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Curing of Polyurethane Elastomers and Foams

Abstract – This paper describes different methodologies and techniques that are used within Dow to 

measure polyurethane curing in various applications. Examples are provided for rigid foams, adhesives, 

and cast elastomers made using a variety of formulations and raw materials. Methods to determine 

curing within a particular subset of materials (foam or solids) are aligned with industry standards.

I. Elastomers

IA. Raw Materials and Formulations

Polyol masterbatches are made with VoranolTM CP 6001 (The Dow Chemical Company), 1,4-butanediol 

(Sigma-Aldrich), and in some cases molecular sieves dispersed in castor oil (UOP). In some instances, 

additional additives are used in the masterbatches to promote property enhancement and aesthetics. 

Catalysts can be either commercially available materials known to promote the reaction of alcohols with 

isocyanates or experimental materials. The isocyanate employed was IsonateTM M340 available from The 

Dow Chemical Company.

IB. Screening Methods

Method A: Into a 20 ml vial, 7 g of polyol master batch is added. To this is added the prescribed amount 

of a catalyst solution. This mixture is stirred with a spatula by hand for one minute. To this is added 3.5 g 

of Isonate M340. This mixture is stirred with a spatula by hand for one minute. After mixing, the vial is 

placed in an insulating foam block, and a thermocouple is inserted in the center of the reaction mixture. 

The tack free gel time is determined as the time when a spatula touched to the surface of the reaction 

mixture pulls away clean without any material sticking to the spatula.

Method B: Method B is similar to Method A but utilizes 100 g of polymer sample. Details of Method B 

can be found in the file titled – “Method B – cold curing.docx”.

Method C: Method C is a hot cure methodology used to make thin polymer sheets (3 mm thickness). 

Polyol masterbatch of Voranol CP 6001, 1,4-butanediol, and UPO L Sieve Paste is prepared and mixed on 

a bottle roller for a minimum of 1 hour. The appropriate amount of catalyst is added to an aliquot of the 

masterbatch and mixed on a FlakTek Speedmixer for 30 seconds at 800 rpm and 1 minute at 2350 rpm. 

The mixture is allowed to cool to room temperature. Isonate M340 is added to the polyol/catalyst 

mixture and mixed on the FlakTek Speedmixer using the identical protocol as above. A sheet mold made 

of aluminum or steel is preheated on a hot plate to 80 °C and the mixture is poured into the mold after 

2.5 minutes from the addition of the Isonate M340. Tack free gel time is determined by tapping the 

surface of the elastomer with a spatula until no more tackiness (stickiness) was noted. Demold time is 

determined as the time when the elastomer could be removed from the mold without tearing or 

deformation. The elastomer is then post-cured in an air-convection oven at 80 °C for 16 hours.
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IC. Elastomer Curing Results

Table 1 shows data pertaining to the tack free gel times obtained with a variety of catalysts. Specific 

details of the catalysts employed here are considered business confidential.

Table 1.

Sample Cure Time 

Methodology

Tack Free Gel Time 

(minutes)

Demold Time (minutes)

2010004277- 1 Method C 4.3 5.5

2010004277- 2 Method C 3.5 4.2

2010004277- 3 Method C 3.8 4.3

2010004277- 4 Method C 3.8 4.5

2010004567-CE1 Method C 4.7 6.0

2010004567-CE2 Method C 3.5 5.0

2010004567-IE1 Method C 4.7 8

2010004567-IE2 Method C 4.0 4.8

2010004567-IE3 Method C 3.8 4.5

2010004567-IE4 Method C 4.0 5.3

2010004567-IE5 Method C 3.8 4.8

2010004567-Run1 Method A 6.2 NA

2010004567-Run2 Method A >10 NA

2010004567-Run3 Method A <5 NA

2010004567-Run4 Method A 5.8 NA

2010004567-Run5 Method A 5.0 NA

2010004567-Run6 Method A 4.8 NA

2010004567-Run7 Method A 5.2 NA

2011012692-1 Method B 8.6 NA

2011012692-2 Method B 8.6 NA

2011012692-3 Method B 8.2 NA

2011012692-1 Method C Not measured 9.5

2011012692-2 Method C Not measured 5.5

2011012692-3 Method C Not measured 5.5

2011015929-L1_1 Method A 2.6 NA

2011015929-L1_2 Method A 3.3 NA

2011015929-L2_1 Method A 4.5 NA

2011015929-L2_2 Method A 3.6 NA

2011015929-L3_1 Method A 3.1 NA

2011015929-L3_2 Method A 1.8 NA

2011015929-A_L1 Method A >10 NA

2011015929-A_L2 Method A >10 NA

2011015929-G_L1 Method A <1 NA

2011015929-G_L2 Method A 2.3 NA

2011015929-G_2L1_F Method A 5.8 NA

2011015929-G_2L2_F Method A 6.6 NA

2011015929-G_3L1_F Method A 7.5 NA
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Sample Cure Time 

Methodology

Tack Free Gel Time 

(minutes)

Demold Time (minutes)

2011015929-G_3L2_F Method A 6.3 NA

2011015929-TDB Method C 3.5 3.7

2011015929-1638-19AB Method C 4.6 10

2011015929-1638-19AC Method C 4.8 10

2011015929-1638-20AB Method C 3.3 6.5

2011015929-1C Method A 7.3 NA

2011015929-1638-2C Method A 6.8 NA

2011015929-1638-3C Method A 9 NA
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II. Adhesives

IA. Raw Materials and Formulations

An isocyanate prepolymer of 1.18 – 1.34% NCO was cured via atmospheric moisture with a variety of 

commercial metal based catalysts.

