Effect of blocking agents on the shock absorbing properties of the films of polyurethane adhesives

Effect of Blocking Agents on the Shock Absorbing Properties of the Films of

Polyurethane Adhesives SURAJIT ROY and ANIL KUMAR*

Department of Chemical Engineering Indian Institute of Technology

Kanpur - 20801 6 India

Various 4,4-diphenylmethane diisocyanate (MDI) based polyurethane adhesives have been developed using polyethylene glycols (PEG) of different molecular weights. The formulations were modified using different blocking agents to inhibit the isocyanate-moisture reaction. Composites of poly(methy1 methacrylate) sheets were developed in which these adhesives gave transparent films and their shatter- ing characteristics were tested in a bullet firing machine developed here. The damaged area was measured and used to characterize the adhesive strength. The molecular weight of the soft segment (diol) and the nature of the blocking agent both were found to affect drastically the urethane film properties.

INTRODUCTION

olyurethane is a class of polymers formed by the P reaction of a diisocyanate with polyols, and the chemistry of polymer formation is well understood (1-4). Since isocyanates can bind with metals, rub- bers, plastics, wood, etc., it has led to the develop ment of large number of polyurethane adhesives (5- 13). As engineering materials, they have been used as is (14-16) as well as in composites (17-20). Polyurethane has excellent modulus even at low temperatures and finds usage in space applications (18, 21). In view of its importance, the kinetics of formation (22), thermal degradation (23-26). and their structure-property relationships (27-33) have been extensively reported.

Polyurethane adhesives are grouped in the follow- ing classes based on how they are applied (2, 3):

1) Single component reactive liquid adhesive where the diisocyanate (NCO) group is the key binding element in a prepolymer base.

2) Solutions of the unpolymerized or polymerized diisocyanate with reactive groups available for bond- ing.

3) Thermoplastic linear polyurethanes with blocked NCO groups.

4) Two component quasi-prepolymer in which one component is the diisocyanate prepolymer.

Polyurethanes are adhesives because isocyanate groups can react with a variety of functional groups.

'To whom correspondence should be addressed.

The polymer has been modeled to have soft (due to diisocyanate groups) and hard (due to polyether or polyester diols) segmented blocks (34-4 1). They are thermodynamically incompatible with each other and stay within separate microdomains.

In this study, we have developed a polyurethane adhesive system giving transparent films on poly(methy1 methacrylate) (PMMA) composite sheets. The usual method of degassing all the ingredients of the adhesive before polymerization did not give satis- factory results, because some carbon dioxide bubbles were always generated, which destroys transparency. As a result, we have proceeded as follows to achieve our goal:

a) We have found a suitable surface treatment method to suppress bubble formation in the PMMA composite sheets.

b) We tested several blocking agents for the suitabil- ity for the purpose of giving transparent film.

c) We have carried out impact damage tests of the PMMA composite plates using an airgun assembly developed in house at controlled projectile velocity. We have evaluated different blocking agents by comparing the damaged area.

d) We have vaned the molecular weight of poly(ethy1- ene glycol) (PEG) and studied its effects on the damaged area under impact testing.

EXPERIMENTAL WORK Chemicals Used

The compound 4,4-diphenylmethane diisocyanate (MDI) was supplied by Merck, Germany, poly(ethy1-

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Effect of Blocking Agents on Shock-Absorbing Properties

ene glycols) (PEG) of molecular weights 200, 300, 400, 600 phenyl, bisphenol-A, hydroquinone by E.

Polyurethane With Glycerine as Crosslinking Agent

Merk, India, and 2-hydroxyethyl methacrylate (HEMA) by Fluka AG Switzerland. A two component polyurethane adhesive was pre-

pared using glycerine as the crosslinking component Preparation of Prepolymer and Application of Adhesive

Prepolymer formation was carried out in a glass reaction vessel with a stirrer under argon atmosphere. The mixing of the two components of the adhesive was done in the open atmosphere. After mixing, the prepolymer was applied on the PMMA sheet in a 1 m3 wooden chamber. This chamber was kept dry using silica gel with temperature regulated using a heater and a variac.

