Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2002-04-11
2004-10-05
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S412100
Reexamination Certificate
active
06800331
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of modifying a polymeric surface of a substrate to control the surface chemistry. The invention also relates to a method of improving the bonding of a substrate having a polymeric surface with adhesives, coatings, functional molecules and other materials.
BACKGROUND OF THE INVENTION
Polymeric or polymer based surfaces are often difficult to wet and bond or to perform other specific functions because of low surface energy, incompatibility, chemical inertness, or the presence of contaminants and weak boundary layers. The lack of adequate adhesion at the substrate/adherent and/or reinforcement/matrix interfaces often results in poor material performance and limits the possible applications of the products of polymeric materials. Effective surface treatments are frequently required to overcome one or more of the above mentioned difficulties in order to achieve controlled or maximized composite performance and controlled level of adhesion with paints, adhesives, coatings, bio-active materials etc.
The durability of interface adhesion of an assembly subjected to high humidity, fluctuation of temperature and UV irradiation are very critical when the products are for out door application, such as painted external plastics components used in automobile industries. The hydrothermal stability of the interface/interphase often determines the success of the surface modification process.
Polymer based materials are often required to provide surface properties such as good adhesion or chemical linkage to another material and at the same time provide a diverse range of physico-chemical properties such as strength, flexibility or elasticity, inertness or reactivity, electrical or heat conductivity, wettability for various applications.
An example of a specific application is the electrostatic painting process on polymer based substrates. The electrostatic painting process has advantages over conventional painting process as up to 80% less paint is used and the VOC can be greatly reduced when less paints are used. To satisfy the electrostatic painting requirements the surface/interface layer of polymer based materials must possess electrical conductivity and good adhesion to both substrate and paints.
Controlled wettability of polymer surfaces is another example of multi-functional surface/interface in practical applications. Solid polymeric material surfaces may also be required to exhibit a specific level or gradient of wettability by organic and/or inorganic liquids or vapours of these liquids. Depending on specific end-applications, the liquid phase or condensate may be required to form a permanent uniform film on the wettable solid's surface, or alternatively it may be required to bead-up on an unwettable liquid-repellent surface, a hydrophobic surface. It is also possible that in some instances, an intermediate level of wettability is desirable. The surface/interface with a specified or well defined wettability must overcome the adverse effects of polymer surface restructure and continuous washing cycles to remain effective.
Polymeric materials used in biomedical field and in the sensor area are another important applications of the multi-functional surface/interface. To design a useful biomedical material, it is important to consider both bulk and surface properties of the material. Historically, selection of a biomedical material for a particular application has been based upon bulk property specifications. However, there is increasing recognition that a biomedical material must exhibit a specific surface chemical behaviour in order to minimise interfacial problems with host tissues and fluids. Thus, the surface of the polymeric material is often chemically modified to control the interface/interphase behaviour with biological systems. To achieve this, the common practice is to activate the polymer surface by grafting chemically reactive groups (such as amines) onto the surface then attach bio-active or bio-compatible molecules to this reactive surface. Molecules that possess recognition ability can be grafted onto the activated polymeric surface to form sensors.
Various surface treatment processes have been developed to achieve different specific requirements. These include chemical oxidation with the use of oxidising agents; surface chemical grafting and various physical-chemical methods such as corona discharge; flame treatment; plasma treatment; and UV irradiation. Simple oxidative treatments by flame treatment, corona discharge, or chemical oxidation generally lead to a noticeable increase in surface hydrophilicity, and bonding ability as a result of the occurrence of oxygenated groups such as carboxyl, hydroxyl and carbonyl on the modified polymer surfaces. Such a modified surface is, however, not stable and the chemistry and increased hydrophilicity is not permanent. This may be due to the partial removal of low molecular weight oxidised material by a polar solvent or water from the oxidised surface. Alternatively, or in addition, it may be due to the reorientation of the surface functional groups, which rotate inwardly into the bulk of the polymer during the storage or use. It is also well know that molecular scission occurs during oxidative treatment which lead a reduction of molecular weight at the surface layer and may reduce the adhesion performance. This becomes more important when the interface/interphase is subjected to a range of conditions that can effect long term durability of the product.
Plasma treatment methods which may involve plasma polymerisation are known to significantly improve bonding ability of the treated polymers and to provide the desired level of wettability. Plasma treatment can achieve these objectives by using a suitable gas or monomer to selectively incorporate different types of chemical species onto the polymer surface under controlled process conditions. However like an oxidised surface the plasma treated polymer surface is not stable in storage because of rotation and migration of the generated surface functional groups into the bulk material and the occurrence of post-chemical reactions at the modified surfaces. An additional drawback with plasma treatment or plasma polymerisation resides in the expensive process apparatus required and the high cost associated with the on-going operations and the difficulties experienced in carrying out the surface treatment continuously.
International Patent Application No. PCT SE89-00187 discloses a method of increasing the hydrophilicity of the polymer surface by a 3-steps process comprising: (1): producing carboxyl, carbonyl and hydroxyl groups on the polymer surface by an oxidation treatment process such as etching with oxidising acid solutions, corona discharge, flame and plasma treatment; (2) reacting the groups on the oxidised polymer surface with a compound belonging to the following groups A and B, wherein group A includes heterocyclic compounds having three or four ring atoms, such as oxiranes, thiiranes, aziridines, azetidinones, oxetanes, and group B includes carbodiimides (R—N═C═N—R′) and isocyanates (R—N═C═O, or N═C═O—R—O═C═N). The reaction according to step (2) has to be carried out in aprotic organic solvents, such as ketones and ethers due to the fact that the compounds in groups A and B are not stable in aqueous solution, and (3) post-treating the polymer material previously treated according to step (2) with further application of compounds containing nucleophilic groups, such as alcohols, water, amines, carboxylic acids and hydroxycarboxylic acids which react with the modified surface either by opening aziridine rings, or reacting with the residual isocyanate groups.
Japanese Patent Publication No. Sho 56-16175 teaches that the poor bonding between an oxidised polyolefin and resorcinol or epoxy adhesive is due to the inability of the adhesive resin molecules to microscopically approach polar groups at an oxidised polymer surface. The method proposed to alleviate this problem involve
Bilyk Alexander
Gutowski Wojciech Stanislaw
Hoobin Pamela Maree
Li Sheng
Russell Lee Joy
Cameron Erma
Commonwealth Scientific and Industrial Research Organisation
Pillsbury & Winthrop LLP
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