TG migrates to the surface of the board (as well as the ends and edges) during the drying process, forming a very thin but concentrated layer. It effectively increases the density of the core at the interface with the paper, but without increasing the overall weight of the board. This dense layer is resistant to over-drying. The added layer of protection allows the boards to be processed more rapidly through a hotter dryer, thus speeding up production.

Increasing starch content can also serve to protect the gypsum crystals from over-drying, but since starch holds water so tightly more energy is required to remove the water from the board, increasing drying times and energy costs. Optibor TG modifies the gypsum crystals, making them larger and thicker. This contributes to the rigidity of the boards, reducing the sagging tendency.

While gypsum board makers are working hard to keep up with the demand, Optibor TG is doing its part by providing increased production rates while maintaining product quality.

Related links

Gypsum boards


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