Improved adhesion of plastics by gas-phase fluoridation

Transcription

1 Gummi Fasern Kunststoffe, 11, 000, p. 78 Improved adhesion of plastics by gas-phase fluoridation G. Krüger* Translation submitted by C. Hinchliffe Selected from International Polymer Science and Technology, 7, 11, 000, reference GK 00/11/78; transl. serial no SUMMARY discontinuous low-pressure plasma treatment plasma treatment at atmospheric pressure (hightemperature plasma) continuous or discontinuous gas-phase Plastics with a low surface energy can be firmly bonded together after gas-phase fluoridation. Gasification with a fluorine/nitrogen mixture considerably improves the wettability and adhesion of glues. The effect of this method of pretreatment was proven with amorphous PET, PP without primers and NR, CR and EPM with different glues. Unlike other industrial pre-treatment methods, gas-phase may be performed by subcontractors, as the activation state is not limited by time. 1. INTRODUCTION Plastics have low surface energy or contain components which are deposited as anti-adhesive layers on the surface. In both cases, paints, printing inks, coatings or adhesives do not bond adequately to surfaces of this kind. Adhesion is particularly poor when the fluids applied themselves have high surface energy. This is particularly the case with so-called waterbased systems, as the surface energy increases with the water content in such systems. In order, nevertheless to be able to use these ecologically interesting paints, varnishes or adhesives, it is necessary to pretreat the majority of plastics. Other reasons for pre-treatment are higher production speeds or new properties due to the improved wettability generally achieved after pre-treatment. Under industrial conditions, the following procedures are used for pre-treatment: flame treatment with controlled metering of air, oxygen and fuel gas continuous or discontinuous, direct or indirect corona treatment However, over the last ten years gas-phase has become an established industrial procedure, so that there are now a large number of practical experiences which we will report on individually in the following.. FULL-SURFACE PRE-TREATMENT BY GASIFICATION Gas-phase exploits the special reactivity of gaseous fluorine to effect a chemical change to the surface of plastics. Fluorine is able to react with individual atoms in the polymer structure (preferably with hydrogen) and form extremely stable covalent C F bonds. Here, the decisive factor is that in suitable process conditions, the polymers carbon structure is subject to virtually no destruction and the fluorine s substitution reaction stops rapidly. This means that complete substitution of the hydrogen atoms is possible (although this may be desirable to create Teflon-like properties in individual cases). In addition, the reaction only takes place in the nanometer range on the surface. With normal industrial times, the thickness of the fluorinated layer is about 10 nm. The fluorine may only be detected with very sensitive procedures, for example ESCA measurements. The fluorine content is less than 0.01 vol. % and hence is below the detection limit of % necessary for EDX measurements. If the conditions are too intensive, corona and lowpressure plasma treatment cause degradation or crosslinking * Dr Georg Krüger, Specialist in Technology and the Material Behaviour of Plastics, Dr Krüger + Partner. For further information, the author may be contacted on Krueger.georg@vdi.de. T/6 International Polymer Science and Technology, Vol. 8, 3, 001

2 reactions so that the technological windows of these procedures are limited. In the event of over-treatment, adhesion even deteriorates. The problem is due to the fact that the status of the treatment, including over-treatment, cannot be measured for a justifiable cost. 'Gasification' with fluorine causes a sharp increase in the surface energy. This effect may be utilised for different applications: to improve the adhesion of adhesives on PE, PP, EPM, EPDM, PA1 1, PET, SI or POM, which are classed as poorly bondable plastics (refs. 1, 3, 5 to 9), to increase the adhesion of adhesion promoters and matrix materials to chemical fibres (ref. ), for the complete activation of geometries with 'complicated' surfaces, such as fabric or foam of components with borings, blind holes etc. (ref. 4), to improve the dispersion or powdery materials in aqueous systems, to improve lubricant or oil adhesion in plastic layers. 