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化学工程作业:玻璃碳混合纱增援工程

时间:2016-01-18 23:08:16 来源:www.ukthesis.org 作者:英国论文网 点击联系客服: 客服:Damien
玻璃碳混合纱增援工程
 
混合纱线是一种最有前途的技术,解决了热塑性复合材料的高粘度生产的主要问题。在这份报告中,我们对一些热塑性复合材料的使用混合纱线主题进行了研究,详细的文献研究已经进行了。
 
混合纱线可以增强热塑性(基质纤维),这些热塑性矩阵纤维应均匀地分布在增强纤维上,使流动距离尽可能短。混合纱往往比其他制造技术如绣结合共同编织或粉末浸渍有更好的浸渍和悬垂性。等各种混合纱线类型开发,并排摩擦旋转,侧共翘曲、捻线和混合。混合过程似乎是更有前途的技术之一。混合纱有夹结构;频率和一些细节的几何形状对最终复合材料的性能有很大的影响。工艺参数的混合空气压力、速度和超喂率对夹结构的影响是很重要的。通常这三个参数对纱线的效应必须综合考虑。 压缩模塑法经常被用于制造复合结构混合纱线。整合被认为是在这个过程中的时间决定的步骤。
 
介绍
 
在本报告中,我们将讨论使用混合纱的热塑性复合材料,本报告的主要重点是对纱线性能和玻璃/热塑性聚合物为基础的混合纱线的生产方法,我们将简短描述来结束该报告的压缩成型过程,以产生实际的复合材料,本报告中讨论的混合纱线可以定义如下。
 
 
玻璃碳混合纱增援工程-Glass Carbon Hybride Yarn Reinforcements Engineering Essay
 
Hybrid yarns are one of the most promising technologies to solve the main problem in the production of thermoplastic composites being the high viscosity. In this report we summarize some of the research done on the topic of hybrid yarns for the use in thermoplastic composites; a detailed literature study has been carried out.
 
The hybrid yarns consist of reinforcing and thermoplastic (matrix fibres) these thermoplastic matrix fibers should be evenly distributed among the reinforcing fibers to make the flow distance as short as possible. Hybrid yarns often give rise to better impregnation and drapability than other preform techniques such as stitch bonding co weaving or powder impregnation. Various hybrid yarn types have been developed such as side by side friction spinning, co-warping, yarn twisting and commingling. The commingling process appears to be one of the more promising techniques. Commingled yarns have a nipped structure; the frequency and geometry of these nips have a large influence on the properties of the final composite. The effect of the commingling process parameters air pressure take up speed and overfeed on the nip structure are therefore important. Often the combined effect of these 3 parameters on the yarn must be taken into consideration. Compression molding is often used to produce composite structures from hybrid yarns. The consolidation is considered to be the time determining step in this process.#p#分页标题#e#
 
介绍-Introduction
 
In this report we will talk about the use of hybrid yarns for thermoplastic composites, the main focus on this report will be on the yarn properties and production methods of glass/thermoplastic polymer based hybrid yarns we will end the report with a short description of the compression molding process to produce the actual composite. The hybrid yarns discussed in this report can be defined as follows.
 
Definition A hybrid yarn is a yarn consisting of bought structural fibres and thermoplastic fibres. These yarns have very interesting properties for the use in composites with thermoplastic matrix materials.
 
In the past decades there has been more and more interest in composite materials using thermoplastic polymers as a matrix material, this is because these thermoplastic polymers offer quite some advantages when compared with thermosetting polymers. These include a potential for a greater fracture toughness, larger elongation at fracture, faster and more automatic processing, unlimited shelf life of the raw material, and most importantly recyclability and a more clean working environment because no organic solvents are involved in the workshop processes[1].
 
