Surface coating deterioration studies

This project is concerned with the deterioration of surface coatings as a result of weathering and exposure to a pollutant gas (in this case nitric oxide). Poly(vinyl chloride) (PVC) plastisol surface coatings have been exposed to natural and artificial weathering and a comparison of the effects of these two types of weathering has been made by use of various analytical techniques. These techniques have each been assessed as to their value in providing information regarding changes taking place in the coatings during ageing, and include, goniophotometry, micro-penetrometry, surface energy measurements, weight loss measurements, thermal analysis and scanning electron microscopy. The results of each of these studies have then been combined to show the changes undergone by PVC plastisol surface coatings during ageing and to show the effects which additives to the coatings have on their behaviour and in particular the effects of plasticiser, pigment and uv and thermal stabilisers. Finally a preliminary study of the interaction between five commercial polymers and nitric oxide has been carried out, the polymers being polypropylene, cellulose acetate butyrate, polystyrene, polyethylene terephthalate and polycarbonate. Each of the samples was examined using infra-red spectroscopy in the transmission mode.

Synthesis and characterisation of vinyl block copolymers

A study has been made of the anionic polymerisation of methyl methacrylate using butyllithium and polystyryl lithium as initiators and the effects of lithium chloride and aluminium alkyls on the molecular weight and molecular weight distributions. Diblock copolymers of styrene-b-methyl methacrylate were synthesised at -78oC in THF in the presence of lithium chloride, and at ambient temperatures in toluene in the presence of aluminium alkyls. Studies in the presence of lithium chloride showed that the polymerisation was difficult to control; there was no conclusive evidence of a living system and the polydispersity indices were between 1.5 and 3. However, using relatively apolar solvents, in the presence of aluminium alkyls, homopolymerisation of methyl methacrylate showed characteristics of a living polymerisation. An investigation of the effects of the structures of the lithium and aluminium alkyls on the efficiency of initiation showed that a t-butyllithium/triisobutylaluminium initiating system exhibited an efficiency of 80%, compared with lower efficiencies (typically 30%) for systems based on butyllithium/triethylaluminium.The polydispersity index was found to decrease from ∼2.2 to ∼1.5 when butyllithium was replaced by t-butyllithium. The efficiency of the initiator was found to be solely dependent on the size of the alkyl group of the aluminium component, whereas the polydispersity index was found to be solely dependent on the size of the alkyl group on the lithium component. The aluminium alkyl is thought to be co-ordinated to the ester carbonyl groups of both the monomer and polymer. There is a critical degree of polymerisation, at which point the rate of polymerisation decreases, which probably relates to a change in structure of the active chain end. Characterisation of poly(styrene )-b-poly(4-vinylpyridine) and poly(styrene)-b-poly(4-vinylpyridine methyl iodide) diblock copolymers using static light scattering techniques, showed the formation of star-shaped 'reverse' micelles when placed in toluene. Temperature effects on micellization behaviour are only exhibited for the unquaternised micelles, which showed characterisically lower aggregation numbers than their quaternised counterparts. A suitable solvent was not obtained for characterisation of the styrene-b-methyl methacrylate diblock copolymers synthesized.

Direct inhibitory effects of an antisense oligodeoxynucleotide upon the volume-sensitive chloride current in rat osteoblast-like (ROS 17/2.8) cells

The effects of a 15-mer antisense c-myc phosphorothioate modified oligodeoxynucleotide (OdN) upon the volume-sensitive Cl- current in ROS 17/2.8 cells were investigated using the whole-cell configuration of the patch clamp technique. At 5 microM, the OdN reversibly inhibited the current in a voltage- and time-dependent fashion. This was evident from the reduction in the peak current as assessed at the termination of each voltage pulse and an acceleration of the time-dependent inactivation present at strongly depolarised potentials. The kinetic modifications induced by the OdN suggest it may act by blocking the pore of open channels when the cell membrane potential is depolarised.

Effects of arachidonic acid upon the volume-sensitive chloride current in rat osteoblast-like (ROS 17/2.8) cells.

1. The effects of arachidonic acid upon the volume-sensitive Cl- current present in cultured osteoblastic cells (ROS 17/2.8) was studied using the whole-cell patch-clamp technique. 2. Arachidonate produced two distinct phases of inhibition, a rapid phase occurring within 10-15 s of application preceding a slower phase that occurred 2 min after onset of arachidonate superfusion. Accompanying the slower inhibitory phase was an acceleration of the time-dependent inactivation exhibited by the current at strongly depolarized potentials (> + 50 mV). The half-maximal inhibitory concentrations (IC50) were 177 +/- 31 and 10 +/- 4 microM for the two phases respectively. 3. Arachidonate was still effective in the presence of inhibitors of cyclo-oxygenase (indomethacin, 10 microM), lipoxygenase (nordihydroguaretic acid, 10-100 microM) and cytochrome P450 (SKF525A, 100 microM; ethoxyresorufin, 10 microM; metyrapone, 500 microM; piperonyl butoxide, 500 microM; cimetidine, 1 mM). The effects of arachidonate could not be produced by another cis unsaturated fatty acid, oleic acid. 4. Measurements of cell volume showed that arachidonate effectively inhibited the regulatory volume decrease elicited by ROS 17/2.8 cells in response to a reduction in extracellular osmolarity. 5. It is concluded that the volume-sensitive Cl- conductance in ROS 17/2.8 cells is directly modulated by arachidonate and may represent a physiological mechanism by which volume regulation can be controlled in these cells.

