GB2065027A

GB2065027A - Bonding materials

Info

Publication number: GB2065027A
Application number: GB7942279A
Authority: GB
Original assignee: Polymer Laboratories Ltd
Current assignee: Polymer Laboratories Ltd
Priority date: 1979-12-07
Filing date: 1979-12-07
Publication date: 1981-06-24

Abstract

Surfaces are bonded together by providing at the interface to be bonded a dielectric heating promoter and exposing the composite to radio frequency or microwave frequency radiation. The dielectric heating promoter comprises a polymer containing ether or thioether linkages having distributed therethrough on a molecular scale an inorganic salt.

Description

SPECIFICATION Bonding materials This invention relates to the bonding of materials.

It is well known to bond composites, e.g. laminates etc. using hot thermoplastic materials which on cooling set to form a secure bond. Thus for example thermoplastic material may be applied to at least one of the surfaces to be bonded in the form of a hot melt adhesive. The surfaces are brought together and the adhesive allowed to cool. Also when one or both of the surfaces to be bonded is itself thermoplastic there may be used a welding technique. Thus one or both surfaces is heated to a sufficiently high temperature that on cooling in contact a secure bond is formed. Good adhesion may be achieved in this manner. However there have in the past been severe practical problems associated with the heating of the thermoplastics material to a sufficient temperature for bonding and with the handling of hot materials.

According to the present invention there is provided a method of bonding two surfaces together wherein at least one of the surfaces to be bonded is provided with a dielectric heating compound (as herein defined), the dielectric heating compound is heated by being subjected to radio frequency or microwave frequency radiation and the compound allowed to cool while the two surfaces are held together.

The dielectric heating compound according to the present invention has a high dielectric loss. When the compound is subjected to the radio frequency or microwave frequency radiation, the compound becomes heated to a high temperature (i.e. it acts as a dielectric absorber) such that on cooling a good bond is formed.

The dielectric heating compound at one of the surfaces may be subject to the radiation before the two surfaces to be bonded are brought into contact.

However more conveniently the two surfaces are first brought into contact and the composite so formed exposed to the radiation. The radiation is then absorbed substantially only by the dielectric heating compound resulting in local heating.

Preferably exposure is to radio waves of frequency 106to 108Hz.

The dielectric heating compound used according to the present invention is a, preferably amorphous (non-crystalline), material comprising a polymer containing ether or thioether linkages having distributed therethrough on a molecular scale an inorganic salt. The precise nature of the material is not completely understood though it is clear that there is a bonding between the polymer linkages and the inorganic salt and at least to some extent there may be complex formation. There must be some ionic mobility in the material to ensure dielectric heating properties.

The dielectric heating compound used according to the present invention is obtainable by removing solvent from a solution of the ether or thioether linkage-containing polymer and the inorganic salt in the solvent at a rate such that the product is predominantly, or preferably wholly, single phase.

Should the rate of removal be too great, the temperature of the solution at which the solvent is removed not properly chosen, or the solvent be not properly chosen, precipitation of unreacted salt or polymer is liable to result. In practice, the salt and polymer are generally dissolved separately in the chosen solvent, which may comprise a plurality of solvent components, typically maintained at a temperature above ambient in order to promote dissolution, and the solutions are then mixed. The mixture is then usually held at a temperature above ambient, typically 100 C or less (generally about 40 C) and the bulk of the solvent is evaporated therefrom for example by a passage of a stream of dry gas such as nitrogen over the surface of the solution. Residual solvent may be removed under vacuum if necessary at elevated temperature.

In general the solventforthe mixture of inorganic salt in the polymer comprises polar organic compounds such as the alcohols, tetrahydrofuran or acetone and may also comprise a less polar cosolvent, such as tolunene. [For example, when the salt is a cobalt halide, e.g. CoCI2, and the polymer a high molecular weight polyalkylene oxide such as polypropylene oxide a carefully chosen mixture of a polar solvent and less polar co-solvent, for example an alcohol such as ethanol and a hydrocarbon such astoluene may be required. Generally however cobalt salts are not preferred because of the difficulty in dispersing them in suitable polymers].

