Final Report Summary - LATEX FILM FORMATION (Investigations on the effect of reactive surfactants on latex film microstructure and properties via tracer diffusion)
The formation of polymer films from aqueous dispersions of latex particles has become the major industrial process for a variety of applications like coatings adhesives etc., as it reduces the emission of volatile organic compounds in comparison to film formation of polymer solutions. The surfactants used in the preparation of these dispersions impart colloidal stability for further processing. As a means to overcome the undesirable features like formation of surfactant cluster, surfactant migration to the interfaces causing deterioration of adhesion and water sensitivity, reactive surfactants were developed. The reactive surfactant or surfmer becomes chemically attached to the particles during the synthesis. Thus, surfactant migration is prevented and relating surfmers of suitable structure and reactivity a homogenous distribution of the surfactant in the polymer film could be achieved with the option of introducing some additional functionality (e.g. for improved mechanical stability.) The studies reported so far concentrated on the macroscopic behaviour of the latex films. This project aimed to monitor the influence of surfmer on the film structure and film formation on a molecular scale. The project was carried out in two phases. The first part of the work was carried out during the phase incoming phase and in the return phase. The main objectives for the incoming phase were synthesis and characterisation of some new surfmers, evaluation in emulsion polymerisation of typical monomers like styrene and butyl methacrylate. During the return phase, evaluation of the material properties and assessing the microstructural properties of the formed polymer was carried out.
During the incoming phase, a new polymerisable acrylate surfmer was synthesised and the performance of the surfmer in the dispersion synthesis and microstructural properties was undertaken using static light scattering and evaluation of the film formation properties. Solution property studies using static light scattering shows supramolecular structure formation originating from a combination of aggregation to inverse micelles and concentration dependent association of these aggregates to long filamentous microstructures. The influence of surfmer on the film formation was studied using tracer diffusion technique and a mechanistic understanding of the surfmer effect on the drying properties was deduced.
In latex films, the surfmer influence had two origins arising from the ion pair of the charged heads (ionomer effect) and the hydrophobic tail of the surfmer (plasticiser effect). The hydrophobic tail was found to act in wet and dry film as an internal plasticiser. The dissociated ion pair of the head group softens the particle surface in wet films, leading to increased polymer segment mobility and thereby enhanced particle deformation. The plasticiser effect was seen microscopically by the significant reduction of MFFT of PBMA above 10 degrees on addition of a slight amount of 0.3 mol% surfmer. This shows that film formation is facilitated without the addition of any volatile organic compounds (VOC) as filming aid. On a molecular scalem the plasticisation is seen as a dramatic enhancement of the dye diffusion in the domains formed by the partially fused particles over that in the particle cores. The effect was amplified in PBMA with only 0.3 mol% surfmer by a factor of about 1000 over the 1 in a typical industrial dispersion and even stronger than in a dispersion with particle surface saturated with the conventional non-reactive surfactant SDS (sodium dodecyl sulphate). In dry films, the ionomer effect and the internal plasticisation effect was found to counteract each other. The former is expected to a strengthening of the polymer by a supramolecular structure formation similar to the one found during solution microstructure studies, where as the latter soften the material.
The study revealed that the use of surfmers is advantageous for improvement of latex formulation in coatings. The significant enhancement of the particle deformation due to the substantially increased hydroplastcisation of the particle interface could allow for reduced amount of film forming aids and that are usually added to the plasticises the latex particle s to promote film formation. By judicious choice of the type and amount of surfmer, the final film may be mechanically strong because of the supramolecular structure formation induced by the surfmer. If surfactant migration is eliminated by the use of chemically attached surfmer, surfactant exudation and surfactant cluster formation can be achieved, which can enhance the resistance against water re-absorption on wetting by avoiding remnant hydrophilic channels in the film.
During the return phase, these effects were further confirmed using DMA and viscometry studies. The SLS studies gave information on chain association and the aggregation number. From detailed analysis of the form factor data, a structural model was derived and it was hypothesised that these copolymers form, a block-like structure with the surfmer-derived charges at one or at both ends. These chains associate to form ionomer-like clusters even in the dilute state with charges inside and uncharged PS chain segments on the outside, forming a daisy-like micro-structure with loops and dangling ends. On increasing the concentration, the clusters associate to form more extended, filament-like structures through bridge formation of the telomeric chains. In summary, studies during the incoming phases revealed that the addition of small amounts of surfmer leads to an improved film formation without detrimental effects on the final film properties and the approach could be used to reduce rest VOCs in coatings. The influence of these structures on the material properties was further revealed from the dynamic mechanical properties.
