Elsevier

Polymer Testing

Volume 28, Issue 7, October 2009, Pages 773-779
Polymer Testing

Short Communication: Analysis Method
Monitoring of oil-based polyurethane synthesis by FTIR-ATR

https://doi.org/10.1016/j.polymertesting.2009.05.013Get rights and content

Abstract

Oil-based polyurethanes were synthesized from triglyceride oil-based hydroxyl containing material and toluene diisocyanate or hexamethylene diisocyanate for wound dressing applications. The reactions were carried out with or without catalyst at 90 °C. The amount of free isocyanate during the reaction was determined by a FTIR-ATR method, and the results were compared with those from a back-titration method. The data obtained were used for determination of kinetic parameters.

Introduction

Polyurethanes are one of the most important subclasses of the family of thermoplastic elastomers. They exhibit excellent mechanical and physical properties, high wear resistance and good blood and tissue compatibility. For this reason, they are widely used for medical applications.

Polyurethanes are generally prepared by step-growth polymerization of diisocyanates with diols (Scheme 1). Toluene-2,4-diisocyanate (TDI), hexamethylene diisocyanate (HMDI) and diphenylmethane diisocyanate (MDI) are the most commonly used isocyanate components in polyurethane synthesis. There is a growing interest in the use of triglyceride oils as a hydroxyl source for polyurethane formulation, due to the economic aspect. Hydroxyl-containing oils, such as castor oil, or partial glyceride mixture prepared from triglyceride oil/fatty acid and glycerol are used to obtain oil-based polyurethanes (urethane oils) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. These types of polymers can be used for biomedical applications as wound dressing material [15], [16].

The reaction for polyurethane formation is widely monitored by two methods; standard dibutylamine back-titration and Fourier transform infrared (FTIR) spectroscopy. Both of them are direct methods which measure the concentration of reactant or products, and can be used to obtain kinetic data for the related reaction [17], [18], [19], [20], [21], [22]. The back-titration method is a well known method for conventionally titrating for free isocyanate content; however, it is both time and chemical consuming. FTIR spectroscopy technique is a good alternative method for kinetic studies, but it has also limited practical use due to the sample preparation requirements. The cast film technique is mainly used for sample preparation. According to this technique, the sample is first dissolved in a non hygroscopic solvent. A defined amount of this solution is placed on KBr cell surface. The solution is then evaporated to dryness, and the formed film is analyzed directly. A bulk polymer sample can also be used in FTIR technique for a kinetic study, but the same amount of sample must be used for each measurement. Consequently, sample preparation for both back-titration and FTIR analysis is time consuming and requires careful work in order to obtain valid data for kinetic studies.

Rheometry and thermal methods are indirect methods and have some difficulties or restrictions [23], [24]. Raman spectroscopy can be also used for investigation of the kinetics for polyurethane polymerization [25], but it is not common.

Although the FTIR in attenuated total reflection (ATR) mode is one of the major techniques for surface characterization, it has been suggested that it is a useful tool for monitoring the formation of lignin based polyurethanes [26]. The aim of the present study is to investigate the use of FTIR-ATR method for determination of the amount of free isocyanate during the reaction of oil-based polyurethanes. The use of the ATR mode allows analysis of smaller samples without requiring any additional sample preparation and, hence, is beneficial over FTIR and back-titration. In this study, first some model mixtures at various isocyanate concentrations were prepared and their free isocyanate contents were determined by both back-titration and FTIR-ATR methods. After comparison of the two methods, the FTIR-ATR method was used to determine free isocyanate content of the reaction mixture during polymer synthesis. The data obtained were evaluated to calculate kinetic parameters.

Section snippets

Materials

Commercially purchased linseed oil was used as the oil component. The main characteristics of triglyceride oil were: refractive index (nD20), 1.4813; acid value, 1.3; saponification value, 196; iodine value, 165.6. Toluene diisocyanate (TDI, Merck), hexamethylene diisocyanate (HMDI, Merck), and anhydrous glycerol (Merck) were used in the synthesis. Calcium hydroxide was used as a catalyst for the preparation of a hydroxyl containing component from linseed oil.

Polymer synthesis

Polymer synthesis was achieved in

Free isocyanate content of model mixture

The first calibration curve (Fig. 1) was drawn from the results of the back-titration method and calculated free isocyanate contents for model mixtures. This yields a straight line with a slope of almost 1, and the regression coefficient (R2) is also very close to 1.

The second calibration curve (Fig. 2) was obtained from the calculated data and the FTIR-ATR measurements. Again this is a straight line and it was used for determination of free isocyanate content of the reaction mixture during the

Conclusions

Polyurethanes from oil-based hydroxyl containing material (HCC) and TDI or HMDI were successfully synthesized in the absence of catalyst by bulk polymerization. FTIR spectroscopy in ATR mode can be used to follow the reaction in the bulk state. The data obtained from the analysis of reaction mixture by FTIR spectrum can be used to calculate the kinetic parameters.

The polyurethane formation between TDI and HCC prepared from linseed oil and glycerol obeys second order rate law. The kinetic study

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