WO2007122417A1

WO2007122417A1 - Adhesive composition

Info

Publication number: WO2007122417A1
Application number: PCT/GB2007/001542
Authority: WO
Inventors: Colin Robert Willis; Brian Alan Beadle; Alan Hugh Clarke
Original assignee: The Secretary Of State For Defence
Priority date: 2006-04-26
Filing date: 2007-04-26
Publication date: 2007-11-01
Also published as: GB0608194D0

Abstract

An adhesive composition comprising an ethylene-vinyl acetate copolymer containing from 1 to 8% by weight of vinyl acetate together with a finely-divided activated carbon at a weight percent in the range from 10 to 30% of the composition is provided. The carbon preferably has a particle size of less than 500, more preferably less than 300, microns and may be a bituminous coal-based (BPL) carbon. The composition is particularly suitable for adhesion of metal surfaces such as stainless or mild steel.

Description

Adhesive Composition

This invention relates to an adhesive composition, particularly a composition suitable for adhering together metals or other non-porous materials, and to a method for formulating such compositions.

Adhesion is often the most effective way of joining materials using relatively simple processes and equipment. A wide selection of polymers possess adhesive properties which can be tailored and/or improved by numerous methods including oxidative cross-linking and inclusion of surface-active substances. Clean, dry surfaces are usually a prerequisite for adhesive bonding. It is known that with non-porous surfaces in particular, such as found on metal and plastics materials, they should preferably be cleaned, degreased, dried and roughened beforehand in order to achieve satisfactory adhesion. Polyethylene itself is a relatively poor adhesive to steel and other metals but its adhesive properties can be significantly improved through oxidation and cross- linking, e.g. Raprex™ are modified polyethylene resins with shear strengths in excess of 650 N/cm² for adhesion to steel.

Ethylene-vinyl acetate copolymers are known examples of the class of so-called hot melt adhesives which are applied as a melt to the surfaces to be bonded together. Bond strength develops as the molten adhesive cools and solidifies. They are used extensively in the fast assembly of structures which are intended to be only lightly loaded.

Ethylene vinyl acetate (EVA) copolymers can be formulated with a range of vinyl acetate contents to give adhesives which have a useful range of melt flow indices and, as they demonstrate good adhesion properties to a variety of substrates, such copolymers are widely used as hot melt adhesives. Typically, such materials have a vinyl acetate content from 18 to 40% by weight and possess melt flow indices in the range 2-400 g/10min. For some applications, such as in bonding electrical cables, graphite or acetylene black has been added to EVA compositions in order to impart electrical conductivity to the composition (see, for example, US patent No. 3 400 020 and French patent No.l 521 006). No reference is made in either of these patents to any possible effect of the added carbon material to the adhesive properties of such compositions.

Applicant has now discovered that the use of activated forms of carbon as an additive to EVA compositions brings about enhanced adhesion properties in the case of such compositions having much lower vinyl acetate content than usual. These compositions, moreover, exhibit good adhesive properties towards surfaces that have received only minimal preparation and consequently represent adhesives which can be quickly and easily applied to surfaces that are to be adhered together.

According to the present invention there is provided an adhesive composition which comprises an ethylene-vinyl acetate copolymer containing from 1 to 8% by weight of vinyl acetate together with a finely-divided carbon at a weight percent in the range from 10 to 30% of the composition.

Ethylene-vinyl acetate copolymers are commercially available, for example Evatane 1005 VN3 (Arkema) which contains 8% by weight of vinyl acetate or Escorene LD361F copolymer (ExxonMobil Chemical Company) which contains 4% by weight of vinyl acetate.

The carbon preferably has a particle size of below 500 microns, most preferably below 300 microns.

In a preferred embodiment the composition contains an activated carbon which may be defined as being a processed carbonaceous product having a porous structure and is most preferably bituminous coal-based (BPL) activated carbon.

Preferably the percentage of carbon within the composition is between 15 and 25 weight %. The invention further provides a method for the preparation of the adhesive composition which comprises the steps of:

(a) heating the ethylene-vinyl acetate copolymer to a temperature of between 5 and 25 ⁰C above its melting temperature; and

(b) adding the activated carbon to the product of step (a) with mixing.

