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Water Resistant-Breathable Hydrophilic Polyurethane Coatings

V. M. DESAI AND V. D. ATHAWALE* Department of Chemistry University of Bombay Vidyanagari, Bombay 400 098 India

ABSTRACT: Polyurethane (PU) coatings on fabric that have high moisture permeability and water repellency properties have been developed. These PU coatings are prepared by reacting ∈-caprolactam-4,4’-diphenyl methane diisocyanate (MDI) adduct with hydrophobic ; subsequently deblocking of ∈-caprolactam was carried out using glycol (PEG) of different molec- ular weights at ambient temperature. The hydrophobic and hydrophilic segment bal- ance was adjusted by changing the molecular weight of PEG to impart optimum moisture permeability properties.

KEY WORDS: ∈-caprolactam, deblocking, polyurethane, moisture permeability and water repellency.

INTRODUCTION

POLYURETHANE (PU) coatings are now successfully used in a wide varietyHE of applications [1]. Polyurethane coatings, however, are becoming increasingly important because of the unique effects that they make possible [2]. In the last few years, major inventions and improvements have been made in waterproof breathable fabrics [2-4]. There are several methods by which breathable-waterproof fabric can be prepared, such as

*Author to whom correspondence should be addressed.

JOURNAL OF COATED FABRICS, Volume 25-July 1995 39 0093-4658/96/01 0039-08 $10.00/0 @1996 Technomic Publishmg Co., Inc 40 microporous, hydrophilic, and the fabric based on microfilaments [5]. The advantage of hydrophilic polyurethane coatings over laminated and micro- porous polyurethane film is that the former has good adhesion on textile substrate, high gloss, water and solvent resistance, high moisture permeable properties and it is less expensive. A tremendous growth in the synthesis and application of hydrophilic polyurethane coatings in various disciplines was recently patented [6-9]. The hydrophilic PUs are manufactured by the com- bination of hydrophobic and hydrophilic polyols with diisocyanate compo- nents to produce optimum moisture vapour transmission properties without loss of other physical properties. E-Caprolactam blocked urethanes have been studied extensively for coating applications by several authors [10-12]. Generally, in one-pack coating system, blocked polyisocyanates are used due to some technical and economical reasons. The traditional way of react- ing a blocked system is that the deblocking of the caprolactam occurs in the first step to form isocyanate, followed by reaction with polyols to form polyurethane. It has been reported in the literature that can be utilized as a deblocking aid with e-caprolactam [13]. Using this approach, in the present study, various polyethylene glycols of molecular weight 400, 1000, 3000 and 6000 were used to prepare hydrophilic polyurethane coatings. These polyurethanes were prepared by the reaction of based on and E-caprolactam-4,4’-diphenyl methane diisocyanate (MDI) block prepolymer. The deblocking of e-cap- rolactam was carried out by polyethylene glycols at ambient temperature. The resultant hydrophilic polyurethane was coated on nylon fabric to get water-repellant moisture permeable properties.

MATERIALS AND METHODS

All the raw materials used in this study are listed in Table 1. The polyols and polyethylene glycols were dried and degassed in vacuum at 100°C prior to use. Castor oil based polyester polyol was prepared by reacting castor oil, adi- pic acid and trimethylol propane by condensation technique under nitrogen atmosphere at 180-220°C. The reaction was monitored by the change in acid value with time. e-Caprolactam was reacted with 4,4’- diphenyl methane diisocyanate at 30°C under nitrogen atmosphere. The reaction was carried out in xylene solvent for 3-4 hours until constant % NCO was obtained. Finally, E-caprolactam-MDI prepolymer was stoichiometrically reacted with polyester polyol to produce e-caprolactam block polyurethane. The completion of the reaction was confirmed by the absence of NCO band at 2270 cm-’ in the infrared (IR) spectrum. 41

Table 1. Raw materials.

Preparation of Hydrophilic Polyurethane The e-caprolactam blocked polyurethane was mixed with polyethylene glycol, xylene and dibutyltin dilurate as catalyst for deblocking reaction. The deblocking of E-caprolactam was carried out at 130°C under nitrogen atmosphere to get hydrophilic PU. The completion of reaction was con- firmed by IR spectroscopy. Application of Polyurethane Coating A processing solution of content 80% was prepared by dissolving e-caprolactam block polyurethane in xylene-MEK (50% v/v) and mixing with stoichiometric quantity of polyethylene glycol 400. The solution was applied to scoured nylon fabric (30 X 30 cm) on one side using laboratory coating table ERNST BENZ A. G. Model 350 KSV/MT-D by knife coating technique to get polyurethane coated fabric (PU1). Similarly PU2, PU3, PU4, Polyurethane coated fabrics were prepared by using polyethylene gly- col 1000, 3000 and 6000 respectively. The specification of uncoated nylon fabric is in Table 2. given ’-,

