Damage zone in PVC and PVC/MBS blends. I. The effect of rubber content and temperature

The damage zone development in poly (vinyl chloride) (PVC) and its transparent blends with methyl methacrylate-butadiene-styrene (MBS) core/shell rubber was studied as a function of temperature and rubber content in a triaxial stress state under slow tensile loading. Failure at the semicircular notch occurred by shear yielding followed by stress whitening. In unmodified PVC, the shear yielded plastic zone size was not affected by temperature in the range between —40 and 40°. In the blends, the plastic zone preceding stress whitening increased in size with temperature and rubber content, shifting the stress-whitened zone further away from the notch root. Below 0°, stress whitening initiated at the notch root and the stress-whitened zones had a crescent shape similar to those of PVC/CPE blends studied previously. In unmodified PVC, stress whitening initiated from the growth of preexisting microvoids at the tip of the shear yielded zone containing two families of curving slip lines emanating from the notch root. In contrast, stress whitening in the blends was more intense and was initiated by the cavitation of the rubber particles.     

Fracture behavior of polypropylene/ ethylene- diene-terpolymer blends : Effect of temperatures,notch radius and rubber content

The mechanical fracture and ductile-brittle transition (DBT) behavior, hysteresis phenomenon and the plastic zone size of polypropylene (PP) / ethylene-propylene-diene terpolymer (PP/EPDM) blends were investigated by varying EPDM content and notch radius under different temperatures. An increase in test temperature or rubber content in the PP/EPDM blend results in lower yield stress and Young's modulus. The ductile-brittle transition temperature (DBTT) of the notched impact strength decreases with the increase of the EPDM content. However, the DBTT is fairly independent of the notch radius. SEM morphologies of the fracture surfaces indicate that two separate modes, localized and mass shear yielding, work simultaneously in these blends. The plane-strain localized shear yielding dominates the brittle failure at lower temperatures, whereas the plane stress mass shear yielding dominates the ductile fracture at higher temperatures. The presence of EPDM rubber decreases the yield stress of the PP/EPDM blend due to the overlapping stress fields of adjacent particles, resulting in higher hysteresis energy. The relationships among the test temperature, hysteresis loss energy and the size of plastic zone are discussed in detail.     

橡胶粉改性沥青制备及性能试验研究

为制备沥青路面用橡胶粉改性沥青,针对胶粉目数、胶粉掺量、剪切时间、制备温度与剪切速率等影响因素,采用5因素4水平正交试验方案,以延度、弹性恢复率、抗剪强度、离析软化点差及粘温回归系数|B|值作为评价指标,遴选出橡胶粉改性沥青制备优方案,并制备出橡胶粉改性沥青;在此基础上,对其作用机理和路用性能进行分析.研究结果表明:采用掺量为24%的30目胶粉、剪切时间60 min、制备温度205 ℃、剪切速率5000 r/min制备出的橡胶粉改性沥青路用性能优异;均匀分散于沥青体系中的多孔状胶粉颗粒吸收沥青中的轻质组分,产生溶胀作用及微弱化学反应并呈空间网状交联结构,是橡胶粉改性沥青性能优异的主要原因.     

Comparative deformational characteristics of poly(styrene-b-ethylene-co-butylene-b-styrene) thermoplastic elastomers and cross-linked natural rubber

Three poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) thermoplastic elastomers (TPEs) are studied mechanically and compared to cross-linked natural rubber. It is observed that subtle alterations in the mid-block of the TPEs affect their mechanical properties significantly. The stress relaxation at room temperature is significantly altered indicating a reduced flow in systems where the ratio of ethylene to butylene segments in the mid-block is greater than one. The cyclic behavior of these systems also shows significant hysteresis. Differential scanning calorimetry suggests that these TPEs crystallize at low temperatures, similar to the observed behavior in cross-linked natural rubber. Results of internal energy changes from deformation calorimetry provide evidence for strain-induced crystallization occurring in certain SEBS systems, similar to the internal energy changes observed for cross-linked natural rubber. Simultaneous WAXD/SAXD measurements on deformed SEBS samples highlight deformation at the nanometer and the molecular length scales. In situ WAXD at different strains further reinforces the evidence for formation of strain-induced crystallites in the selected systems. Strain-induced crystallization occurring in certain TPEs provides a mechanism for reduction of flow at high strains and accounts for the retention of their highly elastic behavior.     

