Modifier impact

Impact modifiers help to improve impact properties by toughening the system, as well as improving the melt index, processability and weatherability. 

As an example, rigid PVC provides an excellent cost/performance balance with inherent flame retardancy. It is used particularly in building and construction applications such as window frames, doors, fencing and plumbing, but it needs improvement in some mechanical and weatherability properties, which is achieved by use of an acrylic-styrene or a butadiene-based, rubbery impact modifier. 

Impact modifiers function by dispersing a damping phase capable of absorbing energy (to stop craze propagation) into the brittle matrix, and in general, elastomerics are preferred and used. 

Acrylic copolymers (i.e., core-shell impact modifiers with a shell of PMMA and a core of butyl acrylate elastomer) have been developed mainly for impact modification of PVC for outdoor applications. Butadiene-styrene copolymers are used exclusively for PVC, PC or styrene-acrylonitrile (SAN). Thermoplastic elastomers in the form of styrenic copolymers, e.g., SBS, are used preferably for styrenics and PA. Polyolefins, like EVA, are used for impact modification of technical polymers. 

Processing aids are usually based on high molecular weight acrylic copolymers (for PVC). They modify the rheology and processing characteristics of the melt. Lubricants are processing aids that function to ease the process and are of two types either internal lubricants (that influence the viscosity, such as calcium stearates) or external lubricants (such as oxidised polyethylene wax). Lead-stabilised PVC lubricants are a part of the stabiliser system. They are important in PVC foam formulations. 

The most commonly encountered impact modified plastic is high impact polystyrene (HIPS). The impact modification is achieved by incorporating discreet domains of a mbber (butadiene) within the polystyrene matrix. The level of butadiene can be varied according to the requirements of the end use. A high level can convert a glassy brittle material into a ductile, tough product. The efficiency of the butadiene to improve impact resistance alone is not solely related to concentration, but is also dependent on the size and 

There are some instances (e.g., rubber impact modification of nylon) where the chemical properties of the two polymers are so dissimilar that solubility differences can be used to isolate and, in this case, quantify the rubber modifier. The test is carried out by taking an accurately weighed sample and refluxing it in formic acid until the plastic is digested and only the rubber phase remains. The mbber is then washed, dried and weighed. 

Other plastics which can contain mbber modifiers (or at least modifiers that have lower glass transition temperatures than the plastic phase) include PET and PVC. In the case of PVC, acrylates and methacrylates are common and the normal approach is to use IR to detect the modifier and then NMR to quantify it. 

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  

CPE impact modifiers contain around 35% of chlorine and are thermoplastic in nature. With a similar melting point to PVC, they initially form a network structure which changes to a particulate structure on processing (134). They also have good processability and excellent weathering performance consequently they are also used for window profiles (287). The effect of residual 

MBS impact modifiers are used in a wide range of applications, particularly for clear packaging, as the refractive indices are similar between PVC and the modifier. They are not suitable for outdoor applications. Their impact behaviour has been studied in terms of the ductile/brittle transition (450). 

Specific ABS terpolymer modifiers also process well in clear applications giving low crease whitening and excellent chemical resistance. The heat distortion temperature of the ABS modified compound can be higher in comparison to MBS modified. They are also not suitable for outdoor applications. 

Butadiene-styrene rubbers in powder form for PVC-U modification have also been proposed (95). 

Such materials essentially contain PS as the matrix polymer and uniformly dispersed in this matrix are elastomeric types of particles, which form the soft phase (3). The soft phase is essentially composed of poly(butadiene) or of block copolymers of butadiene and styrene. This soft phase can be also addressed as the impact modifier for PS. 

High impact poly(styrene) (HIPS) is sometimes also accessed as TPS, which stands for toughened PS. 

Methyl methacrylate-grafted latex rubber particles has been studied for the impact toughening of PS styrenic matrix polymers. The 

The effectiveness of methacrylate-grafted latex rubbers for HIPS was unexpected because of the poor miscibility properties of these type of polymers (5). The situation is different in the case of acrylonitrile-butadiene-styrene (ABS) types that are miscible with methacrylate-grafted latex rubbers. 

Nano-powdered styrene/butadiene rubber has been synthesized by the radiation crosslinking of styrene-butadiene rubber (SBR). Tri-methylolpropane triacrylate can be used as crosslinking agent. This monomer improves the radiation crosslinking of the SBR latex. 