However there are some difficulty's involved in the production of fibre composites with thermoplastic matrices, the most important one being the high melt viscosity, which is up to a 1000 times higher than for thermosetting matrices [1]. This leads to more energy usage, since it is difficult for the molten plastic to penetrate the fibre bundles and ensure a complete wetting of all individual fibres. [1]
 
Several different methods have been studied to overcome this problem (some of them are displayed in the picture below). The basic idea behind all these different approaches is the same; the thermoplastic material should be incorporated in some kind of "pre structure" in which the thermoplastic material (in solid) form is brought in close contact with the reinforcing fibres. So when de thermoplastic fibres melt in later possessing steps the thermoplastic material should only flow over a short distance thereby facilitating the penetration of the molten plastic in the reinforcing fibres.
 
(a) Co weaving: in this technique the reinforcing and thermoplastic yarns are combined in a woven structure, good drapability can be obtained although an uneven distribution of the reinforcement in the matrix is likely to occur [3]
 
(c) Stich bounding of reinforcing and thermoplastic yarns
 
(d) Powder impregnation: the thermoplastic matrix is applied in powder form
 
(b) and (e) are examples of hybrid yarns, namely Co warping yarn (i.e. plied matrix), and commingled yarns (a description of these and other hybrid yarn types is given in the next chapter)
 
The investigations in the field of thermoplastic composites indicate that a solution for the cost-effective production of thermoplastic matrix composite material could be accomplished by using hybrid yarns and their (nonconsolidated) preforms. In such preforms, the matrix fiber and the reinforcing fiber are in close contact providing good and fast impregnation. They have numerous advantages: low specific weight and low cost, fast production cycles, less fiber distortion, the possibility of reformability, and recyclability. Additional advantages of these textile preforms are their flexibility, easy of handling, improved composite properties, etc. [4].
 
More over compared to other intermediate products mentioned above, the flexible (commingled) hybrid yarns can easily be converted into a highly drapable textile fabric (as opposed to powder impregnated fibre bundles), reducing the likelihood of wrinkles during the forming of complex shapes, also the matrix flow distance is usually shorted, which gives better impregnation (as opposed to co woven and stich boned preforms) [5]. Furthermore the cost of hybrid yarns can lower that the cost of powder impregnated fibre bundles.
 
The further processing of these hybrid yarns into composite materials can basically be done as follows. When heat is applied, the thermoplastic component melts and wets the reinforcing component and forms amorphous reinforcing binder. After subsequent cooling, the system is transferred into rigid composite material [6]. Research has shown that composite properties are influenced mainly by the arrangement of the reinforcing fibres and the homogeneity of the fibre distribution in the composite, as well as by impregnation of the fibres with the polymer matrix [7]. These properties are in turn largely affected by the structure of the hybrid yarn itself.
 
In this report we will summarize the most important results of the research done on hybrid yarns for the production of thermoplastic matrix composites. This includes an evaluation of the different hybrid yarn construction types, a more detailed explanation of a very promising production method called commingling. We will conclude with a small paragraph describing the use of compression molding to produce thermoplastic composites form hybrid yarns.
 
Most results represented in this report use glass fibres as a reinforcing fibre however most The basic principles are valid for carbon fibre composites as well.
 
混合纱线的类型-types of hybrid yarns
 
There are several types of hybrid yarns that have been studied in literature; we will now present an overview and comparison of the most important types
 
side- by-side, SBS: parallel arrangement of glass and polyamide fibres
 
'Kemafil' technology, KEM; parallel arrangement of matrix fibres surrounded by parallel glass fibres in the core, sheathed by matrix fibres in the skin.
 
commingled yarn, COM: commingled glass and polyamide fibres made by air texturing.
 
friction spinning (or core spinning), FS parallel arrangement of glass fibres in the core and spun matrix fibres in the skin.
 
Schappe technology, SCH).mixture of glass and matrix fibres, both discontinuous, surrounded by a continuous matrix filament.
 
Fig. Hybride yarn types [7]
 
twisted yarns, (+-90 twists/m)
 
Fig. co warped yarn [6]
 
co warped yarn, In co-wrapping, thermoplastic fibers are wrapped around a core of reinforcing fibers. This provides a better protection for the reinforcing fibers during further processing such as weaving or braiding [8]. The properties of these yarns are comparable with those of the FS yarns [9].
 