地聚物橡胶混凝土的力学、抗冲击性能及强度机理分析

为探究橡胶再生集料在地聚物混凝土中的增韧作用,采用不同比例的橡胶再生集料(粒径为0.15~0.3 mm和1~4 mm)代替地聚物混凝土中的普通集料。首先,采用工业废渣中粒化高炉矿渣粉为原料,水玻璃溶液和NaOH为激发剂制备地聚物胶凝材料,并与普通集料、橡胶再生集料混合制备地聚物橡胶混凝土;然后对地聚物橡胶混凝土的力学性能、抗冲击性能和强度机理进行探究。结果表明:添加适量比例的橡胶再生集料可提高地聚物混凝土的抗压强度,增幅为5%左右,这与橡胶再生集料表面在NaOH碱环境中的氧化和降解作用有关;但地聚物橡胶混凝土在劈裂荷载和弯曲荷载下易断裂,劈裂抗拉强度和抗折强度降低20%~30%,这是地聚物橡胶混凝土的典型特点。通过落锤冲击试验和弯拉冲击试验可知,橡胶再生集料能大幅提高地聚物混凝土的抗冲击性能,最高增幅超过200%。这是由于橡胶再生集料具有的柔性和弹性特征吸收了部分冲击能量,同时延缓了冲击荷载下混凝土初始开裂到失效破坏过程,减少了裂缝开裂应力集中,提高了地聚物混凝土的延性。在地聚物碱性条件下,橡胶颗粒表面发生氧化和降解作用,表面变得粗糙,与地聚物基体结合紧密;橡胶表面在NaOH作用下,产生羟基、羧基等亲水基团,有利于水化产物在橡胶再生集料表面附着。     

Effect of reynolds number on the dispersion of carbon black in natural rubber latex and filler–rubber interfacial interaction in high‐speed jet flow field

A novel strategy of high‐speed impinging jet processing technique was developed to prepare rubber composites by directly mixing natural rubber latex (NRL) with carbon black (CB). The results showed that CB could be more evenly dispersed into natural rubber by jet compounding technique, compared with the traditional dry process. The dispersion of carbon black was improved at higher Reynolds number because of more intensive shear force. Furthermore, a high performance of natural rubber latex/CB composite was gained by jet compounding technique. When compared to the traditional dry process, the mechanical properties and abrasion resistance of the composite prepared by the jet compounding technique were greatly improved because of the improved filler–rubber interaction at higher Reynolds number. POLYM. COMPOS., 34:1071–1075, 2013. © 2013 Society of Plastics Engineers     

Effect of interfacial strengthening in blends of reclaimed rubber and polypropylene

Thermoplastic vulcanizates (TPVs) were prepared from polypropylene (PP) and reclaimed ground tire rubber crumbs. Three types of interfacial strengthening agents—degraded PP, hydrosilylated PP, and hydrosilylated PP grafted onto styrene–butadiene rubber—were prepared in melt via a stepwise series of reactions and employed to generate various degrees of interfacial adhesion in the aforementioned blends. The incorporation of the interfacial agents resulted in improvements in the mechanical properties of these TPVs, and the rubber particle size remained constant. The PP chain length and the functional groups present in the interfacial agents affected the magnitude of the improvement in the mechanical properties. The interfacial agents were primarily present on the surface of the rubber particles in the blends, as shown by energy‐dispersive X‐ray spectra. These interfacial agents in the PP/rubber crumb blends led to a unique preyield kink in their stress–strain curves, a plateau, or a sharp turning point in the region of approximately 3% elongation and approximately 4‐MPa stress. These kinks were interpreted similarly to the cold flow of semicrystalline polymers in tension. The addition of the interfacial modifiers decreased the shear viscosity and increased the entrance pressure drop in flow through capillary dies, and this was attributed to changes in the elongational viscosity of the blends. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

