SSPC AMBIENT CONDITIONS-2004 MONITORING AND CONTROLLING AMBIENT CONDITIONS DURING COATING OPERATIONS《涂装操作期间的环境条件监控》.pdf

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1、7-3Monitoring and Controlling Ambient ConditionsNovember 1, 2004SSPC: The Society for Protective CoatingsMONITORING AND CONTROLLING AMBIENT CONDITIONS DURING COATING OPERATIONS*IntroductionThe locally prevailing air temperature, the moisture content in the air (% relative humidity), and the temperat

2、ure at which moisture will condense on surfaces (dew point) are commonly called the ambient conditions. If these conditions are not within the ranges required by a coating specication during surface preparation or coating application, problems in obtaining a protective lm with a long-term performanc

3、e are likely to occur.Because of concerns for these problems, specications require the monitoring of ambient conditions using specialized instruments and test methods before the start of work and pe-riodically during the work. Measurement of these conditions is especially important when weather cond

4、itions change during the course of a work shift. Common Coating Defects Related to Adverse Ambient ConditionsSome of the most commonly occurring coating defects related to unfavorable ambient conditions are discussed in the next few paragraphs. The tendency for such defects to occur is often related

5、 to changes in local weather after the work has begun.Low TemperaturesAdverse effects of low temperatures are most commonly related to the complete and proper curing of coatings within a reasonable amount of time:Slow curing of coatings. Slow curing will permit the accumulation of wind-borne dirt, m

6、ildew spores, and other undesirable surface contaminants. It will also increase the time during which the painted surfaces must be isolated from trafc or other construction work to avoid damage or contamination.Incomplete curing. Many latex coatings applied below 40 F will not coalesce to form durab

7、le lms. Also, thermoset-ting coatings that cure by chemical reactions of two parts or by reaction with oxygen in the air will have a much longer curing time and may never cure completely.Improper curing. At low temperatures, many thermoset-ting (e.g., two-part) coatings may cure by mechanisms other

8、than that intended by the formulator.Incomplete leveling. Low temperatures may reduce the viscosity (uidity) of a coating applied to a surface, so that it cannot level properly (ow together to form a uniformly thick lm). When brushing, incomplete leveling may result in brush marks in the cured lm; w

9、hen spraying, incomplete leveling may result in orange peel (irregular thicknesses in the cured lm resembling an orange skin).High TemperaturesAdverse effects of high temperatures on coatings are usu-ally related to their accelerated drying or curing rates:Re-rusting. High temperatures accelerate th

10、e rusting of cleaned steel.Rapid drying of lacquers. Coatings called lacquers form a protective lm on substrates merely by evaporation of the solvent in which their resins are dissolved. Accelerated evaporation of the solvent during spray application at high temperatures may result in an inability o

11、f the wet lm to ow together (level) to form a continuous lm of even thickness. This results in dry spray (formation of a rough surface with pinholes or voids).Rapid drying of latex coatings. Latex coatings (disper-sions of resins in water) form a protective lm by coalescence of their resins as the w

12、ater evaporates. If temperatures exceed those recommended by the manufacturer, rapid drying will cause a low quality protective lm to be produced.Rapid curing of coatings. Most coatings cure to form a protective lm by a chemical reaction, either between sepa-rately-packaged paint components, or with

13、 water or oxygen in the air. These chemical reactions are accelerated at high temperatures to cause more rapid curing of coatings. Rapid curing often results in signicant shrinkage, stresses, and other harmful effects on the coating.Effects of exotherm. The reaction of separately-packaged chemically

14、-curing coatings generates a heat called exotherm when the two parts are mixed together. This exotherm, along with a prevailing high temperature, may further accelerate the curing rate to cause harmful effects on coatings. Also, the exotherm may reduce the viscosity signicantly to interfere with nor

15、mal application or curing of the coating. The greater the volumes of Parts A and B that are mixed together for application, the greater will be the exotherm and its harmful effects.* Although this monograph has undergone peer review, it is not an ofcial SSPC standard developed through SSPCs standard

16、s development process.7-4Monitoring and Controlling Ambient ConditionsNovember 1, 2004Reduced induction time. The induction time for two-part, chemically-curing coatings is the time required to produce an exotherm that will cause the chemical curing of the coating to continue to completion. Inductio

