The purpose of the present paper was to test the validity of the nonlinear regression method for calculating the non-isothermal crystallization rate constant of the Nakamura's model of a rotational molding grade LLDPE directly from non-isothermal crystallization experiments carried out in a single cell DSC. Cooling rates of 50, 40, 30, 20, 10 and 5 oC/min were used with samples of 3.0 mg under nitrogen atmosphere. Here, good agreement was observed between the experimental relative crystallinity curves and the simulated ones using the calculated parameters by nonlinear regression. It shows that this method can be used to determine the Nakamura's non isothermal rate constant for using in simulation of the cooling phase of rotational molding. In this paper it was used 10-3, 10-4 and 10-14 as the initial crystallinity in the Nakamura's model. However the best average results for all cooling rates was obtained when 10-4 was used. Average spherulitic dimensions of LLDPE studied in this paper did not change significantly with different cooling conditions.

No presente estudo o empenamento de peças de polietileno linear de baixa densidade (PELBD) moldadas por rotomoldagem foi investigado. O efeito de diferentes fatores como espessura da peça, taxa de resfriamento e diâmetro de tubo de ventilação foi avaliado. Além dos experimentos de rotomoldagem, uma técnica alternativa denominada "Hot Press" foi também aplicada para investigar o empenamento de dois diferentes PELBD sob condições típicas da rotomoldagem quanto ao resfriamento assimétrico. Cristalinidade e morfologia esferulítica ao longo da espessura das peças rotomoldadas foram avaliadas por Calorimetria Exploratória Diferencial (DSC) e Microscopia Ótica de Luz Polarizada. Verificou-se que o grau de empenamento aumenta com a taxa de resfriamento. O aumento do diâmetro do tubo de ventilação é mais efetivo na redução do empenamento principalmente em menores taxas de resfriamento. Nos experimentos de "Hot Press" o PELBD com menor índice de fluidez e maior módulo de flexão apresentou menor empenamento. Nos experimentos de "Hot Press" a espessura das peças afetou o empenamento somente para maiores taxas de resfriamento. Em geral, a cristalinidade e o tamanho de esferulitos se mostraram menores nas posições ao longo da espessura das peças rotomodadas onde as taxas de resfriamento eram mais rápidas.

In the present study the warpage of rotationally molded parts of a linear low density polyethylene (LLDPE) was investigated. The effect from different processing factors such as part thickness, cooling condition and diameter of the venting tube was evaluated. In addition to the rotational molding experiments, an alternative experimental technique, referred to as "Hot Press", was also applied to investigate the warpage of two different grades of LLDPE under typical rotational molding conditions, which means slow cooling from only one side. The crystallinity and spherulitic morphology along the thickness of the rotationally molded parts were studied by Differential Scanning Calorimetry and Polarized Light Optical Microscopy. It is shown that the warpage increases with increasing cooling rate. The increase in diameter of the venting tube is more effective in reducing the warpage of rotationally molded parts, especially for lower cooling rates. In hot press experiments the grade of LLDPE with lower melt flow index and higher flexural strength presented lower warpage. The part thickness affects the warpage in hot press experiments only for faster cooling rates. In general, crystallinity and spherulitic diameter are lower in positions along the cross section of the rotationally molded part where the cooling rate is faster.