When the polymer used has a crystalline form (as in the case of polyethylene oxide) care is required during the separation step to prevent crystal formation if amorphous heating compound is required.

It is usually highly desirable to ensure that the mixture in the solvent is kept anhydrous by for example ensuring that any gas stream employed for evaporation is dry.

The ether or thioether linkages of the starting polymer which may be present either in the polymer backbone or in pendant groups, or in both, usually amount to at least 5 mole % and preferably at least 10 mole % of the polymer and may represent up to 50 mole % thereof. The polymer may be a homopolymer or a copolymer and may be a polyether, polythioether, a copolymer thereof or maybe a different type of polymer, e.g. a polyurethane which may contain ether orthioether linkages. The polymer may for example comprise a homo- or copolymer of an alkylene oxide, e.g. polypropylene oxide, polyethylene oxide, polytetramethylene oxide, of a vinylether, of phenylene oxide, styrene oxide, of formaldehyde, of epichlorohydrin, a polyurethane containing one or more blocks of polyalkylene oxide or any of the above polymers in which ethereal oxygen is wholly or partly substituted by sulphur.

In general the cation of the inorganic salt can be monovalent, although multi-, e.g. di-, valent cations can also be used. It is in general preferred that the anion be monovalent. It is found that the monovalent salts are more readily taken into the polymer without precipitating out. The cation is generally a metal cation though ammonium cations can be used when decomposition difficulties can be avoided.

Preferred cations have been found to be the alkali metal cations, particularly potassium, sodium and lithium. Particularly preferred anions include the halides, e.g. fluoride, chloride, bromide and iodide, and thiocyanate. Preferred metal salts therefore include KSCN, KCI, NaSCN, NaCI, LiSCN, LiCI.

However in addition good results may be obtained with Ca(SCN)2, Ba(SCN)2 and SnCl2 in certain polyethers.

The amount of salt used must be sufficierit to ensure good dielectric heating properties but not so great that those properties are impaired. it is found that if the amount of salt is too low its effect is insufficient but on the other hand if it is too high dielectric properties may become impaired. In general 5 to 20 mole %, based on the ether or thioether linkages, of the salt is used though lower amounts e.g. down to 2 mole % may in some circumstances be suitable. Generally it is not possible to exceed 30 mole % as such a large amount of salt will not usually be assimilated into the polymer.

The dielectric heating compound is itself thermoplastic and may be used alone as thermoplastic adhesive. However preferably it is used as a blend with a thermoplastic polymer, the heated dielectric heating compound heating the surrounding thermoplastic polymer also during the exposure to radiation.

The dielectric heating compound, optionally mixed with thermoplastic polymer, e.g. in particulate form, may be spread over one of the surfaces to be adhered. The dielectric heating compound (and polymer) may be provided in sheet form such that a sheet containing the dielectric heating compound need simply be provided between the two surfaces to be bonded. The compound can also be cast as a liquid on to one of the surfaces to be bonded. Indeed there may be applied the solution of ether or thioether linkage-containing polymer and inorganic salt used in the preparation of the dielectric heating compound and the solvent allowed to evaporate in situ.

The method according to the invention may be used to bond two non-thermoplastic surfaces e.g. in the bonding of fabric and lining materials in the manufacture of clothing and the bonding of lining materials in the manufacture of shoes.

The method may also be used when one or more of the surfaces to be bonded is thermoplastic. In this case there will probably be some local heating of the or both thermoplastic materials at the interface causing a welding of the materials.

In addition one or both of the components to be bonded may comprise a blend of the dielectric heating compound and a thermoplastic polymer, e.g. in film form. In this case it is unnecessary to apply further dielectric heating compound. Exposure to suitable radiation of the two components in direct contact will cause welding of the surfaces.