17、n times will be shortened or may not even be necessary at high temperatures.Reduced pot life. The pot life of two-part, chemically-cur-ing coatings is the period of time after mixing and induction (if any) during which the coating can successfully be applied. If the pot life is exceeded, the mixed p

18、roduct is too viscous (thick) for proper application. In addition to the high ambient temperature, the exotherm will further reduce the pot life.Reduced recoat window for topcoating. Two-part, chemically-curing coatings can only be topcoated successfully in the limited time range specied by their ma

19、nufacturer. If the next coat is applied too soon or too late, harmful results will occur to the protective lm. The recoat window of time will be signicantly reduced at high temperatures.Wrinkling. At high temperatures, alkyd and other drying oil-containing coatings that cure by reaction with oxygen

20、in the air may react rapidly at the coating surface to form a skin that will not permit further oxygen penetration to cure the underly-ing coating. Shrinking of the skin results in wrinkling of the coating surface to form rows and furrows with uncured coating beneath. Wrinkling is more likely to occ

21、ur if these coatings are applied in a thick heavy coat.Pinholes in topcoats of inorganic zinc-rich coatings. When topcoating inorganic zinc-rich coating lms, rising tem-peratures may cause air or solvent vapors entrapped in the pores of the lms to be emitted and rise to the surface of the wet topcoa

22、ts. This will cause pinholes to form in the topcoats.Outgassing from concrete. Rising temperatures during the coating of bare concrete may cause air entrapped in its pores to be emitted and rise to the surface of the wet lm to leave pinholes in it. Sealing of the concrete will reduce the tendency to

23、 form pinholes by outgassing.Low HumiditiesHarmful effects of low humidities are usually associated with changes in coating curing rates:Incomplete curing of inorganic zinc-rich coatings. Solvent-borne inorganic zinc-rich coatings require moisture from the air for curing. On dry days, it may be nece

24、ssary to spray water onto these coatings for complete curing.Curing of polyurethane and polyurea coatings. Single-part polyurethane and polyurea coatings cure by reaction with moisture in the air. Unless the humidity is above 20%, the coatings will not cure satisfactorily.Curing of water-borne coati

25、ngs. On hot, dry days, wa-ter-borne coatings may cure so fast that they do not produce durable lms.High HumiditiesFlash rusting. High humidities will greatly accelerate re-rusting (ash rusting) of cleaned steel. This is why dehumidiers are often used to reduce the humidity in closed spaces.Reduced b

26、onding strength. Moisture condensed on cleaned surfaces, with or without rusting, may reduce the bonding strength of the coating to the surfaces.Blushing of lacquers. High humidities may cause solvent blushing during spray application. The rapidly evaporating sol-vent reduces the temperature at the

27、surface of the wet coating lm to the dew point, and moisture condensation occurs.Blistering. Condensed water on surfaces being coated may cause blistering of the coatings.Improper curing of coatings. If the ambient humidity is over 80%, moisture in the air will react so rapidly with the single-part

28、(moisture-curing) polyurethane and polyurea coat-ings, that these reactions compete with the intended curing reactions, and durable protective lms cannot be achieved. Because of their reactions with moisture, exposure to air of moisture-curing polyurethane and polyurea coatings should occur only dur

29、ing actual application.High Dew Points It is a common rule that the surface temperature must be at least 5 F above the dew point and not falling to prevent moisture condensation. Harmful effects of moisture condensation on protective lms were discussed in the previous paragraph. High WindsHigh winds

30、 (above 15 miles per hour) at job sites may also have harmful effects on coating operations:Contamination of cleaned surfaces. Winds may blow dirt, dust, and other debris onto cleaned surfaces. Unless removed before painting, this contamination will reduce the bond strength of coatings applied to th

31、e contaminated surfaces.Contamination of uncured coatings. Wind may blow dirt, dust, and other debris onto the uncured coating. This will produce an unsightly appearance to nish coats. If a topcoat is to be applied over the contaminated coating, the contami-nation will reduce the bonding strength of

32、 the topcoat to the undercoat.Overspray. High winds make control of overspray more difcult. Wet paint mist may be carried outside the paint area onto automobiles or other unintended surfaces. Exterior application of paint should cease when wind velocity reaches 25 miles per hour or more, either in g

