GB2114585A - Leather-like materials - Google Patents

Abstract

A microporous polyurethane foil or layer characterised by possessing a density in the range of 45% to 75% of the density of a void free film of the said polyurethane, a water vapour permeability of at least 25/g/m<2>/hr, a thickness of at least 100 microns, at least one free surface of the foil or layer having a Taber abrasion loss sum (as defined herein) of not more than 60. The foil may be laminated to a strength imparting substrate, e.g. a leather or hide product, or an artificial product, such as, a needle punched and polymer impregnated felt, or a woven or knitted material. Uses:- Shoe uppers.

Description

SPECIFICATION Leather-like materials The present invention relates to the production of water vapour permeable foils and layers having grain leather-like appearance and improved abrasion resistance. It also relates to the production of leather-like materials having improved abrasion resistance incorporating a strength imparting portion for example derived from natural leather provided with the said foils as an artificial grain appearance layer. The invention extends to the products themselves and to methods of making the products.

The present invention in a preferred aspect is particularly concerned with producing materials having improved abrasion resistance whilst still retaining adequate water vapour permeability.

We have discovered that compressing the surface of a microporous polyurethane layer or foil at least 0.1 mm thick so as to impart a glossy surface or an embossed surface or both results in a reduction of water vapour permeability and an increase in abrasion resistance as might be expected. However when, instead of applying the embossing temperature and pressure briefly from one surface (that which will be the free surface in use) when consolidation is localized at the surface next to the heat source, steps are taken to achieve heating through the thickness of the material, the increases in density and abrasion resistance that can be achieved are very substantial whilst surprisingly the water vapour permeability remains at a significant level useful for shoe upper and other such apparel purposes.

Thus according to one aspect of the present invention a microporous polyurethane foil or layer is characterised by possessing a density in the range of 45% to 75% e.g. 50 to 65% of the density of a void free film of the said polyurethane, a water vapour permeability of at least 25 and preferably at least 30 gr/m2/hr, a thickness of at least 100 and preferably at least 1 50 or more especially at least 200 microns or even at least 300 microns, at least one free surface of the foil or layer having a Taber abrasion loss sum (as defined herein) of not more than 60.

The product of the invention may be a free standing foil which is useful for lamination to a substrate in a subsequent step or may be a layer forming part of a thicker material and thus an integral component or surface zone of such a layer.

Taber abrasion loss sum is defined herein as the weight loss suffered by the said free surface of a sample of the material when subjected to Taber abrasion testing using freshly faced H-22 Calibrade wheels under a 1000 g load and is the sum of the weight loss in milligrams after 200 revolutions of the sample and after 300 revolutions of the sample.

According to another aspect of the present invention a kit of parts for assembly into a grain leather-like material comprises a strength imparting substrate, e.g. a leather substrate, provided on its surface, which will be inside the laminate, with adhesive means, and a microporous polyurethane layer or foil at least 0.1 mm thick, the surface of the foil which will afford an outside surface of the laminate being provided by a layer or foil in accordance with the first aspect of the invention.

The adhesive means may be directly adhesive or may be activatable to afford an adhesive effect.

The layer or foil preferably has a grain appearance which is substantially undamaged by heating to 130"C while stretched by up to 20% for 2 minutes.

Examples of materials useful as the substrates for the foils of the present invention include cowhide splits, damaged grain leathers, side leathers, skivers and goatskin, cabretta, pig or sheep skin or indeed any other leather product or hide product or artificial product, which provides a sufficient strength for the end purpose to which it is intended to put the material but which has a surface appearance which needs upgrading to a more grain leather-like appearance.

Examples of artificial products useful as the substrate include fibre reinforced products such as needle punched and polymer impregnated felts such as disclosed in G.B. specifications 914711, 91 471 2 and 914713, woven or knitted materials or combinations thereof such as disclosed in G.B. 1002225 or fibre free materials such as disclosed in G.B. 1217341.

