## 1 Introduction  
Nowadays the number and amount of polymers produced from renewable resources are increasing considerably. These polymers are also readily available [1]. Global market forecasts predict a fourfold increase until 2019 (7.8 million ton/year) [2]. Polymer foams nowadays are mostly produced from petroleum-based materials. Foamed products, made by physical [3], chemical [4] and bead foaming techniques [5], in addition there are innovative examples of metallic foams as well [6]. Although physical foaming technique produce a lower foam density (density reduction up to 80%), chemical foaming agents can be used without modifying the extruder [7]. Foaming can be divided into four sections. In the initial phase, the gas enters the polymer matrix and the two phases mix. In the next section, the matrix material is saturated with the foaming gas at an elevated temperature and pressure. In the third section, the homogeneous system expands as the pressure decreases. In the final section, the structure stabilizes [8]. An important foaming method is chemical foaming, when the foaming agent requires certain conditions to decompose and in the process a gas phase is generated as a result of a chemical change. It is important to choose the decomposition temperature range of the foaming agent to match the processing temperature of the matrix polymer. Chemical blowing agents can be used in different temperature ranges. Their decomposition products are various gases. The most important is the gas that is produced in the largest quantity [9]. Foaming agents can be endothermic and exothermic. The most often used exothermic foaming agent is azodicarbonamide (ADCA), even though ADCA decomposes at a relatively high temperature, at 230 \({}^{\circ}\)C. When 5% 4,4'-oxybis(benzenesulfonylhydrazide) (OBSH) is added, it reduces the initial temperature of decomposition to 205 \({}^{\circ}\)C. However, this temperature is still higher than the processing temperature of some very common polymers (e.g., P-PVC, U-PVC, EVA and PLA). With the use of activators (zinc stearate, zinc oxide, naphthenate, urea or benzoate), the decomposition temperature range can be reduced to as low as 40 \({}^{\circ}\)C. The decomposition process typically yields 220 cm\({}^{3}\)/g gas. Its decomposition products are nitrogen (N\({}_{2}\)) 65%, carbon monoxide (CO) 24%, carbon dioxide (CO\({}_{2}\)) 5% and ammonia (NH\({}_{3}\)) 5% [10; 11]. It can produce larger cells, greatly reduce density from 1.24 g/cm\({}^{3}\) to 0.599 g/cm\({}^{3}\) (in case of neat, extruded PLA) but the yellowish colour of the foaming agent affects the colour of the product [12]. The most often used endothermic foaming agent is a mixture of sodium bicarbonate and citric acid. It is typically whitish, can produce finer cells and less reduction in density from 1.24 g/cm\({}^{3}\) to 0.645 g/cm\({}^{3}\) in case of neat, extruded PLA [12; 13; 14; 15; 16]. Sodium bicarbonate starts to decompose at around 160 \({}^{\circ}\)C, while citric acid only starts decomposing at around 210 \({}^{\circ}\)C. About 120 cm\({}^{3}\)/g of gas is produced. Foaming agents working in different temperature ranges can be produced by mixing the two components at different ratios [17; 18]. Such CBAs typically have water among their decomposition products. If the polymer matrix is sensitive to moisture (e.g., polycarbonates or polyesters), a CBA should be chosen which does not have water among its decomposition products (or only a very small amount) because the water can hydrolyse the polymer [9]. A great disadvantage of petroleum-based polymeric materials and products made from them is that they are difficult and costly to recycle, cannot be decomposed biologically and are a considerable load on the environment after they lose their function. Renewable resource-based and biodegradable polymers offer an environmentally friendly alternative. Of all biopolymers, poly(lactic acid) (PLA) receives the most attention nowadays [19].