3. GAS-PHASE FLUORINATION INCREASES THE ADHESIVE STRENGTH OF PET Amorphous polyethylene terephthalate is a transparent plastic, which, due to its strength, resistance to scratching, mouldability and resistance to solvents, is gaining popularity in the fields of application of PMMA, PC or glass. Selective crystallisation may be used to improve the mechanical and thermal loadbearing capacity, which means that PET is a highly flexible plastic. Adaptation to a wide variety of requirements enables the number of different plastic types in a component to be reduced and therefore makes PET a 'recyclable' material. Transparent PET is used for glazing, protective devices, linings, signs or packaging. However, for this material to gain acceptance, suitable bonding and sealing methods are required. Welding cannot be used to establish bonds with other materials and so adhesion is preferred. Both adhesion and sealing require the highest possible adhesive strength. With transparent plastics, the adhesive or sealing material must also be extremely lightfast. As the transmission of amorphous 4-mm thick PET plates is 89%, light-curing adhesives may be used. The advantage of adhesives of this kind is their high reaction rate so that it is possible to integrate the adhesive process in production. tests with different transparent adhesives (Table 1) revealed that results in a considerable increase in strength (Table ). With -pack PUR adhesive ( 1), the final strength is only reached after several days. Curing may be accelerated by heat treatment at 40 C. The methyl methacrylate adhesives ( 3) are fast-reacting adhesives. The technologically required strength is achieved after about 60 minutes, the final strength is reached after about 4 hours at room temperature. Table 1 s used base Curing Colour, refractive index 1 Cold-curing, PUR-base, two-pack a t 3 C 3 to 5 h Clear Cold-curing, epoxy resin-base, two pack a t 3 C 4 h, at 65 C 1 h Transparent, Methyl methacrylate a t 3 C 1 h Colourless 4 Cyanoacrylate 5 Acrylate adhesive, UV-light-curing 60 s Transparent, s Transparent, Cold-curing, epoxy resin-base, two-pack a t 3 C 16 h at 40 C 3 h Water-clear, 1553 Table Tensile shear strength of transparent PT with and without for different adhesives, all data in N/mm Without With Explanations Fluorination causes acid surface Material failure during pretreatment Material failure during pretreatment International Polymer Science and Technology, Vol. 8, 3, 001 T/7

3 5 was cured with a UV lamp. With a 3 mm test piece, the duration of the exposure to light was 180 s and the curing energy was 11.7 J/cm at a distance of 45 mm. The two epoxy adhesives (Nos. and 6) differ in their different chemical structures. Cyclo-aliphatic epoxides were used with adhesive 6 to produce extremely light-fast adhesives. The tensile shear strength was determined with reference to DIN EN With an overlap length of 15 mm, a test piece thickness of 3 mm and a test piece width of 5 mm, the stress (superimposed tensile and bending stresses) on the fracture of the PET test pieces reached 7 N/mm. Without pre-treatment, all specimens suffer adhesive failure, with pre-treatment, there is a material failure at the transition between the adhesive surface and the test piece. The material failure occurs under very different forces. The tensile shear strengths are therefore only an indirect indication of the increase in the adhesive strength. In addition to the improved adhesion, the rigidity of the adhesive materials also determines the maximum achievable breaking stress. One noticeable feature is the lower adhesive strength after using cyanoacrylates. The explanation of this effect is that the surface takes on an acid nature which inhibits the reaction of the adhesive. Without pre-treatment, crazing occurs at a shear strength of only.81 N/mm, which means that adequate adhesion is achieved with this adhesive even without. Overall, the investigations reveal that material failure occurs with flexible cold-curing epoxy resins and, i.e. adhesion is considerably increased by the pre-treatment. The same applies to cycloaliphatically structured epoxy resins with high light stability. 