MDI Emissions Reporting Guidelines for the ... - Polyurethanes

MDI Emissions Reporting Guidelines for the Polyurethanes Industry
There are two basic process configurations for coating carpets with polyurethane formulations. The
flow-on method is generally used to apply laminate adhesives or cushion formulations to carpet floor
coverings and reactively binding yarn and fabric. The process consists of pre-mixing a polyol and
inorganic filler and delivering it as a liquid stream to a mixing and frothing device where it is
combined with MDI/PMDI. The formed polyurethane is distributed across the width of the backside of
the textile fabric by a traversing positioning device. A reservoir of the mixture is maintained in
contact with the backside of the fabric and a “doctoring” blade scrapes onto the fabric the desired
amount of the polymerizing mixture. The “doctoring” blade also gauges and controls the gap of the
desired thickness of the laminating layer. Once applied, the carpet is then passed through a heated
oven for curing, cooled and collected on take-up rolls for storage.
The second method takes the pre-mixed polyol/filler and mixes it with MDI in a specially designed
low-pressure mixing head. A reservoir of the mixture is maintained on the belt and the newly formed
polyurethane is spread across the width of a Teflon-impregnated conveyor belt with the use of a
traversing positioning device. The carpet is then continuously laid onto the polymerizing mixture and
conveyed over heated plates where the carpet is heated until the polymerization is complete. The
carpet is the cooled and collected on take-up rolls for further processing.
Process Description
A two-component system is used to produce a carpet backing which is applied to a substrate surface.
Part A, representing 84% of the total formulation, is a mixture of polyol and inorganic filler. Part B,
representing 16% of the total formulation, is a 50% MDI/PMDI isocyanate mixture. The mixture is
continuously applied on a conveyor belt having a width of 8 feet and moving at a speed of 30
feet/minute. The application rate of the formulation is 2 lb/yd 2 . The carpet production rate is
345,600 ft 2 /day, corresponding to a consumption rate of 76,800 lb/day of total formulation (12,288
pound/day of MDI/PMDI). The process temperature is 200 o
F and the air gap above the conveyor belt
is 6 inches. An air blower with a capacity of 20,000 ft 3 /minute discharges air at a temperature of 130
o
F. The unit operates 6,000 hours/year (250 days/year). Estimate the annual emissions of MDI using
the proposed methods.
The methodology developed assumes that the application of the MDI formulation occurs on a
conveyor belt that is essentially enclosed. An air blower maintains a slight negative pressure in the
conveyor belt area by continuously exhausting air from the process area through a stack. The MDI
emissions can be estimated by considering the air exhaust rate, the temperature of the exit gas, the
reaction rate between the isocyanate and polyol resin, and the "effective" MDI concentration in the
gas stream. A block diagram of the carpet coating operation is shown in Figure 2. As a worst-case
scenario, it can be assumed that the air leaving the application area will be in thermodynamic
equilibrium with the MDI present on the carpet. This approach can usually be applied irrespective
of how the polyurethane is applied to the carpet substrate.
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