A great research work has been done in the development of these various types of hybrid yarns as well as in the most promising manufacturing techniques [10,8]. All of these materials could be converted into composites (using e.g. compressing molding).
 
An important parameter to consider when comparing these different hybrid yarn types is the homogeneity of the glass fibre distribution within the matrix, the better the degree of mixing of the reinforcing fibers the shorter the flow distance of the matrix material so the better the impregnation quality of the glass fibers.
 
The thermoplastic yarns should also protect the (often more brittle) reinforcing yarns during mechanical prospecting and composite production. It appears to be hat this last parameter received less attention in resent research probably because most of the hybrid yarns provide sufficient protection of the reinforcing fibers.
 
Lauke et al. [7] have examined the homogeneity of the GF distribution by SEM observation (see fig. 4.) They conclude that the yarns can be ranked according to flow distance as follows SCH, COM, KEM, SBS and FS
 
Fig. Sem of polished area of sbs, com, kem and fs composite [7]
 
Lauke et al. [7] also examined the influence of different fiber arrangements on the interlaminar crack propagation in unidirectional (GF/polyamide 6-based) composites. They have compared SCH, COM, KEM, SBS and FS hybrid yarns. Summarizing all the results, they concluded that the air-textured commingled yarns with optimized fiber sizing provide the highest transverse modulus and strength, as well as the higher crack resistance [7].#p#分页标题#e#
 
Other research indicated that the combination of commingling and co-wrapping may give a yarn with very good matrix/reinforcement distribution and good protection of the reinforcing fibers [9].
 
Composites made from SBS generally show weaker performance that other hybrid yarn types, they show higher void content and lower tensile strength than e.g. those made from commingled yarns, since the GF distribution is rather poor the thermoplastic matrix material has to travel a greater distance across the glass filaments to fill the gaps. [11]
 
In another imaging study E.Klata et al [12] where able to compare the fibre distribution of FS, twisted yarn, COM using optical microscopy. They conclude that, one can observe the shortest mean distance between glass fibres in the friction yarn composite. In the twisted and COM (air textured) yarn composites, the distances between fibres are longer. In the twisted yarn composite the arrangement is more uniform, and almost every fibre is coated by a matrix. In the COM yarn composite , glass fibres often contact, and the distances between them are diversified [12]. The same sudy [12,13] also investigated the cristalintiy of the PA6 matrix of COM FS and twisted yarn using DSC[13] and WAXS. They concluded that we could ranked the yarns according to cristalinity as follows, twisted yarn, COM, FS This fact suggests that the mechanical properties , apart from impact strength, could be ranked in in the same way as well (descending order) [12,13].
 
Fig. POM of cross-sections of composites based on: (a) fr (b) twisted yarn (c) COM [12]
 
From the above discussion we can conclude that hybrid yarns can be manufactured through different ways, all aiming to give uniform distribution of matrix and reinforcement fibers as well as to reduce the damage of reinforcing fibers [14,7]. Some of these methodes appear to be more succesfull in eacheaving this goal then others, in the next part of the report we will focus on one specific production methode namely commingling.
 
混合-commingling
 
From all the hybride yarn types pointed out in the previous chapter most of the reasarchers on hybride yarns beleave that the commingled yarns are one of the most promessing [7,9,15,16,17,18]. Since among these hybrid yarns, commingled yarns provide high potential for thorough blending of matrix-forming filaments and high-performance fibers. This process is versatile and gives soft, flexible, and drapable yarn. This has made commingling technology suitable for textile preforming process to produce high-performance composites [15]
 
Futher more most papers (found in the liturature study) evaluating the use of hybrid yarns in (thermoplastic matrix) composite materials makes use of commingling yarns.
 
#p#分页标题#e#
Now we will take a closer look at the commingeling proces and the parameters involved
 
关键点-Principal
 
Definition mingling: in a mingling process, rapidly moving air in an air jet is used to generate entanglements in and among the filaments.
 
