Polyurethane adhesives

Polyurethaneadhesives Chr. Strdbech (Casco Nobel A/S, Denmark)

This paper reports on the main types, uses and methods of application of polyurethane adhesives, concentrating in particular on the one-component, moisture curing, solventfree and the two-component, hard types. For the latter, the development of strength and hardness during cure is discussed, and the effects of pot life, application time and pressing time are examined. Safety aspects of the use of polyurethane adhesives are also considered, with particular reference to practice in the Scandinavian countries. Key words: adhesives; polyurethanes; strength; pot life; cure; safety

one-component

Due to the versatility of polyurethane chemistry, there are many different types of polyurethane adhesives. This means that when talking about polyurethane adhesives for a specific purpose it is also necessary to be specific about the adhesive type. The most important types are as follows: one-component, one-component, 0 one-component, l one-component, l one-component l two-component, l two-component, l two-component l l

moisture curing, solvent-free; moisture curing, with solvents; moisture curing, elastic; moisture curing, hot melt; contact adhesives, with solvents; hard; elastic; and contact adhesives, with solvents.

Considering that nearly all the types may exist in different variations, such as liquid or thixotropic. fast or slow cure, and fire approved or not fire approved, it is realized that there is a rich variety of adhesives to select from. Apart from some of the one-component, moisture curing, elastic products (sealant adhesives), which usually contain a little (below 0.5%) free toluene diisocyanate (TDI), all the other types of adhesive are based on polymeric methylene diphenyl diisocyanate (MDI), the least volatile of the commonly used isocyanates. Polyurethane

adhesives

in Scandinavia

Health and Product Legislation in the Scandinavian countries has always been very tough, forcing the industry to develop products which are the least harmful to man and environment. In Denmark, for example, the formulations of all polyurethane (and epoxy) products have to be registered with the government, along with labels and 0143-7496/90/030225-04

types; two-component

types;

the material safety data sheets. If found acceptable, the product is given a product registration number which has to appear on labels, safety and technical data sheets. In each factory using chemical products, a safety group has to be established which, together with the management, has responsibility for safety. The safety groups work together with government inspectors and a medical surveillance organization to find the safest products, production methods and equipment. Consequently, the polyurethane products and methods developed in this constantly scrutinizing system are the safest obtainable. As an example, the use of solvent-borne spraying adhesives is virtually unknown in Scandinavia, as special systems and precautions have to be used for spray systems. Even if a closed spray booth is used, it is made difficult to use these adhesives because solvents are released during the handling process, demanding new safety considerations. Instead of the solvent-borne adhesives, fast curing, solvent-free, oneand two-component adhesives have been developed, the use and application of which are described below. Main

uses for polyurethane

adhesives

Due to their good adhesion, toughness, water and chemical resistance, and the ability to tailor polyurethanes to many specific uses, polyurethane adhesives are used in many areas including: all kinds of sandwich element - bulkheads for ships and trains, housing elements, cold store elements, trucks, containers, recreational vehicles; l tools (axes, hammers); 0 constructions - roof elements, metal/wood constructions, bolts in laminated wood structures; l repair (cars, containers, trucks);

l

0 1990 Butterworth-Heinemann Ltd INT.J.ADHESION AND ADHESIVES JULY 1990

225

• • • •

thin plate bonding; weld bonding and rivet bonding; bonding of plastic cars; bonding of reinforced plastic (glass fibre-reinforced plastic (GRP), sheet moulding compound (SM¢), etc).

The properties of adhesives used for the above applications are shown in Tables 1 and 2. Specific adhesive types Of the different types of polyurethane adhesive listed above, only the one-component, moisture curing, solvent-free types and the two-component, hard types will be dealt with more closely, these being the most common adhesives. One-component, moisture curing polyurethane adhesives (solvent-free) When supplied, these prepolymer (pre-reacted) types are normally liquid and may be applied by hand, by spraying (in closed cabins) or preferably by nozzles, the easiest and safest method. Some automatic application systems are up to about 3 m wide and may apply adhesive at a speed of up to 15 m/min in an amount of 100 to 300 g/m 2. Simultaneously with application of the adhesive, water is sprayed onto the adhesive/materials in an amount necessary for the

Table 1.

