Assessment of softening point

188 Principles of Product Design 9.2.1.2 Assessment of softening point 

In the case of crystalline polymers such as types E and F the situation is somewhat more complicated. There is some change in modulus around the which decreases with increasing crystallinity and a catastrophic change around the. Furthermore there are many polymers that soften progressively between the Tg and the due to the wide melting range of the crystalline structures, and the value determined for the softening point can depend very considerably on the test method used. 

Two particular test methods have become very widely used. They are the Vicat softening point test (VSP test) and the heat deflection temperature under load test (HDT test) (which is also widely known by the earlier name of heat distortion temperature test). In the Vicat test a sample of the plastics material is heated at a specified rate of temperature increase and the temperature is noted at which a needle of specified dimensions indents into the material a specified distance under a specified load. In the most common method (method A) a load of ION is used, the needle indentor has a cross-sectional area of 1 mm, the specified penetration distance is 1 mm and the rate of temperature rise is 50°C per hour. For details see the relevant standards (ISO 306 BS 2782 method 120 ASTM D1525 and DIN 53460). (ISO 306 describes two methods, method A with a load of ION and method B with a load of SON, each with two possible rates of temperature rise, 50°C/h and 120°C/h. This results in ISO values quoted as A50, A120, B50 or B120. Many of the results quoted in this book predate the ISO standard and unless otherwise stated may be assumed to correspond to A50.) 

In the deflection temperature under load test (heat distortion temperature test) the temperature is noted at which a bar of material subjected to a three-point bending stress is deformed a specified amount. The load (F) applied to the sample will vary with the thickness (t) and width (tv) of the samples and is determined by the maximum stress specified at the mid-point of the beam (P) which may be either 0.45 MPa (661bf/in ) or 1.82 MPa (264Ibf/in ). 

Where L is the distance between the outer supports (loading points). For details see the relevant standards (ISO 75 BS 2782 method 121 ASTM D648 DIN 53461). 

Whilst the Vicat test usually gives the higher values the differences are quite modest with many polymers (e.g. those of types A, B and C). For example, in the case of the polycarbonate of bis-phenol A (Chapter 20) the heat distortion temperatures are 135-140°C and 140-146°C for the high and low stress levels respectively and the Vicat softening point is about 165°C. In the case of an acetal homopolymer the temperatures are 100,170 and 185°C respectively. With nylon 66 the two ASTM heat distortion tests give values as different as 75 and 200°C. A low-density polyethylene may have a Vicat temperature of 90°C but a heat distortion temperature below normal ambient temperatures. 

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The differences in the assessment of softening point between the tests are clearly largely a matter that the end point of the test measures a different modulus. Figure 1.43 shows that with some materials (e.g., curve A) this will not be of great importance but with other types (e.g., curve E or F) the difference could be very large. 

Compared with atactic polypropylene it has a lower softening point (less than 100°C compared with 154°C when assessed by ball and ring methods), has better resistance to subzero temperatures and is completely soluble in aliphatic hydrocarbons. The molecular mass of atactic polybut-l-ene is about twice that of an atactic polypropylene of similar melt viscosity. 

According to Chalmers [1], the softener forms a film over the entire surface of the fabric, with the fatty chains pointing away from the surface. There is some evidence for the correlation between the soft touch and the reduction of friction, friction between fabric and skin and between the fibers themselves [20], For instance, Roder and colleagues report an excellent correlation between softening efficacy and the interfiber friction coefficient. The latter was assessed by fixing a piece of treated textile to a metal block and measuring the force applied to move the metal along a fixed piece of the same textile [31]. Once the fiber-fiber friction coefficient decreases, the fibers move more independently of one another [3,27], The overall result is a 20% decrease of fiber abrasion [13] and fabrics are perceived as more flexible [3], The friction coefficient, however, increases when the relative humidity rises above 90% [20],... 

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