There are several synonyms used in literature to the term 'mingling like, interlacing, tangling, entangling and intermingling.
 
Definition commingling: A mingling process of two or more yarns to form a single strand of yarn can be defined as commingling.
 
Once again we can point out that in our case the commingled yarn consists of blended combination of reinforcing filament yarn and filament yarn spun from thermoplastic polymers. During commingling the multi-filament yarns are scattered among one another at the filament level [6]. On Fig 6 we can see a simplified representation of the commingling process.
 
We must note that the commingling process is a rather flexible process meaning that by using the commingling process any weavable reinforcing fiber and most spinnable polymer fibers can be combined [16,17].
 
混合纱线结构和工艺参数-Commingling Yarn structure And process parameters
 
Now that we have described how commingling basically works, we want to take a closer look at the structure of the yarns produced by the commingling process
 
Fig. 7 displays the typical structure of commingled yarns; we can see two distinct regions, the nips and the open sections. Several parameters are used to characterize this yarn structure [19,18] these include:
 
nip frequency: the number of nips present per unit length of the commingled yarn, and the average
 
nip length: the average length of nips present in the yarn.
 
The degree of interlacing: defined as the ratio of the total length of nipped portion to the total length of the yarn expressed as a percentage.
 
The effects of the degree of interlacing on the properties of a unidirectional composite laminates produced by compression molding process was studied by R. ALAGIRUSAMY et al [11] they concluded that the laminate properties are related with the degree of interlacing of commingled yarns, the degree of interlacing influences the void content and tensile properties of the composites.
 
The void content of composites made with commingled yarns decreases with the degree of interlacing initially and stabilizes when the degree of interlacing is about 15. As far as the tensile strengths are concerned, there is no significant difference between the composite laminates made from commingled yarns with different degrees of interlacing.#p#分页标题#e#
 
Now we will evaluate the effect of the commingling process parameters air pressure, overfeed, and take up speed on the yarn parameters/properties. Data form past (before 2000, [33-43].) researches has already been reviewed by R. Alagirusamy [6], this work was used to write the following paragraph, it has been completed with additional data and remarks form more recent research [18,19]
 
气压和气流-Air Pressure and airflow
 
Airflow velocity is the main driving force, which is responsible for opening up the filaments and enabling them to entangle with each other. It is evedent that with increase in air pressure the flow velocity must increase as well. The effect of airflow rate is also the same as that of air pressure [20,21]. The non-uniformity and turbulence of the airflow can also be enhanced by increasing the air pressure [22].
 
The air pressure required for comingling is usually up to 3 kg/cm2. With increase in air pressure nip frequency increases, however after a certain limit nip frequency starts to decrease. Increasing the air pressure increases the number of mingling points inserted into the yarn [21]. For higher air pressure the rate of increase in nip frequency is found to be small.
 
Fig. 8 illustrates the effect of air pressure on nip frequency for different types of thermoplastic yarns combined with GF. Fig confirms this trend for a GF/PP yarn
 
Fig. glass/pp infuluance of overfeed air pressure and take up speed on nip freq [18]
 
摄入过多-Overfeed
 
Overfeeding of the multifilament yarns during texturing can be done to move some filaments faster than others. With an increase in overfeed the availability of free length of filaments for forming loops increases [22].
 
With an increase in overfeed texturing efficiency and yarn linear density also increase but loop stability reduces [6]. This has also been confirmed by [16] some researchers have observed a decrease in yarn tenacity with an increase in overfeed [23] However the 2009 paper by H. Mandoki et. al. [18] which evaluates the combination of different commingling parameters on the yarn structure and properties shows that the effect of overfeed on yarn tenacity is more complex, and depends upon the other commingling parameters such as air pressure and take up speed, they conclude that with low take-up speed at constant pressure an increase in overfeed gives better tenacity.
 
In mingling process, yarn overfeed is either zero or very low. Experiments carried out by Imeoto et al. [21] found maximum nip frequency at 1% overfeed. Higher overfeed of the yarn through the mingling jet would reduce the yarn tension and yarn may move out of the potential action of air.
 