As mentioned extensively, PPE is not mainly used as such, but in polymeric blends and copolymers to faciUtate the fabrication. Some of these copolymers act also as impact modifiers for example, block copolymers built from styrene, ethylene, butylene, and propylene. Naturally, the impact can be improved by using high impact poly(styrene) (HIPS) instead of ordinary PS in blends. Other impact modifiers include rubbery materials, such as poly(octenylene), and ethylene propylene diene monomer rubber. 

The above relationships between structure and basic mechanical properties are obtained from the analysis of experimental compounds as well as theoretical analy- 

It can be thus summarized that a range of mechanical behavior can be generated by the appropriate choice of processing conditions, filler size, shape and concentration, and elastomer content and its distribution between the matrix and filler. These choices should lead to results predicted by theory.  

The inclusion of rubber in polymers does, nevertheless, reduce the elastic modulus and the yield stress [53]. The phase separation between the polymer and the rubber is an important requirement, and mechanical resistance increases if the rubber has low elastic modulus in relation to the matrix, good adhesion to the matrix, adequate crosslinking, optimized average particle size and distribution, and low glass transition temperature [54]. The separating distance between the elastomeric zones also plays an important role in the toughening mechanisms. 

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The principal monomer of nitrile resins is acrylonitrile (see Polyacrylonitrile ), which constitutes about 70% by weight of the polymer and provides the polymer with good gas barrier and chemical resistance properties. The remainder of the polymer is 20 to 30% methylacrylate (or styrene), with 0 to 10% butadiene to serve as an impact-modifying termonomer. 

Poly(methyl methacrylate) Cast sheet Impact- modified Heat- resistant ... 

Acrylics. Acetone is converted via the intermediate acetone cyanohydrin to the monomer methyl methacrylate (MMA) [80-62-6]. The MMA is polymerized to poly(methyl methacrylate) (PMMA) to make the familiar clear acryUc sheet. PMMA is also used in mol ding and extmsion powders. Hydrolysis of acetone cyanohydrin gives methacrylic acid (MAA), a monomer which goes direcdy into acryUc latexes, carboxylated styrene—butadiene polymers, or ethylene—MAA ionomers. As part of the methacrylic stmcture, acetone is found in the following major end use products acryUc sheet mol ding resins, impact modifiers and processing aids, acryUc film, ABS and polyester resin modifiers, surface coatings, acryUc lacquers, emulsion polymers, petroleum chemicals, and various copolymers (see METHACRYLIC ACID AND DERIVATIVES METHACRYLIC POLYMERS). 

When monomers of drastically different solubiUty (39) or hydrophobicity are used or when staged polymerizations (40,41) are carried out, core—shell morphologies are possible. A wide variety of core—shell latices have found appHcation ia paints, impact modifiers, and as carriers for biomolecules. In staged polymerizations, spherical core—shell particles are made when polymer made from the first monomer is more hydrophobic than polymer made from the second monomer (42). When the first polymer made is less hydrophobic then the second, complex morphologies are possible including voids and half-moons (43), although spherical particles stiU occur (44). 

MBS polymers are prepared by grafting methyl methacrylate and styrene onto a styrene—butadiene mbber in an emulsion process. The product is a two-phase polymer useful as an impact modifier for rigid poly(vinyl chloride). 

Property Nylon-6,6 Nvlon-6,6 + 30% Glass fiber (wt / wt) Impact modifier Nylon-12 Nylon-12 + plasticizer... 

Mechanical Properties. Properties of typical grades of PBT, either as unfiUed neat resin, glass-fiber fiUed, and FR-grades, are set out in Table 8. This table also includes impact-modified grades which incorporate dispersions of elastomeric particles inside the semicrystalHne polyester matrix. These dispersions act as effective toughening agents which greatly improve impact properties. The mechanisms are not fiiUy understood in all cases. The subject has been discussed in detail (171) and the particular case of impact-modified polyesters such as PBT has also been discussed (172,173). 

Noryl. Noryl engineering thermoplastics are polymer blends formed by melt-blending DMPPO and HIPS or other polymers such as nylon with proprietary stabilizers, flame retardants, impact modifiers, and other additives (69). Because the mbber characteristics that are required for optimum performance in DMPPO—polystyrene blends are not the same as for polystyrene alone, most of the HIPS that is used in DMPPO blends is designed specifically for this use (70). Noryl is produced as sheet and for vacuum forming, but by far the greatest use is in pellets for injection mol ding. 