天然橡胶硫化动力学研究

采用硫化仪测定了2种不同凝固工艺的天然橡胶（NR）的硫化过程,探讨了制胶工艺、配方和温度对硫化动力学参数的影响。研究结果发现,随着温度的上升,硫化速率常数迅速上升。在纯胶体系中,微生物凝固胶的活化能高于酸凝固胶。当加入炭黑后,酸凝固胶的活化能升高,而微生物凝固胶的活化能下降。     

Effect of foaming temperature and rubber grades on properties of natural rubber foams

Foaming temperature and grade of dry natural rubber were varied to evaluate their effects on the morphology and mechanical properties of natural rubber (NR) foams. Three different grades of NR were used; namely ENR‐25, SMR‐L, and SMR‐10. NR foams from these grades were produced at three different foaming temperatures, i.e. 140, 150, and 160°C. The study was carried out using formulated compositions containing sodium bicarbonate as the chemical blowing agent and were expanded using conventional compression molding technique via a heat transfer foaming process. The NR foams were characterized with respect to their relative foam density, density of crosslinking, cell size, compression stress, and compression set. Increase in foaming temperature resulted in lower relative density and larger cell size. It was also discovered that the crosslink density slightly decrease with increasing foaming temperature. For mechanical properties, the highest foam density resulted in the highest compression stress. Compression stress at 50% strain increased with increasing foaming temperature and ENR‐25 foam has the highest compression stress among the produced foams. The results showed that the morphology, physical, and mechanical properties of the rubber foams can be controlled closely by the foaming temperature and rubber grades. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

端氨基丁腈橡胶增韧环氧-聚酰胺胶粘剂的研究

通过DSC,扫描电镜分析及剪切和剥离强度性能测试研究了端氨基液体丁腈橡胶(ATBN)改性环氧-聚酰胺体系的固化动力学,粘接性能及增韧相态。结果表明,根据Ellerstein法和峰值法计算得到的固化反应活化能分别为为73.6 kJ/mol和65.7 kJ/mol,体系最佳固化温度为41～97℃。固化体系中橡胶相粒径大小对胶粘剂性能有较大影响,60℃和室温固化体系分散相粒子平均粒径分别为1～2μm,0.5μm。粒径1～2μm时体系的增韧效果最佳,粘接性能优异。     

Toughening mechanisms in core-shell rubber modified polycarbonate

We have conducted a study of toughening mechanisms in rubber modified polycarbonate systems in order to evaluate the sequence of deformation events which improve fracture toughness. We conclude that cavitation of the rubber particles occurs first, followed by massive shear yielding of the matrix material. The size and shape of the deformation zone created in front of the crack is governed by the mechanical properties of the rubber particles and the stress state at the crack tip. The importance of using a variety of analytical techniques to characterize deformation mechanism is also illustrated.     

Thermoplastic vulcanizates based on epoxidized natural rubber/polypropylene blends: Effect of compatibilizers and reactive blending

