of Low-energy Plastics and Rubbers

Transcription

1 6 Bonding of Low-energy Plastics and Rubbers 6.1 Surface Wetting Silicone rubber, polytetrafluoroethylene (PTFE), Acetal and the polyolefin plastics (polypropylene, polyethylene) are always a challenge to the adhesive engineer due to the low surface energy of these materials. Whilst the detailed consideration of surface tension is more in the province of the physicist than the engineer, wetting (the establishment of contact) plays a significant role in adhesion. Surface tension causes many liquids to behave as an elastic sheet and allows insects, such as the water boatman, to walk on water (Figure 6.1). It also allows small objects, even metal ones such as needles and razor blades, to float on the surface of water and it is the cause of capillary action. For good wetting and therefore good adhesion, the adhesive must be capable of spreading over the solid surface displacing air and any other surface contaminants that may be present. The scientific study of interfacial properties has developed measurement and analytical techniques that give a detailed analysis of components that determine wetting equilibria and surface/interfacial energy. Figure 6.1 The surface tension allows the water boatman to walk on water 93

2 Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Surface tension has the dimension of force per unit length or of energy per unit area. The two are equivalent but when referring to energy per unit of area most engineers use the term surface energy, which is a more general term in the sense that it applies also to solids and not just liquids. For many years surface tension was measured in dynes/cm and many engineers still use this unit today. The modern SI unit however is mn/m (milli-newtons per metre) and is the same as dynes/cm. Since no liquid can exist in a perfect vacuum for very long, the surface of any liquid is an interface between that liquid and some other medium. The top surface of a pond, for example, is an interface between the pond water and the air. Surface tension, then, is not a property of the liquid alone, but a property of the liquid s interface with another medium. Young [1] developed the relationship between the contact angle and the three interfacial tension points that describe a sessile drop, Equation 6.1. γ sv = γ sl + γ lv cosθ (6.1) where γ sv is the solid/vapour point, γ sl is the solid/liquid point, γ lv is the liquid/vapour point and θ is the contact angle (Figure 6.2). θθ Low surface energy θ High surface energy Figure 6.2 Low contact angles favour better wetting 94

3 Bonding of Low-energy Plastics and Rubbers For the common sessile or pendant drop shape the Laplace equation describes the relationship of the two radii of the elliptical sessile drop with the pressure across the surface and the surface tension, Equation 6.2. ( ΔP = σ R1 R2 ) (6.2) where ΔP is the pressure, σ is the surface tension and R1 and R2 are the principal radii of curvature. For an adhesive to wet a surface, it requires a lower surface tension than the surface energy of the solid. If this condition is not met, the liquid does not spread across the surface but forms spherical droplets on the surface. Water has a relatively high surface tension (70 mn/m) and so on a highly polished car bonnet, the water will form droplets (Figure 6.3) because the waxed surface of the metal bonnet will have a lower surface energy than the water and so prevents wetting. Wetting of plastic surfaces is much more complex than wetting clean metal surfaces. Plastics and adhesives are both polymeric materials and thus have similar physical properties, including wetting tensions. Plastic-bonded joints do not have the large difference between the critical surface tension of the substrate and that of the adhesive, Figure 6.3 Water forming droplets on a polished car bonnet [2] 95

4 Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts which ensures wetting for metals. In addition, many plastics have notoriously low critical wetting tensions. Polyethylene (PE) and polypropylene (PP), with critical surface tensions of 31 and 29 mn/m respectively, present serious wetting challenges for most adhesives. The surface tension for an ethyl cyanoacrylate is 33 mn/m and so the surface energy of the solid must be greater than 33 mn/m to achieve good wetting. Other plastics such as polystyrene and polyvinyl chloride (PVC) have higher critical surface tensions and present less of a problem. Table 6.1 shows some surface energy values for a range of materials and it can be seen that PTFE has a surface energy of 18 mn/m and therefore cannot be bonded without surface pre-treatment. PVC, however, has a surface energy of about 38 mn/m and can therefore be bonded. Most industrial adhesives (e.g., cyanoacrylate, epoxy, polyurethane, room-temperaturevulcanising silicone and most acrylic adhesives) do not adhere to PP and PE. Indeed these adhesives are often packaged in PP or PE bottles so that the adhesive itself can be dispensed without sticking to the bottle. Table 6.1 Surface tension values for some plastics Material Surface tension (mn/m) PTFE 18 Acetal 22 Polypropylene 29 Polyethylene 31 Polystyrene Polymethylmethacrylate (acrylic) 39 PVC 39 Polyethylene terephthalate Polycarbonate 46 Nylon 6 46 Polyolefins and fluoropolymers are also difficult to bond for other reasons: Low porosity there is no opportunity for the adhesive to penetrate into the plastic and give mechanical interlocking. No functional groups polyolefins are comprised entirely of carbon and hydrogen atoms and are very non-polar polymers. Most adhesives contain oxygen, nitrogen 96

