Selection of Impact Modifiers for Polymers

What is the need for impact modifiers?

Impact modifiers improve the durability and toughness of a variety of plastic resins. Hence, they are added to plastic compounded materials. Besides impact modification, they can help improve other characteristics of the material such as:

optical and tensile properties
weatherability
processability
flammability
heat distortion

Formulators need to achieve a very different level of impact resistance. This is based on the end-use applications and the polymer's intrinsic resistance. There are various levels of impact modification requirements that are discussed below.

General-purpose impact modification

General-purpose impact modification is a very low level of impact modification. This is, for instance, applied to avoid conditioning of molded polyamide (PA) parts.

It translates to reasonable room temperature impact strength. It does not take into account any requirements for low-temperature (below 0 °C) impact strength.

For most of this type of application, only low levels of impact modifiers are required (<10%). The impact modifier does not necessarily have to contain reactive groups to be acceptable for the application.

Low-temperature impact modification

Low-temperature impact strength is needed for applications that require a certain level of:

low-temperature flexibility and
resistance to break

This is the case for many applications in the appliance area. For this purpose, reactive modifiers are necessary in the levels between 5-15%.

Super toughness impact modification

Super-tough impact strength will be required for applications that should not lead to a failure of the part even if hit at low temperatures (-30 to -40 °C) under high speed. This need can only be fulfilled with high levels (20-25%) of reactive impact modifiers. They have low glass transition temperature (Tg).
Levels of Toughness

Different Levels of Impact Modification Requirements

Key features that make impact modifiers unique

Some of the key features of impact modifiers in plastics are listed below:

Improved impact resistance
Impact modifiers absorb energy during impact and prevent cracks from propagating. Thus, they improve the impact resistance of plastics. This can make plastics more resistant to damage from drops, bumps, and other impacts.

Increased toughness
Impact modifiers make plastics more resistant to deformation and tearing. This helps to increase the toughness of plastics. This can make plastics more durable and able to withstand more wear and tear.

Enhanced flexibility
Impact modifiers can enhance the flexibility of plastics by making plastics more pliable and able to bend without breaking. This can make plastics easier to process and use in a wider range of applications.

Increased UV resistance
Impact modifiers can increase the UV resistance of plastics. This is because they protect them from degradation caused by ultraviolet radiation. This can extend the lifespan of plastics exposed to sunlight.

Enhanced weatherability
Impact modifiers enhance the weatherability of plastics. They make plastics more resistant to degradation caused by weathering factors. For example, rain, wind, and temperature extremes. This can make plastics more durable and last longer in harsh environments.

Reduced cost
Impact modifiers can reduce the cost of plastics by allowing for the use of less expensive base resins. This is because impact modifiers can improve the properties of plastics without increasing their cost.

The amount of impact modifier added to a plastic depends on the desired properties of the final product. For example, if a plastic needs to be very tough, more impact modifier will be added.

How do impact modifiers work?

The elastomeric and rubbery nature of impact modifiers absorbs or dissipates the energy of impact. They can be incorporated:

through polymerization in the reactor, or
as additives in the compounding step

The two mechanisms by which impact modifiers work are discussed below.

Craze propagation

The principle here is to disperse impact modifiers into the brittle matrix. This is a dampening phase. It is capable of absorbing energy and stopping craze propagation.

Mechanism of Craze Propagation

Shear band/cavitation

A second mechanism is the formation of shear bands. These are formed around the elastomeric particle absorbing deformation energy. This mechanism is always accompanied by the cavitation of the dampening particle (apparition of voids). They also absorb the energy. However, the apparition of shear bands absorbs most of the energy.

Mechanism of Shear Band/Cavitation

To be efficient, the dispersed phase needs to have the following properties:

Dampening capability: The elastomeric phase is recommended. Generally, low Tg and low-crystallinity polymers are used. Low Tg is required for low-temperature toughening. Polyolefin copolymers are excellent candidates.

