Method for producing expanded thermoplastic polymers

Abstract

An improved process for fabricating expanded thermoplastic polymers (eTP) starting from non-expanded TP is disclosed whereby said process has improved thermal control, uses preferably environmentally friendly foaming gasses, avoids anisotropy and sticking of the eTP during the processing and minimises the duration of the charging step.

Claims

1. A method for producing expanded thermoplastic polymeric material, said method comprising at least following steps: a) placing a non-expanded thermoplastic polymer material in an autoclave, said autoclave being partly filled with a liquid and wherein the non-expanded thermoplastic polymer material not being in contact with said liquid; b) increasing the pressure in the autoclave by introducing at least one gaseous fluid at a temperature within the autoclave below the melting temperature of the non-expanded thermoplastic polymer material; c) allowing the non-expanded thermoplastic polymer material to reach a saturation state; thereby forming a saturated non-expanded thermoplastic material d) submerging the saturated non-expanded thermoplastic material into the liquid, and then e) decreasing the pressure in the autoclave such that the submerged saturated non-expanded thermoplastic polymer material expands to form the expanded thermoplastic polymer material.

2. The method according to claim 1, wherein the gaseous fluids are selected from N.sub.2 and/or CO.sub.2.

3. The method according to claim 1, wherein the gaseous fluids comprise low thermal conductivity gases selected from Hydro Chloro Fluoro Carbons (HCFC's), Chloro Fluoro Carbons (CFC's), Hydro Chloro Fluoro Olefins (HCFO's), Hydro Fluoro Olefins (HFO's), (cyclo)-alkanes.

4. The method according to claim 1, wherein the liquid in the autoclave is reactive or non-reactive towards the non-expanded thermoplastic polymer material.

5. The method according to claim 1, wherein the non-expanded thermoplastic polymer material is reactive or made reactive towards the liquid in the autoclave.

6. The method according to claim 1, wherein the gaseous fluids in the autoclave further comprise additives which are reactive towards the non-expanded thermoplastic polymer material and can result in modification of the thermoplastic polymer during the charging step.

7. The method according to claim 1, wherein the liquid in the autoclave further comprises additives which are reactive towards the non-expanded thermoplastic polymer material and can result in modification of the non-expanded thermoplastic polymer material during step (d).

8. The method according to claim 1, wherein the thermoplastic polymer material is a thermoplastic polyurethane material.

9. The method according to claim 1, wherein the non-expanded thermoplastic polymer material is a thermoplastic polyurethane pellet having an average diameter in the range 0.2 to 10 mm.

10. The method according to claim 1, wherein the non-expanded thermoplastic polymer thermoplastic polymer material is a thermoplastic polyurethane pellet having an average diameter in the range 0.5 to 5 mm.

11. The method according to claim 1, wherein in step (b) the pressure within the autoclave is above the supercritical limits of the gaseous fluids.

12. The method according to claim 1, wherein the pressure within the autoclave ranges from 1-25 MPa in step (b).

13. The method according to claim 1, wherein the temperature within the autoclave is above the supercritical limits of the gaseous fluids and below the melting temperature of the thermoplastic material.

14. The method according to claim 1, wherein the temperature within the autoclave ranges from 30-250° C.

15. The method according to claim 1, wherein step (c) is performed at controlled pressure and temperature within the autoclave until saturated non-expanded thermoplastic polymer material is achieved.

Description

FIGURES

(1) FIG. 1 illustrates the autoclave set up and process steps to achieve expanded polymer particles according to the invention.

(2) FIG. 2 illustrates the autoclave set up and process steps to achieve expanded polymer particles according to a state of the art process.

(3) FIG. 3 illustrates the autoclave set up and process steps to achieve expanded polymer particles according to a state of the art process.

(4) FIG. 4 illustrates a few embodiments according to the invention to perform the submersion step. FIG. 4B illustrates the dropping mechanism; FIG. 4C illustrates a rotation mechanism.

