Tính kháng hóa chất, dung môi của vật liệu đàn hồi polyurethane

Vật liệu đàn hồi PU nổi bật với tính chất cơ lý kéo, xé, mài mòn tốt, còn kháng tốt với nhiều loại dung môi. Tuy nhiên, vật liệu PU rất dễ bị thủy phân trong môi trường nước. Phần tài liệu sau đây sẽ giới thiệu chung về tính kháng hóa chất của vật liệu PU.

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In certain fluids the polyurethanes, like other rubbers, swell but when removed and allowed to dry out they return to their original dimensions. This is not always so with other elastomers or plastics since they may contain plasticizers which can be leached out by the fluid, resulting in permanent shrinkage. The effect of organic materials on polyurethanes is dependent upon the chemical groups present in these materials.

Alcohols, acids, and ketones and esters tend to cause swelling and degradation, particularly at high temperatures. Aliphatic hydrocarbons and esters are generally inert, but aromatic hydrocarbons are more active and promote swelling at room temperature and gradual breakdown at higher temperatures. Up to service temperatures of about 50°C polyurethane elastomers in contact with these organic fluids, or their greases, can be considered as some of the most resistant materials available when the combined effects of oil or grease and weathering are encountered.

Chlorinated solvents cause swelling and sometimes degradation. The tensile and tear strengths are reduced to about 25% of their initial values after 6 months' immersion in chloroform at room temperature. Methylene chloride causes even more rapid breakdown, whilst carbon tetrachloride and trichloroethylene are relatively inert, although swelling does occur.

Rapid breakdown also occurs upon immersion in 12% sodium hypochlorite and 30% hydrogen peroxide. In 10% hydrogen peroxide, however, the effect is much reduced.

The resistance of polyurethane elastomers to immersion in water has been identified as relatively poor and is directly applicable to immersion in dilute solutions of inorganic materials in water. Provided the inorganic substance has no catalytic effect the solution can be expected to behave as pure water. However, acidic or alkaline media accelerate hydrolytic attack and therefore solutions of salts of weak acids or bases are likely to degrade polyurethanes faster than water. As a generalization, it can be stated that, provided the pH of a solution lies between the values of 5·5 and 8, the action of the solution can be considered similar to the action of water. At higher acidities or higher alkalinities it is advisable to test the effect of the particular solution. As would be expected, strong acids and bases attack polyurethanes rapidly.

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Trích đăng từ sách Polyurethane Elastomers, C. Hepburn, Elsevier Science Publisher, 1992, trang 382 – 387

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Cải thiện tính kháng thủy phân của vật liệu đàn hồi PU

Khác với hầu hết các loại cao su tổng hợp, vật liệu đàn hồi PU bị thủy phân ở những mức độ nhất định. Phần tài liệu sau sẽ so sánh tính kháng thủy phân của các loại vật liệu đàn hồi PU khác nhau và các cách hiệu quả để cải thiện tính chất này.

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With most commercial elastomers the property of hydrolytic stability is not even quoted in data on their properties and they are regarded as possessing excellent resistance to aqueous environments due to their chemical structure being based on all carbon-carbon linked atoms. With polyurethane elastomers, however, the presence of -COC- and -COOC- groups in the main-chain structure results in these materials inevitably undergoing some degree of hydrolytic attack in the course of time. It is primarily the rate of this attack which is considered (it is assumed to occur in principle) as this varies over a very wide range, being dependent upon the chemical structure of the polyurethane. Hence one of the properties most frequently quoted when considering the limitations of polyurethanes is their susceptibility to hydrolytic attack.

It is possible to make some general comments on the relative hydrolytic stability of different polyurethane elastomer structures. The three principal linear polyol series used in synthesizing urethane elastomers can, for hydrolytic stability, be ranked in the following order:

polyether > polycaprolactone > polyester

(highest stability → lowest stability)

Other useful points are:

1. Amine cured polycaprolactones, stabilized with a carbodiimide, have similar resistance to polyethers.

2. Sulphur-cured urethane elastomers have the least resistance to hydrolysis (possibly the typical amine-based sulphur-accelerator combination used accelerates hydrolysis) (see Table 13.5).

3. Carbodiimides (e.g. Staboxal PCD by Bayer) are effective in stabilizing all polyester urethanes against hydrolytic attack; they are not effective when used with polyether-based urethane elastomers.

4. In general it is true that the longer the hydrocarbon chain of the glycol portion of a polyglycol adipate the more resistant is the polyester to hydrolysis.

5. Fungus attack of polyurethanes, especially at relatively low ageing temperatures (15-30°C) occurs as a generalization. The incorporation of a fungicide is effective in preventing this effect.

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Trích đăng từ sách Polyurethane Elastomers, C. Hepburn, Elsevier Science Publisher, 1992, trang 380 – 382

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Ảnh hưởng của loại monomer thứ ba lên tính chất của cao su EPDM

Cao su EPDM được tổng hợp từ 3 monomer chính là ethylene, propylene và monomer thứ ba diene. Mặc dù monomer thứ ba này được sử dụng với một lượng rất nhỏ, nhưng nó ảnh hưởng đáng kể đến cấu trúc và tính chất của cao su EPDM tạo thành. Tài liệu sau đây sẽ trình bày cụ thể vấn đề này.

