Carboxyl-terminated elastomers

Table II. Gardner Impact of ERL-4221/HHPA Modified with Carboxyl Terminated Elastomer (CTBN)...

Modified with Carboxyl Terminated Elastomer (CTBN)... 

If tougher adhesive bonds are required than are attainable from conventional epoxy resins, they can be modified. A typical example involves the carboxyl-terminated elastomer modifications discussed earlier. A modified base resin is first formed as follows (pbw) ... 

CTBN type elastomers (carboxyl-terminated butadiene acrylonitrile) used to toughen this epoxy system. The Tg peak of the elastomer (-30°Q was shifted to higher temperatures suggesting that electron interaction resulted in crosslinking. The fact that the Tg peak disappeared at 10 rads suggests that crosslinking in the elastomer was extensive at high dose levels. 

It is obvious from these data that CTBN (the carboxyl terminated butadiene-acrylonitrile copolymer) is the most effective modifier, and therefore it was selected for further study. As the concentration of the elastomer was increased to levels up to 100 parts, the impact also increased. The data in Table II show that the room temperature impact of ERL-4221 increased from 40 inch-lbs to greater than 320 inch-lbs by adding 100 phr or 33 wt % of CTBN. At very low temperatures ( —160°F) the impact of the system modified with 60 parts, or 23 wt %, of CTBN was 120 inch-lbs. These impact improvements appear to be directly proportional to the concentration of the elastomer modifier. 

Effect of Molecular Configuration of Elastomer. The extent of the impact and strength improvements of ERL-4221 depends on the chemical structure and composition of the elastomer modifier. The data shown in Table I indicate that the carboxyl terminated 80-20 butadiene-acrylonitrile copolymer (CTBN) is the most effective toughening and reinforcing agent. The mercaptan terminated copolymer (MTBN) is considerably less effective as far as tensile strength and heat distortion temperature are concerned. The mercaptan groups are considerably less reactive with epoxides than carboxyls (4), and this difference in the rate of reaction may influence the extent of the epoxy-elastomer copolymerization and therefore the precipitation of the rubber as distinct particles. 

The carboxyl terminated polybutadiene (C-3000) is about equally effective to CTBN in heat distortion temperature and impact but considerably less effective in strength. From the haze data (the percent haze of ERL-4221 modified with 10 phr of CTBN and C-3000 were 17 and 85% respectively) it is quite clear that this elastomer (C-3000) is highly incompatible with the epoxy-hardener system in the cured state. A 2000 molecular weight polybutadiene elastomer, containing no carboxyl groups, was completely incompatible with the epoxy system and segregated in the cured state. 

However, newer adhesives systems having moderate temperature resistance have been developed with improved toughness but without sacrificing other properties. When cured, these structural adhesives have discrete elastomeric particles embedded in the matrix. The most common toughened hybrids using this concept are acrylic and epoxy systems. The elastomer is generally a amine- or carboxyl-terminated acrylonitrile butadiene copolymer (ATBN and CTBN). 

A specialty class of carboxyl containing elastomers are the telechelic ionomers. In these systems the carboxyl functionality terminates both ends of the polymer chain. Such polymers range in molecular weight from 1500 to about 6000. These materials can be prepared via several synthetic routes involving anionic or free radical initiated polymeri-zation(32-34). Recently, telechelic sulfonate ionomers of poly-isobutylene have been synthesized(35). These systems offer an unusual opportunity to assess the influence of chain length, chain architecture, cation type, and the Influence of polar additives on ionomer properties. 

Filled liquid neoprene sealants are cured at room temperature by the addition of lead oxide, tertiary amines or epoxy resins. Neoprene sealants covered by a top coat of white chlorosulfonated polyethlyene are being used on solar collectors.21 Mixtures of carboxyl terminated acrylonitrile elastomers (CTBN) and liquid epoxy resins cure readily at room temperature in the presence of bis phenol A and amines.22 Coal tar-epoxy resin, two component epoxy resin, and epoxypolyester resin systems have been used as sealants.22... 

