Written by:

Grigory Kovalenko1, Elena Bokova1, Maria Pawłowa2
1 Moscow state university of design and technology, Sadovnicheskaya ul., 33-1, Moscow, Russian Federation
2 Uniwersytet Technologiczno-Humanistyczny im. Kazimierza Pułaskiego w Radomiu., Jacka Malczewskiego 29, Radom, Poland

ABSTRACT

The work carried out systematic studies on the development of synthetic footwear and wardrobe membrane materials. The objects used in the study: the solutions of polyurethanes synthesized two-step synthesis, based on 4,4-diphenyl methane diisocyanate and different oligoesters – simple (polytetramethylene glycol) and complex (polyethylene butylene glycol adipate) at a ratio of NCO: OH = 1: 1; and the nonwoven fabric obtained from polyamide solution electrospinning method. The conditions obtaining membrane material with a porosity of 96-98% and a specific surface of 23-24 m2/g. The presence of membrane materials developed in pores with a diameter from 0,01 to 1,41 microns, which is the total contribution of a number of micro-fiber structure, pores formed by phase separation of solutions of polyurethanes in the medium non-solvent, and additional system micropores in local delamination of the polymer from the fiber matrix.

The technology of production of membrane materials for shoes and clothing, with high operational properties: water vapor permeability – 4,4-5,8 mg/(cm2·hour), hygroscopic – 8,6-10%, water yielding – 98-99%, the limit tensile strength – 6-6,4 MPa, elongation – 210-240%.

Keywords: membrane, synthetic leather, polyesterurethane, electrospinning, phase separation, vapor permeability

INTRODUCTION

Membranes are high technology products of inter-branch application, without which the breakthrough development of basic and high-tech sectors of the economy, science, as well as effective solution of the important tasks of social and environmental problemsis impossible. [1].

The largest producers of membrane materials, varying in their properties, in the last five years are such companies as W. L. Gore, Dermizax (Spyder, Descente), AWT (Killy), DIAPLEX (Phenix), Venture (Schoffel), Helly Tex (Helly Hansen), Sensor Tex (Volkl), ATX (EVF).

A sufficiently large segment of the market is occupied by the membrane materials for apparel and shoes production. According to several manufacturers, such membranes represent a sufficiently large range of waterproof and breathable polymer materials designed for extreme and high-functional clothes and shoes. [2]

It is known from literature, that there are various methods for producing (forming) membranes, such as extrusion, laminating, sintering, electrospinning, phase separation, etc. The last two are the most common methods for receipt of apparel and footwearmaterials, as they provide the possibility of a wide variation of morphology and porosityand allow you to form the membrane with a gradient pore system.

Thus, using the method [3] ofthe phase separation of the polyurethane solution in dimethylformamide,the series of membranes for apparel and shoes productionhas been prepared and the effect of temperature of the phase separation on the porous structure, mechanical properties and gas permeabilityhave been studied.

During the process [4] the polyurethane membranes were prepared by electrospinning.The authors have investigated the performance of membrane sanitary properties; they have proved its high oxygen permeability and controlled water vapor permeability.

The authors [5] have obtained waterproof breathable membrane, which is a composite material consisting of a synthetic textile with a coating of a nonwoven fabric.Non-woven fabric was obtained from a solution of a polyesterurethane (PEU) in dimethylacetamideusing the electrospinning method. The composite material showed a high air permeability, vapor permeability and insulating properties, but had lower resistance to water (water resistance) in comparison with a foil membranes prepared by phase separation.

Based on literature data analysis, it is obvious that the membrane material for clothing and footwear should have a number of special properties, such as steam and gas permeability, durability, water resistance, strength, etc. Such a set of operational characteristics can only be achieved by creating composite polymeric materials.

The purpose of the work is development of scientific bases and process solutions to produce PEU membrane materials such as “synthetic leather” on the basis of non-woven fibrous fabric, received by electrospinningmethod.

MATERIALS AND EXPERIMENTAL METHODS

To receive the nonwoven material by electrospinning technique they used the following: polyamide 6/66 synthesized on the basis of the AH-salt (salt of hexamethylenediamine and adipic acid) and ε-caprolactam. The molecular weight of the product was – 30 kDa.

To prepare the impregnating solution, they used polyesterurethane (PEU) of Vitur TM-0533-90 (Russia), obtained by one-step synthesis at interaction with 4,4′-diphenylmethane diisocyanate and polyoxytetramethyleethylene glycol (oligomer tetrahydrofuran) (ratio NCO: OH – 1:1, average molecular weight of product – 40kDa), and polyether urethane (PEU) of Vitur P 0112 (Russia), resulting in a two-step synthesis at interaction with 4,4′ – diphenylmethane diisocyanate and polyethylene butylene glycol adipate (ratio NCO:OH – 3:1, the average molecular weight of the product – 30 kDa). N,N-dimethylformamide was used as the solvent for PEU.