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As has been mentioned, three termonomers are employed in the manufacture of EPDM's: ethylidene norbornene (ENB), 1,4 hexadiene (1,4 HD), and dicyclopentadiene (DCPD). Even though all three introduce unsaturation into the EPDM backbone as vulcanization sites, they impart varying characteristics to the elastomers because of their different structure.

Ethylidene Norbornene (ENB). ENB is the most widely used termonomer employed even though it is the most expensive, the reasons being that it is the most readily incorporated during copolymerization and the double bond introduced has the greatest activity for sulfur vulcanization. This activity is also of such a nature that EPDM's containing ENB have the greatest tendency to be cocured with diene elastomers.⁶ Another unique characteristic of this termonomer is that it makes it possible to prepare linear as well as branched polymers by varying the conditions under which the polymers are synthesized.⁵ Branching has an important role in establishing the rheological properties of a polymer.⁷ Under proper control, it can introduce properties to the EPDM that are beneficial in certain applications.

1,4 Hexadiene (1,4 HD). Polymers containing 1,4 HD exhibit a slower cure rate than ENB but possess certain properties that are superior. One such property is its excellent heat characteristic, which is closest to EPM. Such polymers exhibit a good balance of chain scission and crosslinking reactions.⁸ Polymers prepared with 1,4 HD are normally linear in structure and possess excellent processing characteristics.

Dicyclopentadiene (DCPD). The main advantages of DCPD are its low cost and relative ease of incorporation, in which it is similar to ENB. Of the three termonomers, it has the slowest cure rate. All polymers prepared with it are branched as a result of the slight polymerizability of its second double bond. As mentioned previously, this branching can be beneficial, for example, by imparting ozone resistance to diene rubber blends.

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Trích đăng từ sách Rubber Technology – Third Edition, Maurice Morton, Springer, 1999, trang 264 - 265

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Tính kháng mài mòn của vật liệu polyurethane (PU)

Tính kháng mài mòn tốt là một trong những đặc tính nổi bật của vật liệu đàn hồi PU. Tuy nhiên, đặc tính này bị ảnh hưởng đáng kể bởi tác động tích trữ nhiệt bề mặt, quá trình này có sự khác biệt đáng kể trong trường hợp mài mòn khô và mài mòn ướt. Tài liệu sau đây sẽ phân tích chi tiết về vấn đề này.

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Resistance to wear is usually considered a measure both of resistance to tear and resistance to abrasion. It is generally recognized that most abrasion tests have little practical importance in predicting service but are useful in grading materials of similar hardness. Abrasive wear, acutely in the case of polyurethanes, is considerably affected by heat build-up on the surface and the degree of surface heating. This latter is related to the coefficient of friction of the polyurethane, the relative speed between the two faces, the applied load, and whether the abrading surfaces are dry or wet. The abrasion resistance properties of polyurethane elastomers are good but not exceptional when compared with other rubbers and plastics under dry abrasive conditions. This is primarily due to their relatively high hysteresis, hence surface heat build-up effects, which for an instant of time, over a localized or micro region, drastically soften the polyurethane such that it tears away from the abrading surface. In extreme cases the entire surface of the polyurethane melts for a fraction of time and a small fragment of elastomer is torn off, rolled into a bead which then sticks to the semi-melted polyurethane elastomer surface. This gives rise to a special failure characteristic phenomenon of polyurethanes, as such surfaces can be identified by a multitude of small melted sticky beads adhering to their surface.

These comments apply to abrasion of dry surfaces and must be modified when considering abrasion under wet conditions. In these cases the water or other fluid lubricates the surfaces and reduces the coefficient of friction. Further, any heat generation that does occur is more easily dissipated. It is under these conditions that the polyurethanes are outstanding, since then their inherent abrasion resistance is not diminished by the adverse effect of heat build-up.

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Trích đăng từ sách Polyurethane Elastomers, C. Hepburn, Elsevier Science Publisher, 1992, trang 373 – 375

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Đặc trưng hấp thu năng lượng cơ học của vật liệu đàn hồi PU

Khả năng hấp thu năng lượng cơ học là một đặc tính quan trọng của vật liệu đàn hồi, đặc biệt trong các ứng dụng giảm xóc. Tài liệu này giới thiệu các đặc trưng hấp thu năng lượng cơ học của vật liệu đàn hồi nói chung và PU nói riêng.

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As a generalization urethane elastomers have greater energy absorption properties than other equivalent rubbers and plastics. It is useful to examine the mechanisms of energy absorption in elastomers. These are usually quantified by measurement of resilience, hysteresis energy and damping properties, which are defined in the usual BS and ASTM or ISO standards. When a stress is applied to an elastomeric material there is a small but positive time lag before the material takes up the corresponding strain. This time lag is caused by the need for the intermolecular attractions to be overcome by the vibrational energy of the atoms. The practical result of this time lag in applications involving cyclic deformation (or dynamic applications) is that the stress-strain curve in recovery does not follow the same path as when the stress was applied and there is consequently a loss of energy, or hysteresis, which is converted into heat. This loss of energy can be measured, for example, by rebound resilience. Since the vibrational energy of the atoms increases as the temperature increases, then this time lag decreases as the temperature increases. Thus the rebound resilience for polyurethanes, and other elastomeric materials, increases with increasing temperature. Under dynamic stress there will be heat build-up which in turn will improve the resilience property and result in a lower rate of heat build-up.