Epoxidized natural rubber (ENR) was prepared using the performic epoxidation method. TPVs based on ENR/PP blends were later prepared by melt‐mixing processes via dynamic vulcanization. The effects of blend ratios of ENR/PP, types of compatibilizers, and reactive blending were investigated. Phenolic modified polypropylene (Ph‐PP) and graft copolymer of maleic anhydride on polypropylene molecules (PP‐g‐MA) were prepared and used as blend compatibilizers and reactive blending components of ENR/Ph‐PP and ENR/PP‐g‐MA blends. It was found that the mixing torque, apparent shear stress and apparent shear viscosity increased with increasing levels of ENR. This is attributed to the higher viscosity of the pure ENR than that of the pure PP. Furthermore, there was a higher compatibilizing effect because of the chemical interaction between the polar groups in ENR and PP‐g‐MA or Ph‐PP. Mixing torque, shear flow properties (i.e., shear stress and shear viscosity) and mechanical properties (i.e., tensile strength, elongation at break, and hardness) of the TPVs prepared by reactive blending of ENR/Ph‐PP and ENR/PP‐g‐MA were lower than that of the samples without a compatibilizer. However, the TPVs prepared using Ph‐PP and PP‐g‐MA as compatibilizers exhibited higher values. We observed that the TPVs prepared from ENR/PP with Ph‐PP as a compatibilizer gave the highest rheological and mechanical properties, while the reactive blending of ENR/PP exhibited the lowest values. Trend of the properties corresponds to the morphology of the TPVs. That is, the TPV with Ph‐PP as a blend compatibilizer showed the smallest rubber particles dispersed in the PP matrix, while the reactive blending of ENR/PP‐g‐MA showed the largest particles. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4729–4740, 2006     

Phase morphology and toughening mechanism of polyamide 6/EPDM-g-MA blends obtained via dynamic packing injection molding

One of the most important findings in polymer-toughening is known as the critical matrix ligament thickness (τ_c) theory, which is directly related to both rubber concentration and average size of particles. All these studies assume that rubber particles are spherical and randomly distributed in the matrix. Rubber particles may be stretched and oriented along the shear flow direction in real processing. In this paper the effect of stretched and oriented rubber particles on the impact strength of PA6/EPDM-g-MA blends have been studied via dynamic packing injection molding (DPIM). The impact strength of specimens obtained by DPIM was found substantially increase at all the blends investigated, compared with the one obtained via conventional injection molding. Particularly, more than 30 kJ m⁻² increase of the impact strength was observed for specimens with a higher rubber content (more than 15 wt%). SEM results showed a remarkably decrease of rubber particle size and more uniform dispersion of the dynamic molded specimens. This can be attributed to the shear induced reaction at the interface between polyamide 6 and EPDM-g-MA during the packing stage. The rubber particles were found stretched along the melt shear flow direction when it is content above 15 wt%. A master curve can be also constructed by plotting the impact strength versus the inter-particle distance, indicating that Wu's criterion still works for blends with stretched and oriented rubber particles when the crack propagation direction is perpendicular to the orientation direction of rubber particles. The observed higher impact strength in dynamic specimens could be due to, in part, the enhanced flexural stiffness, which will absorb more energy during impact process when the fracture of IZOD bars is incomplete, but more importantly due to the existence of the stretched and oriented rubber particles, which are more efficient in slowing the velocity of crack propagation and thus cause higher impact resistance when the fracture propagation direction is perpendicular to the rubber oriented direction.     

不同制备方法的橡胶沥青黏度特性对比分析

为了明确制备方法和工艺条件对橡胶沥青黏度特性的影响,在不同制备温度、制备时间、胶粉粒径及胶粉掺量条件下,采用高速剪切和机械搅拌两种方式制备橡胶沥青,对比分析了两种制备体系橡胶沥青黏度变化规律及黏度构成差异。采用Arrhenius方程计算了两种体系橡胶沥青的黏流活化能(E_η),探究了其黏流特性的差异及温度敏感性。结果表明:两种制备体系的橡胶沥青黏度随制备温度、制备时间的增加及胶粉粒径的减小先增大后减小,在135 ℃、150 ℃以及165 ℃测试条件下,剪切橡胶沥青的黏度大于搅拌体系,而在180 ℃条件下呈现出相反规律;随胶粉掺量的增加,二者的黏度不断增大,且搅拌体系的黏度始终大于剪切体系。剪切体系的E_η大于搅拌体系,表明剪切体系橡胶沥青的黏度主要源于胶粉和沥青的交互作用,温度敏感性更强,而搅拌橡胶沥青的黏度主要来自胶粉的位阻效应,自身稳定性更好。     