Poly(vinyl chloride). PVC is one of the most important and versatile commodity polymers (Table 4). It is inherently flame retardant and chemically resistant and has found numerous and varied appHcations, principally because of its low price and capacity for being modified. Without modification, processibiUty, heat stabiUty, impact strength, and appearance all are poor. Thermal stabilizers, lubricants, plasticizers, impact modifiers, and other additives transform PVC into a very versatile polymer (257,258). 

Eor high performance appHcations in the automotive industry, nylon—PPO blends with impact modifiers have been introduced (173,177). 

There is extensive Hterature on PC blends with ABS, and blends of PC with related materials such as SAN, methacrylate-butadiene—styrene (MBS) emulsion-made core-shell mbber modifiers (297—299), and other impact modifiers. One report reviews some of these approaches and compares PC blends based on emulsion vs bulk ABS (229). In PC—ABS blends, no additional compatihili er is used, because of the near-miscihility of the SAN matrix of ABS and PC. 

Also, PBT is blended with poly(ethylene terephthalate) (PET), polysulfone, and SMA (303). PET may also be blended with a number of other engineering polymers, such as PC and impact modifiers. 

Tables 5 and 6 summarize key properties and appHcations for miscible and immiscible blends which are either commercial as of 1996 or were commercialized in the past (2,314—316,342,343). Most of the Hsted blends contain only two primary components, although many are compatibiLized and impact-modified. Consequently, an immiscible system consisting of two primary components or phases may contain impact modifiers for each phase and a compatihilizer copolymer, for a total of five or more components.

By employing additives to improve interfacial adhesion and the cohesive strength of the mbber phase, natural mbber can compete with ethylene—propylene mbbers as an impact modifier for polypropylene. These hard grades, containing between 15 and 25% natural mbber, have the potential for use in the automotive and domestic markets, eg, in bumpers, spoilers, grilles, electrical connectors, and floor tiles. 

Not only ate ABS polymers useful engineering plastics, but some of the high mbber compositions are excellent impact modifiers for poly(vinyl chloride) (PVC). Styrene—acrylonitrile-grafted butadiene mbbers have been used as modifiers for PVC since 1957 (87). 

Styrene-based plastics are used somewhat in blow mol ding but not as much as linear polyethylene and PVC. HIPS and ABS are used in specialty botdes, containers, and furniture parts. ABS is also used as one of the impact modifiers for PVC. Clear, tough bottles with good barrier properties are blow-molded from these formulations. 

The level of technical service support provided for a given product generally tracks in large part where the suppHer considers thek product to be located within the spectmm of commodity to specialty chemicals. Technical service support levels for pure chemicals usually provided in large quantities for specific synthetic or processing needs, eg, ammonia (qv), sulfuric acid (see SuLFURic ACID AND SULFURTRIOXIDe), formaldehyde (qv), oxygen (qv), and so forth, are considerably less than for more complex materials or blends of materials provided for multistep downstream processes. Examples of the latter are many polymers, colorants, flocculants, impact modifiers, associative thickeners, etc. For the former materials, providing specifications of purity and physical properties often comprises the full extent of technical service requked or expected by customers. These materials are termed undifferentiated chemicals (9),... 

The largest volume commercial derivatives of 1-butanol are -butyl acrylate [141-32-2] and methacrylate [97-88-1] (10). These are used principally ia emulsion polymers for latex paints, ia textile appHcations and ia impact modifiers for rigid poly(vinyl chloride). The consumption of / -butanol ia the United States for acrylate and methacrylate esters is expected to rise to 182,000—186,000 t by 1993 (10). 

Considerable amounts of EPM and EPDM are also used in blends with thermoplastics, eg, as impact modifier in quantities up to ca 25% wt/wt for polyamides, polystyrenes, and particularly polypropylene. The latter products are used in many exterior automotive appHcations such as bumpers and body panels. In blends with polypropylene, wherein the EPDM component may be increased to become the larger portion, a thermoplastic elastomer is obtained, provided the EPDM phase is vulcanked during the mixing with polypropylene (dynamic vulcani2ation) to suppress the flow of the EPDM phase and give the end product sufficient set. 

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