Cure studies of network-forming polyurethanes

The polyhydroxy character of lignocellulosics and their natural abundance make them good candidates for the manufacture of polyurethanes. The cure characteristics of hydroxypropyl-cellulose and hydroxypropyl lignin (HPC and HPL, respectively) with polymeric methylene diphenyl diisocyanate (MDI) was studied via dynamic mechanical thermal analysis (DMTA). HPC/MDI and HPL/MDI resins flow at 30°C and proceed to cure at 50°C. The latter has excellent thermal stability over the former. Crosslinking of HPL and HPC with MDI follow an nth order kinetics, with an order of reaction of 2 and an apparent activation energy in the range of 12.9 kcal/mol - 14.7 kcal/mol. The rate of cure with time is higher in HPL-based polymers than HPC-based ones at the initial stage of cure; the difference vanishes at later stages. This demonstrates that the hydroxyl groups in HPC are less accessible to the NCO groups, and that cure rate might be dependent on diffusion limitations at later stages. Degree of cure, under all cure schedules, follows a parallel trend, and has to do with the fact that the hydroxyl groups of HPC are less accessible to isocyanate. Both HPL and HPC react with MDI at a reduced rate in comparison to a synthetic polyol: caprolactone triol. Time-glass transition temperature superposition was used to calculate times to vitrification of the HPL-based polymers, and is presented in a TTT cure diagram. This bio-based polymer displays the s-shaped vitrification pattern characteristics of thermosets. A similar approach did not work with HPC-based polymers. HPC- and HPL-based polymers did not display damping transitions, in isothermal cure, typical of gelation and vitrification. As the isocyanate to hydroxyl ratio (NCO:OH) increased, the glass transition temperature of the polymers increased, and the transition amplitude and width decreased and increased, respectively. In practical terms, this study illustrates that it is advantageous to use a) to use high isocyanate to hydroxyl ratios in order to produce polyurethanes which retain desirable damping behavior over a wider range of temperature. b) to use HPC/MDI resins in those situations where retention of stiffness at temperatures below 230° is required. c) to use HPL where rapid cure is desired. The study also reveals that the relative reactivity of water, HPL and HPC with isocyanate takes the form water > HPL > HPC.

Name thesis:degree_name

Master of Science

Level thesis:degree_level

masters

Discipline thesis:degree_discipline

Forest Products

Department dc:contributor.department

Forest Products

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

1993

Author dc:creator

• Toffey, Ackah

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Dc Identifier Other

etd-03122009-041354

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/41509