An Overview Of The Plasticisation Process
The discussion in this section refers to external plasticisation only.
 Several theories have been proposed in order to explain plasticiser action and some of these are described in detail below. Each theory varies in its detail and complexity. Some involve detailed analysis of polarity, solubility and interaction parameters and the thermodynamics of polymer behaviour, whilst others treat plasticisation as a simple lubrication of chains of polymer, analagous to the lubrication of metal parts by oil. Whilst each theory is not exhaustive, an understanding of the plasticisation process can be gained by combining ideas from each theory and, additionally, an overall theory of plasticisation must include all these aspects.
What must be stated however are the steps involved in the incorporation of a plasticiser into a PVC product. This can be divided into five distinct stages:
- Plasticiser is mixed with PVC resin.
- Plasticiser penetrates and swells the resin particles.
- Polar groups in the PVC resin are freed from each other.
- Plasticiser polar groups interact with the polar groups on the resin.
- The structure of the resin is re-established, with full retention of plasticiser.
Parts 1-2 can be described as a physical plasticisation, and the precise details of how this is carried out depends on the applications technology involved (i.e. suspension or paste PVC).
The rate at which part 2 occurs depends on the physical properties of plasticiser including viscosity, resin porosity and particle size.
Parts 3-4 however, can be described as a chemical plasticisation since the rate at which these processes occur depend on the chemical properties of molecular polarity, molecular volume and molecular weight. An overall mechanism of plasticiser action must give adequate explanations for this as well as the physical plasticisation steps.
The importance of part 5 cannot be stressed too strongly, since no matter how rapidly and easily parts 1-4 occur, if plasticiser is not retained in the final product the product will be rendered useless. As will be described in Section 5 (plasticisation of other polymers) many polymers perform steps 1-4 adequately but are not able to retain the plasticiser in the final product, leading to a product which is totally unacceptable.
The lubricity theory
This is based on the assumption that the rigidity of the resin arises from intermolecular friction binding the chains together in a rigid network. On heating these frictional forces are weakened so as to allow the plasticiser molecules between the chains. Once incorporated into the polymer bulk the plasticiser molecules shield the chains from each other, thus preventing the re-formation of the rigid network. Whilst attractive in its simplicity the theory does not explain the success of some plasticisers and the failure of others.
The gel theory
This extends the lubricity theory in that it deals with the idea of the plasticiser acting by breaking the resin-resin attachments and interactions and by masking these centres of attachment from each other, preventing their reformation. Such a process may be regarded as being necessary but again by itself is insufficient to explain a completely plasticised system since while a certain concentration of plasticiser molecules will provide plasticisation by this process the remainder will act more in accordance with the lubricity theory, with unattached plasticiser molecules swelling the gel and facilitating the movement of plasticiser molecules, thus imparting flexibility.
Molecules acting by this latter action may, on the basis of molecular size measurements, constitute the bulk of plasticiser molecules. If plasticisation took place solely by this method it would not be possible to explain the ability of PVC resins to accept their own weight in plasticiser without exudation i.e. large amounts of additional space ("free volume") are created which other plasticiser molecules can occupy.
The free volume theory
This extends the above ideas and also allows a quantitative assessment of the plasticisation process.
Free Volume of a polymer is described by the equation
Vf = Vt - V0 (1)
In which:
Vf = the free volume of the resin
Vt = specific volume at a temperature t
V0 = Specific volume of an arbitrary reference point, usually taken as zero degrees Kelvin
Free volume is a measure of the internal space available in a polymer for the movement of the polymer chain, which imparts flexibility to the resin. A rigid resin (eg unplasticised PVC) is seen to possess very little free volume whereas resins which are flexible in their own right are seen as having relatively large amounts of free volume. Plasticisers therefore act so as to increase the free volume of the resin and also to ensure that free volume is maintained as the resin-plasticiser mixture is cooled from the melt. Combining these ideas with the gel and lubricity theories, (see above) it can be seen that plasticiser molecules not interacting with the polymer chain must simply fill free volume created by those molecules that do. These molecules may also be envisaged as providing a screening effect preventing interactions between neighbouring polymer chains thus preventing the rigid polymer network reforming on cooling.
For the plasticised resin, free volume can arise from:
motion of the chain ends motion of the side chains motion of the main chain
These motions can be increased in a variety of ways, including:
increasing the number of end groups increasing the length of the side chain increasing the possibility of main group movement by the inclusion of segments of low steric hindrance and low intermolecular attraction Introduction of a lower molecular weight compound which imparts the above properties Raising the temperature
The introduction of a plasticiser, which is a molecule of lower molecular weight than the resin, has the ability to impart a greater free volume per volume of material since (i) there is an increase in the proportion of end groups and (ii) it has a glass transition temperature (Tg) lower than that of the resin itself. A detailed mathematical treatment of this (7) can be carried out to explain the success of some plasticisers and the failure of others. Clearly, the use of a given plasticiser in a certain application will be a compromise between the above ideas and physical properties such as volatility, compatibility, high and low temperature performance, viscosity etc. This choice will be application dependent ie there is no ideal plasticiser for every application.
Solvation-desolvation equilibrium
Due to the observation of migration of plasticiser from plasticised polymers it is clear that plasticiser molecules, or at least some of them, are not bound permanently to the polymer as in an internally plasticised resin, but that an exchange/equilibrium mechanism is present. This would imply that there would not be a stoichiometric relationship between polymer and plasticiser levels, although some quasi-stoichiometric relationships appear to exist (8,9). This idea is extended in the discussion of specific interactions (below).