Characterization of a volume-sensitive chloride current in rat osteoblast-like (ROS 17/2.8) cells

1. During osmotic swelling, cultured osteoblastic cells (ROS 17/2.8) exhibited activation of large amplitude Cl- currents in the whole-cell configuration of the patch-clamp technique. Effects of hypotonic shock on cell volume and membrane conductance were rapidly reversed on return to isotonic conditions. 2. Voltage command pulses in the range -80 to +50 mV produce instantaneous activation of Cl- currents. At potentials more positive than +50 mV the current exhibited time-dependent inactivation. The instantaneous current-voltage relationship was outwardly rectifying. 3. The anion permeability sequence of the induced current was SCN- (2.2) > I- (1.9) > Br- (1.5) > Cl- (1.0) > F- (0.8) > gluconate- (0.2). This corresponds to Eisenman's sequence I. 4. The volume-sensitive Cl- current was effectively inhibited by the Cl- channel blockers 4,4'-diisothiocyanatostilbene-2,2-disulphonic acid (DIDS) and 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB). Outward currents were more effectively suppressed by DIDS than inward currents. The concentrations for 50% inhibition (IC50) of outward and inward currents were 81 and 298 μM, respectively. NPPB was equally effective at inhibiting outward and inward currents (IC50 of 64 μM). The current was relatively insensitive to diphenylamine-2-carboxylate (DPC), 500 μM producing only 22.5 ± 4.0% inhibition. 5. Inhibitors of protein kinase A (H-89, 1 μM) and tyrosine kinase (tyrphostin A25, 200 μM) were without effect upon activation of Cl- currents in response to hypotonic shock. Under isotonic conditions, elevation of intracellular Ca2+ by ionomycin (1 μM) or activation of protein kinase C by 12-O-tetradecanoylphorbol 13-acetate (TPA, 0.1 μM) failed to evoke increases in basal Cl- conductance levels. 6. It is concluded that an outwardly rectifying Cl- conductance is activated upon osmotic swelling and may be involved in cell volume regulation of ROS 17/2.8 cells.

Chloride resistance and durability of cement paste and concrete containing metakaolin

Metakaolin (MK), a calcined clay, was included as a partial cement replacement material, at up to 20% by weight of binder, in cement pastes and concrete, and its influence on the resistance to chloride ingress investigated. Reductions in effective chloride diffusion coefficients through hardened cement paste were obtained for binary blends and by combining OPC, MK and a second cement replacement material of pulverised fuel ash or ground granulated blast furnace slag. Steady state oxygen diffusion measurements through hardened cement pastes measured using an electrochemical cell showed that the interaction between charged species and the pore surfaces is a major factor in determining chloride diffusion rate. Rheology of the binder, particularly at high MK replacement levels, was found to have a dramatic influence on the diffusion performance of cement pastes. It was concluded that plasticising admixtures are essential for adequate dispersion of MK in cement pastes. Chloride concentration profile analysis of the concrete cylinders, exposed to sodium chloride solution for one year, was employed to obtain apparent chloride diffusion coefficients for concrete specimens. MK was found to reduce the depth of chloride penetration into concrete when compared with that of unblended mixes. Corrosion rate and corrosion potential measurements were taken on steel bars embedded in concrete exposed to a saline environment under conditions of cyclic wetting and drying. The initiation time for corrosion was found to be significantly longer for MK blended mixes than for plain OPC systems. The aggregate-paste interfacial zone of MK blended systems was investigated by steady state diffusion of chloride ions through mortar containing glass beads as model aggregate. For the model aggregate specimens tested the work confirmed the hypothesis that properties of the bulk paste are the controlling factors in ionic diffusion through mortar.

