Novel uses of Cyanoacrylate Adhesives - Polycyanoacrylate foams

l%vel of 8 D, Kotzev and V. Kotzev (Chemence Ltd, UK) Cyanoacrylate polymer foams can be obtained by blending cyanoacrylate monomer with an appropriate solvent and a polymerization initiator. Foaming takes place in seconds at room temperature. Various monomers, solvents and initiators were tested and best performance compositions and ratios were determined. The resultant foams are lightweight and can occupy volume up to 30 times that of the original cyanoacrylate monomer. The onset time and temperature of foaming can be regulated in wide ranges. Odourless foaming compositions were obtained. The cyanoacrylate foam and foaming process can be used for a variety of medical applications such as broken-limb support, and blood vessel and Fallopian tube sealing. They are especially suitable for stoppage of fluid flow in industrial pipelines, e.g., gas mains. The foam plug can be collapsed at a later stage and fluM flow restored. Key words: polycyanoacrylates; adhesives; foams; foaming optimization; medical applications; industrial applications Cyanoacrylates are enjoying increased popularity as instant-setting speciality adhesives in industrial and consumer markets I. Commercially known as 'Super Glue' or 'Crazy Glue', they are one-component, catalyst-free and cure in seconds at room temperature. They have exceptionally high adhesion towards most materials -- metals, rubber, plastics, wood, ceramics, glass and even live tissue. The reason behind these unique properties is the chemical structure of the cyanoacrylate monomer, which is the active ingredient of the cyanoacrylate adhesives: CN I CHz=C-COOR A-CH2-C CN AB ~, CN I -COOR e I _-- C H 2 - C - C O O R Q e CN I C H 2 : C -COOR ' -A-CH2- A® : = CN CN I I C - C H 2 - C - COOR I e COOR Polymer where R is usually an alkyl radical but could be an alkenyl or alkoxyalkyl radical, and A is any anionic species. The combined electromeric effects of the nitrile and alkoxycarbonyl groups can cause exceptionally high polarization of the double bond in the presence of weak bases 2. Such polarization is also caused by water. Moisture, which can be found adsorbed on all surfaces, is the major initiator of cyanoacrylate adhesive cure. The monomer undergoes anionic polymerization at a high rate, producing a polymeric adhesive with a molecular weight of around 600 0003. It was recently discovered in the laboratories of Chemence Ltd that when a cyanoacrylate monomer is mixed with an appropriate solvent and a polymerization initiator, the resultant mixture shortly transforms itself into an expanded polymeric foam4. The expansion takes place in seconds at room temperature. The volume of the resultant foam can be up to 30 times the volume of the original cyanoacrylate monomer. The foam adheres to the surfaces it contacts during expansion and can easily be collapsed subsequently, if so desired. The mechanism is quite simple. Under the influence of the initiator the cyanoacrylate molecules begin to polymerize, incorporating into the polymer matrix the molecules of the solvent. The heat generated by the exothermal polymerization becomes sufficient to evaporate the solvent, which, by so doing, expands the polymer matrix. Actually, polymerization and 0 1 4 3 - 7 4 9 6 / 9 2 / 0 3 0 1 5 0 - 0 8 © 1992 Butterworth-Heinemann Ltd 150 INT.J.ADHESION AND ADHESIVES VOL. 12 NO. 3 JULY 1992 expansion take place simultaneously and when the expanding foam reaches an obstacle, such as a containing surface, it contacts the substrate and bonds to it in the same fashion as does a cyanoacrylate adhesive, The present paper reports on work which had the following objectives: (1) to study and optimize the foam-forming reaction: (2) to characterize the cyanoacrylate polymer foam produced: and (3) to make a preliminary assessment of the suitability, of the cyanoacrylate foam for various applications. Materials and m e t h o d s Cyanoacrylate monomers Ethyl-2-cyanoacrylate was obtained by distillation of Anacure 3020 adhesive (Chemence Ltd). All other cyanoacrylate monomers were prepared following wellknown synthetic procedures 