Coagulation of Polyurethanes

Transcription

1 CHAPTER 8 Coagulation of Polyurethanes The polymer most often used for the coagulation process is polyurethane. Nearly all leather substitutes sold in the world consist partially or totally of polyurethanes. Therefore a whole chapter is dedicated to the coagulation of polyurethanes. In 1951 a coagulation process for a polyurethane was published for the first time [1]. Since 1954 [36] additional process steps have been developed to produce microporous polyurethanes [32]. The structure of the soft segments of the polyurethanes influences the coagulation of the polyurethanes remarkably. Soft segments with a higher hydrophilic property result in polyurethanes with a better coagulation behavior. The structure of the hard segments is of minor importance [33]. Only linear polymers have good solubility in organic solvents. Crosslinked polymers are normally insoluble in solvents; due to a certain decomposition crosslinked polyurethanes can be dissolved in heated DMF because hot DMF is able to split branched polymer chains which results in a linearity of the polymer-chains. The NCO/OH ratio determines the linearity of the polyurethane chain. A ratio NCO/OH < 1 results in easily soluble OH-terminated polyurethanes [35], NCO/OH > 1 results in crosslinked, poorly soluble ones (details see Chap. 25.1). The following principally different process types are known: Microporous films may be produced by applying a polymer solution onto a stainless steel band or a glass fiber fabric coated with a fluorine polymer (TEFLON by DuPont) dipped in water, stripped off the band or fabric, washed and dried. The polymer solution may be applied in a direct coating process onto a fabric or a nonwoven and coagulating in a bath of water as a nonsolvent [20, 23, 24, 30]. After washing and drying the coated substrate is ready to sell or only needs an additional finishing step (see Chap. 22). This method is especially suited if the substrate is treated with the coagulation solution too. Sometimes it is difficult to get a smooth surface. Air bubbles may show as open pores on the surface. This may be a disadvantage of the direct coating process. The indirect process involves a releasing fabric which is, usually, coated with a thin polyurethane layer, then with a second top layer or finish. After a short drying of the first layer, the polyurethane solution in DMF is applied and a textile laminated into the wet solution layer. After coagulation, washing

2 CHAPTER 21 Nonwovens According to a Japanese publication, the production of nonwovens has an annual growth rate of more than 10% in Japan. The growth rate for nonwovens based on polyester and polypropylene are responsible for this high growth rate, that for polyamide is stagnant. Polypropylene is cheap, resistant to chemicals and may be used in the medical and hygiene fields [97]. Nonwovens are also important substrates for water vapor permeable products.very often their manufacturing process is linked closely to the production process of vapor permeable top coats or impregnations. In their manufacturing process typical reactions are often used as for the production of water vapor permeable foils or coatings. A general overview of the most important production processes for nonwovens is given. For more detailed information it is necessary to look into surveys [1 7, 13]. Nonwovens can be produced principally in five different ways: mechanically by carding of staple fibers, like paper from a fiber slurry treated in paper making, pneumatically by means of an air stream, by spinning fibers, or by a melt blown process [97]. The production of knitted or woven fabrics needs thread. Nearly all nonwovens are produced with staple fibers. Besides the spinning process all the other processes are carried out with more or less short staple fibers. Carding Process. Cards are machines which have been used for a long time in the processing of wool fibers. For nonwovens staple fibers are unified on a card, possibly mixed with other fibers and orientated in the running direction of the card (Fig. 21-1). The orientated fibers are then laid by a cross lapper (Fig. 21-2) onto a running belt and situated at 90 C at the running direction of the card. The cross lapper moves back and forth over the belt and produces different layers of fibers whose orientations are no longer in the running direction but in angles. The aim of this is to finally get a textile substrate which is anisotropic (see Fig. 21-5) in its physical behavior. The resulting material is needled, shrunk, bonded, split and buffed. These operations will be discussed later.

3 Modification of Physical Properties by Chemical Methods 23.2 Modification of Physical Properties by Physical Methods Directly after production the surface of man-made leathers often needs a physical treatment: Embossing and plating (see Chap. 22) are the usual operations with calenders or embossed rollers. As previously mentioned, this operation reduces the porosity of the material. A presupposition for a good embossing or plating effect is a thermoplasticity of the polymer. By heating thermoplastic, microporous layers lose part of their water vapor permeability. For instance a microporous polyurethane foil with a water vapor permeability of 10 mg/hcm 2 after lamination to the substrate will have a decrease in water vapor permeability to 4 6 mg/hcm 2,after embossing to 2 4 mg/hcm 2,and at the end of the finishing to 1 2 mg/hcm 2 comparable to finished genuine heather. Increasing water vapor permeability reduces the tensile strength of the film (see Fig. 23-1). The same effect occurs with the tear propagation strength. The pore structure also has an influence on the water and air permeability of microporous sheets [6]. To improve the physical properties of microporous sheets a treatment with high frequency [8] or IR rays at C has been discussed. The IR treatment causes a superficial melting of pores which decreases water vapor permeability and eliminates stress in the polymer [5]. Tensite strength and water vapor permeability Tensite strength (kp/cm 2 ) Tear strength and water vapor permeability Tear strength (kp/cm) Water vapor permeability (mg/hcm 2 ) Water vapor permeability (mg/hcm 2 ) Fig The tensile strength and the tear strength of different microporous films with the same composition decreases with increasing water vapor permeability