The leather substrate may comprise leather splits or low substance (low thickness) leather and is preferably substantially free from sizing and/or filling.

The adhesive treated substrate is preferably allowed to dry before lamination of the layer or foil, and the laminated material and its constituent parts prior to lamination are preferably all water vapour permeable.

The microporous polyurethane layer or foil is preferably one which has been preformed or prefinished at a temperature which is greater and preferably at least 1 0,C or 20"C greater and especially at least 30"C greater than the maximum temperature encountered by it during the lamination.

Where the layer or foil is preformed it may have been embossed by a contoured roller or flattened e.g. glossed by a smooth roller. It will preferably have been preformed at a temperature in excess of 120"C.

The polyurethane layer or foil is preferably 0.2 mm to 1.5 mm thick and it may be coloured.

The foil may be coloured by being dyed or pigmented through its bulk and in addition or alternatively it may be given a surface colouration by a surface finishing process, e.g. by a solvent spray finish and heated air blast such as disclosed in British Patent Specification No.

1190551 or by a sprayed or printed or coated thin solvent and polymer containing layer with simuitaneous heating, e.g. by an air blast as in British Patent Specification Nos. 1 325850 and 1 324541 or by a thin coagulated and solvent evaporated coating process such as described in British Patent Specification No. 1 325928 optionally followed by one or both of the two earlier referred to processes.

Such surface colouration processes are preferably carried out prior to the consolidation which is undertaken.

Consolidation may be carried out using embossed rollers with or without an intervening release sheet so as to produce any desired pattern of indentations and lines so as to replicate any natural skin finish or embossed pattern which may be desired. A gloss finish either embossed or plane can be achieved by the interposition of a strippable polymer film and when a plane roller is used this can produce a high gloss finish such as is referred to as a "patent leather" finish. A process for producing such a finish using an interposed smooth film and defined conditions of temperature and pressure is described in British Patent Specification No.

1352512.

In that specification consolidation is restricted to a thin surface zone not in excess of 100 microns thick the material being heated from the free surface only on an embossing roller.

This resulted in a very steep temperature gradient through the material and no doubt resulted in the consolidation being essentially restricted to one face of the material.

We have found that if the back up roller in a roller embossing machine is heated e.g. to temperatures similar to, e.g. preferably within 40"C of, those to which the embossing roller is heated then substantially greater consolidation occurs and the advantages of the present invention are obtained.

When a flat bed press is used the use of longer dwell times achieves the same effect and here the retention of adequate water vapour permeability is all the more surprising.

A further aspect of the invention is that it provides the means for increasing the substance (thickness) of the leathers and splits processed according to the invention to a level which makes them satisfactory for use as upper leather material for the manufacture of footwear. Thin bovine splits at about 1.3-1.4 mm and low substance leathers such as Indian Kips at 0.7-1.1 mm and pigskin which is generally 0.8-1.3 mm thick, are relatively plentiful and inexpensive but despite adequate mechanical strength are considered too low in substance for use in shoe uppers.

As during its lamination to leathers and splits by the process of the invention the polyurethane foil itself undergoes a reduction in thickness that can be allowed for in advance, its thickness can be selected in such a way as to raise the substance of the leathers or splits to be laminated, to the level required. This level is generally 1.5 to 2.2 mm, frequently 1.7 to 2.0 mm.

Another aspect of the present invention is that its products, which can consist largely of leather, can be processed in shoe manufacturing factories in a manner very similar to natural leather.

A still further aspect of the present invention is that its products can combine the hygiene and comfort properties (moisture absorption and permeability to water vapour) associated with natural leathers, with the superior functional properties (scuff- and abrasion resistance, resistance to water penetration and easy clean characteristics) typical of high quality synthetic upper materials.

The invention may be put into practice in various ways and a number of specific examples will be given to illustrate the invention.

EXAMPLE 1Preparation of the microporous foil.

