tailieunhanh - Dynamic Mechanical Analysis part 8

Tham khảo tài liệu 'dynamic mechanical analysis part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Viscosity Poise Temperature c Time seconds FIGURE Effects of staging on the curing of resins. Staging is done to improve handling properties during lay-up but also changes the cure profile. 1999 CRC Press LLC Degree of Cure T c is where the material starts exhibiting behavior characteristic of a high polymer. Glass Transition Temperature shows three region as a function of degree of cure or MW. 1- a linear relationship where the materials acts like a monomeric material 2-a transition zone 3- a region where the material has polymeric properties FIGURE Relationship of Tg to cure time and the stages of a cure. Note that for thermosets it is often difficult to impossible to see the Tg by DSC in the latter half of region 3. Another special area of concern is paints and coatings 9 where the material is used in a thin layer. This can be addressed experimentally by either employing a braid as above or coating the material on a thin sheet of metal. The metal is often run first and its scan subtracted from the coated sheet s scan to leave only the scan of the coating. This is also done with thin films and adhesive coatings. A sample cure profile for a commercial two-part epoxy resin is shown in Figure . From this scan we can determine the minimum viscosity h min the time to h min and the length of time it stays there the onset of cure the point of gelation where the material changes from a viscous liquid to a viscoelastic solid and the beginning of vitrification. The minimum viscosity is seen in the complex viscosity curve and is where the resin viscosity is the lowest. A given resin s minimum viscosity is determined by the resin s chemistry the previous heat history of the resin the rate at which the temperature is increased and the amount of stress or stain applied. Increasing the rate of the temperature ramp is known to decrease the h min the time to h min and the gel time. The resin gets softer faster but also cures faster. The degree of flow limits the type .