5-7. Foam expansion In a polyethylene cylindrical container with diameter of 50 mm were placed 4 ml of freshly distilled cyanoacrylate monomer. A specific amount of liquid foaming agent, containing the polymerization intiator, was added to the cyanoacrylate monomer. The contents were manually mixed for 3 seconds so that a clear solution was produced. The mixture was left static and the time lapse before the onset of expansion and the time interval of actual expansion recorded. The volume of the expanded foam was measured and the coefficient of expansion calculated as the ratio of the volume of the polycyanoacrylate foam to the initial volume of the cyanoacrylate monomer in the composition. tube had an internal diameter (ID) of 18 mm and a wall thickness of 2 ram: a = 40 mm. In the case of human duct occlusion simulation the polyethylene tube had an ID of 2 mm, a wall thickness of 0.5 mm a n d a = 12mm. Foam strength measurement Cyanoacrylate monomer and l,l,2-trichlorotrifluoroethane (TCTFE) in volume ratio of 4:1 and containing 0.01% by weight of N,N-dimethyl-p-toluidine (DMPT) were mixed and injected into a polyethylene tube with an ID of 12.5 ram. Seconds after the injection the composition expanded into polycyanoacrylate foam, plugging the tube. The wall of the tube was carefully cut and the foamed material removed. Test-pieces were cut from it and tested. In order to measure the tensile strength, the foam specimens, which were cylindrical and had a diameter corresponding to the diameter of the pipe into which they were expanded, were bonded with cyanoacrylate adhesive to standard steel specimens for adhesive tensile strength measurements. The steel specimens were placed in the testing machine and were pulled at a cross-head speed of 1 mm min -l. Failure occurred in the volume of the cyanoacrylate foam, rather than at the adhesive bond. Compression testing was carried out using the same cross-head speed. Tensile shear strength data were obtained by expanding the same compositions in the gap between two coaxially inserted stainless steel pipes (Fig. 2). When the plugged assembly was tested in tensile mode (1 mm min -I) the foam was subjected to tensile shear Foaming temperature measurement The experimental set-up is shown in Fig. 1. The thermocouple was placed in the volume of the initial liquid composition where the largest build-up of heat occurs. In the case of expansions simulating preparation of broken-limb support the polyethylene I%t Polyethylene tube = ~I \.= \\1 \\= 1 -~--12 mm--- W Foam Failure line " * - - - - - 2 0 rnm [ ~"~ Volume before expansion Volume after expansion Fig. 1 Experimental set-up for temperature measurement during foaming Fig. 2 Tensile shear strength test of foam expanded between two coaxially placed tubes INT.J.ADHESION AND ADHESIVES JULY 1992 1 51 force. Failure occurred in the volume of the foam. The adhesive strength of the foam towards the surface of the tubes was higher than the strength of the material itself. Results and discussion Evaluation of different foaming agents end optimization of their foaming compositions In the first series of experiments the objective was to evaluate numerous solvents as potential foaming agents and to find the most suitable ones. Ethyl-2cyanoacrylate was used as the cyanoacrylate monomer and DMPT as the polymerization initiator. In the same set of experiments the ratio of cyanoacrylate to foaming agent and the amount of initiator were varied so that the optimum composition for each tested foaming agent was determined. Pentane, hexane, TCTFE, heptane, diethyl ether, cyclohexane, 2-butanone, acetone, ethanol, acetonitrile and methanol were studied. They are ordered in respect of their solubility parameters. Petroleum ether was also tested as a typical example of mixed alkanes. The results are summarized in Table 1. The data for the volume ratio of cyanoacrylate to foaming agent and the concentration of initiator are the optimum values found in preceding experiments. In general, excess of foaming agent over the optimum ratio found either produces soft foams with large pores or the foaming agent splashes out, and part