Method and apparatus for controlling the gas content of dispensed hot melt thermoplastic adhesive foam

• US 4,779,762 A
• Filed: 09/12/1986
• Issued: 10/25/1988
• Est. Priority Date: 05/30/1984
• Status: Expired due to Term

• Current Assignee: Nordson Corporation
• Original Assignee: Nordson Corporation
• Inventors: Schmitkons, James W., Saidman, Laurence B., Klein, Richard G.
• Primary Examiner(s): Skaggs, H. Grant
• Application Number:
  US06/885,960
• Time in Patent Office: 774 Days
• Field of Search: 73/861.71, 73/19, 137/4, 137/88, 366/132, 366/151, 239/61, 422/62, 422/111, 222/52, 222/63, 222/56, 222/57, 222/133, 222/145, 222/318, 222/255, 222/190, 425/135, 425/145
• US Class Current: 222/52
• CPC Class Codes: B05B 12/085 responsive to flow or pressure of liquid or other fluent material to be discharged B05B1/3006, B05B1/323, B05B7/1254 take precedence
• CPC Class Codes: B05B 7/262 a liquid and a gas being brought together before entering the discharge device
• CPC Class Codes: B29B 7/72 Measuring, controlling or regulating
• CPC Class Codes: B29C 44/60 Measuring, controlling or regulating
• CPC Class Codes: G05D 11/136 by sensing the viscosity
• CPC Class Codes: Y10T 137/0335 Controlled by consistency of mixture
• CPC Class Codes: Y10T 137/2499 Mixture condition maintaining or sensing

Abstract

A method and apparatus for controlling the gas content of dispensed hot melt thermoplastic adhesive foam. Air or another relatively inert gas is mixed with molten thermoplastic adhesive, and the mixture is pressurized to form a gas and molten adhesive "solution". When the molten adhesive and gas "solution" is dispensed at atmospheric pressure, the gas is released from "solution" and becomes entrapped in the adhesive in order to form a homogeneous closed cellular adhesive foam. In order to control the gas content of the dispensed adhesive and gas "solution", in accordance with one preferred embodiment, the pressure drop across a restriction in the flow path of the solution is sensed, and a control circuit controls the addition of gas to the adhesive to maintain the sensed pressure drop at a desired level. Other sensing techniques can be used to measure a parameter of the dissolved gas/molten adhesive single phase liquid solution which is correlated to the dissolved gas content of the solution and substantially independent of the molten adhesive content thereof, such as measuring the transmittance of infrared radiation through the gas/adhesive single phase solution at a wavelength relatively heavily absorbed by the gas and unabsorbed by the adhesive.

Claims

1. Apparatus for controlling the gas content of dispensed hot melt thermoplastic adhesive foam comprising:

means for supplying heated thermoplastic adhesive material in a molten state at an outlet;

means for supplying gas at an outlet;

means, coupled to the adhesive material supplying means outlet and to the gas supplying means outlet, for agitating and pressurizing the molten adhesive material in the presence of the gas to force the gas into solution with the molten adhesive material to form a substantially single phase liquid of unfoamed molten adhesive and gas in solution at an outlet, whereby a dispenser may be coupled to the outlet of the agitating and pressurizing means for dispensing the molten adhesive and gas solution at a reduced pressure so that the gas is released from solution and forms a dispensed adhesive foam;

means for sensing a parameter of said substantially single phase liquid of molten adhesive and gas in solution while said solution is in said substantially single liquid phase, unfoamed state, said parameter being correlated to the dissolved gas content of said substantially single phase liquid solution for producing an output dependent upon said sensed parameter; and

means, coupled to the parameter sensing means and to the gas supplying means, for controlling the flow of gas from the gas supplying means to the agitating and pressurizing means dependent upon said parameter sensing means output.

2. The apparatus of claim 1 in which the agitating and pressurizing means has an inlet coupled to the adhesive material supplying means outlet, and further comprising a recirculation path coupling the outlet of the agitating and pressurizing means to the inlet of the agitating and pressurizing means.

