In terms of the use, UPVC (unplasticized polyvinyl chloride) pipe can be mainly divided into water supply pipe, drainage pipe, pipe for chemical use and etc. The characteristics of structure divide them into solid-wall pipe, corrugated pipe, foamed pipe and etc. The fact that different pipe products need various product standard and technical index causes the unlike producing formula and technique. No matter which pipe product, all the materials within the cost of determined formula are expected to have anticipated mechanical strength ( strength of extension, anti-impact strength), a good outside finish, better weather resistance, wonderful thermal properties and less thermal deformation. All the above performance index are measured through hydraulic test, falling weight impact test, Vica softening test, longitude shrinkage test and etc.
CPE and ACR are chiefly involved in the production of UPVC, so this article will talk the specific application effects on perspective of the effects of CPE and ACR to the improvement of performance index of UPVC pipe. ACR ( Acrylic copolymer) , a kind of acrylic emulsion copolymer with core shell structure , is a comprehensively excellent impact-resistant modifier of PVC.
In regard to the UPVC pipe for water supply and chemical use, the most different thing of the material product can be whether the hydraulic test achieve the standard or not. In general, two situations will occur if the test fails to reach the expected standard. The first one is that the pipe materials crack without apparent deformation( increased diameter); The second one is the pipe cracks with apparent and visible deformation. The first situation is called brittle fracture that pipe experiences abrupt tearing fracture and the second one is ductile fracture that pipe materials firstly experience deformation of shape ( increased diameter), then gradually enlarged deformation and lastly crack to be a seemly shape of the vase and a certain period is remained between the deformation and the final fracture.
According to force analysis of plastics pipe under the static hydraulic pressure (P), a relational expression (referring to GB/T6111-2003) among the hoop stress of wall thickness (tensile strength) , external diameter and wall thickness:
In the relational expression, P (Mpa) stands for the hydraulic test pressure; δ means the determined hoop stress ( tensile strength, MPa); dmin（mm）is the measured mean outside diameter; emin（mm) is the minimum wall thickness of measured free length.
For the UPVC pipe of specified product standard and pressure rating, the dmin、emin in the above relational expression is known number, therefore, the hydraulic test pressure of pipe materials in the same size is determined. However the anti-pressure ability of pipe depends on δt which means the strength of extension of pipe materials. Namely, pipe will pass the hydraulic test if δt > δ, or it may crack. Pipe with brittle fracture reveals a lack of tenacity, which usually fails in the falling weight impact test. Here a question appears that which factor the strength of extension is related to?
(1) Firstly, the strength of extension relates to the molecular weight of PVC resin. More molecular weight the resin has, the stronger the strength of extension will be. Secondly, it closely connects with the molecular structure of PVC resin. Generally, the structure of PVC molecular chain mainly is “head-tail”, which is the most stable. Because of the process control, there are possibilities for the “head-head” and “tail-tail “structures which reveal less stability than “head-tail” structure. Besides, some branched chains with double linkage may appear due to disproportionation. These branched chains can easily experience decomposition and are called defect structure. The more defect molecular structure it has, the less strength of extension it indicates
(2) The materials included in the product formula of UPVC pipe such as the composite heat stabilizer, lubricating agent, impact modifier, filler (light calcium carbonate) and etc. will lower the strength of extension of PVC pipe materials at different degrees.
(3) The composition of various materials of UPVC pipe results in that only uniform mixing and gelatin and fusion of all parts can generate excellent strength of extension. Only if the extruder obtains sufficient shear force can it be realized. Obviously, if the formula of PVC resin and UPVC is determined, the degree of plasticification of pipe materials may influence the strength of extension of pipe materials. Actually, it is the main reason of the unsuccessful hydraulic test for many manufactures.
Admittedly, the improved technological conditions such as the reasonably raising technological temperature ,may occasionally contribute to the plasticification of PVC resin but it may lower shear force for extruder with bad plasticizing effect. Besides, because of the easily decomposition of PVC resin, raising processing temperature always lead to the decomposition of PVC materials and bring a series of problems such as process variations, increasing rejection rate, more demand for stabilizer. In fact it is not beneficial for the stable product quality and may increase the producing cost.
In order to achieve the most desired plasticification of PVC resin, it is a best choice to add enough ACR processing agent (such as HF-100、HF-401) . To realize the improvement of PVC degree of plasticification at lower processing temperature, it can lift torsion and enlarge shear of force so as to further improve the strength of extension, perfect the degree of finish, and accelerate the mobility of melt. Moreover, it can lift the productivity and lower the rejection rate and better balance the process control. Comprehensively, supplement of sufficient ACR processing agent actually lessen the producing cost.
There are several factors accounting for the failure of plasticification. More light calcium carbonate , filler and so on can both hinder the processing procedures. Hence , necessary lubricant are needed but more lubricant may produce the slipping of materials. Under this circumstance, PVC mixtures can hardly achieve the needed degree of plasticification without a certain amount of processing agent. In the meantime, adding of light calcium carbonate, fillers bring in the reducing of mobility, raising backpressure and lessening of productivity. In this situation, processing agent ACR can not only accelerate the plasticification of materials but also improve the mobility of materials at the same time. Degree of plasticification closely connects to the shear force of machines. The passing of service life may widen the gap of screw and charging barrel, bring down the shear force and degree of plasticification. At this time, it is needed to add more processing agent, adjust the degree of plasticification to a proper degree and perfect the mobility. Dichloromethane impregnating experiment can be conducted to test if the degree of plasticification achieves the standard.