纱交付速度-Yarn Delivery Speed
 
The structure of textured yarns depends on the way it is removed from the turbulent zone. Yarn should be withdrawn from the air jet at an angle to the path of air stream to redistribute the twist in the yarn [24]. Usually the angle of withdrawal is 90[25]. Resultant forces and torques on the filaments are mainly generated by the relative velocity between the filaments and the surrounding airflow. Therefore, higher forces are exerted on individual filaments at lower texturing speed. However, the expected degree of texturing and loop stability reduced with the increase in texturing speed due to increase in yarn tension [6].
 
With an increase in speed, the limit of interlacing becomes narrow since frequency of yarn traverse inside the jet reduces. With an increase in yarn tension, filaments will not vibrate freely and therefore mingling fails to take place, while too low tension results in early slipping out of filaments from the jet's potential area, again causing failure of intermingling.
 
just like with most commingling parameters it is hard to evaluate the effect of the yarn delivery speed without taking other parameters air pressure and overfeed into consideration. The combined effect of take up speed and these parameters in evaluated in figures 9 to 11
 
In general we can conclude that the main quality parameters of commingled yarn viz. nip frequency, nip stability and nip regularity are mainly affected by interaction value of three processing parameters. It has been observed that high air pressure at 1% overfeed with lower take-up speed gives the best combination in terms of processing glass/polypropylene hybrid yarn [19].
 
Commingled tows are now commercially available, as pointed out in the introduction they offer distinct advantages over powder impregnated tows (notably lower cost and more even dispersion of the matrix and better drapeablity). Materials are usually supplied in the form of a woven fabric, which can be formed into a three-dimensional shape using e.g. compression molding process. A compression molding process basically can be devided in the following steps.
 
Step 1: The first step would be the preheating of the material (to melt the thermoplastic polymer fibres). This should be done in a separate oven, becease the temperature of the bold itself is below the melt temperature of the matrix.
 
Step 2: In the next step the material is formed between matched dies witch operated by a hydraulic pressure.[26] Microscopic flow of matrix wets out the fibers. One must take in mind that a commingled fabric cannot flow macroscopically to fill a mold [27]. Therefor drapbility will hence, largely affect the mechanical properties of such a compression molded composite [28].
 
Step 3: When the mold is closed pressure is appied on the material at a constant rate until the holding pressure is reached. This holding pressure is maintained unthil the material is cool enough for the part to be ejected. During the phase of compression the physical phenomenon of compaction of the reinforcing fibre network and consolidation of the composite takes place.
 
The consolidation process of commingled materials differs from the consolidation process of other prepreg materials like for example pre impregnated sheets. According to A.C. long et. Al. [26] main phases during consolidation are:
 
Coalescence of the thermoplastic matrix occurs as the thermoplastic fibres melt. This generally occurs without applied pressure (happens in step 2 of the compression molding steps).
 
When the pressure is applied in step 3 individual plies will be forced into intimate contact. In this phase air is forced form between the plies.
 
Compaction of the tows, this causes 2 flow types in the material
 
Longitudinal flow of the matrix material present within a tow.
 
Transverse flow for matrix in the inter-tow spaces.
 
The void reduction phase (during cooling), the mechanism of which is a combination of void volume reduction and dissolution of entrapped gases into the matrix.
 
Consolidation, including void reduction, appears to be the rate-determining step of the overall process of compression moulding [26] that is why it has received significant attention from the modeling point of view. May researchers tried to model and predict the consolidation time and void content for given temperature, pressure and rate parameters.
 
Current models tend to focus on a single contribution to consolidation specifically the flow of matrix into a tow (Transverse flow). And example of such a model is described in Fig 12
 
1. The tows contain dry fibres and are surrounded by matrix.
 
2. Pressure forces the tows together and impregnation commences.
 
3. The tows become elliptical and impregnation progresses more rapidly.
 
4. The tows are fully compacted together and fully impregnated.
 
This causes a rise in pressure signalling the end of consolidation.
 