3. The apparatus of claim 2 which further comprises means for reducing the pressure in the recirculation path between the outlet of the agitating and pressurizing means and the inlet of the agitating and pressurizing means such that a substantial fraction of the gas is no longer dissolved in the molten adhesive.

4. The apparatus of claim 3 in which the agitating and pressurizing means comprises a pump operable to pump volumetrically metered amounts of molten adhesive material and molten adhesive and gas solution from the inlet to the outlet of the agitating and pressurizing means and in which the parameter sensing means comprises means for sensing a parameter at least partially dependent upon the flow rate of the adhesive and gas solution from said outlet.

5. The apparatus of claim 4 in which the parameter sensing means comprises means for sensing a parameter dependent upon both the flow rate and the viscosity of the molten adhesive and gas solution formed at the outlet of the agitating and pressurizing means.

6. The apparatus of claim 5 in which the parameter sensing means comprises means for sensing a pressure differential across a restrictor along a flow path coupled from the molten adhesive and gas solution outlet of the agitating and pressurizing means.

7. The apparatus of claim 1 wherein said sensing means includes a source of infrared radiation having a specified wavelength substantially absorbed by said gas and substantially unabsorbed by said adhesive material, and detecting means for sensing radiation at said specified wavelength, said source directing said radiation into said substantially single phase unfoamed molten adhesive and gas solution and said detecting means sensing said radiation emerging from said substantially single phase unfoamed molten adhesive and gas solution.

8. The apparatus of claim 7 wherein said sensing means further includes sampling means defining a sample flow path for passing a sample of said substantially single phase unfoamed molten adhesive and gas solution between said infrared radiation source and said detecting means, said sampling means comprising:

a body having inlet and outlet ports between which is connected an intermediate sample flow region, said intermediate sample flow region having a cross-sectional flow area defined by a width and by a height which is substantially smaller than said width to minimize the pressure drop across said intermediate sample flow region, said inlet and outlet ports and said intermediate sample flow region collectively comprising said sample flow path,

said infrared radiation source passing infrared radiation along a radiation path through said intermediate sample flow path to said detecting means in a direction approximately parallel to said height thereof to maximize transmission of infrared radiation from said source through said intermediate sample flow region to said detecting means.

9. The apparatus of claim 8 wherein said radiation path through said intermediate sample flow region has a width measured parallel to said width of said intermediate sample flow region cross-sectional area which is substantially less than, and approximately centered relative to, said width of said intermediate sample flow region cross-sectional area to minimize sampling errors due to effects at the edge of said intermediate sample flow region.

10. The apparatus of claim 8 wherein said sampling means further comprises:

a heater for heating said body to a temperature approximating the temperature of said molten adhesive material output from said adhesive supply means, and

first and second windows of solid state material transmissive to said infrared radiation located within said body along said radiation path on opposite sides of, and adjacent to, said intermediate sample flow region,

said body having a substantial thermal mass to avoid rapid temperature changes adjacent said first and second radiation windows to minimize the risk of subjecting said windows to thermal shock.

11. The apparatus of claim 10 wherein said windows and said body have significantly different coefficients of thermal expansion, said apparatus further comprising:

a layer of resilient sealant mounting said windows within said body to resiliently mount said windows in sealed relation to said body to accommodate thermally-induced dimensional changes between said body and windows and seal said windows relative to said body to prevent the flow therebetween of said solution from said intermediate sample flow region.

12. Apparatus for dispensing hot melt thermoplastic adhesive foam having a controlled gas content comprising:

a heated thermoplastic adhesive material supply container providing molten adhesive material at an outlet;

means for supplying gas at an outlet;

means, coupled to the outlet of the adhesive material supply and to the outlet of the gas supply, for agitating and pressurizing the molten adhesive material in the presence of the gas to force the gas into solution with molten adhesive material to form substantially single phase liquid of unfoamed molten adhesive and gas in solution at an outlet;

means, coupled to the outlet of the agitating and pressurizing means, for dispensing the molten adhesive and gas solution at a reduced pressure whereby the gas is released from solution and forms a dispensed adhesive foam;

means for sensing a parameter of said substantially single phase liquid of molten adhesive and gas in solution while said solution is in said substantially single liquid phase, unfoamed state, said parameter being correlated to the dissolved gas content of said substantially single phase liquid solution for producing an output dependent upon said sensed parameters; and

means, coupled to the parameter sensing means and coupled between the gas supply outlet and the agitating and pressurizing means, for controlling the flow of gas which is coupled to the agitating and pressurizing means dependent upon said parameter sensing means output.