A homogeneous dispersion of Micronised sodium chloride, mean particle size 10 micron 300 parts Thermoplastic polyurethane polymer solution, 33% solids in dimethylformamide (DMF) 300 parts DMF 100 parts Carbon black 5 parts was extruded onto a moving woven stainless steel endless carrier. The extrudate was next coagulated to sheet form by passing it through a tank of a water salt DMF blend, containing 12% DMF and 8% sodium chloride. The sheet was next stripped off the carrier and residual sodium chloride and DMF leached out by passing the sheet through a series of hot water tanks and pressure nips. The sheet was then dried.

The dried sheet had a microporous open cell structure, was 0.5-0.6 mm thick, weighed 180-200 g per square metre and its water vapour permeability was 1 80 g per square metre per hour. It had a density of about 0.35 g/cc.

The material was then coated as follows: The material was soaked in water at room temperature (using vacuum impregnation to thoroughly and rapidly fill its pores with water). It was then passed between squeeze rolls to express some of its water so that it retained about 80% of water based on the dry weight of the sheet.

While thus wet, it was coated on its upper surface with a brown pigmented solution of elastomeric thermoplastic polyurethane which solution contained 10% solids, the balance being an equal parts mixture of DMF and cyclohexane, the solids content of the solution was made up of pigment and binder (polyurethane), the pigment being a mixture of red iron oxide, yellow iron oxide and a small amount of black to give a brown colouration. The viscosity of the solution was 40 seconds measured with a Ford No. 4 cup at 23"C.

The coating was done on a reverse roll coating machine and the sheet was passed through an oven and dried at 120-130 C to remove the water and solvents. The amount of lacquer solids deposited was 4 to 8 g/sq.metre.

The coloured sheet so produced was 0.5 to 0.6 mm thick and had a water vapour permeability of 55 g per square metre per hour.

The product was then separated into two samples and subjected to different embossing conditions on the same roller embossing machine. This had a heatable embossing roller and a Teflon coated heatable backup roller.

EXAMPLE 2 The material from Example 1 was passed between an engraved heated upper roller, to which the face which was to be the outer or surface face in use was juxtaposed, which roller was heated to a surface temperature of 140 C, and a water cooled Teflon covered rubber padded lower or backup roll, at speed of 3.6 metres per minute.

The steam pressure in the upper roller was 90 p.s.i. and the pressure between the rolls was 80 p.s.i. The upper roll was about 7 inches (17.75 cms) in diameter and the lower roll about 9 inches in diameter. Thus the average pressure between the rolls in the absence of material would be 6.1 kgs/cm linear.

The rubber roll was distorted by the pressure so that it was in contact with upper heated roll over a length on the direction of movement of the roll surfaces of about 3 inch. The material was passed straight through the nip without wrapping round the heated roll either before or after the nip and thus was in contact with the heated roll for about r second.

The thickness of the foil after this embossing was 0.55 mm, it weighed 195 g/square metre, its density was about 0.35 and its water vapour permeability was 45 g per square metre per hour.

EXAMPLE 3 Example 2 was repeated except that the backup roll was heated to 102-104"C with steam, the back surface temperature of the foil rising to about 130-135 C as indicated by a surface contact pyrometer, and the line speed was 2.4 metres/minute.

The thickness of the foil after this embossing was 0.26 mm, it weighed 1 81 9 per square metre, its density was about 0.7 and its water vapour permeability was unchanged from Example 1.

EXAMPLES 4 and 5 The foils produced by Examples 2 and 3 were then laminated to a 1.7 mm thick chrome tanned bovine leather split weighing 1 550 g per square metre using the same laminating conditions.

The thickness of the laminates were 2.1 and 1.95 mms and the water vapour permeabilities 35 and 45 9 per square metre per hour respectively.