of it remains in liquid form during and after foaming. On the other hand, when its quantity is below the optimum ratio the expansion coefficient is reduced due to shrinkage of the foam after its expansion. DMPT concentration has a major influence only on the onset of the reaction, relatively less pronounced is the influence on the expansion coefficient. Generally, higher expansion ratios are obtained when the concentration of DMPT is higher. It creates more sites for initiation of polymerization and the associated exotherm is sufficient for fast simultaneous evaporation of the solvent, which expands the forming polymer. The experimental data show that the foaming ability of the tested compounds depends on a variety of factors. One of these is the boiling point. As a rule. the lower the boiling point, the better the foams produced. A striking example is the pentane/hexane/heptane series of solvents with similar solubility parameters, polarity and structure. Pentane with 35°C boiling point creates foams with 23 times expansion; hexane with 69°C boiling point creates foams with 13 times expansion; while heptane with 98°C boiling point produces foams with only four times expansion. The solubility parameter and foaming agent's polarity also have a major influence on the foams produced. These two factors determine the degree of solubility of the monomer and polymer in the liquid and vapour of the foaming agent. They also determine the degree of permeability of the trapped vapour through the walls of the foam cells. The best results are obtained with non-polar solvents with solubility parameters in the lower end of the scale, i.e., pentane and TCTFE. Solvents with solubility parameters at the other end of the scale, such as methanol, are most unsuitable as foaming agents, producing exceptionally brittle foams. Solvents such as acetone and acetonitrile, which have similar solubility parameters to poty(ethyl2-cyanoacrylate) (8 = 11.2s), are polar and thus are good solvents for the polymer, proved also to be poor foamforming agents. Their vapours or liquid left after the expansion tended to swell and dissolve the polymer, thus shrinking the foam. Furthermore, upon their evaporation, the resultant foams are very. brittle. In most ratios these solvents usually produce gel-like material rather than foam. Solvents in the mid-range of the solubility parameter scale and with boiling point around 80°C, such as cyclohexane, give good resilient Comparison of foaming compositions based on different foaming agents (FA) Table 1. Volume ratio of cyano- Foaming agent Cone. of initiator (wt%) Solubility Polarity parameter of FA of FA Boiling point of FA (°C) Onset time (s) Time of expansion (s) Expansion coefficient acrylate to FA Grading of FA suitability (! -- best, 11 --worst) Pentane 1:1 0.025 7.0 non 35 20 5 23 2 Hexane 2:1 0.017 7.3 non 69 25 5 13 3 TCTFE 4:1 0.010 7.3 non 48 6 6 25 ! Heptane 4:1 0.020 7.4 non 98 10 10 4 8 Diethyl ether 4:1 0.010 7.4 mod 35 19 5 18 4 Cyclohexane 2:1 0.017 8.2 non 81 34 5 10 5 2-Butanone 2:1 0.017 9.3 mot 80 10 15 17 7 Acetone 2:1 0.017 9.9 mod 56 5 10 22* 9 Ethanol 2:1 0.017 10.0 high 78 5 10 10 6 Acetonitrile 2:1 0.017 11.9 high 82 10 10 3 10 Methanol 4:1 0.020 14.5 high 65 5 25 8 11 *Foam collapses after expansion 152 INT.J.ADHESION AND ADHESIVES JULY 1992 foams displaying high adhesion towards the contacted surfaces. Good results are obtained when petroleum ether is used as a foaming agent. The foam formation is better controlled, temperature- and time-wise, and resilient homogeneous foams are produced. This is a result of its wide boiling range, which indicates that blends of different solvents will produce the best required foaming agent for every specific need or application. Evaluation of different polymerization initiators and optimization of their foaming compositions In the second series of experiments, several initiators of the anionic polymerization of cyanoacrylate monomers were evaluated as potential foaming initiators with the objective of finding the most suitable ones. Ethyl-2-cyanoacrylate was used as the cyanoacrylate monomer and TCTFE was used as the