How to choose resin to solve the cause of stone cracking?
Artificial stone made of unsaturated polyester resin and inorganic filler is divided into categories: granite, quartz stone, solid surface material, light stone and jade. Most manufacturers have encountered deformation, cracking or fracture of the product, and the reasons for the formation are different. Taking quartz stone manufacturing as an example, the causes and solutions of stone deformation and cracking are discussed.
In essence, the root cause of deformation and cracking of the sheet is because there is internal stress in the sheet; regardless of the crack, the appearance is crack, irregular curve, divergence, fracture, etc., the fundamental point is stress concentration, that is, sheet The internal stress generated during molding is not uniformly dispersed, and the absolute value of the internal stress is excessive.
Discussion of related influencing factors
1. The influence of resin
In the molding process of the sheet, the unsaturated resin is subjected to the action of the polymerization initiator, from the gel to the solidification, from the liquid to the solid, and the volume is contracted to generate a large internal stress, which is the source of the stress in the sheet.
For the production of the sheet, the higher the reactivity of the resin or the higher the exothermic peak, the more favorable it is to fully cure at a later stage, but the absolute value of the internal stress generated after curing is also greater, so when the resin is selected, the exothermic peak is not suitable. Too high; but not too low, moderately suitable; the content of styrene monomer in the resin should not be too high. At the same time, the larger the amount of the resin, the larger the volume shrinkage; therefore, the resin content should not be too high.
2. Effect of curing system
●The conventional normal temperature curing system of methyl ethyl ketone plus accelerator has many drawbacks:
1) sensitive to changes in temperature and humidity, need to adjust the amount frequently;
2) The reaction rate is not easy to control, and the heat is concentrated; usually, the polymerization rate is initiated quickly, and it is not easy to achieve sufficient curing of the resin in the later stage of the reaction, which requires a certain time and temperature;
3) Gel curing at room temperature, short operation time, and high temperature weather is more difficult to control;
It is recommended to use OT curing agent (tert-butyl peroxy-2-ethylhexanoate) for medium temperature curing: OT addition amount 1-2% (by weight ratio of resin), heat curing at 85 ° C for 2-2.5 hours.
●The advantages of medium temperature curing:
1) The operation time before entering the press is not limited, and the OT curing agent (below 65 ° C) does not initiate polymerization at room temperature;
2) There is basically no requirement for the gel time of the resin; considering the storage stability of the resin itself, the gel time may be selected to be slightly longer;
3) The reaction rate is easy to control, and in the later stage of the reaction, the curing agent can still maintain sufficient activity to ensure the late curing of the resin is sufficient;
At present, there are still some backward processes using a room temperature curing system. In fact, the standard production process has adopted a medium temperature curing system.
3. The effect of the cooling process
The cooling process refers to the temperature change process of the sheet in the drying tunnel from the highest temperature point to the room temperature. Most of the post-cure curing of the resin is done at this stage, so there are certain time and temperature requirements.
Generally speaking, after the polymerization reaction starts and reaches the exothermic peak, the overall heat release of the sheet is obvious. There is no need to heat the external source, and the temperature is gradually lowered in the drying tunnel. After exiting to room temperature, the sheet still has a temperature of about 60 °C. The temperature difference with room temperature is acceptable.
● Need to pay attention to is:
1) Control the temperature of the exit of the drying tunnel and the temperature of the sheet, and the temperature difference between the room and the room temperature should be minimized, and the temperature difference should preferably not exceed 50 °C. If the temperature difference is too large, it is equivalent to “quenching” the hot plate, which increases the internal stress of the plate and exacerbates the stress concentration phenomenon, greatly increases the probability of plate cracking and buryes hidden dangers – the plate is easy to break under external force. .
2) Hot plates should not be directly ground and polished; hot plates will quickly cool down after contact with water, and the surface is prone to whitish and watery lines. Generally, it can be polished when the hand touches the temperature. If it is conditional, it can be treated after the hot plate is left for 24 hours.
4. Effect of coupling agent
The main chemical component of the coupling agent is γ-methacryloxypropyltrimethoxysilane, and the coupling agent can react with the surface of the filler particles to promote better compatibility between the filler and the organic matter; Increases the surface adhesion to the particles as the resin cures. Since a large amount of inorganic sand, powder and the like are used in the plate, the addition of the coupling agent can greatly improve the surface adhesion of the resin and the filler, and reduce the occurrence of "split" of the plate; therefore, the coupling agent is indispensable. The minimum value is not less than 1%, and the suitable addition amount is 2-4% (according to the weight ratio of the resin, adjusted according to the concentration of the coupling agent). This factor is often ignored or ignored by inexperienced manufacturers.
In summary, there are many factors that can affect the cracking of sheet products, but the resin content is controlled by selecting a suitable resin; the medium temperature curing process is applied to reduce stress concentration; the internal stress is evenly dispersed, and the absolute value of internal stress is reduced; Avoid "quick cooling"; add proper amount of coupling agent and other correct operation to avoid cracking of the product.
NEWECO is a large-quality resin supplier in the field of artificial stone. The products are mature and have rich application experience.
NEWECO used quartz stone resin recommended
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