Green strength

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Green strength, or handling strength, can be defined as the strength of a material as it is processed to form its final ultimate tensile strength. This strength is usually considerably lower than the final ultimate strength of a material. The term green strength is usually referenced when discussing non-metallic materials such as adhesives and elastomers (such as rubber). Recently,[ when? ] it has also been referenced in metallurgy applications such as powdered metallurgy.

Contents

Adhesives

A joint made through the use of an adhesive can be referred to as an adhesive joint or bond.

Phases of Epoxy Epoxy-Chemistry-time-v-temp.jpg
Phases of Epoxy

The green strength of adhesives is the early development of bond strength of an adhesive. It indicated "that the adhesive bond is strong enough to be handled a short time after the adherents are mated but much before full cure is obtained." Usually, this strength is significantly lower than the final curing strength. Most adhesives typically have an initial green strength and a final ultimate tensile strength listed for their application. For household adhesives, this data is usually reflected on the packaging. [1]

The best example of this is seen in typical epoxies from a local hardware stores. During curing, the epoxy will travel into an initial curing phase, also called "green phase", when it begins to gel. At that point, the epoxy is no longer workable and will move from being tacky to a firm rubber-like texture. While the epoxy is only partially cured at this point, it has formed a lower green strength. Normally, this process occurs within 30 minutes to 1 hour. At this time, the part in question can be handled, but cannot handle large loads or stress. It typically takes up to 24 hours for a standard epoxy to cure to its final and complete strength. [2] [3]

Temperature is an important factor in the time it takes for an adhesive to form the green strength. While this can vary from adhesive to adhesive, general speaking, heat can speed up the process to form the green strength and the overall curing time. Time-Temperature-Transformation Diagrams exist for various adhesives that relate the time and temperature to the state of the adhesive during curing. This allows for a proper understanding of when the green strength will be reached for an adhesive joint based on certain conditions. [1]

Testing

Mechanical testing can be used to verify the green strength of a material. This will allow the user the understand the amount of load that can be applied in the green phase before final cure. [1]

Tensile loading can be verified by various testing methods. Multiple ASTM specifications exist for the tensile testing of adhesives that are relatively easy to follow. Such tests include the process of attaching the adhesive to two adherents (typically wood or steel) then testing the joint with a pull-type test. One example is the use us ASTM Test Method D2095. In this test, two metal rod ends are polished so it contains no burrs that could affect the adhesive bond. It also machined so the surfaces are perfectly parallel. The rods are then butted against each with the adhesive joining them. As it cures and sets, the fulfillment of green strength can be tested by a pull test, putting the bond in full tension load. [1]

Example of Lap Shear Joint in Load Example of Lap Shear.jpg
Example of Lap Shear Joint in Load

Shear loading can also be tested in respect to green strength. Most adhesive bonds used in design require the bond to typically be in a state of shear, not tensile. Because of this, it is very important to understand the shear loading of a joint in relation to its green strength and final strength. Just like in tensile loading, ASTM provides very specific testing methods for a joint in shear loading.

The standard lap shear specimen test is described in ASTM D1002. This test is the single common and discussed test method for adhesive bonds. In this method, the surface is prepped and cleaned for each specimen. The adhesive is then applied to the area that will be lapped. This lap length is generally 0.5" and the bond width is 1". The bond is then fixed and allowed to cure. For green strength testing, the fixture can be removed, at the appropriate time, and the specimen can be loaded in shear until it finally fails. This will verify the green strength of the material. [1]

Other testing, such as cleavage loading and peel test, can be used to determine both the green strength and final strength of a material. These are typically not reflected on the data sheet for standard adhesives, but can be used for testing of adhesives based on their applications in residential and commercial environments. [1]

Elastomers

Rubber Tire (Elastomer) F1 tire 2012-hard.svg.png
Rubber Tire (Elastomer)

In the elastomer industry, green strength describes the strength of an elastomer in an unvulcanized, uncured state. The most popular referenced type of elastomer is rubber.

For rubber composites, green strength is essential during formation and manufacturing of materials such as radial tires, tank tracks, etc. These rubbers must be stretched from one mill to another during processing to form the final, vulcanized product. Green strength allows these transfers without tearing or wrinkling the workpiece.

