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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.

    pp chart

    The early propylene homopolymer had similar properties to polyethylene but with higher stiffness and a higher temperature working range.  Soon it was discovered that particulate mineral fillers (eg talc) gave even higher stiffness, better elevated temperature performance, reduced mould shrinkage and better dimensional stability.  Coupling agents ensured that glass fibre filled grades not only had high stiffness but also good strength properties.

    The unique ability to form a ‘living hinge’ fascinated designers….and frustrated moulders.   Nucleating agents were added to provide higher transparency, while biaxial orientation during processing gave thin film with high strength and high clarity, quickly displacing packaging film based on cellulosic thermoplastics.  Uniaxial orientation led to polypropylene tape and fibre, replacing jute and other fibres in sacking, carpets and sports surfaces.

    Incorporating ethylene as a comonomer yielded copolymers, both block and random, which greatly improved low temperature impact resistance, the Achilles heel of the polypropylene homopolymer.  Higher ethylene content in the copolymers led to a range of thermoplastic elastomers. These had better chemical resistance than the styrene based TPEs.

    Homopolymer grades are favoured by the film and fibre sectors while copolymer grades are used more in injection moulding.  Today, thermoplastics based on propylene can be considered the workhorses of the plastics industry. However, one wonders why propylene copolymers with higher olefins and more polar monomers have not been more developed.

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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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    What determines friction between thermoplastic components?

    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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    Polyacetal (POM): When to Select Homopolymer or Copolymer?

    The production of stable thermoplastics from formaldehyde was discovered around 60 years ago, using two independent approaches.  To prevent unzipping of the polymer chains at high temperature, DuPont used an end cap mechanism. Celanese opted for incorporation of a comonomer which also stopped the unzipping.  Today you still have a choice between the homopolymer grades […]

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    What makes medical grade plastics so special?

    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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    What causes mouldings (and moulders) to be off-colour?

    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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    Understanding the difference between SBS & SEBS thermoplastic elastomers?

    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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