Good cohesion with the continuous phase: This parameter is the key to efficient toughening. Lack of cohesion can initiate numerous crazes that can then propagate until failure. Good cohesion can be obtained by specific interaction at the surface or by reactivity. The compatibilization occurs by formation, at the interface of "amphiphilic" copolymers. This reduces surface tension and increases adhesion.

Polymer compatibility will also impact the size, regularity, and stability of the dispersion. This positively affects the mechanical performance of the finished part.

Processing techniques

The main processing methods for impact modifiers are explained below:

Extrusion — It is a process in which molten plastic is forced through a die to produce a continuous sheet or profile. Impact modifiers can be added to the plastic melt before it is extruded, or they can be coextruded with the plastic.

Injection molding — It is a process in which molten plastic is injected into a mold to form a part. Impact modifiers can be added to the plastic melt before it is injected into the mold, or they can be incorporated into the mold itself.

Calendering — is a process in which molten plastic is passed between two rollers to produce a thin sheet. Impact modifiers can be added to the plastic melt before it is calendared. They can also be coated onto the surface of the sheet after it has been calendared.

Thermoforming — It is a process in which a sheet of plastic is heated to a temperature that makes it soft and pliable. It is then formed into the desired shape by applying vacuum or pressure. View all thermoforming grades of impact modifiers.

Choose specific impact modifier grades for different processing techniques:

Commonly available physical forms

The specific form of impact modifier used depends on the type of plastic, the desired properties of the final product, and the processing method. They come in a variety of physical forms:

Powders — Powders are the most common form of impact modifier. They are typically added to the plastic melt before it is extruded or molded.

Pellets — Pellets are like powders, but they are larger and have a more regular shape. They are often used in injection molding applications.

Masterbatches — Masterbatches are concentrated mixtures of an impact modifier and a carrier resin. They are typically used in small amounts to achieve a desired level of impact modification.

Liquids — Liquids are typically used in solution blending applications. They are dissolved in a solvent and then mixed with the base polymer.

Elastomers — Elastomers are rubber-like materials that are often used as impact modifiers. They are typically added to the plastic melt in the form of small particles or fibers.

Explore various physical forms of impact modifiers here:

Functionalized Polyolefin Impact Modifiers

To fulfill the industry requirements, several polymers need improved impact resistance. These polymers include polyamide, polyester, PVC, or bioplastics.

Among the impact modification technologies available in the market, polymeric impact modifiers offer a full range of toughening performance. They are also known as functionalized polyolefins. Their performance ranges from general-purpose to super toughening in various polymer systems.

Let's understand the need for some of the key polymers used and how their impact resistance can be improved using this class of impact modifiers.

Polyamide (PA)

A broad range of impact modifiers, based on non-functionalized or functionalized ethylene copolymers or ionomers, are available. They help to meet the unique needs of PA 6, PA 6,6, or glass-reinforced PA compounds. They offer several benefits like:

improved flow for higher productivity
aesthetic properties (Class A surface finish, excellent colorability)
higher graft level to improve efficiency for cost reduction
FDA compliance for direct food contact

It offers industry-leading impact resistance performance. For example, super-tough impact resistance, low-temperature toughness, and intermediate toughness at reduced cost.

Polyesters (PBT, PET)

Polymeric impact modifiers offer a wide range of performance levels. This allows tailored solutions to meet unique requirements. This can be seen in the following applications.

Engineering polymers
Some polymeric impact modifiers provide super-tough impact resistance in virgin and glass fiber-reinforced compounds. This is true for applications with the most demanding requirements. There is a challenge for increasing impact strength while maintaining original properties. This happens when compounding PBT engineering polymers. Among the wide range of offerings, these impact-modifying solutions give compounders a valuable new tool. This allows tailoring the properties of PBT resins to the requirements of electrical and electronic connectors and a range of other industrial and consumer products.