Conventional Taber abrasion weight loss measurements were then carried out using freshly faced H-22 Calibrade wheels under a 1000 g load. A Ford Chisel head test was also carried out.

Table 1 below gives the results.

TABLE 1 Example 4 5 Wt. loss in mg. after 100 15 6 revolutions of the sample beneath the wheels.

200 55 18 300 65 36 400 77 38 500 91 48 600 - 65 Taber abrasion loss sum 1 20 54 (sum of weight loss after 200 and 300 revs.) Ford Chisel head 25 revs. wrinkling of test surface 75 revs. Breaking and wearing through top skin 500 revs. only very slight surface marking EXAMPLE 6 The lamination procedure used in Examples 4 and 5 was repeated using the foil of Example 3 and as the substrate a 1.2 mm thick non-woven material. This was a fibre matt made of a mixture of nylon, polyester and viscose rayon fibres bound with a nitrile rubber latex with a fibre to binder ratio of about 60:40 by weight. The substrate weighed 420 9 per square metre.

The laminate was about 1.45 mm thick.

Taber abrasion resistance tests were then carried out as in Examples 4 and 5 and the results are given in Table 2 below.

TABLE 2 Example 6 Wt. loss in mg after 100 9 revolutions of the sample beneath the wheels.

200 18 300 19 400 22 500 23 600 23 Taber abrasion loss sum 37 (sum of weigh loss after 200 and 300 revs.) EXAMPLE 7 Example 1 was repeated using a smaller amount of larger salt (190 parts, 30 microns) and concentrated polyurethane solution (33% solids; 300 parts) (with no added DMF) to produce a material which before finishing by coating had a thickness of 0.6 mm, a weight of 296 9 per square metre and a water vapour permeability of 100 9 per square metre per hour and a density of about 0.49 g/cc.

After coating as in Example 1 its thickness was 0.58 mms its density 0.51 and its water vapour permeability 50 9 per square metre per hour.

EXAMPLES 8 and 9 Examples 2 and 3 were then repeated using the coated product of Example 7. The material of Example 8 (Example 2 procedure) had a thickness of 0.55 mm a weight of 270 9 per square metre, a density of 0.53 g/cc and a water vapour permeability of 50 9 per square metre per hour. The material of Example 9 (Example 3 procedure) had a thickness of 0.48 mm, a weight of 270 9 per square metre, a density of 0.56 g/cc and a water vapour permeability of 40 g per square metre per hour.

EXAMPLES 10, 11 and 12 The products of Examples 7, 8 and 9 were then laminated to a bovine split as described in Examples 4 and 5.

Conventional Taber abrasion testing wås then carried out as in Examples 4 and 5 and the properties of the laminates and the Taber results are given in Table 3 below.

TABLE 3 Example 10 11 12 Overall thickness (mm) 2.0 1.85 2.1 5 WVP (g/m2/hr) - - 35 Wt. loss in mg. after 100 10 11 8 revolutions of the sample beneath the wheels.

200 34 26 13 300 51 44 20 400 70 58 28 500 91 68 43 600 - - - Taber abrasion loss sum 85 70 33 (sum of weight loss after 200 and 300 revs.) EXAMPLES 13, 14, 15 and 16 A material made as described in Example 1 was subjected to hot plate embossing using a small hydraulically operated press with an electrically heated square flat (unembossed) platen to which the outer surface of the foil was juxtaposed.

Details of the embossing conditions and the properties of the material are given in Table 4 below.

TABLE 4 Example 13 14 15 16 Platen temp. ("C) - 1 30 1 30 1 30 Pressure (p.s.i.) - 680 680 680 Dwell time (secs.) - 5 10 20 Thickness of product (mm) 0.52 0.49 0.36 0.23 Weight (g/m2) 1 89 200 203 204 Density (g/cc) 0.36 0.41 0.56 0.89 WVP (g/m2/hr) - - - 32 EXAMPLE 17 The product of Example 1 6 was laminated to a bovine split as described in Examples 4 and 5 and conventional Taber abrasion tests carried out, the results of which and the properties of the product being shown in Table 5 below.