This energy loss and consequent heat build-up occurs in all elastomers to some degree or other, and since these are poor conductors of heat the temperature can rise sharply. This factor can lead to limitations of use under rapid cycling or in severe dynamic applications and it is necessary to calculate or measure the heat produced under these conditions when designing a specific product. One advantage in the use of polyurethanes is that, due to their high modulus compared with other elastomers, thinner sections can be employed. This helps to inhibit the heat build-up by assisting in heat dissipation. For continuous dynamic conditions a within material temperature of 80^oC may be considered as a maximum working limit for most polyurethanes.

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Trích đăng từ sách Polyurethane Elastomers, C. Hepburn, Elsevier Science Publisher, 1992, trang 372 – 373

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Tính chất cơ lý và ứng dụng của cao su butyl (IIR)

Cao su butyl là loại cao su tổng hợp khá thông dụng, nhưng lượng dùng thấp hơn các loại cao su tổng hợp khác như NBR, SBR, EPDM. Cao su butyl được sử dụng chủ yếu trong các ứng dụng yêu cầu các tính chất đặc trưng của nó như tính không thấm khí, khả năng hấp thu năng lượng cơ học cao. Phần tài liệu sau trình bày chung về tính chất và ứng dụng của cao su butyl.

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The single outstanding physical property of butyl rubber is its impermeability. It does not permit gases like hydrogen or air to diffuse through it nearly as rapidly as ordinary rubber does, and it has excellent resistance to the aging action of air. These properties make butyl rubber valuable in the production of life jackets (inflatable type), life rafts, and inner tubes for tires.

At room temperature, the resiliency of butyl rubber is poor, but as the temperature increases, the resiliency increases. At elevated temperatures, butyl rubber exhibits good resiliency. Its abrasion resistance, tear resistance, tensile strength, and adhesion to fabrics and metals is good. Butyl rubber has a maximum continuous service temperature of 250-350°F (120-150°C) with good resistance to heat aging. Its electrical properties are generally good but not outstanding in any one category. The flame resistance of butyl rubber is poor.

The polymer has excellent flexibility but very low resilience due to its molecular structure. As a result, it absorbs a great deal of any mechanical energy that is put into it. This energy absorption is responsible for butyl rubber’s vibration-damping properties, which are maintained over a broad temperature range. Consequently, butyl rubber is an excellent choice where high vibration damping is required. This property is useful in automobile body mounts and suspension bumpers.

Table 4.15 lists the physical and mechanical properties of butyl rubber.

There are two commercially available halogenated butyl rubber derivatives bromobutyl (BIIR) and chlorobutyl (CIIR). The halogen atoms are incorporated into the polymer on the isoprene units. These compounds have chemical properties that permit the polymers to be covulcanized with other elastomers more readily than IIR. The end-use properties and applications are similar to those for IIR. A typical application is the cover for air conditioning hoses.

Chlorobutyl (CIIR) rubbers have a maximum operating temperature of 300°F (150°C) and can be operated as low as -30°F (-34°C). The other physical and mechanical properties are similar to those of butyl rubber.

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Trích đăng từ sách Mechanical and Corrosion-Resistant Properties of Plastics and Elastomers, Philip A. Schweitzer, CRC Press, 2000, trang 295 – 299

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Tính chất cơ lý của cao su styrene-butadiene (SBR)

Cao su tổng hợp SBR được phát triển trong chiến tranh thế giới II do sự thiếu hụt của cao su thiên nhiên. Sự so sánh tính chất cơ lý của cao su thiên nhiên và cao su SBR được trình bày sau đây.

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In general, Buna-S is very similar to natural rubber although some of its physical and mechanical properties are inferior. It is lacking in tensile strength, elongation, resilience, hot tear, and hysteresis. These disadvantages are offset somewhat by its low cost, cleanliness, slightly better heat-aging properties, slightly better wear than natural rubber for passenger tires, and availability at a stable price. The electrical properties of SBR are generally good but are not outstanding in any one area.

Because of these shortcomings, there are many variations and blends of NR-SBR. Basic is the ratio of butadiene to styrene, temperature of polymerization, and type of chemicals used during polymerization. The NR-SBR blends must be properly formulated to achieve the correct balance of properties. SBR is superior to NR in several of its processing properties. It also has excellent abrasion resistance and resistance to hydraulic brake fluid.

Molded SBR parts have a strong tendency to bloom due to the presence of added stabilizers. The bloom creates an unsightly appearance and can cause migration staining in contact with paint. Consequently, SBR is not generally used in readily visible locations or in contact with painted surfaces.

Buna-S has a maximum operating temperature of 175°F (80^oC), which is not exceptional. At reduced temperatures below 0^oF, Buna-S products are more flexible than those produced from natural rubber.

Butadiene-styrene rubber has poor flame resistance and will support combustion.

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Trích đăng từ sách Mechanical and Corrosion-Resistant Properties of Plastics and Elastomers, Philip A. Schweitzer, CRC Press, 2000, trang 289 – 291

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Chất hóa dẻo dùng cho cao su và phân loại

Chất hóa dẻo được phối trộn vào hỗn hợp cao su, hỗ trợ quá trình gia công. Phần tài liệu dưới đây sẽ giới thiệu về chất hóa dẻo và phân loại chúng theo thành phần hóa học và tính năng.