Table 2. Analysis of nylon fabric. 42

The PU coated fabric was cured at 140°C for 2 minutes in an oven (model ERNST BENZ A.G.). The fabric was further treated with water repellent fluorocarbon (Scotchguard) and cured at 120°C for 1 minute. Measurement of Physical Properties of Coated Fabric (1) Moisture permeability: The moisture permeability of coated fabric was measured by using American Society of Testing Materials (A.S.T.M.) E 96 63-T evaporation method at 37°C. (2) Water repellency: The water repellency was measured by using standard rating test recommended by American Association of Textile Chemists and Colorists (A.ATC.C.) test 22-1977, (1979). (3) Water proofing property: The water resistance hydrostatic pressure head test was carried out according to A.A.T.C.C. test 127-1977, (1979). (4) Washing fastness: The washing fastness of the coated fabric was carried out using A.S.T.M. D-2724 standard method. (5) Tensile strength and elongation: The tensile strength and % elongation of coated nylon fabrics were measured by using tensile testing machine (Master control R&D electronics, India) according to A.STM. D-1775. (6) The cross sectional view of hydrophilic polyurethane material coated on nylon fabric was photographed on photomicroscope, (Carl Zeiss Model, West Germany) at magnification 126 (Figure 1).

FIGURE 1. Cross-sectional view of polyurethane coated nylon fabric at magnification 126. 43

Table 3. Physical properties of polyurethane coated nylon fabric.

RESULTS AND DISCUSSION

The physical properties of polyurethane coated nylon fabric are collec- tively presented in Table 3. Permeability Properties The moisture permeability of hydrophilic polyurethane is a function of the units present in the system. The ethylene oxide concen- tration of the system is proportional to the PEG soft segments [5]. The moisture permeability of the coated fabric is graphically represented in Figure 2 which shows increase in moisture permeability with the increase in molecular weight of polyethylene glycol. The dependence of moisture permeability on molecular weight of polyethylene glycol segment indicates that hydrophilicity of polyurethane can be varied either by increasing the overall content of hydrophilic component or by altering the chain lengths of the hydrophilic segments. The rapid diffusion of water vapour is facilitated by the low binding energy of polyethylene glycol chain segments for water molecules [4]. In the present system on one hand the swelling of the 44 urethane membrane by water vapour is encouraged by hydrophilic soft seg- ments present in polyethylene glycol and on the other hand swelling is re- stricted by hydrophobic chain segments present in the castor oil based polyol to prevent complete dissolution of urethane in water. Water Repellency Wetting of the fabric is a mechanism which depends to a large extent on the difference between the forces between water and the surface in contact and the cohesive force between the molecules of the water. The water repellency of coated nylon fabric gives 50% rating (spray test) due to wetting of surface in contact with the water, because the present polyurethane coating contains both hydrophobic and hydrophilic chain seg- ments in polymer backbone. The castor oil based polyester polyol has hydrophobic chain which imparts water repellency, whereas the soft seg- ments of polyethylene glycol having water absorption tendency results in 50% rating. Therefore, coated fabric treated with flurocarbon emulsion im- proves the repellency up to 90% rating (spray test).

FIGURE 2. Relation between molecular weight of polyethylene glycol and moisture permeability. 45

Washing Fastness The washing fastness of polyurethane coated nylon fabric was determined by using launderometer, in soap-soda solution (1:1) for 30 minute at 60 t 1 °C. The washing fastness was determined as % loss in weight of polymer. The graphical representation shows that (Figure 3) the weight loss of polymer increases with increase in molecular weight of polyethylene gly- col. It indicates that the hydrophilic polyurethane looses its mechanical property, i.e., adhesive strength with increase in the hydrophilic segments.

Tensile Properties The tensile strength of hydrophilic PU coated nylon fabric was found to be on the higher side than uncoated nylon fabrics in both warp and weft direction. The tensile strength is dependent on the thread crimp of fabric, denier, coating quality, location of the coating into the interstices, and hard and soft segments present in the polyurethane coating. The % elongation of

FIGURE 3. Relation between molecular weight of hydrophilic segments and % loss of polyurethane. 46 the PU coated nylon fabrics are found to increase in warp and weft direction of fabric with increase in soft segments of polyethylene glycol. It is logically observed that % elongation of coating is dependent on hardness and stiffness. As stiffness is increased the of coating must lead to reduced mobility of the composite coated structure causing reduction in % elongation of PU coated nylon fabric.

’ CONCLUSION One pack e-caprolactam block polyurethane has been developed, by the optimization of balance between the hydrophobic and hydrophilic chain segments in polymeric backbone. This method has been found to be most convenient to achieve the modified hydrophilic polyurethane coatings. These polyurethane coatings can be utilized for nylon fabric to impart mois- ture permeability and water repellency properties. These fabrics could be the best substitute for synthetic leather cloths used in foul weather garments for civilian and military sectors.

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