(5) FIG. 5 illustrates the charging step (simulations according to example 1 of the invention) in a liquid and in a gaseous environment. FIG. 5A illustrates the amount of CO.sub.2 absorbed in the TPU pellets (g CO.sub.2/kg TPU) as a function of charging duration. FIG. 5B illustrates the density of the TPU beads (kg/m.sup.3) as a function of charging duration.

(6) FIG. 6 illustrates the expansion step (simulations according to example 1 of the invention) in a liquid and in a gaseous environment. FIG. 6A illustrates the amount of CO.sub.2 absorbed in the TPU pellets (g CO.sub.2/kg TPU) as a function of depressurisation duration. FIG. 6B illustrates the density of the TPU beads (kg/m.sup.3) as a function of depressurisation duration. FIG. 6C illustrates the different characteristics of the TPU material (beads) after the expansion step simulations in a liquid (A,B) or in a gaseous (C,D) environment, assuming a fast (A,C) or slow process (B,D). The corresponding points have also been indicated in FIGS. 6A and 6B.

(7) FIG. 7 illustrates the charging step (simulations according to example 2 of the invention) in a liquid and in a gaseous environment. FIG. 7A illustrates the amount of CO.sub.2 absorbed in the TPU pellets (g CO.sub.2/kg TPU) as a function of charging duration. FIG. 7B illustrates the density of the TPU beads (kg/m.sup.3) as a function of charging duration.

(8) FIG. 8 illustrates the expansion step (simulations according to example 2 of the invention) in a liquid and in a gaseous environment. FIG. 8A illustrates the amount of CO.sub.2 absorbed in the TPU pellets (g CO.sub.2/kg TPU) as a function of depressurisation duration. FIG. 8B illustrates the density of the TPU beads (kg/m′) as a function of depressurisation duration.

(9) FIGS. 9A, 9B and 9C: Cross sections of eTPU beads blown according to the invention (obtained through CT-imaging).

EXAMPLES

(10) Simulation examples: illustrating the gas absorption during the charging step and the diffusive leakage during the expansion step (simulations), comparing the effects of a liquid and a gaseous environment.

(11) The numerical experimental setup is a diffusion simulation in a spherical domain of a TPU pellet with a radius of 1 mm surrounded by a liquid layer with a thickness of 5 mm. There is no mixing, no polymer swelling, no nucleation, temperature is held constant, material properties are independent of pressure and dissolved concentration.

Example 1

(12) Input: diffusivity in TPU: 2 10.sup.−9 m.sup.2/s, diffusivity in liquid: 3 10.sup.−9 m.sup.2/s, solubility in TPU: 2 10.sup.−4 mol/m.sup.3 Pa, solubility in liquid: 10.sup.−4 mol/m.sup.3 Pa).

(13) The charging step simulation (FIG. 5) starts from a TPU pellet in equilibrium with a 10.sup.5 Pa CO.sub.2 atmosphere, after which the pressure is ramped up to 2 10.sup.7 Pa CO.sub.2. The expansion step simulation (FIG. 6) starts from a TPU pellet in equilibrium with a 2 10.sup.7 Pa CO.sub.2 atmosphere, after which the pressure is dropped to 10.sup.5 Pa CO.sub.2.

(14) It is shown that the volume averaged CO.sub.2 content (FIG. 5A+6A) in the pellet is much more volatile in the gaseous environment (dashed lines) while its evolution in a liquid environment (full lines) is much slower.

(15) A lower limit for the final bead density (FIG. 5B+6B), which is a measure of bead quality can be derived from CO.sub.2 content—the higher the content at the capture point, the lower the final density can be. For the minimal achievable density calculations, it was assumed that after the indicated charging/depressurization duration all remaining CO.sub.2 content is captured and fully converted into bubble filling gas (this excludes further escape of gas, condensation or any gas remaining in the matrix).

