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Why Are They Important?In the Technical Bulletins,Crystallization of Liquid EpoxyResins (Form No. 296-01652) andAmine Blushing and Blooming ofEpoxy Binder Systems (Form No.296-01656), we discussed some of thepotential effects of temperature andhumidity on liquid epoxy resins andbinder performance. While it is obviousthat laying an epoxy flooring orapplying an epoxy coating cannot bedone in the rain, there are somegeneral requirements that should beconsidered with respect to the humidityand dew-point temperature. Failure toconsider these parameters can result insevere coating defects caused either bycondensation on the substrate beforecoating or condensation on the coatingwhile it is still sensitive to water.

With most formulated epoxy bindersystems, the supplier normallyprovides operating guidelines andinstructions that typically describehow to handle the material as well asrequirements for the quality of thesubstrate and environmentalinfluences.

One might ask then, “What are theoperating guidelines in relation totemperature / humidity for the optimumcuring of ambient curing epoxybinder systems?” This questionshould be answered by the epoxyresin formulator. Most formulatorrecommendations indicate that themaximum humidity tolerated in the

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letinair will depend upon the temperature.

It is generally recommended that theminimum application temperatureshould be at least 3˚C above the dew-point temperature. This prerequisitegenerally ensures that condensationon the substrate and coating, whichwould result in severe coating defects,is avoided.

TemperatureThe minimum (dry-bulb) airtemperature at which an ambientcuring epoxy formulation can beapplied and still provide for asufficient curing rate is an extremelyimportant parameter. The reactivityof an epoxy binder system is reducedby lowering the cure temperature. Thereduced curing rate not only slowsdown the coating job but alsoincreases the risk of coatingdisturbances such as blushing orblooming. A commonly accepted ruleof thumb is that the curing time willincrease by a factor of 2 for every10˚C decrease in the curing temperature.In practice, this means that a coatingsystem which has a through filmdrying time of 6-8 hours undernormal ambient conditions of 25˚Cwill have a drying time of around24-32 hours (factor of 4) at 5˚C.This means that the coating job whichtypically could have been continuedthe next day would require at leastone extra day.

The Effects of Humidity and(Dew-point) Temperature onAmbient Cure Epoxy Coatings

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requirements. Typical commercialambient curing epoxy binderformulations will have a recommendedminimum application temperatureof around 10˚C (mostly based onformulated epoxy-amine adducts).This is, in general, considered asthe low(er) end of the workingtemperature range in ambient cureapplications. However, severalcuring agent technologies, such asMannich-based hardeners, havebeen developed which offer anextended curing range down tozero or sub-zero temperatures.

Humidity and Dew-pointIn order to explain and define thedew-point temperature, it isnecessary to understand somebasics about humidity and relativehumidity.

The amount of water vapor in theair is often expressed as absolutehumidity. The total amount ofwater the air can hold isdependent on the air temperature.The absolute humidity indicatesthe amount of water in a certainvolume of air at a certaintemperature and is subsequentlyexpressed in gr/m3 (@ X ˚C). As thetemperature of the air increases,the volume of water the air canhold also increases. See Table 1.

The minimum applicationtemperature limit needs to bedetermined by the formulator andspecified in his product’s operatingguidelines. The recommendedminimum temperature shouldtake into account the minimumtemperature to achieve cure andshould also consider what willhappen if initial cure takes placeat lower temperatures and thecoating subsequently recovers to“regular” ambient temperatures.Does the system recovercompletely and reach its fullrequired performance properties?

The rate of cure at a certaintemperature is determined by theindividual reactivity of the epoxyresin and of the curing agent.Although the key parameter indetermining the minimumapplication temperature is thecuring agent type, the selection ofthe epoxy resin can also play arole. Epoxy resins which aresolely based on aromatic glycidylethers typically have a higherreactivity with amine-type curingagents than resins which containaliphatic glycidyl ethers. Forexample, the gel time of a 100gram mixture of D.E.R.™ 321Liquid Epoxy Resin, an aromaticglycidyl ether modified bisphenol Aepoxy resin, with a stoichiometricamount of diethylenetetramine(D.E.H.™ 20 Amine Hardener) isapproximately 50 minutes. Asimilar mixture based on D.E.R.324 Epoxy Resin, an aliphaticchain glycidyl ether modifiedbisphenol A epoxy resin, withD.E.H. 20 Amine Hardener wouldrequire around 80 minutes to gel.