Bullet Firing Machine

The impact damage tests on the PMMA composite plates have been done on a setup made in house. The composite plates 180 by 180 mm in size were cut from 0.64 cm PMMA sheet and joined together using the adhesive prepared. These were held between two rigid square shaped mounts and impacted by a mild steel cylindrical projectile with hemispherical nose. The projectile was fired from an airgun assembly and can simulate velocities of bullets fired from conven- tional fire arms. The mild steel projectile is 10 mm in diameter, 20 mm long, and weighs 116 g. The plate mount can withstand an impact of a 75 mm long projectile moving at a velocity of 200 m/s. To stop a high speed projectile which may penetrate through the composite plates, a cylindrical catcher fitted with cotton rags is placed behind the target plate. Further details on construction and operation of the airgun assembly and mount are given in Ref. 13.

instead of the excess MDI. Component A was prepared by reacting 200 meq. (20 ml) of MDI with 80 meq. (14.3 ml) of 400 ( M W ) PEG. The reactants were degassed for 15 min before use. Component B was prepared by a mixture of 76 meq. (13.56 ml) of 400 ( M W ) PEG and 40 meq. (1 ml) of glycerine. Both components A and B are then thoroughly mixed and cured at 70°C in a Petri dish in the wooden chamber for 24 h. mure 2 shows the cured product and is seen to contain bubbles of carbon dioxide. Compared to Fig. 1 there are large domains of gas bubbles. The product is translucent and the bubbles are distinctly visible. Similarly samples were made using 200 ( M W ) PEG. Thus the samples so far have the problem of presence of bubbles and blow holes, which is highly

RESULTS AND DISCUSSION

A two component polyurethane adhesive prepolymer was prepared from polytethylene glycol) and MDI with 2 slight excess of MDI. Component A was prepared by the reaction of 200 meq. (20 ml) of MDI with 80 meq. (14.3 ml) of PEG of M W of 400. The reactants were degassed and the reaction carried out under an inert argon atmosphere. Component B was made of 80 (14.3 ml) meq. of PEG and both components were mixed thoroughly in a Petri dish and cured at 70°C in the wooden chamber for 24 h. m u r e 1 shows the product in which bubbles of carbon dioxide formed by the reaction of MDI with atmospheric moisture is finely dispersed in the sample. As a result, the cured product is opaque and light yellow in color. Similarly, cured samples of 200 (molecular weight M W ) PEG and 300 ( M W ) PEG were prepared and all of them showed a similar distribution of bubbles of carbon dioxide in the cured product. Any amount of degassing of components A and B does not prevent the formation of CO, bubbles. I t is suspected that moisture enters during curing either from the atmosphere or is already adsorbed on the surface of the Petri dish.

Fig. 1 . Polyurethane with excess MDI as the crosslinking agent.

FQ. 2. Polyurethane with glycerin as the crosslinking agent.

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Surajit Roy and Anil Kumar

undesirable. The blocking of MDI was tried in the subsequent experimentation.

Polyurethane With Phenol as Blocking Agent

Phenol was first used as the blocking agent to cap the isocyanate groups of component A. Component A was prepared by reacting 200 meq. (20 ml) of MDI with 60 meq. (10.7 ml) of 400 ( M W ) PEG, and 20 meq. (2.6 ml) of hydroxyethyl methyl acrylate (HEMA). The reactants were degassed for 20 min before the reaction. The excess isocyanate groups were capped by reacting the product obtained with 120 meq. (10.55 ml) of phenol. The prepolymer formation was carried out under an inert argon atmosphere and was allowed to go to completion. Component B was prepared by 76 meq. (13.56 ml) of 400 PEG and 40 meq. (1 .O ml) of glycerin. Both components A and B were thoroughly mixed and allowed to cure at 70°C for 24 h in the wooden chamber in a Petri dish covered with a polyethylene film. The chemistry of capped NCO groups has been discussed (1-3). Beyond 50°C. the NCO groups are released and the curing proceeds without being affected by the blocking agent. The product is shown in Q u r e 3 and the cured polyurethane is dark in color due to phenol but does not contain bubbles of carbon dioxide. This shows that either the polyethylene film or the blocking agents (or both) inhibit the reaction of isocyanate with moisture, but the dark color of the cured product remains a drawback.

Polyurethane With Bisphenol-A as Blocking Agent

Bisphenol-A has been used to block the isocyanate of component A. Component A was prepared by reacting 60 meq. (10.3 ml) of 400 PEG and 20 meq. (2.6 ml) of HEMA with 200 meq. (20 ml) of MDI in excess dried ethyl acetate. All reactants were dried and degassed for 15 min under vacuum before use. The excess isocyanate was then blocked by 120 meq. (13.6968 g.) of Bisphenol-A. The prepolymer was formed as in the Experimental section in a glass vessel under an inert argon atmosphere. Component B was formu-

Fig. 3. Polyurethane with phenol as the blocking agent.

