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Nondestructive testing of defects in adhesive joints

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Studies on pressure sensitive <strong>adhesive</strong>s based on blends <strong>of</strong> natural rubber<br />

and polychloroprene rubber modified with phosphorylated cashew nut<br />

ABSTRACT<br />

shell liquid prepolymer<br />

A.R.R. Menon # , J. Chameswary and J.D. Sudha<br />

National Institute for Interdiscipl<strong>in</strong>ary Science and Technology (NIIST, CSIR)<br />

Thiruvananthapuram – 695 019, Kerala, INDIA<br />

Email : rav<strong>in</strong>dranathamenon@yahoo.co.<strong>in</strong><br />

Effect <strong>of</strong> compositional and process<strong>in</strong>g variations on the T-peel adhesion strength <strong>of</strong> rex<strong>in</strong><br />

specimens bonded with phosphorylated cashew nut shell liquid prepolymer (PCNSL) based pressure<br />

sensitive <strong>adhesive</strong>s (PSAs) conta<strong>in</strong><strong>in</strong>g natural rubber (NR) and polychloroprene rubber (CR) has been<br />

studied. Increase <strong>in</strong> adhesion strength above that <strong>of</strong> a commercial sample <strong>of</strong> PSA has been observed<br />

with the <strong>in</strong>crease <strong>in</strong> solids content <strong>of</strong> the PCNSL based PSA from 15% to 25%. Also, considerable<br />

<strong>in</strong>crease <strong>in</strong> adhesion strength has been obta<strong>in</strong>ed with the <strong>in</strong>crease <strong>in</strong> storage time prior to <strong>test<strong>in</strong>g</strong> <strong>of</strong> the<br />

specimens from 1 to 7 days which <strong>in</strong>dicates the time dependent diffusion behavior <strong>of</strong> PCNSL based<br />

PSA facilitat<strong>in</strong>g wett<strong>in</strong>g and bond formation with the substrates. Irrespective <strong>of</strong> the solids content and<br />

the type <strong>of</strong> PCNSL, a maximum <strong>in</strong> T-peel adhesion strength has been obta<strong>in</strong>ed at a CR dosage <strong>of</strong> 90<br />

phr <strong>in</strong> the blend. Typical cohesive failure along-with high strength have been obta<strong>in</strong>ed with PSAs<br />

based on PCNSL hav<strong>in</strong>g a higher degree <strong>of</strong> phosphorylation / oligomerization. Uniform and almost<br />

complete wett<strong>in</strong>g <strong>of</strong> the substrate has been confirmed by the optical stereo micrographs <strong>of</strong> the failure<br />

surfaces <strong>of</strong> the test specimens and the results on viscosity obta<strong>in</strong>ed from rheometry.<br />

INTRODUCTION<br />

Pressure sensitive <strong>adhesive</strong>s (PSAs) 1 , popularly known as ‘contact <strong>adhesive</strong>s’ hold a ma<strong>in</strong> stay <strong>in</strong><br />

the present consumer and <strong>in</strong>dustrial market. A variety <strong>of</strong> PSAs based on natural rubber (NR) and<br />

polychloroprene rubber (CR) modified with tackifiers and other additives have been <strong>in</strong> use for various<br />

applications such as removable tapes / labels, upholstery, carpentry and footwears. 2-4 It has been<br />

established by the results <strong>of</strong> previous studies that Phosphorylated Cashew Nut Shell Liquid<br />

prepolymer (PCNSL) – an amphiphyllic derivative <strong>of</strong> cashew nut shell liquid (CNSL) could function<br />

as an excellent multifunctional additive for rubber compound<strong>in</strong>g. 5-7 The chemical structure <strong>of</strong> PCNSL<br />

prepolymer is given <strong>in</strong> Fig. 1. The multifunctional role <strong>of</strong> PCNSL as a plasticizer, antioxidant and<br />

tackifier has been made use <strong>of</strong> <strong>in</strong> the design and development <strong>of</strong> a series <strong>of</strong> solvent based PSAs<br />

conta<strong>in</strong><strong>in</strong>g NR and CR. A prelim<strong>in</strong>ary study was made on the effect <strong>of</strong> different compositional and

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