Strength and elasticity

A coating must be able to stretch and change its dimensions so that a minimum of stress is placed on the painted surface underneath. Ideally, the elastic modulus should be lower for the coating than for the paint. In practice, most polymer films have an elastic modulus considerably lower than dried linseed oil paint. A coating must be capable of reasonable elongation so that it remains continuous because if cracks develop, dirt and water vapour can penetrate and cause damage. The material therefore should not be unduly brittle. A safe limit

Table 4.2 Hardness of some

thermoplastic resins

Material

Viscosity grade (centipoise)

Pencil hardness

Sward hardness

PVAC

9

F

63

PVAC

80

H

Poly n-butyl methacrylate

50

2B

30

Poly iso-butyl methacrylate

55

H

65

Mastic and dammar

1.5

>3H

81

Films 0.0381 mm thick baked on window glass approximately 2 mm thick and measured at 21 °C and 50% RH. Source: Information taken from Feller and Curran, 1971

Films 0.0381 mm thick baked on window glass approximately 2 mm thick and measured at 21 °C and 50% RH. Source: Information taken from Feller and Curran, 1971

on brittleness is not precisely defined but a figure of 1—3% elongation at break has been suggested for picture varnish coatings (Feller and Curran, 1985). Knowledge of viscosity grade is useful in comparing polymers in a series based on the same monomer. However, it does not necessarily follow that polymers of different types having the same viscosity grade will have precisely the same strength and brittleness. This applies equally to the hardness of different materials. In general, however, low viscosity grade polymers are more brittle than those of higher viscosity grade. For example, dammar and mastic (viscosity grade 1.5 cp) are much more brittle than Elvacite 2044 (poly (n-butyl methacrylate)) of viscosity grade 48 or AYAF (PVAC) of 80 cp. Brittleness can be reduced by using plasticizers. However, a better result may be achieved by suitable co-polymerization. The inclusion of material which is not part of the polymer structure can result in problems caused by its migration into paint layers. Plasticizers are generally avoided in conservation materials but have been widely used in the manufacture of plastic objects. Plasticizers reduce the modulus of elasticity, effectively reduce Tg, decrease hardness, increase creep and permeability and may result in more rapid deterioration. It is therefore better to use polymers with intrinsically desirable properties.

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Wood Working for Amateur Craftsman

Wood Working for Amateur Craftsman

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