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lated by 76 meq. (13.56 ml) of 400 ( M W ) PEG and 40 meq. (1 .O ml) of glycerin. Both components were then vigorously mixed and cured at 60°C in the wooden chamber for 24 h on a Petri dish covered with polyethylene film. The cured polyurethane obtained is shown in Fig. 4 and the product is found to be light yellow in color and transparent with no bubbles. I t is also rubbery and has excellent glaze. Similar formulations can be developed using 200, 300, and 600 PEG and all have been found to possess identical desirable qualities of clarity and rubbery nature.

Polyurethane With Hydroquinone as Blocking Agent

Hydroquinone was used as the blocking agent. Component A is made by the reaction of 200 meq. (20 ml) of MDI with 60 meq. (10.3 ml) of 400 PEG and 20 meq. (2.6 ml) of HEMA under an argon atmosphere. The reactants were degassed under vacuum before the reaction for 15 min. The isocyanate prepolymer is blocked by 1.5 equivalents (16.8 g.) of hydroquinone and the reaction completed with constant stimng. Component B was formulated by a mixture of 76 meq. (13.56 ml) of 400 PEG and 40 meq. (1.0 ml) of glycerin. Both component A and B were found to have excellent shelf life which we checked by keeping them open to atmosphere for six months. Both components were mixed and cured at 70°C for 24 h on a Petri dish covered with polyethylene sheet; the cured product obtained is shown in FXg. 5. The product is transparent and light yellow in color. It is rubbery but compar- atively harder than the bisphenol-A blocked product in Fig. 4. We feel that hydroquinone capped polyurethane has the qualities desired for an adhe sive formulation and can be used subsequently for further experimentation.

Polyurethane With Novolak Epoxy LY 556 as Crosslinking Agent

In this formulation Novolak epoxy has been used as the crosslinking agent in place of glycerine. Novolak is multifunctional and is expected to produce better

Fig. 4. Polyurethane with bisphenol-A as blocking agent.

POLYMER ENGINEERINGAND SCIENCE, JUNE 1995, Vol. 35, No. 12

Effect of Blocking Agents on Shock-Absorbing Properties

crosslinking. Component A of the prepolymer 200 meq. (20 ml) of MDI was reacted in excess dry ethyl acetate with 60 meq. (10.3 ml) of 400 PEG and 20 meq. (2.6 ml) of HEMA under an argon atmosphere with the excess isocyanate blocked with 120 meq. ( 13.6968 g.) of bisphenol-A. Before crosslinking, component A was dried under vacuum to remove the solvent ethyl acetate and then component B, com- prising 32 g of Novolak epoxy was added. The result- ing formulation was thoroughly mixed and cured at 70°C for 24 h. The cured polyurethane obtained is shown in Fig. 6 and is harder and more brittle having dark spots compared to that of Figs. 4 and 5, though it is transparent with no bubbles. It had excellent glaze and cures very fast. We find that the adhesive properties of this product are not good. We have prepared similar cured blocks in ambient air as well as within the wooden chamber which has controlled humidity, the results are shown in Figs. 7 and 8. It can be seen that silica gel drying of the wooden

Fg. 7. Polyurethane$lrn formed in ambient air.

Flg. 5. Polyurethane with hydroquinone as the blocking agent.

Fig. 8. Polyurethane J l r n formed in controlled atmosphere chamber.

chamber is sufficient for obtaining transparent film of the cured adhesive.

Polyurethane Used for Joining 180 by 180 by 6.4 mm PMMA Plates

Our experiments show that the surface properties of the adherends are extremely important in deter- mining whether film formed is transparent. N o matter how much degassing is done, if the adherend surface is glass, PMMA or teflon, the film obtained is similar to that shown in Fig. 7 which is full of CO, bubbles.

Fig. 6. Polyurethane with Nouolak epoxy LY556 as crosslink- However, if the surfaces are prewashed with ing agent. petroleum ether, bubble formation is suppressed and