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Plasticizer: Any material (primarily liquid) that is added to a compound to facilitate easier mixing, extruding, molding, and curing through modification of the basic physical properties of the uncured and cured compound (e.g., viscosity, hardness, modulus, low-temperature flex).

Monomeric types: It is common to characterize plasticizers generally as either polymeric or monomeric. For our purposes here, monomerics are all plasticizers (glycol esters, monoesters, diesters, and triesters), which are not polymeric. Viscosities range from 4 to 400 cps.

Polymeric types: This group is technically comprised of polyesters with viscosities from 400 to 200,000 cps. To create polyester, a glycol rather than an alcohol is reacted with a fatty acid. For purposes of this chapter we exclude the very high polymers used as polymeric plasticizers such as Nipol 1312, a low-molecular weight NBR elastomer.

Synthetic plasticizers: These are chemically reacted, man-made, from highly specified components to form a plasticizer with consistently reproducible properties. Esters comprise the major portion of this category.

Low-temperature improvement: This is achieved through the use of plasticizers primarily related to their molecular weight as measured by viscosity. A rating system has been applied to a number of plasticizers in Table 12.1. Compatibility is also a factor to be considered, hence the ratings in both polar and semipolar elastomers is given.

High-temperature performance: It is also related to the molecular weight of the plasticizer as measured by volatility, as may be seen in Table 12.2. The relative solubility of a number of plasticizers is also given, which relates to their extractability in various fluids in service. Both of these factors must be considered when designing a specialty elastomer compound.

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Trích đăng từ sách Handbook of Specialty Elastomers, Robert C. Klingender, CRC Press, 2008, trang 389

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Tính kháng hóa chất của cao su chloroprene (CR)

Cao su chloroprene (CR) là một loại cao su tổng hợp thông dụng. Nó có tính kháng dầu mỡ, dung môi hydrocarbon tốt, khắc phục khuyết điểm của cao su thiên nhiên. Ngoài ra, cao su chloroprene còn kháng được nhiều loại hóa chất khác như như axit, kiềm, muối, nước, giúp cho việc sử dụng loại cao su này linh hoạt, phù hợp với nhiều môi trường. Tính kháng hóa chất của cao su chloroprene được trình bày cụ thể bên dưới.

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Neoprene’s resistance to attack from solvents, waxes, fats, oils, greases, and many other petroleum-based products is one of its outstanding properties. Excellent service is also experienced when it is in contact with aliphatic compounds (methyl and ethyl alcohols, ethylene glycols, etc.), aliphatic hydrocarbons, and most Freon refrigerants. A minimum amount of swelling and relatively little loss of strength occur when neoprene is in contact with these fluids.

When exposed to dilute mineral acids, inorganic salt solutions, or alkalies, neoprene products show little if any change in appearance or change in properties.

Chlorinated and aromatic hydrocarbons, organic esters, aromatic hydroxy compounds, and certain ketones have an adverse effect on neoprene, and consequently only limited serviceability can be expected with them. Highly oxidizing acid and salt solutions also cause surface deterioration and loss of strength. Included in this category are nitric acid and concentrated sulfuric acid.

Neoprene formulations can be produced that provide products with outstanding resistance to water absorption. These products can be used in continuous or periodic immersion in either freshwater or saltwater without any loss of properties.

Properly compounded neoprene can be buried underground successfully, since moisture, bacteria, and soil chemicals usually found in the earth have little effect on its properties. It is unaffected by soils saturated with seawater, chemicals, oils, gasolines, wastes, and other industrial byproducts.

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Trích đăng từ sách Mechanical and Corrosion-Resistant Properties of Plastics and Elastomers, Philip A. Schweitzer, CRC Press, 2000, trang 284 – 285

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Tính chất hóa lý chung của cao su chloroprene

Cao su tổng hợp được phát minh để cải thiện các khuyết điểm của cao su thiên nhiên, đáp ứng nhiều ứng dụng thực tế khắc nghiệt. Cao su chloroprene (neoprene hoặc CR) là một trong những loại cao su tổng hợp thông dụng. Tính chất hóa lý chung của cao su chloroprene được trình bày sau đây.

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The properties discussed here are attainable with neoprene but may not necessarily be incorporated into every neoprene product. Nor will every neoprene product perform the same in all environments. The reason for this variation is compounding. By selective addition and/or deletion of specific ingredients during compounding, specific properties can be enhanced or reduced to provide the neoprene formulation best suited for the application. A neoprene compound can be produced that will provide whichever of the properties discussed are desired. When the hardness of neoprene is above 55 Shore A, its resilience exceeds that of natural rubber by approximately 5%. At hardnesses below 50 Shore A, its resilience is not as good as that of natural rubber, even though its resilience is measured at 75%, which is a high value. Because of its high resilience, neoprene products have low hysteresis and a minimum heat buildup during dynamic operations.

Solid neoprene products can be ignited by an open flame but will stop burning when the flame is removed. Because of its chlorine content, neoprene is more resistant to burning than exclusively hydrocarbon elastomers. Natural rubber and many of the other synthetic elastomers will continue to burn once ignited, even if the flame is removed. In an actual fire situation, neoprene will burn. Although compounding can improve the flame resistance of neoprene, it cannot make it immune to burning.

Compared to natural rubber, neoprene is relatively impermeable to gases. Table 4.9 lists typical permeability constants. Because of this impermeability, neoprene can be used to seal against Freon blowing agents, propane, butane, and other gases.

