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    Is Solvent Resistance Predictable ?

    To assess solvent resistance, first consider solvents spread out on a polarity scale. Non-polar solvents are at one end, such as paraffin, petrol and diesel. At the other ‘highly polar’ end we find alcohols and water.  In between are moderately polar solvents such as acetone, esters and chlorinated solvents.     On the same polarity scale we can place the families of thermoplastics. Polyethylene and polystyrene are at the non-polar end, polyamides at the highly polar end. PVC, acrylics and polyesters are somewhere in between in the ’moderately polar’ region.

    Solvent Resistance pic

    Where an amorphous thermoplastics and a solvent match up on the scale, we can expect total solubility, given a bit of time and heat to get the polymer to eventually dissolve.   However, semi-crystalline thermoplastics will not dissolve at ambient temperatures although the amorphous regions would mop up the matching solvent and cause swelling.  If a thermoplastic and solvent mismatch significantly on the scale then there would be little or no interaction, regardless of whether it is amorphous or semi-crystalline.

    This explains why styrene polymers are seriously affected by hydrocarbon solvents. In contrast, polypropylene and polyethylene, being semi-crystalline, show much better resistance at ambient temperatures.  The polarity scale also explains why PVC is credited with ‘good chemical resistance’.   Although PVC, a moderately polar, amorphous thermoplastic, can be attacked and dissolved in moderately polar solvents such as tetrahydrofuran, it is resistant to the solvents at either end of the polarity scale — oil, fuel and water, which of course are the more generally encountered every day chemicals.

    Solubility Parameter Scale

    Non Polar                                                                                                Highly Polar

    Hexane,Oil  Diesel    Toluene       Chloroform      Acetone       Ethanol             Water


    PP/PE                        PS           PMMA         PVC        PET   POM         PA 6.6

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    Coping with weld lines

    In injection moulding, weld lines (knit lines) form when two melt fronts meet. If the melt fronts do not coalesce completely, at best there will be a cosmetic flaw. At worst there will be a mechanical weak-spot, with strengths of the order of 10 - 90 % of the material potential.

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    Friction is an important property for thermoplastics in bearings and gears but also has a part to play in assembly of plastic parts (snap-fit and interference-fit) and ejection during moulding.

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    Polypropylene: the Workhorse of the Plastics Industry

    Polypropylene. Little did Karl Zeigler or Giulio Natta realise, 60 years ago, when they were developing a catalyst system to produce a useful thermoplastic from the inexpensive monomer, propylene, that their work would have such far reaching consequences.

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    Toughness and transparency are important properties for the constituents of intravenous lines.

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    The Long and the Short of Fibre Reinforcement of Thermoplastics

    Fibre Reinforcement. The advantages of adding glass fibre to thermoplastics to increase stiffness (modulus), strength, heat distortion resistance and dimensional stability are well known.

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    When mouldings are not the intended colour, the first thing to check is the raw material, particularly the dosing rate, if you are using masterbatch, and the quality of regrind.

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    Transparent ABS can be a clear winner

    Transparent ABS. Mentioning transparency in the context of ABS moulding materials can raise a few eyebrows. This is because ABS is normally taken to be opaque and indeed the vast majority of grades of ABS are opaque.

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    The difference between SBS and SEBS thermoplastic elastomers explained.

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    Is MFR really much help to moulders these days ?

    Often a moulder has to change material grades. One of the first properties to be consulted on the new data sheet is usually the MFR. This is to establish if the new grade has the same melt viscosity performance.

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    What gives Moulders a Warped View on Life?

    Warping is one of the most frustrating injection moulding faults. This is because it does not always appear immediately, the causes are complex and prevention is difficult.

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