curing process. The most important production process factors of these adhesives are the open time and cure time at room temperature and elevated temperature, since these adhesives may give pressing times down to below 2 min at 70-80°C. One-component polyurethane adhesives are available both as filled and unfilled types. The filled types contain less polyurethane and are consequently used in bulkheads for ships and other places where fire resistance of the adhesive is required (approved by See-Berufsgenossenschaft (SBC)), or where the safest adhesive has to be used. They are normally used for sandwich elements with cores such as stonewool, polyurethane and polystyrene foam and skin materials such as GRP, lacquered metal or galvanized steel. The unfilled types are excellent for bonding wood/wood, even when the moisture content is high. During cure with moisture the one-component polyurethane adhesives liberate carbon dioxide, making the adhesive foam slightly. This gives excellent bonding to porous and uneven surfaces. Some available adhesives are shown in Table 1, ranked according to open time/cure time. As can be seen, it is possible to select an adhesive for continuous production where one element is made at a time, or for stack pressing where a large stack of elements is pressed at a time. Depending on the open time needed for assembling an element, a fast or slow adhesive may be selected. Heat greatly accelerates the cure process.

One-component polyurethane adhesives

Open time at 23 ° C (min)

Press time, water spray, 23°C (rain)

Press time, water spray, 70°C (rain)

Notes

Name/Number Foss Than 1K (Casco)

7 10 10 20 20 30 45 45 200

15 20 20 40 40 60 120 120 12-18 h

1 1~-2 2-3 2-3 4-5 4-5 5-6 10 10 120

Filled, SBG Filled, SBG Non-filled Filled Non-filled Filled, SBG Non-filled Water clear Non-filled

1828 1830 1812 1831 1843 1833 1894 1809 1804

Table 2.

Two-component polyurethane adhesives

Application time (rain)

Open time at Handling time 230C (rain) at 230C

Uses

Application method/ Number Foss Than fire resistance 2K (Casco)

5 12-15 15 20 40 45 60 90 120

10 20 20 40 60 60 120 45 60

50 rain 60 min 120 min 4h 5h 6h 5-6 h

Sandwich elements Sandwich elements Sandwich elements Tools, bolts in wood Metal, car repair, weld bonding Roof elements Sandwich elements

S S T, N, I N B, N N, T

1807 1881 1862 1959 1847 1819

Mounting, repair

N, S, T, SBG B, N, T

1899 1897

8-12 h

Sandwich elements

N, T, R, SBG

1801

150

180

7h

Sandwich elements

B, N, T

1883

240

90

24 h

Acoustic elements

N, T, R, SBG

1867

6h

B - - brush; N - - nozzles (machine application); R - - roller; S - - spraying; T - - t r o w e l (notched) I - - non-combustible. I M C O A 4 7 2 (USCG); SBG - - f i r e approved (See-Berufsgenossenschaft)

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INT.J.ADHESION AND ADHESIVES JULY 1990

Two-component polyurethane adhesives Two-component polyurethane adhesives are used for a large variety of applications (see above). They bond to nearly all materials, either as-delivered or prepared for bonding by proper industrial methods. The twocomponent adhesives are normally applied by hand, brush, nozzles, spray (closed cabin) or roller, and a large range of m o d e m application equipment is available for industrial bonding processes. An automatic application machine for large elements is shown in Fig. 1. For this purpose, the adhesives may be made in different qualities (iiquid-pastelike, fast cureslow cure) to satisfy the requirements of the bonding process. Table 2 shows some typical two-component adhesives grouped according to application time/cure speed. The application methods vary greatly, as do the properties and application of the adhesives. After full cure, they are all hard, tough adhesives, except 1867 which is a soft, sound-dampening adhesive for acoustic panels. Pot life is about double the application time (see later). The adhesive has to be applied within the application time and pressure has to be applied within the open time. Handling time is equal to press time. Final total cure takes days or weeks. For each 10°C temperature increase, the handling time is reduced by about half.

