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Reactivity and Mechanical Performance of Polyurethane Films Made With CASE Dow Polyols and Isocyanates

Whether you’re developing formulations for electrical encapsulants or shoe soles, Dow Polyurethanes is the raw 

material source you can count on for many of your coating, adhesive, sealant, and elastomer (CASE) formulation needs. 

Dow offers a wide range of polyurethane raw materials, including VORANOLTM polyether polyols, VORANATETM TDI,

PAPITM polymeric MDI, and ISONATETM pure and modified MDI. In addition, Dow’s backward integration into the basic 

feedstocks used to produce polyurethane chemicals gives us an important advantage in ensuring high quality and

availability of the products we sell.

Polyurethane chemistry is very versatile, as seen by the multitude of uses from bowling balls to flexible foam and 

windmill adhesives. By changing the polyol and isocyanate components, you can greatly impact the final properties of 

the polyurethane resin. This brochure characterizes a small sampling of the possibilities. 

Adhesives and sealants can be prepared using a single component (1K) system or a two component (2K) system. In a 1K 

system, a prepolymer is prepared in which one or more polyols is reacted with a stoichiometric excess of isocyanate. 

This prepolymer retains isocyanate functionality, but has an increased shelf life and lower volatility compared to pure 

isocyanates. This prepolymer is then mixed with plasticizers, fillers, antioxidants, adhesion promoters, and catalysts. 

When this system is dispensed, the prepolymer reacts with ambient moisture to cure. In a 2K system, the polyol(s) and 

additives are blended together to make up one of the two components. The isocyanate or prepolymer makes up the 

second component. These two components are mixed on site to cure the system. A 2K system typically allows for a 

faster reaction and better control over the curing conditions, but requires the ability to mix separate components at the 

proper ratio.

The data presented in this report was generated using one component systems with polyols and isocyanates only to 

compare resin properties without the influence of additives. A sampling of Dow polyols and isocyanates listed in Table 1 

were used to accomplish this goal. For a more complete listing of the Dow polyols for CASE applications, see the 

brochure entitled “Supplying a Multitude of Products for Measurable Results: Polyols for Coating, Adhesive, Sealant, 

Elastomer, and Electronic Applications.” 

Table 1: Raw Material Information

Polyol Functionality OH Number Average 

Molecular 

Weight

Viscosity at 

25 °C, cSt 

Viscosity at 

100°F, cSt 

Water, % Max

VORANOLTM 220-056N 2 56 2000 300 150 0.03

VORANOLTM 221-050 2 50 2200 360 200 0.04

VORANOLTM 222-056 2 56 2000 321 170 0.04

VORANOLTM 230-056 3 56 3000 480 245 0.03

VORANOLTM 232-034N 3 34 5000 860 415 0.05

VORANOLTM 4000LM 2 28 4000 920 480 0.03

VORANOLTM B-2000 2 56 2000 610 230 0.04

Isocyanate Functionality NCO Content 

by weight, %

I.E. 

Weight

Viscosity, cP Description

ISONATETM 125M 2 33.5 125.5 5 at 43°C Pure MDI

ISONATETM 50 O,P 2 33.5 125.5 10 at 25°C Pure MDI with 50% o,p content

ISONATETM 143L 2.15 29.1 144.5 33 at 25°C CDI modified MDI
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In these formulations, the polyol side was first heated in a round-bottom flask to 50°C. Benzoyl chloride was added to 

stabilize the system. The isocyanate was then weighed into the round-bottom flask and the mixture was heated to 80°C. 

After running the reaction for six hours, the material was analyzed for viscosity and %NCO. Prepolymer films were drawn 

down between 20 and 30mil onto polypropylene sheets and cured with ambient moisture. Samples were taken from 

these films for mechanical property testing following standard ASTM D 1708. Prepolymer and mechanical performance 

data is presented in Table 2.

Table 2: Formulation Data

Component Formulations (parts by weight)

A B C D E F G H I J K

VORANOLTM 220-056N 50 100 100 100 100

VORANOLTM 221-050 100

VORANOLTM 222-056 100

VORANOLTM 232-034N 100

VORANOLTM 230-056 50

VORANOLTM 4000LM 100

VORANOLTM B-2000 100 100

ISONATETM 125M 20.3 14.5 13.0 19.7 25.6 32.2 30.6 32.2 33.0

ISONATETM 50 O,P 20.1

ISONATETM 143L 23.0

Prepolymer Properties

Prepolymer %NCO 2 2 2 2 2 2 3.5 5 5 5 5

Prepolymer Viscosity at 25°c, Pa.s 68.8 81.7 16.6 123.0 95.8 40.4 22.0 10.7 7.3 8.2 14.7