13. The apparatus of claim 12 in which the agitating and pressurizing means has an inlet coupled to the adhesive material supply outlet, and further comprising a recirculation path coupling the outlet of the agitating and pressurizing means to the inlet of the agitating and pressurizing means.

14. The apparatus of claim 13 which further comprises means for reducing the pressure in the recirculation path between the outlet of the agitating and pressurizing means and the inlet of the agitating and pressurizing means such that a substantial fraction of the gas is no longer dissolved in the molten adhesive.

15. The apparatus of claim 14 in which the agitating and pressurizing means comprises a pump operable to pump volumetrically metered amounts of molten adhesive material and molten adhesive and gas solution from the inlet to the outlet of the agitating and pressurizing means and in which the parameter sensing means comprises means for sensing a parameter at least partially dependent upon the flow rate of the adhesive and gas solution from said outlet.

16. The apparatus of claim 15 in which the parameter sensing means comprises means for sensing a parameter dependent upon both the flow rate and the viscosity of the molten adhesive and gas solution formed at the outlet of the agitating and pressurizing means.

17. The apparatus of claim 16 in which the parameter sensing means comprises means for sensing a pressure differential across a restrictor along a flow path coupled from the molten adhesive and gas solution outlet of the agitating and pressurizing means.

18. The apparatus of claim 12 wherein said sensing means includes a source of infrared radiation having a specified wavelength substantially absorbed by said gas and substantially unabsorbed by said adhesive material, and detecting means for sensing radiation at said specified wavelength, said source directing said radiation into said substantially single phase unfoamed molten adhesive and gas solution and said detecting means sensing said radiation emerging from said substantially single phase unfoamed molten adhesive and gas solution.

19. Apparatus for controlling the gas content of dispensed hot melt thermoplastic adhesive foam comprising:

means for supplying heated thermoplastic adhesive material in a molten state at an outlet;

means for supplying gas at an outlet;

means, coupled to the adhesive material supplying means outlet and to the gas supplying means outlet, for agitating and pressurizing the molten adhesive material in the presence of the gas to force the gas into solution with the molten adhesive material to form a substantially single phase unfoamed molten adhesive and gas solution at an outlet, whereby a dispenser may be coupled to the outlet of the agitating and pressurizing means for dispensing the molten adhesive and gas solution at a reduced pressure so that the gas is released from solution and forms a dispensed adhesive foam;

means for sensing a differential pressure across a restrictor along a flow path of the substantially single phase unfoamed molten adhesive and gas solution and for producing an output dependent upon said sensed differential pressure; and

means, coupled to the differential pressure sensing means and to the gas supplying means, for controlling the flow of gas from the gas supplying means to the agitating and pressurizing means dependent upon said differential pressure sensing means output.

20. The apparatus of claim 19 in which the differential pressure sensing means comprises (a) a movable slug having an orifice therethrough positioned in a flow path of the molten adhesive and gas solution and (b) means for producing said differential pressure sensing means output dependent upon the position of the slug in the flow path.

21. The apparatus of claim 20 in which the controlling means comprises means for comparing the output of the differential pressure sensing means, indicative of the slug position, to a desired slug position and for controlling the gas supplying means to couple gas to the agitating and pressurizing means dependent upon said comparison.

22. The apparatus of claim 21 in which the agitating and pressurizing means comprises a variable speed pump, and further comprising means, coupled to the controlling means, for calculating a desired slug position dependent upon the pump speed.

