Improving Flexibility

The room temperature cured epoxy adhesives discussed thus far exhibit a general lack of flexibility, especially when considered next to elastomeric sealants. Flexibility is generally desired when the performance requirements include high peel strength, impact strength, and resistance to thermal shock or thermal cycling. 

TABLE 11.12 Starting Formulations for Rapid-Setting, Room Temperature Curing Epoxy Adhesives17 

Several methods can be used by the formulator to improve the flexibility of room temperature curing epoxy resin adhesive formulations  

High-molecular-weight resins to lower crosslink density 

Polymerized fatty acid modification of resins and/or curing agents (e.g., polyamides, amidoamines) 

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The Cg—0 2 branched, odd and even, linear and internal olefins are used to produce improved flexible poly(vinyl chloride) plastics. Demand for these branched olefins, which are produced from propylene and butylene, is estimated to be increasing at a rate of 2% per year. However, the growth of the linear a-olefins is expected to slow down to a rate of 5% per year from 1992 to 1997 (3), as opposed to growth rates of 7—10% in the 1980s. 

Laminating resins. CNSL resins are added to laminates based on phenol-formaldehyde, epoxy, etc. to reduce brittleness and to improve flexibility of the product. The resins also exhibit better age hardening and improved bonding to the substrate [133,134,137,168]. 

The structure of a titanate coupling agent is expressed by the general formula RO—Ti (OXRY)3. These titanate coupling agents are useful for improving flexibility and processability [70]. Joseph and coworkers [71]... 

An adhesive tape for photographic films can be made by using a coating containing a water- or alkali-soluble polymer and ether carboxylate [211]. A light-sensitive resin composition with ether carboxylic acid salt for flexographic plate material shows an improved flexibility and solvent resistance [212]. 

Epoxidized oils were also used to modify PLA Ali et ah (2009) reported that its use as a plasticizer to improve flexibility. Thermal and scanning electron microscope analysis revealed that epoxidized soybean oil is partially miscible with PLA. Rheological and mechanical properties of PLA/epoxidized soybean oil blends were studied by Xu and Qu (2009) Epoxidized soybean oil exhibited a positive effect on both the elongation at break and melt rheology. Al-Mulla et al. (2010b) also reported that plasticization of PLA (epoxidized palm oil) was carried out via solution casting process using chloroform as a solvent. The results indicated that improved flexibility could be achieved by incorporation of epoxidized palm oil. 

Alternative strategies utilizing the initial introduction of suitable chemical functionality onto the fullerene have been adopted by several research groups, allowing conjugation of the saccharides with complementary functions in the final synthetic steps. This approach has led to improved flexibility with the chemical functions on the carbohydrate moieties, especially the anchoring ones, and a better control on the number of attached saccharide residues. 

Aberration-corrected ETEM/STEM (130) is expected to offer superior (subatomic) resolution under catalytic reaction conditions furthermore, it will provide improved flexibility for tilting the sample to different crystallographic orientations to allow understanding of the geometry of surface structural changes, enable the use of complex sample stages, and perhaps higher gas pressures. 

In this sense, the development of allyl resins with improved flexibility would be required for versatile applications. 

Thus, the effect of introducing flexible crosslinked units into DAP resins was reflected in the improved flexibility. In particular, PEGBAP having a long-chain glycol unit showed a remarkable effect ... 

In conclusion, DAP resins with improved flexibility were successfully obtained by introducing flexible crosslinked units on the surface of the microgel, i.e., by crosslinking flexibly microgels to form the macrogel. This finding may become a useful guideline for the modification of three-dimensional polymers As an extension of... 

Plasticizers are used in the polymer industry to improve flexibility, workability, and general handling properties. Dibutyl sebacate and phthalates, such as dibutyl phthalate, diethyl phthalate, dicyclohexyl phthalate, butylbenzyl phthalate, and diphenyl-2-ethylhexyl phosphate, serve widely as plasticizers in vinylidene chloride copolymers, nitrocellulose-coated regenerated cellulose film, and cellulose acetate (Castle et ah, 1988a). In PVC, di(2-ethylhexyl)... 