Neoprene is used in many electrical applications, although its dielectric characteristics limit its use as an insulation to low voltage (600V) and low frequency (60Hz). Because of its high degree of resistance to indoor and outdoor aging and its resistance to weathering, neoprene is often used as a protective outer jacket to insulation at all voltages. It is also immune to high-voltage corona discharge effects that cause severe surface cutting in many types of elastomers.

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Trích đăng từ sách Mechanical and Corrosion-Resistant Properties of Plastics and Elastomers, Philip A. Schweitzer, CRC Press, 2000, trang 281 – 282

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Tác động đến sức khỏe của dung môi dùng trong công nghiệp cao su

Nhiều loại dung môi được sử dụng phổ biến trong công nghiệp cao su như hydrocarbon no hoặc thơm (benzene, toluene, …) để làm sạch bề mặt cao su, dụng cụ, thiết bị hoặc chlorinated hydrocarbon có trong các loại keo dính chống cháy và các hóa chất khác. Tác động của chúng đến sức khỏe của công nhân được trình bày cụ thể bên dưới.

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5. Solvents

Solvents are used extensively in the rubber industry during the manufacturing process to prevent tackiness. Solvents are also used to degrease and clean workshops. Users must recognise where particular solvents are likely to occur, namely: that aliphatic hydrocarbons are used for the freshening of rubber surfaces; aromatic hydrocarbons (like toluene and xylene) are likely to be found in rubber solutions; carbon disulfide is used in the traditional cold cure process; and, chlorinated hydrocarbons (like methylene chloride) are used in the production of non-flammable adhesives (a.8, a.9).

5.1. Health Effects

The main effects of solvents are irritation to the skin, eyes and lungs, headache, nausea, dizziness and light-headedness. Exposure can impair coordination making workers prone to falling-type accidents. A person may lose concentration or have a reduced reaction time thus affecting judgment of important or difficult tasks. These effects will vary and can be exacerbated by drinking alcohol. Very high exposure, especially where adhesives are used in unventilated, confined spaces, may cause unconsciousness and even death. A person who has been exposed to solvents and feels their health has been adversely affected should seek medical advice. Other effects vary according to the solvent, several are reviewed here.

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5.1.2. Toluene: C₆H₅CH₃

Toluene or methylbenzene is a monomethyl derivative of benzene. Toluene is used to indicate the swelling index of rubber. Acute exposure to concentrations >200 ppm may result in headache, dizziness, irritation of the eyes, nose and throat, paresthesia, incoordination, confusion and narcosis. Chronic exposure may give rise to muscle weakness, abdominal pain, impairment of gait/balance, ataxia, peripheral neuropathy and altered mental state. Enlargement of the liver and neurobehavioural effects have been documented. Effects on the blood system similar to benzene have been attributed to benzene being present as a contaminant in some commercial batches of toluene. The 2000 UK occupational exposure limit for toluene was 50 ppm (8-hour TWA reference period) with a short-term exposure limit of 150 ppm (a.10).

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Trích đăng từ sách Health and Safety in the Rubber Industry, Naesinee Chaiear, Smithers Rapra Press, 2001, trang 15 – 16

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Các yếu tố ảnh hưởng đến sự an toàn khi sử dụng máy cán

Máy cán được sử dụng rộng rãi trong ngành công nghiệp cao su. Các tai nạn liên quan đến máy cán chủ yếu là bị cuốn vào trục cán đang quay trong lúc vận hành, sửa chữa, bảo trì. Nguy cơ xảy ra các tai nạn liên quan đến việc sử dụng máy cán được trình bày cụ thể sau đây.

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The problem of mill safety is not a simple one; there are several major issues involved:

·   mill height,

·   the size of the operator,

·   auxiliary equipment,

·   the way the mill is worked,

·   the tack or stickiness of the stock,

·   stopping distance.

Mill height makes a difference as to where the operator works the mill. For mills less than 1.27 m high, where the height of the operator is greater than 1.68 m, there is a tendency to work too high on the mill or too close to the nip. This allows for a very short reaction time for the automatic safety to stop the mill.

The size of the operator also dictates how close the operator needs to get to the mill face to work the mill. Operators come in many different sizes, and often must operate the same mill. The majority of the time on adjustment is made to the mill safety devices.

Auxiliary equipment such as conveyors or loaders can often conflict with safety cables and ropes. Despite codes to the contrary, often the safety rope or cable is moved to allow for the operation of the auxiliary equipment. This can result in the operator working the mill with the safety cable behind the operator’s head.

While the height of the mill and the auxiliary equipment have a part in the way a mill is worked, there are other factors which enter into the picture. If there is no mixing roll below the mixer to distribute the rubber evenly on the mill, the operator will have to physically move the rubber from one side of the mill to the other by hand. The mixing and moving of the rubber exposes the operator to increased risk of strain or sprain injuries in addition to the hazard of the mill nip.

The tack or stickiness of the stock poses an additional hazard. If the rubber sticks to the mill roll and the operator has to pull it off the roll, a body bar becomes a safety hazard. Operators of mills with hot rubber have to wear gloves. Mill operators use knives. Tacky stock can grab a knife, glove or bare hand and pull it toward the running nip of the mill.

