External plasticisers

The volumetric, elastic and dynamic properties of internally and externally plasticised PVC were studied and compared with those of unplasticised PVC. The glass transition temperature for the plasticised samples was markedly lowered and this decrease was more important for the externally plasticised ones. The positions of the loss peaks from dielectric alpha-relaxation measurements confirmed the higher efficiency of the external plasticisation. However, the shape of the dielectric alpha-relaxation function was altered only for the internally plasticised samples. The plasticisation effect was linked with a decrease in the intensity of the beta-relaxation process but no important changes in the activation energy of this process were observed. The results were discussed. 47 refs. 

The purpose of plasticisation is to decrease hardness and to increase flexibility and elongation. Phthlates (e.g. butyl benzyl phthalate, dioctyl phthalate and butyl phthalate) are used as external plasticisers. Externally plasticised systems have a migratory tendency with ageing. In order to overcome this drawback, systems are being internally plasticised. This process involves the use of comonomers (e.g. acrylates and higher alkyl... 

It is possible to accelerate the rate of the polycondensation reaction by incorporation of special catalysts, resulting in urethane-urea-modified epoxy resin. The low-volatility alkyl phenol liberated in this reaction remains in the film as an external plasticiser. Owing to the different rates of the above two reactions, they have been synchronised to produce suitable polymer segments (Figure 10.9). 

A rigid polymer may be internally plasticised by chemically modifying the polymer chains with structural groups incorporated through a plasticising comonomer, which is a common way of plasticisation or it can be externally plasticised, simply by blending it with the resin. The latter being the most common because of the costs involved. UPVC (PVC-U) can be externally plasticised by use of certain phthalates. 

There are around 25 phthalate compounds, all acting as external plasticisers. Most of them are either persistent organic pollutants (POP) or POP-like compounds. 

External plasticisers can be classified either as primary or secondary , when considering their effects. A primary plasticiser improves mechanical properties (e.g., elongation... 

PVC dominates most of the literature on external plasticisers, mainly phthalate esters of Cg to Cjo, used to flexibilise PVC indoors flooring products to make them easy to roll, store and install. Some external plasticisers have high and low volatilities, but they are mostly toxic and ECD agents [17], and there is the probability that they will be emitted... 

There are also several external plasticisers with almost no proven toxicity, such as, tri-(2-ethylhexyl)trimellitate (TEHTM), di-2-ethylhexyl adipate (DEHA), and acetyl triburyl citrate, (ATBC), which are economically unfeasible for industrial applications, i.e., TEHTM is three times (and for DEHA, four times) as expensive as DEHP. In any case, the use of adipate, mellitate and azoalate type external plasticisers are expected to grow in use at the expense of different phthalate types [19]. Butene based alcohols are also used in the manufacture of flexible PVC [20], whereas polycaprolactone is used as a permanent and safer plasticiser for PVC [21],... 

Plasticisation may occur either internally or externally. In some cases, plasticiser function can be obtained by copolymerising the polymer with the monomer of a low Tg polymer such as PVA, this process is called internal plasticisation, typical external plasticisation is addition of dioctyl phthalate, diisooctyl phthalate and di-2 ethyl... 

One of the most common types of emulsion for decorative paint is based on vinyl acetate internally plasticised with about 20 parts of vinyl versatate. Di-butyl or di-octyl maleate or fumarate are also used as plasticising monomers for vinyl acetate, but butyl acrylate or 2-ethylhexyl acrylate are now preferred. Vinyl acetate homopolymers externally plasticised with di-butyl phthalate are still widely used. 

Obviously, the particle size is important because it affects the capillary and other film formation forces. However, varying particle size has minimal effect on MFFT. The T of the comonomers and the presence of external plasticisers are most important for film formation. 

The MFFT of the emulsion as formulated must be lower than any tempoature at which the paint is likely to be applied. For an indoor paint in the UK it is unlikely that temperatures below 10°C will be encountered, whilst the outdoor limit might be 5°C. Damage to paint films can occur should the ambient temperature fall below the Tg of the film. The Tg of a hard polymer, such as vinyl acetate, styrene or methyl methacrylate, can be reduced either by copolymerisation with a soft monomer or by adding an external plasticiser, such as butyl phthalate. On a cost effective basis external plasticisation is better for reducing the Tg. The snags are, firstly, that plasticisers are slowly lost through evaporation and, secondly, they can also diffuse into the substrate. The use of an internally plasticised polymer overcomes both problems and also saves the processing time necessary to incorporate an external plasticiser. 

The earliest injection moulding machines were of the plunger type as illustrated in Fig. 4.30 and there are still many of these machines in use today. A predetermined quantity of moulding material drops from the feed hopper into the barrel. The plunger then conveys the material along the barrel where it is heated by conduction from the external heaters. The material is thus plasticised under pressure so that it may be forced through the nozzle into the mould cavity. In order to split up the mass of material in the barrel and improve the heat transfer, a torpedo is fitted in the barrel as shown. 

Lubricants can be classified as internal or external lubricants. Internal lubricants should be partially miscible with the polymer at processing temperatures (i.e., behave similar to a plasticiser), but phase separate at ordinary temperatures. Whereas plasticisers are completely miscible with the bulk polymer, lubricants have a limited solubility. 

Traditionally, a low-molecular-weight liquid plasticiser addition method has been used for achieving flexible PLA films. However, the film made with this method was found to be unstable against changes in external factors such as temperature and pressure, resulting in the bleeding out of the liquid plasticiser, which in turn would lead to defects in the film characteristics such as transparency and flexibility, which were altered over time. 

In PVC-P applications, the plasticiser acts as an internal lubricant and external lubricants can be added to increase output etc. Excessive levels or incorrect choice can cause problems with post treatment, e.g., printing or blooming on ageing. 

Lubricants have been classified as either internal or external. Typically, internal lubricants have meant materials that are compatible with PVC that promote flow. The difference between internal lubricants and plasticiser is that the internal lubricant is only soluble at high temperature whereas the plasticiser is soluble and functional at room temperature. External lubricants on the other hand, are said to be incompatible and come to the surface and create metal release. It is reported that there are two distinct types of external lubricants. Internal lubricant efficiency can be readily predicted by simple mathematical formula and that partial substitution of esters for paraffin allows the reduction of modifier or increased filler levels to achieve savings without loss of any physical properties. 

The monograph states that the nature and amount of additive used will depend on the type of polymer, the process used to convert it into a container and the intended purpose of the container. Approved additives include antioxidants, stabilisers, plasticisers, lubricants, colour and impact modifiers. Anti-static and mould release agents can be used only for containers for oral and external preparations. Specific permitted additives are given in the specification for the material within the pharmacopoeial monograph. 

Lubricants may therefore act as either as an external or an internal agent. Depending on their chemical nature, a few may have secondary actions such as dispersing agents, plasticisation, or slip agents. These are usually included up to a 1% level. 

Plasticisers may function either externally or internally in preparation and action, and thus can be of internal or external types. 

Additives are used for a wide variety of purposes, and may be classified as fillers, anti-oxidants, stabilisers, plasticisers, fire retardants, pigments and lubricants. Anti-oxidants and stabilisers are usually used in rather small quantities in order to prevent degradation of the polymer when it is exposed to air, light and heat the intention here is to maintain the properties of the polymer rather than to modify them. Fillers may be used either simply to produce a cheaper product or to improve the properties, in particular the mechanical properties. Lubricants may be used externally, to prevent adhesion of the polymer to the processing equipment, or internally, either to aid flow during processing or to reduce friction between the product and other materials. 

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