(16) Conclusion: the method according to the invention selects the faster charging environment (FIG. 5), i.e. in a gaseous fluid, and the depressurization environment that gives the lowest potential density (FIG. 6), i.e. in a liquid.

Example 2

(17) (Using More Permeable TPU Material)

(18) Input: diffusivity in TPU: 4 10.sup.−9 m.sup.2/s, diffusivity in liquid: 12 10.sup.−9 m.sup.2/s, solubility in TPU: 2 10.sup.−4 mol/m.sup.3 Pa, solubility in liquid: 10.sup.−4 mol/m.sup.3 Pa).

(19) The charging step simulation (FIG. 7) starts from a TPU pellet in equilibrium with a 10.sup.5 Pa CO.sub.2 atmosphere, after which the pressure is ramped up to 2 10.sup.7 Pa CO.sub.2. The expansion step simulation (FIG. 8) starts from a TPU pellet in equilibrium with a 2 10.sup.7 Pa CO.sub.2 atmosphere, after which the pressure is dropped to 10.sup.5 Pa CO.sub.2.

(20) It is also here shown that the volume averaged CO.sub.2 content (FIG. 7A+8A) in the pellet is much more volatile in the gaseous environment (dashed lines) while its evolution in a liquid environment (full lines) is much slower.

(21) A lower limit for the final bead density (FIG. 7B+8B), which is a measure of bead quality can be derived from CO.sub.2 content—the higher the content at the capture point, the lower the final density can be. For the minimal achievable density calculations, it was also here assumed that after the indicated charging/depressurization duration all remaining CO.sub.2 content is captured and fully converted into bubble filling gas (this excludes further escape of gas, condensation or any gas remaining in the matrix).

(22) A similar conclusion can be drawn here: the method according to the invention selects the faster charging environment (FIG. 7), i.e. in a gaseous fluid, and the depressurization environment that gives the lowest potential density (FIG. 8), i.e. in a liquid.

(23) Experimental examples: illustrating the process according to the invention and the obtained expanded thermoplastic (polyurethane) beads.

(24) Three experiments were performed illustrating and confirming the process according to the invention.

(25) Two sizes of largely spherical TPU pellets were used: 1. ‘regular’ pellets with a diameter between 2 and 3 mm (used in experiment A and B), and 2. ‘micropellets’ with a diameter approximately between 0.2 and 0.3 mm (used in experiment C).

(26) For all 3 experiments, two connected (valve separation) cylindrical containers (1 liter) were heated to 130° C. One container was filled with water and pressurized (towards 145 bar CO.sub.2 for experiment A and towards 250 bar CO.sub.2 for experiment B & C).

(27) The other container was loaded with approximately 30 g of pellets/micropellets and charged to the same pressure.

(28) The vessels were kept for 30 minutes (for experiment A and B)/15 minutes (for experiment C) under these conditions to allow pellet saturation.

(29) Subsequently water was transferred to the pellet containing vessel during 1 minute (for experiment A and B)/16 minutes (for experiment C).

(30) In experiment A and C a better CO.sub.2 saturation degree of the water was achieved by adding the water in a thin stream over a longer period while exposed to high pressure CO.sub.2. This did not compromise pellet CO.sub.2 saturation as the micropellets are smaller and thus reach saturation faster than the regular pellets. The overall experiment duration was kept the same to have a comparable thermal history.

(31) Finally, in all 3 experiments, the pressure was released on a timescale of about 5 s, causing bubble nucleation and expansion.

(32) In FIG. 9, CT-analysis images of the results of each experiment are included (black is solid fraction).

(33) FIGS. 9A and 9B illustrate expanded thermoplastic beads originating from ‘regular’ pellets with a diameter between 2 and 3 mm (used in experiment A and B).

(34) FIG. 9C illustrates expanded thermoplastic beads originating from ‘micropellets’ with a diameter approximately between 0.2 and 0.3 mm (used in experiment C).