Types of Asphalt Pavement

The asphalt pavement is commonly divided into asphalt concrete pavement, hot mix asphalt gravel pavement, penetrated asphalt pavement, asphalt surface course, in terms of technology and usage.

Asphalt concrete pavement

Asphalt concrete pavement adopts high quality asphalt, and requires asphalt with higher consistency than other types of asphalt pavement. The adoption of a considerable amount of mineral aggregate is one of its features. Its strength depends on the interlocking and compaction of materials.

The high cohesive force enables the pavement to obtain very high strength to withstand heavy vehicle traffic. However, its allowable tensile strain value is low, which may lead to transverse cracking, thus requiring strong base course. It requires both high and low temperature stability. The smaller porosity makes the asphalt concrete pavement have advantages such as low water permeability, good water stability, high durability, and good resistance to natural factors. The service life of asphalt concrete pavement can be up to 15~20 years.

Hot mix asphalt gravel pavement

This kind of pavement has good high temperature stability. It is not easy to occur ruts in summer and cracks in winter, and not vulnerable to traffic loads. The pavement surface is relatively easy to maintain rough, allowing high-speed driving. The broad requirements for stone grades and asphalt specifications make the formulation design easy to satisfy the needs. Its costs are low, because only a small quantity of asphalt and no mineral aggregate are used. It applies to ordinary roads, but does not apply for high-grade highways. Medium-graded and coarse-graded asphalt gravel is suitable for the bottom layer, binder course or leveling layer of asphalt concrete surface.

Penetrated asphalt pavement

The strength and stability of penetrated asphalt pavement mainly results from stone interlocking. This kind of pavement requires 2~3 weeks for forming. Under vehicle rolling and gravitational effect, asphalt gradually infiltrates into stones and fill in gaps, forming an integral stable structure layer. It has good temperature stability, preventing the pavement from shoving and rutting in hot climates and cracking in cold climates. Sealing materials or mixing layer should be applied at the top of penetrated asphalt pavement. This pavement also can be used as the binder course of asphalt concrete surface.

Asphalt surface course

Asphalt surface course can improve the road conditions for vehicle driving by resisting the driving abrasion and the effect of the atmosphere, thus extending the service life of roads. The thickness of the paved asphalt surface course can be greater than 3 cm, which is generally not counted in the calculation of road thickness. In addition, asphalt surface course can be used as the wear resistant and anti-slipping layer of asphalt pavement.

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How to Prepare SBS for Production of Adhesives?

Styrene-butadiene-styrene triblock copolymer (SBS) possesses features of both thermoplastics and vulcanized rubber owing to the solubility and thermoplasticity of polystyrene (PS) and the toughness and resilience of  polybutadiene (PB). The features result from the incompatibility of thermodynamics between the end of PS molecular chain and the middle of PB molecular chain. The end of PS molecular chain shows glassy state at room temperature, acting as a crosslinking point of rigid network, and displays viscous state at the temperature above viscous flow temperature, enabling plasticity because of the crosslinking destroyed. For this reason, SBS becomes the major material for the production of adhesives.

Solvent-based SBS adhesives have lower viscosity, which enables it to be used for producing adhesive agents with higher solid content and suitable for coating, saving solvent and the energy for solvent volatilization.

Linear SBS is more suitable for being used as the main material of adhesives. The structure of SBS is segmented into linear and radial. Linear SBS possesses low relative molecular weight, good solubility, low viscosity, and lower cohesive strength. Radial SBS has high relative molecular weight, high cohesive strength, high physical crosslinking density, and higher heat resistance and elastic modulus than linear SBS. But if the amount of radial SBS is too large, the shrinkage internal stress of adhesives will increase. The presence of internal stress in adhesive joints will reduce joint strength, resulting in poor toughness and great brittleness of adhesives after curing. In addition, adhesives and adherend molecules need to be made into a solid joint by interdiffusion. Radial SBS are less wettable than linear SBS, and cannot evenly and completely wet bonding materials when bonding.