To improve the green strength of elastomers and prevent issues during forming, various additives and compounds are typically added to the composite. Also, fabrication and forming techniques have been modified to reduce the amount of stress on the material before it is vulcanized. These techniques are a pertinent component of the tire making industry because it is a process that requires much forming, stretching, and bending during fabrication before the final curing is complete. [4]

Metals

Green strength of metals is typically[ weasel words ] referenced in the field of powder metallurgy.

Powder metallurgy refers to the fabrication of materials or components from powders. In powder metallurgy, the initial green strength is formed during compacting and forming. Increased complexity of parts and geometry have created a need for a higher green strength during this process. [5]

There are several limitations that restrict the ability to increase green strength in powder metallurgy components. Characteristics such as particle size and compressibility pose limits on the final green strength. [5]

Various studies have been undertaken to improve the green strength of powder metallurgy. The use of advanced lubricants and the addition of high alloys have shown that it is possible to increase the green strength of these materials. [5]

Related Research Articles

Adhesive Non-metallic material used to bond various materials together

Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation.

Epoxy family of two-part adhesive resins

Epoxy refers to any of the basic components or cured end products of epoxy resins, as well as a colloquial name for the epoxide functional group. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups.

Rebar steel bar or mesh used within concrete

Rebar, known when massed as reinforcing steel or reinforcement steel, is a steel bar or mesh of steel wires used as a tension device in reinforced concrete and reinforced masonry structures to strengthen and aid the concrete under tension. Concrete is strong under compression, but has weak tensile strength. Rebar significantly increases the tensile strength of the structure. Rebar's surface is often "deformed" with ribs, lugs or indentations to promote a better bond with the concrete and reduce the risk of slippage.

Thermosetting polymer polymer material that irreversibly cures

A thermosetting polymer, resin, or plastic, often called a thermoset, is a polymer that is irreversibly hardened by curing from a soft solid or viscous liquid prepolymer or resin. Curing is induced by heat or suitable radiation and may be promoted by high pressure, or mixing with a catalyst. Heat is not necessarily to be applied externally. It is often generated by the reaction of the resin with a curing agent. Curing results in chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.

Fibre-reinforced plastic (FRP) is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass, carbon, aramid, or basalt. Rarely, other fibres such as paper, wood, or asbestos have been used. The polymer is usually an epoxy, vinyl ester, or polyester thermosetting plastic, though phenol formaldehyde resins are still in use.

Cross-link Bond that links one polymer chain to another

In chemistry and biology a cross-link is a bond that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural polymers.

Shear strength strength of a material or component against the type of yield or structural failure when the material or component fails in shear

In engineering, shear strength is the strength of a material or component against the type of yield or structural failure when the material or component fails in shear. A shear load is a force that tends to produce a sliding failure on a material along a plane that is parallel to the direction of the force. When a paper is cut with scissors, the paper fails in shear.

Silicone rubber is an elastomer composed of silicone—itself a polymer—containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers are widely used in industry, and there are multiple formulations. Silicone rubbers are often one- or two-part polymers, and may contain fillers to improve properties or reduce cost. Silicone rubber is generally non-reactive, stable, and resistant to extreme environments and temperatures from −55 to 300 °C while still maintaining its useful properties. Due to these properties and its ease of manufacturing and shaping, silicone rubber can be found in a wide variety of products, including: voltage line insulators, automotive applications; cooking, baking, and food storage products; apparel such as undergarments, sportswear, and footwear; electronics; medical devices and implants; and in home repair and hardware with products such as silicone sealants.

Tire manufacturing

Pneumatic tires are manufactured according to relatively standardized processes and machinery, in around 455 tire factories in the world. With over 1 billion tires manufactured worldwide annually, the tire industry is a major consumer of natural rubber. Tire factories start with bulk raw materials such as synthetic rubber, carbon black, and chemicals and produce numerous specialized components that are assembled and cured.

Wood glue is an adhesive used to tightly bond pieces of wood together. Many substances have been used as glues.

Pressure-sensitive adhesive adhesive which forms a bond when pressure is applied

Pressure-sensitive adhesive is a type of non reactive adhesive which forms a bond when pressure is applied to bond the adhesive with a surface. No solvent, water, or heat is needed to activate the adhesive. It is used in pressure-sensitive tapes, labels, glue dots, note pads, automobile trim, and a wide variety of other products.

