AIM to Impact
About Impact Modifiers
In the initial days of Thermoplastics industry, plastics like Polystyrene(PS), Rigid Polyvinyl Chloride(PVC) and Polymethyl Methacrylate(PMMA) exhibited little or no Impact Strength in their homopolymer form. Thus, they were low cost and easily formed, but had very little practical applications.
To overcome this problem in the initial days, it was thought that modifying the polymer ‘backbone’ could be one of the prime solutions. Classic example of this was Bisphenol-A Polycarbonate(PC) which showed excellent impact strength in its unmodified state. However, this method too posed many challenges.
Firstly, modifying the backbone itself was considered as an ‘art’, and was very difficult to achieve. Secondly, Modifying the polymer backbone would later lead to processing difficulties for the polymer. This left very little room for the manufacturers to process these types of polymers.
However, an alternative approach was devised to overcome the above-mentioned problems. A rubbery additive was added to the polymer while processing which would impart impact strength to the base polymer. These rubbery additives were called Impact Modifiers.
Impact Modifiers are the additives used to enhance the toughness of the material on impact. They are used to provide a proper balance between load bearing and impact resistance. Most commonly used Impact Modifiers are Acrylonitrile-Butadiene-Styrene(ABS),Methacrylate-Butadiene-Styrene(MBS), Acrylic rubbers, Chlorinated Polyethylene and Ethylene-Vinyl Acetate(EVA).
Acrylic Impact Modifiers(AIM)
Acrylic Impact Modifiers are one of the most commonly manufactured Impact Modifiers to be used in processing of PVC. Unlike other Impact modifiers, AIM do not hinder other properties of the base polymer like UV stability, Thermal Stability, etc. Thus, it is suitable for Indoor and Outdoor applications Acrylic Impact Modifier(AIM) are generally manufactured in the form of core-shell type modifier. In these types of modifiers, there is a low Tg rubber core, which in case of AIM is Polybutylene Acrylate. This core is surrounded by a shell, which is a material having a high Tg like PMMA.
These structures are most conveniently synthesised via emulsion polymerisation technique, by use of seeded polymerisation process, it is possible to obtain good control over the particle size of the resulting particles. The core/shell polymers are typically covalently crosslinked to retain their core/shell integrity even after processing. Thus, core/shell type of system provides for a lot of potential for modification, so that an ease of processing is obtained to the manufacturer.
In case of formulation, the amount of AIM ranges broadly from 1 to 20 parts by weight. The formulation factor generally relies on the type of article that is to be formed from the processed PVC. For low-impact toughness, a range of 5-15% of AIM in the formulation would suffice. But for imparting super toughness to the article, a range of 20-25% is preferred.
Acrylic based polymers are specially designed to impart impact strength to rigid PVC formulations, for purpose of outdoor uses. Their excellent processability and ability to not hinder any other physical property of the base polymer, accounts for their high performance-to-cost ratio.
Typical applications of AIM are:
· Window Profiles
· Furniture Sheets
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