The rubber particle size dependence of crazing in polypropylene

The tensile crazing and Charpy impact behavior of polypropylene modified with styrene-butadiene copolymer (SBR) and ethylene-propylene-diene monomer (EPDM) was studied. Various rubber particle size distributions were obtained by varying the relative viscosities between rubbery phase and PP matrix. Transmission electron microscopy and computer-aided image analysis were used to provide particle size information. In general, PP blends with smaller rubber particles are tougher and more ductile than those with larger particles, probably because the former represents a more efficient use of rubbery phase in promoting crazing and/or shear yielding. Samples with average particle diameter D? ≥ 0.5 μm were found to exhibit pronounced crazing. Within a given sample, no crazes appeared to develop around individual rubber particles with D < 0.5 μm. The higher the D, the greater the propensity to form crazes. The behavior of samples with D? ? 0.5 μm appeared to be dominated by shear yielding; very few crazes could be found. That there exists a critical rubber particle size is explained by the requirement that sufficient stress concentration be maintained to a finite radial distance to permit the initiation and growth of a craze, which requires a finite volume. Small particles, inducing smaller stress-enhanced zones, are therefore not effective in initiating crazes.     

用热重分析法研究炭黑分解温度

利用热重(TG)分析法研究炭黑分解温度。结果表明:在相同升温速率下,炭黑粒径越大,炭黑外延起始分解温度越高;对于同一炭黑,升温速率越快,炭黑外延起始分解温度越高。在NBR/炭黑胶料中,随着NBR丙烯腈含量增大,炭黑外延起始分解温度呈降低趋势;随着炭黑用量增大,炭黑外延起始分解温度呈升高趋势,但炭黑用量超过一定值后,炭黑外延起始分解温度基本稳定。在NBR/炭黑/其他配合剂胶料中,炭黑外延起始分解温度受其他配合剂的影响较大。     

Melt rheology of electron-beam-irradiated blends,of polypropylene and ethylene-propylene-diene monomer (EPDM) rubber

A detailed rheologicai analysis over large shear rate intervals has been performed for electron-beam-irradiated blends of polypropylene (PP) and ethylenepropylene-diene monomer (EPDM) rubber. At high frequencies, a lower viscosity results from irradiation compared with unirradiated blends, which implies that the irradiated blends are easily processable via injection molding. At low shear rates, however, the irradiated blends behave like a network, and the viscosity may even exceed the viscosity of the unirradiated blends. This particular behavior can result in the formation of weak weld lines. Aggregation of the dispersed, cross-linked EPDM particles into a skeletal structure is the most probable explanation. In a first attempt, it was tried to correlate the network behavior to the average (shortest) interparticle distance (ID) between two rubber particles, which takes into account both volume fractions and particle size of the dispersed phase. Provided that the EPDM rubber is sufficiently cross-linked, the network behavior becomes more pronounced; i.e., increase in viscosity with decreasing interparticle distance. Above a critical value of the ID, the viscosity does not change and is determined by the PP matrix. As a vast amount of literature indicates, the rheology of blends proves to be difficult to understand. Because of the more stable morphology, compared with usual blends, induced by irradiation, a more valuable interpretation of the rheological behavior is possible.     

Skin-Core morphology and failure of injection-molded specimens of impact-modified polypropylene blends