Cast sheet applications
Increasing productivity while achieving the right impact strength properties is a complex challenge. This occurs with PET-based cast sheet applications. View all impact modifiers for PET.

Benefits in Engineering Polymer Applications	Benefits in Cast Sheet Applications
Toughening and the retention of original properties Higher melt flow Improved processability and better thermo-stability Higher strength retention (tensile strength and flexural modulus) Enhanced hydrolysis resistance	Improved productivity (cycle time, process stability, regrind utilization) Material cost reduction (lower viscosity PET ) (CPET) Food regulations compliant (FDA, European) (APET)

Polyvinyl chloride (PVC)

Different types of PVC resins require different impact modification additives. This depends on the end-use of PVC. It helps in achieving the right performance goals.

Flexible PVC	Rigid PVC
Durable strength and flexibility Better low-temperature properties Greater property retention after heat aging Better flexibility after chemical exposure	Improved flow and fusion characteristics of the compound Enhanced filler compatibility Lower processing temperature Higher throughput Higher filler loading Lower stabilizer content

Polypropylene (PP)

Polypropylene is a semi-crystalline polymer. It exhibits a very attractive cost-performance balance and easy processability. However, to fulfill some industry needs, PP requires improved impact resistance at ambient or low temperatures.

Impact modifiers improve the toughness obtained for PP at room or low temperatures. They offer several benefits like:

enhanced dispersion of pigments, glass fibers, or mineral loads
improved compatibility for PP alloys

A broad portfolio of products is available to offer a unique and customized solution for each situation.

Acrylonitrile butadiene styrene (ABS)

ABS resins perform at a level between engineering plastics, like PC, and commodity materials, like PS. They are widely used in applications such as computer and printer housings, consumer electronics, appliances, garden equipment, automotive parts, and toys. Poor toughness can be encountered while producing ABS compounds. This is true for the production of standard, recycled, or filled grades.

Impact modification in ABS has several benefits like:

high compatibility
high dispersibility (allows in-line modification during processing)

However, it is a highly complex challenge for one specific solution to exist. It depends on the temperature required for general-purpose strength performance.

Polycarbonate blends (PC/ABS, PC/PBT)

Actual requirements concerning polycarbonates are connected with superior low-temperature impact strength. They are also required to maintain good processability, allowing efficient production of highly specific parts and profiles. For example, automotive applications by injection molding. A specific additive is required depending on the polymer used to blend the PC-based resin and the level of toughness needed.

Compared to alternative technology, additional benefits can be found within the following:

Better processability of the compound due to reduced melt viscosity
Better UV and thermo-stability
Higher elongation
Easier handling & processing due to pellet form instead of powder

Find all impact modifier grades that are compatible with various other polymers like:

Core-shell Impact Modifiers

A representation of a typical core-shell impact modifier is provided below.

These materials usually have a low Tg rubber core. For example, butyl acrylate or butadiene, with a polymethyl methacrylate (PMMA) shell. Examples of commercially available core-shell impact modifiers are:

PARALOID™ from Dow Chemical
Clearstrength^® from Arkema

One of the primary advantages offered by the core-shell impact modifier approach is that a pre-determined particle size is provided. However, the impact modifier must be appropriately dispersed in and coupled to the matrix polymer. This facilitates it to be effective for toughening engineering plastics.

Excellent Dispersion of MBS Impact Modifiers

This coupling can result from the physical interaction of the shell matrix with the matrix or by chemical reaction. This can be achieved by combining reactive moieties into the shell chains. This takes place during fabrication by emulsion polymerization. Those reactive moieties, then, subsequently react with the matrix during melt processing.

MBS impact modifiers in polycarbonate

Polycarbonate (PC) is known for several key features during the lifespan of the final article. These include:

transparency,
excellent resistance to impact, and
ability to withstand high temperatures

However, PC's low chemical resistance (gasoline) is an issue for automotive applications. The injection molding of high-viscosity grades is another limitation. This happens especially when high-impact resistance is required. Moreover, the inherent performances of PC, such as impact, are also seriously compromised. This occurs when colored pigments, fillers, or flame retardant additives are used in the compound.