TABLE 5 Example 17 Overall thickness (mm) 1.6 WVP (g/m2/hr) 25 Wt. loss in mg. after 100 4 revolutions of the sample beneath the wheels.

200 7 300 11 400 19 500 40 600 Taber abrasion loss sum 1 8 (sum of weight loss after 200 and 300 revs.) Water vapour permeability was measured by the method described in British Patent Specification No. 1 273524 at page 1 2 lines 44 to 62.

The products had an attractive uniform grain leather-like appearance. The substance of the laminated leathers was 0.25 to 0.55 mm higher than it would have been had the leathers been processed under similar conditions without the polyurethane foil. The products had ample permeability to water vapour for comfort in wear when made into shoes, a scuff resistance substantially better than that of most conventional leathers, a high degree of resistance to water penetration and easy clean properties typical of high quality synthetic upper materials.

Details of the nature of the polyurethane from which the foil is made and of the foil itself prior to preforming or prefinishing are given in British Patent Specification No. 1352512 referred to above to which attention is directed.

Details of how the open cell microporous foil may be made and prefinished are also given in the same specification and attention is directed thereto in those respects as well.

The laminate should desirably permit the passage of water vapour; thus its water vapour transmission should be at least 200 g/m2/24 hours (measured as in ASTM E 96-66, procedure B). Also it is desirable that at least the upper surface of the laminate, after suitable finishing, be resistant to the passage of liquid water, e.g. the finished laminate should have a hydrostatic head (British Standard 2823/1957) of above 100 mm Hg.

All measurements referred to herein are made at room temperature (e.g. 23"C) unless the test method specifies otherwise.

Typical embossing patterns conventionally used in the artificial leather art may be used as the pre-embossing in the present invention.

The kit of parts, product and process of the invention are applicable in the industrial scale production of enhanced appearance leather based products wherein low quality leather materials are given improved appearance and properties and added value.

Claims (12)

1. A microporous polyurethane foil or layer characterised by possessing a density in the range of 45% to 75% of the density of a void free film-of the said polyurethane, a water vapour permeability of at least 25/g/m2/hr, a thickness of at least 100 microns, at least one free surface of the foil or layer having a Taber abrasion loss sum (as defined herein) of not more than 60.

2. A microporous polyurethane foil or layer as claimed in Claim 1 having a density in the range 50 to 65% of the density of a void free film of the said polyurethane.

3. A microporous polyurethane foil or layer as claimed in Claim 1 or Claim 2 having a water vapour permeability of at least 30 g/m2/hr.

4. A microporous polyurethane foil or layer as claimed in Claim 1, 2 or 3 having a thickness of at least 1 50 microns.

5. A microporous polyurethane foil or layer as claimed in Claim 1, 2, 3 or 4 having a thickness of at least 300 microns.

6. A microporous polyurethane foil as claimed in Claim 1 substantially as specifically described herein with reference to any one of Examples 3, 9, 1 5 or 1 6.

7. A microporous polyurethane foil as claimed in any one of Claims 1 to 6 whenever laminated to-a strength imparting substrate.

8. A product as claimed in Claim 7 substantially as specifically described herein with reference to any one of Examples 5, 6, 12 or 1 7.

9. A kit of parts for assembly into a grain leather-like material comprising a leather substrate provided on its surface, which will be inside the laminate, with adhesive means, and a microporous polyurethane foil as claimed in any one of Claims 1 to 6.

10. A kit of parts as claimed in Claim 9 substantially as specifically described herein with reference to the Examples.

11. A grain leather-like material comprising a leather substrate and a microporous polyurethane layer as claimed in any one of Claims 1 to 6 adhered to the substrate.

12. A grain leather-like material substantially as specifically described in any one of Examples 5, 12 or 17.