5 Bonding of Low-energy Plastics and Rubbers and other electron-rich atoms and are polar materials, and if (like polyolefins) the carbon and hydrogen bonds are very unreactive, there is no opportunity for the adhesive to form chemical bonds. Surface weaknesses Some plastics have weak boundary layers due to the low tensile strengths between some of the molecules at the surface of the plastic. Mould release agents can also be the cause of low adhesion if they are silicone or PTFE based and are transferred across from the mould tool. 6.2 Measuring Surface Energy When a designer is selecting an adhesive for a specific application, the engineering properties of the individual plastic will be considered carefully. All too often, however, the data supplied by the plastic manufacturer will include melting point, mould shrinkage, tensile modulus, hardness, dielectric properties, water absorption, density and thermal conductivity but almost never the surface energy of the plastic, which is one of the key properties required for the adhesive application engineer. The use of surface-tension pens is a simple technique to measure surface energy. Each pen contains ink of a known surface tension and if the ink globulates or breaks up, the surface energy of the tested surface is lower than the ink and if the pen is seen to write without the ink breaking into smaller particles, the surface energy of the tested surface is higher than the ink. The use of pens with different inks therefore provides a reasonably accurate measurement of the wetting properties of the tested surface. Most engineering adhesives have a surface tension of approximately 33 mn/m and the plastic needs only to be just above this for the adhesive to wet the surface and therefore bond. The degree of adhesion may well depend on other factors such as surface finish, the gaps between the mating parts and the type of plastic, but once the adhesive starts to wet the surface some degree of adhesion should be obtained. Unlike metals, plastics and elastomers do not have the large difference between the critical surface tension of the substrate and that of the adhesive and so when poor wetting occurs, there are methods to treat the surface for better bonding. 6.3 Surface Treatments Several techniques are in use within the plastics industry, including corona discharge, plasma etching, flame treating and the use of chemical primers to enhance surface energy. 97

6 Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Abrasion One of the easiest forms of surface preparation is simply cleaning and abrading the surface. The most common procedure is a solvent wipe, followed by abrasion and then a final solvent wipe. The solvent selected should not craze or soften the plastic. Grit blasting is the most effective abrasion method, although using aluminium oxide cloth also works well. The final solvent rinse removes residue from abrasion. Using cleaning and abrasion first ensures that wetting problems are not caused by surface contamination. Another potential benefit is that removing the surface layer of plastic may expose material with better wetting characteristics due to a different crystalline microstructure Corona Discharge The corona discharge technique consists of having the polymer film pass over a metal electrode coated with a dielectric material which receives a high voltage from a high-frequency generator (10 20 khz). Normally the voltage increases cyclically until the gas ionises, generating a plasma at atmospheric pressure that is known as corona discharge. This is a highly effective treatment for polyolefins that creates adhesion-enhancing carbonyl groups on the surface and raises the surface energy of the polymer [3] Plasma Treatment Plasma surface treatment increases the surface energy of a substrate by bombarding the substrate surface with ions of a gas such as argon. Plasma treatment can be performed at atmospheric conditions or in a sealed chamber under extremely low pressures. By selecting appropriate gases and exposure conditions, the surface can be cleaned, etched or chemically activated. The results typically show up to a two- or three-fold increase in surface wetting [3] Flame Treatment Flame treatment is often used to change the surface characteristics of plastics. It involves passing the surface of the plastic through the oxidising portion of a natural gas flame. The surface is rapidly melted and quenched by the process; some oxidation of the surface may occur at the same time. Exposure to the flame is only a few seconds. Flame treatment is widely used for PE and PP, but has also been applied to other 98

7 Bonding of Low-energy Plastics and Rubbers plastics, including thermoplastic polyester, polyacetal and polyphenylene sulfide. Specially designed gas burners are available for this process, but butane torches can be used for laboratory trials Use of Primers PTFE and other fluoropolymers have been treated using a solution of sodium in liquid ammonia and other etching solutions [3]. This method dramatically improves surface-wetting characteristics, and the plastic can then readily be bonded using a wide range of adhesives. In the late 1980s primers were introduced that considerably enhance the adhesion of cyanoacrylates to polyolefins. The primer changes the surface condition of the plastic, creating bond sites for the cyanoacrylate adhesive. The effect of a polyolefin primer when used with a cyanoacrylate on polypropylene should not be underestimated. Bond strengths are often 25 to 40 times higher than those achieved when using the same adhesive without primer (Figure 6.4). Note that these polyolefin primers are only suitable for cyanoacrylate adhesives and are not compatible with other technology adhesives. 6.4 Two-part Acrylics The introduction within the last few years of two-part acrylics for the bonding of polyolefins has given the design engineer another option for the bonding of the polyolefin plastics. Bond Strengths (MPa) PVC or PC Bonding with Cyanoacrylates Polypropylene (unprimed) Polypropylene (primed) Figure 6.4 Typical adhesive shear strengths on a selection of materials [4] 99

8 Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts These 10:1 mix ratio acrylics show excellent adhesion to polyethylene and polypropylene with handling strengths in less than 10 minutes. The products contain glass beads or fillers to control the bond-line thickness to 0.2 mm or 0.25 mm and so the joint should be designed to accommodate these fillers. These two-part acrylics do not require any pre-treatment of the joint surfaces or any surface primer and will bond polyethylene, polypropylene and ethylene copolymers with shear strengths in the range 4 8 N/mm 2. They can be used on many other substrates and so can be used as a general-purpose adhesive, although they are not recommended for bonding PTFE or the fluoropolymers. The resistance to water and high humidity environments is good but the mix ratio is critical to avoid unpredictable results. References 1. F. Bashforth and J.C. Adams, An Attempt to Test the Theory of Capillary Action, Cambridge University Press, Cambridge, UK, Henkel Media On-line, The Henkel Brand Database, Industrial Adhesion Problems, Eds., D.M. Brewis and D. Briggs, Orbital Press, Oxford, UK, The Loctite Design Guide for Bonding Plastics, Volume 4, Henkel Ltd, Hatfield, UK,