23. The apparatus of claim 22 in which the desired slug position calculating means calculates a desired slug position which is also dependent upon the temperature of the molten adhesive and gas solution.

24. Apparatus for controlling the gas content of dispensed thermosplastic adhesive foam comprising:

means for supplying heated thermoplastic adhesive material in a molten state at an outlet;

means for supplying gas at an outlet;

means, coupled to the adhesive material supplying means outlet and to the gas supplying means outlet, for agitating and pressurizing the molten adhesive material in the presence of the gas to force the gas into solution with the molten adhesive material to form substantially single phase liquid of unfoamed molten adhesive and gas in solution at an outlet, whereby a dispenser may be coupled to the outlet of the agitating and pressurizing means for dispensing the molten adhesive and gas solution at a reduced pressure, so that the gas is released from solution and forms a dispensed adhesive foam;

means for sensing a parameter of said substantially single phase liquid of molten adhesive and gas in solution while said solution is in said substantially single liquid phase, unfoamed state, said parameter being correlated to the dissolved gas content of said substantially single phase liquid solution for producing an output dependent upon said sensed parameter; and

means, coupled to the parameter sensing means, for comparing the parameter sensing means output with a desired parameter input to produce a comparison signal; and

means, coupled to the comparing means and to the gas supplying means, for controlling the flow of gas from the gas supplying means to the agitating and pressurizing means dependent upon said comparison signal.

25. The apparatus of claim 24 in which the parameter sensing means comprises differential pressure sensing means including (a) a movable slug having an orifice therethrough positioned in a flow path of the molten adhesive and gas solution and (b) means for producing a differential pressure sensing means output dependent upon the position of the slug in the flow path.

26. The apparatus of claim 25 in which the comparing means compares the output of the differential pressure sensing means, indicative of the slug position, with a desired slug position and the controlling means controls the flow of gas to the agitating and pressurizing means dependent upon said comparison.

27. The apparatus of claim 26 in which the agitating and pressurizing means comprises a variable speed pump, and further comprising means, coupled to the comparing means, for calculating a desired slug position dependent upon the pump speed.

28. The apparatus of claim 27 in which the desired slug position calculating means calculates a desired slug position which is also dependent upon the temperature of the molten adhesive and gas solution.

29. The apparatus of claim 24 wherein said sensing means includes a source of infrared radiation having a specified wavelength substantially absorbed by said gas and substantially unabsorbed by said adhesive material, and detecting means for sensing radiation at said specified wavelength, said source directing said radiation into said substantially single phase unfoamed molten adhesive and gas solution and said detecting means sensing said radiation emerging from said substantially single phase unfoamed molten adhesive and gas solution.

30. A method for dispensing hot melt thermoplastic adhesive foam having a controlled gas content comprising the steps of:

agitating and pressurizing heated thermoplastic adhesive material in a molten state in the presence of a gas to force the gas into solution with the molten adhesive material to form a substantially single phase liquid of unfoamed molten adhesive and gas in solution;

sensing a parameter of said substantially single phase liquid of molten adhesive and gas in solution while said solution is in said substantially single liquid phase, unfoamed state, said parameter being correlated to the dissolved gas content of said substantially single phase liquid solution for producing an output dependent upon said sensed parameter;

controlling the flow of said gas into the gas and adhesive material solution dependent upon said sensed parameter; and

dispensing the molten adhesive and gas solution at a reduced pressure so that gas is released from solution and forms a dispensed adhesive foam.

31. The method of claim 30 which further comprises, after said agitating and pressurizing step, the step of:

recirculating at least a portion of the molten adhesive and gas solution for mixing with the heated solid thermoplastic adhesive material in a molten state.

32. The method of claim 31 in which said sensing step comprises sensing a differential pressure across a restriction along a flow path of the molten adhesive and gas solution.

33. The method of claim 31 in which said controlling step comprises the substeps of:

comparing said sensed parameter to a setpoint parameter to produce a comparison signal; and

controlling the gas flow dependent upon said comparison.