When the dielectric heating compound is used in a blend with a thermoplastic polymer, it is finely (on a micron scale) and homogeneously mixed with the polymer. The amount of compound must not be so great as adversely to affect any desirable properties of the polymer but must of course be sufficient to ensure that the thermoplastic polymer is adequately heated during exposure. Generally the compound is used in an amount up to 20%, preferably up to 10% e.g. 0.1 to 5%, by weight.

The mixing to form the blend can be achieved in conventional manner e.g. by mechanical mixing of the ingredients of the blend.

Suitable thermoplastic polymers include acrylonitrile-butadiene-styrene copolymers, polystyrene, thermoelastomers of polystyrene and butadiene and segmented polyether-polyesters.

This invention is further illustrated in the following Examples. In the Examples pph stands for parts per hundred by weight. The mole % of inorganic salt given are based on the ether linkages in the polymer (e.g. 10 mole % sodium thiocyanate in polypropylene glycol contains 1 molecule of sodium thiocyanate per 10 ether linkages in the polymer).

Example I Dielectric heating compound comprising 10 mole % sodium thiocyanate dispersed in a polypropylene glycol of molecular weight 1500 was prepared as follows: 1 0g Anhydrous sodium thiocyanate were dissolved in 50cc dry ethanol and 209 of the polypropylene glycol were dissolved in 1 00cc dry ethanol.

Sufficient salt solution was added to the polymer solution with stirring to provide 10 mole % of salt based on polymer ether linkages. The resulting mixed solution was transferred to an evaporating dish and solvent removed slowly over a period of three days with occasional stirring. The latter was achieved by standing the evaporating dish in a closed vacuum oven, dessicator or similar vessel and passing a slow flow of dry nitrogen gas through the enclosure. The temperature was finally raised to 400C for one day to complete reaction and remove final traces of solvent.

20 pph of the dielectric heating compound so obtained were mixed with acrylonitrile-butadienestyrene copolymer blend (ABS) on a commercial internal mixer (Brabender Plastograph) at a temperature between 1400C and 180"C. The ABS was melted first and then the requisite amount of dielectric heating compound added with mixing over a period of 2 to 5 minutes until the blend appeared homogeneous to the eye.

The blend so obtained was pressed at a temperature between 1 40 C and 1 800C into flat sheets, 0.01 mm to 0.1 mm thick. The sheets were then cut into strips.

One strip was placed on top of another and the composite placed between two extensive 2 mm thick inert silicone rubber sheets and pressed between the jaws of a commercial dielectric heating press (Intertherm Plastics Welder Pressure Type EL 21/1, operating at 38 MHz, power 1.6 kW) and radio frequency radiation applied in normal operating manner. The settings of the press were output 6.5 and overload 7.

After being subjected to the radio frequency radiation in this way for 5 seconds the two strips were securely bonded together.

Example 2 Dielectric heating compound comprising 10 mole % potassium thiocyanate dispersed in a polypropy lene glycol of molecular weight 1500 was prepared as follows: 10g Anhydrous potassium thiocyanate were dis solved in 50cc dry ethanol and 209 of the polypropy lene glycol were dissolved in 100cc dry ethanol.

Sufficient salt solution was added to the polymer solution with stirring to provide 10 mole % of salt based on polymer ether linkages. Solvent was removed from the resulting mixed solution as in Example 1.

20 pph of the dielectric heating compound so obtained were mixed with ABS, the blend so obtained pressed into flat sheets and cut into strips as in Example 1.

Two strips were placed together and subjected to radio frequency radiation in the commercial dielec tric heating press as in Example 1. The settings of the press were output 4.5 and overload 4.5.

After being subjected to radio frequency radiation in this way for 3 seconds the two strips were securely bonded together.