Rubber Technology is the subject dealing with the transformation of rubbers or elastomers into useful products, such as automobile tires, rubber mats and, exercise rubber stretching bands. The materials includes latex, natural rubber, synthetic rubber and other polymeric materials, such as thermoplastic elastomers. Rubber processed through such methods are components of a wide range of items.

Redux is the generic name of a family of phenol–formaldehyde/polyvinyl–formal adhesives developed by Aero Research Limited (ARL) at Duxford, UK, in the 1940s, subsequently produced by Ciba (ARL). The brand name is now also used for a range of epoxy and bismaleimide adhesives manufactured by Hexcel. The name is a contraction of REsearch at DUXford.

Rubber toughening is a process in which rubber nanoparticles are interspersed within a polymer matrix to increase the mechanical robustness, or toughness, of the material. By "toughening" a polymer it is meant that the ability of the polymeric substance to absorb energy and plastically deform without fracture is increased. Considering the significant advantages in mechanical properties that rubber toughening offers, most major thermoplastics are available in rubber-toughened versions; for many engineering applications, material toughness is a deciding factor in final material selection.

The pull-off test, also called stud pull test in which an adhesive connection is made between a stud and the carrier by using a glue, possibly an epoxy or polyester resin, that is stronger than the bond that needs to be tested. The force required to pull the stud from the surface, together with the carrier, is measured. Simple mechanical hand-operated loading equipment has been developed for this purpose. When higher accuracy is required, tests can be performed with more advanced equipment called a bond tester. A bond tester provides more control and possibly automation. Applying the glue automatically and curing with UV light is the next step in automation. This methodology can also be used to measure direct tensile strength or/and the bond strength between two different layers.

Tensile testing Test procedure to determine mechanical properties of a specimen.

Tensile testing, also known as tension testing, is a fundamental materials science and engineering test in which a sample is subjected to a controlled tension until failure. Properties that are directly measured via a tensile test are ultimate tensile strength, breaking strength, maximum elongation and reduction in area. From these measurements the following properties can also be determined: Young's modulus, Poisson's ratio, yield strength, and strain-hardening characteristics. Uniaxial tensile testing is the most commonly used for obtaining the mechanical characteristics of isotropic materials. Some materials use biaxial tensile testing. The main difference between these testing machines being how load is applied on the materials.

Micronized rubber powder (MRP) is classified as fine, dry, powdered elastomeric crumb rubber in which a significant proportion of particles are less than 100 µm and free of foreign particulates. MRP particle size distributions typically range from 180 µm to 10 µm. Narrower distributions can be achieved depending on the classification technology.

Welding of advanced thermoplastic composites is a beneficial method of joining these materials compared to mechanical fastening and adhesive bonding. Mechanical fastening requires intense labor, and creates stress concentrations, while adhesive bonding requires extensive surface preparation, and long curing cycles. Welding these materials is a cost-effective method of joining concerning preparation and execution, and these materials retain their properties upon cooling, so no post processing is necessary. These materials are widely used in the aerospace industry to reduce weight of a part while keeping strength.

Adhesive bonding is a process by which two members of equal or dissimilar composition are joined. It is used in place of, or to complement other joining methods such mechanical fasting by the use nails, rivets, screws or bolts and many welding processes. The use of adhesives provides many advantages over welding and mechanical fastening in steel construction; however, many challenges still exist that have made the use of adhesives in structural steel components very limited.

Solvent bonding is one of several methods of adhesive bonding for joining plastics. Application of a solvent to a thermoplastic material softens the polymer, and with applied pressure this results in polymer chain interdiffusion at the bonding junction. When the solvent evaporates this leaves a fully consolidated bond-line. An advantage to solvent bonding versus other polymer joining methods is that bonding generally occurs below the glass transition temperature of the polymer.

References

  1. 1 2 3 4 5 6 Pocius, Alphonsus (2012). Adhesion and Adhesives Technology: An Introduction (3rd ed.). Hanser. ISBN   978-1-56990-511-1.
  2. "Important Characteristics of Several Common Adhesive Tests". adhesivesmag.com.
  3. "Epoxy Chemistry".
  4. Ma, Liquing. "Method to improve green strength in elastomers". Google Patents.
  5. 1 2 3 Luk, Sydney (1996). "Enhanced Green Strength Material System for Ferrous and Stainless P/M Processing" (PDF). PM2TEC '96 World Congress.