RHEOLOGICAL STUDY OF PVC PLASTISOLS DURING GELATION AND FUSION

Printability during projection-based 3D bioprinting

Since projection-based 3D bioprinting (PBP) could provide high resolution, it is well suited for printing delicate structures for tissue regeneration. However, the low crosslinking density and low photo-crosslinking rate of photocurable bioink make it difficult to print fine structures. Currently, an in-depth understanding of the is lacking. Here, a research framework is established for the analysis of printability during PBP. The gelatin methacryloyl (GelMA)-based bioink is used as an example, and the printability is systematically investigated. We analyze the photo-crosslinking reactions during the PBP process and summarize the specific requirements of bioinks for PBP. Two standard quantized models are established to evaluate 2D and 3D printing errors. Finally, the better strategies for bioprinting five typical structures, including solid organs, vascular structures, nerve conduits, thin-wall scaffolds, and micro needles, are presented.

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The photo-crosslinking mechanism and operation specifications is systematically explored during PBP.

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The mechanism of printing errors are analyzed, and a guide is provided to decrease the printing errors theoretically.

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Two standard models are proposed for evaluating the printing errors.

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Five typical applications are summarized and the suggestion printing strategies are formulated to improve the print quality.

Study of the rheology and foaming processes of poly(vinyl chloride) plastisols with different foaming agents

Poly(vinyl chloride) (PVC) plastisols are widely used in the production of flexible PVC foams. In this study, we investigated the evolution of the complex viscosity of PVC plastisol by dynamic oscillatory tests, the storage modulus of the PVC compound by dynamic mechanical analysis, and the thermal behavior including the decomposition of three chemical blowing agents (CBAs), namely, azodicarbonamide, 4,4′-oxybis(benzenesulfonyl hydrazide), and sodium bicarbonate, by differential scanning calorimetry. Furthermore, the morphology and quality of the foams obtained from the corresponding plastisols were characterized by scanning electron microscopy. The results indicated that the onset decomposition temperature T2(5%) of a CBA in plastisol is the most critical indicator of the foam quality. The temperature difference corresponding to [T2(5%) – Tηmax] was also proved to be another important parameter. When T2(5%) is within the optimum temperature range of a PVC plastisol, the bigger the [T2(5%) – Tηmax] difference, the better the quality of the foams.

Effect of the particulate morphology of resin on the gelation process of PVC plastisols

The gelation process of poly(vinyl chloride) (PVC) plastisol is very important to ensure the quality of the product and is affected by the type of resins, plasticizers, and other additives. In this study, the gelation process of the as-prepared PVC plastisol was characterized by measuring the evolution of vibrational viscosity with temperature or time using a vibrational viscometer. Furthermore, the effect of some commercial resins with different particulate morphologies on the gelation process was investigated by synchronously combining scanning election microscopy and laser particle size analyses. The results of this study proved that the particle size distribution and the aggregation degree of the secondary particles of a resin are the key factors affecting the gelation process. For the resin with bimodal particle size distribution, the closer the aggregation of the secondary particles, the slower the gel speed; however, an opposite behavior was observed for the resins with unimodal particle size distribution.

Epoxidized Esters of Palm Kernel Oil as an Effective Plasticizer for PVC: A Study of Mechanical Properties and Effect of Processing Conditions

One of the most commonly used vegetable oil plasticizer in polyvinyl chloride (PVC) is epoxidized soybean oil (ESBO). On the contrary, epoxidized palm oil is among the least used, because of its low compatibility with PVC. This work reports that epoxidized and esterified palm kernel oil (EEPKO) has the potential to be used as a plasticizer for PVC. In this study, it was found that a maximum of 65 phr of EEPKO could be incorporated within the PVC resin. In addition, subjecting this EEPKO plasticized PVC (p-PVC) at a processing temperature of 200 °C resulted in much reduced plasticizer loss. EEPKO lowers the glass transition temperature of PVC from 79 °C to −23 °C. The optimum mechanical properties, namely tensile strength, elongation at break and hardness, were obtained for p-PVC processed at 200 °C.

Rotational Moulding of PVC Plastisol

The viscosity during the earlier stages of the gelation process of plastisols formulated with 15 different commercial plasticizers has been obtained. Plastisols prepared were rotomoulded and mouldings obtained were characterized according to their wall thickness distribution. This parameter strongly depends on the minimum viscosity and on the rate of viscosity change during gelation (slope of the curves during the viscosity increase), and also on the temperature where no more flow of the plastisol is observed (gelation temperature). These variables are conditioned by the characteristics of the plasticizer and PVC employed, among other variables.

The effect of plate and arm velocity and rotation rate on the wall thickness distribution was also studied.

A simple model has been proposed to correlate the initial stage of the gelation of PVC plastisols formulated with the phthalate plasticizers. The suggested model assumes that the plastisol is a suspension constituted by a continuous phase (initially pure plasticizer) and a discontinuous phase constituted by PVC particles. It has been considered that the viscosity of the plastisol is influenced by the average mass molecular weight of the continuous phase and the temperature. The quantity of PVC dissolved in the continuous phase has been calculated by using a generic n-order kinetic equation for PVC dissolution rate. Additionally, a swelling process of the PVC particle by the plasticizer has also been considered and its effect on the viscosity of the suspension has also been accounted for. The model proposed provides a very good fitting of the sharp viscosity increase observed and is useful to predict the evolution of the viscosity of the parts in a rotomoulding process.