foaming agent. The results summarized in Table 2 clearly indicate that N-(oxydiethylene)benzothiazole-2-sulphenamide (ODEBTSA)and DMPT are by far the best initiators for foaming cyanoacrylate compositions. They dissolve well in the volume of the cyanoacrylate/foaming agent mixture and, shortly after, initiate polymerization, which has the necessary exothermal effect to evaporate the foaming liquid. ODEBTSA has the added advantage of no smell and presents no health hazard. Next in order comes piperidine, which gives slightly longer onset times and produces less expanded foams. A further disadvantage is its noxious smell and toxicity. Triphenyl- and triethylphosphines can be used as initiators when a very long onset time is required. They may prove valuable in expansions of large quantities of foaming compositions, where otherwise a large buildup of heat might be expected. Pyridine may be considered as an unacceptable compound for initiation of cyanoacrylate foaming. The very high reactivity of pyridine (an initiator level of only 1 ppm causes polymerization) prevents the even distribution of the initiator in the bulk of the foaming composition. Table 2. Evaluation of different cyanoacrylate monomers and optimization of their foaming compositions In the next series of experiments the suitability of different cyanoacrylate monomers for producing expanded foams was tested and compared (Table 3). Isobutyl-2-cyanoacrylate produces the highest foam expansion. Further advantages of its foam are the elasticity and homogeneity. Foams based on butyl-2cyanoacrylate are very similar to those based on the isobutyl monomer. Ethyl-2-cyanoacrylate is also an excellent foam-forming monomer, producing high expansion ratios and hard and resilient foams. The alkoxyalkyl monomers produce mid-range expansion foams and are very compatible with diethyl ether, as a result of the common ether bond in their molecules. Their foams are very elastic. Least suitable of the studied monomers is methyl-2-cyanoacrylate. Its foams are very brittle, as a result of the relative stiffness of its corresponding polymer molecule. Regulating the onset time of foaming In an attempt to control the onset time, i.e.. the time interval between mixing of the composition and its expansion into foam, two acids commonly used to stabilize cyanoacrylate monomers against spontaneous anionic polymerization during storage were tested. The results are presented in Table 4. The ethyl-2cyanoacrylate/TCTFE composition of 4:1 ratio, containing 0.01% by weight of DMPT, was used. The results clearly indicate that the onset time of foaming can be successfully regulated by introducing different amounts of p-toluenesulphonic or trifluoromethanesulphonic acid into the cyanoacrylate monomer, or into the composition itself. The minute quantities used do not affect the properties of the foams and do not alter the actual time of foam expansion. When similar experiments were made with hydroquinone, which is most often used as a stabilizer for cyanoacrylate monomers against radical polymerization, it was found, as expected, that it did not have any effect on the onset time of foam Comparison of foaming compositions based on different foaming initiators Initiator Health and safety hazard Optimum concentration of initiator Optimum volume ratio of E-2-CA* to TCTFE Onset time (s) DMPT Irritant 0.01 4:1 6 Pyridine Irritant Flammable 1.7 x 10 -4 4:3 Piperidine Highly toxic Flammable 0.01 Triethylphosphine Pyrophoric Stench Triphenylphosphine ODEBTSA Time of expansion (s) Expansion coefficient Grading of initiator suitability (1 - - best, 6 - - worst) 6 25 2 450 20 7 6 2:1 30 10 18 3 0.003 2:1 73 40 17 4 Irritant 4.8 x 10-4 4:3 298 68 13 5 -- 0.017 2:1 25 5 25 1 * E-2-CA = ethyl-2-cyanoacrylate INT.J.ADHESION AND ADHESIVES JULY 1992 153 Table 3. Foaming compositions based on different cyanoacrylate monomers Cyanoacrylate monomer Foaming agent Volume ratio of cyanoacrylate to foaming agent Expansion coefficient M ethyl-2-cyanoacrylate Diethyl ether 4:1 12 Ethyl-2-cyanoacrylate TCTFE 4:1 25 Butyl-2-cyanoacrylate TCTFE 1: 1 24 Isobutyl-2-cyanoacrylate TCTFE 1: 1 33 