PVC-Plasticiser Interaction

Solvation-desolvation equilibrium

From the observation of migration of plasticiser from polymers, it is possible to conclude that some plasticiser molecules are not bound permanently to the polymer as in an internally plasticised resin. This means that an exchange/equilibrium mechanism is in action implying that there would not be a stoichiometric relationship between polymer and plasticiser levels, although some quasi-stoichiometric relationships appear to exist.  This idea is presented in the discussion of specific interactions

Specific interactions

Ideas on the subject of specific interactions between PVC and a plasticiser molecule, as a basis of plasticisation, can be considered a more detailed form of some of the theories presented in this section.  Clearly, some attraction and interaction mechanisms between PVC and plasticiser must exist for plasticiser to be retained in the polymer after processing. Some recent work has extended these ideas using new analytical techniques, in particular molecular modelling and Solid State Nuclear Magnetic Resonance Spectroscopy.

The study of the role of specific interactions in the plasticisation of PVC has been inspired by the work on specific interactions of esters in solvents (e.g. hydrogenated chlorocarbons) and on blends of polyesters with PVC.  Modes of interaction between the carbonyl functionality of the plasticiser ester or polyester were proposed, mostly on the basis of results from Fourier Transform Infra-red Spectroscopy (FTIR).  Shifts in the absorption frequency of the carbonyl group of the plasticiser ester to lower wave number, indicative of a reduction in polarity (i.e. some interaction between this functionality and the polymer) were reported.  Work performed with dibutyl phthalate suggested an optimum concentration at which such interactions were maximised.  Spectral shifts were in the range 3-8cm. Similar shifts were also reported in blends of PVC with polyesters, showing a concentration dependence on the shift to lower wave number of the ester carbonyl absorption frequency.

Recent developments

Over the past few years these methods have been assessed and extended by many researchers, in particular the Loughborough group. It has been shown that solubility parameters were capable of classifying plasticisers of a given family in terms of their compatibility with PVC but that they were of limited use for comparing plasticisers of different families (e.g. phthalates with adipates).  Polarity parameters provided useful predictions of the activity of monomeric plasticisers but, again, were not able to compare activity of plasticisers from different families.  In all cases, it has not been possible to adequately predict the behaviour of polymeric plasticisers.

Molecular modelling

With recent advances in computer science, the computer modelling of molecules is a rapidly growing branch of chemistry. High resolution graphics and fast computers allow the operator to build molecules in minimum energy configurations and observe their behaviour in real time. This model can be constructed from crystallographic coordinates or by simple intervention from the operator.  Molecular mechanics or quantum mechanics programs are then used to arrive at a likely structure.

A range of plasticiser molecule models, and a model for PVC were generated and energy minimised to observe their most stable conformations. Such models highlighted the free volume increase caused by the mobility of the plasticiser alkyl chains. More detailed models were also produced to concentrate on the polar region of the plasticiser and its possible mode of interaction with the polymer.  These showed the expected repulsion between areas on the polymer and plasticiser of like charge as well as attraction between the negative portions of the plasticiser and positive portions of the PVC.

Solid state nuclear magnetic resonance spectroscopy

Recent advances in technology have made possible the study of solids by Nuclear Magnetic Resonance (NMR) techniques. For the accumulation of solid state, the 13C Cross Polarisation Magic Angle Spinning (CP-MAS) NMR spectra can be used to significantly reduce signal broadening effects present in solid state but not in the liquid state. The technique was used to study the molecular effects of plasticisation by comparing spectral shifts of PVC and plasticiser under various degrees of processing.

For PVC plasticised with DIDP, two different processing temperatures, 130°C and 170°C, were used, representing a low degree and high degree of plasticisation respectively. The comparison of the spectra showed no shift in the resonance frequency of the carbonyl group.  The most significant difference in the two spectra was in the aliphatic carbon resonances. The spectra of the more plasticised sample showed resonance shifts and increased resolution for these carbon atoms. This again shows a strong dependence of successful plasticisation on the conformation of the alkyl chains of the plasticiser ester linked to the increased free volume.

What can be concluded from all of these theories and studies is that plasticiser polarity is important in determining the gelation rate but it does not explain other properties of interest in the final product. What is crucial is the conformation adopted by plasticiser molecules in the polymer matrix in the final product, since this will relate to how many PVC-PVC chain-chain interactions are screened from each other and how much free volume is created. What recent studies have shown is that whilst this conformation is important it is perhaps not so important in samples which have experienced high processing temperatures, since in these samples the separation of the PVC chains and introduction of plasticiser is controlled more by thermal energy than by polarity.  At lower processing temperatures the polarity of the plasticiser has a greater role imparting acceptable physical properties to the final product.