A.C. long et. al. have reviewed these models and have experimentally analyzed the consolidation process, the effects of rate, temperature and holding pressure on the consolidation of a glass/polyproplyene commingled fabric where evaluated. We will not give a detailed description of all the models developed but the interested reader can find them in the reference list [30-36], for those it will also be useful it check [29] since this is a review article about these different models. More recently an other consolidation model was developed by S. Toll et. al. [37, 38]
 
We did however include a summary of A.C. long et al. [26] results:
 
Effects of rate: Increasing rate resulted in increased consolidation pressure as expected, although significant shear thinning occurred even at modest mould closure speeds. Increased rate also led to an increase in void content at the end of the consolidation phase (prior to dissolution into the matrix).
 
Effects of pressure: The application of pressure during cooling resulted in a dramatic decrease in void content, with levels of less than 1% observed for 3 MPa (regardless of the rate of consolidation).
 
Effects of Temperature: Due to supercooling (= delay of solidification to a temperature substantially below the melt temperature). the processing window for a thermoplastic matrix may be extensive. However, the temperature at which consolidation and void elimination occur in non-isothermal moulding must be sufficient for the matrix to be molten. The preheat temperature of the blank can be controlled to satisfy this requirement.
 
Observations of the microstructure at various stages during consolidation suggested that pre-heating of the material resulted in pools of coalesced matrix both within and between the tows. At the end of consolidation the remaining voids were predominantly in the matrix rich regions between tows.
 
An integrated modelling approach is required for consolidation of commingled fabric. Current models concentrate almost solely upon the matrix flow phase. The starting point for consolidation must first be determined, in terms of both the proportion and distribution of reinforcement fibres, coalesced pools of matrix, and dry areas. The effect of inter-fibre spacing on the matrix viscosity requires incorporation into an integrated model to determine consolidation pressure as a function of rate. Modelling of the kinetics of the dissolution of voids into the matrix is required for prediction of the holding time for full consolidation at a given pressure or the prediction of a final void content for given conditions of time at pressure.
 
结论-Conclusion
 
In this report we have reviewed the use of hybrid yarns in composites. Hybrid yarns are one of the most promising technologies to solve the main problem in the production of thermoplastic composites being the high viscosity. Since textile prefabs produced from hybrid yarns contain the matrix material in fibre form these thermoplastic matrix fibers should be evenly distributed among the reinforcing fibers to make the flow distance as short as possible. Hybrid yarns often give rise to better impregnation and drapebility than other preform techniques such as stitch bonding co weaving or powder impregnation.
 
To achieve this these characteristics various hybrid yarn types have been developed such as side by side friction spinning, co-warping, yarn twisting and commingling. The most important hybrid yarn types where described and compared. Most research so far has been done on the commingling process, since this one is considered to be one of the most promising of these production processes. Although good results for some of the other hybrid yarn types where reported as well.
 
The commingling process was studied in more detail and the structure of the commingled yarn was evaluated. Imaging studies have shown that commingled yarns have a nipped structure (alternating open en and compacted fibers), the frequency and geometry of these nips have a large influence on the properties of the final composite. The degree of interlacing has been defined (=ratio of the total length of nipped portion to the total length of the yarn) as a parameter to describe this nip structure a correlation between this parameter and the void content was reported. The void content of composites made with commingled yarns decreases is related to the degree of interlacing; initially it decreases with increasing degree of interlacing and finally stabilizes when the degree of interlacing is about 15.
 
Furthermore a summary of the research done to describe the effect the commingling process parameters air pressure take up speed and overfeed on the nip structure was given. Often the combined effect of these 3 parameters on the yarn must be taken into consideration. It has been observed that high air pressure at 1% overfeed with lower take-up speed gives the best combination in terms of processing glass/polypropylene hybrid yarn.
 
Compression modling is often used to produce composite structures from hybrid yarns, In the final part of this report we described the different steps in this process. The consolidation step is time determining step in this process. We described this process step in more detail and recognized that lots of effort has been made to model these consolidation phenomena.
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