Allyl-2-cyanoacrylate Diethyl ether 1: 1 17 2-Methoxyethyl-2-cyanoacrylate TCTFE 4:3 12 2-Ethoxyethyl-2-cyanoacrylate TCTFE 2:1 14 2-M ethoxyisopropyl-2-cyanoacrylate TCTFE 2:1 15 Table 4. Stabilized foaming compositions Acid Concentration of acid in cyanoacrylate expansion in such a large interval as 0.01-1.0% by weight of the cyanoacrytate monomer. Onset time (s) Monitoring and regulating the temperature during foam formation (wt%) Control 0 p-Toluenesulphonic 0,0003 0.0006 0,0012 0.002 0,005 0.01 14 16 20 37 66 195 Trifluoromethanesulphonic 0.001 0.002 0.005 0.01 In the envisioned medical applications of the foam. such as broken-limb support and Fallopian tube sealing, the heat evolved during foaming, if excessive. may render the methods unacceptable. Temperature measurements (Fig. 1) of expansions likely to be encountered in broken-limb support formation are summarized in Table 5. The following conclusions can be drawn. The temperature of foaming depends on the nature of the cyanoacrylate monomer and decreases with increasing length of the radical R in the molecule, the order being: methyl, ethyl, allyl; isobutyl, butyl, methoxyethyl, ethoxyethyl and methoxyisopropyl. It also depends on the nature of the foaming agent, i.e.. the lower the boiling point of the agent, the lower the temperature of expansion. The 11 24 43 117 Table 5. 10 Temperature of foaming for different compositions Maximum temperature reached (°C) Volume ratio of cyanoacrylate to foaming agent 2-Cyanoacrylate monomer Foaming agant I nitiator Content of initiator in composition (wt%) 4:1 2:1 2:1 Ethyl TCTFE TCTFE TCTF E 0.01 0.017 0.003 95 84 85 Pentane Pentane DM PT ODEBTSA Triethylphosphine DM PT DMPT 0.017 0.025 80 65 2:1 1: 1 2:1 Allyl TCTF E DM PT 0.025 75 2:1 Butyl Pentane DM PT 0.017 66 2:1 1: 1 Isobutyl Pentane Pentane DM PT DMPT 0.017 0.025 67 41 4:1 M ethyl TCTF E DM PT 0.01 106 4:3 Methoxyethyl TCTF E DM PT 0.01 63 1:1 Ethoxyethyl TCTF E DM PT 0.025 58 1: 1 M ethoxyisopropyl TCTF E DM PT 0.025 58 154 INT.J.ADHESION AND ADHESIVES JULY 1992 temperature of foaming depends also on the ratio of monomer/foaming agent and is lowered by increasing the quantity of the foaming agent. The type and quantity of initiator do not have a major effect on the temperature of foaming. These results are very promising in that the recorded temperature values are only the maxima and the generated heat dissipates very rapidly. Considering that the limb would be covered and protected by stockinette, with considerable heat insulating properties, it can be expected that unbearable heat during foaming will not be experienced. A similar set-up (Fig. 1), simulating a human duct occlusion, was used to monitor the temperature of foaming. The time interval during which the temperature stays above 37°C was also recorded. The results presented in Table 6 show that when foamed plugs are used instead of pure cyanoacrylate polymers the temperature that the tissues of the tube, duct or vessel will experience is greatly reduced, which will eliminate any heat-associated necrosis. Furthermore, by choosing the right monomer, foaming agent and composition, very low temperatures of foam expansion can be achieved, even as low as below body temperature. Almost all of the monomers, except methyl-2-cyanoacrylate, can produce acceptable compositions. Monitoring and regulating the odour emitted during foaming During the formation of polycyanoacrylate foams the foaming agent evaporates and can be the main source of odour. Along with it, small amounts of cyanoacrylate monomer and initiator can also be carried away and be a further source of odour. In the Table 6. case of tubular vessel blocking, whether it is industrial pipelines or human ducts, any odour associated with the foaming is unimportant. Only in the case of opentop expansions, such as broken-limb support formation or adhesive bonding or sealing, will the odour emitted be a factor of consideration. Each cyanoacrylate monomer had its own odour, the methyl, allyl and ethyl being the most offensive and with lacrymatory action. Butyl and isobutyl cyanoacrylates had a sweet scent and were less offensive. Methoxyethyl