GB1595431A - Recovery of polyurethane scrap - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THE RECOVERY OF POLYURETHANE SCRAP (71) We, DIAMOND SHAMROCK INDUSTRIAL CHEMICALS LIMITED, formerly known as LANKRO CHEMICALS LIMITED, a British Company, of Emerson House, Albert Street, Eccles, Manchester, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention concerns improvements in or relating to the recovery of polyurethane scrap.

For many years polyurethane resins of various types have been used for a multitude of purposes. One such use has been in the manufacture of microcellular elastomeric shoe soles (the formed and moulded resin contains a myriad of closed cells with a diameter of between 0.01 and 0.1 mm), where the appropriate chemicals - basically a polyol and a polyisocyanate, together with a suitable catalyst, a blowing agent and a cell control agent are injected into a mould and allowed to react and thereby expand to fill the mould.

Because of the imperfect closure of the moulds used, this process invariably results in a shaped product (the shoe sole) which has unwanted bits - known as "flash" or "sprue" around its edges; moreover it is fairly common for the process to go wrong somewhere, so that the formed shoe sole is imperfect and has to be scrapped. Unfortunately, the quantity of scrap can be as much as 10% by weight of the total amount of resin, and if it is not reusable, and must be thrown away, will constitute a significant factor in the cost of the desired product.

Many attempts have in the past been made to recover and re-use polyurethane scrap especially the sort of thermoplastic scrap made during the production of microcellular elastomeric shoe soles. Many of these methods have involved heating the scrap under pressure to form a melt, and then injecting the melt into a mould to make the article desired. Unfortunately, however, none of these methods has been successful; all suffer from the serious drawbacks that the extruded material consistently contains large unwanted air pockets and requires the use of a relatively high temperature in its formation - and. as a result of these two factors, suffers badly from oxidative degradation.

We have now found that, by first pretreating the scrap in an appropriate manner before carrying out this sort of melting and moulding process, there can be obtained a remoulded product which does not suffer, or at the very least does not suffer so badly, from the drawbacks associated with the prior art re-moulded products.

Accordingly, this invention provides a process for the re-use of polyurethane scrap to produce moulded articles therefrom by injecting into an appropriate mould a melt of the scrap, in which process the scrap is first cut, chopped, milled, granulated or otherwise broken into particles all of substantially the same size and having an average maximum dimension of not more than one half inch (1.3 cms) and an average volume of not more than one eighth of a cubic inch (2 cc).

Though there is no intention to be bound by any such explanation, nevertheless the invention will be more readily understood from the following discussion about the possible reasons behind its utility.

As pointed out above, the main factor causing failure of the prior methods for re-using scrap is the formation of large air pockets in the re-moulded material. Now, it seems fairly clear that this air appears in the polyurethane as a result of diffusion into the scrap as the blowing agent originally used - usually carbon dioxide or a fluorohydrocarbon - diffuses out.

When the scrap, in the form of quite sizeable chunks, possibly three or four inches long, one or two inches wide and up to one inch deep, is melted and extruded under pressure, the process is carried out under what are effectively gas-tight conditions; the myriads of air-filled microcells in the scrap pieces coalesce into large bubbles actually within the individual pieces as those pieces melt together, these bubbles are unable to escape out of the melt, and thus get injected into the mould, and so there is formed a moulded article with large air pockets in it. This is all the result of the scrap pieces being so large.

Similarly - and also because the scrap pieces are so large - the mixture of pieces fed into the injection moulding machine packs in such a way as to trap large amounts of air in the mixture; this air is unable to escape during the melting stage (because of the stage's relatively gas-tight conditions), and thus gets injected into the mould to form an article with air pockets within it.