Recycled PCs are a cost-effective solution for compounders. However, the recycling steps reduce the PC's mechanical performance. This makes the use of impact modifiers necessary in recycled PCs to reach the desired level of performance.

Key benefits in PC

Benefits	Description
Exceptional low-temperature impact performance	MBS impact modifiers allow PCs particularly high-flow PCs to achieve high-impact performance at very low temperatures. High-performance impact modifiers based on butadiene rubber (ABS or MBS) are detrimental to the inherent UV and heat stability performances of polycarbonate. In weatherable outdoor applications or high-heat environments, it will be critical to use a very stable impact modifier that will still provide high-impact performances. Impact modifiers are designed to respond to this technical challenge.
Excellent weatherability & heat aging	MBS impact modifiers provide excellent weatherability expected of an all-acrylic impact modifier. This allows impact modifiers to be used in applications that demand good color hold and retention of mechanical properties upon typical outdoor exposure, yielding products that retain long-term performance under severe conditions.
Exceptional mold-in colorability	MBS impact modifiers provide far more superior colorability than most of the acrylic modifiers available in the market, which are known to lower color intensity thus making dark color parts difficult if not impossible to achieve. Based on patented technology, they have opened the door for applications that demand good color hold and unique mechanical properties retention upon exposure to the elements.

MBS impact modifiers in polycarbonate blends (PC/ABS, PC/PBT)

Compounders have designed polymer blends that balance the inherent benefits of PC with the unique cost-performance characteristics of other matrices like ABS or polyesters (PBT). This helps to meet new market requirements, including technical performance and cost.

These polymer blends offer high performance as compared to traditional PCs. These blends help:

Overcome poor flow behavior and embrittlement
To improve flow behavior in opaque applications of high-impact grade PC, a rubbery phase, such as ABS, may be added to the PC matrix. PC/ABS is the fastest-growing PC alloy today. In this, ABS allows to balance of the high impact strength, surface finish, and high flow for better processing. As a drawback, PC/ABS blends often do not meet the new flame retardancy standards. The most common end uses for PC/ABS are automotive parts, office equipment housings, computers, and mobile phones.

Overcome low chemical resistance
PC is known to have very low chemical resistance. This is a critical performance in automotive applications when in contact with oil and gasoline. To overcome this weakness, PC and polyesters like PBT are blended. This results in a high chemical resistance alloy. However, it has a consequent poor impact on the performance characteristics of PBT. But when it comes to recycling, adding fillers, color pigments, or flame retardants loses their primary critical toughness performance.

Key benefits in PC blends

Benefits	Description
Better compatibility	The high-impact performance of PC/ABS alloys will greatly depend on the ability to disperse the various polymer phases (PC, PB, SAN), typically achieved through technical compounds.
Superior low-temperature impact	The low glass transition temperature (Tg < -80 °C) of impact allows them to be used for demanding low-temperature applications to create products that can withstand temperatures as low as -50 °C and still maintain their structural integrity.
Good dispersion	Impact modifiers are easily dispersed using conventional compounding techniques. The resulting engineering plastics compounds flow readily in molding equipment and have exceptional impact strength resistance.

MBS impact modifiers in polyesters

Polyesters like polybutylene terephthalate (PBT) and polyethylene terephthalate (PET) are semi-crystalline polymers. They exhibit very attractive performance such as high heat stability and chemical resistance. Whereas, other polyesters exhibit poor low-temperature impact performances, thus, impact modifiers are needed.

Polyesters are often used in several applications. These include:

automotive such as drive housing for electric window lifts and light-conducting housing
electrical appliances
medical applications.