In the mid-60 s carboxyl-terminated polybutadiene/acrylo-nitrile (CTBN) liquid polymers were introduced for the purpose of epoxy resin modification. These telechellc polymers are essentially macromolecular diacids. They offer processing ease (and therefore advantage) over the solid carboxylic nitrile elastomers. It is no surprise that the epoxy prepreg industry (adhesive and non-adhesive varieties) found the liquid and solid carboxylic nitrile elastomer species useful together in processing liquid and lower molecular weight solid epoxy resins where elastomer modification was needed. 

Later, in 1974, amine reactive versions of the liquid nitrile polymers (ATBN) were issued, thereby offering another way to introduce rubbery segments into a cured epoxy resin network. References are cited which provide detailed discussions of nitrile rubber, carboxylic nitrile rubber and both carboxyl- and amine-terminated nitrile liquid polymers (1-4). Table I illustrates CTBN and ATBN products structurally. Table II provides properties for typical solid carboxylic nitrile elastomers. 

Acrylonitrile-butadiene copolymer, carboxyl-terminated. See Butadiene-acrylonitrile elastomer, carboxyl-terminated... 

Carboxylated butadiene-acrylonitrile copolymer. See Butadiene-acrylonitrile elastomer, carboxyl-terminated 1-(2-Carboxylatoethyl)-4,5-dihydro-3-(2-hydroxyethyl)-2-isoheptadecyl-1H-imidazolium. See Sodium isostearoamphopropionate Carboxylic acid C4. See n-Butyric acid Carboxylic acid C5. See 2-Methylbutyric acid n-Valeric acid... 

Tween 81. See Polysorbate 81 Tween 85 Tween 85LM. See Polysorbate 85 Twinkling Star. See Antimony trioxide Two-stage phenolic resin. See Novolac resin Two-stage resin. See Phenolic resin Ty-lon B11. See Sodium sulfite Tylac 037 Tylac 97-422 Tylac 692 Tylac 757 Tylac 820 Tylac 936 Tylac 979-RG Tylac 68009-00 Tylac 68010-00 Tylac 68012-00 Tylac 68013-00 Tylac 68014-00. See Styrene/butadiene polymer Tylac 68060-00. See Acrylonitrile copolymer Tylac 68073-00 Tylac 68074-00 Tylac 68075-00 Tylac 68076-00. See Butadiene-acrylonitrile elastomer, carboxyl-terminated Tylac 68150-00 Tylac 68151-00. See Butadiene/acrylonitrile copolymer Tylac 68152-00. See Styrene/butadiene polymer... 

Phenoxyethanol Polyazelaic polyanhydride Polytetramethylene ether glycol diamine Trilinoleic acid flexibilizer, epoxy coatings Bis (4-vinyl oxy butyl) hexanediyibiscarbamate flexibilizer, epoxy resin Butadiene-acrylonitrile elastomer, carboxyl-terminated Polyazelaic polyanhydride flexibilizer, inks... 

Bisphenol-F based, standard, undiluted, elastomer-modified resin (carboxyl-terminated butadiene acrylonitrile) Epon 58006 /Resolution 150,000-300,000 330-360... 

The rubber-toughening process has been one of the most successful methods for modifying polymer toughness. The incorporation of small amounts of rubber into polymer matrices has resulted in significantly improved fracture resistance (1). Particularly interesting improvements in the toughness of epoxies have been accomplished by the addition of carboxyl-terminated butadiene/acrylonitrile (CTBN) elastomers (2,3). 

The second ABCP-type materials of commerce are the rubber-toughened epoxy resinsUsually the epoxy resins are based on the diglycidyl ether of bisphenol A (DGEBA). Carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber is the elastomer of preference. A typical composition is ... 

Several methods have been investigated to improve the fracture toughness of BMI resins, the most used technique being the incorporation of a second phase formed by the dispersion of rubber particles into the cross-linked matrix. Carboxyl-terminated butadiene-acrylonitrile elastomers (CTBN), which are very effective with epoxy resins, are also frequently added to BMI adhesives. The general formula of CTBN rubbers is ... 

The use of liquid carboxyl-terminated rubber as a precursor of the elastomer network enables the mixing of more than 100 phr of buffing dust into the compound. 

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