A wide variety of curing agentshave been developed over theyears to address several performance

The more commonly used relativehumidity is the amount of waterin the air expressed as a percentageof the maximum amount of waterthe air can hold at a giventemperature. The relative humidityis the ratio of the water vaporcontent (amount of water vaporactually in the air) compared tothe water vapor capacity(maximum amount of watervapor the air can hold), at thatparticular temperature. Saturatedair at a given temperature can bereferred to as 100 percent relativehumidity.

Table 1

Temperature Maximum Water Vapor(˚C) (gr/m3)

0 4.8

5 6.8

10 9.5

15 12.8

20 17.3

25 23.0

30 30.4

35 39.6

40 51.1

45 65.0

RelativeHumidity (%)

Water Vapor Content (gr/m3)

Maximum Water Vapor Content (gr/m3)X 100%=

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If we say the relative humidity is50%, this indicates that the air isholding half of its maximumpossible amount of moisture at thegiven temperature. If we increasethe temperature, the maximumamount of moisture the air canhold will increase. Since the actualwater content will not change, thismeans the relative humidity willdecrease. In case the temperaturedrops, then the relative humiditywill increase ultimately to the levelwhere the maximum concentrationwill be achieved; 100% relativehumidity. Further reduction of thetemperature will force the water tocondense and water droplets (dew/fog) will form and the dew-pointhas now been reached. The dew-point, or the temperature at whichcondensation occurs, dependson the amount of water vapor inthe air.

The dew-point temperature is thetemperature to which air must becooled for saturation (100% relativehumidity) to occur, provided there

is no change in water content. Thedew-point temperature is animportant measurement used topredict the formation of dew andfog. If the dew-point temperatureand the air temperature are closetogether in the late afternoon whenthe air begins to turn colder, fog islikely to develop during the night.The dew-point temperature is alsoa good indicator of the air’s actualwater vapor content, unlike relativehumidity, which is air temperaturedependent. Since condensationoccurs when the air temperaturereaches the dew-point temperature,and condensation releases heat intothe air, reaching the dew-pointtemperature halts the coolingprocess.

There are some extremely complexequations used to calculate thedew-point temperature. A morepragmatic way to determine thedew-point temperature is to use thevalues shown in Table 2. The (dry-bulb) air temperature (˚C) in thistable is represented by the

horizontal scale and the bulk ofthe table indicates the relativehumidity. The corresponding dew-point temperatures are given in thevertical (bold) columns. Forinstance at a (dry-bulb) airtemperature of 25˚C and a relativehumidity of 50% the dew-pointtemperature would be 14˚C. If the

relative humidity had been78% at the sametemperature the dew-pointtemperature would be21˚C.

A coating job cantypically be performedonly if the temperature isat least 3˚C above thedew-point temperature.For instance, at atemperature of 23˚C and a

relative humidity of 65% the dew-point temperature is 16˚C and thuswe can continue fabrication. Iflater in the afternoon thetemperature drops and approaches19˚C (the relative humidity willincrease), we will have to stop ourapplication process because wewill no longer operate at morethan 3˚C above the dew-pointtemperature.

If the initial relative humidity hadbeen 83% at 23˚C, then the dew-point temperature would havebeen 20˚C and thus fabricationcould not have been initiated.

Note that at a relative humidity of85%, the lowest possible substratetemperature is equal to the roomtemperature. This means that inorder to maintain the safety marginof operating at a temperature of atleast 3˚C above the dew-pointtemperature, a relative humidity of85% cannot be exceeded unless wewarm the substrate.