However, to fulfill some industry requirements, these resins require improved impact performance at ambient or low temperatures. For these applications, the use of impact modifiers is critical.

Key benefits in polyesters

Benefits	Description
Superior low-temperature impact	The low glass transition temperature (Tg < -80°C) of impact modifiers allows them to be used for demanding low-temperature applications, to create products that can withstand temperatures as low as - 50°C while remaining ductile.
Good dispersion	Impact modifiers are easily dispersed using conventional compounding techniques. The resulting engineering plastics compounds flow readily in molding equipment and have an exceptional impact strength resistance.

MBS impact modifiers vs. acrylic impact modifiers

MBS core-shell impact modifiers are designed to provide exceptional cold-temperature impact in a wide range of engineering plastics. These include polycarbonate, polycarbonate alloys (PC/ABS, PC/PBT), and polyesters. When compared to other acrylics available in the market, core-shell impact modifiers offer better properties to PCs like:

low-temperature impact strength,
mold-in colorability, and
thermal stability

Methacrylate Butadiene Styrene (MBS)	Acrylic Impact Modifiers (AIM)

Benefits include: Excellent low-temperature impact Excellent colorability Excellent dispersion in most engineering plastics matrices	Benefits include: Excellent low-temperature impact Excellent UV stability Excellent thermal stability Good colorability
They are used in applications like outdoor (painted) and interior	They are used in applications like outdoor (UV & thermal) and interior (neat heat sources)

TPEs as Impact Modifiers

A thermoplastic elastomer is generally defined as a polymer that can be processed as a thermoplastic material. It also possesses the properties of a conventional thermoset rubber. Some of the general classes of commercial TPEs include:

Styrenic block copolymers
Thermoplastic polyurethanes
Thermoplastic copolyesters
Thermoplastic polyamides

To be classified as a thermoplastic elastomer, a material needs to have the features listed below:

The ability to be stretched to moderate elongations. Upon the removal of the stress, it should return to something close to its original shape
Processability as a melt at elevated temperatures
Absence of significant creep

Some examples of TPE products that have been developed and extensively used are Arnitel^® from DSM, Engage™ from Dow Chemical, Hytrel^® from DuPont, and KRATON™ from Kraton Polymers.

TPEs — Advantages and disadvantages

TPEs are used when conventional elastomers cannot provide the range of physical properties needed in the product. Thus, their use in applications is end-use driven. Specific TPEs are used based on the final need.

This is another example of the need to achieve the appropriate balance of modulus and impact properties with the impact modification of engineering plastics. This is a feature that is relevant and important to all the described approaches.

Advantages	Disadvantages
Recyclable. They have the typical elastic properties of rubbers which are not recyclable Require little or no compounding, with no need to add reinforcing agents, stabilizers, or cure systems Consume less energy	They melt at elevated temperatures and this can limit their utility with certain engineering plastics May require drying before processing There are a limited number of low-modulus materials that can be used in TPEs

Bulk Elastomeric Compounds as Impact Modifiers

The approach of using bulk elastomeric compounds as impact modifiers is different from the use of core-shell materials. This is because the size of the dispersed rubber phase is dependent on the processing conditions that are utilized. This allows the control of the particle size in the final impact-modified product.

Disadvantages of elastomers as impact modifiers

One of the biggest drawbacks of the approach is that the stiffness decreases with the addition of an elastomer. This decrease in stiffness is typically larger than that observed for core-shell modifiers. This means that if the retention of the stiffness offered by the engineering plastic is critical to the application, the concentration of elastomer must be adjusted appropriately.

Consider the example of PBT modification using an elastomeric compound. In this case, processing conditions of the PBT/elastomer mixtures affect the size of the elastomer particles. Hence, the impact modification is achieved. In addition, the relative viscosities of the components will affect the morphology in the final blend. The melt viscosities are directly related to the molecular weights of the polymers. Thus, the molecular weights are important factors in defining the observed impact modifications.

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