The structure-property relationship as well as the failure phenomena of injection molded polypropylene (PP) blends modified with ethylene/propylene/diene terpolymer (EPDM) and thermoplastic polyolefinic rubber (TPO) were investigated. Single and double-gated tensile bars were injection molded by different Injection speeds. Microscopic studies on the failure behavior of knit lines were carried out using microtomed sections taken from the doublegated specimens. It was found that during injection molding, a skin-core morphology is formed in both the continuous PP matrix as well as in the modified PP blends containing rubber particles of various deformation. The characteristics of the latter are in agreement with those described by the Tadmor flow model. The skin consists of a thin pure PP layer, whereas the subsurface layer contains more or less elongated rubbery particles due to the elongational flow at the wall. The deformation of the rubbery particles decreases, but their concentration increases with increasing distance from the skin towards the core. The deformed particles are oriented tengentionally to the flow front profile. Failure during tensile and tensile impact loading is initiated in the shear zone along the skin-core boundary. This zone has a transcrystalline character and favors the formation of crazing. Final fracture of the bars depends, however, on how crazing and shear yielding simultaneously interact. Their interaction is a function of the average particle size of the dispersed phase. Above an average particle size of 0.6 μm, crazing is prevented by shear bands. For injection molding of PP/rubber blends a moderate injection speed is recommended, if the melt viscosities of the components are closely matched. In this way a pronounced dispersion gradient of the rubber particles across the plaque thickness is avoided. However, for the blends modified with rubber of high viscosity ratio and greater melt elasticity, use of higher injection speed is advantageous. Here, the higher shear stress field decreases the average particle size taken into the direction perpen dicular to the lead, since the cross section of the stronger deformed particle decreases.     

CB filled EOC/EPDM blends as a shape-memory material: Manufacturing, morphology and properties

According to theoretical prediction using the Z-model proposed by us a predominant localization of carbon black (CB) in the ethylene propylene diene rubber (EPDM) phase of ethylene octene copolymer (EOC)/EPDM blends was found for a thermodynamic equilibrium state, in case that rheological effects do not play a significant role. In this paper, a special mixing procedure based on the well-known dynamic vulcanization techniques was developed to localize CB in the EOC matrix in order to obtain a low electrical resistivity. Effect of size of the EPDM domains on the electrical resistivity was characterized and it was found that exceeding a critical EPDM domain size of about 30 μm the electrical resistivity decreases exponentially. An inhomogeneous CB distribution formed in the EOC matrix at EPDM domain sizes above 30 μm is the reason for the strong decay of resistivity. The effect of CB network formation during cross-linking process on the resistivity was also characterized by means of the online measured electrical resistance. The electrical resistivity changes strongly after peroxidic cross-linking process, while it remains nearly unchanged after cross-linking with high energy electrons. The Joule heating shape-memory recovery of CB filled EOC/EPDM blends cross-linked by peroxide and by high energy electrons was comparatively characterized and discussed.     

Fracture behavior of poly(vinyl chloride)/methyl methacrylate/butadiene/styrene polymer blends

The fracture mode of poly(vinyl chloride)/methyl methacrylate/butadiene/styrene (PVC/MBS) polymer blends can change from ductile to brittle in accordance with the changes in shape of the test specimen or test conditions. Therefore, the mechanisms of impact energy absorption and the main cause of stress whitening are complicated. The following results on PVC/MBS blends were obtained by carrying out fracture experiments at different test speeds and temperatures:

• (1) The ductile/brittle fracture mode of the PVC and PVC/MBS blends can be explained by σ (the craze initiation stress)/σ_y (the shear yield initiation stress), which depends on the strain rates and temperature.
• (2) The fracture behavior of the PVC/MBS blends can be classified into the following types from the standpoints of fracture mode and whitening degree: Fracture I: ductile fracture without whitening; Fracture II: ductile fracture with whitening; and Fracture III: brittle fracture without whitening.
• (3) The following concepts can be estimated from the measurements of yield stress, specific gravity and SEM, TEM and visual observations. In Fracture I, shear yield occurs mainly. In Fracture II, both shear yield and crazing occur. In Fracture III, deformation of the rubber and local crazing occur.
• (4) The main cause of stress whitening in PVC/MBS blends is light scattering by cavities in the rubber particles.
• (5) In Fracture II, at first, crazes with cavities in the rubber particles occur. Then, shear yield occurs. Finally, crazes are healed by the heat, and only the cavities in the rubber remain.