In the process of the invention exactly the same melting and injecting method is carried out, but the size of the scrap pieces is rigorously controlled to be equal to or less than the defined maximums, and moreover the spread of the scrap pieces' size is also rigorously controlled. The scrap pieces are thereby made much smaller and more uniform than those used in the prior art processes, and in so doing the number of air-filled microcells within each scrap piece, and the size of any air pockets trapped by the packing of the pieces, is considerably reduced.The consequences are two-fold; firstly, the reduction in piece size means that individual pieces melt faster, so that the whole particle has "liquified" before the air-filled microcells therein have time to coalesce (they therefore remain as very small individual bubbles); secondly, even if the microcells in a piece do coalesce, the small size of the piece means that the formed "large" bubble is still relatively small, and so does not cause a moulded product prepared from the melt to have large air-filled voids therein.

The chopping, granulating, milling or other form of breaking-up of the scrap for use in the process of the invention may be carried out using any of chopping, granulating or milling machines currently available and capable of chopping, etc., to the right size. A suitable machine is the "junior" Rotary Cutter as supplied by Blackfriars Rotary Cutters Limited, London.

The scrap must be reduced in size in particles having a maximum dimension equal to or less than one half inch (the particles need not have a regular shape, though that is preferred); if the particles are larger, then the process begins to suffer from all the disadvantages associated with the prior art processes. Indeed, it is most preferred if the particle maximum dimension is not more than one quarter inch (0.7 cm), with an average volume not more than one sixtieth of a cubic inch (0.35 cc), while excellent results are achieved using particles very much smaller.It would seem, in fact, that the smaller the particles the better the product - clearly, if scrap microcellular material containing cells of about one five hundredth of an inch (0.05 mm) is broken down to pieces with a maximum dimension of the same order there will be no cells left at all - though for purely economic reasons there seems little point in using particles of a maximum dimension of less than one tenth inch (0.25 cm), for any further improvements to the quality of the product are not particularly notable.

It is an important feature of the process of the invention that there should be employed therein scrap which has been broken down into particles all of which are roughly the same size, for if there is any major disparity in particle sizes - if, for example, a large proportion is considerably bigger than the average, while another large proportion is considerably smaller then the product will show some of the defects associated with the prior art processes (in particular it will show an undesirable non-uniformity of physical characteristics). By way of indication. it is preferred that at least 50% of the particles have a maximum dimension/volume within 1 50% of the respective average values, with at least 75% of the particles having a maximum dimension/volume within 1 75% of the average values.

Indeed, most preferably the numbers of particles within the 1 50% and + 75% maximum dimension volume limits are af least 75% and 95% respectively.

The sorting of the chopped scrap into pieces of roughly the same size can be done most simply by conventional sieving techniques, and need not be described further here.

The melting injecting and subsequent moulding steps used in the process of this invention may be carried out employing the usual techniques. Typical such machines for the melting, injecting and moulding steps are, for example, the "MANUMOLD" Hydraulic Unit and the DESMA '802' Injection Moulder.

The process of this invention, though particularly suited to the recovery of the sort of semi-thermoplastic scrap produced during the manufacture of microcellular elastomeric polyurethane shoe soles, is in fact capable of being used for the recovery of other sorts of polyurethane scrap, and for the recovery of polyurethane scrap mixed with plastics or other types - provided, always, that these can be melted and injection moulded. A typical semi-thermoplastic, flexible microcellular elastomeric polyurethane used for making moulded shoe soles is prepared by reacting together a polyol (normally a polyester polyol), a chain extender and a diisocyanate, in the presence. of a catalyst, a blowing agent and a cell-control surfactant.

Suitable polyols are polyester reaction products of aliphatic diols with aliphatic dicarboxylic acids, especially those reaction product polyesters having terminal hydroxyl groups and average molecular weights between about 500 and about 6,000, preferably between about 1,000 and about 4,000. Thus, this general class of polyester polyols would include polyesters derived from the condensation reaction between glycols such as ethane-, propane-, butane-, pentane- and hexane-diols, with dicarboxylic acids such as malonic, succinic, glutaric, adipic, pimelic, suberic, azelaic or sebacic acids. Mixtures of different diols and/or different dicarboxylic acids can of course be used, and other polyester polyols (derived from different starting materials, such as polyesters derived from E-caprolactone) may also be used.