4 Other Industrial Applications Modified Lyocell fibers can be used as wound dressings especially for chronically ill persons. These fibers absorb more water than alginate, which is usually used [87]. The material is sold under the name of Hydrocel. Collagen layers are suitable as wound dressings [78] or several layers of collagen fixed on each other [20]. A self-adhesive wound dressing is produced by applying a hydrogel layer to a vapor permeable bacterial barrier [96]. Porosity and compliance of microporous polyurethane based microarterial vessel has an effect on neoarterial wall regeneration [95]. Porous polyurethane films are coated with a layer of a water-absorbing material an epoxide crosslinked hyaluronate foam and a polyurethane top layer. The resulting wound dressing material is soft, resistant to bacteria and is biocompatible [36]. Gels of hydrophilic polyurethanes are also used as wound dressings [47]. Implants produced from aromatic isocyanates should be avoided because under reductive conditions carcinogenic aromatic amines may be formed which can be determined by chemical analysis. According to the author s meaning there is no miracle if, by using polymers based on aromatic isocyanate, carcinogenic aromatic amines are found [38]. Microporous membranes can be used to protect against noise, heat (11.1 [23]) and chemicals [35]. A publication is available which summarizes all types of protective clothing for the medical field [55]. All protection includes measures to avoid penetration by viruses and a good water vapor permeability. This publication also discusses the advantages and disadvantages of one-time use or disposable clothing, e. g. those of Kimberley-Clark and DuPont, as against reusable clothing e. g. of Gore. Clothing and other uses of textile substrates in rooms for surgical operations are examined in a publication in regard to their cleanability and their ability to be sterilized by means of ethylene oxide, autoclave, heat etc. [75]. Incontinence and anti-decubitus articles can be produced with Lyocell fibers and an optional coating [70]. Disposable diapers containing hydrophilic surface layers (12 [39, 40]) are claimed to have a good feel to human skin [81]. Cosmetic buffs may be produced with a coagulated polyurethane with pores of a size of mµ [101]. Veterinary Use. Flea collars for dogs protection are produced from a polyurethane with a density of g/cm 3. The polyurethane is impregnated with an insecticide and laminated onto a textile substrate. As insecticides, pyrethroid, carbamates, organophosphorus compounds etc. are used [52]. Food. Hydrophilic foils can be used in food packaging (18.1 [32], [43]). Other Industrial Applications. Microporous and mesoporous zeolites are used in chemical processes, in catalysis, in Diels Alder reactions, oxidations etc. These products are usually inorganic and of a crystalline structure. They differ from membranes, usually polymeric and elastic, which are the subject of this book [104].

5 CHAPTER 28 Ecology As we have previously seen leather substitutes in most cases consist of a textile substrate with a polymer coating and or impregnations. Therefore, the ecological behavior of the textiles is as important as the polymeric materials used. There are several environmental influences: (1) The impact of the chemicals used in processing during and after the production of the textile and the substitute. (2) The chemicals needed to protect the article during usage, and (3) The disposal of the article when no longer needed. The following figures demonstrate the quantity of products involved: As an example, the textile industry in Germany uses 250,000 tons of water annually which is mainly recycled. 960,000 tons of used clothing are produced of which 300,000 tons are reused. 560,000 tons of used household textiles are disposed [26]. Besides the textiles, dyestuffs of textile substrates are a point of ecological discussion. Since certain dyestuffs are known to contain carcinogenic amines as components, all azo dyes are often regarded as dangerous. Only some dyestuffs contain carcinogenic components. The dyestuffs themselves do not need to be hazardous. Some years ago it was found that such dyestuffs in the human metabolism may resplit by azoreductase in the amines (see Fig. 28-1). Therefore, dyestuffs containing carcinogenic amines should not be used any longer. In most countries today their use is forbidden. All major chemical suppliers test the products they are marketing for toxicological behavior. The safety data sheets [19, 23] which are delivered with the product include information about safe applications of the product so that hazards may be avoided for the workers and the environment [18]. Fig Resplitting of an azo dyestuff

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