Specific production process of magnesium oxide sandwich panels

I. Crucial Aspect—Ingredient Formulation. Ingredient formulation plays a decisive role in the quality of the board:

An unreasonable formulation directly affects the performance of the board (e.g., peeling of the decorative layer due to efflorescence and deformation and cracking caused by the board’s own expansion and contraction).

The fundamental principle of a scientifically sound formulation is to clearly define the quantitative relationship between magnesium oxide, magnesium chloride, and water, that is, to follow the ratio of magnesium oxide to magnesium chloride between 6 and 9:1 (this ratio can fluctuate within this range depending on the activity of magnesium oxide, etc.); and the ratio of water to magnesium oxide between 14 and 19:1.

The Role of Added Modifiers:

Modifiers can improve the stability of the board’s molecular structure and its resistance to bending, water, and efflorescence to a certain extent:

(1) The use of sulfates, phosphoric acid, and phosphates is to block the capillary channels between 5-1-8 molecules to prevent external moisture from entering and causing hydrolysis of the molecular structure, thus losing the original physical properties of the board. It also prevents magnesium hydroxide and other substances inside the board from migrating to the surface with the evaporation of moisture, thus preventing efflorescence.

(2) The use of water-reducing agents. <1> The initial purpose of using water-reducing agents is to reduce the amount of brine used, thereby improving and enhancing the workability of the filler in the slurry pot. This not only facilitates more thorough and uniform mixing of the filler but also improves the casting process of the boards. More importantly, it improves the uniform distribution of the slurry, thus greatly increasing the strength of the boards.

<2> While excessive use of brine can reduce costs by adding more filler, it greatly increases the possibility of brine re-entry and uneven shrinkage of the boards due to uneven evaporation of large amounts of water, causing deformation. Furthermore, excessive water increases the moisture content of the boards; JC/T568-94 clearly stipulates that the moisture content should be between 8% and 10%. In this case, adding an appropriate amount of water-reducing agent can improve the workability of the slurry, which can increase the amount of filler to a certain extent, thereby reducing costs without changing the scientific ratio.

(3) Use of foaming agents.

1. The purpose of adding foaming agents is to generate gas, forming bubbles in the slurry, thereby reducing the density of the slurry and producing lightweight boards. However, excessive foaming agent should not be added blindly in pursuit of lightweight properties, as this will ultimately lead to excessively low board density, thus affecting strength and other properties. (For every 1% increase in porosity, the strength can decrease by 5%-10%, and the decrease in density also leads to a decrease in water resistance.)
2. The role of fillers. Filling with reinforcing materials can change the internal structure of the board, buffering and reducing the volume expansion of magnesia materials; at the same time, it changes the pore structure of the product, increases the density of the product, improves the strength of the product, reduces deformation, and can also comprehensively utilize solid waste, all of which are necessary for reducing product costs.

(1) Active fillers. Active fillers refer to industrial waste containing silica, such as slag, fly ash, and silica fume. We are currently using fly ash. <1> It contains silica, has pozzolanic properties, and is hydraulic. It can hydrate to form hydrated calcium silicate cement, reducing the amount of cementitious material required.

<2> It can improve the later-stage strength of the product, especially beneficial for using glass fiber as a reinforcing material.

<3> It reduces the heat of hydration, making it less prone to material expansion in summer, reducing the use of retarders, reducing volume changes, and reducing drying shrinkage. When using fly ash, the following should be noted:

<1> Loss on ignition should be less than 8%.

<2> Active silica content > 50%, mud content < 1%.

<3> The addition amount is about 15% of the weight of the lightly calcined fly ash. A large addition should not be forced, as it requires a large amount of water, which will affect the brine concentration.

<4> To fully utilize the function of fly ash, we add phosphoric acid and sulfate as activators.

(1) Inert filler. Inert fillers refer to materials that have high strength, good chemical and physical stability, but do not participate in altering the 5-1-8 phase structure.

II. We use perlite and sawdust:

<1> Perlite. Perlite is divided into expanded perlite and closed-cell perlite.

<1> Expanded perlite is inexpensive, but it can absorb 2-9 times its own weight in brine. The absorbed brine penetrates into the pore structure, preventing reaction with magnesium oxide. This causes magnesium chloride in the pores to absorb water, dissolve, evaporate, and migrate to the surface of the product, resulting in efflorescence, reduced strength, and weakened waterproofing. Although it is difficult to control the quality, there are ways to mitigate this: subtract the mass of brine absorbed by the perlite when calculating the proportions, and then add phosphoric acid and ferrous sulfate to form a gel that blocks the pores.

<2> Closed-cell perlite has low moisture absorption and a low brine absorption rate. It has good compressive strength and water resistance, but it is expensive.

