Plastics physical surface treatment

Chemical or physical surface treatments are especially required for structural bonding of low-surface-energy plastics. Low-surface-energy plastics include polyethylene, polypropylene, TPO, and fluorinated polymers. These surface treatments are designed to increase the critical surface tension and improve wetting and adhesion. In addition to increasing the critical surface tension, surface treatments are designed to remove contaminants or weak boundary layers, such as a mold release. 

Figure 12 represents a cross section of a plated sample of ULTEM 1000 having a peel strength of 118 g/mm. Little physical surface change of the plastic has occurred as a result of the pretreatment steps. Figure 13 is a cross section of copper plated ULTEM 2312. While a mechanical component to adhesion is present, it is much less than that found in traditional swell and etch treatment (Figure 2). Physical alteration of the plastic is confined to the outermost 25ii. The bulk properties of the plastic (flexural strength, electrical resistivity) are unaffected by this new process (Table I).

The combination of the processes of the severe plastic deformation of the surface with their physical-chemical treatment can provide the unique opportunity of the controlled formation of nano-sized grain structure for the strength and corrosion stability increasing. By applying both surface plastic deformation and nitriding process simultaneously nanostructured material could be determined. This kind of surface treatment related to refinement of grain could be helpful for considerable modification of material service properties. 

Metal finishing is the name given to a range of processes carried out to modify the surface properties of a metal, for example by the deposition of a layer of another metal or a polymer, or by formation of an oxide film. The origins of the industry lay in the desire to enhance the value of metal articles by improving their appearance, but in modern times the importance of metal finishing for purely decorative reasons has decreased. The trend is now towards surface treatments which will impart corrosion resistance or particular physical or mechanical properties to the surface (e.g. conductivity, heat or wear resistance, lubrication or solderability) and hence to make possible the use of cheaper substrate metals or plastics covered to give them essential metallic surface properties. 

Aliuniniiun trihydrate (ATH) should continue to experience strong growth but there is much competition from mineral suppliers all anxious to launch special precipitated grades and those wilh new surface treatments to make them more effective. The drive is the requirement for lower loadings to preserve the balance of mechanical and physical properties for which the particular plastic system has been chosen. The treatment of hydrophilic ATH particles with hydrophobic coatings, such as fatty acids or silanes, provides for more uniform dispersion in the hydrophobic plastic resins. 

For wetting to occur, the substrate surface has to be chemically or physically altered by some mechanism to raise its surface energy. This is why there are so many prebond surface treatments for plastic substrates. 

From Tables 9.8 and 9.9, it can be forecast that epoxy adhesives will wet dean aluminum or copper surfaces. However, epoxy resin will not wet a substrate having a critical surface tension significantly less than 47 dyn/cm. Epoxies will not, for example, wet either a metal surface contaminated with silicone oil or a clean polyethylene substrate. For wetting to occur, the substrate surface has to be chemically or physically altered by some mechanism to raise its surface energy. This is why there are so many prebond surface treatments for plastic substrates. 

Table 9.11 lists common recommended surface treatments for plastic adherends. These treatments are necessary when plastics Eire to be joined with adhesives. Solvent and heat welding are other methods of fastening plastics that do not require chemical alteration of the surface. Welding procedures win be discussed in another section of this chapter. The effects of plastic suT ce treatments decrease with time. It is necessary to prime or bond soon after the surfaces are treated. Some common plastic materials that require special physical or chemical treatments to achieve adequate surfaces for adhesive bonding are listed in the following sections. 

Kaolin deposits are cored and analyzed before mining to determine quality. Mined clays are then either wet or dry processed by air floatation or water fractionation. Surface-modified clays can be made by treating standard, delaminated, and calcinated grades with surface modifiers. The treatment can be performed by either the supplier or the end user. These surface modifiers include silane, titanate, polyester, and metal hydroxide. The objective of these surface treatments is to increase filler loadings and/or improve physical properties such as melt viscosity, thermal stability, and modulus without loss of physical characteristics. Electrical applications represent the largest use of surface-modified kaolin in plastics. 

As with metallic substrates, the effects of plastic surface treatments decrease with time. It is necessary to prime or bond soon after the surfaces are treated. Listed below are some common plastic materials that require special physical or chemical treatments to achieve adequate surfaces for adhesive bonding. 

There are two classes of surface treatments for plastics physical and chemical (Pocius 1997). Both classes alter the surface chemistry of the plastic but the distinction is that the physical methods involve some form of high-energy radiation while the chemical ones involve wet chemistry. 

Starch is one of the most widely used biopolymer in biocomposites because of its low cost and versatility. A plasticizer like glycol is sometimes used to make it suitable for processing. It is also blended with other polymers like aliphatic polyesters to improve its physical and mechanical properties. Biocomposites based on starch matrices show improved properties, which are comparable to E-glass/epoxy composites. Tensile, flexural, impact, and creep properties of these biocomposites are significantly better than those of neat starch. Various biofiber surface treatments have been shown to improve the properties of starch-based biocomposites. 

Dislocation theory as a portion of the subject of solid-state physics is somewhat beyond the scope of this book, but it is desirable to examine the subject briefly in terms of its implications in surface chemistry. Perhaps the most elementary type of defect is that of an extra or interstitial atom—Frenkel defect [110]—or a missing atom or vacancy—Schottky defect [111]. Such point defects play an important role in the treatment of diffusion and electrical conductivities in solids and the solubility of a salt in the host lattice of another or different valence type [112]. Point defects have a thermodynamic basis for their existence in terms of the energy and entropy of their formation, the situation is similar to the formation of isolated holes and erratic atoms on a surface. Dislocations, on the other hand, may be viewed as an organized concentration of point defects they are lattice defects and play an important role in the mechanism of the plastic deformation of solids. Lattice defects or dislocations are not thermodynamic in the sense of the point defects their formation is intimately connected with the mechanism of nucleation and crystal growth (see Section IX-4), and they constitute an important source of surface imperfection. 

Plastic working of a metal such as steel is the permanent deformation accompHshed by applying mechanical forces to a metal surface. The primary objective is usually the production of a specific shape or si2e (mechanical shaping), although increasingly it also involves the improvement of certain physical and mechanical properties of the metal (mechanical treatment). These two objectives can be readily attained simultaneously. 

Surface modification of a contact lens can be grouped into physical and chemical types of treatment. Physical treatments include plasma treatments with water vapor (siUcone lens) and oxygen (176) and plasma polymerization for which the material surface is exposed to the plasma in the presence of a reactive monomer (177). Surfaces are also altered with exposure to uv radiation (178) or bombardment with oxides of nitrogen (179). Ion implantation (qv) of RGP plastics (180) can greatiy increase the surface hardness and hence the scratch resistance without seriously affecting the transmission of light. 

Following treatment with the alcoholic base and an alcohol rinse, the modified white residues can be removed by immersion in a mixture of an alcohol and an aggressive solvent. SEM and XPS analysis of a polyetherimide sample following treatment in a 50/50 mixture of dimethylformamide/methanol is shown in Figure 10. Little physical change to the plastic surface has occurred. The plastic surface properties, however, have now been altered. The sample is now water wettable. 

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