Electrochemical chloride extraction to halt corrosion of reinforcement

This thesis presents results of experiments designed to study the effect of applying electrochemical chloride extraction (ECE) to a range of different hardened cement pastes. Rectangular prism specimens of hydrated cement paste containing sodium chloride at different concentrations were subjected to electrolysis between the embedded steel cathodes and external anodes of activated titanium mesh. The cathodic current density used was in the range of 1 to 5 A/m2 with treatment periods of 4 to 12 weeks. After treatment, the specimens were cut into sections which were subjected to pore-solution expression and analysis in order to determine changes in the distribution of free and total ionic species. The effect of the ECE treatment on the physical and microstructural properties of the cements was studied by using microhardness and MIP techniques. XRD was employed to look at the possibility of ettringite redistribution as a result of the accumulation of soluble sulphate ions in the cement matrix near the cathode during ECE. Remigration of chloride which remains after the ECE treatment and distribution of other ions were studied by analysing specimens which had been stored for several months, after undergoing ECE treatment. The potentials of the steel cathodes were also monitored over the period to detect any changes in their corrosion state. The main findings of this research were as follows: 1, ECE, as applied in this investigation, was capable of removing both free and bound chloride. The removal process occurred relatively quickly and an equilibrium between free and bound chlorides in the specimens was maintained throughout. At the same time, alkali concentrations in the pore solution near the steel cathode increased. The soluble sulphate ionic concentration near the cathode also increased due to the local increase in the pH of the pore solution. 2, ECE caused some changes in physical and microstructural of the cement matrix. However these changes were minimal and in the case of microhardness, the results were highly scattered. Ettringite in the bulk material well away from the cathode was found not to increase significantly with the increase in charge passed.3, Remigration of chloride and other ionic species occurred slowly after cessation of ECE with a resultant gradual increase in the Cl-/OH- ratio around the steel.4, The removal of chloride from blended cements was slower than that from OPC.

Aspects of the prevention and repair of chloride-induced corrosion of steel in concrete

Sodium formate, potassium acetate and a mixture of calcium and magnesium acetate (CMA) have all been identified as effective de-icing agents. In this project an attempt has been made to elucidate potentially deleterious effects of these substances on the durability of reinforced concrete. Aspects involving the corrosion behaviour of embedded steel along with the chemical and physical degradation of the cementitious matrix were studied. Ionic diffusion characteristics of deicer/pore solution systems in hardened cement paste were also studied since rates of ingress of deleterious agents into cement paste are commonly diffusion-controlled. It was found that all the compounds tested were generally non-corrosive to embedded steel, however, in a small number of cases potassium acetate did cause corrosion. Potassium acetate was also found to cause cracking in concrete and cement paste samples. CMA appeared to degrade hydrated cement paste although this was apparently less of a problem when commercial grade CMA was used in place of the reagent grade chemical. This was thought to be due to the insoluble material present in the commercial formulation forming a physical barrier between the concrete and the de-icing solution. With the test regimes used sodium formate was not seen to have any deleterious effect on the integrity of reinforced concrete. As a means of restoring the corrosion protective character of chloride-contaminated concrete the process of electrochemical chloride removal has been previously developed. Potential side-effects of this method and the effect of external electrolyte composition on chloride removal efficiency were investigated. It was seen that the composition of the external electrolyte has a significant effect on the amount of chloride removed. It was also found that, due to alterations to the composition of the C3A hydration reaction products, it was possible to remove bound chloride as well as that in the pore solution. The use of an external electrolyte containing lithium ions was also tried as a means of preventing cathodically-induced alkali-silica reaction in concretes containing potentially reactive aggregates. The results obtained were inconclusive and further practical development of this approach is needed.

Properties and performance of polymer modified cements and mortars

Polymer modified cements and mortars have become popular for use as patch repair materials. General evidence suggests that these materials offer considerable improvements compared to traditional mortars although the mechanisms for this are not fully understood. This work elucidates the factors which govern some properties and performance of different polymer systems. In view of the wide range of commercial systems available, investigations concentrated on the use of three of the most commonly available groups of polymers. These were: (1) Styrene Butadiene Rubber (SBR), (2) Acrylics and, (3) Ethylene Vinyl Acetates (EVA). The later two were in the form of both emulsions and redispersible powders. Experiments concentrated on: (1) Rheological behaviour of polymer modified cement pastes; (2) Workability of polymer modified mortars; (3) Influence of curing conditions on the pore size distribution and diffusion of chloride ions; (4) Bond strength of polymer modified cement and mortar patches; and (5) Microscopic examination and semi-quantitative analyses of the bulk and interfacial microstructures. The following main conclusions were reached: (1) The addition of polymer emulsions have a considerable influence on the workability of fresh cement pastes, the extent of this depending on the type of system used. (2) The rheological parameters of fresh polymer modified mortars can be established using a two-point workability test which may be used when comparing the properties of different systems at constant workability. (3) Curing conditions affect the properties of polymer modified systems and a wet/dry curing regime was essential for good adhesion of these materials to mortar substrates. (4) In contrast, the wet/dry curing regime resulted in a curing affected zone at the surface of patch materials. This can result in a much coarser pore structure and enhanced diffusion of e.g. chloride ions. (5) The microstructure of polymer modified systems was very different compared with the unmodified cement/mortar and varied depending on curing conditions.