Toluene, dichloroethane, carbon tetrachloride, vinyl acetate, and cyclohexane are good solvents, while n-hexane, heptane, petroleum ether, solvent gasoline, and acetone are poor solvents for SBS. Single solvent is difficult to meet SBS’s comprehensive requirements. Hence, binary or multi-component mixed solvents made of good solvents and poor solvents are generally used. Poor solvents in the mixed solvents must volatilize at a faster rate than good solvents. Otherwise, the surface of adhesive film will be rough, and its bonding strength will decrease.

 

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What is TPE Overmolding?

TPE  overmolding generally refers to the injection molding process where TPE materials are molded onto a second materials, typically a rigid plastic such as PP, PS, ABS, PC and PA. It can increase comfort touch, anti slip and impact resistance performance of finished products.

Two injection molding processes commonly used for the production of overmolded products are insert molding and multi-shot injection molding. Insert molding is the most widely used process where a pre-molded insert is placed into a mold and TPE is shot directly over it. Its advantage is that it uses conventional single shot injection molding machines for TPE overmolding, saving the costs of purchasing new machinery.

Multi-shot injection molding, also known as multiple material or two-shot molding, requires a more advanced injection molding machine that is equipped with two or more barrels, allowing two or more materials to be shot into the same mold during the same molding cycle. Manufacturers will select multi-shot injection molding to reduce cycle times, achieve parts with superior quality and reduce labor costs.

Notices for TPE overmolding:

  1. Take the compatibility of TPE and rigid plastic parts into consideration;
  2. Avoid sharp corners in the product design to improve the adhesion of TPE to rigid plastic parts;
  3. Use appropriateair venting to prevent air from in the mold cavities;
  4. Keep the proportionof TPE’s flow length and product thickness below 150:1;
  5. Achieve balance between TPE’s thickness and expected touch;
  6. Control the temperature of molten TPE to ensureadhesive effect;
  7. Perform special processing for smooth surfaceto increase the contact surface between TPE and rigid plastic parts, thereby strengthening adhesive effect;
  8. Use TPEwith excellent flowability, because the thickness and dimension proportion of TPE overmolding is very small and TPE is usually required to flow through long paths and thin-walled areas to fill molds;
  9. Use good adhesives.

 

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Technology and Applications of SBS-based Adhesives

SBS refers to styrene-butadiene-styrene triblock copolymer. It is a type of thermoplastic elastomer, generally called “thermoplastic styrene-butadiene rubber” and well-known as “the third generation of synthetic rubber”. SBS has both plastic and rubbery properties, showing plasticity at high temperature and elasticity at room temperature. It exhibits properties of thermoplastics without plasticating and properties of vulcanized rubber without vulcanization.

SBS has many advantages such as excellent solubility, the ability to melt at high temperature, low melt viscosity, ease of modification, and good compatibility with a lot of polymers. Thus, it is widely applied in the building decoration, printing and laminating, pressure-sensitive adhesives, woodwork, bags and suitcases, footwear, and other industries.

Currently, the type of building decorative adhesives mainly includes neoprene adhesives and SBS-based adhesives. Due to structural characteristics, SBS-based adhesives are inferior to neoprene adhesives in initial adhesion, bond strength (especially when bonding to polar materials), tack time, heat and weather resistance, and other aspects. This greatly restricts the use of SBS-based adhesives in the building decoration application.

The lack of polar groups in SBS macromolecules results in the failure of SBS-based adhesives to perfectly bond to polar substrates such as aluminum-plastic panels, fireproof boards, PVC panels. There are a lot of researches globally conducted on grafting modification, epoxidation and other aspects of SBS-based adhesives, to improve their bonding properties. However, this will undoubtedly require the increasing investment and industrial costs of adhesive manufacturers, and is difficult to put into operation because of the complicated technique.

 

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SBS Modification by PE / PP / PS

Why SBS needs to be modified by adding plastic products? This mainly results from its poor processing performance and product properties that can’t meet the practical application requirements. In general, SBS is rarely used in the finished products independently. To satisfy the special requirements of applications and reduce costs, SBS must be modified by adding additives, resins and others.