Generalised structure theories and antiplasticisation
In their simplest form these theories attempt to produce a visual representation of the mechanism of plasticiser action, with illustrations appearing in the literature (2-5). The theories are based on the concept that if a small amount of plasticiser is incorporated into the polymer mass it imparts slightly more free volume and gives more opportunity for the movement of macro-molecules. Many resins tend to become more ordered and compact as existing "crystallites" grow or new "crystallites" form at the expense of the more fluid parts of the amorphous material. For small additions of plasticiser, the plasticiser molecules may be totally immobilised by attachment to the resin by various forces. These tend to restrict the freedom of small portions of the polymer molecule so necessary for the absorption of mechanical energy. Therefore it results in a more rigid resin with a higher tensile strength and base modulus than the base polymer itself. This phenomenon is therefore termed antiplasticisation. A study by Horsley (5) used X-Ray Diffraction to show that small amounts of di-octyl phthalate (DOP) progressively increase the order in the PVC. Above these concentrations the order decreases and the polymer becomes plasticised.
This is discussed further under specific interactions (below).
Specific interactions and interaction parameters
Early attempts to describe PVC-plasticiser compatibility were based on the same principles as used to describe solvation i.e. "like dissolves like" (7). To obtain a quantitative measure of PVC-plasticiser compatibility a number of different parameters have been used. These are briefly described below.
The hilderbrand solubility parameter, d
This can be estimated (13) based on data for a set of additive constants for the more common groups in organic molecules to account for the observed magnitude of the solubility parameter. These constants are designated F, for which
d = F/V (2)
In which:
V = Molar volume.
Polarity parameters
These were evaluated by Van Veersen and Meulenberg (13) and despite their apparent simplicity they show a good correlation with plasticiser activity for non-polymeric plasticisers. The parameter is defined as:-
= M(Ap/Po) (3)
1000
In which:
M = Molar Mass of Plasticiser
Ap = Number of Carbon atoms in the Plasticiser excluding Aromatic and Carboxylic Acid Carbon Atoms
Po = Number of Polar (e.g. carbonyl) groups present
The 1000 factor is used to produce values of a convenient number.
The solid-gel transition temperature, Tm
This is a measure of plasticiser activity. It is the temperature at which a single grain of PVC dissolves in excess plasticiser. The more efficient plasticisers will show lower values of Tm as a result of their higher solvating power. This can be correlated with the ease of processing of a given plasticiser, but all measurements should be conducted with a control PVC resin since clearly the choice of resin has an effect here also.
The flory-huggins interaction parameter
These ideas, based on a study of polymer miscibility, were applied to plasticisers by Anagnostopoulos (13) according to the equation:-
1/Tm = 0.002226 + 0.1351(1 - )/V1 (4)
In which:
V1 = Molar volume of the plasticiser, obtained from molar mass figures and density values at Tm
The activity parameter
This is an extension of and was based on work by Bigg (13). It is another measure of plasticiser activity and is defined as
= 1000 (1 - ) (5)
M
Again this gives an indication of the ease of processing for a given plasticiser with a given resin, but will not give estimates of plasticising performance in the final product.
Recent summaries
Over the past few years these methods have been assessed and extended by many workers, in particular the Loughborough group (16, 17) and further work was also described recently (18). It was 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 was not possible to adequately predict the behavior of polymeric plasticisers.
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 ideas already discussed. Clearly some mechanism of attraction and interaction between PVC and plasticiser must exist for plasticiser to be retained in the polymer after processing.
The role of specific interactions in the plasticisation of PVC has been proposed from work on specific interactions of esters in solvents (e.g. hydrogenated chlorocarbons) (19), work on blends of polyesters with PVC (20-25) and work on plasticised PVC itself (26-29). 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 wavenumber, indicative of a reduction in polarity (i.e. some interaction between this functionality and the polymer) were reported (26-28). Work performed with dibutyl phthalate (28) suggested an optimum concentration at which such interactions were maximised. Spectral shifts were in the range 3-8cm1. Similar shifts were also reported in blends of PVC with polyesters (20-26), again showing a concentration dependence of the shift to lower wavenumber of the ester carbonyl absorption frequency.
Recent studies
Some recent work (18) has extended these ideas using new analytical techniques, in particular molecular modelling and Solid State Nuclear Magnetic Resonance Spectrsocopy.
Molecular modelling
With recent advances in computer science, the computer modelling of molecules is a rapidly growing branch of chemistry (30-32). High resolution graphics and fast computers allow the operator to build molecules in minimum energy configurations and view them in real time. This model can be constructed from crystallographic coordinates available from data bases 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 the study of solids by Nuclear Magnetic Resonance (NMR) techniques of considerably greater ease than in previous years. For the accumulation of solid state 13C NMR spectra Cross Polarisation Magic Angle Spinning (CP-MAS) can be utilised 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 (18, 37). For PVC plasticised with DIDP two different processing temperatures, 130oC and 170oC 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 with processing temperature. 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 of the plasticiser but it does not explain other properties of interest in the final product. What clearly is of importance is the conformation adopted by plasticiser molecules in the polymer matrix in the final product, since this will relate to (i) how many PVC-PVC chain-chain interactions are screened from each other and (ii) how much free volume is created. What the 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 ingress of plasticiser, is controlled more by thermal energy than by plasticiser polarity. At lower processing temperatures the polarity of the plasticiser has a greater role in the attainment of acceptable physical properties of the final product.
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