Curing

Fig. 1 Application of two-component polyurethane adhesives on large surfaces

60

Pot life

5O Io

q0

x 3o

"~ 20

Cure starts as soon as the two parts are mixed and the

base and curing agent molecules react. The viscosity of the mixture increases but, as the reaction proceeds, exothermic reaction heat develops which keeps the viscosity down or even lowers it for some time on faster curing products. Fig. 2 shows that the application time is the time from mixing to the time when the adhesive cannot be applied, eg, using a notched trowel. For the specific adhesive shown in Fig. 2 (Foss Than 2K 1899/1821), the viscosity is then about 50 to 70 Pa s or less, depending on the application method. Pot life can be defined as the time when the mixture cannot be stirred at all. The viscosity at this stage is about 500 Pa s (start temperature 22°C). From the figure it can be seen that the application time is about 45-50 min and pot life about 70 min (200 g mixture). As cure continues, the adhesive turns slowly from liquid to a soft, sticky material which slowly hardens. The hardness and strength increase for Foss Than 2K 1899/1821 adhesive is shown in Fig. 3. Strength is measured on Al/A1 joints according to DIN 53283, and Shore D is a measure of hardness. Shear strength takes some 5-7 h to develop to handleable levels, depending on the construction bonded. Slower aftercure then takes place, during which adhesion and water resistance develop. Cure may be accelerated by adding catalysts. Fig. 4 shows the strength during the first few hours of cure for the same adhesive as above but with different pot lives. It can be seen that a reasonable strength can be obtained in less than 30 min if an adhesive with 31 min pot life is used. This time increases for longer pot lives. Heat may greatly increase the cure speed. If it is assumed that breakages in the foam occur in a sandwich element using 60 kg PV¢ foam as a core

A_~n

10

I 0

10

20

30

qo

50

60

70

80

Time (min) Fig. 2 ~scosityincrease w i t h t i m e f o r F o s s T h a n 2K 1 8 9 9 / 1 8 2 1 = 100/22 130[ 120~

~ . - " ~ ' ~ h e a r

strength

,°° r a

80

=~ "

70 /

L

50

/"O""""--

Shoed

hardness

qO ¢. t/1

30 20 10 I

l

I

I

2

q

6

8 10 12 l q

I

I

l

I

I

I

I

I

I

I

I

16 18 20 22 2q 26 28 30

Time ( d a y s ) Fig. 3 Hardness and strength increase with time for Foss Than 2K 1899/1821 = 100/22

and aluminium as a skin at a shear strength of only 1.2 MPa, then the pressing time, defined as the time when the element can be handled without breakage in the foam, is shown in Table 3 as a function of pot life, application time and temperature. Combining these measurements with those of flexural strength changes

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227

Table 3.

Relationship between pot life, application time and pressing time for Foss Then 2K 1899

Pot life (min) 3.5

Application time, approx (min)

Open time, approx (min)

1.5

10 30 50 70

5 15 25 45

a

3

20

5

90 180 240 300

25 45 60 75

3½

0. 4

£ 3 2

.c t~ 1

/J,/, 2

3

4

This means that it is possible to work safely with one- and two-component polyurethanes at room temperature with normal room ventilation, as long as normal hygienic conditions are followed. When heat pressing is performed or the adhesives are sprayed, special precautions have to be taken. The industrial material safety data sheets should always be consulted, and the production methods should be approved by the local health and safety authorities. Conclusion

-

I

Press time at 40°C, approx (min)

12 18 60 120

Pot life (min) 5

Press time, approx (min)

5

I

I

I

6

7

8

Time {h) Fig. 4 Strength during cure of Foss Than 2K 1 8 9 9 / I 821 with different pot lifes

with time of the specific sandwich element gives a clear picture of what is happening during cure in the bondline.

Polyurethane adhesives are widely used in many different industries for bonding sandwich elements, for trucks and containers, tools and constructions, as well as for repair, thin plate bonding and weld and rivet bonding. Modern application equipment is available for applying the adhesives, and likewise effective pressing and curing systems have been developed to meet the demands of the latest one- or two-component polyurethane adhesive technology. Modern application and pressing systems most often have safety equipment as an integral part of their construction.

Safety The base resin of two-component polyurethanes normally consists of polyols, fillers and small amounts of additives. No safety markings are necessary. The curing agent and the one-component, moisture curing adhesives contain polymeric MDI, an isocyanate with a very low vapour pressure. Many measurements in actual industrial use, with hand or nozzle application, show that the level of MDI in the air is much lower than the threshold limit in Scandinavia (and now in many other places) of 0.05 mg m -3, and normally below the detection limit.

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INT.J.ADHESION AND ADHESIVES JULY 1990

Acknowledgement The author wishes to thank the European Commission for supporting the BRITE programme A B C O N

(Adhesive Bonding Technology for Building Construction). Part of the work reported in this paper stems from this BRITE programme. Author Mr Strobech is with Casco Nobel A/S, 2-4 Prmstemosevej, 3480 Fredensborg, Denmark.