4. FIRM BONDING OF POLYPROPYLENE Polypropylene has a surface energy of 9 mn/m and is also a non-polar plastic with a crystallinity of about 60%. This means that PP is usually thought to be non-bondable. The poor adhesive strength of many adhesives is primarily due to poor wettability. In addition, they tend not to interact with the adhesives. A simple, but cost-intensive method would be to use polyolefins, which are applied as intermediate layers Table 4 Dependence of yield stress on temperature Temperature 40 C 3 C 80 C Yield stress 5.1 N/mm 3.9 N/mm 1.1 N/mm (primers). However, xylene and ethyl benzene are used as solvents and both these substances require costly disposal methods and as environmentally hazardous substances should no longer be used. As increases the surface energy of PP from 9 to 65 up to 7 mn/m and the polarity of the surface increases at the same time, this method of activation is also suitable for improving the adhesion of numerous adhesives. The results of bonding tests with PP (crystallite melting point 164 C) are given in Table 3. Once again, the tests were performed with reference to DIN EN The test pieces were mm thick. Without pretreatment, no significant adhesion was measured and the adhesive could be easily removed from the test pieces. With the cold-curing -pack adhesive based on PUR, the adhesive 'fails' at a test temperature of 3 C, i.e. remnants of adhesive are found on both adherend surfaces. With a flexible coldcuring epoxy adhesive, the yield point of PP is exceeded, so that the data in Table 3 relate to apparent shear strength. Lower shear strengths were measured with a brittle type of epoxy resin, where higher stress peaks occur, However, due to the thickness of the test pieces, under tensile stress considerable deformation occurs resulting in high bending stresses. It may be assumed that higher forces may be transmitted in the event of a symmetrical application of stress. For purposes of comparison, the tensile shear strengths at 80 C were also determined. The highest value of.03 N/ mm was achieved with the flexible cold-curing epoxy adhesive. Once again, this is apparent strength, as PP s yield point also falls as the temperature rises (Table 4). No higher values were obtained with the PUR adhesive, as the inherent strength of the adhesive was too low at 80 C. Table 3 Tensile shear (all data in N/mm ) strength of fluorinated PET for different adhesives With Explanations 1 -pack PUR Cohesion failure -pack epoxide Yield stress reached 3 -pack epoxide T est temperature 40 C 4 -pack epoxide, brittle 1.45 High degree of bending stress T/8 International Polymer Science and Technology, Vol. 8, 3, 001

4 To supplement the data in Table 3, other adhesives based on PUR and epoxy resin were investigated. Here, it was found the adhesive strengths are different at the same state of activation, which means that adhesion is not solely determined by the degree of, but also to a decisive degree by the type of adhesive. 5. ADHESION OF RUBBER PROFILES TO PAINTED SHEETS The direct vulcanisation of rubber onto metals with primers is state-of-the-art technology. If the metal surface is varnished and heating is excluded, the various materials cannot be vulcanised without damaging the paint, only bonded with an adhesive. Cyanoacrylate, hot-melt adhesives or adhesive tapes may be used for the bonding, as these adhesives make shorter production times possible. For example, even without special pre-treatment, with the isopropyl cyanoacrylate adhesive Sicomet 3550 (made by Sichel-Werke in Hanover) there was a material failure in the crosspiece of the rubber double T-support in each of the test pieces described in the following. Before bonding, the profiles were degreased with isopropanol and rinsed in a surfactant solution. With high thermal stresses, however, it is necessary to use reactive adhesives based on epoxy resin or PUR. However, compared to painted surfaces or untreated rubber, both types of adhesive have poor adhesion. The use of primers was ruled out due to the solvents used (xylene, ethyl benzene). Therefore, an investigation was performed to see how gas-phase combined with rapid-reacting cold-curing epoxy resin affects adhesion. The investigation used rubber double T-profiles with an adherend surface of 50 mm x 5 mm. The iron counter profiles were stove-enamelled with a polyester varnish. Tensile tests were used to evaluate the adhesion, i.e. the tests were performed with reference to the pull-off test specified in DIN EN The test setup corresponded to the later stress state. The testing speed was 5 mm/min. The tensile forces were used as a comparison. The maximum force was the result of the inherent strength of the rubber crosspiece in the double T-profile. With a crosspiece width of mm, there was a requirement to increase the adhesion to ensure that the rubber profile was destroyed