Suitable chain extenders are glycols such as ethane-diol, 1,2- and 1,3- propane-diol, the isomeric butane-diols, isomeric pentane-diols and isomeric hexane-diols, diethylene glycol, and dipropylene glycol. Preferred diols are those in which both hydroxyl groups are of equal or similar reactivity. Particularly preferred diols are those in which both hydroxyl groups are primary hydroxyl groups.

Suitable diisocyanates are 2,4-tolylenediisocyanate (TDI), 4,4'-diphenyl methane diisocyanate (MDI), hexamethylene diisocyanate, and 4,4'-dicyclohexylmethanediisocyanate (hydrogenated MDI). MDI is particularly preferred.

Suitable catalysts are: tertiary amines, especially trialkylamines such as trimethylamine and triethylamine, and triethylene diamine; and metal compounds, especially fatty acid salts such as stannous octoate, di-n-butyl tin dilaurate, and lead octoate. Mixtures of metallic and amine catalysts can of course be used.

Suitable blowing agents are: low molecular weight halogenated hydrocarbons, especially chlorinated ones (such as methylene chloride) and fluorochlorinated ones (such as trichlorofluoromethane), and carbon dioxide (generated in situ from the reaction of added water with some of the isocyanate groups).

Suitable cell control surface active agents are especially those known as "silicone" surface active agents and which are alkyl-silicon polyoxyalkylene copolymers. Typical such materials are Silicones DC 190 or DC 193 (Dow Corning), Silicone L5420 (Union Carbide), Silicone B1903 (Theodore Goldschmidt) and Silicone LK 443 (Air Products).

Two specific formulations of the type used to make microcellular elastomeric polyurethane for shoe soles are: Formulation A.

Component % wt Polyester polyol [2,000 Mol. Wt.

poly(ethylene butyleneadipate)] 72.6 Chain Extender (1,4-Butane-diol) . 7.3 Diisocyanate (4,4'-diphenyl methane'diisocyanate) . 19.2 Catalyst (Triethylene diamine) . 0.3 Cell Control Agent (Silicone DC 193) . 0.4 Water . 0.2 100.0 Formulation B.

Component % wt.

Polyester polyol [1,000 Mol.Wt.

poly(ethylene butyleneadipate)j ........ ....... 24.55 Polyester polyol [4,000 Mol. Wt.

poly(ethylene butylene-adipate)] ....... ....... 49.75 Chain Extender (1,2-ethane-diol) . ....... 5.1 Diisocyanate (4 ,4'-diphenylmethane diisocyanate) .. 19.7 Catalyst (Triethylene diamine) . 0.15 Catalyst (Dibutyl tin dilaurate) . 0.15 Cell Control Agent (Silicone DC 193) . 0.4 Water . 0.2 100.00 The process of this invention at least partially solves the scrap recovery problem that has bedevilled the polyurethane-using industry for many years. By using the process there may be prepared moulded products which do not contain excessively large and undesirable air pockets, have not been subjected to considerably high temperatures, and do not suffer severely from oxidative degradation.Specific uses for the recovered scrap include shoe soles, shoe heels and shoe stiffening members, and these moulded articles can be used either from recovered polyurethane scrap alone or from a mixture of the scrap together with another thermoplastic such as PVC, polyethylene, polypropylene or ethylene/vinyl acetate copolymer.

The invention extends, of course, to polyurethane products prepared from scrap material recovered according to the defined process.

The following Examples and Test Results are now provided, though by way of illustration only, to show details of various aspects of the invention.

EXAMPLE 1 A. Inventive Process Strips of trimmed waste material from flexible microcellular elastomeric shoe soles prepared from Formulation A were granulated in a laboratory granulator (Junior Rotary Cutter model supplied by Blackfriars Rotary Cutters Limited) until all the granules were of much the same size and shape, and passed through the " diameter holes in the sieve of the machine. The collected granular material was fed into the hopper of a MANUMOLD Hydraulic Injection Moulder and injected into a dumbell test piece mould kept at 40"C.

