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Why is HPMC yellowed and how to solve it?

2018-10-21 Hits:385

HPMC has always known a lot. Today, I want to introduce some information about water resistant HPMC.

Measures to prevent water HPMC from yellowing:

1, seal the bottom with seal agent. Old wall decoration, waterproof HPMC low-grade ordinary HPMC before scraping water-proof powdering, high water-proof hpmc, should be done before the use of technical treatment, the method is: first wall spray wetting, with a spade will be old hpmc, paint all eradicated (until the hard bottom) and clean up, after the wall is dry and clean again and brush the sealing agent seal After bottom treatment, scrape water-resistant hpmc, which can effectively prevent the wall base from alkaline and cause the surface of HPMC yellowing.

2, avoid volatile chemicals and pyrotechnics. In the construction process, especially after the construction of HPMC is not completely dry, do not smoke indoors, fire heating, in three months do not use volatile chemicals indoors, such as water-resistant HPMC paint and its diluents.

3, keep the place dry and ventilated. Do not close the doors and windows tightly until the water-resistant HPMC is not completely dry. Open the windows to ventilate and let the HPMC layer dry as soon as possible.

4. The yellowing of the surface of hpmc, the need for technical treatment, the general treatment is: first in the surface of HPMC brush a primer, then scrape high water-resistant HPMC or brush wall latex paint can be.

Let's take a look at why HPMC turns yellowing.

1. Calcium hydroxide (lime calcium powder) alkaline return causes yellowing calcium hydroxide itching, relative molecular weight 74, melting point 5220, PH value (> 12), strong alkaline, white fine powder, slightly soluble in water, acid, glycerol, sugar, ammonia chloride, soluble in acid release a large amount of heat, relative crack resistance mortar density 2.24, its clarified aqueous solution is colorless. The odorless basic transparent liquid is absorbed gradually in the air and calcium oxide becomes calcium carbonate. Calcium hydroxide is moderately alkaline, its alkalinity and corrosiveness is weaker than sodium hydroxide. Calcium hydroxide and its aqueous solution are non-toxic, but it is corrosive to human skin, clothing and so on. It can not be directly contacted with skin for a long time. Calcium hydroxide is a kind of active filler in water-resistant hpmc, which forms hard film with heavy calcium carbonate and high-gloss rubber powder. Because of its strong alkalinity and high alkali content, some moisture in HPMC will be absorbed by the bottom of plaster mortar cement mortar, or by mortar (lime, sand, a small amount of cement) with the same alkalinity of wall base during construction. The alkaline substances in the mortar and HPMC and some unstable substances (such as divalent iron, trivalent iron, etc.) after hydrolysis will be folded out through the micro-pores of hpmc, and chemical reactions will occur when exposed to air, resulting in yellowing of the surface of hpmc.

2. Volatile organic chemical gases. Such as carbon monoxide, sulfur dioxide, benzene, toluene, xylene, formaldehyde, pyrotechnics and so on. In some engineering cases, the surface yellowing of HPMC is caused by the use of paint or fire for heating in the newly scraped waterproof HPMC room, even incense burning in the HPMC room, and many people smoking at the same time.

3, ventilation and dry condition is not good. When the wall is wet and the waterproof HPMC layer is not completely dry after scraping, it is easy to cause yellowing of the surface of HPMC when the door and window are closed for a long time.

4, grass-roots issues. The bottom of the old wall is generally sand-lime wall (lime, sand, a small amount of cement, and some also mixed with gypsum), this kind of wall base strength is low, high water absorption, easy to pulverize, and the bottom of the new wall is mainly cement mortar, but there are still many areas with lime, gypsum plaster wall. Most of the materials in the base wall are alkaline. When the HPMC contacts the wall, some water will be absorbed by the wall. After hydrolysis and oxidation, some substances, such as alkali and iron, will be folded out through the micro-pores of the flexible HPMC wall. Chemical reactions will occur in the air and cause the yellowing of the surface of the hpmc. HPMC 90% can be polished at dry time, which saves energy and does not stick to sandpaper. After grinding, remove the ash layer on the surface. Manual sanding should be sanded paper (cloth) wrapped in a sanding pad, to the reuse force to push the pad fast, can not only press one or two fingers sanding paper, so as not to affect the smoothness of sanding. Electric grinder is often used in mechanical grinding. Sand paper is clamped on the grinder and pushed gently on the base layer. It is forbidden to press the insulation of the exterior wall to avoid overload and damage of the motor.

At present, there are some improper use of HPMC on the site: the use of ordinary HPMC bottom, and then the use of environmentally friendly water-resistant HPMC cover, from a technical point of view, this is very unreasonable practice. Because the strength of ordinary HPMC is very low, on the basis of batch scraping high-strength environmental protection water-resistant hpmc, so it is easy to cause peeling and shedding phenomenon. The strength of the upper wall is very low after drying with plastering gypsum. On this basis, scraping high-strength environmental protection water-resistant HPMC is easy to cause empty drum, peeling, peeling off phenomenon, and water absorption is too strong, greatly increasing the difficulty of follow-up work of external wall insulation materials. In some corners and cracks, use environmental friendly water resistant HPMC to fill leveling, resulting in cracking. Because the environment-friendly water-resistant HPMC can not scrape too thick in batches at a time, otherwise it will shrink and crack itself, so do not use the environment-friendly water-resistant HPMC to leveling, for uneven walls and cracks should be leveled in advance with gypsum and cement mortar products. It is suggested to brush high quality alkali-resistant sealing primer in color coating to ensure the quality of the project due to the alkalinity of the environment-friendly water-resistant HPMC when it is not completely dried.