<2> Sawdust. When using sawdust, avoid using freshly harvested sawdust, as its high sugar content and high moisture content will cause the pulp to fail to solidify and lose strength. The moisture content of sawdust should not exceed 20%. 1. Fiber Application. The application of fibers can improve the flexibility and impact resistance of the board. Organic fibers, due to their high tensile strength, can be directly added to the brine for thorough mixing. Short-cut glass fibers are easily broken and cannot be directly added to the brine.

In summary: The key to material preparation is:

1. Molar Ratio: Strictly adhere to a molar ratio of magnesium oxide to magnesium chloride between 6 and 9, and a molar ratio of water to magnesium oxide between 14 and 19.
2. Addition Order: The order of adding materials to the slurry pot should be as follows: those that do not react with the brine, such as phosphates and sulfates, should be added first; those that react with the brine, such as fly ash, should be added later. Fly ash is best added to the filling pot.

There is no strict order for adding fillers, but fly ash and glass fibers must be added last, preferably last.

3. Slurry Density Control. Unless otherwise specified, the density is generally controlled at (1.0-1.15) g/cm3. Density control methods: (1) Add lightweight filler such as sawdust without affecting workability and facilitating molding operations. (2) Add an appropriate amount of foaming agent. (3) If the density is too high, use both simultaneously. All the above measurements are taken while weighing the density and adding the filler (experience is very important; with a little more time, you will naturally master the dosage).
4. Mixing time: The slurry pot needs about 5-6 minutes. After the slurry and filler are mixed, it needs about 10 minutes, plus visual inspection until it is evenly mixed (depending on the specific situation).

III. Crucial Step—Forming and Warehousing.

1. The flatness, smoothness, adhesion, and thickness uniformity of the sheet material are all closely related to the forming process.
2. Forming and warehousing mainly includes: placing the template, applying oil to the template, starting the machine, cutting, placing the support, lifting the support, trimming the sheet, and handling.
3. These eight steps are interconnected; the saying “if one fails, all fail; if one prospers, all prosper” aptly describes their relationship. Starting the machine is the most critical link in the entire chain. It controls the pace of the entire forming process. Given a timely and sufficient supply of slurry, the amount of sheet material produced during startup is a major factor. The operator must be observant and attentive to all sides, constantly monitoring the remaining quantity of non-woven fabric and fiberglass cloth, paying particular attention to joints. Strict control over excess material is crucial; too much waste leads to waste, while too little results in uneven sheet thickness. More importantly, close monitoring of the sizing agent and reinforcing fabric ensures they reach the edges; incomplete edges affect sheet cutting dimensions, with serious consequences. The operator must also constantly monitor the movement of sheets along the production line and react quickly to any warnings seen or heard. Failure to do so can lead to serious issues like sheet collisions, necessitating sheet repairs or secondary forming, resulting in delays and lost time.
4. 5. Personnel feeding the mold and applying the release agent must adjust the mold feeding speed according to the machine’s startup speed, ensuring the release agent is distributed as evenly as possible on the mold (neither too much nor too little; too much will reduce the adhesion of the board surface; too little will cause sticking, affecting the appearance of the flat board).
5. Cutting personnel on both sides of the production line must apply moderate cutting force. Excessive force will scratch the mold, eventually causing the mold ends to break. Insufficient force will result in boards remaining connected but not completely severed, especially at higher speeds, easily leading to board collisions. After cutting, the cutting head must be pressed to prevent air bubbles from entering.
6. Personnel placing and lifting the trays must control their speed according to the speed of the sheet material movement. The speed of placing and lifting the trays must be fast; otherwise, production progress will be affected, and a fast-moving production line will inevitably lead to tray collisions.
7. Personnel resting the sheets must promptly and quickly address any creases, air bubbles, or missing material on the sheet surface. Repairing after the sheet material has initially set is difficult.

Packaging personnel must promptly and quickly remove the sheets from the trays and ensure a sufficient supply of trays to avoid affecting production progress.

III. Core – Curing.

Simply put, curing is about ensuring that any unfinished chemical reactions continue. Even with advanced production equipment and a scientifically sound formula, improper curing will prevent the production of the 5-1-8 mechanically contributing components we need, thus hindering the production of high-quality boards. The key to curing is controlling the temperature and humidity of the curing environment: temperature around 25℃, relative humidity around 65% (depending on specific conditions).

IV. Cutting and Shaping.

Cutting is performed strictly according to customer requirements to minimize errors.

V. Packaging.

The cut boards should be cleaned, stacked neatly, protected with paper corner protectors, and then wrapped in plastic film for further curing.