and methoxyisopropyl cyanoacrylates had almost no smell with a faint mouldy scent, while ethoxyethyl cyanoacrylate was completely odourless. A completely odourless foaming process is achieved when TCTFE is used as the foaming agent, alkoxyalkyl cyanoacrylate is used as the monomer and ODEBTSAis used as foaming initiator. Characterization of the foams produced Table 7 summarizes some of the properties of foams based on different cyanoacrylate monomers. The foam based on ethyl-2-cyanoacrylate shows the best strength characteristics. Next come the foams based on butyl-, isobutyl- and allyl-2-cyanoacrylates. Methyl-2cyanoacrylate foams have markedly the poorest properties. It should be noted that the results presented must be treated for comparison purposes only. The structure of the foam (specific gravity, pore size) varies greatly not only with the variation of the components of the composition, but also with the volume available for expansion. When that volume is restricted the foam is denser with smaller pores, harder and tougher. Therefore the physical characteristics of the foam will Temperature of foaming in 2 mm tube for different compositions 2-Cyanoacrylate monomer Foaming agent Volume ratio of cyanoacrylate to foaming agent Maximum temperature reached during foaming (°C) Time interval above 37°C (s) Methyl TCTFE 2:1 58.5 60 Ethyl TCTFE Diethyl ether 1: 1 1: 1 39.6 43.6 37 48 Allyl TCTFE 1: 1 41.5 40 Butyl Diethyl ether 1:1 40.6 35 Isobutyl TCTFE TCTFE Diethyl ether Diethyl ether 2:1 1: 1 2:1 1: 1 51.2 44.7 41.6 36.2 59 54 53 Methoxyethyl TCTFE Diethyl ether 1: 1 1: 1 44.8 45.3 52 32 Ethoxyethyl TCTFE Diethyl ether 2:1 1: 1 57.5 41.7 59 35 Methoxyisopropyl TCTFE Diethyl ether 1: 1 1: 1 40.1 37 36.0 Methyl 70.4 Ethyl 67.3 INT.J.ADHESION AND ADHESIVES JULY 1992 155 Table 7. Physical characteristics of polycyanoacrylate foams 2-Cyanoacrylate monomer Specific gravity (gcm -3) Tensile strength at break (kg cm -2) Elongation at break (%) Tensile shear strength at break (kg cm -2) Compression strength at 10% deformation (kg cm -2) Methyl Ethyl Butyl Isobutyl Allyl 2-M ethoxyethyl 2-Ethoxyethyl 2-Methoxyisopropyl 0.28 O. 13 0.25 0.24 0.18 0.41 0.22 0.28 0.8 10.6 5.7 3.2 5.0 1.6 1,6 1.8 2.5 3.7 6,7 3,7 3.0 2,0 1.7 2.0 0,5 5.2 1 o8 3.0 3.0 1.9 1.5 1.7 5,7 4.9 4.9 5.7 7.3 6.5 5.5 4.0 be dependent on the components, composition and available volume for expansion. It was determined (for the 4:1 ethyl-2-cyanoacrylate/ TCTFE composition) that approximately 50% of the foaming agent escapes during foaming and the rest is trapped into the foam. Approximately 95% of that is in closed-pore cells. The diameter of the pores varies mainly with the volume available for expansion. If that volume is unlimited it is between 0.06 mm and 0.2 mm. When the volume for expansion is restricted the cell size can be below 0.002 mm. The molecular weight of the poly(ethyl-2cyanoacrylate) foam was determined to be in the region of 500 000, which corresponds very well with the molecular weight of this cyanoacrylate's bondline 3. Evaluation of the suitability of the cyanoacrylate polymer foam for orthopaedic casts The results obtained during the course of this work strongly indicate that the cyanoacrylate polymer foam will be suitable for immobilization of broken limbs. The foaming composition can be based on alkoxyalkyl cyanoacrylate monomer, which will have the advantage of no odour and bearable heat build-up during the expansion. As a foaming agent TCTFE can be used, which has the advantage of being odourless, non-toxic, non-irritant and non-flammable; while as an initiator ODEBTSA can be used, which has the advantage of being odourless, non-toxic and non-irritant. The broken limb will be covered with a suitable stockinette, preferably of non-woven polypropylene fabric, and placed in a polymer mould. The foaming composition will be injected into the mould from a cartridge dispenser. It will expand into a foam, thus immobilizing the broken limb. The shell of the mould can stay in place if desired, in which case it can be made from any polymer but polyolefin. If it is desirable to remove the mould it has to be made of polyethylene or polypropylene. When the lightweight and rigid polycyanoacrylate foam support is no longer needed (the fracture has healed), it can easily be removed by soaking with adequate polycyanoacrylate solvent. The actual collapse and removal of the cast will be painless, simple and again accomplished in matter of seconds or minutes. The shell of the polymer mould can be reused. 