【文章內(nèi)容簡介】 d not to increase the ODP and GWP effect. These ABA39。s are needed to produce the most widely used low density PU foams and their replacing cause problems to the foam manufacturers.To obviate this elimination of ABA39。s, some environmentalfriendly systems have been adopted like LCD (liquid carbon dioxide) and CPT (control pressure technology), also known as VPF (Variable Pressure Foaming).CPtechnology was developed some years ago when it was ascertained that ‘slabstock’ foaming plants operating at 2000 meter above sea level about 785 mbar, therefore at low vacuum degree can produce lower foam densities from production formulations made at sea level.No alternative blowing agents are needed in all CP foaming processes, applied to both continuous and discontinuous production of PU blocks either at low or high density.The foam expansion is mechanically obtained by creating above the foaming device either a given, accurate vacuum degree to get foam density lower than 20 kg/m3 reaching even 10 kg/m3 or a certain pressed air pressure degree to get foam density higher than 20 kg/m3.To create such vacuum and/or pressure conditions in these CP plants the foaming process from the cream time up to the block discharge phase occurs in a tunnel where either vacuum or pressed air pressure are kept to a precalculated degree through pertinent equipment.It should be noted that the vacuum ranges from 500 to 1000 absolute mbar, while the pression ranges from 1000 to 1300 mbar.Another important aspect of this innovative technology is that the production plant is entirely environmentallyfriendly being enclosed within an airtight tunnel and all gases developed are retained by activated carbon traps.Last but not least, foams produced under CPT are 3%~5% less expensive than those produced adopting the atmospheric pressure technology.CPT PRINCIPLEThe PU foam rise is based on the gas expansion (CO2 or blowing agent) being influenced by surrounding temperature and pressure degree.Physical laws governing this expansion e from the law governing such gases:Px/Dx＝ Pa/DaFrom which we get:Px ＝ (DxPa)/ DaWhere: Px is the controlled pressure value, Dx is the density at controlled pressure, Pa is the absolute atmospheric pressure, Da is the density at Pa.As a result of this formulation, we can foresee the working pressure without modifying the quantity of the chemical ponents to get the required density.The foam hardness can be calculated by applying the following formula:Hx ＝(Px2/Pa2)HaWhere: Hx is the foam hardness at controlled pressure, Px is the controlled pressure value, Ha is the density at Pa.For example: from a 22 kg/m3 foam density and 120 N hardness it is necessary to work at a 680 mbar to obtain a 15 kg/m3 density and 55,5 N hardness.Though these are formulas for gases, the data obtained for foams are most likely the calculated ones.CONTINUOUS CONTROLLED PRESSURE PROCESS (CCPP)This plant is posed of an airtight tunnel (provided with front access and exit doors) duly dimensioned for reaching an inside pressure of max 1300 mbar and an inside vacuum of max 500 mbar.This tunnel is divided into two sections.The first section of the tunnel houses the continuous production line or so called “dry” equipment: the Trough (2), the Fallplate (6), the production conveyors (9/1), the Unwind and Rewind systems and the Cut Off Machine (8). Afterwards there is a free roller conveyor (10) and a transferring conveyor (9/2) at which end there is the intermediate airlock door (3/1).The second section of the tunnel houses a conveyor (9/3) discharging the foam block through the airlock exit door (3/2) to the block curing system at atmospheric pressure. The colour red highlights the foam block making, whereas the yellow highlights the pressure is at the desired operating value.All metering lines are manufactured with high pressure metering pumps and are located outside the tunnel.The metering of each line is controlled by a pertinent mass flow meter which, through a closed loop system, changes automatically the pumps speed to keep constant their outputs at the desired value.In plants like our Maxfoam (similar to Foamax), the mixing head is fitted outside the tunnel under CP. The connection between the mixing head and the bottom of the trough which is housed in the tunnel is by high pressure hoses.The vacuum and/or pressure values inside each of the sections of the tunnel are set and controlled by suitable independent systems (4/1 and 4/2).A sophisticated electronic control system holds