Viscosity and adhesion properties of NBR/SMR L blend based pressure-sensitive adhesive were investigated using coumarone-indene resin, toluene, and poly(ethylene terephthalate) (PET) as tackifier, solvent, and coating substrate, respectively. Coumarone-indene resin content was fixed at 40 parts per hundred parts of rubber (phr) in the adhesive formulation. The ratio of NBR/SMR L blend used was 0, 20, 40, 60, 80, and 100% of NBR content. Four different thicknesses, that is, 30, 60, 90, and 120 µm, were used to coat the PET film. The viscosity of adhesive was determined by a Brookfield viscometer, whereas loop tack, peel strength, and shear strength were measured using a Lloyd Adhesion Tester operating at 30 cm/min. Result indicates that the viscosity, loop tack, and shear strength of blend adhesives increase with % NBR. However, for peel strength, it indicates a maximum at 40% NBR blend ratio for the three modes of peel tests. In all cases, 120 µm coated sample consistently exhibits the highest adhesion values compared to the other coating thicknesses, an observation which is associated with the higher volume of adhesive in the former system.
Many studies on rubber-based adhesives involve a single component rubber as the elastomer in the adhesive formulation. These include the study of viscoelastic properties of natural rubber pressure-sensitive adhesive using acrylic resin as a tackifier [
One grade of natural rubber (SMR L) and one type of synthetic rubber (NBR) were used as the elastomers. SMR L and NBR were supplied by Rubber Research Institute of Malaysia and Bayer Company, respectively. The bound acrylonitrile content in NBR is 33%. Glass transition temperatures for SMR L and NBR are −72°C and −40°C, respectively. Coumarone indene resin was used as the tackifier. It was freshly supplied by Mukmin Enviro Company, Penang (Malaysia). Toluene was used as the solvent throughout the experiment.
The rubber was masticated using a 2-roll mill for 10 minutes. Different NBR/SMR L blends ratios, that is, 0/5, 1/4, 2/3, 3/2, 4/1, and 5/0 corresponding to 0, 20, 40, 60, 80, and 100% NBR were used in the blend formulation. 30 mL of toluene was added into each rubber blend and stirred evenly. The solution was tightly closed and left for 24 hour to ensure complete dissolution. A fixed amount of 2 g of pulverized coumarone indene resin—corresponding to 40 phr of resin—was added slowly to the solution with constant stirring. The resulting adhesives were left for 3 hours at room temperature (30°C) before testing.
A Brookfield viscometer (model DV-II + Pro) was used to measure the viscosity of the adhesives. Its speed was set at 1 rpm. The spindle (CPE-51) and metal cup (CPE-44Y) were chosen in this testing. The platform and spindle head were cleaned with isopropyl alcohol. A drop of adhesive was put at the middle of the platform and testing was carried out for one minute. The average viscosity was recorded from at least five readings.
A PET film with dimension of 4 cm × 25 cm was coated at the centre of the substrate (4 cm × 4 cm) with different coating thickness using a SHEEN hand coater. The coated sample was conditioned at room temperature for 24 hours before testing. A loop was then formed and the adhesive coated area was gently brought into contact with a glass without any force other than the pushing force on the loop. The testing was conducted using a Lloyd Adhesion Tester (Model LRXPlus with NEXYGEN software) with a testing rate of 30 cm/min to debond the loop from the glass plate. The average debonding force was recorded from the three highest peaks recorded in the test.
Three types of peel test, namely, T-peel, 90°-peel, and 180°-peel tests were employed in this study. For the T-peel and 90° peel test, the dimensions of the substrates were 20 cm × 4 cm. However, the dimensions of the substrate for 180° peel test were 25 cm × 4 cm and 15 cm × 7 cm. The adhesive was coated from the end of PET film at a coating area of 10 cm × 4 cm for various coating thickness using a SHEEN Hand Coater. The face stock was then placed on the coated PET film (base stock) before being conditioned at room temperature for 24 hours prior to testing. A Lloyd Adhesion tester operating at 30 cm/min was used to measure the peel force of the samples. The average peeling force was determined from the three highest peaks computed from the test.
The dimension of the PET film substrate was 20 cm × 4 cm. The substrate was coated 10 cm from the end of the substrate. The end of another uncoated substrate (face stock) was then laid on the coated area of the base stock. The shear sample was then conditioned at room temperature for 24 hours prior to testing on a Lloyd Adhesion Tester operating at 30 cm/min. Shear strength was expressed as the shear force per unit area of testing.
From this study, the effect of % NBR on viscosity, tack, peel strength, and shear strength of adhesive is discussed below.
The dependence of viscosity adhesive on % NBR is shown on Figure
Variation of viscosity with % NBR for NBR/SMR L blend-based adhesive.
The lowest viscosity is recorded for the 0% NBR, that is, 100% SMR L. The increasing viscosity with % NBR is due to the increase in NBR content which has higher viscosity than SMR L-based adhesive. Result shows that the viscosity of NBR-based adhesive is 30.5% higher than that of SMR L-based adhesive. This means that NBR content has significant effect in the NBR/SMR L blend adhesive for higher NBR composition.
Tack may be defined as the property of material which enables it to form a bond of measurable strength immediately upon contact with another surface, usually with low applied pressure [
Variation of loop tack with % NBR for NBR/SMR L blend-based adhesive at various coating thicknesses.
This observation is attributed to the increase in the degree of wettability where the adhesive is able to conform to the irregularities of the adherent; that is, low surface energy condition is observed [
The peel strength (T-peel) of the adhesive is shown in Figure
Variation of peel strength (T-peel test) with % NBR for NBR/SMR L blend-based adhesive at various coating thicknesses.
Variation of peel strength (90° peel test) with % NBR for NBR/SMR L blend-based adhesive at various coating thicknesses.
Variation of peel strength (180° peel test) with % NBR for NBR/SMR L blend-based adhesive at various coating thicknesses.
Dependence of peel strength on coating thickness for various modes of peel tests.
This observation is attributed to the angle of testing which suggests that 90° test requires higher peeling force to separate the mechanical interlocking and anchorage of the adhesion in pores and irregularities in the substrate [
Figure
Variation of shear strength with % NBR for NBR/SMR L blend-based adhesive at various coating thicknesses.
Dependence of shear strength on coating thickness for various % NBR.
The viscosity of NBR/SMR L based adhesive increases with increasing NBR content, an observation which is attributed to the higher viscosity of NBR based adhesive. Loop tack also increases with increase in NBR content. This observation is associated with the increase in wettability where the adhesive is able to conform to the irregularities of the adherent. For the shear strength, the increase is primarily attributed to the increase in cohesive and adhesive strength of the blend adhesive. However, peel strength indicates a maximum value at 40% NBR content due to the culmination of wettability of adhesive which results in mechanical interlocking and anchorage of the adhesive in the pore and irregularities in the adherent. In all cases, the adhesion property increases with coating thickness. This phenomenon is associated with the presence of higher amount of adhesive which enhances the viscoelastic response of the blend adhesive.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors acknowledge the Science Fund provided by the Ministry of Science, Technology, and Innovation (MOSTI), Malaysia for this study.