When the molten powder comes into contact with the sand surface with holes and holes, the melt of Shanghai aluminum profile can penetrate into it and play a role of mechanical anchoring. Although the surface roughening of sand grain can improve the adhesion, we must pay attention to avoid the deep and sharp coating shape (sandpaper-like coating with too low luster of sand grain and too dry surface). This roughened sand grain coating will appear the phenomenon of bottoming out, and the deep and sharp bulge will form an uneven coating, thus generating stress concentration points and reducing the adhesion. When peeling off the back sandblasting coating, it can be seen that the back of the back sandblasting coating is smooth, that is to say, the back sandblasting powder does not really penetrate into the cracks of the first rough sand surface coating when it is melted. If it can't penetrate completely, the contact between the coating and the surface will be smaller than the corresponding geometric area, and there will be a gap between the coating and the substrate. The bubbles in the gap will lead to the accumulation of water vapor, which will eventually lead to the loss of adhesion. This is related to the characteristics of sand grain powder: firstly, the surface tension of the first sand grain coating is lower after curing, and it becomes lower when the coating is over-cured; Secondly, due to the high viscosity of sand grain powder during melting and the "thermal barrier" effect of the first coating during back spraying, the heating temperature of the real workpiece is lower than that of the first spraying, and it is more difficult to reduce the melt viscosity at a lower temperature. The sand grain powder melt with high viscosity and low fluidity is difficult to penetrate into the surface of the first coating at a lower "workpiece temperature", and with the increase of viscosity and coating rigidity, it will gradually form and generate a lot of stress, which will remain in the coating.

Cause analysis of interlayer delamination during back injection

Based on the above theory and the characteristics of grit powder, we can sum up the possible causes of delamination when grit powder is re-blasted:

3.1 polyester resin content is low

If the content of polyester resin is low (the amount of filler is too large), there are few effective film-forming substances, which will lead to incomplete cross-linking of the coating during the curing process, brittle coating after curing, and the coating will fall off when encountering external force.

3.2 Low-grade polyester resin

Polyester resin is formed by polycondensation reaction of step polymerization. Strictly controlling the polycondensation process (controlling the heating rate and vacuum degree in the later stage of polycondensation reaction) can obtain products with good quality with normal molecular weight distribution. If the polycondensation process is not properly controlled (the heating rate is too fast), especially if the vacuum degree is not reached in the later stage of polycondensation, the molecular weight distribution of the obtained polyester resin is wide, which means that there are a large number of heat-resistant low-molecular-weight polycondensates in the resin. At higher curing temperature, these low-molecular-weight polycondensates will decompose and volatilize from the coating and float on the surface of the coating, forming a low-molecular-weight "oil film".

3.3 Improper additives or dosage

Sand grain agent and a certain amount of wax (usually polyethylene wax) should be added to the formula of sand grain powder coating. The main component of sand grain agent is polytetrafluoroethylene, which is the main component of our common Teflon non-stick pan coating. The addition of PTFE and polyethylene wax will reduce the surface tension of the coating, and the appropriate amount will not have a great impact on the adhesion, but if it is used in excess, it will greatly reduce the surface tension of the sand grain coating. A similar non-stick surface with low surface energy is formed, which makes the wetting effect of the coating on its surface worse. It is difficult for the molten back-sprayed powder to completely penetrate into the pores of the undercoat, and there is a gap between the two coatings, which makes it difficult for the back-sprayed sand grain coating to firmly combine with it, and finally leads to unqualified adhesion between the coatings.

3.4 When spraying sand grain powder, too much recycled powder or expired powder is added.

All aluminum profile customers will have a large number of long-term stock powder and recycled powder. In order to consume part of the powder stock and reduce the cost, aluminum profile spraying manufacturers usually mix flat powder or sand grain powder with similar colors that have been in stock for a long time into new powder for spraying. Because the mixed stock powder or recycled powder may have pre-crosslinked reaction or even expired, the proportion of various materials in the sprayed sand grain powder has deviated from the normal new powder formula. When they are used together, the wetting effect of the coating will become worse, and the coating can not completely penetrate into the pores of the bottom sand grain coating, leaving a gap between the back spray coating and the bottom coating, and finally the adhesion is unqualified. Due to the partial pre-crosslinking of polyester resin and curing agent (TGIC or HAA) in expired powder, the effective reaction point of coating curing is reduced (functionality is reduced), which will also seriously affect the adhesion of coating.

3.5 Curing temperature is too low or time is too short

Low curing temperature or short curing time seriously affects the crosslinking reaction between polyester resin and curing agent (TGIC or HAA), and the coating after curing has poor flexibility and poor adhesion.

3.6 Curing temperature is too high or time is too long.

When the coating is cured at a temperature much higher than the original temperature or the baking time is prolonged, the adhesion becomes worse. The main reason is that in these two cases, some coatings are carbonized, which seriously affects the crosslinking between coating molecules. Generally, the curing temperature of polyester powder coatings for aluminum profiles is 180℃(HAA curing) or 200℃(TGIC curing), and the time is 15 ~ 20 minutes. Overcross-linked coating will form a surface with low surface energy, and the high-viscosity melt formed by back blasting coating is difficult to wet its surface with low surface energy.

The reactive carboxyl groups in sand grain powder tend to be more firmly attached to the substrate containing similar groups. If the first coating is cured properly, the remaining small amount of carboxyl groups in the first coating will react with the curing agent TGIC or HAA in the second coating, which is beneficial to bonding the back-sprayed coating with the primer. When the first coating is over-baked (baking time is too long and/or curing temperature is too high), there is almost no carboxyl group left in the first coating, and the adhesion of the back-sprayed coating is obviously weakened, sometimes even no adhesion.

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