Measuring the dilute solution viscosity studies further helped to confirm the aggregation of surfmer modified chains in solution, compared to polystyrene homopolymer prepared without any surfmer. The copolymer prepared with 2.31 and 4.52 mol% surfmer content were insoluble in tetrahydrofuran (THF), anticipated due to stronger association of the polymer chains through sulfonic acid groups. The thermal and dynamic mechanical studies also show signatures of both plasticising effect by the alkyl groups and the polar interactions through sulphonic acid groups operating in such systems. The rheological properties were evaluated in detail for the surfmer modified sample. The chain aggregation through H-bonding of polar sulfonic acid groups and the branching effect were reflected in the melt rheological properties, as an increase of the plateau modulus (Gp) and a shift of the rubbery flow crossover point to lower frequencies. Such results might be anticipated if the existence of aggregated structures, which acts as physical crosslinks whose concentration increase with increasing surfmer content, is assumed. Thus, ionic reactive surfactants due to the specific structural characteristics impart ionomer character as well as internal plasticisation to the formed polymer, due to the polar sulfonate group and the alkyl chains of the surfmer, respectively. The observed results are in direct agreement with the increase in aggregation observed during SLS studies. These finding may help in understanding the enhanced properties of surfmer modified latex films.
The adhesion adhesion properties were evaluated with styrene-butyl acrylate dispersions containing varying amount of the surfmer. The formulated latex dispersions were applied to paper board over a 1 in 2 area and bonded together for the shear sandwhich test on a universal testing machine (UTM) machine as per standard test procdure (ASTM D-3164 (2011)). The results show that the sample with reactive surfmer possesss better strength in comparison to sample prepared without any reactive surfmer and the failure occurs with the substrate rather than failure of the adhesive itself, especially in cases with the highest concentration of the surfmer. For example, in terms of the failure load, the sample without any reactive surfactant failed at 213 N whereas a sample prepared with 3.44 mol% reactive surfactant show break load of 245 N i.e. 15 % increase of break load compared with normal emulsion adhesive. This is anticipated to be a consequence of the chain association through polar sulphonate groups present in the sample prepared with reactive surfactant. In the second project period the synthesis and characterisation of the polymerisable surfactant, sodium maleimido undecanoate (MIU) was also achieved as proposed in Annexure I. Its evaluation in synthesis of emulsions is under progress. The results achieved in this proejct is expected to add vlue to the utilisation of renewable resources and they could be useful in the industrial applciation like adhesisves / coatings.
During the incoming phase, a new polymerisable acrylate surfmer was synthesised and the performance of the surfmer in the dispersion synthesis and microstructural properties was undertaken using static light scattering and evaluation of the film formation properties. Solution property studies using static light scattering shows supramolecular structure formation originating from a combination of aggregation to inverse micelles and concentration dependent association of these aggregates to long filamentous microstructures. The influence of surfmer on the film formation was studied using tracer diffusion technique and a mechanistic understanding of the surfmer effect on the drying properties was deduced.
In latex films, the surfmer influence had two origins arising from the ion pair of the charged heads (ionomer effect) and the hydrophobic tail of the surfmer (plasticiser effect). The hydrophobic tail was found to act in wet and dry film as an internal plasticiser. The dissociated ion pair of the head group softens the particle surface in wet films, leading to increased polymer segment mobility and thereby enhanced particle deformation. The plasticiser effect was seen microscopically by the significant reduction of MFFT of PBMA above 10 degrees on addition of a slight amount of 0.3 mol% surfmer. This shows that film formation is facilitated without the addition of any volatile organic compounds (VOC) as filming aid. On a molecular scalem the plasticisation is seen as a dramatic enhancement of the dye diffusion in the domains formed by the partially fused particles over that in the particle cores. The effect was amplified in PBMA with only 0.3 mol% surfmer by a factor of about 1000 over the 1 in a typical industrial dispersion and even stronger than in a dispersion with particle surface saturated with the conventional non-reactive surfactant SDS (sodium dodecyl sulphate). In dry films, the ionomer effect and the internal plasticisation effect was found to counteract each other. The former is expected to a strengthening of the polymer by a supramolecular structure formation similar to the one found during solution microstructure studies, where as the latter soften the material.
