Surface modification of polymers

Details Surface modification of polymers Surface modification of polymers

1998-12-02

2001-10-09

Gulakowski, Randy (Department: 1746)

Etching a substrate: processes

Adhesive or autogenous bonding of two or more...

Etching improves or promotes adherence of preforms being bonded

C156S272600, C427S536000, C216S067000

Reexamination Certificate

active

06299787

ABSTRACT:

This invention relates to a method for improving the adhesion of materials to polymers including fluorinated polymers such as copolymers of ethylene and tetrafluoroethylene, and especially perfluorinated polymers such as the polytetrafluoroethylene (PTFE), perfluorinated ethylene-propylene polymers FEP, tetrafluoroethylene-ethylene copolymers (PFE-ET), hexafluoropropylene polymers (HFP) or fluorovinylether polymers also known as perfluroalkoxy polymer (PFA).
BACKGROUND TO THE INVENTION
Fluorinated polymers have a unique position in the plastics industry due to their chemical inertness, heat resistance, electrical insulation, low coefficient of friction and water repellancy. Notwithstanding the enormous advantages of fluorinated polymers their inert chemical nature, especially of perfluorinated polymers, makes them extremely hard to bond with adhesives. Fluoropolymers are not amenable to the usual surface modification techniques that have enabled adhesive bonding of hydrocarbon polymers. Surface treatments such as corona discharge and flame treatment, which are well known in the art for introducing bondable polar groups to the surface layers of, for example, polyolefins are ineffective with fluorinated polymers.
The presence of fluorine seems to markedly affect the chemical reactions occurring in polymer surface layers upon surface treatments. It is known that oxygen-containing or nitrogen-containing groups can be incorporated into the surface of, for instance, PTFE (Liston et al.,
Journal of Adhesion Science and Technology
, 7, (1993)) and FEP (Xie et al.,
Journal of Adhesion Science and Technology
, 6 , 1411 (1993)) and the modified surfaces can be wetted, but adhesion is nevertheless poor. This poor adhesion has been attributed to a weak sub-surface structure of PTFE (Liston et al.,
Journal of Adhesion Science and Technology
, 7, (1993)). Moreover the effects of ammonia plasma treatment are greatly reduced over a period of a few days due to surface layer motability (Xie et al).
The surface layers of perfluorinated polymers can be modified also by exposure to an etching solution that contains sodium naphthalene complex. PTFE surfaces etched thus can be adhesively bonded. The “sodium etch” method suffers, however, from considerable disadvantages. The high reactivity of the etch solution makes it dangerous to handle, requiring cumbersome precautions, and causes substantial disposal problems.
Attempts have been made to modify perfluorinated polymers by surface treatment methods such as corona discharge, flame treatment, and low pressure gas plasma (glow discharge) treatments, which pose far lower environmental hazards and are well established. The former two treatments are oxidative in nature and have not given satisfactory bond strengths. Low pressure plasma methods are more versatile in the choice of gas and enable introduction of various chemical groups onto polymer surfaces. However, in spite of many studies on, for instance, the introduction onto PTFE surfaces of amine groups to enable covalent adhesive bonding with cyanoacrylate or epoxy adhesives, no satisfactory process has been found; the bond strength was always found to be considerably below the cohesive strength of the material.
To improve subsequent adhesion perfluorinated polymers have been glow discharge treated in hydrogen
itrogen mixtures (JP-A-6220231) (etching), in silane (JP-A-3164246), fluorocarbons or chlorfluorocarbons (DE-A-3408837) and in Ar, N
2
, O
2
, He or air (JP-A-3064382). Surface modification of PTFE by ammonia plasma treatment and its effect on bonding to nitrile rubber using a phenol-type adhesive was reported by Inagaki et al in J Adhesion Sci Technol Vol 3, no. 8, pp 637-649 (1989). PTFE strips were given a ten minute treatment in a continuous 20 kHz 150 mA plasma in 26Pa NH
3
atmosphere at various sample temperatures; only when the sample temperature reached over 200° C. were bonds of strength exceeding the cohesive strength of the PTFE formed.
SUMMARY OF INVENTION
We have discovered that the use of organic amine vapours in the plasma discharge treatment of fluorinated polymers offers significant improvements in the adhesive bond strength compared with the ammonia plasma treatments described in the prior art.
Accordingly in one aspect the invention provides a method of providing adhesion to fluorinated polymers including exposing at least a portion of the surface of the fluorinated polymer to a plasma discharge in an atmosphere containing an organic amine. The surface of the fluorinated polymer may then be bonded to another material by contacting the surface with a suitable adhesive.
The organic amine used in the surface modification procedure may for example, be a hydrocarbyl amine comprising one or more amino group, a dihydrocarbyl amine or a polyhydrocarbylene polyamine.
When the organic amine is a hydrocarbyl amine or dihydrocarbyl amine the hydrocarbyl group may be branched, straight chain, cyclic-aliphatic or aromatic. Preferred hydrocarbyl are selected from the group consisting of alkyl, cycloalkyl, alkene and aryl. Aliphatic hydrocarbyl are more preferred.
The particularly preferred organic amines contain from one to twelve carbon atoms, more preferably from one to ten.
Specific examples of preferred hydrocarbyl amines and dihydrocarbylamines include methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, heptylamine, allylamine, 1,3-diaminopropane, diamino butane, cyclopentylamine, diisopropylamine, diaminocyclohexane and aniline.
Specific examples preferred polyhydrocarbylene polyamines include diethylenetriamine and triethylenetetramine.
Examples of adhesives which may be used in the process of the invention include cyanoacrylates, epoxy adhesives, structural acrylic adhesives, polyurethane adhesives, silicone adhesives, unsaturated polyester adhesives, contact adhesives or thermoplastic adhesives. The preferred adhesive chosen will depend on the nature of the material to be bonded to the polymer and the intended application of the assembly.
The method of this invention provides an effective pretreatment for achieving strong adhesive bonding. It has minimal environmental impact, and utilises equipment whose commercial use is established. We have found that the method of the invention typically allows an adhesive bond strength to be achieved which is equal to the cohesive strength of the material. Furthermore unlike ammonia plasma treatment of fluoropolymer surfaces the present invention provides a long lasting improvement in bondability of the fluoropolymer surface. This provides an extended shelf life between treatment and bonding.
The invention also provides a fluoropolymer having an enhanced ability to bond to adhesives. Accordingly, in a further aspect the invention provides a fluorinated polymer the surface of which is bonded to a substrate with an adhesive and wherein the surface of the fluoropolymer has been treated with plasma discharge in the presence of an atmosphere containing an organic amine.
In another aspect the invention provides a composite comprising a first article having a surface of fluoropolymer which has been modified according to the above described method of the invention and a second article which has been bonded to the first article by an adhesive disposed between the second article and the modified fluoropolymer surface. The composite may contain two or more components and each may for example be formed of plastic which may optionally be reinforced, ceramic or metal.
The fluoropolymer surface treated by the method may be a portion of the surface area. For example where only part of an article is to be bonded the plasma discharge may be localised to all or part of the area to be bonded. The fluropolymer surface may be part of a film, foil, sheet or rod or can be in any other form such as a complex shape, component or composite.
We have also found that the intermittent operation of the plasma discharge provides an unexpected advantage in increasing the adhesive bond strength when compared with corresponding treatments without inte

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