Even an automatic safety device will not be effective unless the mill can be stopped before the operator reaches the running nip of the mill. Stopping distances must be checked at least weekly and the brakes tested at the beginning of each shift. Dynamic electrical brakes must be checked on a regular basis. If the zero switch is not adjusted properly, the mill will move back and forth and damage to the mill will result. For some situations, disc brakes are preferred. With electrical brakes a problem can arise if the operator has activated the mill stop button and then tried an emergency mill stop. On some mills the emergency stop will not work after the mill stop button has been activated.

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Trích đăng từ sách Health and Safety in the Rubber Industry, Naesinee Chaiear, Smithers Rapra Press, 2001, trang 11 – 13

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Các yếu tố ảnh hưởng đến sự oxy hóa vật liệu đàn hồi

Sự oxy hóa là nguyên nhân phổ biến gây hư hỏng cho vật liệu đàn hồi. Các yếu tố như nhiệt độ, ánh sáng và hàm lượng kim loại trong cao su tăng nhanh quá trình oxy hóa này. Nội dung này được trình bày cụ thể bên dưới.

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EFFECTS OF HEAT

As noted, a small percent of oxygen, combined with an elastomer, can seriously degrade the physical properties of that elastomer. Heat can greatly increase the rate at which oxygen reacts with the polymer. The rate of reaction approximately doubles for each 10^oC increase in temperature. About a 50-fold increase in reaction rate occurs between room temperature and 70^oC. Another perspective would be that a change in physical properties, observed at a service temperature of 150^oC, would require about 8000 times longer for the same change to occur at ambient temperature [9].

EFFECTS OF LIGHT AND WEATHERING

UV light promotes free radical oxidation at the rubber surface which produces a film of oxidized rubber. Heat and humidity then accelerate the formation of a crazing or alligatoring effect, and this oxidized layer can be rubbed off—giving a chalking appearance.

EFFECTS OF METALS

Heavy metal (principally cobalt, copper, manganese, iron) ions are believed to catalyze oxidative reactions in elastomers by influencing the breakdown of peroxides in such a way as to accelerate further attack by oxygen. The first corrective approach is to eliminate all sources of harmful metals. Compounds of copper and manganese, such as stearates and oleates, which are directly soluble in rubber, are particularly active, since they provide a direct source of heavy metal ions. Even the less soluble forms such as the oxides can cause problems by reacting with fatty acids used in compounding to produce more soluble forms.

Although some antioxidants are active against catalyzed oxidation of rubber, in general, the standard antioxidants do not give protection against the heavy metal ions. Since the activity of the metal depends on its being in an ionic form, it is possible to protect compounds by incorporating substances which react with ionic metals to give stable coordination complexes.

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Trích đăng từ sách Handbook of Specialty Elastomers, Robert C. Klingender, CRC Press, 2008, trang 435 – 436

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Quá trình oxy hóa vật liệu đàn hồi

Cũng giống như hầu hết vật liệu khác, vật liệu đàn hồi cũng bị oxy hóa bởi oxy trong không khí, gây ra hư hỏng. Mức độ oxy hóa phụ thuộc vào loại polymer và môi trường tiếp xúc. Quá trình oxy hóa cao su xảy ra theo hai cơ chế khác nhau, quyết định tính chất của sản phẩm cao su bị oxy hóa khác nhau. Nội dung này sẽ được trình bày cụ thể bên dưới.

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Elastomers, like most organic materials, are subject to atmospheric oxidation, even at moderate temperatures. The ease of susceptibility to degradation depends, to a large degree, upon structure and environmental exposure. For example, saturated polymers are inherently more stable than unsaturated polymers because of their stronger bonds, or lack of double bonds in their backbone. Therefore, it would hold that EPDM and butyl rubber would be more stable than SBR or NR against oxidative degradation.

Oxidation is a complex process involving many reactions, each influenced by prevailing conditions such as:

1. Singlet oxygen

2. Ozone

3. Mechanical shear

4. Heat

5. Light

6. Metals

7. Fatigue

Most elastomers are subject to oxidation and it is known that the addition of only 1%–2% combined oxygen will render a rubber article useless.

Oxidation proceeds by two mechanisms:

1. Chain scission: Results from the attack of the polymer backbone which causes softening and weakness. It is the primary mechanism observed for natural rubber and butyl oxidation.

2. Cross-linking: Brittle compounds result because of radical cross-linking reactions, resulting in the formation of new cross-links and a stiffer material. This reaction occurs predominantly with SBR, polychloroprene, NBR, and EPDM.

In most cases, both types of attack occur and the one which prevails determines the final compound properties. It has been found that loss of elongation is the most sensitive criterion for aging measurements regardless of the mechanism, and it is favored over the measurement of tensile loss.

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Trích đăng từ sách Handbook of Specialty Elastomers, Robert C. Klingender, CRC Press, 2008, trang 434 – 435

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Giới thiệu về chất độn carbon nanotubes

Hiện tại, vật liệu cao su nanocomposite – dùng chất độn kích thước nano – thu hút sự quan tâm rộng rãi do tính chất cơ lý tốt. Các chất độn thường được sử dụng trong vật liệu cao su nanocomposite là đất sét, than đen. Phần tài liệu sau đây sẽ giới thiệu chung về chất độn carbon nanotubes.