Injection moulding barrel temperatures within the range from 1300C to 145do were found to be most suitable for injecting the plasticised material at the maximum available pressure of 81.5 kg/cm2. The test pieces so formed were removed from the mould after 1.5 minutes.

They displayed good mould filling, good mould pattern reproduction, and only very slight linear shrinkage (about 0.5%) after removal from the mould. The outer surfaces of the test pieces were for the most part smooth, except for occasional sink marks. Internally. the test piece appeared mainly solid with only a few small voids occurring randomly throughout the structure. The overall average specific gravity of the test piece was found by water displacement to be 1.075.

B. Comparison The scrap material, instead of being granulated, was simply cut with shears into irregular shaped pieces having typical dimensions of about 1" x 1" (2.5 cm x 2.5 cm), the thicknesses of these pieces varying between about 0.004" and 0.04" (0.1 mm and 1.0 mm).

The pieces were injection moulded (in the same way as the granulated material). The moulded test pieces so obtained were of poor surface appearance, displaying many 'sink marks' and small craters. Internally, also they showed many voids, some being very large (about 0.1" [2mm] diameter) in proportion to the thickness of the test piece (0.2" - 5 mm).

Significantly also the 0.2" (5 mm) diameter cylindrical 'sprue' pieces displayed a central hollow core of diameter about 0.1" (2 mm).

The greater extent of voiding was reflected in the average overall specific gravity of these test pieces of 0.97.

EXAMPLE 2.

The scrap material and equipment used in this Example were the same as used in Example 1, except that the " (0.6 cm) granulator sieve was replaced by a similar sieve having holes of approximate diameter 1/32" (0.08 cm). Grinding was continued only for a short time (to prevent overheating). Only a minor amount of the scrap was ground fine enough to pass through the sieve; this was then re-mixed with the retained material, to give granulated scrap with an overall average diameter. of about 1/16" (0.16 cm).

This finely ground material was injection moulded under the same conditions as in Example 1, and test pieces were obtained having a similar general appearance to those of Example 1. Internally, these test pieces appeared solid, as was reflected in their higher overall average specific gravity of 1.20.

The tensile strength of these test pieces was measured, using an Instron Tester (Model TTCM) according to British Standard Test Method BS 2782, and gave: Tensile Strength 115 kg/cm2 Elongation at break 498% EXAMPLE 3 In this Example the granulated scrap material of Example 1 was charged to the hopper of a Desma 802 Injection Moulder operating at a barrel temperature of 175"C. The plasticised material was injected into a children's shoe sole pattern mould at 40"C. After cooling for two minutes the mould was opened and found to have been well filled. The formed shoe sole unit was of good appearance, had reproduced the mould pattern faithfully, and showed no significant dimensional changes after demoulding. Internally its structure was mainly solid with few, randomly distributed very small voids.The shoe sole was judged suitable for use in children's shoes, as an alternative to other commonly used thermoplastic soling materials such as p.v.c., and showed the following test results: Overall Specific Gravity . 0.88 Durometer Hardness (Shore A) . . 81" Graves Tear Strength (kg/cm) . . 35 Flexing Resistance (Ross Flex Tester, - 20"C, % cut growth after 100,000 cycles*) . . nil *Up to 600% Cut Growth under these test conditions is considered an acceptable test result for satisfactory in-service performance.

WHAT WE CLAIM IS: 1. A process for the re-use of polyurethane scrap to produce moulded articles therefrom by injecting into an appropriate mould a melt of the scrap, in which process the scrap is first cut, chopped, milled, granulated or otherwise broken into particles all of substantially the same size and having an average maximum dimension of not more than one half inch (1.3 cms) and an average volume of not more than one eighth of a cubic inch (2 cc).

2. A process as claimed in Claim 1 in which the scrap particles have a substantially regular shape.

3. A process as claimed in Claim 1 or Claim 2 in which the particle maximum dimension is not more than one quarter inch (0.7 cm), with an average volume of not more than one sixtieth of a cubic inch (0.35 cc).