33、usts or at a steady state. Where surfaces can be shielded from wind by articial deection means, work may continue. Ambient Conditions to Monitor/TrackRelative Humidity and Dew PointInstruments used to measure percent relative humidity and dew point are called psychrometers. The three basic types of

34、psychrometer are:Sling psychrometer. A sling psychrometer has two ther-mometers, the bulb of one of which is tted with a cotton sock soaked in water. As the thermometers are whirled through the 7-5Monitoring and Controlling Ambient ConditionsNovember 1, 2004air, water evaporates from the wet sock to

35、 lower the “wet bulb thermometer” below that of the “dry bulb thermometer”. Tables of the U.S. Weather Bureau are used to relate the amount of “temperature depression” to relative humidity and dew point.Battery-powered psychrometer. A battery-powered psychrometer operates on the same principal as th

36、e sling psychrometer, but a fan is used to blow air across the ther-mometers.Electronic psychrometer. Electronic psychrometers with special sensors are much more expensive and easy to use, but many are not suitable for exterior service. Others may perform continuous or intermittent monitoring.Wind V

37、elocityWind meters can be used to determine when wind speeds exceed specication requirements during coating application. The meter is positioned vertically in the wind stream, and its velocity is read directly from the scale. Digital wind meters are also available. Surface TemperatureAlthough not re

38、ally an ambient condition, the surface temperature of substrates is an important local condition that requires periodic measurement to ensure good coating per-formance. The three basic types of instruments for measuring substrate surface temperature are listed below in order of increasing price:Dial

39、 thermometers. Dial thermometers have bi-metallic springs that expand with increasing temperatures to move the indicator needles on the thermometer faces higher on the scales. Magnets secured to the backs of the thermometers hold them in place on vertical steel surfaces.Digital contact thermometers.

40、 Battery-powered digital contact thermometers utilize thermocouples to measure sur-face temperatures. This type of thermometer is usually the most accurate. These thermometers are often combined with electronic psychrometers in a single instrument.Non-contact infrared thermometers. Battery-powered,

41、non-contact thermometers utilize infrared light emitted from the surface to determine temperature. This instrument is also good for measuring the temperature of mixed coatings. Some electronic psychrometers also have surface temperature measuring capabilities.Control of Ambient ConditionsThe easiest

42、 way to conform to ambient condition require-ments during coating operations is to blast and paint only during those times of the day when the conditions meet the specication or coating manufacturers requirements. This may require working at night.Also, at many locations during the winter, ambient c

43、ondi-tions can only be achieved in enclosed shops with climate controls. In these places, exterior painting operations should be scheduled for times of the year when conditions are more appropriate. The atmospheric conditions in tanks and other enclosed spaces can readily be controlled by heating/co

44、oling and hu-midication/dehumidication. Humidication may be required to fully cure inorganic zinc-rich coatings. Control of temperature and dew point can prevent rusting of an abrasive blasted steel surface virtually indenitely. Control of wind, as well as temperature and dew point/relative humidity

45、, on exterior surfaces can be made using containment that isolates the work area. Containment can also control contamination of surrounding environments with particulate debris from abrasive blasting. SSPC-Guide 6, “Guide for Containing Debris Generated During Paint Removal Operations” describes var

46、ious methods of containment and describes four classes or levels of containment.Utilization of Information on Ambient ConditionsInformation on ambient conditions and past histories of local conditions must be effectively utilized to minimize potential coating problems associated with improper ambien

47、t conditions. Coating personnel must be able to anticipate likely changes in ambient conditions and their potentially adverse effects. Measurements of ambient conditions should be made more frequently as any of the measurements approaches a permissible limit. In the morning, typically, both the temp

48、erature and the dew point rise, and the relative humidity falls. As the sun gets higher in the sky, more favorable conditions may be anticipated. In the late afternoon, temperatures are expected to fall and humidities to rise, so that potential adverse effects must be addressed.There are many case h

49、istories where the inspector received acceptable temperature measurement (e.g., 95 F when the maximum temperature is 100 F) in the morning but failed to take additional measurements later in the day as the temperature rose. The higher temperatures in the afternoon exceeded the upper permissible level and resulted in poor quality coating lms. Changes in weather conditions can be anticipated from local weather reports. Also, many locations are susceptible to rapid weather changes (e.g., sudden fogs near coasts, sudden rains in tropical areas, and