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Over recent years, carbon nanotubes (CNT) have inspired scientists for a range of potential applications [7–12]. Carbon nanotubes are allotropes of carbon with a unique atomic structure consisting in covalently bonded carbon atoms arranged in long cylinders with typical diameters in the range 1–50 nm and a wide variety of lengths. Individual carbon nanotubes are characterized by a high aspect ratio (300–1000), high flexibility [13] and unique combination of mechanical, electrical and thermal properties [14, 15]. The combination of these properties with a very low mass density [16] makes them potentially useful as ideal reinforcing fibers for high-performance polymer composites.

Carbon nanotubes are usually produced by three techniques: arc discharge, laser ablation and chemical vapor deposition. The quality and yield of carbon nanotubes strongly depend on the synthesis and purification techniques and the specific growth conditions used [11]. There are two basic types of CNT: single-walled carbon nanotubes (SWCNT) and multiwalled carbon nanotubes (MWCNT). The structure of a SWCNT is characterized by wrapping a one-atom thick layer of graphite called graphene (graphene is a monolayer of sp²-bonded carbon atoms) into a seamless cylinder. The carbon atoms in the cylinder have partial sp³ character that increases as the radius of curvature of the cylinder decreases. SWCNT exhibit important electric properties that are not shared by the multiwalled carbon nanotubes.

MWCNT consist of multiple layers of graphite arranged in concentric cylinders with an interlayer distance close to the distance between graphene layers in graphite (circa 0.34 nm). Of particular interest are the double-walled carbon nanotubes (DWNT) because they combine a similar morphology and properties in relation to SWCNT, while improving significantly their resistance to chemicals.

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Trích đăng từ sách Rubber Nanocomposites: Preparation, Properties and Applications, Sabu Thomas và Ranimol Stephen, Wiley, 2010, trang 147 – 148

Nguồn: www.books.google.com.vn

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Quá trình ozone hóa cao su và sử dụng chất chống ozone

Sản phẩm cao su lưu hóa sẽ phân hủy nhanh khi tiếp xúc với các yếu tố như oxygen, ozone, ánh sáng mặt trời, nhiệt độ cao. Sự phân hủy cao su xảy ra khi tiếp xúc với ozone được cho là do ozone phản ứng với các liên kết đôi của polymer chưa bão hòa. Phần tài liệu bên dưới sẽ trình bày cụ thể quá trình ozone hóa cao su và cơ chế bảo vệ của các chất chống ozone.

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Ozone, a degradant of vulcanized rubber, occurs naturally and is formed in the earth’s atmosphere by the action of the sun’s ultraviolet light on atmospheric oxygen. The ozone is carried into the atmosphere by winds and, depending on the seasons and geographic locations, can be found on the earth in normal concentrations of 6 parts per hundred million to concentrations of as high as 25 parts per hundred million.

The effects of ozone on vulcanized diene rubber are best noted when the rubber is stressed or stretched in use. A series of cracks develop, in time, which are perpendicular to the applied stress. Further exposure to ozone of these cracked surfaces causes the cracks to become wider and deeper until the rubber fails.

The detrimental effects of ozone are noted also on nonstressed rubber surfaces and manifest themselves as frosting, which is the exposure of the non-black fillers resulting from the formation of minute cracks on the rubber surface formed by the severing of the molecular chains. This phenomenon is quite common in footwear compounds [1].

The mechanism proposed by Criegee best describes the degradation initiated by ozone called ozonolysis. Ozone, a very reactive material, reacts at the surface, across the double bond, in an unsaturated polymer to form a trioxolane structure. This structure undergoes decomposition to give a carbonyl compound and a zwitterion, resulting in a severed molecular chain. The zwitterion can recombine to form either an ozonide, diperoxide, or higher peroxide.

Many theories have been proposed to explain the mechanism of how antiozonants protect elastomeric articles from the effects of ozone. Most support the scavenger model, and indicate that an ideal antiozonant should be capable of migrating in a rubber matrix to the surface whenever the equilibrium of the antiozonant concentration in the compound is upset by the formation of ozonized antiozonant at the surface, but yet not freely migrate to the surface and volatilize out of the elastomer without first reacting with ozone at the surface. On the other hand, the antiozonant should not be so slow in migrating through the elastomer matrix that it arrives at the surface after the ozone has already reacted with the elastomeric polymer. It has been stated that ozone is 200 times more reactive with an antiozonant than with the double bonds in an elastomer.

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Trích đăng từ sách Handbook of Specialty Elastomers, Robert C. Klingender, CRC Press, 2008, trang 430 – 431

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Hướng dẫn chung khi lựa chọn chất hóa dẻo dùng cho cao su

Hiện nay, trên thị trường có rất nhiều loại chất hóa dẻo dùng cho cao su. Phần tài liệu dưới đây sẽ cung cấp cho người đọc những hướng dẫn chung để chọn được chất hóa dẻo phù hợp cho một ứng dụng cụ thể.

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SYNTHETIC PLASTICIZERS

1. There are no effective low-temperature synthetic ester plasticizers with viscosities above 50 cps.

2. High-temperature volatility/aging is directly related to the MW of the plasticizer. Low viscosity/low MW equals poor aging and high viscosity/high MW equals good aging.

3. Extraction and swell characteristics are dependent on the chemistry of the extract and plasticizer and the MW. For example, high MW polymerics are relatively immobile and extraction fluids tend to be absorbed by the plasticizer (volume swell), whereas monomerics are much more mobile and easily exchange places with extracts and usually create more or less volume shrinkage depending also on the chemistry involved.