34. The method of claim 33 which further comprises, before said comparing and controlling substeps, the step of:

calculating a setpoint parameter to accommodate system variables which are substantially independent from the gas content of the molten adhesive and gas solution.

35. The method of claim 30 wherein said sensing step includes:

irradiating said substantially single phase unfoamed molten adhesive and gas solution with infrared radiation having a specified wavelength substantially absorbed by said gas and substantially unabsorbed by said adhesive material, and

detecting radiation at said specified wavelength emerging from said substantially single phase unfoamed molten adhesive and gas solution.

36. An apparatus for controlling the gas content of dispensed hot melt thermoplastic adhesive foam comprising:

means for supplying heated thermoplastic adhesive material in a molten state in an outlet;

means for supplying gas at an outlet;

a positive displacement pump having a first inlet coupled to the adhesive material supplying means outlet and having a second inlet coupled to the gas supplying means outlet, for agitating and pressurizing the molten adhesive material in the presence of the gas to force the gas into solution with the molten adhesive material to form a substantially single phase liquid of unfoamed molten adhesive and gas in solution at a pump outlet, whereby a dispenser may be coupled to the pump outlet for dispensing the molten adhesive and gas solution at a reduces pressure, so that the gas is released from solution and forms a dispensed adhesive foam;

means for recirculating the molten adhesive and gas solution from the pump outlet to the first pump inlet;

means for sensing a parameter of said substantially single phase liquid of molten adhesive and gas in solution while said solution is in said substantially single liquid phase, unfoamed state, said parameter being correlated to the dissolved gas content of said substantially single phase liquid solution for producing an output dependent upon said sensed parameter; and

means, coupled to the parameter sensing means and to the gas supplying means, for controlling the flow of gas from the gas supplying means to the second inlet of the pump dependent upon said parameter sensing means output.

37. The apparatus of claim 36 in which the positive displacement pump comprises a two-stage gear pump and in which the first inlet of the pump is an inlet to the first gear pump stage and in which the second inlet of the pump is an inlet between the first and second stages of the gear pump.

38. The apparatus of claim 37 in which the means for recirculating the molten adhesive and gas solution from the pump outlet to the first pump inlet includes means for supplying the recirculated solution to the first pump inlet at a pressure substantially reduced from the pressure of the solution at the pump outlet such that a substantial fraction of the gas is no longer dissolved in the molten adhesive.

39. The apparatus of claim 36 wherein said sensing means includes a source of infrared radiation having a specified wavelength substantially absorbed by said gas and substantially unabsorbed by said adhesive material, and detecting means for sensing radiation at said specified wavelength, said source directing said radiation into said substantially single phase unfoamed molten adhesive and gas solution and said detecting means sensing said radiation emerging from said substantially single phase unfoamed molten adhesive and gas solution.

40. Apparatus for controlling the gas content of dispensed thermoplastic adhesive foam comprising:

means for supplying heated thermoplastic adhesive material in a molten state at an outlet;

means for supplying gas at an outlet;

means, coupled to the adhesive material supplying means outlet and to the gas supplying means outlet, for agitating and pressurizing the molten adhesive material in the presence of the gas to force the gas into solution with the molten adhesive material to form substantially single phase liquid of unfoamedd molten adhesive and gas in solution at an outlet, whereby a dispenser may be coupled to the outlet of the agitating and pressurizing means for dispensing the molten adhesive and gas solution at a reduced pressure, so that the gas is released from solution and forms a dispensed adhesive foam;

means for sensing a parameter of said substantially single phase liquid of unfoamed molten adhesive and gas in solution which is correlated to the dissolved gas content of said substantially single phase liquid solution and for producing an output dependent upon said sensed parameter, said parameter sensing means comprising differential pressure sensing means including (a) a movable slug having an orifice therethrough positioned in a flow path of said substantially single phase liquid of molten adhesive and gas in solution and (b) means for producing a differential pressure sensing means output dependent upon the position of the slug in the flow path;

means, coupled to the parameter sensing means, for comparing the parameter sensing output with a desired parameter input to produce a comparison signal; and

means, coupled to the comparing means and to the gas supplying means, for controlling the flow of gas from the gas supplying means to the agitating and pressurizing means dependent upon said comparison signal.

