What are the causes of powdering in putty?
Release Time:
2016-08-31
Source:
What are the causes of powdering in putty?
What are the causes of powdering in putty?
1. Insufficient bonding strength of putty powder is often due to low bonding strength of the putty adhesive powder or an insufficient dosage, particularly in interior wall putties. The bonding strength—and whether the putty flakes off or not—are closely related to the quality and dosage of the interior wall putty adhesive powder.
2. The quality of hydrated lime is critically important; excessive impurities in the hydrated lime, or an inappropriate CaO to Ca(OH)2 ratio, can adversely affect its performance.
3. The formulation design is unreasonable. The addition levels of talc, hydrated lime powder, hydroxypropyl methylcellulose, and other additives are improperly balanced.
4. The selection of raw materials and the formulation structure are critical in putty recipes. For example, when using HPMC in interior wall putties that are not waterproof, although HPMC is expensive, it has little effect on fillers such as double-fly ash, talc, and wollastonite. If only HPMC is used, powder shedding will occur. In contrast, lower-cost CMC and CMS do not cause powder shedding; however, they cannot be used to formulate waterproof putties or exterior wall putties, because CMC and CMS react with hydrated lime and white cement, which also leads to powder shedding. Similarly, adding polyacrylamide to hydrated lime and white cement to produce a waterproof coating can trigger chemical reactions that result in powder shedding.
5. Defects in the manufacturing process can lead to powder shedding in putty. If the mixing equipment lacks a cleaning function, significant residue will remain, causing the CMC in standard putty to react with the hydrated lime powder in waterproof putty, and the CMC and CMS in interior wall putty to react with the white cement in exterior wall putty, thereby resulting in powder shedding.
6. Variations in the quality of fillers can also easily lead to powder shedding. Interior and exterior wall putties typically contain large amounts of fillers; however, the CO2 and CO3 content—and the pH—of heavy calcium carbonate and talc powders vary from region to region, which can cause putty to lose powder. For example, when using the same putty binder but different talc and heavy calcium carbonate powders, interior wall putty from Chongqing and Chengdu may perform differently: the Chongqing formulation does not shed powder, whereas the Chengdu formulation does; similarly, the Henan and Northeast formulations show no powder shedding, while in some other regions powder shedding occurs.
7. Weather conditions are also a contributing factor to powdering of interior and exterior wall putties. For instance, in some arid regions of northern China with dry climates and good ventilation, the putty film forms quickly and does not powder. In contrast, this is not the case in southern China, particularly in areas prone to the plum rain season, where prolonged humidity impairs the film-forming properties of the putty, leading to powdering. Therefore, in such regions, it is advisable to use water-resistant putties formulated with lime powder.
8. Impure inorganic binders such as lime powder and white cement often contain large amounts of double-fly ash. Many lime powders and white cements on the market are impure; because these impure inorganic binders are used extensively, waterproof putties for both interior and exterior walls are bound to chalk and fail to provide effective waterproofing.
9. In summer, exterior wall putty often lacks sufficient water retention; particularly at high-rise building entrances and window openings—areas subject to high temperatures and strong ventilation—the lime powder and cement may lose moisture before their initial setting time has elapsed, and inadequate curing can further lead to severe powdering.