156 INT.J.ADHESION AND ADHESIVES JULY 1992 Evaluation of the suitability of the cyanoacrylate polymer foam for Fallopian tube sealing and blood vessel occlusion Currently methyl-2-cyanoacrylate is being applied as an agent to occlude the oviducts as a means of permanent, non-reversible sterilization of consenting women9. It is instilled in the fundal region of the uterus, and then is forced into the proximal portion of the oviducts, where it polymerizes, by the action of an expanding balloon. The poly(methyl-2-cyanoacrylate) plug then degrades, releasing degradation products which incite a local inflammatory response, resulting in bilateral occlusion of the Fallopian tubes by fibrous tissue. In two to three months the polymer completely disappears and is replaced by collageneous scar tissue1°. A systemized report N on 652 sterilization cases points out that bilateral tubal closure is obtained in 70.2% of the women after a single application, and in 90.3% after two cyanoacrylate applications. Experiments on blood vessel treatment with cyanoacrylates were performed as early as 196412. Isobutyl cyanoacrylate has been injected by selectively positioned catheters to close off arteriovenous fistula13 The procedure proved generally successful. In-growth of fibrous tissue into the sponge-like mass of adhesive was noted. A life-saving procedure in cases of acute stomach ulcer haemorrhage is the injection of cyanoacrylate into the blood vessel responsible for the haemorrhage 14. The results obtained in the present study without any doubt show that any cyanoacrylate foam will be better than the cyanoacrylate monomer alone. Firstly, it will ensure better plugging and the plug itself will be flexible and elastic, rather than a glassy material; secondly, the heat developed during foaming will be less than that evolved during the block polymerization of pure cyanoacrylate, which will eliminate the associated tissue necrosis; and thirdly, the amount of cyanoacrylate introduced into the vessel or tube will be reduced tenfold, which will ease the burden of eliminating the toxic bio-degradation products. The best foaming composition will be based on isobutyl-2cyanoacrylate, a monomer which has been tested and evaluated extensively and has proven to be amongst the most suitable as surgical adhesives, As most suitable foaming agent diethyl ether is envisioned. An advantage in its favour is the expectation that, along with its excellent foaming ability and wide acceptance as a medical aid, diethyl ether will exercise a local anaesthetic effect and will render the procedure painless. In the case of Fallopian tube blocking, it is our belief that, due to its porosity, the polycyanoacrylate foam plug will degrade faster than a solid polycyanoacrylate plug. Also, because of its structure, it will be very easily intertwined during that process with fibrous tissue, which will take the foam's place in the formation of a permanent plug of body tissue. It might be expected that when a foaming cyanoacrylate composition is used instead of pure cyanoacrylate monomer the success rate of female sterilization will improve from the current 90% to 100% due to its better plugging capacity, and this without the need of second or third applications. creating an efficient plug whose adhesion to the pipe wall was higher than the strength of the foam material itself. The pipes were hermetically sealed by the foam plug and easily withstood pressure of 10 atmospheres. In a following operation acetone was injected with a syringe through the same opening, subsequently plugged with foam. In 5 seconds to 2 minutes, depending on the size of the pipe, the foam collapsed and flow through the pipe was restored. The compositions and foaming method