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Water Resistant-Breathable HydrophilicPolyurethane CoatingsV. M. DESAI AND V. D. ATHAWALE*

Department of ChemistryUniversity of Bombay

Vidyanagari, Bombay 400 098India

ABSTRACT: Polyurethane (PU) coatings on nylon fabric that have high moisturepermeability and water repellency properties have been developed. These PUcoatings are prepared by reacting -caprolactam-4,4-diphenyl methane diisocyanate(MDI) adduct with hydrophobic polyols; subsequently deblocking of-caprolactam was carried out using polyethylene 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 optimummoisture permeability properties.

KEY WORDS: -caprolactam, deblocking, polyurethane, moisture permeabilityand water repellency.

INTRODUCTION

HE POLYURETHANE (PU) coatings are now successfully used in a widevariety of applications [1]. Polyurethane textile coatings, however, arebecoming increasingly important because of the unique effects that theymake possible [2]. In the last few years, major inventions and improvementshave been made in waterproof breathable fabrics [2-4]. There are severalmethods by which breathable-waterproof fabric can be prepared, such as

*Author to whom correspondence should be addressed.

JOURNAL OF COATED FABRICS, Volume 25-July 19950093-4658/96/01 0039-08 $10.00/0

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microporous, hydrophilic, and the fabric based on microfilaments [5]. Theadvantage of hydrophilic polyurethane coatings over laminated and micro-porous polyurethane film is that the former has good adhesion on textilesubstrate, high gloss, water and solvent resistance, high moisture permeableproperties and it is less expensive. A tremendous growth in the synthesis andapplication of hydrophilic polyurethane coatings in various disciplines wasrecently 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 withoutloss of other physical properties. E-Caprolactam blocked urethanes havebeen studied extensively for coating applications by several authors [10-12].Generally, in one-pack coating system, blocked polyisocyanates are useddue to some technical and economical reasons. The traditional way of react-ing a blocked isocyanate system is that the deblocking of the caprolactamoccurs in the first step to form isocyanate, followed by reaction with polyolsto form polyurethane. It has been reported in the literature that polyethyleneglycol can be utilized as a deblocking aid with e-caprolactam [13]. Using thisapproach, in the present study, various polyethylene glycols of molecularweight 400, 1000, 3000 and 6000 were used to prepare hydrophilicpolyurethane coatings. These polyurethanes were prepared by the reactionof polyester polyol based on castor oil and E-caprolactam-4,4-diphenylmethane 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 getwater-repellant moisture permeable properties.

MATERIALS AND METHODS

All the raw materials used in this study are listed in Table 1. The polyolsand polyethylene glycols were dried and degassed in vacuum at 100C priorto use.

Castor oil based polyester polyol was prepared by reacting castor oil, adi-pic acid and trimethylol propane by condensation polymerization techniqueunder nitrogen atmosphere at 180-220C. The reaction was monitored bythe change in acid value with time. e-Caprolactam was reacted with 4,4-diphenyl methane diisocyanate at 30C under nitrogen atmosphere. Thereaction was carried out in xylene solvent for 3-4 hours until constant %NCO was obtained. Finally, E-caprolactam-MDI prepolymer wasstoichiometrically reacted with polyester polyol to produce e-caprolactamblock polyurethane. The completion of the reaction was confirmed by theabsence of NCO band at 2270 cm- in the infrared (IR) spectrum.

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Table 1. Raw materials.

Preparation of Hydrophilic PolyurethaneThe 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 130C under nitrogenatmosphere to get hydrophilic PU. The completion of reaction was con-firmed by IR spectroscopy.

Application of Polyurethane CoatingA processing solution of solid content 80% was prepared by dissolving

e-caprolactam block polyurethane in xylene-MEK (50% v/v) and mixingwith stoichiometric quantity of polyethylene glycol 400. The solution wasapplied to scoured nylon fabric (30 X 30 cm) on one side using laboratorycoating table ERNST BENZ A. G. Model 350 KSV/MT-D by knife coatingtechnique 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 nylonfabric is given in Table 2. - ,

Table 2. Analysis of nylon fabric.

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The PU coated fabric was cured at 140C for 2 minutes in an oven (modelERNST BENZ A.G.). The fabric was further treated with water repellentfluorocarbon (Scotchguard) and cured at 120C 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 37C.

(2) Water repellency: The water repellency was measured by using standardrating test recommended by American Association of Textile Chemistsand Colorists (A.ATC.C.) test 22-1977, (1979).

(3) Water proofing property: The water resistance hydrostatic pressure head testwas carried out according to A.A.T.C.C. test 127-1977, (1979).

(4) Washing fastness: The washing fastness of the coated fabric was carriedout using A.S.T.M. D-2724 standard method.

(5) Tensile strength and elongation: The tensile strength and % elongation ofcoated 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 onnylon 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.

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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 PropertiesThe moisture permeability of hydrophilic polyurethane is a function of

the ethylene oxide units present in the system. The ethylene oxide concen-tration of the system is proportional to the PEG soft segments [5]. Themoisture permeability of the coated fabric is graphically represented inFigure 2 which shows increase in moisture permeability with the increase inmolecular weight of polyethylene glycol. The dependence of moisturepermeability on molecular weight of polyethylene glycol segment indicatesthat hydrophilicity of polyurethane can be varied either by increasing theoverall content of hydrophilic component or by altering the chain lengths ofthe hydrophilic segments. The rapid diffusion of water vapour is facilitatedby the low binding energy of polyethylene glycol chain segments for watermolecules [4]. In the present system on one hand the swelling of the

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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 basedpolyol to prevent complete dissolution of urethane polymer in water.

Water RepellencyWetting of the fabric is a mechanism which depends to a large extent on

the difference between the adhesive forces between water and the surface incontact and the cohesive force between the molecules of the water. Thewater repellency of coated nylon fabric gives 50% rating (spray test) due towetting of surface in contact with the water, because the presentpolyurethane coating contains both hydrophobic and hydrophilic chain seg-ments in polymer backbone. The castor oil based polyester polyol hashydrophobic chain which imparts water repellency, whereas the soft seg-ments of polyethylene glycol having water absorption tendency results in50% 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 moisturepermeability.

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Washing FastnessThe washing fastness of polyurethane coated nylon fabric was determined

by using launderometer, in soap-soda solution (1:1) for 30 minute at60 t 1 C. The washing fastness was determined as % loss in weight ofpolymer. The graphical representation shows that (Figure 3) the weight lossof polymer increases with increase in molecular weight of polyethylene gly-col. It indicates that the hydrophilic polyurethane looses its mechanicalproperty, i.e., adhesive strength with increase in the hydrophilic segments.

Tensile PropertiesThe tensile strength of hydrophilic PU coated nylon fabric was found to

be on the higher side than uncoated nylon fabrics in both warp and weftdirection. The tensile strength is dependent on the thread crimp of fabric,denier, coating quality, location of the coating into the interstices, and hardand soft segments present in the polyurethane coating. The % elongation of

FIGURE 3. Relation between molecular weight of hydrophilic segments and % loss ofpolyurethane.

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the PU coated nylon fabrics are found to increase in warp and weft directionof fabric with increase in soft segments of polyethylene glycol. It is logicallyobserved that % elongation of coating is dependent on hardness andstiffness. As stiffness is increased the elasticity of coating must lead toreduced 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 theoptimization of balance between the hydrophobic and hydrophilic chainsegments in polymeric backbone. This method has been found to be mostconvenient 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 thebest substitute for synthetic leather cloths used in foul weather garments forcivilian and military sectors.

REFERENCES

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