Example 3 Dielectric heating compound comprising 10 mole % lithium thiocyanate dispersed in a polytetramethylene glycol of molecular weight 1000 was prepared as follows: 10g Anhydrous lithium thiocyanate were dis solved in 50cc dry ethanol and 209 of the polytet ramethylene glycol were dissolved in 100cue dry ethanol. Sufficient salt solution was added to the polymer solution with stirring to provide 10 mole % of salt based on polymer ether linkages. Solvent was removed from the resulting mixed solution as in Example 1 except that the temperature was finally raised to 30 C for one day.

10 pph of the dielectric heating compound so obtained were mixed with ABS, the blend so obtained pressed into flat sheets and cut into strips as in Example 1.

Two strips were placed together and subjected to radio frequency radiation in the commercial dielec tric heating press as in Example 1. The settings of the press were output 9 and overload 7.

After being subjected to radio frequency radiation in this way for 8 seconds the two strips were securely bonded together.

Example 4 Dielectric heating compound comprising 20 mole % calcium thiocyanate dispersed in a polytet ramethylene glycol of molecular weight 1000 was prepared as follows: 10g Anhydrous calcium thiocyanate were dis solved in 50cc dry diethyl ether and 20g of the polytetramethylene glycol were dissolved in 100c of dry diethyl ether. Sufficient salt solution was added to the polymer solution with stirring to provide 20 mole % of salt based on polymer ether linkages.

Solvent was removed from the resulting mixed solution as in Example 1 except that the temperature was finally raised to 30 C for one day.

Two strips were placed together and subjected to radio frequency radiation in the commercial dielectric heating press as in Example 1. The settings of the press were output 9 and overload 6.

After being subjected to radio frequency radiation in this way for 10 seconds the two strips were securely bonded together.

Example 5 Dielectric heating compound comprising 15 mole % barium thiocyanate dispersed in a polypropylene oxide of molecular weight approximately 100,000 was prepared as follows: 10g Anhydrous barium thiocyanate were dissolved in 50cc dry diethyl ether and 1 5g of the polypropylene oxide high polymer were dissolved in 200cc dry diethyl ether. Sufficient salt solution was added to the polymer solution with stirring to provide 15 mole % of salt based on polymer ether linkages. The resulting mixed solution was transferred to a wide vessel of glass, polythene or other inert material such that a large surface area of solution was generated. The vessel was sealed and dry nitrogen passed through slowly for three days. The temperature was finally raised to 500C fortwo days to complete reaction and remove final traces of solvent.

15 pph of the dielectric heating compound so obtained were mixed with a segmented polyetherpolyester [Hytrel ex du Pont], the blend so obtained pressed into flat sheets and cut into strips as in Example 1.

Two strips were placed together and subjected to radio frequency radiation in the commercial dielectric heating press as in Example 1. The settings of the press were output 9 and overload 7.

Example 6 Dielectric heating compound comprising 15 mole % stannous chloride (SnCl2) dispersed in a polypropylene oxide of molecular weight approximately 100,000 was prepared as follows: 10g Anhydrous stannous chloride were dissolved in 50cc dry diethyl ether and 1 5g of the polypropylene oxide high polymer was dissolved in 200cc dry diethyl ether. Sufficient salt solution was added to the polymer solution with stirring to provide 15 mole % of salt based on polymer ether linkages. Solvent was removed from the resulting mixed solution as in Example 5.

15 pph of the dielectric heating compound so obtained were mixed with a segmented polyetherpolyester as used in Example 5, the blend so obtained pressed into flat sheets and cut into strips as in Example 1.

Two strips were placed together and subjected to radio frequency radiation in the commercial dielectric heating press as in Example 1. The settings of the press were output 8 and overload 7.

After being subjected to radio frequency radiation in this way for 6 seconds the two strips were securely bonded together.

Example 7 Dielectric heating compound comprising 20 mole % sodium thiocyanate dispersed in a polytetramethylene glycol of molecular weight 1000 was prepared as follows: 10g Anhydrous sodium thiocyanate were dissolved in 50cc dry ethanol and 20g of the polytetramethylene glycol were dissolved in 1 00cc dry ethanol. Sufficient salt solution was added to the polymer solution with stirring to provide 20 mole % of salt based on polymer ether linkages. Solvent was removed from the resulting mixed solution as in Example 1.

