Hardness and Scratch Resistance

Although PP can be scratched with a metal point, its hardness is sufficient to resist the rule of thumb which often distinguishes between the polyolefins. LDPE is easily marred by a thumbnail, HDPE is scratched in this way with difficulty but PP is marked little, if at all. 

Hardness is defined as the resistance of a material to deformation, particularly permanent deformation, indentation or scratching. Hardness is purely a relative term and should not be confused with wear and abrasion resistance of plastics. For example, polystyrene has a high hardness but a poor abrasion resistance. Many tests have been devised to measure hardness. However, Rockwell and Durometer hardness tests are commonly used. The Rockwell hardness test measures the net increase in depth impression as the load on an indentor is increased from a fixed minor load to a major load and then returned to a minor load. The hardness numbers derived are just numbers without units. Rockwell hardness numbers in increasing order of hardness are R, L, M, E and K scales. The higher the number in each scale, the harder is the material. The Durometer hardness test is based on the penetration of a specified indentor forced into 

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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)... 

Thermosetting acrylic resins are used widely in surface coatings. Both acrylic and methacrylic esters are utilized and the term is applied to both of them. Often, such resins are terpolymers or even tetrapolymers, where each monomer is chosen for a special function. One is selected for rigidity, surface hardness, and scratch resistance another for ability to flexibilize the film, and the third for crosslinking it. In addition, not all comonomers are necessarily acrylic or methacrylic esters or acids. For instance, among the monomers that may be chosen for rigidity may be methyl methacrylate. On the other hand, it may be styrene instead, or vinyl toluene, etc. The same is true of the other components. Table 5.12 illustrates some common components that can be found in thermoset acrylic resins. 

Aromatic diacrylates, for example, p-bis(hydroxyalkyl- or hydroxyben-zeneesters dimethacrylates, yield materials with very low water absorption, exceptional transparency, and high hardness and scratching resistance. 

ABS belongs to the group of amorphous plastics. It is impact-resistant and it also exhibits good hardness and scratch-resistance properties. However, it can be used only at temperatures up to 90 °C, and it is therefore not suitable for standard soldering processes. The glass transition temperature is in the range from 85 to 100 °C. Standard processes suffice for chemical metallization. 

Hard and scratch resistant excellent toughness, even at low temperatures excellent dimensional stability. 

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