4. A process as claimed in any one of the preceding claims in which at least 50% of the particles have a maximum dimension/volume within +50% of the respective average values, with at least 75% of the particles having a maximum dimension/volume within 175% of the average values.

5. A process as claimed in Claim 4 in which the numbers of particles within the +50% and 175% maximum dimension/volume limits are at least 75% and 95% respectively.

6. A process as claimed in any one of the preceding claims wherein the polyurethane is a microcellular elastomeric polyurethane.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. Significantly also the 0.2" (5 mm) diameter cylindrical 'sprue' pieces displayed a central hollow core of diameter about 0.1" (2 mm). The greater extent of voiding was reflected in the average overall specific gravity of these test pieces of 0.97. EXAMPLE 2. The scrap material and equipment used in this Example were the same as used in Example 1, except that the " (0.6 cm) granulator sieve was replaced by a similar sieve having holes of approximate diameter 1/32" (0.08 cm). Grinding was continued only for a short time (to prevent overheating). Only a minor amount of the scrap was ground fine enough to pass through the sieve; this was then re-mixed with the retained material, to give granulated scrap with an overall average diameter. of about 1/16" (0.16 cm). This finely ground material was injection moulded under the same conditions as in Example 1, and test pieces were obtained having a similar general appearance to those of Example 1. Internally, these test pieces appeared solid, as was reflected in their higher overall average specific gravity of 1.20. The tensile strength of these test pieces was measured, using an Instron Tester (Model TTCM) according to British Standard Test Method BS 2782, and gave: Tensile Strength 115 kg/cm2 Elongation at break 498% EXAMPLE 3 In this Example the granulated scrap material of Example 1 was charged to the hopper of a Desma 802 Injection Moulder operating at a barrel temperature of 175"C. The plasticised material was injected into a children's shoe sole pattern mould at 40"C. After cooling for two minutes the mould was opened and found to have been well filled. The formed shoe sole unit was of good appearance, had reproduced the mould pattern faithfully, and showed no significant dimensional changes after demoulding. Internally its structure was mainly solid with few, randomly distributed very small voids.The shoe sole was judged suitable for use in children's shoes, as an alternative to other commonly used thermoplastic soling materials such as p.v.c., and showed the following test results: Overall Specific Gravity . 0.88 Durometer Hardness (Shore A) . . 81" Graves Tear Strength (kg/cm) . . 35 Flexing Resistance (Ross Flex Tester, - 20"C, % cut growth after 100,000 cycles*) . . nil *Up to 600% Cut Growth under these test conditions is considered an acceptable test result for satisfactory in-service performance. WHAT WE CLAIM IS:

1. A process for the re-use of polyurethane scrap to produce moulded articles therefrom by injecting into an appropriate mould a melt of the scrap, in which process the scrap is first cut, chopped, milled, granulated or otherwise broken into particles all of substantially the same size and having an average maximum dimension of not more than one half inch (1.3 cms) and an average volume of not more than one eighth of a cubic inch (2 cc).

2. A process as claimed in Claim 1 in which the scrap particles have a substantially regular shape.

3. A process as claimed in Claim 1 or Claim 2 in which the particle maximum dimension is not more than one quarter inch (0.7 cm), with an average volume of not more than one sixtieth of a cubic inch (0.35 cc).

4. A process as claimed in any one of the preceding claims in which at least 50% of the particles have a maximum dimension/volume within +50% of the respective average values, with at least 75% of the particles having a maximum dimension/volume within 175% of the average values.

5. A process as claimed in Claim 4 in which the numbers of particles within the +50% and 175% maximum dimension/volume limits are at least 75% and 95% respectively.

6. A process as claimed in any one of the preceding claims wherein the polyurethane is a microcellular elastomeric polyurethane.

7. A process as claimed in Claim 1, substantially as hereinbefore described.

8. A process as claimed in Claim 1, substantially as hereinbefore described in any one of the specific Examples.

9. Polyurethane products prepared from scrap material recovered according to a process as claimed in any of the preceding claims.