in each case, i.e. the adhesive strength had to be at least 1000 N. The results of the measurements shown in Table 5 show that with rapid-reacting epoxy resin, the required adhesive strength of 1000 N is attained after and that the other adhesive systems are unsuccessful. EP1 is a slow-reacting adhesive which sets within 4 h. The reaction rate may be reduced by heating for about 30 minutes. EP is a fast-reacting adhesive that is predominantly set after 5 minutes. The final strength is achieved after about 4 h. A special advantage which cannot be expressed in absolute values is the low scatter of all measurements after. In practice, this means less risk when using bonded joints. In addition to the double T-profiles, we also investigated EPDM distance pieces. The parts measured 1 mm x 1 mm x 5 mm and were required to bond firmly to metal surfaces. The glass transition temperature of the EPDM determined from DSC measurements was 46.9 C. Table 5 strengths of the rubber/painted sheet steel with different adhesives and pre-treatment Pretreatment strength Failure -pack EP1 370 to rubber -pack EP1, conditioned Degreaded 590 to rubber -pack EP1, after boiling test 430 to rubber -pack EP pack EP1, after boiling test 110 -pack EP 480 to rubber -pack EP 1140 Table 6 Tensile forces of EPDM parts on metal surfaces Type of adhesive Pressure-sensitive adhesive, pure acrylate, adhesive film thickness 0.05 mm Pressure-sensitive adhesive, modified acrylate, adhesive film thickness 0.05 mm Homopolymer PVAC dispersion, adhesive film thickness 0.03 mm EP adhesive, cold-curing adhesive film thickness 0.15 mm Without pre-treatment part International Polymer Science and Technology, Vol. 8, 3, 001 T/9

5 Due to the production process, pressure-sensitive adhesives, a PVAC-based dispersion adhesive and a two-pack epoxy adhesive were investigated. The distance pieces were bonded between two metal punches and then loaded in a tensile test in accordance with DIN 539 with a testing speed of 10 mm/min until the bond failed. To exclude the possibility of uncontrolled bending stresses, cardan joints were used on both clamps. Due to the thickness of the test pieces and the elasticity of the parts, the scatter of the measurements was much lower than that with rigid compound structures less than ±8% in all the investigations. Table 6 shows the maximum tensile forces achieved with the adhesives. In the case of epoxy adhesives, resulted in the cohesive failure of the EPDM. In other cases, adhesion was increased by about 100%. 6. CONCLUSIONS Gas-phase is an interesting alternative to the known industrial pre-treatment procedures such as flame treatment as corona treatment. The improvement of the adhesion of different plastics and plastic surfaces is easy to determine. Fluorination is characterised by the following: the activation effect is stable for several months, the increase in the surface energy is reproducible, small items, such as bulk solids in drum systems, may be treated, different plastic parts may be treated in one process (efficient system utilisation), no structural impact as a result of a partially high temperatures is possible, due to the better adhesion of PET, PP or EPDM, the utilisation level of the materials in bonded joints increases. This procedure is offered by several subcontractors so that there are no 'security problems' in the use of the potential. REFERENCES 1. O.-D. Hennemann and G. Krüger, Gas-phase for the pre-treatment of plastics. Kunststoffberater, 5, 199, pp M. Weber and D. Schilo, Impact of fluorine-gas treatment of chemical fibres, 35th International Chemical Fibre Conference, Dornbirn, A. Pahl, Direct in practical use, Kleben & dichten, Adhäsion, 40, 3,1996, pp R. Milker and A. Koch, Surface Pretreatment of Polymer Webs by Fluorine. Coating Technology Handbook, (Ed. D. Satas), Marcel Dekker, New York, Basle, Hong Kong, D.M. Brewis, Pretreatments of Hydrocarbon and Fluorocarbon Polymers, J. Adhesion, Vol. 37, 199, pp G. Krüger, Making Plastics Bondable, Kleben & dichten, Adhäsion, 4, 10, 1988, pp I. Brass., D.M. Brewis, I. Sutherland and R. Wiktorowitcz, The Effect of Fluorination on Adhesion to Polyethylene. Int. J. Adhesion and s, Vol. 11, 1991, pp R. Milker, Pretreating Surfaces with Fluorine, Kunststoffe, 8, 10, 199, pp G. Kranz, R. Lüschen, A. Pahl and O.-D. Hennemann, Surface activation of PE and PP with fluorine, Kleben & dichten, Adhäsion, 38, 6, 1994, pp. 1 4 (No date given) T/10 International Polymer Science and Technology, Vol. 8, 3, 001