41. The apparatus of claim 40 in which the comparing means compares the output of the differential pressure sensing means, indicative of the slug position, with a desired slug position and the controlling means controls the flow of gas to the agitating and pressurizing means dependent upon said comparison.

42. The apparatus of claim 41 in which the agitating and pressurizing means comprises a variable speed pump, and further comprising means, coupled to the comparing means, for calculating a desired slug position dependent upon the pump speed.

43. The apparatus of claim 42 in which the desires slug position calculating means calculates a desired slug position which is also dependent upon the temperature of the molten adhesive and gas solution.

44. Apparatus for controlling the gas content of dispensed hot melt thermoplastic adhesive foam comprising:

means for supplying heated thermoplastic adhesive material in a molten state at an outlet;

means for supplying gas at an outlet;

means, coupled to the adhesive material supplying means outlet and to the gas supplying means outlet, for agitating and pressurizing the molten adhesive material in the presence of the gas to force the gas into solution with the molten adhesive material to form a substantially single phase liquid of unfoamed molten adhesive and gas in solution at an outlet, whereby a dispenser may be coupled to the outlet of the agitating and pressurizing means for dispensing the molten adhesive and gas solution at a reduced pressure so that the gas is released from solution and forms a dispensed adhesive foam;

means for sensing a parameter of said substantially single phase liquid of unfoamed molten adhesive and gas in solution which is correlated to the dissolved gas content of said substantially single phase liquid solution for producing an output dependent upon said sensed parameter, said sensing means including a source of infrared radiation having a specified wavelength substantially absorbed by said gas and substantially unabsorbed by said adhesive material, and detecting means for sensing radiation at said specified wavelength, said source directing said radiation into said substantially single phase liquid of unfoamed molten adhesive and gas in solution and said detecting means sensing said radiation emerging from said substantially single phase liquid of unfoamed molten adhesive and gas in solution; and

means, coupled to the parameter sensing means and to the gas supplying means, for controlling the flow of gas from the gas supplying means to the agitating and pressurizing means dependent upon said parameter sensing means output.

45. Apparatus for dispensing hot melt thermoplastic adhesive foam having a controlled gas content comprising:

a heated thermoplastic adhesive material supply container providing molten adhesive material at an outlet;

means for supplying gas at an outlet;

means, coupled to the outlet of the adhesive material supply and to the outlet of the gas supply, for agitating and pressurizing the molten adhesive material in the presence of the gas to force the gas into solution with molten adhesive material to form substantially single phase liquid of unfoamed molten adhesive and gas in solution at an outlet;

means, coupled to the outlet of agitating and pressurizing means, for dispensing the molten adhesive and gas solution at a reduced pressure whereby the gas is released from solution and forms a dispensed adhesive foam;

means for sensing a parameter of said substantially single phase liquid of unfoamed molten adhesive and gas in solution which is correlated to the dissolved gas content of said substantially single phase liquid solution for producing an output dependent upon said sensed parameter, said sensing means including a source of infrared radiation having a specified wavelength substantially absorbed by said gas and substantially unabsorbed by said adhesive material, and detecting means for sensing radiation at said specified wavelength, said source directing said radiation into said substantially single phase liquid of unfoamed molten adhesive and gas in solution and said detecting means sensing radiation emerging from said substantially single phase liquid of unfoamed molten adhesive and gas in solution; and

means, coupled to the parameter sensing means and coupled between the gas supply outlet and the agitating and pressurizing means, for controlling the flow of gas which is coupled to the agitating and pressurizing means dependent upon said parameter sensing means output.