With excellent elasticity, SBS elastomers can be mixed with other polymer materials in the molten state, extruded and cut, then cooled to prepare the modified SBS thermoplastic elastomers. This improves SBS’s flowability and mechanical properties such as hardness, abrasion resistance and tear strength, and expands its application range. The major polyolefin plastic polymers used for SBS modification include PE, PP, PS, etc. They share common characteristic in the good compatibility with SBS, thus the modified SBS has excellent bonding property.

PE / PP: Both polyethylene (PE) and polypropylene (PP) have outstanding properties, including ease of processing, excellent electrical properties, excellent flexural strength, low temperature flexibility, excellent corrosion resistance, and low permeability. The use of PE and PP for SBS modification can improve the abrasion resistance, hardness, weatherability, tear strength and melt flow rate of SBS elastomers, while maintain high tensile strength and elongation at break.

PS: Polystyrene (PS) has the advantages such as easy molding, low shrinkage, low hygroscopicity, and good thermal performance. PS has good compatibility with SBS. Strong bonding force exists at the phase interface of two polymers, and increases as the rising number of styrene chain segments in SBS. The tensile strength and elongation at break of the SBS modified by PS will decrease as the increase of PS content, while the hardness will rise slightly.

SBS modified by plastics is mainly applied in footwear, wires and cables, automotive, household appliances, medical devices, and other sectors.

 

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Requirements for Ingredients in Production of SBS Granules for Shoe Soles

SBS, the major raw material for SBS-based TPR footwear, provides strength, elasticity and other performance for footwear. SBS grade YH-815,   SBS T171 are frequently used for production of SBS-based TPR footwear. However, the specific grades should be selected based on manufacturers’ requirements for footwear, including costs, performance, sole type, etc.

Softening Oil

Softening oil is mainly used for improving the flowability, reducing costs in the production of SBS granules for shoe soles. The flowability of SBS granules for shoe soles is generally measured by melt flow rate (MFR). The hardness of SBS granules for shoe soles can be reduced by adding softening oil.

The softening oil can be chosen based on two cases: a. The selected softening oil has good compatibility with polystyrene, the rigid phase in SBS; b. The selected softening oil has good compatibility with polybutadiene, the soft phase in SBS. The former one is not recommended.

Resin

Resin plays the role of improving performance and filling in the production of SBS granules for shoe soles. The selection of resin should consider the compatibility with SBS first, generally following the principle of “SBS is compatible with resin when their solubility parameters are approximate.” General-purpose polystyrene (GPPS) is the most common organic filler, which can enhance hardness and elasticity modulus of SBS granules for shoe soles with slight reduction of abrasion and tear resistance. The commonly used resins also include ethylene vinyl acetate copolymer (EVA) and polyethylene (PE). EVA can enhance weatherability, ozone resistance, aging resistance and solvent resistance.

Fillers

The addition of fillers in the production of SBS granules for shoe soles is to reduce costs.

 

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What is the White Substance on the Surface of TPE/TPR Products?

The white substance on the surface of TPE/TPR products is small molecules caused by migration and precipitation.

Sometimes, certain low molecular weight additives, such as brighteners, releasing agents, may be added into the modification process of TPE/TPR products. Owing to the poor compatibility with substrates, these additives are generally added by 2~3‰. If excessive additives are added, the migration of small molecules will occur as time goes by.

The temperature is high while blending and modifying TPE/TPR. Hence, the additives show relatively even dispersed state (unstable) in the blending system. As time goes by, the oversaturated additives will migrate to the surface of TPE/TPR products owing to the low solubility of low molecular weight additives in the blending system. In daily life, we may find white substance exists on the surface of TPE/TPR finished products after a period of time, which can be rubbed off by hand. This is the migration of excessive low molecular weight additives.

Acidification, frosting, whitening and other phenomena commonly found in the production of TPE/TPR products are all caused by the migration of constituents that have poor compatibility in the blending system.

As a result, the amount of added additives with low solubility should be strictly controlled in the modification process, to prevent TPE/TPR products from consequent migration. In the actual production process, the migration of additives also occur in other modified engineering plastics and rubber products in addition to TPE/TPR products.

 

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