The analysis of the cross-link density of prevulcanised natural rubber latex using near infrared spectroscopy was conducted using a Fourier transform near infrared spectrometer in diffuse reflection mode over the wavenumber range of 12500–3600 cm−1. As the density of cross-link is an indication of the degree of cure, hence the properties of the latex products, the proposed method is useful for industrial purposes. For samples of prevulcanised latex of 50% total solids content (i.e. PV 50%) at 100% extension (prevulcanisate relaxed modulus 100%), the best model was developed using the partial least squares regression from the spectra, which were pre-treated using the first derivative method, where the coefficient of determination (r2), root mean square error of prediction and bias were 0.66, 6.06 × 104 Nm−2 and 1.63 × 104 Nm−2, respectively. The ratio of standard error of prediction to the standard deviation of the reference data in the prediction sample set was 1.8. This model could be used for screening. For samples at 300% extension (prevulcanisate relaxed modulus 300%) for PV 50%, the best model was developed using spectra pre-treated for scattering correction: r2, root mean square error of prediction and bias were 0.88, 6.74 × 104 Nm−2 and 1.35 × 104 Nm−2, respectively and the ratio of prediction to deviation was 3.0. Hence, the near infrared spectroscopy technique can be utilised as a rapid screening method for estimating the cross-link densities of prevulcanised natural rubber latex.

Toluene swell or equilibrium swelling is universally used by rubber factories to measure the degree of crosslink of their compounded or prevulcanized latices at different stages of production. To apply near infrared spectroscopy for rapid and accurate quality control, spectral acquisition of prevulcanized latex, thin film and thick film was performed using a Fourier transform near infrared spectrometer in diffuse reflection mode across the wavenumber range of 12,500-3600 cm À1. For prevulcanized latex an effective model was developed using partial least squares regression with preprocessing (first derivative þ straight line subtraction method). The coefficient of determination (r 2), root mean square error of cross validation and bias of the validation set were 0.71, 3.93% and À0.005%, respectively. For the thin film model the r 2 , root mean square error of cross validation and bias were 0.65, 4.01% and À0.028%, respectively. Whereas for the thick film model the r 2 , root mean square error of cross validation and bias were 0.70, 4.00% and À0.006%, respectively. Three models including prevulcanized latex, thin film and thick film were validated by 23 unknown samples, providing standard error of prediction and bias of 5.357 and 2.494, 4.565 and 1.001 and 3.641 and À0.961%, respectively, for prevulcanized latex, thin film and thick film. The model developed for the thick film spectra gave the best results.

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Volatile Fatty Acid number (VFA no.) is one of the parameters indicating the state of quality of Para rubber latex at that particular time. Most factories analyze this parameter using standard analytical method as in ISO 506:1992(E). Nevertheless, this procedure is complicated, chemical and time consuming, as well as skilled analyst required. Therefore, near infrared (NIR) spectroscopy which is rapid, accurate and nonchemicals method was applied to determine the VFA no. in¯eld latex and concentrated latex based on quanti¯cation and discriminant model. The best calibration equation was obtained from standard normal variate (SNV) spectra in the region of 6109.7-5770.3, 4613.1-4242.9 cm À1 with R ¼ 0:832, SECV ¼ 0:036 and no bias. From the performance check, statistically it was found that SECV and bias were low enough for practical acceptance and the predicted VFA no. was not di®erent signi¯cantly from actual VFA no. at 95% con¯dence intervals. In addition, discriminant model was developed to separate good quality latex from the deteriorated latex using VFA no. at 0.06 as standard as in ISO 2004ISO :2010. The discriminant model can be used to screen the latex with overall accuracy of 91.86% in validation set.

The parameters including physico-chemical properties such as dry rubber content (DRC), total solids content (TSC), volatile fatty acid (VFA) number, alkalinity (ammonia content) and potassium hydroxide (KOH) number and physical properties such as viscosity and cross-link density are important to the trading and processing of natural rubber latex and its products. Traditionally these properties of field and concentrated latices and their products including thin and thick films are evaluated with chemicals and time-consuming methods and with the need of technical experts. The near-infrared (NIR) spectroscopy is a nondestructive, rapid, accurate and environment-friendly technique. It needs no chemical. This chapter describes the application of NIR spectroscopy on evaluation of the properties of natural rubber latex and its products.