Investigation of the structure of nonwoven materials was determined by scanning electron microscopy instrument «PHENOM» (USA).The surface was measured according to ISO 9864-90, bulk density – acc. to ISO 9073-2:1995. The study of the structure of membrane materials was carried out: by low-temperature nitrogen adsorption with the help of the instrument Gemini VII 2390, manufactured by the company «Micromeritics» (USA).Water vapor permeability of the membrane was determined acc. to ISO 2528, hygroscopicity and water-yielding capacity – acc. to ISO 811-81, sorption capacity – acc. to ISO 12500-2-2009. Physical and mechanical properties of the membrane materials were measured acc. to ISO-4674.

RESULTS

One of the main load-bearing elements of modern membrane materials such as “synthetic leather” are non-woven fibrous bases produced by the aerodynamic method of forming the canvas from a mixture of polyester and polypropylene (70:30) fibers, followed by needling and heat shrinkage. Such materials have a sufficiently large thickness of about 7 mm, so it necessary to skive it for 2-3 layers. The linear density of individual fibers in such fabrics ranges from 0.33 to 0.44 tex, surface density – 350-700 g/m2, bulk – 150-220 kg/m3, the total porosity – 76-80%.

Currently, the multifibrillar fibers with linear density 0.1 – 0.01 tex and diameter of several micrometers are gaining increasing importancein the production of synthetic leather;they are obtained from bicomponent fibers “matrix” (e.g., high pressure polyethylene) – “fibril” (e.g. polyester) by extracting the “matrix” boiling organic solvent (e.g., xylene, toluene, etc.) in the already formed and treated with polymeric binder nonwoven base.

This technological process, pretty complicated from ecological safety viewpoint, is aimed at reducing the dimensional features of the fibers in the nonwoven fabric, changing the structure of nonwoven webs, increasing their total porosity (90-95%), geteroporosity, specific surface, vapor permeability without a change in hydrophilicity and sorption activity with respect to water vapor.

To solve the similar problems the electrospinning method for nonwoven materials as one of possible waysto obtain highly porous structures of a wide range of polymers including those of hydrophilic nature has been used.

When choosing a spinning solution for electrospinning,the necessity ofinsolubility condition of nonwoven webs in dimethylformamide (DMFA) was a precondition, taking into account its subsequent impregnation with a solution of polyether urethane (PEU), as well as providing hydrophilic materials prepared for its comfortable operating in contact with the human.Polyamide solution PA 6/66 in a mixture of ethanol (70 wt.%) – water (30 wt.%) was chosen from the wide range of analyzed polymer solutions, used to prepare the materials by the electrospinning method, and satisfying these requirements.

Nonwoven fabricshave been obtained in the laboratory of aerosols of Karpov Scientific Research Institute of Physics and Chemistry (NIFKhl) with a help of NanospiderTM (Elmarco, Czech Republic) using the following parameters: concentration of the polyamide solution – 15%, the viscosity of the solution – 0.4 Pa•s, conductivity – 0.11 cm/m, the voltage – 30V, the volumetric flow rate of 30 ml/hour, the distance between the electrodes – 20 cm. The structure of the formed fabric is shown in Fig. 1

Fig. 1. Microphotographof nonwoven material from solution of PA 6/66 Image magnification 2500x.
Fig. 1. Microphotographof nonwoven material from solution of PA 6/66
Image magnification 2500x.

The diameter of the fibers in such material ranges from 0.8 to 1.3 microns. Bulk density is 100-110 kg/m3, surface density – 25-30 g/m2, which is about 25 times less than in the nonwoven webs obtained by needling technology.

Analysis of modern synthetic leathers demonstrates that the total contribution in the structure, properties and behavior in the operation are made by nonwoven base, the polymeric binder and the final morphology of the material formed as a result of impregnation, phase separation and subsequent washing and drying operations [6].

15% solutions PEU of Vitur TM-1413-85 and Vitur 0533-90-TM in DMFA were used for impregnating the nonwoven bases. 30% DMFA solution in water at 20 ± 5 °C was used as the coagulation bath. Washing was conducted with water at a T = 20 ± 5 °C, and the drying – in heat chamber at a temperature of 100 ± 10 °C.When selecting a temperature of the phase separation the undesired shrinkage of the hydrophilic nonwoven baseobserved at the stage of impregnation and phase separation was taken into account.

Fig. 2 shows microphotographs of the structure of membrane materials such as “synthetic leathers”.

Fig. 2. The microphotographs of structure of membrane materials such as "synthetic leathers": a - on the basis of PEU TM-0533-90 (image magnification 1500x) c, d - on the basis of PEU TM-1413-85 (image magnification 1500x, and 5000x, respectively). Composition of the precipitation bath - 30% solution of DMFA in water. The temperature of phase separation – 20 °С, the drying temperature – 100 °С.
Fig. 2. The microphotographs of structure of membrane materials such as “synthetic leathers”: a – on the basis of PEU TM-0533-90 (image magnification 1500x) c, d – on the basis of PEU TM-1413-85 (image magnification 1500x, and 5000x, respectively). Composition of the precipitation bath – 30% solution of DMFA in water. The temperature of phase separation – 20 °С, the drying temperature – 100 °С.

