Understanding the mechanical strength of microcapsules and their adhesion on fabric surfaces

There is a growing interest to incorporate melamine formaldehyde (MF) microcapsules containing perfume oil in detergents, which can be delivered to consumers at end-use applications. The microcapsules should have desirable properties including optimum mechanical strength and capability to adhere on fabric surfaces after laundry. They should be strong enough to withstand a serious of engineering processes including pumping, mixing, drying etc, but be weak enough to be ruptured by consumers in post-laundry handling. For this purpose, the mechanical strength of MF microcapsules made by different processing conditions, with additional coating, after being dried using different methods and being exposed to various suspending liquids were characterised in this work. Moreover, the adhesion of single MF microcapsules or single MF microspheres on flat fabric films in air or in liquids with different concentrations of detergent, surfactants, pH etc was investigated. The mechanical strength of MF microcapsules produced using an in-situ polymerisation technique were characterised by a micromanipulation technique. Conventionally, the mechanical strength parameters include microcapsule diameter, rupture force, deformation at rupture and nominal rupture stress (the ratio of the rupture force to the initial cross-sectional area of individual microcapsule). It was found that larger microcapsules in a sample on average had greater rupture force but small ones had higher nominal rupture stress. Since the rupture force or nominal rupture stress depends on the size of microcapsules, which is not easy to use particularly for comparison of the mechanical strength of microcapsules in different samples, a new strength parameter nominal wall tension at rupture has been proposed in this work, which is defined as the ratio of the rupture force to the circumference of individual microcapsule. The results from micromanipulation measurements showed that the increase of core/capsule ratio in weight percentage reduced the nominal wall tension of microcapsules. The use of silicate coating on surface of MF microcapsules increased the nominal wall tension of microcapsules and made microcapsules more brittle. The nominal wall tension of microcapsules did not differ significantly when the pH of their suspending liquid ranged from 2 to 11 for a duration of 25 hours. It has also been shown that the prolonged polymerisation time alone or combined with the elevated polymerisation temperature increased the nominal wall tension of MF microcapsules. Furthermore, there was no significant change in the nominal wall tension of microcapsules after being oven dried, fluidised bed dried or freeze dried. However, there was a significant increase in the nominal rupture tension of microcapsules after being spray dried, which resulted from destroying weak (in general large) microcapsules in the drying process. Modelling of the force versus displacement data from micromanipulation has been attempted in order to determine intrinsic mechanical property parameters, such as Young’s modulus, yield stress and stress at rupture that requires to know the contact area between a compressed microcapsule and force probe at rupture. The mean Young’s modulus of MF microcapsules Ec predicted from the Hertz model was found to be 32±4 MPa which represents the modulus of single whole microcapsule. In addition, the Young’s modulus of MF microcapsule wall material Ew was found to be 8±1 GPa by applying finite element analysis with a linear elastic model. A correlation describing the relationship between E\(_c\) and E\(_w\) has been developed based on the modelled results, wall thickness and diameter of microcapsules. The Hertz model and Johnson’s plastic model were further applied to determine the rupture stress of single MF microcapsules, which take their rupture deformation into consideration. The models help to determine the mechanical strength of microcapsules precisely. Real fabric surface can be very rough, and quantification of the adhesion of single microcapsules on such rough surface can be difficult so that flat fabric surface was fabricated. Cotton films were successfully generated by dissolving cotton powder and their properties were also characterised including their surface roughness, thickness, contact angle and purity. The adhesive forces between MF microcapsules/MF microparticles and cotton films under ambient condition at air RH above 40% were measured using an AFM technique, which was considered to be dominated by capillary forces. It was also found that there was little adhesion between MF microparticle and cotton films in detergent or surfactant solution. Instead, repulsion between them was observed and reduced with the increase of detergent/surfactant concentration and the decrease in solution pH. It was suggested that the repulsion was contributed from two mechanisms of steric interaction and electrostatic repulsion. It is believed that this work can be used to guide formulation and processing of MF microcapsules with desirable mechanical strength. The studies on the adhesion between MF microcapsules/microparticles and cotton films under ambient condition or in the detergent solutions should be beneficial to the future work to enhance adhesion of microcapsules on fabric surface via modification of the surface compositions and morphology of microcapsules.