Step (b) conveniently involves adding the activated carbon to the copolymer while subject to milling in a rolling mill, preferably a heated rolling mill, most preferably a heated double rolling mill. Alternatively, other methods of incorporation of the carbon can be used such as compounding or batch mixing. The mixing is continued for a period sufficient to achieve a consistent blend of the ingredients, for example for a period of 5 to 10 minutes.

Although milling will usually produce a sheet of material which is sufficiently thin and consistent enough to be used for bonding, ideally the milled sheet is subject to a further treatment of pressing to give an even more consistent (flatter) sheet. Any suitable method of pressing the milled sheet can be used though, conveniently, a platen press is used. In this case, the temperature of the press is preferably between 2O⁰C and 6O⁰C above the defined melting temperature of the EVA composition being used. As will be readily understood by the skilled person, the conditions in the press (temperature and time of exposure) should be such as to avoid any degradation of the composition while it is in the press. For the compositions of this invention it has been found convenient to operate the platen press at a temperature of 21O⁰C and at a pressure of between 50 and lOObar for 30 sec, followed by quench cooling to room temperature for 60 sec. This gives an overall time for processing in the press of around 1.5 minutes.

The thickness of the sheets of adhesive composite material thus produced is conveniently between 0.01 mm and 100 mm, most preferably between 0.1 mm and 10 mm. The composite has use as an adhesive or cement for any material, but is particularly useful as an adhesive for metals such as aluminium, steel and brass, ceramics or glass.

The composite is conveniently applied to the surfaces which are to be adhered together in the form of a piece of sheet which corresponds in size to the area of the interface between the two surfaces that are to be joined. The surfaces are then brought into contact under pressure and with the application of a degree of heating in order to make the bond. Typically, these conditions are applied for a period of around 4 to 5 minutes followed by a period of steady cooling but longer periods of pressing may however be advantageous when bonding certain materials and depending on the exact composition being used.

It is possible to use the adhesive composition of the invention on surfaces which have not been rigorously prepared beforehand i.e. other than by simple pre-cleaning involving degreasing, washing and/or roughening of the surfaces to be adhered. Thus it has been found to be unnecessary. to subject the surfaces to any other type of pre- treatment such as shot blasting or plasma/flame treatment before the compositions of the invention are applied thereto.

Specific embodiments of the invention will now be described by way of example.

Ethylene-vinyl acetate copolymers and bituminous coal-based activated carbon (BPL) composites were prepared on a Dr Collin WlOOT heated double roll mill (front roller 145°C, back roller 150°C). Before milling, the BPL carbon was ground in a pestle and mortar. The copolymers used were Evatane 1005 VN3 (Arkema) containing 8% by weight of vinyl acetate and Escorene LD361F copolymer (ExxonMobil Chemical Company) which contains 4% by weight of vinyl acetate.

Sheets of composite material were prepared from the compositions produced by milling using a Dr Collin P200P platen press operated under the following conditions: pressing for 30 seconds at a temperature of 21O⁰C and a pressure of between 50 and 100 bar followed by quench cooling for 60 seconds to room temperature. Example 1

Lap shear experiments were performed using an Instron 5564 with Merlin 5.51 software and according to BS ISO 4587:2003 "Adhesives - Determination of tensile lap shear strength of rigid-to-rigid bonded assemblies". The adhesive bonds for testing were obtained as follows. Stainless steel bars were prepared for the bonding tests by degreasing with acetone and abrading the ends of the bars. A 25mm x 30mm piece of composite material was cut from milled sheet and placed between the abraded ends of two bars giving an area for each bond of 750mm . A frame was placed around the steel bars to encourage uniform thickness of the adhesive layer and the bars were then pressed together for 4 minutes at a temperature of 210°C at lbar pressure after which the samples were cooled to 6O⁰C at a rate of 0.5°C/sec.

Three samples of each composition were tested and the average maximum load obtained, divided by the bond area, was taken as the bond strength for that particular composition. For an 8% by weight vinyl acetate content composition, bond strengths of 4.91, 5.26 and 4.46 N/mm² were obtained respectively with activated carbon contents of 10%, 20% and 30% by weight. For a 4% by weight vinyl acetate, 20% by weight activated carbon content composition, a bond strength of 6.3 N/mm² was recorded.