A distinctive feature of the membranes obtained during the process is the presence of a strongly marked fibrous structure in the microporous PEU matrix, as well as the absence of the expressed surface gradient layer in almost all the samples.This is due to both, the impregnation features of ultrathin nonwoven webs, and kinetic factors in the process of structure formation at the stage of phase separation of PEU solutions. Fast uniform impregnation, virtually simultaneous phase separation throughout the nonwoven webs consisting of a number of micro-fibers, results in materials of similar appearance and organoleptic to natural leather.

During the investigations, it was established that as the result of processing nonwoven base with a polymeric binder by the impregnationmethod followed by the phase separation in a non-solvent medium, the diameter of the fibers in the semi-finished product of synthetic leather after the drying process practically does not change and is about 1.3-1.5 microns.

However, the photographs clearly show the presence of additional microvoids between the microfibers and astabilizated polymeric binder, which is a consequence of detachment of the hydrophilic fibers from hydrophilic-hydrophobic polymer matrix during the drying process. Specific surface of membrane materials and pore sizes were measured by low-temperature nitrogen adsorption.For samples based on solutions PEU Vitur TM-1413-85, specific surface area is 24.1 m2/g with a correlation 0.989, and for the samples based on PEU TM-0533-90 – 23.9 m2/g with a correlation 0.992. The presence of pores are determined in developed synthetic leathers with a diameter from 0.01 to 1.41 microns, which is the total contribution of pores to the structure of micro fibers formed as result of PEU solutions astabilization, as well as an additional system of micropores, which are formed during the drying process due to the multi-directional action of the forces of capillary contraction.

The Table 1 shows the indicators of performance properties of membrane materials such as “synthetic leather”.

 

Indicators

Membrane material based on Vitur TM-1413-85Membrane material based on Vitur TM-1413-90

Thickness, mm

1.3

1.5

Vapor permeabilty, mg/(cm2∙hour)

4.4

5.8

Hygroscopicity, %

10

8.6

Water-yielding capacity, %

9.8

7.4

Sorption capacity, g/g

0.2

0.25

Tensile strength, MPa

3.5

4.1

Tensile strain at break, %

210

240

 

High levels of vapor permeability, hygroscopicity, water-yielding capacity of synthetic membrane materials obtained in the work are related to the formation of a highly open-cell structure consisting of the hydrophilic polymer microfibers. High rates of tensile strength at low thickness of these materials are provided with the greater binder weight gain in experimental synthetic membrane materials (up to 2 g/g), the presence of the reinforcing component in the form of a solid, as compared with polypropylene and polyester, polyamide fibers, a different mechanism of deformation and destruction of electroformed webs, compared with needle-punched fabrics.

In general, the availability of received materials together with heteroporous structure, developed system of micropores, hydrophilic fibrous skeleton, as well as high levels of hygienic properties, can be used to create high-performance “smart” materials, working closely with a human.

CONCLUSIONS

  1. The systematic study and science-based approach were conducted to receipt membrane materials such as “synthetic leather” on the non-woven base, formed by the electrospinning method, and PEU solutions impregnated into the structure of the nonwoven fabric by phase separation in non-solvent medium.

  2. The conditions of the preparation of membranes on the non-woven base from the polyamide solution 6/66 and new brands of PEU (Vitur TM-1413-85 and Vitur TM-0533-90) were also developed.

  3. It was revealed that the porosity of the received membrane material (~ 96-98%) corresponds to the porosity of synthetic leather comprising microfibrillar fibers, and the specific surface area was 23-24 m2/g, which is much higher than that of industrial analogue.

  4. The modified production technology of membrane materialswas also proposed, allowing to expand the range of competitive synthetic leathers with geteroporous structure, leather-like organoleptic properties and high levels of operational properties, such as: vapor permeability – 4.4-5.8 mg/(cm2 per hour), hygroscopicity – 8.6-10%, water yielding capacity – 98-99%, tensile strength – 6-6.4 MPa, and elongation – 210-240% withno extra cost and increase in environmental risks.

REFERENCES
  1. http://www.memtech.ru/index.php/ru/glavnaya/publications/98-membrany-i-nanotekhnologii. Last accessed date: 20.01.2016
  2. http://www.keeptex.ru/dealer/membrany_porelle/. Last accessed date: 21.01.2016
  3. Oprea S., Ciobanu C. 2008. Effect of the Temperature of Polyurethane Wet-Casting Membrane Formation on the Physico-Mechanical Properties. High Performance Polymers, 20(2):208-220
  4. Myung-Seob Khil, Dong-Il Cha, Hak-Yong Kim, In-Shik Kim, Narayan Bhattarai. 2003. Electrospun nanofibrous polyurethane membrane as wound dressing. Journal of Biomedical Materials Research, Part B: Applied Biomaterials, Volume 67B, Issue 2: 675–679.
  5. Yun K. K., Hee Ch., Kim J., Kang T. J. 2007. Application of electrospun polyurethane web to breathable water-proof fabrics. Fibers and Polymers, Volume 8, Issue 5:564-570.
  6. Bokovа E. S., Kovalenko G. M., Lavrent’ev A.V., Kalinin M.V. 2015. Targeted Control of the Structure Formation Process in Production of New Synthetic Leathers. Fibre Chemistry, Volume 46, Issue 5: 312-316.

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