10 pph of the dielectric heating compound so obtained were mixed with a segmented polyetherpolyester as used in Example 5, the blend so obtained pressed into flat sheets and cut into strips as in Example 1.

Example 8 Dielectric heating compound comprising 10 mole % lithium thiocyanate dispersed in a polypropylene glycol of molecular weight 1500 was prepared as follows: 10g Anhydrous lithium thiocyanate were dissolved in S0ccdrydiethyl ether and 209 of the polypropylene glycol were dissolved in 1 00cc dry diethyl ether. Sufficient salt solution was added to the polymer solution with stirring to provide 10 mole % of salt based on polymer ether linkages. Solvent was removed from the resulting mixed solution as in Example 1 except that the temperature was finally raised to 30 C for one day.

20 pph of the dielectric heating compound so obtained were mixed with SBS (a thermoelastomer of polystyrene and polybutadiene sold under the trade name Shell K 1101), the blend so obtained pressed into flat sheets and cut into strips as in Example 1.

After being subjected to radio frequency radiation in this way for 5 seconds the two strips were securely bonded together.

10 pph of the dielectric heating compound so obtained were mixed with polystyrene, the blend so obtained pressed into flat sheets and cut into strips as in Example 1.

Example 10 Dielectric heating compound comprising 20 mole % potassium thiocyanate dispersed in polypropylene glycol of molecular weight 1500 was prepared as follows: 10g Anhydrous potassium thiocyanate were dissolved in 50cc dry ethanol and 20g of the polypropylene glycol were dissolved in 1 00cc dry ethanol.

Sufficient salt solution was added to the polymer solution with stirring to provide 20 mole % of salt based on polymer ether linkages. Solvent was removed from the resulting mixed solution as in Example 1.

20 pph of the dielectric heating compound so obtained were mixed with a segmented polyetherpolyester as used in Example 5 and pressed into sheets as described in Example 1.

The blend sheet so obtained was used to bond leather, 2 mm thick to PVC coated paper fabric shoe lining. The blend sheet is placed between the leather and lining to be bonded (the blend sheet being in contact with the rough side of the leather and the paper backing of the lining). The composite is subjected to radio frequency radiation in a commercial dielectric heating press as in Example 1. The settings of the press were output 8 and overload 7.

After being subjected to radio frequency radiation in this way for 5 seconds the leather and lining were securely bonded.

Claims

1. A method of bonding two surfaces together wherein at least one of the surfaces to be bonded is provided with a dielectric heating compound (as herein defined), the dielectric heating compound is heated by being subjected to radio frequency or microwave frequency radiation and the compound allowed to cool while the two surfaces are held together.

2. A method according to claim 1 wherein radio frequency radiation of 106 to 1 0s Hz is used.

3. A method according to claim 1 or 2 wherein a sheet comprising the dielectric heating compound is placed the two surfaces to be bonded and the composite so formed exposed to the radiation.

4. A method according to claim 3 wherein the sheet comprises a blend of the dielectric heating compound and a thermoplastic polymer.

5. A method according to claim 1 or 2 for weiding two components at least one of which comprises a blend of the dielectric heating compound and a thermoplastic polymer wherein the two components are brought into direct contact and the composite so formed exposed to the radiation.

6. A method according to claim 4 or 5 wherein the blend contains 0.1 to 20% by weight of the dielectric heating compound.

7. A method according to any one of the preceding claims wherein the dielectric heating compound comprises a polymer containing ether or thioether linkages having distributed therethrough on a molecular scale an alkali metal halide orthiocyanate.

8. A method according to claim 7 wherein the dielectric heating compound contains 5 to 20 mole %, based on the ether or thioether linkages, of the alkali metal halide orthiocyanate.

9. A composite formed by the method claimed in any one of the preceding claims.