Moisture Cured Film Properties

100% Modulus, psi 104 162 61 170 123 179 155 308 310 344 410

Tensile Strength, psi 953 275 168 342 379 320 978 700 473 685 1082

Max Elongation, % 1459 227 1980 281 505 693 1945 1051 1078 1298 755

Shore A Hardness 44 49 30 47 42 51 47 62 67 68 67

In moisture cured films such as these, the free NCO groups on the prepolymer react with atmospheric water to form

urea crosslinks. If one desires to use these prepolymers in a two component elastomer, the atmospheric water could be 

substituted with a chain extender such as 1,4-butanediol. 

As highlighted in formulations A and K, VORANOLTM B-2000 based resins provide excellent mechanical properties in 

addition to their excellent hydrophobicity. The VORANOLTM 4000LM resin shown in formulation C achieves a maximum 

elongation of nearly 2000% at 2% NCO, which is difficult to do with conventional polyols. Formulation C also has a 

significantly lower prepolymer viscosity than other typical formulations at 2% NCO. As a whole, the formulations in Table 

2 show that with increasing isocyanate content (%NCO), the maximum elongation tends to decrease while the 100% 

modulus tends to increase.

In creating these films of varying compositions, a correlation drawn between Shore A Hardness and 100% Modulus is 

shown in Figure 1. This allows for a quick screening of test formulations using Shore A Hardness as a comparison without 

immediately having to run the microtensile tests. The data used for the correlation is a series of samples that includes, 

but is not exclusive to, the data from the formulations in Table 2.
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Figure 1: Comparison Between Shore A Hardness and 100% Modulus

System reactivity depends on four main factors: polyol reactivity, isocyanate reactivity, acid/base balance, and catalyst

type/loading. System reactivity is very important for several reasons: open/handling time, tack-free time, and because 

the morphology of the system can organize differently depending on how quickly it reacts. A slower reacting system may 

react so slowly that as the system reacts, there is too much phase separation between the polyol soft segment and the 

growing urethane hard segment. Depending on the system, it may not be visible immediately, but mechanical properties

could suffer. 

One way of controlling the system reactivity is by changing the polyol backbone. Typical polyether polyols are 

polypropylene oxide with either some or no ethylene oxide (EO) capping. Increasing the level of EO capping on a polyol

increases the primary hydroxyl content, which subsequently increases the reactivity. 

Another consequence of increased EO content is a higher affinity for water. If one makes a prepolymer using a polyol 

with a high EO content, such as VORANOLTM 222-056, that prepolymer will have a much higher affinity for water than a 

prepolymer made using all-PO polyols such as VORANOLTM 220-056N. This high affinity for water will draw in more water 

in a moisture-cured system, significantly increasing the rate of cure of the prepolymer and subsequently decreasing the 

open time. VORANOLTM B-2000 is a very hydrophobic polyol, so prepolymers made with this polyol will take longer to

bring in atmospheric moisture and cure.

0

10

20

30

40

50

60

70

80

0 50 100 150 200 250 300 350 400 450

Shore A Hardness

100% Modulus, psi
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The following reactivity comparison was done by making low %NCO prepolymers on a rheometer using ISONATETM 50 

OP, Dabco 33LV catalyst, and several VORANOLTM ~2000MW diols. Figure 2 shows viscosity growth as a function of time, 

which can be correlated to the reactivity of the system. There is a clear difference between the reactivity of the higher 

EO-capped polyols, VORANOLTM 221-050 and VORANOLTM 222-056, the all PO polyol, VORANOLTM 220-056N, and the 

hydrophobic polyol, VORANOLTM B-2000. The prepolymer made with VORANOLTM 221-050 has a much lower final 

viscosity because VORANOLTM 221-050 has a slightly higher molecular weight than the other polyols, at ~2200g/mol. 

Figure 2: Viscosity Build for Prepolymers with Different Polyols

Because of the differences in final viscosity, it is helpful to view the data in terms of percentage of final viscosity versus

time. As shown in Figure 3, this gives us a normalized picture of how each of the polyols reacts.
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Figure 3: Normalized Viscosity Build for Prepolymers with Different Polyols

From this, the picture is a little more straightforward. VORANOLTM 221-050 and VORANOLTM 222-056 are more reactive 

than VORANOLTM 220-056N and VORANOLTM B-2000 because of their EO capping. On the other hand, VORANOLTM B-2000 

has a slower reactivity profile than the other polyols and will provide for significantly longer open times in the base case.

Form Number: 109-01857-1011