4. There is a hold-in characteristic of some plasticizers—especially high MW types—effectively increasing compatibility is related not only to size of the molecule (see 3 above) relative to a co-plasticizer or swell medium but also to the polarity of the plasticizers involved (see also castor factice below).

VULCANIZED VEGETABLE OIL/FACTICE

1. In highly polar elastomers use castor-based factice for highest compatibility, best stress–strain properties, and lowest volume swells.

2. Compression set is adversely affected by all sulfur-cured factice and should be monitored carefully.

3. Improve compatibility of highly plasticizer-loaded compounds by substituting 0.5–1.5 phr of factice for each phr of liquid plasticizer.

4. Depending on the formulation and loading, factice can have positive or negative effects on heat-aged properties in semipolar or polar compounds where use of factice is indicated.

RUBBER PROCESS OILS

1. Use hydrocarbon process oils sparingly in specialty elastomers.

2. Never use the paraffinic types in polar or semipolar elastomers.

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Trích đăng từ sách Handbook of Specialty Elastomers, Robert C. Klingender, CRC Press, 2008, trang 407 – 408

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Các loại lưu huỳnh kết mạng cao su

Hai loại lưu huỳnh thường được sử dụng trong kết mạng cao su là lưu huỳnh tan và lưu huỳnh không tan. Ưu và khuyết điểm của từng loại lưu huỳnh được mô tả bên dưới.

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Elemental sulphur is the most widely used vulcanisation agent in the rubber industry and is effective in elastomers containing some degree of unsaturation. Ground sulphur is most widely used, often referred to as rhombic sulphur or rubber makers’ sulphur. The molecular structure of rhombic sulphur comprises an eight membered ring and is crystalline in nature. It has a melting point of 115 °C and is soluble to a limited degree in elastomers; for example, around 1% w/w in natural rubber at room temperature, increasing to a level of the order of 7% at 100 °C. The relatively low solubility of sulphur in rubber at ambient temperature is the cause of so-called ‘sulphur bloom’. It appears as an off-white powdery coating on the surface of the uncured compound due to migration from the bulk compound when the limit of solubility is exceeded. If present in excess it has an unfavourable effect on the building tack of green components. Sulphur bloom can also occur in vulcanisates but here the disadvantage is largely cosmetic.

Sulphur bloom can be prevented by substituting rubber makers’ sulphur with so called insoluble sulphur. This is a crystalline, polymeric form of sulphur [1] and is insoluble in solvents and elastomers. It should be processed at temperatures not exceeding 110 °C, preferably 105 °C, in order to prevent excessive conversion into the rhombic form. During vulcanisation it is converted into rhombic sulphur allowing the vulcanisation process to proceed as normal. The following advantages are claimed with regard to the use of insoluble sulphur [1, 2]:

·         elimination of sulphur bloom,

·         prevention of sulphur migration between green components during storage,

·         reduced bin-scorch during the storage of green compounds.

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Trích đăng từ sách Rubber Technologist’s Handbook, Sadhan K. De và Jim R. White, Smithers Rapra Technology, 2001, trang 167 – 168

Nguồn: www.books.google.com.vn

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Sử dụng polymer, copolymer cải thiện tính chất cơ lý của cao su blend

Các vật liệu đàn hồi không tương thích cao, như NR và NBR, rất khó tạo thành hỗn hợp có cơ tính tốt. Nhiều yếu tố ảnh hưởng như kỹ thuật cán luyện, hệ kết mạng đã được khảo sát. Trong đó, biện pháp thêm vào các polymer, copolymer tăng sự tương thích giữa hai pha cao su mạng lại hiệu quả cao. Phần tài liệu bên dưới sẽ cung cấp thêm thông tin về vấn đề này.

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Blends of highly incompatible elastomers may sometimes be improved by the addition of small amounts of another polymer. Setua and White [1] applied this technique to improve the homogeneity of binary and ternary blends of CR, NBR and EPM. When a small amount of chlorinated polyethylene is added to NBR/EPM or CR/EPDM blends, they mix more rapidly. The chlorinated polyethylene acts as a compatibilizing agent which appears to form a skin on the EPM particles that helps the larger NBR chunks adhere to them. This increased adhesion and polarity at the EPDM surface increases the compatibility. The presence of block or graft copolymers can also alleviate blending of incompatible elastomers as they can alter interfacial properties [2-6]. Ideally the block or graft component should contain a segment which is chemically identical to one of those in the respective phases, but the desired effect may still be achieved if one polymer of the graft is miscible with, or adhered to, one of the phases.

It is possible to obtain considerable improvement in the phase morphology and tensile properties of NR/NBR gum blend vulcanizates by incorporating typically 5-20% of methyl methacrylate grafted natural rubber (e.g. Heveaplus MG30), or chloroprene rubber (CR) (Chapter 7). MG30 is a mixture of graft copolymer with ungrafted NR and poly(methyl methacrylate), PMMA, present as a homopolymer. It was considered that there would be sufficient interaction between the PMMA graft chains and the acrylonitrile repeat units of the NBR for the graft copolymer to locate at the NR/NBR interface and thus reduce interfacial tension and hence phase size.

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Trích đăng từ sách Blends of Natural Rubber: Novel Techniques for Blending with Specialty Polymers, Andrew J. Tinker và Kevin P. Jones, Springer, 1998, trang 68 – 69

Nguồn: www.books.google.com.vn

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