Technical Bulletin

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Commonwealth Bureau of MeteorologyConversion Chart

Dew-point to Relative HumidityDEW DEWPT PT

˚C 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 ˚C

30 100 94 89 84 80 75 30

29 100 94 89 84 80 75 71 29

28 100 94 89 84 79 75 71 67 28

27 100 94 89 84 79 75 71 67 63 27

26 100 94 89 84 79 75 71 67 63 60 26

25 100 94 89 84 79 75 70 67 63 60 57 25

24 100 94 89 84 79 74 70 66 63 59 56 53 24

23 100 94 89 84 79 74 70 66 63 59 56 53 50 23

22 100 94 89 83 79 74 70 66 62 59 56 53 50 47 22

21 100 94 89 83 78 74 70 66 62 59 55 52 49 47 44 21

20 100 94 88 83 78 74 70 66 62 58 55 52 49 46 44 42 20

19 100 94 88 83 78 74 69 65 62 58 55 52 49 46 44 41 39 19

18 100 94 88 83 78 73 69 65 61 58 55 52 49 46 43 41 39 37 18

17 100 94 88 80 78 73 69 65 61 58 54 51 48 46 43 41 38 36 34 17

16 100 94 88 83 78 73 69 65 61 57 54 51 48 45 43 40 38 36 34 32 16

15 100 94 88 83 78 73 69 64 61 57 54 51 48 45 43 40 38 36 34 32 30 15

14 100 94 88 82 77 73 68 64 60 57 54 50 48 45 42 40 38 36 34 32 30 28 14

13 100 94 88 82 77 73 68 64 60 57 53 50 47 45 42 40 37 35 33 31 30 28 27 13

12 100 94 88 82 77 72 68 64 60 56 53 50 47 44 42 39 37 35 33 31 29 28 26 25 12

11 100 94 88 82 77 72 68 64 60 56 53 50 47 44 41 39 37 36 33 31 29 28 26 25 23 11

10 100 94 88 82 77 72 68 63 59 56 53 49 46 44 41 39 37 34 32 31 29 27 26 24 23 22 10

9 100 93 87 82 77 72 67 63 59 56 52 40 46 43 41 38 36 34 32 30 29 27 26 24 23 22 20 9

8 100 93 87 82 76 72 67 63 59 55 52 49 46 43 41 38 36 34 32 30 28 27 25 24 23 21 20 19 8

7 100 93 87 82 76 71 67 63 59 55 52 49 46 43 40 38 36 34 32 30 28 26 25 24 22 21 20 19 18 7

6 100 93 87 81 76 71 67 62 58 55 51 48 45 43 40 38 35 33 31 30 28 26 25 23 22 21 20 19 18 17 6

5 93 87 81 76 71 66 62 58 55 51 48 45 42 40 37 35 33 31 29 28 26 24 23 22 21 19 18 17 16 16 5

4 87 81 76 71 66 62 58 54 51 48 45 42 39 37 35 33 31 29 27 26 24 23 22 20 19 18 17 16 15 14 4

3 81 76 71 66 62 58 54 51 47 44 42 39 37 34 32 30 29 27 25 24 23 21 20 19 18 17 16 15 14 13 3

2 75 70 66 61 57 54 50 47 44 41 39 36 34 32 30 28 27 25 24 22 21 20 19 18 17 16 15 14 13 13 2

1 70 66 61 57 54 50 47 44 41 39 36 34 32 30 28 26 25 23 22 21 20 18 17 16 15 15 14 13 12 12 1

Table 2

Dry-Bulb (˚C)

Technical Bulletin

5Form No. 296-01667-1207X-TD

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Notice: No freedom from any patent owned by Dow or others is to be inferred. Dow assumes no obligation or liability for the information in this document. The informationprovided herein is presented in good faith and is based on the best of Dow’s knowledge, information, and belief. Since use conditions at non-Dow facilities are beyond Dow’scontrol and government requirements may differ from one location to another and may change with time, it is solely the Buyer’s responsibility to determine whether Dow’sproducts are appropriate for the Buyer’s use, and to assure the Buyer’s workplace, use, and disposal practices are in compliance with applicable government requirements.Consequently, Dow assumes no obligation or liability for use of these materials and makes no warranty, express or implied. The user of the information provided is solelyresponsible for compliance with any applicable government requirements. NO WARRANTIES ARE GIVEN; ALL IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FORA PARTICULAR PURPOSE ARE EXPRESSLY EXCLUDED. Dow does not endorse any third party products referenced in this document.

® Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of DowPublished December 2007

Technical Bulletin