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Surajit Roy and Anil K u m r

the film obtained is transparent. Glass, teflon, and PMMA are believed to have moisture adsorped on their surfaces. Even when the adhesive prepolymer is blocked, on heating the NCO groups are released and react with the adsorbed moisture. On washing with petroleum ether, the adsorbed water appears to be removed. After thoroughly washing with petroleum ether, the PMMA plates are dried in the wooden chamber. The adhesive with Novolak epoxy as the crosslinking agent showed poor adhesion. On the other hand formulations of bisphenol-A and hydrc- quinone as the blocking agent and glycerine as the crosslinking agent have shown excellent adhesive ability for the PMMA sheets. Figure 9 shows two strips of PMMA sheet joined together with bisphenol-A and hydroquinone capped polyurethane adhesives, respectively, both giving clear films. For component A, the prepolymer has been prepared under argon atmosphere with ethyl acetate as the reaction medium. Component B is made of PEG and glycerin and the total NCO content was kept in slight excess. Component A was dried thoroughly under vacuum before it was mixed with component B. The formulation obtained was then spread between two 15 by 15 cm PMMA plates and pressed between the mild steel press plates. The specimen (Fig. 10) was cured for 24 h at 70°C.

Impact Testing of PMMA Composites Plates in the Airgun Assembly

The target was fxed to the frame of the bullet firing machine. For low speeds, the plates only shatter and it is difficult to measure and compare the damage of different composite plates. As a consequence the bullet speed has been raised till it completely passes through the specimen. At this speed we varied the molecular weight of the polyethylene glycol for different blocking agents. Some of the damaged specimens

FTg. 9. PMMA strips jointed using polyurethane adhesive.

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are shown in Figs. 1 I , 12, and 13. The damaged area is well defined at this speed and almost circular in nature, with the farther plate damaging into a smaller circle. We would have liked to determine the exit velocities of the bullet in order to determine the exact impact energy, but due to space constraints of our airgun assembly, we could not do so. We have plotted the percent damaged area us. molecular weight in Figs. 14 and 15, and it is evident that different adhe sives can be compared based on the damaged area in spite of this limitation.

For a given bullet speed, as the molecular weight of the poldethylene glycol) was increased, the percent of damaged area was reduced. The result is expected because of the two phase morphology of the polyurethanes (35-41). With the increase in the molecular weight of polyethylene glycol, the lengh of the softer segments increases. Since these segments are entangled, they can stretch to absorb the shock of the sudden loading better. We have tested the adhesive based on bisphenol A blocking agent for two bullet speeds and for higher speed (83 m/s), the results are similar except that the percent damaged area is marginally higher. In Fig. 15, we have compared the performance of adhesives based on hydre quinone and bisphenol A blocking agents. We find that for the latter, the damaged area is always less and this study indicates that bisphenol A should be used for blocking the NCO groups.

CONCLUSION

In order to obtain transparent films of polyurethane adhesives on PMMA, we must treat the surface with petroleum ether and use blocking agents such as hydroquinone and bisphenol-A in order to suppress water reacting with isocyanate groups. On capping, near ambient conditions, the blocking agents are known to form complexes with isocyanate groups but

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Effect ofBlocking Agents on Shock-Absorbing Properties

Fig. 10. Target specimen prepared using PU adhesive.

Fg. 12. Damaged target specimen using 200 (MW) PEG and bisphenol A capped PU adhesive {bullet speed = 83 m/sl .

Rg. 1 1 . Damaged target specimen using 400 (MW) PEG and bisphenol A capped PV adhesive {bullet speed = 64 rn/s).

on heating beyond 40 to 50°C, they undergo Rg. 13. Damaged target specimen using 300 (MW) PEG and crosslinking giving transparent film. We have found conditions for joining two PMMA plates with complete clarity and have camed out impact testing of these composite plates at controlled speeds to evaluate different adhesives successfully.

The results indicate that the damaged area of the specimen decreases with increasing molecular weight

hydroquinone capped PU adhesive {bullet speed = 64 m/sl.

of the polyethylene glycol. This can be explained due to the increase in the length of the soft segment with the increase in molecular weight. As a result, the film can absorb the shock of sudden loading.

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Surajit Roy and Anil Kumar

Speed otbul let .mls A 64 0 93

I I I I 0 200 400 600 800

Mol. w t . o f peg.

Rg. 14. Damaged area us. molecular weight of PEG when bisphenol A is used as the capping agent.

I I I I I 0 200 400 600 800

Mol. wt. o f peg.

I%J. 15. Effect of capping agent on damaged area us. molece lar weight of PEG when the bullet speed is 64 m/s.

The bisphenol-A blocked polyurethane adhesive specimens show less damaged area compared to the adhesive blocked with hydroquinone.

ACKNOWLEDGMENT

The authors wish to thank the Aeronautical Re- search and Development Board, Bangalore, India, for partial financial support.

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Effect of blocking agents on the shock absorbing properties of the films of polyurethane adhesives

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