Types of Impact Modifier

Rubbers, often based on poly diene rubbers or else copolymers of dienes like 1,3-butadiene, were the first successful toughening additives, and they are effective partly because they have a low modulus, 100 to 500 times lower than that of most thermoplastic polymers. Unfortunately polydienes introduce chemical double bonds which are susceptible to UV, thermal and oxidative degradation. Hydrogenation removes some of them. Acrylic compounds and ethylene copolymers are also popular impact modifiers, and they do not necessarily introduce double bonds. 

So-called core-shell impact modifiers have a less damaging effect on the modulus and HDT. They are made by emulsion graft polymerisation, and consist of two parts. As their name implies, one part is an outer shell of a hard acrylic polymer-like PMMA, in direct contact with the polymer that needs toughening the two must be compatible. (In the case of SAN copolymers, the compatibility depends on the polarity, and hence on the nitrogen content of the copolymer.) 

Inside the shell is a core of a more resilient, energy-absorbing polymer such as crosslinked polybutadiene, natural rubber, butadiene-styrene copolymer or butyl acrylate. The rubbery phase is thus prevented from mixing with the continuous phase (the polymer being toughened) and lowering its modulus. 

There is an optimum rubber particle size for toughening, dependent on the polymer being upgraded. If crazing is to be promoted, ABS requires a low particle size of around 500 mn to 1 micron, whereas HIPS needs 2 to 4 microns, and can benefit from even larger particles. Bimodal particle size distributions may be beneficial in some instances to facilitate two energyabsorbing mechanisms at the same time. 

If the polymer is not compatible with the impact modifier, a compatibiliser (see earlier in this chapter) may be needed. The choice will depend on both the main constituents. Maleated ethylene-octene and maleated SEBS are often used, since they also contribute to the impact modifying action. When polyamide-6 is mixed with ABS, the compatibiliser can be styrene-maleic anhydride copolymer, poly(methyl methacrylate co-maleic anhydride) or poly(methyl methacrylate co-glycidyl methacrylate). 

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Unmodified PVC-U has relatively poor impact strength at, and below, ambient temperatures and the inclusion of an impact modifier significantly improves its performance. Examples of the different generic types of impact modifier that can be added to PVC formulations are as follows ... 

The effect of the type of impact modifier on the melt flow of a PVC window profile formulation as a function of shear rates encountered during extrusion was investigated and the relationship between the melt flow and mechanical properties of the profiles evaluated. A Rheoplast Capillary Rheometer with a pre-shearing device was employed to investigate the melt viscoelastic properties of the formulations and the performance of the formulations in terms of post-extrusion shrinkage, surface gloss and enthalpy relaxation discussed. 2 refs. 

Details are given of the effect of processing on mechanical properties of PVC window profiles by altering the temperature profile set on the extruder and by varying the shear heating phenomena using different lubrication balances. The effect of filler level and type of impact modifier on the impact properties of extruded profiles with various levels of free volume are presented. 7 refs. 

Two general types of impact modified acryhc resins have been developed commercially, viz. (a) weatherable, impact modified, transparent acryhc resins for outdoor use in signs and automobiles ... 

The impact modification of aliphatic PAs has been known for a long time. Impact modifiers for PPA are acrylic polymers composed from of ethylene, n-butylacrylate, acrylic acid, and maleic anhydride. Other types of impact modifiers are ethylene propylene rubbers, grafted with maleic anhydride. ... 

Table 19 1 Some common reactive rubber and other types of impact modifiers used for meltblending and impact modification of polyamides...

TABLE 4.15 Major Types of Impact Modifiers by Resin... 

Ethylene vinyl acetate (EVA). Ethylene vinyl acetate modifiers have minor usage compared to other types of impact modifiers. EVA finds use in limited segments of the flexible PVC sheet business. 

Other types of impact modifier for epoxies include (i) so-called hyperbranched additives that mix with the polymer but phase-separate on cining, and (ii) core-shell polybutadiene-aciylic modifiers. 

Table 1.2 Major Types of Impact Modifiers by Resin (Ref Kattas, L, et. al., Plastics Additives , Modern Plastics Handbook, C.A. Harper, ed., McGraw-Hill, New York, 2000)... 

The major types of impact modifiers are acrylics, styrenics including methacrylate-butadiene-styrene (MBS) copolymers and Acrylonitrile-Butadiene-Styrene Polymers, chlorinated polyethylene (CPE), EVA copolymers, and the ethylene-propylene copolymers and terpolymers (EPR and EPDM respectively). The major market for impact modifiers is in PVC, although they are used in a wide range of other polymers such as polyolefins and engineering polymers. 

Luran S BASF SAN acrylics one of the most stable modified resins mosUy due to the type of impact modifier... 

The previous discussion points the need for different types of impact modifiers. Some are used to prevent flaws, and some minimize the effects of these flaws even though they do not prevent flaws. Some impact modifiers modify the effects of flaws that are caused by other additives. There are many materials to serve these many different functions. 

Impact modifiers are usually prepared by grafting methylmetacrylate and styrene on a styrene-butadiene rubber in an emulsion process (29,30). The preparation and properties of these polymers has been reviewed (31). Impact efficiency is achieved by controlling the rubber substrate particle size between 1000 and 3000 A in a narrow distribution range. The solubility-incompability balance determines the performance of impact modifiers. Typical opaque compound impact modifiers show high efficiency. High-efficiency impact modifiers are used in PVC conduit, injection molding components and calendered opaque films and sheets. Various types of impact modifiers are available (32,33). 

Impact Strength Versus Impact Behavior Characterization Volume of Rubber Types of Impact Modifiers... 

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