A material, such as (a) a terpolymer of propylene, ethylene and butene-1, (b) a polyolefin composition, which includes about 31 to 39% of a copolymer of propylene and ethylene and about 58 to 72% of a terpolymer of propylene, ethylene and butene-1 or (c) a polyolefin composition, which includes about 30 to 65% of a copolymer of propylene and butene-1 and about 35 to 70% of a copolymer of propylene and ethylene, is irradiated and extruded through a die in the presence of a physical expanding agent and a cell nucleating agent to produce a structure having a density, which is at least 10 times less than the initial density of the material. The foam articles exhibit improved flexibility and low temperature toughness compared to conventional propylene polymer materials. 

Polymer Plasticizer. Nylon, cellulose, and cellulose esters can be plasticized using sulfolane to improve flexibility and to increase elongation of the polymer (130,131). More importantly, sulfolane is a preferred plasticizer for the synthesis of cellulose hollow fibers, which are used as permeability membranes in reverse osmosis (qv) cells (131—133) (see Hollow-FIBERMEMBRANES). In the preparation of the hollow fibers, a molten mixture of sulfolane and cellulose triacetate is extruded through a die to form the hollow fiber. The sulfolane is subsequently extracted from the fiber with water to give a permeable, plasticizer-free, hollow fiber. 

Long-chain aliphatic acids such as adipic acid (qv) [124-04-9] are generally used to improve flexibility and enhance impact properties, demonstrating subtle improvements over resins modified with the ether glycols (diethylene glycol) and polyether glycols (polypropylene glycol) (see Polyethers). 

Nitrocellulose-based lacquers often contain short or medium oil alkyds to improve flexibility and adhesion. The principal applications are furniture coalings, top lacquer for printed paper, and automotive refinishing primers. 

Acrylic Copolymers. A widely used method to modify the physical and chemical properties of polymers is to prepare copolymers that contain monomer units chosen to give the desired properties. For example, copolymers of MMA are used in thermoplastic coatings where improved flexibility or resistance to degradation are needed (15). 

The fact that a major proportion of the resin consists of phenol serves a two-fold purpose (1) it reduces cost considerably because of the lower cost of phenol (2) it controls the extent of final cross-linking in the glue film thus improving flexibility and strength. 

The introduction of fluorine into the elastomeric macromolecule generally produces materials exhibiting an improved retention of properties at high temperatures, an improved flexibility at low temperatures, and an improved resistance to solvents. Essentially, there are two groups of fluoroelastomers fluoro-inorganic elastomers and fluorocarbon (or fluorohydrocarbon) elastomers. 

Self-managed (empowered). Whilst a consequence of the flatter management organisations that exist in most companies. These types of team are now the cornerstones of the management of many R D projects. Self-managed teams are designed to improve flexibility, drive customer service improvement, speed up product delivery and maintain motivation. 

The polyglycol diepoxies are used primarily as flexibilizers and reactive diluents. Commercial products are available where n varies from 2 to 7. Generally, these are used in the range of 10 to 30 percent by weight with DGEBA or epoxy novolac resins to improve flexibility without a significant loss in physical properties. 

The higher-MW semisolid (EEW of 225 to 280) and solid epoxy resins (EEW > 450) may be blended into lower-MW resins to improve flexibility and decrease reactivity. They also improve adhesion due to the higher concentration of hydroxyl groups along the molecular chain. Ten percent of a higher-MW epoxy resin blended into a conventional liquid epoxy resin (EEW of 190) can significantly improve flexibihty, but a reactive diluent frequently needs to be added to the formulation to counteract the increased viscosity caused by the addition. 

Examples of epoxy resins modified for improved flexibility include the fohowing ... 

Polyamide cured DGEBA epoxies provide improved flexibility, moisture resistance, and adhesion over aliphatic amines alone. However, polyamide cured epoxies are generally inferior in thermal resistance and shear strength due to the reduction in crosslink density. Polyamide cured epoxies lose structural strength rapidly with increasing temperatures and... 

The improved flexibility results in improved adhesion and thermal shock resistance, but at the sacrifice of elevated-temperature performance. A TETA cured FIGURE 6.4 Chemical structure of dibutyl epoxy (EEW = 190) plasticized with 17 pph phthalate. 0f chbutyl phthalate exhibited a heat distor-... 

These hybrids generally yield improved flexibility and toughness, but they are more commonly used in epoxy coating formulations than in adhesive systems. 

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