Five series of acrylic water-borne pressure-sensitive adhesives (PSA's) were prepared with controlled particle morphologies, including core-shell and continually-varying composition. All latex adhesives were prepared at 50 % solids content by emulsion polymerisation of n-butyl acrylate (BA) and acrylic acid (AA) as the main monomers and 1,6-hexanediol diacrylate (HDA) as a crosslinking agent. Seed particles of poly(butyl acrylate) (PBA) were grown by an in-situ batch process to produce a particle of diameter 110 nm. Direct growth of these particles by a semi-continuous process under monomerstarved conditions was employed to give a final particle diameter of approximately 310 nm. Preliminary investigations were aimed at preparation of a latex with a narrow particle size distribution and a low level of coagulum. Initial work using 2-ethylhexyl acrylate (EHA) as the main monomer was not successful because a stable 50 % solids latex could not be produced with controlled particle growth. Hence a formulation was developed using BA and AA. Three series of latexes were prepared with a core-shell particle morphology. Series 1 involved the investigation of the level of crosslinker in the core, at fixed 45:55 core:shell weight ratio, with HDA levels ranging from 0 to 37.5 mol%. Series 2 investigated the volume fraction of crosslinked core (containing 33.3 mol% HDA) over the range 55:45 to 15:85 core:shell weight ratio. Series 3 investigated the effects of the thickness of a crosslinked shell (9.1 mol% HDA) for core-shell particles with a noncrosslinked core (at 55:45, 75:25 and 90:10 core:shell weight ratios). Series 4 latexes were prepared using power-feed processes, one linear power-feed and three based on Series 1 and 2 core-shell adhesive compositions. An additional latex was prepared with a particle profile in which the composition changed linearly with particle radius. Series 5 adhesives were blends of two latexes, such that the mixture had the same composition as Series 2 core-shell adhesives and consisted of the 'core' of the core-shell adhesive blended with a latex with the same composition as the shell of the respective core-shell adhesive. Latex preparation was controlled and monitored by measuring the particle size and the conversion of monomer to polymer at intervals during the preparation. Thermal properties of the adhesives were investigated for thick film samples by dynamic mechanical analysis {DMA}to measure the glass transition temperature {Tg}. Differential scanning calorimetry {DSC}was also used to measure Tgfor comparison. DMA showed two Tg's corresponding to the core and the shell. As the level of crosslinker was increased, the two Tg's became more discrete due to the Tg of the crosslinked phase increasing. The magnitude of the peaks in loss tangent {tan 5} for the core and shell material changed in accordance with the ratio of core:shell. Power-feed adhesives showed a broad glass transition region, which spanned the regions between the Tg's of the equivalent core-shell adhesive. The peaks in tan 5 for the blended systems were more discrete than for the equivalent core-shell adhesives. Adhesive properties were assessed using shear resistance and 1800 peel adhesion tests. Static shear tests were inadequate for testing highly-crosslinked adhesives; hence a dynamic shear resistance test was developed. In comparison to a uniform {noncrosslinked} poly(butyl acrylate-eo-acrylic acid} (PBAlAA) latex, inclusion of crosslinker led to vastly reduced peel adhesion. However, as the level of crosslinker was increased in the Series 1 adhesives, both the peel adhesion and the shear resistance increased. Series 2 and 3 latexes showed that the peel adhesion increases as the amount of non-crosslinked phase is increased. Series 2 adhesives exhibited a maximum in the shear resistance, while Series 3 adhesives showed a decreased in shear resistance as amount of non-crosslinked! ehe core was increased. Adhesive properties of Series 4 power-feed latex polymers showed no dependence on overall crosslink density, but revealed that there is a dependence on the distribution of crosslinker through the particle.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:594755 |
| Date | January 2001 |
| Creators | Errington, Nicola |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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