IB. Screening Methods

Gel Time Test: The gel time is tested by an ARES-G2 from TA Instruments and determined by the 

crossover point of the storage modulus (G’) and the loss modulus (G”). The test conditions are: 

temperature of 23 °C; humidity of 50%; and a strain of 2%.

Tack Free Time: A bead of sample is placed on paper and tested with polyethylene film over time for 

tackiness. Tack free time was determined when no uncured material is transferred to the polyethylene 

film when the film is pressed onto the sample bead and pulled away. Tack Free results are dependent on 

humidity and temperature. Work is done in an environmentally controlled room to lay up the beads so 

that the bead is exposed to the correct humidity and temperature during the tack free curing process. 

IIC. Curing Results

Table 2 shows data pertaining to the gel and tack free times obtained with a variety of catalysts. Specific 

details of the catalysts employed here are considered business confidential.

Table 2.

Sample Name Gel Time (seconds) Tack Free Time (Minutes)

2011018341-GB-1 2620 90

2011018341-GB-2 4687 120

2011018341-GB-3 7085 150

2011018341-GB-4 29330 420

2011018341-GB-5 18830 420

2011018341-GB-6 17770 290

2011018341-GB-7 21640 300

2011018341-GB-8 35280 420

2011018341-GB-9 28410 320

2011018341-GB-10 19870 240
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III. Rigid Foams

IA. Raw Materials and Formulations

Results below are based on a number of different rigid foam formulations used in both the appliance 

and construction industries. A typical formulation includes polyether and/or polyester polyols; 

surfactants; flame retardants; a physical blowing agent; catalyst(s); and polymeric methylene diphenyl 

diisocyanate (p-MDI). 

IB. Screening Methods

Cream Time; Stringing Gel Time; and Tack Free Time: Curing in rigid foams is measured by first preparing 

a masterbatch of polyol and all additives except the isocyanate and mixing thoroughly. Then, the 

isocyanate is dosed in to a predefined amount of masterbatch and this mixture is further mixed for a 

defined amount of time (5 – 30 seconds). The cream time is measured as a visual observation of when 

the reacting mixture turns from a clear, dark brown liquid to an opaque light colored liquid. Stringing gel 

time is defined as the time when the reacting material forms a string or fiber when an implement (i.e. a 

tongue depressor) is inserted into the reacting mixture and quickly pulled away. Tack free time is the 

point when the surface of the foam is no longer sticky to the touch. It is important to note that not all 

three measurements are taken in every foaming experiment. Definitions within this method can be 

found in ASTM D7487.

Also, it should be noted that cure time targets can vary over a relatively broad range (seconds to 

minutes) depending on the targeted application.

IIC. Curing Results

Table 3 shows data pertaining to the cream, string, and tack free times obtained with a variety of 

formulations. Specific details of the catalysts employed here are considered business confidential.

Table 3.

Sample Cream Time (Seconds) String or Gel Time 

(Seconds)

Tack Free Time (Seconds)

2011016161-245fa_10 Not Measured (NM) 84 NM

2011016161-HBA-2_10 NM 83 NM

2011016161-FEA 1100_10 NM 80 NM

2011016161-AFA-L1_10 NM 80 NM

2011016161-CR961_12 NM 110 NM

2011016161-245fa_12 NM 110 NM

2011016161-HBA-2_12 NM 113 NM

2011016161-FEA 1100_12 NM 103 NM

2011016161-AFA-L1_12 NM 113 NM

2011016161-245fa_18 22 78 NM

2011016161-HBA-2_18 26 78 NM

2011016161-FEA 1100_18 27 79 NM

2011016161-245fa_21 13 123 NM
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Sample Cream Time (Seconds) String or Gel Time 

(Seconds)

Tack Free Time (Seconds)

2011016161-HBA-2_21 16 121 NM

2011016161-FEA 1100_21 20 125 NM

2011016161-245fa_23 NM 95 NM

2011016161-HBA-2_23 NM 93 NM

2011016161-FEA 1100_23 NM 95 NM

2011015267-3080-15 4 45 55

2011015267-3081-15 4 35 38

2011015267-3082-15 3 – 4 40 45

2011015267-3083-15 3 37 40

2011015267-3084-15 4 34 38

2011015267-3084-bis-15 3 – 4 36 38

2011015267-3085-15 3 36 38

2011015267-3086-15 3 35 39

2011015267-3083-21 4 45 55

2011015267-3084-21 4 35 38

2011015267-3085-21 3 36 38

2011015267-3087-21 3 35 38

2011015267-3088-21 3 34 38

2010003500-CM659 14 43 NM

2010003500-5814-1 12 41 NM

2010003500-5814-2 12 42 NM

2010003500-5814-3 13 47 NM

2010003500-5814-4 12 45 NM

2010003500-5814-5 13 45 NM

2010003500-5814-6 13 47 NM

2010003500-6008 6 31 56

2010003500-6009 5 29 58

2010003500-6011 5 27 50

2010003500-6010 6 30 52

2010003500-5999 6 32 51

2009003152-274-08 13 27 59

2009003152-275-02 22 50 74

2009003152-274-07 12 26 59

2009003152-275-19 22 45 102

2009003152-275-20 12 24 48

2009003152-275-18 22 50 100

2008005552-24-110 7.8 71.4 124

2008005552-24-115 7.9 72.9 122

2008005552-24-120 8.2 75.5 122

2008005552-24-125 8.1 73.5 121

2008005552-24-130 8.4 75.5 121

2008005552-25-16/16 12.2 91.2 140

2008005552-25-19/19 9.9 85.2 133

2008005552-25-22/22 8.2 75.5 122

2008005552-25-25/25 7.0 71 121

2008005552-25-28/28 6.1 65.9 120