Development of high-performance glass fibre-polypropylene composite laminates: Effect of fibre sizing type and coupling agent concentration on mechanical properties

Abstract

A selection of woven roving and woven filament yarn E-glass fabrics, equipped with different sizing and finish formulae, were tested for compatibility with maleic anhydride functionalised polypropylene (MAH-g-PP) matrices. Glass fibre-polypropylene laminates (GF-PP) were prepared at 50% fibre volume fraction via film stacking and flat panel compression moulding. The effectivity of the utilised coupling agent concentration on interfacial adhesion was characterised through indirect method in 3-point flexural tests. Besides aminosilane A-1100 and PP-compatible fabrics, FK144 Volan “A” chromium complex finished fabrics showed exceptional compatibility and performance in combination with MAH-g-PP modified PP matrices, which has not been reported in the literature before. Mean flexural properties of FK144 laminates peaked at 450 MPa for strength and 21 GPa for modulus at 0.10% MAH loading and a fibre volume fraction of 53%. Possible coupling mechanisms were discussed, which may explain the basic compatibility between the chromium complex and PP.

Introduction

In recent years, industrial interest has been focusing on finding weight reduction measures for transportation means, thereby improving upon energy efficiency and fuel economy. Through re-design and material substitution of specific components with lighter materials, substantial efforts are being undertaken towards reduced environmental impacts across vehicles life cycles. Continuous fibre-reinforced thermoplastics represent a lightweight structural material group, which are gaining popularity in various technical applications, especially in the automotive sector. The possibility of sub-one-minute thermoforming cycles makes these materials very attractive for high-volume production. At the same time, fibre-reinforced thermoplastics meet the requirements of circular economy concepts due to general recyclability and therefore higher sustainability compared to thermoset composites.

Pre-impregnated glass fibre-polypropylene (GF-PP) laminates, also known as “GF-PP organo sheets”, come in a favourable cost to performance ratio as an engineering material. They are suited for use in mild temperature conditions with typical areas of application below 100 °C. Chemical resistance, moisture resistance and basically unlimited shelf life, in addition to excellent thermoforming characteristics are just a few advantages of polypropylene (PP) composite laminates. However, material development of high-quality pre-impregnated laminates poses many challenges in terms of base material selection, given by the large quantity of available fibrous products, polypropylene types, additives and combinations thereof.

Owing to the nonpolar nature of polypropylene, glass fibre-reinforced polypropylene composites made of unfunctionalised matrices, suffer from poor fibre adhesion and therefore low mechanical properties [1]. To promote fibre adhesion, a coupling agent such as maleic anhydride grafted polypropylene (MAH-g-PP) may be introduced into the matrix, which provides some degree of polarity, reactivity and compatibilisation of the two phases [2]. The addition of up to 10 wt% MAH-g-PP concentrate in the matrix phase for short-, long- or continuous fibre glass reinforced PP is common practice documented in literature [3], [4], [5], [6], [7]. The amount of coupling agent to be blended with the PP base polymer depends, besides the graft level, viscosity and fibre content, in practice mainly on the cost factor and the mechanical requirements of the composite. For this reason, PP composites can in a certain performance range be mechanically fine-tuned fairly easily compared to other polymer systems. However, high mechanical performance of GF-PP composites cannot be expected solely by the addition of MAH-g-PP. A fundamental prerequisite for an MAH-g-PP modified PP matrix to yield high-performance composites is the ability to properly wet the reinforcement fibre surface, chemically react with it and entangle (interdiffusion) with the bulk of the polymer [8], [9]. In general, adhesion at the fibre interface occurs mainly due to: physical adhesion related to wettability and compatibility which are controlled by the surface energies of the materials, and chemical bonding which depends on the chemical functionality of the matrix and the fibre coatings [10]. Mechanical interlocking (mechanical adhesion) due to surface roughness is less important in the case of glass fibre reinforcement.

Continuous glass fibre strands which are used for fabric weaving can be differentiated by their fibre diameter and linear densities into roving and filament yarn. Roving refers to strands with filaments typically over 13 µm in diameter and linear densities of 300–4800 Tex. Filament yarn refers to strands with filaments typically under 14 µm in diameter and linear densities under 300 Tex. Roving fabrics and filament yarn fabrics can be further differentiated by the method of coating application, either by sizing or finishing. It is important to note that the terms sizing and finish shall not be used interchangeably. Direct roving fabrics, which are used in this work, are equipped with a permanent coating immediately applied after melt spinning to the rovings, called a direct sizing. Apart from film formers and the important silane adhesion promoters, direct sizings may contain several other additives, including antistatics and lubricants which facilitate weaving and prevent fibre breakage [11]. Sizing formulations are proprietary and no information on the exact adhesion promoter is given.

Filament yarns are used to produce tightly woven fabrics, usually with lower grammage compared to roving fabrics. In certain cases, filament yarns may be equipped with a direct sizing for immediate use after weaving, however, more common is the use of filament yarns for the finishing process [12]. Filament yarns used for finishing contain a specially formulated protective coating which needs removal after weaving. This woven (loomstate) product is cleaned by either heat or chemical treatment, followed by re-coating of the fabric with a chemically pure adhesion promoter, thereafter, called a finished fabric. Silane-finished fabrics used in this work are silane-only coatings and do not contain any other additives. The active adhesion promoter applied in a finish is generally disclosed, e.g. “Z6040 epoxypropylsilane” [12], [13], [14]. A finish can be tailored to yield compatibility towards certain resins. Fibre wettability is an important topic and adhesion to fibres may be impeded if an incompatible coating is selected in terms of surface energy. For bonding to occur, which is important for effective stress transfer, the fibres and matrix must be brought into intimate contact; wettability can therefore be regarded as an essential prerequisite for chemical bonding [10], [15]. An optimised fibre coating can make the difference between a low- or high-performance composite as for example Sambale, Schönreich and Stommel concluded [16]. A wide range of fibre adhesion promoters are available, most of which were specifically developed for thermoset resin systems in the form of reactive silanes. The majority of silane-based fibre coatings are highly heat resistant (200–300 °C) in oxidising atmospheres, which makes them also suitable for thermoplastic composite processing [17], [18]. A lot of research has gone into aminosilane coated fibres, which are most commonly referred to as very good adhesion promoters for a variety of thermoplastics, especially for maleated polypropylene [11]. Silanised fabrics with organofunctionalities other than amino, such as epoxy, vinyl, or methacrylic and non-silane-based fabrics containing chromium complexes are easily available, but not sufficiently analysed for polypropylene compatibility [14], [19].

Nevertheless, especially for compression moulded continuous GF-PP composites, the influence of fibre coatings and coupling agent was not investigated in detail until now. Therefore, the aim of this work was to investigate the influence of MAH coupling agent content in combination with different fibre coating types on the mechanical properties and to reach laminate strengths at least in the range of an industrial GF-PP laminate product. Initially two fabrics were compared, a roving fabric with a PP-compatible sizing and a filament yarn fabric containing a chromium methacrylate complex finish called Volan. Laminates made of these fabrics were tested in 3-point flexural tests in an MAH-concentration range between 0.02 and 0.10% in 0.02% steps. Through these measurements, a reasonable amount of coupling agent could be assessed which was then used for a fibre coating study comparing eight further coating types. Apart from mechanical optimisation and characterisation, a processing method is presented in the final chapter of the paper for drastically improved laminate surface quality comparable to industrial standards.