Scratch resistance, hardness

Chem. Descrip. Tetraethyl orthosilicate CAS 78-10-4 EINECS/ELINCS 201-083-8 Uses Binder in precision investment castings as sec. backup casting coating mfg. of electronics, computer components silicon dioxide source in films deposited on silicon substrates catalyst intermediate component in zinc-based primer coatings for corrosion-resist, paints crosslinking agent in mfg. of sealants raw material in sol-gel systems scratch resist, hard coat on glass and plastic lens... 

The resulting films are usually hard and scratch-resistant but often show considerable stress (152). Such films can be used as corrosion-resistant coatings on metals or as membranes in gas sensor devices (153,154)... 

Hard lenses can be defined as plastic lenses that contain no water, have moduli in excess of 5 MPa (500 g/mm ), and have T well above the temperature of the ocular environment. Poly(methyl methacrylate) (PMMA) has excellent optical and mechanical properties and scratch resistance and was the first and only plastic used as a hard lens material before higher oxygen-permeable materials were developed. PMMA lenses also show excellent wetting in the ocular environment even though they are hydrophobic, eg, the contact angle is 66°. 

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. 

Abrasion Resistance. Porcelain enamel is the most scratch resistant and hardest of commercial coatings (see Hardness). This property is used to distinguish between porcelain enamel and organic enamel or painted coatings. The rate of abrasive wear in surface abrasion increases with time, and the subsurface abrasion which follows exhibits a higher, but constant rate of wear. Abrasion resistance can be evaluated by loss of gloss or weight (35). 

Surface hardness Low - scratches easily Much higher. Scratch resistant. 

Hardness is closely related to strength, stiffness, scratch resistance, wear resistance, and brittleness. The opposite characteristic, softness, is associated with ductility. There are different kinds of hardness that measure a number of different properties (Fig. 5-5). The usual hardness tests are listed in three categories (a) to measure the resistance of a material to indentation by an indentor some measure indentation with the load applied, some the residual indentation after it is removed, such as tests using Brinell hardness,... 

Vickers and Knoop indentors, Barcol hardness, and Shore durometers (2) (b) to measure the resistance of a material to scratching by another material or by a sharp point, such as the Bierbaum hardness or scratch-resistance test and the Moh one for hardness and (c) to measure rebound efficiency or resilience, such as the various Rockwell hardness tests. The various tests provide different behavior characteristics for plastics, as described by different ASTM standards such as D 785. The ASTM and other sources provide different degrees of comparison for some of these tests. 

Mohs hardness It is a measure of the scratch resistance of a material. The higher the number, the greater scratch resistance with number 10 being termed diamond. 

Although hardness is a somewhat nebulous term, it can be defined in terms of the tensile modulus of elasticity. From a more practical side, it is usually characterized by a combination of three measurable parameters (1) scratch resistance (2) abrasion or mar resistance and (3) indentation under load. To measure scratch resistance or hardness, an approach is where a specimen is moved laterally under a loaded diamond point. The hardness value is expressed as the load divided by the width of the scratch. In other tests, especially in the paint industry, the surface is scratched with lead pencils of different hardnesses. The hardness of the surface is defined by the pencil hardness that first causes a visible scratch. Other tests include a sand-blast spray evaluation. 

Hard layer and soft layer combined together can reduce the intrinsic stress of the whole coating [17,18,22-27]. Samples 4, 5, and 6 have higher critical load than that of monolayer A and B. For Samples 5 and 6, no obvious crack occurs during the scratch test. Sample 5 has the highest hardness and reduced elastic modulus among the multilayer samples, and the interfaces in Sample 5 also contribute to scratch resistance. So it has the best micromechanical properties here. 

Sample 6 has the highest critical cracking load among the samples. The main reason is that there are layers A(2 A)/B(3 A)/... A/B and the hardness of the whole multilayer is not too high. Previous study on multilayers showed that the interfaces of multilayers have a great contribution to the multilayers scratch resistance [28]. 

The polymerization reaction (Equation 5) leads to materials with extremely hard surfaces. Based on this, a new type of scratch resistant coating for different purposes could be developed. Table I gives some data of the high scratch resistant coating developed for CR 39 plastic eye glass lenses (39). 

Scratch resistance depends on the hardness of the added particles. The problem of a lack of this property can be addressed by adding chemically identical particles of different crystal modification and Mohs hardness. The preferred additives are silica, alumina, layered silicates such as kaolin, titania, barium sulfate and calcium carbonate. The latter is only suitable for the DMT process owing to side reaction caused by acidity during the terephthalic acid (TPA) route. 

UV and/or heat stabilized improved colour and gloss retention cold-temperature ductility and flexibility soft touch food contact low hardness conductive high stiffness high-impact ultra-high impact high modulus colourable scratch resistant paint performance electrostatic painting. .. 

Resistance to abrasion is measured by determining the weight loss at the abraded surface. Scratch resistance may be measured by using materials of known hardness to scratch the polymer surface. Pencils with known degrees of hardness may be used for this test. 

Each material was evaluated for enhance initial drying rate, gloss, and hardness or scratch resistance. 

Parts molded from melamine compounds have a hard scratch resistant surface, practically no odor or taste, good electrical properties, fair arc track resistance, decorative appearance, and color fast characteristics. 

The results thus obtained enabled them to plot the relation between frequency and Young s modulus the conventional lettered hardness symbols (Fig. 9.7) and also the Young s modulus-scratch resistance relation (Fig. 9.8). Young s modulus was calculated from the formula... 

Articles made of the Md>E-type aminoplast have a hard, scratch-resistant surface and are generally stronger than articles made of the Kd>A-aminoplast. Heat stability on exposure to moisture, solvents and domestic chemicals and also good electric properties permit utilization of the MOE-type aminoplast for making electrotechnical parts intended for operation at elevated humidity and temperature. 

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