46. Apparatus for controlling the gas content of dispensed thermoplastic adhesive foam comprising:

means for supplying heated thermoplastic adhesive material in a molten state at an outlet;

means for supplying gas at an outlet;

means, coupled to the adhesive material supplying means outlet and to the gas supplying means outlet, for agitating and pressurizing the molten adhesive material in the presence of the gas to force the gas into solution with the molten adhesive material to form a substantially single phase liquid of unfoamed molten adhesive and gas in solution at an outlet, whereby a dispenser may be coupled to the outlet of the agitating and pressuring means for dispensing the molten adhesive and gas solution at a reduced pressure so that the gas is released from solution and forms a dispensed adhesive foam;

means for sensing a parameter of said substantially single phase liquid of unfoamed molten adhesive and gas in solution which is correlated to the dissolved gas content of said substantially single phase liquid solution for producing an output dependent upon said sensed parameter, said sensing means including a source of infrared radiation having a specified wavelength substantially absorbed by said gas and substantially unabsorbed by said adhesive material, and detecting means for sensing radiation at said specified wavelength, said source directing said radiation into said substantially single phase liquid of unfoamed molten adhesive and gas in solution and said detecting means sensing said radiation emerging from said substantially single phase liquid of unfoamed molten adhesive and gas in solution;

means, coupled to the parameter sensing means, for comparing the parameter sensing means output with a desired parameter input to produce a comparison signal; and

means, coupled to the comparing means and to the gas supplying means, for controlling the flow of gas from the gas supplying means to the agitating and pressurizing means dependent upon said comparison signal.

47. An apparatus for controlling the gas content of dispensed hot melt thermoplastic adhesive foam comprising:

means for supplying heated thermoplastic adhesive material in a molten state in an outlet;

means for supplying gas at an outlet;

a positive displacement pump having a first inlet coupled to the adhesive material supplying means outlet and having a second inlet coupled to the gas supplying means outlet, for agitating and pressurizing the molten adhesive material in the presence of the gas to force the gas into solution with the molten adhesive material to form a substantially single phase liquid of unfoamed molten adhesive and gas in solution at a pump outlet, whereby a dispenser may be coupled to the pump outlet for dispensing the molten adhesive and gas solution at a reduced pressure, so that the gas is released from solution and forms a dispensed adhesive foam;

means for recirculating the molten adhesive and gas solution from the pump outlet to the first pump inlet;

means for sensing a parameter of said substantially single phase liquid of unfoamed molten adhesive and gas in solution which is correlated to the dissolved gas content of said substantially single phase liquid solution for producing an output dependent upon said sensed parameter, said sensing means including a source of infrared radiation having a specified wavelength substantially absorbed by said gas and substantially unabsorbed by said adhesive material, and detecting means for sensing radiation at said specified wavelength, said source directing said radiation into said substantially single phase liquid of unfoamed molten adhesive and gas in solution and said detecting means sensing said radiation emerging from said substantially single phase liquid of unfoamed molten adhesive and gas in solution; and

means, coupled to the parameter sensing means and to the gas supplying means, for controlliing the flow of gas from the gas supplying means to the second inlet of the pump dependent upon said parameter sensing means output.

48. A method for dispensing hot melt thermoplastic adhesive foam having a controlled gas content comprising the steps of:

agitating and pressurizing heated thermoplastic adhesive material in a molten state in the presence of a gas to force the gas into solution with the molten adhesive material to form a substantially single phase liquid of unfoamed molten adhesive and gas in solution;

sensing a parameter of said substantially single phase liquid of unfoamed molten adhesive and gas in solution which is correlated to the dissolved gas content of said substantially single phase liquid solution for producing an output dependent upon said sensed parameter, said sensing step including;

irradiating said substantially single phase liquid solution of unfoamed molten adhesive and gas in solution with infrared radiation having a specified wavelength substantially absorbed by said gas and substantially unabsorbed by said adhesive material, and

detecting radiation at said specified wavelength emerging from said substantially single phase liquid of unfoamed molten adhesive and gas in solution;

controlling the flow of said gas into the gas and adhesive material solution dependent upon said sensed parameter; and

dispensing the molten adhesive and gas solution at a reduced pressure so that the gas is released from solution and forms a dispensed adhesive foam.