The study revealed that the use of surfmers is advantageous for improvement of latex formulation in coatings. The significant enhancement of the particle deformation due to the substantially increased hydroplastcisation of the particle interface could allow for reduced amount of film forming aids and that are usually added to the plasticises the latex particle s to promote film formation. By judicious choice of the type and amount of surfmer, the final film may be mechanically strong because of the supramolecular structure formation induced by the surfmer. If surfactant migration is eliminated by the use of chemically attached surfmer, surfactant exudation and surfactant cluster formation can be achieved, which can enhance the resistance against water re-absorption on wetting by avoiding remnant hydrophilic channels in the film.
During the return phase, these effects were further confirmed using DMA and viscometry studies. The SLS studies gave information on chain association and the aggregation number. From detailed analysis of the form factor data, a structural model was derived and it was hypothesised that these copolymers form, a block-like structure with the surfmer-derived charges at one or at both ends. These chains associate to form ionomer-like clusters even in the dilute state with charges inside and uncharged PS chain segments on the outside, forming a daisy-like micro-structure with loops and dangling ends. On increasing the concentration, the clusters associate to form more extended, filament-like structures through bridge formation of the telomeric chains. In summary, studies during the incoming phases revealed that the addition of small amounts of surfmer leads to an improved film formation without detrimental effects on the final film properties and the approach could be used to reduce rest VOCs in coatings. The influence of these structures on the material properties was further revealed from the dynamic mechanical properties.
Measuring the dilute solution viscosity studies further helped to confirm the aggregation of surfmer modified chains in solution, compared to polystyrene homopolymer prepared without any surfmer. The copolymer prepared with 2.31 and 4.52 mol% surfmer content were insoluble in tetrahydrofuran (THF), anticipated due to stronger association of the polymer chains through sulfonic acid groups. The thermal and dynamic mechanical studies also show signatures of both plasticising effect by the alkyl groups and the polar interactions through sulphonic acid groups operating in such systems. The rheological properties were evaluated in detail for the surfmer modified sample. The chain aggregation through H-bonding of polar sulfonic acid groups and the branching effect were reflected in the melt rheological properties, as an increase of the plateau modulus (Gp) and a shift of the rubbery flow crossover point to lower frequencies. Such results might be anticipated if the existence of aggregated structures, which acts as physical crosslinks whose concentration increase with increasing surfmer content, is assumed. Thus, ionic reactive surfactants due to the specific structural characteristics impart ionomer character as well as internal plasticisation to the formed polymer, due to the polar sulfonate group and the alkyl chains of the surfmer, respectively. The observed results are in direct agreement with the increase in aggregation observed during SLS studies. These finding may help in understanding the enhanced properties of surfmer modified latex films.
The adhesion adhesion properties were evaluated with styrene-butyl acrylate dispersions containing varying amount of the surfmer. The formulated latex dispersions were applied to paper board over a 1 in 2 area and bonded together for the shear sandwhich test on a universal testing machine (UTM) machine as per standard test procdure (ASTM D-3164 (2011)). The results show that the sample with reactive surfmer possesss better strength in comparison to sample prepared without any reactive surfmer and the failure occurs with the substrate rather than failure of the adhesive itself, especially in cases with the highest concentration of the surfmer. For example, in terms of the failure load, the sample without any reactive surfactant failed at 213 N whereas a sample prepared with 3.44 mol% reactive surfactant show break load of 245 N i.e. 15 % increase of break load compared with normal emulsion adhesive. This is anticipated to be a consequence of the chain association through polar sulphonate groups present in the sample prepared with reactive surfactant. In the second project period the synthesis and characterisation of the polymerisable surfactant, sodium maleimido undecanoate (MIU) was also achieved as proposed in Annexure I. Its evaluation in synthesis of emulsions is under progress. The results achieved in this proejct is expected to add vlue to the utilisation of renewable resources and they could be useful in the industrial applciation like adhesisves / coatings.