discussed can also be used as instant, foaming adhesives and sealants as well as in numerous, not so obvious, applications. Evaluation of the suitability of the cyanoacrylate polymer foam for permanent or temporary blockage of fluid flow in pipelines Lightweight polycyanoacrylate foams can be obtained instantly by combining cyanoacrylate monomer with foaming agent and polymerization initiator. The foams have high adhesion and instantly bond to the surfaces they contact during expansion. This new property of cyanoacrylates opens new vistas of potential applications as adhesives and sealants in medicine and industry. Currently polyurethane-based foam is used to stop gas flow in gas mains. The foam is produced by mixing polyether or polyester diol, containing tertiary amine as catalyst with diphenylmethane-4,4-diisocyanate. By drilling a hole in the gas main and using special membranes and cartridges, the foam is injected into the pipe 15. Very often, however, the foam or pre-foam material flows along the pipe and/or its expansion (only four times) is insufficient to block and seal the pipe. In an improvement of the method 16 a special bag has been devised which is inserted into the pipe and has the purpose of holding the material 'in place'. It is porous, thereby permitting some of the foam to 'ooze' through the bag, contact the wall of the pipe and provide a seal. A special device is needed in turn to hold the bag in the correct position during this procedure. It is obvious that the current foam and methods of blocking gas mains are in no way ideal. The ideal case would require: (1) a simple method of foam injection without the use of special bags and devices; (2) the foam to adhesively bond itself to the walls of the pipe, so that complete sealing and blockage are ensured; and (3) the foam to be easily removed at a later stage if desired, so that fluid flow through the pipe can be reinstated. All of these conditions are met by the foaming compositions and method described in this work. The foams with best physical properties are based on ethyl2-cyanoacrylate. A variety of foaming agents can be used, but TCTFE has the advantage of being nonflammable. As initiator both DMPT and ODEBTSAare adequate. Tubes and pipes made of cast iron, stainless steel, copper, polycarbonate, polystyrene, polyvinylchloride, polypropylene and polyethylene having inside diameter from 1 mm to 100 mm were plugged with foam by injecting through a specially drilled opening a foaming composition consisting of ethyl-2-cyanoacrylate and TCTFE in volume ratio 4:1 and containing 0.01% by weight DMPT. The composition expanded into a foam, Conclusions Acknowledgement This work was supported by Feasibility Award FA-90285 in the BRITE/EURAM Programme of the Commission of the European Communities, 1990. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 "JapanAdhesives Industry Annual Book" (Tokyo, Ja!3an, 1985) p 57 Coover, Jr, H.W. in "Handbook of Adhesives" edited by I. Skeist (Van Nostrand-Reinhold, New York, NY, USA, 1977) p 569 Guthrie, J., Ottenburn, M.S., Rooney, J.M. and Tsang, C.N. J Appl Polyrn Sci 30 (1985) pp 2863-2867 International Patent Application WO 92/09651 (1992) Joyner, F.B. and Sheerer, N.H. US Pat 2 721 858 (1955) Hawkins, G.F. and McCurry, H.F. US Pat 325 411 (1966) Danchav, Z., Kotzev, D. and Serafimov, B. J Adhesion Sci Technol 2 No 3 (1988) pp 157-165 Donnelly, E.F. end Pepper, D.C. Makrornol Chem, Rapid Commun 2 (1981) pp 439-442 Nauwirth, R.S. ArnJObstet Gynecol 136 (1968) p 951 Nightingale, J.A., Hoffman, A.S. and Harbert, S.A. Polyrn Preprints (Am Chem Soc, Div Polym Chern) 24 No 1 (1983) pp 28-29 Neuwirth, R.S. and Richart, R.M. Contraception 31 No 3 (1985) pp 243-252 Weissberg, D. and Goetz, R.H. Surgery, Gynecol Obstetr 119 (1964) pp 1248-1252 Zanetti, P.H. and Sherman, F.E. J Neurosurg 36 (1972) pp 72-79 Akimova, A. (VNIIMT, Moscow) private communication Vick, S.M. UKPet 2 123919 (1986) ~ck, S.M. UKPat2157390{1987) Authors The authors are with Chemence Ltd, Princewood Road, Corby, Northants NN17 2XD, UK. Correspondence should be directed to D. Kotzev. INT.J.ADHESION AND ADHESIVES JULY 1992 157