It was also noted in the latter instance from the load vs. displacement graph obtained during testing that the bond formed by the composition of this invention sustained higher loads over a longer displacement after the maximum load had been reached than was the case with a non-carbon containing control where, instead, the sample bonds failed dramatically after the maximum load point and were unable to sustain any load thereafter.

A number of samples of mild steel bars bonded together were prepared in the same way as the stainless steel samples described above. Using an 8% VA composition, a bond strength of 5.9 N/mm² was obtained under like conditions of testing as with the stainless steel samples. Longer pressing (10 minutes) resulted in increased bond strengths for both stainless steel and mild steel with this same composition, of 5.51 and 7.63 N/mm² respectively.

Example 2

A number of peel strength experiments was performed on stainless steel bars using compositions with 20% activated carbon and, respectively, 4% and 8% vinyl acetate. Steel bars were pressed together, one bar on top of another to form a complete overlap with a sample of the composition 40mm long at the interface. After pressing, the unbonded ends were bent at 90° angles to produce T-shaped test pieces. Maximum load was divided by the test piece width (20mm) to provide peel strength data. While the 8%VA composition showed a lower peel strength than with a simple 8% VA non- carbon containing adhesive (2.8 vs. 3.3 N/mm²), the 4% VA composition according to the invention produced a much enhanced peel strength as compared to the non-carbon containing equivalent (4.91 vs. 2.15 N/mm²).

Overall, therefore, with the 4 % VA composition in particular, the combination of enhanced bond and peel strengths gives a better bond which maintains higher strengths over longer displacements than similar compositions lacking the presence of the activated carbon component.

Claims

1. An adhesive composition which comprises an ethylene -vinyl acetate copolymer containing from 1 to 8% by weight of vinyl acetate together with a finely-divided activated carbon at a weight percent in the range from 10 to 30% of the composition.

2. A composition as claimed in Claim 1 wherein the carbon has a particle size of less than 500 microns.

3. A composition as claimed in Claim 2 wherein the carbon has a particle size of less than 300 microns.

4. A composition as claimed in any of Claims 1 to 3 wherein the percentage of carbon is between 15 wt % and 25 wt %.

5. A composition as claimed in any of Claims 1 to 4 wherein the activated carbon is bituminous coal-based (BPL) activated carbon.

6. A composition as claimed in any preceding Claim wherein the percentage of vinyl acetate in the composition is between 1.5 wt % and 4 wt %.

7. A method for the preparation of the polymer/carbon composition of Claim 1 , comprising the steps of (a) heating an ethylene- vinyl acetate copolymer containing from 1 to 8% by weight of vinyl acetate to a temperature of between 5 and 25⁰C above its melting temperature; and (b) adding a finely divided activated carbon at a weight percent in the range from 10 to 30% to said polymer with mixing.

8. A method as claimed in Claim 7 wherein the mixing in step (b) is continued until the ingredients are consistently blended. S

9. A method as claimed in Claim 7 or Claim 8 wherein the mixing in step (b) is provided by a heated rolling mill.

10. A method as claimed in Claim 9 wherein the heated rolling mill is a heated double rolling mill.

11. A method as claimed in any of Claims 7 to 10 wherein the composition is further processed to produce sheets of composite adhesive material.

12. A method as claimed in Claim 11 wherein the sheets of composite are produced by use of a platen press.

13. A method as claimed in Claim 12 wherein the platen press is applied at a temperature of 210 ⁰C and a pressure of between 50 and 100 bar for a period of about 30 seconds, followed by quench cooling for 60 seconds.

14. A method as claimed in any of Claims 11 to 13 wherein the thickness of composite sheet is between 0.01 mm and 100 mm.

15. A method as claimed in Claim 14 wherein the thickness of composite sheet is between 0.1 mm and 10 mm.

16. A sheet of a composite adhesive material produced by the method of any one of Claims 11 to 13.

17. Use of a composition as claimed in any of Claims 1 to 6 as an adhesive.

18. Use of a composition as claimed in any of Claims 1 to 6 for adhering together metal surfaces.

19. Use of a composition as claimed in Claim 18 wherein the metal surfaces comprise stainless steel.

20. Use of a composition as claimed in Claim 19 wherein the metal surfaces comprise mild steel.

21. Use of a composition as claimed in any of Claims 17 to 20 wherein the composition is in the form of a sheet of the composite adhesive material of Claim 16.