Considering the information mentioned above, some other factor may influence the strength of extension of UPVC pipe materials. Impact modifier cannot be neglected.
CPE and ACR are the dominant members among all the impact modifiers in the application of pipe materials. Several experiences from many manufactures prove that with extrusion formula, adding of CPE reveals worse strength of extension than the adding of ACR. The results of experiments prove that using 6 portions of ACR instead of 10 portions of CPE may lift the strength of extension by 10% with the same or slightly higher impact strength.
Drop-weight test, an important index of the application of pipe products, intends to measure the impact resistance of pipe materials. The main reasons of the impact resistance below the mark are as follows:
①Degree of plasticification of UPVC pipe materials
Degree of impact resistance also connects to the degree of plasticification just as the strength of extension does and displays more influence to the degree of impact strength. The pipe materials without better degree of plasticification are easily cracked and cannot pass the drop-weight test. Just as mentioned above, it is a vital choice to add sufficient amount of ACR processing agent (such as HF-100、HF-401) to improve the plasticification of materials.
②. the performance , amount and dispensability of impact modifier
Brittleness is the shared characteristic of PVC resin, hence some impact modifiers are expected to added to increase the degree of impact resistance. The efforts of impact modifiers greatly rely on its the performance, amounts and dispensability. It is proved that ACR has better impact resistance than CPE. With the same basic formula, only more amounts of CPE can achieve the same degree but with less amount of ACR. Besides, there are different impact resistance mechanisms between CPE and ACR. ACR are globularly scattered in the PVC materials and depends on the shear zone and crazing to abort energy so as to realize the impact resistance. In order to achieve this goal, it also greatly depends on the dispensability of ACR besides the dependency on structure of ACR. ACR is made of countless spherical particles and can easily crack into smaller spherical particles and disperse in the basic materials with the working of high mixing machine and screw shearing. CPE particle consists of ten billion of linear CPE molecules twisting with each other, which cannot disperse evenly in the PVC. Foreign high extruder rarely adopts CPE as impact modifiers but adopts ACR. With the high efficient extruding condition, piling phenomenon of CPE in the PVC can influence the actual impact resistance efforts but also bring uneven basis materials and greatly reduce the tensile strength in some areas. It easily cracks in the hydraulic pressure test of UPVC and the application of pipe materials.
Obviously, ACR can use a slightly add to reach better degree of impact resistance and exerts less influence on the degree of impact resistance of PVC, which is the incomparable features for CPE.
Vica softening point
Vica softening point represents the thermal properties of plastic materials. Materials with high vica softening point reveal less thermal deformation and strength at a raising temperature than the materials with low vica softening point. Therefore, many plastic products all determine the vica softening standard of materials with the aim of guaranteeing that products won’t deform and lose its strength and then abandon its application performance within the determined application range.
With the determined PVC resin, impact modifier mainly influences the vica softening point. CPE and ACR of resistance all belong to rubber elastomer with a tendency to reduce the vica softening point of PVC products. CPE is of “sea-island” structure. But the modular structure of ACR exerts difference with the linear CPE as the“core-shell” structure , which won’t have apparent impact on vica softening point as the CPE does. This is the reason why a majority of manufacturers primarily choose ACR as impact modifier.
Longitudinal shrinkage rate represents the longitudinal size changes from a high temperature to a low temperature. If the longitudinal shrinkage rate exaggerates, pipe materials will produce great internal stress due to the lessening temperature. The internal stress overlays with other stresses may accelerate the destroy of materials. Long laying pipe lines may lead to the unplugged pipe line because of the phenomenon of “thermal expansion and contraction when cold” without backfilling. According to the experiments, less necessary amount of ACR in the UPVC pipe materials than the amount of CPE make it less influential to longitudinal shrinkage rate. Besides, longitudinal shrinkage rate is closely connected to mold matches and processing control.
Metachromasim and strength decrease property are two aspects of the weather resistance of UPVC pipe materials. It is obvious that the metachromasim closely relate to the outside appearance and the latter influences the service life of materials. There do exist the same problems in the door and window profiles and other products. In regrading to the current weather resistance tests and massive experiment results, ACR reveals unparalleled weather resistance and the application of ACR displays postponing potential economic benefits.
During the processing, the melt strength of UPVC exerts apparent negative effects on the highly filled light calcium carbonate and other fillers. With more fillers , decreasing melt strength , the melt fracture phenomenon easily appear and form surface defects with the coordinate working of high shear and traction. It finally leads to the decreasing mechanical property of UPVC materials. Thanks to large amount of modular particles and good compatibility, ACR can contribute to the improvement of melt strength so as to get high strength UPVC products. In general, it is advised to add 1.5-3 portion of ACR to match 1-2 portions of ACR during the production of UPVC. Then a comprehensive excellent UPVC product can be get. With the people’s deep exploration of PVC on its toughening modification, more new method and thoughts will spring up to make PVC resin widely used among all the fields.